Dual Pressure Swing Adsorption Units for Gas Separation and Purification Carlos A. Grande* and Richard Blom Department of Process Chemistry, SINTEF Materials and Chemistry, P.O. Box 124 Blindern, N0314 Oslo, Norway ABSTRACT: This work presents a new application of pressure swing adsorption (PSA) technology. Using the dual PSA concept it is possible to separate a gaseous stream and obtain two high purity products. The dual PSA concept resembles distillation in its spatial distribution with two well-defined subsections: stripping and rectifying. After the gas mixture passes through the stripping section, the most-adsorbed gas is produced with high purity and the less-adsorbed gas is produced as top product from the rectifying section. Stripping and rectifying sections can operate under different operating conditions, have different adsorbents, and, most importantly, have different cycle scheduling. 1. INTRODUCTION The design of a pressure swing adsorption (PSA) unit is a challenging issue. 1-6 The most important decisions are related to the selection of the adsorbent and the strategy to regenerate it. The different treatments that are done in order to proceed to a fast and efficient regeneration constitute the different steps of the PSA cycle. Owing to the large diversity of possibilities or arranging such steps, developing new applications for PSA technology may be an iterative and tedious process since the best combination of adsorbent-cycle is rarely the first one. Furthermore, the theoretical solutions of a PSA that were developed can only be applied in limited cases (and normally not the ones you should solve). 4,7,8 There is not a generic theoretical framework to design a PSA cycle, so the cycle scheduling can be so diverse that radically different cycles can be realized. 4,9-12 Introducing the feed in an intermediary position was also suggested 9 resembling the PSA process with the Petlyuk concept used in distillation. 13 Some attempts were also developed to give a theoretical framework for a PSA cycle. 14-17 Another approach that has been proposed to schedule PSA cycles is to use a “super-cycle” and then optimize the time of each steps: 18 a series of possible steps is arranged and the result of time = 0 should be obtained if one or more steps are not necessary. Most commercial PSA units are designed for the purification of the less-adsorbed gas from a multicomponent mixture. There are very few industrial applications of purification of the most- adsorbed gas (CO 2 from steel mills) and most of the work is still research based. 4,11,19-31 One problem that was recently placed to PSA developers is the design of PSA units with not only purity specifications in the top product (less-adsorbed product), but also in the bottom product (most-adsorbed gas). The most common example is related to the capture of CO 2 in the production of energy and fuels. The design of the PSA unit is more complex when the feed pressure is high and the proportion of gases is very different (H 2 purification from steam-methane reforming off- gas and natural gas upgrading are good examples). For the sake of simplicity, from this point on, a binary mixture will be used as example. To keep high purity of the top product, the heavy component should be prevented from breaking through the top of the column and similarly, some measures should be taken to ensure that the less-adsorbed gas will not abandon the column with the bottom product. In Skarstrom-type PSA cycles, 32 the light product can be purified to a large extent but some losses of the light gas are observed in the bottom product end from the blowdown and purge steps. Thus, the purity of the heavy component taken from those streams is not very high. The most common way to solve the problem is to introduce some additional steps before the blowdown in order to displace the light gas from the column. Rinsing with a purified stream of the heavy gas was proposed by several authors (heavy gas recycle). 9-11,18,23,24,26-28,31,33,34 When a rinse step is employed, part of the heavy component can be lost in the top product end reducing the purity of the less-adsorbed gas. 24,31 Furthermore, this heavy-gas recycle introduces more heavy gas in the column and the addition of more steps also increases the total cycle time: the result is a direct reduction in the unit productivity. To circumvent the problem of reduction of unit productivity, a lead-trim cycle was recently suggested to increase the concentration of the heavy gas in the column without directly using a rinse step. 35 One solution to obtain two purified products is the utilization of a second PSA unit. This ″dual PSA concept″ was presented in literature some time ago, 37-45 but a simple explanation of its operation principle is missing. This work intends to provide such explanation and furthermore it presents the possibility of deeper integration for improved recycling performance. The idea is to minimize the number of variables to evaluate when designing a PSA process for a new application where eventually new adsorbent materials should be tested. Received: November 16, 2011 Revised: May 28, 2012 Accepted: June 15, 2012 Published: June 15, 2012 Research Note pubs.acs.org/IECR © 2012 American Chemical Society 8695 dx.doi.org/10.1021/ie300341v | Ind. Eng. Chem. Res. 2012, 51, 8695-8699