Ion Exchange (IEX) Process

Transcription

1 Ion Exchange (IEX) Process Mohammad Mahdi Bazri Chemical and Biological Engineering Department University of British Columbia Vancouver, BC, CANADA December 2011-Tehran-Iran

2 Water analysis 2 2

3 Introduction 3 Initially developed around 5o s A powerful technology to soften and demineralise water Industrial and domestic applications Industrial and drinking water treatment applications Chemistry and chromatography

4 Applications Water Treatment Food Industry Sugar industry Dairy products Chemical Industry Metal recovery Selective removal of various elements Pharmaceutical Industry Extract and purification of drug Other Applications Mining Enzyme immobilization Analytical chromatography

5 Fundamentals 5 Exchange of the ions on the resin structure with the ones present in the solution Schematic cation and anion exchange resin beads

6 Fundamentals 6 Exchange of the ions on the resin structure with the ones present in the solution Schematic cation and anion exchange resin beads

7 Resin Structure 7 Polystyrene DVB Polyacrylic Weakly anionic Strongly anionic

8 Resin Types 8 Remove hardness/alkalinity and other cations: Weakly Acidic Cation Exchange Resins (-COOH) Strongly Acidic Cation Exchange Resins (-HSO 3- ) Remove anions (NO 3, SO 4, HCO 3 ) and NOM: Weakly Basic Anion Exchange Resins (-N(CH 3 ) 2 ) Strongly Basic Anion Exchange resins (-N(CH 3 ) 3 OH) o Type I (trimethyl amine) o Type II (dimethyl ethanol amine, lower basicity, better regenerability)

9 Terminologies: capacity Exchange capacity (eq/l) The number of ion exchange sites Operating Capacity (useful capacity) The number of ion exchange sites where exchange has really taken place during the loading run. Operating capacity < Total capacity

10 The typical operating capacity of a weak base anion exchange resin is 70 to 90 % of the total capacity. For weak acid cation resin, operating capacity depends on several parameters, so there is no such simple estimate. The typical operating capacity of SAC and SBA resins is about 40 to 60 % of their total capacity.

11 Operating capacity depends on: Concentration and type of ions to be adsorbed Service flow rate Temperature Type, concentration and quantity of regenerant Type of regeneration process (co-flow, reverse flow) Bed depth (reverse flow regeneration only) Particle size of the ion exchange resins Performance data and computer programs

12 Terminologies: exchange zone The area between fully regenerated and fully exhausted resin is called exchange zone or reaction zone. Ion leakage endpoint of run

13 Weak acid and weak base resins are sensitive to flow rate, higher flow rate longer reaction zones. SAC and SBA resins are less sensitive to flow rate. Shorter reaction zone, higher utilization of total capacity, higher achievable operating capacity. Fine resins have generally higher kinetics: shorter path for the ions to travel inside the resin beads

14 Water Softening Remove hardness mainly Ca +2 and Mg +2 Strongly acidic cation exchange resin in Na + form Applications Domestic and industrial water boilers Laundries, Dish washer Soft drink plants Resins used Amberlite IR120 Na; SR1L Na, Amberjet TM 1000 Na Treated water quality Residual hardness < 0.02 meq/l (1 mg/l as CaCO 3 ) with reverse flow regeneration Regeneration: brine (NaCl as a 10 % solution)

15 Water Softening SAC (Na + )

16 Water Softening The water salinity is unchanged, only the hardness is replaced by sodium. Sodium salts have much higher solubility, so they don't form scale or deposits. A small residual hardness is still there, its value depends on regeneration conditions.

17 Water De-alkalization Remove bicarbonate and temporary associated hardness Weakly acidic cation exchange resin in H + form Applications Beverages, Soft drink plants, municipal water Resins used Amberlite IRC86 (industrial), Amberlite PWC13 (municipal), Imac TM HP333 and HP335 (household filter cartridges) Treated water quality Endpoint at 10 % of the raw water alkalinity Contains CO 2 which requires degasifier Regeneration: Acid (preferably HCl at 5 % concentration)

18 Degasification 2 R-H + Ca (HCO 3 ) 2 R 2 -Ca+2 H HCO 3 - H + + HCO 3 CO 2 + H 2 O (CO 2 25 : 1.5 g/l) Reduce the ionic load thus regenerant CO 2 residual 10 mg/l

19 Degasifier

20 Degasifier Atmospheric degasser (bicarbonate + CO 2 < 0.6 meq/l) Forced draft degasser Thermal degasser, (O 2 -CO 2 ) Vacuum degasser, (1-5 kpa, O 2 -CO 2 ) Membrane degasser Small size, for RO permeates with low ph and high free CO 2, small demineralization systems

21 Water Demineralization Remove all the ions from water CIX (H + ) - AIX (OH - ) Treated water contains only traces of sodium and silica Applications Beverages, soft drink plants, municipal water Resins used Amberlite IRC86 (WAC), Amberlite IR120 or Amberjet 1000 or 1200 (SBA), Amberlite IRA96 or IRA67 (WBA), Amberlite IRA402 or Amberjet 4200 or 4600 (SBA) Treated water quality (lower than RO or Distillation) Conductivity: 0.2 to 1 µs/cm Residual silica 5 to 50 µg/l Regeneration: strong acid and caustic soda

22 Operational aspect Column Operation Co-flow regeneration Counter flow regeneration Batch (well-mixed) MIEX, other IEX resins Fluidized or Suspended MIEX or any other IEX resin

23 23 Column Operation 23

24 Column operation Freeboard columns Air or water hold down vessels (requires inert resin) Stratified (weak/strong mix) vessels Split flow regeneration unit (avoid disturb, hard to adjust) Packed bed columns Very little extra space, Smaller and cheaper Usually higher depth Floating bed with almost not fluidization WAC/SAC and WBA/SBA are separated via a plate with nozzles Mix beds Hard to regenerate Not very efficient due to shallow depth

25 25 Practical limitations Affinity difference Effective for low concentrations (brackish or sea water ) Only ionized targets can be eliminated Regeneration (cost, environmental burden)

26 26 Practical limitations Affinity difference Effective for low concentrations (brackish or sea water ) Only ionized targets can be eliminated Regeneration (cost, environmental burden)

27 27 Regenerant NaCl or KCl (10% ) for SAC and SBA HCl for nitrate removal Demineralization (SAC) o HCl (5%), efficient no precipitation o H 2 SO 4 (0.7-6%), cheaper and easier to store, less efficient, potential for precipitation o HNO 3, exothermic reaction, dangerous, not recommended De-alkalization WAC, HCl, H 2 SO 4 (0.7%) SBA, NaOH (4%) WBA, NaOH, NH 3, bicarbonate 27

28 28 Regeneration Counter-flow regeneration Higher reg. Efficiency, lower reg. quantity Lower elution leakage Lower resin inventory Improved water quality Hard to keep the resin bed consolidated Resin mixing Purity of the regenerant Co-flow regeneration Reverse exchange (displacement of low affinity with high affinity ions) Requires large excess of solution to fully regenerate Leakage in the next run 28

29 Regeneration steps 29 Backwash the bed to remove the suspended solids (co-flow) Regenerant injection at low flow-rate (20-40 minutes) Rinsing Washing with service water Regeneration ratio Amberjet 1200 regenerated with 55 g HCl per litre operating capacity : 1.20 eq/l 55 g/l HCl = 55/36.5 = eq/l Regenerant ratio = 1.507/1.20 = 1.26 = 126 % 29

30 Regeneration ratio 30 WAC resins: A safe number is 105 to 110 %. WBA resins: 115 to 140 %, due to presence of some strongly basic functional groups. SAC resins: Regenerated with sulphuric acid, larger excess (at least 40%) than those regenerated with HCl. SBA resins: Depends on the type of SBA resin (styrenic type I, type II or acrylic resins). Not economical to regenerate the resin totally. SAC and SBA resins: Larger excess than their weak counterparts. 30

31 31 Thoroughfare regeneration 31

32 32 Throughput Volume produced until the cartridge has to be replaced Salinity of the feed water Volume of resin in the cartridge (throughput is approximately proportional to resin volume) Type of resin used Quality and efficiency of the off-site regeneration process Endpoint (conductivity at which the unit is considered exhausted) With good resins and good regeneration, the throughput can be approximately calculated as: Throughput [L] = 500 (Resin volume [L]) / (Salinity [meq/l])

33 Kinetics 33

34 34 Kinetics In normal practice, solution is withdrawn at the outlet of the column so the equilibrium is permanently shifted to the right. R-A + + B + R-B + + A + Na + -form SAC resins initially remove divalent cations from water, but not other monovalent cations (selectivity) 2 R-Na + Ca +2 (HCO 3 ) 2 R 2 -Ca + 2 Na + HCO 3

35 35 Selectivity WAC (H-form) resin removes only hardness, and only when alkalinity is present. For softening WAC resin must first be converted to the Naform with an alkali. OH form resins are not used to remove bicarbonate or carbonate from neutral water when it contains hardness (precipitation risk)

36 36 Selectivity OH-form resin reacts with very weak acids, such as silica (SiO 2 ) or boric acid (H 3 BO 3 ). R-OH + H + HSiO 3 R-HSiO 3 + H + OH Special SBA resins with different functional groups are used with an increased nitrate, sulphate, perchlorate selectivity.

37 Selectivity 37

38 Selectivity 38

39 Selectivity 39

40 Selectivity 40

41 Selectivity 41

42 Selectivity 42

43 Selectivity 43

44 44 Nitrate removal Methemoglobinemia, also known as blue baby syndrome R SBA -Cl + NO 3- R SBA -NO 3 + Cl - SAB (Cl - ) Amberlite PWA5 PWA15 Purolite A520E SIR-100-HP 44

45 45 Nitrate removal Anion resins are less dense and require a backwash flow rate about 1/3 that of softening resin. salt concentrations are typically applied at 4 percent to 8 percent. Regenerant levels must be high enough in nitrate removal to ensure that nitrate leakages are kept to acceptably low levels. 45

46 46 Natural Organic Matter Hydrophobic Hydrophilic Very Complex Mixture Neutral Removal Mechanisms by IEX resins : Adsorption (blocking exchange sites) Size Exclusion (blocking exchange sites) Ion exchange (favourable) Easier regeneration (lower conc., lower flow rate/contact time) 46

47 47 IEX for NOM removal A relatively recent concept Polystyrene resins are more hydrophobic /selective than Polyacrylic resins. Polystyrene resins have shown stronger affinity with aromatic moieties. Competition kinetics exist between NOM and inorganic Ions SO 4 > NO 3 > NOM > Cl > HCO 3 > OH > F

48 Important characteristics of resin 48 Functional group (strong or weak basic) Backbone structure (acrylic or styrene) Resin water content Physical form (gel or macroporous) Bead size Treatment process

49 49 IEX for NOM removal Efficient reactor design Enforce IEX as the dominant removal mechanism Shorter contact time Combined different IEX resins Enhance the NOM removal Combination of weakly-strongly basic IEX resin Combination of Polystyrene-Polyacrylic IEX resins Microbial growth on IEX resins Disinfection set-up configuration