Method for separating methanol-normal hexane-tetrahydrofuran-water azeotropic system through extractive distillation

Abstract

The invention provides a method for separating a methanol-normal hexane-tetrahydrofuran-water azeotropic system through extractive distillation, and relates to the technical field of methods for separating the methanol-normal hexane-tetrahydrofuran-water azeotropic system through extractive distillation. A mixture of methanol, n-hexane, tetrahydrofuran and water to be separated respectively passes through a first extractive distillation tower, a first solvent desorption tower, a side column and a second extractive distillation tower, a solvent is a mixture of dimethyl sulfoxide, N-methyl pyrrolidone and ethylene glycol, and the compound solvent has good selectivity on components to be separated and is high in separation efficiency. Compared with the prior art, the method effectively increases the relative volatility of multiple azeotropes to the internal components of the system, and is matched with the extractive distillation process design, so that not only can high-purity methanol, n-hexane, tetrahydrofuran and other products be obtained and the circulation volume of the solvent be reduced, but also the energy consumption and fixed investment of the device can be obviously reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of methods for extractive distillation separation of methanol-n-hexane-tetrahydrofuran-water azeotropic systems, and more specifically relates to a method for extractive distillation separation of methanol-n-hexane-tetrahydrofuran-water azeotropic systems. Background Art

[0002] Methanol and tetrahydrofuran (THF) are excellent organic solvents widely used in the production of pharmaceuticals, coatings, anti-corrosion coatings, printing inks, and thin-film coatings. They are particularly common in the production of polyimide (PI), pharmaceuticals, and emerging polymer materials, generating a wastewater mixture containing methanol, n-hexane, THF, and water. Effectively recovering methanol, n-hexane, and THF not only reduces production costs but also minimizes the environmental impact of these wastewaters. N-hexane is a widely used organic solvent in the chemical, food processing, electronics, rubber, and pharmaceutical industries. Market demand for n-hexane is expected to rise slowly, with demand expected to increase in three key sectors: vegetable oil extraction, polyolefins, and rubber synthesis. Therefore, recovering methanol, cyclopentane, and THF from wastewater has high practical value.

[0003] Methanol, n-hexane, tetrahydrofuran, and water form multiple azeotropic pairs at atmospheric pressure: tetrahydrofuran-water, n-hexane-water, methanol-n-hexane, n-hexane-tetrahydrofuran, and methanol-tetrahydrofuran. These pairs all form azeotropes with very close azeotropic points, all around 61°C. The detailed azeotropic compositions are shown in Table 1. As can be seen from the data in this table, separation is difficult to achieve using conventional distillation or a single specialized distillation. Azeotropic or near-boiling mixtures are commonly separated industrially using azeotropic and extractive distillation. Extractive distillation involves adding an extractant to increase the relative volatility of the system to be separated, thereby enabling separation. Azeotropic distillation, on the other hand, involves adding a third component to the system to be separated, forming either a lower or higher azeotrope, which allows for separation and recycling. Compared to azeotropic distillation, extractive distillation is easier to select than the third component used in azeotropic distillation. It also consumes less energy and offers greater operational flexibility and controllability. Therefore, for the process of extractive distillation, developing efficient, green and economical extractants is a research hotspot in extractive distillation.

[0004] Table 1 Composition of azeotropic pairs in methanol-n-hexane-tetrahydrofuran-water system Chinese patent CN103193579A discloses a method for separating a mixture of n-hexane and tetrahydrofuran by extractive distillation, using N-substituted lactam compounds as extractants.

[0005] Chinese patent CN107903223A discloses a solvent extraction and distillation separation method and process for a tetrahydrofuran-methanol system using ethylene glycol as an extractant and tetrahydrofuran and methanol as products, while recycling the extractant.

[0006] Through a large number of experimental studies, extractive distillation has a high separation efficiency in separating azeotropic systems. However, when there are multiple azeotropic pairs in the mixture, the difficulty of screening the extractant and designing the extraction process is significantly increased. Summary of the Invention

[0007] In order to solve the above technical problems, the present invention provides a method for separating a methanol-n-hexane-tetrahydrofuran-water azeotropic system by extractive distillation to solve the above problems.

[0008] A method for separating a methanol-n-hexane-tetrahydrofuran-water azeotropic system by extractive distillation adopts an extractive distillation process using a composite solvent as an extractant, wherein the composite solvent is a mixture of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol, wherein the mass percentage of dimethyl sulfoxide is 70%-85%, the mass percentage of N-methylpyrrolidone is 8%-20%, and the mass percentage of ethylene glycol is 7%-15%.

[0009] Preferably, the raw material to be separated is a mixture of methanol, n-hexane, tetrahydrofuran and water, and the mass percentages of each component are: 24.8% methanol, 31.2% n-hexane, 27.8% tetrahydrofuran, and 16.2% water. First, the material stream S01 to be separated enters the lower part of the first extractive distillation tower, and the composite solvent stream S02 is adjusted to a suitable temperature through a heat exchanger and then enters the upper part of the tower. A mixture stream S03 of n-hexane and tetrahydrofuran is obtained at the top of the tower, and a stream rich in methanol, n-hexane and tetrahydrofuran is extracted from the bottom of the tower. Logistics S04 of water and composite solvent enters the first solvent desorption tower from the middle, and methanol logistics S05 with a purity of ≥99.2% is obtained at the top of the tower. Logistics S06 of composite solvent and water is produced from the side line at the lower part of the tower, and logistics S07 of desorbed composite solvent is obtained at the bottom of the tower and recycled after the temperature is adjusted by the heat exchanger. Logistics S06 enters the side line tower from the middle, and separated water logistics S08 is obtained at the top of the tower. Logistics S09 of desorbed composite solvent is obtained at the bottom of the tower, which is combined with logistics S07 and recycled.

[0010] Preferably, logistics S03 enters the second extractive distillation tower from the middle and lower part, the composite solvent logistics S12 is adjusted to a suitable temperature by a heat exchanger and enters the upper part of the tower, and a tetrahydrofuran logistics S10 with a content of more than 99.2% is obtained at the top of the tower, and a mixed flow S11 of composite solvent and n-hexane is obtained at the bottom of the tower. Logistics S11 enters the second solvent desorption tower from the middle part, and a tetrahydrofuran logistics S14 with a content of more than 99.2% is obtained at the top of the tower, and a desorbed composite solvent logistics S15 is obtained at the bottom of the tower. The temperature is adjusted by the heat exchanger and then recycled. The operating conditions of the first extractive distillation tower are: the number of theoretical plates is 40-90, the feed temperature of the flow to be separated S01 is 30-50°C, the feed temperature of the composite solvent S02 is 40-55°C, the bottom temperature is 80-130°C, the top temperature is 45-70°C, the reflux ratio is 1.3-3.5, the ratio of the composite solvent to the material to be separated is 2.2-5.6:1 (mass ratio), and the operating pressure is 86-200kPa.

[0011] Preferably, the operating conditions of the first solvent desorption tower are: 10-40 theoretical plates, 56-79°C tower top temperature, 130-210°C tower bottom temperature, 1.5-3.0 reflux ratio, and 80-150 kPa operating pressure; the operating conditions of the side column are: 15-30 theoretical plates, 90-110°C tower top temperature, 130-200°C tower bottom temperature, 1.8-2.6 reflux ratio, and 85-140 kPa operating pressure; the operating conditions of the second extractive distillation tower are: 50- 85 blocks, the feed temperature of composite solvent S12 is 40-55°C, the top temperature is 60-75°C, the bottom temperature is 140-180°C, the reflux ratio is 1.8-2.9, the ratio of composite solvent to material to be separated is 2.5-7.1:1 (mass ratio), and the operating pressure is 80-130kPa; the operating conditions of the second solvent desorption tower are: the theoretical plate number is 10-40 blocks, the top temperature is 60-74°C, the bottom temperature is 140-200°C, the reflux ratio is 1.8-3.0, and the operating pressure is 75-150kPa.

[0012] Compared with the prior art, the present invention has the following beneficial effects: This patent uses extractive distillation to separate the azeotropic system of methanol, n-hexane, tetrahydrofuran and water. The extractant is a high-efficiency composite solvent, which is a mixture of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol. The composite solvent has good selectivity for the components to be separated and effectively increases the relative volatility of multiple azeotropic components to the internal components of the system. Combined with the extractive distillation process design, not only can high-purity methanol, n-hexane, tetrahydrofuran and other products be obtained, but the circulation volume of the solvent can also be reduced, and the energy consumption and fixed investment of the device can be significantly reduced.

[0013] Efficient separation and high-purity products: A specific ratio of dimethyl sulfoxide, N-methylpyrrolidone, and ethylene glycol as an extractant effectively breaks the complex azeotropic system formed by methanol, n-hexane, tetrahydrofuran, and water, resulting in excellent extraction efficiency and selectivity for each component. Through the extractive distillation process, the final yields of methanol, n-hexane, and tetrahydrofuran all exceed 97%, and the purity of all four products reaches over 99.2% (mass percentage). This achieves efficient separation and high-purity product production, meeting the stringent quality requirements of industrial production and enhancing the value of resource recovery.

[0014] Cost Reduction and Solvent Recycling: The composite solvent components are simple and readily available, significantly reducing solvent costs. Furthermore, the optimized process reduces the agent-to-material ratio, significantly reducing operating costs and fixed investment. The solvent can be recycled throughout the entire process. For example, the composite solvent desorbed from the first solvent desorption tower, the side-line tower, and the second solvent desorption tower kettle can be recycled back to the first and second extractive distillation towers after temperature adjustment via heat exchangers. This effectively improves solvent utilization, further reduces production costs, aligns with the concepts of green chemistry and sustainable development, and enhances the economic competitiveness of this method in industrial applications.

[0015] Wide Applicability and Environmental Advantages: This composite solvent has relaxed requirements for the individual content of methanol, n-hexane, tetrahydrofuran, and water. It can be used with quaternary mixtures containing varying amounts of these substances, broadening its application scope and enabling the treatment of mixed wastewater from a variety of sources. The separated water can directly meet national emission standards or be used as process water, reducing wastewater treatment complexity and environmental pollution. This achieves resource recovery while also protecting the environment, resulting in significant environmental and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a process flow diagram of the method of the present invention.

[0017] In the figure, the correspondence between the component names and the drawing numbers is: T101, first extractive distillation tower; T102, first solvent desorption tower; T103, side-line tower; T104, second extractive distillation tower; T105, second solvent desorption tower; E101, first composite solvent heat exchanger; E102, second composite solvent heat exchanger. DETAILED DESCRIPTION

[0018] The following embodiments of the present invention are described in further detail with reference to the accompanying drawings and examples. The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0019] See also Figure 1The present invention provides an extractive distillation process using a composite solvent as an extractant. The solvent is a mixture of dimethyl sulfoxide, N-methylpyrrolidone, and ethylene glycol, wherein the weight percentage of dimethyl sulfoxide is 70-85%, the weight percentage of N-methylpyrrolidone is 8-20%, and the weight percentage of ethylene glycol is 7-15%. Using this composite extractant, the final yields of methanol, n-hexane, and tetrahydrofuran can all exceed 97%, and the purity of all four products can reach over 99.2% (weight percentage).

[0020] In combination with the above technologies, the present invention is achieved through the following technical solutions: a method for extractive distillation separation of an azeotropic system of methanol, n-hexane, tetrahydrofuran and water, comprising the following steps: (1) The raw material to be separated is a mixture of methanol, n-hexane, tetrahydrofuran, and water, with the mass percentages of each component being: 24.8% methanol, 31.2% n-hexane, 27.8% tetrahydrofuran, and 16.2% water. First, the material stream S01 to be separated enters the lower middle portion of the first extractive distillation tower T101, and the composite solvent stream S02 is adjusted to a suitable temperature by E101 and then enters the upper portion of the T101 tower. A mixture stream S03 of n-hexane and tetrahydrofuran is obtained at the top of the tower, and a stream S04 rich in methanol, water, and composite solvent, after removing n-hexane and tetrahydrofuran, is extracted from the bottom of the tower. (2) Logistics S04 enters the first solvent desorption tower T102 from the middle, and methanol logistics S05 with a purity of ≥99.2% is obtained at the top of the tower. Logistics S06 of composite solvent and water is produced from the side line in the lower part of the T102 tower. Logistics S07 of desorbed composite solvent is obtained at the bottom of the tower and is recycled after the temperature is adjusted by heat exchanger E101; (3) Logistics S06 enters the side-line tower T103 from the middle, and the separated water logistics S08 is obtained at the top of the tower. The desorbed composite solvent logistics 09 is obtained at the bottom of the tower and combined with logistics S07 for recycling; (4) Logistics S03 enters the second extractive distillation tower T104 from the middle and lower part, and the composite solvent logistics S12 is adjusted to a suitable temperature by E102 and enters the upper part of the T104 tower. The top of the tower obtains the tetrahydrofuran logistics S10 with a content of more than 99.2%, and the bottom of the tower obtains the mixed logistics S11 of composite solvent and n-hexane; (5) Logistics S11 enters the second solvent desorption tower T105 from the middle, and a tetrahydrofuran logistics S14 with a content of more than 99.2% is obtained at the top of the tower. The desorbed composite solvent logistics S15 is obtained at the bottom of the tower and is recycled after the temperature is adjusted by the heat exchanger E102.

[0021] In the above process, the operating conditions of each operating unit are as follows: (1) Operating conditions of the first extractive distillation tower T101: the number of theoretical plates is 40 to 90, the feed temperature of the stream to be separated S01 is 30 to 50°C, the feed temperature of the composite solvent S02 is 40 to 55°C, the bottom temperature is 80 to 130°C, the top temperature is 45 to 70°C, the reflux ratio is 1.3 to 3.5, the ratio of the composite solvent to the stream to be separated is 2.2 to 5.6:1 (mass ratio), and the operating pressure is 86 to 200 kPa; (2) Operating conditions of the first solvent desorption tower T102: theoretical plate number 10-40, tower top temperature 56-79°C, tower bottom temperature 130-210°C, reflux ratio 1.5-3.0, operating pressure 80-150 kPa; (3) Operating conditions of the side column T103: the number of theoretical plates is 15 to 30, the top temperature is 90 to 110°C, the bottom temperature is 130 to 200°C, the reflux ratio is 1.8 to 2.6, and the operating pressure is 85 to 140 kPa; (4) The operating conditions of the second extractive distillation tower T103 are as follows: the number of theoretical plates is 50 to 85, the feed temperature of the composite solvent S12 is 40 to 55°C, the tower top temperature is 60 to 75°C, the tower bottom temperature is 140 to 180°C, the reflux ratio is 1.8 to 2.9, the ratio of the composite solvent to the material to be separated is 2.5 to 7.1:1 (mass ratio), and the operating pressure is 80 to 130 kPa; (5) The operating conditions of the second solvent desorption tower T105 are as follows: the number of theoretical plates is 10 to 40, the tower top temperature is 60 to 74 °C, the tower bottom temperature is 140 to 200 °C, the reflux ratio is 1.8 to 3.0, and the operating pressure is 75 to 150 kPa; (6) Fresh composite solvent flow is added as S13 to supplement the solvent for the first and second extraction distillation towers.

[0022] The present invention provides a method for extractive distillation separation of an azeotropic system of methanol, n-hexane, tetrahydrofuran and water, which has the following characteristics: (1) The composite solvent is composed of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol. The components are simple and easy to obtain, which greatly reduces the solvent cost; (2) For the complex azeotropic system formed by methanol, n-hexane, tetrahydrofuran and water, the composite solvent used in the present invention can effectively break the azeotropic pairs, have good extraction effect and selectivity for each component, reduce the agent-to-material ratio, and significantly reduce operating costs and fixed investment; optimize the process flow to recover heat, effectively reduce operating energy consumption, and realize the full process recycling of solvents; the yield of the three organic compounds can reach more than 97%, and the purity can reach more than 99.2% (mass percentage).

[0023] (3) The composite solvent has no strict requirements on the content of each component of methanol, n-hexane, tetrahydrofuran and water, and can be applied to quaternary mixtures of different contents of the above substances; (4) The separated water can directly meet national emission standards or be used as process water.

[0024] The raw material to be separated is the mixed waste liquid generated in the production process of polyimide (PI), pharmaceuticals and emerging polymer materials. The main components of the mixed waste liquid are methanol, n-hexane, tetrahydrofuran and water, which are recorded as logistics S01. First, the material logistics S01 to be separated is pumped into the middle and lower part of the first extractive distillation tower T101, and the composite solvent logistics S02 is added from the upper part of the tower. A mixture of n-hexane and tetrahydrofuran is obtained at the top of the tower, which is recorded as logistics S03. The composite solvent, methanol and water mixture is extracted from the bottom of the tower, which is recorded as logistics S04 and enters the first solvent desorption tower T102 from the middle part; methanol with a purity of ≥99.2% is obtained at the top of T102, and the desorbed dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol in the bottom of the tower form a composite solvent. After the temperature is adjusted by the first composite solvent heat exchanger E101, it is recycled. The composite solvent and water logistics S06 extracted from the side line in the middle and lower part enter the side line tower T103 from the middle; the removed water logistics S04 is obtained at the top of T103. 08, the composite solvent stream S09 obtained in the bottom of the tower is combined with S07 for recycling. A mixture of n-hexane and tetrahydrofuran (THF) S03 enters the second extractive distillation tower T104 from the middle and lower portion, and composite solvent stream S12 enters T104 from the upper portion. A tetrahydrofuran stream S10 with a purity of ≥99.2% is obtained at the top of this tower. A composite solvent stream S11, composed of dimethyl sulfoxide, N-methylpyrrolidone, and ethylene glycol, and tetrahydrofuran are obtained in the bottom of the tower and enter the second solvent desorption tower T105 from the middle portion. A n-hexane stream S14 with a purity of ≥99.2% is obtained at the top of T105, and a decomposed composite solvent stream S15 is obtained in the bottom of the tower. This stream is temperature-controlled in the second composite solvent heat exchanger E102 and recycled. When the composite solvent level in the bottoms of towers T101 and T104 is low, fresh composite solvent can be replenished from stream S13.

[0025] The present invention will be further described below by way of examples.

[0026] Example: Example 1: according to Figure 1 The process flow adopts a mixture of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol as a composite solvent, wherein the mass percentage content of dimethyl sulfoxide is 70%, the mass percentage content of N-methylpyrrolidone is 20%, and the mass percentage content of ethylene glycol is 10%.

[0027] The mixed waste liquid of methanol, n-hexane, tetrahydrofuran and water generated in the production process of polyimide (PI), pharmaceuticals and emerging polymer materials has the following mass percentages: methanol 24.8%, n-hexane 31.2%, tetrahydrofuran 27.8% and water 16.2%.

[0028] (1) First, the material stream S01 to be separated enters the 50th plate of the first extractive distillation tower T101 from the middle and lower part (the tower has 90 theoretical plates. Calculated from top to bottom, the top is the 1st plate, the same below). The composite solvent of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol enters T101 from the 6th plate. A mixture of n-hexane and tetrahydrofuran with a content of ≥99.6% is obtained at the top of the tower, which is recorded as stream S03. A mixture rich in composite solvent, methanol and water, from which n-hexane and tetrahydrofuran have been removed, is obtained at the bottom of the tower, which is recorded as stream S04. (2) Logistics S04 enters the first solvent desorption tower T102 from the 20th plate (a total of 30 plates), and methanol with a purity of ≥99.33% is obtained at the top of the tower, which is recorded as logistics S05. A mixture of water and composite solvent is produced from the side line of the 26th plate, which is recorded as logistics S06. Logistics S07 rich in composite solvent is obtained at the bottom of the tower, and the temperature is adjusted by the first composite solvent heat exchanger E101 before entering T101 for recycling; (3) Logistics S06 enters the side-line tower T103 from the 15th tray (25 trays in total), and the water logistics S08 is removed from the top of the tower; (4) Logistics S03 enters the second extractive distillation tower T104 from the 60th plate (85 plates in total), and the composite solvent logistics S12 enters the T104 tower from the 5th plate. The top of the tower obtains n-hexane with a purity of ≥99.43%, which is recorded as logistics S10. The bottom of the tower obtains a mixture of composite solvent and tetrahydrofuran, which is recorded as logistics S11. (5) Logistics S11 enters the second solvent desorption tower T105 from the 10th plate (a total of 25 plates), and tetrahydrofuran with a purity of ≥99.56% is obtained at the top of the tower, which is recorded as logistics S14. The desorbed composite solvent is obtained at the bottom of the tower. After the temperature is adjusted by the second composite solvent heat exchanger E102, it enters the T104 tower and is recycled; (6) When the amount of composite solvent in the bottoms of towers T101 and T104 is small, fresh composite solvent is added through logistics S13.

[0029] In the above process, the operating conditions of each operating unit are as follows: (1) Operating conditions of the first extractive distillation tower T101: the number of theoretical plates is 90, the feed temperature of the stream to be separated S01 is 45°C, the feed temperature of the composite solvent S02 is 50°C, the bottom temperature is 121.5°C, the top temperature is 63.8°C, the reflux ratio is 2.4, the mass ratio of the composite solvent to the stream to be separated is 4.0:1, and the operating pressure is 103 kPa; (2) Operating conditions of the first solvent desorption tower T102: theoretical plate number 30, bottom temperature 201.3°C, top temperature 64.9°C, reflux ratio 2.1, operating pressure 105 kPa; (3) Operating conditions of side column T103: theoretical plate number 25, bottom temperature 158.4°C, top temperature 100.8°C, reflux ratio 2.0, operating pressure 104 kPa; (4) Operating conditions of the second extractive distillation column T104: theoretical plate number 85, feed temperature of composite solvent S12 48°C, bottom temperature 165.5°C, top temperature 65.8°C, reflux ratio 3.0, mass ratio of composite solvent to material to be separated 5.2:1, operating pressure 105 kPa; (5) The operating conditions of the second solvent desorption tower T105 are as follows: the theoretical number of plates is 25, the top temperature is 69.1°C, the bottom temperature is 196.7°C, the reflux ratio is 2.8, and the operating pressure is 105 kPa.

[0030] Combine Figure 1 According to the process, the yield of methanol was 98.3% and the purity was 99.33% (mass percentage, the same below); the yield of n-hexane was 98.4% and the purity was 99.56%; the yield of tetrahydrofuran was 98.1% and the purity was 99.43%.

[0031] Table 1 Raw material composition and product quality Example 2: according to Figure 1 The process flow adopts a mixture of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol as a composite solvent, wherein the mass percentage content of dimethyl sulfoxide is 85.0%, the mass percentage content of N-methylpyrrolidone is 8.0%, and the mass percentage content of ethylene glycol is 7.0%.

[0032] The mixed waste liquid of methanol, n-hexane, tetrahydrofuran and water generated in the production process of polyimide (PI), pharmaceuticals and emerging polymer materials has the following mass percentages: methanol 31.91%, n-hexane 18.96%, tetrahydrofuran 28.46% and water 20.67%.

[0033] (1) First, the material stream S01 to be separated enters the 50th plate of the first extractive distillation tower T101 from the middle and lower part (the tower has 85 theoretical plates. Calculated from top to bottom, the top is the 1st plate, the same below). The composite solvent of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol enters T101 from the 7th plate. A mixture of n-hexane and tetrahydrofuran with a content of ≥99.7% is obtained at the top of the tower, which is recorded as stream S03. A mixture rich in composite solvent, methanol and water, from which n-hexane and tetrahydrofuran have been removed, is obtained at the bottom of the tower, which is recorded as stream S04. (2) Logistics S04 enters the first solvent desorption tower T102 from the 25th plate (a total of 40 plates), and methanol with a purity of ≥99.42% is obtained at the top of the tower, which is recorded as logistics S05. A mixture of water and composite solvent is produced from the side line of the 26th plate, which is recorded as logistics S06. Logistics S07 rich in composite solvent is obtained at the bottom of the tower, and the temperature is adjusted by the first composite solvent heat exchanger E101 before entering T101 for recycling; (3) Logistics S06 enters the side-line tower T103 from the 15th tray (a total of 20 trays), and the water logistics S08 is removed from the top of the tower, which can meet the discharge standards; (4) Logistics S03 enters the second extractive distillation tower T104 from the 65th plate (80 plates in total), and the composite solvent logistics S12 enters the T104 tower from the 6th plate. The top of the tower obtains n-hexane with a purity of ≥99.51%, which is recorded as logistics S10. The bottom of the tower obtains a mixture of composite solvent and tetrahydrofuran, which is recorded as logistics S11. (5) Logistics S11 enters the second solvent desorption tower T105 from the 15th plate (a total of 20 plates), and tetrahydrofuran with a purity of ≥99.49% is obtained at the top of the tower, which is recorded as logistics S14. The desorbed composite solvent is obtained at the bottom of the tower. After the temperature is adjusted by the second composite solvent heat exchanger E102, it enters the T104 tower and is recycled; (6) When the amount of composite solvent in the bottoms of towers T101 and T104 is small, fresh composite solvent is added through logistics S13.

[0034] In the above process, the operating conditions of each operating unit are as follows: (1) Operating conditions of the first extractive distillation tower T101: the number of theoretical plates is 85, the feed temperature of the stream to be separated S01 is 40°C, the feed temperature of the composite solvent S02 is 45°C, the bottom temperature is 119.3°C, the top temperature is 61.7°C, the reflux ratio is 2.1, the mass ratio of the composite solvent to the stream to be separated is 3.5:1, and the operating pressure is 96 kPa; (2) Operating conditions of the first solvent desorption tower T102: theoretical plate number 40, bottom temperature 197.5°C, top temperature 60.4°C, reflux ratio 2.3, operating pressure 95 kPa; (3) Operating conditions of side column T103: theoretical plate number 20, bottom temperature 189.3°C, top temperature 99.6°C, reflux ratio 2.3, operating pressure 100 kPa; (4) Operating conditions of the second extractive distillation tower T104: theoretical plate number 80, feed temperature of composite solvent S12 45°C, bottom temperature 152.6°C, top temperature 58.6°C, reflux ratio 2.8, mass ratio of composite solvent to material to be separated 6.1:1, operating pressure 80 kPa; (5) The operating conditions of the second solvent desorption tower T105 are as follows: the theoretical number of plates is 30, the tower top temperature is 65.2°C, the tower bottom temperature is 190.9°C, the reflux ratio is 3.0, and the operating pressure is 95 kPa.

[0035] Combine Figure 1 According to the process, the methanol yield was 99.1% and the purity was 99.42% (mass percentage, the same below); the n-hexane yield was 98.2% and the purity was 99.51%; the tetrahydrofuran yield was 98.5% and the purity was 99.49%.

[0036] Table 2 Raw material composition and product quality Example 3: according to Figure 1 The process flow adopts a mixture of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol as a composite solvent, wherein the mass percentage content of dimethyl sulfoxide is 80.0%, the mass percentage content of N-methylpyrrolidone is 10.0%, and the mass percentage content of ethylene glycol is 10.0%.

[0037] The mixed waste liquid of methanol, n-hexane, tetrahydrofuran and water generated in the production process of polyimide (PI), pharmaceuticals and emerging polymer materials has the following mass percentages: methanol 32.17%, n-hexane 26.08%, tetrahydrofuran 30.52% and water 11.23%.

[0038] (1) First, the material stream S01 to be separated enters the 60th plate of the first extractive distillation tower T101 from the middle and lower part (the tower has 78 theoretical plates. Calculated from top to bottom, the top is the 1st plate, the same below). The composite solvent of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol enters T101 from the 5th plate. A mixture of n-hexane and tetrahydrofuran with a content of ≥99.6% is obtained at the top of the tower, which is recorded as stream S03. A mixture rich in composite solvent, methanol and water, from which n-hexane and tetrahydrofuran have been removed, is obtained at the bottom of the tower, which is recorded as stream S04. (2) Logistics S04 enters the first solvent desorption tower T102 from the 25th plate (a total of 35 plates), and methanol with a purity of ≥99.37% is obtained at the top of the tower, which is recorded as logistics S05. A mixture of water and composite solvent is produced from the side line of the 26th plate, which is recorded as logistics S06. Logistics S07 rich in composite solvent is obtained at the bottom of the tower, and the temperature is adjusted by the first composite solvent heat exchanger E101 before entering T101 for recycling; (3) Logistics S06 enters the side-line tower T103 from the 10th tray (a total of 18 trays), and the water logistics S08 is removed from the top of the tower, which can meet the discharge standards; (4) Logistics S03 enters the second extractive distillation tower T104 from the 58th plate (75 plates in total), and the composite solvent logistics S12 enters the T104 tower from the 5th plate. The top of the tower obtains n-hexane with a purity of ≥99.45%, which is recorded as logistics S10. The bottom of the tower obtains a mixture of composite solvent and tetrahydrofuran, which is recorded as logistics S11. (5) Logistics S11 enters the second solvent desorption tower T105 from the 25th plate (40 plates in total), and tetrahydrofuran with a purity of ≥99.52% is obtained at the top of the tower, which is recorded as logistics S14. The desorbed composite solvent is obtained at the bottom of the tower. After the temperature is adjusted by the second composite solvent heat exchanger E102, it enters the T104 tower and is recycled; (6) When the amount of composite solvent in the bottoms of towers T101 and T104 is small, fresh composite solvent is added through logistics S13.

[0039] In the above process, the operating conditions of each operating unit are as follows: (1) Operating conditions of the first extractive distillation tower T101: the number of theoretical plates is 78, the feed temperature of the stream to be separated S01 is 60°C, the feed temperature of the composite solvent S02 is 48°C, the bottom temperature is 113.7°C, the top temperature is 56.2°C, the reflux ratio is 2.3, the mass ratio of the composite solvent to the stream to be separated is 5.5:1, and the operating pressure is 80 kPa; (2) Operating conditions of the first solvent desorption tower T102: theoretical plate number 35, bottom temperature 193.4°C, top temperature 57.8°C, reflux ratio 2.0, operating pressure 80 kPa; (3) Operating conditions of side column T103: theoretical plate number 18, bottom temperature 180.4°C, top temperature 84.9°C, reflux ratio 1.8, operating pressure 85 kPa; (4) Operating conditions of the second extractive distillation column T104: theoretical plate number 75, feed temperature of composite solvent S12 48°C, bottom temperature 154.9°C, top temperature 60.6°C, reflux ratio 2.1, mass ratio of composite solvent to material to be separated 4.9:1, operating pressure 85 kPa; (5) The operating conditions of the second solvent desorption tower T105 are as follows: the theoretical number of plates is 40, the top temperature is 57.7°C, the bottom temperature is 188.2°C, the reflux ratio is 2.0, and the operating pressure is 75 kPa.

[0040] Combine Figure 1 According to the process, the yield of methanol was 99.6% and the purity was 99.37% (mass percentage, the same below); the yield of n-hexane was 98.6% and the purity was 99.45%; the yield of tetrahydrofuran was 98.2% and the purity was 99.52%.

[0041] Table 3 Raw material composition and product quality The embodiments of the present invention are presented for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art. The embodiments are chosen and described in order to better illustrate the principles of the invention and its practical application and to enable those skilled in the art to understand the invention and design various embodiments with various modifications as suited for specific applications.

Claims

1. A method for separating a methanol-n-hexane-tetrahydrofuran-water azeotropic system by extractive distillation, characterized in that: The invention discloses an extractive distillation process using a composite solvent as an extractant, wherein the composite solvent is a mixture of dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol, wherein the mass percentage of dimethyl sulfoxide is 70%-85%, the mass percentage of N-methylpyrrolidone is 8%-20%, and the mass percentage of ethylene glycol is 7%-15%.

2. The method according to claim 1, characterized in that The raw material to be separated is a mixture of methanol, n-hexane, tetrahydrofuran and water, with the mass percentages of each component being: 24.8% methanol, 31.2% n-hexane, 27.8% tetrahydrofuran and 16.2% water. First, the material flow S01 to be separated enters the lower middle part of the first extractive distillation tower (T101), and the composite solvent flow S02 is adjusted to a suitable temperature through a heat exchanger and then enters the upper part of the tower (T101). A mixture flow S03 of n-hexane and tetrahydrofuran is obtained at the top of the tower, and a flow S04 rich in methanol, water and composite solvents, from which n-hexane and tetrahydrofuran have been removed, is extracted from the bottom of the tower.

3. The method according to claim 2, characterized in that Logistics S04 enters the first solvent desorption tower (T102) from the middle, and methanol logistics S05 with a purity of ≥99.2% is obtained at the top of the tower. Logistics S06 of composite solvent and water is produced from the side line at the lower part of the tower (T102). The desorbed composite solvent logistics S07 is obtained at the bottom of the tower and recycled after the temperature is adjusted by the heat exchanger.

4. The method according to claim 3, characterized in that Logistics S06 enters the side-line tower (T103) from the middle, and the separated water logistics S08 is obtained at the top of the tower, and the desorbed composite solvent logistics S09 is obtained at the bottom of the tower, which is combined with logistics S07 and recycled.

5. The method according to claim 4, characterized in that Logistics S03 enters the second extractive distillation tower (T104) from the middle and lower part, and the composite solvent logistics S12 enters the upper part of the tower (T104) after being adjusted to the appropriate temperature by the heat exchanger. The top of the tower obtains tetrahydrofuran logistics S10 with a content of more than 99.2%, and the bottom of the tower obtains a mixture flow S11 of composite solvent and n-hexane.

6. The method according to claim 5, characterized in that Logistics S11 enters the second solvent desorption tower (T105) from the middle, and tetrahydrofuran logistics S14 with a content of more than 99.2% is obtained at the top of the tower. The desorbed composite solvent logistics S15 is obtained at the bottom of the tower and recycled after the temperature is adjusted by a heat exchanger.

7. The method according to any one of claim 6, characterized in that The operating conditions of the first extractive distillation tower (T101) are as follows: the number of theoretical plates is 40-90, the feed temperature of the stream to be separated S01 is 30-50°C, the feed temperature of the composite solvent S02 is 40-55°C, the bottom temperature is 80-130°C, the top temperature is 45-70°C, the reflux ratio is 1.3-3.5, the ratio of the composite solvent to the material to be separated is 2.2-5.6:1 (mass ratio), and the operating pressure is 86-200kPa.

8. The method according to any one of claim 6, characterized in that The operating conditions of the first solvent desorption tower (T102) are: the number of theoretical plates is 10-40, the top temperature is 56-79°C, the bottom temperature is 130-210°C, the reflux ratio is 1.5-3.0, and the operating pressure is 80-150kPa.

9. The method according to any one of claim 6, characterized in that The operating conditions of the side column (T103) are: the number of theoretical plates is 15-30, the top temperature is 90-110℃, the bottom temperature is 130-200℃, the reflux ratio is 1.8-2.6, and the operating pressure is 85-140kPa.

10. The method according to any one of claim 6, characterized in that: The operating conditions of the second extractive distillation tower (T104) are as follows: the number of theoretical plates is 50-85, the feed temperature of the composite solvent S12 is 40-55°C, the top temperature is 60-75°C, the bottom temperature is 140-180°C, the reflux ratio is 1.8-2.9, the ratio of the composite solvent to the material to be separated is 2.5-7.1:1 (mass ratio), and the operating pressure is 80-130 kPa; the operating conditions of the second solvent desorption tower (T105) are as follows: the number of theoretical plates is 10-40, the top temperature is 60-74°C, the bottom temperature is 140-200°C, the reflux ratio is 1.8-3.0, and the operating pressure is 75-150 kPa.

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