The absorption efficiencies of CO2 in ceramic hollow-fiber membrane contactors using monoethanolamine (MEA) absorbent under both cocurrent- and countercurrent-flow operations were investigated theoretically and experimentally; various MEA absorbent flow rates, CO2 feed flow rates, and inlet CO2 concentrations were used as parameters. Theoretical predictions of the CO2 absorption flux were analyzed by developing the mathematical formulations based on Happel's free surface model in terms of mass transfer resistances in series. The experiments of the CO2 absorption were conducted by using alumina (Al2O3) hollow-fiber membranes to confirm the accuracy of the theoretical predictions. The simplified expression of the Sherwood number was formulated to calculate the mass transfer coefficient of the CO2 absorption incorporating experimental data. The data were obtained numerically using the fourth-order Runge-Kutta method to predict the concentration distribution and absorption rate enhancement under various fiber packing configurations accomplished by the CO2/N2 stream passing through the fiber cells. The operations of the hollow-fiber membrane contactor encapsulating N = 7 fiber cells and N = 19 fiber cells of different packing densities were fabricated in this work to examine the device performance. The accuracy derivation between experimental results and theoretical predictions for cocurrent- and countercurrent-flow operations were 1.31×10-2≤E≤4.35×10-2 and 3.90×10-3≤E≤2.43×10-2, respectively. A maximum of 965.5% CO2 absorption rate enhancement was found in the module with embedding multiple fiber cells compared with that in the device with inserting single-fiber cell. Implementing more fiber cells offers an inexpensive method of improving the absorption efficiency, and thus the operations of the ceramic hollow-fiber membrane contactor with implementing more fiber cells propose a low-priced design to improve the absorption rate enhancement. The higher overall CO2 absorption rate was achieved in countercurrent-flow operations than that in cocurrent-flow operations.
Keywords: Happel’s free surface model; MEA absorbent; Sherwood number; carbon dioxide absorption; ceramic hollow-fiber membrane contactor.