An online cryogenic purification process for o-fluorotoluene synthesis tail gas

Abstract

The invention discloses an online cryogenic purification process for o-fluorotoluene synthesis tail gas. The process comprises the following steps: generating a synthetic tail gas in a synthesis reactor during the o-fluorotoluene synthesis process; the synthetic tail gas is indirectly cooled to below -80°C by a cryogenic refrigerant at -100°C in a cryogenic unit; condensing and intercepting more than 99wt% of the o-fluorotoluene, nitrogen dioxide, and more than 90wt% of the hydrogen fluoride components; and returning the condensate to the synthesis reactor to continue participating in the synthesis reaction. The cryogenic tail gas exiting the cryogenic unit enters an activated carbon adsorption unit for further adsorption of hydrogen fluoride, and then enters an SCR denitration unit for denitration before being discharged. The process route of the invention is simple, clean, environmentally friendly, stable, efficient, low in operating cost, and high in defluorination and denitration efficiency.

Description

Technical Field

[0001] The present invention belongs to the field of chemical tail gas treatment, and relates to the treatment of tail gas pollutants generated in the production process of the organic fluorine chemical industry. Specifically, it is an online cryogenic purification process for o-fluorotoluene synthesis tail gas. Background Art

[0002] Organic fluorine materials possess excellent thermal and oxygen stability, chemical resistance, aging resistance, non-stick properties, electrical insulation, and a very low coefficient of friction. They are now widely used in a wide range of industries and fields, including aviation, aerospace, navigation, chemicals, petroleum, automobiles, machinery, electronic information, biomedical materials, construction, and environmental protection. However, due to the extreme rarity of naturally occurring fluorine-containing organic molecules, the vast majority of required organic fluorine-containing substances must be synthesized, which has led to the rapid development of the fluorine chemical industry.

[0003] One of the most important emerging downstream applications for organic fluorine is the new energy vehicle industry. For example, the application of o-fluorotoluene in lithium-ion batteries significantly improves the thermal stability of the electrolyte and the safety of the battery. This helps increase the reduction potential of solvent molecules on the carbon anode surface, optimizes the solid electrolyte interface, and improves the compatibility between the electrolyte and activated carbon materials, thereby stabilizing the electrochemical performance of the electrode. [Song Xin, Wu Zesheng, Yang Qiangqiang, et al. Research Progress on New Fluorine-Containing Additives for Lithium-Ion Battery Electrolytes. Power Sources Technology, Issue 10, 2017]

[0004] During the synthesis of o-fluorotoluene, the tail gas contains high concentrations of hydrogen fluoride and nitrogen oxides, and also produces a large amount of fluorine-containing wastewater. HF is very harmful to human health. Long-term inhalation of low-concentration HF can cause chronic poisoning, vomiting, dizziness and other symptoms. When the HF concentration is greater than 30μg / m 3 It can cause acute poisoning. To date, there is no efficient and low-cost method to solve the HF and NOx pollution problems in the production process. To recover HF from the exhaust gas, Lv Zhimin et al. [Lv Zhimin, Zhang Yuqing, Tang Anping et al. Clean Production Process of o-Fluorotoluene. Chemical World, 2002.12] studied a water washing recovery process. They used a three-stage water circulation to absorb HF from the exhaust gas, and coagulation sedimentation and activated carbon adsorption to treat the fluorine-containing wastewater. Although some HF can be recovered, the recovery process is long and there are still wastewater pollutants. After treatment, a certain amount of HF pollution components still exist in the exhaust gas. In order to minimize the HF pollution components in the exhaust gas, there are also studies and applications using alkaline solution absorption. However, due to the low solubility of sodium fluoride, the system scales and clogs, seriously affecting the normal operation of the system and generating a large amount of fluorine-containing wastewater.

[0005] In addition, there is a side reaction in the synthesis process of o-fluorotoluene: 2HNO2→NO+NO2+water, which produces a large amount of nitrogen oxides and is discharged with the synthesis tail gas. The concentration of nitrogen oxides reaches 20,000-40,000 mg / Nm3. Traditional SCR denitrification catalysts cannot withstand the impact of such high concentrations of nitrogen oxides.

[0006] The present invention aims at the above-mentioned problem and develops an online cryogenic purification process for o-fluorotoluene synthesis tail gas. Summary of the Invention

[0007] The purpose of the present invention is to solve the above technical problems and provide an online cryogenic purification process for o-fluorotoluene synthesis tail gas with a simple process route, clean and environmentally friendly, stable and efficient, low operating cost and high defluorination and denitrification efficiency.

[0008] The method of the present invention comprises the following steps: the synthesis tail gas generated in a synthesis reactor during the synthesis of o-fluorotoluene is indirectly cooled to below -80°C by a low-temperature refrigerant at -100°C in a cryogenic unit, wherein more than 99% of o-fluorotoluene, nitrogen dioxide and more than 90% of hydrogen fluoride components are condensed and intercepted and returned to the synthesis reactor along with the condensate to continue participating in the synthesis reaction; the cryogenic tail gas exiting the cryogenic unit enters an activated carbon adsorption unit for adsorption and further dehydrogenation, and then enters an SCR denitration unit for denitration and is discharged.

[0009] The cryogenic unit is composed of multiple cryogenic heat exchangers installed on the cover of the synthesis reactor. The multiple cryogenic heat exchangers take turns to perform condensation and defrosting processes. The synthesis tail gas generated by the synthesis reactor enters the cryogenic heat exchanger in the condensation stage from the bottom of the cryogenic heat exchanger for condensation.

[0010] The cryogenic heat exchanger has multiple enhanced cooling sections, and each enhanced cooling section uses a refrigerant circulation pump to return the refrigerant from the downstream to the upstream.

[0011] The cryogenic tail gas from the cryogenic unit is divided into two streams, one of which is fed into the activated carbon adsorption unit, and the other is fed back into the synthesis reactor to adjust the reactor liquid temperature and stir the reactor liquid.

[0012] The cryogenic tail gas enters the adsorption unit, and o-fluorotoluene and hydrogen fluoride are adsorbed. The adsorbed tail gas after the removal of o-fluorotoluene and hydrogen fluoride enters the SCR denitrification unit for denitrification; the adsorption unit includes at least two independent activated carbon adsorption towers, one for adsorption and the other for desorption, which are carried out alternately; 10-20% by volume of the adsorbed tail gas from another activated carbon adsorption tower after adsorption is introduced into the desorbed activated carbon adsorption tower to cool the carbon adsorption bed until the set temperature, and then enters the subsequent denitrification unit together with the remaining adsorbed tail gas.

[0013] The desorption medium of the activated carbon adsorption tower is superheated steam, and the desorption tail gas drawn out from the adsorption unit is either sent to the synthesis reactor to adjust the reactor liquid temperature, or condensed by a condensing device and reused as defrosting gas of the cryogenic unit or sent to the synthesis reactor to adjust the reactor liquid temperature.

[0014] The condensing device includes a spray cooling tower and an analytical tail gas condenser connected in sequence. The desorbed tail gas is sprayed and cooled by low-temperature water in the spray cooling tower and then sent to the analytical tail gas condenser for condensation. The low-temperature cooling water in the spray cooling tower is cooled by a low-temperature water heat exchanger and then circulated and sprayed to absorb and condense part of the HF component in the desorbed tail gas.

[0015] According to the mass of hydrogen fluoride adsorbed by the activated carbon adsorption tower, the temperature of the superheated steam is controlled, the regeneration time is controlled, and the amount of superheated steam used for regeneration is controlled; the amount of condensed water introduced by the superheated steam for desorption is controlled to ensure that the mass concentration of hydrogen fluoride in the low-temperature water is above 40%.

[0016] The SCR denitration unit includes a denitration heat exchanger, an exhaust gas heater, an ammonia adding device, a pipeline mixer and a denitration reactor; the adsorbed exhaust gas from the adsorption unit is heated by heat exchange with the purified exhaust gas in the denitration heat exchanger, and then heated to above 170°C by the exhaust heater, ammonia is added by the ammonia adding device, and finally mixed evenly by the pipeline mixer before entering the denitration reactor for denitration purification. The obtained purified exhaust gas is then sent to the denitration heat exchanger for heat exchange with the adsorbed exhaust gas.

[0017] The purified tail gas coming out of the denitrification heat exchanger is divided into two parts. One part is directly mixed with the adsorbed tail gas after being heated out of the denitrification heat exchanger, participates in the tail gas circulation, and dilutes the nitrogen oxide concentration in the tail gas entering the denitrification reactor. The other part is discharged through the chimney after heat exchange with the desorbed tail gas in the purified tail gas heat exchanger for cooling and dehumidification.

[0018] The synthesis reaction of o-fluorotoluene is generally completed in a synthesis reactor. The production process is discontinuous. After one reactor is completed, the next round of synthesis reaction process begins. Therefore, the temperature of the reactor liquid in the synthesis reactor needs to be adjusted frequently, which causes a large amount of reaction mixture components in the reactor liquid to be vaporized and discharged together with the synthesis tail gas, and the concentration is high, which greatly reduces the utilization rate of raw materials. In order to improve the utilization rate of raw materials, the present invention proposes a technical solution for online deep cooling of the synthesis tail gas, which changes the traditional technical solution of drawing the tail gas into a deep cold heat exchanger for condensation and recovery. The specific improvement measures are as follows:

[0019] 1) The synthetic tail gas generated by the reaction in the synthesis reactor directly rises through the top of the reactor and enters the cryogenic reactor, reducing the energy consumption of pipeline installation and transportation;

[0020] 2) After most of the nitrogen dioxide in the synthesis tail gas is condensed during the cryogenic process, it falls directly into the synthesis reactor through the bottom of the cryogenic reactor, increasing the concentration of nitrogen dioxide in the reactor liquid, inhibiting the occurrence of the side reaction (2HNO2→NO+NO2+water), and improving the utilization rate of HNO2;

[0021] 3) After dehydration, the cryogenic tail gas from the cryogenic unit contains high concentrations of HF, NO and a small amount of NO2 and o-fluorotoluene. At this time, a part of it is separated and returned to the synthesis reactor. On the one hand, it can enter the cryogenic heat exchanger again together with the synthetic tail gas in the reactor for condensation and recovery of HF, NO2 and o-fluorotoluene. On the other hand, it can also serve the purpose of stirring and humidity adjustment before entering the reactor.

[0022] 4) A cryogenic heat exchanger refrigerant internal circulation solution: Exhaust gas enters the cryogenic heat exchanger for counter-heat exchange with the refrigerant. This forced refrigerant circulates within the cryogenic heat exchanger, creating 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 downstream to the upstream section, creating a condensation section with a small temperature difference corresponding to the melting point of the pollutants. This ensures that the pollutants are fully condensed and prevents solidification and blockage.

[0023] By adjusting the circulation volume, different cooling effects can be obtained, and the condensation rates of different pollutant components can be achieved, thereby improving the condensation interception effect and slowing down the impact of frosting.

[0024] In the synthesis tail gas, nitrogen dioxide has a melting point of −11°C and a boiling point of 21°C; nitric oxide has a melting point of −163.6°C and a boiling point of −151°C; o-fluorotoluene has a melting point of −62°C and a boiling point of 112-113°C; and HF has a melting point of −83°C and a boiling point of 19.54°C. When the tail gas is cryogenically cooled to below −80°C, HF, nitrogen dioxide, and o-fluorotoluene are almost completely condensed. During the condensation process, nitrogen dioxide and o-fluorotoluene can form frost in the cryogenic heat exchanger, blocking the tail gas flow path. To mitigate this frost blockage, higher-temperature gas is circulated within the tail gas purification system for defrosting.

[0025] (1) The exhaust gas enters the cryogenic heat exchanger and exchanges heat with the refrigerant in reverse. 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. A refrigerant circulation pump is set up to reflux the refrigerant at the higher temperature downstream to the upstream, forming a condensation section with a smaller temperature difference corresponding to the melting point of the pollutant component, so as to ensure that the pollutant component is fully condensed and does not solidify and block.

[0026] (2) The higher temperature gas circulation in the tail gas purification system is used for defrosting, and the higher temperature desorbed tail gas desorbed from the activated carbon adsorption unit is used as the defrosting heat source, which not only recovers the o-fluorotoluene in the desorbed tail gas but also achieves efficient defrosting.

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

[0028] (1) Condensation is performed step by step to recover pollutants, slowing down the frosting and blockage in the cryogenic process and improving the stability of system operation.

[0029] (2) By forced circulation of refrigerant, the cooling capacity of the refrigerant is fully utilized, the temperature of the refrigerant leaving the cryogenic heat exchanger is increased, and then heat is exchanged with the cryogenic exhaust gas at a lower temperature (-50℃) after cryogenic cooling, thereby improving the utilization efficiency of the refrigerant cooling capacity.

[0030] (3) Fully recover the frosting cold energy without introducing external defrosting medium, achieving efficient defrosting.

[0031] 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 condensing device, where it is spray-cooled with low-temperature water to recover the hydrogen fluoride component and obtain a hydrofluoric acid product with a 40wt% HF concentration.

[0032] Since the concentration of nitrogen oxides in the synthesis tail gas reaches 20,000-40,000 mg / Nm3, in order to slow down the impact of such a high concentration of tail gas entering the denitration reactor on the catalyst, the present invention sets an activated carbon absorption unit before the denitration reactor. The main components of the cryogenic tail gas are HF, NOx, o-fluorotoluene, nitrogen and a small amount of low-molecular VOCs components. Since HF has the highest polarity, it is in a competitive adsorption advantage, followed by NO, then o-fluorotoluene and NO2, and nitrogen is hardly adsorbed. Therefore, the activated carbon adsorption tower is also a concentration unit for HF, NO, and o-fluorotoluene. The desorbed tail gas after adsorption contains almost no nitrogen. After dehydration, it is introduced into the synthesis reactor and is easily recovered by cryogenic cooling.

[0033] Due to the high polarity of NO, the activated carbon adsorption tower partially adsorbs NO while adsorbing o-fluorotoluene and HF, thereby reducing the NO flow rate entering the SCR denitration catalyst and mitigating the reaction shock. The adsorbed NO then reenters the synthesis reactor with the desorbed tail gas, increasing the NO concentration in the reactor liquid and suppressing the side reaction (2HNO2 → NO + NO2 + water).

[0034] Because the desorption tail gas contains high concentrations of HF, NO and a small amount of NO2 and o-fluorotoluene after dehydration, it returns to the synthesis reactor and enters the deep cold heat exchanger again together with the synthesis tail gas in the reactor to condense and recover HF, NO2 and o-fluorotoluene.

[0035] Furthermore, the desorbed tail gas from the activated carbon adsorption tower adopts two treatment schemes according to the requirements 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 desorbed 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 desorbed tail gas enters the condensing device for cooling, dehydration and recovery of the hydrogen fluoride component. The remaining gas phase can be sent to the cryogenic unit as a defrosting medium or can enter the synthesis reactor to participate in the synthesis reaction. The HF, NO and other components in the gas phase are all reaction components of the synthesis reaction.

[0036] The effects of adopting the above technical improvement measures are as follows:

[0037] (1) Completely eliminate the damage to the SCR denitration catalyst caused by trace amounts of HF remaining in the cryogenic exhaust gas. Since HF has a greater polarity than NO, it is preferentially adsorbed by the cryogenic exhaust gas after entering the activated carbon adsorption tower, essentially eliminating the damage to the SCR denitration catalyst caused by HF.

[0038] (2) Recover trace amounts of HF components in the tail gas. The HF adsorbed by the activated carbon is desorbed by the superheated steam, washed with cooling water, and dissolved in water to obtain the hydrofluoric acid product.

[0039] (3) Reduce the impact of high-concentration nitrogen oxides on the SCR denitrification catalyst. After the deep-cooled exhaust gas enters the activated carbon adsorption tower, the high-concentration NO in the exhaust gas is partially adsorbed, reducing the NO flow entering the catalyst bed.

[0040] (4) Economical and efficient. The manufacturing cost of SCR denitrification catalyst is high and more expensive than activated carbon. At the same time, the tolerance of SCR denitrification catalyst to acid and alkali is worse than that of activated carbon. The installation of activated carbon adsorption tower in front of the denitrification reactor is equivalent to adding a protective measure for the denitrification reactor, which is conducive to extending the life of the SCR denitrification catalyst.

[0041] (5) The heat source of the system's internal medium is fully utilized. By adding the desorbed tail gas to the reactor for stirring, the heat enthalpy required to heat and decompose the synthesis reactor liquid is saved.

[0042] (6) Fully recycle and utilize pollutants such as o-fluorotoluene and nitrogen oxides contained in the exhaust gas.

[0043] Furthermore, the present invention innovatively proposes a technical solution for purifying tail gas circulation when performing SCR denitration treatment on cryogenic tail gas. This solution divides the purified tail gas from the denitration heat exchanger into two parts. One part is directly mixed with the heated adsorbed tail gas and participates in the tail gas circulation. The other part is cooled and dehydrated by the purified tail gas heat exchanger with the cryogenic tail gas before being discharged through the chimney. The effects of this technical solution are as follows:

[0044] (1) In terms of system operation stability, by circulating the purified exhaust gas, the fluctuation range of the NOx content in the adsorbed exhaust gas entering the denitrification reactor is reduced, and the impact of the NOx concentration fluctuation in the intake air on the catalyst is slowed down, which is beneficial to the stability of the SCR denitrification reaction process.

[0045] (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 operation stability. The improvement of system operation stability can effectively reduce the system operation cost and extend the service life of the catalyst.

[0046] (3) By online monitoring of the NOx concentration in the purified exhaust gas and adjusting the amount of ammonia injected into the adsorbed exhaust gas, the circulation of the purified exhaust gas also controls the fluctuation of the ammonia content in the exhaust gas entering the denitrification reactor, which is beneficial to the stability of the SCR denitrification reaction process, improves the denitrification effect, and reduces ammonia escape.

[0047] (4) The purified exhaust gas of each row is exchanged with the cryogenic exhaust gas at a lower temperature to condense part of the water in the purified exhaust gas. 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 during the SCR denitrification process.

[0048] (5) By circulating the purified exhaust gas, the concentration of nitrogen oxides is diluted, avoiding violent local reactions in the reactor and damage to the catalyst.

[0049] The present invention can fully recover nitrogen dioxide, o-fluorotoluene and fluorine components in the synthesis tail gas, fully recover waste heat and waste cooling, and has a relatively simple process route, is environmentally friendly, economical and efficient, and has good stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a process flow chart of the present invention.

[0051] Among them: synthesis reactor HC1, refrigerant circulation pump HC2, cryogenic heat exchanger HC3, reactor cover HC7, purified tail gas heat exchanger HC15;

[0052] Activated carbon adsorption tower HX1, reflux cooling fan HX3, spray cooling tower HX4, low-temperature water heat exchanger HX5, analytical tail gas condenser HX7, hydrofluoric acid product tank HX9, high-concentration hydrofluoric acid product tank HX10;

[0053] Denitrification heat exchanger XT3, circulating fan XT4, exhaust gas circulation regulating valve XT5, exhaust gas heater XT8, denitrification reactor XT9, pipeline mixer XT10, ammonia addition device XT11. DETAILED DESCRIPTION

[0054] Take a factory with an annual production capacity of 2,000 tons of o-fluorotoluene as an example. The production line includes 24 synthesis reactors, and the amount of synthetic tail gas is 1,000-2,000 Nm 3 / h, the main components of tail gas are: HF: 160∽170g / Nm 3 ; o-Fluorotoluene: 1000∽2000mg / Nm 3 ; NOx: 12000∽20000mg / Nm 3 A small amount of VOCs components; the rest is nitrogen. The main process of o-fluorotoluene synthesis includes salt formation, diazotization and thermal decomposition reaction steps. These reaction processes are all completed in the synthesis reactor. The process is as follows:

[0055] (1) Salt formation

[0056] Low-temperature hydrofluoric acid liquid (-15°C) is pumped from a hydrofluoric acid storage tank to a hydrogen fluoride metering tank. Chilled brine (CaCl2 solution comes from a refrigeration room) is introduced into the metering tank for cooling to maintain the temperature below 10°C. The low-temperature hydrofluoric acid liquid is then poured from the metering tank into a synthesis kettle and continued to be cooled with chilled brine to reduce the temperature of the synthesis kettle to between 4 and 6°C. The discharge valve of the o-toluidine metering tank is opened under stirring, and o-toluidine is slowly added dropwise while maintaining the temperature between 5 and 8°C. The reaction process is an excess reaction of hydrofluoric acid with a slightly negative pressure. After the addition is completed, the amount of chilled brine is adjusted and the reaction is completed by keeping the temperature at around 6°C for 30 minutes. The reaction time is 3 hours to obtain an o-toluidine hydrofluoride salt mixture.

[0057] (2) Diazotization

[0058] Lower the temperature of the circulating frozen brine in the synthesis kettle to 0~-2℃, and slowly add sodium nitrite solid using an automatic closed feeder under stirring. The feeding temperature is controlled at 0~5℃ and the pressure is slightly negative. After the addition is completed, keep warm for 30 minutes to complete the reaction. The reaction time is 6 hours to obtain diazo liquid.

[0059] There is a side reaction in the diazotization reaction process: 2HNO2→NO+NO2+water.

[0060] (3) Pyrolysis

[0061] Adjust the temperature of the chilled brine and slowly raise the temperature in stages (5-15°C, 15-25°C, 25-38°C) while stirring, increasing the temperature by approximately 1.2°C per hour until it reaches 38°C. Thermal decomposition is carried out under negative pressure for 16 hours. After the reaction, the contents of the kettle are placed in a standing tank for separation. The organic layer, a mixture of o-fluorotoluene, is sent to the neutralization kettle; 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 kettle.

[0062] The reaction temperature is different in different reaction stages and needs to be adjusted according to the reaction period.

[0063] During the above reaction process, a large amount of tail gas is generated. The salt formation stage produces tail gas containing hydrogen fluoride and a small amount of o-toluidine; the diazotization stage produces tail gas containing hydrogen fluoride, nitrogen dioxide, nitric oxide, and o-toluidine; and the pyrolysis stage produces tail gas containing nitrogen, hydrogen fluoride, o-toluidine, and o-fluorotoluene.

[0064] The reaction temperature is different in different reaction stages and needs to be adjusted according to the reaction period.

[0065] See attached Figure 1 The structure and cryogenic process of 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 synthesis reactor cover HC7. The tube-side air inlet of one end of the cryogenic heat exchanger HC3 is directly connected to the reactor cover HC7 and communicates with the synthesis reactor HC1. The tube-side air outlet at the other end is connected to the induced draft fan inlet through a pipeline.

[0067] 2. The cryogenic heat exchangers HC3 are used in parallel. During normal operation, at least one unit is defrosted while the others are in cryogenic operation. Under the pressure of the kettle and the suction of the induced draft fan, the synthetic tail gas enters the tube side of the cryogenic heat exchanger HC3, exchanges heat with the -70°C low-temperature refrigerant introduced into the shell side, and forms cryogenic tail gas with a temperature below -50°C, which enters the subsequent purification unit.

[0068] 3. Set the outlet temperature limit of the cryogenic tail gas of the cryogenic heat exchanger HC3 to -50∽-55°C, and also set the pressure difference limit between the tail gas inlet and outlet. By real-time monitoring the temperature and pressure difference, the operating mode of the cryogenic heat exchanger HC3 is regulated. When the outlet cryogenic tail gas temperature is lower than -55°C, stop supplying the -70°C low-temperature refrigerant to the cryogenic heat exchanger HC3. The cryogenic heat exchanger HC3 enters the defrost mode and automatically defrosts by generating the higher temperature synthetic tail gas in the kettle. During the defrost phase, stop supplying the -70°C low-temperature refrigerant to the cryogenic heat exchanger HC3.

[0069] 4. The "cryogenic-defrosting" working mode of the cryogenic heat exchanger HC3 can also be controlled in a timing manner, that is, the time ratio of cryogenic and defrosting in one "cryogenic-defrosting" cycle of the cryogenic heat exchanger HC3 can be set, and the "cryogenic" and "defrosting" modes of the cryogenic heat exchanger HC3 can be staggered and used alternately;

[0070] 5. Defrosting cryogenic heat exchanger HC3 can also be done using the desorbed tail gas from the activated carbon adsorption unit. The desorbed tail gas originally entering the synthesis reactor HC1 is diverted to the outlet of the cryogenic heat exchanger HC3 to be defrosted. The outlet valve of this cryogenic heat exchanger HC3 is closed, and the frost is defrosted using the thermal enthalpy of the desorbed tail gas, while the frost blocks are impacted using the pressure of the desorbed tail gas.

[0071] Although the amount and composition of tail gas discharged from each synthesis reactor HC1 are different at different synthesis reaction stages, the flow rate and composition of the discharged cryogenic tail gas are basically stable due to the large number of synthesis reactors HC1.

[0072] One set of "adsorption unit + denitrification unit" corresponds to multiple sets of "synthesis reactor HC1 + cryogenic unit".

[0073] The following is a further explanation of the process of online cryogenic purification of o-fluorotoluene synthesis tail gas with reference to the accompanying drawings:

[0074] Refer to the attached figure, the amount of synthetic tail gas discharged without cryogenic condensation is 1000∽2000Nm 3 / h, the main components of tail gas are: HF: 160∽170g / Nm 3 ; o-Fluorotoluene: 1000∽2000mg / Nm 3 ; NOx: 12000∽20000mg / Nm 3 ; A small amount of VOCs components; the rest is nitrogen.

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

[0076] The cryogenic unit consists of multiple cryogenic heat exchangers mounted on the lid HC7 of synthesis reactor HC1. These cryogenic heat exchangers HC3 rotate through the condensation and defrosting processes. Synthesis tail gas generated by synthesis reactor HC1 enters the condensation stage of cryogenic heat exchanger HC3 from its bottom. The cryogenic tail gas exiting the cryogenic unit is split into two streams: one stream is fed into the activated carbon adsorption unit for adsorption, and the other stream is returned to synthesis reactor HC1 to regulate 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 limit values ​​for the exhaust gas pressure at the outlet of the cryogenic heat exchanger HC3. When the exhaust gas pressure at the outlet of the cryogenic heat exchanger HC3 is lower than the lower limit value, the refrigerant is cut off and the cryogenic heat exchanger HC3 enters the defrosting stage; when the exhaust gas pressure at the outlet of the cryogenic heat exchanger HC3 is higher than the upper limit value, the refrigerant is introduced and the cryogenic heat exchanger HC3 enters the cryogenic working stage, and the process is repeated alternately.

[0078] The cryogenic heat exchanger HC3 features three enhanced cooling sections, each of which uses a refrigerant circulation pump HC2 to return refrigerant from downstream to upstream. The synthesis tail gas and condensate are subjected to countercurrent contact, with hydrogen fluoride, o-fluorotoluene, and nitrogen dioxide components condensed in each of the three enhanced cooling sections. This improves the condensation and interception of pollutants in the synthesis tail gas. The condensate containing nitrogen dioxide generated in the cryogenic heat exchanger HC3 falls directly through the reactor cover HC7 into the synthesis reactor HC1, and then directly into the synthesis reactor HC1 through the bottom of the cryogenic reactor HC3. This increases the nitrogen dioxide concentration in the reactor liquid, suppresses the side reaction (2HNO2 → NO + NO2 + water), and improves HNO2 utilization.

[0079] The cryogenic tail gas enters the adsorption unit, and 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 includes at least two independent activated carbon adsorption towers HX1, one for adsorption and the other for desorption, which are carried out alternately; 10-20wt% of the adsorbed tail gas after adsorption from another activated carbon adsorption tower is introduced into the desorbed activated carbon adsorption tower HX1 to cool the carbon adsorption bed until the set temperature, and then 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 drawn out from the activated carbon adsorption unit is either fed into the synthesis reactor HC1 to adjust the reactor liquid temperature; or condensed by the condensing device and returned to the defrost gas pipeline as the defrost gas of the cryogenic unit or fed into the synthesis reactor HC1 to adjust the reactor liquid temperature.

[0081] The condensing device includes a spray cooling tower HX4 and an analytical tail gas condenser HX7 connected in sequence. The desorbed tail gas is sprayed and cooled by low-temperature water in the spray cooling tower HX4 and then sent to the analytical 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 part of the HF component in the desorbed tail gas. The collected and drawn part of the circulating cold cooling water and the condensate of the low-temperature water heat exchanger HX5 are sent to the hydrofluoric acid product tank HX9 to obtain a hydrofluoric acid product with a hydrogen fluoride mass concentration of more than 40wt%; the condensate of the analytical tail gas condenser HX7 is sent to the high-concentration hydrofluoric acid product tank HX10 to obtain a high-concentration hydrofluoric acid product.

[0082] See also Figure 1The denitration unit includes a denitration heat exchanger XT3, an exhaust heater XT8, an ammonia device XT11, a pipeline mixer XT10, an ammonia addition device XT11, and a denitration reactor XT9. The adsorbed exhaust gas from the adsorption unit is heated by heat exchange with the purified exhaust gas in the denitration heat exchanger XT3, then heated to above 170°C by the exhaust heater XT8. After ammonia is added by the ammonia addition device XT11, it enters the denitration reactor XT9 for denitration and purification. The resulting purified exhaust gas is divided into two parts. One part is directly mixed with the heated adsorbed exhaust gas exiting the denitration heat exchanger XT3 through a circulation fan XT4 and an exhaust circulation regulating valve XT5 and sent to the exhaust heater XT8 to participate in the exhaust gas circulation, thereby diluting the nitrogen oxide concentration in the exhaust gas entering the denitration reactor. The other part is cooled and dehumidified by heat exchange with the desorbed exhaust gas through the circulation fan XT4 and the purified exhaust heat exchanger HC15, and then discharged through a 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 less than 100mg / Nm 3 .

[0084] After purification, the annual recovery of HF is about 2,000 tons and o-fluorotoluene is about 10 tons.

Claims

1. An online cryogenic purification process for o-fluorotoluene synthesis tail gas, comprising the synthesis tail gas generated in a synthesis reactor during the o-fluorotoluene synthesis process, characterized in that: The synthesis tail gas is indirectly cooled to below -80°C by a cryogenic unit through a low-temperature refrigerant at -100°C, wherein more than 99wt% of o-fluorotoluene, nitrogen dioxide and more than 90wt% of hydrogen fluoride components are condensed and intercepted and returned to the synthesis reactor with the condensate to continue participating in the synthesis reaction. The cryogenic tail gas leaving the cryogenic unit is divided into two streams, one of which is sent to an activated carbon adsorption unit for further dehydrogenation and then to an SCR denitration unit for denitration and then discharged, and the other is returned to the synthesis reactor to adjust the reactor liquid temperature and stir the reactor liquid; The cryogenic unit is composed of multiple cryogenic heat exchangers installed on the cover of the synthesis reactor. The multiple cryogenic heat exchangers take turns to perform condensation and defrosting processes. The synthesis tail gas generated by the synthesis reactor enters the cryogenic heat exchanger in the condensation stage from the bottom of the cryogenic heat exchanger for condensation.

2. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 1, characterized in that: The cryogenic heat exchanger has multiple enhanced cooling sections, and each enhanced cooling section uses a refrigerant circulation pump to return the refrigerant from the downstream to the upstream.

3. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 1 or 2, characterized in that: The cryogenic tail gas enters the adsorption unit to adsorb o-fluorotoluene and hydrogen fluoride, and the adsorbed tail gas after removing o-fluorotoluene and hydrogen fluoride enters the SCR denitration unit for denitration; the adsorption unit includes at least two independent activated carbon adsorption towers, one for adsorption and the other for desorption, which are performed alternately; 10-20% by volume of the adsorbed tail gas from another activated carbon adsorption tower after adsorption is introduced into the desorbed activated carbon adsorption tower to cool the carbon adsorption bed until the set temperature, and then enters the subsequent denitration unit together with the remaining adsorbed tail gas.

4. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 3, characterized in that: The desorption medium of the activated carbon adsorption tower is superheated steam. The desorption tail gas drawn out from the adsorption unit is condensed by a condensing device and divided into two streams. One stream is reused as the defrosting gas of the cryogenic unit, and the other stream is sent to the synthesis reactor to adjust the reactor liquid temperature.

5. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 4, characterized in that: The condensing device includes a spray cooling tower and an analytical tail gas condenser connected in sequence. The desorbed tail gas is sprayed and cooled by low-temperature water in the spray cooling tower and then sent to the analytical tail gas condenser for condensation. The low-temperature cooling water in the spray cooling tower is cooled by a low-temperature water heat exchanger and then circulated and sprayed to absorb and condense part of the HF component in the desorbed tail gas.

6. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 4, characterized in that: According to the mass of hydrogen fluoride adsorbed by the activated carbon adsorption tower, the temperature of the superheated steam is controlled, the regeneration time is controlled, and the amount of superheated steam used for regeneration is controlled; the amount of condensed water introduced by the superheated steam for desorption is controlled to ensure that the mass concentration of hydrogen fluoride in the low-temperature water is above 40%.

7. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 1, characterized in that: The SCR denitration unit includes a denitration heat exchanger, a tail gas heater, an ammonia adding device, a pipeline mixer and an SCR denitration reactor; the adsorbed tail gas from the adsorption unit is heated by heat exchange with the purified tail gas in the denitration heat exchanger, and then heated to above 170°C by the tail gas heater, ammonia is added by the ammonia adding device, and finally mixed evenly by the pipeline mixer before entering the SCR denitration reactor for denitration purification. The obtained purified tail gas is then sent to the denitration heat exchanger for heat exchange with the adsorbed tail gas.

8. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 7, characterized in that: The purified exhaust gas coming out of the denitrification heat exchanger is divided into two parts. One part is directly mixed with the adsorbed exhaust gas after being heated out of the denitrification heat exchanger, participates in the exhaust gas circulation, and dilutes the nitrogen oxide concentration in the exhaust gas entering the SCR denitrification reactor. The other part is discharged through the chimney after heat exchange with the desorbed exhaust gas in the purified exhaust gas heat exchanger for cooling and dehumidification.

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