Synthetic method of nitrobenzene

Abstract

The invention relates to a synthetic method of nitrobenzene, which comprises the following steps: under adiabatic conditions, introducing a first stream of mixed acid and benzene into the first stage of a multi-stage stirring reactor for nitration reaction, then introducing a second stream of mixed acid into the second stage to the Nth stage in batches for continuous nitration reaction, and separating nitration reaction liquid to obtain nitrobenzene, wherein the number of stages of the multi-stage stirring reactor is M, N is larger than 2 and smaller than M, the first mixed acid is obtained by mixing a first sulfuric acid, a nitric acid and a tetravalent cerium compound at the temperature of 50-60 DEG C, the mass fraction of the nitric acid is 0.5%-1.5%, the mass fraction of the sulfuric acid is 65%-68%, the second mixed acid is obtained by mixing a second sulfuric acid, a nitric acid and a tetravalent cerium compound, and the mass fraction of the second sulfuric acid is 0.5%-1.5%. The temperature of the second stream of sulfuric acid is lower than that of the first stream of sulfuric acid, the mass fraction of the nitric acid is 3.0%-5.0%, the mass fraction of the sulfuric acid is 65%-68%, and meanwhile, the molar ratio of the nitric acid in the first stream of mixed acid to the nitric acid in the second stream of mixed acid is controlled to be 2: 98-15: 85, so that the content of nitrophenol compounds can be effectively reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical synthesis, in particular to a method for synthesizing nitrobenzene. Background Art

[0002] Nitrobenzene is an important intermediate in the chemical industry. The current mainstream process for producing nitrobenzene involves the adiabatic reaction of a mixed acid consisting of sulfuric acid and nitric acid with benzene. This process utilizes the exothermic heat of the reaction to maintain the system temperature, resulting in low energy consumption and high efficiency. However, due to the strong oxidizing properties and high reaction temperature of the mixed acid, side reactions such as overnitration and oxidation can occur. Nitrophenols are a major byproduct, typically present in the reaction solution at levels of 2000-3000 ppm. These compounds primarily include nitrophenol, dinitrophenol, and trinitrophenol (picric acid). These nitrophenols not only reduce product purity but are also highly toxic and unstable, leading to increased wastewater treatment costs and process safety risks.

[0003] At present, in order to reduce the impact of nitrophenol compounds on the nitration process system, measures such as lowering the reaction temperature, changing the strength of the mixed acid, and controlling the ratio of raw materials are usually taken to reduce the production of nitrophenol compounds.

[0004] For example, patent CN101759568A reduces the degree of nitration of benzene by controlling the molar ratio of benzene to nitric acid to be greater than 1.42:1, thereby controlling the formation of nitrophenol compounds. However, as shown in its examples, although the molar ratio of benzene to nitric acid is 1.51 to 2.1 and the nitrophenol content can be reduced to 117 to 168 ppm, the conversion rate of nitric acid is only 88.29 to 98.76%. This will lead to an increase in the unit consumption of nitric acid and the energy consumption for separating nitrobenzene and benzene.

[0005] Patents WO2010 / 051616A1 and EP0436443A2 reduce the formation of nitrophenols by lowering the initial reaction temperature. However, since the temperature rise reaches 20°C during the adiabatic reaction, the content of nitrophenols in the nitration product still reaches approximately 430 ppm even if the initial reaction temperature is lowered to 57-58°C.

[0006] Patents EP0436443B1 and EP0771783B1 disclose the effect of the mixed acid ratio on the nitration system. Under the conditions of adiabatic nitration at 97-120°C, a nitric acid concentration of about 0.5-3.0% in the mixed acid, and a water content of 27-33%, the content of nitrophenol compounds in the nitration product is about 1700 ppm. Summary of the Invention

[0007] Based on this, it is necessary to provide a method for synthesizing nitrobenzene to address the above problems, which can effectively reduce the content of nitrophenol compounds, thereby reducing the difficulty and cost of post-processing and improving the safety and environmental protection of the process.

[0008] A method for synthesizing nitrobenzene comprises the following steps:

[0009] Under adiabatic conditions, the first stream of mixed acid and benzene are introduced into the first stage of a multi-stage stirred reactor for nitration reaction;

[0010] Then, the second mixed acid is introduced into the second to Nth stages of the multi-stage stirred reactor in batches to continue the nitration reaction, and after the reaction is completed, a nitration reaction liquid is obtained, and the nitration reaction liquid is separated to obtain nitrobenzene;

[0011] The number of stages of the multi-stage stirred reactor is M, 2<N<M, the first mixed acid is obtained by mixing a first stream of sulfuric acid, nitric acid and a tetravalent cerium compound, the temperature of the first sulfuric acid is 50°C~60°C, the mass fraction of nitric acid in the first mixed acid is 0.5%~1.5%, and the mass fraction of sulfuric acid is 65%~68%, the second mixed acid is obtained by mixing a second stream of sulfuric acid, nitric acid and a tetravalent cerium compound, the temperature of the second sulfuric acid is lower than the temperature of the first sulfuric acid, the mass fraction of nitric acid in the second mixed acid is 3.0%~5.0%, and the mass fraction of sulfuric acid is 65%~68%, and the molar ratio of nitric acid in the first mixed acid to the second mixed acid is 2:98~15:85.

[0012] In one embodiment, the molar ratio of the total amount of nitric acid in the first mixed acid and the second mixed acid to the benzene is 1:1.05 to 1:1.1.

[0013] In one embodiment, the temperature of the second stream of sulfuric acid is 40°C to 50°C.

[0014] In one embodiment, when the second mixed acid is introduced into the second to N stages of the multi-stage stirred reactor in batches to continue the nitration reaction, the difference between the temperature of each stage and the temperature of the first stage is controlled to be less than or equal to 4°C.

[0015] In one embodiment, the mass of the cerium compound is 0.01%-0.5% of the total mass of sulfuric acid in the first mixed acid and the second mixed acid.

[0016] In one embodiment, the cerium compound is selected from at least one of Ce(NO3)4, Ce(SO4)2, Ce(OH)4, and CeO2.

[0017] In one embodiment, 8≤M≤30.

[0018] In one embodiment, the total time of the nitration reaction is 10 min to 30 min.

[0019] In one embodiment, the step of separating the nitration reaction liquid also produces spent sulfuric acid, which is subjected to a first evaporation treatment to obtain a sulfuric acid phase. A portion of the sulfuric acid phase is recycled as the first sulfuric acid, and the remaining portion of the sulfuric acid phase is further subjected to a second evaporation treatment and recycled as the second sulfuric acid, and oxygen-containing gas is introduced into both the first evaporation treatment and the second evaporation treatment.

[0020] In one embodiment, 15% to 50% of the sulfuric acid phase is recycled as the first sulfuric acid by mass fraction.

[0021] In the synthesis method of the present invention, first, the mixed acid is divided into two strands and the second strand is fed in batches again to avoid the side reaction caused by the excessively high local concentration of nitric acid in the reaction system. Secondly, by controlling the temperature of the first strand of sulfuric acid used in the first strand of mixed acid and the concentration of nitric acid and sulfuric acid in the first strand of mixed acid, it is ensured that the initial stage of the nitration reaction is not too intense and the temperature rise is relatively gentle. The second strand of mixed acid fed in batches can reduce the local concentration of nitric acid and control the temperature rise to not be too high. At the same time, the second strand of sulfuric acid used in the second strand of mixed acid has a low temperature and can absorb the reaction heat, thereby making the temperature rise of the entire nitration reaction process relatively gentle. In addition, the present invention adds a high-valent cerium compound to the first strand of mixed acid and the second strand of mixed acid, which can react with nitrous acid to convert it into nitric acid. Therefore, the synthesis method of the present invention can reduce the content of nitrophenol compounds to less than 100ppm, thereby reducing the difficulty and cost of post-processing and improving the safety and environmental protection of the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the conventional technology, the following briefly introduces the drawings required for use in the embodiments or the conventional technology descriptions. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 Schematic diagram of the process for synthesizing nitrobenzene of the present invention.

[0024] In the figure: 1, multi-stage stirred reactor; 2, separator; 3, first evaporation tower; 4, second evaporation tower; 11, benzene feed pipe; 12, first mixed acid feed pipe; 13, second mixed acid feed pipe; 14, first oxygen-containing gas feed pipe; 15, second oxygen-containing gas feed pipe; 16, product outlet pipe; 12a, first sulfuric acid circulation pipe; 12b, first nitric acid feed pipe; 13a, second sulfuric acid circulation pipe; 13b, second nitric acid feed pipe. DETAILED DESCRIPTION

[0025] To facilitate understanding of the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, the purpose of providing these embodiments or examples is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the technical field of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments or embodiments and are not intended to limit the present invention. The optional scope of the term "and / or" used herein includes any one of two or more related listed items, and also includes any and all combinations of related listed items, including any two related listed items, any more related listed items, or the combination of all related listed items.

[0027] In the present invention, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Furthermore, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0028] During the synthesis of nitrobenzene, nitric acid and recycled sulfuric acid are typically mixed in a certain proportion to form a mixed acid, with the mass fraction of nitric acid generally being around 5% and the mass fraction of sulfuric acid generally being around 68%. The mixed acid is then pumped into a reactor for a continuous nitration reaction with benzene to produce nitrobenzene. While measures such as lowering the reaction temperature, varying the strength of the mixed acid, and controlling the ratio of raw materials can reduce the production of nitrophenols, the cause of their formation remains unclear.

[0029] Through systematic research, the present invention has found that high temperatures, high nitric acid concentrations, and high nitrous acid content all promote the formation of nitrophenol compounds. The nitrous acid is partially derived from the raw nitric acid and partially from the oxidative side reaction of benzene by nitroxyl cations during the nitration process. The reaction process is as follows:

[0030]

[0031]

[0032] When nitrous acid is used as a nitrating agent, a series of oxidation-reduction reactions will occur, significantly accelerating the production of nitrophenol compounds. The reaction process is as follows:

[0033]

[0034] To this end, the present invention provides a method for synthesizing nitrobenzene, comprising the following steps:

[0035] Under adiabatic conditions, the first stream of mixed acid and benzene are introduced into the first stage of a multi-stage stirred reactor for nitration reaction;

[0036] Then, the second mixed acid is introduced into the second to Nth stages of the multi-stage stirred reactor in batches to continue the nitration reaction. After the reaction is completed, a nitration reaction liquid is obtained, and the nitration reaction liquid is separated to obtain nitrobenzene.

[0037] The number of stages of the multi-stage stirred reactor is M, 2<N<M, the first mixed acid is obtained by mixing a first stream of sulfuric acid, nitric acid and a tetravalent cerium compound, the temperature of the first sulfuric acid is 50°C~60°C, the mass fraction of nitric acid in the first mixed acid is 0.5%~1.5%, and the mass fraction of sulfuric acid is 65%~68%, the second mixed acid is obtained by mixing a second stream of sulfuric acid, nitric acid and a tetravalent cerium compound, the temperature of the second sulfuric acid is lower than the temperature of the first sulfuric acid, the mass fraction of nitric acid in the second mixed acid is 3.0%~5.0%, and the mass fraction of sulfuric acid is 65%~68%, and the molar ratio of nitric acid in the first mixed acid to the second mixed acid is 2:98~15:85.

[0038] In the synthesis method of the present invention, a first stream of mixed acid undergoes a nitration reaction with benzene in the first stage of a multi-stage stirred reactor, and the reaction liquid flows sequentially to the next stage of the multi-stage stirred reactor. At this time, a second stream of mixed acid enters the second to Nth stages of the multi-stage stirred reactor in batches to continue the nitration reaction. This method of dividing the mixed acid into two streams and feeding the second stream in batches again can avoid side reactions caused by excessively high local concentrations of nitric acid in the reaction system.

[0039] It should be noted that in the nitration reaction of the present invention, the amount of sulfuric acid used is much greater than that of nitric acid, so the temperature of the sulfuric acid basically determines the temperature of the first mixed acid and the second mixed acid. Therefore, by controlling the temperature of the first sulfuric acid used in the first mixed acid and the concentration of nitric acid and sulfuric acid in the first mixed acid, it can be ensured that the initial stage of the nitration reaction is not too intense and the temperature rise is relatively gentle. The batch feeding method of the second mixed acid can reduce the local concentration of nitric acid and control the temperature rise from being too high. At the same time, the second sulfuric acid used in the second mixed acid is low in temperature and can absorb the reaction heat, thereby making the temperature rise of the entire nitration reaction process relatively gentle. Specifically, the nitration reaction of the present invention can be carried out at a temperature of 50°C-70°C, and the reaction temperature does not change much.

[0040] In addition, the present invention adds a high-valent cerium compound to the first mixed acid and the second mixed acid to generate an oxidation reaction with nitrous acid to convert it into nitric acid. The reaction process is as follows:

[0041]

[0042] Therefore, the synthesis method of the present invention can reduce the content of nitrophenol compounds to below 100 ppm, thereby reducing the difficulty and cost of post-processing and improving the safety and environmental protection of the process.

[0043] When nitration reaction of nitric acid and benzene is carried out to prepare nitrobenzene, the excess of benzene is generally controlled to avoid the formation of dinitrobenzene and ensure the yield of nitrobenzene. Therefore, in the present invention, the molar ratio of the total amount of nitric acid in the first mixed acid and the second mixed acid to the benzene is preferably 1:1.05 to 1:1.1.

[0044] In order to better absorb the heat generated by the endothermic reaction and control the temperature rise, the temperature of the second stream of sulfuric acid is preferably 40°C to 50°C.

[0045] In the present invention, when the second mixed acid is introduced into the second to N stages of the multi-stage stirred reactor in batches to continue the nitration reaction, the temperature rise can also be controlled by controlling the feed amount of each batch. Preferably, the difference between the temperature of each stage and the temperature of the first stage is controlled to be less than or equal to 4°C, more preferably less than or equal to 3°C, which is conducive to further reducing the formation of nitrophenol compounds.

[0046] Optionally, the multi-stage stirred reactor has 8 to 30 stages, i.e., 8≤M≤30, more preferably, 10≤M≤20. Thus, when the second stream of mixed acid is fed in batches, the batch selection can be more diverse and more flexible, reducing the batch feed amount of each stage and controlling the temperature rise. It should be noted that when controlling 2<N<M, it can be ensured that at least one stage will not be pumped with mixed acid. For example, the number of stages M of the multi-stage stirred reactor is 10, and N can be 3, 4, 5, 6, 7, 8, or 9. Thus, at least one stage is guaranteed not to be pumped with mixed acid and only undergoes insulation reaction, ensuring that the nitration reaction can proceed completely.

[0047] Optionally, the total time of the nitrification reaction is 10 min to 30 min.

[0048] The concentration of the cerium compound in the first mixed acid and the second mixed acid is not limited, as long as it can ensure that nitrous acid is fully converted into nitric acid. Preferably, the mass of the cerium compound is 0.01%-0.5% of the total mass of sulfuric acid in the first mixed acid and the second mixed acid.

[0049] Wherein, the cerium compound is selected from at least one of Ce(NO3)4, Ce(SO4)2, Ce(OH)4, and CeO2.

[0050] It is understood that the step of separating the nitration reaction liquid also produces spent sulfuric acid, which is subjected to a first evaporation treatment to obtain a sulfuric acid phase. A portion of the sulfuric acid phase can be recycled as the first stream of sulfuric acid, and the remaining portion of the sulfuric acid phase is further subjected to a second evaporation treatment and recycled as the second stream of sulfuric acid. Alternatively, 15% to 50% of the sulfuric acid phase can be recycled as the first stream of sulfuric acid, and more preferably, 20% to 35% of the sulfuric acid phase can be recycled as the first stream of sulfuric acid.

[0051] Furthermore, during the first evaporation step, an oxygen-containing gas is introduced. More preferably, during both the first and second evaporation steps, an oxygen-containing gas is introduced. The oxygen-containing gas is preferably air. This can oxidize the reduced cerium to a high-valent state, thereby restoring the function of the cerium. The specific reaction process is as follows:

[0052]

[0053] At this time, if Figure 1As shown, benzene is pumped into the first stage of the multistage stirred reactor 1 through the benzene feed pipe 11, and the first mixed acid is pumped into the first stage of the multistage stirred reactor 1 through the first mixed acid feed pipe 12 to undergo a nitration reaction with benzene. The reaction liquid flows sequentially to the next stage of the multistage stirred reactor 1, and the second mixed acid is pumped into the second to Nth stages of the multistage stirred reactor 1 through the second mixed acid feed pipe 13 to continue the nitration reaction with benzene. The nitration reaction liquid obtained from the multistage stirred reactor 1 enters the stratifier 2 for stratification, and nitrobenzene is obtained at the product outlet pipe 16 of the stratifier 2. The nitrobenzene enters the refining system for refining. At the same time, the spent sulfuric acid obtained by stratification enters the first evaporation tower 3 for the first evaporation treatment, and the sulfuric acid phase is obtained at the bottom of the first evaporation tower 3. At this time, a controlled portion of the sulfuric acid phase enters the first sulfuric acid circulation pipe 12a as the first sulfuric acid, and the remaining portion of the sulfuric acid phase enters the second evaporation tower 4 for the second evaporation treatment. The product obtained at the bottom of the second evaporation tower 4 enters the second circulating sulfuric acid pipe 13a as the second sulfuric acid.

[0054] Furthermore, the first sulfuric acid circulation pipe 12a is also connected to the first nitric acid feed pipe 12b, and nitric acid enters the first sulfuric acid circulation pipe 12a from the first nitric acid feed pipe 12b and mixes with sulfuric acid to obtain a first mixed acid. Similarly, the second sulfuric acid circulation pipe 13a is also connected to the second nitric acid feed pipe 13b, and nitric acid enters the second sulfuric acid circulation pipe 13a from the second nitric acid feed pipe 13b and mixes with sulfuric acid to obtain a second mixed acid.

[0055] Furthermore, the bottom of the first evaporation tower 3 is further provided with a first oxygen-containing gas feed pipe 14, through which oxygen-containing gas such as air enters the first evaporation tower 3 to oxidize the reduced cerium into a high-valent cerium. Similarly, the bottom of the second evaporation tower 4 is further provided with a second oxygen-containing gas feed pipe 15, through which oxygen-containing gas such as air enters the second evaporation tower 4 to oxidize the reduced cerium into a high-valent cerium.

[0056] It can be understood that the gas obtained at the top of the first evaporation tower 3 and the second evaporation tower 4 is cooled by circulating cooling water, and the condensed liquid is separated into an aqueous phase and an organic phase by a separator, and the uncondensed gas is sucked out by a vacuum system and sent to the exhaust gas treatment unit.

[0057] In summary, in the synthesis method of the present invention, by controlling the temperature, nitric acid concentration and nitrous acid content during the nitration reaction, the content of nitrophenol compounds can be reduced to below 100 ppm, thereby reducing the difficulty and cost of post-processing and improving the safety and environmental friendliness of the process.

[0058] Hereinafter, the synthesis method of nitrobenzene will be further described through the following specific examples.

[0059] Example 1

[0060] The multi-stage stirred reactor used in this embodiment is a 10-stage reactor. The molar ratio of the total amount of nitric acid in the first and second mixed acids to benzene is 1:1.1, and the reaction residence time is 15 minutes. The reaction process is as follows:

[0061] Under adiabatic conditions, benzene and a first mixed acid were introduced into the first stage of a multi-stage stirred reactor for a nitration reaction at an average reaction temperature of 60°C. The first mixed acid was obtained by mixing a first stream of sulfuric acid, nitric acid, and Ce(NO₃)₄, with the mass fraction of nitric acid controlled to be 1.5%, the mass fraction of sulfuric acid controlled to be 67%, and the amount of nitric acid used being 15% of the total molar amount. A second mixed acid was then introduced into the second through sixth stages of the multi-stage stirred reactor for a nitration reaction, each of which was controlled to not exceed 63°C. The second mixed acid was obtained by mixing a second stream of sulfuric acid, nitric acid, and Ce(NO₃)₄, with the mass fraction of nitric acid controlled to be 3.64%, the mass fraction of sulfuric acid controlled to be 65%, and the amount of nitric acid used being 85% of the total molar amount. The total mass of Ce(NO₃)₄ in the first and second mixed acids was 0.24% of the total mass of sulfuric acid in the first and second mixed acids. The reaction was then continued at a temperature below 63°C in stages 7 through 10 of the multi-stage stirred reactor until the nitric acid was fully converted, yielding a nitration reaction solution.

[0062] The nitration reaction liquid is separated into nitrobenzene and spent sulfuric acid in the separator. The spent sulfuric acid enters the first evaporation tower for vacuum distillation to remove water. The temperature drop is controlled at 5°C. At the same time, air is introduced to Ce 3+ Oxidation proceeds, producing a sulfuric acid phase at the bottom of the tower. 30 wt% of this phase is recycled as the first sulfuric acid stream at 58°C. The remaining 70 wt% enters the second evaporation tower for further evaporation at a controlled temperature of 8°C. Air is introduced simultaneously to further oxidize the cerium compound. The product is recycled from the bottom of the tower as the second sulfuric acid stream at 50°C. After the nitration reaction stabilizes, the yield of nitrobenzene is 99.98%, and the content of nitrophenols in nitrobenzene is 82 ppm.

[0063] Example 2

[0064] The multi-stage stirred reactor used in this embodiment is 20 stages. The molar ratio of the total amount of nitric acid in the first and second mixed acids to benzene is 1:1.05, and the reaction residence time is 12 minutes. The reaction process is as follows:

[0065] Under adiabatic conditions, benzene and a first mixed acid were introduced into the first stage of a multi-stage stirred reactor for a nitration reaction at an average reaction temperature of 67°C. The first mixed acid was obtained by mixing a first stream of sulfuric acid, nitric acid, and Ce(NO₃), with the mass fraction of nitric acid controlled to be 0.75%, the mass fraction of sulfuric acid controlled to be 68%, and the amount of nitric acid used being 5% of the total molar amount. Then, a second mixed acid was introduced into the second through tenth stages of the multi-stage stirred reactor for a nitration reaction, with the reaction temperature never exceeding 70°C. The second mixed acid was obtained by mixing a second stream of sulfuric acid, nitric acid, and Ce(NO₃)₄, with the mass fraction of nitric acid controlled to be 3.56%, the mass fraction of sulfuric acid controlled to be 65%, and the amount of nitric acid used being 95% of the total molar amount. The total mass of Ce(NO₃)₄ in the first and second mixed acids was 0.31% of the total mass of sulfuric acid in the first and second mixed acids. The reaction was then continued at a temperature below 70°C in stages 11 through 20 of the multi-stage stirred reactor until the nitric acid was fully converted, yielding a nitration reaction solution.

[0066] The nitration reaction liquid is separated into nitrobenzene and spent sulfuric acid in the separator. The spent sulfuric acid enters the first evaporation tower for vacuum distillation to remove water. The temperature drop is controlled at 10℃. At the same time, air is introduced to Ce 3+ Oxidation proceeds, producing a sulfuric acid phase at the bottom of the tower. 20 wt% of this phase is recycled as the first sulfuric acid stream at 60°C. The remaining 80 wt% enters the second evaporation tower for further evaporation at a controlled temperature of 10°C. Air is introduced simultaneously to further oxidize the cerium compound. The product is recycled from the bottom of the tower as the second sulfuric acid stream at 50°C. After the nitration reaction stabilizes, the yield of nitrobenzene is 99.98%, and the content of nitrophenols in nitrobenzene is 98 ppm.

[0067] Example 3

[0068] The multi-stage stirred reactor used in this embodiment is 15 stages, wherein the molar ratio of the total amount of nitric acid in the first mixed acid and the second mixed acid to benzene is 1:1.08, and the reaction residence time is 20 minutes. The reaction process is as follows:

[0069] Under adiabatic conditions, benzene and a first mixed acid were introduced into the first stage of a multi-stage stirred reactor for a nitration reaction at an average reaction temperature of 50°C. The first mixed acid was obtained by mixing the first stream of sulfuric acid, nitric acid, and Ce(SO₄)₂, with the mass fraction of nitric acid controlled to be 1.03%, the mass fraction of sulfuric acid controlled to be 68%, and the amount of nitric acid used being 12% of the total molar amount. Then, a second mixed acid was introduced into the second through eighth stages of the multi-stage stirred reactor for a nitration reaction, each of which was controlled to not exceed 53°C. The second mixed acid was obtained by mixing the second stream of sulfuric acid, nitric acid, and Ce(SO₄)₂, with the mass fraction of nitric acid controlled to be 4.06%, the mass fraction of sulfuric acid controlled to be 65%, and the amount of nitric acid used being 88% of the total molar amount. The total mass of Ce(NO₃)₄ in the first and second mixed acids was 0.23% of the total mass of sulfuric acid in the first and second mixed acids. The reaction was then continued at a temperature below 53°C in stages 9 through 15 of the multi-stage stirred reactor until the nitric acid was fully converted, thereby obtaining a nitration reaction solution.

[0070] The nitration reaction liquid is separated into nitrobenzene and spent sulfuric acid in the separator. The spent sulfuric acid enters the first evaporation tower for vacuum distillation to remove water. The temperature drop is controlled at 3°C. At the same time, air is introduced to Ce 3+ Oxidation proceeds, producing a sulfuric acid phase at the bottom of the tower. 35 wt% of this phase is recycled as the first sulfuric acid stream at 50°C. The remaining 65 wt% enters the second evaporation tower for further evaporation at a controlled 10°C drop. Air is introduced simultaneously to further oxidize the cerium compound. The product is recycled from the bottom of the tower as the second sulfuric acid stream at 40°C. After the nitration reaction stabilizes, the yield of nitrobenzene is 99.98%, and the nitrophenol content in nitrobenzene is 76 ppm.

[0071] Comparative Example 1

[0072] The multi-stage stirred reactor used in this comparative example is a 10-stage reactor, wherein the molar ratio of nitric acid to benzene is 1:1.1, and the reaction residence time is 15 minutes. The reaction process is as follows:

[0073] Under adiabatic conditions, benzene and a mixed acid are introduced into the first stage of a multi-stage stirred reactor for a nitration reaction to produce a nitration reaction solution. The mixed acid is obtained by mixing sulfuric acid and nitric acid, with the mass fraction of nitric acid controlled to be 3% and the mass fraction of sulfuric acid controlled to be 68%. At this time, the reaction temperature of the first to tenth stages is 60°C to 80°C.

[0074] The nitration reaction liquid is separated into nitrobenzene and spent sulfuric acid in a separator. The spent sulfuric acid enters an evaporation tower to evaporate water and cool it down, with a controlled temperature drop of 20°C. Sulfuric acid at 50°C is obtained at the bottom of the tower and recycled. After the nitration reaction stabilizes, the nitrobenzene yield is 99.95%, and the nitrophenol content in nitrobenzene is 395 ppm.

[0075] Comparative Example 2

[0076] The multi-stage stirred reactor used in this comparative example is 20 stages, wherein the molar ratio of nitric acid to benzene is 1:1.05, and the reaction residence time is 12 minutes. The reaction process is as follows:

[0077] Under adiabatic conditions, benzene and a mixed acid are introduced into the first stage of a multi-stage stirred reactor for a nitration reaction to produce a nitration reaction solution. The mixed acid is obtained by mixing sulfuric acid and nitric acid, with the mass fraction of nitric acid controlled to be 3% and the mass fraction of sulfuric acid controlled to be 66%. At this time, the reaction temperature of the first to 20th stages is 70°C to 90°C.

[0078] The nitration reaction liquid is separated into nitrobenzene and spent sulfuric acid in a separator. The spent sulfuric acid enters an evaporation tower to evaporate water and cool it down, with a controlled cooling amplitude of 20°C. Sulfuric acid at 60°C is obtained at the bottom of the tower and recycled. After the nitration reaction stabilizes, the nitrobenzene yield is 99.92%, and the nitrophenol content in nitrobenzene is 607 ppm.

[0079] Comparative Example 3

[0080] The only difference between Comparative Example 3 and Example 1 is that no cerium compound is used. After the nitration reaction stabilizes, the reaction yield of nitrobenzene is 99.97%, and the content of nitrophenol compounds in nitrobenzene is 152 ppm.

[0081] Comparative Example 4

[0082] The only difference between Comparative Example 4 and Example 2 is that no cerium compound is used. After the nitration reaction stabilizes, the reaction yield of nitrobenzene is 99.97%, and the content of nitrophenol compounds in nitrobenzene is 163 ppm.

[0083] Comparative Example 5

[0084] Comparative Example 5 differs from Comparative Example 1 only in that the mixed acid contains Ce(NO₃)₄, with the mass of Ce(NO₃)₄ accounting for 0.063% of the mass of sulfuric acid in the mixed acid. After the nitration reaction stabilizes, the yield of nitrobenzene is 99.98%, and the content of nitrophenols in nitrobenzene is 142 ppm.

[0085] Comparative Example 6

[0086] Comparative Example 6 differs from Comparative Example 2 only in that the mixed acid contains Ce(NO₃)₄, with the mass of Ce(NO₃)₄ accounting for 0.5% of the mass of sulfuric acid in the mixed acid. After the nitration reaction stabilizes, the yield of nitrobenzene is 99.96%, and the content of nitrophenols in nitrobenzene is 258 ppm.

[0087] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. 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.

[0088] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for synthesizing nitrobenzene, characterized in that: The following steps are involved: Under adiabatic conditions, the first stream of mixed acid and benzene is introduced into the first stage of a multi-stage stirred reactor for nitration reaction; Then, the second mixed acid is introduced into the second to Nth stages of the multi-stage stirred reactor in batches to continue the nitration reaction, and after the reaction is completed, a nitration reaction liquid is obtained, and the nitration reaction liquid is separated to obtain nitrobenzene; The number of stages of the multi-stage stirred reactor is M, 2<N<M, the first mixed acid is obtained by mixing a first stream of sulfuric acid, nitric acid and a tetravalent cerium compound, the temperature of the first sulfuric acid is 50°C~60°C, the mass fraction of nitric acid in the first mixed acid is 0.5%~1.5%, and the mass fraction of sulfuric acid is 65%~68%, the second mixed acid is obtained by mixing a second stream of sulfuric acid, nitric acid and a tetravalent cerium compound, the temperature of the second sulfuric acid is lower than the temperature of the first sulfuric acid, the mass fraction of nitric acid in the second mixed acid is 3.0%~5.0%, and the mass fraction of sulfuric acid is 65%~68%, and the molar ratio of nitric acid in the first mixed acid to the second mixed acid is 2:98~15:

85.

2. The method for synthesizing nitrobenzene according to claim 1, wherein The molar ratio of the total amount of nitric acid in the first mixed acid and the second mixed acid to the benzene is 1:1.05 to 1:1.

1.

3. The method for synthesizing nitrobenzene according to claim 1, wherein The temperature of the second stream of sulfuric acid is 40°C to 50°C.

4. The method for synthesizing nitrobenzene according to claim 1, wherein When the second mixed acid is introduced into the second to Nth stages of the multi-stage stirred reactor in batches to continue the nitration reaction, the difference between the temperature of each stage and the temperature of the first stage is controlled to be less than or equal to 4°C.

5. The method for synthesizing nitrobenzene according to claim 1, wherein The mass of the cerium compound is 0.01%-0.5% of the total mass of sulfuric acid in the first mixed acid and the second mixed acid.

6. The method for synthesizing nitrobenzene according to claim 1, wherein The cerium compound is selected from at least one of Ce(NO3)4, Ce(SO4)2, Ce(OH)4, and CeO2.

7. The method for synthesizing nitrobenzene according to claim 1, wherein 8≤M≤30。 8. The method for synthesizing nitrobenzene according to claim 1, wherein The total time of nitrification reaction is 10min~30min.

9. The method for synthesizing nitrobenzene according to any one of claims 1 to 8, wherein The step of separating the nitration reaction liquid also produces spent sulfuric acid, which is subjected to a first evaporation treatment to produce a sulfuric acid phase. A portion of the sulfuric acid phase is recycled as the first sulfuric acid, and the remaining portion of the sulfuric acid phase is subjected to a second evaporation treatment and recycled as the second sulfuric acid. Oxygen-containing gas is introduced into both the first evaporation treatment and the second evaporation treatment.

10. The method for synthesizing nitrobenzene according to claim 9, characterized in that: 15% to 50% of the sulfuric acid phase is recycled as the first sulfuric acid by mass fraction.

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