METHOD OF CLEANING AN APPARATUS USED IN THE CONCENTRATION OF A MINERAL ACID

Abstract

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.

Claims

1. A method for preparing an apparatus for use in a process for concentrating a mineral acid by evaporation of water, wherein the apparatus comprises a device which is resistant to the mineral acid, wherein the 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 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 flushing the device 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.

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 apparatus comprises a heat exchanger for heating the mineral acid to be concentrated, a distillation apparatus for removing organic compounds from the mineral acid to be concentrated, a vaporization apparatus, a heat exchanger for cooling the mineral acid which has been concentrated and/or a vessel for accommodating the mineral acid which has been concentrated.

5. The method as claimed in claim 4, wherein the apparatus is a heat exchanger which is designed for heating the mineral acid to be concentrated and/or for cooling the mineral acid which has been concentrated.

6. The method as claimed in claim 4, wherein the apparatus comprises a heat exchanger for cooling the mineral acid which has been concentrated and a vessel downstream thereof for accommodating the mineral acid which 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 is used to concentrate sulfuric acid, wherein the concentrated sulfuric acid is used as reaction medium in a nitration of an aromatic compound with nitric acid to obtain an aromatic nitro compound, and wherein the concentrated sulfuric acid is diluted by the nitration, back to its concentration originally used in the nitration.

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, wherein the concentrated sulfuric acid is used as desiccant for drying a gas, and wherein the concentrated sulfuric acid is diluted by the drying, back to its concentration originally used in the drying.

11. The method as claimed in claim 10, wherein the gas comprises chlorine.

12. The method as claimed in claim 1, wherein the device comprises a coating comprising a perfluorinated polymer or wherein the 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 device comprises a coating comprising steel enamel, silicon carbide, glass, niobium and/or tantalum or wherein the 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 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 device.

Description

EXAMPLES

[0047] A plant for concentrating waste acid from a process for nitration of toluene to give dinitrotoluene (DNT) is shown in simplified form in FIG. 1. It consists of a plurality of recuperators (1), a plurality of vaporizers (2) which are each connected to columns (3) and condensers (4) (just one of these plant components is shown in each case). The inflow of the waste acid is denoted by A, the acid which has been concentrated by F and the process condensate by D. Severe fouling of the heat exchangers (1) was found, so that the hot concentrated acid, denoted by E, from the vaporizer (2) could no longer flow out freely, as is necessary for operation, through the heat exchangers (1). In addition, it was observed that the heat transfer through the heat exchangers (1) was considerably reduced, which could be seen from a reduced temperature of the stream B, an increased temperature of the stream F and an increased energy consumption by the vaporizer (2).

[0048] The heat exchangers (1) were firstly flushed with water and subsequently with 10% strength sodium hydroxide solution. For this purpose, the parts of the two pipes B and E which are in each case connected to nozzles on the last of the heat exchangers (1) were disconnected and the two nozzles were connected by a hose. During this operation, the nozzles were inspected and large amounts of solid deposits were found. The sodium hydroxide solution which had been heated to 65° C. was subsequently pumped from a reservoir for 24 hours through the flushing nozzles located on the heat exchanger (1) in the pipe F and taken off at a flushing nozzle located in the pipe A and returned to the reservoir. After another brief flushing with water, the hose was disconnected and the nozzles were inspected again. No fouling was observed any longer; accordingly, cleaning was complete. The pipes which had been disconnected were connected again and the plant was started up again. It was found here that the acid could again flow out freely and the heat transfer through the heat exchangers (1) corresponds to the design value.