Selective separation of CO2 and CH4 for biogas upgrading on zeolite NaKA and SAPO-56

Abstract

Several commercial and potential adsorbents were investigated for the separation of CO2 from CH4, which is relevant for the upgrading of raw biogas. The main focus of the paper was on the working capacities and selectivities of the adsorbents for a generic vacuum swing adsorption (VSA) process. Zeolites 4A and 13X had good estimated CO2-over-CH4 selectivities and reasonably high working capacities for the removal of CO2. A variant of zeolite A – |Na12−xKx|-LTA (with 1.8⩽x⩽3.2), had at least the same working capacity as zeolite 4A but with a significantly improved selectivity. Hence, the environmentally important “CH4 slip” can be minimized with this |Na12−xKx|-LTA sorbent. If a high working capacity for CO2 removal is the most important characteristic for a VSA process, then silicoaluminum phosphate, specifically SAPO-56, appeared to be the best candidate among the studied sorbents. In addition, SAPO-56 had a substantially high estimated CO2-over-CH4 selectivity with a value between ∼20 and 30.