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Purification of biogas by vacuum pressure swing adsorption was evaluated to produce fuel grade methane (FGM). Two different adsorbents were employed to evaluate the process performance with equilibrium- and kinetic-based adsorbents. Carbon molecular sieve 3K was employed as the example of a kinetic adsorbent because methane diffusion is extremely small. A five-step cycle configuration (feed, intermediate depressurization, blowdown, purge, and pressurization) was employed to compare with results obtained with a four-step cycle without intermediate depressurization. The effect of different operating variables like step times and pressure conditions of the cycle was analyzed. We have employed zeolite 13X as the equilibrium-based adsorbent (much higher capacity for CO2 with fast diffusion of both gases). Different operating conditions as well as a new cycle configuration were evaluated. Methane with purity over 98% was obtained with both adsorbents. The CMS 3K adsorbent showed much higher methane recovery (close to 80%) compared to those obtained with zeolite 13X (close to 60%) because of low adsorption in the pressurization step and because the stream exiting the intermediate depressurization step can be considered as being a product.
Grande et al. (Wed,) studied this question.