Continuous preparation method of 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, using a stoichiometric excess of benzene based on the nitric acid; b) firstly separating a gas phase comprising benzene and gaseous secondary components from the crude nitration process product in a gas separator provided specifically for this purpose, and then separating the liquid phase obtained thereby, which is depleted in gaseous components and comprises nitrobenzene and sulfuric acid, into a sulfuric acid phase and a nitrobenzene phase in a phase separation device arranged downstream; c) after-treatment of the nitrobenzene phase to obtain nitrobenzene. The invention also relates to a production plant suitable for carrying out the process according to the invention.

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, using a stoichiometric excess of benzene based on the nitric acid; b) firstly separating a gas phase comprising benzene and gaseous secondary components from the crude nitration process product in a gas separator provided specifically for this purpose, and then separating the liquid phase obtained in this way, which is depleted in gaseous components and comprises nitrobenzene and sulfuric acid, into a sulfuric acid phase and a nitrobenzene phase in a phase separation device 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 types of processes are distinguished here, the "isothermal" mode and the "adiabatic" mode:

[0004] In isothermal mode, the (considerable) heat of reaction of the nitration is removed as much as possible by indirect cooling with the aid of a heat transfer medium. US 3,092,671 B1 describes an isothermal process for preparing nitrobenzene in which a reaction loop is used. In this process - see Figure 1 and explanatory text paragraph - Benzene and a mixture of sulfuric acid and nitric acid are pumped through a nitration reactor (4) by means of a centrifugal pump (1) and reacted. The nitration reactor (4) is designed as a heat exchanger in which the reaction temperature is kept constant 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 is fed to an equalization vessel (11) which is operated without pressurization (see discharge line 19). The first part of the liquid product mixture leaving the equalization vessel (11) - without separation of the acid phase - is returned to the reaction via a first discharge line (16). Thus, the equalization vessel (11) together with the discharge line (19) ensures that the centrifugal pump (1) is also supplied with the same amount of returned product mixture in the event of fluctuations in the production process (at least for a limited time) and that no impermissibly high pressures are formed. No gas separation in the true sense takes place in this equalization vessel, since due to the low reaction temperature, the formation of any significant proportion of the gas phase would not be expected at all; finally, the boiling point of benzene at standard pressure is 80°C, which is significantly higher than the maximum temperature given of 65.6°C. The described arrangement of the equalization vessel with unpressurized operation is therefore due to a special (circulation) mode.

[0005] The second part of the liquid product mixture leaving the equalization vessel (11) is conveyed via a second discharge line (13) having a siphon equipped with a siphon breaker (15) and introduced via an overflow line (14) into a phase separation device ( Figure 1The siphon breaker (15) is obviously intended to prevent the contents of the equilibration vessel (11) from being accidentally sucked out by reducing the suction effect. completely Drain into a phase separation apparatus.

[0006] In the case of adiabatic mode, which is more common at present and is also used in the present invention, the cooling of the nitration reactor is omitted, so once the inevitable heat loss is ignored, the exothermic heat of the reaction is quantitatively reflected in the temperature difference between the temperature when entering the nitration reactor and the temperature of the product mixture of the complete reaction (the so-called adiabatic temperature jump). In order not to make this temperature rise too large, a very large excess of sulfuric acid is usually used in the method of adiabatic operation. The circulation mode described in US 3,092,671 B1 is not feasible in this adiabatic mode without at least partially abandoning the economic advantage of the method, because here due to the adiabatic temperature jump, the temperature of the reaction product is significantly higher than the temperature of the mixed reactants before the reaction starts. The relatively high temperature of the reaction product (after it is separated into an acid phase and a nitrobenzene phase) is required for the expansion evaporation of the water contained in the acid phase. As described in US 3,092,671 B1, returning a portion of the reaction product before phase separation will require cooling the returned portion, which will damage the energy balance of the method and therefore damage its economy.

[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 single 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 unreacted excess benzene is separated from the nitrobenzene as "return benzene" in the final distillation 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] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] Operating practice 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 black precipitates). These problems occur to a greater extent when the crude process products of several nitration reactors, which are operated in parallel and can be adjusted independently of one another (i.e. in the case of reactions in several parallel reaction tubes (also called reaction lines)), are fed to a common phase separation device. This practice is not uncommon in practice. Multi-channel reactions in combination with single-channel work-up usually prove to be the best compromise between the requirements of minimizing investment costs on the one hand and maximizing production flexibility on the other.

[0014] 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 Internals ), 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.

[0015] Therefore, there is a need for further improvement in the preparation of nitrobenzene, in particular with regard to the efficiency of separating the nitration reaction product into two liquid phases and one gaseous phase. It is particularly desirable to design the discharge of the gaseous component and the separation of the two liquid phases from one another as optimally as possible with regard to the quality of the separation as well as the process technology and the equipment design. The present invention takes this need into account from the perspective of process technology and equipment technology.

[0016] It was surprisingly found that a problem that was consistently observed in liquid-liquid phase separations is related to Simultaneously, the gas phase emissionRelated. Depending on the proportion of the gas phase, its presence can lead to significantly higher velocities and turbulence in the liquid phase in the phase separation device, which makes the separation of the two liquid phases difficult. Therefore, in the event of ratio fluctuations or an increase in the gas phase, the liquid phase separation may be insufficient, so that even a larger proportion of organic matter may enter the acidic aqueous phase. The following conclusion is drawn: The presence of a gas phase in the separator generally leads to high velocities (also leading to higher velocities of the liquid phase) because the gas phase moves at a significantly higher speed compared to the liquid due to the lower density. In addition, the presence of the gas phase and the resulting three-phase gas-liquid-liquid separation also impairs the separation efficiency of the two liquid phases. The rising bubbles make the stratification of the liquid phase difficult, because mixing constantly occurs again at the liquid-liquid phase boundary, and the liquid phase with a higher density can be entrained into the liquid phase with a lower density together with the bubbles.

[0017] Therefore, a first subject of the present invention is a process for the continuous preparation of nitrobenzene, wherein

[0018] a) reacting a benzene-containing stream (a.1) with sulfuric acid (a.2) and nitric acid (a.3) in a reactor under adiabatic conditions, using a stoichiometric excess of benzene based on the nitric acid (a.3), thereby obtaining a process product (a.4, hereinafter also referred to as nitrobenzene, benzene and sulfuric acid) Crude nitration process products );

[0019] b) (i) the process product of the reaction in step a) comprising nitrobenzene, benzene and sulphuric acid is first introduced into a gas separator, in which a gas phase (b.1) comprising benzene (which is used in excess and partially evaporates) and gaseous secondary components is separated from the process product, wherein a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene (and the unevaporated portion of benzene which has not reacted due to the excess use) and sulphuric acid remains, and the liquid phase is then

[0020] (ii) introduced into a phase separation apparatus and subjected thereto to phase separation, wherein a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene are removed from the phase separation apparatus;

[0021] c) Working up the liquid organic phase (b.4) comprising nitrobenzene from step b) gives nitrobenzene (c.1).

[0022] 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:

[0023] a) a reactor for the adiabatic reaction of a benzene-containing stream (a.1) with sulfuric acid (a.2) and nitric acid (a.3), using a stoichiometric excess of benzene, based on the nitric acid (a.3);

[0024] b) (i) a gas separator arranged downstream of the reactor of a) for separating the process product from a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid;

[0025] (ii) a phase separation device arranged downstream of the gas separator of b)(i) for separating the process product from b)(i) into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene;

[0026] c) an apparatus for working up the liquid organic phase (b.4) comprising nitrobenzene from b)(ii) to give nitrobenzene (c.1), wherein the apparatus comprises in particular the following apparatus:

[0027] (i) an apparatus for single-stage or multi-stage washing of the liquid organic phase (b.4) comprising nitrobenzene and for separating off unreacted benzene;

[0028] (ii) an apparatus for recycling the separated benzene obtained from c)(i) to the reactor of a) as a component of the benzene-comprising stream (a.1) used there;

[0029] d) optionally, an apparatus for concentrating the liquid aqueous phase (b.3) comprising sulfuric acid from b)(ii) by evaporating water and for recycling the liquid aqueous phase (d.1) thus obtained, which comprises a higher concentration of sulfuric acid than phase (b.3), into the reactor of a) as a component of the sulfuric acid (a.2) used there.

[0030] 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.

[0031] In the method according to the invention, the Depleted gaseous components containing nitrobenzene and liquid phase of sulfuric acid (b.2) The phase separation of step b) (ii) is not carried out, that is, part of the liquid phase (b.2) is not returned to the reaction in step a). The reaction circuit as described in the prior art for isothermal processes is not the subject of the process according to the invention. Of course, the same applies to the production device according to the invention; it is not used for the gas separator [b) (i)] obtained Liquid phase depleted of gaseous components containing nitrobenzene and sulfuric acid (b.2) Equipment for recycling to reactor [a)].

[0032] In the attached picture:

[0033] Figure 1 A vertically arranged gas separator is shown, wherein the process product (a.4) is fed in at the side, the gas phase (b.1) is discharged at the top and the liquid phase (b.2) is discharged at the bottom;

[0034] Figure 2 A vertically arranged gas separator is shown, wherein the process product (a.4) is fed in at the bottom, the gas phase (b.1) is discharged at the top and the liquid phase (b.2) is discharged from the side;

[0035] Figure 3 A vertically arranged gas separator is shown, wherein the process product (a.4) is fed in at the top, the gas phase (b.1) is discharged at the side and the liquid phase (b.2) is discharged at the bottom;

[0036] Figure 4 A possible design of the production device according to the present invention is shown;

[0037] Figure 5 A possible design of a production plant according to the invention with two reactors operated in parallel is shown;

[0038] Figure 6 shows a grid used for computational fluid dynamics (CFD) calculations of an embodiment;

[0039] Figure 7 The volume ratio of the three phases (upper: aqueous phase, middle: organic phase, lower: gas phase) in the CFD simulation of Example 1 (comparative example; no degassing before entering the phase separation device) is shown;

[0040] Figure 8 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 present invention; there is degassing before entering the phase separation device) are shown.

[0041] Below first Brief Overview Various possible implementations.

[0042] In a first embodiment of the process according to the invention, which can be combined with all other embodiments, in step d), the liquid aqueous phase (b.3) comprising sulfuric acid obtained in step b) (ii) is concentrated by evaporating water to form a liquid aqueous phase (d.1) comprising a higher concentration of sulfuric acid than phase (b.3), this liquid aqueous phase (d.1) is returned to step a) and used as a component of the sulfuric acid (a.2) used there.

[0043] In a second embodiment of the process according to the invention, which can be combined with all other embodiments, the workup of the liquid organic phase (b.4) comprising nitrobenzene in step c) comprises the following steps:

[0044] (i) washing the liquid organic phase (b.4) containing nitrobenzene (single or multiple stages) and separating off unreacted benzene (c.2),

[0045] (ii) using the separated benzene (c.2) as a component of the benzene used in step a).

[0046] In a third embodiment of the process according to the invention, which can be combined with all other embodiments, the stoichiometric excess of benzene in step a) based on nitric acid is set to a value of 2.0 to 40% of theory, preferably 3.0 to 30% and particularly preferably 4.0 to 25% of theory.

[0047] In a fourth embodiment of the process according to the invention, which can be combined with all other embodiments, the temperature in the reactor of step a) is kept between 98°C and 140°C.

[0048] In a fifth embodiment of the process according to the invention, which can be combined with all other embodiments, step a) is carried out in a plurality of, preferably in 2 to 5, particularly preferably in 2 to 3, reactors operated in parallel, wherein

[0049] (a) combining the process products (a.4) comprising nitrobenzene, benzene and sulfuric acid obtained in each of these reactors before carrying out step b) (i);

[0050] or

[0051] (β) the process products (a.4) obtained in each of these reactors, comprising nitrobenzene, benzene and sulfuric acid, are introduced in step b) (i) into a respective dedicated gas separator and the liquid phase (b.2) obtained there, which is depleted in gaseous components and comprises nitrobenzene and sulfuric acid, is introduced into the respective dedicated gas separator.

[0052] (β)(1) In the phase separation device of step b)(ii)

[0053] or

[0054] (β)(2) Before carrying out step b)(ii)

[0055] merge;

[0056] or

[0057] (γ) In step b)(i), the process products (a.4) obtained in each of the reactors and comprising nitrobenzene, benzene and sulfuric acid are introduced into a common gas separator without prior combination.

[0058] In a sixth embodiment of the process according to the invention, which can be combined with all other embodiments, the gas separator used is a gravity separator or a centrifugal separator.

[0059] In a seventh embodiment of the process of the present invention, which is a special design of the sixth embodiment, the gas separator used is a gravity separator.

[0060] In an eighth embodiment of the process of the present invention, which is a special design of the seventh embodiment, a horizontally or vertically arranged gravity separator is used as the gravity separator to separate the process product (a.4) comprising nitrobenzene, benzene and sulfuric acid.

[0061] A gravity separator is fed from the side or from the bottom, wherein a gas phase (b.1) comprising benzene and gaseous secondary components is removed from the gravity separator as a top stream, and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid is removed from the gravity separator at the bottom or at the side as a bottom stream, or

[0062] A gravity separator is fed from the top, a gas phase (b.1) comprising benzene and gaseous secondary components is removed from the gravity separator at the side, and a liquid phase (b.2) depleted in gaseous constituents comprising nitrobenzene and sulfuric acid is removed from the gravity separator at the bottom.

[0063] In a ninth embodiment of the process according to the invention, which is a special design of the sixth embodiment, a centrifugal separator is used as the gravity separator.

[0064] In the tenth embodiment of the process of the present invention, which is a special design of the ninth embodiment, a vertically arranged cylindrical, conical or cylindrical-conical cyclone separator is used as a centrifugal separator, and the process product (a.4) containing nitrobenzene, benzene and sulfuric acid is guided through the centrifugal separator to generate a vortex, in which a gas phase (b.1) containing benzene and gaseous secondary components is discharged upward, and a liquid phase (b.2) containing nitrobenzene and sulfuric acid depleted in gaseous components is discharged downward.

[0065] In an eleventh embodiment of the process according to the invention, which can be combined with all other embodiments, a tubular reactor is used as the reactor in step a), which is preferably arranged vertically and has a plurality (preferably 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 (a.1), sulfuric acid (a.2) and nitric acid (a.3) are fed to the vertically arranged tubular reactor at the bottom and the process product (a.4) comprising nitrobenzene, benzene and sulfuric acid is removed from the tubular reactor at the top). If the process comprises the use of a plurality of reactors, as in the fifth embodiment, this preferably applies to all reactors.

[0066] exist Production device of the present invention In a first embodiment, it has the following:

[0067] a) a plurality of, preferably two to five, particularly preferably two to three, reactors operated in parallel (and in particular adjustable independently of one another) for the adiabatic reaction of a benzene-containing stream (a.1) with sulfuric acid (a.2) and nitric acid (a.3), using a stoichiometric excess of benzene, based on the nitric acid (a.3), and

[0068] In variant (α),

[0069] b) (i-0) a device arranged downstream of the reactor of a) for combining the process products obtained in these reactors, comprising nitrobenzene, benzene and sulfuric acid, into a mixed stream and for introducing the mixed stream into

[0070] (i) a gas separator arranged downstream of the device for combining and introducing b)(i-0) for separating the mixed flow from b)(i-0) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and

[0071] (ii) a phase separation device arranged downstream of the gas separator of b)(i), for separating the liquid phase (b.2) depleted in gaseous components and comprising nitrobenzene and sulfuric acid into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene;

[0072] Or, in variant (β),

[0073] b) (i) a plurality of gas separators operated in parallel arranged downstream of the reactors of a) for separating the process product from each of the reactors of a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and

[0074] (ii) a phase separation device arranged downstream of the gas separator of b)(i) for separating the liquid process product from b)(i) into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene,

[0075] wherein optionally between the plurality of gas separators operated in parallel of b)(i) and the phase separation device of b)(ii) a device for combining the liquid phases (b.2) obtained in the gas separators and depleted in gaseous components and comprising nitrobenzene and sulfuric acid is arranged;

[0076] Or, in variant (γ),

[0077] b) (i) a gas separator (common to all reactors) arranged downstream of the reactor of a) for separating the process product of the reactor of a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and

[0078] (ii) a phase separation device arranged downstream of the gas separator of b)(i), for separating the liquid phase (b.2) containing nitrobenzene and sulfuric acid depleted in gaseous components from b)(i) into a liquid aqueous phase (b.3) containing sulfuric acid and a liquid organic phase (b.4) containing nitrobenzene.

[0079] In a second embodiment of the production plant according to the invention, which can be combined with all other embodiments, the reactor of a) is a tubular reactor which is preferably arranged vertically and has a plurality of (preferably 2 to 15, particularly preferably 4 to 12, not including mixing devices for the initial mixing of benzene with nitric acid and sulfuric acid) dispersing elements, wherein particularly preferably the flow through the tubular reactor is from bottom to top (i.e. the starting material benzene-containing stream (a.1), sulfuric acid (a.2) and nitric acid (a.3) are fed to the vertically arranged tubular reactor at the bottom and the process product (a.4) comprising nitrobenzene, benzene and sulfuric acid is removed from the tubular reactor at the top). If the production plant comprises a plurality of reactors, as in the first embodiment, this preferably applies to all reactors.

[0080] 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.

[0081] Step a) of the method of the invention, The benzene-containing stream (a.1) is nitrated in a reactor with sulfuric acid (a.2) and nitric acid (a.3). Based on nitric acid (a.3), a stoichiometric excess of benzene is used. In principle, this can be carried out by all adiabatically operated nitration processes known from the prior art.

[0082] 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.

[0083] 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.

[0084] 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 (c.2) is recovered before or after, in particular after, the 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%.

[0085] 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 reactor is preferably well insulated to reduce heat losses to a minimum. In the case of adiabatic nitration, the reaction temperature of the mixture reacted in the reactor is therefore increased from the "starting temperature" immediately after the first mixing of the reactants to the "final temperature" after the maximum conversion, and is preferably always maintained 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 a tubular reactor (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.

[0086] 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] .

[0087] 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 this regard, in which a plurality of dispersing elements are distributed over the length of the tubular reactor, thereby ensuring intensive 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 mixing equipment 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 forms of dispersing elements that can be used are described, for example, in EP 1 291 078 A2 (see there). Figure 1 ).

[0088] In step b) of the process according to the invention, the process product (a.4) of step a) comprising nitrobenzene, benzene and sulfuric acid (and secondary components which may be present in the gas phase or dissolved) is first subjected to (i) Depleted gaseous components and subsequently (ii) The phases separate in the separation device.

[0089] Step b)(i), The gas-liquid phase separation is carried out in a gas separator. In principle, all separators known to the person skilled in the art which are capable of achieving a gas-liquid separation can be used as gas separators. The possible equipment for separating gaseous and liquid streams is 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 Manning, Francis S. and Thompson, Richard E., Oilfield Processing, Crude Oil, Vol. 2, Chapter 6, pp. 79 to 112, 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 the basic principle can also be applied to gas-liquid phase separation. Essential to the invention is the separation of: (i) Separate (at least most of) the two liquid phases gas phase and (ii) Separate the two liquid phases from each otherTherefore, according to the invention, 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.

[0090] Preference is given to using a gravity separator or a centrifugal separator as the gas separator.

[0091] In one embodiment of the present invention, the gas separator is a horizontally or vertically arranged gravity separator, and the process product (a.4) of the reaction of step a) comprising nitrobenzene, benzene and sulfuric acid

[0092] A gravity separator is fed from the side or from the bottom, wherein a gas phase (b.1) comprising benzene and gaseous secondary components is removed from the gravity separator as a top stream, and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid is removed from the gravity separator at the bottom or at the side as a bottom stream, or

[0093] A gravity separator is fed from the top, a gas phase (b.1) comprising benzene and gaseous secondary components is removed from the gravity separator at the side, and a liquid phase (b.2) depleted in gaseous constituents comprising nitrobenzene and sulfuric acid is removed from the gravity separator at the bottom.

[0094] The expression “horizontally or vertically arranged” relates here to the longitudinal axis of the essentially cylindrical device. Figures 1 to 3 A vertically arranged gravity separator that can be used in step b) (i) is shown:

[0095] In the gas separator according to FIG. 1 , the process product ( a.4 ) is fed in from the side, the gas phase ( b.1 ) is discharged at the top and the liquid phase ( b.2 ) is discharged at the bottom.

[0096] In accordance with Figure 2 In a gas separator of this type, the process product (a.4) is fed in at the bottom, the gas phase (b.1) is removed at the top and the liquid phase (b.2) is removed at the side.

[0097] In the gas separator according to FIG. 3 , the process product ( a.4 ) is fed in at the top, the gas phase ( b.1 ) is removed at the side and the liquid phase ( b.2 ) is removed at the bottom.

[0098] Best Figure 2 design plan.

[0099] However, it is also possible to use a centrifugal separator. Preference is given to vertically arranged cylindrical, conical or cylindrical-conical cyclone separators, through which the process product (a.4) comprising nitrobenzene, benzene and sulfuric acid is guided, generating a vortex, in which a gas phase 5 (b.1) comprising benzene and gaseous secondary components is discharged upwards and a liquid phase (b.2) 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 Gulf Equipment Guides: Gas-liquid and liquid-liquid Separators, Stewart & Arnold, 2009, Gulf Professional Publishing). Figure 3 .20).

[0100] The liquid-liquid phase separation in step b)(ii) can be carried out in a phase separation apparatus known in principle to the person skilled in the art according to methods known per se from the prior art. The aqueous phase ((b.3) comprises essentially (due to the formation of water of reaction and water introduced into the reaction by the nitric acid used) diluted sulfuric acid and inorganic impurities, the organic nitrobenzene phase (b.3) comprises essentially nitrobenzene and excess benzene and organic impurities. The phase separation apparatus preferably has a gas outlet, through which residual proportions of gaseous components that are optionally present can be discharged. The gas outlet of the gas separator of step b)(i) and the gas outlet of the phase separation apparatus of step b)(ii) preferably lead to a common exhaust gas aftertreatment apparatus. The gas separator is preferably not temperature-controlled, whereby the temperature in the gas separator is generated by the temperature of the inflowing reaction mixture (see step a)). The gas separator is preferably operated at a pressure slightly elevated 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. The phase separation apparatus of step b) (ii) is also preferably not temperature-controlled and is preferably operated under a slightly positive pressure, wherein in particular the same values ​​as in the gas separator are adhered to (ie 50 to 100 mbar above ambient pressure, for example 80 mbar, measured in the gas space).

[0101] Independent of the exact design and exact design of the reactor in step a) and the gas separator and the phase separation device in step b), the liquid aqueous phase (b.3) containing sulfuric acid obtained in step b) is preferably concentrated by evaporating water to a liquid aqueous phase (d.1) containing sulfuric acid at a higher concentration than phase (b.3), which is recycled to 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) thus contains the recycled sulfuric acid (d.1) and in certain embodiments can even consist of it. 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.

[0102] Working up the liquid phase (b.4) (crude nitrobenzene) obtained in step b) (ii) in step c) of the process according to the invention, Obtain nitrobenzene (c.1) The post-treatment can in principle be carried out as known in the prior art. A preferred process is described as follows:

[0103] First, the organic phase (b.4) is subjected to a single-stage or multi-stage washing ( Step c) (i) ).exist The first sub-step of the washing In 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 off, 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.

[0104] Then, the organic phase thus obtained Second Son In the step, a single-stage or multi-stage washing is carried out with an aqueous solution of an alkali in the 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 the methods of the prior art, for example, according to the teachings of EP 1 593 654 A1 and EP 1 132 347 A2.

[0105] Finally, the organic phase thus obtained is The third sub-step In the neutral washing, the washing is carried out in a single or multiple stages with water and then separated from the aqueous phase by phase separation; in the case of multiple washing stages, the aqueous 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 steam condensate is used. It is preferred to use an electrophoretic method in the last neutralization stage of the neutral washing (see WO 2012 / 013678 A2).

[0106] The nitrobenzene washed in this way is then further treated by (Step c)(ii) Remove dissolved water, unreacted 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 vessel. 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. 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, hydrogenation of aniline) as (pure) nitrobenzene (c.1).

[0107] As an alternative to the procedure described here, it is also conceivable to separate off excess benzene before the washing.

[0108] As already mentioned, preferably in step d) The liquid aqueous phase comprising sulfuric acid obtained in step b)(ii) is (b.3) is concentrated by evaporation of water into a liquid aqueous phase (d.1) containing a higher concentration of sulfuric acid than phase (b.3), which is partially Or it can be recycled in its entirety into step a) and used as a component of the sulphuric acid (a.2) used there. This concentration of the aqueous phase (b.3) can in principle be carried out as known in the prior art. Preferably, the sulfuric acid in the aqueous phase (b.3) 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.3) containing sulfuric acid can be heated so strongly with the reaction heat 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] .

[0109] The reactor can be operated in parallel (and in particular independently of one another) (i.e. in a plurality of so-called Reaction pipeline or reaction pipeline In this case, the nitration in step a) can be carried out.

[0110] (a) combining the process products (a.4) comprising nitrobenzene, benzene and sulfuric acid obtained in each of these reactors before carrying out step b) (i);

[0111] or

[0112] (β) the process products (a.4) obtained in each of these reactors, comprising nitrobenzene, benzene and sulfuric acid, are introduced in step b) (i) into a respective dedicated gas separator and the liquid phase (b.2) obtained there, which is depleted in gaseous components and comprises nitrobenzene and sulfuric acid, is introduced into the respective dedicated gas separator.

[0113] (β)(1) In the phase separation device of step b)(ii)

[0114] or

[0115] (β)(2) Before carrying out step b)(ii)

[0116] merge,

[0117] or

[0118] (γ) In step b)(i), the process products (a.4) obtained in each of the reactors and comprising nitrobenzene, benzene and sulfuric acid are introduced into a common gas separator without prior combination.

[0119] Preferably, in this embodiment, two to five reactors, particularly preferably two to three reactors, are operated in parallel.

[0120] Preferred are variants (α), (β)(2) and (γ). Homogenization of the nitrated reaction solutions from the individual reaction lines before the liquid-liquid phase separation in step b)(ii) carried out in these variants leads to a reduction or avoidance of turbulence when entering the phase separation device of step b)(ii). Undesirable flows in the phase separation device, such as cross-flow and reflux and eddy formation, which are formed due to different proportions of the three phases (aqueous phase, organic phase, gas phase) in the incoming reaction solution can be reduced or eliminated. Variants (α), (β)(2) are particularly preferred because these advantages in these variants can also be achieved in the gas separator of (b)(i). Combining the nitrated reaction solutions from the individual reaction lines before step b)(i) is the simplest in terms of equipment, so variant (α) is very particularly preferred.

[0121] In variants (α), (β)(2) and (γ), a mixed stream of the liquid phase (b.2) depleted in gaseous components and comprising nitrobenzene and sulfuric acid originating from all reactors operating in parallel is fed to the phase separation device of step b)(ii). The mixed stream can be fed to the phase separation device via a (single) inlet connection. However, it is also possible to redistribute the mixed stream (b.2) into a plurality of (preferably 2 to 5, particularly preferably 2 to 3) substreams and feed these substreams to the phase separation device at spatially different locations via a number of inlet connections corresponding to the number of substreams. If the nitrated reaction solutions from the individual reactors operating in parallel are subjected to the above-mentioned homogenization once, there is only one single homogenous process product (mixed stream) with a given temperature and chemical composition. By redistributing the homogenous process product into substreams, the temperature and composition of the individual substreams no longer change relative to the single homogenous process product, so that this approach does not impair the above-mentioned advantages of the homogenization. Therefore, in this approach, the substreams fed into the phase separation device are also present in a uniform form with regard to their velocity, temperature and chemical composition at the inlet of the phase separation device. The approach of using a plurality of inlet connections into the phase separation device also has the advantage that the velocity at the individual inlet connections (at the same diameter) and generally the inflow and mixing process is significantly reduced, and the phase separation can be initiated more quickly.

[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 4 A possible embodiment of the production device according to the present invention is shown.

[0124] 1000: Reactor

[0125] 2100: Gas Separator

[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 unreacted benzene (especially distillation columns)

[0132] In a particular embodiment, the production device according to the invention comprises a plurality of parallel operation (and reactors that are independently adjustable (i.e., multiple so-called Reaction pipeline or reaction pipeline ). In this case, there are several possibilities for further designing the production device:

[0133] (α) The production device may have a device arranged downstream of the reactors operated in parallel for combining the products obtained in the reactors. Process products containing nitrobenzene, benzene and sulfuric acid of equipment. Such equipment is, for example, a container which is connected via a pipeline to the outlet openings of the reactors for the crude nitration process product. The crude nitration process product is combined in the container and then introduced into a gas separator (common to all reactors operating in parallel). Instead of a container, the individual pipelines of the outlet openings of the reactors for the crude nitration process product can also be simply fed via a connecting piece (in the simplest case of two reactors, a T-piece) into a common line leading to the gas separator. Starting from the gas separator, the production plant in this embodiment is constructed in the same way as when using a single reactor (except at most for a larger plant design, since if particularly large amounts of nitrobenzene are to be produced, a plurality of reactors operating in parallel are usually used).

[0134] (β) However, it is also possible to connect a plurality of gas separators operating in parallel downstream of a plurality of reactors operating in parallel, whose liquid outlets lead to the phase separation device (common to all gas separators operating in parallel) of b)(ii). This can be achieved (1) in such a way that a pipeline leads from the liquid outlet of each gas separator to the phase separation device, so that the liquid phases (b.2) containing nitrobenzene and sulfuric acid, which are depleted in gaseous components, are not combined until the phase separation device. However, it is preferred that (2) these liquid phases are already combined before entering the phase separation device. This is achieved by a corresponding device for combining, which can be constructed exactly as described above for the design with a single gas separator.

[0135] (γ) The process products (a.4) comprising nitrobenzene, benzene and sulfuric acid obtained in each of the multiple reactors can also be led to the product without prior combination. into a common Gas separator.

[0136] Preference is given to the variants (α), (β)(2) and (γ), particular preference to the variants (α) and (β)(2).

[0137] In the embodiment with a plurality of reaction tubes, the production plant according to the invention therefore preferably comprises

[0138] a) a plurality of reactors operated in parallel (and in particular adjustable independently of one another) for the adiabatic reaction of a benzene-containing stream (a.1) with sulfuric acid (a.2) and nitric acid (a.3), using a stoichiometric excess of benzene, based on the nitric acid (a.3), and

[0139] In variant (α),

[0140] b) (i-0) a device arranged downstream of the reactor of a) for combining the process products obtained in these reactors, comprising nitrobenzene, benzene and sulfuric acid, into a mixed stream and for introducing the mixed stream into

[0141] (i) a gas separator arranged downstream of the device for combining and introducing b)(i-0) for separating the mixed flow from b)(i-0) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and

[0142] (ii) a phase separation device arranged downstream of the gas separator of b)(i), for separating the liquid phase (b.2) depleted in gaseous components and comprising nitrobenzene and sulfuric acid into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene;

[0143] Or, in variant (β),

[0144] b) (i) a plurality of gas separators operated in parallel arranged downstream of the reactors of a) for separating the process product from each of the reactors of a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and

[0145] (ii) a phase separation device arranged downstream of the gas separator of b)(i) for separating the liquid process product from b)(i) into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene,

[0146] wherein optionally (and preferably) between the gas separators operated in parallel of b)(i) and the phase separation device of b)(ii) a device for combining the liquid phase (b.2) obtained in the gas separators depleted in gaseous components and comprising nitrobenzene and sulfuric acid is arranged;

[0147] Or, in variant (γ),

[0148] b) (i) a gas separator (common to all reactors) arranged downstream of the reactor of a) for separating the process product of the reactor of a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and

[0149] (ii) a phase separation device arranged downstream of the gas separator of b)(i), for separating the liquid phase (b.2) containing nitrobenzene and sulfuric acid depleted in gaseous components from b)(i) into a liquid aqueous phase (b.3) containing sulfuric acid and a liquid organic phase (b.4) containing nitrobenzene.

[0150] Preferably, in all designs of this embodiment, the production plant comprises two to five reactors operated in parallel, particularly preferably two to three reactors operated in parallel. In variant (β), the number of gas separators operated in parallel corresponds to the number of reactors operated in parallel.

[0151] Variant (α) is very particularly preferred and is Figure 5 In the figure, two reactors 1001 and 1002 are shown as an example. For the sake of simplifying the drawing, regulating valves and the like are not shown. By combining the various process products (a.4) in a common pipeline before the gas separator (2110) and therefore also before the phase separation device, the phase separation is facilitated because the undesirable flows in the device such as cross flow and reflux and vortex formation formed due to the different proportions of the three phases (aqueous phase, organic phase, gas phase) in the incoming reaction solution are reduced or eliminated. As discussed above in conjunction with the discussion of the method according to the present invention, Figure 5 Unlike the arrangement shown, it may be advantageous to redistribute stream b.2 into a plurality of (preferably 2 to 5, particularly preferably 2 to 3) substreams and to feed these substreams into the phase separation device at spatially different locations via a corresponding number of inlet connections. This can avoid the situation where there are a plurality of inlet connections into the phase separation device. No prior homogenization This can be a problem when there are multiple inlet openings (in the case of multiple inlet openings, varying throughputs and reaction conditions in the individual lines lead to varying and strongly different velocities at the inlet and therefore to unknown flow conditions in the phase separation device).

[0152] The method according to the present invention has at least the following advantages:

[0153] i) By degassing the reaction solution upstream of the phase separation apparatus, the velocity and turbulence in the inlet region of the phase separation apparatus are reduced, whereby the separation efficiency can be significantly increased.

[0154] ii) The phase separation time in the phase separation apparatus is minimized, whereby the investment outlay for the apparatus becomes lower or expansion of production in existing plants becomes easier.

[0155] iii) Through improved phase separation, the carryover of organics into the sulfuric acid concentration evaporator is reduced, which reduces energy consumption and reduces the problems caused by these organics.

[0156] iv) 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.

[0157] Since less sulphuric acid enters the wastewater in step c), wastewater pollution is reduced.

[0158] v) Flow-smoothing internals in phase separation apparatuses, which are susceptible to disruptive fouling and agglomeration, can generally be omitted.

[0159] vi) Optional homogenization of the reaction solution: By means of a single feed into the phase separation device, the turbulence can be better controlled and optimized than by means of multiple feeds of the reaction solution with fluctuating process conditions.

[0160] The following is intended to illustrate the present invention by way of examples.

[0161] Example:

[0162] The following two examples are intended to clarify the negative influence of the presence of a gas phase on the phase separation (Example 1) and the advantages of an upstream gas separation (Example 2). For this purpose, the phase separation device ( Figure 4 The three-phase flow behavior in the device 2200 in the reactor was simulated by computational fluid dynamics. In the example discussed, three reactors are operated in parallel. For the simulation, it is assumed that the flows leaving the three reactors (process products a.4, here including three flows a.4.1, a.4.2, a.4.3) are identical in temperature, flow rate and composition, and each contains 250 t / h of aqueous phase (sulfuric acid phase), 14 t / h of organic phase (nitrobenzene phase) and 0.15 t / h of gas phase (mainly benzene), and has a temperature of 130°C. The phase separation device is operated at an absolute pressure of about 1.1 bar (in the gas phase).

[0163] Figure 6 The 3D grid used for the phase separation device is shown in FIG. , which has 800 000 computational cells.

[0164] 100: Inlet connection pipe

[0165] 200: Gas phase outlet

[0166] 300: Organic phase outlet

[0167] 400: Water phase outlet

[0168] 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.3) is located at the lower end. The gas phase (b.1) 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.

[0169] 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.

[0170] In the embodiments, the case without a gas separator (Example 1) and the case with a gas separator (device 2100, Example 2) are considered respectively.

[0171] Embodiment 1 (comparative example):

[0172] In comparative example 1, 250 t / h of aqueous phase, 14 t / h of organic phase and 0.15 t / h of gas phase (mainly benzene) - process product (a.4) - flowed from each of the three reactors into the phase separation apparatus.

[0173] Figure 7 The volume ratios of the three phases are shown in grayscale (top: 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 identified 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] The three-phase mixture flows in at the inlet connection on the left in the image. As can be seen from the upper image, although a continuous aqueous phase is formed immediately after entering, it settles downwards. In the middle of the phase separation device there is a large area in which all three phases are present (the "dispersed" phase). This can also be seen in the middle and lower images, where the larger volume proportions of the organic and gas phases in this area can also be seen. The continuous organic phase is only formed at the end of the phase separation device (middle image, the continuous organic phase is on the right before the outlet for the organic phase). In the lower image showing the volume proportion of the gas phase, it can be seen that the gas phase rises upwards, but a part of it is still entrained into the device to a great distance. Due to the low density of the gas phase (approximately 3 kg / m³), the volume proportion of the gas phase in the inlet area of ​​the decanter and in the middle part is still high despite the low mass proportion of the gas phase (0.15 t / h in 250 t / h). Here, this high gas proportion also leads to higher velocities in the liquid phase in the inlet area (up to 2 m / s) and eddies in the liquid-liquid phase separation area. For such runs, entrainment of foreign phase at the individual outlets cannot be excluded, especially if in reality a smaller ratio of droplet and bubble sizes than the 1 mm diameter simulated here is present, which would require more time for separation.

[0179] In actual production, the phase boundary can be observed through a sight glass installed in the phase separation device. Under the above conditions, in actual operation, a clear fluctuation (± 100 mm) of the liquid-liquid phase boundary was always observed on the right side below the organic phase outlet. In addition, rising bubbles were always observed through the sight glass. Thus, the simulation was confirmed by observation 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 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 any problems with conventional degassing devices). In addition, in the 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 Figure 8(Image top: volume proportion of aqueous phase, middle: volume proportion of organic phase, bottom: volume proportion of gas phase). In contrast to Example 1, it is shown that a stable continuous aqueous phase and organic phase are formed immediately 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 smooth, so that an ascent and phase separation can be carried out. For such an operation, entrainment of foreign phases at the respective outlets can be substantially excluded.

[0182] 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, wherein a) reacting a benzene-containing stream (a.1) adiabatically with sulfuric acid (a.2) and nitric acid (a.3) in a reactor using a stoichiometric excess of benzene, based on the nitric acid (a.3), thereby obtaining a process product (a.4) comprising nitrobenzene, benzene and sulfuric acid, wherein the temperature in the reactor is maintained at 98° C. to 140° C.; b) (i) The process product (a.4) comprising nitrobenzene, benzene and sulfuric acid is first introduced into a gas separator, in which a gas phase (b.1) comprising benzene and gaseous secondary components is separated from the process product, wherein a liquid phase (b.2) depleted in gaseous components and comprising nitrobenzene and sulfuric acid is retained, and then the liquid phase is (ii) introduced into a phase separation apparatus and subjected thereto to phase separation, wherein a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene are removed from the phase separation apparatus; c) Working up the liquid organic phase (b.4) comprising nitrobenzene from step b) to give nitrobenzene (c.1).

2. A process according to claim 1, wherein in step d), the liquid aqueous phase (b.3) comprising sulfuric acid obtained in step b)(ii) is concentrated by evaporating water to form a liquid aqueous phase (d.1) comprising sulfuric acid at a higher concentration than phase (b.3), and this liquid aqueous phase (d.1) is returned to step a) and used as a component of the sulfuric acid (a.2) used there.

3. The method according to claim 1 or 2, wherein the post-treatment of the liquid organic phase (b.4) containing nitrobenzene in step c) comprises: (i) washing the liquid organic phase (b.4) comprising nitrobenzene and separating off unreacted benzene (c.2), (ii) using the separated benzene (c.2) as a component of the benzene used in step a).

4. The process according to claim 1 or 2, wherein the stoichiometric excess of benzene in step a) based on nitric acid is set to a value of 2.0% to 40% of theory.

5. The process according to claim 1 or 2, wherein step a) is carried out in a plurality of reactors operated in parallel and (a) the process products (a.4) comprising nitrobenzene, benzene and sulfuric acid obtained in each of these reactors are combined before carrying out step b) (i); or (β) the process products (a.4) obtained in each of these reactors, comprising nitrobenzene, benzene and sulfuric acid, are introduced in step b)(i) into a respective dedicated gas separator and the liquid phase (b.2) (β)(1) obtained there, which is depleted in gaseous components and comprises nitrobenzene and sulfuric acid, is introduced into a phase separation device in step b)(ii) or (β)(2) combined before carrying out step b)(ii); or (γ) In step b)(i), the process products (a.4) obtained in each of the reactors and comprising nitrobenzene, benzene and sulfuric acid are introduced into a common gas separator without prior combination.

6. The method according to claim 1 or 2, wherein a gravity separator or a centrifugal separator is used as the gas separator.

7. The method of claim 6, wherein the gas separator is a gravity separator.

8. The method according to claim 7, wherein the gravity separator is a horizontally or vertically arranged gravity separator, and the process product (a.4) comprising nitrobenzene, benzene and sulfuric acid is separated into The gravity separator is fed from the side or from the bottom, a gas phase (b.1) comprising benzene and gaseous secondary components is removed from the gravity separator as a top stream, and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid is removed from the gravity separator at the bottom or at the side as a bottom stream, or The gravity separator is fed from the top, a gas phase (b.1) comprising benzene and gaseous secondary components is withdrawn from the gravity separator at the side, and a liquid phase (b.2) depleted in gaseous constituents comprising nitrobenzene and sulfuric acid is withdrawn from the gravity separator at the bottom.

9. The method of claim 6, wherein the gas separator is a centrifugal separator.

10. The method according to claim 9, wherein the centrifugal separator is a vertically arranged cylindrical, conical or cylindrical-conical cyclone separator, through which the process product (a.4) comprising nitrobenzene, benzene and sulfuric acid is guided to generate a vortex, wherein a gas phase (b.1) comprising benzene and gaseous secondary components is discharged upward, and a liquid phase (b.2) comprising nitrobenzene and sulfuric acid depleted in gaseous components is discharged downward.

11. The process according to claim 1 or 2, wherein a tubular reactor is used as the reactor in step a).

12. A production plant for carrying out the process for the continuous preparation of nitrobenzene according to any one of the preceding claims, wherein the production plant comprises the following equipment a) a reactor for the adiabatic reaction of a benzene-containing stream (a.1) with sulfuric acid (a.2) and nitric acid (a.3), using a stoichiometric excess of benzene, based on the nitric acid (a.3); b) (i) a gas separator arranged downstream of the reactor of a) for separating the process product from a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid; (ii) a phase separation device arranged downstream of the gas separator of b)(i) for separating the process product from b)(i) into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene; c) an apparatus for working up the liquid organic phase (b.4) comprising nitrobenzene from b) (ii) to give nitrobenzene (c.1), d) optionally, an apparatus for concentrating the liquid aqueous phase (b.3) comprising sulfuric acid from b)(ii) by evaporating water and for recycling the liquid aqueous phase (d.1) thus obtained, which comprises a higher concentration of sulfuric acid than phase (b.3), into the reactor of a) as a component of the sulfuric acid (a.2) used there.

13. The production device according to claim 12, comprising a) a plurality of reactors operated in parallel for the adiabatic reaction of a benzene-containing stream (a.1) with sulfuric acid (a.2) and nitric acid (a.3), using a stoichiometric excess of benzene, based on the nitric acid (a.3), and In variant (α), b) (i-0) a device arranged downstream of the reactor of a) for combining the process products obtained in these reactors, comprising nitrobenzene, benzene and sulfuric acid, into a mixed stream and for introducing the mixed stream into (i) a gas separator arranged downstream of the device for combining and introducing b)(i-0) for separating the mixed flow from b)(i-0) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and (ii) a phase separation device arranged downstream of the gas separator of b)(i), for separating the liquid phase (b.2) depleted in gaseous components and comprising nitrobenzene and sulfuric acid into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene; Or, in variant (β), b) (i) a plurality of gas separators operated in parallel arranged downstream of the reactors of a) for separating the process products from the individual reactors of a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and (ii) a phase separation device arranged downstream of the gas separator of b)(i) for separating the liquid process product from b)(i) into a liquid aqueous phase (b.3) comprising sulfuric acid and a liquid organic phase (b.4) comprising nitrobenzene, wherein optionally between the plurality of gas separators operated in parallel of b)(i) and the phase separation device of b)(ii) a device for combining the liquid phases (b.2) obtained in the gas separators and depleted in gaseous components and comprising nitrobenzene and sulfuric acid is arranged; Or, in variant (γ), b) (i) a gas separator common to all reactors, arranged downstream of the reactor of a), for separating the process product of the reactor of a) into a gas phase (b.1) comprising benzene and gaseous secondary components and a liquid phase (b.2) depleted in gaseous components comprising nitrobenzene and sulfuric acid, and (ii) a phase separation device arranged downstream of the gas separator of b)(i), for separating the liquid phase (b.2) containing nitrobenzene and sulfuric acid depleted in gaseous components from b)(i) into a liquid aqueous phase (b.3) containing sulfuric acid and a liquid organic phase (b.4) containing nitrobenzene.

14. The production plant according to claim 12 or 13, which has a tubular reactor as the reactor.

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