Adsorption PSA cycle

ADSORBED PRODUCT PLUS LESS-ADSORBED PRODUCT PURGE GAS FIGURE 12.4-6 Pressure-swing adsorption (PSA) cycle. 

The repressurization step returns the adsorber to feed pressure and completes the steps of a PSA cycle. Pressurization is carried out with product and/or feed. Pressurizing with product is done countercurrent to adsorption so that purging of the product end continues indeed it may be merely a continuation of the purge step but with the bed exit valve closed. Pressurizing with feed cocurrent to adsorption in effect begins adsorption without producing any product. 

Other Cycle Steps A PSA cycle may have several other steps in addition to the basic adsorption, depressurization, and repressuriza-tion. Cocurrent depressurization, purge, and pressure-equalization steps are normally added to increase efficiency of separation and recoveiy of product. At the end of the adsorption step, the more weakly adsorbed species have been recovered as product, but there is still a significant amount held up in the bed in the inter- and intra-... 

Applications PSA cycles are used primarily for purification of wet gases and of hydrogen. One of the earhest applications was the original Skarstrom two-bed cycle (adsorption, countercurrent blowdown, countercurrent purge, and cocurrent repressurization) to diy air stream to less than 1 ppm H9O [Skarstrom, ibid.]. Instrument-air diyers stiU use a PSA cy(de similar to Skarstrom s with activated alumina or sihca gel [Armond, in Townsend, The Propei ties and Applications of Zeolites, The Chemical Society, London, pp. 92-102 (1980)]. 

Although PSA is a batchwise process, by using multiple beds in a sequential manner the overall process is operated in a continuous fashion. Each bed may contain layers of different adsorbent materials selective for specific contaminants in the hydrogen gas stream to be purified. Each bed undergoes a sequence of four basic steps in a PSA cycle adsorption, depressurization, purge at low pressure, and repressurization. This sequence of cyclic operations for each bed is shown schematically for a four-bed PSA process in Figure 8.4 (Yang, 1987 Cassidy, 1980 Miller and Stocker, 1999). 

A pressure-swing adsorption (PSA) process cycle is one in which desorption takes place at a pressure much lower than adsorption. Reduction of pressure is used to shift the adsorption equilibrium and affect regeneration of the adsorbent. Figure 16-39 depicts a simplified pressure-swing cycle. Feed fluid containing an adsorbate at a molar concentration of y i = pi /Pi is passed through an adsorbent at conditions... 

Other Cycle Steps A PSA cycle may have several other steps in addition to the basic adsorption, depressurization, and repressurization. 

Gas bulk-separation processes consist mostly of pressureswing adsorption (PSA) variations and displacement-purge and Inert-purge processes Recently, however, a chromatographic cycle has been commercialized Below, these cycles will be discussed ... 

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 ... 

Although the simple two-bed PSA cycle is widely used in small-scale units, to achieve economic operations on a larger scale it is necessary to improve the energy efficiency of the process. This can be accomplished by using multiple-bed systems in which blowdown and repressurization take place in several stages in such a way that the high-pressure enriched gas at the end of the adsorption step in column 1 is used to pressurize partially column 2 and so on. 

Pressure swing adsorption (PSA) utilizing copper adsorbent is used to separate carbon monoxide from blast furnace and coke oven gases. The copper adsorbent is finely dispersed on a high-surface-area support such as alumina or carbon. Dining the adsorption cycle carbon monoxide is selectively adsorbed... 

Another interesting example of using a chemisorbent (Na20 supported on alumina) in a PSA process is direct removal and recovery of bulk CO2 from a wet hi temperature feed gas without pre-drying or cooling the feed [21]. A gas stream at 200 C containing 10 mole % CO2 and an inert component (dry basis), and which is saturated with water, can be treated to simultaneously produce a CO2 depleted stream (<3% CO2) and a CO2 enriched gas (99+%) without removing the water. The PSA cycle steps consist of (a) adsorption at... 

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