The problem of hydrogenation catalyst reduction
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Dear experts from Haichuan, there is a question that has troubled me for many years: whether hydrogenation catalysts can be reduced. Most available information suggests that catalysts are prone to reduction, but I have always had doubts about this. I have discussed it with researchers from Fushun Institute as well as professors from petroleum universities, but to no avail. Let me share my views first. I believe that catalysts in their oxidized state are indeed easy to reduce, which is why the temperature during sulfurization is specified to be below 180 degrees – there’s no disagreement on this point. However, as for catalysts in their sulfided state, they are not necessarily easy to reduce. During thermal hydrogen treatment, the temperatures are quite high – 360 degrees for diesel and around 400 degrees for wax oil – yet the catalysts are not reduced under these conditions. Liquid-phase hydrogenation also takes a very long time. Another issue is that people generally think it’s necessary to maintain a certain sulfur content in the recycle hydrogen. I believe this isn’t necessary in hydrogenation refining units, because even if the sulfur content in the recycle hydrogen is 0, the sulfur content inside the reactor, especially around the catalyst, remains high. In hydrocracking processes, a higher sulfur content in the recycle hydrogen is required, as acidic centers play a role in these reactions, and hydrogen sulfide can enhance acidity, thereby increasing the conversion rate. If any expert has theoretical information regarding the reduction of sulfided catalysts, I would appreciate it if they could share it.
Now, the hydrogen sulfide level during catalyst stripping is not as high as it used to be required.
Why is the temperature for thermal hydrogen stripping set at 400°C for wax oil and 360°C for diesel?
The experiences and views of scholars***
1. When hot hydrogen is used with oil, it is still necessary to control the hydrogen sulfide content; in a pure hydrogen environment, there is definitely a possibility of reduction occurring. 2. During normal production, if the sulfur content in the raw materials is high enough, the hydrogen sulfide content in the recycled hydrogen becomes less important, as the hydrogen sulfide produced by the reaction may have already been converted into ammonium salts and removed through washing. The hydrogen sulfide content in the reactor can be inferred from the total sulfur in acidic water, as long as the hydrogen sulfide level in the reactor is sufficient.
I have also been troubled by this issue: when liquid-phase hydrogenation is shut down and hot hydrogen contains oil, there is no recycle hydrogen compressor, so the amount of gas in circulation is low; as a result, the time during which oil is present will definitely be long. What should be done if hydrogen sulfide levels decrease during this process? Please, experts, give me some advice!
Generally, the H2S concentration will decrease. In my opinion, as the temperature drops, some of the H2S gets absorbed into the catalyst; to be safe, it is advisable to add a suitable amount of sulfiding agent. If the concentration remains above 500 ppm, the catalyst can remain safe and stable.
The 2005 article in the Journal of Catalysis titled “Simultaneous Catalytic Reduction of SO2 and NO over Sulfided NiWAl2O3 Catalysts Ⅰ. Decomposition and Reduction of NO” mentioned that H2-TPR characterization of sulfided NiWAl2O3 revealed the release of a small amount of H2S at 250°C. So, reduction is possible
Before shutting down, it is necessary to add a vulcanizing agent to raise the hydrogen sulfide concentration in the system; once a certain concentration is reached, the temperature is lowered, allowing hot hydrogen to carry oil with it. Yes?
1. I fully agree with the original poster’s analysis; it’s very thorough! 2. Regarding this issue, I first heard about it around 2005; I hadn’t heard of it before, nor had any supplier put forward such a requirement! I discussed this with many suppliers later on, and the reply was the same: it’s possible, but note that “possible” is an adverb indicating degree! Anything is possible! Personally, I analyze that the emergence of this problem is related to the large number of catalysts from abroad entering the Chinese market; they have many strict requirements, and others tried to keep up, but why exactly it happened remains unclear! 3. For this reason, I have also looked up some information; the most clear source of information is the explanation in \"Hydrogenation Processes\": it will not be reduced (to a sulfide state) below 230°C. My understanding of this statement is that experts’ experiments were conducted only up to this temperature point, so it is determined that reduction does not occur below this temperature. Since no experiments were carried out above this temperature, it does not mean that reduction must occur there! 4. Practical experience: I agree with the original poster once again – the temperature used in hot hydrogen stripping is very high, but I have never seen a case where the catalyst was reduced! I run it in the same way, and the catalyst performance has always been excellent! The refined catalysts that have been used the most are over 10 years old ； 5. Regarding the question about wangqiudong1987: Is there no hydrogen circulation compressor when hot hydrogen is introduced during shutdown? How is the cyclic power provided? 6. Once again, I would like to remind everyone: what has been discussed is the issue of “possibilities,” not inevitability! There are no quantitative data at all! Personal theoretical knowledge and practical experience: I fully agree with the poster’s analysis – fresh and regenerated oxidized-state catalysts can be easily reduced, so the temperature at which they are exposed to hot hydrogen needs to be kept below 175°C (this is when the cold hydrogen blind plate comes in use). However, I have not observed any impact on the performance of catalysts in the sulfided state at temperatures below 400°C; no sulfur supplementation is required during the hot hydrogen stripping process. This is just my personal opinion, intended for reference in practical operations! Additionally: Friend Zirulinwu, could you please provide this journal article for everyone’s research and reference? Thank you very much!