How Does Azeotropic Distillation Work?
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 Explained: Methods and Applications
1. What is an azeotropic mixture as per the CBSE Class 12 Chemistry syllabus?
An azeotropic mixture, or simply an azeotrope, is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This occurs because the vapour produced by boiling the liquid has the same composition as the liquid itself. A classic example studied in the NCERT syllabus is the mixture of 95.6% ethanol and 4.4% water by mass, which boils at a constant temperature of 351.15 K.
2. What is the fundamental principle of azeotropic distillation?
The principle of azeotropic distillation is to alter the volatility of the components in an azeotropic mixture by adding a third component, known as an entrainer or a separating agent. This entrainer forms a new, lower-boiling azeotrope with one or more of the original components. This new azeotrope is then distilled off, effectively breaking the original azeotrope and allowing for the separation of the initial components.
3. How does azeotropic distillation differ from fractional distillation?
The key difference lies in their applicability to different types of mixtures:
- Fractional Distillation is used to separate liquid mixtures where the components have different boiling points and obey Raoult's Law. It works by repeatedly vaporising and condensing the mixture to enrich the vapour with the more volatile component.
- Azeotropic Distillation is a specialised technique used specifically for mixtures that form azeotropes (constant boiling mixtures) and do not obey Raoult's Law. It requires adding an external substance (an entrainer) to change the molecular interactions and break the azeotrope, which is not a requirement for fractional distillation.
4. What are the two main types of azeotropes? Give an example for each.
Azeotropes are classified based on their boiling points relative to their components:
- Minimum-Boiling Azeotropes: These are formed by liquid pairs that show a large positive deviation from Raoult's law. The boiling point of the azeotrope is lower than the boiling points of either of its pure components. Example: The ethanol-water mixture is a minimum-boiling azeotrope.
- Maximum-Boiling Azeotropes: These are formed by liquid pairs that show a large negative deviation from Raoult's law. The boiling point of the azeotrope is higher than the boiling points of its pure components. Example: A mixture of 68% nitric acid and 32% water by mass.
5. Why exactly can't an azeotrope be separated by simple or fractional distillation?
An azeotrope cannot be separated by these methods because, at the azeotropic composition, the liquid and vapour phases have the exact same mole fraction of each component. In normal distillation, separation is possible because the vapour phase is always richer in the more volatile component. In an azeotrope, this difference disappears, and the mixture boils at a constant temperature as if it were a single pure substance. Therefore, no further enrichment or separation is possible through boiling alone.
6. How does the choice of an entrainer, like benzene or toluene, help separate the ethanol-water azeotrope?
When an entrainer like toluene is added to the ethanol-water azeotrope, it forms a new, ternary (three-component) azeotrope with a boiling point lower than any of the other components or the original azeotrope. For instance, the water-ethanol-toluene azeotrope boils at a lower temperature than the water-ethanol azeotrope. This ternary azeotrope, containing most of the water, is distilled off first. This process removes the water from the mixture, breaking the original azeotrope and leaving behind nearly pure, anhydrous ethanol.
7. What are some important industrial applications of azeotropic distillation?
Beyond the common example of producing anhydrous ethanol for fuel or as a solvent, azeotropic distillation has several key industrial uses:
- Acetic Acid Purification: It is used to dehydrate acetic acid, as water and acetic acid form an azeotrope.
- Solvent Recycling: It helps in breaking azeotropes formed during chemical reactions to recover and recycle valuable solvents like isopropanol.
- Esterification Reactions: It is used to remove water produced during esterification, which is often an equilibrium reaction. Removing the water drives the reaction towards completion.
