Method for producing nitrobenzene

Abstract

The present invention relates to a process for producing nitrobenzene by adiabatic nitration of benzene (2) with nitric acid (1) in the presence of sulfuric acid (7), the process comprising the following steps: (a) Supplying benzene (2), nitric acid (1) and sulfuric acid (7) to a reaction zone (R); (b) Reacting benzene with nitric acid in the presence of sulfuric acid in the reaction zone (R) to produce a reaction mixture (4) containing water, nitrobenzene and sulfuric acid; and (c) Separating the reaction mixture into an aqueous phase (6) containing water and sulfuric acid and an organic phase (5) containing crude nitrobenzene by phase separation. The process includes maintaining the concentration of titanium in the reaction mixture (4) at less than 170 mg / kg.

Description

Technical Field

[0001] The present invention relates to a process for producing nitrobenzene by adiabatic nitration of benzene and nitric acid in the presence of sulfuric acid, the process comprising the following steps: (a) Supplying benzene, nitric acid and sulfuric acid to a reaction zone; (b) Reacting benzene and nitric acid in the presence of sulfuric acid in the reaction zone to produce a reaction mixture comprising water, nitrobenzene and sulfuric acid; and (c) Separating the reaction mixture by phase separation into an aqueous phase comprising water and sulfuric acid and an organic phase comprising crude nitrobenzene. It includes.

Background Art

[0002] Producing nitrobenzene by adiabatic nitration of benzene with nitric acid in the presence of sulfuric acid is a well-known method and is described, for example, in DE-A102017110084, WO-A2014 / 177450, or EP-B2070907.

[0003] One drawback of known processes is the formation of a black precipitate. This black precipitate is described, for example, in US-A2016 / 0083332 and CN-A112939781 and is found in apparatuses for phase separation in which crude nitrobenzene is separated from an aqueous phase comprising sulfuric acid and water, or in heat exchangers used to cool the crude nitrobenzene discharged from apparatuses for phase separation. Furthermore, when the reaction is carried out such that the reaction mixture exits the reactor as a gas, the black precipitate is also found in condensers for the reaction mixture and in pipelines connecting the plurality of apparatuses in a plant for producing nitrobenzene.

[0004] According to US-A2016 / 0083332 and CN-A112939781, an oxidizing agent or a reducing agent is added to the circulating sulfuric acid. However, adding an oxidizing agent or a reducing agent requires installing additional apparatuses for storing and adding the oxidizing agent / reducing agent in the plant, thus increasing the overall installation cost and the complexity of operation.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0006] Accordingly, an object of the present invention is to provide a method for producing nitrobenzene by adiabatic nitration of benzene and nitric acid in the presence of sulfuric acid, in which the formation of black precipitate is reduced or prevented and no additional equipment needs to be added to the plant.

Means for Solving the Problems

[0007] This object is achieved by a method for producing nitrobenzene by adiabatic nitration of benzene and nitric acid in the presence of sulfuric acid, comprising the following steps: (a) Supplying benzene, nitric acid and sulfuric acid to a reaction zone; (b) Reacting benzene and nitric acid in the presence of sulfuric acid in the reaction zone to produce a reaction mixture containing water, nitrobenzene and sulfuric acid; and (c) Separating the reaction mixture by phase separation into an aqueous phase containing water and sulfuric acid and an organic phase containing crude nitrobenzene. The method is achieved by maintaining the titanium concentration in the reaction mixture below 170 mg / kg. is achieved thereby.

Brief Description of the Drawings

[0008]

Figure 1

Modes for Carrying Out the Invention

[0009] In the context of the present invention, the concentration unit “mg / kg” refers to milligrams of titanium per kilogram of liquid, and the mass of the liquid includes all components contained in the liquid including dissolved and / or dispersed solids such as salts. Thereby, titanium is present in the liquid as metal particles, as titanium ions, or as metal particles and ions.

[0010] Surprisingly, it has been found that keeping the titanium concentration in the reaction mixture below 170 mg / kg can reduce or avoid black precipitates. More preferably, the titanium concentration in the reaction mixture is kept below 85 mg / kg, and particularly below 25 mg / kg.

[0011] Keeping the titanium concentration in the reaction mixture below 170 mg / kg can be achieved by any suitable means known to those skilled in the art, for example, by supplying nitric acid having a titanium concentration of less than 1 mg / kg, or by entrainment of at least 1.8 g of H2SO4 per kg of nitrobenzene into the organic phase in the phase separation of step (c).

[0012] Titanium enters the process, especially as an impurity in the nitric acid supplied to the process. Therefore, it is preferable to supply nitric acid having a titanium concentration of less than 1 mg / kg, more preferably less than 0.5 mg / kg, and particularly less than 0.1 mg / kg. Titanium enters the nitric acid, especially during the production of nitric acid, for example, through the corrosion of plant facilities in contact with nitric acid during chemical reactions. The rate of chemical reaction is strongly affected by temperature. The higher the temperature, the faster the reaction rate. In nitric acid plants and storage tanks, the faster the reaction rate, the faster the corrosion rate, and consequently, the higher the titanium concentration in the nitric acid. Therefore, it is advantageous to cool the nitric acid as soon as possible after production and store the nitric acid in the tank at a sufficiently low temperature to limit chemical reactions with the plant facilities, especially corrosion.

[0013] The temperature at which the nitric acid is cooled after production and the resulting storage temperature depend on the method of producing the nitric acid and the material of the plant facilities.

[0014] In addition to reducing the amount of titanium entering the nitric acid during the production process or during storage by decelerating the reaction rate by cooling and storing the nitric acid at a sufficiently low temperature, thereby reducing corrosion caused by the entry of titanium into the nitric acid, it is also possible to treat the nitric acid after production to reduce the concentration of titanium in the nitric acid.

[0015] However, regardless of how the concentration of titanium in the nitric acid is set, according to the first option of the present invention, it is important that the concentration of titanium in the nitric acid supplied to the production process of nitrobenzene is less than 1 mg / kg.

[0016] In the prior art, the sulfuric acid used in the production process of nitrobenzene is reconcentrated and reused in the reaction. When sulfuric acid is reused, titanium is usually concentrated in the aqueous phase. Therefore, in order to avoid the accumulation of titanium until the concentration in the reaction mixture reaches more than 170 mg / kg, more preferably more than 85 mg / kg, and particularly more than 25 mg / kg, it is necessary to limit the amount of titanium in the sulfuric acid reused in the reaction.

[0017] By thus restricting the titanium concentration in sulfuric acid that is reused in the reaction, keeping the titanium concentration in the reaction mixture below 170 mg / kg is achieved, for example, by entrainment of the aqueous phase into the organic phase. Preferably, the amount of entrainment of the aqueous phase into the organic phase is such that at least 1.8 g of H2SO4 is entrained into the organic phase per 1 kg of nitrobenzene. More preferably, the amount of the aqueous phase entrained into the organic phase is such that at least 3.6 g of H2SO4 is entrained into the organic phase per 1 kg of nitrobenzene. This can be achieved, for example, by restricting the size of the phase separator and shortening the available separation time.

[0018] This entrainment of H2SO4 into the organic phase is in contrast to this process where the entrainment of H2SO4 into the organic phase is restricted. This is because this entrainment results in an increase in sulfates in the wastewater, an increase in the amount of fresh sulfuric acid added, and an increase in the consumption of neutralizing agents such as caustic soda when neutralizing the wastewater. However, these drawbacks are not as significant compared to the formation of black precipitates (which requires the process to be periodically stopped to clean the equipment, resulting in a loss of plant capacity due to a decrease in the operating rate).

[0019] On the other hand, if the titanium concentration in the reaction mixture below 170 mg / kg is achieved by another suitable means, for example, by supplying nitric acid with a titanium concentration below 1 mg / kg and without entrainment of sulfuric acid into the organic phase, the amount of fresh sulfuric acid supplied to the process is maintained at a low level such as in this process.

[0020] "Per kilogram of nitrobenzene" here means the amount of pure nitrobenzene in the organic phase. The amount of organic phase is therefore usually even higher, since it may contain organic impurities, by-products from the reaction, and further entrained aqueous phase. This means that the total amount of aqueous phase entrained with the organic phase during phase separation depends on the concentration of sulfuric acid and is usually higher than the above-mentioned amount of H2SO4, since besides H2SO4, water and further impurities dissolved in water are also entrained in the organic phase. The amount of H2SO4 entrained with the organic phase during phase separation is preferably selected at a level that ensures that the concentration of titanium in the concentrated sulfuric acid recycled to the reaction zone remains below 170 mg / kg, more preferably below 85 mg / kg and in particular below 30 mg / kg of titanium present in the reaction mixture.

[0021] The H2SO4-containing aqueous phase (hereinafter also referred to as sulfuric acid or aqueous sulfuric acid) is entrained with the organic phase during phase separation, so that titanium contained in the aqueous sulfuric acid is also entrained with the organic phase. When the sulfuric acid entrained with the organic phase is withdrawn from the process, the titanium entrained with the organic phase is also withdrawn by this entrainment. In order to obtain a sufficient amount of sulfuric acid for the reaction, the sulfuric acid entrained with the organic phase is replaced with fresh sulfuric acid. Usually, fresh sulfuric acid does not contain titanium as an impurity, so that the fresh sulfuric acid does not introduce additional titanium into the process.

[0022] Usually, sulfuric acid is not consumed during the reaction and can therefore be returned to the process. However, since water is formed during the reaction, the sulfuric acid obtained as the aqueous phase is diluted by phase separation of the reaction mixture. For this reason, if it is intended to reuse the sulfuric acid, it must be concentrated by removing water from the aqueous phase before it can be reused in the reaction.

[0023] Removing water from the aqueous phase to concentrate sulfuric acid can be carried out by any process known to those skilled in the art. Such processes are described, for example, in WO-A2014 / 177450 or US4,091,042. Usually, sulfuric acid is concentrated by flash evaporation, preferably removing water in a single evaporation step. The concentration at which sulfuric acid is concentrated preferably corresponds to the concentration at which sulfuric acid is fed to the reaction zone and is preferably in the range of 60-96% by mass.

[0024] The concentrated sulfuric acid thus obtained can be reused in the reaction. For reuse, sulfuric acid is preferably mixed with nitric acid and then fed to the reaction zone.

[0025] The reaction zone may be any suitable reactor capable of carrying out nitration. Nitration can be carried out batchwise or continuously, with continuous being preferred. The reactor used to produce nitrobenzene may be, for example, a stirred tank reactor or a cascade of stirred tank reactors, a loop reactor or a tubular reactor. Preferably, the reactor is a tubular reactor, and particularly preferably, the reactor is a tubular reactor having a dispersion element as described, for example, in US4,994,242, US-A2003 / 0055300, EP-A1272268 or DE-A102017110084.

[0026] The temperature at which the reaction is carried out is preferably in the range of 70-145°C. The nitric acid fed to the reaction zone preferably has a concentration in the range of 60-98% by mass, and the sulfuric acid has a concentration in the range of 60-96% by mass. Benzene is preferably fed in a stoichiometric amount, but particularly preferably, benzene is fed in the range of 2-10% relative to the stoichiometrically required amount.

[0027] The reaction mixture obtained in the reaction zone is fed to an apparatus for phase separation and separated into an organic phase containing crude nitrobenzene and an aqueous phase containing water and sulfuric acid. As the reaction, the phase separation may be carried out batchwise or continuously, and continuous phase separation is preferred. The phase separation apparatus used for separating the organic phase containing crude nitrobenzene and the aqueous phase containing water and sulfuric acid can be any phase separation apparatus known to those skilled in the art.

[0028] Next, the organic phase containing crude nitrobenzene is further processed to obtain nitrobenzene of higher purity as the product. Particularly preferably, by treating the organic phase containing crude nitrobenzene, nitrobenzene having a purity of 99 to 99.99% by mass is obtained. The treatment of the organic phase usually includes adding an aqueous phase for washing to remove dissolved and suspended acids, and then further phase-separating the organic phase containing nitrobenzene and the aqueous phase containing sulfate and titanium. The aqueous phase obtained by phase separation is withdrawn from the process as wastewater and is usually supplied to wastewater purification treatment.

[0029] The organic phase containing nitrobenzene obtained by phase separation may optionally be subjected to further purification steps such as extraction and distillation steps to remove low-boiling and high-boiling substances.

[0030] When the concentration of titanium in the reaction mixture is kept below 170 mg / kg by using nitric acid having a titanium concentration of less than 1 mg / kg, preferably less than 0.5 mg / kg and particularly less than 0.1 mg / kg, the phase separation of the reaction mixture may be carried out as in known processes. However, even when using such nitric acid, it is further possible to additionally entrain an amount of sulfuric acid of at least 1.8 g of H2SO4 per kg of nitrobenzene in the organic phase.

[0031] One embodiment of the present invention is shown in the figures and will be described in more detail in the following description.

[0032] In the figure: Figure 1 shows a flowchart of the process of adiabatic nitration of benzene and nitric acid in the presence of sulfuric acid.

[0033] The only figure schematically shows the process of adiabatic nitration of benzene with nitric acid.

[0034] To produce nitrobenzene, nitric acid 1 having a concentration of 60 - 98% by mass is mixed with sulfuric acid 7 having a concentration in the range of 60 - 96% by mass. After mixing the nitric acid and sulfuric acid, benzene 2 is added to this mixture. Benzene 2 is preferably added in stoichiometric amount or in excess, preferably 2 - 10% in excess with respect to the stoichiometric amount.

[0035] The mixture of nitric acid, sulfuric acid and benzene is fed into reactor R. In reactor R, nitric acid and benzene react in the presence of sulfuric acid to form nitrobenzene and water. The reaction is exothermic, and when the reaction is carried out adiabatically, the temperature rises, and the reaction mixture containing nitrobenzene, water and sulfuric acid has a higher temperature than the mixture of nitric acid, sulfuric acid and benzene fed into the reactor. To carry out the reaction adiabatically, the reactor is insulated to minimize the dissipation of the reaction heat to the environment.

[0036] The reaction mixture 4 obtained in the reactor is fed into phase separator S. In phase separator S, the reaction mixture 4 is separated into an organic phase 5 containing crude nitrobenzene and an aqueous phase 6 containing sulfuric acid.

[0037] The organic phase 5 is further processed to purify nitrobenzene. For this purpose, the organic phase still containing a small amount of sulfuric acid is washed with water, and then the aqueous phase containing sulfate is subjected to further phase separation to separate it from the organic phase containing nitrobenzene. Then, the organic phase is further purified by removing low-boiling and high-boiling substances, for example, by extraction and distillation, to obtain pure nitrobenzene. The entire purification process is carried out as known to those skilled in the art.

[0038] The aqueous phase obtained by phase separation in the phase separator S is supplied to the concentrator C, and sulfuric acid is reconcentrated by removing water from the aqueous phase. The concentrator C may include one or more stages for reconcentrating sulfuric acid. Preferably, water is removed from the aqueous phase by evaporation to concentrate sulfuric acid. For this purpose, the concentrator C may include at least one evaporator in which water evaporates and is removed as vapor 8. The vapor 8 is then condensed and withdrawn from the process as wastewater and subjected to wastewater purification before being released to the environment.

[0039] Next, the reconcentrated sulfuric acid 7 obtained in the concentrator C is reused in the reaction and mixed with nitric acid 1. Since a part of the sulfuric acid is entrained in the organic phase in the phase separator S, fresh sulfuric acid 3 is added to the sulfuric acid 7 to be reused before mixing with nitric acid 1. However, in addition to mixing nitric acid after adding fresh sulfuric acid 3 to the reused reconcentrated sulfuric acid 7, it is also possible to mix the reused sulfuric acid 7, fresh sulfuric acid 3, and nitric acid 1 simultaneously.

[0040] In order to minimize or preferably avoid the formation of black precipitate, according to the present invention, the amount of titanium in the reaction mixture is maintained below 170 mg / kg, more preferably below 85 mg / kg, and particularly below 30 mg / kg. In one embodiment of the present invention, nitric acid 1 having a titanium concentration of less than 1 mg / kg, more preferably less than 0.5 mg / kg, and particularly less than 0.1 mg / kg is added to the process to achieve these concentrations.

[0041] In an alternative embodiment, the amount of sulfuric acid entrained in the organic phase in the phase separator S is at least 1.8 g of H2SO4 per kilogram of nitrobenzene 1, and particularly at least 3.6 g of H2SO4 per kilogram of nitrobenzene 1. This small amount of sulfuric acid entrained in the organic phase of the phase separator S is sufficient to achieve the intended concentration of titanium in the reaction mixture and to minimize or even avoid the formation of black precipitate.

[0042] As an alternative to keeping the titanium concentration in the reaction mixture below 170 mg / kg, nitric acid having a titanium concentration of less than 1 mg / kg is used, or in addition to accompanying 1.8 g or more of H2SO4 per kg of nitrobenzene, nitric acid having a titanium concentration of less than 1 mg / kg, more preferably less than 0.5 mg / kg, and particularly less than 0.1 mg / kg is used, and at least 1.8 g of H2SO4 per kg of nitrobenzene, and particularly at least 3.6 g of H2SO4 per kg of nitrobenzene is also accompanied in the organic phase of the phase separator S.

Examples

[0043] Comparative Example 24.1 t / h of nitric acid, 371 t / h of sulfuric acid and 22.4 t / h of benzene were fed to a reactor for adiabatic nitration to produce nitrobenzene. The resulting reaction mixture was fed to a phase separator to obtain an organic phase of 34.1 t / h containing nitrobenzene and an aqueous phase of 383.4 t / h containing water and sulfuric acid. Water was evaporated from the aqueous phase, and the thus reconcentrated sulfuric acid was reused in the reactor.

[0044] The amount of sulfuric acid reused was 371 t / h and it contained 400 mg / kg of titanium. In the phase separator, only 0.9 g of H2SO4 per kilogram of nitrobenzene was accompanied.

[0045] In this process, black precipitates were significantly formed and it was necessary to frequently stop and wash the plant.

[0046] Example 1 A production plant for producing nitrobenzene was operated under the same operating conditions as in the comparative example, except that a phase separator in which a larger amount of sulfuric acid was accompanied in the organic phase was used. The amount of sulfuric acid accompanied in the organic phase was 3.6 g of H2SO4 per kg of nitrobenzene.

[0047] The titanium concentration in the sulfuric acid reused in the reactor was less than 50 mg / kg. Furthermore, the amount of black substance formed was much less than in the comparative example.

[0048] Example 2 A production plant for nitrobenzene production was operated under the same conditions as in the comparative example and Example 1, but this time the titanium concentration in the nitric acid supplied to the nitrobenzene production process was less than 0.1 mg / kg.

[0049] The concentration of titanium in the sulfuric acid reused in the reaction was 100 mg / kg. This amount of titanium was significantly lower in the formation of black precipitate than in the comparative example, but still higher than in Example 1.

Claims

Claim 1 A process for producing nitrobenzene by adiabatic nitration of benzene (2) with nitric acid (1) in the presence of sulfuric acid (7), comprising the following steps: (a) Supplying benzene (2), nitric acid (1) and sulfuric acid (7) to a reaction zone (R); (b) Reacting benzene with nitric acid in the presence of sulfuric acid in the reaction zone (R) to produce a reaction mixture (4) containing water, nitrobenzene and sulfuric acid; and (c) Separating the reaction mixture into an aqueous phase (6) containing water and sulfuric acid and an organic phase (5) containing crude nitrobenzene by phase separation. The process is characterized in that the concentration of titanium in the reaction mixture (4) is maintained below 170 mg / kg. Claim 2 The process according to claim 1, wherein the titanium concentration in the reaction mixture (4) is maintained below 170 mg / kg by supplying nitric acid (1) having a titanium concentration of less than 1 mg / kg. Claim 3 The method according to claim 1 or 2, wherein the titanium concentration in the reaction mixture (4) is kept below 170 mg / kg by entrainment of H ２ SO ４ into the organic phase in the phase separation of step (c). Claim 4 The process according to claim 1 or 2, wherein water is removed from the aqueous phase (6) containing water and sulfuric acid to obtain concentrated sulfuric acid. Claim 5 The process according to claim 4, wherein the concentrated sulfuric acid (7) is reused in the reaction zone. Claim 6 The process according to claim 5, wherein the titanium concentration in the concentrated sulfuric acid (7) reused in the reaction zone is less than 200 mg / kg. Claim 7 The process according to claim 1 or 2, wherein the crude nitrobenzene (5) is treated by washing with an aqueous phase and phase-separating the organic phase containing nitrobenzene from the aqueous phase containing sulfate and titanium.

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