Plasma Grafted Composite Membranes For The Pervaporation Separation Of Aromatic/Aliphatic Compounds

The separation of aromatic/aliphatic hydrocarbons is of great importance in petrochemical industry. The "pore filling" composite membranes have a very thin dense selective layer with smaller mass transfer resistance and higher flux, in addition, the effective grafted layer is covalently anchored to the inert porous substrate membrane, which restrains functional filling polymer from swelling, and hence ensures the integrity and mechanical strength of the composite membranes. Therefore, the "pore filling" composite membrane is one of the most promising ones for the pervaporation separation aromatic/aliphatic hydrocarbons. The atmospheric pressure dielectric barrier discharge (DBD) plasma was used to fabricate the "pore filling" composite membrane herein. The effects of preparation parameters and operation conditions on pervaporation performances of composite membranes were discussed systematically. The mass transfer mechanism was preliminarily explored by molecular dynamics simulation and adsorption experiments.The double-plasma grafting strategy was performed to prepare the pervaporation composite membrane with an aromatic selective permeability. The macromolecules poly(ethylene glycol) methacrylate was plasma graft-filling polymerized onto the porous polyacrylonitrile ultrafiltration membrane. Attenuated total reflection fourier transform infrared spectroscopy, field emission scanning electron microscopy and surface contact angle were used to characterize the composite membrane. The "pore filling" structure of the composite membrane was identified. Using toluene/n-heptane (1:4 in mass) mixture as feed, the effects of preparation parameters on the separation performances were investigated. The results showed that the optimal dip coating temperature was 45℃. The addition of initiator K2S2O8 into the monomer solution could improve the pervaopration performance of the composite membranes. The composite membrane made from hydroxyl-terminated monomer (PEO500OHMA) possessed better pervaporation performance. Using argon gas as discharge gas, PEO500OHMA (0.45 mol/L) as graft monomers and methyl acrylate (0.1 mol/L) as graft comonomers, the composite membranes was successfully prepared, which had the separation factor and permeate flux were 7.87 and 0.49 kg/(m2·h), respectively.The impacts of the operation conditions on the pervaporation performance were studied. The results showed that both the permeate flux and separation factor increased with the increasing operating temperature and the dowmstream pressure. With the increasing of toluene concentration in feed, the separation factor decreased and permeate flux increased.The diffusion coefficient of toluene and n-heptane in polymers were calculated by using the molecular dynamics simulation method. The results exhibited that hydroxyl-terminated polymers has no diffusion selectivity for toluene and n-heptane mixtures, and the ether-terminated polymers has a diffusion selectivity for n-heptane at 353 K. Therefore, for the composite membranes investigate herein, it was dissolution (adsorption) selectivity that largely determined the separation performance for aromatic/aliphatic pervaporation. Furthermore, the adsorption isothermal experiment showed that the hydroxyl did facilitate the adsorption of toluene in polymer, which confirmed the results of the molecular simulation.