Mixed dinitrobenzene extraction separation process applied to nitration reaction

Abstract

The invention belongs to the technical field of nitration reaction product separation, and particularly relates to a mixed dinitrobenzene extraction separation process applied to nitration reaction. Through three-section gradient dilution and accurate concentration control, the problem of incomplete separation caused by high solubility of high-concentration sulfuric acid to nitrobenzene is effectively solved, meanwhile, the hidden danger of pipeline blockage caused by dinitro compound crystallization is eliminated, and the production safety and the product purity are remarkably improved. According to the invention, a complete material closed-loop system is established, the extractant-nitrobenzene is directly reused for nitration reaction after deep dehydration and impurity removal, and the waste sulfuric acid is completely returned to a production system for recycling after being treated, so that the utilization rate of raw materials is greatly improved, and the discharge of waste acid is reduced. According to the method, key parameters such as temperature, concentration and flow are monitored in real time, so that the nitration reaction yield is stabilized at 97.2% or above, the operation cycle of the device is effectively prolonged, and reliable technical guarantee is provided for industrial production.

Description

Technical Field

[0001] This invention belongs to the field of nitration reaction product separation technology, specifically relating to an extraction and separation process for mixed dinitrobenzene in nitration reactions. Background Technology

[0002] In the production of phenylenediamine, the separation of mixed dinitrobenzenes is a critical step. Current processes typically send the products of the secondary nitration reaction to a nitration separator to separate the mixed dinitrobenzenes from sulfuric acid. However, the resulting sulfuric acid has a concentration as high as 86-88%, at which point the solubility of the mixed dinitrobenzenes exceeds 20%, resulting in a large amount of dinitrobenzene failing to separate effectively and returning to the upstream nitration system with the sulfuric acid. This not only reduces the yield of the target product but also causes frequent wall adhesion and blockage in the pipelines due to the tendency of dinitrobenzenes to crystallize and precipitate as the temperature decreases, seriously affecting continuous production safety and increasing equipment maintenance costs.

[0003] To alleviate the above problems, existing technologies have attempted to dilute high-concentration sulfuric acid or employ extraction methods, but significant limitations remain. Single dilution methods struggle to achieve precise concentration control, resulting in large fluctuations in the solubility of dinitrobenzene in sulfuric acid and poor separation stability. Conventional extraction processes often have limited efficiency or require the introduction of external solvents, leading to difficulties in subsequent separation and potential secondary pollution. Furthermore, to achieve sulfuric acid reuse, existing processes often require the concentration of waste acid, a process that is energy-intensive and accompanied by acid mist emissions, creating environmental pressure.

[0004] Therefore, there is an urgent need to develop a new process for the extraction and separation of mixed dinitrobenzene that can balance separation efficiency and operational safety, in order to break through the existing technological bottlenecks and achieve efficient, stable and clean operation of the nitration production process. Summary of the Invention

[0005] The purpose of this invention is to address existing problems by providing a process for the extraction and separation of mixed dinitrobenzene in nitration reactions.

[0006] This invention is achieved through the following technical solution:

[0007] An extraction and separation process for mixed dinitrobenzene used in nitration reactions includes the following steps:

[0008] S1, Precise ratio of two-stage waste acid:

[0009] The 68-72% concentration waste sulfuric acid obtained from the primary nitration reaction and the 86-88% concentration nitrate-containing sulfuric acid obtained from the nitration reaction are introduced into a static mixer and mixed to obtain a uniformly mixed acid solution.

[0010] S2, Three-stage gradient dilution and temperature-coordinated regulation:

[0011] The mixed acid solution in step S1 is sequentially passed through a three-stage dilution system to achieve a target concentration of 81-82%.

[0012] S3, Multi-stage countercurrent extraction separation:

[0013] The acid solution with the concentration adjusted in step S2 is introduced into a multi-stage countercurrent extraction system, and the mononitrobenzene generated by the first-stage nitration reaction is used as the extractant to perform 5 to 8 stages of countercurrent extraction.

[0014] S4, Closed-loop material circulation:

[0015] After extraction, the mononitrobenzene extract phase rich in mixed dinitrobenzene is dehydrated and directly refluxed to the secondary nitration reactor as raw material, while the raffinate phase containing 82-83% sulfuric acid is directly transported to the primary nitration system for recycling.

[0016] Furthermore, when the 68-72% concentration waste sulfuric acid and the 86-88% concentration nitrate-containing sulfuric acid are mixed in step S1, the mass ratio is 1:(1-1.3).

[0017] During mixing, the mixing temperature should be controlled at 70~80℃ and the mixing time at 10~15min.

[0018] Furthermore, the three-stage dilution described in step S2 is as follows: the first stage is pre-diluted to 83-84% with hot water at 85-90℃; the second stage is moderately diluted to 82-82.5% with warm water at 70-75℃; and the third stage is finely diluted to the target concentration of 81-82% with cold water at 50-55℃, while the acid temperature is lowered to 50-60℃.

[0019] Furthermore, the three-stage dilution system described in step S2 adopts flow interlock control. By monitoring the sulfuric acid concentration and temperature at each stage outlet in real time, the flow rate and temperature of the dilution water at each stage are automatically adjusted to ensure that the sulfuric acid concentration fluctuation range is ≤ ±0.3% and the temperature fluctuation range is ≤ ±2℃.

[0020] Further, in step S3, the volume ratio of the extractant to the acid solution is 1:(1.5~2.0);

[0021] The extraction conditions were as follows: 4-6 stages of countercurrent extraction were carried out at 0.02-0.08 MPa and 45-55℃, with a total extraction time of 20-30 min.

[0022] Furthermore, the extractant, nitrobenzene, is dehydrated by modified molecular sieve before use, with the water content controlled below 100 ppm, and accumulated trace impurities are periodically removed by adsorbent during use.

[0023] Furthermore, the preparation of the modified molecular sieve includes the following steps:

[0024] (1) Place ZSM-5 molecular sieve into a muffle furnace and calcine at 500~600℃ for 4~5h. After cooling to room temperature, add deionized water at a ratio of 1g:(4.5~5.5)mL, stir to disperse and form a suspension. Adjust the pH to 3.0~4.0 with 1mol / L hydrochloric acid, reflux at 80~85℃ for 2~3h, filter, wash with deionized water until the pH of the filtrate is neutral, and then dry at 100~110℃ for 12~15h to obtain the pretreated ZSM-5 carrier;

[0025] (2) The pretreated ZSM-5 carrier was added to a 0.1 mol / L LiNO3 aqueous solution at a solid-liquid ratio of 1 g: (8~10) mL, stirred to form a suspension, and then stirred at 60~65℃ for 4~5 h. After washing with deionized water 3~4 times, it was dried at 100~110℃ for 8~10 h to obtain the hydrophilic modified intermediate.

[0026] (3) Dissolve methyltriethoxysilane (MTES) in anhydrous ethanol at a mass-volume ratio of 1g:(25~30)mL, stir evenly, adjust the pH to 8.0~9.0 with 1mol / L ammonia water to obtain a modified solution, then add the hydrophilic modification intermediate to the above modified solution at a solid-liquid ratio of 1g:(5~6)mL, stir at a constant temperature of 50~55℃ for 2~3h, filter, wash 2~3 times with anhydrous ethanol, and then dry at 100~105℃ for 6~8h;

[0027] (4) Place the dried molecular sieve into a tube furnace and keep it at 140~160℃ for 3~4h under N2 atmosphere (N2 flow rate is 50mL / min). Then raise the temperature to 190~210℃ and keep it for 2~3h. After cooling to room temperature, sieve to obtain the modified molecular sieve.

[0028] Furthermore, the adsorbent is a microporous adsorbent material with a BET specific surface area of ​​not less than 1000 m². 2 / g, and after being soaked in a 5wt% sulfuric acid aqueous solution at 60℃ for 24h, its crystal structure remains stable and the specific surface area loss is less than 20%.

[0029] Furthermore, the adsorbent is a metal-organic framework material UiO-66 (BASF Basolite Z1200).

[0030] Furthermore, the dehydration treatment described in step S4 adopts a combination of coalescence separation and membrane dehydration process. First, coarse dehydration is carried out through a polypropylene coalescence separator at 40~60℃ and 0.2~0.5MPa; then, deep fine dehydration is carried out through a hydrophilic molecular sieve membrane module at 60~80℃, so that the water content of the extract phase is stabilized below 50ppm, and then it is reused in the secondary nitration reaction.

[0031] The hydrophilic molecular sieve membrane is a NaA-type zeolite membrane.

[0032] Furthermore, it also includes S5, online monitoring and feedback control:

[0033] Online concentration analyzers, temperature sensors, and flow meters are installed at key points throughout the process, and the entire process is automatically controlled through a DCS system to ensure that process parameters remain stable within the optimal range.

[0034] The present invention has the following advantages over the prior art:

[0035] 1. This invention controls the sulfuric acid concentration fluctuation range within ±0.2% through three-stage gradient dilution and precise concentration control, greatly reducing the residual amount of mixed dinitrobenzene in sulfuric acid and increasing the extraction efficiency to over 95.6%. It effectively solves the problem of incomplete separation caused by the high solubility of nitrobenzene in high-concentration sulfuric acid, while eliminating the risk of pipeline blockage caused by dinitrobenzene crystallization, and significantly improving production safety and product purity.

[0036] 2. This invention establishes a complete closed-loop material system. The extractant, nitrobenzene, is directly recycled to the nitration reaction after deep dehydration and impurity removal. Waste sulfuric acid is treated and returned to the production system for recycling, which greatly improves the utilization rate of raw materials and reduces the discharge of waste acid.

[0037] 3. By monitoring key parameters such as temperature, concentration, and flow rate in real time, this invention effectively prevents the risk of pipeline crystallization blockage and equipment corrosion, keeps the yield of nitro nitration reaction stable at over 97.2%, effectively extends the operating cycle of the device, and provides reliable technical support for industrial production. Detailed Implementation

[0038] To further explain the present invention, the following specific embodiments are described.

[0039] Example 1

[0040] An extraction and separation process for mixed dinitrobenzene used in nitration reactions includes the following steps:

[0041] S1, Precise ratio of two-stage waste acid:

[0042] The 68% concentration waste sulfuric acid obtained from the primary nitration reaction and the 88% concentration nitrate-containing sulfuric acid obtained from the nitration reaction were introduced into a static mixer at a mass ratio of 1:1. The mixing temperature was controlled at 70℃ and the mixing time was 10 min to obtain a uniformly mixed acid solution.

[0043] S2, Three-stage gradient dilution and temperature-coordinated regulation:

[0044] The mixed acid solution in step S1 is passed through a three-stage dilution system. The first stage is pre-diluted to 83% with 85°C hot water, the second stage is moderately diluted to 82% with 70°C warm water, and the third stage is finely diluted to the target concentration of 81% with 50°C low-temperature water, while the acid solution temperature is lowered to 50°C.

[0045] The three-stage dilution system adopts flow interlock control. By monitoring the sulfuric acid concentration and temperature at each stage outlet in real time, it automatically adjusts the flow rate and temperature of the dilution water at each stage to ensure that the sulfuric acid concentration fluctuation range is ≤±0.3% and the temperature fluctuation range is ≤±2℃.

[0046] S3, Multi-stage countercurrent extraction separation:

[0047] The acid solution with the concentration adjusted in step S2 was introduced into a multi-stage countercurrent extraction system. The mononitrobenzene generated by the first-stage nitration reaction was used as the extractant. The volume ratio of the extractant to the acid solution was 1:1.5. Four-stage countercurrent extraction was carried out at 0.02 MPa and 45°C for a total extraction time of 20 min.

[0048] The multi-stage countercurrent extraction system includes four extraction units connected in series. Each unit is equipped with a dedicated stirring system and a phase separation interface control system to control the axial flow velocity of the continuous phase to 0.05 m / s and the axial flow velocity of the dispersed phase to 0.02 m / s.

[0049] The extractant mononitrobenzene is dehydrated by modified molecular sieve before use, and the water content is controlled below 100 ppm. During use, the accumulated trace impurities are removed periodically by the adsorbent metal-organic framework material UiO-66 (BASF Basolite Z1200).

[0050] The preparation of the modified molecular sieve includes the following steps:

[0051] (1) ZSM-5 molecular sieve was placed in a muffle furnace and calcined at 500℃ for 4 hours. After cooling to room temperature, deionized water was added at a ratio of 1g:4.5mL. The mixture was stirred and dispersed to form a suspension. The pH was adjusted to 3.0 with 1mol / L hydrochloric acid and refluxed at 80℃ for 2 hours. After filtration, the filtrate was washed with deionized water until the pH of the filtrate was neutral. The filtrate was then dried at 100℃ for 12 hours to obtain the pretreated ZSM-5 carrier.

[0052] (2) The pretreated ZSM-5 carrier was added to a 0.1 mol / L LiNO3 aqueous solution at a solid-liquid ratio of 1 g: 8 mL, and stirred to form a suspension. After stirring at 60 °C for 4 h, it was washed three times with deionized water and dried at 100 °C for 8 h to obtain the hydrophilic modified intermediate.

[0053] (3) Methyltriethoxysilane (MTES) was dissolved in anhydrous ethanol at a mass-volume ratio of 1g:25mL, stirred evenly, and the pH was adjusted to 8.0 with 1mol / L ammonia water to obtain the modified solution. Then, the hydrophilic modification intermediate was added to the above modified solution at a solid-liquid ratio of 1g:5mL. After stirring at 50℃ for 2h, the solution was filtered, washed twice with anhydrous ethanol, and then dried at 100℃ for 6h.

[0054] (4) Place the dried molecular sieve into a tube furnace and, under N2 atmosphere (N2 flow rate of 50 mL / min), first keep it at 140℃ for 3 h, then raise the temperature to 190℃ and keep it at 190℃ for 2 h, then cool it to room temperature and sieve it to obtain the modified molecular sieve.

[0055] S4, Closed-loop material circulation:

[0056] After extraction, the mononitrobenzene extract phase rich in mixed dinitrobenzene is dehydrated and directly refluxed to the secondary nitration reactor as raw material, while the raffinate phase of 82% sulfuric acid is directly sent to the primary nitration system for recycling.

[0057] The dehydration of the extract phase employs a combined process of coalescence separation and membrane dehydration. First, coarse dehydration is carried out using a polypropylene coalescence separator at 40°C and 0.2 MPa; then, deep fine dehydration is performed using a hydrophilic molecular sieve membrane module at 60°C, ultimately stabilizing the water content of the extract phase to below 50 ppm, which is then reused in the secondary nitration reaction. The hydrophilic molecular sieve membrane is a NaA type zeolite membrane.

[0058] S5. Online monitoring and feedback control:

[0059] Online concentration analyzers, temperature sensors, and flow meters are installed at key points throughout the process, and the entire process is automatically controlled through a DCS system to ensure that process parameters remain stable within the optimal range.

[0060] Example 2

[0061] An extraction and separation process for mixed dinitrobenzene used in nitration reactions includes the following steps:

[0062] S1, Precise ratio of two-stage waste acid:

[0063] The 70% concentration waste sulfuric acid obtained from the primary nitration reaction and the 87% concentration nitrate-containing sulfuric acid obtained from the nitration reaction were introduced into a static mixer at a mass ratio of 1:1.2. The mixing temperature was controlled at 75℃ and the mixing time was 12 minutes to obtain a uniformly mixed acid solution.

[0064] S2, Three-stage gradient dilution and temperature-coordinated regulation:

[0065] The mixed acid solution in step S1 is passed through a three-stage dilution system. The first stage is pre-diluted to 83.5% with 88°C hot water, the second stage is moderately diluted to 82% with 73°C warm water, and the third stage is finely diluted to the target concentration of 81.5% with 53°C low-temperature water, while the acid solution temperature is lowered to 55°C.

[0066] The three-stage dilution system adopts flow interlock control. By monitoring the sulfuric acid concentration and temperature at each stage outlet in real time, it automatically adjusts the flow rate and temperature of the dilution water at each stage to ensure that the sulfuric acid concentration fluctuation range is ≤±0.3% and the temperature fluctuation range is ≤±2℃.

[0067] S3, Multi-stage countercurrent extraction separation:

[0068] The acid solution with the concentration adjusted in step S2 was introduced into a multi-stage countercurrent extraction system. The mononitrobenzene generated by the first-stage nitration reaction was used as the extractant. The volume ratio of the extractant to the acid solution was 1:1.8. The five-stage countercurrent extraction was carried out at 0.05 MPa and 48 °C for a total extraction time of 25 min.

[0069] The multi-stage countercurrent extraction system includes five extraction units connected in series. Each unit is equipped with a dedicated stirring system and a phase separation interface control system, which controls the axial flow velocity of the continuous phase to be 0.07 m / s and the axial flow velocity of the dispersed phase to be 0.03 m / s.

[0070] The extractant mononitrobenzene is dehydrated by modified molecular sieve before use, and the water content is controlled below 100 ppm. During use, the accumulated trace impurities are removed periodically by the adsorbent metal-organic framework material UiO-66 (BASF Basolite Z1200).

[0071] The preparation of the modified molecular sieve includes the following steps:

[0072] (1) ZSM-5 molecular sieve was placed in a muffle furnace and calcined at 550℃ for 4.5h. After cooling to room temperature, deionized water was added at a ratio of 1g:5mL. The mixture was stirred and dispersed to form a suspension. The pH was adjusted to 3.5 with 1mol / L hydrochloric acid. The suspension was refluxed at 83℃ for 2.5h. After filtration, the filtrate was washed with deionized water until the pH of the filtrate was neutral. The filtrate was then dried at 105℃ for 13h to obtain the pretreated ZSM-5 carrier.

[0073] (2) The pretreated ZSM-5 carrier was added to a 0.1 mol / L LiNO3 aqueous solution at a solid-liquid ratio of 1 g: 9 mL, and stirred to form a suspension. After stirring at 63 °C for 4.5 h, it was washed three times with deionized water and dried at 105 °C for 9 h to obtain the hydrophilic modified intermediate.

[0074] (3) Methyltriethoxysilane (MTES) was dissolved in anhydrous ethanol at a mass-volume ratio of 1g:28mL, stirred evenly, and the pH was adjusted to 8.5 with 1mol / L ammonia water to obtain a modified solution. Then, the hydrophilic modification intermediate was added to the above modified solution at a solid-liquid ratio of 1g:5.5mL. After stirring at 53℃ for 2.5h, the solution was filtered, washed twice with anhydrous ethanol, and then dried at 102℃ for 7h.

[0075] (4) Place the dried molecular sieve into a tube furnace and, under N2 atmosphere (N2 flow rate of 50 mL / min), first keep it at 150℃ for 3.5 h, then raise the temperature to 200℃ and continue to keep it at 2.5 h, then cool it to room temperature and sieve it to obtain the modified molecular sieve.

[0076] S4, Closed-loop material circulation:

[0077] After extraction, the mononitrobenzene extract phase rich in mixed dinitrobenzene is dehydrated and directly refluxed to the secondary nitration reactor as raw material, while the raffinate phase of 82.5% sulfuric acid is directly sent to the primary nitration system for recycling.

[0078] The dehydration of the extract phase employs a combination of coalescence separation and membrane dehydration processes. First, coarse dehydration is carried out using a polypropylene coalescence separator at 50°C and 0.35 MPa; then, deep fine dehydration is performed using a hydrophilic molecular sieve membrane module at 70°C, ultimately stabilizing the water content of the extract phase to below 50 ppm, which is then reused in the secondary nitration reaction. The hydrophilic molecular sieve membrane is a NaA type zeolite membrane.

[0079] S5. Online monitoring and feedback control:

[0080] Online concentration analyzers, temperature sensors, and flow meters are installed at key points throughout the process, and the entire process is automatically controlled through a DCS system to ensure that process parameters remain stable within the optimal range.

[0081] Example 3

[0082] An extraction and separation process for mixed dinitrobenzene used in nitration reactions includes the following steps:

[0083] S1, Precise ratio of two-stage waste acid:

[0084] The 72% concentration waste sulfuric acid obtained from the primary nitration reaction and the 86% concentration nitrate-containing sulfuric acid obtained from the nitration reaction were introduced into a static mixer at a mass ratio of 1:1.3. The mixing temperature was controlled at 80℃ and the mixing time was 15 minutes to obtain a uniformly mixed acid solution.

[0085] S2, Three-stage gradient dilution and temperature-coordinated regulation:

[0086] The mixed acid solution in step S1 is passed through a three-stage dilution system. The first stage is pre-diluted to 84% with 90°C hot water, the second stage is moderately diluted to 82.5% with 75°C warm water, and the third stage is finely diluted to the target concentration of 82% with 55°C low-temperature water, while the acid solution temperature is lowered to 60°C.

[0087] The three-stage dilution system adopts flow interlock control. By monitoring the sulfuric acid concentration and temperature at each stage outlet in real time, it automatically adjusts the flow rate and temperature of the dilution water at each stage to ensure that the sulfuric acid concentration fluctuation range is ≤±0.3% and the temperature fluctuation range is ≤±2℃.

[0088] S3, Multi-stage countercurrent extraction separation:

[0089] The acid solution with the concentration adjusted in step S2 was introduced into a multi-stage countercurrent extraction system. The mononitrobenzene generated by the first-stage nitration reaction was used as the extractant. The volume ratio of the extractant to the acid solution was 1:2.0. Six-stage countercurrent extraction was carried out at 0.08 MPa and 55 °C for a total extraction time of 30 min.

[0090] The multi-stage countercurrent extraction system includes six extraction units connected in series. Each unit is equipped with a dedicated stirring system and a phase separation interface control system, which controls the axial flow velocity of the continuous phase to be 0.08 m / s and the axial flow velocity of the dispersed phase to be 0.04 m / s.

[0091] The extractant mononitrobenzene is dehydrated by modified molecular sieve before use, and the water content is controlled below 100 ppm. During use, the accumulated trace impurities are removed periodically by the adsorbent metal-organic framework material UiO-66 (BASF Basolite Z1200).

[0092] The preparation of the modified molecular sieve includes the following steps:

[0093] (1) ZSM-5 molecular sieve was placed in a muffle furnace and calcined at 600℃ for 5h. After cooling to room temperature, deionized water was added at a ratio of 1g:5.5mL. The mixture was stirred and dispersed to form a suspension. The pH was adjusted to 4.0 with 1mol / L hydrochloric acid. The suspension was refluxed at 85℃ for 3h. After filtration, the filtrate was washed with deionized water until the pH of the filtrate was neutral. The filtrate was then dried at 110℃ for 15h to obtain the pretreated ZSM-5 carrier.

[0094] (2) The pretreated ZSM-5 carrier was added to a 0.1 mol / L LiNO3 aqueous solution at a solid-liquid ratio of 1 g: 10 mL, and stirred to form a suspension. After stirring at 65 °C for 5 h, it was washed 4 times with deionized water and dried at 110 °C for 10 h to obtain the hydrophilic modified intermediate.

[0095] (3) Dissolve methyltriethoxysilane (MTES) in anhydrous ethanol at a mass-volume ratio of 1g:30mL, stir evenly, adjust the pH to 9.0 with 1mol / L ammonia water to obtain the modified solution, then add the hydrophilic modification intermediate to the above modified solution at a solid-liquid ratio of 1g:6mL, stir at 55℃ for 3h, filter, wash 3 times with anhydrous ethanol, and then dry at 105℃ for 8h.

[0096] (4) Place the dried molecular sieve into a tube furnace and, under N2 atmosphere (N2 flow rate of 50 mL / min), first keep it at 160℃ for 4 hours, then raise the temperature to 210℃ and keep it at that temperature for 3 hours, then cool it to room temperature and sieve it to obtain the modified molecular sieve.

[0097] S4, Closed-loop material circulation:

[0098] After extraction, the mononitrobenzene extract phase rich in mixed dinitrobenzene is dehydrated and directly refluxed to the secondary nitration reactor as raw material, while the raffinate phase of 83% sulfuric acid is directly sent to the primary nitration system for recycling.

[0099] The dehydration of the extract phase employs a combined process of coalescence separation and membrane dehydration. First, coarse dehydration is carried out using a polypropylene coalescence separator at 60°C and 0.5 MPa; then, deep fine dehydration is performed using a hydrophilic molecular sieve membrane module at 80°C, ultimately stabilizing the water content of the extract phase to below 50 ppm, which is then reused in the secondary nitration reaction. The hydrophilic molecular sieve membrane is a NaA type zeolite membrane.

[0100] S5. Online monitoring and feedback control:

[0101] Online concentration analyzers, temperature sensors, and flow meters are installed at key points throughout the process, and the entire process is automatically controlled through a DCS system to ensure that process parameters remain stable within the optimal range.

[0102] Comparative Example 1

[0103] Compared with Example 2, Comparative Example 1 uses "single-stage dilution" instead of "three-stage gradient dilution and temperature-coordinated control". The mixed acid solution is directly diluted to 81.5% with 73°C warm water. There is no graded temperature control or flow interlock control. Other steps are the same as in Example 2.

[0104] Comparative Example 2

[0105] Compared with Example 2, Comparative Example 2 uses "conventional 3A molecular sieve" instead of "modified ZSM-5 molecular sieve", while the other steps are the same as in Example 2.

[0106] Comparative Example 3

[0107] Compared with Example 2, Comparative Example 3 uses "single-stage extraction" instead of "5-stage countercurrent extraction". The extraction method is single-stage stirred extraction, the extraction time is 25 min, the reagent-liquid ratio is 1:1.8, the pressure is 0.05 MPa, and the other steps are the same as in Example 2.

[0108] Comparative Example 4

[0109] Compared with Example 2, Comparative Example 4 omits the "UiO-66 adsorbent for impurity removal", and the mononitrobenzene is only dehydrated without an online impurity removal step. The other steps are the same as in Example 2.

[0110] Indicator Test

[0111] The separation process parameters of Examples 1-3 and Comparative Examples 1-4 were tested. All groups used the same experimental apparatus and detection methods. The sulfuric acid concentration was detected according to GB / T 11198.1-2018; the mixed dinitrobenzene content was detected according to GB / T 23961-2009; the water content was detected according to GB / T 6283-2008; and the impurity content was detected according to high performance liquid chromatography.

[0112] The specific test results are shown in Table 1 below.

[0113] Table 1

[0114] Sulfuric acid concentration fluctuation range (%) Extraction rate of mixed dinitrobenzene (%) Residual amount of dinitrobenzene in sulfuric acid (%) Moisture content (ppm) of mononitrobenzene after pretreatment Total mass of impurities in mononitrobenzene (ppm) Secondary nitration yield (%) Example 1 ±0.2 95.6 9.6 80 8.8 97.2 Example 2 ±0.2 96.5 8.2 85 8.5 98.1 Example 3 ±0.1 97.3 7.9 75 8.3 98.0 Comparative Example 1 ±0.8 82.3 15.3 83 9.2 92.5 Comparative Example 2 ±0.2 95.8 8.7 186 9.0 96.3 Comparative Example 3 ±0.2 75.1 22.6 84 8.8 90.7 Comparative Example 4 ±0.2 96.3 8.3 87 126 93.8

[0115] As can be seen from Table 1 above, Comparative Example 1 uses single-stage dilution, while Example 2 uses three-stage gradient dilution + flow interlock control, which precisely controls the sulfuric acid concentration fluctuation within ±0.2%, far superior to the single-stage dilution of Comparative Example 1. The small concentration fluctuation directly reduces the solubility fluctuation of mixed dinitrobenzene, thereby increasing the extraction rate by 14.2%. This shows that the three-stage gradient dilution + flow interlock control of the present invention can effectively improve the separation efficiency.

[0116] Comparative Example 2 uses a conventional 3A molecular sieve instead of the modified ZSM-5 molecular sieve in Example 2. The water content in Example 2 is controlled at 85 ppm, while the water content in Comparative Example 2 is as high as 186 ppm. Excessive water content will lead to a decrease in the yield of the secondary nitration reaction. The oleophilic-hydrophilic dual-functional design of the modified molecular sieve of this invention solves the problem of poor dispersibility and incomplete dehydration of conventional molecular sieves in the organic phase, thus verifying the necessity of modification.

[0117] Comparative Example 3 used single-stage extraction, while Example 2 used 5-stage countercurrent extraction. It can be seen that compared with Comparative Example 3, the extraction rate of mixed dinitrobenzene in Example 2 was increased by 21.4%. This may be because the multi-stage extraction design enhances mass transfer through multiple countercurrent contacts, which can achieve efficient separation at a low agent-to-liquid ratio, overcoming the limitation of low efficiency at high agent-to-liquid ratios in single-stage extraction.

[0118] Comparative Example 4 omits the UiO-66 adsorbent for impurity removal. Compared to Example 2, Comparative Example 4 has an impurity content as high as 126 ppm due to the absence of an impurity removal step, which is much higher than that of Example 2. The accumulation of impurities will lead to a decrease in the yield of the secondary nitration reaction, indicating that the presence of trace impurities will have a negative impact on the stability of the reaction. The targeted impurity removal capability of UiO-66 in this invention solves the industry pain point of impurity interference in the nitration reaction.

[0119] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A process for the extraction and separation of mixed dinitrobenzene in a nitration reaction, characterized in that, Includes the following steps: S1, Precise ratio of two-stage waste acid: The 68-72% concentration waste sulfuric acid obtained from the primary nitration reaction and the 86-88% concentration nitrate-containing sulfuric acid obtained from the nitration reaction are introduced into a static mixer and mixed to obtain a uniformly mixed acid solution. S2, Three-stage gradient dilution and temperature-coordinated regulation: The mixed acid solution in step S1 is sequentially passed through a three-stage dilution system to achieve a target concentration of 81-82%. S3, Multi-stage countercurrent extraction separation: The acid solution with the concentration adjusted in step S2 is introduced into a multi-stage countercurrent extraction system, and the mononitrobenzene generated by the first-stage nitration reaction is used as the extractant to perform 5 to 8 stages of countercurrent extraction. S4, Closed-loop material circulation: After extraction, the mononitrobenzene extract phase rich in mixed dinitrobenzene is dehydrated and directly refluxed to the secondary nitration reactor as raw material, while the raffinate phase containing 82-83% sulfuric acid is directly transported to the primary nitration system for recycling.

2. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, When the 68-72% concentration waste sulfuric acid and the 86-88% concentration nitrate-containing sulfuric acid are mixed in step S1, the mass ratio is 1:(1-1.3). During mixing, the mixing temperature should be controlled at 70~80℃ and the mixing time at 10~15min.

3. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, The three-stage dilution described in step S2 is as follows: the first stage is pre-diluted with hot water at 85~90℃ to 83~84%, the second stage is moderately diluted with warm water at 70~75℃ to 82~82.5%, and the third stage is finely diluted with cold water at 50~55℃ to the target concentration of 81~82%, while the acid temperature is lowered to 50~60℃.

4. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, The three-stage dilution system described in step S2 adopts flow interlock control. By monitoring the sulfuric acid concentration and temperature at each stage outlet in real time, the flow rate and temperature of the dilution water at each stage are automatically adjusted to ensure that the sulfuric acid concentration fluctuation range is ≤ ±0.3% and the temperature fluctuation range is ≤ ±2℃.

5. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, The volume ratio of the extractant to the acid solution in step S3 is 1:(1.5~2.0); The extraction conditions were as follows: 4-6 stages of countercurrent extraction were carried out at 0.02-0.08 MPa and 45-55℃, with a total extraction time of 20-30 min.

6. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, The extractant, nitrobenzene, is dehydrated by modified molecular sieve before use, with the water content controlled below 100 ppm. During use, accumulated trace impurities are periodically removed by an adsorbent.

7. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 6, characterized in that, The preparation of the modified molecular sieve includes the following steps: (1) Place ZSM-5 molecular sieve into a muffle furnace and calcine at 500~600℃ for 4~5h. After cooling to room temperature, add deionized water at a ratio of 1g:(4.5~5.5)mL, stir to disperse and form a suspension. Adjust the pH to 3.0~4.0 with 1mol / L hydrochloric acid, reflux at 80~85℃ for 2~3h, filter, wash with deionized water until the pH of the filtrate is neutral, and then dry at 100~110℃ for 12~15h to obtain the pretreated ZSM-5 carrier; (2) The pretreated ZSM-5 carrier was added to a 0.1 mol / L LiNO3 aqueous solution at a solid-liquid ratio of 1 g: (8~10) mL, stirred to form a suspension, and then stirred at 60~65℃ for 4~5 h. After washing with deionized water 3~4 times, it was dried at 100~110℃ for 8~10 h to obtain the hydrophilic modified intermediate. (3) Dissolve methyltriethoxysilane in anhydrous ethanol at a mass-volume ratio of 1g:(25~30)mL, stir evenly, adjust the pH to 8.0~9.0 with 1mol / L ammonia water to obtain a modified solution, then add the hydrophilic modification intermediate to the above modified solution at a solid-liquid ratio of 1g:(5~6)mL, stir at a constant temperature of 50~55℃ for 2~3h, filter, wash 2~3 times with anhydrous ethanol, and then dry at 100~105℃ for 6~8h; (4) Place the dried molecular sieve into a tube furnace and, under N2 atmosphere, first keep it at 140~160℃ for 3~4h, then raise the temperature to 190~210℃ and continue to keep it at 2~3h, then cool it to room temperature and sieve it to obtain the modified molecular sieve.

8. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 6, characterized in that, The adsorbent is a microporous adsorbent material with a BET specific surface area of ​​not less than 1000 m². 2 / g, and after being soaked in a 5wt% sulfuric acid aqueous solution at 60℃ for 24h, its crystal structure remains stable and the specific surface area loss is less than 20%.

9. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, The dehydration process described in step S4 adopts a combination of coalescence separation and membrane dehydration. First, coarse dehydration is carried out through a polypropylene coalescence separator at 40~60℃ and 0.2~0.5MPa. Then, deep fine dehydration is carried out through a hydrophilic molecular sieve membrane module at 60~80℃, so that the water content of the extract phase is stabilized below 50ppm and then reused in the secondary nitration reaction. The hydrophilic molecular sieve membrane is a NaA-type zeolite membrane.

10. The extraction and separation process for mixed dinitrobenzene in a nitration reaction according to claim 1, characterized in that, It also includes S5, online monitoring and feedback control: Online concentration analyzers, temperature sensors, and flow meters are installed at key points throughout the process, and the entire process is automatically controlled through a DCS system to ensure that process parameters remain stable within the optimal range.