Discussion on preventing coking in the slurry system of catalytic units

The above explanation seems very comprehensive to me. Coking in the slurry system is primarily prevented by strict control of operational parameters, especially the temperature at the bottom of the tower; it is absolutely essential to avoid overheating, with the temperature generally kept between 350–355°C. Prolonged overheating leads to the formation of large amounts of polycyclic aromatic hydrocarbons, which further condense to form coke. Additionally, the liquid level at the bottom of the distillation tower must also be well controlled; avoid operating at high or full liquid levels, and try to minimize the residence time of the liquid at the bottom of the tower. A distillation column is generally controlled by upward and downward flow in the column. A orifice plate is required for steam injection at the bottom of the tower to control the amount of steam.

Increase the dosage of the scale inhibitor during operation. Last edited by yfq4308 on 2009-3-9 08:17.]

Increase steam stirring and replace the steam with superheated steam

Construction quality. During the shutdown for maintenance or emergency repairs, stricter management of the coking removal team is required; the coked areas must be thoroughly cleaned to ensure that the settler, oil and gas main lines, and the fractionated slurry system are completely free of coke, with no dead corners remaining ； Additionally, during the commissioning phase of the unit, the external circulation time of the crude oil tower should be appropriately extended to remove as much of the remaining small coke particles from the system as possible. Reduce the reprocessing ratio and increase the amount of slurry discharged outside. Measures to reduce the reaction reprocessing ratio: First, increase the reaction severity to boost the one-pass conversion rate; second, keep the slurry density at or below 980 kg/m3 and increase the amount of heavy slurry removed. The heavier the crude oil, the lower the reprocessing ratio should be controlled, and the greater the amount of slurry to be discharged. In response to changes in the raw materials, it is necessary to adjust the amount of slurry discharged in order to reduce coking in the settler and the slurry system, thereby ensuring the proper operation of the slurry system. Furthermore, the amount of slurry to be discharged should be determined based on the principle of preferring more rather than less, and it should not be decided simply by referring to the density of the slurry. 　 Reduce the solid content in the slurry. The solid catalyst in the slurry has a significant impact on coking in the slurry system. The catalyst deposits at the bottom of the distillation tower; its adsorptive effect reduces the number of sites where oil slurry droplets can accumulate. Moreover, the collisions between the solid catalyst particles in the oil slurry accelerate their settling rate, resulting in a large amount of catalyst particles depositing at the bottom of the distillation tower, where coking occurs under suitable conditions. The relative density of the slurry reflects its properties; a higher relative density indicates the presence of more high-boiling-point, large-molecule components. It has a high coking potential, which can easily lead to coking in the slurry system; at the same time, it increases coking of the reprocessed slurry in the riser. Use a lower bottom temperature of the tower. Reduce the temperature of the circulating slurry returning to the tower, and increase the flow rate at the inlet located lower in the tower. Control the residence time. Maintain a short residence time at the bottom of the distillation tower, try to keep the flow rate at the upper limit of the slurry pump in order to achieve the maximum slurry circulation volume, and keep the liquid level in the distillation tower low to prevent coking of the slurry under high temperatures. The residence time at the bottom of the distillation tower should be kept within an appropriate range (3–5 minutes). Increase the flow rate. To prevent coking in the pipes and equipment of the slurry circulation system, the flow velocity of the slurry in the pipes should be no less than 15–20 m/s, while it is advisable to keep it between 12–20 m/s inside the tubes of the heat exchanger. The original design called for four vaporization heat exchangers for the unit’s slurry, but now it is changed to two in operation and two on standby ； In normal operation, maintain the maximum circulation rate of the slurry; the use of bypass lines for the slurry heat exchanger should be done with caution, ensuring that the flow velocity inside the heat exchanger remains at least 12 m/s, in order to prevent scaling due to the increase in viscosity as the oil temperature drops during the heat exchange process. Add a scale inhibitor. Select an appropriate slurry scale inhibitor and inject it continuously from the start of operation of the unit, to prevent insoluble substances in the slurry from adhering to the tube walls of the heat exchanger. Avoid large fluctuations in processing capacity to reduce operational instability of the equipment. The unit significantly increases or decreases the processing capacity, which directly affects the stable operation of the reaction-regeneration system. When in standby mode, the unit operates at a low load; as a result, the linear velocity at the inlet of the single cyclone separator in the settler is low, and some fine catalyst particles end up in the distillation system, where they can accumulate in the slurry system. It is necessary to avoid prolonged operation of the unit at low load. The processing capacity of the unit reaches the design load, and the feed to the riser remains relatively stable, which helps to ensure proper separation in the settler and single cyclone separator. To ensure stable operation of the unit, the reaction-regeneration system should avoid large fluctuations in load, maintain the separation efficiency of the cyclone separator, and reduce the amount of fine catalyst powder that enters the distillation tower. During operation, it is necessary to ensure stability as much as possible to reduce coking caused by operational fluctuations. When making various operational adjustments, efforts should be made to find the optimal operating conditions in order to prevent catalyst from escaping from the settler, which could lead to coking in the bottom of the distillation tower and in the slurry system. If operational fluctuations occur, it is necessary to increase the amount of slurry discharged promptly to keep the solid content in the slurry at a low level. When the unit starts up, we need to enhance catalyst fluidization before fuel injection, raise the temperature of the settler as quickly as possible, and reduce the formation of excessive start-up coke in the settler. Additionally, at the start of operation of the unit, the distillation system should establish a three-way circulation as early as possible to recycle the coking solids within the system. Ensure proper maintenance of the entire installation in order to avoid or minimize unplanned shutdowns, thereby preventing coking from occurring as a result. The coking or shedding of coke masses resulting from unplanned shutdowns has a significant impact on the long-term operation of the plant. Strengthen the management of catalytic materials; more thorough analysis of these materials is needed, using compelling indicators such as group composition, rather than simply relying on residual carbon as a criterion for adjusting them. This ensures the stability of the properties of the catalyst material.