Method and device for preparing nitrobenzene

Abstract

The invention relates to a continuously operated process for preparing nitrobenzene, comprising the following steps: a) nitrating benzene with sulfuric acid and nitric acid under adiabatic conditions in a plurality of reactors operated in parallel, using a stoichiometric excess of benzene based on the nitric acid; b) first combining the crude nitration process products from the reactors operated in parallel in an apparatus provided specifically for this purpose to form a mixed stream, which is then separated into a sulfuric acid phase and a nitrobenzene phase in a phase separation apparatus arranged downstream; c) working up the nitrobenzene phase to obtain nitrobenzene. The invention further relates to a production plant suitable for carrying out the process.

Description

[0001] The invention relates to a continuously operated process for preparing nitrobenzene, comprising the following steps: a) nitrating benzene with sulfuric acid and nitric acid under adiabatic conditions in a plurality of reactors operated in parallel, using a stoichiometric excess of benzene based on the nitric acid; b) first combining the crude nitration process products from the reactors operated in parallel in an apparatus provided specifically for this purpose to form a mixed stream, which is then separated into a sulfuric acid phase and a nitrobenzene phase in a phase separation apparatus arranged downstream; c) working up the nitrobenzene phase to obtain nitrobenzene.

[0002] The invention also relates to a production plant suitable for carrying out the process according to the invention.

[0003] The nitration of benzene with nitric acid in the presence of sulfuric acid to nitrobenzene and water has been the subject of numerous publications and patent applications. Two basic process types are distinguished here, the "isothermal" mode and the "adiabatic" mode.

[0004] In isothermal mode, the (considerable) reaction heat of the nitration is removed as far as possible by indirect cooling with the aid of a heat transfer medium.

[0005] US 3,092,671 B1 describes an isothermal process for preparing nitrobenzene in which a reaction loop is used. In this process - see FIG. 1 and the explanatory text - benzene and a mixture of sulfuric acid and nitric acid are pumped by means of a centrifugal pump (1) through a nitration reactor (4) and reacted. The nitration reactor (4) is designed as a heat exchanger in which the reaction temperature is maintained at 120° F. (48.9° C.) to 150° C. (65.6° C.) by thermostating (see the examples and patent claim 4). The reaction product obtained (which, in addition to nitrobenzene, also contains an acid phase) is partially - without separating out the acid phase - fed back into the reaction. The remainder of the liquid product mixture is introduced into a phase separation device (not shown in FIG. 1 ), in which a phase separation occurs into crude nitrobenzene and an acid phase.

[0006] In the case of the adiabatic mode, which is currently more common and is also used in the present invention, the cooling of the nitration reactor is omitted, so that once the inevitable heat losses are neglected, the exotherm of the reaction is quantitatively reflected in the temperature difference between the temperature on entry into the nitration reactor and the temperature of the completely converted product mixture (the so-called adiabatic temperature jump). In order not to increase this temperature too much, a very large excess of sulfuric acid is usually used in adiabatically operated processes. A continuous process for preparing nitrobenzene by adiabatic nitration of benzene using a mixture of sulfuric acid and nitric acid (the so-called mixed acid) was first claimed in 1941 in US Pat. No. 2,256,999, where a device consisting of four stirred tanks arranged in parallel, which are supplied with reactants in sequence, is described. A circulation process as described in US Pat. No. 3,092,671 B1 is not feasible in this adiabatic mode without at least partially abandoning the economic advantages of the process, because here, due to the adiabatic temperature jump, the temperature of the reaction products is significantly higher than the temperature of the mixed reactants before the start of the reaction. The relatively high temperature of the reaction product (after it is separated into the acid phase and the nitrobenzene phase) is required for the expansion evaporation of the water contained in the acid phase. As described in US 3,092,671 B1, the return of a portion of the reaction product before phase separation would require cooling of the returned portion, which would impair the energy balance of the process and therefore its economic viability.

[0007] The reaction mode in adiabatic operation is usually carried out in such a way that nitric acid and sulfuric acid are combined to form a so-called nitrating acid (also called mixed acid). Benzene is metered into the nitrating acid. This approach is also preferably used in the method according to the invention. The reaction products are essentially water and nitrobenzene. In the nitration reaction, benzene is used at least in a stoichiometric amount, but preferably in an excess of 2% to 10%, based on the molar amount of nitric acid, so that the process product produced in the nitration is essentially free of nitric acid. The process product is returned to the phase separation device, in which two liquid phases are formed, an organic phase and an aqueous phase. The organic phase is called crude nitrobenzene and is essentially composed of nitrobenzene, benzene and a certain amount of sulfuric acid and water dissolved in nitrobenzene. The aqueous phase is called spent acid and is essentially composed of water, sulfuric acid and nitrobenzene dissolved in sulfuric acid. In addition to these liquid components, the process products of the nitration also contain gaseous components, more precisely on the one hand organic components such as evaporated benzene and low-boiling non-aromatic secondary components (commonly referred to as low boilers), and on the other hand inorganic components such as nitrous gases (NO x). According to the prior art, these gaseous components are separated from the two liquid phases in a phase separation device and discharged as an offgas stream via a separate outlet. This offgas stream from the phase separation device can be combined with various offgas streams from other parts of the plant and subjected to post-treatment, wherein, as described in patent application EP 2 719 682 A1, benzene can be recovered and nitrous gases can be converted into nitrous acid. In this way, the recovered benzene and nitrous acid can be recycled and supplied again to the nitration.

[0008] The crude nitrobenzene formed in the reaction unit and separated from the acid phase in the phase separation device is subjected to washing and distillation work-up according to the prior art. The characteristic of this work-up is that the unconverted excess benzene is separated from the nitrobenzene in the final distillation as "return benzene" after washing. This return benzene, which contains a portion of low-boiling non-aromatic organic compounds (low boilers) in addition to the gas phase discharged in the phase separation device, is reused in the nitration reaction.

[0009] International patent application WO 2015 / 197521 A1 relates to a process for the continuous preparation of nitrobenzene by nitration of benzene with a mixture of nitric acid and sulfuric acid, wherein during production downtimes the entire production plant is not shut down, but the production plant is operated completely or at least partially in a "circulation". In addition, the patent application relates to a plant for the preparation of nitrobenzene and a method for operating a plant for the preparation of nitrobenzene. The plant for the preparation of nitrobenzene can have a plurality of nitration reactors connected in parallel or in series.

[0010] Patent application US 2017 / 152210 A1 (also published as WO 2015 / 197522 A1) relates to a method for preparing a chemical product, wherein one or more raw materials are converted into a chemical product or a chemical composition, and to an apparatus for carrying out such a method. The method and the apparatus are characterized in that during production interruptions, at least one raw material is not introduced into the reaction and the parts of the apparatus that are not involved in inspection, maintenance, repair or cleaning measures continue to operate in a so-called circulation mode. The apparatus can have reactors connected in parallel or in series. As an example of a method and an apparatus to which the invention can be applied, nitrobenzene preparation is mentioned.

[0011] Patent application EP 0 696 574 A1 relates to a process for the hydrogenation of nitroaromatics to aromatic amines in the gas phase over a stationary catalyst, wherein heat is neither supplied to the catalyst nor removed from the catalyst from the outside, ie the process is operated adiabatically. Figure 2A production unit with three parallel reactors (II, III and IV) is shown. The reaction products (6, 7, 8) of the three reactors are combined in a common pipeline and sent to a distillation column (VIII) to produce steam after being cooled in a heat exchanger (V). A water / aniline vapor mixture (12) is obtained at the top of the distillation column (VIII), which is condensed in a condenser (IX). The first part of the condensate (13) is returned to the distillation column (VIII) as a reflux, and the second part is sent to a separation container (X). In this separation container, water-containing aniline (14) is separated from aniline-containing water (16). Water-containing aniline (14) is combined with the bottom flow (11) of the distillation column (VIII) which also contains water-containing aniline and is sent to further post-treatment. The fixed catalyst is placed on or between the gas-permeable walls in the form of a catalyst charge. It is also feasible to use a honeycomb or corrugated layer that is given catalytic activity by applying a suitable metal compound instead of a catalyst charge. Such reactors for gas phase reactions equipped with stationary catalysts are not suitable for the nitration of benzene with nitric acid in the presence of sulfuric acid, wherein the sulfuric acid - like the other reactants - flows through the reactor and, in addition to its function as catalyst, also serves to absorb the heat of reaction.

[0012] Chinese patent application CN 1789235 A relates to the use of a tubular reactor in a nitration reaction.

[0013] Patent application DE 10 2009 005324 A1 deals with the problems associated with a high low boiler content in the return benzene and describes in this context a process for preparing nitrobenzene by adiabatic nitration of benzene, wherein the benzene / low boiler mixture obtained in the purification of nitrobenzene is recycled to the nitration and the separation of the crude nitrobenzene from the sulfuric acid is carried out under pressure after the reaction.

[0014] The treatment of the offgas from an adiabatically carried out nitration reaction involving nitrous gases is described in EP 0 976 718 A2. The offgas from the acid circuit and crude nitrobenzene are removed, combined and treated via NO x The nitric acid is then transported to the absorber to recover the dilute nitric acid and return it to the reaction. The circulating sulfuric acid is concentrated in the flash evaporator and is essentially free of organic matter. High-boiling organics such as nitrobenzene, dinitrobenzene and nitrophenols remain in trace amounts in the circulating acid and are therefore also returned to the reaction.

[0015] Patent application WO 2014 / 016292 A1 describes how the nitrobenzene process can be better started by keeping the content of aliphatic organic compounds in the feed benzene at a low level (less than 1.5% by mass) during the startup time. This is achieved by adjusting the ratio of fresh benzene to return benzene during the startup time, depending in particular on the purity of the return benzene, so as not to exceed the required maximum content of aliphatic organic compounds in the feed benzene. Here, the proportion of return benzene during the startup time can also be zero; then only fresh benzene of sufficient purity is supplied to the nitration reactor during the startup time. Patent application WO 2014 / 016289 A1 describes how the continuous nitration of benzene to nitrobenzene can be improved in conventional operation by limiting the content of aliphatic organic compounds in the feed benzene to a mass ratio of less than 1.5%. In one embodiment, this is achieved by discharging low boilers with the gas phase of the phase separation device. Both patent applications are particularly concerned with improving product quality and optimizing the washing of crude nitrobenzene; however, the influence of low boilers in the phase separation device is not taken into account.

[0016] The phase separation device (also called decanter) has not only the important task of separating the nitration process product into an acidic aqueous phase and an organic phase containing crude nitrobenzene. In addition, as already mentioned, a gas phase containing benzene, low boilers and nitrous gases is also discharged in the phase separation device. It is therefore necessary to provide a sufficiently long residence time in the phase separation device so that these physical processes (separation of the crude nitration process product into two liquid phases and a gas phase) can take place without negatively affecting the production capacity of the plant. Due to the presence of a gas phase in the device, the separation device must be designed to be significantly larger than in the case of pure liquid-liquid separation.

[0017] The efficiency of gas-liquid or liquid-liquid phase separation devices can be increased according to the prior art by special internal components or a special design of the entry device. This also applies to the phase separation in the nitrobenzene process (phase separation after the reaction and phase separation within the washing range). Internal components such as plate internal components, knitted fabrics, foils and filler bodies can make the flow uniform and smooth and increase the surface area, so that phenomena such as coalescence and segregation of droplets and gas bubbles proceed more quickly. The entry into the phase separation device can be carried out by baffles or deflectors, which smooth the flow or direct it to the wall of the device with the aim of increasing the residence time in the device and thus increasing the separation efficiency. For example, Maurice Stewart and Ken Arnold in Gulf Equipment Guides, Gas-Liquid and Liquid-Liquid Separators, Chapter 3.5( Vessel Internais), pp. 84-89, mid-2009 and Arthur J Kidnay, William R. Parrish and Daniel G. McCartney, Fundamentals of Natural Gas Processing , Chapter 5, pages 105 to 117, 2011. The variants described in the mentioned document are partially explained using the example of gas-liquid phase separation, but, in terms of the basic principle, this can also be applied to liquid-liquid separation or three-phase gas-liquid-liquid separation. A disadvantage of these methods of the prior art is that, due to the smoothness of the flow and the nature of the internal components, deposits and scaling may form. For example, knitted fabrics and foils become clogged over time and form deposits on the plates. The internal components may be damaged in the event of pressure shocks or excessively high flow rates. Due to the corrosive media, the interior of the phase separation device is usually made of enamel. The device may be damaged by internal components and the maintenance or repair of the device becomes more complicated.

[0018] Operating experience shows that problems often occur in the phase separation of the crude nitration process product. These manifest themselves, for example, in inadequate phase separation (e.g. entrainment of organic matter into the acid phase or formation of a black precipitate). These problems occur to an even greater extent when the crude process products of several nitration reactors, which are operated in parallel and can be adjusted independently of one another, that is to say in the case of reactions in several parallel-operating reaction tubes (also called reaction tubes), are fed to a common phase separation device. This practice is not uncommon in practice. Multiple-pass reactions in combination with single-pass work-up have generally proven to be the best compromise between the requirements of minimizing investment costs on the one hand and maximizing production flexibility on the other.

[0019] Therefore, there is a need for further improvement in the preparation of nitrobenzene in a plurality of reaction circuits operating in parallel, in particular with regard to the efficiency of the separation of the nitration reaction product into two liquid phases and one gaseous phase. It is particularly desirable to design the removal of the gaseous components and the separation of the two liquid phases from one another as optimally as possible with regard to the quality of the separation and the process technology and equipment design. Different loads and reaction conditions in the individual reaction circuits should also not lead to a deterioration in the separation efficiency. The present invention takes this need into account from the perspective of process technology and equipment technology.

[0020] It was surprisingly found that a problem that was consistently observed in liquid-liquid phase separations is related to In a common Separation equipment Combining the process products of each reaction pipeline (i.e. mixing and phase separation) In one and the same device and When mixing and phase separation occur separately, these problems can be solved or at least significantly reduced. For example, with the fluctuation of the demand for nitrobenzene product, the throughput in each reaction line can change, so that the amount entering the phase separation device from each line is different, which can cause the separation efficiency to decrease. Since each line usually enters the separation device at different locations, different local velocities and undesirable cross-flow and reflux are caused due to different amounts, which adversely affects the separation efficiency, and in the case of excessive load of the reaction line, also causes high turbulence and further efficiency loss. Different reaction conditions such as pressure, temperature and concentration also lead to mixing and equilibrium processes in the equipment, which must be carried out in parallel and slow down the stratification of the liquid phase. In addition, problems can be generated by simultaneously exporting the gas phase. Depending on the ratio of the gas phase, its presence can lead to significantly higher speeds and turbulence in the liquid phase in the phase separation device, which makes the separation of two liquid phases difficult. Therefore, in the case of ratio fluctuations or gas phase increase, liquid phase separation may be insufficient, so that even a larger proportion of organic matter may enter the acidic aqueous phase. The following conclusions were drawn: The presence of a gas phase in the separator generally leads to high velocities (and also to higher velocities of the liquid phase), since the gas phase moves at a significantly higher velocity compared to the liquid due to its lower density. Furthermore, the presence of the gas phase and the resulting three-phase gas-liquid-liquid separation also impair the separation efficiency of the two liquid phases. Rising gas bubbles make the stratification of the liquid phases difficult, since mixing constantly reoccurs at the liquid-liquid phase boundary and the liquid phase with a higher density can be entrained together with the gas bubbles into the liquid phase with a lower density.

[0021] Therefore, a first subject of the present invention is a process for the continuous preparation of nitrobenzene, comprising the following steps:

[0022] a) nitrating benzene with sulfuric acid and nitric acid under adiabatic conditions in n parallel reactors, using a stoichiometric excess of benzene based on the nitric acid, wherein n is a natural number of 2 to 5, thereby obtaining n process products comprising nitrobenzene, benzene and sulfuric acid (hereinafter also referred to as crude nitration process products);

[0023] b) (i) combining n process products comprising nitrobenzene, benzene and sulfuric acid into a mixed stream comprising nitrobenzene, benzene and sulfuric acid, optionally additionally comprising depletion of gaseous components after (α), before (β) or during (γ) the combining process,

[0024] (ii) introducing the mixed stream, which is optionally depleted in gaseous components, in its unchanged entirety or divided into a plurality, in particular n, preferably 2 to 3, substreams, into a phase separation apparatus, in which the mixed stream is separated into a liquid aqueous sulfuric acid phase and a liquid organic nitrobenzene phase;

[0025] c) working up the nitrobenzene phase from step b) to obtain nitrobenzene;

[0026] And choose

[0027] d) evaporating water in the sulfuric acid phase obtained in step b) to obtain a concentrated sulfuric acid phase, and using the concentrated sulfuric acid phase as a component of the sulfuric acid used in step a).

[0028] A second subject of the present invention is a production plant for carrying out the process according to the invention for the continuous preparation of nitrobenzene, wherein the production plant comprises the following equipment:

[0029] a) n parallel reactors for adiabatic nitration of benzene with sulfuric acid and nitric acid, using a stoichiometric excess of benzene based on the nitric acid, wherein n is a natural number from 2 to 5, to obtain n process products comprising nitrobenzene, benzene and sulfuric acid;

[0030] b) (i) an apparatus arranged downstream of the reactor of a) for combining n process products comprising nitrobenzene, benzene and sulfuric acid into a mixed stream comprising nitrobenzene, benzene and sulfuric acid,

[0031] (ii) a phase separation device arranged downstream of the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid, for separating the resulting mixed stream into a liquid aqueous sulfuric acid phase and a liquid organic nitrobenzene phase;

[0032] c) an apparatus for working up the liquid organic nitrobenzene phase from b)(ii) to give nitrobenzene, wherein the apparatus comprises in particular the following:

[0033] (i) Equipment for washing the liquid organic nitrobenzene phase and for separating off unconverted benzene;

[0034] (ii) means for recycling the separated benzene obtained in c)(i) to the reactor in a) as a component of the benzene used therein;

[0035] d) Optionally, a device for concentrating the sulphuric acid phase from b) (ii) by evaporating water and for recycling the concentrated sulphuric acid phase thus obtained to the n reactors of a).

[0036] In the technical terminology of the present invention, the term "gaseous secondary components" includes at least the low boilers already mentioned above, wherein low boilers are understood to mean all non-aromatic organic secondary components of the process product of the nitration (= step a)) whose boiling point at standard pressure (1013 mbar) is below that of nitrobenzene. Typical low boilers are n-heptane, dimethylcyclopentane, 3-ethylpentane, cyclohexane, the isomeric dimethylpentanes, n-hexane, cyclopentane, n-pentane, trimethylcyclopentane, methylcyclohexane, ethylcyclopentane and octane. In addition, inorganic secondary components may also be present, such as, in particular, the nitrous gases already mentioned.

[0037] In the process according to the invention, the mixed stream obtained in step b)(i) is fed to the phase separation in step b)(ii), that is to say no part of this mixed stream is returned to the conversion in step a). The same applies to the phase separation of the mixed stream before it is combined to form the mixed stream. n process products containing nitrobenzene, benzene and sulfuric acid ; these are also not returned to step a). Reaction loops as described in the prior art for isothermal processes are not the subject of the process according to the invention. Of course, the same applies to the production plant according to the invention; it has no device for returning n process products comprising nitrobenzene, benzene and sulfuric acid or the mixed stream obtained in the device for combining [b)(i)] n process products comprising nitrobenzene, benzene and sulfuric acid to one, more or all of the n reactors [a)].

[0038] In the attached picture:

[0039] FIG. 1 shows two possible designs of a plant for combining n process products comprising nitrobenzene, benzene and sulfuric acid ( Figure 1a and Figure 1b );

[0040] Figure 2 A vertically arranged gas separator is shown with lateral feed of the input stream (b.1) and discharge of the gas phase (b.3) in the upper part and of the liquid phase (b.2) in the lower part;

[0041] Figure 3 A vertically arranged gas separator is shown with a feed of an input stream (b.1) at the bottom and a discharge of a gas phase (b.3) at the top and a discharge of a liquid phase (b.2) at the side;

[0042] Figure 4 A vertically arranged gas separator is shown with an upper feed of an input stream (b.1) and a lateral discharge of a gas phase (b.3) and a lower discharge of a liquid phase (b.2);

[0043] Figure 5 A possible design of the production device according to the invention is shown in the case of n = 2;

[0044] Figure 6 A possible design of a production plant according to the invention in combination with an additional gas-liquid separation is shown for n=2;

[0045] Figure 7 shows a grid used for computational fluid dynamics (CFD) calculations of an embodiment;

[0046] Figure 8The volume ratios of the three phases (upper: aqueous phase, middle: organic phase, lower: gas phase) in the CFD simulation of Example 1 (comparative example; the crude process product was not combined (= homogenized) and was not degassed before entering the phase separation device) are shown;

[0047] Fig. 9 The volume ratios of the three phases (upper: aqueous phase, middle: organic phase, lower: gas phase) in the CFD simulation of Example 2 (an example according to the invention; with combined (=homogenized) raw process products and no degassing before entering the phase separation device) are shown;

[0048] Fig.10 The volume proportions of the three phases (top: aqueous phase, middle: organic phase, bottom: gas phase) in the CFD simulation of Example 3 (example according to the invention; with combined (=homogenized) crude process products and degassing before entering the phase separation device) are shown.

[0049] Below first Brief Overview Various possible implementations.

[0050] In a first embodiment of the process according to the invention, which can be combined with all other embodiments, the workup of the nitrobenzene phase in step c) comprises the following:

[0051] (i) washing the nitrobenzene phase and separating off unconverted benzene,

[0052] (ii) using the separated benzene as a component of the benzene used in step a).

[0053] In a second embodiment of the process according to the invention, which can be combined with all the other embodiments, benzene is used in step a) in a stoichiometric excess of 2.0 to 40%, preferably 3.0 to 30%, particularly preferably 4.0 to 25%, based on the theoretical nitric acid.

[0054] In a third embodiment of the process according to the invention, which can be combined with all the other embodiments, the temperature in each of the n reactors of step a) is maintained between 98°C and 140°C.

[0055] In a fourth embodiment of the method of the invention, which may be combined with all other embodiments, the method comprises the following:

[0056] (a) after the combination in step b)(i), the mixed stream comprising nitrobenzene, benzene and sulphuric acid is introduced into a gas separator, in which a gas phase comprising benzene and gaseous secondary components is separated off and a liquid phase comprising nitrobenzene and sulphuric acid, which is depleted in gaseous components, is retained and fed to step b)(ii);

[0057] or

[0058] (β) after step a) and before the combination in step b(i), the n process products comprising nitrobenzene, benzene and sulfuric acid are introduced into n gas separators, in which n gas phases comprising benzene and gaseous secondary components are separated off and n liquid phases depleted in gaseous components comprising nitrobenzene and sulfuric acid are retained, which liquid phases are subsequently fed to step b(i);

[0059] or

[0060] (γ) For the combination according to step b)(i), the n process products comprising nitrobenzene, benzene and sulfuric acid from step a) are introduced into a common gas separator, in which a gas phase comprising benzene and gaseous secondary components is separated off, and a mixed stream of the liquid phase comprising nitrobenzene and sulfuric acid, which is depleted in gaseous components, is retained and fed to step b)(ii).

[0061] In a fifth embodiment of the process according to the invention, which is a special design of the fourth embodiment, a gravity separator or a centrifugal separator is used for separating off the gas phase comprising benzene and gaseous secondary components.

[0062] In a sixth embodiment of the process of the present invention, which is a special design of the fifth embodiment, a gravity separator is used.

[0063] In a seventh embodiment of the process of the present invention, which is a special design of the sixth embodiment, a horizontally or vertically arranged gravity separator is used to separate the process product containing nitrobenzene, benzene and sulfuric acid or the mixed stream containing nitrobenzene, benzene and sulfuric acid into separate streams.

[0064] A gravity separator is fed from the side or from below, wherein the gas phase is removed from the gravity separator as a top stream and the liquid phase is removed from the gravity separator at the bottom or at the side as a bottom stream, or

[0065] The gravity separator is fed from above, with the gas phase being removed from the gravity separator at the side and the liquid phase being removed from the gravity separator at the bottom.

[0066] In an eighth embodiment of the method of the present invention, which is another special design of the fifth embodiment, a centrifugal separator is used.

[0067] In a ninth embodiment of the process of the present invention, which is a special design of the eighth embodiment, a vertically arranged cylindrical, conical or cylindrical-conical cyclone separator is used as a centrifugal separator, and a process product comprising nitrobenzene, benzene and sulfuric acid or a mixed stream comprising nitrobenzene, benzene and sulfuric acid is guided individually through the centrifugal separator to generate a vortex, in which the gas phase is discharged upward and the liquid phase is discharged downward.

[0068] In the tenth embodiment of the process according to the invention, which can be combined with all other embodiments, as long as they do not provide for the division of the mixed stream obtained in step b)(i), all the mixed stream obtained in step b)(i) is fed to the phase separation device of step b)(ii) at one point.

[0069] In an eleventh embodiment of the process according to the invention, which can be combined with all other embodiments, as long as they do not provide for feeding the entire mixed stream obtained unchanged in step b)(i) to the phase separation device at a single point, the mixed stream obtained in step b)(i) is divided into a plurality of (in particular into 2 to n, preferably into 2 to 3) substreams and these substreams are fed to the phase separation device of step b)(ii) at different points.

[0070] In a twelfth embodiment of the process according to the invention, which can be combined with all the other embodiments, the n reactors in step a) are capable of being adjusted independently of one another.

[0071] In a thirteenth embodiment of the process according to the invention, which can be combined with all the other embodiments, in step a), a tubular reactor is used as the reactor, which is preferably arranged vertically and each has a plurality (preferably in each case 2 to 15, particularly preferably 4 to 12, not including mixing devices for the initial mixing of benzene with nitric acid and sulfuric acid) of dispersing elements, wherein particularly preferably the flow through the tubular reactor is from bottom to top (i.e. the starting material benzene-containing stream, sulfuric acid and nitric acid are each fed to the vertically arranged tubular reactor at the bottom and the process product comprising nitrobenzene, benzene and sulfuric acid is in each case removed from the tubular reactor at the top).

[0072] In a first embodiment of the production plant according to the invention, which can be combined with all other embodiments, the production plant has the following:

[0073] In variant (α),

[0074] b) a gas separator arranged downstream of the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid and upstream of the phase separation device for separating the mixed stream from b)(i) into a gas phase comprising benzene and gaseous secondary components and a mixed stream comprising nitrobenzene, benzene and sulfuric acid which is depleted in gaseous components,

[0075] Or, in variant (β),

[0076] b) n gas separators arranged downstream of the n reactors of a) and upstream of a device for combining n process products comprising nitrobenzene, benzene and sulfuric acid, operated in parallel, for separating the process products of the n reactors of a) into n gas phases comprising benzene and gaseous secondary components and n process products depleted in gaseous components comprising nitrobenzene, benzene and sulfuric acid,

[0077] Or, in variant (γ),

[0078] b) an apparatus for combining n process products comprising nitrobenzene, benzene and sulfuric acid, arranged downstream of the n reactors of a), which also serves as a gas separator for separating the process products of the n reactors of a) into a gas phase comprising benzene and gaseous secondary components and a mixed stream comprising nitrobenzene, benzene and sulfuric acid which is depleted in gaseous components.

[0079] In a second embodiment of the production plant according to the invention, which can be combined with all other embodiments, the n reactors of a) can be adjusted independently of one another.

[0080] In a third embodiment of the production plant according to the invention, which can be combined with all other embodiments, provided that they do not provide for a division of the mixed streams from b)(i), the phase separation device has a single inlet nozzle for introducing all mixed streams.

[0081] In a fourth embodiment of the production plant according to the invention, which can be combined with all other embodiments as long as they do not provide for feeding the entire mixed stream obtained unchanged from b)(i) to the phase separation device at a single point, a distributor system is arranged between the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid and the phase separation device, for distributing the mixed stream to a plurality (in particular 2 to n, preferably 2 to 3) of inlet nozzles arranged on the phase separation device.

[0082] In a fifth embodiment of the production plant according to the invention, which can be combined with all the other embodiments, the reactors in a) are tubular reactors which are preferably arranged vertically and each have a plurality (preferably in each case 2 to 15, particularly preferably 4 to 12, not including mixing devices for the initial mixing of benzene with nitric acid and sulfuric acid) of dispersing elements, wherein particularly preferably the flow through the tubular reactor is from bottom to top (i.e. the starting material benzene-containing stream, sulfuric acid and nitric acid are each fed to the vertically arranged tubular reactor at the bottom and the process product comprising nitrobenzene, benzene and sulfuric acid is in each case removed from the tubular reactor at the top).

[0083] In the following More details in The above briefly describes the embodiments and further possible designs of the present invention. Here, the above embodiments and further possible designs can be combined with each other arbitrarily, unless the context indicates otherwise.

[0084] Step a) of the method of the invention, In n reactors, sulfuric acid (hereinafter also referred to as a.2) and nitric acid (hereinafter also referred to as For a.3) nitration of a benzene-containing stream (hereinafter also referred to as a.1), a stoichiometric excess of benzene,In principle, all adiabatically operated nitration processes known from the prior art can be used. According to the invention, two to five reactors, preferably two to three reactors, are operated in parallel.

[0085] Preferably, first nitric acid (a.3) and then the benzene-containing stream (a.1) are metered into sulfuric acid (a.2). The premixing of nitric acid (a.3) and sulfuric acid (a.2) produces a so-called mixed acid, into which the benzene-containing stream (a.1) is then metered in this embodiment. In this case, the mixed acid used contains preferably at least 2.0% by mass of nitric acid and at least 66.0% by mass of sulfuric acid, particularly preferably 2.0% to 4.0% by mass of nitric acid and 66.0% to 75.0% by mass of sulfuric acid, based on the total mass of the mixed acid.

[0086] The stoichiometric excess of benzene based on nitric acid (a.3) is preferably set at a value of 2.0% to 40% of theory, particularly preferably 3.0% to 30%, very particularly preferably 4.0% to 25%. Theoretically, 1 mol HNO 3 reacts with 1 mol of benzene. Therefore, benzene has a 3 An excess of x% is equivalent to The molar ratio n (benzene) / n (HNO 3 ) ( n = amount of substance), i.e., for example, in the case of a 2% excess of benzene = 1.02 or, for example, in the case of a 40% excess of benzene = 1.40.

[0087] The excess benzene is preferably recovered and used partly or completely as a component of the benzene-containing stream (a.1). In this case, the excess benzene is recovered before or after, in particular after, a single-stage or multi-stage washing of the crude nitrobenzene; further details can be found in the discussion of step c) below. The benzene-containing stream (a.1) is therefore preferably a mixture of benzene freshly fed to the reaction (so-called fresh benzene) and recycled benzene (so-called return benzene). In each case, the reaction conditions are particularly selected so that the mass proportion of benzene in the benzene-containing stream (a.1), based on the total mass of the benzene-containing stream (a.1), is at least 90.0%, preferably at least 95.0%, particularly preferably at least 98.5%.

[0088] According to the invention, step a) is carried out under adiabatic conditions. In the case of an adiabatic reaction mode, the reactor used in step a) is neither heated nor cooled; the reaction temperature results from the temperature of the reactants used and the mixing ratio between them. The n reactors are preferably well insulated to reduce heat losses to a minimum. In the case of adiabatic nitration, the reaction temperature of the mixture reacted in each of the n reactors thus rises from the "starting temperature" immediately after the first mixing of the reactants to the "final temperature" after the maximum conversion, and preferably always remains at a value of 98° C. to 140° C. The starting temperature results from the temperature of the starting materials benzene, sulfuric acid and nitric acid, the concentration of the acids used, their quantitative ratio to one another and the volume ratio of the organic phase (benzene) to the aqueous phase (sulfuric acid and nitric acid), the so-called phase ratio. The phase ratio is also decisive for the final temperature: the smaller the phase ratio (and therefore the more sulfuric acid present), the lower the final temperature. In the case of the preferred use of tubular reactors (see below), the temperature increases due to the increasing conversion along the longitudinal axis of the reactor. On entering the reactor, the temperature is in the lower region of the temperature range of 98°C to 140°C, and on leaving the reactor, the temperature is in the upper region of the temperature range.

[0089] Step a) is preferably carried out according to the process scheme as described in DE 10 2008 048 713 A1, in particular in paragraph

[0024] .

[0090] The reactors suitable for step a) are in principle all reactors known in the prior art for adiabatic nitration, such as stirred tanks (especially stirred tank cascades) and tubular reactors. Tubular reactors are preferred. Tubular reactors are particularly preferred in which a plurality of dispersing elements are distributed over the length of the tubular reactor, thereby ensuring an intense mixing of benzene, nitric acid and sulfuric acid. It is particularly preferred to use a vertically arranged tubular reactor in which a plurality of (preferably 2 to 15, particularly preferably 4 to 12, not including a mixing device for the initial mixing of benzene with nitric acid and sulfuric acid) dispersing elements are distributed over the length of the tubular reactor. It is very particularly preferred to flow through such a tubular reactor from bottom to top. Such a reactor and the form of available dispersing elements are, for example, described in EP 1 291 078 A2 (see FIG. 1 there).

[0091] Particularly preferably, step a) is designed so that the n reactors can be adjusted independently of one another, ie can be operated independently of one another. This allows production capacity to be achieved by shutting down individual reactors when demand for the nitrobenzene product decreases.

[0092] In step b) of the process according to the invention, firstly n process products of step a) comprising nitrobenzene, benzene and sulfuric acid (and secondary components which may be present as gaseous phase or dissolved) (hereinafter also referred to as a.4.1, a.4.2, ..., a.4.n) are reacted in a Step b)(i)The nitrobenzene, benzene and sulfuric acid are combined into a mixed stream (hereinafter also referred to as b.1) comprising nitrobenzene, benzene and sulfuric acid. This is carried out in an apparatus for combining the process products (a.4.1, a.4.2, ... a.4.n) obtained in n reactors and comprising nitrobenzene, benzene and sulfuric acid. Such an apparatus is, for example, a container which is connected via pipelines to the outlet openings of the reactors for the crude nitration process product (at Figure 1a In the example shown in Figure 1, n = 2 is used). In this container, n crude nitration process products are combined. This combination can also be carried out in a pipeline, into which n streams (a.4.1, a.4.2, ... a.4.n) flow together and the mixed stream (b.1) is fed to step c) (in Figure 1b The required thorough mixing (= homogenization) of the n crude process products can also be promoted by using static internals or stirrers that promote mixing in the apparatus for combining the n crude process products.

[0093] In a preferred embodiment, a gas-liquid phase separation for depleting the gaseous components is carried out before the phase separation in step b) (ii). The gas-liquid phase separation is carried out in a gas separator. In principle, all separators known to the person skilled in the art that are capable of achieving gas-liquid separation can be used as gas separators. The possible equipment for separating gaseous and liquid streams is well known to the person skilled in the art. Detailed information on the various processes and equipment for separating gaseous and liquid streams can be found in the specialist literature, for example in Oilfield Processing, Crude Oil, Vol. 2, Chapter 6, pages 79 to 112, Manning, Francis S. and Thompson, Richard E., 1995, or in Maurice Stewart and Ken Arnold. Gulf Equipment Guides, Gas-Liquid and Liquid-Liquid Separators , Chapters 3.3 to 3.5, pages 72 to 103, mid-2009. The variants described in the cited document are partially illustrated by way of example for three-phase gas-liquid-liquid separation, but, in principle, can also be used for gas-liquid phase separation. This preferred embodiment is characterized in that separation step (i) Separation of (at least most of) the gas phase from the two liquid phases and (ii) Separate the two liquid phases from each other Thus, in this embodiment, these steps are carried out in two apparatuses, a gas separator and a phase separation apparatus. However, with regard to the design of the apparatus, commonalities can exist between the gas separator and the phase separation apparatus.

[0094] The gas separator is preferably not temperature-controlled, whereby the temperature in the gas separator is generated by the temperature of the reaction mixture flowing in. The gas separator is preferably operated at a slightly elevated pressure relative to the ambient pressure ("overpressure"), wherein the pressure in the gas space of the gas separator is 50 mbar to 100 mbar above the ambient pressure, for example 80 mbar.

[0095] Preference is given to using a gravity separator or a centrifugal separator as the gas separator.

[0096] Gas-liquid phase separation can be achieved in a variety of ways.

[0097] In variant (α), in step b)(i) above Combine n crude nitration products A gas-liquid phase separation is then carried out. Here, the mixed stream (b.1) comprising nitrobenzene, benzene and sulfuric acid is introduced into a gas separator, in which a gas phase comprising benzene and gaseous secondary components (hereinafter also referred to as b.3) is separated and a liquid phase comprising nitrobenzene and sulfuric acid depleted in gaseous components (hereinafter also referred to as b.2) remains and is fed to step b) (ii) as a mixed stream to be separated into a sulfuric acid phase and a nitrobenzene phase.

[0098] In variant (β), the gas-liquid phase separation is carried out after step a) (and in step b)(i) Combine n crude nitrates Chemical Process Products Here, n process products comprising nitrobenzene, benzene and sulfuric acid (a.4.1, a.4.2, ... a.4.n) are fed to n gas separators, in which n gas phases comprising benzene and gaseous secondary components (hereinafter also referred to as b.3.1, b.3.2, ... b.3.n) are separated, and n liquid phases depleted in gaseous components comprising nitrobenzene and sulfuric acid (hereinafter also referred to as b.2.1, b.2.2, ... b.2.n) remain, which are then fed to the step b) (i) of Combine n crude nitration process products .

[0099] In variant (γ), the gas-liquid phase separation and the combination of the n nitration crude process products are carried out in a common apparatus, i.e. the gas-liquid phase separation is a step of step b)(i). Combine n crude nitration process productsA special embodiment of the process of step a) is provided. Here, n process products (a.4.1, a.4.2, ... a.4.n) comprising nitrobenzene, benzene and sulfuric acid from step a) are introduced into a common gas separator, in which a gas phase comprising benzene and gaseous secondary components is separated and a mixed stream of a liquid phase comprising nitrobenzene and sulfuric acid, which is depleted in gaseous components, is retained and fed to step b) (ii). In this variant, spontaneous evaporation may occur if the individual liquid components of the process products (a.4.1, a.4.2, ... a.4.n) are mixed and have significantly different compositions and / or temperatures. Such spontaneous evaporation can interfere with homogenization and gas-liquid phase separation.

[0100] Variants (α) and (β) are therefore preferred. Homogenization of the nitration reaction solution from the individual reaction lines prior to liquid-liquid phase separation leads to a reduction or avoidance of turbulence when entering the phase separation device of step b) (ii). Undesirable flows in the device, such as cross-flow and reflux and eddy formation, which are formed due to different ratios of the three phases (aqueous phase, organic phase, gas phase) in the incoming reaction solution can be reduced or eliminated. The same applies to the gas separator accordingly. In terms of the device, the merging before the gas-liquid phase separation is the simplest, so variant (α) is particularly preferred.

[0101] Independently of the variant selected, in one embodiment of the invention, a horizontally or vertically arranged gravity separator is used as a gas separator, the process products comprising nitrobenzene, benzene and sulfuric acid (a.4.1, a.4.2, ... a.4.n) or the mixed stream comprising nitrobenzene, benzene and sulfuric acid (b.1) are each separated into

[0102] A gravity separator is fed from the side or from below, a gas phase comprising benzene and gaseous secondary components being removed from the gravity separator as a top stream, and a liquid phase depleted in gaseous components comprising nitrobenzene and sulfuric acid being removed from the gravity separator at the bottom or from the side as a bottom stream, or

[0103] A gravity separator is fed from above, a gas phase comprising benzene and gaseous secondary components being removed from the gravity separator at the side, and a liquid phase depleted in gaseous constituents comprising nitrobenzene and sulfuric acid being removed from the gravity separator at the bottom.

[0104] The expression “horizontally or vertically arranged” relates here to the longitudinal axis of the essentially cylindrical device. Figures 2 to 4 A vertically arranged gravity separator that can be used in the gas-liquid separation step is shown. The flows in the figure are considered as variant (α) (input flow = b.1; gas phase = b.3, retained liquid phase = b.2):

[0105] In the gas separator according to FIG. 2 , the process product ( b.1 ) is fed in from the side, the gas phase ( b.3 ) is discharged at the top and the liquid phase ( b.2 ) is discharged at the bottom.

[0106] In the gas separator according to FIG. 3 , the process product ( b.1 ) is fed in at the bottom, the gas phase ( b.3 ) is discharged at the top and the liquid phase ( b.2 ) is discharged laterally.

[0107] In the gas separator according to FIG. 4 , the process product ( b.1 ) is fed in from the top, the gas phase ( b.3 ) is discharged from the side and the liquid phase ( b.2 ) is discharged from the bottom.

[0108] Best Figure 2 design plan.

[0109] However, centrifugal separators can also be used. Preference is given to vertically arranged cylindrical, conical or cylindrical-conical cyclones, through which the process product comprising nitrobenzene, benzene and sulfuric acid is guided, creating a vortex, in which a gas phase comprising benzene and gaseous secondary components is discharged upwards and a liquid phase comprising nitrobenzene and sulfuric acid, which is depleted in gaseous components, is discharged downwards. The term "vertical arrangement" here again relates to the longitudinal axis of the device. The vortex can be generated by tangentially arranged inlet nozzles or baffles (see Figure 3.20 in Gulf Equipment Guides: Gas-liquid and liquid-liquid Separators, Stewart & Arnold, 2009, Gulf Professional Publishing).

[0110] exist Step b)(ii) , the mixed stream obtained in step b)(i) is introduced into the phase separation device. In the simplest design of this step, this can be achieved by feeding the mixed stream into the phase separation device via a single inlet nozzle (i.e., all the mixed stream obtained in step b)(i) is fed into the phase separation device of step b)(ii) at one point, such as Figure 5 and Figure 6 In this case, it is advisable to dimension the inlet nozzle sufficiently large to be able to feed a single (compared to the individual flows in the uncombined mode) larger mixed flow into the phase separation device without high flow velocities and turbulence occurring in the phase separation device.

[0111] If the procedure according to the invention is to be subsequently introduced into an existing production plant having n reactors connected in parallel and therefore also having n inlet lances for n crude process products arranged at different spatial locations in the phase separation device, it is preferred to continue to use the n inlet lances and the associated pipelines of the phase separation device already present by connecting the device for combining the n crude process products (a.4.1, a.4.2, ... a.4.n) to be used according to the invention between the n reactor outlets and the n inlets into the phase separation device. In this case, the mixed flow (b.1) leaving the device for combining the n crude process products (a.4.1, a.4.2, ... a.4.n) can be divided again into n sub-flows or, in contrast to FIG. 1 , the device for combining the n process products has n outlet lances (in this case it should be ensured that the n crude process products are thoroughly mixed (= homogenized) on reaching the outlet lances, for example by dimensioning the device sufficiently large to provide a sufficient residence time and / or by using static internals or stirrers which promote mixing). That is to say, if the combination according to the invention is carried out in the sense of step b)(i), at the end of this step there is only one single homogeneous process product (mixed stream) with a given temperature and chemical composition. By redistributing this homogeneous process product into substreams, the temperature and composition of the individual substreams no longer change relative to the single homogeneous process product, so that this approach does not impair the concept of the invention. Therefore, in this approach, the substreams fed to the phase separation device are also in a homogeneous form with regard to their velocity, temperature and chemical composition at the inlet of the phase separation device.

[0112] The use of a plurality of inlet nozzles into the phase separation device also has the advantage that the velocity at the individual inlet nozzles (at the same diameter) and in general the inflow and mixing process is significantly reduced and the phase separation can be initiated more quickly. It may therefore also be useful when planning new production plants to divide the mixed stream comprising nitrobenzene, benzene and sulfuric acid obtained in step b) (i) into a plurality, in particular 2 to n, preferably 2 to 3 substreams (in the same manner as described above) and to feed them into the phase separation device at spatially separated points.

[0113] Regardless of whether the mixed stream is fed into the phase separation device at one point in its unchanged entirety or is divided into a plurality of substreams and fed at a plurality of different points, the liquid-liquid phase separation in step b) (ii) can be carried out in a phase separation device known in principle to the person skilled in the art according to methods known per se from the prior art. The aqueous sulfuric acid phase (hereinafter also referred to as b.5) essentially contains (due to the formation of reaction water and water introduced into the reaction by the used nitric acid) diluted sulfuric acid and inorganic impurities. The organic nitrobenzene phase (hereinafter also referred to as b.4) essentially contains nitrobenzene and excess benzene and organic impurities. The phase separation device preferably has a gas outlet, through which the optionally present gaseous components can be discharged (as long as these components have not been separated out beforehand in the preferred gas-liquid separation). The gas outlet of the optionally present gas separator and the gas outlet of the phase separation device of step b) (ii) preferably lead to a common exhaust gas aftertreatment device. The phase separation device of step b) (ii) is preferably not temperature-controlled and preferably operates at a slightly positive pressure (preferably 50 mbar to 100 mbar above ambient pressure, for example 80 mbar, measured in the gas space).

[0114] Independently of the exact design and exact design of the reactor and the device for combining n process products in step a), the optionally present gas separator and the phase separation device of step b), the aqueous liquid phase containing sulfuric acid (hereinafter also referred to as b.5) obtained in step b) is preferably concentrated by evaporation of water into an aqueous liquid phase containing sulfuric acid at a higher concentration than phase (b.5) (hereinafter also referred to as d.1), which is recycled into step a) and partially or completely used as a component of the sulfuric acid (a.2) used there. In this case, the sulfuric acid (a.2) used in step a) therefore contains the recycled sulfuric acid (d.1) and can even consist of it in certain embodiments. This preferred process variant is referred to as step d) in the technical terminology of the present invention and will be explained in more detail below.

[0115] In step c) of the process according to the invention, the liquid phase obtained in step b) (ii) (hereinafter also referred to as b.4) (crude nitrobenzene) is worked up, Obtaining nitrobenzene (hereinafter also referred to as c.1) The post-treatment can in principle be carried out as known in the prior art. A preferred process is described as follows:

[0116] First, the organic phase (b.4) is subjected to one or more washing stages ( Step c) (i) ).exist The first sub-step of the washingIn the process, the organic phase (b.4), which usually still contains traces of acid, is washed in one or more stages with an aqueous washing liquid and then separated from the acidic aqueous phase obtained by phase separation; in the case of multiple washing stages, after each individual washing stage, the acidic aqueous phase obtained is separated. In the process, the acid residues contained in the crude nitrobenzene (b.4) are washed out, so that this process step is also called acid washing. This step is well known in the prior art and is therefore only briefly outlined here. Preferably, the aqueous stream generated in the operation is recycled for the acid washing.

[0117] Then, the organic phase thus obtained Second Son In the step, an aqueous solution of an alkali is used to perform one or more stages of washing in an alkaline washing, wherein the alkali is preferably selected from sodium hydroxide, sodium carbonate or sodium bicarbonate, and then separated from the alkaline washing water obtained by phase separation; in the case of multiple washing stages, the alkaline washing water is separated from the alkaline washing water obtained after each individual washing stage. It is particularly preferred to use an aqueous sodium hydroxide solution as the alkaline aqueous solution. This step is well known in the prior art and is therefore only briefly outlined here. The pH value of the sodium hydroxide solution used and its mass ratio to the organic phase are adjusted so that acidic impurities (e.g., nitrophenols formed as by-products and acidic residues not completely removed in the first sub-step) are neutralized in the alkaline washing. The subsequent post-treatment of the alkaline wastewater can be carried out by methods of the prior art, for example, according to the teachings of EP 1 593 654 A1 and EP 1 132 347 A2.

[0118] Finally, the organic phase thus obtained is The third sub-step In the neutral washing, one or more washing stages are carried out with water, and then separated from the water phase by phase separation; in the case of multiple washing stages, the water phase is separated after each individual washing stage. In principle, this can be achieved by all common methods in the prior art. The washing water used here is preferably fully desalted water (VE water), particularly preferably a mixture of VE water and steam condensate (i.e., a condensate of steam obtained by heat exchange of water with any exothermic process step), and very particularly preferably using steam condensate. It is preferred to use an electrophoretic method in the last neutralization stage of the neutral washing (see WO 2012 / 013678 A2).

[0119] The nitrobenzene washed in this way is then further treated by (Step c)(ii)Remove dissolved water, unconverted benzene and optional organic impurities. The post-treatment is preferably carried out by distillation, wherein the vapor of water and benzene and optional organic impurities are removed from the top. The vapor is cooled and sent to a separation container. The separated water settles in the lower phase. Benzene and low-boiling substances are located in the upper phase, which are fed back into the reaction as return benzene (c.2). If necessary, a portion of the upper phase can be discharged (i.e. not recycled) to avoid excessive accumulation of low-boiling substances. It is also possible to separate low-boiling substances from the upper phase and feed the return benzene depleted of low-boiling substances into the reaction. The distillation apparatus used is preferably a rectifying column. The bottom product of the distillation, optionally after further distillation to obtain nitrobenzene as a distillate (i.e. as an overhead or side stream product), is fed into further applications (such as, in particular, the hydrogenation of aniline) as (pure) nitrobenzene (c.1).

[0120] As an alternative to the procedure described here, it is also conceivable to separate off excess benzene before the washing.

[0121] As already mentioned, preferably in step d) The aqueous liquid phase comprising sulfuric acid obtained in step b) (ii) (b.5) is concentrated by evaporating water into an aqueous liquid phase (hereinafter also referred to as d.1) containing a higher concentration of sulfuric acid than phase (b.5), It is recycled partly or completely into step a) and used as a component of the sulphuric acid (a.2) used there. This concentration of the aqueous sulfuric acid phase (b.5) can in principle be carried out as known in the prior art. Preferably, the sulfuric acid in the aqueous phase (b.5) is concentrated in an expansion evaporator (also say In the adiabatic mode provided according to the invention, under the condition of correctly selecting the reaction conditions, the aqueous phase (b.5) containing sulfuric acid can be heated so strongly with the heat of reaction of the exothermic reaction in step a) that the concentration and temperature of the aqueous phase containing sulfuric acid can be set in the expansion evaporator at the same time before the reaction with benzene and nitric acid when entering the reactor space, i.e. (d.1) corresponds to (a.2) in terms of temperature and concentration. This is described in EP 2 354 117 A1, in particular in paragraph

[0045] .

[0122] As already mentioned, the second subject of the present invention is to provide a production plant for implementing the process of the present invention for continuously preparing nitrobenzene. The preferred embodiments and designs of the process of the present invention are also applicable to the production plant of the present invention accordingly. For example, the production plant of the present invention preferably comprises a tubular reactor as a reactor.

[0123] Attached photos Figure 5 Taking n = 2 as an example, a possible implementation scheme of the production device according to the present invention is shown.

[0124] 1001, 1002: Reactor

[0125] 2100: Equipment for combining process products obtained in reactors

[0126] 2200: Phase separation equipment

[0127] 3000: Sulfuric acid concentration equipment (evaporator)

[0128] 4000: Sulfuric acid tank

[0129] 5000: Crude Nitrobenzene Tank

[0130] 6000: Equipment for single-stage or multi-stage washing of crude nitrobenzene

[0131] 7000: Equipment for separating unconverted benzene (especially distillation columns)

[0132] In a particular embodiment, the production plant according to the invention further comprises one or more gas separators. In this case, as already described above with respect to the method according to the invention, there are a number of possibilities for further designing the production plant:

[0133] (α) The production plant can have a gas separator arranged downstream of the device for combining the process products obtained in the n reactors and comprising nitrobenzene, benzene and sulfuric acid (and upstream of the phase separation device).

[0134] (β) However, it is also possible to connect n gas separators operated in parallel downstream of the n reactors operated in parallel, whose liquid outlets open into a device for combining the process products obtained in the n reactors, which comprise nitrobenzene, benzene and sulfuric acid.

[0135] (γ) Finally, the device for combining the process products obtained in the n reactors and comprising nitrobenzene, benzene and sulfuric acid can be designed so that it jointly fulfils the function of combining the n crude nitration process products and the depleted gaseous components.

[0136] In an embodiment with an additional gas-liquid separation, the production plant according to the invention therefore preferably comprises

[0137] In variant (α),

[0138] b) a gas separator arranged downstream of the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid and upstream of the phase separation device for separating the mixed stream from b) (i) into a gas phase (b.3) comprising benzene and gaseous secondary components and a (liquid) mixed stream (b.2) depleted in gaseous components comprising nitrobenzene, benzene and sulfuric acid,

[0139] Or, in variant (β),

[0140] b) n gas separators arranged downstream of the n reactors of a) and upstream of a device for combining n process products comprising nitrobenzene, benzene and sulfuric acid, operated in parallel, for separating the process products of the n reactors of a) into n gas phases (b.3.1, b.3.2, ... b.3.n) comprising benzene and gaseous secondary components and n (liquid) process products depleted in gaseous components comprising nitrobenzene, benzene and sulfuric acid (b.2.1, b.2.2, ... b.2.n),

[0141] Or, in variant (γ),

[0142] b) an apparatus for combining n process products comprising nitrobenzene, benzene and sulfuric acid, which is arranged downstream of the reactor of a) and which also serves as a gas separator (common to all n reactors) for separating the process products (a. 4.1, a.4.2, ..., a.4.n) of the n reactors of a) into a gas phase (b.3) comprising benzene and gaseous secondary components and a (liquid) mixed stream (b.2) depleted in gaseous components and comprising nitrobenzene, benzene and sulfuric acid.

[0143] Variant (α) is particularly preferred (see above for further corresponding statements regarding the description of the process according to the invention) and in Figure 6 In the figure, two reactors 1001 and 1002 are shown as examples. For the sake of simplifying the drawings, regulating valves, etc. are not shown. By combining the various process products (a.4.1, a.4.2) in the container (2100) before the gas separator (2110) and therefore also before the phase separation device (2200), the construction of the phase separation device is simplified (there is only one opening instead of at least two for metering the liquid phase). In addition, due to the reduction or elimination of the undesirable flows such as cross flow and reflux and vortex formation in the device formed due to the different proportions of the three phases (aqueous phase, organic phase, gas phase) in the incoming reaction solution, phase separation is facilitated. In addition, in the case of multiple entry openings, the varying throughput and reaction conditions in each pipeline lead to varying and strongly different velocities at the inlet, and therefore lead to unknown flow conditions in the phase separation device. These effects can be better controlled by pre-homogenization and metering together at one site.

[0144] If the depletion of the gaseous components is omitted according to one of the variants (α), (β) or (γ), then in a preferred embodiment, the gaseous components are separated to a certain extent in the phase separation in step b)(ii) by providing a phase separation device with a gas outlet through which the gaseous components are discharged. Figure 5This is indicated by the arrow "b.3" at the upper end of the phase separation device (2200). The gas outlet of the phase separation device in step b) (ii) preferably leads to an exhaust gas aftertreatment device.

[0145] In all embodiments of the production plant according to the invention, it is particularly preferred to design the production plant in such a way that the n reactors can be regulated independently of one another, i.e. can be operated independently of one another. This allows the production capacity to be achieved by shutting down individual reactors when the demand for the product nitrobenzene decreases. The equipment required for this, in particular the regulating valves and their corresponding controllers, are well known to the person skilled in the art.

[0146] In the simplest embodiment of the introduction of the mixed flow into the phase separation device, the phase separation device has a single inlet nozzle for the mixed flow.

[0147] If the mixed stream obtained in the device (2100) for combining the process products obtained in the reactors, as described above in connection with the description of the process according to the invention as a possible embodiment, is Figure 5 and Figure 6 In the embodiment of the present invention, the phase separation device (2200) is divided into a plurality of (particularly 2 to n, preferably 2 to 3) sub-streams and fed to the phase separation device (2200) at different locations, and the production device according to the present invention has a distributor system downstream of the device for combining the process products (2100) obtained in the reactor, the distributor system has a plurality of outlets corresponding to the number of sub-streams, and the phase separation device (2200) has a plurality of inlet nozzles, which are connected to the outlet of the distributor system and the number of which corresponds to the number of sub-streams. Such a distributor system can be realized in a simple manner by passing the pipeline for discharging the mixed flow obtained from the device for combining the process products (2100) obtained in the reactor into a plurality of pipelines, the number of which corresponds to the number of required sub-streams, or by having the device for combining the process products (2100) obtained in the reactor have a number of outlet nozzles corresponding to the number of sub-streams, and the outlet nozzles are connected to the inlet nozzles of the phase separation device through pipelines.

[0148] The method according to the present invention has at least the following advantages:

[0149] i) By combining the individual reaction products of the n lines upstream of the phase separation device and the associated homogenization, undesirable flows and turbulences in the phase separation device can be minimized.

[0150] ii) The n reactors can be operated independently of each other at different process conditions (throughput, pressure, temperature) without negatively affecting the separation performance of the phase separation device.

[0151] iii) By degassing the reaction solution upstream of the phase separation device in a preferred embodiment of the invention, the velocity and turbulence in the inlet region of the phase separation device are reduced, whereby the separation efficiency can be significantly increased.

[0152] iv) The phase separation time in the phase separation apparatus is minimized, whereby the investment outlay for the apparatus becomes lower or the expansion of production in existing plants becomes easier.

[0153] v) Through improved phase separation, carryover of organics into the sulfuric acid concentration evaporator is reduced, which reduces energy consumption and reduces problems otherwise caused by these organics.

[0154] vi) Due to the improved phase separation, the sulfuric acid entrained in the crude nitrobenzene sent to the work-up is reduced. This saves raw material, since the sulfuric acid losses in the work-up are lower.

[0155] vii) Reduced wastewater pollution due to less sulfuric acid entering the wastewater of step c).

[0156] viii) Flow-smoothing internals in phase separation apparatuses, which are susceptible to disruptive fouling and agglomeration, can generally be omitted.

[0157] The following is intended to illustrate the present invention by way of examples.

[0158] Example:

[0159] The following two examples are intended to demonstrate the positive influence of the homogenization of the input stream on the phase separation. Figure 5 The three-phase flow behavior in the device 2200 in the embodiment discussed was simulated by computational fluid dynamics. In the embodiment discussed, three reactors are operated in parallel, and their outflow flows flow into the phase separation device at three different locations. Since the three reactors are operated with different production capacities, the simulation assumes that three reaction products (a.4.1, a.4.2, a.4.3) are produced in different amounts. The reaction product of the first reactor contains 300 t / h of aqueous phase (sulfuric acid phase), 17 t / h of organic phase (nitrobenzene phase) and 0.18 t / h of gas phase (mainly benzene), while 225 t / h of aqueous phase (sulfuric acid phase), 13 t / h of organic phase (nitrobenzene phase) and 0.13 t / h of gas phase are formed by the reaction of the other two reactors respectively. The phase separation device is operated at an absolute pressure of about 1.1 bar (in the gas phase), and the temperature of the input flow is 130°C.

[0160] Figure 7 The 3D grid used for the phase separation device is shown in FIG. , which has 800 000 computational cells.

[0161] 100: Inlet nozzle (3x; the third one is behind the viewing plane and not visible in the picture)

[0162] 200: Gas phase outlet

[0163] 300: Organic phase outlet

[0164] 400: Water phase outlet

[0165] For simplicity, the phase separation device is depicted as a cylinder, without taking into account the rounded shape of the side covers. Due to the axisymmetry, only half of this phase separation device is modeled. The process products of the three reactors (a.4) flow in through the inlet nozzles on the left. The outlet for the organic phase (b.4) is located in the middle of the right side. The outlet for the aqueous phase (b.5) is located at the lower end. The gas phase (b.3) can be taken off at the top. The three-phase flow is simulated using the Euler-Euler method, where the aqueous phase is described as the continuous phase and the organic and gas phases as the dispersed phases. The momentum equations are solved for continuity and conservation for all phases within the scope of the simulation. The turbulence model used is the k-epsilon model. The equations are solved instantaneously, where the time step varies between 0.1 and 0.001 seconds.

[0166] Since the exact droplet / bubble size entering the phase separation device or in the phase separation device itself is unknown, a constant droplet / bubble diameter of 1 mm is assumed for both phases. Since in reality the droplets and bubbles follow a certain size distribution and breakup and coalescence processes occur in the device, the actual particle size and the resulting phase ratios in the device may vary. The purpose of the CFD simulation is to qualitatively describe the influence of the gas phase on the flow properties and ultimately on the separation efficiency of the phase separation device.

[0167] In the examples, the cases without homogenization (Example 1) and with homogenization (Apparatus 2100, Examples 2 and 3) are considered respectively.

[0168] Embodiment 1 (comparative example):

[0169] In Comparative Example 1, each line enters the phase separation device at three inlet nozzles with its own volume.

[0170] 300 t / h aqueous phase, 17 t / h organic phase and 0.18 t / h gas phase from reactor 1;

[0171] From reactors 2 and 3 in each case 225 t / h of aqueous phase, 13 t / h of organic phase and 0.13 t / h of gas phase.

[0172] The process product of the reactor with the greater load flows in laterally at the central inlet. The process products of the two reactors with the lesser load flow in laterally at the nozzles.

[0173] Figure 8 The volume ratios of the three phases are shown in grayscale (top image: volume ratio of the aqueous phase, middle: volume ratio of the organic phase, bottom: volume ratio of the gas phase). In addition, in the image, each phase is marked as

[0174] 10: Water phase,

[0175] 20: organic phase,

[0176] 30: gas phase, and

[0177] 40: Dispersed phase that has not yet been fully mixed.

[0178] As can be seen from the figure at the top, a continuous aqueous phase is formed immediately after entering, which settles downward. However, here, the aqueous phase is located very far above the inlet nozzle and is first entrained upward by the rising airflow. The continuous organic phase is only formed at the very end of the outlet (300) of the organic phase, wherein the water-organic phase boundary is located at the lower end of the outlet (middle image, the right side of the continuous organic phase is before the outlet 300 of the organic phase), thereby causing a large amount of entrained aqueous phase. There is a large area in the middle of the phase separation device, wherein all three phases exist ("dispersed" phase) and eddies can be seen. It is worth noting that the organic phase is only very finely distributed here and hardly reaches a volume ratio greater than 5%. In the figure at the bottom showing the volume ratio of the gas phase, it is seen that the rising gas phase entrains the aqueous phase and the organic phase upward. Thus, separation occurs later in the device, which can explain the high volume ratio of the aqueous phase and the almost invisible ratio of the organic phase. In summary, for such runs, a substantial entrainment of the foreign phase at the various outlets must be assumed, especially if a smaller ratio of droplet and bubble sizes than the 1 mm diameter simulated here is actually present, which would require more time for separation.

[0179] In actual production, the phase boundaries can be observed by means of a sight glass installed in the phase separation device. Under the conditions described above, very clear fluctuations (±200 mm) of the liquid-liquid phase boundary were always repeatedly observed on the right side below the organic phase outlet in actual operation. In addition, rising bubbles were observed through the sight glass. The greater the turbulence observed in the device, the greater the load differences of the individual reactors. In this mode, an aqueous phase was found in the crude nitrobenzene tank (5000). Thus, the simulation was confirmed by observations of the real device.

[0180] Example 2 (according to the present invention):

[0181] In Example 2 according to the invention, the operation of the phase separation device of Example 1 was simulated taking into account the upstream homogenization, that is to say it was assumed that the process products flowing into the phase separation device through the three inlet nozzles were identical in terms of temperature, composition and flow. Therefore, in Example 2, the process products of the individual reactors entered the phase separation device at the three inlet nozzles in the same proportions, more precisely:

[0182] 3×250 t / h aqueous phase, 14 t / h organic phase, and 0.15 t / h gas phase.

[0183] The results are shown in Fig. 9 (the image arrangement and reference numerals are the same as Figure 8 ). Although the continuous organic phase still only forms at the end of the phase separation device (center image, the continuous organic phase is on the right before the outlet for the organic phase). However, this region has become significantly larger and the aqueous-organic phase boundary is no longer located in the region of the outflow nozzle. In addition, there is a large region in the middle of the phase separation device in which all three phases are present (the "dispersed" phase). In this region, the proportion of the organic phase has increased significantly, so that phase separation has already occurred here. This can also be seen in the middle and lower images, where the larger volume proportions of the organic and gas phases in this region can also be seen. Overall, the organic and gas phases are no longer so finely distributed in the device, and the aqueous phase no longer reaches so far up into the device, but settles down more quickly. In the lower figure showing the volume proportion of the gas phase, it can be seen that the gas phase rises upward, but a part of it is still entrained into the device to a great distance. Due to the low density of the gas phase (approx. 3 kg / m³), the volume proportion of the gas phase in the inlet region of the decanter and in the middle section is still high despite the low mass proportion of the gas phase (0.15 t / h in 250 t / h). Here, this high proportion of gas also leads to higher velocities in the liquid phase (up to 2 m / s) in the inlet region and to eddies in the liquid-liquid phase separation region. For such an operation, entrainment of foreign phases at the individual outlets cannot still be ruled out, especially if in reality there is a smaller ratio of droplet and bubble sizes than the 1 mm diameter simulated here, which would require more time for separation.

[0184] Overall, the phase separation in the apparatus is significantly improved by homogenization. Example 3 shows that this result can be further improved if a gas separation is additionally carried out.

[0185] Example 3 (according to the present invention):

[0186] In Example 3 according to the invention, the operation of the phase separation device of Example 2 was simulated taking into account the upstream degassing. For this purpose, the proportion of the gas phase in the three process products (a.4.1, a.4.2, a.4.3) was reduced to 0.012 t / h each (the simulation therefore assumed that > ​​90% of the gas phase was separated off, which can be achieved without problems with conventional degassing devices), which corresponds to variants (α) or (β) in actual operation. In addition, in this simulation, 250 t / h of aqueous phase and 14 t / h of organic phase flowed into the phase separation device from each reactor. The volume ratios of the three phases are shown in Fig.10 (the image arrangement and reference numerals are as follows Figure 8 and 9 In contrast to Example 2, it was shown that a stable continuous aqueous phase and an organic phase were formed directly after entering the phase separation device. Due to the low proportion of the gas phase, it no longer interferes with the separation process. The velocity in the inlet region is also significantly reduced (< 1 m / s). Even for small droplet diameters, the flow is stable so that an increase and phase separation are possible. For such an operation, entrainment of foreign phases at the individual outlets can be substantially excluded.

[0187] The positive effects were also shown in actual operation: after installing the gas separator, the phase boundaries in the phase separation device could be stabilized and rising gas bubbles were no longer visible near the outlet.

Claims

1. A method for continuously preparing nitrobenzene, comprising the following steps a) nitrating benzene with sulfuric acid and nitric acid under adiabatic conditions in n parallel reactors, using a stoichiometric excess of benzene based on the nitric acid, wherein n is a natural number from 2 to 5, to obtain n process products comprising nitrobenzene, benzene and sulfuric acid, wherein the benzene is used in a theoretical stoichiometric excess of 2.0% to 40% based on the nitric acid; b) (i) combining the n process products comprising nitrobenzene, benzene and sulfuric acid into a mixed stream comprising nitrobenzene, benzene and sulfuric acid, Includes depleted gaseous components, of which The step (α) includes: (a) after the combination in step b)(i), the mixed stream comprising nitrobenzene, benzene and sulphuric acid is introduced into a gas separator, in which a gas phase comprising benzene and gaseous secondary components is separated off and a liquid phase comprising nitrobenzene and sulphuric acid which is depleted in gaseous components is retained and this liquid phase is fed to step b)(ii); Or in step (β) including: (β) after step a) and before the combination in step b(i), the n process products comprising nitrobenzene, benzene and sulfuric acid are introduced into n gas separators, in which n gas phases comprising benzene and gaseous secondary components are separated and n liquid phases depleted in gaseous components comprising nitrobenzene and sulfuric acid are retained, which liquid phases are subsequently fed to step b(i); Or in step (γ) include: (γ) for the purpose of combining in step b)(i), the n process products from step a) comprising nitrobenzene, benzene and sulphuric acid are introduced into a common gas separator, in which a gas phase comprising benzene and gaseous secondary components is separated off, and a mixed stream of a liquid phase comprising nitrobenzene and sulphuric acid which is depleted in gaseous components is retained and fed to step b)(i). b)(ii), wherein the term "gaseous secondary components" comprises at least low boilers, wherein low boilers are understood to mean all non-aromatic, organic secondary components of the process product of step a) whose boiling point at standard pressure is below the boiling point of nitrobenzene; (ii) introducing the mixed stream depleted in gaseous components into a phase separation device, wherein the mixed stream is separated into a liquid aqueous sulfuric acid phase and a liquid organic nitrobenzene phase; c) working up the nitrobenzene phase from step b) to obtain nitrobenzene; And choose d) evaporating water in the sulfuric acid phase obtained in step b) to obtain a concentrated sulfuric acid phase, and using the concentrated sulfuric acid phase as a component of the sulfuric acid used in step a).

2. The method according to claim 1, wherein the post-treatment of the nitrobenzene phase in step c) comprises: (i) washing the nitrobenzene phase and separating off unconverted benzene, (ii) using the separated benzene as a component of the benzene used in step a).

3. The process according to claim 1 or 2, wherein the temperature in each of the n reactors of step a) is maintained at 98°C to 140°C.

4. The process according to claim 1 or 2, wherein all mixed streams obtained in step b) (i) are fed at one point to the phase separation device of step b) (ii).

5. The process according to claim 1 or 2, wherein the mixed stream obtained in step b)(i) is divided into a plurality of substreams, and these substreams are fed to the phase separation device of step b)(ii) at different locations.

6. The process according to claim 1 or 2, wherein the n reactors in step a) can be adjusted independently of one another.

7. The process according to claim 1 or 2, wherein the n reactors in step a) are tubular reactors.

8. A production plant for carrying out the method for continuously preparing nitrobenzene according to any one of claims 1 to 7, wherein the production plant comprises the following equipment: a) n parallel reactors for adiabatically nitrifying benzene with sulfuric acid and nitric acid, using a stoichiometric excess of benzene based on the nitric acid, wherein n is a natural number from 2 to 5, to obtain n process products comprising nitrobenzene, benzene and sulfuric acid; b) (i) an apparatus arranged downstream of the reactor of a) for combining n process products comprising nitrobenzene, benzene and sulfuric acid into a mixed stream comprising nitrobenzene, benzene and sulfuric acid, (ii) a phase separation device arranged downstream of the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid, for separating the obtained mixed stream into a liquid aqueous sulfuric acid phase and a liquid organic nitrobenzene phase; c) an apparatus for working up the liquid organic nitrobenzene phase from b)(ii) to give nitrobenzene; d) Optionally, a device for concentrating the sulphuric acid phase from b) (ii) by evaporating water and for recycling the concentrated sulphuric acid phase thus obtained to the n reactors of a).

9. The production device according to claim 8, comprising In variant (α), b) a gas separator arranged downstream of the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid and upstream of the phase separation device for separating the mixed stream from b)(i) into a gas phase comprising benzene and gaseous secondary components and a mixed stream comprising nitrobenzene, benzene and sulfuric acid which is depleted in gaseous components, Or, in variant (β), b) n gas separators operated in parallel, arranged downstream of the n reactors of a) and upstream of a device for combining n process products comprising nitrobenzene, benzene and sulfuric acid, for separating the process products of the n reactors of a) into n gas phases comprising benzene and gaseous secondary components and n process products depleted in gaseous components comprising nitrobenzene, benzene and sulfuric acid, Or, in variant (γ), b) an apparatus for combining n process products comprising nitrobenzene, benzene and sulfuric acid, arranged downstream of the n reactors of a), which also serves as a gas separator for separating the process products of the n reactors of a) into a gas phase comprising benzene and gaseous secondary components and a mixed stream comprising nitrobenzene, benzene and sulfuric acid which is depleted in gaseous components.

10. The production plant according to claim 8 or 9, wherein the n reactors of a) can be adjusted independently of one another.

11. The production plant according to claim 8 or 9, wherein the phase separation device has a single inlet nozzle for introducing all mixed flows.

12. The production device according to claim 8 or 9, wherein a distributor system is arranged between the device for combining n process products comprising nitrobenzene, benzene and sulfuric acid and the phase separation device for distributing the mixed flow to a plurality of inlet nozzles arranged on the phase separation device.

13. The production device according to claim 8 or 9, wherein the n reactors are tubular reactors.

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