What are the measures to reduce coking in catalytic cracking?

1. The catalyst has too high activity, resulting in excessive reaction depth. 2. Catalyst poisoning leads to reduced selectivity. 3. The reaction temperature is too high

I. Coking in the settler and the internal rotation area: Hazards of coking: Large, dense, and hard coke masses can form on the walls and vault of the settler, as well as in the stripping section. These coke masses can sometimes be sent to the regenerator through the raw catalyst slurry tubes to be burned away, but at other times they may block the grids in the stripping section without any warning, reducing the flow area and thus decreasing the circulation rate of the system. In severe cases, this can lead to catalyst bridging and the regenerator becoming filled with air. Coking inside the internal rotator, as well as blockages in the material legs and jamming of the wing valves when these particles fall off, can all lead to a decrease in the efficiency of the centrifugal separator or even its complete failure. This results in an increase in catalyst loss, a higher solid content in the slurry, which in turn causes wear and damage to the impellers of the slurry pumps, the pump bodies, heat exchangers, pipelines, etc., increasing the risk of fire accidents. In severe cases, it can also cause blockages at the bottom of the fractionation tower, leading to vacuum conditions in the slurry pump as well as blockages in the slurry pipelines. Severe coking in any one of these areas will prevent production from continuing, resulting in unplanned shutdowns ； It not only requires shutdown for maintenance but also increases the repair costs of the equipment, while posing significant risks to its long-term operation. Therefore, it is an important task to analyze the causes of coking in different locations where coking occurs, to adopt practical preventive and control measures, to optimize operating conditions, and to minimize coking as well as unplanned shutdowns. Therefore, how to prevent coking and ensure long-term operation is the key to plant renovation. Reasons for coking: 1. High proportion of sludge oil in the catalytic cracking feed, resulting in heavy fractions; the mixing temperature between the feed and the regenerator is low, insufficient for complete vaporization, so part of the feed remains in liquid form, which facilitates the formation of liquid coke. 2. The use of inefficient atomization nozzles results in low steam pressure for atomization, insufficient amount of vapor for atomization, and the formation of large liquid droplets ； Or the nozzle position is improperly arranged, resulting in uneven feed distribution. 3. The temperatures on the inner surfaces of the equipment in the reactive sedimentation system and on the inner walls of the oil and gas pipelines are low, falling below the dew point of the oil and gas, which leads to their condensation and coking. 4. The rapid separation system of the settler was poorly designed; as a result, oil and gas stayed in the settler for too long, undergoing secondary cracking, which led to extensive coking within the system ； It is generally believed that poor atomization and vaporization of heavy feedstocks, as well as the condensation and polymerization reactions of dienes and large molecular aromatic hydrocarbons generated during the reaction process, are the main mechanisms behind coking. An unreasonable internal structure of the settler is the main cause of coking. The oil and gas produced by heavy oil catalytic cracking contain large amounts of heavy aromatic compounds, gums, asphaltenes, and other heavy components, which are precursors to the formation of coke. To reduce coking in the settler, it is necessary to shorten the residence time of the oil and gas within the settler. 5. Severe coking occurs when the temperature at the exit of the lift pipe remains too low for an extended period, or when the temperature in the settler does not reach its peak value during startup. 6. There are frequent unit accidents, continuous start-ups and shutdowns, as well as repeated interruptions in feed supply, all of which exacerbate coking in the unit. 7. Poor material discharge in the regeneration line, large fluctuations in reaction temperature, and uneven contact between the catalyst and the raw materials all contribute to an increased formation of liquid coke in the settler. 8. The poor quality of the raw materials, high levels of heavy metals, and catalyst poisoning, along with low catalyst activity, all contribute to an increased tendency to form coke. 9. Poor fluidization quality in the pre-raising section, along with an excessively low reagent-to-oil ratio, can also lead to coking in the riser. 10. The pressure difference between the two vessels is low, resulting in a small amount of steam injection; this causes the oil and gas to stay in the lift pipe for too long, leading to secondary cracking and extensive coking within the system.

1. Improve the atomization and vaporization of the feed oil. Since residue is in a gas-liquid mixture state during catalytic cracking, when liquid residue comes into contact with the hot catalyst, it is adsorbed by the catalyst and enters the micropores within the catalyst particles, where it is cracked into coke. This leads to an increase in the amount of coke produced and a decrease in the catalytic activity. It can therefore be seen that, in order to reduce the amount of coking on the catalyst, it is necessary to minimize the proportion of the liquid phase as much as possible; hence, it is essential to enhance the misting and evaporation processes during the early stages of catalytic cracking, increase the gasification rate, and reduce liquid-solid reactions. 2. Use a higher reaction temperature and a shorter reaction time. When the reaction temperature is increased, the cracking of the raw materials accelerates significantly, while the coking reaction accelerates to a lesser extent. Meanwhile, as the temperature increases, it promotes an intensification of the thermal cracking reaction, resulting in an increase in C1 and C2 compounds and a decrease in C3 and C4 compounds in the gas produced by the catalytic cracking of heavy oil. Therefore, a higher reaction temperature and a shorter reaction time should be employed.

This post was last edited by Shuichangshou on 2009-12-28 21:28 to increase the amount of anti-scorch steam. The anti-scorching steam uses a two-stage orifice nozzle, with the nozzle directed toward the oil and gas retention space in the settler to avoid dead zones. A new type of rapid separation device is used to reduce the residence time of the reaction gas and oil in the settler. By using methods such as rough spinning, three-blade design, and closed direct-connected rapid separators, the residence time of the reaction gas mixture can be reduced from 20–30 to 4–9. The riser reactor terminator technology is employed to reduce unsaturated dienes generated by over-crackling reactions, and the structure of the settler is optimized to eliminate flow dead zones in the reaction gas stream. Operate smoothly to avoid large fluctuations in the temperature and pressure of the settler.

1. Replace the nozzle； 2. Reduce the reprocessing ratio ； 3. Adjust and increase the steam supply and the misting steam volume ； 4. Appropriately control the reaction temperature

1. Reasons for coking in the lift pipe. 2. When the temperature of the coking vessel becomes too high, it is reasonable to reduce the reaction temperature in the lift pipe in order to lower the temperature of the coking vessel. Are there any other methods? 2. Reasons for coking in the lift pipe: 1. Poor quality of raw materials, resulting in poor atomization by the nozzle ； 2. The nozzle itself has poor atomization performance, and heavy oil adheres to the surface of the catalyst ； 3. The temperature in the vaporization section is low, so the heavy oil on the catalyst surface does not vaporize and cokes on the wall of the reactor ； 4. Nozzle installation issues, such as inaccurate angle ； 5. In an accident scenario, low-temperature feed ； Or frequent interruption of feeding leads to frequent fluctuations in the temperature of the feeding section, resulting in coking ； When the temperature in the coking vessel becomes too high, attempting to lower this temperature by reducing the reaction temperature in the lift pipe is not a valid approach, as there is a limit to how much the reaction temperature in the lift pipe can be reduced. It is necessary to analyze the reasons behind the excessively high temperature in the coking vessel and then take appropriate measures, such as increasing the fuel-to-oil ratio to reduce the amount of coke produced, enhancing external heat removal, replacing the catalyst, or altering the selectivity of the coke formation ； Reduce the feed rate ； Adjust the main air supply, reserve distribution, and the ratio of re-burning and coking, etc ； 3. Additional point from the second floor: Poor fluidization quality in the pre-raising section can also affect coking in the riser ； An excessively low oil-to-agent ratio can also affect coking in the riser ； Too low a preheating temperature of the raw materials can also exacerbate coking. 4. 1) The catalytic cracking feed contains a high amount of sludge oil; the feed fraction is heavy, and the mixing temperature with the regenerator is low, insufficient for complete vaporization. As a result, a considerable amount of the feed remains in liquid form, which facilitates the formation of liquid coke. (2) The use of inefficient atomization nozzles results in insufficient vapor atomization, leading to the formation of liquid droplets larger than 80 microns; furthermore, improper placement of the nozzles causes uneven feed distribution, which results in coking on the nozzles and the inner walls of the upper μ lift tubes. (3) Low temperatures on the inner surfaces of the equipment in the reactive sedimentation system and on the inner walls of oil and gas pipelines, below the dew point of the oil and gas, cause condensation and coking of the oil and gas. Severe coking occurs in situations such as an excessively low temperature at the outlet of the lift pipe, or when the temperature of the sedimentator does not reach the predetermined level during startup. (4) There are frequent unit accidents, continuous start-stop operations, and repeated interruptions in feed supply, which exacerbate coking in the unit, and so on. (5) A higher oil-to-agent mixing temperature should be used; especially when the raw material fraction is heavy, large molecules are difficult to vaporize. Therefore, it is necessary to use a higher oil-to-agent ratio and higher temperatures in order to mix with the catalyst and ensure thorough vaporization, thereby reducing the amount of unvaporized liquid droplets and minimizing the tendency to coking. The regeneration temperature should be increased appropriately; it can approach 710°C. By raising the regeneration temperature from 660°C to 710°C, an increase of 50°C, the hydrogen content in the coke drops from about 9% to around 7%. 5. Coking in the lift pipe is mainly caused by the nozzle; it is recommended to check the installation of the nozzle and the condition of the atomizing steam, or replace it with a new nozzle. Lowering the reaction temperature reduces the depth of the reaction and decreases the amount of coke formed; this is a temporary solution. It is recommended to reduce the amount of slag added, and when there is still a surplus, increase the main air flow to enhance the capacity for burning coke. Reasons for riser coking: 1. High content of sludge oil in the catalytic cracking feed, resulting in heavy fractions; the mixing temperature between the feed and the regenerator is low, insufficient for complete gasification, so part of the feed remains in liquid form, which facilitates the formation of liquid coke ； 2. The use of inefficient atomization nozzles results in insufficient vapor atomization, leading to the formation of large liquid droplets ； The improper placement of the nozzles results in uneven feed distribution ； 3. Poor fluidization quality in the pre-rising section, along with an excessively low reagent-to-oil ratio, can also lead to coking in the riser ； 4. The pressure difference between the two vessels is small, resulting in a low steam injection rate; this causes the oil and gas to remain in the lift pipe for too long, leading to secondary cracking and extensive coking within the system ； 5. High vapor and oil partial pressures increase the tendency to form coke ； 7. There are frequent device failures, continuous start-ups and shutdowns, as well as frequent interruptions in feed supply, which exacerbates coking in the lift pipe ； 8. Severe coking can occur when the temperature at the outlet of the lift pipe remains too low for an extended period, or when the temperature of the lift pipe does not reach its peak value during startup ； It is unreasonable to lower the temperature of the coking drum by reducing the reaction temperature in the lift pipe when the temperature of the coking drum becomes too high; lowering the temperature is necessary

The brothers are really thorough; the concepts of charring and coking at floors 3, 5, and 7 aren’t the same, but they still go into quite a lot of detail. 1. A good nozzle is important ； 2. Ensure proper operation of these sections: the pre-raising section, the feed vaporization section, the reaction section, the rear system, and the stripping section are all very important. It’s quite general, but when expanded it covers a wide range; still, the basic function of each section can be understood.

An excessively high yield of coke in the reaction will cause overheating of the regenerator bed, a decrease in catalyst activity and specific surface area; in severe cases, it can damage the equipment and lead to an excess of heat in the system. Daily measures to reduce coking include: (1) Controlling the quality of the pre-raising steam and dry gas. When the high-temperature, low-carbon catalyst is fed from the regenerator to the bottom of the riser, it first comes into contact with a mixture of water vapor and light hydrocarbons present in the dry gas. The decomposition of these light hydrocarbons results in a slight coating of carbon on the active heavy metals on the catalyst surface, which helps to suppress the dehydrogenation activity of nickel and renders the heavy metals on the catalyst less active. This also facilitates faster transport of the catalyst within the riser, allowing for better mixing and contact between the catalyst and the feed materials. Under other identical conditions, this approach can reduce the production of gases and coke. Furthermore, when components with a molecular weight of C3 or higher in the dry gas come into contact with high-temperature catalysts, they decompose and form coke, thereby reducing the catalyst’s activity; therefore, it is effective to keep the proportion of such components in the dry gas below 6%, with a H2 content of around 20%. (2) Improving the atomization effect of the feedstock: Since the feedstock is in a gas-liquid mixture state during catalytic cracking, when the liquid feedstock comes into contact with the hot catalyst, it is adsorbed by the catalyst and enters the micropores within the catalyst particles, where it is then cracked into coke. This leads to an increase in the amount of coke produced and a decrease in catalytic activity. It can therefore be seen that, in order to reduce the amount of coking on the catalyst, it is necessary to minimize the proportion of the liquid phase as much as possible; hence, the misting and evaporation processes during the early stages of catalytic cracking need to be enhanced to increase the gasification rate and reduce liquid-solid reactions. To improve heat transfer efficiency, the feed oil should be misted into small particles as much as possible. Some foreign companies believe that when the size of the aerosol particles is 60μm, just 4 to 5 catalyst particles are sufficient to vaporize them ； However, for 250μm particles, thousands of catalyst particles are required, so the atomization effect is very important. (3) Slightly increase the regeneration temperature. Although increasing the regeneration temperature has no direct effect on coke, it accelerates the atomization and vaporization of the feed, thereby reducing the carbon content in the regenerated catalyst. Moreover, as the circulation volume decreases, the catalyst-to-oil ratio is reduced, which helps to lower the overall coke formation rate. (4) Improving stripping efficiency: The stripping efficiency depends on the amount of steam used, the catalyst circulation rate, and the structure of the stripping section. When the catalyst circulation rate is constant, more steam can be used to extract more hydrocarbons. The amount of stripping steam required per unit of catalyst is generally 1.5–3.5 kg/t catalyst. When the feed oil is heavier, a slightly higher amount of stripping steam is required; for example, in heavy oil catalysis technology, this amount is 5.2–6.5 kg per ton of catalyst. The method to determine the appropriate amount of stripping steam is to set the flow rate at the specified value first, and then gradually reduce it at a rate of 200 kg/h while keeping an eye on the temperature of the regenerator. If the temperature remains constant and does not rise after one hour, then reduce the amount of water vapor by 200 kg/h and proceed in this manner until an increase in the regenerator temperature is observed. At this point, the amount of water vapor should be increased slightly to ensure proper stripping. Another method is to determine the hydrogen content in the catalyst coke. According to the data, if the hydrogen content in the catalyst coke reaches 5.9%, it indicates good stripping efficiency. (5) Improving the separation efficiency of the cyclone separator at the riser outlet: To reduce secondary reactions, the cyclone separator at the riser outlet quickly separates the catalyst from the reaction gases and oils, thereby bringing the reaction to an end rapidly. In this way, the coke yield can be reduced, yielding a product distribution with high selectivity. (6) External slurry discharge. The composition of the oil slurry depends on the composition of the feedstocks and the reaction conditions, and it is a fraction with an uncertain range. The greater the condensation amount of the fractionator slurry, the more its fractions resemble reprocessed oil. Whether it is reprocessed oil or slurry, it is more difficult to crack than the crude oil, and it results in a high coke yield. According to domestic data analysis, 3.9% coking occurs in Daqing wax oil, while 20.9% coking occurs in clarified oil ； Coking rate of Daqing normal residue is 8.7%, and that of clarified oil is 38.3% ； Coking from pipeline oil (31.5% residue removal) was 11.9%, and coking from clarified oil was 42.4% (relative density 0.9584). Therefore, it is beneficial to discharge a portion of the oil slurry, but the light oil yield will decrease. In recent years, crude oils have become increasingly heavier and their compositional profiles have changed significantly; in some refineries, the content of polycyclic aromatic hydrocarbons in the sludge reaches 55%, making the practice of separating this sludge one of the essential methods for maintaining production. Of course, the amount of external oil slurry discharged must be determined based on factors such as the composition of the feed oil, the degree of reaction, and the production plan. (7) Appropriate reaction temperature: Increasing the reaction temperature increases the reaction rate. The activation energy for the catalytic cracking reaction is approximately 41.8–125.4 kJ/mol; for every 10%–20% increase in temperature, the reaction rate increases by about 10%–20%. As the reaction temperature increases, the reaction rate at which gasoline is converted into gas increases the most; the reaction rate at which the raw material is converted into gasoline increases to a lesser extent; while the reaction rate at which the raw material is converted into coke increases the least. Therefore, as the reaction temperature increases, if the conversion rate remains unchanged, the gasoline yield is low, the gas yield increases, while the coke yield decreases slightly. (8) Appropriate oil-to-agent ratio. Due to the presence of catalyst active centers, **the reaction rate is increased. Therefore, increasing the oil-to-catalyst ratio relatively increases the number of active centers, thereby accelerating the reaction rate and allowing for more thorough contact between the feed oil and the catalyst. At the same time, the increase in the catalyst circulation rate reduces the carbon difference between the raw catalyst and the regenerated catalyst, thereby increasing the effective active centers of the catalyst and enhancing the reaction conversion depth. As the oil-to-agent ratio increases, the conversion rate rises, along with the amounts of gas, gasoline, and coke. In particular, the ratio of coke to conversion increased significantly. This is because an increased oil-to-agent ratio leads to an increase in the amount of hydrocarbons adsorbed on the raw catalyst, and a shorter stripping time results in an increased coke yield. At the same reaction temperature, changes in the preheating temperature of the feed oil affect the agent-to-oil ratio, as well as the amount of coke formed.

This post was last edited by 660 on 2011-7-24 at 14:30. Making good use of stripping steam to reduce the oil load on the catalyst helps to decrease the amount of main air used for burning, which in turn contributes to energy savings.

Which brother is using a device with that large-prelift section? How does one make adjustments when fluidization isn’t good under normal conditions? Thank you