Pervaporation process

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

R. Bakish, ed.. Proceedings of the International Conference of Pervaporation Processes, Heidelberg, Germany, 1991, and Ottawa, Canada, 1992. 

Table 6. Alcohol Reduction Using Pervaporation Process...

The simplifying assumptions that make Tick s law useful for other processes are not valid for pervaporation. The activity gradient across the membrane is far more important than the pressure gradient. Equation (20-98) is generally used to describe the pervaporation process ... 

A very high separation factor has been obtained in phenol dehydration by using pervaporation process and PVA/PAA as membranes. The membrane composition and the process characteristics are presented in table 1. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

Based on experimental results and a model describing the kinetics of the system, it has been found that the temperature has the strongest influence on the performance of the system as it affects both the kinetics of esterification and of pervaporation. The rate of reaction increases with temperature according to Arrhenius law, whereas an increased temperature accelerates the pervaporation process also. Consequently, the water content decreases much faster at a higher temperature. The second important parameter is the initial molar ratio of the reactants involved. It has to be noted, however, that a deviation in the initial molar ratio from the stoichiometric value requires a rather expensive separation step to recover the unreacted component afterwards. The third factor is the ratio of membrane area to reaction volume, at least in the case of a batch reactor. For continuous opera-... 

H.E.A. Bruschke, State-of-the-Art of Pervaporation Processes in the Chemical Industry, in Membrane Technology in the Chemical Industry, (eds. S.P. Nunes,... 

Fig. 19.4 Aspects of optimisation of the pervaporation process, apart from the membrane material 1 module design for optimum upstream and downstream conditions 2 condensation temperature(s) or aroma capture strategy 3 vacuum applied and type of vacuum pump. All aspects of the optimisation are interdependent in pervaporation and therefore need to be tackled as a whole, rather than individimlly...

Bengtson G, Pingel H, Boddeker KW (1991) In Bakish R (ed) Proceedings of the 5th international conference on pervaporation processes in the chemical industry, Heidelberg, 11-15 March 1991. Bakish Material Corporation, Englewood... 

Figure 1.5 Schematic diagram of the basic pervaporation process...

This equation explicitly expresses the driving force in pervaporation as the vapor pressure difference across the membrane, a form of the pervaporation process... 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

The pervaporation process to separate liquid mixtures is shown schematically in Figure 9.1. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapor from the other side. Transport through the membrane is induced by the vapor pressure difference between the feed solution and the permeate vapor. This vapor pressure difference can be maintained in several ways. In the laboratory, a vacuum pump is usually used to draw a vacuum on the permeate side of the system. Industrially, the permeate vacuum is most economically generated by cooling the permeate vapor, causing it to condense condensation spontaneously creates a partial vacuum. 

Pervaporation systems are now commercially available for two applications. The first and most important is the removal of water from concentrated alcohol solutions. GFT, now owned by Sulzer, the leader in this field, installed the first pilot plant in 1982 [9]. The ethanol feed to the membrane contains about 10 % water. The pervaporation process removes the water as the permeate, producing a residue of pure ethanol containing less than 1 % water. All... 

Figure 9.1 In the pervaporation process, a liquid mixture contacts the membrane, which preferentially permeates one of the liquid components as a vapor. The vapor enriched in the more permeable component is cooled and condensed, spontaneously generating a vacuum that drives the process...

Both of the current commercial pervaporation processes concentrate on the separation of VOCs from contaminated water. This separation is relatively easy, because organic solvents and water have very different polarities and exhibit distinct membrane permeation properties. No commercial pervaporation systems have yet been developed for the separation of organic/organic mixtures. However, current membrane technology makes pervaporation for these applications possible, and the process is being actively developed by a number of companies. The first pilot-plant results for an organic-organic application, the separation of methanol from methyl tert-butyl ether/isobutene mixtures, was reported by Separex in 1988 [14,15], This is a particularly favorable application... 

The separation factor, /3pervap, contains contributions from the intrinsic permeation properties of the membrane, the composition and temperature of the feed liquid, and the permeate pressure of the membrane. The contributions of these factors are best understood if the pervaporation process is divided into two steps, as shown in Figure 9.3 [18]. The first step is evaporation of the feed liquid to form a saturated vapor in contact with the membrane the second step is diffusion of this vapor through the membrane to the low-pressure permeate side. This two-step description is only a conceptual representation in pervaporation no vapor phase actually contacts the membrane surface. Nonetheless, the representation of the process shown in Figure 9.3 is thermodynamically completely equivalent to the actual pervaporation process shown in Figure 9.F... 

Figure 9.3 The pervaporation process shown in Figure 9.1 can be described by the thermodynamically equivalent process illustrated here. In this model the total pervaporation separation /9pervap is made up of an evaporation step followed by a membrane permeation step [18]...

Figure 9.7 Schematics of potential pervaporation process configurations that have been suggested but not necessarily practiced...

Most of the early solvent dehydration systems were installed for ethanol dehydration. More recently pervaporation has been applied to dehydration of other solvents, particularly isopropanol used as a cleaning solvent. Dehydration of other solvents, including glycols, acetone and methylene chloride, has been considered. Schematics of pervaporation processes for these separations are shown in Figure 9.13. 

P. Aptel, N. Challard, J. Cuny and J. Neel, Application of the Pervaporation Process to Separate Azeotropic Mixtures, J. Membr. Sci. 1, 271 (1976). 

R.C. Schucker, Separation of Organic Liquids by Perstraction, in Proceedings of Seventh International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 321-332 (1995). 

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