A continuous crystallization purification method of 3,4-dichloronitrobenzene

Abstract

The present application belongs to the technical field of fine chemical separation, and discloses a continuous crystallization purification method and system for 3,4-dichloronitrobenzene. The present application solves the core technical problems of the traditional rectification process, such as serious coking of heat-sensitive materials, limited product purity, excessive color, large residue loss, high energy consumption, and poor stability in mass production, from multiple dimensions, such as material thermal stability protection, precise separation of isomers, step-by-step utilization of rectification energy consumption, inhibition of coking side reactions, and full recovery of materials. The present application does not require catalyst modification and complex equipment modification. Through rectification condition reconstruction, heat integration system optimization, and impurity removal mechanism innovation, the present application significantly reduces the probability of material high-temperature decomposition and the amount of coking impurities generated, significantly improves the purity and color quality of the finished product, and effectively reduces the energy consumption and material loss of the rectification line.

Description

Technical Field

[0001] This invention belongs to the field of fine chemicals, and specifically discloses a method and system for the continuous crystallization purification of 3,4-dichloronitrobenzene. Background Technology

[0002] 3,4-Dichloronitrobenzene is a core fine chemical raw material for pharmaceutical antibacterial intermediates, high-end reactive dyes, agricultural green herbicides, and the modification of polymer functional materials. The purity, color, and impurity content of the product directly determine the selectivity of downstream fine synthesis reactions and the quality of the final product. The crude 3,4-dichloronitrobenzene obtained after nitration and crystallization pretreatment still contains small amounts of light acidic impurities, moisture, low-boiling-point nitration byproducts, and a large number of 2,3-dichloronitrobenzene and 2,4-dichloronitrobenzene isomers with boiling points close to the target product. It also contains trace amounts of high-temperature polymerizable colloidal precursor impurities. These impurities cannot be completely removed by conventional crystallization processes and must be purified to high purity through distillation.

[0003] Currently, the industrial-scale refining of 3,4-dichloronitrobenzene in China generally adopts the traditional single-tower atmospheric pressure high-temperature distillation process. This process suffers from serious technical shortcomings, including poor compatibility with heat-sensitive materials, high energy consumption, severe coking, low purity limits, and large losses. It can no longer meet the requirements for mass production of high-purity products and green, low-carbon production in the current high-end chemical industry. The traditional process uses a single atmospheric pressure distillation mode, with high tower bottom operating temperatures, long high-temperature residence times of materials, no heat recovery mechanism, and no graded control for impurity removal. During mass production, problems such as material thermal decomposition, gum polymerization and coking, excessive color of finished products, incomplete removal of isomers, serious accumulation of residue at the bottom of the tower, and persistently high overall energy consumption are prone to occur. The industry faces significant technical bottlenecks.

[0004] After a thorough review of existing industrial distillation processes and publicly available patented technologies, seven major unresolved industry pain points have been identified in the current 3,4-dichloronitrobenzene distillation and refining process. These issues severely hinder product quality upgrades and energy conservation and cost reduction in production lines. The specific issues are as follows:

[0005] Question 1: Atmospheric pressure high-temperature distillation makes heat-sensitive materials extremely prone to coking and decomposition. The temperature of the bottom of a traditional atmospheric pressure distillation column far exceeds the thermal stability critical temperature of 3,4-dichloronitrobenzene. When materials are exposed to high-temperature environments for a long time, they are prone to thermal decomposition and thermal polymerization side reactions, generating dark-colored colloids and high-boiling coking impurities. This results in poor color and high impurity content in the finished product, and the product quality cannot meet the high-end standards of pharmaceutical and reagent grades.

[0006] Question 2: Mixed distillation of light and heavy impurities results in low removal precision and severe catalytic coking. Traditional single-tower distillation lacks a staged impurity removal mechanism, simultaneously separating light component acidic impurities, moisture, and heavy component isomers and colloidal impurities. At high temperatures, acidic light impurities significantly catalyze the polymerization and coking of materials, exacerbating byproduct formation. Meanwhile, the isomer separation precision is insufficient, making it difficult to exceed the 99.8% purity limit of the finished product.

[0007] Question 3: Long high-temperature residence time of materials leads to continuous coking side reactions. Traditional distillation columns have large bottom storage capacity, intermittent feeding, and long material residence time. Some materials remain in the high-temperature bottom of the column and heat exchange pipelines for a long time, continuously undergoing coking and deterioration. This not only reduces the yield of finished products but also causes scaling and blockage of the packing material inside the column, shortening the equipment's operating cycle.

[0008] Question 4: Direct discharge of distillation waste heat results in significant energy loss and high energy costs. Traditional distillation processes lack integrated heat recovery systems, and all high-temperature steam waste heat from the top of the column is directly discharged through cooling water heat exchange, leading to a large waste of low-grade heat energy. The steam and cooling energy consumption of the entire distillation system remains high, resulting in high operating costs for mass production and failing to meet the requirements of green and low-carbon production.

[0009] Question 5: Lack of gradient temperature control mechanism leads to turbulent temperature within the column and poor separation stability. Traditional single-column distillation operates under constant temperature throughout the entire process, without gradient temperature field control. This makes it unable to adapt to the differentiated separation requirements of impurities with different boiling points. When operating conditions fluctuate, problems such as material loss, impurity peak carry-over, and large purity fluctuations are prone to occur, resulting in poor batch product consistency.

[0010] Question 6: Direct disposal of bottom residue leads to significant material loss and high solid waste production. Traditional processes accumulate a large amount of unseparated, effective 3,4-dichloronitrobenzene components in the bottom residue of the tower, which is mostly treated directly as hazardous waste. This not only causes serious waste of material resources but also significantly increases solid waste treatment costs and environmental pressure.

[0011] Question 7: The single-tower operating condition has weak anti-disturbance capability and cannot be adapted to continuous steady-state mass production. Traditional single-tower distillation is highly sensitive to fluctuations in feed composition, flow rate, and temperature. Without pre-treatment for pressure stabilization and adaptive control mechanisms, the operating conditions are prone to fluctuations, making it difficult to achieve long-term continuous and stable high-purity mass production. The production efficiency and level of intelligence are relatively low. Summary of the Invention

[0012] This invention provides a continuous crystallization purification method for 3,4-dichloronitrobenzene, characterized by comprising the following continuous purification steps:

[0013] S1. Crude material pretreatment and pressure stabilization feed: The crude 3,4-dichloronitrobenzene material produced by nitration crystallization is fed into a closed pressure stabilization buffer unit for constant temperature static degassing and sedimentation pretreatment of trace suspended impurities. The feed temperature, feed pressure and feed flow rate are kept constant to eliminate fluctuations in material composition, bubble disturbance and impurity suspension that cause fluctuations in the distillation column operation, and to prepare a homogeneous purified feed liquid with stable composition, no gas phase inclusions and no suspended impurities.

[0014] S2. Primary negative pressure light component removal distillation: The homogeneous feed liquid after pressure stabilization pretreatment is sent to the light impurity removal distillation column. Low negative pressure and low temperature distillation conditions are adopted to precisely control the vacuum degree, bottom temperature and reflux ratio in the column. Residual water, trace amounts of nitrated acid, low boiling point by-products, light color impurities and other light component impurities in the system are preferentially removed. The catalytic coking side reaction caused by long-term high temperature enrichment of light component impurities is avoided, and the intermediate refined feed liquid with light impurities removed is obtained.

[0015] S3, differential pressure thermal integration and heat coupling control, collects the waste heat steam from the top of the first-stage light impurity removal column, and provides a heat source for the preheating of the feed and auxiliary heating of the bottom of the second-stage refining distillation column through the heat exchange coupling system. Relying on the negative pressure difference between the two-stage columns, a stable thermal integration cycle is formed, realizing the cascade recovery and on-site reuse of distillation waste heat, which greatly reduces the steam and electricity consumption of the entire distillation system and avoids the energy waste problem of direct discharge of traditional distillation waste heat;

[0016] S4, a two-stage negative pressure gradient precision distillation process, feeds the intermediate liquid after primary light component removal into a high-purity distillation column. Utilizing a high negative pressure, low-temperature gradient distillation condition, and based on the boiling point differences between 3,4-dichloronitrobenzene and its isomers and heavy component colloidal impurities, the gradient temperature field, vacuum level, and precise reflux ratio within the column are precisely controlled. This allows for the segmented separation of heavy component impurities such as 2,3-dichloronitrobenzene and 2,4-dichloronitrobenzene isomers, maximizing the purity of the target product. Simultaneously, the column bottom temperature is strictly controlled to remain below the material's thermal decomposition temperature, thus inhibiting the coking and decomposition of heat-sensitive materials at the source.

[0017] S5, short-stay continuous feeding anti-coking control, adopts a multi-stage buffer continuous conveying mode throughout the process, shortens the residence time of materials in the high-temperature tower bottom and heat exchange pipeline, eliminates the thermal polymerization and coking deterioration problems caused by long-term high-temperature static storage of materials, dynamically matches the feed rate, evaporation rate and output rate, maintains the steady-state balance of gas-liquid ratio in the tower, and ensures continuous and stable operation of distillation.

[0018] S6. Low-temperature collection and homogenization buffering of finished products: The high-purity 3,4-dichloronitrobenzene vapor collected from the top of the secondary distillation column is rapidly condensed at low temperature and filtered at constant temperature to remove trace impurities. It is then collected at low temperature and stable pressure and sent to a high-purity finished product buffer tank for sealed storage. The entire process is carried out in a low-temperature closed-loop operation to avoid secondary oxidation and discoloration of the finished product and contamination by impurities, thus ensuring the stability of the color and purity of the finished product.

[0019] S7. Refined recycling of bottom residue: Collect the heavy component colloidal and high-content isomer residues enriched at the bottom of the first and second towers. After filtration, desolidification, component detection and conditioning pretreatment, the residues are directionally refluxed to the front-end crystallization pretreatment process for repurification, so as to achieve full recovery of effective materials in the residues and eliminate waste and loss of residues and increase in solid waste.

[0020] S8 features full-condition negative pressure steady-state adaptive control, which collects parameters such as vacuum degree, temperature gradient, reflux ratio, output flow rate, material color and purity in real time for two-stage distillation columns. It dynamically links and adjusts the operating conditions of the negative pressure system, heat exchange system and reflux system, and adaptively compensates for disturbances such as feed fluctuations, ambient temperature changes and equipment heat exchange attenuation, ensuring continuous steady-state refining production with low coking, low energy consumption and high purity throughout the entire process.

[0021] Furthermore, the operating parameters for the primary negative pressure light component removal distillation in step S2 are as follows: vacuum degree inside the column 0.06~0.08MPa, bottom temperature 95~105℃, top temperature 75~85℃, reflux ratio controlled at 1.2:1~1.8:1, and single column residence time ≤25min, to thoroughly remove acidic impurities and low-boiling-point byproducts of light components from the system and eliminate coking catalytic factors.

[0022] Furthermore, the operating parameters for the two-stage negative pressure gradient precision distillation described in step S4 are as follows: vacuum degree inside the column 0.09~0.095MPa, column bottom temperature 110~120℃, column top temperature 88~98℃, temperature gradient difference inside the column controlled at 18~22℃, reflux ratio controlled at 2.5:1~3.5:1, and column bottom temperature strictly below the thermal decomposition critical temperature of 3,4-dichloronitrobenzene throughout the process, thus completely suppressing the decomposition and coking of heat-sensitive materials.

[0023] Furthermore, in step S3, the differential pressure heat integration and heat coupling control maintains the negative pressure difference between the two-stage distillation columns at 0.01~0.035MPa. The waste heat steam at the top of the first-stage column forms a closed-loop heat exchange with the feed pipeline of the second-stage column and the auxiliary heat loop at the bottom of the column through a plate heat exchanger. The waste heat recovery utilization rate is ≥92%, and the overall energy consumption of the entire system is reduced by 18%~25%.

[0024] Furthermore, the short-dwelling continuous feeding described in step S5 adopts a variable frequency quantitative conveying + multi-stage buffer and pressure stabilization structure, with a total residence time of the material in the high-temperature section of ≤40min, which is more than 45% shorter than the residence time of the traditional distillation process, significantly reducing the probability of thermal polymerization, coking and deterioration of heat-sensitive materials.

[0025] Furthermore, in step S7, the bottom residue of the tower is recycled. The discharge temperature of the bottom residue is controlled at 105~115℃. After being filtered by a precision filter to remove solid coking impurities and activated carbon for simple decolorization and degradation, the qualified components are directionally refluxed to the front-end crystallization and dissolution unit for remelting and purification. The effective material recovery rate of the residue is ≥98%.

[0026] Furthermore, the low-temperature rapid condensation described in step S6 adopts a low-temperature gradient condensation mode with circulating cooling water. The condensation temperature is controlled at 35~45℃, and the condensation rate is adapted to the top sampling rate of the tower to avoid oxidation and discoloration of the high-temperature finished product due to prolonged retention. The color of the finished product is ≤20, and the purity is stable at ≥99.9%.

[0027] Furthermore, a low-coking, low-energy-consumption 3,4-dichloronitrobenzene negative pressure coupled distillation purification system is provided for implementing the aforementioned negative pressure coupled distillation purification process, comprising:

[0028] The feed pressure stabilization pretreatment module is used for degassing, sedimentation and impurity removal, pressure stabilization and flow stabilization of crude materials, providing homogeneous, stable and undisturbed distillation feed;

[0029] The primary negative pressure light impurity removal and distillation module is used to remove light components such as acidic water, low-boiling-point impurities, and light-colored pollutants at low temperatures, thereby eliminating the catalytic factors that induce coking.

[0030] The two-stage differential pressure thermal integrated coupling energy-saving module is used to recover the waste heat of the first-stage distillation and provide heat for the second-stage distillation, realizing the cascade utilization of energy and energy saving and consumption reduction throughout the entire process;

[0031] The two-stage negative pressure gradient precision refining module is used for the precise separation of isomers, heavy components, and colloidal impurities, and for low-temperature anti-coking high-purity refining.

[0032] The short-dwelling continuous conveying coke control module is used to shorten the high-temperature residence time of materials, dynamically match the distillation conditions, and suppress thermal polymerization and coking side reactions.

[0033] The finished product low-temperature condensation purification buffer module is used for rapid condensation, impurity removal, and low-temperature sealed storage of finished products to ensure the stability of the color and purity of the finished products.

[0034] The bottom residue recycling module is used for residue filtration and conditioning, component detection, and directional reflux, achieving full material recovery and reducing losses.

[0035] The full-process negative pressure steady-state intelligent control module is used for multi-parameter linkage adaptive adjustment to ensure long-term, low-energy-consumption, low-coking, and high-purity steady-state operation of the system.

[0036] Furthermore, the differential pressure thermal integrated coupling energy-saving module is equipped with a closed-loop heat exchange circulation loop and a waste heat energy storage buffer structure, which can adaptively adjust the heat exchange capacity according to the fluctuations in the operating conditions of the two-stage tower, ensuring continuous and stable reuse of waste heat and eliminating direct waste heat discharge losses.

[0037] Furthermore, the secondary negative pressure gradient precision refining module is equipped with a multi-point temperature gradient acquisition array and an independent PID temperature control unit, ensuring that the temperature gradient inside the tower is uniform and controllable, accurately matching the boiling point requirements for isomer separation, while locking in low-temperature safety conditions throughout the process to prevent thermal decomposition and coking of materials.

[0038] Beneficial effects:

[0039] Compared to the traditional atmospheric pressure high temperature single-tower distillation process for 3,4-dichloronitrobenzene, this invention achieves comprehensive technological breakthroughs in distillation operating mechanisms, impurity separation modes, energy utilization methods, material control systems, and anti-disturbance mass production capabilities. It completely overcomes seven long-standing technical bottlenecks in the industry and possesses five quantifiable and highly industrializable significant benefits:

[0040] 1. This invention pioneers a two-stage negative pressure staged low-temperature distillation system, completely avoiding the coking defects of heat-sensitive materials in traditional atmospheric pressure high-temperature distillation. Through the first stage of low-temperature removal of acidic catalytic impurities and the second stage of gradient low-temperature precise purification, the thermal decomposition, thermal polymerization and discoloration side reactions of materials are suppressed from the source. The color of the finished product is greatly optimized, coking impurities are almost eliminated, and the purity of the finished product is stably exceeded 99.9%. It can stably produce pharmaceutical grade and reagent grade ultra-high purity products, completely solving the industry pain points of low quality upper limit and poor color of traditional processes.

[0041] 2. This invention features an original differential pressure heat integration waste heat coupling energy-saving technology, which enables on-site closed-loop reuse of waste heat at the top of the distillation column, with a waste heat recovery utilization rate of ≥92%. The overall energy consumption of the entire distillation system is reduced by 18%~25%, eliminating the need for a large amount of external steam and cooling medium replenishment. This significantly reduces the energy consumption cost of industrial mass production, aligning with the development trend of green energy saving, low carbon and cost reduction in the fine chemical industry.

[0042] 3. This invention adopts a short-dwelling continuous feeding anti-coking mechanism, which significantly shortens the high-temperature residence time of materials, effectively avoids high-temperature static accumulation of materials, polymerization coking and scaling and blockage of the packing in the tower, extends the long-term stable operation time of the equipment, reduces the frequency of equipment cleaning and maintenance and operation and maintenance costs, and at the same time greatly improves the effective material yield and significantly improves the stability of mass production.

[0043] 4. This invention constructs a refined closed-loop recycling system for residual materials, realizing the full recovery and repurification of effective materials from the bottom of the tower, with an effective material recovery rate of ≥98%, and a significant reduction in solid waste production of more than 85%. It completely solves the problems of high material loss, high hazardous waste production, and high environmental pressure in traditional distillation, and achieves near-zero raw material loss and green ultra-low emission mass production.

[0044] 5. This invention is equipped with a full-condition adaptive intelligent control system. Through a multi-parameter linkage mechanism of feed pressure stabilization, negative pressure linkage, gradient temperature control, and reflux adaptation, it significantly improves the anti-disturbance capability of the distillation system, realizes long-cycle continuous steady-state mass production, and improves production efficiency by more than 30% compared with traditional batch distillation. It has low dependence on manual labor, high level of intelligence, and can be directly adapted to modern intelligent chemical production lines. It has strong industry technology substitution and application prospects. Detailed Implementation

[0045] Example 1

[0046] Addressing the seven core technological bottlenecks in existing atmospheric distillation processes for 3,4-dichloronitrobenzene—namely, severe high-temperature coking, low impurity removal accuracy, long material residence time, high energy consumption, significant residue waste, poor operating stability, and inability to achieve continuous high-purity mass production—this invention aims to provide a continuous crystallization purification method and system for 3,4-dichloronitrobenzene. This system utilizes two-stage negative pressure fractional distillation, differential pressure heat integration with waste heat coupling for energy saving, short residence time to prevent coking and continuous feeding, gradient temperature field for precise separation, and refined residue closed-loop processing. The system incorporates six brand-new original core technologies, including reuse and full-condition adaptive steady-state control, which completely overturn the outdated process mode of traditional atmospheric pressure high-temperature single-tower distillation. From multiple dimensions such as thermal protection of heat-sensitive materials, precise removal of impurities by classification, energy cascade utilization, full material recovery, and steady-state optimization of operating conditions, it thoroughly solves long-standing technical problems in the industry, and achieves low coking, low energy consumption, high purity, zero residue, and continuous green refining mass production of 3,4-dichloronitrobenzene. It fills the gap in the industry for negative pressure coupling energy-saving distillation technology for heat-sensitive nitrobenzene materials.

[0047] This invention is completely different from conventional improvement schemes involving traditional crystallization purification, single-tower distillation, and simple parameter fine-tuning. It adopts a novel original process system combining negative pressure staged distillation, differential pressure thermal integration for energy saving, short residence time to prevent coking, gradient precision separation, closed-loop recycling of residues, and intelligent steady-state control. This system forms six entirely new core technological innovations, precisely addressing seven major industry pain points in the background technologies, and possesses strong novelty, inventiveness, and industrial applicability.

[0048] I. Two-stage negative pressure fractional distillation innovation to prevent coking: The original two-stage negative pressure distillation architecture of pre-removal of light impurities + high-purity refining abandons the traditional atmospheric pressure high temperature single tower distillation mode. The first stage of low temperature negative pressure is specially used to remove acidic light components, moisture and low boiling point impurities, eliminating the catalytic factors of coking; the second stage of high negative pressure low temperature gradient distillation separates isomers and heavy component colloids, and the whole process is far below the thermal decomposition temperature of the material, which completely inhibits the side reactions of coking, decomposition and discoloration of heat-sensitive materials from the source.

[0049] II. Differential Pressure Heat Integration and Waste Heat Coupling Energy Saving Innovation: The first two-stage tower differential pressure and negative pressure matching heat integration system utilizes the vacuum pressure difference between the two-stage distillation towers to form a stable temperature difference. The waste heat steam from the top of the first-stage tower is accurately recovered and used for feed preheating and bottom auxiliary heating of the second-stage tower, realizing the on-site closed-loop reuse of distillation waste heat without the need for additional heat source replenishment. This significantly reduces the steam and cooling energy consumption of the entire line and solves the core problems of waste heat and high energy consumption in traditional distillation.

[0050] III. Short-Dwelling Continuous Feeding Coking Innovation: The original multi-level buffer continuous frequency conversion feeding mechanism constructs a dynamic balance between feeding-evaporation-collection conditions, significantly shortening the residence time of materials in the high-temperature section, preventing high-temperature static polymerization and coking of materials, while avoiding material accumulation at the bottom of the tower and scaling and blockage of the packing, effectively extending the long-cycle operation time of the equipment and improving material yield.

[0051] IV. Innovative Precision Separation with Gradient Temperature Field within the Column: The first-ever multi-point gradient temperature control system for a two-stage distillation column constructs a top-down stepped temperature field, precisely matching the boiling point differences of light components, target components, and heavy components. This enables the orderly removal of different impurities in stages, completely solving the problems of low separation accuracy, chaotic operating conditions, and large purity fluctuations in traditional isothermal distillation, and significantly improving the isomer removal accuracy and product purity.

[0052] V. Innovative Refined Closed-Loop Recycling of Bottom Components: An original full-process recycling mechanism for bottom residues, including filtration and impurity removal, decolorization and quality improvement, component detection, and directional reflux, is used to finely pre-treat the heavy component residues enriched at the bottom of the tower. After removing ineffective coking impurities, the residues are refluxed back to the front-end processes for repurification, achieving near-zero loss of effective materials and a significant reduction in solid waste, which meets the needs of green and low-carbon mass production.

[0053] VI. Innovation of all-condition negative pressure steady-state adaptive control: Construct a multi-parameter intelligent linkage control system with feed pressure stabilization, negative pressure linkage, temperature control gradient, reflux adaptation, and output matching. Adaptively compensate for mass production interferences such as feed fluctuations, environmental disturbances, and equipment heat exchange attenuation, ensuring long-term, low-fluctuation, high-quality continuous steady-state operation of the two-stage distillation system, and adapting to large-scale intelligent mass production in industrial applications.

[0054] Overall Technical Principles and Solutions Overview

[0055] The core innovation mechanism of this invention focuses on five core dimensions to reconstruct the distillation and refining process: low-temperature protection of thermosensitive materials, precise separation of impurities through classification, tiered energy-saving utilization of waste heat, full material recovery, and steady-state optimization of operating conditions. First, feed disturbances are eliminated through pressure stabilization pretreatment, ensuring stable distillation conditions. Second, acidic light impurities are removed through primary low-temperature negative-pressure distillation, eliminating the catalytic causes of coking at the source. Waste heat is recovered using two-stage differential pressure heat integration technology, achieving efficient energy reuse and overall energy saving and consumption reduction. Third, isomers and heavy component gum impurities are precisely removed under low-temperature safe conditions through secondary high-pressure gradient temperature field distillation, preventing high-temperature coking decomposition of materials. Fourth, a short-dwelling continuous feeding mode minimizes the high-temperature residence time of materials, suppressing thermal polymerization side reactions. Fifth, zero material loss and reduced solid waste are achieved through refined recycling of residual materials. Finally, multi-parameter intelligent adaptive control enables low-coking, low-energy consumption, high-purity, continuous, and steady-state refining production throughout the entire process. The present invention has strong process adaptability, low modification cost, and significant quality and efficiency improvement effects. It can directly replace the traditional high-energy-consuming and high-coking distillation process and has extremely high industrialization and promotion value.

[0056] Example 2

[0057] Step 1: Pretreatment of crude material under constant temperature and pressure: Take the crude 3,4-dichloronitrobenzene after industrial crystallization and purification, and send it into a closed pressure-stabilized buffer unit. Control the pretreatment temperature at 55℃ and let it stand at a constant temperature for 25 minutes to complete degassing, sedimentation of suspended impurities and homogenization of components. Maintain constant feed flow rate, temperature and pressure to eliminate feed fluctuations, gas phase inclusions and impurity disturbances, and prepare a distillation feed liquid with uniform composition and stable operating conditions, laying the foundation for steady-state operation of two-stage distillation.

[0058] Step 2, Primary Negative Pressure Light Component Removal Distillation: The pretreated homogenized feed solution is fed into a primary light impurity removal distillation column. The vacuum degree inside the column is controlled at 0.07 MPa, the bottom temperature at 100℃, the top temperature at 80℃, and the reflux ratio at a stable 1.5:1. The residence time of the material in a single column is strictly controlled to be ≤25 min. Through low-temperature negative pressure operation, residual moisture, trace amounts of nitrated acidic impurities, low-boiling-point byproducts, and light-colored suspended impurities in the material are completely removed. This completely eliminates the core inducing factor of high-temperature catalytic coking of acidic impurities, resulting in a refined intermediate feed solution free of light impurity contamination.

[0059] Step 3, Differential Pressure Heat Integration and Waste Heat Coupling for Energy Saving: Maintain a negative pressure difference of 0.025MPa between the first-stage and second-stage towers, utilize the high-temperature steam at the top of the first-stage tower as a heat source, and continuously provide a stable heat source for the preheating of the feed and auxiliary heating at the bottom of the second-stage tower through a closed-loop plate heat exchange system. The waste heat recovery utilization rate reaches 93%, eliminating the need to turn on an additional steam heat source, effectively offsetting the energy consumption of the second-stage distillation system, reducing the overall energy consumption of the entire process by 22%, and realizing efficient cascade reuse of waste heat.

[0060] Step 4, Two-stage negative pressure gradient precision refining: The feed liquid after the first-stage removal of light impurities is fed into a two-stage high-purity refining distillation column. The vacuum degree inside the column is controlled at 0.095 MPa, the bottom temperature at 115℃, the top temperature at 93℃, and the gradient temperature difference inside the column is kept stable at 20℃. The reflux ratio is 3.0:1. By precisely adapting the gradient temperature field to the boiling point differences of different impurities, 2,3-dichloronitrobenzene, 2,4-dichloronitrobenzene isomers, and heavy component colloidal impurities are removed in stages. Throughout the process, the bottom temperature of the column is far below the critical temperature for thermal decomposition of the material, thus eliminating the problems of coking, decomposition, and discoloration of heat-sensitive materials at the source.

[0061] Step 5, Short Residence Continuous Operation Control: Start the variable frequency continuous feeding and multi-stage buffer pressure stabilization system to dynamically match the feed rate, evaporation rate in the tower and the top output rate, maintain the steady-state balance of gas-liquid ratio in the tower, and control the total residence time of the material in the high-temperature section to within 35 minutes, which is 48% shorter than traditional distillation. This completely avoids thermal polymerization and coking caused by the accumulation of material at high temperature, and keeps the packing in the tower clean and free from scaling and blockage.

[0062] Step 6: Low-temperature condensation and product buffering: The high-purity material steam at the top of the secondary tower is rapidly condensed at a low temperature gradient of 40℃. After condensation, it is precisely filtered to remove trace suspended impurities. It is continuously collected at low temperature and stable pressure and sent to a sealed high-purity product buffer tank for constant temperature storage to prevent high-temperature oxidation and secondary contamination of the product, and to ensure that the product has a clean color and stable purity.

[0063] Step 7: Refined Reuse of Bottom Material: Collect the heavy component residues enriched at the bottom of the two-stage towers, control the discharge temperature at 110℃, filter through a precision filter to remove solid coking impurities, and perform low-temperature decolorization and quality treatment with activated carbon. After component testing, qualified residues are directed to the front-end crystallization and dissolution unit for re-dissolution, crystallization, and purification. The effective material recovery rate reaches 98.5%, with no effective material waste, significantly reducing solid waste output.

[0064] Step 8: Full-condition adaptive steady-state optimization: The system collects real-time data on vacuum degree, gradient temperature, reflux ratio, output flow rate, finished product purity and color of the two-stage tower, and dynamically adjusts the parameters of the negative pressure unit, heat exchange system, variable frequency feed and reflux system. It adaptively compensates for disturbances such as feed fluctuations, ambient temperature changes, and equipment heat exchange attenuation during mass production, ensuring long-cycle, low-coking, low-energy consumption, and high-purity continuous steady-state mass production.

[0065] The innovative efficiency-enhancing principle of this embodiment is as follows: This embodiment integrates all the six core innovative technological advantages of this invention. It completely eliminates coking causes and precisely removes impurities through two-stage negative pressure fractional distillation; achieves significant energy savings through differential pressure thermal integration; prevents material thermal deterioration through short residence conditions; improves separation accuracy through gradient temperature fields; achieves zero material loss through residue recycling; and ensures stable mass production through intelligent control. The final product has a purity ≥99.92%, a color ≤18, a 22% reduction in overall energy consumption, and a residue loss rate ≤1.5%. It perfectly meets the continuous mass production needs of pharmaceutical-grade and reagent-grade high-end products, possessing multiple advantages including high quality, low energy consumption, and environmental friendliness.

[0066] Example 3

[0067] Step 1: Enhance pre-treatment cleaning: Extend the time for raw material to stand, degas and settle, add two stages of precision filtration, thoroughly remove trace amounts of suspended colloids and solid impurities from the raw material, reduce the introduction of coking precursors from the source, and build an extremely clean distillation feed system.

[0068] Step 2: Lower the operating temperature of the first-stage distillation column: Optimize the operating conditions of the first-stage column, increase the vacuum to 0.08MPa, and reduce the column bottom temperature to 95℃. The low temperature maximizes the removal of acidic light impurities, completely eliminates the catalytic polymerization reaction of acidic impurities, and inhibits the formation of dark gum from the source.

[0069] Step 3, Extremely Short Dwell Time Control: Optimize the variable frequency feeding parameters to compress the total high-temperature dwell time of the material to within 30 minutes, minimize the high-temperature heating time of the material, further reduce the probability of thermal decomposition and thermal polymerization, and prevent discoloration of the finished product and increase in impurities.

[0070] Step 4: Refined gradient temperature control separation: Refine the temperature gradient range of the secondary tower, stabilize the temperature field uniformity inside the tower, accurately remove trace isomer impurities, and further improve the purity and color cleanliness of the finished product.

[0071] The principle of this embodiment for enhancing efficiency: This embodiment focuses on the core innovations of low-temperature anti-coking and short-term residence to suppress side reactions, and specifically solves the problems of poor color, high coking impurities, and low purity upper limit of traditional distillation products. The produced product has a color of ≤15 and a stable purity of ≥99.95%, which can meet the stringent quality standards of ultra-high purity reagent grade and pharmaceutical intermediates.

[0072] Example 4

[0073] Step 1: Optimize differential pressure heat integration parameters: accurately match the negative pressure difference between the two-stage towers to 0.02MPa, stabilize the heat exchange temperature difference, improve the waste heat exchange efficiency, increase the waste heat recovery utilization rate to 95%, and minimize the input of external steam.

[0074] Step 2, Working Condition Linkage Energy Saving Control: Activate the load adaptive energy saving mode, dynamically adjust the power of the negative pressure unit, heat exchange flow rate and reflux ratio according to changes in feed flow rate and composition, avoid ineffective operation of equipment under high load, and reduce power consumption and cooling energy consumption.

[0075] Step 3, Waste Heat Storage Buffer and Stabilization: Activate the waste heat storage buffer structure to avoid the problem of unstable waste heat supply caused by fluctuations in operating conditions, ensure the continuous and stable operation of the heat exchange system, and achieve low-energy steady-state production throughout the process.

[0076] The efficiency enhancement principle of this embodiment is as follows: This embodiment focuses on the core innovation of differential pressure heat integration and waste heat coupling for energy saving, maximizing the utilization value of distillation waste heat, reducing the overall energy consumption of the entire system by more than 25%, and significantly reducing the mass production operating cost while ensuring the stability of the finished product quality, with significant green and energy-saving advantages.

[0077] Example 5

[0078] Step 1, Precise Grading and Processing of Residual Materials: Refine the processes of residual material filtration, decolorization, and conditioning to accurately separate ineffective coking impurities from effective materials, avoid the effective materials being filtered out and discarded, and improve the purification and recovery rate of residual materials.

[0079] Step 2, Precise Conditioning of Recycled Components: Fine-tune the composition of the recycled residue to match the front-end crystallization feed standard, avoid fluctuations in feed composition caused by the recycling of residue, and ensure stable operation of the entire line.

[0080] Step 3, Extreme Reduction of Solid Waste: Only the ineffective impurities from complete coking are treated to render them harmless, and all other residues containing effective materials are recycled in a closed loop, achieving an effective material recovery rate of ≥99%, and reducing solid waste production by more than 85% compared to traditional processes.

[0081] The efficiency enhancement principle of this embodiment is as follows: This embodiment focuses on the core innovation of refined closed-loop recycling of residual materials, which completely solves the problems of high material loss, high solid waste production and high environmental protection costs in traditional distillation, and achieves near-zero material loss, green and low-carbon mass production, significantly reducing raw material procurement costs and hazardous waste treatment costs.

Claims

1. A method for continuous crystallization purification of 3,4-dichloronitrobenzene, characterized in that, Includes the following continuous refining steps: S1. Crude material pretreatment and pressure stabilization feed: The crude 3,4-dichloronitrobenzene material produced by nitration crystallization is fed into a closed pressure stabilization buffer unit for constant temperature static degassing and sedimentation pretreatment of trace suspended impurities. The feed temperature, feed pressure and feed flow rate are kept constant to eliminate fluctuations in material composition, bubble disturbance and impurity suspension that cause fluctuations in the distillation column operation, and to prepare a homogeneous purified feed liquid with stable composition, no gas phase inclusions and no suspended impurities. S2. Primary negative pressure light component removal distillation: The homogeneous feed liquid after pressure stabilization pretreatment is sent to the light impurity removal distillation column. Low negative pressure and low temperature distillation conditions are adopted to precisely control the vacuum degree, bottom temperature and reflux ratio in the column. Residual water, trace amounts of nitrated acid, low boiling point by-products, light color impurities and other light component impurities in the system are preferentially removed. The catalytic coking side reaction caused by long-term high temperature enrichment of light component impurities is avoided, and the intermediate refined feed liquid with light impurities removed is obtained. S3, differential pressure thermal integration and heat coupling control, collects the waste heat steam from the top of the first-stage light impurity removal column, and provides a heat source for the preheating of the feed and auxiliary heating of the bottom of the second-stage refining distillation column through the heat exchange coupling system. Relying on the negative pressure difference between the two-stage columns, a stable thermal integration cycle is formed, realizing the cascade recovery and on-site reuse of distillation waste heat, which greatly reduces the steam and electricity consumption of the entire distillation system and avoids the energy waste problem of direct discharge of traditional distillation waste heat; S4, a two-stage negative pressure gradient precision distillation process, feeds the intermediate liquid after primary light component removal into a high-purity distillation column. Utilizing a high negative pressure, low-temperature gradient distillation condition, and based on the boiling point differences between 3,4-dichloronitrobenzene and its isomers and heavy component colloidal impurities, the gradient temperature field, vacuum level, and precise reflux ratio within the column are precisely controlled. This allows for the segmented separation of heavy component impurities such as 2,3-dichloronitrobenzene and 2,4-dichloronitrobenzene isomers, maximizing the purity of the target product. Simultaneously, the column bottom temperature is strictly controlled to remain below the material's thermal decomposition temperature, thus inhibiting the coking and decomposition of heat-sensitive materials at the source. S5, short-stay continuous feeding anti-coking control, adopts a multi-stage buffer continuous conveying mode throughout the process, shortens the residence time of materials in the high-temperature tower bottom and heat exchange pipeline, eliminates the thermal polymerization and coking deterioration problems caused by long-term high-temperature static storage of materials, dynamically matches the feed rate, evaporation rate and output rate, maintains the steady-state balance of gas-liquid ratio in the tower, and ensures continuous and stable operation of distillation. S6. Low-temperature collection and homogenization buffering of finished products: The high-purity 3,4-dichloronitrobenzene vapor collected from the top of the secondary distillation column is rapidly condensed at low temperature and filtered at constant temperature to remove trace impurities. It is then collected at low temperature and stable pressure and sent to a high-purity finished product buffer tank for sealed storage. The entire process is carried out in a low-temperature closed-loop operation to avoid secondary oxidation and discoloration of the finished product and contamination by impurities, thus ensuring the stability of the color and purity of the finished product. S7. Refined recycling of bottom residue: Collect the heavy component colloidal and high-content isomer residues enriched at the bottom of the first and second towers. After filtration, desolidification, component detection and conditioning pretreatment, the residues are directionally refluxed to the front-end crystallization pretreatment process for repurification, so as to achieve full recovery of effective materials in the residues and eliminate waste and loss of residues and increase in solid waste. S8 features full-condition negative pressure steady-state adaptive control, which collects parameters such as vacuum degree, temperature gradient, reflux ratio, output flow rate, material color and purity in real time for two-stage distillation columns. It dynamically links and adjusts the operating conditions of the negative pressure system, heat exchange system and reflux system, and adaptively compensates for disturbances such as feed fluctuations, ambient temperature changes and equipment heat exchange attenuation, ensuring continuous steady-state refining production with low coking, low energy consumption and high purity throughout the entire process.

2. The continuous crystallization purification method for 3,4-dichloronitrobenzene according to claim 1, characterized in that, The operating parameters for the primary negative pressure light component removal distillation in step S2 are as follows: vacuum degree inside the column 0.06~0.08MPa, bottom temperature 95~105℃, top temperature 75~85℃, reflux ratio controlled at 1.2:1~1.8:1, and single column residence time ≤25min, to thoroughly remove acidic impurities and low-boiling-point byproducts of light components from the system and eliminate coking catalytic factors.

3. The continuous crystallization purification method for 3,4-dichloronitrobenzene according to claim 1, characterized in that, The operating parameters for the two-stage negative pressure gradient precision distillation described in step S4 are as follows: vacuum degree inside the column 0.09~0.095MPa, column bottom temperature 110~120℃, column top temperature 88~98℃, temperature gradient difference inside the column controlled at 18~22℃, reflux ratio controlled at 2.5:1~3.5:1, and column bottom temperature strictly below the thermal decomposition critical temperature of 3,4-dichloronitrobenzene throughout the process, so as to completely inhibit the decomposition and coking of heat-sensitive materials.

4. The continuous crystallization purification method for 3,4-dichloronitrobenzene according to claim 1, characterized in that, In step S3, the differential pressure heat integration and heat coupling control maintains the negative pressure difference between the two-stage distillation columns at 0.01~0.035MPa. The waste heat steam at the top of the first-stage column forms a closed-loop heat exchange with the feed pipeline of the second-stage column and the auxiliary heat loop at the bottom of the column through a plate heat exchanger. The waste heat recovery utilization rate is ≥92%, and the overall energy consumption of the whole system is reduced by 18%~25%.

5. The continuous crystallization purification method for 3,4-dichloronitrobenzene according to claim 1, characterized in that, The short-dwelling continuous feeding described in step S5 adopts a variable frequency quantitative conveying + multi-stage buffer and pressure stabilization structure. The total residence time of the material in the high-temperature section is ≤40min, which is more than 45% shorter than the residence time of the traditional distillation process, and significantly reduces the probability of thermal polymerization, coking and deterioration of heat-sensitive materials.

6. The continuous crystallization purification method for 3,4-dichloronitrobenzene according to claim 1, characterized in that, The bottom residue recycling process described in step S7 involves controlling the discharge temperature of the bottom residue at 105~115℃. After being filtered through a precision filter to remove solid coking impurities and activated carbon for simple decolorization and degradation, the residue is directionally refluxed to the front-end crystallization and dissolution unit for remelting and purification if the components are qualified. The effective material recovery rate of the residue is ≥98%.

7. The continuous crystallization purification method for 3,4-dichloronitrobenzene according to claim 1, characterized in that, The low-temperature rapid condensation described in step S6 adopts a low-temperature gradient condensation mode with circulating cooling water. The condensation temperature is controlled at 35~45℃, and the condensation rate is adapted to the top collection rate of the tower to avoid oxidation and discoloration of the high-temperature finished product due to prolonged retention. The color of the finished product is ≤20, and the purity is stable at ≥99.9%.

8. A low-coking, low-energy-consumption 3,4-dichloronitrobenzene negative pressure coupled distillation purification system, characterized in that, To implement the negative pressure coupled distillation purification process according to any one of claims 1-7, comprising: The feed pressure stabilization pretreatment module is used for degassing, sedimentation and impurity removal, pressure stabilization and flow stabilization of crude materials, providing homogeneous, stable and undisturbed distillation feed; The primary negative pressure light impurity removal and distillation module is used to remove light components such as acidic water, low-boiling-point impurities, and light-colored pollutants at low temperatures, thereby eliminating the catalytic factors that induce coking. The two-stage differential pressure thermal integrated coupling energy-saving module is used to recover the waste heat of the first-stage distillation and provide heat for the second-stage distillation, realizing the cascade utilization of energy and energy saving and consumption reduction throughout the entire process; The two-stage negative pressure gradient precision refining module is used for the precise separation of isomers, heavy components, and colloidal impurities, and for low-temperature anti-coking high-purity refining. The short-dwelling continuous conveying coke control module is used to shorten the high-temperature residence time of materials, dynamically match the distillation conditions, and suppress thermal polymerization and coking side reactions. The finished product low-temperature condensation purification buffer module is used for rapid condensation, impurity removal, and low-temperature sealed storage of finished products to ensure the stability of the color and purity of the finished products. The bottom residue recycling module is used for residue filtration and conditioning, component detection, and directional reflux, achieving full material recovery and reducing losses. The full-process negative pressure steady-state intelligent control module is used for multi-parameter linkage adaptive adjustment to ensure long-term, low-energy-consumption, low-coking, and high-purity steady-state operation of the system.

9. The low-coking, low-energy-consumption 3,4-dichloronitrobenzene negative pressure coupled distillation purification system according to claim 8, characterized in that, The differential pressure thermal integrated coupling energy-saving module is equipped with a closed-loop heat exchange circulation loop and a waste heat energy storage buffer structure. It can dynamically adjust the heat exchange capacity to adapt to fluctuations in the operating conditions of the two-stage tower, ensuring continuous and stable reuse of waste heat without direct discharge of waste heat.

10. The low-coking, low-energy-consumption 3,4-dichloronitrobenzene negative pressure coupled distillation purification system according to claim 8, characterized in that, The two-stage negative pressure gradient precision refining module is equipped with a multi-point temperature gradient acquisition array and an independent PID temperature control unit. The temperature gradient inside the tower is uniform and controllable, accurately matching the boiling point requirements for isomer separation. At the same time, it locks in low-temperature safety conditions throughout the process, preventing thermal decomposition and coking of materials.