(26a) Improving FCC Performance with Low Cost Tower Internals Modifications

A recent revamp of the FCC unit at the Marathon Petroleum Robinson refinery demonstrates the ability to achieve significant improvement in unit performance with relatively low cost tower internals modifications.    Proper internals selection and design proved critical to the performance of the main fractionator, primary absorber, and slurry stripper columns.

The major objective of the revamp was to recover lost capacity in the main fractionator due to endpoint changes on product specifications.  The undercutting caused a 7.5% decrease in feed rate to the FCC, which was a significant economic impact.  The tower was already converted to structured packing in a 1988 revamp, so the recovery of the lost capacity was challenging.  Careful analysis of transition zones between beds indicated a problem with an orifice deck distributor.  The distributor was replaced along with a change to the packing, which resulted in a 13% increase in feed rate.

During the turnaround, the packing in the primary absorber was replaced with the wrong size due to a documentation error.  During troubleshooting of this tower it was discovered that naphtha was being bypassed around the tower due to flooding.  The investigation identified a transition zone restriction that when fixed added an additional 7% of propane recovery and 11% of propylene recovery.

A new slurry stripper column was added to increase the flash of the slurry going to storage.  Special fouling resistant trays were installed due to the concern for fouling.    The tower operation has been trouble-free and has been successful in meeting the desired flash point specifications.