Why does the amount of recycled hydrogen increase at the end of the reaction in a hydrocracking unit?

The Hydrocracking Unit is an important component in refineries for converting heavy crude oils into lighter ones; through hydrogenation and cracking chemical reactions, the feed oil is transformed under high pressure and in the presence of catalysts. During hydrocracking, recycled hydrogen serves to supply hydrogen, stabilize the reaction, remove heat, and dilute oil vapors. Changes in the amount of recycle hydrogen are closely related to the operating conditions of the unit. The following are several reasons that may lead to an increase in the amount of recycle hydrogen toward the end of the hydrocracking process: 1. Carbon deposition in the reactor: As the reaction proceeds, carbon deposits may gradually accumulate on the surface of the catalyst, resulting in a decrease in its activity. To maintain the desired conversion rate, operators may increase the amount of recycle hydrogen to raise the hydrogen/oil ratio, thereby enhancing catalyst regeneration and the hydrodehydrogenation of carbon. 2. Reaction progress: As the reaction proceeds, some of the reformed components are converted into lighter components, resulting in a decrease in hydrogen solubility within the reactor. To maintain an effective hydrogen concentration, it may be necessary to increase the amount of circulating hydrogen. 3. Heat conduction and pressure retention: The heat generated during the reaction process needs to be removed by circulating hydrogen in order to maintain the temperature of the apparatus. At the same time, recycled hydrogen also helps maintain a high-pressure environment, facilitating the progress of the hydrogenation reaction. As the reaction progresses, more cyclic hydrogen may be required to meet these demands. 4. System leakage: Devices that operate for extended periods may experience aging or leakage, resulting in hydrogen loss from the reaction system. To maintain system pressure and hydrogen concentration, it is necessary to increase the amount of supplementary recycled hydrogen. 5. Change in feedstock: If the heavy fraction in the feedstock composition increases or its quality deteriorates, more hydrogen may be required to achieve sufficient hydrocracking effects. Increasing the amount of recycled hydrogen can help maintain or improve the efficiency of hydrocracking, but it also increases operating costs. Therefore, it is necessary to balance the amount of recycled hydrogen with economic benefits during operation. It should be noted that the specific situation needs to be analyzed based on real-time data from device operation and technical requirements. .

This is because the activity of the catalyst inside the reactor has declined; to maintain a good hydrogenation state, it is necessary to increase the hydrogen partial pressure.

The amount of circulating hydrogen increases on its own; there is no manual adjustment needed

That’s impossible; there must be some parameter requirements, like temperature or something, for automatic control

I’ll give you a direction for thinking: the measurement principle of flow meters, and consider which variables contribute to it

Since the catalyst activity in the final reactor has decreased, the degree of hydrocracking reaction becomes less intense and hydrogen consumption drops; if the amount of fresh hydrogen supplied remains unchanged, the amount of recycled hydrogen will increase automatically.

In the later stages, the catalyst has low activity and high temperature, resulting in a decline in reaction rate; an increased amount of hydrogen circulation is used to promote the reaction to proceed in the desired direction