Pervaporators membranes

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

Fig. 42. Integrated distillation/pervaporation plant for ethanol recovery from fermentors. The distillation columns concentrate the ethanol—water mixture from 5 to 80%. The pervaporation membrane produces a 99.5% ethanol product stream and a 40—50% ethanol stream that is sent back to the distillation...

A. H. BaHweg and co-workers, "Pervaporation Membranes," Proceedings of the Fifth InternationalMlcohol Fuels Symposium, Auckland, New Zealand, May 13—18, John Mclndoe, Dunedin, New Zealand, 1982. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

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... 

FIG. 22-80 Simplified flow schematic for a pervaporation system. Heated feed enters from left through a feed pump. Heaters in a recirculating feed loop may he required (not shown). Stripped liquid exits at the top of the pervaporation membrane. Vapor exits at the bottom to a condenser. Liquid and noncondensibles are removed under vac-imm. Cowiesif Hoechst Celanese. )... 

This equation is not particularly useful in practice, since it is difficult to quantify the relationship between concentration and ac tivity. The Floiy-Huggins theory does not work well with the cross-linked semi-ciystaUine polymers that comprise an important class of pervaporation membranes. Neel (in Noble and Stern, op. cit., pp. 169-176) reviews modifications of the Stefan-Maxwell approach and other equations of state appropriate for the process. 

FIG. 22-81 Permeant -concentration profile in a pervaporation membrane. 1— Upstream side (swollen). 2—Convex curvature due to concentration-dependent permeant diffiisivity. 3—Downstream concentration gradient. 4—Exit surface of membrane, depleted of permeant, thus unswollen. (Couttesy Elseoier )... 

FIG. 25-20 Ethanol dehydration using pervaporation membrane. (SOURCE Redrawn from Ref. 24.)... 

Membrane Morphology—Pores, Symmetric, Composite Only nucleopore and anodyne membranes have relatively uniform pores. Reverse osmosis, gas permeation, and pervaporation membranes have nonuniform angstrom-sized pores corresponding to spaces in between the rigid or agamic membrane molecules. Solute-membrane molecular interactions are very high. Ultrafiltration membranes have nonuniform nanometer sized pores with some solute-membrane interactions. For other microfiltration membranes with nonuniform pores on the submicrometer to micrometer range, solute-membrane interactions are small. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

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... 

So far, the separation of azeotropic systems has been restricted to the use of pressure shift and the use of entrainers. The third method is to use a membrane to alter the vapor-liquid equilibrium behavior. Pervaporation differs from other membrane processes in that the phase-state on one side of the membrane is different from the other side. The feed to the membrane is a liquid mixture at a high-enough pressure to maintain it in the liquid phase. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, maintaining it in the vapor phase. Dense membranes are used for pervaporation, and selectivity results from chemical affinity (see Chapter 10). Most pervaporation membranes in commercial use are hydrophyllic19. This means that they preferentially allow... 

The ease with which chitosan hydrocolloid gels can be casted and functionalised has prompted their use as matrices for zeolites in pervaporation membranes and as binders of adsorbents and catalysts [3,4]. 

The use of silicones in membrane applications is relatively new. It is, however, a rapidly growing area as evidenced by a number of original papers and reviews published recently. Pervaporation with the use of polymer membranes has been recognized as a versatile separation process in the chemical industry.458 A study of PDMS as an active layer in the composite pervaporation membranes for separation of alcohols and esters has been reported.459 Two-dimensional... 

Figure 8.21. Comparison of selectivity of pervaporation membranes and liquid-vapour equilibrium for...

Miura and Yoshida also investigated the changes in the microstructure of 1100 EW Nafion sulfonate membranes, in alkali, ammonium, and alkylammonium cation forms, that were induced by swelling in ethanol using DSC, dynamic mechanical analysis (DMA), SAXS, and electron probe microanalysis (EPMA). These studies were performed within the context of liquid pervaporation membranes that could potentially be used to separate ethanol from water... 

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] ... 

The resulting spectra from El usually contain a number of fragments, providing extensive structural information about the analyte. A disadvantage of the observed fragmentation is eventually occurring isobaric overlay from different compounds in the analysis of sample mixtures, which often requires a separation step prior to the MS analysis. For this purpose the coupling of a GC with the ion source of the mass spectrometer via capillary inlet is a well established technique. Volatiles can be selectively introduced into El mass spectrometers via pervaporation membranes. The principle and application of this technique, called membrane introduction (MI) MS was recently reviewed [45]. The accuracy of intensity ratio measurements by El MS is about 0.1 -0.5% [4,8]. 

When the driving force is maximum, i.e. whenpj.p 0, then the pervaporative membrane selectivity a between two components can be expressed as... 

Table 19.1 Selectivity of a pervaporation membrane (based on poly(octylmethylsiloxane) [7]) for ethyl hexanoate and isobutyl alcohol with respect to water...

Purification of hydrazine by pervaporation techniques could be an alternative process. Since hydrazine is highly alkaline, the proper selection of the polymer for the pervaporation membrane is important. 

In this section the solution-diffusion model is used to describe transport in dialysis, reverse osmosis, gas permeation and pervaporation membranes. The resulting equations, linking the driving forces of pressure and concentration with flow, are then shown to be consistent with experimental observations. 

Figure 2.12 Chemical potential, pressure, and activity profiles through a pervaporation membrane following the solution-diffusion model...

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