Process and apparatus for recovery of nitrated aromatics from wastewaters

Abstract

A process to recover nitrated aromatic species and aromatic reactant species from wastewater generated during the production of nitrated aromatic compounds. The process comprises a liquid-liquid extraction step wherein the nitrated aromatic species present in the wastewater phase is extracted into an organic phase comprising primarily the aromatic reactant. In a second step, the wastewater, now substantially free of the nitrated aromatic species, but still containing aromatic reactant, is fed to a flash evaporation step to recover the remaining aromatic reactant. The resulting wastewater is free of the nitrated aromatic compound as well as the aromatic reactant.

Description

[0001] Process and Apparatus for Recovery of Nitrated Aromatics from Wastewaters

[0002] Field of the Invention

[0003] The invention pertains to processes and apparatuses for recovery of nitrated aromatic species which are present in wastewater generated during the production of nitrated aromatic species.

[0004] Background of the Invention

[0005] The adiabatic nitration of various aromatic species is a known process in the art. During the adiabatic production of nitrated aromatic species, sulfuric acid and nitric acid are combined to form a mixed or “nitrating” acid. This nitrating acid is combined with the aromatic reactant, which is typically fed in stoichiometric molar excess with respect to the nitric acid, and the mixture is fed together to the inlet of a nitration reactor. The sulfuric acid provides dual functionality, serving as a catalyst to increase the rate of the nitration reaction, as well as absorbing the resulting heat released by the exothermic nitration reaction. As the nitration reactor operates adiabatically, heat is not removed from the reaction mixture over the course of the reaction. The conserved heat is used to re-concentrate the diluted or spent sulfuric acid, thus increasing the energy efficiency of the process compared to isothermally operated aromatic nitration processes. Examples of adiabatic nitration processes in the patent literature are disclosed in US 2,256,999 (Castner), US, 4,021,498 (Alexanderson et al.), US 5,313,009 (Guenkel et al.), US, 8, 357, 827 (Munnig et al.), US 8,604,256 (Berretta et al.), US 8,692,035 (Berretta) for mononitration of benzene; in US 5,648,565 (Konig et al.) and US 8,907,144 (Gattrell) for mononitration of toluene; and in US 4,453,027 (Vaidyanathan) and US 6,586,645 (Demuth et al.) for mononitration of chlorobenzene.

[0006] At the outlet of the nitration reactor, a liquid separator is used to separate the nitrated aromatic species and the spent acid phase. The spent acid phase contains the sulfuric acid and water introduced at the inlet of the nitration reactor, but also contains anyexcess of water that is introduced via the nitric acid feed, water produced during aromatic nitration reaction as well as, to a lesser extent, dissolved nitrated aromatics and reactant aromatics. In adiabatic nitration processes, it is desirable to re-concentrate and recycle the sulfuric acid back to the nitration reactor to minimize the quantity of fresh sulfuric acid consumed in the process. To obtain the required concentration of sulfuric acid, the spent sulfuric acid is directed to a flash evaporator where water as well as dissolved nitrated and feed organic species are evaporated using the heat released over the course of the adiabatic aromatic nitration reaction. The acid concentration step is carried out under vacuum pressure to lower the sulfuric acid boiling point. Water and dissolved organics that evaporate from the acid are subsequently condensed in a heat exchanger and the resulting condensate is recovered. The condenser which is used to condense the water and organics from the evaporator is operated at the same pressure as the evaporator. A vacuum system, typically connected at the outlet of the condenser, is used to generate the sub-atmospheric operating pressure necessary for the reconcentration of sulfuric acid that is ultimately recycled to the nitration reactor.

[0007] The condensate exiting the condenser consists of two immiscible liquids, namely, an aqueous phase consisting primarily of water separated from the sulfuric acid in the acid re-concentration step, and an organic phase consisting primarily of nitrated aromatic species and to a much lesser extent aromatic reactant species. Although the phases are immiscible, some dissolution of water in the organic phase will still occur up to the saturation limit, and vice versa for the nitrated organic species in the aqueous phase. These phases are separated, and the aqueous phase obtained may be used for other downstream processing steps for the purification of the nitrated aromatic compound. These processing steps can include, for example, acidic, alkaline or neutral washing of the crude nitrated aromatic compound. Ultimately the aqueous phase is rejected from the production plant as a wastewater stream, whereby the dissolved organics are first recovered and recycled to the nitration process prior to further processing to meet environmental regulatory requirement.The organic phase, consisting primarily of the nitrated aromatic species, as well as any unreacted aromatic reactant species, is blended with the balance of the organic phase obtained from the nitration reactor separator and directed to a series of washing steps to remove impurities. This typically involves an acid washing step to remove entrained and dissolved nitrating acids from the nitrated aromatic species, and an alkaline washing step to remove nitro-hydroxy-aromatic byproducts formed over the course of the aromatic nitration reaction process. Additional neutral washing steps may also be included after the alkaline washing stage to extract any residual salt that may be carried over in the nitrated organic phase from the alkaline washing step. Examples of the washing steps used in adiabatic nitration processes are disclosed in US 6,936,741 (Munnig et al.), US 7,326,816 (Knauf et al.), US 7,344,650 (Knauf et al.), and US 9,040,758 (Berretta). Outputs from these washing steps include crude nitrated aromatic species free of nitrating acids and organic acid by-products, an acidic wastewater stream characterized with a pH between 0-7 containing primarily water and the extracted nitrating acids, and an alkaline wastewater stream characterized with a pH of 7-14 containing primarily nitro-hydroxy-aromatic byproducts and / or their alkali salts. The washed organic phase may then be distilled or stripped to recover the unreacted aromatic reactant for recycling back to the nitration reactor.

[0008] Washing steps are commonly carried out by dispersing the crude nitrated aromatic within a continuous recirculating stream of wash water, typically by means of a mechanical or static mixer. Dispersing the organic phase increases its interfacial area which accelerates the mass transfer of impurities from the nitrated aromatic wash water. The dispersion is then separated, and the individual phases are recovered. A key outcome of the energy-intensive mixing process is that the wash water not only extracts impurities from the crude nitrated aromatic stream but also dissolves a small portion of the nitrated aromatic species. Likewise, the crude nitrated aromatic bulk phase exiting the wash step will saturate with water.

[0009] Separation of the immiscible wash water and nitrated aromatic phases is commonly accomplished using gravity separators, which allow for coalescence and setting of thedroplets generated in the dispersing stage. Separators are sized to remove a target droplet diameter, such that separation efficiency is 100% for droplets greater than the target diameter. Below the target droplet diameter, separation efficiency diminishes as droplet size decreases. When droplet sizes fall below approximately 30pm, gravity separation becomes increasingly challenging, as the settling force approaches the magnitude of forces caused by turbulent fluctuations within the bulk flow. As a result, some entrainment of the nitrated organic phase inevitably occurs in the form of droplets, which along with the dissolved nitrated aromatic, results in an overall residual organic content which must be removed from the wastewater in a separate separation step.

[0010] To maintain optimal separation efficiency and minimize phase entrainment, various process variables must be carefully monitored and controlled. Variables that impact bulk liquid phase separation include mixing energy input at the dispersion stage which impacts the dispersed droplet size, feed flow rates which impact separator residence time, and temperature which affects both viscosity and density, and therefore separation characteristics of the liquids. US 8,604,256 (Berretta et al.) also describes the effect of pH on separation performance, with a pH near neutral conditions leading to difficult phase separation. Operational disturbances, including sudden load changes, prolonged idle conditions, surface-stabilizing impurities, and variations in the composition of incoming feeds to the separators, can arise during the process. These factors can adversely affect key variables, resulting in reduced separation efficiency and increased phase entrainment. Prolonged suboptimal separation can result in excessive entrainment of the nitrated aromatic phase, posing significant environmental and safety risks to downstream processing units. Additionally, it can lead to severe equipment fouling, potentially causing plant shutdowns.

[0011] The wastewater streams generated in the washing steps as well as the sulfuric acid reconcentration steps may be blended or may remain separated for further processing. Commonly, the acidic wastewater stream obtained from the acidic washing step may be blended with the aqueous phase obtained in the sulfuric acid re-concentration process, but kept separate from the alkaline wastewater obtained from the alkaline washing step.This is advantageous as the alkaline wastewater contains dissolved nitro-hydroxy-aromatic byproducts which are toxic to biomass employed for biological treatment of the plant’s wastewater. Hence the alkaline wastewater stream typically requires further pretreatment prior to discharge into a biological effluent plant. Commonly employed pretreatment steps include thermal degradation of the nitro-hydroxy-aromatic species under high temperature, high pressure conditions, either in the presence or absence of oxygen. Examples of such pre-treatment steps are disclosed in EP 0953546 (Papkalla et al), US 4,230,567 (Larbig), US 6,288,289 (Boyd et al.), US 8,801,932 (Gattrell), US 7,344,650 (Knauf et. al.) and US 5,250,193 (Sawicki et al.). Maintaining a separate alkaline wastewater stream minimizes the flow rate entering the thermal degradation process, and therefore reduces capital and operating costs associated degradation of the nitro-hydroxy-aromatic byproducts.

[0012] Whether the various wastewater streams are blended or maintained as separate streams, the wastewater generated in the nitration plant contains residual nitrated aromatic species and to a lesser extent residual feed aromatic species both in the dissolved and entrained form, which it is desirable to separate and recover in order to maximize the plant’s product yield, as well as to reduce the carbon and nitrogen loading on further wastewater treatment processes such as biological treatment.

[0013] Several processes have been proposed and employed for recovering nitrated aromatic species as well as aromatic reactant species from wastewater. One method includes stripping the wastewater using steam. Although a relatively simple process to employ, the disadvantages of steam stripping are: a) high energy consumption in the form of steam, b) the use of steam generates additional wastewater, and c) it requires the installation of a considerable number of process equipment including pressure vessels, pumps, heat exchangers, columns and column internals. A further disadvantage is realized for cases where the acidic and alkaline wastewater streams are kept separate for further postprocessing of the nitro-hydroxy-aromatic contained in the alkaline wastewater. In this case, two separate stripping column trains are required to handle the acidic and alkaline wastewater individually. Steam stripping equipment for the acidicwastewater stripping column must be constructed of a material that is corrosion-resistant to dilute sulfuric and nitric acid concentrations at or above 100°C, or, alternatively, the wastewater stream must first be neutralized with a base such as sodium hydroxide to increase the pH prior to steam stripping. Both options further increase the cost of recovery of dissolved nitrated aromatic compounds from the wastewater generated in the aromatic nitration production process. An example of a process for removal of organic species from alkaline wastewater is disclosed in US 7,344,650 (Knauf et al.).

[0014] A further disadvantage is realized in the event the crude nitrated aromatic washing steps are operated outside the prescribed operating range, resulting in excessive organic entrainment in the wastewater phase. Under these conditions, steam stripping alone cannot remove the free phase nitrated aromatic species without a substantial increase to the steam flow to the stripping column, leading to potential contamination of biological treatment processes. If thermal degradation is employed for removal of nitro-hydroxy-aromatic species, the presence of free phase nitrated aromatics can lead to a significant safety hazard due to exposure of the nitrated aromatic to temperatures near or potentially exceeding the onset temperature for an extended period.

[0015] There is a need in the adiabatic aromatic nitration art for a process that recovers nitrated aromatic species and aromatic reactant species from the alkaline and / or acidic wastewater streams generated during the adiabatic nitration of aromatic species in such a way that additional thermal energy input for recovery of nitrated aromatic species and reactant aromatic species is minimized or eliminated, while simultaneously reducing the cost of the associated equipment required for the recovery process. Furthermore, there is a need for a process that effectively removes both dissolved organic compounds and excess entrained organic droplets, particularly in the event of a process upset.

[0016] Summary of the Invention

[0017] The present invention provides a continuous process for the removal of nitrated aromatic species and aromatic reactant species from wastewater generated in theproduction of nitrated aromatic products. For convenience of description, references to wastewater in the following disclosure typically refer to either acidic or alkaline wastewater generated in the aromatic nitration process. However, the invention applies any wastewater stream generated within a production plant for nitrated aromatic species that contain dissolved and entrained nitrated and aromatic reactant species to be recovered, including but not limited to acidic wastewater produced from acid wash of the nitrated aromatic species, alkaline wastewater produced from alkaline wash of the nitrated aromatic species, neutral wastewater produced from neutral wash of the nitrated aromatic species, aqueous condensate generated by re-concentration of sulfuric acid required for the adiabatic aromatic nitration reactor, and any combination thereof. The removal of nitrated aromatic and aromatic reactant species from wastewater generated as a by-product of the adiabatic nitration of aromatic species is accomplished using a liquid extraction process with the aromatic reactant as the solvent followed by a flash evaporation process under sub-atmospheric conditions. Both the nitrated aromatic species and the aromatic reactant species are recovered, thereby minimizing yield loss. The use of an extraction stage recovers both dissolved and entrained nitrated aromatic species from the wastewater, while the use of a sub-atmospheric flash evaporation step after the extraction step recovers the aromatic reactant, thereby reducing feed losses and energy consumption compared to prior art processes. The process of the invention can be embedded within an existing production process, and it can utilize equipment already required for the adiabatic nitration reaction process.

[0018] In the liquid extraction step, a wastewater stream saturated in nitrated aromatic compound and potentially containing entrained free phase nitrated aromatic droplets is extracted with the aromatic reactant compound used for the nitration reaction. The use of the aromatic reactant as the solvent is advantageous for several reasons. First, the nitrated aromatic compound is fully soluble in the aromatic reactant phase, but only sparingly soluble in the aqueous phase. This promotes the nearly complete transfer of nitrated aromatic species from the aqueous phase to the aromatic reactant. As well, the aromatic reactant is typically less soluble in the aqueous phase compared to thenitrated aromatic species, resulting in reduced overall organic content in the wastewater to be removed at the conclusion of the extraction step. The aromatic reactant also has a significantly higher vapor pressure compared to the nitrated aromatic species, which promotes easier separation by thermal means such as evaporation. Both of these characteristics result in a reduced energy requirement for subsequent recovery of the aromatic reactant from the extracted wastewater. Further, the use of aromatic reactant as a solvent eliminates the requirement to use additional chemicals not typically present in the adiabatic nitration process for the extraction process, which would otherwise require a dedicated separation process for solvent recovery, and which can lead to other product or wastewater contamination issues.

[0019] Finally, in the event of a process upset, whereby excessive entrainment of free phase nitrated aromatic species may develop after the completion of a washing step, the use of the aromatic reactant as an extraction solvent promotes the removal of the entrained nitrated aromatics from the wastewater phase, which may not be effectively eliminated using conventional separation means such as gravity settling vessels nor steam stripping columns. This minimizes the possibility of free-phase organics from entering downstream processing units and leads to improved operability and safety.

[0020] The extraction process is accomplished by mixing the wastewater with a stream of aromatic reactant compound for a suitable period of time to allow for mass transfer between phases, followed by separation of the two phases. As the nitrated aromatic compound is more soluble in the aromatic reactant compound than in the aqueous wastewater phase, dissolved nitrated aromatic will preferentially transfer from the aqueous phase to the organic phase. Additionally, entrained free phase nitroaromatic initially present in the wastewater phase will be absorbed into the organic phase. At the end of the extraction step, the organic phase will contain nearly all the nitrated aromatic compound, and the aqueous phase will be substantially free of the nitrated aromatic compound. The organic phase may then be blended with fresh aromatic reactant feed and fed to the nitration reactor.During the extraction process, the close interaction between the wastewater and the aromatic reactant solvent causes a portion of the solvent to dissolve into the wastewater up to its saturation limit. As cost of the aromatic reactant is one of, if not the highest, cost of operation to produce nitrated aromatics, the wastewater stream obtained after the extraction step still retains significant economic value to be recovered. Therefore, after the extraction step, the wastewater is introduced to a flash evaporation step. In this step, the pressure is reduced below atmospheric pressure, promoting the evaporation of the aromatic reactant compound. After the flash evaporation step, the resulting wastewater stream will contain a significantly lower concentration of both the nitrated aromatic compound and the aromatic reactant compound and will be suitable for further treatment steps well known to those skilled in the art such as nitro-hydroxy-aromatic degradation, neutralization and biological treatment (in the case of alkaline wastewater) or neutralization and biological treatment (in the case of acidic wastewater).

[0021] Surprisingly, the inventors have determined that if the flash evaporation step is carried out at the same pressure as the sulfuric acid re-concentration step employed for the adiabatic nitration reaction process, a significant reduction, or in some cases, complete elimination of heat input can be achieved to recover the aromatic reactant as well as meet the target wastewater specification in comparison with the existing art.

[0022] The inventors have further found that the use of the same condenser and vacuum system used in the sulfuric acid re-concentration step offers another advantage, in that this equipment may be used to condense the aromatic reactant vapor produced in the wastewater flash evaporator with only marginal increase to the condenser’s duty and vacuum system’s vapor load. This is due to the organic phase already present in the sulfuric acid evaporator condenser, which inherently consists of 95-99 percent by weight nitrated aromatic. The presence of the nitrated aromatic in the organic condensate stream promotes absorption of the aromatic reactant vapor, allowing complete or nearly complete condensation of the aromatic reactant without the need for chilled water, excessive heat transfer area, or additional vacuum capacity that would otherwise be required to condense the aromatic reactant vapor generated in the wastewater flashevaporator. The aromatic reactant is ultimately recovered from the organic condensate in a downstream distillation or steam stripping process that is typically included in the nitrated aromatic production process. It is recycled back to the nitration reactor.

[0023] According to one aspect of the invention there is provided a process to recover nitrated aromatic species and aromatic reactant species from wastewater generated in a plant for the nitration of the aromatic reactant species, comprising the steps of: (a) extracting the nitrated aromatic species from the wastewater by means of: (i) mixing the wastewater with a stream of the aromatic reactant species to produce an aqueous phase and an organic phase, and allowing the nitrated aromatic species to transfer into the organic phase; and (ii) separating the aqueous phase and the organic phase; (b) feeding the aqueous phase produced in step (a) to a flash evaporator and operating it under sub-atmospheric pressure to evaporate the aromatic reactant species in the aqueous phase, thereby producing a vapour stream comprising the aromatic reactant and a liquid stream comprising purified wastewater; and (c) feeding the organic phase produced in step (a) to a nitration reactor.

[0024] According to another aspect of the invention there is provided an apparatus for recovering nitrated aromatic species and aromatic reactant species from wastewater generated in a plant for the nitration of the aromatic reactant species, comprising:

[0025] (a) a mixer arranged to receive and mix a stream comprising an organic aromatic reactant phase and a stream comprising an aqueous wastewater phase;

[0026] (b) a separator vessel arranged to receive and separate the aqueous phase and the organic phase;

[0027] (c) a flash evaporator arranged to receive the aqueous phase from the separator vessel and to operate under sub-atmospheric pressure to produce a vapour stream comprising the aromatic reactant and a liquid stream comprising purified wastewater;

[0028] (d) a vacuum system operatively connected to the flash evaporator and to a sulfuric acid re-concentration evaporator and condenser in the plant; and

[0029] (e) a nitration reactor in the plant arranged to receive a stream of the organic phase from the separator vessel.Further aspects of the invention and features of specific embodiments of the invention are described below.

[0030] Brief Description of the Drawings

[0031] Exemplary embodiments of the invention are illustrated in the drawings. The embodiments and figures disclosed herein are to be considered illustrative of the invention rather than restrictive.

[0032] Figure 1 is a schematic block diagram of an embodiment of the invention integrated into an adiabatic aromatic nitration process.

[0033] Figure 2 is a schematic block diagram of the main unit operations of an embodiment of the apparatus of the invention.

[0034] Figure 3 is a schematic block diagram of the invention according to one embodiment.

[0035] Figure 4 is a schematic block diagram of the invention according to a further embodiment.

[0036] Figure 5 is a schematic block diagram of the invention according to an embodiment having two extraction stages.

[0037] Figure 6 is a graph showing a sensitivity analysis of operating conditions according to an example application in a mononitrobenzene plant.

[0038] Detailed Description of the Invention

[0039] Throughout this description and the claims:

[0040] • The term “nitrated aromatic”, “nitrated aromatic species” or “nitrated aromatic compound” refers to chemical species consisting of a benzene ring and one ormore nitro functional groups bonded to the benzene ring. The benzene ring may be further substituted prior to its nitration, for example with one or more methyl groups or one or more halogen groups. Examples of nitrated aromatic species include mononitrobenzene, dinitrobenzene, mononitrotoluene, and nitrochlorobenzene.

[0041] • The term “aromatic reactant species” or “aromatic reactant” or “unreacted aromatic” refers to the organic reactant fed to one or more reactors to generate the nitrated aromatic product. The organic reactant is typically benzene, a methylated substituent of benzene such as toluene or xylene, or a halogenated substituent of benzene such as chlorobenzene.

[0042] • The term “wastewater” refers to an aqueous waste stream generated during the nitration of aromatic reactants that contains dissolved nitrated aromatic species and aromatic reactant species and optionally entrained nitrated and reactant aromatic species that are to be removed prior to discharge of the wastewater to downstream processing. Examples include acidic wastewater produced from acid wash of the nitrated aromatic species, alkaline wastewater produced from alkaline wash of the nitrated aromatic species, neutral wastewater produced from neutral wash of the nitrated aromatic species, aqueous condensate generated by re-concentration of sulfuric acid required for the adiabatic aromatic nitration reaction, or any combination thereof.

[0043] • The term “purified wastewater” refers to an aqueous waste stream generated during the nitration of aromatic reactants that is depleted or substantially depleted of dissolved nitrated aromatic species and aromatic reactant species and is of an acceptable quality for the discharge to downstream processing. • The term “flash evaporator” or “flash evaporation” refers to a process where a feed stream partially vaporizes after a reduction in pressure, the vapor and liquid in equilibrium with each other are separated, whereby the more volatile component becomes more concentrated in the vapor phase.

[0044] Figure 1 illustrates an embodiment of the invention in combination with an adiabatic nitration process. Blocks outlined in dashed lines in the figures represent unit operationsor equipment that may be found within an existing adiabatic nitration process, whereas blocks outlined in solid lines represent new unit operations introduced in accordance with the present invention. As shown in Figure 2, the new unit operations are a mixer 100, a wastewater liquid separator 101, and a wastewater flash evaporator 102.

[0045] For the purpose of illustration of one embodiment, the acidic wash process will be referenced in the description that follows; but it will be understood that the same concept can be applied to any other wastewater generated in the adiabatic aromatic nitration production process, including alkaline wastewater formed in an alkaline wash process, neutral wastewater formed in a neutral wash process, a blend of alkaline and acidic wastewater, aqueous condensate generated through the re-concentration of sulfuric acid, and any other wastewater stream generated in the adiabatic nitration process, and any combination thereof.

[0046] Referring to Figure 1 , in a process 10 to recover nitrated aromatic species and aromatic reactant species, a stream of fresh aromatic reactant 20 enters the adiabatic nitration process, and a slip stream 5 of the aromatic reactant is first taken for the liquid extraction process. The balance 21 of the fresh aromatic reactant 20 is mixed, for example in a feed vessel 200, with the aromatic reactant 7 which is obtained as explained below from the extraction separator 101, forming a stream of combined aromatic reactant 22. The feed vessel 200 may also receive aromatic reactant from other sources in the nitration plant, such as other extraction processes or overheads from the distillation or stripping of the washed nitrated aromatic species. In some embodiments, a feed vessel is not used. The combined aromatic reactant 22 is fed to the nitration reactor 201 along with fresh nitric acid reactant 23, re-concentrated sulfuric acid 27, and optionally with fresh sulfuric acid and recovered nitric acid obtained from treatment of the plant’s vent gas.

[0047] The product mixture 24 from the nitration reactor 201 is discharged into a nitration phase separator 202, where an organic phase 1 consisting of crude nitrated aromatics and unreacted aromatic reactant is separated from the spent acid 26. A stream of thespent acid 26 enters a sulfuric acid evaporator 203, where water and dissolved organic species 28 such as nitrated aromatic and aromatic reactant are evaporated from the spent acid 26 at reduced pressure, thereby re-concentrating the sulfuric acid required for the nitration reaction. The re-concentrated acid 27 produced by the sulfuric acid evaporator 203 is circulated back to the inlet of the nitration reactor 201, along with the blended aromatic reactant 22 and fresh nitric acid 23. The vapors 28 formed in the sulfuric acid evaporator 203 are condensed in a condenser 204 which operates at or near the same pressure as the sulfuric acid evaporator. Residual, non-condensed gaseous components 31 at the outlet of the condenser 204 are introduced to a vacuum system 205, which produces the sub-atmospheric pressure required for the sulfuric acid evaporator 203.

[0048] Condensate 30 consisting of an aqueous liquid phase and an organic liquid phase from the condenser 204 as well as condensate 32 generated within the vacuum system 205 are mixed and the mixture is subjected to a liquid phase separation, where an organic condensate stream and an aqueous condensate stream are produced (not shown). A portion of the aqueous condensate, consisting primarily of water and dissolved nitrated aromatic and aromatic reactant species, may be used as wash water for acidic and alkaline washing of the nitrated aromatic species, or it may be directly added to the plant’s overall wastewater effluent stream. The organic condensate may be blended with the crude nitrated aromatic 1 and the blend is then further worked up to meet the product quality specification. Non-condensable components 33 exiting the acid concentrator vacuum system 204 are treated in the production plant’s vent treatment system.

[0049] The crude nitrated aromatic 1 that exits the nitration phase separator 202 is routed to an acid wash 300 where wash water 2 is contacted with the crude nitrated aromatic 1 to remove entrained and dissolved mineral acids from the crude nitrated aromatic.

[0050] Optionally, the crude nitrated aromatic 1 exiting the separator 202 may first be cooled by exchanging heat with the aromatic reactant 22 entering the nitration reactor 201. The nitrated aromatic species 3 exits the acid wash 300. It may be fed to washing stepssuch alkaline or neutral washing (not shown). A distillation or steam stripping step (not shown) may be used to recover unreacted aromatic reactant from the washed nitrated aromatic 3, and the recovered aromatic reactant is blended with the fresh feed aromatic reactant 20 and subsequently recycled to the nitration reactor 201.

[0051] The wastewater stream 4 generated during the acid wash 300 exits the acid wash containing dissolved nitrated aromatic species and may also contain entrained droplets of nitrated aromatic species, which it is desirable to recover. The wastewater 4 is combined with the aromatic reactant slip stream 5 and mixed thoroughly in a wastewater mixer 100, creating an immiscible two-phase liquid dispersion 6. The nitrated aromatic compound, having a much higher solubility in the aromatic reactant phase than in the aqueous phase, preferentially transfers from the aqueous phase to the organic phase within the wastewater mixer 100.

[0052] The two-phase liquid dispersion 6 from the wastewater mixer 100 is discharged to a wastewater liquid separator 101. The separator 101 produces an aqueous stream 8 from which the nitrated aromatic has been removed, and an organic stream 7 consisting of the aromatic reactant along with the extracted nitrated aromatic. The organic stream 7, containing some dissolved nitrated aromatic compound, is mixed with the balance of the fresh aromatic reactant feed 21 and fed to the nitration reactor 201.

[0053] The aqueous phase 8 exiting the liquid separator 101 still contains some dissolved aromatic reactant, which it is desirable to recover and reuse within the nitration process. The aqueous phase 8 is fed to a wastewater flash evaporator 102 where the dissolved aromatic reactant and a portion of the wastewater is flashed under reduced pressure into a vapor phase 11. The purified wastewater stream 9 exiting the wastewater flash evaporator 102 is now free of the nitrated aromatic compound as well as the aromatic reactant and is ready for further workup, for example by neutralization and subsequent biological treatment.The vapor 11 generated in the wastewater flash evaporator 102 is fed to the vapor side inlet of the same condenser 204 that is used to condense the vapor 28 generated in the sulfuric acid evaporator 203. In the condenser 204, the vapor 11 comprising water vapor and aromatic reactant evaporated in the wastewater flash evaporator 102 is condensed along with the vapor 28 from the sulfuric acid evaporator 203 consisting of water vapor and nitrated aromatic vapor. The organic phase generated in the condenser 204 effectively absorbs the aromatic reactant from the vapor 11, resulting in substantially complete condensation of the aromatic reactant. As the vacuum system 205 is hydraulically connected to the sulfuric acid evaporator 203, the condenser 204 and the wastewater flash evaporator 102, the vacuum system 205 serves to provide the necessary operating pressure for both sulfuric acid reconcentration and wastewater flash evaporation simultaneously. Thus, in this embodiment, the vapor 11 generated by the wastewater flash evaporator 102 is handled using equipment that is already employed in the existing adiabatic aromatic nitration process, so further new equipment for condensation and vacuum generation is not required.

[0054] Figure 3 illustrates one embodiment of the arrangement of equipment and process streams required to achieve the target residual nitrated aromatic and aromatic reactant composition in the purified wastewater stream, wherein the target residual content of each of the nitrated aromatic and aromatic reactant species are < 10 ppm by weight. The process conditions required to meet the target concentration are also described below. With reference to Figure 3, the fresh aromatic reactant stream 20 entering the plant is split into two streams 21, 5. One stream 21 is routed directly to a feed vessel 200. The feed vessel is an optional component of the apparatus, which can be used to provide buffer capacity for the aromatic feed to the nitration reactor and also serve as a mixing vessel for fresh aromatic reactant 20 and recycled aromatic reactant obtained from downstream distillation or steam stripping of the nitrated aromatic. The aromatic reactant slip stream 5 is mixed with wastewater 4 from the acid wash and directed to a mixer 100, where droplets of either the aromatic reactant or the wastewater are generated to increase the interfacial area of the immiscible liquids and thereby increasethe rate of mass transfer of nitrated aromatics from the aqueous wastewater phase to the organic aromatic reactant phase.

[0055] Although Figure 3 shows pure aromatic reactant is fed to the mixer 100, many alternative options exist for the source of aromatic reactant. For example, the blended aromatic reactant stream 22 can be recirculated upstream of the mixer 100, allowing for any desired feed rate of aromatic reactant to the mixer 100. Alternatively, aromatic reactant may be sourced from the overhead distillate of the nitrated aromatic distillation or steam stripping separation process, or it may be a blend of fresh aromatic reactant, distillate from the distillation or stripping process, or any other combination of aromatic reactant streams that are known to persons skilled in the art to exist in the adiabatic nitration process. Thus, the selection of fresh aromatic reactant in a once-through flow pattern as in Figure 3 is used in one embodiment of the invention but is not a general limitation. However, to limit the concentration of nitrated aromatic compounds entering the liquid extraction process, the preferred source of aromatic reactant is the fresh feedstock 20 to the nitration production plant.

[0056] In one embodiment, as in Figure 3, the mixer 100 is a single static mixer followed by a pipe. In other embodiments, multiple static mixers may be arranged in series, or other mixing devices may be used, such as a pump, agitator, orifice, plurality of orifices, perforated plate or plurality of perforated plates, or any other mixing device or combination of devices known to those skilled in the art of liquid extraction processes. In a preferred embodiment, a static mixer is used to generate a narrow droplet size range, allowing for greater control of the mass transfer rate as well as simpler design of the downstream separator vessel. Regardless of the mixing technology used, an energy input of 10-750 W / kg is used to generate sufficiently small droplets to ensure that complete mass transfer occurs within a reasonable mixing and contacting volume prior to phase separation. Once the dispersed phase is generated in the mixer 100, a residence time in the range of 1-150 seconds is used, alternatively 15-45 seconds, to allow sufficient time for the mass transfer of dissolved nitrated aromatic species as wellas any residual droplets of nitrated aromatic species from the wastewater phase to the aromatic reactant phase.

[0057] The wastewater mixer 100 and wastewater liquid separator 101 are depicted as separate entities in the embodiments illustrated in the figures; however, they may also be designed to be a single vessel, whereby the separator contains a compartment that provides sufficient dispersed phase contact time and an additional compartment for phase separation. Additionally, in other embodiments of the invention, the wastewater mixer 100 does not need to be located outside the separator vessel 101, but may be integrated into the same vessel, so long as there is sufficient mixing and contacting time for the dispersed droplets in the continuous phase for mass transfer and there is sufficient time for separation of the phases. The liquid separator vessel 101 may be a gravity settling vessel or any other apparatus known to those skilled in the art of liquid phase separation, such as a centrifuge.

[0058] In a preferred embodiment of the invention as illustrated in Figure 3, the aromatic reactant and wastewater are mixed in a static mixer 100. The static mixer is connected to a gravity separator by a pipe. The static mixer in conjunction with the liquid flow provides the energy required for dispersion of the aromatic reactant phase within the aqueous wastewater phase. The static mixer volume in conjunction with the connecting pipe volume provides the contact time required for mass transfer to occur. The dispersed flow 6 enters a horizontal separator 101 where the dispersed organic phase is separated from the aqueous phase via gravity separation. The gravity separator 101 may optionally contain internal components such as a wire mesh or parallel plate packing to promote coalescence of the dispersed phase, and therefore promote a shorter phase separation time.

[0059] After sufficient contact time in the mixer 100 is provided for the dispersed wastewater and aromatic reactant phases, the dispersion 6 is fed to the wastewater liquid separator 101, thereby generating a wastewater phase 8 containing a lower concentration of nitrated aromatic species compared to the inlet wastewater stream 4, and an organicphase 7 consisting primarily of the aromatic reactants as well as the extracted nitrated aromatic species 7. The organic phase 7 may be mixed with fresh aromatic reactant 20 as depicted in Figure 3. Alternatively, the organic phase 7 may be fed directly to the nitration reactor or may be blended with any other stream within the nitration process that consists primarily of the aromatic reactant.

[0060] The aqueous stream 8 is routed from the wastewater liquid separator 101 to the wastewater flash evaporator 102. The pressure in the wastewater flash evaporator 102 is set by the requirements of the sulfuric acid re-concentration process within the adiabatic aromatic nitration reaction process. Typical operating pressures range from 4 mbarto 20 mbar, with 6-10 mbar being preferred. However, the pressure is ultimately dependent on the type of aromatic nitration being carried out, the required sulfuric acid concentration for the nitration reactor and the minimum cooling water temperature available at the plant production site. The low pressure, in combination with the high relative volatility of the dissolved aromatic reactant with respect to water leads to a significant reduction in dissolved aromatic reactant in the liquid phase. The evaporated aromatic reactant 11 is then condensed in the condenser 204 used for the condensation of vapor formed in the sulfuric acid re-concentration process as described above. The treated wastewater phase 9 exits the wastewater flash evaporator 102 with little or no nitrated aromatic species or aromatic reactant species and may be fed to any further wastewater processing steps required to meet site requirements.

[0061] Depending on the temperature of the organic reactant slip-stream 5 or the wastewater stream 4, the resulting temperature of the aqueous stream 8 obtained after mixing and liquid phase separation may not be sufficient for depressurization alone to achieve the required aromatic reactant concentration in the wastewater stream 9. Factors that may affect the temperature of aqueous stream 8 include changes to ambient conditions, or low operating temperature of the upstream process generating wastewater stream 4. Alternatively, depending on site requirements, more stringent organic concentration targets for the purified wastewater stream may need to be achieved. Therefore, depending on these additional factors, it may be necessary to preheat the wastewaterstream 8 exiting the liquid separator 101 prior to its entrance to the wastewater flash evaporator 102.

[0062] An inherent advantage of the invention is that, unlike atmospheric steam stripping, the wastewater feed to the flash evaporation step need not be preheated to temperatures at or exceeding 100°C to reach the required operating temperature. This is due to the boiling point suppression of the wastewater under the sub atmospheric operating pressure of the flash evaporator 102. Therefore, the aqueous stream 8 may be heated using waste heat obtained from elsewhere in the nitrated aromatic nitration process. For example, waste heat from the distillation or steam stripping process used to separate the aromatic reactant from the nitrated aromatic may be used to increase the temperature of the aqueous stream 8 before it enters the wastewater flash evaporator 102. In the preferred embodiment, the temperature of the aqueous stream 8 enters the flash evaporator 102 at a temperature between 30-95°C, preferably 60-80°C.

[0063] The wastewater flash evaporator 102 may be designed in accordance with standard design practices known to those skilled in the art of liquid / vapor separation. Optionally, a demister is included within the wastewater flash evaporator 102 above the feed location of the aqueous stream 8 to minimize liquid entrainment to the evaporated reactant aromatic stream 11 connected to the condenser 204. Preferably, in the event the extraction process is carried out separately for both acidic wastewater and alkaline wastewater, a demister is included in the wastewater flash evaporator 102 utilized for the alkaline wastewater flash step to avoid alkaline wastewater droplet carryover into the condenser 204.

[0064] In some embodiments, liquid extraction technology other than that depicted in Figure 3 is used to achieve the required extraction target. For example, a packed extraction column, a spray column, a perforated plate column, a pulsed perforated plate column, a rotating disk column, or any other mass transfer apparatus known to those skilled in the art of liquid extraction technology, may be employed.Figure 4 depicts an embodiment of the process in which blended reactant aromatic is circulated through the liquid extraction step, as opposed to the once-through configuration illustrated in Figure 3. In the embodiment of Figure 4, a slip stream 5 of the blend of fresh aromatic reactant 20 and the organic stream 7 from the wastewater liquid separator 101 are circulated to the mixer 100.

[0065] Figure 5 depicts a further embodiment of the liquid extraction process in which two extraction stages A and B are used in a countercurrent arrangement. Between one and four extraction stages may be incorporated depending on the plant’s specific wastewater requirements. The extraction stages may be operated in a countercurrent or crossflow arrangement.

[0066] Referring to Figure 5, two mixers, 100a and 100b as well as two separators 101a and 101b are provided. In this embodiment, the fresh aromatic reactant 5b is fed to the second extraction phase B first, where it is contacted with wastewater 4b exiting the separator 101a of the first extraction stage A. The remaining extraction is carried out to minimize the nitrated aromatic species contained in the wastewater before it is sent to the wastewater flash evaporator 102 for aromatic reactant recovery. The aromatic reactant solvent 5a from extraction stage B, now containing a small quantity of nitrated aromatic, is routed to extraction stage A where it contacts and extracts the bulk nitrated aromatics from the wastewater 4a entering from an upstream process, such as an acid or alkaline wash. The reactant aromatic phase 7 exiting the separator 101a of extraction phase A is blended with the balance 21 of the reactant aromatic 20 to form the aromatic reactant feed 22 for the nitration reaction.

[0067] Figure 6 shows a sensitivity analysis to some process parameters in a specific example mononitrobenzene plant. The graph correlates the concentration of mononitrobenzene in acidic wastewater at various levels of free-phase organic entrainment with the benzene to wastewater mass ratio used for the extraction step. The concentration of mononitrobenzene remaining in the wastewater is reported after the flash evaporation step, which occurs at a feed temperature of 70°C and an operating pressure of 10 kPa.Generally, it is beneficial to achieve residual nitroaromatic concentrations in wastewater of less than 10ppm at the outlet of the flash evaporation step. The data represented graphically in Figure 6 is shown in the following table.

[0068]

[0069] Figure 6 shows that for operation with little to no entrainment of nitroaromatic in the wastewater, a mass ratio of aromatic reactant 5 to wastewater 4 fed to the mixer 100 may be in the range of 0.2: 1 to 4: 1 , preferably 0.25: 1 to 0.5: 1 , to reduce the nitroaromatic concentration to the desired level. In this case, the continuous phase in mixer 100 would be the aqueous phase, while the organic phase would be present as the dispersed phase. For more stringent wastewater nitroaromatic limits, for example 1ppm nitroaromatic in the wastewater, or to ensure complete removal of entrained nitroaromatic in the wastewater, the mass ratio of aromatic reactant 5 to wastewater 4 fed to the mixer may be increased to a range of 1 :1 to 4:1.

[0070] For extraction processes carried out using alkaline wastewater however, it may be preferable to operate the mixer 100 with a mass ratio of aromatic reactant 5 to wastewater 4 in the range of 1.5:1 to 4:1 to generate a continuous organic phase and a dispersed aqueous phase. Operating in this range helps ensure complete transfer of both dissolved and potentially entrained free-phase nitrated aromatic droplets from the wastewater phase to the organic phase and prevents carryover of the free-phase nitrated aromatic to downstream thermolysis, should this process be utilized in the nitration plant.

[0071] Examples

[0072] The following examples are simulations done using Aspen Plus version 14 . Examples 1 and 2 are comparative simulations based on prior art methods. Examples 3 and 4 are simulations in accordance with the present invention.

[0073] Example 1 (Comparative)

[0074] An acidic wastewater stream consisting of wash water used for acidic washing of crude nitrobenzene as well as condensate obtained from re-concentration of sulfuric acid used for the reaction of benzene and nitric acid was fed to a live steam stripping column. The blended wastewater stream was saturated in nitrobenzene (approximately 2400 ppm percent by mass at 38°C) and contained no entrained nitrobenzene. The stripping column was operated at atmospheric pressure and was equipped with a pre-feed interchanger to recover heat from the stripped wastewater and preheat the feed to 80°C, as well as a condenser to condense the overhead water and organic vapor and a cooler to reduce the bottoms temperature to that required for downstream biological treatment. The column was specified for ten equilibrium stages. A steam flow of 0.095 kg / kg of feed was required to achieve a final nitrobenzene concentration of 2 ppm bymass in the bottoms. All steam added to the column was ultimately rejected from the plant and condensed as additional wastewater.

[0075] Example 2 (Comparative)

[0076] The same stripping column, heat exchanger configuration, feed flow rates and operating conditions were selected as described Example 1 , but in addition to dissolved nitrobenzene, the wastewater contained an additional 2 percent by mass free phase nitrobenzene droplets. The steam flow to the column was kept constant at 0.095 kg / kg feed. After cooling the stripper bottoms to 40°C, the wastewater remained saturated with nitrobenzene (approximately 2400 ppm by weight) and contained 1 percent by weight free phase nitrobenzene.

[0077] Example 3 (Invention)

[0078] An acidic wastewater stream consisting of wash water used for acidic washing of crude nitrobenzene as well as condensate obtained from re-concentration of sulfuric acid used for the reaction of benzene and nitric acid was fed to a liquid extractor. The blended wastewater stream was saturated in nitrobenzene (approximately 2400 ppm percent by mass at 38°C) and contained no entrained nitrobenzene. Benzene was added to the wastewater at a ratio of 0.45 kg per kg of wastewater. The resulting wastewater obtained from phase separation contained 6 ppm nitrobenzene and was saturated in benzene (approximately 1900 ppm by mass at an extraction temperature of 36°C). The wastewater was heated to 70°C using waste heat from the bottoms of the benzene distillation column located elsewhere in the nitrobenzene plant and its pressure was reduced in a flash evaporator to 10 kPa absolute, resulting in partial evaporation of the wastewater and benzene. The selected pressure coincided with the operating pressure of the sulfuric acid evaporator used for re-concentration of the sulfuric acid used for the nitrobenzene reaction. The wastewater obtained after pressure reduction contained 2 ppm nitrobenzene and 7 ppm benzene. Aside from energy recovered from the distillednitrobenzene, no further steam or other form of thermal energy was added to the process, and no additional wastewater was generated.

[0079] Example 4 (Invention)

[0080] The same extraction process and operating conditions were selected as Example 3, but in addition to dissolved nitrobenzene, the wastewater contained an additional 2 percent by mass free phase nitrobenzene droplets. At the outlet of the extraction process, the wastewater contained 6 ppm nitrobenzene and was saturated in benzene (approximately 1900 ppm by mass at an extraction temperature of 36°C). After partial evaporation of the wastewater obtained from the extraction step, the resulting wastewater contained 2 ppm nitrobenzene and 7 ppm benzene by mass.

[0081] Throughout the foregoing description and the drawings, in which corresponding and like parts are identified by the same reference characters, specific details have been set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

[0082] As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the following claims.

Claims

Claims1. A process (10) to recover nitrated aromatic species and aromatic reactant species from wastewater generated in a plant for the nitration of the aromatic reactant species, comprising the steps of:(a) extracting the nitrated aromatic species from the wastewater by means of:(i) mixing (100) the wastewater (4) with a stream (5) of the aromatic reactant species (20) to produce an aqueous phase and an organic phase, and allowing the nitrated aromatic species to transfer into the organic phase; and(ii) separating (101) the aqueous phase (8) and the organic phase (7); (b) feeding the aqueous phase (8) produced in step (a) to a flash evaporator (102) and operating it under sub-atmospheric pressure to evaporate the aromatic reactant species in the aqueous phase, thereby producing a vapour stream (11) comprising the aromatic reactant and a liquid stream (9) comprising purified wastewater; and(c) feeding the organic phase (7) produced in step (a) to a nitration reactor (201).

2. The process according to claim 1 , further comprising:(d) recycling the aromatic reactant in the vapour stream (11 ) produced in step (b) to the nitration reactor (201).

3. The process according to claim 1 or 2, wherein step (a) is carried out in a single extraction stage.

4. The process according to claim 1 or 2, wherein step (a) is carried out in up to four extraction stages.

5. The process according to any one of claims 1 to 4, wherein the flash evaporator (102) is operatively connected to a condenser (204) and vacuum system (205) used for sulfuric acid re-concentration (203) in the plant.

6. The process according to any one of claims 1 to 5, wherein the aqueous phase produced in step (a)(ii) contains less than 10 ppm of nitrated aromatic.

7. The process according to any one of claims 1 to 6, wherein the purified wastewater (9) produced in step (b) contains less than 10 ppm of aromatic reactant and less than 10 ppm of nitrated aromatic species.

8. The process according to any one of claims 1 to 7 wherein no additional heat input is used to carry out the vacuum flash evaporation of step (b).

9. The process according to any one of claims 1 to 8, wherein a residence time of step (a)(i) is in the range of 1 to 150 seconds, alternatively in the range of 15 to 45 seconds.

10. The process according to any one of claims 1 to 9, wherein an energy input in step (a)(i) is in the range of 10 to 750 W / kg11. The process according to any one of claims 1 to 10, where a mass ratio of aromatic reactant to wastewater in step (a)(i) is in the range of 0.2:1 to 4:1 , alternatively in the range of 0.25: 1 to 0.5: 1.

12. The process according to any one of claims 1 to 10, wherein step (a)(i) produces a continuous aqueous phase and a dispersed organic phase.

13. The process according to any one of claims 1 to 10, wherein step (a)(i) produces a continuous organic phase and a dispersed aqueous phase.

14. The process according to any one of claims 1 to 7, wherein the aqueous phase produced in step (a)(ii) is heated to a temperature in the range of 30-95°C, alternatively in the range of 60-80°C, prior to step (b).

15. The process according to any one of claims 1 to 14, wherein the evaporation in step (b) is carried out at a pressure in the range of 4 to 20 mbar, alternatively in the range of 6 to 10 mbar.

16. The process according to any one of claims 1 to 15, wherein the nitrated aromatic species is mononitrobenzene.

17. The process according to any one of claims 1 to 15, wherein the nitrated aromatic species is a species selected from the group consisting of dinitrobenzene, mononitrotoluene, dinitrotoluene, mononitrochlorobenzene, and dinitrochlorobenzene.

18. The process according to any one of claims 1 to 15, wherein the wastewater (4) has a pH in the range of 0-14.

19. The process according to any one of claims 1 to 15, wherein the wastewater (4) is acidic, having a pH below 7.

20. The process according to any one of claims 1 to 15, wherein the wastewater (4) is alkaline, having a pH above 7.

21. The process according to any one of claims 1 to 20, wherein the aromatic reactant used in step (a)(i) is sourced from fresh feed to the nitration production plant.

22. The process according to any one of claims 1 to 20, wherein the aromatic reactant used in step (a)(i) is a blend of fresh reactant and recycled aromatic reactant recovered from within the nitration production process.

23. The process according to any one of claims 1 to 20, wherein the aromatic reactant used in step (a)(i) is recycled aromatic reactant recovered from within the nitration production process.

24. The process according to any one of claims 1 to 23, wherein the aqueous phase of the wastewater (4) is saturated in the nitrated aromatic species and contains up to 5 percent by weight of an entrained nitroaromatic phase.

25. An apparatus for recovering nitrated aromatic species and aromatic reactant species from wastewater generated in a plant for the nitration of the aromatic reactant species, comprising:(a) a mixer (100) arranged to receive and mix a stream (5) comprising an organic aromatic reactant phase and a stream (4) comprising an aqueous wastewater phase;(b) a separator vessel (101) arranged to receive and separate the aqueous phase and the organic phase;(c) a flash evaporator (102) arranged to receive the aqueous phase from the separator vessel (101) and to operate under sub-atmospheric pressure to produce a vapour stream (11) comprising the aromatic reactant and a liquid stream (9) comprising purified wastewater;(d) a vacuum system (205) operatively connected to the flash evaporator (102) and to a sulfuric acid re-concentration evaporator and condenser (203) in the plant; and (e) a nitration reactor (201 ) in the plant arranged to receive a stream of the organic phase (7) from the separator vessel (101).

26. The apparatus of claim 25, wherein the nitration reactor (201 ) is arranged to receive a recycled stream of the aromatic reactant from the flash evaporator (102).

27. The apparatus according to claim 25 or 26, wherein the mixer (100) and separator vessel (101) comprise an extraction stage.

28. The apparatus according to claim 27, wherein the apparatus comprises a single extraction stage.

29. The apparatus according to claim 27, wherein the apparatus comprises up to four extraction stages.

30. The plant according to any one of claims 25 to 29, wherein the mixer (100) comprises an in-line static mixer.

31. The apparatus according to any one of claims 25 to 30, wherein the flash evaporator (102) comprises a vacuum vessel with a demister.

32. The apparatus according to any one of claims 25 to 31 , wherein the plant is a mononitrobenzene plant.

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