A Complete Understanding of the Key-Factors in Distillation

A Complete Understanding of the Key-Factors in Distillation

Distillation is the preferential separation of a more volatile component from the less volatile. It is based on the idea of relative volatility.

The important thing to understand here is that it is a physical operation. It does not involve the formation or breaking of molecules.

In a refinery, the engineers separate mixtures of different hydrocarbons by distillation.

We should not confuse it with evaporation. In evaporation, the vapours have only one component which is volatile and the other component in the mixture is non-volatile.

While in the distillation, all components are volatile. But their volatilities are different. So we separate them on the basis of their different volatilities.

The following example will help you better understand the difference.

Suppose we want to concentrate glycerine from its dilute aqueous solution. Up to its 80 % concentration, water as a single component would be present in the vapour phase. Therefore, it is evaporation.

Beyond 80% concentration, the vapour phase contains both glycerine and water. So the operation would be distillation.

Reference: Mass Transfer Principles and Operations by A.P. Sinha & Parameswar De

A Complete Understanding of the Key-Factors in Distillation

Different Terminologies used in the Distillation Process

1. Relative Volatility

Relative volatility is the ratio of mole fractions in the vapour phase to the ratio of mole fractions in the liquid phase at equilibrium. For example, in a mixture of two components A & B

Relative Volatility (αAB)= (yA/yB)/(xA/xB)

where ‘y’ represents the mole fractions of the vapour phase

and ‘x’ represents the mole fractions of the liquid phase

For an ideal case, relative volatility can also be defined as:

αAB = pvA/pvB

where pvA and PvB are the vapour pressures of components A and B respectively

From the Antoine Equation, we know that relative volatility is the function of temperature. It tells us about the ease of separation.

By decreasing the temperature, relative volatility increases. This means we get more mole fractions of the lighter component in the vapour phase.

2. Vapour Pressure

Suppose we have a closed container that is partially filled with a liquid. Some of the molecules escape the liquid at any given temperature (evaporation).

In this way, they form a vapour space above the liquid. The vapour molecules that strike the liquid surface, stick to it. When the number of molecules leaving the liquid and returning back become equal, we say the gas and liquid are in equilibrium with each other.

The pressure that vapours exert when the vapour and liquid phase are in equilibrium is called the vapour pressure of the liquid.

By increasing the temperature, the vapour pressure increases until a new state of equilibrium gets established.

This phenomenon is the basis of understanding the difference between evaporation and boiling and also the impact of pressure on the boiling point.

If we heat the liquid in a container the vapour pressure increases as discussed above.

If we continue heating it so that the vapour pressure equals the atmospheric pressure, evaporation gets rapid and boiling occurs.

Thus we can increase or decrease the boiling point of a substance by just increasing or decreasing the pressure above it.

3. Partial Pressure

When we mix two or more gases with each other in a container, each gas exerts some pressure on the walls of the container.

The total pressure of the mixture is the sum of the pressure exerted by each gas. Thus the pressure of an individual gas in a mixture is called its partial pressure.

Raoult’s law gives its mathematical explanation as:

The partial pressure of a component (at a constant temperature) is equal to the product of its mole fraction in the liquid and the vapour pressure of the pure component at that temperature.

A = pAxA

where pˊA is the partial pressure of component A

pA is its vapour pressure

xA is its mole fraction

So, for the systems obeying Raoult’s law, we can use it to generate equilibrium data for distillation.

4. Vapour Liquid Equilibrium (VLE) & Txy Pxy Diagrams

We present the vapour-liquid equilibrium data calculated either through experimentation or mathematical methods (Raoult’s law, relative volatility) in the form of plots.

This tells us about the distribution of different components in both vapour and liquid phases. The VLE plot gives information about the following three things in special.

  • Bubble Point: The point at which first bubble/vapor is formed.
  • Dew Point: The point where the first droplet (liquid) forms.
  • Two-Phase: A region on the plot where both liquid and vapours exist simultaneously.

There are generally two types of VLE diagrams. One with constant pressure i.e, Txy diagram and the other with constant temperature i.e, Pxy diagram.

Types of Distillation

Batch Distillation

Batch distillation as the name suggests is carried out in batches. There is no continuous stream of the feed coming in or the products going out.

As the distillation starts, the more volatile component first starts going out. Then if it is further continued, the less volatile goes out also.

So we’ve different components with different compositions in the batch distillation. We use batch distillation when the relative volatility of the different components is quite large.

Flash Distillation

We generally use flash distillation for generating the vapour-liquid equilibrium data especially. What we do is partially vaporise the liquid mixture. Then keep the mixture together for a long enough time so that an equilibrium establishes. It can be batch-wise or continuous.

Continuous/ Fractional Distillation

In continuous/fractional distillation, a mixture is continuously fed into the distillation column and is separated into more refined products.

A typical refinery column, separating crude oil into different products like gasoline, naphtha, diesel, etc. in a continuous operation is a good example.

Steam Distillation

We use steam distillation for a mixture that contains two components that are heat sensitive. By introducing steam into the mixture the individual partial pressure of the other substances decrease. And is carried away with steam.

This makes the volatile component boil at a temperature lower than its boiling point.

Steam must be immiscible with the substances because the downstream separation would be difficult otherwise.

Vacuum Distillation

We see it mostly in the refineries where the heavy residue of the fractionating tower is distilled. The reason is, those heavy components are heat sensitive. By applying vacuum, their boiling points are decreased which helps in their separation prior to the decomposition temperature.

Azeotropic Distillation/Extractive Distillation

An azeotrope is a liquid mixture that boils at a constant temperature. It behaves like, it is an individual component. Ordinary distillation in such a case does not work.

The composition of both vapour and liquid phases remains the same. A third component usually an”entrainer” is used.

If it combines with the low boiling liquid and we recover it in the distillate. This is azeotropic distillation.

And if the entrainer is less volatile itself and dissolves the heavier component into it. It is recovered in the residue and is called extractive distillation.

Read about: The Easy way to comprehend the Pump Curves.


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Umer Farooq

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