CO2 absorption at elevated pressures using a hollow fiber membrane contactor

Recently, hollow fiber membrane gas–liquid contactor-based processes have gained an increasing attention. Compared to conventional processes, these processes have numerous advantages. The membrane contactors provide a very high interfacial area per unit volume, independent regulation of gas and liquid flows and are insensitive to module-orientation, which make them very effective in comparison with conventional equipment for offshore application. However, the research done so far is mainly limited to atmospheric pressures applications using aqueous absorption solvents. In this study, the use of hollow fiber membrane contactor for gas absorption at elevated pressure is investigated. CO2 absorption in propylene carbonate was used as a model system in combination with polypropylene (PP) hollow fiber membrane. The absorption experiments were carried out in a semi-batch mode with the solvent continuously flowing through the fiber. The experiments were carried out in a single hollow fiber membrane contactor at elevated pressures up to 20 bar. Higher CO2 pressures result in the increase of the driving force and thus enable higher rates of removal. The study shows that the decrease in the binary gas phase diffusivity and hence the membrane mass transfer coefficient due to increase in the gas pressures does not have a significant effect on the overall mass transfer coefficient. Thus the overall mass transfer coefficient Ko is controlled by the liquid film resistance even at elevated pressures. The overall mass transfer coefficients, at all pressures investigated, can be given by the Graetz–Leveque solution. The long time duration experiments at atmospheric pressure suggest that Accurel PP Q3/2 hollow fiber membrane was subject to morphological changes when used with propylene carbonate as an absorbing solvent. These changes resulted possibly in the wetting of the fiber. It was found that by applying slight over-pressure on the gas-side wetting of the fiber could be avoided and stability of the operation over the long period of application can be obtained.