DEVICE FOR THE PROCESSING OF MIXED ACID AND WASTEWATER FROM THE NITRITION OF AROMATES

Description

[0001] The invention relates to a device for processing waste stream components from the nitration of aromatics, in which the nitric acid contained therein is converted by reaction with an aromatic under adiabatic conditions. The production of nitroaromatic compounds by nitration with mixed acid generates various waste streams. These include the mixed acid used in the reaction, which is diluted during the reaction; acidic wash water from the processing of the crude nitroaromatics; and dilute nitric acid, which is recovered during the exhaust gas treatment.

[0002] DE 196 36 191 A1 already discloses a process for the purification and concentration of used, contaminated sulfuric acid, which arises during the nitration of aromatic hydrocarbons in the presence of sulfuric acid. Here, the steam-volatile compounds are completely removed by decomposing the nitrogen-containing compounds, and the thus purified sulfuric acid is concentrated. The contaminated sulfuric acid is preheated and freed of steam-volatile compounds in countercurrent with the vapors from the first concentration stage at pressures between 200 and 1000 mbar. The sulfuric acid is fed to a first concentration stage, where it is concentrated under indirect heat input at the same pressure. The sulfuric acid is then concentrated to 88 to 97 wt.% in a single- or multi-stage vacuum concentration at a pressure lower than that in the first concentration stage.

[0003] The crude nitroaromatic resulting from nitration is contaminated with residues of the nitrating acid and byproducts and must be washed several times. First, the acid residues are removed by a process known as "acidic washing." The wastewater resulting from this process contains large amounts of nitric acid and sulfuric acid, as shown in EP 0 736 514 A1 using the example of dinitrotoluene.

[0004] DE10 2006 013 579 B3 describes a process for reducing the amount of wastewater in the production of dinitrotoluene (DNT) and simultaneously optimizing wastewater quality by reducing the proportion of organic contaminants. In a first step, the waste acid produced during nitration is preheated to 150 to 170 °C under atmospheric pressure conditions and then stripped in a column in countercurrent with steam generated by concentrating the sulfuric acid effluent from the column. A stripping steam quantity of between 0.25 and 10 wt.%, based on the amount of waste acid, is used, thereby producing nitric acid with 20 to 40 wt.% HNO3 at the top of the column. The nitric acid is then recycled back into the nitration process, either directly or after further concentration.In a second step, the pre-purified waste acid from the atmospheric stripping is stripped in a column at a vacuum of between 200 and 600 mbar in countercurrent with steam, which is generated by concentrating the sulfuric acid flowing out of the column. A stripping steam quantity of between 5 and 10% by weight based on the incoming pre-purified waste acid quantity is used, thereby obtaining a condensate at the top of the column which can be reused in the acid scrubbing of the DNT. The sulfuric acid flowing out of the vacuum stripping is concentrated in a downstream sulfuric acid concentration in one or more stages at a vacuum of between 150 and 30 mbar, preferably between 100 and 50 mbar, to concentrations of between 85 and 98% H 2 SO 4, thereby obtaining a condensate which, in addition to small amounts of sulfuric acid, contains only traces of nitroaromatic compounds in concentrations < 100 ppm.Nitrous exhaust gases from the individual process steps are cleaned by absorbing the NOx in countercurrent with water, thereby recovering nitric acid, which is then fed back into the nitration process directly or after concentration.

[0005] EP 2 295 375 B1 describes a process for the processing of waste acid from the production of nitroaromatics, in particular the production of dinitrotoluene (DNT) or trinitrotoluene (TNT), to obtain concentrated and purified sulfuric acid and nitric acid, wherein in a first stage the preheated waste acid, which in addition to up to 80 mass% sulfuric acid and water contains as further components nitric acid (HNO3), nitrosylsulfuric acid (as HNO2) and nitroorganics, in particular DNT and mononitrotoluene (MNT), is separated in a stripping column in countercurrent to steam obtained from the bottom of the stripping column by heating the preconcentrated sulfuric acid, into at least one vaporous phase containing nitric acid with or without nitroorganics and a preconcentrated sulfuric acid.In subsequent process steps, (i) the pre-concentrated sulphuric acid obtained from the bottom of the stripping column is subjected to further purification to remove nitroorganics and to higher concentrations, and (ii) the nitric acid and the nitroorganics obtained from the vaporous nitric acid phase, including the nitroorganics obtained during the further purification and concentration of the pre-concentrated sulphuric acid, are processed and returned to the nitration process.This process is further characterized in that, in the first process stage, in addition to stripping the preheated waste acid in a stripping column in countercurrent to the vapor from the sulfuric acid enrichment (V1), the nitric acid present in the stripping vapor is concentrated in countercurrent to additional purified and optionally fresh concentrated sulfuric acid with a concentration in the range of 75 to 97 mass% and preferably 80 to 96 mass%. The nitric acid vapors obtained from the top of the first-stage column are condensed directly to a highly concentrated nitric acid suitable for recycling to the nitration process.

[0006] US 4,496,782 A describes processes for recovering nitric acid from the spent acid phase of a mixed acid mononitration reaction, comprising adding a sufficient amount of nitric acid to provide at least about 2 wt. % nitric acid concentration in the spent acid phase from the mononitration reaction. By adiabatic reaction of a mononitroaromatic hydrocarbon in greater than a stoichiometric amount with the nitric acid in the spent acid phase, a dinitroaromatic hydrocarbon product and a nitric acid concentration of less than about 0.25 wt. % in the spent acid phase are then obtained.

[0007] US 4 650 912 A describes a process for the denitrification of the sulfuric acid and nitrous acid-containing spent acid phase from the nitration of an aromatic hydrocarbon by the mixed acid process, comprising the formation of a denitrification reaction medium by contacting the spent acid phase with an aromatic hydrocarbon under nitration reaction conditions to recover the nitric acid by forming a nitroaromatic hydrocarbon.This involves adding an amount of aromatic hydrocarbon slightly less than or equal to the stoichiometric amount required to degrade the spent acid phase of the nitric acid, and photometrically monitoring the denitrification reaction medium for the appearance of a dark red to black color. Upon detection of such a color, the molar ratio of aromatic hydrocarbon to nitric acid in the denitrification reaction medium is adjusted to eliminate the color.

[0008] US Pat. No. 8,907,144 B2 describes a process for the continuous adiabatic nitration of toluene to mononitrotoluene (MNT). The process yields a product quality of MNT comparable to that of isothermal production. The process uses excess toluene, with the reaction rate controlled to maintain a residual nitric acid content of 0.003–0.102 wt.% in the spent acid and an orange-red color of the spent acid. Further process conditions include reconcentrated sulfuric acid at 83–99 °C with a sulfuric acid concentration of 66–70.5 wt.%. This is mixed with nitric acid to form a mixed acid containing 1.0–3.8 wt.% nitric acid, and toluene is added at a rate of 1.1–1.71 mol toluene / mol nitric acid.

[0009] Although the processes described in US 4,496,782 A and US 4,650,912 A should be significantly cheaper than processes involving stripping because they convert the nitric acid, which therefore does not require reconcentration, they have not been adopted. Both processes add the aromatic compound to the nitric acid only in a substoichiometric to maximum stoichiometric ratio, so that the nitric acid is not fully converted. US 4,650,912 A even describes the occurrence of a dark red to black coloration, which occurs when the aromatics are added in excess of stoichiometric amounts and makes safe, continuous operation of these processes difficult. Both processes are also limited to the processing of the mixed acid from the nitration reaction, and the other waste streams containing nitric acid must be treated separately.In the adiabatic nitration process described in US Pat. No. 8,907,144 B2, toluene is added in stoichiometric excess, but here, too, the reaction is controlled so that not all of the nitric acid is converted. Furthermore, this process is neither a process for the adiabatic production of MNT nor a process for the processing of waste acid from the isothermal nitration of aromatics.

[0010] DE 10 2017110084 A1 describes a production plant for the nitration of organic aromatics, but does not address the treatment of resulting mixed acids or wastewater. The production plant includes a mixing unit, a pump, an injection unit, a tubular reactor, and a separator. Heat exchangers are not used.

[0011] DE 39 03 661 A1 describes a device for the treatment of waste acid containing nitric and sulfuric acid using heat exchangers.

[0012] EP 0 937 680 A1 describes a device for concentrating sulfuric acid and, if necessary, for purifying the sulfuric acid, consisting of at least one natural circulation evaporator system formed by a two-part vapor dome and a heat exchanger.

[0013] The invention is based on the object of providing a device with which nitric acid-containing waste streams arising from the nitration of aromatics can be effectively processed. Preferably, all relevant components, namely the mixed acid used, which is diluted during the reaction, the acidic wash water from the processing of the crude nitroaromatics, and the dilute nitric acid arising from the exhaust gas treatment, are processed together.

[0014] This object is achieved according to the invention by a device for processing a waste acid and acidic wash water from a system comprising a nitration of aromatics, a NO x absorption process, a nitroaromatic wash and optionally a di-nitration process, as defined in claims 1-8.

[0015] Furthermore, a process for the processing of waste streams from the nitration of aromatics is described herein, which can be carried out in the device according to the invention and in which nitric acid contained therein is converted by reaction with an aromatic under adiabatic conditions, which is characterized in that a) at least one waste stream component selected from waste acid (mixed acid) obtained during the nitration, acidic wash water from the processing of crude nitroaromatics and dilute nitric acid obtained during off-gas treatment in the course of the nitration is provided, b) the at least one waste stream component is mixed with reconcentrated sulfuric acid, c) an aromatic is metered into the mixture in a stoichiometric excess based on the nitric acid, d) the resulting reaction mixture is reacted in an adiabatically operated reactor, e) the resulting organic phase is separated from the sulfuric acid phase in a separator, f) the sulfuric acid phase is concentrated in vacuo, and g) at least a partial stream of the reconcentrated sulfuric acid from step g) is used in step b).

[0016] Preferably, at least two waste stream components and particularly preferably all of the above-mentioned waste stream components are premixed in step a) and then mixed with the reconcentrated sulfuric acid.

[0017] The process enables the complete removal of nitric acid from nitration processes containing nitric acid, preferably both, by adiabatic nitration and reaction with an aromatic or nitroaromatic compound. The reaction conditions and reactor are designed in such a way that the problems described in similar processes do not occur and the process is also viable in practical operation. This can be achieved surprisingly easily by using a continuously operated reactor and recycling a partial stream of the reconcentrated, purified sulfuric acid. This procedure surprisingly allows the aromatic compound to be used in a stoichiometric excess relative to the nitric acid contained in the mixture, allowing the nitric acid to react completely.The problems with over-stoichiometric dosing described in US 4 650 912 A are surprisingly not observed in the process described here.

[0018] The process is described below using the example of the nitration of toluene to dinitrotoluene. All pressures are given as absolute pressure. The process is, of course, also applicable to all other aromatic compounds that can be nitrated singly, doubly, or multiply. Examples include the nitration of benzene to nitrobenzene or dinitrobenzene; of toluene to mononitro(MNT), dinitro(DNT), or trinitrotoluene (TNT); of nitrochlorobenzene (NCB) to mononitrochlorobenzene (MNCB) or dinitrochlorobenzene (DNCB), etc. The aromatics listed here serve only as examples and do not limit the scope of the process. Therefore, all information and preferred embodiments also apply to the nitration of other aromatics.

[0019] Figure 1 shows a simplified block flow diagram for a process for producing DNT with acid processing according to the process described herein. In the mononitration, toluene is nitrated to mononitrotoluene (MNT) with the addition of nitric acid and sulfuric acid from the dinitration. The resulting crude MNT, a mixture of the various isomers with a proportion of DNT, is then further nitrated to DNT in the dinitration with further addition of nitric acid and reconcentrated sulfuric acid. The resulting crude dinitrotoluene is largely freed of adhering nitric acid and sulfuric acid in an acidic wash.

[0020] Preferably, the waste acid 1 resulting from the mono-nitration is mixed with the acidic wash water from the acidic scrubbing and the recovered nitric acid from a NOx absorption process typically included in nitrations. The mixing can take place in a simple container. Alternatively, state-of-the-art mixing devices such as stirrers, static mixers, or similar devices can be used.

[0021] The mixture is preferably heated by recovering the energy from the reconcentrated sulfuric acid 2 obtained during the sulfuric acid concentration, which is cooled in the process. However, heating can also be achieved by indirect steam heating or a combination of steam heating and energy recovery. The preheating of the mixture is adjusted so that the temperature after addition of the reconcentrated sulfuric acid 1 is in the range of 70 °C to 130 °C. Uncooled reconcentrated sulfuric acid 1 is then added. The reconcentrated sulfuric acid 1 used for adiabatic nitration can have the same or a lower concentration than the reconcentrated sulfuric acid 2 used in the dinitration.The quantity and concentration of the reconcentrated sulfuric acid 1 are selected to produce a mixed acid with a sulfuric acid content between 60% and 70% H 2 SO 4 and a nitric acid content between 1% and 5% HNO 3 . The required concentration and temperature in the concentration stage can be determined by the vacuum set during the concentration stage. The expert can refer to the current literature on the physical properties of sulfuric acid, such as Perry's Chemical Engineers' Handbook (McGraw Hill).

[0022] If no reconcentrated sulfuric acid from a previous process is available in the start-up phase of the reaction, fresh sulfuric acid of the appropriate concentration can be used instead.

[0023] The temperature at the reactor inlet is set in the range 70 °C to 130 °C, preferably in the range 90 °C to 110 °C, so that the reaction starts immediately after the aromatic compound is added. Using a special dosing system, the aromatic compound, for example toluene or MNT, is metered into the circulating sulfuric acid-nitric acid-water mixture and finely dispersed. As an alternative to pure MNT as a single isomer or as an isomer mixture, crude MNT from mono-nitration, which contains a proportion of DNT, can also be used. In the adiabatic nitration, the nitric acid present in the mixture is almost completely converted with the aromatic compound to form the nitroaromatic compound in an adiabatically operated reactor. With toluene, MNT is predominantly obtained as an isomer mixture; when MNT is used, DNT is predominantly obtained in the reaction.

[0024] A tubular reactor is preferred as the reactor. Stirred reactors are also possible. However, the tubular reactor has the advantage of requiring significantly less space. The tubular reactor is preferably modular in design and contains static mixing elements for remixing the reaction media. According to the invention, the number of static mixing elements is between 2 and 20. The mixing elements result in a pressure drop across the reactor. The operating pressure at the reactor inlet is in the range of 1 bar to 10 bar, preferably in the range of 3 bar to 6 bar.

[0025] The aromatic compound is added in a stoichiometric excess of 1% to 20%, preferably 3% to 10%, based on the amount of nitric acid to ensure complete reaction. A higher excess would also be possible and would not interfere with the reaction, but is not desirable, as the organic substances are separated again in the next step.

[0026] After the nitric acid has been converted in the adiabatic reactor, the sulfuric acid and the organic product phase are separated. This is preferably done by exploiting the different densities of the sulfuric acid and the resulting nitroaromatic, for example, a toluene-MNT or an MNT-DNT mixture. Simple state-of-the-art containers with or without internals, or even centrifuges, can be used as separators. The pressure in the separator is preferably between 1 bar and 2 bar. This prevents the product mixture of aromatics and nitroaromatics from partially evaporating, which would impede separation. The pressure loss across the reactor corresponds to the introduced mixing energy.The separated organic phase can then be fed separately or together with the organic compounds recovered during sulfuric acid concentration into the mono- and / or di-nitration for further nitration or alternatively be processed directly.

[0027] Surprisingly, the color phenomena described in US Pat. No. 4,650,912 A, which occur when the aromatic compound is added in excess of stoichiometric amounts, were not observed in the process described here. This could be due to the elevated temperature and the different acid composition resulting from the recycling of purified, reconcentrated sulfuric acid.

[0028] The separated waste sulfuric acid 2 has absorbed the reaction energy through the adiabatic process and has thus heated by 10 to 40°C. The waste acid 2 is flashed into an evaporator, which is preferably operated at a pressure between 30 mbar and 500 mbar, and thus pre-concentrated. Further concentration of the sulfuric acid is carried out using conventional state-of-the-art equipment for concentrating sulfuric acid and can be carried out in one or more stages, depending on the plant's capacity. If necessary, organic solvent can also be metered into the condensation system of the sulfuric acid concentration in accordance with the state of the art to prevent deposits of nitroaromatics with higher melting points. The reconcentrated sulfuric acid 1 is recycled for adiabatic nitration.The remaining sulfuric acid can, if necessary, be further concentrated using state-of-the-art processes and is used for di-nitration as reconcentrated sulfuric acid 2. Further concentration is carried out at a vacuum between 150 and 30 mbar, preferably between 100 and 50 mbar, to the desired final concentration of between 85 and 98% H 2 SO 4 . Depending on the concentration used in the respective nitration process, any sulfuric acid concentration required for the nitration process can be set for the reconcentrated sulfuric acid 2. The concentrations specified are examples only and are not intended to limit the process.

[0029] The process condensate from the sulfuric acid concentration can be partially used for acid scrubbing. All waste gases generated in the process steps are directed to NO x absorption, and the nitrous gases they contain are recovered as nitric acid. The resulting nitric acid is mixed with waste acid 1 and fed into the adiabatic nitration plant for conversion.

[0030] The process described here offers the main advantage over the prior art that all of the nitric acid is converted and does not need to be reconcentrated. In the processes currently used according to DE 10 2006 013 579 B3 and EP 2 295 375 B1, in which the nitric acid is stripped from the waste sulfuric acid, energy is required for this stripping and subsequent concentration of the nitric acid. In the process described here, however, no energy input is required; on the contrary, the adiabatic operation even makes the released reaction energy usable for the reconcentration of the sulfuric acid. Furthermore, the process described here does not require any columns for stripping, which means that the building required to set up the apparatus for carrying out the process can be constructed much smaller and thus the investment costs correspondingly lower.

[0031] The present invention relates to an apparatus for carrying out the above method as defined in claims 1-8.

[0032] The device consists of a mixing unit in which the waste acid from the nitration, the recovered dilute nitric acid from the absorption of nitrous gases during the process, and the acidic wash water from the acidic wash of the nitroaromatics are mixed. Furthermore, the device comprises at least one heat exchanger for preheating the resulting mixture, which is to be processed by the process described herein, and a pump with which this mixture is mixed with a first reconcentrated sulfuric acid from the described process and which can generate the pressure required at the reactor inlet. Furthermore, the device comprises an injection unit in which the aromatic to be nitrated is metered into the mixture via nozzles and finely distributed.The device further comprises a modular tubular reactor with between 2 and 20 static mixers, in which the aromatic nitration takes place, as well as a separator downstream of the tubular reactor, in which the mixture emerging from the tubular reactor is separated into an organic phase and an acid phase. Furthermore, the device according to the invention comprises a flash evaporator and a concentration unit with an indirectly heated heat exchanger and an associated evaporator with associated vapor condensation, as well as a vacuum unit.

[0033] Within the scope of the present invention, a single- or multi-stage sulfuric acid concentration unit is additionally installed, in which residual sulfuric acid not used for adiabatic nitration is further concentrated. According to the invention, the sulfuric acid concentration unit is designed to provide the first reconcentrated sulfuric acid.

[0034] Figure 2 shows the simplified structure of an apparatus according to the invention for carrying out the process described herein. The apparatus according to the invention consists of a mixing unit M1 in which the waste acid from the nitration, the recovered nitric acid from the NO x absorption, and the acidic wash water from the nitroaromatics wash are mixed. A simple tank without internals can serve as the mixing unit M1. Alternatively, a tank with a stirring device or an apparatus with static mixers can be used.

[0035] The mixture is then preheated in a heat exchanger W1, preferably by indirect heat exchange with the reconcentrated sulfuric acid 2 flowing from the sulfuric acid concentration and with a heat exchanger W2 indirectly heated with steam. Optionally, only one of the two preheaters can be used. The preheated mixture is added to the suction side of pump P1 together with reconcentrated sulfuric acid from flashing F1 and concentration K1. Pump P1 mixes the two streams and generates the liquid pressure required at the reactor inlet. The mixture flows through injection unit I1, where the aromatic to be nitrated is metered into the mixture via nozzles and finely distributed. The reaction mixture then flows through tubular reactor R1. This has a modular design and consists of pipe sections of different lengths.According to the invention, between 2 and 20 static mixers are installed between the pipeline sections to ensure the appropriate remixing of the reaction mixture. Reactor R1 is followed by a separator S1, in which the organic phase is separated from the acid phase. The resulting sulfuric acid phase is then fed into a flash evaporator F1, which is operated under vacuum. The relative positioning of separator S1 to F1 is selected such that, at the set vacuum, the acid phase overflows independently from S1 to F1 due to the difference in pressure. Here, water, the organic components still contained in the sulfuric acid (depending on its solubility), and HNO2 spontaneously evaporate from the sulfuric acid through flash evaporation until the acid has cooled to boiling temperature corresponding to the set vacuum.Subsequently, or alternatively integrated into the flash evaporator, the sulfuric acid is indirectly heated and concentrated to the desired concentration of the reconcentrated sulfuric acid 1 using the concentration unit K1, which, when combined with F1, consists only of a corresponding indirectly heated heat exchanger. In the case of a separate concentration unit following F1, K1 consists of an indirectly heated heat exchanger and an associated evaporator. The portion of the reconcentrated sulfuric acid required for adiabatic nitration from the flashing and concentration F1 + K1 is fed to the suction side of pump P1. The water vapor obtained during evaporation in F1 + K1 is condensed either separately or together with the vapors from the subsequent sulfuric acid concentration (process condensate). Inert gases are removed by a vacuum unit.An indirectly cooled vacuum pump is preferably used as the vacuum unit, since both cooling by direct water supply and the use of steam-driven vacuum jets would increase the total wastewater volume.

[0036] The remaining sulfuric acid not used for adiabatic nitration is fed into a connected sulfuric acid concentration unit. In this unit, the acid is concentrated in one or more stages under vacuum, according to the state of the art. Here, too, an indirectly cooled vacuum pump (or several, depending on performance) is preferably used as the vacuum unit, since both cooling by direct water supply and the use of steam-driven vacuum jets would increase the total wastewater volume. If F1+K1 and the sulfuric acid concentration unit are operated at the same vacuum, a shared vacuum unit can also be used. All exhaust gases from the individual process steps are directed to NOx absorption to recover any nitrous exhaust gases as nitric acid.

[0037] Common evaporator types, such as natural circulation evaporators, forced circulation evaporators, horizontal evaporators, etc., are used to heat and concentrate the acid. Corresponding processes for sulfuric acid concentration are well known and will not be discussed in detail here. Corrosion-resistant materials such as glass or PTFE-lined steel are used for reactor R1, separator S1, flash evaporator F1, the evaporator at K1, and the evaporators in the sulfuric acid concentration. For steam-heated heat exchangers such as W2, the heater in K1, and the heaters in the sulfuric acid concentration, corrosion-resistant materials such as tantalum, which are commonly used for sulfuric acid concentration, are used. Materials such as silicon carbide or tantalum are used for acid-acid heat exchangers such as W1.Corrosion-resistant materials such as enamel or PTFE-lined steel are used for pipelines in contact with hot sulfuric acid. Suitable stainless steels are used for equipment and machinery in contact with process condensate.

[0038] The following examples further illustrate the invention.

[0039] Example 1 illustrates how the individual mass flows can interact with each other. The individual mass flows can, of course, also vary in their quantitative ratios and composition, depending on the process from which the mass flows originate and how the respective nitration, scrubbing, and NOx absorption are operated. Example 1 is intended to describe the process in more detail, but the values ​​given are not intended to limit the process. All percentages refer to weight percent.

[0040] In a nitration process for the production of DNT, 9,859 kg / h of waste acid are produced with the following composition: 71% H 2 SO 4 , 0.8% HNO 3 , 1.6% HNO 2 , 0.5% DNT, 26.1% H 2 O. Furthermore, 2,260 kg / h of wash water are produced with the following composition: 8% H 2 SO 4 , 17% HNO 3 , 1% HNO 2 , 1% DNT, 73% H 2 O. From the NO x absorption, 811 kg / h of nitric acid with 55% HNO 3 and 45% H 2 O are recovered. The three streams are mixed and preheated to 90 °C. Subsequently, 16,086 kg / h of uncooled, reconcentrated sulfuric acid 1 containing 75% H 2 SO 4 and 25% water at approximately 112 °C are added to the mixture, which corresponds to the boiling point of the sulfuric acid in the associated reconcentration stage at 80 mbar. 1,462 kg / h of toluene are injected into the resulting mixture, which then has a temperature of approximately 102 °C, and the mixture is fed into the adiabatic tubular reactor.In the adiabatic tubular reactor, the mixture is further mixed by successive static mixers to achieve a complete reaction. In Example 1, 10 mixing elements were used. At the outlet of the adiabatic reactor, a mixture is obtained containing 63.14% H 2 SO 4 , 0.59% HNO 2 , 6.45% MNT, 0.24% DNT, 0.44% toluene and 29.1% H 2 O. HNO 3 has reacted and is only present in traces, if at all. The temperature of the reaction mixture has increased to approximately 130 °C due to the reaction energy released and the adiabatic reaction. The reaction mixture is passed into a separator. The organic phase separates at the surface and is drawn off. The sulfuric acid phase is then adulterated in an evaporator at 80 mbar and pre-concentrated accordingly by the resulting water evaporation.The acid is then further concentrated to 75% H2SO4 by indirect heating at 80 mbar using standard state-of-the-art methods for concentrating sulfuric acid. During the concentration of the sulfuric acid, the dissolved organic compounds and the HNO2 contained therein are evaporated from the sulfuric acid together with the water. The vapors are condensed, and the organic phase is separated from the resulting aqueous phase using standard state-of-the-art technology. To prevent deposits of DNT, a solvent, in this case MNT, is added to the condensate system using standard state-of-the-art technology. The remaining sulfuric acid not recycled for adiabatic nitration is further concentrated to 93% H2SO4 in a vacuum of 80 mbar using standard steam heating using standard state-of-the-art methods for concentrating sulfuric acid.This reconcentrated sulfuric acid 2 is then cooled, using part of the energy for preheating the incoming mixture as described above, and then fed to di-nitration.

[0041] Example 2should also show how the individual mass flows can relate to one another. Here, MNT is used instead of toluene as the aromatic for the conversion of nitric acid. In a nitration process for the production of DNT, 24,951 kg / h of waste acid are produced with the following composition: 72% H 2 SO 4 , 1% HNO 3 , 1.3% HNO 2 , 0.6% DNT, 25.1% H 2 O. Furthermore, 5,650 kg / h of wash water are produced with the following composition: 6% H 2 SO 4 , 15% HNO 3 , 1% HNO 2 , 1% DNT, 77% H 2 O. From the NO x absorption, 1,598 kg / h of nitric acid with 55% HNO 3 and 45% H 2 O are recovered. The three streams are mixed and preheated to 85 °C. 50,033 kg / h of uncooled, reconcentrated sulfuric acid 1 containing 74% H 2 SO 4 and 26% water are then added to the mixture at approximately 114 °C, which corresponds to the boiling point of the sulfuric acid in the associated reconcentration stage at 100 mbar. The resulting mixture, which then has a temperature of approximately102.6 °C, 3,630 kg / h of toluene are injected and the mixture is fed into the adiabatic tubular reactor. In the adiabatic tubular reactor, the mixture is further mixed by successive static mixers in order to achieve a complete reaction. In the example, 15 mixing elements were used. At the outlet of the adiabatic reactor, a mixture is obtained containing 63.78% H 2 SO 4 , 0.44% HNO 2 , 0.25% MNT, 6.82% DNT and 28.71% H 2 O. HNO 3 has reacted and is only present in traces, if at all. The temperature of the reaction mixture has increased to approx. 125 °C due to the reaction energy released and the adiabatic reaction. The reaction mixture is fed into a separator. The organic phase separates on the surface and is drawn off. The sulfuric acid phase is then falsified in an evaporator at 100 mbar and pre-concentrated accordingly by the resulting water evaporation.The acid is then further concentrated to 74% H 2 SO 4 at 100 mbar by indirect heating, for example, with steam, using conventional state-of-the-art methods for sulfuric acid concentration. The sulfuric acid not recycled for adiabatic nitration is further concentrated to 94% H 2 SO 4 in a vacuum of 50 mbar by indirect heating with steam, using conventional state-of-the-art methods for sulfuric acid concentration.

Claims

1. An apparatus for processing a waste acid and acid wash water from a system comprising a nitration of aromatics, a NOx absorption process, a nitroaromatic wash and optionally a dinitration process, comprising: a mixing unit configured to receive a waste acid from the nitration of aromatics, a recovered dilute nitric acid from the NOx absorption and an acidic wash water from the nitroaromatic wash and to dispense a mixture; at least one heat exchanger configured to preheat the mixture from the mixing unit to produce a preheated mixture; a pump configured to mix the preheated mixture with a first reconcentrated sulfuric acid; an injection unit downstream of the pump comprising one or a plurality of nozzles configured to meter and finely disperse an aromatic to be nitrated into the preheated mixture and the sulfuric acid; a modular tubular reactor comprising from 2 to 20 static mixers configured to receive the preheated mixture with the sulfuric acid and the aromatic to be nitrated; a separator downstream of the modular tubular reactor configured to separate an organic phase from an acid phase; and a sulfuric acid concentration downstream of the separator comprising an indirectly heated heat exchanger and an evaporator with associated vapor condensation and a vacuum unit, wherein the sulfuric acid concentration is configured to provide the first reconcentrated sulfuric acid.

2. Apparatus according to claim 1, wherein the pump is moreover configured to generate an operating pressure required at the inlet of the modular tubular reactor.

3. Apparatus according to any one of the preceding claims further comprising a flash evaporator downstream of the separator.

4. Apparatus according to any one of the preceding claims, further comprising an additional single or multi-stage sulfuric acid concentrator downstream of the sulfuric acid concentration, in which residual first reconcentrated sulfuric acid which is not used for adiabatic nitration is further concentrated to a second reconcentrated sulfuric acid.

5. Apparatus according to claim 4, wherein the at least one heat exchanger is configured to preheat the mixture from the mixing unit by indirect heat exchange with the second reconcentrated sulfuric acid.

6. Apparatus according to any one of the preceding claims, further comprising a supply of the organic phase separated by the separator, to the nitration of aromatics and / or to the dinitration process.

7. Apparatus according to any one of the preceding claims, further comprising a supply of first reconcentrated sulfuric acid from the sulfuric acid concentration to the dinitration process.

8. Apparatus according to any one of the preceding claims, further comprising a supply of at least a portion of the first reconcentrated sulfuric acid from the sulfuric acid concentration to the nitroaromatic wash.