Method of cleaning an apparatus used in the concentration of a mineral acid
The present invention relates to a method for preparing an apparatus (or a plant comprising a plurality of apparatuses), for example a heat exchanger, a vaporizer or a distillation column, for use in a process (in particular a continuously operated process) for concentrating a mineral acid by evaporation of water, wherein the apparatus or the plant comprises (at least) a device which is resistant to the mineral acid and the device is flushed with an aqueous alkali metal hydroxide solution having a concentration by mass of alkali metal hydroxide in the range from 1% to 30% at a temperature in the range from 40° C. to 90° C. for a time of from 2 hours to 7 days.
1. A method for preparing and using an apparatus comprising an acid-resistant device,
wherein the acid-resistant device has a coating comprising steel enamel, silicon carbide, glass, tantalum, niobium, a perfluorinated polymer, or a composite of two or more thereof,
or
wherein the acid-resistant device is made of one of steel enamel, silicon carbide, glass, tantalum, niobium, a perfluorinated polymer, or a composite of two or more thereof,
the method comprising:
(a) flushing the acid-resistant device with an aqueous alkali metal hydroxide solution having a concentration by mass of alkali metal in the range from 1% to 30% at a temperature in the range from 40° C. to 90° C. for a time of from 2 hours to 7 days; and then subsequently
(b) using the apparatus comprising the acid-resistant device flushed in step (a) in a process of concentrating a mineral acid by evaporation of water, wherein said process comprises contacting the acid-resistant device with the mineral acid during the concentrating of the mineral acid or wherein said process comprises contacting the acid-resistant device with the mineral acid while the mineral acid has a concentration that was increased as a result of said concentrating of the mineral acid.
2. The method as claimed in claim 1 , wherein the alkali metal hydroxide solution comprises sodium hydroxide solution, potassium hydroxide solution, or a mixture thereof.
3. The method as claimed in claim 1 , wherein the flushing is carried out for a time of from 8 hours to 30 hours.
4. The method as claimed in claim 1 , wherein the acid-resistant device comprises a heat exchanger for heating the mineral acid, a distillation apparatus for removing organic compounds from the mineral acid, a vaporization apparatus, a heat exchanger for cooling the mineral acid after the mineral acid has been concentrated and/or a vessel for accommodating the mineral acid after the mineral acid has been concentrated.
5. The method as claimed in claim 4 , wherein the acid-resistant device is a heat exchanger which is designed for heating the mineral acid and/or for cooling the mineral acid after the mineral acid has been concentrated.
6. The method as claimed in claim 4 , wherein the acid-resistant device comprises a heat exchanger for cooling the mineral acid after the mineral acid has been concentrated and a vessel downstream thereof for accommodating the mineral acid after the mineral acid has been concentrated.
7. The method as claimed in claim 1 , wherein the mineral acid comprises sulfuric acid, nitric acid, or a mixture thereof.
8. The method as claimed in claim 7 , wherein the process of concentrating the mineral acid by evaporation of water comprises concentrating sulfuric acid that is used as a catalyst and diluent in a nitration of an aromatic compound with nitric acid to obtain an aromatic nitro compound.
9. The method as claimed in claim 8 , wherein the aromatic nitro compound comprises nitrobenzene or dinitrotoluene.
10. The method as claimed in claim 7 , wherein the process of concentrating the mineral acid by evaporation of water is used for concentrating sulfuric acid that has been diluted by use of the sulfuric acid as a desiccant for drying a gas.
11. The method as claimed in claim 10 , wherein the gas comprises chlorine.
12. The method as claimed in claim 1 , wherein the acid-resistant device comprises a coating comprising a perfluorinated polymer or wherein the acid-resistant device is made at least partly of a perfluorinated polymer, where the perfluorinated polymer is mechanically stabilized with an inert material.
13. The method as claimed in claim 1 , wherein the acid-resistant device comprises a coating comprising steel enamel, silicon carbide, glass, niobium and/or tantalum or wherein the acid-resistant device is made at least partly of steel enamel, silicon carbide, glass, niobium and/or tantalum.
14. The method as claimed in claim 1 , wherein the acid-resistant device is not disassembled for the purpose of carrying out the flushing.
15. The method as claimed in claim 14 , wherein the apparatus is not disassembled for the purpose of carrying out the flushing of the acid-resistant device.