Thermal management in vacuum pressure swing adsorption processes using phase change materials

Abstract

Vacuum pressure swing adsorption (VPSA) is a mature separation process; however, given the current energy situation, further improvements in terms of process operations are required. To this end, the use of a thermal energy storage system during VPSA cycles appears promising, as the exothermic heat and endothermic heat generated by adsorption and desorption are known to reduce the working capacity. In this study, we investigated thermal management by using a commercial phase change material (PCM) composed of phenolic resin and microencapsulated paraffin wax in the VPSA process to purify an equimolar CH₄/CO₂ gas mixture using zeolite 13X. Here, VPSA processes were simulated using our one-dimensional nonisothermal column model considering the melting and solidification of the PCM. The influence of the PCM mixing ratios was evaluated based on the specifications of CO₂ products and the column length, wherein the VPSA processes were designed, such that the purity and recovery of CH₄ products met the prescribed criteria to achieve a reasonable comparison. The temperature variation (270–351 K) in the cyclic steady state owing to the exothermic and endothermic heat produced during the adsorption and blowdown steps, respectively, was suppressed to 276–344 K by the latent heat of the PCM, resulting in low CH₄ co-adsorption and improved purity of the CO₂ product. Finally, we proposed a method involving the intensive installation of a PCM in the central part of the column and demonstrated that the advantages of improved adsorbent performance realized by mixing the PCM outweighed the disadvantages of increased PCM volume, resulting in a shorter column length than that in the case without the PCM. These results demonstrate the superiority of PCM-installed VPSA systems.