These occur through ionic chain reactions (Figure 8.7) initiated by donation of a proton from the acid catalyst to an olefin to produce a carbocation that reacts with iso-butane to form a tert-butyl cation. Some examples of desired alkylation reactions (combination of iso-paraffins with olefins) are given in Figure 8.6. Good mixing of acid with hydrocarbons is essential for high conversions. The reactions are run at sufficiently high pressures to keep the hydrocarbons and the acid in the liquid phase. Important operating variables include acid strength, reaction temperature, iso-butane/olefin ratio, and olefin space velocity. In both processes, the volume of acid used is approximately equal to the volume of liquid hydrocarbon feed. HF can be more easily regenerated than H 2SO 4 in the alkylation process, and HF alkylation is less sensitive to temperature fluctuations than H 2SO 4 alkylation. Early commercial units used H 2SO 4, but more recently, HF alkylation has been used more commonly in petroleum refineries. By careful selection of the operating conditions, a high proportion of products can fall in the gasoline boiling range with motor octane numbers (MONs) of 88–94 and RONs of 94–99. Alkylate does not contain any olefinic or aromatic hydrocarbons.Īlkylation reactions are catalyzed by strong acids (i.e., sulfuric acid and hydrofluoric acid ) to take place more selectively at low temperatures of 70☏ for H 2SO 4 and 100☏ for HF. With the recent restrictions on benzene and the total aromatic hydrocarbon contents of gasoline by environmental regulations, alkylation has gained favor as an octane number booster over catalytic reforming. The alkylation process was developed in the 1930s and 1940s to initially produce high-octane aviation gasoline, but later it became important for producing motor gasoline because the spark ignition engines have become more powerful with higher compression ratios that require fuel with higher octane numbers. Iso-butane and C 3–C 4 olefins are produced as by-products from FCC and other catalytic and thermal conversion processes in a refinery. The alkylation process combines light iso-paraffins, most commonly isobutane, with C 3–C 4 olefins, to produce a mixture of higher molecular weight iso-paraffins (i.e., alkylate) as a high-octane number blending component for the gasoline pool.
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