IMPROVED OUTGASSING FOR HYDROPROCESSING VESSELS

Abstract The increasing use of high-temperature, high-pressure hydroprocessing has stimulated the need to minimize process unit turn-around time. Both the restreaming of active catalyst at turn-around and the discharging of spent catalyst must be preceded by a cooling of the vessel wall to ambient temperature. Consequently, temperature-time and pressure-time profiles must be selected which transform the unit from operating temperature and hydrogen pressure to ambient temperature and zero hydrogen pressure without: (1) generation of excessive thermal stress in vessel walls, (2) coke formation on catalyst, (3) overlay disbonding or (4) crack initiation and growth due to base-metal hydrogenembrittlement. This paper describes a method for selecting the temperature-time and pressure-time profiles which produce outgassing in minimum time consistent with these constraints. The outgassing time computed by this method for a particular unit is a theoretical minimum in the sense that a shorter time is not possible w...