[Q&A Question 148] 2018.05.28

The liquid level at the bottom of the tower is one of the important parameters in the operation of an entire distillation tower. The level of the liquid at the bottom of the tower has a significant impact on the operation of the unit. An excessively low liquid level can cause the slurry pump to experience vacuum conditions, disrupting the thermal balance throughout the tower and leading to interruptions in the slurry circulation; this in turn can result in accidents such as tower overpressure and overheating ； An excessively high liquid level can flood the reaction gas feed port, causing pressure buildup in the reaction system and leading to serious consequences. Therefore, the liquid level in the distillation tower must be strictly controlled. The level of the liquid at the bottom of the distillation tower is essentially determined by the reaction process; the degree of reaction has a significant impact on the liquid level at the bottom of the distillation tower. Therefore, when the liquid level changes, it is necessary to contact the reaction unit to make adjustments; the distillation system can also be adjusted by modifying the amount of slurry sent outside or by adjusting the temperature of the baffle plates. Generally, it is not necessary to adjust the amount of reprocessed oil sent back to the tower or the opening degree of the flow control valve for the slurry flowing back to the tower in order to regulate the liquid level. As for distillation itself, changes in the liquid level at the bottom of the tower reflect changes in the material balance throughout the tower, and this material balance is determined by the equilibrium of temperature, flow rate, and pressure, as well as by the balance between the amount of recycled oil and the amount of slurry. The bottom liquid level of the fractionation tower and the level of the reprocessed oil both belong to the heavy oil levels, and the height of these levels serves as an indicator of the equilibrium in heavy oil cracking. If the amount of slurry discharged remains unchanged while both liquid levels rise or fall simultaneously, it indicates that the conversion rate of heavy oil is decreasing or increasing. The influencing factors include changes in the properties of the feed oil, alterations in the reaction severity, and adjustments to the amount of catalyst added. If the two liquid levels are at different heights, it indicates that the separation point between the slurry and the reprocessed oil has changed, meaning that the temperature on the wedge-shaped baffle has changed. If one of the two liquid levels changes while the other remains constant, the change in the bottom liquid level of the distillation column is usually due to the amount of slurry discharged, whereas the change in the level of the reprocessed oil is usually caused by the amount of diesel distillate produced. The design of this unit involves controlling the amount of slurry discharged from the bottom of the distillation tower based on the liquid level there. However, in order to increase the yield of light oil and thus boost efficiency, the amount of slurry discharged is generally not used as a control parameter for the liquid level at the bottom of the tower; instead, the density or solid content of the slurry is used to determine the amount of slurry to be discharged. At this point, the liquid level at the bottom of the fractionation tower needs to be adjusted according to the situation based on the degree of reaction. Factors affecting it: 1. Change in reaction depth: As the reaction depth increases, the liquid level decreases. 2. Changes in processing volume or raw material properties: An increase in processing volume or a heavier raw material leads to an elevation of the liquid level at the bottom of the tower. 3. Change in the temperature of the slurry returning to the tower: A decrease in the return temperature leads to an increase in the liquid level at the bottom of the tower. 4. Change in slurry circulation volume: An increase in circulation volume leads to an elevation of the liquid level at the bottom of the tower. 5. Change in the amount of reprocessed oil returning to the tower: An increase in the amount returning to the tower leads to an elevation of the liquid level. 6. An increase in the amount of slurry recycled or discharged leads to a drop in the liquid level. 7. Reaction to pressure changes: As pressure increases, the liquid level rises. 8. Failures in the slurry pump, temperature control valves, flow control valves, and instruments can cause changes in the liquid level. 9. As the terminator flow increases, the liquid level drops. 10. As the temperature drops further, the liquid level falls. 11. The amount of fresh catalyst added increases, causing the liquid level to drop. 12. As the temperature of the diagonal baffle rises, the liquid level drops. At the same temperature of the diagonal baffle, a decrease in the internal return flow leads to a drop in the liquid level. 13. The carbon content reduced due to the regenerator, and the liquid level dropped. 14. Overflow occurs when the crude oil tank and the reprocessed oil tank are full, causing the liquid level to rise. 15. The temperature rises below the first tray of the distillation column, and the liquid level drops. 16. The backflow increases, causing the liquid level to rise. 17. An increase in steam injection at the bottom of the distillation tower results in a decrease in the liquid level.

Reasons: (1) Changes in reaction temperature; higher temperatures result in a lower liquid level. (2) Changes in the amount of residue reprocessed; a lower amount of reprocessing results in a higher liquid level. (3) Change in reaction depth: a greater reaction depth results in a lower liquid level. (4) Changes in feed rate: a higher feed rate results in a higher liquid level. (5) The change in the opening degree of the lower valve for returning the slurry to the tower: as the lower valve opens wider, the liquid level drops. (6) Changes in slurry circulation rate and tower return temperature. (7) Overflow in the re-refining cylinder, with the liquid level rising. Handling: (1) Carefully monitor the feed rate to the stabilizing distillation column; if there are changes in the feed rate, adjust the liquid control valve to stabilize the liquid level at the bottom of the column. (2) Operate the heating furnace properly to maintain a stable outlet temperature and circulation rate for it. (3) Adjust the temperature at the top of the column, ensure stable operation to keep the pressure at the top of the column under control, and inspect and maintain the pressure control systems as well as the level indicators and control valves. (8) Pressure changes in the tower: higher pressure corresponds to a higher liquid level.

(1) Changes in reaction temperature: as the temperature increases, the liquid level decreases. (2) Changes in the amount of residue reprocessed; a lower amount of reprocessing results in a higher liquid level. (3) Change in reaction depth: a greater reaction depth results in a lower liquid level. (4) Changes in feed rate: a higher feed rate results in a higher liquid level. (5) The change in the opening degree of the lower valve for returning the slurry to the tower: as the lower valve opens wider, the liquid level drops. (6) Changes in slurry circulation rate and tower return temperature. (7) Overflow in the re-refining cylinder, with the liquid level rising. (8) Pressure changes in the tower: higher pressure corresponds to a higher liquid level. Actions to take: 1. Make an accurate assessment of the reasons for fluctuations in the liquid level in the distillation tower. 2. Stabilize the feed rate to the distillation tower; if there are changes in the feed rate, adjust the liquid control valve to maintain a stable liquid level at the bottom of the tower. 3. Improve the operation of the heating furnace to keep its outlet temperature and flow rate stable. 4. Adjust the temperature at the top of the tower, ensure stable operation to maintain pressure there, and conduct inspections and repairs on the pressure control systems as well as the liquid level indicators and control valves

1. The feed rate to the tower and the flow rate of liquid drawn from the bottom of the tower are not well controlled. Solution: Stabilize the flow rates; if there is a fault with the control valve or the extraction pump, switch to a backup pump immediately； If it is due to issues upstream or downstream in the pipeline network, report it promptly so that the headquarters can take action ； 2. Changes in the composition of the feed were not accompanied by timely adjustments. Solution: By monitoring tower pressure, the amount of material drawn from the top of the tower, or analysis data, it is possible to detect changes in the feed composition in a timely manner; accordingly, the heat source at the bottom of the tower should be adjusted – increasing it if the composition becomes heavier, and reducing it if it becomes lighter ； 3. The heat source at the bottom of the tower was not adjusted in a timely manner, resulting in fluctuations in the temperature at that location. Solution: Stabilize the reflux flow rate and the heat source at the bottom of the tower. 4. Fluctuations in tower pressure. Solution: Stabilize the liquid level and pressure in the reflux tank

Answer: Reasons: ① Changes in feed composition and feed volume. ②Load variation of the bottom heater. ③Changes in feed temperature and tower top temperature. ④Changes in tower top pressure. ⑤Failures in liquid level indication and flow control can cause changes in the liquid level at the bottom of the tower. Treatment method: ① Maintain a stable feed rate; if the composition of the feed changes, adjust the liquid control valve to stabilize the liquid level at the bottom of the tower. ②Strengthen the operation of the heating furnace to stabilize the outlet temperature and flow rate. ③Adjust the tower top temperature to ensure smoother operation. ④Stabilize the top pressure of the tower. ⑤Inspect the level gauges and control valves to prevent false levels.

Fluctuations in feed flow rate, reflux flow rate, bottom of tower (reboiler) temperature, and inaccurate level gauges – address the issues according to their respective causes

Reason: ① Changes in feed composition and feed volume. ②Load variation of the bottom heater. ③Changes in feed temperature and tower top temperature. ④Changes in tower top pressure. ⑤Failures in liquid level indication and flow control can cause changes in the liquid level at the bottom of the tower. Treatment method: ① Maintain a stable feed rate; if the composition of the feed changes, adjust the liquid control valve to stabilize the liquid level at the bottom of the tower. ②Strengthen the operation of the heating furnace to stabilize the outlet temperature and flow rate. ③Adjust the tower top temperature to ensure smoother operation. ④Stabilize the top pressure of the tower. ⑤Inspect the level gauges and control valves to prevent false levels.

1. Changes in reaction temperature: as the temperature increases, the liquid level decreases. 2. Changes in the amount of residue reprocessed; a lower amount of reprocessing results in a higher liquid level. 3. Change in reaction depth: a greater reaction depth results in a lower liquid level. 4. Changes in the feed rate: a higher feed rate results in a higher liquid level. 5. Pressure changes in the tower: higher pressure corresponds to a higher liquid level. 6. Change in the reflux volume: higher reflux means a higher liquid level. 7. Changes in slurry circulation rate and reflux temperature.

Reasons: (1) Changes in reaction temperature; higher temperatures result in a lower liquid level. (2) Changes in the amount of residue reprocessed; a lower amount of reprocessing results in a higher liquid level. (3) Change in reaction depth: a greater reaction depth results in a lower liquid level. (4) Changes in feed rate: a higher feed rate results in a higher liquid level. (5) The change in the opening degree of the lower valve for returning the slurry to the tower: as the lower valve opens wider, the liquid level drops. (6) Changes in slurry circulation rate and tower return temperature. (7) Overflow in the re-refining cylinder, with the liquid level rising. Handling: (1) Carefully monitor the feed rate to the stabilizing distillation column; if there are changes in the feed rate, adjust the liquid control valve to stabilize the liquid level at the bottom of the column. (2) Operate the heating furnace properly to maintain a stable outlet temperature and circulation rate for it. (3) Adjust the temperature at the top of the column, ensure stable operation to keep the pressure at the top of the column under control, and inspect and maintain the pressure control systems as well as the level indicators and control valves. (8) Pressure changes in the tower: higher pressure corresponds to a higher liquid level.

Reason: ① Changes in feed composition and feed volume. ②Load variation of the bottom heater. ③Changes in feed temperature and tower top temperature. ④Changes in tower top pressure. ⑤Failures in liquid level indication and flow control can cause changes in the liquid level at the bottom of the tower. Treatment method: ① Maintain a stable feed rate; if the composition of the feed changes, adjust the liquid control valve to stabilize the liquid level at the bottom of the tower. ②Strengthen the operation of the heating furnace to stabilize the outlet temperature and flow rate. ③Adjust the tower top temperature to ensure smoother operation. ④Stabilize the top pressure of the tower. ⑤Inspect the level gauges and control valves to prevent false levels.