A partitioned dry vacuum system and process for adipic acid crystallization purification

Abstract

The application discloses a partition dry vacuum system and process for refining adipic acid crystallization. The system comprises a multi-stage mechanical booster pump, a pre-pump heat exchanger, a post-pump heat exchanger and a liquid seal tank; the crystallization chambers of a twelve-chamber crystallizer are divided into multiple stages, each crystallization chamber is connected with a corresponding mechanical booster pump through a pre-pump heat exchanger, and each mechanical booster pump is connected with the liquid seal tank through a post-pump heat exchanger; the gas outlet of the post-pump heat exchanger corresponding to the upper stage is connected with the pump cavity of the mechanical booster pump corresponding to the stage and the pump cavity of the mechanical booster pump of the next stage; each mechanical booster pump is connected with two nitrogen shaft sealing pipelines, and nitrogen is introduced into the gap between the pump cavity of the mechanical booster pump and the bearing box oil seal and the gear box oil seal. The process comprises nitrogen shaft sealing, multi-stage partition precooling of gas-phase materials, partition and stage compression and condensation of gas-phase materials, and nitrogen purging. The system and process do not consume medium-pressure steam and cooling water, and have remarkable energy-saving, consumption-reducing, and environmental protection effects.

Description

Technical Field

[0001] This invention belongs to the technical field of adipic acid production process, specifically relating to a partitioned dry vacuum system and process for the crystallization and purification of adipic acid. Background Technology

[0002] Adipic acid is an important chemical raw material for the production of products such as nylon 66, polyurethane, and plasticizers. Industrially, crude adipic acid is typically prepared by nitric acid oxidation, followed by purification through vacuum crystallization. During crystallization, a stable negative pressure environment must be maintained within the crystallizer to allow continuous evaporation of moisture, carrying away heat and achieving cooling crystallization of adipic acid. The gaseous material produced by evaporation contains nitrite, saturated water vapor, trace amounts of nitric acid mist, and adipic acid vapor, exhibiting strong corrosiveness and high humidity.

[0003] Currently, all existing adipic acid crystallization processes use wet vacuum systems and generally adopt a process route of steam ejector connected in series with liquid ring pumps, which has obvious technical defects: (1) The steam ejector continuously consumes a large amount of medium-pressure steam, and the liquid ring pump requires a large flow of circulating cooling water, resulting in high operating costs and energy consumption; (2) The working fluid continuously absorbs nitric acid and nitrite components in the gas phase, generating a large amount of nitrogen-containing acidic wastewater, which is difficult and costly to treat later; (3) The system vacuum degree is significantly affected by the fluctuation of steam pressure and circulating water temperature, and the large vacuum fluctuation range leads to uneven distribution of crystallization particle size and decreased product purity; (4) Nitrite gas and dilute nitric acid continuously corrode the equipment, resulting in severe equipment corrosion, short service life, frequent maintenance and spare parts replacement, and high risk of unplanned shutdown. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a partitioned dry vacuum system and process for adipic acid crystallization and refining, which solves the three major problems of corrosion, scaling, and overheating of dry pumps under high humidity and strong corrosion conditions, and achieves energy saving, consumption reduction, zero wastewater discharge, and long-term stable operation in the adipic acid crystallization section.

[0005] The present invention addresses the aforementioned technical problem by adopting the following technical solution: A partitioned dry vacuum system for the crystallization and purification of adipic acid, characterized in that it includes a multi-stage mechanical booster pump, a pre-pump heat exchanger, a post-pump heat exchanger, and a liquid seal tank. The twelve crystallization chambers of the twelve-chamber crystallizer are divided into multiple zones according to the principle of similar vacuum levels. The higher the zone level, the lower the corresponding vacuum level. Each zone level corresponds to a mechanical booster pump and a post-pump heat exchanger. Each crystallization chamber in each zone level is connected to the corresponding mechanical booster pump through a pre-pump heat exchanger. Each mechanical booster pump is connected to the liquid seal tank through a post-pump heat exchanger. The outlet of the post-pump heat exchanger of the previous zone level is connected to the pump chamber of the mechanical booster pump of that zone level and the pump chamber of the next level mechanical booster pump. The outlet of the post-pump heat exchanger of the last zone level is connected to the exhaust pipe of the original vacuum system through a silencer. Each stage of the mechanical booster pump is connected to two nitrogen shaft seal lines. One nitrogen shaft seal line introduces nitrogen into the gap between the pump chamber of the mechanical booster pump and the bearing housing oil seal, and the other nitrogen shaft seal line introduces nitrogen into the gap between the pump chamber of the mechanical booster pump and the gearbox oil seal. The air inlet of all nitrogen shaft seal lines is connected to a nitrogen source. The multi-stage mechanical booster pump and the flow-through components of each pre-pump and post-pump heat exchanger are all made of duplex stainless steel.

[0006] A partitioned dry vacuum process for the crystallization and purification of adipic acid, characterized by comprising the following steps: Step 1: Before evacuation, nitrogen gas is introduced into the gaps between the pump chamber of each mechanical booster pump and the oil seals of the bearing housing and gearbox to achieve nitrogen shaft sealing; Step 2: The gaseous material generated in each crystallization chamber of the twelve-chamber crystallizer is pre-cooled in the corresponding pre-pump heat exchanger. Step 3: The gaseous materials, after being pre-cooled in each zone, enter the corresponding mechanical booster pump for compression. The compressed gaseous materials then enter the heat exchanger after the pump for condensation. The dilute nitric acid produced by condensation flows by gravity to the liquid seal tank. A portion of the non-condensable gas flows back to the pump chamber of the current mechanical booster pump to cool the pump body, while the other portion of the non-condensable gas enters the pump chamber of the next mechanical booster pump for further compression. Finally, the non-condensable gas produced is discharged into the exhaust pipe of the original vacuum system after being reduced in noise by a silencer. Step 4: After the vacuuming is completed, the gaseous material generated by the twelve-chamber crystallizer stops being discharged. Nitrogen is then continuously introduced into each stage of the mechanical booster pump for purging until all residual gas is discharged.

[0007] Compared with the prior art, the beneficial effects of the present invention are: 1. Based on the principle of similar vacuum levels, the crystallization chambers of the twelve-chamber crystallizer are divided into zones, so that gaseous materials with similar vacuum levels enter the corresponding level of post-pump heat exchanger for condensation. Under the same operating conditions, zoned vacuuming requires lower pumping speed from the mechanical booster pump and consumes less electrical energy and cooling water.

[0008] 2. Nitrogen gas is introduced into the gaps between the pump chamber and gearbox, and between the pump chamber and bearing housing oil seal of the mechanical booster pump, forming a positive pressure gas seal barrier. This prevents corrosive nitrite gas from entering the bearing housing and gearbox and contaminating the lubricating oil, while also preventing lubricating oil leakage into the pump chamber and contaminating the adipic acid product, achieving full nitrogen protection. The flow-through components of the mechanical booster pump and heat exchanger are made of duplex stainless steel to avoid corrosion.

[0009] 3. Significant energy-saving effect: No medium-pressure steam is required. Based on an annual production of 200,000 tons of adipic acid, approximately 22,000 tons of medium-pressure steam are saved annually. The elimination of the circulating water system saves approximately 1.8 million tons of water annually.

[0010] 4. Outstanding environmental benefits: Zero discharge of wastewater from the crystallization section; no oil pollution or exhaust gas emissions, completely solving the secondary pollution problem of traditional wet vacuum systems.

[0011] 5. Significantly improved product quality: The vacuum degree is stabilized at 2kPa.A±0.5kPa, the crystal particle size is uniform, and the product purity is increased to over 99.98%; there is no oil or water pollution, meeting production requirements. Attached Figure Description

[0012] Figure 1 This is a system structure diagram of the present invention; Figure 2 A schematic diagram showing the introduction of nitrogen gas between the pump chamber and the bearing housing oil seal of a mechanical booster pump; Wherein: P100~P103 are primary mechanical booster pumps to quaternary mechanical booster pumps; E1~E12 are pre-pump heat exchangers; E13~E16 are post-pump heat exchangers; V1 is a liquid seal tank; X1 is a silencer; K1~K12 are pressure regulating valves; K13~K17 are shut-off valves; K18 is a butterfly valve; K19 is a ball valve; SV1 is a solenoid valve; RV1~RV8 are pressure reducing valves; CV1~CV9 are check valves; T1~T4 are temperature gauges; PI1~PI5 are pressure gauges; FI1~FI8 are flow meters. Detailed Implementation

[0013] Specific embodiments are given below with reference to the accompanying drawings. These specific embodiments are only used to describe the technical solution of the present invention in detail and are not intended to limit the scope of protection of this application.

[0014] See Figure 1 The present invention provides a partitioned dry vacuum system for the crystallization and purification of adipic acid, comprising a multi-stage mechanical booster pump, a pre-pump heat exchanger, a post-pump heat exchanger, and a liquid seal tank. Based on the principle of similar vacuum levels, the twelve crystallization chambers of the twelve-chamber crystallizer are divided into multiple levels of zones, each level of zone being independent of the others. The higher the zone level, the lower the vacuum level. Each crystallization chamber in each level of zone is connected to the corresponding level of mechanical booster pump via a pre-pump heat exchanger. Each level of mechanical booster pump is connected to the liquid seal tank via a post-pump heat exchanger. Each level of zone corresponds to one mechanical booster pump and one post-pump heat exchanger. The outlet of the post-pump heat exchanger of the previous level of zone is connected to the pump chamber of the mechanical booster pump of that level and the pump chamber of the next level of mechanical booster pump. The outlet of the post-pump heat exchanger of the last level of zone is connected to the exhaust pipe of the original vacuum system via a silencer. Each stage of the mechanical booster pump is connected to two nitrogen shaft seal lines. One nitrogen shaft seal line introduces nitrogen into the gap between the pump chamber of the mechanical booster pump and the bearing housing oil seal (see...). Figure 2 Another nitrogen shaft seal line introduces nitrogen into the gap between the pump chamber of the mechanical booster pump and the oil seal of the gearbox, forming a positive pressure gas seal barrier to prevent corrosive nitrite gas from entering the bearing housing and gearbox and contaminating the lubricating oil, while also preventing lubricating oil from leaking into the pump chamber and contaminating the adipic acid product; the air inlets of all nitrogen shaft seal lines are connected to a nitrogen source.

[0015] The multi-stage mechanical booster pump and the flow-through components of each pump's upstream and downstream heat exchangers are made of duplex stainless steel to prevent corrosion.

[0016] A partitioned dry vacuum process for the crystallization and purification of adipic acid aims to treat the gaseous material generated in a twelve-chamber crystallizer with partitioned, fully dry negative pressure to reduce the vacuum level of the twelve-chamber crystallizer and improve the crystallization effect of adipic acid. The process includes the following steps: Step 1: Nitrogen shaft seal Before vacuuming, nitrogen gas is introduced into the gaps between the pump chamber of each mechanical booster pump and the oil seals of the bearing housing and gearbox through each nitrogen shaft seal pipeline to form a positive pressure gas seal barrier, preventing corrosive nitrite gas from entering the bearing housing and gearbox and contaminating the lubricating oil, while also preventing lubricating oil from leaking into the pump chamber and contaminating the gaseous materials. Step 2: Precooling of gaseous materials The crude adipic acid solution from the nitric acid oxidation process enters the twelve-chamber crystallizer for crystallization. The gaseous materials (including nitrite, water vapor and trace amounts of adipic acid vapor) generated in each crystallization chamber are pre-cooled in the corresponding pre-pump heat exchangers to reduce the heat load on the mechanical booster pump. Step 3: Partitioned and graded compression and condensation After pre-cooling in each stage, the gaseous material enters the corresponding mechanical booster pump for compression. The compressed gaseous material then enters the heat exchanger after the pump for condensation. The dilute nitric acid produced by condensation flows by gravity to the liquid seal tank and is used in the nitric acid oxidation process to achieve resource recycling. A portion of the non-condensable gas flows back to the pump chamber of the current stage mechanical booster pump to cool the pump body and prevent high-temperature decomposition of nitrite and overheating deformation of the pump body. The non-condensable gas reflux ratio is 15%~25%, and the maximum pump body temperature is ≤65℃. Another portion of the non-condensable gas enters the pump chamber of the next stage mechanical booster pump and is compressed together with the gaseous material pre-cooled in the next stage. After multiple stages of compression and condensation, the final non-condensable gas is discharged into the exhaust pipe of the original vacuum system after noise reduction by a silencer. Step 4: Nitrogen purging After the vacuuming is completed, the gaseous material generated by the twelve-chamber crystallizer is stopped from being fed into the mechanical booster pumps at all levels; nitrogen is then continuously introduced into the mechanical booster pumps at all levels for purging to completely remove the residual nitrite gas in the system, preventing equipment corrosion and personnel poisoning during maintenance.

[0017] Example This example uses a 200,000-ton-per-year adipic acid crystallization and refining section of a chemical enterprise as an example. Figure 1 As shown, the twelve-chamber crystallizer is divided into four levels according to the principle of similar vacuum degree. Crystallization chamber #1 is the fourth level, crystallization chambers #2 to #7 are the third level, crystallization chambers #8 to #10 are the second level, and crystallization chambers #11 to #12 are the first level. The vacuum degree of each level decreases sequentially. The No. 1 crystallization chamber is connected to the inlet of the four-stage mechanical booster pump P103 via the pre-pump heat exchanger E1. The output side of the pre-pump heat exchanger E1 is equipped with a pressure regulating valve K1. The exhaust port of the four-stage mechanical booster pump P103 is connected to the inlet of the post-pump heat exchanger E16. The exhaust port of the post-pump heat exchanger E16 is connected to the exhaust pipe of the original vacuum system via a silencer X1. A pressure gauge PI5, a check valve CV9, and a butterfly valve K18 are installed between the exhaust port of the post-pump heat exchanger E16 and the exhaust pipe of the original vacuum system. The drain port of the post-pump heat exchanger E16 is connected to the liquid seal tank V1 via a shut-off valve K16. Crystallization chambers #2 to #7 are connected to the inlet of the three-stage mechanical booster pump P102 via heat exchangers E2 to E7 before the pump. Pressure regulating valves K2 to K7 are installed on the output side of the heat exchangers E2 to E7 respectively. The exhaust port of the three-stage mechanical booster pump P102 is connected to the inlet of the heat exchanger E15 after the pump. The exhaust port of the heat exchanger E15 after the pump is connected to the pump chamber of the three-stage mechanical booster pump P102 and the inlet of the four-stage mechanical booster pump P103 respectively. A temperature gauge T4 and a pressure gauge PI4 are installed between the exhaust port of the heat exchanger E15 after the pump and the inlet of the four-stage mechanical booster pump P103. The drain port of the heat exchanger E15 after the pump is connected to the liquid seal tank V1 via a shut-off valve K15. Crystallization chambers #8 to #10 are connected to the inlet of the secondary mechanical booster pump P101 via heat exchangers E8 to E10 before the pump. Pressure regulating valves K8 to K10 are installed on the output side of heat exchangers E8 to E10. The exhaust port of the secondary mechanical booster pump P101 is connected to the inlet of the post-pump heat exchanger E14. The exhaust port of the post-pump heat exchanger E14 is connected to the pump chamber of the secondary mechanical booster pump P101 and the inlet of the tertiary mechanical booster pump P102. A thermometer T3 and a pressure gauge PI3 are installed between the exhaust port of the post-pump heat exchanger E14 and the inlet of the tertiary mechanical booster pump P102. The drain port of the post-pump heat exchanger E14 is connected to the liquid seal tank V1 via a shut-off valve K14. Crystallization chambers 11# and 12# are connected to the inlet of the first-stage mechanical booster pump P100 via heat exchangers E11 and E12 before the pump, respectively. Pressure regulating valves K11 and K12 are installed on the output side of heat exchangers E11 and E12 before the pump, respectively. The output side of the first-stage zone is equipped with a thermometer T1 and a pressure gauge PI1. The exhaust port of the first-stage mechanical booster pump P100 is connected to the inlet of the post-pump heat exchanger E13. The exhaust port of the post-pump heat exchanger E13 is connected to the pump chamber of the first-stage mechanical booster pump P100 and the inlet of the second-stage mechanical booster pump P101, respectively. A thermometer T2 and a pressure gauge PI2 are installed between the exhaust port of the post-pump heat exchanger E13 and the inlet of the second-stage mechanical booster pump P101. The drain port of the post-pump heat exchanger E13 is connected to the liquid seal tank V1 via a shut-off valve K13. The inlet of the liquid seal tank V1 is connected to the external water supply port via ball valve K19, and the drain pipe of the liquid seal tank V1 is connected to the nitric acid oxidation process pipe. One nitrogen shaft seal line of the first-stage mechanical booster pump P100 introduces nitrogen into the gap between the pump chamber and the bearing housing oil seal. This nitrogen shaft seal line is equipped with a pressure reducing valve RV1, a flow meter FI1, and a check valve CV1. Another nitrogen shaft seal line of the first-stage mechanical booster pump P100 introduces nitrogen into the gap between the pump chamber and the gearbox oil seal. This nitrogen shaft seal line is equipped with a pressure reducing valve RV2, a flow meter FI2, and a check valve CV2. Nitrogen gas is introduced into the gap between the pump chamber and the bearing housing oil seal through one nitrogen shaft seal line of the secondary mechanical booster pump P101. This nitrogen shaft seal line is equipped with a pressure reducing valve RV3, a flow meter FI3, and a check valve CV3. Nitrogen gas is introduced into the gap between the pump chamber and the gearbox oil seal through another nitrogen shaft seal line of the secondary mechanical booster pump P101. This nitrogen shaft seal line is equipped with a pressure reducing valve RV4, a flow meter FI4, and a check valve CV4. One nitrogen shaft seal line of the three-stage mechanical booster pump P102 introduces nitrogen into the gap between the pump chamber and the bearing housing oil seal. This nitrogen shaft seal line is equipped with RV5, flow meter FI5, and check valve CV5. The other nitrogen shaft seal line of the three-stage mechanical booster pump P102 introduces nitrogen into the gap between the pump chamber and the gearbox oil seal. This nitrogen shaft seal line is equipped with pressure reducing valve RV6, flow meter FI6, and check valve CV6. One nitrogen shaft seal line of the four-stage mechanical booster pump P103 introduces nitrogen into the gap between the pump chamber and the bearing housing oil seal. This nitrogen shaft seal line is equipped with a pressure reducing valve RV7, a flow meter FI7, and a check valve CV7. The other nitrogen shaft seal line of the four-stage mechanical booster pump P103 introduces nitrogen into the gap between the pump chamber and the gearbox oil seal. This nitrogen shaft seal line is equipped with a pressure reducing valve RV8, a flow meter FI8, and a check valve CV8. All nitrogen shaft seal pipeline inlets are connected to a nitrogen source, and a solenoid valve SV1 is installed at the nitrogen inlet to control the nitrogen flow.

[0018] All multi-stage mechanical booster pumps are dry-type Roots booster pumps, with flow-through components made of 2507 duplex stainless steel; the single-stage mechanical booster pump P100 is driven by a variable frequency motor with a frequency range of 20~75Hz and a rated pumping speed of 20000 Am. 3 / h, suction pressure 1.8kPa.A, suction temperature 10°C, discharge pressure 4.0kPa.A, discharge temperature 80°C; the secondary mechanical booster pump P101 is driven by a variable frequency motor with a frequency range of 0~50Hz and a rated pumping speed of 7800am. 3 / h, suction pressure 4.0 kPa.A, suction temperature 16°C, discharge pressure 10.0 kPa.A, discharge temperature 55°C; the three-stage mechanical booster pump P102 is driven by a power frequency motor, direct start, rated pumping speed 2600 mA. 3 / h, suction pressure 10.0 kPa.A, suction temperature 20°C, discharge pressure 28.0 kPa.A, discharge temperature 50°C; the four-stage mechanical booster pump P103 is driven by a power frequency motor, directly started, with a rated pumping speed of 1500 mA. 3 / h, inhalation pressure 28.0 kPa.A, inhalation temperature 23°, exhaust pressure 101.3 kPa.A, exhaust temperature 50°.

[0019] The partitioned dry vacuum process for the crystallization and purification of adipic acid includes the following steps: Step 1: Nitrogen shaft seal Before evacuation, adjust flow meters FI2~FI8 to control the nitrogen flow rate between 0.5-1.0 m³ / h. 3 / h, adjust the pressure reducing valves RV2~RV8 to adjust the nitrogen pressure to 0.05MPa.G; introduce nitrogen into the gap between the pump chamber of each mechanical booster pump and the oil seal of the bearing box and gearbox through the nitrogen shaft seal pipeline to form a positive pressure gas seal barrier to prevent corrosive nitrite gas from entering the bearing box and gearbox and contaminating the lubricating oil, and at the same time prevent lubricating oil from leaking into the pump chamber and contaminating the gaseous material; Step 2: Precooling of gaseous materials The crude adipic acid solution from the nitric acid oxidation process enters the twelve-chamber crystallizer for crystallization. The gaseous materials generated in each crystallization chamber (including nitrite, water vapor and trace amounts of adipic acid vapor) enter the corresponding pre-pump heat exchangers for pre-cooling to reduce the heat load on the mechanical booster pump. Step 3: Partitioned and graded compression and condensation Multi-stage mechanical booster pumps must strictly adhere to graded interlocking startup procedures, and direct startup at atmospheric pressure is prohibited. Before startup, confirm that the shaft seal pressure and flow rate have reached the set values. Subsequently, start the fourth-stage mechanical booster pump P103. When the system vacuum reaches 40 kPa·A, interlock the startup of the third-stage mechanical booster pump P102. When the system vacuum reaches 10 kPa·A, interlock the startup of the second-stage mechanical booster pump P101, with an initial operating frequency of 25 Hz. Once the operating frequency of the second-stage mechanical booster pump P101 reaches 50 Hz and the system vacuum reaches 4 kPa·A, interlock the startup of the first-stage mechanical booster pump P100, completing the full startup of the unit. Primary zone compression and condensation: The gaseous material generated in crystallization chambers 11# and 12# is pre-cooled by heat exchangers E11 and E12 before the pump. The pre-cooled gaseous material enters the primary mechanical booster pump P100 for compression. The suction pressure of the primary mechanical booster pump P100 is 1.8 kPa·A, the suction temperature is about 10°C, the discharge pressure is 4.0 kPa·A, and the discharge temperature is 80°C. The compressed gaseous material enters the post-pump heat exchanger E13 for cooling. The dilute nitric acid generated by condensation flows by gravity to the liquid seal tank V1. The non-condensable gas is divided into two paths: about 80% of the non-condensable gas enters the secondary mechanical booster pump P101 for further compression, and about 20% of the non-condensable gas flows back to the pump chamber of the primary mechanical booster pump P100 for gas cooling of the pump body. Secondary zone compression and condensation: The gaseous material generated in crystallization chambers #8 to #10 is pre-cooled by heat exchangers E8 to E10 before the pump. The pre-cooled gaseous material is then compressed in the secondary mechanical booster pump P101. The suction pressure of the secondary mechanical booster pump P101 is 4.0 kPa·A, the suction temperature is 16°C, the discharge pressure is 10.0 kPa·A, and the discharge temperature is 55°C. The compressed gaseous material is then cooled in the post-pump heat exchanger E14. The dilute nitric acid generated during condensation flows by gravity to the liquid seal tank V1. The non-condensable gas is divided into two paths: approximately 80% of the non-condensable gas enters the tertiary mechanical booster pump P102 for further compression, and approximately 20% of the non-condensable gas flows back to the pump chamber of the secondary mechanical booster pump P101 for gas cooling of the pump body. Three-stage zone compression and condensation: The gaseous material generated in crystallization chambers 2# to 7# is pre-cooled by heat exchangers E2 to E7 before the pump. The pre-cooled gaseous material enters the three-stage mechanical booster pump P102 for compression. The suction pressure of the three-stage mechanical booster pump P102 is 10.0 kPa·A, the suction temperature is 20°C, the discharge pressure is 28.0 kPa·A, and the discharge temperature is 55°C. The compressed gaseous material enters the heat exchanger E15 after the pump for cooling. The dilute nitric acid generated by condensation flows by gravity to the liquid seal tank V1. The non-condensable gas is divided into two paths: about 85% of the non-condensable gas enters the four-stage mechanical booster pump P103 for further compression, and about 15% of the non-condensable gas flows back to the pump chamber of the three-stage mechanical booster pump P102 for gas cooling of the pump body. Four-stage zoned compression and condensation: The gaseous material generated in crystallization chamber #1 is pre-cooled by the pre-pump heat exchanger E1. The pre-cooled gaseous material enters the four-stage mechanical booster pump P103 for compression. The suction pressure of the four-stage mechanical booster pump P103 is 28 kPa·A, the suction temperature is 23°C, the discharge pressure is 101.3 kPa·A, and the discharge temperature is 50°C. The compressed gaseous material enters the post-pump heat exchanger E16 for cooling. The dilute nitric acid generated by condensation flows by gravity to the liquid seal tank V1. The non-condensable gas is discharged into the original vacuum system exhaust pipe after noise reduction by the silencer X1. The concentration of dilute nitric acid in the liquid-sealed tank V1 is about 5-8%, and it is all recycled in the nitric acid oxidation process. Therefore, there is no waste liquid discharged during the entire process.

[0020] Step 4: Nitrogen purging After vacuuming is completed, the gaseous material generated by the twelve-chamber crystallizer is stopped from flowing into the mechanical booster pumps at each stage; adjust the pressure reducing valve RV1 to adjust the pressure to 0.3-0.4 MPa.G, then open the solenoid valve SV1 and adjust the flow meter FI1 to control the flow rate at 5-10 m³ / s. 3 / h, nitrogen is introduced into each level of mechanical booster pump for purging. After purging for 15 minutes, the solenoid valve SV1 is closed to completely remove the residual nitrite gas in the system, preventing equipment corrosion and personnel poisoning during maintenance.

[0021] The crude adipic acid feed concentration is 55 wt%, the feed temperature is 85℃, and the total vapor evaporation rate in the crystallization section is approximately 18 t / h. The system's target vacuum level is 2.0 kPa·A, and the annual operating time is estimated at 8000 hours. In this embodiment, the system operated continuously and stably for 1680 hours, and the test data are as follows: Vacuum stability: The vacuum level fluctuates within a range of ±0.4 kPa.A, and the interval is 1.6~2.4 kPa.A. Under the operating condition of 38℃ ambient temperature in summer, the vacuum level does not decrease significantly and always meets the process requirements.

[0022] Energy consumption indicators: The system consumes no medium-pressure steam, and the total operating power of the unit is about 76.3kW; based on 8,000 hours of operation per year, the annual power consumption is about 610,000 kWh; compared with the original wet vacuum system, it saves 22,000 tons of medium-pressure steam of 0.8MPa.G per year and 1.8 million tons of circulating cooling water per year.

[0023] Environmental protection and material recycling: No nitrogen-containing acidic wastewater is discharged during operation. A total of about 720 tons of dilute nitric acid (average concentration of about 6wt%) is recovered and reused in the nitric acid oxidation process, realizing a closed-loop material circulation.

[0024] Product quality: The adipic acid produced by crystallization has an average purity of 99.985%, uniform particle size distribution, and a particle size variation coefficient that is 32% lower than that of the original system, meeting the quality requirements of high-end nylon 66 polymerization raw materials.

[0025] Equipment corrosion status: After continuous operation, the pump chamber and flow components were disassembled and inspected. There were no obvious corrosion pits on the surface, and no scale deposits of adipic acid and nitrite decomposition products. The average corrosion rate of the flow components was measured to be ≤0.02mm / year, which meets the requirements for long-term stable operation.

[0026] Comparative Example The comparative example used a two-stage steam jet + liquid ring vacuum system commonly used in adipic acid crystallization processes. The feed conditions and production load were the same as in the previous example. The working steam pressure was stabilized at 0.8 MPa.G, the circulating water inlet temperature was 32℃, and the system's target vacuum degree was 2.0 kPa.A. The detection data during continuous stable operation are as follows: Vacuum stability: The vacuum level fluctuates within ±8 kPa.A, and is significantly affected by fluctuations in steam pressure and changes in circulating water temperature. During the high-temperature period in summer, the vacuum level drops to a minimum of 3.5 kPa.A. Energy consumption indicators: Annual consumption of medium-pressure steam is 22,000 tons, and annual consumption of circulating water is 1.8 million tons; annual discharge of nitrogen-containing acidic wastewater is approximately 300,000 tons.

[0027] Environmental protection and material recycling: During operation, nitrogen-containing acidic wastewater is discharged, with a cumulative annual discharge of 16,000 tons of nitrogen-containing wastewater.

[0028] Product quality: The adipic acid produced by crystallization has an average purity of 99.9% and a uniform particle size distribution, meeting the quality requirements of high-end nylon 66 polymerization raw materials.

[0029] Equipment corrosion status: After continuous operation, the corrosion of the equipment was inspected. There were no obvious corrosion marks on the surface of the impeller, pump body and other flow parts of the liquid ring pump. There was residual nitric acid material. The average corrosion rate of the flow parts was measured to be about 0.026 mm / year, which basically meets the usage requirements.

[0030] In summary, the vacuum fluctuation range of the embodiment is significantly smaller than that of the comparative example. Based on 8000 hours of operation per year, the embodiment saves 22,000 tons of steam and 1.8 million tons of water annually. At a steam price of 160 yuan / ton and a water price of 0.5 yuan / ton, the annual comprehensive benefit is approximately 4.42 million yuan. Therefore, the process of this invention is significantly superior to traditional wet vacuum systems in terms of energy saving and environmental protection, and has enormous potential for widespread application.

[0031] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A partitioned dry vacuum system for the crystallization and purification of adipic acid, characterized in that, It includes a multi-stage mechanical booster pump, a pre-pump heat exchanger, a post-pump heat exchanger, and a liquid seal tank; The twelve crystallization chambers of the twelve-chamber crystallizer are divided into multiple zones according to the principle of similar vacuum levels. The higher the zone level, the lower the corresponding vacuum level. Each zone level corresponds to a mechanical booster pump and a post-pump heat exchanger. Each crystallization chamber in each zone level is connected to the corresponding mechanical booster pump through a pre-pump heat exchanger. Each mechanical booster pump is connected to the liquid seal tank through a post-pump heat exchanger. The outlet of the post-pump heat exchanger of the previous zone level is connected to the pump chamber of the mechanical booster pump of that zone level and the pump chamber of the next level mechanical booster pump. The outlet of the post-pump heat exchanger of the last zone level is connected to the exhaust pipe of the original vacuum system through a silencer. Each stage of the mechanical booster pump is connected to two nitrogen shaft seal lines. One nitrogen shaft seal line introduces nitrogen into the gap between the pump chamber of the mechanical booster pump and the bearing housing oil seal, and the other nitrogen shaft seal line introduces nitrogen into the gap between the pump chamber of the mechanical booster pump and the gearbox oil seal. The air inlet of all nitrogen shaft seal lines is connected to a nitrogen source. The multi-stage mechanical booster pump and the flow-through components of each pre-pump and post-pump heat exchanger are all made of duplex stainless steel.

2. A partitioned dry vacuum process for the crystallization and purification of adipic acid, wherein the process uses the system of claim 1 for partitioned vacuuming, characterized in that, Includes the following steps: Step 1: Before evacuation, nitrogen gas is introduced into the gaps between the pump chamber of each mechanical booster pump and the oil seals of the bearing housing and gearbox to achieve nitrogen shaft sealing; Step 2: The gaseous material generated in each crystallization chamber of the twelve-chamber crystallizer is pre-cooled in the corresponding pre-pump heat exchanger. Step 3: The gaseous materials, after being pre-cooled in each zone, enter the corresponding mechanical booster pump for compression. The compressed gaseous materials then enter the heat exchanger after the pump for condensation. The dilute nitric acid produced by condensation flows by gravity to the liquid seal tank. A portion of the non-condensable gas flows back to the pump chamber of the current mechanical booster pump to cool the pump body, while the other portion of the non-condensable gas enters the pump chamber of the next mechanical booster pump for further compression. Finally, the non-condensable gas produced is discharged into the exhaust pipe of the original vacuum system after being reduced in noise by a silencer. Step 4: After the vacuuming is completed, the gaseous material generated by the twelve-chamber crystallizer stops being discharged. Nitrogen is then continuously introduced into each stage of the mechanical booster pump for purging until all residual gas is discharged.

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