Distillation, azeotropic

Azeotropic distillation deals with the separation of mixtures involving one or several azeotropes. This problem, which in the past was tackled by means of experience and intuition, is today approached by means of systematic methods based on a deeper thermodynamic analysis. Here, we review the indispensable aspects for process synthesis. More details can be found in recent specialized books [8, 14]. 

Azeotropic distillation. In some cases two or more liquids form constant-boiling mixtures, or azeotropes. Azeotropic mixtures are most likely to be found with components which readily form hydrogen bonds or are otherwise highly associated, especially when the components are dissimilar, for example an alcohol and an aromatic hydrocarbon, but have similar boiling points. 

Examples where the boiling point of the distillate is a minimum (less than either pure component) include  

Water with ethanol, n-propanol and isopropanol, tcrt-butanol, propionic acid, butyric acid, pyridine, methanol with methyl iodide, methyl acetate, chloroform, 

Although less common, azeotropic mixtures are known which have higher boiling points than their components. These include water with most of the mineral acids (hydrofluoric, hydrochloric, hydrobromic, perchloric, nitric and sulfuric) and formic acid. Other examples are acetic acid-pyridine, acetone-chloroform, aniline-phenol, and chloroform-methyl acetate. 

The following azeotropes are important commercially for drying ethanol  

Materials are sometimes added to form an azeotropic mixture with the substance to be purified. Because the azeotrope boils at a different temperature, this facilitates separation from substances distilling in the same range as the pure material. (Conversely, the impurity might form the azeotrope and be removed in this way.) This method is often convenient, especially where the impurities are isomers or are otherwise closely related to the desired substance. Formation of low-boiling azeotropes also facihtates distiUation. 

The combination of distillation and decantation is used in the well-known azeotropic distillation process shown in Fig. 11.4-1. The separation of the homogeneous azeotropic feed (e.g., ethanol/water) is achieved by the admixture of an entrainer (e.g., toluene) which forms a large mixing gap with one of the feed components (here water). In colunrn C-1, water is recovered as bottoms B. The azeotropic overhead fraction t is mixed with the fraction S2 (toluene-rich) from the decanter. In colunrn C-2 pure ethanol is recovered as bottoms B2. The overhead fraction D2 is condensed, subcooled, and split into the fractions 51 (water-rich) and 52 (toluene-rich) in the decanter. Both fractions are recycled within the pro- 

Water/ethanol Benzene Acetic acid/formic acid Chloroform 

For the separation of narrow-boiling mixtures, which have usually been prepared by a previous countercurrent distillation  

For the separation of mixtures forming an azeotrope, generally having a composition close to the azeotropic point. 

As a rule the compound to be added is so chosen that it forms an azeotrope of minimum boiling point with one of the components. But it is also possible to select an entrainer forming a binary or ternary minimum azeotrope with both of the components to be separated in the latter case it is necessary for the proportion of the components in the new azeotropes to be different from their initial proportions. Discus.sing extensive investigations of various types of phase diagrams and of the elaboration of column schemes Sharov and Serafiniov [35a] have treated the problems specific to the countercurrent distillation of azeotropic multicomponent mixtures. 

A particularly striking example of the treatment of narrow-boiling mixtures i.s the mixture indole-diphenyl, which can be separated with diethyleneglycol as entrainer. At atmospheric pressure indole and diphenyl differ by only 0.6 deg. C in boiling point by the addition of diethyleneglycol this difference can be increased to 

Commercial p-picoline is a mixture of x-picoline, p-picoline and 2.6-lutidine. The difference between the boiling points of p-picoline and 2.6-lutidine is 0.15 deg. C at atmospheric pressure. By means of azeotropic distillation, using acetic or propionic acid as additive, one can se] arate the individual bases in a purity of 95—98% [37]. Further processes employed in industry were reported by Dummett [38]. 

Data of Azeotropes. The choice of azeotropic entrainer for a desired separation is much more restricted than that of solvents for extractive distillation, although many azeotropic data are known. The most extensive compilation is that of Ogorodnikov, Lesteva, and Kogan (Handbook of Azeotropic Mixtures (in Russian), 1971). It contains data of 21,069 systems, of which 1274 are ternary, 60 multicomponent, and the rest binary. Another compilation Handbook of Chemistry and Physics, 60th ed., CRC Press, Boca Raton, FL, 1979) has data of 685 binary and 119 ternary azeotropes. Shorter lists with grouping according to the major substances also are available in Lange s Handbook of Chemistry 

Commercial Examples. The small but often undesirable contents of water dissolved in hydrocarbons may be removed by distillation. In drying benzene, for instance, the water is removed overhead in the azeotrope, and the residual benzene becomes dry enough for processing such as chlorination for which the presence of water is harmful. The benzene phase from the condenser is refluxed to the tower. Water can be removed from heavy liquids by addition of some light hydrocarbon which then is cooked out of the liquid as an azeotrope containing the water content of the original heavy liquid. Such a scheme also is applicable to the breaking of aqueous emulsions in crude oils from tar sands. After the water is removed 

Ordinary rectification for the dehydration of acetic acid requires many trays if the losses of acid overhead are to be restricted, so that azeotropic processes are used exclusively. Among the entrainers that have been found effective are ethylene dichloride, n-propyl acetate, and n-butyl acetate. Water contents of these azeotropes are 8, 14, and 28.7 wt%, respectively. 

Accordingly, the n-butyl acetate is the most thermally efficient of these agents. The n-propyl acetate has been used in large installations, in the first stage as solvent for extraction of acetic acid and then as azeotropic entrainer to remove the accompanying 

Liquid on equilibrium-stage number (from bottom) 

Formic acid can be dehydrated with propyl formate as entrainer. Small contents of formic acid and water in acetic acid can be entrained away with chloroform which forms binary azeotropes with water and formic acid hut no other azeotropes in this system. 

Figme 13.24. Composition profiles and flowsketches of two extractive distillation processes, (a) Separation of methylcyclohexane and toluene with phenol as solvent data calculated by Smith, 1963). (b) Separation of aqueous ethanol and isopropanol, recovering 98% of the ethanol containing 0.2 mol % isopropanol, employing water as the solvent. Flow rates are in mols/hr data calculated by Robinson and Gilliland, 19S0). 

Vapor-liquid equilibrium curve for the ethanol-water system 

The residue curve map graphs the liquid composition paths that are solutions to the following set of ordinary differential equations  

A residue curve map of the propyl amine-acetonitrile-water system. 

Despite the advances in the thermodynamics for predicting azeotropic mixture, feasible distillation boundaries, and sequence of cuts, the azeotropic batch distillation system is still incipient in terms of design, optimization, and optimal control. 

Solution Since the curve from propyl amine to the ACN-water azeotrope is distillation barrier, there are two distillation regions in this system. For the left distillation region, the product sequence is propyl amine, ACN-water azeotrope, and water. For the right region, the sequence is propyl amine, ACN-water azeotrope, and ACN. This example shows that conventional distillation caimot obtain pure water and pure ACN cuts at the same time. 

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Mixtures with low relative volatility or which exhibit azeotropic behavior. The most common means of dealing with the separation of low-relative-volatility and azeotropic mixtures is to use extractive or azeotropic distillation. These processes are considered in detail later. Crystallization and liquid-liquid extraction also can be used. 

In the first class, azeotropic distillation, the extraneous mass-separating agent is relatively volatile and is known as an entrainer. This entrainer forms either a low-boiling binary azeotrope with one of the keys or, more often, a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation of the overhead vapor results in two liquid phases, one of which contains the bulk of one of the key components and the other contains the bulk of the entrainer. A t3q)ical scheme is shown in Fig. 3.10. The mixture (A -I- B) is fed to the column, and relatively pure A is taken from the column bottoms. A ternary azeotrope distilled overhead is condensed and separated into two liquid layers in the decanter. One layer contains a mixture of A -I- entrainer which is returned as reflux. The other layer contains relatively pure B. If the B layer contains a significant amount of entrainer, then this layer may need to be fed to an additional column to separate and recycle the entrainer and produce pure B. 

Figure 3.10 A typical azeotropic distillation using an entrainer.

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. ... 

As with azeotropic distillation, the separation is possible in extractive distillation because the extraneous mass-separating agent interacts more strongly with one of the components than the other. This in turn alters in a favorable way the relative volatility between the key components. 

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. 

Wastewater leaves the process from the bottom of the second column and the decanter of the azeotropic distillation column. Although both these streams are essentially pure water, they will nevertheless contain small quantities of organics and must be treated before final discharge. This treatment can be avoided altogether by recycling the wastewater to the reactor inlet to substitute part of the freshwater feed (see Fig. 10.36). 

Absolute Ethanol.. Supplies ot iibsolutc cthnnol, which is frec ucntly required in <>rj> mic chemiciil work, nrc now freely iiwiihible commercially as a result of azeotropic distillation methods. If however it should be... 

The formation of ethyl isopropylidene cyanoacetate is an example of the Knoevenagel reaction (see Discussion before Section IV,123). With higher ketones a mixture of ammonium acetate and acetic acid is an effective catalyst the water formed is removed by azeotropic distillation with benzene. The essential step in the reaction with aqueous potassium cyanide is the addition of the cyanide ion to the p-end of the ap-double bond ... 

Nitromethane is a very common material. Just go down to your local drag strip and pick up a gallon or two for doping your high performance cars fuel. It s also available up to 40% pure in RC model fuels. Simply fractionally distill the nitromethane (bp 101°C) out of the model fuel mixture and you re ready to go. If methanol Is present in the fuel formulation, some will azeotropically distill over with the nitromethane lowering its boiling point slightly, but this does not present a problem. 

Diols that bear two hydroxyl groups m a 1 2 or 1 3 relationship to each other yield cyclic acetals on reaction with either aldehydes or ketones The five membered cyclic acetals derived from ethylene glycol (12 ethanediol) are the most commonly encoun tered examples Often the position of equilibrium is made more favorable by removing the water formed m the reaction by azeotropic distillation with benzene or toluene... 

Because of this parallel with liquid-vapor equilibrium, copolymers for which ri = l/r2 are said to be ideal. For those nonideal cases in which the copolymer and feedstock happen to have the same composition, the reaction is called an azeotropic polymerization. Just as in the case of azeotropic distillation, the composition of the reaction mixture does not change as copolymer is formed if the composition corresponds to the azeotrope. The proportion of the two monomers at this point is given by Eq. (7.19). 

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). 

Maleic acid can be thermally dehydrated to maleic anhydride (69) or dehydrated through azeotropic distillation. Solvents such as xylenes (70) or dibutyl phthalate [84-74-2] (71) are preferred but conditions must be carefully adjusted to avoid isomerization to fumaric acid. 

Azeotropic and Extractive Distillations. Effective as they are for producing various Hquid fractions, distillation units generally do not produce specific fractions. In order to accommodate the demand for such products, refineries have incorporated azeotropic distillation and extractive distillation in their operations (see Distillation, azeotropic and extractive). 

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