I like your line of logic. I’ll add my 2 cents. I see the petcoke as much like the precipitate in a chemistry experiment.

http://en.wikipedia.org/wiki/Precipitate

Could it be that the petcoke is removed as precipitate at the last stage of premelt, then precipitate free liquid plastic/supernate is transferred to the smaller more efficient kiln for precracking?

I’m not yet comfortable with asserting anything without knowing which component goes where in the physical plant. I.e. the small ‘kiln’ or the large ‘kiln’ first. Also, is the smaller unit an addition to the larger units, or a replacement for one of them?