Method for producing aromatic amines in the liquid phase

1. A process for the production of an aromatic amine represented by formula (I)

in which

R1 and R2 independently of one another represent hydrogen, a methyl group or ethyl group, and wherein R1 can additionally represent NH 2

comprising hydrogenating a nitroaromatic compound represented by formula (II)

in which

R2 and R3 independently of one another represent hydrogen, a methyl group or ethyl group, and wherein R3 can additionally represent NO 2 ,

in the presence of a catalyst in the liquid phase in at least two series-connected reaction spaces arranged in one reactor or in a plurality of reactors, wherein

at least one reaction space is operated isothermally and at least one reaction space connected downstream thereof is operated adiabatically.

2. The process of claim 1 in which progress of the hydrogenation reaction is determined by monitoring a characteristic parameter of the hydrogenation carried out in at least one adiabatically operated reaction space the characteristic parameter being selected from color, temperature, and concentration of compounds having nitro or nitroso groups.

3. The process of claim 2 in which at least one process parameter is changed when the characteristic parameter has reached or exceeded its critical value so that the characteristic parameter falls below the critical value.

4. The process of claim 3 in which the characteristic parameter is the temperature T adiabatic measured in at least one adiabatically operated reaction space.

5. The process of claim 3 in which the characteristic parameter is the temperature T adiabatic measured in the product outlet from at least one adiabatically operated reaction space.

6. The process claim 4 in which the critical value of the parameter T adiabatic is from 200° C. to 250° C.

7. The process claim 5 in which the critical value of the parameter T adiabatic is from 200° C. to 250° C.

8. The process of claim 3 in which the process parameter changed is at least one of the following:

a) supply of fresh catalyst is adjusted;

b) amount of used catalyst supplied is increased;

c) amount of fresh nitroaromatic compound fed to the hydrogenation reaction is gradually reduced;

d) pressure in at least one of the reaction spaces is increased; or

e) temperature in at least one of the reaction spaces is increased.

9. The process of claim 2 in which the concentration of compounds containing nitro and nitroso groups is determined by chemical or spectroscopic analysis.

10. The process of claim 2 in which the characteristic parameter is the temperature difference ΔT adiabatic =T adiabatic −T isothermal between at least one adiabatically operated reaction space and the isothermally operated reaction space preceding it, and

T adiabatic is measured in the adiabatically operated reaction space product outlet or in the adiabatically operated reaction space

and

T isothermal is measured in the preceding isothermally operated reaction space or in the adiabatically operated reaction space inlet.

11. The process of claim 10 in which the critical value of the parameter ΔT adiabatic (ΔT adiabatic, crit. ) is from −10 K to +8 K.

12. The process of claim 1 in which temperatures of from 50° C. to 250° C. and absolute pressures of from 6 bar to 101 bar are maintained in all the reaction spaces.

13. The process of claim 12 in which the ratio of the pressure in an arbitrary adiabatically operated reaction space to the pressure in the isothermally operated reaction space preceding it is greater than 0.5.

14. The process of claim 1 in which dinitrotoluene (in formula II: R2=methyl and R3=NO 2 ) is hydrogenated to produce toluenediamine (in formula I: R1=NH 2 and R2=methyl).

15. The process of claim 1 in which from 2 to 10 reaction spaces are connected in series.

16. The process of claim 1 in which the first reaction space in the direction of flow of the nitroaromatic compound is operated isothermally and the last is operated adiabatically.