Temperature swing adsorption processes for gas separation
Pahinkar, Darshan Gopalrao
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Natural gas has become increasingly important as a fuel source with lower environmental impact; therefore, there is a growing need for scalable natural gas purification systems with small footprints. Current industrial purification systems are based on absorption, membrane separation, or adsorption techniques; however, each of these technologies requires large capital costs or suffers from scalability issues. Adsorption-based separation techniques are categorized into pressure-swing adsorption (PSA) and temperature-swing adsorption (TSA). Among adsorption-based gas purification techniques, PSA has typically been preferred over TSA due to the ease of operation and reliability. TSA processes have not commonly been used for industrial gas separation due to low thermal conductivity of the adsorbent bed, which causes difficulty in desorbing impurities and regenerating the adsorbent. However, the high heat and mass transfer coefficients possible with microchannels open the possibility of using the TSA process for gas purification. This work investigated the fluid mechanics and coupled heat and mass transfer processes within a microchannel monolith with a polymer-adsorbent matrix coating the inner walls of the microchannels during TSA-based gas separation. The concept involved separation of carbon dioxide from methane by passing the feed gas through microchannels, followed by sequential flow of desorbing hot liquid, cooling liquid, and purge gas through the same microchannels. It was found that for selected operating conditions and geometries, the process showed merit when compared to current technologies. A combination of spatially- and temporally-resolved analyses was conducted to assess these processes and select optimal configurations and process parameters. Experimental validation followed, wherein the adsorption stage of the separation process in adsorbent-coated microchannels was measured and analyzed using mass spectrometry. The combination of measurements and analyses was used to develop validated models and provide design guidance for TSA processes.