Chapter INTRODUCTION

Transcription

1 INTRODUCTION

2 Chapter INTRODUCTION Nitrogen and oxygen, respectively second and third largest man-made commodity chemicals today, are mainly produced from air which is rich in nitrogen (78.084mol %) and oxygen (20.94mol %). In fact, air is the only source used for the production of these gases ever since Linde achieved large-scale liquefaction of air in 1895 (1). This development gave rise to a large cryogenic industry for the separation of oxygen. World over, oxygen is mainly used for steel making ethylene oxide production, coal gasification etc. (Table-I). The oxidation process for the ethylene-to-ethylene oxide production consumes the largest amount of oxygen among petrochemicals. The largest single site oxygen facility is in South Africa for coal gasification at SASOL II consuming 1560 tons per day of oxygen. A substantial quantity of oxygen or oxygen-enriched air is used for the treatment of municipal wastes. During the last forty years, the demand for oxygen-free nitrogen for inert gas atmosphere led to the development of nitrogen generation plants. The use of N2 as an inert gas for blanketing started gaining ground in the 1950s. The use for purging the tanks and vessels that store hydrocarbons and corrosive liquids gave an impetus to N2 applications in chemical industries. The hydrocarbon vapors accumulate in the residual space at the top of the containers storing hydrocarbons. The atmospheric air is drawn into the tank during emptying of these containers. The practice of nitrogen blanketing in these containers has become prevalent in hydrocarbon industries to prevent hazardous air-hydrocarbon mixtures. Nitrogen blanketing while storing corrosive liquids like sulphuric acid minimizes the corrosion of vessels by avoiding oxygen. Similarly, in metal industries nitrogen blanketing is used to prevent metal oxidation during smelting. Another growing application for nitrogen is for maintaining dust free and inert atmosphere in the electronic industry. Nitrogen gas production, during last years, has become (2) a major industry as a consequence of these diverse applications (Table-2). Argon gas is mainly used in industries as an inert gas for creating inert atmosphere, where a completely inert atmosphere is required. Usually an argon or argon-hydrogen mixture is used in the production of high-purity iron. Argon is also used in welding, 2

3 Chapter I cutting, and spraying of metals, depending on the welding process, the noble gases are used pure, as a mixture, or in combination with oxygen, hydrogen, or carbon dioxide. Argon / argon-hydrogen mixtures (>5% H 2 ) are used as protective gases for plasma welding. Table-l Commercial Applications of Oxygen gas Application Metal industry Steel manufacturing, metal fabrication, Welding Chemical industry Production of ethylene oxide, acetylene, Titanium oxide, propylene oxide, vinyl acetate. Partial oxidation processes Coal gasification Others Pollution control; Industrial & municipal waste water treatment Fish farming Paper industry Bleaching & treatment of pulp Medical uses Breathing gas for patients suffering from Pulmonary disorder Glass industry % Share Process 60 Cryogenics & PSA 20 Cryogenics 10 Cryogenics 10 PSAICryogenics Cryogenics PSAICryogenics PSAICryogenics Research work on new argon applications is being done in number of laboratories it is expected that this work will lead to increased consumption of argon in the future (3). Table- 2 Commercial Applications of Nitrogen gas Application Chemical industry Blanketing of vessels having flammable or toxic chemicals Oil industry Well fracturing, enhanced oil recovery Electronic industry For inert dust free atmosphere Metal industry Cooling mould, blanket gas Food industry Food preservation, soft drinks % Share Process CryogenicslPSA Cryogenics IPSAI Combustion Cryogenics CryogenicslPSA CryogenicslPSA 3

4 Chapter METHODS FOR THE SEPARATION OF OXYGEN, NITROGEN AND ARGON FROM AIR Theoretically, oxygen, nitrogen, and argon can be obtained both from the air and water. Because water electrolysis is expensive, virtually all oxygen is produced from air. The various physical properties of oxygen, nitrogen and argon, are given in Table-3 and form the basis of separation of these gases from air. Gaseous oxygen required for commercial applications is obtained predominantly using cryogenic fractionation due to difference in boiling points of nitrogen ( C), argon ( C) and oxygen (-I C). In the 1950s and 1960s, combustion of hydrocarbons in air to produce nitrogen was an accepted method. This gave relatively pure nitrogen the only impurities being carbon dioxide and moisture. Later, cryogenics based processes came into practice for production of highly pure and dry nitrogen. Cryogenic industries, today, has become the predominant source for nitrogen supply to many industries. The late 1970s and early 1980s saw the advent of membrane and adsorption technology for air separation and made their niche in many industries for the separation of oxygen and nitrogen from air (4-7). In the following sections, we shall briefly discuss the principles of cryogenic and membrane adsorption processes and other methods used for production of oxygen and nitrogen from air. Table -3. Physical Properties of Oxygen, Nitrogen and Argon Property Value N2 O2 Ar Atomic weight Boiling point, DC at kPa Quadrapole moment A < Polarizability, cm 3 X I Lennerad-Jones force constant, Elk, K Lennerad-Jones force constant,o', A Molecular length, A

5 Chapter I Molecular width, AO Kinetic diameter, A Critical diameter, A Vander wall radii, A CHEMICAL PROCESS FOR PRODUCTION OF OXYGEN In the late 1800s, various attempts were made to separate oxygen from air on a commercial scale employing chemical processes in which solid chemicals were reacted with atmospheric oxygen to store oxygen in them. Thus stored oxygen was later recovered thermally by heating the solid at higher temperature (8). The most successful processes involved heating barium oxide in air at 590 C under pressure, so that it accumulated oxygen to form barium peroxide. The peroxide, in tum, was heated to ca. 870 C, at which point it released half of its oxygen, which was collected at ca. 95% purity. This was subsequently improved by maintaining the barium oxide at 600 C, while oxygen was absorbed from air supplied at 170 kpa and subsequently desorbed under vacuum. 1.4 CRYOGENIC AIR SEPARATION Cryogenic air separation first introduced by Carl von Linde in 1901 based on distillation ofliquid air involves three steps ~ Purification of the incoming air to remove particles, carbon dioxide, hydrocarbon and water. ~ Refrigeration and economization of refrigeration values contained in the product and waste streams. >- Separation by distillation. The vapor pressure curves of air constituents indicate that air condenses under ambient pressure (l bar) at a temperature of 81.5 K (;-192 0c), whereas at 6bar only 1010K (-172 C) is required to do so. To cool 1 kg of air from y15 to ;-196 C requires ca

6 Chapter J kj of energy. The refrigeration required could be provided by expansion in a throttle, by the Joul!'!;-Thomson effect, or by using a turbo expander, which produces a greater amount of refrigeration energy. Starting at ambient temperature, neither effect is in itself sufficient to produce liquid air; therefore, countercurrent pre-cooling must be used to lower the temperature to a suitable level. Earlier distillation columns were inefficient, however, by 1910; Linde had introduced a double column, one above the other, with a condense.r evaporator in between. This remains the concept employed for producing oxygen by cryogenic air separation. Over the years, oxygen production plants have continued evolving into larger, more efficient process systems. To date, the largest modular plant is capable of producing 2250 tpd (9). 10 i ~ 5.7 <U.= 5 l!? 4 => <I) 3 <I) ~ Q. 2 5 ~ ~ -,L- ~ Nitrogen ",i',/ Oxygen,,,-,,,,,,,,,, " AT==2.5K Temperature, c ~_ Temperature, K Figure-I Outlines the relation between the vapor pressures of oxygen and nitrogen at difference temperature Separation of air components is performed by cryogenic rectification. The simplest system is the single-column process. It is used to produce gaseous nitrogen and small amounts of liquid nitrogen. If both nitrogen and oxygen products are required, the double-column process is used. The principle of double-column rectification combines the condenser of the high-pressure column (HP column) with the reboiler of the lowpressure column (LP column) in a single heat-transfer apparatus. To provide a sufficient 6

7 Chapter 1 heat-transfer rate, a temperature difference of about 2 K is required. Therefore, the columns are operated at different pressures. Therefore, the relations between the vapor pressures of oxygen and nitrogen, temperature difference and resulting operating pressures at which most double-column processes are more suitable to operate (10, II). 1.5 MEMBRANE AIR SEPARATION The development of membranes with high permeability and selectivity for one gas over others in early 1980's led to increased research for the use of membranes for air separations (12). Although gas membrane technology is still young, it is proving to be one of the most significant new unit operations to appear in the past two decades. Membrane technology for the separation of liquid-liquid and liquid-solid streams has been practiced in industry for many years (e.g., reverse osmosis, ultra filtration, micro filtration, and other membrane-based processes.). Advances in membrane technology in these other areas gradually gave impetus to the development of membranes suitable for industrial gas separations also. Separation of oxygen and nitrogen is being practiced by membrane separation processes. However, at present, gas separation membranes do not offer a separation solution or product gas that is particularly unique over existing processes. Gas separation membranes, therefore, must compete primarily on the basis of overall economics and convenience. The basic principle for membrane air separation is described below: Gases permeate through dense nonporous membranes by solution diffusion. The permeability coefficient, Pi' which is a measure of the capacity of the membrane to transport gas species i, is the product of the Henry's law constant S and the diffusivity I coefficient D. for component i in the membrane material. The volumetric flux of the I component through a membrane of surface area A and thickness I, whose two faces are kept at pressures p 1 and P2 is given by where /;;p is (he difference in partial pressures (p,l :-P 2 ) across the membrane. The ability of a membrane to separate a pair of gases is described by the ratio of permeability's (I) 7

8 Chapter 1 I>~P IPf' which is called the selectivity or separation factor for gaseous components i and j. Typical pemleability coefficient and 0/N 2 selectivity for a membrane fiber is given in Table-4. Virtually all membranes are more permeable to oxygen than they are to nitrogen. Hence, the nitrogen product from a membrane separation stage is the retentive on the high-pressure feed side (13). Economical membrane operation requires a high flow rate (of oxygen, in the case of nitrogen production) and O/N z selectivity of at least 4. Furthermore, a thin membrane is essential to give a high gas flux. Thin polymeric films by themselves are too weak to withstand the high differential gas pressures required. The commercial breakthrough of membranes in gas separation came with the fabrication of asymmetric and thin-film composite membranes. Table-4 The state of the art is membrane fibers with properties as listed. 0/N 2 selectivity permeability, Barrer 2.0 Skin thickness, nm 50 Membrane cost, $/m Feed pressure, bar I I Permeate pressure, bar I Fiber geometry, J.lm 250 x 500 Length, m 2 The separation membrane can be arranged either in spiral wound modules or in the shape of hollow fibers; the latter are normally mounted together in bundles (14). The thin membrane must be virtually defecting free because a higher defect area is detrimental to the selectivity Ci owing to Knudsen diffusion. Surface defects can be eliminated by over coating the membrane with a 500 nm layer of a highly selective material such as silicone rubber (15). 8

9 Chapter J Early membrane systems used single-stage processes for N2 production, which are best suited for low-purity nitrogen. More sophisticated, multi-stage processes are better suited for higher purities and larger capacities. Further, processes that are currently used for nitrogen production are shown in Figure-2. Nitrogen containing less than 10 ppm by volume of oxygen can be produced economically by using a membrane/deoxo hybrid system; oxygen is removed by reducing it to water over a catalyst with hydrogen. The gas leaving the catalyst bed is cooled, dried, and delivered as product. The main components of a commercial membrane air separation module typically include the compressor, the feed-conditioning package, the membrane modules and the system controls. Air is compressed to ~;-15 bar, and conditioned to remove contaminants and to provide temperatures and humidity levels for optimal membrane performance. The separation unit typically comprises one or more separation stages, each consisting of one or more membrane modules in parallel. Prepackaged systems for producing 9S% N2 range In wide range of capacity are commercially available. The largest nitrogen membrane units produce 600 tid of nitrogen, which corresponds to an hourly capacity of m 3 (STP) (16). Current commercial experience indicates unscheduled down times below 1-1.;>%. Given protection from organic contaminants and liquid water, membrane lifetimes may exceed 10 years or more. 9

10 Chapter I Process schematic,.---+ Product N2 '----~ O 2 enriched waste Description 1 - stage Low purity Product N2 2-stage Waste Medium purity Product N2 3 - stage High purity Waste Air Membrane process DEOXO Product N2 Membrane + DEOXO Waste '02 free' nitrogen Figure-2 Configurations of membrane processes for nitrogen production 1.6 ION TRANSPORT METHOD Ion transport membranes (ITMs) are solid inorganic oxide ceramic materials that produced oxygen by the passage of oxygen ions through the ceramic crystal structure. These systems operate at high temperatures, generally over 593 C. Oxygen molecules are Bhavn3:J,]r University Library. BHAVNAGAR. 10

11 Chapter 1 converted to oxygen ions at the surface of the membrane and transported through the membrane by an applied electrical voltage or oxygen partial pressure difference, then reform oxygen molecules after passing through the membrane material. Membrane material can be fabricated into flat sheets or tubes. For large energy conversion processes the pressure difference transport driving force is the method of choice. Membranes, which operate by a pressure difference, are referred to as mixed conducting membranes since they conduct both oxygen ions and electrons. The oxygen ions travel through the ITM at very high flow rates and produce nearly pure oxygen on the permeate side of the membrane. The oxygen can be separated as a pure product, or another gas can be used to sweep on the permeate side of the membrane to produce a lower purity product (17). 1.7 ADSORPTION SEPARATION Adsorption separation like membrane processes is one of the emerging alternatives to the traditional cryogenic distillation and is being increasingly used for the separation of oxygen and nitrogen from air for the last three decades. It is estimated (18) that presently around 20% of air separation in the world's is accomplished by adsorption processes. Adsorption based pressure swing or vacuum swing processes are more cost effective than the cryogenic distillation for producing oxygen at volumes of 200 tons per day, particularly when the required oxygen purity is up to 95%. However, the maximum attainable oxygen purity by adsorption processes is around 95% with separation of mole percent argon present in the air being a limiting factor to achieve 100% oxygen purity PlUNCIPLE OF ADSORPTION SEPARATION Separation of these gases is based on the selective physical adsorption of one or more components of a gas mixture on the surface of a microporous solid. When a gaseous mixture is exposed to an adsorbent for a sufficient time, equilibrium is established between the adsorbed and gas phase. Adsorbed phase often has a composition different from that of bulk phase. The gas phase becomes richer in the less selectively adsorbed 11

12 Chapter J component. The attractive forces responsible for adsorption are of the van der Waals type. Desorption can be achieved either by increasing the temperature of the system (TemperalUre Swing) or by reducing the adsorbate pressure (Pressure Swing). Desorption step also regenerates the adsorbent surface for reuse during the subsequent adsorption step. Therefore, the adsorption separation processes consist of a cyclic sequence of adsorption and desorption steps (19). In a typical adsorption isotherm as shown in Figure-3, the amount of gas adsorbed for a component gas decreases along the curve from point A to point B with a decrease in the partial pressure of that component. The adsorption capacity available for separation in pressure swing adsorption (PSA) is the difference in the capacities for the given component at the two pressures. However, in actual practice, due to the heat of adsorption, there is an associated change in the temperature of the system and working adsorption capacity is slightly different as shown in the Figure-3. Adsorption selectivity for a component A over B in a gaseous mixture is defined as (2) 1 q,ads o ~ ~ S? q.,ads ~ z :::> ~ «A T SWING Pads PRESSURE- Tl Figure-3 Adsorption isotherms showing pressure and temperature swings where, X and Y are the adsorbed and the gas phase concentrations. There are following factors used singly or in combination for achieving the desired selectivity: 12

13 Chapter 1 Steric factors, such as difference in the shape and the size of the adsorbate molecules. Equilibrium effect, when the adsorption isotherms of the components of the gas mixture differ appreciably. Kinetic effect, when the components have substantially different adsorption rates PRESSURE SWING ADSORPTION (PSA) Adsorption processes commercially used for the separation of oxygen and nitrogen from air is pressure swing adsorption (PSA) type process. This process, in its simplest form consists of two adsorption beds (Figure-4a), which are alternately pressurized and depressurized. The basic steps involved (6, 7) in the process shown in the Figure-4b are: as follows FEED PRESSURIZATION The feed gas, i.e., air in compressed into an adsorbent bed at a pressure of 5-7 atmosphere from one end with the other bed end being closed. The gas freed from the adsorbed gas, for example nitrogen in case oxygen emichment process is accumulated at the closed end PRODUCT RELEASE At the higher pressure, the adsorbates free gas for example oxygen in case of oxygen emichment process is withdrawn from the far end of the bed while airflow is maintained. The distinct regions as shown in Figure and described below develop in side adsorbent bed. At the air inlet end, adsorbent bed is saturated with the adsorbate and the gas phase will have the composition of the air. At the outlet end, adsorbent bed is still adsorbate free and the gas phase is enriched in oxygen. The adsorbent bed region in between is known as mass transfer zone and it is in this zone adsorption is occurring and air composition changes rapidly along the axial position. Adsorption front moves along the bed as more air is introduced and eventually the 13

14 Chapter I adsorbent is completely saturated with nitrogen and bed is said to be to "breakthrough" into enriched oxygen stream DEPRESSURIZA non This involves reducing the air pressure from 7-8 atmospheres to I atmosphere. During this step, adsorbed nitrogen is largely desorbed into the gas phase and gets released as a waste stream LOW PRESSURE PURGE To complete the regeneration, adsorbent bed is further purged with product quality oxygen from the product end at a low pressure usually in the countercurrent to the feed airflow. A typical PSA unit consists of 2-4 adsorbent beds, solenoid valves, piping and an electronic timer for automatic valve switches. The steps of adsorption and desorption are repeated in each bed and finally the production of a continuous stream of enriched oxygen results at the product end. The total cycle time may vary from a few seconds to a few minutes. Product Gas VI Orific"e-p==,! V2 Zeolite mole~ular sieve,..--'--=:' Bed " Bed B Exhaust Compressed air Figure-4a Schematic diagram of a two- bed Pressure Swing Adsorption process. 14

15 Chapter 1 1 I. Feed pressurization 3. Depressurization 2. Product release 4. Low pressure purge Figure-4b The basic steps indicating mass transfer zones in a PSA process VACUUM SWING ADSORPTION (VSA) The PSA processes carry out the feed step at pressure higher than the ambient and adsorbent regeneration at pressure close to ambient; the VSA processes on the other hand, carry out the feed step at pressure close to the ambient and the adsorbent regeneration at sub-atmospheric pressure. Since selectivity of nitrogen over oxygen decreases as pressure increases, a greater amount of oxygen is co-adsorbed and then lost during the regeneration step in the in the PSA than in the VSA processes. This results in lower oxygen recovery from the PSA than the VSA processes. Also, the entire feed air stream has to be compressed in the PSA processes as compared with the evacuation of only the waste gas in the VSA processes. Those two factor result in higher energy consumption per unit of oxygen production for the PSA processes than for the VSA processes. The PSA process is inherently simpler and therefore has a capital advantage over VSA processes. At present PSA is the process of choice for production of less than 15 tpd of oxygen. Above this, VSA is generally the process of choice for producing low purity oxygen. In the last 25 years, oxygen VSA has transformed from a laboratory curiosity to a formidable industrial process. This has been possible by evolving more efficient and simpler cycles as well as developing higher efficiency adsorbents. To move this technology to the next stage would require increasing the range of its application by innovative process designs, and developing higher capacity and efficiency vacuum 15

16 Chapter I pumps. Of course, new discoveries in adsorbent area will have a major impact on this technology. Adsorbents, such as oxygen selective materials may change the landscape of this technology forever (20) PRESSVREIV ACVVM SWING ADSORPTION (PVSA) Union Carbide Industrial Gases Technology Corporation has introduced a further advancement in adsorption-based system. This new system termed Pressure Vacuum Swing Adsorption (PVSA) utilizes state of the art zeolite molecular sieve along with an improved adsorption cycle to further reduce the cost of oxygen production. PVSA process is a hybrid of PSA and VSA. During operation of the system, the feed compressor supplies air to one of the adsorption vessels, while the other vessel undergoes regeneration. The adsorbent material present in the vessel preferably adsorbs the nitrogen, water and carbon dioxide present in the air, while allowing the majority of the oxygen and argon to pass through. Once the vessel is saturated with nitrogen and can no longer produce oxygen of acceptable purity, the vessels switch operation. Regeneration occurs via a pressure blowdown step followed by evacuation and then a purge with product quality gas. The PVSA system is able to achieve an efficient turn down because of the batch type of process utilized with equilibrium based adsorbents. Since these features allow for a discontinuous operating of the cycle, turn down can be accomplished by simply lengthening the cycle at discrete times, in proportion to the decrease in product demand. During this turn down period, the positive displacement compressors and vrcuum pumps are vented to the atmosphere at low-pressure ratio, minimizing the energy consumption. 1.8 COMPARISONS OF PROCESS ALTERNATIVES Adsorption and polymeric membrane processes will continue to improve in both cost and energy efficiency through ongoing research and development of adsorbents and membrane materials. Neither technology is expected to challenge cryogenics for large tonnage production of oxygen, especially at high purities. Both adsorption and membrane systems produce by-product nitrogen containing significant amounts of oxygen. If high 16

17 Chapler I purity nitrogen IS required, an-add on deoxo or other purification system must be employed to upgrade the quality of the nitrogen. Neither process has the ability to directly produce argon or rare gases. Production of liquid oxygen or nitrogen for systems back up requires an add-on cryogenic unit or delivery of product form a merchant facility. On the other hand, adsorption and membrane processes are less complex and more passive than cryogenic technology. Chemical processes offer the potential for continuous operation and economies of scale through large production output from single trains, but to date have not been able to overcome material corrosion problems. lim technology is currently foreseen as the best candidate to challenge cryogenics for the production of high purity, tonnage quantitites of oxygen. As with the other non-cryogenic processes, ITM has limitations in regard to production of pure by-products and liquids for storage and backup. lim is also at an embryonic state of development, so the potential for improvement is large Table-4a compares the merits of the vanous technologies based on the following categories. Status is the degree to which the technology has been commercialized, varying from mature for cryogenics through developing for lim. Economic Range is the typical production rang where the technology is currently economically feasible. Table-4a Technology comparisons table Process Status Economic range Byproduct Purity Start-up (stpd) capability limit(vol.%) time Adsorption Semi-mature < 150 Poor 95 Minutes Chemical developing Undetermined Poor 99 + Hours Cryogenic mature >20 Excellent 99 + Hours Membrane Semi-mature <20 Poor -40 Minutes ITM developing undetermined Poor 99+ Hours The less efficient but less complex adsorption and polymeric membrane systems are favored at the lower production rates. By-product Capability is a measure of the ability of the process to produce relatively pure nitrogen or argon streams without add-on deoxo or cryogenic systems. Purity limit is the maximum purity that can be economically 17

18 Chapter 1 produced using the specific technology. All the technologies are capable of production lower purities than the maximum by adjusting process conditions or by blending purer oxygen product with air. Start-up time is a measure of the time required to restart the process and reach purity after a shutdown (17). 1.9 ADSORBENTS Adsorbent is ccntral to the separation process as adsorption selectivity and capacity determines the product purity and the adsorbent bed size design. Micro porous solids like zeolites and carbon molecular sieves are employed as adsorbent for oxygen and nitrogen separation from air respectively. From the process simulation studies (21), it is reported that the minimum adsorption selectivity for a desired component of 3 is required for a separation process to be economical. It is well known that the intrinsic equilibrium and kinetic properties of the adsorbent can strongly influence the efficiency of a particular separation process. Still, most of the focus in the literature is on process innovation and relatively little on new adsorbent development. Effect of adsorbent development on the process economics of an adsorption process for oxygen enrichment can be clearly signified from the data in Table-5. Therefore, developments in the area of adsorbent~ used for oxygen and nitrogen separation are discussed in the following sections NITROGEN SELECTIVE ADSORBENTS As observed from the Table-3, nitrogen molecule possesses a quadrupole moment (0.3 I A 02) higher than oxygen molecule «0. I I A 02) and is expected to interact strongly with polar surfaces. This difference in adsorption affinities with polar surface forms the basis for separation of these gases from air wherein nitrogen is retained on the adsorbent surface and oxygen enriched air is recovered as a product. In principle, all polar adsorbents such as alumina, silica gel and zeolite molecular sieve can be used for 02 enriclm1ent of air by PSA. However, zeolite based adsorbents are prominently used adsorbents for this separation. The different zeolites used for 02 enrichment of air include (2,22), NaA, CaA, LiX, NaX, cax and other cation exchanged X, CaY, Mordenite, 18

19 Chapter I Chabazite, Erionite. To have a better insight, a brief introduction of zeolite structure, type of interactions involved between nitrogen and oxygen with zeolite surface and the total energy of physical adsorption are discussed below. Tablc-S Impact of improved adsorbent on PSA process for oxygen enrichment from air Earlier Present adsorbent adsorbent Property NaCaA LiX UN2/ Adsorbent beds 4 4 Valves 28 6 Pressure, atm. 3.3/10 1.1/0.2 O 2 recovery, % Power, kjlmol product Cycle Time, min 6 4 Working capacity of Adsorbent Adsorbent inventory in tones for 200 Nm 3 /h 1 product ZEOLITE MOLECULAR SIEVES Zeolites are crystalline alumino silicates (24) with a framework based on extensive tbreedimensional network of Al0 4 and Si0 4 tetrahedral. The Al0 4 in the structure gives rise to anionic charge for the framework, which is balanced by cations that occupy nonframework positions. A general chemical formula of zeolite is represented as (3) Where M is the exchangeable cation and n is its valence. The value of x is equal to or greater than 2 because Al l + does not occupy adjacent tetrahedral sites. Mainly alkali and 19

20 Chapter J alkaline earth cations form the exchangeable cations. The structure of the crystal framework contains voids (cavities, pores) and channels of molecular dimensions. The pore or channel openings range from 3 to load, depending on the structure. Framework structures of zeolite A and X, the most widely used adsorbents are shown in Figure-S. Zcolite-A Zeolitc-X ~I -_.- SUFE R CAG E 'J.L~..I,-(?I. Figure-S Structure of zeolite A and X 1.10 ENERGETIC OF PHYSICAL ADSORPTION OF N 2 /0 2 IN ZEOLITE The total energy of physical adsorption between nitrogen/oxygen molecules and zeolite surface is comprised of dispersion ~D' polarization ~p, field-dipole interactions ~F~. fieldquadrupole interactions ~FQ' close-range repulsion ~R' and sorbate-sorbate interactions ~sp. Thus, ~ is given as 20

21 Chapter I (4) The adsorbate nitrogen and oxygen being nonpolar and also adsorption coverage being small, the term ~F~ and ~sp can be ignored while calculating interaction energy. Hence the above equation for total energy becomes (5) Substituting the terms for various interactions (24) the equation becomes ~ = -1I4Q(~F/~R)- 112 (~s/k) F2- 'LAlr 6 + 'LB/r I2 (6) Where Q is quadrupole moment, k is a constant (in SI units, k=9 X 10 9 ), F is the field, and ~s is the polarizability of a adsorbate molecule. The values of the constants A and B are calculated by the method described by Barrer (25). As, in zeolites Al and Si atoms are buried inside the center of the tetrahedral and are not directly exposed to nitrogen/oxygen molecules and also possess small polarizability values, their interactions with the adsorbates molecules can be ignored. Therefore, the principle interactions of oxygen and nitrogen molecules with the zeolite structure are through the lattice oxygen atoms and accessible extra framework cations. Reasonable idea of these interactions can be had from the heats of adsorption data at zero coverage for N2, O2 and Ar in zeolite mordenite, A and X shown in Table 6. In mordenite, ZM-060, there is electrostatic and polarization interactions between extra framework Na+ cations and the adsorbate molecules. Lattice oxygen interacts with the adsorbates molecules through dispersion forces. As in ZM-210 and ZM-510, there are no extra framework Na + ions, heats of adsorption reflect only dispersion interactions between zeolite oxygen and adsorbate molecules. Furthermore, argon (3.7A') molecule having zero quadrupole moment and size almost similar to N 2 /0 2 molecule can be presumed to interact through dispersion and polarization forces only with the zeolite oxygen. Therefore, the difference in the heat of adsorption of nitrogen and argon will give an idea about electrostatic contribution to the total N2 interaction energy. This difference for Nz as seen from the Table-6 is 9.9kJ/mol for ZM-060 compared to 1.4 and 1.2 kj/mol for ZM-2l0 and ZM-S10 respectively. The higher interaction energy for ZM- 21

22 Chapter I 060 is clearly arising because of electrostatic interaction of Na + ions with N2 molecules. The contribution of the quadrupole interaction energy for N2 with an isolated Na + ion has been theoretically calculated as 13.9kJfmol. Lower value for this interaction when Na+ ions are inside zeolite cavity is expected due to screening effect of framework oxygen on Na+ ions. The values for ZM-210 and ZM-510 reflect that dispersion interactions for argon are marginally higher than those for oxygen. Heats of adsorption values for NaA, NaX and Na-mordenite (Table-6) show that these values for the three adsorbates do not differ much for zeolite A and X. However, ZM-060 shows substantially higher value because contributions from electrostatic as well as dispersion & polarization interactions are higher for zeolite mordenite. Higher interactions of adsorbate molecules in mordenite have been attributed (26) to restricted rotational freedom of adsorbed molecules inside mordenite side pockets. Table-6 Heats of adsorption of N 2 O 2 and Ar at zero coverage in various zeolites Zeolite -LIH kjfmor l [LlHo(N2)- N2 O2 Ar LlHo(Ar)] kjfmor i Mordenite ZM-060 (Si fal = 5.5) ZM-210 (SifAl= 5.5) ZM-510 (SifAl= 5.5) NaA NaX FACTORS EFFECTING ADSORPTION CAPACITY AND SELECTIVITY The following are the major factors that influence the nitrogen capacity and selectivity of a given zeolite molecular sieve. 22

23 Chapter J TYPE OF ZEOLITE AND SiiAI RATIO Both natural and synthetic zeolites have been reportedly used for oxygen generation using adsorption. Most of adsorbent development research has been confined to post synthesis modification particularly of zeolite A and X. Other zeolites and zeolite type microporous solids also hold potential for new adsorbent for this application. Natural zeolite reported (21) suitable for this includes chabazite, mordenite, clinoptilolite etc. For example, Itado mordenite from Japan adsorbs about 30 cm 3 N 2 1g at room temperature and atmospheric pressure. Oxygen generator using this zeolite has also been reported (21). However, natural zeolites have problems of purity and availability. Coe et al. (27) have reported that synthetic Lithium exchanged Chabazite (Sil AI= 2.6) exhibits 26.5 and 6.7cm 3 STPIg N2 and 02 adsorption capacity respectively and UN2/02= 5.9. With suitable isotherm shape at ambient conditions to increase the working adsorption productivity beyond the standard zeolites used for air separation, this adsorbent is reported to hold potential for oxygen generation. Most of the present commercial PSA units employ synthetic zeolites with NaCaA, NaX, CaX LiX and mordenite for oxygen generation. The adsorption capacity of an adsorbent depends on the number of cations. However, the strength of the cation - sorbate interaction is relatively independent of the number of the cations present (28). Hence, the adsorption capacity for weakly interacting adsorbates such as nitrogen is directly related to the number of sorbate-accessible cations in a zeolite. Therefore, it can be increased if the numbers of cations are increased in a given zeolite. Further, the number of cations in a zeolite depends on its Si! Al ratio. Coe et ai. (29) have developed a unique way of increasing Al content in the form of low silica X zeolite (LSX) with Sil Al ratio equal to 1 (the typical value being 1.25). CaLSX contains about 18.5% more accessible cations over cax having Si/AI as This results in a 20% increase in the nitrogen adsorption capacity. It is claimed that LSX possesses air separation properties superior to any other pelletized nitrogen selective adsorbent (29). In a similar manner, a low silica zeolite X in which more than 95% of cations can be exchanged with lithium is patented (30), which exhibits an extraordinarily high adsorption capacity and selectivity towards nitrogen from its mixture with oxygen. For 23

24 Chapter 1 example, at 700torr, the nitrogen adsorption capacity of 99% Li exchanged LiX (SiJAI =1)) is 32% higher than the 94% lithium exchanged LiX (Si/AI =1.25). The adsorption data for binary mixtures of oxygen and nitrogen (30) also show that LiX (2.0) has higher nitrogen selectivity than LiX 2.5 with the same exchange level has. It has been shown (30) that at room temperature and at 1 atm. N 2 /0 2 separation factor of LiX (SiJAI=I.O) is II compared to 6.4 oflix (Si/AI=1.25) and 3.2 ofnax. -; "" 2. ~ -, '" -=r? - '" <l I 22 I. JO Cs' K' <a) S.so "" 40 CAY ION CHARGe O "~TY / C.M -I.. 10 I() Figure-6 Dependence of Heats of adsorption, L'.Ho, at zero coverage with extra framework cation charge density NATURE AND LOCATIONS OF CATIONS In general, in a given zeolite the interaction energy and the adsorption capacity for nitrogen are expected to increase with the charge density of the cation (31-34). However, adsorption data given in Figure-6 and Table-8 show that there are factors other than cation charge density, which play significant role in adsorbate-adsorbent interactions. For example, among monovalent cations, lithium shows abnormally high values for heats of adsorption, specific retention volume and adsorption selectivity. Furthermore, bivalent cations show values lower that that expected by extending the heats of adsorption versus cation charge density plot for monovalent cations. These observations have been attributed in terms of different sites these cations occupy inside zeolite structure (Figure-5). 24

25 Chapter 1 The distribution of cations in NaX is of 4 type I sites, 32 type j' sites, 31 type II sites and 8 type III sites with 6 ions unlocated. However, the cations, which are accessible and interact with the adsorbate nitrogen/oxygen molecules, are site II and site III cations only. Lithium are reported (35,36) to preferentially occupy sites j' and II in zeolite X. Li+ ion due to its smaller size (0.68A 0) reside almost in the plane of the six member ring, while the larger Na+ cation (0.95AO) is held above the plane in trigonal-pyramidal coordination to the Oz molecules. As a result, site II Lt cations are less accessible to adsorbates than site Na + ions and show weaker interactions and heats of adsorption. Once the entire Na + cations of site II are replaced, Na+ ions from site III or unlocated positions are replaced by Lt ions and increase in heats of adsorption is observed. It has been reported (37) that the sharp rise in the heat value observed above 90% Li exchange is due Li+ ions occupying accessible site III at the expense ofna+ ions from site I or unlocated position. On the basis of (20) Li-NMR spectra ( Herden et ai., 1987), it has been shown that for zeolite LiNaX [NaI43Li674(AI02)8L7(siOz)II03],32 Li+ ions/unit cell are in site I', 32 Li+ ions are in the site II position, and 4 Lt ions with 14 Na+ ions/unit cell are in site III in the super cage. NMR lines corresponding to Li+ ions at site III are only observed when Lt exchange is higher than 64 Lt ions/unit cell. This clearly shows that site III Na+ are replaced with Lt at nearly 80 % Lt exchange. This means that the increase in the sorption properties of Nz and O 2 observed nearly this exchange level is due to the site III Li+ ion interaction with sorbate molecules. However, the sharp rise in sorption properties at lithium exchange above 90 % indicates that, at those Lt exchange values, the four site I or six unallocated Na + ions start getting removed. However, as the site I position is energetically unfavorable (Mortier et ai., 1972; Mortier and Bosmans, 1971; Herden et ai., 1987) the in comings Lt ions occupy site III instead of site J. Thus, there is an increase in the number of accessible site III Li+ ions corresponding to those Na+ ions, which are removed from site I/unallocated site. The abnormally high values of sorption properties observed for LiX-98 compared to NaX, KX-90 and CsX-70 could be explained in terms of the availability of these additional sites III Li+ ions compared to other monovalent ions. The lower heats of adsorption observed for bivalent Ca, Ba and Sr cation in zeolite X compared to values extrapolated from monovalent cations is also r I Hh;) Jll<l(~!,-! ' ~)r'~\}". { : lhr~: r 'I L ~'.,/,', oj;' '1.. I. '--, _.J 25

26 Chapter I resulting from the fact that bivalent cations occupy site II only (22,38) where monovalent cations occupy sites II and III. Site III is more accessible to adsorbate molecules and show higher interaction. Similar conclusions have been made about the effect of cation locations on the adsorption of nitrogen/oxygen in zeolite A (39) particularly in calcium exchanged zeolite A where abnormally high adsorption capacity & selectivity was observed with calcium exchange of>90% (Figure-7 and Table-7). 5 o. r AcmiATtON D II ACTIYATlOIII 4 2., PERCENT ION E~C~ANGE 100 Figure-7 Dependence of nitrogen adsorption selectivity and Heats of adsorption calcium exchange in NaCaA zeolite at 300K This has been attributed to the highly accessible site near the pore opening of the zeolite A occupied by calcium cations at this exchange level. Far IR studies (40,41) have shown that at higher than 90% calcium exchange in zeolite A, a major part of Ca 2 + ions are located at site B, i.e., in the six-ring but displaced into the a-cage and hence more accessible to adsorbate molecules. The remaining cations are located at the inaccessible site C, in the six-ring displaced in the ~-cage. However, at calcium exchange level, less than 75% ci+, Ca 2 + ions are located at site A, i.e., in the plane of the six-ring. Therefore, it is highly significant to understand cation locations in the zeolite structure to develop the desired adsorbent. The commercial implications of having high selectivity 26

27 Chapter 1 adsorbent can be seen from the data given III Table-S wherein the commercial performance comparison of LiX and NaX I cax adsorbent is given. Choudary et al. (42) have extended the study of N 2 /0 2 adsorption to rare earth exchanged zeolite X and the adsorption selectivity and capacity values are shown in Table-S. However, it has been demonstrated (39, 43-45) that the adsorption capacities for zeolites having multivalent cations are more sensitive to the activation procedure followed. o+---~----~--~-- --~ M Z 10 un a UX o KX 8.. CsX cax.. Figure-S Dependence of nitrogen adsorption selectivity,un2j02 with percentage cation exchange Ag EXCHANGED ZEOLITE A ADSORBENT Study of N2 adsorption in zeolite A having varied silver content shows nonlinear dependence both for adsorption capacity and heats of adsorption for nitrogen (Figure-9). Both heat of adsorption tl.had and nitrogen adsorption capacity of the silver zeolite A sharply increases after exchanging around 70% of the extra framework Na+ cations with silver ions (96) Nitrogen adsorption isotherm data measured at 77.35K also shown increase in micropore area (Radushkevich) from 2.20 to m 2 g'! as the Ag content increases from 0 to 100% indicating enhanced ingress of nitrogen inside zeolite A cavity at higher silver content. This can be explained in terms of different locations Na and Ag 27

28 Chapter I cations occupy inside zeolite cavity. Three Na+ cations present at site II in the 8-ring partially blocks the pore aperture and thus restricting the ingress of nitrogen inside the pores ofnaa (20). Form our adsorption data, it is apparent that up to 70% Na+ exchange, Ag + exchanges those sodium cations, which are present at sites I (inside 6-ring) and III (4-ring inside main cage) only. It is only after 70% exchange; Na+ ions located at site II in the 8-ring are replaced. Variation of Adsorption Capacity & Heat of Adsorption with Percentage of Silver 25, ,45 "I': 20 z':'... 'OJ) 15 ~ e c '0 g Jl 10 -< E " 0.g 5 o l-- ~ ;--~~ + 0 o Percentage of Silver ~ '- 25.;g ~ 20 :: " 15 ~ 10 ~ 5 Adsorption Capacity --+- Heat of adsorption Figure-9 Variation of adsorption capacity and heat of adsorption with percentage of Ag content DEGREE OF CATION EXCHANGE In general, the nitrogen adsorption capacity and selectivity increase with the increase in the degree of bivalent ion exchange (44-45) as shown in Figure-7 & 8 (39). For example, Henry constant for N2, which is indicative of adsorption capacity, increases with calcium exchange with steep rise at 90% exchange level. However, Henry constants for oxygen are not affected significantly by calcium exchange. Similarly, Heats of adsorption as seen in Table 7 for N2 increases form 18 to 25KJ/mol as the percent calcium exchange increase to 97 whereas heat of adsorption for O2 remains at Kllmol. In the case of zeolite X, the effect of Ca-exchange on N2 adsorption capacity and selectivity is more 28

29 Chapter I pronounced than in zeolite A as shown in Fig. 7. We have observed three fold increase in nitrogen selectivity as calcium exchange increase from 0 to 95% level ACTIVATION OF THE ADSORBENT AND PRES ORBED MOISTURE In any application of zeolites for adsorption purposes, the first step is to activate/regenerate the adsorbent in order to drive off the residual water. This is generally carried out by heating the adsorbent at an elevated temperature (up to 450 C). Both for zeolite A (39) and X (43-45), it has been shown that an activation procedure has an influence on the adsorption behavior of Ca exchanged zeolites. It has further been emphasized that in the case of polyvalent cation exchanged zeolites, thermal activation step should be carefully controlled failing which zeolite undergoes substantial degree of hydroxylation. Our studies (39) in zeolite A has demonstrated that the presence of moisture and CO 2 in the air from which nitrogen is adsorbed has also negative effect on the adsorption capacity & selectivity. This effect is more pronounced even with less than I mmol/g of adsorbed water for highly calcium-exchanged zeolite A. Effect of C02 is more pronounced than moisture. CO2 may be physiosorbed or chemisorbed in NaCaA zeolites and thus effecting the adsorption behavior. Effect of pres orbed water can be explained in terms of hydroxylation of the bivalent cations. Hydration is an equilibrium reaction represented by Where M is a cation having valancy n (usually 2 or 3), x is I to 6 and e is or 2. Products on the right hand side of the equation are detrimental to the zeolite capacity and stability. Hydroxylated multivalent cations, such as Ca(OH)+' are known to be ineffective sites for the selective adsorption especially for nitrogen. Additionally, the zeolite framework is unstable towards H+. The equilibrium may be shifted towards left by minimizing the amount of water present at any given temperature particularly above 150 C during activation. 29

30 Chapter 1 Table-7: Effect of Presorbed Water on!!jio of Nitrogen and Oxygen in Different Adsorbent Samples. t'lho, kj/mol, for presorbed water, mmollg of zeolite Adsorbate NaA N O NaCaA-24 N O I NaCaA-43 N O NaCaA-60 N O NaCaA-75 N O NaCaA-90 N O NaCaA-97 N2 25. I I

31 Chapter 1 Table-8: Adsorption data for oxygen 1 nitrogen gaseous mixture on different adsorbents at 30 C. Adsorbent Ilcnry Constant, Adsorption Heats of adsorption K/mmol.g.k.Pa- 1 Selectivity kj mor l O2 N2 U021N2 O2 N2 NaX NaY CeY \ CeX CaX LiXE OXYGEN SELECTIVE ADSORBENTS CARBON MOLECULAR SIEVES The production of nitrogen from air by PSA is largely based on the kinetic separation for oxygen due to the faster diffusion of the latter in carbon molecular sieve. From the sorption uptake curves for oxygen and nitrogen, as given in Fig. 8, it is observed that oxygen is adsorbed to its equilibrium value much faster than nitrogen (7,46). This is because oxygen molecule that is smaller in size (kinetic diameter 3.46 A'J diffuses much faster in carbon molecular sieve than nitrogen (kinetic diameter 3.64 A 'J. The diffusion time constant (D/r2) for oxygen and nitrogen in carbon molecular sieve, as calculated from sorption uptake, is reported (47) to be 1.7 x IO-4 S-I+ and 7 x 1O- 6 s- l, respectively. Hence, oxygen gets selectively adsorbed in carbon molecular sieve adsorbent and nitrogen is collected as the high-pressure product. 31