Nitration Technology for Aromatics As Described in the Patent Literature

Chapter 6

Nitration Technology for Aromatics As Described in the Patent Literature Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1155.ch006

Johannes Duehr* Am Holzbruch 19, 47802 Krefeld, Germany *E-mail: [email protected]

Several trends in nitration technology have emerged as ascertained by examination of the patent literature. The adiabatic process has become state of the art for benzene nitration. Nitration waste water treatment has moved from extraction processes to thermal processes (for safety reasons). Though many patents claim adiabatic toluene nitration, analogous to the adiabatic nitration of benzene, most new dinitrotoluene plants continue to use isothermal processes; this is most likely to control isomer ratios and to avoid high temperatures for safety concerns. Several patents focus on the improvement of the washing area to isolate the byproducts as well as to recover nitric acid. The field of gas-phase nitration is active; however this process is not competitive with liquid-phase processes using sulfuric acid. The challenge remains to find an efficient catalyst for the gas-phase nitrations.

Introduction The nitration of organic molecules has been practiced for almost 200 years. For example, the first nitration of the benzene molecule to mononitrobenzene (MNB) was recorded by Mitcherlich in 1834. Today MNB, mononitrotoluene (MNT), and dinitrotoluene (DNT) have become versatile precursors to many other commodity chemicals such as aniline which is used to produce dyes, rubber chemicals, and polyurethanes. The production of these nitrated species continues to increase to meet market needs. There has been a continuous effort to improve the nitration processes during

© 2013 American Chemical Society In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

the periods of growth. The process development has focused on safety concerns, economical aspects (costs), and environmental challenges. The number of patents in the field of nitration is large. The most relevant were selected to review here, and in particular the industrially relevant patents related to MNB and MNT/DNT technologies will be emphasized. This paper discusses how these patents have shaped the current MNB and MNT/DNT industries, and what they suggest for future process trends. These are very high volume products and are mostly feed materials for the production of polyurethanes.

Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1155.ch006

Process Overview and Trend Areas for MNB and MNT/DNT The methods to manufacture these materials are comprised essentially of the same major process steps, as shown in Figure 1.

Figure 1. Scope and Definition of the Trend Areas. The first step is the nitration reaction, which provides the crude product. The reaction is followed by a product purification process, which involves a liquid-liquid extraction step of the organic product phase with a water or basic aqueous wash. The aqueous extract must be treated before discharge to the environment; the quality of the discharged water must meet strict purity specifications as mandated by governmental agencies. Similarly, the vent gases from the plant have to be treated. In most cases, where a mixture of sulfuric acid and nitric acid are used as the nitrating media, the spent acid is diluted by the water created in the reaction, and must be reconcentrated before being fed back to the nitration step. The author has assumed that the reader is familiar with the basic chemistry and process engineering for the mixed acid nitration of benzene and toluene to afford nitrobenzene and MNT/DNT, respectively (1).

Patents The patents reviewed tend to focus in a particular key area. Some focus on heat integration, using the reaction enthalpy to supply the energy demand for reconcentrating spent sulfuric acid. Other patents focus on lowering by-product formation, such as nitrophenols, nitrocresols, dinitrobenzene (DNB), and in some 72 In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

cases DNT. Some patents show methods of maximizing favorable ratios of desired isomeric products. There are also patents concerning the reduction of emissions, and there are others which describe gas-phase nitration technology.


Summary of Patents Related to the Nitration of Benzene A review of the patent literature for MNB production suggests that the adiabatic nitration process has almost completely replaced the isothermal process, making the adiabatic reaction the state of the art. The first full-scale plants for adiabatic mixed acid (sulphuric acid and nitric acid) nitration of benzene to produce MNB were constructed in the U.S. in the late 1970’s and were equipped with cascades of stirred reactor vessels. The flow of the reaction media in the reactors was upwards so that the discharge from the reactors was under lower pressure. The nitration vessels were pressurized with nitrogen to avoid evaporation of residual benzene at the higher temperatures present at the tail end of the reaction vessel train. The concept of the continuous MNB process was first presented by Castner (2) and further refined by Alexanderson et al. (3, 4). The reaction rate of nitration of benzene with nitric acid is limited by mass transfer. The reaction itself occurs at the surface of the organic (benzene) droplets as nitric acid reaches the interface between the mixed acid phase and the organic phase. The surface area, over which the reaction takes place, is maximized by creating an emulsion between the mixed acid continuous phase and the organic (benzene) dispersed phase as small droplets. A design of a mixing device for a plug flow reactor was described by Evans (5), with the intention to increase the interfacial surface area between the mixed acid and organic phases using high shear forces and turbulence. An aromatic substrate and/or mixed acid can be forced through an annulus to form fine droplets of each phase when the substrate and mixed acid are contacted with one another in a reaction chamber. Guenkel et al. (6) claim a nitration feed condition, for the concentration of nitric acid in the mixed acid, which is outside the operating range previously mentioned in the literature. Maintaining the nitric acid concentration below 3% favours the complete dissociation of nitric acid to nitronium ions and thus accelerates the reaction rate. Mixing elements are used to generate a fine emulsion of organic droplets in the mixed acid. Thus the mass transfer is enhanced and the overall reaction rate increases. A significant benefit of the rate increase was that the desired nitration reaction was favoured and the production of impurities was minimized. An additional advantage of these operating conditions was improved safety due to lower operating temperatures and lower nitric acid concentrations. The reactor type was a plug flow reactor. The invention of Larbig (7) relates to a process for treating the basic aqueous extracts from the nitrated product washing step that contains nitrophenolic compounds. The nitrophenolics are dissolved in the water as their corresponding phenate salts. These salts are bactericides and cannot be released untreated into a conventional biological plant. Larbig claimed a process to treat the effluent water under pressure at temperatures above 170 °C resulting in the decomposition of 73 In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


the nitrophenolics to carboxylic acids, which can then be successfully treated in a subsequent biological wastewater treatment plant. The treatment is done in a liquid filled system without the addition of air. Lailach et al. (8) published a patent for a SAC (sulfuric acid concentration) plant in an isothermal nitration plant, using a horizontal evaporator using a steam heated exchanger constructed with tantalum. The evaporator is operated under vacuum conditions. Rae et al. (9) introduced a plug flow reactor with built-in devices to form and regenerate the organic droplets in an emulsion from mixed acid and organics. The reactor has been called the jet impingement reactor, because of the formation of jet streams which impinge at a wall to afford fine droplets of the dispersed phase. McCall (10) describes another technology to use the heat of the nitration reaction in the process. Benzene was vaporized and fed to the nitration reactor along with the mixed acid, where the reaction takes place in the liquid phase at a temperature of about 120 – 160 °C. The heat of reaction is used to evaporate the water formed in the reaction as well as the excess benzene as an azeotropic vapour phase. The reaction resembles an adiabatic process. The vapour leaving the reactor is condensed. The condensate forms 2 phases, a lighter benzene phase and a water phase. Gas-phase Nitration is the subject of the patent of Sato et al. (11). It is only one example of numerous patents describing a gas-phase nitration. The motivation to switch to gas-phase nitration is to avoid the large quantities of waste sulfuric acid generated in a mixed acid nitration. The nitrating agents are nitrogen oxides NO2 or N2O4. The reaction occurs in the presence of a solid catalyst comprised of acidic mixed oxides such as WO3, MO3, TiO2 and optionally SiO2 or ZnO. Other patents of Sato mention catalysts comprised of acidic clay minerals ion exchanged with polyvalent metals. The process has been tested on a laboratory scale. Brereton et al. (12) integrated the vent gas treatment with the nitration plant. He describes a pressurised absorption column for NOx gases to produce a weak nitric acid which is recycled to the reactor as a separate feed. Thus the yield of nitric acid can be improved. The NOx content remaining in the vent gas is small, and in some instances can be discharged without further treatment. If organic is present in the gas stream off the adsorption column, then the stream can be sent to a volatile organic oxidizer before being discharged to the atmosphere. Hermann et al. (13) describe a mixing apparatus used to mix nitric acid, sulphuric acid and an aromatic organic to achieve a rotating main flow in a central mixing tube at the entrance region of the reactor to form an emulsion. Gillis et al. (14) present a tubular plug flow reactor for nitration having builtin static mixing elements separated by coalescing zones. The mixing elements provide for efficient nitration in a two-phase system (organic and mixed acid) due to the formation of small droplets in the mixing zones. It is claimed that lower levels of impurities result in this process configuration. Knauf et al. (15) claim the use of an electrophoresis device to facilitate the separation of organics from the wash water. The washed organic phase is passed through an electric field created by the electrophoresis device. The organics still contain small droplets of conductive water which are electrically charged and migrate to charged metal plates where they coagulate and can be separated. The 74 In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


purity of the organic product can be improved. This is an electrostatic coalesce of sorts. Boyd (16) describes a method to treat waste water from mixed acid nitration. A concentrated alkaline aqueous extract of the organic product, containing dissolved sodium nitrophenates is treated in a process involving supercritical water oxidation. Eiermann et al. (17) describes a liquid phase nitration of an aromatic hydrocarbon using NOx and oxygen gas in the presence of a heterogeneous oxide catalyst, in the presence of at least 0.1 mol% water (with respect to the aromatic hydrocarbon). Use of the catalyst in the liquid phase reaction avoids the corrosive medium of hot sulfuric acid. Berretta (18) defines other process conditions for the adiabatic reaction to form MNB in mixed acid media. A process is claimed wherein the nitric acid concentrations is kept as low as reasonably possible (less than 3% in the mixed acid), and to start the reaction at a low temperature (60°C – 96°C). The temperature rise in the reactor is limited to about 20°C. Limiting the temperature was the main factor identified that limited the formation of the typical by-products (

Nitration Technology for Aromatics As Described in the Patent Literature

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