Auto-refrigeration processes

Because of its large reactor volume, the auto-refrigerated process can operate at very low alkene space velocities of about 0.1 h-1 LHSV (WHSV ca. 0.03 h 1). This design helps in increasing the octane number of the product and lowering acid consumption. The reaction temperature is maintained at about 278 K to minimize side reactions. Spent acid is withdrawn as 90-92 wt% acid. The isobutane concentration in the hydrocarbon phase is kept between 50 and 70 vol%. 

Fig. 12. ExxonMobil auto-refrigerated alkylation process. Adapted from Ref. (227).

A dense gas is defined as any gas whose density is greater than the density of the ambient air through which it is being dispersed. This result can be due to a gas with a molecular weight greater than that of air, or a gas with a low temperature due to auto-refrigeration during release, or other processes. 

Vapors from depressurization valves are typically routed to a pipe header and then to the flare to safely remove the vapors from the area and dispose of them without impact to the environment. A special concern when high levels of pressurized gases are released into a piping system is the possibihty of auto-refrigeration of the piping material that may cause a brittle fracture. A process engineer should verify which pipe materials and flow rates, specified for the depressurization system, are suitable for the pressures, flows, and gases under consideration. 

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