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1. What quality indicators of motor gasoline can be controlled by atmospheric and vacuum distillation units? The atmospheric and vacuum distillation unit can control the boiling range of automotive gasoline, including the 10% point, 50% point, 90% point, and dry point (final boiling point). The temperatures at each boiling point are specified according to the requirements for use in automotive gasoline. It specifies that the distillation temperature at 10% volume fraction should not exceed 70°C, which is necessary to ensure the engine’s performance during cold starts. The 50% distillation temperature is an important parameter for ensuring a uniform evaporation of gasoline, thereby achieving good acceleration and stability, as well as maximizing power and climbing performance; it is specified that this temperature should not exceed 120°C. The 90% distillation temperature is an indicator for controlling the heavy components in automotive gasoline; it ensures good evaporation and complete combustion, prevents carbon buildup and the formation of acidic substances, and also helps to avoid diluting the engine oil. For general automotive gasoline, the 90% distillation temperature must not exceed 190°C to ensure complete vaporization and combustion. Dry point is an indicator that ensures vehicle gasoline does not suffer from incomplete combustion due to the presence of heavy components, which could lead to coking and carbon buildup in the combustion chamber; it is also an indicator that ensures the lubricating oil is not diluted. It is more important for cars that stop and start frequently. However, the straight-run gasoline produced by atmospheric and vacuum distillation units has a low octane rating, typically around 50–60; therefore, it needs to be blended with high-octane components from other units before it can be sold as finished gasoline. 2. What quality indicators of light diesel oil can be controlled by atmospheric and vacuum distillation units? The atmospheric and vacuum distillation unit can control parameters such as the boiling range, freezing point, and flash point of light diesel. The diesel distillation range is an important quality indicator. The higher the speed of a diesel engine, the stricter the requirements for the fuel’s boiling range. Generally, fuels with lighter fractions have better starting performance and faster evaporation and combustion rates. However, if the fuel fraction is too light, its auto-ignition point is high, the combustion delay period is long, and excessive evaporation can lead to knocking in the cylinder. Excessive fuel weight leads to poor atomization of the spray, slow evaporation; the partially unburned fuel decomposes under high temperatures, forming carbon residue that contaminates engine components, results in black smoke in the exhaust, and increases the fuel consumption per unit of distance traveled. Therefore, the specifications for light diesel require that the 50% distillation temperature be no higher than 300°C, and the 95% distillation temperature be no higher than 365°C. The distillation range of diesel is also closely related to its freezing point and flash point. The freezing point is also an important quality indicator for diesel fuel. Light diesel is classified into six grades based on its freezing point: Grade 10, Grade 0, Grade -10, Grade -20, Grade -35, and Grade -50. Generally, the lighter the distillation range of diesel, the lower its freezing point. The flash point of light diesel is an important parameter specified based on safety and fire prevention requirements. The flash point of diesel is specified in the standards to be no less than 65°C. The lower the distillation range of diesel, the lower its flash point. 3. What quality indicators of heavy diesel oil can be controlled by atmospheric and vacuum distillation units? The atmospheric and vacuum distillation unit can control properties of heavy diesel such as boiling range, density, flash point, and viscosity. The boiling range of heavy diesel is roughly 300–400°C; that is, crude oil from the third or fourth fractionation stage, as well as oil from the vacuum distillation first stage, can be used to produce heavy diesel. The density of heavy diesel should not be too high; if it is too high, there will be too much asphaltene and gum, making complete combustion difficult ； When the density is too low, there are excessive light fractions, which results in a low flash point and fails to ensure safe use. The flash point of heavy diesel is determined by its content of light fractions. The flash point should be no lower than 65°C; if there is a high content of light fractions, the flash point will be lower, making it unsafe for storage and transportation. Especially heavy diesel with a high freezing point needs to be preheated before use, which requires a high flash point. To ensure the safe use of heavy diesel, it is stipulated that the preheating temperature must not exceed two-thirds of the flash point. Heavy diesel is used in low and medium-speed diesel engines. When its viscosity is too high, it causes the pressure in the oil pump to drop, leading to bubbling in the oil delivery pipes, oil resistance, and poor fuel injection as well as inadequate atomization. This results in incomplete combustion and the emission of black smoke, which not only wastes fuel but also pollutes the environment ； When the viscosity is too low, it causes the fuel injection distance to be too short and poor atomization and mixing, thereby affecting combustion. Therefore, the minimum viscosity of heavy diesel fuel for general large and medium-sized low-speed diesel engines should be maintained at 8.6 mm2/s or above. The density, flash point, and viscosity of heavy diesel are controlled by the separation of fractions during the operation of atmospheric and vacuum distillation units; generally, the lighter the fraction, the lower its density, as well as its flash point and viscosity. 4. Which quality parameters of atmospheric heavy oil are controlled by the atmospheric and vacuum distillation unit? When atmospheric residue oil is used as feedstock in a heavy oil catalytic cracking unit, the atmospheric and vacuum distillation unit needs to control the sodium ion content in the atmospheric residue oil. Heavy oil catalytic cracking units require that the sodium content in the feedstock be below 1–2 ppm. This is because sodium deposited on the catalyst “neutralizes” the catalyst’s acid sites and forms a low-melting eutectic mixture with the catalyst matrix, resulting in permanent deactivation of the catalyst. Therefore, deep desalination is required for the atmospheric and vacuum distillation units. Typically, when the desalination level in a atmospheric and vacuum distillation unit reaches 3 mg/L, it is sufficient to meet the requirement that the sodium ion content in atmospheric heavy oil be less than 1 ppm. 5. What are the requirements for vacuum wax oil as a feedstock in catalytic cracking? When the carbon residue in the reduced-pressure wax oil is too high, excessive coking occurs during catalytic cracking, which overloads the regenerator and may even lead to overheating. However, if the amount of residue carbon is too low, it will result in insufficient heat in the regenerator, leading to a lack of heat for the reaction, and fuel must be added to the regenerator. The heavy metals in vacuum residue wax oil deposit on the catalyst during catalytic cracking, deactivating it and leading to an increase in dehydration reactions as well as higher amounts of gas and coke formation. Therefore, each refinery has certain requirements regarding the quality of catalytic cracking feedstock. When catalytic cracking employs a process involving the blending of residue oil (such as in heavy oil catalytic cracking processes), parameters such as the carbon residue and heavy metal content of vacuum wax oil mainly affect the amount of residue oil that can be blended in. If the residue and heavy metal content of the reduced wax oil are low, more residue can be blended in ； If the reduced wax oil has high carbon residue and heavy metal content, only a small amount of residue oil can be blended in. Therefore, the heavy oil catalytic cracking process also has certain requirements regarding the residue and heavy metal content of the feed oil. 6. What is the important role of the atmospheric and vacuum distillation unit in the overall processing flow of the entire plant? The atmospheric and vacuum distillation unit separates crude oil into components with different boiling point ranges through distillation, in order to meet the requirements of products and downstream processing units for raw materials. Atmospheric and vacuum distillation is the first process in a refinery for processing crude oil, representing its initial treatment; it plays a crucial role in the overall refining process and is often referred to as the \"key\" unit. Generally speaking, after being processed in a vacuum distillation unit, crude oil can yield products such as straight-run gasoline, jet fuel, kerosene, light and heavy diesel oil, and fuel oil. For certain types of crude oil rich in resins and asphaltenes, road asphalt can be directly produced through extensive vacuum distillation. Among the aforementioned products, except for gasoline, which no longer serves as a direct product due to its low octane number, the rest can generally be shipped as products either directly or after appropriate refining. Another main function of the atmospheric and vacuum distillation unit is to provide high-quality raw materials for downstream secondary processing units or chemical plants. For example, feedstocks for reforming, ethylene cracking, catalytic cracking, hydrocracking, or lubricant processing units, as well as feedstocks for coking, oxidized asphalt, solvent deasphalting, or visbreaking units. In recent years, with the development of heavy oil catalytic cracking technology, the heavy oil from the atmospheric distillation bottom of certain crude oils can also be used directly as feedstock for catalytic cracking units. Therefore, the operation of the atmospheric and vacuum distillation units directly affects the production conditions of the downstream secondary processing units as well as those of the entire plant. 7. What is the function of a stripping tower? The purpose of the stripping tower is to use direct steam stripping or indirect heating on the side stream product in order to remove the low-boiling-point components from it, thereby ensuring that the product’s flash point and boiling range meet the specified requirements. The most commonly used stripping method is direct stripping with water vapor at a temperature higher than that of the side-stream extraction. The amount of stripping steam used is generally 2–4% (by weight) of the product volume. The product temperature after stripping is approximately 5–10°C lower than the extraction temperature. The gas from the top of the stripping tower is returned to the gas phase section of the side draw layer. 8. What are the necessary conditions for the distillation process? ①The distillation process mainly relies on multiple vaporizations and condensations to separate liquid mixtures. Therefore, a significant difference in the relative volatility of the various components in the liquid mixture is the primary condition for carrying out a distillation process. When the volatilities of the components in a mixture are very similar (such as in a C4 fraction mixture), separation can be achieved by adding a solvent to form an non-ideal solution, using azeotropic distillation or extractive distillation; at this point, there are significant differences in the relative volatilities of the various components within the resulting non-ideal solution. ②Reflux liquid with a high concentration of light components is introduced at the top of the tower, while hot steam is generated at the bottom by heating or stripping. ③The tower is equipped with trays or packing, so that the steam with a higher temperature and a higher content of heavy components at the lower part comes into contact with the liquid phase with a lower temperature and a higher content of light components at the upper part, thereby enabling heat and mass transfer to occur. The heavy components in the steam are condensed into a liquid; the heat released thereby causes the light components in the liquid to vaporize. The vapor stream inside the tower passes through multiple condensation stages from bottom to top, resulting in a decreasing concentration of light components; thus, a high-concentration heavy product can be obtained at the bottom of the tower. 9. What is the reflux ratio? What impact does its size have on distillation operations? The reflux ratio refers to the ratio of the reflux volume L0 to the overhead product D, that is: R = L0/D. The size of the reflux ratio is determined by the ease of separating the various components (i.e., the relative volatility) as well as the requirements regarding product quality. For binary or multicomponent systems, it is determined by the calculations of the distillation process. In China, the petroleum distillation process is mainly designed using empirical or semi-empirical methods; the reflux ratio is primarily determined by the overall heat balance of the column. During the production process, the number of trays or the theoretical number of trays in the distillation column remains constant; increasing the reflux ratio raises the concentration of the light components at the top of the column, thereby improving their quality. For a simple tower in which one product is obtained at each of the top and bottom, when increasing the reflux ratio, it is necessary to also increase the evaporation rate of the reboiler at the bottom of the tower ； For composite towers with multiple side streams, when increasing the reflux ratio, it is necessary to adjust the opening degrees of each side stream in order to maintain a proper material balance and the quality of the side stream products. 10. What is the impact of pressure levels during atmospheric distillation on the distillation process? How to select the operating pressure? The overhead product of an atmospheric distillation tower is usually a gasoline fraction or a feedstock for reforming. When water is used as the cooling medium and the product is cooled to around 40°C, with the reflux tank operating at a pressure of 0.11–0.3 MPa, almost all of the oil is condensed. Therefore, crude oil distillation is generally carried out under pressure slightly higher than atmospheric pressure, which is how the term \"atmospheric pressure column\" came about. When the content of non-condensable gases in crude oil is high, increasing the pressure can reduce the amount of light gasoline lost along with the inert gases emitted. Appropriately increasing the tower pressure can enhance the tower’s processing capacity; when the operating pressure of the tower is raised from 0.11 MPa to 0.3 MPa, its production capacity can increase by 70%. After the pressure in the tower is increased, the overall operating temperature of the tower also rises, which is beneficial for the heat exchange between the side-draw fractions, the intermediate circulating reflux, and the crude oil. The disadvantage is that as pressure increases, the relative volatility decreases and separation becomes more difficult; to achieve the same level of separation precision, it is necessary to increase the reflux ratio at the tower top, which in turn increases the load on the overhead condenser. Furthermore, since the furnace exit temperature cannot be increased arbitrarily, the normal-pressure drawing rate tends to decrease as pressure rises. Most crude oils in our country contain low levels of light fractions; therefore, to ensure normal pressure distillation yields and high light oil recovery rates, lower operating pressures are typically chosen. When processing crude oils with a high content of light distillate oil, it is advisable to use a higher tower pressure. 11. How to choose the amount of stripping steam reasonably? The stripping of side-line products is primarily aimed at evaporating the light components, thereby increasing the flash point, initial boiling point, and 10% point of the products. Bottom steam stripping in an atmospheric pressure tower is primarily aimed at reducing the content of fractions below 350°C in the heavy oil at the bottom of the tower, increasing the yield of light oils, and reducing the load on the vacuum distillation tower. For a vacuum distillation column, the purpose of bottom stripping is mainly to reduce the oil vapor partial pressure in the vaporization section, thereby maximizing the vaporization fraction of the feed under the allowable temperature and vacuum conditions. The amount of stripping steam used is related to the content of light fractions that need to be stripped out. In China, it is common practice to use an amount of stripping steam equivalent to 2–4% of the weight of the oil being stripped. The amount of stripped distillate from side-stream products is approximately 3–4.5% (by weight) of the oil volume, while that from the heavy residue at the bottom of the tower is about 1–2% (by weight). If the amount to be stripped needs to be 6–10% or more, this should be addressed by adjusting the operation of the distillation tower. Excessive stripping steam will increase the vapor load on the distillation column, as well as the energy consumption required to generate superheated steam and for condensation at the column top. The stripping steam used in refineries is superheated steam with a pressure of 0.3–0.4 MPa and a temperature of 400–450°C. 12. Why is the vacuum distillation tower designed to be narrow at both ends and thicker in the middle? In the upper part of the vacuum tower, since the vapor and liquid phase loads are relatively low, the corresponding tower diameter is also small. At the bottom of the vacuum distillation tower, high temperatures and a long residence time of the product at that location can lead to chemical reactions such as cracking, condensation, and coking, which affect product quality and are detrimental to long-term safe operation. To reduce the residence time of the product at the bottom of the tower, a smaller diameter is used for the stripping section as well. In the vast majority of cases, the diameter of the stripping section at the lower part of the vacuum tower and the reduced-diameter section at the upper part is the same, which facilitates the manufacturing and installation of the tower. The middle part of the vacuum tower is due to vapor. The liquid phase load is relatively high, so a larger diameter was chosen accordingly. Thus, it results in the external shape of the pressure reduction tower being narrow at both ends and thick in the middle. 13. What problems can occur if the vapor and liquid phase loads in a packed tower are too low or too high? In a packed tower, as the gas flow velocity increases, the pressure drop across the bed increases, and the liquid holdup in the packing layer also increases accordingly. When the gas flow rate increases to a certain value, it becomes difficult for the liquid to flow downward, resulting in a phenomenon known as flooding; the operation of the tower is completely disrupted at this point, and the gas flow rate at this value is referred to as the flooding velocity. The appropriate operating gas velocity for a packed tower is generally 60~80% of the flooding velocity. The level of the bubble velocity in a packed tower is primarily determined by the physical properties of the vapor and liquid phases, such as specific gravity and viscosity, as well as the flow rates of these two phases and the porosity of the packing layer. If the liquid flow rate is too low, it may result in some of the surfaces of the packing tower not being adequately wetted. Heat and mass transfer between the vapor and liquid phases within the packing tower occur primarily through the surfaces of the packing that are wetted by the liquid; if some of the packing is not wetted, the surface area available for heat and mass transfer decreases, which inevitably leads to a reduced separation efficiency. When the liquid flow rate in the packed tower is too low, efforts should be made to increase the flow rate of the recirculation stream in that section. 14. What are the special structural requirements for the vaporization section? In a vacuum distillation unit, the feed to each column is in a vapor-liquid mixture state and at a high flow rate. To reduce the inlet pressure drop and minimize the impact on the column walls that could cause vibration, large vacuum columns employ low-speed feed lines that deliver the feed vertically along the centerline of the column. Although measures such as tangential feeding are also employed to achieve high-speed separation of gas and liquid in the vaporization section, the height of this section must be greater than the usual plate spacing in order to provide more space for gas-liquid separation and reduce mist entrainment. Since the feed section of the vacuum tower is precisely the hemispherical diameter-changing zone where the distillation section and the stripping section meet, the space in this feed section of the vacuum tower is particularly large. Nevertheless, due to the high gas flow velocity in the vacuum tower, and in order to reduce foam entrainment, some vacuum towers are equipped with a foam breaker above the vaporization section. The temperature and pressure of the feed are important parameters for on-site operation, as well as the basis for heat balance calculations in the distillation column; therefore, temperature and pressure measuring instruments should generally be installed in the vaporization section. 15. What is the impact of tower installation on distillation operations? It is hoped that newly built and renovated towers will meet the requirements of high separation capacity, large production capacity, and stable operation. To this end, the following requirements are imposed on the installation quality: ① Tower body: The tower body must be vertical. The inclination should not exceed one in a thousand; otherwise, dead zones will form on the tray, reducing the distillation efficiency of the tower. ②Plate: The plates must be level, with a levelness deviation of no more than ±2 millimeters. If the levelness of the plates does not meet this requirement, it will result in uneven liquid layer heights on the plates. This allows the rising gas phase inside the tower to pass through areas where the liquid layer is shallower, preventing the gas and liquid phases from achieving the desired heat and mass transfer effects on the plates, thereby reducing the efficiency of those plates. For sieve plate towers, special attention must be paid to the horizontal alignment requirements of the sieve plates. For coiled tray plates, floating jet tray plates, inclined hole tray plates, etc., attention must also be paid to the installation position of the tray plates, ensuring that the direction of the openings is consistent with the flow direction of the liquid on that layer of tray plates. ③Overflow port: The distance between the overflow port and the lower tray should be determined based on the production capacity and the height of the overflow weir on the lower tray. However, it is necessary to ensure that the weir plate can be inserted into the liquid in the lower liquid receiving tray, so as to provide sufficient passage for the liquid phase in the upper layer to flow downward, and to create a liquid seal that prevents steam from rising in the lower layer, thus avoiding short-circuiting of the gas phase. Additionally, it is important to pay attention to whether the tear holes are unobstructed, as well as the installation and maintenance of components such as the liquid collection tank, oil collection tank, and vent pipe. Different types of trays have varying installation requirements; only by installing them as specified can the production efficiency of the tower be ensured. 16. How are the heights of the footings for the initial distillation column, atmospheric column, and vacuum column determined? Both the initial distillation tower and the atmospheric pressure tower operate at pressures slightly above atmospheric pressure. If the bottom pump is switched or if the bottom oil pump is restarted after evacuation, the pressure inside the tower allows the liquid to flow directly into the pump and fill it, so no difficulties arise when starting the pump. The height of its skirt mainly ensures that the height difference between the product extraction port at the bottom of the tower and the pump inlet is greater than the cavitation margin of the pump at the bottom of the tower, thereby preventing damage to the pump due to cavitation. The cavitation margin of domestically produced centrifugal pumps is generally around 4 m; therefore, the skirt height of the initial distillation tower and the atmospheric pressure tower is usually between 4 and 5 m. The vacuum distillation tower operates under negative pressure; if the liquid level at the bottom of the tower is not high enough, it becomes impossible to start the pump located at the bottom of the tower and get it operational. To provide sufficient pumping head, the head difference between the liquid level at the bottom of the tower and the pump inlet is generally between 7 and 10 meters. 17. Which three equilibria should be understood in the operation of a distillation column? The operation of a distillation tower requires an understanding of material balance, vapor-liquid equilibrium, and heat balance. Material balance means that the amount of material entering the tower per unit time should equal the sum of the amounts of material leaving the tower. Material balance reflects the production capacity of the tower, which is primarily regulated by the feed rate and the outlet rates at the top and bottom of the tower. During operation, changes in material balance are specifically reflected in the liquid level at the bottom of the tower. When the operation of the tower does not conform to the overall material balance equation, this can be reflected in changes in the tower pressure difference. For example, if more material enters than exits, the pressure difference across the tower increases. For a fixed distillation column, the column pressure difference should remain within a certain range. The pressure difference across the tower is too large; the speed of the rising steam inside the tower is excessive, leading to severe entrainment of mist, and in some cases flooding occurs, which disrupts normal operation ； If the pressure difference across the tower is too low, the speed of the rising steam inside the tower is too slow, which reduces the efficiency of mass transfer between vapor and liquid on the tray surfaces; in some cases, liquid leakage occurs, further **reducing the efficiency of the trays. Poor control of material balance can lead to chaos in the operation of the entire tower; mastering material balance is a key aspect of tower operation. If the normal material balance is disrupted, it will affect the other two balances: the vapor-liquid phase balance will not achieve the desired results, and the heat balance will also be disrupted, requiring readjustment. Vapor-liquid equilibrium mainly reflects the quality of the product and the amount of loss. This is achieved by adjusting the operating conditions of the tower (temperature, pressure) and the vapor-liquid contact on the tray. Only when temperature and pressure are fixed is there a definite vapor-liquid equilibrium composition. When temperature and pressure change, the composition determined by the vapor-liquid equilibrium changes, and as a result, the quality of the product as well as any losses associated with it also change. Vapor-liquid equilibrium is closely related to mass balance. When mass balance is properly managed, the upward velocity of steam in the tower is appropriate and vapor-liquid contact is good, resulting in high efficiency in heat and mass transfer as well as high tray efficiency. Of course, temperature and pressure also change as the material balance changes. Heat balance refers to the equilibrium between the heat entering the tower and the heat leaving it, which is reflected in the temperature at the top of the tower. Heat balance is the foundation upon which material balance and vapor-liquid phase balance are achieved, and in turn it depends on them. Without a hot vapor phase and a cold reflux, the entire distillation process cannot take place ； Changes in the operating pressure and temperature of the tower (i.e., changes in the composition of the vapor-liquid equilibrium) result in corresponding changes in the heat released due to vapor condensation on each tray, as well as the heat absorbed during liquid vaporization; this is reflected in changes in the heat supplied to the feed and the heat removed from the tower top. Mastering material balance, vapor-liquid equilibrium, and heat balance is key to distillation operations; these three equilibria influence and restrict one another. In operation, control of material balance is usually the main focus, with heat balance adjusted accordingly, in order to ultimately achieve vapor-liquid phase equilibrium. To maintain a stable equilibrium of the liquid level at the bottom of the tower, it is necessary to keep stable: ① the feed rate and feed temperature ； ②Top, side line, and bottom extract rates ； ③Top pressure of the tower. To maintain a stable top temperature, it is necessary to stabilize: ① feed rate and feed temperature ； ②Flow rates and temperatures at various intermediate stages of top reflux and circulating reflux ； ③Top pressure ； ④Stirring steam volume ； ⑤The raw materials and reflux are free of water. By closely monitoring the top temperature of the tower and the liquid level at the bottom, analyzing the causes of fluctuations, and making timely adjustments, it is possible to maintain the three balances of the tower and ensure its proper operation. 18. What are the reasons for changes in the bottom liquid level of the initial distillation tower? The small diameter of the primary distillation tower, along with a high flow rate per unit volume and slight fluctuations in the crude oil flow rate, can cause changes in the liquid level at the bottom of the primary distillation tower. Reasons for changes in the bottom liquid level of the primary distillation tower: ① Changes in the feed flow rate to the primary distillation tower and the discharge volume of the pump at the bottom of the tower ； When adjusting the processing capacity or the bottom liquid level of the initial distillation tower, the flow rates in and out of the tower are not properly balanced. ②Changes in the properties of crude oil will cause fluctuations in the bottom liquid level of the tower. For example, the crude oil becomes heavier, and the liquid level at the bottom of the tower rises ； The crude oil becomes lighter, and the liquid level at the bottom of the tower drops. ③When the feed temperature to the initial distillation tower changes and the top temperature is not adjusted in a timely manner, a higher feed temperature leads to a decrease in the liquid level ； A low feed temperature causes the liquid level to rise. ④The level of pressure and temperature at the top of the tower affects changes in the liquid level at the bottom of the tower. The high temperature at the top of the tower and low pressure reduce the vaporization rate of oil within the tower; the unvaporized oil reaches the bottom of the tower, causing the liquid level there to rise ； The temperature at the top of the tower is high and the pressure is low, while the liquid level at the bottom of the tower decreases. 19. How can a distillation tower be operated smoothly? The operation of the distillation tower is the most important aspect of a vacuum distillation unit; it plays a key role in ensuring the stable operation of the entire facility and is responsible for controlling the quality of the main products. The distillation tower should operate smoothly to minimize fluctuations, and to achieve this it is necessary to: ① stabilize the feed and discharge flow rates of each tower. In addition to the flow rate pumped out by the bottom pump at the product discharge point, it is necessary to pay attention to changes in the oil extraction rates from each side stream; these adjustment amounts should be small and determined based on the quality and yield of each respective product. ②The temperature of each material should be maintained stable; when there are changes in temperature, it should be adjusted appropriately based on product quality. ③The initial distillation column, the atmospheric pressure column, and the vacuum column are distillation columns operated in series one after another; if the operation of the preceding column is unstable and the properties of the oil at its bottom change, this will inevitably affect the operation of the subsequent columns, so close attention must be paid during operation. When the properties of crude oil change, adjustments should first be made to the initial distillation tower, followed by necessary adjustments to the atmospheric distillation tower and the vacuum distillation tower in sequence. ④In the operation of towers, a stable liquid level at the bottom of the tower and a stable amount of stripping steam are important conditions for the stable operation of each tower. ⑤When the operating pressure of the tower changes, the extraction temperatures at various side streams also change accordingly; these must be adjusted promptly to ensure product quality. The vacuum level at the top of the vacuum distillation tower has a significant impact on its extraction efficiency and the separation of fractions, so it is essential to maintain stable pressure. ⑥Changes in the water content of crude oil have a significant impact on the initial distillation tower; when there are substantial changes in water content, operational adjustments should be made promptly. 20. What are the symptoms of water in the tower top reflux oil, and how to deal with it? The overhead reflux oil is drawn from the overhead reflux oil tank. If the water level in this tank is not properly controlled or fails to function correctly, and the water level rises above the normal range, exceeding the level of the gasoline extraction pipe, then the reflux oil will contain water and be sent to the tower top. Additionally, if the tubes of the tower top water cooler corrode and perforate, allowing large amounts of cooling water to leak into the reflux oil tank before the water can be removed, this can also result in the reflux oil containing water. High water content in crude oil, improper operation of the electrodesalination and dehydration tanks, and tripping of the electrical system can also lead to an excessive amount of water in the reflux oil tank at the top of the distillation tower; inadequate dehydration results in the reflux oil containing water. The oil returning with water enters the top of the tower. Since the vaporization heat of water is more than 4 times greater than that of oil, and the volume of water vapor is 10 times larger than that of oil vapor, this leads to an increase in pressure at the top of the tower and a decrease in its temperature. Subsequently, the temperature of the main product stream also drops, causing supercooling in the upper part of the tower. No oil flows out from the side streams, resulting in pump cavitation; or the oil in the main stream contains water. If this is not addressed promptly, the pressure at the top of the tower can rise sharply, leading to a collapse of the tower, and the safety valve may open. When it is observed that the temperature at the top of the tower drops significantly, along with a decrease in the distillation temperature of the first column and the pump on that column running dry, it can be preliminarily concluded that the reflux oil contains water. It is necessary to promptly check whether the control level for the oil-water interface in the reflux tank is set too high; by opening an exhaust valve below the control valve, it is possible to directly observe whether the reflux oil contains water, thereby allowing for an accurate determination. If the water level in the return oil tank is too high, resulting in the return oil containing water, the following measures should be taken: ① Resolve any issues with instrument control, open the dehydration valve or the auxiliary line valve to increase the water removal rate, thereby rapidly lowering the water level. ②If there is a leak in the cooler tube bundle, stop using it and carry out repairs promptly. ③Appropriately increase the top temperature of the tower to accelerate the evaporation of water inside it. ④An increase in the pressure at the top of the tower can trigger the operation of the air-cooling fan, while reducing the valve controlling the steam supply at the bottom of the tower in order to decrease the amount of steam flowing into the tower. If the electrical operation of the electrodesalination tank is abnormal and the crude oil contains too much water, which then enters the distillation tower, this can result in water being carried back in the reflux stream from the top of the distillation tower. In such cases, the water injection into the crude oil desalination tank should be stopped; the electrical faults in the tank must be resolved as soon as possible so that power can be restored to the tank. The amount of demulsifier added to the crude oil can be increased, depending on the degree of water removal, to facilitate the proper operation of the electrodesalination tank. When backflow oil contains water, it is necessary to make an early diagnosis and take prompt action to remove the water from the oil; this will allow normal operations to be resumed quickly. Delaying detection and slow response pose a serious threat to safe production. 21. Why doesn’t adjusting the tower top temperature using the tower top reflux flow sometimes achieve an effective control effect? Under normal operating conditions, the top temperature of the tower is regulated by the amount of reflux at the tower top; when the load on the tower top is too high, the reflux can no longer effectively regulate the top temperature. An excessive load on the tower top can be caused by the following reasons: ① The nature of the crude oil becomes lighter, especially when the gasoline components increase or the crude oil contains a high amount of water. ②The large-capacity fractionation tower for crude oil processing is operating at its upper limit load; the reflux flow at the intermediate stage is low, and the water content in the crude oil is too high. When the top of the tower becomes overloaded for the reasons mentioned above, its temperature rises. Increasing the reflux flow rate can only have a short-term effect on lowering this temperature; it will rise again soon. If the reflux flow rate is increased further, not only will the temperature at the top of the tower not decrease, but the liquid level of gasoline in the reflux tank there will also rise suddenly. If measures are not taken promptly to increase the flow rate of gasoline out of the unit and thus reduce the liquid level in the tank, the tank will fill up with gasoline, resulting in pressure buildup. The reason for this phenomenon is that when the reflux at the top of the tower vaporizes inside the tower, it increases the load at the top further, creating a vicious cycle; as a result, the reflux fails to effectively regulate and control the temperature at the top of the tower. In such situations, efforts should be made to reduce the load at the top of the tower, lower the reflux temperature, increase the reflux flow rate in the middle section, and reduce the stripping flow rate at the bottom of the tower. If the amount of crude oil processed is too high, or if there are too many light components in the crude oil, it is possible to reduce the amount of crude oil processed; when the water content in the crude oil is too high, proper electrodialysis and dehydration procedures must be carried out. 22. What are the symptoms when salt scale forms and blocks the distillation column during production operations? How to remove salt without stopping work? The salts contained in crude oil (mainly chlorides) are treated by an electrodialysis unit, but a small amount of residual salt still enters the distillation tower. To prevent corrosion of the equipment caused by the hydrolysis of chloride solutions, anti-corrosion measures such as the injection of alkali and ammonia are employed. Injecting an excessive amount of alkali (Na2CO3, NaOH) into crude oil can result in the formation of alkaline scale, which deposits on the bottom tray. Ammonium salts are produced by injecting ammonia at the top of the tower; they return to the tower along with the reflux oil from the top and can accumulate in the downcomer. Nitrogen-containing compounds in crude oil decompose at high temperatures to form ammonium salts. Sediment and other impurities mixed in crude oil can also deposit on the trays of the distillation column. For the above reasons, during production, the crude oil distillation tray plates are prone to salt scaling and tower blockage. Salt scaling on the distillation column trays blocks these trays, leading to the following phenomena: ① The salt scaling actually reduces the porosity of the trays, which deteriorates the mass and heat transfer between the oil and vapor phases. As a result, the top temperature and the temperatures of the side streams experience regular fluctuations. Due to uneven distribution of the vapor load, the top pressure often changes suddenly. ②The distillation efficiency of the tower deteriorates, the quality of the oils from each side stream becomes worse, the boiling ranges overlap, the resulting oils do not meet the required standards, and in severe cases, black oil is produced. ③As the pressure drop across the distillation tray increases, identifying the location where this increase occurs allows one to determine the approximate location of salt deposition on the tray. When salt deposits clog the trays in a distillation column, a method for handling this situation without shutting down the plant is as follows: The salt deposits on the trays are generally soluble in water; therefore, during normal operation, fresh water can be pumped in using the top reflux pump to dissolve the salt and wash it away. Fresh water enters the tower, where the salt dissolves in it. The salt-containing water can then flow through a side stream outlet into that side stream, or it can be extracted from the pump outlet or sent to the pipeline for unqualified oil. Points to note for water-washing tray plates: ① Identify the areas where salt accumulation occurs inside the tower. ②Reduce the crude oil processing volume to 60~80% of the normal level. ③Increasing the mid-section reflux flow and the top reflux flow reduces the top temperature, rendering the temperature control via top reflux ineffective. When water is pumped into the tower using the top reflux pump, the top temperature will definitely drop to 100°C, thereby preventing water from vaporizing and causing excessive pressure at the top of the tower. The water flow rate must not be too high, as this can drive the temperature down too low; when water flows downward onto the trays where no salt has formed, it has a significant impact on the operation. ④After water enters the top of the tower, it is discharged through the side-line pump and waste water outlet; the flow of water to the washing trays is monitored. The amount of water supplied by the top pump is used to strictly control the temperature at the side-line distillation outlet at 103–105°C. If the temperature is too high, the water vaporizes, preventing the removal of salts; if the temperature is too low, the water flows downward to the trays below the extraction point, contaminating those trays. ⑤As water is detected at the discharge port of the side-line pump, analysis of the Cl-1 content in the water sample begins, and this process continues until the Cl-1 level in the sample no longer decreases significantly, the wastewater becomes clean, and the washing of the tower trays is complete. ⑥Once the washing trays have been cleaned, normal operation can be resumed: the water flow should be reduced gradually, and the reflux flow increased step by step, so as to allow the temperature inside the tower to rise slowly. Do not raise the temperature too quickly. Then, the other operating conditions were gradually adjusted to normal levels, restoring normal production. 23. How to operate the oil-water separation tank at the top of the vacuum distillation tower properly? In the vacuum operation of the vacuum distillation tower’s top oil-water separation tank, its functions are twofold: first, it condenses the medium extracted by the ejector, and separates this condensate into oil and water within the tank; second, thanks to its structure, it creates a certain level of water in the tank, which acts as a water seal for the atmospheric leg, preventing air from entering the vacuum system and thus avoiding a loss of vacuum pressure as well as the risk of explosion. During operation, special attention must be paid to controlling the level of the water interface. If this level is too high, water will overflow into the oil separation and storage tanks, resulting in water being carried along with the oil that is sent out. If the water level is too low, oil and water do not have enough time to separate, and the wastewater discharged will contain oil or emulsified oil-in-water mixture, posing a burden on wastewater treatment. The water level is generally controlled using instruments or a reverse \"U\"-shaped tube device designed to prevent siphoning. It is necessary to check whether the valve at the top of the reverse \"U\"-shaped tube is open and in communication with the atmosphere, so as to ensure that siphoning is indeed prevented; otherwise, if siphoning occurs in this device, it will cause the water level in the container to drop automatically, leading to serious consequences. It is important to regularly check the actual oil-water level and ensure that it matches the value indicated by the instruments, in order to avoid false readings of the liquid level. 24. How to adjust low yield of vacuum distillate oil? The feed to the vacuum distillation tower consists of the heavier fractions of crude oil. Vacuum distillation is used to separate these heavier fractions into various distillates. Whether these distillates are intended to be used as feedstock for cracking or as raw materials for lubricants, it is important to maximize the yield, provided that they meet the specified quality standards. Improving the yield should be based on meeting quality standards. If the yield is low, the following methods can be adopted: ① Increase the vacuum level of the tower. It can reduce the temperature of the cooled fluid in the condensers at each stage of pressure reduction. Systems equipped with multiple stages and numerous injectors allow for an increase in the number of injectors used. If some injectors are not functioning properly, such as when there is steam leakage, efforts should be made to adjust them so that they can operate efficiently and fully utilize their capabilities. ②Appropriately increase the outlet temperature of the vacuum furnace, raise the amount of stripping steam at the bottom of the tower, and improve the vaporization rate of the oil. ③Optimize the distribution of the product and the proportion of heat extraction through mid-stage reflux, so as to prevent excessive local pressure drops on the trays inside the tower; this increases the vacuum level in the vaporization section, thereby raising the yield of distillate oil. Improving the distillation efficiency of the tower allows heavier components to end up in the distillate oil, further increasing the product yield. 25. What is the impact of the vacuum level in the vacuum distillation tower on the operating conditions? The normal and stable operation of a vacuum distillation tower must take place under a stable vacuum level; the degree of vacuum has a significant impact on the vapor-liquid load within the tower as well as on its stable operation. With the oil temperature at the outlet of the vacuum furnace, the feed oil flow rate, the steam flow rate for stripping at the bottom of the tower, and the reflux rate all remaining constant, a decrease in vacuum level alters the pressure-temperature equilibrium of the oil inside the tower. This increases the saturated vapor pressure of the oil, thereby raising its partial pressure and increasing its boiling point; as a result, the vaporization rate of the feed oil decreases, leading to a lower yield. In terms of operation, as the vaporization rate decreases, the reflux in the tower is reduced and the temperatures at each distillation outlet rise. Therefore, when controlling the operating conditions at these outlets, in addition to adjusting the product yield based on changes in vacuum level, it is also necessary to adjust the distillation outlet temperatures accordingly: the temperature can be lowered slightly when the vacuum level is high, while it should be increased slightly when the vacuum level is low. 26. How should sudden interruptions in the crude oil supply to the unit be handled? A sudden interruption in the supply of crude oil to the unit can be caused by the following reasons: ① Low oil level in the crude oil tank, resulting in the crude oil pump running dry. ②When feeding crude oil to the unit for tank transfer, errors made by the operators in the tank area when switching valves, or failures of the valves themselves, along with low external temperatures in winter that cause the crude oil to solidify within the pipelines and prevent smooth flow, can all result in the crude oil pump running dry. ③The crude oil pump stopped due to mechanical and electrical faults. When there is a sudden interruption in the supply of crude oil, the feed to the tower stops, while the pump at the bottom of the tower continues to draw out material. As a result, the liquid level at the bottom of the tower drops sharply. If this is not addressed promptly, the oil pump at the bottom of the tower will run out of fluid, leading to interruptions in the feed to the heater and a sharp increase in the temperature of the oil at the outlet of the heater. In the event of a disruption in crude oil supply, urgent action must be taken to restore the flow of crude oil as soon as possible. This may involve contacting the tank farm to have tanks with higher liquid levels used for processing, and it is necessary to thoroughly check whether there are any issues with the valves and pipelines connected to these tanks. In the event of a pump failure, emergency startup of the backup crude oil pump is carried out, etc. Due to the high flow rate of crude oil, the residence time of oil in the tower is short; once the supply of crude oil is interrupted, it is necessary to reduce the flow rate of oil drawn from the bottom of the tower, lower the temperature of the burners in the heater, and prepare for shutting down the system. Due to the long interruption of crude oil supply, the plant switched to circulation mode. 27. What phenomena occur when the oil-water separation tank at the top of the tower is filled with gasoline? How to handle it? Issues such as a failure in the liquid level control of the oil-water separation tank at the top of the tower, blockages in the pipelines leading out of the unit that prevent gasoline from being delivered, excessively high temperatures at the top of the tower resulting in large amounts of distillate, or the tripping of the pump motor used to transfer the oil from the top of the tower going unnoticed, can all cause the oil-water separation tank to fill up with gasoline, leading to a sudden increase in pressure at the top of the tower. The low-pressure gas at the top of the tower is fed through pipes from the top of the oil-water separation tank to the heater for combustion. Once the tank is filled with gasoline, it also enters the heater to burn, causing the temperature inside the heater to rise sharply. Black smoke is emitted from the heater’s chimney, and gasoline leaks beneath the burner and catches fire, leading to an explosion. Upon discovering that the oil-water separation tank at the top of the tower is filled with gasoline, first close the low-pressure gas valve leading to the heating furnace and switch to direct venting; immediately increase the flow rate of gasoline leaving the facility. If this approach does not work, modify the defective tank to reduce the gasoline level inside it as quickly as possible. In the event of a pump failure, promptly activate a backup pump to lower the temperature at the top of the tower and thereby reduce the amount of gasoline distilled. Once operations return to normal, drain all the oil remaining in the low-pressure gas tank; thereafter, the heater can once again use low-pressure gas as fuel.