Raw MaterialsPetroleum crude oils are complex mixtures of hydrocarbons, chemical compounds composed only of carbon (C) and hydrogen (H). The simplest of the hydrocarbon molecules is methane, which has one carbon atom and four hydrogen atoms per molecule. The next simplest, ethane, 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 CnH2n+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) 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. 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, has double bonds linking every other carbon molecule. 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.
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