Anhydrous ethanol azeotropic distillation

Fig. 9. Extractive distillation sequence cost as a function of the feed ratio for the production of anhydrous ethanol from azeotropic ethanol using ethylene glvcol at reflux ratios of A, 1.15 r O, 1.2 r and 1.3 r (39). Point A represents a previously pubhshed design for the same mixture (37).

Ethanol [64-17-5] M 46.1, b 78.3 , d 0.79360, d 0.78506, n 1.36139, pK 15.93. Usual impurities of fermentation alcohol are fusel oils (mainly higher alcohols, especially pentanols), aldehydes, esters, ketones and water. With synthetic alcohol, likely impurities are water, aldehydes, aliphatic esters, acetone and diethyl ether. Traces of benzene are present in ethanol that has been dehydrated by azeotropic distillation with benzene. Anhydrous ethanol is very hygroscopic. Water (down to 0.05%) can be detected by formation of a voluminous ppte when aluminium ethoxide in benzene is added to a test portion. Rectified... 

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. 

The liquid product streams are fed to a distillation system to remove the light impurities and to recover the ethanol as a 95% volume ethanol—water azeotrope. To produce anhydrous ethanol, the ethanol—water azeotrope is fed to a dehydration system. 

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. 

FIGURE 12.15 Flow diagram of azeotropic distillation system to obtain anhydrous ethanol. 

Consider producing anhydrous ethanol from a feed stream containing 85 mole-% C2H5OH and the remainder water. Since this system exhibits a minimum-boiling azeotrope at 78.1°C containing 89.43 mole-% CjHjOH (at 1 atm), the separation is impossible by ordinary distillation. 

An organic synthesis requires 50 mL of anhydrous ethanol as a solvent. Only 95 % ethanol is available. Removing the water by storing over molecular sieves or using a drying agent such as anhydrous sodium sulfate is not considered economical because too much water is present and time is short. An azeotropic distillation is decided upon. You get 150 mL of 95% ethanol and prepare the desired product. 

The imidic ester hydrochloride (1 mole) is dissolved in an eight- to ten-fold weight of anhydrous ethanol, treated with 2-(diethylamino)ethylamine (1.05 moles), and stirred for 8 h at 40-45°. The alcohol is then removed in a vacuum and the residue is dissolved in water and acidified to Congo Red by hydrochloric acid. After short warming the mixture is cooled and all non-basic material is removed by filtration or by extraction with ether. The solution is then evaporated to dryness at 40° in a vacuum and the last traces of water are distilled off azeotropically with benzene-ethanol. The yield is 75-80%. 

For the production of fuel-grade ethanol, the ethanol has to be dried . Anhydrous ethanol cannot be produced by simple distillation because ethanol forms an azeotropic mixture with water. The maximum ethanol content achievable by distillation is approximately 97.2vol.%, which is usually not sufficient for the application as fuel-ethanol. The residual water can be removed either by azeotropic distillation by the addition of, e.g., cyclohexane or by the application of molecular sieves. Today, state-of-the-art plants operate with molecular sieves which provide considerable advantages in terms of investment and operating costs. 

In a fortunate case, distillation aimed at purification may also result in the drying of the solvent. Distillation may be particularly effective in the removal of moisture if azeotropic mixtures with low boiling points are formed. For instance, the first step in the dehydration of ethanol is distillation after the addition of benzene, when water is removed in the volatile ternary azeotrope. Other solvents may also be dehydrated by distillation, e.g., benzene, chloroform, carbon tetrachloride, ethylene dichloride, heptane, hexane, toluene and xylene. In distillations with the aim of dehydration, the apparatus must be fitted with moisture traps (containing calcium chloride, silica gel or some other drying agent). It must be borne in mind that many anhydrous organic solvents are hygroscopic. 

Absolute alcohol n. Ethyl alcohol that has been refined by azeotropic distillation to 99.9% purity (200 proof). Other commercial ethanols contain about 5% water and may contain denaturants that make the alcohol undrinkable. Pure anhydrous ethyl alcohol (ethanol). The term is used to distinguish it from the several varieties of alcohol which are available, and which contain varying amounts of water and/or other impurities. 

Anhydrous ethanol means an ethyl alcohol that has a purity of >99%, exclusive of added denaturants, meeting the requirements of ASTM D4806. Hydrous (or wet, also sometimes known as azeotropic) ethanol is the most concentrated grade of ethanol that can be produced by simple distillation, without the further dehydration step necessary to produce anhydrous (or dry) ethanol. 

Besides the methods illustrated so far in this book, there are other ways for separating azeotropes. One way is to react the azeotrope away in a reactive distillation column to form other useful products. The design and control of various reactive distillations have been extensively studied in a recent book by Luyben and Yu. Another way commonly used in ethanol dehydration is to use the hybrid distillation-adsorption process. In this process, distillation is used to purify the mixture to a composition near the ethanol-water azetrope, and then an adsorption unit (e.g., molecular sieves) is used to adsorb the remaming water so that anhydrous ethanol can be obtained. The key technology in this process is the performance of the adsorbent material in removing water from the mixture and is beyond the scope of this book. 

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