Why are preventive measures against spontaneous combustion required when removing pre-hydrogenation catalysts that have been used but not regenerated? How should they be removed?

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When discharging used catalysts that have not been regenerated, special preventive measures must be taken, as iron sulfide (FeS) present in the reactor and catalyst bed can catch fire easily upon contact with air. The temperature generated by this combustion is quite high; if not controlled, it may damage the catalyst, the components inside the reactor, and the equipment used for catalyst handling. To prevent the catalyst from coming into contact with air and being damaged, the entire process of removing and screening the catalyst must be carried out under nitrogen protection. The procedure is roughly as follows: (1) Shutdown of the unit – After a normal shutdown, the feed oil is removed from the reactor, and after thorough flushing with hydrogen, the system’s pressure is increased and the temperature at the reactor inlet is raised to around 350°C in order to remove the hydrocarbons adsorbed on the catalyst. During this period, attention should be paid to draining oil from the lowest points. Once the catalyst bed reaches 300°C and remains stable for 2 hours (with hydrogen continuously flowing through the bed), the catalyst bed is further cooled down to below 65°C, preferably to 40–50°C. After purging with nitrogen, the pressure is reduced to prepare for the removal of the catalyst. (2) Catalyst removal: During the entire catalyst removal process, the reactor should be covered with nitrogen to prevent air from entering and causing the catalyst [containing Fe] to burn. Open the reactor; those entering it must wear protective gear. Take out the ceramic balls and baskets to separate the catalyst from them, and remove any hard deposits that may be present on top of the catalyst, so as to prevent blockages in the flow channels during catalyst removal. Remove the agent under nitrogen protection. If the catalyst has not been regenerated, it is necessary to sieve out the ceramic balls and powder at the bottom of the reactor, and this should also be carried out under a nitrogen atmosphere. When the catalyst is poured into the tank, nitrogen flow should be maintained. If air accidentally enters the reactor and causes FeS to catch fire, the unloading process should be stopped and nitrogen should be introduced until the combustion ceases.