Ethanol/water mixture, azeotropes

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. 

Absolute (100%) ethanol is often made by adding benzene to the ethanol -water binary azeotrope (two components), to make a ternary azeotrope (three components). This ternary alcohol-water-benzene (18.5 7.4 74.1) azeotrope comes over until all the water is gone, followed by a benzene-ethanol mixture. Finally, absolute ethanol gets its chance to appear, marred only slightly by traces of benzene. 

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. 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

By the reaction of concentrated solutions of nickel acetate and Hacac in an ethanol-water mixture the bis-aqua adduct [Ni(acac)2(H20)2] is obtained. An improved synthesis of the same compound has been devised starting from NiO(OH) which was reduced with Hacac at room temperature.1549 The green anhydrous Ni(acac)2 derivative is obtained by azeotropic distillation with toluene of the aqua complex or by its sublimation in vacuo. 

Fig. 4.7 shows a temperature versus composition diagram for an ethanol-water mixture. It is slightly idealized to make a pedagogical point. If one starts with initial composition Q in the liquid (point A in Fig. 4.7), the vapor mixture will have the composition B. If this vapor is condensed, the liquid mixture will have composition C2 at point C the vapor phase will have the composition C3 at point D. Further cycles will achieve ever smaller increases in ethanol liquid content until the azeotrope (constant-boiling) composition is reached at 95.6 mass% and 78.2°C... 

As it is well known, fermentation processes generally produce aqueous solutions of ethanol, the separation of which necessitates a series of distillation processes. Distillation, however, is a very expensive operation and in addition, ethanol-water mixture forms an azeotrope which further increases the cost of purification. Therefore, novel separation methods must be investigated to make the process more feasible and economical. 

FIG. 13-52 Azeotropic distillation tower for distillation of an ethanol-water mixture using benzene as a mass separating agent. [Ajier Prokopakis and Seider (op. cit.).]... 

A good example of separation on the basis of affinity is the separation of alcohol/ water mixtures using a hydrophobic, silicalite membrane. Pervaporation of an ethanol/ water mixture through such a membrane resulted the removal of the alcohol from the mixture [16]. The separation selectivities achieved are between 10 and 60, depending on temperature and the alcohol content in the feed. In this way azeotropes can be broken. The reason for this is that the principle of separation, namely, differences in adsorptive behavior, is different from separation based on vapor pressure differences, used in distillation. Another example of such a separation is the pervaporation of an acetic acid/water mixture through a silicalite membrane, resulting in the removal of acetic acid [17]. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

By far, the most common prejudice (sometimes completely overlooking the other alternative) is to propose the most volatile composition (low-boiling node) as distillate (Fig. 28). In this region containing the decant aqueous layer, the lightest composition is the ternary azeotrope. With enough stages, all of the azeotropic composition in the decant aqueous layer may be recovered, and the underflow will contain only a binary ethanol-water mixture. If the distillate of... 

Working with azeotropes - not all liquid mixtures can be separated by distillation. In some cases an azeotrope, a mixture of the liquids of definite composition, which boils at a constant temperature, is formed. For example, an azeotrope containing 95.5% ethanol and 4.5% water boils at 78.15 C, which is below the boiling point of pure ethanol (78.3 C). Therefore 100% ethanol cannot be obtained by distillation of ethanol-water mixtures, even though their boiling points are about 22 C apart. 

Fig. 7.27 presents the separation of the ethanol-water mixture using ethylene glycol. Column C-1 removes the excess of water and supplies a mixture with the composition close to azeotrope with 90% mol ethanol. The top distillate is sent to the second column... 

Combinations of distillation and pervaporation (membrane separation with a vapor permeate stream) have been applied to many separation problems. One example is the separation of ethanol-water mixtures. Ethanol and water form an azeotrope and therefore conventional distillation is not feasible. Using a combination including pervaporation, pure ethanol and water can be obtained as can be seen from Fig. 3.2-10. Water can be obtained from the bottom of the distillation column. The distillate stream consists of the azeotropic mixture which is fed into the pervaporation unit The highly selective polyvinyl alcohol membrane is capable of separating water from ethanol. Pure ethanol can be obtained from the retentate, and the permeate stream, which consists of ethanol and water, is recycled into the distillation column. 

Since vapor and liquid have the same composition at a homogeneous azeotrope, the occurrence of an azeotrope prevents a separation by simple distillation. Once an azeotrope forms on a stage or plate of a distillation column, no further separation occurs and the azeotropic mixture becomes one of the product streams. For example, simple distillation cannot be used to extract pure-grain alcohol from ethanol-water mixtures because an azeotrope forms at atmospheric pressure. 

Pandey and Shahi [98] prepared functionalized silica (sodium 2-formylbenzenesulfonatepolysiloxane [SBAPTS])-NSBC (modified CS derivative A,0-sulfonic acid benzyl CS, Figure 16.16) hybrid membranes by sol-gel technique followed by cross-linking using formaldehyde for pervaporation separation of water/ethanol azeotrope. The prepared hybrid membrane was assessed to be very suitable for the separation of water from azeotrope of water-ethanol with 5282 selectivity and 0.59 L/m h total flux at 30°C in ethanol/ water mixture (90 wt.%). 

B = 4.0 X 10 mmHg. 9.22 Ethanol water mixture is a nonidetil solution that exhibits positive deviations from Raoult s law. Therefore, this system has a minimum boihng point and will form a low boiling azeotrope that cemnot be separated by fractioneil distillation. 9.24 1.3X10 g. 9.26 128 g. 9.28 0.59 m. 

The fermentation is inhibited by its end product ethanol it is not possible to prepare solutions containing more than 10-15% ethanol by this method. More concentrated ethanol can be isolated by fractional distillation. Ethanol and water form an azeotropic mixture consisting of 95% ethanol and 5% water by weight, which is the most concentrated ethanol that can be obtained by fractionation of dilute ethanol-water mixtures. 

An example of heterogeneous azeotropic distillation is the system ethanol and water with benzene as entrainer (Figure 3.3.20). In a first column (without entrainer, column I in Figure 3.3.20b), the binary ethanol-water mixture is separated by normal distillation. An azeotrope with about 90 mol.% ethanol (96wt%) leaves the column on top (A) while water forms the bottom product. The azeotrope is fed to a second column where benzene (recycle of a phase rich in benzene from the separator of the top products of column II and III) is added as entrainer. A new low-boiling heterogeneous azeotrope (B) leaves column II as distillate, and pure ethanol remains as bottom product. After condensation, the heterogeneous azeotrope separates into two phases rich in either benzene (C) or water (D). The phase rich in benzene is recycled back into column II while the phase rich in water is reconditioned in a third column by distillation. The small amount of benzene is separated as top product (azeotrope B), and a mixture of ethanol and water (E) is recycled into column I. 

Sometimes the new azeotrope which is formed contains all three constituents. The dehydration of ethanol-water mixture with benzene as added substance is an example. Dilute ethanol-water solutions can be continuously rectified to give at best mixtures containing 89.4 mole percent ethanol at atmospheric pressure, since this is the composition of the minimum-boiling azeotrope in the binary system. By introducing benzene into the top of a column fed with an ethanol-water mixture, the ternary azeotrope containing benzene (53.9 mol %), water (23.3 mol %), ethanol (22.8 mol %), boiling at 64.9°C, is readily separated from the ethanol (bp — 78.4 C), which leaves as a residue product. In this case also the azeotropic overhead product separates into two liquid layers, one rich in benzene which is returned to the top of the column as reflux, the other rich in water which is withdrawn. Since the latter contains appreciable quantities of both benzene and ethanol, it must be rectified separately. The ternary azeotrope contains nearly equal molar proportions of ethanol and water, and consequently dilute ethanol-water solutions must be given a preliminary rectification to produce substantially the alcohol-rich binary azeotrope which is used as a feed. 

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. 

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