Distillation Theory Part VI – Fractional Distillation

In the previous post, simple distillation was described.  Simple distillation is very useful when removing solvent from a much larger product, or separating components that have very different boiling points.

However, simple distillation is not sufficient when high purities are needed and the starting feed material contains components with similar boiling points.  For example, the methanol water mixture we have been using cannot be separated using simple distillation.  For these situations, chemists and chemical engineers employ fractional distillation.

The most basic form of fractional distillation consists of a vessel containing the liquid mixture to be purified, a condenser to catch the vapors, a packed column between the vessel and the condenser, and a receiving vessel to hold the distillate.  The column is placed vertically between the vessel and condenser.  The column can be packed with all different types of media, or the packing can be solid mesh structures.  Each packing type has a specific use that will not be discussed in this simple explanation of fractional distillation.

When the vapors rise from the boiling liquid, they encounter the bottom of the column.  The column is cool, so the vapors condense in the bottom portion of the column.  Taking the example of 0.4 mole ratio methanol in water from earlier posts, the mole ratio of the condensed vapors would be 0.78 methanol.  This liquid is enriched in methanol, and will boil easier than the original mixture.  As this liquid starts to drip and fall back down to the vessel, it is met by more vapors from below.  These vapors are hot, and they provide enough energy to boil the new liquid mixture.

Now, the new liquid mixture is 78% methanol, meaning that the methanol will make up even more of the new vapors that form.  The vapors will be 95% methanol, and they will rise a little further up the column, where they will encounter a cold section of column and condense.  The process will repeat, this new liquid will be boiled, and the new vapors will be even more enriched in methanol.  By the time the vapors reach the condenser at the top of the column, they will consist almost entirely of methanol.  Very pure methanol can be collected.

Each spot of condensing and boiling on the column is called a theoretical plate.  Engineers can calculate based on packing, column height and other factors exactly how many plates a column has, and how many are needed to perform a given separation.  Fractional distillation is a very powerful tool in performing complex and challenging distillations.

In the next post, I will talk about wiped film distillation.

Post 6 in theory of distillation series.