Combined mass-transfer chemical engineering processes using pervaporation

Almost all chemical and associated industrial processes have certain implementation limitations, a significant part of which relates to specific properties of the materials being treated. In this context, traditional technologies of various products, approved over many years, can be markedly improved by combining several processes with simultaneous reduction of energy and materials consumption and, in some cases, by simplifying production equipment design and technology. All combined processes are conventionally divided into three groups: stage-by-stage processes, i.e., several stages interlinked essentially by direct flows; conjugate processes, i.e., individual stages interlinked by both direct and reverse flows; integrated processes, i.e., two or more stages executed simultaneously in a common apparatus [1]. Membrane technologies, one version of which is pervaporation, i.e., separation of liquid systems into pure or enriched components by permeation and evaporation through a nonporous (diffusion) membrane, are a fairly promising line of development of chemical, petrochemical, food, and other industries [2, 3]. Pervaporation separation is based on differential diffusional permeation of components of a liquid mixture through a membrane. The essential feature of pervaporation consists in membrane separation of liquid systems that combines dissolution of the volatile components of the system in the surface layer of a selective nonporous membrane, molecular diffusion of these substances through the membrane, and their subsequent evaporation on the opposite (reverse) side of the membrane. The components that passed through the membrane in vaporous form, owing to evacuation on the permeate side (less commonly, with a gas-carrier stream), are led away from the membrane apparatus and condensed in an external condenser. If the vapor condenses on the cooled surface right inside the membrane apparatus, the process is called thermal pervaporation or thermopervaporation. Pervaporation is distinguished from other widely used membrane processes (reverse osmosis, ultrafiltration, electrodialysis, etc.) by low energy consumption and simplicity of apparatus design. In this regard, besides pervaporation as a separate process, some combined mass-transfer processes where pervaporation is involved may also be quite effective. Let us examine some possible combined mass-transfer processes, one stage of which is pervaporation. Stage-by-Stage Distillation–Pervaporation Process. Distillation of liquid systems involves heating of the mixture to be separated to the boiling points and its subsequent partial vaporization and condensation of the vapors formed. This method not only entails substantial energy consumption but also does not ensure separation of azeotropic systems without changing process implementation conditions. These deficiencies can be obviated by providing, for example, conventional Chemical and Petroleum Engineering, Vol. 48, Nos. 1–2, May, 2012 (Russian Original Nos. 1–2, Jan.–Feb., 2012)