Cooling of the hydrocracking cycle
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I would like to ask about the normal shutdown procedure for the first stage of the hydrocracking process. During the cooling phase of the hot nitrogen purge cycle, all the nitrogen was used for mixing with hydrogen; the valves for cold hydrogen used in purification and cracking were kept manually closed. At that time, the system pressure was between 40 and 50 kilograms, and the temperature at the furnace outlet was still around 100°C. Attempts to reduce this temperature by adjusting the furnace flue dampers, the preheating and recovery systems, as well as the relevant heat exchangers, did not yield any significant effect! I would like to ask now whether it is possible to lower the temperature of the last catalyst bed by adjusting the opening degree of the cold hydrogen valve, thereby reducing the temperature at the furnace outlet and ultimately cooling down the reactor. Also, if this approach is used, it could cause the catalyst to be dried out and damaged; after all, there are ceramic balls both above and below the catalyst. Moreover, cold hydrogen should be distributed evenly, right? I would appreciate some guidance from the teachers!
Cooling with cold hydrogen is possible; as long as the cooling rate is controlled, the problem you mentioned will not occur.
When cooling down, the cold hydrogen valve can be opened, which allows several beds to cool down simultaneously, preventing the situation where the inlet temperature is low while the outlet temperature is high. Just keep the speed reduction within the required range. The temperature has dropped below 100°C; why not feed hydrogen directly into the catalyst bed? Moreover, when hydrogen is introduced from the inlet, it also comes into direct contact with the catalyst.