Method for producing aniline

Abstract

The present invention relates to (a) reacting hydrogen and nitrobenzene in the presence of a catalyst in a hydrogenation reactor to form a crude reaction product; (b) working up the crude reaction product to obtain pure aniline; The present invention relates to a process for producing aniline by catalytic hydrogenation of nitrobenzene, comprising: Here, the composition of the crude reaction product is analyzed continuously or at regular time intervals before starting the work-up of step (b) to determine the proportion of by-products, and in case of an increased amount of by-products, measures are taken to prevent breakthrough of nitrobenzene in the hydrogenation reactor.

Description

[Technical field]

[0001] The present invention relates to a process for producing aniline by catalytic hydrogenation of nitrobenzene, wherein hydrogen and nitrobenzene are reacted in a hydrogenation reactor in the presence of a catalyst to form a crude reaction product, and the crude reaction product is purified to obtain purified aniline. [Background technology]

[0002] The hydrogenation of nitrobenzene to obtain aniline can be carried out in the gas phase or in the liquid phase. When the hydrogenation is carried out in the gas phase, the catalyst used in the hydrogenation reaction can be in a fixed bed, as described for example in US 4,740,621 or EP-A 748 790, or in a fluidized bed, as described for example in US 3,136,818. The hydrogenation of nitrobenzene to obtain aniline in the liquid phase is disclosed for example in US 4,415,754 or US 3,270,057.

[0003] The crude reaction product obtained by hydrogenation of nitrobenzene contains aniline as product, hydrogen, water and further by-products such as phenol, aminophenols and phenazines. Depending on the type of reaction, the crude reaction product may also contain unreacted nitrobenzene.

[0004] When the reaction is carried out in the gas phase, the crude reaction product is usually condensed, where this condensation may be carried out in one stage, or in two or more stages, with or without heat integration.

[0005] The crude reaction product obtained in the liquid phase reaction or the condensed crude reaction product obtained after condensation of the crude reaction product in the gas phase reaction is then subjected to a gas / liquid separation, in which more than 95% of the water of reaction and the aniline are condensed and the hydrogen-containing gas phase is removed.

[0006] The liquid phase thus obtained is subjected to a liquid / liquid separation, which can be carried out in one or more stages. The liquid / liquid separation gives an organic phase containing aniline and an aqueous phase. The organic phase is further worked up by distillation to remove low boilers, in particular residual water, and high boilers, to give pure aniline as product. Since the high boiler-containing stream removed from the process, which is usually incinerated, still contains aniline, WO-A 2014 / 057053 describes a method for working up the crude aniline, which further reduces the loss of aniline.

[0007] However, a problem with all processes for producing aniline is that the activity of the catalyst decreases over time and towards the end of the run time or catalyst life, the amount of unreacted nitrobenzene increases, resulting in nitrobenzene breakthrough, lower quality aniline, and lower conversion and yield. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] US 4,740,621 [Patent Document 2] EP-A 748 790 [Patent Document 3] US 3,136,818 [Patent Document 4] US 4,415,754 [Patent Document 5] US 3,270,057 [Patent Document 6] WO-A 2014 / 057053 Summary of the Invention [Problem to be solved by the invention]

[0009] It is therefore an object of the present invention to provide a process for the preparation of aniline by catalytic hydrogenation of nitrobenzene, in which, by timely recognition of catalyst deactivation, deterioration of the quality of aniline due to nitrobenzene breakthrough can be avoided and only a short interruption of the process allows sufficient time to plan for the regeneration or replacement of the catalyst. [Means for solving the problem]

[0010] The purpose of this is to (a) reacting hydrogen and nitrobenzene in the presence of a catalyst in a hydrogenation reactor to form a crude reaction product; (b) working up the crude reaction product to obtain pure aniline; This is achieved by a method for producing aniline by catalytic hydrogenation of nitrobenzene, comprising: Here, the composition of the crude reaction product is analyzed continuously or at regular time intervals before starting the work-up of step (b) to determine the proportion of by-products, and in case of an increased amount of by-products, measures are taken to prevent breakthrough of nitrobenzene in the hydrogenation reactor. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows a schematic diagram of a process for producing aniline in the gas phase. [Diagram 2] FIG. 2 shows the concentration gradient of the by-products over time. [Diagram 3] FIG. 3 shows the concentration gradient of the by-products over time. [Figure 4] FIG. 4 shows the concentration gradient of the by-products over time. [Diagram 5] FIG. 5 shows the concentration gradient of the by-products over time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Surprisingly, it was shown that the amount of by-products in the crude reaction product begins to increase several days before catalyst deactivation is observed by analysis of pure aniline, which is the normal process. Most of these by-products that cause early observation of catalyst deactivation are removed from the crude reaction product during the normal purification steps to produce pure aniline and are therefore not present in the pure aniline product.

[0013] Since the catalyst lifetime can be between several weeks and several months depending on the catalyst system, if the crude reaction product is not analyzed continuously but at regular time intervals, the regular time interval for analyzing the crude reaction product is a maximum of 24 hours, preferably in the range of 18 hours to 10 hours, in particular in the range of 10 hours to 6 hours. A regular time interval of 24 hours means that the composition of the crude reaction product is analyzed only once a day during normal operation of the process.

[0014] By detecting the deactivation of the catalyst early, it is possible to take some measures to continue the production of pure aniline with the required properties. In particular, a short interruption of the production process allows the process parameters to be modified in order to continue the production of pure aniline at the required low level of nitrobenzene contained therein until the catalyst can be regenerated or replaced. Taking such early measures also has the advantage that the quality of the pure aniline produced in the hydrogenation reactor is always the same and that even at the end of the operating time or shortly before the end of the catalyst life, no pure aniline is produced that does not meet the required properties for the majority of the applications in which it is used, such as for example the production of methylene diphenyl diisocyanate (MDI).

[0015] On the other hand, as is known from conventional processes, a delay in the analytical detection of pure aniline leaves little time to stop the production process to prevent the production of pure aniline that does not meet the required properties, such as nitrobenzene concentration, which may lead to unscheduled interruptions of production in downstream production plants that use aniline as a feedstock, due to supply shortages.

[0016] If the production process is not stopped in time, further deactivation of the catalyst will significantly increase the nitrobenzene concentration in the pure aniline produced, causing the pure aniline to no longer meet the required properties. This pure aniline will require expensive and energy-intensive treatment to remove the nitrobenzene below the required concentration. Furthermore, customers may demand financial compensation if they receive pure aniline with a concentration of nitrobenzene that exceeds the required concentration.

[0017] Measures that can be taken to prevent breakthrough of nitrobenzene in the hydrogenation reactor include, in particular, at least one of the following: - scheduling catalyst regeneration or replacement and foreseeing sufficient pure aniline inventory to cover plant downtime during said regeneration or replacement; - Reducing the load on the plant to slow down catalyst deactivation; - temporarily increasing the time interval for analyzing the composition of the crude reaction product; - Raise the reaction temperature to increase reactivity and delay catalyst deactivation.

[0018] "Plant loading" in this context refers to the nitrobenzene feed rate to the hydrogenation reactor and thus the production rate of the plant. Thus, a reduction in the plant loading means a reduction in the feed rate of nitrobenzene to the hydrogenation reactor.

[0019] In the process of the invention, impending deactivation is detected up to several days before nitrobenzene breakthrough, so that sufficient time remains to schedule a regeneration or replacement of the catalyst and, in particular, to foresee a sufficient stock of pure aniline, which can be achieved, for example, by increasing the production rate of another aniline plant on the same site, by importing aniline from another production site, by purchasing aniline from another manufacturer, or by reducing the sale of aniline up to or during the regeneration or replacement of the catalyst.

[0020] If regeneration or replacement of the catalyst is not possible within the remaining life determined by by-product analysis, it is preferable to reduce the load on the plant, which slows catalyst deactivation and provides a longer time before catalyst regeneration or replacement is required.

[0021] As an alternative or additional measure to delay catalyst deactivation, the reaction temperature can be temporarily increased. By increasing the temperature, the reactivity is also increased, which in turn delays catalyst deactivation. In order to increase the reactivity and thereby delay catalyst deactivation, it is preferred to increase the reactor temperature by at least 2° C., in particular by more than 5° C. The resulting temperature increase depends on the reaction type, the reactor type, the operation of the reactor, in particular isothermal or adiabatic, and the overall plant setup.

[0022] Moreover, since it has been shown that the increase in by-products changes from a linear to an exponential increase as the catalyst approaches deactivation, it is even more preferable to temporarily shorten the time interval and, therefore, to increase the frequency of analyzing the composition of the crude reaction product. By shortening the time interval in this way, it is possible to stop the reaction close to the final deactivation of the catalyst, thus allowing the reaction to continue as long as possible while avoiding nitrobenzene breakthrough, and to regenerate or replace the catalyst very short of final deactivation. Therefore, regardless of what measures are taken to delay the deactivation of the catalyst or to perform the regeneration or replacement of the catalyst on schedule, it is particularly preferable to further temporarily reduce the time interval for analyzing the composition of the crude reaction product, if the analysis is not performed continuously.

[0023] The hydrogenation reaction to produce aniline in the presence of a catalyst can be any type of reaction between hydrogen and nitrobenzene, in particular mononitrobenzene (MNB), known to those skilled in the art. The reaction between hydrogen and nitrobenzene can be carried out, for example, in the liquid or gas phase.

[0024] When the reaction is carried out in the liquid phase, liquid nitrobenzene is usually fed to a hydrogenation reactor where it is contacted with hydrogen. The hydrogenation reactor contains a catalyst, which is in the form of small particles and forms a slurry with the liquid nitrobenzene. The reaction is carried out at a temperature in the range of 100-300 ° C. Depending on the temperature of the hydrogenation reactor, the crude reaction product may be in the vapor phase, which is removed from the hydrogenation reactor and then condensed. Since in addition to aniline water is also produced, it is necessary to separate the aniline from the water, and for this purpose a phase separation can be carried out in which the aqueous phase containing the water is separated from the organic phase containing the aniline and corresponding to the crude reaction product.

[0025] When the reaction is carried out in the gas phase, unlike when it is carried out in the liquid phase, nitrobenzene is vaporized before being fed to the hydrogenation reactor. The catalyst in the gas phase reaction may be in the form of a fluidized bed or in the form of a fixed bed. Regardless of the form of a fluidized or fixed bed, it is preferred to mix hydrogen and vaporized nitrobenzene before feeding them to the hydrogenation reactor. The mixing of hydrogen and vaporized nitrobenzene can be carried out in any manner known to those skilled in the art. When the catalyst is in the form of a fluidized bed, it is fluidized by the gaseous reactant mixture containing hydrogen and nitrobenzene. When the catalyst is in the form of a fixed bed, the reactant mixture is fed to the catalyst bed, in which hydrogen and nitrobenzene react. As the reaction is exothermic, the crude reaction product is also in the gas phase, which is removed from the reactor and condensed. Thus, the condensation can be carried out in only one stage, or in two or more stages.

[0026] To obtain pure aniline, the crude reaction mixture must be worked up to remove by-products formed during the reaction and to further remove impurities that may be contained in the reaction mixture and thus entrained therein.

[0027] In the catalytic reaction of hydrogen with nitrobenzene, even though it is carried out in the liquid or gas phase, in addition to aniline, water is also produced. Therefore, it is necessary to separate water and aniline. To remove the water, the crude reaction product taken from the reactor is preferably at least partially condensed. Since the boiling point of aniline is higher than that of water, aniline starts to condense first, so that the water remaining in the gas phase can be removed. Usually, only a part of the aniline is condensed in the first step, so that the vapors removed from the first condensation still contain the product. For this reason, further condensation steps are provided, which are carried out in two or more stages, in each stage aniline is condensed from the vapor, and the gaseous water with a low aniline content after each condensation step is fed to the next condensation step or is removed from the last condensation step and then removed from the process.

[0028] Since in each condensation step, besides aniline, water and also higher boiling by-products and impurities may be condensed, the condensed phases must be further worked up. If the condensation is carried out in several condensation steps, the condensed phases of all steps are combined and fed to the work-up step (b). The work-up of the crude reaction product is carried out by known processes, in particular by distillation, to remove the low boiling components remaining from the several steps and the higher boiling by-products and impurities.

[0029] In the context of the present invention, "high boiling" means that the boiling point of the respective component is higher than the boiling point of aniline, and "low boiling" means that the boiling point of the respective component is lower than the boiling point of aniline.

[0030] If the crude reaction product is removed from the reactor in the gas phase, it is possible to analyze the composition of the gaseous crude reaction product. However, preferably, the composition of the liquid crude reaction product, i.e. the at least partially condensed crude reaction product, is analyzed. For this purpose, if the condensation is carried out in only one stage, the condensed crude reaction product withdrawn from the condensation is analyzed. If the condensation is carried out in at least two condensation steps, it is possible to analyze the composition of the crude reaction product obtained in any of the condensation steps. If the condensed phases of all condensation steps are combined, it is also possible to analyze the composition of the combined crude reaction product.

[0031] To detect catalyst deactivation, the crude reaction product is analyzed for by-products which are the result of side reactions in the hydrogenation reactor which increasingly occur due to catalyst deactivation. These by-products are in particular high boiling components.

[0032] In particular, the by-products of the portion analyzed are at least one of aminophenols, n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, phenazine, phenol, nitrobenzene, and unidentifiable high boiling components. An "unidentifiable high boiling component" is one for which the analytical method used gives a signal, but the signal cannot be associated with a specific component, and although a component is detected, it is not possible to identify this specific component.

[0033] Since the high boiling by-products and impurities start to condense first, the amount of these high boiling by-products is greatest in the condensed phase of the condensation step, and therefore it is particularly preferred to analyze the partially condensed crude reaction product obtained in the first condensation step.

[0034] During normal operation, when the catalyst has normal activity, the concentration of the by-products is stable. Subsequent deactivation of the catalyst is indicated by an increase in the concentration of the by-products of 10% or more compared to the stable concentration. More preferably, subsequent deactivation of the catalyst is indicated by an increase in the concentration of the by-products of more than 20%, especially more than 30%, compared to the stable concentration. Furthermore, subsequent deactivation of the catalyst is often indicated by an additional combination of a transition from a linear increase in the amount of by-products to an exponential increase.

[0035] The increase in by-product concentration and the transition from linear to exponential can usually be observed several days before the catalyst is completely deactivated and nitrobenzene begins to break through. It is therefore particularly preferred to take measures to prevent nitrobenzene breakthrough in the hydrogenation reactor when the increase in the amount of by-products in the crude reaction product shows a transition from linear to exponential increase.

[0036] The analysis of the composition of the crude reaction product can be carried out by any analytical method capable of analyzing the composition of a stream known to the skilled artisan. Particularly suitable analytical methods are chromatographic or spectroscopic techniques. Chromatographic techniques include, for example, gas chromatography (GC) or high performance liquid chromatography (HPLC), with gas chromatography being particularly preferred. Spectroscopic techniques that can be used to analyze the composition of the crude reaction product include, in particular, for example, infrared spectroscopy (IR), near infrared spectroscopy (NIR), nuclear magnetic resonance spectroscopy (NMR) or Raman spectroscopy. Preferred spectroscopic techniques are near infrared spectroscopy or Raman spectroscopy.

[0037] The analysis of the crude reaction product can be performed by an online analytical process, whether it is performed continuously or at regular time intervals. If the analysis of the crude reaction product is performed at regular time intervals, it is also possible to take samples and analyze the samples to analyze the composition of the crude reaction product.

[0038] When using an on-line analytical process to analyze the composition of the crude reaction product, it is necessary to use an analytical method that can be operated continuously, for example gas chromatography is suitable for this purpose.

[0039] If a sample of the crude reaction product is taken and the sample is analysed, it is particularly preferred to use gas chromatography to analyse the crude reaction product for its composition.

[0040] An embodiment of the invention is illustrated in the drawings and explained in more detail in the following description.

[0041] In the figure, FIG. 1 shows a schematic of a process for producing aniline in the gas phase; Figures 2-5 show the concentration gradients of several by-products over time.

[0042] FIG. 1 shows a process for producing aniline in the gas phase.

[0043] To produce aniline, nitrobenzene 1 and hydrogen 3, preferably recycled hydrogen 5, are fed to a hydrogenation reactor 7. In the hydrogenation reactor, a crude reaction product 9 is formed. The crude reaction product 9 comprises aniline, water, residual unreacted nitrobenzene and hydrogen, and by-products formed during the reaction. Typical by-products include, for example, phenol, phenazine, n-phenyl-1,2-benzenediamine, and n-phenyl-1,4-benzenediamine.

[0044] The crude reaction product 9 is transferred to a first condensate 11. In the first condensate 11, the crude reaction product 9 is cooled to a temperature below the boiling point of aniline and above the boiling point of water, so that aniline condenses and water remains in the gas phase. From the first condensate 11, a gaseous component 13 containing water, aniline and low boiling products and a liquid stream 15 containing crude aniline and high boiling by-products are removed.

[0045] The gaseous components 13, which still contain aniline, are transferred to a second condensate 17. The second condensate 17 is carried out in such a way that crude aniline is withdrawn as a first liquid stream 19 and waste water is withdrawn as a second liquid stream 21. Further gaseous components, in particular unreacted hydrogen, are removed from the second condensate 17 as gas stream 23.

[0046] The hydrogen-containing gas stream may be at least partially recycled to the hydrogenation reactor 7 as recycled hydrogen 5. If no hydrogen is recycled, or if only a portion of the hydrogen is recycled, the non-recycled portion is removed as hydrogen off-gas 25. Since gas stream 23 may contain further gaseous components other than hydrogen, it is preferred to recycle only a portion of gas stream 23 to the hydrogenation reactor 7 and to remove a portion of gas stream 23 as hydrogen off-gas 25 in order to avoid the accumulation of further gaseous components contained in gas stream 23.

[0047] The first liquid stream 19 containing the crude aniline obtained in the second condensation and the liquid stream 15 containing the crude aniline obtained in the first condensation are preferably combined into a combined stream 27. The combined stream 27 is then fed to a crude aniline purification 29. In the crude aniline purification 29, which can be carried out in one or more distillation steps, pure aniline 31, a low boiling product 33 and a high boiling product 35 are obtained.

[0048] Furthermore, it is preferred to separate water from the low boiling products 33 and remove water 37 separately. The water 37 obtained in the crude aniline purification 29 and the water obtained in the second condensation 17 as the second liquid stream 21 are preferably combined and removed from the process as waste water 39. This waste water 39 is preferably transferred to a water purification plant for further treatment.

[0049] According to the invention it is preferred to analyse the composition of the crude aniline obtained as liquid stream 15 in the first condensate 11. However, besides the analysis of the liquid stream 15 it is also possible to analyse the combined stream 27 containing the crude aniline obtained in the first condensate 11 and in the second condensate 17, or the first liquid stream 19 containing the crude aniline obtained in the second condensate 17. However it is most preferred to analyse the first liquid stream 19 obtained in the second condensate. EXAMPLES

[0050] 2 to 5, which show the results of the examples, the horizontal axis 100 indicates time, the left vertical axis indicates the concentration of by-products (mg / kg), and the right vertical axis 103 indicates the concentration of nitrobenzene (mg / kg). Furthermore, the curves of the graphs are assigned as follows: 105 Phenol 107 Phenazine 109 n-Phenyl-1,2-benzenediamine 111 n-Phenyl-1,4-benzenediamine 113 Nitrobenzene.

[0051] Example 1 Aniline was produced in a plant as shown in Figure 1. During the production process, the concentrations of several by-products were monitored. Figure 2 shows the results for the last 550 hours before catalyst deactivation.

[0052] From the graph timestamps 150 to 200 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine are stable, indicating that the catalyst is still operating at normal activity.

[0053] At timestamp 300 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine increase to more than 10% above their stable values, indicating a decrease in catalyst activity and the subsequent breakthrough of nitrobenzene.

[0054] At timestamp 450 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine and phenazine increased to more than 20% above the stable values. The increase in concentrations also changed from linear to exponential. Both indicate a further decrease in catalyst activity and a breakthrough of nitrobenzene within the next 4-5 days. A catalyst regeneration was scheduled and sufficient stocks of pure aniline were expected to continue supplying customers during the catalyst regeneration. If this action had not been taken, downstream production plants using aniline as a feedstock would have had to shut down due to lack of supply during catalyst regeneration. Additionally, increasing the sampling frequency would allow for more regular monitoring of breakthrough.

[0055] At timestamp 546 hours, production was stopped as scheduled before measuring breakthrough of nitrobenzene in the crude reaction product and measuring nitrobenzene in pure aniline.

[0056] Example 2 A further manufacturing process was carried out in a plant as shown in Figure 1. The observed concentrations of by-products are shown as a function of time in the graph of Figure 3.

[0057] From the graph timestamps 150 to 200 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine are stable, indicating that the catalyst is still operating at normal activity.

[0058] At timestamp 350 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine increase to more than 10% above their stable values, indicating a decrease in catalyst activity and the subsequent breakthrough of nitrobenzene.

[0059] At timestamp 500 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine and phenazine increased to more than 30% above the stable values. The increase in concentrations also changed from linear to exponential. Both indicate a further decrease in catalyst activity and a breakthrough of nitrobenzene within the next 4-5 days. A catalyst regeneration was scheduled and sufficient stocks of pure aniline were expected to continue supplying customers during the catalyst regeneration. If this action had not been taken, downstream production plants using aniline as a feedstock would have had to shut down due to lack of supply during the catalyst regeneration. Additionally, increasing the sampling frequency would allow for more regular monitoring of breakthrough.

[0060] Besides scheduling catalyst regeneration, it is also possible to reduce the load on the plant and continue production until a planned shutdown in a few days. Alternatively, the reaction temperature can be increased to delay the breakthrough of nitrobenzene. However, this also increases the rate of catalyst deactivation, as the higher temperatures increase the rate of formation of heavy deposits (coke) on the catalyst, which must be further removed during catalyst regeneration.

[0061] Example 3 A further manufacturing process was carried out in a plant as shown in Figure 1. The observed concentrations of by-products are shown as a function of time in the graph of Figure 4.

[0062] Up until the 250 hour timestamp on the graph, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine are stable, indicating that the catalyst is still operating at normal activity.

[0063] At timestamp 380 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine increase to greater than 10% above their stable values, indicating a decrease in catalyst activity and the subsequent breakthrough of nitrobenzene.

[0064] At timestamp 450 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine and phenazine have risen to approximately 20% above the stable values. Production was continued with the plant load reduced from ±100% to ±80% until a scheduled shutdown occurred a few days later at timestamp 660 hours. At that timestamp, the increase in concentrations was still linear. The vertical axis 103 on the right side of Figure 4 further shows the plant load in [% / 10]. The graph of plant load is assigned the reference number 115.

[0065] Alternatively, the reaction temperature can be increased to delay nitrobenzene breakthrough, but this also increases the rate of catalyst deactivation because the higher temperatures increase the rate of formation of heavy deposits (coke) on the catalyst, which must be further removed during catalyst regeneration.

[0066] Example 4 A further manufacturing process was carried out in a plant as shown in Figure 1. The concentrations of the by-products observed are shown as a function of time in the graph of Figure 5.

[0067] Up until the 200 hour timestamp on the graph, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine are stable, indicating that the catalyst is still operating at normal activity.

[0068] At timestamp 300 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, and phenazine increase to more than 10% above their stable values, indicating a decrease in catalyst activity and the subsequent breakthrough of nitrobenzene.

[0069] At timestamp 400 hours, the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine and phenazine had increased to more than 30% above their stable values. The concentration increase also changed from linear to slightly exponential. Both indicate further decline in catalyst activity and that nitrobenzene breakthrough will occur within the next 4-5 days. Catalyst regeneration was scheduled and sufficient stocks of pure aniline were anticipated to continue supplying customers during the catalyst regeneration. Additionally, increasing the sampling frequency would allow for more regular monitoring of breakthrough.

[0070] At timestamp 528 hours, the reactor temperature increased by 3°C, resulting in a decrease in the concentrations of n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine and phenazine, indicating that the breakthrough of nitrobenzene was postponed. However, in this case, the catalyst deactivation rate also increased, since the high temperature increases the rate of formation of heavy deposits (coke) on the catalyst, which must be further removed during catalyst regeneration.

[0071] The increase in reactor temperature is shown on the right-hand vertical axis 103 and the graph of temperature increase is assigned the reference numeral 117 .

Claims

1. (a) A step of reacting hydrogen and nitrobenzene in a hydrogenation reactor in the presence of a catalyst to form a crude reaction product; (b) A step of working up the crude reaction product to obtain pure aniline A method for producing aniline by catalytic hydrogenation of nitrobenzene, comprising: The composition of the crude reaction product is analyzed continuously or at regular time intervals before starting the work-up in step (b) to determine the proportion of by-products. When the amount of by-products increases, measures are taken to prevent breakthrough of nitrobenzene in the hydrogenation reactor.

2. Measures taken to prevent breakthrough of nitrobenzene in the hydrogenation reactor include: - Planning for catalyst regeneration or replacement and foreseeing a sufficient stock of pure aniline; - Reducing the load of the plant to delay catalyst deactivation; - Temporarily increasing the time interval for analyzing the composition of the crude reaction product; - Increasing the reaction temperature to enhance reactivity and delay catalyst deactivation The method according to claim 1, comprising at least one of the above.

3. The method according to claim 1 or 2, wherein the reaction in step (a) is carried out in the gas phase or the liquid phase.

4. The method according to claim 1 or 2, wherein the reaction in step (a) is carried out in the gas phase and the crude reaction product is at least partially condensed before starting the work-up in step (b).

5. The method according to claim 4, wherein the composition of the at least partially condensed crude reaction product is analyzed.

6. The method according to claim 4, wherein at least partial condensation of the crude reaction product is carried out in at least two condensation steps.

7. The method according to claim 6, wherein the composition of the at least partially condensed crude reaction product obtained in the first condensation step is analyzed.

8. When the amount of by-products in the crude reaction product increases by more than 10% compared to the concentration during normal operation, measures are taken to prevent breakthrough of nitrobenzene in the hydrogenation reactor. The method according to claim 1 or 2.

9. The method according to claim 1 or 2, wherein the analysis of the composition of the crude reaction product is carried out by an on-line analysis process.

10. The method according to claim 1 or 2, wherein a sample is taken and analyzed to analyze the composition of the crude reaction product.

11. The method according to claim 1 or 2, wherein the composition of the crude reaction product is analyzed at regular time intervals shorter than 24 hours.

12. The method according to claim 1 or 2, wherein the by-product whose ratio is analyzed is at least one of aminophenol, n-phenyl-1,2-benzenediamine, n-phenyl-1,4-benzenediamine, phenazine, phenol, nitrobenzene, and high-boiling components that cannot be specified.

13. The method according to claim 1 or 2, wherein the analysis of the composition of the crude reaction product is performed by chromatography techniques or spectroscopic techniques.

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