Distillation

Distillation is the method of separating mixtures, in which the conversion of a liquid into vapour is afterwards condensed back to liquid form. distillation method is used for the purification of metals. Distillation is preferable where both solid and liquid have to be extracted from the solution. Also known as simple distillation, it is based on the differences noticed in the volatility and corresponding vapour pressures observed in the components of a mixture.

In this article, we will learn about, Distillation Definition, Distillation Process, Types of Distillation and others in detail.

What is Distillation?

Distillation is a separation technique that is used to extract a mixture of solids in a liquid. It is basically the process of heating the liquid to form vapors, and then condensing the vapors in order to get back the liquid. The liquid that is obtained by performing the condensation of vapor is called the distillate. 

Miscible liquids mix together to form a solution, for instance, a mixture of ethanol and water. Immiscible liquids don’t mix well together. For example, oil and water. The two liquids that are miscible with each other in all proportions can be termed a binary mixture of liquid. This technique is used for the segregation of components of a mixture of two miscible liquids with sufficient difference in boiling points (B.P), that can be boiled without undergoing decomposition. The volatile liquid evaporates on heating which can be recovered by cooling its vapours by the process of condensation. The diagram for distillation is shown in the image attached below:

Distillation Process

Distillation Process is proceed in the following ways,

• Homogeneous mixture containing a solid and liquid are heated together in a closed distillation flask.
• Liquid escapes the distillation flask forming vapours since they are volatile.
• These vapours are cooled by passing them through the condenser.
• Pure liquid is obtained by the condensation process in the given vessel.
• Non-volatile solids are left as residue.

Raoult’s Law and Dalton’s Law in Distillation

For a solution containing a mixture of liquids, the distillation process is dependent on Dalton’s law and Raoult’s law. 

Raoult’s law states that the partial pressure of any specific liquid in an ideal liquid mixture equals the product of the vapour pressure of the pure liquid component and its mole fraction. Also, Dalton’s law of partial pressures states that the total pressure exerted by the mixture of gases is equal to the sum of the partial pressures of all the individual gases collectively.

Combining both laws we can state that, if a mixture of liquids is heated, the vapour pressure of the individual components of liquid increases, which increases the overall vapour pressure. Thus, it is safe to say that, a mixture cannot have multiple boiling points. This concept is used to separate liquid mixtures.

Equipment needed for Distillation

The process of distillation basically uses the following apparatus: 

• Still: A vaporizing chamber is used to place the material to be distilled. It is heated in order to provide vaporization of the volatile constituents. It is also attached to a condenser and a trap is fixed in between.
• Condenser: It is used to condense the vapour. It is kept cold by circulating water/air through the jacket. Condensers are available as single-surface condensers and multi-surface condensers.
• Receiver: It is used to collect the distillate.

Principle of Distillation

The principle of Distillation is explained by the points added below:

• Distillation is carried out at the solvent’s boiling points. Boiling takes place when the vapour pressure is equivalent to the atmospheric pressure.
• Separation of the components of the mixture takes place better in case of the higher relative volatility of a liquid.
• On supplying heat to the liquid, the vapour boils and then condensation takes place.

Why is it Impossible to Completely Purify a Mixture by Distillation?

It is impossible to completely purify a mixture by Distillation as at the boiling point of the mixture of liquids, all the volatile constituents in the mixture boil and the quantity of any constituent in the resulting mixture of vapour is based on the contribution of the total vapour pressure of the mixture. Thus, the compounds with higher partial pressures are concentrated in vapour phase whereas the compounds with low partial pressures are concentrated in the liquid phase.

Since we know that no component in the mixture can have zero partial pressure, it is nearly impossible to obtain a completely pure sample of a component from the mixture via the distillation process.

Types of Distillation

There are various types of Distillation that are,

• Simple distillation
• Fractional distillation
• Steam distillation
• Vacuum distillation
• Air-Sensitive vacuum distillation
• Short path distillation
• Zone distillation

Now let’s learn about them in detail.

Simple Distillation

Simple distillation is the basic process of distillation it involves heating the liquid mixture to its boiling point and then condensing the resulting vapours immediately.
This method of distillation is only applicable when the difference between the boiling points of the liquid is a considerably large minimum difference of 25°C is required at least. Raoult’s Law helps to purify the liquid separated.

Fractional Distillation

Fractional Distillation is used to separate miscible liquids that are volatile in nature. The boiling points of these liquids are close enough. The fractionating column apparatus is used to simulate the separation. Also known as rectification, since the vapour is condensed partially and returned as a liquid. It is basically a process where the vaporization of a liquid mixture gives rise to a mixture of constituents, followed by the extraction of the required component in its pure form. 

However, this technique can be only used to separate miscible liquids, which form PURE azeotropic mixtures.

Principle of Fractional Distillation

• On distillation of the liquid mixture, the partial condensation of the vapour occurs in a fractionating column.
• In the column, advancing vapour from the still comes in touch with the condensing vapour that returns back to the still. This causes the enrichment of vapour with the more volatile component.
• By constant condensation and heating of the vapour, an equilibrium is achieved between liquid and vapour. This results in the extraction of a more volatile component from the mixture.

Applications of Fractional Distillation

Fractional distillation can be used to separate the mixtures like: 

• Acetone and Water
• Chloroform and Benzene
• Separation of Gases From Air

Steam Distillation

Steam distillation separates the heat-sensitive components of the mixtures.
This process is achieved by passing steam through the heated mixture which vaporizes some mixture. Thus a high heat-transfer rate is established without requiring higher temperatures. Resulting vapour is then condensed to the distillate. This process helps to obtain essential oils and herbal distillates from various aromatic flowers and natural herbs.

Vacuum Distillation

Vacuum distillation is used for separating mixtures of liquids with high boiling points. For boiling such mixtures heating them to high temperatures is very inefficient. Therefore, boiling such mixtures is achieved by lowering the pressure of the surroundings. Lowering the pressure helps to reduce the boiling of the mixture and as soon as the vapour pressure of the mixture equals the vapour pressure of the surroundings the mixture changes to vapour.
The vapours are then collected and then condensed as the distillate. Vacuum distillation method obtains a high-purity sample of compounds that decompose at high temperatures.

Air-Sensitive Vacuum Distillation

Compounds that easily react with air cannot be directly distilled. They are distilled using the vacuum distillation process.  This process is carried out in vacuum but the vacuum is replaced with inert gas as soon as the process is completed. This type of distillation process is called air-sensitive vacuum distillation.

Short Path Distillation

Short-path distillation purifies a small quantity of the compound which is unstable at high temperatures. This process is done under low pressure. In this process the distillate travels a very short distance before being collected hence, this process is called Short Path Distillation.

Zone Distillation

Zone distillation process involves the partial melting of the given substance and then condensing the resultant vapours to obtain a pure distillate. This process is carried out in long containers with the help of zone heaters.

Applications of Distillation

Some of the Applications of Distillation are:

• Purification of organic solvents-absolute alcohol (100%).
• Separation of non-volatile substances from volatile ones.
• Purification of drugs manufactured using the chemical process.
• Refining of petroleum products – Petroleum Ether 60,80.
• Recovery of solvents – synthesis.
• Remove salt from the water to prepare drinking water.
• Separation of Methanol or ethanol from water
• The concentration of oils and beverages in the food industry.

Simple Distillation vs Fractional Distillation

The difference between Simple Distillation and Fractional Distillation are tabulated below:

Simple Distillation	Fractional Distillation
Vapour is directly passed through the condenser.	Vapour must pass through a fractionating column in which partial condensation of vapour occurs.
Condensate is collected directly onto the receiver.	Condensation occurs in the fractionating column, which results in a part of the condensing vapour returning to the still.

FAQs on Distillation

1. Define Distillation?

Distillation is the process of selectively boiling a liquid mixture and then subsequently condensation of the component of the liquid mixture is done. In this technique, the concentration of the liquid mixture increases. It is also used to obtain the pure substance from the mixture.

2. What is Fractional Distillation?

Fractional distillation is a technique for separating individual elements from the mixture using the concept that each component boils at a different temperature.

3. What is the Use of Fractional Distillation?

Fractional distillation is used in separating liquid with near-equal boiling points.

4. What is Water Distillation?

In water distillation, impure water is evaporated leaving the soil impurities in form of inorganic salt. The vapour is then condensed to obtain pure water.

5. What is Destructive Distillation?

Pyrolysis is also referred to as destructive distillation or carbonization. It is the process of thermal decomposition of organic matter at high temperatures, about 900°C.

6. What is Steam Distillation?

Steam Distillation is a technique that is used to separate heat-sensitive components of a mixture. Steam Distillation is achieved by passing the steam through the mixture.

7. What is Vaccum Distillation?

Vacuum Distilation is a type of distillation process that is used to separate mixture with high boling points.

8. What is Zone Distillation?

Zone Distillation is a distillation technique that uses partial melting of substances and then condensating the vapous to obtain pure distillate which further distilled to obtain pure substance.