Method and system for recycling waste acid from nitration process

Abstract

The invention relates to a method for recycling waste acid from a nitration process, which comprises the following steps: (1) carrying out denitration and steam stripping on the waste acid discharged in the nitration process to obtain waste sulfuric acid, concentrating the waste sulfuric acid to obtain finished sulfuric acid, and returning a part of the finished sulfuric acid to be used in the nitration process; (2) diluting the other part of the finished product sulfuric acid to obtain diluted sulfuric acid, and removing iron ions in the diluted sulfuric acid to obtain diluted sulfuric acid without the iron ions; and (3) adding the diluted sulfuric acid from which the iron ions are removed into the waste sulfuric acid in the step (1), concentrating to obtain a finished product sulfuric acid again, and repeating the steps. According to the method, the problem of salt deposition caused by iron ions is solved, and the operation stability of the device is improved.

Description

Technical Field

[0001] The invention belongs to the field of nitrate production, and in particular relates to a method and a system for recycling waste acid from a nitration process. Background Art

[0002] DNT is an important chemical raw material for the production of polyurethane materials, fuels, explosives and other supplies. In industry, DNT is produced by mixed acid nitration process. Waste acid is inevitably produced in the production process of nitration process. After the waste acid is concentrated, it is reused in the nitration reaction system. It is necessary to remove nitric acid and organic matter from it, and concentrate the sulfuric acid. Steam stripping is generally used in the industry to remove nitric acid and DNT from the waste acid. Due to the corrosion of metal equipment such as nitration reactors and sulfuric acid tanks, iron ions are contained in the waste acid. During the waste acid concentration process, iron salt particles are continuously precipitated and accumulated, resulting in reduced heat exchange efficiency of the heat exchanger and pipeline blockage. The industry needs to stop the waste acid concentration system regularly (30-40 days) to clean it. Since glass-lined equipment is often used in the waste acid concentration process, frequent parking and cleaning also leads to poor operating stability of glass-lined equipment and high porcelain explosion rate. At present, how to solve the salt formation problem in the waste acid concentration process is an industry problem in the nitration process. Summary of the invention

[0003] The object of the present invention is to provide a method and system for recycling waste acid from a nitration process, so as to solve the salt formation problem in the waste acid concentration process in the nitration process.

[0004] One aspect of the present invention is to provide a method for recycling waste acid from a nitration process, the method comprising the following steps:

[0005] (1) denitrifying and stripping the waste acid discharged in the nitration process to obtain waste sulfuric acid, concentrating the waste sulfuric acid to obtain finished sulfuric acid, and returning a portion of the finished sulfuric acid to the nitration process;

[0006] (2) taking another portion of the finished sulfuric acid for dilution to obtain diluted sulfuric acid, removing iron ions from the diluted sulfuric acid, and obtaining diluted sulfuric acid from which the iron ions have been removed; and

[0007] (3) returning the diluted sulfuric acid after the iron ions are removed to the waste sulfuric acid, concentrating it to obtain finished sulfuric acid again, and repeating the above steps.

[0008] Another aspect of the present invention is to provide a system for implementing the above method, the system comprising:

[0009] - a nitration reaction zone, which generates nitration products and produces spent acid;

[0010] - a denitrification tower, the denitrification tower receives the waste acid and denitrates the waste acid;

[0011] - a stripping tower, wherein the denitrification tower strips the denitrated waste acid to produce waste sulfuric acid;

[0012] - a concentration zone, which concentrates the waste sulfuric acid from the stripping tower to produce finished sulfuric acid, and returns a portion of the finished sulfuric acid to the nitration reaction zone; and

[0013] - an iron ion removal zone, the iron ion removal zone comprising a dilution section and an adsorption section;

[0014] The dilution unit dilutes another part of the finished sulfuric acid to obtain diluted sulfuric acid;

[0015] The adsorption section removes iron ions from the diluted sulfuric acid to obtain the diluted sulfuric acid after the iron ions are removed, and returns the diluted sulfuric acid to the concentration zone to be mixed with the waste sulfuric acid.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The present invention proposes a method for recycling waste acid from a nitration process, which dilutes a portion of the finished sulfuric acid, removes the iron ions in the diluted sulfuric acid, and mixes the diluted sulfuric acid from which the iron ions are removed with the waste sulfuric acid to be concentrated, so as to dilute the iron ions in the waste sulfuric acid to reduce the iron ion concentration, so that the iron ion content in the waste sulfuric acid is always below the saturated solubility during the concentration process. The method realizes the real-time dynamic removal of iron ions, solves the problem of salt formation during the concentration process, and improves the stability of the device operation.

[0018] 2. The present invention combines the introduction rate of Fe ions in the nitration reaction unit, takes a small amount of finished sulfuric acid for dilution and removes iron ions, so that the introduction rate of iron ions is not higher than the removal rate, ensuring that the iron ions in the sulfuric acid are always in an unsaturated state during the concentration and cooling process.

[0019] 3. The present invention utilizes a packed tower to provide a large amount of rough specific surface area to achieve the removal of tiny salt solid impurities in sulfuric acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The present invention is a schematic diagram of a process flow involved in one embodiment of the present invention. DETAILED DESCRIPTION

[0021] The present invention is further described below by means of specific embodiments. The embodiments of the present invention are only used as illustrations of the present invention and do not mean that the scope of the present invention is limited thereto.

[0022] The key to solving the salt formation problem in the waste acid concentration process in the nitration process is to remove the iron ions present in the waste acid. Usually, due to the high concentration of sulfuric acid, it is extremely difficult to remove the iron ions. For example, the sulfuric acid concentration in DNT waste acid is 72-93%, and the iron ion concentration is about 100-150ppm.

[0023] In order to solve the above problems, one aspect of the present invention provides a method for recycling waste acid from a nitration process, the method comprising the following steps:

[0024] (1) denitrifying and stripping the waste acid discharged in the nitration process to obtain waste sulfuric acid, concentrating the waste sulfuric acid to obtain finished sulfuric acid, and returning a portion of the finished sulfuric acid to the nitration process;

[0025] (2) taking another portion of the finished sulfuric acid for dilution to obtain diluted sulfuric acid, removing iron ions from the diluted sulfuric acid, and obtaining diluted sulfuric acid from which the iron ions have been removed; and

[0026] (3) returning the diluted sulfuric acid after the iron ions are removed to the waste sulfuric acid, concentrating it to obtain finished sulfuric acid again, and repeating the above steps.

[0027] Preferably, the nitration process is a nitration process for producing DNT.

[0028] The inventors have noticed that in the DNT production process, the introduction of iron ions in the acid reaction liquid is mainly due to the corrosion of metal equipment such as stainless steel or carbon steel by mixed acid or sulfuric acid in the nitration process. In the DNT production process, the corrosion rate of acid on metal equipment is generally <0.01mm / a, and the process is a slow process. However, with the recycling of sulfuric acid, the content of iron ions in the acid reaction liquid will continue to rise until the iron ion concentration reaches saturation during the subsequent concentration and / or cooling process, resulting in the continuous precipitation of iron ions in the form of salts, causing equipment abnormalities. The method of the present invention continuously removes the iron ions in part of the finished sulfuric acid, and then returns this part of the sulfuric acid after the iron ions are removed to the waste sulfuric acid, thereby diluting the iron ion concentration in the waste sulfuric acid, reducing or avoiding the precipitation of iron ions, and improving the operating stability of the equipment.

[0029] Further, in order to ensure that the iron ion concentration in the acid reaction solution will never reach saturation and salt out under various operating conditions (especially during the concentration of sulfuric acid and the subsequent cooling process), it is necessary to avoid the accumulation of iron ions in the acid reaction solution. For this reason, the introduction rate of iron ions must not be higher than the removal rate.

[0030] In some embodiments, before step (2), the method further comprises the following steps:

[0031] (2-1) determining the rate of introduction of iron ions into the waste acid, and adjusting the rate of iron ion removal so that the rate of iron ion removal is greater than or equal to the rate of introduction of iron ions.

[0032] In the present invention, the method for determining the iron ion introduction rate is not particularly limited and can be measured or calculated by a suitable method, for example, it can be obtained by the following formula (i):

[0033] Iron ion introduction rate (kg / h) = corrosion area (m 2 )*Corrosion rate (mm / h)*Density of iron-containing equipment material (kg / m 3 )*Iron content in iron-containing equipment material (wt%) / 1000 (i),

[0034] in,

[0035] Corrosion area: the area where all iron-containing equipment involved in the nitration process (which may also include metal pipelines) are in contact with the acid reaction liquid in the reaction system, such as mixed acid or sulfuric acid;

[0036] Corrosion rate: the thickness of the equipment reduced per unit time under acid reaction liquid working conditions. The corrosion rate can be obtained by consulting the relevant corrosion manuals for simple systems; for complex systems, the corrosion rate can be determined experimentally (such as by weight method, volumetric method, polarization curve method and other commonly used methods in this field); the corrosion rate can also be obtained by measuring the thickness of the equipment in long-term operation.

[0037] Density of ferrous equipment material: the density of the ferrous equipment material itself.

[0038] Iron content in iron-containing equipment materials: specifically refers to the mass fraction of iron contained in the metal materials of iron-containing equipment (which may also include metal pipelines) in contact with the acid reaction liquid;

[0039] In the case of different equipment materials and / or acid reaction liquids, different units can be divided according to the combination of materials and acid reaction liquids, and the iron ion introduction rate can be calculated by the following formula:

[0040] Iron ion introduction rate = iron ion introduction rate of unit 1 + iron ion introduction rate of unit 2 ... + iron ion introduction rate of unit 1 of unit n.

[0041] In the present invention, the iron ion removal rate can be obtained by the following formula (ii):

[0042] Iron ion removal rate (kg / h) = sulfuric acid circulation volume (t / h) * iron ion concentration in finished sulfuric acid (ppm) * finished sulfuric acid extraction ratio (%) * iron ion removal rate (%) / 1000 Formula (ii),

[0043] Sulfuric acid circulation volume: refers to the mass flow rate of finished sulfuric acid generated after concentration.

[0044] Iron ion concentration in finished sulfuric acid: refers to the iron ion concentration in finished sulfuric acid, measured in mass concentration.

[0045] Finished sulfuric acid extraction ratio: the ratio of the part of finished sulfuric acid extracted for dilution and removal of iron ions to the total amount of finished sulfuric acid.

[0046] Iron ion removal rate: the ratio of the difference in iron ion content in the diluted sulfuric acid before and after iron ion removal to the iron ion content in the diluted sulfuric acid before iron ion removal, that is: (iron ion content in the diluted sulfuric acid before iron ion removal - iron ion content in the diluted sulfuric acid after iron ion removal) / iron ion content in the diluted sulfuric acid before iron ion removal.

[0047] It should be noted that, in general, the above calculations can be calculated by selecting approximate values ​​that can meet the actual application requirements. In addition, it should be noted that the main contribution to salt formation during the corrosion of iron-containing equipment is iron, while other metal elements such as chromium and nickel contribute less. For example, the sum of the salt content of other metal elements such as chromium salts and nickel salts in salt formation is less than 1%.

[0048] In order to prevent the precipitation of iron ion salt, it is necessary to make the iron ion concentration below the saturated solubility under various operating conditions. Therefore, in the finished sulfuric acid obtained after concentration, especially in the concentrated sulfuric acid after cooling, the iron ion concentration needs to be below the saturated solubility. Usually, the concentration of the finished sulfuric acid can be 90wt%-95wt%. At this time, in order to ensure that no iron salt is precipitated under various operating conditions, it is necessary to make the Fe ion concentration <80ppm, preferably <77ppm.

[0049] In one embodiment, if the Fe ion content in the acid reaction solution, especially the finished sulfuric acid, is low when the iron ion is removed, and the concentration of salting out is not reached, as long as the iron ion introduction rate is less than or equal to the iron ion removal rate. In one embodiment, if the Fe ion content in the acid reaction solution is high when the iron ion is removed, and the concentration of salting out has been reached or is about to be reached, then it is necessary to make the iron ion introduction rate < the iron ion removal rate at the beginning, at this time, as the iron ion is removed, the Fe ion content in the acid reaction solution decreases, and as the Fe ion content in the acid reaction solution decreases, the Fe ion removal rate gradually decreases, and finally the iron ion introduction rate = the iron ion removal rate is made, and the dynamic balance of the iron ion content is reached.

[0050] Since the extracted sulfuric acid needs to be diluted and then returned to the waste sulfuric acid for concentration, this part of sulfuric acid should be appropriate in terms of process cost. Therefore, in some embodiments, the other part of the finished sulfuric acid used for dilution accounts for 1%-5% of the total mass of the finished sulfuric acid.

[0051] In some embodiments, another portion of the finished sulfuric acid is diluted to a concentration of 40 wt% to 60 wt%.

[0052] In some embodiments, the diluted sulfuric acid is adsorbed by an ion exchange resin to remove iron ions. It should be understood that iron ions can also be removed by other methods known in the art. The adsorption temperature of the ion exchange resin is 20-50°C and the air velocity is 0.1-3BV; preferably, the iron ion removal rate is ≥90%.

[0053] It should be pointed out that as the resin adsorption capacity approaches saturation, its adsorption rate decreases. At this time, the Fe ion introduction rate may be greater than the Fe ion removal rate, so the resin needs to be replaced regularly.

[0054] In some embodiments, the finished sulfuric acid is further filtered to remove tiny solid impurities in the sulfuric acid, such as precipitated crystalline salt particles, and then a portion of the finished sulfuric acid is returned to the nitration process, and another portion of the finished sulfuric acid is taken for dilution.

[0055] In some embodiments, the filtration is carried out in a packed tower, which uses structured packing. The material of the packing is non-metal, preferably PTFE, ceramic, silicon carbide; the height of the structured packing is 1-10m, preferably 2-6m, and the air velocity is 1-3BV.

[0056] In some embodiments, the denitrification is carried out in a denitrification tower, the waste acid is preheated to 120-160°C, preferably 140-160°C, and then enters the denitrification tower to remove nitric acid and nitrous acid therein; the top pressure of the denitrification tower is 25-40 kPaa, the bottom temperature is 130-155°C, and the number of theoretical plates is 5-10.

[0057] In some embodiments, the stripping is carried out in a stripping tower, and the waste acid after denitration enters the stripping tower to remove the nitration products therein to obtain waste sulfuric acid; the top pressure of the stripping tower is 20-30 kPaa, the bottom temperature is 170-190°C, and the number of theoretical plates is 10-20.

[0058] In a preferred embodiment, the waste acid after denitration is subjected to stripping treatment to remove the nitration products, such as DNT, dissolved in the waste acid after denitration. The stripping is carried out in a stripping tower, the waste acid after denitration is fed from the top of the stripping tower, water and nitration products are produced from the top of the stripping tower, and after the water and nitration products are separated, the nitration products are recovered to the nitration product washing unit, the water phase is treated as waste water, and the waste sulfuric acid whose main component is sulfuric acid is obtained at the bottom of the tower.

[0059] In a preferred embodiment, the waste sulfuric acid is concentrated by 1-4 stages, preferably 2-3 stages, to obtain the finished sulfuric acid, and the gas phase produced by the concentration is condensed and discharged as wastewater. The last stage of concentration has a pressure of 3-5 kPaa and a concentration temperature of 175-186°C, preferably 180-183°C; then the finished sulfuric acid is cooled to 30-50°C, preferably 35-45°C. The finished sulfuric acid obtained after concentration can also be first heat exchanged with waste acid, waste acid before steam generation and / or waste acid before denitration, and then cooled by heat exchange with circulating water. Usually, the concentration of the finished sulfuric acid is 90wt%-95wt%.

[0060] In a preferred embodiment, the waste acid is first heat exchanged with high-temperature finished sulfuric acid and then preheated by steam. After preheating, the waste acid is fed from the top of the denitrification tower, and the nitric acid and nitrous acid in the waste acid are removed by steam stripping. The gas phase at the top of the tower is condensed and sent to the nitration reaction as a raw material, and the non-condensable gas is sent to the NOx absorption unit for treatment.

[0061] In some embodiments, the concentration of sulfuric acid in the waste acid is 70-75 wt %, the concentration of nitric acid is 0.5-1 wt %, and the residual content of nitration products is 4000-12000 ppm.

[0062] One aspect of the present invention provides a system for the above method, characterized in that the system comprises:

[0063] - a nitration reaction zone, which generates nitration products and produces spent acid;

[0064] - a denitrification tower, the denitrification tower receives the waste acid and denitrates the waste acid;

[0065] - a stripping tower, which strips the denitrated waste acid to produce waste sulfuric acid;

[0066] - a concentration zone, which concentrates the waste sulfuric acid from the stripping tower to produce finished sulfuric acid, and returns a portion of the finished sulfuric acid to the nitration reaction zone; and

[0067] - an iron ion removal zone, the iron ion removal zone comprising a dilution section and an adsorption section;

[0068] The dilution unit dilutes another part of the finished sulfuric acid to obtain diluted sulfuric acid;

[0069] The adsorption section removes iron ions from the diluted sulfuric acid to obtain the diluted sulfuric acid after the iron ions are removed, and returns the diluted sulfuric acid to the concentration zone to be mixed with the waste sulfuric acid.

[0070] Preferably, the nitration product is DNT.

[0071] In a preferred embodiment, the iron ion removal zone further comprises a filtration unit, which filters the finished sulfuric acid. The filtration unit can be a packed tower, and the packed tower is as described above.

[0072] In some embodiments, the nitration reaction zone further comprises:

[0073] a spent acid storage tank that stores the spent acid and delivers the spent acid to the denitration tower; and

[0074] A sulfuric acid storage tank, wherein a portion of the finished sulfuric acid is returned to the nitration reaction zone and stored in the sulfuric acid storage tank for standby use.

[0075] It should be understood that the method and apparatus of the present invention can also be applied to the removal of other metal ions (such as chromium, nickel and other metal ions) other than iron through equivalent substitution or modification, and these equivalent substitutions, modifications or various forms of combinations should also be included in the scope of the present invention.

[0076] The present invention is further described below by means of specific examples. The examples described in the present invention are only used to illustrate the present invention and do not limit the scope of the present invention.

[0077] In such Figure 1 The system shown recycles waste acid from the nitration process. Where the system is not specifically described, the above description may be referred to. The system comprises: a nitration reaction zone (1-1), a denitration tower (2), a stripping tower (3), a concentration zone (4), and an iron ion removal zone (5). A waste acid storage tank (1-2) is provided at the outlet of the nitration reaction zone (1-1), and a sulfuric acid storage tank (1-3) is provided at the entrance of the nitration reaction zone (1-1). The denitration tower (2), the stripping tower (3), and the concentration zone (4) are connected in sequence; an iron ion removal zone (5) is provided downstream of the concentration zone (4), and the iron ion removal zone (5) is connected to the sulfuric acid storage tank (1-3). The iron ion removal zone (5) comprises a filtering section (5-1), a dilution section (5-2), and an adsorption section (5-3). The nitration reaction zone (1-1) generates waste acid, which is stored in a waste acid storage tank (1-2) or sent to a downstream denitration tower (2), a stripping tower (3), and a concentration zone (4) in sequence for treatment to obtain finished sulfuric acid. The finished sulfuric acid flowing out of the concentration zone (4) enters a filtering section (5-1) to remove particulate impurities, and a portion of the finished sulfuric acid after filtration is returned to the nitration reaction zone (1-1) or temporarily stored in a sulfuric acid storage tank (1-3) for standby use, and another portion of the finished sulfuric acid after filtration enters a dilution section (5-2) for dilution, and then enters an adsorption section (5-3) to remove iron ions through ion exchange resin adsorption, and finally returns to the concentration zone (4) to mix with the waste sulfuric acid and re-concentrate.

[0078] The resin involved in the present invention adopts conventional heavy metal resin purchased from Zhengzhou Xidian Xi'an Lanxiao.

[0079] The present invention relates to an analysis method for Fe ions: using an ICP-OES725 inductively coupled plasma emission spectrometer to analyze the Fe ion content in sulfuric acid, the specific method is to dilute concentrated sulfuric acid with ultrapure water to an acid concentration of 2wt%, then feed analysis, select a Fe element wavelength of 238.204nm, and use a 1000ppm Fe ion standard solution to draw a marking line (0.01-1ppm).

[0080] Calculation of iron ion introduction rate:

[0081] Iron ion introduction rate (kg / h) = corrosion area (m 2 )*Corrosion rate (mm / h)*Density of iron-containing equipment material (kg / m 3 )*Iron content in iron-containing equipment materials (wt%) / 1000,

[0082] According to the different reaction liquids in each area of ​​the reaction system and the materials of the iron-containing equipment, the system is divided into multiple units, and the iron ion introduction rate is calculated by the following formula:

[0083] Iron ion introduction rate = iron ion introduction rate of unit 1 + iron ion introduction rate of unit 2 ... + iron ion introduction rate of unit 1 of unit n.

[0084] The corrosion rate of each unit is determined by consulting the "Corrosion Data and Material Selection Handbook" (Chemical Industry Press) or experiments according to the actual situation. The corrosion area of ​​each unit is calculated based on the actual size of the equipment.

[0085] The materials used for iron-containing equipment are mainly carbon steel and 316L stainless steel. In industrial calculations, the density of carbon steel is 7.85g / cm 3 , 316L stainless steel density is 7930kg / m 3 The iron content of iron-containing equipment is determined according to the equipment material. In industrial calculations, the iron content of carbon steel equipment is 98wt%, and the iron content of 316L stainless steel equipment is 68wt%.

[0086] The calculation of iron ion removal rate, the definitions of the following parameters are as described above:

[0087] Iron ion removal rate (kg / h) = sulfuric acid circulation volume (t / h) * iron ion concentration in sulfuric acid (ppm) * sulfuric acid extraction ratio (%) * iron ion removal rate (%) / 1000.

[0088] [Example 1]

[0089] The sulfuric acid concentration in waste acid 1 is 70wt%, the nitric acid concentration is 0.5wt%, the DNT content is 4000ppm, and the waste acid volume is 55t / h. Due to the corrosion of the nitration reaction system, after considering the iron content in the equipment material, the Fe ion introduction rate in the waste acid is calculated to be 0.03kg / h. The waste acid is preheated to 120°C and enters from the top of the denitration tower. The top pressure of the denitration tower is 25kpaa, the bottom temperature is 130°C, and the number of theoretical plates is 10. The discharge from the bottom of the denitration tower enters the stripping tower from the top of the tower. The top pressure of the stripping tower is 20kpaa, the bottom temperature is 170°C, and the number of theoretical plates is 10. After stripping, the waste acid is concentrated in the first stage, with a concentration pressure of 3kpaa and a temperature of 175°C. After concentration, the sulfuric acid is cooled to 35°C. At this time, the Fe ion content in the concentrated sulfuric acid is 80ppm, the sulfuric acid concentration is 93%, and the flow rate is 41.4t / h. After cooling, sulfuric acid is filtered through a packed bed. The packing is silicon carbide structured packing, with a packing height of 3m and an air velocity of 1BV. After filtration, 3% of the total mass of concentrated sulfuric acid is extracted and diluted to a concentration of 40wt%. Ion exchange resin is used for adsorption at 20°C, with an air velocity of 1.5BV and an Fe ion removal rate of 98%. At this time, the Fe ion removal rate is 0.1kg / h, and the Fe ions in sulfuric acid show a downward trend until reaching an equilibrium state. At the equilibrium state, the Fe ion content in the concentrated sulfuric acid is 24ppm, which is lower than its saturation concentration. The frequency of acid concentration and salt washing is reduced to 0.

[0090] [Example 2]

[0091] The sulfuric acid concentration in waste acid 1 is 75wt%, the nitric acid concentration is 1wt%, the DNT content is 12000ppm, and the waste acid volume is 70t / h. Due to the corrosion of the nitration reaction system, after considering the iron content in the equipment material, the Fe ion introduction rate in the waste acid is calculated to be 0.06kg / h. The waste acid is preheated to 160°C and enters from the top of the denitration tower. The top pressure of the denitration tower is 40kpaa, the bottom temperature is 155°C, and the number of theoretical plates is 5. The discharge from the denitration tower bottom enters the stripping tower from the top of the tower. The top pressure of the stripping tower is 30kpaa, the bottom temperature is 190°C, and the number of theoretical plates is 20. After stripping, the waste acid is concentrated in 4 stages, with a concentration pressure of 5kpaa and a temperature of 186°C. After concentration, the waste acid is cooled to 50°C. At this time, the Fe ion content in the concentrated sulfuric acid is 50ppm, the sulfuric acid concentration is 92%, and the flow rate is 57.1t / h. After cooling, sulfuric acid is filtered through a packed bed. The packing is ceramic structured packing, with a packing height of 10m and an air velocity of 2BV. 5% of the total mass of concentrated sulfuric acid is extracted and diluted to a concentration of 60wt%, and resin adsorption is used at 50°C with an air velocity of 3BV. The Fe ion removal rate is 90%, and the Fe ion removal rate is 0.13kg / h. The Fe ions in sulfuric acid show a downward trend until reaching equilibrium. At equilibrium, the Fe ion content in concentrated sulfuric acid is 23ppm, which is lower than its saturation concentration. The frequency of acid concentration and salt washing is reduced to 0.

[0092] [Example 3]

[0093] The sulfuric acid concentration in waste acid 3 is 73wt%, the nitric acid concentration is 0.5wt%, the DNT content is 8000ppm, and the waste acid volume is 65t / h. Due to the corrosion of the nitration reaction system, after considering the iron content in the equipment material, the Fe ion introduction rate in the waste acid is calculated to be 0.07kg / h. The waste acid is preheated to 140°C and enters from the top of the denitration tower. The top pressure of the denitration tower is 25kpaa, the bottom temperature is 140°C, and the number of theoretical plates is 7. The discharge from the bottom of the denitration tower enters the stripping tower from the top of the tower. The top pressure of the stripping tower is 35kpaa, the bottom temperature is 180°C, and the number of theoretical plates is 15. After stripping, the waste acid is concentrated in two stages, and the last stage of concentration pressure is 4kpaa and the temperature is 180°C. After concentration, the waste acid is cooled to 30°C. At this time, the Fe ion content in the concentrated sulfuric acid is 90ppm, the sulfuric acid concentration is 92%, and the flow rate is 51.6t / h. After cooling, the sulfuric acid is filtered through a packed bed. The packing is a ceramic structured packing with a packing height of 1m and an air velocity of 3BV. 2% of the total mass of the concentrated sulfuric acid is extracted and diluted to a concentration of 50wt%. Resin adsorption is used at 30°C with an air velocity of 2BV. The Fe ion removal rate is 95%. At this time, the Fe ion removal rate is 0.09kg / h. The Fe ions in the sulfuric acid show a downward trend until they reach a balance state. At the balance state, the Fe ion content in the concentrated sulfuric acid is 73ppm, which is lower than its saturation concentration. The frequency of acid concentration and salt washing is reduced to 0.

[0094] [Example 4]

[0095] The sulfuric acid concentration in waste acid 4 is 72wt%, the nitric acid concentration is 0.7wt%, the DNT content is 6000ppm, and the waste acid volume is 30t / h. Due to the corrosion of the nitration reaction system, after considering the iron content in the equipment material, the Fe ion introduction rate in the waste acid is calculated to be 0.03kg / h. The waste acid is preheated to 150°C and enters from the top of the denitration tower. The top pressure of the denitration tower is 35kpaa, the bottom temperature is 150°C, and the number of theoretical plates is 9. The discharge from the bottom of the denitration tower enters the stripping tower from the top of the tower. The top pressure of the stripping tower is 28kpaa, the bottom temperature is 186°C, and the number of theoretical plates is 17. After stripping, the waste acid is concentrated in three stages, and the last stage of concentration pressure is 3kpaa and the temperature is 183°C. After concentration, the waste acid is cooled to 45°C. At this time, the Fe ion content in the concentrated sulfuric acid is 70ppm, the sulfuric acid concentration is 95%, and the flow rate is 22.7t / h. 4% of the total mass of concentrated sulfuric acid was extracted and diluted to a concentration of 55wt%, and resin adsorption was used at 45°C, with a space velocity of 2BV and a Fe ion removal rate of 92%. At this time, the Fe ion removal rate was 0.06kg / h, and the Fe ion in sulfuric acid showed a downward trend until it reached a balance state. At the balance state, the Fe ion content in the concentrated sulfuric acid was 35ppm, which was lower than its saturation concentration. The frequency of acid concentration and salt washing was reduced to 0.

[0096] [Comparative Example 1]

[0097] The sulfuric acid concentration in waste acid 4 is 72wt%, the nitric acid concentration is 0.7wt%, and the DNT content is 6000ppm. The amount of waste acid is 30t / h. Due to the corrosion of the nitration reaction system, after considering the iron content in the equipment material, the Fe ion introduction rate in the waste acid is calculated to be 0.03kg / h. The waste acid is preheated to 150°C and enters from the top of the denitration tower. The top pressure of the denitration tower is 35kpaa, the bottom temperature is 150°C, and the number of theoretical plates is 9. The discharge from the denitration tower bottom enters the stripping tower from the top of the tower. The top pressure of the stripping tower is 28kpaa, the bottom temperature is 186°C, and the number of theoretical plates is 17. After stripping, the waste acid is concentrated in three stages, and the last stage of concentration pressure is 3kpaa and the temperature is 183°C. The frequency of acid concentration and salt washing is 12 times / year.

Claims

1. A method for recycling waste acid from a nitration process, characterized in that: The method comprises the following steps: (1) denitrifying and stripping the waste acid discharged in the nitration process to obtain waste sulfuric acid, concentrating the waste sulfuric acid to obtain finished sulfuric acid, and returning a portion of the finished sulfuric acid to the nitration process; (2) taking another portion of the finished sulfuric acid for dilution to obtain diluted sulfuric acid, removing iron ions from the diluted sulfuric acid, and obtaining diluted sulfuric acid from which the iron ions have been removed; and (3) adding the diluted sulfuric acid after the iron ions are removed to the waste sulfuric acid in step (1), concentrating it to obtain the finished sulfuric acid again, and repeating the above steps.

2. The method according to claim 1, characterized in that Before step (2), the method further comprises the following steps: (2-1) determining the iron ion introduction rate in the waste acid, and adjusting the iron ion removal rate so that the iron ion removal rate is greater than or equal to the iron ion introduction rate; Preferably, wherein Iron ion introduction rate (kg / h) = corrosion area (m 2 )*Corrosion rate (mm / h)*Density of iron-containing equipment material (kg / m 3 )*Iron content in iron-containing equipment materials (wt%) / 1000, Iron ion removal rate (kg / h) = sulfuric acid circulation volume (t / h) * iron ion concentration in finished sulfuric acid (ppm) * finished sulfuric acid extraction ratio (%) * iron ion removal rate (%) / 1000, Here, "sulfuric acid circulation rate" refers to the mass flow rate of finished sulfuric acid generated after concentration. "Fe ion concentration in finished sulfuric acid" refers to the iron ion concentration in the finished sulfuric acid. "Finished sulfuric acid extraction ratio" refers to the ratio of the part of the finished sulfuric acid taken out for dilution and removal of iron ions to the total amount of the finished sulfuric acid. "Iron ion removal rate" refers to the ratio of the difference in iron ion content in diluted sulfuric acid before and after iron ion removal to the iron ion content in diluted sulfuric acid before iron ion removal.

3. The method according to claim 1 or 2, characterized in that: In step (2), the other part of the finished sulfuric acid used for dilution accounts for 1% to 5% of the total mass of the finished sulfuric acid; And / or, diluting another portion of the finished sulfuric acid to a concentration of 40 wt % to 60 wt %.

4. The method according to any one of claims 1 to 3, characterized in that: The diluted sulfuric acid is adsorbed by an ion exchange resin to remove iron ions therein, preferably the adsorption temperature of the ion exchange resin is 20-50° C., and the space velocity is 0.1-3 BV; More preferably, the iron ion removal rate is ≥ 90% by adsorption with ion exchange resin.

5. The method according to any one of claims 1 to 4, characterized in that: The finished sulfuric acid is further filtered, and then a portion of the finished sulfuric acid is returned to the nitration process and another portion of the finished sulfuric acid is taken for dilution.

6. The method according to any one of claims 1 to 5, characterized in that: The denitration is carried out in a denitration tower, the waste acid is preheated to 120-160°C, preferably 140-160°C, and then enters the denitration tower to remove nitric acid and nitrous acid therein; the top pressure of the denitration tower is 25-40 kPaa, the bottom temperature is 130-155°C, and the number of theoretical plates is 5-10; And / or, the stripping is carried out in a stripping tower, the waste acid after denitration enters the stripping tower to remove the nitration product therein to obtain waste sulfuric acid; the top pressure of the stripping tower is 20-30 kPaa, the bottom temperature is 170-190°C, and the number of theoretical plates is 10-20; And / or, the waste sulfuric acid is concentrated by 1-4 stages, preferably 2-3 stages, to obtain the finished sulfuric acid, wherein the final stage of concentration has a pressure of 3-5 kPaa and a concentration temperature of 175-186° C., preferably 180-183° C.; and then the finished sulfuric acid is cooled to 30-50° C., preferably 35-45° C.; Preferably, the spent acid is heat exchanged with the finished sulfuric acid before preheating.

7. The method according to claim 5, characterized in that The filtration is carried out in a packed tower, which uses structured packing. The material of the packing is non-metal, preferably PTFE, ceramic, silicon carbide; the height of the structured packing is 1-10m, preferably 2-6m, and the air velocity is 1-3BV.

8. The method according to any one of claims 1 to 7, characterized in that: The waste acid has a sulfuric acid concentration of 70-75wt%, a nitric acid concentration of 0.5-1wt%, and a residual content of nitration products of 4000-12000ppm; And / or, the concentration of sulfuric acid in the finished sulfuric acid is 90wt%-95wt%.

9. A system for implementing the method according to any one of claims 1 to 8, characterized in that: The system comprises: - a nitration reaction zone, which generates nitration products and produces spent acid; - a denitrification tower, the denitrification tower receives the waste acid and denitrates the waste acid; - a stripping tower, which strips the denitrated waste acid to produce waste sulfuric acid; - a concentration zone, which concentrates the waste sulfuric acid from the stripping tower to produce finished sulfuric acid, and returns a portion of the finished sulfuric acid to the nitration reaction zone; and - an iron ion removal zone, the iron ion removal zone comprising a dilution section and an adsorption section; The dilution unit dilutes another part of the finished sulfuric acid to obtain diluted sulfuric acid; The adsorption section removes iron ions from the diluted sulfuric acid to obtain the diluted sulfuric acid after the iron ions are removed, and returns the diluted sulfuric acid to the concentration zone to be mixed with the waste sulfuric acid; Preferably, the iron ion removal zone further comprises a filtering section, and the filtering section filters the finished sulfuric acid.

10. The system according to claim 9, characterized in that The nitration reaction zone also includes: a spent acid storage tank that stores the spent acid and delivers the spent acid to the denitration tower; and A sulfuric acid storage tank, wherein a portion of the finished sulfuric acid is returned to the nitration reaction zone and stored in the sulfuric acid storage tank for standby use.

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