A high-stability complex extraction system and extraction distillation method for aromatic extraction

Abstract

This invention discloses a highly stable compound extraction system and extractive distillation method for aromatic hydrocarbon extraction. The compound extraction system comprises the following components by mass percentage: 10–89.9% sulfolane, 10–89.9% furfural and / or tetraethylene glycol, and 0.1–2% stabilizer. The extractive distillation method involves countercurrent contact between the system and aromatic hydrocarbon-containing distillate oil in a distillation unit. An aromatic hydrocarbon-rich phase is obtained at the bottom of the unit, and high-purity aromatic hydrocarbons and a regenerated system are obtained through multi-stage distillation and stripping. The regenerated system is recycled and is suitable for catalytic reformed gasoline with a total aromatic hydrocarbon content ≥50%. The compound extraction system of this invention exhibits no pyrolysis or component self-polymerization at 80–200℃, and produces no black slag after use at 200℃. Sulfolane does not produce sulfonic acid, and furfural and tetraethylene glycol do not form polymers. This invention achieves synergistic extraction of sulfolane with furfural / tetraethylene glycol, with single-stage separation performance superior to single-solvent extraction, effectively improving the aromatic hydrocarbon extraction recovery rate and the long-term stability of the process.

Description

Technical Field

[0001] This invention relates to the field of high-efficiency purification technology of aromatics, and in particular to a highly stable compound extraction system and extraction distillation method for aromatic extraction. Background Technology

[0002] Aromatic compounds have wide applications in various industries such as petroleum, chemical, and pharmaceutical, serving as raw materials or intermediates for many important chemicals. Therefore, improving the extraction capacity of aromatics can not only enhance product purity and quality, meeting market demand for high-quality aromatic compounds, but also increase production efficiency, reduce production costs, and thus enhance enterprise competitiveness. This has significant economic and social implications for promoting the development of related industries such as petroleum, chemical, and pharmaceutical, and for facilitating industrial upgrading and transformation.

[0003] Currently, sulfolane is the primary solvent used in aromatic hydrocarbon extraction. However, sulfolane has several significant drawbacks as a solvent, such as limited extraction efficiency, strong corrosiveness to equipment, and complex recovery and processing. These issues not only affect the stability and reliability of aromatic hydrocarbon extraction processes but also limit their further development and application. Therefore, researching new solvent systems, such as mixed solvents, has become an urgent need. According to literature review, mixed solvents have great application potential and research value in aromatic hydrocarbon extraction compared to sulfolane. First, mixed solvents can be customized according to the characteristics of aromatic compounds, achieving higher selectivity and extraction efficiency by adjusting the proportions of different components. Second, some single solvents may have high toxicity, while using them in combination can reduce overall toxicity, minimize harm to the environment and operators, reduce damage to equipment, and make the process safer and more environmentally friendly.

[0004] For example, Chinese invention patent CN 117946729 A provides a composite solvent for aromatic extraction. The composite solvent contains alkyl sulfolane, sulfolane, chain sulfone and a small amount of glycol ether. It also proposes a method for extracting aromatics, in which aromatic distillate oil is contacted countercurrently with the composite solvent in an extractive distillation column to perform extractive distillation, and extract material and raffinate are obtained. The extract material is then distilled in a solvent recovery column to obtain aromatic products and crude circulating solvent.

[0005] For example, Chinese invention patent CN 105219422 A provides a composite extractant suitable for coal-based naphtha. The composite extractant is composed of sulfolane, propylene carbonate, triethylene glycol ether and water. The compound solvent has a synergistic effect, which ensures high selectivity while maintaining good solubility, reducing non-aromatic content, and achieving fast phase separation and high extraction efficiency.

[0006] However, none of the above methods have solved the problem of the decomposition of sulfolane, the main component of the extraction system, at high temperatures of 150℃-200℃. Long-term high-temperature operation will lead to the accumulation of sulfonic acid produced by the pyrolysis of sulfolane, which will corrode the equipment, reduce the extraction rate, and affect the stability of the process.

[0007] In summary, developing a novel, highly stable mixed solvent extraction system to replace the traditional single sulfolane system is of great significance for improving the recovery rate and process stability of aromatic hydrocarbons. Summary of the Invention

[0008] To address the problems of poor stability of the extraction system and the decay of extraction efficiency over time during aromatic hydrocarbon extraction, this invention provides a highly stable compound extraction system and extraction distillation method for aromatic hydrocarbon extraction.

[0009] In a first aspect, the present invention provides a highly stable compound extraction system for aromatic hydrocarbon extraction, which is achieved by the following technical solution.

[0010] A highly stable compound extraction system for aromatic hydrocarbon extraction comprises the following components by mass percentage: 10–89.9% sulfolane, 10–89.9% furfural and / or tetraethylene glycol, and 0.1–2% stabilizer.

[0011] Furthermore, the stabilizer may be selected from one or more of monoethanolamine, diethanolamine, triethanolamine, and N-methylpyrrolidone.

[0012] By adopting the above technical solution, the compound extraction system of the present invention does not exhibit the phenomenon of sulfonic acid production by pyrolysis of sulfolane within the temperature range of 80~200℃, and does not exhibit the phenomenon of self-polymerization of tetraethylene glycol or furfural into dimers or polymers, and no black residue is produced after high-temperature use.

[0013] Secondly, the present invention provides an extractive distillation method for aromatic hydrocarbon extraction, which is achieved by the following technical solution.

[0014] An extractive distillation method for aromatic hydrocarbon extraction, employing the aforementioned complex extraction system, includes the following steps: S1. The aromatic distillate oil and the compound extraction system are fed into the extractive distillation unit, and the aromatic distillate oil and the compound extraction system are contacted countercurrently to carry out extractive distillation. During the extractive distillation, the non-aromatic components are extracted from the top of the unit in the form of steam, and the aromatic components are enriched at the bottom of the unit with the compound extraction system to obtain an aromatic-rich phase. S2. The aromatic-rich phase obtained in step S1 is subjected to multi-stage distillation-stripping separation to obtain high-purity aromatic products and regenerated compound extraction systems, respectively. S3. The regenerated compound extraction system obtained in step S2 is recycled back to the extraction distillation unit for reuse.

[0015] Furthermore, in step S1, the aromatic fraction oil is catalytic reformed gasoline with a total aromatic content ≥50% and a distillation temperature range of 60~200℃.

[0016] Furthermore, in step S1, the mass ratio of the compound extraction system to the feed oil containing aromatics is (1~10):1, preferably (5~10):1.

[0017] Furthermore, in step S1, when the extractive distillation apparatus uses an extractive distillation column, the theoretical number of trays in the extractive distillation column is 25 to 45, and the column is equipped with trays or packing, the packing including structured packing, bulk packing and combinations thereof.

[0018] Furthermore, in step S1, the feed temperature of the compound extraction system is 80℃~150℃, preferably 100℃~120℃; the feed temperature of the aromatic distillate oil is 80℃~120℃, preferably 90℃~110℃; the feed position of the compound extraction system is at the top of the column, on the 3rd to 10th theoretical plates; the feed position of the aromatic distillate oil is at the middle of the column, on the 15th to 25th theoretical plates.

[0019] Furthermore, the top temperature of the extractive distillation column is 80–150 °C, preferably 90–120 °C; the bottom temperature is 160–220 °C, preferably 170–200 °C; the operating pressure is 0.02–0.10 MPa (absolute pressure), preferably 0.03–0.06 MPa; and the reflux ratio is 1.2–3.5, preferably 1.5–2.5.

[0020] Furthermore, in step S2, when the multi-stage distillation-stripping separation uses a distillation column and a stripping column, the distillation column has 25 to 50 theoretical plates, the feed position is the 10th to 25th theoretical plate, the top pressure is 0.04 to 0.10 MPa, the reflux ratio is 1.2 to 3.5, and the bottom temperature is 130 to 220 ℃; the stripping column has 3 to 15 theoretical plates, the operating pressure is 0.02 to 0.08 MPa, the bottom temperature is 90 to 120 ℃, and the stripping medium is steam or nitrogen.

[0021] In the multi-stage distillation-stripping separation process of this invention, the recovery rate of the compound extraction system is ≥99%. Moreover, the component content of the regenerated compound extraction system deviates from that of the fresh compound extraction system by ≤±2%, and the extraction performance remains stable after being recycled ≥50 times.

[0022] This application has the following beneficial effects: (1) Compared with a single extractant, under the same mass ratio, the compound extraction system requires fewer theoretical distillation columns in the aromatic extraction process, that is, the compound extraction system has a better single-stage separation effect than the single extractant sulfolane. (2) By combining sulfolane, tetraethylene glycol, stabilizers, etc., the synergistic effect of sulfolane and tetraethylene glycol is demonstrated, which is better than the single extractant. The addition of stabilizers can solve the problems of sulfolane decomposition at high temperature producing sulfonic acid that corrodes the equipment and tetraethylene glycol undergoing dimerization at high temperature. Attached Figure Description

[0023] Figure 1 This is a structural diagram of the ROSE gas-liquid phase equilibrium vessel used for solvent screening of aromatic hydrocarbon extraction in the compound extraction system of this invention. Figure 2 This is a comparison diagram of the properties of the compound extraction system of this invention and sulfolane after high-temperature use; Figure 3 This is a gas chromatography-mass spectrometry (GC-MS) characterization of the self-polymerization of tetraethylene glycol without the addition of a stabilizer. Detailed Implementation

[0024] The present patent application will be further described below with reference to the embodiments.

[0025] The raw material composition used in the following examples is shown in Table 1.

[0026] Example 1

[0027] The key issue in separating aromatics and alkanes through aromatic extraction is the thermodynamic limit of the aromatic-alkane vapor-liquid phase equilibrium under the influence of the extractant during extractive distillation. Therefore, this invention addresses this issue by... Figure 1 The effect of the extractant was examined by using the vapor-liquid phase equilibrium experiment results of the ROSE reactor.

[0028] The compound extraction system used in this embodiment consists of 60% sulfolane, 39% tetraethylene glycol, and 1% monoethanolamine by mass fraction. The feed mass ratio of the extractant to the catalytic reforming gasoline feedstock is set to 1:1. The experimental apparatus is as follows: Figure 1As shown, a vapor-liquid phase equilibrium experiment was conducted under normal pressure. The aromatic distillate oil and the compound extraction system were brought into full contact and reached phase equilibrium at a temperature of 150℃, a stirring speed of 450 r / min, an equilibrium contact time of 60 min, and a settling and phase separation time of 30 min. This resulted in the enrichment of aromatic components in the compound extraction system, yielding an aromatic-rich phase. Gas chromatography analysis of the liquid sample showed that all benzene series components were efficiently enriched, with benzene accounting for 31.58% by mass, toluene for 41.08%, ethylbenzene for 10.51%, and xylene for 15.46%. The total proportion of the four core benzene series components reached 98.63%. The removal of non-aromatic hydrocarbons was particularly significant. The original content of C5NA (0.52%) and C6NA (9.45%) in the feedstock was 100% removed, with only trace amounts of C7NA (0.28%), C8NA (0.54%), and C9+NA (0.58%) detected in the liquid phase. The total non-aromatic hydrocarbon content was less than 1.4%. The final total aromatic hydrocarbon mass fraction in the liquid phase reached 98.63%, an increase of 16.42 percentage points compared to the feedstock's 82.21%. The single-stage separation efficiency far exceeded that of existing conventional solvent systems.

[0029] High-temperature stability tests showed that the system remained clear and transparent after use at 200℃, with no black residue generated. Gas chromatography-mass spectrometry (GC-MS) analysis revealed no dimers or polymers generated by the self-polymerization of tetraethylene glycol, nor any corrosive sulfonic acid substances generated by the pyrolysis of sulfolane. This fully demonstrates that monoethanolamine, as a stabilizer, can neutralize potential acidic decomposition products and inhibit the self-polymerization reaction of tetraethylene glycol, achieving a dual optimization of extraction performance and thermal stability.

[0030] Example 2

[0031] This embodiment uses a low-sulfolane ratio compound system, consisting of 20% sulfolane, 79.9% tetraethylene glycol, and 0.1% triethanolamine by mass fraction. The feed ratio of extractant to raw material is increased to 8:1, and the operation is carried out under normal pressure. Figure 1The ROSE gas-liquid phase equilibrium vessel was used as the evaluation equipment. Aromatic distillate oil and the compound extraction system were brought into full contact and reached phase equilibrium under the conditions of 155℃, stirring speed 500 r / min, equilibrium contact time 60 min, and settling time 30 min. This allowed the aromatic components to be enriched in the compound extraction system, resulting in an aromatic-rich phase. Gas chromatography analysis of the liquid phase products, as shown in Example 2 of Table 1, revealed a significant enrichment effect of benzene series compounds. The mass fractions of benzene were 32.17%, toluene 39.99%, ethylbenzene 10.51%, and xylene 15.81%, with a uniform distribution of each benzene series compound, demonstrating the system's balanced extraction capability for aromatics with different boiling points. Regarding the removal of non-aromatic hydrocarbons, C5NA and C6NA were also completely removed, with only trace amounts of non-aromatic components detected in the liquid phase. Specifically, C7NA accounted for 1.09%, C8NA for 0.4%, and C9+NA for only 0.04%, indicating that a higher solvent ratio resulted in better separation of heavier non-aromatic hydrocarbons. The final total aromatic hydrocarbon mass fraction in the liquid phase reached 98.48%, an improvement of 16.27 percentage points compared to the raw material, and was close to the effect of the high-sulfolane ratio system in Example 1. This demonstrates that the compound system of this invention maintains excellent extraction performance across a wide ratio range.

[0032] In high-temperature durability testing, after prolonged operation at 200℃, the system showed no turbidity or black slag. Gas chromatography-mass spectrometry (GC-MS) analysis revealed no tetraethylene glycol self-polymers or sulfolane pyrolysis products. Furthermore, only 0.1% triethanolamine was required for stabilization, significantly reducing stabilizer costs. Simultaneously, the regenerated extraction system showed a component content deviation of only ±0.8% from the fresh system. After 50 cycles, the total aromatic hydrocarbon content in the liquid phase remained above 98%, fully meeting the requirements for long-term continuous industrial operation.

[0033] Comparative Example 1 This comparative example uses industrially common single sulfolane as the extractant. The feed-to-extractant mass ratio, operating temperature, and pressure are completely consistent with those of Example 1 to ensure comparability. Liquid phase component analysis results show that the extraction performance of single sulfolane has significant shortcomings. The enrichment of benzene compounds is much lower than that of the compound system. The mass fraction of benzene is 29.3%, toluene 37.51%, ethylbenzene 10.75%, and xylene 15.33%, with the total proportion of the four major benzene compounds being only 92.89%. Incomplete removal of non-aromatic hydrocarbons is the core problem. Although C5NA was 100% removed, a large amount of C6NA (0.8%), C7NA (2.87%), C8NA (1.91%), and C9+NA (1.53%) still remain in the liquid phase, with the total non-aromatic hydrocarbon content reaching as high as 7.11%, far exceeding that of Examples 1 and 2. The final total aromatic hydrocarbon mass fraction in the liquid phase was only 92.89%, which was 5.74 percentage points lower than that in Example 1, and could not meet the production requirements for high-purity aromatic hydrocarbons.

[0034] More importantly, after high-temperature use, the single sulfolane system produced a large amount of black residue, and the liquid phase became extremely turbid, such as... Figure 2 As shown in the image, gas chromatography-mass spectrometry (GC-MS) confirmed the presence of sulfonic acid compounds generated during the high-temperature pyrolysis of sulfolane. These compounds caused significant corrosion to the stainless steel sample, which is the fundamental reason for the severe equipment corrosion and high maintenance costs associated with using sulfolane as a single solvent in industrial applications. Furthermore, due to the formation of pyrolysis products, the effective content of sulfolane continuously decreased, leading to a rapid decline in extraction performance over time, highlighting the limitations of a single solvent system.

[0035] Comparative Example 2 This comparative example used tetraethylene glycol as the extractant, and all operating conditions were consistent with those of Example 1 and Comparative Example 1. The focus was on investigating the aromatic hydrocarbon extraction capacity and thermal stability of tetraethylene glycol alone. Liquid phase component analysis results showed that the extraction performance of tetraethylene glycol alone was the worst among all experimental groups, with significantly insufficient enrichment of benzene series compounds. The mass fraction of benzene was 29.28%, toluene only 34.58%, ethylbenzene 10.49%, and xylene 13.97%, with the total proportion of the four major benzene series compounds being only 88.32%. The removal of non-aromatic hydrocarbons was extremely poor, with a large amount of various non-aromatic components remaining in the liquid phase. The C6NA content was as high as 4.53%, C7NA 3.77%, C8NA 1.81%, C9+NA 1.31%, and even 0.28% of C5NA remained. The total non-aromatic hydrocarbon content exceeded 11.6%, severely affecting the purity of the aromatic hydrocarbon product. The final total aromatic hydrocarbon mass fraction in the liquid phase was only 88.32%, an increase of only 6.11 percentage points compared to the raw materials, which is in stark contrast to the purity of over 98% of the compound system of this invention.

[0036] In high-temperature stability testing, after the single tetraethylene glycol system was used at 200°C, gas chromatography-mass spectrometry (GC-MS) revealed a large number of tetraethylene glycol self-polymerization products, such as... Figure 3 As shown, these self-polymers not only reduce extraction efficiency but also clog equipment pipes and trays, increasing operational risks in industrial production. This result fully demonstrates that single tetraethylene glycol lacks both excellent aromatic hydrocarbon extraction selectivity and structural stability at high temperatures, making it unsuitable as a high-efficiency solvent for aromatic hydrocarbon extraction on its own.

[0037] Comparative Example 3 This comparative example uses a compound system without stabilizers, consisting of 20% sulfolane and 80% tetraethylene glycol by mass fraction. The feed ratio of extractant to catalytic reforming gasoline feedstock is set at 1:1, and the operating conditions are the same as in Example 1, aiming to verify the core role of stabilizers in the compound system. Liquid phase component analysis results show that the extraction effect of this stabilizer-free system is better than that of a single solvent, but far lower than that of the compound system with stabilizers in this invention. The mass fraction of benzene is 32.81%, toluene is 36.68%, ethylbenzene is 9.41%, xylene is 14.5%, and the total proportion of the four major benzene series compounds is 93.4%. There are obvious deficiencies in the removal of non-aromatic hydrocarbons. In addition to C6NA (0.38%), C7NA (3.4%), C8NA (1.35%) and C9+NA (1.46%), 0.01% of C5NA was also detected in the liquid phase. The total content of non-aromatic hydrocarbons reached 6.6%, and the final total aromatic hydrocarbon mass fraction in the liquid phase was 93.4%, which is 5.23 percentage points lower than 98.63% in Example 1.

[0038] The high-temperature stability test results are even more crucial. After use at 200℃, the liquid phase showed slight turbidity. Although no large amount of black slag was produced, gas chromatography-mass spectrometry (GC-MS) clearly detected oligomers formed by the self-polymerization of tetraethylene glycol, and also detected trace amounts of sulfonic acid substances produced by the pyrolysis of sulfolane. This result fully demonstrates that the simple combination of sulfolane and tetraethylene glycol can only improve the extraction efficiency to a certain extent, but cannot solve the problems of pyrolysis and self-polymerization at high temperatures. The addition of a stabilizer is a necessary condition for suppressing the above-mentioned side reactions and ensuring the long-term stable operation of the compound system, and is also one of the core innovations of this invention.

[0039] A comparison of the experimental results of Example 1 and Comparative Example 1 shows that the aromatic hydrocarbon content in the bottom liquid phase of the compound extraction system of Example 1 (sulfolane-tetraethylene glycol-stabilizer) is higher than that of Comparative Example 1 (sulfolane). Furthermore, no black slag appeared in the compound extraction system after the experiment. Figure 2 As shown.

[0040] Comparison of Comparative Examples 1, 2 and 3 revealed that sulfolane and tetraethylene glycol alone were of average effectiveness as extractants, but the effect was significantly better when sulfolane-tetraethylene glycol were combined.

[0041] A comparison of Examples 1, 2, and 3 revealed that by adding the stabilizers monoethanolamine and triethanolamine, tetraethylene glycol did not appear in the gas chromatography-mass spectrometry (GC-MS) results. Figure 3 The aggregation phenomenon in [the context].

[0042] Table 1 Summary of data from examples and comparative examples

[0043] Comparing the experimental data and performance of the comparative examples and various comparative examples, the high-stability compound extraction system of the present invention exhibits significant technical and application advantages, primarily reflected in three aspects: extraction efficiency, high-temperature stability, and system compatibility. In terms of extraction performance, the total aromatic hydrocarbon content in the liquid phase of the system of the present invention is significantly higher than that of single sulfolane and tetraethylene glycol, and far superior to the sulfolane-tetraethylene glycol compound system without stabilizers. It also achieves 100% removal of C5NA and C6NA, reducing the residual non-aromatic hydrocarbons to below approximately 1.6%. The synergistic effect of sulfolane and furfural / tetraethylene glycol significantly improves the single-stage separation effect. High-temperature stability is the core advantage of the present invention. After use at 200°C, the system is clear and free of black slag, sulfolane does not exhibit pyrolysis producing sulfonic acid, and tetraethylene glycol does not generate self-polymers. This solves the industry pain points of single sulfolane corroding equipment and single tetraethylene glycol causing severe self-polymerization. Furthermore, only 0.1% to 1% of stabilizer is required, ensuring stability while controlling usage costs. Furthermore, the system of this invention has a wide range of compatibility, and can maintain excellent performance with both high and low sulfolane ratios. It can be directly adapted to existing catalytic reforming gasoline aromatics extraction processes without large-scale equipment modifications, thus possessing both economic advantages and industrial promotion value.

[0044] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A highly stable compound extraction system for aromatic hydrocarbon extraction, characterized in that: It includes the following components by mass percentage: 10-89.9% sulfolane, 10-89.9% furfural and / or tetraethylene glycol, and 0.1%-2% stabilizer.

2. The highly stable compound extraction system for aromatic hydrocarbon extraction according to claim 1, characterized in that: The stabilizer is selected from one or more of monoethanolamine, diethanolamine, triethanolamine, and N-methylpyrrolidone.

3. An extractive distillation method for aromatic hydrocarbon extraction, characterized in that: The complex extraction system described in claim 1 or 2 includes the following steps: S1. The aromatic distillate oil and the compound extraction system are fed into the extractive distillation unit, and the aromatic distillate oil and the compound extraction system are contacted countercurrently to carry out extractive distillation. During the extractive distillation, the non-aromatic components are extracted from the top of the unit in the form of steam, and the aromatic components are enriched at the bottom of the unit with the compound extraction system to obtain an aromatic-rich phase. S2. The aromatic-rich phase obtained in step S1 is subjected to multi-stage distillation-stripping separation to obtain high-purity aromatic products and regenerated compound extraction systems, respectively. S3. The regenerated compound extraction system obtained in step S2 is recycled back to the extraction distillation unit for reuse.

4. The extractive distillation method for aromatic hydrocarbon extraction according to claim 3, characterized in that: In step S1, the aromatic fraction oil is catalytic reformed gasoline with a total aromatic content ≥50% and a distillation temperature range of 60~200℃.

5. The extractive distillation method for aromatic hydrocarbon extraction according to claim 3, characterized in that: In step S1, the mass ratio of the compound extraction system to the feed oil containing aromatics is (1~10):

1.

6. The extractive distillation method for aromatic hydrocarbon extraction according to claim 3, characterized in that: In step S1, when the extractive distillation apparatus uses an extractive distillation column, the theoretical number of trays in the extractive distillation column is 25 to 45, and the column is equipped with trays or packing, the packing including structured packing, bulk packing and combinations thereof.

7. The extractive distillation method for aromatic hydrocarbon extraction according to claim 6, characterized in that: The top temperature of the extractive distillation column is 80–150 ℃, the bottom temperature is 160–220 ℃, the operating pressure is 0.02–0.10 MPa, and the reflux ratio is 1.2–3.

5.

8. The extractive distillation method for aromatic hydrocarbon extraction according to claim 6, characterized in that: In step S1, the feed temperature of the compound extraction system is 80℃~150℃; the feed temperature of the aromatic distillate oil is 80℃~120℃.

9. The extractive distillation method for aromatic hydrocarbon extraction according to claim 3, characterized in that: In step S2, when multi-stage distillation-stripping separation uses a distillation column and a stripping column, the distillation column has 25 to 50 theoretical plates, the feed position is the 10th to 25th theoretical plate, the top pressure is 0.04 to 0.10 MPa, the reflux ratio is 1.2 to 3.5, and the bottom temperature is 130 to 220 ℃; the stripping column has 3 to 15 theoretical plates, the operating pressure is 0.02 to 0.08 MPa, the bottom temperature is 90 to 120 ℃, and the stripping medium is steam or nitrogen.

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