Hydrorefining of coal tar distillates: Exploration of the role of sulfur supplementation in cracking processes?
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In the production processes of diesel and naphtha via coal tar hydrogenation refining and cracking, in addition to catalyst sulfidization before starting up, sulfur supplementation is required throughout normal operation as well (carbon disulfide used for catalyst pre-sulfidization). This sulfur is injected before the high-pressure feed pump and enters the system along with the crude oil. What is the role of sulfur supplementation during normal operation? In reality, it has been observed that excessive reactor temperature rise occurs due to long maintenance times for the sulfur injection pump, which results in prolonged interruptions in sulfur injection
Well, this is necessary to control the reaction; sulfidation is what renders the catalyst inert! Moreover, the catalyst needs to be sulfided continuously. Although most of our catalysts are sulfided during the startup phase, they are only sulfided to 70-80% level; the remaining portion needs to be further sulfided during the production process! To control the reaction!
Hello, I still don’t understand. By sulfidation, it means converting the catalyst in its oxidized state into a sulfided state, which is what enables it to have catalytic properties. How should I understand what makes the catalyst inert? I’m a bit confused
The hydrogenation catalysts initially loaded into the reactor exist in the monovalent state and are not reactive; they only exhibit hydrogenation activity, as well as stability and selectivity, when in the sulfide state. By inertness referred to here, it is meant that the catalyst just added to the reactor has very high activity, which can easily cause the temperature to rise sharply when oil is introduced for the first time; therefore, its activity needs to be reduced through sulfidation.
During normal production, sulfur on the catalyst is lost, resulting in a decrease in its activity; replenishing sulfur is done to maintain a certain level of activity.
Thank you. My understanding is that once the sulfur on that catalyst is lost, its activity decreases, which means the reaction activity also drops. Then why does the bed temperature increase after stopping the supply of sulfur? Logically, if the reactivity is lower, shouldn’t the bed temperature also decrease?
The bed temperature is increased artificially, because after the loss of activity occurs, it is necessary to raise the temperature in order to maintain a certain yield and product quality, thereby compensating for the decrease in activity resulting from sulfur loss.
Sulfur supplementation is necessary when the raw materials contain low sulfur or high nitrogen, resulting in a low hydrogen sulfide concentration in the system, in order to maintain catalyst activity. Sulfur addition was interrupted due to a fault in the sulfur addition pump, resulting in a decrease in catalyst activity and insufficient conversion rates; therefore, it was necessary to increase the temperature, which contradicts what the original poster said, namely that the long interruption in sulfur addition caused the reactor temperature to rise too much.
Coal tar and crude oil are both low-sulfur, right? This is to maintain the hydrogen sulfide concentration in the circulating hydrogen and prevent the concentration from being too low, which could damage the catalysts.
The friend upstairs is absolutely right; the activity decreases after stopping sulfur supplementation. To maintain the conversion rate and product quality, it is necessary to raise the temperature, which in turn accelerates catalyst deactivation and reduces selectivity as well. Therefore, sulfur needs to be added.
During normal production. The hydrogen sulfide content in the recycled hydrogen should be around 1,000–2,000 ppm, depending on the requirements of the catalyst. If the hydrogen sulfide content is low, it will cause desulfurization of the catalyst, so sulfur supplementation is required. The catalyst exhibits good activity only in its sulfided state.