Process for working up waste water from nitrobenzene preparation

Abstract

The present invention provides a process for working up alkaline waste water which is formed during washing of crude nitrobenzene obtained by nitration of benzene, wherein (i) the alkaline waste water is heated to a temperature of from 150° C. to 500° C. under an increased pressure with respect to atmospheric pressure with exclusion of oxygen; (ii) a base is added to the waste water obtained in (i); and (iii) the waste water obtained in (ii) is purified further by stripping with a stripping gas and the stripping gas stream loaded with impurities is then cooled to a temperature of from 10° C. to 60° C.

Claims

1. A process for working up alkaline waste water which is formed during washing of crude nitrobenzene obtained by nitration of benzene, comprising: (i) heating the alkaline waste water to a temperature of from 150° C. to 500° C. under an increased pressure with respect to atmospheric pressure with exclusion of oxygen; (ii) adding a base to the waste water obtained in (i); and (iii) further purifying the waste water obtained in (ii) by stripping with a stripping gas and then cooling the stripping gas stream loaded with impurities to a temperature of from 10° C. to 60° C.

2. The process of claim 1, comprising in heating the alkaline waste water in step (i) under an absolute pressure of from 50 bar to 350 bar.

3. The process of claim 1, comprising heating the alkaline waste water in step (i) for a period of from 5 minutes to 120 minutes.

4. The process of claim 3, comprising, after the heating, cooling the alkaline waste water to a temperature of from 60° C. to 100° C.

5. The process of claim 1, in which the base used in step (ii) is an aqueous solution of a base selected from the group consisting of lithium hydroxide, sodium hydroxide, potassium hydroxide and rubidium hydroxide.

6. The process of claim 1, comprising adding the base in step (ii) such that a pH of at least 12 is established.

7. The process of claim 1, in which the stripping in step (iii) is carried out under an absolute pressure of from 0.5 bar to 2 bar and at a temperature of from 80° C. to 120° C.

8. The process of claim 1, in which the stripping gas is steam.

9. The process of claim 1, comprising purging organic constituents out of the stream obtained in step (iii) after cooling of the stripping gas stream loaded with impurities to a temperature of 10° C. to 60° C., to obtain a stream depleted in organic constituents.

10. The process of claim 9, comprising partially to completely returning the stream depleted in organic constituents to the stripping (iii).

11. The process of claim 9, comprising sending the stream depleted in organic constituents, where it is not returned to the stripping (iii), directly to a waste water treatment.

12. The process of claim 1, comprising sending the stripped waste water obtained in step (iii) directly to a waste water treatment.

13. The process of claim 1, in which the alkaline waste water used in step (i) originates from step c) or step d) or step e) the following process steps: a) nitration of benzene with nitric acid or a mixture of nitric acid and sulfuric acid and separating off of the aqueous phase; b) washing of the organic process product obtained in step a); c) alkaline washing of the washed organic process product obtained in step b); d) optional separation of benzene and/or nitrobenzene out of the alkaline waste water obtained in step c); and e) optional stripping of the alkaline waste water obtained in step c) or step d).

Description

DETAILED DESCRIPTION

(1) Embodiments of the invention are described in more detail in the following. In this context, various embodiments can be combined with one another as desired, if the opposite does not clearly emerge from the context.

(2) In a particularly preferred embodiment, the overall process includes the following steps: a) nitration of benzene with nitric acid or—preferably—a mixture of nitric acid and sulfuric acid (also called mixed acid in the following) and separating off of the aqueous phase; b) washing of the organic process product obtained in step a); c) alkaline washing of the washed organic process product obtained in step b), an alkaline waste water comprising benzene in a concentration of from 100 ppm to 3,000 ppm and nitrobenzene in a concentration of from 1,000 ppm to 10,000 ppm preferably being obtained; d) optional separation of benzene and/or nitrobenzene out of the alkaline waste water obtained in step c); e) optional stripping of the alkaline waste water obtained in step c) or step d) for removal of residual amounts of benzene and/or nitrobenzene; f) work-up of the alkaline waste water obtained in step c) or step d) or step e), including steps (i) to (iii) described above.

(3) The nitration of benzene to give nitrobenzene with nitric acid or a mixture of nitric acid and sulfuric acid (mixed acid) in step a) is carried out in this context by any desired process from the state of the art which is known to the person skilled in the art, as described e.g. in EP 0 436 443 B1, EP 0 771 783 B1, U.S. Pat. No. 6,562,247 B2 or in EP 0 156 199 B1. Since a crude nitrobenzene which comprises excess acid, unreacted benzene, water and organic by-products is obtained in all the processes of the state of the art, the purification according to the invention of the crude nitrobenzene obtained in step a) can in principle be applied to all processes. For example, the nitration can be carried out with dissipation of the heat of reaction (i.e. isothermally or approximately isothermally) or also without dissipation of the heat of reaction in preferably isolated reactors (i.e. adiabatically). However, the reaction of benzene with a mixture of nitric acid and sulfuric acid using an adiabatic process procedure, such as is described in particular in DE 10 2008 048 713 A1, and there in particular in paragraph [0024], is preferred. The crude nitrobenzene prepared in step a) is finally separated from excess acid (if mixed acid is used substantially sulfuric acid) in a separating tank.

(4) The organic phase, which conventionally still comprises traces of acid, obtained in step a) after the phase separation is washed in step b) in one to two, preferably one wash(es), and is then separated from the acid aqueous phase by phase separation (in the case of several washes after each individual wash). In step b) the acid residues which the crude nitrobenzene contains are washed out; this process step is therefore also called an acid wash. Preferably, the procedure in this context is such that a pH of <5 (measured at 20° C.) is established in the aqueous phase obtained after the phase separation. Any type of water, e.g. deionized water or steam condensate, can be employed as the wash liquid in step b). The water can also contain dissolved salts. Preferably, aqueous streams obtained in the plant are recycled for carrying out step b).

(5) The organic phase obtained in this way is then washed in step c) in preferably one to two, particularly preferably one alkaline wash(es) with a base, preferably an aqueous solution of a base chosen from the group consisting of sodium hydroxide, sodium carbonate and sodium bicarbonate, and is then separated from the alkaline wash water by phase separation (in the ease of several washes after each individual wash). Sodium hydroxide solution is preferably used as the aqueous base solution. The alkaline wash is described in the following by means of sodium hydroxide solution; it is an easy matter for the person skilled in the art to make appropriate modifications if necessary if other bases are used.

(6) Preferably, the sodium hydroxide solution used has a pH of between 9.0 and 14 (measured at 20° C.). The weight ratio of sodium hydroxide solution to organic phase (substantially nitrobenzene) depends on the benzene excess employed in step a) and is preferably between 1:80 and 1:500. The pH of the sodium hydroxide solution used and its weight ratio to the organic phase are established such that acid impurities (e.g. nitrophenols formed as byproducts and acid residues which have not been completely removed in step b)), are largely to completely, preferably completely, neutralized in step c).

(7) In the optional step d), undissolved benzene and/or nitrobenzene which are still present in the alkaline waste water from step c) are separated off. The benzene and/or nitrobenzene separated off in this way are then preferably fed back to the nitration process, particularly preferably into the crude nitrobenzene. In this context, the separating off of the nitrobenzene present in undissolved form can be carried out by separators, settling tanks or other phase separation apparatuses. A settling tank is preferably used. Preferably, an alkaline waste water which comprises benzene in a concentration of from 100 ppm to 1,000 ppm and nitrobenzene in a concentration of from 1,200 ppm to 3,000 ppm is obtained in step d). It is preferable to carry out step d).

(8) Optionally, in step e) benzene and if appropriate residual nitrobenzene can then be removed by stripping from the alkaline waste water obtained from step c) and, depending on whether or not step d) is carried out, from step d). In this context the stripping is preferably carried out in a stripping column by stripping off the residual amounts of benzene and nitrobenzene overhead with steam. The vapours obtained, comprising benzene and nitrobenzene, are then preferably recycled into the alkaline wash in step c). Malfunction of the stripping column can be monitored, for example, by redundant safety equipment. Preferably, an alkaline waste water which still comprises benzene only in a concentration of up to 10 ppm and nitrobenzene in a concentration of up to 10 ppm is obtained in step e).

(9) In step f) (i) (the TPD) the alkaline waste water, which is obtained from steps c), d) and e) and is still loaded with organic salts of the nitro-hydroxyaromatics, is heated to a temperature of from 150° C. to 500° C., preferably from 250° C. to 350° C., particularly preferably from 270° C. to 290° C., under an increased pressure with respect to atmospheric pressure, preferably under an absolute pressure of from 50 bar to 350 bar, particularly preferably from 50 bar to 200 bar, very particularly preferably from 70 bar to 130 bar, with exclusion of oxygen. It is also possible for the alkaline waste water to be heated under an inert gas atmosphere or under an inert gas admission pressure of, for example, 0.1 bar to 100 bar. Suitable inert gases are e.g. nitrogen and/or argon. Depending on the temperature and where appropriate the inert gas admission pressure, the abovementioned pressures are preferably established during heating of the waste waters. The heating of the alkaline waste water and thermal pressure decomposition of the organic constituents such as benzene, nitrobenzene and nitrohydroxyaromatics is conventionally carried out in this context for 5 minutes to 120 minutes, preferably 15 minutes to 30 minutes. Preferably, the alkaline waste water is then cooled such that it leaves the TPD with a temperature of from 60° C. to 100° C.

(10) Steps d), e) and f) (i) can be carried out according to the state of the art, preferably according to the disclosure of EP 1 593 654 A1.

(11) In step f) (ii) a base, preferably an aqueous solution of a base chosen from the group consisting of lithium hydroxide, sodium hydroxide, potassium hydroxide and rubidium hydroxide, particularly preferably an aqueous sodium hydroxide solution, is added to the waste water obtained in f) (i), and that preferably in such a way that a pH of at least 12, preferably of from 12 to 13, is established. Hereby it is ensured that carbonates present in the waste water obtained in step f) (i) remain in the waste water, and indeed preferably to the extent of >99.999% and no carbon dioxide is formed. The process according to the invention renders it possible to limit the carbon dioxide content in the waste water obtained in step f) (ii) to <10 ppm, based on the total weight of the waste water obtained in step f) (ii). Due to the high pH in step f) (ii), ammonia is liberated from ammonium carbonate. In contrast to ammonium carbonate, ammonia can be easily removed by stripping.

(12) In step f) (iii) the waste water obtained in f) (ii) is finally purified further by stripping and the stripping gas stream loaded with impurities is then cooled to a temperature of from 10° C. to 60° C., preferably from 20° C. to 50° C., particularly preferably from 25° C. to 45° C., very particularly preferably from 30° C. to 40° C. Ammonia and organic constituents still present are removed by this procedure. Preferably, the stripping in the context of the present invention is carried out in counter-current in a stripping column; the gas stream of the stripping gas used (preferably steam) and the volatile constituents stripped off preferably emerging at the head of the stripping column (vapours) and the stripped waste water preferably being removed at the bottom of the stripping column. The gas stream removed at the head of the stripping column (vapours) is cooled in a condenser to a temperature of from 10° C. to 60° C., preferably from 20° C. to 50° C., particularly preferably from 25° C. to 45° C., very particularly preferably from 30° C. to 40° C., and the liquid stream formed in this way is preferably then transferred into a phase separation apparatus, from which the organic substances, which separate out, are purged. The stream obtained in this way, which is largely freed from organic substances (in the preferred embodiment with steam as the stripping gas, the so-called vapour water), is preferably led back partially to completely to the head of the stripping column. The remaining part of the stream which has been largely freed from organic substances is preferably fed directly (without further intermediate purification steps) to a waste water treatment, preferably a biological treatment plant. The stripping column is preferably a tubular device with several baffles (e.g. packed beds, structured packings or mass transfer trays) for intensive mass transfer of the gaseous and liquid phase. Appropriate processes and columns are known to the person skilled in the art and are described e.g. in W. Meier, Sulzer, Kolonnen für Rektifikation und Absorption, in: Technische Rundschau Sulzer, 2 (1979), page 49 to 61. Preferably, the stripping according to step f) (iii) is carried out under an absolute pressure of from 0.1 bar to 5 bar, particularly preferably 0.5 bar to 2 bar, and preferably at a temperature of from 40° C. to 160° C., particularly preferably 80° C. to 120° C.

(13) The ammonia stripped in step f) (iii) can be driven into the waste gas system by means of an inert gas, preferably nitrogen, by bubbling in at the head of the stripping column, and fed to a technical exhaust air decontamination.

(14) According to the state of the art, the person skilled in the art would always keep the temperatures in all the steps after the TPD higher than 60° C., in order to avoid the problem of precipitating ammonium carbonate. As a result, nevertheless, the possibility of separating off the organic substances at the head of the stripper in the phase separation apparatus is impaired, because organic substances dissolve better in water at elevated temperature and an increased level of organic constituents is consequently established in the waste water sent off to the biological treatment plant.

(15) The process according to the invention is characterized by a number of features which are not found in the prior art. Thus, in the work-up according to the invention of the alkaline waste water, after adding base after the TPD and therefore ensuring a very high pH, carbon dioxide is kept in the aqueous phase as carbonate. In the further work-up step in the stripping, therefore, no ammonium carbonate, which can lead, for example, to blockages in the exhaust gas system of the stripping column, can form by reaction of ammonia with carbon dioxide. Furthermore, the vapours at the head of the stripping column can now be cooled without forming troublesome ammonium carbonate. A better phase separation between the organic phase and aqueous phase comprising ammonia is achieved in the phase separation apparatus of the stripping column.

(16) Waste water purified according to the invention (i.e. the waste water stripped in step (iii), which in the preferred embodiment is removed from the bottom of the stripping column) can be sent without problems directly, i.e. without intermediate further purification steps, to a waste water treatment, preferably a biological treatment plant.

EXAMPLES

(17) Measurement Methods

(18) Content of organic components: gas chromatography (GC), area % are stated.

(19) Ammonium ion content: photometric measurement according to DIN 38406; weight contents in ppm are stated.

(20) General Conditions for the Preparation of Nitrobenzene

(21) Nitrobenzene was prepared in an adiabatic process as described in EP 2 168 942 A1. The waste water obtained in the last alkaline wash in this procedure was used in the following examples.

Example 1 (Comparison—High Condensation Temperature, No Addition of Base after TPD)

(22) The waste water from the alkaline wash was led into a settling tank in which undissolved benzene and nitrobenzene were separated out. 3.5 tonnes per hour of waste water which had a content of nitrobenzene of 2,293 ppm, of benzene of 212 ppm and of nitrophenols of 16,244 ppm and a pH of 13.4 (2% NaOH excess compared with the content of nitrophenols) were led from there into the TPD and treated with a residence time of 20 min, a temperature of 290° C. and an absolute pressure of 90 bar. The waste water formed was then cooled to 88° C. after the TPD. This waste water had a pH of 10.3 (0.1% NaOH, based on the waste water) and a very high content of free ammonia (2,300 ppm) and of carbonate (1.58%). This waste water was then stripped with direct steam. A strewn of 3.9 tonnes per hour which substantially comprised water, ammonia, carbon dioxide and organics was obtained in the bottom of the stripping column under an absolute pressure of 1.02 bar 2.0 in the stripper column. The head product of the stripping column was condensed and cooled to 65° C. A purge stream of organic substances was purged from the condensate. 0.25 tonne per hour of the aqueous condensate stream depleted in organic substances was recycled as reflux into the stripping column. The content of organic substances, measured as TOC (total organic carbon), in the waste water obtained (bottom stream of the stripping column), which were sent to a biological treatment plant, was 5,945 ppm. The content of ammonium in this waste water was 182 ppm.

Example 2 (Comparison—Low Condensation Temperature, No Addition of Base after TPD)

(23) Example 2 was carried out as Example 1, with the difference that the head product of the stripping column was condensed and cooled to 35° C. The content of organic substances in the waste water obtained, which was sent to a biological treatment plant, was 5,661 ppm. There were massive problems with blockages in the exhaust gas system of the stripping column, which were to be attributed to deposits of ammonium carbonate on valves and in the pipelines of the exhaust as system. The content of ammonium in this waste water was 212 ppm.

Example 3 (Comparison—High Condensation Temperature, Addition of Base after TPD)

(24) Example 3 was carried out as Example 1 up to and including the process step of TPD. 30% strength sodium hydroxide solution (100 l/h) was added to the waste water from the TPD, which had been cooled to 88° C. The pH thereafter was 12.5. Thereafter, the waste water was stripped with direct steam. A stream of 4.0 tonnes per hour which substantially comprised water, ammonia, carbon dioxide and organics was obtained in the bottom of the stripping column under an absolute pressure of 1.02 bar. The top product was condensed and cooled to 65° C. A purge stream of organics was purged from the condensate. 0.25 tonne per hour of the aqueous condensate stream depleted in organics was recycled as reflux into the stripping column. The content of organics in the waste water, which was sent to a biological treatment plant, was 5,894 ppm. The content of ammonium was less than 91 ppm.

Example 4 (According to the Invention—Low Condensation Temperature, Addition of Base after TPD)

(25) Example 4 was carried out as Example 3, with the difference that the head product of the stripping column was condensed and cooled to 35° C. The content of organics in the waste water obtained, which was sent to a biological treatment plant, was 4,726 ppm. The content of ammonium in the waste water was less than 87 ppm. There were no problems at all with deposits in the area of the exhaust gas of the stripping column.

(26) The examples show that only the procedure according to the invention with addition of a base (step (ii)) and a low condensation temperature (step (iii)) leads to low contents of organics of below 5,000 ppm and low ammonium contents of below 90 ppm.