Recovery of methanol from a catalyst slurry by pervaporation membrane technique.

摘要

This work was undertaken to study and develop a pervaporation membrane separation methodology for the recovery of methanol from a catalyst slurry of a novel process for methanol synthesis from synthetic gas. The methanol concentration in the reaction mixture (the solvent was triglyme, triethylene glycol dimethyl ether) was 2-4 wt%. In a screening study, poly(tetrafluoroethylene-co-perfluoro(alkoxy vinyl ether)) (Nafion) proved to be the best membrane material for this separation. A unique technique was developed to prepare a composite membrane of Nafion on porous PTFE support, with top layer thickness of the order of about 3 {dollar}mu{dollar}m. In the feed concentration range of interest, the Nafion composite provided a separation factor of about 500 and a flux of roughly 1 kg/m{dollar}sp2{dollar}h at low feed flowrates, where concentration polarization is severe for this thin membrane. In the extreme case, i.e., in the absence of concentration polarization, the flux can approach about 4 kg/m{dollar}sp2{dollar}h according to the model. However, the higher the Reynolds number, the higher the pumping energy. Economically, a compromise between the two might be more appropriate. The transport behavior of methanol and triglyme through Nafion is discussed, wherein it was found that both sorption and diffusion favored methanol. A mathematical model of methanol transport through Nafion has been derived for the conditions of this work. The intrinsic diffusivity and the plasticizing constant of methanol in Nafion, as well as the activation energy for methanol, were determined. The effect of concentration polarization on pervaporation has also been studied. The experimental data proved that for the methanol-triglyme-Nafion system, due to its high permeability and selectivity, even at moderate feed concentrations, concentration polarization does occur which becomes especially severe for thin membrane composite. The combination of Grober's equation, which is a valid description of the transport in the liquid boundary layer in the flat channel crossflow pervaporation cell used in this system, and resistance-in-series model was used to study the boundary layer effect. With the determined model parameters, the model yielded a reasonable prediction of methanol flux in the feed concentration range of interest.