What is the impact on olefins and isobutane production when slurry oil is recycled?
Ralph Ragsdale, Ragsdale Refining Courses, firstname.lastname@example.org
For max gasoline operation, a low flow of slurry recycle is practiced to return settled cat fines to the reactor and to keep the line open in case larger flow rates are needed during abnormal operation. One way to increase propylene production is to recycle something: Light Cycle Oil, a heavy cycle oil cut, slurry, or naphtha to the middle of the riser or to a separate riser. Slurry is a poor choice as it is very refractory, yielding mostly coke and dry gas. Not a good choice in cases of limited coke burning capacity. If fresh feed has to be reduced, overall yield of olefins and isobutane will likely be reduced, not increased. If there is sufficient regenerator capacity for increasing slurry recycle without reducing fresh feed or conversion, expect an increase in dry gas and olefins, but not isobutane.
Before the development of zeolitic catalysts, heavy gas oil and slurry were recycled in significant quantities to maximize the yield of octane barrels. A bygone era.