What causes the increase in reactor bed pressure drop?

Gasoline carries alkaline solutions into the fixed-bed reactor; the accumulation of salts and impurities on the catalyst surface, as well as fluctuations in the processing volume, perturb the bed layer

If there is a sudden increase, it could be due to an increase in the feed rate, or an increase in the reaction pressure; water entering the reactor may cause the catalyst to break down; impurities in the feed may accumulate; the catalyst may gradually coking; or the pressure difference gauge may not be accurate

High circulating hydrogen volume and high space velocity lead to catalyst coking and bed collapse

High circulating hydrogen volume and high space velocity lead to catalyst coking and bed collapse

A decrease in the purity of the recycled hydrogen or sludge coking in the catalyst bed can be addressed by adding some fresh hydrogen to the recycled hydrogen

I) Influence of the properties of the feed oil 　　1. If the feed contains a high amount of naphthenic acids, then the iron present in the oil is mainly in the form of naphthenic acid iron. When the iron ion concentration is high, it comes into contact with hydrogen sulfide in the recycle hydrogen inside the reactor, leading to the reaction: Fe + H2S = FeS + H2. This results in the formation of iron sulfide deposits within the pores of the catalyst; in severe cases, these deposits can block the catalyst’s micropores, causing it to lose its activity. Moreover, it is difficult to remove these deposits through regeneration methods, resulting in irreversible and permanent poisoning of the catalyst. The attraction between iron sulfides is strong, causing them to aggregate and cover the upper layer of the catalyst bed, which leads to caking at the top of the bed and an increase in the pressure drop across it. At the same time, this iron sulfide can promote the coking reaction of the coking precursor at high temperatures, thereby accelerating the blockage of the bed. From this perspective, the iron content in the crude oil and the FeS formed due to equipment corrosion are key factors contributing to bed clogging and increased pressure drop. Furthermore, nickel can enter the pores of the catalyst, while vanadium enters the pores’ openings, thereby reducing the catalyst’s surface area. 　　2. Among the various indicators for which control is required regarding raw materials, the nitride content is also particularly important. Due to the increase in nitrogen content, particularly basic nitrides, which can strongly adsorb onto the acidic sites of cracking catalysts and thereby reduce their activity, this has an inhibitory effect on the performance of acidic cracking catalysts. Moreover, the nitrides themselves are unstable and tend to condense to form coke, leading to catalyst deactivation and affecting product quality. 　　3. The feed oil may contain coke particles from upstream units, or polymers formed as a result of polymerization reactions of olefins in the feed before entering the reactor; these large molecular particles deposit on the surface of the catalyst. When their accumulation reaches a certain level, it leads to an increase in the pressure drop across the reactor bed. During the third operating cycle of the unit, on July 2, 2010, a failure in the backwash filter was detected and shut down. Monitoring of the parameters showed that the pressure drop across the first bed of the reactor was significantly affected, rising from 0.06 KPa; by July 7, this pressure drop increased to 0.07 KPa. 　　(II) Effect of reaction temperature The reaction temperature is the most important and flexible operating parameter in hydrogenation processes, and it is also the main means for controlling product quality. From a kinetic perspective, both the hydrorefining reaction and the hydrocracking reaction are exothermic reactions; increasing the reaction temperature is not conducive to the forward progression of these reactions ； However, too low a temperature will slow down the reaction rate, reduce the desulfurization and denitrification efficiencies, and fail to achieve the desired conversion rate. Denitration requires a higher temperature than desulfurization; excessively high reaction temperatures exacerbate the cracking reaction. Besides reducing the liquid yield, this also leads to catalyst carbon deposition and an accelerated increase in the pressure drop across the reactor bed. The pressure drop increases. At high temperatures, some condensation and polycondensation reactions also occur, especially when the content of polycyclic aromatic hydrocarbons, asphaltenes, and non-hydrocarbon compounds in the feedstock is high. The polymers resulting from these polycondensation reactions serve as precursors for carbon deposition; the formation and accumulation of such carbon lead to the loss of active sites on the catalyst, resulting in deactivation and an increase in the pressure drop across the bed. 　　When the reaction temperature is increased during the hydrorefining process, the degree of hydrogen saturation, desulfurization, and denitration improves; however, if the temperature is too high, the catalyst’s cracking activity increases, resulting in an increase in the bromine value of the product. As carbon deposition during the process and nitrogen compounds in the feed oil reduce the acidity and activity of the catalyst, in order to maintain the desired degree of reaction, the reaction temperature needs to be increased gradually toward the end of operation. Excessively high reaction temperatures have three main adverse effects: First, coking in the feed heat exchanger increases; as the reaction temperature rises, the temperature of the raw materials entering the heat exchanger also increases. In the presence of the aforementioned coking-forming substances as well as Fe and trace amounts of oxygen, such relatively high temperatures accelerate the coking process ； Secondly, coking intensifies from the outlet of the feed heat exchanger to the furnace tubes. Some of this coked material enters the reactor, while some adheres to the tube walls. When there are fluctuations in the operation of the plant or after it stops operating and resumes, changes in the system conditions cause the coke adhering to the tube walls to fall off and be carried into the reactor ； Thirdly, an increase in reaction temperature can easily lead to carbon deposition reactions of large molecular hydrocarbons on the catalyst bed, resulting in coking and an increase in the pressure drop across the bed. 　　(III) Effects of hydrogen purity and hydrogen-to-oil ratio High levels of CO and CO2 in the fresh hydrogen compete for adsorption on the active sites of the catalyst, leading to methanation reactions: CO + CO2 + H2 = CH4 + H2O. This reaction releases a large amount of heat, causing the reaction temperature to rise rapidly. Additionally, the hydrogen partial pressure decreases; these two factors are the main reasons for catalyst carbon deposition and coking. 　　The hydrogen-to-oil ratio should generally be kept within the designed range; a higher hydrogen-to-oil ratio can prevent catalyst carbon deposition, as a higher hydrogen concentration helps to stop the formation of carbon. A high hydrogen partial pressure needs to be maintained in the hydrogenation system, and increasing the hydrogen-to-oil ratio can raise the hydrogen partial pressure. This helps to increase the vaporization rate of the feed oil and reduce the thickness of the oil film on the catalyst surface, thereby increasing the conversion rate; it also slows down the rate of catalyst coking. However, an excessively high hydrogen-to-oil ratio not only increases energy consumption but also inhibits the hydrocracking and hydrorefining reactions. If the hydrogen-to-oil ratio in the catalyst bed does not meet the design requirements, or is even too low, it will lead to increased coking in the bed. Therefore, we must carry out production operations in accordance with the hydrogen-to-oil ratio specified in the design requirements. 　　(IV) Significant fluctuations in operating conditions When there are fluctuations in the operation of the device, such as sudden changes in pressure, the coke adhering to the tube walls will fall off and be carried into the reactor, causing a sharp increase in the pressure drop across the catalyst bed. Among these factors, the greatest impact on pressure drop is caused by significant fluctuations in the hydrogen flow rate. When the hydrogen flow rate suddenly decreases or is interrupted, the hydrogen-to-oil ratio drops rapidly, the amount of gas circulating decreases, and the heat generated by the hydrogenation reaction cannot be removed from the bed, leading to overheating of the bed and increased carbon deposition. 　　During normal production, abnormalities such as overheating or excessive temperature rise, low purity of circulating hydrogen, low hydrogen-to-oil ratio, low hydrogen partial pressure, and low space velocity can occur, leading to catalyst coking. The bed pressure drop increases.