Distillation Theory Part V – Simple Distillation

In Part IV, we learned how to determine the partial vapor pressure of each component in a liquid mixture.  In this post, we will learn how that translates to distillate composition.  For the purpose of this post, we will ignore the phenomenon of azeotrope.  That will be discussed in a future post.

First, let’s talk about condensing.

When a liquid mixture reaches boiling, the vapors rise above the liquid freely.  These vapors fill the gas phase and will reach every corner of the container that they are in.  Whenever they come in contact with a surface that is cooler than the boiling temperature, the vapors condense on that surface.  This means the vapors return to the liquid phase on that cool surface.

Equipment manufacturers take advantage of this and place a heat exchanger above the boiling vessel to collect the vapors.  It is positioned in such a way that the vapors condense on the cool heat exchanger and the liquid (known as distillate) runs off into a separate vessel.  This is how a distillation is run.  A liquid mixture is placed in a boiling vessel and controlled to a specific pressure.  Heat is applied and the mixture starts to boil.  The vapors rise to the condenser and are collected as distillate.

The composition of the distillate is determined by the ratio of the partial vapor pressures.  In our example from Part IV, we discussed a binary liquid mixture of methanol and water.   The mole ratio of the water was 0.6, and the mole ratio of the ethanol was 0.4.  We used the partial vapor pressure equation to calculate a partial vapor pressure of water at 20°C of 11 mmHg and methanol of 38 mmHg.

We take these two pressures and sum them to get 49 mmHg.  Then, we divide each partial pressure by 49 mmHg to determine the mole ratio of each component in the collected distillate liquid.

We find that the water mole ratio is 0.22 and the methanol mole ratio is 0.78.  So, even though 60% of the molecules in the pot (boiling vessel) are water, the distillate is 78% methanol.  This is because the methanol boils at a lower temperature and is more volatile than water.

This is simple distillation.  It is process of bringing a liquid mixture to boiling and collecting the vapors that form.  It is very useful at stripping solvent away from a much higher boiling material, but not useful at separating components that are closer in boiling point than 60-100°C if purity is important.

For example, if you had a mixture that was mostly water and you wanted to strip the methanol away, simple distillation could work.  You would lose some water, but you could always add it back.  But if you are trying to produce pure methanol, this is not a good option.

In the next post, I will describe fractional distillation.

Part 5 in theory of distillation series.