Method for cleaning equipment for concentrating mineral acids

Abstract

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

Description

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

[0002] In processes where mineral acids are used and the mineral acids are diluted but not (at least not completely) consumed during the application, equipment for concentrating the mineral acids (e.g. heat exchangers, evaporators or distillation columns) is required. In order to achieve the reuse of the mineral acid, it must be concentrated again to the original concentration (and optionally purified). For example, sulfuric acid is used in various fields of application where sulfuric acid is not chemically consumed but diluted with water (e.g. used as a desiccant or catalyst in reactions that form water). Certain components of the equipment for concentrating mineral acids are inevitably in direct contact with the mineral acids during their operation. Therefore, such equipment always also includes devices made of acid-resistant materials or at least coated with acid-resistant materials. According to experience, in particular the acid-resistant devices of such equipment must be cleaned regularly, in particular in order to remove solid deposits.

[0003] For this purpose, the equipment or even the acid-resistant device itself is usually at least partially disassembled and cleaned with high-pressure water where necessary. However, since the required acid-resistant materials necessarily have a certain sensitivity to mechanical loads, this can lead to damage. Moreover, cleaning methods in which only individual parts of the equipment need to be mechanically disassembled are of course cumbersome and associated with a high time consumption.

[0004] Cleaning with water alone is almost impossible without disassembly of at least individual components and without mechanical loading. However, the use of cleaning chemicals is limited by the resistance of the materials used.

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

[0006] VDI-Wärmeatlas (10th edition, Springer Verlag 2006, ISBN-10 3-540-25504-4) discloses, as a general treatment method, firstly removing organic deposits with lye, then washing them off with water, subsequently loosening or dissolving them with acid, rinsing them again with water and subsequently passivating them, on page 23 of Od. It is also stated in Example 9 that in certain cases, treatment of deposits containing silicon dioxide and calcium phosphate with a combination of aqueous sodium hydroxide solution and a wetting agent at 100° C. makes them more soluble. This is followed by treatment with hydrochloric acid and ammonium bifluoride at 70° C., wherein the ammonium bifluoride produces a small amount of hydrofluoric acid, which dissolves the silicon dioxide.

[0007] Korean patent application KR 20190004557A relates to a method for improving the acid resistance of a shell and tube heat exchanger in which air or water should be indirectly heated by a heating gas. Here, the air to be heated or the water to be heated flows through the inside of the heating tube, while the heating gas flows around the heating tube on the outside. Because a gas containing acidic components should be used as the heating gas, there is a danger that these acidic components are deposited on the outside of the heating tube due to the cooling of the heating gas and thus cause corrosion. According to the teaching of KR 20190004557, this corrosion is prevented by a special coating on the outside of the tube. For this purpose, the heating tube is coated with a glaze and then calcined. A mixture containing silicon dioxide, linearly structured nanometals, a variety of metal oxides and metal salts, and charcoal obtained by pyrolysis of biomass, a particularly inorganic pigment, and silicone oil is used as a glaze. In order to prepare for the application of the glaze, the surface of the heating tube can be treated with an etching solution containing sulfuric acid, ascorbic acid, and an anion exchange resin. The document does not involve concentrating the inorganic acid in the case of heating the inorganic acid. Instead, the acid-containing heated gas is cooled in the process. This document also does not deal with the cleaning of the heat exchanger to be used, but only with how to design the heat exchanger so as to be as little susceptible to corrosion as possible.

[0008] Japanese patent application JP 2018151132A relates to a method for cleaning a heat exchanger, wherein the first cleaning liquid is used for cleaning, and then the second cleaning liquid is used for cleaning. The method also provides a scheme in particular, with which the cleaning process can be controlled and is particularly suitable for those heat exchangers that are prone to forming calcium-containing or silicon-containing deposits during operation, such as water heaters or heat exchangers for concentrating semiconductor clean water. In particular, an aqueous solution containing carboxylic acid, aminosulfonic acid, methanesulfonic acid and / or its salt is suitable as the first cleaning liquid. As the second cleaning liquid, in one embodiment, hydrogen difluoride is described, such as an aqueous solution of ammonium difluoride, potassium difluoride or sodium difluoride, and in another embodiment, an alkaline solution such as sodium hydroxide or potassium hydroxide is provided. Which embodiment should preferably be used depends on the material of the heat exchanger. Therefore, for a heat exchanger composed of stainless steel or titanium, it is recommended to use an alkaline cleaning solution using the embodiment mentioned later. The document does not involve a heat exchanger in which an inorganic acid is to be concentrated.

[0009] International patent application WO 2011 / 032659A1 relates to a method and a plant for the post-treatment, i.e. separation, purification and concentration of spent and diluted sulfuric acid ("spent acid") from a nitration process using nitric acid as a nitration medium in the presence of sulfuric acid. The method is characterized in particular in that in a first stage, the preheated spent acid is separated in a stripping column into at least one vaporous phase comprising nitric acid and optionally nitro-organics and pre-concentrated sulfuric acid, and in a downstream process stage (i) the pre-concentrated sulfuric acid is sent to further purification for separation of nitro-organics and high concentration; and (ii) the nitric acid and nitro-organics obtained from the vaporous nitric acid phase, including the nitro-organics obtained in the further purification and concentration of the pre-concentrated sulfuric acid, are post-treated and sent back to the nitration process, wherein in the first stage of the method, in addition to stripping the preheated spent acid, the nitric acid contained in the stripping vapor is also concentrated, and wherein the nitric acid vapor obtained from the top of the column of the first stage is condensed and the nitric acid is obtained directly therein in a highly concentrated form suitable for recirculation to the nitration process.

[0010] The prior art described above does not solve the problems described at the beginning of this article. Therefore, there is a need for further improvements in the field of cleaning of equipment used in the concentration of mineral acids. In particular, it is desirable to provide a cleaning method that can be carried out simply, as far as possible without disassembling the parts of the equipment to be cleaned and as far as possible without damaging the acid-resistant materials due to mechanical and chemical loads.

[0011] Taking this need into account, the subject of the present invention is a method for preparing a plant (or a facility comprising a plurality of plants) for use in a process (in particular continuously operated) for concentrating mineral acids by evaporating water (i.e. a method for cleaning a plant or a facility or a part thereof so that it can be used in such a process for concentrating mineral acids),

[0012] wherein the apparatus comprises (at least) one mineral acid resistant device,

[0013] wherein the device has a coating of a material selected from the group consisting of steel enamel, silicon carbide, glass (especially borosilicate glass), tantalum, niobium, perfluorinated plastics (such as especially polytetrafluoroethylene and / or perfluoroalkoxy polymers) and composites of two or more of these materials,

[0014] or

[0015] wherein the device is made of (at least) one of the materials mentioned,

[0016] The method comprises the step of flushing the device with an alkali metal hydroxide aqueous solution having a mass concentration of 1% to 30% (based on the total mass of the alkali metal hydroxide solution) at a temperature of 40° C. to 90° C. for a period of 2 hours to 7 days.

[0017] In the terminology of the present invention, an acid-proof device is understood to be a component of a plant that comes into direct contact with the mineral acid to be concentrated, i.e., for example, the tube interior or the tube exterior of a tube bundle heat exchanger. The term "plant" according to the present invention also includes the case where a plurality of individual components (apparatus), which are each characterized as plants in the sense used herein, are combined into a complex of a plurality of plants, also referred to as a facility. In this case, the facility may of course also include other components in addition to the plant to be cleaned.

[0018] First briefly summarize various possible embodiments of the present invention below:

[0019] In a first embodiment of the invention, which is combinable 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.

[0020] In a second embodiment of the invention, which may be combined with all other embodiments, the mass concentration of alkali metal hydroxide in the alkali metal hydroxide solution is 3% to 20%.

[0021] In a third embodiment of the present invention, which is a particular embodiment of the second embodiment, the mass concentration of the alkali metal hydroxide in the alkali metal hydroxide solution is 5% to 15%.

[0022] In a fourth embodiment of the invention, which can be combined with all other embodiments, rinsing is carried out at a temperature of 50 to 80°C.

[0023] In a fifth embodiment of the present invention, which is a particular embodiment of the fourth embodiment, rinsing is performed at a temperature of 55°C to 70°C.

[0024] In a sixth embodiment of the invention, combinable with all other embodiments, said flushing is performed for a period of 8 to 30 hours.

[0025] In a seventh embodiment of the present invention, which is a particular embodiment of the sixth embodiment, the flushing is performed for a period of 12 hours to 24 hours.

[0026] In an eighth embodiment of the invention, which can be combined with all other embodiments, the apparatus comprises a heat exchanger for heating the inorganic acid to be concentrated, a distillation assembly for removing organic impurities from the inorganic acid to be concentrated, an evaporation assembly (wherein a plurality of evaporation assemblies can be connected in series), a heat exchanger for cooling the concentrated inorganic acid and / or a container for containing the concentrated inorganic acid.

[0027] In a ninth embodiment of the invention, which is a particular embodiment of the eighth embodiment, the device is a heat exchanger designed for heating the mineral acid to be concentrated and / or for cooling the concentrated mineral acid.

[0028] In a tenth embodiment of the present invention, which is another particular embodiment of the eighth embodiment, the apparatus comprises a heat exchanger for cooling the concentrated inorganic acid and a container for containing the concentrated inorganic acid connected downstream of the heat exchanger.

[0029] In an eleventh embodiment of the present invention, combinable with all other embodiments, the inorganic acid is selected from the group consisting of sulfuric acid, nitric acid and mixtures thereof.

[0030] In the twelfth embodiment of the present invention, which is a particular embodiment of the eleventh embodiment, the process for concentrating the mineral acid by evaporating water is used to re-concentrate the sulfuric acid used as a reaction medium in the nitration of aromatic compounds with nitric acid to obtain aromatic nitro compounds and diluted by nitration to the concentration originally used in the nitration. In other words, in this twelfth embodiment, the process for concentrating the mineral acid by evaporating water is a component of the method for preparing aromatic nitro compounds by nitrating aromatic compounds with nitric acid in the presence of sulfuric acid, wherein the process for concentrating the mineral acid is used to re-concentrate the sulfuric acid diluted in the nitration process to the concentration originally used in the nitration.

[0031] In the thirteenth embodiment of the present invention, which is a particular embodiment of the twelfth embodiment, the aromatic nitro compound is nitrobenzene or dinitrotoluene, preferably dinitrotoluene.

[0032] In the fourteenth embodiment of the present invention, which is a special embodiment of the eleventh embodiment, the process for concentrating the inorganic acid by evaporating water is used to re-concentrate the sulfuric acid used as a desiccant for drying the gas and diluted by the drying to the concentration originally used in the drying. In other words, in this fourteenth embodiment, the process for concentrating the inorganic acid by evaporating water is a component of the method for drying the gas using sulfuric acid as a desiccant, wherein the process for concentrating the inorganic acid is used to re-concentrate the sulfuric acid diluted in the drying process to the concentration originally used in the drying.

[0033] In a fifteenth embodiment of the present invention, which is a particular embodiment of the fourteenth embodiment, the gas is chlorine.

[0034] In a sixteenth embodiment of the invention, which can be combined with all other embodiments, the device has a coating which comprises or consists of a perfluorinated plastic, such as in particular polytetrafluoroethylene and / or a perfluoroalkoxy polymer, or the device is at least partially made of a perfluorinated plastic, such as in particular polytetrafluoroethylene and / or a perfluoroalkoxy polymer, wherein the perfluorinated plastic (in both cases) is mechanically stabilized with inert substances, in particular glass spheres, glass beads and clays containing aluminates or silicates.

[0035] In a seventeenth embodiment of the invention, which can be combined with all other embodiments, as long as they are not limited to the use of perfluorinated plastics as material resistant to inorganic acids, the device has a coating, which comprises or consists of steel enamel, silicon carbide, glass (especially borosilicate glass), niobium and / or tantalum, or the device is at least partially made of steel enamel, silicon carbide, glass (especially borosilicate glass), niobium and / or tantalum.

[0036] In an eighteenth embodiment of the invention, combinable with all the other embodiments, the device is not disassembled for said flushing purposes.

[0037] In a nineteenth embodiment of the invention, which is a special embodiment of the eighteenth embodiment, the (entire) device is not disassembled for the purpose of flushing the device.

[0038] The embodiments and other possible implementations of the present invention briefly described above are explained in detail below. Unless the person skilled in the art clearly derives the opposite from the context, the different embodiments can be combined with each other at will.

[0039] Preferably, sodium hydroxide solution and / or potassium hydroxide solution are used as the alkali metal hydroxide solution. Regardless of the type of the alkali metal hydroxide solution used, the mass concentration of the alkali metal hydroxide is preferably 3%-20%, particularly preferably 5%-15%.

[0040] Flushing with an alkali metal hydroxide solution is preferably carried out at a temperature of 50 to 80° C., particularly preferably 55 to 70° C., for a period of preferably 8 to 30 hours, particularly preferably 12 to 24 hours.

[0041] The method according to the invention can in principle be used for all devices for concentrating mineral acids. For example, these can be heat exchangers for heating the mineral acid to be concentrated, distillation modules for removing organic impurities from the mineral acid to be concentrated, evaporation modules (wherein a plurality of evaporation modules can be connected in series), heat exchangers for cooling the concentrated mineral acid and / or containers for containing the concentrated mineral acid. In this case, here and below, expressions such as "heat exchanger", "distillation module" etc. naturally include the possibility that a plurality of such modules are present and are cleaned when using the method according to the invention.

[0042] In a preferred embodiment of the invention, the cleaning method according to the invention is used to clean a heat exchanger designed for heating the mineral acid to be concentrated and / or for cooling the concentrated mineral acid. The cleaning method according to the invention is also suitable for cleaning an installation comprising a heat exchanger for cooling the concentrated mineral acid and a container for containing the concentrated mineral acid connected downstream of the heat exchanger.

[0043] In principle, the cleaning method according to the invention can be applied to plants for concentrating any inorganic acids. Sulfuric acid, nitric acid and mixtures thereof are particularly preferred. Sulfuric acid and nitric acid are used for preparing aromatic nitro compounds by nitration of aromatic compounds. Nitric acid serves as a source of nitro groups, while sulphuric acid serves as a catalyst and diluent. In adiabatically operated processes, sulphuric acid also absorbs (considerable) heat of reaction.

[0044] Preferably, in the aromatic compound nitration process, the aromatic compound to be nitrated is used in excess, so that the nitric acid (except for possible traces) is completely consumed. However, the sulfuric acid is only diluted and, except for a possible fraction discharged for the removal of impurities, is returned to the nitration process after concentration to the concentration originally used in the nitration.

[0045] Particularly suitable aromatic compounds to be nitrated are benzene and toluene. Here, benzene is mononitrated to nitrobenzene and toluene is dinitrated to dinitrotoluene. The process according to the invention is preferably used to prepare dinitrotoluene.

[0046] Another area of ​​application of 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 this process, the sulfuric acid, after absorbing a certain amount of water, must be concentrated again to the concentration originally used in the drying. The drying of wet chlorine gas may be mentioned as an example of such an application.

[0047] If perfluorinated plastics are used as acid-resistant materials, polytetrafluoroethylene and / or perfluoroalkoxy polymers are preferred. In particular, when such plastics are not only used as coatings, but also when devices that can come into contact with inorganic acids are made of such plastics, it is preferred to mechanically stabilize the perfluorinated plastics with inert substances. Suitable inert substances are preferably glass spheres or glass beads or clays containing aluminates or silicates.

[0048] Other suitable acid-resistant materials are steel enamel, silicon carbide, glass (especially borosilicate glass), niobium and / or tantalum. They can be used as coatings or as materials for the manufacture of devices that come into contact with inorganic acids.

[0049] 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, are suitable as materials for the device which comes into contact with the mineral acid during operation of the plant and to which the coating is applied.

[0050] The method according to the present invention provides a great advantage, that is, it can be carried out without disassembling the device to be flushed. However, it may be necessary to disassemble the pipes connecting the various devices to each other or connecting the device to other components or other components of the equipment. The alkali metal hydroxide solution can be introduced through the inlet opening that must exist (for example, for supplying the inlet opening of the inorganic acid to be concentrated when the device is operating normally), and can be discharged through the outlet opening that must exist (for example, for discharging the concentrated inorganic acid when the device is operating normally). However, it is certainly also possible to provide a closable inlet opening and outlet opening that are specifically provided for flushing purposes (and closed during the normal operation of the device) at a suitable position of the device, so that it only has to be opened for the purpose of flushing. In this case, the device may not even need to be partially disassembled (that is, all pipes that exist in normal operation can also remain installed).

[0051] The method according to the present invention will be explained in more detail below with reference to examples.

[0052] Example:

[0053] Figure 1In the simplified diagrammatic representation of the plant for concentrating spent acid from the nitration process of toluene to form dinitrotoluene (DNT), it is shown that it consists of a plurality of recuperators (1), each of which is connected to a column (3) and a condenser (4), a plurality of evaporators (2) (only one of these components is shown in each case). The feed of spent acid is denoted by A, the concentrated acid is denoted by F and the process condensate is denoted by D. It was found that the heat exchanger (1) was severely fouled, so that the hot, concentrated acid, denoted by E, could no longer flow freely from the evaporator (2) through the heat exchanger (1) as required for operation. In addition, a significant reduction in the heat transfer through the heat exchanger (1) was observed, which can be perceived by the reduction in the temperature of the material stream B, the increase in the temperature of the material stream F and the increase in the energy consumption of the evaporator (2).

[0054] The heat exchanger (1) was first flushed with water and then with a 10% sodium hydroxide solution. For this purpose, the sections of the two pipes B and E, each connected to the nozzle of the last heat exchanger (1), were removed and the two nozzles were connected by a hose. During this work, the nozzles were checked and a large amount of solid deposits were found. Subsequently, sodium hydroxide solution heated to 65° C. was pumped from the container through the flushing nozzle located in line F at the heat exchanger (1) for 24 hours and was removed from the flushing nozzle located in line A and returned to the container. After a brief re-flushing with water, the hose was removed and the nozzle was re-checked. No scaling was observed, so the cleaning was completed. The removed pipes were reinstalled and the plant was put back into operation. It was found that the acid could flow freely again and the heat transfer through the heat exchanger (1) was in accordance with the design value.

Claims

1. A method for preparing an apparatus for use in a process for concentrating sulfuric acid by evaporating water, wherein the concentration process is used to reconcentrate sulfuric acid used as reaction medium in the nitration of aromatic compounds with nitric acid to obtain aromatic nitro compounds and diluted by the nitration to the concentration originally used in the nitration, wherein the apparatus comprises devices resistant to sulfuric acid, wherein the devices have a coating selected from the group consisting of steel enamel, silicon carbide, glass, tantalum, niobium, perfluorinated plastics and composites of two or more of the above materials, or wherein said device is made of one of said materials, The method for preparing the device comprises the step of flushing the device with an aqueous alkali metal hydroxide solution having a mass concentration of 1% to 30% of alkali metal hydroxide at a temperature of 40°C to 90°C for a period of 2 hours to 7 days.

2. The 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. The method according to claim 1 or 2, wherein the flushing is performed for a period of 8 hours to 30 hours.

4. The process according to claim 1 or 2, wherein the apparatus comprises a heat exchanger for heating the sulfuric acid to be concentrated, a distillation assembly for removing organic impurities from the sulfuric acid to be concentrated, an evaporation assembly, a heat exchanger for cooling the concentrated sulfuric acid and / or a container for containing the concentrated sulfuric acid.

5. The process according to claim 4, wherein the device is a heat exchanger designed for heating the sulfuric acid to be concentrated and / or for cooling concentrated sulfuric acid.

6. The method according to claim 4, wherein the apparatus comprises a heat exchanger for cooling the concentrated sulfuric acid and a container for containing the concentrated sulfuric acid connected downstream of the heat exchanger.

7. The method according to claim 1 or 2, wherein the aromatic nitro compound is nitrobenzene or dinitrotoluene.

8. The method according to claim 1 or 2, wherein the device has a coating, the coating comprising or consisting of a perfluorinated plastic, or wherein the device is at least partially made of a perfluorinated plastic, wherein The perfluorinated plastic is mechanically stabilized with inert substances.

9. The method according to claim 1 or 2, wherein the device has a coating which comprises or consists of steel enamel, silicon carbide, glass, niobium and / or tantalum, or wherein the device is at least partially made of steel enamel, silicon carbide, glass, niobium and / or tantalum.

10. The method of claim 1 or 2, wherein the device is not disassembled for the purpose of performing the flushing.

11. A method according to claim 10, wherein the device is not disassembled for the purpose of flushing the apparatus.

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