Resource recycling system for fluorine component in fluorobenzene production tail gas

Abstract

The invention discloses a resource recycling system for fluorine components in fluorobenzene production tail gas, which comprises a synthesis reaction kettle, the top of the synthesis reaction kettle is provided with a copious cooling unit, and the copious cooling unit is connected with a washing unit; wherein the washing unit comprises a multi-stage water washing tower, a first-stage alkaline washing tower and a water washing demisting tower which are connected in series, the alkaline washing period of the first-stage alkaline washing tower is set, or the concentration limit value of sodium fluoride in spraying alkaline liquor in the first-stage alkaline washing tower is set, and the first-stage water washing tower and the first-stage alkaline washing tower in the multi-stage water washing tower are periodically switched and alternately used. The system is simple, clean, environment-friendly, stable, efficient, controllable in component, high in fluorine element recovery rate and long in service life.

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 to a resource recovery system for fluorine components in the tail gas of fluorobenzene production. Background Art

[0002] Fluorobenzene (FB) contains F - , which can reduce the surface tension of the electrolyte on the surface of the active material and increase the wetting speed. At the same time, the benzene ring has a protective effect on the positive electrode and can effectively prevent Fe 2+ The dissolution of fluorinated surfactants can improve the cycle performance of batteries; trace amounts of fluorinated surfactants can effectively reduce the surface tension of the electrolyte on the surface of the active material and increase the wetting speed, while having little effect on the electrochemical performance. At the same time, fluorobenzene is an important chemical intermediate, at the top of the entire fluorine chemical industry value chain, and its application has received widespread attention. In particular, with the development of the new energy industry, the demand for the application of fluorobenzene is increasing.

[0003] At present, the mainstream production method of fluorobenzene is the anhydrous hydrogen fluoride method (AHF). The technical core of this method is that the synthesis reaction is carried out in a liquid anhydrous hydrogen fluoride system. Since the polarity of hydrogen fluoride molecules and the electronegativity of fluorine are the strongest, it is not only an excellent solvent for electrolytes, but also a superacid system. In anhydrous hydrogen fluoride, not only can aniline be dissolved by salt formation, but also sodium nitrite can be dissolved in the solvent, which is easy to undergo diazotization reaction; since the decomposition reaction of the diazonium salt is carried out in anhydrous hydrogen fluoride, the formation of by-product phenol is greatly suppressed, and the fluorobenzene yield of the AHF method reaches more than 80%. Therefore, the anhydrous hydrogen fluoride method (AHF) fluorobenzene production process has become the mainstream process for the current synthesis of fluorobenzene. Due to the constraints of the process itself, a large amount of HF, NOx and other atmospheric pollutants will be generated during the synthesis of fluorobenzene by the AHF method. HF is very harmful to human health. Long-term inhalation of low-concentration HF by people will cause chronic poisoning, vomiting, dizziness and other symptoms. When the HF concentration is greater than 30μg / m 3 It can cause acute poisoning.

[0004] Due to the complex components and high concentration of pollutants in the synthesis of fluorobenzene, there is no efficient and low-cost method to solve the HF and NOx pollution problems in the production process so far. 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 some 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 of o-fluorotoluene: 2HNO 2 →NO+NO 2 + water, a large amount of nitrogen oxides are produced and discharged with the synthetic tail gas, and the nitrogen oxide concentration reaches 20000-40000mg / Nm 3 , 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. Summary of the invention

[0007] The purpose of the present invention is to solve the above technical problems and to provide a fluorine component resource recovery system in fluorobenzene production tail gas which is simple in system, clean and environmentally friendly, stable and efficient, has controllable components, high fluorine element recovery rate and long equipment service life.

[0008] The system of the present invention comprises a synthesis reactor, a cryogenic unit is arranged on the top of the synthesis reactor, and the cryogenic unit is connected to a washing unit;

[0009] The washing unit comprises a multi-stage water washing tower, a primary alkali washing tower and a water washing and demisting tower connected in series, the alkali washing cycle of the primary alkali washing tower is set, or the sodium fluoride concentration limit in the sprayed alkali solution in the primary alkali washing tower is set, and the first-stage water washing tower and the primary alkali washing tower in the multi-stage water washing tower are switched regularly and used alternately.

[0010] The tail gas outlet of the deep cooling unit is respectively connected to the air inlet of the first-stage water washing tower and the air inlet of the first-stage alkali washing tower in the multi-stage water washing tower, the air outlet of the first-stage water washing tower and the air outlet of the first-stage alkali washing tower in the multi-stage water washing tower are both connected to the air inlet of the water washing and demisting tower, the air outlet of the first-stage water washing tower and the air outlet of the first-stage alkali washing tower in the multi-stage water washing tower are connected to the air inlet of the second-stage water washing tower through a pipeline, and switching valves are provided on the connecting pipes to realize a single series connection between towers.

[0011] Two circulating liquid tanks are correspondingly arranged at the bottom of the primary water washing tower and the primary alkali washing tower, one is a circulating alkali liquid tank, and the other is a circulating spray water tank, both of which are connected to the tower through a circulating spray pipe.

[0012] The circulating alkali solution tank is connected with the circulating spray water tank through an alkali drainage pump.

[0013] The washing water outlets in the storage tanks under the water washing and demister towers and the multi-stage water washing towers except the first-stage water washing tower are connected to a tee via a water washing circulation pump, and one end of the tee is connected to the tower, and the other end is connected in series in the order from the water washing and demister tower to the multi-stage water washing tower.

[0014] The washing unit is connected to the denitration unit via the adsorption unit;

[0015] Wherein, the adsorption unit includes at least two resin adsorption towers, each of which is provided with an exhaust gas inlet, a superheated steam inlet and an exhaust gas outlet, and a desorption exhaust gas outlet. The resin adsorption towers are connected in parallel, and the exhaust gas inlet of each resin adsorption tower is connected to the exhaust gas outlet of the washing unit. The exhaust gas outlets of each resin adsorption tower are connected to the denitrification unit after merging; the exhaust gas output of the adsorption unit is also connected to the exhaust gas inlet of each resin adsorption tower through a reflux cooling fan.

[0016] The desorbed tail gas outlet of the adsorption unit is connected to a condensing device and a purified tail gas heat exchanger, and then respectively connected to a defrosting gas pipeline of a cryogenic unit and a gas inlet at the bottom of a synthesis reactor.

[0017] The denitration unit comprises a denitration heat exchanger, a tail gas heater, a pipeline mixer, an ammonia device and an SCR denitration reactor which are connected in sequence.

[0018] The purified tail gas outlet of the denitration reactor is connected to the chimney via the denitration heat exchanger, the circulating fan and the purified tail gas heat exchanger.

[0019] The purified tail gas outlet of the denitration reactor is also connected to the tail gas heater through a denitration heat exchanger, a circulating fan and a tail gas circulation regulating valve.

[0020] The cryogenic unit comprises a plurality of cryogenic heat exchangers installed on the cover of the synthesis reactor, the bottom air inlets of the plurality of cryogenic heat exchangers are connected to the tail gas outlet of the synthesis reactor, the air outlets are connected in series and parallel through pipelines, and the switching valves arranged on the connecting pipes are used to switch between the parallel use and the series use of the cryogenic heat exchangers, and condensation and defrosting are performed alternately.

[0021] The shell side of the cryogenic heat exchanger where the refrigerant flows is provided with multiple enhanced cooling sections from the refrigerant inlet to the outlet. A refrigerant outlet is provided downstream of each enhanced cooling section and is connected to the upstream refrigerant inlet via a refrigerant circulation pump.

[0022] The cryogenic tail gas outlet of the cryogenic unit is also connected to the gas inlet at the bottom of the synthesis reactor.

[0023] The multiple groups of cryogenic units are connected in parallel and then connected to the washing unit.

[0024] Aiming at the problem that the pollution components of synthetic tail gas are complex and the concentration of pollutants is high, if it is purified, it will not only be costly but also waste resources. The present invention breaks through the traditional governance concept and proposes a technical solution of deep cold condensation and direct reuse. The synthetic tail gas is cooled to below -50°C by a -70°C low-temperature refrigerant, and nitrogen dioxide, organic fluorinated compounds, hydrogen fluoride, etc. are recovered and returned to the production process to participate in the synthesis reaction again to achieve resource utilization. The synthetic tail gas is deep-cooled to below -50°C, and almost all nitrogen dioxide is condensed, more than 70% of hydrogen fluoride is also condensed, and more than 90% of organic fluorinated compounds are also condensed. Through deep cooling, the pollutant components can be timely and on-site resource utilization, with high utilization efficiency and low utilization cost. The condensed pollutant components come from the raw materials required for the synthesis process of organic fluorinated compounds, or reaction products, or side reaction products. These components are condensed and returned to the reaction system, which can effectively inhibit the side reactions and improve the conversion rate of the reactants. In addition, the present invention creatively sets the deep cooling unit directly on the top of the synthesis reactor, which has the following advantages:

[0025] 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;

[0026] 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;

[0027] 3) After dehydration, the cryogenic tail gas from the cryogenic unit contains high concentrations of HF, NO and a small amount of NO2 and fluorobenzene. 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 fluorobenzene. On the other hand, it can also serve the purpose of stirring and humidity adjustment in the reactor.

[0028] In addition to the HF component, the fluorobenzene synthesis tail gas also contains a large amount of fluorobenzene, nitrogen oxides, and a small amount of VOCs and other pollutants. In order to remove or recycle these pollutants and obtain anhydrous HF products at the source, according to the physical properties and characteristics of these components, the present invention creatively proposes a cryogenic heat exchanger refrigerant internal circulation scheme based on the deep cooling of the tail gas, and forms an enhanced cooling section in the cryogenic heat exchanger. By adjusting the circulation amount, different cooling effects can be obtained, the condensation rate of different pollutants can be achieved, and the condensation interception effect of the synthetic tail gas pollutants can be improved.

[0029] Among these pollutants, 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 fluorobenzene is -42°C and the boiling point is 85°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 -50°C, nitrogen dioxide and fluorobenzene are basically completely condensed, and more than 70% of HF is also condensed. During the condensation process, nitrogen dioxide and fluorobenzene will appear in the deep-cooling heat exchanger to frost and block the tail gas flow channel. In order to alleviate the problem of frosting and clogging, the present invention has made the following improvements:

[0030] (1) The exhaust gas enters the cryogenic heat exchanger and exchanges heat with the refrigerant in reverse. The refrigerant is forced to circulate in the cryogenic heat exchanger to form enhanced cooling sections corresponding to hydrogen fluoride, fluorobenzene 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.

[0031] (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 resin adsorption unit is used as a defrosting heat source, which not only recovers the fluorobenzene in the exhaust gas but also provides efficient defrosting.

[0032] The effects of this improvement are as follows:

[0033] (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.

[0034] (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.

[0035] (3) Fully recover the frosting cold energy.

[0036] (4) No need to introduce external defrosting medium to achieve efficient defrosting.

[0037] In order to efficiently recover the HF component in the tail gas and eliminate the scaling and clogging problems in the absorption process of sodium hydroxide solution, the primary water washing tower and the primary alkali washing tower are regularly switched and used alternately in the washing unit. By setting the cycle of the alkali washing stage of the alkali washing tower, the functions of the alkali washing tower and the first-stage water washing tower are regularly switched, that is, the "alkali washing" and "water washing" functions are switched. The effect of this improvement is as follows:

[0038] (1) Completely eliminate the problem of scaling and clogging. Since the solubility of sodium fluoride in water is only 3.85% (10°C), it is easy to adhere to the surface of the packing and the contact surfaces such as the pump impeller and the inner wall of the absorption tower, forming a scale layer that blocks the equipment. By switching, the original circulating alkali solution is replaced by circulating spray water, and the original low-concentration hydrogen fluoride tail gas is replaced by high-concentration hydrogen fluoride tail gas. With the circulation of the circulating spray water, the concentration of hydrogen fluoride becomes higher and higher, and the pH value of the absorption liquid becomes lower and lower. The sodium fluoride scale layer originally adhered to the surface of the packing and the contact surfaces such as the pump impeller and the inner wall of the absorption tower is dissolved into the hydrogen fluoride aqueous solution, achieving online descaling without the need to introduce external descaling agents.

[0039] (2) Obtain high-concentration HF products and improve the recovery rate of fluorine. Because in the process of washing and absorbing HF in the tail gas, as the HF in the tail gas decreases from level 1 to level 4, the water washing absorption efficiency will decrease. The ionization process of HF in a dilute solution can be expressed as HF+H 2 O→H 3 O + +F - , due to F - is a strong proton acceptor, while H 3 O + It is a strong proton donor, H 3 O + With F - Through hydrogen bonding, a relatively stable ion pair is generated: this ion pair is difficult to ionize. HF shows weak acidity in dilute solution. There is a gas-liquid equilibrium concentration of HF in the absorption process, so there is a certain amount of HF residual in the tail gas. The HF component in the tail gas can be absorbed more thoroughly through alkaline washing, and then the fluorine element in the scale layer can be further dissolved and recovered through the solution of absorption equipment function conversion.

[0040] Furthermore, according to the HF concentration in the tail gas entering the washing unit, the amount of deionized water added to the water washing and demister tower is controlled, thereby controlling the amount of washing liquid discharged from the water washing and demister tower to the multi-stage water washing tower in sequence, so as to ensure that the washing liquid finally entering the first-stage water washing tower absorbs the HF in the tail gas and reaches a concentration of more than 40%, thereby ensuring the liquid phase balance of the system.

[0041] When too much deionized water is added to the water washing and demister tower, the amount of liquid discharged from the washing unit will increase, and the HF content in the discharged liquid product will be low. At this time, the amount of deionized water added should be reduced; if the amount of deionized water added is too low, it will affect the dehydrofluorination effect of the previous water washing tower and increase the dehydrofluorination load of the alkali washing tower.

[0042] Furthermore, while converting the function of the absorption equipment, two circulating liquid tanks are cleverly set for the first-level water washing tower and the alkali washing tower, one is a circulating alkali liquid tank, and the other is a circulating spray water tank. When used as a first-level water washing tower, the circulating spray liquid returns to the circulating spray water tank; when used as an alkali washing tower, the circulating spray liquid returns to the circulating alkali liquid tank. When washing, the circulating alkali liquid in the alkali washing stage is continuously and evenly discharged into the circulating spray water tank in use. When the discharge volume reaches 50-60% of the total circulating alkali liquid volume, the discharge is stopped, and fresh sodium hydroxide solution is added to the circulating alkali liquid tank to the original liquid level. The effect of this improvement is:

[0043] (1) It can ensure the full recovery of the fluorine components in the system and zero discharge of fluorine components.

[0044] (2) No fluorine-containing wastewater is generated, which saves wastewater treatment investment and operating costs and greatly simplifies the treatment process of fluorine components in exhaust gas.

[0045] Furthermore, the present invention innovatively proposes the following two improvements in the fluorobenzene limit recovery resin adsorption unit:

[0046] (1) Circulation cooling of adsorbed tail gas. 10-20% volume percentage of the washing tail gas from another resin adsorption tower is refluxed into the resin adsorption tower after desorption to cool the resin adsorption layer. The washing tail gas after heat exchange and temperature rise in the resin layer is mixed with the remaining 80-90% adsorption tail gas and enters the subsequent denitration unit. The reason for cooling the resin adsorption layer after desorption is that the lower the temperature during resin adsorption, the more conducive it is to adsorption, so as to avoid the washing tail gas being heated when entering the resin adsorption layer due to the high temperature in the initial stage of adsorption, resulting in a fast tail gas flow rate, short adsorption time, and decreased adsorption rate of fluorobenzene; the reason for using partial adsorption tail gas reflux cooling is to take into account the use of a lower cooling rate to cool the resin to avoid rapid cooling affecting the service life of the resin.

[0047] (2) The desorbed tail gas (mainly composed of fluorobenzene and nitrogen oxides) after desorption by the resin is condensed and dehydrated, and then heated by the tail gas heat exchanger with the purified tail gas after SCR denitration, and then returned to the synthesis reactor for stirring and temperature adjustment.

[0048] The effects of this improvement are as follows:

[0049] (1) Fully utilize the cold and heat sources of the system's internal medium. The cold capacity of the adsorbed tail gas at a lower temperature is recovered by cooling the desorbed resin adsorption tower with the reflux of the adsorbed tail gas; the heat enthalpy of the purified tail gas at a higher temperature is recovered by heat exchange between the desorbed tail gas and the purified tail gas after SCR denitration; the desorbed tail gas after heat exchange and heating is stirred in the reactor, saving the heat enthalpy required for heating and pyrolysis of the synthesis reactor liquid.

[0050] (2) Fully recycle and utilize pollutants such as fluorobenzene and nitrogen oxides contained in exhaust gas.

[0051] Furthermore, when the adsorbed tail gas after resin adsorption is subjected to SCR denitration treatment, the present invention innovatively proposes a technical solution for purifying tail gas circulation, 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 heating and participates in the tail gas circulation, and the other part is discharged through the chimney after heat exchange with the desorbed tail gas through the purified tail gas heat exchanger for cooling and dehydration. The effects of adopting this technical solution are as follows:

[0052] (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 SCR denitrification 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 denitrification reaction process.

[0053] (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.

[0054] (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 SCR denitrification reactor, which is beneficial to the stability of the SCR denitrification reaction process, improves the denitrification effect, and reduces ammonia escape.

[0055] (4) The purified exhaust gas of each row is heat exchanged with the desorbed exhaust gas at a lower temperature to condense part of the water in the purified exhaust gas. The condensation process also absorbs the ammonia component in the flue gas, reducing ammonia escape.

[0056] (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.

[0057] The present invention can realize the full recovery of fluorobenzene and fluorine components in the synthetic tail gas, fully recover the waste heat and waste cold, and has a relatively simple process route, is environmentally friendly, and has low equipment investment and operating costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 This is a system diagram of the present invention.

[0059] Among them: synthesis reactor HC1, refrigerant circulation pump HC2, cryogenic heat exchanger HC3, reactor cover HC7, purified tail gas heat exchanger HC15;

[0060] Washing induced draft fan XD1, primary alkali washing tower XD2A, primary water washing tower XD2B, secondary water washing tower XD3, tertiary water washing tower XD4, quaternary water washing tower XD5, water washing demister tower XD6, under-tower storage tank XD8, water washing circulating pump XD11, alkali drainage pump XD12, circulating alkali solution tank XD20, circulating spray water tank XD21;

[0061] Resin adsorption tower SX1, reflux cooling fan SX3, spray cooling tower SX4, analytical tail gas condenser SX5, gas-liquid separator SX7, oil-water separation tank SX11, fluorobenzene product tank SX12;

[0062] Denitrification heat exchanger XT3, circulating fan XT4, tail gas circulation regulating valve XT5, tail gas heater XT8, pipeline mixer XT10, ammonia adding device XT11, SCR denitrification reactor XT9, DETAILED DESCRIPTION

[0063] The system of the present invention is further explained below in conjunction with the accompanying drawings:

[0064] A cryogenic unit is provided on the top of the synthesis reactor HC1, and the cryogenic unit is sequentially connected to a washing unit, an adsorption unit and a denitration unit;

[0065] The cryogenic unit includes a plurality of cryogenic heat exchangers HC3 installed on the cover of the synthesis reactor HC1, the bottom air inlets of the plurality of cryogenic heat exchangers HC3 are all connected to the tail gas outlet of the synthesis reactor HC1, the air outlets are connected in series and parallel through pipelines, and the switching valves provided on the connecting pipes are used to switch between the parallel use and the series use of the plurality of cryogenic heat exchangers HC3, and condensation and defrosting are performed alternately. The cryogenic tail gas outlet of the cryogenic unit is also connected to the bottom gas inlet of the synthesis reactor HC1. The shell side of the cryogenic heat exchanger HC3 where the refrigerant flows is provided with multiple enhanced cooling sections from the refrigerant inlet to the outlet, and a refrigerant outlet is provided downstream of each enhanced cooling section and connected to the refrigerant inlet upstream through a refrigerant circulation pump HC2.

[0066] The washing unit comprises four-stage water washing towers (a primary water washing tower XD2B, a secondary water washing tower XD3, a tertiary water washing tower XD4, and a quaternary water washing tower XD5) connected in series, a primary alkali washing tower XD2A, and a water washing and demisting tower. The alkali washing cycle of the primary alkali washing tower XD2A is set, or the sodium fluoride concentration limit in the sprayed alkali solution in the primary alkali washing tower XD2A is set, and the primary water washing tower XD2B and the primary alkali washing tower XD2A in the multi-stage water washing tower are regularly switched and used alternately.

[0067] The tail gas outlet of the deep cooling unit is respectively connected to the air inlet of the first-stage water washing tower XD2B and the air inlet of the first-stage alkali washing tower in the multi-stage water washing tower, and the air outlet of the first-stage water washing tower XD2B and the air outlet of the first-stage alkali washing tower XD2A in the multi-stage water washing tower are connected to the air inlet of the water washing and demisting tower XD6, and the air outlet of the first-stage water washing tower XD2B and the air outlet of the first-stage alkali washing tower XD2A in the multi-stage water washing tower are connected to the air inlet of the second-stage water washing tower XD3 through a pipeline, and a switching valve is set on the connecting pipe to realize a single series connection between towers. Two circulating liquid tanks are correspondingly set at the bottom of the first-stage water washing tower XD2B and the first-stage alkali washing tower XD2A, one is a circulating alkali liquid tank XD20, and the other is a circulating spray water tank XD21, both of which are connected to the tower through a circulating spray pipe, and the circulating alkali liquid tank XD20 is connected to the circulating spray water tank XD21 through the alkali pump XD12.

[0068] The washing water outlets in the storage tanks under the water washing and demister tower XD6 and the multi-stage water washing towers except the first-stage water washing tower are connected to a tee via a water washing circulation pump, and one end of the tee is connected to the tower, and the other end is connected in series in the order of the water washing and demister tower XD6 to the multi-stage water washing towers.

[0069] The adsorption unit includes at least two resin adsorption towers SX1, each of which is provided with a tail gas inlet, a superheated steam inlet and a tail gas outlet, and a desorption tail gas outlet. The resin adsorption towers SX1 are connected in parallel, and the tail gas inlet of each resin adsorption tower SX1 is connected to the tail gas outlet of the washing unit. The tail gas outlets of each resin adsorption tower SX1 are connected to the denitration unit after merging; the tail gas outlet of the adsorption unit is also connected to the tail gas inlet of each resin adsorption tower SX1 through a reflux cooling fan.

[0070] The desorption tail gas outlet of the adsorption unit is connected to the condensing device (connected gas-liquid separator SX7, spray cooling tower SX4, desorption tail gas condenser SX5), the purified tail gas heat exchanger HC15, and then connected to the defrost gas pipeline of the cryogenic unit and the bottom gas inlet of the synthesis reactor CH1.

[0071] The denitration unit includes a denitration heat exchanger XT3, a tail gas heater XT8, an ammonia device XT11, a pipeline mixer XT10, an ammonia adding device XT11, and an SCR denitration reactor XT9, which are connected in sequence. The purified tail gas outlet of the XT3 denitration reactor XT9 is connected to the chimney through the denitration heat exchanger XT3, a circulating fan XT4, and a purified tail gas heat exchanger HC15; the purified tail gas outlet of the XT3 denitration reactor XT9 is also connected to the tail gas heater XT8 through the denitration heat exchanger XT3, a circulating fan XT4, and a tail gas circulation regulating valve XT5.

[0072] Taking the tail gas production of fluorobenzene in a certain project as an example, the project production line contains 60 HC1 synthesis reactors, producing 4,000 tons of fluorobenzene annually.

[0073] Each synthesis reactor HC1 is in intermittent production. The main process of fluorobenzene synthesis includes salt formation, diazotization and pyrolysis reaction steps, all of which are completed in the synthesis reactor HC1.

[0074] (1) Salt formation

[0075] Low-temperature hydrofluoric acid liquid (-15°C) is measured and put into the synthesis reactor HC1, and the temperature of the synthesis reactor HC1 is continuously cooled with frozen brine to reduce the temperature to between 4 and 6°C. Aniline is slowly measured and added dropwise into the synthesis reactor HC1 under stirring. 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 warm at about 6°C for 30 minutes. The reaction time is 6 hours to obtain an aniline hydrofluoride mixture.

[0076] (2) Diazotization

[0077] The temperature of the circulating frozen brine in the synthesis reactor HC1 was lowered to 0-2°C, and solid sodium nitrite was slowly added under stirring. The feeding temperature was controlled at 0-5°C and the pressure was slightly negative. After the addition was completed, the reaction was kept warm for 30 minutes and the reaction was completed. The reaction time was 10 hours to obtain a diazo liquid.

[0078] There is a side reaction in the diazotization reaction: 2HNO 2 →NO+NO 2 + Water.

[0079] (3) Pyrolysis

[0080] Adjust the temperature of the frozen brine, and slowly heat the liquid in the synthesis reactor HC1 in stages (5-15°C, 15-25°C, 25-38°C) under stirring, and heat it up by about 1.2°C per hour until it reaches 38°C, with a slightly negative pressure, and perform thermal decomposition for 32 hours. After the reaction is completed, the materials in the reactor are placed in a standing tank for stratification, and the organic layer is a fluorobenzene mixture; the inorganic layer contains hydrofluoric acid and sodium fluoride, etc.

[0081] The reaction temperature is different in different reaction stages and needs to be adjusted according to the reaction period.

[0082] See attached Figure 1 The structure and cryogenic process of the fluorobenzene synthesis tail gas cryogenic unit are as follows:

[0083] 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, and the tube side air outlet of the other end is connected to the inlet of the induced draft fan through a pipeline.

[0084] 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;

[0085] 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;

[0086] 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;

[0087] 5. The "defrosting" of the cryogenic heat exchanger HC3 can also be done by using the desorbed tail gas from the 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.

[0088] 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.

[0089] The amount of synthetic tail gas discharged without cryogenic condensation is 1000∽2000Nm3 / h, the main components of 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.

[0090] One set of “washing unit + adsorption unit + denitrification unit” corresponds to multiple sets of “synthesis reactor HC1 + cryogenic unit”.

[0091] See attached Figure 1 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, where it is indirectly cooled to between -50 and -55°C by a -70°C low-temperature refrigerant. After more than 99.99% of fluorobenzene, nitrogen dioxide and more than 70% of hydrogen fluoride are cryogenically condensed, the cryogenic tail gas formed is divided into two streams. One stream enters the washing unit to further recover the remaining hydrogen fluoride component in the cryogenic tail gas, and then enters the washing unit to remove hydrogen fluoride, the adsorption unit to remove defluorobenzene, and the denitration unit to remove nitrogen and purify before being discharged; the other stream is sent to the synthesis reactor HC1 to adjust the temperature in the reactor, stir the reactor liquid, and recycle the hydrogen fluoride component.

[0092] 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 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 process is performed alternately.

[0093] 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, organic fluorinated compounds 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 pollution components of the synthetic tail gas. 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 kettle liquid and inhibiting the side reaction of 〈2HNO 2 →NO+NO 2 + water〉, increasing HNO 2 utilization rate.

[0094] Continue to see Figure 1In this embodiment, the washing unit includes four-stage water washing towers XD2B, XD3, XD4, XD5, a primary alkali washing tower XD2A and a water washing and demisting tower XD6 connected in series. The deep-cold tail gas after condensation and recovery of most of the organic fluorinated compounds and hydrogen fluoride is passed through the washing induced draft fan XD1 and sequentially passed through the four-stage water washing towers XD2B, XD3, XD4, and XD5 for water washing, the primary alkali washing tower XD2A for alkali washing and the water washing and demisting tower XD6 for demisting to remove 99.99% of hydrogen fluoride, and then enters the adsorption unit to remove organic fluorinated compounds.

[0095] Here, through the control of pipelines and valves, the primary water washing tower XD2B and the primary alkali washing tower XD2A in the multi-stage water washing tower can be switched alternately, that is, the primary water washing tower XD2B can be switched to the primary alkali washing tower, and the primary alkali washing tower XD2A can be switched to the water washing tower. The bottoms of the primary water washing tower XD2B and the primary alkali washing tower XD2A are respectively provided with two circulating liquid tanks, one is a circulating alkali liquid tank XD20, and the other is a circulating spray water tank XD21. When working as a primary water washing tower, the circulating spray liquid is collected by the corresponding circulating spray water tank XD21 and then returned to the tower. When working as a primary alkali washing tower, the circulating spray liquid is collected by the circulating alkali liquid tank XD21 and then returned to the tower. The above control can be completed by corresponding pipeline connection and valve switching, which is a prior art and will not be described in detail.

[0096] The specific preferred method is: monitor the sodium fluoride concentration in the circulating alkali liquid tank of the tower working as the primary alkali washing tower XD2A, when it exceeds 3wt%, the deep cold tail gas entering the original primary water washing tower is directly switched to be introduced into the tower, and at the same time, the circulating liquid tank of the tower is switched, and the circulating spray water in the circulating spray water tank XD21 is sprayed into the tower for circulating spraying, and finally the tower is switched as the primary water washing tower;

[0097] At the same time, the original first-stage water washing tower is switched to a first-stage alkali washing tower, and the tail gas from the last-stage water washing tower is introduced into the tower. At the same time, the circulating liquid tank of the tower is switched, and the circulating alkali liquid spray in the circulating alkali liquid tank XD20 is introduced into the tower for circulating spraying. Since the solubility of sodium fluoride in water is only 3.85wt% (10°C), it is easy to adhere to the surface of the packing and the contact surfaces such as the pump impeller and the inner wall of the absorption tower, forming a scale layer to block the equipment. By switching, the original circulating alkali liquid is replaced by circulating spray water, and the original low-concentration hydrogen fluoride tail gas is replaced by high-concentration hydrogen fluoride tail gas. With the circulating spraying of the circulating spray water, the concentration of hydrogen fluoride becomes higher and higher, and the sodium fluoride scale layer originally adhered to the surface of the packing and the contact surfaces such as the pump impeller and the inner wall of the absorption tower is dissolved into the hydrogen fluoride aqueous solution, realizing online descaling, and there is no need to introduce external descaling agents, while ensuring the full recovery of the fluorine components of the system and zero external discharge.

[0098] When used as a primary water washing tower, the circulating alkali liquid is continuously and evenly discharged from the corresponding circulating alkali liquid tank XD20 and enters the corresponding circulating spray water tank XD21 through the alkali discharge pump XD1. When the discharge volume reaches 50-60% of the total circulating alkali liquid volume, the discharge is stopped, and fresh sodium hydroxide solution is added to the circulating alkali liquid tank XD21 to the original liquid level. Deionized water is evenly and continuously added to the water washing and demisting tower XD6, and part of the washing water discharged from the bottom of the tower is reversely entered into the multi-stage water washing tower in turn as circulating spray water.

[0099] The external washing liquid phase introduction pipe of the washing unit is respectively connected to the circulating alkali liquid tank XD20 of the primary alkali washing tower and the tower storage tank under the water washing and demisting tower XD6. The circulating spray water tank XD21 of the primary water washing tower XD2B and the primary alkali washing tower XD2A is provided with an external liquid phase outlet pipe, which is used as the primary water washing tower XD2B to lead out the hydrofluoric acid product with HF mass concentration of more than 40wt%.

[0100] See also Figure 1 The adsorption unit includes at least two resin adsorption towers SX1, one for adsorption and the other for desorption, which are performed alternately; the resin adsorption tower is desorbed by superheated steam, and 10-20% volume percentage of adsorbed tail gas from another resin adsorption tower is introduced into the desorbed resin adsorption tower to cool the resin adsorption layer until the set temperature, and then enters the subsequent denitration unit together with the remaining adsorbed tail gas. The desorbed tail gas drawn out of the adsorption unit is cooled to -5∽0℃ and dehydrated by the condensing device, and then indirectly heat-exchanged with the purified tail gas by the purified tail gas heat exchanger SX13 and is divided into two streams, one of which is used as the defrost gas of the cryogenic unit and is recycled to the defrost gas pipeline of the cryogenic unit, and the other is returned to the bottom of the synthesis reactor HC1 to adjust the temperature in the reactor.

[0101] The condensing device comprises a gas-liquid separator SX7, a spray cooling tower SX4, and an analytical tail gas condenser SX5 which are connected in sequence. The desorbed tail gas is condensed in the gas-liquid separator SX7, the spray cooling tower SX7, and the analytical tail gas condenser SX5 in sequence and then sent to the purified tail gas heat exchanger HC15; the liquid phase (or condensate) drawn out from the gas-liquid separator SX7, the spray cooling tower SX4, and the analytical tail gas condenser SX5 enters the oil-water separation tank SX11 to separate and recover fluorobenzene and sends it to the fluorobenzene product tank SX12 to obtain the fluorobenzene product.

[0102] See also Figure 1The denitration unit includes a denitration heat exchanger XT3, a tail gas heater XT8, an ammonia device XT11, a pipeline mixer XT10, an ammonia adding device XT11, and an SCR denitration reactor XT9; the adsorbed tail gas from the adsorption unit is heated by heat exchange with the purified tail gas in the denitration heat exchanger XT3 and then heated to above 170°C by the tail gas heater XT8, and then enters the SCR denitration reactor XT9 for denitration purification after ammonia is added by the ammonia adding device XT11. The obtained purified tail gas is divided into two parts, one part is directly mixed with the adsorbed tail gas after the temperature is increased after leaving the denitration heat exchanger XT3 through the circulating fan XT4 and the tail gas circulation regulating valve XT4, and is sent to the tail gas heater XT8 to participate in the tail gas circulation, so as to dilute the nitrogen oxide concentration in the tail gas entering the SCR denitration reactor, and the other part is discharged through the chimney after heat exchange with the desorbed tail gas through the circulating ring fan XT4 and the purified tail gas heat exchanger HC15, and then cooled and dehumidified.

[0103] After purification, the HF recovery rate is above 99.99%; the organic fluorinated compound recovery rate is above 99.9%; the NOx removal rate is above 99.95%, and the outlet NOx is less than 100mg / Nm 3 .

[0104] After purification, about 2,000 tons of HF and about 20 tons of organic fluorinated compounds are recovered annually.

Claims

1. A system for recycling fluorine components in tail gas from fluorobenzene production, comprising a synthesis reactor, characterized in that: A cryogenic unit is provided on the top of the synthesis reactor, and the cryogenic unit is connected to a washing unit; The washing unit comprises a multi-stage water washing tower, a primary alkali washing tower and a water washing and demisting tower connected in series, the alkali washing cycle of the primary alkali washing tower is set, or the sodium fluoride concentration limit in the sprayed alkali solution in the primary alkali washing tower is set, and the first-stage water washing tower and the primary alkali washing tower in the multi-stage water washing tower are switched regularly and used alternately.

2. The fluorine component resource recovery system in fluorobenzene production tail gas according to claim 1, characterized in that: The tail gas outlet of the deep cooling unit is respectively connected to the air inlet of the first-stage water washing tower and the air inlet of the first-stage alkali washing tower in the multi-stage water washing tower, the air outlet of the first-stage water washing tower and the air outlet of the first-stage alkali washing tower in the multi-stage water washing tower are both connected to the air inlet of the water washing and demisting tower, the air outlet of the first-stage water washing tower and the air outlet of the first-stage alkali washing tower in the multi-stage water washing tower are connected to the air inlet of the second-stage water washing tower through a pipeline, and switching valves are provided on the connecting pipes to realize a single series connection between towers.

3. The fluorine component resource recovery system in the tail gas of fluorobenzene production according to claim 2 comprises a synthesis reactor, characterized in that: Two circulating liquid tanks are correspondingly arranged at the bottom of the primary water washing tower and the primary alkali washing tower, one is a circulating alkali liquid tank, and the other is a circulating spray water tank, both of which are connected to the tower through a circulating spray pipe.

4. The system for recycling fluorine components in tail gas from fluorobenzene production as claimed in claim 3, characterized in that: The circulating alkali solution tank is connected with the circulating spray water tank through an alkali drainage pump.

5. The system for recycling fluorine components in tail gas from fluorobenzene production as claimed in claim 4, characterized in that: The washing water outlets in the storage tanks under the water washing and demister towers and the multi-stage water washing towers except the first-stage water washing tower are connected to a tee via a water washing circulation pump, and one end of the tee is connected to the tower, and the other end is connected in series in the order from the water washing and demister tower to the multi-stage water washing tower.

6. The system for recycling fluorine components in tail gas from fluorobenzene production according to any one of claims 1 to 5, characterized in that: The washing unit is connected to the denitration unit via the adsorption unit; Wherein, the adsorption unit includes at least two resin adsorption towers, each of which is provided with an exhaust gas inlet, a superheated steam inlet and an exhaust gas outlet, and a desorption exhaust gas outlet. The resin adsorption towers are connected in parallel, and the exhaust gas inlet of each resin adsorption tower is connected to the exhaust gas outlet of the washing unit. The exhaust gas outlet of each resin adsorption tower is connected to the denitrification unit after merging; the exhaust gas outlet of the adsorption unit is also connected to the exhaust gas inlet of each resin adsorption tower through a reflux cooling fan.

7. The system for recycling fluorine components in tail gas from fluorobenzene production as claimed in claim 6, characterized in that: The desorbed tail gas outlet of the adsorption unit is connected to a condensing device and a purified tail gas heat exchanger, and then respectively connected to a defrosting gas pipeline of a cryogenic unit and a gas inlet at the bottom of a synthesis reactor.

8. The system for recycling fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: The denitration unit comprises a denitration heat exchanger, a tail gas heater, a pipeline mixer, an ammonia device and an SCR denitration reactor which are connected in sequence.

9. The system for recycling fluorine components in tail gas from fluorobenzene production as claimed in claim 8, characterized in that: The purified tail gas outlet of the denitration reactor is connected to the chimney via the denitration heat exchanger, the circulating fan and the purified tail gas heat exchanger.

10. The system for recycling fluorine components in tail gas from fluorobenzene production according to claim 9, characterized in that: The purified tail gas outlet of the denitration reactor is also connected to the tail gas heater through a denitration heat exchanger, a circulating fan and a tail gas circulation regulating valve.

11. The system for recycling fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: The cryogenic unit comprises a plurality of cryogenic heat exchangers installed on the cover of the synthesis reactor, the bottom air inlets of the plurality of cryogenic heat exchangers are connected to the tail gas outlet of the synthesis reactor, the air outlets are connected in series and parallel through pipelines, and the switching valves arranged on the connecting pipes are used to switch between the parallel use and the series use of the cryogenic heat exchangers, and condensation and defrosting are performed alternately.

12. The fluorine component resource recovery system in the tail gas of fluorobenzene production according to claim 11, comprising a synthesis reactor, characterized in that: The shell side of the cryogenic heat exchanger where the refrigerant flows is provided with multiple enhanced cooling sections from the refrigerant inlet to the outlet. A refrigerant outlet is provided downstream of each enhanced cooling section and is connected to the upstream refrigerant inlet via a refrigerant circulation pump.

13. The system for recycling fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: The cryogenic tail gas outlet of the cryogenic unit is also connected to the gas inlet at the bottom of the synthesis reactor.

14. The system for recycling fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: The multiple groups of cryogenic units are connected in parallel and then connected to the washing unit.

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