(161d) Details Matter When Designing Hot Vapor Bypasses

Valero’s Meraux refinery has a Hydrocracker that is a world-class high conversion unit which converts heavy gas oil (HVGO) and Deasphalted oil (DAO) into higher value products; primarily ultra-low sulfur diesel (ULSD).

In 2014, the unit was upgraded to improve operating margins with changes to the unit design. Modifications were completed in the feed system, a reactor was added, and major changes in the fractionation system. Fractionator changes included changes to improve capacity and fractionation efficiency to handle the new product slates. As part of the revamp, additional heat removal was added to the condenser circuit that increased the capacity by over 50%.

After startup, it was apparent that the hot vapor bypass system was not operating as designed. Primary fractionator pressure control was unstable, along with major process fluctuations in circuit temperatures and drum level. The instability was severe enough that the refinery considered a shutdown of the newly re-started hydrocracker to avoid potential equipment damage.

Field troubleshooting identified the problems with the overhead drum design which resulted in hot vapor bypass instabilities with a highly subcooled system. After identification, the overhead trim condenser duty was reduced by reducing water flow to limit subcooling. This eliminated the instability until a pad gas system was installed.

The author will present operating data that substantiates a previously unseen phenomena with hot vapor bypass pressure control configurations. Design parameters and recommendations will be made to improve future hot vapor bypass system designs.