Azeotropic distillation 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 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 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],... 

Van HV, Vanden AL, Buekenhoudt A, Dotremont C, and Ley sen R. Economic comparison between azeotropic distillation and different hybrid systems combining distillation with pervaporation for the dehydration of isopropanol. Sep. Purific. Tech. 2004 37(l) 33-49. 

Distillation is a unit operation based on the relative volatility of the components in the mixture this unit operation encounters many problems in the separation of azeotropic mixmres, close boiling point mixtures, isomer separation, and removal of thermally sensitive compounds. Azeotropic distillation is an alternative however, it adds a third component to break the azeotrope and this solution it is not environmental friendly and cost effective. Pervaporation could overcome these drawbacks and it is presented as a solution for the separation of these kinds of mixtures. In this section, we discuss the separation of different azeotropic and close boiling point mixmres. 

Debottlenecking distillations Breaking azeotropes Continuous pervaporation and vapor permeation Various Growing... 

Methanol forms azeotropes with many substances, particularly esters, and often cannot be recovered from spent solvents or from reaction mixtures by simple distillation. Pervaporation provides a simple way to break these azeotropes. Used alone or in combination with distillation, such units provide an economical and reliable route to recover or remove methanol (Fig. 26). 

Acetone Does not form an azeotrope with water but requires a large reflux when distilled. Pervaporation is ideal for final dehydration or for de-bottlenecking existing distillation systems. Acetonitrile Forms an azeotrope with water fully miscible with water. Can easily be dehydrated to low water concentrations. Avoids messing with contaminated salt solution andre-distilla-tion of salt-contaminated organic phase. 

Of course, there are many different feasible designs. If a higher permeate concentration is desired, the ethanol-water feed mixture in Example 17-10 would probably be concentrated in an ordinary distillation column to a concentration much closer to the concentration of the azeotrope (see Figure 17-14B). The retentate would typically be recycled to the distillation column. Pervaporation, azeotropic distillation rchapter 8 and adsorption tChapter 181 are all used commercially to break the ethanol-water azeotrope. 

As we saw in the previous examinations among technologies 1, 2, and 3 for the removal of water fiom the fermentation broth, the most promising is absorption, then azeotropic distillation, and finally pervaporation. Process synthesis is also capable of... 

Figure VI - 27. Schematic drawing of a hybrid distillation/pervaporation for the separation of a SO/SO azeotropic mixture.

There are many important industrial applications of azeotropic separations, which employ a variety of methods. In this book we discuss several of these chemical systems and demonstrate the application of alternative methods of separation. The methods presented include pressure-swing distillation, azeotropic distillation with a light entrainer, extractive distillation with a heavy entrainer (solvent), and pervaporation. The chemical systems used in the numerical case studies included ethanol-water tetrahydrofuran (THF)-water, isopropanol-water, acetone-methanol, isopentane-methanol, n-butanol-water, acetone-chloroform, and acetic acid-water. Economic and dynamic comparisons between alternative methods are presented for some of the chemical systems, for example azeotropic distillation versus extractive distillation for the isopropanol-water system. 

Many papers point out the economic advantages of pervaporation over conventional processes. A quantitative economic comparison between azeotropic distillation and hybrid column-pervaporation is presented by Guerreri," whose results show higher capital cost but much lower energy cost for the ethanol-water separation. A review of industrial applications of pervaporation, coupled with either distillation columns or chemical reactors, is... 

Efficient enzymatic conversion can be achieved even though most of the reactants are present as solids, provided that there is a liquid phase in which the reaction can occur. This approach has been successfully used for carbohydrate ester synthesis with synthesis of glucose esters of fatty acids between C12 and C18 as typical examples [34]. It is important that the substrates dissolve during the reaction, and often the products precipitate as they are formed, which can be an advantage due to a favourable effect on the equilibrium position. Candida antarctica lipase B is an efficient catalyst in this system and solvents used (in moderate amounts) include ethyl methyl ketone, acetone or dioxane. In order to increase the ester yield, water formed in the reaction can be removed by azeotropic distillation and the solvent (e.g. ethyl metyl ketone) can after condensation be dried by pervaporation, giving a practically useful complete process [35]. 

It is quite common in the chemical process industry to use various kinds of salts to dehydrate complex mixtures of solvents to avoid azeotropic distillation. These salts can cause many serious problems in the whole process. They can also cause a msting problem in the whole piping system. The disposal of such toxic wastewater is a colossal issue for the chemical industry. The AZEO SEP pervaporation hybrid system can eliminate the use of salt, and it can breakdown the azeotropes for further process or use. 

Fig. 18.4-2. Distillation versus pervaporation. The vapor-liquid equilibrium is given by the dotted line and the pervaporation into vacuum is given by the solid line. The data are for ethanol-water using a polyvinyl alcohol membrane. Note there is no azeotrope for pervaporation.

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. 

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