Azeotropic distillation technique

Introduction The term azeotropic distillation has been apphed to a broad class of fractional distillation-based separation techniques in that specific azeotropic behavior is exploited to effect a separation. The agent that causes the specific azeotropic behavior, often called the entrainer, may already be present in the feed mixture (a self-entraining mixture) or may be an added mass-separation agent. Azeotropic distillation techniques are used throughout the petro-... 

Initially, the coupling of digitoxigenin (1) with (70a) in dry 1,2-di-chloroethane was accomplished87 by employing an azeotropic distillation technique to remove the water. The silver carbonate used in the reaction had been prepared several days previously, and had been stored under anhydrous conditions in the dark at 06. After completion of the reaction, the products were saponified, followed by extraction, giving solid material which consisted of two components as disclosed by paper chromatography. 

The preparation of (71) is given in the following experiment, which serves to typify the synthesis of a 2-deoxycardenolide, starting with a 2-deoxy sugar. The experiment also embodies a Meystre-Miescher type of azeotropic distillation technique for removing the water, and saponificar tion catalyzed by potassium hydrogen carbonate for isolation of the unsubstituted cardenolide. 

Method 5031 describes an azeotropic distillation technique for the determination of nonpurgeable, water-soluble VOCs that are present in aqueous environmental samples. The sample is distilled in an azeotropic distillation apparatus, followed by direct aqueous injection of the distillate into a GC or GC MS system. The method is not amenable to automation. The distillation is time-consuming and is limited to a small number of samples. 

Distillation (qv) is the most widely used separation technique in the chemical and petroleum industries. Not aU. Hquid mixtures are amenable to ordinary fractional distillation, however. Close-boiling and low relative volatihty mixtures are difficult and often uneconomical to distill, and azeotropic mixtures are impossible to separate by ordinary distillation. Yet such mixtures are quite common (1) and many industrial processes depend on efficient methods for their separation (see also Separation systems synthesis). This article describes special distillation techniques for economically separating low relative volatihty and azeotropic mixtures. 

As mentioned earlier the ease or difficulty of separating two products depends on the difference in their vapor pressures or volatilities. There are situations in the refining industry in which it is desirable to recover a single valuable compound in high purity from a mixture with other hydrocarbons which have boiling points so close to the more valuable product that separation by conventional distillation is a practical impossibility. Two techniques which may be applied to these situations are azeotropic distillation and extractive distillation. Both methods depend upon the addition to the system of a third component which increases the relative volatility of the constituents to be separated. 

This is an equilibrium process and two techniques are used to drive the reaction to completion. One is to use a large excess of the alcohol, which is feasible for simple and inexpensive alcohols. The second method is to drive the reaction forward by irreversible removal of water, and azeotropic distillation is one way to accomplish this. Entries 1 to 4 in Scheme 3.5 are examples of acid-catalyzed esterifications. Entry 5 is the preparation of a diester starting with an anhydride. The initial opening of the anhydride ring is followed by an acid-catalyzed esterification. 

When choosing a separation technique (azeotropic distillation, absorption, stripping, liquid-liquid extraction, etc.), the use of extraneous mass-separating agents should be avoided for the following reasons ... 

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. 

Acetic acid is an important intermediate organic tonnage chemical that may be produced by the petroleum industry and fermentation. The latter process requires the recovery of acetic acid from water solutions, and several techniques have been applied to this separation, including solvent extraction, azeotropic distillation, and extractive distillation. A comparison of economics between azeotropic distillation and solvent extraction combined with azeotropic distillation (Table 10.3) shows that the introduction of... 

The problem of concentrating toluene above the 50 to 70% value obtainable by fractional distillation of hydroformates was solved by the development of azeotropic and extractive distillation techniques and by improvement in the sulfur dioxide extraction... 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

Resolution. The technique is applicable to systems containing components with very similar boiling points. By choosing a selective liquid phase or the proper adsorbent one can separate molecules that are very similar physically and chemically. Components that form azeotropic mixtures in ordinary distillation techniques may be separated by GC. 

Many new developments have occurred over the last 20 years, as discussed in Section 1.2.3, particularly for the production of strong nitric acid ( > 90% by weight). Strong nitric acid production employs the technique of super-azeotropic distillation to concentrate weaker acid beyond theazeotropic point (68% by weight). However, it is not necessary to discuss this technology further because the market... 

In the absence of a catalyst, the reaction between a polyhydric alcohol and a carbonyl compound may proceed only as far as the formation of a semi-acetal 14 however, if water is removed as it is produced in the reaction mixture, the true acetal is obtained. For example, azeotropic distillation with a water-immiscible liquid, such as benzene, toluene or xylene, has been employed in the synthesis of butylidene, methylene and furfurylidene acetals.15 In the latter case, this technique is particularly useful because of the marked instability of furfuraldehyde under acidic conditions.15... 

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