Separation by Extractive Distillation

Typical mixtures that can be separated by extractive distillation in processes similar to the one described above include cyclohexane and benzene, and toluene and methylcyclohexane, both using phenol as the solvent. In another process, isobutane and 1-butene are separated using furfural as the solvent. 

The relative rate of solvent to feed determines the sharpness of separation between the key components, much as the absorbent rate or reflux ratio influences separation in absorbers or conventional columns. An estimate of the required solvent rate may be obtained based on vapor-liquid equilibrium data at different solvent concentrations, as described later in this section. 

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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 hydroly2ed to acetic acid. The resulting acetic acid solution is concentrated by extraction or a2eotropic distillation. 

The C4 stream from steam crackers, unlike its counterpart from a refinery, contains about 45% butadiene by weight. Steam crackers that process significant amounts of Hquid feedstocks have satellite faciUties to recover butadiene from the stream. Conventional distillation techniques are not feasible because the relative volatihty of the chemicals in this stream is very close. Butadiene and butylenes are separated by extractive distillation using polar solvents. 

A mixture of acetone and chloroform is to be separated into pure products [Hostrup et al. (1999)]. Since they also form an azeotrope, one alternative to satisfy the separation objective is to find a suitable solvent for separation by extractive distillation. This type of problem in product design is usually encountered during the purification or recovery of products, by-products, reactants or removal of undesirable products from the process. Also, it can be noted that failure to find a suitable solvent may result in the discard of the product. Alternatively, a functional chemical product manufacturer may be interested to find, design and develop a new solvent. In this case, the solvent is the chemical product. 

Because of their very similar boiling points and azeotrope formation, the components of the C4 fraction cannot be separated by distillation. Instead, other physical and chemical methods must be used. 1,3-Butadiene is recovered by complex formation or by extractive distillation.143-146 Since the reactivity of isobutylene is higher than that of n-butenes, it is separated next by chemical transformations. It is converted with water or methyl alcohol to form, respectively, tert-butyl alcohol and tert-butyl methyl ether, or by oligomerization and polymerization. The remaining n-butenes may be isomerized to yield additional isobutylene. Alternatively, 1-butene in the butadiene-free C4 fraction is isomerized to 2-butenes. The difference between the boiling points of 2-butenes and isobutylene is sufficient to separate them by distillation. n-Butenes and butane may also be separated by extractive distillation.147... 

Processes of separation by extraction, distillation, crystallization, or adsorption sometimes are equally possible. Differences in solubility, and hence of separability by extraction, are associated with differences in chemical structure, whereas differences in vapor pressure are the basis of separation by distillation. Extraction often is effective at near-ambient temperatures, a valuable feature in the separation of thermally unstable natural mixtures or pharmaceutical substances such as penicillin. 

BTX (benzene-toluene-xylene) mixtures are an important petroleum refinery stream that is separated by extractive distillation (Fig. 1) from a hydrocarbon stream, usually a reformate, and followed by downstream fractionation for isolation of the pure materials for further treatment and use (Fig. 2). 

Several plants have been built operating according to the two-stage technology (Figure 4) [48, 49] quite analogously to the acetaldehyde process described above, with air as oxidant and a catalyst cycle. An important by-product in acetone manufacture is propionaldehyde, which is separated by extractive distillation... 

EXAMPLE 10.6 ACETONE/CHLOROFORM SEPARATION BY EXTRACTIVE DISTILLATION... 

Iso-butane and 1-butene can be separated by extractive distillation, using furfural as the solvent. This agent alters the relative volatility of isobutane to 1-butene according to the following equation ... 

While the use of nitriles and dinitriles as selective solvents for hydrocarbon separation by extractive distillation has been described (13), no... 

A mixture of n-heptane and toluene (T) is separated by extractive distillation with... 

Toluene and n-heptane are to be separated by extractive distillation with phenol. One proposed specification for the operation is shown below. Use the Naphtali-Sandholm SC method, with the Wilson equation for activity coefficients, to calculate the product compositions, stage temperatures, interstage flow rates and compositions, and condenser and reboiler duties. Constants for the Wilson equation can be determined readily for the van Laar constants An developed in Example 5.5 by computing from these constants the infinite-dilution activity... 

Extractive distillation is not limited to the separation of binary mixtures, but is also capable of removing particular classes of substances from multicomponent inixtiire.s, as for instance benzene from mineral oil fractions. Mixtures of saturated and imsaturated hydrocarbons having closely similar boiling points can be separated by extractive distillation with ketoesters [73]. Recently, the sei)aration of lower hydrocarbons CyCa has been gaining ground [74]. Garner et al. [75] studied the efficiency of packed columns in the extractive distillation of the system iiictliyl cyclohexane-toluene with derived equations for this process. 

In spite of its wide use, there are still three major problems with the Wittig reaction. 1) The stereochemistry often cannot be controlled. 2) Ketones and hindered aldehydes fail to react with phosphoranes that are hindered or are stabilized by strongly electron withdrawing substituents. 3) The by-product triphenylphosphine oxide can be difficult to separate from the product alkene. Often the alkene and the triphenylphosphine oxide cannot be separated by extraction, distillation, or crystallization, and column chromatography is required. 

A mixture of cyclohexane bp 80.8 "C) and benzene hp 80.1 "C) can, for example, be separated by distillation after adding aniline, since the interaction between benzene and aniline is greater than that between cyclohexane and aniline. Azeotropic mixtures can be separated similarly by extractive distillation (e.g.. water-ethanol by adding glycerol). Hydrocarbons with similar boiling points can be separated by extractive distillation in the presence of polar liquids (nitrobenzene, phenol, furfurol). 

For example, for a mixture containing components 1 and 2 to be separated by distillation, with a separation factor of a = 1.5, separation by extractive distillation or LLE is economic with separation factors of 2 or 6, respectively. 

In most cases not binary but multicomponent systems have to be separated. Sometimes an additional component is needed as an entrainer e.g. for the separation by extractive distillation. As an example selected separation factors of 12 for the system benzene (l)-cyclohexane (2)-NMP (3) calculated using modified UNIFAC and default UNIQUAC parameters from a simulator are shown in Figures 11.8 and 11.9, respectively. As benzene and cyclohexane form an azeotrope, the main task of the entrainer NMP is to shift the separation factor between benzene and cyclohexane as far from unity as possible this means au 1 or au 1- In practice, typical entrainer concentrations of 50-80 mol% are employed to achieve satisfying separation factors. A higher entrainer concentration usually improves the separation factor. 

McLaughlin, D.R and Stoltz, R.A. (1988) Zirconium and hafnium tetrachloride separation by extractive distillation with molten zinc chloride lead chloride solvent. US Patent 4737 244. 

Such a process depends upon the difference in departure from ideally between the solvent and the components of the binary mixture to be separated. In the example given, both toluene and isooctane separately form nonideal liquid solutions with phenol, but the extent of the nonideality with isooctane is greater than that with toluene. When all three substances are present, therefore, the toluene and isooctane themselves behave as a nonideal mixture and then-relative volatility becomes high. Considerations of this sort form the basis for the choice of an extractive-distillation solvent. If, for example, a mixture of acetone (bp = 56.4 C) and methanol (bp = 64.7°Q, which form a binary azeotrope, were to be separated by extractive distillation, a suitable solvent could probably be chosen from the group of aliphatic alcohols. Butanol (bp = 117.8 Q, since it is a member of the same homologous series but not far removed, forms substantially ideal solutions with methanol, which are themselves readily separated. It will form solutions of positive deviation from ideality with acetone, however, and the acetone-methanol vapor-liquid equilibria will therefore be substantially altered in ternary mixtures. If butanol forms no azeotrope with acetone, and if it alters the vapor-liquid equilibrium of acetone-methanol sufficiently to destroy the azeotrope in this system, it will serve as an extractive-distillation solvent. When both substances of the binary mixture to be separated are themselves chemically very similar, a solvent of an entirely different chemical nature will be necessary. Acetone and furfural, for example, are useful as extractive-distillation solvents for separating the hydrocarbons butene-2 and a-butane. 

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