CO₂ Capture In A Polypropylene Hollow Fiber Membrane Contactor

Global warming caused by CO2 from fossil fuel combustion is threatening the ecological environment and human survival conditions. Carbon dioxide capture and storage has been examined as one of technically viable options for stabilizing or mitigating atmospheric CO2 levels, having significance on reducing the risks of climate changes. Membrane gas absorption process is a new hybrid technology that combines the advantages of membrane separation and conventional gas absorption processes. It has been identified as a promising alternative to conventional technologies for CO2 removal. As a new developed technology, there are still some research gaps. In this thesis, the mass transfer process and long-term stability operation of membrane gas absorption system, the feasibility of simultaneous removal of acid gases, the membrane-absorbent interaction mechanism, as well as the improvement of membrane hydrophobicity have been systemically investigated in this contribution. The main contents and conclusions are as follows:The experimental was set up to remove CO2 from simulated flue gas in a polypropylene hollow fiber membrane contactor to evaluate the CO2 absorption performance in the membrane contactor. The dependency of system mass transfer process on various operating parameters was investigated. Long-term stability of membrane contactor was also studied. Experimental results indicated that high CO2 removal efficiency and mass transfer rate could be achieved by membrane gas absorption. But the increase of membrane mass transfer resistance due to membrane wetting had been a big concern and a primary drawback for the long-term industrial application of membrane contactors in CO2 capture. It was found that the membrane performance could be temporarily retrieved by applying slight over-pressure on the gas side.The feasibility of simultaneous removal of CO2 and SO2 and influence of SO2 on CO2 absorption were investigated by MEA solution in a polypropylene hollow fiber membrane contactor. It was found that desulphurization and decarbonization could be achieved effectively and simultaneously by membrane gas absorption process. SO2 competed with CO2 and consumed a part of effective component in the absorbent, resulting in a slight influence on the CO? absorption in the liquid.The intrusion of polypropylene membrane fibers by absorbents was investigated by various membrane characterization methods. The absorption-swelling mechanism was proposed to illustrate the interaction process between the membrane and absorbents. Methods to smooth membrane wetting phenomenon was also discussed. According to the results, membrane swelling resulted from the intrusion of absorbent molecules into the polymer chains leads to membrane wetting. Membrane-absorbent interaction and membrane wetting were two critical factors affecting the mass transfer. It was concluded that developing new absorbents with high surface tension or improving the membrane hydrophobicity may be an alternative to smooth or eliminate the membrane wetting phenomenon.The polypropylene hollow fibers were modified by depositing a rough layer on the membrane surface to improve their hydrophobicity. The influences of non-solvent addition on membrane surface morphologies and properties were investigated. In addition, the effects of modification treatment on the membrane hydrophobicity and contactor performance were studied. Weighing the coating homogeneity, hydrophobicity and modification process efficiency, the mixture of cyclohexanone and methylethyl ketone system was considered as the best non-solvent. The modification treatment effectively enhanced the hydrophobicity and anti-wettability of membrane fibers. It was concluded that the hydrophobicity and prolonged performance stability of membrane contactor could be improved by modifying the porous hollow fibers with a superhydrophobic surface from a long-term view.