Alkylation unit

Also known as: alky

In refining, the alkylation unit produces a high-quality gasoline blendstock by combining two LPG-range molecules to form one gasoline-range molecule. This involves reacting isobutane with some type of light olefin, typically either propylene or butylene coming from the FCC.

The economic value of the alkylation unit comes from its ability to upgrade low-value light ends (C3s and C4s) to a high-value gasoline blendstock, one that has a premium quality with relatively high octane properties and no aromatic components. Alkylation units have become more popular as aromatics limits on gasoline have been tightened.

The alky relies on, and complements, the FCC. Most, if not all, of the light olefins generated in a refinery come from the FCC unit. The capacity and/or throughput of the alky are typically limited by the availability of the olefin to feed it.

How it works

A mixture of isobutane and light olefins (either C4s, C3s, or a mix) is injected into a series of reaction chambers filled with an acid catalyst. The acid is either sulfuric acid (H2SO4) or hydrofluoric acid (HF). The catalyst causes the isobutane to combine with the olefin to produce either a C8 or C7 naphtha-range product called alkylate.

The isobutane used to feed the alky is produced throughout the refinery as a product of other conversion processes. Typically, this is supplemented with isobutane produced intentionally in a C4 isomerization unit.

The olefin feed for the alky primarily comes from the FCC unit. In general, refiners will preferentially feed the alky with butylene, because this yields the highest quality alkylate (octane and vapor pressure). Also, butylene has a lower-valued alternative use than propylene.

Propylene will also be used whenever there is insufficient volume of butylene available and/or it is not possible to sell the propylene as a separate product.

C5 olefins (amylenes) are also used at times as a supplemental feed. However, this is less common.

The primary product from an alkylation unit is C5+ alkylate. The volume and quality of the alkylate will depend on the olefin feed used (butylene or propylene) and the operating conditions for the unit, but all alkylates benefit from having a zero level of aromatic components.

The alky unit will also yield any saturated propane and/or butane that was present in the feed. The alky produces a small amount of alkylate bottoms.

Author: Tim Fitzgibbon, Refining Industry Sr. Expert

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