Chemical distillation is a process whereby a homogeneous (meaning only one solution phase) solution mixture consisting of 2 or more chemical species are attempted to be separated. Students learn from high school science that distillation is a physical process, meaning there is no chemical reaction taking place, just like the melting of ice; the components or molecules making up the solid ice and liquid water are the same.
Distillation works on a very simple principle: the difference in chemicals’ melting points.
BEHAVIOUR of PURE SUBSTANCES
Consider we have a beaker of pure water (we know it boils at 100 degrees Celsius at standard atmosphere and pressure) and a beaker of pure ethanol (which boils at 78 degrees Celsius). If we were to heat these 2 beakers separately, we would find that they would boil at their respective boiling points (i.e. 78 degrees Celsius for ethanol and 100 degrees Celsius for pure water). Even if more heat is applied to the beakers, the boiling temperatures of the pure substances will remain CONSTANT.
BEHAVIOUR of MIXTURES
Now, we consider a mixture of water and ethanol. This mixture will start boiling at about 78 degrees Celsius and the temperature will CONTINUE to increase when more heat is applied to the mixture. Here we see that the boiling temperature of the mixture is NOT CONSTANT.
MIXTURE BOILING
During mixture boiling (for example at 85 degrees Celsius), a vapor phase is generated above the boiling liquid phase. There is an important note that this vapor phase is a mixture, while the liquid phase is another mixture. Quite reasonably, the vapor mixture must be richer in the ethanol and the liquid mixture must be richer in the less volatile species (water). It is in nature that complete separation between the 2 species rarely occurs; due to the interactions of the water molecules and ethanol molecules, not all the ethanol molecules get boiled off during boiling at 85 degrees Celsius and some water molecules are able to gain enough energy to escape the liquid phase even below their pure-state boiling point.
STAGED OPERATIONS
As illustrated in the previous point, complete separation does not usually occur and additional separation operations are required when purer chemicals are desired. This means the vapor mixture and liquid mixture will be going through another heating operation separately to generate more vapor-liquid mixtures. It must be realized that the new vapor mixture obtained from the original vapor mixture (from first round of heating) is purer in ethanol content compared to the original vapor mixture and likewise for the new liquid mixture, the water content must be higher.
COLUMN DISTILLATION
It should be noted that staged operations as described in the previous point would be laborious to conduct. Scientists found a method to simplify the staged operations – column distillation. There are plates or trays lined in the column, and each of this tray or plates represent a single stage. Beginning from the bottom of a column, the mixture M is boiled, to give the vapor mixture V1 and liquid mixture L1. This vapor mixture V1 rises up the column and condense on the upper tray or plate and it is another stage where it will then produce vapor mixture V2 and the liquid mixture L2. This V2 mixture will continue to move up and get separated and get increasing pure in ethanol concentration, while the liquid will drip down to the lower plates and water coming out from the bottom of the column is higher in water concentrations.
APPLICATIONS in CHEMICAL INDUSTRIES
Distillation is used extensively in the chemical industries, for the separation of chemicals in a reactant-products mixture, or in crude oil distillation, where we get the natural gas, petrol and diesel that based the development of human civilizations. Of course, application in the industrial scale is not as simple as we think. Intricate calculations and planning must be involved to design the process, such that wastes and losses due to miscalculations and deviations is reduced to the minimum.
