The emission of carbon dioxide from cement industry and power plants that burn fossil fuels is the major cause for the accumulation of CO2 in the atmosphere, which causes long-range environmental problems. One option to mitigate the emission of CO2 is to capture it from the emission sources and store it to the ocean or depleted oil field or use it for enhanced oil recovery. CO2 recovery has been achieved by gas absorption employing solutions of carbonates and alkanolamines. However, this process is energy-intensive for the regeneration of solvent and also faces problems due to corrosion. Recently, the pressure swing adsorption (PSA) process has been considered as an alternative to the absorption process. PSA processes have been widely applied for the removal of CO2 from various feed mixtures, such as CO2 in the steam reformer off gas, landfill gas and natural gas [1]. In all these commercial PSA cycles, the weakly adsorbed component in the mixture is the desired product and enriching the strongly adsorbed CO2 is not a concern. On the other hand, for the capture of CO2 for sequestration, it is necessary to concentrate the CO2 to a high purity to reduce the compression and transportation cost. Thus, it is necessary to develop a PSA cycle by which a high-purity product for the strongly adsorbed component with a high recovery is obtained.

A multitude of PSA cycles and adsorbents have been developed for producing highly pure heavy component (CO2) from feedstock with low CO2 concentration. Kikkinides et al. [2] suggested a 4-bed 4-step process with activated carbon as the sorbent and could recover 68% of CO2 at 99.997% purity. Chue et al. [3] compared activated carbon and zeolite 13X on a 3-bed 7-step process and concluded that the latter is better than the former for CO2 recovery. However, the CO2 recovery was low in their process due to the lack of a countercurrent step in the chosen cycle. Choi et al. [4] reported more than 70% CO2 recovery at more than 90% purity for a modified 3-bed 7-step cycle. However, they solved a very small two variable optimization problem, thus being a specialized case. Zhang et al. [5] have given justifications for using a specific cyclic component step in the adsorption cycle in the context of CO2 capture by using a simplistic mathematical model for the PSA process. Reynolds et al. [6] have suggested a variety of stripping PSA cycles for CO2 recovery at high temperature using a hydrotalcite-like adsorbent.

In this study, a two-bed superstructure of the PSA process has been developed to optimally synthesize an appropriate cycle for CO2 capture. The superstructure considers all the possible operating steps in a PSA cycle with two beds. An optimal control problem with a PDE-based model for PSA system has been formulated in which different steps within a cycle are realized with the help of control variables changing with time. The optimization problem has been solved for three different cases of maximizing CO2 recovery (for a given purity), maximizing feed throughput and minimizing specific power (for a given level of CO2 purity and recovery). Current results indicate the superstructure-based approach as a promising technique for deriving optimal PSA cycles. Different cases with different number of control variables indicate convergence to a particular kind of PSA cycle with over 99% purity and recovery of CO2. The results obtained from optimization problem will also be compared with the optimal PSA cycle simulated more accurately in a dynamic simulation environment.

References

[1] Sircar, S., Separation of methane and carbon dioxide gas mixtures by pressure swing adsorption. Sep. Sci. Technolog., 1988, 23, 519.

[2] Kikkinides, E. S., Yang, R. T., Cho, S. H., Concentration and recovery of CO2 from flue gas by pressure swing adsorption. Ind. Eng. Chem. Res., 1993, 32, 2714.

[3] Chue, K. T., Kim, J. N., Yoo, Y. J., Cho, S. H., Yang, R. T., Comparison of activated carbon and zeolite 13X for CO2 recovery from flue gas by pressure swing adsorption. Ind. Eng. Chem. Res., 1995, 34, 591.

[4] Choi, W. K., Kwon, T. I., Yeo, Y. K., Lee, H., Song, H. K., Na, B. K., Optimal operation of the pressure swing adsorption process for CO2 recovery. Korean J. Chem. Eng., 2003, 20, 617.

[5] Zhang, J., Webley, P. A., Cycle development and design for CO2 capture from flue gas by vacuum swing adsorption. Environ. Sci. Technol., 2008, 42, 563.

[6] 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., 2006, 45, 4278.