Azeotropic pervaporation

Alcohol is a clean energy source that can be produced by the fermentation of biomass. However, it needs to be highly concentrated. In general, aqueous alcohol solutions are concentrated by distillation, but an azeotrope (96.5 wt% ethanol) prevents further separated by distillation. Pervaporation, a membrane separation technique, can be used for separation of these azeotropes pervaporation is a promising membrane technique for the separation of organic liquid mixtures such as azeotropic mixtures [34] or close-boiling point mixtures. 

Pervaporation is a membrane process in which a liquid is maintained on the feed side of a membrane and permeate is removed as a vapor on the downstream side of the membrane. Pervaporation is used, because of its low energy consumption and low cost, to separate dissolved organics from water, purify waste water or volatile chemicals, and break azeotropes. Pervaporation plants range from processing a few grams per hour up to thousands of tons per year. For waste water treatment flow of less than 76 L min pervaporation is more cost-effective than other treatment options, such as chemical oxidation, ultraviolet destruction, air stripping followed by carbon adsorption, steam stripping, or distillation/incineration [262]. 

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. 

Because a preconcentration step is probably needed to make the final sequence more economical, it is logical to start with the opportunistic separation. This separation produces one of the products, pure water, as the underflow and a concentrated distillate appropriate for feed into either strategic separation. Arbitrarily choosing pervaporation first, the retentate has a composition on the 2-propanol-rich side of the azeotrope, whereas the permeate is pure water. No further strategic separations are required. 

Of these five methods all but pressure-swing distillation can also be used to separate low volatiUty mixtures and all but reactive distillation are discussed herein. It is also possible to combine distillation and other separation techniques such as Hquid—Hquid extraction (see Extraction, liquid-liquid), adsorption (qv), melt crystallization (qv), or pervaporation to complete the separation of azeotropic mixtures. 

An important characteristic of pervaporation that distinguishes it from distillation is that it is a rate process, not an equilibrium process. The more permeable component may be the less-volatile component. Perv oration has its greatest iitihty in the resolution of azeotropes, as an acqiinct to distillation. Selecting a membrane permeable to the minor corTiponent is important, since the membrane area required is roughly proportional to the mass of permeate. Thus pervaporation devices for the purification of the ethanol-water azeotrope (95 percent ethanol) are always based on a hydrophihc membrane. 

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... 

Volkov (1994) has given a state-of-the-art review on pervaporation. A number of industrial plants exist for dehydration of ethanol-water and (.vwpropanol-water azeotropes, dehydration of ethyl acetate, etc. There is considerable potential in removing dissolved water from benzene by pervaporation. The recovery of dis.solved organics like CH2CI2, CHCI3, CCI4, etc. from aqueous waste streams also lends itself for pervaporation and pilot plants already exist. 

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. 

Membranes can also be used to alter the vapor-liquid equilibrium behavior and allow separation of azeotropes. The liquid mixture is fed to one side of the membrane, and the permeate is held under conditions to maintain it in the vapor phase. Most separations use hydrophyllic membranes that preferentially pass water rather than organic material. Thus, pervaporation is commonly used for the dehydration of organic components. 

One of the most common liquid-liquid separation methods investigated is pervaporation. This technique is a low-energy alternative for the separation of mixtures that are difficult and expensive to separate by traditional means such as azeotropes and isomers [35]. In pervaporation a liquid feed is vaporized as ittravels... 

Another potential application for zeolite/polymer mixed-matrix membranes is the separation of various liquid chemical mixtures via pervaporation. Pervapora-tion is a promising membrane-based technique for the separation of liquid chemical mixtures, especially in azeotropic or close-boihng solutions. Polydime thy 1-siloxane (PDMS), which is a hydrophobic polymer, has been widely used as the continuous polymer matrix for preparing hydrophobic mixed-matrix membranes. To achieve good compatibility and adhesion between the zeolite particles and the PDMS polymer, ZSM-5 was incorporated into the PDMS polymer matrix, the resulting ZS M -5/ P DM S mixed-matrix membranes showed simultaneous enhancement in selectivity and flux for the separation of isopropyl alcohol from water. It was demonstrated that the separation performance of these membranes was affected by the concentration of the isopropyl alcohol in the feed [96]. 

Pervaporation forms an interesting alternative to separate water from the reaction mixture instead of distillation, especially in systems with azeotropes or low boiling reactants. Some examples of pervaporation-assisted esterifications are given in Tab. 13.3. Additionally, pervaporation can also be used for the production of polycondensation esters [36, 37]. 

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

Fig. 4. Distillation of aqueous isopropyl alcohol (IPA), where A is the azeotrope, combined with (a) pervaporation and (b) adsorption.

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. 

Pervaporation is a membrane separation process where the liquid feed mixture is in contact with the membrane in the upstream under atmospheric pressure and permeate is removed from the downstream as vapor by vacuum or a swept inert gas. Most of the research efforts of the pervaporation have concentrated on the separation of alcohol-water system [1-20] but the separation of acetic acid-water mixtures has received relatively little attention [21-34]. Acetic acid is an important basic chemical in the industry ranking among the top 20 organic intermediates. Because of the small differences in the volatility s of water and acetic acid in dilute aqueous solutions, azeotropic distillation is used instead of normal binary distillation so that the process is an energy intensive process. From this point of view, the pervaporation separation of acetic acid-water mixtures can be one of the alternate processes for saving energy. 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Several hundred plants have been installed for the dehydration of ethanol by pervaporation. This is a particularly favorable application for pervaporation because ethanol forms an azeotrope with water at 95 % and a 99.5 % pure product is needed. Because the azeotrope forms at 95 % ethanol, simple distillation does not work. A comparison of the separation of ethanol and water obtained by various pervaporation membranes and the vapor-liquid equilibrium line that controls separation obtained by distillation is shown in Figure 9.9 [40], The membranes... 

The third application area for pervaporation is the separation of organic/organic mixtures. The competitive technology is generally distillation, a well-established and familiar technology. However, a number of azeotropic and close-boiling organic mixtures cannot be efficiently separated by distillation pervaporation can be used to separate these mixtures, often as a combination membrane-distillation process. Lipnizki et al. have recently reviewed the most important applications [53],... 

Figure 9.20(b) illustrates the use of pervaporation with two distillation columns to break a binary azeotrope such as benzene/cyclohexane. The feed is supplied at the azeotropic composition and is split into two streams by the pervaporation unit. The residue stream, rich in cyclohexane, is fed to a distillation column that produces a pure bottom product and an azeotropic top stream, which is recycled to the pervaporation unit. Similarly, the other distillation column treats the benzene-rich stream to produce a pure benzene product and an azeotropic mixture that is returned to the pervaporation unit. 

P. Aptel, N. Challard, J. Cuny and J. Neel, Application of the Pervaporation Process to Separate Azeotropic Mixtures, J. Membr. Sci. 1, 271 (1976). 

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