Pressure-swing adsorption processes

Waldron, W.E. and S. Sircar, Parameteric study of a pressure swing adsorption process, Adsorption, 6,179-188, 2000. 

Xu, J., D.L. Rarig, T.A. Cook, K.K. Hsu, M. Schoonover, and R. Agrawal, Pressure Swing Adsorption Process with Reduced Pressure Equalization Time, U.S. Patent 2003/0015091 Al, 2003. 

Because of the modular nature of fuel cells, they are attractive for use in small portable units, ranging in size from 5 W or smaller to 100 W power levels. Examples of uses include the Ballard fuel cell, demonstrating 20 hour operation of a portable power unit (32), and an IFC military backpack. There has also been technology transfer from fuel cell system components. The best example is a joint IFC and Praxair, Inc., venture to develop a unit that converts natural gas to 99.999% pure hydrogen based on using fuel cell reformer technology and pressure swing adsorption process. 

A cell model is presented for the description of the separation of two-component gas mixtures by pressure swing adsorption processes. Local equilibrium is assumed with linear, independent isotherms. The model is used to determine the light gas enrichment and recovery performance of a single-column recovery process and a two-column recovery and purification process. The results are discussed in general terms and with reference to the separation of helium and methane. 

The equations for a local equilibrium cell model of pressure swing adsorption processes with linear isotherms have been derived. These equations may be used to describe any PSA cycle composed of pressurization and blowdown steps and steps with flow at constant pressure. The use of the equations was illustrated by obtaining solutions for a single-column recovery process and a two-column recovery and purification process. The single-column process was superior in enrichment and recovery of the light component at large product cuts. The two-column process was superior at small cuts ... 

Early work of Barrer (3), McKee (4) and Domine and Hay (5) showed that calcium A, calcium X, mordenite and several types of natural zeolites could be used to enrich air by a selective adsorption of nitrogen. Several pressure-swing-adsorption processes utilizing zeolite adsorbents have been developed which yield a product containing up to 95% oxygen at rates of 20 tons per day (6,7). 

A pressure swing adsorption process (PSA) has been described with high efficiency for separation and capture of C02 in N2 at content from 16 to 25% (22). High purity C02 (> 99%) was recovered with efficiency ranging from 53% to 70% depending on C02 concentration. The selectivity and sorption capacity of zeolite 13X (FAU type) was much better than those of activated carbon. However, the influence of H20 on process efficiency was not reported. It is clear that H20, always present in flue gases from combustion, should first be separated to prevent inhibition of the zeolite. 

Specific properties of molecular sieves are the reason for which these materials found application as adsorbents for separation of gas mixtures, especially air [2]. In fact, one can say that carbon-based molecular sieves played fundamental role in commercialisation of the pressure swing adsorption process (PSA) used for separation of nitrogen from air [3]. 

The larger the selectivity, the easier the separation of component i from component j by adsorption. Zeolites with a selectivity as high as 10 for nitrogen relative to oxygen are used in pressure-swing adsorption processes" to produce oxygen from air. The specific amount of each component adsorbed for an ideal solution is given by... 

Malek, A. and Farooq, S. Study of a six bed pressure swing adsorption process. AIChE Journal, 1997, 43, 2509. 

Zhou, L., Lue, C.Z., Bian, S.J., and Zhou, Y.P. Pure hydrogen from the dry gas of refineries via a novel pressure swing adsorption process. Industrial Engineering Chemistry Research, 2002, 41, 5290. 

Pressure swing adsorption processes are also designed to produce high-purity (99.95+ %) H2 products from refinery-off gases containing H2 (65-90%) and C1-C5 hydrocarbon impurities with high H2 recoveries ( 86+ %). Silica gel and activated carbons are used as adsorbents. 

Ko, D. Siriwardane, R. Biegler, L.T. Optimization of a pressure-swing adsorption process... 

Lacava, A.I. and Lemcoff, N.O. (1996). Single bed pressure swing adsorption process to generate high purity nitrogen. Gas. Sep. Purif, 10(2), 113-15. 

In a pressure swing adsorption process, water vapor is adsorbed by a zeolithic adsorbent and, after saturation of the adsorbent, it is switched over to a second adsorber, HUed with fresh adsorbent. Meanwhile the gas is cleaned in the second adsorber, the pressure inside the first adsorber is reduced, whereby water vapor desorbs from the adsorbent material. 

Sato, M., GOTO, M., Kunishima, N., Kodama, A. and Hirose, T. (1997) Pressure swing adsorption process in supercritical carbon dioxide for the fractionation of citrus oil. Proceedings of the 4th International Symposium on Supercritical Fluids - Vol. B, Tohoku University Press, Sendai, Japan, pp. 629-632. 

R. L. Jones, G. E. Keller and R. C. Wells, Rapid Pressure Swing Adsorption Process with High Enrichment Factor, US Patent No. 4 194 892 (1980). 

Carbon molecular sieves (CMS) have played a critical role in the commercialization of the pressure swing adsorption process for the separation of nitrogen from air. They differ from activated carbon mainly in the pore size distribution and surface area. While activated carbons have a broad range of pores, with a typical average pore diameter of 20 A, carbon molecular sieves have a more narrow pore size distribution, with pore sizes in the range of 3 - 5 A. A molecular probe method is one of the best approaches to determine the effective micropore size distribution of carbon molecular sieves [3,4]. Typical surface areas for a carbon molecular sieve are in the range of 250-400 m /g, while the micropore volume is about 0.15-0.25 cm Vg [2,5]. 

---