PTA process waste heat recovery system, PTA process system and PTA process

Abstract

This invention discloses a PTA process waste heat recovery system, a PTA process system, and a PTA process. The PTA process waste heat recovery system includes a waste heat recovery unit, a waste heat condensate treatment unit, and a non-condensable component treatment unit. The low-temperature waste heat generated by the PTA process is exchanged through the waste heat recovery unit, and the generated waste heat condensate enters the waste heat condensate treatment unit for separation. The separated non-condensable components are discharged through the non-condensable component treatment unit. The PTA process system includes the aforementioned waste heat recovery system. This waste heat recovery system fully utilizes various types of waste heat generated by the process, especially low-temperature waste heat, improving energy utilization efficiency and reducing pollution emissions. Combining the process system containing the aforementioned waste heat recovery system with corresponding process methods can reduce the overall system energy consumption by more than 4%. Furthermore, this system can be easily implemented by modifying existing systems without requiring additional complex equipment.

Description

Technical Field

[0001] This invention relates to a PTA process waste heat recovery system, a PTA process system, and a PTA process, and more particularly to a PTA process waste heat recovery system, a PTA process system, and a PTA process that can improve energy utilization and reduce pollution emissions. Background Technology

[0002] PTA is short for purified terephthalic acid, which is obtained by oxidizing and purifying p-xylene (PX) and then hydrogenating it. During PTA preparation, the refining section generates a certain amount of low-temperature heat sources. These heat sources are typically low-temperature solutions and low-pressure steam, such as the mother liquor and washing liquid produced during the solid-liquid separation process in the PTA refining section, as well as the low-pressure saturated steam generated by the flash evaporation of these mother liquor and washing liquid.

[0003] The existing process mainly involves flash evaporation of the low-temperature mother liquor and washing liquid at atmospheric pressure. The waste heat steam generated by flash evaporation is directly discharged. The waste heat from the low-temperature mother liquor, washing liquid, and low-pressure steam is not recovered, resulting in a waste of heat energy and water resources. In addition, since the waste heat steam contains a small amount of PT acid, direct discharge will also cause environmental pollution. Summary of the Invention

[0004] Purpose of the invention: In view of the problems of insufficient energy utilization and pollution in existing PTA production, the present invention aims to provide a PTA process waste heat recovery system, PTA process system and PTA process that can improve energy utilization and reduce pollution emissions.

[0005] Technical solution: As the first aspect of the present invention, the PTA process waste heat recovery system of the present invention includes a waste heat recovery unit, a waste heat condensate treatment unit and a non-condensable component treatment unit; the low-temperature waste heat of the PTA process is heat exchanged through the waste heat recovery unit, and the generated waste heat condensate enters the waste heat condensate treatment unit for separation, and the separated non-condensable components are discharged through the non-condensable component treatment unit.

[0006] The main design concept of the PTA process waste heat recovery system of this invention is to collect the low-temperature mother liquor and low-pressure steam generated in the PTA refining unit during the PTA production process and send them to the waste heat recovery system. The system converts the low-grade heat in the low-temperature mother liquor and low-pressure steam into high-grade heat, and uses this high-grade heat to generate low-pressure steam, which is ultimately sent to the PTA unit's steam network for heating materials, thereby reducing the overall energy consumption of the PTA process system. The recovered low-temperature waste heat condenses to form clean waste heat condensate, which is collected and returned to the process system for recycling. Trace amounts of non-condensable components are discharged to a safe location.

[0007] The waste heat recovery unit is a heat pump unit, which is equipped with a regenerator or evaporator with a shell-and-tube heat exchanger.

[0008] Some waste heat steam contains a very small amount of hydrogen (about 0.1 wt%), which may affect the heat exchange effect and pose a safety hazard. Therefore, the regenerator or evaporator inside the heat pump unit is designed as a shell-and-tube heat exchanger with a gas-liquid separation function in the rear end. After the hydrogen slowly accumulates in the upper part of the rear end, it is discharged from the unit according to the pressure control and sent to the combustion device. The liquid phase in the lower part of the rear end enters the downstream equipment.

[0009] Furthermore, the waste heat steam pipeline of the regenerator or evaporator is an internally threaded pipeline and / or coated with a hydrophilic coating.

[0010] In view of the special case that the recovered low-temperature waste heat steam contains trace amounts of non-condensable gases (mainly easily crystallizing and corrosive PT acid), this invention improves the low-temperature steam pipeline of the heat pump unit in order to recover the low-temperature steam waste heat generated by the PTA refining unit.

[0011] First, measures such as increasing the pipe diameter and regularly performing alkaline washing, water washing, and steam purging are implemented to ensure the heat exchange efficiency of the heat pump unit. Second, the waste heat steam pipelines of the regenerator and evaporator within the heat pump unit are replaced with high-efficiency internally threaded pipes to increase the inner surface area of ​​the pipes. Simultaneously, a hydrophilic coating can be applied to the inner wall of the low-temperature steam pipeline to enhance its hydrophilicity, thereby improving the heat transfer coefficient of the waste heat steam pipeline and increasing the recovery of low-temperature steam waste heat, overcoming the adverse effects of trace amounts of non-condensable gases in the low-temperature waste heat steam. Furthermore, the heat pump unit, in conjunction with a waste heat condensate treatment unit and a non-condensable component treatment unit, separates and removes trace amounts of non-condensable gases, preventing their accumulation within the waste heat steam condensate treatment unit. More specifically, the waste heat steam condensate enters a waste heat condensate buffer tank for flash evaporation; the flash steam enters a vapor phase condenser for partial condensation and separation of non-condensable gases, which are then extracted and removed by a vacuum pump.

[0012] Furthermore, the waste heat recovery unit is equipped with a thermosiphon flash evaporation tank.

[0013] Hot water supply utilizes the siphon effect instead of conventional forced circulation, which can further save on pump equipment investment and pump power consumption.

[0014] As a second aspect of the present invention, the PTA process system of the present invention includes an oxidation unit and a refining unit, and also includes the above-mentioned PTA process waste heat recovery system for recovering the low-temperature waste heat of the refining unit, specifically the low-temperature mother liquor and low-pressure steam generated by the refining unit.

[0015] Furthermore, the refining unit is equipped with a waste heat steam venting pipeline, and the waste heat steam venting pipeline has a branch connected to the PTA process waste heat recovery system.

[0016] The present invention extends a new pipeline from the original venting pipeline to transport the waste heat steam to be treated to a waste heat recovery system for heat recovery.

[0017] Furthermore, the waste heat steam venting pipeline and the branch line connected to the PTA process waste heat recovery system are equipped with interlocking control devices.

[0018] The original venting pipeline was equipped with a remotely controlled on / off valve (FO). During waste heat steam recovery operations, this valve was closed; under abnormal circumstances, it opened as a backup switching process to ensure stable operation of upstream process equipment. This invention additionally installs a remotely controlled on / off valve (FC) at the inlet of the heat pump unit to control the waste heat steam feed. A pressure alarm is installed on the waste heat steam source pipeline to monitor the waste heat steam pressure. When the waste heat steam pressure in this pipeline section is too high or too low, the alarm is triggered, further initiating interlocking control.

[0019] Furthermore, the PTA process waste heat recovery system is equipped with a liquid phase inlet and a vapor phase inlet.

[0020] Based on the different vapor-liquid phases and cleanliness levels of the materials to be recovered, the oxidation reaction mother liquor and low-pressure steam separated by the filter in the refining unit are sent to two inlets of the waste heat recovery system. The low-cleanliness oxidation reaction mother liquor recovers its sensible heat through the heat pump unit in the low-temperature waste heat recovery unit and generates high-grade low-pressure steam. The oxidation reaction mother liquor after heat recovery is returned to the original process system for further purification. At the same time, the high-cleanliness low-pressure steam recovers its sensible and latent heat through the heat pump unit in the low-temperature waste heat recovery unit and generates high-grade low-pressure steam. The low-pressure steam after heat recovery is condensed into steam condensate, and non-condensable components are removed. It is then directly returned to the refining solvent system for recycling.

[0021] Similarly, based on the different vapor-liquid phases and cleanliness levels of the materials to be recovered, the hydrogenation reaction mother liquor and low-pressure steam separated by the centrifugal separator in the refining unit are respectively sent to the two inlets of the waste heat recovery system. The low-cleanliness hydrogenation reaction mother liquor recovers its sensible heat through the heat pump unit in the waste heat recovery unit and generates high-grade low-pressure steam. The hydrogenation reaction mother liquor after heat recovery is returned to the original process system for further purification. At the same time, the high-cleanliness low-pressure steam recovers its sensible and latent heat through the heat pump unit in the waste heat recovery unit and generates high-grade low-pressure steam. The low-pressure steam after heat recovery is condensed into steam condensate, and non-condensable components are removed. It is then directly returned to the refining solvent system for recycling.

[0022] As a third aspect of the present invention, the process for preparing PTA using the above-mentioned PTA process system includes an oxidation process and a refining process, and further includes a waste heat recovery process for recovering the low-temperature mother liquor and low-pressure steam from the refining process.

[0023] The pressure of the low-pressure steam is -7 kPaG to 12 kPaG, preferably -6 kPaG to 11 kPaG, and more preferably -5 kPaG to 10 kPaG.

[0024] The method for recovering the low-temperature mother liquor and low-pressure steam from the refining process is as follows: the operating temperature of the low-temperature mother liquor and low-pressure steam is 91-103°C, the operating pressure is 0.073-0.113 MPaA, and the mass ratio of the low-temperature mother liquor to the low-pressure steam is 200-600:1.

[0025] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0026] (1) The PTA waste heat recovery system makes full use of the various waste heat generated by the process, especially the low-temperature waste heat. At the same time, the system avoids the adverse factors of energy recovery through equipment improvement, improves energy utilization, reduces energy consumption, and also reduces pollution emissions.

[0027] (2) When this PTA process system is combined with the corresponding process method, the overall energy consumption of the system can be reduced by more than 4%;

[0028] (3) The process system can be modified on the original system, which is convenient to implement and does not require the addition of complex equipment. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the PTA process waste heat recovery system of the present invention;

[0030] Figure 2 This is a schematic diagram of the regenerator and evaporator structure of the present invention;

[0031] Figure 3 This is a schematic diagram of the PTA process system structure of the present invention. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0033] Example 1: PTA Waste Heat Recovery System

[0034] like Figure 1As shown, the PTA process waste heat recovery system of the present invention includes a waste heat recovery unit 2, a waste heat condensate treatment unit, and a non-condensable component treatment unit. The waste heat recovery unit 2 is a heat pump unit, specifically comprising a flash tank 21, a regenerator 22, an evaporator 23, a heat exchanger 24, a condenser, and an absorber. The heat pump unit is a lithium bromide Class II heat pump unit. The waste heat condensate treatment unit mainly includes a waste heat condensate buffer tank 3, a vapor phase condenser 4, and a waste heat condensate pump 6. The non-condensable component treatment unit mainly includes a vacuum pump 5, which is a liquid ring vacuum pump. The waste heat condensate buffer tank 3 is designed for full vacuum, with its top connected to the vacuum pump 5 via the vapor phase condenser 4 to maintain a certain negative pressure. The normal operating pressure is -20 to -12 kPaG, preventing the pressure of the waste heat steam source, which operates at atmospheric pressure, from rising and causing fluctuations in upstream production.

[0035] To achieve effective waste heat recovery, in the aforementioned heat pump units, to avoid the influence of hydrogen, such as Figure 2 As shown, the regenerator 22 and evaporator 23's rear end caps utilize shell-and-tube heat exchangers 25. These heat exchangers, with their gas-liquid separation function, allow trace amounts of hydrogen from the waste heat steam to be slowly enriched at the upper part of the rear end caps. This hydrogen is then discharged from the unit under pressure control and sent to the combustion unit, while the liquid phase at the lower part of the rear end caps enters downstream equipment. Simultaneously, the heat pump unit's piping uses Φ25mm x 2mm stainless steel heat exchange tubes, which are regularly cleaned with alkali, water, and steam. The waste heat steam pipes of the regenerator 22 and evaporator 23 are replaced with high-efficiency internally threaded pipes, or further coated with hydrophilic coatings to improve the heat transfer coefficient of the heat pump unit's waste heat steam piping, increase the recovery of low-temperature steam waste heat, and overcome the adverse effects of trace amounts of non-condensable gases in the low-temperature waste heat steam. Furthermore, the heat pump unit is also equipped with a thermosiphon flash tank 21 to further reduce system energy consumption.

[0036] The working principle of the above-mentioned PTA waste heat recovery system is as follows:

[0037] The PTA unit's boiler water system supplies boiler water 10 (temperature: 130-150°C) to the thermosiphon flash tank 21 in the waste heat recovery unit 2. The recovered low-temperature waste heat is used to heat the flash boiler water, generating high-grade steam 11 (pressure: 0.27 MPaG), which is then sent to the PTA unit's steam network. The circulating water 12 supplied by the PTA unit's circulating water system condenses the chilled water in the waste heat recovery unit 2 (circulating water temperature: 33°C), and the circulating return water 13 returns it to the circulating water system.

[0038] After heat recovery, the waste heat vapor condensate 14 flows by gravity into the waste heat condensate buffer tank 3, where it flashes under negative pressure. The flash vapor 15 enters the vapor phase condenser 4 for condensation, and the condensate 16 (temperature: 40℃) flows by gravity back to the waste heat condensate buffer tank 3. The non-condensable gas 17 is extracted by the vacuum pump 5 and discharged to a safe location 18. The liquid phase 19 at the bottom of the waste heat condensate buffer tank 3 is pressurized by the waste heat condensate pump 6 and returned to the PTA unit's refining solvent system 20 for reuse.

[0039] Example 2: PTA Process System

[0040] like Figure 3 As shown, the PTA process system of the present invention includes an oxidation unit and a refining unit, and also includes the aforementioned PTA process waste heat recovery system 325. The oxidation unit mainly includes a raw material mixing tank 311 and an oxidation reactor 312, while the refining unit mainly includes a first crystallizer 313, a filter 314, a first dryer 315, a scrubber 316, a hydrogenation reactor 321, a second crystallizer 322, a centrifuge 323, and a second dryer 324. Figure 1 As shown, the PTA process waste heat recovery system 325 is connected to the PTA process system via a branch 9 on the waste heat venting pipeline 8 of the refining unit. The venting pipeline 8 is equipped with a remote control switch valve (FO), and a remote control switch valve (FC) is installed at the inlet of the heat pump unit, controlled by an interlocking device. Based on the material phase and cleanliness, the heat pump unit is equipped with a liquid phase inlet and a vapor phase inlet to receive the oxidation mother liquor separated by the filter 314, the hydrogenation reaction mother liquor separated by the centrifuge 323, and the low-pressure vapors generated by each.

[0041] The working principle of the above PTA process system is as follows:

[0042] Paraxylene feedstock A2, main catalyst A3 (cobalt acetate and manganese acetate solution, mass ratio of cobalt acetate:manganese acetate = 0.34–1.66:1), co-catalyst A4 (tetrabromoethane, concentration 46–88 wt%), and solvent A5 (acetic acid, concentration 34–80 wt%) from outside the boundary are fed into raw material mixing tank 311 for material mixing. The mixed process material B1 and air A1 from outside the boundary are sent into oxidation reactor 312 for oxidation reaction (reaction temperature: 194–248℃, reaction pressure: 2.0–3.2 MPaA, residence time: 0.6–1.8 hours). The reacted process material B2 is sent to the first crystallizer 313 for material crystallization separation. The crystallized process material B3 is sent to the filter 314 for material filtration separation. The filtered process material B4 is sent to the first dryer 315 for material drying treatment. The dried process material B5 is sent to the scrubber 316 for material washing treatment.

[0043] The washed process material B6 and hydrogen gas A6 from outside the boundary are fed into the hydrogenation reactor 321 for hydrogenation reaction (reaction temperature: 274~292℃, reaction pressure: 7.1~7.9MPaA, hydrogen partial pressure: 0.6~1.2MPaA, catalyst: palladium on carbon catalyst). The process material B7 after the reaction is sent to the second crystallizer 322 for material crystallization separation. The crystallized process material B8 is sent to the centrifuge 323 for centrifugal separation. The separated process material B9 is sent to the second dryer 324 for material drying treatment. The dried terephthalic acid product D2 is sent outside the boundary.

[0044] The oxidation reaction mother liquor and low-pressure steam C1 separated by filter 314, and the hydrogenation reaction mother liquor and low-pressure steam C2 separated by centrifuge 323, are sent to the PTA process waste heat recovery system 325. A heat pump unit converts the low-grade heat in the low-temperature mother liquor and low-pressure steam into high-grade heat, generating high-grade steam 11, which is then sent to the external PTA unit steam network. The treatment of the waste heat steam condensate 14 after heat recovery is the same as the corresponding process in the PTA process waste heat recovery system described above. The operating temperature of the low-temperature mother liquor and low-pressure steam used for PTA unit waste heat recovery is 91–103°C, the operating pressure is 0.073–0.113 MPaA, and the mass ratio is low-temperature mother liquor: low-pressure steam = 200–600:1. The operating temperature of the high-grade steam 11 generated by the PTA process waste heat recovery system 325 is 135–145°C, and the operating pressure is 0.32–0.42 MPaA.

[0045] To prevent overpressure in the waste heat steam system due to operational malfunctions, misoperation, or pipeline blockage, or negative pressure in the waste heat steam system due to vacuum system control failure, the original waste heat steam venting system is retained, and an interlock control circuit is set up. Upon interlock triggering, the waste heat steam is vented, the vacuum pump is shut down, and the waste heat steam heat recovery system is disconnected to ensure safe operation of the unit. Specifically, during waste heat steam heat recovery operations in the aforementioned PTA process system, the remote control switch valve (FO) on the waste heat steam venting pipeline 8 of the refining unit is in the closed state; under abnormal circumstances, this control valve opens as a backup switching process to ensure stable operation of the upstream process equipment 1. A branch line 9 is led out from the original venting pipeline to transport the waste heat steam to be processed to the waste heat recovery unit 2 for heat recovery. A remote control switch valve (FC) is installed at the inlet of the heat pump unit on branch line 9 to control the waste heat steam feed to the heat pump unit. The waste heat steam source pipeline 7 is equipped with a pressure alarm device to monitor the waste heat steam pressure. When the waste heat steam pressure in this pipeline section is too high or too low, an alarm will be triggered, which will further trigger interlock control.

[0046] Example 3

[0047] The nominal capacity of the terephthalic acid (PTA) unit is 225,000 tons / year, with an annual operating time of 8,000 hours. The low-temperature waste heat steam emitted from the PTA unit is collected and converted into high-grade steam using a lithium bromide Class II heat pump unit for efficient recovery and heating. This high-grade steam is then fed into the unit's steam network, saving on fresh steam consumption. Simultaneously, the condensate from the recovered waste heat steam is returned to the circulating solvent tank of the PTA unit's refining solvent system, saving on fresh deionized water consumption. The process parameters for this embodiment are as follows: waste heat flash steam pressure -3 kPaG, waste heat flash steam flow rate 4.78 tons / hour, recovered steam condensate temperature 93°C, supplementary boiler feedwater temperature 133°C, supplementary boiler feedwater flow rate 1.97 tons / hour, high-grade steam generation pressure 0.27 MPaG, and high-grade steam generation temperature 140°C. Using the PTA process described in this embodiment, a high-grade steam generation rate of 1.97 tons / hour and a steam condensate recovery rate of 4.78 tons / hour were obtained.

[0048] Example 4

[0049] The process flow in this embodiment is the same as in Embodiment 3, except for the change in nominal capacity and process parameters. The nominal capacity of the terephthalic acid (PTA) unit is 800,000 tons / year, and the annual operating time is 8,000 hours. The process parameters in this embodiment are as follows: waste heat flash steam pressure 0 kPaG, waste heat flash steam flow rate 17.00 tons / hour, recovered steam condensate temperature 94℃, supplementary boiler feedwater temperature 137℃, supplementary boiler feedwater flow rate 7.00 tons / hour, high-grade steam generation pressure 0.27 MPaG, and high-grade steam generation temperature 140℃. Using the PTA process method described in this embodiment, a high-grade steam generation rate of 7.00 tons / hour and a recovered steam condensate flow rate of 17.00 tons / hour are obtained.

[0050] Example 5

[0051] The process flow in this embodiment is the same as in Embodiment 3, except for the change in nominal capacity and process parameters. The nominal capacity of the terephthalic acid (PTA) unit is 1.2 million tons / year, and the annual operating time is 8000 hours. The process parameters in this embodiment are as follows: waste heat flash steam pressure 3 kPaG, waste heat flash steam flow rate 25.50 tons / hour, recovered steam condensate temperature 96℃, supplementary boiler feedwater temperature 143℃, supplementary boiler feedwater flow rate 10.50 tons / hour, high-grade steam generation pressure 0.27 MPaG, and high-grade steam generation temperature 140℃. Using the PTA process method described in this embodiment, a high-grade steam generation rate of 10.50 tons / hour and a recovered steam condensate flow rate of 25.50 tons / hour are obtained.

[0052] Example 6

[0053] The process flow in this embodiment is the same as in Embodiment 3, except for the change in nominal capacity and process parameters. The nominal capacity of the terephthalic acid (PTA) unit is 2.5 million tons / year, and the annual operating time is 8000 hours. The process parameters in this embodiment are as follows: waste heat flash steam pressure 9 kPaG, waste heat flash steam flow rate 53.13 tons / hour, recovered steam condensate temperature 97°C, supplementary boiler feedwater temperature 147°C, supplementary boiler feedwater flow rate 21.88 tons / hour, high-grade steam generation pressure 0.27 MPaG, and high-grade steam generation temperature 140°C. Using the PTA process method described in this embodiment, a high-grade steam generation rate of 21.88 tons / hour and a recovered steam condensate flow rate of 53.13 tons / hour are obtained.

[0054] Example 7

[0055] The process flow in this embodiment is the same as in Embodiment 3, except for the change in nominal capacity and process parameters. The nominal capacity of the terephthalic acid (PTA) unit is 800,000 tons / year, and the annual operating time is 8,000 hours. The process parameters in this embodiment are as follows: waste heat flash steam pressure -7 kPaG, waste heat flash steam flow rate 16.83 tons / hour, recovered steam condensate temperature 92°C, supplementary boiler feedwater temperature 130°C, supplementary boiler feedwater flow rate 6.93 tons / hour, high-grade steam generation pressure 0.27 MPaG, and high-grade steam generation temperature 140°C. Using the PTA process method described in this embodiment, a high-grade steam generation rate of 6.93 tons / hour and a recovered steam condensate flow rate of 16.83 tons / hour are obtained.

[0056] Example 8

[0057] The process flow in this embodiment is the same as in Embodiment 3, except for the change in nominal capacity and process parameters. The nominal capacity of the terephthalic acid (PTA) unit is 800,000 tons / year, and the annual operating time is 8,000 hours. The process parameters in this embodiment are as follows: waste heat flash steam pressure 12 kPaG, waste heat flash steam flow rate 17.20 tons / hour, recovered steam condensate temperature 98℃, supplementary boiler feedwater temperature 150℃, supplementary boiler feedwater flow rate 7.08 tons / hour, high-grade steam generation pressure 0.27 MPaG, and high-grade steam generation temperature 140℃. Using the PTA process method described in this embodiment, a high-grade steam generation rate of 7.08 tons / hour and a recovered steam condensate flow rate of 17.20 tons / hour are obtained.

[0058] Comparative Example 1

[0059] The nominal capacity of the existing terephthalic acid (PTA) unit is 75,000 tons / year. The low-temperature mother liquor and washing liquid are subjected to atmospheric pressure flash evaporation. The waste heat steam generated by flash evaporation is treated by direct discharge. The waste heat of the low-temperature mother liquor, washing liquid, and low-pressure steam is not recovered. The process parameters are shown in Table 1. As a result, the steam consumption of the PTA unit is 12.6 tons / hour, and the comprehensive energy consumption of the PTA unit is 259.2 kg·oil / t·PTA.

[0060] Example 9

[0061] The nominal capacity of the terephthalic acid (PTA) unit is 75,000 tons / year. The low-grade heat in the low-temperature mother liquor and low-pressure steam is converted into high-grade heat by a lithium bromide heat pump unit, and the generated low-pressure steam is sent to the steam network of the PTA unit. The process parameters are shown in Table 2. As a result, the steam consumption of the PTA unit is reduced to 11.3 tons / hour, and the comprehensive energy consumption of the PTA unit is 250.6 kg·oil / t·PTA, which is a relative reduction of 3.32% in the comprehensive energy consumption of the PTA unit.

[0062] Comparative Example 2

[0063] The nominal capacity of the existing terephthalic acid (PTA) unit is 400,000 tons / year. The low-temperature mother liquor and washing liquid are subjected to atmospheric pressure flash evaporation. The waste heat steam generated by flash evaporation is treated by direct discharge. The waste heat of the low-temperature mother liquor, washing liquid and low-pressure steam is not recovered. The process parameters are shown in Table 1. As a result, the steam consumption of the PTA unit is 45.0 tons / hour, and the comprehensive energy consumption of the PTA unit is 174.4 kg·oil / t·PTA.

[0064] Example 10

[0065] The nominal capacity of the terephthalic acid (PTA) unit is 400,000 tons / year. The low-grade heat in the low-temperature mother liquor and low-pressure steam is converted into high-grade heat by a lithium bromide heat pump unit, and the low-pressure steam is sent to the steam network of the PTA unit. The process parameters are shown in Table 2. As a result, the steam consumption of the PTA unit is reduced to 39.8 tons / hour, and the comprehensive energy consumption of the PTA unit is 167.7 kg·oil / t·PTA, which is a relative reduction of 3.84% in the comprehensive energy consumption of the PTA unit.

[0066] Comparative Example 3

[0067] The nominal capacity of the existing terephthalic acid (PTA) unit is 1 million tons / year. The low-temperature mother liquor and washing liquid are subjected to atmospheric pressure flash evaporation. The waste heat steam generated by flash evaporation is treated by direct discharge. The waste heat of the low-temperature mother liquor, washing liquid and low-pressure steam is not recovered. The process parameters are shown in Table 1. As a result, the steam consumption of the PTA unit is 92.8 tons / hour, and the comprehensive energy consumption of the PTA unit is 145.8 kg·oil / t·PTA.

[0068] Example 11

[0069] This embodiment relates to a system and method for reducing the overall energy consumption of a PTA plant. The nominal capacity of the terephthalic acid PTA plant is 1 million tons / year. The low-grade heat in the low-temperature mother liquor and low-pressure steam is converted into high-grade heat by a lithium bromide heat pump unit, and the low-pressure steam is sent to the steam network of the PTA plant. The process parameters are shown in Table 2. As a result, the steam consumption of the PTA plant is reduced to 80.7 tons / hour, and the overall energy consumption of the PTA plant is 139.7 kg·oil / t·PTA, which is a relative reduction of 4.23% in the overall energy consumption of the PTA plant.

[0070] Table 1. Process parameters and results of Comparative Examples 1-3

[0071]

[0072]

[0073] Table 2. Process parameters and results of Examples 9-11

[0074]

[0075]

[0076] Comparative Example 4

[0077] The nominal capacity of the existing terephthalic acid PTA plant is 1 million tons / year. The low-temperature mother liquor and washing liquid are subjected to atmospheric pressure flash evaporation. The waste heat steam generated by flash evaporation is treated by direct discharge. The waste heat of the low-temperature mother liquor, washing liquid and low-pressure steam is not recovered. The process parameters are shown in Table 3. As a result, the steam consumption of the PTA plant is 94.9 tons / hour, and the comprehensive energy consumption of the PTA plant is 146.0 kg·oil / t·PTA.

[0078] Example 12

[0079] The nominal capacity of the terephthalic acid (PTA) unit is 1 million tons / year. The low-grade heat in the low-temperature mother liquor and low-pressure steam is converted into high-grade heat by a lithium bromide heat pump unit, and the generated low-pressure steam is sent to the steam network of the PTA unit. The process parameters are shown in Table 4. As a result, the steam consumption of the PTA unit is reduced to 83.6 tons / hour, and the comprehensive energy consumption of the PTA unit is 140.5 kg·oil / t·PTA, which is a relative reduction of 3.76% in the comprehensive energy consumption of the PTA unit.

[0080] Comparative Example 5

[0081] The nominal capacity of the existing terephthalic acid (PTA) unit is 2.4 million tons / year. The low-temperature mother liquor and washing liquid are subjected to atmospheric pressure flash evaporation. The waste heat steam generated by flash evaporation is treated by direct discharge. The waste heat of the low-temperature mother liquor, washing liquid and low-pressure steam is not recovered. The process parameters are shown in Table 3. As a result, the steam consumption of the PTA unit is 219.0 tons / hour, and the comprehensive energy consumption of the PTA unit is 142.6 kg·oil / t·PTA.

[0082] Example 13

[0083] The nominal capacity of the terephthalic acid (PTA) unit is 2.4 million tons per year. The low-grade heat in the low-temperature mother liquor and low-pressure steam is converted into high-grade heat by a lithium bromide heat pump unit, and the generated low-pressure steam is sent to the steam network of the PTA unit. The process parameters are shown in Table 4. As a result, the steam consumption of the PTA unit is reduced to 190.8 tons / hour, and the comprehensive energy consumption of the PTA unit is 136.7 kg·oil / t·PTA, which is a relative reduction of 4.11% in the comprehensive energy consumption of the PTA unit.

[0084] Comparative Example 6

[0085] The nominal capacity of the existing terephthalic acid (PTA) unit is 3.3 million tons / year. The low-temperature mother liquor and washing liquid are subjected to atmospheric pressure flash evaporation. The waste heat steam generated by flash evaporation is treated by direct discharge. The waste heat of the low-temperature mother liquor, washing liquid and low-pressure steam is not recovered. The process parameters are shown in Table 3. As a result, the steam consumption of the PTA unit is 284.6 tons / hour, and the comprehensive energy consumption of the PTA unit is 135.0 kg·oil / t·PTA.

[0086] Example 14

[0087] The nominal capacity of the terephthalic acid (PTA) unit is 3.3 million tons per year. The low-grade heat in the low-temperature mother liquor and low-pressure steam is converted into high-grade heat by a lithium bromide heat pump unit, and the low-pressure steam is sent to the steam network of the PTA unit. The process parameters are shown in Table 4. As a result, the steam consumption of the PTA unit is reduced to 245.9 tons / hour, and the comprehensive energy consumption of the PTA unit is 129.2 kg·oil / t·PTA, which is a relative reduction of 4.34% in the comprehensive energy consumption of the PTA unit.

[0088] Appendix Table 3: Process parameters and results for Comparative Examples 4–6

[0089] terephthalic acid PTA unit Comparative Example 4 Comparative Example 5 Comparative Example 6 PTA unit nominal capacity (10,000 tons / year) 100 240 330 Oxidation reactor (12) reaction temperature (°C) 248 223 235 Oxidation reactor (12) reaction pressure (MPaA) 3.2 2.7 3.0 Oxidation reactor (12) residence time (hours) 1.8 1.3 1.5 Hydrogenation reactor (21) reaction temperature (°C) 292 284 288 Hydrogenation reactor (21) reaction pressure (MPaA) 7.9 7.6 7.7 Hydrogen partial pressure (MPaA) of hydrogen in hydrogenation reactor (21) 1.2 1.0 1.1 Mother liquor and steam operating temperatures (°C) 103 98 100 Mother liquor and steam operating pressure (MPaA) 0.113 0.097 0.105 Low-temperature mother liquor : low-pressure vapor ratio (m : m) 600 420 500 Unit consumption of circulating water (tCW / tPTA) 236 231 218 Circulating water consumption (t / hr) 29854 69300 89925 Comprehensive energy consumption per unit of circulating water (kgoil / tPTA) 14.16 13.86 13.08 Electricity consumption per unit (kWh / tPTA) 340 333 316 Electricity consumption (kWh / hr) 43010 99900 130350 Comprehensive energy consumption per unit of electricity (kgoil / tPTA) 74.8 73.3 69.5 Steam consumption per unit (tST / tPTA) 0.75 0.73 0.69 Steam consumption (t / hr) 94.9 219.0 284.6 Comprehensive energy consumption per unit of steam (kgoil / tPTA) 57.0 55.5 52.4 Comprehensive energy consumption of PTA unit (kgoil / tPTA) 146.0 142.6 135.0

[0090] Table 4. Process parameters and results of Examples 12-14

[0091]

[0092]

Claims

1. A PTA process system comprising an oxidation unit and a refining unit, characterized in that, It also includes a PTA process waste heat recovery system for recovering the low-temperature waste heat of the refining unit; the PTA process waste heat recovery system includes a waste heat recovery unit, a waste heat condensate treatment unit, and a non-condensable component treatment unit. The low-temperature waste heat generated by the PTA process is exchanged through the waste heat recovery unit, and the resulting waste heat condensate enters the waste heat condensate treatment unit for separation. The separated non-condensable components are discharged through the non-condensable component treatment unit. The PTA process waste heat recovery system is equipped with a liquid phase inlet and a vapor phase inlet. The oxidation reaction mother liquor and low-pressure steam separated by the filter of the refining unit filter, and the hydrogenation reaction mother liquor and low-pressure steam separated by the centrifugal separator of the refining unit are respectively sent to the two inlets.

2. The PTA process system according to claim 1, characterized in that, The waste heat recovery unit is a heat pump unit, which is equipped with a regenerator (22) or an evaporator (23) with a shell-and-tube heat exchanger (25).

3. The PTA process system according to claim 2, characterized in that, The waste heat steam pipeline of the regenerator (22) or evaporator (23) is an internally threaded pipeline and / or coated with a hydrophilic coating.

4. The PTA process system according to claim 1, characterized in that, The waste heat recovery unit is equipped with a thermosiphon flash evaporator (21).

5. The PTA process system according to claim 1, characterized in that, The refining unit is provided with a waste heat steam venting pipeline (8), and the waste heat steam venting pipeline (8) is provided with a branch (9) connected to the PTA process waste heat recovery system.

6. The PTA process system according to claim 5, characterized in that, Interlocking control devices are provided on the waste heat steam venting pipeline (8) and the branch (9) connected to the PTA process waste heat recovery system.

7. A process for preparing PTA using the process system described in any one of claims 1 to 6, comprising an oxidation step and a refining step, characterized in that, It also includes a waste heat recovery process for recovering the low-temperature mother liquor and low-pressure steam from the refining process, wherein the pressure of the low-pressure steam is -7 kPaG to 12 kPaG; the method for recovering the low-temperature mother liquor and low-pressure steam from the refining process is that the operating temperature of the low-temperature mother liquor and low-pressure steam is 91 to 103°C, the operating pressure is 0.073 to 0.113 MPaA, and the mass ratio of the low-temperature mother liquor to the low-pressure steam is 200 to 600:

1.

8. The process for preparing PTA according to claim 7, characterized in that, The pressure of the low-pressure steam is -6 kPaG to 11 kPaG.

9. The process for preparing PTA according to claim 8, characterized in that, The pressure of the low-pressure steam is -5 kPaG to 10 kPaG.

10. The process for preparing PTA according to claim 7, characterized in that, The high-grade steam generated in the waste heat recovery process of the PTA process operates at a temperature of 135–145°C and an operating pressure of 0.32–0.42 MPaA.

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