What are the reasons that shorten the lifespan of pre-hydrogenation catalysts? How can the service life of pre-hydrogenation catalysts be extended?

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The reasons that shorten the lifespan of the pre-hydrogenation catalyst include: (1) excessively high reactor temperature; (2) excessively high dry point of the feed; (3) excessively low feed space velocity; (4) insufficient purity of the hydrogen in the recycle gas, resulting in a decrease in hydrogen partial pressure; (5) excessively low hydrogen-to-oil ratio, etc. To extend the service life of the pre-hydrogenation catalyst, it is necessary to keep the five main factors mentioned above, which cause catalyst coking and a reduction in its service life, within acceptable limits: (1) Control of reaction temperature – At all times, the reaction temperature must be maintained at the minimum level required to achieve the desired levels of desulfurization and denitration. Increasing the reaction temperature accelerates the formation of coke, thereby shortening the operating time between regenerations. Similarly, if the feed rate is changed and the reaction temperature changes as a result, it is common practice to lower the reaction temperature first before reducing the feed rate, or to increase the feed rate before raising the temperature. (2) Control the feedstock boiling range if the dry point of the feedstock increases. The sulfur and nitrogen contents will increase, which requires higher temperatures for removal. The feed of heavy distillates contains a relatively large amount of coke precursors, which increases catalyst coking. Therefore, it is essential to control the boiling range of the raw materials carefully, to prevent materials with a too high dry point from entering the reactor. (3) Control the feed rate. If the feed amount decreases significantly, the residence time of the crude oil in the reactor (pseudo-reaction time) increases, leading to an increase in the amount of coke produced ； At the same time, if the feed rate (i.e., space velocity) is too low, hydraulic factors become more problematic; the distribution of the flow within the reactor may become uneven, leading to edge effects and a reduced efficiency of the catalyst. This requires higher reaction temperatures to compensate, which in turn increases carbon deposition and shortens the catalyst’s lifespan. To avoid this situation, it is generally required that the feed rate should not be lower than 70% of the design value. (4) Controlling hydrogen purity (hydrogen partial pressure) If the hydrogen purity (hydrogen partial pressure) decreases, the hydrogenation reaction cannot be carried out effectively; the tendency to form coke increases, and the catalyst’s activity declines. As a result, operators have to compensate by raising the reaction temperature, which only makes the problem worse. To extend the catalyst's service life and minimize the formation of coke, it is not advisable to operate the plant under conditions where the hydrogen purity is below 70% (molar). The hydrogen purity of the system should be improved as much as possible by increasing the purity of the supplementary hydrogen and/or by supplying more fresh hydrogen. (5) Controlling an appropriate hydrogen-to-oil ratio: An appropriate hydrogen-to-oil ratio is a fundamental condition for ensuring the service life of the catalyst. If the unit operates at a hydrogen/oil ratio lower than the design value, both the temperature of the catalyst bed and the wall temperature of the heating tubes will increase, thereby accelerating coking formation. A disruption in hydrogen supply is even more dangerous. Therefore, usually when the hydrogen/oil ratio drops to a predetermined minimum, the heating furnace should be shut down to cool down, or other emergency measures (interlocks) should be taken.