Is hydrogen sulfide easily removed from the catalyst surface?
Thread Content
During this shutdown period, we spent nearly 10 days replacing the contents of the reaction system, but hydrogen sulfide was not completely removed from it. After the system was restarted, a leak was detected at one of the flanges, so we planned to replace the gasket; however, we found that the hydrogen sulfide level in the system had risen again. I believe this is because hydrogen sulfide does not easily get removed from the catalyst bed. As the temperature of the catalyst bed decreases, the hydrogen sulfide content in the circulating hydrogen drops gradually. But as the temperature rises, the hydrogen sulfide adsorbed on the surface of the catalyst is released back into the system, which slows down the process of purging the system. I hope everyone can offer some advice. We have a total of 7 catalyst beds.
How did you carry out the replacement? Did you use a vacuum pump to reduce the system pressure? A few attempts were enough, right?
Every possible method has been tried: draining the system, releasing gas on-site, and venting the system – yet the hydrogen sulfide level remained high. Even when the plant was shut down, the hydrogen sulfide level was reduced to below 20 ppm. During this restart process, pressure was increased due to a leak in one of the flanges, which caused pressure loss and required new gaskets to be installed. When analyzing the situation, the hydrogen sulfide level was over 1000 ppm; I suspect that this was because the hydrogen sulfide absorbed by the catalyst bed was being released. While fixing the leak in the flange, the hydrogen sulfide level was around 300 ppm. Once the system pressure reached 10 MPa, the hydrogen sulfide level remained almost unchanged, indicating that it hadn’t been diluted by large amounts of hydrogen. Therefore, I think it’s possible that the hydrogen sulfide in the catalyst bed still hasn’t been completely released. Since the catalyst we use is a newly developed reforming catalyst, I believe this issue might be related to the properties of that catalyst
What about later on? Have you found the reason?
This post was last edited by xgog on 2016-12-7 at 12:53. Hydrocracking catalysts are generally in an oxidized state outside the catalyst; through sulfidation, they become in a sulfided state under normal operating conditions. Once the H2S concentration is reduced to 20 ppm, the sulfided catalyst is reduced by hydrogen, with the sulfur elements in the catalyst combining with hydrogen to form H2S, which leads to an increase in the H2S concentration thereafter. In process design, the hydrogen circulation concentration is set between 500 and 1000 ppm; too low a H2S concentration in the circulating hydrogen can reduce the catalyst’s activity, and this might be the reason for that. My humble opinion
During shutdown, the replacement in the reaction system involves mainly the replacement of the catalyst bed; in fact, no catalyst is loaded or unloaded, and H2S levels are not closely monitored – it is primarily CO and flammable gases that are of concern
Personally, I think it’s mainly because the oil in the catalyst wasn’t cleaned properly, which led to the release of hydrogen sulfide from that oil
This possibility cannot be ruled out, but the liquid level has stopped rising; moreover, no oil flowed out when the reactor’s outlet flange was opened. The gas discharged under nitrogen protection also contained no oil. Therefore, I believe that this catalyst has a strong adsorption capacity for hydrogen sulfide, and it is difficult to release it at lower temperatures and pressures.
Since the high levels of cold and hot liquids are no longer rising, it indicates that the displacement has been thorough. During the sulfidation phase of the new catalyst, the H2S concentration in the circulating hydrogen can exceed 40,000 ppm; for a long time at the beginning of production, the H2S level remains high. This H2S comes not only from the feedstock but is also released by the catalyst itself, and it is only after some time of operation that the H2S concentration begins to decrease. Hydrogen displacement is employed; when the H2S concentration is low, it displaces some of the sulfur atoms in the sulfided catalyst, resulting in the formation of H2S that is released
It should be hydrogen sulfide produced as a reaction product.