Aviation alkylate

In the alkylation of isobutane with butenes, several variables have an important bearing on the quality of the alkylate produced. The most important is the concentration of isobutane in the reactor. Although theoretically only equimolecular ratios of isobutane and butene are required for the reaction, a large excess of isobutane in the reaction zone has been found necessary to suppress undesirable side reactions which result in loss of yield and octane number. Over-all isobutane-olefin ratios of 5 to 1 or higher are necessary for the production of high quality aviation alkylate. 

When World War II ended, the demand for aviation alkylate dropped, and refiners had excess alkylate which they started using in motor gasoline. About half of the alkylation units were soon shut down, and some of the units were dismantled because the octane levels of motor gasoline were not high enough to justify the continued use of this relatively expensive process (6). 

The Korean War caused the demand for aviation alkylate to jump again in 1950, but only about 35 alkylation units were in operation at the... 

World War II when the first commercial isomerization processes were introduced to manufacture isobutane as a feedstock for aviation alkylate. The Friedel-Crafts catalysts are highly active at 100°-200°F whereas the conventional platinum-alumina reforming catalysts give reasonable reaction rates only above 850°F. 

After the war the need for aviation alkylate declined rapidly, and most of the isomerization units closed down. During the motor gasoline octane race in the 1950 s, a number of butane isomerization units were placed on stream. Several pentane isomerization units were placed on stream in the 1960 s, and it is believed that only one or two plants today are being used to isomerize a C5/C6 straight run cut (41). 

The first catalyst to be used widely in commercial alkylation for the production of aviation alkylate was sulfuric acid. The catalyst in the reaction zone is normally 88 to 92 wt. % H2SO4 with the makeup acid usually having a strength of 98 % or higher. 

Typical sulfuric and hydrofluoric aviation alkylates are shown in Table IV. 

Typical Aviation Alkylates from Refrigerated Sulfuric and Water-Cooled Hydrofluoric Acid Units... 

Aviation alkylate made from mixtures of propylene, butylenes, and amylenes is sometimes made in areas where isobutane is available at relatively low cost. 

Fig. 7. Kellogg cascade type of sulfuric acid alkylation for aviation alkylate using autorefrigeration.

A tremendous growth in alkylation is expected for many years in the production of both motor alkylate and petrochemicals. The production of aviation alkylate is not expected to increase in the future, because of the increase in jet-type airplanes on the other hand, it is not expected to decrease to any great extent. 

Before the naphtha is redistilled into a number of fractions with boiling ranges suitable for aliphatic solvents, the naphthas are usually treated to remove sulfur compounds, as well as aromatic hydrocarbons, which are present in sufficient quantity to cause an odor. Aliphatic solvents that are specially treated to remove aromatic hydrocarbons are known as deodorized solvents. Odorless solvent is the name given to heavy alkylate used as an aliphatic solvent, which is a by-product in the manufacture of aviation alkylate. 

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