Abstract

Petroleum is a complex mixture, consisting primarily of hydrocarbons, derived from ancient living organisms deposited as geological sediments. The composition of petroleum conveniently described in terms of refinery distillation varies greatly from one source to another. The naphtha, middle-distillate, vacuum gas oil, and vacuum resid fractions increase in complexity and molecular weight. The elemental and molecular composition of these fractions show a general shift from simple saturated hydrocarbons in the naphtha to more complex aromatics and eventually sulfur-, nitrogen-, and oxygen-containing hydrocarbons in the heavier distillates. The nondistillable vacuum resid includes still larger molecules that appear to contain several groups of the benzologues of smaller molecular structures linked together plus the largest portion of the S, N, O, and metal compounds. Complete compositional analysis of petroleum is possible only for low boiling fractions. For high boiling fractions, composition is usually determined by compound types and carbon number ranges. For nonvolatile fractions, only average structure can be obtained. Elaborate separation into well-defined fractions can aid in compositional analysis by sophisticated analytical techniques.