On-line cryogenic purification system for o-fluorotoluene synthesis tail gas

Abstract

The present application relates to a kind of o-fluorotoluene synthesis tail gas online cryogenic purification system, including synthesis reactor, the tail gas outlet of synthesis reactor is connected in sequence cryogenic unit, activated carbon adsorption unit and SCR denitration unit, the tail gas outlet of cryogenic unit is also connected with synthesis reactor.The present application system is simple, clean and environmentally friendly, stable and efficient, low operating cost, good purification effect.

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 an online cryogenic purification method for o-fluorotoluene synthesis waste gas. Background Technology

[0002] o-Fluorotoluene is an important fluorinated compound widely used in the pharmaceutical and pesticide industries. With the development of downstream markets such as pharmaceuticals and pesticides and the expansion of new application areas, the demand for o-fluorotoluene is expected to increase, thereby driving the growth of o-fluorotoluene production. Existing preparation methods generally use o-toluidine, anhydrous hydrogen fluoride, and sodium nitrite as raw materials, and produce qualified o-fluorotoluene through steps such as salt formation, diazotization, and pyrolysis. The process is as follows:

[0003] (1) Salt formation

[0004] Low-temperature hydrofluoric acid liquid (-15℃) is pumped from the hydrofluoric acid storage tank to the hydrogen fluoride metering tank. The metering tank is cooled by introducing chilled brine (CaCl2 solution from the refrigeration room) to maintain the temperature below 10℃. Then, the low-temperature hydrofluoric acid liquid is poured from the metering tank into the synthesis reactor and cooled further with chilled brine until the temperature of the synthesis reactor drops to between 4 and 6℃. Under stirring, the discharge valve of the o-toluidine metering tank is opened, and o-toluidine is slowly added dropwise while maintaining the temperature between 5 and 8℃. The reaction process is an excess reaction of hydrofluoric acid under a slightly negative pressure. After the dropwise addition is completed, the amount of chilled brine is adjusted, and the temperature is maintained at around 6℃ for 30 minutes to complete the reaction. The reaction time is 3 hours, yielding an o-toluidine hydrofluoric acid mixture.

[0005] (2) Diazotization

[0006] Lower the temperature of the circulating chilled brine in the synthesis reactor to 0 to -2℃. Under stirring, slowly add solid sodium nitrite using an automatic closed feeder. Control the feeding temperature at 0 to 5℃ and maintain a slight negative pressure. After feeding, keep the temperature at that temperature for 30 minutes to complete the reaction. The reaction time is 6 hours, yielding a diazonium solution.

[0007] The diazotization process involves a side reaction: 2HNO2 → NO + NO2 + water.

[0008] (3) Pyrolysis

[0009] Adjust the temperature of the chilled brine and slowly raise the temperature in stages (5-15℃, 15-25℃, 25-38℃) with stirring, increasing the temperature by approximately 1.2℃ per hour until it reaches 38℃. Maintain negative pressure for thermal decomposition, which takes 16 hours. After the reaction, the material in the reactor is transferred to a settling tank for stratification. The organic layer, a mixture of o-fluorotoluene, is sent to the neutralization reactor; the inorganic layer, a mixed acid solution containing hydrofluoric acid and sodium fluoride, is sent to the mother liquor tank and then to the hydrogen fluoride recovery reactor.

[0010] The synthesis process is carried out in an excess of anhydrous hydrogen fluoride solution, resulting in high reactant conversion and product yield. This process has become the mainstream technology for the synthesis of o-fluorotoluene. However, the production process generates tail gas containing components such as hydrogen fluoride, nitrogen oxides, nitrogen, and VOCs. The concentrations of hydrogen fluoride and nitrogen oxides in the tail gas are high, making treatment difficult. There is an urgent need to find efficient and low-cost solutions to the HF and NOx pollution problems. Summary of the Invention

[0011] The purpose of this invention is to solve the above-mentioned technical problems and provide an online cryogenic purification system for o-fluorotoluene synthesis tail gas that is simple, clean and environmentally friendly, stable and efficient, low in operating cost and has good purification effect.

[0012] The system of the present invention includes a synthesis reactor, the tail gas outlet of which is sequentially connected to a cryogenic unit, an activated carbon adsorption unit and an SCR denitrification unit, and the tail gas outlet of the cryogenic unit is also connected to the synthesis reactor.

[0013] The cryogenic unit consists of multiple cryogenic heat exchangers installed on the lid of the synthesis reactor. The exhaust gas outlet on the lid of the synthesis reactor is connected to the bottom outlet of the cryogenic heat exchangers. The multiple cryogenic heat exchangers take turns performing condensation and defrosting processes.

[0014] The cryogenic heat exchanger has multiple enhanced cooling sections, and the refrigerant outlet downstream of each enhanced cooling section is connected to the refrigerant inlet upstream via a refrigerant circulation pump.

[0015] The exhaust gas outlet of the cryogenic unit is connected to the activated carbon adsorption unit and the conditioning liquid of the synthesis reaction vessel, respectively.

[0016] The adsorption unit comprises at least two independent activated carbon adsorption towers, one for adsorption and the other for desorption, alternating between the two. Each activated carbon adsorption tower is equipped with a tail gas inlet, a superheated steam inlet, a tail gas outlet, and a desorption tail gas outlet. The resin adsorption towers are connected in parallel. The tail gas inlet of each activated carbon adsorption tower is connected to the tail gas outlet of the washing unit. The tail gas outlets of each activated carbon adsorption tower are combined and then connected to the SCR denitrification unit. The tail gas outlet of the adsorption unit is also connected to the tail gas inlet of each activated carbon adsorption tower via a reflux cooling fan.

[0017] The desorption tail gas outlet of the adsorption unit is connected to the synthesis reactor and the condensation device, and the tail gas outlet of the condensation device is connected to the defrost gas pipeline of the cryogenic unit and the synthesis reactor.

[0018] The condensation device includes a spray cooling tower and a desorption tail gas condenser connected in sequence. The cooling water outlet at the bottom of the spray cooling tower is connected to the cooling water inlet at the top via a low-temperature water heat exchanger.

[0019] The SCR denitrification unit includes a denitrification heat exchanger, a tail gas heater, an ammonia addition device, a pipeline mixer, and a denitrification reactor connected in sequence.

[0020] The purified exhaust gas outlet of the denitrification reactor is connected to a chimney via the denitrification heat exchanger, the circulating fan, and the purified exhaust gas heat exchanger.

[0021] The purified exhaust gas outlet of the denitrification reactor is also connected to the exhaust gas heater via a denitrification heat exchanger, a circulating fan, and an exhaust gas circulation regulating valve.

[0022] The synthesis of o-fluorotoluene is generally carried out in a synthesis reactor, and the production process is discontinuous, with one reactor completed before the next cycle begins. Therefore, the temperature of the reactor liquid needs frequent adjustment, leading to a large amount of reaction mixture components being vaporized and discharged with the synthesis tail gas at high concentrations, significantly reducing the utilization rate of the raw materials. To improve the utilization rate of raw materials, this invention proposes an online cryogenic treatment solution for the synthesis tail gas, changing the traditional method of leading the tail gas into a cryogenic heat exchanger for condensation and recovery. Specific improvements are as follows:

[0023] 1) The synthesis tail gas generated in the synthesis reactor rises directly through the top of the reactor and enters the cryogenic reactor, reducing the energy consumption of pipeline installation and transportation.

[0024] 2) During the cryogenic process, most of the nitrogen dioxide in the synthesis tail gas is condensed and falls directly into the synthesis reactor through the bottom of the cryogenic reactor, which increases the nitrogen dioxide concentration in the reactor liquid, inhibits the occurrence of the side reaction <2HNO2→NO+NO2+water>, and improves the utilization rate of HNO2.

[0025] 3) The cryogenic tail gas from the cryogenic unit contains high concentrations of HF, NO and small amounts of NO2 and o-fluorotoluene after dehydration. At this time, a portion of it is separated and sent back to the synthesis reactor. On the one hand, it can re-enter the cryogenic heat exchanger with the synthesis tail gas in the reactor to condense and recover HF, NO2 and o-fluorotoluene. On the other hand, it can also serve the purpose of stirring and humidifying in the reactor.

[0026] 4) Cryogenic heat exchanger refrigerant internal circulation scheme: Exhaust gas enters the cryogenic heat exchanger and exchanges heat with the refrigerant in a counter-current manner. Within the cryogenic heat exchanger, the refrigerant undergoes forced recirculation, forming enhanced cooling sections corresponding to hydrogen fluoride, o-fluorotoluene, and nitrogen dioxide, respectively. A refrigerant circulation pump is installed to return the higher-temperature refrigerant from downstream to upstream, forming a condensation section with a small temperature difference corresponding to the melting point temperature of the pollutant components. This ensures sufficient condensation of the pollutant components and prevents solidification and blockage.

[0027] By adjusting the circulation volume, different cooling effects can be obtained, achieving condensation rates for different contaminants, thereby improving condensation interception while mitigating the effects of frost formation.

[0028] In the synthesis tail gas, 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℃; o-fluorotoluene has a melting point of -62℃ and a boiling point of 112-113℃; and HF has a melting point of 8-3℃ and a boiling point of 19.54℃. When the tail gas is cryogenically cooled to below -80℃, almost all of the HF, nitrogen dioxide, and o-fluorotoluene condenses. During the condensation process, nitrogen dioxide and o-fluorotoluene will frost up inside the cryogenic heat exchanger, blocking the tail gas flow channels. To mitigate the frost clogging problem, a higher-temperature gas circulation system within the tail gas purification system is used for defrosting.

[0029] (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, o-fluorotoluene, 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.

[0030] (2) The higher temperature gas circulation defrost is used in the exhaust gas purification system. The higher temperature desorbed exhaust gas desorbed from the activated carbon adsorption unit is used as the defrost heat source, which not only recovers o-fluorotoluene in the desorbed exhaust gas, but also defrosts efficiently.

[0031] The effects of this improvement are as follows:

[0032] (1) Stepwise condensation and recovery of contaminant components reduces frost blockage during the cryogenic process and improves system operational stability.

[0033] (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 (-50℃) after cryogenic cooling, thereby improving the efficiency of refrigerant cold energy utilization.

[0034] (3) Fully recovers the cold energy of frost formation, eliminating the need to introduce external defrosting media and achieving efficient defrosting.

[0035] Furthermore, in order to efficiently recover the hydrogen fluoride component in the synthesis tail gas, the activated carbon adsorption tower is desorbed and regenerated, and the desorbed tail gas enters the spray cooling tower of the condensation unit, where low-temperature water spray cooling is used to recover the hydrogen fluoride component and obtain a hydrofluoric acid product with a concentration of 40wt% HF.

[0036] Because the concentration of nitrogen oxides in the synthesis tail gas reaches 20,000-40,000 mg / Nm3, in order to mitigate the impact of such a high concentration of tail gas on the catalyst when entering the denitrification reactor, this invention sets up an activated carbon adsorption unit before the denitrification reactor. The main components of the cryogenic tail gas are HF, NOx, o-fluorotoluene, nitrogen, and a small amount of low-molecular-weight VOCs. Since HF has the highest polarity, it has a competitive adsorption advantage, followed by NO, then o-fluorotoluene and NO2, while nitrogen is hardly adsorbed. Therefore, the activated carbon adsorption tower is also the concentration unit for HF, NO, and o-fluorotoluene. The desorbed tail gas after adsorption contains almost no nitrogen, and after dehydration, it is introduced into the synthesis reactor for easy cryogenic recovery.

[0037] Because NO is highly polar, the activated carbon adsorption tower adsorbs some NO along with o-fluorotoluene and HF, thereby reducing the NO flow rate entering the SCR denitrification catalyst and mitigating the reaction shock. The adsorbed NO enters the synthesis reactor with the desorption tail gas, increasing the NO concentration in the reactor liquid and inhibiting the occurrence of the side reaction <2HNO2→NO+NO2+water>.

[0038] Because the desorption tail gas contains high concentrations of HF, NO and small amounts of NO2 and o-fluorotoluene after dehydration, it returns to the synthesis reactor and, together with the synthesis tail gas inside the reactor, re-enters the cryogenic heat exchanger to condense and recover HF, NO2 and o-fluorotoluene.

[0039] Furthermore, the desorption tail gas from the activated carbon adsorption tower is treated in two ways according to the needs of the o-fluorotoluene synthesis process. When the synthesis process in the synthesis reactor enters the pyrolysis stage and the reactor liquid needs to be heated, the desorption tail gas can be directly introduced into the synthesis reactor to adjust the temperature and stir the reactor liquid. When the synthesis process is in the salt formation or diazotization stage, the desorption tail gas enters the condenser to cool, dehydrate, and recover the hydrogen fluoride component. The remaining gas phase can be sent to the cryogenic unit as a defrosting medium or enter the synthesis reactor to participate in the synthesis reaction. The components such as HF and NO in the gas phase are all reaction components of the synthesis reaction.

[0040] The effects of adopting the above-mentioned technical improvement measures are as follows:

[0041] (1) Completely eliminate the damage of trace amounts of HF remaining in the cryogenic tail gas to the SCR denitrification catalyst. Since HF is more polar than NO, it is preferentially adsorbed after the cryogenic tail gas enters the activated carbon adsorption tower, thus basically eliminating the damage of HF to the SCR denitrification catalyst.

[0042] (2) Recover trace amounts of HF components from the exhaust gas. The HF adsorbed by activated carbon is desorbed by superheated steam, washed with cooling water and dissolved in water to obtain hydrofluoric acid product.

[0043] (3) Mitigating the impact of high concentrations of nitrogen oxides on the SCR denitrification catalyst. After the cryogenic tail gas enters the activated carbon adsorption tower, some of the high concentrations of NO in the tail gas are adsorbed, reducing the NO flow rate entering the catalyst bed.

[0044] (4) Economical and efficient. SCR denitrification catalysts are expensive to manufacture and more expensive than activated carbon. At the same time, SCR denitrification catalysts are less resistant to acids and alkalis than activated carbon. Setting up an activated carbon adsorption tower before the denitrification reactor is equivalent to adding a protective measure to the denitrification reactor, which is conducive to extending the life of SCR denitrification catalysts.

[0045] (5) The heat source of the internal medium of the system is fully utilized. By desorbing the tail gas and stirring it in the reactor, the enthalpy required for heating and pyrolysis of the synthesis reaction liquid is saved.

[0046] (6) Fully recover and utilize pollutants such as o-fluorotoluene and nitrogen oxides contained in the exhaust gas.

[0047] Furthermore, this invention innovatively proposes a purified exhaust gas recirculation technology for SCR denitrification treatment of cryogenic exhaust gas. This involves dividing the purified exhaust gas from the denitrification heat exchanger into two parts. One part is directly mixed with the heated adsorption exhaust gas and participates in the exhaust gas recirculation. The other part is cooled and dehydrated by exchanging heat with the cryogenic exhaust gas in the purified exhaust gas heat exchanger before being discharged through a chimney. The effects of this technology are as follows:

[0048] (1) In terms of system operation stability, by circulating the purified tail gas, the fluctuation range of NOx content in the adsorbed tail gas entering the denitrification reactor is reduced, the impact of NOx concentration fluctuation in the inlet gas on the catalyst is mitigated, which is conducive to the stability of the SCR denitrification reaction process.

[0049] (2) Through the circulating dilution effect of the purified tail gas, the temperature fluctuation of the adsorbed tail gas entering the reactor will also be reduced, thereby improving the system's operational stability. The improved system operational stability can effectively reduce system operating costs and extend the catalyst's service life.

[0050] (3) By monitoring the NOx concentration in the purified tail gas online and adjusting the amount of ammonia injected into the adsorption tail gas, the circulation of the purified tail gas also controls the fluctuation of ammonia content in the tail gas entering the denitrification reactor, which is conducive to the stability of the SCR denitrification reaction process, improves the denitrification effect, and reduces ammonia escape.

[0051] (4) The purified exhaust gas of each row is exchanged with the cryogenic exhaust gas at a lower temperature, so that some of the water in the purified exhaust gas is condensed. The condensation process also absorbs the ammonia component in the flue gas, reducing ammonia escape. After the heat exchange, the temperature of the cryogenic exhaust gas is greatly increased, which greatly saves the energy consumption of heating the exhaust gas in the SCR denitrification process.

[0052] (5) By circulating the purified exhaust gas, the concentration of nitrogen oxides is diluted, thus avoiding violent local reactions in the reactor and damage to the catalyst.

[0053] This invention can achieve full recovery of nitrogen dioxide, o-fluorotoluene and fluorine components in the synthesis tail gas, fully recover waste heat and waste cooling capacity, and has a relatively simple process route, is environmentally friendly, economical and efficient, and has good stable operation. Attached Figure Description

[0054] Figure 1 This is a system diagram of the present invention.

[0055] Among them: synthesis reactor HC1, refrigerant circulation pump HC2, cryogenic heat exchanger HC3, strong reactor lid HC7, and purified tail gas heat exchanger HC15.

[0056] Activated carbon adsorption tower HX1, reflux cooling fan HX3, spray cooling tower HX4, low temperature water heat exchanger HX5, desorption tail gas condenser HX7, hydrofluoric acid product tank HX9, high concentration hydrofluoric acid product tank HX10.

[0057] Denitrification heat exchanger XT3, circulating fan XT4, tail gas circulation regulating valve XT5, tail gas heater XT8, denitrification reactor XT9, pipeline mixer XT10, ammonia addition device XT11. Detailed Implementation

[0058] The invention will be further explained below with reference to the accompanying drawings:

[0059] The tail gas outlet of the synthesis reactor HC1 is sequentially connected to the cryogenic unit, the activated carbon adsorption unit, and the SCR denitrification unit. The tail gas outlet of the cryogenic unit is also connected to the synthesis reactor HC1.

[0060] The cryogenic unit consists of multiple cryogenic heat exchangers HC3 mounted on the lid HC7 of the synthesis reactor HC1. The exhaust gas outlet on the lid of the synthesis reactor HC1 is connected to the bottom outlet of the cryogenic heat exchangers HC3. The multiple cryogenic heat exchangers HC3 take turns performing condensation and defrosting processes. The cryogenic heat exchangers HC3 have multiple enhanced cooling sections, and the refrigerant outlet downstream of each enhanced cooling section is connected to the refrigerant inlet upstream via a refrigerant circulation pump HC2.

[0061] The adsorption unit comprises at least two independent activated carbon adsorption towers HX1, one for adsorption and the other for desorption, alternating between the two. Each activated carbon adsorption tower HX1 is equipped with a tail gas inlet, a superheated steam inlet, a tail gas outlet, and a desorption tail gas outlet. The resin adsorption towers are connected in parallel. The tail gas inlet of each activated carbon adsorption tower HX1 is connected to the tail gas outlet of the washing unit. The tail gas outlets of each activated carbon adsorption tower HX1 are combined and then connected to the SCR denitrification unit. The tail gas outlet of the adsorption unit is also connected to the tail gas inlet of each activated carbon adsorption tower HX1 through a reflux cooling fan HX3.

[0062] The desorption tail gas outlet of the adsorption unit is connected to the synthesis reactor HC1 and the condensation device. The tail gas outlet of the condensation device is connected to the defrost gas pipeline of the cryogenic unit and the synthesis reactor HC1. The condensation device includes a spray cooling tower HX4 and a desorption tail gas condenser HX7 connected in sequence. The cooling water outlet at the bottom of the spray cooling tower HX4 is connected to the cooling water inlet at the top via a low-temperature water heat exchanger HX5.

[0063] The denitrification unit comprises a denitrification heat exchanger XT3, a tail gas heater XT8, an ammonia device XT11, a pipeline mixer XT10, an ammonia addition device XT11, and a denitrification reactor XT9, connected in sequence. The purified tail gas outlet of the denitrification reactor XT9 is connected to a chimney via the denitrification heat exchanger XT3, a circulating fan XT4, and a purified tail gas heat exchanger HC15; the purified tail gas outlet of the denitrification reactor XT9 is also connected to the tail gas heater XT8 via the denitrification heat exchanger XT3, the circulating fan XT4, and a tail gas circulation regulating valve XT5.

[0064] The reaction temperature varies at different stages of the reaction and needs to be adjusted according to the reaction time.

[0065] See appendix Figure 1 The structure and cryogenic process of the o-fluorotoluene synthesis tail gas cryogenic unit are as follows:

[0066] 1. The cryogenic unit 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 HC1. The other end has its tube-side air outlet connected to the inlet of the induced draft fan through a pipe.

[0067] 2. 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 enters the tube side of the cryogenic heat exchanger HC3 under the pressure of the reactor and the suction of the induced draft fan, where it exchanges heat with the -70℃ low-temperature refrigerant introduced into the shell side, forming cryogenic tail gas with a temperature below -50℃, which then enters the subsequent purification unit.

[0068] 3. Set the cryogenic tail gas outlet temperature limit for cryogenic heat exchanger HC3 to -50 to -55℃, and simultaneously set the pressure difference limit between the tail gas inlet and outlet. Regulate the operating mode of cryogenic heat exchanger HC3 by real-time monitoring of temperature and pressure difference. When the outlet cryogenic tail gas temperature is below -55℃, stop supplying -70℃ low-temperature refrigerant to cryogenic heat exchanger HC3. Cryogenic heat exchanger HC3 will then enter defrost mode, automatically defrosting using the higher-temperature synthesis tail gas generated inside the reactor. During the defrost phase, stop supplying -70℃ low-temperature refrigerant to cryogenic heat exchanger HC3.

[0069] 4. The "deep cooling-defrosting" working mode control of the cryogenic heat exchanger HC3 can also be adopted by timed method, 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 alternated.

[0070] 5. The defrosting of the cryogenic heat exchanger HC3 can also be achieved using desorption tail gas from the activated carbon 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.

[0071] 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.

[0072] One set of "adsorption unit + denitrification unit" corresponds to multiple sets of "synthesis reactor HCl + cryogenic unit".

[0073] The process of an online cryogenic purification system for o-fluorotoluene synthesis tail gas is further explained below with reference to the accompanying drawings:

[0074] Referring to the attached diagram, the amount of synthesis exhaust gas discharged without cryogenic condensation is 1000–2000 Nm³. 3 / h, the main components of the exhaust gas are: HF: 160~170g / Nm 3 o-Fluorotoluene: 1000–2000 mg / Nm 3 NOx: 12000–20000 mg / Nm³ 3 It contains a small amount of VOCs; the rest is nitrogen.

[0075] The synthesis tail gas generated in the synthesis reactor HC1 rises and passes through the reactor lid HC7 at the top of the synthesis reactor HC1, directly entering the cryogenic heat exchanger HC3 of the cryogenic unit. The synthesis tail gas is indirectly cooled to below -80°C by a -100°C low-temperature refrigerant in the cryogenic unit. More than 99% of the o-fluorotoluene, nitrogen dioxide, and more than 90% of the hydrogen fluoride components are condensed and intercepted, and returned to the synthesis reactor HC1 with the condensate to continue participating in the synthesis reaction. The cryogenic tail gas exiting the cryogenic unit enters the activated carbon adsorption unit for further defluorination of hydrogen fluoride, and then enters the SCR denitrification unit for denitrification before being discharged.

[0076] The cryogenic unit consists of multiple cryogenic heat exchangers installed on the lid HC7 of the synthesis reactor HC1. Multiple cryogenic heat exchangers HC3 alternately perform condensation and defrosting processes. The synthesis tail gas generated by the synthesis reactor HC1 enters the bottom of cryogenic heat exchanger HC3, which is in the condensation stage, for condensation. The cryogenic tail gas exiting the cryogenic unit is divided into two streams: one stream is sent to the activated carbon adsorption unit for adsorption, and the other stream is returned to the synthesis reactor HC1 to adjust the reactor liquid temperature and stir the reactor liquid.

[0077] The condensation-condensation-defrosting process of the cryogenic heat exchanger HC3 can also be controlled by setting upper and lower limits for the outlet exhaust gas pressure. When the outlet exhaust gas pressure of the cryogenic heat exchanger HC3 is lower than the lower limit, the refrigerant is cut off, and the cryogenic heat exchanger HC3 enters the defrosting stage. When the outlet exhaust gas pressure of the cryogenic heat exchanger HC3 is higher than the upper limit, the refrigerant is introduced, and the cryogenic heat exchanger HC3 enters the cryogenic working stage. This process is repeated alternately.

[0078] The cryogenic heat exchanger HC3 has three enhanced cooling sections. Each enhanced cooling section uses a refrigerant circulation pump HC2 to return the refrigerant from the downstream to the upstream. The synthesis tail gas and condensate come into counter-current contact. In the three enhanced cooling sections SL14, hydrogen fluoride, o-fluorotoluene, and nitrogen dioxide components are condensed and condensed in an enhanced manner, improving the condensation and interception effect of pollutants in the synthesis tail gas. The nitrogen dioxide-containing condensate generated in the cryogenic heat exchanger HC3 falls directly through the reactor lid HC7 into the synthesis reactor HC1. It falls directly into the synthesis reactor HC1 through the bottom of the cryogenic reactor HC3, increasing the nitrogen dioxide concentration in the reactor liquid, inhibiting the occurrence of the side reaction <2HNO2→NO+NO2+water>, and improving the utilization rate of HNO2.

[0079] The cryogenic tail gas enters the adsorption unit, where o-fluorotoluene and hydrogen fluoride are adsorbed. The adsorbed tail gas after removing o-fluorotoluene and hydrogen fluoride enters the SCR denitrification unit for denitrification. The adsorption unit consists of at least two independent activated carbon adsorption towers HX1, one for adsorption and one for desorption, alternating between the two. 10-20 wt% of the adsorbed tail gas from the other activated carbon adsorption tower is introduced into the desorbed activated carbon adsorption tower HX1 to cool the activated carbon adsorption bed until the set temperature is reached. Then, it enters the subsequent denitrification unit together with the remaining adsorbed tail gas.

[0080] The desorption medium of the activated carbon adsorption tower HX1 is superheated steam. The desorption tail gas from the activated carbon adsorption unit is either sent into the synthesis reactor HC1 to adjust the temperature of the reactor liquid; or it is condensed by the condenser and sent back to the defrost gas pipeline of the cryogenic unit or sent into the synthesis reactor HC1 to adjust the temperature of the reactor liquid.

[0081] The condensation device includes a spray cooling tower HX4 and a desorption tail gas condenser HX7 connected in sequence. The desorption tail gas is cooled by low-temperature water spraying in the spray cooling tower HX4 and then sent to the desorption tail gas condenser HX7 for condensation. The low-temperature cooling water in the spray cooling tower HX4 is cooled by the low-temperature water heat exchanger HX5 and then circulated and sprayed to absorb and condense some of the HF component in the desorption tail gas. The collected portion of the circulating cooling water and the condensate from the low-temperature water heat exchanger HX5 are sent to the hydrofluoric acid product tank HX9 to obtain hydrofluoric acid product with a hydrogen fluoride mass concentration of more than 40 wt%. The condensate from the desorption tail gas condenser HX7 is sent to the high-concentration hydrofluoric acid product tank HX10 to obtain high-concentration hydrofluoric acid product.

[0082] See Figure 1 The denitrification unit includes a denitrification heat exchanger XT3, a tail gas heater XT8, an ammonia device XT11, a pipeline mixer XT10, an ammonia addition device XT11, and a denitrification reactor XT9. The adsorption tail gas from the adsorption unit is heated by exchanging heat with the purified tail gas in the denitrification heat exchanger XT3, and then heated to above 170°C by the tail gas heater XT8. After ammonia is added by the ammonia addition device XT11, it enters the denitrification reactor XT9 for denitrification and purification. The purified tail gas is divided into two parts. One part is directly mixed with the adsorption tail gas heated by the denitrification heat exchanger XT3 and sent to the tail gas heater XT8 through the circulating fan XT4 and the tail gas circulation regulating valve XT4 to participate in the tail gas circulation, diluting the nitrogen oxide concentration in the tail gas entering the denitrification reactor. The other part is cooled and dehumidified by exchanging heat with the desorbed tail gas through the circulating fan XT4 and the purified tail gas heat exchanger HC15, and then discharged through the chimney.

[0083] After purification, the HF recovery rate is above 99.99%; the o-fluorotoluene recovery rate is above 99.9%; the NOx removal rate is above 99.95%, and the outlet NOx is below 100 mg / Nm³. 3.

[0084] After purification, approximately 2,000 tons of HF and 10 tons of o-fluorotoluene can be recovered annually.

Claims

1. An online cryogenic purification system for o-fluorotoluene synthesis tail gas, comprising a synthesis reactor, characterized in that, The tail gas outlet of the synthesis reactor is sequentially connected to the cryogenic unit, the activated carbon adsorption unit, and the SCR denitrification unit. The tail gas outlet of the cryogenic unit is also connected to the synthesis reactor to regulate the temperature and stir the reactor liquid. The cryogenic unit consists of multiple cryogenic heat exchangers installed on the reactor lid. The upper opening of the synthesis reactor is tightly connected to the reactor lid via a reactor body positioning frustum and a reactor lid positioning frustum. The tail gas outlet on the reactor lid is connected to the bottom outlet of the cryogenic heat exchangers. The multiple cryogenic heat exchangers take turns performing condensation and defrosting processes. The synthesis tail gas is indirectly cooled to below 80°C by a -100°C cryogenic refrigerant in the cryogenic unit. More than 99% of the o-fluorotoluene, nitrogen dioxide, and more than 90% of the hydrogen fluoride components are condensed and intercepted, and returned to the HCl synthesis reactor with the condensate to continue participating in the synthesis reaction.

2. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 1, characterized in that, The cryogenic heat exchanger has multiple enhanced cooling sections, and the refrigerant outlet downstream of each enhanced cooling section is connected to the refrigerant inlet upstream via a refrigerant circulation pump.

3. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 1 or 2, characterized in that, The adsorption unit comprises at least two independent activated carbon adsorption towers, one for adsorption and the other for desorption, alternating between the two. Each activated carbon adsorption tower is equipped with a tail gas inlet, a superheated steam inlet, a tail gas outlet, and a desorption tail gas outlet. The activated carbon adsorption towers are connected in parallel. The tail gas inlet of each activated carbon adsorption tower is connected to the tail gas outlet of the cryogenic unit. The tail gas outlets of each activated carbon adsorption tower are combined and then connected to the SCR denitrification unit. The tail gas outlet of the adsorption unit is also connected to the tail gas inlet of each activated carbon adsorption tower through a reflux cooling fan.

4. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 3, characterized in that, The desorption tail gas outlet of the adsorption unit is connected to the synthesis reactor and the condensation device. The tail gas outlet of the condensation device is connected to the defrost gas pipeline of the cryogenic unit and the temperature regulation tank liquid inside the synthesis reactor.

5. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 4, characterized in that, The condensation device includes a spray cooling tower and a desorption tail gas condenser connected in sequence. The cooling water outlet at the bottom of the spray cooling tower is connected to the cooling water inlet at the top via a low-temperature water heat exchanger.

6. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 1, characterized in that, The SCR denitrification unit includes a denitrification heat exchanger, a tail gas heater, an ammonia addition device, a pipeline mixer, and an SCR denitrification reactor connected in sequence.

7. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 6, characterized in that, The purified exhaust gas outlet of the denitrification reactor is connected to a chimney via the denitrification heat exchanger, the circulating fan, and the purified exhaust gas heat exchanger.

8. The online cryogenic purification system for o-fluorotoluene synthesis tail gas as described in claim 7, characterized in that, The purified exhaust gas outlet of the denitrification reactor is also connected to the exhaust gas heater via a denitrification heat exchanger, a circulating fan, and an exhaust gas circulation regulating valve.

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