Celso Pajaro - Sulzer Chemtech
Typically LCO maximization is done by reducing the FCC naphtha end point and minimizing LCO in the slurry. Lowering the naphtha end point reduces the column top temperature which favors the precipitation of ammonium chloride salts which lead to plugging and/or corrosion of the top column internals. Reducing LCO in slurry requires a combination of minimizing slurry reflux rate along with increasing the amount of quench to keep the slurry temperature below 680oF.
With these changes, the possibility of coking must be considered. Some units have experienced coke formation in the slurry pumparound section when they have tried to reduce the slurry. Other units that have a wash section between the slurry and the HCO pumparounds have experienced coke formation due to a sharp reduction on the wash liquid coming from the HCO section.
The proper selection of column internals like anti-fouling trays combined with either permanent or temporary water wash facilities can reduce the impact of salt deposition. Adjustments in the operation of the FCC main fractionator bottom section along with monitoring the liquid wash rate have successfully allowed the recovery of LCO without plugging/coking problems.