What is azeotropic distillation principle types and examples
Ethanol Water Azeotrope Distillation
Azeotropic distillation is the process of separating all the components of an azeotropic mixture by the process of distillation. An azeotropic mixture consists of two or more liquids which cannot be separated through simple distillation since the vapours that are formed via boiling the azeotropic mixtures consist of the same proportions of the liquids as the mixture itself. Hence, the azeotropic distillation is a specialized distillation that involves using specific techniques in breaking the azeotropes.
One of the most common methods to break an azeotrope includes adding a material separation agent which has the capability for changing the molecular interactions between all the components of the azeotrope. Adding the material separation agent alters the activity of the coefficient of the compounds of the azeotropic mixture. It hence changes the relative volatility of the whole azeotropic mixture. Today, we will learn about azeotropic distillation, ethanol-water azeotrope distillation, and the difference between azeotropic distillation and extractive distillation.
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Material Separation Agent
Let us first learn about what is a material separation agent.
The addition of a material separation agent like benzene to a mixture of ethanol/water tends to change the molecular interactions and, in turn, eliminates the azeotrope. When added in the liquid phase, the new component tends to alter the activity coefficient of several compounds in several ways, hence altering the relative volatility of the mixture. Higher deviations from the Raoult's law tends to ease out achieving significant changes in the relative volatility along with the addition of the other component.
In an azeotropic distillation, the volatility of the newly added component is the same as that of the mixture. A new azeotrope gets formed with one or more components depending on differences in polarity. If the material separation agent is opted for forming azeotropes having more than one component in the feed, it is called an entrainer. The added entrainer must be recovered by decantation, distillation, or another separation method and then returned to the top of the original column.
Distillation of Water/Ethanol
Let us now discuss the distillation of ethanol and water.
A historical example of an azeotropic distillation is its usage to dehydrate water and ethanol mixtures. To achieve this, a nearly azeotropic mixture gets delivered to the final column in which the azeotropic distillation tends to take place. Different entrainers are used in carrying out this process such as benzene, hexane, cyclohexane, pentane, acetone, isooctane, heptane, and diethyl ether. Out of all these compounds, benzene and cyclohexane have been into use the most widely. However, since benzene has been found to have carcinogenic properties, there was a decline in its usage. While this was the standard way to dehydrate ethanol previously, it lost its favour significantly because of the higher energy and capital cost that is associated with it. Another lesser toxic and favourable method than the usage of benzene for breaking the ethanol-water azeotrope is the usage of toluene.
Difference Between Extractive Distillation and Azeotropic Distillation
Now, let us look at the difference between extractive distillation and azeotropic distillation.
The main difference between extractive distillation and azeotropic distillation lies in the process followed during the separation of the mixture. In extractive distillation, a particular separation solvent has to be used in each mixture which should not form an azeotrope. Considering this, extractive distillation is considered a comparatively easier method when it is compared to the azeotropic distillation.
FAQs on Azeotropic Distillation in Physical Chemistry
1. What is azeotropic distillation?
Azeotropic distillation is a separation technique used to break an azeotrope by adding a third component (entrainer) to alter the vapor–liquid equilibrium. An azeotrope is a mixture that boils at a constant temperature and has the same composition in liquid and vapor phases, making ordinary fractional distillation ineffective. In azeotropic distillation, the added substance forms a new azeotrope with one component, allowing separation based on differences in boiling points and volatility.
2. What is an azeotrope in chemistry?
An azeotrope is a liquid mixture that boils at a constant temperature with vapor having the same composition as the liquid. Because the vapor and liquid compositions are identical, the mixture cannot be separated by simple or fractional distillation. A common example is ethanol–water, which forms a minimum-boiling azeotrope at about 95.6% ethanol by mass.
3. Why can't azeotropes be separated by simple distillation?
Azeotropes cannot be separated by simple distillation because the vapor phase has the same composition as the liquid phase at the azeotropic point. During boiling, both components vaporize in the same ratio, so repeated condensation and vaporization do not change the composition. This constant boiling behavior prevents further purification by ordinary distillation methods.
4. How does azeotropic distillation work?
Azeotropic distillation works by adding an entrainer that forms a new azeotrope with one of the original components, enabling separation. The process typically involves:
- Adding a suitable third component (entrainer).
- Formation of a new azeotrope with altered boiling point.
- Distillation of the new azeotrope.
- Separation and recovery of the entrainer for reuse.
5. What are the types of azeotropes?
The two main types of azeotropes are minimum-boiling azeotropes and maximum-boiling azeotropes.
- Minimum-boiling azeotrope: Boils at a temperature lower than either pure component (e.g., ethanol–water).
- Maximum-boiling azeotrope: Boils at a temperature higher than either pure component (e.g., nitric acid–water).
6. What is the difference between azeotropic distillation and extractive distillation?
The main difference is that azeotropic distillation forms a new azeotrope with an entrainer, while extractive distillation uses a high-boiling solvent that does not form an azeotrope.
- Azeotropic distillation: Entrainer forms a new azeotrope and is distilled off.
- Extractive distillation: Solvent changes relative volatility without forming an azeotrope and remains in the liquid phase.
7. What is an entrainer in azeotropic distillation?
An entrainer is a substance added to an azeotropic mixture to modify vapor–liquid equilibrium and enable separation. The entrainer selectively interacts with one component to form a new azeotrope or alter relative volatility. For example, benzene was historically used as an entrainer to dehydrate ethanol by forming a ternary azeotrope with water.
8. What is a common example of azeotropic distillation?
A common example of azeotropic distillation is the dehydration of ethanol (C2H5OH) to obtain absolute ethanol. The ethanol–water mixture forms a minimum-boiling azeotrope at about 95.6% ethanol. By adding an entrainer such as benzene or cyclohexane, a new azeotrope forms and water can be removed, yielding nearly pure ethanol.
9. What are the industrial applications of azeotropic distillation?
Azeotropic distillation is widely used in industry to separate difficult liquid mixtures that form azeotropes. Major applications include:
- Production of absolute ethanol for fuels and pharmaceuticals.
- Purification of organic solvents.
- Separation of close-boiling petrochemical fractions.
- Removal of water from chemical reaction mixtures.
10. How is azeotropic distillation different from fractional distillation?
Azeotropic distillation uses an added entrainer to break an azeotrope, whereas fractional distillation separates components based only on boiling point differences.
- Fractional distillation: Effective when components have different boiling points and no azeotrope is formed.
- Azeotropic distillation: Required when a constant-boiling azeotrope prevents further separation.
