E-book materials - catalytic cracking

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I. Overview of the Unit The catalytic cracking process involves the catalytic cracking of feed oil (atmospheric residue oil, vacuum or hydrogenated wax oil) under the action of high-temperature catalysts to produce cracked gas and oil. This cracked gas and oil are then separated into dry gas, liquefied gas, stable gasoline, diesel, reprocessed oil, and slurry through methods such as distillation, absorption, and desorption. The main reactions in catalytic cracking include decomposition, isomerization, hydrogen transfer, aromatization, condensation, and coking. Compared to thermal cracking, it has a high yield of light oils, high octane number gasoline, good diesel stability, and produces olefin-rich liquefied gas as a by-product. II. Device Types There are various types of fluidized bed catalytic cracking units, which can be divided into two main categories based on the relative arrangement of the reactor (or settler) and the regenerator: ① Parallel type with the reactor and regenerator arranged separately ； ②A coaxial type where the reactor and regenerator are stacked together. The parallel type is further divided into two categories: equal-height parallel type and height-different parallel type, depending on the different heights of the reactor (or settler) and the regenerator. The main features of the parallel arrangement at the same height are: ① The catalyst is transported in a dense phase through U-tubes; ② The catalyst circulation between the reactor and the regenerator is regulated primarily by changing the catalyst density at both ends of the U-tubes ； ③The catalyst transported from the reactor to the regenerator, rather than passing through the regenerator’s distribution plate, is directly fed from the dense-phase lift pipe into the fluidized bed on the distribution plate, which reduces wear on the distribution plate. l The parallel high-low type is characterized by a short reaction time, which reduces secondary reactions ； The catalyst circulation is controlled by a slide valve, which offers good flexibility. The characteristics of the coaxial-type configuration are as follows: ① Catalyst transfer between the reactor and the regenerator is controlled by plug valves; ② Vertical lift pipes and 90° corrosion-resistant elbows are used; ③ The feedstock is injected into the lift pipes through multiple nozzles. III. Process of the catalytic unit (taking the vertical lift tube catalytic cracking unit shown in the figure below as an example): In the reaction-regeneration section, fresh feed oil is mixed with recycled slurry after heat exchange; it is then heated to 180–320°C in a heater before being sent to the nozzles at the bottom of the catalytic cracking lift tube reactor. There, the feed oil is atomized by steam and sprayed into the lift tube, where it comes into contact with the high-temperature catalyst (600–750°C) from the regenerator, vaporizes, and undergoes reactions. The residence time of oil and gas in the riser is very short, usually only a few seconds. The reaction product leaves the settler for the fractionation tower after the entrained catalyst has been separated out by a cyclone separator. The catalyst loaded with coke (referred to as raw catalyst) falls from the settler into the stripping section below. The stripping section is equipped with multiple layers of herringbone baffles, and superheated steam is introduced at the bottom; the oil and gas present in the space between the oil and gas adsorbed on the raw catalyst and the particles are displaced by the steam and sent back upward. The stripped green catalyst enters the regenerator through the green inclined tube. The main function of the regenerator is to burn off the carbon deposits that form on the catalyst as a result of reactions, thereby restoring the catalyst’s activity. The air for regeneration is supplied by the main fan, and it enters the fluidized bed layer through the auxiliary combustion chamber and distribution pipes located below the regenerator. In heat-balanced systems, the auxiliary combustion chamber is used only during startup and heating up; it does not burn fuel oil during normal operation. The regenerated catalyst (referred to as the regenerated catalyst) is dropped into the flooded tube and sent back to the reactor via the regeneration inclined tube for reuse. After the regenerated flue gas has had the entrained catalyst separated out by a cyclone separator, it is discharged into the atmosphere through a double-acting slide valve. When processing feedstocks with a high coking rate, such as those containing residue oil, the high coke yield results in excess heat in the regenerator; therefore, heat removal facilities must be installed in the regenerator to remove this excess heat. The temperature of the regenerated flue gas is very high. Many catalytic cracking units are equipped with flue gas energy recovery systems that utilize the thermal and pressure energy of the flue gas (when such a system is installed, the operating pressure of the regenerator should be set at a higher level) to perform work and drive the main blower, thereby saving electrical energy. In some cases, any excess electricity generated can even be fed back into the grid. For some devices that do not regenerate completely, the regenerated flue gas contains 5%-10% (by volume) of CO; a CO boiler can be used to burn this CO completely in order to recover energy. During the production process, catalysts are lost and become deactivated; to maintain the amount of catalysts present in the system as well as their activity, it is necessary to regularly or frequently supply or replace them with fresh catalysts. To this end, at least two catalyst storage tanks should be provided within the unit. A dilute-phase conveying method is used for loading and unloading the catalyst, with compressed air as the conveying medium. In the automatic control system of a fluidized catalytic cracking unit, in addition to automatic control systems for temperature, pressure, flow rate, etc., which are similar to those in other refining units, there is also a complete set of automatic control systems to maintain proper catalyst circulation, as well as an automatic protection system in case of abnormal fluidization. This system generally includes multiple safety systems, such as the reactor feed low flow rate safety system, the main fan outlet low flow rate safety system, the differential pressure safety system for the two vessels, and so on. Take the low-feed-rate safeguard system for the reactor feed as an example: when the feed rate falls below a certain threshold, it becomes impossible to achieve a sufficiently low density within the riser. This disrupts the normal pressure balance between the two reactors; the catalyst fails to circulate along its designated path. Additionally, catalyst backflow occurs, causing large amounts of oil and gas to be carried into the regenerator, thereby leading to accidents. At this point, the low-flow feed auto-protection system automatically takes the following actions: it shuts off the reactor feed and returns it to the crude oil tank (or intermediate tank), and introduces emergency steam into the lift pipe to maintain the fluidization and circulation of the catalyst. l Fractionation system: A typical catalytic cracking fractionation system is shown in the figure above. The reaction products (oil and gas) from the reactor enter the fractionation tower from the bottom; after passing through the de-superheating section at the bottom, they are separated into several intermediate products in the fractionation section: at the top of the tower are rich gas and gasoline, while on the side draws there are light diesel oil, heavy diesel oil, and recycle oil. The product at the bottom of the tower is slurry. Light diesel and heavy diesel are stripped separately, and then pass through heat exchange and cooling before leaving the plant. The fractionation tower in a catalytic cracking unit has several characteristics: ① The feed is superheated oil and gas containing catalyst dust; therefore, a de-superheating section is installed at the bottom of the fractionation tower, where the oil and gas are cooled to saturated condition using cooled slurry, and the entrained dust is removed in order to facilitate fractionation and prevent blockage of the tray plates. ②The remaining heat in the entire tower is high, and the requirements for precise product separation are relatively easy to meet. Therefore, multiple circulation returns are generally provided: tower top circulation return, 1–2 mid-stage circulation returns, and slurry circulation. ③At the top of the tower, recycle reflux is used instead of cold reflux. The main reason for this is that the oil vapor entering the fractionation tower contains a considerable amount of inert gases and non-condensable gases, which can affect the efficiency of the overhead condenser/cooler. Using recycle reflux in place of cold reflux can reduce the pressure drop from the top of the fractionation tower to the inlet of the compressor, thereby increasing the compressor’s inlet pressure and decreasing its power consumption. The absorption-stabilization system – the absorption-stabilization system is mainly composed of an absorption tower, a reabsorption tower, a desorption tower, and a stabilization tower. The rich gas coming out of the overhead oil-gas separator of the fractionator contains gasoline components, while C3 and C4 components are dissolved in the crude gasoline. The function of the absorption stabilization system is to use absorption and distillation methods to separate the rich gas and crude gasoline into dry gas (≤C2), liquefied gas (C3, C4), and stabilized gasoline with acceptable vapor pressure. The liquefied gas is then processed using distillation; through a gas fractionation unit, propylene and butylene are separated from it for use in chemical industries. The fractionation system and absorption-stabilization system of catalytic cracking units generally show little difference among various catalytic cracking plants. 1- Absorber tower ； 2-Analysis Tower ； 3-Reabsorption tower ； 4-Stabilizer Tower ； 5-Balancing tank ； 6-Condenser or cooler ； 7-Heat exchanger ； 8-reboiler ； 9-Reflux tank ； Product refining desulfurization system (alkaline washing desulfurization method) – Gasoline desulfurization and denaturation section: Principle of desulfurization: This process makes use of the neutralization reaction between an alkaline solution and the acidic substances present in the oil, thereby removing hydrogen sulfide. The reaction equations are as follows: H2S + NaOH = NaHS + H2O; NaHS + H2S = Na2S + H2O. When the concentration of the alkaline solution is low, sodium hydrosulfide, an acid salt, is primarily formed; whereas when the concentration of the alkaline solution is high, sodium sulfide, a normal salt, is produced. Principle of mercaptan removal: Under the action of a catalyst in an alkaline medium, mercaptans in petroleum products are oxidized to form sulfur dioxide, which dissolves in gasoline. This process effectively removes mercaptans from gasoline. Desulfurization section for dry gas and liquefied petroleum gas: The liquefied petroleum gas and dry gas produced by the fluid catalytic cracking unit contain harmful impurities such as hydrogen sulfide and sulfur dioxide, which affect the usability of the products. Therefore, desulfurization must be carried out prior to their use. The commonly used method for hydrogen sulfide removal is the alkanolamine absorption method, in which a weak organic base (amine solution) is used as the absorbent; this absorbent comes into countercurrent contact with the gas in both the LPG desulfurization tower and the dry gas desulfurization tower. Hydrogen sulfide in liquefied petroleum gas and dry gas is absorbed by an amine solution, thereby purifying the gas. The absorption of hydrogen sulfide by amine solution is a reversible process. The amine-rich liquid that has absorbed hydrogen sulfide decomposes when heated under low pressure, releasing hydrogen sulfide again. By utilizing this reversible chemical reaction, the amine-rich liquid is regenerated into an amine-poor liquid in a solvent regeneration tower, while acidic gas containing hydrogen sulfide is produced. The lean amine solution is recycled as an absorbent, while the acidic gas is sent to downstream units for treatment. LPG desulfurization and dewaxing section: In LPG desulfurization and dewaxing, a catalyst and alkaline solution are used to extract thiols from the LPG; these are then oxidized and regenerated, followed by the separation and removal of disulfides, with the catalyst and alkaline solution being reused. V. Main components of the catalytic unit: 1. Reaction-regeneration section: Both the cracking of the feed oil and the regeneration of the catalyst take place in this section; the yield of the product is determined in the reaction section. l Fractionation unit: Its task is to change the mixed oil vapor coming from the settler from a superheated state to a saturated state, wash the catalyst carried by the reacted oil vapor, and separate it into gas-rich fraction, crude gasoline, light diesel oil, heavy diesel oil, recycle oil, and slurry according to boiling point ranges. l Absorption stabilization unit: Its task is to further separate crude gasoline and compressed rich gas into dry gas, liquefied gas, and stabilized gasoline, to maintain pressure balance in the fuel gas system, to ensure the unobstructed operation of the high- and low-pressure gas flare systems, and to allow the flares to burn properly. Refined desulfurization unit: Its task is to carry out alkaline washing desulfurization of gasoline, diesel, and liquefied gas products to ensure that the product quality meets standards. With the development of manufacturing processes, catalytic gasoline and diesel are no longer subjected to direct alkaline washing; instead, they go through hydrogenation units for desulfurization. Unit shift: The task of the main fan is to supply air for the coking and regeneration of the catalyst in the reaction system. The compressor compresses the rich gas from the top of the distillation tower and sends it to the stabilization system. The booster pump increases the pressure of a portion of the main air, providing the motive air needed for the fluidization of the external heat exchanger and the regeneration sleeve. VI. Functions of the main equipment in each system of the catalytic unit
Reaction and regeneration system
1. Pre-lift section: This ensures that the catalyst forms a “plug flow” in the lower and middle portions of the lift pipe, with optimal velocity and density, thereby facilitating its thorough contact with the feedstock. l Feed nozzle: At an appropriate preheating temperature of the feed oil, the feed oil is atomized into small droplets through the action of vaporization, so as to ensure full contact with the catalyst. l Lift pipe reactor: It is the place where feed oil comes into contact with a high-temperature catalyst for catalytic cracking. The structural features of the riser reactor enable better control over the reaction time, preventing backmixing and reducing secondary reactions. Coarse spinning rapid separation and rapid oil-gas extraction technologies are used in the oil-gas-catalyst separation device at the outlet of the riser, to reduce the contact time between oil gas and catalyst, prevent secondary reactions, and minimize over-cracking and thermal cracking. Stirring section: The catalyst, after being separated by rough stirring, descends to the stirring section where it comes into contact with stripping steam in the opposite direction; this process displaces the oil and gas contained within the catalyst particles as well as those attached to the catalyst itself, thereby increasing the yield of light oils and reducing the production of dry gas and coke. l Regenerator: Its main function is to burn off the carbon deposits on the surface of the catalyst to be regenerated, restore the catalyst’s activity, and provide the heat required for the cracking reaction of the feedstock. l Auxiliary combustion chamber: fueled by gas (diesel), with primary and secondary air supplied to it. Primary air enters the combustion chamber to provide the oxygen required for combustion, while secondary air cools the furnace through the jacket. After mixing, primary and secondary air together help control the temperature at the furnace outlet. The auxiliary combustion chamber is used only to heat the regenerator during startup; during normal operation, it serves merely as a passage for the main air flow. l Main air distribution pipe: Its primary function is to ensure that the main air is distributed evenly across the entire cross-section of the bed, thereby creating favorable conditions for fluidization and combustion. Cyclone separator: As the catalyst moves from the two-phase state to the dilute phase, it carries some of the catalyst with it. The flue gas containing the catalyst enters the cyclone separator in a tangential direction; this creates a rotating outer vortex between the rising gas duct and the shell. From top to bottom, up to the bottom of the cone, the catalyst suspended in the flue gas is forced toward the wall by centrifugal force, while also rotating downward along with the flue gas flow. Eventually, it falls into the ash hopper and is returned to the regenerator’s dense phase through the material leg. The purified flue gas forms an upward internal vortex and is discharged through the rise pipe. l External heat exchanger: The catalyst is drawn out from the two-phase bed, enters the top of the external heat exchanger via inclined tubes, and under the action of the fluidizing air it passes through the external heat exchanger’s shell side, where it exchanges heat with the deoxygenated water in the tube side before returning to the bottom of the coking tank. This mechanism serves to remove the excess heat from the regeneration system, and it is the main means of regulating the temperature of the first phase and that of the second phase. l Waiting valve: Its function is to control the inventory in the stripping section. l Double-acting spool: Its function is to control the regenerator pressure. l Regeneration slide valve: Its function is to control the flow rate of the regenerated catalyst in order to adjust the reaction temperature. l Waste heat boiler: The flue gas from the regenerator (at around 680°C) enters the shell side of the waste heat boiler, where it exchanges heat with the fluid in the superheating section, evaporation section, and preheating section of the economizer. After being cooled to below 180°C, it is discharged into the chimney. The use of waste heat boilers allows for the full utilization of the thermal energy in flue gas, thereby reducing the energy consumption of the facility. Fractionation system: Fractionation tower – By utilizing the principles of distillation, and with the use of reflux, the vapor-liquid phases inside the tower come into contact with each other multiple times on the tray surfaces, allowing for mass and heat transfer between the phases. Through repeated processes of partial vaporization and partial condensation, the lighter components accumulate at the top of the tower, while the heavier components accumulate at the bottom; thus, the various components in the mixture can be effectively separated. l Diesel stripper: In the stripper, stripping steam enters from the bottom and comes into countercurrent contact with diesel oil. This process strips the light components from the diesel oil, thereby controlling its flash point. l Slurry filter: The slurry passes through the filter elements in the slurry filter to have the catalyst it contains removed, and then is cooled in the slurry cooling tank before being sent to the storage area as fuel for combustion. Rinse the filter element with reprocessed oil; the backwash oil is sent to the reaction feed nozzle and the emergency bypass line. Assume that the two filter tanks operate intermittently. Stabilization system l: Stabilization tower – uses absorption, desorption, and distillation methods to separate compressed rich gas and crude gasoline into dry gas, liquid hydrocarbons, and stabilized gasoline of satisfactory quality. Unit system l Main fan: Air must be supplied to the bottom of the regenerator in the reaction and regeneration system, and the fan that compresses and delivers this air is the main fan. The main fan is extremely important in the catalytic unit; it is the core equipment of the unit and must operate continuously. If the main fan stops working, the catalytic unit will cease production. From a process perspective, the main function of the main fan is: A: to provide the oxygen required for coking. B: Ensure that the catalysts in the regenerator and coking drum are in a fluidized state. l Compression unit: The compression unit is primarily used to process the rich gas coming from the overhead vapor separator of the distillation tower – a gas with a pressure of 0.06 MPa and a temperature of 40°C. After being compressed in the first stage of the compressor (to a pressure of 0.4 MPa and a temperature of 100°C), it passes through an intermediate cooler before entering the intermediate liquid separation tank for further separation. The gas enters the second stage, where the pressure is increased further (to 1.25 Mpa and the temperature to 107°C), and then it is sent to the stabilization unit via the outlet of the second stage. While compressing the gas, this unit is also responsible for controlling the reaction pressure. Under normal conditions, the unit speed is adjusted through the reaction pressure to achieve control of the reaction pressure. Additionally, there are two flare valves of different sizes at the compressor inlet, used to assist in regulating the reaction pressure. There is a reverse flutter valve at the compressor outlet to prevent the compressor from fluttering. l Pressurizer: It pressurizes a portion of the main air, providing air for lifting and fluidization to the external heat exchanger, as well as air for fluidization to the raw catalyst sleeve. VII. Main adjustment methods and influencing factors of the catalytic unit 1. Control of reaction temperature and its influencing factors: Reaction temperature is an important process parameter in the reaction section; an appropriate reaction temperature ensures that the unit can achieve maximum yields or production of the target product. During normal operation, the reaction temperature is controlled by adjusting the opening degree of the regeneration slide valve, thereby controlling the catalyst circulation rate. When the lift medium remains constant, the higher the reaction temperature, the greater the reaction depth; as a result, the yields of gasoline, liquefied gas, dry gas, and coke increase while the yield of diesel decreases. There are many factors that affect the reaction temperature, such as: changes in the opening degree of the single-acting slide valve; changes in the total feed volume to the reaction; changes in the preheating temperature of the feed; changes in the temperature of the regenerator bed; moisture content in the raw materials; excessive amount of steam used for pre-heating or atomizing the feed, or steam containing moisture; use of a small-sized feeder; blockages or coking in the lift pipe; moisture content in the terminator gas or liquid carried in the dry gas; changes in the amount and temperature of the terminator; changes in the amount of dry gas used for pre-heating; large fluctuations in the pressure difference between the two vessels; malfunctions of instruments and the single-acting slide valve. 2. Control of the pressures in the two vessels and the factors that influence them. The pressures of these two vessels refer to the pressure in the settler and the pressure in the regenerator. During normal operation, the pressure in the regenerator is 40 KPa higher than that in the settler. The pressure difference between these two vessels serves as an important driving force for catalyst circulation. If this pressure difference reverses, it will cause the catalyst to flow in the reverse direction, with the reaction gases entering the regenerator, which can lead to equipment overheating or even serious accidents. During normal production, the pressure in the settler is controlled by the speed of the air compressor and the opening degree of the anti-surge valve. The main factors affecting this pressure include changes in the medium flowing into the lift pipe, variations in the liquid level of the fractionation tower or the cold reflux, fluctuations in the speed of the air compressor, and water content in the crude oil. The regenerator pressure is controlled by the opening degree of the double-acting slide valve; factors affecting it include changes in the medium entering the regenerator, catalyst contamination with oil, and failures in the external heat exchangers. 3. Control of the inventory in the stripping section and its influencing factors. The catalyst inventory in the stripping section refers to the amount of catalyst present in the equipment of that section. Controlling this inventory is intended to facilitate better displacement of the oil and gas from the catalyst to be regenerated, as well as to prevent the main air flow from entering the settler. The inventory index in the stripping section is controlled by the opening degree of the raw material stop valve; if the pressure in this section drops, main air will flow into the settler, leading to serious accidents. 4. Principles for handling accidents at the reaction station. Under any circumstances, the catalyst reserves in the reaction-regeneration systems must not exhaust each other. The reaction-regeneration system must be fed with a fluidizing medium to prevent collapse or dead bed. l Once the main air supply is interrupted, the feed must be stopped immediately, and the combustion oil injection must be halted. Under feeding conditions, the exit temperature of the riser must not be lower than 480°C; if it is not possible to increase this temperature temporarily, the feed should be stopped. When the main fan is stopped and the feed is cut off, the temperature of both reactors must remain above 400°C; if it drops below this level, the catalyst should be removed. l In a reaction-regeneration system with catalyst circulation, oil slurry circulation must be maintained. If the slurry circulation is interrupted for an extended period, it should be switched to single-regenerator fluidization or, depending on the circumstances, the catalyst should be discharged. The feed should be cut off in case of severe overheating in the regenerator or a fire accident in the reaction regeneration system. In the reverse-regeneration system, even if the feed is cut off and the main fan stops operating, as long as there is still catalyst in the system, the backblowing air and loosening air (steam) must continue to be supplied without interruption. 5. Control of the distillation column bottom liquid level and its influences. The level of the liquid at the bottom of the distillation tower reflects changes in the material balance throughout the tower, and this material balance is determined by pressure, temperature, and flow rates. Additionally, the liquid level at the bottom of the tower serves as an indicator of the degree of reaction completion. An excessively low liquid level can lead to cavitation in the slurry pump, disrupting the circulation of the slurry and resulting in accidents such as tower overpressure and overheating ； High liquid levels cause the slurry to remain in the bottom of the tower for a longer time, leading to coking at that location. Additionally, it is possible for the feed inlet for the overheated reaction gases and oils to be submerged, resulting in pressure buildup in the reaction system and serious consequences. The influencing factors include: l changes in reaction depth; as the depth increases, the liquid level decreases. l Changes in processing capacity: as the processing capacity increases, the liquid level rises. l Changes in the properties of the raw material: it becomes lighter, and the liquid level drops. l Changes in the temperature of the slurry returning to the tower: as the temperature decreases, the liquid level rises. l Changes in the amount of slurry recycled and the amount discarded; as the amount recycled and discarded increases, the liquid level decreases. l Changes in the pressure of the fractionation tower: as pressure increases, the liquid level rises. l Changes in the bottom temperature of the fractionation tower: as the temperature rises, the liquid level decreases. l Pump malfunctions; instruments fail. l Changes in the amount of slurry flowing up and down the tower. l Variations in the circulation rates and heat extraction of each stream in the fractionation tower, or pump cavitation. 6. Control of the solid content in oil slurry and its influencing factors. During normal production, the solid content in the slurry should be kept at ≤8.0 g/l. If the solid content in the slurry is high, it can cause equipment wear or pipeline blockages, leading to production accidents; therefore, the solid content in the slurry must be controlled during normal operation. The level of solid content in the slurry depends primarily on the separation efficiency of the cyclone separators in the reaction settler. The influencing factors include: large pressure fluctuations in the settler, which cause the oil and gas to carry excessive catalyst. The efficiency of the swirl separator inside the settler deteriorates or it malfunctions. The catalyst strength is not high or there is a lot of dust. l The oil slurry is not discharged externally for a long time, or the amount recycled is too small. l The liquid level at the bottom of the tower is high, and the residence time is too long. l The temperature at the bottom of the tower is high, resulting in severe coking. 7. Control of the dry point of crude gasoline. During normal production, the dry point of crude gasoline is primarily controlled by the temperature at the top of the distillation tower; the higher this temperature, the higher the dry point of the crude gasoline. The factors that influence it include the temperature at the top of the distillation tower, pressure changes, changes in the amount of material recycled to the middle section as well as the distillation temperature of light diesel, changes in the volume of material circulated to the top of the tower and the temperature of the material returning to the tower, as well as changes in the degree of reaction and the volume of material processed. 8. Control of diesel dry point and influencing factors. The dry point of diesel is primarily controlled by the temperature in the middle section of the distillation tower and the opening degree of the valve at the outlet for the lower fractions of diesel. The factors that influence it include changes in the degree of reaction, variations in the properties of the feed oil, changes in the amount of material returning to the middle section of the tower and in the temperature of that return flow, as well as changes in the pressure within the distillation tower. 9. Control of the content of components above C3 in dry gas. It has a high gas content, low absorption dose, and poor absorption efficiency. The cooling effect of E2301 and E2302 is poor; the rich gas enters the tower, and the temperature of the crude gasoline or stabilized gasoline entering the tower is high. In stage l, the amount of material is low, or the cooling effects of E2303 and E2304 are poor, resulting in high temperatures inside the tower. The pressure in the absorption tower is low or experiences significant fluctuations. The high temperature in the desorption tower causes a large amount of C3 and C4 to be desorbed, increasing the absorption load. l The temperature in the reabsorption tower is high or the absorption dose is low. l Low pressure or significant fluctuations in T2303. l Absorption tower flooding. l The liquid level at the bottom of the reabsorption tower is too high. 10. Control of the content of components above C5 in liquefied gas. l The reflux flow at the top of the stabilizer is low, and the temperature at the top of the tower is high. The pressure in the stabilizer tower is low. l The temperature of the reboiler return stream to the bottom of the stabilizer is high. l The thermal bypass adjustment range for gases is large. The feed temperature to the stabilizer tower is high, with the feed inlet located high up.