Assessing the performance of adsorbents for CO2/CH4 separation in pressure swing adsorption units: A review

This review article assesses the performance of various adsorbents for CO2/CH4 separation, a critical step in the final phase of a biogas upgrading plant. The focus is particularly on the use of pressure swing adsorption (PSA) units for this purpose. Four types of adsorbents, including activated carbons (ACs), carbon molecular sieves, metal organic frameworks, and zeolites, are thus evaluated based on their selectivity in CO2/CH4 separation and PSA working capacity ratio. This selectivity is estimated from pure component isotherms through the Ideal Adsorbed Solution Theory (IAST), and the PSA working capacity ratio is evaluated for a PSA cycle with depressurization at atmospheric and sub-atmospheric pressure. The results show that ACs exhibit the lowest selectivity, while zeolites have the highest selectivity and PSA working capacity ratio. The article also includes a review of simulation studies on CO2/CH4 separation by PSA. In addition, PSA units are compared based on their productivity, energy consumption, purity, and recovery. To calculate energy consumption and productivity, parameters that are often overlooked, a mass balance is applied to the PSA cycle described in each relevant literature study. Our study suggests that the PSA cycle with pressure equalization and sub-atmospheric pressure depressurization is the most optimal, providing valuable insights into the selection and optimization of adsorbents and PSA units for CO2/CH4 separation.