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Separation of Binary Mixture By Using Pervaporation Chapter 1 INTRODUCTION TO PERVAPORATION 1.1 BACKGROUND: Compared with traditional separation processes, such as distillation, extraction and filtration, membrane technology is a relatively new method that has been developed in the past few decades, but it has been widely adopted in many industries. The membrane processes have the following distinguishing characteristics [Mulder 1991]: 1) Continuity and simplicity of the processes, 2) Adjustability of the separation properties, 3) Feasibility of incorporation into hybrid processes, 4) Low energy consumption and moderate operating conditions. Developments in membrane formation techniques and materials science accelerate the research and applications of membrane technology. Now commercial membrane applications have successfully displaced some conventional processes, and membrane technology has become an indispensable component in many industrial fields and our daily life. Figure 1.1 Schematic membrane separation processes Figure 1.1 shows a schematic membrane process [Mulder 1991; Baker 2004]. Separation membranes are located between the feed side and the permeate side. In most membrane processes, such as gas separation, reverse osmosis and ultra filtration, both the feed and the permeate sides are in the same phases, gas or liquid, while in 1Separation of Binary Mixture By Using Pervaporation pervaporation, the liquid feed is separated into vaporous permeates with the aid of vacuum or a purge gas in the downstream side. Pervaporation has become a very important technique to separate azeotropes, close- boiling mixtures, and recover volatile organic chemicals from liquid mixtures, and now it has emerged as a good choice for separating heat sensitive products. The phenomenon of pervaporation was first discovered in 1917 by Kober [1995], but no extensive research was carried out until in the 1950s by Binning et al. [1961]. In pervaporation processes with functional polymer membranes, the non-porous dense membranes are essential. By choosing proper membranes, pervaporation has great advantages as an alternative separation method in the following separation tasks: 1) Dehydration of organic solvents, 2) Removal of organics from water, 3) Separation of organic liquids. Non-porous dense membranes can also be applied in other separation processes such as gas separation. Furthermore, both gas separation and pervaporation can be interpreted with the solution diffusion mechanism for mass transport in membranes. Membrane-based pervaporation or vapor permeation is a promising alternative to distillation since it is an energy-saving one-step separation process. If the proper membrane material is selected, pervaporation can separate azeotropic mixtures and close boiling mixtures that traditional distillation has difficulties in processing [3]. 1.2 MEMBRANE BASED PERVAPORATION SEPARATION: Pervaporation, in its simplest form, is an energy efficient combination of membrane permeation and evaporation. Liquid mixtures can be separated by partial vaporization through a non-porus permselective membrane. This technique, which was srcinally called \u201cLiquid permeation\u201d has subsequently been termed \u201cpervaporation\u201d in order to emphasized the fact that permeate undergoes a phase change, from liquid to vapor, during the transport through the barrier. 2Separation of Binary Mixture By Using Pervaporation It s considered an attractive alternative to other separation methods for a variety of processes. For example, with the low temperatures and pressures involved in pervaporation, it often has cost and performance advantages for the separation of constant-boiling azeotropes. Pervaporation is also used for the dehydration of organic solvents and the removal of organics from aqueous streams. Additionally, pervaporation has emerged as a good choice for separation heat sensitive products. Pervaporation involves the separation of two or more components across a membrane by differing rates of diffusion through a thin polymer and an evaporative phase change comparable to a simple flash step. A concentrate and vapor pressure gradient is used to allow one component to preferentially permeate across the membrane. A vacuum applied to the permeate side is coupled with the immediate condensation of the permeated vapors. Pervaporation is typically suited to separating a minor component of a liquid mixture, thus high selectivity through the membrane is essential. Figure 1.2 Overview of Pervaporation process In addition, a pervaporation unit can be integrated into a bioreactor to improve bioconversion rate and reduce downstream processing costs, if membranes can selectively remove volatile inhibitory substances from fermentation broths [7]. Compared to the relatively easy separation of non-aggressive chemicals from water in industry, very few commercial systems have been developed to separate aggressive organics-water systems [8-11]. The most significant opportunity to use pervaporation is in splitting an azeotrope or a close boiling-temperature mixture, where distillation is less efficient due to the huge amount of energy consumption. Theoretically, if a liquid feed contacts a nonporous membrane with vacuum downstream, the vaporization rate of each component in the liquid is limited by the membrane permeability. In other words, the concentration 3Separation of Binary Mixture By Using Pervaporation distribution of each component in the liquid and vapor is not only controlled by the thermodynamic equilibrium [12], but also is governed by the membrane permeability. In this case, the membrane is sometimes referred to as a “mass separating agent”. Nevertheless, the membrane-mediated evaporation is generally regarded as pervaporation. In order to maximize the driving force, i.e. an activity difference between a feed liquid and permeate vapor, heating the feed liquid at the boiling temperature on one side of the membrane and pulling a vacuum or cooling the permeate vapor to condense on the other side are generally applied in the pervaporation process [3]. Figure 1.2.1: Membrane-based pervaporation separation processes Vacuum Operation Pervaporation can used for breaking azeotropes, dehydration of solvents and other volatile organics, organic/organic separations such as ethanol or methanol removal, and wastewater purification. Characteristics of the pervaporation process include: 1. Low energy consumption 2. No entrainer required, no contamination 3. Permeate must be volatile at operating conditions 4. Functions independent of vapor/liquid equilibrium 1.2.1 1.3 MEMBRANE MATERIAL: Tobacco plants require fertile well-drained moist soil and warm temperatures. Most types of tobacco are grown in full sun to counteract these problems; tobacco farmers grow strains of tobacco that resist disease and insects. By rotating crops (planting tobacco one year and different crop in the same field next year i.e. change of crop successively) farmers keep the population of tobacco pests in check by depraving them of tobacco plants on alternate years. 4
