Discussions related to butadiene extraction

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1. What are the requirements for the extractant in extractive distillation? Answer: 1) High selectivity. It enables the relative volatility between the original components to increase significantly. 2) High solubility. It is miscible with solutions at any concentration to prevent stratification. 3) The boiling point should be appropriate, much higher than that of any individual component, to prevent it from mixing into the overhead product and to facilitate separation from the first component. 4) Meet general process requirements. It is thermally stable, non-corrosive, non-toxic, not prone to explosion, easily available, and inexpensive. 2. What is the impact of changes in the operating pressure of the distillation tower on product quality? Answer: Changing the operating pressure will alter the composition of the vapor-liquid equilibrium on each tray. As pressure increases, the concentration of light components in the overhead distillate rises, but their quantity decreases relatively; meanwhile, the concentration of light components in the bottom liquid increases, and the volume of the bottom liquid also increases. As the pressure decreases, the concentration of light components at the tower top drops, and the concentration of light components in the reactor broth also decreases. Under normal operation, a constant pressure should be maintained. If abnormal operations lead to an increase in the concentration of heavy components in the product at the top of the tower, the operating pressure can be appropriately increased to bring it back to acceptable levels. 3. How to confirm on-site whether the bottom filter of the stripping tower is blocked or not? Answer: 1) First, check the reading of the pressure gauge on the solvent circulation line before it enters the solvent reboiler. If the pressure there is high, the filter’s pressure indicator will inevitably show a high value; it is not possible to determine whether the filter is clogged. At this point, slightly open the valve on the line connecting the solvent reboiler; if the pressure on the solvent line at the inlet of the solvent reboiler decreases while the pressure across the filter remains high, it can be concluded that the filter is clogged. 2) If the pressure reading before the solvent reboiler is low, the pressure in the filter is high, and the liquid level in the stripping tower cannot be adjusted, it can be concluded that the filter is clogged. 4. How to switch the operation of the stripping tower solvent reboiler A/B on-site (taking the switch from A to B as an example)? Answer: Once it is confirmed that the heat exchange capacity of A is insufficient, a switch should be made. 1) Check whether the backflow valve on the shell side of backup reboiler B is in the closed position, and check whether the valves on the condensate drain line are closed. 2) Slowly open the valve on the liquid phase line of standby reboiler B. Once the liquid level in the raw material evaporation tank reaches 50%, open the inlet and outlet valves for the hot solvent, adjust the liquid level in the raw material evaporation tank, and maintain stability of the liquid level. 3) Close the hot solvent inlet and outlet manual valves of A, and then close its gas-liquid phase line valve ； Open the pipe bank vent valve again to discharge C4 ； Open the drain valve on Shell Side A to empty the solvent. The cut A piece is connected to N2 for replacement and handover for maintenance. 5. Is a higher solvent ratio in the first extraction distillation column more favorable for extraction? Answer: Not necessarily. As the solvent ratio increases, although it has a more significant effect on altering the relative volatility between the components of the substances to be separated, which is beneficial for separation, this increase is limited. When the amount of solvent reaches a certain level, its effect becomes less noticeable. Moreover, as the solvent ratio rises, the vapor-liquid load increases, and consequently the circulation volume in extractive distillation, the amount of heating steam required, and the amount of solvent that needs to be regenerated all increase, leading to higher energy consumption in the operation. During normal production, selecting an appropriate solvent ratio ensures that the extraction efficiency is not affected while also reducing operational costs. 6. How should indoor adjustments be coordinated during the on-site switching operation of stripping tower reboilers A and B? Answer: When it is determined that a switch of the stripping tower reboiler is necessary, the liquid level in the stripping tower should be maintained at 70–80% level, and the level control valve should be switched to manual mode with the valve position set lower than usual. If it is observed that the liquid level in the stripping tower drops rapidly during the switching process, the level control valve should be closed further, and its position gradually adjusted back to its normal setting, after which it should be switched to automatic mode. Meanwhile, during the switching process, the steam consumption of the reboiler should be adjusted appropriately to keep the temperature of the stripping tower’s sensitive plate above the specified range. 7. What is the reflux ratio of the distillation column under full reflux? Why is full reflux operation used at the start of operation? Answer: At full reflux, the reflux ratio is infinite; this results in the minimum number of theoretical plates required to achieve a given degree of separation. When the actual number of operating plates is fixed, there is more flexibility, which facilitates process stability and control. 8. What impact will changes in the feed composition have on the product quality of the distillation process? Answer: As the concentration of the heavy components in the feed increases, the load on the distillation section rises. For a column with a fixed number of distillation section trays, this results in the heavy components reaching the top of the column, causing the top product to be of substandard quality. If the concentration of light components in the feed increases, the load on the stripping section rises, which may result in incomplete evaporation of the light components in that section and an increased loss of light components in the bottom liquid. 9. What is the impact of changes in the operating pressure of a distillation tower on product quality? Answer: Changing the operating pressure will alter the composition of the vapor-liquid equilibrium on each tray. As pressure increases, the concentration of light components in the overhead distillate rises, but their quantity decreases relatively; meanwhile, the concentration of light components in the bottom liquid increases, and the volume of the bottom liquid also increases. As the pressure decreases, the concentration of light components at the tower top drops, and the concentration of light components in the reactor broth also decreases. Under normal operation, a constant pressure should be maintained. If abnormal operations lead to an increase in the concentration of heavy components in the product at the top of the tower, the operating pressure can be appropriately increased to bring it back to acceptable levels. 10. How to prevent the \"water hammer\" phenomenon when using steam from a reboiler to raise the temperature? Answer: Before introducing steam, the condensate must be drained first. Slowly open the manual valve on the steam line to preheat the pipeline (by draining condensate before the control valve). Until the condensate is drained, the steam is discharged and led to the control valve. When putting the reboiler into operation, first open the backflow valve in front of the trap, then slowly open the control valve (with the backflow valve closed in front of it) or the valve on the bypass line of the control valve to introduce steam into the reboiler, with the condensate being discharged on-site. When the temperature of the condensed water discharge rises to 80–90°C, close the backflow valve before the steam trap and send the condensed water to the condensate tank. Following the steps above can prevent the \"water hammer\" phenomenon. 11. What impact will it have on product quality if the operation of the distillation tower does not comply with material balance? Answer: 1) The amount of light components recovered exceeded the amount permitted by material balance, causing the composition of the material inside the tower to become heavier. As a result, the temperature of the entire tower gradually increased, and the concentration of heavy components in the distillate at the top of the tower rose, leading to products that did not meet quality standards. 2) The recovery of the heavier components exceeds the amount permitted by material balance; as the operation proceeds, the material composition of the entire tower gradually becomes lighter, the temperature of the tower decreases, and the concentration of lighter components in the bottom liquid increases. 12. How to add fresh solvent to the solvent tank during normal production? Answer: During the production process, solvent loss can occur due to reasons such as the emptying of the reboiler, leaks at sealed points during maintenance, and high water content in the system leading to solvent decomposition; therefore, it is necessary to add fresh solvent to the solvent system on a regular basis. Steps for adding fresh solvent: 1) Transport the iron tank or tank truck containing the fresh solvent to the pneumatic diaphragm pump, and check whether the amount delivered matches that specified on the receipt sheet. 2) Connect the rubber hose from the pneumatic diaphragm pump to the iron tank or tank truck. 3) Open the inlet valve of the pneumatic diaphragm pump and start it ； 4) Open the outlet valve of the pneumatic diaphragm pump and the valve of the solvent tank, and feed the solvent into the solvent tank. 5) After adding the solvent, close all valves that were opened. 13. What should be noted when adjusting the load on the first extraction distillation tower? Answer: 1) When increasing or decreasing the load, the amount of solvent to be adjusted and the reflux volume should be determined based on the process requirements. 2) Before reducing the load, first decrease the amount of C4, and then reduce the solvent feed rate and recycle rate. 3) Before increasing the load, first increase the solvent feed and reflux rate, then raise the C4 feed amount. 4) Based on the material balance principle, sequentially adjust the control instruments of the first extractive distillation column: the C4 feed rate, the solvent feed rate, and the reflux rate. Adjust the steam volume of the steam reboiler at the same time. 5) The process parameters of the distillation system shall be adjusted accordingly. 14. How can it be understood that the drop in steam pressure in the reboiler of a stripping tower is caused by pressure buildup at the bottom of the tower? Answer: If there is a pressure buildup in the bottom of the stripping tower, that is, if the pressure at the bottom of the tower is high, the boiling point of the solvent increases due to this higher pressure, resulting in a reduced temperature difference for heat transfer between the steam and the solvent. This requires a greater driving force for heat transfer, and it can lead to an increase in the pressure in the shell side of the reboiler at the bottom of the stripping tower, thereby causing a decrease in the amount of steam used. 15. How to remove and replace the solvent reboiler in the first extractive distillation column? Answer: Once it is confirmed that the shell side of the solvent reboiler in the first extraction distillation tower is blocked, it should be removed and replaced. The replacement process should be carried out as follows: 1) Open the bypass valve and close the inlet valve of the solvent line to the solvent reboiler in the first extraction distillation tower, so that the solvent can flow into the backup reboiler. 2) Close the outlet valve of the solvent line at the solvent reboiler of the first extraction distillation column; close the vapor line and liquid line valves to sever the connection with the column. 3) Open the valve of the liquid-phase line drain line in the shell side of the solvent reboiler of the first extraction distillation column, as well as the valve of the liquid-line drain line, to discharge the solvent into the underground tank. 4) After emptying, purge with N2 to ensure there is no material in the tube side. 5) After the N2 purging is completed successfully, stop the N2 flow, close the valves on the draining line, install blind flanges, obtain confirmation and signatures, fill out the equipment maintenance form, and submit it. 6) Replace it with a new spare reboiler. 16. How to deal with high pressure difference in the first extractive distillation column? Answer: 1) If the tower operation is unstable, measure the pressure difference at various points to determine whether there is blockage inside the tower. If there is a blockage inside the tower, shutdown for maintenance is required in severe cases. 2) Avoid excessive load fluctuations in the first extraction distillation tower; check whether the flow rate of the C4 feed control valve is stable, and switch to manual mode automatically if necessary. If the instrument malfunctions, it should be taken to the instrument specialist for repair. 3) The maximum feed load to the first extraction distillation column must not exceed the design value to avoid excessive load. 4) Check the operating condition of the differential pressure transmitter and the situation of the heating steam. 17. How to deal with the excessive level of butadiene in the carbon tetrafluoride drawn from the top of the first extractive distillation column? Answer: 1) Adjust the solvent feed rate; based on changes in the contents of the three key components in the C4 feedstock, re-adjust the ratio between the solvent feed rate and the C4 feed rate, as well as the recycle flow rate. 2) Recalculate the feed rate for the second extractive distillation column based on material balance. Prevent feeding adjustment errors. If the feed control valve fails, it should be repaired by an instrumentation technician. 18. How to deal with high steam emission from the steam condensate tank? Answer: 1) Check the opening degree of the bypass valve on the device’s steam trap to reduce the flow of steam through the bypass line. 2) Check the levels of the steam reboiler in the first extraction tower and the reboiler condensate tank in the first stripping tower to see if they are too low. If it is too low, raise the liquid level using the indoor control valve ； If there is a problem with level control, consult an instrumentation technician. 19. How to deal with material leakage caused by broken glass plates in glass plate level gauges? Answer: 1) Close the upper and lower protection valves of the level gauge. 2) Open the discharge valve at the bottom of the level gauge to drain all the material from it. 3) Remove the fastening bolts that hold down the glass plate of the liquid level gauge. 4) Remove the glass plate and clean the old gasket. 5) Prepare gaskets and glass plates that meet the specified requirements, and apply a uniform layer of butter or molybdenum disulfide grease on the gaskets. 6) Install the shims in sequence, followed by the glass plate and the pressing plate. 7) Install the bolts and nuts, then tighten them gently in a symmetrical manner; do not use too much force to prevent the glass panel from breaking. 20. Shutdown procedures for the raw material evaporation tank. Answer: 1) Close the feed valve of the raw material tank and shut off the boundary zone valve. 2) As the liquid level in the tank drops, slowly close the level control valve and open the manual valve on the bypass pipe of the solvent heating control valve; once the liquid level is low, close the level control valve completely. 3) The solvent inlet valve of the raw material tank evaporator is closed to drain the shell-side solvent into an underground tank. 4) Open the raw material evaporation tank; close the valve connecting the evaporator of the raw material tank to the flare tank to prevent carbon tetrafluoride from being released into the flare, and arrange for purging with N2 or water. 21. How to increase the reflux flow rate of a stripping tower? Answer: Although the reflux flow rate in the stripping tower is automatically adjusted based on the liquid level in the reflux tank, the magnitude of the reflux flow rate in the stripping tower mainly depends on changes in the heat transfer load of the reboiler. Therefore, there are two main ways to increase the reflux flow rate: one is to increase the amount of steam supplied to the reboiler, thereby providing more heat to the solvent in the tower and accelerating its vaporization. Second is to reduce the operating pressure of the stripping tower. Additionally, reducing solvent recovery can also increase the flowback rate accordingly. 22. What should be done if the level of vinyl acetylene at the top of the second extractive distillation column exceeds the specified limit during operation? Answer: When the level of vinyl acetylene at the top of the second extractive distillation tower exceeds the specified limit, it affects the quality of butadiene; therefore, during operation, the level of vinyl acetylene should be kept at no more than 5 kg/kg. If the level of vinyl acetylene at the top of the second extraction distillation tower is too high, it should be analyzed and addressed based on different causes: 1) A low temperature in the stripping vessel of the stripper can result in incomplete separation of components such as C4 acetylenes and C5 compounds within the solvent. When this solvent is reintroduced into the second extraction distillation tower, it increases the difficulty of separating these components, leading to an elevated level of VA (vinyl acetylene) at the tower top. At this time, the operation of the stripping tower should be intensified to eliminate factors that result in incomplete solvent removal. 2) The solvent ratio changes with the concentration of vinyl acetylene in the feedstock; when the concentration of vinyl acetylene in the feedstock is high, the solvent feed and recycle rates should be increased appropriately. 3) The temperature at the bottom of the recovery tower is high, the ethylene and acetylene content in the returned gas is high, and the acetylene content in the feed to the second extraction distillation tower is high, increasing the difficulty of separation. The bottom temperature of the recovery tower should be reduced based on the relationship between the vinyl acetylene content in the feed and that temperature. 4) The bottom temperature of the second extractive distillation column is high. The temperature in the second extractive distillation column is too high, causing vinyl acetylene to end up in the overhead distillate. Therefore, the bottom temperature of the tower changes with the operating pressure of the tower. If the column pressure decreases, the temperature at the bottom of the second extractive distillation column should be reduced; if the column pressure increases, the bottom temperature should be increased as well. 5) If the top condenser becomes clogged, the tower pressure cannot be controlled, which increases the difficulty of operating the tower; in severe cases, it is necessary to shut down the unit for maintenance. 23. What are the benefits of controlling the plate temperature in a stripping tower? Answer: The sensitive plate is highly responsive to temperature changes, allowing it to promptly detect changes in the composition of the material on the tray. If the temperature of the sensitive plate decreases, it indicates an increasing trend in the content of light components. Based on this, it is possible to anticipate changes in the material at the bottom of the tower and adjust the steam supply in advance, thereby preventing light components from entering the liquid phase and ensuring thorough solvent stripping. 24. How should steam be shut off for a short period of time to keep the first extraction distillation tower running? Answer: A short-term shutdown of steam can be compensated for by using the heat in the system to keep the first extraction distillation tower operating. The specific procedure is to reduce the feed rate of the solvent to the first extraction distillation column after stopping the steam supply. Appropriately reduce the operating load of the unit and decrease the feed rate to the second extractive distillation column; under normal circumstances, as long as operations remain stable, a short-term shutdown of the steam supply is sufficient, without the need to shut down the first extractive distillation column. 25. Explain why the level of sensitivity of a tray’s plate temperature reflects changes in the composition of the material on that tray Answer: The temperature of the sensitive plate changes significantly even with minor variations in the composition of the material. On this tray, the temperature corresponds to the value at which the various components reach vapor-liquid equilibrium. When components with higher boiling points make up a larger proportion, the temperature of the sensitive plate is high; whereas when such components account for a smaller proportion, the temperature of the tray is lower. Thus, by examining the level of the sensitive plate temperature, it is possible to determine how much of the heavier components rise to the top of the tower and how much of the lighter components settle at the bottom. 26. How to verify whether the indication of the level gauge in the reflux tank of the first extraction distillation unit on site is correct? Answer: 1) Close the upper and lower isolation valves of the level gauge. 2) Open the drain valve at the bottom of the level gauge to empty the material inside it. 3) Open the cut-off valve at the top and observe whether gas is discharged. If gas is discharged with a certain pressure, it indicates that the connection between the upper part of the level gauge and the equipment is unobstructed; if the pressure is low due to insufficient gas flow, it suggests that there is some blockage. 4) Close the upper shut-off valve and open the lower shut-off valve; liquid material will flow out from the emptying valve area, and a liquid column will appear in the level gauge. A small outflow with no liquid column indicates that there is a blockage at the connection point between the lower part and the equipment. 5) During inspection, pay attention to the valve at the top of the level gauge and ensure that the fittings are properly sealed; air leakage from the upper part can lead to inaccurate level readings. 6) Blockages at the upper and lower parts of the level gauge can both lead to incorrect readings of the liquid level; judge whether the level gauge is giving accurate readings based on these conditions. 27. How to perform the switching operation of the vapor tower bottom liquid pump? Answer: 1) Check whether the oil level in the tank of the standby pump is at 50% of the level indicated by the sight glass. 2) Open the coupling guard and manually turn the shaft to check whether the pump shaft rotates smoothly. 3) Check and close the valves on the pump body and the inlet pipeline leading to the underground drainage tank. 4) Open the closed solvent inlet valve. 5) Open the pump inlet valve, vent steam at the outlet, and check that there are no leaks at the mechanical seal flanges and other areas. 6) Confirm that the motor is powered on and in good working condition. 7) Start the standby pump; once the pressure rises and stabilizes, slowly open the outlet valve. 8) Close the outlet valve of the original pump, press the pump stop button to halt its operation, and close the inlet valve. 9) Close the sealant inlet valve of the original pump. 10) Pumps that require maintenance must be emptied, powered off, and handed over for maintenance after confirmation and signature. 28. How to clean the filter at the inlet of the pump connecting the two columns in the first extractive distillation column? Answer: 1) Shut off the intermediate connection pump of the first extraction distillation tower, close the solvent inlet valve, and open the drain valves of the pump body and the inlet line leading to the underground tank. 2) Connect the rubber hose to the exhaust valve at the pump outlet, and use nitrogen to push the material inside the pump into the underground tank. 3) Stop nitrogen purging and open the rear cover of the filter. 4) Remove the filter and clean out any blockages on it. 5) Insert the cleaned filter (or a new filter of the same specification) into the pump. 6) Tighten the bolts/compress the gaskets/perform a leak test with N2, then set it aside for use. 29. How to carry out the solvent cooling operation for the first extractive distillation tower and stripping tower? Answer: 1) Before starting the cold transport, activate the sealed solvent system. 2) Arrange the process according to the solvent cooling process flowchart in the operating procedure. 3) When arranging the process, make sure to check whether the backflow valves of each solvent pump are fully closed, whether the inlet and outlet valves are closed, and whether the protective valves before and after the control valves are open. 4) Feed the solvent into the seal solvent of the solvent pump system, adjust the flow rate to an appropriate level, and then start the solvent pump; for the initial feeding, set the solvent dosage at 200–240 t/h. 5) When the liquid level in the first extractive distillation column begins to rise and reaches 50%, the bottom liquid pump is started to send the solvent to the stripping column. Manual control is used to stabilize the liquid level in the first extraction distillation tower. 6) When the liquid level in the lower section of the first extractive distillation column rises to 50%, start the kettle liquid pump to send the solvent to the stripping column, and manually adjust to stabilize the liquid level in the lower section of the first extractive distillation column. 7) When the liquid level in the stripping tower rises to 50%, start the kettle liquid pump to begin solvent circulation. During the circulation back to the solvent tank according to the established procedure, manually control the liquid level gauge in the bottom of the stripping tower to maintain a normal liquid level there. 8) According to the circulating solvent route, N2 should be discharged from the shell side of each unit. 30. How to operate the second extractive distillation column during startup? Answer: 1) Feeding to the second extractive distillation column begins when the content of 2-methylbutene in the overhead material of the stripping tower is below 0.002 kg/kg. 2) The pressure in the second extractive distillation column rises, and non-condensable gas is discharged from the top condenser until a liquid level is established in the reflux tank and the pressure is stabilized at the specified value. 3) When the liquid level in the reflux tank rises, start the reflux pump to establish reflux. Adjust the flow control valve to the specified range. 4) Establish a feedback process for the second extraction distillation column to return the butadiene in the reflux tank to the outlet area. 5) Maintain the normal values of the process conditions in the double extraction tower. 31. How should high head pressure in the top of the first extraction distillation column be addressed? Answer: 1) If the content of the C3 components in the C4 feed exceeds the specified limit, open the vent valve on the reflux tank to discharge the C3 light components. 2) Check whether the circulating water temperature is below 32°C. If the water temperature is high, contact the dispatch team to adjust it. 3) Check the operation of the control valve of the heat exchanger to see if it is malfunctioning. In the event of a failure, switch the control valve to manual mode and increase its opening degree appropriately, then contact the instrumentation technician to handle the valve. 4) Check the heat exchange conditions of the tower top condenser and take corrective actions. 32. What should be noted when pre-coating heat exchange equipment using circulating water? Answer: 1) During the pre-coating treatment phase, after receiving the dispatch instructions, the staff members check the external connections of the circulating water system according to the supervisor’s assignments and close them. They then open all the inlet and outlet valves of the devices related to the circulating water system, and adjust the flow rate so that all devices have access to water. 2) The cold heat exchange equipment after pre-coating can be prepared for use, but the circulating water must not be stopped before the next maintenance cycle arrives. 33. Briefly describe the steps for putting a cooler and a condenser into operation? Answer: 1) Complete condensation; the cooler should be put into operation following the principle of introducing cold material first and then hot material. 2) After water is introduced, check for exhaust at the high points of the water flow in each device until water is discharged. 3) Once the supply pressure/flow rate/temperature of the cooling water medium is normal, the inlet and outlet valves for the hot material can be opened on the other side to start operation. 34. Describe the emptying and replacement steps for the light-end tower and the heavy-end tower after chemical cleaning with diethylhydroxylamine. Answer: 1) After the chemical cleaning is completed, all the C4 fractions from the light ends distillation column and the heavy ends distillation column are emptied and sent out of the facility. 2) After all the liquid C4 has been returned, first open the line leading to the flare to evacuate the system until the pressure drops to 0.02 MPa. Once it is confirmed that there is no more C4 in both towers, proceed with the N2 displacement process to reduce the combustible content in the system to below 0.001 kg/kg, maintain a pressure of 0.1 MPa, and then wait for the official start-up of feeding. 35. What impact will changes in the feed composition have on the product quality of the distillation process? Answer: As the concentration of heavy components in the feed increases, the load on the distillation section rises. For a column with a fixed number of distillation section trays, this results in the heavy components reaching the top of the column, causing the top product to be of substandard quality. If the concentration of light components in the feed increases, the load on the stripping section rises, which may result in incomplete evaporation of the light components in that section and an increased loss of light components in the bottom liquid. 36. Describe the chemical cleaning steps prior to the major overhaul of the stripping tower. Answer: 1) Arrange the chemical cleaning process reasonably and open all valves on the pipelines. Outlet valve of the light-end tower bottom liquid pump – Protection valve of the light-end tower bottom liquid level control valve – Outlet valve of the light-end reflux pump – Check valve and double-line valve of the light-end reflux pump – Inlet valve of the light-end reflux pump – Light-end reflux tank – Top condenser of the light-end tower – Tower top of the light-end tower. 2) Depending on the required amount of solution, some C4 is added from the boundary zone through the C4 scrubber line into the dewaxing reflux tank to prepare the chemical cleaning solution. The prepared diethylhydroxylamine is transferred from the degassing tower system, via a degassing reflux pump and through the butadiene extraction network, along a small-circulation return line, to the feed plate of the stripping tower. 3) When the diethylenethanolamine and C4 solution from the stripping tower are sufficient for circulation, stop the supply of C4 to the stripping tower. 4) Start the bottom liquid pump of the light-end removal tower, and circulate for 24 hours following the route: light-end removal tower – bottom liquid pump of the light-end removal tower – light-end removal tower reflux tank – condenser of the light-end removal tower – top of the light-end removal tower. 37. Describe the chemical cleaning steps prior to the major overhaul of the degassing tower. Answer: 1) According to the chemical cleaning plan, open all valves on the pipelines of the chemical cleaning process. Close the main valve from the reflux tank to the reflux pump, as well as the safety valve of the product extraction control valve. Open the outlet valve of the reflux pump and the bypass valve on the pipeline from the outlet to the reflux tank – reflux tank – top condenser – top of the stripping column – bottom of the stripping column – liquid line from the bottom reboiler to the reflux tank – inlet valve of the reflux pump. 2) The diethylhydroxylamine tetrahydrocarbon solution prepared in the degassing tower system is pumped by a reflux pump through the reflux tank and condenser to the top of the degassing tower; the tetrahydrocarbon solution flows naturally to the bottom of the tower, and then returns to the reflux pump for circulation via the liquid phase line of the steam reboiler. 3) Open the backflow valve on the exhaust pipeline of the top condenser to release the nitrogen inside the equipment, so that it can be thoroughly cleaned. 4) When there is insufficient C4 in the system, C4 is replenished from the boundary zone through the C4 washing column line. 5) After maintaining the cycle for 24 hours, send tetrahydrocarbon back to the exit area. 38. Describe the steps for emptying and replacing the acetonitrile regeneration tower system. Answer: 1) Stop feeding steam to the reboiler. 2) Stop adding reflux when the temperature of the sensitive plate in the acetonitrile regeneration tower is below 100°C, and close the condensate inlet valve. 3) After the tower has cooled sufficiently, pour the water-containing solvent inside the tower into an underground tank. 4) Add water to the water chamber of the backflow tank, push all the dimers in the water chamber into the oil chamber, and then drain the water from the water chamber into the sewage system. 5) Transfer all the oil from the oil chamber to the fuel oil tank. 6) Disconnect the acetonitrile regeneration system from other systems and close the isolation valve. 7) After the column has been thoroughly emptied, water is added from the reflux pump into the reflux line at the top of the solvent purification column to wash away the residual acetonitrile in the column. 8) Open the exhaust valve and manhole at the top of the tower; select one condensate discharge point each at the lower section of the liquid phase line in the tower bottom and at the bottom of the reflux tank. Connect a temporary steam hose at the inlet of the reflux pump. 9) Steam is also supplied to the temporary rubber hose of the reflux pump in the tower bottom cooking line, so as to discharge the combustible gases in the tower/reflux tank from the top of the tower. 39. Describe the steps for emptying and handing over the reflux pump of the second distillation column for maintenance. Answer: The maintenance of the pump is determined based on the maintenance plan and the operating condition of the equipment on site. The emptying before maintenance should be carried out in accordance with the following procedures: 1) Close the inlet and outlet valves of the pump, and open the check valve’s bypass valve. 2) Open the emptying valves of the pump unit and the inlet pipeline, and open the main valve of the emptying pipeline from the pump to the flare tank. 3) Check whether the outlet pressure gauge of the pumps has dropped; when the pressure approaches zero, close the main drain valves for the two pumps leading to the flare tank. 4) Open the local discharge valve on the discharge main to check whether the pump has discharged completely. 5) After confirming that the pump has been drained completely, fill out the power shutdown/start-up notification form and contact the electrician to cut off the power. 6) Notify the maintenance personnel to confirm and sign before proceeding with the maintenance. 40. Describe the startup procedures for the degassing tower. Answer: 1) After feeding begins in the light-end stripping column, the heavy-end stripping column must prepare for operation by arranging the corresponding processes for feeding, the top of the column, and the bottom of the column. 2) Check whether the circulating water of the condenser at the top of the degassing tower is operating properly. Switch the pressure control for the top of the tower to manual mode and set the valve position at around 50%. 3) Check whether the condensate and solvent inlet/outlet valves of the bottom reboiler are open. 4) Manually adjust the liquid level and temperature in the reactor bottom; supply a small amount of hot water and hot solvent to the reboiler at the bottom of the tower. 5) Organize the feeding of the degassing tower. Monitor process parameters such as the pressure and liquid level in the tower, paying close attention to any changes. Once the liquid level in the tower bottom rises, it is necessary to adjust the amount of hot water and solvent used to maintain a stable liquid level there. 6) When the pressure exceeds 0.4 MPa, non-condensable gases should be vented from the top of the tower in a timely manner to keep the pressure at 0.4 MPa. 7) When the liquid level in the reflux tank rises to 30%, start the reflux pump to establish reflux in the degassing tower. The amount of reflux should be increased gradually, so as to keep the liquid level in the reflux tank rising slowly, eventually stabilizing at around 50%. 8) Once the reflux, pressure, and liquid level are normal, adjust the butadiene product extraction valve to transfer the butadiene to the feed tank. 9) During startup, the heat supply to the reboiler at the bottom of the tower should be adjusted promptly to maintain a stable liquid level in the tower bottom. 10) Organize the extraction from the tower bottom; at the beginning of operation, the discharge rate can be increased appropriately to shorten the time required for the product to meet the specifications. 41. The dibutene content at the bottom of the degassing tower should be less than or equal to 0.15 kg/kg. What corrective actions should be taken when this parameter deviates from the specified value? Answer: The dibutylene content in the bottom liquid of the degassing tower is an important indicator for controlling loss amounts and product quality in the tower’s bottom liquid; too high a content affects the dibutylene yield of the plant, while too low a level over an extended period impacts product quality. Excessive adjustment: 1) Switch the liquid level control at the bottom of the tower to manual mode, and temporarily increase the heat supply to the tower in order to lower its liquid level. 2) Reduce the bottom product withdrawal from the tower. Under-adjustment: 1) Switch the liquid level control at the bottom of the tower to manual mode, temporarily reduce the heat supply to the tower, so as to raise the liquid level in the tower slightly. 2) Increase the bottom draw rate of the tower. Adjusting the liquid level can yield immediate effects, but fundamentally it is necessary to address the issue by altering the material balance of the tower. In practice, it is essential to adjust the discharge volume of the liquid at the bottom of the tower in accordance with changes in the composition of the feed, that is, by changing the amount of liquid taken out from the bottom of the tower. 42. How to confirm that the indication of the on-site degassing reflux level gauge is correct? Answer: 1) Close the upper and lower isolation valves of the level gauge. 2) Open the discharge valve at the bottom of the level gauge to drain the material inside it. 3) Open the upper shut-off valve and observe whether gas is discharged. The discharge of gas along with a certain pressure indicates that there are no issues with the connection between the top of the level gauge and the equipment; insufficient gas flow and low pressure suggest some blockage. 4) Close the upper shut-off valve and open the lower shut-off valve; liquid C4 should flow out at the drain valve, and a liquid level should appear in the level gauge. A low flow rate and the absence of a liquid level indicate that there is blockage at the connection point between the lower part and the tower equipment. 5) During inspection, special attention should be paid to whether the valves and fittings at the upper part of the level gauge are properly sealed; air leakage in that area can easily lead to inaccurate readings from the level gauge. 6) Based on the above inspection results, it is possible to determine whether the actual indication of the level gauge is correct. 43. Procedure for handling high liquid level alarms in the degassing reflux tank. Answer: First, check whether the product withdrawal rate is normal. If the flow rate is normal, the following reasons may be at play: 1) The pressure in the tower has dropped significantly, causing a large amount of material within the tower to distill off and resulting in an increase in the liquid level. The original operating pressure of the tower should be restored; if pressure reduction is necessary, the withdrawal rate should be increased. 2) The reflux decreases rapidly; the new thermal equilibrium of the column has not yet been established, and the distillates within the column have not decreased, causing the liquid level to rise. It is necessary to increase the draw rate to restore the liquid level. If the flow rate decreases or is absent, it may be due to: 1) A blockage inside the primary measuring instrument of the control valve leading to the tank area, which can also prevent the product from being delivered; in such cases, the secondary circuit of the primary instrument should be activated promptly to arrange for maintenance. 2) If the liquid level control valve of the reflux tank fails, the bypass line should be activated and the instrumentation department should be contacted for handling. 3) Issues such as a full butadiene intermediate tank or problems in the process should be checked and addressed promptly. 44. How to handle the situation where the liquid at the bottom of the degassing tower cannot be removed? Answer: A decrease in the discharge flow at the bottom of the tower and an increase in the valve position should draw the attention of the operators. Handle it as follows: 1) Check whether the tank liquid pump is under pressure; if not, clean the outlet filter of the degassing tower. 2) Check the waste C4 and C5 storage tanks sowie the processes leading to the external area for issues such as full tanks, pressure buildup, or blocked flows, and address them promptly. 3) Switch the liquid level control in the tower bottom to manual mode and fix the valve position, so that the heating capacity of the tower’s heating system does not adjust according to fluctuations in the liquid level, thereby ensuring the quality of the butadiene product. 4) When the liquid level in the tower rises to a level that makes it impossible to maintain production, the liquid in the reactor can be temporarily transferred to a flare tank and sent to the flare system. 5) Contact the instrumentation team to clean and inspect the flow meter for effluent from the tower bottom, as well as the primary instruments and their associated filtering devices. 1) When all the above measures are ineffective, reduce the circulation in the extraction system and switch the light-end stripping tower to recirculation mode; identify the cause, address it, and then resume production.