Reynolds et al. [1] recently studied six different stripping pressure swing adsorption (PSA) cycles over a wide range of process conditions for concentrating CO2 from a stack gas effluent at high temperature using K-promoted HTlc. Using an adsorption process simulator, they showed that the three stripping PSA cycles with a heavy reflux (HR) step outperformed the three stripping PSA cycles without a HR step, with the 5-bed 5-step stripping PSA cycle with light reflux (LR) and HR from LR purge (i.e., a dual reflux, cycle) providing the best process performance. No matter the CO2 recovery or feed throughput, they also showed that it was not possible to obtain a much higher CO2 purity, even with this very effective stripping PSA cycle. Yet, a CO2 purity approaching 100% is desirable for CO2 sequestration or sale.

However, there are different ways to run the heavy reflux step that they did not investigate. In every heavy reflux cycle they examined [1], the light gas effluent produced during the heavy reflux step was taken as part of the light product. Operating the cycle in this manner necessarily limited the recovery of the CO2, as some of it was lost in the light product. If this CO2 could be recycled back into the process, not only might the CO2 recovery increase but also the CO2 purity might increase.

Two ways have been utilized in the literature for recycling the light gas effluent back into the process. The first way was simply to blend this light gas effluent with the feed during the feed step. This method did not add another step to the cycle. The second way was to feed this light gas effluent to the heavy end of the bed either just after the feed step or just after the heavy reflux step depending on its composition. This method did add another step to the cycle, which was referred to as a recovery step. However, a paucity of studies have operated the heavy reflux steps in this way, with very limited results.

This presentation will discuss these relatively new ways to operate the heavy reflux step and provide an overview of the results obtained from them for the concentration of CO2 from a stack gas effluent at high temperature using K-promoted HTlc. It is anticipated that the CO2 purity and CO2 recovery will both increase at the expense of a slight decrease in the feed throughput. It is also anticipated that these novel heavy reflux steps will allow the CO2 purity and the CO2 recovery to both exceed 90%.

[1] S. P. Reynolds, A. D. Ebner, and J. A. Ritter, “Stripping PSA Cycles for CO2 Recovery from Flue Gas at High Temperature Using a Hydrotalcite-Like Adsorbent,” Ind. Eng. Chem. Res., in press on line (2006).