【Q&A Question 124】2017.05.04

The hydrogen partial pressure determines the depth of the reaction: a high hydrogen partial pressure results in a lower depth of reaction, while a low hydrogen partial pressure also leads to a lower depth of reaction, which affects product quality!

Increasing the hydrogen partial pressure is beneficial for catalytic reactions; on one hand, it can suppress coking reactions and reduce the catalyst deactivation rate； On the other hand, it can improve the removal rates of HDS, HDN, and HDM, while also promoting the hydrogenation and saturation reaction of polycyclic aromatics. Therefore, the hydrogen partial pressure in the reaction system should be increased as much as possible within the limits permitted by the equipment and operations.

Increasing the hydrogen partial pressure is beneficial for hydrogenation catalytic reactions; on one hand, it can suppress coking reactions and reduce the catalyst deactivation rate； On the other hand, it can improve the removal rates of HDS, HDN, and HDM, while also promoting the hydrogenation and saturation reaction of polycyclic aromatics. Therefore, the hydrogen partial pressure in the reaction system should be increased as much as possible within the limits permitted by the equipment and operations.

The hydrogenation reaction is an exothermic process that consumes hydrogen and results in a reduction in volume. Increasing the hydrogen partial pressure helps to shift the chemical equilibrium in the desired direction, thereby enhancing the depth of the hydrogenation reaction, preventing catalyst coking, improving product quality, and extending the catalyst’s service life. If the hydrogen partial pressure is low, the effect is opposite. The rate of catalyst coking and carbon deposition increases.

The effect of reaction pressure is manifested through the hydrogen partial pressure. The hydrogen partial pressure in the system is determined by the operating pressure, hydrogen-to-oil ratio, purity of the recycled hydrogen, and the vaporization rate of the feedstock. Providing a hydrogen partial pressure facilitates the progress of the hydrogenation reaction and accelerates its rate. An increase in hydrogen partial pressure is beneficial for catalytic reactions. Providing a hydrogen partial pressure can, on the one hand, suppress coking reactions and reduce the catalyst deactivation rate ； On the other hand, it can improve the removal rate of impurities such as sulfur, nitrogen, and metals, while simultaneously promoting the hydrogenation and saturation of polycyclic aromatic hydrocarbons. Therefore, the hydrogen partial pressure in the reaction system should be increased as much as possible within the limits permitted by the equipment and operations. For the hydrodesulfurization of sulfides and the hydrosaturation of olefins, a high degree of conversion is achieved at relatively low pressures. When gasoline is hydrotreated at a hydrogen partial pressure of 2.5–3.0 MPa, the degree of purification is not governed by thermodynamic equilibrium, but rather by the reaction rate and reaction time. Under hydrorefining conditions, gasoline is generally in the gas phase; increasing the pressure prolongs the residence time of gasoline, thereby enhancing the degree of its refinement. When the hydrogen partial pressure exceeds 3.0 MPa, the hydrogen concentration on the catalyst surface reaches saturation; if the operating pressure remains unchanged, increasing the hydrogen partial pressure by adjusting the hydrogen-to-oil ratio results in a decrease in the degree of purification. Because this will lower the partial pressure of the crude oil at this time. The effect of pressure on the hydrorefining of diesel fractions (180–360°C) is more complex. Diesel fractions can be in a gas phase or a liquid phase under hydrorefining conditions. In the gas phase, increasing the pressure prolongs the reaction time, thereby increasing the reaction depth. Increasing the reaction pressure enhances the degree of purification; it promotes both the hydrodesulfurization and denitration reactions, but to different extents. Increasing the reaction pressure significantly boosts the nitrogen removal rate, with only a minor effect on the sulfur removal rate; this is because the nitrogen removal reaction is slower while the sulfur removal reaction is faster, and sufficient reaction time is available at lower pressures. For feed materials with a high content of purified nitrogen, it is necessary to increase the pressure or reduce the space velocity in order to achieve a certain denitration rate. If the conditions remain unchanged and the reaction pressure is increased to a certain value, a liquid phase will appear in the reaction system; further increasing the pressure will instead degrade the effectiveness of hydrorefining. In the presence of a liquid phase, the rate at which hydrogen diffuses across the liquid film to the catalyst surface is often the controlling factor affecting the reaction rate. This diffusion rate is proportional to the hydrogen partial pressure, and decreases as the thickness of the liquid layer on the catalyst surface increases. Therefore, after the formation of a liquid phase, increasing the reaction pressure thickens the liquid layer on the catalyst surface, thereby reducing the reaction rate. If the pressure remains constant, increasing the hydrogen partial pressure by raising the hydrogen-to-oil ratio will result in a maximum depth of hydroprocessing, yielding better effects.

Increasing the hydrogen partial pressure is beneficial for catalytic reactions; on one hand, it can suppress coking reactions and reduce the catalyst deactivation rate； On the other hand, it can improve the removal rates of HDS, HDN, and HDM, while also promoting the hydrogenation and saturation reaction of polycyclic aromatics. Therefore, the hydrogen partial pressure in the reaction system should be increased as much as possible within the limits permitted by the equipment and operations.

The hydrogen partial pressure depends on the pressure of the reaction system and the purity of hydrogen. An increase in the hydrogen partial pressure is beneficial for catalytic reactions, as it can suppress coking reactions and reduce the rate of catalyst deactivation； On the other hand, it can improve the removal rates of HDS, HDN, and HDM, while also promoting the hydrogenation and saturation reaction of polycyclic aromatics. When the hydrogen partial pressure is low, the coking of the feed oil and the accumulation of carbon on the catalyst accelerate, resulting in a significant decrease in the depth of reaction. Therefore, the hydrogen partial pressure in the reaction system should be increased as much as possible within the limits permitted by the equipment and operations.

The hydrogen partial pressure can increase hydrogen purity, as well as the reaction rate and reaction depth

The hydrogenation reaction is an exothermic process that consumes hydrogen and results in a reduction in volume. Increasing the hydrogen partial pressure helps to shift the chemical equilibrium in the desired direction, thereby enhancing the depth of the hydrogenation reaction, preventing catalyst coking, improving product quality, and extending the catalyst’s service life. As for the hydrogen partial pressure, the effect is the opposite: the coking of the feed oil and the accumulation of carbon on the catalyst accelerate.