Operating conditions during sulfiding of hydrotreating catalyst depend upon the catalyst provider guidelines. Each catalyst vendor has its own procedure of sulfiding to ensure maximum performance after sulfiding. There are two reasons to control the pressure below the normal operating pressure.
1. The reactor pressure should never be increased above the temperature embrittlement limit, the reactor skin temperature needs to be increased above the Minimum pressurization temperature (MPT).
2. If the hydrogen partial pressure is high and the temperature is also above the defined limit then catalyst metal oxide will be reduced to metal and which is useless and non-catalytic.
Further, in our UOP licensed Hydrotreating plant catalyst provided by Albemarle, we maintain the same operating pressure during both stages of sulfiding. Pressure remains low when we are below the MPT skin temperature of the reactor. After crossing the skin temperature safe limit we increase the system pressure up to operating pressure then we go for pre-wetting and sulfiding steps.