Aromatic extraction training

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Principles of Aromatic Extraction 1. Introduction The aromatic extraction unit serves as a bridge from oil refining to the chemical industry. It can improve high-purity basic organic chemical raw materials such as B, T, X, etc. The principle of the aromatic extraction process is to separate aromatics from non-aromatics through solvent extraction. They are mainly divided into the Udex method (glycol solvent), Sulfolane method (cyclosulfurone solvent), Arosolvan method (N-methylpyrrolidone solvent), DMSO method (dimethyl sulfoxide solvent), and Formex method (N-formylmorpholine solvent). Old plants in our country use the Udex method, while most new plants use the Sulfolane method. In recent years, with the increasing demand for mono-aromatic compounds (mainly pure benzene), an extraction distillation process has developed rapidly; the RIPP patented process has been put into industrial use at several refineries in China. For this project, the scale of our company’s aromatic extraction unit is 350,000 tons per year (based on the feed volume). The extraction process utilizes the Sulfolane method – which involves the use of sulfolane as a solvent – and this is the same process used in the existing continuous reforming units. It is a mature technology with extensive operational experience behind it. Product requirements: Serial number | Item | Value 1 | Benzene yield, % | 99.92 | Toluene yield, % | 99.53 | Quality of benzene product | Grade A according to GB3405-89 4 | Quality of toluene product | Grade A according to GB3406-90 Main feed and output materials for the aromatic hydrocarbon extraction unit: Serial number | Feed | Output 1 | Reformate C5–C7 | Mixed aromatics (benzene, toluene) from the xylene unit 2 | Isomerization | Light hydrocarbons, non-aromatic compounds 3 | Adsorption separation | Toluene from *C5 fraction*; toluene from adsorption separation, which is then fed into the mixed aromatics tank along with the aromatics obtained through extraction, and sent to the disproportionation unit. Schematic diagram of the aromatic extraction unit: Section 1 – Principles of aromatic extraction. Extraction, also known as liquid-liquid extraction, is a method of separation that takes advantage of the differences in solubility of various components in a liquid mixture within a certain solvent. Aromatic extraction is a process of separating aromatics from hydrocarbons using liquid-liquid extraction. Like adsorption and distillation, extraction is also a physical separation method. The material to be extracted is a mixture; upon the addition of sulfolane, the aromatics in the oil dissolve into the solvent, thereby forming two liquid phases with different compositions and densities – namely the oil phase and the solvent phase. The oil phase contains a small amount of aromatics and has a low density, while the solvent phase contains a large amount of aromatics and has a high density. By carrying out multiple successive countercurrent contact extractions using a sieve tray tower, high-purity aromatics can be obtained. The main factors affecting the extraction process There are many factors that influence the extraction process, which can be summarized into three elements: the crude oil to be extracted, the solvent, and the methods used (equipment, operating conditions, etc.). After the solvent and equipment structure are selected, operating conditions play an important role. Below, in conjunction with the aromatic extraction process, the impact of the above three factors on the extraction process is discussed separately. 1 Influence of solvent properties 1.1 Distribution coefficient kc In the extraction process, the distribution coefficient is often used to represent the relationship between the concentrations of the solute in the two coexisting phases at equilibrium. The distribution coefficient kc is defined as: kc = CE/CR, where CE is the concentration of the solute in the extraction phase (E) at equilibrium ； CR — the concentration of the solute in the raffinate phase (R) at equilibrium. It is clear from the above equation that a large distribution coefficient KC facilitates extraction; therefore, we should choose a solvent extractant with a high distribution coefficient. 1.2. Solvent’s solvating power The solvating power refers to the affinity between the solute and the solvent. Currently, solubility parameters are widely used in industry to represent the solvating power of solvents. There are van der Waals forces between liquid molecules, and it is these forces that enable them to aggregate into a liquid state; this force is also known as cohesive force. For one mole of liquid, the molar cohesive energy ΔE = H – RT, where ΔH is the molar heat of vaporization (calories per mole) ； ΔE —— Molar cohesive energy (cal/mol) ； RT —— The work done due to the expansion of steam volume during vaporization. The cohesive energy per unit volume of a liquid is called the cohesive energy density; thus, cohesive energy density = ΔE√ΔV = (H – RT)D/M. The solubility parameter δ is given by δ = √ΔE/ΔV = √(H – RT)d/M. Here, V represents the molar volume (ml/mole); R is the universal gas constant, equal to 1.987 (cal/K·mole); T is the absolute temperature; M is the molecular weight (g); and D is the density (g/ml). Since the dissolution of two substances involves attraction between different molecules, and the solubility parameter is a measure of the cohesive energy density of a substance (i.e., the intermolecular forces), the solubility parameter can be used to determine whether two substances will dissolve in each other. When the solubility parameters δ of the solute and solvent are similar, it indicates that the intermolecular forces between them are similar to those within the same molecule, and therefore the solvent has a stronger ability to dissolve that solute. In the aromatic extraction process, the solvating power of a solvent is often expressed as the reciprocal of the activity of the aromatics in that solvent. The so-called activity coefficient is a measure of the deviation of a solution from an ideal solution. The activity coefficient r is the ratio of the actual concentration C of the solution to its effective concentration a (i.e., activity), that is, r = a/C. When the solute and solvent form an ideal solution, the activity coefficient r=1 ； When there is a positive deviation from an ideal solution, the activity coefficient > 1 ； If there is a deviation from an ideal solution, the activity coefficient r << 1. Systems that can form partially miscible systems are all systems with positive deviation from ideal solutions. The greater the positive deviation, the smaller the reciprocal of the activity coefficient, meaning the lower the solubility. In the aromatic extraction process, 1/r toluene is generally used to represent the solvating power of the solvent. Additionally, the concept of hydrocarbon load is also commonly used in industry. The hydrocarbon load of the solvent indicates the fraction of the total hydrocarbons present in the solvent/hydrocarbon mixture. Its difference from solubility lies in the fact that solubility is merely an assumption made when there is equilibrium between a pure toluene phase and the solvent. Q+BW hydrocarbon load: HCL=2ZHC= S+Q+BW. Where: BW – backwashing dosage; Q – material to be extracted; S – amount of solvent used. 1.3 Selectivity of the solvent: As can be seen from the above, using only the distribution coefficient is not sufficient to assess the performance of an extractant; it is closely related to the extractant’s selective solvating ability, namely the selectivity coefficient β (which has properties similar to relative volatility in distillation). It is defined as: β=（YAE/YBE）/XAR /XBR =KA /KB, where YAE/YBE represents the ratio of the concentration of solute (A) to that of the original solvent (B) in the extract phase (E) ； XAR/XBR —— the ratio of the solubility of solute (A) to that of the original solvent (B) in the raffinate phase (R) ； KA —— distribution coefficient of solute (A) ； KB —— Distribution coefficient of the original solvent (B) ； When β=1, then K = K; the extract E and the raffinate R will have the same composition as the feed solution, and thus cannot be separated by extraction. When β > 1, then KA > KB, and extraction is possible; the larger the β value, the easier the extraction separation becomes. When β < 1, then KA < KB, and the extraction process can still proceed; however, what is extracted is not the solute A but rather the original solvent B. If the desired product is A, then β < 1 does not meet the requirements. In the aromatic extraction process, two concepts are used when the selectivity coefficient β is applied in practice: group selectivity and light/heavy selectivity. The group selectivity of solvents depends on the polarity of the solvent molecules and the order of affinity for different types of hydrocarbons; this order of affinity is the same for all solvents. The order of decreasing affinity is: polycyclic aromatics > monocyclic aromatics > polycyclic alkanes > alkenes > alkanes. However, the order of decreasing affinity corresponds exactly to the order of increasing activity coefficients (r). Therefore, the selectivity β (A/P) of aromatics relative to alkanes can be conveniently defined as the ratio of the activity coefficient of a non-aromatic compound to that of an aromatic compound: Group selectivity: β (A/P) = rP/rA. Next, the relative affinity of hydrocarbons within the same group can be referred to as light/heavy selectivity β (L/H); the selectivity of light hydrocarbons over heavy hydrocarbons can also be defined as the ratio of the activity coefficient of heavy hydrocarbons to that of light hydrocarbons: Light/heavy selectivity: β (L/H) = rHeptane/rHexane. Generally, affinity decreases as the number of carbon atoms increases. 1.4. Selection of solvent For the extraction process, the selection of solvent is very important. Choosing a suitable solvent is key to the success of the extraction process. Moreover, this solvent should be easy to recover, have low consumption of utility resources, and be less corrosive. Therefore, when selecting a solvent, the following aspects should generally be taken into consideration: the solvent’s selectivity should be high enough; the higher, the better ； The density difference between the solvent and the aromatic hydrocarbons should be large to prevent emulsification, which facilitates counter-current operation ； The solvent should have a high solubility for aromatics in order to reduce the amount of solvent needed and the operational costs ； The solvent itself must possess good chemical stability, thermal stability, and antioxidant stability, and must not undergo chemical reactions with the raw materials, so as to ensure its reuse ； The difference between the boiling point of the solvent and that of the solution should be large, so as to prevent the formation of azeotropes and to enable the separation of the product from the solvent using simple distillation methods ； The latent heat of vaporization and specific heat of the solvent should be low to reduce heat consumption during solvent recovery ； The interfacial tension between the solvent and the slurry should be high to facilitate the coalescence and stratification of droplets ； The solvent should be non-foaming, not corrosive to equipment, non-toxic, not prone to explosion or ignition, inexpensive, and readily available. The viscosity of the solvent should not be too high to facilitate mass transfer. Among the above factors, the most important indicators are the solvent’s ability to dissolve aromatics and its selectivity. The solvent used in this device is sulfolane, which boasts advantages such as a high density, high boiling point, low specific heat, strong thermal stability, and low corrosivity to carbon steel. Sulfolane is a highly polar solvent with excellent chemical and stability properties; it can mix with water and is also a good solvent for many organic compounds as well as various common polymers. Molecular formula: C4H8SO2; molecular weight: 120. 17 Density (200°C): 1270 Kr/m3. Boiling point at atmospheric pressure: 2850°C. Decomposition temperature: 2000°C. Flash point: 1770°C. Freezing point: 27.80°C. Critical pressure: 5.3 MPa. Critical temperature: 4270°C. Chemical properties of sulfolane: Sulfolane generally does not react with chemicals such as acids, thiol compounds, and dienes; it does not polymerize or decompose in the presence of acids or bases. When sulfolane was heated with sodium carbonate, sodium acetate, an 25% sodium hydroxide aqueous solution, as well as copper and iron at reflux temperature for five hours, no reaction between them was observed. At 140–150°C, a reaction occurs between sulfolane and 93% sulfuric acid; it can also react with aluminum chloride and hydrogen sulfide to produce hydrogen chloride and hydrogen sulfide. Sulfolane is not reduced by metallic zinc and acetic acid hydrochloride; however, it can be reduced to sulfur by lithium aluminum hydride. Sulfolane exhibits good thermal stability below 220°C; at this temperature, it slowly produces sulfur dioxide and unsaturated compounds (possibly polymers), causing the solution to turn brown. Sulfolane was subjected to corrosion testing at 200°C, and the results showed that its corrosive effect on carbon steel is negligible (about 1 mm/year). Sulfolane is a slightly toxic compound that does not irritate the skin. Its oral toxicity, as indicated by the LD50 (lethal dose for 50% of the population), is 500–50,000 mg/kg in mice, 1,900–2,500 ml/kg in rats, and greater than 2,820 ml/kg in rabbits. Exposure of rabbit skin to sulfolane for 24 hours does not cause any irritation. Effect of sulfolane on clay: The concentration of sulfolane in the extracted product is less than 200 ppm, so it does not affect the activity of the clay nor is it adsorbed by it; only when the concentration exceeds 500 ppm is some sulfolane adsorbed by the clay. If white clay is introduced for a long time, it will lose its activity. For short-term passage, higher sulfolane concentrations may also be possible. Such experiments were conducted during production: material containing 1% sulfolane was fed into an alumina tower for several hours, and then material with a lower sulfolane concentration was passed through the alumina for some time; the alumina was still able to regain its activity. Generally speaking, when a dose of 2000 pm is reached, only minor effects occur, and increasing the temperature of the white clay can improve these effects. Currently, people take advantage of the insensitivity of sulfolane to bentonite by reducing the reflux flow in the solvent recovery tower, thereby saving on utility consumption. It is estimated that a 25% reduction in the reflux flow, with a sulfolane content in the tower top distillate ranging from 100 to 200 ppm, results in a 5% reduction in utility consumption. Quality specifications for sulfolane: Density (at 30°C): 1260–1270 kg/m3; Sulfur (by weight): 26.0–27.0%; Thermal stability: ≯20 mg SO2; Water (by weight): ≯3%; Ash content (by weight): ≯0.1%; Sulfolene-2 (by weight): ≯0.3%. Handling and storage of sulfolane: Sulfolane is quite easy to handle and store; in most cases, carbon steel containers are sufficient for storage. Due to the presence of trace amounts of sulfur dioxide, it forms a weakly acidic solution with a pH of 3–4. Therefore, a small amount of alkylolamine must be added during storage to maintain a pH level above 7. Sulfolane has a relatively high freezing point of 27.8°C. For ease of loading, unloading, and storage, 3% H2O can be added to lower the freezing point, and the storage container should be properly insulated to prevent storage below the freezing point. 2. Influence of the hydrocarbon composition of the extraction feed oil 2.1. Influence of the hydrocarbon composition of the extraction feed oil Generally speaking, as the water content in the solvent increases, the dissolving capacity decreases while selectivity increases; whereas as the extraction temperature rises, the dissolving capacity increases and selectivity decreases. Since the selectivity difference between aromatics and naphthenes is smaller than that between aromatics and alkanes, the higher the content of naphthenes in the feedstock, the more difficult the separation becomes, and the greater the number of theoretical plates required. 2.2. Influence of the distillation composition of the feed oil In addition to the composition of the feed hydrocarbons, the distillation composition of the feed also has a significant impact on the extraction process. During aromatic extraction, since lighter components exhibit higher selectivity, the heavier the feedstock, the lower the yield of aromatics. For example, when the initial boiling point of the feedstock increases from 65°C to 78°C, the yield of aromatics drops from 95% to 88%. A low initial boiling point indicates an increased content of light hydrocarbons (such as alkanes) in the crude oil. Since light hydrocarbons have a high solubility in solvents, they can displace heavy non-aromatic hydrocarbons; these heavy non-aromatic hydrocarbons can be easily stripped out in a stripping tower, which facilitates the recovery of aromatics and ensures their purity. The drawback of a too high initial boiling point is that heavy non-aromatic compounds are not easily displaced. To ensure the quality of aromatics, it is necessary to sacrifice some of the aromatics recovery rate; therefore, the actual maximum achievable aromatics recovery rate in production is slightly lower. However, the lower the initial boiling point of the crude oil, the higher the solvent ratio required to achieve the same level of aromatic recovery; therefore, the initial boiling point of the crude oil should not be too low, and it is advisable to keep it at around 65°C. 3. Influence of operational factors on the extraction process 3.1 Influence of temperature The extraction process can take place because the addition of the extractant creates a two-phase region; therefore, the size of this two-phase region has a significant impact on the extraction process, and the size of the two-phase region is related to the operating temperature of the system. Generally speaking, an increase in temperature increases solubility, reducing the size of the two regions. When the temperature rises to certain critical values, the two-phase region can disappear and complete miscibility is achieved; in this case, extraction separation becomes impossible. Therefore, an increase in temperature is clearly unfavorable for the extraction process. At the same time, an increase in temperature also brings the concentrations in the two-phase region closer together, reducing the density difference and increasing the likelihood of flooding. A decrease in temperature increases the two-phase region, which is favorable for extraction. However, if the temperature is lowered too much, it may lead to a second or third phase of mutual solubility in some systems; typically, this results in the solvent and solute no longer being completely miscible. Therefore, an excessively low operating temperature is also unfavorable for extraction. 3.2 Effect of pressure: Pressure has a minimal effect on the phase diagram and can be ignored. It is generally desirable to operate at atmospheric pressure, but in order to keep the system in a liquid state, the operating pressure must be higher than the saturated vapor pressure of the substance. Moreover, the operating pressure of the extraction column is closely related to interface control. The extraction column should operate at constant pressure; the pressure within the column is intended to ensure that all types of hydrocarbons involved in the extraction process remain in a liquid state at the operating temperature – in other words, above the bubble point pressure of the non-aromatic compounds at that temperature. Otherwise, vaporization will occur within the column, which will reduce the efficiency of extraction. Pressure itself does not affect the solubility of aromatics in the solvent; therefore, operational measures should be taken to prevent sudden fluctuations in the pressure of the extraction tower. 3.3 Effect of solvent ratio The ratio of the amount of solvent to the amount of feed is called the solvent ratio, which represents the amount of solvent used to process a unit amount of feed. For a given feed rate, a higher solvent ratio means an increased amount of solvent circulating in the equipment, which raises the cost of the solvent recovery system. However, the solvent ratio has a significant impact on the separation efficiency of each extraction stage: a higher solvent ratio enhances the separation efficiency at each stage, allowing for a reduced number of stages required to achieve a certain level of separation, or it enables a higher degree of separation with the same extraction equipment, thereby increasing the recovery rate of the solute. However, an excessively high solvent ratio will also increase the solubility of the original solution in the solvent, thereby affecting the purity of the solute; therefore, it is important to choose an appropriate solvent ratio during the extraction process. The suitable solvent ratio for extraction devices is generally between 4 and 5. 3.4 Effect of the backwash ratio The ratio of the amount of reflux extract to the amount of extracted product is called the backwash ratio, which is similar to the reflux ratio in distillation operations. As the backwash ratio increases, the number of stages required to achieve the desired degree of separation can be reduced; however, the cost of solvent recovery increases. The opposite is true when the backwash ratio decreases. In production, it is common to adjust the temperatures at both ends of the extraction tower; this causes a change in the solubility of the solute by the solvent, resulting in the release of some of the solutes that were previously dissolved in the solution. This acts as a form of backwashing, allowing the backwashing ratio to be reduced and the purity of the product to be increased. Section 2: Adjustment of the extraction process The adjustment of this process must achieve two main objectives: obtaining a product of high purity and a high recovery rate. Low utility consumption. What is discussed in this section is how to ensure that the above two objectives are achieved. As for the solvent ratio, we know that it is an important factor in ensuring the recovery rate of aromatics; however, a higher solvent ratio does help improve aromatic recovery but reduces the purity of these aromatics. Therefore, while striving to recover as much aromatic as possible, it is also necessary to take into account the impact on their purity. The appropriate solvent ratio for this system should be between 4 and 5. If only one main solvent is used, it will inevitably lead to significant waste, as the main solvent needs to be thoroughly cooled before it can enter the top of the extraction tower; at the same time, the size of the extraction tower must be increased, and it is generally designed based on the minimum requirements. The solvent added to the feed of the stripping tower helps to maintain the purity of aromatics in the recovery tower; therefore, its concentration is generally set at a relatively high level. This approach also reduces the workload on the backwashing section. Adding the solvent to the stripping tower is less costly than adding it to the backwashing area. New installations usually do not employ a second solvent. When the aromatic content in the extraction feed is 50%, a shortage of solvent occurs in the backwash section, resulting in a high hydrocarbon load; in such cases, the amount of main solvent should be increased slightly to avoid complicating the process involving the third solvent. When the hydrocarbon content in the rich solvent exceeds 25–35%, it is advisable to use a second solvent; when the aromatic content in the feed is greater than 80%, it is appropriate to use the first, second, and third solvents separately. 2. Extraction column temperature: When selecting the temperature for the extraction column, it is necessary to consider factors that enhance selectivity and solubility. An increase in the extraction column temperature raises the solubility of hydrocarbons in the solvent, but it reduces the selectivity between aromatics and non-aromatics. To maintain stable operation, the temperature of the extraction tower should be kept essentially constant. However, when there are significant changes in the boiling range of the feed, raising the operating temperature of the extraction tower can help address the issue of low solubility of the raw material. The temperature of the extraction tower should be controlled between 70 and 80°C. 3. Pressure in the extraction tower: The pressure in the extraction tower should be set at a level that maintains the material inside the tower in a liquid state, while allowing the material at the bottom of the tower to be pumped to the stripping tower under its own pressure. If vaporization occurs within the extraction tower, it will reduce the efficiency of extraction and limit the flow rate within the tower. However, the pressure itself does not affect the solubility and selectivity of the solvent in the tower. To prevent pressure fluctuations, sudden changes in the flow rate of material entering and leaving the extraction tower should be avoided, with the tower pressure being controlled at around 0.48 MPa. 4. Backwashing of the extraction tower: To achieve the desired purity of the extracted product, it is necessary to adjust the flow rate of the backwash liquid. This amount of backwash liquid is controlled by the quantity of vapor emitted from the top of the stripping tower. The amount of backwash liquid should not be too large, as this would increase the solvent load in the backwashing section and reduce the selectivity of the solvent. When increasing the amount of backwash liquid, it is also necessary to increase the solvent ratio in order to compensate for any decrease in solvent selectivity that may occur as a result. 5 Operation of the stripping tower: In the stripping tower, it is necessary to remove non-aromatic compounds from the solvent-rich mixture. The tower must have a high enough vapor flow rate at its top in order to ensure that alkanes and cycloalkanes are completely removed. As the hydrocarbon content in the feed to the stripping tower increases, the selectivity decreases; in such cases, an additional amount of solvent must be added to reduce the hydrocarbon concentration in the feed. Foaming can occur easily in the stripping tower, with a large amount of solvent being carried to the water stripping tower as well. The causes of foaming are generally due to the presence of insoluble hydrocarbons in the feed, high hydrocarbon concentrations, and the self-pressure of the equipment. To prevent foaming, an antifoaming agent can be added to the solvent-rich stream, at a concentration of 1–2 ppm based on the amount of feed to the stripping tower. 6 Operation of the recovery tower The recovery tower is used to separate the extract from the solvent. To completely remove the hydrocarbons from the solvent, the bottom temperature of the recovery tower and the stripping rate must be high enough, but the bottom temperature should not be too high to prevent the solvent from mixing into the extract, thereby contaminating the purity of the product and increasing solvent loss. A sufficiently high reflux rate is set at the tower top to achieve good separation and ensure the purity of the extract. The higher the ratio of reflux liquid to extract, the lower the solvent loss, but the utility consumption also increases; an appropriate reflux ratio lies between 0.3 and 0.5. To prevent the thermal decomposition of the solvent, the column operates under reduced pressure, and the temperature at the bottom of the column must not exceed the solvent’s decomposition temperature of 200°C. At the same time, the tower must have an adequate supply of stripping water, which is used to completely vaporize all the hydrocarbons from the solvent. Too much stripping water leads to heat losses in the recovery tower, while too little results in hydrocarbons remaining in the lean solvent that flows into the extraction tower, causing losses of aromatics. The amount of water used for stripping should be slightly higher than the higher of the following two requirements: it must be sufficient to recover the solvent from the raffinate, or it must be able to strip the aromatics from the solvent in the recovery tower. Generally, meeting the requirement for stripping water also means meeting the requirement for washing water. The stripping water ratio (weight ratio to the lean solvent) is controlled at around 0.022. Both the water content and hydrocarbon content of the lean solvent are determined by the operation of this tower. The water content in the lean solvent depends on the bottom temperature and pressure of the recovery tower; water reduces the solubility of hydrocarbons in the solvent, with the water content in the lean solvent generally ranging from 0.4 to 0.8 WT%. The aromatic content in the lean solvent depends on the temperature of the recovery tower, but it is primarily determined by the ratio of stripping steam to lean solvent. Most plants use large amounts of stripping steam in order to improve the recovery rate; meanwhile, reducing the pressure in the recovery tower is also highly effective in lowering the aromatic content in the lean solvent. 7. Washing of the raffinate oil: The raffinate oil that comes out of the top of the extraction tower generally contains 1% (WT) of sulfolane. A small amount of water is sufficient to recover the solvent from this raffinate oil; therefore, the amount of water needed for washing can be determined based on the stripping volume in the recovery tower. However, when the aromatic content in the feed is very low (30–40%), the water volume in the system must be determined by the amount of water used for washing. The bottom wash tower for the raffinate oil is designed to mix a portion of the solvent-containing water leaving the bottom with the raffinate oil before the cooler, in order to reduce the possibility of tiny dispersed sulfolane droplets forming a stable suspension in the hydrocarbon continuous phase. 8. Operation of the water stripping tower: 10% of the total water volume in the feed to the water stripping tower is stripped to the top of the tower in order to remove any non-aromatic compounds (which may be present in the water during the refluxing process in the top of the stripping tower or during the washing with raffinate oil). The remaining 90% enters the bottom of the recovery tower in either vapor or liquid form. 9. Operation of the solvent regeneration tower: To remove impurities and degraded products from the recycled solvent, a portion of the solvent is sent to the solvent regeneration tower for regeneration. In the sulfolane process, solvent management is an important aspect; solvent control is primarily carried out in three areas: strict control of dissolved oxygen in the feed and air leakage in the vacuum system. Maintaining the pH of the circulating solvent at 5.5–6.0 requires an increase in monoethanolamine, indicating an increase in dissolution oxidation. Feed the solvent into the regeneration tower at the maximum flow rate.