A process for recycling fluorine components in tail gas from fluorobenzene production

Abstract

The invention discloses a process for resource recovery of fluorine components in tail gas of fluorobenzene production, including synthetic tail gas generated in the process of fluorobenzene synthesis, wherein the synthetic tail gas is cryogenically condensed by a cryogenic unit to form a cryogenic tail gas after 70wt% or more of hydrogen fluoride is formed, and then enters a washing unit to deeply recover the HF component in the cryogenic tail gas, and generate a hydrofluoric acid product containing more than 45wt% of the F component; wherein, in the washing unit, the cryogenic tail gas is sequentially washed by a multi-stage water washing tower and a primary alkali washing tower to further absorb the remaining hydrogen fluoride component in the cryogenic tail gas, the alkali washing cycle of the primary alkali washing tower is set, or the sodium fluoride concentration limit in the spray 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 regularly switched and used alternately. The invention has simple process, clean and environmental protection, stable and efficient, controllable composition, high fluorine element recovery rate, and long equipment 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 is a resource recovery process for fluorine components in the tail gas of fluorobenzene production. Background Art

[0002] Fluorobenzene is an important chemical intermediate, which is widely used in the pharmaceutical, dye and other industries. Fluorobenzene production has high profits, but in addition to a large amount of concentrated waste HF, the production process also produces a large amount of fluorine-containing wastewater, waste gas and waste residue pollutants. HF is very harmful to human health. Long-term inhalation of low-concentration HF can cause chronic poisoning, vomiting, dizziness and other symptoms. When the HF concentration is greater than 30μg / m 3 It can cause acute poisoning. So far, there is no efficient and low-cost method to solve the HF pollution problem in the fluorobenzene production process. In order to recover HF in the exhaust gas, Lv Zhimin et al. [Lv Zhimin, Zhang Yuqing, Tang Anping et al. Fluorobenzene Clean Production Process. Chemical World, 2002.12] studied the water washing recovery process, using three-stage water circulation to absorb HF in the exhaust gas, coagulation sedimentation and activated carbon adsorption to treat fluorine-containing wastewater. Although part of HF can be recovered, the recovery process route is long, and there are still wastewater pollutants. There is still a certain amount of HF pollution components in the tail gas after treatment. In order to reduce the HF pollution components in the exhaust gas as much as possible, there are also studies and applications using alkaline solution absorption, but 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 fluorine-containing wastewater.

[0003] With the development of fluorine chemical industry, the pollution problem of HF becomes more and more prominent, which seriously restricts the development of fluorine chemical industry.

[0004] The present invention is aimed at the above problems and develops a process for resource recovery of fluorine components in tail gas from fluorobenzene production. Summary of the invention

[0005] The purpose of the present invention is to solve the above technical problems and to provide a process for resource recovery of fluorine components in fluorobenzene production tail gas, which is simple in process, clean and environmentally friendly, stable and efficient, has controllable components, high fluorine element recovery rate, low operating cost and long equipment service life.

[0006] The method of the present invention comprises the synthesis tail gas generated in the process of fluorobenzene synthesis, characterized in that the synthesis tail gas is cryogenically condensed by a cryogenic unit to obtain more than 70wt% of hydrogen fluoride to form a cryogenic tail gas which enters a washing unit to deeply recover the HF component in the cryogenic tail gas to generate a hydrofluoric acid product containing more than 45wt% of the F component;

[0007] Among them, in the washing unit, the deep-cold tail gas is washed by a multi-stage water washing tower and a primary alkali washing tower in sequence to further absorb the remaining hydrogen fluoride components in the deep-cold tail gas, the alkali washing cycle of the primary alkali washing tower is set, or the sodium fluoride concentration limit in the alkali solution sprayed 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.

[0008] The washing unit comprises at least 4-stage water washing towers, a primary alkali washing tower and a water washing and demisting tower connected in series. The deep-cooled tail gas after condensation is sequentially washed by water in the 4-stage water washing tower, alkaline washed by the primary alkali washing tower and demisted by the water washing and demisting tower.

[0009] The bottoms of the first-stage water washing tower and the first-stage alkali washing tower in the four-stage water washing tower are respectively provided with two circulating liquid tanks, one is a circulating alkali liquid tank, and the other is a circulating spray water tank. When the first-stage water washing tower of the four-stage water washing tower works, the circulating spray liquid is collected in the circulating spray water tank and then returned to the tower. When the first-stage alkali washing tower works, the circulating spray liquid is collected in the circulating alkali liquid tank and then returned to the tower; a hydrofluoric acid product containing more than 45wt% of F component is drawn out from the bottom of the circulating alkali liquid tank.

[0010] Monitor the sodium fluoride concentration in the circulating alkali liquid tank of the tower used as the first-stage alkali washing tower. When it exceeds 3wt%, switch the cryogenic tail gas entering the washing unit directly to the tower, and switch the circulating liquid tank of the tower to introduce the circulating spray water in the circulating spray water tank into the tower for circulating spraying, that is, switch the alkali washing function of the tower to the first-stage water washing function;

[0011] 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 4th-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 in the circulating alkali liquid tank is introduced into the tower for circulating spraying.

[0012] When used as the first-stage water washing tower, the circulating alkali liquid is continuously and evenly discharged from the corresponding circulating alkali liquid tank into the corresponding circulating spray water tank. When the discharge volume reaches 50-60% volume percentage 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.

[0013] Deionized water is evenly and continuously added to the water washing and demisting tower, and part of the washing water discharged from the bottom of the tower is reversed and sequentially enters the multi-stage water washing tower.

[0014] The washing tail gas after HF removal from the washing unit enters the adsorption unit for defluorinated benzene and the denitration unit for denitration and purification before being discharged;

[0015] Wherein, the adsorption unit includes at least two resin adsorption towers, one for adsorption and the other for desorption, which are performed alternately; 10-20% volume percentage of adsorbed tail gas from another resin adsorption tower after adsorption is introduced into the resin adsorption tower that has completed desorption to cool the resin adsorption layer until the set temperature, and then enters the subsequent denitration unit together with the remaining adsorbed tail gas.

[0016] The desorbed tail gas drawn out from the adsorption unit is condensed by a condensing device and then heat exchanged by a purified tail gas heat exchanger and is divided into two streams, one of which is reused as defrosting gas of a cryogenic unit and the other is sent to a synthesis reactor to adjust the reactor liquid temperature.

[0017] The denitration unit comprises a denitration heat exchanger, an exhaust gas heater, an ammonia adding device, a pipeline mixer and an SCR denitration reactor; the adsorbed exhaust gas from the adsorption unit is heated by heat exchange with the purified exhaust gas in the denitration heat exchanger and then heated to above 170°C by the exhaust heater, ammonia is added by the ammonia adding device, and finally mixed evenly by the pipeline mixer before entering the SCR denitration reactor for denitration purification, and the obtained purified exhaust gas is then sent to the denitration heat exchanger for heat exchange with the adsorbed exhaust gas.

[0018] The purified exhaust gas coming out of the denitration heat exchanger is divided into two parts. One part is directly mixed with the adsorbed exhaust gas after the temperature is increased after leaving the denitration heat exchanger, participates in the exhaust gas circulation, and dilutes the nitrogen oxide concentration in the exhaust gas entering the SCR denitration reactor. The other part is discharged through the chimney after heat exchange with the desorbed exhaust gas through the purified exhaust gas heat exchanger for cooling and dehumidification.

[0019] The cryogenic unit is composed of a plurality of cryogenic heat exchangers installed on the cover of the synthesis reactor. The plurality of cryogenic heat exchangers perform condensation and defrosting processes in turn. The synthetic tail gas generated by the synthesis reactor enters the cryogenic heat exchanger in the condensation stage from the bottom of the cryogenic heat exchanger for condensation.

[0020] The cryogenic heat exchanger has multiple enhanced cooling sections, and each enhanced cooling section uses a refrigerant circulation pump to return the refrigerant from the downstream to the upstream.

[0021] A plurality of sets of the cryogenic units correspond to one set of washing unit, adsorption unit and denitration unit.

[0022] The cryogenic tail gas from the cryogenic unit is divided into two streams, one stream enters the washing unit, and the other stream is returned to the synthesis reactor to adjust the reactor liquid temperature and stir the reactor liquid.

[0023] 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 70wt% of hydrogen fluoride is also condensed, and more than 90wt% of organic fluorinated compounds are also condensed. Through deep cooling, the pollution components are timely and on-site resource utilization, with high utilization efficiency and low utilization cost. The condensed pollution 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:

[0024] 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;

[0025] 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;

[0026] 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.

[0027] 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.

[0028] 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:

[0029] (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.

[0030] (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.

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

[0032] (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.

[0033] (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.

[0034] (3) Fully recover the frosting cold energy.

[0035] (4) No need to introduce external defrosting medium to achieve efficient defrosting.

[0036] 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:

[0037] (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.

[0038] (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+H2O→H3O + +F - , due to F - is a strong proton acceptor, while H3O + It is a strong proton donor, H3O + 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.

[0039] 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.

[0040] 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.

[0041] 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:

[0042] (1) It can ensure the full recovery of the fluorine components in the system and zero discharge of fluorine components.

[0043] (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.

[0044] Furthermore, the present invention innovatively proposes the following two improvements in the fluorobenzene limit recovery resin adsorption unit:

[0045] (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% volume percentage of adsorbed 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.

[0046] (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.

[0047] The effects of this improvement are as follows:

[0048] (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.

[0049] (2) Fully recycle and utilize pollutants such as fluorobenzene and nitrogen oxides contained in exhaust gas.

[0050] 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:

[0051] (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.

[0052] (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.

[0053] (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.

[0054] (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.

[0055] (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.

[0056] 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

[0057] Figure 1 It is a process flow chart of the present invention.

[0058] Among them: synthesis reactor HC1, refrigerant circulation pump HC2, cryogenic heat exchanger HC3, reactor cover HC7, purified tail gas heat exchanger HC15;

[0059] 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;

[0060] 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;

[0061] 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

[0062] The process of the present invention is further explained below in conjunction with the accompanying drawings:

[0063] Taking the production of fluorobenzene in a certain enterprise as an example, the production line contains 60 HC1 synthesis reactors, producing 4,000 tons of fluorobenzene annually.

[0064] Each synthesis reactor HC1 is produced intermittently. The main process of fluorobenzene synthesis includes salt formation, diazotization and pyrolysis reaction steps. These reaction processes are all carried out in the synthesis reactor. HC1 Completed within.

[0065] (1) Salt formation

[0066] 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.

[0067] (2) Diazotization

[0068] 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.

[0069] There is a side reaction in the diazotization process: 2HNO2→NO+NO2+water.

[0070] (3) Pyrolysis

[0071] 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.

[0072] The reaction temperature is different in different reaction stages and needs to be adjusted according to the reaction period.

[0073] See attached Figure 1 The structure and cryogenic process of the fluorobenzene synthesis tail gas cryogenic unit are as follows:

[0074] 1. The cryogenic part 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 HC, and the tube side air outlet of the other end is connected to the inlet of the induced draft fan through a pipeline.

[0075] 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;

[0076] 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;

[0077] 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;

[0078] 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.

[0079] 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.

[0080] The amount of synthetic tail gas discharged without deep condensation is 1000∽2000Nm3 / h. The main components of the tail gas are: HF: 160∽170g / Nm3; fluorobenzene: 2000∽3000mg / Nm3; NOx: 12000∽20000mg / Nm3; a small amount of VOCs components; the rest is nitrogen.

[0081] One set of "washing unit + adsorption unit + denitrification unit" corresponds to multiple sets of "synthesis reactor HC1 + cryogenic unit".

[0082] 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.

[0083] 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.

[0084] 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, thereby improving the condensation interception effect of the synthetic tail gas pollution components. The condensate containing nitrogen dioxide generated in the cryogenic heat exchanger HC3 directly falls through the kettle cover HC7 into the synthesis reactor HC1, and directly falls into the synthesis reactor HC1 through the bottom of the cryogenic reactor HC3, thereby increasing the concentration of nitrogen dioxide in the reactor liquid, inhibiting the occurrence of the side reaction <2HNO2→NO+NO2+water>, and improving the utilization rate of HNO2.

[0085] Continue to see Figure 1 In 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.

[0086] 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.

[0087] 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 3%wt, 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;

[0088] 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 of the 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 circulating spray in the tower. 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 to block the equipment. Through the switch, 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 spray 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.

[0089] 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.

[0090] 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 concentration of more than 40wt%.

[0091] 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 10-20wt% of the adsorbed tail gas from another resin adsorption tower is introduced into the resin adsorption tower after desorption 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 HC15 and is divided into two streams, one of which is returned to the defrost gas pipeline of the cryogenic unit as the defrost gas of the cryogenic unit, and the other is returned to the bottom of the synthesis reactor HC1 to adjust the temperature in the reactor.

[0092] The condensing device comprises a gas-liquid separator SX7, a spray cooling tower SX7, 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 SX13; the liquid phase (or condensate) drawn out from the gas-liquid separator SX7, the spray cooling tower SX7, 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.

[0093] See also Figure 1 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; 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 ring 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.

[0094] 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 .

[0095] After purification, about 2,000 tons of HF and about 20 tons of organic fluorinated compounds are recovered annually.

Claims

1. A process for recycling fluorine components in tail gas from fluorobenzene production, comprising the synthetic tail gas generated during the synthesis of fluorobenzene, characterized in that: The synthetic tail gas is cryogenically condensed by a cryogenic unit to produce more than 70 wt% of hydrogen fluoride to form a cryogenic tail gas which enters a washing unit to deeply recover the HF component in the cryogenic tail gas to generate a hydrofluoric acid product containing more than 45 wt% of the F component; The washing unit comprises at least four water washing towers, a primary alkali washing tower and a water washing and demisting tower connected in series, and the deep cold tail gas after condensation is sequentially washed by water in the four-stage water washing tower, alkaline washed by the primary alkali washing tower and demisted by the water washing and demisting tower; Among them, in the washing unit, the cryogenic tail gas is sequentially washed by a 4-stage water washing tower and a first-stage alkali washing tower to further absorb the remaining hydrogen fluoride component in the cryogenic tail gas, the alkali washing cycle of the first-stage alkali washing tower is set, or the sodium fluoride concentration limit in the spray alkali solution in the first-stage alkali washing tower is set, and the first-stage water washing tower and the first-stage alkali washing tower in the multi-stage water washing tower are regularly switched and used alternately; The bottoms of the first-stage water washing tower and the first-stage alkali washing tower in the four-stage water washing tower are respectively provided with two circulating liquid tanks, one is a circulating alkali liquid tank, and the other is a circulating spray water tank. When the first-stage water washing tower works as the four-stage water washing tower, the circulating spray liquid is collected in the circulating spray water tank and then returned to the tower. When the first-stage alkali washing tower works as the first-stage alkali washing tower, the circulating spray liquid is collected in the circulating alkali liquid tank and then returned to the tower; a hydrofluoric acid product containing more than 45wt% of F component is drawn out from the bottom of the circulating alkali liquid tank.

2. The process for resource recovery of fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: Monitor the sodium fluoride concentration in the circulating alkali liquid tank of the tower used as the first-stage alkali washing tower. When it exceeds 3wt%, switch the cryogenic tail gas entering the washing unit directly to the tower, and switch the circulating liquid tank of the tower to introduce the circulating spray water in the circulating spray water tank into the tower for circulating spraying, that is, switch the alkali washing function of the tower to the first-stage water washing function; 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 4th-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 in the circulating alkali liquid tank is introduced into the tower for circulating spraying.

3. The process for resource recovery of fluorine components in tail gas from fluorobenzene production as claimed in claim 2, characterized in that: When used as the first-stage water washing tower, the circulating alkali liquid is continuously and evenly discharged from the corresponding circulating alkali liquid tank into the corresponding circulating spray water tank. When the discharge volume reaches 50-60% volume percentage 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.

4. The process for resource recovery of fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: Deionized water is evenly and continuously added to the water washing and demisting tower, and part of the washing water discharged from the bottom of the tower is reversed and sequentially enters the multi-stage water washing tower.

5. The process for resource recovery of fluorine components in tail gas from fluorobenzene production according to any one of claims 1 to 4, characterized in that: The washing tail gas after HF removal from the washing unit enters the adsorption unit for defluorinated benzene and the denitration unit for denitration and purification before being discharged; Wherein, the adsorption unit includes at least two resin adsorption towers, one for adsorption and the other for desorption, which are performed alternately; 10-20% volume percentage of adsorbed tail gas from another resin adsorption tower after adsorption is introduced into the resin adsorption tower that has completed desorption to cool the resin adsorption layer until the set temperature, and then enters the subsequent denitration unit together with the remaining adsorbed tail gas.

6. The process for resource recovery of fluorine components in tail gas from fluorobenzene production as claimed in claim 5, characterized in that: The desorbed tail gas drawn out from the adsorption unit is condensed by a condensing device and then heat exchanged by a purified tail gas heat exchanger and is divided into two streams, one of which is reused as defrosting gas of a cryogenic unit and the other is sent to a synthesis reactor to adjust the reactor liquid temperature.

7. The process for resource recovery of fluorine components in tail gas from fluorobenzene production as claimed in claim 5, characterized in that: The denitration unit comprises a denitration heat exchanger, an exhaust gas heater, an ammonia adding device, a pipeline mixer and an SCR denitration reactor; the adsorbed exhaust gas from the adsorption unit is heated by heat exchange with the purified exhaust gas in the denitration heat exchanger and then heated to above 170°C by the exhaust heater, ammonia is added by the ammonia adding device, and finally mixed evenly by the pipeline mixer before entering the SCR denitration reactor for denitration purification, and the obtained purified exhaust gas is then sent to the denitration heat exchanger for heat exchange with the adsorbed exhaust gas.

8. The process for resource recovery of fluorine components in tail gas from fluorobenzene production as claimed in claim 7, characterized in that: The purified exhaust gas coming out of the denitration heat exchanger is divided into two parts. One part is directly mixed with the adsorbed exhaust gas after the temperature is increased after leaving the denitration heat exchanger, participates in the exhaust gas circulation, and dilutes the nitrogen oxide concentration in the exhaust gas entering the SCR denitration reactor. The other part is discharged through the chimney after heat exchange with the desorbed exhaust gas through the purified exhaust gas heat exchanger for cooling and dehumidification.

9. The process for resource recovery of fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: The cryogenic unit is composed of a plurality of cryogenic heat exchangers installed on the cover of the synthesis reactor. The plurality of cryogenic heat exchangers perform condensation and defrosting processes in turn. The synthetic tail gas generated by the synthesis reactor enters the cryogenic heat exchanger in the condensation stage from the bottom of the cryogenic heat exchanger for condensation.

10. The process for resource recovery of fluorine components in tail gas from fluorobenzene production according to claim 9, characterized in that: The cryogenic heat exchanger has multiple enhanced cooling sections, and each enhanced cooling section uses a refrigerant circulation pump to return the refrigerant from the downstream to the upstream.

11. A process for recycling fluorine components in tail gas from fluorobenzene production as claimed in claim 5, characterized in that: A plurality of sets of the cryogenic units correspond to one set of washing unit, adsorption unit and denitration unit.

12. The process for resource recovery of fluorine components in tail gas from fluorobenzene production according to claim 1, characterized in that: The cryogenic tail gas from the cryogenic unit is divided into two streams, one stream enters the washing unit, and the other stream is returned to the synthesis reactor to adjust the reactor liquid temperature and stir the reactor liquid.

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