Biogas to fuel by vacuum pressure swing adsorption I. Behavior of equilibrium and kinetic-based adsorbents

Abstract

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 CO 2 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. © 2007American Chemical Society.