Separation of a Propanol–Water Mixture by Membrane Pervaporation

Abstract

Methods for the separation of liquid mixtures and the production of pure substances occupy an elevated position in the chemical, foodstuff, and pharmaceutical branches of industry. In addition to such traditional methods as rectification, distillation, sorption, and others, methods for the separation of liquid mixtures using semi-permeable membranes have come into widespread use in the past 50 years: reverse osmosis, and ultra- and microfiltration. Over the last 20 years, scientists have, together with baromembrane processes, expressed interest in processes that are not classed among the former. Diffusion evaporation through a membrane ‐ pervaporation, which is still not well understood [1, 2], occupies a special position along with membrane distillation, dialysis, and electrodialysis. Depletion of natural resources and degradation of the environment due to human activity have supplied the impetus for a search for methods of more effective utilization of available energy, and for development of new means of protecting the environment. Interest in the pervaporation process has been continuously on the increase, therefore, since the start of the 1980s. Pervaporation is a membrane separation process for liquid mixtures in which the initial solution comes in contact with the internal surface of a membrane module, whereas permeate in the form of vapors with a low partial pressure is removed from its outer surface (the substances passed and retained by the membrane are called the permeate and retant, and the extracted component the penetrant).The selective layer of the membrane may alter the equilibrium of the vapor‐liquid system [2]. Pervaporation is frequently referred to, therefore, as an extractive distillation process in which the membrane plays the role of the third component. Separation of liquid mixtures in the pervaporation process, however, is not based on equilibrium of the vapor‐liquidsystem, as in distillation, but on the differences in the solubility coefficient and diffusivity of the components of the mixture. Here, the equilibrium of the vapor‐liquidsystem exerts a direct influence on the motive force of the process, and, consequently, on the characteristics of the separation [1]. In general form, the mass-transfer equation can be written in the following manner: J = kΔ, where J is the specific output of the membrane, k is the mass-transfer coefficient, and Δ is the motive force of the process. The mass-transfer coefficient can be written as where β x and β y are mass-output coefficients; δ m and δ back are the thicknesses of the selective layer of the membrane and backing; and L m and L back are the coefficients of mass conductivity in the selective layer of the membrane and backing. The motive force of mass transfer through the membrane during pervaporation is the gradient of the chemical potential across the thickness of the membrane