The effect of hydrogen sulfide in circulating hydrogen on diesel hydrogenation reactions

An excessively high H2S partial pressure has no significant effect on the hydrodenitrogenation and cracking activities of sulfurized hydrocracking catalysts, but it significantly inhibits their hydrodesulfurization activity and aromatic saturation capacity. After the hydrocracking products leave the cracking bed, the trace amounts of olefins present in them still react with H2S to form thiols, thereby increasing product corrosion. If the sulfur content in the raw material is too high, not only will NH4HS be formed and clog the system, but the corrosion rate of the equipment will also increase. Typically, when the H2S level in the system exceeds 2%, a circulating hydrogen desulfurization unit is required.

The sulfur and nitrogen compounds in the feedstock are mostly converted to H2S and NH3 during hydrocracking. During the reaction, H2S and NH3 are partially dissolved in the oil phase, while the rest is sometimes released outside the plant through exhaust gases. A portion of the H2S also reacts with ammonia in the stream to form (NH4)2S and NH4HS, which are then discharged after washing. The presence of H2S has both advantages and disadvantages. Since non-precious metal catalysts are used in the majority of hydrocracking processes, it is necessary to maintain a certain H2S partial pressure in the system to prevent the sulfided catalysts from being reduced. An excessively high H2S partial pressure has no significant effect on the hydrodenitration and cracking activities of sulfurized hydrocracking catalysts, but it significantly inhibits their hydrodesulfurization activity and aromatic saturation capacity. In particular, precious metal catalysts revert to a sulfided state at high H2S partial pressures, resulting in a decrease in their activity. After the hydrocracking products leave the cracking bed, the trace amounts of olefins present in them still react with H2S to form thiols, thereby increasing product corrosion. If the sulfur content in the raw material is too high, not only will NH4HS be formed and clog the system, but the corrosion rate of the equipment will also increase. Generally, when the H2S level in the system exceeds 2%, desulfurization measures must be taken to remove H2S from the high-pressure system. When the H2S concentration in the recycled hydrogen is too low, it causes the metal components of the catalyst to be reduced, thereby reducing the catalyst’s hydrogenation activity and accelerating its deactivation. When processing low-sulfur, high-nitrogen VGO, a situation of too low sulfur content in the recycle hydrogen occurs. Our country has encountered this issue when processing Daqing and Liaohe oils, with the H2S concentration in the recycled hydrogen sometimes being as low as 200 ppm or even lower. The consequence of the above situation is a reduction in hydrodesulfurization activity and an accelerated deactivation rate of the catalyst. When the H2S concentration in the recycled hydrogen is below 300 ppm, measures must be taken to add sulfur to the feed oil in order to maintain the H2S concentration within the range of 300–500 ppm. There are two methods for supplementing sulfur in low-sulfur feedstocks: one is to add some high-sulfur VGO if conditions permit ； Another method is to directly add sulfides such as CS2, elemental sulfur, thiols, or dimethyldisulfide to the feedstock during the processing of low-sulfur oil.