Process for the continuous synthesis of dnr in microchannels

Abstract

The present application relates to the field of PBO fiber monomer aromatic dinitro compound synthesis, disclose a kind of microchannel continuous synthesis DNR method, comprising: with DMSO as solvent, raw material resorcinol and mixed acid nitration agent consisting of sulfuric acid and nitric acid are pumped into microchannel reactor respectively and are carried out nitration reaction, then the crude product of microchannel reactor output is sent to ice water, obtain DNR crude crystalline product;Wherein, microchannel reactor is the microchannel reactor with heat exchange channel with fluid splitting composite, such as series tank reactor, corning heart-shaped reactor or branch type distributor, the mononitration and dinitration of resorcinol are realized by two sets of microchannel reactor, the cooling circulating water flow and temperature of two sets of microchannel reactor heat exchange channel are controlled, and the selectivity and yield of the step-by-step nitration of resorcinol are regulated.

Description

Technical Field

[0001] This invention relates to the field of synthesis of aromatic dinitro compounds from poly(p-phenylenebenzobisoxazole) (PBO) cellulose monomers, and more specifically to a method for continuous microchannel synthesis of DNR. Background Technology

[0002] Poly(p-phenylenebenzobisoxazole) (PBO) fiber is a high-performance synthetic fiber that plays an important role in many cutting-edge fields due to its excellent properties such as high strength and high thermal stability. 4,6-Dinitroresorcinol (DNR) is an important intermediate in the synthesis of PBO fiber, mainly prepared from resorcinol through a three-step reaction involving sulfonation, nitration, and hydrolysis. Currently, patent reports (such as CN201910853516.3, CN201110257600.2, and CN200810054708.X) all adopt the preparation method of Chinese patent ZL00117956.X authorized by Yukihiro Kumamoto in 2004. This method stabilizes the nitration rate by using the SO3H group from the first step of sulfonation to prevent excessive nitration into a trinitration product. Despite this, the strong exothermic effect of the system results in relatively high selectivity for both mononitration and trinitration of resorcinol, making the preparation process relatively complex, with a long reaction cycle and relatively high energy consumption.

[0003] ZL201610929583.5 first reported a process for synthesizing 4,6-dinitroresorcinol by hydrolysis using a microchannel reactor combined with a batch reactor, achieving a total DNR yield of up to 87%. However, the total reaction time is relatively long, and the losses of raw materials and products in the semi-continuous and semi-batch reaction, as well as the energy consumption, are relatively large. Summary of the Invention

[0004] The applicant of this invention, through further analysis and research on the nitration mechanism of resorcinol, identified the system's thermal effect and moisture content as the decisive factors affecting the mononitration, dinitration, and trinitration of resorcinol. A method for controlling the system's thermal effect and moisture content in microchannel continuous synthesis is proposed, thereby regulating the selectivity and yield of the main product (dinitrohydration). This further saves energy and reduces consumption, achieving green and economical synthesis of PBO monomer intermediates and enhancing my country's competitiveness in the field of high-performance fiber monomer synthesis.

[0005] This invention provides a method for continuous synthesis of DNR via microchannels to improve the selectivity and yield of DNR in nitration systems. The method is carried out in a microchannel reaction system, which includes a heat transfer unit and at least two microchannel reaction units connected in series. The microchannel reaction units are capable of mixing and reacting at least two reactants.

[0006] The method includes: feeding a mixed acid nitrifying agent into each microchannel reaction unit, and feeding a resorcinol-solvent raw material into the first-stage microchannel reaction unit, wherein the resorcinol-solvent raw material is gradually mixed with the fed mixed acid nitrifying agent in at least two microchannel reaction units connected in series to carry out a nitration reaction, generating a final nitration product, cooling, and recrystallizing to obtain a crude DNR crystal product.

[0007] Specifically, the heat removal unit is used to remove the heat of reaction in each of the microchannel reaction units.

[0008] Preferably, the microchannel reaction unit includes a microchannel reactor with a fluid splitting composite mixing channel, and the heat transfer unit includes a heat exchange channel for removing the heat of reaction from each of the microchannels.

[0009] More preferably, the microchannel reactor is selected from at least one of a series tank reactor, a Corning heart-shaped reactor, or a branched distributor.

[0010] Through the above technical solution, this invention, based on the stepwise nitration mechanism and reaction characteristics of resorcinol, discovers that controlling the amount of nitration-active intermediate phase generated in the nitrate-sulfur mixed acid can effectively control the stepwise nitration rate of resorcinol, thereby effectively regulating the dinitration selectivity and yield of resorcinol. This simplifies the reaction process from the traditional three-step sulfonation, nitration, and hydrolysis reaction to a single direct nitration reaction, shortening the production cycle and improving operational safety. At the same time, the acidic solvent can be directly recovered and reused after dehydration, which is beneficial for industrial-scale production. Attached Figure Description

[0011] Figure 1 This is a flowchart illustrating a method for continuous microchannel synthesis of DNR according to one embodiment of the present invention.

[0012] Figure 2 yes Figure 1 A schematic diagram of the microchannel reactor in the image.

[0013] Explanation of reference numerals in the attached figures

[0014] 1 and 2 are constant temperature baths; 6 and 7 are circulating pumps; 3 is a DMSO solution of resorcinol; 4 and 5 are DMSO solutions of concentrated sulfuric acid / concentrated nitric acid; 8, 9 and 10 are horizontal flow pumps; 11 and 12 are heat exchangers; 13 and 14 are microchannel reactors; 15 is an ice-water crystallizer. Detailed Implementation

[0015] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0016] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0017] In this invention, unless otherwise stated, directional terms such as "upper," "lower," "left," and "right" generally refer to the upper, lower, left, and right as shown in the accompanying drawings; "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0018] It should be noted that 4,6-dinitroresorcinol (DNR) is an important intermediate in the synthesis of PBO fibers. It is mainly prepared by resorcinol through three steps of sulfonation, nitration, and hydrolysis. Currently, sulfonation, nitration, and hydrolysis reactions are all carried out in batch reactors. However, sulfonation and nitration are both strongly exothermic reactions. Traditional batch operation cannot achieve precise temperature control and it is difficult to achieve uniform mixing of materials. This not only results in poor safety and low reaction yield, but also unstable product quality between batches.

[0019] To solve the above problems, such as Figures 1-2 As shown, the present invention provides a method for continuous synthesis of DNR via microchannels. The method is carried out in a microchannel reaction system, which includes a heat transfer unit and at least two microchannel reaction units connected in series. The microchannel reaction units are capable of mixing and reacting at least two reactants.

[0020] The method includes: feeding a mixed acid nitrifying agent into each microchannel reaction unit, and feeding a resorcinol-solvent feedstock into the first-stage microchannel reaction unit, wherein the resorcinol-solvent feedstock is gradually mixed with the fed mixed acid nitrifying agent in at least two microchannel reaction units connected in series to carry out a nitration reaction, generating a final nitration product, cooling, and recrystallizing to obtain a crude DNR crystal product; wherein a heat removal unit is used to forcibly remove the heat of reaction in each microchannel reaction unit.

[0021] In this invention, the goal of removing the heat of reaction is to control the temperature rise of the system to be less than 5-10°C.

[0022] Traditional methods often involve the vigorous and exothermic nitration of resorcinol, which readily induces side reactions. Therefore, sulfonic acid groups are used as protecting groups to weaken the nitration rate. However, this necessitates the use of alkaline reagents to neutralize the acidity of the system for subsequent hydrolysis and product collection. Overall, the process is relatively complex, has a long reaction cycle, and consumes a significant amount of acid. This invention, based on the stepwise nitration mechanism and reaction characteristics of resorcinol, discovers that controlling the amount of the active intermediate phase generated in the nitrate-sulfuric acid mixture can effectively control the stepwise nitration rate of resorcinol. This allows for effective regulation of the dinitration selectivity and yield of resorcinol, simplifying the traditional three-step sulfonation, nitration, and hydrolysis reaction into a single direct nitration reaction. This shortens the production cycle, improves operational safety, and allows the acidic solvent to be directly recovered and reused after dehydration, which is beneficial for large-scale industrial production.

[0023] Compared to batch reactors, microchannel reactors operate continuously, have a small reaction space and a large heat transfer area, and can accurately control the material concentration during the reaction. Therefore, they can not only achieve stable and safe operation, but also significantly improve product yield.

[0024] In this invention, each microchannel reaction unit includes a microchannel reactor with fluid splitting and recombination mixing channels. This microchannel reactor allows the fluid to be mixed to continuously split, recombine, stretch, and break up, increasing the contact area and improving fluid distribution, thereby achieving rapid mixing and efficient reaction. In some embodiments of this invention, the heat exchange channels of the microchannel reaction unit and the heat transfer unit are combined in the microchannel reactor, which has the following characteristics: Figure 2 The left figure shows a series-connected microchannel tube 16. Multiple parallel, spaced heat exchange channels are added to both sides of the series-connected microchannel tube 16 for the flow of heat exchange medium. The inlet and outlet of each heat exchange channel are connected to a thermostatic bath containing the heat exchange medium, and the heat exchange medium is transported by a circulating pump. Figure 1 As shown, two sets of the above-mentioned microchannel reactors are connected in series to realize the mononitration and dinitration of resorcinol. The flow rate and temperature of the cooling circulating water in the heat exchange channels of the two sets of microchannel reactors are controlled to achieve the selectivity and yield of the stepwise nitration of resorcinol.

[0025] In this invention, the microchannel reactor is preferably selected from at least one of a series tank reactor (e.g., ZL202110500782.5), a Corning heart-shaped reactor, or a branched distributor (e.g., ZL 202110500297.8).

[0026] To improve the selectivity and yield of resorcinol dinitration, according to a preferred embodiment of the present invention, the flow rate of the heat exchange medium in the heat exchange channel is F > (500~2500)f / Ke, in mL / min; where f is the total flow rate of the fluid in the microchannel of the microreactor, in mL / min; and Ke is the effective heat transfer coefficient of the heat exchange channel, in W / m³. -2 K -1 In this invention, the heat exchange channel is made of stainless steel, alloy or glass. The microchannel reactor has an internal structure with fluid splitting-combining action. If the external heat exchange channel is mainly made of glass, the flow rate of the heat exchange medium in the heat exchange channel is F>(2000~2500)f / Ke; if it is mainly made of stainless steel and alloy, the flow rate of the heat exchange medium in the heat exchange channel is F>(500~1000)f / Ke.

[0027] This invention does not have any special requirements for the flow direction of the heat exchange medium. The following is an illustrative description, but it does not limit the scope of the invention. According to one embodiment of the invention, the flow direction of the reactants in the microchannel reactor is opposite to the flow direction of the heat exchange medium in the heat exchange channel of the microchannel reactor.

[0028] This invention applies to all conventional heat exchange media. The following is an illustrative description, but it does not limit the scope of the invention. According to one embodiment of the invention, the heat exchange medium is selected from at least one of water, cooling brine, or an aqueous solution of ethylene glycol.

[0029] According to a preferred embodiment of the present invention, the liquid holdup within each split-combination mixing module of the single-stage microchannel reactor is 0.1–0.5 mL. The split-combination module in this invention refers to, for example, a... Figure 2 A single mixing tank in a series tank reactor (ZL202110500782.5); or the last-stage branching structure of the fluid mixing channel in a branched distributor (e.g., ZL 202110500297.8).

[0030] To improve the selectivity and yield of resorcinol dinitration, according to a preferred embodiment of the present invention, the liquid holding capacity within the microchannel reactor volume is >6 ml.

[0031] In this invention, the characteristic dimension of fluid mixing at the inlet and outlet of the microchannel reactor is <1 mm. In this invention, the characteristic dimension of fluid mixing at the inlet and outlet refers to the minimum diameter of the mixing channel in the microreactor, such as... Figure 2 The neck channel of the trough reactor has the smallest diameter. In this invention, the smaller this characteristic size, the more conducive it is to the generation of the active phase of the nitration reaction, and the more conducive it is to improving the yield and selectivity.

[0032] According to a preferred embodiment of the present invention, the flow rate of the resorcinol-solvent feedstock fed into the first-stage microchannel reaction unit is 2 to 5 mL / min.

[0033] According to a preferred embodiment of the present invention, the solution concentration of resorcinol-DMSO raw material is preferably 1 to 6 mol / L.

[0034] In this invention, the mixed acid nitrifying agent includes sulfuric acid, nitric acid, and DMSO solvent. This invention does not have any special restrictions on the source of sulfuric acid, nitric acid, and DMSO solvent. Commercially available products of the above substances can be used, or they can be prepared by oneself according to the technical solutions for preparing the above substances well known to those skilled in the art.

[0035] In this invention, preferably, in the mixed acid nitrifying agent, the sulfuric acid is fuming sulfuric acid or concentrated sulfuric acid with a concentration of 70-98 wt%, and the nitric acid is fuming nitric acid or concentrated nitric acid with a concentration of ≥65 wt%.

[0036] In this invention, preferably, the flow rate of the mixed acid nitrifying agent fed into the first-stage microchannel reaction unit is 1-4 mL / min.

[0037] In this invention, preferably, the volume ratio of sulfuric acid, nitric acid and DMSO solvent in the mixed acid nitrifying agent fed to the first-stage microchannel reaction unit is (1-4):1:(0.4-2).

[0038] According to a preferred embodiment of the present invention, the flow rate of the mixed acid nitrifying agent fed into the microchannel reaction unit after the first stage is 2 to 8 mL / min.

[0039] According to a preferred embodiment of the present invention, in the mixed acid nitrifying agent fed into the microchannel reaction unit after the first stage, the volume ratio of sulfuric acid, nitric acid and DMSO solvent is (6-8):1:(1-2).

[0040] According to a preferred embodiment of the present invention, the liquid holding capacity of the microchannel reactor in the first-stage microchannel reaction unit is 6-8 mL.

[0041] According to a preferred embodiment of the present invention, the feed temperature of the mixed acid nitrifying agent and resorcinol-solvent raw materials fed into the microchannel reactor in the first-stage microchannel reaction unit is 20-30°C.

[0042] According to a preferred embodiment of the present invention, the temperature of the heat exchange medium flowing in the heat exchange channel of the microchannel reactor in the first-stage microchannel reaction unit is 5-15°C, and the flow rate of the heat exchange medium is 100-120 mL / min.

[0043] According to a preferred embodiment of the present invention, the liquid holding capacity of the microchannel reactor in the reaction unit after the first stage is 12-22 mL.

[0044] According to a preferred embodiment of the present invention, the feed temperature of the mixed acid nitrifying agent in the microchannel reactor of the reaction unit after the first stage is 45-60°C.

[0045] According to a preferred embodiment of the present invention, the temperature of the heat exchange medium flowing in the heat exchange channel after the first stage is 20-30°C, and the flow rate of the heat exchange medium is 100-150 mL / min.

[0046] The advantages of the present invention will be illustrated by the following examples, but the present invention is not limited thereto.

[0047] The following examples are as follows Figures 1-2 The microchannel reaction system shown includes two microchannel reactors connected in series. These microchannel reactors are the series-connected trough-type microreactors proposed by the applicant in patent ZL202110500782.5. They are named the first-stage microchannel reactor 13 and the second-stage microchannel reactor 14 along the overall material flow direction. The characteristic dimension of the inlet and outlet fluid mixing of the first-stage microchannel reactor 13, the reaction inlet 2, and the reaction outlet of the second-stage microchannel reactor 14 is 0.8 mm.

[0048] The method includes: feeding a mixed solution of resorcinol and DMSO as one feed 3, mixing a mixed acid nitrifying agent composed of sulfuric acid and nitric acid with DMSO solvent as another feed 4, and feeding the two feeds to the reaction temperature through a heat exchanger 11 before feeding them into a first-stage microchannel reactor 13 to carry out a first-stage nitration reaction to generate a first-stage nitration product. The heat exchange channel of the first-stage microchannel reactor 13 is cooled by a constant temperature bath 2.

[0049] The outlet of the first-stage microchannel reactor 13 is connected to the second-stage microchannel reactor 14. The other feed is mixed acid nitrifying agent 5, which is heated to the reaction temperature by the heat exchanger 12 and then enters the second-stage microchannel reactor 14 to carry out a secondary nitration reaction with the primary nitration product from the outlet of the first-stage microchannel reactor 13. The cooling medium is introduced into the heat exchange channel of the second-stage microchannel reactor 14 through the constant temperature bath 1 for heat transfer. The product from the outlet of the second-stage microchannel reactor 14 is directly fed into the ice water crystallizer 15 containing ice water for cooling and recrystallization to obtain the crude DNR crystal product. It should be noted that the crystallized material includes the target material and the by-product (the sulfonation product of DNR).

[0050] It should be noted that the fluid flow rate formula of the heat exchange channel of the present invention is limited to the resorcinol nitration reaction conditions proposed in this embodiment, and there are no strict restrictions on the structure of the microchannel reactor (the branch distributor proposed in the patent ZL202110500297.8 granted by the applicant of the present invention also has the same effect);

[0051] The method for calculating the yield of the crystalline product is Y = m / (n DNR M DNR +n 副 M 副 ), where m is the mass of the crystalline substance and M is the molar mass;

[0052] The method for calculating DNR selectivity is S DNR =n DNR / (n DNR +n 副 ), where n is the molar quantity.

[0053] Example 1

[0054] The DMSO solution for resorcinol was prepared at a concentration of 3 mol / L and a flow rate of 5 mL / min. The mixed acid nitrifying agent consisted of commercially available 98 wt% concentrated sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 1:1:2, with a flow rate of 2 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 80 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 6:1:1, with a flow rate of 4 mL / min. In the first-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 7 mL, a feed temperature of 25°C, and a liquid holdup of 0.112 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 10°C, and the flow rate was 110 mL / min. In the second-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 14 mL, a feed temperature of 55°C, and a liquid holdup of 0.224 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 25°C, and the flow rate was 140 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer, and the yield of the crystals was determined to be 94%, with a DNR selectivity of 97%.

[0055] Example 2

[0056] The DMSO solution for resorcinol was prepared at a concentration of 1 mol / L and a flow rate of 2 mL / min. The mixed acid nitrifying agent consisted of commercially available 70 wt% concentrated sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 4:1:0.4, with a flow rate of 4 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 100 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 8:1:1, with a flow rate of 8 mL / min. In the first-stage microchannel reaction unit, the liquid holdup of the microchannel reactor was 10 mL, the feed temperature was 30°C, the liquid holdup in each splitting and compounding mixing module was 0.112 mL, the circulating water temperature in the heat exchange channel was 15°C, and the flow rate was 120 mL / min. In the second-stage microchannel reaction unit, the liquid holdup of the microchannel reactor was 22 mL, the feed temperature was 60°C, the liquid holdup in each splitting and compounding mixing module was 0.448 mL, the circulating water temperature in the heat exchange channel was 30°C, and the flow rate was 100 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer, and the yield of the crystals was determined to be 97%, with a DNR selectivity of 91%.

[0057] Example 3

[0058] The DMSO solution for resorcinol was prepared at a concentration of 6 mol / L and a flow rate of 4 mL / min. The mixed acid nitrifying agent consisted of 80 wt% concentrated sulfuric acid, 100 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 2:1:1, with a flow rate of 1 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 100 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 7:1:2, with a flow rate of 2 mL / min. In the first-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 6 mL, a feed temperature of 20°C, and a liquid holdup of 0.224 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 5°C, and the flow rate was 100 mL / min. In the second-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 12 mL, a feed temperature of 45°C, and a liquid holdup of 0.112 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 20°C, and the flow rate was 150 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer, and the yield of the crystals was determined to be 99%, with a DNR selectivity of 94%.

[0059] Example 4

[0060] The DMSO solution for resorcinol was prepared at a concentration of 2 mol / L and a flow rate of 3 mL / min. The mixed acid nitrifying agent consisted of 20 wt% commercially available fuming sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 1:1:0.4, with a flow rate of 1 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 6:1:2, with a flow rate of 5 mL / min. In the first-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 8 mL, a feed temperature of 20°C, and a liquid holdup of 0.448 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 10°C, and the flow rate was 100 mL / min. In the second-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 16 mL, a feed temperature of 50°C, and a liquid holdup of 0.112 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 20°C, and the flow rate was 150 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer, and the yield of the crystals was determined to be 97%, with a DNR selectivity of 95%.

[0061] Example 5

[0062] Unlike Example 1, the DMSO solution for preparing resorcinol was prepared at a concentration of 3 mol / L and a flow rate of 10 mL / min. The mixed acid nitrifying agent consisted of commercially available 98 wt% concentrated sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 6:1:5, with a flow rate of 10 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 80 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 2:1:5, with a flow rate of 10 mL / min. The microchannel reactor outlet was connected to an ice-water crystallizer, and the yield of the crystals was determined to be 86%, with a DNR selectivity of 83%.

[0063] Example 6

[0064] Unlike Example 1, the first-stage microchannel reaction unit had a liquid holdup of 4 mL in the microchannel reactor, a feed temperature of 25°C, and a liquid holdup of 0.112 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 10°C, and the flow rate was 80 mL / min. The second-stage microchannel reaction unit had a liquid holdup of 20 mL in the microchannel reactor, a feed temperature of 55°C, and a liquid holdup of 0.224 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 25°C, and the flow rate was 180 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer. The yield of the crystals was determined to be 92%, and the DNR selectivity was 66%.

[0065] Comparative Example 1

[0066] The DMSO solution for resorcinol was prepared at a concentration of 3 mol / L and a flow rate of 5 mL / min. The mixed acid nitrifying agent consisted of commercially available 98 wt% concentrated sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 1:1:2, with a flow rate of 2 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 80 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 6:1:1, with a flow rate of 4 mL / min. The liquid holdup of the microchannel reactor in the first-stage microchannel reaction unit was 7 mL, the feed temperature was 25 °C, and the liquid holdup in each splitting and compounding mixing module was 0.112 mL; the liquid holdup of the microchannel reactor in the second-stage microchannel reaction unit was 14 mL, the feed temperature was 55 °C, and the liquid holdup in each splitting and compounding mixing module was 0.224 mL. The outlet of the microchannel reactor was connected to an ice-water crystallizer. The yield of the crystals was determined to be 60% (microchannel reactor blockage occurred in the later stage of the reaction), and the DNR selectivity was 60%.

[0067] Comparative Example 2

[0068] The DMSO solution for resorcinol was prepared at a concentration of 6 mol / L and a flow rate of 4 mL / min. The mixed acid nitrifying agent consisted of 80 wt% concentrated sulfuric acid, 100 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 2:1:1, with a flow rate of 1 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 100 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 7:1:2, with a flow rate of 2 mL / min. In the first-stage microchannel reaction unit, the liquid holdup of the microchannel reactor was 6 mL, the feed temperature was 20 °C, and the liquid holdup in each splitting and compounding mixing module was 0.224 mL. In the second-stage microchannel reaction unit, the liquid holdup of the microchannel reactor was 12 mL, the feed temperature was 45 °C, and the liquid holdup in each splitting and compounding mixing module was 0.112 mL. The circulating water temperature in the heat exchange channel was 20 °C, and the flow rate was 150 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer. The yield of the crystals was determined to be 88%, and the DNR selectivity was 74%.

[0069] Comparative Example 3

[0070] The DMSO solution for resorcinol was prepared at a concentration of 2 mol / L and a flow rate of 3 mL / min. The mixed acid nitrifying agent consisted of 20 wt% commercially available fuming sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 1:1:0.4, with a flow rate of 1 mL / min. The second-stage mixed acid nitrifying agent consisted of 20 wt% fuming sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 6:1:2, with a flow rate of 5 mL / min. In the first-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 8 mL, a feed temperature of 20°C, and a liquid holdup of 0.448 mL in each splitting and compounding mixing module. The circulating water temperature in the heat exchange channel was 10°C, and the flow rate was 100 mL / min. In the second-stage microchannel reaction unit, the microchannel reactor had a liquid holdup of 16 mL, a feed temperature of 50°C, and a liquid holdup of 0.112 mL in each splitting and compounding mixing module. The outlet of the microchannel reactor was connected to an ice-water crystallizer, and the yield of the crystals was determined to be 82%, with a DNR selectivity of 71%.

[0071] Comparative Example 4

[0072] Unlike Example 1, only a first-stage microchannel reactor was used. The DMSO solution of resorcinol was prepared at a concentration of 3 mol / L and a flow rate of 5 mL / min. The mixed acid nitrifying agent consisted of commercially available 98% concentrated sulfuric acid, 65 wt% concentrated nitric acid, and DMSO solvent in a volume ratio of 1:1:2, with a flow rate of 2 mL / min. The liquid holdup in the microchannel reactor of the first-stage microchannel reaction unit was 7 mL, the feed temperature was 25°C, the liquid holdup in each splitting and compounding mixing module was 0.112 mL, the circulating water temperature in the heat exchange channel was 10°C, and the flow rate was 110 mL / min. The outlet of the microchannel reactor was connected to an ice-water crystallizer. The yield of the crystals was determined to be 75%, and the DNR selectivity was 98%.

[0073] Comparative Example 5

[0074] A batch synthesis reaction was carried out in a water-jacketed reactor. 100 mL of a 3 mol / L resorcinol (DMSO) solution was added to the reactor. The reactor temperature was maintained at 25°C using the water jacket. A mixed acid was prepared by mixing commercially available 98 wt% concentrated sulfuric acid and 65 wt% concentrated nitric acid, with a molar ratio of 2:1:1 to resorcinol. The mixed acid was added dropwise to the reactor at a rate of 0.3 mL / min, with stirring at 500 rpm. After the addition was complete, the mixture was kept at the constant temperature for 1 hour. Subsequently, the mixture was crystallized in an ice-water crystallizer. The yield of the crystals was determined to be 90%, and the DNR selectivity was 15%.

[0075] This invention achieves mononitration and dinitration of resorcinol using two sets of microchannel reactors. By controlling the flow rate and temperature of the cooling circulating water in the heat exchange channels of the two microchannel reactors, the selectivity and yield of the stepwise nitration of resorcinol can be regulated. According to the method of this invention, DNR selectivity of up to 98% and yield of up to 99% can be rapidly achieved within a short residence time (3-15 minutes). Furthermore, this invention is green, economical, safe, and efficient.

[0076] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various specific technical features in any suitable manner. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately. However, these simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for the continuous synthesis of DNR in microchannels, characterized in that, The method is carried out in a micro-channel reaction system, which comprises a heat removal unit and at least two micro-channel reaction units connected in series, and the micro-channel reaction units are capable of mixing and reacting at least two reactants; The method comprises: feeding a mixed acid nitration agent to each micro-channel reaction unit, and feeding a resorcinol-solvent raw material to the first-stage micro-channel reaction unit, wherein the resorcinol-solvent raw material is gradually mixed with the mixed acid nitration agent fed to each of the at least two micro-channel reaction units in series to carry out a nitration reaction, to generate a final nitration product, which is cooled and recrystallized to obtain a DNR crude crystalline product; The heat of reaction in each of the micro-channel reaction units is removed by using the heat removal unit; The flow rate of the resorcinol-solvent raw material fed to the first-stage micro-channel reaction unit is 2-5 mL / min, and the solution concentration of the resorcinol-solvent raw material is 1-6 mol / L; The flow rate of the mixed acid nitration agent fed to the first-stage micro-channel reaction unit is 1-4 mL / min; In the mixed acid nitration agent fed to the first-stage micro-channel reaction unit, the volume ratio of sulfuric acid, nitric acid and DMSO solvent is (1-4):1:(0.4-2); In the micro-channel reaction units after the first stage: the flow rate of the mixed acid nitration agent fed is 2-8 mL / min, and in the mixed acid nitration agent fed, the volume ratio of sulfuric acid, nitric acid and DMSO solvent is (6-8):1:(1-2); The liquid holdup of the micro-channel reactor in the first-stage micro-channel reaction unit is 6-8 mL; The feed temperature of the mixed acid nitrating agent and the resorcinol-solvent feedstock to the microchannel reactor in the first stage microchannel reaction unit is 20 to 30 o C; In the micro-channel reaction units after the first stage: the liquid holdup of the micro-channel reactor is 12-22 mL; The feed temperature of the mixed acid nitrating agent is 45 to 60 o C.

2. The method of claim 1, wherein, The micro-channel reaction unit comprises a micro-channel reactor with a fluid splitting and complex mixing channel, and the heat removal unit comprises a heat exchange channel for removing the heat of reaction of each micro-channel reactor.

3. The method according to claim 2, wherein, The micro-channel reactor is selected from at least one of a series tank reactor, a Corning heart-shaped reactor or a dendritic distributor.

4. The method of claim 2, wherein, The flow direction of the reactant in the micro-channel reactor is opposite to the flow direction of the heat exchange medium in the heat exchange channel of the micro-channel reactor. And / or The heat exchange medium is selected from at least one of water, cooled brine or an aqueous ethylene glycol solution.

5. The method of claim 2 or 3, wherein, The liquid holdup in each splitting and complex mixing module of a single-stage micro-channel reactor is 0.1-0.5 mL. And / or The total liquid holdup in the volume of a single-stage micro-channel reactor is >6 mL.

6. The method of any one of claims 1-4, wherein, The mixed acid nitration agent comprises sulfuric acid, nitric acid and DMSO solvent, and in the mixed acid nitration agent, the sulfuric acid is oleum or concentrated sulfuric acid with a concentration of 70-98 wt%, and the nitric acid is oleum or concentrated nitric acid with a concentration of ≥65 wt%.

7. The method according to any one of claims 1-4, wherein, The temperature of the heat exchange medium flowing in the heat exchange channel of the micro-channel reactor of the first-stage micro-channel reaction unit is 5-15 o C, and the flow rate of the heat exchange medium is 100-120 mL / min.

8. The method of any one of claims 1-4, wherein, In the micro-channel reaction units after the first stage: The temperature of the heat exchange medium flowing in the heat exchange channel is 20-30 o C, and the flow rate of the heat exchange medium is 100-150 mL / min.

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