METHOD FOR CLEANING A DEVICE USED IN THE CONCENTRATING OF A MINERAL ACID

Description

[0001] The present invention relates to a method for preparing a device (or a plant comprising a plurality of devices), for example a heat exchanger, an evaporator or a distillation column, for use in a (in particular continuously operated) process for concentrating a mineral acid by evaporating water, wherein the device or the plant comprises (at least) one device resistant to the mineral acid and the device is rinsed with an aqueous alkali metal hydroxide solution with a mass concentration of alkali metal hydroxide in the range of 1% to 30% at a temperature in the range of 40 °C to 90 °C for a period of 2 hours to 7 days.

[0002] Devices used to concentrate mineral acids (e.g., heat exchangers, evaporators, or distillation columns) are required in processes where a mineral acid is used, and the mineral acid is diluted but not (at least not completely) consumed. To enable reuse of the mineral acid, it must be concentrated back to its original concentration (and purified if necessary). Sulfuric acid, for example, is used in various applications where it is not chemically consumed but diluted with water, such as when used as a desiccant or as a catalyst in reactions that produce water. Certain components of the devices used to concentrate mineral acids inevitably come into direct contact with the mineral acid during operation.Therefore, such devices always include components made of acid-resistant materials or at least coated with such materials. Experience has shown that the acid-resistant components of such devices, in particular, must be cleaned regularly, especially to remove solid deposits.

[0003] For this purpose, it is common practice to at least partially disassemble the device or even the acid-resistant components themselves and, where necessary, clean them with high-pressure water. However, since the required acid-resistant materials are quite sensitive to mechanical stress, this can lead to damage. Furthermore, a cleaning process that requires mechanical disassembly of even individual components of the device is naturally cumbersome and time-consuming.

[0004] Cleaning without disassembling at least individual components and without subjecting them to mechanical stress is virtually impossible with water alone. However, the durability of the materials used limits the use of cleaning chemicals.

[0005] CN 104745337 A describes a neutral chemical cleaning agent for removing hard deposits from an air preheater of a desulfurization system and a manufacturing process of the neutral chemical cleaning agent.

[0006] The VDI Heat Atlas (10th edition, Springer Verlag 2006, ISBN-10 3-540-25504-4) discloses on page 0D 23, paragraph 5.1.1, that the general procedure is to first remove organic deposits with an alkali, then wash them out with water, followed by loosening or dissolving them with an acid, further rinsing with water, and then passivation. Example 9 also states that, in a specific case, treating silica and calcium phosphate deposits with a combination of caustic soda and a wetting agent at 100°C makes them more easily soluble. The treatment is then followed by hydrochloric acid and ammonium bifluoride at 70°C, with the ammonium bifluoride generating a small amount of hydrofluoric acid, which dissolves the silica.

[0007] Korean patent application KR 2019 0004557 A relates to a method for improving the acid resistance of a shell-and-tube heat exchanger in which air or water is to be indirectly heated by a heating gas. The air or water to be heated flows through the interior of the heating tubes, while the heating gas flows around them from the outside. Since the heating gas used contains acidic components, there is a risk that the cooling of the heating gas will cause these acidic components to precipitate on the exterior of the heating tubes, resulting in corrosion. According to the teaching of KR 2019 0004557, such corrosion is prevented by a special coating on the exterior of the tubes. For this purpose, the heating tubes are coated with a glaze and then calcined.The glaze consists of a mixture comprising silicon dioxide, a linear nanometal, various metal oxides and metal salts, as well as charcoal obtained from the pyrolysis of biomass, a pigment, particularly an inorganic one, and silicone oil. To prepare for the application of the glaze, the surface of the heating tubes can be treated with an etching liquid containing sulfuric acid, ascorbic acid, and an anion exchange resin. This document does not address the concentration of mineral acids by heating them. Rather, the acidic heating gas is cooled in the described process. This document also does not address the cleaning of the heat exchanger to be used, but only how it should be designed to minimize its susceptibility to corrosion.

[0008] Japanese patent application JP 2018 151132 A relates to a method for cleaning a heat exchanger, in which cleaning is first carried out with a first cleaning liquid, followed by cleaning with a second cleaning liquid. In particular, the method also provides a method with which the cleaning progress can be monitored and is particularly suitable for heat exchangers that tend to form calcium- or silicon-containing deposits during operation, for example, water heaters or heat exchangers for concentrating cleaning water for semiconductors. An aqueous solution containing a carboxylic acid, sulfamic acid, methanesulfonic acid, and / or salts thereof is particularly suitable as the first cleaning liquid.One embodiment describes aqueous solutions of a hydrogen difluoride such as ammonium, potassium, or sodium hydrogen difluoride as the second cleaning fluid, while another embodiment proposes the use of alkaline solutions such as sodium or potassium hydroxide. Which embodiment is preferred depends on the heat exchanger material. The latter embodiment, using alkaline cleaning fluids, is recommended for heat exchangers made of stainless steel or titanium. The document does not address heat exchangers in which mineral acids are to be concentrated.

[0009] The international patent application WO 2011 / 032659 A1 relates to a process and a plant for the processing, i.e. separation, purification, and concentration of spent and diluted sulfuric acid from nitration processes ("waste acid"), in which nitric acid in the presence of sulfuric acid is used as the nitration medium. The process is characterized in particular in that, in a first stage, the preheated waste acid is separated in a stripping column into at least one vapor phase containing nitric acid and optionally nitroorganics, as well as a preconcentrated sulfuric acid, and in subsequent process stages (i) the preconcentrated sulfuric acid is fed to a further purification to separate nitroorganics and to a higher concentration, and (ii) the nitric acid obtained from the vapor nitric acid phase and the nitroorganics,including the nitroorganics obtained during the further purification and concentration of the pre-concentrated sulphuric acid, is processed and returned to the nitration process, whereby in the first stage of the process, in addition to stripping the preheated waste acid, a concentration of the nitric acid contained in the stripping vapor is carried out, and whereby the nitric acid vapors obtained from the top of the column of the first stage are condensed, thereby obtaining nitric acid directly in a highly concentrated form suitable for recirculation to the nitration process.

[0010] The previously described state of the art does not solve the problems outlined above. Therefore, there was a need for further improvements in the field of cleaning devices used in the concentration of mineral acids. In particular, it was desirable to provide a cleaning process that is easy to perform, eliminates the need to disassemble components of the device to be cleaned as much as possible, and causes as little damage as possible to the acid-resistant materials, either through mechanical or chemical stress.

[0011] Taking this need into account, the present invention provides a A method for preparing a device (or a system comprising several devices) for use in a (particularly continuously operated) process for concentrating a mineral acid by evaporating water (i.e. a method for cleaning the device or the system or a part thereof such that the device or the system can be used in such a process for concentrating a mineral acid), wherein the device comprises (at least) one device resistant to the mineral acid, wherein the device has coatings selected from the group consisting of the materials steel enamel, silicon carbide, glass (in particular borosilicate glass), tantalum, niobium, perfluorinated plastics (such as in particular polytetrafluoroethylene and / or perfluoroalkoxy polymers) and a composite of two or more of these materials, or wherein the device is made of (at least) one of these materials, comprising a step of rinsing the device with an aqueous alkali metal hydroxide solution of a mass concentration of alkali metal hydroxide (based on the total mass of the alkali metal hydroxide solution) in the range of 1% to 30% at a temperature in the range of 40°C to 90°C for a period of 2 hours to 7 days.

[0012] In the terminology of the present invention, an acid-resistant Furnishings a component of the Device, which comes into direct contact with the mineral acid to be concentrated, for example the inside or outside of a tube bundle heat exchanger. The inventive term device This also includes the case where several individual devices, each of which is device in the sense used here, to form a group of several devices, also as Attachment are connected together. A system can, of course, include other devices in addition to the equipment to be cleaned.

[0013] First, there follows a Short summary various possible Embodiments of the invention: In one first embodiment of the invention, which can be combined with all other embodiments, the alkali metal hydroxide solution is selected from the group consisting of sodium hydroxide solution, potassium hydroxide solution and mixtures thereof.

[0014] In one second embodimentof the invention, which can be combined with all other embodiments, the mass concentration of alkali metal hydroxide in the alkali metal hydroxide solution is in the range of 3% to 20%.

[0015] In one third embodiment of the invention, which is a particular embodiment of the second embodiment, the mass concentration of alkali metal hydroxide in the alkali metal hydroxide solution is in the range of 5% to 15%.

[0016] In one fourth embodiment of the invention, which can be combined with all other embodiments, rinsing is carried out at a temperature in the range of 50 °C to 80 °C.

[0017] In one fifth embodiment of the invention, which is a particular embodiment of the fourth embodiment, the rinsing is carried out at a temperature in the range of 55 °C to 70 °C.

[0018] In one sixth embodimentof the invention, which can be combined with all other embodiments, the rinsing is carried out for a period of 8 hours to 30 hours.

[0019] In one seventh embodiment of the invention, which is a particular embodiment of the sixth embodiment, the rinsing is carried out for a period of 12 hours to 24 hours.

[0020] In one eighth embodiment of the invention, which can be combined with all other embodiments, the device comprises a heat exchanger for heating the mineral acid to be concentrated, a distillation apparatus for removing organic impurities from the mineral acid to be concentrated, an evaporation apparatus (whereby several evaporation apparatuses can be connected in series), a heat exchanger for cooling the concentrated mineral acid and / or a container for receiving the concentrated mineral acid.

[0021] In one ninth embodimentof the invention, which is a particular embodiment of the eighth embodiment, the device is a heat exchanger designed to heat the mineral acid to be concentrated and / or to cool the concentrated mineral acid.

[0022] In one tenth embodiment of the invention, which is a further particular embodiment of the eighth embodiment, the device comprises a heat exchanger for cooling the concentrated mineral acid and a container downstream of this for receiving the concentrated mineral acid.

[0023] In one eleventh embodiment of the invention, which can be combined with all other embodiments, the mineral acid is selected from the group consisting of sulfuric acid, nitric acid and mixtures thereof.

[0024] In one twelfth embodimentof the invention, which is a particular embodiment of the eleventh embodiment, the process for concentrating the mineral acid by evaporating water is used to reconcentrate sulfuric acid, which is used as a reaction medium in a nitration of an aromatic compound with nitric acid to obtain an aromatic nitro compound and is diluted by the nitration, to its original concentration used in the nitration.In other words, the process for concentrating the mineral acid by evaporating water in this twelfth embodiment is part of a process for producing an aromatic nitro compound by nitrating an aromatic compound with nitric acid in the presence of sulfuric acid, wherein the process for concentrating the mineral acid is used to reconcentrate the sulfuric acid diluted during the nitration to its original concentration used in the nitration.

[0025] In one thirteenth embodiment of the invention, which is a particular embodiment of the twelfth embodiment, the aromatic nitro compound is nitrobenzene or dinitrotoluene, preferably dinitrotoluene.

[0026] In one fourteenth embodimentIn the invention, which is a further specific embodiment of the eleventh embodiment, the process for concentrating the mineral acid by evaporating water is used to reconcentrate sulfuric acid, which is used as a desiccant for drying a gas and is diluted by drying, back to the concentration originally used in the drying process. In other words, the process for concentrating the mineral acid by evaporating water in this fourteenth embodiment is part of a method for drying a gas in which sulfuric acid is used as a desiccant, wherein the process for concentrating the mineral acid is used to reconcentrate the sulfuric acid diluted during drying back to the concentration originally used in the drying process.

[0027] In one fifteenth embodimentof the invention, which is a particular embodiment of the fourteenth embodiment, the gas is chlorine.

[0028] In one sixteenth embodiment of the invention, which can be combined with all other embodiments, the device has coatings which comprise or consist of perfluorinated plastics (such as in particular polytetrafluoroethylene and / or perfluoroalkoxy polymers), or the device is at least partially made of perfluorinated plastics (such as in particular polytetrafluoroethylene and / or perfluoroalkoxy polymers), the perfluorinated plastics (in both cases) being mechanically stabilized with inert materials (in particular glass spheres, glass beads and alumina or silicate-containing clays).

[0029] In one seventeenth embodimentof the invention, which can be combined with all other embodiments, provided that these are not limited to the use of perfluorinated plastics as materials resistant to the mineral acid, the device has coatings which comprise or consist of steel enamel, silicon carbide, glass (in particular borosilicate glass), niobium and / or tantalum, or the device is at least partially made of steel enamel, silicon carbide, glass (in particular borosilicate glass), niobium and / or tantalum.

[0030] In one eighteenth embodiment of the invention, which can be combined with all other embodiments, the device is not disassembled for the purpose of carrying out the rinsing.

[0031] In one nineteenth embodiment of the invention, which is a particular embodiment of the eighteenth embodiment, the (entire) device is not disassembled for the purpose of performing the flushing of the device.

[0032] The previously briefly described embodiments and further possible embodiments of the invention are described below explained in more detail . Different embodiments can be combined with one another in any way, unless the context clearly indicates the opposite to a person skilled in the art.

[0033] Sodium hydroxide solution and / or potassium hydroxide solution are preferably used as the alkali metal hydroxide solution. Regardless of the type of alkali metal hydroxide solution used, its mass concentration of alkali metal hydroxide is preferably in the range of 3% to 20%, and particularly preferably in the range of 5% to 15%.

[0034] Rinsing with the alkali metal hydroxide solution is preferably carried out at temperatures in the range of 50 °C to 80 °C, particularly preferably in the range of 55 °C to 70 °C, over a period of preferably 8 hours to 30 hours, particularly preferably 12 hours to 24 hours.

[0035] The process according to the invention is, in principle, applicable to all devices used in the concentration of mineral acids. For example, it may be a heat exchanger for heating the mineral acid to be concentrated, a distillation apparatus for removing organic impurities from the mineral acid to be concentrated, an evaporation apparatus (whereby several evaporation apparatuses can be connected in series), a heat exchanger for cooling the concentrated mineral acid, and / or a container for receiving the concentrated mineral acid. Here and in the following, formulations such as "a heat exchanger", "a distillation apparatus" and the like, of course, includes the possibility that several such apparatuses are present and are cleaned using the method according to the invention.

[0036] In a preferred embodiment of the invention, the cleaning method according to the invention is used for cleaning a heat exchanger designed to heat the mineral acid to be concentrated and / or to cool the concentrated mineral acid. The cleaning method according to the invention is also suitable for cleaning a system that comprises a heat exchanger for cooling the concentrated mineral acid and a container downstream of the heat exchanger for receiving the concentrated mineral acid.

[0037] In principle, the purification process according to the invention can be applied to devices for concentrating any mineral acid. Sulfuric acid, nitric acid, and mixtures thereof are particularly preferred. Sulfuric and nitric acids are used in the production of aromatic nitro compounds by nitration of aromatic compounds. The nitric acid serves as a source of the nitro group, while the sulfuric acid acts as a catalyst and diluent. In adiabatically operated processes, the sulfuric acid also absorbs the (considerable) heat of reaction.

[0038] In aromatic nitration processes, the aromatic to be nitrated is preferably used in excess, so that the nitric acid is completely consumed (except for possible trace amounts). However, the sulfuric acid is merely diluted and, with the exception of any portions that are removed to remove impurities, is to be returned to the nitration process after concentration to the concentration originally used in the nitration.

[0039] Benzene and toluene are particularly suitable aromatics for nitration. Benzene is mononitrated to nitrobenzene, and toluene is dinitrated to dinitrotoluene. The process according to the invention is preferably used in the production of dinitrotoluene.

[0040] Another area of ​​application for the method according to the invention is in processes in which sulfuric acid is used as a desiccant for gas drying and is diluted in the process. In such processes, the sulfuric acid must be reconcentrated to the concentration originally used in the drying process after absorbing a certain amount of water. An example of such an application is the drying of moist chlorine gas.

[0041] If perfluorinated plastics are used as acid-resistant materials, polytetrafluoroethylene and / or perfluoroalkoxy polymers are preferred. Especially if such plastics are not used merely as coatings, but the devices that may come into contact with the mineral acid are made of such plastics, it is preferable to mechanically stabilize the perfluorinated plastics with inert materials. Glass spheres or beads, or aluminate- or silicate-based clays, are preferred inert materials.

[0042] Other suitable acid-resistant materials include steel enamel, silicon carbide, glass (especially borosilicate glass), niobium, and / or tantalum. These can be used as coatings or as the material from which the equipment that comes into contact with the mineral acid is made.

[0043] If the acid-resistant material is used only as a coating, all materials familiar to the person skilled in the art, in particular stainless steel or black steel, can be used as materials for the devices that come into contact with the mineral acid during operation of the device and to which the coatings are applied.

[0044] The process according to the invention offers the great advantage that it can be carried out without disassembling the device to be flushed. However, it may be necessary to disassemble pipes that connect individual devices to one another or the device to other components of the device or other apparatus. The alkali metal hydroxide solution can be introduced through existing inlet openings (e.g., the inlet opening for supplying the mineral acid to be concentrated during regular operation of the device) and discharged through existing outlet openings (e.g., for discharging the concentrated mineral acid during regular operation of the device).However, it is of course also possible to install inlet and outlet openings specifically designed for flushing purposes (and closed during regular operation of the device) at suitable locations on the device, so that they only need to be opened for flushing. In such a case, it is possible that the device does not even need to be partially disassembled (i.e., all existing piping can remain installed during normal operation).

[0045] The process according to the invention is explained in more detail below using examples. Examples:

[0046] A plant for the concentration of waste acid from a nitration process from toluene to dinitrotoluene (DNT) is shown in simplified form in FIG. 1shown. It consists of several recuperators (1) and several evaporators (2), each connected to columns (3) and condensers (4) (only one of these devices is shown). The waste acid feed is labeled A, the concentrated acid F, and the process condensate D. Severe fouling of the heat exchangers (1) was detected, so that the hot, concentrated acid, labeled E, from the evaporator (2) could no longer flow freely through the heat exchangers (1) as required for operation. In addition, it was observed that the heat transfer through the heat exchangers (1) was significantly reduced, which was noticeable by a reduced temperature of stream B, an increased temperature of stream F, and increased energy consumption of the evaporators (2).

[0047] The heat exchanger (1) was first rinsed with water and then with 10% caustic soda. For this purpose, the sections of the two pipes B and E, each connected to the nozzles of the last heat exchanger (1), were dismantled and the two nozzles connected with a hose. During this process, the nozzles were inspected and large amounts of solid deposits were found. The caustic soda, heated to 65 °C, was then pumped from a reservoir for 24 hours through the flushing nozzle located on the heat exchanger (1) in line F, withdrawn from a flushing nozzle located in line A and returned to the reservoir. After another brief rinse with water, the hose was dismantled and the nozzles inspected again. No further contamination was observed, thus the cleaning was complete. The removed pipes were reinstalled and the system put back into operation.It was found that the acid could drain freely again and the heat transfer through the heat exchangers (1) corresponded to the design value.

Claims

1. 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 coatings selected from the group consisting of the materials steel enamel, silicon carbide, glass, tantalum, niobium, perfluorinated polymers and a composite of two or more of these materials, or wherein the device is made of one of these materials, comprising a step of flushing of 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. Method according to Claim 1, wherein the alkali metal hydroxide solution is selected from the group consisting of sodium hydroxide solution, potassium hydroxide solution and mixtures thereof.

3. Method according to either of the preceding claims, wherein flushing is carried out for a time of from 8 hours to 30 hours.

4. Method according to any of the preceding claims, wherein the apparatus comprises a heat exchanger for heating the mineral acid to be concentrated, a distillation apparatus for removing organic contaminants 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. Method according to 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. Method according to 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. Method according to any of the preceding claims, wherein the mineral acid is selected from the group consisting of sulfuric acid, nitric acid and mixtures thereof.

8. Method according to Claim 7, wherein the process of concentrating the mineral acid by evaporation of water is used to concentrate sulfuric acid, which is used as reaction medium in a nitration of an aromatic compound with nitric acid to obtain an aromatic nitro compound and is diluted by the nitration, back to its concentration originally used in the nitration.

9. Method according to Claim 8, wherein the aromatic nitro compound is nitrobenzene or dinitrotoluene.

10. Method according to Claim 7, wherein the process of concentrating the mineral acid by evaporation of water is used for concentrating sulfuric acid, which is used as desiccant for drying a gas and is diluted by the drying, back to its concentration originally used in the drying.

11. Method according to Claim 10, wherein the gas is chlorine.

12. Method according to any of the preceding claims, wherein the device has coatings which comprise or consist of perfluorinated polymers or wherein the device is made at least partly of perfluorinated polymers, where the perfluorinated polymers are mechanically stabilized with inert materials.

13. Method according to any of Claims 1 to 11, wherein the device has coatings which comprise or consist of 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. Method according to any of the preceding claims, wherein the device is not disassembled for the purpose of carrying out the flushing.

15. Method according to Claim 14, wherein the apparatus is not disassembled for the purpose of carrying out the flushing of the device.