Insights into a membrane contactor based demonstration unit for CO2 capture

A continuously operated CO2 capture unit, based on absorption in a membrane contactor and low-temperature vacuum desorption, is demonstrated. The major advantage of membrane contactors is their high specific interfacial area per unit volume. The unit is designed to be modular to allow different absorption membrane modules and stripping units to be tested, with the aim of capturing CO2 from simulated flue gases at concentrations down to the ambient concentration. In addition, desorption can be performed under vacuum to improve the desorption efficiency. The experimental unit incorporates comprehensive measurements and a high level of automation, with heat integration and continuous measurement of electricity consumption providing real-time estimates of the energy consumed in the capture process. In preliminary tests, the results of which are described herein, a 3 M Liqui-Cel (TM) polypropylene hollow-fiber membrane module and a glass vacuum chamber were used for absorption and desorption, respectively, along with a potassium glycinate amino acid salt absorbent solution. This solution has high surface tension and is fully compatible with the polypropylene membrane unit used. In preliminary tests, the highest observed CO2 flux was 0.82 mol m(-2 )h(-1), with a CO2 product purity of above 80%. The calculated overall mass transfer coefficient was comparable to reference systems. The performance of the unit in its current setup was found to be limited by the desorption efficiency. Due to the low desorption rates, the measured specific energy consumption was exceedingly high, at 4.6 MJ/mol CO2 (29.0 MW h/t) and 0.8 MJ/mol CO2 (5.0 MW h/t) of heat and electricity, respectively. Higher desorption temperatures and lower vacuum pressures enhanced the desorption efficiency and reduced the specific energy consumption. The energy efficiency could be improved via several methods in the future, e.g., by applying ultrasound radiation or by replacing the current vacuum chamber stripping unit with a membrane module or some other type of desorption unit.