Deactivation and activation of hydrocracking catalysts

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This post was last edited by jbcyf on 2012-10-13 08:05 1 Analysis of causes of deactivation of hydrocracking catalysts The deactivation mechanism of catalysts is divided into three categories: poisoning, coking and sintering. The poisoning of hydrocracking catalysts mainly refers to the chemical adsorption of basic nitrogen such as pyridine compounds on the acidic center, which not only makes the catalyst inactive but also blocks the pores and internal channels; coking is the formation of carbon on the surface of the catalyst, covering the active center. A large amount of coking causes pore blockage, preventing reactant molecules from entering the active center in the pores; sintering causes the catalyst structure to change and lose the active center. For hydrocracking catalysts, it refers to the accumulation of small metals or the enlargement of crystals. Coking and acid-base neutralization caused by chemical adsorption of impurities are temporary poisoning, and the activity can be restored by air charring. Jilin Chemical Hydrocracking Unit used 3825 hydrocracking catalyst when it was put into operation in 1996. After a period of time, it was found that the temperature of the first bed layer did not rise. At first, because the reaction temperature was low (only about 330°C), it was considered that the temperature was not high enough and the catalyst activity requirements had not yet been met. However, as the temperature increased, no temperature rise was found, and the maximum temperature at the entrance reached 376°C. This shows that the 3825 catalyst in the first bed has been deactivated. The hydrocracking reaction is maintained by the 3825 catalyst in the last three beds. After 3 years of operation, it was found that the temperature rise of the second bed 3825 catalyst also gradually decreased and its activity became worse. The crude oil processed by Jilin Chemical is Daqing oil, which has fewer impurities and better properties. The production has been relatively stable, the reaction temperature is not high, and the reactor pressure drop is small. Therefore, the possibility of coking and sintering is ruled out as the cause of catalyst deactivation. It was determined that the reason was due to improper use of the catalyst during start-up or production control. The nitrogen content of the refined oil exceeded the standard, causing chemical adsorption of basic nitrogen compounds on the acidic center, resulting in catalyst poisoning. In June 2000, the equipment was overhauled. 8.32t of 3825 catalyst in the first bed of the hydrocracking reactor was unloaded for regeneration, and 8.7t of fresh 3976 catalyst was loaded. After being put into operation, the 3976 catalyst was running well. But less than half a month later, the DCS power supply of the device failed three times in a row, causing a complete emergency shutdown of the device. Frequent starts and stops caused a great impact on the new catalyst. Unqualified hydrorefined oil entered the cracking reactor, causing the 3976 catalyst to be poisoned and deactivated due to the adsorption of basic nitrogen on the acidic center. The second layer of 3825 catalyst was also severely affected and lost its activity. By July 2000, among the four beds of the hydrocracking reactor, the first and second beds had lost their activity. Since only the last two beds were undergoing cracking reactions, higher temperatures must be maintained to continue production. However, such production operation is difficult, product distribution and product properties are not ideal, and the catalyst deactivation rate is fast. 2 Research on the recovery of catalyst activity shows that the nitrogen poisoning phenomenon of hydrocracking catalysts is reversible to a certain extent. When the nitrogen content of the refined oil is reduced, even if the reaction temperature in the cracking section remains unchanged, the conversion rate can rebound and continue to increase with the extension of the operation time. Because low-nitrogen oil has a certain elution effect on organic nitrogen compounds originally adsorbed on the active center of the catalyst. In addition, increasing the reaction temperature at the entrance to the cracking section is also beneficial to the desorption of nitrogen compounds on the catalyst and reduces catalyst poisoning. Increasing the bed inlet temperature can also "activate" the catalyst activity, allowing its activity to gradually recover at high temperatures. According to these theories, in production control, the nitrogen content of the oil produced by hydrorefining is minimized, and the total nitrogen content of the refined oil is less than 5ppm within a period of time. However, the activity of the catalyst did not improve, indicating that low-nitrogen oil alone was not enough to elute the poisoned catalyst. Starting from November 2000, the inlet temperature of the first and second bed catalysts in the hydrocracking reactor was increased to 380°C, and high temperatures were used to "activate" the catalysts. After about 30 days, the temperature of the two beds began to rise significantly. The temperature rise of the first bed could reach about 2℃, and the temperature of the second bed * * Around 4°C, the catalyst activity begins to recover. Later, for a period of time, due to the increase in the dry point of the equipment's feed oil and the increase in the nitrogen content of the refined oil, the bed temperature rose by 1 to 2°C. It shows that the catalyst activity cannot be restored by raising the temperature alone, and it is necessary to raise the temperature and reduce the nitrogen content at the same time. In 2001, the unit always adhered to this principle in production control, trying to reduce the nitrogen content of hydrorefined oil, increase the temperature of the first and second beds of the cracking reactor, and reduce the reaction temperature of the third and fourth beds. After a long period of operation, the effect is very good, and the catalyst activity gradually recovers. In particular, the maximum temperature rise of the 3825 catalyst in the second bed can reach 8°C during normal production, while the maximum temperature rise of the 3976 catalyst in the first bed is also 4 to 5°C. By analyzing the temperature distribution of the catalyst bed during normal production, it was found that the activity recovery effect of the 3825 catalyst in the second bed * * It is better than the 3976 catalyst in the first bed because it is located below the 3976 catalyst and is less poisoned. Judging from its inlet temperature and temperature rise, the activity of the 3825 catalyst in the second bed is significantly different from that in the third and fourth beds, indicating that complete recovery of activity is difficult to achieve. Moreover, when the catalyst in the first and second beds is deactivated, the reaction temperature in the third and fourth beds is higher, and the average temperature of the reactor is also higher. However, under these conditions, the reaction conversion rate is not high, and the product distribution and product properties are not ideal. After the catalyst activity of the first two beds became better, the inlet temperature of the last two beds dropped by about 10°C, but the reaction conversion rate increased. Under this condition, the product properties and product distribution were better than the former. In 2001, the liquid yield of the hydrocracking unit increased by 1% compared with 2000, and the overall life of the catalyst was extended. According to this situation, it is expected to be able to operate until May 2002 for maintenance. 3 Conclusion Hydrocracking catalysts may be poisoned due to excessive alkaline nitrogen in the raw materials during production operations, resulting in catalyst deactivation. However, during production operations, methods such as increasing the operating temperature of the catalyst and reducing the nitrogen content in the feed oil can be used to "activate" the catalyst to increase its activity. In this way, unplanned shutdown of the device and external regeneration of the catalyst can be avoided, thereby extending the production cycle and achieving significant economic benefits.