Method for preparing high-purity parachlorotoluene through current pulse-assisted continuous suspension melt crystallization

Abstract

The invention belongs to the field of chemical purification, and relates to a method for preparing high-purity parachlorotoluene through current pulse-assisted continuous suspension melt crystallization, which is characterized in that current pulse with specific parameters is applied in the suspension melt crystallization process for assistance, and a pulse electric field effect and directional energy input are utilized, so that the crystallization induction period can be remarkably shortened, and the high-purity parachlorotoluene can be prepared. The crystallization time is shortened by 50%-95% compared with that of a traditional method, the purity, the yield and the particle size are improved, and finally pure parachlorotoluene with the purity larger than or equal to 99.9% is obtained. No additional solvent is needed in the whole process, and the method has the advantages of being high in controllability, low in energy consumption, high in efficiency, capable of achieving continuity and the like, and is a green and efficient melt crystallization strengthening technology.

Description

Technical Field

[0001] This invention belongs to the field of chemical purification and relates to a method for purifying p-chlorotoluene, particularly a method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization. Background Technology

[0002] 4-Chlorotoluene, also known as 4-chlorotoluene, is an organic compound with the chemical formula C7H7Cl. It is a colorless, transparent liquid, insoluble in water, soluble in ethanol, chloroform, and acetic acid, and miscible with diethyl ether. 4-Chlorotoluene is an important fine chemical intermediate and a crucial raw material for the production of pesticides (insecticides, fungicides, herbicides, plant growth regulators), dyes, pharmaceutical intermediates, and textile finishing agents.

[0003] With technological advancements, many new applications are constantly being developed. Domestic and international literature and patent reports indicate that the industrial production of p-chlorotoluene primarily employs the toluene liquid-phase chlorination method, yielding a mixture typically containing o-chlorotoluene, p-chlorotoluene, m-chlorotoluene, and polychlorinated toluenes. Existing patent reports describe two stages in the preparation of p-chlorotoluene: the first stage is the toluene liquid-phase chlorination reaction, and the second stage is separation. Currently, the main separation methods for the second stage include: molecular sieve adsorption separation, falling film cryogenic crystallization, sulfonation separation, solvent extraction separation, and distillation separation.

[0004] Chinese patent CN116444341A discloses a process for preparing p-chlorotoluene. The process involves simultaneously introducing chlorinating liquid and fuming sulfuric acid into the first micro-reaction module of a microchannel reactor, controlling the temperature at 105-115℃ and the residence time at 10-15 min; simultaneously introducing a primary sulfonate and fuming sulfuric acid into the second micro-reaction module of the microchannel reactor, controlling the temperature at 115-125℃ and the residence time at 10-15 min; feeding a secondary sulfonate into the third micro-reaction module of the microchannel reactor, controlling the temperature at 125-130℃ and the residence time at 5-10 min; cooling the tertiary sulfonate to 12-15℃, maintaining the temperature, and allowing it to crystallize until no crystals precipitate; performing a first-stage pressure filtration; rinsing the first filter cake with dichloromethane; and performing a second-stage pressure filtration; purifying the second filter cake with o-chlorotoluene to obtain o-chlorotoluene; combining the rinsing liquid, the first-stage filtrate, and the second-stage filtrate and purifying them with p-chlorotoluene to obtain p-chlorotoluene. The purity of the obtained p-chlorotoluene was 99.90-99.92 wt%, and the yield was 99.2-99.6%. This technique is inefficient and cannot be continuously implemented due to the static crystallization process.

[0005] Chinese patent CN110092706A discloses a method for separating p-chlorotoluene and o-chlorotoluene. This method utilizes a bis(diethoxy)[n]aromatic crystalline material to adsorb and separate a mixture of p-chlorotoluene and o-chlorotoluene. The bis(diethoxy)[n]aromatic crystalline material is placed in a mixed vapor atmosphere of o-chlorotoluene and p-chlorotoluene at a temperature not exceeding 80°C. Vacuum heating or reduced-pressure heating is used to remove the p-chlorotoluene and o-chlorotoluene mixture adsorbed on the surface of the bis(diethoxy)[n]aromatic crystalline material. Regeneration of the bis(diethoxy)[n]aromatic crystalline material is achieved through heating desorption, reducing energy consumption and production costs. However, this technology does not report the purity of the p-chlorotoluene and o-chlorotoluene products and cannot achieve continuous operation. Furthermore, the entire adsorption-preheating-regeneration system has a long operation time, high energy consumption, and relies on specific molecular sieve materials, resulting in high costs.

[0006] Chinese patent CN116143582A relates to a method for preparing p-chlorotoluene, comprising the following steps: primary chlorination, secondary chlorination, evaporation, detoluene removal, and separation. Dry toluene is fed into the chlorination reactor from the bottom, mixed with a catalyst, and stirred for a certain time. The chlorine feed flow rate is controlled, and the secondary chlorination solution is evaporated to obtain an evaporation condensate after condensation of the evaporated light components. The evaporation condensate is then detoluene-removed to obtain a detoluene-removed solution. This solution is further purified by distillation, followed by distillation of o-chlorotoluene and p-chlorotoluene to obtain o-chlorotoluene and p-chlorotoluene. The distillation equipment requires a large investment, incurs high operating costs, consumes a large amount of steam, and has a high overall cost.

[0007] Chinese patent CN101492344A discloses a process for purifying p-chlorotoluene, which includes the following steps: crystallization, solid-liquid separation, heating, leaching, heating again, and finished product. The crystallization step includes: adding p-chlorotoluene with a purity of 99.4% or higher into an enamel-jacketed reactor equipped with a refrigeration coil; opening the valves of the jacket and refrigeration coil for chilled brine; simultaneously introducing chilled brine at a pressure of 0.3±0.02MPa and maintaining a temperature of -8±2℃; cooling the p-chlorotoluene in the reactor; and maintaining the temperature of the p-chlorotoluene in the reactor at 7.6℃ for 1 hour to allow 60-70% of the material to crystallize. The p-chlorotoluene produced by this invention has a purity increased from 99.4% to over 99.8%. However, this crystallization process has low separation efficiency, and the purity of the obtained product cannot reach 99.9% or higher, and the crystallization process cannot be continuous.

[0008] Chinese patent CN114505016A relates to a post-treatment device and method for o- and p-chlorotoluene chlorination products. The post-treatment method involves filtering the chlorinated product using a ceramic membrane filter with a pore size of 50-100 μm, which removes most of the catalyst-insoluble matter. A reinforced pipeline mixer is used for forced mixing and washing, reducing water consumption in the washing stage by 60-80%. Simultaneously, thorough washing avoids catalyst residue, reducing tar formation in the distillation stage by 90%. The distillation stage has fewer side reactions, resulting in high product yield and a product purity of not less than 99.5%. However, this method produces a product with low purity, consumes a large amount of heat energy, and increases costs.

[0009] Existing technologies suffer from several drawbacks: solvent extraction separation methods result in low product purity and high solvent recovery costs, making industrial implementation difficult; molecular sieve adsorption separation methods result in low product purity, require frequent preheating and regeneration of the entire system, and involve expensive molecular sieves; intermittent crystallization methods suffer from low separation efficiency, cannot achieve high product purity, and cannot be implemented continuously; and distillation operations involve high energy consumption, large equipment investment, and high operating costs. Therefore, the industry urgently needs to develop a green, environmentally friendly, and low-energy-consumption continuous process technology for preparing high-purity p-chlorotoluene. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization. The method produces p-chlorotoluene products with a purity of ≥99.9% by using current pulse-assisted continuous suspension melt crystallization, which has the advantages of not adding solvents, not involving solvent recovery and drying, green and environmentally friendly process, low energy consumption, and high product purity.

[0011] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, comprising the following steps: (1) Current pulse assisted suspension melting crystallization: Liquid phase chlorotoluene raw material is continuously fed into the suspension melting crystallization device at a feed rate of 1-5 kg / h, and continuous cooling suspension melting crystallization is carried out at -10 - 5℃. At the same time, current pulse assisted crystallization is applied. The crystallization residence time is 1-8h to obtain solid liquid crystal paste. The parameters of the current pulse are: pulse voltage 50-120 V, frequency 1-10 Hz, pulse width 0.1-10 ms, and duration 5-90 min; (2) Solid-liquid separation and mother liquor treatment: After centrifugation and solid-liquid separation, the solid liquid slurry is obtained with a mass percentage of ≥99.9% p-chlorotoluene crystals and a mass percentage of ≥90% primary mother liquor. The primary mother liquor is continuously fed into the suspension melting crystallization device for recycling until the mass percentage of p-chlorotoluene in the mother liquor is <65% and discharged from the system.

[0012] Furthermore, the parameters of the current pulse are: pulse voltage of 60 V-110 V, frequency range of 1 Hz-8 Hz, pulse width of 5-10 ms, and duration of 10-80 min.

[0013] Furthermore, the liquid has a mass percentage content of 95% relative to the chlorotoluene feedstock.

[0014] Furthermore, the D50 particle size of the p-chlorotoluene product is 255-274 μm.

[0015] Furthermore, the feed rate is 1-3 kg / h.

[0016] Furthermore, the crystallization temperature is -2 to 5°C.

[0017] Furthermore, the crystallization residence time is 2-5 hours.

[0018] The method of this invention for separating and purifying p-chlorotoluene is applicable to crude p-chlorotoluene with a content ≥95%. After the raw materials enter the separation process, the materials with a p-chlorotoluene content ≥65% obtained in each step can be recycled. The p-chlorotoluene in the entire system of this invention can be recovered and repurified, with a total recovery rate of up to 88%.

[0019] Advantages and beneficial effects of the present invention: 1. This invention employs a current pulse-assisted continuous suspension melt crystallization process, which can directly obtain p-chlorotoluene with a purity of ≥99.9% from crude product with a purity of ≥95%. The entire process does not involve the addition of external solvents, and separation is achieved through crystallization-centrifugation. It has outstanding advantages such as green process, safe operation, low energy consumption, high product purity, and the ability to achieve continuous production.

[0020] 2. The present invention employs current pulse assistance, which means that a series of electrical pulses are applied to the crystallization slurry through electrodes during the crystallization process. The pulse electric field effect and directional energy input can significantly shorten the induction period, reduce the crystallization time of p-chlorotoluene by 50%-95%, and at the same time improve the purity and total yield, and increase the product particle size. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the process flow of the method of the present invention. Detailed Implementation

[0022] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the preferred embodiments.

[0023] Example 1 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1500.24g of crude p-chlorotoluene (95.00% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and the liquid phase temperature was controlled at -3℃ for suspension melt crystallization. Immediately afterward, a current pulse device was added, and a current pulse was applied with the following parameters: pulse voltage 80V, frequency 3Hz, pulse width 10ms, and pulse duration 20min. Within 20 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. The crystallization residence time was 3h, yielding a solid-liquid slurry. The slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.90% and a primary mother liquor with a mass fraction of 90.10%. The D50 particle size of the p-chlorotoluene crystal product was 256.46 μm. The primary mother liquor was recycled into a continuous suspension melt crystallizer until the mass percentage of p-chlorotoluene in the mother liquor was <65%, at which point it was discharged from the system. The total yield was 87.50%.

[0024] Comparative Example 1 The difference from Example 1 lies in removing the current pulse assistance, the specific method of which is as follows: In a 2L suspension melt crystallizer, 1500.24g of crude p-chlorotoluene (95.00% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and the liquid phase temperature was controlled at -3℃. Within 80 minutes of the start of the cooling program, uniform crystals visible to the naked eye began to appear in the system. The crystallization residence time was 3h, resulting in a solid-liquid slurry. Solid-liquid separation was performed to obtain p-chlorotoluene crystals with a mass fraction of 97.50% and a primary mother liquor with a mass fraction of 92.50%. The D50 particle size of the crystal product was 51.12 μm. The primary mother liquor was recycled into a continuous suspension melt crystallizer until the mass percentage of p-chlorotoluene in the mother liquor was <65% and discharged from the system. The total yield was 80.68%.

[0025] Example 2 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1600.92g of crude p-chlorotoluene (95.42% by mass) after complete crystallization and melting at 8℃ was continuously fed at a feed rate of 2kg / h. The mixture was stirred until homogeneous, and the liquid phase temperature was controlled at -2℃ for crystallization. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: pulse voltage 80V, frequency 3Hz, pulse width 8ms, and pulse duration 20min. Within 20 minutes after the application of the current pulse, uniform crystals visible to the naked eye began to appear in the system. The crystallization residence time was 3h, resulting in a solid liquid crystal paste. The solid liquid crystal paste was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.92% and a primary mother liquor with a mass fraction of 90.92%. The D50 particle size of the crystal product was 263.57 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mass percentage of p-chlorotoluene in the mother liquor was <65% and discharged from the system. The total yield was 88.44%.

[0026] Comparative Example 2 The difference from Example 2 lies in removing the current pulse assistance, the specific method of which is as follows: In a 2L suspension melt crystallizer, 1600.92g of crude p-chlorotoluene (95.42% by mass) completely melted at 8℃ was continuously fed at a feed rate of 2kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at -2℃. Within 70 minutes of the start of the cooling program, uniform crystals visible to the naked eye began to appear in the system. The crystallization residence time was 3h, resulting in a solid liquid crystal paste. The solid liquid crystal paste was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 98.40% and a primary mother liquor with a mass fraction of 92.44%. The D50 particle size of the crystal product was 57.58 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mass percentage of p-chlorotoluene in the mother liquor was <65% and discharged from the system. The total yield was 81.80%.

[0027] Example 3 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1650.5g of crude p-chlorotoluene (95.69% by mass) completely melted at 8℃ was continuously fed at a feed rate of 2kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of -2℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 3 Hz, pulse width 5 ms, and pulse duration 20 min. Within 20 minutes after the application of the current pulse, uniform crystals visible to the naked eye began to appear in the system. The residence time was 3 h, resulting in a solid liquid crystal paste. The solid liquid crystal paste was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.95% and a primary mother liquor with a mass fraction of 91.44%. The D50 particle size of the crystal product was 271.82 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mass percentage of p-chlorotoluene in the mother liquor was <65% and discharged from the system. The total yield was 89.46%.

[0028] Comparative Example 3 The difference from Example 3 lies in removing the current pulse assistance, the specific method of which is as follows: In a 2L suspension melt crystallizer, 1650.5g of crude p-chlorotoluene (95.69% by mass) completely melted at 8℃ was continuously fed at a feed rate of 2kg / h. The mixture was stirred evenly, and crystallization was carried out at a liquid phase temperature of -2℃. Within 70 minutes after the cooling program was started, uniform crystals visible to the naked eye began to appear in the system. The crystallization residence time was 3h, resulting in a solid liquid crystal paste. The solid liquid crystal paste was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 98.80% and a primary mother liquor with a mass fraction of 92.58%. The D50 particle size of the crystal product was 62.41 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mass percentage of p-chlorotoluene in the mother liquor was <65% and discharged from the system. The total yield was 81.65%.

[0029] Example 4 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1524.79g of crude p-chlorotoluene (95.21% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred evenly, and crystallization was carried out at a liquid phase temperature of -1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 3 Hz, pulse width 5 ms, and pulse duration 20 min. Within 20 minutes after the application of the current pulse, uniform crystals visible to the naked eye began to appear in the system. The residence time was 4 h, resulting in a solid-liquid slurry. The solid-liquid slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.91% and a primary mother liquor with a mass fraction of 90.51%. The D50 particle size of the crystal product was 268.84 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mother liquor content was <65% and discharged from the system. The total yield was 88.36%.

[0030] Comparative Example 4 The difference from Example 4 is that the current pulse assistance is replaced with ultrasonic assistance, and the specific method is as follows: In a 2L suspension melt crystallizer, 1524.79g of crude p-chlorotoluene with a mass percentage of 95.21% was continuously fed at a feed rate of 3kg / h after complete melting at 8℃. The mixture was stirred evenly, and crystallization was carried out at a liquid phase temperature of -1℃. Then, an ultrasonic generator was turned on for ultrasonic-assisted crystallization. The ultrasonic time was 20 minutes and then turned off. Within 40 minutes after the ultrasonic start-up, uniform crystals visible to the naked eye began to appear in the system. The residence time was 4 hours to obtain a solid liquid crystal paste. The solid liquid crystal paste was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass percentage of 99.23% and a primary mother liquor with a mass percentage of 91.19%. The D50 particle size of the crystal product was 174.64 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mother liquor was discharged from the system at <65%. The total yield was 86.03%.

[0031] Example 5 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1550.5g of crude p-chlorotoluene (95.85% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 3 Hz, duration 20 min, and pulse width 5 ms. Within 20 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. After a residence time of 3 h, a solid-liquid slurry was obtained. The slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.98% and a primary mother liquor with a mass fraction of 91.72%. The D50 particle size of the crystal product was 273.57 μm. The primary mother liquor was continuously recycled back into the suspension melt crystallizer until <65% was discharged from the system, with a total yield of 88.14%.

[0032] Comparative Example 5 The difference from Example 5 is that the voltage of the current pulse is 122V, and the specific method is as follows: In a 2L suspension melt crystallizer, 1550.5g of crude p-chlorotoluene (95.85% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 122 V, frequency 3Hz, duration 20 min, and pulse width 5 ms. Within 50 minutes after the application of the current pulse, uniform crystals visible to the naked eye began to appear in the system. The residence time was 3 h, resulting in a solid-liquid slurry. The solid-liquid slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 98.91% and a primary mother liquor with a mass fraction of 92.79%. The D50 particle size of the crystal product was 170.56 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mother liquor content was <65% and discharged from the system. The total yield was 82.67%.

[0033] Example 6 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1610.5g of crude p-chlorotoluene (95.57% by mass), completely melted at 8℃, was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 3 Hz, duration 20 min, and pulse width 5 ms. Within 20 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. After a residence time of 3 h, a solid-liquid slurry was obtained. The slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.95% and a primary mother liquor with a mass fraction of 91.19%. The D50 particle size of the crystal product was 267.63 μm. The primary mother liquor was continuously recycled back into the suspension melt crystallizer until <65% was discharged from the system, with a total yield of 88.41%.

[0034] Comparative Example 6 The difference from Example 6 is that the voltage of the current pulse is 48V, and the specific method is as follows: In a 2L suspension melt crystallizer, 1610.5g of crude p-chlorotoluene (95.57% by mass), completely melted at 8℃, was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 48V, frequency 3Hz, duration 20min, and pulse width 5ms. Within 40 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. After a residence time of 3h, a solid-liquid slurry was obtained. The slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 98.75% and a primary mother liquor with a mass fraction of 92.39%. The D50 particle size of the crystal product was 180.98 μm. The primary mother liquor was continuously recycled back into the suspension melt crystallizer until <65% was discharged from the system, resulting in a total yield of 82.21%.

[0035] Example 7 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1745.5g of crude p-chlorotoluene (95.31% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 3 Hz, duration 20 min, and pulse width 5 ms. Within 20 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. The residence time was 3 h, resulting in a solid-liquid slurry. The solid-liquid slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.91% and a primary mother liquor with a mass fraction of 90.71%. The D50 particle size of the crystal product was 255.73 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mother liquor content was <65% and discharged from the system. The overall yield was 87.82%.

[0036] Comparative Example 7 The difference from Example 7 is that the frequency of the current pulse is 11Hz, and the specific method is as follows: In a 2L suspension melt crystallizer, 1745.5g of crude p-chlorotoluene (95.31% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were 80 V voltage, 11 Hz frequency, 20 min duration, and 5 ms pulse width. Within 50 minutes after the application of the current pulse, uniform crystals visible to the naked eye began to appear in the system. The residence time was 3 h, resulting in a solid-liquid slurry. The solid-liquid slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 98.81% and a primary mother liquor with a mass fraction of 91.81%. The D50 particle size of the crystal product was 163.41 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mother liquor content was <65% and discharged from the system. The total yield was 81.27%.

[0037] Example 8 A method for preparing high-purity p-chlorotoluene using current pulse-assisted continuous suspension melt crystallization, the method comprising the following steps: In a 2L suspension melt crystallizer, 1659.5g of crude p-chlorotoluene (95.38% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 3 Hz, duration 20 min, and pulse width 5 ms. Within 20 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. The residence time was 3 h, resulting in a solid-liquid slurry. The solid-liquid slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 99.92% and a primary mother liquor with a mass fraction of 90.84%. The D50 particle size of the crystal product was 263.75 μm. The primary mother liquor was continuously fed into the suspension melt crystallizer for recycling until the mother liquor content was <65% and discharged from the system. The total yield was 89.35%.

[0038] Comparative Example 8 The difference from Example 8 is that the frequency of the current pulse is 0.8Hz, and the specific method is as follows: In a 2L suspension melt crystallizer, 1659.5g of crude p-chlorotoluene (95.38% by mass) completely melted at 8℃ was continuously fed at a feed rate of 3kg / h. The mixture was stirred until homogeneous, and crystallization was carried out at a liquid phase temperature of 1℃. Immediately afterward, a current pulse device was added, and a current pulse was applied simultaneously. The parameters of the current pulse were: voltage 80 V, frequency 0.8Hz, duration 20 min, and pulse width 5 ms. Within 60 minutes of the current pulse application, uniform crystals visible to the naked eye began to appear in the system. After a residence time of 3 h, a solid-liquid slurry was obtained. The slurry was then subjected to solid-liquid separation to obtain p-chlorotoluene crystals with a mass fraction of 98.89% and a primary mother liquor with a mass fraction of 91.87%. The D50 particle size of the crystal product was 178.97μm. The primary mother liquor was continuously recycled back into the suspension melt crystallizer until <65% was discharged from the system, resulting in a total yield of 83.21%.

[0039] The data results of the examples and comparative examples are shown in Table 1.

[0040] Table 1

[0041] As shown in Table 1, by comparing continuous suspension melting crystallization with and without current pulse assistance, and using ultrasonic assistance instead of current pulse assistance, continuous suspension melting crystallization with different voltages and frequencies, it can be seen that continuous suspension melting crystallization without current pulse assistance, continuous suspension melting crystallization with ultrasonic assistance, and continuous suspension melting crystallization with different voltages and frequencies result in lower product purity, lower overall yield, smaller crystal particle size, and longer crystallization time. If the same purity is required, multi-stage suspension melting crystallization is necessary, but this will also lead to higher investment and operating costs.

[0042] The above description is only a preferred example of the present invention. For those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. The content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for the production of high purity p-chlorotoluene by current pulse assisted continuous suspension melt crystallization, characterized in that The method comprises the following steps: (1) current pulse assisted suspension melt crystallization: continuously feeding liquid phase p-chloromethylbenzene raw material into a suspension melt crystallization device at a feeding rate of 1-5 kg / h, continuously cooling the suspension melt crystallization at -10-5 ℃, and applying a current pulse for assisted crystallization, with a crystallization residence time of 1-8 h, to obtain a solid-liquid crystal slurry, wherein the parameters of the current pulse are: pulse voltage 50-120 V, frequency 1-10 Hz, pulse width 0.1-10 ms, and duration 5-90 min; (2) solid-liquid separation: after centrifugation and solid-liquid separation of the solid-liquid crystal slurry, p-chloromethylbenzene crystals with a mass percentage of ≥99.9% and a first-stage mother liquor with a mass percentage of p-chloromethylbenzene of ≥90% are obtained.

2. The method of claim 1, wherein, The parameters of the current pulse are: pulse voltage 60-100 V, frequency 1-8 Hz, pulse width 5-10 ms, and duration 10-80 min.

3. The method of claim 1, wherein, The mass percentage of the liquid phase p-chloromethylbenzene raw material is ≥95%.

4. The method of claim 1, wherein, The D50 particle size of the p-chloromethylbenzene product is 255-274 μm.

5. The method of claim 1, wherein, The feeding rate is 1-3 kg / h.

6. The method of claim 1, wherein, The crystallization temperature is -2-5 ℃.

7. The method of claim 1, wherein, The crystallization residence time is 2-5 h.

8. The method of claim 1, wherein, The first-stage mother liquor continuously enters the suspension melt crystallization device for recycling until the mass percentage of p-chloromethylbenzene in the mother liquor is <65% and is discharged from the system.

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