Oil Processing: Delayed Coking

Is it necessary to steam-strip the heavy coker gas oil (HCGO) product?

Heavy Coker Gas Oil (HCGO) is routed to downstream conversion units, typically FCC or Gas Oil Hydrocracker. For energy optimization, HCGO transfer is direct, without intermediate storage tanks. Since the coker fractionator typically achieves high flash point temperature for this stream, it’s tempting to say that steam stripping is unnecessary.  Nonetheless, modern coker design includes an HCGO stripper, although stripping steam flow may be shut off except during a coker unit startup or shutdown.

Normal operation

With no stripping steam flow, the HCGO stripper vessel simply provides liquid surge volume for the HCGO pumps. Any dissolved or entrained light gases carry through with HCGO to the referenced downstream process units, and are removed in back-end stripping or fractionating systems.

Shutdowns (for example)

When either the delayed coker or downstream conversion unit is to be shut down, HCGO product must be cooled and routed to low-pressure storage. At this point the HCGO must be stripped to remove dissolved H2S.  Stripping steam is required only for such times.  In normal operation, avoidance of stripping steam saves both utility costs and sour water treating costs.