(15d) Acid Strength Control in a Sulfuric Acid Alkylation Unit

Sulfuric acid alkylation units are characterized by the interaction between two very different and competing process behaviors: The hydrocarbon side, where iso-butane and C3/4 olefins are reacted to form alkylate, and the sulfuric acid side, which acts to catalyze the reaction. On the hydrocarbon side the reaction occurs very quickly, while the acid side literally takes days to settle-out after a load change.

Alkylation units where feed compositions and rates vary require constant manipulation of the acid addition rate to control both the acid material balance and the acid strength. The long response times make it difficult for operations to run these units at the optimum acid consumption while at the same time controlling the acid strength and the acid inventories. Unit conditions can change in a few hours making it difficult for the operator to account for these changes on the acid side of the process, resulting in higher than necessary acid consumption.

This paper discusses the application of multi-variable control to the sulfuric acid unit at LyondellBasell's Channelview Complex in Channelview Texas. The application was designed to minimize acid consumption, stabilize spent acid strength and manage acid inventories. The challenges presented by this unique process, as well as the methods used to exploit dynamic and steady-state process behavior are discussed. Performance results from this application are presented.