Isobutane is alkylated with C₃-C₅ olefins to produce the highest-quality and cleanest-burning gasolines. More than 1.1 million barrels of alkylate/day are produced in the United States (P. Pyror, personal communication). The amounts of alkylate produced in the remainder of the world are considerably less, but they are increasing at a significant rate. In the United States, about 11-13% of the total gasoline pool is alkylate. This percentage depends on the location in the United States and on the season. Alkylate production will likely increase substantially in the future.

Alkylation was first practiced for gasoline production about 60 yr ago. At that time, most of the alkylate was used as fuel for the airplanes used in World War II. Four quite distinct reactors were developed in which isobutane and olefins were introduced as liquids to the reactor. In the reactor, the hydrocarbon liquids are contacted with either liquid sulfuric acid or liquid hydrofluoric acid (HF), which acts as a catalyst. Dispersions of these two relatively immiscible liquids are formed. The alkylate product formed is a mixture of mainly C₅-C₁₆ isoparaffins. Alkylate products often have research octane numbers (RONs) varying from 93 to 98 (the motor octane numbers tend to be two to three units lower).

The alkylates and by-products produced using sulfuric acid and HF have compositions that differ in several respects, as discussed later. About 50 yr ago, considerably more alkylate was produced using sulfuric acid. For the next 30-35 yr, the relative importance of the two processes using HF as catalyst increased until essentially equal amounts of alkylate were produced with each acid. In the late 1980s, however, two events occurred that indicated safety concerns with HF needed to be re-examined. First tests were conducted in which 1000 gallons of liquid HF were ejected into the atmosphere.1 An aerosol cloud formed that contained lethal levels of HF at ground level for 5-6 mi downwind. Alkylation units at the time sometimes had much larger liquid HF inventories. Second, a major release of gaseous HF occurred in an American refinery as the result of an accident. No human fatalities occurred probably because the release was gaseous HF and no aerosol cloud formed. Many residents living within several blocks of the refinery, however, required medical care. Liquid HF often results in serious skin burns and even damage to bones. Also in the late 1980s, two workers in an alkylation plant were killed because of an HF release. Since that time, no grassroot alkylation plants have been built in the United States, but several with sulfuric acid have been.

In current processes that use either sulfuric acid or HF, isobutane in large excess and olefins are introduced as liquids into the reactor. After completion of the reactions, the liquid-liquid dispersions are separated by decanting. The alkylate product is separated by distillation or stripping from the unreacted isobutane, which is recirculated to the reactor. This entry reviews the chemistry, physicochemical phenomena, current processes, and finally suggests methods to improve significantly the alkylation process.