Method for producing nitrobenzene

Abstract

The invention relates to a method for continuous production of nitrobenzene by means of nitration of benzene with nitric acid and sulfuric acid, in which load change (i.e. a reduction or increase in the quantity of nitric acid supplied to the process per time interval) is particularly advantageously developed. The invention particularly relates to a method in which, in the case of a load reduction, the ratio of the masses of benzene and nitric acid supplied per time interval is significantly increased compared to said ratio before the load change and/or the ratio of the masses of nitric acid and sulfuric acid supplied per time interval is significantly reduced compared to said ratio before the load change. In the event of a load increase, the reverse is carried out.

Claims

1. A continuously operated process for preparing nitrobenzene, comprising nitrating benzene with nitric acid and sulfuric acid, wherein (i) the nitration is supplied with a stream containing benzene and having a proportion by mass of benzene w.sub.1 with a mass flow rate of {dot over (m)}.sub.10, a stream containing nitric acid and having a proportion by mass of nitric acid w.sub.2 with a mass flow rate of {dot over (m)}.sub.20, and a stream containing sulfuric acid and having a proportion by mass of sulfuric acid w.sub.3 with a mass flow rate of {dot over (m)}.sub.30; (ii) {dot over (m)}.sub.10 and {dot over (m)}.sub.20, for given values of w.sub.1 and w.sub.2, are chosen such that benzene is in a stoichiometric excess relative to nitric acid; and (iii) at least one change is made to the amount of nitric acid supplied to the nitration via {dot over (m)}.sub.20 from a starting state A defined by a mass flow rate of nitric acid {dot over (m)}.sub.2 (A)={dot over (m)}.sub.20 (A).Math.w.sub.2 (A), a mass flow rate of benzene {dot over (m)}.sub.1 (A)={dot over (m)}.sub.10 (A).Math.w.sub.1 (A) selected with regard to (ii) and a mass flow rate of sulfuric acid {dot over (m)}.sub.3 (A)={dot over (m)}.sub.30 (A).Math.w.sub.3 (A), to a final state E defined by a desired mass flow rate of nitric acid {dot over (m)}.sub.2 (E)={dot over (m)}.sub.20 (E).Math.w.sub.2 (E), a mass flow rate of benzene {dot over (m)}.sub.1 (E)={dot over (m)}.sub.10 (E).Math.w.sub.1 (E) selected with regard to (ii) and a mass flow rate of sulfuric acid {dot over (m)}.sub.3 (E)={dot over (m)}.sub.30 (E).Math.w.sub.3 (E), {dot over (m)}.sub.20 and w.sub.2 being chosen to establish the desired value for {dot over (m)}.sub.2 (E) wherein the at least one change in the amount of nitric acid supplied to the nitration via {dot over (m)}.sub.20 is (1) a decrease to a value {dot over (m)}.sub.2 (E)<0.95.Math.{dot over (m)}.sub.2 (A) for more than 0.50 hour or (2) an increase to a value {dot over (m)}.sub.2 (E)>1.05.Math.{dot over (m)}.sub.2 (A) for more than 0.50 hour, wherein, in case (1) (a) the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is increased compared to {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) such that: 1.03.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E)1.50.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and the ratio {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is altered to a maximum degree compared to the ratio {dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) such that: 0.98.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A){dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E)1.02.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) or (b) the ratio {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is decreased compared to the ratio {dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) such that: 0.45.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A){dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E)0.97.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) and the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is altered to a maximum degree compared to the ratio {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) such that: 0.98.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E)1.02.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) or (c) the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is increased compared to {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) such that: 1.03.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E)1.50.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and the ratio {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is decreased compared to the ratio {dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) such that: 0.45.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.3 (E)0.97.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); and wherein, in case (2) (a) the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is decreased compared to {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) such that: 0.45.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E)0.97.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and the ratio {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is altered to a maximum degree compared to the ratio {dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) such that: 0.98.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.3 (E)1.02.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) or (b) the ratio {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is increased compared to the ratio {dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) such that: 1.03.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A){dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E)1.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) and the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is altered to a maximum degree compared to the ratio {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) such that: 0.98.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E)1.02.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) or (c) the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is decreased compared to {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) such that: 0.45.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A){dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E)0.97.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and the ratio {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is increased compared to the ratio {dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) such that: 1.03.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A){dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E)1.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A).

2. The process of claim 1, wherein nitrating benzene to nitrobenzene is performed adiabatically.

3. The process of claim 2, wherein nitrating benzene to nitrobenzene comprises: (I) introducing a benzene-containing stream with a mass flow rate of {dot over (m)}.sub.10, a nitric acid-containing stream with a mass flow rate of {dot over (m)}.sub.20 and a sulfuric acid-containing stream with a mass flow rate of {dot over (m)}.sub.30 into a reactor to form a reaction mixture; (II) separating phases of the reaction mixture from step (I) in a phase separation apparatus into an aqueous sulfuric acid-containing phase and an organic nitrobenzene-containing phase; (III) concentrating the aqueous phase obtained in step (II) by evaporating water in an evaporation apparatus to give an aqueous sulfuric acid-containing phase having elevated sulfuric acid concentration, and recycling the concentrated sulfuric acid-containing aqueous phase into step (I) as a constituent of the sulfuric acid-containing stream; (IV) washing, in at least two stages, the organic nitrobenzene-containing phase obtained in step (II) and separating the aqueous phase off after each stage; (V) distilling the organic nitrobenzene-containing phase obtained in the last stage of step (IV), and removing and recycling unconverted benzene into step (I) as a constituent of the benzene-containing stream.

4. The process of claim 3, further comprising: (VI) working up wastewater from a first wash stage of step (IV) by cleaning the wastewater from the first wash stage in an apparatus for distillation or stripping, (VII) working up wastewater from a second wash stage of step (IV) by cleaning wastewater from the second wash stage in an apparatus for distillation or stripping, wherein an apparatus for thermal pressure decomposition is connected upstream and/or downstream from the apparatus for distillation or stripping.

5. The process of claim 3, further comprising: (IVa) performing a first wash stage by washing the organic nitrobenzene-containing phase obtained in step (II) in at least one wash, then separating the phases into an aqueous phase and an organic nitrobenzene-containing phase; (IVb) performing a second wash stage by washing the organic phase obtained in step (IVa) in at least one alkaline wash with an aqueous solution of a base, then separating the phases into an aqueous phase and an organic nitrobenzene-containing phase; (IVc) performing a third wash stage by washing the organic phase obtained in step (IVb) in at least one neutral wash with water, then separating the phases into an aqueous phase and an organic nitrobenzene-containing organic phase.

6. The process of claim 1, wherein the ratio {dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) is in the range from 1.26 to 1.74.

7. The process of claim 2, wherein: 0.40.Math.{dot over (m)}.sub.2 (A){dot over (m)}.sub.2 (E)2.50.Math.{dot over (m)}.sub.2 (A).

8. The process of claim 7, wherein variant (a) is conducted, wherein, in case (1), the following relationships are applicable: when {dot over (m)}.sub.2 (E) is in the range from 0.80.Math.{dot over (m)}.sub.2 (A) to <0.95.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 1.03.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 1.20.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A); when {dot over (m)}.sub.2 (E) is in the range from 0.65.Math.{dot over (m)}.sub.2 (A) to <0.80.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from >1.20.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 1.40.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A); and when {dot over (m)}.sub.2 (E) is in the range from 0.40.Math.{dot over (m)}.sub.2 (A) to <0.65.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from >1.40.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 1.50.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A); or wherein, in case (2), the following relationships are applicable: when {dot over (m)}.sub.2 (E) is in the range from >1.05.Math.{dot over (m)}.sub.2 (A) to 1.50.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 0.97.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 0.75.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A); when {dot over (m)}.sub.2 (E) is in the range from >1.50.Math.{dot over (m)}.sub.2 (A) to 2.00.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 0.55.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to <0.75.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A); and when {dot over (m)}.sub.2 (E) is in the range from >2.00.Math.{dot over (m)}.sub.2 (A) to 2.50.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 0.45.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to <0.55.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A).

9. The process of claim 7, wherein variant (b) is conducted, wherein, in case (1), the following relationships are applicable: when {dot over (m)}.sub.2 (E) is in the range from 0.80.Math.{dot over (m)}.sub.2 (A) to <0.95.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is adjusted to a value in the range from 0.80.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 0.97.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); when {dot over (m)}.sub.2 (E) is in the range from 0.65.Math.{dot over (m)}.sub.2 (A) to <0.80.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is adjusted to a value in the range from 0.65.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to <0.80.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); and when {dot over (m)}.sub.2 (E) is in the range from 0.40.Math.{dot over (m)}.sub.2 (A) to <0.65.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is adjusted to a value in the range from 0.40.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to <0.65.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); or wherein, in case (2), the following relationships are applicable: when {dot over (m)}.sub.2 (E) is in the range from >1.05.Math.{dot over (m)}.sub.2 (A) to 1.50.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is adjusted to a value in the range from 1.03.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 1.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); when {dot over (m)}.sub.2 (E) is in the range from >1.50.Math.{dot over (m)}.sub.2 (A) to 2.00.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is adjusted to a value in the range from >1.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 2.00.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); and when {dot over (m)}.sub.2 (E) is in the range from >2.00.Math.{dot over (m)}.sub.2 (A) to 2.50.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) is adjusted to a value in the range from >2.00.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 2.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A).

10. The process of claim 7, wherein variant (c) is conducted, wherein, in case (1), the following relationships are applicable: when {dot over (m)}.sub.2 (E) is in the range from 0.80.Math.{dot over (m)}.sub.2 (A) to <0.95.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 1.03.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 1.20.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) to a value in the range from 0.80.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 0.97.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); when {dot over (m)}.sub.2 (E) is in the range from 0.65.Math.{dot over (m)}.sub.2 (A) to <0.80.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from >1.20.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 1.40.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) to a value in the range from 0.65.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to <0.80.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); and when {dot over (m)}.sub.2 (E) is in the range from 0.40.Math.{dot over (m)}.sub.2 (A) to <0.65.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from >1.40.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 1.50.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) to a value in the range from 0.40.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to <0.65.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); or wherein, in case (2), the following relationships are applicable: when {dot over (m)}.sub.2 (E) is in the range from >1.05.Math.{dot over (m)}.sub.2 (A) to 1.50.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 0.75.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to 0.97.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) to a value in the range from 1.03.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 1.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); when {dot over (m)}.sub.2 (E) is in the range from >1.50.Math.{dot over (m)}.sub.2 (A) to 2.00.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 0.55.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to <0.75.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) to a value in the range from >1.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 2.00.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A); and when {dot over (m)}.sub.2 (E) is in the range from >2.00.Math.{dot over (m)}.sub.2 (A) to 2.50.Math.{dot over (m)}.sub.2 (A), {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) is adjusted to a value in the range from 0.45.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) to <0.55.Math.{dot over (m)}.sub.1 (A)/{dot over (m)}.sub.2 (A) and {dot over (m)}.sub.2 (E)/{dot over (m)}.sub.3 (E) to a value in the range from >2.00.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A) to 2.50.Math.{dot over (m)}.sub.2 (A)/{dot over (m)}.sub.3 (A).

11. The process of claim 9, wherein 0.98.Math.{dot over (m)}.sub.3 (A){dot over (m)}.sub.3 (E)1.02.Math.{dot over (m)}.sub.3 (A).

12. The process of claim 1, wherein w.sub.1 (A)=w.sub.1 (E) and/or w.sub.2 (A)=w.sub.2 (E) and/or w.sub.3 (A)=w.sub.3 (E).

13. The process of claim 12, wherein w.sub.1 (A)=w.sub.1 (E), w.sub.2 (A)=w.sub.2 (E) and w.sub.3 (A)=w.sub.3 (E).

14. The process of claim 1, comprising decreasing the amount of nitric acid supplied to the nitration via {dot over (m)}.sub.20 in case (1), wherein decreasing is conducted by variant (b).

15. The process of claim 1, comprising increasing the amount of nitric acid supplied to the nitration via {dot over (m)}.sub.20 in case (2), wherein increasing is conducted by variant (b).

Description

EXAMPLES

(1) Content figures in ppm or % are parts by mass based on the total mass of the respective material/stream. Analysis values, unless stated otherwise, have been determined by means of high-performance liquid chromatography (HPLCnitrophenols) and gas chromatography (GCother by-products and benzene) and.

(2) A. General Conditions for the Preparation of Nitrobenzene in Regular Operation at Nameplate Load

(3) Into a nitration reactor are metered a sulfuric acid stream ({dot over (m)}.sub.30=210 t/h; w.sub.3=0.713), a nitric acid stream ({dot over (m)}.sub.20=10 000 kg/h; w.sub.2=0.685) and a benzene stream ({dot over (m)}.sub.10=9800 kg/h; w.sub.1=0.989) consisting of 95% by mass of fresh benzene and 5% by mass of return benzene. A 14.14% excess of benzene is used, based on nitric acid. On completion of conversion of the nitric acid with the benzene to give nitrobenzene in an adiabatic reaction regime, the reaction product, now at about 130 C., is fed to a phase separation apparatus in which the reaction product separates into an organic phase (=crude nitrobenzene, also containing benzene as well as nitrobenzene) and an aqueous phase (=waste acid, also containing small proportions of nitrobenzene and benzene as well as sulfuric acid). The aqueous phase comprising mainly sulfuric acid is subjected to a flash evaporation of water by abruptly lowering the pressure in the evaporator, and concentrated in this way. The concentrated sulfuric acid is stored in the sulfuric acid tank for reuse. After being removed in the phase separation apparatus, the crude nitrobenzene is cooled down to about 50 C. in the crude nitrobenzene cooling operation and sent to the washing operation. This wash comprises an acidic wash stage, an alkaline wash stage and a neutral wash stage.

(4) The stream of purified crude nitrobenzene which has been substantially freed of nitrophenols and salts and has been obtained in this way is heated up again and, in a distillation column, freed of water, benzene and other low boilers which are removed overhead, giving dried pure nitrobenzene. The condensed top product from the distillation column is fed to a phase separation apparatus in which the top product separates into an organic phase (comprising benzene) and an aqueous phase. The organic phase is stored intermediately in a buffer tank and thence run back, as already described above, into the feed of the nitration reactor for reaction.

(5) The wastewater obtained in the alkaline wash is worked up as follows:

(6) The wastewater from the alkaline wash is run into a settling tank in which undissolved benzene and nitrobenzene are separated out. 3.5 tonnes per hour of alkaline wastewater which has, on average, a nitrobenzene content of 2870 ppm, a benzene content of 409 ppm and a nitrophenols content of 11 809 ppm and a pH of 12.8 (1.8% excess of NaOH compared to the starting content of nitrophenols prior to the alkaline wash) are conducted into a stripping column in order to remove benzene and nitrobenzene from this alkaline wastewater overhead by stripping with steam. For this purpose, 500 kg/h of 6 bar steam are used. The pressure in the top of the column is 1.05 bar (absolute), and the temperature is 99.5 C. The top of the stripping column is equipped with a vertical condenser in which the benzene- and nitrobenzene-containing vapors are condensed out and then recycled into the acidic wash. The moist offgas at 99 C. from the stripping column is guided directly into the condenser and quenched by spraying with acidic water at 30 C. from the acid water tank. This prevents the possible deposition of ammonium nitrate and/or ammonium nitrite, which can form in the dry region of a conventional offgas conduit used for the separate conduction of the offgas out of the condenser (the ammonium salts mentioned may form from ammonia and nitrogen oxides present in the alkaline wastewater). The acidic water is fed to the acidic wash together with the condensed vapors. Any malfunction of the stripping column can be monitored, for example, by means of redundant safety devices. After the stripping, an alkaline wastewater that contains benzene only in a concentration of up to 10 ppm and nitrobenzene in a concentration of up to 10 ppm is obtained. Subsequently, the alkaline wastewater thus treated is treated in a plant for thermal pressure decomposition with a residence time of 20.Math.min, a temperature of 290 C. and an absolute pressure of 90 bar. The wastewater that arises here is cooled down to 80 C. Thereafter, the wastewater is stripped with direct steam. In the bottoms from the stripping column, a stream of 4.0 tonnes per hour is obtained at an absolute pressure of 1.02 bar, which contains essentially water, ammonia, carbon dioxide and organics. The top product is condensed and cooled down to 35 C. A purge stream of organics is discharged from the condensate. 0.25 tonne per hour of the aqueous condensate stream depleted of organics is recycled into the stripping column as reflux. The proportion of organics in the wastewater obtained, which is sent to a biological water treatment plant, is 4726 ppm. The ammonium content in the wastewater is less than 87 ppm. In general, there are no problems at all with deposits in the region of the offgas from the stripping column.

(7) Nitrobenzene prepared in this way has, on average, a purity of about 99.96% (GC), a residual benzene content of 0.0028% (GC) and a water content of 0.0079% (determined according to Karl Fischer). The following table summarizes the operating conditions at nameplate load:

(8) TABLE-US-00007 TABLE 1 Operating conditions at nameplate load {dot over (m)}.sub.10/ {dot over (m)}.sub.20/ {dot over (m)}.sub.30/ (kg h.sup.1) w.sub.1 (kg h.sup.1) w.sub.2 {dot over (m)}.sub.1/{dot over (m)}.sub.2 (t .Math. h.sup.1) w.sub.3 {dot over (m)}.sub.2/{dot over (m)}.sub.3 9800 0.989 10 000 0.685 1.41 210 0.713 0.0457 Notes: {dot over (m)}.sub.1/{dot over (m)}.sub.2 = {dot over (m)}.sub.10/{dot over (m)}.sub.20 .Math. w.sub.1/w.sub.2; {dot over (m)}.sub.2/{dot over (m)}.sub.3 = {dot over (m)}.sub.20/{dot over (m)}.sub.30 .Math. w.sub.2/w.sub.3.
B. Preparation of Nitrobenzene at Lower than Nameplate Load

(9) All proportions by mass w.sub.i were kept the same with respect to operation at nameplate load.

Example 1 (Comparative): Decrease in Nitric Acid Load to about 93% of Nameplate Load with Corresponding Decrease in {dot over (m)}.SUB.10 .and Corresponding Decrease in {dot over (m)}.SUB.30

(10) Proceeding from production at nameplate load as described above under A, the mass flow rate {dot over (m)}.sub.20 was reduced to 9301 kg/h (corresponding to {dot over (m)}.sub.20 (E)/{dot over (m)}.sub.20 (A)={dot over (m)}.sub.2 (E)/{dot over (m)}.sub.2 (A)=0.9301). The mass flow rate {dot over (m)}.sub.10 was reduced to 8954 kg/h, i.e. the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) was essentially maintained at 1.39 compared to the starting state.

(11) The mass flow rate {dot over (m)}.sub.30 was reduced to 195 t/h, i.e. the ratio {dot over (m)}.sub.2/{dot over (m)}.sub.3 was essentially maintained at 0.0458.

Example 2 (Inventive): Decrease in Nitric Acid Load to about 93% of Nameplate Load with Decrease in {dot over (m)}.SUB.10 .and Retention of {dot over (m)}.SUB.30

(12) Proceeding from production at nameplate load as described above under A, the mass flow rate {dot over (m)}.sub.20 was reduced to 9298 kg/h (corresponding to {dot over (m)}.sub.20 (E)/{dot over (m)}.sub.20 (A)={dot over (m)}.sub.2 (E)/{dot over (m)}.sub.2 (A)=0.9298). The mass flow rate {dot over (m)}.sub.10 was reduced to 8937 kg/h, i.e. the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) was essentially maintained at 1.39 compared to the starting state. The slight nominal differences in {dot over (m)}.sub.10 (E) and {dot over (m)}.sub.20 (E) by comparison with example 1 do not impair comparability.

(13) The mass flow rate {dot over (m)}.sub.30 was kept constant, i.e. the ratio {dot over (m)}.sub.2/{dot over (m)}.sub.3 was decreased to 0.0425.

Example 3 (Comparative): Decrease in Nitric Acid Load to about 60% of Nameplate Load with Corresponding Decrease in {dot over (m)}.SUB.10 .and Corresponding Decrease in {dot over (m)}.SUB.30

(14) Proceeding from production at nameplate load as described above under A, the mass flow rate {dot over (m)}.sub.20 was reduced to 6052 kg/h (corresponding to {dot over (m)}.sub.20 (E)/{dot over (m)}.sub.20 (A)={dot over (m)}.sub.2 (E)/{dot over (m)}.sub.2 (A)=0.6052). The mass flow rate {dot over (m)}.sub.10 was reduced to 5956 kg/h, i.e. the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) was essentially maintained at 1.42 compared to the starting state.

(15) The mass flow rate {dot over (m)}.sub.30 was reduced to 127 t/h, i.e. the ratio {dot over (m)}.sub.2/{dot over (m)}.sub.3 was kept essentially constant at 0.0458 compared to the starting state.

Example 4 (Inventive): Decrease in Nitric Acid Load to about 60% of Nameplate Load with Corresponding Decrease in {dot over (m)}.SUB.10 .and (Essentially) Retention of {dot over (m)}.SUB.30

(16) Proceeding from production at nameplate load as described above under A, the mass flow rate {dot over (m)}.sub.20 was reduced to 6005 kg/h (corresponding to {dot over (m)}.sub.20 (E)/{dot over (m)}.sub.20 (A)={dot over (m)}.sub.2 (E)/{dot over (m)}.sub.2 (A)=0.6005). The mass flow rate {dot over (m)}.sub.10 was reduced to 5945 kg/h, i.e. the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) was essentially maintained at 1.43 compared to the starting state. The slight nominal differences in {dot over (m)}.sub.10 (E) and {dot over (m)}.sub.20 (E) by comparison with example 3 do not impair comparability.

(17) The mass flow rate {dot over (m)}.sub.30 was reduced only minimally to 207 t/h, i.e. the ratio {dot over (m)}.sub.2/{dot over (m)}.sub.3 was decreased to 0.0279 compared to the starting state.

Example 5 (Inventive): Decrease in Nitric Acid Load to about 60% of Nameplate Load with Increase in {dot over (m)}.SUB.1./{dot over (m)}.SUB.2 .and (Essentially) Retention of {dot over (m)}.SUB.30

(18) Proceeding from production at nameplate load as described above under A, the mass flow rate {dot over (m)}.sub.20 was reduced to 6011 kg/h (corresponding to {dot over (m)}.sub.20 (E)/{dot over (m)}.sub.20 (A)={dot over (m)}.sub.2 (E)/{dot over (m)}.sub.2 (A)=0.6011). The mass flow rate {dot over (m)}.sub.10 was reduced to 6064 kg/h, i.e. the ratio {dot over (m)}.sub.1 (E)/{dot over (m)}.sub.2 (E) was increased to 1.46 compared to the starting state.

(19) The mass flow rate {dot over (m)}.sub.30 was reduced only minimally to 206 t/h, i.e. the ratio {dot over (m)}.sub.2/{dot over (m)}.sub.3 was decreased to 0.0280 compared to the starting state.

(20) The following table compares the by-product contents to one another:

(21) TABLE-US-00008 TABLE 2 By-product contents of nitrobenzene {dot over (m)}.sub.1/{dot over (m)}.sub.2 {dot over (m)}.sub.2/{dot over (m)}.sub.3 compared compared C.sub.1,3-DNB/ C.sub.DNP/ C.sub.PS/ Nitrobenzene from to A to A ppm ppm ppm Operation at nameplate load (A) 250 1819 137 Ex. 1 (comparative) ess. the ess. the 316 2388 215 same same Ex. 2 (inventive - variant (b)) ess. the decreased 284 1841 148 same Ex. 3 (comparative) ess. the ess. the 522 2479 321 same same Ex. 4 (inventive - variant (b)) ess. the decreased 301 1945 162 same Ex. 5 (inventive - variant (c)) increased decreased 289 1928 157 Notes: ess. = essentially; c = concentration; DNB = dinitrobenzene; DNP = dinitrophenol; PS = picric acid.

(22) It is immediately apparent that the reduction in load in all cases leads to increased by-product formation. However, the rise in the by-product content is considerably smaller in the case of the procedure of the invention than in the comparative experiments.