Method and system for producing nitrobenzene

1. A process for the production of nitrobenzene by means of adiabatic nitration of benzene with nitric acid in the presence of sulfuric acid, in which

a) the product mixture obtained in the nitration is separated into spent acid and crude nitrobenzene and

b) at least the water contained in the spent acid is partially separated from the sulfuric acid contained therein and

c) the sulfuric acid so concentrated is fed to the nitration again,

which process is characterised in that

i) the separation according to step b) is carried out by evaporation in two stages with an absolute pressure that falls from the first stage to the second stage from 100 mbar to 300 mbar to 50 mbar to 150 mbar;

ii) a proportion of at least 90% of the heat for the evaporation comes from the reaction enthalpy of the adiabatic nitration reaction and

iii) the product mixture obtained in the nitration has a temperature of at least 110° C.

2. The process according to claim 1 , characterised in that the product mixture obtained in the nitration has a temperature of at least 120° C.

3. The process according to claim 1 , characterised in that the heat for the evaporation according to step ii) comes solely from the reaction enthalpy of the adiabatic nitration reaction.

4. The process according to any one of the preceding claims, characterised in that the feed of spent acid to the first stage according to i) contains from 60 to 75% by mass H 2 SO 4 .

5. The process according to claim 1 , 2 or 3 , characterised in that the steam separated in each stage according to i) is liquefied by condensation.

6. The process according to claim 5 , characterised in that the liquefied steam separated off is fed to a phase separator.

7. The process according to claim 1 , 2 or 3 , characterised in that the concentrated sulfuric acid according to step c), after passing through both stages of step i), has a content by mass of H 2 SO 4 of not less than 65%.