Method for concentrating dilute sulfuric acid

Abstract

The present invention relates to a process for concentrating dilute sulfuric acid (10) which can contain at least one nitroaromatic compound and / or nitric acid as impurities, comprising: (a) feeding the dilute sulfuric acid (10) to a first stage (1) in which low boilers are removed by evaporation and / or stripping to obtain a first concentrated sulfuric acid (12); (b) optionally feeding the first concentrated sulfuric acid (12) to a second evaporation stage (2) to obtain a second concentrated sulfuric acid (14); (c) if step (b) is carried out, feeding the second concentrated sulfuric acid (14) to a third evaporation stage (3), or if step (b) is not carried out, feeding the first concentrated sulfuric acid (12) to a third evaporation stage (3) to obtain as product concentrated sulfuric acid (16), wherein an oxidizing agent (17) and / or a precursor of an oxidizing agent is fed to the third evaporation stage (3).

Description

[0001] This invention relates to a method for concentrating dilute sulfuric acid, wherein the dilute sulfuric acid may contain at least one nitroaromatic compound as an impurity and / or nitric acid, the method comprising:

[0002] (a) Dilute sulfuric acid is fed into the first stage, where low-boiling-point substances are removed by evaporation and / or...

[0003] Alternatively, it can be stripped to obtain the first type of concentrated sulfuric acid;

[0004] (b) Optionally, the first concentrated sulfuric acid is fed into the second evaporation stage to obtain a second concentrated sulfuric acid;

[0005] (c) If step (b) is performed, the second type of concentrated sulfuric acid is fed into the third evaporation stage; or if step (b) is not performed, the first type of concentrated sulfuric acid is fed into the third evaporation stage.

[0006] To obtain concentrated sulfuric acid as a product.

[0007] Sulfuric acid may contain at least one nitroaromatic compound as an impurity and / or nitric acid, for example derived from a method for preparing nitroaromatic compounds by nitration of aromatic compounds with nitric acid and sulfuric acid.

[0008] These nitroaromatic compounds are, in particular, nitrobenzene or nitrotoluene. In the method, sulfuric acid is used as a catalyst. After the reaction is complete, the reaction mixture is separated into an organic phase containing the nitroaromatic compound and an aqueous phase containing dilute sulfuric acid. After separating the organic and aqueous phases, the aqueous phase is concentrated to recover the sulfuric acid, which can be reused in the method for preparing the nitroaromatic compound.

[0009] The aqueous phase typically still contains residues of nitroaromatic compounds or organic byproducts prepared in the process, as well as unreacted nitric acid. These impurities are primarily removed in the first stage. However, during the subsequent evaporation stage, the sulfuric acid still contains trace amounts of organic impurities, which can lead to the formation of solid carbonaceous precipitates, especially at higher temperatures. Since the formation of solid precipitates causes scaling on the equipment, thereby impairing equipment performance, shutting down the equipment for cleaning purposes is necessary. In cases of localized organic accumulation, particularly in dead zones at higher temperatures, there is a risk of spontaneous decomposition of organic impurities, potentially leading to pressure shocks and equipment damage.

[0010] WO-A 2014 / 177450 describes a method for preparing nitrobenzene via adiabatic nitration of benzene with nitric acid and sulfuric acid, wherein dilute sulfuric acid, separated from the organic phase containing nitrobenzene, is concentrated as an aqueous phase by distillation. After concentration, an oxidizing agent is added to the sulfuric acid at least one minute before it is re-contaminated with fresh nitric acid. However, this method cannot avoid the formation of solid precipitates and the spontaneous decomposition of organic impurities.

[0011] The purpose of this invention is to provide a method for concentrating dilute sulfuric acid, which avoids the formation of solid carbonaceous precipitates and the spontaneous decomposition of organic impurities, thus preventing scaling, pressure shocks, and equipment damage.

[0012] This objective is achieved by a method for concentrating dilute sulfuric acid, which may contain at least one nitroaromatic compound as an impurity and / or nitric acid, the method comprising:

[0013] (a) Dilute sulfuric acid is fed to the first stage, in which low-boiling substances are removed by evaporation and / or stripping to obtain a first concentrated sulfuric acid;

[0014] (b) Optionally, the first concentrated sulfuric acid is fed into the second evaporation stage to obtain a second concentrated sulfuric acid;

[0015] (c) If step (b) is performed, the second type of concentrated sulfuric acid is fed into the third evaporation stage; if step (b) is not performed, the first type of concentrated sulfuric acid is fed into the third evaporation stage to obtain concentrated sulfuric acid as a product.

[0016] The oxidant and / or its precursor are fed into the third evaporation stage.

[0017] Surprisingly, it has been shown that by feeding the oxidant and / or its precursor into the third evaporation stage, which typically operates at temperatures where solid carbonaceous precipitates begin to form and nitroaromatic compounds begin to decompose, both scaling due to solid precipitates and spontaneous decomposition of nitroaromatic compounds, which can lead to pressure shocks and equipment damage, can be avoided. Furthermore, by adding the oxidant and / or its precursor to the third evaporation stage, total organic carbon (TOC) can also be minimized.

[0018] According to the invention, if step (b) is performed, the oxidant and / or its precursor can be fed into the third evaporation stage, for example, by adding the oxidant and / or its precursor to the second concentrated sulfuric acid; if step (b) is not performed, the oxidant is fed into the first concentrated sulfuric acid before either the second or the first concentrated sulfuric acid is fed into the third evaporation stage. Alternatively or alternatively, the oxidant and / or its precursor can be fed directly into the third evaporation stage, into the equipment of the third evaporation stage, or into the loop through which the evaporated liquid circulates.

[0019] The dilute sulfuric acid and / or nitric acid, which may contain at least one nitroaromatic compound, are derived, for example, from methods for preparing nitroaromatic compounds. Such methods include, for example, methods for preparing nitrobenzene by nitrifying benzene with nitric acid and sulfuric acid, or methods for preparing nitrotoluene by nitrifying toluene with nitric acid and sulfuric acid. Particularly preferred is the use of the method of the present invention to concentrate dilute sulfuric acid containing nitrotoluene as an impurity. If the dilute sulfuric acid contains nitrotoluene as an organic impurity, the nitroaromatic compound is particularly at least one of mononitrotoluene, dinitrotoluene, and / or dinitrocresol. If the dilute sulfuric acid originates from a method for preparing nitroaromatic compounds, it is particularly preferred to recycle the concentrated sulfuric acid obtained by the method of the present invention back into the method for preparing nitroaromatic compounds, especially back into the method for preparing nitrotoluene.

[0020] The total amount of nitroaromatic compounds in dilute sulfuric acid is preferably 0.01 to 10.0% by weight, more preferably 0.1 to 5.0% by weight, and particularly 0.3 to 3.0% by weight, each based on the total amount of dilute sulfuric acid.

[0021] If the dilute sulfuric acid also contains nitric acid, the concentration of nitric acid is typically 0.0 to 10.0% by weight, more preferably 0.1 to 5.0% by weight, and particularly 0.4 to 3.0% by weight, each based on the total amount of dilute sulfuric acid. In addition to nitroaromatic compounds and nitric acid, the dilute sulfuric acid may contain other impurities, such as nitrous acid. The amount of other impurities may be 0.0 to 5.0% by weight, more preferably 0.0 to 2.0% by weight, and particularly 0.1 to 1.0% by weight, each based on the total amount of dilute sulfuric acid.

[0022] The concentration of dilute sulfuric acid may be 60 to 85% by weight, more preferably 65 to 79% by weight, and particularly 70 to 75% by weight.

[0023] The concentration of concentrated sulfuric acid obtained as a product from the third evaporation step is preferably 80 to 99% by weight, more preferably 85 to 97% by weight, and particularly 90 to 95% by weight.

[0024] Therefore, the concentrations of dilute sulfuric acid and concentrated sulfuric acid are defined as the amount of H2SO4 based on the volume of H2SO4 and water, respectively. Impurities in dilute sulfuric acid are not considered when determining the concentration.

[0025] For concentration, dilute sulfuric acid is fed to the first stage in step (a). In the first stage, low-boiling substances are removed from the dilute sulfuric acid. The low-boiling substances removed in the first stage may include, for example, nitric acid, as well as most of the organic components contained in the dilute sulfuric acid. The organic compounds removed from the dilute sulfuric acid in the first stage are, for example, mononitrotoluene isomers and dinitrotoluene isomers.

[0026] The first stage preferably includes simple evaporation, stripping, and / or a combination of evaporation and stripping. If low-boiling-point substances are removed by evaporation, the temperature of the dilute sulfuric acid increases, and the low-boiling-point substances evaporate and can be removed in gaseous form. For evaporation of low-boiling-point substances, the dilute sulfuric acid is preferably fed into an evaporation apparatus and / or an evaporation and stripping apparatus at a temperature of 10 to 150°C, more preferably 15 to 120°C, and particularly 30 to 100°C. In the evaporation apparatus and / or the evaporation and stripping apparatus, the temperature is preferably 130 to 200°C, more preferably 150 to 200°C, and particularly 150 to 190°C. The pressure inside the evaporation apparatus and / or the evaporation and stripping apparatus is preferably 800 to 1200 mbar (absolute pressure), more preferably 900 to 1100 mbar (absolute pressure), and particularly 950 to 1050 mbar (absolute pressure).

[0027] Alternatively, or as an alternative, low-boiling-point substances can be removed in the first stage by stripping. In this case, it is preferable to preheat the dilute sulfuric acid to a temperature of 30 to 180°C before feeding it into the stripping unit. For stripping, any stripping equipment known to those skilled in the art can be used. Suitable stripping equipment is, for example, a stripping tower, which may contain external materials, such as structured or random packing.

[0028] For stripping, dilute sulfuric acid is brought into contact with stripping gas, particularly steam. Preferably, the stripping gas is fed to the bottom of the stripping column and the dilute sulfuric acid is at the top. In addition to using a separate stripping gas, a portion of the steam obtained from evaporating dilute sulfuric acid can also be used as the stripping gas. For this purpose, for example, the dilute sulfuric acid can be evaporated at the bottom of the column and fed at the top. In this method, the steam obtained from the evaporation of dilute sulfuric acid flows countercurrently to the liquid dilute sulfuric acid fed at the top of the stripping column, and low-boiling-point substances are stripped by the steam.

[0029] The equipment used for evaporation and / or stripping can be any equipment suitable for evaporation and / or stripping and known to those skilled in the art. Typically, such equipment for evaporation includes a vessel with an internal heat exchange device or a vessel with an external heat exchanger and a circulation loop. If an evaporation and stripping equipment is used, the equipment includes a stripping section, preferably a tower with internal packing material such as random or structured packing, and a heat exchange device at the bottom. Diluted acid is filled to the top of the vessel, circulation loop, and / or stripping tower. Unevaporated liquid is collected at the bottom of the vessel and / or stripping tower, and vapor is removed through a vapor line connected to the top of the vessel and / or stripping tower. A portion of the vapor removed from the equipment for evaporation and / or stripping is condensed and then released into the surrounding environment. However, since vapor often contains components that may be harmful to the environment, it is particularly preferable to feed the vapor to a gas purification unit before releasing it into the surrounding environment. The gas purification unit can be any gas purification unit suitable for removing unwanted components from the vapor and known to those skilled in the art.

[0030] In the first stage, a first concentrated sulfuric acid is obtained. The concentration of the first concentrated sulfuric acid is higher than that of the dilute sulfuric acid, but lower than that of the concentrated sulfuric acid obtained as a product. Preferably, the concentration of the first sulfuric acid is 70 to 86% by weight, more preferably 72 to 84% by weight, and particularly 76 to 81% by weight.

[0031] In one embodiment, a first concentrated sulfuric acid is fed to a second evaporation stage. The second evaporation stage is preferably operated under vacuum conditions at a temperature that does not allow the formation of solid carbonaceous precipitates and / or the spontaneous decomposition of organic impurities still contained in the first concentrated sulfuric acid. To establish vacuum conditions, the vapor outlet of the evaporation unit used in the second evaporation stage is preferably connected to a vacuum unit, such as a vacuum pump. During the operation of the second evaporation stage, a portion of the vapor obtained in the evaporation unit of the second evaporation stage is condensed, for example by direct cooling in a countercurrent tower, particularly using sulfuric acid as a coolant, and / or by indirect cooling in a heat exchanger using a cooling medium (e.g., water). The portion of the vapor that remains in gaseous form is discharged to the vacuum unit and extracted from the method, preferably after passing through a gas purification unit. In the vacuum unit, the pressure of the second evaporation stage is set.

[0032] The gas purification unit can be the same as the unit used to extract vapor from the first stage.

[0033] The operating pressure of the second evaporation stage is preferably 10 to 800 millibars (absolute pressure), more preferably 20 to 500 millibars (absolute pressure), and particularly 50 to 200 millibars (absolute pressure). The temperature of the second evaporation stage is preferably 120 to 200°C, more preferably 140 to 190°C, and particularly 140 to 180°C.

[0034] The second evaporation stage may be carried out in only one evaporation unit or in more than one evaporation unit connected in series. Preferably, the second evaporation stage is carried out in 1 to 4 evaporation units, more preferably in 1 to 3 evaporation units, and particularly in 1 to 2 evaporation units. Particularly preferably, the second evaporation stage is carried out in one evaporation unit.

[0035] If the second evaporation stage is carried out in more than one evaporation unit connected in series, the temperature in the evaporation unit increases from the first evaporation unit connected in series to the last evaporation unit connected in series and / or the pressure decreases from the first evaporation unit connected in series to the last evaporation unit connected in series, wherein each evaporation unit operates under the conditions of the aforementioned second evaporation unit.

[0036] In the second evaporation stage, water is evaporated and the concentration of sulfuric acid is increased. A second concentrated sulfuric acid is obtained in the second evaporation stage. The concentration of the second concentrated sulfuric acid is higher than that of the first concentrated sulfuric acid but lower than that of the concentrated sulfuric acid obtained as a product of the method. Preferably, the concentration of the second concentrated sulfuric acid is 78 to 94% by weight, more preferably 80 to 92% by weight, and particularly 84 to 89% by weight.

[0037] If a second evaporation stage is performed, the second type of concentrated sulfuric acid is fed into the third evaporation stage; otherwise, the first type of concentrated sulfuric acid is fed into the third evaporation stage. The third evaporation stage is also preferably performed under vacuum conditions. However, unlike the second evaporation stage, the third evaporation stage is performed at a temperature at which the formation of solid carbonaceous precipitates and / or the spontaneous decomposition of organic impurities contained in the first or second type of concentrated sulfuric acid can occur.

[0038] The pressure for carrying out the third evaporation stage is preferably 10 to 800 millibars (absolute pressure), more preferably 20 to 500 millibars (absolute pressure), particularly 50 to 200 millibars (absolute pressure), and at a temperature of 180 to 300°C, more preferably 190 to 250°C, and especially at 190 to 230°C.

[0039] The third evaporation stage may include one evaporation unit or more evaporation units connected in series. Preferably, the third evaporation stage includes 1 to 4 evaporation units, more preferably 1 to 3 evaporation units, and particularly 1 to 2 evaporation units. Particularly preferably, the third evaporation stage is carried out in one evaporation unit.

[0040] The third evaporation stage is preferably operated in the same manner as the second evaporation stage. Specifically, a portion of the vapor obtained in the evaporation unit of the third evaporation stage is condensed, for example by direct cooling in a countercurrent tower, particularly using sulfuric acid as a coolant, and / or by indirect cooling in a heat exchanger using a cooling medium (e.g., water). The uncondensed portion of the vapor, remaining in the gaseous phase, is transferred to a vacuum unit, such as a vacuum pump, and removed from the method. As vapor is removed from the first and second evaporation stages, the vapor removed from the third evaporation stage is also preferably passed through a gas purification unit before being released into the environment.

[0041] A separate gas purification unit can be provided for each evaporation stage or even for each evaporation unit. Alternatively, the vapors from at least two evaporation units can be combined, more preferably the vapors from at least two evaporation stages, and especially all evaporation stages, and the combined vapors can be fed into the gas purification unit.

[0042] The evaporation units used in the second (if performed) and third evaporation stages can be any type of evaporator known to those skilled in the art. Suitable evaporators are, for example, falling film evaporators, circulating evaporators, or horizontal evaporators. Particularly preferred are circulating evaporators and / or horizontal evaporators used as evaporation units in the second and third evaporation stages.

[0043] In particular, for concentrating large quantities of dilute sulfuric acid, it may be necessary to use more than one concentration unit, which includes equipment for a first stage, an optional second evaporation stage, and a third evaporation stage. In this case, depending on the amount of dilute sulfuric acid and the size of the equipment, at least two concentration units are operated in parallel, preferably all concentration units are designed and operated in the same manner.

[0044] According to the present invention, the oxidant and / or the precursor of the oxidant are fed into the third evaporation stage.

[0045] The oxidant fed into the third evaporation stage can be any oxidant that can controllably react with organic impurities to selectively decompose them into volatile components. This reaction prevents the formation of solid carbonaceous precipitates and scaling on equipment surfaces. It further prevents the accumulation of organic impurities in, for example, dead zones or over extended periods, and avoids the spontaneous decomposition of accumulated organic impurities, which could lead to equipment damage. The volatile components prepared by the reaction of organic impurities with the oxidant are removed from the third evaporation stage along with the uncondensed steam. Examples of volatile components include nitrogen oxides, carbon monoxide, and / or carbon dioxide. To prevent the release of volatile components (especially nitrogen oxides) into the environment, these components are removed from the steam in the gas purification unit.

[0046] Suitable oxidants for decomposing nitroaromatic compounds, which are organic impurities, are nitric acid, nitrous acid, nitrosylsulfonic acid, and combinations thereof. Nitric acid is particularly preferred. If nitric acid is used as the oxidant, it is particularly preferable to draw the nitric acid from the same nitric acid storage container as that used for the nitration reaction.

[0047] In addition to feeding an oxidant, a precursor for the oxidant can also be fed into the third evaporation stage. If the precursor is fed into the third evaporation stage, the oxidant is formed in situ through a chemical reaction of the precursor. A suitable precursor is sodium nitrite. By adding sodium nitrite to sulfuric acid, nitrous acid, which acts as the oxidant, is generated.

[0048] To fully decompose organic impurities, particularly nitroaromatic compounds still contained in the first or second type of concentrated sulfuric acid fed into the third evaporation stage, the amount of oxidant fed into the third evaporation stage and / or formed in situ by converting the oxidant precursor in the third evaporation stage is preferably 0.01 to 100 g per kg of concentrated sulfuric acid, more preferably 0.1 to 50 g per kg of concentrated sulfuric acid, and particularly 0.5 to 30 g per kg of concentrated sulfuric acid. The oxidant dosage per kg of concentrated sulfuric acid depends on the assumed pure oxidant and is independent of the amount of aqueous solution.

[0049] To avoid diluting the sulfuric acid, it is even more preferable if the concentration of the oxidant fed to the third evaporation stage is 30 to 100% by weight, more preferably 65 to 100% by weight, and particularly 85 to 100% by weight, respectively, based on the amounts of oxidant and water. If the precursor of the oxidant is fed to the third evaporation stage, the amount of the precursor is selected such that the amount of oxidant generated in situ through the chemical reaction of the precursor is within the above-mentioned range.

[0050] To feed the oxidant or its precursor to the third evaporation stage, if the second evaporation stage is not performed, the first concentrated sulfuric acid can be mixed with the oxidant; or if the second evaporation stage is performed, the second concentrated sulfuric acid can be mixed with the oxidant, and the mixture of the first or second concentrated sulfuric acid with the oxidant can be fed to the third evaporation stage.

[0051] The mixing of the first or second type of concentrated sulfuric acid with the oxidant can be carried out, for example, by feeding the oxidant or a precursor of the oxidant into a feed line through which the first or second type of concentrated sulfuric acid is fed to the third evaporation stage. Alternatively, a mixing unit for mixing the first or second type of concentrated sulfuric acid with the oxidant or a precursor of the oxidant may also be provided. The mixing unit can be any static or dynamic mixer that allows the mixing of two liquids.

[0052] Besides mixing the first or second type of concentrated sulfuric acid with the oxidant or its precursor, the oxidant or its precursor can also be fed directly into the third evaporation stage. If the oxidant or its precursor is fed directly into the third evaporation stage, it can be fed into the evaporation unit of the third evaporation stage. However, especially if a circulating evaporator is used as the evaporation unit, it is preferable to feed the oxidant or its precursor into the evaporation loop. The evaporation loop can be, for example, a pipeline between a vessel and a heat exchanger for gas-liquid separation, or a pipeline through which the liquid phase circulates during evaporation.

[0053] The oxidant or its precursor may be added to the third evaporation stage continuously or discontinuously. However, it is preferred to feed the oxidant or its precursor continuously to the third evaporation stage.

[0054] To ensure controlled feeding of the oxidant or oxidant precursor into the third evaporation stage, a metering unit for feeding the oxidant or oxidant precursor into the third evaporation stage is particularly preferred. The metering unit can be any metering unit known to those skilled in the art for pre-quantitative feeding. Particularly preferred is a mass control valve.

[0055] Illustrative embodiments of the present invention are shown in the figures and explained in more detail in the following description.

[0056] The only illustration shows a flowchart of the method of the present invention for concentrating sulfuric acid.

[0057] To concentrate dilute sulfuric acid, which may contain nitroaromatic compounds as organic impurities, dilute sulfuric acid 10 is fed to the first stage 1. The first stage 1 is preferably a combination of an evaporation and stripping section and / or an evaporation and stripping section, wherein nitric acid and most of the organic impurities are stripped from the dilute sulfuric acid 10. Nitric acid and most of the organic impurities are removed as a gaseous waste gas stream 11.

[0058] In the first stage 1, a first type of concentrated sulfuric acid 12 is obtained, which can be fed into the second evaporation stage 2. The second evaporation stage may include n evaporation units, where n is a number between 1 and 5. However, the second evaporation stage may also be omitted, and the first type of concentrated sulfuric acid may be fed into the third evaporation stage 3.

[0059] In the second evaporation stage 2, the first concentrated sulfuric acid is further concentrated under vacuum conditions. Evaporation in the second evaporation stage yields partially condensed acidic vapor 13, and the uncondensed vapor is discharged into the vacuum unit. The second evaporation stage is operated at a temperature of 120 to 200°C, more preferably 140 to 190°C, particularly 140 to 180°C, and a pressure of 10 to 800 mbar (absolute pressure), more preferably 20 to 500 mbar (absolute pressure), and particularly 50 to 200 mbar (absolute pressure), wherein the temperature is below the limit temperature at which solid carbonaceous precipitates form or organic impurities spontaneously decompose, even if organic impurities accumulate in equipment areas such as dead zones and persist for a considerable period.

[0060] In the second evaporation stage 2, a second concentrated sulfuric acid 14 is obtained and fed into the third evaporation stage 3. In the third evaporation stage 3, the sulfuric acid is concentrated to a final concentration of 80 to 99% by weight. Preferably, the concentrated sulfuric acid 16 obtained as a product in the third evaporation stage 3 is recycled to a nitration method for preparing nitroaromatic compounds, preferably nitrotoluene.

[0061] Similar to the second evaporation stage 2, the third evaporation stage 3 is also carried out under vacuum conditions. Part of the acidic vapor 15 obtained by evaporation in the third evaporation stage will be condensed and recycled to the third evaporation stage, while the uncondensed vapor will be discharged into the vacuum unit.

[0062] The third evaporation stage 3 operates at a temperature of 180 to 300°C, more preferably 190 to 250°C, and particularly 190 to 230°C, and at a pressure of 10 to 800 mbar (absolute pressure), more preferably 20 to 500 mbar (absolute pressure), and particularly 50 to 200 mbar (absolute pressure). Because the operating temperature is above 180°C, there is a risk of forming solid carbonaceous precipitates that deposit on the equipment surface, causing scaling. Furthermore, because the operating temperature is above 180°C, especially if organic impurities accumulate in equipment areas such as dead zones for extended periods, there is a further risk of spontaneous organic decomposition. This spontaneous organic decomposition can cause pressure shocks and equipment damage.

[0063] To prevent the formation of solid carbonaceous precipitates or the accumulation of organic matter, oxidant 17 or a precursor of oxidant is fed into the third evaporation stage 3. Due to oxidation, organic impurities still contained in the first or second type of concentrated sulfuric acid fed into the third evaporation stage 3 are selectively decomposed into volatile components before they begin to form solid precipitates or accumulate. These volatile components, such as nitrogen oxides, carbon monoxide, and / or carbon dioxide, are discharged into the vacuum unit as part of acidic vapor 15.

[0064] The third evaporation stage may include 1 to 5 evaporation units. Suitable evaporation units may be, for example, circulating evaporators, horizontal evaporators, or falling film evaporators. Depending on the type of evaporator and the method design, the oxidant 17 or its precursor may be fed into the evaporation loop, for example, on the suction side of the circulating pump, or before being fed into the first or second concentrated sulfuric acid 12, 14.

[0065] Comparative Example 1

[0066] A dilute sulfuric acid mixture comprising 71 wt% sulfuric acid, 1 wt% nitric acid, 0.5 wt% dinitrotoluene, and the remainder water is fed into the first evaporation stage at a feed temperature of 135°C. The first evaporation stage is operated at a pressure of 1 bar and a bottom temperature of 180°C.

[0067] The resulting concentrated sulfuric acid is extracted from the bottom of the first evaporation stage and fed into a second evaporation stage at a feed temperature of 180°C. The second evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 160°C. A second concentrated sulfuric acid is obtained at the bottom of the second evaporation stage and then fed into a third evaporation stage at a feed temperature of 160°C. The third evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 210°C, and the concentrated sulfuric acid obtained at the bottom of the third evaporation stage is extracted as a product.

[0068] In this embodiment, no oxidant was added.

[0069] The product has a TOC of 0.023, a density of 1.833 g / ml, and a sulfuric acid concentration of 96% by weight. No solids were observed in the product, and the product is yellow.

[0070] Comparative Example 2

[0071] A dilute sulfuric acid mixture comprising 71 wt% sulfuric acid, 1 wt% nitric acid, 0.5 wt% dinitrotoluene, and the remainder water is fed into the first evaporation stage at a feed temperature of 135°C. The first evaporation stage is operated at a pressure of 1 bar and a bottom temperature of 180°C.

[0072] The resulting concentrated sulfuric acid is extracted from the bottom of the first evaporation stage and fed into a second evaporation stage at a feed temperature of 180°C. The second evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 160°C. A second concentrated sulfuric acid is obtained at the bottom of the second evaporation stage and then fed into a third evaporation stage at a feed temperature of 160°C. The third evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 210°C, and the concentrated sulfuric acid obtained at the bottom of the third evaporation stage is extracted as a product.

[0073] In this embodiment, 5g of 99.8% nitric acid per kg of feed stream is added to the second stage as an oxidant.

[0074] The product has a TOC of 0.025, a density of 1.826 g / ml, and a sulfuric acid concentration of 95% by weight. No solids were observed in the product, and the product is yellow.

[0075] Example 1

[0076] A dilute sulfuric acid mixture comprising 71 wt% sulfuric acid, 1 wt% nitric acid, 0.5 wt% dinitrotoluene, and the remainder water is fed into the first evaporation stage at a feed temperature of 135°C. The first evaporation stage is operated at a pressure of 1 bar and a bottom temperature of 180°C.

[0077] The resulting concentrated sulfuric acid is extracted from the bottom of the first evaporation stage and fed into a second evaporation stage at a feed temperature of 180°C. The second evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 160°C. A second concentrated sulfuric acid is obtained at the bottom of the second evaporation stage and then fed into a third evaporation stage at a feed temperature of 160°C. The third evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 210°C, and the concentrated sulfuric acid obtained at the bottom of the third evaporation stage is extracted as a product.

[0078] Add 5g of 99.8% nitric acid per kg of feed stream as an oxidant to the third stage.

[0079] The product has a TOC of 0.015, a density of 1.837 g / ml, and a sulfuric acid concentration of 96% by weight. No solids were observed in the product, and the product is colorless.

[0080] Example 2

[0081] A dilute sulfuric acid mixture comprising 71 wt% sulfuric acid, 1 wt% nitric acid, 0.5 wt% dinitrotoluene, and the remainder water is fed into the first evaporation stage at a feed temperature of 135°C. The first evaporation stage is operated at a pressure of 1 bar and a bottom temperature of 180°C.

[0082] The resulting concentrated sulfuric acid is extracted from the bottom of the first evaporation stage and fed into the second evaporation stage at a feed temperature of 180°C. The second evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 160°C. A second concentrated sulfuric acid is obtained at the bottom of the second evaporation stage and then fed into the third evaporation stage at a feed temperature of 160°C. The third evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 180°C, and the concentrated sulfuric acid obtained at the bottom of the third evaporation stage is extracted as a product.

[0083] In this embodiment, 5g of 99.8% nitric acid per kg of feed stream is added as an oxidant to the third stage.

[0084] The product has a TOC of 0.042, a density of 1.823 g / ml, and a sulfuric acid concentration of 94% by weight. No solids were observed in the product, and the product is colorless.

[0085] Example 3

[0086] A dilute sulfuric acid mixture comprising 71 wt% sulfuric acid, 1 wt% nitric acid, 0.5 wt% dinitrotoluene, and the remainder water is fed into the first evaporation stage at a feed temperature of 135°C. The first evaporation stage is operated at a pressure of 1 bar and a bottom temperature of 180°C.

[0087] The resulting concentrated sulfuric acid is extracted from the bottom of the first evaporation stage and fed into the third evaporation stage at a feed temperature of 180°C. The third evaporation stage operates at a pressure of 0.09 bar and a bottom temperature of 210°C, and the concentrated sulfuric acid obtained at the bottom of the third evaporation stage is extracted as a product.

[0088] In this embodiment, 5g of 99.8% nitric acid per kg of feed stream is added as an oxidant to the third stage.

[0089] The product has a TOC of 0.024, a density of 1.826 g / ml, and a sulfuric acid concentration of 95% by weight. No solids were observed in the product, and the product is colorless.

Claims

1. A method for concentrating dilute sulfuric acid (10) that may contain at least one nitroaromatic compound and / or nitric acid as impurities, comprising: (a) Dilute sulfuric acid (10) is fed to the first stage (1), in which low-boiling substances are removed by evaporation and / or stripping to obtain a first concentrated sulfuric acid (12); (b) Optionally, the first concentrated sulfuric acid (12) is fed to the second evaporation stage (2) to obtain the second concentrated sulfuric acid (14); (c) If step (b) is performed, the second type of concentrated sulfuric acid (14) is fed into the third evaporation stage (3), or if step (b) is not performed, the first type of concentrated sulfuric acid (12) is fed into the third evaporation stage (3) to obtain concentrated sulfuric acid (16) as the product. The third evaporation stage (3) is operated at a temperature of 180 to 250°C, and the oxidant (17) and / or the precursor of the oxidant are fed into the third evaporation stage (3).

2. The method according to claim 1, wherein at least one of the at least one nitroaromatic compound is mononitrotoluene and / or dinitrotoluene.

3. The method according to claim 1 or 2, wherein the oxidant is selected from nitric acid, nitrous acid, nitrosylsulfonic acid and combinations thereof, and / or the precursor of the oxidant is sodium nitrite.

4. The method according to claim 1 or 2, wherein the amount of oxidant is from 0.01 to 100 g per kg of concentrated sulfuric acid.

5. The method according to claim 1 or 2, wherein the first stage (1) comprises evaporation, which is operated at a temperature of 130 to 200°C and an absolute pressure of 800 to 1200 mbar.

6. The method according to claim 1 or 2, wherein the second evaporation stage (2) is operated at an absolute pressure of 10 to 800 mbar and a temperature of 120 to 200°C.

7. The method according to claim 1 or 2, wherein if a second evaporation stage (2) is performed, it comprises 1 to 5 evaporation units.

8. The method according to claim 1 or 2, wherein the third evaporation stage (3) is operated at an absolute pressure of 10 to 800 mbar and a temperature of 180 to 250°C.

9. The method according to claim 1 or 2, wherein the third evaporation stage (3) comprises 1 to 5 evaporation units.

10. The method according to claim 1 or 2, wherein the evaporation unit used in the third evaporation stage (3) is a circulating evaporator, a horizontal evaporator, or a falling film evaporator.

11. The method according to claim 1 or 2, wherein the oxidant and / or the precursor of the oxidant is fed into the evaporation circuit.

12. The method according to claim 1 or 2, wherein if step (b) is performed, the oxidant and / or the precursor of the oxidant is mixed with the second concentrated sulfuric acid (14), or if step (b) is not performed, the oxidant and / or the precursor of the oxidant is mixed with the first concentrated sulfuric acid (12) before being fed to the third evaporation stage (3).

13. The method according to claim 1 or 2, wherein the concentration of the dilute sulfuric acid (10) is 60 to 85% by weight.

14. The method according to claim 1 or 2, wherein the concentration of the concentrated sulfuric acid (16) is 80 to 99% by weight.

15. The method according to claim 1 or 2, wherein the dilute sulfuric acid (10) is obtained in a method for preparing nitrotoluene.

16. The method of claim 15, wherein the concentrated sulfuric acid (16) is recycled to the method for preparing nitrotoluene.

Images