(75b) Modified ‘Molecular-Gate' PSA for CO2 Capture From Flue Gas

Keller II and Kuo in 1982 proposed a dual-piston Pressure Swing Adsorption (PSA) for separation of mixtures and termed it as molecular gate PSA. It consisted of a single adsorbent bed connected with two piston-cylinder assemblies at the ends of the bed to impose cyclic flow of gas with feed entering at an intermediate location along the bed. High and low pressures in the bed are varied by manipulating the displacement volume of pistons. They claim that high purities of both products in the range 99 mol% and high productivity can be achieved at moderate vacuum. However, its application is limited by the size of piston-cylinder assembly that could be used in practice, for example, for CO₂ capture from flue gas. To overcome this limitation, we propose a modified molecular gate PSA replacing the piston-cylinders with two surge tanks, a compressor, a vacuum pump and mass flow controllers. The cycle essentially has only two steps, pressurization and depressurization in which cyclic flow of gas is maintained through the bed. The lead/lag functionality of pistons in molecular gate PSA has been realized using the mass flow controllers. An additional modification of molecular gate has been proposed in timing products withdrawal by passing tanks during the steps to enhance purity and productivity. We have carried out simulation studies on separation of synthetic flue gas containing 20 mol% CO₂ and 80 mol% N₂, using Zeolite 13x. The results indicate that it is possible to get CO₂ purity and recovery in excess of 99% with high pressure of 1 bar and low pressure 0.08 bar with productivity of 43.9 LSTP/h/kg a performance superior to the performance of conventional PSA. The productivity can be further enhanced by 3.5 times upon relaxing the purity specification from 99 mol % to 95 mol%.

Keywords: modified molecular gate, CO₂ capture, flue gas

Reference:

Keller II, G.E. and Kuo, C.H.A.: Enhanced Gas Separation by Selective Adsorption, US Patent 4,354,859 (1982).