Overcoming Azeotrope ii

Since the previous method does not work with all azeotropes, we need to have a look at other methods.

The second method to be dealt with in this entry is:

2) Extractive distillation : Unlike pressure swing, there is no need to alter the operating pressure. All we have to do is to introduce a third component ( in case of having a mixture of two liquids ). This third component is also called separating agent. It is non-volatile and has normally a high boiling point than the key components. Besides that it is also important to mention that this separating agent must not form any azeotrope with the key components.

Let us assume we have a mixture of two liquids A and B.

A: low-boiling point
B: high-boiling point

A and B form an azeotrope at a certain composition.
By introducing a third component, C, the order of boiling point will look like this:

A: lowest-boiling point
B: intermediate-boiling point
C: highest-boiling point

The function of the separating agent is to alter the relative volatility of the mixture. Normally this method is used when we have a mixture whose relative volatility is very low or close to one. There are other factors that we have to consider in choosing the right separating agent.

1) It must not be toxic and harmful
2) It must be cheap
3) It must be easily and readily available
4) It must not react chemically with the key components

Column 1, C1 is able to separate A after adding C as the relative volatility changes or increases.
The bottom product from C1 is fed into the second column where B and C is separated as distillate and bottom product respectively. C is the recycled into the first column. The second column, C2 is also known as the solvent recovery column.