(28e) Installation of Secondary Pumparound Resolves Resid Entrainment from Fluid Coker Scrubber

Valero Benicia refinery operates a 28 MBD Fluid Coker.  The reactor effluent vapor passes through a single cyclone discharging to a Scrubber that contains 6 rows of 8-pass shed decks.  The Scrubber bottoms liquid is pumped both as a recycle stream to the reactor and as pumparound liquid to the top of the sheds fed through a slotted pipe distributor with laterals.  The top section of the Scrubber contains a 5 ft bed of structured grid packing washed with heavy gas oil.  After the passing through the reactor and scrubber, heavy and light gas oil products are separated in the Fractionator.

While coking and fouling of the Scrubber occurs throughout the 3-year run cycle, at the midpoint of the run heavy gas oil product quality should be maintainable without undercutting.  At 1.5 years into the current cycle, significant residue carryover into the heavy gas oil was observed, along with an accelerated growth in Scrubber grid pressure drop.  Temperature instruments located at the top shed level indicated that vapor and liquid temperatures were equal, whereas previously the vapor indicator read 50 degF higher than the liquid indicator.

The product quality and operating data suggested an increased entrainment of resid above the shed section, most likely related to a change in distribution quality of the pumparound return.  Skin temperature surveys at multiple levels of column elevation supported the data from the TI’s, indicating temperature maldistribution focused in one quadrant of the Scrubber.  The unit responded positively to increasing pumparound flow through use of the open-tee start-up header located above the normal shed lateral pipe distributor, and thus undercutting the heavy gas oil.

A project was implemented to install piping to feed additional shed pumparound liquid through lances installed in the Scrubber above the shed section.  Positioning of the lances and flow have been optimized to target quenching of the hot quadrant.  Post-commissioning data indicates that HKGO product quality has decreased from 6-7% resid to 4% resid, and temperatures have dropped by approx. 40degF in the hot zone with grid DP stabilizing.  The authors will describe the troubleshooting methodology, discuss the unique piping and lance design, and present post-commissioning data that will show that the new secondary pumparound injection was successful.