Preparation And Optimization Of ILs-PAN Composite Membranes For CO₂Separation

Carbon dioxide separation and capture process has great significance for reducing greenhouse effect and improving utilization rate of resources. Compared to traditional technology, membrane separation technology has such advantages as low energy consumption, small capital cost, simple operation and environmentally-friendly. Room-temperature ionic liquids (RTILs) have such advantages as low vapor pressure, tunable-structure and good thermal stability. Many imidazolium-based ionic liquids show good solution selectivity for CO2. Thus, RTILs are excellent candidates in CO? membrane separation. In recent years, ionic liquids have been frequently used in preparing RTIL-polymer supported liquid membranes (SILMs) due to the volatilization of the liquid. However, in such operation condition as relatively high cross-membrane pressure difference, the separation performance and age of the membrane would decrease. The development of polymerized ionic liquids membrane decreases loss of membrane liquid effectively, but the corresponding membranes have low permeance and few monomers could be used for polymerization. Based on former researches, preparation of ionic liquid-polymer blend membrane could improve the stability of the membrane and decrease the loss in higher operating pressure.In this research, solvent-evaporation method was chosen to prepare composite blend membrane. In early experiments, polypropylene(PP) micropore membranes were chosen as supporting layer, l-butyl-3-methy limidazoliumtetrafluoroborate([bmim][BF4]) and polyacrylonitrile (PAN) were dissolved in N, N-dimethylformamide (DMF) and then the homogeneous solution was casted and dried on the supported layer of polypropylene(PP) membrane. Main factors (content of ionic liquids, concentration of polymer and drying temperature) affect membrane performance and their importance were determined by orthogonal experiments. Range analysis showed the content of ionic liquid contributed the most to permeability and selectivity of the membranes.Afterwards, single factor experiments were performed to optimize the membrane performance. Permeabilities of the composite blend membranes with up to60wt.%[bmim][BF4] were measured under0.4MPa pressure at room temperature. The results showed addition of ionic liquids increased membrane performance dramatically in [bmim][BF4]-PAN (Mw=20,000)/PP composite blend membrane, both permeability and selectivity increased with the increase of ionic liquid content, CO2/N2selectivity increased from7.15to54.32and pressure normailized flux of CO2increased from5.29GPU to13.58GPU. Experiments on different operation conditions revealed that the separation performance of membranes increase as the increase of crossmembrane pressure difference and operation temperature in certain ranges.To improve the performance of composite membrane,[bmim][Tf2N]-PAN (Mw=90,000)/PP membrane were prepared cause the permeability of [bmim][Tf2N] is much higher than other ionic liquids. The permeance results showed [bmim][Tf2N]-PAN/PP membranes exhibit better separation performance with CO2permeance up to34.76GPU and selectivity of CO2/N2and CO2/CH4reach to49.51and24.14, respectively.