DISTILLATION COLUMN - HOW IT WORK AND IT'S COMPONENTS

 DISTILLATION COLUMN - HOW TO WORK AND IT'S COMPONENTS

What is a Distillation Column? 

• A distillation column is an essential item used in the distillation of liquid mixtures to separate the mixture into its component parts or fractions, based on the differences in volatilities. 

• Fractionating columns are used in small scale laboratory distillations as well as large scale industrial distillations.

Main Components of Distillation Columns:

• Distillation columns are made up of several components, each of which is used either to tranfer heat energy or enhance material transfer.

A typical distillation contains several major components:

• a vertical shell where the separation of liquid components is carried out
• column internals such as trays/plates and/or packings which are used to enhance component separations
• a reboiler to provide the necessary vaporisation for the distillation process
• a condenser to cool and condense the vapour leaving the top of the column
• a reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column

The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column.

HOW COLUMN WORK? 

• Distillation is a process used to separate a mixture of two (or more) components into its virgin state by heating the mixture to a temperature between their respective boiling points.

• For example, at atmospheric pressure, water boils at 212ºF and ethanol boils at about 176ºF. If the mixture of water and ethanol is heated to about 195ºF, the ethanol will boil and change into vapour which is then collected and condensed. The water will separate and remain as a liquid.

A fractionating distillation column is used to make this process more efficient. 

• The distillation column is made up of a series of stacked plates. A liquid feed containing the mixture of two or more liquids enters the column at one or more points. 

• The liquid flows over the plates, and vapor bubbles up through the liquid via holes in the plates. As liquid travels down the column, the vapor comes in contact with it several times due to the multiple plates – a critical process in distillation columns. 

• The liquid and vapor phases are brought into contact because as one molecule of higher boiling material converts from vapor to liquid phase by energy release, another molecule of the low boiling material utilizes the free energy to convert from liquid to vapor phase.

• The base of the distillation column contains a large volume of liquid consisting mostly of the liquid with higher boiling point (in our example, this would be water). Out of the base flows some of this liquid, some of which is heated in the re boiler and returned to the column. This is called the boil up.

• Some vapor escapes from the top of the column and is returned to a liquid state in the condenser. Some of this liquid is returned to the column as reflux, and the remainder is the top product or distillate. Vapor and liquid phases on a given plate approach thermal pressure and composition equilibrium to an extent depending upon the efficiency of the plate.

• In essence, the hot mixture is pumped into the bottom. The tower acts as a heat exchanger, removing heat from the vapors as they rise. Some of them condense back into liquids and fall back down the column. 

• The temperature gradually decreases as you go up the column. Different groups of hydrocarbons condense at different heights – the heaviest at the bottom, the lightest at the top. The final product is in its virgin state.