Method of operating a hydrogenation reactor

Abstract

The present invention relates to a method of operating a reactor for continuous hydrogenation of aromatic nitro compounds, specifically for closed-loop control thereof with regard to the completeness of conversion in the event of increases in throughput, for example in the startup phase after startup of the reactor up to attainment of a steady operating state. The method is more particularly characterized in that, over a period Δt10 within which an increase in throughput is implemented, (1) the flow rate of aromatic nitro compound N(Ar(NO2)m) fed to the reactor and (2) the difference between the flow rate of hydrogen fed to the reactor and the flow rate of any hydrogen removed from the reactor, ΔN(H2) = N(H2) - N AUS(H2), is determined n times, where n is a natural number in the range from 10 to 3000, wherein the supply of aromatic nitro compound to the reactor is stopped when a theoretical value, calculated by the summation (I) elucidated hereinafter after each of the n determinations, for an (absolute) amount of non-hydrogenated aromatic nitro compound N FIK(Ar(NO2)m) in the liquid phase (F) reaches or exceeds a maximum value N MAX FIK(Ar(NO2)m) fixed beforehand: (I), in which i is a serial index with which the n determinations are numbered, N i (Ar(NO2)m) is reported as the mass flow rate ṅ i (Ar(NO2)m) (for example in mol • h-1), S represents a stoichiometric factor, with S = 3 • m, ΔN I(H2) is reported as the mass flow rate Δṅi(H2) (for example in mol • h-1), and Δti(Ar(NO2)m) and Δti(H2) each represent the period between two successive determinations.

Description

[0001] METHOD FOR OPERATING A HYDROGENATION REACTOR

[0002] The present invention relates to a process for operating a reactor for the continuous hydrogenation of aromatic nitro compounds, namely its control with regard to the completeness of the conversion when throughput increases, for example in the start-up phase after commissioning of the reactor until a steady operating state is reached.

[0003] Continuously operated chemical reactions are characterized by the fact that, during normal operation, a certain flow of reactants is continuously fed into a reactor (the reactor is "loaded" with a certain flow of reactants, i.e., operated with a certain "load" of reactants) and a corresponding flow of products is continuously removed from the reactor. Continuously operated chemical reactors are designed and optimized for operation at a certain flow of reactants; this flow is often referred to as the "nominal load." During the operation of such a reactor, there are regularly various periods in which, in order to increase the resulting flow of products (hereinafter referred to as "production quantity"), the flow of reactants fed into the reactor must be increased (i.e.an increase in the "load", i.e. an increase in throughput, is carried out), for example when starting up a reactor after a period of downtime or when a reactor has been operated temporarily (e.g. due to a lack of demand for product) at a lower than "nominal load" and the "load" is to be increased to "nominal load". The method according to the invention deals with such periods of throughput increase in order to increase the production quantity. The method is characterized in particular in that during a period Atw within which a throughput increase is carried out,.

[0004] (1) the mass flow of aromatic nitro compound fed to the reactor / V(Ar(NO2) m ) and

[0005] (2) the difference between the mass flow of hydrogen supplied to the reactor and the mass flow of hydrogen removed from the reactor, A / V(H2) = / V(H2) - / V AUS(H2), n-fold, where n is a natural number in the range from 10 to 3000, wherein the supply of aromatic nitro compound to the reactor is interrupted when a fictitious value for an (absolute) amount of non-hydrogenated aromatic nitro compound / V calculated according to the summation (I) explained below after each of the n determinations FIK (Ar(NO2) m ) in the liquid phase (F) a previously defined maximum permissible value / VMAx FIK (Ar(NO2) m ) reaches or exceeds: ni FIK (Ar(NO2) m ) = !:?{ / i(Ar(NO2) m ) • Ati(Ar(NO2) m )} - S • Zl=?{A / i( H2) • At,(H2)} (I), where

[0006] / stands for a running index with which the n determinations are numbered, / Vi(Ar(NO2) m ) as mass flow rate i(Ar(NO2) m ) (e.g. in mol • h -1), S stands for a stoichiometric factor, with S = 3 • m, A / Vj(H2) as the mass flow Ahi(H2) (e.g. in mol • h -1 ) is specified, and

[0007] Ati(Ar(NO2) m ) and Ati(H2) each represent the period between two consecutive determinations.

[0008] The hydrogenation of aromatic nitro compounds to the corresponding aromatic amines according to the equation

[0009] Ar(NO2) m + 3 m H2^ Ar(NH2) m+ 2 m H2O is an important reaction in preparative organic chemistry and is also of great importance in industrial production. The aromatic amines aniline (ANL) and toluenediamine (TDA), which are probably the most important in terms of annual production volume, are both produced predominantly or entirely by hydrogenation of the corresponding nitro compounds, mononitrobenzene (MNB; Ar = CeHs; m = 1) and dinitrotoluene (DNT; Ar = CH3CeH3; m = 2). For this purpose, hydrogenation reactors with a large production capacity (and consequently a high throughput) must be operated continuously, which poses challenges for process control technology. Both safety and quality aspects must be considered. In particular, insufficient conversion of the aromatic nitro compound can be problematic in both respects.

[0010] WO 2006 / 089906 A1 describes a process for producing aromatic amines by hydrogenating nitroaromatics or aliphatic amino alcohols by hydrogenating nitroalcohols in the presence of catalysts. A fluid reaction mixture containing amines or amino alcohols is formed in a reactor, from which the catalysts are separated. The process is characterized in particular by measuring the absorption of UV / VIS radiation by the reaction mixture after the catalysts have been separated to determine the concentration of nitro and nitroso compounds in the reaction mixture. Monitoring the conversion using UV / VIS spectroscopy is naturally associated with complex measurement techniques.In addition, such devices require regular monitoring, since the precision of such optical measuring methods can decrease over time, for example due to contamination of the measuring devices used, the windows required for the measurement can become dirty, and the like. EP 1 077 921 A1, published as WO 99 / 59956 A1, describes a process for preparing an optionally substituted 4-aminodiphenylamine, comprising reacting an optionally substituted aniline and an optionally substituted nitrobenzene in the presence of water and a base, wherein the water content is controlled such that a molar ratio of water to the base used of not less than about 4:1 at the beginning of the coupling reaction and not less than about 0.6:1 at the end of the coupling reaction is ensured, in order to produce 4-nitrodiphenylamine and / or 4-nitrosodiphenylamine and / or salts thereof.The coupling reaction is followed by a hydrogenation reaction in which the coupling reaction product is hydrogenated in the presence of a hydrogenation catalyst and added water, ensuring a molar ratio of total water to base of at least about 4:1 at the end of the hydrogenation. Aqueous and organic phases are obtained, and the optionally substituted 4-aminodiphenylamine is recovered from the organic phase. Only batch processes are specifically described.

[0011] EP 2 523 933 A2, published as WO 2011 / 086050 A2, describes a process for producing aromatic amines in the liquid phase by catalytic hydrogenation of the corresponding nitroaromatics in at least two reaction chambers arranged in series, wherein at least one reaction chamber is operated isothermally and at least the reaction chamber downstream of it is operated adiabatically. In preferred embodiments, the adiabatic temperature jump is used to control the reaction, which is difficult or even impossible when the production quantity changes. The described procedure is obviously not applicable to processes that are operated purely isothermally or that provide only one reaction chamber. Furthermore, this process requires that the reaction mixture exhibit a certain reactivity.If this is not the case, for example due to the complete absence of catalyst due to incorrect operation or a technical defect, there is no adiabatic temperature jump, which makes appropriate reaction control impossible.

[0012] EP 2 812 308 A1, published as WO 2013 / 117622 A1, describes a continuous process for preparing at least one aromatic amine by hydrogenating at least one nitroaromatic with hydrogen, wherein a liquid phase comprising at least the aromatic amine and a gas phase comprising at least hydrogen are present, in the presence of a catalyst suspended in the liquid phase at a temperature of 50 to 250 °C and a pressure of 5 to 50 bar. Reactors as disclosed in WO 00 / 35852 A1 are described as being particularly suitable. In the process according to WO 2013 / 117622 A1, the pressure in the reactor is kept essentially constant by continuously adjusting the amount of hydrogen fed to the reactor. The total amount of hydrogen fed to the reactor is monitored, and the feed of the at least one nitroaromatic is interrupted if the hydrogen uptake by the reactor does not reach at least 50 mol.-%, preferably at least 70 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%, in particular at least 98 mol%, of the amount of hydrogen required for the stoichiometric conversion of the at least one nitroaromatic to the at least one aromatic amine. A disadvantage of this process is that it is not suitable for transient operating conditions, such as those encountered in particular with throughput increases, for example during start-up of the hydrogenation reactor from a shutdown state.

[0013] A hydrogenation reactor is typically started up with flow rates of nitro compound and hydrogen that are significantly below the corresponding target flow rates. These initial flow rates are then gradually increased (e.g., continuously increased) until the desired target flow rates are reached. Only then does the hydrogenation reactor reach a steady-state operating state in the sense that—barring any possible operational disruptions—continuously defined and (unintentional fluctuations notwithstanding) constant flow rates of aromatic nitro compound and hydrogen are supplied, and defined, constant flow rates of aromatic amine and water are removed. However, in the intermediate state from the initial supply of aromatic nitro compound and hydrogen until the target flow rates are reached (start-up time), the flow rates are subject to constant fluctuations.Even during periods in which no change is intended ("plateau phases" with a gradual increase in flow rates), fluctuations can occur that are large compared to fluctuations in steady-state operation. In this sense, the start-up period is a transient operating condition of the hydrogenation reactor. Monitoring hydrogenation in this condition presents particular challenges, which the present invention has addressed. In particular, it is of great importance to provide plant operators with a criterion that can be used, possibly automatically, to decide when a start-up process deviates so significantly from the desired procedure that the supply of aromatic nitro compound must be interrupted in order to meet product quality requirements or even safety requirements.The aforementioned aspects are important not only when starting up a hydrogenation reactor from a shutdown, but also when increasing throughput during regular operation. For example, it is conceivable that a hydrogenation plant may be operated for a certain period of time after startup at a throughput that does not correspond to the maximum possible throughput for the plant in question (e.g., because demand for the hydrogenation product is not high enough). If demand increases at a later point in time and the throughput is increased accordingly, similar problems arise as when starting up from a shutdown.

[0014] There was therefore a need for further improvements in the hydrogenation of nitroaromatics. In particular, it would be desirable to be able to operate hydrogenation reactors in such a way that reliable monitoring of the reaction, particularly with regard to the presence of unreacted nitro compound in the liquid phase, is ensured even in transient conditions such as those encountered during throughput increases, for example, during start-up. This should improve the reaction control from a quality and, in particular, safety perspective, ideally without requiring extensive measurement technology.

[0015] Taking this need into account, the present invention relates to a process, in particular a computer-implemented process, for operating a reactor (R) for the continuous hydrogenation of an aromatic nitro compound (Ar(NO2) m) in times of increased throughput, wherein the hydrogenation is carried out in the presence of a catalyst with hydrogen (H2) to obtain an aromatic amine (Ar(NH2)m) according to

[0016] Ar(NO2) m + 3 m H2^ Ar(NH2) m + 2 m H2O, where Ar is an aryl radical and m is 1 or 2. In the process according to the invention, a liquid phase (F) containing a suspension of the catalyst and a gas phase (G) containing hydrogen are present in the reactor during the hydrogenation. The aromatic nitro compound is fed to the reactor at a flow rate of / V(Ar(NO2) m ) and the hydrogen is fed in at a rate of / (H2), and while retaining the catalyst, a product stream (PS) containing the aromatic amine and optionally non-hydrogenated aromatic nitro compound is withdrawn. Optionally, hydrogen is added at a rate of / V AUS(H2) is removed from the reactor (so-called purge). A constant pressure is maintained in the gas phase (G).

[0017] The process comprises (at least) an increase in throughput within a period of time Atw, starting at a time to and ending at a time ti. In the context of the present invention, an increase in throughput is understood to mean an increase in the mass flow rates of reactants, i.e. of aromatic nitro compound and hydrogen, fed to the reactor. In a throughput increase in the sense of the invention, the mass flow / V(Ar(NO2) m ) of a value / V t =to(Ar(N02)m) to a value A / t=ti(Ar(NO2)m) > / V t =to(Ar(N02)m) and the mass flow N( H2) of a value / t=to(H2) to a value / t=ti(H2) > / t=to(H2) continuously or at intervals (i.e. ti is reached when the mass flows of aromatic nitro compound and hydrogen have each reached the value targeted for the end of the throughput increase, whereby it is not necessary that both mass flows reach the respective value simultaneously). A throughput increase in the sense of the invention means in particular an increase of / V(Ar(NO2) m ) by at least 0.5%, particularly preferably by at least 1% of the initial value, so that in particular / t=tl (Ar(NO2) m ) > 1.005 • / t=t0 (Ar(NO2) m ) and particularly preferred / t=tl (Ar(NO2) m ) > 1.01 • / t=t0 (Ar(NO2) m). The flow rate of hydrogen A / (H2) fed into the reactor is increased so that, taking into account any hydrogen purge, at least the stoichiometric amount of hydrogen required for hydrogenation is always available. The maximum throughput increase is limited only by the maximum production capacity of the reactor used.

[0018] The method according to the invention is characterized in that during the period Atio

[0019] (1) the mass flow of aromatic nitro compound fed to the reactor / V(Ar(NO2)m) and

[0020] (2) the difference between the flow rate of hydrogen supplied to the reactor and the flow rate of hydrogen removed from the reactor, A / V(H2) = / V(H2) - A / AUS(H2), are determined n-fold, in particular via a sensor unit n-fold, where n is a natural number in the range from 10 to 3000, wherein the supply of aromatic nitro compound to the reactor is interrupted when a fictitious value for an (absolute) amount of non-hydrogenated aromatic nitro compound / V calculated according to the summation explained below after each of the n determinations FIK (Ar(NO2) m ) in the liquid phase (F) a previously defined maximum permissible value / VMAx FIK (Ar(NO2) m ) is reached or exceeded. It is useful to represent the summation using quantities of substance (n), so that / V FIK (Ar(NO2) m ) as mass flow n FIK (Ar(NO2) m ), where of course the permissible maximum value is given in the same unit, i.e. as the amount of substance nMAx FIK (Ar(NO2) m ) is to be expressed as: ni FIK (Ar(NO2)m ) = !:?{ / i(Ar(NO2) m ) • Ati(Ar(NO2) m )} - S • Z^=^A / Vi(H2) • At,(H2)} (I), where

[0021] / stands for a running index with which the n determinations are numbered, / Vi(Ar(NO2) m ) as mass flow rate hi(Ar(NO2) m ) (e.g. in mol • h -1 ), S stands for a stoichiometric factor, with S = 3 • m,

[0022] A / Vj(H2) as the mass flow rate Ahi(H2) (e.g. in mol • h -1 ) and Ati(Ar(NO2)m) and At Fh) each represent the period between two consecutive determinations.

[0023] Further objects of the present invention are a computer system for controlling the conversion of aromatic nitro compound (Ar(NC>2)m) in the process according to the invention, a computer program product and a production plant for the continuous hydrogenation of an aromatic nitro compound (Ar(NC>2)m) with hydrogen (H2) in the presence of a catalyst comprising the computer system according to the invention.

[0024] Quite surprisingly, it was found that determining a fictitious value for the amount of unhydrogenated aromatic nitro compound as explained above provides a suitable criterion for deciding when the supply of aromatic nitro compound to the hydrogenation reactor needs to be interrupted for quality and / or safety reasons during a throughput increase (e.g., during start-up). A suitable permissible maximum value can be easily determined by a person skilled in the art under given boundary conditions (type of reactor, its operating conditions, particularly with regard to temperature and pressure, as well as the size of the desired target mass flows for aromatic nitro compound and hydrogen).A significant to complete independence from the aforementioned boundary conditions is achieved if, to determine a criterion for interrupting the supply of aromatic nitro compound, one uses not the absolute value, but rather the concentration of the permissible maximum value of non-hydrogenated aromatic nitro compound relative to the mass of the liquid phase (ITIF). This is the subject of a preferred embodiment and will be explained in more detail below.

[0025] The process according to the invention relates to the control of the reactor with regard to the completeness of the conversion during times of a throughput increase. Such a throughput increase comprises a period Atio, within which, starting at a time t0, the mass flow of aromatic nitro compound fed to the reactor and the mass flow of hydrogen fed to the reactor are increased (= throughput increase). The time at which both mass flows have reached the respectively desired increased values ​​is designated ti (thus: Atio = ti - t0). It is not necessary for the mass flows of nitroaromatic and hydrogen to reach the respectively desired increased values ​​at the same time. If one of the two streams (in particular the hydrogen stream N(H2)) reaches the value desired for the end of the throughput increase later than the other stream, this later time corresponds to time ti.

[0026] According to the invention, the reactor is operated continuously. This means that the reactants (i.e., aromatic nitro compound and hydrogen) and any additives used, such as solvents, are continuously fed into the reactor during a production cycle, and the products formed (i.e., aromatic amine and water) as well as additives (solvents) are continuously removed from the reactor. This does not necessarily apply to the catalyst used. This can, for example, be retained by a filter in the reactor and only replaced after the end of a production cycle (or several production cycles), namely when the catalytic activity has decreased too much. However, a catalyst change (or even just the addition of fresh catalyst) during continuous operation is also encompassed by the process according to the invention.In the terminology of the present invention, a production cycle refers to the period from the start of continuous production to its end. The start of continuous production is understood to be the point in time at which hydrogen and aromatic nitro compound are first fed to the reactor (tß, a special case of t0). The end of continuous production is determined by shutting off the supply of the aromatic nitro compound (t2). (Generally, after shutting off the supply of aromatic nitro compound, the hydrogen is allowed to continue running for a while before the reactor is shut down completely to ensure complete reaction of any aromatic nitro compound still present in the reactor.)

[0027] Mass flows, symbolized by N in the terminology of the present invention, can generally be specified as mass, volume, or substance flows. In operational practice, nitroaromatic flows / V(Ar(NC>2)m) are often specified as mass flows rh(Ar(NO2)m), e.g., in kg • h -1 Hydrogen flows / (H2) (regardless of whether hydrogen is added or removed) are often given as volume flows Q(H2), e.g. in Nm 3 • h 1 (Nm 3 = standard cubic meter; the volume converted to the standard state of a pressure of 101325 Pa and a temperature of 273.15 K). Both can also be easily applied within the scope of the present invention. For the sake of simplicity, the above equation (I) is formulated for mass flow rates hi, so that the fictitious value for the amount of non-hydrogenated aromatic ni compound is expressed as the mass ni FIK (Ar(NO2) m). In the already mentioned embodiment, using the maximum permissible (fictitious) concentration of non-hydrogenated aromatic nitro compound, this concentration then results in a specific partial substance amount <7MAx FIK (Ar(NO2) m ). It is easy for a person skilled in the art to reformulate equation (I) in operational practice for other units of mass flow, adjusting the stoichiometric factor S accordingly, and to specify the preferred concentrations in quantities other than the specific amount of partial substance. This does not depart from the scope of the present invention.

[0028] In the attached drawings: FIG 1: a reactor including peripheral devices, which is produced by means of the inventive

[0029] procedure can be operated;

[0030] FIG. 2 shows a schematic representation of the mass flows / Van reactants of a hydrogenation during a production cycle;

[0031] FIG. 3: a schematic representation of the relationships between the determined fictitious values ​​of the amount of unreacted aromatic nitro compound and the interruption of the supply of aromatic nitro compound;

[0032] FIG. 4: the relative flow rates (in % of the nominal load) of hydrogen and dinitrotoluene

[0033] (DNT) and the fictive concentration of non-hydrogenated aromatic nitro compound during a regular start-up of a DNT hydrogenation plant; and

[0034] FIG. 5: the relative flow rates (in % of the nominal load) of hydrogen and dinitrotoluene

[0035] (DNT) and the fictitious concentration of non-hydrogenated aromatic nitro compound during a start-up process of a DNT hydrogenation plant in which no catalyst is fed to the hydrogenation reactor.

[0036] First, a brief summary of various possible embodiments of the invention follows:

[0037] In a first embodiment of the process according to the invention, which can be combined with all other embodiments, the specific partial substance amount related to the mass m / w_MAx(F) of the liquid phase is <7MAx FIK (Ar(NO2) m ) of the permissible maximum value n M Ax FIK (Ar(NO2) m ), in the range from 0.006 mol / kg to 0.550 mol / kg, preferably 0.010 mol / kg to 0.200 mol / kg, particularly preferably 0.013 mol / kg to 0.029 mol / kg, where m / w_MAx(F) is the mass of the liquid phase at the time of the determination / at which ni FIK (Ar(NO2) m ) equal to or greater than nMAx for the first time FIK (Ar(NO2) m ) is (and where each of the values ​​for the specific partial substance amount <7MAx FIK (Ar(NO2) m) specified ranges can be combined with all other embodiments of the invention).

[0038] In a second embodiment of the process according to the invention, which can be combined with all other embodiments, n is in the range from 80 to 3000 and the period Atio is 5 min to 100 min. Preferably, n is in the range from 100 to 2500 and Atio is 7.5 min to 80 min. Particularly preferably, n is in the range from 500 to 2200 and Atio is 15 min to 60 min. Very particularly preferably, n is in the range from 1000 to 2000 and Atio is 20 min to 40 min.

[0039] In a third embodiment of the process according to the invention, which can be combined with all other embodiments, the nitroaromatic is mononitrobenzene (Ar = CeHs; m = 1) or dinitrotoluene (Ar = CH3C6H3; m = 2).

[0040] In a fourth embodiment of the process according to the invention, which can be combined with all other embodiments, the hydrogenation is carried out at a temperature in the range from 80 °C to 200 °C, preferably 110 °C to 180 °C and at a pressure in the range from 5.0 bar to 120 bar, preferably 10 bar to 100 bar.

[0041] In a fifth embodiment of the process according to the invention, which can be combined with all other embodiments, the catalyst comprises Pt, Pd, Rh, Ru, Ni, Co, Cu or mixtures thereof, with catalysts containing Pt, Pd, Ni or Cu being preferred.

[0042] In a sixth embodiment of the process according to the invention, which can be combined with all other embodiments, the reactor is a stirred tank reactor, a falling film reactor or a bubble column reactor.

[0043] In a seventh embodiment of the process according to the invention, which can be combined with all other embodiments, the suspension of the catalyst is a suspension in an organic solvent and / or in aromatic amine (Ar(NH2)m)) (from a previous production cycle).

[0044] In an eighth embodiment of the process according to the invention, which can be combined with all other embodiments, the throughput increase comprises starting the hydrogenation from a state in which the reactor (R) is not in operation.

[0045] The embodiments briefly described above and other possible configurations of the invention are explained in more detail below. All of the embodiments described above and the other configurations of the invention described below can be combined with one another as desired, unless the context clearly indicates otherwise to a person skilled in the art or unless expressly stated otherwise.

[0046] Mononitrobenzene (Ar = CeHs; m = 1) and dinitrotoluene (Ar = CH3C6H3; m = 2) are particularly suitable as aromatic nitro compounds. The hydrogenation of the aromatic nitro compound to the corresponding aromatic amine is carried out using hydrogen or a mixture of hydrogen and inert gases as the hydrogenation reagent. The hydrogenation can be carried out at temperatures and pressures customary in the art. Preferred temperatures are in the range from 80 °C to 200 °C, in particular 110 °C to 180 °C. Preferred pressures are in the range from 5.0 bar to 120 bar, in particular 10 bar to 100 bar. (Here and below, all pressures are to be understood as absolute pressures.) Temperature measurement is carried out using devices known to those skilled in the art, such as thermocouples or resistance, semiconductor, or infrared thermometers. Pressure measurement is preferably carried out using mechanical manometers or electronic pressure sensors.

[0047] All catalysts commonly used for catalytic hydrogenation are suitable. Preference is given to catalysts containing noble metals such as Pt, Pd, Rh, Ru or non-ferrous metals such as Ni, Co or Cu or mixtures thereof. Particular preference is given to catalysts containing Pt, Pd, Ni or Cu, in particular as a suspension in water. In the case of noble metal catalysts, these are applied to supports such as activated carbon, SiO2 or Al2O3; in the case of Ni catalysts, either supported Ni or a Ni skeletal catalyst can be used. The concentration of the catalyst in the liquid phase present in the reactor is preferably between 0.01 mass% and 20 mass%, more preferably between 0.50 mass% and 10 mass%, based on the total mass of the liquid phase.

[0048] If mixtures of hydrogen and inert gases are used, preferred inert gases are ammonia, noble gases, and / or nitrogen. Hydrogen or the mixture of hydrogen and inert gases is added in such a way that a constant pressure is established in the reactor, i.e., the amount of hydrogen added corresponds to the sum of the hydrogen consumed chemically and, if appropriate, the hydrogen discharged as a purge stream. When using a mixture of hydrogen and inert gases as the hydrogenation reagent, the ratio of hydrogen to inert gas in the supplied hydrogenation reagent is preferably gradually increased to prevent the reactor contents from becoming depleted of hydrogen.

[0049] In the process according to the invention, hydrogen is preferably used in excess of the amount required for the hydrogenation of the nitro groups to amino groups. In particular, the excess of hydrogen relative to the amount of substance required for the hydrogenation of the nitro groups to amino groups is at least 0.01%, preferably at least 0.10%, and at most 10%. These values ​​apply to steady-state conditions outside of periods of increased throughput. While these values ​​can generally also be used during periods of increased throughput, deviations from these values ​​may also occur, particularly during the throughput increase for reactor startup (commissioning).

[0050] Optionally, inert solvents can be used under the reaction conditions, such as alcohols such as methanol, propanol, isopropanol, or ethers such as dihydrofuran or tetrahydrofuran. To increase the economic efficiency of the process, a low solvent concentration is generally advantageous. This is typically between 1.0 and 50 mass%, preferably between 20 and 35 mass%, based on the total mass of the liquid phase.

[0051] Furthermore, in the process according to the invention, 0.10% to 10%, preferably 0.20% to 5.0% of the mass flow of the hydrogen used can be discharged (purge hydrogen / V AUS(H2)). This discharge can prevent the accumulation of inert compounds or gaseous by-products. These values ​​apply to steady-state conditions outside of periods of throughput increase. While these values ​​can generally be used during periods of throughput increase, deviations may occur, particularly during the throughput increase for reactor startup (commissioning).

[0052] FIG. 1 shows an example of a reactor that can be operated using the process according to the invention. It shows a stirred tank reactor. Other suitable reactor types include, in particular, falling-film reactors and bubble column reactors.

[0053] To facilitate understanding of the present invention, the operation of such a reactor in the steady state (i.e. in regular operation with constant throughput, in particular with "nominal load") is described below:

[0054] The aromatic nitro compound (e.g., molten dinitroluene) is fed to the reactor (R) at a flow rate of / VsoLi.(Ar(NO2)m) via a nitroaromatic line (1), and hydrogen is fed to the reactor (R) at a flow rate of / VSOLL(H2) via a hydrogen line (2). The liquid phase (F) of the reactor (R) contains a mixture of aromatic amine (in the selected example, toluenediamine) and water, as well as the catalyst, for example, a nickel catalyst. The reactor (R) can be equipped with a device for supplying fresh catalyst during operation (not shown in the figure). The aromatic nitro compound, in the selected example, dinitrotoluene, is hydrogenated using hydrogen at a temperature of, for example, 130 °C and a pressure of, for example, 20 bar.

[0055] A portion of the liquid phase (F) comprising the catalyst is circulated from the reactor (R) by means of a pump (P) via lines 3, 4, 5, and 6. Line 4 leads to a cooler (K), in which the reaction mixture is cooled. From there, the cooled reaction mixture is fed via line 5 to a cross-flow filter (Q), from which the product stream (PS) containing the aromatic amine, in the selected example toluenediamine, is discharged free of catalyst via product line (7). The remainder of the reaction mixture, including catalyst, is fed back to the reactor (R) via line 6, thus closing the cycle.

[0056] / VsoLL(Ar(NO2)m) is set so that an average residence time in the reactor (R) of, for example, 120 min results. / VSOLL(H2) is set so that the pressure in the reactor (R) (measured in the gas space (G) above the liquid phase (F)) remains constant. Hydrogen is continuously discharged from the reactor (R) at a flow rate / VSOLL AUS (H2) is discharged via line 8 (so-called purge). A / AUS (H2) for example 1% of / VSOLL(H2).

[0057] FIG. 2 shows a schematic representation of the reactant flow rates N of a hydrogenation during a production cycle, with the throughput increase being illustrated using the example of the increase in the flow rates of aromatic nitro compound and hydrogen from the start of the hydrogenation (t = tß = t0) until reaching a (possibly first) steady-state operating state with constant throughput (from t = t1). Thus, a throughput increase for starting the hydrogenation is shown, starting from a state in which the reactor (R) is not in operation (reactor start-up), for which the process according to the invention is particularly suitable.

[0058] The hydrogenation should be started at time tß=t0, i.e., from this time onwards, hydrogen and nitroaromatic are fed to the reactor. In preparation for the actual start-up (before time t0; not shown in FIG. 2), the reactor is preferably charged with a suspension of the catalyst (i) in one of the above-mentioned organic solvents inert under the hydrogenation conditions, (ii) in water and / or (iii) in aromatic amine (Ar(NH2)m)) (from a previous production cycle) and heated to the desired reaction temperature. Preferably, the hydrogen feed is also started before t0, in such a way that the operating pressure in the gas phase (G) desired for the steady state from t1 is established. If a purge stream / V AUS (H2) is provided, which is preferred, this will already be set at this time.

[0059] From to, the aromatic nitro compound is then additionally fed into the reactor. The initial values ​​for both flow rates, / t =to(H2) and / V t =to(Ar(N02)m), are adjusted within a period Atio to the desired values ​​N t =ti(H2) and / V t=ti(Ar(NO2)m). In the illustration, the mass flow of aromatic nitro compound reaches the value intended for the (possibly first) steady-state operating condition earlier than the mass flow of hydrogen. However, time ti is only considered to be reached when both flows have reached the intended value. The time period Atio is preferably 5 min to 100 min, particularly preferably 10 min to 80 min, very particularly preferably 15 min to 60 min, and extraordinarily very particularly preferably 20 min to 40 min. At time ti, the mass flows desired for the (possibly first) steady-state operating condition are present. Aromatic nitro compound is now continuously produced in the reactor until the end of the production cycle at time t2. The illustration is not to scale; in general, the time period t2 - ti will be a multiple of the time period ti - t0 (= Atio).At t2, the supply of aromatic nitro compound is shut off, for example because the demand for aromatic amine has decreased or the catalyst is exhausted. This shutdown does not have to be abrupt as shown in the figure. A gradual reduction is of course also possible. The time period from t0 to t2 is referred to in the context of the present invention as the production cycle. It is possible for a production cycle to comprise several different stationary operating states of constant throughput. For example, it is possible for the throughput of aromatic nitro compound and hydrogen present at ti to correspond to a first stationary operating state, and for a second stationary operating state with a further increased throughput to be established after a certain period of time has elapsed. In such a case, the production cycle therefore comprises two time periods Atio, Atio(1) and Atio(2).In such a case where there are several periods of throughput increase, the method according to the invention is applied in at least one and preferably in all of these periods.

[0060] The procedure according to the invention during at least one period Atio includes that

[0061] (1) the mass flow of aromatic nitro compound fed to the reactor / V(Ar(NO2) m ) and

[0062] (2) the difference between the mass flow of hydrogen supplied to the reactor and the mass flow of hydrogen removed from the reactor, A / V(H2) = / V(H2) - A / AUS (H2), n-fold, where n is a natural number in the range from 10 to 3000. The determination is carried out in particular via a sensor unit, for example a commercially available flow meter.

[0063] The most suitable value for n depends on the duration of the period Atio. Preferably, n is in the range of

[0064] 80 to 3000, if Atio is 5 min to 100 min,

[0065] 100 to 2500, if Atio is 7.5 min to 80 min,

[0066] 500 to 2200, if Atio is 15 min to 60 min and

[0067] 1000 to 2000 if Atio is 20 min to 40 min.

[0068] The mass flow of aromatic nitro compound fed to the reactor / V(Ar(NO2) m) at a specific time is, since it is set to a specific value, generally known and can advantageously be controlled with a flow meter for liquids and generally read directly from the process control system. The hydrogen is fed into the reactor under pressure control, i.e. a pressure drop resulting from hydrogen consumption as the hydrogenation progresses is counteracted by feeding in an appropriate flow rate of hydrogen. This supplied flow rate of hydrogen, A / (H2), is determined using a flow meter for gases. In principle, therefore, it is not necessary to remove hydrogen during a production cycle. However, as already mentioned, it has proven useful to remove some hydrogen from the reactor as a purge stream in order to remove inert substances and gaseous by-components. The amount of hydrogen removed with the purge stream / V AUS(H2)) is determined using a gas flow meter or set to a defined value using a perforated orifice plate or valve position.

[0069] According to the invention, after each of the n determinations, a fictitious value for the amount of non-hydrogenated aromatic nitro compound (= / V FIK (Ar(NC>2)m)). The resulting individual values ​​ / Vj FIK (Ar(NC>2)m) are added up and as soon as the sum reaches a previously defined maximum permissible value for the amount of non-hydrogenated aromatic nitro compound (= / VMAx FIK (Ar(NO2) m )) is reached or exceeded, the supply of aromatic nitro compound is interrupted. This is shown schematically in FIG. 3:

[0070] Hydrogenation begins at t0 (as shown in FIG. 2). The throughput increase period, in the selected example the start-up period, initially proceeds smoothly. Although small fictitious quantities of unhydrogenated aromatic nitro compound are determined, these are initially well below the previously defined maximum value. At time t3, however, this maximum value is reached, and the supply of aromatic nitro compound is shut off. The shutdown time is designated t3 rather than t2 to distinguish it from the scheduled shutdown at the end of a production cycle.

[0071] As already mentioned, it is advantageous to express the maximum permissible value of non-hydrogenated aromatic nitro compound as a concentration, since such a maximum value is essentially independent of plant-specific parameters such as reactor size, etc. The mass of the liquid phase (F) has proven to be a suitable reference value. For this purpose, the volume of the liquid phase, and from this, its mass, can be calculated from the fill level (which can be adjusted using a weir installed at a suitable height or a level control system) and the known reactor dimensions.

[0072] In this case, the maximum permissible value of non-hydrogenated aromatic nitro compound is defined as the specific partial substance amount <7MAx FIK (Ar(NO2) m ). It has proven particularly useful that q MA x FIK (Ar(NO2) m ) = n M Ax FIK (Ar(NO2) m ) / m / W MAx(F) (II), in the range from 0.006 mol / kg to 0.550 mol / kg, preferably 0.010 mol / kg to 0.200 mol / kg, particularly preferably 0.013 mol / kg to 0.029 mol / kg, where in equation (II) m / w_MAx(F) is the mass of the liquid phase at the time of the determination / at which ni FIK (Ar(NO2) m ) equal to or greater than nMAx for the first time FIK (Ar(NO2) m ) is (and where each of the values ​​for the specific partial substance amount <7MAx FIK (Ar(NO2) m ) can be combined with all other embodiments of the invention). Of course, the numerical ranges mentioned are for the specific partial substance amount qMAx FIK(Ar(NO2)m) should not be understood as limiting in the sense that the application of other concentration quantities (e.g., mass fractions) in operational practice would therefore depart from the scope of the invention. Rather, the numerical ranges mentioned are to be regarded as decisive in the sense that concentrations, regardless of the unit used in operational practice, fall within the numerical ranges mentioned when converted to specific partial substance quantities.

[0073] Another object of the present invention is a computer system for controlling the conversion of aromatic nitro compound (Ar(NO2) m ) in the method according to the invention described above, wherein the computer system comprises: an interface unit configured to transmit the results of the n-fold determination of / V(Ar(NO2) m) and A / VfFh), in particular from the sensor unit described above; and a processor configured to process the read values ​​for / V(Ar(NO2) m ) and A / VfFh) the amount of substance ni FIK (Ar(NO2) m ) and compare it with the permissible maximum value nMAx stored and retrievable in a database that is communicatively connected to the processor FIK (Ar(NO2) m ) and when the permissible maximum value nMAx is reached or exceeded FIK (Ar(NO2) m ) to interrupt the supply of aromatic nitro compound to the reactor.

[0074] The computer system according to the invention is preferably integrated into the process control system of the production plant.

[0075] The interruption of the supply of aromatic nitro compound to the reactor when the permissible maximum value nMAx is reached or exceeded FIK (Ar(NO2) m) is carried out in particular in such a way that an interruption signal is emitted and received by a receiving unit (e.g. a valve control), whereupon the supply of aromatic nitro compound is interrupted (e.g. by closing an inlet valve).

[0076] A further subject of the present invention is a computer program product comprising instructions which, when executed by the above-described computer system according to the invention, cause the above-described computer system to execute the above-described method according to the invention. In particular, it is a computer program product which, when loaded into the computer system according to the invention, is executed by the processor of the computer system and, in the method according to the invention, uses the results of the n-fold determination of / V(Ar(NO2) m ) and A / V(H2) periodically or continuously and with the permissible maximum value nMAx FIK (Ar(NO2)m ) and when the permissible maximum value nMAx is reached or exceeded FIK (Ar(NO2) m ) interrupts the supply of aromatic nitro compound to the reactor, in particular as described above by issuing an interruption signal which is received by a receiving unit (e.g. a valve control), whereupon the supply of aromatic nitro compound is interrupted (e.g. by closing an inlet valve).

[0077] Finally, the present invention relates to a production plant for the continuous hydrogenation of an aromatic nitro compound (Ar(NO2) m ) with hydrogen (H2) in the presence of a catalyst to give an aromatic amine (Ar(NH2)m)) according to

[0078] Ar(NO2) m + 3 m H2^ Ar(NH2) m + 2 m H2O, wherein Ar is an aryl radical and m is 1 or 2, comprising:

[0079] (I) a reactor (R) for carrying out the hydrogenation;

[0080] (II) a device for providing the aromatic nitro compound and introducing it into the reactor;

[0081] (III) a device for providing the hydrogen and introducing it into the reactor;

[0082] (IV) a device for discharging a product stream (PS) containing the aromatic amine and optionally non-hydrogenated aromatic nitro compound;

[0083] (V) a device for retaining the catalyst during discharge of the product stream (PS); and

[0084] (VI) the computer system according to the invention.

[0085] With regard to the devices mentioned under (I) to (V), reference can be made to the above description of the process according to the invention and the literature cited therein.

[0086] The invention is explained in more detail below with the aid of examples. A plant for the production of toluenediamine (TDA) from dinitrotoluene (DNT), comprising a stirred tank and a device for separating and recycling the catalyst from the product stream, was filled with the mixture of TDA and water resulting from the hydrogenation (obtained from a previous production cycle). The plant was brought to the required temperature, and catalyst was added in an average concentration of approximately 1% by mass, based on the total mass of the reaction mixture. Hydrogen was then added to the plant using a pressure-maintaining device until an absolute pressure of 25 bar was established in the gas phase. At time t = 0, a temporally increasing amount of DNT was added (see FIG. 4) until, after approximately 4.0 min, a value of approximately 35% of the maximum possible DNT flow for this plant was reached.After initial fluctuations, the hydrogen flow also reaches a largely stable value after approximately 7.5 minutes (= ti), corresponding to approximately 37% of the maximum possible value for this system. The calculated concentration is <7. FIK (Ar(NO2) m ) of unreacted dinitrotoluene is at all times well below 0.005 mol / kg.

[0087] Example 2:

[0088] FIG. 5 shows the curve obtained when, for example, no catalyst is added to the reactor due to operator error. Here, the calculated concentration of unreacted dinitrotoluene increases over time and exceeds a value of 0.020 mol / kg after just 4.0 min.

[0089] Example 1 shows that in this case, pure ratio monitoring can only become active after approximately 7.5 minutes due to fluctuations in the pressure maintenance control loop. In contrast, Example 2 shows that with the present method, when choosing a maximum permissible concentration of <7MAx FIK (Ar(NO2) m ) of 0.020 mol / kg, the impending dangerous accumulation of unreacted nitroaromatics is detected in almost half the time.

Claims

1. A process for operating a reactor (R) for the continuous hydrogenation of an aromatic nitro compound (ArfNChJm) during periods of increased throughput, wherein the hydrogenation is carried out in the presence of a catalyst with hydrogen (H2) to obtain an aromatic amine (Ar(NH2)m)) according to Ar(NO2) m + 3 m H2^ Ar(NH2) m + 2 m H2O, where Ar is an aryl radical and m is 1 or 2, wherein during the hydrogenation in the reactor a liquid phase (F) containing a suspension of the catalyst and a gas phase (G) containing hydrogen are present and the aromatic nitro compound is fed to the reactor with a mass flow / V(Ar(NO2) m ) and the hydrogen are fed in at a flow rate A / (H2) and, while retaining the catalyst, a product stream (PS) containing the aromatic amine and optionally non-hydrogenated aromatic nitro compound is withdrawn, optionally hydrogen being fed in at a flow rate / V AUS(H2) is discharged from the reactor and wherein a constant pressure is set in the gas phase (G), the method comprising a throughput increase within a period Atw, starting at a time to and ending at a time ti, at which throughput increase the mass flow / V(Ar(NO2) m ) of a value / V t =to(Ar(N02)m) to a value A / t=ti(Ar(NO2)m) > / V t =to(Ar(N02)m) and the mass flow N( H2) of a value / t =to(H2) to a value / t =ti(H2) > / V t =to(Ar(N02)m) continuously or at intervals, whereby during the period Atw (1) the mass flow of aromatic nitro compound fed to the reactor / V(Ar(NO2) m ) and (2) the difference between the flow rate of hydrogen supplied to the reactor and the flow rate of hydrogen removed from the reactor, A / V(H2) = / V(H2) - A / AUS(H2), are determined n times, where n is a natural number in the range from 10 to 3000, and wherein the supply of aromatic nitro compound to the reactor is interrupted when a fictitious value for a substance amount ni calculated according to the following equation after each of the n determinations FIK (Ar(NO2) m ) of non-hydrogenated aromatic nitro compound in the liquid phase (F) a previously defined maximum permissible value nMAx FIK (Ar(NO2) m ) reaches or exceeds: no FIK (Ar(NO2)m) = gz?{^(Ar(NO2) m ) ■ Ati(Ar(NO2) m )} - S ■ gz?{A^(H2) ■ Ati(H2)}, where / stands for a running index with which the n provisions are numbered, / Vi(Ar(NO2)m) as mass flow rate hi(Ar(NO2) m ) is specified, S stands for a stoichiometric factor, with S = 3 • m, AA / j(H2) is given as mass flow Anj(H2), and Ati(Ar(NO2)m ) and Ati(H2) each represent the period between two consecutive determinations.

2. Process according to claim 1, wherein the specific partial substance quantity related to the mass m / w_MAx(F) of the liquid phase is <7MAx FIK (Ar(NO2) m ) of the permissible maximum value n M Ax FIK (Ar(NO2) m ), in the range of 0.006 mol / kg to 0.550 mol / kg, where m / w_MAx(F) is the mass of the liquid phase at the time of the determination / , at which ni FIK (Ar(NO2) m ) equal to or greater than nMAx for the first time FIK (Ar(NO2) m ) is, stands.

3. The method according to claim 1 or 2, wherein n is in the range of 80 to 3000 and the period Atio is 5 min to 100 min.

4. A process according to claim 1 or 2, wherein n is in the range of 100 to 2500 and Atio is 7.5 min to 80 min.

5. A process according to claim 1 or 2, wherein n is in the range of 500 to 2200 and Atio is 15 min to 60 min.

6. The process according to claim 1 or 2, wherein n is in the range of 1000 to 2000 and Atio is 20 min to 40 min.

7. A process according to any one of claims 1 to 6, wherein the nitroaromatic is mononitrobenzene (Ar = CeHs; m = 1) or dinitrotoluene (Ar = CH3C6H3; m = 2).

8. A process according to any one of the preceding claims, wherein the hydrogenation is carried out at a temperature in the range of 80 °C to 200 °C.

9. A process according to any one of the preceding claims, wherein the catalyst comprises Pt, Pd, Rh, Ru, Ni, Co, Cu or mixtures thereof.

10. A process according to any one of the preceding claims, wherein the reactor is a stirred tank reactor, a falling film reactor or a bubble column reactor.

11. A process according to any one of the preceding claims, wherein the suspension of the catalyst is a suspension in an organic solvent and / or in aromatic amine (Ar(NH2)m)).

12. A process according to any one of the preceding claims, wherein the throughput increase comprises starting the hydrogenation from a state in which the reactor (R) is not in operation.

13. A computer system for controlling the conversion of aromatic nitro compound (ArfNChJm) in a process according to any one of claims 1 to 13, comprising: an interface unit configured to process the results of the n-fold determination of / V(Ar(NO2) m ) and A / V(H2); and a processor configured to process the read values ​​for / V(Ar(NO2) m ) and A / V(H2) is the amount of substance ni FIK (Ar(NO2) m) and compare it with the permissible maximum value nMAx stored and retrievable in a database that is communicatively connected to the processor FIK (Ar(NO2) m ) and when the permissible maximum value nMAx is reached or exceeded FIK (Ar(NO2) m ) to interrupt the supply of aromatic nitro compound to the reactor.

14. A computer program product comprising instructions which, when the computer program product is executed by the computer system according to claim 13, cause the computer system to carry out the method according to any one of claims 1 to 12.

15. Production plant for the continuous hydrogenation of an aromatic nitro compound (Ar(NO2) m ) with hydrogen (H2) in the presence of a catalyst to give an aromatic amine (Ar(NH2)m)) according to Ar(NO2) m + 3 m H2^ Ar(NH2) m+ 2 m H2O, wherein Ar is an aryl radical and m is 1 or 2, comprising: a reactor (R) for carrying out the hydrogenation; a device for providing the aromatic nitro compound and introducing it into the reactor; a device for providing the hydrogen and introducing it into the reactor; a device for discharging a product stream (PS) containing the aromatic amine and optionally unhydrogenated aromatic nitro compound; a device for retaining the catalyst during the discharge of the product stream (PS); and a computer system according to claim 13.

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