Alkylation Feedstock and Products

To run a refinery alkylation unit isobutane and light olefins are required as feedstock. However, the composition of the olefin stream varies significantly with the local refinery situation and this requires careful adjustment of the process conditions. The most commonly used olefins are butenes and propene but sometimes the use of pentenes is also considered. New gasoline specifications and the Chan Air Act (a United States federal law) amendments make it necessary to remove pentenes from the gasoline pool, because of their potential for atmospheric pollution. The main sources of olefins are catalytic cracking and coking processes. The isobutane feed for alkylation units is mainly obtained from hydrocrackers, catalytic crackers, and catalytic reformers. Additional amounts of isobutane are directly available from crude distillation and natural gas processing. Moreover, n-butane can be 

Before entering the reactor, the alkylation feed needs to be purified from sulfur -to avoid corrosion problems of process equipment - and from all impurities that would lead to higher consumption of the acid catalyst. To illustrate this point, Table 6.10.1 gives a Ust of common impurities in alkylation feedstock together with the amount of add consumption they cause. 

In addition to the desired isoparaffins from alkylation, the product stream leaving the reactor contains undesired lights (e.g., propane and n-butane), as well as certain quantities of tar produced by polymerization reactions (see also Example 6.10.1). 

Example 6.10.1 Important side reactions occurring in refinery allqrlations 

1) Oligomerization and polymerization reactions take place 4) Self-alkylation leads to the formation of trimethylpentanes 

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