Zeolite Molecular Sieves

Transcription

1 An Introduction to Zeolite Molecular Sieves ry Purify Separate Dry Purify Separate Dry Purify Sep

2 What are zeolite molecular sieves? Hundreds of systems for the drying and purification of liquids and gases rely on the high adsorption efficiency of zeolite molecular sieves. These unique adsorbents are a result of synthetically produced crystalline metal aluminosilicates that have been activated for adsorption by removing their water of hydration. Since little or no change in structure occurs during this dehydration, highly porous adsorbents are formed that have a strong affinity for water and specific molecules. Unlike other adsorbents, zeolite molecular sieves have precisely uniform pore sizes and molecular dimensions. This translates into a sieve-like selectivity where molecules of varying size and polarity may be readily adsorbed, slowly adsorbed or completely excluded. This selectivity, combined with a high capacity over a wide range of operating conditions, gives each zeolite molecular sieve a high level of adsorption efficacy. Why they are used Use of zeolite molecular sieves to dry, purify and separate liquids and gases prevents unwanted side reactions, helps meet product specifications, and avoids costly complications from equipment corrosion and freeze-up. Other beneficial performance characteristics include: Dehydration to water content less than 0.1ppm High capacity for water above 200 F (93 C) Used successfully in hundreds of commercial systems for drying and purifying liquids and gases, zeolite molecular sieves are the most universally applicable adsorbents in the process industries.

3 Purification and dehydration in one operation Dehydration without adsorbing valuable product or altering the composition High product recovery Numerous purification and dehydration cycles are possible due to the reversible adsorption process High cyclic capacity with sufficient thermal or pressure swing purging Specific, uniform pore size is the key to adsorbent efficiency and selectivity Based on size and charge distribution in a molecule, zeolite molecular sieves can adsorb individual molecules readily, slowly or not at all. Table of Contents Page 2 What are zeolite molecular sieves? Page 4 Zeolite research and synthetic production Page 6 Crystal structure and molecular sieve types Page 8 Adsorption based on molecular size, polarity and degree of unsaturation Page 12 Zeolite molecular sieves and adsorption efficiency Page 13 Page 15 Page 17 Page 18 Zeolite molecular sieves and co-adsorption Regeneration cycles Applications Put UOP s experience and technology to work for you TM MOLSIV TM Adsorbents

4 Salt Steam Ion Exchange Tank Sodium Silicate Sodium Aluminate Makeup Tank Crystallization Tank Steam Crystal Slurry Wash Water Filter Zeolite Crystals Ion-Substituted Zeolite Weigh Hopper Clay Binder Naturally occurring crystalline zeolites, a subset of molecular sieves, were first noted two centuries ago. Their ability to release water when heated and readsorb upon cooling was known at that time, but their capacity to selectively adsorb molecules other than water was not recorded until the 1920s. In the early 1930s, X-ray diffraction studies revealed the zeolites as crystalline materials with precisely arrayed cavities and pores within each crystal. Since zeolites found in nature have a high degree of chemical and physical variability, these products were not viable for commercial separation processes. In the early 1950s, a division of Union Carbide Corporation, that is now part of UOP, was searching for an adsorbent to separate atmospheric gases and to be used in other industrial applications. As a result of this research, structures of silicon and aluminum oxides with uniform pore sizes and precise molecular dimensions were synthesized and patented. The synthetic zeolites sieve-like selectivity offered the consistent performance necessary for commercial use. By 1953, more than 30 pure zeolite species had been prepared. Their crystal structures and adsorption properties had been characterized, and researchers had learned how to regenerate them for repeated use in commercial applications. Zeolite research and synthetic production Zeolite research spawns commercial adsorption technology Once engineers recognized the incredible potential zeolite molecular sieves had for commercial use, they began to delve into adsorption technology and design processes that could rely on these new materials. Extensive QC testing insures superior product quality and consistency As a consequence of their research, zeolite molecular sieves were substituted into existing dryer and simple adsorber systems with amazing results. The use of zeolite molecular sieves improved the drying and purifying of various gas and liquid process streams with minimal changes in technology. For more advanced uses, however, additional process engineering knowledge was required. To address this problem, Union 4

5 Wash Water Filter Manufacturing process for the production of activated zeolite molecular sieves. Mixer Particle Forming Dryer Screen Kiln Activated Molecular Sieve Product Carbide formed a large, process engineering group to develop new and comprehensive adsorption technology and design guidelines. Starting with fundamental adsorbent data, the researchers studied adsorption equilibria, adsorption kinetics, deactivation phenomena, cyclic life and scale-up factors. After much research, the group discovered how to economically manufacture the zeolites in commercially useful forms without adversely affecting their adsorption properties. In November of 1954, Union Carbide announced the availability of the first limited commercial quantities. The pure zeolites were then used within the chemical, manufacturing and petroleum refining industries to solve difficult gas purification and dehydration problems. Today, by altering existing crystalline structures for improved functionality, UOP continues to manufacture many types of zeolites for a myriad of industries. How zeolite products are manufactured Sodium silicate, alumina trihydrate and sodium hydroxide are batch-weighed into mix tanks and stirred until homogenous. The mixture forms a gel that is pumped into a crystallization tank where it is monitored under closely controlled conditions. Filter, wash and exchange After crystallization is complete, a rotary filter separates and washes the zeolite crystal slurry. For cationic exchange to take place (calcium, potassium or other cations substituted for sodium in the crystal), the filter cake is transferred to a heated tank where it will be mixed with a solution of the appropriate metal salt. The exchanged forms will then be washed and filtered in the same manner as the original crystal slurry. Forming final product Once separated and washed, the filter cake is conveyed to hoppers. To form commercial 1/16-in and 1/8-in (about 1.6-mm and 3.2-mm) pellets (extrudates) or beads (spheres), crystals from the filter are mixed with specially formulated clay binders. The crystals are then fed through forming equipment to produce pellets or beads. The various product forms are then dried, screened and fired in a rotary kiln to drive out the water and activate the zeolite molecular sieves. The adsorbents are then immediately packaged to prevent any moisture pick up. Many tests are used to determine product quality from crystallization to final firing. Examples include x-ray diffraction, McBain- Bakr adsorption, loss on ignition, crush strength, density and particle size. Quality control techniques including Statistical Process Control and adherence to ISO 9000 standards ensure that crystallization and other manufacturing processes achieve exact specifications. 5

6 Structural model of a zeolite. Crystal structure and molecular sieve types The basic formula for zeolite molecular sieves is M2/nO Al2O3 xsio2 yh2o where M is a cation of n valence. The fundamental building block of the molecular sieve crystal structure is a tetrahedra with four oxygen anions surrounding a smaller silicon or aluminum cation. Sodium ions or other cations make up the positive charge deficit in the alumina tetrahedra, and each of the four oxygen anions is shared, in turn, with another silica or alumina tetrahedron to extend the crystal lattice in three dimensions. In all molecular sieve types, the sodium ion can be exchanged to form other functional products. The crystal structure of zeolite molecular sieves is honeycombed with relatively large cavities. Each cavity is connected through apertures or pores. The water of hydration is contained within these cavities. Before product is used, the water of hydration is removed by heating. Skeletal Tetrahedron Packed Spheres Solid Tetrahedron Illustrations of the rigid, three-dimensional framework of SiO 4 and Al0 4 tetrahedra 1 2 The crystallization of molecular sieve Type A from a hydrous gel as seen through the electron microscope. Photo 1 shows development of crystallization after two hours at 100º C. Photo 2 shows completely crystallized A. 6

7 Commercially useful zeolite species In general, the elasticity and kinetic energy of incoming molecules allows for easy passage of molecules of up to 0.5 angstroms larger than the free diameter of the aperture. In addition, the size and position of the exchangeable cations may affect the free aperture size in any type of molecular sieve. The zeolite molecular sieves that are most commonly used include Types A and X. Unit cell formulas and structural details for each type are outlined below. Type A Na 12 [(AlO 2 ) 12 (SiO 2 ) 12 ] 27H 2 O Note: Na + (sodium) can be replaced by other cations. Type A contains roughly spherical cavities that are approximately 11 angstroms in diameter and about 925 cubic angstroms in volume. They account for nearly half of the total crystalline volume that is available for adsorption. 1 2 The Type A molecular sieve has a framework composed of truncated octahedra joined in a cubic array. The result is a central truncated cube-octahedron with an internal cavity 11 angstroms in diameter (alpha cage). Each central cavity, or alpha cage, is entered through six circular apertures formed by a nearly regular ring of eight oxygen atoms with a free diameter of 4.2 angstroms. The cavities are arranged in a continuous three-dimensional pattern forming a system of unduloid-like channels with a maximum diameter of 11 angstroms and a minimum of 4.2 angstroms. The truncated octahedra enclose a second set of smaller cavities 6.6 angstroms in internal diameter (beta cages). The smaller cavities are connected to the larger cavities via a distorted ring of six oxygen atoms of 2.2 angstroms free diameter. Type 3A Type 3A crystals are produced when some of the sodium ions are replaced by potassium ions. Since potassium ions are larger than sodium ions, the pore size is effectively reduced to about 3.2 angstroms. Type 4A Type 4A sodium-bearing crystals have a free aperture size of 3.5 angstroms in diameter. At typical operating temperatures, molecules with an effective diameter of up to four angstroms may be passed through this aperture. 4.2 A 11.4 A 6.6 A 4 A 2.6 A Above: Two adjacent unit cells of Type 4A light circles represent oxygen ions and dark circles represent sodium cations. (1) Truncated octahedron. (2) Face of cubic array of truncated octahedra. 7

8 Type 5A When some of the sodium ions in Type 4A are replaced with calcium ions,type 5A is produced. It features the largest pore opening of the A types, with a free aperture size of 4.2 angstroms. Type X Na 86 [(AlO 2 ) 86 (SiO 2 ) 106 ] 264H 2 O Note: Na + (sodium) can be replaced by other cations. Although Type X is based on the same building blocks as Type A, the beta cages are linked tetrahedrally instead of in a cubic arrangement. The Type X crystal has a larger, elliptical-shaped internal cavity of 13 angstroms in diameter with a pore diameter of approximately 8 angstroms for the sodium form. High silica molecular sieves Like Types A and X, high silica zeolites selectively adsorb molecules based on their size. However, they differ from Types A and X in that they have a significantly higher proportion of SiO 2 to AlO 2 in their molecular structure. With the reduced amount of AlO 2 and the corresponding reduction in cation density, the high silica zeolites are hydrophobic and organophilic adsorbents. The high silica zeolites are also stable at low ph ranges and high temperatures up to 1,292ºF (700ºC). Adsorption based on molecular size, polarity and degree of unsaturation Numerous zeolite species that differ in chemical composition, crystal structure and adsorption properties are known. By selecting the appropriate adsorbent one that allows entry of those molecules small enough to pass into the pore system and by choosing the proper operating conditions, zeolite molecular sieves can be adapted to suit specific applications. While the external surface area of the molecular sieve crystal is available for adsorption of molecules of all sizes, the internal area is available only to those Zeolite molecular sieve characteristics and applications Type Nominal Pore Common Bulk Density Heat of Adsorption Equilibrium Molecules Diameter Form lb/cu-ft (max) btu/lb H 2 O H 2 O Capacity* Adsorbed** Angstroms (gm/cc) (kcal/kg H 2 O) wt-% 3A 3 Powder 35 (0.56) Molecules with an effective 1/16-inch Pellets 40(0.64) (1,000) 21 diameter <3 angstroms 1/8-inch Pellets 40 (0.64) 21 including H 2 O and NH 3 8 x 12 Beads 44 (0.71) 21 4 x 8 Beads 44 (0.71) 21 4A 4 Powder 32 (0.51) Molecules with an effective 1/16-inch Pellets 44 (0.71) (1,000) 22 diameter <4 angstroms 1/8-inch Pellets 44 (0.71) 22 including ethanol, H 2 S, CO 2, SO 2, 8 x 12 Beads 44 (0.71) 22 C 2 H 4, C 2 H 6 and C 3 H 6 4 x 8 Beads 44 (0.71) x 30 Mesh 44 (0.71) 22 5A 5 Powder 32 (0.51) Molecules with an effective 1/16-inch Pellets 44 (0.71) (1,000) 21.5 diameter <5 angstroms including 1/8-inch Pellets 44 (0.71) 21.5 n-c 4 H 9 OH, n-c 4 H 10, C 3 H 8 to C 22 H 46, R-12 13X 8 Powder 27(0.43) Molecules with an effective 1/16-inch Pellets 40 (0.64) (1,000) 26 diameter <8 angstroms 1/8-inch Pellets 40 (0.64) 26 including C 6 H 6, C 7 H 8 8 x 12 Beads 40 (0.64) 26 4 x 8 Beads 40 (0.64) 26 Chart depicts basic molecular sieve types only. In all applications, these basic forms are customized for specific use. *Lbs H 2 O/100 lbs activated adsorbent at 17.5 torr H 2 O at 25ºC. **Each type adsorbs listed molecules plus those of preceding type.

9 molecules small enough to enter the pores. The external area is about one percent of the total surface area. Materials that are too large to be adsorbed internally will typically be adsorbed externally to the extent of 0.2 to 1 weight percent. Zeolites will preferentially adsorb molecules based on polarity and degree of unsaturation in organic molecules, in addition to selectivity based on size and configuration. In a mixture of molecules small enough to enter the pores, the molecules with lower volatility, increased polarity, and a greater degree of unsaturation will be more tightly held within the crystal. Molecules Excluded Molecules with an effective diameter >3 angstroms (ethane) Molecules with an effective diameter >4 angstroms (propane) Molecules with an effective diameter >5 angstroms (iso compounds and all 4-carbon rings) Molecules with an effective diameter >8 angstroms (C 4 F 9 ) 3 N The role of cations The strong adsorptive forces in zeolite molecular sieves are primarily due to the cations that are exposed within the crystal lattice. They act as sites of strong localized positive charges that electrostatically attract the negative end Exposed cations within the crystal structure act as sites of strong localized positive charge. These sites electrostatically attract the negative end of polar molecules. of polar molecules. The greater the polarity of the molecule, the more strongly it will be attracted and adsorbed. Polar molecules are generally those that are asymmetrical and contain O, Applications Preferred adsorbent for commercial dehydration of unsaturated hydrocarbon streams (cracked gas, propylene, butylene and acetylene) Dries polar liquids such as methanol and ethanol Static desiccant in household refrigeration systems Adsorbent for static dehydration in a closed gas or liquid system Used in the packaging of drugs, electronic components and perishable chemicals Water scavenger in paint and plastic systems Used commercially in drying saturated hydrocarbon streams Separates normal paraffins from branched-chain and cyclic hydrocarbons through a selective adsorption process Pressure swing purification of hydrogen Used commercially for general gas drying, air plant feed purification (simultaneous removal of H 2 O and CO 2 ), and liquid hydrocarbon and natural gas sweetening (H 2 S and mercaptans removal) S, Cl or N atoms. Carbon monoxide, for example, will be adsorbed in preference to argon. In fact, under the influence of localized, strong positive charges on the cations, polarity may be induced in the molecules. The polarized molecules are then adsorbed strongly due to the electrostatic attraction of the cations. In hydrocarbons, the more unsaturated the molecule, the more polarizable it is and the more strongly it is adsorbed. As an example, zeolite molecular sieves will effectively remove acetylene from olefins and ethylene or propylene from saturated hydrocarbons. Adsorption, desorption and hysteresis Since zeolite molecular sieves rely on strong physical forces rather than chemisorption to retain adsorbates, their adsorption is characterized by a Langmuir-type isotherm (the amount of a given compound adsorbed increases rapidly to a saturation value as its pressure or concentration increases in the external bulk phase). Any further increase in pressure at constant temperature causes no further increase in the amount adsorbed. With zeolite molecular sieves, this equilibrium saturation value typically corresponds to a complete filling of the internal void volume with the adsorbate. When adsorbed molecules are desorbed via heat or by displacement with another material, the crystal s chemical state remains unchanged.

10 Capacity wt-% Molecular Sieve Type A Water Vapor Adsorption at 25 C (Equilibrium Data) Silica Gel Activated Alumina Relative Humidity Percent Adsorption on zeolite molecular sieves produces a Langmuir-type isotherm. With zeolite molecular sieve powders, no hysteresis occurs during desorption. Adsorption and desorption are completely reversible with their respective isothermal curves coinciding completely. However, with zeolite molecular sieve pellets or beads, further adsorption may occur at pressures near the saturation vapor pressure. This can occur as a result of condensation in the pellet or bead voids external to the zeolite crystals. In addition, hysteresis may take place during desorption of the adsorbate in the macro-pore region of the binder. A brief review of adsorption principles and systems The rate at which molecules are adsorbed into formed zeolite molecular sieves depends on the following four variables: 1. The rate at which molecules being adsorbed can diffuse to activated crystals within the pellet or bead 2. The relative size of molecules and molecular sieve pores 3. The strength of adsorptive forces between molecular sieves and adsorbate 4. Adsorption temperature Fundamental adsorption systems Depending on the type of operation, zeolite molecular sieves may be used in one of three basic types of adsorption systems: Multiple-bed adsorption Single-bed adsorption Static adsorption Multiple-bed adsorption Multiple bed adsorption is ideal for most commercial, large-scale fluid purification operations. Conventional fixed-bed, heat-regenerated adsorption systems are commonly used. A typical dual-bed installation places one bed on-stream Out Liquid Stream In Adsorption 10 Desorption General flow chart for liquid drying. to purify the fluid while the other bed is being heated, purged and cooled. When the process design requires less than six hours for the adsorption step, additional beds can be added to permit continuous processing of the feed. Single-bed adsorption Single-bed adsorption can be used when interrupted product flow can be tolerated. When the adsorption capacity of the bed is reached, it can be regenerated for further use either in place or at another location. Alternatively, it can be discarded if economically feasible. Static adsorption When manufactured into various physical forms, zeolite molecular sieves can be used as static desiccants in closed gas or liquid systems. In Purge Gas Out Cooler Condenser Heater Out Cooling Gas Multiple bed adsorption for H 2 0 and C0 2 removal from natural gas before methane liquification. In

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12 Zeolite molecular sieves and adsorption efficiency Zeolite molecular sieves are employed in numerous installations and operations due to their exceptional adsorption efficiency. The following details typical conditions where they are effectively used. When very dry streams are required In industry, drying by adsorption is favored due to its ability to produce a much drier liquid or gas than other commercial methods. When extremely dry streams are required, zeolite molecular sieves are selected because they can reduce water concentrations to less than 0.1 ppm. In addition, they are effective over a wide range of operating conditions. When operating at high temperatures Zeolite molecular sieves are also a good choice when drying streams at high temperatures. In fact, they are the only adsorbents that remain effective under very hot conditions. For example, at 200ºF (93ºC) and above, zeolite molecular sieves have more than 13 weight-percent capacity while other adsorbents have none. The isobars plotted below illustrate zeolite molecular sieve performance over a spectrum of operating temperatures. The solid lines assume the use of completely regenerated adsorbents. The capacity is lowered by any residual water left on the adsorbent, a factor of particular importance in high temperature drying operations. As an example, the dotted line isobars show the effect of two percent residual water at the start of adsorption on silica gel, zeolite molecular sieves and activated alumina. In some applications, this residual water can completely consume the adsorption capacity of silica and alumina type adsorbents. For this reason, it is best to use silica and alumina type adsorbents for the bulk separation of water. They are very effective for this purpose and offer the additional benefit of extending the life of zeolite molecular sieves. After bulk separation processes have taken place, zeolite molecular sieves can then be used to achieve very low dew point levels. Water Adsorbed wt-% (-18) 100 (38) Water Vapor Adsorption Isobars at 10mm Hg Partial Pressure (Equilibrium Data) Silica Gel Activated Alumina 200 (93) Zeolite Molecular Sieves 300 (149) Temperature F ( C) 400 (204) 500 (260) Drying power of silica gel, zeolite molecular sieves and activated alumina under various operating temperatures. 12

13 When purifying acidic streams The chemical stability of zeolite molecular sieves allows them to dry, purify and separate numerous types of materials including inorganic gases, hydrocarbons, halogenated Zeolite molecular sieves and co-adsorption hydrocarbons, alcohols, esters, ethers, amines, amides, ketones and others. Zeolite molecular sieves are alkaline in nature with a ph range in water slurry of 9 to 11. Most types are stable in solutions within a ph range of 5 to 12, and a few are stable in solutions having a ph as low as 3. They are stable in most organic streams, however in vapor phase processes, gases that will hydrolize to form strong acids will readily react with the adsorbents. In some drying applications, components other than water may be adsorbed. In many chemical process streams, this altering of stream composition, or co-adsorption, can cause serious problems. When product composition is critical, zeolite molecular sieves can be used to solve these co-adsorption difficulties. Co-adsorption and pore size Co-adsorption can be avoided through proper selection of zeolite molecular sieve type. The zeolite molecular sieve should have a critical pore diameter small enough to prevent all stream components except water from being admitted to the active inner surfaces of the adsorption cavities. In this way, co-adsorption of molecules other than water (including polar and unsaturated components), is eliminated. By eliminating co-adsorptions the molecular sieve will provide maximum capacity for water and reduce outlet water concentrations to less than 10 ppm. Co-adsorption and affinity for water Zeolite molecular sieves feature an extremely high adsorptive attraction for water. This affinity is so strong that water will normally displace any other material that is already adsorbed. To further enhance this selectivity for water, the temperature of the adsorbent bed can be raised. Although the rate of adsorption will be somewhat reduced if the water has to displace another material before it can be adsorbed, zeolite molecular sieves still offer better performance when compared to other adsorbents. Due to the ability of zeolite molecular sieves to produce a drier liquid or gas, industry operations typically favor drying by adsorption over other commercial methods. 13 Zeolite molecular sieves strong attraction for water prevents co-adsorption problems in chemical process streams.

14 Capacity wt-% Carbon Dioxide Pressure, mm Hg Carbon Dioxide Capacity at 25 C Molecular Sieve Type A (Equilibrium Data) Hydrogen Sulfide Capacity at 25 C Molecular Sieve Type A (Equilibrium Data) One-step drying and purifying In addition to water, impurities in a process stream can be removed via proper operating conditions and appropriate zeolite molecular sieve selection. Since zeolite molecular sieves adsorb water more strongly than other material, the adsorbed water concentrates at the inlet end of the bed. Here, it displaces other impurities that have been previously adsorbed. These desorbed impurities are then re-adsorbed farther down the column. The desorbed impurities will begin to appear in the effluent stream as displacement continues. This displacement can be allowed to continue until little adsorbate, other than water, is left on the bed. However, it is possible to design and operate a zeolite molecular sieve adsorption system so that impurities are retained on the adsorbent rather than re-entering the purified stream. To accomplish this, sufficient bed must be provided to contain the impurities in addition to the water. See the figure below for an example of a co-adsorption system. Capacity wt-% Capacity wt-% Hydrogen Sulfide Pressure, mm Hg Ammonia Capacity at 25 C Sulfur Dioxide Capacity at 25 C Molecular Sieve Type A (Equilibrium Data) Sulfur Dioxide Ammonia Pressure, mm Hg Adsorption Sweet LPG Product Adsorption (Desulfurization Step) Adsorption (Desulfurization Step) Cooling Line Sour LPG Feed Regeneration (Heating Step) Regeneration (Heating Step) Pad Gas Regeneration Gas In Heater Cooler Regeneration Fuel Liquids Separator Typical co-adsorption system. Since zeolite molecular sieves have the ability to adsorb hydrogen sulfide, mercaptans and water, the propane feed is simultaneously purified (sweetened) and dried. These three graphs depict the equilibrium capacity of zeolite molecular sieves for various gas impurities. Through co-adsorption, zeolite molecular sieves will remove these materials in addition to water. 14

15 Dew Point, F ( C) Regeneration cycles Cyclic regeneration, or desorption, can be classified into four types. Used separately or in combination, the major adsorptiondesorption cycles are: Thermal swing Pressure swing Purge gas stripping Displacement Thermal swing Thermal swing cycles reactivate the sieve by elevating the temperature. Typically, the operating temperature is increased to ºF ( ºC). The bed is heated either by direct heat transfer via hot fluid in contact with the bed or by use of indirect heat transfer through a 120 (+49) +80 (+27) +40 (+4) 0 (-18) -40 (-40) -80 (-62) -120 (-84) -160 (-107) -200 (-129) 0 (-18) Residual Loading After Regeneration Minimum Obtainable Dew Point 4.0 WT % 3.2 WT % 2.3 WT % 1.7 WT % 1.0 WT % 0 WT % 100 (38) 200 (93) (Dynamic Data) 300 (149) 400 (204) 500 (260) Bed Temperature, F ( C) 600 (316) surface. Once the reactivation temperature is reached, the bed is flushed with a dry purge gas or reduced in pressure. It is then returned to adsorption conditions. As a result, high loadings of water and impurities on the adsorbent can be obtained, following a cooling step. Pressure swing Pressure swing cycles, operating at nearly isothermal conditions, use either a lower pressure or a vacuum to desorb the bed. Advantages of this technique include fast cycling with reduced adsorber dimensions and adsorbent inventory, direct production of a high purity product and the ability to use gas compression as the main source of energy. 700 (371) Purge gas stripping This method uses non-adsorbing purge gas. The purge gas desorbs the bed by reducing the partial pressure of the adsorbed component. The higher the operating temperature and the lower the operating pressure, the more efficient the stripping. The use of a condensable purge gas offers the following advantages: Reduced power requirements gained by using a liquid pump instead of a blower An effluent stream that may be condensed to separate the desorbed material by simple distillation Displacement cycles Displacement cycles use an adsorbable purge to displace the previously adsorbed material. The stronger the adsorption of the purge media, the more completely the bed is desorbed. In this case, lesser amounts of purge can be used, but it is consequently more difficult to remove the adsorbed purge. This graph is used to find the minimum obtainable dew point as a function of residual loading and effluent gas temperature during adsorption. Also shown is residual loading after regeneration as a function of regeneration temperature and purge gas dew point. 15

16 Air dryers with a desiccant-type in-line filtration system supplies clean, dry air to truck air brake systems aiding in the prevention of air line freezeups. Zeolite molecular sieves keep dual pane windows free of moisture and vapors. Zeolite molecular sieves are used to purify industrial gases and for the bulk separation of oxygen from air. 16

17 The chart below provides a brief review of how and where zeolite molecular sieves are used in industry today. Application Air dryers Oxygen concentrators for respiratory patients Air brakes Insulated glass (dual-pane windows) Polymer formulations Radioactive cleanup Refrigeration and air-conditioning (A/C) systems Deodorization Package dehydration Air separation Natural gas Petroleum refining Petrochemicals Volatile organic compound removal Role of zeolite molecular sieves Dehydration of plastic pellets before they are molded Dehydration for instrument air Dehydration of room air with molecular sieve impregnated dessicant wheels Adsorption of nitrogen from compressed air using a pressure or vacuum swing system to obtain oxygen purity up to 95% Dehydration of compressed air on brake systems of heavy- and medium-duty trucks, buses and trains Pressure swing dryers are used to reduce the dew point of air in the brake reservoir below ambient temperature to prevent freeze-up and corrosion Removal of initial trapped moisture inside the dual-pane window and the moisture that will permeate during the life of the unit to prevent fogging Removal of vapors from organic sealing materials, paint and cleaning solvents introduced during window manufacture Dehydration of moisture-sensitive formulations added to poly coatings, epoxies and urethanes to control the curing process and coatings, adhesives, sealants, elastomers, metal-rich paints and vinyl foams to eliminate unwanted water reactions Removal of radioactive nucleotides by ion exchange cesium and strontium are exchanged preferentially into the zeolite molecular sieves to greatly reduce the volume of liquid waste Dehydration of automotive A/C, transport refrigeration, home refrigerators, freezers, residential A/C, heat pumps and commercial refrigerants to prevent freeze-up and corrosion Dehydration to protect system materials from adverse chemical reactions Removal of odor or taste from personal-care products and plastics with high silica (hydrophobic) zeolite molecular sieves. Odors are adsorbed, not masked Dehydration with zeolite molecular sieves when very low humidity conditions are required. Small desiccant packets or tablets protect products such as pharmaceuticals, medical diagnostic reagent kits, vitamins, food, candy, batteries, dry fuel propellants, machine parts, film and instruments Removal of water and carbon dioxide from air before liquefaction and cryogenic separation of nitrogen, oxygen and other atmospheric gases Separation of oxygen and nitrogen with pressure swing or vacuum swing adsorption systems Dehydration before cryogenic recovery of hydrocarbon products and helium Dehydration of high acid gas content (CO 2 and H 2 S) natural gas and natural gas condensate streams Removal of sulfur compounds from ethane, propane and butane Removal of water and CO 2 before methane liquefaction Removal of water and sulfur compounds to protect gas transmission pipelines Dehydration of natural gas liquids Desulfurization of feed streams for ammonia and other chemical plants Removal of mercury, preventing damage to aluminum heat exchangers Dehydration of alkylation feed, refinery gas streams prior to cryogenic separation, naphtha and diesel oil Purification of feedstocks to protect isomerization catalysts Removal of water, HCl and H 2 S from reformer streams Removal of oxygenates from etherification raffinate streams and alkylation feed Removal of nitriles from etherification feed Dehydration of ethanol Dehydration and desulfurization of LPG streams Separation of normal paraffins from branched chain and cyclic compounds Purification by pressure swing adsorption for upgrading hydrocarbon streams Dehydration and purification of NGL/ethane/propane feed Dehydration of cracked gas, C 2 and C 3 splitter feed and hydrogen Dehydration and purification of salt-dome-stored ethylene, propylene and various other feedstocks Removal of water, carbon dioxide, methyl alchohol and other oxygenates, hydrogen sulfide and sulfur compounds, ammonia and mercury from ethylene, propylene, butylenes, amylenes and various solvents and co-monomers Removal of trace volatile organic compounds from air streams Removal of volatile organic compounds from moisture-laden process streams 17

18 Put UOP's experience and technology to work for you UOP's expertise and innovation extends from research and development to manufacturing and from application product selection to technical services. To meet customer needs, UOP offers the broadest portfolio of molecular sieve and activated alumina products in the world. With sales, 18 technical support staff, and manufacturing facilities located around the globe, UOP continues to lead the industry through our commitment to our customers. Whether you are looking to dry, purify or separate, you'll find the adsorbent solution with UOP. urify

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