On the issue of ammonia injection for pre-sulfiding hydrogenation catalysts
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The catalyst used in our diesel upgrading reaction contains molecular sieve catalysts; therefore, ammonia must be injected when the pre-sulfidation temperature is raised to 250°C. The sulfurizing agent is DMDS. What I want to ask is what the mechanism behind ammonia injection is – is it to adjust the sulfur concentration, or to suppress the acidic sites of the molecular sieve and prevent exothermic reactions? If it is to suppress the acidic centers, then why does the amount of ammonia injected decrease as the temperature rises? Shouldn’t the reaction intensify with an increase in normal temperature? Should more ammonia be added then? Looking forward to an answer! !
Injecting ammonia to passivate and stabilize the reforming catalyst
Injecting ammonia serves to deactivate the catalyst and prevent the reactor temperature from rising too rapidly. As the reactor temperature increases, the amount of ammonia injected is gradually reduced; once the reactor temperature reaches a certain level, no more ammonia needs to be injected, allowing the reactor to return to its normal operating conditions.
What causes this rise in temperature? Is it the acidic sites of the molecular sieve reacting with the sulfurized oil, releasing heat, or something like that? If that’s the case, as the temperature rises, shouldn’t the reaction between the molecular sieve and the carrier oil be more intense? Should more ammonia then be injected?
Injecting ammonia suppresses the activity of the cracking catalyst
The temperature rises due to the exothermic nature of the reaction. Ammonia is introduced at the beginning in order to deactivate the catalyst and prevent a sudden increase in temperature. In the subsequent reactions, the amount of ammonia added should be gradually reduced so that the device can return to normal operating conditions. If, as you say, the amount of ammonia added should be increased, then wouldn’t the reaction rate become too low?
Ammonia injection is used to prevent the temperature from rising too rapidly; thereafter, the amount of ammonia injected is gradually reduced, as the reaction releases heat and at the same time requires heat as well. The high heat generated by the reaction serves as the heat source in a normal reaction, and it cannot be restricted.
Ammonia is not lost in the recycled hydrogen; as long as its concentration in the recycled hydrogen meets the technical requirements during the pre-sulfurization stage, no additional ammonia needs to be added. Otherwise, a large amount of waste hydrogen will have to be discharged for replacement later
Could I ask what the mechanism of passivation is? How does ammonia manage to passivate the catalyst? Is it simply by reducing the temperature, thereby preventing too rapid an increase in temperature and thus passivating the catalyst?
What causes this “excessive increase”? There is no feed during the presulfurization process; then what is causing the temperature of the molecular sieve catalyst to rise so rapidly? Could it be the sulfurized oil? Is it still due to the heat from presulfurization? If it is the heat from presulfurization, then is the purpose of injecting ammonia to reduce the sulfur uptake rate of the catalyst?