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COMPLEX DISTILLATION METHODS

Simple distillation columns are not able to completely separate mixtures when azeotropes occur(e.g. ethanol water). If the relative volatility of a binary mixture is low (e. g. acetic acid andwater) continuous rectification of the mixture to give nearly pure products will require high refluxratios and correspondingly high heat requirements as well as towers of large cross section andnumber of trays.

Extractive distillation, azeotropic distillation and two-pressure distillation are methods to modifythe equilibrium to separate these complex mixtures.

Azeotropic DistillationAzeotropic distillation is often performed by adding a solvent or entrainer that forms anazeotrope with one or both of the components in the mixture.

This is most convenient if the azeotrope formed is binary and heterogeneous; that is azeotropewith two components only (entrainer and one component from the feed) and the vapour of theazeotrope will condense to form two liquid phases that are immiscible e.g. Acetic acid and watersolutions can be separated using butyl acetate as the entrainer (figure 1). Butyl acetate is onlyslightly soluble in water and forms a heterogeneous azeotrope (b.p.90.2 0C) with it.

When a homogeneous azeotrope is formed (i.e. does not separate into two layer oncondensation) special means of separating it such as liquid extraction must be provided, but thisis less desirable. e.g. the separation of butadiene from butylenes using liquid ammonia as theentrainer. At 40 0C the azeotrope is homogeneous. But the temperature below 20 0C two liquidphases are formed. Therefore successful separation can be achieved by operating theazeotrope separator at 40 0C. However this involves extensive heat exchange.

Some times the azeotrope contains all three components (ternary azeotrope) e.g. separation ofethanol-water using benzene. This ternary heterogeneous azeotrope formed contains 53.9mol% benzene, 23.3 mol% water and rest ethanol (b.p. 64.9 0C). Due to toxicity of benzene inethanol water separation it has been replaced by diethyl ether, n - pentane and n hexane. Theazeotropes formed are also heterogeneous.

Selecting a solvent for azeotropic distillation is often more difficult. There are usually fewersolvents that form azeotropes and boil at low enough temperature to be easy to remove in thedistillate or boil at a high enough temperature to be easy to remove in the bottoms. In additionthe binary or ternary azeotrope formed must be easy to separate. In practice, this requirement ismet by heterogeneous azeotropes and by azeotropes that are easy to separate with waterwash. The chosen entrainer must also satisfy the usual requirements of being non-toxic,noncorrosive, chemically stable, available and inexpensive.

Extractive distillationIn Extractive distillation a solvent, which alters the relative volatility of the components, is addedto the distillation mixtures. Consider the distillation of a binary mixture containing components Aand B. the solvent is selected so that one of the components, say B is selectively attracted to it.Since the solvent is usually chosen to have a significantly higher boiling point than thecomponents being separated, the attached component B has its volatility reduced. Thus theother component A, becomes relatively more volatile and is easy to remove in the distillate.Then a another column is required to separate the solvent and component B.

e.g.

(i) Separation of toluene and iso-octane using phenol (see figure 2).(ii) Butane and butene can be separated using furfural.(iii) HCl & HNO3 separation from water using H2SO4.

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Selection of the solvent is extremely important. The following factors should be considered.

(i) Ability to alter the vapour-liquid equilibria of the original mixture sufficiently to permitits easy separation.

(ii) The solvent should not form an azeotrope with any components (if so it becomesazeotropic distillation).

(iii) Low volatility in order to prevent vaporization of the solvent with the overheadproduct.

(iv) The solvent must be readily separated from the mixture.(v) Cost, toxicity, corrosiveness, chemical stability, freezing point and viscosity should

also be considered.

Two pressure distillationPressure effects the vapour-liquid equilibrium and in systems that from azeotropes it will affectthe composition of the azeotrope.

e.g. at 1 atm ethanol-water forms an azeotrope at 0.8943 mole fraction ethanol. When thepressure is reduced azeotropic concentration increases and at 70 mmHg the azeotropedisappears entirely. Unfortunately use of this disappearance of the azeotrope for the separation

of ethanol-water is not economical because the column requires a large number of trays andhas a large diameter.

Even though the azeotrope may not disappear, in general pressure affects the azeotropiccomposition. If this shift in composition is large enough a tow column process using two differentpressures can be used to completely separate the binary mixture.

e.g. Separation of Methyl ethyl ketone (MEK) and water.

At 1 atm the azeotrope contains 35% water and at 100 psi the azeotrope is at 50% water.

If the feed containing more than 35% water is fed to the first column the bottom will be purewater. The distillate form the atmospheric column will be 35% azeotrope. When this azeotrope

is sent to the high-pressure column an azeotrope containing 50% water comes off as thedistillate. This distillate is recycled to column 1 (see figure 3). Since this feed contains less waterthan this distillate the bottoms of the column 2 is pure MEK. Note that the water is less volatile incolumn 1 (P1) and MEK is less volatile in column 2 (P2).

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Azeotropic Distillation

Twp Pressure Distillation

Extractive Distillation