CN117771873A - Method for recycling fluorobenzene synthesis tail gas pollution components based on activated carbon assisted denitration - Google Patents

Abstract

The invention discloses a resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification, which includes the synthesis tail gas from the synthesis reactor during the fluorobenzene synthesis process. It is characterized in that the synthesis tail gas undergoes cryogenic heat exchange After the cryogenic defluorination of benzene, nitrogen dioxide and part of hydrogen fluoride, it enters the activated carbon adsorption tower for adsorption. Ammonia and air are periodically sprayed into the cryogenic exhaust gas before entering the activated carbon adsorption tower to perform adsorption and denitrification on the activated carbon adsorption tower. Desorption, the desorption tail gas is condensed and dehydrated and then returned to the synthesis reactor for reuse. The process of the invention is simple, clean, environmentally friendly, stable and efficient, and has high denitrification efficiency.

Description

Resource utilization of polluting components in tail gas of fluorobenzene synthesis based on activated carbon-assisted denitrification Method

Technical field

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 to a method for resource utilization of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification.

Background technique

In the past 10 years, the country has stepped up air pollution control, and the technology for removing nitrogen oxides from industrial flue gas has become increasingly perfect. The main technologies include SCR (selective catalytic reduction), SNCR (selective non-catalytic reduction), oxidation absorption, complex absorption, adsorption, activated carbon and other technical solutions, and the application process is becoming increasingly mature. Comparing different denitrification technologies, as listed in the table below, although the denitrification effects can meet the national emission standards, they all have certain shortcomings. Among them, the activated carbon method has the dual effects of SCR and adsorption in principle.

Table 1: Comparison of NO _X terminal control technologies

Since the amount of nitrogen oxide waste gas generated by the semiconductor and chemical industries is small and the concentration is high, the advanced oxidation method is generally used at present. This method is based on the principle that nitrogen dioxide in nitrogen oxides dissolves in water to generate nitric acid and nitrogen monoxide. The nitrogen monoxide in the waste gas is oxidized to nitrogen dioxide by an oxidant and then absorbed in the liquid phase to remove nitrogen oxides. This solution requires less investment and has a mild reaction process, but the denitrification efficiency is low and the oxidant is highly corrosive.

In recent years, with the development of the fluorine chemical industry, the problem of nitrogen oxide control has become increasingly prominent. Fluorobenzene is an important chemical intermediate, which is widely used in the pharmaceutical, dye and other industries. The profit of fluorobenzene production is relatively high, but due to the side reaction in the production process: 2HNO ₂ →NO+NO ₂ + water, a large amount of nitrogen oxides are generated and discharged with the synthetic tail gas. The concentration of nitrogen oxides reaches 20000-40000mg/Nm ^3. Traditional SCR denitrification catalysts cannot withstand such a high concentration of nitrogen oxides.

What is more serious is that the exhaust gas also contains a large amount of HF, fluorobenzene and a small amount of VOCs, which further increases the difficulty of removing nitrogen oxides. The problem of nitrogen oxide control has seriously restricted the development of the fluorine chemical industry.

In view of the above problems, the present invention develops a resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification.

Contents of the invention

The purpose of the present invention is to solve the above technical problems and provide a resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitration that has a simple process route, is clean and environmentally friendly, stable, efficient, and has high denitration efficiency.

The method of the present invention includes sequentially adding anhydrous HF, aniline, _NaNO2 and other components into a synthesis reaction kettle to undergo salt formation, diazotization, pyrolysis and other reactions to generate fluorobenzene and gas containing hydrogen fluoride, fluorobenzene, nitrogen oxides, and nitrogen. Synthetic tail gas with equal components. The synthetic tail gas is cryogenically defluorinated benzene, nitrogen dioxide and partial hydrogen fluoride in a cryogenic heat exchanger and then enters the activated carbon adsorption tower. Ammonia is periodically sprayed into the cryogenic tail gas entering the activated carbon adsorption tower. Gas and air are used to create different process environments for adsorption and denitration of the activated carbon adsorption tower. The activated carbon adsorption tower after adsorption and denitrification is desorbed. The desorbed tail gas is condensed and dehydrated and returned to the synthesis reactor for reuse.

The synthetic tail gas is indirectly cooled to below -85°C through a cryogenic heat exchanger through a low-temperature refrigerant below -100°C. More than 99.99% of fluorobenzene, nitrogen dioxide, hydrogen fluoride and part of nitric oxide are condensed and intercepted and returned to the synthesis reaction. The resources in the kettle are utilized. The condensed cryogenic tail gas is heated to 100-120°C through denitrification heat exchanger. It is then adsorbed by the activated carbon adsorption tower and assists in denitrification, and then enters the denitrification reactor for further denitrification and purification.

The flow rate of ammonia and air injected into the cryogenic exhaust gas entering the activated carbon adsorption tower is 20-30% of the molar flow rate of nitrogen oxides entering the tower; the cryogenic exhaust gas is mixed with the injected ammonia gas and air. After adsorption by the activated carbon adsorption tower, a small amount of fluorobenzene, nitrogen dioxide, hydrogen fluoride and part of nitric oxide remaining in the cryogenic exhaust gas after cryogenic cooling are adsorbed. At the same time, part of the adsorbed nitrogen oxides are reduced to nitrogen. After adsorption, The exhaust gas enters the denitrification reactor.

The activated carbon adsorption towers are at least used alternately in parallel; by setting the adsorption-desorption cycle of the activated carbon adsorption towers, they are switched regularly; superheated steam is used to desorb the activated carbon adsorption towers after completing an adsorption cycle, and the desorption tail gas is condensed and dehydrated. Pass it into the synthesis reactor to participate in the synthesis reaction, or enter the cryogenic heat exchanger to defrost and then enter the synthesis reactor to participate in the synthesis reaction.

In the first 80-90% time period of an adsorption cycle, spray ammonia and air into the cryogenic tail gas entering the activated carbon adsorption tower. In the 10-20% time period before the end of a cycle, stop injecting ammonia gas and air into the deep-cooled tail gas entering the tower. Ammonia and air are injected into the cold tail gas to gradually consume the remaining ammonia and oxygen in the activated carbon adsorption tower.

After the desorption of the activated carbon adsorption tower is completed, 10-20% volume percentage of cryogenic tail gas is introduced, which directly contacts the activated carbon adsorption bed for heat exchange, cools the activated carbon adsorption bed, and the heated tail gas enters the denitrification reactor.

The upper mouth of the synthesis reaction kettle is tightly connected to the kettle lid through the kettle body positioning truncated platform and the kettle lid positioning truncated platform. At least 3 of the cryogenic heat exchangers are installed on the kettle lid. The lower end of the tube side of the cryogenic heat exchanger is directly connected to the kettle lid. The synthesis reaction kettle is connected, and the upper end of the tube side of the cryogenic heat exchanger is connected to the induced draft fan through a pipeline.

The three cryogenic heat exchangers are used alternately for condensation-condensation-defrost. By setting the upper and lower pressure limits of the exhaust gas at the outlet of the cryogenic heat exchanger, the condensation-condensation-defrost process is controlled; when the cryogenic heat exchanger When the pressure of the cryogenic exhaust gas at the outlet of the cryogenic heat exchanger is lower than the lower limit, the refrigerant is cut off and the cryogenic heat exchanger enters the defrosting stage; when the pressure of the cryogenic exhaust gas at the outlet of the cryogenic heat exchanger is higher than the upper limit, the refrigerant is introduced and the cryogenic heat exchanger enters the defrosting stage. The cryogenic heat exchanger enters the cryogenic working stage and proceeds alternately.

The desorbed tail gas desorbed from the activated carbon adsorption bed enters the low-temperature water spray cooling tower, is cooled by low-temperature water spray at about 7°C, and then enters the condenser for cooling and dehydration. The condensate returns to the spray cooling tower; by controlling the superheat Steam temperature, reduce the amount of condensed water introduced by desorption steam, and ensure that the mass concentration of hydrogen fluoride in low-temperature water is above 40%.

The tail gas from the activated carbon adsorption tower is heated to the denitration temperature window by a heater before entering the denitration reactor, and then supplemented with sufficient air and ammonia by an ammonia adding device before entering the denitration reactor for denitration purification.

The denitrified purified exhaust gas from the denitrification reactor is divided into two parts after exchanging heat with the cryogenic exhaust gas in the denitrification heat exchanger. One part is directly mixed with the cryogenic exhaust gas from the denitrification heat exchanger and circulates into the activated carbon adsorption tower for adsorption and denitrification. Part of it enters the purified exhaust gas heat exchanger, exchanges heat with the cryogenic exhaust gas, condenses and removes part of the moisture, and then is discharged through the chimney.

Each of the cryogenic heat exchangers has multiple enhanced cooling sections, and each enhanced cooling section uses a refrigerant circulation pump to return the downstream refrigerant to the upstream.

Since the concentration of nitrogen oxides in the synthetic tail gas reaches 20,000-40,000 mg/Nm ³ , in order to slow down the impact of such a high concentration of tail gas entering the denitrification reactor on the catalyst, the present invention creatively sets up an activated carbon adsorption tower in front of the denitrification reactor for pre-denitration. This is based on the fact that activated carbon has the dual functions of adsorption and denitrification. By creating conditions conducive to the adsorption or denitrification of activated carbon, the function conversion of activated carbon denitrification or adsorption can be achieved to achieve a pre-denitrification effect and slow down the impact of high concentrations of nitrogen oxides in the exhaust gas on the SCR denitrification catalyst. . The technical improvement measures taken are as follows:

(1) Before the cryogenic tail gas enters the denitration reactor, it first enters the activated carbon adsorption tower for pre-denitrification. The cryogenic tail gas from the cryogenic heat exchanger is sucked by the denitration induced draft fan and then sprayed with a small amount of ammonia by the primary ammonia feeder. It then enters the denitration heat exchanger and exchanges heat with the purified tail gas at a temperature of 220-260°C after denitrification. The temperature rises to 100-120°C and enters the activated carbon adsorption tower for pre-denitrification. At the same time, under the adsorption effect of the activated carbon, the trace HF components remaining in the cryogenic tail gas are preferentially adsorbed (HF molecules have a large polarity and are preferentially adsorbed), and a large amount of nitric oxide is also adsorbed.

(2) A small amount of ammonia is sprayed into the deep-cooled tail gas entering the activated carbon adsorption tower. In the primary ammonia feeder, ammonia is sprayed into the tail gas at a ratio of 25-30% of the molar flow rate of nitrogen oxides in the tail gas, and the mixed tail gas enters the activated carbon adsorption tower for pre-denitrification.

(3) Use at least two activated carbon adsorption towers, and adsorption-pre-denitration and desorption are used alternately. Two activated carbon adsorption towers, one for adsorption and denitration, and the other for desorption and standby, are used alternately on a regular basis.

(3) Ammonia is sprayed periodically on the cryogenic tail gas entering the activated carbon adsorption tower. During the first 80-90% of an adsorption pre-denitrification cycle, ammonia and air are sprayed into the cryogenic tail gas entering the activated carbon adsorption tower. During the 10-20% period before the end of a cycle, the spraying of ammonia and air into the cryogenic tail gas entering the tower is stopped.

(4) Regularly desorb the activated carbon adsorption tower. After an adsorption pre-denitrification cycle of the activated carbon adsorption tower is completed, superheated steam is introduced into the tower to desorb and regenerate the activated carbon adsorption bed. The flow direction of the superheated steam into the tower is opposite to the flow direction of the tail gas. The desorption steam leaving the activated carbon adsorption tower enters the spray cooling tower, is cooled by low-temperature water spray, and the hydrogen fluoride component is recovered. The uncondensed gas enters the desorption tail gas condenser for further cooling, dehydration and recovery of the hydrogen fluoride component. The remaining gas phase is sent to the cryogenic heat exchanger as a defrosting medium through the condensing tail gas fan. After the cryogenic heat exchanger is defrosted, it enters the synthesis reactor to participate in the synthesis reaction, or directly enters the synthesis reactor to participate in the synthesis reaction. Components such as HF and NO in the gas phase are all reaction components of the synthesis reaction.

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

(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.

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

(3) Slow down the impact of high concentrations of nitrogen oxides on the SCR denitration catalyst. After the cryogenic exhaust gas enters the activated carbon adsorption tower, the high concentration of NO in the exhaust gas is partially adsorbed and partially reduced, reducing the flow of NO entering the catalyst bed.

(4) The speed of the pre-denitrification reaction is controllable, which is beneficial to adjusting the concentration of nitrogen oxides in the exhaust gas entering the denitrification reactor. By adjusting the amount of ammonia and air injected into the exhaust gas, the pre-denitration reaction speed of the activated carbon adsorption tower is controlled, thereby regulating the NO flow rate into the exhaust gas of the denitration reactor.

(5) Eliminate possible residual ammonia and oxygen in the desorption tail gas, improve the purity of the desorption tail gas, and eliminate the impact of residual ammonia and oxygen on the synthesis reaction. In the 10-20% period before the end of a cycle of activated carbon adsorption tower adsorption, stop spraying ammonia and air into the cryogenic exhaust gas entering the tower, and use the deep-cooled exhaust gas heated to 100-120°C introduced during this period. The high concentration of nitrogen oxides in the cold exhaust gas gradually consumes the residual ammonia and air in the activated carbon adsorption tower.

(6) Economical and efficient. The manufacturing cost of SCR denitrification catalyst is high and more expensive than activated carbon. At the same time, the SCR denitrification catalyst has worse acid and alkali tolerance than activated carbon. Setting up an activated carbon adsorption tower in front of the denitrification reactor greatly adds a protective measure to the denitrification reactor. It is beneficial to extend the life of SCR denitration catalyst.

Furthermore, in order to improve the condensation and interception effect of cryogenic cooling on pollutant components in synthetic exhaust gas, the present invention proposes a refrigerant internal circulation scheme in the cryogenic heat exchanger to form an enhanced cooling section in the cryogenic heat exchanger. In order to condense the hydrogen fluoride, fluorobenzene and nitrogen dioxide components in the synthetic tail gas, three enhanced cooling sections are formed in the cryogenic heat exchanger. By adjusting the circulation volume, different cooling effects are obtained and the condensation of different pollution components is achieved. speed, while improving the condensation interception effect and slowing down the impact of frost.

Among these pollutants, the melting point of nitrogen dioxide is -11°C and the boiling point is 21°C; the melting point of nitric oxide is -163.6°C and the boiling point is -151°C; the melting point of fluorobenzene is -42°C and the boiling point is 85°C, and the melting point of HF is -83°C. The boiling point is 19.54℃. When the exhaust gas is cryogenically cooled to below -80°C, HF, nitrogen dioxide and fluorobenzene are basically completely condensed. During the condensation process, nitrogen dioxide and fluorobenzene will appear in the cryogenic heat exchanger and form frost to block the exhaust gas flow channel. In order to alleviate the problem of frost clogging, the present invention has made the following improvements:

(1) The exhaust gas enters the cryogenic heat exchanger and exchanges heat with the refrigerant in reverse, and the refrigerant is forced to circulate in the cryogenic heat exchanger to form enhanced cooling sections corresponding to hydrogen fluoride, fluorobenzene and nitrogen dioxide respectively. By setting up a refrigerant circulation pump, the higher-temperature refrigerant in the downstream flows back to the upstream, forming a condensation section with a small temperature difference corresponding to the melting point temperature of the pollutant components to ensure that the pollutants are fully condensed and will not solidify and block.

(2) Utilize the higher temperature gas circulation defrosting in the exhaust gas purification system. First, the higher-temperature desorption tail gas desorbed from the activated carbon adsorption tower is used as a defrosting heat source, which not only recovers the fluorobenzene in the desorption tail gas, but also performs efficient defrosting; second, the original synthetic tail gas is used as a defrosting heat source, which not only initially cools the synthetic tail gas , recover the frosting cooling capacity, and also achieve the defrosting effect, introducing part of the original synthetic exhaust gas into the cryogenic heat exchanger that needs to be defrosted, and then return to the cryogenic heat exchanger for further condensation after defrosting.

The effect of such improvement is as follows:

(1) Step-by-step condensation recovers polluted components, slows down frost blockage in the cryogenic process, and improves system operation stability.

(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 (-80°C) cryogenic exhaust gas after cryogenic cooling to improve the utilization efficiency of the refrigerant cooling capacity.

(3) Fully recover the frosting cooling capacity.

(4) No need to introduce external defrosting medium to achieve efficient defrosting.

(5) During the cryogenic process, most of the nitrogen dioxide is condensed back into the synthesis reactor, increasing the nitrogen dioxide concentration in the reactor liquid, inhibiting the occurrence of the side reaction 〈2HNO ₂ →NO+NO ₂ +water〉, and improving the utilization rate of HNO ₂ .

Furthermore, the present invention combines the fluorobenzene synthesis process and cryogenic defrosting needs to make full use of the component value and waste heat value of the desorption tail gas. When the synthesis process requires heating and pyrolysis (pyrolysis reaction stage), the desorption tail gas does not undergo cooling, Condensate and directly enter the synthesis reaction kettle, stir and adjust the temperature of the kettle liquid.

The effects of this improvement are as follows:

(1) Make full use of the heat source of the internal medium of the system. By entering the desorbed tail gas into the kettle for stirring, the heat enthalpy required to heat up and pyrolyze the synthesis reaction kettle liquid is saved.

(2) Fully recycle and utilize polluting components such as fluorobenzene and nitrogen oxides contained in the desorbed exhaust gas.

Furthermore, when performing SCR denitration treatment on cryogenic exhaust gas, the present invention innovatively proposes a technical solution for purifying exhaust gas circulation, that is, the purified exhaust gas coming out of the denitrification heat exchanger is divided into two parts, one part is mixed with the cryogenic exhaust gas, and then enters the denitrification reaction again. The purified exhaust gas heat exchanger and the cryogenic exhaust gas exchange heat, cool down and dehydrate, and then are discharged through the chimney.

The plan to participate in the exhaust gas circulation is adjusted according to the purified exhaust gas temperature of the denitrification heat exchanger. When the temperature of the purified exhaust gas leaving the denitrification heat exchanger is higher than 120°C, the purified exhaust gas participating in the cycle is introduced downstream of the activated carbon adsorption tower to mix with the adsorbed exhaust gas and then enter the denitrification reactor. That is, the cryogenic exhaust gas is heat exchanged by the denitrification heat exchanger. Enter the activated carbon adsorption tower separately; when the temperature of the purified exhaust gas leaving the denitrification heat exchanger is lower than 100°C, the purified exhaust gas participating in the cycle is directly mixed with the cryogenic exhaust gas exchanged from the denitrification heat exchanger and enters the activated carbon adsorption tower together.

The effects of adopting this technical solution are as follows:

(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.

(2) Through the cyclic dilution effect of purified tail gas, the temperature fluctuation range 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 system operation costs and extend the service life of the catalyst.

(3) By online monitoring of the NOx concentration in the purified exhaust gas, the amount of ammonia injected into the adsorbed exhaust gas is adjusted. The cycle of purified exhaust gas also controls the fluctuation of ammonia content in the exhaust gas entering the denitration reactor, which is beneficial to the stability of the SCR denitration reaction process. , improve the denitrification effect and reduce the escape of ammonia.

(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.

(5) Through the circulation of purified exhaust gas, the concentration of nitrogen oxides is diluted to avoid severe local reactions in the reactor and damage to the catalyst.

(6) By adjusting the circulation method of purifying exhaust gas, the system operation stability is improved and the denitrification efficiency is improved. In order to control the reaction temperature of the activated carbon bed during activated carbon adsorption and denitrification, as well as the temperature rise amplitude of the reaction process, the temperature of the tail gas entering the activated carbon adsorption tower is strictly controlled.

Furthermore, in order to completely recover the waste heat of the purified exhaust gas after denitration, use the waste heat to fully heat the cryogenic exhaust gas, and simultaneously reduce the humidity of the purified exhaust gas discharged, two "reverse" heat exchanges are performed between the cryogenic exhaust gas and the purified exhaust gas.

First, heat is exchanged with the purified exhaust gas in the external exhaust part. The cryogenic exhaust gas is heated from -80∽-85°C to 20°C, and the temperature of the purified exhaust gas is dropped to 60°C. Secondly, the cryogenic exhaust gas at 20°C is mixed with a certain amount (20-30% of the molar flow rate of nitrogen oxides in the cryogenic exhaust gas) of ammonia and air and then exchanged with the purified exhaust gas at about 260°C exiting the SCR denitrification reaction tower. Heat, the temperature rises to 100∽120℃, and the purified exhaust gas temperature drops to 160∽170℃.

The invention can realize full recovery of nitrogen dioxide, fluorobenzene and fluorine components of synthetic exhaust gas, fully recover waste heat and waste cooling, and has a relatively simple process route, is environmentally friendly, cost-effective, and has stable operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: process flow chart of the present invention.

Among them, HC1-synthesis reactor; HC2-refrigerant circulation pump; HC3-cryogenic heat exchanger; HC4-cryogenic tail gas outlet; HC5-kettle cover positioning round table; HC6-kettle cover air inlet; HC7-kettle cover; HC8-kettle cover feeding port; HC9-kettle body positioning round table; HC10-valve; HC11-kettle body; HC12-kettle body discharge port; HC13-kettle bottom air inlet; HC14-cryogenic tail gas circulation fan; HC15-cryogenic tail gas heat exchanger;

XT1-denitrification induced draft fan; XT2-primary ammonia adder; XT3-denitrification heat exchanger; XT9-denitrification reactor; XT10-pipe mixer; XT11-ammonia adding device; -Low temperature water heat exchanger; XT17-40% hydrofluoric acid product tank; XT18-low temperature water circulation pump; XT19-desorption tail gas induced draft fan;

Detailed ways

The main process of fluorobenzene synthesis includes reaction steps such as salt formation, diazotization and pyrolysis. These reaction processes are all completed in the synthesis reactor HC1.

(1) Salt formation

The low-temperature hydrofluoric acid liquid (-15°C) is metered into the synthesis reactor HC1, and continues to be cooled with frozen brine to reduce the temperature of the synthesis reactor HC1 to between 4 and 6°C. Stir and slowly drop the aniline after metering. Add it to the synthesis reaction kettle HC1, and keep the temperature between 5 and 8°C during the dropwise addition. The reaction process is an excess reaction of hydrofluoric acid, and the pressure is slightly negative. After the dropwise addition, adjust the amount of frozen brine and keep it warm at about 6°C. The reaction ended in 30 minutes, and the reaction time was 6 hours, and an aniline hydrofluoride mixture was obtained.

(2) Diazotization

Reduce the temperature of the HC1 circulating frozen brine in the synthesis reactor to 0~-2℃, slowly add sodium nitrite solid under stirring, control the feeding temperature at 0~5℃, use a slightly negative pressure, and keep it warm for 30 minutes after the addition is completed. When the reaction is completed, the reaction time is 10h, and diazo liquid is obtained.

There is a side reaction during the diazotization reaction: 2HNO ₂ →NO + NO ₂ + water.

(3) Pyrolysis

Adjust the temperature of the frozen brine, and slowly heat up the liquid in the synthesis reactor HC1 in stages (5-15°C, 15-25°C, 25-38°C) while stirring, by about 1.2°C per hour until it reaches 38°C. The pressure is slightly negative and the thermal decomposition is carried out. The pyrolysis time is 32 hours. After the reaction is completed, the materials in the kettle are put into a static tank and stratified. The organic layer is a fluorobenzene mixture; the inorganic layer contains hydrofluoric acid, sodium fluoride, etc.

In different reaction stages, the reaction temperatures are different and need to be adjusted according to the reaction period.

Take the fluorobenzene production line of a factory as an example. The factory contains 60 fluorobenzene synthesis reactors HC1, with an annual production of 4,000 tons of fluorobenzene. Each synthesis reactor HC1 is produced intermittently. The main process of fluorobenzene synthesis includes reaction steps such as salt formation, diazotization and pyrolysis. These reaction processes are all completed in the synthesis reactor HC1.

The process of resource utilization of pollutant components in fluorobenzene synthesis tail gas based on activated carbon-assisted denitrification will be further explained below with reference to the accompanying drawings:

The cryogenic unit structure and cryogenic process of fluorobenzene synthesis tail gas are as follows:

1. The cryogenic part consists of at least three shell-and-tube cryogenic heat exchangers HC3 installed on the lid HC7 of the synthesis reaction kettle. One end of the tube side air inlet of the cryogenic heat exchanger HC3 is directly connected to the kettle lid HC7. It is connected to the synthesis reaction kettle, and the pipe outlet at the other end is connected to the induced draft fan inlet through a pipeline.

2. The cryogenic heat exchangers HC3 are used in parallel. During normal operation, at least one is defrosted and the rest are cryogenically working. Under the suction action of the kettle pressure and the induced draft fan HX3, the synthetic exhaust gas enters the tube side of the cryogenic heat exchanger HC3, and exchanges heat with the -70°C low-temperature refrigerant introduced in the shell side, forming a deep refrigerant with a temperature below -80°C. The cold exhaust gas enters the subsequent purification unit.

3. Set the cryogenic exhaust gas outlet temperature limit value of the cryogenic heat exchanger HC3 -80∽-85℃, and set the upper and lower pressure limits of the cryogenic heat exchanger HC3 exhaust gas outlet, and monitor the temperature and pressure in real time , regulate the working mode of cryogenic heat exchanger HC3. When the outlet cryogenic exhaust gas temperature is lower than -85°C, the flow of -100°C low-temperature refrigerant to the cryogenic heat exchanger HC3 is stopped, and the cryogenic heat exchanger HC3 enters the defrost mode and generates higher temperature refrigerant in the kettle. Synthetic exhaust gas is used for automatic defrosting. During the defrosting stage, stop flowing -100°C low-temperature refrigerant into the cryogenic heat exchanger HC3.

4. The "deep cooling-defrosting" working mode control of the cryogenic heat exchanger HC3 can also adopt a timing method, 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.

5. The "defrost" of the cryogenic heat exchanger HC3 can also use the desorbed exhaust gas from the adsorption unit. Switch the desorbed tail gas that originally entered the synthesis reactor HC1 to the outlet of the cryogenic heat exchanger HC3 that needs to be "defrosted", and close the outlet valve of the cryogenic heat exchanger HC3 to defrost with the help of the heat enthalpy of the desorbed tail gas. The pressure impacts the frost block.

Although the volume and composition of the exhaust gas discharged by each synthesis reactor HC1 is different at different synthesis reaction stages, due to the large number of synthesis reaction HC1 reactors, the flow rate and composition of the cryogenic exhaust gas discharged are basically stable.

The amount of synthetic tail gas discharged when cryogenic condensation is not carried out is 1000∽2000Nm ³ /h. The main components of the tail gas are: HF: 160∽170g/Nm ³ ; fluorobenzene: 2000∽3000mg/Nm ³ ; NOx: 12000∽20000mg/Nm ³ ; A small amount of VOCs components; the rest is nitrogen.

The amount of cryogenic exhaust gas discharged after cryogenic condensation is 1000∽2000Nm ³ /h. The exhaust gas is mainly composed of NO and nitrogen, among which: NO: 10000∽12000mg/Nm ³ and the rest is nitrogen.

Referring to Figure 1, one set of "adsorption units" corresponds to multiple sets of "synthesis reactor HC1 + cryogenic units (at least 3 cryogenic heat exchangers)".

60 synthesis reactors HC1 carry out the synthesis reaction of fluorobenzene. The synthetic tail gas produced is sucked by the induced draft fan/reflux cooling fan HX3, and then cooled and condensed by the deep cold heat exchanger HC3. Almost all HF, fluorobenzene, _NO2 and VOCs components are condensed and intercepted. At the same time, part of NO is also condensed and intercepted and directly returned to the synthesis reactor HC1.

The shell side of the cryogenic heat exchanger HC3 is the refrigerant channel. On the shell side, along the flow direction of the refrigerant from the inlet to the outlet, three sections of enhanced cooling are formed in sequence through the refrigerant circulation pump HC2. The refrigerant from the downstream is circulated to the upstream by the refrigerant circulation pump HC2. By adjusting the circulation volume, different cooling effects are obtained to achieve the condensation rate of different pollutant components, while improving the condensation interception effect and slowing down the impact of frosting. The circulation volume is adjusted according to the melting points of HF, fluorobenzene, NO ₂ and VOCs components.

The cryogenic tail gas from the cryogenic heat exchanger HC3 has a low temperature (-80∽-85℃) and contains trace amounts of HF, fluorobenzene, NO ₂ and VOCs. The main component is nitrogen and also contains NO, with a concentration of 10000∽ 12000mg/Nm ³ .

The cryogenic exhaust gas coming out of the cryogenic heat exchanger HC3 first enters the cryogenic exhaust gas heat exchanger HC15 to exchange heat with the purified exhaust gas in the external exhaust part. The temperature rises to about 20°C, and then is mixed with a certain amount (deep) in the primary ammonia adder XT2. Ammonia and air (20-30% of the molar flow rate of nitrogen oxides) in the cold tail gas enter the denitrification heat exchanger , adsorb trace amounts of HF, fluorobenzene, NO ₂ and VOCs components in cryogenic exhaust gas, and perform pre-denitration at the same time.

The temperature of the purified exhaust gas from the denitrification heat exchanger XT3 drops from 260°C to 160∽170°C. Part of the purified exhaust gas participates in the circulation in the denitrification reactor XT9, and the remaining part passes through the cryogenic exhaust gas heat exchanger HC15 and the denitrification heat exchanger XT3. The cryogenic exhaust gas is heat exchanged, and the temperature drops to 50°C and is discharged outside. The power for the cryogenic exhaust gas denitration and purification exhaust gas circulation processes is provided by the cryogenic exhaust gas circulation fan HC14.

According to the temperature of the cryogenic exhaust gas exiting the denitrification heat exchanger XT3, the purified exhaust gas circulation method of the denitrification heat exchanger XT3 is adjusted. When the temperature of the purified tail gas exiting the denitrification heat exchanger After heating, it enters the activated carbon adsorption tower separately; when the temperature of the purified exhaust gas leaving the denitrification heat exchanger XT7.

The adsorption-desorption cycle of the two activated carbon adsorption towers XT7 is set to 22 hours for adsorption and 2 hours for desorption. The ammonia and air spraying time in the adsorption stage is 16-18 hours. At the beginning of an adsorption cycle, ammonia and air are sprayed into the deep-cold tail gas at the same time. When one of the activated carbon adsorption towers XT stops spraying ammonia and air, the other activated carbon adsorption tower XT7 that has completed desorption is started to introduce about 10% of the deep-cold tail gas (without ammonia and air spraying) from the denitration heat exchanger XT3.

The activated carbon adsorption tower XT7 is desorbed after adsorption, and superheated steam is used in the desorption process. Superheated steam is introduced from the tail gas outlet of the activated carbon adsorption tower XT7, that is, the desorbed gas is reversed to the adsorbed gas. The desorbed tail gas is divided into two parts under the suction action of the condensing tail gas fan XT12 and the desorbed tail gas induced draft fan TX19, one part enters the spray cooling tower TX14, and the other part is bubbled from the bottom air inlet HC13 into the synthesis reactor HC1 that needs to be heated in the pyrolysis stage, to heat the reactor liquid temperature and stir the reactor liquid at the same time. The nitrogen oxide components rich in the desorbed tail gas also have the effect of inhibiting the side reaction "2HNO2→NO+NO2+water", or enter the deep cold heat exchanger HC3 for defrosting. HF, NO and other components in the gas phase are all reactant components of the synthesis process.

The desorption tail gas introduced into the spray cooling tower HX4 by the desorption tail gas induced draft fan TX19 uses low-temperature water below 7°C. Under the circulation and cooling provided by the low-temperature water circulation pump XT18 and the low-temperature water heat exchanger TX16, the desorption tail gas is circulated and sprayed to cool the desorption tail gas and absorb it. Desorb HF components in exhaust gas. The effect of condensation interception of hydrogen fluoride components in the cryogenic process and the concentration of hydrogen fluoride in the desorption tail gas can control the temperature of the superheated steam and reduce the usage of superheated steam in the desorption process, thus controlling the amount of condensed water entering the spray cooling tower TX14 to ensure the hydrogen fluoride in the condensate. The mass concentration is greater than 40%.

Through the implementation of the fluorobenzene clean production process of the present invention, the HF recovery rate is above 99.99%; the fluorobenzene recovery rate is above 99.9%; the NOx removal rate is above 99.95%, the outlet NOx is lower than 100mg/ ^Nm3 , and about 2000 tons of HF and about 20 tons of fluorobenzene are recovered annually.

Denitrification reactor.

Claims (12)

1. A resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification, including the synthesis tail gas from the synthesis reactor during the fluorobenzene synthesis process, which is characterized in that the synthesis tail gas is deep-sealed through a cryogenic heat exchanger. After cold defluorination of benzene, nitrogen dioxide and part of hydrogen fluoride, it enters the activated carbon adsorption tower for adsorption. Ammonia and air are periodically sprayed into the cryogenic exhaust gas before entering the activated carbon adsorption tower to desorb the adsorbed and denitrified activated carbon adsorption tower. The desorption tail gas is condensed and dehydrated and then returned to the synthesis reactor for reuse. 2. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 1, characterized in that the synthesis tail gas is indirectly cooled to a temperature of 100 ℃ by a cryogenic heat exchanger through a low-temperature refrigerant below -100°C. Below -85℃, more than 99.99% of fluorobenzene, nitrogen dioxide, hydrogen fluoride and part of nitrogen monoxide are condensed and intercepted and returned to the synthesis reactor. The condensed cryogenic tail gas is heated to 100-100 by denitrification heat exchanger. 120℃, and then adsorbed by the activated carbon adsorption tower and assisted denitrification, and then enter the denitrification reactor for further denitrification and purification. 3. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 1, characterized in that the ammonia gas and air flow rate injected into the cryogenic tail gas entering the activated carbon adsorption tower are respectively It is 20-30% of the molar flow rate of nitrogen oxides entering the tower; after the cryogenic tail gas is mixed with the injected ammonia and air and adsorbed by the activated carbon adsorption tower, a small amount of fluorobenzene remains in the cryogenic tail gas after cryogenic cooling , nitrogen dioxide, hydrogen fluoride and part of nitrogen monoxide are adsorbed, and part of the adsorbed nitrogen oxides are reduced to nitrogen. After adsorption, the tail gas enters the denitrification reactor. 4. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 1, characterized in that the activated carbon adsorption tower is alternately used by at least two parallel connections; the adsorption by setting the activated carbon adsorption tower -Desorption service life cycle, switch regularly; superheated steam is used to desorb the activated carbon adsorption tower after completing an adsorption cycle. The desorption tail gas is condensed and dehydrated and then passed into the synthesis reactor to participate in the synthesis reaction, or enters the cryogenic heat exchanger after defrosting Enter the synthesis reactor to participate in the synthesis reaction. 5. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 4, characterized in that, in the first 80-90% time period of the one adsorption cycle, the fluorobenzene synthesis tail gas entering the activated carbon adsorption tower Inject ammonia and air into the cryogenic exhaust gas before the cycle ends. In the 10-20% period before the end of a cycle, stop injecting ammonia and air into the cryogenic exhaust gas entering the tower, and gradually consume the residual ammonia in the activated carbon adsorption tower. Chi and air. 6. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 4, characterized in that, after the desorption of the activated carbon adsorption tower is completed, the cryogenics with a volume percentage of 10-20% is introduced. The exhaust gas directly contacts the activated carbon adsorption bed for heat exchange, cooling the activated carbon adsorption bed, and the heated exhaust gas enters the denitrification reactor. 7. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 1, characterized in that at least three of the cryogenic heat exchangers are installed on the lid of the synthesis reaction kettle. , the lower end of the tube side of the cryogenic heat exchanger is directly connected to the synthesis reactor, and the upper end of the tube side of the cryogenic heat exchanger is connected to the induced draft fan through a pipeline. 8. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 7, characterized in that the three cryogenic heat exchangers are used alternately for condensation-condensation-defrost. Set the upper and lower limits of the pressure of the exhaust gas at the outlet of the cryogenic heat exchanger to control the condensation-condensation-defrost process; when the pressure of the cryogenic exhaust gas at the outlet of the cryogenic heat exchanger is lower than the lower limit, the refrigerant is cut off and the cryogenic exchanger The heater enters the defrosting stage; when the pressure of the cryogenic exhaust gas at the outlet of the cryogenic heat exchanger is higher than the upper limit, refrigerant is introduced, and the cryogenic heat exchanger enters the cryogenic working stage, which is performed alternately. 9. The method for resource utilization of fluorobenzene synthesis tail gas pollution components based on activated carbon assisted denitrification as claimed in claim 4, characterized in that the desorbed tail gas desorbed from the activated carbon adsorption bed enters a low-temperature water spray cooling tower, and after being cooled by low-temperature water spray, enters a desorbed tail gas condenser for cooling and dehydration, and the condensate returns to the spray cooling tower; by controlling the temperature of the superheated steam, the amount of condensed water introduced into the activated carbon adsorption tower by the superheated steam is reduced, ensuring that the mass concentration of hydrogen fluoride in the low-temperature water is above 40%. 10. The resource utilization method of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as claimed in claim 2 or 3 or 6, characterized in that the adsorption tail gas exiting the activated carbon adsorption tower is heated before entering the denitrification reactor. After the reactor is heated to the denitration temperature window, sufficient air and ammonia are replenished through the ammonia adding device and then enter the denitration reactor for denitration and purification. 11. The method for resource utilization of fluorobenzene synthesis tail gas pollution components based on activated carbon assisted denitrification as described in claim 2 or 3, characterized in that the purified tail gas coming out of the denitrification reactor is divided into two parts after heat exchange with the cryogenic tail gas in the denitrification heat exchanger, one part is directly mixed with the cryogenic tail gas coming out of the denitrification heat exchanger and circulated into the activated carbon adsorption tower for adsorption and denitrification, and the other part enters the purified tail gas heat exchanger and the cryogenic tail gas for heat exchange condensation to remove part of the moisture and then is discharged through the chimney. 12. The method for resource utilization of fluorobenzene synthesis tail gas pollution components based on activated carbon-assisted denitrification as described in claim 1, 7 or 8, characterized in that each of the deep cold heat exchangers 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.