Abstract
A new design of gas absorption that winds the permeable membrane onto an inner concentric tube to conduct a concentric circular gas–liquid membrane module has been studied theoretically in the fully developed region. An analytical formulation, referred to as conjugated Graetz problems, is developed to predict the concentration distribution and Sherwood numbers for the absorbent fluid flowing in the shell side and CO2/N2 gas mixture flowing in the tube side under various designs and operating parameters. The analytical solutions to the CO2 absorption efficiency were developed by using a two-dimensional mathematical modeling, and the resultant conjugated partial differential equations were solved analytically using the method of separation variables and eigen-function expansion in terms of power series. The predictions of CO2 absorption rate by using Monoethanolamide (MEA) solution in concentric circular membrane contactors under both concurrent- and countercurrent-flow operations are developed theoretically and confirmed with the experimental results. Consistency in both a good qualitative and quantitative sense is achieved between the theoretical predictions and experimental results. The advantage of the present mathematical treatment provides a concise expression for the chemical absorption of CO2 by MEA solution to calculate the absorption rate, absorption efficiency, and average Sherwood number. The concentration profiles with the mass-transfer Graetz number, inlet CO2 concentration, and both gas feed and absorbent flow rates are also emphasized. Both theoretical predictions and experimental results show that the device performance of the countercurrent-flow operation is better than that of the concurrent-flow device operation. The availability of such simplified expressions of the absorption rate and averaged Sherwood as developed directly from the analytical solutions is the value of the present study.
Highlights
Membrane contactor modules were applied to gas/liquid absorption process in aiming to avoid the existence of foaming, unloading and flooding in packed towers, bubble columns, and spray towers, which is performed in conventional gas absorption processes to remove CO2 by absorbing with the gas mixtures dispersed into an aqueous amines solution
The experimental setup of the membrane absorption of CO2 by using MEA absorbent flowing into the concentric circular gas–liquid membrane contactor is illustrated by Figure 2
The eigenvalues in the membrane contactor are solved from Equation (24), the associated eigen-functions obtained from Equations (25) and (26)
Summary
Membrane contactor modules were applied to gas/liquid absorption process in aiming to avoid the existence of foaming, unloading and flooding in packed towers, bubble columns, and spray towers, which is performed in conventional gas absorption processes to remove CO2 by absorbing with the gas mixtures dispersed into an aqueous amines solution. Implementing hydrophobic microporous membranes [2,3] to overcome the disadvantage of non-dispersive contact allows the soluble gas to be absorbed on the membrane surface in the pore mouth adjacent to the solvent phase. The benefits of both membrane reactor and gas/liquid absorption processes were combined together in chemical absorption processes, which are widely utilized due to the high selectivity of amines towards CO2 absorption. Theoretical investigation of two-dimensional concentration distributions in the gas/liquid concentric circular membrane extractor modules is the value of the present study. The theoretical results of absorption efficiency and absorption rate for concurrent- and countercurrent-flow patterns are compared to the experimental data to confirm the two-dimensional theoretical model in practical manners
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