Esterification waste heat utilization system of three-kettle polyester device

Abstract

An esterification waste heat utilization system of a three-kettle polyester device, comprising a reaction module, a slurry configuration module and an EG-water separation module; the slurry configuration module comprises a slurry configuration tank, a CPEG tank and an esterification EG tank, the CPEG tank sends CPEG to the slurry configuration tank, the esterification EG tank sends esterification EG to the slurry configuration tank, and the slurry configuration tank is connected with an input PTA; the reaction module comprises an esterification kettle and an esterification heat exchanger, the esterification kettle is connected with the esterification heat exchanger in a circulation mode, and the slurry configuration tank sends configured slurry to the esterification heat exchanger; the CPEG tank sends CPEG to the bottom of a process tower, the esterification EG tank sprays esterification EG to the bottom of the process tower, and the esterification EG tank is provided with an esterification EG return pipeline connected with the bottom of the process tower. The esterification EG solid content is reduced, the slurry molar ratio and the esterification EG tank liquid level are stably controlled.

Description

Technical Field

[0001] This invention relates to the field of polyester production technology, and in particular to a waste heat utilization system for esterification in a three-reactor polyester plant. Background Technology

[0002] PET, short for polyethylene terephthalate, is currently mainly produced by the reaction of purified terephthalic acid (PTA) and ethylene glycol (EG). During the esterification stage of the polymerization process, PTA, recycled esterified EG, and CPEG are prepared into a slurry with the correct molar ratio in a slurry preparation tank. The slurry, at approximately 80°C, is fed into an esterification heat exchanger and heated to approximately 280°C. The slurry circulates within the esterification heat exchanger and esterification vessel via thermosiphon for esterification, producing esterified products and water. Esterification is a reversible reaction; therefore, to continuously shift the esterification reaction in a direction favorable to the forward reaction and achieve a higher esterification rate, water must be continuously distilled from the reaction solution. However, when distilling water, excess EG is also distilled off along with it. To allow the distilled EG to flow back into the reactor to continue participating in the reaction, a process tower is installed above the reactor to separate water and EG. The separated EG enters the esterification EG tank for slurry preparation, and the recovered esterification wastewater is sent to a stripping tower for wastewater treatment.

[0003] The high-temperature wastewater steam generated by the esterification reaction and the excess esterified EG distilled off are cooled by circulating cooling water for recycling and wastewater treatment. This results in a very large energy loss in the system. The heat energy generated by the esterification system is not only not recovered and utilized, but also consumes a large amount of circulating cooling water and the electrical energy consumed by the unit.

[0004] In Chinese patent literature, patent number CN 2015206179458, authorized and announced on January 27, 2016, describes a waste heat recovery system for esterification steam in a polyester plant. This system uses heat exchange to preheat the slurry with steam from the top of the process tower. However, the prior art still has the following unresolved technical problems: 1. The existing three-reactor polyester plant with one start and two ends has a high solids content in the esterified EG, posing a risk of failure in the process tower system and slurry preparation system; 2. When preparing the slurry, esterified EG and CPEG are used, and the raw material molar ratio is adjusted by changing the CPEG flow rate. The esterified EG flow rate is fixed, and due to the large temperature difference between CPEG and esterified EG, the temperature of the slurry preparation system fluctuates greatly, leading to unstable control of the PTA and EG raw material molar ratio. Summary of the Invention

[0005] To overcome the above-mentioned shortcomings in the prior art, the present invention provides an esterification waste heat utilization system for a three-reactor polyester unit. The system utilizes the waste heat from the esterification of EG and water vapor in the esterification reactor to raise the temperature of CPEG to 160°C, thereby reducing the heating energy required for the esterification reaction and reducing the electrical energy consumed by the circulating cooling water and the unit. At the same time, the solid content of the esterified EG is reduced by diluting it with the added CPEG, and the liquid level in the esterification EG tank is stably controlled by adjusting the flow rate of the added CPEG.

[0006] The second objective of this invention is to use esterified EG and CPEG for ratio adjustment and control during slurry preparation, so that esterified EG and CPEG are prepared in a set ratio, the slurry tank temperature is stable, and the stability of slurry molar ratio control is improved.

[0007] To achieve the above objectives, the present invention adopts the following technical solution.

[0008] A waste heat recovery system for esterification in a three-reactor polyester unit, characterized in that it includes a reaction module, a slurry preparation module, and an EG-water separation module;

[0009] The slurry preparation module includes a slurry preparation tank, a CPEG tank, and an esterified EG tank. The CPEG tank supplies CPEG to the slurry preparation tank, and the esterified EG tank supplies esterified EG to the slurry preparation tank. The slurry preparation tank is connected to an input PTA.

[0010] The reaction module includes an esterification reactor and an esterification heat exchanger. The esterification reactor is connected to the esterification heat exchanger in a loop, and the slurry preparation tank delivers the prepared slurry into the esterification heat exchanger.

[0011] The EG-water separation module includes a process tower, an esterification wastewater heat exchanger, and an esterification wastewater tank. The top of the process tower is connected to the esterification wastewater heat exchanger and the esterification wastewater tank in sequence. The esterification wastewater tank is equipped with a process tower esterification wastewater reflux pump connected to the process tower.

[0012] The CPEG tank delivers CPEG to the bottom of the process tower, while the esterified EG tank sprays esterified EG to the bottom of the process tower. The esterified EG tank is equipped with a reflux line for esterified EG at the bottom of the process tower.

[0013] By utilizing the residual heat from the esterified EG and steam in the esterification reactor to raise the CPEG to 160°C, the heating energy, circulating cooling water, and electrical energy consumed by the unit in the esterification reaction are reduced. After the CPEG added to the bottom of the process tower absorbs heat, the heat of the esterification steam rising to the process tower is reduced, the flow rate of esterification wastewater required for the process tower to separate esterified EG and esterification wastewater is reduced, and the amount of esterification wastewater steam exiting the top of the process tower is reduced. Therefore, the flow rate of cooling water required to cool the esterification wastewater heat exchanger is reduced accordingly, saving cooling energy. The CPEG at the bottom of the process tower flows into the esterification EG tank together with the sprayed esterified EG and the cooled esterified EG. The esterified EG in the esterification EG tank is diluted, and its solid content is reduced from more than 10% to less than 6%, reducing the risk of failure of the process tower system and slurry preparation system caused by esterified EG.

[0014] Preferably, the CPEG tank is connected to a CPEG delivery pump, which is equipped with a room-temperature CPEG pipeline connected to the slurry preparation tank. The esterified EG tank is connected to an esterified EG delivery pump, which is equipped with an esterified EG delivery pipeline connected to the slurry preparation tank. The CPEG delivery pipeline is equipped with a CPEG regulating valve and a CPEG flow meter. The esterified EG delivery pipeline is equipped with an esterified EG regulating valve and an esterified EG flow meter. The CPEG regulating valve, the CPEG flow meter, the esterified EG regulating valve, and the esterified EG flow meter are connected to the esterified EG and CPEG proportional regulators.

[0015] The 160℃ esterified EG and room temperature CPEG are mixed with PTA in a set ratio, stabilizing the temperature of the slurry tank, eliminating the interference of slurry temperature changes on molar ratio control, and improving the stability of slurry molar ratio control.

[0016] Preferably, a bypass CPEG line is connected to the ambient temperature CPEG line, which leads to the process tower. The bypass CPEG line is equipped with a CPEG regulating valve, which works in conjunction with an esterification EG tank level controller connected to the esterification EG tank. The esterification EG tank level SV value is set via the esterification EG tank level controller. Based on the deviation from the actual level PV value, the CPEG regulating valve is output to control the CPEG flow rate to the bottom of the process tower, i.e., to control the esterification EG flow rate to the esterification EG tank, thus achieving stable control of the esterification EG tank level.

[0017] Preferably, a spray esterification EG delivery pipeline is bypassed on the esterification EG delivery pipeline, which is connected to the bottom of the process tower. The esterification EG tank sprays esterification EG onto the process tower through the spray esterification EG delivery pipeline.

[0018] Preferably, the esterification wastewater heat exchanger is surrounded by circulating cooling water to cool the esterification steam.

[0019] Preferably, the esterification heat exchanger is equipped with a heat transfer medium to ensure reliable heating of the slurry during the esterification reaction.

[0020] As a preferred method, the liquid level of the esterified EG tank is stabilized by setting the liquid level SV value of the esterified EG tank through the liquid level controller. Based on the deviation from the actual liquid level value PV, the CPEG regulating valve is output to control the CPEG flow rate from the bypass CPEG pipeline to the bottom of the process tower, that is, to control the esterified EG flow rate to the esterified EG tank.

[0021] As a preferred option, the temperature of the slurry preparation tank is stably controlled: during slurry preparation, the ratio values ​​of the esterified EG and CPEG ratio adjusters are set so that the esterified EG and room temperature CPEG are prepared with PTA in the set ratio, and the temperature of the slurry tank is stable.

[0022] The beneficial effects of the present invention are: (1) The residual heat of the esterification steam in the esterification kettle is used to heat CPEG, which reduces the amount of heating required for the esterification reaction and saves energy;

[0023] (2) Add CPEG cooling esterification steam to the bottom of the process tower to reduce the electrical energy required to cool the waste heat of the esterification reaction and save energy;

[0024] (3) Dilute the esterified EG to reduce its solid content and reduce the risk of esterified EG causing failure to the process tower system and slurry preparation system;

[0025] (4) The liquid level control in the esterification EG tank is more stable, reducing control risks;

[0026] (5) The temperature control of the slurry tank is stable, and the stability of the slurry molar ratio control is improved. Attached Figure Description

[0027] Figure 1 This is a simplified process flow control diagram of the present invention.

[0028] In the picture:

[0029] 1-Bypass CPEG line

[0030] 2-CPEG regulating valve

[0031] 3-Esterification kettle

[0032] 4 process towers

[0033] 5-Esterified EG reflux line

[0034] 6-Esterified EG container

[0035] 7-Esterification EG Transfer Pump

[0036] 8-Esterification wastewater heat exchanger

[0037] 9-Cooling water

[0038] 10- Overflow esterification wastewater

[0039] 11-Esterification wastewater tank

[0040] 12-Process Tower Esterification Wastewater Recirculation Pump

[0041] 13-Esterification EG Tank Level Controller

[0042] 14-Esterified EG, CPEG ratio adjuster

[0043] 15-Configure CPEG flow meter

[0044] 16-Configure CPEG regulating valve

[0045] 17- PTA

[0046] 18-Configure the esterification EG control valve

[0047] 19-Configure esterified EG flow meter

[0048] 20-Slurry Preparation Tank

[0049] 21-Slurry Transfer Pump

[0050] 22-Slurry delivery pipeline

[0051] 23-Heat Media

[0052] 24-Esterification Heat Exchanger

[0053] 25-CPEG tank

[0054] 26-CPEG transfer pump

[0055] 27-Ambient Temperature CPEG Pipeline

[0056] 28-Esterified EG Delivery Pipeline

[0057] 29-Spray Esterification EG Delivery Pipeline

[0058] 30-Process tower reflux esterification wastewater

[0059] 31-Esterification wastewater vapor. Detailed Implementation

[0060] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0061] Example 1:

[0062] like Figure 1 As shown, an esterification waste heat utilization system for a three-reactor polyester unit includes a reaction module, a slurry preparation module, and a wastewater treatment module.

[0063] The slurry preparation module includes a slurry preparation tank 20, a CPEG tank 25, and an esterified EG tank 6. The CPEG tank 25 supplies CPEG to the slurry preparation tank 20, and the esterified EG tank 6 supplies esterified EG to the slurry preparation tank 20. The slurry preparation tank 20 is connected to an input PTA. CPEG tank 25 is connected to CPEG transfer pump 26, which is connected to a room-temperature CPEG pipeline 27 connected to slurry preparation tank 20. Esterification EG tank 6 is connected to esterification EG transfer pump 7, which is connected to an esterification EG transfer pipeline 28 connected to slurry preparation tank 20. CPEG transfer pipeline 28 is equipped with a CPEG regulating valve 16 and a CPEG flow meter 15. Esterification EG transfer pipeline 28 is equipped with an esterification EG regulating valve 18 and an esterification EG flow meter 19. These components are connected to an esterification EG / CPEG proportional controller 14. Slurry preparation tank 20 temperature stabilization control: During slurry preparation, the ratio value of the esterification EG / CPEG proportional controller 14 is set to ensure that esterification EG and room-temperature CPEG are mixed with PTA 17 at the set ratio, thus stabilizing the slurry tank temperature.

[0064] The reaction module includes an esterification reactor 3 and an esterification heat exchanger 24. The esterification reactor 3 is circulated and connected to the esterification heat exchanger 24. A slurry preparation tank delivers the prepared slurry into the esterification heat exchanger 24. The esterification heat exchanger 24 is equipped with a heat transfer medium 23. The slurry preparation tank is equipped with a slurry delivery pump 21, which is connected to a slurry delivery pipeline 22 that connects to the esterification heat exchanger 24. A bypass CPEG pipeline 1, which connects to the process tower 4, is connected to a room temperature CPEG pipeline 27. A CPEG regulating valve 2 is installed on the bypass CPEG pipeline 1, which is equipped with an esterification EG tank level controller 13LIC connected to the esterification EG tank 6. Esterification EG Tank 6 Level Stabilization Control: The EG level controller 13 sets the SV value of EG level in EG tank 6. Based on the deviation from the actual level value PV, the CPEG regulating valve 2 controls the CPEG flow rate from the bypass CPEG line 1 to the bottom of process tower 4, thus controlling the EG flow rate in EG tank 6. A spray EG delivery line 29 bypasses the CPEG line 1 to the EG delivery line 28. The spray EG delivery line 29 connects to the bottom of process tower 4, and EG tank 6 sprays EG onto process tower 4 via the spray EG delivery line 29. CPEG tank 25 delivers CPEG to the bottom of process tower 4. EG tank 6 is equipped with an EG return line 5 connected to the bottom of process tower 4.

[0065] The EG-water separation module includes a process tower 4, an esterification wastewater heat exchanger 8, and an esterification wastewater tank 11. The top of the process tower 4 is sequentially connected to the esterification wastewater heat exchanger 8 and the esterification wastewater tank 11. Cooling water 9 circulates around the esterification wastewater heat exchanger 8. Esterification wastewater vapor 31 output from the process tower is condensed and liquefied after passing through the heat exchanger and collected in the esterification wastewater tank. The esterification wastewater tank 11 is equipped with a process tower esterification wastewater reflux pump 12 connected to the process tower 4; the process tower esterification wastewater reflux pump 12 can deliver process tower reflux esterification wastewater 30 to the top of the process tower. The esterification wastewater tank 11 discharges overflow esterification wastewater 10.

[0066] This application has the following beneficial effects:

[0067] Automatic control: Utilize the original DCS control system, add a LIC level control program for the esterification EG tank 6; add a XIC ratio control program for esterification EG and CPEG to improve the level of automation.

[0068] Esterification waste heat utilization: The CPEG diverted from the original CPEG transfer pump 26 is delivered to the bottom of process tower 4, where it flows together with the sprayed esterified EG from the bottom of process tower 4 and the cooled esterified EG into the esterified EG tank 6 via the esterified EG return line 5. During this process, the CPEG absorbs the waste heat from the esterification steam, raising its temperature from 40℃ to 160℃. The flow rate of high-temperature esterified EG delivered to the slurry preparation tank 20 via the esterified EG transfer pump 7 increases. Since a portion of the original CPEG is diverted to the bottom of process tower 4, the flow rate of ambient-temperature CPEG entering the slurry preparation tank 20 decreases accordingly. Ultimately, the slurry temperature in the slurry preparation tank 20 rises from 80℃ to over 100℃. The heated slurry is then pumped into the esterification heat exchanger 24 via the slurry transfer pump 21, reducing the flow rate of the heat transfer medium required for the esterification reaction and saving heating energy.

[0069] Energy saving through steam cooling of esterification wastewater: After the CPEG added to the bottom of process tower 4 absorbs heat, the heat of the esterification steam rising to process tower 4 is reduced. The flow rate of esterification wastewater required for process tower 4 to separate esterified EG and esterification wastewater is reduced. Finally, the amount of esterification wastewater steam discharged from the top of process tower 4 is reduced. Therefore, the flow rate of cooling water 9 required to cool the esterification wastewater heat exchanger 8 is reduced accordingly, saving cooling energy.

[0070] Reduce the solids content of esterified EG: CPEG flows into the esterified EG tank 6 together with the sprayed esterified EG29 and the cooled esterified EG at the bottom of process tower 4. The esterified EG in the tank is diluted, and its solids content is reduced from more than 10% to less than 6%. The risk of esterified EG causing failure to the process tower 4 system and slurry preparation system is reduced.

[0071] Stable control of the liquid level in esterification EG tank 6: By controlling the liquid level of esterification EG tank 6 through LIC, the SV value of the liquid level in esterification EG tank 6 is set. Based on the deviation from the actual liquid level value PV, the CPEG flow rate controlled by the CPEG regulating valve 2 is controlled to the bottom of the process tower 4, that is, the esterification EG flow rate in esterification EG tank 6 is controlled, thereby achieving stable control of the liquid level in esterification EG tank 6.

[0072] Stabilizing slurry tank temperature: Add an esterified EG and CPEG ratio control program XIC to the DCS control system. When preparing the slurry, set the flow ratio control XIC value so that 160℃ esterified EG and room temperature CPEG are prepared with PTA 17 in the set ratio. This stabilizes the slurry tank temperature, eliminates the interference of slurry temperature changes on molar ratio control, and improves the stability of slurry molar ratio control.

Claims

1. A waste heat recovery system for esterification in a three-reactor polyester unit, characterized in that, It includes a reaction module, a slurry preparation module, and an EG-water separation module; The slurry preparation module includes a slurry preparation tank, a CPEG tank, and an esterified EG tank. The CPEG tank supplies CPEG to the slurry preparation tank, and the esterified EG tank supplies esterified EG to the slurry preparation tank. The slurry preparation tank is connected to an input PTA. The reaction module includes an esterification reactor and an esterification heat exchanger. The esterification reactor is connected to the esterification heat exchanger in a loop, and the slurry preparation tank delivers the prepared slurry into the esterification heat exchanger. The EG-water separation module includes a process tower, an esterification wastewater heat exchanger, and an esterification wastewater tank. The top of the process tower is connected to the esterification wastewater heat exchanger and the esterification wastewater tank in sequence. The esterification wastewater tank is equipped with a process tower esterification wastewater reflux pump connected to the process tower. The CPEG tank delivers CPEG to the bottom of the process tower, and the esterified EG tank sprays esterified EG to the bottom of the process tower. The esterified EG tank is equipped with an esterified EG reflux pipeline connected to the bottom of the process tower. The esterified EG tank is equipped with a level controller, which dynamically adjusts the CPEG flow rate input to the process tower to keep the solid content of the esterified EG stable below 6%.

2. The esterification waste heat utilization system of a three-reactor polyester unit according to claim 1, characterized in that, The CPEG tank is connected to a CPEG delivery pump, which is connected to a room-temperature CPEG pipeline that connects to the slurry preparation tank. The esterified EG tank is connected to an esterified EG delivery pump, which is connected to an esterified EG delivery pipeline that connects to the slurry preparation tank. The CPEG delivery pipeline is equipped with a CPEG regulating valve and a CPEG flow meter. The esterified EG delivery pipeline is equipped with an esterified EG regulating valve and an esterified EG flow meter. The CPEG regulating valve, the CPEG flow meter, the esterified EG regulating valve, and the esterified EG flow meter are connected to an esterified EG and a CPEG proportional regulator.

3. The esterification waste heat utilization system of a three-reactor polyester unit according to claim 2, characterized in that, The ambient temperature CPEG pipeline is connected to a bypass CPEG pipeline that leads to the process tower; the bypass CPEG pipeline is equipped with a CPEG regulating valve, which is in conjunction with a liquid level controller for the esterification EG tank connected to the esterification EG tank.

4. The esterification waste heat utilization system of a three-reactor polyester unit according to claim 2, characterized in that, The esterified EG delivery pipeline is bypassed by a sprayed esterified EG delivery pipeline, which connects to the bottom of the process tower. The esterified EG tank sprays esterified EG onto the process tower through the sprayed esterified EG delivery pipeline.

5. The esterification waste heat utilization system of a three-reactor polyester unit according to claim 1, characterized in that, The esterification wastewater heat exchanger is surrounded by circulating cooling water.

6. The esterification waste heat utilization system of a three-reactor polyester unit according to claim 1, characterized in that, The esterification heat exchanger is equipped with a heat medium.

7. A waste heat recovery system for esterification in a three-reactor polyester unit according to claim 3, characterized in that, The liquid level stability control of the esterification EG tank is as follows: the liquid level SV value of the esterification EG tank is set by the liquid level controller of the esterification EG tank. According to the deviation from the actual liquid level value PV, the CPEG regulating valve is output to control the CPEG flow rate from the bypass CPEG pipeline to the bottom of the process tower, that is, to control the esterification EG flow rate to the esterification EG tank.

8. A waste heat recovery system for esterification in a three-reactor polyester unit according to claim 2, characterized in that, Temperature stability control of the slurry preparation tank: During slurry preparation, the ratio values ​​of the esterified EG and CPEG ratio adjusters are set so that the esterified EG and room temperature CPEG are prepared with PTA in the set ratio, and the temperature of the slurry tank is stable.

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