Vapor Permeation and Pervaporation

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

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B. Will, R.N. Lichtenthaler, Comparison of the separation of mixtures by vapor permeation and pervaporation using PVA composite membranes. Part I. Binary alcohol—water systems, J. Membr. Sci. 68 (1992) 119-125. 

Figure 9.3 illustrates the concept of permeation from a saturated vapor phase in equilibrium with the feed liquid as a tool to obtain Equation (9.5). A number of workers have experimentally compared vapor permeation and pervaporation separations and have sometimes shown that permeation from the... 

Flanders CL, Tuan VA, Noble RD, and Falconer JL. Separation of C-6 isomers by vapor permeation and pervaporation through ZSM-5 membranes. J Membr Sci 2000 176 43-53. 

Okamoto K, Tanihara N, Watanabe H, Tanaka K, Kita H, Nakamura A, Kusuki Y, and Nakagawa K. Vapor permeation and pervaporation separation of water ethanol mixtures through polyimide membranes. J Membr Sci 1992 68(l-2) 53-63. 

Vapor permeation (VP) and pervaporation (PV) are membrane separation processes whose only difference lies in the feed fluid being a vapor (VP) or a liquid (PV), respectively. This difference has impHcations for feed fluid handling as well as the nature of the transport phenomena occurring in the feed stream, as in VP the feed fluid is compressible whilst in PV it is effectively not however, this does not in any way affect the transport phenomena across and after the membrane barrier. For this reason, vapor permeation and pervaporation will be discussed simultaneously, with differences being expHcitly emphasized where necessary. 

Ishida, M., Tasaka, Y. and Aseada, M. 2005. A study on vapor permeation and pervaporation of acetic acid/water mixtures by porous silica membranes, 38(11) ... 

One of them employs membrane-based separation processes connected to the esterification reaction. In this respect, vapor permeation and pervaporation process have been tested and dn-ee different layouts have been reported for ethyl lactate production. In one of them, membrane module is located outside the reactor unit and the retenate is recirculated to the reactor." " In another scheme, the membrane module is placed inside the reactor, but the membrane does not participate in the reaction directly and simply acts as a filter," " and in the third configuration, membrane itself participates in die reaction catalysis (catalytic membrane reactor)." Different hydrophilic membranes, such as polymeric, ceramic, zeolites and organic-inorganic hybrid membranes were tested. ... 

K., Kondo, M., and NaA, Z. (2001) membrane preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures. Ind. Eng. Chem. Res., 40, 163-175. 

Figure 9.12 Isothermal vapor permeation and multistage pervaporation with intermediate heating. GFT poly (vinyl alcohol) membranes [45]...

Pervaporation and vapor permeation (and gas permeation) are closely related processes and are characterized by generating a permeate in the vapor state. In this situation, the driving force for permeation of a particular component approximates very closely to the difference in partial vapor pressure of that component across the membrane. Because the pressure on the back-side of the membrane is low, almost all of the faster permeating component can be removed from the feed. The process purifies the feed by removing the faster permeating component. The product from the process is the retentate and the concentrated impurity is the permeate. 

Equation (9) illustrates that in comparison to the selectivity of a simple liquid-vapor equilibrium based on the ratio of the respective volatilities, here expressed in terms of the Henry coefficients of solutes i andj, respectively, the membrane introduces a further selectivity given by the ratio of the permeabilities of the respective compounds i andj (assuming all the above assumptions to be valid). In other words, whilst the driving force for solute transport is identical in processes based on the vapor-liquid equilibrium and in vapor permeation or pervaporation, the latter can exceed the vapor-liquid equilibrium selectivity whenever... 

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. 

Pervaporation, vapor permeation and gas permeation are very closely related processes. In aU three cases the driving force for the transport of matter through the membrane is a gradient in the chemical potential that can best be described by a gradient in partial vapor pressure of the components. The separation is governed by the physical-chemical affinity between the membrane material and the species to be passed through and thus by sorption and solubility phenomena. The transport through the membrane is affected by diffusion and the differences in the diffiisivities of the different components in the membrane can play an important role for the separation efficiency, too. All three processes are best described by the solution-diffusion mechanism , their main differences are determined by the phase state and the thermodynamic conditions of the feed mixture and the condensability of the permeate. 

Velterop F (2011), The potential of the HybSi ceramic membrane in process intensification , Programme booklet of the International Scientific Conference on Pervaporation, Vapor Permeation and Membrane Distillation, Torun (Poland)... 

Fig. 18.4-1. A schematic representation of pervaporation. The membrane, represented by the diagonal, separates a liquid feed into a vapor permeate and a liquid retentate.

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

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. 

T. Kataoka, T. Tsuro, S.-I. Nakao and S. Kimura, Membrane Transport Properties of Pervaporation and Vapor Permeation in an Ethanol-Water System Using Polyacrylonitrile and Cellulose Acetate Membranes, J. Chem. Eng. Jpn 24, 326 (1991). 

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