2009 Annual Meeting
(338s) Process Intensification in CO2 Capture by PSA: Comparison of Variants of Skarstrom Cycle and Duplex PSA Cycles
Author
Carbon dioxide is a major source of global warming. Power plant flue gas typically contains 10-15% carbon dioxide, which amounts to nearly 30% of the total carbon dioxide emitted to the atmosphere every year. CO2 capture and sequestration is then crucial in reducing its level in the atmosphere.
Pressure swing adsorption (PSA) is one of the promising methods for CO2 capture. Several PSA cycles, which are variants of the Skarstrom cycle (Reynolds et.al. 2005, 2006 and 2008), have been proposed involving four basic steps, namely, pressurization, adsorption, blowdown and purge. During the adsorption and purge steps the mass transfer zone formation takes place. The driving force for mass transfer on either side of the zone is negligible. Therefore, the overall driving force is small. This sets constraint on allowable throughput which leads to low productivity and hence bulky PSA units. The volume reduction of PSA units by orders of magnitude, that is, process intensification is desirable as the throughputs for the CO2 capture are very high.Hirose, (1991) and Leavitt, (1992) independently proposed a novel two-bed PSA cycle. The former named it as dual reflux PSA and the latter termed it as duplex PSA. Sivakumar, (2007) showed that the duplex PSA superficially resembles the Skarstrom cycle but differs entirely in the mechanism of separation. In duplex PSA, the mass transfer zone extends over the entire bed and the overall driving force can be set much higher than those feasible with the variants of the Skarstrom cycle, thus leading to process intensification. Various studies on duplex cycle showed that the purity of products are limited to 94 mol% but at low throughputs. Kumar et. al. (2006) proposed a modified duplex PSA which yields products of purity in excess of 99 mol% at high throughputs.
In this work, we propose a variant of Skarstrom cycle in which the zone extends over the entire bed. Simulation studies have been carried out on the possible process intensification using the modified duplex PSA and the proposed cycle. Table 1 presents the comparison of the performance of various cycles. The results suggest that the proposed and modified duplex cycles yields the CO2 purity in excess of 98.0 mol% and productivities of about 20 times that of the variants of Skarstrom cycle reported in literature. Though the product purity in case of the modified duplex PSA is higher than those of proposed cycle, the energy requirement is significantly lower.
Table 1: The optimum performance of different PSA cycles for CO2 capture
References:
Diagne, D., Goto, M. & Hirose, T., ?Parametric Studies on CO2 Separation and Recovery by a Dual Reflux PSA Process Consisting of Both Rectifying and Stripping Section,? Ind.
Hirose, T., ?A Simple Design Method of a New PSA Process Consisting of both Rectifying and Stripping Sections,? Proceedings of the 2nd China-Japan-USA Symposium on Adsorption, pp.123, (1991).
Leavitt, F.W., ?Duplex Adsorption Process,? US Patent 5,085,674, (1992).
Kumar Praveen, Rao, D.P., Sivakumar, S.V., ?New Duplex Adsorption Process for Fraction of Gas Mixture,? Indian Patent number 1567/DEL/2006, (2006).
Reynolds, S.P., Ebner, A.D., Ritter, J.A., ?New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration,? Adsorption, 11, pp.531-536, (2005).
Reynolds, S.P., Ebner, A.D., Ritter, J.A., ?Stripping PSA Cycles for CO2 Recovery from Flue Gas at High Temperature Using a Hydrotalcite Like Adsorbent,? Ind. Eng. Chem. Res., 45,pp. 4278-4294,( 2006a).
Reynolds, S.P., Mehrotra, A., Ebner, A.D., Ritter, J.A., ?Heavy Reflux PSA Cycles for CO2 Recovery from Flue Gas:
Skarstrom,
Sivakumar.S.V., ?Sharp Separation and Process Intensification in Adsorptive Separation Processes,? Ph.D. Thesis, Indian Institute of Technology