WO2025045698A1

WO2025045698A1 - Reactor for the nitration of aromatic compounds, production plant comprising said reactor and nitration process using said reactor

Info

Publication number: WO2025045698A1
Application number: PCT/EP2024/073486
Authority: WO
Inventors: Jan Busch; Gordon FRANKEN; Alfred Guenkel; Michael Gattrell
Original assignee: Covestro Deutschland AG; Noram International Ltd
Current assignee: Covestro Deutschland AG; Noram International Ltd
Priority date: 2023-08-25
Filing date: 2024-08-21
Publication date: 2025-03-06
Anticipated expiration: 2026-02-25

Abstract

The present invention relates to a reactor (1000) configured to nitrate a stoichiometric excess of an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in a continuous process under adiabatic conditions, the reactor (1000) comprising a plurality of static dispersion elements (310) arranged in a spaced-apart relationship to form a plurality of reaction chambers (400) inside the reactor (1000), wherein at least one of the reaction chambers (400) has a largest cross-sectional area (420') that is larger than that of the in flow direction preceding reaction chamber (400), and at least two of the reaction chambers (400) have an internal volume that is larger than that of the in flow direction preceding reaction chamber (400). The present invention also relates to a production plant for producing a nitrated aromatic compound, comprising the reactor (1000) according to the invention, and to a continuous process for producing a nitrated aromatic compound, comprising adiabatically reacting an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in the reactor of the invention (1000) or in the production plant of the invention.

Description

REACTOR FOR THE NITRATION OF AROMATIC COMPOUNDS, PRODUCTION PLANT COMPRISING SAID REACTOR AND NITRATION PROCESS USING SAID REACTOR

The present invention relates to a reactor (1000) configured to nitrate a stoichiometric excess of an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in a continuous process under adiabatic conditions, the reactor (1000) comprising a plurality of static dispersion elements (310) arranged in a spaced-apart relationship to form a plurality of reaction chambers (400) inside the reactor (1000), wherein at least one of the reaction chambers (400) has a largest cross-sectional area (420’) that is larger than that of the in flow direction preceding reaction chamber (400), and at least two of the reaction chambers (400) have an internal volume that is larger than that of the in flow direction preceding reaction chamber (400). The present invention also relates to a production plant for producing a nitrated aromatic compound, comprising the reactor (1000) according to the invention, and to a continuous process for producing a nitrated aromatic compound, comprising adiabatically reacting an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in the reactor of the invention (1000) or in the production plant of the invention.

The nitration of aromatic compounds is practiced industrially for the production of valuable intermediate chemicals. Important examples are the nitration of benzene to mononitrobenzene (MNB) and of toluene to mononitrotoluene (MNT) and the further nitration of MNT to dinitrotoluene (DNT). Example end products include polyurethanes, dyes, additives for rubber, and agricultural chemicals.

The nitration of a starting aromatic compound is thought to involve the reaction with the nitronium ion (shown below for the example of benzene):

C₆H₆ + H₂O + NO₂ ⁺ -> C₆H₅NO₂ + H₃O⁺ (nitration reaction) [1]

See also EP 0 436 443 A2 for a more detailed discussion. The nitronium ion is generated by an equilibrium between nitric acid and a strong acid, typically sulphuric acid:

H₂SO₄ + HNO₃ NO₂ ⁺ + HSOzf + H₂O (nitronium ion formation) [2]

A mixture of nitric and sulphuric acids with a composition chosen to generate nitronium ions for nitration is commonly referred to in industry as “mixed acid” or “nitrating acid”. The overall reaction is then given by:

C₆H₆ + HNO₃ -> C₆H₅NO₂ + H₂O (overall reaction) [3]

So, the overall reaction converts the starting aromatic compound and nitric acid into a nitrated aromatic compound and water. The reaction is also highly exothermic and significant heat is released. Also, in the example above, the sulphuric acid is not consumed and so acts as a catalyst in the reaction. Though, it should be noted that the sulphuric acid is diluted by the water generated in the reaction and any water in the feed nitric acid.

An important characteristic of the reaction is that the starting aromatic compound and the resulting nitrated aromatic compounds are mildly soluble in the mixed acid phase. Because of this, the reaction relies on the small amount of aromatic compound that dissolves into the mixed acid phase for the reaction to occur. Under typical industrial conditions, the chemical reaction rate is quite fast and so the overall rate involves a mixture of mass transfer and chemical reaction with the reaction being essentially completed within the mass transfer boundary layer (Hatta number is greater than 2). The standard relationship for such a mixed mode reaction is given by:

Where: a = the interfacial area between the organic and acid phases (m²/m³), xorg = the mole fraction of reactant in the organic phase,

Corg* = the solubility of organic in the acid phase (kmole/m³), kprod = the overall or apparent chemical rate constant for product formation (m³/(kmole s)),

C_HNO3 = the bulk concentration of nitric acid (kmole/m³), and

Dorg = the diffusion coefficient of organic in the acid phase (m²/s).

Factors such as the solubility of the aromatic compound in the acid phase, the overall reaction rate and the diffusion coefficient of the aromatic compound in the acid phase are influenced by temperature and the composition of the mixed acid phase. A key factor in the achieved reaction rate is the interfacial area between the organic phase and the acid phase, generated by mechanically mixing the two phases together. Historically, this was done isothermally in cooled, stirred reactors. However, the use of a series of adiabatic, stirred reactors with a controlled amount of reaction and so a controlled temperature rise allowed the heat of reaction to be used to re-concentrate and so re-use the sulphuric acid, avoiding the disposal of large amounts of acid and so greatly improving the economics of the process. See for example US 2,256,999 (1941), US 4,021,498 (1977) and US 4,091,042 (1978).

For such stirred reactors at steady state with a low volume fraction of dispersed phase (< ~10%), the interfacial area is related to:

Where: = the organic phase volume fraction, p = is the acid phase density (kg/m³), o = is the interfacial tension between the organic and acid phases (N/m or J/m²),

Ci = is a constant for the mixing device, and

E = turbulent energy dissipation (W/kg or m²/s³).

At steady state, the average turbulent energy dissipation equals the inputted mixing power per kg of solution.

More recently, through the careful choice of reaction conditions, the required reaction time and so reactor volume has been decreased allowing the use of smaller volume, pipe-type reactors with static mixers resulting in improved conversions, significantly higher production rates per unit volume and minimal by-products. See for example the disclosure EP 0 436 443 A2 mentioned above.

Note that because of the highly corrosive environment due to the hot, mixed acid, the pipe-type nitration reactors used are typically fabricated from glass-lined steel and the mixing elements from tantalum. Because of the high cost of tantalum, designs therefore typically use periodic static mixers followed by reaction zones. While the use of pipe-type reactors with periodic static mixers has resulted in greatly improved performance for the nitration of aromatic compounds, the understanding and optimization of such reactors is complex. In the mixing elements, drops of the organic phase are broken into smaller sizes resulting in increased interfacial area (a) and a higher reaction rate (equation 4). In unmixed areas, drops of the organic phase slowly coalesce resulting in a decreasing interfacial area and so a slowing of the reaction rate, requiring the periodic remixing of the two phases.

Similar factors also have an impact on the relative production of undesirable, over-nitrated byproducts. Taking as an example the nitration of toluene to mononitrotoluene (MNT), for some applications the over nitration to dinitrotoluene (DNT) is undesirable. If the reaction conditions are chosen to give a fast mixed mode reaction (i.e. combined mass transfer and chemical reaction) of toluene to MNT, the further nitration of MNT to DNT will be a slow reaction occurring in the bulk of the acid phase. This reaction would then be given by: rate_DNT — k_DNTx_MNTC_MNT C_HN03 [6]

Where:

XMNT = is the mole fraction of MNT in the organic phase,

CMNT* = the solubility of pure MNT in the acid phase (kmole/m³), k_DNT = is the overall or apparent chemical rate constant for nitration DNT formation,

(m³/(kmole s)), and

= the bulk concentration of nitric acid (kmole/m³).

And so, considering equations 4 and 6, the fraction of by-product DNT produced per product MNT would be given by:

In the first bracket, the chemical rate constants, solubilities and the diffusion coefficient are affected by temperature and mixed acid composition with the net effect being complex to predict. However, a clear impact is predicted for interfacial area (a), with increased area decreasing the DNT produced per product MNT. And so, controlling the variation of interfacial area (a) as reactants move through a pipe-type reactor with periodic static mixers is also important for minimizing over-nitrated by-products.

Tantalum static mixing elements are, however, expensive and so an excessive number of mixing elements is typically avoided. Further, the required turbulent energy dissipation for breaking drops and increasing the interfacial area is related to the pressure drop across the static mixing elements. The reactor feed system must therefore provide sufficient pressure to overcome the combined pressure drops of the static mixing elements, which represents a power demand for the process as well as causing mechanical demands on pumps, seals and the reactors themselves if high pressures are required. Therefore, there is a desire to minimize the total pressure drop of the pipe-type reactor with periodic static mixers while maintaining good performance (i.e. high conversion and low by-products). Achieving this goal, requires a careful consideration of the number, location and pressure drop of each static mixing element in the pipe-type reactor.

An additional complexity arises if pipe-type reactors are operated in a horizontal configuration. The buoyancy of the organic phase relative to the mixed acid phase will cause the organic phase to accumulate at the top of the pipe, increasing the speed of the coalescence. A horizontal configuration is described in WO 01/64333 A2, which discloses a tubular reactor comprising a tube having an inlet end into which a reaction mixture enters the tubular reactor, an outlet end from which a product stream emerges, and, located in said tube between said inlet and outlet ends, a sequence of short static mixing elements separated by coalescing zones, wherein (a) the length of each static mixing element is no greater than 6 times the diameter of that static mixing element, and (b) the length of each of said coalescing zones is at least 4 times the diameter of that coalescing zone. In such a situation, the organic phase collects at the top of the horizontal pipe-type reactors requiring special, asymmetric static mixing elements to properly re-disperse the two phases. For benzene nitration to nitrobenzene, this buoyancy effect also gives rise to large differences in performance between the start and the end of the pipe-type reactor because of the large density difference between the starting benzene (approximately 0.88 kg/L) and the product MNB (approximately 1.2 kg/L) versus that of the mixed acid (approximately 1.5 kg/L). The horizontal design therefore requires large changes in the spacing of the re-mixing locations as the reaction proceeds between the inlet and the outlet of the pipe-type reactor.

This buoyancy issue can be avoided by arranging the pipe-type reactors in a vertical, up-flow orientation (so buoyant, rising organic phase drops do not move in a different direction than the mixed acid phase). Such a vertical reactor configuration is described in, for example, US 2003/0055300 A1. US 2003/0055300 A1 relates to an optimized tubular reactor for adiabatically mononitrating aromatics, halogenated aromatics and halogenated hydrocarbons, in which the tubular reactor is divided into from 4 to 12 chambers by plates which have openings and effect a pressure drop of from 0.5 to 4 bar per plate.

WO 2020/021323 A1 describes an improvement that overcomes the hydrostatic and plant layout limitations of prior art nitration reactors of the type in which fluids flow predominantly in a vertically upward direction WO 2020/021323 A1 relates to a nitration reactor incorporating sections of upward flow alternating with downward flow for use in preparing nitrated organic compounds. In this way it avoids the inefficiencies associated with organic phase drop buoyancy operating perpendicular to the flow direction. Also, through the use of the sections of upward flow alternating with downward flow, sufficient reactor residence time can be achieved with a small diameter reactor without an excessive total height and so an excessive hydrostatic head. However, such a reactor design unavoidably has local high and low points along the fluid path. This would require special valving to properly fill the reactor without leaving gas pockets and to completely drain the reactor.

US 5,616,818 describes a polynitration of an aromatic compound in a continuous process in a single apparatus under adiabatic conditions in an emulsion as the reaction medium. From 1 .3 to 3.5 mol of HNO₃ per mol of aromatic compound are introduced in the form of a nitronium ion solution into the reactor with the aromatic compound under conditions such that an emulsion forms. The emulsion, which has a tendency to coalesce, is maintained by repeated dispersion. The first dispersion of the liquid streams to produce the emulsion takes place in less than one second. At least 20% of the total amount of HNO₃ to be used should generally be present during this first dispersion. It is preferred, however, that the total amount of nitronium ion solution to be used be present at the time the aromatic compound and nitronium ion solution are first dispersed.

US 8,592,637 B2 relates to a process for continuously preparing a mononitrated organic compound, especially a process for preparing mononitrobenzene. The invention relates more particularly to an improved continuous adiabatic process for preparing nitrobenzene. The process comprises (a) feeding an aromatic compound and a mixture comprising nitric acid, sulphuric acid and water into a tubular reactor, to obtain a first reaction mixture, wherein the tubular reactor has a plurality of vertical tube bends having a deflection angle of 160° to 200°, which are arranged over a total length of the tubular reactor at a distance which corresponds to 30 to 70 times a diameter of the tubular reactor, and a number of static mixing elements which are arranged such that a Bodenstein number is less than 5 in a first part of the tubular reactor and a Bodenstein number is greater than 5 in a second part of the tubular reactor, (b) converting the first reaction mixture in the tubular reactor under an adiabatic condition to obtain a second reaction mixture, and (c) separating the second reaction mixture into an organic phase and an aqueous phase. The tubular reactor in which the process is performed comprises a number of static mixing elements which are arranged such that, in interplay with the tube bends, high backmixing exists in a first part of the tubular reactor and lower backmixing in a second part of the tubular reactor. The example reactor is 130 m long with 12 vertically oriented 180° tube bends distributed homogeneously over the length. Thus, while vertically oriented 180° tube bends are used to help keep the organic phase dispersed, the reactor is still an essentially horizontal flow reactor and so it does not avoid the inefficiencies associated with organic phase drop buoyancy operating perpendicular to the flow direction.

Further, it has been found that even in the case of an essentially vertical, up-flow orientation, where no density-driven separation of the phases occurs, the required changing of the spacing and/or the intensity of the periodic static mixing elements is less dramatic, but still important to achieving high volumetric efficiency, high production with minimal total pressure drop, and minimizing over-nitrated by-products. This is likely due to changing physical properties of the organic and acid phases as the on-going reactions cause changes in the compositions of the two phases and an increase in the temperature. These changes in physical properties will impact the ability to create small drops (higher interfacial area (see Equation 4)) and the rate of drop coalescence and so in turn influence the optimal number, location and pressure drop of static mixing elements in the pipe-type reactor.

Also, to provide high volumetric efficiency in a flow reactor the design should provide as close to plug-flow operation as possible and avoid back-mixing. Back-mixing causes dilution of the feed concentrations and so decreases the volumetric efficiency of a reactor. Also, for the case of nitration of benzene to mononitrobenzene, WO 01/64333 A2 states that back-mixing is believed to contribute to the formation of impurities such as nitrophenols (oxidation by-products).

Thus, all of the described prior art nitration reactors suffer from some kind of drawback. None of them is completely satisfactory in terms of high volumetric efficiency, high production with minimal total pressure drop, and minimal over-nitrated by-products. Thus, a need exists for further improvements in this area. In particular, it would be desirable to improve the efficiency of a nitration reactor and a nitration reaction without having to increase the overall volume of the nitration reactor.

Considering the above, the present invention provides, according to a first aspect, the following:

A reactor (1000) configured to nitrate (in particular, to mononitrate or dinitrate, preferably, to mononitrate benzene, toluene or mononitrotoluene, or to dinitrate toluene) a stoichiometric excess of an aromatic compound (1) (in particular, benzene, toluene or mononitrotoluene) with nitric acid (2) in the presence of sulfuric acid (3) in a continuous process under adiabatic conditions, thereby producing a product mixture (5) comprising an organic, unreacted aromatic compound and nitrated aromatic compound-containing phase (the nitrated aromatic compound being in particular mononitrobenzene, mononitrotoluene or dinitrotoluene) and an aqueous, sulfuric acidcontaining phase, the reactor (1000) comprising: (a) an inlet side (100) configured to continuously introduce the aromatic compound (1), nitric acid (2) and sulfuric acid (3) into the reactor (1000), and (b) an outlet side (200) configured to continuously withdraw the product mixture (5) from the reactor (1000), wherein the inlet side (100) comprises a first static dispersion element (300) which is configured to disperse the aromatic compound (1) in the nitric acid (2) and the sulfuric acid (3) to produce a reaction mixture (4) that is in a dispersed state; the reactor (1000) being configured for a continuous flow of the reaction mixture (4) from the inlet side (100) to the outlet side (200), during which nitration of the aromatic compound (1) in the reaction mixture (4) takes place, thereby producing the product mixture (5); the reactor (1000) further comprising (c) a plurality of further static dispersion elements (310) arranged in the direction of the continuous flow of the reaction mixture (4) downstream of the first static dispersion element (300) and which are configured to maintain the reaction mixture in the dispersed state or to restore the reaction mixture (4) to the dispersed state, the further static dispersion elements (310) being arranged in the reactor (1000) in a spaced-apart relationship to form a plurality of reaction chambers (400) inside the reactor (1000), wherein each reaction chamber (400) has an internal volume that is defined by: (i) a first further static dispersion element (310’), (ii) a second further static dispersion element (310”) arranged downstream of the first further static dispersion element (310’) in the direction of the continuous flow of the reaction mixture (4), the second further static dispersion element (310") being spacedapart from the first further static dispersion element (310’) (for example, by a distance d as shown in the drawings), and (iii) a surrounding wall (410), the internal volume having a cross-sectional area (420) in the direction of the continuous flow of the reaction mixture (4) which cross- sectional (420) area may vary along the direction of flow of the reaction mixture (i.e. may vary over the distance d); wherein at least one of the reaction chambers (400) has a largest cross-sectional area (420’) that is larger than that of the, in the direction of the continuous flow of the reaction mixture (4), preceding (i.e. immediately preceding) reaction chamber (400), and at least two of the reaction chambers (400) have an internal volume that is larger than that of the, in the direction of the continuous flow of the reaction mixture (4), preceding reaction chamber (400).

According to a second aspect, the present invention is directed at a production plant for producing a nitrated aromatic compound, comprising the reactor (1000) according to the invention.

According to a third aspect, the present invention is directed at a continuous process for producing a nitrated aromatic compound, comprising adiabatically reacting a stoichiometric excess of an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in the reactor of the invention (1000) or in the production plant of the invention.

Surprisingly it has been found that good performance of a nitration reactor can be achieved, thereby solving or at least mitigating the problems of the prior art, by using a reactor configuration comprising several reactions chambers, the internal volume of which increases due to an increase of the reaction chambers’ largest cross-sectional area in the direction of the continuous flow of the reaction mixture. It has been found that, in particular, in a vertical reactor arrangement, it is extremely beneficial to increase the internal volume, and hence residence time, of at least a subset (i.e. at least two of the reaction chambers have an internal volume that is larger than that of the preceding reaction chamber) of the individual reaction chambers in the direction of flow from the inlet of the reactor to the outlet, by increasing the cross sectional area of said reaction chambers, rather than increasing the length of said reaction chambers with a constant cross-sectional area. This allows the reactor volume to be uniquely distributed along the length of the reactor based on the change in physical properties of the acid and organic phases as the reaction proceeds, while ensuring the overall height of the reactor remains within the height limitation of the plant structure. In this way, a reactor may be constructed with an equal overall volume, overall height and overall operating pressure compared to the vertical reactor arrangements described in the prior art but will result in improved overall volumetric efficiency and conversion while minimizing the production of over-nitrated by-products.

In the context of the present invention, the aromatic compound in the feed is used in stoichiometric excess. In mononitrations, theoretically 1 mol of HNO₃ reacts with 1 mol of aromatic compound (e.g. benzene). A stoichiometric excess of aromatic compound means that more than 1 mol of aromatic compound is used per mol of HNO₃. Likewise, in dinitrations, theoretically 2 moles of HNO₃ react with 1 mol of aromatic compound (e.g. toluene dinitration to dinitrotoluene, DNT). A stoichiometric excess of aromatic compound means that more than 0.5 moles of aromatic compound are used per mol of HNO₃. The stoichiometric excess of aromatic compound is usually given in % in relation to HNO₃. A stoichiometric excess of aromatic compound of x% therefore

-(i + — ) corresponds to a molar ratio n(aromatic compound)/n(HNO₃) (n = molar amount) of - — , wherein m is the number of nitro groups to be introduced into the aromatic compound. Preferably, for mononitrations the aromatic compound is used in a stoichiometric excess of from 0,5 % to 50 %. Preferably, for dinitrations the aromatic compound is used in a stoichiometric excess of from 0,5 % to 50 %.

In the appended drawings:

FIG. 1a shows a first possible configuration of a reactor(IOOO) according to the invention.

FIG. 1 b shows an enlarged view of a reaction chamber (400).

FIG. 1c shows another enlarged view of a reaction chamber (400).

FIG. 2 shows a second possible configuration of a reactor (1000) according to the invention.

FIG. 3 shows a third possible configuration of a reactor (1000) according to the invention. FIG’s. 4a-d show examples of individual modules configured to be assembled together to form a reactor (1000) according to the invention. There follows firstly a brief summary of various possible embodiments.

In a first embodiment of the reactor according to the invention, which can be combined with all other embodiments, the spacing-apart of the second further static dispersion elements (310") from the first further static dispersion elements (310’) successively increases in the direction of the continuous flow of the reaction mixture (4).

In a second embodiment of the reactor according to the invention, which can be combined with all other embodiments, at least some of the reaction chambers (400) have the shape of a cylinder.

In a third embodiment of the reactor according to the invention, which can be combined with all other embodiments, at least some of the reaction chambers (400) have the shape of a truncated cone.

In a fourth embodiment of the reactor according to the invention, which can be combined with all other embodiments, at least some of the reaction chambers (400) have the shape of a double truncated cone.

In a fifth embodiment of the reactor according to the invention, which can be combined with all other embodiments unless these exclude the reactions chambers (400) having different shapes, at least some of the reaction chambers (400) have the shape of a cylinder and at least some of the reaction chambers (400) have the shape of a double truncated cone.

In a sixth embodiment of the reactor according to the invention, which is a particular configuration of the fifth embodiment, the reaction chambers (400) having the shape of a double truncated cone are arranged downstream, in the direction of the continuous flow of the reaction mixture (4), of the reaction chambers (400) having the shape of a cylinder.

In a seventh embodiment of the reactor according to the invention, which can be combined with all other embodiments, the reactor (1000) is arranged for a vertical flow of the reaction mixture (4) through the reaction chambers (400).

In an eighth embodiment of the reactor according to the invention, which is a particular configuration of the seventh embodiment, the reactor comprises two or more vertically arranged reactor sections each comprising two or more reaction chambers (400), wherein at least two reactor sections are connected to each other by horizontally arranged pipes. In a ninth embodiment of the reactor according to the invention, which can be combined with all other embodiments, each further static dispersion element (310) is configured to produce a pressure drop that decreases in the direction of the continuous flow of the reaction mixture (4).

In a tenth embodiment of the reactor according to the invention, which can be combined with all other embodiments, the first static dispersion element (300) comprises an inlet manifold, a spray injector, a perforated plate, a static mixing element, or a combination of any two thereof.

In an eleventh embodiment of the reactor according to the invention, which can be combined with all other embodiments, the further static dispersion elements (310) comprise one or more jet impingement devices, one or more orifice plates, one or more perforated plates, one or more static mixers, one or more baffles, one or more structured packings, or a combination of any two or more thereof.

In a twelfth embodiment of the reactor according to the invention, which can be combined with all other embodiments unless these are directed at a nitration other than the mononitration of benzene, the reactor (1000) is configured to mononitrate benzene (i.e. benzene is the aromatic compound (1) and the reactor (1000) is configured for mononitration).

In a thirteenth embodiment of the reactor according to the invention, which can be combined with all other embodiments unless these are directed at a nitration other than the dinitration of toluene, the reactor (1000) is configured to dinitrate toluene (i.e. toluene is the aromatic compound (1) and the reactor (1000) is configured for dinitration).

In a fourteenth embodiment of the reactor according to the invention, which can be combined with all other embodiments unless these are directed at a nitration other than the mononitration of toluene, the reactor (1000) is configured to mononitrate toluene (i.e. toluene is the aromatic compound (1) and the reactor (1000) is configured for mononitration).

In a fifteenth embodiment of the reactor according to the invention, which can be combined with all other embodiments unless these are directed at a nitration other than the mononitration of mononitrotoluene, the the reactor (1000) is configured to mononitrate mononitrotoluene (i.e. mononitrotoluene is the aromatic compound (1) and the reactor (1000) is configured for mononitration). In a first embodiment of the production plant according to the invention, which can be combined with all other embodiments, the production plant comprises: a device configured to separate the product mixture (5) into the organic, nitrated aromatic compound-containing phase and the aqueous, sulfuric acid-containing phase; a device configured to concentrate the aqueous, sulfuric acid-containing phase by evaporation of water to obtain a concentrated aqueous, sulfuric acid-containing phase; a device configured to recirculate the concentrated aqueous, sulfuric acid-containing phase to the reactor; a device configured to wash the organic, nitrated aromatic compound-containing phase to obtain a washed organic, nitrated aromatic compound-containing phase; a device configured to distill or strip the washed organic, nitrated aromatic compoundcontaining phase to obtain a fraction comprising the unreacted aromatic compound and a fraction comprising the nitrated aromatic compound; and a device configured to recirculate the fraction comprising the unreacted aromatic compound to the reactor (1000).

In a first embodiment of the process for producing a nitrated aromatic compound according to the invention, which can be combined with all other embodiments, the process comprises guiding the reaction mixture (4) through the reactor (1000) under plug-flow conditions.

In a second embodiment of the process for producing a nitrated aromatic compound, which can be combined with all other embodiments, the aromatic compound (1) comprises benzene and the reactor (1000) is configured to mononitrate the benzene, or the aromatic compound (1) comprises toluene and the reactor (1000) is configured to dinitrate the toluene, or the aromatic compound (1) comprises toluene and the reactor (1000) is configured to mononitrate the toluene, or the aromatic compound (1) comprises mononitrotoluene and the reactor (1000) is configured to mononitrate the mononitrotoluene.

The embodiments briefly outlined above and further possible configurations of the invention are elucidated in detail hereinafter. The abovementioned embodiments and further possible configurations may be combined with one another as desired, unless the opposite is stated explicitly or is clearly apparent for those skilled in the art from the context.

FIG. 1a shows a first possible configuration of a reactor according to the invention. The reactor (1000) shown is a vertical reactor having a conical shape with an increasing cross- sectional area from bottom to top. An arrangement of the reactor (1000) such that the reaction mixture (4) flows vertically through the reaction chambers (400) is generally preferred, regardless of the precise configuration of the reactor in other aspects. The overall conical shape of the reactor shown results in a succession of reaction chambers each of which is a truncated cone.

For reasons of mechanical stability it may be desirable to construct the reactor in several vertical sections that are connected to each other by (essentially horizontal) pipes (not shown in FIG. 1a; see FIG. 2 for an example). Such an arrangement is for the purposes of the present invention considered as a vertical arrangement in the aforementioned meaning and of course does not leave the scope of this embodiment.

An aromatic compound (1), nitric acid (2) and sulfuric acid (3) are fed continuously into the inlet side (100) that is arranged in the bottom of the reactor (1000) and are initially mixed in the first static dispersion element (300), thereby dispersing the aromatic compound (1) in the nitric acid (2) and the sulfuric acid (3) to produce a reaction mixture (4) that is in a dispersed state. The reaction mixture (4) travels through the reactor from bottom to top and is re-dispersed in the further dispersion elements (310). Thereby product mixture (5) is formed that is withdrawn from the reactor (1000) at the outlet side (200). Product mixture (5) essentially comprises the nitrated aromatic compound, the excess of aromatic compound used, and sulphuric acid that has been diluted as a result of the formation of water during the nitration. The space between two of these further static dispersion elements (310) is referred to in the terminology of the present invention as reaction chamber (400). This is shown in more detail in the enlargement of FIG. 1b:

Each reaction chamber (400) has an internal volume that is defined by: (i) a first further static dispersion element (310’), (ii) a second further static dispersion element (310”) arranged downstream of the first further static dispersion element (310’) in the direction of the continuous flow of the reaction mixture (4), the second further static dispersion element (310") being spaced from the first further static dispersion element (310’) by a distance d, and (iii) a surrounding wall (410). For the reaction chamber shown, the first (upstream) further dispersion element is the lower one in the drawing (labelled as 310’) and the second (downstream) further dispersion element is the upper one in the drawing (labelled as 310”). It should be noted that for the reaction chamber that follows the one shown, the upper dispersion element shown in the drawing is the first (upstream) dispersion element.

Further details of a reaction chamber (400) within the meaning of the present invention can be seen in the enlargement of FIG. 1c. The reaction chamber has the form of a truncated cone resulting in a perpetual increase of its cross-sectional area (420), the cross sectional area (420) being largest (i.e. 420=420’) at the upper (downstream) end of the reaction chamber. The distance d is the distance from the point where the reaction mixture (4) leaves the first further static dispersion element (310’) to the point where it enters the second further static dispersion element (310") (nominal distance). Due to the overall conical shape of the reactor, the largest cross- sectional area of the reaction chamber that follows (i.e. is downstream of) the one shown in FIG. 1 c is larger than area 420’ of FIG. 1c, thereby resulting in an increase of internal volume (hydraulic volume).

FIG. 2 shows another possible reactor configuration comprising three vertically arranged reactor segments being connected to each other at positions “X” and “Y” by essentially horizontal pipes. It should be noted that three reactor segments were chosen by way of example only. Each reactor segment comprises reaction chambers of the shape of double truncated cones. The largest cross- sectional area (420’) is constant for the reaction chambers of each reactor segment. However, the largest cross-sectional area (420’) of the reaction chambers of the downstream reactor segments is larger than that of the preceding, upstream reactor segment. It should be noted in this context that the invention does not require that every reaction chamber has a largest cross- sectional area (420’) that is larger than that of the preceding (immediately upstream) reaction chamber. Whilst this is the case in the embodiment of FIG. 1 , it is not required to realize the invention. Successions of reaction chambers having the same largest cross-sectional area (420’) do not leave the scope of the invention if only the inventive concept of an (overall) increasing internal volume of the reaction chambers due to an increase of the largest cross-sectional area is realized in other parts of the reactor. It is, for example, also possible that some of the reaction chambers (400) have the shape of a cylinder and some of the reaction chambers (400) have the shape of a double truncated cone, the latter ones being used to realize the increase in largest cross-sectional area. In this case it is preferred that the reaction chambers (400) having the shape of a double truncated cone are arranged downstream, in the direction of the continuous flow of the reaction mixture (4), of the reaction chambers (400) having the shape of a cylinder.

The reactor (1000) according to the invention may also encompass reaction chambers (400) having the shape of a cylinder as shown in FIG. 3. The reactor shown in this drawing shows a preferred embodiment in which the spacing-apart of the second further static dispersion elements (310") from the first further static dispersion elements (310’) successively increases in the direction of the continuous flow of the reaction mixture (4). This is realized in a first cylindrical part having reaction chambers of successively increasing distance d, thereby also increasing the internal volume of the reaction chambers in this cylindrical part (albeit without realizing the inventive concept of increasing largest cross-sectional areas in this part of the reactor). In the upper part the reactor comprises reaction chambers having a double truncated cone-like structure (with a cylindrical section in the middle part of each reaction chamber). The reactor (1000) of claim 1 , wherein the spacing-apart of the second further static dispersion elements (310") from the first further static dispersion elements (310’) successively increases in the direction of the continuous flow of the reaction mixture (4).

FIG’s. 4a-d show examples of individual modules configured to be assembled together to form a reactor according to the invention. FIG. 4a shows a cylindrical module of the length L and diameter di. FIG. 4b shows a truncated cone-like structure with a cylindrical section in the middle part of the module. In this configuration, the largest cross-sectional area is defined by diameter d₂. The diameter in the bottom and top part of the module is the same as di in FIG. 4a so as to allow assembling individual modules together. FIG. 4c shows a module as shown in FIG. 4b with the exception of having a larger diameter d₂ (i.e. d₂(FIG. 4c) > d₂(FIG. 4b)). The internal volume can also be increased by increasing the angle 01 and/or 0₂ (see FIG. 4d).

Regardless of the precise reactor configuration it is preferred that each further static dispersion element (310) is configured to produce - under given reaction conditions - a pressure drop that decreases in the direction of the continuous flow of the reaction mixture (4). This does not necessarily imply that every further static dispersion element produces a pressure drop lower than that immediately preceding it; successions of a limited number of further static dispersion elements producing the same pressure drop are not per se excluded in this embodiment. It does, however, mean that there is a significant decrease in the produced pressure drop from the further static dispersion elements near the inlet side (100) to those near the outlet side (200).

Regardless of the precise reactor configuration it is preferred that the first static dispersion element (300) comprises an inlet manifold, a spray injector, a perforated plate, a static mixing element, or a combination of any two thereof.

Regardless of the precise reactor configuration it is preferred that the further static dispersion elements (310) comprise one or more jet impingement devices, one or more orifice plates (plates having one hole), one or more perforated plates (plate having several holes), one or more static mixers, one or more baffles, one or more structured packings, or a combination of any two or more thereof.

The reactor according to the invention is suitable for the nitration of all kind of aromatics used in industry. In particular, it is preferred to use the reactor according to the invention for the mononitration of benzene, the dinitration of toluene, the mononitration of toluene or the mononitration of mononitrotoluene.

The reactor according to the invention (1000) can, according to the second aspect of the invention, easily be integrated into a production plant for producing a nitrated aromatic compound. The production plant preferably comprises in addition to the reactor (1000): a device configured to separate the product mixture (5) into the organic, nitrated aromatic compound-containing phase and the aqueous, sulfuric acid-containing phase; a device configured to concentrate the aqueous, sulfuric acid-containing phase by evaporation of water to obtain a concentrated aqueous, sulfuric acid-containing phase; a device configured to recirculate the concentrated aqueous, sulfuric acid-containing phase to the reactor; a device configured to wash the organic, nitrated aromatic compound-containing phase to obtain a washed organic, nitrated aromatic compound-containing phase; a device configured to distill or strip the washed organic, nitrated aromatic compoundcontaining phase to obtain a fraction comprising the unreacted aromatic compound and a fraction comprising the nitrated aromatic compound; and a device configured to recirculate the fraction comprising the unreacted aromatic compound to the reactor (1000).

The reactor according to the invention and the production plant according to the invention allow for carrying out - according to the third aspect of the invention - a continuous process for

(continuous nitration), comprising adiabatically reacting an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in the reactor (1000) or in the production plant, wherein the aromatic compound (1) is used in stoichiometric excess over the nitric acid (2). All aspects described above for the reactor according to the invention and the production plant according to the invention likewise apply to the process according to the invention and are not repeated here for the sake of brevity.

The process according to the invention is preferably carried out such that the reaction mixture (4) is guided through the reactor (1000) under plug-flow conditions, this being the most efficient way of producing the nitrated aromatic compounds with a minimum of secondary components. Generally, the continuous nitration can be carried out in the reactor according to the invention as known in the art for other reactors. By way of example, reference is made to the mononitration of benzene which has been exhaustively described in the art, for example in US 2010/076230 A1 (see in particular paragraphs [0027] to [0037]), US 2009/187051 A1 (see in particular paragraphs [0015] to [0023]), US 2011/196177A1 (see in particular paragraph [0022]), US 2015/175521 A1 (see in particular paragraphs [0019] to [0027]), US 2015/166460 A1 (see in particular paragraphs [0020] to [0025]), US 2015/175522 A1 (see in particular paragraphs [0020] to [0025] and [0031] to [0032]), US 2018/346405 A1 (see in particular paragraphs [0016] to [0024]), and/or US 2020/017434 A1 (see in particular paragraphs [0016] to [0067]). The aforementioned methods of the art can be used within the context of the present invention (i.e. employing the inventive reactor (1000)) as well.

A preferred embodiment of the inventive process is briefly as follows:

The continuous nitration is carried out in reactor (1000) yielding product mixture (5), followed by phase separation thereof into an organic, nitrated aromatic compound-containing phase and an aqueous, sulfuric acid-containing phase. The aqueous, sulfuric acid-containing phase is then concentrated by evaporation of water to obtain a concentrated aqueous, sulfuric acid-containing phase, which is recirculated to the reactor (1000). The organic, nitrated aromatic compoundcontaining phase is washed (usually in several stages comprising an acidic, an alkaline and a neutral wash stage) to obtain a washed organic, nitrated aromatic compound-containing phase. This phase is then sent to distillation or stripping thereby obtaining a fraction comprising the unreacted aromatic compound and a fraction comprising the nitrated aromatic compound. The fraction comprising unreacted aromatic compound is recirculated to the reactor (1000). Optionally, if a very high purity is desired, the fraction comprising the nitrated aromatic compound can be further purified by distillation.

In the case of mononitration of benzene, the mixed acid used in the nitration contains preferably from 64 % by mass to 71 % by mass sulfuric acid and from 2 % by mass to 8 % by mass nitric acid; most preferably from 66 % by mass to 69 % by mass sulfuric acid and from 2.5 % by mass to 5 % by mass nitric acid, the remainder to 100 % by mass preferably being water and the percentages by mass being based on the total mass of the mixed acid. The concentration of the sulfuric acid used is preferably from 65 % by mass to 80 % by mass and that of the nitric acid used is preferably from 62 % by mass to 70 % by mass, in each case based on the total mass of the acid in question. The ratio of the mixed acid stream (in mass of mixed acid added per hour) to the benzene stream (in mass of benzene added per hour) is also referred to as the phase ratio and is preferably from 15:1 to 35:1 , more preferably from 22:1 to 28:1.

In the present invention, the nitration reaction takes place in the reactor (1000) under adiabatic conditions, i.e., no technical measures are taken to supply heat to the reaction mixture or to remove heat from the reaction mixture. An important feature of the adiabatic nitration of aromatic hydrocarbons is that the temperature of the reaction mixture increases proportionally to the progress of the reaction, i.e., proportionally to the nitric acid conversion. A temperature difference is thereby obtained between the temperature of the mixed reactants, the aromatic compound and the mixed acid, before the start of the reaction (which is determined by thermodynamic calculations known to the person skilled in the art and is referred to below as the "start temperature") and the temperature of the reaction mixture after at least 99% nitric acid conversion (referred to below as the "reaction end temperature"). The reaction start temperatures are preferably from 90°C to 105°C, more preferably from 100°C to 102°C. The reaction end temperatures in the reactor are preferably from 120°C to 160°C, more preferably from 125°C to 140°C.

Examples:

The following examples are based on simulation results carried out with a rigorous equation based process simulation and relate to the nitration of benzene to mononitrobenzene. The process simulation includes the continuous nitration reaction, phase separation and acid concentration. In addition to the nitration reaction of benzene, the formation of by-products and decomposition reactions are also considered.

In all simulations, a vertically-arranged reactor comprising at its lower end an inlet side (100) with an internal volume of 0.021 m³, followed by eleven cylindrical reaction chambers (400) of the same length and having internal volumes as described in Table 1 below was used. In all examples, the total internal reactor volume was kept constant (2.96 m³ including the inlet side (100)). A benzene feed of 12000 kg/h (98 wt.% C₆H₆), a nitric acid feed of 12716 kg/h (68 wt.% HNO₃), and a mass ratio of benzene to nitric acid of 1.36 (calculated as /77(C₆H6)//?7(HNO₃), i.e. considering the mass of “pure C₆H₆” without the impurities present and the mass of “pure HNO₃” without the water present) was applied in all examples. The results are summarized in Table 2 below.

Table 1 : Reactor configurations used in the examples

Table 2: Results

It can be seen that the inventive reactor configuration (Example 3) gives better results in terms of conversion, by-product formation, and space time yield than both comparative examples (Example 1 and Example 2). Surprisingly, using only one reaction chamber (400) that has an internal volume that is larger than that of the preceding reaction chamber (Example 2) gives worse results than Example 1 in which the inner volume of all reaction chambers was kept constant.

Claims

Claims:

1 . A reactor (1000) configured to nitrate a stoichiometric excess of an aromatic compound (1 ) with nitric acid (2) in the presence of sulfuric acid (3) in a continuous process under adiabatic conditions, thereby producing a product mixture (5) comprising an organic, unreacted aromatic compound and nitrated aromatic compound-containing phase and an aqueous, sulfuric acidcontaining phase, the reactor (1000) comprising: (a) an inlet side (100) configured to continuously introduce the aromatic compound (1), nitric acid (2) and sulfuric acid (3) into the reactor (1000), and (b) an outlet side (200) configured to continuously withdraw the product mixture (5) from the reactor (1000), wherein the inlet side (100) comprises a first static dispersion element (300) which is configured to disperse the aromatic compound (1) in the nitric acid (2) and the sulfuric acid (3) to produce a reaction mixture (4) that is in a dispersed state; the reactor (1000) being configured for a continuous flow of the reaction mixture (4) from the inlet side (100) to the outlet side (200), during which nitration of the aromatic compound (1) in the reaction mixture (4) takes place, thereby producing the product mixture (5); the reactor (1000) further comprising (c) a plurality of further static dispersion elements (310) arranged in the direction of the continuous flow of the reaction mixture (4) downstream of the first static dispersion element (300) and which are configured to maintain the reaction mixture in the dispersed state or to restore the reaction mixture (4) to the dispersed state, the further static dispersion elements (310) being arranged in the reactor (1000) in a spaced-apart relationship to form a plurality of reaction chambers (400) inside the reactor (1000), wherein each reaction chamber (400) has an internal volume that is defined by: (i) a first further static dispersion element (310’), (ii) a second further static dispersion element (310”) arranged downstream of the first further static dispersion element (310’) in the direction of the continuous flow of the reaction mixture (4), the second further static dispersion element (310") being spaced-apart from the first further static dispersion element (310’), and (iii) a surrounding wall (410), the internal volume having a cross-sectional area (420) in the direction of the continuous flow of the reaction mixture (4) which cross-sectional (420) area may vary along the direction of flow of the reaction mixture (4); wherein at least one of the reaction chambers (400) has a largest cross-sectional area (420’) that is larger than that of the, in the direction of the continuous flow of the reaction mixture (4), preceding reaction chamber (400), and at least two of the reaction chambers (400) have an internal volume that is larger than that of the, in the direction of the continuous flow of the reaction mixture (4), preceding reaction chamber (400).

2. The reactor (1000) of claim 1 , wherein the spacing-apart of the second further static dispersion elements (310") from the first further static dispersion elements (310’) successively increases in the direction of the continuous flow of the reaction mixture (4).

3. The reactor (1000) of claim 1 or 2, wherein at least some of the reaction chambers (400) have the shape of a cylinder.

4. The reactor (1000) of any one of claims 1 to 3, wherein at least some of the reaction chambers (400) have the shape of a truncated cone.

5. The reactor (1000) of any one of claims 1 to 4, wherein at least some of the reaction chambers (400) have the shape of a double truncated cone.

6. The reactor (1000) of claim 1 or 2, wherein at least some of the reaction chambers (400) have the shape of a cylinder and at least some of the reaction chambers (400) have the shape of a double truncated cone.

7. The reactor (1000) of claim 6, wherein the reaction chambers (400) having the shape of a double truncated cone are arranged downstream, in the direction of the continuous flow of the reaction mixture (4), of the reaction chambers (400) having the shape of a cylinder.

8. The reactor (1000) of any one of claims 1 to 7, wherein the reactor (1000) is arranged for a vertical flow of the reaction mixture (4) through the reaction chambers (400).

9. The reactor (1000) of claim 8, wherein the reactor comprises two or more vertically arranged reactor sections each comprising two or more reaction chambers (400), wherein at least two reactor sections are connected to each other by horizontally arranged pipes.

10. The reactor (1000) of any one of claims 1 to 9, wherein each further static dispersion element (310) is configured to produce a pressure drop that decreases in the direction of the continuous flow of the reaction mixture (4).

11 . The reactor (1000) of any one of claims 1 to 10, wherein the first static dispersion element (300) comprises an inlet manifold, a spray injector, a perforated plate, a static mixing element, or a combination of any two thereof.

12. The reactor (1000) of any one of claims 1 to 11 , wherein the further static dispersion elements (310) comprise one or more jet impingement devices, one or more orifice plates, one or more perforated plates, one or more static mixers, one or more baffles, one or more structured packings, or a combination of any two or more thereof.

13. A production plant for producing a nitrated aromatic compound, comprising the reactor (1000) of any one of claims 1 to 12.

14. A continuous process for producing a nitrated aromatic compound, comprising adiabatically reacting an aromatic compound (1) with nitric acid (2) in the presence of sulfuric acid (3) in the reactor (1000) of any one of claims 1 to 12 or in the production plant of claim 13, wherein the aromatic compound (1) is used in stoichiometric excess over the nitric acid (2).

15. The process of claim 14, wherein the aromatic compound (1) comprises benzene and wherein the reactor (1000) is configured to mononitrate the benzene, or wherein the aromatic compound (1) comprises toluene and wherein the reactor (1000) is configured to dinitrate the toluene, or wherein the aromatic compound (1) comprises toluene and wherein the reactor (1000) is configured to mononitrate the toluene, or wherein the aromatic compound (1) comprises mononitrotoluene and wherein the reactor (1000) is configured to mononitrate the mononitrotoluene.