The alkylation process combines light iso-paraffins, most commonly isobutane, with C3–C4 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. Iso-butane and C3–C4 olefins are produced as by-products from FCC and other catalytic and thermal conversion processes in a refinery. 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. 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. Alkylate does not contain any olefinic or aromatic hydrocarbons.
Alkylation reactions are catalyzed by strong acids (i.e., sulfuric acid [H2SO4] and hydrofluoric acid [HF]) to take place more selectively at low temperatures of 70°F for H2SO4 and 100°F for HF. 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 [15]. Early commercial units used H2SO4, but more recently, HF alkylation has been used more commonly in petroleum refineries. HF can be more easily regenerated than H2SO4 in the alkylation process, and HF alkylation is less sensitive to temperature fluctuations than H2SO4 alkylation [3]. In both processes, the volume of acid used is approximately equal to the volume of liquid hydrocarbon feed. Important operating variables include acid strength, reaction temperature, iso-butane/olefin ratio, and olefin space velocity. The reactions are run at sufficiently high pressures to keep the hydrocarbons and the acid in the liquid phase. Good mixing of acid with hydrocarbons is essential for high conversions.
Some examples of desired alkylation reactions (combination of iso-paraffins with olefins) are given in Figure 8.6. 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. Subsequent propagation reactions involve the reactions of a tert-butyl cation with olefins to form larger iso-paraffin cations that lead to final products through reactions with iso-butane to form a new tert-butyl cation to sustain the chain reaction [3]. The alkylation reaction is highly exothermic; therefore, cooling the reactor contents during alkylation is important.
Figure 8.8 shows a flow diagram for a UOP HF alkylation process [4]. Olefin and iso-butane feed streams are dried to remove water before they are mixed with the iso-butane recycle stream. The mixture is fed to the reactor, where it is highly dispersed into an incoming stream of acid catalyst. Conversion of reactants to high-quality alkylate takes place quickly, and the mixture flows up to the settling zone. In the settler, the catalyst is separated out as a bottom phase and flows, by gravity, through the catalyst cooler and returns to the reactor. The hydrocarbon phase from the settler, which contains propane, recycled iso-butane, normal butane, and alkylate, is charged to the main fractionator. High-purity propane is sent overhead to pass through the HF- propane stripper, de-fluorinator, and potassium hydroxide (KOH) treater before it is recovered. Recycled iso-butane is drawn from the side of the fractionator and returned to the entrance of the reactor after it is mixed with the dried olefin and isobutane feed. The n-butane product is taken from the side of the fractionators as vapor, condensed and KOH- treated before recovery. The alkylate product is obtained from the bottom of the fractionator. The HF catalyst is regenerated onsite in the regeneration section, where heavy oils (tars) are removed from the catalyst.
Transporting and working with concentrated acids pose safety risks. In particular, HF tends to form a vapor cloud that is difficult to disperse. The major licensor of the HF alkylation processes is UOP, whereas ExxonMobil and Stratford Engineering Corporation license H2SO4 alkylation processes. A newly designed alkylation process by UOP uses a solid catalyst called Alkylene®. Advantages of this new process over traditional HF alkylation processes (liquid acid technology) include no acid transportation, no acid spills, no corrosion, and reduced maintenance cost. Efforts to develop alternative processes that use solid acid catalysts instead of concentrated HF and H2SO4 for alkylation are underway.