A method for preparing nitrobenzene using a microreactor under low acid conditions

Abstract

The present invention relates to the technical field of nitrobenzene production, and specifically to a method for preparing nitrobenzene by using a microreactor under low-acid conditions. The method for preparing nitrobenzene is as follows: a nitrating agent and raw benzene are introduced into a microreactor for nitration reaction to obtain nitrobenzene, the microreactor is provided with at least four reaction modules in series, the nitrating agent and raw benzene are fed into the first reaction module, and concentrated sulfuric acid is added to at least one reaction module in other reaction modules except the first reaction module during the nitration reaction; the effective concentration of sulfuric acid in the nitrating agent is not higher than 77%. The method for preparing nitrobenzene solves the production problems of poor reaction stability of benzene in a microreactor system under low-acid conditions, incomplete conversion, large residual amount, affecting dinitration, and large amount of waste acid generated. Finally, the application process of dinitration waste acid and acidic benzene is realized, and the production process is safe and reliable, with large operation flexibility and strong stability.

Description

Technical Field

[0001] The invention relates to a method for preparing nitrobenzene by using a microreactor under low-acid conditions, and belongs to the technical field of nitrobenzene production. Background Art

[0002] Nitrobenzene is an organic synthesis intermediate and the main raw material for the production of aniline. It is used in the production of dyes, fragrances, explosives and other organic synthesis industries. Nitrobenzene is mainly prepared by nitration of benzene with nitric acid and sulfuric acid. At present, the manufacturers that use benzene as a raw material and nitrate it with a nitrating agent to produce nitrobenzene adopt an intermittent production process. The intermittent production method mainly has problems such as small production capacity, poor process stability, backward production environment, complex production process, and the safety of on-site personnel cannot be guaranteed. A large amount of sulfuric acid is required as a catalyst in the intermittent reaction process, which produces a large amount of waste acid and wastewater, corrodes equipment, and pollutes the environment. Among them, the most prominent is the safety risk, that is, the heat release in the synthesis of nitrobenzene is large, the reaction temperature of the materials is uncontrollable and continues to rise, which will cause nitric acid to decompose and a large amount of by-product nitrophenols to be generated. These phenols are also the main cause of accidents in the production of nitrobenzene. At present, the nitration industry also generally has the problems of large sulfuric acid consumption, large amount of waste acid generation, and poor waste acid resource utilization. Environmental protection requirements are becoming increasingly stringent, and the harmless disposal and resource utilization of waste sulfuric acid are urgent.

[0003] In recent years, micro-reaction technology has been rapidly developed in various fine chemical industries, especially the nitration industry has paid more attention to intrinsically safe production models and equipment. Microreactors have been applied in the development. For example, Chinese patent applications CN115197067A and CN117123179A use microreactors to carry out nitration reactions, which can effectively improve the reaction efficiency.

[0004] At present, in order to improve the utilization rate of key atoms in the nitration reaction, the mononitration reaction is basically carried out using a mononitration mixed acid prepared with acidic benzene and dinitration waste acid as raw materials. The acidic benzene is the organic phase obtained by extracting the waste acid of the mononitration reaction with fresh benzene, wherein the acidic benzene contains 0.1-1.0% nitrobenzene product and a trace amount of mononitration waste acid wrapped in nitrobenzene; the dinitration waste acid is the large amount of waste acid produced in the process of preparing the required dinitrobenzene by dinitration reaction of the mononitration product nitrobenzene, the sulfuric acid mass concentration of the dinitration waste acid is 70-85%, and the dinitration waste acid also contains 1-15% organic matter (such as o-dinitrobenzene, m-dinitrobenzene, p-dinitrobenzene, nitrobenzene), and 1-3% nitric acid. The processing cost of concentrating and purifying dinitration waste acid in industry is very high. If the recycling of dinitration waste acid can be directly realized in the production process, it will have important economic value. The important problem currently faced is that if nitrobenzene is prepared according to conventional nitration reaction conditions and using a microreactor, that is, when the reaction raw materials are acidic benzene and dinitrogen waste acid, although a recycling process is achieved, the presence of organic matter in the dinitrogen waste acid will cause the temperature and pressure in the microreactor to be uncontrollable, and especially cause the temperature to rise significantly, produce a large amount of by-products (nitrophenols), and lead to a decrease in the quality of the target product nitrobenzene. In addition, it is easy to cause the risk of explosion, posing a production safety hazard.

[0005] Therefore, it is of great value to use microreactors to recycle dinitrogen waste acid and acidic benzene, achieve efficient heat transfer, accurately control reaction temperature and reaction time, effectively reduce by-products, improve product yield, enhance product selectivity, improve the safety of the production process, enable enterprises to achieve safe production, and achieve the goal of reducing costs and increasing efficiency. Summary of the invention

[0006] The present invention aims at the deficiencies in the prior art and provides a method for preparing nitrobenzene by using a microreactor under low-acid conditions. The method for preparing nitrobenzene can achieve effective control of reaction temperature, reaction time and pressure in the microreactor to obtain high-quality nitrobenzene products, and can also achieve the reuse of acidic benzene and dinitrile waste acid, thereby solving the problems of large amount of dinitration waste acid generated, high concentration cost, production and environmental protection, etc.

[0007] The technical solution of the present invention to solve the above technical problem is as follows: a method for preparing nitrobenzene using a microreactor under low-acid conditions, wherein the method for preparing nitrobenzene is:

[0008] A nitrating agent and a raw benzene are introduced into a microreactor for nitration reaction to obtain nitrobenzene, wherein the microreactor is provided with at least four reaction modules connected in series, the nitrating agent and the raw benzene are fed into the first reaction module, and concentrated sulfuric acid is added to at least one reaction module of the other reaction modules except the first reaction module during the nitration reaction;

[0009] The effective concentration of sulfuric acid in the nitrating agent is no more than 77%.

[0010] Furthermore, the nitrating agent is prepared using dinitrogen waste acid, sulfuric acid, nitric acid and water; and the raw benzene includes acidic benzene.

[0011] Furthermore, in terms of mass fraction, the dinitrogen waste acid includes 70-85% sulfuric acid, 1-3% nitric acid, and 1-15% organic matter.

[0012] Furthermore, the acidic benzene contains 0.1-1.0% by mass of nitrobenzene and 0.1-0.5% by mass of a mixed acid of sulfuric acid and nitric acid.

[0013] Furthermore, in the nitrating agent, the mass concentration range of sulfuric acid is 47% to 55%, the mass concentration range of nitric acid is 27% to 36%, and the water content is 5% to 20%.

[0014] Furthermore, the molar ratio of benzene in the acidic benzene to sulfuric acid in the nitrating agent is (0.9-2):1; the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1-1.1):1.

[0015] Furthermore, the feed flow rate of the nitrating agent into the first reaction module is 230-290 g / min, the feed flow rate of the raw benzene into the first reaction module is 80-110 g / min; and the feed flow rate of the additional concentrated sulfuric acid is 15-45 g / min.

[0016] Preferably, the feed flow rate of the nitrating agent into the first reaction module is 230-250 g / min, 240-260 g / min, 250-270 g / min or 260-280 g / min.

[0017] The feed flow rate of the raw benzene into the first reaction module is 80-110 g / min, 80-100 g / min or 80-90 g / min.

[0018] Preferably, the feed rate of the additional concentrated sulfuric acid is 15-25 g / min, 25-35 g / min or 35-45 g / min.

[0019] Furthermore, the heat exchange temperature range of the microreactor is 20-40°C.

[0020] The beneficial effects of the present invention are:

[0021] The method for preparing nitrobenzene of the present invention can use dinitrate waste acid and acidic benzene as raw materials, ensure that nitrobenzene is prepared by a microreactor under low acid conditions (the effective concentration of sulfuric acid in the nitrating agent is low), and compensate for the problem of reduced reaction efficiency caused by the dilution of the effective sulfuric acid concentration in the nitrating agent by the generated water by adding concentrated sulfuric acid as a catalyst at the rear end of the reaction, so that the reaction of acidic benzene is in a relatively stable acid environment and is as completely converted as possible; the appropriate material feed amount in the synergistic preparation method determines the optimal amount of dinitrate waste acid and nitric acid, which can effectively control the outlet temperature of the material reaction to be less than 100°C, the conversion rate of acidic benzene is as high as 100%, and the entire preparation process is safe and controllable. At the same time, the stable recycling process of dinitrate waste acid in the microreactor system is realized to solve the production problems such as the pressure of waste acid concentration and high concentration cost. It meets the high standardization and production requirements of safe and reliable production process, large operation flexibility and strong stability.

[0022] In the method for preparing nitrobenzene described in the present invention, dinitrobenzene waste acid with an effective concentration of sulfuric acid not higher than 77% is used, that is, a nitrifying agent with a low effective concentration of sulfuric acid in the dinitrobenzene waste acid. The dissolved content of dinitrobenzene in the nitrifying agent can be effectively reduced under the low effective concentration of sulfuric acid, thereby controlling the influence of dinitrification waste acid on the reaction temperature, reaction pressure and product quality during the nitration reaction, and ensuring that the entire nitration reaction is safer and controllable. If dinitrobenzene waste acid with a high effective concentration of sulfuric acid is directly used, since it contains a relatively high amount of mixed dinitrobenzene, if nitration is carried out under the premise of safe reaction, the output of nitrobenzene will inevitably be affected (in order to ensure safe reaction, the feed flow rate needs to be reduced), if full load operation is carried out under the premise of ensuring that the output of nitrobenzene is not affected, the reaction residence time will be shortened, and the material will react extremely vigorously in the micro-reaction system. Heat release, there is a safety risk of uncontrollable reaction temperature, and at the same time, accompanied by the generation of more by-product nitrophenols at high temperature, it not only affects the quality of the target product, but also brings potential safety hazards to the nitration process. However, if low-acid conditions are directly used, sulfuric acid cannot ensure that nitric acid decomposes more nitroxyl cations, resulting in incomplete benzene reaction and a large amount of residual benzene; at the same time, during the reaction, the water produced by the reaction will further reduce the effective concentration of sulfuric acid in the system, and the reaction effect will gradually deteriorate. In the method for preparing nitrobenzene of the present invention, sulfuric acid is added during the reaction to maintain the effective concentration of sulfuric acid in the reaction system feed liquid within a certain stable range, ensuring that high-yield nitrobenzene is obtained under safe and controllable operating conditions, and the reuse of acidic benzene and dinitro waste acid can be achieved, so that the entire production process can be smoothly carried out, the production cost investment can be reduced, and the atomic utilization rate can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the structure of a microreactor used in preparing nitrobenzene in an embodiment of the present invention;

[0024] In the figure, 1. a mixing kettle for preparing a nitrating agent; 2. an intermediate tank for the nitrating agent; 3. an intermediate tank for acidic benzene; 4. a microreactor; 5. a first reaction module; 6. a second reaction module; 7. a third reaction module; 8. a fourth reaction module. DETAILED DESCRIPTION

[0025] The specific implementation of the present invention is described in detail below. The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used are only for describing specific embodiments and are not intended to limit the present invention.

[0027] A method for preparing nitrobenzene using a microreactor 4 under low acid conditions, the method for preparing nitrobenzene is:

[0028] The nitrating agent and the raw benzene are introduced into the microreactor 4 for nitration reaction to obtain nitrobenzene. The microreactor 4 is provided with at least four reaction modules connected in series. The nitrating agent and the raw benzene are fed into the first reaction module 5. During the nitration reaction, concentrated sulfuric acid is added to at least one reaction module of the other reaction modules except the first reaction module 5.

[0029] The effective concentration of sulfuric acid in the nitrating agent is not higher than 77% (the effective concentration is m (sulfuric acid) / m (sulfuric acid+water)).

[0030] Specifically, the nitrating agent is prepared using dinitrogen waste acid, sulfuric acid, nitric acid and water; the raw material benzene includes acidic benzene.

[0031] Specifically, in terms of mass fraction, the dinitrogen waste acid includes 70-85% sulfuric acid, 1-3% nitric acid, and 1-15% organic matter.

[0032] More specifically, the effective concentration of sulfuric acid in the dinitrate waste acid is 70%-77%, 70%-74%, 70%-73%, 73%-77% or 75%-77%.

[0033] Specifically, the acidic benzene contains 0.1-1.0% by mass of nitrobenzene and 0.1-0.5% by mass of a mixed acid of sulfuric acid and nitric acid.

[0034] Specifically, in the nitrating agent, the mass concentration of sulfuric acid (mass concentration is m (sulfuric acid) / m (sulfuric acid + nitric acid + water)) ranges from 47% to 55%, the mass concentration of nitric acid ranges from 27% to 36%, and the water content is 5% to 20%.

[0035] Specifically, the molar ratio of benzene in the acidic benzene to sulfuric acid in the nitrating agent is (0.9-2):1; the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1-1.1):1.

[0036] Preferably, in practical applications, the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1-1.02):1, (1-1.04):1, (1-1.06):1, (1-1.08):1, (1.02-1.05):1, (1.02-1.08):1, (1.04-1.06):1 or (1.05-1.08):1.

[0037] Specifically, the feed flow rate of the nitrating agent into the first reaction module 5 is 230-290 g / min, the feed flow rate of the raw benzene into the first reaction module 5 is 80-110 g / min; the feed flow rate of the additional concentrated sulfuric acid is 15-45 g / min.

[0038] Preferably, the feed flow rate of the nitrating agent into the first reaction module 5 is 230-250 g / min, 240-260 g / min, 250-270 g / min or 260-280 g / min.

[0039] Preferably, the feed flow rate of concentrated sulfuric acid is 15-25 g / min, 25-35 g / min, or 35-45 g / min.

[0040] Specifically, the heat exchange temperature range of the microreactor 4 is 20-40°C.

[0041] Preferably, the heat exchange temperature of the microreactor 4 is 20-25°C, 20-30°C or 20-35°C.

[0042] More specifically, Figure 1As shown, the microreactor 4 described in the embodiment of the present invention is composed of a first reaction module 5, a second reaction module 6, a third reaction module 7 and a fourth reaction module 8 connected in series. After the dinitrogen waste acid, 98% sulfuric acid, 98% nitric acid and water are mixed in a mixing kettle 1 with a nitrating agent, they enter the intermediate tank 2 of the nitrating agent, and then pass into the first reaction module 5 of the microreactor 4. The acidic benzene enters the intermediate tank 3 of the acidic benzene, and then passes into the first reaction module 5 of the microreactor 4 to react with the nitrating agent, and then reacts through the second reaction module 6, the third reaction module 7 and the fourth reaction module 8 in turn. Among them, during the reaction process, concentrated sulfuric acid (mass concentration is 98%) is added to the second reaction module 6, the third reaction module 7 or the fourth reaction module 8. Among them, Figure 1 A is the feed inlet of the first reaction module 5 of the microreactor 4 for acidic benzene; B is the feed inlet of the first reaction module 5 for the nitrating agent to enter the microreactor 4; C is the feed inlet of the third reaction module 7 for 98% sulfuric acid to enter the microreactor 4; D is the feed inlet of the second reaction module 6 for materials to enter the microreactor 4; E is the feed inlet of the third reaction module 7 for materials to enter the microreactor 4; F is the feed inlet of the fourth reaction module 8 for materials to enter the microreactor 4; and J is the discharge port of the fourth reaction module 8 for materials to exit the microreactor 4.

[0043] More specifically, the volume of each reaction module of the microreactor 4 used in the embodiment of the present invention ranges from 25 mL to 35 mL.

[0044] More specifically, since the mononitration reaction system for preparing nitrobenzene will be directly used in the subsequent dinitration reaction, the nitrobenzene is not purified for analysis, detection and evaluation after the mononitration reaction in the embodiment, but the reaction is directly evaluated by the conversion rate to determine the optimal mononitration process. The specific embodiments and comparative examples are as follows.

[0045] Example 1

[0046] The nitrating agent is prepared using dinitro waste acid, concentrated sulfuric acid, concentrated nitric acid and water, wherein the mass concentration of sulfuric acid in the nitrating agent is 48.4%, and the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1 ~1.02):1.

[0047] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a stable flow rate of 96 / min; a nitrating agent was pumped into the inlet B of the first module of the microreactor 4 at a stable flow rate of 230g / min; at a constant heat exchange temperature of 20-30°C, after 2-4s, the flow rate of 98% sulfuric acid was adjusted to 15-25g / min, 25-35g / min, and 35-45g / min, respectively, and pumped into the inlet C of the third reaction module 7 of the microreactor 4. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the fourth reaction module 8 of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 1 below.

[0048] Table 1 Sampling analysis results of Example 1

[0049]

[0050] Note: Sample numbers 1, 2, and 3 in Table 1 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0051] It can be seen from Table 1 that under the condition of a certain feed amount, gradually increasing the feed amount of concentrated sulfuric acid is beneficial to replenishing the sulfuric acid concentration diluted by the generated water, and the conversion rate of benzene is gradually improved within a safe reaction temperature.

[0052] Comparative Example 1

[0053] The same nitrating agent as in Example 1, i.e., acidic benzene, was used to react at a constant heat exchange temperature of 20-30°C. In this comparative example, 98% sulfuric acid was not added during the reaction process. A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a stable flow rate of 96 g / min; and a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a stable flow rate of 230 g / min. The reaction temperature of the material at the outlet J of the fourth reaction module 8 of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 2 below.

[0054] Table 2 Sampling analysis results of comparative example 1

[0055]

[0056] From the comparison of the experimental results of Example 1 and Comparative Example 1, it can be seen that if concentrated sulfuric acid is not added during the reaction, the conversion rate of nitrobenzene decreases significantly.

[0057] Comparative Example 2

[0058] According to the acid preparation ratio of Example 1, fresh 98% sulfuric acid, 98% nitric acid and water are used to prepare the nitrating agent for the benzene nitration reaction, and the benzene is fresh industrial benzene with a purity of >99.9%, and other process parameters are the same as those of Example 1. The specific analysis results are shown in Table 3 below.

[0059] Table 3 Sampling analysis results of Comparative Example 2

[0060]

[0061] Note: Sample numbers 5, 6, and 7 in Table 3 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0062] After analysis, the remaining amount of benzene in the three groups of samples of this comparative example 2 is higher than that of the corresponding samples 1, 2, and 3, and the corresponding reaction temperature is slightly lower, because under the same feed amount, the raw materials of Example 1 contain certain nitrobenzene and mixed dinitrobenzene, which occupy a certain volume but do not participate in the reaction during the process, and the actual reaction residence time is shortened compared with comparative example 2, and the mixing effect of the materials in the micro-reaction system is more intense, and the heat release in a short time cannot be removed in time, and the higher the temperature, the more conducive to the conversion of benzene, so the conversion rate and reaction temperature of benzene in Example 1 are slightly higher than those in comparative example 2. Secondly, by comparison, it can be seen that at a lower reaction temperature, the reaction of only mononitration is carried out using pure raw materials, and mixed dinitrobenzene (m-dinitrobenzene, o-dinitrobenzene, p-dinitrobenzene) is not generated, and the reaction effect is poor. Although the actual recycled acidic benzene waste acid contains a certain amount of mixed dinitrobenzene and nitrobenzene impurities, it has little effect on the temperature of the reaction as a whole and ensures the conversion of benzene under safe reaction conditions, and at the same time, the presence of a certain amount of impurities enhances the reaction effect of mononitration.

[0063] Example 2

[0064] The nitrating agent is prepared using dinitro waste acid, concentrated sulfuric acid, concentrated nitric acid and water, wherein the mass concentration of sulfuric acid in the nitrating agent is 48.4%, and the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1.02-1.04):1.

[0065] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 242 g / min; at a constant heat exchange temperature of 20-30°C, after 4-6 seconds, the flow rate of 98% sulfuric acid was adjusted to 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively, and pumped into the inlet C of the third reaction module 7 of the microreactor 4. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 4 below.

[0066] Table 4 Sampling analysis results of Example 2

[0067]

[0068] Note: Sample numbers 8, 9, and 10 in Table 4 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0069] From the data comparison of Example 1 and Example 2, it can be seen that under the condition of a certain feed amount, within a safe reaction temperature, gradually increasing the feed amount of concentrated nitric acid is beneficial to the reaction of benzene. Because the material contains nitrobenzene and mixed dinitrobenzene, the actual reaction residence time of the material becomes shorter, a large amount of reaction heat cannot be removed in time, the reaction of benzene is more complete, and its residual amount gradually decreases. Under the condition of less mixed dinitrobenzene and nitrobenzene content, the impact on the reaction temperature is small, and the materials react at a safe reaction temperature, further illustrating that the material application under the effective sulfuric acid concentration is stable and safe.

[0070] Comparative Example 3

[0071] The nitrating agent prepared in Example 2 was used to directly react with acidic benzene at a constant heat exchange temperature of 20-30°C without supplementing concentrated sulfuric acid, and the acidic benzene was pumped into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min using a plunger pump; the nitrating agent was pumped into the inlet B of the second reaction module 6 of the microreactor 4 at a steady flow rate of 242 g / min. The specific analysis results are shown in Table 5 below.

[0072] Table 5 Sampling analysis results of comparative example 3

[0073]

[0074] It can be seen from the comparative example that when only acidic benzene and a nitrating agent are used for the reaction without adding sulfuric acid, the residual amount of benzene is large and the reaction effect is poor.

[0075] Comparative Example 4

[0076] According to the acid preparation ratio of Example 2, fresh 98% sulfuric acid, 98% nitric acid and water are used to prepare the nitration acid for the benzene nitration reaction, and the benzene is fresh industrial benzene with a purity of >99.9%, and the other process parameters are the same as those of Example 2. The specific analysis results are shown in Table 6 below.

[0077] Table 6 Sampling analysis results of Comparative Example 4

[0078]

[0079] After analysis, the reaction phenomenon and results are similar to those of Comparative Example 2. Compared with Example 2, since fresh raw materials without nitrobenzene and mixed dinitrobenzene are used for the mononitration reaction, the actual reaction residence time is longer, the heat exchange effect of the reaction system is more sufficient, and a large amount of reaction heat is removed, which makes the reaction effect worse. The reaction is not as obvious as the material effect of Example 2, resulting in a large amount of residual benzene.

[0080] Example 3

[0081] The nitrating agent is prepared using dinitro waste acid, concentrated sulfuric acid, concentrated nitric acid and water, wherein the mass concentration of sulfuric acid in the nitrating agent is 48.4%, and the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1.04-1.06):1.

[0082] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 253 g / min; at a constant heat exchange temperature of 20-30°C, after 6-8 seconds, 98% sulfuric acid was adjusted to be pumped into the inlet C of the third reaction module 7 of the microreactor 4 at a steady flow rate of 15-25 g / min, 25-35 g / min, and 35-45 g / min. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 7 below.

[0083] Table 7 Sampling analysis results of Example 3

[0084]

[0085] Note: Sample numbers 15, 16, and 17 in Table 7 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0086] After analysis, it was found that under the condition of determined feed amount, gradually increasing the feed amount of concentrated sulfuric acid within a safe reaction temperature is beneficial to the reaction of benzene.

[0087] Comparative Example 5

[0088] The nitrating agent and material flow rate prepared in Example 3 were used to directly react with acidic benzene at a constant heat exchange temperature of 20-30° C. without adding concentrated sulfuric acid. The specific analysis results are shown in Table 8 below.

[0089] Table 8 Sampling analysis results of Comparative Example 5

[0090]

[0091] After analysis, under the condition of not adding concentrated sulfuric acid, compared with Example 3, the residual amount of benzene is higher and the actual reaction effect is poor.

[0092] Comparative Example 6

[0093] According to the acid preparation ratio of Example 3, fresh 98% sulfuric acid, 98% nitric acid and water are used to prepare the nitration acid for the benzene nitration reaction, and the benzene is fresh industrial benzene with a purity of >99.9%, and the other process parameters are the same as those of Example 3. The specific analysis results are shown in Table 9 below.

[0094] Table 9 Sampling analysis results of Comparative Example 6

[0095]

[0096] Note: Sample numbers 19, 20, and 21 in Table 9 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0097] After analysis, the reaction phenomenon and results are similar to those of Comparative Example 2, indicating that the raw materials contain nitrobenzene and mixed dinitrobenzene impurities, and the reaction still proceeds in a safe reaction temperature range for nitration reaction. At the same time, due to the presence of the impurities, the actual reaction residence time is shortened, so that the reaction effect is better.

[0098] Example 4

[0099] The nitrating agent is prepared using dinitro waste acid, concentrated sulfuric acid, concentrated nitric acid and water, wherein the mass concentration of sulfuric acid in the nitrating agent is 48.4%, and the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1.06-1.08):1.

[0100] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 265 g / min; at a constant heat exchange temperature of 20-30°C, after 8-10 seconds, 98% sulfuric acid was adjusted to be pumped into the inlet C of the third reaction module 7 of the microreactor 4 at a steady flow rate of 15-25 g / min, 25-35 g / min, and 35-45 g / min. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 10 below.

[0101] Table 10 Sampling analysis results of Example 4

[0102]

[0103] Note: Sample numbers 22, 23, and 24 in Table 10 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0104] After analysis, under the condition of determined feed amount, further increasing the feed amount of concentrated nitric acid is beneficial to the reaction of benzene, especially further increasing the flow rate of concentrated sulfuric acid to 35~45g / min, and the residual amount of benzene is 0.08%, which basically meets the experimental effect.

[0105] Comparative Example 7

[0106] The nitrating agent and material flow rate prepared in Example 4 were used to directly react with acidic benzene at a constant heat exchange temperature of 20-30° C. without adding concentrated sulfuric acid. The specific analysis results are shown in Table 11 below.

[0107] Table 11 Sampling analysis results of Comparative Example 7

[0108]

[0109] After analysis, it was found that without adding concentrated sulfuric acid, the amount of residual benzene in the mononitration reaction using acidic benzene and dinitration waste acid as raw materials would be greater than that in Example 4.

[0110] Comparative Example 8

[0111] According to the acid preparation ratio of Example 4, fresh 98% sulfuric acid, 98% nitric acid and water are used to prepare the nitration acid for the benzene nitration reaction, and the benzene is fresh industrial benzene with a purity of >99.9%, and the other process parameters are the same as those of Example 4. The specific analysis results are shown in Table 12 below.

[0112] Table 12 Sampling analysis results of Comparative Example 8

[0113]

[0114] Note: Sample numbers 26, 27, and 28 in Table 12 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0115] After analysis, compared with Example 4, the reaction effect and law are basically consistent with Comparative Examples 2, 4 and 6.

[0116] Example 5

[0117] The nitrating agent is prepared using dinitro waste acid, concentrated sulfuric acid, concentrated nitric acid and water, wherein the mass concentration of sulfuric acid in the nitrating agent is 48.4%, and the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1.08-1.1):1.

[0118] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the second reaction module 6 of the microreactor 4 at a steady flow rate of 271 g / min; at a constant heat exchange temperature of 20-30°C, after 10-12 seconds, 98% sulfuric acid was adjusted to be pumped into the inlet C of the third reaction module 7 of the microreactor 4 at a steady flow rate of 15-25 g / min, 25-35 g / min, and 35-45 g / min. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 13 below.

[0119] Table 13 Sampling analysis results of Example 5

[0120]

[0121] Note: Sample numbers 29, 30, and 31 in Table 13 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0122] After analysis, it was found that under the condition of a certain feed amount, gradually increasing the feed amount of concentrated sulfuric acid is beneficial to the reaction of benzene, especially when the concentrated sulfuric acid is increased from 15~25g / min to 35~45g / min, the minimum residual amount of benzene is 0.007%, and the reaction effect is quite obvious.

[0123] Comparative Example 9

[0124] The nitrating agent and material flow rate prepared in Example 5 were used to directly react with acidic benzene at a constant heat exchange temperature of 20-30° C. without adding concentrated sulfuric acid. The specific analysis results are shown in Table 14 below.

[0125] Table 14 Sampling analysis results of Comparative Example 9

[0126]

[0127] After analysis, under the condition of not supplementing concentrated sulfuric acid, the remaining amount of benzene is higher than that of Example 5.

[0128] Comparative Example 10

[0129] According to the acid preparation ratio of Example 5, fresh 98% sulfuric acid, 98% nitric acid and water are used to prepare the nitration acid for the benzene nitration reaction, and the benzene is fresh industrial benzene with a purity of >99.9%, and the other process parameters are the same as those of Example 5. The specific analysis results are shown in Table 15 below.

[0130] Table 15 Sampling analysis results of Comparative Example 10

[0131]

[0132] Note: Sample numbers 32, 33, and 34 in Table 15 are the sample names of the reaction solution when the sulfuric acid flow rates are 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0133] After analysis, the reaction phenomenon is basically similar to that of Comparative Examples 2, 4, 6 and 8, and the reaction law is the same.

[0134] Combining Examples 1-5 and Comparative Examples 1-10, under the same low effective concentration of sulfuric acid, a nitration mixed acid prepared from acidic benzene and dinitration waste acid is used as a raw material for nitration reaction, and the excess amount of nitric acid and the amount of concentrated sulfuric acid added are gradually increased. Under a safe reaction temperature, the remaining amount of benzene gradually decreases, and the reaction effect is more obvious; and when comparing the nitration reaction states of concentrated sulfuric acid with and without supplementation, the change in the remaining amount of benzene is more obvious, further indicating that the method of supplementing concentrated sulfuric acid under low effective concentration of sulfuric acid is more conducive to the complete conversion of benzene; wherein, the molar ratio of benzene in the nitric acid agent and the acidic benzene is (1.06-1.08): 1, that is, under the condition of an excess amount of nitric acid of 6%-8%, the amount of concentrated sulfuric acid added is 35-45 g / min, the conversion rate of benzene is the best, and the outlet temperature of the material is also controlled within a safe range of 100°C, meeting the process requirements.

[0135] At the same time, the biggest difference between the nitration reaction in the micro-reactor system using dinitrate waste acid containing 1-15% organic matter and acidic benzene containing 0.1-1.0% organic matter and fresh raw materials is: the reaction temperature of nitration increases, that is, at the same feed amount, the actual reaction residence time is reduced due to the presence of nitrobenzene and mixed dinitrobenzene impurities. Although the nitration reaction temperature increases, the reaction is carried out at a safe reaction temperature. On the contrary, the reaction effect of benzene is better and the residual amount is less.

[0136] Example 6, Example 7, Comparative Example 11

[0137] Using the nitrating agent and material flow rate prepared in Example 4, at a constant heat exchange temperature of 20-30°C, changing the feed amount of acidic benzene and nitrating agent to carry out Example 6, Example 7 and Comparative Example 11, the supplementary catalyst 98% sulfuric acid is injected into the inlet C of the third reaction module 7 of the microreactor 4 at a stable flow rate of 35-45g / min. After the system reaction is stable, the reaction temperature of the material at the outlet J of the microreactor 4 is recorded and sampled for analysis. The specific analysis results are shown in Table 16 below.

[0138] Table 16 Sampling analysis results of Example 6, Example 7 and Comparative Example 11

[0139]

[0140] Note: Sample No. 35 in Table 16 is the sampling of Example 6, sample No. 36 is the sampling of Example 7, and sample No. 37 is the sampling of Comparative Example 11.

[0141] From the above data comparison, it can be seen that when the feed rate of acidic benzene is controlled between 86 and 96 g / min, the remaining amount of benzene after the reaction will be less than 1%, and the reaction temperature will be less than 100°C; when the feed rate of acidic benzene exceeds 96 g / min, the reaction temperature exceeds the safety control range by more than 100°C, and at the same time, the remaining amount of acidic benzene is large, which is not conducive to the reaction.

[0142] Comparative Example 12

[0143] The experiment was conducted using the same nitrating agent and acidic benzene as in Example 4, except that the flow rate of 98% sulfuric acid was increased in Comparative Example 12, and the flow rate of concentrated sulfuric acid added in Comparative Example 12 was 50-70 g / min (higher than the flow rate specified in the present invention). The specific process is as follows:

[0144] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 265 g / min; at a constant heat exchange temperature of 20-30°C, after 8-10 seconds, 98% sulfuric acid was adjusted to a steady flow rate of 50-70 g / min and pumped into the inlet C of the third reaction module 7 of the microreactor 4. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 17 below.

[0145] Table 17 Sampling analysis results of Comparative Example 12

[0146]

[0147] From the data comparison between Comparative Example 12 and Example 4, it can be seen that if too much concentrated sulfuric acid is added during the reaction, the remaining amount of benzene does not change significantly, but the key is that the nitration reaction temperature exceeds the safe control range, which poses certain safety hazards. At the same time, due to the high reaction temperature of the material, a trace amount of nitrophenols is produced. Therefore, it is more conducive to achieving a higher conversion rate of benzene under the concentrated sulfuric acid supplement amount defined in the present invention.

[0148] Comparative Example 13

[0149] The experiment was conducted using the same nitrating agent and acidic benzene as in Example 4, except that in this comparative example 13, the added concentrated sulfuric acid was directly introduced into the first reaction module 5 of the microreactor 4. The specific process is as follows:

[0150] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 265 g / min; 98% sulfuric acid was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 35-45 g / min; at a constant heat exchange temperature of 20-30°C. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 18 below.

[0151] Table 18 Sampling analysis results of Comparative Example 13

[0152]

[0153] From the data comparison of comparative example 13 and embodiment 4, it can be seen that: if concentrated sulfuric acid is directly added in the first reaction module 5 of microreactor 4, materials will be concentrated in the first module of microreactor, the reaction exothermic heat will increase sharply, and the heat exchange system cannot remove the heat generated by the reaction in time. Although the conversion rate of benzene is relatively complete, the content of mixed dinitrobenzene is significantly increased, the output of nitrobenzene decreases, and more by-product nitrophenols appear at the same time, which brings safety hazards to the reaction. It has been verified in the embodiment of the present invention that the stable reaction of dinitro waste acid and acidic benzene can be achieved by adding concentrated sulfuric acid in the third reaction module 7. In addition, in the actual experimental process, by adding concentrated sulfuric acid in the second reaction module 6 or the fourth reaction module 8, similar results to those of embodiment 1-embodiment 6 can also be obtained, which will not be listed in detail one by one here.

[0154] Comparative Example 14

[0155] The nitrating agent is prepared using dinitro waste acid, concentrated sulfuric acid, concentrated nitric acid and water, wherein the mass concentration of sulfuric acid in the nitrating agent is 57.0% (higher than the concentration specified in the present invention), and the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1.06-1.08):1.

[0156] A plunger pump was used to pump acidic benzene into the inlet A of the first reaction module 5 of the microreactor 4 at a steady flow rate of 96 g / min; a nitrating agent was pumped into the inlet B of the first reaction module 5 of the microreactor 4 at a steady flow rate of 265 g / min; at a constant heat exchange temperature of 20-30°C, after 8-10 seconds, 98% sulfuric acid was adjusted to be pumped into the inlet C of the third reaction module 7 of the microreactor 4 at a steady flow rate of 15-25 g / min, 25-35 g / min, and 35-45 g / min. After the system discharge was stable, the reaction temperature of the material at the discharge port J of the microreactor 4 was recorded and sampled for analysis. The specific analysis results are shown in Table 19 below.

[0157] Table 19 Sampling analysis results of Comparative Example 14

[0158]

[0159] Note: Sample numbers 40, 41, and 42 in Table 10 are samples obtained when the sulfuric acid flow rate is 15-25 g / min, 25-35 g / min, and 35-45 g / min, respectively.

[0160] From the data comparison of Comparative Example 14 and Example 4, it can be seen that the effective concentration of sulfuric acid is increased to increase the dissolved content of mixed dinitrobenzene. Although the residual amount of raw benzene is slightly reduced after the reaction is completed, the reaction temperature is significantly increased, resulting in a slight increase in the production of mixed dinitrobenzene and a slight decrease in the output of nitrobenzene; the reaction temperature is also increased due to the increase in the amount of sulfuric acid supplement, accompanied by the appearance of trace nitrophenols. This experiment did not achieve a relatively ideal reaction effect, but increased the nitration reaction temperature and the danger of the reaction. Therefore, the method for preparing nitrobenzene according to the present invention is more conducive to realizing the reuse of dinitro waste acid and acidic benzene, and at the same time realizing the stable, safe and efficient production of nitrobenzene.

[0161] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0162] For those skilled in the art, several modifications and improvements may be made without departing from the concept of the present invention, all of which belong to the protection scope of the present invention. The protection scope of the present invention shall be based on the attached claims.

Claims

1. A method for preparing nitrobenzene using a microreactor under low acid conditions, characterized in that: The method for preparing nitrobenzene is: A nitrating agent and a raw benzene are introduced into a microreactor for nitration reaction to obtain nitrobenzene, wherein the microreactor is provided with at least four reaction modules connected in series, the nitrating agent and the raw benzene are fed into the first reaction module, and during the nitration reaction, concentrated sulfuric acid is added to at least one reaction module of the other reaction modules except the first reaction module; The nitrating agent is prepared by using dinitrogen waste acid, sulfuric acid, nitric acid and water; the raw material benzene includes acidic benzene; The dinitrification waste acid is: a large amount of waste acid produced in the process of preparing the required dinitrobenzene by dinitrification of the mononitration product nitrobenzene; the acidic benzene is: the organic phase obtained by extracting the waste acid of the mononitration reaction with fresh benzene; In the nitrating agent, the mass concentration range of sulfuric acid is 47% to 55%, the mass concentration range of nitric acid is 27% to 36%, and the water content is 5% to 20%; The feed flow rate of the nitrating agent into the first reaction module is 230-290 g / min, the feed flow rate of the raw benzene into the first reaction module is 80-100 g / min; the feed flow rate of the additional concentrated sulfuric acid is 15-45 g / min.

2. The method for preparing nitrobenzene using a microreactor under low acid conditions according to claim 1, characterized in that: Calculated by mass fraction, the dinitrogen waste acid includes 70-85% sulfuric acid, 1-3% nitric acid, and 1-15% organic matter.

3. The method for preparing nitrobenzene using a microreactor under low acid conditions according to claim 1, characterized in that: The acidic benzene contains nitrobenzene with a mass fraction of 0.1-1.0% and a mixed acid of sulfuric acid and nitric acid with a mass fraction of 0.1-0.5%.

4. The method for preparing nitrobenzene using a microreactor under low acid conditions according to claim 1, characterized in that: The molar ratio of benzene in the acidic benzene to sulfuric acid in the nitrating agent is (0.9-2):1; the molar ratio of nitric acid in the nitrating agent to benzene in the acidic benzene is (1-1.1):

1.

5. The method for preparing nitrobenzene using a microreactor under low acid conditions according to claim 1, characterized in that: The feed flow rate of the nitrating agent into the first reaction module is 230-250 g / min, 240-260 g / min, 250-270 g / min or 260-280 g / min.

6. The method for preparing nitrobenzene using a microreactor under low acid conditions according to claim 1, characterized in that: The feed rate of concentrated sulfuric acid is 15-25 g / min, 25-35 g / min or 35-45 g / min.

7. The method for preparing nitrobenzene using a microreactor under low acid conditions according to claim 1, characterized in that: The heat exchange temperature range of the microreactor is 20-40°C.

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