Nitric acid industry, nitric acid production method

There are five existing dilute nitric acid production methods: Atmospheric Pressure Method and medium pressure method (0.25 ~ 50MPa), high pressure method (0.7 ~ 2MPa), synthesis method (oxidation is atmospheric pressure, absorption is pressurized) and double addition pressure (oxidation is medium pressure, absorption is high pressure ). normal Pressure Oxidation and absorption are both carried out under normal pressure, and the Investment and power consumption of equipment are relatively low, but the concentration of nitric acid produced is not high, which is only 45% ~ 52%. in the exhaust gas, the content of NOx (NO, NO2 and other nitrogen compounds) is high. A processing device must be added before it can be discharged into the atmosphere after treatment. medium pressure method, high pressure method and double pressure method are both oxidized and absorbed under pressure. equipment investment and power consumption are high, but the acid concentration is high, which can reach 65 ~ 72%. The NOx content in the exhaust gas is relatively low, which is easy to handle or directly discharged. the Method of Double-pressure pressurization is reasonable, and the absorption rate is 99.5%. NOx in the exhaust gas is only about 180 ppm, which can be directly discharged into the atmosphere through the chimney. Therefore, it is a production method worthy of vigorous promotion. the synthesis method oxidation is carried out under normal pressure and absorption is carried out under pressurization. The concentration of nitric acid can still reach 65% ~ between equipment investment and power consumption between normal pressure and pressurization ~ 72%. the consumption quota of the above five processes is shown in Table 3-1-08. table 3-1-08 consumption quota of five production methods (measured in 1 ton of 100% nitric acid) Project Atmospheric Pressure Method Medium Pressure Method High Pressure Method Synthesis Method Double Pressure Method ammonia oxidation pressure/MPa (absolute) NO2 absorption pressure/MPa (absolute) Ammonia [W (NH 3) = 100%]/ton platinum/gram electricity/degree Cooling Water/m3 by-product Steam/ton product acid concentration/% oxidation rate/% absorption rate/% atmospheric pressure 0.296 0.06 125 150 45-52 97 98 0.4-0.6 0.4-0.6 0.281 0.13 12 143 0.74 67 95-97 98.7 0.8 0.8 0.288 0.18 1.65 0.9 55-67-0.1 0.35 0.281 0.04 26 155 50 97 0.25-0.5 0.8-1.0 0.281 0.10 141 0.20 55-67 97 99.5 '(2) There are three methods for producing concentrated nitric acid in the production process of concentrated nitric acid: direct method, indirect method, and hyper-boiling acid distillation method. The direct method is to directly synthesize concentrated nitric acid from ammonia, with a large investment in infrastructure, it is only used in large domestic and foreign factories. the indirect method is to first produce dilute nitric acid and then try to remove the water from the dilute nitric acid, increase the concentration to the co-boiling acid concentration or above, and finally obtain 98% of the finished nitric acid by distillation. this production method is suitable for medium and small concentrated nitric acid production units and is widely used at home and abroad. hyperboiling acid distillation is a method that removes a large amount of water from the oxidation gas, so that NOx can directly generate hyperboiling acid and then produce concentrated nitric acid by distillation, it is regarded as a good method for making concentrated nitric acid. A direct method is used to directly synthesize concentrated nitric acid from ammonia and air by oxidation. The key to production is to remove the water generated by the reaction. the reaction has gone through the following five steps: ① the production of nitric oxide ammonia and air are oxidized to nitric oxide at high temperatures through a platinum network catalyst, and rapidly cooled to 40 ~ 50 ℃, so that the generated water vapor is removed by condensation. ② the reaction of nitrogen dioxide and oxygen in the air. After the formation of NO2, the residual NO and concentrated nitric acid with a concentration greater than 98% are completely oxidized to nitrogen dioxide: ③ separation of nitrogen dioxide at low temperatures with concentrated nitric acid (> 98%) absorption of nitrogen dioxide into smoke nitric acid, cannot be absorbed by the inert gas (N2, etc.) Discharge System for further processing. (4) Pure NO2 is produced and then condensed and aggregated into liquid tetroxide nitrogen and heated to smoke nitric acid. It is decomposed to release nitrogen dioxide, and then the pure NO2 is condensed into liquid tetroxide nitrogen: ⑤ High-Pressure Kettle reaction to produce concentrated nitric acid mixing liquid nitrogen oxide with dilute nitric acid (requires the water in dilute nitric acid to a certain proportion of liquid n204) into the high-pressure kettle, under 5. 0MPa pressure into oxygen, the reaction between nitrogen oxide and water (from dilute nitric acid) and oxygen directly generates 98% concentrated nitric acid. in order to speed up the reaction, the added liquid n204 should be more than the theoretical amount. In this way, the white concentrated nitric acid containing a large amount of free nitrogen dioxide (I .e. smoke nitric acid) should be prepared and placed in the Bleaching Tower, the air is used to blow out free NO2 and produce 98% concentrated nitric acid. nitrogen Dioxide recovery and Condensation And then sent to the high-pressure kettle for use. if the oxidation of ammonia does not use air, and pure oxygen is used (water vapor must be added to dilute to prevent explosion), the concentration of nitric oxide can be higher, which is advantageous for the acid production operation in the future. however, oxygen production devices need to be built to increase power consumption. the agents used by Indirect Method B include sulfuric acid, magnesium nitrate, calcium nitrate and zinc nitrate. after years of production practices, almost all of them are now using magnesium nitrate. figure 3-1-15 mg (NO3) 2-h2o system the crystallization curve of magnesium nitrate is a colorless crystal in the Tri-oblique system. After the crystal turns into an aqueous solution, different concentrations can form a variety of crystallization hydrate, figure 3-1-15 shows the crystallization curve of Mg (NO3) 2-h2o system. in the figure, the D point is the critical dissolution temperature. When the concentration of magnesium nitrate solution is 57.8%, the crystallization temperature is 90 ℃. At this time, Mg (NO3) 2 · 6H2O crystals are precipitated. the F point is the melting point, that is, when the concentration of magnesium nitrate solution is 81.1%, the crystallization temperature is 130.9 ℃. At this time, Mg (NO3) 2 and Mg (NO3) 2 · 2H2O crystals are precipitated together. therefore, when selecting the operating temperature of magnesium nitrate, we should avoid these highest points to avoid crystallization of the solution. when magnesium nitrate is dissolved When the concentration is greater than 67.6%, the crystallization temperature increases rapidly with the increasing concentration of the solution. When the concentration of the solution exceeds 81%, the crystallization temperature rises linearly. Under this concentration, the operation can easily cause blockage of the pipeline. therefore, the dehydration Effect of magnesium nitrate concentration is not good, but it is difficult to operate if it is too high. In actual production, it is generally controlled at 64% ~ Between 80%, that is, the concentration of magnesium nitrate cannot exceed 80% (generally 72%), and the outlet of the heater (that is, the concentration of dilute nitric acid after water absorption) is not lower than 64%. The principle of magnesium nitrate concentration is as follows: the concentration is 72% ~ When 74% of magnesium nitrate solution is added to dilute nitric acid, the water in dilute nitric acid is immediately absorbed, and the nitric acid concentration is increased to more than 68.4%. The concentration of magnesium nitrate is reduced to about 65% due to water absorption, at this time, the concentration of HNO3 in the gas phase of the mixed solution of nitric acid and magnesium nitrate is more than 80%, and then the latter is rectified to produce the finished concentrated nitric acid. the HNO3 Vapor Composition produced by the boiling of Mg (NO3) 2-hno3-h2o three-element mixture can be obtained from Figure 3-1-16. so as to determine the minimum amount of magnesium nitrate required for concentrated nitric acid. figure 3-1-16 mg (NO3) 2-hno3-h2o the dehydration of magnesium nitrate at the equilibrium concentration in the gas phase of the ternary system was carried out in a vacuum. the boiling points of Magnesium Nitrate Aqueous Solution at different vacuum levels are shown in Table 3-1-09. table 3-1-09 boiling point of Magnesium Nitrate Aqueous Solution under different vacuum degree % vacuum degree/kPa 80.0 82.7 85.3 88.0 90.7 boiling point/℃ pure magnesium nitrate containing impurity magnesium nitrate 93.3 71.21 73.94 69.69 71.52 72.72 137.7 145.2 143.6 147.1 151.4 134.6 142.0 140.5 143.1 148.0 131.3 138.5 137.0 139.7 144.6 127.3 135.0 133.6 136.0 140.4 124.0 131.2 129.0---132.4--* magnesium nitrate contains Ca (NO3) 2, the method for rectification of Al (NO3) 3, Fe (NO3) 3, and other C hyperboiling acid was developed by espimdesa in Spain, the key technology is to require that the moisture in the gas after the ammonia oxidation reaction be removed as much as possible (the concentration of condensation acid is lower than 2% HNO3 ), the concentration of nitric acid generated in the system after dehydration exceeds the common boiling point of dilute nitric acid. the dehydration reaction gas is oxidized by boiling acid in the oxidation tower, so that no is converted into NO2, and the concentration of 80% ~ is absorbed in the ultra-boiling acid absorption tower ~ 90% of HNO3, and then into the ultra-boiling acid distillation tower, hot acid bleaching White Tower to produce 98% of the finished acid. this method has the advantages of large scale, saving investment, and low operating costs. It is currently the most economical method. The NOx content in the exhaust gas is less than 200 ppm, so it can be directly discharged, it will not cause environmental pollution. the variable costs of the three concentrated nitric acid production methods are compared in Table 3-1-10. in the table (1), the ultra-boiling acid distillation method is used. (2) The dilute nitric acid is produced by the full pressure method (pressure 45MPa), and then concentrated nitric acid is produced by Magnesium nitrate. (3) dilute nitric acid was prepared by double pressurization (oxidation 45MPa, absorption 15MPa) and then concentrated by Magnesium nitrate. as can be seen from table 3-1-10, the variable cost of the hyperboiling acid distillation method is the lowest, which is 118.24 RMB/t lower than that of the full pressure method and 87.83 RMB/t lower than that of the double pressure method, for a plant that produces 50 thousand tons of concentrated nitric acid in a year, the ultra-composite acid distillation can save the production cost by 42 ~ $6 million/. table 3-1-10 variable costs of three concentrated nitric acid production 2. principle of Contact Oxidation of ammonia (1) Chemical Equilibrium of ammonia oxidation the Contact Oxidation of ammonia may generate different products with different reaction conditions and catalysts: apart from the above reaction, it may also produce the following side reactions: Decomposition of Ammonia: Decomposition of 2nh 3 = n2 + 3h2-91.8 kJ/mol nitric oxide: 2no = n2 + O2-180.3 kJ/mol ammonia and nitric oxide for mutual phase use: 4nh 3 + 6no = 5n2 + 6H2O + 1804 kJ/mol (3-1-08 )~ (3-1-09) equilibrium constants of the three reactions at 900 ℃. according to the formula of nermst, the calculation results are as follows: The obtained equilibrium constants kp1, kp2, and kp3 are huge, it indicates that these three reactions are actually three irreversible reactions. kp3 is extremely huge. If there is no catalyst, the ammonia oxidation results will mainly generate nitrogen and water vapor. (2) The Catalysts Used for ammonia oxidation and catalytic mechanism are platinum alloys, commonly used Pt-Rh, Pt-Rh-Pd binary and ternary alloys, and some platinum alloys are also added with Co, metals such as Ni and Mo can reduce the catalyst cost and reduce the volatility consumption of platinum at a high temperature of 1500 ℃. the diameter of the commonly used platinum wire is 0.040 ~ 0.10mm. The free area occupied by platinum wire is 50% ~ 60%. the surface of the platinum net is smooth, elastic, and active. Therefore, it must be activated. the method is to bake with a hydrogen flame to make it loose and rough, thus increasing the contact surface area. dust (various metal oxides) in the air and impurities such as rust and oil may be carried in the ammonia gas, which will be covered on the platinum mesh surface, resulting in temporary poisoning of the catalyst, but water vapor is not toxic to the platinum net, which only reduces the temperature of the platinum net. therefore, raw gas must be purified before mixing. even so, the catalyst is in use 3 ~ Regeneration is still needed after 6 months. The regeneration method is to remove the platinum net from the oxidation furnace and soak it in 10% ~ In the hydrochloric acid solution of 50% ~ Constant Temperature at 70 ℃ 1 ~ 2 h, then separated and then washed to neutral distilled water, and finally the platinum net after drying and burning activation in the hydrogen flame, loaded into the oxidation furnace to participate in the reaction again. the reaction mechanism of catalytic oxidation of ammonia is similar to that of SO2 and O2 on the catalyst of Al2O3, including the following steps: ① due to the strong oxygen adsorption capability of platinum, the carbon bond in the oxygen molecules adsorbed on the platinum catalyst surface is destroyed to generate two oxygen atoms. ② the platinum catalyst surface absorbs ammonia molecules from the gas, and then the nitrogen and hydrogen atoms in the ammonia molecules are combined with oxygen atoms; ③ The electrons are redischarged to generate nitric oxide and water molecules. ④ platinum has little adsorption ability to no and water molecules. They are desorption on the surface of the platinum catalyst and enter the gas phase. studies show that the diffusion of ammonia molecules in the gas phase to the platinum net surface is the control stage of the entire catalytic oxidation process, that is, the entire reaction is controlled by the External Diffusion. (3) Reaction Kinetics of catalytic oxidation of ammonia according to the above reaction mechanism. invalid. jemkin and others export 800 ~ Macro reaction kinetics on the Pt-Rh network between 900 ℃: In formula: concentration of ammonia in C0-ammonia air mixture, %; c1-concentration of ammonia in nitrogen oxide gas after platinum net, %; specific surface area of S-platinum net (activated surface cm2/platinum net cross-sectional area cm2); number of layers of M-platinum net; d-platinum wire diameter, cm; V0-standard gas flow, L/h · cm2 platinum network sectional area. in actual production, where C0, S, M, and D are known, the C1 values in different V0 values can be obtained through the (3-1-11) formula, in order to obtain the reaction conversion rate X: x = (C0-C1)/C0 x including the conversion of ammonia in the main and side reactions, it is greater than the above oxidation rate α-no. the following formula can be used to calculate the time when the ammonia molecule spreads to the platinum net surface. the average length of the Z2/2D (3-1-12) formula and the diffusion coefficient of D-ammonia in air. if ammonia is oxidized at 700 ℃, the platinum wire used is 0.009 cm, and the number of platinum wires with a length of 1 cm is 32, then Z equals 0.010, and D equals 1cm2/s, by (3-1-12 The formula calculates the diffusion time of 5x10-4s. in practice, the reaction temperature is around 800 ℃, and the diffusion time is shortened. this data shows that the reaction speed of no is extremely fast, which can be completed within 10-4 seconds. (4) When selecting the ammonia Contact Oxidation Process for the catalytic oxidation process, first of all, we must ensure a high oxidation rate, which can reduce the consumption of ammonia and the production cost of nitric acid, the oxidation rate can reach 97% ~ under normal pressure ~ 98.5%, up to 96% ~ 97%; second, high production intensity and low platinum consumption should be considered to maximize the working time of the platinum network, so as to achieve operational stability and production continuity. although the formation of Nitric Oxide by ammonia oxidation at a temperature starts at 145 ℃, it reaches 300 ~ At 400 ℃, the amount of nitrogen (N2) and water vapor is still very small, and the yield of Nitric Oxide must reach 97% ~ 98%, the reaction temperature must be no less than 780 ℃. however, the reaction temperature is too high. Due to the decomposition of nitric oxide, the yield of nitric oxide not only does not increase, but may also decrease. When the reaction temperature is higher than 920 ℃, the loss of platinum will be greatly increased (mainly due to the increased volatility of platinum at high temperatures ). generally, the catalytic oxidation temperature of ammonia is controlled to 780 ~ 840 ℃, under pressurization 870 ~ 900 ℃. B pressure from the reaction itself, the operating pressure has no effect on the yield of nitric oxide, pressurization oxidation (such as in 0.8 ~ 0MPa operation) the oxidation rate is 1% to lower than that of normal-pressure oxidation ~ 2%, but the production intensity of platinum catalysts was greatly increased. for example, under normal pressure, each kilogram of platinum catalyst can only oxidized 1.5 tons of ammonia every day and night, while 10 tons of ammonia can be oxidized at 9MPa. the production capacity of the same equipment can be increased by 5 ~ 6 times. But the pressure is too high, which increases the impact of the gas on the platinum net. The mechanical loss of the platinum net (friction, after collision into the powder) increases, so it is generally used 0.3 ~ 5MPa, up to 0MPa abroad. the time when C contact time mixed gas passes through the platinum catalyst layer is called the contact time. in order to ensure that the oxidation rate of ammonia reaches the maximum value, the contact time cannot be too long (that is, the air flow speed is too slow), because this reduces the production capacity of the equipment, and the ammonia is easy to be decomposed into nitrogen, reduce the oxidation rate. the contact time cannot be too short. If the ammonia is too short for oxidation, it will leave the platinum catalyst layer, which will also reduce the oxidation rate. production practices have proved that the contact time should be about 10-4 s under normal pressure, and the pressure should be about 1.55 × 10-4 s. figure 3-1-17 relationship between ammonia oxidation rate and oxygen-ammonia ratio in the ammonia-air mixture composition of the mixture increases the oxygen concentration in the mixture, that is, increases the Atomic Oxygen Concentration on the catalyst surface, it not only strengthens the ammonia oxidation reaction, but also facilitates the oxidation of nitric oxide into nitrogen dioxide. however, the ammonia oxidation reaction is accelerated and the reaction heat increases. If the temperature is not properly controlled, the catalyst will be burned out, and even lead to an explosion accident. figure 3-1-17 shows the relationship between the mixture ratio and the ammonia oxidation rate. as shown in the figure, line 1 shows the theory of completely generating nitric oxide reaction, and line 2 shows the actual production. we can see from curve 2 that Gamma = O2 After the/NH 3 reaches 1.7, the oxidation rate increases little, so the GAMMA value is generally maintained at 1.7 ~ The oxygen surplus is about 2.2 higher than the theoretical value. however, when the reaction temperature is high, such as reaching 800 ℃, the reaction speed is fast enough, and the gamma value can be slightly lower, take 1.5 ~ 1. 6. if a non-platinum catalyst is used, due to its small activity, the gamma value should be greater than 2 to maintain sufficient ammonia oxidation rate, otherwise the oxidation rate will decrease sharply. according to the Gamma value, the concentration of ammonia in the mixed gas can be obtained from 9.5% ~ Range: 11.5%. E explosion and prevention measures when the concentration of ammonia in the mixture reaches a certain level, it may cause an explosion. explosion limits of NH3-O2-N2 hybrid gases are shown in Table 3-1-11. water vapor is incorporated into the mixture to narrow the explosion limit or even disappear. for example, when a mixture contains more than 10% of water vapor, there is no explosion risk at a temperature of 45 ℃. therefore, a certain amount of water vapor is usually added in production, even if the ammonia concentration is increased to 13% ~ 14% is also safe. in order to prevent explosion, the operating conditions must be strictly controlled so that the gas uniformly passes through the platinum network; the contact oxidation equipment should be properly designed; the water vapor should be added; the hidden danger of detonation should be eliminated (such as the equipment should be well grounded, there is no need for iron percussion pipelines and equipment, no shoes with nails, no smoking in the workshop, etc ). table 3-1-11 explosion limit of NH3-O2-N2 Mixture) o2 + N2 mixture in O2 % 20 30 40 60 80 100 lowest 22 31 17 46 18 57 19 64 19 64 18 77 13.5 82 3. the following describes four nitric acid production processes. (1) The typical process of this method is shown in Figure 3-1-18. the oxidation and thermal energy of ammonia A are collected into the filters for ammonia and air to remove impurities harmful to the ammonia oxidation catalyst, such as solid dust and oil mist contained in the gas, after the Purified Gas is mixed by the mixer (the ammonia content in the mixed gas is about 9.5% (V), it enters the ammonia oxide device and is in contact with the platinum-Rh net, 96% ~ 97% (V) of ammonia is oxidized to nitric oxide, and the temperature of the gas also rises ~ At 860 ℃, the temperature of this gas is about 400 ℃ after the steam superheater at the bottom of the ammonia oxide device and the heat recovery from the waste heat boiler. figure 3-1-18 oxidation of NO in the dilute nitric acid process B by double pressurization and absorption of nitric oxide gas from the waste heat boiler and heat recovery by the economizer, It is cooled to about 156 ℃. when the temperature drops, no in the gas is oxidized to NO2, then enters the water cooler (I), and further cools down to 40 ℃. here, nitrogen oxide (NOx) gas reacts with condensate to generate a dilute nitric acid with a concentration of about 34%. the acid gas mixture is separated by a separator, And the dilute nitric acid is sent to the absorption tower. the nitrogen oxide gas from the water cooler (I) is mixed with the secondary air from the White Tower and then compressed to 0MPa (table ). after the gas is cooled to 126 ℃ by the heat exchanger and further cooled to 40 ℃ by the water cooler (II), the nitrogen oxide gas and condensate acid are sent to the oxidation device at the bottom of the absorption tower for further oxidation, the nitrogen oxide gas in the tower is absorbed by water to generate nitric acid, and a cooling coil is arranged on the plate of the absorption tower to remove the absorption heat and oxidation heat. When the liquid in the tower is flowing down by plate, the nitrogen oxide gas is fully exposed, increasing acid concentration and collecting at the bottom of the tower The acid concentration is 65% ~ 67%. c bleaching from the absorption tower ~ 65% ~ Many NOx gases are dissolved in 67% of nitric acid and sent to the top of the floating White Tower. the NOX gas is blown out of nitric acid by the secondary air. The resulting acid concentration is 60%, containing hno2 <0.01%, the temperature is 62 ℃. After being cooled to about 50 ℃, it is sent to the acid storage tank of the finished product. the exhaust gas from the top of the absorption tower is heated to about 360 ℃ by the tail gas preheater. The hot gas enters the exhaust gas turbine, and about 60% of the total compression work can be recycled. Finally, the exhaust gas cylinder is discharged into the atmosphere. the NOx content in the exhaust gas discharged into the atmosphere is about 180 ppm. (2) The direct process of producing concentrated nitric acid is shown in Figure 3-1-19. the following is a brief description: Figure 3-1-19 is a direct method for the synthesis of the contact reaction of the ammonia of concentrated nitric acid A and the preliminary oxidation of ammonia of nitric oxide on the platinum catalyst, it is oxidized with oxygen in the remaining air to generate nitrogen dioxide with an oxidation rate of over 90%. the heat generated during oxidation is taken away with nitric acid with a concentration of about 65% in the upper part of the smoke nitric acid absorption tower (washing Section). nitric acid is diluted to 55% and sent to the mixing tank. the reoxidation of Nitric Oxide B and absorption of nitrogen dioxide smoke absorption tower are divided into three sections. the lower part is In the heavy oxidation stage, the nitric oxide in the gas is oxidized to almost all nitrogen dioxide by nitric acid with a concentration of 98%. At the same time, nitric acid is diluted to 75%, and the middle stage is the absorption segment of smoke nitric acid, use 98% concentrated nitric acid, which is cooled to-10 ℃ as absorbent. After absorption by concentrated nitric acid, it becomes smoke nitric acid containing 30% free NO2. smoke nitric acid is sent to the White Tower at the bottom of the middle section, and the frozen brine in the upper tube of the sieve plate is taken away for reaction heat. the upper part is the washing section. After the nitrate foam in the tail gas is washed with condensate, it becomes 65% dilute nitric acid. This acid is sent to the oxidation tower. the tail gas also contains 0.2% (2000 ppm) of nitrogen oxide, which is sent to the dilute nitric acid production system for energy recovery. After treatment, the chimney is discharged into the atmosphere. C Nitrogen Dioxide desorption contains 30% free NO2 of concentrated nitric acid to the White Tower, heat desorption, the release of NO2. concentrated nitric acid discharged from the bottom of the tower, after cooling can be used for smoke Nitric Acid Absorption segments for recycling. the top of the tower is pure nitrogen dioxide. After the preliminary cooler is cooled by water to remove the acid mist, it enters the tetroxide two nitrogen condenser, and uses frozen brine to compress the liquid tetroxide two nitrogen and sends it to the mixing tank. D synthesis of concentrated nitric acid mixing tank with liquid nitrogen oxide and dilute nitric acid from various places, with n204: HNO3: H 2O = mixture, in the case of full mixing with pumping to the high-pressure kettle, 5. 0MPa pressure and 70 deg C, the mixture and pure oxygen reaction to generate concentrated nitric acid. therefore, the discharged concentrated nitric acid Contains 25% n204 (hot acid), which is sent to the middle of the Bleaching Tower for desorption. Finally, it becomes the final acid (98% concentrated nitric acid ). (3) The indirect production of concentrated nitric acid process the indirect production of concentrated nitric acid process includes two parts: Manufacturing of dilute nitric acid and concentration of dilute nitric acid. The preceding five production methods can be used together. figure 3-1-20 shows the concentrated process of dilute nitric acid. The dehydrating agent used is magnesium nitrate. the description is as follows. figure 3-1-20 process flow of concentration of dilute nitric acid by Magnesium nitrate method 72% ~ 76% of the concentrated magnesium nitrate solution and dilute nitric acid are 4 ~ The 6ratio is added to the top of the Self-extract distillation tower after mixing in the mixer 7. This tower is a stuffing tower with a tower temperature of 115 ~ 130 ℃, the required heat is provided by the heater 13 located in the lower part of the tower. Contains 80% ~ 90% HNO3 steam escaped from the distillation tower and entered the rectification column 11. The tower is also a stuffing tower, and the temperature of the tower is 80 ~ 90 deg C 98% HNO3 steam, the nitric acid condenser 1 into the acid distributor 9, here the gas is discharged to the dilute nitric acid system by the fan 23, part of the liquid (2/3) for reflux, part (1/3) after Bleaching, 98% of the acid is obtained. The dilute magnesium nitrate solution from the bottom of the distillation tower enters the heater 13. heating with 3MPa indirect steam, in 174 ~ Denitration (NO2 removal) at 177 ℃. The generated steam is used as the heat source of the distillation tower. The concentration of dilute magnesium nitrate in the discharge heater 13 is 62% ~ 67%, with a liquid pump into the membrane evaporator 16 for evaporation. After concentration, magnesium nitrate concentration to 72% ~ 76% flow into magnesium nitrate storage tank 22 for recycling. the steam from the membrane evaporator is directly cooled into the atmosphere condenser with 15 water. The cooling water enters the circulating pool 21, the exhaust gas is pumped out by the jet pump, and the excess acid water in the circulating pool is sent to the wastewater treatment system. in recent years, many nitric acid manufacturers in China have improved the process shown in Figure 3-1-20 and achieved good results: ① change atmospheric condenser 15 from direct condenser cooling to indirect condensation cooling, the waste water volume ranges from 140 ~ 180 T/T acid is reduced to 1.2 T/T acid, and the waste acid concentration reaches 2.5%. It can be sent to the dilute nitric acid absorption tower for water absorption, or to the waste acid processing system for processing. Due to the small amount, the concentration is high, the neutralization processing device is small and efficient. ② cancel the centrifugal pump 8 for transporting dilute magnesium nitrate and use vacuum suction. feeding with a centrifugal pump requires power consumption. When the heavy nitric acid system stops, the dilute magnesium nitrate feeding pump is required to be emptied (otherwise, after the liquid cooling, magnesium nitrate will crystallization and precipitation, blocking the pipeline and valve parts ), this will not only increase the consumption of dilute magnesium nitrate, but also pollute the surrounding environment. change to vacuum loading. Once the nitric acid system stops, the vacuum can be destroyed and the materials can flow back to the dilute nitric acid storage tank. ③ Change the floating White Tower 14 to the hot Denitration tower. although the air Denitration (also called cold Denitration) process is simple, the wet content in the air has an effect on the concentration of concentrated nitric acid products, resulting in a large amount of nitrate (NO2) tail gas, not easy to handle. now, an Denitration segment (or a Denitration Tower) is added in the upper part (or next to) of the rectification tower, and 80 ~ HNO3 steam above 90 ℃ is used to remove the NO2 from the nitric acid distributor, while the nitric acid steam carries the NO2 into the nitric acid condenser, And the nitric acid in the denitration liquid is the finished acid delivery system. ④ change fan 23 to a water jet pump. In this way, there will be no exhaust gas containing nitrate in the concentrated nitric acid production system for processing, and a dilute acid with a concentration of 5.0% can be obtained, which can be treated as ①. through the above measures, the emissions of tail gas and dilute acid are greatly reduced, operation costs are reduced, and product quality is improved. (4) The udaic process of udaic by udaic acid is shown in Figure 3-1-21. it is divided into oxidation process, absorption and distillation process two parts. raw liquid ammonia is extracted from liquid ammonia storage tank (v101) with a pump (p102), filtered by Liquid Ammonia filter (s103), and then sent to the sub-boiling absorption tower (T202) as the cooling medium, at the same time, the liquid ammonia is converted into gas ammonia. the air enters the air compressor section of the "Four in one" unit (c118) through the air filter (s104, s105), and the outlet air pressure is 45MPa MPa. After the gas heat exchanger (e115, enter the ammonia air mixer (s106) together with the gas ammonia and enter the oxidation furnace through the mixed filter (s107) (R108). No is generated under the catalysis of Pt-Rh catalyst. The reaction gas temperature of the oxidation furnace is about 820 ~ 850 ℃, after the exhaust gas heater (e109), waste heat boiler (E110) and quick pot cooler (E111), the gas is cooled to 49 ℃, the water produced during the reaction is condensed. condensation water contains a small amount of nitric acid and NOX gas. After separation by the condensate acid separator (v114), the dilute acid water is removed from the White Tower (t121 ). the NOx dissolved in acid is extracted from the secondary air sent to the bottom of the tower, and the gas is combined with the output gas of the oxidation tower (t116. the acid water is decompressed and sent to the acid water storage tank (v122). Some of the acid water pumps (P203) are used in a co-boiling absorption tower (T202) for recycling, and the rest are sent to the acid water treatment tank. the gases separated from the condensate acid separator (v114) are heated to 160 ℃ by the gas heat exchanger (e115) to enter the oxidation tower (t116). The dilute nitric acid with a concentration of 58% is sent to the top of the oxidation tower, gas-Liquid in contact with the oxidation Tower, no is oxidized to NO2, nitric acid is decomposed into NO2. after the reaction of nitric acid concentration reduced to 25%, with the acid pump (p117) into the co-boiling tower (T202 ), 58% of the acid is recycled to the oxidation tower (t116 ). in a co-boiling absorption tower, no is oxidized to NO2, while NO2 is absorbed by water to generate nitric acid. This exothermic reaction is carried out at 15MPa. The tower is equipped with a snake tube cooler, and the cooling medium is liquid ammonia. liquid Ammonia gasification absorbs reaction heat at 6MPa pressure, so that the top temperature of the tower is controlled at 20 ℃, the NOx content in the exhaust gas is less than 200 ppm (the emission standard set by the state is 500 ppm ). the exhaust gas of the exit tower passes through the gas heat exchanger (e120), enters the exhaust gas heater (e109), and then enters the exhaust gas turbine Recovery Section of the "Four in one" unit (c118) for energy recovery, and passes through the chimney (s119) discharged into the atmosphere. the NO2-rich gas from the top of the oxidation tower (t116) is similar to the acid water Bleaching Tower (t121), the acid Bleaching Tower (t204), and the acid Bleaching Tower (t208) after bleaching, the NOx-containing secondary air is mixed and the mixture enters the oxidation nitrogen compression section of the "Four in one" unit (c118) and is compressed to 2MPa, the gas is then cooled by the gas exchanger (e120) and water cooler (e207), and then sent to the ultra-co-boiling absorption tower (t201 ). here, the NO2 in the gas is absorbed, and the remaining gases (NO, NO2, etc.) are sent to the next co-boiling absorption tower (T202) for absorption. the nitric acid concentration at the bottom of the vacuum distillation column (t212) is 70%, which is higher than the common boiling point (68.8%). It is pumped by heat exchanger (e211), (e209) (p210) it is sent to the top of the ultra-boiling absorption tower (t201) to generate nitric acid by using NO2 in the gas in the absorption tower. the material at the bottom of the ultra-co-boiling absorption tower (t201) is nitric acid with a concentration of 80%, which contains partially dissolved NO2. After decompression to 4MPa, it enters the strong acid bleaching.