Method and system for recovering industrial-grade n-heptane from toluene-containing n-heptane waste liquid

Abstract

The invention discloses a method and a system for recovering industrial-grade n-heptane from toluene-containing n-heptane waste liquid, and belongs to the technical field of solvent recovery and purification. The method comprises the following steps: mixing a toluene-containing n-heptane crude product with concentrated sulfuric acid in a static mixer, carrying out sulfonation reaction, enabling toluene to generate p-toluenesulfonic acid, dissolving the p-toluenesulfonic acid in sulfuric acid, and carrying out hydrocyclone separation, water washing, secondary water separation and rectification treatment to obtain the toluene-free high-purity n-heptane. The method has the advantages of continuous process, simplicity and convenience in operation, capability of thoroughly removing toluene and meeting the requirements of zero toluene products, suitability for the recovery requirements of high-purity non-polar solvents in pharmaceutical and fine chemical industries, low energy consumption, low cost, environmental friendliness and the like.

Description

Technical Field

[0001] This invention relates to the field of chemical separation and solvent recovery technology, specifically to a method and system for recovering high-purity n-heptane from mixed waste liquid containing toluene, which is particularly suitable for the resource recovery and high-value purification of n-heptane waste liquid generated in the pharmaceutical and fine chemical industries. Background Technology

[0002] n-Heptane, as an important nonpolar solvent, has wide applications in organic synthesis, especially in pharmaceuticals and fine chemicals. Due to its strong inertness and high boiling point, using n-heptane as a solvent can significantly improve the safety of active substances such as n-butyllithium during storage and transportation. However, n-heptane wastewater generated in pharmaceutical and chemical production processes often contains impurities such as toluene, severely affecting its recycling value. Toluene is reactive and easily participates in or interferes with synthetic reactions, especially in pharmaceutical processes, potentially leading to complex side reactions and uncontrollable impurity structures, thereby affecting product quality and the accuracy of pharmacological analysis.

[0003] In existing technologies, the separation and purification of toluene-containing n-heptane systems mainly employs methods such as extractive distillation or azeotropic distillation. For example, while extractive distillation using phenol as the extractant can improve the purity of n-heptane, it cannot completely remove trace amounts of toluene, and the equipment investment and operating costs are high. Similarly, azeotropic distillation using ethanol as the azeotropic agent also suffers from toluene residue, and subsequent steps such as water washing and secondary distillation are required, making the process complex and energy-intensive. Furthermore, conventional distillation, due to the azeotropic relationship between n-heptane and toluene, makes it difficult to reduce the toluene content to below 0.1% even with repeated operations, failing to meet the stringent "zero toluene" standard for high-purity industrial-grade n-heptane.

[0004] Therefore, developing a method for the complete removal of toluene, with a simple process flow, low energy consumption, and suitability for industrial-scale n-heptane recovery has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the deficiencies in the existing technology and provide a method and system for recovering industrial-grade n-heptane from toluene-n-heptane-containing waste liquid. This method aims to: Complete removal of toluene: This addresses the technical bottleneck that traditional methods such as distillation, extractive distillation, or azeotropic distillation cannot completely remove trace amounts of toluene, thus preventing products from meeting the high standard requirement of "zero toluene".

[0006] Achieving process simplification and energy saving: Providing a purification process that is compact, continuous in operation, and has significantly lower energy consumption than traditional multiple distillation.

[0007] Improve safety and economy: The system utilizes the selectivity of chemical reactions to achieve efficient separation, and the by-product waste acid can be recycled, reducing treatment costs; at the same time, the system design fully considers the antistatic requirements of n-heptane, improving the intrinsic safety level.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for recovering industrial-grade n-heptane from toluene-n-heptane-containing waste liquid.

[0009] The core of this method lies in the combination of chemical transformation and physical separation, specifically including the following steps: (1) Sulfonation reaction: Crude n-heptane containing toluene and concentrated sulfuric acid are continuously passed into a static mixer and thoroughly mixed and reacted at a temperature of 70–97°C. Toluene undergoes a sulfonation reaction with concentrated sulfuric acid to produce p-toluenesulfonic acid, which is soluble in the sulfuric acid phase, while n-heptane does not participate in the reaction due to its chemical inertness. The amount of concentrated sulfuric acid added is 2–5 times the mass of toluene in the raw material.

[0010] (2) Acid phase separation: The reaction mixture of step (1) is subjected to first-stage hydrocyclone separation to achieve rapid and efficient separation of the light phase (rich in n-heptane) and the heavy phase (waste sulfuric acid containing p-toluenesulfonic acid).

[0011] (3) Washing: The separated light phase n-heptane crude product is washed with water to remove trace amounts of sulfuric acid that are entrained or dispersed therein.

[0012] (4) Dehydration: The washed material is subjected to a second-stage hydrocyclone separation to remove the free water (clear water).

[0013] (5) Refining: The dehydrated wet n-heptane is fed into a distillation column for atmospheric distillation. The light component containing water is collected from the top of the column, while the bottom of the column yields dry, pure, and toluene-free industrial-grade n-heptane.

[0014] Preferably, the static mixer is of type SV, consisting of 40 to 50 mixing units connected in series, with an aspect ratio L / D=15, to ensure sufficient mixing and reaction residence time (approximately 1 minute).

[0015] Preferably, the distillation column has 40 theoretical plates, the feed inlet is located between the 15th and 20th theoretical plates, and the operating reflux ratio is 1 to 5.

[0016] More preferably, in step (1), the amount of concentrated sulfuric acid added is 4.5 times the mass of toluene, and the reaction temperature is 95°C; in step (5), the reflux ratio of the distillation column is 1.5.

[0017] Secondly, the present invention provides a system for implementing the above-described method. The system includes: [list of components connected sequentially via pipes]. Static mixer: Equipped with a raw material inlet and a concentrated sulfuric acid inlet, it is used to achieve continuous mixing and sulfonation reaction of materials. It is preferably equipped with a jacket for steam heating.

[0018] First hydrocyclone separator: The inlet is connected to the outlet of the static mixer, and it is used to separate the waste acid heavy phase and the n-heptane light phase in the mixture after the reaction.

[0019] Washing tank: The inlet is connected to the light phase outlet of the first hydrocyclone separator and is equipped with a washing water inlet for washing and removing trace amounts of acid.

[0020] Second hydrocyclone separator: The inlet is connected to the light phase outlet of the washing tank, used to remove free water from the material.

[0021] Distillation column: The feed inlet is connected to the light phase outlet of the second hydrocyclone, which is used to finally remove dissolved water and obtain high-purity n-heptane product.

[0022] Preferably, the washing tank is a slender tank with a length-to-diameter ratio of not less than 5, to facilitate sufficient contact and sedimentation separation of the oil and water phases.

[0023] Preferably, the distillation column has 40 theoretical trays, and the feed inlet is located between the 15th and 20th trays.

[0024] Preferably, the distillation column is equipped with a reboiler temperature control device, and the controlled temperature is 102±0.5℃.

[0025] Preferably, the system further includes a plunger metering pump for conveying concentrated sulfuric acid, connected to the concentrated sulfuric acid inlet b of the static mixer.

[0026] Preferably, the distillation column is equipped with a condenser at the top, and the top outlet q is used to output aqueous n-heptane.

[0027] Preferably, the heavy phase outlet of the first hydrocyclone is connected to a wastewater treatment system so that the waste acid can be used for pH adjustment in pretreatment processes such as Fenton oxidation, thereby achieving resource utilization.

[0028] Preferably, all equipment and pipelines in the system that come into contact with n-heptane are made of metal and reliably grounded to eliminate the hazards of static electricity.

[0029] The beneficial effects of this invention are as follows: 1. By introducing a concentrated sulfuric acid sulfonation reaction, toluene is chemically converted into p-toluenesulfonic acid, which is soluble in the acid phase, fundamentally overcoming the physical separation limit imposed by the n-heptane-toluene azeotropic system. This method can completely remove toluene, achieving a toluene content in the product below the detection limit (e.g., 5 ppm), truly meeting the "0 toluene" standard and satisfying the stringent requirements for high-purity non-polar solvents in the pharmaceutical, high-end fine chemical, and other fields.

[0030] 2. The integrated process of "static mixing reaction + hydrocyclone separation + water washing + distillation" avoids the complex processes of traditional multiple distillations or extractive distillations. The reaction proceeds continuously in a tubular static mixer with a short residence time (approximately 1 minute); the distillation column is used only to remove trace amounts of dissolved water, with a small number of theoretical plates (40) and a low reflux ratio (1-5), significantly reducing steam consumption and equipment size. Overall energy consumption is reduced by approximately 30-50% compared to traditional processes.

[0031] 3. The system features a fully enclosed design, effectively preventing the leakage of volatile organic compounds; all equipment and pipelines in contact with n-heptane are made of metal and reliably grounded, completely eliminating the risk of static electricity buildup. Process parameters (such as sulfuric acid dosage, reaction temperature, reflux ratio, etc.) can be adjusted within a wide range, accommodating fluctuations in toluene content in the raw materials between 0.1% and 3%, ensuring continuous and stable operation.

[0032] 4. The waste acid (including p-toluenesulfonic acid) generated from the sulfonation reaction can be directly used for pH adjustment before Fenton oxidation of wastewater, achieving "waste treatment with waste" and reducing the cost of sulfuric acid purchase and wastewater treatment. The entire process produces no harmful gas emissions, and the washing wastewater can be treated after simple neutralization, which conforms to the concepts of green chemical engineering and circular economy.

[0033] 5. The system features a modular design, conventional equipment selection, smooth process integration, and easy automation control. Examples demonstrate that this method can stably produce qualified products within a wide range of operating conditions, exhibiting good process flexibility and scalability, making it particularly suitable for the resource recovery of n-heptane waste liquid in industries such as pharmaceuticals, pesticides, and electronic chemicals. Attached Figure Description

[0034] Figure 1 This is a process flow diagram of the method of the present invention. Detailed Implementation

[0035] The present invention will be further described in detail below with reference to embodiments, but the scope of protection of the present invention is not limited to the content described.

[0036] This invention provides a method for recovering industrial-grade n-heptane from n-heptane waste containing toluene. The core of this method lies in utilizing the sulfonation reaction between concentrated sulfuric acid and toluene to convert toluene, which is difficult to completely remove through physical separation, into p-toluenesulfonic acid, which is soluble in the acid phase, thereby achieving "zero residue" (0 toluene) of toluene in n-heptane. The entire system adopts a modular and continuous design, ensuring the high efficiency and stability of the process.

[0037] As attached Figure 1As shown, this method employs an integrated system, mainly comprising: a static mixer 1 for the sulfonation reaction, a first hydrocyclone 2 for acid phase separation, a water washing tank 3 for washing residual acid, a second hydrocyclone 4 for removing free water, and a distillation column 5 for final dehydration. All equipment and pipelines in contact with n-heptane are made of metal and reliably grounded to prevent static electricity buildup.

[0038] The process flow of this invention is as follows: 1. Crude n-heptane enters through port a of static mixer 1, and concentrated sulfuric acid is added through port b. The amount of concentrated sulfuric acid added is 2 to 5 times the mass of toluene, with 4.5 times being optimal. For static mixers, a higher flow rate results in better mixing.

[0039] The combined static mixer can be regarded as a tubular reactor in which toluene and concentrated sulfuric acid react to produce p-toluenesulfonic acid; Static mixer 1 has 40-50 groups to ensure sufficient reaction time (50 groups are optimal). The static mixer 1 is equipped with a jacket, which can introduce steam to provide the heat required for the reaction to accelerate the reaction process. The reaction temperature is 70~97℃ (optimal 95℃). The SV type is preferred for static mixers, but other types can also be used, although more sets may be required. The length-to-diameter ratio of the static mixer is L / D=15.

[0040] 2. After the material completes the chemical reaction in the static mixer 1, it enters from port c of the first hydrocyclone 2 at a speed of 0.5~1m / s (optimal 1m / s). After separation, the light phase leaves from port d of the first hydrocyclone 2 and enters the washing tank 3, while the heavy phase flows out from port e of the first hydrocyclone 2. The main component of the heavy phase is waste acid, which contains p-toluenesulfonic acid. It is sent to the wastewater station for acid adjustment before Fenton. The concentrated sulfuric acid is used almost regardless of cost.

[0041] 3. The light phase from the first hydrocyclone separator 2 enters the water washing tank 3 through port f, the clean water enters through port g, the light phase exits through port h, and the heavy phase, mainly water, is discharged from the bottom port j of the first hydrocyclone separator.

[0042] The purpose of water washing is to remove trace amounts of sulfuric acid dispersed in crude n-heptane. Water washing tank 3 should be made into a slender shape with a length-to-diameter ratio of not less than 5.

[0043] 4. The light phase flowing out of the water washing tank 3 enters through port k of the second hydrocyclone 4 and exits through port m. It then enters through port p of the distillation column 5. The heavy phase, consisting of a small amount of residual clear water, flows out through port n of the second hydrocyclone 4. The function of the second hydrocyclone 4 is to further remove water.

[0044] 5. After washing and water separation, the n-heptane contains a certain amount of saturated water, which enters from port p of distillation column 5. After distillation, the water-containing n-heptane at the top of the column is collected from port q, and the n-heptane product is collected from port r at the bottom of the column.

[0045] The number of trays in distillation column 5 is 40, and the feed inlet is located at the number of trays 15-20, with 15 being optimal. The operating conditions for distillation column 5 are: atmospheric pressure, reflux ratio of 1 to 5, with an optimal ratio of 1.5.

[0046] Example 1 (Preferred Condition Example) Raw material: crude n-heptane: flow rate 1000 kg / h, composition of n-heptane 98.5% (w / w), toluene 1.2% (w / w), and trace amounts of other impurities (such as moisture).

[0047] Concentrated sulfuric acid: 98% (w / w).

[0048] Equipment and process conditions: Sulfonation process: Crude n-heptane and concentrated sulfuric acid (added at a rate of 6 kg / h, i.e., 4.5 times the mass of toluene) are separately pumped through pipelines to SV-type static mixer 1. This mixer consists of 50 units connected in series (L / D=15), with 0.3 MPa steam introduced through the jacket to control the reaction temperature at 95℃. The residence time of the materials in the mixer is approximately 1 minute.

[0049] Acid phase separation: The reaction mixture enters the first hydrocyclone 2 tangentially at a flow rate of 1 m / s. After separation, the light phase (n-heptane) is collected from the top outlet, and the heavy phase (waste acid) is discharged from the bottom and sent to the wastewater treatment plant for pH adjustment before the subsequent Fenton oxidation process.

[0050] Washing process: Light phase n-heptane enters from the bottom of washing tank 3, while clean water (flow rate approximately 50 kg / h) is sprayed down from the top of the tank. The washing tank is a vertical, slender type with a length-to-diameter ratio of 5.5, ensuring sufficient hydraulic contact and sedimentation separation. The washed n-heptane overflows from the top of the tank.

[0051] Dehydration process: The washed material enters the second hydrocyclone separator 4 to further separate the entrained water.

[0052] Distillation process: Dehydrated wet n-heptane (flow rate approximately 1050 kg / h) enters distillation column 5, a 40-theoretical-plate column, from the 15th theoretical plate. Operating conditions are atmospheric pressure, with a reflux ratio controlled at 1.5. The column top temperature is maintained at 85°C, and aqueous n-heptane is collected (flow rate approximately 300 kg / h, which can be recycled or processed separately). The column bottom temperature is precisely controlled at 102.0 ± 0.5°C, and n-heptane is continuously collected from the column bottom.

[0053] Results: The heptane product from the bottom of the column was consistently produced at a flow rate of approximately 800 kg / h. Gas chromatography-mass spectrometry (GC-MS) analysis showed no toluene characteristic peaks in the product (detection limit below 5 ppm), achieving "zero toluene". The product meets relevant standards for industrial-grade heptane (such as distillation range, purity, and moisture content) and can be directly used as a solvent for reagents such as n-butyllithium.

[0054] Example 2 (Example with lower reaction temperature and sulfuric acid dosage) This embodiment aims to illustrate that, even under suboptimal conditions, this method can still effectively remove toluene.

[0055] Raw material: crude n-heptane: flow rate 800 kg / h, toluene content 0.8% (w / w).

[0056] Concentrated sulfuric acid: 98% concentration.

[0057] Equipment and process condition adjustments: Sulfonation process: The amount of concentrated sulfuric acid added is 3.0 times the mass of toluene (i.e., 800 × 0.8% × 3.0 = 19.2 kg / h). Static mixer 1 uses 45 units, the jacket steam pressure is reduced, and the reaction temperature is controlled at 80℃.

[0058] Acid phase separation and post-treatment: The feed flow rate of the first hydrocyclone 2 is adjusted to 0.7 m / s. The water washing and dewatering processes are the same as in Example 1.

[0059] Distillation process: The feed inlet position of distillation column 5 is adjusted to the 18th theoretical plate, the reflux ratio is adjusted to 2.0, and other conditions are the same as in Example 1.

[0060] Results: The heptane product from the bottom of the distillation column was approximately 650 kg / h. GC-MS analysis showed that the toluene content in the product was below 50 ppm. Although it did not reach the instrument's "zero detection" level, it was far below the 0.1% (1000 ppm) residue limit of conventional distillation methods, fully meeting the requirements of most high-demand industrial applications.

[0061] Example 3 (Example of treating raw materials with high toluene content) This embodiment illustrates that the method of the present invention has strong adaptability to raw materials, and can effectively process even high initial toluene content by adjusting the process.

[0062] Raw material: crude n-heptane: flow rate 1200 kg / h, toluene content 2.5% (w / w).

[0063] Concentrated sulfuric acid: 98% concentration.

[0064] Equipment and process condition adjustments: Sulfonation process: To ensure complete reaction, concentrated sulfuric acid is added at 5.0 times the mass of toluene (i.e., 1200 × 2.5% × 5.0 = 150 kg / h). Static mixer 1 still uses 50 units, and the reaction temperature is controlled at 97℃.

[0065] Acid phase separation and post-treatment: Due to the large amount of sulfuric acid added, the flow rate of the waste acid phase separated by the first hydrocyclone 2 increased significantly. The washing water flow rate in the washing tank 3 was correspondingly increased to 80 kg / h to ensure effective acid washing. The operating parameters of the second hydrocyclone 4 were fine-tuned to accommodate a larger total material throughput.

[0066] Distillation process: The feed rate was increased, and the reflux ratio of distillation column 5 was increased to 3.0 to maintain a stable gas-liquid load and separation efficiency within the column. The reboiler temperature was still controlled at 102.5℃.

[0067] Results: The flow rate of n-heptane from the bottom of the distillation column was approximately 1000 kg / h. Testing revealed that the residual toluene content in the product was below 5 ppm. These results indicate that even with feedstocks containing high toluene content, the method of this invention can still produce n-heptane of extremely high purity by proportionally increasing the amount of sulfuric acid and optimizing the distillation parameters.

[0068] The above embodiments demonstrate that the recovery method provided by this invention, through a combined process of "sulfonation reaction + distillation," can stably and efficiently purify toluene-containing n-heptane wastewater to an industrial-grade product with "0 toluene" or near-zero toluene content. This process exhibits good adaptability to toluene content in the raw materials (0.1%-3%). By adjusting key parameters such as the amount of sulfuric acid used (2-5 times the mass of toluene) and the reaction temperature (70-97℃), an optimized balance between energy and material consumption can be achieved while ensuring the removal effect. The entire system has a compact design, continuous and stable operation, and promising prospects for industrial application.

Claims

1. A method for recovering industrial-grade n-heptane from toluene-containing n-heptane wastewater, characterized in that, Includes the following steps: (1) Pass the crude n-heptane containing toluene and concentrated sulfuric acid into a static mixer in a certain proportion and react at 70-97℃ to sulfonate toluene and generate p-toluenesulfonic acid which dissolves in the sulfuric acid phase; (2) The mixture obtained in step (1) is subjected to a first hydrocyclone separation to obtain light phase crude n-heptane and heavy phase waste acid; (3) The light phase n-heptane crude product is washed with water to remove the trace amount of residual sulfuric acid; (4) Perform a second hydrocyclone separation on the washed material to further separate the clear water; (5) The oil phase obtained in step (4) is fed into a distillation column and distilled under normal pressure. Industrial grade n-heptane product is obtained from the bottom of the column, and water-containing n-heptane is collected from the top of the column.

2. The method according to claim 1, characterized in that, The amount of concentrated sulfuric acid added is 2-5 times the mass of toluene.

3. The method according to claim 1, characterized in that, The static mixer is of type SV, with 40-50 sets and an aspect ratio L / D=15.

4. The method according to claim 1, characterized in that, The static mixer is equipped with a jacket, and steam is introduced to control the reaction temperature at 95°C.

5. The method according to claim 1, characterized in that, The distillation column has a theoretical number of 40 trays, with the feed inlet located on trays 15-20; the reflux ratio is 1-5.

6. The method according to claim 1, characterized in that, The material from the outlet of the first hydrocyclone separator enters a washing tank, and the length-to-diameter ratio of the washing tank is not less than 5.

7. The method according to claim 1, characterized in that, The waste acid heavy phase is used to adjust the acidity of wastewater before the Fenton reaction.

8. The method according to claim 1, characterized in that, The crude n-heptane contains 0.1-3% toluene.

9. The method according to claim 1, characterized in that, The distillation column has a bottom operating temperature of 102±0.5℃ and a top temperature of 85℃.

10. A system for recovering industrial-grade n-heptane from toluene-n-heptane-containing wastewater, characterized in that, It includes the following components connected in sequence: a static mixer (1), a first hydrocyclone separator (2), a water washing tank (3), a second hydrocyclone separator (4), and a distillation column (5); The static mixer (1) is provided with a raw material inlet (a) and a concentrated sulfuric acid inlet (b), and its outlet is connected to the inlet (c) of the first hydrocyclone (2). The first hydrocyclone separator (2) is provided with a light phase outlet (d) and a heavy phase outlet (e), and the light phase outlet (d) is connected to the inlet (f) of the washing tank (3); The washing tank (3) is provided with an inlet (g) and a light phase outlet (h), and the light phase outlet (h) is connected to the inlet (k) of the second hydrocyclone (4); The second hydrocyclone separator (4) is provided with a light phase outlet (m) and a heavy phase outlet (n), and the light phase outlet (m) is connected to the feed inlet (p) of the distillation column (5); The distillation column (5) is provided with a top outlet (q) and a bottom outlet (r).

11. The system according to claim 10, characterized in that, The static mixer (1) is an SV type static mixer, with a quantity of 40-50 sets and an aspect ratio of L / D=15.

12. The system according to claim 10, characterized in that, The static mixer (1) is equipped with a jacket for introducing steam for heating and controlling the reaction temperature to 70-97℃.

13. The system according to claim 10, characterized in that, The heavy phase outlet (e) of the first hydrocyclone (2) is connected to the wastewater treatment system for supplying waste acid to the acid conditioning process before the Fenton reaction.

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