A method for thermal cleaning of a polycarbonate polycondensation reactor

Abstract

The application relates to the technical field of reactor cleaning, and particularly discloses a polycarbonate polycondensation reactor heat cleaning method, which comprises the following operation steps: a polycarbonate polycondensation reactor heat cleaning method is characterized by comprising the following operation steps: (1) after polycarbonate melt in the polycondensation reactor is discharged, the polycondensation reactor is depressurized and then pressurized again, and the depressurizing and pressurizing process is repeated 2 times or more; (2) the polycondensation reactor is cooled; (3) phenol is added into the polycondensation reactor; (4) the polycondensation reactor is boiled with the phenol, the liquid level of the phenol is observed, and the phenol is supplemented; (5) after boiling, the phenol is recovered by using a phenol recovery system; (6) the phenol discharged from the polycondensation reactor in the step (5) is detected to determine whether the boiling is qualified, and if qualified, the cleaning process is ended, and if not qualified, the steps (4)-(5) are repeated once. The polycarbonate polycondensation reactor heat cleaning method can clean the polycondensation reactor of polycarbonate more efficiently.

Description

Technical Field

[0001] This invention relates to the technical field of reactor cleaning, and more specifically, to a thermal cleaning method for a polycarbonate polycondensation reactor. Background Technology

[0002] Polycarbonate (PC) is a thermoplastic engineering plastic with excellent comprehensive properties. The main representative is bisphenol A polycarbonate. Due to its unique structure, polycarbonate possesses excellent light transmittance, impact toughness, electrical insulation, and weather resistance, and is widely used in aerospace, electrical lighting, and translucent building materials. Industrial production mainly employs interfacial polycondensation or melt transesterification.

[0003] The melt transesterification process for producing polycarbonate involves two stages: prepolymerization and step-growth polycondensation. The polycondensation reaction first takes place in a multi-stage falling film evaporator, and then continues in a multi-stage polycondensation reactor. As the reaction progresses, increasingly higher temperatures and vacuum levels are required for step-growth polycondensation to obtain polycarbonate melt of the appropriate molecular weight. After the reaction, the polycarbonate melt is mixed with additives, filtered to remove impurities, and then pelletized to obtain granular polycarbonate products.

[0004] In the melt transesterification process of polycarbonate production, the polycondensation reactor is mainly a horizontal reactor equipped with a disc agitator. The operating temperature of the polycondensation reactor is around 280-310℃, and the operating pressure is around 100-1000 Pa·A.

[0005] During the production process, some of the oligomers in the liquid phase of the polycondensation reactor are carried out by the gas phase and adhere to the inner wall of the gas phase of the reactor. Under long-term high temperature and high vacuum, these oligomers further cross-link and polymerize to form brownish polymers. Due to vacuum pressure fluctuations, they fall into the liquid phase and cannot melt. They can only swell and break into small particles. The extremely fine particles cannot be intercepted by the filtration device and enter the polycarbonate product, which appears as "black spots" in downstream applications, affecting the appearance and coating. Summary of the Invention

[0006] The purpose of this invention is to provide a thermal cleaning method for polycarbonate polycondensation reactors, which can clean polycarbonate polycondensation reactors more efficiently.

[0007] This invention is achieved through the following technical solution: The thermal cleaning method for the polycarbonate polycondensation reactor of this invention includes the following operating steps:

[0008] (1) After discharging the polycarbonate melt from the polycondensation reactor, adjust the temperature of the polycondensation reactor to 275-285℃; depressurize and then repressurize the polycondensation reactor, repeating the depressurization and repressurization process two or more times; the depressurization parameters are: depressurize to 50-1000 Pa.A, depressurization rate ≥1.25 kPa / h; the repressurization parameters are: repressurize to 100-200 kPa.A, repressurization rate ≥5 kPa / h. Through pressure changes, the polymer on the inner wall of the gas phase can be initially stripped.

[0009] (2) Cool the polycondensation reactor; cool the polycondensation reactor to 180-190℃ at a rate of 5-10℃ / h. Further strip the polymer from the inner wall of the gas phase by changing the temperature.

[0010] (3) Add phenol to the polycondensation reactor; when adding phenol, control the temperature of the polycondensation reactor to 180-190℃, the pressure to >5kPa.G, and the injection amount of phenol to 20-60% of the liquid level of the polycondensation reactor. The purity of the phenol used is >99.5%, and the color is ≤20Hazen.

[0011] (4) Phenol is used to heat-cook the polycondensation reactor. The phenol level is observed, and phenol is added as needed. During the heating process, the temperature of the polycondensation reactor is controlled at 183-200℃, the pressure at 5-60 kPa.G, and the heating time is 2-4 hours. During the heating process, the vaporization rate of liquid phenol in the polycondensation reactor is controlled to be <50% by pressure control, and the liquid level in the polycondensation reactor is increased to >20% by adding phenol. The gaseous phenol permeates and peels off the brown polymer on the inner wall of the polycondensation reactor. The detached brown polymer further swells and melts in the liquid phenol, and the color of the phenol deepens accordingly.

[0012] (5) After the boiling process is completed, a phenol recovery system is used to recover the phenol. The phenol recovery system includes a phenol evaporator, a crude phenol collection tower, and a phenol distillation tower connected in sequence. The temperature of the phenol evaporator is 185-200℃. The pressure at the top of the phenol distillation tower is 1.5-2.5 kPa·A, the reflux rate at the top is 6000-10000 kg / h, the temperature at the bottom is 175-190℃, and the circulation rate at the bottom is 100-150 t / h. The heavy component waste liquid remaining at the bottom of the phenol evaporator and the phenol distillation tower is sent for incineration.

[0013] (6) Test the phenol discharged from the polycondensation reactor outlet in step (5) and determine whether the hot cooking is qualified. If it is qualified, end the cleaning process. If it is not qualified, repeat steps (4)-(5) once. The criteria for determining whether the hot cooking is qualified are: if the purity of the phenol discharged from the polycondensation reactor outlet is <98.5% or the color is >200 Hazen, it is unqualified; if the purity of the phenol discharged from the polycondensation reactor outlet is >98.5% and the color is <200 Hazen, it is qualified.

[0014] The technical solution of this invention has at least the following advantages and beneficial effects: The thermal cleaning method for the polycarbonate polycondensation reactor of this invention can clean the brownish polymer on the inner wall of the reactor's gas phase space, reducing the risk of black spots entering the product and significantly improving the quality of the polycarbonate product. It can also clean residual melt inside the reactor, enabling the stirrer to be rotated even in a cold state, facilitating stirrer maintenance. Detailed Implementation

[0015] Example 1

[0016] This embodiment provides a method for thermal cleaning of a polycarbonate polycondensation reactor, including the following steps:

[0017] (1) After the residual polycarbonate melt in the polycondensation reactor is discharged, the melt valve in front of the melt pump is closed, the temperature of the polycondensation reactor is maintained at 280°C, the pressure of the polycondensation reactor is pumped down to 100 Pa.A using the vacuum system at a rate of 1.3 kPa / h, the vacuum valve is closed, nitrogen is used to break the vacuum to 160 kPa.A at a rate of 5.3 kPa / h, and this process is repeated twice. Through the rapid change in pressure, the polymers attached to the gas phase inner wall of the polycondensation reactor are initially stripped off.

[0018] (2) The temperature of the polycondensation reactor is reduced to 180°C at 7°C / h, and the polymer on the gas phase inner wall of the polycondensation reactor is further stripped off by the temperature change.

[0019] (3) Adjust the pressure of the polycondensation reactor to 110 kPa.A. Set a phenol inlet at the feed end. Phenol (purity 99.6%, color ≤20 Hazen) is pumped into the polycondensation reactor and the liquid level in the polycondensation reactor is set to 30%.

[0020] (4) Raise the temperature of the polycondensation reactor to 184-186℃ and control the pressure at 110 kPa·A to vaporize some of the phenol inside the reactor. Heat the phenol for 2 hours, and when the liquid level drops to 18%, continue to add phenol until it reaches 30%, and continue heating for another 2 hours. During this period, manually rotate the agitator of the polycondensation reactor. Once you can rotate it, rotate it once every half hour.

[0021] (5) After the heating process is complete, open the hand valve on the outlet pipeline of the polycondensation reactor to discharge the material from the polycondensation reactor to the phenol evaporator. Simultaneously, take a sample at the sampling point. The phenol purity is 96%, and the color is greater than 400 Hazen. The phenol evaporator is heated by a heat transfer oil coil and continuously stirred by a stirrer. The temperature is controlled at 190℃, and the pressure is controlled at 3kPa.G. After evaporation, the phenol is condensed and recovered in the phenol condenser to the crude phenol collection tank. When the liquid level in the crude phenol collection tank reaches more than 50%, it is pumped to the phenol distillation column. The pressure of the phenol distillation column is controlled at 1.6kPa.A, the top reflux rate is 7000kg / h, the bottom temperature is 180℃, and the bottom circulation rate is 120t / h. After purification at the top of the column, part of the phenol is refluxed, and part is recycled for use in applications requiring high phenol purity, such as catalyst preparation and the working medium of the phenol vacuum system. Part of the bottom liquid from the phenol distillation column is collected and sent for incineration. After phenol recovery, the remaining material from the phenol evaporator is sent for incineration as waste liquid.

[0022] (6) Repeat steps (4) and (5) to perform phenol heating and phenol recovery. Samples were taken again at the sampling point when the phenol was discharged into the phenol evaporator. Analysis results: phenol purity 98.8%, color 150 Hazen, cleaning qualified. The amount of phenol added to the polycondensation reactor was calculated: 10.8 tons of phenol were used, and 9.9 tons of phenol were recovered from the phenol distillation column, resulting in a recovery rate of 92%.

[0023] Example 2

[0024] This embodiment provides a method for thermal cleaning of a polycarbonate polycondensation reactor, including the following steps:

[0025] (1) After the residual polycarbonate melt in the polycondensation reactor is discharged, the melt valve in front of the melt pump is closed, the temperature of the polycondensation reactor is maintained at 280°C, the pressure of the polycondensation reactor is pumped to 200 Pa.A using the vacuum system, the pressure reduction rate is 1.3 kPa / h, the vacuum valve is closed, nitrogen is used to break the vacuum to 150 kPa.A, the pressure increase rate is 5.1 kPa / h, and this process is repeated three times. Through the rapid change of pressure, the polymers attached to the gas phase inner wall of the polycondensation reactor are initially stripped.

[0026] (2) The temperature of the polycondensation reactor is reduced to 180°C at a rate of 5°C / h, and the polymer on the gas phase inner wall of the polycondensation reactor is further stripped off by the temperature change.

[0027] (3) Adjust the pressure of the polycondensation reactor to 115 kPa.A. Set a phenol inlet at the feed end. Phenol (purity 99.6%, color ≤20 Hazen) is pumped into the polycondensation reactor and the liquid level in the polycondensation reactor is set to 30%.

[0028] (4) Raise the temperature of the polycondensation reactor to 186-187℃ and control the pressure at 115 kPa·A to vaporize some of the phenol inside the reactor. Heat the phenol for 2 hours, and when the liquid level drops to 20%, continue to add phenol until it reaches 30%, and continue heating for another 2 hours. During this period, manually rotate the agitator of the polycondensation reactor. Once you can rotate it, rotate it once every half hour.

[0029] (5) After the heating process is complete, open the hand valve on the outlet pipeline of the polycondensation reactor to discharge the material from the polycondensation reactor to the phenol evaporator. Simultaneously, take a sample at the sampling point. The phenol purity is 97%, and the color is greater than 400 Hazen. The phenol evaporator is heated by a heat transfer oil coil and continuously stirred by a stirrer. The temperature is controlled at 195℃, and the pressure is controlled at 2kPa.G. After evaporation, the phenol is condensed and recovered in the phenol condenser to the crude phenol collection tank. When the liquid level in the crude phenol collection tank reaches more than 50%, it is pumped to the phenol distillation column. The pressure of the phenol distillation column is controlled at 2.0kPa.A, the top reflux rate is 8000kg / h, the bottom temperature is 185℃, and the bottom circulation rate is 120t / h. After purification at the top of the column, part of the phenol is refluxed, and part is recycled for use in applications requiring high phenol purity, such as catalyst preparation and the working medium of the phenol vacuum system. Part of the bottom liquid from the phenol distillation column is collected and sent for incineration. After phenol recovery, the remaining material from the phenol evaporator is sent for incineration as waste liquid.

[0030] (6) Repeat steps (4) and (5) twice to perform phenol heating and phenol recovery. Samples were taken at the sampling point during the final discharge to the phenol evaporator. Analysis results showed: phenol purity 98.9%, color 150 Hazen, and the cleaning was qualified. The amount of phenol added to the polycondensation reactor was calculated: 18.4 tons of phenol were used, and 16.7 tons of phenol were recovered from the phenol distillation column, resulting in a recovery rate of 91%.

[0031] (7) Raise the temperature of the polycondensation reactor to 200℃, evacuate the reactor to 100 Pa·A, then break the vacuum to 100 kPa·A with nitrogen, repeat this process 4 times, and then cool it down to room temperature. During the cooling process, pressurize the reactor with positive pressure and replace the residual phenol with nitrogen. After cooling to room temperature, shut off the blind flange isolation system, replace the reactor with factory air, and after the analysis is qualified, hand it over to maintenance personnel for maintenance.

[0032] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. 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 method for thermal cleaning a polycarbonate polycondensation reactor, characterized in that, The following steps are included: (1) After the polycarbonate melt in the polycondensation reactor is discharged, the pressure of the polycondensation reactor is reduced and then increased, and the pressure reduction and increase process is repeated 2 times or more. (2) Cool the polycondensation reactor; (3) Add phenol to the polycondensation reactor; (4) Use phenol to heat the polycondensation reactor, observe the phenol level, and add phenol as needed; (5) After the hot cooking is completed, the phenol is recovered using a phenol recovery system; (6) Detect the phenol discharged from the polycondensation reactor outlet in step (5) and determine whether the hot cooking is qualified. If it is qualified, end the cleaning process. If it is not qualified, repeat step (4)-(5) once.

2. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 1, characterized in that, In step (1), the temperature of the polycondensation reactor is 275-285℃; The pressure reduction parameters are: pressure reduction to 50-1000 Pa·A, pressure reduction rate ≥ 1.25 kPa / h; The boosting parameters are: boosting to 100-200 kPa.A, with a boosting rate ≥ 5 kPa / h.

3. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 1, characterized in that, In step (2), the polycondensation reactor is cooled to 180-190℃ at a rate of 5-10℃ / h.

4. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 1, characterized in that, In step (3), when phenol is added, the temperature of the polycondensation reactor is controlled at 180-190℃, the pressure is >5kPa.G, and the amount of phenol injected is 20-60% of the liquid level of the polycondensation reactor.

5. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 4, characterized in that, The phenol used in step (3) has a purity of >99.5% and a color of ≤20 Hazen.

6. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 1, characterized in that, During the hot cooking process in step (4), the temperature of the polycondensation reactor is controlled at 183-200℃, the pressure is 5-60kPa.G, and the cooking time is 2-4 hours.

7. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 6, characterized in that, In step (4), the vaporization rate of liquid phenol in the polycondensation reactor is controlled to be <50% by pressure control, and the liquid level in the polycondensation reactor is >20% by adding phenol.

8. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 1, characterized in that, In step (5), the phenol recovery system includes a phenol evaporator, a crude phenol collection tower, and a phenol distillation tower connected in sequence; The temperature of the phenol evaporator is 185-200℃; The phenol distillation column has a top pressure of 1.5-2.5 kPa·A, a top reflux rate of 6000-10000 kg / h, a bottom temperature of 175-190℃, and a bottom circulation rate of 100-150 t / h.

9. The thermal cleaning method for a polycarbonate polycondensation reactor according to claim 1, characterized in that, In step (6), the criteria for determining whether the hot cooking is qualified are as follows: if the purity of phenol discharged from the outlet of the polycondensation reactor is <98.5% or the color is >200 Hazen, it is unqualified; if the purity of phenol discharged from the outlet of the polycondensation reactor is >98.5% and the color is <200 Hazen, it is qualified.