Composition by volatility

Distillation is a common method for the fractionation of petroleum that is used in the laboratory as well as in refineries. The technique of distillation has been practiced for many centuries, and the stills that have been employed have taken many forms (Speight, 1999). Distillation is the first and the most fundamental step in the refining process (after the crude oil has been cleaned and any remnants of brine removed) (Bland and Davidson, 1967 Speight, 1999, and references cited therein Speight and Ozum, 2002, and references cited therein), which is often referred to as the primary refining process. Distillation involves the separation of the various hydrocarbon compounds that occur naturally in a crude oil into a number of different fractions (a fraction is often referred to as a cut). 

Fractions from the atmospheric and vacuum towers are often used as feedstocks to these second-stage refinery processes that break down the fractions or bring about a basic chemical change in the nature of a particular hydrocarbon compound to produce specific products. 

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