Pervaporation diffusion

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. 

Figure 20-48 shows Wijmans s plot [Wijmans et al.,/. Membr. Sci., 109, 135 (1996)] along with regions where different membrane processes operate (Baker, Membrane Technology and Applications, 2d ed., Wiley, 2004, p. 177). For RO and UF applications, Sj , < 1, and c > Cl,. This may cause precipitation, fouling, or product denatura-tion. For gas separation and pervaporation, Sj , >1 and c < ci. MF is not shown since other transport mechanisms besides Brownian diffusion are at work. 

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. 

FIG. 20-77 Permeant-concentration profile in a pervaporation membrane. 1— Upstream side (swollen). 2—Convex curvature due to concentration-dependent permeant diffusivity. 3—Downstream concentration gradient. 4—Exit surface of membrane, depleted of permeant, thus unswollen. Courtesy Elsevier.)... 

Dense membranes are used for pervaporation, as for reverse osmosis, and the process can be described by a solution-diffusion model. That is, in an ideal case there is equilibrium at the membrane interfaces and diffusional transport of components through the bulk of the membrane. The activity of a component on the feed side of the membrane is proportional to the composition of that component in the feed solution. 

The composition at the permeate-phase interface depends on the partial pressure and saturation vapour pressure of the component. Solvent composition within the membrane may vary considerably between the feed and permeate sides interface in pervaporation. By lowering the pressure at the permeate side, very low concentrations can be achieved while the solvent concentration on the feed-side can be up to 90 per cent by mass. Thus, in contrast to reverse osmosis, where such differences are not observed in practice, the modelling of material transport in pervaporation must take into account the concentration dependence of the diffusion coefficients. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

Many other methods for separating isotopes have been described. A partial list includes membrane and membrane pervaporation, thermal diffusion of liquids, mass diffusion, electrolysis and electro-migration, differential precipitation, solvent extraction, biological microbial enrichment, and more. Although not discussed in... 

Finally it should be noted that inorganic membranes suitable for separation by multilayer diffusion and capillary condensation are also appropriate for performing pervaporation (a technique in which the feed is liquid and the permeate is gas) and distillation at reduced pressure (where gases with overlapping condensation regions are separated). 

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.

From Fig. 19.3a-c, and as opposed to purely sorption controlled processes, it can be seen that during pervaporation both sorption and diffusion control the process performance because the membrane is a transport barrier. As a consequence, the flux 7i of solute i across the membrane is expressed as the product of both the sorption (partition) coefficient S, and the membrane diffusion coefficient Di, the so-called membrane permeability U, divided by the membrane thickness f and times the driving force, which maybe expressed as a gradient of partial pressures in place of chemical potentials [6] ... 

Fig. 19.3 The solution-diffusion transport model in pervaporation. a Solution of compounds from the feed phase into the membrane surface, b Diffusion across the membrane barrier, c Desorption from the membrane permeate (downstream) side into the permeate phase...

In comparison with adsorptive/absorptive techniques for aroma recovery from bioconversions, the disadvantage of pervaporation is the fact that both sorption and diffusion determine the overall selectivity. While the sorption selectivity is very high (equal to that of adsorptive/absorption), the diffusion selectivity favours water owing to the simple fact that water is a smaller molecule than aroma compounds and thus sterically less hindered during diffusion (Table 19.1). The overall (perm)selectivity P=SD) is therefore lower than in strictly sorption controlled processes, although it is still favourable compared with that for evaporation. This shortcoming compares, however, with operational advantages of pervaporation as outlined before. 

Additionally in pervaporation separation follows the solution-diffusion mechanism. Therefore the molecular size of the permeating molecules becomes very important to characterize the permeation behavior [43], It is known that acetic acid has larger molecular size (0.40 nm) than water molecules (0.28 nm). As the amount of acetic acid increases in the feed mixture it becomes difficult for acetic acid molecules to diffuse through the less swollen membrane, so separation factor increases at high acid concentrations. 

U.S. Toti, M.Y. Kariduraganavar, K.S. Soppimath, T.M. Aminabhavi, Sorption, diffusion and pervaporation separation of water/acetic acid mixtures through the blend membranes of sodium alginate and guar gum-g-poly(acrylamide), J. Appl. Polym. Sci. 83 (2002) 259-272. 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

The solution-diffusion model applies to reverse osmosis, pervaporation and gas permeation in polymer films. At first glance these processes appear to be very... 

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...

Figure 2.14 The effect of feed and permeate pressure on the flux of hexane through a rubbery pervaporation membrane. The flux is essentially independent of feed pressure up to 20 atm but is extremely sensitive to permeate pressure [18]. The explanation for this behavior is in the transport equation (2.79). Reprinted from J. Membr. Sci. 2, F.W. Greenlaw, W.D. Prince, R.A. Shelden and E.V. Thompson, Dependence of Diffusive Permeation Rates by Upstream and Downstream Pressures, p. 141, Copyright 1977, with permission from Elsevier...

Sorption data were used to obtain values for A" L. As pointed out by Paul and Paciotti, the data in Figure 2.17 show that reverse osmosis and pervaporation obey one unique transport equation—Fick s law. In other words, transport follows the solution-diffusion model. The slope of the curve decreases at the higher concentration differences, that is, at smaller values for c,eimi because of decreases in the diffusion coefficient, as the swelling of the membrane decreases. 

In the case of dehydration of ethanol by pervaporation illustrated in Figure 4.3(b), the water concentration cio adjacent to the membrane surface is lower than the bulk solution concentration clb because the pervaporation membrane preferentially permeates water and retains ethanol. Water and ethanol are brought towards the membrane surface by the flow of solution through the membrane. Water and a little ethanol permeate the membrane, but most of the ethanol is retained at the membrane surface. Ethanol accumulates at the membrane surface until a sufficient gradient has formed to allow it to diffuse back to the bulk solution. An equal and opposite water gradient must form thus, water becomes depleted at the membrane surface. 

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. 

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... 

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... 

Pervaporation is a concentration-driven membrane process for liquid feeds. It is based on selective sorption of feed compounds into the membrane phase, as a result of differences in membrane-solvent compatibility, often referred to as solubility in the membrane matrix. The concentration difference (or, in fact, the difference in chemical potential) is obtained by applying a vacuum at the permeate side, so that transport through the membrane matrix occurs by diffusion in a transition from liquid to vapor conditions (Figure 3.1). Alternatively, a sweep gas can be used to obtain low vapor pressures at the permeate side with the same effect of a chemical potential gradient. 

Polymeric membranes with a less porous structure, pervaporation membranes as well as nanofiltration membranes, can be described by a solution-diffusion mecha-... 

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