PSA systems

Fig. 15. Four-bed PSA system cycle sequence chart (64). EQ, equalization C D A, cocurrent depressurization C D T, countercurrent depressurization R, repressurization A, cocurrent flow T, countercurrent flow. Courtesy of American Institute of Chemical Engineers.

Because RPSA is appHed to gain maximum product rate from minimum adsorbent, single beds are the norm. In such cycles where the steps take only a few seconds, flows to and from the bed are discontinuous. Therefore, surge vessels are usuaHy used on feed and product streams to provide unintermpted flow. Some RPSA cycles incorporate delay steps unique to these processes. During these steps, the adsorbent bed is completely isolated and any pressure gradient is aHowed to dissipate (68). The UOP Polybed PSA system uses five to ten beds to maximize the recovery of the less selectively adsorbed component and to extend the process to larger capacities (69). 

Pressure sensitive adhesives typically employ a polymer, a tackifier, and an oil or solvent. Environmental concerns are moving the PSA industry toward aqueous systems. Polymers employed in PSA systems are butyl mbber, natural mbber (NR), random styrene—butadiene mbber (SBR), and block copolymers. Terpene and aUphatic resins are widely used in butyl mbber and NR-based systems, whereas PSAs based on SBR may require aromatic or aromatic modified aUphatic resins. 

DWN [Druckwechsel nitrogen] A proprietary PSA system for separating nitrogen from air. Developed and offered by Linde. See also DWO. 

MOLPSA-nitrogen [Molecular sieve pressure swing adsorption] A version of the PSA process for separating nitrogen from air, developed by Kobe Steel. Most PSA processes for nitrogen use molecular sieve carbon as the adsorbent, but this one uses zeolite X. Water and carbon dioxide are first removed in a two-bed PSA system, and then the nitrogen is concentrated and purified in a three-bed system. 

In most hydrogen plants, a pressure-swing adsorption (PSA) system is used for hydrogen purification. In these plants, a major portion of reformer fuel is PSA offgas with a hydrocarbon stream for makeup fuel. 

In the simplest type of PSA system, an adsorber separates the desired product from the feed mixture at feed pressure and ambient temperatures. When the adsorbent bed has been saturated with impurities, it is depressurized to a lower pressure and in a direction countercurrent to the feed flow direction. The depressurization (blowdown) causes desorption and removal of much of the impurities from the adsorbent. The bed is then purged, also countercurrently to the feed flow direction, with product gas to complete the removal of impurities to a low residual level. Alternately, the bed may be evacuated to a sub-atmospheric pressure to remove impurities. The adsorber is then pressurized with product gas and it is put back on line to begin the adsorption step once again. 

Improved cycles were developed to increase the efficiencies of PSA systems. The innovation consisted of withdrawing product-quality gas from an adsorber that has completed its adsorption step by depressurizing the bed cocurrently, in the direction of feed and product flow, to some intermediate pressure. By control of the mass-transfer fronts of the impurities, the purity of this lower-pressure gas is maintained at... 

A cycle sequence chart for a PSA system incorporating these innovations is shown in Figure 1. The gas for pressurization of adsorbers is transferred via equalizations in pressure between the bed supplying the gas and the bed receiving it. The system is arranged so that one adsorber is always "on-line" supplying product gas. 

The advantages of PSA systems over other processing schemes are ... 

A review of conventional hydrogen production via steam reforming is useful to appreciate the advantages of the POLYBED PSA system. The conventional system consists of a feed desulfurizer, reforming furnace, high-temperature and low-temperature shift converters, C02 removal system and a methanator (see Figure 2). 

The POLYBED PSA system replaces several of the units associated with hydrogen purification in a conventional plant — specifically, the low temperature shift converter, the C02 removal system and the methanator (see Figure 2). The tail gas from the PSA system is used as fuel to the reformer and it is supplemented by an external supply. However, the amount of fuel consumed is greatly reduced. Because some hydrogen... 

The POLYBED PSA system is clean and non-polluting. The only materials leaving the unit are product and a tail gas which is used for fuel or subsequent processing. The system requires no water, combustion air, or fuel, and discharges nothing to the atmosphere. Existing commercial units are distinguished by their clean appearance and freedom from odor and corrosion problems. 

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