A polycarbonate plant solvent recovery system and use thereof

Abstract

The present application relates to the field of wastewater recovery process, especially to a polycarbonate factory solvent recovery system and application thereof. The polycarbonate factory solvent recovery system, the system structure at least includes a flash tower, a stripping tower, a decanter, a steam nozzle and a storage tank; the system structure is fixedly connected through a steam pipeline. The polycarbonate factory solvent recovery system provided in the application can improve the overall separation capacity of the system, reduce the content of dichloromethane in wastewater, and also reduce the content of other organic impurities in wastewater, improve the separation efficiency of the overall system and reduce the steam and energy consumption.

Description

Technical Field

[0001] This invention relates to the field of wastewater recycling processes, and more particularly to a solvent recovery system for polycarbonate plants and its application. Background Technology

[0002] Polycarbonate (PC) is a high molecular weight polymer containing carbonate groups in its molecular chain. Based on the different ester groups, polycarbonate can be classified into several different types. As a non-crystalline engineering plastic, polycarbonate is widely used in the automotive, electrical appliance, and healthcare product industries due to its good heat resistance and transparency.

[0003] Among existing preparation methods, the phosgene process for preparing polymeric carbonates is a relatively common process. However, due to the process conditions of the phosgene process, a significant amount of brine and wastewater containing organic matter is generated. Factories and businesses typically employ wastewater / solvent recovery systems to separate and recover dichloromethane from the brine separated by centrifuges and various wastewater sources. The feed stream for solvent recovery usually contains 13–15 wt% NaCl and approximately 1 wt% Na₂CO₃. After stripping off the dichloromethane, the wastewater stream is further purified before being recycled at a chlor-alkali plant or sent to a public wastewater treatment plant. The purpose of the solvent recovery system is to reduce the concentration of dichloromethane to below 200 ppb wt (0.2 ppm wt).

[0004] Chinese patent CN217887031U provides a polycarbonate dichloromethane recovery system. The system structure mainly includes a devolatilization tower, a scrubbing tank, an air cooler, a circulating water cooler, and a liquid-liquid separator. It claims that it can ensure heat exchange efficiency, improve the efficiency of cooling gaseous dichloromethane into liquid, and ultimately improve the recovery rate of dichloromethane. However, it also claims to significantly enhance energy utilization and recovery efficiency.

[0005] Therefore, in order to further solve the above problems, this application provides a solvent recovery system for polycarbonate plants. Summary of the Invention

[0006] To address the aforementioned problems, the first aspect of this invention provides a solvent recovery system for a polycarbonate plant. The system structure includes at least a flash tower, a stripping tower, a decanter, a steam nozzle, and a storage tank; the system structure is fixedly connected via steam pipelines.

[0007] As a preferred embodiment, the flash tower includes an intermediate tower body, an extension tower body, and a bottom tower body; the extension tower body includes a feed inlet and a steam outlet; the bottom tower body includes a steam inlet, a reflux steam inlet, and a bottom discharge outlet.

[0008] As a preferred embodiment, the diameter ratio of the extension section to the intermediate section of the flash tower is 1.2 to 1.4:1.

[0009] As a preferred embodiment, the diameter ratio of the extension section to the intermediate section of the flash tower is 1.25:1.

[0010] As a preferred embodiment, the length of the extension section of the flash tower is 8 to 12 times the diameter of the intermediate tower.

[0011] As a preferred embodiment, the length of the extension section of the flash tower is 10 times the diameter of the intermediate tower.

[0012] As a preferred embodiment, the decanter is fixedly connected to the feed inlet pipe in the flash tower, which transports the material to be processed to the flash tower.

[0013] As a preferred embodiment, the flash tower has 6 to 12 flash tower plates arranged at equal heights inside the middle tower body, and the flash tower plates are any type of anti-fouling tower plates.

[0014] As a preferred embodiment, the flash tower has 11 flash tower plates arranged at equal heights inside the middle tower body.

[0015] This application presents a flash distillation tower design that improves the overall separation capacity of the system, reduces the dichloromethane content in wastewater, and also reduces the content of other organic impurities in the wastewater, thereby increasing the overall system separation efficiency and reducing steam and energy consumption. With sufficient trays, the flash distillation tower can effectively reduce the need for a stripping tower, or even eliminate it altogether, and also effectively reduce steam usage. However, to achieve the aforementioned technical effects, it is necessary to know the Henry's law constant for dichloromethane and brine, as well as the maximum solubility of dichloromethane in brine.

[0016] The applicant obtained the relationship between Henry's constant and temperature and salt water concentration through experimental testing, and as follows: Figure 3 As shown, the data regression yielded the following function formula: H = 78.2 – 9.25 * T + 33.5 * C + 0.182 * T², where H is the Henry's constant, T is the temperature in °C, and C is the salt concentration in wt%.

[0017] In addition, the maximum solubility of dichloromethane in salt water was experimentally determined, as shown in Table 1 below:

[0018] Table 1

[0019]

[0020] Furthermore, the data regression method yields the following transformation equation:

[0021] The maximum solubility of CH2Cl2 is calculated as 1.5675 - 0.05325 * C, where C represents the mass percentage concentration of the brine.

[0022] Furthermore, based on past operating experience, the equilibrium time for CH2Cl2 / water (the time between CH2Cl2 addition and pressure stabilization) is approximately 10–15 minutes; however, for brine, the equilibrium time is very slow, approximately 2–3 hours. The diffusion rate of CH2Cl2 in brine is much slower than in water; simultaneously, the solubility of CH2Cl2 in brine is less than its solubility in water. Therefore, system design will essentially be a trade-off between these two parameters (solubility and diffusivity); this applies not only to equipment such as stripping towers but also to flash evaporators or decanters, where low diffusivity may mean it cannot provide separation during the equilibrium phase, but rather is only part of it.

[0023] As a preferred embodiment, the flash tray is Tray, Koch-Glitch.

[0024] Because the process medium in this application system is moderately to heavily fouled, with polycarbonate aggregates and NaCl and Na2CO3 compounds that can solidify into microcrystals, it is necessary to select trays with excellent anti-fouling properties. Considering the above factors, the optimal technical solution in this application is to adopt... Tray, Koch-Glic; The tray includes a fixed valve with a special conical shape that effectively contacts the vapor and liquid and minimizes liquid stagnation areas. The tray can operate between 45% and 100% of the specified flow rate.

[0025] As a preferred embodiment, the extended section of the flash tower is equipped with 3 to 6 baffle plates at equal heights inside the tower body.

[0026] As a preferred embodiment, the extended section of the flash tower is equipped with five baffle plates at equal heights inside the tower body.

[0027] As a preferred embodiment, 0 to 10.5 bar of steam is introduced into the steam inlet of the flash tower.

[0028] As a preferred embodiment, 0 bar steam is introduced at the steam inlet of the flash tower.

[0029] As a preferred embodiment, the bottom outlet of the flash tower is fixedly connected to the inlet pipe of the stripping tower; the top of the stripping tower includes a reflux steam outlet, which is fixedly connected to the reflux steam inlet pipe of the flash tower to provide reflux steam to the flash tower.

[0030] As a preferred embodiment, the stripping tower further includes a low-pressure flash inlet, which is fixedly connected to a steam nozzle pipeline, and the steam nozzle supplies a total of 10 to 15 bar of steam to the stripping tower.

[0031] As a preferred embodiment, the steam outlet of the flash tower and the bottom discharge port of the stripping tower are both fixedly connected to the storage tank pipeline.

[0032] A second aspect of the present invention provides an application of the above-mentioned solvent recovery system for a polycarbonate plant, including its application in the waste solvent recovery process for polymer resin preparation.

[0033] Beneficial effects:

[0034] 1. This application provides a solvent recovery system for a polycarbonate plant, which can improve the overall separation capacity of the system, reduce the content of dichloromethane in wastewater, and reduce the content of other organic impurities in wastewater by designing the flash tower, thereby improving the overall separation efficiency of the system and reducing steam and energy consumption.

[0035] 2. This application provides a solvent recovery system for a polycarbonate plant. By selecting specific trays and combinations of tray numbers, the use of steam in the overall recovery system can be significantly reduced, thereby greatly enhancing the system's energy utilization and recovery efficiency.

[0036] 3. This application provides a solvent recovery system for polycarbonate plants, which simplifies the complexity of recovery operations, reduces the time and labor costs of recovery processing, and has excellent market prospects.

[0037] 4. This application provides a solvent recovery system for a polycarbonate plant. By designing the tray structure and selecting the tray type, as well as calculating the Henry's constant for dichloromethane and brine, the minimum number of trays required for stable system operation is clearly determined, making the recovery system more stable and preventing system operation errors or insufficient processing capacity.

[0038] 5. This application provides a solvent recovery system for a polycarbonate plant, which can effectively prevent the generation of heavy dirt, ensure the smooth operation of the system, and effectively reduce the frequency of dirt generation, thereby ensuring good equipment cleanliness and avoiding equipment damage and excessive maintenance costs due to the presence of dirt. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the system structure of this application.

[0040] Figure 2 This is a schematic diagram of the flash tower in this application.

[0041] Figure 3 This is a line graph showing the relationship between the Henry's constant and temperature and brine concentration in this application.

[0042] In the picture:

[0043] V001-Flash tower, V002-Stripping tower, V004-Storage tank, V006-Decanter, H001-Condenser, 1-Bottom tower body, 2-Intermediate tower body, 3-Extension tower body, 4-Flash tower plate, 5-Baffle tower plate. Detailed Implementation

[0044] Example 1

[0045] Example 1 provides a solvent recovery system for a polycarbonate plant. The system structure includes at least a flash tower V001, a stripping tower V002, a decanter V006, a steam nozzle, and a storage tank V004. The system structure is fixedly connected by steam pipelines.

[0046] The flash tower V001 includes an intermediate tower body 2, an extension tower body 3, and a bottom tower body 1; the extension tower body 3 includes a feed inlet and a steam outlet; the bottom tower body 1 includes a steam inlet, a reflux steam inlet, and a bottom discharge outlet.

[0047] The diameter ratio of the extension section 3 to the intermediate section 2 in flash tower V001 is 1.25:1.

[0048] The length of the extension section 3 in flash tower V001 is 10 times the diameter of the intermediate section 2.

[0049] The decanter V006 is fixedly connected to the feed inlet pipe in the flash tower V001, which transports the material to be processed to the flash tower V001; after the decanter V006, there is also a condenser H001 for heat exchange of the material.

[0050] The intermediate tower body 2 of flash tower V001 has 11 flash tower plates 4 arranged at equal height intervals.

[0051] The applicant obtained the relationship between Henry's constant and temperature and salt water concentration through experimental testing, and as follows: Figure 3 As shown, the data regression yielded the following function formula: H = 78.2 – 9.25 * T + 33.5 * C + 0.182 * T², where H is the Henry's constant, T is the temperature in °C, and C is the salt concentration in wt%.

[0052] In addition, the maximum solubility of dichloromethane in salt water was determined experimentally, as shown in Table 1 above.

[0053] The data regression method yields the following transformation equation:

[0054] The maximum solubility of CH2Cl2 is calculated as 1.5675 - 0.05325 * C, where C represents the mass percentage concentration of the brine.

[0055] Flash tray 4 is Tower plate, Koch-Glitch.

[0056] The extension section of the flash tower V001 has five baffle plates 5 arranged at equal heights inside the tower body 3.

[0057] 0 bar steam is introduced into the steam inlet of flash tower V001.

[0058] The bottom outlet of the flash tower V001 is fixedly connected to the inlet pipe of the stripping tower V002; the top of the stripping tower V002 includes a reflux steam outlet, which is fixedly connected to the reflux steam inlet pipe of the flash tower V001 to provide reflux steam for the flash tower V001.

[0059] The stripping tower V002 also includes a low-pressure flash inlet, which is fixedly connected to a steam nozzle pipeline. The steam nozzle supplies a total of 12 bar of steam to the stripping tower V002.

[0060] The steam outlet of flash tower V001 and the bottom discharge port of stripping tower V002 are both fixedly connected to the pipeline of storage tank V004.

[0061] Performance Evaluation

[0062] With the maximum value of the equipment at the time of design as 100%, the process parameter values ​​of the system equipment obtained in Example 1 during stable operation are recorded, as shown in Table 2 below.

[0063] Table 2

[0064]

[0065] CH2Cl2 and triethylamine in the flash distillation column feed and bottom product samples were analyzed by gas chromatography using PerkinElmer headspace gas chromatography. 10 ml samples were heated in vials at 90°C for 8 minutes; the gas was then injected into the chromatograph. One sample was taken every half hour during the run, and the results are recorded in Table 3.

[0066] Table 3

[0067]

[0068]

Claims

1. A solvent recovery system for a polycarbonate plant, characterized in that: The system structure includes at least a flash tower, a stripping tower, a decanter, a steam nozzle, and a storage tank; the system structure is fixedly connected by steam pipelines. The flash tower comprises, from bottom to top, a bottom tower body, a middle tower body, and an extension tower body; the extension tower body includes a feed inlet and a steam outlet; the bottom tower body includes a steam inlet, a reflux steam inlet, and a bottom discharge outlet. The decanter is fixedly connected to the feed inlet pipe in the flash tower, and it transports the material to be processed to the flash tower. The bottom outlet of the flash tower is fixedly connected to the inlet pipe of the stripping tower; the top of the stripping tower includes a reflux steam outlet, which is fixedly connected to the reflux steam inlet pipe of the flash tower to provide reflux steam for the flash tower. The flash tower has 6 to 12 flash tower plates arranged at equal heights inside the middle tower body, and the flash tower plates are any of the anti-fouling tower plates. The extended section of the flash tower has 3 to 6 baffle plates spaced at equal heights inside the tower body. The diameter ratio of the extension section to the intermediate section of the flash tower is 1.2 to 1.4:

1. The length of the extended section of the flash tower is 8 to 12 times the diameter of the intermediate tower. The stripping tower also includes a low-pressure flash inlet, which is fixedly connected to a steam nozzle pipeline. The steam nozzle supplies a total of 10-15 bar of steam to the stripping tower. The steam outlet of the flash evaporator and the bottom discharge port of the stripping tower are both fixedly connected to the storage tank pipeline.

2. The solvent recovery system for a polycarbonate plant according to claim 1, characterized in that: The flash evaporator is supplied with 0-10.5 bar of steam at its steam inlet.

3. An application of the solvent recovery system for a polycarbonate plant according to any one of claims 1 to 2, characterized in that: This includes the application of the solvent recovery system in the production of polycarbonate using the phosgene process.

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