Preparation And Modification Of EC-based Gas Composite Membranes

Asymmetric structure is the primary structure for gas separation membrane, because high gas permeation and permselectivity are the necessity for membrane to separate of industrial gases. Composite membrane consists of an asymmetric membrane and a coating layer. Both the micro structure of the asymmetric membrane and the property of coating materials can affect the performance of the composite membrane. Ethyl cellulose (EC) is a good material for separating CO2and non-condensable gases (e. g. N2and CH4), which has been attributed to the strong interaction between the high concentration of polar ether oxygen atoms and CO2, but the permeance of its homogeneous membrane is minimal due to its large thickness. Therefore, it is of great importance to conduct research on the preparation of EC composite membrane for industrial CO2capture, of which the membrane’s coating layer is EC and the structure is asymmetric. In this thesis, EC composite membranes were prepared by two kinds of porous membranes, polymetherimide (PEI) membrane and polypropylene (PP) membrane.Firstly, ethanol was chosen as the optimal solvent for EC coating solution based on the analysis of the solubility parameter and solvents boiling point, and the data of gas permeation experiment also confirmed that. The CO2permeance of EC/PEI composite membrane can be reached at10.75GPU (1GPU=1×10-6cm3cm-2s-1cmHg-1) and the separation factor of CO2/CH4, CO2/N2and O2/N2can be respectively reached to66.4,27.2and5.7with the coating solution concentration ranged from6wt.%to10wt.%and the solvent evaporation temperature ranged from40℃to70℃.In addition, commercialized polypropylene microporous membrane (PP) with a larger pore size and porosity was chosen as the substrate to reduce mass transfer resistance of EC composite membrane. And meanwhile, the lower EC coating solution concentration was used to improve the permeance of composite membrane too. Similarly, the effect of EC coating solution concentration and solvent evaporation temperature on the performance of membrane was studied and heat treatment was used to modify the EC/PP composite membrane. The results indicated that the optimal EC concentration is at a range of2-3wt.%,30℃is a better solvent evaporation temperature and heat treatment is not suitable for the performance improvement of EC/PP composite membrane. Further study on variation of permeance and permselectivity through EC/PP membrane with different operating conditions showed that low-temperature and high-pressure were good for the improvement of separation factor in EC/PP composite membrane, and higher-pressure was also in favor of enhancing the permeance. The CO2permeance of EC/PP composite membrane can be obtained at94.62GPU and the separation factor of CO2/N2, CO2/CH4and O2/N2can respectively reach a level of27.7,12.8and3.8under the technology of2wt.%EC concentration,30℃evaporation temperature and the operating conditions of25℃,0.6MPag feed gas pressure.Meanwhile, an improved Henis model for gas permeation in composite membranes was utilized to analyse these two EC composite membranes. As the substrated membrane, PEI membrane contributed separation performance to EC/PEI composite membrane, while PP scarcely contributed separation performance to EC/PP composite membrane. To EC/PP composite membrane, EC played a leading role in the gas separation.Lastly, two kinds of ionic liquids,1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BmimTf2N), were respectively blended with EC to improve the CO2/light gases separation performance. The permeance of modified membrane decreased with the increased ILs content, whereas the separation factor of CO2/light gases increased with the increased ILs content. The modified EC/PP membrane prepared by2wt.%EC concentration containing75wt.%BmimBF4(BmimBF4:EC mass ratio equal3) showed the separation factor of37.7and16.9for CO2/N2and CO2/CH4, respectively. And the modified2wt.%EC/PP membrane containing75wt.%BmimTf2N showed the separation factor of38.0and19.9for CO2/N2and CO2/CH4, respectively. Meanwhile, the CO2permeance of these two ILs can be correspondingly maintained at a level of55GPU and40GPU. Micro phase separation between EC and ILs may be benefit to the gas permeability of EC-ILs membrane, and the solubility selectivity of ILs is the key factor in determining the permeselectivity of blending membrane.