Thermodynamic analyses and optimizations of extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor

Abstract

Extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor is studied in this paper by using finite time thermodynamics or entropy generation minimization theory. Analytical formulae of the extraction rate of CO2 and entropy generation rate (EGR) caused by mass transfer (MT) and the flow of the process are obtained when the acidic seawater (liquid phase) flows in the lumen side and the H2 (gaseous phase) flows in the shell side conversely. Relations between the concentration of CO2 and contactor length in both liquid phase and gaseous phase, between the MT flux and contactor length, between the pressure of the two phases and the contactor length, as well as between the local EGR and contactor length are obtained by numerical examples. The influences of the volume flow rates of the acidic seawater and the H2 at the inlet, the number and the length of hollow fibers on the extraction rate of CO2 and EGR per MT rate are also analyzed. The results show that the extraction rate of CO2 can reach up to more than 98%. The EGRs caused by MT and fluid flow account for 85.99% and 14.01% of the total EGR, respectively. Furthermore, the total EGR caused by MT and fluid flow is minimized by using nonlinear programming method with fixed extraction rate of CO2 when the gas mixture of H2 and CO2 flows in the shell side and the concentration of CO2 in the gaseous phase is completely controllable. The EGR of the minimum EGR MT strategy can be decreased by 36.57% than that of the MT strategy that H2 flows in shell side, in which the EGR caused by MT is decreased by 42.52% and the EGR caused by the flow remain basically unchanged. The EGR caused by MT is minimized by using optimal control theory and the analytical solution of the optimal concentration configuration of CO2 in the gaseous phase is obtained.