Question

Explain how the composition of the column material varies from the bottom to the top of the distilling column.

Answer 1

After the mixture to be distilled is fed to the column, liquid flows down the column and across the collecting trays in either cross direction or in countercurrent direction.

A reboiler located at the bottom separates the stream into a vapor stream (that comes back to the column) and a liquid form of stream

-The vapor stream flows upward across the trays and comes into contact with the down-flowing liquid stream, enabling the separation to occur.

-At the top of the column, the vapor is condensed into a condenser. The condensed stream is split into a product stream called the "distillate", and a reflux stream that returns to the top of the column.

In essence:

The column which contains 'Beds' of packing material which make the rising vapours to come into contact with falling liquid within the tower. The heat supplied to the mixture before entering the column partially evaporates the distillation mixture and the vapours rise up the column and slowly cool down. The liquid falls down to the bottom of the column where more heat is suppliee to the liquid by a 'Reboiler' which may be steam generated or fuel generated.The supply of heat here makes more vapours to be produced which rise up the column. As the two phases of the distillation mixture - falling liquid and rising vapour - come into contact and mix with each other, lighter components of the mixture are eliminated out of the liquid and enter into the gas phase while heavier components of the mixture in the vapour are condensed into the liquid phase at the bottom

In this way, as the vapour rises and gradually cools, it becomes lighter and, as the liquid falls, it becomes hotter and heavier.

With this type of distillation column there is generally only a top and bottom product.