Experimental study on the capture of CO2 from flue gas using adsorption combined with membrane separation

The removal of carbon dioxide from flue gas streams can be carried out using standard separation techniques, such as adsorption or membrane separation. However, due to the low CO2 concentrations (below 20 vol.%), the adequate purity and recovery of CO2 in the product can usually be achieved by using two-stage systems, in which the high recovery is commonly obtained by minimizing CO2 content in the gas leaving stage 1 and recycling the CO2 that remains after stage 2 to the inlet of the installation. The hybrid technique is an alternative to the two-stage adsorptive or membrane systems, both simpler and, potentially, more economical. In the hybrid process, proposed in this paper stage one includes a four-column VSA (vacuum swing adsorption) unit, whereas stage two is a membrane module. Extensive experiments were performed in a demonstration hybrid installation processing 5-10 m3 (STP)/h of dry flue gas showing the possibility to increase CO2 content from the initial 12 vol.% to over 95 vol.% with a 100% recovery over a wide range of the operating parameters. The experimental energy consumption (in excess of 4.1 MJ/kg of CO2 removed) can be reduced, for higher capacities, to a value as low as 1.7 MJ/kg of CO2, and thus, lower than that for a number of one- and two-stage adsorption processes. The detailed investigation of the hybrid technique, so far inadequately explored in the context of CO2 capture from flue gas, has provided a wealth of data that shows the principal directions in which further studies should proceed.