IMPROVED OUTGASSING FOR HYDROPROCESSING VESSELS

The increasing use of high-temperature, high-pressure hydroprocessing has stimulated the need to minimize process unit turn-around time. Bot h the restreaming of active catalyst at turn-around and the dischargin g of spent catalyst must be preceded by a cooling of the vessel wall t o ambient temperature. Consequently, temperature-time and pressure-tim e profiles must be selected which transform the unit from operating te mperature and hydrogen pressure to ambient temperature and zero hydrog en pressure without: (1) generation of excessive thermal stress in ves sel walls, (2) coke formation on catalyst, (3) overlay disbonding or ( 4) crack initiation and growth due to base-metal hydrogen embrittlemen t. This paper describes a method for selecting the temperature-time an d pressure-time profiles which produce outgassing in minimum time cons istent with these constraints. The outgassing time computed by this me thod for a particular unit is a theoretical minimum in the sense that a shorter time is not possible without violating one of the constraint s; this procedure can therefore be used as a standard of reference aga inst which other procedures can be compared. Conversely, the procedure is also practical and can be implemented on actual process units. Imp roved outgassing methods can result in significant cost savings by red ucing equipment downtime. The U.S. Patent and Trademark Office has adv ised the authors that this outgassing procedure is patentable and that a patent will be issued in due course.