CO2 Capture by Cold Membrane Operation

Abstract

Air Liquide is developing a cost effective hybrid CO2 capture process based on sub-ambient temperature operation of a hollow fiber membrane in combination with cryogenic separation. These membranes, when operated at temperatures below -20˚C, show two to four times increase in CO2/N2 selectivity with minimal CO2 permeance loss compared to ambient temperature values. Long term (6 month) bench-scale testing with CO2/N2 mixtures at sub-ambient conditions has verified the enhanced separation performance seen at lab scale translated to commercial membrane modules [1,2].A relatively high CO2 capture rate is required to drive down the cost per tonne of captured CO2 as it valorizes the cost of the flue gas pre-treatment and compression prior to the membrane unit. However, as the CO2 recovery increases, the productivity of the membrane module decreases, thereby driving up the membrane system capital cost. The main reason for this is a “pinch effect”: the CO2 driving partial pressure differential across the membrane decreases as CO2 recovery proceeds. Computational fluid dynamics modelling shows that this effect can be partially off-set by a sweep operation where a small fraction (<5%) of the N2- enriched retentate gas is fed into the permeate chamber. Experimental measurements were made with a commercial 12” membrane module, as a function of CO2 recovery, in both sweep and non-sweep (baseline) mode. At the desired 90% CO2 recovery level, sweep operation resulted in 30% higher membrane productivity with negligible effect on permeate purity. This would result in 30% lower membrane system cost with negligible change in specific energy for CO2 capture.Bench-scale process optimization work with synthetic CO2/N2 mixtures is currently being performed within Air Liquide's Delaware Research & Technology Center, USA; this will be followed by field testing at the National Carbon Capture Center (Wilsonville, Alabama, USA) with pre-treated flue gas from air-fired coal combustion.