Process for separating nitroaromatics from wastewater

Abstract

A process for the electrochemical treatment of wastewater from the production of mononitrotoluene, which comprises the steps: a) introducing a wastewater stream from the production of mononitrotoluene into an undivided electrolysis cell, wherein the electrolysis cell comprises at least one anode and at least one cathode and wherein at least one anode comprises a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf), and the coating comprises boron-doped diamond; b) carrying out an electrolysis at a power density in the range from 0.1 to 10 kA / m2 and a cell potential in the range from 1 to 15 V.

Description

[0001] Process for separating nitroaromatics from wastewater

[0002] Description

[0003] The present invention relates to a process for the electrochemical treatment of wastewater from the production of mononitrotoluene and dinitrotoluene, in particular an electrolytic process for treating alkaline process wastewater, e.g. from a process for the nitration of toluene to form mononitroaromatics and dinitroaromatics. A substantial part of the aromatic nitro compounds and nitrites comprised in the process wastewater are reacted or destroyed by anodic oxidation or by means of anodically generated, oxidizing compounds. The process makes essentially complete oxidation of the aromatic nitro compounds to carbon dioxide and nitrate possible.

[0004] Mononitrotoluene (MNT) and dinitrotoluene (DNT) are typically produced by nitrating toluene using a mixture of concentrated nitric and sulfuric acid. This process results in an organic phase containing the raw nitration product and an aqueous phase largely comprised of sulfuric acid, water and reaction water. Following the separation of these phases, the aqueous sulfuric acid phase is reused for further nitration after being mixed with fresh nitric acid. However, some sulfuric acid must be discharged to prevent impurity build-up. The nitration reaction results in a crude product that comprises the desired nitrotoluenes, along with small amounts of undesirable by-products. These byproducts, such as mononitrophenols, dinitrophenols, trinitrophenols (summarized as nitrophenols), mononitrocresols, dinitrocresols, trinitrocresols (summarized as nitrocresols), mononitroxylenols, dinitroxylenols, trinitroxylenols (summarized as nitroxylenols) and mononitrobenzoic and dinitrobenzoic acids (summarized as nitrobenzoic acids), must be removed before the crude product can be used further. This is achieved by washing the organic phase with acidic, alkaline, and / or neutral washing liquids. The washing process produces alkaline wastewater containing the by-products in the form of their water-soluble salts. These substances, including nitrophenols, nitrocresols, nitroxylenols, and nitrobenzoic acids, are highly toxic and potentially explosive, necessitating their removal before the wastewater can be discharged to the environment. Furthermore, due to the biocidal or bactericidal properties of the aromatic nitro compounds, the wastewater must undergo purification before it can be treated in a conventional wastewater treatment facility with a microbiological purification stage.

[0005] Numerous methods for removing nitrophenols, nitrocresols, nitroxylenols, nitrobenzoic acids, and neutral nitroaromatics from process wastewater have been described, including extraction, adsorption, oxidation, and thermolysis. The "Encyclopedia of Chemical Technology" (Kirk-Othmer, Fourth Edition 1996, Vol. 17, p. 138) details an extraction process for nitrobenzene using benzene. EP-A 005 203 describes a thermal process for treating wastewater with hydroxy-nitroaromatics by heating under pressure. Ullmanns Enzyklopadie der technischen Chemie (4th edition, 1974, volume 17, page 386) discusses acid medium extraction. DE-A 197 48 229 and EP-A 0 808 920 describe electrochemical cathodic reduction processes, which often suffer from incomplete removal and severe foaming. US 6,953,869 and US 4,604,214 describe oxidative processes using concentrated nitric acid and Fenton’s reagent, respectively. CN-A 1 600 697 combines UV light, Fenton’s reagent, and anodic oxidation. "J. Hazardous Materials" (Vol. 161, No. 2-3, 2009, pp. 1017-1023) details the electrochemical removal of dinitrotoluene and trinitrotoluene using hydrogen peroxide generated in situ. The "Proceedings of the 1992 Incineration Conference" (1992, pp. 167-174) reports indirect anodic oxidation using Ag2+ ions. CN-A 1 850 643 describes the removal of aniline and nitrobenzene via electrochemical oxidation with a titanium-based anode. These methods often require complicated separation steps, large excesses of oxidants, and often have unsatisfactory operational periods.

[0006] US6533916B1 aims to produce boron-doped diamond-coated electrodes suitable for electrochemical reactions involving organic compounds. The document mentions the use of these electrodes for the electrochemical oxidation of nitrotoluenes, such as m-, o-, p-nitrotoluene, and 2,4- and 2,6-dinitrotoluene, to their methoxylated derivatives, demonstrating the electrodes' effectiveness in transforming nitrotoluenes into valuable chemical products.

[0007] WO2011144594A1 describes a process and apparatus for the electrochemical treatment of aromatic nitro compounds in wastewater, particularly from nitration processes. The objective is to achieve complete oxidation of these compounds to carbon dioxide and nitrate, ensuring the treated wastewater is free of toxic, environmentally harmful, and explosive substances. The method involves introducing an aqueous composition containing aromatic nitro compounds into the anode space of a divided electrolysis cell, which is separated from the cathode space by a separator. The electrolysis is carried out using anodes coated with boron-doped diamond or platinum at specific power densities and cell potentials.

[0008] A publication by Szopinska et al. discloses the removal of 2,4,6-trinitrotoluene (TNT) from industrial / military wastewater using anodic oxidation on boron-doped diamond electrodes (Scientific Reports (2024)14:4802; https: / / doi.org / 10.1038 / s41598-024-55573-w).

[0009] The object of the present invention was to provide an improved treatment of wastewater from production of mononitrotoluene and dinitrotoluene by electrooxidation.

[0010] The present invention therefore relates to a process for the electrochemical treatmentof wastewater from mononitrotoluene or dinitrotoluene production, which comprises the steps of: a) introducing a wastewater stream from the production of mononitrotoluene into an undivided electrolysis cell, wherein the electrolysis cell comprises at least one anode and at least one cathode and wherein at least one anode comprises a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf), and the coating comprises boron-doped diamond; b) carrying out an electrolysis at a power density in the range from 0.1 to 10 kA / m2 and a cell potential in the range from 1 to 15 V. The process according to the present invention has the advantage allowing a simple setup and compact design which is easy to assemble or operate. The process according to the present invention further has the advantage of a lower cell resistance compared to an electrolysis cell with a membrane dividing the cell allowing for higher current densities. This is particularly advantageous when coupled with renewable energy sources like wind and solar energy allowing for better utilization of energy spikes that otherwise may not be processable by the system. The process according to the present invention may also allow for lower operational costs. A further advantage of the process according to the present invention is the improved cleaning and maintenance of the electrolysis cell. The cells according to the present invention avoid the issue of membrane fouling and clogging. The electrolysis cells of the present invention also allow for easier disassembling, e.g. for cleaning.

[0011] According to the process of the present invention a wastewater from the production of mononitrotoluene or dinitrotoluene is introduced into an undivided electrolysis cell.

[0012] The wastewater from the production of mononitrotoluene or dinitrotoluene preferably comprises at least one aromatic nitro compound selected from the group consisting of mononitrotoluene (MNT), dinitrotoluene (DNT) and trinitrotoluene (TNT), mononitrophenols, dinitrophenols, trinitrophenols (summarized as nitrophenols), mononitrocresols, dinitrocresols, trinitrocresols (summarized as nitrocresols), mononitroxylenols, dinitroxylenols, trinitroxylenols (summarized as nitroxylenols) and mononitrobenzoic and dinitrobenzoic acids (summarized as nitrobenzoic acids).

[0013] These compounds are preferably converted to the desired products of the electrolchemical treatment, which are CO2 and nitrate.

[0014] More preferably, the wastewater from mononitrotoluene production comprises at least one aromatic nitro compound selected from the group consisting of mononitrotoluene (MNT), dinitrotoluene (DNT), mononitrocresol and dinitrocresol.

[0015] In the further preferred embodiment, the wastewater from MNT-production does not comprise trinitrotoluene (TNT) or TNT is an amount of 1 weight percent of less, based on the weight of the wastewater introduced into the electrolysis cell.

[0016] The aromatic nitro compound(s) in the wastewater can be present in dissolved, emulsified or suspended form in the aqueous composition. In particular, the invention provides a process as described above for the electrochemical treatment, in particular the electrochemical oxidation, of aromatic nitro compounds, where the aromatic nitro compound(s) is / are present in at least two of the abovementioned forms.

[0017] Preference is also given to a process as described above, wherein the wastewater comprises (optionally in addition to at least one dissolved nitro compound) at least one aromatic nitro compound, preferably in suspended form.

[0018] In a preferred embodiment of the invention, the wastewater (electrolyte) introduced into the electrolysis cell comprises at least one aromatic nitro compound in an amount in the range from 0.1 to 5% by weight, preferably in the range from 0.3 to 2.5% by weight (based on the total aqueous composition). In a preferred embodiment of the invention, the wastewater introduced into the electrolysis cell comprises not only the aromatic nitro compound but also additional components, in particular in a concentration of from 0.001 to 30 g / l, preferably in a concentration of from 0.01 to 10 g / l. Preferably, the aqueous composition can also comprise inorganic nitrites in addition to the aromatic nitro compound.

[0019] As further additional components, water-soluble salts can be added to or are present in the aqueous composition in order to increase the conductivity. These salts are preferably selected from among water-soluble inorganic salts, in particular salts comprising nitrate, sulfate and / or carbonate, in particular alkali metal salts comprising nitrate, sulfate and / or carbonate. The abovementioned salts for increasing the conductivity can be preferably comprised in a concentration in the range from 0.1 to 30 g / l, preferably in the range from 0.1 to 10 g / l, particularly preferably in the range from 1 to 10 g / l (based on the wastewater introduced into the electrolysis cell).

[0020] For the purposes of the present invention, a water-soluble salt is generally a salt having a solubility in water of greater than or equal to 1 mol / l.

[0021] Preferably, no further additional components, other than the ones already present in the wastewater, are added to the aqueous composition as the additional components already present in the wastewater ensure a sufficient conductivity.

[0022] The present invention further provides a process as described above, wherein the wastewater from MNT- or DNT- production comprising at least one aromatic nitro compound additionally comprises a (at least one) redox mediator. The redox mediator can be comprised in a concentration in the range from 0.001 to 0.2 mol / l, in particular in the range from 0.01 to 0.05 mol / l (based on the wastewater introduced into the electrolysis cell).

[0023] The redox mediator can be, in particular, at least one compound selected from among inorganic salts comprising e.g.,; an inorganic salts of cerium (Ce) or praseodymium (Pr), in particular nitrates, sulfates and / or hydrogenphosphate salts of cerium or praseodymium.

[0024] In one embodiment of the invention, the wastewater from MNT-or DNT-production comprises from 0.1 to 10 g / l, preferably from 1 to 5 g / l, of cerium and / or praseodymium ions.

[0025] The wastewater from MNT-or DNT-production preferably has a pH in the range from 4 to 14, in particularly in the range from 4 to 12, in particularly from 4 to 11 .

[0026] The wastewater from MNT-or DNT-production is preferably obtained by a process comprising the steps of:

[0027] (i) contacting toluene with a mixture of nitric acid and sulfuric acid,

[0028] (ii) separating the organic phase obtained in step (i);

[0029] (iii) washing the organic phase separated in step (ii) with an aqueous washing liquid to produce the alkaline wastewater. In step (i) toluene is nitrated by contacting toluene with a mixture of concentrated nitric and sulfuric acid. This process results in an organic phase containing the raw nitration products, such as MNT and DNT and the undesired byproducts mentioned above, and an aqueous phase largely comprised of sulfuric acid, water and reaction water.

[0030] In step (ii), the organic and the aqueous phase formed in step (i) are separated. Following the separation of these phases, the aqueous sulfuric acid phase is usually reused for further nitration after being mixed with fresh nitric acid. However, some sulfuric acid must be discharged to prevent impurity build-up.

[0031] The organic phase comprises the desired nitrotoluenes, along with small amounts of undesirable by-products. These by-products, such as mononitrophenols, dinitrophenols, trinitrophenols (summarized as nitrophenols), mononitrocresols, dinitrocresols, trinitrocresols (summarized as nitrocresols), mononitroxylenols, dinitroxylenols, trinitroxylenols (summarized as nitroxylenols) and mononitrobenzoic and dinitrobenzoic acids (summarized as nitrobenzoic acids), must be removed before the crude product can be used further.

[0032] This is achieved in a step (iii) by washing the organic phase with aqueous acidic, alkaline, and / or neutral washing liquids.

[0033] The washing process produces alkaline wastewater containing the by-products in the form of their water-soluble salts.

[0034] The wastewater from MNT-production is introduced into an undivided electrolysis cell.

[0035] An undivided electrolysis cell is a type of electrochemical cell where the cathode and anode are housed in the same compartment, without any physical separation between the electrodes, such as a separator or a membrane. The electrolyte solution is in direct contact with both electrodes, allowing ions to move freely between them. The simplified design of the undivided cell reduces the number of components required, as there is no need for a membrane, diaphragm, or other physical barrier separating the electrodes. This results in a more straightforward construction and potentially lower manufacturing costs compared to divided electrolysis cells. The absence of a separator also contributes to a lower internal resistance within the undivided cell. This can enable the use of higher current densities and allowing the improvement of the overall energy efficiency of the electrochemical process.

[0036] In principle, it is possible to use any undivided electrolysis cells for the process of the present invention. Typical cell geometries and reactor designs are described in the chapters “Electrochemical Reactors” and “Electrochemistry, 1. Fundamentals” of Ullmann's Encyclopedia of Industrial Chemistry (https: / / doi.org / 10.1002 / 14356007.l09J01.pub2 and https: / / doi.org / 10.1002 / 14356007.a09_183.pub4).

[0037] Preferably, the undivided electrolysis cell is a flow cell which allows the electrolyte solution to continuously get introduced to the inlet and to let the electrolyte flow through the cell to the outlet.

[0038] Preferably, the electrodes are in a plane-parallel arrangement, because in this embodiment a homogeneous current distribution is given with a small electrode gap (preferably from 0.5 mm to 30 mm, more preferably from 2 to 10 mm). Preferably in this case the electrodes can be used individually or stacked in a plurality. In a preferred embodiment the electrolysis cell is a parallel plate and frame cell, which usually comprises a series of parallel plates arranged in a frame and wherein the plates are separated by a gap. The electrolyte usually flows through the gap between the plates, allowing for electrochemical reactions to occur. The gap size (distance between two plates) is preferably in the range of 0.5 to 30 mm, more preferably in the range or 2 to 10 mm.

[0039] The electrodes can be connected either in a monopolar or in a bipolar manner.

[0040] In a monopolar configuration, all the anodes are usually connected to each other, and all the cathodes are also usually connected to each other. This creates a parallel electrical circuit where the electric current flows through each plate individually, but in parallel with the other plates of the same polarity. This means that each plate operates independently, with the same current flowing through each plate.

[0041] In a bipolar configuration, each plate usually acts as both an anode and a cathode. The plates are usually arranged in a stack. The electric current usually flows through each plate in series, with the anode side of one plate in contact to the cathode side of the adjacent plate. This means that each plate has both an anode and a cathode side, the anode side of one plate is electrically connected to the cathode side of the adjacent plate and the current flows through each plate in series

[0042] According to the present invention, the electrolysis cell comprises at least one anode comprising a support material and a coating.

[0043] The support material of the anode comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf).

[0044] In a most preferred embodiment, the metal is niobium.

[0045] In a most preferred embodiment, the support material essentially consists of niobium.

[0046] The support material preferably has a thickness of from 1 to 4 mm, in particular from 2 to 3 mm.

[0047] The coating of the anode comprises boron-doped diamond.

[0048] Preferably, the coating comprises boron-doped diamond in an amount of from 90 to 100% (based on the electrochemically active electrode area).

[0049] Preference is given to using anodes which have a corresponding coating on two sides, preferably on all sides. In particular, the boron-doped diamond has a dopant content of from 0.01 to 3%, in particular from 0.1 to 0.5%. Preferably, the coating of boron-doped diamond has a layer thickness in the range from 1 to 50 pm, more preferably from 2 to 40 pm and most preferably from 4 to 20 pm.

[0050] A preferred methods for producing a boron-doped-diamond electrodes are described US6533916B1 .

[0051] The wastewater from MNT-production is introduced into the undivided electrolysis cell.

[0052] The aqueous composition is preferably provided as a continuous feed to an inlet of the electrolysis cell and then flows through the gaps or spacers separating the plates to an outlet. In a preferred embodiment, a part of the effluent from the outlet of the electrolysis cell can be recycled to the feed inlet, e.g. with a recycling ratio in the range from 80 to 98%, preferably in the range from 90 to 95%.

[0053] Step b) Electrolysis

[0054] The wastewater from MNT-production is subjected to an electrolysis in the undivided electrolysis cell.

[0055] The temperature during the electrolysis is preferably in the range from 30 to 90°C, particularly preferably in the range from 40 to 70°C.

[0056] According to the invention, the electrolysis is carried out at a power density range from 0.1 to 10 kA / m2 and a cell potential in the range from 1 to 15 V.

[0057] More preferably the electrolysis is carried out at a power density is in the range of 0.2 to 8 kA / m2 and even more preferably in the range of 0.5 to 5 kA / m2 .

[0058] It is further preferred that the electrolysis is carried out a cell potential in the range of 1 to 14 V and more preferably in the range of 4 to 13 V.

[0059] The electrolysis is preferably carried out in electrolysis cells having a low resistance.

[0060] In a preferred embodiment of the invention, the cathode and the anode of the electrolysis have substantially the same composition. More preferable, the anode and the cathode are substantially identical and made from the same material, including the same support material and the same coating. Preferably, both electrodes comprise niobium as support material and a coating of boron-doped diamond. More preferably, both electrodes consist of niobium as support material and a coating of boron-doped diamond.

[0061] In a preferred embodiment, the electric current for operating the electrolysis cell is at least partially generated by renewable energy sources, such as water-driven turbines, wind turbines, and solar cells, with a preference for solar and wind. Electricity from these sources can vary in availability and intensity due to factors like time of day and weather. The undivided electrolysis cells in this invention can potentially handle these fluctuations better than divided cells, thanks to their wider operating range. This adaptability potentially allows them to adjust more effectively to changes in power supply. Additionally, without a dividing membrane, undivided cells can often operate at higher current densities, potentially enabling them to achieve higher capacities during sunny and windy conditions compared to divided cells.

[0062] The process according to the present invention has the advantage of allowing for a simple setup and compact design which is easy to assemble or operate. The process according to the present invention further has the advantage of a lower cell resistance which allows for higher current densities which is particularly advantageous when coupled with renewable energy sources like wind and solar energy allowing for better utilization of energy spikes that otherwise may not be processable by the system. The process according to the present invention may also allow for lower operational costs, in particularly in connection with fluctuating energy sources, such as renewables. A further advantage of the process according to the present invention is the improved cleaning and maintenance of the electrolysis cell. The cells according to the present invention also avoid the issue of membrane fouling and clogging. In addition, the electrolysis cells of the present invention allow for easier disassembling, e.g. for cleaning.

[0063] The invention is illustrated by the following examples.

[0064] Example 1

[0065] The wastewater from a toluene nitration plant, which was characterized as follows: 130 ppm nitrate, 680 ppm nitrite, 580 ppm sulfate 8880 ppm nitrocresols, 343 ppm 2-nitrotoluene, 26 ppm 3-nitrotoluene, 200 ppm 4-nitrotoluene was electrolyzed in an undivided cell with a current density of 100 mA / cm2 (=1 kA / m2) The cell is part of a circuit, and the wastewater is pumped through cell and circuit for this batch electrolysis. As anode as well as cathode a boron doped diamond electrode of an area of 63,6 cm2 was used having a 5 pm boron doped diamond layer on a niobium substrate. The gap between the electrodes was 3 mm. After 6h of electrolysis the TOC value was decreased from 4500 ppm to 2500 ppm, after 12 h of electrolysis a value 550 ppm was reached and after 30h the TOC value was even at 30 ppm. Analyzing the taken samples more in detail it was observed that after 6 h neither toluidine nor 2-nitrotoluene, 3-nitrotoluene or 4-nitrotoluene were detectable anymore. The nitrocresols content was drastically reduced to 200 ppm.

[0066] Example 2:

[0067] With a second sample from the same wastewater stream as in example 1 the electrolysis was repeated and after 5 h electrolysis time the current density was increased from 100 mA / cm2 to 150 mA / cm2 (1-1 .5 kA / m2). After the first 5 hours electrolysis time the TOC value was reduced from 4613 ppm to 2379 ppm, the nitrocresols content decreased to 140 ppm, the 2-nitrotoluene content was at 2 ppm, the value for 3-nitrotoluene was below 0.1 ppm and the 4- nitrotoluene content was at 0.7 ppm. After 16 h the nitrotoluenes were not detectable anymore and the nitrocresols content was at 11 ppm.

Claims

Claims1 . A process for the electrochemical treatment of wastewater from the production of mononitrotoluene, which comprises the steps: a) introducing a wastewater stream from the production of mononitrotoluene into an undivided electrolysis cell, wherein the electrolysis cell comprises at least one anode and at least one cathode and wherein at least one anode comprises a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf), and the coating comprises boron-doped diamond; b) carrying out an electrolysis at a power density in the range from 0.1 to 10 kA / m2 and a cell potential in the range from 1 to 15 V.

2. A process according to claim 1 , wherein the support material comprises or consists of niobium (Nb).

3. A process according to claim 2, wherein the coating consists of boron-boron doped diamond.

4. A process according to a least one of claims 1 to 3, wherein the anode and the cathode have the substantially same composition.

5. A process according to at least one of claims 1 to 4, wherein the electric current for the electrolysis cell is at least partially generated by a renewable energy source.

6. A process according to at least one of claims 1 to 5, wherein the undivided electrolysis cell is an undivided plate and frame cell or a capillary gap cell.

7. The process according to any of claims 1 or 6, wherein the wastewater from mononitrotoluene production comprises at least one compound selected from the group consisting of nitrobenzene (NB), dinitrobenzene (DNB), trinitrobenzene (TNB), mononitrotoluene (MNT), dinitrotoluene (DNT), trinitrotoluene (TNT), nitrochlorobenzene (NCB), mononitroxylenes, dinitroxylenes, trinitroxylenes, mononitrocresol, dinitrocresol, trinitrocresol, mononitrophenol, dinitrophenol, trinitrophenol, mononitrobenzoic acid, dinitrobenzoic acid, trinitrobenzoic acid, mononitroxylenols, dinitroxylenols and trinitroxylenols, with all isomeric forms of the compounds mentioned being encompassed.

8. The process according to any of claims 1 to 7, wherein the wastewater from mononitrotoluene production comprises at least one aromatic nitro compound in suspended form.

9. The process according to any of claims 1 to 8, wherein the wastewater from mononitrotoluene production comprises a redox mediator.

10. The process according to any of claims 1 to 9, wherein the temperature of the wastewater from mononitrotoluene production in the anode space in the electrolysis is in the range from 30 to 90°C.

11. The process according to any of claims 1 to 10, wherein the wastewater from mononitrololuene production is an alkaline wastewater.

12. The process according to any of claims 1 to 11 , wherein the wastewater from mononitrololuene production is produced by a process comprising the steps of:(i) Contacting toluene with a mixture of nitric acid and sulfuric acid,(ii) Separating the organic phase obtained in step (i);(iii) Washing the organic phase separated in step (ii) with an aqueous washing liquid to produce the alkaline wastewater.