Dual Pressure Swing Adsorption Units for Gas Separation and

DOI: 10.1021/ie300341v. Publication Date (Web): June 15, 2012. Copyright © 2012 American Chemical Society. *Tel.: +47 93207532. Fax: +47 22067350...
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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 suggested9 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 mostadsorbed gas (CO2 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 CO2 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 (H2 purification from steam-methane reforming offgas 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 © 2012 American Chemical Society

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: Revised: Accepted: Published: 8695

November 16, 2011 May 28, 2012 June 15, 2012 June 15, 2012 dx.doi.org/10.1021/ie300341v | Ind. Eng. Chem. Res. 2012, 51, 8695−8699

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2. DUAL PSA The Dual PSA Concept. The simple explanation of the dual PSA concept was that it was developed to resemble the design of a distillation tower to separate liquid mixtures. In a distillation column, the feed is inserted in an intermediate position of the column. The most volatile (light) compound vaporizes in the column and is obtained concentrated in gaseous form in the top of the column. The less volatile (heavy) compound is obtained as liquid in the bottom. Within the column there is an ascending movement of vapor and a descending movement of liquid that is controlled by thermodynamic gas−liquid equilibrium at the temperature of the position in the column. Two important parts of the distillation column are the reboiler and the condenser. In the reboiler, a portion of the heavy compound is vaporized and return to the column (boil-up or heavy recycle). In the condenser, the gas exiting the column is condensed to get a liquid and part of it is also recycled (reflux or light recycle).36 A simple sketch of a distillation column is shown in Figure 1a.

ideally, the streams leaving the steps of one subunit can be used either in the same subunit or in the other. A graphical sketch of the dual PSA technology is shown in Figure 1b to enhance its similarities to distillation. Resembling distillation, the PSA subunit where the less-adsorbed (light) product is recovered was termed as rectifying PSA while the subunit where the most-adsorbed compound (heavy) is obtained was termed as stripping PSA. Depending on the operating conditions of the feed and the product specifications, the feed stream can feed the stripping or the rectifying PSA subunits: if the feed is rich in the heavy component, it should be processed in the stripping PSA. In the rectifying PSA, the extent of utilization of the purge (light gas recycle) will then dominate the adsorbent regeneration and will definitively establish the extent of regeneration of the entire subunit. Since this is the “more common” type of PSA units, initially, the proposed cycle is a Skarstrom cycle including one (or more) pressure equalizations (modified Skarstrom). Although more complex cycles can be used, to keep the simplicity only this cycle will be considered in this work. The cycle thus comprises feed, depressurization(s), blowdown, purge, pressure equalization(s), and repressurization. The streams taken in the blowdown and purge streams have concentrated the most-adsorbed compound (relatively to feed amount) and are recycled to the stripping unit. The objective of the stripping PSA is to produce a purified stream of the most-adsorbed component. For this reason, and also resembling distillation, a recycle of the heavy compound (rinse step) will be performed in this unit. Furthermore, and contrary to most PSA units reported in literature, no purge step will be employed in the stripping PSA. The lack of a purge step implies that part of the heavy compound will be lost in the top of the column of the stripping PSA. This stream should be recycled to the rectifying PSA subunit. Note that the two internal streams that are recycled in both subunits resemble the gas going up and liquid moving down in the distillation column. The cycle to be employed in the stripping PSA unit comprises feed, depressurization(s), rinse, blowdown, pressure equalization(s), and repressurization. Again, this cycle will be considered as fixed to keep the design of the PSA unit simple. Note that the dual PSA technology proposed in this work does not intend to be used as the generic approach to design a PSA unit. The main goal is to limit the number of possibilities to design a PSA cycle into a finite (and small) number: two subunits with a relatively fixed operating cycle. Depending on the specific conditions employed in the design of the dual PSA unit, some ancillary equipment may have to be employed in the interface of the two PSA subunits. The cycle may not be entirely fixed because we are not specifying how many pressure equalizations will be employed in each of the units and this should be done case by case. Furthermore, depending on the operating conditions and on the product specifications, the repressurization steps can be either done with product or with feed or with other stream coming from the other subunit. In the next section, the generic explanation of the design of the dual PSA is provided and afterward, the results of two examples are shown. Design of the Dual PSA Unit. Instead of providing a specific example with numerical simulation of the results, the design of the dual PSA unit will be presented as generic as possible. The objective is to separate a binary mixture A−B where A is the less-adsorbed compound (light) and B is the most-adsorbed

Figure 1. Simplified scheme of (a) distillation column and (b) dual PSA concept. The feed location in the dual PSA depends on the amount of most-adsorbed gas.

The section above the feed is called the rectification section while the section below the feed is the stripping section. In a simplified way it can be said that the condenser controls the separation conditions in the rectification zone while the reboiler controls the conditions of the stripping zone. The traditional strategy followed to design a PSA unit to achieve high purity in both streams did not follow the same “spatial distribution” but a different and more complex “time distribution”. In a PSA unit, the purge (light recycle) affects the entire fixed-bed column during a certain period of time. If existing, the rinse (heavy recycle) also affects the entire column performance, but over a different period of time. The resulting effect is loss of light component in the blowdown step decreasing the purity of the heavy component and leaks of heavy compound in the top product if a rinse step is used. Furthermore, as the cycle takes more time until completion, more columns are required to process continuous feed. An alternative way to use PSA technology is employing the same concept of “spatial distribution” of distillation. This is done by using columns that belong to two different subunits of the entire process: the rectifying PSA and the stripping PSA subunits. Each of these two subunits possesses a different PSA cycle, with its own operating conditions and eventually a different adsorbent. The design of the dual PSA technology consider that all the columns belong to the same process since, 8696

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gas (heavy) into two valuable products with specifications in purity for each exiting stream of the PSA. As mentioned before, the PSA subunit that receives the feed stream depends fundamentally on gas composition and operating conditions. Three different cases can be identified and their schemes are presented in Figure 2.

Figure 3. Example of application of the dual PSA concept: (a) cycle of the rectifying PSA subsection; (b) schematic design of the 6-column unit; (c) cycle of the stripping PSA subsection.

to the stripping PSA subunit. From the stripping PSA subunit, a stream having some component B is also recycled to the rectifying PSA subunit. As mentioned before, the cycles are fixed, and for sake of simplicity only one pressure equalization will be used in the initial design. Since no numerical simulation is performed on the process, it is immaterial to define the adsorbent, but it can be mentioned that two different materials can be used. However, note that in the process scheme, there are two compressors marked with dotted lines. Depending on the adsorbent and on product specifications, the two subunits may operate at different pressures and this should be taken into account. Case 3 (Figure 2c) is a new and more advanced utilization of the dual PSA concept where the steps of the two different subcycles have a deeper interconnection. We envisage this as an improved utilization of the dual PSA concept since the recompression energy can be minimized. The position of the feed always depends on the content of component B. An example of a simple modification of the unit shown before is presented in Figure 4. The operating cycle is the same, but the exit of the rinse step is used in the rectifying unit for the pressurization. Examples of Utilization of Dual PSA. As mentioned before, Case 1 and Case 2 are the most simple and direct utilization of the dual PSA concept. They can be seen as two PSA units operating together. However, only in limited cases are the PSA units designed to recover two high-purity products from a binary mixture.39,44,45 The first example is the separation of an equimolar mixture of propane−propylene using zeolite 4A. In a previous work, a PSA cycle was developed using a rinse step where polymer grade propylene (purity >99.5%) was obtained as product.31 However, the propylene recovery of this cycle was ∼86% and losses of propylene were observed in the feed step. Using the dual PSA concept (as shown in case 1), a rectifying section was used to treat the propane-rich feed and recover propylene that

Figure 2. Different configurations of the dual PSA concept: (a) feed rich in component B (case 1); (b) feed rich in component A (case 2); (c) generic case of dual PSA concept with multiple internal recycles (case 3).

In case 1 (Figure 2a), the mixture has a substantial amount of component B and thus is fed to the stripping PSA subunit.37−42,45 From the feed step a stream is sent to the rectifying section where A is obtained as product. From the rectifying section, in the regeneration steps a mixture rich in B is obtained and after compression is mixed with the feed stream and treated again in the stripping PSA subsection. In case 2 (Figure 2b), the mixture is rich in the light gas and thus the feed is processed in the rectifying PSA.43,44 In the rectifying section, A is obtained as top product. From the blowdown and purge steps, a mixture where B is more concentrated than in the feed is obtained and sent to the stripping PSA where B is obtained as product. From the stripping PSA the top product is a mixture that is recycled and mixed with the feed stream. One possible implementation of case 2 is shown in Figure 3. From the blowdown and purge streams, a mixture rich in component B (compared to feed stream) is obtained and sent 8697

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comply with pipeline specifications and CO2 should have purity over 95% to be sequestered.

3. CONCLUSIONS An explanation of the dual pressure swing adsorption (PSA) concept was presented. With this concept it is possible to design PSA units with straightforward cycle scheduling to separate gas mixtures and obtain two products with high purity. In the dual PSA, the unit is composed of two different subsections: stripping and rectifying. In the stripping section, the most-adsorbed gas is produced with a cycle that uses rinse (heavy gas recycle) and no purge with the less-adsorbed gas. In the rectifying section, the less-adsorbed gas is produced as top product using a simple modified Skarstrom cycle (including as many pressure equalizations as required). Stripping and rectifying subsections can run at different operating conditions and use different adsorbents depending on the product specifications. A new and more advanced utilization of the dual PSA concept relies on a deeper interlink of steps of stripping and rectifying sections to reduce compression energy of the recycle streams.

Figure 4. Modification of the dual PSA concept including multiple internal recycles. The cycles of both subsections are the same as the ones shown in Figure 3.

was recycled to the stripping PSA. Using the dual PSA concept, polymer grade propylene was obtained with a total propylene recovery higher than 96%. The flexibility of the concept is demonstrated in this example since in the stripping PSA subsection, the diffusion of gas within the zeolite 4A crystals was controlling the process (kinetic adsorbent) while in the rectifying section, smaller crystals were used to ensure a fast diffusion of gases. The second example corresponds to case 2 where the content of heavy gas in the feed stream is 15%: CO2−N2 separation in the context of postcombustion capture of CO2.44 In this example, the objective is to remove CO2 from the flue gas, but also to obtain high-purity CO2 for sequestration. It was observed also by several researchers that by using a single PSA unit it was not possible to obtain a very high purity of CO2,46,47 and dual bed PSA was already suggested for this application.48 In fact, the purity of CO2 was around 40−70% which is far away from the desired 95% for sequestration. In this case, the raw flue gas was admitted to the rectifying PSA subunit obtaining high-purity N2 on the top and CO2 purity greater than 96% and recovery of ∼91%. In this example, the streams were not recycled but composition of the exit of the rectifying section was very close to the composition of the feed stream. A commercial application which is close to the dual PSA concept is termed as dual-stage PSA and is commercialized by LNI Schmidin SA (Switzerland) for N2 generation from air.49 In this process, N2 is concentrated from air until it reaches a purity of 98%, and then routed to a second PSA where it is concentrated to high-purity N2 (99.999%). The main difference of this commercial application as opposed to the concept presented in this work is that in the already commercial technology the objective is to obtain very high purity of one gas and not the existence of specifications on both streams. The dual-stage PSA results only in a net increase of performance in the purification of N2. As shown, the applicability of the dual PSA concept is generic and may help to reduce the number of possible PSA configurations that should be tested for a specific new application. So far, we are working in a PSA cycle corresponding to case 3, which is more interlinked and potentially more integrated. We envision that this kind of technology is particularly interesting for rapid PSA units where simple and straightforward cycles should be employed. An emergent application of this technology is in the removal of CO2 from methane where high purity methane is required to

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +47 93207532. Fax: +47 22067350. E-mail: carlos. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication is based on the results from the research project “A Green Sea”, performed under the Petromaks program. The author(s) acknowledge the partners: Statoil, Total, Gassco, Petrobras and the Research Council of Norway (200455/S60) for their support.

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REFERENCES

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