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Anything that increases the residence time in this “transfer” line would increase the temperature drop between furnace and drum: longer line, larger diameter, removal of coke formed, etc. The temp drop is caused more by coking reactions than heat loss. As operation continues, coke will likely deposit in this line reducing residence time and reducing the temp drop. Check your past data after a maintenance shutdown, particularly after one where this lined was cleaned of coke deposits.
Without physical modifications to the line or changing process conditions, the only way to reduce temp drop is to add more flow to the coking coils via more feed, more recycle, or more velocity steam or water. All of these will effect coke morphology in the drum, possibly increasing loose coke formation at the bottom.
I have no experience with insulation “in” this line.
Regards, Steve Sparks