53 Molecular Sieve Adsorbent Applications State of the Art R. A. ANDERSON
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Union Carbide Corp., Molecular Sieve Department, Tarrytown Technical Center, Tarrytown, N.Y. 10591
ABSTRACT
The use of molecular sieves as a highly versatile means of performing difficult separations has become firmly established. In addition to a review of both regenerative and non-regenerative applications, it is shown that the invention of new molecular sieves, tailored to provide an almost limitless variety of product performance characteristics, promises the continued discovery and development of new and exciting separation applications. Introduction The purpose of this paper is to review the impact of zeolite molecular sieve adsorption technology upon the commercial world in which we live. T h i s unique family of materials has gained broad acceptance and brought the unit operation of adsorption to maturity. A comprehensive review of the range of adsorptive separations being performed on a commercial s c a l e in the world today will be provided along with some quantitative market information. T o begin, it is usually appropriate to describe the phenomena of adsorption, the unit operation as practiced today, and to describe zeolite molecular s i e v e s . Adsorption is a phenomenon whereby molecules in a fluid phase spontaneously concentrate on a solid surface without any chemical change. Adsorption takes place due to unsatisfied forces in a surface which attract and hold the molecules of the fluid surrounding the surface. The adsorption energy determines the strength with which any given molecule is adsorbed relative to other molecules in the system. The range of separations practiced covers gases from gases, liquids from liquids, and solutes from solutions. Adsorbents have been developed for a wide variety of separations. Commercial materials are usually provided as pellets, granules or beads, although powders are o c c a s i o n a l l y used. The adsorbent may be used once and discarded, or, as is more common, it is employed on a regenerative basis and used for many, many c y c l e s . They are generally used in cylindrical v e s s e l s through which the stream to be treated is p a s s e d . In the regenerative mode, two or more beds are usually employed with suitable valving, e t c . , with at least one bed being in the regeneration mode to allow for continuous processing. Regeneration can be car-
637 In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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ried out by means of a thermal c y c l e , a pressure c y c l e , a displacement purge c y c l e , an Inert purge stripping c y c l e or combinations of these. The selection of the appropriate regeneration c y c l e is an important factor in the design of any system. Adsorbents are used in applications requiring from a few ounces to over a million pounds in one plant. T h e s e applications are all based on the unique adsorptive properties of the crystalline z e o l i t e s . Adsorption is unique in a number of respects. In some c a s e s , the separation performed involves the accomplishment of hundreds of mass transfer units. In others, the properties of the adsorbent allow the selective removal of one com ponent from a mixture, based on molecular s i z e differences, that would be near ly impossible to perform by any other means. In addition, removal of contami nants from fluid streams can be performed which achieves virtually undetectable levels of the impurity in the product. The advent of zeolite molecular s i e v e s has brought adsorption to the forefront as a major tool of the chemical proces sing industry. T h i s unit operation has undergone a major development in the last 15 to 20 years and the future growth of this unique operation is unques tioned. Molecular Sieves are a unique c l a s s of synthetic zeolites which are charac terized by a highly ordered, uniform crystal structure. They are basically hydrated crystal I ine metal alumino-si I icates and are characterized by uniformly smal I sized pores leading from the exterior surface to an internal three-dimensional cagework formed of interconnecting s i l i c a and alumina tetrahedra. E s s e n t i a l l y all of the adsorption takes place internally as opposed to the amorphous sorbents which sorb on their external surface. In some c a s e s , molecules which are small enough to pass through these pores and be adsorbed, can be separated from larger molecules, which are too big to pass t h r o u g h - h e n c e , the name Mo lecular S i e v e s . Molecular Sieves have a strong affinity for polar or polarizable molecules. T h i s property combined with the internal adsorption characteristics allows for purifications and separations to be performed that were not even con sidered 20 years ago. In addition, the synthetic zeolites can be altered still further by ion exchange to provide a nearly limitless variety of products and potential u s e s . The commerical application of Molecular Sieve adsorbents has grown into two major areas: Purification and Bulk Separation. T h i s paper will d i s c u s s the range of Molecular Sieve adsorbent applications along these two major lines. Purification The major gas and liquid purification processes utilizing Molecular Sieves can be c l a s s i f i e d by either the type of stream requiring purification or by the type of impurity removed. Because much of the technology centers upon the im purity to be removed (adsorbate) and because of the large varieties of streams processed in the petroleum, petrochemical and chemical industry, the d i s c u s sion of the applications in terms of the adsorbate is l o g i c a l . A) Water. T h e first industrial gas purification applications for Molecular Sieves were dehydration of natural gas and dehydration of air. Because of their high adsorptive selectivity and high capacity at low water partial pressures, Molecular Sieves were an obvious processing choice for total front-end water removal for cryogenic extraction of helium from natural gas and cryogenic air
In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
53.
ANDERSON
Molecular Sieve Adsorbent Applications
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Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch053
separation of oxygen, nitrogen and the rare atmospheric g a s e s . The process design aspects of dry bed Molecular Sieve dehydration were e a s i l y adapted to existing s i l i c a gel and activated alumina systems. In fact, many systems originally designed around other adsorbents were converted to Molecular Sieves for increased drying efficiencies when Molecular Sieves were first made commercially available in the late 1950*s. Today, all of the major helium recovery plants in the United States use Molecular Sieves to dehydrate a total of over 3.5 billion S C F D of natural g a s . Additional units are under construction e l s e where in the world to increase that figure by 50%. Total dehydration for cryogenic processing continues to be a major market for Molecular Sieves today. Two recent cryogenic applications which represent significant potential in the years ahead are associated with natural gas processing. F i r s t , Molecular Sieve dehydration is now used almost exclusively in the cryogenic production of liquified natural gas ( L N G ) , for both the relatively small peak demand type storage facilities found throughout the United States, and the giant base load f a c i l i t i e s currently under very active development around the world. T h e base load units provide the L N G being shipped in super tankers from the gas producing countries to the major industrial nations. Molecular Sieves dehydrate over six billion cubic feet of natural gas per day in this one area alone. A typical L N G " t r a i n " will employ about 60 tons of Molecular Sieve to dehydrate 300 M M S C F D . Secondly, the trend toward deep ethane recovery from natural gas, utilizing the cryogenic turboexpander process, has necessitated low dew point dehydration of the feed g a s . While several of the earlier plants have used methanol injection, high efficiency glycol systems or dry bed s i l i c a g e l , Molecular Sieves have proven to be the most popular and effective dehydration route. It is expected that the majority of all future cryogenic ethane recovery plants will be designed with front-end Molecular Sieve dehydration. Currently over 7-1/2 b i l lion S C F D of natural gas being fed to cryogenic liquids recovery processes are being dried with Molecular S i e v e s . Another area in which Molecular Sieves have found widespread use in recent years is the dehydration of cracked gases prior to low temperature fractionation in ethylene plants for olefin production. In this application, the small pore Type 3A Molecular Sieve crystal was developed which is selective for water molecules and will not co-adsorb the larger olefin molecules. A s a result, many of the problems associated with the use of non-selective activated aluminas, including hydrocarbon " h o l d - u p " and coking (causing loss of dehydration capacity), have been effectively reduced or eliminated. While activated alumina has been used in this application for a number of years, current industry trends indicate Molecular Sieve as the preferred desiccant. At present, there are over 55 Molecular Sieve dehydration systems operating in olefin serv i c e . Molecular Sieves are a l s o employed for drying finished product ethylene. propylene, and acetylene following salt dome or conventional storage. New grass roots ethylene plants typically employ about 150 tons of Molecular S i e v e . An interesting application which is somewhat s p e c i f i c , but which demonstrates the unique features of Molecular Sieves as dehydration agents, is the removal of water from natural gas streams containing high percentages of A c i d G a s e s , (i.e., hydrogen sulfide and carbon dioxide). While other dry bed adsorbents degrade rapidly in highly a c i d i c environments, special acid-resistant Molecular Sieves have been developed which maintain their dehydration c a -
In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch053
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pacities over long periods of on-stream use. Acid-resistant Molecular Sieves are also used to dehydrate various industrial gas and refinery gas streams which contain corrosive components like chlorine, sulfur dioxide, and hydrogen chloride. A flow sheet of a typical Molecular Sieve dehydrator appears in Figure 1. After physical separation of entrained s o l i d s and liquids, the inlet gas is simply passed through a tower containing the adsorbent. When the Molecular Sieve approaches saturation, the inlet stream is switched to a second tower, while the adsorbent in the first is regenerated by flowing heated, dry gas counterflow to the direction of the stream that was being dried. After leaving the tower, the warm, moist regeneration gas is cooled and much of the water is condensed, separated and removed from the system. The regeneration gas is then either mixed with the wet inlet gas to the adsorbing tower (closed c y c l e operation), or returned to a lower pressure distribution line (open c y c l e opera tion). Once regenerated, the tower must be cooled by a flow of c o o l , dry gas before being placed back in s e r v i c e . Molecular Sieves have a l s o found wide use in dehydration of liquid phase streams. Both batch type and continuous processes have been developed for drying a variety of hydrocarbon and chemical liquids including alkylation feed, isomerization feed, natural gas condensates, absorber o i l , kerosine, solvents, alcohols, aromatics and halogenated hydrocarbons. The processing is e s s e n t i a l ly the same as for gas phase operation except for the necessary draining and filling steps. There are, of course, a number of other less general Molecular Sieve dehy dration applications in industrial use today. The advantages and features of using Molecular Sieve for all low dew point dehydration applications include: low system pressure drops, no liquids carryover or make-up, simple unattended operation, and low operating c o s t s . Non-regenerative drying employing Molecular Sieves is also practiced. In this c a s e , the Molecular Sieve unit is sized to last for the lifetime of the unit. A typical example in this area is in refrigerant drying and purification. The re frigerant system has two problems: water, the first, some of which will be pre sent upon initial filling and, of course, that which will inevitably diffuse in; the second is the decomposition products of the refrigerant. The former can res u l t in system failures due to freeze-ups in the expansion valve (or capillary). The latter will cause corrosion of the hardware. A suitably s i z e d cartridge of the proper Molecular Sieve, installed in the circulating refrigerant stream, will adequately protect the refrigeration system for the life of the unit by adsorbing these impurities. Another non-regenerative drying application for Molecular Sieve is its use as a desiccant and solvent adsorbent for dual-pane insulated g l a s s windows. In this c a s e , the proper Molecular Sieve is loaded into the spacer frame used to separate the panes Once the window has been s e a l e d , the Molecular Sieve will maintain extremely low hydrocarbon and water dew points within the enclosed space for the lifetime of the unit. Consequently, no condensation or fogging will occur within this space which would foul the window. e
The technology has been developed and commercially proven to allow the design of gas and liquid dehydrators employing Molecular Sieves to provide product gas streams of
