Pervaporation solvent dehydration

In coupled transport and solvent dehydration by pervaporation, concentration polarization effects are generally modest and controllable, with a concentration polarization modulus of 1.5 or less. In reverse osmosis, the Peclet number of 0.3-0.5 was calculated on the basis of typical fluxes of current reverse osmosis membrane modules, which are 30- to 50-gal/ft2 day. Concentration polarization modulus values in this range are between 1.0 and 1.5. 

The three current applications of pervaporation are dehydration of solvents, water purification, and organic/organic separations as an alternative to distillation. Currently dehydration of solvents, in particular ethanol and isopropanol, is the only process installed on a large scale. However, as the technology develops, the other applications are expected to grow. Separation of organic mixtures, in particular, could become a major application. Each of these applications is described separately below. 

Most of the early solvent dehydration systems were installed for ethanol dehydration. More recently pervaporation has been applied to dehydration of other solvents, particularly isopropanol used as a cleaning solvent. Dehydration of other solvents, including glycols, acetone and methylene chloride, has been considered. Schematics of pervaporation processes for these separations are shown in Figure 9.13. 

J. Bergdorf, Case Study of Solvent Dehydration in Hybrid Processes With and Without Pervaporation, in Proceedings of Fifth International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 362-382 (1991). 

In spite of all these hurdles, there are already industrial-scale applications of zeolite membranes for solvent dehydration [106] by pervaporation plants using tubular zeolite A membranes with 0.0275 m of permeation area each (see Section 10.2.3). Li et al. [280] have prepared large area (0.0260 m ) ZSM-5 membranes on tubular a-alumina supports. This work is also interesting from the industrial point of view because the authors used inexpensive n-butylamine as template. Indeed, the cost required for industrial modules, on a general basis, is still far from clear. However, it must be noted that most of the costs can be ascribed to the module, and only 10%-20% to the membrane itself [3]. This underlines again the importance of preparation of zeolite membranes on cheaper, alternative supports that can also pack more area per unit volume. 

Solvent dehydration Breaking azeotropes Batch and continuous pervaporation, vapor permeation—often coupled with distillation Hydrophilic, e.g., PVA polymer composite, ceramic Well-established... 

No introduction of additional chemicals—complete solvent dehydration by pervaporation membranes irrespective of azeotrope formation—no possibility of contamination. 

These developments will have a wide impact. Reaction enhancement will be a major beneficiary, but a look at the simpler field of solvent dehydration shows that the innovation process is very application dependant. Pervaporation (with vapor permeation) is progressively displacing other techniques in solvent dehydration. Replacing entrainer distillation for drying ethanol and isopropanol, pervaporation at initial stages is always now preferred to techniques, where a third component must be added to shift equlibria. The handling of entrainers and/or calcium chloride or caustic with the attendant environmental risks and costs is no longer a viable option. 

This application is based on the hydrophilicity of the ion exchange membrane. Though it is not essential to use an ion exchange membrane,219 they show excellent performance in pervaporation for dehydration of organic solvents. Pervaporation is the separation of solvents on the basis of their different affinities for the membrane and different permeation speeds through the membrane phase. The system consists of a liquid mixture to be separated, which contacts one side of the membrane, and a gas phase to permeate under reduced pressure, which is on the other side of the membrane (Figure 6.37). Membrane performance is evaluated by a permeability coefficient (flux) and separation factor (selectivity coefficient). The permeability coefficient, Q, is the permeated solvent through the membrane per unit area and unit time (kgm 2 h1). When a mixed solvent composed of components A and B is separated, the separation factor, a, is defined as... 

Because pervaporation is suitable for separation azeotropic mixtures, such as dehydration of an azeotropic mixture of ethanol-water (ca. 94%), economic comparison of the process with distillation has been reported.228 Besides separation of azeotropic mixtures of organic solvents, dehydration of nitric acid (azeotropic point ca. 68 wt.%) has been tried using a perfluorocarbon ion exchange membrane for the chlor-alkali process nitric acid is concentrated up to... 

Pervaporation is a contraction of the terms permeation and evaporation because the feed is a liquid, and vapor exits the membrane on the permeate side. Pervaporation is a membrane process for liquid separation, and today, it is considered as a basic unit operation for the separation of organic-organic liquid mixtures because of its efficiency in separating azeotropic and close-boiling mixtures, isomers, and heat-sensitive compounds. It allows separations of some mixtures that are difficult to separate by distillation, extraction, and sorption. Pervaporation is one such type of membrane separation process with a wide range of uses such as solvent dehydration and separation of organic mixtures. When a membrane is in contact with a liquid mixture, one of the components can be preferentially removed from the mixture due to its higher affinity and quicker diffusivity in the membrane. 

Zeolite membranes show high thermal stability and chemical resistance compared with those of polymeric membranes. They are able to separate mixtures continuously on the basis of differences in the molecular size and shape [18], and/or on the basis of different adsorption properties [19], since their separation ability depends on the interplay of the mixture adsorption equilibrium and the mixture. Different types of zeolites have been studied (e.g. MFI, LTA, MOR, FAU) for the membrane separation. They are used still at laboratory level, also as catalytic membranes in membrane reactors (e.g. CO clean-up, water gas shift, methane reforming, etc.) [20,21]. The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation [22], Some other pervaporation plants have been installed since 2001, but no industrial applications use zeolite membranes in the GS field [23]. The reason for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. 

Pervaporation np Ap 1/v Separation of azeotropic mixtures solvent dehydration... 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

To attain this goal, a pervaporation technique has been proposed, using a PVA composite membrane, made by casting of a mixture of PVA aqueous solution and a GA one on a polyethersulfone (PES) porous support, solvent evaporation and thermic curing [72], Excellent dehydration performance has been obtained (separation factor 320 and permeation flux 1.5 kg m 2 h 1, for 90 wt% TFEA in the feed and 80 °C). 

Pervaporation (PV) is a membrane-based process used to separate aqueous, azeotropic solvent mixtures. This is done using a hydrophihc, non-porous membrane that is highly selective to water. Figure 3.9 shows a typical PV system that produces a dehydrated solvent stream (retentate) from a solvent/water feed. 

A third possibility, illustrated in Figure 9.7(c), is to sweep the permeate side of the membrane with a counter-current flow of carrier gas. In the example shown, the carrier gas is cooled to condense and recover the permeate vapor, and the gas is recirculated. This mode of operation has little to offer compared to temperature-gradient-driven pervaporation, because both require cooling water for the condenser. However, if the permeate has no value and can be discarded without condensation (for example, in the pervaporative dehydration of an organic solvent with an extremely water-selective membrane), this is the preferred mode of operation. In this case, the permeate would contain only water plus a trace of organic solvent and could be discharged or incinerated at low cost. No permeate refrigeration is required [36],... 

Figure 9.13(b) shows the use of pervaporation to dry a chlorinated solvent, in this case water-saturated ethylene dichloride containing 2000 ppm water. A poly(vinyl alcohol) dehydration membrane can easily produce a residue containing less than 10 ppm water and a permeate containing about 50 wt% water. On condensation the permeate vapor separates into two phases, a very small water... 

Separation of isopropanol (IPA) and water by pervaporation has also reached production scale. Much of the current capacity is devoted to azeotrope breaking and dehydration during IPA synthesis. Recently, anhydrous isopropanol has become a preferred drying solvent in the semiconductor industry, where chip wafers are first washed with ultrapure water, then rinsed with the alcohol to promote uniform drying. The water-laden isopropanol generated can be conveniently reused after dehydration by pervaporation. Unlike with pressure-driven membrane processes such as RO or UF, particulates and nonvolatile substances such as salts are not carried over during pervaporation. This helps maintain the effectiveness of contamination control. 

Solvent Recovery The largest current industrial use of pervaporation is the treatment of mixed organic process streams that have become contaminated with small (10 percent) quantities of water. Pervaporation becomes very attractive when dehydrating streams down to less than 1 percent water. The advantages result from the small operating costs relative to distillation and adsorption. Also, dis-... 

Fleming. H.L.. 1990. Dehydration and recovery of organic solvents by pervaporation. in Proc. 8th Annual BBC Mcmbr. Planning Conf.. Cambridge. MA. USA, p. 293. 

Asada T. Dehydration of organic solvents. Some acmal results of pervaporation plants in Japan. In Backish R. ed.. Proceedings of the Third International Conference on Pervaporation Processes in Chemical Industry. Nancy, France, September 1988 Englewood, NJ Bakish Materials Corporation, 1988 379-386. 

Burshe MC, Netke SA, Sawant SB, Joshi JB, and Pangarkar VG. Pervaporative dehydration of organic solvents. Sep. Sci. Tech. 1997 32(8) 1335-1349. 

Pervaporation and vapor permeation are typical membrane processes with high application potential in chemical industry due to their high efficiency in the separation or the dehydration of organic solvents. Developed initially with organo-polymeric... 

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