Typically, the overhead fraction from the distillation column consists predominantly of the more volatile component and some small amount of the less volatile component and the bottoms fraction consists predominantly of the less volatile component and some small amount of the more volatile component.Ī liquid mixture containing many components is called a multi-component mixture. When a multi-component mixture is is distilled, the overhead fraction and the bottoms fraction typically contain much more than one or two components. A bottoms fraction containing predominantly the less volatile components ranging from isobutane (having four carbon atoms) to decanes (ten carbon atoms).An overhead fraction containing predominantly the more volatile components ranging from methane (having one carbon atom) to propane (having three carbon atoms).For distilling such a mixture, the distillation column may be designed (for example) to produce: For example, some intermediate products in an oil refinery are multi-component liquid mixtures that may contain the alkane, alkene and alkyne hydrocarbons ranging from methane having one carbon atom to decanes having ten carbon atoms.
Such a distillation column is typically called a depropanizer. The designer would designate the key components governing the separation design to be propane as the so-called light key (LK) and isobutane as the so-called heavy key (HK). Thus, for the distillation of any multi-component mixture, the relative volatility is often defined as In that context, a lighter component means a component with a lower boiling point (or a higher vapor pressure) and a heavier component means a component with a higher boiling point (or a lower vapor pressure).
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