| CO2 separation is of great significance on reducing greenhouse effect, improving energy efficiency and achieving carbon resource recycle. Membrane separation technique shows promising potential for CO2 separation due to its advantages such as simple equipment, low investment, low consumption of energy and environment friendly. Poly(ethylene oxide) (PEO) is one of the most potential materials for CO2 separation, because ether groups have an affinity for CO2 owing to the quadrupole-dipole interactions. However, PEO with high molecular weight has a strong tendency to crystallize, which is deleterious for gas permeability. PEO with low molecular weight, also known as poly(ethylene glycol) (PEG), is difficult to form defect-free membrane on its own because of its poor mechanical property. Blending PEG and a suitable polymer can provide a simple and effective way to prepare membranes with high CO2 separation performance.Supporting membrane, polymer material and molecular weight of PEG were determined at first. Different polymers were chosen to blend with PEG and the ternary solution of polymer, solvent and PEG was casted on porous polypropylene (PP) supporting membrane with excellent mechanical property. Polymer-PEG/PP blend composite membrane was prepared by solvent evaporation method. It was found that PVDF-PEG/PP membrane exhibited promising gas separation performance. Moreover, the performance of PVDF-PEG/PP membrane decreased while the molecular weight of PEG increased. Therefore, PEG with average molecular weight of 200 was chosen as the blend material.The effects of PEG content, solvent evaporation temperature and PVDF concentration on the morphology and CO2 separation performance of PVDF-PEG/PP membranes were investigated orderly according to the orthogonal test results, and the membrane stability was further studied. FT-IR spectrum indicated that PVDF and PEG were liable to form hydrogen bond interaction and DSC curves showed that the melting point of PVDF in the PVDF-PEG blend membranes decreased. The membranes prepared at 60℃exhibited reticular structure of spherical particles whose diameter became smaller with increasing PEG content. The CO2 permeation rate and CO2/N2 selectivity first increased and then decreased when the PEG content was simplely increased. Membrane with PEG content of 45% achieved great performance with CO2 permeation rate and CO2/N2 selectivity equal to 3.97 GPU and 36.0, respectively. Solvent evaporation temperature had a remarkable effect on the structure and CO2 permeation rate of PVDF-PEG/PP membranes. The loose structure that accumulated by spherical particles was widely existed in the cross-section of PVDF-PEG45/PP membranes prepared at 60℃and 80℃, but dense structure appeared when the membranes were prepared at 40℃and 100℃. Improvement of solvent evaporation temperature made PVDF change fromβcrystal toαcrystal and CO2 permeation rate decrease while the CO2/N2 selectivity hardly changed. Lower PVDF concentration in the blend membrane led to lower crystallinity and larger spherical particle on the surface. The membrane with PVDF concentration of 14% and solvent evaporation temperature at 40℃exhibited the highest CO2/N2 selectivity of 42.7 with CO2 permeation rate of 6.29GPU. The long-term stability of PVDF-PEG45/PP membrane at room temperature was excellent. After 30 days operation, the CO2 permeation rate kept almost constant without obvious decline of CO2/N2 selectivity.The thermodynamics properties and crystallization kinetics of PVDF-DMAc-PEG system was analysed. The cloud point of this system didn't exactly follow the linearized cloud point (LCP) relation, indicating the coexistence of liquid-liquid demixing and solid-liquid demixing. The structure of blend membranes was significantly affected by kinetics process. When the solvent evaporation temperature was around the maximum crystallization rate temperature of PVDF, the solid-liquid demixing took hold and loose spherical particles structure could be formed. Otherwise, liquid-liquid demixing became dominant and more dense membrane was developed. The results of kinetics analysis were consistent with the SEM and microscope images.
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