Alkylation unit

An Alkylation unit is one of the conversion processes used in petroleum refineries. It is used to convert isobutane and low-molecular-weight alkenes (primarily a mixture of propene and butene) into alkylate, a high octane gasoline component. The process occurs in the presence of a strong acting acid such as sulfuric acid or hydrofluoric acid (HF) as catalyst. Depending on the acid used, the unit takes the name of SAAU (Sulphuric Acid Alkylation Unit) or HFAU (Hydrofluoric Acid Alkylation Unit).

Since crude oil generally contains only 10 to 40 percent of hydrocarbon constituents in the gasoline range, refineries typically use a fluid catalytic cracking unit (FCCU) process to convert high molecular weight hydrocarbons into smaller and more volatile compounds, which are then converted into liquid gasoline-size hydrocarbons. Alkylation processes transform low molecular-weight alkenes and iso-paraffin molecules into larger iso-paraffins with a high octane number. While FCCUs are a very common unit in modern oil refineries, it is not uncommon for a refinery not to have an alkylation unit. Indeed, as of 2010 there are some countries in the world without any installed alkylation units.

The product of the unit, the alkylate, is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons (mostly isoheptane and isooctane). Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. The octane number of the alkylate depends mainly upon the kind of alkenes used and upon operating conditions. For example, isooctane results from combining butylene with isobutane and has an octane rating of 100 by definition. There are other products in the alkylate effluent, however, so the octane rating will vary accordingly.

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