Azeotropic distillation

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Any of several processes by which liquid mixtures containing azeotropes may be separated into their pure components with the aid of an additional substance (called the entrainer, the solvent, or the mass separating agent) to facilitate the distillation. Distillation is a separation technique that exploits the fact that when a liquid is partially vaporized the compositions of the two phases are different. By separating the phases, and repeating the procedure, it is often possible to separate the original mixture completely. However, many mixtures exhibit special states, known as azeotropes, at which the composition, temperature, and pressure of the liquid phase become equal to those of the vapor phase. Thus, further separation by conventional distillation is no longer possible. By adding a carefully selected entrainer to the mixture, it is often possible to “break” the azeotrope and thereby achieve the desired separation. See also Azeotropic mixture; Distillation.

Entrainers fall into at least four distinct categories that may be identified by the way in which they make the separation possible. These categories are: (1) liquid entrainers that do not induce liquid-phase separation, used in homogeneous azeotropic distillations, of which classical extractive distillation is a special case; (2) liquid entrainers that do induce a liquid-phase separation, used in heterogeneous azeotropic distillations; (3) entrainers that react with one of the components; and (4) entrainers that dissociate ionically, that is, salts. See also Salt-effect distillation.

Within each of these categories, not all entrainers will make the separation possible, that is, not all entrainers will break the azeotrope. In order to determine whether a given entrainer is feasible, a schematic representation known as a residue curve map for a mixture undergoing simple distillation is created. The path of liquid compositions starting from some initial point is the residue curve. The collection of all such curves for a given mixture is known as a residue curve map (see illustration). These maps contain exactly the same information as the corresponding phase diagram for the mixture, but they represent it in such a way that it is more useful for understanding and designing distillation systems.

Schematic representation of the residue curve maps for ternary mixtures with one minimum-boiling binary azeotrope. (a) Azeotrope between the lowest-(L) and highest-boiling (H) pure components. (b) Azeotrope between the intermediate-(I) and highest-boiling components. (c) Azeotrope between the intermediate- and lowest-boiling components.

Mixtures that do not contain azeotropes have residue curve maps that all look the same. The presence of even one binary azeotrope destroys the structure. If the mixture contains a single minimum-boiling binary azeotrope, three residue curve maps are possible, depending on whether the azeotrope is between the lowest- and highest-boiling components, between the intermediate- and highest-boiling components, or between the intermediate- and lowest-boiling components.

Nonazeotropic mixtures may be separated into their pure components by using a sequence of distillation columns because there are no distillation boundaries to get in the way. The situation is quite different when azeotropes are present, as can be seen from the illustration. It is possible to separate mixtures that have residue curve maps similar to those shown in illus. a and c by straightforward sequences of distillation columns. This is because these maps do not have any distillation boundaries. These, and other feasible separations for more complex mixtures, are referred to collectively as homogeneous azeotropic distillations. Without exploiting some other effect (such as changing the pressure from column to column), it is impossible to separate mixtures that have residue curve maps like illus. b.

A large number of mixtures have residue curve maps similar to illus. c, and therefore the corresponding distillation is given the special name extractive distillation.

Heterogeneous entrainers cause liquid-liquid phase separations to occur in such a way that the composition of each phase lies on either side of a distillation boundary. In this way, the entrainer allows the separation to “jump” over a boundary that would otherwise be impassable.

In chemistry, azeotropic distillation^[1] is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, azeotropic distillation usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g. producing two, immiscible liquid phases), such as the example below with the addition of benzene to water and ethanol. This practice of adding an entrainer which forms a separate phase is a specific sub-set of (industrial) azeotropic distillation methods, or combination thereof. In some senses, adding an entrainer is similar to extractive distillation.

Contents 1 Material Separation Agent 2 Example - distillation of ethanol/water 3 Pressure-swing distillation 4 Molecular sieves 5 See also 6 References

Material Separation Agent

The addition of a Material Separation Agent, such as benzene to an ethanol/water mixture, changes the molecular interactions and eliminates the azeotrope. Added in the liquid phase, the new component can alter the activity coefficient of various compounds in different ways thus altering a mixture's relative volatility. Greater deviations from Raoult's law make it easier to have significant changes in relative volatility with the addition of another component. In azeotropic distillation the volatility of the added component is the same as the mixture, and a new azeotrope is formed with one or more of the components based on differences in polarity.^[2] If the material separation agent is selected to form azeotropes with more than one component in the feed then it is referred to as an entrainer. The added entrainer should be recovered by distillation, decantation, or another separation method and returned near the top of the original column.^[3]

Example - distillation of ethanol/water

A common historical example of azeotropic distillation is its use in dehydrating ethanol and water mixtures. For this, a near azeotropic mixture is sent to the final column where azeotropic distillation takes place. Several entrainers can be used for this specific process: benzene, pentane, cyclohexane, hexane, heptane, isooctane, acetone, and diethyl ether are all options as the mixture.^[2] Of these benzene and cyclohexane have been used the most extensively. However, because benzene has been discovered to be a carcinogenic compound, its use has declined. While this method was the standard for dehydrating ethanol in the past, it has lost favor due to the high capital and energy costs associated with it. Another favourable method and less toxic than using benzene to azeotrope water, is to utilise toluene instead.

Pressure-swing distillation

Another method, pressure-swing distillation, relies on the fact that an azeotrope is pressure dependent. An azeotrope is not a range of concentrations that cannot be distilled, but the point at which the activity coefficients of the distillates are crossing one another. If the azeotrope can be "jumped over", distillation can continue, although because the activity coefficients have crossed, the water will boil out of the remaining ethanol, rather than the ethanol out of the water as at lower concentrations.

To "jump" the azeotrope, the azeotrope can be moved by altering the pressure. Typically, pressure will be set such that the azeotrope will be closer to 100% concentration. For ethanol, that may be 97%. Ethanol can now be distilled up to 97%. It will actually be distilled to something slightly less, like 96.5%. The 96.5% alcohol is then sent to a distillation column that is under a different pressure, one that pulls the azeotrope down, maybe to 96%. Since the mixture is already above the 96% azeotrope, the distillation will not get "stuck" at that point and the ethanol can be distilled to whatever concentration is needed.

Molecular sieves

Main article: molecular sieve

For the distillation of ethanol for gasoline addition, the most common means of breaking the azeotrope is the use of molecular sieves. Ethanol is distilled to 96%, then run over a molecular sieve which adsorbs water from the mixture. The concentration is now above 96% and can be further distilled. The sieve is heated to remove the water and reused.

See also

• Azeotrope
• Theoretical plate

References

1. ^ Kister, Henry Z. (1992). Distillation Design (1st Edition ed.). McGraw-Hill. ISBN 0-07-034909-6. 
2. ^ ^{a b} Kumar, Santosh et. al (2010), "Anhydrous ethanol: A renewable source of energy.", Renewable and Sustainable Energy Reviews 
3. ^ Treybal (1980). Mass-Transfer Operations (3rd Edition ed.). McGraw-Hill. 

v t e Distillation Principles Raoult's law Dalton's law Reflux Fenske equation McCabe-Thiele method Theoretical plate Partial pressure Vapor-liquid equilibrium Industrial processes Batch distillation Continuous distillation Fractionating column Spinning cone Laboratory methods Alembic Kugelrohr Rotary evaporator Spinning band distillation Still Techniques Azeotropic Catalytic Destructive Dry Extractive Fractional Reactive Salt-effect Steam-based Vacuum-based

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