Debottlenecking FCCU's and Alky's

FCCU’s are another unit which are typically debottlenecked significantly beyond their original design capacities. Advances in riser technology have allowed higher reactor charge rates and conversion. The main fractionator or the other fractionating towers can often become limiting as reactor capacity is increased. Debottlenecking options in the gas plant have kept pace.

Packing is a typical solution to increasing the fractionating capacity. Packing is a proven application in FCCU main fractionator and other towers. Even if only the pumparound zones are packed, the main fractionator capacity typically can be increased by approximately 15%. Also, the pumparound loadings can be modified to significantly increase column capacity. For example, increasing the slurry pumparound duty can greatly unload the rest of the main fractionator. As with all debottlenecking revamps, the ramifications of the changed unit operation upon the equipment and the product specifications need to be rigorously evaluated.

Most plants with a FCCU have a downstream Alkylation unit. Often the reaction section is designed conservatively, and the charge rate can be increased with little impact upon the alkylate octane. Sometimes the fractionating towers become the limit. Sometimes in tall towers with narrow boiling range feedstocks such as the Alky deisobutanizer or debutanizer, the optimum feed location has a greater effect than normal. For example, optimizing the deisobutanizer feed location can improve tower capacity by up to 10%. Also, high capacity trays have increased Alky deisobutanizer capacity by over 15%.