Azeotropes ethanol-water

The potential of ILs in fhe field of exfracfive distillation was shown in Refs 126 and 146 for the separation of THF-wafer and ethanol-water azeotropes using [C2Qlm][BFJ. This IL easily breaks the azeotropic ethanol-water phase behavior by interacting selectively with water. 

Go back to the temperature-mole fraction diagram for the isopropyl alcohol-isobutyl alcohol system (Fig. 175). The composition of the vapor is always different from that of the liquid, and we can separate the two compounds. If the composition of the vapor is the same as that of the liquid, that separation is hopeless. Since we ve used the notions of an ideal gas in deriving our equations for the liquid and vapor compositions (Clausius-Clapeyron, Dalton, and Raoult), this azeotropic behavior is said to result from deviation from ideality, specifically deviations from Raoult s law. Although you might invoke certain interactive forces in explaining nonideal behavior, you cannot predict azeotrope formation a priori. Very similar materials form azeotropes (ethanol-water). Very different materials form azeotropes (toluene-water). And they can be either minimum-boiling azeotropes or maximum-boiling azeotropes. 

Let s examine the case of breaking the azeotrope ethanol/water with ethylene glycol. The flowsheet was given in Fig. 7.28. The RCM plot is given in Fig. 9.13-left. It can be observed that both products are saddles, which is typical for extractive distillation. 

The azeotrope ethanol/water can be broken by azeotropic distillation with benzene. Examine by simulation the feasibility of a two-column alternative. Study the influence of different design elements on the achievable purity. 

The ethanol column (C-1) has practically only stripping zone. Fig. 9.31-left shows composition profile both for liquid and vapour phase. The examination of the composition profiles highlights the role of the entrainer. In the zone close to the top the benzene extracts the ethanol in the liquid phase, and as a result increases the volatility of water, so that on lower stages the water is completely removed. In the lower part practically only the binary ethanol/benzene remains. The distillation trajectory starts from the ternary azeotrope, goes along the ethanol/ benzene saddle and terminates in the ethanol vertex. Because the boiling point of the azeotrope ethanol-water is close to the pure ethanol, the profile could easily jump to the ethanol/water azeotrope. Consequently, the design and operation of the column (C-1) is very sensitive. 

The relationship (6.3.182) between the separation factor Uij for the pervaporation process and the separation factors a and a . for the hypothetical thermodynamically equivalent process for Figure 6.3.30(b) can be conceptually illustrated via Figure 6.3.31(a). This figure is based on a similar one from Wijmans and Baker (1993) showing vapor-Uquid equilibrium of the azeotropic ethanol-water system at 60 °C. The plot illustrates total permeate pressure vs. alcohol concentration in the liquid phase (both feed liquid and permeate vapor). Feed liquid having molar concentrations of Cif and Cjf (mole fractions are in equi-... 

Wash with rectified spirits (azeotrope of Ethanol + water, also known as "grain alcohol") to dissolve the Safrole, leaving the Phellandrene behind. Of course, one should use only the amount of alcohol necessary to dissolve the expected yield of Safrole. 1 1 seems to work, but could be an excessive amount. 

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. 

Fig. 15. Isobaric vapor—liquid—liquid (VLLE) phase diagrams for the ethanol—water—benzene system at 101.3 kPa (D-D) representHquid—Hquid tie-lines (A—A), the vapor line I, homogeneous azeotropes , heterogeneous azeotropes Horsley s azeotropes, (a) Calculated, where A is the end poiat of the vapor line and the numbers correspond to boiling temperatures ia °C of 1, 70.50 2, 68.55 3, 67.46 4, 66.88 5, 66.59 6, 66.46 7, 66.47, and 8, the critical poiat, 66.48. (b) Experimental, where A is the critical poiat at 64.90°C and the numbers correspond to boiling temperatures ia °C of 1, 67 2, 65.5 3, 65.0 ...

Fig. 18. Separation of ethanol from an ethanol—water—benzene mixture using benzene as the entrainer. (a) Schematic representation of the azeo-column (b) material balance lines where I denotes the homogeneous and the heterogeneous azeotropes D, the end points of the Hquid tie-line and A, the overhead vapor leaving the top of the column. The distillate regions, I, II, and III, and the boundaries are marked. Other terms are defined in text.

Fig. 19. Separation of ethanol and water from an ethanol—water—benzene mixture. Bottoms and are water, B is ethanol, (a) Kubierschky three-column sequence where columns 1, 2, and 3 represent the preconcentration, azeotropic, and entrainer recovery columns, respectively, (b) Material balance lines from the azeotropic and the entrainer recovery columns, A and E, respectively, where represents the overall vapor composition from the azeo-column, 2 1SP Hquid in equiUbrium with overhead vapor composition from the azeo-column, Xj, distillate composition from entrainer...

In summary, for systems of the ethanol—water—benzene type, the three most attractive sequences for carrying out azeotropic distHlation are the Kubierschky three-column sequence, the Kubierschky two-column sequence, and the Ricard-AHenet three-column sequence. For each of these there is the added possibHity of putting a Hquid—Hquid extraction step after the azeo-column. 

Podebush Sequence forPthanol—Water Separation. When ethyl acetate is used as the entrainer to break the ethanol—water azeotrope the residue curve map is similar to the one shown in Figure 21d, ie, the ternary azeotrope is homogeneous. Otherwise the map is the same as for ethanol—water—benzene. In such... 

Three types of binary equilibrium cui ves are shown in Fig. 13-27. The y-x diagram is almost always plotted for the component that is the more volatile (denoted by the subscript 1) in the region where distillation is to take place. Cui ve A shows the most usual case, in which component 1 remains more volatile over the entire composition range. Cui ve B is typical of many systems (ethanol-water, for example) in which the component that is more volatile at lowvalues of X becomes less volatile than the other component at high values of X. The vapor and liquid compositions are identical for the homogeneous azeotrope where cui ve B crosses the 45° diagonal. A heterogeneous azeotrope is formed with two liquid phases by cui ve C,... 

In Region II, the high- and low-boihng nodes are MIPK and the MEK-water azeotrope, respectively. The more complicated cyclo-hexane-ethanol-water system (Fig. 13-58c) has three separatrices and three regions, all of which share the ternaiy azeotrope as the low-boiling node. 

Ethanol-water Minimum boiling azeotrope None Alternative to extractive distillation,... 

Ethanol-water Minimum-hoiling azeotrope Ethylene glycol, acetate salts for salt process Alternative to azeotropic distillation, pressure swing distillation... 

FIG. 13-108 Initial steady state for dynamic azeotropic distillation of ethanol-water with benzene. 

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. 

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. 

Water and ethanol form a low boiling point azeotrope. So, water cannot be completely separated from ethanol by straight distillation. To produce absolute (100 per cent) ethanol it is necessary to add an entraining agent to break the azeotrope. Benzene is an effective entrainer and is used where the product is not required for food products. Three columns are used in the benzene process. Column 1. This column separates the ethanol from the water. The bottom product is essentially pure ethanol. The water in the feed is carried overhead as the ternary azeotrope of ethanol, benzene and water (24 per cent ethanol, 54 per cent benzene, 22 per cent water). The overhead vapour is condensed and the condensate separated in a decanter into, a benzene-rich phase (22 per cent ethanol, 74 per cent benzene, 4 per cent water) and a water-rich phase (35 per cent ethanol, 4 per cent benzene, 61 per cent water). The benzene-rich phase is recycled to the column as reflux. A benzene make-up stream is added to the reflux to make good any loss of benzene from the process. The water-rich phase is fed to the second column. 

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. 

The acid in this step clearly functions as a catalyst (acids are known to catalyze the esterification of silanols) (21). The toluene, when employed, serves to drive the esterification to completion by forming a water-toluene-ethanol azeotrope (12% water) (22). It also renders the reaction solution a poor solvent for the byproduct salts and thus facilitates the separation of these salts (the pentane, when used, serves this same function). 

The ethanol-water azeotrope (95%ethanol-5%water) is an example of a minimum boiling azeotrope. Its boiling point is lower than that of the components (Fig. 143). If you ve ever fermented anything and distilled the results in the hopes of obtaining 200 proof (100%) white lightning, you d have to content yourself with getting the azeotropic 190 proof mixture, instead. Fermentation usually stops when the yeast die in their own 15% ethanol solution. At room temperature, this is point A on our phase diagram. When you heat the... 

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