A fluorobenzene synthesis tail gas nitrogen oxide resource recycling system

Abstract

The application discloses a fluorobenzene synthesis tail gas nitrogen oxide resource recycling system, which comprises a synthesis reaction kettle, wherein the upper opening of the synthesis reaction kettle is sequentially connected with a kettle cover, a deep cooling heat exchanger, a deep cooling tail gas heat exchanger and an activated carbon adsorption tower, and the activated carbon adsorption tower is connected with an exhaust tail gas pipeline through an air draught fan / return cooling fan. The system is simple, clean, environmentally friendly, stable, efficient and high in nitrogen oxide recycling efficiency.

Description

Technical Field

[0001] This invention belongs to the field of chemical waste gas treatment, and relates to the treatment of waste gas pollutants generated in the production process of the organic fluorine chemical industry. Specifically, it is a resource-based full recovery system for nitrogen oxides in fluorobenzene synthesis waste gas. Background Technology

[0002] In the synthesis of fluorobenzene using NaNO2 as one of the raw materials, the side reaction 2HNO2→NO+NO2+water results in high concentrations of nitrogen oxides (NOx) in the tail gas. Nitrogen oxides are one of the main causes of environmental problems such as acid rain, ozone depletion, and photochemical smog. NOx reacts with ozone in the ozone layer, consuming large amounts of ozone. NOx then reacts with ozone in the stratosphere to produce nitric oxide and oxygen. Nitric oxide further reacts with ozone to produce nitrogen dioxide and oxygen, thus disrupting the ozone balance, reducing ozone concentration, and leading to ozone layer depletion. The harm of nitrogen oxides to humans mainly occurs through irritating the lungs, making people more susceptible to respiratory illnesses such as colds. People with respiratory problems, such as asthma patients, are more vulnerable to nitrogen dioxide. Nitric acid and nitrite, produced by the reaction of nitrogen oxides with water in the air, are components of acid rain. Nitric oxide in vehicle exhaust reacts with hydrocarbons under ultraviolet radiation to form toxic smog, known as photochemical smog. Photochemical smog has a distinctive odor, irritates the eyes, damages plants, and reduces atmospheric visibility. Therefore, the state has established strict emission limits for nitrogen oxide (NOx) concentrations in waste gas and flue gas.

[0003] The concentration of nitrogen oxides (NOx) in industrial flue gas is relatively low, and the treatment processes are quite mature, mainly focusing on SCR, SNCR, and activated carbon methods. However, the concentration of NOx in the tail gas of fluorobenzene synthesis is dozens of times higher than that in industrial flue gas, and traditional industrial flue gas NOx treatment equipment cannot withstand such a high concentration of NOx. Based on the characteristics of nitrogen dioxide (NOx) being easily soluble in water and nitric oxide (NOx) being easily oxidized, some enterprises use "alkaline absorption + oxidation" to treat the high-concentration, low-volume tail gas of fluorobenzene synthesis. First, a certain amount of air is added to the NOx waste gas entering the purification tower to ensure that there is sufficient air for the oxidation of nitric oxide. In the first stage, water is used as the absorbent. NO2 is absorbed by water to generate nitric acid and nitric oxide, i.e.: 3NO2 + H2O → 2HNO3 + NO. In this reaction, 2 / 3 of the NO2 is converted into HNO3, and 1 / 3 of the NO2 is converted into NO, which then reacts with oxygen to generate NO2, which is then absorbed by water. However, water absorption can only serve as a preliminary pretreatment. After water absorption, soda ash or sodium hydroxide solution is used as an absorbent for further absorption. But because the process of water absorbing nitrogen dioxide is a redox reaction, only 1 / 3 of the nitrogen dioxide is absorbed, while almost no nitric oxide is absorbed, resulting in exhaust gas that cannot meet emission standards. On the other hand, the "alkaline absorption + oxidation" treatment process is lengthy, and the sodium nitrate-containing wastewater generated during the absorption process requires further treatment, making this technology difficult to promote and apply [Lü Zhimin, Zhang Yuqing, Tang Anping, et al. Clean Production Process of Fluorobenzene. Chemical World, 2002.12].

[0004] On the other hand, the synthesis of fluorobenzene is carried out in an excess of anhydrous hydrogen fluoride solution, and the synthesis tail gas also contains a high concentration of HF component, which further increases the difficulty of treating the tail gas. The effective treatment of this tail gas has become an urgent problem for fluorobenzene production enterprises. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned technical problems and provide a complete recovery system for nitrogen oxides in fluorobenzene synthesis tail gas that features a simple process route, is clean and environmentally friendly, stable and efficient, and recovers nitrogen oxides.

[0006] The method of the present invention includes a synthesis reactor, the upper opening of which is connected to an activated carbon adsorption tower via a reactor cover, a cryogenic heat exchanger, a cryogenic tail gas heat exchanger, and then to an external exhaust gas pipeline via an HX3-induced draft fan / reflux cooling fan.

[0007] The activated carbon adsorption tower consists of at least three connected in parallel to form an activated carbon adsorption unit. The top of the activated carbon adsorption tower is provided with a purified exhaust gas outlet, a superheated steam inlet, and a cooling gas outlet. The cooling gas outlet is connected to the purified exhaust gas outlet. The purified exhaust gas outlets of all activated carbon adsorption towers are connected in parallel and then connected to an induced draft fan / return cooling fan.

[0008] The activated carbon adsorption tower is equipped with a cryogenic tail gas inlet and a desorption tail gas outlet at the bottom. The desorption tail gas outlet is connected to a spray cooling tower via a desorption tail gas induced draft fan.

[0009] The lower part of the spray cooling tower is provided with a desorption tail gas inlet and a low-temperature spray water outlet; the upper part of the spray cooling tower is provided with a desorption tail gas outlet and a low-temperature spray water inlet; and the low-temperature spray water outlet is connected to the low-temperature water circulation pump and the low-temperature water heat exchanger through the channel; the desorption tail gas outlet is connected to the HX6-condensing tail gas fan through the desorption tail gas condenser.

[0010] The outlet of the condensate exhaust fan is connected to the bottom air inlet of the synthesis reactor, the cryogenic exhaust outlet of the cryogenic heat exchanger, and the nitric oxide purifier, respectively.

[0011] The nitric oxide purifier has a superheated steam inlet and a nitric oxide gas outlet at one end, and a desorption tail gas inlet, a secondary desorption tail gas outlet, and a cooling gas inlet at the other end.

[0012] The nitric oxide gas outlet is connected to the nitric oxide storage tank via a gas compressor, and the gas compressor outlet is also connected to the cooling gas inlet of the nitric oxide purifier.

[0013] Each of the aforementioned synthesis reactors corresponds to at least three of the aforementioned cryogenic heat exchangers. The upper opening of the synthesis reactor is connected to the reactor lid via a positioning frustum on the reactor body and a positioning frustum on the reactor lid. The three aforementioned cryogenic heat exchangers are installed on the reactor lid to form a cryogenic unit. The lower end of the tube side of the cryogenic heat exchanger is directly connected to the synthesis reactor, and the upper end of the tube side of the cryogenic heat exchanger is connected to the activated carbon adsorption tower via the HC15-cryogenic tail gas heat exchanger.

[0014] Three cryogenic heat exchangers operate in an alternating condensation-condensation-defrost cycle. The condensation-condensation-defrost process is controlled by setting upper and lower limits for the pressure difference between the inlet and outlet exhaust gases of the cryogenic heat exchanger. When the pressure difference between the inlet and outlet exhaust gases of the cryogenic heat exchanger is higher than the upper limit, the refrigerant is cut off, and the cryogenic heat exchanger enters the defrost stage. When the pressure difference between the inlet and outlet exhaust gases of the cryogenic heat exchanger is lower than the lower limit, the refrigerant is introduced, and the cryogenic heat exchanger enters the cryogenic operation stage. This alternation occurs continuously.

[0015] Each cryogenic heat exchanger is equipped with multiple enhanced cooling sections, and the upstream and downstream of each enhanced cooling section are connected by a refrigerant circulation pump.

[0016] The outlet of the cryogenic heat exchanger is also connected to the bottom air inlet of the synthesis reactor via a cryogenic exhaust gas circulation fan. The cryogenic exhaust gas is bubbled in to stir and adjust the temperature of the synthesis reactants.

[0017] Each activated carbon adsorption unit corresponds to multiple synthesis reactors and multiple cryogenic units.

[0018] Three activated carbon adsorption towers are used in parallel and alternately for adsorption, desorption, and standby, respectively. By setting an alternating use cycle, the towers are switched periodically. Superheated steam is used to desorb the activated carbon adsorption towers after completing one adsorption cycle. The desorption tail gas is condensed and dehydrated before being fed into the synthesis reactor to participate in the synthesis reaction, or it can be defrosted in a cryogenic heat exchanger before being fed into the synthesis reactor to participate in the synthesis reaction.

[0019] The desorbed tail gas enters a low-temperature water spray cooling tower, and after being cooled by low-temperature water spray, it enters a condenser for further cooling and dehydration. The condensate is then circulated back into the spray cooling tower. By controlling the temperature of the superheated steam, the amount of condensate introduced by the superheated steam is reduced. Through circulation and concentration, hydrofluoric acid product with a hydrogen fluoride mass concentration of over 40% is obtained.

[0020] The desorption tail gas is divided into two parts. In addition to the part that is recycled back into the synthesis reactor, the part is led out by the gas compressor and purified by the nitric oxide purifier before being stored in the nitric oxide storage tank as a nitric oxide product to maintain the balance of tail gas volume and composition in the production process.

[0021] The synthesis tail gas is indirectly cooled to below -80°C via a cryogenic heat exchanger using a low-temperature refrigerant below -100°C. This process condenses and intercepts over 99.99% of fluorobenzene, nitrogen dioxide, and over 90% of hydrogen fluoride, as well as some nitric oxide. The cryogenic tail gas exiting the cryogenic heat exchanger enters an activated carbon adsorption tower. After adsorption in the activated carbon adsorption tower, nitric oxide, hydrogen fluoride, and residual trace amounts of fluorobenzene and nitrogen dioxide in the cryogenic tail gas are adsorbed. The adsorbed tail gas after adsorption in the activated carbon adsorption tower is divided into two parts. One part is circulated back into the activated carbon adsorption tower after desorption to cool the higher-temperature activated carbon bed after desorption. The remaining part is directly discharged.

[0022] The refrigerant circulation pump enhances flow, the refrigerant is circulated in stages, the refrigerant circulation flow rate is controlled, and the temperature of the cryogenic exhaust gas exiting the cryogenic heat exchanger and the refrigerant temperature exiting the cryogenic heat exchanger are adjusted. The temperature of the exhaust gas exiting the cryogenic heat exchanger is controlled below -80℃, and the temperature of the refrigerant exiting the cryogenic heat exchanger is controlled at around -30℃. In order to deeply recover the low-temperature cold energy of the process, the cryogenic exhaust gas exiting the cryogenic heat exchanger at below -80℃ is exchanged with the refrigerant at around -30℃, and then circulated back to the refrigeration unit after the heat exchange.

[0023] This invention breaks through the traditional concept of nitrogen oxide treatment, transforming traditional removal and purification into recycling. It creatively proposes a technical solution for cryogenic separation and adsorption recovery of fluorobenzene synthesis tail gas. Specifically, the synthesis tail gas is cryogenically cooled, adsorbed by activated carbon, and then discharged. Saturated activated carbon is then desorbed, and the desorbed tail gas is purified and recovered, achieving resource recovery of nitric oxide. Other gases are returned to the synthesis reactor for reuse. The main technical solutions are as follows:

[0024] (1) Utilizing the differences in physical properties of pollutants in synthesis tail gas for cryogenic separation and reuse. Through cryogenic cooling below -80℃, almost all components such as NO, HF, fluorobenzene, and NO2 in the synthesis tail gas are condensed and returned to the synthesis reactor to participate in the synthesis reaction again, or recovered as a product. Nitric oxide is also partially condensed.

[0025] (2) Nitric oxide is recovered by adsorbing the components of the synthetic tail gas in stages based on their polarity differences. The first adsorption of the "escaped gas" is purified tail gas (mainly nitrogen) for discharge, and the second adsorption of the "escaped gas" is high-purity nitric oxide, which is recovered as a product.

[0026] The pollutants in the synthesis tail gas have different polarities. Nitric oxide, nitrogen dioxide, fluorobenzene, and hydrogen fluoride are all polar molecules, with hydrogen fluoride being the most polar (the polarity of a covalent bond between two atoms depends on the difference in electronegativity between the two atoms; the greater the difference in electronegativity, the more polar the covalent bond). Nitric oxide has a linear molecular configuration and relatively low polarity; the greater the polarity, the easier it is to be adsorbed. Because the concentration of nitric oxide in the desorbed tail gas is high, accounting for the majority, trace amounts of nitrogen dioxide, fluorobenzene, and hydrogen fluoride are adsorbed during secondary adsorption, resulting in high purity nitric oxide after secondary adsorption.

[0027] The effects of this improvement are as follows:

[0028] (1) Turning waste into treasure to obtain high-purity nitric oxide products. Activated carbon is used to adsorb the cryogenic tail gas after cryogenic treatment. The highly polar nitric oxide in the cryogenic tail gas, as well as trace amounts of HF, fluorobenzene, NO2 and other components remaining from cryogenic treatment, are also adsorbed. The remaining nitrogen gas is directly discharged. The adsorbent is then desorbed, and the desorbed tail gas is further purified to obtain nitric oxide products.

[0029] (2) Obtain high-concentration HF products and improve the recovery rate of fluorine. By adjusting the process steam temperature used in the desorption process and reducing the amount of condensate introduced, high-concentration HF products are obtained by cooling and condensing the desorption tail gas.

[0030] (3) Zero emission of pollutants in the synthesis tail gas. After the desorption tail gas is cooled and condensed to recover HF products, the remaining components, except for the obtained nitric oxide product, are all reused in the synthesis reactor.

[0031] (4) Improved system operational stability. By monitoring the gas phase pressure within the system, the output of nitric oxide products is adjusted to maintain stable tail gas pressure and a balanced tail gas volume within the system. After condensation, almost all nitrogen dioxide is intercepted and enters the synthesis reactor. A small amount of nitrogen dioxide and most of the nitric oxide leave the synthesis reactor and are finally adsorbed by the activated carbon adsorption tower. Through desorption and condensation, some of the adsorbed nitrogen dioxide dissolves in low-temperature water and generates a certain amount of nitric oxide, which enters the desorption tail gas. At the same time, all the adsorbed nitric oxide enters the desorption tail gas. Therefore, there is a surplus of nitric oxide, and a portion needs to be extracted and stored as product to maintain the balance of tail gas volume and composition in the production process.

[0032] (5) Improve the utilization and conversion rate of raw materials. During the cryogenic process, most of the nitrogen dioxide is condensed back into the synthesis reactor, increasing the nitrogen dioxide concentration in the reactor liquid, inhibiting the occurrence of the side reaction <2HNO2→NO+NO2+water>, improving the utilization rate of HNO2, and thus improving the conversion rate of the raw material NaNO2. At the same time, HF is also condensed back into the synthesis reactor, improving its utilization rate.

[0033] Furthermore, since the cryogenic temperature of the synthesis exhaust gas is lower than or close to the melting points of nitrogen dioxide, fluorobenzene, and HF during cryogenic treatment, this invention creatively proposes an internal refrigerant circulation scheme for the cryogenic heat exchanger to avoid or mitigate system blockage caused by frost, based on the cryogenic treatment of the exhaust gas. This forms an enhanced cooling section within the cryogenic heat exchanger. By adjusting the circulation rate, different cooling effects can be obtained, achieving the condensation rate of different pollutant components and improving the condensation and interception effect of pollutant components in the synthesis exhaust gas.

[0034] Among these pollutants, nitrogen dioxide has a melting point of -11℃ and a boiling point of 21℃; nitric oxide has a melting point of -163.6℃ and a boiling point of -151℃; fluorobenzene has a melting point of -42℃ and a boiling point of 85℃; and HF has a melting point of -83℃ and a boiling point of 19.54℃. When the exhaust gas is cryogenically cooled to below -80℃, HF, nitrogen dioxide, and fluorobenzene almost completely condense. During the condensation process, HF, nitrogen dioxide, and fluorobenzene will frost up inside the cryogenic heat exchanger, blocking the exhaust gas flow channels. To mitigate the frost clogging problem, this invention makes the following improvements:

[0035] (1) The exhaust gas enters the cryogenic heat exchanger and exchanges heat with the refrigerant in the opposite direction. The refrigerant is forced to circulate back in the cryogenic heat exchanger, forming enhanced cooling sections corresponding to hydrogen fluoride, fluorobenzene and nitrogen dioxide respectively. By setting up a refrigerant circulation pump, the higher temperature refrigerant downstream is returned to the upstream, forming a condensation section with a small temperature difference corresponding to the melting point temperature of the pollutant components, so as to ensure that the pollutant components are fully condensed and do not solidify and blockage.

[0036] (2) Defrosting is achieved by circulating the higher-temperature gas within the exhaust gas purification system. The higher-temperature desorbed exhaust gas from the activated carbon adsorption tower serves as the defrosting heat source, which not only recovers the fluorobenzene in the desorbed exhaust gas but also provides efficient defrosting.

[0037] The effects of this improvement are as follows:

[0038] (1) Stepwise condensation and recovery of contaminant components reduces frost blockage during the cryogenic process and improves system operational stability.

[0039] (2) Forced circulation of refrigerant is carried out to make full use of the cold energy of refrigerant, increase the temperature of refrigerant exiting the cryogenic heat exchanger, and then exchange heat with the cryogenic tail gas at a lower temperature (-80℃) after cryogenic cooling, thereby improving the efficiency of refrigerant cold energy utilization.

[0040] (3) Fully recover the cold energy from frost formation.

[0041] (4) No external defrosting medium is required to achieve efficient defrosting.

[0042] (5) During the cryogenic process, most of the nitrogen dioxide condenses back into the synthesis reactor, which increases the concentration of nitrogen dioxide in the reactor liquid, inhibits the occurrence of the side reaction <2HNO2→NO+NO2+water>, and improves the utilization rate of HNO2.

[0043] Furthermore, this invention innovatively proposes the following three improved technical solutions for the activated carbon adsorption unit in the extreme recovery of fluorobenzene:

[0044] (1) Adsorption tail gas circulation cooling. 10-20% by volume of the adsorption tail gas (purified tail gas) from the activated carbon adsorption tower is refluxed back into the activated carbon adsorption tower after desorption to cool the activated carbon adsorption layer. The adsorption tail gas, after being heated by heat exchange in the activated carbon layer, is mixed with the remaining 80-90% of the adsorption tail gas before being discharged. The reason for cooling the activated carbon adsorption layer after desorption is that the lower the temperature of the activated carbon during adsorption, the more favorable the adsorption is. This avoids the situation where, in the early stage of adsorption, due to the high temperature of the activated carbon adsorption layer, the cryogenic tail gas is heated upon entry, resulting in a fast tail gas flow rate, short adsorption time, and a decrease in the adsorption rate of fluorobenzene, etc. The reason for using 20-30% of the adsorption tail gas for reflux cooling is to cool the activated carbon at a lower cooling rate, avoiding rapid cooling that would affect the service life of the activated carbon.

[0045] (2) After the desorption tail gas (mainly composed of fluorobenzene and nitrogen oxides) after desorption by activated carbon is condensed and dehydrated, it is then heated by heat exchange with the purified tail gas after denitrification by SCR, and then returned to the synthesis reactor for stirring and temperature adjustment.

[0046] (3) Control the temperature of the superheated steam used for desorption.

[0047] The effects of this improvement are as follows:

[0048] (1) The cold and heat sources of the internal medium of the system are fully utilized. The cold energy of the adsorption tail gas at a lower temperature is recovered by cooling the activated carbon adsorption tower after desorption through the reflux of the adsorption tail gas; the heat enthalpy of the purified tail gas at a higher temperature is recovered by exchanging heat with the desorption tail gas and the purified tail gas after SCR denitrification; the heat enthalpy required for heating and pyrolysis of the synthesis reaction vessel liquid is saved by stirring the desorption tail gas after heat exchange and heating.

[0049] (2) Fully recover and utilize pollutants such as fluorobenzene and nitrogen oxides contained in exhaust gas.

[0050] (3) Obtain a high concentration of hydrofluoric acid solution. During the desorption and regeneration of the activated carbon adsorption tower, control the temperature of the superheated steam used for desorption and reduce the amount of condensate introduced to obtain the hydrofluoric acid product with the required HF concentration.

[0051] Furthermore, this invention takes advantage of the varying temperatures of the reactor liquid at different stages of the fluorobenzene synthesis process. A portion of the desorption tail gas is directly introduced into the synthesis reactor, while a portion of the cryogenic tail gas is also introduced. The reactor liquid temperature is regulated by these two gases, saving the amount of refrigerant required for temperature control via the reactor jacket in existing processes. It also simplifies the complex adjustment methods used in existing processes, which rely on adjusting the temperature of the introduced refrigerant. Existing synthesis reactors have a jacketed structure, with a -15°C refrigerant circulated within the jacket to cool the reactants. The circulating cryogenic tail gas not only provides stirring but also cooling; the introduced desorption tail gas provides both stirring and heating.

[0052] By circulating two exhaust gases into the reactor, the temperature adjustment process is simple, fast, and energy-efficient, while also recovering and utilizing pollutants from the exhaust gases.

[0053] Furthermore, by cooling and condensing the desorption tail gas to recover residual HF, a hydrofluoric acid solution with a mass concentration of over 40% is generated, achieving complete recovery of the system's fluorine components and zero external discharge. This is because, although some HF is absorbed during spray cooling, the desorption tail gas still contains a high concentration of HF, NO, and small amounts of NO2 and fluorobenzene after dehydration. After dehydration, the gas returns to the synthesis reactor and, along with the synthesis tail gas, re-enters the cryogenic heat exchanger for condensation and recovery of HF, NO2, and fluorobenzene.

[0054] This invention can achieve full recovery of polluting components in synthesis tail gas and generate high-value-added product nitric oxide gas. It also fully recovers waste heat and waste cooling capacity. The process route is relatively simple, environmentally friendly, and has low equipment investment and operating costs. Attached Figure Description

[0055] Figure 1 : This is a system diagram of the present invention.

[0056] Among them, HC1 - Synthesis reactor; HC2 - Refrigerant circulation pump; HC3 - Cryogenic heat exchanger; HC4 - Cryogenic tail gas outlet; HC5 - Reactor lid positioning frustum; HC6 - Desorption tail gas injection port; HC7 - Reactor lid; HC8 - Reactor lid feeding port; HC9 - Reactor body positioning frustum; HC10 - Valve; HC11 - Reactor body; HC12 - Reactor body discharge port; HC13 - Reactor bottom air inlet; HC14 - Cryogenic tail gas circulation fan; HC15 - Cryogenic tail gas heat exchanger; HX 1-Activated carbon adsorption tower; HX2-Regulating valve; HX3-Exhaust fan / recirculation cooling fan; HX4-Spray cooling tower; HX5-Low-temperature water heat exchanger; HX6-Condensation tail gas fan; HX7-Desorption tail gas condenser; HX8-Low-temperature water circulating pump; HX9-Hydrofluoric acid product tank; HX10-High-concentration hydrofluoric acid product tank; HX11-Nitric oxide purifier; HX12-Gas compressor; HX13-Nitric oxide storage tank; HX14-Desorption tail gas exhaust fan. Detailed Implementation

[0057] Taking a certain factory's 60-unit HC1 production line for fluorobenzene synthesis reactors as an example, the annual production of fluorobenzene products is 4,000 tons.

[0058] The following explanation, in conjunction with the accompanying drawings, further illustrates a system for the complete recovery of nitrogen oxides from fluorobenzene synthesis tail gas:

[0059] The synthesis process of fluorobenzene is as follows:

[0060] Each synthesis reactor (HC1) produces in batches. The main processes of fluorobenzene synthesis include salt formation, diazotization, and pyrolysis, all of which are completed within the synthesis reactor (HC1).

[0061] (1) Salt formation

[0062] Low-temperature hydrofluoric acid liquid (-15℃) was metered into the synthesis reactor HC1, and then cooled with chilled brine until the temperature of the synthesis reactor HC1 dropped to between 4 and 6℃. Aniline was then slowly added dropwise to the synthesis reactor HC1 with stirring, while maintaining the temperature between 5 and 8℃ during the addition. The reaction process was an excess reaction of hydrofluoric acid under a slightly negative pressure. After the addition was completed, the amount of chilled brine was adjusted, and the temperature was maintained at around 6℃ for 30 minutes to complete the reaction. The reaction time was 6 hours, yielding a mixture of aniline hydrofluoric acid salts.

[0063] (2) Diazotization

[0064] Lower the temperature of the HCl circulating chilled brine in the synthesis reactor to 0 to -2℃, and slowly add solid sodium nitrite while stirring. Control the feeding temperature at 0 to 5℃ and maintain a slight negative pressure. After the feeding is completed, keep the temperature for 30 minutes to end the reaction. The reaction time is 10 hours, and diazonium solution is obtained.

[0065] The diazotization process involves a side reaction: 2HNO2 → NO + NO2 + water.

[0066] (3) Pyrolysis

[0067] Adjust the temperature of the chilled brine and, with stirring, slowly raise the temperature of the HCl solution in the synthesis reactor in stages (5-15℃, 15-25℃, 25-38℃), increasing the temperature by approximately 1.2℃ per hour, until it reaches 38℃. Maintain a slightly negative pressure to initiate thermal decomposition, which takes 32 hours. After the reaction, the material in the reactor is transferred to a settling tank to separate into layers: the organic layer is a mixture of fluorobenzene; the inorganic layer contains hydrofluoric acid and sodium fluoride, etc.

[0068] The reaction temperature varies at different stages of the reaction and needs to be adjusted according to the reaction time.

[0069] See appendix Figure 1 The structure and cryogenic process of the fluorobenzene synthesis tail gas cryogenic unit are as follows:

[0070] 1. The cryogenic section consists of at least three shell-and-tube cryogenic heat exchangers HC3 installed on the lid HC7 of the synthesis reactor. One end of the cryogenic heat exchanger HC3 has its tube-side air inlet directly connected to the lid HC7 and connected to the synthesis reactor. The other end has its tube-side air outlet connected to the inlet of the induced draft fan through a pipe.

[0071] 2. The cryogenic heat exchangers HC3 are used in parallel. During normal operation, at least one unit defrosts while the others operate in cryogenic mode. The synthesis tail gas, under the pressure of the reactor and the suction of the induced draft fan HX3, enters the tube side of the cryogenic heat exchanger HC3, where it exchanges heat with the -70℃ low-temperature refrigerant introduced into the shell side, forming cryogenic tail gas with a temperature below -80℃, which then enters the subsequent purification unit.

[0072] 3. Set the cryogenic exhaust gas outlet temperature limit for cryogenic heat exchanger HC3 to -80 to -85℃, and simultaneously set the upper and lower pressure limits for the exhaust gas outlet of cryogenic heat exchanger HC3. Regulate the operating mode of cryogenic heat exchanger HC3 by real-time monitoring of temperature and pressure. When the outlet cryogenic exhaust gas temperature is below -85℃, stop supplying -100℃ low-temperature refrigerant to cryogenic heat exchanger HC3. Cryogenic heat exchanger HC3 will then enter defrost mode, automatically defrosting using the higher-temperature synthesis exhaust gas generated inside the reactor. During the defrost phase, stop supplying -100℃ low-temperature refrigerant to cryogenic heat exchanger HC3.

[0073] 4. The "deep cooling-defrosting" working mode control of the cryogenic heat exchanger HC3 can also be adopted by timed mode, that is, the ratio of deep cooling and defrosting time in one cycle of the cryogenic heat exchanger HC3 is set, and the "deep cooling" and "defrosting" of the cryogenic heat exchanger HC3 are staggered and used alternately.

[0074] 5. The defrosting of the cryogenic heat exchanger HC3 can also be achieved using desorption tail gas from the adsorption unit. The desorption tail gas originally entering the synthesis reactor HC1 is switched to the outlet of the cryogenic heat exchanger HC3 that needs to be defrosted, and the outlet valve of the cryogenic heat exchanger HC3 is closed. Defrosting is achieved by using the enthalpy of the desorption tail gas and by using the pressure of the desorption tail gas to impact the frost.

[0075] Although the amount and composition of the exhaust gas discharged from each HC1 synthesis reactor differ at different stages of the synthesis reaction, the flow rate and composition of the discharged cryogenic exhaust gas are basically stable due to the large number of HC1 synthesis reactors.

[0076] Without cryogenic condensation, the amount of synthesis tail gas discharged is 1000-2000 Nm3 / h. The main components of the tail gas are: HF: 160-170 g / Nm3; fluorobenzene: 2000-3000 mg / Nm3; NOx: 12000-20000 mg / Nm3; a small amount of VOCs; and the remainder is nitrogen.

[0077] The cryogenic exhaust gas discharged after cryogenic condensation has a volume of 1000-2000 Nm3 / h. The main components of the exhaust gas are NO and nitrogen, of which NO: 10000-12000 mg / Nm3, and the remainder is nitrogen.

[0078] See appendix Figure 1 One "adsorption unit" corresponds to multiple "synthesis reactor HCl + cryogenic unit (at least 3 cryogenic heat exchangers)".

[0079] There are 60 synthesis reactors HC1. Each synthesis reactor HC1 is connected to a cryogenic unit consisting of at least 3 cryogenic heat exchangers HC3 installed through the reactor cover HC7. The 60 synthesis reactors HC1 correspond to 60 cryogenic units. The cryogenic exhaust gas outlets of the 60 cryogenic units are merged and then connected to the cryogenic exhaust gas heat exchanger HC15, the activated carbon adsorption tower HX1, and then to the induced draft fan / recirculation cooling fan HX3.

[0080] The activated carbon adsorption tower HX1 is equipped with a purified exhaust gas outlet, a superheated steam inlet, and a cooling gas outlet at the top; the cooling gas outlet is connected to the purified exhaust gas outlet; all the purified exhaust gas outlets of the activated carbon adsorption towers HX1 are connected in parallel and then connected to the induced draft fan / return cooling fan HX3.

[0081] The activated carbon adsorption tower HX1 has a cryogenic tail gas inlet and a desorption tail gas outlet at the bottom. The desorption tail gas outlet is connected to the spray cooling tower HX4 through the desorption tail gas induced draft fan HX14.

[0082] The lower part of the spray cooling tower HX4 is equipped with a desorption tail gas inlet and a low-temperature spray water outlet; the upper part of the spray cooling tower HX4 is equipped with a desorption tail gas outlet and a low-temperature spray water inlet, and is connected to the low-temperature spray water outlet via a low-temperature water circulation pump HX8 and a low-temperature water heat exchanger HX5; the desorption tail gas outlet is connected to the HX6 condenser tail gas fan via a desorption tail gas condenser HX7.

[0083] The outlet of the condenser exhaust fan HX6 is connected to the bottom air inlet HC13 at the bottom of the synthesis reactor HC1, the cryogenic exhaust outlet of the cryogenic heat exchanger HC3, and the nitric oxide purifier HX11.

[0084] The HX11 nitric oxide purifier has a superheated steam inlet and a nitric oxide gas outlet at one end, and a desorption tail gas inlet, a secondary desorption tail gas outlet, and a cooling gas inlet at the other end.

[0085] The nitric oxide gas outlet is connected to the nitric oxide storage tank HX13 via the gas compressor HX12, and the outlet of the gas compressor HX12 is also connected to the cooling gas inlet of the nitric oxide purifier HX11.

[0086] Each synthesis reactor HC1 corresponds to at least 3 of the aforementioned cryogenic heat exchangers HC3. The upper opening of the synthesis reactor HC1 is tightly connected to the reactor cover HC7 via the reactor body positioning frustum HC9 and the reactor cover positioning frustum HC5.

[0087] Three cryogenic heat exchangers HC3 are installed on the reactor cover HC7 to form a cryogenic unit. The lower end of the tube side of the cryogenic heat exchanger HC3 is directly connected to the synthesis reactor HC1, and the upper end of the tube side of the cryogenic heat exchanger HC3 is connected to the activated carbon adsorption tower HX1 via the cryogenic tail gas heat exchanger HC15.

[0088] Each cryogenic heat exchanger HC3 is equipped with multiple enhanced cooling sections, and the upstream and downstream of each enhanced cooling section are connected by a refrigerant circulation pump HC2.

[0089] The outlet of the cryogenic heat exchanger HC3 is also connected to the bottom air inlet HC13 of the synthesis reactor HC1 via the cryogenic exhaust gas circulation fan HC14.

[0090] Sixty synthesis reactors (HC1) conduct the synthesis reaction of fluorobenzene. The resulting synthesis tail gas is drawn in by the induced draft fan / recirculation cooling fan (HX3) and cooled and condensed in the cryogenic heat exchanger (HC3). Almost all HF, fluorobenzene, NO2, and VOCs are condensed and intercepted, while some NO is also condensed and intercepted, and is directly returned to the synthesis reactor (HC1). The cryogenic tail gas exiting the cryogenic heat exchanger (HC3) enters the HC15-cryogenic tail gas heat exchanger (HC15) to exchange heat with the refrigerant exiting the cryogenic heat exchanger (HC3). After the refrigerant temperature decreases, it returns to the refrigeration unit for circulation.

[0091] The shell side of the cryogenic heat exchanger HC3 serves as the refrigerant channel. Along the refrigerant flow direction from inlet to outlet, three enhanced cooling sections are sequentially formed by the refrigerant circulation pump HC2. The refrigerant is circulated from downstream to upstream via HC2. By adjusting the circulation rate, different cooling effects are achieved, thus controlling the condensation rate of different pollutant components. This improves condensation interception while mitigating the effects of frosting. The circulation rate is adjusted according to the melting points of HF, fluorobenzene, NO2, and VOCs.

[0092] The heated cryogenic tail gas enters the activated carbon adsorption tower HX1. Simultaneously, the method of extracting the cryogenic tail gas is adjusted according to the stage of the synthesis reaction in the synthesis reactor HC1. For example, when the synthesis reaction is in the "diazotization" stage, the reactor liquid needs to be cooled. In this case, a portion of the cryogenic tail gas is bubbled into the synthesis reactor HC1 through the bottom inlet HC13 to cool the reactor liquid and also to agitate it.

[0093] The adsorption cycle of each activated carbon adsorption tower HX1 is set to 24 hours. Two of the three activated carbon adsorption towers HX1 are simultaneously in the adsorption stage, while the third is in the desorption stage. The adsorption start time of the three activated carbon adsorption towers HX1 is delayed by 12 hours, meaning that each activated carbon adsorption tower HX1 enters the desorption stage after 24 hours of adsorption, and the desorption stage lasts for 12 hours. This "adsorption-desorption" cycle is used alternately. The exhaust gas from the activated carbon adsorption tower HX1 mainly consists of nitrogen and a small amount of oxygen, and is directly discharged.

[0094] A portion of the exhaust gas is introduced into the desorbed activated carbon adsorption tower HX1 via an induced draft fan / recirculation cooling fan HX3, where it is cooled. The cooled gas then merges into the exhaust gas.

[0095] The activated carbon adsorption tower HX1 is equipped with an online nitrogen oxide monitoring device at the adsorption tail gas outlet, and a nitrogen oxide concentration emission limit is set. When the nitrogen oxide concentration exceeds the limit, the desorption rate of the activated carbon adsorption tower HX1 is increased, the desorption time of the activated carbon adsorption tower HX1 is shortened, and the adsorption stage time is extended.

[0096] Superheated steam is used as the desorption medium. The superheated steam is introduced from the adsorption tail gas outlet of the activated carbon adsorption tower HX1, meaning the desorption gas and adsorption gas flow in opposite directions. Under the suction of the desorption tail gas induced draft fan HX14, the desorption tail gas is divided into two parts: one part enters the spray cooling tower HX4, and the remaining part is bubbled into the synthesis reactor HC1, which is in the pyrolysis stage and requires temperature increase, through the bottom inlet HC13. This heats the reactor liquid and simultaneously stirs it. The nitrogen oxide components abundant in the desorption tail gas also help suppress the side reaction "2HNO2→NO+NO2+water".

[0097] The desorption tail gas entering the spray cooling tower HX4 is circulated and sprayed with cryogenic water below 7°C, provided by the cryogenic water circulation pump HX8 and the cryogenic water heat exchanger HX5, cooling the desorption tail gas and absorbing the HF component in it. The effectiveness of the cryogenic process in condensing and intercepting hydrogen fluoride components and the concentration of hydrogen fluoride in the desorption tail gas are controlled. By controlling the superheated steam temperature and reducing the amount of steam used in the desorption process, the amount of condensate entering the spray cooling tower HX4 is controlled to ensure that the hydrogen fluoride concentration in the condensate is greater than 40%.

[0098] The desorption tail gas from the spray cooling tower HX4 enters the HX7 desorption tail gas condenser for further cooling, condensation and dehydration. After dehydration, it is divided into three parts. One part enters the cryogenic heat exchanger HC3 as a defrosting medium for defrosting. Another part enters the synthesis reactor HC1 through the bottom air inlet HC13 according to the stage of the synthesis reaction. The remaining part is introduced into the nitric oxide purifier HX11 for further adsorption and purification of the desorption tail gas.

[0099] At least two HX11 nitric oxide purifiers should be used, one for purification and the other for cleaning, alternating between them. Monitor the time when nitric oxide appears at the exhaust outlet of the HX11 nitric oxide purifier, set the purification cycle for each HX11 nitric oxide purifier, and switch between the two.

[0100] The nitric oxide gas from the nitric oxide purifier HX11 is compressed by the gas compressor HX12 and then enters the nitric oxide storage tank HX13 for product storage and sale.

[0101] When the nitric oxide purifier HX11 needs to be cooled after cleaning, some of the gas from the gas compressor HX12 can be drawn into the nitric oxide purifier HX11 for cooling. After cooling, the gas enters the cryogenic heat exchanger HC3 for defrosting.

[0102] Through the implementation of the fluorobenzene clean production process of the present invention, the HF recovery rate is above 99.99%; the fluorobenzene recovery rate is above 99.9%; the NOx removal rate is above 99.95%, and the annual recovery and production of NO products is about 100 tons, HF about 2000 tons, and fluorobenzene about 20 tons.

Claims

1. A system for the complete recovery of nitrogen oxides from fluorobenzene synthesis tail gas, comprising a synthesis reactor, characterized in that, The synthesis reactor is connected to an activated carbon adsorption tower via a reactor lid, a cryogenic heat exchanger, and a cryogenic tail gas heat exchanger. The activated carbon adsorption tower, through which an induced draft fan / reflux cooling fan connects to an exhaust gas pipeline. The activated carbon adsorption tower has a cryogenic tail gas inlet and a desorption tail gas outlet at its bottom. The desorption tail gas outlet is connected to a spray cooling tower via a desorption tail gas induced draft fan. The spray cooling tower has a desorption tail gas inlet and a low-temperature spray water outlet at its lower part. The spray cooling tower has a desorption tail gas outlet and a low-temperature spray water inlet at its upper part, and is connected to the low-temperature spray water outlet via a low-temperature water circulation pump and a low-temperature water heat exchanger. The desorption tail gas outlet is connected to a condensing tail gas fan via a desorption tail gas condenser. The outlet of the condensing tail gas fan is connected to the bottom air inlet of the synthesis reactor, the cryogenic tail gas outlet of the cryogenic heat exchanger, and a nitric oxide purifier. The outlet of the cryogenic heat exchanger is also connected to the bottom air inlet of the synthesis reactor via a cryogenic tail gas circulation fan.

2. The complete resource recovery system for nitrogen oxides from fluorobenzene synthesis tail gas as described in claim 1, characterized in that, The activated carbon adsorption tower consists of at least three connected in parallel to form an activated carbon adsorption unit. The top of the activated carbon adsorption tower is provided with a purified exhaust gas outlet, a superheated steam inlet, and a cooling gas outlet. The cooling gas outlet is connected to the purified exhaust gas outlet. The purified exhaust gas outlets of all activated carbon adsorption towers are connected in parallel and then connected to an induced draft fan / return cooling fan.

3. The complete resource recovery system for nitrogen oxides from fluorobenzene synthesis tail gas as described in claim 1, characterized in that, The nitric oxide purifier has a superheated steam inlet and a nitric oxide gas outlet at one end, and a desorption tail gas inlet, a secondary desorption tail gas outlet, and a cooling gas inlet at the other end.

4. The complete resource recovery system for nitrogen oxides from fluorobenzene synthesis tail gas as described in claim 3, characterized in that, The nitric oxide gas outlet is connected to the nitric oxide storage tank via a gas compressor, and the gas compressor outlet is also connected to the cooling gas inlet of the nitric oxide purifier.

5. The complete resource recovery system for nitrogen oxides from fluorobenzene synthesis tail gas as described in claim 1, characterized in that, Each of the aforementioned synthesis reactors corresponds to at least three of the aforementioned cryogenic heat exchangers. The three cryogenic heat exchangers are installed on the reactor lid to form a cryogenic unit. The lower end of the tube side of the cryogenic heat exchanger is directly connected to the synthesis reactor, and the upper end of the tube side of the cryogenic heat exchanger is connected to the activated carbon adsorption tower via the cryogenic tail gas heat exchanger.

6. The complete resource recovery system for nitrogen oxides from fluorobenzene synthesis tail gas as described in claim 1 or 5, characterized in that, Each cryogenic heat exchanger is equipped with multiple enhanced cooling sections, and the upstream and downstream of each enhanced cooling section are connected by a refrigerant circulation pump.

7. The complete resource recovery system for nitrogen oxides from fluorobenzene synthesis tail gas as described in claim 2, characterized in that, Each activated carbon adsorption unit corresponds to multiple synthesis reactors and multiple cryogenic units.

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