Alcohol-water ternary azeotropes

Because ethyl alcohol forms an azeotrope with water that is a constant boiling mixture, i.e. both the ethyl alcohol and the water, in a ratio of 95/5, boil together at a temperature different than either separately. Other examples in this chapter are the ternary azeotrope, ethyl alcohol - water -benzene and DIPE - isopropyl alcohol - water. An azeotrope mentioned earlier is MEK - water - toluene rafFmate used for toluene extraction. 

LYL ALCOHOL. FORMS TERNARY AZEOTROPES WITH 17, 5 Water 81.4... 

The boiling point temperature rankings in Table 7.4 reveal that BuAc differs from IPAc in that the acetate is the highest boiler. This implies that we can remove the acetate from the column base. Moreover, the minimum boiling azeotrope between BuOH and H2O is also a heterogeneous one (Fig. 7.1ternary composition space. Figure l.ld also reveals that the two-liquid zone in the ternary system constitutes more than 50% of the composition space. However, the BuAc and IPAc systems also share a common characteristic a minimum boiling ternary azeotrope exists in the acetate/alcohol/water ternary system, and it is located inside the... 

Isoprene [78-79-5] (2-methyl-1,3-butadiene) is a colorless, volatile Hquid that is soluble in most hydrocarbons but is practically insoluble in water. Isoprene forms binary azeotropes with water, methanol, methylamine, acetonitrile, methyl formate, bromoethane, ethyl alcohol, methyl sulfide, acetone, propylene oxide, ethyl formate, isopropyl nitrate, methyla1 (dimethoxymethane), ethyl ether, and / -pentane. Ternary azeotropes form with water—acetone, water—acetonitrile, and methyl formate—ethyl bromide (8). Typical properties of isoprene are Hsted in Table 1. 

Esters of medium volatility are capable of removing the water formed by distillation. Examples are propyl, butyl, and amyl formates, ethyl, propyl, butyl, and amyl acetates, and the methyl and ethyl esters of propionic, butyric, and valeric acids. In some cases, ternary azeotropic mixtures of alcohol, ester, and water are formed. This group is capable of further subdivision with ethyl acetate, all of the ester is removed as a vapor mixture with alcohol and part of the water, while the balance of the water accumulates in the system. With butyl acetate, on the other hand, all of the water formed is removed overhead with part of the ester and alcohol, and the balance of the ester accumulates as a high boiler in the system. 

The checkers found this distillation to require about 15 hours. The distillate is the ternary azeotrope. It consists of 8.5% water, 9.2% allyl alcohol, and 82.2% benzene, and boils at 68.2°. The aqueous layer contains some allyl alcohol, though this loss is insignificant, since only about 15 ml. of the aqueous layer is obtained from each mole of lactic acid used. 

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 answer is like fighting fire with fire—another azeotrope is formed. When benzene is added to ethyl alcohol and water a ternary azeotrope, a mixture of three compounds that boil at a single temperature, is formed. The ternary azeotrope has the composition of 68% benzene, 24% ethyl alcohol, and 6% water, and it boils at a temperature lower than the binary ethyl alcohol/water azeotrope. So, when a little benzene is added to the ethyl alcohol/water mixture and then put through a distillation column, the ternary azeotrope, in a 68-24-6 composition will come off the top, talcing with it all the benzene, all the water, but just some of the ethyl alcohol. Out the bottom comes whats left, the rest of the ethyl alcohol in nearly pure form. Slick. None of this, by the way, is shown in Figure 13—2. 

The ternary azeotrope is liquefied, which causes it to phase separate. That is, it separates into two layers of liquid, one of benzene plus ethylene alcohol and one of water. The benzene/ethyl alcohol is drawn off and split in another column to create pure ethyl alcohol and a benzene recycle stream. 

Like ethyl alcohol, the absolute (99 %) grade is made by forming a ternary azeotrope. In this case, DIPE (di-isopropyl ether) is used to form the ternary with water and IPA. But the idea is exactly the same. 

Allyl alcohol is a colorless liquid having a pungent odor its vapor may cause severe irritation and injury to eyes, nose, throat, and lungs. It is also corrosive. Allyl alcohol is freely miscible with water and miscible with many polar organic solvents and aromatic hydrocarbons, but is not miscible with n-hexane. It forms an azeotropic mixture with water and a ternary azeotropic mixture with water and organic solvents. Allyl alcohol lias both bacterial and fungicidal effects. Properties of allyl alcohol are shown in Tabic 1. 

KEYES PROCESS. A distillation patcess involving the addition of benzene to a constant-boiling OS, alcohol-water solution to obtain ahsolute 1100 ) alcohol. On distillation, a ternary azeotropic mixture containing all iltree components leaves the lop of the column while anhydrous alcohol leaves the bottom. The azeotrope tvvhich separates into two layers) is redistilled separately for recovery and reuse ol the henzene and alcohol... 

Ethyl Acetate. The production of ethyl acetate by continuous esterification is an excellent example of the use of azeotropic principles to obtain a high yield of ester (2). The acetic acid, concentrated sulfuric acid, and an excess of 95% ethyl alcohol are mixed in reaction tanks provided with agitators. After esterification equilibrium is reached in the mixture, it is pumped into a receiving tank and through a preheater into the upper section of a bubblecap plate column (Fig. 5). The temperature at the top of this column is maintained at ca 80°C and its vapor (alcohol with the ester formed and ca 10% water) is passed to a condenser. The first recovery column is operated with a top temperature of 70°C, producing a ternary azeotrope of 83% ester, 9% alcohol, and 8% water. The ternary mixture is fed to a static mixer where water is added in order to form two layers and allowed to separate in a decanter. The upper layer contains ca 93% ethyl acetate, 5% water, and 2% alcohol, and is sent to a second recovery or ester-drying column. The overhead from this column is 95—100% ethyl acetate which is sent to a cooler and then to a storage tank. This process also applies to methyl butyrate. 

TABLE 4.8 Ternary Azeotropic Mixtures A. Ternary azeotropes containing water and alcohols... 

Ternary acetate-alcohol-water systems (propyl, butyl, amyl, hexyl) 178 Azeotrope problem ... 

Allyl alcohol forms an azeotropic mixture with water, and the mixture is a homogeneous liquid. Therefore, to obtain dry allyl alcohol, ternary azeotropic distillation and dehydration are required. 

The esterification with propanols raises the problem of breaking the azeotrope that the alcohol forms with water. The solution of this problem passes by the use of an entrainer forming a heterogeneous ternary azeotrope. Suitable entrainers are hydrocarbons and oxygenated species, as esters and ethers. The solution is in principle similar for u-propanol with the notable difference that the reaction rate is much slower for the last when using the same catalyst. 

Surprisingly, there is limited nonproprietary experimental data on methanol esterification with acetic acid (29). Studies have been confined to liquid-phase systems distant from equilibrium (30), in regions where hydrolysis is unimportant. A physical study of the ternary methanol—methyl acetate—water system is useful for design work (31). Methyl acetate and methanol form an azeotrope which boils at 53.8°C and contains 18.7% alcohol An apparent methanol—water azeotrope exists, boiling at 64.4°C and containing about 2.9% water. These azeotropes seriously complicate methyl acetate recovery. Methyl acetate is quite soluble in water, and very soluble in water—methanol mixtures, hence two liquid phases suitable for decanting are seldom found. 

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. 

Water is probably the easiest product to separate, because it is immiscible with many organic solvents and may often be removed from the reaction mixture as an azeotrope. As a typical example, consider the formation of an ethyl ester. The acid, somewhat more than an equivalent amount of ethyl alcohol, and about three times the volume of benzene are placed in a flask equipped with a Dean-Stark water trap (Fig. 5-21). The solution is heated to reflux, and the ternary azeotrope (7.4 per cent water, 18.5 per cent alcohol, and 74.1 per cent benzene, bp 65°) is vaporized and transferred to the trap. The liquid upon cooling separates into two layers— the lower, a solution of ethanol in water, and the upper, a solution of ethanol in benzene. The upper layer returns to the flask to be recycled, and the amount of water may be noted in the graduated lower part of the trap. As this fills up, the water may be withdrawn. The completion of the reaction is taken as the point at which water no longer collects in the trap. In order to ensure completion of the reaction, a small additional amount of ethanol may be added to the reaction flask and the heating continued. If water still does not collect in the trap, the reaction is complete, and an essentially quantitative yield of product should be obtained. 

Any higher alcohols that may have formed in the process from traces of higher olefins present in the propylene feed are absorbed from the azeotrope into mineral oil, in which isopropanol is insoluble. Pure isopropanol is obtained by ternary distillation of the cleaned water azeotrope with the appropriate proportion of added di-isopropyl ether. The ternary azeotrope (Table 19.2) is the top product from the column, and pure isopropanol is removed from the bottom (see Section 16.4 for related information). 

Azeotropic distillation involves either an embedded azeotrope, present in the feed mixture, or a contrived azeotrope, formed by the addition of an extraneous component called an entrainer. Benzene-water may be separated into high-purity benzene and the benzene-water azeotrope this is frequently practiced to remove water from benzene when very dry benzene is needed for chemical processing. More commonly encountered are distillation separations that are enhanced through the addition of an entrainer to form an azeotrope. Perhaps the best known separation of this type is the production of anhydrous ethanol from the ethanol-water azeotrope. Here, benzene is added as the entrainer, with the result that a low-boiling ternary azeotrope is formed between benzene, ethanol, and water. This permits the higher-boiling ethanol to be taken from the bottom of the column. The distillate condenses to a heterogeneous mixture of benzene and alcohol-water phases. 

An example of azeotropic distillation is the use of benzene to permit the complete separation of ethanol and water, which forms a minimum-boiling azeotrope with 95.6 weight percent alcohol. The alcohol-water mixture with about 95 percent alcohol is fed to the azeotropic distillation column with a benzene-rich stream added at the top. The bottom product is nearly pure alcohol, and the overhead vapor is a ternary azeotrope. The overhead vapor is condensed and separated into two phases. The organic layer is refluxed, and the water layer is sent to a benzene recovery column. All the benzene and some alcohol is taken overhead and sent back to the first column, and the bottoms stream is distilled in a third column to give pure water and some of the binary azeotrope. 

---