The simplest of the hydrocarbon molecules is methane (CH₄), which has one carbon atom and four hydrogen atoms per molecule. The next simplest, ethane (C₂H₆), has two carbon atoms and six hydrogen atoms. A whole class of hydrocarbons can be defined by expanding upon the relationship between methane and ethane. Known as the paraffins, this is a family of chainlike molecules with the chemical formula C_nH_{2n + 2}. These molecules are also referred to as saturated, since each of the four valence electrons on a carbon atom that are available for bonding is taken up by a single hydrogen or carbon atom. Because these “single” bonds leave no valence electron available for sharing with another atom, paraffin molecules tend to be chemically stable.

Paraffins can be arranged either in straight chains (normal paraffins, such as butane; see figure) or branched chains (isoparaffins). Most of the paraffin compounds in naturally occurring crude oils are normal paraffins, while isoparaffins are frequently produced in refinery processes. The normal paraffins are uniquely poor as motor fuels, while isoparaffins have good engine-combustion characteristics. Longer-chain paraffins are major constituents of waxes.

Once a hydrocarbon molecule contains more than four carbon atoms, the carbon atoms may form not a branched or straight chain but a closed-ring structure known as a cyclo-compound. Saturated cyclo-compounds are called naphthenes. Naphthenic crudes tend to be poor raw materials for lubricant manufacture, but they are more easily converted into high-quality gasolines than are the paraffin compounds.

Unsaturated molecules

Two other chemical families that are important in petroleum refining are composed of unsaturated molecules. In unsaturated molecules, not all the valence electrons on a carbon atom are bonded to separate carbon or hydrogen atoms; instead, two or three electrons may be taken up by one neighbouring carbon atom, thus forming a “double” or “triple” carbon-carbon bond. Like saturated compounds, unsaturated compounds can form either chain or ring molecules. Unsaturated chain molecules are known as olefins. Only small amounts of olefins are found in crude oils, but large volumes are produced in refining processes. Olefins are relatively reactive as chemicals and can be readily combined to form other longer-chain compounds.

The other family of unsaturated compounds is made up of ring molecules called aromatics. The simplest aromatic compound, benzene (C₆H₆), has double bonds linking every other carbon molecule (see figure). The double bonds in the benzene ring are very unstable and chemically reactive. Partly for this reason, benzene is a popular building block in the petrochemical industry.

Unsaturated hydrocarbons generally have good combustion characteristics, but their reactivity can lead to instability in storage and sometimes to environmental emission problems.

Fractional distillation

The primary process for separating the hydrocarbon components of crude oil is fractional distillation. Crude oil distillers separate crude oil into fractions for subsequent processing in such units as catalytic reformers, cracking units, alkylation units, or cokers. In turn, each of these more complex processing units also incorporates a fractional distillation tower to separate its own reaction products.

Modern crude oil distillation units operate continuously over long periods of time and are much larger than the fractional distillation units employed in chemical or other industries. Process rates are commonly delineated in American barrels; units capable of processing 100,000 barrels per day are commonplace, and the largest units are capable of charging more than 200,000 barrels per day.

The principles of operation of a modern crude oil distillation unit are shown in the figure. Crude oil is withdrawn from storage tanks at ambient temperature and pumped at a constant rate through a series of heat exchangers in order to reach a temperature of about 120 °C (250 °F). A controlled amount of fresh water is introduced, and the mixture is pumped into a desalting drum, where it passes through an electric field and a saltwater phase is separated. (If the salt were not removed at this stage, it would be deposited later on the tubes of the furnace and cause plugging.) The desalted crude oil passes through additional heat exchangers and then through steel alloy tubes in a furnace. There it is heated to a temperature between 315 and 400 °C (600 and 750 °F), depending on the type of crude oil and the end products desired. A mixture of vapour and unvaporized oil passes from the furnace into the fractionating column, a vertical cylindrical tower as much as 45 metres (150 feet) high containing 20 to 40 fractionating trays spaced at regular intervals. The most common fractionating trays are of the sieve or valve type. Sieve trays are simple perforated plates with small holes about 5 to 6 mm (0.2 to 0.25 inch) in diameter. Valve trays are similar, except the perforations are covered by small metal disks that restrict the flow through the perforations under certain process conditions.

The oil vapours rise up through the column and are condensed to a liquid in a water- or air-cooled condenser at the top of the tower. A small amount of gas remains uncondensed and is piped into the refinery fuel-gas system. A pressure control valve on the fuel-gas line maintains fractionating column pressure at the desired figure, usually near one standard atmosphere pressure, measured as approximately 1 bar, 100 kilopascals (KPa), or 15 pounds per square inch (psi). Part of the condensed liquid, called reflux, is pumped back into the top of the column and descends from tray to tray, contacting rising vapours as they pass through the slots in the trays. The liquid progressively absorbs heavier constituents from the vapour and, in turn, gives up lighter constituents to the vapour phase. Condensation and reevaporation takes place on each tray. Eventually an equilibrium is reached in which there is a continual gradation of temperature and oil properties throughout the column, with the lightest constituents on the top tray and the heaviest on the bottom. The use of reflux and vapour-liquid contacting trays distinguishes fractional distillation from simple distillation columns.

Intermediate products, or “sidestreams,” are withdrawn at several points from the column, as shown in the figure. In addition, modern crude distillation units employ intermediate reflux streams. Sidestreams are known as intermediate products because they have properties between those of the top or overhead product and those of products issuing from the base of the column. Typical boiling ranges for various streams are as follows: light straight-run naphtha (overhead), 20–95 °C (70–200 °F); heavy naphtha (top sidestream), 90–165 °C (195– 330 °F); crude kerosene (second sidestream), 150–245 °C (300–475 °F); light gas oil (third sidestream), 215–315 °C (420–600 °F).

Unvaporized oil entering the column flows downward over a similar set of trays in the lower part of the column, called stripping trays, which act to remove any light constituents remaining in the liquid. Steam is injected into the bottom of the column in order to reduce the partial pressure of the hydrocarbons and assist in the separation. Typically a single sidestream is withdrawn from the stripping section: heavy gas oil, with a boiling range of 285–370 °C (545–700 °F). The residue that passes from the bottom of the column is suitable for blending into industrial fuels. Alternately, it may be further distilled under vacuum conditions to yield quantities of distilled oils for manufacture into lubricating oils or for use as a feedstock in a gas oil cracking process.