Basic knowledge for the catalytic fractionation position

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1. What are the initial boiling point, dry point, and flash point? When an oil product is heated and distilled using an Enneberg distillation apparatus, the first molecules to vaporize and be distilled off are those hydrocarbon molecules with low boiling points. The vapor temperature at which the first drop of condensate is formed is called the initial boiling point. During the distillation process, hydrocarbon molecules evaporate gradually in order of their boiling points, and the temperature of the vapor phase rises accordingly. The highest temperature reached at the end of distillation is known as the final boiling point or dry point. The flash point (also known as the ignition point) is the lowest temperature at which a mixture of vapors from flammable liquids such as hydrocarbons and petroleum products and air can undergo a brief flash fire when brought near a flame. 2. What are the freezing point and auto-ignition point? The freezing point of a oil is the temperature at which it loses its fluidity, under specific testing conditions in a particular instrument. And the so-called loss of liquidity is also entirely conditional. When the oil is cooled to a certain temperature and the test tube containing it is tilted at 45 degrees, if no visual change in the level of the liquid inside the tube can be observed after 1 minute, the oil is considered to be solidified. The highest temperature at which this phenomenon occurs is known as the freezing point of this oil. If the oil is preheated to a very high temperature and then exposed to air, it can ignite on its own due to intense oxidation, burning spontaneously – this is known as the spontaneous combustion of oil. The lowest oil temperature at which spontaneous combustion can occur is called the autoignition point. 3. What do the 90% distillation temperature and dry point of gasoline indicate? The 90% distillation temperature and dry point show the degree to which gasoline evaporates completely in an engine. This temperature is too high, and the manual specifies too many heavy components, which results in reduced engine power and efficiency. 4. What is the impact of blending residue oil on the quality of light diesel? As the amount of residue oil in the feed for catalytic cracking increases, the quality of light diesel deteriorates, which is manifested in aspects such as increased gum content, poor stability of the fuel, darker color, and lower cetane number. 5. What is the pattern of cetane numbers of various hydrocarbons in light diesel? (1) Normal alkanes have the highest cetane numbers among all types of hydrocarbons, and this value increases as the chain length increases. (2) The cetane number of isoparaffins is lower than that of n-paraffins, and the more branched the chain, the lower its cetane number. (3) The cetane number of n-olefins is only slightly lower than that of n-alkanes, and the effect of branching is similar to that in alkanes. (4) Cycloalkanes without side chains have a lower cetane number than n-alkanes and n-alkenes ； Aromatic hydrocarbons without side chains, especially polycyclic aromatics, have the lowest cetane number among all types of hydrocarbons. (5) Naphthenes and aromatics with long side chains can increase their cetane number; when the side chains are branched, their cetane number decreases accordingly. 6. What is cetane number? The cetane number is an indicator used to assess the anti-knock properties of diesel fuel, and it is determined by comparing it with a standard fuel. The cetane number of pure hexadecane is 100, while that of pure a-methylnaphthalene is 0. By mixing them in different proportions, standard fuels with various octane numbers ranging from 0 to 100, corresponding to different anti-knock properties, can be obtained, which are then compared with the diesel fuel under test on a single-cylinder testing machine of a specific design. 7. Why is the cetane number of catalytically processed light diesel lower than that of straight-run diesel? Based on the patterns regarding the cetane numbers of different hydrocarbon fractions in light diesel, it can be seen that straight-run diesel has the highest cetane number; whereas catalytically processed diesel contains more polycyclic aromatic hydrocarbons, which results in a lower cetane number. The diesel cetane number of paraffinic crude oils should be higher than that of naphthenic diesel. 8. What is the basic principle of distillation? The basic principle of distillation is to utilize the difference in relative volatility of various components in a vapor-liquid mixture for separation. In the tower, steam rises from the bottom to the top, while the liquid descends from the top to the bottom. When the vapor and liquid phases come into contact on each plate, part of the vapor phase condenses, and part of the liquid phase vaporizes. Due to the partial vaporization of the liquid, light components in the liquid phase diffuse into the vapor phase, increasing the amount of light components in the steam ； The partial condensation of the vapor causes the heavier components in the vapor to diffuse into the liquid phase, increasing the concentration of these heavy components in the liquid phase; this in turn helps to bring the vapor and liquid phases in contact on the same plate into equilibrium. 9. How to determine the distillation efficiency of a distillation column? The separation accuracy, also known as distillation precision in the distillation process, is used to indicate the distillation efficiency of the column. For binary systems, it refers to whether an effective separation is achieved between the light and heavy components. For multi-component systems, it refers to the degree of separation between the light and heavy key components. For complex systems, the distillation accuracy between two adjacent fractions is usually expressed by the relationship between their fractional compositions or their distillation curves. If the initial boiling point of the heavier fraction is higher than the final boiling point of the lighter fraction, then it is said that there is a certain \"gap\" between these two fractions" ； The opposite is called \"overlap\". Overlap means that some of the light fractions end up in the heavy fractions, or some of the heavy fractions end up in the hydrocarbon fractions. As a result, it reduces both the yield of light fractions and their quality. Clearly, overlap is caused by poor fractionation accuracy, while gaps indicate higher separation accuracy. The larger the gap, the higher the precision of fractionation. 10. What are the basic conditions for the distillation process? (1) There must be a place where gas and liquid can come into full contact for interphase heat and mass transfer, namely the tray (or packing). (2) On each tray, both phases – the gas with an unbalanced composition (rising oil and gas) and the liquid (descending reflux) – must be present simultaneously. To ensure that there is rising vapor and oil on each tray in the distillation column, a heat source is required ； To achieve descending reflux on the trays, external reflux must be introduced below the top of the tower and at each full extraction hopper, so as to create internal reflux on the other trays. (3) To achieve distillation, not only are trays necessary, but a certain number of trays is also required ； Not only must there be backflow, but there must also be an appropriate amount of it. 11. Why are baffles used in distillation towers? The feed temperature to catalytic cracking distillation towers is above 450 degrees, and the superheated oil and gas contain catalyst dust, which is what sets them apart from other types of distillation towers. The lower part of the catalytic fractionation tower contains an oil slurry heat exchange section. The circulating slurry is drawn out from the bottom of the tower, and after heat exchange and cooling, it is returned to the tower where it comes into counterflow contact with the rising oil and gas. This not only rapidly cools the oil and gas to prevent coking but also washes away any catalyst dust carried along with it. In the slurry heat exchange section, due to the high temperatures and the presence of catalyst dust, baffles are generally used instead of trays. 12. What is the difference between a catalytic distillation tower and other distillation towers? (1) It has a heat exchange section for removing superheat and washing away catalyst dust. The feed to a catalytic distillation tower is high-temperature oil gas containing catalyst dust. Therefore, slurry circulation is provided at the bottom of the tower to cool the superheated oil-gas and wash the catalyst. (2) High residual heat in the entire tower: The feed to the catalytic fractionation tower is high-temperature superheated oil and gas at over 450 degrees. Therefore, while meeting the separation requirements, efforts should be made to minimize the amount of heat taken from the top reflux, and instead increase the amount of heat taken from the oil slurry at higher temperatures as well as from the mid-stage circulating reflux, so as to make full use of the high-energy heat for heat exchange and steam generation. (3) Low system pressure drop requirement: To increase the inlet pressure of the compressor, reduce its energy consumption, and enhance its processing capacity, it is necessary to minimize the pressure drop in the fractionation system – including the pressure drop in the large oil and gas pipelines, across each tray, in the overhead oil and gas pipelines and condensers, as well as from the oil and gas separator to the compressor inlet. Strive to reduce system pressure drop. 13. What is the difference between a stripping tower and a fractionating tower? Superheated steam is introduced at the bottom of the stripping tower; under constant temperature and total pressure, this reduces the vapor pressure of the oil-gas mixture and increases the rate of vaporization. As a result, more light components are extracted from the side products, thereby reducing the content of these light components in the final product. The catalytic fractionator is fed at the bottom, with only a distillation section and no stripping section; the stripper is fed at the top, serving as the stripping section for the side-product stream. 14. Why are vortex eliminators and foam breakers installed? Vortex eliminators are placed at the liquid outlet of the equipment to prevent vortex formation when the liquid material flows out, which could otherwise draw gas into the pump and cause it to empty; they are also used to stabilize the liquid level. In the oil and gas separation unit at the top of the distillation tower, as well as in the petroleum coke analysis separator, an anti-vortex device is installed at the gasoline extraction port to stabilize the liquid interface between water and gasoline. In areas where it is not allowed for gaseous materials to carry mist, such as the overhead gas-liquid separator in a distillation tower, if the rich gas contains oil, it can damage the compressor. To ensure that the gaseous material exits without any mist, a foam breaker is installed at the gaseous outlet of the equipment. 15. Why is a filter screen needed in a distillation tower? Since coking inevitably occurs at the bottom of the distillation tower, it is necessary to install a filter screen at the oil slurry extraction point at the bottom of the tower in order to prevent coking particles and lumps from entering the oil slurry pump, which could cause wear to the pump or even affect its operation. 16. Why is a three-way merging valve used? A three-way merging valve is employed to adjust the ratio of cold to hot flow through the heat exchanger while keeping the total flow rate constant, thereby regulating the outlet temperature of the heat exchanger. 17. How to switch a three-way merging valve between manual and automatic mode? (1) Change from “remote control” to “local manual”: ① Turn the handwheel until the valve stem moves slightly, then stop turning it. ②Please ask the instrumentation technician to turn off the instrument air. ③Turn the handwheel to position the three-way valve at the desired opening. (2) Change “manual on-site” to “remote control”. ① Turn the handwheel to position the valve stem in the desired position, and use this position to estimate an output current value. ②Set the output current to an approximate current value. ③Please turn on the instrument air for the instrument to operate. ④Turn the handwheel to the middle position. ⑤If necessary, change “remote control” to the “automatic” mode of the regulator. 18. What is cold reflux? What is its function? Strictly speaking, cold reflux refers to reflux whose temperature is lower than the equilibrium temperature of the tray to which it is sent, such as top reflux and recycle reflux. However, the term \"cold reflux\" is generally used to refer to the subcooled reflux at the top of the tower; that is, the product from the top of the tower is drawn out, condensed and cooled outside the tower, and then returned to the top of the tower as reflux for the uppermost tray. The function of cold reflux is, first, to serve as the reflux for the topmost tray; thereby, each subsequent tray also has internal reflux ； Secondly, it undertakes the task of cooling and heat extraction to maintain the thermal balance of the entire tower. Control the temperature at the top of the tower to ensure product quality. 19. What is bottom reflux? What is its function? Bottom reflux refers to the circulation of liquid at the bottom of the tower; it involves taking a portion of this liquid, cooling it, and then returning it to the tower. In a catalytic fractionation tower, the feed is high-temperature superheated oil and gas from the reactor, which carries a large amount of heat as well as significant amounts of catalyst powder. Therefore, the bottom-loop reflux allows a large amount of high-temperature heat to be extracted from the bottom of the tower for reuse, thereby significantly reducing the load on the upper part of the tower ； Moreover, a large amount of circulating oil slurry can wash away solid particles in the oil vapor, preventing blockages in the upper trays. 20. What are the reasons for the vacuum condition in the top reflux pump? How to address it? (1) Reasons: ① Insufficient load on the top of the distillation column, or excessively low temperature and pressure in the middle section. ②Cold reflux carries water. ③The temperature at the top of the fractionation tower is too high, and due to inadequate adjustment, the amount of oil stored in the oil collection tank from the top reflux stream decreases. ④Pump failure, instruments malfunctioning. (2) Treatment methods: ① When the top reflux flow fluctuates, the flow rate should be reduced promptly, and the cold reflux should be increased appropriately to ensure the dry point of crude gasoline. ②Appropriately increase the temperature of the light diesel extraction tray to gradually raise the top load. ③Switch to the backup pump in a timely manner. Cold reflux should be introduced first to keep the top temperature in check, but care must be taken regarding system pressure. After increasing the flow rate on the pump, it should be increased gradually to the normal level. If the temperature at the top of the distillation column is not under control and this leads to column flooding, it may be necessary to reduce the amount of reactant fed in, followed by taking measures to address the situation. ④If the instrument fails, switch to a manual operator or use a backup line promptly, and contact maintenance for repair. 21. What are the causes and solutions for fluctuations in the reflux flow rate in the middle section and pump cavitation? (1) Causes: ① Significant fluctuations in reaction conditions. ②Excessive extraction of heavy diesel and high temperature in the light diesel extraction tray result in insufficient load in the middle section. ③The temperature at the top of the distillation tower is too high. ④The seal oil of the mid-section reflux pump contains water. ⑤Pump failure, instruments malfunctioning. (2) Treatment methods: ① Appropriately adjust the amount of heavy diesel extracted. ②When the backflow volume in the middle section fluctuates, it is necessary to reduce the extraction volume from that middle section promptly and increase the top temperature slightly to prevent the pump from running dry. ③Reduce the top temperature of the tower, appropriately increase the steam flow at the bottom of the tower, and gradually increase the load in the middle section. ④Strengthen oil sealing and dehydration. ⑤Appropriately increase the amount of absorbed oil returned to the tower. ⑥When the intermediate section volume is severely insufficient or the pump is under vacuum, light diesel is added to the inlet of the intermediate section reflux pump. ⑦When the pump is under vacuum, the amount of light diesel extracted can be appropriately reduced to increase internal recirculation. ⑧In the event of a pump failure, switch to the backup pump promptly. ⑨If the instrument fails, switch to manual control promptly or use a backup line for operation. 22. Why are upper and lower ports used for the return of slurry to the tower? After the reaction oil vapor enters the tower, the slurry is used to remove excess heat and wash the catalyst powder; at the same time, it causes a small amount of the heaviest fractions – residue – to condense into a liquid state, which then flows to the bottom of the tower along with the catalyst powder. This residue is continuously extracted for reprocessing or use as a product. Therefore, under normal circumstances, to ensure overheating prevention and the flushing of the catalyst slurry, it is necessary for the fluid to enter the tower from above the chevron baffle, which is the upper inlet. The temperature at the bottom of the tower must be strictly controlled below 370–380 degrees, depending on the density of the raw material; otherwise, coking will occur. In addition to controlling the circulation volume and temperature of the slurry, the adjustment measures also involve injecting a small amount of the slurry returning to the tower directly into the bottom liquid level of the tower, thereby cooling the mixture through the combination of the cooler-slurry with the slurry at the bottom of the tower. At this point, it is necessary to use the oil slurry inlet; if cooling is achieved by increasing the amount of circulating oil slurry entering through the upper inlet, it will also lead to increased condensation of residue, causing the liquid level at the bottom of the tower to rise. 23. What impact does the distillation unit have on other units? (1) If the liquid level at the bottom of the distillation tower is higher than that of the inlet pipeline for the reaction gas, it will cause the reaction pressure to become excessively high. (2) The opening degree of the slurry-primary oil heat exchanger bypass affects the temperature of the primary oil entering the furnace. (3) When there are problems with the crude gasoline (interruptions, high water content), it will affect the gasoline injection in the riser. (4) Excessive cold reflux in the distillation tower can cause the reactor pressure to become extremely high. (5) Cold reflux carrying water can cause the reactor pressure to become excessively high. (6) An excessively high liquid level in the overhead oil-gas separator can affect the reaction pressure and the operation of the compressor. (7) Refueling the distillation tower will affect the reaction feed rate. (8) Water in crude gasoline increases the load on the absorption-stabilization system; it can also cause the gasoline to become unsuitable for use and lead to an increase in waste alkaline solution. The unqualified crude gasoline affects the quality of the stabilized gasoline. (9) Significant adjustments to the absorption oil can affect the pressure in the reabsorption tower; in severe cases, it can also lead to oil carried in the dry gas. (10) An insufficient amount of light diesel will affect the sealing oil supply to the hot oil pump. (11) An excessively high or low temperature of the crude gasoline affects the operation of the absorption-desorption tower. It also affects the composition of the rich gas, thereby impacting the operation of the compressor. In (12), mid-stage reflux fluctuations affect not only the operation of the distillation column and thus the reaction unit, but also the operation of the absorption stabilization system. 24. What impact do other positions have on the distillation position? (1) Reaction depth. (2) Total reaction feed amount. (3) An accident occurs in the reaction. (4) The amount of reaction stripping steam and other steams (such as emergency steam) directly affects the liquid level in the overhead oil-gas separator, as well as the dry point of the crude gasoline. (5) Reaction temperature. (6) Abnormalities in the gas compressor unit affect the pressure in the distillation tower. Large fluctuations affect the operation of the entire tower and the quality of the product, as well as the reaction process. (7) When the condensate oil from the reabsorption tower is sent to the vapor-liquid separator at the top of the tower, it affects the liquid level in the vessel (in some systems, it is sent to the distillation tower) as well as the pressure in the distillation tower. (8) Driving the flare into the condensed oil affects the liquid level in the overhead gas oil separator and the pressure in the fractionating tower. (9) When the absorption and stabilization system malfunctions, it affects the outlet of crude gasoline, as well as the reflux rates and temperatures in the first and second stages, the amount of rich absorption oil in the reabsorption tower, and the pressure in the distillation tower. 25. What are the operating principles and main tasks of the distillation unit? (1) The operating principles of the distillation unit are to maintain stable liquid levels in all areas, control the reflux rates and temperatures at each stage, adjust the heat balance appropriately, ensure smooth operation, and produce products that meet quality standards. (2) The main task of the distillation unit is to, under stable operating conditions, separate the mixed oil and gas fed from the reactor into fractions such as rich gas, crude gasoline, light diesel, heavy diesel, reprocessed oil, and sludge, based on their boiling points, and to ensure that the quality of each product meets the specified requirements. 26. Why is it necessary to analyze the solid content in the slurry? The lower the solid content (catalyst powder) in the slurry, the better, as a high solid concentration can cause severe wear on the equipment, especially in those parts that operate at high speeds, such as slurry pumps. Too high a concentration can also lead to serious coking and blockage incidents. Therefore, based on experience, the solid content in the slurry should be kept at no more than 6 grams per liter, and ideally at no more than 2 grams per liter. Currently, the feedstocks used in catalytic cracking units are becoming increasingly heavier; some units even use residue oil, which leads to coking at the bottom of the distillation columns. The oil slurry contains solids, and in addition to catalyst powder, it often includes coke particles as well. Therefore, the existing methods for analyzing solid content need to be improved, and alternative methods should be explored. 27. How to control the solid content in the slurry? (1) Ensure a constant circulation rate of the slurry; it should not vary greatly, and the amount entering through the upper inlet must be maintained to ensure that the catalyst is washed away in the de-overshooting section. (2) The amount of recycled oil slurry must be kept above a certain level; it should not remain at a very low level all the time, in order to prevent catalyst powder from accumulating at the bottom of the distillation tower. (3) If an increase in the solid content of the slurry is detected, it is necessary to promptly analyze the conditions of the reaction process, increase the amount of slurry discharged, and sometimes increase the amount of material recycled. 28. What issues should be considered when adjusting the cold reflux at the top of the fractionation tower? (1) Dehydration. Rapid vaporization of water increases the tower pressure, which in turn causes significant fluctuations in the reaction pressure and can lead to the backflow pump running dry. (2) The amplitude cannot be too large. To prevent large fluctuations in reaction pressure. (3) Analyze the dry point of crude gasoline in a timely manner, and adjust the top temperature based on the analysis results. 29. Why is it necessary to control the liquid level at the bottom of the distillation tower? When the quality of the feedstock remains constant, the degree of reaction is fixed, and the operation of the distillation tower is stable, any imbalance between the amount of residue removed and the amount of condensate can cause fluctuations in the liquid level. If the liquid level rises too high, it can easily overflow the oil and gas inlet of the tower; the resistance created by the liquid column can increase the pressure in the reactor, potentially leading to overpressure and accidents ； If the liquid level is too low, the slurry pump will experience vacuum conditions, which will interrupt the return flow of the slurry. This can also lead to excessive temperature and pressure inside the tower, affecting the reaction system as well. Furthermore, fluctuations in the liquid level at the bottom of the tower also affect the thermal balance and the gas-liquid phase loads in the upper part of the tower, thereby impacting the overall operation of the distillation column. Therefore, the liquid level at the bottom of the tower is a very important operational parameter in the distillation system, and it is necessary to constantly ensure that it remains stable. 30. How to control the liquid level at the bottom of the distillation tower? If the liquid level drops, it can be adjusted by increasing the heat absorption from the slurry reflux in order to increase the amount of residue that condenses, or by reducing the amount of slurry discharged ； When the liquid level drops too low and adjustment is not possible in time, to prevent the slurry pump from running dry, the amount of residue fed back for reprocessing should be reduced. However, reducing the amount of residue reprocessed too much can affect the stable operation of the reaction system; therefore, if necessary, crude oil (or reactor feed) can be temporarily added directly to the bottom of the tower to maintain the liquid level. If the liquid level is too high, do the opposite. It is also possible to increase the discharge volume of the oil slurry; if necessary, part of the oil slurry can be temporarily vented urgently (the unit must be equipped with emergency venting facilities) to ensure that the flow of oil and gas into the tower is not affected. Under normal conditions, it is adjusted using the amount of circulating slurry and the temperature of the slurry returning to the tower. What is emphasized here is the fluctuation of the liquid level in the fractionation tower; first, it is necessary to check whether it is caused by the reaction unit, and if so, close cooperation with that unit is required to address the issue. 31. What to do if the liquid level at the bottom of the distillation tower rises sharply? A sharp rise in the liquid level of a distillation tower is a very dangerous sign of potential trouble, and every effort must be made to address it. (1) When there is a risk of blocking large oil and gas pipelines due to a sudden rise in the liquid level, it is necessary to promptly reduce the reaction volume and increase the reaction depth. (2) Increase the discharge volume of oil slurry, the amount of oil slurry recycled for reprocessing, and the extraction volume of heavy diesel. (3) Reduce the heat taken from the bottom of the tower for circulation; however, it should be noted that the reduction in the slurry circulation rate must be done on the condition that the catalyst powder can still be washed and the operation in the upper part of the distillation tower remains stable. (4) If necessary, the inlet at the bottom of the tower for the oil slurry can be enlarged. (5) Appropriately increase the tower top temperature and the light diesel extraction temperature. (6) If the slurry pump becomes vacuumed, the cause should be identified promptly and addressed immediately. (7) When regulatory means are lost, a device equipped with emergency venting facilities can urgently vent part of the slurry for a short period of time. 32. Why does the slurry system coking? Apart from the high temperature at the bottom of the tower, slurry coking is closely related to its inherent properties. From the perspective of the coking pathway, the olefins and polycyclic aromatics contained in the oil slurry are the most prone to coking. Oil slurry is a cracking product that contains not only olefins and aromatics but also many aromatic rings with few long side chains; this is why oil slurry cokes more easily than vacuum wax oil, and even more so than vacuum residue. In recent years, the feedstock for catalytic cracking has become increasingly heavy. Changes in the properties of the feedstock have led to changes in the properties of the catalytic cracking slurry; specifically, there is an increase in tricyclic and tetracyclic aromatics, while the content of straight-chain alkanes and naphthenes decreases, which increases the likelihood of coking in the slurry. The main chemical process behind slurry coking is the polymerization of polycyclic aromatic hydrocarbons. The precursor to coke formation is closely related to the presence of non-volatile liquids adhering to the surface in the oil slurry; subsequently, carbonization occurs rapidly to form graphitic coke. In addition to the above reasons, another important factor is the residence time of the slurry at the bottom of the tower and within the system. It tends to coking over time. 33. How to reduce coking in the slurry system? There are generally three ways to reduce coking in the slurry system: increasing the slurry circulation rate and lowering the liquid level at the bottom of the distillation tower in order to reduce the residence time ； Reduce the temperature of the circulating slurry returning to the tower, increase the flow rate at the inlet located below that point in the tower, and use quench oil slurry for rapid cooling in order to lower the temperature at the bottom of the distillation tower ； Adjust the slurry discharge volume to reduce the relative density of the slurry. 34. What is the function of the overhead vapor-liquid separator in a distillation tower? The overhead vapor-liquid separator is a vessel used for separating three types of materials. Rich gas and crude gasoline are separated at the top, while crude gasoline and water are separated at the bottom. The overhead gas-liquid separator of the fractionation tower has two liquid levels that need to be controlled: the oil-water interface and the gas-liquid interface. No matter which interface it is, if not controlled properly, it can lead to accidents in severe cases, or environmental pollution in less serious cases. 35. What should be done if the liquid level in the overhead oil-gas separator of a distillation tower rises sharply? A sharp increase in the liquid level of this separator is a very dangerous sign of potential trouble; if not addressed promptly, it can lead to overpressure in the reactor, oil entering the compressor, or even a shutdown of the feed supply. Therefore, strict prevention measures are necessary, along with rapid and proper handling of such situations. (1) When the pump is under vacuum, the pump body should first be vented and water should be poured into it to increase the flow rate; if the flow rate still does not increase, it is necessary to switch to the backup pump promptly. (2) If the liquid level rises rapidly, both pumps should be started to pump oil out simultaneously; in emergency situations, other connected pumps can be used to transfer crude gasoline into the sludge tank. (3) If a certain group of the distillation tower top condenser indeed leaks water, it should be shut down promptly. (4) When the compressor creates a negative pressure, it is necessary to promptly coordinate with the reactor in order to increase the flow rate of reactants, thereby maintaining the pressure at the compressor inlet and preventing air from being drawn in due to the negative pressure. This also helps to avoid oil contamination in the gas at the inlet. (5) If the instrument fails, manual control or a backup line should be used promptly. (6) If, even after taking certain measures, the liquid level still rises sharply and endangers the gas compressor and reaction pressure, it may be necessary to drain a large amount of water in a short period of time, even including oil, in order to get through the dangerous situation. But emergency safety measures should be taken at this time to prevent accidents and potential hazards. 36. What is oil slurry rejection? What are its advantages? Oil slurry rejection refers to the process of sending a portion of the oil slurry to the oil tank after cooling it. Discharging oil slurry can reduce coking amount. Due to the high content of polycyclic aromatic hydrocarbons in the slurry, its coking rate is very high; reducing the amount of slurry used in reprocessing can lower this coking rate and thus decrease the regeneration temperature. The externally discharged slurry, after passing through the catalyst powder separation unit, can be used as high-quality raw material for producing needle coke, as well as a feedstock for units such as hydrodesulfurization and hydrocracking. 37. Why is it necessary to establish an open-loop large circulation at the start of operation? This is because a thorough purging process is required at that time, and water may accumulate in the pipes and equipment; if this water comes into contact with hot oil, boiling will occur, leading to damage to the pipes and equipment. To this end, an open-loop circulation is required; on the one hand, it removes water accumulated in the equipment and pipelines while reducing startup time, and on the other hand, it allows the equipment to heat up gradually to prevent damage. 38. Why is diesel needed to be filled in the middle section during startup? What precautions should be taken? During startup, there is water that condenses after steam is used for line cleaning in the pipes of the middle section as well as in the heat exchangers. If this water enters the distillation tower, the high temperature can cause sudden boiling, leading to a sharp increase in pressure within the distillation tower and consequently a rise in the reaction pressure. Therefore, the pipes need to be filled with light diesel to drive out the water inside them. Additionally, at the start of operation, the load on the tower is very low; the middle section of the pipeline and the equipment are filled with diesel, which allows the pump in that section to operate at full capacity, enabling the middle section to be established as quickly as possible and thus facilitating stable operation of the distillation tower. When filling with light diesel, care should be taken to ensure no dead corners remain and that all surface water is drained. The specific procedure is to fill the tank with diesel and let it stand for a few hours; then drain the surface water from the lowest point. After a few hours, drain the water again and refill the tank with diesel. This process is especially important when establishing a mid-stage circulation system, as the water should be drained completely one last time. During dehydration, one must not leave the area to prevent oil leakage. 39. Why is gasoline used to fill the overhead reflux line at startup? What precautions should be taken? During startup, water condenses after steam is used to clean the overhead reflux pipeline; if this water enters the water distillation tower, it can boil suddenly due to the high temperature, causing a sharp rise in pressure within the distillation tower and thus an abrupt increase in the reaction pressure as well. Therefore, gasoline must be used to drive the water out of the pipes. Furthermore, at the start of operation, the load on the tower is very low; filling the top reflux line with gasoline allows the top reflux pump to increase its flow rate more quickly, thereby establishing top reflux as soon as possible and enabling smoother operation of the distillation tower. When filling with gasoline, make sure there are no dead corners and that all surface water is removed. The specific procedure is as follows: fill it with gasoline, let it sit for a few hours, then drain all the surface water from the lowest point; after a few more hours, drain the water again and fill it with gasoline once more. Especially when about to establish a top reflux cycle, drain all the water for the final time. During dehydration, one must not leave the area to prevent oil leakage. 40. What is the spontaneous combustion of iron sulfide? How can it be prevented? The phenomenon in which iron sulfide catches fire when exposed to air is called the spontaneous combustion of iron sulfide. The method of prevention is to prevent air from coming into contact with iron sulfide. During shutdown, it is necessary to maintain the reaction pressure above the regeneration pressure at all times to prevent air from entering the distillation system, which could lead to spontaneous combustion of iron sulfide in the distillation tower, oil-gas separator, and associated pipelines. If spontaneous combustion of iron sulfide occurs, the air supply should be cut off immediately, steam should be blown in, and water should be poured in to rapidly cool down the area. During maintenance, the iron sulfide removed from the equipment and inside the tower should be buried in the ground at a certain humidity level, to prevent it from catching fire on contact with air. 41. What should be noted when conducting line inspections? The following points should be taken into account when performing line inspections: (1) Strengthen communication with external organizations. (2) Strengthen the connection and coordination between systems; line cleaning must be carried out in accordance with the established procedures and protocols. (3) Before feeding steam, all condensate in the steam must be removed; it is not allowed to feed water-containing steam into the hot oil pipeline. (4) Before cleaning, the inlet and outlet valves of the shell side or tube side of the heat exchanger that are not to be cleaned should be kept at a certain opening degree to avoid pressure buildup. For the cooler, the water supply should be stopped, the inlet and outlet valves closed, and the vent opened. For the container, it is necessary to ensure that all oil inside has been drained before performing line cleaning; the bottom drain valve should be opened first, followed by the top vent valve. After scanning, the vent should be opened to allow proper drainage of condensate. When cleaning the pump body with a centrifugal pump, it is essential to turn on the pump’s cooling water. After sweeping, open the pump body to remove blockages and drain the cooling water. (5) At the end of the line cleaning, the relevant connection valves should be closed first, followed by shutting off the steam, to strictly prevent cross-contamination. (6) Before shutting off the steam, an evacuation check should be carried out to ensure complete clearance before shutdown, and condensate should be drained afterward. (7) During the line cleaning process, stricter inspections should be carried out on the oil tank containing contaminated oil; attention should be paid to potential overfilling and dehydration of the tank, and regular communication should be maintained with the dispatch team and relevant personnel. Proper handling of tank changes and oil transfer should be ensured in accordance with the HS coding for petroleum coke. (8) Based on the steam supply volume, focus efforts on cleaning several pipelines in sequence; after they are cleaned, proceed to clean more pipelines, without dispersing the steam supply. (9) Proper records must be kept; those who have cleaned the pipelines during their shift should sign at the end of the pipeline cleaning process. (10) The handover must be clear. 42. What is the principle for scanning pipelines? (1) Scan the heavy oil pipelines first, then the light oil pipelines. (2) The heat exchanger should first be purged through the main line, and then through the secondary line. (3) The cooler first shuts off the water supply, and the water remaining is drained through the petroleum coke valve located after the water inlet valve; thereafter, steam is used to clean the lines. (4) The control valve first purges the bypass line, then the main line of the control valve. (5) The three-way union valve moves repeatedly during cleaning. (6) The gasoline and condensed oil systems should first be flushed with water to remove all the oil, and then purged with steam. (7) Use the pressure-holding scanning method for scanning. (8) After the fractionation purging is complete, use 80-degree hot water to flush out any oil remaining in the pipelines and equipment. After draining the oil, drain the water completely before proceeding with the distillation tower. (9) The stabilization system should follow the principle of washing first and then purging with steam. (10) It is prohibited to discharge oil onto the ground. When draining oil into the oily wastewater system, it must be rinsed with hot water to prevent the oily wastewater drains from being clogged by wax oil. 43. Why is washing necessary for the distillation system during shutdown? During shutdown, the washing of the distillation system takes place in two stages: the light oil section, which includes the top reflux, crude gasoline, and tower top vapor systems ； The heavy oil section includes six systems: light diesel, middle distillate stage 1, heavy diesel, reprocessed oil, middle distillate stage 2, slurry oil, and crude oil. (1) In the light oil fraction, since gasoline vaporizes easily, its volume expands sharply after vaporization ； When gas is heated, its volume also increases significantly. Therefore, crude gasoline and the top reflux system should first be washed with cold water, and then line cleaning should be carried out ； In the oil and gas system at the top of the tower, there is a small amount of unremoved gasoline and gas, which also need to be washed with cold water first, followed by steam cleaning. (2) In the case of heavy oil, because the oil is dense, it adheres to the equipment (especially towers, vessels, the shell side of heat exchangers, furnace tubes, etc.) and cannot be removed by steam; therefore, hot water is required to clean it. 44. What should be noted during the water washing of the distillation system when it is shut down? When washing the distillation system, the following points should be taken into account: (1) For the light oil fraction, ① water must be introduced from below and discharged from above in order to displace the oil. ②No dead corners should remain; rotate the three-way valve to switch between the main and auxiliary lines of the heat exchanger. ③Attention should be paid to the opening and closing status of the critical valve to prevent oil leakage or blockages in the flow. ④For air coolers, they should be cleaned thoroughly piece by piece. ⑤It should be noted that the operating pressure must not exceed the value permitted by the equipment. ⑥Carefully check that all visible areas are drained. ⑦When water accumulates in the overhead oil and gas system of the tower, the distillation systems for various heavy oils cannot purge the tower. (2) Heavy oil section: ① The furnace outlet temperature should be between 100 and 110 degrees; it should not be too low to prevent pipeline condensation, nor too high to avoid water hammer caused by vaporization. ②Ensure that the branch flow rate and temperature difference of the heating furnace are within acceptable limits. ③All systems should be fully circulated without any dead zones; the three-way valves should be adjusted, and the main and auxiliary lines of the heat exchanger should be switched. ④Try to keep the temperatures of various systems (including distillation towers, stripping towers, re-refining tanks, etc.) between 70 and 90 degrees. ⑤In the distillation tower, from bottom to top, as the liquid level rises, the oil in the re-refining tank, the heavy diesel stripping tower, and the light diesel stripping tower should be pumped out one after another. Water can only be introduced upward or the water injection stopped after it has been confirmed that there is no oil left, so as to prevent heavier oils from entering the upper, lighter sections of the system. ⑥When filling with water, the liquid level must not exceed the light diesel extraction plate to prevent heavier oils from entering the lighter systems above. ⑦At no time should the water supply to the heating furnace be interrupted, to prevent dry burning that could damage the furnace tubes. 45. How to get the slurry pump up to full capacity as soon as possible at startup? (1) Direct the pump outlet flow toward the heat exchanger. (2) Sweep from the bottom of the tower to the pump inlet. (3) Drain all water from the pipeline. (4) Switch during pump preheating. (5) The bottom temperature of the tower is greater than 300 degrees. 46. Why is ammonia injected at the top of the fractionation tower? In the oil and gas pipelines at the top of the fractionation tower in some catalytic cracking units (especially those for residue catalytic cracking), ammonia water with a concentration of 0.5–3.0% is injected to neutralize acidic substances such as H2S, RSH, HCl, and HCN present in the oil and gas from the top of the fractionation tower. These acidic substances are primarily generated during the catalytic cracking process due to sulfur, nitrogen compounds, and salts contained in the feedstock. The ammonium salt, which is the product of the neutralization reaction, is soluble in water and can be separated from the vapor stream as sulfur-containing wastewater in the overhead gas oil separator of the distillation tower. Ammonia injection at the top of the tower involves the following neutralization reactions: (1) H2S(g) + 2NH3·H2O → (NH4)2S + 2H2O; (2) RSH(g) + NH3·H2O → RSNH4 + H2O; (3) HCl + NH3·H2O → NH4Cl + H2O; (4) HCN + NH3·H2O → NH4CN + H2O. 47. What are tower flooding, tower submersion, and leakage? (1) Tower flooding: Excessive vapor load causes recombined substances within the tower to be carried to the top of the tower, resulting in defective product. (2) Tower flooding: Tower flooding occurs when the liquid level in the downcomer is too high, spreading to the upper tray and causing the liquids on the upper and lower trays to mix together, thereby completely destroying the distillation effect. (3) Leakage: When the processing volume is small and the gas velocity in the tower is very low, most of the liquid flows from the downcomer to the lower tray without passing through the tray plates, but directly from the rear holes or valve holes to the lower tray. 48. Why does a hot oil pump need to be preheated? (1) High temperature. Hot oil pumps typically operate at temperatures of around 200–350 degrees. If they are not heated and remain cold, the pump body will be subject to significant temperature differences when hot oil enters it. Such extreme changes can cause the pump to fail, leading to leaks, cracks, damage to components, and issues resulting from the different thermal expansion coefficients of various parts, which in turn affects the pump’s operation. (2) High viscosity. The media pumped by hot oil pumps have high viscosity; they solidify at room temperature and can even block the pump’s pipelines, resulting in an inability to build pressure during startup, low flow rates and head, as well as vibration in the pump. 49. How to start and stop a heat exchanger correctly? Proper starting and stopping: To extend the lifespan of the heat exchanger and ensure smooth operation, it is necessary to start, stop, and switch it on and off correctly. (1) When using or switching, the cold flow should be introduced first, followed by the hot flow; at the same time, the vent valve should be opened to release the gas inside the device, after which the vent valve should be closed. (2) In some heavy oil heat exchangers, to prevent the heavy oil from solidifying upon initial operation, water vapor is used first for preheating and cleaning before normal operation can begin. (3) When shutting down the heat exchanger, first shut off the hot flow and then the cold flow, while simultaneously performing line flushing and evacuation. 50. What are the advantages and disadvantages of air coolers? Advantages: they save water, reduce operating costs, are easy to adjust, and can also help minimize water pollution in industrial areas. Disadvantages: The biggest drawback of using air as a coolant is its poor heat transfer properties. To compensate for this, air coolers employ finned tubes and fans to enhance heat transfer on the air side, which increases the heat transfer area and turbulence on that side, thereby improving the heat transfer process. 51. What are the anti-coking measures for the slurry system in terms of operational procedures? (1) Control the appropriate slurry circulation rate to ensure that the linear velocity inside the heat exchanger tubes exceeds 1.4 m/s. (2) The temperature at the bottom of the distillation tower should be maintained between 350 and 355 degrees; it should not exceed 360 degrees. Keeping the temperature at this upper limit can slow down the rate of coking, while maintaining it at the lower limit can reduce the content of light components in the oil slurry that is discharged. (3) The liquid level at the bottom of the distillation tower should not exceed 60%, in order to keep the average residence time of the slurry at the bottom of the distillation tower below 7 minutes. (4) Increase the stirring steam volume at the bottom of the distillation tower. 52. What should be noted when using slurry scale inhibitors? (1) They should be used from the start of operation, rather than waiting until scaling occurs; otherwise, descaling processes may cause blockages in the heat exchangers. (2) Maintain a continuous injection; do not stop. (3) Maintain the concentration of the effective antiscalant component in the slurry system at around 200×10-6.