| Membrane contactor is an effective separation device that achieves gas/liquid or liquid/liquid mass transfer on interface which is provided by non-selective micoporous membrane. Membrane contactor could perform with very high separation efficiency and better operation flexibility as a consequence of the inherent huge area per volume and the independent fluids. In distillation, physical and chemical characteres of membrane play a critical role and the ideal membrane structured packing should be hydrophobic and with good chemical and thermal stability. For preparing composite membrane packing with high performance, PDMS coating method was used firstly. The key parameters including PDMS, crosslinker (TEOS), catalyst (dibutyltin dilaurate) and coating time were explored. Three types of composite membrane packings (PAN, PS and PVDF) were prepared with the optimum coating compostion and then were constructed to membrane modules for the distillation of alcohols/water solution. Affections of PDMS coating layer on membrane material in distillation of different mixtures were analysed. Finally, a membrane packing with good chemical and thermal stability was prepared by thermal treatment and the affections of temperature and duration were tested. The tests on material stability and distillation separation of IPA/water were also studied.For ensuring the optimum composition of PDMS coating, N2 permeance and contact angle were tested. The N2 permeance increased with PDMS. crosslinker and catalyst content firstly then decreased with these three parameters. For example, N2 permeance increased from 0.00034 to 0.00093 cm3(STP)cmHg-1cm-2s-1 when the PDMS content in the range of 0.5-2 wt%. However, it would decreased to 0.0007 cm3(STP)cmHg-1cm-2s-1 when the PDMS content increased from 2 wt% to 5 wt%. The affectoion of TEOS and dibutyltin dilaurate on N2 permeance presented similar tendency. The N2 permeance reached maximum when the constitutent of TEOS and dibutyltin dilaurate at 1 wt% and 0.5 wt% respectively. N2 permeance decreased with soaking time while the hydrophobility of membrane increased with it. In summary, the optimum parameters could be ensured as follows:PDMS (2 wt%), TEOS (1 wt%), dibutyltin dilaurate (0.5 wt%) and soaking time (8 min). The permeance of composite membrane packing made with above mentioned optium parameter is about 10-3 cm3(STP)cmHg-1cm-2s-1 which is 2-40 times higher than the permeance of PDMS composite membrane in PV process of ethanol/water. Generally, the contact angle of PDMS composite membrane is about 90°-105°. After PDMS coating, contact angle of our membrane packing could be improved from 75.6° to 99.1°.The PDMS/PAN composite membrane had smoother surface and thickness of PDMS layer is about 1-2μm which was confirmed by SEM. Signal intensity at 1100 cm-1 represents the formation of Si-O-Si bond in PDMS crosslinking reaction.PAN, PS and PVDF hollow fiber packing were coated with above mentioned optimum PDMS solution for obtaining better membrane packing with good gas permeance and hydrophobility. In distillation of alcohols/water, all membrane modules could operate normally at vapor velocity which above several times flooding velocity. The affection of PDMS coating layer on membrane packings are different. Separation performance of membrane packings, PAN and PVDF, was elevated significantly after PDMS coating while PS performed versely. In all membrane packings, PDMS/PAN showed best performance and the highest distillate concentrion could reach about 92%、 72% and 65% respectively with only 20 cm module length. The HETP value was kept stablely in the range of 0.07-0.12m when F-factor during 0.2-1.4 m/s(kg/m3)0.5.That is a very high performance which could be even better than Mellapak 752Y structured packing. The contribution of individual mass transfer was discussed for clarifying the role of dry membrane, wet membrane and composite membrane. An obvious decrease (>50%) of km was found even when membrane was only wetted lightly(<10%). The rate of km/km’is versely proportional to rm/rm,wet. For preparing a good composite membrane packing, the thichness should be confined to the "proper thickness’. It is not suitable for PDMS coating on a membrane with thickness less than 30 μm or a hydrophobic dense membrane. Based on model of the contribution of individual mass transfer, the extra mass transfer resistance which is derived from PDMS layer is only the 1/4-1/3 of rmwet. For explaining the difference among alcohols/water distillation with different packings, theoretical model was simplied to the discussion on the resistance of wetting membrane, PDMS coating and liquid boundary layer. Considering the difference of Hansen parameters between solvents and polymers, parameter (?) which is proportional to the experimetail mass transfer coefficent was proposed and the experimental phenomenon could be explained reasonably. After correction of fluids in tube and shell side with Gaussian distribution and Happel free surface model, deviation between the theoretical KG and experimental KG was narrowed from 200-300% to less thanl00%. However, obvious gaps still exited. That mighe be induced by the underestimation of Leveque solution on tube side mass transfer as the existence of local turbulence and transflow.During the research on preparing a novel membrane packing with surperhydrophobility and robust stability, PAN performed specially as its porours structure could be sustained at 200-300℃ while other membrane, such as PES, PS and PVDF, melted and porous structure dispeared. This is due to the generation of ladder structure which could prevent the melt of polymer chain effectively. It has shown that both the membrane dimensions and weight decreased with increasing heat-treatment time (0.5h-12h) and temperature (200℃-350℃). The tensile stress at break and strain at break decreased from 5.5 Mpa and 13.9% to 2.6MPa and 3.5% respectively. Meanwhile, the porosity decreased from 73% to 50%. The FTIR was used to explore the change of chemical bonds and found that the dehydrogenation. cyclization and crosslinking reactions occurred in thermal treatment. Compared with original PAN membrane, the hydrophobicity of heat-treated membranes was improved obviously. The heat-treated membrane S2 (250℃,6h) was selected and immersed in various boiling solvents for 24h to test material stability. Excellent thermal and chemical stability was proved with membrane packing S2 in various solvents, such as acetone (57℃), toluene (111℃) hexamethylene (81℃) and N.N-dimethylacetamide (166.1℃). Finally, membrane packings PAN and S2 were chosen for packing membrane contactor modules which were used for the distillation of isopropanol-water solution. During 10 days’operation, although module PAN performed better than module S2, module S2 still showed a comparative high separation efficiency with the height of mass transfer unit (HTU) and larger overall mass transfer coefficients in the range of 0.1-0.18m and 2.5-3.2cm/s respectively. That might be benefited from the smaller hollow fiber dimensions and improvement of hydrophobility. Based on the theoretical estimation, the contribution of mass transfer resistance of wetted membrane to overall resistance will increase from 18.8-30.2% to 69.6-81% and that will lead to an increase of overall resistance with 167%-268%. |
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