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

Abstract

The invention discloses an o-fluorotoluene synthesis tail gas on-line cryogenic purification process, which comprises the following steps: synthesizing synthesis tail gas generated in a reaction kettle in the o-fluorotoluene synthesis process, and indirectly cooling the synthesis tail gas to below-80 DEG C by a cryogenic unit through a low-temperature refrigerant at-100 DEG C; wherein 99 wt% or more of o-fluorotoluene, nitrogen dioxide and 90 wt% or more of hydrogen fluoride components are condensed and intercepted and return to the synthesis reaction kettle along with condensate to continue to participate in the synthesis reaction, and copious cooling tail gas from the copious cooling unit enters the activated carbon adsorption unit to be adsorbed for further dehydrofluorination, then enters the SCR denitration unit to be denitrated and then is discharged. The method is simple in process route, clean, environment-friendly, stable, efficient, low in operation cost and high in defluorination and denitration efficiency.

Description

Technical Field

[0001] The 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, and specifically is an online deep-cold purification process for o-fluorotoluene synthesis tail gas. Background Art

[0002] Organic fluorine materials have excellent thermal oxygen stability, chemical corrosion resistance, aging resistance, non-stickiness, electrical insulation and extremely small friction coefficient, and are now widely used in many industries and fields such as aviation, aerospace, navigation, chemical industry, petroleum, automobile, machinery, electronic information, biomedical materials, construction, environmental protection, etc. However, since natural fluorine-containing organic molecules are very rare, most of the organic fluorine-containing substances required must be synthesized artificially, which has also led to the rapid development of the fluorine chemical industry.

[0003] Organic fluorine chemicals are at the top of the entire fluorine chemical industry value chain. Its downstream applications are mainly concentrated in special environments that require high temperature resistance and corrosion resistance. The material performance requirements are relatively high, so the added value at the end of the industrial chain is relatively high. One of the most important emerging downstream application areas of organic fluorine is the new energy vehicle industry. For example, the application of o-fluorotoluene in the field of lithium-ion batteries has greatly improved the thermal stability of the electrolyte and the safety of lithium batteries, which is conducive to increasing the reduction potential of solvent molecules on the surface of the carbon negative electrode, optimizing the solid electrolyte interface film, and improving the compatibility between the electrolyte and the activated carbon material, thereby stabilizing the electrochemical properties of the electrode [Song Xin, Wu Zesheng, Yang Qiangqiang, et al. Research Progress on New Fluorine-containing Additives for Lithium-ion Battery Electrolytes. Power Technology, Issue 10, 2017].

[0004] However, organic fluorine materials will produce a large amount of air pollutants during the production process, and the concentration of pollutants is high. For example, during the synthesis of o-fluorotoluene, the synthetic tail gas contains high concentrations of hydrogen fluoride and nitrogen oxide pollution components, 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 3It can cause acute poisoning. So far, there is no efficient and low-cost method to solve the HF and NOx pollution problems in the production process. Although some enterprises have taken end-of-pipe treatment measures, which have played a certain role in alleviating local environmental problems, end-of-pipe treatment only focuses on the purification and treatment of pollutants, and cannot prevent and eliminate the generation and discharge of waste from the source and the production process, which is a temporary solution but not a fundamental solution. The most critical thing is that due to the large investment in end-of-pipe treatment infrastructure and high operating costs, enterprises only invest but do not produce, and cannot achieve the unity of economic, environmental and social benefits. In order to recover HF in waste gas, Lv Zhimin et al. [Lv Zhimin, Zhang Yuqing, Tang Anping et al. Clean production process of o-fluorotoluene. Chemical World, 2002.12] studied the water washing recovery process, using three-stage water circulation to absorb HF in waste gas, coagulation sedimentation and activated carbon adsorption to treat fluorine-containing wastewater. Although part of HF can be recovered, the recovery process route is long, and there are still wastewater pollutants. After treatment, there is still a certain amount of HF pollution components in the tail gas. In order to reduce the HF pollution components in the exhaust gas as much as possible, there are also studies and applications of using alkaline solution absorption. However, due to the low solubility of sodium fluoride, the system is scaled and blocked, which seriously affects the normal operation of the system and also produces a large amount of fluoride-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 synthetic 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] With the development of fluorine chemical industry, the pollution problems of HF and NOx have become more and more prominent, which has seriously restricted the development of organic fluorine chemical industry.

[0007] The present invention is aimed at the above problems and develops an online deep-cold purification process for o-fluorotoluene synthesis tail gas. Summary of the invention

[0008] The purpose of the present invention is to solve the above technical problems and to provide an online deep-cold 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.

[0009] The method of the 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 cryogenic unit through a low-temperature refrigerant at -100°C, 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 with the condensate to continue to participate in the synthesis reaction; the cryogenic tail gas from the cryogenic unit enters an activated carbon adsorption unit for adsorption and further dehydrofluorination, and then enters an SCR denitration unit for denitration and then is discharged.

[0010] The cryogenic unit is composed of a plurality of cryogenic heat exchangers installed on the cover of the synthesis reactor. The plurality of cryogenic heat exchangers perform condensation and defrosting processes in turn. The synthetic 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.

[0011] 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.

[0012] The cryogenic tail gas from the cryogenic unit is divided into two streams, one of which is sent to the activated carbon adsorption unit, and the other is returned to the synthesis reactor to adjust the reactor liquid temperature and stir the reactor liquid.

[0013] 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 denitration unit for denitration; the adsorption unit comprises 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 activated carbon adsorption tower after desorption to cool the carbon adsorption bed until the set temperature, and then enters the subsequent denitration unit together with the remaining adsorbed tail gas.

[0014] 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 is condensed by a condensing device and reused as the defrosting gas of the cryogenic unit or sent to the synthesis reactor to adjust the reactor liquid temperature.

[0015] The condensing device comprises 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.

[0016] 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, so as to control the amount of superheated steam used for regeneration; 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%.

[0017] The SCR denitration unit comprises 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, and the obtained purified exhaust gas is then sent to the denitration heat exchanger for heat exchange with the adsorbed exhaust gas.

[0018] The purified exhaust gas coming out of the denitration heat exchanger is divided into two parts. One part is directly mixed with the adsorbed exhaust gas after the temperature is increased after leaving the denitration heat exchanger, participates in the exhaust gas circulation, and dilutes the nitrogen oxide concentration in the exhaust gas entering the denitration 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.

[0019] 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 will begin. Therefore, the temperature of the kettle liquid in the synthesis reactor needs to be adjusted frequently, which causes a large amount of reaction mixture components in the kettle liquid to be gasified and discharged together with the synthetic tail gas, and the concentration is high, which greatly reduces the utilization rate of the raw materials. In order to improve the utilization rate of the raw materials, the present invention proposes a technical solution for online deep cooling of the synthetic tail gas, which changes the traditional technical solution of leading the tail gas into a deep cold heat exchanger for condensation and recovery. The specific improvement measures are as follows:

[0020] 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;

[0021] 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;

[0022] 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 sent back 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 to condense and recover HF, NO2 and o-fluorotoluene, and on the other hand, it can also be used for stirring and humidity adjustment in the reactor.

[0023] 4) In the cryogenic heat exchanger, the exhaust gas enters the cryogenic heat exchanger for reverse heat exchange with the refrigerant, and the refrigerant is forced to circulate in the cryogenic heat exchanger to form enhanced cooling sections corresponding to hydrogen fluoride, o-fluorotoluene and nitrogen dioxide. A refrigerant circulation pump is set to reflux the refrigerant with a higher temperature downstream to the upstream, forming a condensation section with a small temperature difference corresponding to the melting point temperature of the pollutant component, so as to ensure that the pollutant component is fully condensed and does not solidify and block.

[0024] 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.

[0025] In the synthetic tail gas, the melting point of nitrogen dioxide is -11°C and the boiling point is 21°C; the melting point of nitrogen monoxide is -163.6°C and the boiling point is -151°C; the melting point of o-fluorotoluene is -62°C and the boiling point is 112-113°C; the melting point of HF is 8-3°C and the boiling point is 19.54°C. When the tail gas is deep-cooled to below -80°C, HF, nitrogen dioxide and o-fluorotoluene are almost completely condensed. During the condensation process, nitrogen dioxide and o-fluorotoluene will appear in the deep-cold heat exchanger to frost and block the tail gas flow channel. In order to alleviate the frosting and clogging problem, the higher temperature gas circulation in the tail gas purification system is used for defrosting.

[0026] (1) The tail gas enters the cryogenic heat exchanger for reverse heat exchange with the refrigerant, and the refrigerant is forced to circulate in the cryogenic heat exchanger to form enhanced cooling sections corresponding to hydrogen fluoride, o-fluorotoluene and nitrogen dioxide. A refrigerant circulation pump is set to reflux the refrigerant with a higher temperature downstream to the upstream, forming a condensation section with a small temperature difference corresponding to the melting point temperature of the pollutant component, so as to ensure that the pollutant component is fully condensed and does not solidify and block.

[0027] (2) The relatively high temperature gas circulation in the exhaust gas purification system is used for defrosting, and the relatively high temperature exhaust gas desorbed from the activated carbon adsorption unit is used as a defrosting heat source, thereby recovering the o-fluorotoluene in the desorbed exhaust gas and achieving efficient defrosting.

[0028] The effects of this improvement are as follows:

[0029] (1) Condensation and recovery of polluted components step by step can slow down the frosting and blockage in the cryogenic process and improve the stability of system operation.

[0030] (2) By forcedly circulating the 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 relatively low temperature (-50°C) cryogenic exhaust gas after cryogenic cooling to improve the utilization efficiency of the refrigerant cooling capacity.

[0031] (3) Fully recover the frosting cold energy, without the need to introduce external defrosting media, to achieve efficient defrosting.

[0032] Furthermore, in order to efficiently recover the hydrogen fluoride component in the synthetic tail gas, the activated carbon adsorption tower is desorbed and regenerated, and the desorbed tail gas enters the spray cooling tower of the condensation device, and is spray-cooled with low-temperature water to recover the hydrogen fluoride component and obtain a hydrofluoric acid product with a 40wt% HF concentration.

[0033] Since the concentration of nitrogen oxides in the synthetic tail gas reaches 20000-40000 mg / Nm3, in order to reduce 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 in front of the denitration reactor. The main components of the deep cold tail gas are HF, NOx, o-fluorotoluene, nitrogen and a small amount of low molecular weight VOCs components. Since HF has the largest 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 desorption tail gas after adsorption contains almost no nitrogen. After dehydration, it is introduced into the synthesis reactor and is easy to be recovered by deep cold.

[0034] 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 entering the SCR denitration catalyst and slowing down the reaction impact. The adsorbed NO enters the synthesis reactor with the desorbed tail gas, increasing the NO concentration in the reactor liquid and inhibiting the occurrence of the side reaction 〈2HNO2→NO+NO2+water〉.

[0035] 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 synthetic tail gas in the reactor to condense and recover HF, NO2 and o-fluorotoluene.

[0036] Furthermore, the desorbed tail gas from the activated carbon adsorption tower is treated in two ways 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 condensation device for cooling, dehydration and recovery of the hydrogen fluoride component. The remaining gas phase can be sent to the deep cooling unit as a defrosting medium or 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.

[0037] The effects of adopting the above technical improvement measures are as follows:

[0038] (1) Completely eliminate the damage of trace HF remaining in the cryogenic exhaust gas to the SCR denitration catalyst. Since HF has a greater polarity than NO, the cryogenic exhaust gas is preferentially adsorbed after entering the activated carbon adsorption tower, which basically eliminates the damage of HF to the SCR denitration catalyst.

[0039] (2) Recovering 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.

[0040] (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.

[0041] (4) Economical and efficient. The manufacturing cost of SCR denitration catalyst is high, which is more expensive than activated carbon. At the same time, the tolerance of SCR denitration catalyst to acid and alkali is worse than that of activated carbon. The activated carbon adsorption tower is set in front of the denitration reactor, which is equivalent to adding a protective measure to the denitration reactor, which is conducive to extending the life of the SCR denitration catalyst.

[0042] (5) The heat source of the system's internal medium is fully utilized. By desorbing the tail gas and stirring it in the reactor, the heat enthalpy required for heating and pyrolysis of the synthesis reactor liquid is saved.

[0043] (6) Fully recycle and utilize pollutants such as o-fluorotoluene and nitrogen oxides contained in the desorption exhaust gas.

[0044] Furthermore, when the present invention performs SCR denitration treatment on cryogenic tail gas, an innovative technical solution for purifying tail gas circulation is proposed, that is, the purified tail gas from the denitration heat exchanger is divided into two parts, one part is directly mixed with the adsorbed tail gas after the temperature is increased and participates in the tail gas circulation, and the other part is discharged through the chimney after heat exchange and dehydration with the cryogenic tail gas through the purified tail gas heat exchanger. The effects of adopting this technical solution are as follows:

[0045] (1) In terms of system operation stability, by purifying the circulation of exhaust gas, the fluctuation range of NOx content in the adsorbed exhaust gas entering the denitration reactor is reduced, and the impact of NOx concentration fluctuation in the intake air on the catalyst is mitigated, which is beneficial to the stability of the SCR denitration reaction process.

[0046] (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.

[0047] (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.

[0048] (4) The purified exhaust gas of each row is exchanged with the deep-cold 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 to reduce ammonia escape. After the heat exchange, the temperature of the deep-cold exhaust gas is greatly increased, which greatly saves the energy consumption of heating the exhaust gas in the SCR denitrification process.

[0049] (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.

[0050] The present invention can realize the full recovery of nitrogen dioxide, o-fluorotoluene and fluorine components in the synthetic tail gas, fully recover the waste heat and waste cold, and the process route is relatively simple, environmentally friendly, economical and efficient, and has good stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a process flow chart of the present invention.

[0052] Among them: synthesis reactor HC1, refrigerant circulation pump HC2, cryogenic heat exchanger HC3, reactor cover HC7, purified tail gas heat exchanger HC15;

[0053] 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;

[0054] DeNOx heat exchanger XT3, circulating fan XT4, exhaust gas circulation regulating valve XT5, exhaust gas heater XT8, deNOx reactor XT9, pipeline mixer XT10, ammonia adding device XT11. DETAILED DESCRIPTION

[0055] 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 with a synthetic tail gas volume of 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 pyrolysis reaction steps, all of which are completed in the synthesis reactor. The process is as follows:

[0056] (1) Salt formation

[0057] Low-temperature hydrofluoric acid liquid (-15°C) is pumped from a hydrofluoric acid storage tank to a hydrogen fluoride metering tank, and frozen brine (CaCl2 solution comes from a refrigeration room) is introduced into the metering tank for cooling to keep the temperature below 10°C. Then, the low-temperature hydrofluoric acid liquid is put into a synthesis kettle from the metering tank, and the frozen brine is continued to be used for cooling to reduce the temperature of the synthesis kettle to between 4 and 6°C. Under stirring, the discharge valve of the o-toluidine metering tank is opened, and o-toluidine is slowly added dropwise. The temperature is kept between 5 and 8°C during the addition. The reaction process is an excess reaction of hydrofluoric acid and a slightly negative pressure. After the addition is completed, the amount of frozen brine is adjusted, and the reaction is completed by keeping the temperature at about 6°C for 30 minutes. The reaction time is 3 hours to obtain an o-toluidine hydrofluoride salt mixture.

[0058] (2) Diazotization

[0059] Lower the temperature of the circulating frozen brine in the synthesis kettle to 0-2°C, slowly add solid sodium nitrite using an automatic closed feeder under stirring, control the feeding temperature at 0-5°C, and slightly negative pressure. After the addition is completed, keep warm for 30 minutes to complete the reaction. The reaction time is 6 hours to obtain diazo liquid.

[0060] There is a side reaction in the diazotization process: 2HNO2→NO+NO2+water.

[0061] (3) Pyrolysis

[0062] Adjust the temperature of the frozen brine, slowly raise the temperature in stages (5-15°C, 15-25°C, 25-38°C) under stirring, raise the temperature by about 1.2°C per hour until it reaches 38°C, and the pressure is negative pressure, and perform thermal decomposition for 16 hours. After the reaction is completed, the materials in the kettle are placed in a standing tank for stratification, and the organic layer is a mixture of o-fluorotoluene sent to the neutralization kettle; the inorganic layer is a mixed acid solution containing hydrofluoric acid and sodium fluoride, etc., which is sent to the mother liquid tank and enters the hydrogen fluoride recovery kettle.

[0063] The reaction temperature is different in different reaction stages and needs to be adjusted according to the reaction period.

[0064] During the above reaction process, a large amount of tail gas is generated. The salt-forming 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; the pyrolysis stage produces tail gas containing nitrogen, hydrogen fluoride, o-toluidine, and o-fluorotoluene.

[0065] The reaction temperature is different in different reaction stages and needs to be adjusted according to the reaction period.

[0066] See attached Figure 1 The structure and cryogenic process of the o-fluorotoluene synthesis tail gas cryogenic unit are as follows:

[0067] 1. The cryogenic unit is composed of at least three shell and tube cryogenic heat exchangers HC3 arranged on the cover HC7 of the synthesis reactor. The tube side air inlet of one end of the cryogenic heat exchanger HC3 is directly connected to the cover HC7 and communicated with the synthesis reactor HC1, and the tube side air outlet of the other end is connected to the inlet of the induced draft fan through a pipeline.

[0068] 2. Cryogenic heat exchangers HC3 are used in parallel. During normal operation, at least one unit is defrosted, and the rest 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℃ low-temperature refrigerant introduced into the shell side, and forms cryogenic tail gas with a temperature below -50℃ that enters the subsequent purification unit;

[0069] 3. Set the outlet temperature limit of the cryogenic tail gas of the cryogenic heat exchanger HC3 to -50∽-55℃, and set the pressure difference limit between the inlet and outlet of the tail gas. By real-time monitoring the temperature and pressure difference, adjust the working mode of the cryogenic heat exchanger HC3. When the outlet cryogenic tail gas temperature is lower than -55℃, stop introducing -70℃ low-temperature refrigerant into the cryogenic heat exchanger HC3, and the cryogenic heat exchanger HC3 enters the defrost mode, and automatically defrosts by generating a relatively high temperature synthetic tail gas in the kettle. During the defrost stage, stop introducing -70℃ low-temperature refrigerant into the cryogenic heat exchanger HC3;

[0070] 4. The “deep cooling-defrosting” working mode control of the cryogenic heat exchanger HC3 can also be controlled in a timing manner, that is, the time ratio of deep cooling and defrosting in one cycle of the cryogenic heat exchanger HC3 “deep cooling-defrosting” is set, and the “deep cooling” and “defrosting” of the cryogenic heat exchanger HC3 are staggered and used alternately;

[0071] 5. The "defrosting" of the cryogenic heat exchanger HC3 can also be done by using the desorbed tail gas from the activated carbon adsorption unit. The desorbed 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. The frost is defrosted by the heat enthalpy of the desorbed tail gas, and the frost blocks are impacted by the pressure of the desorbed tail gas.

[0072] Although the amount and composition of tail gas discharged from each synthesis reactor HC1 are different at different synthesis reaction stages, due to the large number of synthesis reactors HC1, the flow rate and composition of the discharged deep-cold tail gas are basically stable.

[0073] One set of "adsorption unit + denitrification unit" corresponds to multiple sets of "synthesis reactor HC1 + cryogenic unit".

[0074] The following is a further explanation of the process of online cryogenic purification of o-fluorotoluene synthesis tail gas in conjunction with the accompanying drawings:

[0075] 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.

[0076] The synthetic tail gas generated by the reaction in the synthesis reactor HC1 rises and passes through the kettle cover HC7 on the top of the synthesis reactor HC1 and directly enters the cryogenic heat exchanger HC3 of the cryogenic unit. The synthetic tail gas is indirectly cooled to below -80°C by a low-temperature refrigerant of -100°C in the cryogenic unit, and 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 to participate in the synthetic reaction. The cryogenic tail gas leaving the cryogenic unit enters the activated carbon adsorption unit for adsorption and further dehydrofluorination, and then enters the SCR denitration unit for denitration and then is discharged.

[0077] The cryogenic unit is composed of multiple cryogenic heat exchangers installed on the lid HC7 of the synthesis reactor HC1. The multiple cryogenic heat exchangers HC3 perform condensation and defrosting processes in turn. The synthesis tail gas generated by the synthesis reactor HC1 enters the cryogenic heat exchanger HC3 in the condensation stage from the bottom of the cryogenic heat exchanger HC3 for condensation. The cryogenic tail gas leaving the cryogenic unit is divided into two streams, one of which is sent to the activated carbon adsorption unit for adsorption, and the other is sent back to the synthesis reactor HC1 to adjust the reactor liquid temperature and stir the reactor liquid.

[0078] The condensation-condensation-defrosting process of the cryogenic heat exchanger HC3 can also be controlled by setting upper and lower limit values ​​of 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 this is performed alternately.

[0079] The cryogenic heat exchanger HC3 has three sections of enhanced cooling, and each section of the enhanced cooling uses a refrigerant circulation pump HC2 to return the refrigerant from the downstream to the upstream. The synthetic tail gas is in reverse contact with the condensate, and the hydrogen fluoride, o-fluorotoluene and nitrogen dioxide components are respectively enhanced and condensed in the three sections of enhanced cooling section SL14, thereby improving the condensation interception effect of the synthetic tail gas pollution components. The condensate containing nitrogen dioxide generated in the cryogenic heat exchanger HC3 directly falls through the kettle cover HC7 into the synthesis reactor HC1, and directly falls into the synthesis reactor HC1 through the bottom of the cryogenic reactor HC3, thereby 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.

[0080] 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 denitration unit for denitration; the adsorption unit comprises at least two independent activated carbon adsorption towers HX1, one for adsorption and the other for desorption, which are performed 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 denitration unit together with the remaining adsorbed tail gas.

[0081] The desorption medium of the activated carbon adsorption tower HX1 is superheated steam, and the desorption tail gas drawn out from the activated carbon adsorption unit is either sent to 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 sent to the synthesis reactor HC1 to adjust the reactor liquid temperature.

[0082] The condensing device comprises a spray cooling tower HX4 and an analytical tail gas condenser HX7 which are 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 a 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 a hydrofluoric acid product tank HX9 to obtain a hydrofluoric acid product with a mass concentration of hydrogen fluoride of more than 40wt%; the condensate of the analytical tail gas condenser HX7 is sent to a high-concentration hydrofluoric acid product tank HX10 to obtain a high-concentration hydrofluoric acid product.

[0083] See also Figure 1 The denitration unit includes a denitration heat exchanger XT3, an exhaust heater XT8, an ammonia device XT11, a pipeline mixer XT10, an ammonia adding 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 and then heated to above 170°C by the exhaust heater XT8, and then enters the denitration reactor XT9 for denitration purification after ammonia is added by the ammonia adding device XT11. The obtained purified exhaust gas is divided into two parts, one part is directly mixed with the adsorbed exhaust gas after the temperature is increased after leaving the denitration heat exchanger XT3 through the circulating fan XT4 and the exhaust gas circulation regulating valve XT4, and is sent to the exhaust heater XT8 to participate in the exhaust gas circulation, so as to dilute the nitrogen oxide concentration in the exhaust gas entering the denitration reactor, and the other part is discharged through the chimney after heat exchange with the desorbed exhaust gas through the circulating fan XT4 and the purified exhaust heat exchanger HC15, and then cooled and dehumidified.

[0084] 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 .

[0085] 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 synthetic 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 to participate in the synthesis reaction. The cryogenic tail gas from the cryogenic unit enters the activated carbon adsorption unit for adsorption and further dehydrofluorination, and then enters the SCR denitration unit for denitration and then is discharged.

2. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 1, characterized in that: The cryogenic unit is composed of a plurality of cryogenic heat exchangers installed on the cover of the synthesis reactor. The plurality of cryogenic heat exchangers perform condensation and defrosting processes in turn. The synthetic 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.

3. The online cryogenic purification process for o-fluorotoluene synthesis tail gas as claimed in claim 2, 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.

4. The method for recycling nitrogen oxides from o-fluorotoluene synthesis tail gas as claimed in claim 1, characterized in that: The cryogenic tail gas from the cryogenic unit is divided into two streams, one of which is sent to the activated carbon adsorption unit, and the other is returned to the synthesis reactor to adjust the reactor liquid temperature and stir the reactor liquid.

5. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to any one of claims 1 to 4, 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 comprises 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 activated carbon adsorption tower after desorption to cool the carbon adsorption bed until the set temperature, and then enters the subsequent denitration unit together with the remaining adsorbed tail gas.

6. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 5, 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 of which is reused as the defrosting gas of the deep cooling unit, and the other is sent to the synthesis reactor to adjust the reactor liquid temperature.

7. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 6, characterized in that: The condensing device comprises 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.

8. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 5, 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, so as to control the amount of superheated steam used for regeneration; 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%.

9. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 1, characterized in that: The SCR denitration unit comprises a denitration heat exchanger, an exhaust gas heater, an ammonia adding device, a pipeline mixer and an SCR 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 SCR denitration reactor for denitration purification, and the obtained purified exhaust gas is then sent to the denitration heat exchanger for heat exchange with the adsorbed exhaust gas.

10. The online cryogenic purification process for o-fluorotoluene synthesis tail gas according to claim 9, characterized in that: The purified exhaust gas coming out of the denitration heat exchanger is divided into two parts. One part is directly mixed with the adsorbed exhaust gas after the temperature is increased after leaving the denitration heat exchanger, participates in the exhaust gas circulation, and dilutes the nitrogen oxide concentration in the exhaust gas entering the SCR denitration reactor. The other part is discharged through the chimney after heat exchange with the desorbed exhaust gas through the purified exhaust gas heat exchanger for cooling and dehumidification.

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