Method for producing nitrobenzene

Abstract

The invention relates to a process for the production of nitrobenzene by adiabatic nitration of benzene (2) and nitric acid (1) in the presence of sulfuric acid (7), the process comprising: (a) feeding benzene (2), nitric acid (1) and sulfuric acid (7) into a reaction zone (R); (b) producing a reaction mixture (4) comprising water, nitrobenzene and sulfuric acid by reaction of the benzene and the nitric acid in the presence of the sulfuric acid in the reaction zone (R); and (c) separating the reaction mixture by phase separation into an aqueous phase (6) comprising water and sulfuric acid and an organic phase (5) comprising crude nitrobenzene; wherein the concentration of titanium in the reaction mixture (4) is kept below 170 mg / kg.

Description

[0001] describe

[0002] 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:

[0003] (a) feeding benzene, nitric acid and sulfuric acid into a reaction zone;

[0004] (b) producing a reaction mixture comprising water, nitrobenzene and sulfuric acid by reacting benzene and nitric acid in the presence of sulfuric acid in a reaction zone; and

[0005] (c) separating the reaction mixture into an aqueous phase comprising water and sulfuric acid and an organic phase comprising crude nitrobenzene by phase separation.

[0006] The production of nitrobenzene by adiabatic nitration of benzene with nitric acid in the presence of sulfuric acid is a well-known process and is described, for example, in DE-A 10 2017 110084, WO-A 2014 / 177450 or EP-B 2 070 907.

[0007] A disadvantage of the known processes is the formation of black precipitates. These black precipitates are described, for example, in US-A 2016 / 0083332 and CN-A 112939781 and are found in phase separation devices in which crude nitrobenzene is separated from an aqueous phase comprising sulfuric acid and water, or in heat exchangers for cooling crude nitrobenzene discharged from the phase separation device. In addition, if the reaction is carried out in such a way that the reaction mixture leaves the reactor as a gas, black precipitates can also be found in the condenser of the reaction mixture and in the pipes connecting several devices of the plant for producing nitrobenzene.

[0008] According to US-A 2016 / 0083332 and CN-A 112939781, an oxidizing agent or a reducing agent is added to the circulating sulfuric acid. However, the addition of an oxidizing agent or a reducing agent requires the installation of additional equipment in the plant for storing and adding the oxidizing agent / reducing agent, and thus increases the overall installation cost and operational complexity.

[0009] It is therefore an object of the present invention to provide a process for producing nitrobenzene by adiabatic nitration of benzene and nitric acid in the presence of sulfuric acid, wherein the formation of black precipitates is reduced or prevented, without requiring the addition of additional equipment to the plant.

[0010] This object is achieved by a process for producing nitrobenzene by adiabatic nitration of benzene and nitric acid in the presence of sulfuric acid, comprising:

[0011] (a) feeding benzene, nitric acid and sulfuric acid into a reaction zone;

[0012] (b) producing a reaction mixture comprising water, nitrobenzene and sulfuric acid by reacting benzene and nitric acid in the presence of sulfuric acid in a reaction zone; and

[0013] (c) separating the reaction mixture into an aqueous phase comprising water and sulfuric acid and an organic phase comprising crude nitrobenzene by phase separation;

[0014] The titanium concentration in the reaction mixture was kept below 170 mg / kg.

[0015] In the context of the present invention, the concentration unit "mg / kg" refers to milligrams of titanium per kilogram of liquid, wherein the mass of the liquid includes all components contained in the liquid, including dissolved and / or dispersed solids, such as salts. Thus, titanium can be present in the liquid as metal particles, as titanium ions, or as metal particles and ions.

[0016] Surprisingly, it has been shown that black precipitates can be reduced or even avoided when the titanium concentration in the reaction mixture is kept below 170 mg / kg. More preferably, the titanium concentration in the reaction mixture is kept below 85 mg / kg, and in particular below 25 mg / kg.

[0017] Keeping the titanium concentration in the reaction mixture below 170 mg / kg can be achieved by any suitable measures known to the person skilled in the art, for example by feeding nitric acid having a titanium concentration of less than 1 mg / kg or by entraining at least 1.8 g H2SO4 per kg of nitrobenzene into the organic phase during the phase separation in step (c).

[0018] When titanium enters the process, particularly as an impurity in the nitric acid fed to the process, it is preferred that the feed titanium concentration is below 1 mg / kg, more preferably below 0.5 mg / kg, and in particular below 0.1 mg / kg of nitric acid. Titanium enters nitric acid, particularly during the production of nitric acid, for example by chemical reactions, such as corrosion of plant equipment with which the nitric acid comes into contact. The rate of chemical reactions is strongly affected by temperature. The higher the temperature, the faster the reaction rate. For nitric acid plants and storage tanks, higher reaction rates mean higher corrosion rates, and therefore higher titanium concentrations in the nitric acid. It is therefore advantageous to cool the nitric acid as quickly as possible after production, and to store the nitric acid in tanks that is cold enough to limit chemical reactions with plant equipment, particularly corrosion.

[0019] The temperature at which the nitric acid is cooled after production and the storage temperature thus depends on the production process of the nitric acid and the materials of the manufacturing plant equipment.

[0020] In addition to reducing the amount of titanium that enters the nitric acid during the production process or during storage by cooling and storing the nitric acid at sufficiently low temperatures to slow the reaction rate and therefore slow the corrosion of the titanium that may enter the nitric acid, the nitric acid can also be treated after production to reduce the titanium concentration in the nitric acid.

[0021] However, independently of how the titanium concentration in the nitric acid is set, according to the first alternative of the invention, it is important that the titanium concentration in the nitric acid fed to the nitrobenzene production process is below 1 mg / kg.

[0022] It is currently state of the art to re-concentrate the sulfuric acid used in the production of nitrobenzene and recycle it to the reaction. If the sulfuric acid is recycled, titanium will usually be concentrated in the aqueous phase. Therefore, in order to avoid the accumulation of titanium until its concentration in the reaction mixture exceeds 170 mg / kg, more preferably exceeds 85 mg / kg and in particular exceeds 25 mg / kg, it is necessary to limit the amount of titanium in the sulfuric acid recycled to the reaction.

[0023] This limitation of the titanium concentration in the sulfuric acid recycled to the reaction, and thus keeping the titanium concentration in the reaction mixture below 170 mg / kg, can be achieved, for example, by entraining the aqueous phase into the organic phase. Preferably, an amount of the aqueous phase is entrained into the organic phase such that at least 1.8 g of H2SO4 per kg of nitrobenzene is entrained into the organic phase. More preferably, the amount of the aqueous phase entrained into the organic phase is such that at least 3.6 g of H2SO4 per kg of nitrobenzene is entrained into the organic phase. This can be achieved, for example, by limiting the size of the phase separator to reduce the available separation time.

[0024] This entrainment of H2SO4 into the organic phase is in contrast to the process of the present invention, in which the entrainment of H2SO4 into the organic phase is limited, since this entrainment leads to an increase in sulfates in the wastewater, an increased addition of fresh sulfuric acid and, in the case of neutralized wastewater, a higher consumption of neutralizing agents such as caustic soda. However, these disadvantages are less significant than the formation of black precipitates, which requires regular shutdown of the process for cleaning of the equipment and, therefore, a loss of plant capacity due to lower availability.

[0025] On the other hand, if the titanium concentration in the reaction mixture is kept below 170 mg / kg by another suitable measure, for example by feeding nitric acid having a titanium concentration of less than 1 mg / kg and not by entraining sulfuric acid into the organic phase, the amount of fresh sulfuric acid fed to the process can be kept at such a low level as in the process of the present invention.

[0026] "Per kg of nitrobenzene" in this context means the amount of pure nitrobenzene in the organic phase. The amount of organic phase is therefore usually greater, since the organic phase may additionally contain organic impurities, by-products obtained in the reaction and entrained aqueous phase. This means that the total amount of aqueous phase entrained into the organic phase during the phase separation depends on the concentration of sulfuric acid and is usually higher than the above-specified amount of H2SO4, since, in addition to H2SO4, water and further impurities dissolved in water are also entrained into the organic phase. The amount of H2SO4 entrained into the organic phase during the phase separation is preferably chosen such that the titanium concentration in the concentrated sulfuric acid recycled to the reaction zone is at a level that ensures that the titanium present in the reaction mixture remains below 170 mg / kg, more preferably below 85 mg / kg and in particular below 30 mg / kg.

[0027] By entraining the aqueous phase containing H2SO4 (hereinafter also denoted as sulfuric acid or aqueous sulfuric acid) into the organic phase during phase separation, the titanium contained in the aqueous sulfuric acid is also entrained into the organic phase. As the sulfuric acid entrained into the organic phase is extracted from the process, the titanium entrained into the organic phase by this entrainment is also extracted. In order to have a sufficient amount of sulfuric acid in the reaction, the sulfuric acid entrained into the organic phase is replaced by fresh sulfuric acid. Since fresh sulfuric acid generally does not contain titanium as an impurity, no additional titanium is fed into the process via fresh sulfuric acid.

[0028] Sulfuric acid is usually not consumed during the reaction and can therefore be returned to the process. However, due to the formation of water in the reaction, the sulphuric acid obtained as an aqueous phase by phase separation of the reaction mixture is diluted. For this reason, if the intention is to reuse the sulphuric acid, the water must be removed from the aqueous phase to concentrate the sulphuric acid before recycling it into the reaction.

[0029] Removing water from the aqueous phase to concentrate sulfuric acid can be carried out by any method known to the technician. Such methods are described, for example, in WO-A2014 / 177450 or US 4,091,042. Typically, sulfuric acid is concentrated by flash evaporation, preferably removing water in one evaporation step. The concentration of sulfuric acid concentration preferably corresponds to the concentration of sulfuric acid fed to the reaction zone, and is preferably in the range of 60% by weight to 96% by weight.

[0030] The concentrated sulfuric acid thus obtained can then be recycled into the reaction. For recycling, the sulfuric acid is preferably mixed with nitric acid and then fed into the reaction zone.

[0031] The reaction zone can be any suitable reactor in which the nitration can be carried out. The nitration can be carried out batchwise or continuously, wherein a continuous process is preferred. The reactor for producing nitrobenzene can be, for example, a stirred tank reactor or a cascade stirred tank reactor, a loop reactor or a tubular reactor. Preferably, the reactor is a tubular reactor and particularly preferably, the reactor is a tubular reactor with dispersed elements, as described, for example, in US 4,994,242, US-A 2003 / 0055300, EP-A 1 272 268 or DE-A 10 2017 110 084.

[0032] The temperature at which the reaction is carried out is preferably in the range of 70° C. to 145° C. The nitric acid fed into the reaction zone preferably has a concentration in the range of 60% to 98% by weight, and the sulfuric acid preferably has a concentration in the range of 60% to 96% by weight. Benzene is preferably fed in stoichiometric amounts, however it is particularly preferred that benzene is fed in an amount of 2% to 10% compared to the amount required by stoichiometry.

[0033] The reaction mixture obtained in the reaction zone is fed into a phase separation device, and the mixture is separated into an organic phase containing crude nitrobenzene and an aqueous phase containing water and sulfuric acid. As a reaction, phase separation can be carried out batchwise or continuously, preferably continuous phase separation. The phase separation device for separating the organic phase containing crude nitrobenzene and the aqueous phase containing water and sulfuric acid can be any phase separation device known to the skilled person.

[0034] The organic phase containing crude nitrobenzene is then further processed to obtain a nitrobenzene product with a higher purity. Particularly preferably, nitrobenzene with a purity of 99% to 99.99% by weight is obtained by processing the organic phase containing crude nitrobenzene. The processing of the organic phase generally comprises washing by adding an aqueous phase to remove dissolved and suspended acids, followed by further phase separation into an organic phase containing nitrobenzene and an aqueous phase containing sulfate and titanium. The aqueous phase obtained in the phase separation is extracted from the process as waste water and is generally fed to a waste water purification process.

[0035] The nitrobenzene-comprising organic phase obtained in the phase separation can optionally be subjected to further purification steps, such as extraction and distillation steps, in order to remove low and high boilers.

[0036] If the titanium concentration in the reaction mixture is kept below 170 mg / kg by using nitric acid having a titanium concentration below 1 mg / kg, preferably below 0.5 mg / kg and in particular below 0.1 mg / kg, the phase separation of the reaction mixture can be carried out as in known methods. However, if such nitric acid is used, sulfuric acid can also be entrained into the organic phase in an amount of at least 1.8 g H2SO4 per kg of nitrobenzene.

[0037] Embodiments of the invention are shown in the drawings and are explained in more detail in the following description.

[0038] In the attached picture:

[0039] Figure 1 A flow chart showing the adiabatic nitration process of benzene and nitric acid in the presence of sulfuric acid;

[0040] The single FIGURE schematically shows the adiabatic nitration process of benzene and nitric acid.

[0041] To produce nitrobenzene, nitric acid 1 in a concentration range of 60% to 98% by weight is mixed with sulfuric acid 7 in a concentration range of 60% to 96% by weight. After the nitric acid and sulfuric acid are mixed, benzene 2 is added to the mixture. Benzene 2 is preferably added in a stoichiometric amount or in excess, preferably in an excess of 2% to 10% compared to the stoichiometric amount.

[0042] A mixture of nitric acid, sulfuric acid and benzene is fed into the reactor R. In the 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 temperature of the reaction mixture comprising nitrobenzene, water and sulfuric acid is higher than the temperature of the mixture of nitric acid, sulfuric acid and benzene fed into the reactor. In order to carry out the reaction adiabatically, the reactor is insulated to minimize the heat of reaction dissipated to the environment.

[0043] The reaction mixture 4 obtained in the reactor is fed to a phase separator S. In the phase separator S, the reaction mixture 4 is separated into an organic phase 5 comprising crude nitrobenzene and an aqueous phase 6 comprising sulfuric acid.

[0044] The organic phase 5 is further processed to purify the nitrobenzene. For this purpose, the organic phase which still contains small amounts of sulfuric acid is washed with water and subsequently subjected to a further phase separation, in which the aqueous phase comprising the sulfate is separated from the organic phase comprising the nitrobenzene. The organic phase can then be further purified, for example by extraction and distillation to remove low and high boilers, to obtain pure nitrobenzene. The entire purification process is carried out as known to the skilled person.

[0045] The aqueous phase obtained by the phase separation in the phase separator S is fed to the concentrator C for re-concentrating the sulfuric acid by removing water from the aqueous phase. The concentrator C may comprise one or more stages for re-concentrating the sulfuric acid. The water is preferably removed from the aqueous phase by evaporation for concentrating the sulfuric acid. For this purpose, the concentrator C may comprise at least one evaporator in which the water is evaporated and removed as steam 8. The steam 8 is then condensed and extracted from the process as waste water and subjected to waste water purification before being released into the environment.

[0046] The reconcentrated sulfuric acid 7 obtained in the concentrator C is then recycled to the reaction and mixed with the nitric acid 1. As some sulfuric acid is entrained into the organic phase in the phase separator S, fresh sulfuric acid 3 is added to the recycled sulfuric acid 7 before mixing with the nitric acid 1. However, in addition to adding fresh sulfuric acid 3 together with the recycled reconcentrated sulfuric acid 7 before mixing with the nitric acid, it is also possible to mix the recycled sulfuric acid 7, fresh sulfuric acid 3 and nitric acid 1 simultaneously.

[0047] In order to minimize or preferably avoid the formation of black precipitates, according to the present invention, the amount of titanium in the reaction mixture is kept below 170 mg / kg, more preferably below 85 mg / kg, and in particular below 30 mg / kg. In one embodiment of the present invention, to achieve these concentrations, nitric acid 1 is added to the process at a titanium concentration of less than 1 mg / kg, more preferably less than 0.5 mg / kg, and in particular less than 0.1 mg / kg.

[0048] In an alternative embodiment, the amount of sulfuric acid entrained into the organic phase in the phase separator S is at least 1.8 g H2SO4 per kg of nitrobenzene, and in particular at least 3.6 g H2SO4 per kg of nitrobenzene. This small amount of sulfuric acid entrained into the organic phase in the phase separator S is sufficient to achieve the desired titanium concentration in the reaction mixture and to minimize or even avoid the formation of black precipitates.

[0049] As an alternative to using nitric acid with a titanium concentration of less than 1 mg / kg or an entrainment of more than 1.8 g of H2SO4 per kg of nitrobenzene to keep the titanium concentration in the reaction mixture below 170 mg / kg, it is also possible to use nitric acid with a titanium concentration of less than 1 mg / kg, more preferably less than 0.5 mg / kg and in particular less than 0.1 mg / kg and to entrain at least 1.8 g of H2SO4 per kg of nitrobenzene and in particular at least 3.6 g of H2SO4 per kg of nitrobenzene into the organic phase in the phase separator S. Example

[0050] Comparative Example

[0051] 24.1 t / h of nitric acid, 371 t / h of sulfuric acid and 22.4 t / h of benzene are fed to a reactor for adiabatic nitration to produce nitrobenzene. The resulting reaction mixture is fed to a phase separator, in which 34.1 t / h of an organic phase containing nitrobenzene and 383.4 t / h of an aqueous phase containing water and sulfuric acid are obtained. Water is evaporated from the aqueous phase and the sulfuric acid thus reconcentrated is recycled to the reactor.

[0052] The amount of recycled sulfuric acid was 371 t / h and contained 400 mg / kg of titanium. In the phase separator, only 0.9 g of H2SO4 was entrained per kg of nitrobenzene.

[0053] The process showed the formation of large amounts of black precipitate, requiring frequent plant shutdowns for cleaning.

[0054] Example 1

[0055] The production plant for producing nitrobenzene was operated under the same operating conditions as in the comparative example, except that a phase separator was used, in which a large amount of sulfuric acid was entrained into the organic phase. The amount of sulfuric acid entrained into the organic phase was 3.6 g H2SO4 per kg of nitrobenzene.

[0056] The titanium concentration in the sulfuric acid recycled to the reactor was less than 50 mg / kg. In addition, the amount of black matter formed was much smaller than that in the comparative example.

[0057] Example 2

[0058] The production plant for producing nitrobenzene was operated under the same conditions as in the comparative example and example 1, but this time the titanium concentration in the nitric acid fed to the nitrobenzene production process was below 0.1 mg / kg.

[0059] The titanium concentration in the sulfuric acid recycled to the reaction was 100 mg / kg. This amount of titanium showed significantly less formation of black precipitate than the comparative example, but still higher than that of Example 1.

Claims

1. A method for producing nitrobenzene by adiabatic nitration of benzene (2) and nitric acid (1) in the presence of sulfuric acid (7), the method comprising: (a) feeding benzene (2), nitric acid (1) and sulfuric acid (7) into a reaction zone (R); (b) producing a reaction mixture (4) comprising water, nitrobenzene and sulfuric acid by reacting the benzene and the nitric acid in the presence of the sulfuric acid in the reaction zone (R); and (c) separating the reaction mixture into an aqueous phase (6) comprising water and sulfuric acid and an organic phase (5) comprising crude nitrobenzene by phase separation; The concentration of titanium in the reaction mixture (4) is maintained below 170 mg / kg.

2. The process according to claim 1, wherein the titanium concentration in the reaction mixture (4) is kept below 170 mg / kg by feeding nitric acid (1) having a titanium concentration below 1 mg / kg.

3. A process according to claim 1 or 2, wherein the titanium concentration in the reaction mixture (4) is maintained below 170 mg / kg by entraining at least 1.8 g H2SO4 per kg of nitrobenzene into the organic phase during the phase separation of step (c).

4. The process according to any one of claims 1 to 3, wherein water is removed from the aqueous phase (6) comprising water and sulfuric acid to obtain concentrated sulfuric acid.

5. The process according to claim 4, wherein the concentrated sulfuric acid (7) is recycled into the reaction zone.

6. The process according to claim 5, wherein the titanium concentration in the concentrated sulfuric acid (7) recycled to the reaction zone is lower than 200 mg / kg.

7. The process according to any one of claims 1 to 6, wherein the crude nitrobenzene (5) is treated by washing with an aqueous phase and phase separation into an organic phase comprising nitrobenzene and an aqueous phase comprising sulfate and titanium.

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