Chemical Analysis and Production
Gasoline is produced in oil refineries. Material that is separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet the required specifications for modern engines (in particular octane rating; see below), but will comprise part of the blend.
The bulk of a typical gasoline consists of hydrocarbons with between four and 12 carbon atoms per molecule (commonly referred to as C4-C12).
The various refinery streams blended to make gasoline have different characteristics. Some important streams are:
- straight-run gasoline is distilled directly from crude oil. Once the leading source of fuel, its low octane rating required lead additives. It is low in aromatics (depending on the grade of crude oil), containing some naphthenes (cycloalkanes) and no olefins. About 0-20% of gasoline is derived from this material, in part because the supply of this fraction is insufficient and its RON is too low.
- reformate, produced in a catalytic reformer with a high octane rating and high aromatic content, and very low olefins (alkenes). Most of the benzene, toluene, and xylene (the so-called BTX) are more valuable as chemical feedstocks and are thus removed to some extent.
- cat cracked gasoline or cat cracked naphtha, produced from a catalytic cracker, with a moderate octane rating, high olefins (alkene) content, and moderate aromatics level.
- hydrocrackate (heavy, mid, and light) produced from a hydrocracker, with medium to low octane rating and moderate aromatic levels.
- alkylate is produced in an alkylation unit, involving the addition of isobutane to alkenes giving branched chains but low aromatics.
- isomerate is obtained by isomerizing low octane straight run gasoline to iso-parafins (like isooctane).
The terms above are the jargon used in the oil industry, but terminology varies.
Overall, a typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), and olefins (alkenes). The actual ratio depends on:
- the oil refinery that makes the gasoline, as not all refineries have the same set of processing units;
- crude oil feed used by the refinery;
- the grade of gasoline, in particular, the octane rating.
Currently, many countries set limits on gasoline aromatics in general, benzene in particular, and olefin (alkene) content. Such regulations led to increasing preference for high octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries to add processing units to reduce benzene content.
Gasoline can also contain other organic compounds, such as organic ethers (deliberately added), plus small levels of contaminants, in particular organosulfur compounds, but these are usually removed at the refinery.
Read more about this topic: Gasoline
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