Pervaporation, material selection

Factors governing Pa and Pb are understood, so the key scientific issues in pervaporation materials selection are similar to those in gas separation. 

T.Q. Nguyen, A. Essamri, R. Clement, J. Nell, Synthesis of membranes for the dehydration of water—acetic acid mixture by pervaporation. Part 1. Polymer material selection, Makromol. Chem. 188... 

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. 

The selective separation of water from aqueous solutions of isopropanol or the dehydration of isopropanol can be carried out with different membranes, which contain polar groups, either in the backbone or as pendent moieties. For the dehydration of such a mixture, poly(vinyl alcohol) (PVA) and PVA-based membranes have been used extensively. PVA is the primary material from which the commercial membranes are fabricated and has been studied intensively for pervaporation because of its excellent film forming, high hydrophilicity due to -OH groups as pendant moieties, and chemical-resistant properties. On the contrary, PVA has poor stability at higher water concentrations, and hence selectivity decreases remarkably. 

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). 

Fig. 23.4 Organophilic pervaporation (PV) for in situ recovery of volatile flavour compounds from bioreactors. The principle of PV can be viewed as a vacuum distillation across a polymeric barrier (membrane) dividing the liquid feed phase from the gaseous permeate phase. A highly aroma enriched permeate is recovered by freezing the target compounds out of the gas stream. As a typical silicone membrane, an asymmetric poly(octylsiloxane) (POMS) membrane is exemplarily depicted. Here, the selective barrier is a thin POMS layer on a polypropylene (PP)/poly(ether imide) (PEI) support material. Several investigations of PV for the recovery of different microbially produced flavours, e.g. 2-phenylethanol [119], benzaldehyde [264], 6-pentyl-a-pyrone [239], acetone/buta-nol/ethanol [265] and citronellol/geraniol/short-chain esters [266], have been published...

The composite materials have been used to form selective membranes for the separation of liquid mixtures [181]. The membranes should consist of a polymer which is soluble in the liquid components) to be separated, as the dispersed phase-derived polymer, and a continuous phase-derived polymer which is insoluble in all components of the liquid mixture. Thus, membranes consisting of polystyrene in polyacrylamide will separate toluene from cyclohexane, and those comprising polyacrylamide in crosslinked polystyrene can be used for water removal from ethanol. Due to the very thin films of polymer which separate the polyhedral dispersed phase cells, the permeation rates, which are measured by pervaporation, are relatively high. 

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... 

H.H. Nijhuis, M.V.H. Mulder and C.A. Smolders, Selection of Elastomeric Membranes for the Removal of Volatile Organic Components from Water, in Proceedings of Third International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 239-251 (1988). 

At low permeate pressures typical of ideal pervaporation, the same form for membrane selectivity applies as for gas or vapor separation of components A and B, which will be discussed later. This simple selectivity equals the inverse ratio of the resistances of the membrane to permeation of components A and B [S2B / 2A from Eq. (1)]. This ratio can be shown to simply equal to the ratio of permeabilities of the two components in the material comprising the selective layer of the membrane. 

As a special case there is pervaporation, in which the feed material is a liquid, but the permeate becomes a gas or vapor. That is, the temperature and pressure of the permeate produced are such that the permeate product will exist in the gaseous or vapor phase. Conceivably and conversely, the feed stream could be a gas but the permeate conditions would be such that the components selectively obtained would constitute a liquid phase. In any event, compositional phase changes (e.g., flash vaporizations) can affect the outcome. 

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. 

Separation from mixtures is achieved because the membrane transports one component more readily than the others, even if the driving forces are equal. The effectiveness of pervaporation is measured by two parameters, namely flux, which determines the rate of permeation and selectivity, which measures the separation efficiency of the membrane (controlled by the intrinsic properties of the polymer used to construct it). The coupling of fluxes affecting the permeability of a mixture component can be divided into two parts, namely a thermodynamic part expressed as solubility, and a kinetic part expressed as diffusivity. In the thermodynamic part, the concentration change of one component in the membrane due to the presence of another is caused by mutual interactions between the permeates in the membrane in addition to interactions between the individual components and the membrane material. On the other hand, kinetic coupling arises from the dependence of the concentration on the diffusion coefficients of the permeates in the polymers [155]. 

Pervaporation (PV) membranes were developed for the dehydration of ethanol and other organic solvents. Therefore, the dense selective layer is made of polyvinyl alcohol, which is one of the most hydrophilic materials (see Table 2). Water is preferentially sorbed to polyvinyl alcohol and also preferentially transported. To suppress the excessive swelling of polymer in water, polyvinyl alcohol is partially cross-linked by dialdehydes such as glutaraldehyde. ... 

Reverse osmosis and pervaporation are able to separate molecules of similar size, such as sodium chloride and water. In such cases, the affinity between the membrane and the target component is important, as the speed of permeation through the membrane. Components that have a greater affinity for the membrane material dissolve in the membrane more easily than other components, cansing the manbrane material acts as an extraction phase. Differences in diffnsion coefficients of components throngh the membrane allow the separation. According to the theory of solution diffusion, solubility and diffusivity together will control the manbrane selectivity. The mechanism by which NF membranes act is... 

Called pervaporation its name implies a combination of permeation (of the water through the membrane) and evaporation (from the membrane surface) to maintain the driving force which, by promoting selective permeation of water through the material of the membrane dries the solvent (Fig. 7.8). 

The phenomenon of solvent transport through solid barriers has three aspects which discussed under the heading of permeability. These are the permeation of solvent through materials (films, containers, etc.) the use of pervaporation membranes to separate organic solvents from water or water from solvents the manufacture of permeate selective membranes. 

Buckley-Smith, M.K. 2006. The use of solubility parameters to select membrane materials for pervaporation of organic mixtures. Ph.D. Thesis, University of Waikato, Hamilton, New Zealand. 

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