Analysis and optimal design of membrane processes for flue gas CO2 capture

Membrane separation technology is a potential low-cost flue gas CO2 capture technology to cope with increasing CO2 content in the atmosphere. This paper analyzes the effects of different driving force generation strategies, membrane separation performance and water vapor on operating energy consumption and CO2 capture cost. Then membrane processes are optimized and designed under a wide range of separation requirements. The energy consumption of feed compression combined with permeate vacuum is the lowest when the stage cut is larger than 33.8%, but from the perspective of CO2 capture cost, the vacuum operation is suitable for membranes with high CO2 permeance and moderate selectivity, such as the CO2 permeance above 4000 GPU and the CO2/N-2 selectivity below 100, to reduce the investment cost of membrane-related equipment. Since only improving the CO2/N-2 selectivity results in an enlarged membrane area and consequently limits the reduction of CO2 capture cost, the development trend of CO2 permeance with increasing CO2/N2 selectivity is proposed to restrain the expansion of membrane area. The water vapor in flue gas can improve the mass transport driving force of CO2 and reduce the membrane area and the capture cost. For water-facilitated membranes, it is recommended to use segmented humidification to replenish the water vapor content of the residue side, especially for the membrane process with a high stage cut, such as the first stage of a two-stage membrane process. Finally, the optimal membrane process and operating pressure under different separation targets, specifically 50-95% dry basis CO2 purity and 50-90% CO2 recovery rate, are obtained by the techno-economic analyses.