Separation and enrichment of CH4 and CO2 from a dry biogas using a thermally regenerative adsorbent-packed heat exchanger

Thermal swing adsorption (TSA) using a heat exchanger packed with an adsorbent material was examined as a means of removing CO2 from a simulated biogas. TSA driven by low-temperature waste heat or solar energy represents an environmentally-friendly gas separation process. In this process, the adsorbent material can be heated indirectly by circulating hot water through a heat exchanger and can operate with a smaller amount of regeneration air than in a conventional TSA system to significantly increase the CO2 concentration at the regeneration outlet. In the present work, carbon molecular sieves (CMS) and a high-silica zeolite (HSZ) were examined with regard to their CO2 adsorption from a simulated biogas containing 60% CH4 and 40% CO2. The effects of the hot water temperature supplied to the adsorber and the regeneration air flow rate on the separation performance were investigated. Increasing the regeneration temperature was found to improve the separation performance, and the HSZ was observed to be more selective for CO2 during the adsorption process. However, the CO2 concentration in the desorption outlet gas was not increased when the regeneration air flow was equal to that of the feed gas. Reducing the regeneration air flow rate to one-tenth the original value significantly increased the CO2 concentration at the desorption outlet while only slightly lowering the CH4 concentration in the product gas. A TSA process incorporating an adsorbent-packed heat exchanger is evidently an effective means of processing biogas, based on poor adsorption of CH4 and strong adsorption of CO2.