Dehydration of caprolactam–water mixtures through cross-linked PVA composite pervaporation membranes

Abstract

ɛ-Caprolactam (CPL) is the monomer of nylon-6 used extensively in the manufacture of high quality nylon-6 fibers and resins. Since CPL is a very heat-sensitive substance and water is the most important impurity in the final CPL purification, reduced pressure distillation through triple-effect evaporation sets is widely adopted by chemical industry. But high energy consumption, high wastage and condensate pollution have restricted the wide-scale production of CPL and the commercial profit of the manufactures. To improve or develop a new CPL dehydration process, pervaporation separation of CPL–water mixtures was investigated using composite membranes consisting of a selective poly(vinyl alcohol) (PVA) membrane as top layer and a porous polyacrylonitrile (PAN) substrate. The selective layer was formed by PVA matrix followed by the cross-linking reaction of PVA with glutaraldehyde. The membranes were characterized by SEM, FTIR and TGA. Due to the formation of more compact cross-linked structure, cross-linked PVA composite membranes exhibited enhanced thermal stability. Through evaluating the pervaporation performance, we introduced a pervaporation transport equation to normalized permeation fluxes in terms of water permeance, CPL permeance and selectivity. The evaluated results have revealed that the separation performances of PVA composite membranes are strongly related to its hydrophilic/hydrophobic nature as well as the operating parameters, such as feed concentration and operating temperature. Experimental data showed that the PVA composite membranes had superior pervaporation separation performances for dehydration of caprolactam–water mixtures, which provided a new way for CPL dehydration.