Evaluation of binderless LTA and SAPO-34 beads as CO2 adsorbents for biogas upgrading in a vacuum pressure swing adsorption setup

Abstract

Biogas upgrading by selective adsorption of CO2 using vacuum pressure swing adsorption (VPSA) is a technology that can enable the utilization of the isolated biomethane as a direct replacement for natural gas. In this work, we report for the first time the investigation of LTA and SAPO-34 macroscopic beads with hierarchical porosity as CO2 adsorbents in a VPSA setup. While a binder is generally required to shape zeolites and zeotypes into the macroscopic format (e.g. beads, pellets) needed for application in a VPSA column, in this work binderless LTA and SAPO-34 beads were studied and compared with commercial binder-containing zeolite 4A beads. Binary breakthrough experiments were conducted with a gas mixture mimicking biogas (i.e. 40 vol% CO2 and 60 vol% CH4) in a single adsorption column up to 4 bar. The SAPO-34 beads displayed a slightly steeper breakthrough with less significant tailing compared to the LTA beads, which was ascribed to faster intra-crystalline diffusion due to the different framework structure and the lower adsorption strength of CO2 on SAPO-34 compared to LTA. Notably, both the binderless LTA and SAPO-34 beads displayed a slightly sharper breakthrough and less significant tailing compared to commercial 4A beads. This was attributed to the open and accessible hierarchical pore structure of the binderless beads. The CO2 adsorption capacity for the SAPO-34 beads was relatively stable over 5 cycles, while the LTA and commercial 4A beads displayed a significant decrease in adsorption capacity from the first to the second cycle. For the SAPO-34 beads, a cyclic adsorption capacity at breakthrough around 2 mmol g−1 and a CO2 productivity > 3 mol kg−1 h−1 were achieved. These values are significantly higher than those of the LTA and commercial 4A beads, making the SAPO-34 beads a promising candidate for industrial application in VPSA.