In the previous article fractional distillation has been discussed. It again utilizes the different boiling points of the components of crude oil, enabling their separation through distillation. These fractions are then collected for further processing and refinement. One must take note that fractional distillation is an efficient but an imperfect process – the isolated compounds may contain not only one pure substance, or could contain a substance with various other impurities combined; hence they require further treatment. Also, one could process the other products and convert them into other hydrocarbons depending on need (converting diesel to gasoline). Chemical processing achieves all these other goals.

In chemical processing, a refinery could combine small hydrocarbons and transform them into a longer or larger hydrocarbon chain, also known as unification. The major step is catalytic reforming wherein a platinum or platinum-rhenium mixture speeds up the aggregation of smaller hydrocarbons into larger ones (for example, naphtha into aromatics). Hydrogen gas belongs to the significant products that are produced by this process.

A long hydrocarbon chain subsequently could be broken down into smaller hydrocarbon components, termed cracking. Cracking may be done thermally (for example, by steam: this breaks down ethane, butane and naphtha converting them into ethylene and benzene), or by speeding the breakdown through catalysis as well (using aluminum hydrosilicate, bauxite, or alumina-silica).

And in a certain fraction the hydrocarbons could be reformed to become another type of hydrocarbon, and this processed is called alteration. One of the processes is alkylation (technically an formation of another bond in double-bonded hydrocarbons making them a triple bond) – the mix of butylene and propylene in the presence of hydrofluoric or sulphuric acid yields alkynes that are hydrocarbons with high octane ratings, and hence blended into gasoline mixtures.

Impurities, such as water, metals, inorganic salts and compounds of sulphur and nitrogen, are removed via treatment using an absorption column (for water), sulphuric acid column (unsaturated hydrocarbons) and hydrogen sulphide scrubbers among others.

Treated fractions could be then fractionally distilled to yield separate compounds. All these chemical processes are done in refineries, as well as fractional distillation, to deliver the needed petroleum products.