>> I continue to think using HTF is related to the petcoke buildup issue and running the processors continuously.<<

That's my wild guess as well.  Makes the 3rd kiln make sense as well.  Flush the residue/petcoke out into the 3rd kiln.  Remember back in the day the intention was to hot tap the liquid residue straight out.  Apparently some of it cakes on the kilns and isn't liquid.  Perhaps the HTF keeps it liquid and keeps it from sticking and the whole thing is drained into kiln 3.

I could be completely dead wrong.  But I do find the timing interesting of HTF at the same time as P3 being built so maybe they go together.