by Alexius Tan

Created on: December 15, 2009

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

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