Zeolite ZSM-5 for Humid CO2 Adsorption Processes

Adsorption processes for CO2 separation can be heavily impacted by feed gas humidity, especially when hydrophilic adsorbents such as zeolite 13X are used. In this work, a hydrophobic ZSM-5 zeolite is studied for CO2 separation from postcombustion flue gas (CO2/N2) and off-gas from electrochemical CO2 capture and conversion processes (CO2/O2) without extensive feed gas drying. The binary CO2/H2O equilibrium was probed, demonstrating that more than 90% of the CO2 adsorption capacity of zeolite 13X is lost under humid conditions, whereas ZSM-5 zeolite retains 83% of its CO2 adsorption capacity at a relative humidity up to 62%. The ternary CO2/N2/H2O and CO2/O2/H2O equilibria were modeled and pressure-vacuum swing adsorption (PVSA) processes for CO2 separation from humid flue gases and electrolysis gases were optimized. Purity ≥ 95% and recovery ≥ 90% can be achieved for both feed gases under high humidity conditions using ZSM-5 zeolite. For CO2 separation from electrolysis gases, with a higher CO2 content, purity-recovery requirements can be met in PVSA processes operating at mild vacuum pressure (100 mbar) with an energy consumption down to 0.084 kWh/kg CO2.