Azeotropic distillation solvent separation

T xtractive and azeotropic distillation in different types of chemical industry has become more important as more separations of close-boiling mixtures and azeotropic ones are encountered. Extractive distillation is used more because it is generally less expensive, simpler, and can use more solvents than azeotropic distillation. Solvent selection for azeotropic distillation has recently been discussed by Berg (I). Therefore, solvent screening for extractive distillation is discussed here. 

There are many instances in which the objective of a separation process is to recover a given compound at a very high level of purity. When this is the case, it is sometimes necessary to lake a different approach to the porticalar separation. For example, the ethanol-water azeotrope precludes the recovery of pure ethanol using ordinary distillation. For higher purities, azeotropic distillation, solvent extraction, or the use of molecular sieves can be used. Clearly, the necessity to recover ethanol in concentrations above the azeotrope requires an alternative separations strategy,... 

The second class of distillation operation using an extraneous mass-separating agent is extractive distillation. Here, the extraneous mass-separating agent is relatively involatile and is known as a solvent. This operation is quite different from azeotropic distillation in that the solvent is withdrawn from the column bottoms and does not form an azeotrope with any of the components. A typical extractive distillation process is shown in Fig. 3.11. ... 

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

The choice of separation method to be appHed to a particular system depends largely on the phase relations that can be developed by using various separative agents. Adsorption is usually considered to be a more complex operation than is the use of selective solvents in Hquid—Hquid extraction (see Extraction, liquid-liquid), extractive distillation, or azeotropic distillation (see Distillation, azeotropic and extractive). Consequentiy, adsorption is employed when it achieves higher selectivities than those obtained with solvents. 

Anhydrous Acetic Acid. In the manufacture of acetic acid by direct oxidation of a petroleum-based feedstock, solvent extraction has been used to separate acetic acid [64-19-7] from the aqueous reaction Hquor containing significant quantities of formic and propionic acids. Isoamyl acetate [123-92-2] is used as solvent to extract nearly all the acetic acid, and some water, from the aqueous feed (236). The extract is then dehydrated by azeotropic distillation using isoamyl acetate as water entrainer (see DISTILLATION, AZEOTROPIC AND EXTRACTIVE). It is claimed that the extraction step in this process affords substantial savings in plant capital investment and operating cost (see Acetic acid and derivatives). A detailed description of various extraction processes is available (237). 

The equihbrium shown in equation 3 normally ties far to the left. Usually the water formed is removed by azeotropic distillation with excess alcohol or a suitable azeotroping solvent such as benzene, toluene, or various petroleum distillate fractions. The procedure used depends on the specific ester desired. Preparation of methyl borate and ethyl borate is compHcated by the formation of low boiling azeotropes (Table 1) which are the lowest boiling constituents in these systems. Consequently, the ester—alcohol azeotrope must be prepared and then separated in another step. Some of the methods that have been used to separate methyl borate from the azeotrope are extraction with sulfuric acid and distillation of the enriched phase (18), treatment with calcium chloride or lithium chloride (19,20), washing with a hydrocarbon and distillation (21), fractional distillation at 709 kPa (7 atmospheres) (22), and addition of a third component that will form a low boiling methanol azeotrope (23). 

As a starting point for identifying candidate solvents, all compounds having boiling points below that of any component in the mixture to be separated should be eliminated. This is necessary to yield the correct residue curve map for extractive distillation, but this process implicitly rules out other forms of homogeneous azeotropic distillation. In fact, compounds which boil as much as 50°C or more above the mixture have been recommended (68) in order to minimize the likelihood of azeotrope formation. On the other hand, the solvent should not bod so high that excessive temperatures are required in the solvent recovery column. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

Extractive distillation is a method of rectification similar in purpose to azeotropic distillation. To a binary mixture which is difficult or impossible to separate by ordinary means, a third component, termed a solvent, is added which alters the relative volatility of the original constituents, thus permitting the separation. The added solvent is, however, of low volatility and is itself not appreciably vaporised in the fractionator. 

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... 

Performance Constraints A major use of solvents facilitates separation via liquid-liquid extraction, azeotropic distillation, crystallization, and absorption. A solvent s capacity, selectivity, and distribntion can all be related to its activity coefficients. 

The determination of the exact composition of feed and product streams is also receiving attention. By precision fractionation in the laboratory, often at 100 plus plates and reflux ratios of equal magnitude, narrow boiling fractions are obtained that are further resolved by separation procedures such as azeotropic distillation, extractive distillation, chromatography, solvent extraction, and crystallization. In addition, the instrumental methods of analysis are increasing our information on the composition of complex hydrocarbon fractions. 

The separation art is now a continuous band extending from simple distillation through azeotropic distillation, extractive distillation, vapor phase adsorption, and solvent extraction to liquid phase adsorption. 

Both processes involve the addition of a new material, the solvent, to the mixture. The solvent is chosen so as to increase the relative volatility of the components to be separated. During extractive distillation, the solvent is generally added near the top of the column, and because it has a low volatility it is withdrawn with the product at the bottom. In azeotropic distillation, the solvent is withdrawn as an azeotrope with one or more of the components to be separated—usually in the overhead product. If the ratio of the components to be separated is different in the withdrawn azeotrope from their ratio in the feed to the column, then at least a partial separation has been achieved. In both processes it is necessary to separate the solvent from the product. This can be accomplished, for example, by distillation, solvent extraction, or even gravity settling, depending on the characteristics of the components involved. 

In the nitrate route Nd(N03)3 is dissolved in water and the Nd-carboxylate is extracted from the aqueous phase by an organic solvent which contains the respective lithium-, sodium-, potassium- or ammonium-carboxylates. After the completion of the extraction Nd-carboxylate is present in the organic phase and the lithium-, sodium-, potassium- or ammonium nitrates are left in the aqueous phase. The two phases are separated and azeotropic distillation is applied to the organic phase in order to remove water [219,220]. 

Solvent Recovery. A mixture of methanol and methyl acetate is obtained after saponification. The methyl acetate can be sold as a solvent or converted back into acetic acid and methanol using a cationic-exchange resin such as a cross-linked styrene—sulfonic acid gel (273—276). The methyl acetate and methanol mixture is separated by extractive distillation using water or ethylene glycol (277—281). Water is preferred if the methyl acetate is to be hydrolyzed to acetic acid. The resulting acetic acid solution is concentrated by extraction or azeotropic distillation. 

The stripped solvent is practically free from water. The water undergoes overhead azeotropic distillation and is produced as separate subphase in the reflux drum. This water is then fed to the solvent recovery stage of the extraction process. 

Simple distillation cannot separate aromatics from noD -aromatic, because the relative volatilities are very low, and many azeotropes are formed. Azeotropic distillation is based on the formation of an azeotrope betu een the non-aromatic hydrocarbons and a low boiling polar solveat It is select among the hrst terms of the series of alcohols, ketones, aldehydes and nitriles, and is employed pure or mixed with water. If the solvent forms a hetero-azeotrope, its recovery is accordbgly facilitated. The )aeld is not limited in principle. The impurity content of the feedstock and the composition of the azeotrope determine the amount of solvent required. Cuts rich in aromatics can be treated in this way fairly economically. However, any variation in the type of impurity to be removed, and consequently in the composition of the azeotrope, may lead to less perfect purification. Furthermore, this method can be applied only to a narrow cut which contains... 

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