Alan R. English
Diolefins and high potential gum values are often seen in FCC gasoline product when riser outlet temperatures (ROT) exceed 1000 F (538 C). They are less commonly observed in the LCO but this is likely only because potential gum is not an LCO product specification.
Polymer-like formations are often observed in the stripper, debutanizer or LPG tower reboilers when HCO or slurry is used as the heating medium. As Evan suggested, in this case the high tube wall temperatures which result from using hotter-heavier reboiling medium dramatically increase the rate of diolefin polymerization.
Based on these observations, I believe you have a few courses of action. Reducing ROT or increasing MAT activity or catalyst-to-oil ratio will shift the balance of reactions away from thermal cracking and toward catalytic cracking. I am surprised that you are seeing polymer in the sponge column since this column typically operates at low temperature (less than 100 F (38 C)). Is the oil heated elsewhere where the polymer could form and then be carried into the column after cooling?
Another possible, but rare, mechanism could occur if you are drastically overdosing liquid metals passivators such as antimony. If your metals passivator deposition efficiency is less than 70% you may be in an overdosing situation. Testing for metals in the deposits is also recommended.