Ion Exchangers. FRANÇOIS DE DARDEL, Rohm and Haas, Paris, France THOMAS V. ARDEN, Cobham, United Kingdom

Transcription

1 Ion Exchangers FRANÇOIS DE DARDEL, Rohm and Haas, Paris, France THOMAS V. ARDEN, Cobham, United Kingdom 1. Introduction Structures of Ion-Exchange Resins Polymer Matrices Functional Groups Cation-Exchange Resins Anion-Exchange Resins Other Types of Ion-Exchange Resins Adsorbent Resins and Inert Polymers Properties Degree of Cross-Linking and Porosity Exchange Capacity Stability and Service Life Density Particle Size Moisture Content Ion-Exchange Reactions Cation Exchange Anion Exchange Cation and Anion Exchange in Water Treatment Ion-Exchange Equilibria Dissociation and pk Value Mono Monovalent Exchange Mono Divalent Exchange (Water Softening) General Case Exchange Kinetics Principles Kinetic Curves Strongly Acidic or Strongly Basic Resins Film Diffusion Particle Diffusion Weakly Acidic or Weakly Basic Resins Practical Consequences of Ion-Exchange Equilibrium and Kinetics Operating Capacity, Regeneration Efficiency, and Regenerant Usage Permanent Leakage Water Analysis Calculations in the Design of Ion- Exchange Plants for Water Purification Example of Calculation Principle Basic Data Demineralization Unit Polishing Unit Industrial Use of Ion Exchange Description of the Ion-Exchange Cycle Methods for Overcoming Equilibrium Problems Ion-Exchange Resin Combinations Pretreatment Softening Demineralization (Primary System) Polishing Choice of Resin Plant Design General Considerations Fixed-Bed Ion-Exchange Units Column Diameter and Bed Depth Small-Scale Units Industrial Co- and Counterflow Plants Mixed Beds Other Ion-Exchange Polishers Continuously Circulated Ion-Exchange Resins External Valves and Pipework Control Systems Special Processes in Water Treatment Removal of Organic Matter Treatment of Potable Water Treatment of Brackish Water Processes Involving Sea Water Treatment of Condensates Conventional Resins Powdered Resins Water Treatment in the Nuclear Industry Production of Ultrapure Water Special Applications of Ion Exchange Processing Steps Purification Ion Substitution Recovery and Concentration Separation Diffusion Catalysis Dehydration Coalescence on Oleophilic Resins Liquid Ion Exchangers Ion-Exchange Membranes Technical Considerations References Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: / a14_393.pub2

2 474 Ion Exchangers Vol. 19 Abbreviations and Acronyms: BV: bed volume DG: atmospheric degasser DM: dry matter DVB: divinylbenzene EMA: equivalent mineral acidity eq: equivalents FMA: free mineral acidity FR: flow rate IN: inert resin MHC: moisture-holding capacity R: resin SAC: strongly acidic cation exchanger SBA: strongly basic anion exchanger SBC: strongly basic capacity TAlk: total alkalinity TDS: total dissolved solids TH: total hardness TOC: total organic carbon U.C.: uniformity coefficient WAC: weakly acidic cation exchanger WBA: weakly basic anion exchanger Symbols: C P : weight capacity, eq/l C V : volume capacity, eq/l D: diffusion coefficient, mol/cm 3 J: ion flux, mol s 1 cm 2 k: mass-transfer coefficient, cm/s K: equilibrium constant K B A : selectivity coeffient between components A and B N: fouling factor P: weight of resin Q: flow rate, m 3 /h r: radius of ion-exchange bead S: salinity, meq/l t: operating time, h V: volume of resin, m 3 X: mole fraction a B A : separation factor between components A and B g: activity coefficient d: thickness of Nernst film 1. Introduction Definition and Principles. In ion exchange, ions of a given charge (either cations or anions) in a solution are adsorbed on a solid material (the ion exchanger) and are replaced by equivalent quantities of other ions of the same charge released by the solid. The ion exchanger may be a salt, acid, or base in solid form that is insoluble in water but hydrated. Exchange reactions take place in the water retained by the ion exchanger; this is generally termed swelling water or gel water. The water content of the apparently dry material may constitute more than 50 % of its total mass. Figure 1 shows the partial structure of a cation exchanger; each positive or negative ion is surrounded by water molecules. Ion exchange forms the basis of a large number of chemical processes which can be divided into three main categories: substitution, separation, and removal of ions. 1. Substitution. A valuable ion (e.g., copper) can be recovered from solution and replaced by a worthless one. Similarly, a toxic ion (e.g., cyanide) can be removed from solution and replaced by a nontoxic ion. 2. Separation. A solution containing a number of different ions passes through a column containing beads of an ion-exchange resin. The ions are separated and emerge in order of their increasing affinity for the resin. 3. Removal. By using a combination of a cation resin (in the H þ form) and an anion resin (in the OH form), all ions are removed and replaced by water (H þ OH ). The solution is thus demineralized. Historical Aspects. The discovery of ion exchange dates from the middle of the nineteenth century when THOMSON [1] and WAY [2] noticed that ammonium sulfate was transformed into calcium sulfate after percolation through a tube filled with soil. In 1905, GANS [3] softened water for the first time by passing it through a column of sodium aluminosilicate that could be regenerated with sodium chloride solution. In 1935, LIEBKNECHT [4] and SMIT [5] discovered that certain types of coal could be sulfonated to give a chemically and mechanically stable cation exchanger. In addition,

3 Vol. 19 Ion Exchangers 475 Figure 1. Structure of a cation exchanger that exchanges H þ for Na þ ions Swelling water is represented in the inset. ADAMS and HOLMES [6] produced the first synthetic cation and anion exchangers by polycondensation of phenol with formaldehyde and a polyamine, respectively. Demineralization then became possible. At present, aluminosilicates and phenol formaldehyde resins are reserved for special applications and sulfonated coal has been replaced by sulfonated polystyrene. Polystyrene Resins. The first polystyrenebased resin was invented by D ALELIO in 1944 [7]. Two years later, MCBURNEY produced polystyrene anion-exchange resins by chloromethylation and amination of the matrix [8]. The anion exchangers known until then were weakly basic and took up only strong mineral acids. The new resins produced by the McBurney process were stronger bases and could adsorb weak acids such as carbon dioxide or silica, allowing complete demineralization of water with a purity previously obtainable only by multiple distillation in platinum. Even today, ion exchange is still the only process capable of producing the water quality needed for highpressure boilers. Reverse osmosis and electrodialysis can demineralize solutions with % efficiency. Only ion exchange can polish the predemineralized solution with a demineralization efficiency of % Macroporous Resins. Two of the problems encountered in the use of ion-exchange resins are the fouling of the resin by natural organic acids present in surface waters and the mechanical stress imposed by plants operating at high flow rates. To cope with these, three manufacturers [9 11] invented resins with a high degree of cross-linking but containing artificial open pores in the form of channels with diameters up to 150 nm that can adsorb large molecules. Resins in which the polymer is artificially expanded by the addition of a nonpolymerizable compound that is soluble in the monomer are known as macroporous or macroreticular resins (see Section 3.1). Other naturally porous resins are known as gel resins. Polyacrylic Anion Exchangers. Between 1970 and 1972, a new type of anion-exchange resin with a polyacrylic matrix appeared on the market. This possesses exceptional resistance to organic fouling and a very high mechanical stability due to the elasticity of the polymer. Uniform Size Resins. In the 1980s and 1990s, several producers developed new manufacturing

4 476 Ion Exchangers Vol. 19 technologies aimed at producing resins with particles of almost identical size. 2. Structures of Ion-Exchange Resins An ion exchanger consists of the polymer matrix and the functional groups that interact with the ions. This article deals only with organic ion exchangers; inorganic ion exchangers are of minor importance and are primarily layer silicates and zeolites (! Silicates,! Zeolites). Other Types of Matrix. matrix include Other types of 2.1. Polymer Matrices Polystyrene Matrix. (! Polystyrene and Styrene Copolymers). The polymerization of styrene [ ] (vinylbenzene) under the influence of a catalyst (usually an organic peroxide) yields linear polystyrene [ ]. Linear polystyrene is a clear moldable plastic which is soluble in certain solvents (e.g., styrene or toluene) and has a well-defined softening point. If a proportion of divinylbenzene is mixed with styrene, the resultant polymer becomes cross-linked and is then completely insoluble. 1. Phenol formaldehyde resins (! Phenolic Resins) which show interesting adsorption properties 2. Polyalkylamine resins, obtained from polyamines by condensation with epichlorohydrin, which gives an anion exchanger directly in a single step Functional Groups Cation-Exchange Resins Cation-exchange resins in current use can be separated into two classes according to their active groups: 1. Strongly acidic (sulfonic groups) 2. Weakly acidic (carboxylic groups) Strongly Acidic Cation-Exchange Resins. Chemically inert polystyrene beads are treated with concentrated sulfuric or chlorosulfonic acid to give cross-linked polystyrene 3-sulfonic acid. This material is the most widely used cationexchange resin and is strongly acidic. In the manufacture of ion-exchange resins, polymerization generally occurs in suspension. Monomer droplets are formed in water and, upon completion of the polymerization process, become hard spherical beads of the polymer. Polyacrylic Matrix. Matrices for ion exchangers can also be obtained by polymerizing an acrylate, a methacrylate, or an acrylonitrile, any of which can be cross-linked with divinylbenzene [ ] (DVB) (! Polyacrylamides and Poly(Acrylic Acids));! Polyacrylates, Section 3.1. Examples: Amberlite IR 120, Dowex HCR, Lewatit S100.

5 Vol. 19 Ion Exchangers 477 Weakly Acidic Carboxylic Cation-Exchange Resins. The weakly acidic resins are almost always obtained by hydrolysis of polymethylacrylate or polyacrylonitrile to give a poly (acrylic acid) matrix. Examples: Amberlite IRC 86, Lewatit CNP Anion-Exchange Resins Polystyrene Materials. Cross-linked polystyrene beads are treated with chloromethyl methyl ether under anhydrous conditions, with either aluminum chloride or tin(iv) chloride as catalyst. Chloromethylated polystyrene is obtained: In a second stage, the chlorine in the chloromethylated group can be replaced by an amine or even by ammonia. Depending on the reaction selected, the anion exchanger obtained may be strongly to weakly basic. The degree of basicity can be made to measure because of the large number of amines available. The anion exchangers listed below are arranged in order of decreasing basicity: Resins with quaternary ammonium groups are strongly basic. Those with benzyltrimethylammonium groups are known as type 1 and are the most strongly basic, whereas those with benzyldimethylethanolammonium groups are known as type 2 and are slightly less basic. Type 1 resins are used when total removal of anions, even those of weak acids (including silica), is essential. Type 2 resins are also basic enough to remove all anions, but they release the anions more easily during regeneration with caustic soda; as a result, they have a high exchange capacity and a better regeneration efficiency (see Section 7.1). Unfortunately, they are chemically less stable and produce greater silica leakage than type 1 resins. Resins whose active group is an amine are generally denoted as weakly basic, although their basicity may vary considerably. Tertiary amines are sometimes called medium-base or intermediate-base resins, whereas primary amines are very weakly basic and are rarely used. The most widely used weakly basic resins contain tertiary amino groups and adsorb any strong acids present in the solution to be treated but do not affect neutral salts or weak acids. Manufacturers do not always indicate the chemical structure of their exchangers in their literature. Care should therefore be taken not to assume that resins are chemically identical merely because they have similar general characteristics. Secondary and Tertiary Cross-Linking. During chloromethylation, a side reaction may occur in which the chloromethyl group of a chloromethylated benzene ring reacts with an unconverted ring, to yield a methylene bridge. These bridges form additional cross-links in the polystyrene matrix: where R can be CH 2 N þ (CH 3 ) 3 Cl (type 1 resin) CH 2 N þ (CH 3 ) 2 CH 2 CH 2 OHCl (type 2 resin) CH 2 N(CH 3 ) 2 e.g., Amberlite IRA402 e.g., Amberlite IRA410 e.g., Amberlite IRA96 The amount of this secondary cross-linking can be adjusted by varying the conditions (quantity and type of catalyst, temperature) of the chloromethylation reaction. Most strongly basic and weakly basic polystyrene resins have some degree of secondary cross-linking.

6 478 Ion Exchangers Vol. 19 Furthermore, during the amination of weakly basic resins, another type of cross-linking may be produced. This is called tertiary cross-linking and yields strongly basic quaternary groups in addition to the weakly basic tertiary groups. Polyacrylic Resins. Polyacrylic resins are manufactured in a manner analogous to that used for polystyrene resins. Beads are prepared from an acrylic ester copolymerized with divinylbenzene by using suspension polymerization and free-radical catalysis. The polyacrylate formed is then given active groups by reaction with a polyfunctional amine containing at least one primary amino group and one secondary or, more frequently, tertiary amino group. The primary amino group reacts with the polyester to form an amide, whereas the secondary or tertiary amino group forms the active group of the anion exchanger. This method always yields a weakly basic exchanger, which can be further treated with chloromethane or dimethyl sulfate to give a quaternary strongly basic resin: chosen as the starting material and the polyamine used for activation. In practice, the range is limited by the availability and cost of raw materials Other Types of Ion-Exchange Resins By using polymerization and activation methods analogous to those described above, a wide variety of functional groups can be grafted onto a given polymer. Some of these groups can be used for selective uptake of ions, principally metals (Table 1). The thiol group forms very stable bonds with certain metals, particularly mercury. The iminodiacetic, aminophosphonic, and amidoxime groups form metal complexes whose stability depends mainly on the ph of the solution. Selective adsorption of certain metals can thus be achieved by varying the ph. These types of material are known as chelating or complexing resins. The N-methylglucamino group is used to make resins specific for boric acid, which is taken up as a complex Adsorbent Resins and Inert Polymers Strictly speaking, adsorbent resins are not ion exchangers but resemble them very closely. They have a high porosity and are used for the adsorption of nonionic or weakly ionized species as a complement to ion exchange. They may have cation- or anion-exchange groups or no ion- Table 1. Principal active groups of ion exchangers used for selective uptake of metals Active group Formula * Example Thiol SH Ambersep GT74 Iminodiacetic acid CH 2 N(CH 2 COOH) 2 Lewatit TP207 Aminophosphonic acid CH 2 NHCH 2 CH 2 PO 3 H Amberlite IRA747 Amidoxime Duolite ES346 N-Methylglucamine Amberlite IRA743 In principle, a wide range of anion-exchange resins can be obtained by varying the type of ester * The active groups are substituents (R) of polystyrene with the following formula:

7 Vol. 19 Ion Exchangers 479 exchange groups at all. The latter are ionically inert. In order of decreasing polarity, adsorbent resins can be classified in the following manner: 1. Ionized adsorbents are strongly basic exchangers used in chloride form for color removal from sugar juices or as organic scavengers (see Section 11.1, e.g., Amberlite IRA958). 2. Phenolic adsorbents contain weakly basic amine and phenolic groups or phenolic groups, only. They are used to remove color bodies (colored impurities) from solutions of organic acids and food-processing streams (e. g., Duolite A561, XAD761). 3. Inert adsorbents are macroporous copolymers of styrene and divinylbenzene with a very high degree of cross-linking and a large surface-to-volume ratio. These resins are used to remove organic, weakly ionized, or nonionic substances, such as phenols, chlorinated solvents, antibiotics, and complexing agents, from aqueous or organic solutions (e.g., Amberlite XAD4, Diaion HP20). Inert polymers without measurable porosity and without active groups can be used either to separate two resin layers or to keep a resin separate from a collector system. 3. Properties 3.1. Degree of Cross-Linking and Porosity An increase in the degree of cross-linking (i.e., the weight percentage of DVB related to the total amount of monomer prior to polymerization) produces harder, less elastic resins. Resins with higher degrees of cross-linking show more resistance to oxidizing conditions that tend to de-crosslink the polymer. Above % DVB, however, the structure becomes too hard and dense. Activation (i.e., chemical transformation of the inert copolymer into an ion-exchange resin) becomes difficult because access to the interior of the bead is hindered by the high density of the matrix. In addition, osmotic stress cannot be absorbed by the elasticity of the structure, which causes the bead to shatter. Finally, the rate of exchange increases in proportion to the mobility of the ions inside the exchanger bead: if the structure is too dense, ionic motion is slowed down, thus reducing the operating capacity of the resin. For sulfonic resins, maximum operating capacity (Section 3.2) is obtained with approximately 8 % DVB. Cross-Linking and Affinity. The greater the ionic mobility in the resin, the poorer is the differentiation between the adsorption of ionic species with the same charge. Consequently, the degree of cross-linking in the resin must be increased when greater differences in ionic affinity are required. In water treatment, the sulfonated polystyrene resins usually have a DVB concentration of ca. 8 %. Resins with a slightly higher degree of cross-linking (10 12 %) are sometimes used to increase the retention of mineral ions when water of very high purity is being produced. Nonuniformity in the Matrix. Cross-linking reduces the retention of water in ion-exchange resins (Section 3.6). The volume occupied by this water is a measure of the resin s porosity. Crosslinking is not uniform because the DVB DVB reaction is more rapid than that between DVB and styrene. Polymerization begins to occur around the catalyst molecules, and polymer growth is faster at sites rich in DVB than at those rich in styrene. Material with an average of 8 % DVB may contain local microscopic regions with more than 20 % DVB, whereas other regions may have less than 4 %. Macroporous resins are made by mixing the monomers with a compound (e.g., heptane, saturated fatty acids, C 4 C 10 alcohols or polyalcohols, or low molecular mass linear polystyrene) which expands the resin. The substance does not itself polymerize and, thus, although it acts as a solvent for the monomers, it causes the polymer to precipitate from the liquid. Channels are formed inside the beads, producing an artificially high porosity. Resins containing such channels are described as macroporous, whereas other resins with natural porosity are known as gel resins (Fig. 2). Macroporous resins have a higher degree of cross-linking than gel resins to strengthen the matrix and compensate for voids left by the added solvent. The porosity and mechanical strength of the resin can be modified by varying the degree of cross-linking or the amount of

8 480 Ion Exchangers Vol. 19 Table 2. Typical capacities of ion-exchange resins * Type Amberlite C p, eq/kg C v, eq/l Figure 2. Arrangement of structural units in gel (A) and macroporous (B) resins solvent added. Therefore, various macroporous resins are available, with different moistureholding capacities and internal structures. The pore diameter is ca. 100 nm in a macroporous resin and ca. 1 nm in a gel resin. The macropores form a network of channels filled with free water, and large molecules can move freely in the resin into the center of a bead. Once inside the resin, ions generally have a much shorter distance to travel before they encounter an active group: ca. 100 nm in macroporous resins and up to 500 mm in gel resins. Exchange is thus faster in a macroporous resin. Macroporous resins are highly resistant to physical stress and generally withstand osmotic shock very well. They are therefore used in systems where mechanical and osmotic stress would otherwise cause gel resins to deteriorate rapidly, such as those involving circulation of resin, fluidized beds, high flow rates, oxidizing conditions, concentrated solutions, and short cycles. Finally, macroporous resins are used when reversible uptake of large molecules is necessary, without fouling the resin. The adsorbents described in Section 2.3 have a macroporosity that allows selective retention of various molecules Exchange Capacity Total Capacity. The total exchange capacity of a resin, expressed in equivalents per unit weight (or per unit volume), represents the number of active sites available. In polystyrene exchangers, the maximum number of active sites corresponds to the grafting of one active group per benzene ring. The capacity is expressed in equivalents (eq) per kilogram of dry resin (the weight capacity C p ) or equivalents per liter of wet settled resin (the volume capacity C v ). Total capacity values for some of the most common resins are given in Strongly acidic gel, IR (Na) 2.05 (Na) 8 % DVB Weakly acidic gel IRC (H) 4.2 (H) Strongly basic, type 1 IRA (Cl) 1.3 (Cl) Strongly basic, type 2 IRA (Cl) 1.3 (Cl) Strongly basic acrylic IRA (Cl) 1.3 (Cl) Weakly basic styrene IRA (free base) 1.25 (free base) Weakly basic acrylic IRA (free base) 1.6 (free base) * Values depend to some extent on the analytical method used for capacity determination. Macroporous strongly acidic resins have a dry weight capacity of eq/kg. Their volume capacity depends greatly on their porosity, which can be adjusted within relatively broad limits. Similarly, macroporous strong base resins have dry weight capacities close to their gel counterparts, but generally lower volume capacities. Table 2. Equivalent resins of other brands have similar capacity values (see Table 10). Operating Capacity. The operating capacity is defined as the proportion of total capacity used during the exchange process. It can amount to a large or small proportion of the total capacity and depends on a number of process variables including 1. Concentration and type of ions to be absorbed 2. Rate of percolation 3. Temperature 4. Depth of resin bed 5. Type, concentration, and quantity of regenerant In a packed column, reaction between the ions in solution and those in the resin occurs over a welldefined region of the resin bed known as the reaction zone. When the selectivity of a resin for a dissolved ion is high (see Chap. 5), a sharp exchange wavefront is formed which moves toward the column outlet, making maximum use of the resin. The depth of the reaction zone depends on factors such as flow rate (kinetics) and ionic concentration, but is independent of column length [12]. When selectivity is low, a diffuse wavefront develops. The depth of the reaction zone then depends on the selectivity coefficient (Section 5.2) and also on the bed depth. The longer the column, the deeper is the reaction zone and the greater is the operating capacity of the resin. Figure 3 illustrates this: the top of the column contains the completely exhausted resins (a), whereas the reaction zone (b) contains partially exhausted resin. The time at which the lowest point of this zone reaches the bottom of the

9 Vol. 19 Ion Exchangers 481 Figure 3. Reaction zone in a resin column during percolation a) Exhausted resin; b) Reaction zone; c) Regenerated resin column (i.e., when the adsorbed ions break through the bottom of the bed) is generally taken to be the time at which the service phase is complete. A proportion of the resin is still not exhausted at the time of breakthrough. In practice, strongly acidic and strongly basic resins are never 100 % regenerated at the beginning of a cycle. The operating capacity thus represents the difference between the available capacity at the beginning of a cycle and that remaining at the end point. The most important factor is the amount of regenerant used to convert the resin to the regenerated form required at the beginning of the service cycle (c). The calculation of capacity is described in Chapter 7. oxidants. However, 1 mg/kg of chlorine oxidizes the polymer at a rate dependent on temperature; this breaks down the cross-linking, releases sulfonated organic compounds and causes the resin to swell until it softens, resulting in excessive head loss (sloughage) [13]. When oxidizing agents are present, highly cross-linked resins with a greater resistance to oxidation, such as the macroporous resins, should be used. Degradation products from a cation-exchange resin may foul anion resins [14, 15]. This is particularly critical in processes designed to produce ultrapure water (Fig. 4). Oxidants break the cross-links to produce soluble, short-chain oligomers that can be measured as the total organic carbon (TOC) in the treated water (Fig. 4 A). The 3.3. Stability and Service Life Because ion-exchange resins must give several years of service, their stability over long periods of time is of prime importance. Chemical Stability of the Matrix. Industrially available resins have a degree of crosslinking high enough to make them insoluble. A new resin may release minute quantities of shortchain polymers or other soluble substances, but this effect is short-lived. Highly oxidizing conditions (presence of chlorine or chromic acid) can attack the matrix and destroy cross-linking. A sulfonated polystyrene cation-exchange resin with 8 % DVB crosslinking withstands 0.2 mg/kg of chlorine at ambient temperature for several years and is also completely stable at 120 C in the absence of Figure 4. Suitability of resins for producing ultrapure water in mixed beds [14] Resins were tested at ambient temperature during first chlorine exposure by using an influent with 0.30 mg/l active chlorine dosed as NaOCl and mg/l TOC. Resins: a) Gel cation porous gel anion; b) Gel cation standard gel anion; c) Macroporous cation porous gel anion; d) Macroporous cation standard gel anion; e) Macroporous cation macroporous anion; f) Macroporous cation developmental anion; g) Gel cation developmental anion

10 482 Ion Exchangers Vol. 19 oligomers bear ionized active groups and thus decrease the resistivity of the treated water (Fig. 4 B). Under normal conditions of water treatment, resins can operate continuously for many years (sometimes up to 20 years) without deterioration of their physical and chemical properties. Thermal Stability of Active Groups. [16] The sulfonic group of cation-exchange resins is extremely stable. Anion-exchange resins, on the other hand, are temperature-sensitive. When heated, Hofmann degradation may transform quaternary ammonium groups (strongly basic) into tertiary amines (weakly basic) or even destroy the active group completely. Because this reaction occurs under alkaline conditions, anion exchangers aremorestableintheformofasaltthanasabase. Strongly basic type 1 resins are the most stable; the Hofmann degradation reaction becomes significant only above 50 C (Fig. 5). At ambient temperature, these resins can last for five to seven years or more. Figure 5. Half-life of Amberlite IRA 402 as a function of temperature Half-life is the contact time of resin with hot solution required to reduce total capacity by 50 %. Type 2 resins are more liable to undergo Hofmann degradation because the hydroxyethyl group weakens the bond: At 15 C these resins lose ca. 50 % of their strongly basic groups in five years; the same effect occurs in about one year at 50 C. (A resin in which 50 % of the strongly basic groups have been converted into weak groups, often has an unchanged operating capacity because the weakly basic groups are fully capable of taking up mineral acids). Mechanical Stability. Polycondensationtype resins that are manufactured in bulk and broken up into irregular grains are comparatively fragile and used only in fixed beds (Section 10.2). Polystyrene and polyacrylic resins made by suspension polymerization are perfect spheres and suffer little damage when used in continuous moving-bed ion-exchange plants. However, mechanical strength can vary considerably from one product to another, and resin beads which are seen to have many internal cracks under the microscope are more likely to break under mechanical stress than crack-free products. Gel-type anion resins are generally weaker than cation materials and are particularly poor at withstanding compression. However, new polymerization techniques produce a more uniformly structured polystyrene matrix. As a result, gel resins with high physical stability are now available. To prevent fragile resins from breaking, it is important to keep the bed clean by frequent backwashing because the water pressure on a layer of fine debris can be as much as 60 kpa. Acrylic resins are more elastic than polystyrene materials and can normally withstand any mechanical stress encountered in practice. Macroporous

11 Vol. 19 Ion Exchangers 483 resins are often the strongest of all and are used widely for the most severe stress conditions. The less elastic resins, i.e., those with the highest degree of cross-linking (gel resins with > 8% DVB and macroporous resins with >15 % DVB), have the disadvantage that, when they do break, they explode into minute fragments, whereas other resins break into two or three usable pieces. Osmotic Stability. During ion exchange, the configuration around each active group in the resin changes: the adsorbed ion generally has a different size and, more important, a different hydration layer than the displaced ion. The resin bead may therefore swell or contract appreciably during the reaction. The stresses to which the resin is subjected during these volume changes are known as osmotic forces. They are very intense and can produce local pressures of several thousand kilopascals much greater than purely mechanical stress. Resins for industrial use must be able to withstand hundreds of cycles of exhaustion and regeneration. The nature and, hence, the strength of osmotic shock vary according to the ionic species in solution and their concentration. Higher mechanical and osmotic strengths are obtained with resins whose matrix is sufficiently strong to withstand physical shock (attrition) but sufficiently flexible and porous to deform without breaking under the effect of osmotic shock. Some macroporous resins combine both of these qualities. Resistance to Drying. Repeated drying and rewetting produce stresses analogous to those due to osmotic shock and can lead to fragmentation of most gel resins. Resins must therefore be kept permanently moist. By choosing suitable particle sizes, several different types of resin can be used in the same column. If necessary, they can be kept separate by an upflow of water. This is used mainly in layered beds (see Section 9.3). Resin density can be increased artificially, for example, by attaching chlorine or bromine atoms to the matrix. This yields high-density anion exchangers, which are useful when fluidized-bed operation is required. However, such high-density resins are not available commercially Particle Size For industrial use, particle size is a compromise between the speed of the exchange reaction (which is greater with small beads) and high flow rates (which require coarse particles to minimize the head loss). The size of the polymer droplets formed during polymerization, and hence the size of the resin beads, is determined by the polymerization technology, the suspension medium and, the monomer concentration. Traditional polymerizations are carried out in batches in a stirred reactor. The beads produced in this way have a range of particle sizes rather than a uniform size (Table 3). The population of a resin sample (i.e., the number of beads classified according to bead diameter) has an approximately Gaussian distribution. Because the volume of a fraction is a logarithmic function of the number of beads, the particle-size distribution is often represented by a straight line on probability graph paper (Fig. 6). For Gaussian distribution, the particle-size distribution is defined by 3.4. Density Resin density is an important property because it determines the hydrodynamic behavior in counterflow systems. Resin density normally lies in the following ranges (figures in parentheses are the most common values for standard resins): Strongly acidic cation exchangers (1.28) Weakly acidic cation exchangers (1.18) Strongly basic anion exchangers (1.10) Weakly basic anion exchangers (1.05) Table 3. Particle-size distribution of ion-exchange resins * Noncumulative total Cumulative total Sieve aperture, mm (between sieves), % (passing through), % < > * Effective particle size 0.50 mm, mean diameter 0.68 mm, uniformity coefficient 0.73/0.50 ¼ 1.46

12 484 Ion Exchangers Vol. 19 Several producers offer ion-exchange resins with a very uniform particle size distribution, are produced with different polymerization technologies. In one technique, the monomers are injected into the suspension medium through a plate perforated with thousands of very small holes. Droplets of the monomer mixture with an almost identical volume are expelled. The uniformity coefficient of resins produced in this way is always less than 1.2, often in the range of Examples of such resins are Amberjet (Rohm and Haas), Dowex Monosphere (Dow Chemical), and Lewatit Monoplus (Lanxess) Moisture Content Ion-exchange resins carry both fixed and mobile ions which are always surrounded by water molecules located in the interior of the resin beads. The water retention capacity governs the kinetics, exchange capacity, and mechanical strength of ion-exchange resins. The moisture content or moisture-holding capacity (MHC) is defined as Figure 6. Typical particle-size distribution in a normal resin 1. The mean diameter (corresponding to the mesh size of a sieve allowing 50 % of the beads to pass) 2. The uniformity coefficient, U.C. (given by the ratio between the aperture of a sieve allowing 60 % of the beads to pass and that of a sieve allowing 10 % to pass. The latter theoretical sieve is known as the effective size. U:C: ¼ x 60% =x 10% 3. The closer the uniformity coefficient is to unity, the narrower is the Gaussian curve and hence the smaller is the range of particle size. Resins produced with the traditional stirredreactor process usually have a uniformity coefficient of 1.5 to 1.9. MHC ¼ðP Hydr P Dry Þ=P Hydr where P Hydr is the weight of the hydrated resin sample and P Dry the weight of the same sample after drying. The MHC of an ion-exchange resin is an inverse function of the degree of cross-linking unless the porosity or degree of cross-linking in the polymer is artificially increased (as in macroporous resins). Figure 7 shows how the moisture content varies with the proportion of DVB for gel-type sulfonic polystyrene resins. Another useful quantity is the dry matter (DM) defined as DM ¼ P Dry =V Hydr where P Dry is the weight of the dry resin sample and V Hydr the volume of the sample before drying. Although they are closely connected, no simple arithmetical relationship exists between dry matter and moisture content. In all cases, the ionic form of the resin at the time of measurement should be quoted. 4. Ion-Exchange Reactions 4.1. Cation Exchange General cation exchange is used widely to remove undesirable ions from a solution without changing the total ionic concentration or ph. The resin can be used in many ionic forms, but the sodium form is usually preferred because the resin has a relatively low affinity for sodium, which facilitates the adsorption of other metals. Furthermore, sodium chloride is an inexpensive regenerant.

13 Vol. 19 Ion Exchangers 485 is sometimes called salt splitting. Regeneration is carried out with a mineral acid. Hydrogen Exchange in Weakly Acidic Resins. Carboxylic resins are such weak acids that they ionize only slightly under acid conditions. However, they have such a high affinity for divalent metal ions that in the presence of these metal ions, they are forced to ionize and therefore remain active under slightly acidic conditions down to ca. ph RCOOHþMgðOHÞ 2!ðRCOOÞ 2 Mgþ2H 2 O 2 RCOOHþNa 2 CO 3!2 RCOONaþH 2 OþCO 2 RCOOHþNaCl No reaction 2 RCOOHþCaCl 2 ðrcooþ 2 Caþ2 HCl Figure 7. Variation of moisture content (A) and total capacity (B) with the degree of cross-linking in a sulfonated polystyrene resin in sodium form The following reaction is used to treat wine (R denotes the resin): R Na þ þk þ ðhtartrateþ!r K þ þna þ ðhtartrateþ The reaction used in water softening is 2R Na þ þca 2þ ðhco 3 Þ 2!ðR Þ 2 Ca 2þ þ2na þ HCO 3 In each case, the resin is regenerated by reversal of the reaction with sodium chloride solution. Hydrogen Exchange in Strongly Acidic Resins. The replacement of metal ions with hydrogen ions leads to a reduction of the total dissolved solids in solution and the production of free acid: R H þ þna þ Cl!R Na þ þh þ Cl This reaction is used as the first stage in the demineralization of water and other solutions. It However, H þ form carboxylic resins cannot remove significant amounts of metal ions from solutions of mineral acid salts because the acid produced quickly lowers the ph and prevents further ion exchange. In this case, ion exchange is controlled by the basicity of the anion in solution. A strongly nucleophilic anion tears off hydrogen from the carboxylic group, which takes up the associated cation in exchange, whereas a stable anion does not react significantly. Because the resin has a very high affinity for divalent ions (the effect of chelation), but only moderate affinity for monovalent ions, it has a high capacity for removing calcium and magnesium from bicarbonate solution but takes up only a small amount of sodium. This occurs because sufficient carbonic acid is formed to suppress the exchange of monovalent ions: RCOOHþNaHCO 3 RCOONaþH 2 OþCO 2 2 RCOOHþCaðHCO 3 Þ 2!ðRCOOÞ 2 Caþ2H 2 Oþ2CO 2 Carboxylic exchangers (weakly acidic) are selective: they preferentially remove divalent or trivalent cations until competition arises from alkaline anions present in solution. Selective Exchange in Weakly Acidic and Complexing Resins. Because carboxylic resins take up multivalent cations in preference to monovalent ones and are regenerated very easily, they can be used for selective removal of divalent

14 486 Ion Exchangers Vol. 19 and trivalent metals, even when high concentrations of alkaline cations are present. To achieve this, the exchanger is first converted to the monovalent sodium form: RCOOHþNaOH!RCOONaþH 2 O which allows exchange of neutral salts: 2 RCOONaþZnSO 4!ðRCOOÞ 2 ZnþNa 2 SO 4 At the end of the cycle, the resin is regenerated with acid: ðrcooþ 2 ZnþH 2 SO 4!2 RCOOHþZnSO 4 and is reconverted to the monovalent sodium form by reaction with sodium hydroxide. Acid Absorption in Weakly Basic Resins. Weakly basic resins, in which the active groups are usually amines, do not have a true hydroxide form. They ionize only under acidic conditions: RNðCH 3 Þ 2 þh þ Cl!RN þ HðCH 3 Þ 2 Cl Under alkaline conditions, they exist as free bases and can therefore adsorb acids in the same way that free ammonia reacts with hydrochloric acid to form ammonium chloride. In practice, they are called the chloride form. Weakly basic resins can only adsorb strong acids. Neutral salts are not split : RNðCH 3 Þ 2 þnacl No reaction 4.2. Anion Exchange General Anion Exchange. The most widely used resin for general anion exchange is a strongly basic exchanger in chloride form: R þ Cl þna þ NO 3!Rþ NO 3 þnaþ Cl This process is used to remove natural organic acids (e.g., humic acid, see Section 7.3) and nitrate from water and in hydrometallurgy to selectively adsorb metals that form anionic complexes: n R þ Cl þðna þ Þ n Humate n!ðr þ Þ n Humate n þn Na þ Cl This is because no H þ ion exists to which the nucleophilic base can give its electrons and thus balance the anion. Similarly, weak acids do not have dissociated H þ ions, so they are taken up only in very small amounts, if at all: RNðCH 3 Þ 2 þco 2 þh 2 ORN þ HðCH 3 Þ 2 þhco 3 RNðCH 3 Þ 2 þsio 2 No reaction Weakly basic exchangers can be regenerated with ammonia or sodium carbonate: RNðCH 3 Þ 2 HClþNH 3!RNðCH 3 Þ 2 þnh 4 Cl RNðCH 3 Þ 2 HClþNa 2 CO 3!RNðCH 3 Þ 2 þnaclþnahco 3 R þ Cl þna þ ½AuðCNÞ 2 Š!R þ ½AuðCNÞ 2 Š þna þ Cl The resins are regenerated by reversal of the reactions with sodium chloride solution. Acid absorption in strongly basic resins is the most widely used form of anion exchange. When it follows hydrogen exchange in strongly acidic resins (Section 4.1), it completes the demineralizing process: R þ OH þh þ Cl!R þ Cl þh 2 O 4.3. Cation and Anion Exchange in Water Treatment (see also! Water, Ultrapure, Section 3.1.) Figure 8 summarizes the way in which the various forms of ion exchange described in Sections 4.1 and 4.2 are used in water treatment. The composition of raw water is described in detail in Section 7.3. R þ OH þco 2!R þ HCO 3 Regeneration of the bicarbonate form of the resin requires two equivalents of OH ions per equivalent of HCO 3 taken up, because half the OH neutralizes the bicarbonate and converts it to carbonate. The same applies to silica. 5. Ion-Exchange Equilibria 5.1. Dissociation and pk Value Dissociation of the acid group in a cation-exchange resin is described by the equilibrium reaction

15 Vol. 19 Ion Exchangers 487 Figure 8. Summary of the kinds of ion exchange used in water treatment SAC ¼ strongly acidic cation exchangers; SBA ¼ strongly basic anion exchangers; WAC ¼ weakly acidic cation exchangers; WBA ¼ weakly basic anion exchangers R. HR þh þ where R is the co-ion fixed in the matrix structure. The acidity of the resin is defined by its degree of dissociation at equilibrium K ¼½R Š. ½H þ Š=½RHŠ where K is the equilibrium constant; the quantities in square brackets represent concentrations, and underlines indicate the resin phase rather than the aqueous phase. The pk is defined as pk ¼ logk Cation-exchange resins can be titrated (Fig. 9) with sodium hydroxide in the presence of sodium chloride. Uptake of Na þ by the cation-exchange resin in H þ form occurs: R H þ þna þ!r Na þ þh þ ð1þ The H þ ions released by the resin combine immediately with the OH ions from the alkali

16 488 Ion Exchangers Vol. 19 gives ½Na þ Šg Na ½H þ Šg H ¼ K ½Naþ Šg Na ½H þ Šg H ð2þ Figure 9. Titration curves of cation-exchange resins a) Amberlite IR120, sulfonic acid resin; b) Amberlite IRC86 carboxylic acid resin H þ þoh!h 2 O and drive the reaction shown in Equation 1 to completion. At the beginning of titration, the strongly acidic (sulfonic) resin Amberlite IR120 exchanges sodium ions from sodium chloride, and released H þ ions lower the ph of the aqueous phase (Fig. 9, curve a). The ph remains low as long as the resin releases H þ ions to replace the H þ ions picked up from the added sodium hydroxide solution. As soon as all H þ ions have been replaced in the resin, further addition of sodium hydroxide raises the ph sharply. The weakly acidic (carboxylic) resin Amberlite IRC86 behaves differently. Because its acidic groups are weakly dissociated, the exchange reaction (Eq. 1) remains incomplete, and the ph of the solution increases progressively even during the early stages of titration (Fig. 9, curve b). A sharp rise is noticed, however, when the resin is converted completely to the Na þ form. In both cases, the total capacity of the resin can be read from the titration curves and corresponds to the amount of sodium hydroxide that produces the sharp rise in ph. In Figure 9, Amberlite IR120 has a total capacity of 2 meq/ml, whereas Amberlite IRC86 has a capacity > 4 meq/ml. Similar considerations apply to anion-exchange resins Mono Monovalent Exchange The law of mass action applied to the reaction R H þ þna þ R Na þ þh þ where [Na þ ] and [H þ ] are the equivalent concentrations in the liquid phase and g Na and g H are the corresponding activity coefficients. The concentrations and coefficients for the resin are indicated by underlines. Thus, K g Na g H g H g Na ¼ ½Naþ Š ½H þ Š ½Hþ Š ½Na þ Š The activity coefficients may be assumed constant, so that ½NaŠ ½HŠ ¼ KNa H ½NaŠ ½HŠ (the þ signs are omitted for simplicity). The parameter KH Na is known as the selectivity coefficient for the Na þ /H þ exchange. Selectivity coefficients differ for each pair of ions because the affinity of the resin for an ion is governed by the size of its hydrated form. The larger the ion, the more the resin bead must expand to accommodate it. Expansion is opposed by the restraining cross-links, so that large ions require a greater force to penetrate the resin than small ones. Table 4 lists the relative selectivities of sulfonic resins for mono- and divalent cations, and Table 5 gives the selectivities of strongly basic type 1 and type 2 resins for monovalent anions. Values increase with the degree of cross-linking and tend to unity as the cross-linking tends to zero. Data are only approximate and merely demonstrate the scale of selectivities. The degree of cross-linking in resins is not uniform, so that an ion entering a resin first seeks the most favorable regions. All exchange reactions become less favorable as they progress toward completion. The practical effects of this general phenomenon on the exchange rate, exchange capacity, and leakage from resins are considered in Chapter Mono Divalent Exchange (Water Softening) In the exchange reaction 2R Na þ þca 2þ ðr Þ 2 Ca 2þ þ2na þ

17 Vol. 19 Ion Exchangers 489 Table 4. Relative selectivities of sulfonic resins for cations Degree of cross-linking, % DVB Cation Monovalent H * Li Na NH K Rb Cs Cu Ag Divalent Mn Mg Fe Zn Co Cu Cd Ni Ca Sr Hg Pb Ba * Reference value Table 5. Relative selectivities of quaternary ammonium exchangers for monovalent anions Resin Anion Type 1 Type 2 Hydroxide * Benzenesulfonate > Salicylate Iodide Phenolate Bisulfate Chlorate Nitrate 65 8 Bromide 50 6 Cyanide 28 3 Bisulfite 27 3 Bromate 27 3 Nitrite 24 3 Chloride Bicarbonate Iodate Formate Acetate Propionate Fluoride * Reference value ½CŠ ¼½NaŠþ½CaŠ the law of mass action gives ½CaŠg Ca ½NaŠ 2 g 2 Na ¼ K ½CaŠg Ca ½NaŠ 2 g 2 Na If the activity coefficients are constant, then ½CaŠ ½NaŠ 2 ¼ ½CaŠ KCa Na ½NaŠ 2 The equivalent fraction X i of an ion i in a solution with a total concentration of ions [C ] is defined by X i ¼½iŠ=½CŠ Similarly, the equivalent fraction of ion i in the resin is ½X i Š¼½iŠ=½CŠ The total concentration of ions in the resin [C] (in equivalents per liter) is the same as the total capacity (defined in Section 3.2). In exchange involving only two ions (e.g., Na þ and Ca 2þ ), the following equations apply to the solution: X Na þx Ca ¼ 1 Similar equations hold for the resin. Equation (2) thus becomes X Ca ð1 X Ca Þ 2 ¼ KCa Na ½CŠ ½CŠ X Ca ð1 X Ca Þ 2 ð3þ where [C] is constant and KNa Ca is approximately constant at low concentrations. Equation (3) shows that as the solution becomes more dilute (decreasing C ) with a constant fraction of calcium (X Ca constant), X Ca increases. The resin takes up more calcium as the total cation concentration of the solution decreases. This effect is illustrated in Figure 10, which shows the equilibrium curves for a given resin at various total concentrations of the solution. Each point on a curve corresponds to the equivalent fraction of calcium in the resin at equilibrium, i.e., the curve gives the proportion of active sites in the resin in Ca 2þ form as a function of the proportion of calcium in solution. These curves are called ion-exchange isotherms.

18 490 Ion Exchangers Vol. 19 Figure 10. Mono divalent equilibrium curves for Na þ Ca 2þ solutions of different total concentration Total concentration of Na þ and Ca 2þ, eq/l: a) 0.005; b) 0.01; c) 0.1; d) 1; e) 5 Water Softening. For water with a total salinity of 5 meq/l, i.e. [C ] ¼ N, Figure 10 shows that water containing only 5 % Ca 2þ (X Ca ¼ 0.05 or [Ca] ¼ 0.25 meq/l) is in equilibrium with a resin loaded with 95 % calcium (X Ca ¼ 0.95). In other words, the resin is capable of removing calcium from the water even if the concentration accounts for only 5 % of total cations (X Ca ¼ 0.05), as long as the resin contains more than 5 % sodium ( X Ca < 95 %). At high concentrations, the affinity of the resin for calcium over sodium decreases until sodium becomes favored. This is the effect that makes regeneration of the resin possible. Figure 11 shows the complete softening cycle and regeneration with 12 % brine (ca. 2 N NaCl) using curves representing successive equilibrium states of the system. At the starting point O, all of the resin is in the sodium form. The water, with a total sodium and calcium content of 5 meq/l (0.005 N), comes in contact with the resin, and establishment of an equilibrium leads to the uptake of calcium by the resin and its removal from the water. The first upward-pointing arrow on the curve indicates the displacement of the equilibrium. A new equilibrium is then established at P 0 where the resin loaded with 20 % calcium is in equilibrium with completely softened water. The equilibrium Figure 11. Successive equilibria in the water-softening cycle then shifts to P, where resin loaded with 40 % calcium is still in equilibrium with completely softened water, and finally to Q where the resin loaded with 90 % calcium is in equilibrium with water which is about 98 % softened (X Ca ¼ 0.02) and therefore contains ¼ 0.1 meq/l of calcium. At this point, known as the end point, a new stage of the process is initiated to regenerate the resin (to continue as before would lead to the removal of less calcium). The system is displaced to point R on the other curve as the sodium chloride concentration changes from to 2 N. At R, the resin is in equilibrium with a solution containing about 95 % calcium and therefore tends to release calcium ions so as to produce this concentration in the liquid. In this regeneration process, the resin is converted continuously into the sodium form by exchanging the calcium ions adsorbed during the first stage for sodium ions from the regenerating brine. As the system moves toward the bottom of the curve, the equilibrium becomes less favorable: at R, 95 % of the sodium chloride in the solution is used up in regeneration of the resin (because X Ca ¼ 0.95), whereas at point S the proportion is already less than 60 % and at point T, 25 %. At the latter point, the resin is 60 % regenerated (X Ca ¼ 0.4) and the process stops because any attempt to reach point O would need increasing amounts of brine, only a small proportion of which would be used in actual

19 Vol. 19 Ion Exchangers 491 regeneration. The resin is therefore rinsed at point T and another new stage is initiated by displacing the system to P and beginning a second cycle in which the water is again almost completely softened. All subsequent cycles follow the path PQRT. The regeneration efficiency (percentage of brine exchanged) can be evaluated at each point on the curve RSTO. Between R and T the mean regeneration efficiency is 60 %. Regeneration was carried out between X Ca ¼ 0.9 and 0.4, so that half the total capacity of the resin was used. With a total capacity of 2 eq/l, the operating capacity would thus be 1 eq/l. A regeneration efficiency of 60 % is assumed, 1/0.6 ¼ 1.67 eq of sodium chloride would be used, i.e., 97.5 g per liter of resin. Figure 12. Ion-exchange isotherms for mono monovalent ions 5.4. General Case In the general exchange reaction between ions A and B of ionic valences a and b, respectively, the selectivity coefficient KA B is defined as K B A ¼ ½BŠb ½AŠ a ½AŠa ½BŠ b K B A depends on experimental conditions such as concentration and temperature. Another useful quantity is the separation factor, defined as a B A ¼ ½BŠ ½AŠ ½AŠ ½BŠ ¼ X B 1 X B 1 X B X B For mono monovalent exchange, the separation factor a B A and selectivity coefficient K B A are identical and practically independent of the total salt content. For mono divalent exchange, a B A is a function of the term K [C]/[C]. A separation factor a B A greater than unity means that the resin will take up ion B in preference to ion A. Figure 12 shows ion-exchange isotherms for two monovalent ions A and B. The upper curve (a ¼ 2) represents a case in which the resin has a higher affinity for B than for A; the lower curve (a ¼ 0.2) represents the opposite case. For any point on a given isotherm, the ratio between the two hatched areas is equal to the separation factor a. In many cases, simple selectivity calculations show whether proposed ion-exchange processes will function [17, 18]. 6. Exchange Kinetics 6.1. Principles [19, 20] Mass action equations apply only to systems in equilibrium. In industrial practice where a solution flows through the resin, equilibrium is not necessarily reached and the results are influenced by kinetic considerations. In fully ionized systems, the rate-determining step of ion exchange is the diffusion of the mobile ions toward, from, and in the resin phase, rather than the chemical reaction between fixed ions of the resin and mobile counterions. If a cation-exchange resin with negative fixed ions (e.g., sulfonate ions) is used as an example, the cation concentration in the resin is much greater than that in solution. However, any cations diffusing out of the resin into the dilute solution create a net negative charge in the solid phase and a net positive charge in solution. The resulting potential difference is called the Donnan potential; it prevents anions from penetrating the (negatively charged) resin. This phenomenon is called Donnan exclusion.

20 492 Ion Exchangers Vol. 19 Diffusion through the film and in the solid phase occurs at different rates and two steps may be rate-determining: 1. Diffusion of ions within the resin (particle diffusion) 2. Diffusion in the Nernst film (film diffusion) The slower step controls the overall ionexchange rate. A criterion has been established by HELFERRICH [19] to determine which process is rate-determining: Figure 13. Diffusion through a film and inside a particle Therefore, the co-ion (i.e., the anion in the case of cation exchange) does not participate in the ionexchange process. Figure 13 illustrates the uptake of Na þ ions by a bead of H þ -form resin. The bulk solution contains a large excess of available Na þ ions, with an effectively constant concentration. A static layer of solution, known as the Nernst film, surrounds the bead. This film is defined such that it is unaffected by convection (i.e., flow) around the bead; ion transport takes place by diffusion only. Strong convection (i.e., high flow rate) decreases film thickness. The ion concentration is practically constant outside the Nernst film, and a concentration gradient occurs within it (Fig. 14). H ¼ðC Dd=CDrÞð5þ2 a B A Þ where. C ¼ total ion concentration in solution. C ¼ total ion concentration in the solid phase (total capacity). D, D ¼ diffusion coefficients. H ¼ Helfferich constant. d ¼ thickness of Nernst film. r ¼ radius of ion-exchange bead. a ¼ separation factor (defined in Section 5.4) When H 1, film diffusion is rate-limiting; when H 1, particle diffusion is rate-limiting. The definition of H shows that film diffusion is favored by the following:. High resin capacity C. Thick Nernst film d (i.e., low flow rate). Low concentration C in solution. Small resin beads. High selectivity (high a) In general, forward and reverse exchange rates differ (Fig. 15). In this example of H þ /Na þ exchange in a conventional strongly acidic cation-exchange resin, forward exchange (uptake of Na þ ions) is faster than reverse exchange (regeneration). Because the H þ ion generally is more mobile, exchange is faster when the faster ion is in the resin initially. The rates of sodium calcium exchange are several times slower. For a given ion, mass-transfer flux through the film can be described by Fick s law: J ¼ DgradC where Figure 14. Nernst film. J ¼ ion flux, mol s 1 cm 2

21 Vol. 19 Ion Exchangers 493 Figure 15. Forward and reverse exchange rates in a polystyrene sulfonic resin a) Conversion from H þ to Na þ ; b) Conversion from Na þ to H þ. grad C ¼ ion concentration gradient, mol/cm 3. D ¼ diffusion coefficient cm 2 /s When D is constant, the ion concentration independent of C, and the concentration gradient linear, the relationship becomes J ¼ kðc 1 C 0 Þ where k is the mass-transfer coefficient (cm/s) given by D/d. Measurement of the mass-transfer coefficient of new and used resins can provide useful information about their ability to operate under critical conditions [21]. Diffusion in the resin phase is always slower than that in solution, due to the obstruction created by the resin matrix. Highly cross-linked materials have smaller diffusion coefficients. For a comprehensive treatment of diffusion and rate laws in ion exchange, the reader is referred to [19, 20]. The remainder of this chapter deals with practical consequences of the above kinetic principles. Figure 16. Kinetics of Duolite A 101U in uranium extraction a sulfuric acid leaching solution in a stirred tank with strongly basic exchangers in the sulfate form. Duolite A 101U is a high-capacity polystyrene material. Kinetic Leakage. The concentration of ions that is not taken up by the resin but passes into the treated solution is termed the ion leakage. Kinetic ion leakage can be measured as a function of specific flow rate, which is the ratio of the flow rate of the solution to be treated to the volume of resin. The specific flow rate is expressed as cubic meters of solution per hour per cubic meter of resin or as bed volume (BV) per hour. In the example shown in Figure 17, new and used macroporous strongly acidic cation-exchange resins were tested for their ability to remove sodium from dilute sodium chloride solutions percolated through a mixed bed made of the cation 6.2. Kinetic Curves As with ionic equilibria, exchange kinetics are difficult to appreciate from a purely mathematical point of view. However, exchange rates for different resins can be compared under the same conditions. Rate of Ion Uptake (Exhaustion Rate). The exhaustion rate of a resin through which a given solution flows is measured under standard conditions. In the general case of a single ion species, the exhausted fraction X i of the resin is plotted vertically against time (Fig. 16). The example illustrates the recovery of uranium from Figure 17. Kinetic leakage: conductivity vs. flow rate for two resin mixtures at two concentrations a) Used cation resin, NaCl 16 mg/l; b) New cation resin, NaCl 16 mg/l; c) Used and new resins, NaCl 1.7 mg/l

22 494 Ion Exchangers Vol. 19 resin and of a new, highly regenerated nucleargrade anion resin. The conductivity of effluent water was measured. Both mixtures give identical low leakage for the more dilute sodium chloride solution (1.7 mg/l, curve c). However, the mixture containing used cation resin is more sensitive to the flow rate than that with new cation resin when the sodium chloride concentration is increased to 16 mg/l (curves a and b); both mixtures then become strongly sensitive to flow rate. This type of kinetic test is useful to assess resin performance, particularly when very high-purity water is required [22, 23] Strongly Acidic or Strongly Basic Resins Film Diffusion At concentrations up to 10 meq/l and flow rates up to 120 m/h used in water treatment, diffusion rates through the resin mass are much greater than through the surrounding film. The film thus controls the rate of exchange, and the process exhibits film-controlled kinetics. When sodium chloride solution passes through a column of resin, originally in H þ form, the concentration of the ion under consideration in the emergent liquor exhibits variations of the form shown in Figure 18. If the flow rate is slow enough, equilibrium is established as the solution reaches a new layer of the resin. The concentration in the emergent liquor is represented by the curve OFP. At F, the breakthrough point, the concentration reaches its maximum permitted value and flow stops. If it were continued up to P where the concentration in Figure 18. Film kinetics: exhaustion curves and volumes passed at maximum leakage For explanation see text. the emergent liquor equals that of the raw solution, the resin would be 100 % exhausted. In industrial practice, flow is stopped when the concentration of the ion under consideration in the emergent liquor reaches a small fraction (e.g., 1 %) of the concentration in raw solution. At F, a volume A of the solution has flown through the column. The capacity used is given by the area OFA 0 Z, which differs very little from the area OFPZ representing the total capacity of the resin. Figure 18 can also be taken to represent the progress of the exhaustion wavefront through a resin column. In the slow flow described above, the wavefront is only slightly diffuse, each successive layer of resin being almost completely exhausted before leakage occurs. As flow rate increases, equilibrium is no longer reached and the exhaustion curve OGQ is obtained. If the process is continued beyond the breakthrough point G as far as Q, the area OGQZ gives another measure of the total capacity of the resin. If, however, the process is stopped at G, the capacity used is only that given by OGB 0 Z, which is less than the total capacity. If the flow rate is increased further, a curve such as OHR is obtained, which has a still smaller operating capacity at the new breakthrough point H. Because the Nernst film thickness is an inverse function of flow rate, the film becomes thinner as the flow rate increases. Thus, doubling the flow rate does not mean that the leakage curve is spread out over twice the distance. In filmcontrolled kinetics, capacity depends very little on the rate of presentation of ions or kinetic load (i.e., the product of flow rate and concentration in the solution to be treated). In the complete absence of kinetic effects, the operating capacity is entirely independent of flow rate; therefore, the percolated volume of the exhaustion stage is inversely proportional to the ionic concentration. As described in Section 5.3, the operating capacity of a strongly acidic resin depends mainly on the amount of regenerant. For a given quantity of regenerant, a corresponding capacity is available which is less than the total capacity. If the exhaustion time is too short (i.e., if resin volume is small and flow rate per unit volume is high), the operating capacity is less than the available capacity. This reduction in operating capacity (which is mainly confined to strongly and weakly basic anion resins) can increase as the resin ages due to partial blocking of its pores with organic matter absorbed during several

23 Vol. 19 Ion Exchangers 495 years of service. Fouling of this type reduces ion diffusion rates. In designing a demineralizing plant, the anion-exchange column should have sufficient capacity to allow the plant to run for at least 8 h at the maximum loading rate Particle Diffusion As the concentration of ions in solution increases, the mass-transfer rate through the film rises until it exceeds the diffusion rate through the resin beads. Diffusion through the resin then becomes the controlling factor, and the system is said to exhibit particle-controlled kinetics. This condition occurs mainly during regeneration of resins with solutions having concentrations between 1 and 3 N. Breakthrough curves are similar to those in Figure 18, except that the length of the wavefront is a linear function of flow rate. Because of the high concentration gradient through the resin, the whole process is much faster than the exhaustion stage. Virtually complete equilibrium can be achieved in 15 min, but if a shorter regeneration time is used, operating capacity can be significantly reduced. Nevertheless, small automatic water softeners sometimes have brine injection times as short as 5 min Weakly Acidic or Weakly Basic Resins Weakly acidic or weakly basic resins are not fully ionized in the regenerated form. As a result, the adsorption process involves not only diffusion through the resin but also the formation or destruction of covalent bonds, which are slower processes. The exchange rate of these resins is therefore controlled by slow particle diffusion and the resin cannot be exhausted as quickly as with strongly ionized materials. Weakly acidic and weakly basic resins are said to have slow kinetics: their capacity depends strictly on the rate of presentation of ions. As shown in Figure 19, the carboxylic resin Amberlite IRC 86 requires a cycle time of over 24 h to utilize its full capacity (in this example, the ions to be removed from solution have a concentration of 5 meq/l). In practice, usually less than half the total capacity of weakly acidic resins is utilized. The kinetic effect is not as Figure 19. Effect of cycle time on usable capacity for a weakly acidic cation resin (a) and two weakly basic anion resins (b, c) a) Amberlite IRC 86 (weakly acidic); b) Amberlite IRA 67 (weakly basic, acrylic); c) Amberlite IRA 96 (weakly basic, styrenic) dramatic for weakly basic resins, which are usually operated at ca. 75 % of their total capacity. 7. Practical Consequences of Ion- Exchange Equilibrium and Kinetics As shown in Chapter 6, under normal conditions the exchange process is always incomplete in both the service and the regeneration stages, which means that the total capacity of the resin can never be fully used Operating Capacity, Regeneration Efficiency, and Regenerant Usage Calculation of operating capacity must take into account the following:. Raw water analysis. Required quality of treated water (acceptable leakage). Service flow rate

24 496 Ion Exchangers Vol. 19. Temperature of water to be treated. Type and amount of regenerant. Regeneration flow rate. Regenerant temperature. Required duration of cycle The concept of operating capacity has been introduced in Section 3.2 and developed in Section 5.3 for the special case of water softening (Na þ Ca 2þ equilibrium), in both instances starting from 100 % regenerated resin. In practice, this is seldom the situation because an enormous amount of regenerant would be required to remove all the ions from the resin that were taken up during the previous cycle. The variable that has the greatest effect on operating capacity is therefore the amount of regenerant used. Regeneration efficiency is defined as the ratio of the operating capacity to the amount of regenerant used; both expressed in equivalents per liter. The reciprocal of this efficiency is known as the regenerant usage or regenerant ratio and is always >1. In practice, these quantities are often expressed as percentages. With most strongly acidic or strongly basic resins more than 60 % of their total capacity is seldom used. As an example, the total capacity of the strongly basic type 1 resin shown in Figure 20 is ca. 1.2 eq/l. A basic operating capacity of 0.46 eq/l can be obtained by using 60 g of caustic soda per liter of resin (1.5 eq/l). In this case, the regeneration efficiency (0.46/ 1.5) barely exceeds 30 %. By doubling the amount of regenerant, a basic operating capacity of eq/l can be achieved: this is only onethird greater than before, and regeneration efficiency falls to 21 %. Thus, such a resin is seldom operated at more than 50 % of its total capacity. However, these very strongly basic type 1 materials are less easily regenerated than type 2 resins and therefore yield a lower operating capacity for the same amount of regenerant than the latter. Weakly acidic and weakly basic resins, on the other hand, can be completely regenerated, but their operating capacity is limited by their poor kinetics (Section 6.4) Permanent Leakage Strongly Acidic and Strongly Basic Resins. When strongly basic or strongly acidic resins are returned to service after regeneration, they are only partially regenerated. When coflow operation is used (i.e., when flow is in the same direction in the service and regeneration stages), the most poorly regenerated layer is usually the lower one through which the treated solution emerges. This affects the quality of the treated solution due to self-regeneration. In practice, the bed does not consist of two distinct (completely regenerated, completely exhausted) regions, but has a continuously varying level of regeneration along the column. Figure 21 shows the exchange reactions that occur in the Figure 20. Operating capacity of a strongly basic type 1 resin as a function of regenerant dosage (caustic soda) Numbers on the curves indicate percentage of regenerant efficiency. Figure 21. Ion leakage due to self-regeneration in a coflow regenerated column For explanation see text.

25 Vol. 19 Ion Exchangers 497 column during the service stage, in which salts from the solution (represented here by NaCl) are retained by the regenerated resin layers. Each Na þ ion that is taken up ejects an H þ ion from the resin. The H þ ions move down the column and reach a region where the resin is only partially regenerated. Reverse exchange then takes place (self-regeneration), leading to leakage of Na þ ions into the treated water. Figure 22 shows the operating capacity (A) and sodium leakage (B) of Amberlite IR 120 (a standard, gel-type, strongly acidic cation-exchange resin) as a function of regenerant level. In raw water, the main cations are usually sodium, calcium, and magnesium. The selectivity of strongly acidic resins for these cations increases in the order Na þ < Mg 2þ < Ca 2þ (Section 5.2). Therefore, chromatographic separation occurs in the resin column, and the sodium ion is the first to emerge in the treated solution, both other cations being more strongly held by the resin. This explains the high leakage obtained for water having only sodium as a cation (Fig. 22 B, curve a). Because the quality of demineralized water is generally expressed in terms of its electrical conductivity, sodium leakage from the cationexchange column is the main contributor to the conductivity of treated water. A sodium leakage of 1 mg/l corresponds to 9 ms/cm conductivity (as NaOH). In coflow regenerated systems, the level of regenerant is chosen according to the desired conductivity. For high-sodium water, the only way to obtain low leakage is to use counterflow regeneration (see Section 8.2). Weakly Acidic and Weakly Basic Resins. Weakly acidic and weakly basic resins are almost 100 % regenerated at the start of the service stage, but their exchange rate is relatively low so that some ions are not taken up and pass through the bed (i.e., kinetic leakage; see Section 6.2) Water Analysis Calculations for designing water treatment plants that use ion-exchange resins involve some simple concepts related to the composition of the raw water. Units. Ion concentration is usually measured in equivalents per liter (eq/l), most often in milliequivalents per liter (meq/l). The unit mole is not suitable in practice because the resins have to deal with the number of charges of each ion, i.e. its valence. The following units are also used to express ionic concentrations in water: Figure 22. Capacity (A) and sodium leakage (B) of a standard strongly acidic cation resin (Amberlite IR 120) regenerated in coflow mode. Sodium (NaCl) content with respect to total cation concentration: a) 100 %; b) 50 %; c) 25 %Flow rate, 10 BV/h; total salinity of raw water, 10 meq/l; acid dosage expressed as 100 % HCl per liter of resin; water temperature, 20 C.. the French degree: 1 f ¼ 0.2 meq/l. the German degree: 1 dh ¼ meq/l (which corresponds to 10 mg of CaO per liter) The following units are used in Great Britain and other English-speaking countries:. 1 g of CaCO 3 per liter ¼ 0.02 eq/l. 1 mg of CaCO 3 per liter ¼ 0.02 meq/l

26 498 Ion Exchangers Vol. 19 Although resin capacities are internationally expressed in equivalents per liter, in the United States capacity values are widely given in kilograins (kgr) where 1 kgr as calcium carbonate per cubic foot corresponds to eq/l. Water Composition. For ion-exchange calculations, the exact composition of the solution to be treated must be known. The composition of a typical water is given in Figure 23. The total concentration of all the anions and cations in solution is known as the total dissolved solids (TDS). Ions normally encountered in water treatment are the cations Na þ,ca 2þ,Mg 2þ, and the anions OH, CO 2 3, HCO 3,Cl, NO 3, SO2 4. Other ions may be present (K þ, NH þ 4,Mn2þ, Fe 2þ ), but their concentration in natural water is usually very low. For ionic equilibrium, the total cation and anion concentrations measured as equivalents per liter must be equal. Ca 2þ and Mg 2þ ions (and, possibly, Sr 2þ, Ba 2þ,Fe 2þ and Mn 2þ ) are classified as ions producing hardness, and their concentration gives the total hardness (TH) measured in milliequivalents per liter. In water softening, Ca 2þ and Mg 2þ are exchanged for Na þ.thecl, NO 3, and SO 2 4 ions are grouped together, and their total concentration is called the equivalent mineral acidity (EMA), or free mineral acidity (FMA) after cation exchange. The total concentration of OH, CO 2 3, and HCO 3 ions, measured in equivalents per liter gives the total alkalinity (TAlk). This is often referred to as m-alk or the m value (alkalinity to methyl orange): TAlk ¼ m ¼½HCO 3 Šþ½CO2 3 Šþ½OH Š Figure 23. Composition of water When TH exceeds TAlk, the difference (TH TAlk) is called the permanent hardness, whereas TAlk is known as the temporary (or bicarbonate) hardness. When TH is lower than TAlk, only temporary hardness (no permanent hardness) exists. For water with alkaline ph values, the caustic alkalinity, usually described as p-alk or the p value (alkalinity to phenolphthalein) is measured separately. Carbonate Equilibrium. The p value includes all anions with a pk > 8.3, i.e., in practice the OH ions and a value arithmetically equal to half the total carbonate present because CO 2 3 is first neutralized to HCO 3, which itself has a pk of only 6.7: p ¼½OH Šþ½CO 2 3 Š=2 In a demineralization system, where the first step is usually cation exchange, ions of p alkalinity must not be considered in the anion-exchange balance, because they are neutralized by the H þ ions produced in the cation-exchange part of the process: OH þh þ!h 2 O CO 2 3 þ2hþ!co 2 þh 2 O As demonstrated in Section 4.2, carbon dioxide is taken up by the strongly basic resin as monovalent bicarbonate, but must be regenerated as a divalent species: RHCO 3 þ2 NaOH!ROHþNa 2 CO 3 þh 2 O Because in the series OH, CO 2 3, HCO 3, and CO 2, only two adjacent species can coexist in water, raw water does not contain free carbon dioxide when p is positive. In practice, the equivalent concentration of carbon dioxide to be considered after cation exchange (and before the degasifier, if any) is ½CO 2 Š¼m pþ½free CO 2 Š The case in which p ¼ 0 is the most common one, except when the raw water is lime-decarbonated. Besides, free carbon dioxide exists only with a ph close to or lower than neutral. The concentrations of nonionized substances must also be known; these include free silicon dioxide and carbon dioxide. The amount of organic substances present in the raw water should be

27 Vol. 19 Ion Exchangers 499 considered on the anion side, because they are mainly acidic. Concentration is usually expressed in terms of the quantity of potassium permanganate needed to oxidize these substances (milligrams of KMnO 4 per liter of solution). The amount of organic matter determines the choice of anionexchange resin (see Section 10.1). The permanganate value obtained in the oxidizing test depends on the test method. Here, oxidation is assumed to be performed with N potassium permanganate in the presence of sulfuric acid, the solution being boiled for 10 min. The composition of a typical water is given in Figure 23. Fouling with Organic Material. The most common organic constituents in natural waters are the high molecular mass carboxylic acids, humic and fulvic acid. These large molecules enter the ion-exchange resins and are trapped in the most highly cross-linked regions, their chains becoming tangled with the resin matrix. This eventually reduces the capacity of anion exchangers. Rinsing becomes more difficult, and problems arise with the quality of treated water because the carboxylic character of the organic matter means that, as the ph varies, caustic soda is initially taken up and then leached out. The problem is aggravated by synthetic ionic surfactants. Although qualitative and quantitative assessment of organic materials is difficult, their concentration is usually expressed in milligrams of potassium permanganate required per liter of water to oxidize them under the given conditions. The fouling factor N is the quantity of organic matter (in milligrams per liter of KMnO 4 ) divided by the total anion concentration (in milliequivalents per liter). Anion resins can be listed in order of their resistance to fouling, starting with the least resistant: Polystyrene, gel type 1 2 Polystyrene, macroporous type 1 4 Polyamine, weakly basic 4 Polystyrene, gel type 2 6 Polystyrene, macroporous type 2 8 Polyacrylic, strongly basic 20 Polystyrene, weakly basic 20 Polyacrylic, weakly basic 40 These values merely indicate the fouling risk of each individual resin and give no information about the amount of organic material taken up. Anion resins, even those with high fouling resistance, do not remove all organic matter from the influent solution. Reversible uptake can range from 20 to 80 %; a reasonably safe figure is 50 %. If N is too large (e.g. > 8 for a strongly basic polystyrene material), the risk of fouling can be reduced by increasing the volume of resin. The working capacity of the resin will be less than its potential operating capacity. Weakly basic resins can also protect strongly basic resins against fouling Calculations in the Design of Ion- Exchange Plants for Water Purification General Method. The various types of combination process, the criteria used in choice of resin, and the recommended operational conditions are considered in Chapter 9. The operating capacity for water softening is greater when the amount of regenerant is large, the sodium ion (self-regenerant) concentration is low, and the flow rate is low. Resin manufacturers provide standard charts and curves for each type of resin, enabling calculations to be made of the volume of each resin, amount of regenerant, operating capacity, and leakage. The starting point is normally provided by a curve that applies to standard conditions, and corrections are made for each condition that differs from the set standard. The method of calculation is too complicated to be described here; only the main factors are considered, here in an example of total water deionization. Weakly acidic carboxylic resins have a very high capacity for divalent ions (Ca 2þ and Mg 2þ ): they remove temporary hardness (see Section 7.3) and are normally used only when the temporary hardness is high and the total hardness of the water is even greater (TH > TAlk). Their relatively slow exchange rate means that they are very sensitive to the specific flow rate. Regeneration is carried out with an amount of acid calculated to be slightly in excess of the design capacity, with a regenerant ratio on the order of %. These resins are therefore very efficient.

28 500 Ion Exchangers Vol. 19 Strongly Acidic Sulfonic Resins. In hydrogen exchange, capacity depends on the type of acid used for regeneration: hydrochloric acid is the most efficient because sulfuric acid is not completely dissociated at the concentration normally used. In addition (this also applies to carboxylic resins), if the resin has taken up a lot of calcium, the sulfuric acid must be highly diluted to avoid precipitation of calcium sulfate during regeneration. The amount of regenerant is chosen according to the permitted ion leakage: in other words, it depends on the permissible level of electrical conductivity in the treated water (see Section 7.2). The greater the amount of regenerant, the lower is the electrical conductivity and the greater is the operating capacity (see Fig. 22). The operating capacity of the resin depends on the cation composition of the water, more in relation to proportions than to absolute values. Reference should be made to manufacturers curves. Degasser. After cation exchange, any bicarbonate or carbonate ions in the feed water are converted to free carbon dioxide, which can be removed by an atmospheric or membrane degasser (DG) to reduce the load on the anion-exchange resin. In the absence of such a degasser, the carbon dioxide, which is a weak acid, must be removed by the strongly basic resin. When a combination of weakly and strongly basic resins is used in separate columns, the degasser can be placed either before the weak base resin or between the weakly and the strongly basic resin. Each method has its own benefit: degassing is more efficient when the water is more acidic, i.e., when the degasser is before the weakly basic resin, but the operating capacity of the resin is higher when the degasser is placed downstream. In most modern plants, however, the weakly basic and strongly basic resins are in the same ion exchange column, so the degasser will be located before the weakly basic resin. Weakly basic resins also have slow kinetics and are rate-sensitive. In addition, their capacity depends on the composition of the water, increasing as the concentration of strong acids decreases. Only small quantities of weak acids are removed. Weakly basic acrylic resins have a higher pk value, so that they effectively remove carbon dioxide from water, and if an atmospheric degasser is used, it should be located before the weak base resin. The quantity of regenerant normally used is on the order of 130 % of the operating capacity of the resin. Strongly basic resins remove strong and weak acids. Because the uptake of silica is poorer than that of other anions, it is the first to leak. The regenerant level thus depends on the acceptable silica leakage. In addition, because silica has a tendency to polymerize on the resin, regenerating with hot sodium hydroxide is sometimes worthwhile. However, only type 1 materials withstand high temperature (see Section 3.3). In practice, regenerant requirements are from 150 to ca. 500 % (sometimes > 1000 % when the strongly basic resin is preceded by a weakly basic one), so the efficiency of these resins is only moderate. However, a new type of strongly basic resins was developed by Duolite International, Bayer, and Rohm and Haas in the mid 1980s. These are partially quaternized weakly basic resins, which thus exhibit a strong base capacity of ca. 40 % (i.e., enough to efficiently remove silica from waters containing up to about 15 % SiO 2 ), whereas they offer a regenerability close to that of weakly basic resins. Examples of such resins are Amberlite IRA478 and Lewatit AP246. Calculation of Resin Volume. From the calculated operating capacities, the volume of each type of resin can be determined as a function of the operating time chosen between two regenerations: V ¼ðQtSÞ=C where. V ¼ resin volume, m 3. Q ¼ flow rate, m 3 /h. t ¼ operating time, h. S ¼ salinity to be adsorbed by the resin, eq/m 3 of water or meq/l. C ¼ operating capacity of the resin, eq/m 3 of resin Service Water. The operating cycle for water treatment includes backwashing, regeneration, and rinsing stages, all of which consume water. The salinity of this service water is thus

29 Vol. 19 Ion Exchangers 501 added to the ionic load of the production cycle and reduces resin capacity. The volume of resin must therefore be increased in proportion to the amount of service water used. The additional volume depends on the salinity of the raw water, which varies between 5 % (low-tds water) and 35 % (semibrackish water). Specific Flow Rate and Cycle Duration. The specific flow rate should not be too low because a very even flow must always be obtained; irregularities in flow may cause channeling in the resin bed, thereby impairing complete exchange. Too high a rate produces an excessive head loss. The specific flow rate Q/V should be between 4 and 40 BV/h. If this is not the case, the volume V must be appropriately adjusted by shortening or lengthening the operating time t Example of Calculation In this section, the calculation of a system for demineralizing water, based on Amberlite resins is described. One solution of the problem is fully developed here, further options are discussed in Section 9.3 and column design in Section Principle A set of curves for each type of resin is loaded into a computer, equipped with a program designed to yield the optimum system, given the characteristics required for the treated water and the consumption of regenerants. The calculation can also be carried out by hand. Basic data include the following:. An analysis of the raw water. Flow rate to be treated. Required quality of the treated water. Combination system required. Types of resin to be used. Length of the production cycle. Types of regenerant. Temperature of the caustic soda regenerant The calculation leads to a determination of the volume, operating capacity, and regeneration rate for each resin. The diameter of each filter is also calculated as a function of its volume. A set of limiting criteria controls the calculation. Examples of these are the following: 1. The specific flow rates for each column must lie between 4 and 40 BV/h. 2. The linear velocity of flow should not exceed a value leading to high head loss across the bed; this linear flow rate is the ratio between the flow rate (in cubic meters per hour) and the area of the column (in square meters). The recommended head loss limit is 100 to 150 kpa. 3. The depth of the resin beds must be between 0.7 and 2.5 m. 4. The amounts of organic matter determine the choice of anion-exchange resins (see Section 7.3). The above values should serve as guidelines for industrial water treatment. For special cases (e.g., condensate polishing or specific ion-exchange systems), the process limits are considerably different. The demineralization of 80 m 3 /h of water with a salt content of 8 meq/l can be used as an example. Each of the items is treated in the set of basic data (Table 6) and in the results of the calculation [demineralizing (Table 7) and polishing (Table 8) units] Basic Data Basic data are summarized in Table 6. Analysis of Water. The water analysis must be available in detail and be ionically balanced. Required Quality. Quality is expressed in terms of the electrolytic conductivity of the treated solution and the residual concentration of silica, which is important in boiler feedwaters. Combination Process for Demineralization. The type of ion-exchange line and regeneration system is chosen according to the composition of the raw water, the flow rate to be processed, and the required water quality (see Chap. 9). Each of the pairs strong acid weak acid and strong

30 502 Ion Exchangers Vol. 19 Table 6. Examples of design data for a plant calculation 1) Water analysis, meq/l Ca 2þ 5.0 Cl 3.0 P-Alk 0.5 Mg 2þ 1.0 NO M-Alk 4.0 Na þ þ K þ þ NH þ SO Total cations 8.0 EMA 4.0 Silica 12 mg/l as SiO 2 (¼ 0.20 meq/l) Organic matter 10 mg/l as KMnO 4 Residual CO 2 (after degassing) 0.30 meq/l Water temperature 15 C 2) Production Running time (production) ¼ 16 h Net flow rate 80 m 3 /h Gross flow rate 82.7 m 3 /h Net throughput 1280 m 3 Gross throughput 1324 m 3 3) Required water quality Before polishing After polishing Conductivity ms/cm SiO 2, mg/l base weak base resins is regenerated in thoroughfare, i.e., the weakly functional resin is regene-rated with the excess from the strongly functional resin. Role of Each Resin. 1. Amberlite IRC86, a weakly acidic cation resin, removes the temporary hardness, i.e. the calcium and magnesium ions, up to the limit of TAlk (4 meq/l). 2. Amberlite IR120, a strongly acidic cation resin, removes the rest of the cations (Ca 2þ þ Mg 2þ þ Na þ ), i.e., 8 4 ¼ 4meq/L. 3. Amberlite IRA96, a weakly basic anion resin, removes the EMA (Cl þ NO 3 þ SO2 4 ), i.e., 4 meq/l. 4. Amberlite IRA458, a strongly basic anion resin, removes the other anions after degassing together with residual carbon dioxide (0.3 meq/l) and the silica (0.2 meq/l). In addition, the two anion-exchange resins remove a considerable proportion of the organic matter contained in the water. Since the initial amount is quite large, a strongly basic polyacrylic material is chosen to avoid fouling Demineralization Unit Duration of Production. The figure of 16 h was chosen for hydraulic reasons because the specific flow rate must not exceed 40 BV/h. Table 7. Calculated results for the demineralization unit Ion-exchange resin type Parameter Amberlite IRC86RF Amberjet 1200 Na Amberlite IRA96RF Amberlite IRA458RF Volume per line, L Gross ionic load, eq Overrun, % of ionic load 0 30 Operating capacity, eq/l Regeneration with HCl HCl NaOH (25 C) NaOH (25 C) Regenerant ratio, % Regenerant level, g/l Concentration, wt % Regenerant mode: Amberpack Amberpack Amberpack Amberpack Suggested diameter, mm Resin height, mm Linear velocity, m/h Specific flow-rate BV/h Pressure drop, kpa

31 Vol. 19 Ion Exchangers 503 Table 8. Calculated results for the polishing unit Parameter Amberjet 1200 H Amberjet 4200 Cl Volume per line, L Production time, h Max. throughput, m Regeneration with HCl NaOH (25 C) Regenerant level, g/l Concentration, wt % Suggested diameter, mm Resin height, mm Linear velocity, m/h Specific flow rate, BV/h Pressure drop, kpa Total Ionic Load. This is the product of the gross production (percolated volume in cubic meters, taking into account the water required for regenerant dilution and rinse) and the salt content taken up by each exchange resin (in equivalents per cubic meter). Operating Capacity. This is calculated for each resin from the curves supplied by the manufacturer. Volume. The volume of each resin can be determined as a function of the operating time chosen between two regenerations from the formula: V ¼ðQtSÞ=C where (Q t S) is the ionic load and C the operating capacity of the resin. The volume of resin is calculated by dividing the ionic load by the operating capacity. Amount of Regenerant. This is determined from the manufacturer s curves as a function of the raw water composition and the required quality of the treated water. Both resin couples (WAC SAC and WBA SBA) are regenerated in thoroughfare (see Section 9.3), i.e., the fresh regenerant first passes through the strong resin, then through the weak resin. Regenerant Temperature. To improve elution of the silica, the caustic soda regenerant may be heated to 45 C, but only if the strongly basic exchange resin is of type 1. For type 2 or polyacrylic materials, the temperature should not exceed 30 C. Since Amberlite IRA 458 is a polyacrylic resin, regeneration at 25 C is preferred. With counterflow regeneration (see Section 8.2), hot caustic is only necessary in case of high silica load. Acid Concentration, Flow Rate, and Contact Time. Hydrochloric acid is generally used at a concentration of between 5 and 8 wt % and caustic soda at 4 wt %. Recommended contact time for all regenerants is between 15 and 60 min. If sulfuric acid is used, care must be taken that its strength is low enough to avoid precipitation of calcium sulfate on the cation exchange resins. Regeneration in this case is often carried out in several stages with sulfuric acid concentration increasing from 0.8 to 7 wt %. Column Diameter. The diameter is chosen according to the volume of resin and flow rate so as to obtain a bed depth between 0.7 and 2.5 m of resin and a pressure drop of less than ca. 120 kpa across the bed. Overrunning the Weakly Basic Resin. Calculating resin volumes on the basis of the ionic loads (see Role of Each Resin) results in a large discrepancy between the volume of the weakly basic resin and that of the strongly basic resin. This is because the quantity of weak acids (carbon dioxide and silica) is small compared to that of strong acids. The volume of strongly basic resin would be less than 1000 L, and the corresponding specific flow rate (ca. 80 BV/h) would exceed the maximum recommended value of 40 BV/h. A better solution, always used in real projects, is to overrun the WBA resin. The volume of the WBA resin is made deliberately too small, so that part of the ionic load of the strong acids must be removed by the SBA resin, the amount of which, as a result, is made larger. In practice, the resin volumes are balanced in such a way that they are approximately equal. When the bicarbonate hardness in the feedwaterisveryhigh(say75%ormoreof the dissolved solids), overrunning of the WAC resin is also possible. Two Resins in One Column. As is discussed in Chapter 10, modern ion-exchange systems use packed beds in which the WAC/SAC and the WBA/SBA resin couples are placed in the same column. This reduces considerably equipment costs.

32 504 Ion Exchangers Vol. 19 Results of The Calculation. Using the overrun technique and packed bed design, and taking into account the higher capacity of the Amberjet resin and a specific rinse procedure that reduces the amount of waste, the final results of the calculation are summarized in Table Polishing Unit Calculated results for the polishing unit are listed in Table 8. The mixed bed for polishing is calculated in such a way as to have a specific flow rate of about 40 BV/h. Regeneration is carried out with g of acid per liter of cation resin and g of caustic soda per liter of anion resin. The operating capacity is usually limited by the silica content, because the anion-exchange resin in the mixed bed almost entirely removes the silica that has leaked from the primary system. In addition, the proportion of anionexchange resin in the mixture is generally selected between 50 and 65 %. The diameter of a mixed bed unit is calculated to give a bed depth between 0.9 and 1.5 m and a linear flow rate between 30 and 60 m/h. Because the influent to the mixed bed varies during the service run of the primary line, the run length of the mixed bed is only approximate, and most users decide to regenerate their polishing unit after a fixed running time. 8. Industrial Use of Ion Exchange The practical use of ion exchangers, based on the general principles discussed in the previous chapters, is described here. Although many examples refer to water treatment, other fluids are treated in a very similar way Description of the Ion-Exchange Cycle Figure 24. An industrial ion-exchange column a) Distributor; b) Resin; c) Collector Figure 24 shows a vertical section through a coflow ion-exchange column. The solution to be treated is introduced into the column through a distributor. It passes through the resin and emerges through a collector or collection system. In a conventional plant, the resin fills only half the available space so that the bed can be decompacted by an upward flow of water. This expands the resin and removes suspended matter and fragments of resin accumulated during the previous cycle. The ion-exchange cycle is divided into four stages: 1. exhaustion (or service), 2. decompaction (or backwash), 3. regeneration, and 4. rinsing. Exhaustion (or Service). The solution to be treated passes through the resin bed and exhausts it. As soon as the quantity of ions taken up reaches the operating capacity (i.e., the breakthrough point) and leakage reaches a predetermined limiting value, the service stage is stopped. Backwash. After the service stage, the resin is decompacted for about 15 min by an upward current of water. This treatment also removes any particles deposited on the surface of the bed, together with any fragments of resin. The bed is then allowed to settle. Regeneration. The regenerant solution is introduced, usually at a concentration of a few percent, and slowly percolates through the bed. The injection of the regenerant takes min. Rinsing. The regenerant is then displaced by water at a low flow rate until the resin bed contains no more than traces of regenerant. This

33 Vol. 19 Ion Exchangers 505 Figure 25. Effects of coflow and counterflow regeneration on the condition of resin beds during loading and regeneration Hatched areas indicate 100 % exhausted resin, and white areas, regenerated resin. displacement or slow rinse stage is followed by a rapid rinse stage at a higher flow rate to remove the last traces of regenerant. The concentration of the residual regenerant is measured at the end of the operation and, as soon as this falls to an appropriate limiting value, the next cycle begins. When water is being demineralized, the electrolytic conductivity of the emergent liquor is measured Methods for Overcoming Equilibrium Problems Counterflow Regeneration [24, 25]. In the traditional method, the resin is regenerated by flow in the same direction as that during the service stage (coflow). Figure 21 illustrates the exhaustion of the bed in this case. For economically acceptable quantities of regenerant, the bottom of the bed is only partially regenerated, which results in high leakage (see Section 7.2). This effect is of major importance in the demineralization of water. The leakage problem can be overcome by regenerating in the reverse direction (i.e., from bottom to top). The lowest resin layers reach equilibrium with fresh, uncontaminated acid and are therefore completely regenerated. This minimizes permanent leakage during the next cycle. Several counterflow regeneration systems are described in Section Among these, some have an upflow exhaustion phase with downflow regeneration. The bed must be kept perfectly compacted when operating in upflow mode to maintain contact between the ions in solution and the resin beads. Reverse flow regeneration is often called countercurrent, which implies two streams flowing in the opposite direction. The terms counterflow or reverse flow are used here, both meaning only that the direction of percolation is reversed between service and regeneration. The difference between coflow and counterflow regeneration is illustrated in Figure 25, and their effects on treated water quality are shown in Figure 26. Two-bed demineralization with counterflow regeneration produces water with a conductivity 2.0 ms/cm, even from feedwater with a high sodium content, and the consumption of regenerant is much lower than with coflow regeneration. Figure 25 also shows that in counterflow regeneration, ions taken up by the resin are desorbed over the shortest path instead of passing through the whole bed; the lower layers are always regenerated well. Figure 26. Effects of coflow and counterflow regeneration on ionic leakage

34 506 Ion Exchangers Vol. 19 In order not to disturb the arrangement of resin layers, the bed is only backwashed every cycles. Rinse Recycling. Considerable economy can be achieved by recycling the rinse water to remove traces of salts and regenerant, which slowly diffuse out of the resin beads into the treated water and thus interfere with its quality during the early part of the service run. Each column is first rinsed until the conductivity of the rinse water decreases to a value equal to or less than that of the feedwater. The rinse water is recycled through the two columns in series in a closed circuit, so that each resin removes the last traces of regenerant from the other. The amount of rinse water used is reduced significantly and the quality of the treated water is improved. Buffer Column. The final ion leakage at the output from the demineralization units consists of sodium hydroxide and silica (see Section 7.4). The ph is slightly alkaline, normally between 7 and 9. If a neutral effluent is required, sodium hydroxide leakage can be taken up by a cationexchange material, either weakly or strongly acidic. This is known as a buffer column or cation polisher. Since the reaction is a pure neutralization, it is very fast and efficient. The resin can be regenerated in thoroughfare with the cation exchanger of the demineralization system after each of the resin s service runs. The other method is to wait until the cation polisher is exhausted to sodium, which can take several weeks. Weakly acidic resins loaded with sodium ions may swell considerably, so an osmotically stable carboxylic resin is required for this application, or a strongly acidic resin is used. Mixed Beds. When beads of a cation- and an anion-exchange resin (R C and R A, respectively) are mixed intimately, the normal equilibrium for each resin is displaced: R C Hþ þna þ Cl þr þ A OH!R C Naþ þr þ A Cl þh 2 O This reaction produces water. The exchange process is no longer reversible and continues until completion. Mixed-bed resins in welldesigned equipment reduce the concentration of dissolved salts to 0.01 mg/l, giving a conductivity of ms/cm at 25 C, equal to that of completely pure water. Regeneration is carried out after the two resins have been separated (see Section ). The advantages of mixed beds are twofold: (1) they produce water of excellent quality in a single stage, and (2) final rinsing is very rapid because the regenerants neutralize each other. However, the operating capacities of the resins are quite low, and the end of the cycle (breakthrough) is very abrupt. If the ionic concentration of the feedwater is low (i.e., for polishing), the limiting factor is the specific flow rate. Ordinary gel-type resins are used up to about 40 BV/h and macroporous resins up to BV/h. The amounts of regenerant used are on the order of 100 g of acid per liter of cation-exchange resin and 100 g of sodium hydroxide per liter of anion-exchange resin. The ratios of resin volumes used (R A OH : R C H) vary between 2 : 1 and 1 : 1. If the feedwater contains neutral salts, the 2 : 1 ratio is chosen; if only traces of sodium hydroxide from the demineralization train are present, the 1 : 1 ratio is selected. In mixed beds for general use where the feedwater has a high ionic concentration, each of the exchange resins is calculated as if it were in a separate bed. The capacities are multiplied by a reduction factor of about 0.8 to allow for imperfect regeneration caused by the resin mixing zone after backwash and separation, as well as by the mutual diluting effect of the resins. Continuous Systems. In continuous systems (always used with counterflow regeneration), the exhausted resin layers are extracted continuously or, more frequently, semicontinuously at intervals of a few minutes. They are replaced by freshly regenerated resin introduced at the point where the treated liquid emerges from the column. The total volume of resin is reduced by rapid circulation. In large industrial demineralization plants, a constant supply of treated water is often required. With fixed-bed systems, this necessitates two alternately operating lines or a large treated-water tank to secure water supply during regeneration period. With continuous systems, a single unit can be used, which ensures that the quality of the treated liquid is virtually constant. The plant must be completely automated. The main disadvantage of these systems lies in the complexity of their construction. Examples are described in Chapter 10.

35 Vol. 19 Ion Exchangers Ion-Exchange Resin Combinations Many combinations of ion-exchange units are employed in water treatment; only the most widely used are considered here and summarized in Figure 27. They include pretreatment, softening, demineralization (primary system), and polishing. Softening exchanges Ca 2þ and Mg 2þ for Na þ ions, whereas demineralization exchanges all cations for H þ and all anions for OH ions. Polishing removes the cations and anions that have escaped the first-stage demineralization Pretreatment Before being allowed to percolate through an ion-exchange column, the solution to be treated must be thoroughly filtered to free it from suspended matter. Organic matter can also be partly removed by flocculation, by using ferric chloride or aluminum sulfate with the possible addition of polyelectrolytes. When the raw water contains a high proportion of bicarbonate hardness (see Section 7.3), the classical process of lime softening may be used to remove alkaline salts as calcium carbonate. This process, known as de-alkalization or carbonate removal, reduces the ionic load on subsequent ion-exchange units Softening Pretreatment can be followed by softening (Figure 27, route a) with a single strongly acidic cation-exchange resin regenerated with sodium chloride. Carbonate removal can also be combined with softening (Fig. 27, route b) to reduce the salinity of hard alkaline water and remove the hardness for use in low-pressure boilers and as cooling water. The ph may have to be increased from 4.5 to ca. 8 with sodium hydroxide Demineralization (Primary System) The abbreviations used in this section are defined in Figure 27. Degasser. As soon as the alkalinity of the raw water exceeds ca. 1 meq/l, an atmospheric degasser should be used in the system. When the solution flows through the cation-exchange resin, bicarbonate ions (and carbonate ions if any) are converted into carbon dioxide, which is removed in the degasser instead of passing to the strongly basic resin and being taken up there. A degasser is assumed to be present in all systems except those with mixed beds. The cation-free water at the degasser outlet contains meq of free carbon dioxide per liter. SAC WBA DG System (Fig. 27, route c). If the raw water is hard, the SAC WBA DG system produces water with < 5 mg of dissolved salts per liter, together with silica and ca. 15 mg of carbon dioxide per liter. If the raw water is soft, however, sodium leakage can be quite high unless the SAC is regenerated in reverse flow mode. This type of arrangement is used to produce water for medium-pressure boilers and for chemical processes. The degasser may be placed after the weakly basic anionexchange unit to obtain maximum capacity from the resin (see Section 7.4). SAC DG SBA System (Fig. 27, route d). This simple system produces very pure water suitable for most applications. The water quality obtained depends on the regeneration system and the amount of regenerant. With coflow regeneration, a conductivity of 5 25 ms/cm and a residual silica content of mg/l can be achieved, depending on the quality of raw water. With counterflow regeneration, the corresponding values can reach 1 ms/cm and 5 25 mg of silica per liter. Small plants are often built without a degasser. General-Purpose Mixed Bed (SACþS- BA). A working mixed bed system (Fig. 27, route e) produces very pure water (0.1 1 ms/ cm; mg of SiO 2 per liter), but its regeneration efficiency is moderate because it is impossible to fit a degasser. The system is therefore restricted to small laboratory units or water with very low salinity, such as permeate from a reverse osmosis plant. SAC WBA DG SBA System (Fig. 27, route f). This system produces the same quality

36 508 Ion Exchangers Vol. 19 Figure 27. Combination ion-exchange systems for water treatment DG ¼ atmospheric degasser; IN ¼ inert resin; SAC ¼ strongly acidic cation exchanger; SBA ¼ strongly basic anion exchanger; WAC ¼ weakly acidic cation exchanger; WBA ¼ weakly basic anion exchanger

37 Vol. 19 Ion Exchangers 509 of water as the SAC DG SBA system (route d). Its advantage lies in the higher regeneration efficiency due to the weakly basic resin. The anion exchangers are regenerated in series in the direction from SBA to WBA, so that an excess of clean caustic soda passes through the final resin, usually in sufficient amounts to regenerate the weak resin. WAC SAC WBA DG SBA System (Fig. 27, route g). A weakly acidic cation resin can be used in all the above systems for appropriate types of raw water (i.e., high alkalinity and even greater total hardness; see Section 7.4). The system considered here should be used for large plants dealing with highly saline water in order to use the high efficiency of the WAC and WBA resins to reduce the cost of regeneration. Acid regeneration is carried out in the direction from SAC to WAC, and sodium hydroxide flows from SBA to WBA as in route f. Layered Beds. Weakly and strongly acidic resins can be superposed in the same column, because a carboxylic resin is less dense than a sulfonic resin. In addition, fine particle size is used for the WAC and coarse particle size for the SAC so that the two layers are still wellseparated after the backwash. The same applies to anion exchangers. Ion-exchange units containing superposed weakly and strongly active resins are called layered or stratified beds. These dual resin systems (Fig. 27, routes h j) have a high efficiency only if counterflow regeneration is used. They then produce water of excellent quality (1 ms/cm, 5 20 mg of SiO 2 per liter) with low regenerant consumption (regeneration efficiencies close to 100 %) all in a plant with only two exchange columns. Calculation of the capacities and maximization of the performance of the system are complex operations, but they can be carried out rapidly on a suitably programmed computer. Due to the inevitable partial mixing of the two resin layers after some time of operation, layered beds are not as efficient as physically separated resin beds, such as those in multi-compartment packed bed systems Polishing To remove the last traces of salts, a polishing unit can be installed after the primary demineralization system. A final buffer column containing a sulfonic, or sometimes a carboxylic exchanger (Fig. 27, route k) can remove sodium leakage from an anionexchange resin (see Section 8.2, buffer column). Two-column polishing (Fig. 27, route l) was often used before counterflow regeneration became widespread. The sodium and silica leakages were taken up by a strongly acidic and a strongly basic exchanger in series. Regeneration was carried out from SBA2 to SBA1 and from SAC2 to SAC1, a primitive kind of counterflow system, often referred to as thoroughfare regeneration. Today, separate-bed polishing is again used in multicompartment columns with reverse flow regeneration (see Section ). The mixed-bed polisher (Fig. 27, route m) is the most widely used polishing system, with its three-component variation (Fig. 27, route n; see also Section ). With a mixed bed, a theoretical conductivity of pure water (0.055 ms/cm at 25 C) can be achieved; ion and silica leakages are ca. 1 mg/l. Because the mixed-bed polishing system has to deal with only a low ionic load, it has a long service run, and is regenerated only every cycles of the primary system. If the weak acid concentration in the raw water is low, the SAC WBA DG MB system can be used and all the weak acids are then retained in the strongly basic resin of the mixed-bed polishing system Choice of Resin General Selection Criteria. To ensure efficient operation of an ion-exchange installation, minimize running costs, and obtain a maximum resin lifetime, the resins must be selected carefully. Before individual resins are selected, the following questions should be answered: 1. Is the solution to be treated aggressive? Does it contain oxidants? 2. What is the operating temperature? 3. Is the flow velocity high or moderate? 4. Is the resin bed deep or shallow?

38 510 Ion Exchangers Vol Is a high operating capacity required? 6. Is the expected leakage very low? 7. Are the cycles long or short? 8. Is the regenerant hot? 9. Is the regenerant very concentrated? 10. Are the resins used in a fixed, packed, or fluidized-bed unit or in a continuous system? 11. Are the resins used in stratified-bed or in mixed-bed units? 12. Does the regeneration process have a resin transfer step? 13. Is an effluent TOC strictly limited? 14. Is the application in the food-processing or nuclear industry? The answers to these questions provide useful hints for resin selection: 1. In the presence of oxidants, a resin with a high degree of cross-linking should be used (see Section 3.3). 2. High operating temperature is critical for the choice of strongly basic resins: above 40 C, only type 1 resins should be used (see Section 3.3). 3. For a high flow rate, physically stable resins and a relatively coarse particle size should be used. 4. With deep beds, coarse resins should be used to minimize the pressure drop. 5. If a high capacity is required, gel-type resins are preferred over macroporous types; type 2 or acrylic anion resins give a higher capacity than type 1. For condensate polishing applications, or for strongly basic resins placed downstream from a weakly basic resin, a high operating capacity is not of prime importance, but flow rate is a dominating factor. 6. For low leakage, resins with high crosslinking (i.e., high selectivity) associated with high regenerant dosage will give the lowest concentration of residuals. 7. Long cycles increase possible fouling risks and tend to compress resin beds and create large pressure drops; mechanically strong resins are thus advisable. Short cycles mean frequent osmotic shocks; in this case, macroporous resins are generally more stable. 8. For a hot regenerant, see item Concentrated regenerant also mean stronger osmotic shocks; see item Packed beds, fluidized beds, and continuous systems call for special particle sizes. 11. In stratified-bed or mixed-bed units, the particle size and density of the resins must be selected to obtain perfect resin separation after backwash. 12. Resin transfers result in mechanical stress: tough resins must be selected. 13. For low effluent TOC, special resin grades must be used that give rise to a minimum of leachables. On the other hand, if the influent solution contains organics, the anion resin must be capable of removing them reversibly so that it does not become fouled (see Section 7.3). 14. For food processing or nuclear waste treatment, special resin grades are prepared by major resin manufacturers to comply with applicable regulations or specifications. Weakly Acidic Cation Exchangers. For water dealkalization or as a first unit in a series of demineralization steps, a high-capacity carboxylic resin should be selected. The osmotic stress for such a resin operating between the H þ and the Ca 2þ or Mg 2þ forms is generally moderate, and most weakly acidic resins are mechanically very stable. In applications involving the Na þ or NH þ 4 forms, however, osmotic stability is essential; the best choice is a resin with limited swelling and a macroporous structure. Strongly Acidic Cation Exchangers. For softening or demineralization of water, gel-type cation-exchange resins are generally chosen. Sulfonic resins have a maximum capacity for a degree of cross-linking corresponding to 8 % DVB (Section 3.1). Gel resins with 6 % DVB can be used for water softening, and 10 % DVB resins for the production of highly pure water. Macroporous resins are restricted to special applications, continuous processes, and the treatment of condensates. Weakly Basic Anion Exchangers. A wide variety of weakly basic anion exchangers are available with different matrices and porosities. The main types are as follows: 1. Macroporous polystyrene resins are the most widely used and the strongest, but the capacity of the standard grades is not very high. For high ionic loads and low flow rates, some

39 Vol. 19 Ion Exchangers 511 Figure 28. Operating capacities of weakly basic resin (SO 2 4 : Cl ¼ 1 : 1) a) Polystyrene resin; b) Polyacrylic resin K * ¼ 10 4 ½CO2Š2 where FMA is the free mineral acidity ðfmaþðfrþ and FR is the flow 2 rate in bed volumes per hour. manufacturers offer special grades with lower moisture and higher volume capacities. 2. Polyacrylic resins have a higher capacity and excellent resistance against organic fouling. They are, however, more expensive and sometimes difficult to rinse after regeneration. 3. Phenol formaldehyde resins are generally restricted to special applications, but they have a strong adsorptive power for organic matter. Figure 28 illustrates the comparative operating capacities of types 1 and 2. The capacity is plotted against an arbitrary factor K * which takes into account the water salinity and the flow rate. 1. Polystyrene resins type 1 are very strongly basic and therefore achieve extremely low silica leakage. They withstand high temperature quite well and can be regenerated with sodium hydroxide up to 50 C. However, their regeneration efficiency is only moderate, so that their capacity is rather low and they are subject to irreversible fouling by organic matter. 2. Polystyrene resins type 2 are slightly less basic and have a better regeneration efficiency and higher capacity (about 20 % higher than type 1). They become fouled less quickly but withstand high temperature less well (see Section 3.3). Their silica leakage is higher. 3. Polyacrylic resins combine some of the advantages of types 1 and 2 polystyrene resins: low silica leakage, good capacity, moderate regeneration efficiency, high physical stability, and good elution of organic matter adsorbed. However, they are 25 % more expensive than polystyrene resins and can withstand only moderately high temperature ( 35 C). The partially quaternized acrylic resins described in Section 7.4 (strongly basic resins) have a particularly high capacity. Figure 29 shows the basic operating capacity of the three types of resin for water with a 1 / 1 Cl /SO 2 4 ratio with coflow regeneration. Figure 30 indicates the regenerability of the resins. One or Two Anion Resins? Most water to be demineralized contains salts of both strong and weak acids, the former frequently in high proportion. The economic advantage of using weakly and strongly basic anion-exchange resins in series, due to considerable savings in caustic regenerant, is demonstrated in Section 9.3. Strongly Basic Anion Exchangers. Like cation exchangers, strongly basic anion exchangers are gel-type resins which are widely used for water treatment because of their high capacity. Macroporous resins are restricted to continuous systems, condensate polishing and special applications. A distinction is made between the following three types: Figure 29. Basic operating capacity of strongly basic anion exchangers as a function of the amount of regenerant a) Polystyrene resin type 1; b) Polyacrylic resin; c) Polystyrene resin type 2; d) Dual base acrylic resins

40 512 Ion Exchangers Vol The rate at which treated water is required 2. The specified quality of treated water 3. The capital cost of the plant 4. The running costs of the plant Figure 30. Regenerability of strongly basic anion exchangers a) Polystyrene resin type 1; b) Polyacrylic resin; c) Polystyrene resin type 2 For small and medium-sized plants (up to ca. 40 m 3 /h), capital cost considerations are generally predominant, so that a series of the type SAC DG SBA is often selected. Because of its relatively good regeneration efficiency, a type 2 polystyrene resin is a good candidate for the anion exchanger. Dual base acrylic resins (see Section 7.4) offer an even higher operating capacity (0.8 to >1 eq/l) with low regenerant consumption if the influent water contains less than 5 % silica. The regenerant usage is 140 % of the capacity in reverse flow regeneration. Layered-bed systems are used for bigger plants. In very large installations, however (>100 m 3 /h), hydraulic considerations often call for separate weak base and strongly basic resin columns or double-compartment units. The strongly basic resin is generally a type 1 or an acrylic resin with a strong base functionality. 10. Plant Design For a detailed discussion, see [27] General Considerations The size of the water treatment plant, number of columns, choice of resins, and type and amount of regenerant are largely determined by Small, mass-produced ion-exchange plants generally use large amounts of regenerant to maximize water quality and operating capacity. Running costs are high, but the user benefits from low capital cost and operational simplicity. Where very large volumes of water are required (e.g., plants for municipal softening and nitrate removal or in heavy industry), emphasis is placed on the reduction of running costs, leading to the systematic use of weak and strong resin combinations, even if this increases capital cost a little due to a greater complexity of the system. With a judicious choice of resin combination, reverse flow regeneration and polishing units, large quantities of ultrapure water can be produced in efficient, compact plants. The water is used, for instance, to feed high pressure boilers in the power industry, or to clean sensitive parts in the production of semi-conductors and electronic devices (! Water, Ultrapure, Section 3.1). A complete treatment cycle consists of four phases (Section 8.1): 1. Service, i.e., the phase during which the resin is exhausted 2. Decompaction of the bed by backwash 3. Regeneration 4. Rinsing In units regenerated by reverse flow, decompaction is only carried out occasionally so as not to disturb the resin layers and lose the advantages associated with counterflow operation. Some systems provide for cleaning the upper layer each cycle by backwashing only the top 15 cm of the bed Fixed-Bed Ion-Exchange Units Column Diameter and Bed Depth Column diameters vary from a few centimeters (in laboratory units) to 5 m. Industrial units usually have diameters of m. The depth of the resin bed also varies from 10 cm to about 3.5 m. For standard industrial units, bed depths

41 Vol. 19 Ion Exchangers 513 should generally not be less than 70 cm. The maximum depth depends on the head loss that can be tolerated. Most units have resin beds from 1 to 2 m in depth. The main problem lies in producing even flow across the bed, because the liquid to be treated and the regenerant must be in uniform contact with the whole volume of resin to allow stable horizontal wavefronts to develop and move vertically through the beds Small-Scale Units Figure 31. Small-scale water softener a) Control unit; b) Resin; c) Solid salt; d) Saturated brine Figure 31 shows a typical column designed for domestic water softening with a diameter up to about 50 cm. The outer cylinder is constructed from polyester reinforced with fiber glass. The water and regenerant inlet and the backwash outlet are combined in a single large strainer formed by a plastic cylinder with slits ca mm wide through which resin beads cannot pass. The treated water outlet is formed by a pipe concentric with the input strainer and fitted with a polyamide screen at the bottom. This type of unit is fully automatic; a multiport valve is fitted directly on top of the cylinder so as to form a compact arrangement with the system of inlet and outlet pipes. Small counterflow units, such as those normally used for domestic softeners, can be extremely simple, consisting of a column similar to that shown in Figure 31 but completely filled with resin. Although backwashing is not possible, the units operate satisfactorily because any dirt is filtered out at the top of the resin bed and washed out during reverse-flow regeneration Industrial Co- and Counterflow Plants Industrial demineralization units are typically rubber-lined and fabricated from steel. A rubber layer about 3 mm thick is needed to protect the steel against acid corrosion. Various distribution and recycling systems are used to ensure even flow across the resin. Four examples are illustrated in Figure 32. In coflow regenerated units, a regenerant distributor should be located just above the resin bed, as shown in Figure 32 A. In some simple columns, the regenerant is introduced at the top of the column. Regeneration is less efficient, as the regenerant is diluted before it reaches the resin. Because the columns are constantly filled with water during the whole cycle, space above the resin bed must be sufficient to allow for expansion of the bed during the backwash phase. When introduced at the same level as the upper surface, dilution is negligible. Uniformity of flow is controlled by the collection system at the bottom of the column rather than by the inlet. In Figure 32 A, the collection system is buried in successive layers of gravel, or pieces of crushed flint (h, i) or anthracite from 2 to 10 mm in size; the coarser layers are at the bottom and the finer at the top. The collection system consists of a main header pipe onto which a set of perforated lateral pipes is welded or screwed, the whole system being supported by a base plate. The space between this plate and the convex bottom of the column is filled with concrete (m) for strength and rigidity. This long-established system is still used for softeners or simple demineralizers and gives a very uniform flow. Figure 32 B shows a unit with counterflow (upflow) regeneration. The treated water collector (j) has lateral pipes fitted with small plastic filtering nozzles or strainers (two types of strainer are illustrated in Fig. 33). Slits in the strainers allow water to pass but retain the resins. To obtain uniform collection, the lateral pipes must be spaced at intervals of less than 15 cm and form a network at the bottom of the column. This

42 514 Ion Exchangers Vol. 19 Figure 32. A) Coflow regenerated unit; B) Counterflow regenerated unit with plunging strainers; C) Counterflowregenerated unit with nozzle plate; D) Floating bed unit with two nozzle plates a) Inlet for raw water and outlet for backwash water; b) Regenerant inlet; c) Outlet for treated water; d) Resin; e) Manhole; f) Sight glass; g) Breather or vent; h) Inlet for backwash water; i) Support layer; j) Collector system with strainers; k) Nozzle plate; m) Concrete; n) Splash plate; o) Pillar support; p) Resin transfer port network can retain traces of regenerant that subsequently leak into the treated water. In large industrial units, the particles filtered out of the resin bed must be removed with a backwash. Many systems are available to keep the bed compact during service and regeneration while allowing it to expand during an occasional backwash. A broad distinction can be made between designs with a large freeboard, in which a backwash can be carried out in the ionexchange vessel itself, and packed-bed designs, in which the ion-exchange vessel is fully packed with resin, without a freeboard. Buried collector systems, as shown in Figures 32 B and 32 C, are widely used in designs that have a freeboard. Figure 34 shows how they are operated. During the exhaustion phase (A), the solution to be treated flows from the top to the bottom of the unit; regeneration is carried out by

43 Vol. 19 Ion Exchangers 515 part in the ion-exchange process because it is not regenerated. Air-holddown systems use an inert polymer instead of this inactive resin layer, to protect the ion-exchange resin and prevent its drying out during regeneration. Figure 33. Examples of strainers used for industrial watersoftening plants a) Simple plate strainer (Johnson); B) Double strainer for two-chamber floating beds (Kleemeier, Schewe & Co, Herford, Germany) introducing the regenerant at the bottom and extracting it through the collector (C); the resin bed is maintained in a compact state by a current of water from the top of the column or by compressed air which leaves with the spent regenerant through the collector. This type of construction enables the upper resin layer to be backwashed from the collector without disturbing the lower layers (B). In a variation of this process, the buried collector is replaced by a number of downpipes with strainers at their ends (see Fig. 32 B); these long, candle-shaped tubes penetrate the resin. This system restricts the water flow less and gives a more even collection. In both systems, the layer of resin above the collector does not take Nozzle plates, as shown in Figures 32 C and D, offer a very efficient collection method and have become standard for all packed-bed systems. The plates consist of a flat perforated steel plate fitted with screwed-on strainers. The number of strainers per square meter is determined as a function of the flow rate. Usually the interval between the individual strainers is about 15 cm. The plate must be supported by L-beams or small pillars to withstand the considerable operating pressure. The perforated plate and the space underneath must be lined with vulcanized rubber. A manhole or fisthole is necessary in the dished end for the fitting of the individual nozzles. Packed-Bed Column. A packed bed column is shown in Figure 32 D: the resin is kept between two nozzle plates (k), and there is almost no freeboard. The particular design here is a floating bed unit (Amberpack) with upflow loading. Packed bed units have several advantages:. Absence of internals such as regenerant collectors causing obstruction and flow disturbances. Better utilization of space (no empty volume) Figure 34. Buried collector system for counterflow regeneration A) Service (exhaustion) phase; B) Surface backwash phase; C) Regeneration phase

44 516 Ion Exchangers Vol. 19 collector (b) that is buried below the bed surface at about one-third of the resin bed depth. The regenerant distributor (a) is located just above the bed surface. The regenerant is introduced simultaneously through the distributor and at the bottom of the column. The top stream keeps the bed compacted and regenerates the most exhausted part of the resin, whereas the bottom stream ensures that the lowest layers are always completely regenerated. This system also allows the upper part of the bed to be backwashed without disturbing the lower layers. Figure 35. Stratified-bed unit a) Weakly acidic or weakly basic resin; b) Strongly acidic or strongly basic resin; c) Regenerant collector; d) Nozzle plate. Absence of inactive resin: the complete bed is regenerated. Simplicity of construction Stratified-Bed Units. In upflow regenerated columns using the air- or water-holddown technology, a combination of weakly and strongly active resins can be used in the form of a layered or stratified bed (Figure 35). Selected resin grades with a suitable particle size and density are required to prevent mixing of the layers. Split-Flow System [28]. As shown in Figure 36, the split-flow system has a regenerant Figure 36. Split-flow system a) Regenerant distributor; b) Regenerant collector; c) Resin; d) Nozzle plate Econex System. In the Econex system of Davy Bamag (Fig. 37) [29], the upper part of the column is filled with inert material (b), which is used for compaction and can be removed to decompact the bed. Floating-Bed Unit. The floating bed, invented and patented by Bayer in 1963 [30], is a reverse counterflow system (Fig. 38). The unit consists of a vessel fitted with an upper and a lower nozzle plate. During the service phase, the flow of water is from bottom to top, thus pushing the whole resin bed up and compacting it against the top nozzle plate. The column is regenerated from top to bottom, the bed sitting on the bottom plate without the need of a compacting method. To prevent fluidization of the resin during service, a minimum flow rate is required (about 25 m/h at 15 C) and the vessel is filled almost completely with resin. This type of unit has been quite successful thanks to its excellent performance, simplicity and compact design. It is, however, sensitive to the presence of suspended solids in the feed. A layer of light, inert, unbreakable resin granules may be used to prevent the fine particles or pieces of active resin from clogging the upper strainers. A variation of the floating bed has two chambers that are separated by a floor fitted with double strainers (d) and allows two resins to be placed in a single column (Fig. 39). The resins are regenerated in the direction from SAC to WAC or from SBA to WBA (see Section 9.3). Amberpack System. The Amberpack system of Rohm and Haas [31] is an improved floating-bed unit. It has one or two small ancillary backwash columns into which portions of the resin beds can be transferred for the

45 Vol. 19 Ion Exchangers 517 Figure 37. Econex system a) Water distributor and regenerant collector; b) Coarse inert resin for bed compaction; d) Ionexchange resin; e) Nozzle plate; f) Tank for inert resin during backwash removal of fines or suspended solids, as shown in Figure 40. UFD System. In the UFD system of Degremont (Fig. 41) [32], the resin fills the column completely (as in the floating bed), but percolation occurs in the conventional direction (downflow), and regeneration in the floating-bed mode (upflow). UpCoRe System. This system, proposed by Esmil [33] and licensed to Dow Chemical under the name UpCoRe, is very similar to the UFD system. It is less sensitive to the presence of suspended solids than the floating-bed system. However, regeneration of the bed at a high flow rate makes it slightly less efficient. Recoflo System. The Recoflo system is a shallow-bed process offered by Eco-Tec (Fig. 42) [34], using fine mesh resins (particle size mm) between two distributing and collecting plates. The bed depth is usually 15 cm, and the run length is typically only 5 30 min. Regeneration time is correspondingly short. Flat Bottom. For the treatment of valuable solutions (see Chap. 12), special units with a flat bottom and a precisely engineered collection system are frequently used to minimize the void Figure 38. Floating-bed unit a) Inert floating resin; b) Ion-exchange resin; c) Void space; d) Nozzle plate

46 518 Ion Exchangers Vol. 19 Figure 40. Amberpack system a) Nozzle plate; b) Resin; c) Nozzle plate with double nozzles; d) Lower transfer port; e) Upper transfer port; f) Flexible transfer hose; g) Backwash column; h) Backwash collector Figure 39. Double-compartment floating bed a) Outlet nozzle plate; b) Inert resin for protection of nozzles; c) Strongly acidic or strongly basic resin; d) Separation plate with double nozzles; e) Weakly acidic or weakly basic resin; f) Inlet nozzle plate with double nozzles; g) Sight glass volume and the risk of regenerant hide-out, i.e., a dead zone from which spent regenerant is difficult to wash out during rinsing steps. Hide-out is usually due to poor hydraulic distribution (channeling) or to an imperfect collecting system Mixed Beds General Case. The mixed-bed principle was described in Section 8.2. Successful operation depends on the fact that the resins are separated by backwashing and settling before each component is regenerated. The densities of the resins normally used are for the strongly basic resin and for the strongly acidic resin. These are sufficiently different for the resin to be well-separated by backwashing, provided the cation exchanger does not contain too many fine particles or the anion exchanger too many large beads. Uniform particle size resins (see Section 3.5) are ideally suited for mixed-bed units. A typical mixed-bed design is shown in Figure 43. Untreated water enters at the top of the column through a strainer that also acts as the backwash outlet and prevents loss of resin. A Figure 41. UFD system a) Inert resin for protection of collector; b) Water distributor and regenerant collector; c) Void space; d) Resin; e) Nozzle plate with counterflow valves

47 Vol. 19 Ion Exchangers 519 Figure 42. Recoflo system a) Distributor collector; b) Bed of fine resin, depth ca. 15 cm sodium hydroxide distributor (c) is placed just above the bed top, and an intermediate central collector (d) is located at the interface between the two resins. The operational cycle is illustrated in Figure 44 and consists of the following stages: 1. Service (Fig. 44 A) 2. Backwash for bed separation and subsequent settling (B) 3. Regeneration of the anion exchanger (C), followed by a rinse (the spent regenerant and rinse water leave via the central collector) 4. Regeneration of the cation exchanger with acid introduced through the bottom of the column and extracted through the central collector (D) followed by a rinse 5. Re-mixing of the resins with compressed air (E), followed by a final rinse Figure 43. Mixed-bed ion-exchange system a) Breather; b) Raw water distributor; c) Sodium hydroxide distributor; d) Intermediate collector; e) Strainer rack A small counterflow of water is advisable during the two regeneration stages to restrict the diffusion of acid into the anion exchanger and of sodium hydroxide into the cation exchanger. The two resin beds can also be regenerated simultaneously, with both spent regenerants leaving through the central collector. Although this system is very efficient, problems may arise due Figure 44. Operational phases of a mixed-bed cycle A) Service; B) Separation; C) Regeneration of anion exchanger; D) Regeneration of cation exchanger; E) Remixing with air o Anion-exchange resin;. Cation-exchange resin

48 520 Ion Exchangers Vol. 19 prevents cross-contamination, improves regeneration efficiency, and reduces ionic leakage [36, 37]. However, the inert beads, being less hydrophilic than ion exchange resins, can cause operational problems. The availability of resins with uniform particle size and sophisticated regeneration technologies have gradually reduced the popularity of the Triobed concept. Figure 45. Triobed process a) Anion-exchange resin; b) Inert resin; c) Cation-exchange resin to precipitation of calcium carbonate or ferric hydroxide when the acid and the alkaline effluents meet. However, this situation is encountered mainly in general-purpose mixed beds and only rarely in polishing systems. Three-Component Mixed Beds. Threecomponent mixed beds, known under the trade name Triobed, overcome the problem of mixing of resins at the interface [35]. A layer of inert polymer beads, with a density intermediate between those of the cation- and anionexchange resins, is mixed with both. Backwashing causes the mixture to split into three distinct layers, giving complete separation between the two ion exchangers (Fig. 45). This External Regeneration. In the treatment of condensates or in polishing that follows a demineralizing system, the water to be treated by the mixed bed has a very low salinity. As a result, only a small exchange capacity is required and the units are therefore made as small as possible. They operate at very high specific flow rates of up to 130 BV/h. Any internal obstruction in the column can cause high pressure; therefore, these units are constructed as simply as possible (one rack of strainers, bed depths of ca. 1 m to avoid excessive head loss). To prevent accidental inlet of regenerant chemicals into the condensate circuit, the resins are transferred to separate regeneration units that operate at much lower flow rates. A single regeneration station can serve a series of mixed-bed units, as shown in Figure 46. It may consist of several separate columns, specially designed to produce maximum separation of the resins and the best possible regeneration efficiency: 1. A separation column, also used for regenerating the cation exchanger Figure 46. Mixed-bed external regeneration station MB: Mixed-bed unit; CRT: Cation-regeneration tower; ART: Anion-regeneration tower; MSV: Mixing and storage vessel

49 Vol. 19 Ion Exchangers 521 Figure 47. Condensate polishing unit a) Condensate distributor; b) Mixed resin; c) Condensate collector; d) Clean resin return; e) Resin transfer ports; f) Dirty resin transfer 2. A column to which the anion exchanger is transferred for regeneration 3. A mixing and waiting column An example of a mixed-bed unit as used for condensate polishing is shown in Figure Other Ion-Exchange Polishers In the Amberpack Sandwich polisher of Rohm and Haas (Figure 48), a strongly acidic cationexchange resin is combined with a strongly basic Figure 48. Amberpack Sandwich polisher a) Strongly acidic resin; b) Strongly basic resin; c) Nozzle plate; d) Separation plate with double nozzles; e) Regenerant inlet 1 and regenerant outlet 2; f) Transfer port; g) Regenerant outlet 1; h) Regenerant inlet 2 anion-exchange resin in two separate compartments of a column. The two compartments communicate via a plate fitted with double nozzles. A collecting device located just below the separation plate serves as a regenerant collector for the upper compartment and a regenerant distributor for the lower compartment. In the Tripol process of Permutit-Boby (now Veolia Water) [38, 39], three resins, (SAC1/ SBA/SAC2) are placed in three shallow compartments of a single column. The second cationexchange resin compartment, which is always very highly regenerated, acts as a final sodium trap. The Multistep process of Bayer [40] is essentially the same as the Tripol process but operates in the opposite direction, in the floating-bed mode Continuously Circulated Ion- Exchange Resins [41] As discussed in Section 8.2, automatic units in which the resin circulates continuously or intermittently should offer the following advantages:. Smaller floor areas. Smaller resin volume. Continuous flow of regeneration liquids In all resin-circulating units the resin flows counter to the solution to be treated. The exhaustion, regeneration, and rinsing stages occur simultaneously in different columns or in different sections of the same column. The difficulties associated with circulation of the resin, its more rapid exhaustion than in fixedbed units, and the complex construction of continuous systems have restricted their successful application. The counterflow processes described in Section have now replaced these continuous processes. However, mention should be made of some examples that are still in operation: 1. The Asahi process [42], invented in Japan and finally developed in France by Degremont (Fig. 49); 2. The Kontimat process [43] developed by Hager und Els asser; and 3. The Higgins process of Chem-Seps [44].

50 522 Ion Exchangers Vol. 19 Figure 49. Mixed-bed demineralization (Asahi process) a) Service column; b) Backwash and separation column; c) Anion exchanger regeneration column; d) Cation exchanger regeneration column; e) Regenerated cation-exchange resin; f) Regenerated anion-exchange resin; g) Mixing chamber; h) Valve water; mixed resin; cation exchanger; anion exchanger External Valves and Pipework A typical arrangement of external valves and pipework is shown in Figure 50. In the case of automatic softeners, separate valves can be replaced by a single multiport valve actuated by means of an electronic scheduler. For large systems, separate pneumatically or hydraulically controlled valves are used Control Systems The quality of water produced by a plant must be monitored. As soon as it falls below a previously specified level, the service stage is stopped and the regeneration stage initiated. This section briefly describes the measuring instruments used in monitoring. Timers. For domestic water softeners, daily consumption is assumed to be constant, and the softener is scaled to require regeneration at fixed intervals (e.g., once every three days). A timer initiates the process automatically, and regeneration frequency can be varied according to changing requirements. The method is simple and reliable, but wasteful, because regeneration is always initiated before the softener is exhausted. Quantity Measurement. In communal buildings or laundries, where a cheaper and more flexible method is required, a contacting-head flow meter can be installed which automatically initiates regeneration as soon as a given volume of water has passed. This method is accurate as long as the hardness of the water supply does not vary. Automatic Hardness Detector. A colorimetric hardness analyzer can be installed at the output of the softener to initiate regeneration when the allowed level of leakage is reached. This avoids waste of regenerant, but the analyzer is relatively expensive and the method is used chiefly in large industrial installations. Figure 50. External pipework and valves a) Raw water inlet; b) Treated water outlet; c) Regenerant inlet; d) Regenerant outlet; e) Recycle outlet; f) Fast rinse outlet; g) Sample point for quality analysis Conductivity Meters. Demineralizers are normally controlled by measuring electrical conductivity. Simple, reliable instruments can

51 Vol. 19 Ion Exchangers 523 beinstalledtoinitiateregenerationwhenthe conductivity of the treated water rises above a preset alarm value (e.g., 0.25 ms/cm for mixed-bed units or 10 ms/cm for two-bed units). The breakthrough of silica, which has no detectable conductivity, can cause a problem. A common, reasonably reliable way of dealing with this is to place a conductivity probe above the bottom of the bed. The position of the probe is calculated in such a way that the wavefront of chlorides moving down the bed reaches the probe before the silica wavefront reaches the bottom. However, this method is not reliable enough to be used for high-pressure boiler water, in which the absence of silica is vital. Silicometers are automatic colorimetric analyzers that operate like hardness detectors. They normally analyze water taken from a sampling point located above the bottom to ensure that no silica leaves the unit. They require very careful maintenance. Differential Conductivity. For monitoring hydrogen-ion exchangers, a small separate cation-exchange column is set up and maintained in a highly regenerated condition by passing a large excess of acid through it at each regeneration of the main unit. During the service stage, a continuous sample of water from the main unit is passed through the test column and the difference in conductivity is monitored continuously. The difference is small and can stay constant or decrease during the run. A sudden increase marks the end of the cycle, indicating breakthrough from the main unit (see Section 6.3). ph Measurement. [45]. Although ph is easily measured, it is not a very useful parameter for plant control because unbuffered demineralized water tends to produce inconsistent readings. Furthermore, the change in ph at the end of the cycle is less distinct and more subject to random changes than the variation in conductivity. Sodium Electrodes. For cation exchangers, sodium-sensitive electrodes afford an extremely sensitive and accurate method of control. 11. Special Processes in Water Treatment This chapter deals with special ion-exchange processes used in water treatment applications Removal of Organic Matter In certain water with low salt concentrations, the fouling factor N (see Section 7.3) may become very high. To protect the anion-exchange resins in a demineralization plant, a special resin column can be installed at the head of the system to trap the organic matter. These resins are known as organic scavengers. Two kinds are available: 1. Highly porous strongly basic polystyrene or polyacrylic resins used in the chloride form for neutral or alkaline water, and regenerated with a mixture of sodium chloride and sodium hydroxide (e.g., Amberlite IRA900, Amberlite IRA958) 2. Weakly basic phenol formaldehyde resin (e.g., Duolite A7) for water with high sulfate concentration, regenerated with sodium hydroxide (this resin is also capable of retaining certain detergents). These scavenger resins have a limited efficiency, and their popularity decreased after the development of acrylic anion resins, which have excellent resistance to organic fouling. Activated carbon can also be used as organic scavenger. It is relatively cheap and also removes excess chlorine from water. Other technologies, such as ultrafiltration, are also available for removing organics from water Treatment of Potable Water Ion-exchange resins do not transform chemically or bacteriologically contaminated water into drinking water. Other processes are necessary, such as coagulation, filtration, chlorination, or ozonization. However, ion exchange can be used to reduce or remove excess hardness, alkalinity, nitrate or sulfate content, heavy metals, or radioactive substances from water destined for human or animal consumption. Dealkalization with carboxylic resins is frequently used in northern

52 524 Ion Exchangers Vol. 19 European breweries to prepare water that will make good beer. Many private houses, apartment buildings, or small communities have water softeners for domestic use. Most ion-exchange resins, although totally insoluble, release minute amounts of residual organic raw materials into the treated water, particularly when they are new [46]. These impurities may not be allowed if the treated water (or solution in food-processing applications) is to be consumed by humans. Several countries now have regulations that must be observed by suppliers of potable water treatment equipment, including ion-exchange resins. In the United States [47], France [48], and Germany [49], these regulations specify the amount of impurities that may be released by the resin into the water. The composition of authorized resins is also described. In general, ion-exchange is not used for the general production of drinking water, but it is a very helpful technology to remove specific contaminants. Removal of Nitrate. [50]. In Europe and the United States, a trend toward higher nitrate concentration has been observed in both surface and groundwater. Nitrate levels higher than 50 mg/l endanger children s health. Any strongly basic resin in chloride form can remove nitrate from water. Unfortunately, the efficiency of standard strongly basic resins is only moderate because these resins also absorb sulfate and some bicarbonate, both of which are present in higher concentration than nitrate. This represents a waste of capacity and regenerant, and causes large variations in the quality of the treated water with regard to its sulfate (almost completely removed), bicarbonate (reduced), and chloride (increased) concentrations. These disadvantages can be partly alleviated by regenerating the resin with a mixture of sodium chloride, sulfate, and bicarbonate to reduce the variations in water quality [51]. Resins with a higher affinity for nitrates than for sulfates were developed in 1985 (Imac HP 555). They offer an advantage over conventional strongly basic resins when the water to be purified contains a high proportion of sulfate. Borate/Boric Acid Removal [52]. Borate can be a problem in areas where the underground water contains high levels of borate, as well as in areas where drinking water is produced by desalination using reverse osmosis. The RO membranes have a poor rejection of boric acid, so that ion-exchange is an ideal complement to remove borate selectively where required. Borate is also undesirable in water used for the irrigation of certain crops, notably citrus. Borate-selective resins are available based on a methylglucamine functional group (see Section 2.2.3). The polyol part of the functional groups forms a complex with boric acid. After exhaustion of the resin, the complex is eluted with a strong acid, and the resin is converted to its free-base form using caustic soda. A resin approved for the treatment of drinking water is Amberlite PWA10. Heavy Metals. Lead, cadmium, and other heavy metal ions can be removed from drinking water in dealkalization plants, using weakly acidic cation-exchange resins, regenerated with hydrochloric acid. These resins have a high affinity for lead and several other divalent cations. If the water does not contain much alkalinity, these resins can also be used in the sodium form, but then they require a caustic soda conversion after the normal acid regeneration (see the Carbosoft Process in Section 11.3). Amberlite PWC13 can be used for this purpose. Radium and Barium. The sulfonate groups of strongly acidic cation-exchange resins have a high affinity for radium and barium. Both can therefore be removed with very low residuals, either in a normal softening process, when calcium and magnesium also have to be removed, or otherwise operating the resin beyond the calcium breakthrough. Regeneration with magnesium chloride is also possible. Removal of Natural Organic Matter (NOM). Surface water sometimes contains organic compounds that give undesirable color to the drinking water. These substances can be removed with macroporous acrylic anionexchange resins in the same way as described in Section An example of resin is Amberlite PWA9. Carix Process. This was developed in Germany by the Kernforschungszentrum (Nuclear Research Center) in Karlsruhe [53]. It is unique in

53 Vol. 19 Ion Exchangers 525 that it allows partial demineralization of water using carbonic acid (CO 2 þ H 2 O) as the sole regenerant. The salinity of the environment is not increased as is the case when a mineral acid and sodium hydroxide are used for regeneration. Moreover, in a mixed bed, the cation and anion resins do not require separation for regeneration, and the carbonic acid produced during service can be recovered and pressurized for use in the next regeneration sequence. Two special acrylic resins are the key to the process: a weakly acidic cation exchanger (Amberlite PWC11) and a strongly basic anion exchanger (Amberlite PWA12). At the beginning of the service run, the cation resin is in its carboxylic acid form, whereas the anion resin is in the bicarbonate form. The principle of the Carix process can be described by the following reactions: 2R þ A HCO 3 þca2þ SO 2 4 ðrþ A Þ 2 SO2 4 þca2þ ðhco 3 Þ 2 2R C COO H þ þca 2þ ðhco 3 Þ 2 ðr CCOO Þ 2 Ca 2þ þ2co 2 þ2h 2 O The combined mechanism of both resins produces the demineralization effect. The weakly acidic resin can exchange cations only if the water is alkaline. The alkalinity is produced by the anion resin, which is converted from the bicarbonate to the sulfate, nitrate, or chloride form. Regeneration is carried out by a mechanism in exactly the opposite order. High carbon dioxide pressure displaces the second equation to the left; bicarbonate salts are formed in relatively high concentration and regenerate the anion resin. Being reversible, the reactions are never complete, but the Carix process results in a % reduction of raw water salinity. The system is particularly effective for reducing the concentration of hardness, bicarbonate, sulfate, and nitrate. Typical breakthrough curves are shown as a function of time in Figure 51. A schematic diagram of the first full-scale Carix installation is illustrated in Figure 52. The Carix process is not limited to the treatment of potable water. It is very economical in terms of regenerant usage and very environment-friendly. However, it brings about only a partial demineralization of water. Figure 51. Carix process profile of treated water in a partial demineralization test aimed at softening water Cation : anion resin volume ratio ¼ 2 : 1. Dashed lines indicate concentrations in raw water. Solid lines indicate concentrations in product water Disinfection of Water and Resins. Normal ion-exchange resins do not sterilize water. Moreover, they may promote microbial growth during shutdown periods. If a solution contaminated with microorganisms flows steadily through a resin bed, most of the insoluble contaminants are filtered out on the resin surface. However, as soon as flow stops, the immobilized silt provides an ideal feedstock for incoming bacteria. The bacteria proliferate, contaminating the treated solution when service is resumed. Both the solution and the resin bed must therefore be disinfected. Influent water can normally be disinfected with chlorine or ozone. Unfortunately, this may often not be sufficient, because these oxidants are almost totally consumed by the silt and debris deposited on the bed surface and by the resin itself. The resin bed must therefore also be disinfected. This can be done at regular intervals by using chemical bactericides such as chlorine, quaternary ammonium salts, or peracetic acid. Resin manufacturers provide some guidance in this respect.

54 526 Ion Exchangers Vol. 19 Figure 52. Simplified flow sheet of the large-scale Carix plant in Bad Rappenau (Germany) a) Filter in operation regeneration; b) Reaction tank; c) Vacuum degasifier; d) CO 2 storage tank; e) CO 2 vacuum compressor unit; f) CO 2 degasifier The first two columns are shown in production and the third in regeneration. Water softeners can be disinfected with chlorine produced by electrolysis of the brine used for regeneration. Another method involves mixing a small percentage (usually 1 3 %) of a special resin containing precipitated silver with the actual softening resin. The precious metal diffuses within the resin bed during standby period and prevents the growth of bacteria. Unfortunately, some strains become progressively resistant to silver. Triiodide resins, which are strongly basic anion exchangers loaded with a large amount of the triiodide ion, can disinfect treated water. They are used in India and other tropical countries Treatment of Brackish Water For many years, ion-exchange systems, such as those with SAC WBA pairs (see Chap. 9), were used to demineralize brackish water containing up to 2500 mg (ca. 50 meq) of salt per liter. However, reverse osmosis has almost completely replaced ion exchange, which is now limited for economic reasons to water containing < 20 meq/l. The Desal Process. Developed by Rohm and Haas in 1964 [54], the Desal system uses three columns: the first and third contain medium-basicity acrylic resins in the bicarbonate and hydroxide forms, respectively; the center column is filled with a carboxylic resin. Exchange reactions for the three columns are: R þ A HCO 3 þnaþ Cl!R þ A Cl þna þ HCO 3 R C Hþ þna þ HCO 3!R C Naþ þh 2 OþCO 2 R þ A OH þco 2!R þ A HCO 3 During the process, the third column is converted to the bicarbonate form. Thus, after regeneration of the first column to the hydroxide form with a cheap alkali such as a calcium hydroxide suspension, and a normal regeneration

55 Vol. 19 Ion Exchangers 527 of the cation unit with acid, the next cycle can proceed in the opposite direction, the third column now becoming the first. This system has major limitations: sodium leakage from the center column is very high, so that desalination rarely exceeds 50 %. Bicarbonate is also lost from the system, unless the incoming water is strongly alkaline. Today, the development of reverse osmosis has made this process practically obsolete. The Carbosoft Process. The softening of brackish water with a sulfonic acid resin in the sodium form is difficult if the influent contains a large proportion of sodium, due to its selfregenerating effect (see Section 7.2). Carboxylic resins have a much better selectivity for multivalent ions than sulfonic resins. The Carbosoft process exploits this property, and is still popular because it is selective, like many ion-exchange processes. A weakly acidic resin in the sodium form selectively removes Ca 2þ and Mg 2þ ions from highly saline water with high efficiency and acceptably low leakage: 2 RCOO Na þ þca 2þ Cl 2!ðRCOO Þ 2 Ca 2þ þ2na þ Cl This system requires a two-step regeneration because the carboxylic resin cannot be regenerated with brine. The first step uses an acid: ðrcoo Þ 2 Ca 2þ þ2h þ Cl!2 RCOO H þ þca 2þ Cl 2 The resin is converted to the sodium form in a second step with dilute alkali: RCOO H þ þna þ 2 CO2 3!2 RCOO Na þ þco 2 þh 2 O Resins for the Carbosoft process must be able to withstand frequent sodium hydrogen osmotic shock. In practice, only highly stable macroporous resins can be used. This process is widely employed to soften water injected as steam into oil wells to enhance the recovery of oil Processes Involving Sea Water Sea water typically contains 35 g of sodium chloride per liter (i.e., 0.6 eq/l) and other minerals in lower concentration. This makes industrialscale desalination by ion exchange totally impractical: only one bed volume of sea water would exhaust the resins, and more than one bed volume of treated water is needed for regenerant dilution and resin rinse. Therefore sea water desalination is performed exclusively by membrane methods (reverse osmosis! Water, Section and electrodialysis! Membranes and Membrane Separation Processes, Section 3.7. Membranes and Membrane Separation) or distillation. However, ion-exchange resins are well-suited to treat sea water in special cases. Survival Kits. Many armed forces and airlines equip their soldiers and aircraft with emergency cartridges containing fine ion-exchange materials and silver salts to produce a few liters of potable water for immediate survival. Softening of Sea Water. Sea water evaporators are subject to scaling with calcium sulfate. The Carbosoft process (Section 11.3) can be used to soften sea water. Sea Water as a Regenerant. Because sea water contains essentially sodium chloride, it can be used to regenerate softening resins. However, the sodium chloride concentration of sea water (ca. 3.5 %) is lower than the optimum regenerant concentration (10 %), and the presence of other mineral salts is detrimental to hardness leakage and operating capacity Treatment of Condensates Returned condensate from modern high-pressure boiler turbine condenser systems is not of suitable quality for recycling as boiler feedwater. It contains erosion and corrosion products, mainly insoluble iron oxides with traces of soluble salts of copper and other metals, together with salts arising from pinhole leaks in the condenser system which cause the entry of cooling water. The problem of condenser leakage is particularly serious in coastal power stations where condensers are cooled by sea water. Boiler and turbine manufacturers keep up with the advances in ion exchange and demand increasingly strict standards for the quality of feedwater used in their equipment. The acceptable limit for sodium and chloride content is often expressed in parts per trillion or nanograms per liter.

56 528 Ion Exchangers Vol. 19 As long as condenser leaks are small, raw condensates contain only very low concentrations of dissolved and suspended minerals, often a few micrograms per liter. During startup periods or with serious condenser leakage, concentrations may rise to 1 5 mg/l. The condensate is sometimes conditioned with mg/l of ammonia or several milligrams of morpholine or other amines to raise the ph and reduce corrosion. Special problems are associated with the filtration and demineralization of such condensates, and abundant literature exists on the subject [57, 58]. The processes fall into two main categories: one uses standard resins and plants similar to those used for demineralization; the other employs finely ground resins applied to precoated filters (see Section ) Conventional Resins In the majority of plants, conventional resins in bead form are used in mixed beds (Section ) which often operate with very high specific flow rates ( BV/h) and very long cycles. For condensates conditioned with ammonia, the mixed bed is sometimes preceded by a cation exchanger which removes ammonia so that the mixed bed does not become saturated too quickly and can work at neutral ph where it is more efficient. To ensure that ionic leakage is kept very low, especially at high ph, special techniques are available for the regeneration of mixed beds, mainly with a view to limiting crosscontamination between resins. Such processes include the following: 1. The Seprex process of Graver [59], in which the resins are separated by density differences in ca. 15 wt % sodium hydroxide solution. 2. The Ammonex process of Cochrane [60], in which an ammonia solution is circulated after regeneration of the anion exchanger to displace sodium from the contaminating cation resin. 3. The Conesep process of Kennicott, where separation is greatly improved with a conical column and monitored with a conductivity device. 4. The Interface Isolation technique, shown in Figure 53, makes sure that the resin that cannot be completely separated and thus fully regenerated is not returned to the mixed bed unit Powdered Resins The process launched by Graver under the name Powdex [61, 62] uses completely regenerated ion-exchange resins that are ground into a powder with particle sizes between 20 and 200 mm. Figure 53. Regeneration with interface isolation MB: mixed-bed unit; CRT: cation regeneration tower; ART: anion regeneration tower; MSV: mixing and storage vessel; IIV: interface isolation vessel 1) Exhausted resin transfer to regeneration station; 2) Transfer of interface resin into CRT; 3) Backwash for separation; 4) Anion resin transfer to ART; 5) Transfer of mixed interface to IIV; 6) Cation resin regeneration with acid; 7) Anion resin regeneration with caustic soda; 8) Cation resin transfer to MSV; 9) Anion resin transfer to MSV; 10) Air or nitrogen mixing of resins; 11) Return of mixed resins to mixed bed unit

57 Vol. 19 Ion Exchangers 529 The cation-exchange powder in H þ form (sometimes NH þ 4 form) and the anion-exchange powder in OH form are mixed with water to yield a large flaky mass which is applied to candle columns (Fig. 54) or plate columns (Fig. 55). About 1 kg dry weight of mixed exchanger is used per square meter of filter surface. Several manufacturers of powdered resins have devised ready-made mixtures that are easier to use [63]. The advantage of powdered resins is that they filter suspended matter from the condensate very efficiently. Moreover, if radioactive substances are absorbed (e.g., in boiling water nuclear reactors, see Section 11.6), subsequent storage is easier because they are attached to a solid. The major disadvantage of powdered resins is their low demineralization capacity when condenser leakage is considerable. Figure 55. Plate filter a) Distribution crown; b) Plate covered with resin cake Powdered resins are never regenerated. The caked layer of used resin is removed by a stream of compressed air, and a new layer is coated onto the surface. The criterion used to judge the end point is the head loss produced by compaction of the filtering layer from the effect of flow rate and the filtered matter: this head loss is normally limited to 180 kpa. In the case of condenser leakage, the end point occurs through exhaustion of the resin, whose operating exchange capacity is about 2 eq per kilogram of dry anion exchanger Water Treatment in the Nuclear Industry Figure 54. Candle filter a) Filter candle covered with resin precoat; b) Emptying outlet for upper compartment; c) Candle outlet In nuclear power plants, special problems occur because water circuits may contain radioactive elements. Water suspected of containing such products cannot be rejected to the public sewage system before being decontaminated. Similarly, spent regenerants of ion-exchange resins used in such circuits cannot be rejected either. Therefore, many nuclear circuits are treated with resins on a once-through basis: once exhausted, these resins are disposed of as nuclear waste (i.e., enclosed in cement, plastic, or glass as solid blocks; put in drums; and stored in underground nuclear dumps). Nuclear-grade resins must meet very strict purity specifications and are supplied in a highly regenerated form. Because the storage

58 530 Ion Exchangers Vol. 19 and disposal cost of the solid waste is many times higher than the resin price, nuclear power stations use resins with higher cross-linking, providing low leakage and high operating capacity Production of Ultrapure Water [64] (! Water, Ultrapure, Section 3.1) In the semiconductor industry, the chips used for integrated circuits are washed with demineralized water and dried. Any mineral or organic impurity in the water may form a bridge between adjacent conductors which are separated by a fraction of a micrometer. This causes a short circuit and makes the part unusable. Therefore, extremely pure water must be used in this process. Typical water quality specifications are Resistivity 18.2 M W cm at 25 C (Conductivity < 0.55 ms/cm at 25 C) TOC < 3 mg/l Microorganisms < 2/mL Particles (>0.2 mm) < 10/mL Many other ions are specified in concentrations smaller than 20 ng/l (20 ppt). The production of such water requires the combination of several technologies, typically filtration, reverse osmosis, treatment with activated carbon, primary ion exchange, UV sterilization, microfiltration, final ion exchange, and ultrafiltration. Not only should the plant remove all impurities from the feedwater, but also none of its components must add any additional impurities to the treated water. Therefore, special materials with nearabsolute insolubility must be used. The primary ion-exchange step often consists of two mixedbed units in series. At the time of commissioning new resins, these are thoroughly cleaned by cycling. In the final ion-exchange step, special resin grades of ultimate purity are used in nonregenerable point-of-use cartridges. These so-called semiconductor-grade resins are available from several producers and must comply with very strict specifications: 1. Extremely low content of leachable substances 2. Degree of regeneration close to 100 % 3. Absence of small resin fragments 4. Quick start-up rinse to specified resistivity and TOC value 12. Special Applications of Ion Exchange Ever since their discovery, ion-exchange resins have been used primarily for water treatment. In 2006, % of the resins produced were used for the softening or demineralization of water. The properties of ion-exchange resins were soon discovered to be advantageous in many areas other than water treatment. Since the end of the 1940s, resins have been used to treat sugar syrup or to extract precious metals. The number of such special applications has been constantly increasing because ion-exchange resins provide an efficient and elegant method for solving a large number of problems in chemical engineering. In many purification processes, the principles involved are similar to those described in previous chapters concerned with water treatment. However, specific properties of the solutions to be treated will affect the selection of resins: the feed solutions are often hot, colored, or viscous. Consequently, particle size distribution of the resin is adjusted to give acceptable pressure drop values, and the porosity of the polymer is designed to prevent resin fouling or enhance the removal of color bodies or other large molecules from solution. Resins with uniform particle size (see Section 3.5) which have always been required for some applications, such as chromatographic separation, are now routinely available and have improved the efficiency of many processes. Additionally, new regulations are being implemented to govern the use of ion exchange resins in several applications, particularly in the food processing industry. Many resins are authorized for the treatment of beverages and food, but regulations differ geographically. There are also cases, such as catalysis, separation, or diffusion, where resins are used in a very different way, requiring specific columns and reactors. An exhaustive list of actual and potential applications cannot be given in this article. Table 9 lists in alphabetical order several uses of ion-exchange resins in the food, chemical, and pharmaceutical industries, and in hydrometallurgy. A few key

59 Vol. 19 Ion Exchangers 531 Table 9. Special uses of ion-exchange resins a Product Application b Recommended resins c,d Acetic acid purification Amberlyst 15Wet Amberlite IRA67 Adipic acid catalyst recovery Amberlyst 40Wet Alkaloids separation and purification Amberlite FPC14, CR1320 K, IRC76, FPA42 and FPA51 Alkylphenols catalysis Amberlyst 15dry, 35dry, 36dry Amino acids separation Amberlite FPC3500, FPC11 Na, FPC22 H, FPA53, FPA91, Duolite A7 Ammonium nitrate recovery and concentration Amberlyst 40Wet Amberlite IRA67 Anthocyanines recovery Amberlite FPX66 Antibiotics purification (several processes) Amberlite FPC3500, XAD16, XAD18, FPA90, FPC14 Apple juice removal of hydroxymethylfurfural Amberlite FPX66 Biodiesel purification Amberlite BD10Dry Bisphenol A catalysis Amberlyst 31Wet, 33, 121Wet, 131Wet Boric acid removal from water or solution Amberlite IRA743, PWA10 Brine hardness removal Amberlite IRC748, Amberlite IRC747, Amberlite IRC747UPS Cadmium removal from waste Ambersep GT74, Amberlite IRC748 Caprolactam purification Amberlite IRA402 Amberlyst 15Wet, Amberlite IRA910 Carobs sugar recovery Amberlite FPC14 FPA42 Casein precipitation Amberlite FPC22 H Catalysis (acidic) many processes Amberlyst 15Wet, 16Wet, 31Wet, 35Wet, 36Wet, 39Wet, 70 Cephalosporin extraction and purification Amberlite XAD1600, XAD18, FPA53 Chlorinated solvents removal from waste Amberlite XAD4 Cholesterol drug for hypercholesterolhemia Cholestyramine (Duolite AP143) Chromates recovery from cooling circuits Amberlyst A21 Chromic acid iron removal Amberlyst 15Wet Chromic acid recovery and concentration Amberlyst 16Wet, 40Wet A21 Citric acid purification Amberlite FPC22 Duolite A561 Citrus juices de-bittering Amberlite FPX66 Cobalt recovery Amberlite IRC76 Copper recovery and concentration Amberlite IRC747, IRC748 Copper recovery from adipic acid Amberlyst 40Wet Cyanides removal from waste Amberlite IRA958 Dates sugar recovery Amberlite FPC22 FPA54, IRA96 Electroplating rinse water recycling Amberlyst 16Wet A21 A26 Enzymes immobilization Duolite A568, Amberlite FPA98, XAD1180. Esters esterification catalysis Amberlyst 15Wet, 36Wet, 39Wet, 46, 70, 131Wet Formaldehyde acid removal Amberlyst A21 Fructose chromatographic separation Amberlite CR1320 Ca Fruit juices acid removal Duolite A7, Amberlite FPA53 Fruit juices removal of fungicides Amberlite FPX66, Gasoline octane enhancers (MTBE, ETBE, TAME) Amberlyst 35Wet, 15Wet Gelatin demineralization Amberlite FPC14 FPA42, FPA53 Glucose demineralization Amberlite FPC22, FPA51 Glucose fructose chromatographic separation Amberlite CR1310 Ca Glycerol demineralization and color removal Amberlite FPC14 IRA96 FPA90 Glycol purification Amberlyst A21, Amberlyst 15Wet Glyoxal nitrate removal Amberlite IRA67 Gold recovery from plating solutions Amberlite IRA900, Amberjet 4400 Grape must rectification and sugar recovery Amberlite FPC14 FPA51 FPA90 & FPX66 Hydrochloric acid purification (iron and zinc removal) Amberlyst A24 Hydroculture resin as fertilizer Lewatit HD5 Hydrogen peroxide purification Amberlite XAD4, Amberlyst 15Wet Isomerase demineralization Amberlite FPC22 Duolite A561 Lactic acid demineralization Amberlite FPC22, FPC23, FPA54, FPA53 Lactose calcium removal Imac HP336 Lactose demineralization Imac HP1110 Amberlite FPA55 Lactose hydrolysis Amberlite FPC12 H Lysine extraction and purification Amberlite FPC11 Lysozyme recovery from egg white Amberlite FPC3500 Malic acid recovery from apple juice Duolite A7 Mercury recovery or removal from waste Ambersep GT74 Methanol ammonia and sodium removal Amberlyst 15Wet (Continued)

60 532 Ion Exchangers Vol. 19 Table 9 (Continued) Product Application b Recommended resins c,d Milk sodium removal Amberlite FPC14 Molasses betaine recovery Amberlite CR1310 K Molasses sugar recovery Amberlite CR1310 K Monosodium glutamate purification Amberlite FPC11 Na, Nickel iron and cobalt removal Amberjet 4400, Amberlite IRC747 Nickel lead removal Amberlite IRA96 Nickel recovery from waste Amberlite IRC748, Amberlyst 35 Wet Oxygen removal of traces from water Lewatit K7333 Pentaerythritol catalysis Amberlite IRA958, IRA900 Phenol acid removal from phenol Amberlyst A21 Phenol purification Amberlyst 16Wet, Amberlyst 36Wet Phenol removal from waste Amberlite XAD4 Phosphoric acid purification Amberlyst 15Wet Pickling baths rejuvenation (Fe and Zn removal) Amberjet 4200 Platinum recovery Amberlite IRA402 Polyols demineralization (sugar industry) Amberlite FPC22, Amberlite FPA51, Amberlite IRA92 Polyols separation (sugar industry) Amberlite CR1310 Ca Polyphenols recovery Amberlite FPX66, XAD7HP, XAD761 Semiconductor chips final rinse Amberlite UP6040 Silica gels removal of sodium Amberlite IRC84SP Silver recovery Amberlyst A24, Amberjet 4200 Sorbitol demineralization Amberlite FPC22 FPA55, FPA90 Imac HP333 Streptomycin purification Amberlite FPC3500, FPA40 Sucrose hydrolysis into glucose and fructose Amberlite FPC12 H Sugar alkalization Amberlite FPA98, FPA90 Sugar calcium removal (Gryllus process) Amberlite FPC22 Sugar calcium removal (NRS process) Amberlite FPC14 Sugar color removal from cane sugar Amberlite FPA98, FPA90 Sugar hydrogenation catalyst recovery Imac HP336 Sugar Quentin process Amberlite FPC23 Sugars chromatographic separation Amberlite CR1320 Ca Sweeteners final polishing (purification of traces) Amberlite FPC22/FPA90, Tartaric acid purification Amberlite FPC14, FPA53 Uranium mining (recovery from leach liquors) Amberjet 4400, Amberlite IRA910U, Ambersep 920U Vanadium recovery from adipic acid Amberlyst 40Wet Vitamins extraction Amberlite XAD1180, XAD16, FPC3500 Vitamins rurification Amberlite FPC22, FPA51 Whey calcium removal Imac HP336 Whey demineralization Imac HP336 HP1110 HP661 Xylitol purification Amberlite FPA90 a All listed applications are commercial b For food-processing applications, the prospective user must check whether the suggested resin meets local regulations c For other producers, see Table 10 d Dashes between resin names indicate that several resins are used in series; commas mean that several options are available examples are discussed in more detail. Some principal manufacturers of ion-exchange resins are listed in Table Processing Steps The uses of ion-exchange resins for various processing steps are described below. Resin manufacturers can provide more detailed information about specific processes and the corresponding ion exchange and adsorbent materials Purification Ion exchange is used for the removal of impurities from an aqueous or organic solution. The term impurities covers here a large spectrum of chemical species. Deacidification. A solution containing acids is passed through a strongly basic anion exchanger in OH form or a weakly basic exchanger in free base form, depending on whether all acids or only strong acids are to be removed.

61 Vol. 19 Ion Exchangers 533 Table 10. Equivalent resins a,b Rohm and Haas Lanxess Dow Chemical (Dowex) Mitsubishi Purolite (Purolite) Weakly acidic cation resins Water treatment Amberlite IRC50 Diaion WK10 C115 Amberlite IRC76 Lewatit CNP80 Amberlite IRC86 Lewatit CNP80 MAC3 Diaion WK40 C105 Amberlite IRC86RF Lewatit CNP80WS Upcore MAC3 C105FL Amberlite IRC86SB MAC3LB C105DL Food processing and drinking water Imac HP333 Lewatit S8227 Imac HP336 Lewatit S8528 Relite CND C107E Amberlite FPC3500 Diaion WK100 Strongly acidic cation resins Water treatment Amberjet 1200 H Lewatit Monoplus S100 H Marathon C H PFC100 H Amberjet 1200 Na Lewatit Monoplus S100 Marathon C PFC100 Amberjet 1300 Na Lewatit Monoplus S110 Amberjet 1500 H Lewatit Monoplus S110 H Monosphere 650C Amberlite IR120 Na Ionac C249 HCRS Diaion SK1B C100 Amberlite IR120 H Ionac C267 C100 H Amberlite 200C Na Diaion PK228 Amberlite 252 H MSC H C150 H Amberlite 252 Na MSC Na Diaion PK220 C150 Amberlite 252RF H Lewatit Monoplus SP112 H Marathon MSC C150FL Food processing and drinking water Amberlite SR1L Ionac C249NS HCRS/S C100E, PF100E Imac HP1110 Na Lewatit S1468 Amberlite FPC12 Lewatit S1428 Amberlite FPC14 Diaion SKL10 Amberlite FPC22 Lewatit S Diaion PK220 Amberlite FPC23 Relite CFZ C155S Weakly basic anion resins Water treatment Amberlite IRA67 Lewatit VPOC1072 Diaion WA10 A845, A847 Amberlite IRA67RF A845FL Amberlite IRA96 Ionac A328 MWA1 Diaion WA20 A100 Amberlite IRA96RF Lewatit Monoplus MP64 Monosphere WB500 A100FL Amberlite IRA96SB MWA1LB A100DL Food processing and drinking water Amberlite FPA51 Lewatit S Diaion WA30 Amberlite FPA53 Lewatit VPOC1072 Relite RAM2 A845 Strongly basic anion resins Water treatment Amberjet 4200 Cl Lewatit Monoplus M500 Monosphere A625 PFA400 Amberjet 4400 Cl Lewatit Monoplus M800 Monosphere 550A Amberjet 4600 Cl Lewatit Monoplus M600 Monosphere A2 500 PFA300 Amberjet 9000 OH Monosphere MP725A Amberlite IRA402 Cl Ionac ASB1P SBRP Diaion SA10A A400 Amberlite IRA405 Cl 11 Diaion SA11A A420S, A444 Amberlite IRA410 Cl M600, M610 SAR Diaion SA20A A200, A300 Amberlite IRA458 Cl VPOC1071 A850 Amberlite IRA478RF Cl VPOC1073 A870 Amberlite IRA900 Cl MSA1 Diaion PA308 A500 Amberlite IRA900RF Cl Lewatit Monoplus MP500 Marathon MSA A500FL Amberlite IRA910 Cl Lewatit Monoplus MP600 MSA2 Diaion PA412 A510 Amberlite IRA958 Cl Lewatit VPOC1074 A860 Ambersep 900 SO4 Marathon MSA A500TL Food processing and drinking water Amberlite PWA5 Ionac SR6 A520E Amberlite PWA6 Lewatit GW66 Diaion SAF11AL Amberlite PWA9 Lewatit S6368 Diaion PAF308L (Continued)

62 534 Ion Exchangers Vol. 19 Table 10 (Continued) Rohm and Haas Lanxess Dow Chemical (Dowex) Mitsubishi Purolite (Purolite) Amberlite FPA90 Lewatit S6368A Relite D182 Amberlite FPA98 Lewatit VPOC1074 Mixed resins (ready for use) Amberlite MB6113 Ionac NM68 MB500 IND Amberlite MB20 Ionac NM60 MB400 Amberlite MB9L Ionac NM91 MB46 Nuclear-grade resins Amberlite IRN77 Lewatit Monoplus S100KR Monosphere 650C NG Diaion SKN1 NRW100 Amberlite IRN78 Lewatit Monoplus M500KR Monosphere 550A LC NG Diaion SAN1 NRW400 Amberlite IRN97 H Lewatit Monoplus S200KR Monosphere 500C NG Amberlite IRN150 Monosphere MR3 LC NG Diaion SMN1 NRW37 Amberlite IRN160 Monosphere MR575 LC NG Semiconductor-grade resins Amberjet UP1400 Lewatit Ultrapure 1213MD Monosphere 650C UPW Diaion SKNUP Amberjet UP4000 Lewatit Ultrapure 1243MD Monosphere 550A UPW Diaion SANUP Amberjet UP6040 Lewatit Ultrapure 1294MD Monosphere MR450 UPW Diaion SMNUP EXMB Amberjet UP6150 Lewatit Ultrapure 1293MD Monosphere MR3 UPW Inert spacer resins Amberlite RF12 IF62 IP1 Amberlite RF14 Lewatit IN42 IF59 IP4 Ambersep 349 Monosphere 600BB IP5 Chelating resins Amberlite IRA743 Lewatit MK51 XUS Diaion CRB01 S108 Amberlite IRC748 Lewatit TP207 A1 Diaion CR11 S930 Amberlite IRC747 Lewatit TP260 APA1 S940 Ambersep GT74 Lewatit TP214 S920 Polymeric adsorbents Amberlite XAD16 Diaion HP20 AP400 Amberlite XAD4 Lewatit R255K XFS4022 Diaion HP10 AP250 Amberlite FPX66 Sepabeads SP70 a Only the most current resins are listed b Correspondence of resins is only approximate. For detailed advice, consult the resin manufacturers Since this process is a neutralization reaction, the product of reaction with a strongly basic resin is water, and with a weakly basic resin the acid is taken up as a whole, so that the solution is effectively purified: RNðCH 3 Þ 2 þh þ Cl!½RNHðCH 3 Þ 2 Š þ Cl or RNðCH 3 Þ 2 HCl Examples: Removal of formic acid from formaldehyde, deacidification of fruit juice, and removal of mineral acids from alcohols. Demineralization. All cations and anions may be removed (total demineralization) or only some of them, generally the strongly dissociated ions (partial demineralization). This is achieved by passing a solution successively through a cation-exchange resin in H þ form and an anion-exchange resin either in OH or free base form. Non-ionized substances in the solution are largely unaffected by this treatment. This type of demineralization is sometimes called deashing. Examples: Treatment of sugar syrups, glucose, antibiotics, glycerol, cheese whey, alcohols, polyols (e.g., sorbitol), and glycols. In a variation of this process, organic acids are purified by using strongly acidic and weakly basic resins. Inorganic salts are removed, whereas Table 11. Principal producers of ion-exchange resins Company Location Principal trademarks Lanxess GmbH Leverkusen, Germany Lewatit, Ionac Dow Chemical Company Midland, Michigan, USA Dowex, Kastel Mitsubishi Chemical Tokyo, Japan Diaion, Relite Purolite Company Bala Cynwyd, Pennsylvania, USA Purolite Rohm and Haas Company Philadelphia, Pennsylvania, USA Amberlite, Amberjet, Duolite, Imac

63 Vol. 19 Ion Exchangers 535 weakly dissociated organic acids are not taken up by the anion-exchange resins. If the organic acid has a pk value lower than that of the resin, it is taken up during the first part of the run but then displaced by the stronger mineral acids, so that losses of the valuable organic acid are negligible or minimal. As weakly basic resins with different structures and active groups have different pk values, proper choice of resin will greatly enhance separation. Examples: Demineralization of citric and lactic acids. Selective Removal of Impurities. Harmful ions must be removed, while other constituents of the solution are affected as little as possible. Various sorts of selective resins are available. Examples: The commonest applications are 1. Selective removal of toxic metals from industrial effluents by using complexing (chelating) resins (see Section 2.2.3) such as Amberlite IRC748 for transition metals and Ambersep GT74 for mercury, cadmium and other heavy metals 2. Nitrate removal from drinking water by using the Cl or HCO 3 form of anion-exchange resins (see Section 11.2) 3. Calcium removal from saturated brine with an aminophosphonic resin (e.g., Amberlite IRC747) that can remove Ca 2þ in a concentration of 10 4 g/l from brine containing >10 2 g/l of Na þ 4. Removal of borate from drinking water with a resin having glucamino groups (e.g., Amberlite IRA743) Decolorizing differs from selective removal in that it normally combines ion exchange with adsorption. Resins have partially replaced bone char or activated carbon for this application, as they are easier to handle and to regenerate. Decolorizing is applied mainly to solutions of sugar or other organic compounds. Examples of the color bodies to be removed from solution are sugar degradation products, flavonoids, polyphenols, melanoidins, tannins, and anthocyanines. Because impurities responsible for the color of these solutions are often acidic, the resins used are normally highly porous anion exchangers, sometimes in combination with carbon, inorganic adsorbents, or nonionic resins. Examples: 1. Strongly basic polystyrene materials in chloride form, strongly basic polyacrylic materials in chloride form are used for color removal from cane sugar, and regenerated with a sodium chloride brine 2. Weakly basic or nonionic phenol formaldehyde resins are used for citric or tartaric acid 3. Acrylic weakly basic resins simultaneously decolorize and deacidify antibiotics 4. Aromatic polymeric adsorbents are used for the removal of polyphenols or anthocyanines Adsorption of Impurities. True adsorption is not strictly speaking an ion-exchange process. However, adsorbent resins are so similar to ionexchange resins that the process can be mentioned here. The adsorbent materials used are macroporous polystyrene such as the Amberlite XAD series or phenol formaldehyde resins without active groups. Examples: Current applications include the removal of impurities from effluents containing phenol derivatives, pesticides, chlorinated solvents, or dyes. Macroporous phenol formaldehyde resins have also been used in cigarette filters to remove tar, aldehydes, and heavy nitriles Ion Substitution All ion-exchange processes are ion substitutions. However, a distinction should be made between processes that remove ions while producing water or a pure solution of nonionic substances and those that leave the total ion concentration unchanged, merely replacing an undesirable ion by an acceptable one. Softening. The principle of softening is the same as that used in water treatment: Ca 2þ, Mg 2þ, and possibly Fe 2þ and Mn 2þ, are replaced by Na þ ions. Example: The most widespread application is in the beet sugar industry, where the crude juice containing a high concentration of calcium is softened to prevent scaling in the evaporators. Variations on the basic softening process have been developed to save regenerant and reduce waste. One of the most efficient is the NRS process, in which regeneration of the cation-exchange resin

64 536 Ion Exchangers Vol. 19 is carried out with a mixture of the concentrated softened juice and a little caustic soda, so that no additional salt is required and the process produces no waste. Desodation is a kind of reverse softening: the sodium in a solution is replaced by calcium, magnesium, or potassium, and the cation resin used is regenerated by salts of one or more of the latter metals. Examples: Sodium removal from milk (Na Ca, Na K exchange) and the Quentin process in sugar refining (Na Mg, K Mg exchange). Production of Organic Acids and Salts. Organic acids can be prepared from their salts simply by passage through a cation-exchange resin in H þ form. This process has an excellent yield and is used widely to manufacture modified, water-soluble ethylenediaminetetraacetic acids and some amino acids from their sodium salts. Conversely, an organic acid can be converted into its salt. One organic salt can also be converted into another. Example: In the pharmaceutical industry, cephalosporin C is converted into its potassium salt by passage through a cation-exchange resin in K þ form Recovery and Concentration In recovery and concentration, the principle is the same as in selective purification, but the aim is different. Here, the objective is to recover a valuable substance from solution, so that the stage during which elution from the resin takes place is particularly important. In most cases, the substance recovered from the dilute solution is simultaneously concentrated during the regeneration stage. Examples: 1. Recovery of traces of precious metals, most often in the form of metal complexes, in the metal plating industry (jewelry) and silver recovery from photographic baths 2. Isolation of pharmaceutical compounds during the manufacturing process (extraction and purification) 3. Recovery of ammonium nitrate in fertilizer plants 4. Recovery of sugar and amino acids from molasses in sugar refining by ion exclusion 5. Recovery and recycling of water by demineralizing the rinse water from surface treatment plants 6. Recovery of uranium or gold in the mining industry Separation A special type of ion-exchange resin (with fine, uniform particles and an accurately defined moisture content) is used in columns several meters tall for various industrial separation methods that are similar to those in chromatography. This technique allows separation of a mixture of ions, an electrolyte from a nonelectrolyte, and a mixture of nonelectrolytes (see also! Process- Scale Chromatography;! Basic Principles of Chromatography). For separations involving nonelectrolytes, a special type of ion-exchange resin with fine, uniform particles and an accurately defined moisture content is used in columns several meters tall. Cation exchange resins with a mean diameter of 250 to 350 mm and a uniformity coefficient of less than 1.1 (see Section 3.5) are used in many processes, in the appropriate ionic form, usually Ca, K, or Na. In the pharmaceutical field, chromatographic separations are carried out with polymeric adsorbents of similarly uniform particle size. As shown in Tables 4 and 5, ions can be arranged in order of their relative affinity for a resin. For a sulfonated polystyrene exchange material, the following monovalent ions are listed as examples in order of increasing affinity: Li þ < H þ < Na þ < NH þ 4 < Kþ < Rb þ < Cu þ < Ag þ Separation by Displacement. A small amount of the mixture to be separated is passed through a column (e.g., a mixture containing K þ and Cs þ is passed through a resin in Na þ form). The ions taken up are then displaced by means of another ion with a greater affinity (e.g., Ag þ ). The emergent liquor is thus fractionated because the silver ions, in turn, displace the sodium, potassium, and cesium ions. Separation by selective displacement is a variation of the last process. A small amount of the mixture to be separated (e.g., Li þ and Cs þ )is

65 Vol. 19 Ion Exchangers 537 Figure 56. Separation of Li þ and Cs þ by selective displacement in a resin initially in NH þ 4 form passed through a column containing a resin in a form with intermediate affinity (e.g., NH þ 4 ). This is followed by a solution containing NH þ 4 which displaces only the lithium. Lithium is therefore completely separated from cesium. Cesium can then be displaced by silver (Fig. 56). This technique is called displacement chromatography. It can be applied to the separation of amino acids which are taken up by an H þ form of cation-exchange resin column and displaced with sodium hydroxide or ammonium hydroxide. Separation by Elution. A column is prepared as above but, instead of ions being displaced by an ion with greater affinity than those taken up, they are now eluted with an ion of lower affinity. This is normally the ion with which the resin was initially loaded (e.g., H þ ). The ions to be separated are displaced toward the bottom of the column by the eluent (HCl in the example shown in Fig. 57). The ions always emerge in order of their increasing affinity for the resin and are usually separated well. Separation by Ion Exclusion. An electrolyte can be separated from a non-electrolyte in an ion-exchange column by a sorption process (Fig. 58). The ion with which the resin column Figure 58. Separation of an electrolyte from an organic product by ion exclusion a) non-electrolyte; b) Organic compound is loaded is normally the same as one of those in the electrolyte to be separated. For instance, an aqueous solution of a mixture of sodium chloride and glycerol can be passed through a column containing a cation-exchange resin in Na þ form. Cation exchange does not take place because the column and the solution contain the same cation. The Cl anion cannot penetrate the resin because it encounters the Donnan potential which guarantees electrical neutrality within the resin (see Section 6.1). The electrolyte is thus excluded from the resin. The non-electrolyte, on the other hand, can penetrate the resin by means of adsorption until its concentration is the same inside the beads as outside. When this equilibrium state has been reached, pure water is passed down the column and drives out the electrolyte more quickly than the glycerol, which must diffuse out of the beads. This process of alternating input of the mixture and displacement by water is repeated without the need to regenerate the resin, and successive fractions containing pure glycerol are obtained. Examples: An exclusion process is used industrially to recover sugar from sugar beet molasses [65]. Figure 59 shows the successive fractions Figure 57. Separation of Na þ, K þ, Rb þ, and Cs þ in a solution by elution with 0.1 N HCl through a resin initially in H þ form Figure 59. Recovery of sugar from molasses by ion exclusion a) Conductivity; b) Sugar concentration Fractions: AB nonsugars (residual molasses); BC recycled; CD sugar produced; DA recycled

66 538 Ion Exchangers Vol. 19 Figure 61. Acid retardation by a strong anion resin Figure 60. Separation of glucose from fructose in highfructose corn syrup a) Glucose; b) Fructose; c) Oligosaccharides obtained, alternately rich in sugar and nonsugars. The same process can be applied to the separation of strong and weak electrolytes (e.g., hydrochloric acid from organic acids). Chromatography of Nonelectrolytes. Another property of resins becomes evident during separation by ion exclusion: if different nonelectrolytes are present in solution, the process not only separates out the mineral salts but also the nonelectrolytes from each other because of their different rate of uptake and release by the resin. Example: Separation of glucose from fructose is carried out commercially with fine bead sulfonic resins, generally in the calcium form [66]. A typical elution profile is shown in Figure 60. Separation by Acid Retardation. Somewhat surprisingly, an acid can be separated from its salts by using a column containing a strongly basic anion-exchange resin of suitable porosity and particle size [67]. This occurs because, at high concentration, the acid crosses the Donnan potential barrier (Donnan invasion) and is taken up by the resin, whereas the salts are excluded from it. The acid is thus retarded, and the salts are allowed through. Pure fractions can be obtained at the output by passing concentrated electrolyte solution and water alternately down the column. Example: Acid retardation is now used industrially for purifying and recycling acids that have been used for metal pickling. An elution curve is shown in Figure Diffusion An ion-exchange resin already loaded with ions releases them slowly into a solution. This property is used in two special applications. Hydroponics. (Soilless Cultivation). A resin loaded with fertilizing elements can be used for the hydroculture of plants instead of liquid fertilizers [68]. The elements are released gradually into the water as needed by the plant (Fig. 62). Delayed-Action Drugs. Instead of administering a pure drug directly, it can be absorbed onto a suitable resin and taken in this form. The active substance is then released more slowly in the stomach. Figure 62. Hydroculture a) Hydroponic resin; b) Beads of blown clay; c) Water level; d) Inner perforated pot; e) Outer pot

67 Vol. 19 Ion Exchangers Catalysis (! Heterogeneous Catalysis and Solid Catalysts) Acid or Alkaline Catalysis. [69]. Because standard ion-exchange resins are insoluble acids or bases, they can be used with advantage in many organic reactions where an acidic or basic catalyst is required. Special macroporous sulfonic resins are used, for example, in esterification (e.g., the production of butyl acetate), synthesis (e.g., of methyl tert-butyl ether), or the hydrolysis of sucrose to glucose and fructose. Catalysis with ion-exchange resins has the following advantages over the use of sulfuric acid: 1. A higher local concentration of H þ (or OH ) ions 2. No corrosion 3. Possibility of use in continuous processes 4. Fewer secondary reactions 5. Easy separation from the reaction medium (by simple filtration) In the petrochemical industry, a large user of resin catalysts, most reactions occur at high temperature, usually > 100 C. Halogenated catalysts offer a better temperature stability, whilst polysulfonated resins have a higher acid strength. Catalyst Carrier. Some resins can be loaded with a metallic catalyst (palladium, silver, nickel, etc.), which is otherwise difficult to use because of its high solubility. Enzyme Immobilization. Some ion-exchange resins can be loaded with enzymes to enable enzymatic reactions to take place continuously. Glucoseisomerase or b-galactosidase, for example, can be taken up by weakly basic phenol formaldehyde resins (see also! Biocatalysis, 2. Immobilized Biocatalysts). Drying of Gases or Solvents. Previously dried sulfonic resins can be used to lower the humidity of air, gases, or organic solvents. The resins take up their own weight of water. Dehydration of Alcohols. The affinity for water is so great that sulfonic resins can be used to dehydrate alcohols, (e.g., in converting ethylene glycol into dioxane or cyclohexanol into cyclohexene Coalescence on Oleophilic Resins An oleophilic resin can be obtained by loading a resin permanently with an organic substance consisting of long-chain molecules [70]. In practice, a sulfonic resin loaded with an organic cation is used (e.g., an aromatic quaternary ammonium ion): or a pyridinium ion: The cationic part of the oleophilic molecule is taken up almost irreversibly by the cation-exchange resin. An oleophilic resin bead of this type is illustrated in Figure 63. Water containing oils or fats passes through a column containing an oleophilic resin. The oil particles are taken up by the resin beads in the form of microscopic droplets. The droplets coalesce and grow in size as more oil is taken up. Beyond a certain size, the drops become detached from the resin and are then decanted in the normal way. This process has been developed on an industrial scale as the Elf ANVAR process Dehydration Ion-exchange resins are strongly hydrophilic. When dried, they tend to take up their quota of swelling water. This property is exploited in the two following applications. Figure 63. Schematic representation of an oleophilic resin bead a) Sulfonic resin bead; b) Ionic end of oleophilic molecule; c) Oleophilic end of molecule

68 540 Ion Exchangers Vol. 19 Figure 64. Elf coalescing deoiler a) Oil water mixture; b) Oleophilic resin; c) Decanted oil; d) Level controller operating the oil outlet valve [71]. A typical coalescing deoiling unit is shown in Figure Liquid Ion Exchangers (! Liquid Liquid Extraction) Liquid ion exchangers are high molecular mass, water-insoluble acids and bases that are soluble in oil and in organic solvents. The compounds have a hydrophobic structure and an ionogenic group. They are usually phosphonic acids and amines substituted with long-chain (C 8 C 24 ) aliphatic groups. They are used for liquid liquid extraction of electrolytes from aqueous solutions. An example of cation exchange is the following reaction with a polyalkyl phosphonic acid: ðr 3 PO 4 HÞ org þm þ ðr 3 PO 4 MÞ org þh þ An example of anion exchange is the following reaction with a polyalkylamine in salt form: ðr 3 NHAÞ org þb ðr 3 NHBÞ org þa where M þ represents a metal cation, A and B represent two different anions, R is an alkyl substituent, and the subscript org denotes the organic phase. Recovery of the extracted component from the organic phase does not require distillation as is often the case in processes based on solubility differences. Instead, the liquid exchanger is stripped in a second liquid liquid extraction, similar to the regeneration of solid ion exchangers. The regenerant (stripping agent) is an acid, base, or salt in aqueous solution. In practice, the liquid ion exchanger is used as a solution in an organic solvent (frequently kerosene), with an organic : aqueous phase ratio of between 1 : 1 and 1 : 4. In the exchange process, both immiscible phases are thoroughly mixed mechanically in conventional liquid liquid extraction equipment (mixer settlers, centrifugal contactors, and pulsed and nonpulsed extraction columns). The advantages of liquid ion exchangers are the relative simplicity of their production, the possibility to adjust their concentration in the organic phase according to the concentration of the aqueous solution, and the possibility of continuous countercurrent operation. Their main disadvantage is the inevitable losses of solvent and ion exchanger during the extraction process. The main industrial application of liquid ion exchangers is hydrometallurgy. Large plants are in operation for the recovery of uranium from sulfuric acid leach liquors. The uranyl sulfate anion is extracted with a dialkylamine and subsequently stripped with sodium carbonate, resulting in a water-soluble uranyl carbonate complex which can be precipitated (e.g., by addition of sodium hydroxide) to produce a yellow cake of uranium oxide. Examples: N-lauryl-N-trialkylmethylamine Ion-Exchange Membranes (! Membranes and Membrane Separation Processes, 1. Principles) Ion-exchange membranes are materials with ionexchange properties that can be used as a separation wall between two solutions. They show pronounced differences in permeability toward counterions, co-ions, and neutral molecules. The membranes are permselective. Cationic membranes are permeable to cations and impermeable to anions. Anionic membranes are permeable to anions and impermeable to cations. Ion-exchange membranes are classified as hetero- or homogeneous: Heterogeneous membranes are made of finely ground ion-exchange resin particles embedded

69 Vol. 19 Ion Exchangers 541 in an inert porous binder [polyethylene, poly (vinyl chloride)] in the form of flat sheets. The resin is usually sulfonated polystyrene for cationic membranes and polystyrene with quaternary ammonium groups for anionic membranes. To increase dimensional stability, the mixture is often reinforced with a woven fabric. In homogeneous membranes the ion-exchange groups are grafted directly to the membrane polymer structure. Their manufacture is similar to that of bead-form ion-exchange resins. They are somewhat less stable than heterogeneous membranes, but have a more uniform structure. In the absence of electric current, the membrane is slightly permeable to nonelectrolytes and impermeable to electrolytes because the Donnan potential prevents co-ions from passing through it. Furthermore, the principle of electroneutrality makes it impossible for counterions to concentrate on one side of the membrane. When an electric potential is applied, however, cations are attracted by the cathode and anions by the anode. In the case of a cation-exchange membrane, only cations can migrate through it. An example of application is the electrolysis of brine using a cation-exchange membrane (chlor-alkali process) to produce chlorine and caustic soda (! Chlorine, Chap. 7.). Sodium chloride and water are introduced into the anode chamber where oxidation of the chloride ions to chlorine takes place. Water in the cathode chamber is reduced to hydrogen and hydroxyl ions. Sodium ions migrate through the cationexchange membrane into the cathode compartment and combine with the hydroxyl ions to form sodium hydroxide. Another important example is electrodialysis, where both cation- and anionexchange membranes are used for demineralization of water (Fig. 65). If all three chambers are filled with sodium chloride solution, sodium passes through the cation-exchange membrane Figure 65. Electrodialysis for demineralization of water and produces NaOH in the cathode chamber, chloride goes through the anion-exchange membrane and produces HCl and the central chamber is progressively demineralized. In industrial practice, a large number of alternating cation- and anion-exchange membranes are combined for demineralization of water, especially brackish or sea water. Electrodialysis is also used for the demineralization of cheese whey to recover valuable proteins and lactose. Membrane electrodes operating according to the principle of permselectivity can be used for analytical purposes. Ion-specific membranes are now available for measuring. Important ion-exchange membrane manufacturers are Asahi Chemical (Japan), Asahi Glass (Japan), Du Pont (USA), Ionics (USA), Rhône- Poulenc (France), Sybron (USA), and Tokuyama Soda (Japan) Technical Considerations For special applications in softening, purification, demineralization, or recovery, the plants are often similar to those used in water treatment. Counterflow regeneration is sometimes used to improve regeneration efficiency and reduce leakage. Continuous systems (see Section 10.3) can be employed, e.g., in the softening of sugar syrup or the treatment of rinse water in electroplating. However, many special applications require special operating conditions. Flow Rates. Concentrated solutions must frequently be dealt with, which requires relatively large amounts of resin. The specific flow rate is often low: BV/h instead of the BV/h used in water treatment. Regenerant Recycling. Regenerant consumption can be reduced by recycling. The tails, which are relatively uncontaminated, can be used at the head of the next regeneration stage. Merry-go-round Systems. To increase the treatment capacity of a plant and improve the quality of the treated liquid, a so-called merrygo-round system can be used. This involves passing the solution through several identical columns in series (at least two, but usually more),

70 542 Ion Exchangers Vol. 19 form are removed or recovered. In the next step, the resin is converted into another ionic form more suitable for the service stage. In the selective uptake of metals, for example, a carboxylic or chelating resin (see Section 2.2.3) is often used in Na þ form in order not to lower the ph of the solution to be treated. However, the resin must be regenerated with acid. The H þ form must then be converted to the Na þ form with sodium hydroxide: Service: 2 RCOONaþZnSO 4!ðRCOOÞ 2 ZnþNa 2 SO 4 Regeneration: ðrcooþ 2 ZnþH 2 SO 4!2 RCOOHþZnSO 4 Figure 66. Extraction of streptomycin with weakly acidic resins (three columns in series) so that the one at the head is exhausted as completely as possible. When this state of exhaustion is reached, the next column is placed at the head of the series, and the one just regenerated is used for polishing while the exhausted column is regenerated. The cycle shifts by one column as each column at the head of the chain is exhausted. In this way, the product is obtained in the most concentrated form possible (Fig. 66). Sweetening Off and On. In most of the purification processes described in Section , the liquid to be treated must be removed from the resin before the latter is regenerated. This involves inserting a rinsing stage before regeneration, a stage generally known as sweetening off because of its similarity to the treatment of sugar syrup. In the same way, after this stage and a rinse of the water from the exchangers, the water filling the column is replaced by the liquid to be treated, a stage known as sweetening on. The complete cycle thus consists of sweetening on, the service stage, sweetening off, decompaction, regeneration (or elution), and rinsing. Decompaction is sometimes carried out prior to sweetening off, with the liquid to be treated. Multistage Regeneration. The resin must sometimes be regenerated in several steps. In the first step, the substances taken up in one ionic Conversion to Na þ form: RCOOHþNaOH!RCOONaþH 2 O Temperature Control. Some viscous liquids, particularly concentrated organic solutions (e.g., heavy sugar syrups), must be decolorized when hot (70 80 C) so that they remain fluid and do not crystallize. Other liquids, such as light sugar syrups, must be demineralized when cool (12 C) so that they are not hydrolyzed when they come in contact with the cation exchanger in H þ form. Temperature control thus plays a major role in many applications. Special Plants. Some applications require special types of plant, e.g., 1. Very long columns up to 10 m tall for chromatographic separations 2. Columns in the shape of truncated cones widening at the top to enable resins to swell during service or regeneration 3. More rarely, fluidized-bed systems, sometimes using high-density resins to treat liquids containing suspended matter Simulated moving beds (SMBs) are progressively replacing the tall columns of the first industrial plants. Whilst the old plants had to be operated in a sequential, discontinuous way, alternating the feed solution to be separated and an eluent, the new SMB technology allows the separation of two or more compounds in a

71 Vol. 19 Ion Exchangers 543 principle of operation in two dimensions. Simulated moving beds can be used for purification of fermentation broth, sugar syrup deashing, decolorization of various solutions, separation of metals and other applications. References Figure 67. ISEP carousel (simplified) a) Rotating resin columns; b) Stationary inlet ports; c) Stationary outlet ports quasicontinuous, stepwise manner: the columns (typically 30 of them) are arranged in a carousel. The major system of this type is called ISEP [72] and is provided by the U.S. company AST (Advanced Separation Technologies). The feed and elution solutions are connected to a stationary upper distributor fitted with typically 20 ports, and the raffinate and the extract are connected to a lower stationary connector fitted with the same number of ports. The columns themselves are mounted on a rotating frame. The carousel rotates continuously at a speed of 0.1 to 1.5 revolutions per hour, and the ports are thus successively connected to all columns. A simplified scheme is shown in Fig 67 with only eight resin columns and six inlet and outlet ports. Figure 68 shows the Figure 68. ISEP: two-dimensional view 1 H. S. Thompson: On the adsorbent power of soils, J. R. Agric. Soc. Engl. 11 (1850) J. T. Way, J. R. Agric. Soc. Engl. 11 (1850) R. Gans, Jb. Preuss. Geol. Landesamt 26 (1905) O. Liebknecht, US , P. Smit, US , B. A. Adams, E. L. Holmes, J. Soc. Chem. Ind. London 54 (1935) 1. 7 G. F. D Alelio, US , C. H. Mc Burney, US , The Permutit Co. Ltd., GB , , 1960 (J. R. Millar). 10 Farben Fabriken Bayer., DEI , Rohm and Haas Co., GB , GB , R. E. Anderson: Fundamentals of column ion-exchange in fully-ionised system, 18th Annual Liberty Bell Corrosion Course (USA), S. Fisher, G. Otten: Sloughage of organic materials from field-decross-linked sulfonic acid cation exchange resins Proceedings of the 42nd International Water Conference, Pittsburgh, Pennsylvania, B. Hoffman, M. Kasahara, M. Gavaghan: The effects of chlorine on mixed beds in ultrapure water systems, Sixth annual semiconductor pure water conference, D. C. Auerswald: Effects of cation resin leachables on condensate polisher system performance, Ion Exchange for Industry, Ellis Horwood Ltd, Chichester 1988, p R. Kunin: Thermal stability of anion exchange resins. Amber-Hi-Lites 139, 1974 (published by Rohm and Haas, Philadelphia). 17 R. E. Anderson: Estimation of ion exchange process limits by selectivity calculations, AIChE Symp. Ser. 152 (1973) no. 71, R. E. Anderson: Basics of column ion exchange with strong acid and strong base resins, Third Annual Convention Am. FESAAC, Mexico, F. G. Helfferich: Ion Exchange, McGraw Hill, New York F. G. Helfferich: Ion exchange Kinetics Evolution of a Theory, Mass Transfer and Kinetics of Ion Exchange, NATO ASI Series, Series E N 71, Martins Nijhoff, The Hague 1983, J. T. McNulty, M. Eumann, C. A. Bevan, V. C. Tan: Anion exchange resin kinetic testing: an indispensable diagnostic tool for condensate polisher troubleshooting, Proceedings of the 47th International Water Conference, Pittsburg, Pennsylvania, 1986.