Pervaporation membrane materials

Pervaporation operates under constraints similar to those for low-pressure gas separation. Pressure drops on the permeate side of the membrane must be small, and many pervaporation membrane materials are rubbery, so both spiral-wound modules and plate-and-frame systems are in use. Plate-and-frame systems are competitive in this application despite their high cost, primarily because they can be operated at high temperatures with relatively aggressive feed solutions, conditions under which spiral-wound modules might fail. 

Table 9.1 Widely used pervaporation membrane materials...

Pereira CC, Rufino JRM, Habert AC, Noberga R, Cabral LMC, and Borges CP. Aroma compounds recovery of tropical fruit juice by pervaporation Membrane material selection and process evaluation. J. Food Eng. 2005 66(l) 77-87. 

C. Schepers and D. Hofmann. Molecular simulation study on sorption and diffusion processes in polymeric pervaporation membrane materials. Mol. Simul. 32, 2006, 2. 

Nonporous Dense Membranes. Nonporous, dense membranes consist of a dense film through which permeants are transported by diffusion under the driving force of a pressure, concentration, or electrical potential gradient. The separation of various components of a solution is related directiy to their relative transport rate within the membrane, which is determined by their diffusivity and solubiUty ia the membrane material. An important property of nonporous, dense membranes is that even permeants of similar size may be separated when their concentration ia the membrane material (ie, their solubiUty) differs significantly. Most gas separation, pervaporation, and reverse osmosis membranes use dense membranes to perform the separation. However, these membranes usually have an asymmetric stmcture to improve the flux. 

Pervaporation Membranes Pervaporation has a long history, and many materials have found use in pervaporation experiments. Cellulosic-based materials have given way to polyvinyl alcohol and blends of polyvinyl alcohol and acrylics in commercial water-removing membranes. These membranes are typically solution cast (from... 

The membranes used for pervaporation are similar to reverse osmosis membranes, i.e. both are composite membranes consisting of a very thin dense permselective film on top of a nonselective porous support. In pervaporation, however, the membrane is highly swollen at the feed side and relatively dry at the permeate side. Two different types of pervaporation membranes based on polymeric materials were developed at about the same time in the early 1980s [31] ... 

Figure 19.3 schematically describes in more detail the transport phenomena occurring during pervaporation. First, solutes partition into the membrane material according to the thermodynamic equilibrium at the liquid-membrane interface (Fig. 19.3a), followed by diffusion across the membrane material owing to the concentration gradient (Fig. 19.3b). A vacuum or carrier gas stream promotes then continuous desorption of the molecules reaching the permeate side of the membrane (Fig. 19.3c), maintaining in this way a concentration gradient across the membrane and hence a continuous transmembrane flux of compounds.

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...

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

Equation (9.11) identifies the three factors that determine the performance of a pervaporation system. The first factor, pevAp, is the vapor-liquid equilibrium, determined mainly by the feed liquid composition and temperature the second is the membrane selectivity, G-men, an intrinsic permeability property of the membrane material and the third includes the feed and permeate vapor pressures, reflecting the effect of operating parameters on membrane performance. This equation is, in fact, the pervaporation equivalent of Equation (8.19) that describes gas separation in Chapter 8. 

The selectivity (amcm) of pervaporation membranes critically affects the overall separation obtained and depends on the membrane material. Therefore, membrane materials are tailored for particular separation problems. As with other solution-diffusion membranes, the permeability of a component is the product of the membrane sorption coefficient and the diffusion coefficient (mobility). The membrane selectivity term amem in Equation (9.11) can be written as... 

Concentration polarization plays a dominant role in the selection of membrane materials, operating conditions, and system design in the pervaporation of VOCs from water. Selection of the appropriate membrane thickness and permeate pressure is discussed in detail elsewhere [50], In general, concentration polarization effects are not a major problem for VOCs with separation factors less than 100-200. With solutions containing such VOCs, very high feed velocities through... 

C. Streicher, P. Kremer, V. Tomas, A. Hubner and G. Ellinghorst, Development of New Pervaporation Membranes, Systems and Processes to Separate Alcohols/Ethers/ Hydrocarbons Mixtures, in Proceedings of Seventh International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 297-309 (1995). 

A list of typical commercial pervaporation membranes [23] is given in Table 3.1. Commercial hydrophilic membranes are very often made of polyvinyl alcohol (PVA), with differences in the degree of crosslinking. Commercial hydrophobic membranes often have a top layer in polydimethyl siloxane (PDMS). However, a wide variety of membrane materials for pervaporation can be found in the literature, including polymethylglutamate, polyacrylonitrile, polytetrafluoroethylene, polyvinylpyrrolidone, styrene-butadiene rubber, polyacrylic acid, and many others [24]. A comprehensive overview of membrane materials for pervaporation is given by Semenova et al. [25],... 

ACS, Polymeric Materials Science Engineering Fall Meeting 1999. Volume 81. Conference proceedings. New Orleans, La., 22nd-26th Aug.1999, p.542-3 SILANE-MODIFIED POLYVINYL CHLORIDE PERVAPORATION MEMBRANES Silverstein M S Sluszny A Narkis M Technion-Israel Institute of Technology (ACS,Div.of Polymeric Materials Science Engng.)... 

Heinzebnann W. Eabrication methods for pervaporation membranes. In Bakish R. ed.. Proceedings of the Fifth International Conference on Pervaporation Processes in the Chemical Industry. Nancy, Erance, September 19-22 Englewood, NJ Bakish Materials Corporation, 1991 22-30. 

Streicher C, Kermer P, Tomas V, Hubner A, and EUinghorst G. Development of new pervaporation membrane, system and processes to separate alcohols/ethers/hydrocarbone mixture. In Bakish. R., ed. Proceedings of the 7th International Conference on Pervaporation Process in the Chemical Industry. Englewood, NJ Bakish Material Corporation, 1995 297-309. 

The experimental procedures are quite similar to and often confused with pervaporation. The main difference between VMD and pervaporation is the nature of the membrane used, which plays an important role in the separations. While VMD uses a porous hydrophobic membrane and the degree of separation is determined by vapor-liquid equilibrium conditions at the membrane-solution interface, pervaporation uses a dense membrane and the separation is based on the relative perm-selectivity and the diffusivity of each component in the membrane material. 

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