【Q&A Question 277】2017.10.05

Sulfur can inhibit the hydrolysis activity of the catalyst and improve its selectivity

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. Excessively high H2S partial pressures significantly suppress the hydrodesulfurization activity and aromatic saturation capacity of catalysts; in particular, precious metal catalysts turn into sulfided states at high H2S partial pressures, resulting in a decrease in their activity. After the hydrocracking products leave the cracking bed, the very small amount of olefins present still reacts 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%, desulfurization measures must be taken to remove H2S from the high-pressure system. When the H2S concentration in the circulating 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. When the H2S concentration in the recycle hydrogen is only 200 ppm or lower, the result is a decrease 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, thioethers, or dimethyldisulfide to the feedstock during the processing of low-sulfur oil.

A high sulfur content has an inhibitory effect on the hydrogenation reaction

Answer: Sulfur content: The sulfides in the feedstock convert into hydrogen sulfide during the hydrogenation process, and this is necessary to maintain the activity of the non-precious metal catalysts used. Feedstocks with high sulfur content can lead to excessive hydrogen sulfide concentrations in the recycle hydrogen, which not only reduces the hydrogen partial pressure but also causes equipment corrosion.

In crude oil, if the sulfur content is too low, it results in a low hydrogen sulfide level in the circulating hydrogen, which leads to sulfur loss from the catalysts and ultimately affects their activity; on the other hand, a high sulfur content increases corrosion of the equipment

As the sulfur content of the feed oil increases, the sulfur content in the product, as well as the H2S content in the recycle hydrogen before desulfurization, the H2S content in the hydrogen-rich gas, and the sulfur content in the wastewater increase accordingly. When the sulfur content in the feed oil is between 1.228% and 1.590%, and the reaction temperature is between 330 and 347°C, the sulfur content in the product is between 0.128% and 0.187%. When the sulfur content in the feed oil is between 1.029% and 1.702%, and the amount of amine solution used is between 31 and 55 tons, the H2S concentration in the recycled hydrogen after desulfurization is between 45 and 6,390 mg/m3. When the sulfur content in the feed oil is between 1.279% and 1.664%, and the amount of amine solution used in the hydrogen-enriched gas desulfurization tower is between 29 and 37 tons, the H2S concentration in the hydrogen-enriched gas after desulfurization is no more than 50 mg/m3. When the sulfur content in the feed oil is between 1.029% and 1.664%, and the water injection rate is between 31 and 48 tons per hour, the sulfur concentration in the wastewater is between 15,448.44 and 40,791.04 mg/L. The system addresses low-temperature corrosion by adjusting the scale inhibitor and water injection volume, performing alkaline cleaning promptly after shutdown, and implementing appropriate safety measures.

A high sulfur content in the raw materials has a certain positive effect on catalyst activity. However, if it is too high, it will cause the sulfur content in the product to exceed the limit, resulting in substandard product quality.

It can increase the strength of the catalyst.

Sulfur content: Sulfides in the feedstock convert to hydrogen sulfide during the hydrogenation process, and this is necessary to maintain the activity of the non-precious metal catalysts used. Feedstocks with high sulfur content can lead to excessive hydrogen sulfide concentrations in the recycle hydrogen, which not only reduces the hydrogen partial pressure but also causes corrosion of the equipment.

When the sulfur content in the feedstock increases, it has the following effects on the hydrocracking process: (1) It increases the difficulty of hydrodesulfurization; the reaction conditions become more stringent, and the rate of catalyst deactivation accelerates; (2) An increase in sulfur content can lead to a significant rise in reactor temperature, making it easier for subsequent beds to exceed their optimal temperature limits; (3) An increased sulfur content results in the generation of large amounts of hydrogen sulfide after desulfurization, which reduces the hydrogen partial pressure and increases equipment corrosion. The sulfur content in hydrocracking feedstocks is generally controlled at 0.3% to 3.0%.