Pervaporation organic from water

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. 

Organic from Water An area where pervaporation may become important is in flavors, fragrances, and essential oils. Here, high-value materials with unique properties are recovered from aqueous or alcohol solutions. 

The membrane s intrinsic enrichment Ea also affects concentration polarization. If the membrane is completely unselective, Ea = 1. The relative concentrations of the components passing through the membrane do not change, so concentration gradients are not formed in the boundary layer. As the difference in permeability between the more permeable and less permeable components increases, the intrinsic enrichment E0 achieved by the membrane increases, and the concentration gradients that form become larger. As a practical example, in pervaporation of organics from water, concentration polarization is much more important when the solute is toluene (with an enrichment Ea of 5000 over water) than when the solute is methanol (with an enrichment Ea less than 5). 

K.W. Boddeker and G. Bengtson, Selective Pervaporation of Organics from Water, in Pervaporation Membrane Separation Processes, R.Y.M. Huang (ed.), Elsevier, Amsterdam, pp. 437-460 (1991). 

Yeom CK, Dickson JM, and Brook MA. A characterization of PDMS pervaporation membranes for the removal of trace organic from water. Korean J. Chem. Engg. 1996 13(5) 482-488. 

Pervaporation is a membrane process in which a liquid is maintained on the feed side of a membrane and permeate is removed as a vapor on the downstream side of the membrane. Pervaporation is used, because of its low energy consumption and low cost, to separate dissolved organics from water, purify waste water or volatile chemicals, and break azeotropes. Pervaporation plants range from processing a few grams per hour up to thousands of tons per year. For waste water treatment flow of less than 76 L min pervaporation is more cost-effective than other treatment options, such as chemical oxidation, ultraviolet destruction, air stripping followed by carbon adsorption, steam stripping, or distillation/incineration [262]. 

A large variety of applications using either vapor permeation or pervaporation has been reported. These include the use of pervaporation for the removal of toxic organics from water (Schnabel et al., 1998) and wastewater streams (Moulin et al., 2002), sometimes using hybrid approaches with adsorptive techniques the use of pervaporation membranes in direct methanol fuel cells (Pivovar et al., 1999) and, more recently, the resolution of isomeric mixtures (Kusumocahyo etal., 2004) and membrane-assisted enantiomer enrichment (Paris et al., 2004), in both cases using membranes containing specific complexation agents such as cyclodextrins. 

Yamaguchi, T., Yamahara, S., Nakao, S., and Kimura, S. (1994). Preparation of pervaporation membranes for removal of dissolved organics from water by plasma-graft filling polymerization,... 

The different applications of these zeolites in pervaporation include alcohol dehydration water removal in acid solutions organic dehydration separations such as water/ tetrahydrofuran (THF), water/dioxane, or water/dimethyl-formamide (DMF) removal of organics from water and organic/organic separations such as methanol/methyl-tert-butyl ether (MTBE) or p-xylene/o-xylene. In the following subsections, the mechanism of pervaporation in zeolite membranes will be briefly described and we will provide the details about the different applications. 

The commercial success of pervaporation has been a disappointment to many process developers. Current pervaporation sales worldwide are probably less than US 10 million almost all are for dehydration of ethanol or isopropanol solutions using water-permeable poly (vinyl alcohol) or equivalent membranes. A smaller market also exists for the separation of volatile organics from water using silicone-rubber membranes. 

Psaume, R., Aptel, Ph., AureUe, Y., Mora, J.C., and Bersillon, J.L., Pervaporation importance of concentration polarization in the extraction of trace organics from water, J. Membr. Set, 36, 373-384, 1988. 

Lee, G.T., Krovvidi, K.R. and Greenberg, D.B. 1989. Pervaporation of trace chlorinated organics from water through irradiated polyethylene membrane, 47 183-202. 

As usual with membrane separations, the membrane is critical for success. Currently, two different classes of membranes are used commercially for pervaporation. To remove traces of organics from water a hydrophobic membrane, most commonly silicone rubber is used. To remove traces of water from organic solvents a hydrophilic membrane such as cellulose acetate, ion exchange men )rane, polyacrylic acid, polysulfone, pol5 inyl alcohol, composite membrane, and ceramic zeolite is used. Both types of membranes are nonporous and operate by a solution-diffusion mechanism Selecting a membrane that will preferentially permeate the more dilute conponent will usually reduce the membrane area required. Membrane life is typically about four years tBaker. 20041. 

Wigmans, J. G., R. W. Baker, and A. L. Aythayde. Pervaporation Removal of Organics from Water and Organic/Organic Separations. In Membrane Processes in Separation and Purification, ed. J. G. Crespo and K. W. Boddeker. Dordrecht, the Netherlands Kluwer Academic Publishers, 1994. 

Wijmans, J.G., Baker, R.W., and Athayde, A.L., Pervaporation Removal of organics from water and organic/otganic separations , in "Membrane Processes in Separation and Purification , Crespo, J.C., and Boddeker. K.W. (Eds.). Kluwer, Dordrecht, 1994. p. 283... 

Thinly coated PDMS films exhibit a higher amount of the chain aggregates [29]. Thus, a thinner membrane has a relatively loose structure, which could be responsible for low selectivity but high flux in the separation of organics from water by pervaporation. Thin film composite membranes... 

W. Kujawski, Pervaporative removal of organics from water using hydrophobic membranes. Binary mixtures. Separation Science and Technology 35 (2000) 89-108. 

Pervaporation has been commercialized for two appHcations. The first and most developed is the separation of water from concentrated alcohol solutions. GFT of Neunkirchen, Germany, the leader in this field, installed their first important plant in 1982. More than 100 plants have been installed by GFT for this appHcation (90). The second appHcation is the separation of small amounts of organic solvents from contaminated water (91). In both of these apphcations, organics are separated from water. This separation is relatively easy, because organic compounds and water, due to their difference in polarity, exhibit distinct membrane permeation properties. The separation is also amenable to membrane pervaporation because the feed solutions are relatively nonaggressive and do not chemically degrade the membrane. 

Pervaporation membranes are of two general types. Hydrophilic membranes are used to remove water from organic solutions, often from azeotropes. Hydrophobic membranes are used to remove organic compounds from water. The important operating charac teris-tics of hydrophobic and hydrophihc membranes differ. Hydrophobic membranes are usually used where the solvent concentration is about... 

The mechanical properties of these membranes were improved by including a crosslinker, methylene bisacrylamide, in the aqueous phase, and by using a styrene/butyl acrylate (BA) mixture as the continuous phase [185]. The styrene/BA mixture had to be prepolymerised to low conversion to allow HIPE formation. The permeation rate of the membrane was improved by including a porogen (hexane) in the organic phase, generating a permanent porous structure [186]. The pervaporation rate was indeed increased, however a drop in selectivity for water from water/ethanol mixtures was also observed. 

Figure 4.11 shows that ultrafiltration and pervaporation for the removal of organic solutes from water are both seriously affected by concentration polarization. In ultrafiltration, the low diffusion coefficient of macromolecules produces a concentration of retained solutes 70 times the bulk solution volume at the membrane surface. At these high concentrations, macromolecules precipitate, forming a gel layer at the membrane surface and reducing flux. The effect of this gel layer on ultrafiltration membrane performance is discussed in Chapter 6. 

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

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