Efficient concentration process of waste acid in phenylenediamine production

Abstract

This invention belongs to the field of waste acid treatment technology, specifically relating to a high-efficiency concentration process for waste acid from phenylenediamine production. This high-efficiency concentration process for waste acid from phenylenediamine production combines multi-stage concentration with resource recovery. Impurities are removed through pretreatment with a composite adsorbent, and magnetic separation enables adsorbent recovery and regeneration. Through primary vacuum, secondary vacuum, and pot-type tertiary concentration, the waste acid concentration is increased from 70-75% to 95.5-96%, which can be reused in the second-stage nitration process, achieving recycling and reducing raw material consumption. The process utilizes the high-temperature tail gas from the pot-type concentration as a heat source for primary vacuum concentration to recover waste heat, reducing external energy consumption. Dilute acid water is separated by a pervaporation membrane, and the recovered water is used for washing and concentrating the dilute acid back for pretreatment, forming a closed-loop cycle for emission reduction, meeting the requirements of green, low-carbon, and circular economy. Simultaneously, the composite adsorbent magnetic separation, the application of corrosion-resistant materials, and high-efficiency composite membrane technology improve system stability and efficiency, extend equipment life, and reduce maintenance costs.

Description

Technical Field

[0001] This invention belongs to the field of waste acid treatment technology, specifically relating to a high-efficiency concentration process for waste acid from phenylenediamine production. Background Technology

[0002] With the advancement of science and technology and the widespread promotion of the circular economy concept, waste acid resource utilization technology will be more widely applied and developed, contributing to the construction of a green, low-carbon, and circular modern industrial system. Through technological research and development and the promotion of waste acid recycling and resource utilization, not only can enterprises enjoy tangible economic benefits, but it also has a profound impact on environmental protection, the circular economy, and sustainable economic and social development.

[0003] In the production of phenylenediamine, the first-stage nitration reaction generates waste sulfuric acid with a mass concentration of 70%–75%. Current technologies typically employ a combination of vacuum concentration and pot-type concentration to concentrate the waste sulfuric acid to 95% before recycling it for the second-stage nitration. While this method has some effectiveness, it still suffers from high energy consumption, a heavy burden on wastewater treatment, and incomplete removal of organic impurities. Furthermore, the inability to effectively recycle heat in existing processes leads to high operating costs, and the environmental performance needs improvement.

[0004] Based on this, we propose a highly efficient concentration process for waste acid from phenylenediamine production, hoping to address the shortcomings of existing technologies. Summary of the Invention

[0005] The purpose of this invention is to address existing problems by providing a highly efficient process for concentrating waste acid from phenylenediamine production.

[0006] This invention is achieved through the following technical solution:

[0007] A highly efficient process for concentrating waste acid from phenylenediamine production includes the following steps:

[0008] S1. Waste acid pretreatment:

[0009] Waste acid with a mass concentration of 70-75% generated from the first stage nitration of phenylenediamine is sent to a pretreatment tank, a composite adsorbent is added, and after stirring and adsorption treatment, a magnetic field is applied to make the composite adsorbent settle and separate rapidly to obtain pretreated waste acid.

[0010] S2, First-stage vacuum concentration:

[0011] The pretreated waste acid obtained from S1 is sent to a primary vacuum concentrator for vacuum concentration to obtain primary concentrated sulfuric acid with a mass concentration of 78-82%.

[0012] S3, Secondary Vacuum Concentration:

[0013] The primary concentrated sulfuric acid obtained in step S2 is sent to a secondary vacuum concentrator for further concentration and removal of water until the sulfuric acid mass concentration is 85-87%, thus obtaining secondary concentrated sulfuric acid.

[0014] At the same time, collect the dilute acid water with a mass concentration of 5-8% that has been distilled out;

[0015] S4, Pot-type Deep Concentration:

[0016] The secondary concentrated sulfuric acid obtained from S3 is sent to a pot-type concentration system for deep pot-type concentration to obtain a finished concentrated sulfuric acid with a mass concentration of 95.5~96%.

[0017] S5. Resource recovery of dilute acid water:

[0018] The dilute acid water collected in S3 is separated by a pervaporation membrane module to obtain recycled water and concentrated dilute acid with a mass concentration of 12-15%.

[0019] Furthermore, the amount of the composite adsorbent used in step S1 is 0.8~1.5% of the mass of the waste acid;

[0020] The preparation of the composite adsorbent includes the following steps:

[0021] S101. Agricultural straw or forestry processing residues are crushed to a particle size of 0.5~1mm and subjected to oxygen-limited temperature-controlled pyrolysis under N2 protection. The heating rate is 10℃ / min, the final temperature is 500~600℃, and the temperature is maintained for 2~3h. After cooling, porous biochar is obtained.

[0022] S102. Add biochar to a mixed solution of FeCl3·6H2O and CoCl2·6H2O (molar ratio of 3:1, total concentration of 0.2mol / L), with a solid-liquid ratio of 1:20~25 (g:mL). Impregnate with ultrasonic assistance at 300W for 30~40min. Then add 1mol / L NaOH solution to adjust the pH to 10. Stir in a water bath at 60~65℃ for 1~2h. After standing to precipitate, wash with deionized water until neutral and dry at 60~70℃ to obtain magnetic biochar.

[0023] S103. Add the above magnetic biochar to a mixed solution containing 0.5 mol / L H2SO4 and 0.1 mol / L Na2S2O8, with a solid-liquid ratio of 1:15~20 (g:mL), transfer to a hydrothermal reactor, control the reaction temperature at 100~130℃, the reaction pressure at 0.3~0.5MPa, the stirring rate at 50~80rpm, and the reaction time at 3~5h. After the reaction is completed, collect the composite material by magnetic separation, wash with deionized water until the pH of the filtrate is 5~6, and dry at 70~80℃ to obtain the composite adsorbent.

[0024] Furthermore, the agricultural straw mentioned in step S101 is rice husk or wheat straw, and the forestry processing residue is wood chips.

[0025] Furthermore, in step S1, the temperature during stirring and adsorption is controlled at 30~45℃, the stirring speed is 180~200rpm, and the stirring time is 1~1.5h.

[0026] The magnetic field strength is controlled to be 0.3~0.5T when the magnetic field is applied.

[0027] Furthermore, in step S2, the high-temperature exhaust gas of 180~220℃ generated by the subsequent pot-type concentration is used as a heat source for vacuum concentration, and vacuum concentration is carried out at a vacuum degree of -0.085~-0.095MPa and a temperature of 75~90℃.

[0028] The high-temperature exhaust gas and waste acid exchange heat through a shell-and-tube heat exchanger. After heat exchange, the exhaust gas temperature drops to 80~100℃ and is discharged in compliance with standards after being absorbed by spraying.

[0029] The tube side of the shell-and-tube heat exchanger is made of 316L stainless steel with a DuPont Teflon™ PFA 440HP coating, with a coating thickness of 50~80μm, while the shell side is made of Q345R carbon steel lined with glass flakes.

[0030] Furthermore, in step S3, the further concentration is carried out using low-pressure steam heating at 0.3~0.5MPa, under vacuum conditions of -0.092~-0.098MPa and temperature of 85~100℃.

[0031] The vacuum level is controlled at -0.092 to -0.098 MPa, and the temperature is controlled at 85 to 100℃.

[0032] Furthermore, in step S4, the pot-type deep concentration is heated by high-pressure steam at 0.8~1.2MPa. Under normal pressure and 160~180℃ conditions, the secondary concentrated sulfuric acid is exchanged heat countercurrently with the sulfuric acid gas generated by heating the concentration pot and vaporized to remove residual moisture.

[0033] The finished concentrated sulfuric acid is cooled and sent to the finished acid tank for recycling in the second stage of phenylenediamine nitration;

[0034] The inner wall of the concentration tower of the pot-type concentration system is lined with 2-3mm thick tantalum material, the bottom of the concentration pot is made of Hastelloy C276 material, the outer shell of the tower / pot body is Q345R carbon steel, and the space between the outer shell and the lining is filled with ceramic insulation material with a thermal conductivity ≤0.15W / (m·K).

[0035] Furthermore, the recycled water mentioned in step S5 is reused in the production process, and the concentrated dilute acid is returned to the pretreatment tank of step S1 for reprocessing.

[0036] Further, in step S5, the separation membrane used in the pervaporation membrane module is a ZIF-8@GQDs / PDMS composite membrane; the pervaporation membrane module operates at 40~60℃ and an operating pressure of 0.1~0.3MPa, and the water flux of the membrane module is 2.0~2.8kg / (m³). 2 ·h).

[0037] Furthermore, the preparation of the ZIF-8@GQDs / PDMS composite membrane includes the following steps:

[0038] S501. Dissolve citric acid and urea in deionized water at a mass ratio of 5:2, with a solid-liquid ratio of 1:8~10 (g:mL), and perform a hydrothermal reaction at 180~190℃ for 3~4h to obtain a carboxyl-containing graphene quantum dot (GQDs) solution.

[0039] S502. Dissolve 0.5 mmol Zn(NO3)2·6H2O and 2 mmol 2-methylimidazole in methanol, stir at room temperature for 20-26 h, and collect ZIF-8 nanoparticles by centrifugation.

[0040] S503. Disperse ZIF-8 nanoparticles in GQDs solution and sonicate at 200W for 1~1.5h. The ZIF-8@GQDs hybrid is formed by self-assembly through electrostatic interaction. Then, centrifuge and dry for later use.

[0041] S504. Add ZIF-8@GQDs hybrid (5-8% of the mass of PDMS (polydimethylsiloxane)) to a mixture of PDMS prepolymer and tetraethyl orthosilicate (TEOS, crosslinking agent) (mass ratio 10:1), and stir and sonicate until uniformly dispersed.

[0042] Casting solution was applied to an acid-resistant PVDF substrate (pore size 0.22 μm) using a doctor blade (50 μm gap), allowed to stand at room temperature for 30 min, and then crosslinked and cured in an 80℃ oven for 2 h to form an active separation layer of 1~2 μm thickness, namely ZIF-8@GQDs / PDMS composite membrane.

[0043] The present invention has the following advantages over the prior art:

[0044] 1. The high-efficiency concentration process for waste acid from phenylenediamine production in this invention significantly improves the recovery efficiency and resource utilization rate of waste acid by combining multi-stage concentration with resource recovery. The process first uses a composite adsorbent to pretreat the waste acid, effectively removing impurities, and then achieves rapid recovery and regeneration of the adsorbent through magnetic separation. Subsequently, a three-stage concentration process (primary vacuum, secondary vacuum, and pot-type concentration) is used to increase the waste acid concentration from an initial 70-75% to a finished concentrated sulfuric acid of 95.5-96%, which can be directly reused in the second-stage nitration process of phenylenediamine, realizing the recycling of waste acid and reducing raw material consumption.

[0045] 2. This invention uses the high-temperature exhaust gas generated by the pot-type concentration as the heat source for the first-stage vacuum concentration, thus realizing waste heat recovery and significantly reducing external energy consumption. At the same time, the dilute acid water is separated through a pervaporation membrane module, the recovered water is used for the production washing process, and the concentrated dilute acid is returned to the pretreatment, forming a closed loop, reducing wastewater discharge. The overall process meets the requirements of green, low-carbon and circular economy.

[0046] 3. The magnetic separation of the composite adsorbent, the application of corrosion-resistant materials (such as tantalum lining and Hastelloy), and the high water permeability of the ZIF-8@GQDs / PDMS composite membrane in this invention improve the stability and operational efficiency of the system. This integration of technologies not only extends equipment lifespan but also reduces maintenance costs, providing an efficient, economical, and environmentally friendly solution for waste acid treatment in phenylenediamine production. Detailed Implementation

[0047] To further explain the present invention, the following specific embodiments are described.

[0048] Example 1

[0049] A highly efficient process for concentrating waste acid from phenylenediamine production includes the following steps:

[0050] S1. Waste acid pretreatment:

[0051] Waste acid with a mass concentration of 70-75% generated from the first-stage nitration of phenylenediamine is fed into a pretreatment tank. A composite adsorbent is added, and the mixture is stirred and adsorbed at 30°C and 180 rpm for 1 hour. After stirring is stopped, an electromagnet (magnetic field strength 0.3T) is attached to the outer wall of the pretreatment tank, and the mixture is allowed to stand for 5 minutes. Under the action of the magnetic field, the composite adsorbent quickly settles to the bottom of the tank, and the supernatant is the pretreated waste acid. The bottom valve of the tank is opened, and the supernatant is first discharged to the first-stage vacuum concentrator. Then the electromagnet is closed, and the adsorbent that has been saturated with adsorption is discharged (for regeneration).

[0052] The amount of the composite adsorbent used is 0.8% of the mass of the waste acid;

[0053] The preparation of the composite adsorbent includes the following steps:

[0054] S101. The rice husks were crushed to a particle size of 0.5 mm and subjected to oxygen-limited temperature-controlled pyrolysis under N2 protection. The heating rate was 10℃ / min, the final temperature was 500℃, and the temperature was maintained for 2 hours. After cooling, porous biochar was obtained.

[0055] S102. Add biochar to a mixed solution of FeCl3·6H2O and CoCl2·6H2O (molar ratio of 3:1, total concentration of 0.2mol / L), with a solid-liquid ratio of 1:20. Soak under ultrasonic assisted impregnation for 30 min at 300W. Then add 1mol / L NaOH solution to adjust the pH to 10. Stir in a 60℃ water bath for 1 h. After standing to precipitate, wash with deionized water until neutral and dry at 60℃ to obtain magnetic biochar.

[0056] S103. The above magnetic biochar was added to a mixed solution containing 0.5 mol / L H2SO4 and 0.1 mol / L Na2S2O8 at a solid-liquid ratio of 1:15. The mixture was then transferred to a hydrothermal reactor. The reaction temperature was controlled at 100℃, the reaction pressure at 0.3 MPa, the stirring rate at 50 rpm, and the reaction time at 3 h. After the reaction was completed, the composite material was collected by magnetic separation, washed with deionized water until the pH of the filtrate was 5, and dried at 70℃ to obtain the composite adsorbent.

[0057] S2, First-stage vacuum concentration:

[0058] The pretreated waste acid obtained in step S1 is sent to a primary vacuum concentrator. The high-temperature tail gas generated by the subsequent pot-type concentration at 180°C is used as a heat source. Vacuum concentration is carried out at a vacuum degree of -0.085MPa and a temperature of 75°C to remove some water and obtain primary concentrated sulfuric acid with a mass concentration of 78~82%.

[0059] The high-temperature exhaust gas and waste acid exchange heat through a shell-and-tube heat exchanger. After heat exchange, the exhaust gas temperature drops to 80°C and is then discharged in compliance with emission standards after being absorbed by spraying.

[0060] The tube side of the shell-and-tube heat exchanger uses DuPont Teflon. TM PFA 440HP coated 316L stainless steel with a coating thickness of 50μm, and Q345R carbon steel with glass flake lining for the shell side.

[0061] Note: During the first run, S4 has not yet generated exhaust gas, so a temporary backup heat source (such as low-pressure steam) needs to be configured. After S4 is running stably, it can be switched to exhaust gas waste heat to avoid the S2 process from stopping.

[0062] S3, Secondary Vacuum Concentration:

[0063] The primary concentrated sulfuric acid obtained in step S2 is fed into a secondary vacuum concentrator and heated with low-pressure steam at 0.3 MPa. The concentrator is then further concentrated under vacuum conditions of -0.092 MPa and a temperature of 85°C to remove water until the sulfuric acid mass concentration is 85-87%, thus obtaining secondary concentrated sulfuric acid.

[0064] At the same time, collect the dilute acid water with a mass concentration of 5-8% that has been distilled out;

[0065] S4, Pot-type Deep Concentration:

[0066] The secondary concentrated sulfuric acid obtained in step S3 is fed into a pot-type concentration system. The pot is heated by high-pressure steam at 0.8 MPa. Under normal pressure and 160°C conditions, the secondary concentrated sulfuric acid and the sulfuric acid gas generated by heating the pot exchange heat countercurrently and vaporize to remove residual moisture, thereby obtaining a finished concentrated sulfuric acid with a mass concentration of 95.5~96%. The finished concentrated sulfuric acid is cooled and then sent to a finished acid tank for recycling in the second stage of phenylenediamine nitration.

[0067] The inner wall of the concentration tower of the pot-type concentration system is lined with 2mm thick tantalum material, the bottom of the concentration pot is made of Hastelloy C276 material, the outer shell of the tower / pot body is Q345R carbon steel, and the space between the outer shell and the lining is filled with ceramic insulation material (thermal conductivity ≤0.15W / (m·K)).

[0068] S5. Resource recovery of dilute acid water:

[0069] The dilute acid water collected in step S3 is fed into a pervaporation membrane module and separated at 40°C and an operating pressure of 0.1 MPa using a hydrophobic modified silicone rubber membrane ZIF-8@GQDs / PDMS composite membrane to obtain recycled water and concentrated dilute acid with a mass concentration of 12-15%. The recycled water is recycled for the water washing process in the production of phenylenediamine, and the concentrated dilute acid is returned to the pretreatment tank in step S1 for reprocessing.

[0070] The membrane area of ​​the pervaporation membrane module is 50m². 2 The membrane module housing is made of acid-resistant fiberglass, and the water flux of the membrane module is 2.0 kg / (m²). 2 ·h);

[0071] The preparation of the ZIF-8@GQDs / PDMS composite membrane includes the following steps:

[0072] S501. Citric acid and urea are dissolved in deionized water at a mass ratio of 5:2, with a solid-liquid ratio of 1:8. The mixture is then subjected to a hydrothermal reaction at 180℃ for 3 hours to obtain a GQDs solution containing carboxyl groups.

[0073] S502. Dissolve 0.5 mmol Zn(NO3)2·6H2O and 2 mmol 2-methylimidazole in methanol, stir at room temperature for 20 h, and collect ZIF-8 nanoparticles by centrifugation.

[0074] S503. Disperse ZIF-8 nanoparticles in GQDs solution, sonicate at 200W for 1 hour, and form ZIF-8@GQDs hybrids through electrostatic self-assembly. Centrifuge and dry for later use.

[0075] S504. Add ZIF-8@GQDs hybrid (5% of PDMS mass) to a mixture of PDMS prepolymer and TEOS (mass ratio 10:1), and stir and sonicate until uniformly dispersed.

[0076] On an acid-resistant PVDF substrate, a casting solution is applied by scraping with a doctor blade, allowed to stand at room temperature for 30 minutes, and then crosslinked and cured in an 80℃ oven for 2 hours to form a 1μm thick active separation layer, namely the ZIF-8@GQDs / PDMS composite membrane.

[0077] Example 2

[0078] A highly efficient process for concentrating waste acid from phenylenediamine production includes the following steps:

[0079] S1. Waste acid pretreatment:

[0080] Waste acid with a mass concentration of 70-75% generated from the first-stage nitration of phenylenediamine is fed into a pretreatment tank. A composite adsorbent is added, and the mixture is stirred and adsorbed at 38°C and 200 rpm for 1 hour. After stirring is stopped, an electromagnet (magnetic field strength 0.4T) is attached to the outer wall of the pretreatment tank, and the mixture is allowed to stand for 8 minutes. Under the action of the magnetic field, the composite adsorbent quickly settles to the bottom of the tank, and the supernatant is the pretreated waste acid. The bottom valve of the tank is opened, and the supernatant is first discharged to the first-stage vacuum concentrator. Then the electromagnet is closed, and the adsorbent that has been saturated with adsorption is discharged (for regeneration).

[0081] The amount of the composite adsorbent used is 1.2% of the mass of the waste acid;

[0082] The preparation of the composite adsorbent includes the following steps:

[0083] S101. The rice husks were crushed to a particle size of 0.8 mm and subjected to oxygen-limited temperature-controlled pyrolysis under N2 protection. The heating rate was 10℃ / min, the final temperature was 550℃, and the temperature was maintained for 2.5 h. After cooling, porous biochar was obtained.

[0084] S102. Add biochar to a mixed solution of FeCl3·6H2O and CoCl2·6H2O (molar ratio 3:1, total concentration 0.2mol / L), with a solid-liquid ratio of 1:22. Impregnate with ultrasonic assistance at 300W for 35min. Then add 1mol / L NaOH solution to adjust the pH to 10. Stir in a 62℃ water bath for 1.5h. After standing to precipitate, wash with deionized water until neutral and dry at 65℃ to obtain magnetic biochar.

[0085] S103. The above magnetic biochar was added to a mixed solution containing 0.5 mol / L H2SO4 and 0.1 mol / L Na2S2O8 at a solid-liquid ratio of 1:18. The mixture was then transferred to a hydrothermal reactor. The reaction temperature was controlled at 120℃, the reaction pressure at 0.4 MPa, the stirring rate at 60 rpm, and the reaction time at 4 h. After the reaction was completed, the composite material was collected by magnetic separation, washed with deionized water until the pH of the filtrate was 5.5, and dried at 75℃ to obtain the composite adsorbent.

[0086] S2, First-stage vacuum concentration:

[0087] The pretreated waste acid obtained in step S1 is sent to a primary vacuum concentrator. The high-temperature tail gas generated by the subsequent pot-type concentration at 200°C is used as a heat source. Vacuum concentration is carried out at a vacuum degree of -0.09MPa and a temperature of 80°C to remove some water and obtain primary concentrated sulfuric acid with a mass concentration of 78~82%.

[0088] The high-temperature exhaust gas and waste acid exchange heat through a shell-and-tube heat exchanger. After heat exchange, the exhaust gas temperature drops to 90°C and is then discharged in compliance with emission standards after being absorbed by spraying.

[0089] The tube side of the shell-and-tube heat exchanger uses DuPont Teflon. TM PFA 440HP coated 316L stainless steel with a coating thickness of 60μm, and Q345R carbon steel with glass flake lining for the shell side.

[0090] Note: During the first run, S4 has not yet generated exhaust gas, so a temporary backup heat source (such as low-pressure steam) needs to be configured. After S4 is running stably, it can be switched to exhaust gas waste heat to avoid the S2 process from stopping.

[0091] S3, Secondary Vacuum Concentration:

[0092] The primary concentrated sulfuric acid obtained in step S2 is fed into a secondary vacuum concentrator and heated with low-pressure steam at 0.4 MPa. The concentrator is then further concentrated under vacuum conditions of -0.095 MPa and a temperature of 90°C to remove water until the sulfuric acid mass concentration is 85-87%, thus obtaining secondary concentrated sulfuric acid.

[0093] At the same time, collect the dilute acid water with a mass concentration of 5-8% that has been distilled out;

[0094] S4, Pot-type Deep Concentration:

[0095] The secondary concentrated sulfuric acid obtained in step S3 is fed into a pot-type concentration system. The pot is heated by high-pressure steam at 1 MPa. Under normal pressure and 170°C, the secondary concentrated sulfuric acid and the sulfuric acid gas generated by heating the pot exchange heat countercurrently and vaporize to remove residual moisture, thereby obtaining a finished concentrated sulfuric acid with a mass concentration of 95.5~96%. The finished concentrated sulfuric acid is cooled and then sent to a finished acid tank for recycling in the second stage of phenylenediamine nitration.

[0096] The inner wall of the concentration tower of the pot-type concentration system is lined with 2mm thick tantalum material, the bottom of the concentration pot is made of Hastelloy C276 material, the outer shell of the tower / pot body is Q345R carbon steel, and the space between the outer shell and the lining is filled with ceramic insulation material (thermal conductivity ≤0.15W / (m·K)).

[0097] S5. Resource recovery of dilute acid water:

[0098] The dilute acid water collected in step S3 is fed into a pervaporation membrane module and separated at 50°C and an operating pressure of 0.2 MPa using a hydrophobic modified silicone rubber membrane ZIF-8@GQDs / PDMS composite membrane to obtain recycled water and concentrated dilute acid with a mass concentration of 12-15%. The recycled water is recycled for the water washing process in the production of phenylenediamine, and the concentrated dilute acid is returned to the pretreatment tank in step S1 for reprocessing.

[0099] The membrane area of ​​the pervaporation membrane module is 80m². 2 The membrane module housing is made of acid-resistant fiberglass, and the water flux of the membrane module is 2.5 kg / (m²). 2 ·h);

[0100] The preparation of the ZIF-8@GQDs / PDMS composite membrane includes the following steps:

[0101] S501. Citric acid and urea are dissolved in deionized water at a mass ratio of 5:2, with a solid-liquid ratio of 1:9. The mixture is then subjected to a hydrothermal reaction at 185℃ for 3.5 hours to obtain a GQDs solution containing carboxyl groups.

[0102] S502. Dissolve 0.5 mmol Zn(NO3)2·6H2O and 2 mmol 2-methylimidazole in methanol, stir at room temperature for 23 h, and collect ZIF-8 nanoparticles by centrifugation.

[0103] S503. Disperse ZIF-8 nanoparticles in GQDs solution, sonicate at 200W for 1 hour, and form ZIF-8@GQDs hybrids through electrostatic self-assembly. Centrifuge and dry for later use.

[0104] S504. Add ZIF-8@GQDs hybrid (6% of PDMS mass) to a mixture of PDMS prepolymer and TEOS (mass ratio 10:1), and stir and sonicate until uniformly dispersed.

[0105] On an acid-resistant PVDF substrate, a casting solution is applied by scraping with a doctor blade, allowed to stand at room temperature for 30 minutes, and then crosslinked and cured in an 80℃ oven for 2 hours to form a 1μm thick active separation layer, namely the ZIF-8@GQDs / PDMS composite membrane.

[0106] Example 3

[0107] A highly efficient process for concentrating waste acid from phenylenediamine production includes the following steps:

[0108] S1. Waste acid pretreatment:

[0109] Waste acid with a mass concentration of 70-75% generated from the first-stage nitration of phenylenediamine is fed into a pretreatment tank. A composite adsorbent is added, and the mixture is stirred and adsorbed at 45℃ and 220 rpm for 1.5 hours. After stirring is stopped, an electromagnet (magnetic field strength 0.5T) is attached to the outer wall of the pretreatment tank, and the mixture is allowed to stand for 10 minutes. Under the action of the magnetic field, the composite adsorbent quickly settles to the bottom of the tank, and the supernatant is the pretreated waste acid. The bottom valve of the tank is opened, and the supernatant is first discharged to the first-stage vacuum concentrator. Then the electromagnet is closed, and the adsorbent that has been saturated with adsorption is discharged (for regeneration).

[0110] The amount of the composite adsorbent used is 1.5% of the mass of the waste acid;

[0111] The preparation of the composite adsorbent includes the following steps:

[0112] S101. The rice husks were crushed to a particle size of 1 mm and subjected to oxygen-limited temperature-controlled pyrolysis under N2 protection. The heating rate was 10℃ / min, the final temperature was 600℃, and the temperature was maintained for 3 hours. After cooling, porous biochar was obtained.

[0113] S102. Add biochar to a mixed solution of FeCl3·6H2O and CoCl2·6H2O (molar ratio of 3:1, total concentration of 0.2mol / L), with a solid-liquid ratio of 1:25. Impregnate with ultrasonic assistance at 300W for 40 min. Then add 1mol / L NaOH solution to adjust the pH to 10. Stir in a water bath at 65℃ for 2 h. After standing to precipitate, wash with deionized water until neutral and dry at 70℃ to obtain magnetic biochar.

[0114] S103. The above magnetic biochar was added to a mixed solution containing 0.5 mol / L H2SO4 and 0.1 mol / L Na2S2O8 at a solid-liquid ratio of 1:20. The mixture was then transferred to a hydrothermal reactor. The reaction temperature was controlled at 130℃, the reaction pressure at 0.5 MPa, the stirring rate at 80 rpm, and the reaction time at 5 h. After the reaction was completed, the composite material was collected by magnetic separation, washed with deionized water until the pH of the filtrate was 6, and dried at 80℃ to obtain the composite adsorbent.

[0115] S2, First-stage vacuum concentration:

[0116] The pretreated waste acid obtained in step S1 is sent to a primary vacuum concentrator. The high-temperature tail gas generated by the subsequent pot-type concentration at 220°C is used as a heat source. Vacuum concentration is carried out at a vacuum degree of -0.095MPa and a temperature of 90°C to remove some water and obtain primary concentrated sulfuric acid with a mass concentration of 78~82%.

[0117] The high-temperature exhaust gas and waste acid exchange heat through a shell-and-tube heat exchanger. After heat exchange, the exhaust gas temperature drops to 100°C and is then discharged in compliance with emission standards after being absorbed by spraying.

[0118] The tube side of the shell-and-tube heat exchanger uses DuPont Teflon. TM PFA 440HP coated 316L stainless steel with a coating thickness of 80μm, and Q345R carbon steel with glass flake lining for the shell side.

[0119] Note: During the first run, S4 has not yet generated exhaust gas, so a temporary backup heat source (such as low-pressure steam) needs to be configured. After S4 is running stably, it can be switched to exhaust gas waste heat to avoid the S2 process from stopping.

[0120] S3, Secondary Vacuum Concentration:

[0121] The primary concentrated sulfuric acid obtained in step S2 is fed into a secondary vacuum concentrator and heated with low-pressure steam at 0.5 MPa. The concentrator is then further concentrated under conditions of vacuum of -0.098 MPa and temperature of 100°C to remove water until the sulfuric acid mass concentration is 85-87%, thus obtaining secondary concentrated sulfuric acid.

[0122] At the same time, collect the dilute acid water with a mass concentration of 5-8% that has been distilled out;

[0123] S4, Pot-type Deep Concentration:

[0124] The secondary concentrated sulfuric acid obtained in step S3 is fed into a pot-type concentration system. The pot is heated by high-pressure steam at 1.2 MPa. Under normal pressure and 180°C conditions, the secondary concentrated sulfuric acid and the sulfuric acid gas generated by heating the pot exchange heat countercurrently and vaporize to remove residual moisture, thereby obtaining a finished concentrated sulfuric acid with a mass concentration of 95.5~96%. The finished concentrated sulfuric acid is cooled and then sent to a finished acid tank for recycling in the second stage of phenylenediamine nitration.

[0125] The inner wall of the concentration tower of the pot-type concentration system is lined with 3mm thick tantalum material, the bottom of the concentration pot is made of Hastelloy C276 material, the outer shell of the tower / pot body is Q345R carbon steel, and the space between the outer shell and the lining is filled with ceramic insulation material (thermal conductivity ≤0.15W / (m·K)).

[0126] S5. Resource recovery of dilute acid water:

[0127] The dilute acid water collected in step S3 is fed into a pervaporation membrane module and separated at 60°C and an operating pressure of 0.3 MPa using a hydrophobic modified silicone rubber membrane ZIF-8@GQDs / PDMS composite membrane to obtain recycled water and concentrated dilute acid with a mass concentration of 12-15%. The recycled water is recycled for the water washing process in the production of phenylenediamine, and the concentrated dilute acid is returned to the pretreatment tank in step S1 for reprocessing.

[0128] The membrane area of ​​the pervaporation membrane module is 100m². 2 The membrane module housing is made of acid-resistant fiberglass, and the water flux of the membrane module is 2.8 kg / (m³). 2 ·h);

[0129] The preparation of the ZIF-8@GQDs / PDMS composite membrane includes the following steps:

[0130] S501. Citric acid and urea are dissolved in deionized water at a mass ratio of 5:2, with a solid-liquid ratio of 1:10. The mixture is then subjected to a hydrothermal reaction at 190℃ for 4 hours to obtain a GQDs solution containing carboxyl groups.

[0131] S502. Dissolve 0.5 mmol Zn(NO3)2·6H2O and 2 mmol 2-methylimidazole in methanol, stir at room temperature for 26 h, and collect ZIF-8 nanoparticles by centrifugation.

[0132] S503. Disperse ZIF-8 nanoparticles in GQDs solution, sonicate at 200W for 1.5h, and form ZIF-8@GQDs hybrids through electrostatic self-assembly. Centrifuge and dry for later use.

[0133] S504. Add ZIF-8@GQDs hybrid (8% of PDMS mass) to a mixture of PDMS prepolymer and TEOS (mass ratio 10:1), and stir and sonicate until uniformly dispersed.

[0134] On the acid-resistant PVDF substrate, the casting solution is applied by scraping with a doctor blade, allowed to stand at room temperature for 30 minutes, and then crosslinked and cured in an 80℃ oven for 2 hours to form a 2μm thick active separation layer, namely ZIF-8@GQDs / PDMS composite membrane.

[0135] Comparative Example 1

[0136] Compared with Example 2, Comparative Example 1 omits the use of composite adsorbent, the pretreatment only involves stirring and no adsorption, and the remaining steps and parameters are the same as in Example 2.

[0137] Comparative Example 2

[0138] Compared with Example 2, in the preparation of the composite adsorbent, step S102 of Comparative Example 2 omits the use of CoCl2·6H2O, while the remaining steps and parameters are the same as in Example 2.

[0139] Comparative Example 3

[0140] Compared with Example 2, Comparative Example 3 uses a primary vacuum concentration heat source and low-pressure steam throughout the process (without using the waste heat from the boiler-type concentration of exhaust gas). The remaining steps and parameters are the same as in Example 2.

[0141] Comparative Example 4

[0142] Compared with Example 2, Comparative Example 4 uses a pure PDMS membrane (without ZIF-8@GQDs hybrid) for pervaporation membrane, and the remaining steps and parameters are the same as in Example 2.

[0143] Comparative Example 5

[0144] Compared with Example 2, Comparative Example 5 omits the secondary vacuum concentration stage and directly performs pot-type concentration after the primary concentration stage. The remaining steps and parameters are the same as in Example 2.

[0145] The pretreatment impurity removal rate, final product concentrated sulfuric acid concentration, unit product energy consumption, recycled water purity, and concentrated dilute acid concentration of each embodiment and comparative example were tested and calculated. The experimental comparison results are shown in Table 1 below.

[0146] Table 1

[0147] Pretreatment impurity removal rate (%) Final product concentration of concentrated sulfuric acid (%) Energy consumption per unit product (kJ / kg) Purity of recycled water (μS / cm) Concentration of concentrated dilute acid (%) Example 1 89.2 95.6 2850 8.6 13.1 Example 2 94.5 95.8 2520 8.3 14.2 Example 3 97.8 95.9 2680 7.1 14.8 Comparative Example 1 32.6 94.2 3180 28.7 10.5 Comparative Example 2 81.3 95.1 2750 15.6 12.3 Comparative Example 3 - 95.7 3460 8.5 14.1 Comparative Example 4 - 95.8 2530 35.2 9.8 Comparative Example 5 - 94.7 3020 8.4 14.3

[0148] As can be seen from Table 1 above, the overall performance of Examples 1-3 is significantly better than that of the comparative examples. Compared with Example 2, Comparative Example 1 has no adsorbent, and the impurity removal rate is only 32.6%, which is much lower than the 94.5% of Example 2. The presence of impurities will cause the equipment corrosion rate to increase sharply. Moreover, because of the presence of impurities, they adhere to the heat transfer surface, reducing the heat transfer efficiency, resulting in the final concentration dropping to 94.2% and the energy consumption increasing to 3180kJ / kg. Due to the decrease in heat transfer efficiency, more energy is required, which shows the necessity of composite adsorbents for this invention.

[0149] Compared to Example 2, Comparative Example 2 showed an impurity removal rate of 81.3%, which was lower than that of Example 2, indicating that Co 2+ with Fe 3+ The resulting composite magnetic centers can enhance adsorption activity and magnetic separation efficiency.

[0150] Compared with Example 2, Comparative Example 3 uses low-pressure steam throughout the process. The concentration and impurity removal rate are similar to those of Example 2, but the unit energy consumption increases to 3460 kJ / kg, an increase of 37.3%. This shows that Example 2 uses boiler-type concentrated tail gas as a primary heat source, which can effectively recover waste heat, significantly reduce energy consumption, and does not affect process stability.

[0151] Compared with Example 2, Comparative Example 4 used a pure PDMS membrane, and the purity of the recovered water was only 35.2 μS / cm (compared to 8.3 μS / cm in Example 2). The concentration of concentrated dilute acid was 9.8% (not reaching the target of 12-15%). This is because the porous structure of the ZIF-8@GQDs hybrid can enhance the selective permeability of water, while the hydrophobic properties and separation efficiency of the pure PDMS membrane are insufficient, resulting in a deterioration in the resource recovery effect of dilute acid water.

[0152] Compared to Example 2, Comparative Example 5 omits the secondary concentration stage, directly feeding the primary concentrated acid (78-82%) into the pot concentrater. The final concentration drops to 94.7%, and the energy consumption increases to 3020 kJ / kg, compared to 2520 kJ / kg in Example 2. This is because the secondary concentration stage can pre-raise the acid concentration to 85-87%, reducing the water load on the pot concentrater, avoiding excessive acid vaporization loss at high temperatures, and simultaneously reducing the energy consumption of the pot heating system.

[0153] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A highly efficient concentration process for waste acid from phenylenediamine production, characterized in that, Includes the following steps: S1. Waste acid pretreatment: Waste acid with a mass concentration of 70-75% generated from the first stage nitration of phenylenediamine is sent to a pretreatment tank, a composite adsorbent is added, and after stirring and adsorption treatment, a magnetic field is applied to make the composite adsorbent settle and separate rapidly to obtain pretreated waste acid. The preparation of the composite adsorbent includes the following steps: S101. Agricultural straw or forestry processing residues are crushed to a particle size of 0.5~1mm and subjected to oxygen-limited temperature-controlled pyrolysis under N2 protection. The heating rate is 10℃ / min, the final temperature is 500~600℃, and the temperature is maintained for 2~3h. After cooling, porous biochar is obtained. S102. Add biochar to a mixed solution of FeCl3·6H2O and CoCl2·6H2O. The molar ratio of FeCl3·6H2O to CoCl2·6H2O in the mixed solution is 3:1, the total concentration is 0.2 mol / L, and the solid-liquid ratio is 1:20~25. Impregnate with ultrasonic assistance at 300W for 30~40 min. Then add 1 mol / L NaOH solution to adjust the pH to 10. Stir in a water bath at 60~65℃ for 1~2 h. After standing and precipitation, wash with deionized water until neutral and dry at 60~70℃ to obtain magnetic biochar. S103. Add the above magnetic biochar to a mixed solution containing 0.5 mol / L H2SO4 and 0.1 mol / L Na2S2O8, with a solid-liquid ratio of 1:15~20. Transfer the solution to a hydrothermal reactor and control the reaction temperature at 100~130℃, the reaction pressure at 0.3~0.5MPa, the stirring rate at 50~80rpm, and the reaction time at 3~5h. After the reaction is completed, collect the composite material by magnetic separation, wash it with deionized water until the pH of the filtrate is 5~6, and dry it at 70~80℃ to obtain the composite adsorbent. S2, First-stage vacuum concentration: The pretreated waste acid obtained from S1 is sent to a primary vacuum concentrator for vacuum concentration to obtain primary concentrated sulfuric acid with a mass concentration of 78-82%. S3, Secondary Vacuum Concentration: The primary concentrated sulfuric acid obtained in step S2 is sent to a secondary vacuum concentrator for further concentration and removal of water until the sulfuric acid mass concentration is 85-87%, thus obtaining secondary concentrated sulfuric acid. At the same time, collect the dilute acid water with a mass concentration of 5-8% that has been distilled out; S4, Pot-type Deep Concentration: The secondary concentrated sulfuric acid obtained from S3 is sent to a pot-type concentration system for deep pot-type concentration to obtain a finished concentrated sulfuric acid with a mass concentration of 95.5~96%. S5. Resource recovery of dilute acid water: The dilute acid water collected in S3 is separated by a pervaporation membrane module to obtain recycled water and concentrated dilute acid with a mass concentration of 12-15%. The separation membrane used in the pervaporation membrane assembly is a ZIF-8@GQDs / PDMS composite membrane; The preparation of the ZIF-8@GQDs / PDMS composite membrane includes the following steps: S501. Dissolve citric acid and urea in deionized water at a mass ratio of 5:2, with a solid-liquid ratio of 1:8~10, and perform a hydrothermal reaction at 180~190℃ for 3~4 hours to obtain a GQDs solution containing carboxyl groups. S502. Dissolve 0.5 mmol Zn(NO3)2·6H2O and 2 mmol 2-methylimidazole in methanol, stir at room temperature for 20-26 h, and collect ZIF-8 nanoparticles by centrifugation. S503. Disperse ZIF-8 nanoparticles in GQDs solution and sonicate at 200W for 1~1.5h. The ZIF-8@GQDs hybrid is formed by self-assembly through electrostatic interaction. Then, centrifuge and dry for later use. S504. Add the ZIF-8@GQDs hybrid to the mixture of PDMS prepolymer and tetraethyl orthosilicate, and stir and sonicate until uniformly dispersed. The ZIF-8@GQDs hybrid accounts for 5-8% of the PDMS mass; The mass ratio of the PDMS prepolymer to tetraethyl orthosilicate is 10:1; On an acid-resistant PVDF substrate, a casting solution is applied by scraping with a doctor blade, allowed to stand at room temperature for 30 minutes, and then crosslinked and cured in an 80℃ oven for 2 hours to form an active separation layer of 1~2μm thickness, namely ZIF-8@GQDs / PDMS composite membrane.

2. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, The amount of composite adsorbent used in step S1 is 0.8 to 1.5% of the mass of waste acid.

3. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 2, characterized in that, The agricultural straw mentioned in step S101 is rice husk or wheat straw, and the forestry processing residue is wood chips.

4. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, In step S1, the temperature is controlled at 30~45℃, the stirring speed is 180~200rpm, and the stirring time is 1~1.5h during the stirring adsorption process. The magnetic field strength is controlled to be 0.3~0.5T when the magnetic field is applied.

5. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, The vacuum concentration described in step S2 uses the high-temperature tail gas of 180~220℃ generated by the subsequent pot-type concentration as a heat source, and performs vacuum concentration at a vacuum degree of -0.085~-0.095MPa and a temperature of 75~90℃. The high-temperature exhaust gas and waste acid exchange heat through a shell-and-tube heat exchanger. After heat exchange, the exhaust gas temperature drops to 80~100℃ and is discharged in compliance with standards after being absorbed by spraying. The tube side of the shell-and-tube heat exchanger is made of 316L stainless steel with a DuPont Teflon™ PFA 440HP coating, with a coating thickness of 50~80μm, while the shell side is made of Q345R carbon steel lined with glass flakes.

6. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, In step S3, the further concentration is carried out using low-pressure steam heating at 0.3~0.5MPa, under vacuum conditions of -0.092~-0.098MPa and temperature of 85~100℃. The vacuum level is controlled at -0.092 to -0.098 MPa, and the temperature is controlled at 85 to 100℃.

7. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, In step S4, the pot-type deep concentration is heated by high-pressure steam at 0.8~1.2MPa. Under normal pressure and 160~180℃ conditions, the secondary concentrated sulfuric acid is heated by countercurrent heat exchange and vaporization with the sulfuric acid gas generated by heating the concentration pot, thereby removing residual moisture. The finished concentrated sulfuric acid is cooled and sent to the finished acid tank for recycling in the second stage of phenylenediamine nitration; The inner wall of the concentration tower of the pot-type concentration system is lined with 2-3mm thick tantalum material, the bottom of the concentration pot is made of Hastelloy C276 material, the outer shell of the tower / pot body is Q345R carbon steel, and the space between the outer shell and the lining is filled with ceramic insulation material with a thermal conductivity ≤0.15W / (m·K).

8. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, The recycled water mentioned in step S5 is reused in the production process, and the concentrated dilute acid is returned to the pretreatment tank in step S1 for reprocessing.

9. The high-efficiency concentration process for waste acid from phenylenediamine production according to claim 1, characterized in that, In step S5, the pervaporation membrane module operates at 40~60℃ and 0.1~0.3MPa, with a water flux of 2.0~2.8 kg / (m³). 2 ·h).