Multi-section continuous micro-channel nitration process and device for preparing mononitrotoluene

Abstract

The invention provides a multi-section continuous micro-channel nitration process and equipment for preparing mononitrotoluene, which are characterized in that firstly, acid is mixed on line, then, two-stage micro-channel rapid nitration is performed, and finally, curing is completed in a vertical jacketed reactor with a flow limiting hole, a reducer pipe and high-speed stirring. After continuous discharging of the system, closed acid distribution, three-stage temperature control washing and waste acid application are realized. The invention provides a continuous nitration process and a matched device which are intrinsically safe, efficient in mass and heat transfer and easy in industrial amplification, and meet the comprehensive requirements of high conversion rate, low byproduct, high safety and low energy consumption of the mononitrotoluene nitration process.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, and in particular to a multi-stage continuous microchannel nitration process and apparatus for preparing mononitrotoluene. Background Technology

[0002] Mononitrotoluene (MNT, including ortho, meta, and para isomers) is an indispensable intermediate in the fine chemical industry, including pharmaceuticals, pesticides, dyes, and explosives. Industrially, it typically employs a "mixed acid nitration" route, using nitric acid as the nitrating agent and concentrated sulfuric acid as the catalyst and dehydrating agent to react with toluene in a liquid-liquid two-phase reaction, followed by neutralization, washing, and fractionation to obtain the product. However, existing technologies still face the following prominent challenges in terms of production scale-up, safety control, and product quality: The reactor has a large liquid holdup and is highly exothermic. Traditional batch or continuous stirred-tank nitration reactors typically have volumes ranging from several cubic meters to tens of cubic meters, with high single-pass liquid holdup and concentrated exothermic reaction (ΔH≈-145 kJ / mol). ¹), which is highly susceptible to side reactions or even explosions due to localized overheating. Although multi-stage series reactor processes can partially alleviate this problem, the equipment is large, the temperature gradient is large, and the total amount of material online remains high, resulting in insufficient inherent safety.

[0003] Byproduct control is difficult

[0004] Under batch conditions, uneven mixing and wide residence time distribution easily lead to the formation of byproducts such as dinitrotoluene (DNT) and nitrophenol. DNT not only reduces the yield of dinitrotoluene (MNT) but also forms highly sensitive tar during subsequent distillation, increasing safety risks; nitrophenol is a highly toxic and explosive impurity that must be strictly controlled. Existing processes typically require additional alkaline washing or extraction steps, resulting in large amounts of waste and increased costs.

[0005] Long reaction time and high energy consumption

[0006] Traditional low-temperature (<40 ℃) dropping processes require several hours to complete the reaction. Although continuous batch processing can shorten the time to 30~60 minutes, low-temperature operation is still required to prevent side reactions. This necessitates the configuration of a high-power ice machine or cryogenic system, resulting in high energy consumption and large equipment investment.

[0007] Low degree of continuous and automated operation

[0008] Existing microchannel nitration technologies are mostly used in laboratories or kilogram-level experiments. When scaled up to ton-level, they suffer from problems such as large pressure drop, blockage, and limited heat transfer. Moreover, most of them adopt a single microchannel straight-through structure, which cannot meet the requirements of rapid reaction and subsequent curing, thus limiting their industrial application.

[0009] Waste acid treatment and environmental pressure

[0010] Traditional processes use large amounts of sulfuric acid (the mass ratio of sulfuric acid to nitric acid is often >4:1), requiring a large amount of water for dilution and phase separation after the reaction, resulting in wastewater with high nitrate and acid content, and high treatment costs. Some companies use adiabatic nitration to reduce acid consumption, but the single-pass conversion rate is low, requiring a large amount of toluene to be recycled, and the overall energy consumption is not reduced.

[0011] In summary, existing technologies cannot simultaneously meet the comprehensive requirements of "high conversion rate, low by-products, high safety, and low energy consumption." Therefore, developing a continuous nitration process and supporting equipment that combines intrinsic safety, efficient mass and heat transfer, and ease of industrial scale-up has become a key technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0012] This invention proposes a multi-stage continuous microchannel nitration process and apparatus for the preparation of mononitrotoluene, solving the problems mentioned in the background art. The technical solution of this invention is implemented as follows: A multi-stage continuous microchannel nitration process for preparing mononitrotoluene includes the following steps: Step S1: Mix nitric acid, sulfuric acid and water in a certain proportion to form a mixed acid nitrifying agent; Step S2: The mixed acid nitrifying agent and toluene are continuously fed into a multi-stage microchannel nitration reactor, respectively, and undergo nitration reaction sequentially through three reaction zones; the three reaction zones are as follows: First stage: Microchannel reactor, with an inner diameter of 300~500μm, a length of 1~10 m, a temperature control of 40~55℃, and a residence time of 0.2~1 s; The second stage: a microchannel reactor with an inner diameter of 300~500μm, a length of 1~10 m, a temperature control of 70~80℃, and a residence time of 0.2~1 s; The third stage: a vertical stirred reactor with jacketed cooling, with a temperature control of 50~65℃, a stirring blade diameter-to-height ratio of 8~20:1, a rotation speed of 300~600 rpm, and a residence time of 5~8 min; Step S3: Discharge and post-processing. In the three reaction zones, each zone is equipped with an independent temperature control system and a material distributor. After the reaction in step S2 is completed, the crude mononitrotoluene is obtained by acid-oil two-phase separation. The separation temperature is controlled at 45~55 ℃ so that the toluene conversion rate is ≥97%, and the by-products are dinitrotoluene ≤0.3% and nitrophenol ≤0.003%.

[0013] Furthermore, the mass ratio of nitric acid: sulfuric acid: water in the mixed acid nitrifying agent is 12:62:26~18:58:24.

[0014] Furthermore, the molar ratio of nitric acid to sulfuric acid is 1:2 to 1:6.

[0015] Furthermore, the molar ratio of toluene to nitric acid is 1:1.01 to 1:1.05.

[0016] Furthermore, the nitric acid concentration is 50-98%, preferably 60-70%.

[0017] Furthermore, the reaction does not require refrigeration; temperature control is achieved solely through circulating cooling water.

[0018] A multi-stage continuous nitration apparatus for the above process includes: The first microchannel reactor is equipped with a toluene inlet, a mixed acid inlet, a cooling water inlet / outlet, and a reaction product outlet. The second microchannel reactor is connected in series with the outlet of the first section and is equipped with a reactant inlet, a cooling water inlet and outlet, and a reaction product outlet. The third stage of the stirred reactor is a vertical jacketed structure with a stirring motor at the top. The interior, from top to bottom, contains: Variable diameter reaction tube, used to extend the reaction path; The stirring paddle has a diameter-to-height ratio of 8:1 to 20:1 and a rotation speed of 300 to 600 rpm. Stirring baffles are used to enhance turbulent mixing; A flow-limiting orifice is used to control the residence time; A heat exchange jacket surrounds the outer wall of the reactor and is equipped with cooling water inlet and outlet. The three reactors are connected in series to form a continuous flow path, and each section is independently temperature controlled.

[0019] Furthermore, the first and second microchannel reactors are coiled or variable diameter tube structures with a diameter of 300-500 μm and a length of 1-10 m.

[0020] Furthermore, the variable-diameter reaction tube of the third stage stirred reactor is a tapered structure that gradually narrows from top to bottom, with a flow-limiting orifice located at the bottom of the reactor to precisely control the material residence time of 5 to 8 minutes.

[0021] Furthermore, the entire device is an ice-free continuous system, which achieves temperature control in each section solely through circulating cooling water, reducing energy consumption by ≥50%.

[0022] Compared with existing technologies, this solution has the following advantages: (1) High conversion rate and selectivity. The toluene conversion rate is ≥97%, and the byproducts dinitrotoluene and nitrophenol are ≤0.3% and ≤0.003%, respectively, which are significantly better than the traditional batch process. (2) High safety. The amount of material carried online is reduced by more than 65%; the microchannel has a very small liquid holding capacity, and combined with segmented temperature control (35–90 ℃) and an emergency interlock system, the risk of overheating is completely eliminated; (3) High efficiency and energy saving. The entire process is continuous, and the total reaction time is shortened to 1–10 min; no ice machine is needed, and temperature control can be met by circulating cooling water alone, reducing energy consumption by ≥50%; (4) Structural innovation. The microchannel-variable diameter reaction tube-stirred tank are connected in series, and the flow-limiting orifice plate, stirring baffle and jacket heat exchange are integrated in the third section to achieve enhanced mass / heat transfer and precise control of residence time; (5) Environmentally friendly. The closed negative pressure discharge + tail gas alkaline washing prevents nitrate leakage; the waste acid is directly reused after vacuum concentration, achieving near-zero emissions. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a process flow diagram for synthesizing mononitrotoluene according to the present invention; Figure 2 This is a schematic diagram of the structure of a nitration reactor of the present invention; Figure 3 This is a schematic diagram of the two-stage nitration reactor of the present invention; Figure 4 This is a schematic diagram of the vertical stirring reaction structure used in the two-stage nitration reaction of this invention.

[0025] 1- Variable diameter reaction tube; 2- Stirring baffle; 3- Stirring paddle; 4- Heat exchange jacket; 5-Flow restrictor orifice; 6-Stirring motor Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] Reference Figures 1-4 This invention provides a multi-stage continuous microchannel nitration process for preparing mononitrotoluene, comprising the following steps: Step S1: Mix nitric acid, sulfuric acid and water in a certain proportion to form a mixed acid nitrifying agent; Step S2: The mixed acid nitrifying agent and toluene are continuously fed into a multi-stage microchannel nitration reactor, respectively, and undergo nitration reaction sequentially through three reaction zones; the three reaction zones are as follows: First stage: Microchannel reactor, with an inner diameter of 300~500μm, a length of 1~10 m, a temperature control of 40~55℃, and a residence time of 0.2~1 s; The second stage: a microchannel reactor with an inner diameter of 300~500μm, a length of 1~10 m, a temperature control of 70~80℃, and a residence time of 0.2~1 s; The third stage: a vertical stirred reactor with jacketed cooling, with a temperature control of 50~65℃, a stirring blade diameter-to-height ratio of 8~20:1, a rotation speed of 300~600 rpm, and a residence time of 5~8 min; Step S3: Discharge and post-processing. In the three reaction zones, each zone is equipped with an independent temperature control system and a material distributor. After the reaction in step S2 is completed, the crude mononitrotoluene is obtained by acid-oil two-phase separation. The separation temperature is controlled at 45~55 ℃ so that the toluene conversion rate is ≥97%, and the by-products are dinitrotoluene ≤0.3% and nitrophenol ≤0.003%.

[0028] It also provides a multi-stage continuous nitration apparatus for the above process, comprising: The first microchannel reactor is equipped with a toluene inlet, a mixed acid inlet, a cooling water inlet / outlet, and a reaction product outlet. The second microchannel reactor is connected in series with the outlet of the first section and is equipped with a reactant inlet, a cooling water inlet and outlet, and a reaction product outlet. The third stage of the stirred reactor is a vertical jacketed structure, with a stirring motor 6 installed at the top. The interior, from top to bottom, contains: Variable diameter reaction tube 1, used to extend the reaction path; Agitator 3, diameter-to-height ratio 8:1~20:1, rotation speed 300~600 rpm; Stirring baffle 2 is used to enhance turbulent mixing; Flow restrictor 5 is used to control the residence time; Heat exchange jacket 4 surrounds the outer wall of the reactor and is equipped with cooling water inlet and outlet; The three reactors are connected in series to form a continuous flow path, and each section is independently temperature controlled.

[0029] Furthermore, the first and second microchannel reactors are coiled or variable diameter tube structures with a diameter of 300-500 μm and a length of 1-10 m.

[0030] Furthermore, the variable-diameter reaction tube 1 of the third stage stirred reactor is a tapered structure that gradually narrows from top to bottom, and the flow-limiting orifice 5 is located at the bottom of the reactor to precisely control the material residence time of 5 to 8 minutes.

[0031] Furthermore, the entire device is an ice-free continuous system, which achieves temperature control in each section solely through circulating cooling water, reducing energy consumption by ≥50%.

[0032] The following three specific examples will provide a detailed explanation: Example 1.

[0033] like Figure 1 As shown, a process for synthesizing mononitrotoluene using a multi-stage continuous microchannel nitration reactor includes the following steps: 1) Fuming nitric acid is pumped from the nitric acid storage tank to the cooled static mixer M1 at a rate of 1.04 kg / h using a nitric acid pump. Concentrated sulfuric acid is pumped from the concentrated sulfuric acid storage tank to the cooled static mixer M1 at a rate of 4.66 kg / h using a sulfuric acid pump. Water is pumped to the cooled static mixer M1 at a rate of 2.07 kg / h using a water pump. The mixed acid nitrifying agent is discharged from the static mixer M1 and enters the mixed acid nitrifying agent raw material tank. 3) Input the mixed acid nitrifying agent and toluene obtained in step 1) into the multi-stage continuous microchannel nitration reactor for nitration reaction: Use a toluene solution transfer pump to input the mixed toluene from the raw material tank into the multi-stage continuous microchannel nitration reactor at a rate of 1.5 kg / h, and use a mixed acid transfer pump to input the mixed acid nitrifying agent from the raw material tank into the multi-stage continuous microchannel nitration reactor at a rate of 7.77 kg / h. The two materials undergo nitration reaction in the multi-stage continuous microchannel nitration reactor to obtain mononitrotoluene.

[0034] In this embodiment, the reaction temperature in step 3) is controlled to be 35-90℃, with different temperature controls for each stage. It is particularly preferred that the first stage is 40℃, the second stage is 75℃, and the third stage is 55℃.

[0035] The concentration of nitric acid is 98%, the concentration of sulfuric acid is 98%, and the molar ratio of nitric acid:sulfuric acid:water is 1:4.5:2; the molar ratio of toluene to nitric acid is 1:1.01.

[0036] like Figure 2-4 As shown, the multi-stage continuous microchannel nitration reactor includes a first-stage microchannel variable-diameter reaction tube 1, within which a distributor is installed. The second-stage microchannel reactor includes a remixer and a distributor. The third-stage reactor is a jacketed vertical tubular reactor with a high-speed stirrer, arranged in series. The material enters the microchannel main reactor from the outlet and then from the bottom into the jacketed vertical tubular reactor with a high-speed stirrer. The first two microchannel reactors have a tube diameter of 500 μm, a length of 10 m, and a residence time of 0.3 s. The third-stage stirring tube has a blade diameter-to-height ratio of 0.9, a stirring speed of 300 r / min, and a residence time of 8 min, with discharge from the top. Each stage employs a different individually controlled temperature and heat exchange system.

[0037] The first and second stage microchannel reactors are equipped with material distributors at their material inlets; the microchannel reactors are equipped with jacketed heat exchangers. The overall reaction time is 0.3 s for the first and second stages and 8 min for the third stage, yielding products with a toluene conversion rate of 97.5%, dinitrotoluene of 0.24%, and nitrophenol of 0.003%.

[0038] Example 2.

[0039] like Figure 1 As shown, a process for synthesizing mononitrotoluene using a multi-stage continuous microchannel nitration reactor includes the following steps: 1) Fuming nitric acid is pumped from the nitric acid storage tank to the cooled static mixer M1 at a rate of 1.24 kg / h using a nitric acid pump. Concentrated sulfuric acid is pumped from the concentrated sulfuric acid storage tank to the cooled static mixer M1 at a rate of 5.59 kg / h using a sulfuric acid pump. Water is pumped to the cooled static mixer M1 at a rate of 2.49 kg / h using a water pump. The mixed acid nitrifying agent is discharged from the static mixer M1 and enters the mixed acid nitrifying agent raw material tank. 3) Input the mixed acid nitrifying agent and toluene obtained in step 1) into the multi-stage continuous microchannel nitration reactor for nitration reaction: Use a toluene solution transfer pump to input the mixed toluene from the raw material tank into the multi-stage continuous microchannel nitration reactor at a rate of 1.8 kg / h, and use a mixed acid transfer pump to input the mixed acid nitrifying agent from the raw material tank into the multi-stage continuous microchannel nitration reactor at a rate of 9.32 kg / h. The two materials undergo nitration reaction in the multi-stage continuous microchannel nitration reactor to obtain mononitrotoluene.

[0040] In this embodiment, the reaction temperature in step 3) is controlled to be 35-90℃, with different temperature controls for each stage. It is particularly preferred that the first stage is 40℃, the second stage is 75℃, and the third stage is 55℃.

[0041] The concentration of nitric acid is 98%, the concentration of sulfuric acid is 98%, and the molar ratio of nitric acid:sulfuric acid:water is 1:4.5:2; the molar ratio of toluene to nitric acid is 1:1.01.

[0042] The multi-stage continuous microchannel nitration reactor includes a first microchannel reactor 1, which contains a distributor; a second microchannel reactor 2, which contains a remixer and a distributor; and a third reactor, a jacketed vertical tubular reactor with a high-speed stirrer, arranged in series. The material enters the main microchannel reactor from its outlet and then from the bottom into the jacketed vertical tubular reactor with the high-speed stirrer. The first two microchannel reactors have a tube diameter of 300 μm, a length of 10 m, and a residence time of 0.2 s. The third stage has a stirrer blade diameter-to-height ratio of 0.8, a stirring speed of 400 r / min, a residence time of 6 min, and discharges from the top. Each stage employs a different individually controlled temperature and heat exchange system.

[0043] The first and second stage microchannel reactors are equipped with material distributors at their material inlets; the microchannel reactors are equipped with jacketed heat exchangers. The overall reaction time is 0.2 s for the first and second stages and 6 min for the third stage, yielding products with a toluene conversion rate of 97.8%, dinitrotoluene of 0.28%, and nitrophenol of 0.003%.

[0044] Example 3.

[0045] like Figure 1 As shown, a process for synthesizing mononitrotoluene using a multi-stage continuous microchannel nitration reactor includes the following steps: 1) Fuming nitric acid is pumped from the nitric acid storage tank to the cooled static mixer at a rate of 1.38 kg / h using a nitric acid pump. Concentrated sulfuric acid is pumped from the concentrated sulfuric acid storage tank to the cooled static mixer at a rate of 6.19 kg / h using a sulfuric acid pump. Water is pumped to the cooled static mixer at a rate of 2.75 kg / h using a water pump. The mixed acid nitrifying agent is discharged from the static mixer and enters the mixed acid nitrifying agent raw material tank. 3) Input the mixed acid nitrifying agent and toluene obtained in step 1) into the multi-stage continuous microchannel nitration reactor for nitration reaction: Use a toluene solution transfer pump to input the mixed toluene from the raw material tank into the multi-stage continuous microchannel nitration reactor at a rate of 1.99 kg / h, and use a mixed acid transfer pump to input the mixed acid nitrifying agent from the raw material tank into the multi-stage continuous microchannel nitration reactor at a rate of 10.32 kg / h. The two materials undergo nitration reaction in the multi-stage continuous microchannel nitration reactor to obtain mononitrotoluene.

[0046] In this embodiment, the reaction temperature in step 3) is controlled to be 35-90℃, with different temperature controls for each stage. It is particularly preferred that the first stage is 40℃, the second stage is 75℃, and the third stage is 55℃.

[0047] The concentration of nitric acid is 98%, the concentration of sulfuric acid is 98%, and the molar ratio of nitric acid:sulfuric acid:water is 1:4.5:2; the molar ratio of toluene to nitric acid is 1:1.01.

[0048] like Figure 2-4 As shown, the multi-stage continuous microchannel nitration reactor includes a first microchannel reactor with a distributor inside; a second microchannel reactor with a remixer and distributor; and a third jacketed vertical tubular reactor with a high-speed stirrer, arranged in series. The material enters the microchannel reactor from the outlet of the main microchannel reactor and then from the bottom into the jacketed vertical tubular reactor with a high-speed stirrer. The first two microchannel reactors have a tube diameter of 300 μm, a length of 8 m, and a residence time of 1 second. The third stage has a stirrer blade diameter-to-height ratio of 0.9, a stirring speed of 600 r / min, a residence time of 5 min, and discharges from the top. Each stage uses a different individually controlled temperature and heat exchanger.

[0049] The first and second stage microchannel reactors are equipped with material distributors at their material inlets; the microchannel reactors are equipped with jacketed heat exchangers. The overall reaction time is 1 second for the first and second stages and 5 minutes for the third stage, yielding products with a toluene conversion rate of 97.3%, dinitrotoluene of 0.14%, and nitrophenol of 0.002%.

[0050] In fact, under different reaction conditions, the reaction time can be as short as 1-10 minutes. Sampling and testing of the mononitrotoluene obtained in Examples 1-3 showed that the toluene conversion rate in each example reached over 97%, and the byproduct dinitrotoluene was less than 0.3%. This continuous production process not only improves production efficiency but also results in a higher purity product compared to other process routes.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multi-stage continuous microchannel nitration process for preparing mononitrotoluene, characterized in that, Includes the following steps: Step S1: Mix nitric acid, sulfuric acid and water in a certain proportion to form a mixed acid nitrifying agent; Step S2: The mixed acid nitrifying agent and toluene are continuously fed into a multi-stage microchannel nitration reactor, respectively, and undergo nitration reaction sequentially through three reaction zones; the three reaction zones are as follows: The first stage: a microchannel reactor with an inner diameter of 300~500μm, a length of 1~10 m, a temperature control of 40~55℃, and a residence time of 0.2~1s; The second stage: a microchannel reactor with an inner diameter of 300~500μm, a length of 1~10 m, a temperature control of 70~80℃, and a residence time of 0.2~1s; The third stage: a vertical stirred reactor with jacketed cooling, with a temperature control of 50~65℃, a stirring blade diameter-to-height ratio of 8~20:1, a rotation speed of 300~600 rpm, and a residence time of 5~8 min; Step S3: Discharge and post-processing. In the three reaction zones, each zone is equipped with an independent temperature control system and a material distributor. After the reaction in step S2 is completed, the crude mononitrotoluene is obtained by acid-oil two-phase separation. The separation temperature is controlled at 45~55 ℃ so that the toluene conversion rate is ≥97%, and the by-products are dinitrotoluene ≤0.3% and nitrophenol ≤0.003%.

2. The process according to claim 1, characterized in that, The mass ratio of nitric acid: sulfuric acid: water in the mixed acid nitrifying agent is 12:62:26 ~ 18:58:

24.

3. The process according to claim 1, characterized in that, The molar ratio of nitric acid to sulfuric acid is 1:2 to 1:

6.

4. The process according to claim 1, characterized in that, The molar ratio of toluene to nitric acid is 1:1.01 to 1:1.

05.

5. The process according to claim 1, characterized in that, The concentration of nitric acid is 50-98%, preferably 60-70%.

6. The process according to claim 1, characterized in that, The reaction does not require refrigeration; temperature control is achieved solely through circulating cooling water.

7. A multi-stage continuous nitration apparatus for implementing the process described in any one of claims 1 to 6, characterized in that, include: The first microchannel reactor is equipped with a toluene inlet, a mixed acid inlet, a cooling water inlet and outlet, and a reaction product outlet. The second microchannel reactor is connected in series with the outlet of the first stage and is equipped with a reactant inlet, a cooling water inlet and outlet, and a reaction product outlet. The third stage of the stirred reactor is a vertical jacketed structure with a stirring motor (6) at the top. The interior, from top to bottom, contains: A variable diameter reaction tube (1) is used to extend the reaction path; Agitator (3), diameter-to-height ratio 8:1~20:1, rotation speed 300~600 rpm; Stirring baffle (2) is used to enhance turbulent mixing; The flow-limiting orifice (5) is used to control the residence time; A heat exchange jacket (4) surrounds the outer wall of the reactor and is provided with cooling water inlet and outlet; The three reactors are connected in series to form a continuous flow path, and each section has independent temperature control.

8. The apparatus according to claim 7, characterized in that, The first and second microchannel reactors are coil-type or variable-diameter tube-type structures with a diameter of 300~500μm and a length of 1~10 m.

9. The apparatus according to claim 7, characterized in that, The variable diameter reaction tube (1) of the third stage stirred reactor is a tapered structure that gradually narrows from top to bottom. The flow restriction hole (5) is located at the bottom of the reactor to precisely control the material residence time of 5 to 8 minutes.

10. The apparatus according to any one of claims 7 to 9, characterized in that, The device is an ice-free continuous system that uses only circulating cooling water to control the temperature of each section, reducing energy consumption by ≥50%.

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