EP3956290A1

EP3956290A1 - Process and device for producing nitrobenzene

Info

Publication number: EP3956290A1
Application number: EP20716854.3A
Authority: EP
Inventors: Thomas Knauf; Murat Kalem; Christian Drumm; Alexandre Racoes
Original assignee: Covestro Deutschland AG; Covestro Intellectual Property GmbH and Co KG
Current assignee: Covestro Deutschland AG
Priority date: 2019-04-17
Filing date: 2020-04-14
Publication date: 2022-02-23
Also published as: WO2020212334A1; US20220162151A1; CN113924284A

Abstract

The invention relates to a continuously operating process for producing nitrobenzene, comprising the following steps: a) nitriding benzene in adiabatic conditions with sulfuric acid and nitric acid, using a stoichiometric excess of benzene in relation to the nitric acid, in multiple parallel reactors; b) first combining the raw process products of the nitridation from the parallel reactors to form a mixed flow in a device provided specifically for this purpose, then separating the mixed flow into a sulfuric acid phase and a nitrobenzene phase in a downstream phase separation apparatus; and c) processing the nitrobenzene phase, obtaining nitrobenzene. The invention also relates to a production plant suitable for carrying out the claimed process.

Description

METHOD AND DEVICE FOR THE PRODUCTION OF NITROBENZENE

The invention relates to a continuously operated process for the production of nitrobenzene comprising the steps: a) nitration of benzene under adiabatic conditions with sulfuric acid and nitric acid using a, based on nitric acid, stoichiometric excess of benzene in several reactors operated in parallel; b) first of all combining the crude process products of the nitration from the reactors operated in parallel to form a mixed flow in a device specially provided for this purpose, followed by separation of the mixed flow into a sulfuric acid phase and a nitrobenzene phase in a downstream phase separation apparatus; c) Working up the nitrobenzene phase to obtain nitrobenzene.

The invention also relates to a production plant suitable for carrying out the method according to the invention.

The nitration of benzene with nitric acid in the presence of sulfuric acid to form nitrobenzene and water has already been the subject of numerous publications and patent applications. A distinction is made between two basic types of process, the "isothermal" and the "adiabatic" mode of operation:

In the isothermal mode of operation, the (considerable) heat of reaction of the nitration is dissipated as far as possible by indirect cooling using a heat transfer medium.

An isothermal process for the production of nitrobenzene, in which a reaction ^ c / i / an / fe is used, is described in US Pat. No. 3,092,671 B1. In this process - see FIG. 1 and the explanatory text passages - benzene and a mixture of sulfuric and nitric acid are pumped through a nitration reactor (4) using a centrifugal pump (1) and reacted. The nitration reactor (4) is designed as a heat exchanger in which the reaction temperature is kept between 120 ° F (48.9 ° C) and 150 ° C (65.6 ° C) by thermostatting (cf. the examples and claim 4). The reaction product obtained (comprising not only nitrobenzene but also an acid phase) is partially returned to the reaction without separating off the acid phase. The remaining part of the liquid product mixture is passed into a phase separation apparatus (not shown in FIG. 1) in which a phase separation into crude nitrobenzene and acid phase takes place.

In the adiabatic mode of operation, which is more common today and is also used in the present invention, there is no cooling of the nitration reactor, so that the exothermicity of the reaction, apart from unavoidable heat losses, is quantified in the temperature difference between the temperature at the entry into the nitration reactor and the temperature of the completely converted product mixture (so-called adiabatic temperature jump). So that this temperature rise does not become too great, a very large excess of sulfuric acid is usually used in adiabatically operated processes. A continuous process for the production of nitrobenzene by an adiabatically operated nitration of benzene using a mixture of sulfuric and nitric acid (so-called mixed acid) was first claimed in 1941 in US 2,256,999, where an arrangement of four stirred tanks arranged in parallel is described, successively with the reactants are charged. A cycle as described in US Pat. No. 3,092,671 B1 is not possible with such an adiabatic procedure without at least partially giving up the economic advantages of this process, since here, due to the adiabatic temperature jump, the reaction product has a temperature which is well above the temperature of the mixed reactants before use the reaction lies. The comparatively high temperature of the reaction product is required (after it has been separated into an acid phase and a nitrobenzene phase) for the flash evaporation of water contained in the acid phase. Recirculating part of the reaction product before the phase separation, as described in US Pat. No. 3,092,671 B1, would make it necessary to cool the recirculated portion, which would impair the energy balance of the process and thus its economic viability.

In the adiabatic mode of operation, the reaction is generally carried out in such a way that the nitric acid and sulfuric acid are combined to form what is known as the nitrating acid (also called mixed acid). Benzene is dosed into this nitrating acid. This procedure is also preferably used in the process according to the invention. The reaction products are essentially water and nitrobenzene. In the nitration reaction, benzene is used in at least a stoichiometric amount based on the molar amount of nitric acid, but preferably in a 2% to 10% excess, so that the process product obtained in the nitration is essentially nitric acid-free. This process product is fed to a phase separator in which two liquid phases are formed, one organic and one aqueous. The organic phase is called crude nitrobenzene and consists essentially of nitrobenzene, benzene and a certain amount of sulfuric acid and water dissolved in the nitrobenzene. The aqueous phase is referred to as waste acid and consists essentially of water, sulfuric acid and nitrobenzene dissolved in the sulfuric acid. In addition to these liquid components, the nitration process product also contains gaseous components, on the one hand organic components such as evaporated benzene and low-boiling, non-aromatic secondary components (usually referred to as low boilers) and on the other hand inorganic components such as nitrous gases (NO _x ) in particular, caused by side reactions of the nitric acid used. According to the state of the art, these gaseous components separate from the two liquid phases in the phase separator and are discharged as an exhaust gas stream via a separate outlet. This exhaust gas flow from the phase separator can be combined with the various exhaust gas flows from other parts of the system are brought together and worked up, whereby, as described in patent application EP 2 719 682 A1, benzene can be recovered and the nitrous gases can be converted to nitrous acid. In this way, the recovered benzene and nitrous acid can be recycled and made available again for nitration.

The crude nitrobenzene formed in the reaction apparatus and separated from the acid phase in the phase separation apparatus is subjected to washing and working up by distillation according to the prior art. It is characteristic of this work-up that unreacted excess benzene is separated off as return benzene in a final distillation after washing from the nitrobenzene. This return benzene, which - in addition to the gas phase discharged in the phase separator - also contains some of the low-boiling, non-aromatic organic compounds (low boilers), is used again in the nitration reaction.

The international patent application WO 2015/197521 A1 relates to a method for the continuous production of nitrobenzene by nitrating benzene with a mixture of nitric acid and sulfuric acid, in which the entire production plant is not shut down during a production standstill, but the production plant completely or at least partially "in a circle “Is driven. This patent application also relates to a plant for the production of nitrobenzene and a method for operating a plant for the production of nitrobenzene. The plant for the production of nitrobenzene can have several nitration reactors connected in parallel or in series.

The patent application US 2017/152210 A1 (also published as WO 2015/197522 A1) relates to processes for the production of chemical products in which the input material (s) are converted into a chemical product or a chemical composition, as well as systems for carrying out such processes . The processes and systems are characterized by the fact that during an interruption in production there is no entry of at least one input material into the reaction and the system parts not affected by an overhaul, maintenance, repair or cleaning measure continue to be operated in a so-called cycle mode. The plants described can have reactors connected in parallel or in series. Nitrobenzene production is mentioned as an example of processes and systems to which the described invention can be applied.

Patent application EP 0 696 574 A1 deals with a process for the hydrogenation of nitroaromatics to aromatic amines in the gas phase over stationary catalysts, in which neither heat is supplied to the catalyst from the outside nor heat is withdrawn, that is, the process is operated adiabatically. FIG. 2 shows a production plant with three reactors connected in parallel (II, III and IV). The reaction products (6, 7, 8) of the three reactors are combined in a common line and after cooling in a heat exchanger (V) for Steam generation passed into a distillation column (VIII). At the top of the distillation column (VIII), a water / aniline vapor mixture (12) is obtained which is condensed in a condenser (IX). A first part of the condensate (13) is returned to the distillation column (VIII) as reflux, while a second part is fed into a separation vessel (X). In this separation vessel, water-containing aniline (14) is separated from aniline-containing water (16). The water-containing aniline (14) is combined with the bottom stream (11) of the distillation column (VIII), which likewise contains water-containing aniline, and fed to further work-up. The stationary catalysts are attached in the form of catalyst beds on or between gas-permeable walls. The use of honeycombs or corrugated layers, which have been made catalytically active by applying suitable metal compounds, instead of catalyst beds is also possible. Such reactors designed for gas phase reactions with stationary catalysts are not suitable for the nitration of benzene with nitric acid in the presence of sulfuric acid, the sulfuric acid - just like the other reactants - flowing through the reactor and, in addition to its effect as a catalyst, also serves to absorb the heat of reaction.

The Chinese patent application CN 1789235 A deals with the use of a tubular reactor in nitration reactions.

The patent application DE 10 2009 005324 A1 deals with problems that the high low boiler content of the backbenzene can bring with it and describes in this context a process for the production of nitrobenzene by adiabatic nitration of benzene, in which the benzene obtained in the purification of the nitrobenzene / Low boiler mixture is recycled for nitration and the separation of the crude nitrobenzene from the sulfuric acid is carried out under pressure after the reaction.

The treatment of the exhaust gas from the adiabatically carried out nitration reaction with regard to nitrous gases is described in EP 0 976 718 A2. The exhaust gas from the acid cycle and the crude nitrobenzene are drawn off, combined and sent through an NO _x absorber in order to recover dilute nitric acid, which is returned to the reaction. The circulating sulfuric acid is concentrated in a flash evaporator and largely freed from organic substances. There remain high-boiling organic substances such. B. nitrobenzene, dinitrobenzene and nitrophenols in traces in the circulatory acid and are thus also reduced to the reaction.

The patent application WO 2014/016292 A1 describes how the nitrobenzene process can be started up better by keeping the content of aliphatic organic compounds in the starting benzene low (mass fraction less than 1.5%) during the start-up time. This is achieved by adjusting the ratio of fresh benzene to reverse benzene during the start-up time, depending in particular on the purity of the reverse benzene, so that the required maximum content of aliphatic organic compounds in the feed benzene is not exceeded becomes. The proportion of reverse benzene can also be zero during the start-up time; then only fresh benzene of sufficient purity is fed to the nitration reactor during the start-up period. Patent application WO 2014/016289 A1 describes how the continuous nitration of benzene to nitrobenzene can be improved in regular operation by limiting the content of aliphatic organic compounds in the feed benzene to a mass fraction of less than 1.5%. In one embodiment, this is achieved by discharging low boilers with the gas phase of the phase separation apparatus. Both patent applications relate in particular to improved product quality and optimized washing of the crude nitrobenzene; however, the influence of low boilers in the phase separator is not taken up.

The phase separator (also called decanter) not only has the important task of separating the nitration process product into an aqueous acidic 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 drawn off in the phase separator. It is therefore important to provide a sufficiently long residence time in the phase separation apparatus so that these physical processes (separation of the crude process product of nitration into two liquid phases and one gas phase) can be carried out without adversely affecting the production capacity of the plant. Due to the presence of the gas phase in the apparatus, the separation apparatus must be designed to be significantly larger than would be the case with a pure liquid-liquid separation.

According to the state of the art, the efficiency of gas-liquid or liquid-liquid phase separation apparatus can be increased by means of special internals or a special design of the inlet into the apparatus. This also applies to the phase separations in the nitrobenzene process (phase separation after the reaction and phase separations during the washes). Fixtures such as built-in panels, knitted fabrics, lamellas and fillers can even out and calm the flow and increase the surface area so that phenomena such as coalescence and the separation of droplets and bubbles take place more quickly. Entry into the phase separation apparatus can take place via baffles or baffles, which calm the flow or direct it to the apparatus wall, with the aim of increasing the residence time in the apparatus and thus increasing the separation efficiency. Common variants are described, for example, in Gulf Equipment Guides, Gas-Liquid and Liquid-Liquid Separators, Chapter 3.5 (Vessel Internais) on pages 84 to 89, year 2009, by Maurice Stewart and Ken Arnold and in Fundamentals of Natural Gas Processing, Chapter 5, pages 105 to 117, year 2011, by Arthur J Kidnay, William R. Parrish and Daniel G. McCartney. The variants described in the literature mentioned are partially explained using the example of gas-liquid phase separations, but, as far as the basic principles are concerned, they can also be used for liquid-liquid or three-phase gas-liquid-liquid separations. The disadvantage of the prior art method is that the flow calming and The condition of the fixtures, deposits and fouling can occur. For example, knitted fabrics and lamellas clog over time, and deposits form on the panels. In the event of pressure surges or excessive flow velocities, the built-in components can be damaged. Due to the corrosive media, the phase separators are usually made of enamel inside. The equipment can be damaged by the built-in components and the maintenance or repair of the equipment becomes more complex.

Operational practice has shown that problems can arise again and again in the phase separation of the crude process product of the nitration. This manifests itself, for example, in insufficient phase separation (e.g. entrainment of organic substances in the acid phase or formation of black precipitates). These problems occur to an even greater extent when the crude process products of several nitration reactors operated in parallel, in particular independently controllable, ie. H. when carrying out the reaction in several reaction lines operated in parallel (also referred to as reaction lines), are fed into a common phase separation apparatus. This approach is not uncommon in practice. A multi-line reaction in connection with a single line processing has often proven to be the best compromise between the requirements of minimizing investment costs on the one hand and the greatest possible flexibility in production on the other.

There was therefore a need to further improve the production of nitrobenzene in several reaction lines operated in parallel, in particular with regard to the efficiency of the separation of the nitration reaction product into two liquid phases and a gaseous phase. In particular, it would be desirable to design the removal of the gaseous fraction and the separation of the two liquid phases from one another as optimally as possible, both in terms of the quality of the separation and the design of the process and equipment. Also, different loads and reaction conditions on the individual reaction lines should not worsen the separation efficiency. The present invention takes this need into account both in terms of process engineering and in terms of device engineering.

Surprisingly, it was found that repeatedly observed problems in the liquid-liquid phase separation in connection with the merging of the process products of the individual reaction lines in a common phase separation apparatus (i.e. the implementation of the mixing and phase separation in one and the same apparatus) and that these problems can be dissolved or at least noticeably reduced if the mixing takes place separately from the phase separation. With fluctuating demand for the product nitrobenzene z. B. the throughputs in the individual reaction lines varied, so that different amounts from the individual lines get into the phase separation apparatus, which can lead to a reduced separation efficiency. Since the individual roads usually enter the separator at different points, the different amounts cause it different local velocities, and undesired cross and backflows which have a negative effect on the separation efficiency, if the load on a reaction line is too high, there is also high turbulence and further losses in efficiency. Different reaction conditions such as pressures, temperatures and concentrations also lead to mixing and equalization processes in the apparatus, which must run in parallel and slow down the separation of the liquid phases. In addition, problems can arise from the simultaneous removal of the gas phase. Depending on the proportion of the gaseous phase, its presence can lead to significantly higher speeds and turbulence in the liquid phases in the phase separator, which makes the separation of the two liquid phases more difficult. In the case of fluctuating proportions or an increase in the gas phase, the liquid phases can therefore be inadequately separated, so that even larger proportions of organic substances can get into the aqueous acidic phase. The following conclusions were drawn: The presence of the gas phase in the separator generally leads to high speeds (also to higher speeds of the liquid phases), since the gas phase moves at significantly higher speeds due to its lower density compared to the liquid. 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 it more difficult for the liquid phases to separate, since mixing takes place constantly at the liquid-liquid phase boundary and the liquid phase with the higher density can be carried along with the gas bubbles into the liquid phase with the lower density.

A first object of the present invention is therefore a process for continuous

Production of nitrobenzene, comprising the steps a) nitrating benzene under adiabatic conditions with sulfuric acid and nitric acid using a stoichiometric excess of benzene, based on nitric acid, in reactors connected in re parallel, re being a natural number in the range from 2 to 5, so that process products containing re nitrobenzene, benzene and sulfuric acid (henceforth also referred to as crude process products of nitration) are obtained; b) (i) Combining the process products containing nitrobenzene, benzene and sulfuric acid to form a mixed stream containing nitrobenzene, benzene and sulfuric acid, optionally additionally comprising a depletion of gaseous constituents (a) after, (β) before or (g) during the combining process ,

(ii) Introducing the mixed stream, optionally depleted in gaseous constituents (either in its unchanged totality or divided into several, in particular re, preferably 2 to 3, partial streams) into a phase separator in which 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 optionally d) evaporation of water from the sulfuric acid phase obtained in step b) while obtaining a concentrated sulfuric acid phase and using concentrated sulfuric acid phase as a component of the sulfuric acid used in step a).

A second object of the present invention is a production plant for

Implementation of the process according to the invention for the continuous production of

Nitrobenzene, the production plant comprising the following devices: a) n reactors connected in parallel for the adiabatic nitration of benzene with sulfuric acid and nitric acid using a stoichiometric excess of benzene based on nitric acid, where n is a natural number in the range from 2 to 5 , to obtain process products containing n nitrobenzene, benzene and sulfuric acid; b) (i) arranged downstream of the reactors from a), a device for combining the process products containing n-nitrobenzene, benzene and sulfuric acid to form a mixed stream containing nitrobenzene, benzene and sulfuric acid,

(ii) arranged downstream of the device for combining the process products containing n-nitrobenzene, benzene and sulfuric acid, a phase separation apparatus for separating the mixed stream obtained into a liquid aqueous sulfuric acid phase and a liquid organic nitrobenzene phase; c) a device for processing the liquid organic nitrobenzene phase from b) (ii) to nitrobenzene, this device comprising in particular the following devices:

(i) means for washing the liquid organic nitrobenzene phase and means for separating unreacted benzene;

(ii) Devices for recycling benzene separated off from c) (i) into the reactor from a) as part of the benzene used there; d) optionally, devices for concentrating the sulfuric acid phase from b) (ii) by evaporation of water and devices for recycling the concentrated sulfuric acid phase obtained thereby into the n reactors from a).

In the terminology of the present invention, the term “gaseous secondary components” encompasses at least the low boilers already mentioned above, with low boilers being understood to mean all non-aromatic, organic secondary components of the nitration process product (= step a)) whose boiling points at atmospheric pressure (1013 mbar) are 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 can also be included, such as in particular the nitrous gases already mentioned.

In the process according to the invention, the mixed stream obtained in step b) (i) is fed to the phase separation from step b) (ii), specifically without recirculating part of this mixed stream into the reaction from step a). The same applies to the process products containing n nitrobenzene, benzene and sulfuric acid before they are combined to form a mixed stream; these are also not returned to step a). A reaction loop, as described in the prior art for isothermal processes, is not the subject of the process according to the invention. The same applies, of course, to the production plant according to the invention; This does not have facilities for recycling the process products containing n nitrobenzene, benzene and sulfuric acid or the mixed stream obtained in the device for combining the process products [b) (i) J containing n nitrobenzene, benzene and sulfuric acid, into one, several or all of the n Reactors [a)].

In the attached drawings show:

FIG. 1 shows two possible configurations (FIG. La and FIG. Lb) of the device for combining the process products containing n nitrobenzene, benzene and sulfuric acid;

FIG. 2 a vertically arranged gas separator with a lateral supply of the inlet stream (b.l) and discharge of the gas phase (b.3) above and discharge of the liquid phase (b.2) below;

FIG. 3 a vertically arranged gas separator with supply of the inlet stream (bl) below and discharge of the gas phase (b.3) above and discharge of the liquid phase (b.2) on the side; FIG. 4 a vertically arranged gas separator with supply of the input stream (bl) above and discharge of the gas phase (b.3) on the side and discharge of the liquid phase (b.2) below;

FIG. 5 shows a possible embodiment of a production plant according to the invention for the case n = 2;

FIG. 6 shows a possible embodiment of a production plant according to the invention for the case re = 2 in connection with an additional gas-liquid separation;

FIG. 7 shows the grid used for the Computational Fluid Dynamics (CFD) calculations of the examples;

FIG. 8 the volume fractions of the three phases (top: aqueous phase, middle: organic phase, bottom: gas phase) in the CFD simulation of Example 1 (comparative example; without combining (= homogenization) of the crude process products and without degassing before entering the phase separation apparatus) ;

FIG. 9 the volume fractions of the three phases (top: aqueous phase, middle: organic phase, bottom: gas phase) in the CFD simulation of example 2 (inventive example; with combination (= homogenization) of the crude process products and without degassing before entering the phase separation apparatus );

FIG. 10 the volume fractions of the three phases (top: aqueous phase, middle: organic phase, bottom: gas phase) in the CFD simulation of example 3 (inventive example; with combination (= homogenization) of the crude process products and with degassing before entering the phase separation apparatus ).

A brief summary of various possible embodiments follows.

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

(i) washing the nitrobenzene phase and separating off unreacted benzene,

(ii) Using separated benzene as part of the benzene used in step a).

In a second embodiment of the process according to the invention, which can be combined with all other embodiments, in step a) benzene is added in a stoichiometric excess based on nitric acid in the range from 2.0% to 40%, preferably 3.0% to 30%, particularly preferably 4.0% to 25% of theory are used. In a third embodiment of the process according to the invention, which can be combined with all other embodiments, the temperature in each of the n reactors from step a) is kept in the range from 98.degree. C. to 140.degree.

In a fourth embodiment of the method according to the invention, which can be combined with all other embodiments, the method comprises the following:

(a) After combining step b) (i), introducing the mixed stream containing nitrobenzene, benzene and sulfuric acid into a gas separator in which a gaseous phase comprising benzene and gaseous secondary components is separated off and a phase which is depleted in gaseous constituents comprises nitrobenzene and sulfuric acid liquid phase remains, which is fed to step b) (ii); or

(ß) after step a) and before combining step b) (i), introducing the process products containing n nitrobenzene, benzene and sulfuric acid into n gas separators, in which n gaseous phases comprising benzene and gaseous secondary components are separated off and n gaseous ones Ingredients depleted, nitrobenzene and sulfuric acid comprising liquid phases remain, which are then fed to step b (i); or

(G) to carry out the combining of step b) (i), introducing the process products containing n nitrobenzene, benzene and sulfuric acid from step a) into a common gas separator in which a gaseous phase comprising benzene and gaseous secondary components is separated off and the Mixed stream remains as a liquid phase which is depleted in gaseous constituents and comprises nitrobenzene and sulfuric acid and is fed to step b) (ii).

In a fifth embodiment of the method according to the invention, which is a particular embodiment of the fourth embodiment, gravity separators or centrifugal separators are used to separate the gaseous phase (s) comprising benzene and gaseous secondary components.

In a sixth embodiment of the method according to the invention, which is a special embodiment of the fifth embodiment, gravity separators are used.

In a seventh embodiment of the method according to the invention, which is a special embodiment of the sixth embodiment, horizontally or vertically arranged gravity separators are used, which are nitrobenzene, benzene and sulfuric acid containing process products or the nitrobenzene, benzene and sulfuric acid-containing mixed stream are fed in from the side or from below, the gaseous phase being taken from the gravity separator as a top stream and the liquid phase from the gravity separator as a bottom stream below or from the side or being supplied from above, with the gaseous phase Phase can be taken from the gravity separator on the side and the liquid phase from the gravity separator below.

In an eighth embodiment of the method according to the invention, which is a further special embodiment of the fifth embodiment, centrifugal separators are used.

In a ninth embodiment of the method according to the invention, which is a special embodiment of the eighth embodiment, vertically arranged, cylindrical, conical or cylindrical-conical cyclones are used as centrifugal separators, through which the nitrobenzene, benzene and sulfuric acid-containing process product or the nitrobenzene, benzene and Sulfuric acid mixed stream is passed through each with a swirl generation, the gaseous phase being discharged upwards and the liquid phase downwards.

In a tenth embodiment of the method according to the invention, which can be combined with all other embodiments, provided they do not provide for a division of the mixed flow obtained in step b) (i), the entire mixed flow obtained in step b) (i) is removed from the phase separation apparatus Step b) (ii) fed at one point.

In an eleventh embodiment of the method according to the invention, which can be combined with all other embodiments, provided that these do not provide for the supply of the unchanged totality of the mixed flow obtained in step b) (i) to the phase separation apparatus at a single point, the in step b ) (i) the mixed stream obtained is divided into several (in particular into 2 to re, preferably into 2 to 3) substreams, and these substreams are fed to the phase separation apparatus from step b) (ii) at different points.

In a twelfth embodiment of the process according to the invention, which can be combined with all other embodiments, the re reactors in step a) can be regulated independently of one another.

In a thirteenth embodiment of the process according to the invention, which can be combined with all other embodiments, the reactors used in step a) are tubular reactors, which are preferably arranged vertically and each have a plurality (each preferably 2 to 15, particularly preferably 4 to 12, excluding the mixing device used for the initial mixing of benzene with nitric and sulfuric acid) having dispersing elements, wherein the tubular reactors are particularly preferably flowed through from bottom to top (ie the starting materials benzene-containing stream, sulfuric acid and nitric acid are fed to the vertically arranged tubular reactors at the bottom, and the process product containing nitrobenzene, benzene and sulfuric acid are removed from the tubular reactors at the top).

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

In a variant (a), b) arranged downstream of the device for combining the process products containing n nitrobenzene, benzene and sulfuric acid and upstream of the phase separator, a gas separator for separating the mixed flow of b) (i) into a gaseous, benzene and gaseous secondary component comprehensive phase and a mixed stream depleted in gaseous constituents, nitrobenzene, benzene and sulfuric acid, or, in a variant (β), b) downstream to the n reactors from a) and upstream to the device for combining the n nitrobenzene, benzene and sulfuric acid containing process products arranged, n gas separators operated in parallel for separating the process products of the n reactors from a) into n gaseous phases comprising benzene and gaseous secondary components and n process products containing nitrobenzene, benzene and sulfuric acid, depleted in gaseous constituents, or, in a variant (g ), b) downstream z u the n reactors from a) arranged such a device for combining the process products containing n nitrobenzene, benzene and sulfuric acid, which also acts as a gas separator for separating the process products of the n reactors from a) into a gaseous phase comprising benzene and gaseous secondary components and a mixed flow comprising nitrobenzene, benzene and sulfuric acid depleted in gaseous constituents acts.

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

In a third embodiment of the production plant according to the invention, which is compatible with all other embodiments, provided that these do not involve a division of the mixed flow from b) (i) provide, can be combined, the phase separator has a single inlet port for introducing the entire mixed flow.

In a fourth embodiment of the production plant according to the invention, which can be combined with all other embodiments, provided that these do not provide for the supply of the unchanged totality of the mixed flow from b) (i) to the phase separation apparatus at a single point, the device for combining the re nitrobenzene, benzene and sulfuric acid-containing process products and the phase separation apparatus, a distributor system for distributing the mixed flow to several (in particular 2 to re, preferably in 2 to 3) inlet nozzles attached to the phase separation apparatus.

In a fifth embodiment of the production plant according to the invention, which can be combined with all other embodiments, the reactors are from a) tubular reactors, which are preferably arranged vertically and each have several (each preferably 2 to 15, particularly preferably 4 to 12, excluding those for the initial mixing of benzene with nitric and sulfuric acid used mixing device) have dispersing elements, whereby the tubular reactors are particularly preferably flowed through from bottom to top (i.e. the starting materials benzene-containing stream, sulfuric acid and nitric acid are fed to the vertically arranged tubular reactors at the bottom, and the nitrobenzene, benzene and sulfuric acid-containing process product is taken from the top of each tubular reactor).

The embodiments briefly described above and further possible configurations of the invention are explained in more detail below. The aforementioned embodiments and other possible configurations can be combined with one another as desired, unless the context indicates otherwise.

Step a) of the process according to the invention, the nitration of a benzene-containing stream (henceforth also al) in re reactors with sulfuric acid (henceforth also a.2) and nitric acid (henceforth also a.3) using a, based on nitric acid (henceforth also a.3), stoichiometric excess of benzene, can in principle be carried out by all adiabatically operated nitration processes known from the prior art. According to the invention, two to five reactors, preferably two to three reactors, are operated in parallel.

It is preferred to first meter the nitric acid (a.3) and then the benzene-containing stream (a1) into the sulfuric acid (a.2). The prior mixing of nitric acid (a.3) and sulfuric acid (a.2) gives the so-called mixed acid, into which the benzene-containing stream (a1) is then metered in in this embodiment. The mixed acid used contains, based on the total mass of the mixed acid, preferably at least 2.0 mass% nitric acid and at least 66.0 mass% sulfuric acid, particularly preferably 2.0 mass% to 4.0 mass% nitric acid and 66.0 mass% up to 75, 0 mass% sulfuric acid.

The stoichiometric excess of benzene based on nitric acid (a.3) is preferably in the range from 2.0% to 40%, particularly preferably in the range from 3.0% to 30%, very particularly preferably in the range from 4, 0% to 25% of theory set. Theoretically, 1 mole of HNO3 reacts with 1 mole of benzene. A benzene excess of x% compared to HNO3

1 + - therefore corresponds to a molar ratio «(benzene) / re (HNO) (re = amount of substance) of - j ¹²² , ie

1 + - 1 + - for example - 1.02 with 2% benzene excess or for example - 1.40 with

40% excess benzene.

It is preferred to recover excess benzene and use part or all of it as part of the benzene-containing stream (a.l). The excess benzene is recovered before or after, in particular after, a single or multi-stage washing of the crude nitrobenzene; for further details, reference is made to the discussion of step e) below. The benzene-containing stream (a.l) is therefore preferably a mixture of benzene freshly added to the reaction (so-called fresh benzene) and recycled benzene (so-called reverse benzene). In any case, the reaction conditions are selected in particular so that the mass fraction of benzene in the benzene-containing stream (a1), based on the total mass of the benzene-containing stream (a1), is at least 90.0%, preferably at least 95.0%, particularly preferably at least 98.5%.

According to the invention, step a) is carried out under adiabatic conditions. When the reaction is carried out adiabatically, the reactor used in step a) is neither heated nor cooled; the reaction temperature results from the temperature of the reactants used and their mixing ratio to one another. The re reactors are preferably well insulated in order to reduce heat losses to a minimum. If the nitration is carried out adiabatically, the reaction temperature of the mixture reacting in each of the reactors therefore rises from the “start temperature” immediately after the first mixing of the reactants to the “end temperature” after maximum conversion and is preferably always in the range of Maintained 98 ° C to 140 ° C. The starting temperature results from the temperatures of the starting materials benzene, sulfuric and nitric acid, the concentrations of the acids used, their relative proportions and the volumetric ratio of organic phase (benzene) to aqueous phase (sulfuric and nitric acid), the so-called phase ratio. The phase ratio is also decisive for the final temperature: the smaller the phase ratio (i.e. the more sulfuric acid is present), the lower the final temperature. In the preferred use of a tubular reactor (see below), the temperature rises as a result increasing conversion along the longitudinal axis of the reactor. At the entry into the reactor, the temperature is in the lower range of the stated temperature range of 98 ° C. to 140 ° C., and at the exit of the reactor the temperature is in the upper range of the stated temperature range.

Step a) is preferably carried out in a process as described in DE 10 2008 048 713 A1, in particular paragraph [0024].

Suitable reactors for step a) are in principle all reactors known in the prior art for adiabatic nitrations, such as stirred kettles (in particular stirred kettle cascades) and tubular reactors. Tubular reactors are preferred. Particularly preferred is a tubular reactor in which several dispersing elements are arranged distributed over the length of the tubular reactor, which ensures intensive mixing of benzene, nitric acid and sulfuric acid. A vertically arranged tubular reactor is particularly preferably used in which several (preferably 2 to 15, particularly preferably 4 to 12, excluding the mixing device used for the initial mixing of benzene with nitric and sulfuric acid) dispersing elements are arranged distributed over the length of the tubular reactor. Such a tubular reactor is very particularly preferably flowed through from bottom to top. Such a reactor, as well as the form of dispersing elements which can be used, is described, for example, in EP 1 291 078 A2 (see FIG. 1 there).

It is particularly preferred to design step a) so that the n reactors can be regulated independently of one another, that is to say can be operated independently of one another. This makes it possible, with reduced demand for the product nitrobenzene, to enable production output by shutting down individual reactors.

In step b) of the process according to the invention, the process products containing n nitrobenzene, benzene and sulfuric acid (as well as secondary components which can be present as a gas phase or in solution) (henceforth also a.4.1, a.4.2, ... of step a) initially combined in a step b) (i) to form a mixed stream containing nitrobenzene, benzene and sulfuric acid (henceforth also bl). This takes place in a device for combining the process products containing nitrobenzene, benzene and sulfuric acid (a.4.1, a.4.2, ... a.4.n) obtained in the n reactors. Such a device is, for example, a container which is connected via lines to the outlet openings for the crude process products of the nitration of the reactors (shown in FIG. 1a using the example n = 2). The n crude process products of the nitration are brought together in this container. The combination can also take place in a pipeline into which the n streams (a.4.1, a.4.2, ... a.4.re) run together and their mixed stream (bl) is fed to step c) (in FIG. Lb shown using the example n = 2). It is also possible to achieve the desired mixing (= homogenization) of the n crude process products by using static internals that promote mixing or a stirrer in the device for bringing together the n crude process products.

In a preferred embodiment, before the phase separation in step b) (ii), a gas-liquid phase separation takes place to reduce the concentration of gaseous constituents. This gas-liquid phase separation takes place in a gas separator. In principle, all separators known to the person skilled in the art that enable gas-liquid separation can be used as gas separators. Apparatus possibilities for separating gaseous and liquid streams are generally known to the person skilled in the art. Details of 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, 1995 year, by Manning, Francis S. . and Thompson, Richard E. or in Gulf Equipment Guides, Gas-Liquid and Liquid-Liquid Separators, Chapters 3.3 to 3.5 on pages 72 to 103, year 2009, by Maurice Stewart and Ken Arnold. The variants described in the cited literature are partially explained using the example of three-phase gas-liquid-liquid separations, but with regard to the basic principles can also be used for gas-liquid phase separations. This preferred embodiment is distinguished by the separation of the steps (i) separation (at least most of) the gas phase from the two liquid phases and (ii) separation of the two liquid phases from one another. In this embodiment, these steps are therefore carried out in two apparatuses, the gas separator and the phase separator. With regard to the design of the apparatus, however, there may well be commonalities between the gas separator and the phase separation apparatus.

The gas separator is preferably not temperature-controlled, as a result of which the temperatures in the gas separator result from the temperature of the incoming reaction mixture. The gas separator is preferably operated at a pressure that is slightly higher than ambient pressure (“overpressure”), the pressure in the gas space of the gas separator being 50 mbar to 100 mbar, e.g. B. 80 mbar, above ambient pressure.

Gravity separators or centrifugal separators are preferably used as gas separators.

The gas-liquid phase separation can be implemented in various ways.

In a variant (a), the gas-liquid phase separation takes place after the above-described combining of the n crude process products of the nitration from step b) (i). The mixed stream (bl) containing nitrobenzene, benzene and sulfuric acid is introduced into a gas separator in which a gaseous phase comprising benzene and gaseous secondary components (henceforth also b.3) is separated off and a liquid phase which is depleted in gaseous components and comprises nitrobenzene and sulfuric acid (henceforth also b.2), which as in a sulfuric acid phase and a nitrobenzene phase to be separated mixed stream is fed to step b) (ii).

In a variant (β), the gas-liquid phase separation takes place after step a) (and before combining the n crude process products of the nitration from step b) (i)). The process products containing re nitrobenzene, benzene and sulfuric acid (a.4.1, a.4.2, ... a.4.re) are passed into re gas separators, in which re gaseous phases, benzene and gaseous secondary components (henceforth also b. 3.1, b.3.2, ... b.3.re) are separated and re liquid phases containing nitrobenzene and sulfuric acid, which are depleted in gaseous components (henceforth also b.2.1, b.2.2, ... b.2.re) remain, which are then fed to the combining of the re crude process products of the nitration of step b (i).

In a variant (g) the gas-liquid phase separation and the combination of the raw process products of the nitration take place in a common apparatus, i.e. H. the gas-liquid phase separation is a special embodiment of the combination of the re crude process products of the nitration from step b) (i). The process products containing re nitrobenzene, benzene and sulfuric acid (a.4.1, a.4.2, ... a.4.re) from step a) are passed into a common gas separator in which a gaseous phase comprising benzene and gaseous secondary components separated off and the mixed stream remains as a liquid phase depleted in gaseous constituents, comprising nitrobenzene and sulfuric acid, which is fed to step b) (ii). With this variant, spontaneous evaporation may occur if the individual liquid components of the process products (a.4.1, a.4.2, ... a.4.re) mix and have significantly different compositions and / or temperatures. Such spontaneous evaporation could disrupt both the homogenization and the gas-liquid phase separation.

Variants (a) and (β) are therefore preferred. The homogenization of the nitrated reaction solutions from the individual reaction lines before the liquid-liquid phase separation leads to a reduction or avoidance of turbulence on entry into the phase separation apparatus from step b) (ii). Undesired currents in the apparatus such as cross and back currents as well as the formation of eddies, which form due to different proportions of the three phases (aqueous, organic, gas) in the incoming reaction solutions, can be reduced or eliminated. The same applies to the gas separator. A combination before the gas-liquid phase separation is simplest in terms of apparatus; variant (a) is therefore particularly preferred.

Regardless of the variant selected, in one embodiment of the invention horizontally or vertically arranged gravity separators can be used as gas separators, which process products containing nitrobenzene, benzene and sulfuric acid (a.4.1, a.4.2, ... a.4.re) or the mixed stream containing nitrobenzene, benzene and sulfuric acid (bl) are each fed in from the side or from below, the gaseous phase comprising benzene and gaseous secondary components being the top stream and the The liquid phase depleted in gaseous components, nitrobenzene and sulfuric acid, can be taken from the gravity separator as a bottom stream below or from the side or fed from above, the gaseous phase comprising benzene and gaseous secondary components to the side of the gravity separator and the phase depleted in gaseous components comprising nitrobenzene and sulfuric acid liquid phase can be taken from the gravity separator below.

The expression “arranged horizontally or vertically” refers to the longitudinal axis of the essentially cylindrical apparatus. FIG. 2 to FIG. 4 show vertically arranged gravity separators which can be used in the gas-liquid separation step. For the sake of simplicity, the flows in the drawings are designated as for variant (a) (feed flow = b.l; gaseous phase = b.3, remaining liquid phase = b.2):

In the gas separator according to FIG. 2, the process product (b.l) 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.

In the gas separator according to FIG. 3, the process product (b.l) 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.

In the gas separator according to FIG. 4, the process product (b.l) is fed in at the top, the gas phase (b.3) is discharged to the side and the liquid phase (b.2) is discharged below.

An embodiment according to FIG. 2.

However, a centrifugal separator can also be used. Preference is given to a vertically arranged, cylindrical, conical or cylindrical-conical cyclone through which the process product containing nitrobenzene, benzene and sulfuric acid is passed with swirl generation, the gaseous phase comprising benzene and gaseous secondary components at the top and the phase depleted in gaseous constituents, Nitrobenzene and sulfuric acid comprising liquid phase is discharged downwards. The term “vertically arranged” again refers to the catch axis of the apparatus. The twist can either be generated by a tangentially arranged inlet nozzle or a baffle plate (see Fig.3.20 in Gulf Equipment Guides: Gas-liquid and liquid-liquid Separators, Stewart & Arnold, 2009, Gulf Professional Publishing).

In step b) (ii) the mixed stream obtained in step b) (i) is passed into a phase separation apparatus. This can be done in the simplest embodiment of this step, in that the mixed flow is fed to the phase separation apparatus via a single inlet port (i.e. the entire mixed flow obtained in step b) (i) is fed to the phase separation apparatus from step b) (ii) at one point, as in FIG. 5 and FIG. 6). In this case, it is advisable to dimension the inlet connection sufficiently large in order to be able to feed the single larger mixed flow (compared to the individual flows when operating without a combination) to the phase separation apparatus without high flow velocities and turbulence in the phase separation apparatus.

If the procedure according to the invention is to be introduced retrospectively into an already existing production plant with reactors connected in parallel and consequently also with inlet nozzles for the raw process products arranged in different spatial locations of the phase separator, it is preferred to use the existing inlet nozzles of the phase separator and the associated ones Pipelines can be used further by switching the device to be used according to the invention for bringing together the re crude process products (a.4.1, a.4.2, ... a.4.n) between the right reactor outlets and the right inlets into the phase separation apparatus. In this case, the mixed flow (bl) leaving the device for merging the re raw process products (a.4.1, a.4.2, ... a.4.n) can either be divided again into right partial flows, or the device for merging the re process products, unlike FIG. 1, provided with right outlet nozzle (in which case it must be ensured that the right raw process products are sufficiently mixed (= homogenized) when they reach the outlet nozzle, e.g. by dimensioning the apparatus sufficiently to provide sufficient residence time, and / or by using static built-in components that promote mixing or a stirrer). 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 a single, standardized process product of a given temperature and chemical composition (the mixed flow). By again dividing this standardized process product into substreams, the temperature and composition of the individual substreams are no longer changed compared to the single standardized process product, so that this procedure is not detrimental to the concept of the invention. Therefore, with this procedure, too, the partial flows fed to the phase separation apparatus lie with regard to their speeds, temperatures and chemical compositions at the inlets of the phase separator in homogenized form.

The procedure with several inlet nozzles in the phase separation apparatus also has the advantage that the speeds at the individual inlet nozzles (with the same diameter) and generally inlet and mixing processes are significantly reduced and the phase separation can begin more quickly. Therefore, even when planning a new production plant, it can be useful to divide the mixed stream containing nitrobenzene, benzene and sulfuric acid obtained in step b) (i) into several, in particular 2 to 3, preferably 2 to 3, substreams (in the same way as before described) and feed them to the phase separator at spatially separate locations.

Regardless of whether the mixed flow is fed to the phase separation apparatus in its unchanged entirety at one point or distributed over several substreams at several different points, the liquid-liquid phase separation in step b) (ii) can be carried out according to the prior art Process take place in a phase separation apparatus known in principle to the person skilled in the art. The aqueous sulfuric acid phase (henceforth also b.5) essentially contains (as a result of the formation of reaction water and the entrainment of water in the reaction from the nitric acid used) dilute sulfuric acid in addition to inorganic impurities. The organic nitrobenzene phase (henceforth also b.4) essentially contains nitrobenzene in addition to excess benzene and organic impurities. The phase separation apparatus is preferably provided with a gas outlet, via which any gaseous constituents that may be present can be discharged (if not previously separated in the preferred gas-liquid separation). The gas outlet of the gas separator, if present, and the gas outlet of the phase separation apparatus from step b) (ii) preferably open into a common exhaust gas processing device. The phase separation apparatus from step b) (ii) is preferably not temperature-controlled and is preferably operated at a slight excess pressure (preferably 50 mbar to 100 mbar, for example 80 mbar, above ambient pressure, measured in the gas space).

Regardless of the exact mode of operation and the exact design of the reactor in step a) and the device for combining the re process products, the gas separator, if present, and the phase separator from step b), it is preferred to include the liquid aqueous sulfuric acid obtained in step b) Phase (henceforth also b.5) by evaporation of water to a liquid aqueous phase (henceforth also dl) comprising sulfuric acid in a higher concentration compared to phase (b.5), to be returned in step a) and partly or completely as a component the sulfuric acid (a.2) used there. In this case, the sulfuric acid (a.2) used in step a) contains recycled sulfuric acid (dl) and can even consist of this in certain embodiments. This preferred process management is referred to in the terminology of the present invention as step d) and is explained in more detail below.

In step c) of the process according to the invention, the liquid phase obtained in step b) (ii) (henceforth also b.4) (the crude nitrobenzene) is worked up to obtain nitrobenzene (henceforth also c.l). This work-up can in principle be carried out as known from the prior art. A preferred procedure is outlined below:

First, the organic phase (b.4) is washed in one or more stages (step c) (i)). In a first partial step of this wash, 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; with several washing stages after each individual washing stage. During this process, the acid residues contained in the crude nitrobenzene (b.4) are washed out; therefore this process step is also referred to as acid washing. This step is well known from the prior art and is therefore only outlined briefly here. In order to carry out this acidic wash, it is preferred to recycle any aqueous streams that occur during operation.

The organic phase obtained in this way is then washed in a second partial step in an alkaline wash with an aqueous solution of a base, preferably selected from sodium hydroxide, sodium carbonate or sodium hydrogen carbonate, in one or more stages and then separated from the alkaline wash water by phase separation; with several washing stages after each individual washing stage. Sodium hydroxide solution is particularly preferably used as the aqueous base solution. This step is well known from the prior art and is therefore only outlined briefly here. The pH value of the sodium hydroxide solution used and its mass ratio to the organic phase are set so that acidic impurities (e.g. nitrophenols formed as by-products and acid residues that were not completely removed in the first step) are neutralized in the alkaline wash. The subsequent work-up of the alkaline waste water can be carried out according to the methods of the prior art, e.g. B. in accordance with the teaching of EP 1 593 654 A1 and EP 1 132 347 A2.

The organic phase thus obtained is finally washed in a third sub-step in a neutral wash in one or more stages with water and then separated from the aqueous phase by phase separation; with several washing stages after each individual washing stage. In principle, this can be done by any of the methods customary in the prior art. The wash water is preferably fully demineralized water (deionized water), particularly preferably a mixture of deionized water and steam condensate (ie a condensate of water vapor obtained by heat exchange of water with any exothermic process steps was) and very particularly preferably steam condensate used. A procedure is preferred in which electrophoresis is used in the last neutral stage of the neutral wash (see WO 2012/013678 A2).

The nitrobenzene washed in this way is finally freed from dissolved water, unreacted benzene and possibly organic impurities by further work-up (step c) (ii)). This work-up is preferably carried out by distillation, the vapors water and benzene and possibly organic impurities being driven off overhead. The vapors are cooled and fed into a separation tank. In the lower phase, water settles out and is separated off. The upper phase contains benzene and low boilers, which are returned to the reaction as return benzene (c.2). If necessary, part of this upper phase can be discharged (that is, not returned) in order to avoid excessive accumulation of low boilers. It is also possible to separate low boilers from this upper phase and to feed a low boilers-depleted return benzene to the reaction. A rectifying column is preferably used as the distillation apparatus. The bottom product of the distillation is, if necessary after a further distillation, in which nitrobenzene is obtained as a distillate (i.e. as top or side stream product), as (pure) nitrobenzene (c.l) for further applications (such as in particular hydrogenation to aniline).

As an alternative to the procedure presented here, it is also conceivable to separate off excess benzene before washing.

As already mentioned, it is preferred in a step d) to convert the liquid aqueous phase (b.5) comprising sulfuric acid obtained in step b) (ii) by evaporation of water into a liquid aqueous phase compared to phase (b.5 ) Concentrate phase (henceforth also dl) comprising sulfuric acid in a higher concentration, recirculate it partially or completely in step a) and use it as a component of the sulfuric acid (a.2) used there. This concentration of the aqueous sulfuric acid phase (b.5) can in principle take place as is known from the prior art. An embodiment is preferred in which the sulfuric acid in the aqueous phase (b.5) is concentrated in a flash evaporator (also referred to as a flash evaporator) by evaporating water into an area of reduced pressure. In the case of the adiabatic mode of operation provided according to the invention, with the correct choice of reaction conditions, it is possible in step a) to use the heat of reaction of the exothermic reaction to heat the aqueous phase (b.5) containing sulfuric acid to such an extent that the concentration and again simultaneously in the flash evaporator The temperature of the aqueous phase containing sulfuric acid can be set which it had before the reaction with benzene and nitric acid on entry into the reactor space, ie (dl) corresponds to (a.2) in terms of temperature and concentration. This is described in EP 2 354 117 A1, in particular paragraph [0045].

As already mentioned, a second subject of the present invention is a production plant for carrying out the process according to the invention for the continuous production of nitrobenzene. Preferred embodiments and configurations of the method according to the invention also apply in a corresponding manner to the production plant according to the invention. For example, the production plant according to the invention preferably comprises tubular reactors as reactors.

The accompanying drawing FIG. 5 shows a possible embodiment of the production plant according to the invention using the example n = 2.

1001, 1002: reactors

2100: device for merging the obtained in the reactors

Process products

2200 phase separator

3000 facility for sulfuric acid concentration (evaporator)

4000 sulfuric acid tank

5000 raw nitrobenzene tank

6000 devices for single or multi-stage washing of the crude nitrobenzene

7000 facility for the separation of unreacted benzene (in particular

Rectifying column)

In a particular embodiment, the production plant according to the invention additionally comprises one or more gas separators. In this case, as already described above in connection with the method according to the invention, there are several options for further designing the production plant:

(a) The production plant can have a downstream to the device for combining the nitrobenzene, benzene and obtained in the n reactors

Process products containing sulfuric acid (and upstream of the phase separation apparatus) have gas separators.

(ß) But it is also possible to follow the n reactors operated in parallel n gas separators operated in parallel, the liquid outlets of which into the device for merging the nitrobenzene, benzene and obtained in the n reactors

Process products containing sulfuric acid open.

(g) Finally, the device for bringing together the process products obtained in the n reactors and containing nitrobenzene, benzene and sulfuric acid can be designed in this way that it fulfills the functions of combining the re crude process products of nitration and the depletion of gaseous components together.

In the embodiment with additional gas-liquid separation, the production plant according to the invention thus preferably comprises, in a variant (a), b) downstream of the device for combining the process products containing n nitrobenzene, benzene and sulfuric acid and arranged upstream of the phase separation apparatus, a gas separator for separation of the mixed stream from b) (i) into a gaseous phase (b.3) comprising benzene and gaseous secondary components and a (liquid) mixed stream (b.2) which is depleted in gaseous components and comprises nitrobenzene, benzene and sulfuric acid, or, in one Variant (β), b) arranged downstream of the n reactors from a) and upstream of the device for combining the n nitrobenzene, benzene and sulfuric acid-containing process products, n gas separators operated in parallel to separate the process products of the n reactors from a) into n gaseous ones , Phases comprising benzene and gaseous secondary components (b.3.1, b.3.2, ... b.3. re) and re (liquid) process products (b.2.1, b.2.2, ... b.2. re), or, in a variant (g), b) downstream, containing nitrobenzene, benzene and sulfuric acid and depleted in gaseous components Arranged to the reactors from a), such a device for combining the process products containing re nitrobenzene, benzene and sulfuric acid, which can also be used as a gas separator (common to all re reactors) for separating the process products (a.4.1, a.4.2, ... , a.4.re) the re reactors from a) into a gaseous 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 acts.

Variant (a) is particularly preferred (see the corresponding statements above in connection with the description of the method according to the invention) and in FIG. 6 using the example of two reactors 1001 and 1002. Control valves and the like are not shown for reasons of simplicity of the drawing. By combining the individual process products (a.4.1, a.4.2) in a container (2100) in front of the gas separator (2110) and thus also upstream of the phase separation apparatus (2200), the construction of the phase separation apparatus is simplified (only one instead of at least two openings for metering in the liquid phase). Furthermore, the phase separation is facilitated, since undesired flows in the apparatus such as cross and back flows as well as vortex formation, which form due to different proportions of the three phases (aqueous, organic, gas) in the incoming reaction solutions, are reduced or eliminated. In addition, varying throughputs and reaction conditions in the individual streets with several inlet openings lead to varying and very different speeds at the inlets and thus to unknown flow conditions in the phase separation apparatus. These influences can be better controlled by prior homogenization with joint metering at one point.

If the depletion of gaseous constituents according to one of the variants (a), (β) or (g) is dispensed with, in a preferred embodiment gaseous constituents are separated to a certain extent in the phase separation of step b) (ii) by the phase separation apparatus is provided with a gas outlet through which gaseous components are discharged. This is shown in FIG. 5 indicated by the arrow “b.3” at the upper end of the phase separation apparatus (2200). The gas outlet of the phase separation apparatus from step b) (ii) preferably opens into an exhaust gas processing device.

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, that is, can be operated independently of one another. This makes it possible, with reduced demand for the product nitrobenzene, to enable production output by shutting down individual reactors. The equipment required for this (in particular control valves and their corresponding controls) are sufficiently known to the person skilled in the art.

In the simplest embodiment of the introduction of the mixed flow into the phase separation apparatus, the phase separation apparatus has a single inlet connection for the mixed flow.

If the mixed stream obtained in the device for combining the process products (2100) obtained in the reactors, as described in connection with the description of the process according to the invention as a possible embodiment, deviating from the representations in FIG. 5 and FIG. 6 are divided into several (in particular 2 to right, preferably 2 to 3) substreams and fed to the phase separation apparatus (2200) at different points, the production plant according to the invention has a distribution system downstream of the device for combining the process products (2100) obtained in the reactors with one of the outlets corresponding to the number of partial flows, and the phase separator (2200) has several inlet nozzles which are connected to the outlets of the distribution system and the number of which corresponds to the number of partial flows. Such a distribution system can be realized in a simple manner that the line to Removal of the mixed flow from the device for merging the process products (2100) obtained in the reactors opens into several lines, the number of which corresponds to the number of the desired partial flows, or by the fact that the device for merging the process products (2100) obtained in the reactors via a the number of outlet nozzles corresponding to the number of partial flows, which are connected via lines to the inlet nozzle of the phase separation apparatus.

The procedure according to the invention results in at least the following advantages: i) By bringing together the individual reaction products of the n streets in front of the phase separation apparatus and the associated homogenization, undesired flows and turbulence in the phase separation apparatus can be minimized.

ii) The n reactors can be operated independently of one another under different process conditions (throughput, pressure, temperature) without this having a negative effect on the separation performance of the phase separation apparatus.

iii) The degassing of the reaction solutions upstream of the phase separation apparatus, which is carried out in a preferred embodiment of the invention, reduces velocities and turbulence in the inlet area of the same, whereby the separation efficiency can be increased significantly.

iv) The phase separation times in the phase separation apparatus are minimized, as a result of which the investment outlay for this apparatus is lower and a production expansion in an existing plant is facilitated.

v) The improved phase separation reduces the entrainment of organic substances in the evaporator of the sulfuric acid concentration, which reduces energy consumption and the problems that are otherwise caused by these organic substances.

vi) Due to the improved phase separation, the entrainment of sulfuric acid in the crude nitrobenzene, which is sent to work-up, is reduced. This saves input materials because the sulfuric acid losses in the work-up are lower.

vii) The wastewater pollution is reduced since less sulfuric acid gets into the wastewater from step c).

viii) Flow-calming internals in the phase separation apparatus, which are susceptible to disruptive contamination and caking, can generally be dispensed with.

The present invention is illustrated below by means of examples. Examples:

In the following two examples, the positive influence of a homogenization of the incoming streams on the phase separation should be made clear. To this end, computational fluid dynamics simulations were carried out inside the three-phase flow behavior in the phase separation apparatus (apparatus 2200 in FIG. 5). In the examples discussed, three reactors are operated in parallel, the exiting streams of which enter the phase separation apparatus at three different points. For the simulation it was assumed that the three reaction products (a.4.1, a.4.2, a.4.3) occur in different amounts, since the three reactors are operated with different production capacities. While the reaction product of the first reactor contains 300 t / h aqueous phase (sulfuric acid phase), 17 t / h organic phase (nitrobenzene phase) and 0.18 t / h gas phase (especially benzene), the other two reactors emerge 225 t / h aqueous phase (sulfuric acid phase), 13 t / h organic phase (nitrobenzene phase) and 0.13 t / h gas phase from the reaction. The phase separator is operated at an absolute pressure (in the gas phase) of approx. 1.1 bar, and the incoming streams have a temperature of 130 ° C.

The 3D grid used by the phase separation apparatus with 800,000 computing cells is shown in FIG. 7 shown. It means:

100: Inlet connection (3x; the third is behind the viewing plane and is in the

Drawing not visible)

200: exit gas phase

300: Organic phase drain

400: Aqueous phase drain

For the sake of simplicity, the phase separator was depicted as a cylinder without taking into account the rounding of the side covers. Due to the axis symmetry, only half of the phase separation apparatus has to be modeled. The process products of the three reactors (a.4) flow in via inlet connections on the left side. The flow of the organic phase (b.4) is in the middle on the right-hand side. The drainage of the aqueous phase (b.5) is at the lower end. The gas phase (b.3) can be withdrawn at the top. The three-phase flow was simulated with an Euler-Euler approach, the aqueous phase being described as the continuous phase and the organic phase and the gas phase as the disperse phase. The continuity and conservation of momentum equations for all phases were solved in the simulation. A k-epsilon model was used as the turbulence model. The equations were solved unsteadily, the time steps being varied between 0.1 s and 0.001 s.

Since the exact droplet or bubble sizes at the entry into the phase separator or in the phase separator itself are unknown, a constant droplet or bubble diameter of 1 mm was assumed for both phases. Because drops and bubbles in reality of a certain amount Size distribution and if disintegration and coalescence processes occur in the apparatus, the actual particle sizes and the resulting phase proportions in the apparatus may differ. The aim of the CFD simulation is to qualitatively describe the influence of a gas phase on the flow conditions and ultimately the separation efficiency of the phase separation apparatus.

In the examples, the case without homogenization (example 1) and with homogenization (apparatus 2100, examples 2 and 3) is considered.

Example 1 ( ) :

In comparative example 1, the individual streets enter the phase separator with their respective quantities at the three inlet connections.

300 t / h aqueous phase, 17 t / h organic phase and 0.18 t / h gas phase from reactor 1; 225 t / h aqueous phase, 13 t / h organic phase and 0.13 t / h gas phase from reactors 2 and 3.

The process product of the reactor with the greater load runs to the side of the central inlet. The process products of the two reactors with the low load run to the side of the nozzle.

In FIG. 8 the volume proportions of the three phases are shown in gray scales (figure above: volume proportions aqueous phase, middle: volume proportions organic phase, bottom: volume proportions gas phase). In the figures, the individual phases are also marked with

10: aqueous phase,

20: organic phase,

30: gas phase and

40: disperse phase in which complete mixing has not yet taken place.

In the picture above it can be seen that a continuous aqueous phase forms immediately after entry, which settles downwards. The aqueous phase is, however, very far above the inlet connection and is initially entrained upwards by the rising gas flow. A continuous organic phase does not form until the very end at the outlet (300) of the organic phase, the aqueous-organic phase boundary being at the lower end of the sequence (middle figure, coherent organic phase on the right before the outlet of the organic phase 300), so that massive entrainment of the aqueous phase occurs. In the middle of the phase separator there is a large area in which all three phases are present ("disperse" phase) and turbulences are visible. It is noticeable that the organic phase is only very fine here is distributed and hardly reaches volume proportions above 5%. In the figure below, which shows the volume fractions of the gas phase, you can see that the ascending gas phase carries the aqueous and organic phases upwards. As a result, the separation takes place late in the apparatus and the high volume fraction of aqueous phase and the barely visible proportions of organic phase can be explained in this way. In total, massive entrainment of foreign phase at the individual outputs must be assumed for such an operation, especially if in reality there are proportions of smaller droplets and gas bubble sizes than the 1mm diameter simulated here, which require more time for separation.

In real production, the phase boundary can be observed through a sight glass attached to the phase separator. Under the conditions described above, very clear fluctuations in the liquid-liquid phase boundary (± 200 mm) on the right-hand side below the exit of the organic phase were observed again and again in real operation. Furthermore, rising gas bubbles are observed at the sight glass. The observed turbulence in the apparatus is greater, the more the loads of the individual reactors differ. In this procedure, an aqueous phase was solidified in the crude nitrobenzene tank (5000). The simulation is thus confirmed by the observations on the real apparatus.

Example 2 (crfindungsgcmäfi):

In example 2 according to the invention, the operation of the phase separation apparatus from example 1 was simulated taking into account an upstream homogenization, i.e. That is, it was assumed that the process products flowing into the phase separation apparatus via the three inlet nozzles are identical with regard to temperature, composition and flow rate. In example 2, the process products of the individual reactors therefore enter the phase separation apparatus in equal proportions at the three inlet connections, namely:

3 x 250 t / h aqueous phase, 14 t / h organic phase and 0.15 t / h gas phase.

The results are shown in FIG. 9 (arrangement of the figures and designations as in FIG. 8). A continuous organic phase does not form until the end of the phase separator (middle figure, coherent organic phase on the right before the organic phase runs out). However, the area has become significantly larger, and the aqueous-organic phase boundary is no longer in the area of the outlet connection. In the middle of the phase separator there is still a large area in which all three phases are present ("disperse" phase). In this area, the proportion of organic phase has increased significantly, so that phase separation already takes place here. This can also be seen in the middle and lower figure, where larger volume fractions can also be seen in this area for the organic and gas phase. Overall, the organic and gas phases are no longer so fine Distributed in the apparatus and the aqueous phase no longer reaches the top of the apparatus, but settles down more quickly. In the figure below, in which the volume fractions of the gas phase are shown, you can see that the gas phase rises upwards, but a part is also carried along far into the apparatus. Due to the low density of the gas phase (approx. 3 kg / m ³ ), despite the low mass fraction of the gas phase (0.15 t / h to 250 t / h), there is a high volume fraction in the area of the inlet and in the middle part of the decanter . The high gas content also leads to higher velocities in the liquid phases (up to 2 m / s) in the area of entry and to turbulence in the area of liquid-liquid phase separation. For such an operation, entrainment of foreign phase at the individual outlets cannot be ruled out, especially if in reality there are proportions of drops and gas bubble sizes smaller than the 1 mm diameter simulated here, which require more time for separation.

All in all, the phase separation in the apparatus has improved significantly as a result of the homogenization. Example 3 shows that this result can be further improved if gas separation is also carried out.

Example 3 (crfindungsgcmäfi):

In example 3 according to the invention, the operation of the phase separation apparatus from example 2 was simulated taking into account an 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 (the simulation therefore assumes that> 90% of the gas phase is separated, which can be easily implemented with conventional degassing devices), which corresponds to variant (a) or (ß) in real operation. In the simulation, 250 t / h of aqueous phase and 14 t / h of organic phase from each reactor continue to flow into the phase separation apparatus. The volume fractions of the three phases are shown in FIG. 10 shown (arrangement of the figures and designations as in FIGS. 8 and 9). In contrast to Example 2, it can be seen that a stable, continuous aqueous and organic phase is formed immediately after entry into the phase separation apparatus. Due to the low gas phase content, this no longer interferes with the separation process. The speeds in the area of the entrance are also significantly reduced (<1m / s). Even for small droplet diameters, the flow is so calm that an ascent and a phase separation can take place. For such an operation, entrainment of foreign phase at the individual outputs can be largely excluded.

The positive influence was also shown in real operation, where after the installation of the gas separator, the phase boundary in the phase separator could be calmed and no rising gas bubbles were visible near the outlet.

Claims

Patent claims:

1. A process for the continuous production of nitrobenzene, comprising the steps of a) nitration of benzene under adiabatic conditions with sulfuric acid and nitric acid using a, based on nitric acid, stoichiometric excess of benzene in n reactors connected in parallel, where n is a natural number in the range of 2 to 5, so that process products containing n nitrobenzene, benzene and sulfuric acid are obtained; b) (i) combining the process products containing n nitrobenzene, benzene and sulfuric acid to form a mixed stream containing nitrobenzene, benzene and sulfuric acid,

optionally comprising a depletion of gaseous constituents (a) after, (ß) before or (g) during the process of combining,

(ii) introducing the mixed stream, optionally depleted in gaseous constituents, 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; c) working up the nitrobenzene phase from step b) to obtain nitrobenzene; and optionally d) evaporation of water from the sulfuric acid phase obtained in step b) while obtaining a concentrated sulfuric acid phase and using concentrated sulfuric acid phase as a component of the sulfuric acid used in step a).

2. The method according to claim 1, in which the working up of the nitrobenzene phase in step c) comprises:

(i) washing the nitrobenzene phase and separating off unreacted benzene,

(ii) Using separated benzene as part of the benzene used in step a).

3. The method according to claim 1 or 2, in which in step a) benzene is used in a stoichiometric excess based on nitric acid in the range from 2.0% to 40% of theory.

4. The method according to any one of the preceding claims, wherein the temperature in each of the n reactors from step a) is maintained in the range from 98 ° C to 140 ° C.

5. The method according to any one of the preceding claims, comprising:

(g) to carry out the combining of step b) (i), introducing the process products containing n nitrobenzene, benzene and sulfuric acid from step a) into a common gas separator in which a gaseous phase comprising benzene and gaseous secondary components is separated off and the mixed stream as The liquid phase depleted in gaseous constituents and comprising nitrobenzene and sulfuric acid remains, which is fed to step b) (ii).

6. The method according to any one of the preceding claims, in which the entire mixed stream obtained in step b) (i) is fed to the phase separation apparatus from step b) (ii) at one point.

7. The method according to any one of claims 1 to 5, in which the mixed stream obtained in step b) (i) is divided into several substreams and these substreams are fed to the phase separation apparatus from step b) (ii) at different points.

8. The method according to any one of the preceding claims, in which the n reactors in step a) can be regulated independently of one another.

9. The method according to any one of the preceding claims, in which the n reactors in step a) are tubular reactors.

10. Production plant for carrying out a process for the continuous production of nitrobenzene according to one of the preceding claims, wherein the production plant comprises the following devices: a) n parallel-connected reactors for the adiabatic nitration of benzene with sulfuric acid and nitric acid using a stoichiometric based on nitric acid Excess of benzene, where n is a natural number in the range from 2 to 5, to obtain n process products containing nitrobenzene, benzene and sulfuric acid; b) (i) arranged downstream of the reactors from a), a device for combining the process products containing n-nitrobenzene, benzene and sulfuric acid to form a mixed stream containing nitrobenzene, benzene and sulfuric acid,

(ii) arranged downstream of the device for combining the process products containing n-nitrobenzene, benzene and sulfuric acid, a phase separation apparatus for separating the mixed stream obtained into a liquid aqueous sulfuric acid phase and a liquid organic nitrobenzene phase; c) a device for working up the liquid organic nitrobenzene phase from b) (ii) to nitrobenzene; d) optionally, devices for concentrating the sulfuric acid phase from b) (ii) by evaporation of water and devices for recycling the concentrated sulfuric acid phase obtained thereby into the n reactors from a).

11. Production plant according to claim 10, comprising in a variant (a), b) arranged downstream of the device for combining the process products containing n nitrobenzene, benzene and sulfuric acid and upstream of the phase separation apparatus, a gas separator for separating the mixed stream from b) (i) into a gaseous phase comprising benzene and gaseous secondary components and a mixed flow comprising nitrobenzene, benzene and sulfuric acid depleted in gaseous components, or, in a variant (β), b) downstream to the n reactors from a) and upstream to the device for Combine the n containing nitrobenzene, benzene and sulfuric acid Process products arranged, n gas separators operated in parallel for separating the process products of the n reactors from a) into n gaseous phases comprising benzene and gaseous secondary components and n process products containing nitrobenzene, benzene and sulfuric acid, depleted in gaseous components, or, in a variant (g) , b) arranged downstream of the n reactors from a), such a device for combining the process products containing n nitrobenzene, benzene and sulfuric acid, which also acts as a gas separator for separating the process products of the n reactors from a) into a gaseous, benzene and gaseous form Phase comprising secondary components and a mixed stream depleted in gaseous constituents, comprising nitrobenzene, benzene and sulfuric acid, functions.

12. Production plant according to claim 10 or 11, in which the n reactors from a) can be regulated independently of one another.

13. Production plant according to one of claims 10 to 12, in which the phase separation apparatus has a single inlet connection for introducing the entire mixed flow.

14. Production plant according to one of claims 10 to 13, in which between the device for combining the process products containing n nitrobenzene, benzene and sulfuric acid and the phase separation apparatus, a distributor system for distributing the mixed flow to a plurality of inlet nozzles attached to the phase separation apparatus is arranged.

15. Production plant according to one of claims 10 to 14, in which the n reactors are tubular reactors.