Let’s work through an example where the a heavy crude revamp increases coke yield to the point at which existing drum coke bed height (to avoid carryover to the fractionator) limits the fill cycle time to 8 hours. In two-drum operation, an 8-hour fill cycle leaves only the next 8 hours to complete all decoking operations required for cooling, emptying and reheating this drum in preparation for return to coke filling service. Sixteen hours is the recommended minimum drum decoking cycle time. A third coke drum (same dimensions as existing drums) will be installed to remove this decoking cycle bottleneck.
Hot feed from the coker furnace is routed through a single coke drum, while the remaining two drums are undergoing various stages of decoking functions. Decoking functions (steaming, quenching, draining, coke drilling, reheating) are staggered (offset by 8 hours) such that the existing quench and coke cutting equipment can be used by each of the off-line drums without interference. Therefore, each of the three drums proceeds though repeated cycles of on-line filling, steaming / quench, and drain / coke cutting / reheat.