HK1070349A - Process for preparing aromatic amines - Google Patents

Description

Process for preparing aromatic amines

Technical Field

The present invention relates to a process for producing aromatic amines from aromatic hydrocarbons, wherein a nitrated aromatic compound obtained by nitrating the corresponding aromatic hydrocarbon is washed with reaction water generated during the hydrogenation of a nitroaromatic compound.

Background

Aromatic amines and diamines are used in large amounts as chemical intermediates, for example for the preparation of dyes or plastics; industrially, it is prepared by nitrating an aromatic hydrocarbon to form a nitroaromatic compound, and then hydrogenating the nitroaromatic compound. In this case, in the nitration with nitric acid, 1 mol of water is produced per mole of nitro groups; during the hydrogenation, 2 moles of water are produced per mole of amine groups. This water eventually leaves the process as waste water. Other waste waters are also produced, in particular during the treatment of the nitroaromatics, which is generally carried out by separating the acid phase and then washing the nitroaromatics with water and alkaline solution, both generally stepwise and sometimes alternately. According to the relevant regulations, this waste water must be purified in a suitable manner to enable it to be discharged into the receiving water. Thus, for example, U.S. Pat. No. 6,506,948 describes A process for purifying the waste water produced in the preparation of dinitrotoluene.

EP-A-236839 describes ｃA distillation process for working up aqueous amine solutions, such as are produced by the hydrogenation of nitroaromatic compounds. According to EP-A-236839, the water produced in the distillation can be reused in the chemical process. However, the concentration of organic impurities (especially amines) in the water produced during distillation is from 10 to 500 ppm. Thus, the use of such water in chemical processes is often problematic because these impurities can cause difficulties in the process where the impurities are present in the regeneration water. For example, such regenerant water can produce undesirable side reactions.

In particular, diaminotoluene reacts with dinitrotoluene, undesirable by-products being formed, in other words, the thermal stability of dinitrotoluene is reduced in particular.

Disclosure of Invention

Accordingly, it is an object of the present invention to reduce the amount of waste water generated during the production of aromatic amines and to improve the economic efficiency of the aromatic amine production process.

It has surprisingly been found that water having a concentration of aromatic amine of up to 1000ppm or more can be used to wash the corresponding crude nitroaromatic compound without causing any problems and without affecting the nitration process or the subsequent hydrogenation reaction to give aromatic amine.

Detailed Description

The present invention relates to a process for producing aromatic amines from aromatic hydrocarbons, as follows:

a) reacting an aromatic hydrocarbon with a mixture of nitric acid and sulfuric acid to form a two-phase reaction mixture; the mixture consists of an acid aqueous phase and an organic phase comprising nitroaromatic compounds,

b) separating the aqueous acid phase from the biphasic reaction mixture,

c) purifying the organic phase comprising nitroaromatic compound(s) by washing the organic phase, removing waste water or solution and optionally separating the nitroaromatic compound(s),

d) hydrogenating the nitroaromatic compound in the presence of a catalyst to give the corresponding aromatic amine and to form reaction water,

e) separating the water of reaction formed in step d) from the aromatic amine.

An important feature of the present invention is the use of the reaction water separated in step e) to wash the organic phase comprising the nitroaromatic compound in step c).

Any industrially important aromatic amine can be prepared by the process according to the invention. Examples of diamines produced by the process of the invention are diaminotoluene and examples of monoamines are aniline and ring-substituted anilines such as toluidine and chloroaniline. The process of the present invention is particularly suitable for the preparation of diaminotoluene (2, 4-and 2, 6-diaminotoluene).

In step a), any aromatic hydrocarbon, preferably benzene or toluene, is nitrated in a known manner using a mixture of nitric acid and sulfuric acid. The resulting biphasic reaction mixture is separated in step b) into an organic phase and an acid phase. The phases are separated in a static separator or a centrifugal separator. The organic phase consists essentially of the crude nitroaromatic compound, which is preferably purified by a plurality of washings before the hydrogenation.

In the present invention, the reaction water recovered from the hydrogenation mixture in the subsequent step e) may be used as wash water in step c). Typically, the organic phase is washed several times. In the first washing, residual sulfuric acid is removed with water. The organic phase is washed with 1 to 100 parts by weight of water, preferably 5 to 50 parts by weight of water, per 100 parts by weight of the organic phase. For this purpose, the water of reaction produced in the hydrogenation of the nitro compound and in the separation step e) can be used directly as washing water. However, the wash water of the subsequent step in the process, preferably the wash water separated in the last wash step, may also be used as wash water for the first wash step. However, when the acid concentration of the organic phase is low, the first washing step with water can be omitted.

After washing the organic phase with water, or directly after separation of the aqueous acid phase (if the acid concentration in the organic phase is sufficiently low, the first water washing step is omitted), the organic phase is typically washed with an alkaline wash solution, such as dilute caustic soda or soda solution, to remove weakly acidic secondary products, such as phenols or organic acids. In this case, 0.1 to 2 parts by weight of caustic soda solution (100%) or soda are generally used per 100 parts by weight of organic phase having a concentration of 0.5 to 20% or being completely saturated. In the present invention, the reaction water recovered in step e) may also be used as the water for dissolution or dilution of the alkaline washing solution.

After the alkaline washing step, the third and further optional washing steps are carried out using water. In the third step and any further additional washing steps, from 1 to 100 parts by weight of water, preferably from 5 to 50 parts by weight of water, are used per 100 parts by weight of organic phase. The reaction water recovered in step e) may be used in the third step as well as in other optional washing steps. All or some of the water separated in these washing steps for washing may be used as the washing water for the first washing step to wash the organic phase produced in the subsequent nitration reaction.

The reaction water produced in the hydrogenation of the nitroaromatic compound and separated in step e) is preferably used as wash water for the third and/or further optional washing steps of the organic phase, and the water from these washing steps is preferably used as wash water for the first washing step for washing the organic phase produced in the subsequent nitration reaction.

When preparing aniline, it is generally possible to use about 29 parts by weight of reaction water from step e) per 100 parts by weight of nitrobenzene as wash water. When diaminotoluene is prepared, it is generally possible to use about 40 parts by weight of reaction water from step e) per 100 parts by weight of dinitrotoluene as washing water. Such usable washing water can be used completely, for example, in the third and/or further optional washing steps or be distributed over all washing steps. If the amount of wash water required exceeds the reaction water that can be used from step e), pure water can additionally be used.

All washing steps can be carried out in one or more stages in industrially customary mixer/separators or washing and extraction columns. When the washing step is carried out in several stages, the washing liquid is preferably passed in countercurrent.

If the individual aqueous phases produced in the washing step are no longer used as wash water for the subsequent washing step, they are generally passed as waste water to a waste water treatment unit. After all washing steps, the organic phase obtained is the nitroaromatic compound of technical grade purity.

In step d), the nitroaromatic compound is reacted with hydrogen in the presence of a catalyst to form the aromatic amine. Depending on the boiling point and stability of the starting components, this hydrogenation may be carried out in the gas phase or in the liquid phase, with or without the use of auxiliary solvents. The reaction conditions, the design of the apparatus and the choice of catalyst are known from the prior art (see, for example, U.S. Pat. No. 3, 6,080,890 (aniline) or DE-A-4435839 (diaminotoluene)). After the reaction and optionally after separation of the catalyst, the reaction mixture obtained is an amine/water mixture, which, according to the amine, may be present in a single phase or in two phases.

In step e), the reaction water and the amine/water mixture are separated. If the amine/water mixture is present in two phases (for example when preparing aniline), the mixture is usually first separated into a water-rich phase and an amine-rich phase. In this case, the phase separation is usually carried out in a separation tank. The dissolved water, as well as some amine, is typically distilled from the organic phase and the distillate is returned to the separation tank. The organic components (essentially amines) are removed from the aqueous phase in a stripping column to reduce the concentration of organic components to preferably less than 0.1 wt%, most preferably less than 100 ppm. For example, in the case of aniline, the amount of water obtained is preferably about 39 parts by weight per 100 parts by weight of amine. The water purified in this way can be used in step c) of the present invention to wash the nitroaromatic compound.

If the amine/water mixture is present as a single phase (in the preparation of diaminotoluene), after separating off all optionally present auxiliary solvents, the mixture is separated by distillation into high-boiling amines (product remaining at the bottom of the column), volatile secondary products and water (as intermediate fraction). A process similar to that of, for example, EP-A-236839 can be used. Furthermore, an aqueous phase is prepared having a concentration of organic components (essentially amine) of preferably less than 0.1% by weight, most preferably less than 100 ppm. In the case of diaminotoluene, for example, the amount of water obtained is about 59 parts by weight per 100 parts by weight of amine. The water purified in this way is then used in step c) of the present invention for washing nitroaromatics.

Having thus described the invention, the following examples are given to illustrate the invention.

Examples

Example 1

Separating the reaction water from the reaction solution obtained from the hydrogenation reaction

A 50mm diameter bubble column with 30 plates and a transverse discharge on the 20 th plate was used. This column was operated at an absolute pressure of 3 bar and a bottom temperature of about 200 ℃. A reaction mixture of about 60% by weight of a mixture of isomeric Toluenediamines (TDA) having a concentration of 0.5% by weight of organic secondary products and a water content of about 40% by weight is obtained by catalytic hydrogenation of Dinitrotoluene (DNT) with hydrogen. This reaction mixture was fed to the 5 th plate of the column at a rate of 2 kg/h. By heating at the bottom of the column 1100g/h of vapour were produced at the top of the column. These vapors were condensed and returned to the column as reflux and 15g/h of distillate, containing the volatile secondary products, were discharged. 1250g of TDA isomer mixture having a water content of 4% by weight are continuously removed as bottom product. Furthermore, 735g/h of water were taken at the lateral discharge point on the 20 th plate. The residual concentration of organic amine in this water was 300ppm (0.03 wt.%) and was used to wash the crude DNT.

Example 2

Washing the crude DNT with the reaction water from the hydrogenation reaction

In this example three mixer/separators in series were used, each consisting of a 1-liter stirred flask and a 1-liter separating vessel. 1900g/h of crude DNT (analogous to the mixture obtained from the two-stage reaction of toluene and nitrating acid) having a sulfuric acid concentration of 1.2% by weight, a nitric acid concentration of 0.8% by weight and cresol and organic acid concentrations of 0.6% by weight were placed in this mixer/separator battery at 70 ℃. The washed DNTs from each stage were added as a lower phase from each separation vessel to the next stirred vessel in the series by siphoning. From the last separation vessel, technical-grade purity DNT was obtained.

In the first stirred vessel, 200g/h of reaction water from example 1 were added as wash liquor. In a second stirred vessel, 200g/h of a 5% strength by weight soda solution were added. In the third stirred vessel 535g/h of the remaining reaction water from example 1 were added. The upper aqueous phase of all three separation vessels was run off as waste water.

The washed DNT obtained in this way meets all purity requirements of technical grade products, i.e. the concentration of neutral pH, organic acids and cresols is below 200 ppm.

Although the invention has been described in detail in the foregoing application, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the scope and spirit of the invention except as it may be limited by the claims.

Claims (6)

1. A process for preparing an aromatic amine, the process comprising:

a) reacting an aromatic hydrocarbon with a mixture of nitric acid and sulfuric acid to form a two-phase reaction mixture,

b) separating the reaction mixture into an aqueous acid phase and an organic phase comprising the nitroaromatic compound,

c) washing the organic phase to purify the nitroaromatic compound,

d) hydrogenating the nitroaromatic compound in the presence of a catalyst to produce an aromatic amine and water,

e) separating the water produced in step d) from the aromatic amine,

wherein the organic phase comprising nitroaromatic compounds described in step c) is washed with water separated in step e).

2. The process of claim 1, wherein the water separated in step e) has a concentration of organic compounds of less than 0.1 wt.%.

3. The process of claim 1, wherein the water of reaction separated in step e) has a concentration of organic compounds of less than 100 ppm.

4. The method according to claim 1, wherein step c) is carried out in two or more stages.

5. The process as claimed in claim 4, characterized in that all or part of the reaction water separated off in step d) is used in the final washing stage.

6. The method as claimed in claim 5, characterized in that the wash water from the last stage is used as wash water for the first washing stage.