CO2 absorption by using PVDF hollow fiber membrane contactors with various membrane structures

Seven kinds of asymmetric poly(vinylidene fluoride) (PVDF) hollow fiber membranes with considerable different structures at the outer surfaces were prepared via thermally induced phase separation (TIPS) method and applied for CO2 absorption as gas–liquid membrane contactors. A commercial microporous poly(tetrafluoroethylene) (PTFE) hollow fiber membrane was also used as a highly hydrophobic membrane. Experiments on the absorption of pure CO2 into monoethanolamine (MEA) solutions were performed and the effects of membrane structure and inner diameter on the membrane performance were investigated. The module efficiencies were compared for the two patterns of operation, i.e. case 1 operation where liquid flows in the tube side while gas in the shell side and case 2 operation where liquid flows in the shell side while gas in the tube side. CO2 absorption fluxes observed in case 1 operation were inversely proportional to the inner diameter of the hollow fiber membrane, and CO2 absorption rate per fiber was almost the same for all membranes. CO2 absorption rates observed in case 2 operation were much smaller than those in case 1 operations. The performances of the PVDF membranes were comparable with that of the commercial PTFE membrane.A mathematical model for pure CO2 absorption in a membrane contactor, which assumes that the membrane resistance is negligibly small and the total membrane area is effective for gas absorption, was proposed to simulate CO2 absorption rates. Experimental results in case 1 operation were satisfactorily simulated by the model for all membranes with different structures. Experimental results obtained in case 2 operation were also simulated by the model except for a membrane with a very low surface porosity on its outer surface. CO2 absorption flux increased with increasing the MEA concentration up to the concentration of 2 mol/dm3, however, the CO2 absorption flux hardly increased with further increase in MEA concentration. This behavior is discussed based on the decrease in the effective gas–liquid contacting area with increasing MEA concentration. A method to estimate CO2 solubility and diffusivity in MEA solutions, which are essential for calculating CO2 absorption flux by the model, is also described.