Process Design Aspects of Alkylation
In designing alkylation processes, several aspects are especially important.
The level of agitation and the ability to provide intimate contact between the two liquid phases is always of great importance. For alkylation with H2S04, the main reactions occur at, or at least near, the interfaces between the liquid phase. The following design factors are important:
- Both the type of agitator and the rate of agitation. In one comparison using H2S04 and 2-butenes, the alkylate quality increased by about 7.5 RON as the agitation speed increased from 1,000 to 3,000 rpm.
- Baffling and the location of the impeller in the reactor vessel.
- Continuous liquid phase in reactor. With H2S04, acid-continuous suspensions produce alkylates having higher quality by about 1-3 RON than hydrocarbon-continuous emulsions.
- The olefin feed should preferentially be premixed with the isobutane before entering the reactor.
High levels of agitation are less important for alkylation using HF because HF has much lower viscosities than H2S04, and HF is slightly soluble in the hydrocarbon phase.
Alkylation reactions are highly exothermic, necessitating cooling. When H2S04 is used, improved quality alkylates are generally obtained as the reaction temperatures are reduced to about 41° F.
At lower temperatures, the viscosity of the acid increases and effective agitation is a problem. Refrigeration costs of course increase as the reaction temperatures are decreased.
When HF is used, alkylation occurs normally at 86-113° F, and cooling water is generally used as a coolant. This means a substantial cost saving compared to alkylation using H2S04.
Lower temperatures substantially improve the quality of the alkylates produced from 2-butenes and isobutylene when HF is used, but the quality is greatly decreased in the case of 1-butene.
Higher ratios of isobutane to olefins in the feed streams to the reactor minimize the undesired polymerization reactions. The quality of the alkylate hence increases as the ratios increase (especially when the ratios are relatively small). Costs of separating and recycling the unreacted isobutane and cooling both become larger as the ratio increases. Alkylation plants employing H2S04 as the catalyst often operate in the range of 5:1 to 8:1. Plants employing HF generally operate at higher ratios, such as 10:1 to 15:1.
Slightly higher quality alkylates and lower acid consumption normally occur if the residence time of the hydrocarbon-acid suspension in the reactor, is increased, i.e., the space velocities are decreased. Longer residence times, however, reduce the capacity of a reactor and increase operating expenses.
Plant results show that several additives to H2S04 result in improved production rates, improved alkylate quality, and/or decreased acid consumption. These additives, which presumably accumulate at the acid-hydrocarbon interface, result in improved hydride ion transfer and in improved interracial surface tensions. Fluorosulfonic acid promoters gave beneficial results when added to HF.
In summary, improved quality alkylate and generally lowered acid composition can be obtained by several operation changes, but unfortunately all tend to increase operating costs. Operating conditions for an alkylation plant should obtain maximum profits.
Reference used: L. F. Albright: Alkylation will be key process in reformulated gasoline era, Oil & Gas Journal, 77, 79-92, 1990.