Ind. Eng. Chem. Res. 2002, 41, 1389-1392
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COMMENTARIES Pressure Swing Adsorption Introduction Pressure swing adsorption (PSA) is a very versatile technology for separation and purification of gas mixtures. Some of the key industrial applications include (a) gas drying, (b) solvent vapor recovery, (c) fractionation of air, (d) production of hydrogen from steammethane reformer (SMR) and petroleum refinery offgases, (e) separation of carbon dioxide and methane from landfill gas, (f) carbon monoxide-hydrogen separation, (g) normal isoparaffin separation, and (h) alcohol dehydration. There are several hundred thousand PSA units operating around the world servicing these and other applications. In fact, PSA has become the stateof-the-art separation technology for application areas a-d listed above, and the sizes of these units range from very small (∼300 SCFD units for the production of 90% O2 from air for personal medical use) to very large (∼100 MMSCFD units for the production of 99.999+% hydrogen from an SMR). Many of these processes are described in the published books and review papers on the subject.1-13 The growth in the research and development of PSA technology has been phenomenal since the first U.S. patent on the subject, authored by C. W. Skarstrom, was granted in 1960.14 A recent survey showed that ∼600 U.S. patents on PSA were issued in the application areas a, c, and d alone during 1980-2000, while the number of published papers with PSA as the keyword exceeded 800 during the period of 1970-2000.15 The concept of PSA for gas separation is relatively simple. Certain components of a gas mixture are selectively adsorbed on a microporous-mesoporous solid adsorbent at a relatively high pressure by contacting the gas with the solid in a packed column of the adsorbent in order to produce a gas stream enriched in the less strongly adsorbed components of the feed gas. The adsorbed components are then desorbed from the solid by lowering their superincumbent gas-phase partial pressures inside the column so that the adsorbent can be reused. The desorbed gases are enriched in the more strongly adsorbed components of the feed gas. No external heat is generally used for desorption. Many different nomenclatures are used to describe these concepts. A PSA process carries out the adsorption step at a superambient pressure, and the desorption is achieved at a near-ambient pressure level. A vacuum swing adsorption (VSA) process undergoes the adsorption step at a near-ambient pressure level, and the desorption is achieved under vacuum. A pressurevacuum swing adsorption (PVSA) process utilizes the benefits of both concepts. Although simple in concept, a practical PSA/VSA process can be fairly complex because it involves a multicolumn design where the adsorbers operate under a cyclic steady state using a series of sequential nonisothermal, nonisobaric, and non-steady-state steps. These include adsorption, de-
sorption, and a multitude of complementary steps which are designed to control the product gas purity and recovery and to optimize the overall separation performance.9 The research trends are to (a) produce purer products at higher recovery, (b) lower adsorbent inventory and energy of separation, and (c) increase the scale of application at a lower overall cost. Unique PSA cycles are also designed to simultaneously produce two pure products from a multicomponent feed gas (e.g., O2 and N2 from air, CO2 and H2 from SMR off-gas, and CO2 and CH4 from landfill gas) as well as to produce a product gas containing a component which is not initially present in the feed gas (ammonia synthesis gas from SMR off-gas). These examples demonstrate the wide flexibility of PSA process designs.10,16,17 The key reasons for such growth in this technology are as follows:18 (a) An extra degree of thermodynamic freedom for describing the adsorption process introduces immense flexibility in PSA process design as compared with other conventional separation tools such as distillation, extraction, or absorption. (b) Numerous microporous-mesoporous families of adsorbents (new or modified) like activated carbons, zeolites, aluminas, silica gels, and polymeric sorbents exhibiting different adsorptive properties for separation of gas mixtures (equilibria, kinetics, and heats) are available. (c) The optimum marriage between a material and a process in designing the PSA separation scheme promotes innovations. (d) Many PSA process paths can be designed for the same separation objectives. A good example of items a-d listed above can be found in the area of air fractionation by adsorption. Approximately 390 U.S. patents were issued on the subject during the last 20 years.15 The processes are designed to (a) separately produce ∼23-95% O2 from air, (b) separately produce 98-99.99+% N2 from air, and (c) simultaneously produce ∼90+% O2 and 99+% N2 from air. Many different zeolites having different thermodynamic selectivities and capacities for adsorption of N2 over O2 are employed, and the processes are tailormade to fit the zeolite properties in order to produce the specific product demands for cases a and c.11 Many different carbon molecular sieves having different kinetic selectivities and capacities for adsorption of O2 over N2 are employed for case b with appropriate process designs for controlling the N2 product purity and recovery.19,20 These processes use many different designs of PSA, VSA, or PVSA cycles and operating conditions in order to achieve the final goals. A key success in this area has been the lowering of the specific power (
