Petrochemicals are generally chemical compounds derived from petroleum either by direct manufacture as by-products from the variety of processes that are used during the refining of petroleum. Gasoline, kerosene, fuel oils, lubricating oils, waxes, asphalts, and the true sense, chemical compounds but are in fact intimate mixtures of hydrocarbons.
The classification of materials such as petrochemicals is used to indicate the source of the chemical compounds, but it should be remembered that many common petrochemicals can be made from other sources and the termonology is therefore a matter of source identification.
The starting materials fro the petrochemical industry are obtained from crude petroleum in one of two general ways. They may be present in the virgin petroleum and as such, are isolated by physical methods, such as distillation or solvent extraction. On the other hand, they may be present in trace amounts and are synthesized during the refining operations. In fact, unsaturated hydrocarbons, which are not usually present in virgin petroleum, are nearly always manufactured as intermediates during the varous refining sequences.
The manufacture of chemicals from petroleum is based on the ready response of the various compounds to basic chemical reactions, such as oxidation, halogenation, nitration, dehydrogenation, polymerization, and alkylation. The low-molecular-weight paraffins and olefins, as found in natural gas and refinery gases, and the simple aromatic hydrocarbons have so far been of the most interest because it is individual species that can readily be isolated and dealt with. A wide range of compounds is possible, manyof which are being manufactured, and we are now progressing the stage in which a sizable group of products is being prepared from the heavier fractions of petroleum. For example, the various reactions of petroleum heavy ends, in particular the asphaltenes, indicate that these materials may be regarded as chemical entities and are able to participate in numerous chemical or physical conversions to, perhaps, more useful materials. The overall effect of these modifications is the production of materials that either afford good-grade aromatic cokes comparatively easily or form products bearing functional groups that may be employed as a nonfuel material.