Permselectivities Pervaporation

Jiang XJ, Shi YQ, and Chen GW. Permselective pervaporation membrane for the separation of organic solutions. Gongneng Gaofenzi Xuebao. 2000 13(2) 233-239. 

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

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. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

Evapomeation is a new membrane-separation technique for liquids mixtures, which eliminates some disadvantages of the pervaporation technique such as the decreasing of membrane permselectivity, due to its swelling by the direct contact with the feed solution. In evapomeation technique the membrane is not in direct contact with the feed solution, only with the solution s vapors. In this way the swelling of the membrane could be suppressed and consequently, the permeation rates in evapomeation are smaller than those in pervaporation, but the separation factor is greater [83],... 

The blend membranes are permselective for different organic isomers. So, these could be used for the separation of n-propanol from a mixture of n-propanol (n-PrOH) and i-propanol (i-PrOH) [84] and the separation of p-xylene from a p-xylene and o-xylene mixture [35], It was evidenced that, in both cases, the separation was better by applying the evapomeation technique than that of the pervaporation. 

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] ... 

Membrane pervaporation (permselective evaporation of liquid molecules) is the term used to describe the extraction of volatile organics from an aqueous matrix to a gas phase through a semipermeable membrane. 

A process referred to as vapor-arbitrated pervaporation addresses these issues by manipulating the transmembrane activity gradients of water and ethanol in a pervaporation system. Using a permeate side sweep stream that contains water vapor at a partial pressure corresponding to the activity of water on the feed side, permeation of water is halted while ethanol continues to diffuse through the membrane into the sweep stream and is removed. In this way, the native permselectivity of the membrane system can be altered in a controlled fashion to extract one or more volatile components from a solution. 

It is important to clarify why only poly[l-(trimethylsilyl)-l-propyne] is ethanol-permselective among substituted polyacetylenes. Further investigation of pervaporation through substituted polyacetylenes is desired. 

Figure 11 is a plot of a versus R for ethanol-water pervaporation through substituted polyacetylenes. If the a value is larger than unity, the membrame permeates ethanol preferentially. Previously, with the exception of poly(dimethylsiloxane), no polymer had exhibited ethanol permselectivity. Now, we know that poly(4a) permeates ethanol preferentially (30). The a and R values of poly(4a) are close to those of poly(dimethylsiloxane). In contrast, other polyacetylenes exhibit practically no permselectivity or allow water to permeate preferentially. 

Cabasso I. Organic liquid mixtures separation by permselective polymer membranes. 1. Selection and characteristics of dense isotropic membranes employed in the pervaporation process. Ind Eng Chem Prod Res Dev 1983 22 313-319. 

A water-permselective membrane is useful in accelerating chemical reactions in which water has to be removed. For example, the reaction, R-OH + R -COOH R-OOC-R + H20, is accelerated by the removal of H20 from the reaction bath by pervaporation. Figure 6.40 shows that pervaporation to remove water substantially increases the conversion in esterification of oleic acid with ethanol.231 This technique has been used in the esterification of oleic acid with methanol and ethanol.232 The concept is also applicable in accelerating chemical reactions near the equilibrium state. 

Pervaporation is a method of separating liquid mixtures, especially ethanol-water mixtures, withtheuseofapolymermembrane. Poly(TMSP)membranehasprovedhighlypermselective for ethanol in the ethanol-water pervaporation. Thus, the separation factor, a(EtOH/ H2O), reaches 17 at 10 wt% ethanol in the feed. This value is similar to that for poly(dimethylsiloxane), a well-known ethanol-permselective membrane. In contrast, other substituted poly acetylenes are nonpermselective or rather water-permselective. On the basis of its excellent adsorptivity for organic compounds, poly(TMSP) has been applied to a regenerate column packing which removes trace organic solvents from water ". ... 

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. 

Separation of IPA and Water by Pervaporation. In the PV membrane separation unit, the water permselective membrane is used for dehydration. Water in the feed... 

As permselective barriers, synthetic membranes have been employed in a variety of applications, which include dialysis, mirofiltration, ultrafiltration, reverse osmosis, pervaporation, electrodialysis, and gas separation. Synthetic membranes also find special applications as permselective barriers for ion-spedfic electrodes, biosensors, controlled release, and tissue-culture growth. Some commercial polymer membranes are listed in Table 5.20. 

Matsuda, H., Yanagishita, H., Kitamoto, D. et al. 2001. Preparation of silicate pervaporation membrane with ethanol permselectivity by qa 2-step hydrothermal synthesis. Sep. Sci. Technol. 36(15) 3305-3310. 

BagneU L, CaveU K, Hodges A M, Man AW-H and Seen A J (1993), The use of cata-lyticaUy active pervaporation membranes in esterification reactions to simultaneously iucrease product yield, membraue permselectivity aud flux , / Membr 5 d, 85,291-299. 

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