Organic chemicals from oil and natural gas

Up to the early 1940s the production of chemicals from petroleum was confined to North America. This was due in no small measure to the policy of locating refineries adjacent to the oilfields. From 1950 this policy was reversed and the crude oil was transported from oilfields (the major ones then being located in the Middle East) to the refineries, now located in the areas occupied by the major users of the end products. This coincided with, or led to, the development of the European petrochemical industry, followed in the 1960s by Japan. Oil-producing countries, such as Saudi Arabia, have continued 

Crude oil consists principally of a complex mixture of saturated hydrocarbons—mainly alkanes (paraffins) and cycloalkanes (naphthenes) with smaller amounts of alkenes and aromatics—plus small amounts ( 5% in total) of compounds containing nitrogen, oxygen or sulphur. The presence of the latter is undesirable since many sulphur-containing compounds, e.g. mercaptans, have rather unpleasant odours and also, more importantly, are catalyst poisons and can therefore have disastrous effects on some refinery operations and downstream chemical processes. In addition, their combustion may cause formation of the air pollutant sulphur dioxide. They are therefore 

In practice the feedstock, being a crude-oil fraction such as gases or naphtha, is a mixture and therefore the product consists of a number of unsaturated and saturated compounds (cf. p. 358). Cracking is used to break down the longer-chain alkanes, which are found in (say) the gas-oil fraction, producing a product akin to a naphtha (gasoline) fraction. This process was developed because of the great demand for naphtha and the relatively low demand for gas oil, particularly in the U.S. 

Aromatics are made from alkanes and cycloalkanes by a process aptly named reforming, which may be represented, again in over-simplified terms, as 

As in cracking, the feedstock is a mixture of compounds. A substantial conversion (c. 50%) to aromatic compounds is achievable. The principal components, benzene, toluene and the xylenes, are separated for further processing. 

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