Microwave assisted vacuum regeneration for CO₂ capture from wet flue gas

Small scale processing of flue gas with the goal of enriching the stream in CO₂ for sequestration or use is an interesting application area for adsorption technology. For example, boiler flue gas which may contain up to 10 % (v/v) CO₂ in air can be readily enriched to a stream containing >70 % CO₂ which may be ideal for use within a process such as acidification, precipitation, stripping, etc. The challenge in these applications is producing high purity CO₂ without excessive energy use and handling high concentrations of water vapor without the added complication of a pre-drying stage. In this study we have examined the use of microwave assisted vacuum as a way of rapidly directing thermal energy to the adsorbent surface to liberate water and CO₂. Preliminary “proof-of-concept” pump down experiments were conducted on a small transparent adsorption column of 13X zeolite pre-saturated with a 12 % CO₂ in N₂ gas mixture. Both wet and dry gas tests were conducted. The addition of microwave radiation improved the rapid desorption of CO₂ and water and improved the integrated CO₂ purity in the blowdown stream from 60 to 80 %. In the case of dry CO₂ mixtures, the enhancement is due to microwave heating of the 13X zeolite facilitated by the high cation density in the faujasite structure. In the case of water and CO₂ desorption, the temperature rise of the adsorbent upon microwave heating was much lower than that predicted by simple heating suggesting that the microwave radiation is absorbed primarily by the adsorbed water. A simplified energy analysis suggests that brief exposure of an adsorbent to microwave radiation will raise the required vacuum level for regeneration of high humidity flue gas streams and may lead to an overall lower energy penalty. The selective ability of microwave radiation to target different species provides scope for optimized, compact, flue gas treatment systems.