Melting device for refining DMT

Abstract

The utility model relates to the field of recovery of waste PET (polyethylene glycol terephthalate), and discloses a melting device for refining DMT, ethylene glycol in a DMT crude product is removed by arranging a gas-liquid separator, nitrogen is continuously introduced into the bottom of a melting tank in the melting process, methanol is completely removed, and the purity of the product is ensured.

Description

Technical Field

[0001] The utility model relates to the field of recycling waste polyester PET (polyethylene terephthalate), in particular to a melting device for refining DMT. Background Art

[0002] DMT is the main product of chemical recycling of waste PET. When refining DMT, the crude DMT product needs to undergo crystallization, centrifugation, melting, distillation and other operations. Therefore, the DMT melting process largely determines the efficiency and yield of subsequent DMT refining.

[0003] Currently, the equipment used in the melting process includes melting tanks, melting furnaces, melting tanks, etc. In order to increase the turbulence of the material in the melting equipment, it is mainly achieved by setting a stirring device inside or a reflux pump outside. The heating method is mainly to set a heating tube or heater inside the device.

[0004] Due to the high melting point of DMT, using conventional melting equipment for the DMT melting process can easily lead to uneven melting of the DMT, resulting in significant heat loss and reduced production efficiency. In addition to methanol, the feed also contains ethylene glycol, a product of transesterification. If the melted liquid contains ethylene glycol, during the distillation of DMT, ethylene glycol will be recovered as a light component at the top of the tower along with DMT, affecting product quality. Although the temperature within the melting equipment is higher than the boiling point of methanol, it is difficult for methanol vapor at the bottom of the melting tank to break through the liquid layer and overflow from the gas phase outlet. Therefore, the melting process and equipment proposed in this design focus on solving the above problems, thereby reducing energy consumption, improving production efficiency, and ensuring the purity of the DMT product. Summary of the Invention

[0005] In order to solve the problems of uneven DMT melting, which causes a large amount of heat loss and reduces production efficiency, the presence of ethylene glycol in the product affects product quality, and the difficulty of methanol vapor at the bottom of the melting tank to break through the liquid layer and overflow from the gas phase outlet, the utility model provides a melting device for refining DMT.

[0006] The specific technical solution of the utility model is: a melting device for recovering DMT from waste PET, comprising:

[0007] DMT melting tank;

[0008] The DMT slurry supplier is connected to a centrifuge, which is connected to the material inlet at the top of the DMT melting tank;

[0009] The spiral heating pipes distributed on both sides of the DMT melting tank include a vertical inner coil inside the DMT melting tank, a horizontal inner coil located at the bottom of the DMT melting tank, and an outer coil located outside the DMT melting tank;

[0010] A gas-liquid separator connected to the gas-liquid separator inlet at the bottom of the DMT melting tank;

[0011] A nitrogen supplier connected to the nitrogen inlet at the bottom of the DMT melting tank.

[0012] The DMT slurry supplier continuously pumps the DMT slurry into the centrifuge. After centrifugation, the solid phase and a small amount of liquid methanol enter the DMT melting tank. The centrifuged material is heated and melted by spiral heating pipes installed on both sides of the DMT melting tank to prevent DMT from condensing on the wall and the nozzle and causing blockage. When the temperature in the DMT melting tank reaches 150-155℃, the main components inside the DMT melting tank are molten DMT, DMI, ethylene glycol and vaporized methanol that has not completely escaped. In order to ensure the purity of the product, a gas-liquid separator is installed next to the DMT melting tank. The separator controls the pressure and temperature in the gas-liquid separator to discharge part of the ethylene glycol in the original components in the gas phase, and the remaining ethylene glycol reacts with DMT to form the high-boiling-point product HMT. HMT will be separated into the kettle residue in the subsequent distillation process. The nitrogen supplier connected to the nitrogen inlet at the bottom of the DMT melting tank fills the tank bottom with nitrogen during the entire melting process, entraining gaseous ethanol to break through the liquid film resistance to completely remove the methanol in the components. The above device reduces the probability of DMT crystallization and blockage in the entire device during the DMT distillation process, and reduces the frequency of pipeline and equipment cleaning.

[0013] Preferably, the nitrogen inlet is connected to a gas distributor to ensure that the gas ejected from the gas distributor is in an irregular direction, thereby enhancing the turbulence effect of the gas-liquid two phases and promoting the methanol gas phase at the bottom of the melting tank to escape faster with the nitrogen.

[0014] Preferably, the gas distributor is an umbrella-shaped gas distributor composed of a plurality of elliptical cylinders.

[0015] Preferably, a separation pump is installed between the DMT melting tank and the gas-liquid separator to pump the material into the gas-liquid separator.

[0016] Preferably, a pressure regulating valve e is provided on the gas-liquid separator to adjust the pressure in the gas-liquid separator to 10-20 kPa.

[0017] The circulating material discharge port at the bottom of the DMT melting tank is connected to a heater through a circulating material discharge pipe, and the heater is connected to the circulating material feed port at the top of the DMT melting tank through a circulating material feed pipe. While the material is fluid in the circulating pipeline, the DMT slurry can be further heated and melted.

[0018] Preferably, the heater is a double-pass heater to accelerate the melting process.

[0019] Preferably, a melting tank circulation pump is provided on the circulating material discharge pipe to provide power so that the molten DMT can re-enter the DMT melting tank from the circulating material feed port.

[0020] Preferably, the parameter range of the melting tank circulation pump is: flow rate is 70-80 m 3 / h, with a head of 25-30m, to control the flow rate of molten DMT and speed up the melting process.

[0021] The circulating material discharge pipe is provided with a circulation valve b and a circulation valve c. When the temperature of the DMT melting tank does not reach 150-155°C, the circulation valve b and the circulation valve c need to be opened. By continuously circulating the slurry inside the melting tank, the turbulence of the slurry in the melting tank is increased, thereby increasing the heat transfer rate between the material in the tank and the heat medium in the inner coil. When the temperature of the DMT melting tank reaches 150-155°C, the circulation valve b and the circulation valve c are closed and subsequent operations are carried out.

[0022] The top of the gas-liquid separator is connected to the gas phase outlet pipe B through the separator gas phase outlet, and the bottom of the gas-liquid separator is connected to the downstream of the circulation valve b on the circulating material discharge pipe through the separator liquid phase outlet. When the operating temperature of the material in the gas-liquid separator reaches 180-190°C, ethylene glycol escapes as a gas phase and is discharged from the system through the gas phase outlet pipe B, thereby reducing the pressure in the system and enhancing operational safety. The liquid phase outlet of the separator facilitates the entry of the purified DMT into the next process.

[0023] Specifically, the material is maintained at 180-190° C. for 30 min-60 min. Based on the gas chromatography / mass spectrometry (GC / MS) data of the on-site material, under this condition, the EG in the material can be completely reacted to produce methanol and the heavy component HMT.

[0024] A liquid phase discharge pipe A is connected upstream of the circulation valve b of the circulating material discharge pipe, and a liquid phase discharge pipe B is connected downstream of the circulation valve c of the circulating material discharge pipe. The liquid phase discharge pipe B is connected to the DMT discharge system to facilitate the transportation of the refined DMT to the next refining process.

[0025] The liquid phase discharge pipe A is provided with a discharge valve a, and the liquid phase discharge pipe 2 is provided with a discharge valve d. The downstream of the discharge valve a of the liquid phase discharge pipe 1 is connected to the downstream of the discharge valve d of the liquid phase discharge pipe B. When the temperature of the DMT melting tank does not reach 150-155°C, the discharge valves a and d need to be closed to ensure that the slurry inside the melting tank can be continuously circulated, thereby improving the heat transfer rate between the material in the tank and the heat medium in the inner coil. When the temperature of the DMT melting tank reaches 150-155°C, the discharge valves a and d are opened to transport the refined DMT to the next process.

[0026] The top of the DMT melting tank is connected to the gas phase outlet pipe B through the melting tank gas phase outlet, and the nitrogen introduced from the bottom and the ethylene glycol that breaks through the liquid film with the nitrogen are discharged into the system to ensure that the pressure in the DMT melting tank is within the normal range and ensure operational safety.

[0027] The centrifuge is connected to the liquid phase methanol outlet to discharge the upper liquid phase ethanol obtained after centrifugation.

[0028] The heating medium of the vertical inner coil and the vertical inner coil is heat-conducting oil, and the heating medium of the heater is heat-conducting oil. The liquid is convenient for controlling the heating process at any time.

[0029] Preferably, the temperature of the heat transfer oil is 240° C.-260° C., which is higher than the liquefaction temperature of DMT, so as to melt the DMT into a liquid state.

[0030] The heating medium of the outer coil is high temperature steam, which heats the outer wall of the melting tank to prevent DMT from crystallizing on the wall and the pipe mouth.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1. By setting up a gas-liquid separator, the ethylene glycol in the material is vaporized or reacted with DMT to generate a high-boiling point product HMT (diethylene glycol terephthalate), removing the ethylene glycol and improving the product purity.

[0033] 2. By filling nitrogen at the bottom of the DMT melting tank, the methanol removal time is shortened and the methanol in the material is completely removed.

[0034] 3. A material circulation pipe is set up to continuously circulate the material in the DMT melting tank, thereby increasing the turbulence of the slurry in the melting tank and greatly reducing the consumption of heating medium in the DMT melting tank.

[0035] 4. Set up a drain line to clear the molten material in the circulation pipeline and heat exchanger, reduce the probability of DMT crystallization and blockage in the heater and circulation pipeline, and reduce the frequency of pipeline and equipment cleaning. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a perspective view of the DMT melting tank of the present invention.

[0037] Figure 2 It is a top view of the DMT melting tank of the present invention.

[0038] Figure 3 This is a schematic diagram of the position of the gas distributor in the DMT melting tank.

[0039] Figure 4 It is a schematic diagram of the gas distributor structure.

[0040] Figure 5 It is the distribution diagram of tongue-shaped holes on a single elliptical cylinder of the gas distributor.

[0041] Figure 6 It is a schematic diagram of the openings on both sides of the elliptical cylinder AA and BB on the gas distributor.

[0042] Figure 7 This is a schematic diagram of the DMT melting process of the utility model.

[0043] The accompanying drawings are marked as follows: 1. DMT slurry supplier; 2. External coil heat supplier; 3. Internal coil heat recovery device; 4. Internal coil heat supplier; 5. External coil heat recovery device; 6. Nitrogen supplier; 7. Liquid methanol outlet; 8. Melting tank gas outlet pipe; 9. Separator gas outlet pipe; 10. Heat supplier for heater; 11. Heat recovery device for heater; 12. DMT discharge system; 13. Centrifuge; 14. DMT melting tank; 15. Gas-liquid separator; 16. Circulation pump; 17. Discharge pump; 18. Heater; 19. Melting tank thermometer; 20. Separator pump; 21. Separator gas outlet pipe; 22. Separator liquid outlet pipe; 23. Liquid discharge pipe B; 24. Gas-liquid Separator thermometer; 25. Discharge valve a; 26. Circulation valve b; 27. Circulation valve c; 28. Discharge valve d; 29. Pressure regulating valve e; 30. Temperature control valve f; 31. Temperature control valve g; 32. External coil; 33. Vertical inner coil; 34. Horizontal inner coil; 35. Circulation material outlet pipe; 36. Circulation material inlet pipe; 37. Pressure detector; 38. Liquid phase discharge pipe A; 39. Gas distributor; 391. One side of the small-sized tongue-shaped hole; 392. One side of the large-sized tongue-shaped hole; N1. Material inlet; N2. Melting tank gas phase outlet; N3. Circulation material feed port; N4. Nitrogen inlet; N5. Circulation material discharge port; N6. Gas-liquid separator inlet. DETAILED DESCRIPTION

[0044] The present invention will be further described below with reference to the following embodiments. Unless otherwise specified, the devices, connection structures and methods involved in the present invention are all well known in the art.

[0045] Example 1

[0046] like Figure 1 As shown, inside the DMT melting tank 14, there are wrapped vertical inner coils 33 and vertical inner coils 34, and outside the DMT melting tank 14 there is an outer coil 32 wound around the material inlet N1 and the material circulation outlet N5.

[0047] Specifically, the bottommost outlet of the inner coil 32 is connected to the inner coil heat supplier 4 , and the topmost inlet is connected to the inner coil heat recovery device 3 .

[0048] Specifically, a melting tank thermometer 19 is provided at the bottom of the DMT melting tank 14 , and a gas-liquid separator thermometer 24 is provided on the gas-liquid separator 15 .

[0049] like Figure 2 As shown, the outer coil 32 is connected to the DMT melting tank 14 to heat the pipe mouth.

[0050] like Figure 3-6 As shown, the nitrogen inlet N4 is connected to a gas distributor 39. This is an umbrella-shaped gas distributor composed of eight elliptical cylinders, fixed to the bottom of the DMT melting tank 14. This ensures that the position and structure of the gas distributor 39 do not affect the discharge of the molten DMT and that no molten DMT residue remains on the gas separator. The ellipse of the gas distributor 39 has a major axis of 50-60 mm, a minor axis of 25-30 mm, and an angle of 15-20° with the horizontal. The length of the elliptical cylinder is 550-720 mm. Each elliptical cylinder of the gas distributor has a number of tongue-shaped holes arranged on either side of the cylinder: a small-sized tongue-shaped hole side 391 (13-18 holes) and a large-sized tongue-shaped hole side 392 (10-15 holes). The tongue-shaped holes on each side are of different sizes, but the holes on the same side are the same size. The holes are located 20-30 mm from the gas inlet. For the smaller tongue-shaped holes (20-1), the angle γ between the centerline of the upper circular arc and the axis of the base of the elliptical cylinder is 120-135°. The intersection of the tongue's base and the tongue's centerline lies on the cross-sectional centerline of the elliptical cylinder. The radius of the arc, d, is 15-20 mm, and the hole spacing (the distance between the arc centers of the two tongue-shaped holes) is t, which is 40-45 mm. For the larger tongue-shaped holes (20-2), the angle β between the centerline of the upper circular arc and the axis of the base of the elliptical cylinder is 45-60°. The intersection of the tongue's base and the tongue's centerline lies on the cross-sectional centerline of the elliptical cylinder. The radius of the arc, d´, is 25-30 mm, and the hole spacing (the distance between the arc centers of the two tongue-shaped holes) is t´, which is 50-55 mm. The tongue-shaped hole's upper tongue-shaped hole angle, α, is the angle between the tongue and the AA axis and ranges from 20-35°. The tongue-shaped holes provided inside the gas distributor 39 can ensure that the gas ejected from the gas distributor 39 is in an irregular direction, thereby enhancing the turbulent effect of the gas-liquid two-phase flow and promoting the methanol gas phase at the bottom of the melting tank to escape faster with the nitrogen.

[0051] like Figure 7As shown: A melting device for refining DMT, the specific structure includes a DMT slurry supplier 1, a centrifuge 13 connected to the DMT slurry supplier 1, one side of the centrifuge 13 is connected to the liquid methanol outlet 7, and the other side of the centrifuge 13 is connected to the material inlet NI at the top of the DMT melting tank 14. The top of the DMT melting tank 14 is also provided with a melting tank gas phase outlet N2 and a circulating material inlet N3. The melting tank gas phase outlet N2 is connected to the melting tank gas phase outlet pipe 8. At the bottom of the DMT melting tank 14, there is a nitrogen inlet N4, a circulating material discharge port N5 and a gas-liquid separator inlet N6, wherein the nitrogen inlet N4 is connected to the nitrogen supplier 6 to supply nitrogen to the bottom of the DMT melting tank 14 during the melting process; the circulating material discharge port N5 is connected to the circulating material outlet pipe 35, and a discharge port 35 is installed on the circulating material outlet pipe 35. Valve a25, the circulation pump 16 is connected to the downstream of the discharge valve a25 on the circulating material outlet pipe 35, the circulation pump 16 is connected to the circulation valve c27 downstream, the heater 18 is connected to the downstream of the circulation valve c27, and the other end of the heater is connected to the circulating material inlet N3 at the top of the DMT melting tank 14; the gas-liquid separator inlet N6 is connected to the separator pump 20, the separator pump 20 is connected to the downstream of the pressure regulating valve e29, the pressure detector 37 is connected downstream of the pressure regulating valve e29, the pressure detector 37 is directly connected to the gas-liquid separator 15, the top of the gas-liquid separator 15 is connected to the separator gas phase outlet pipe 9, the lower end of the gas-liquid separator 15 is connected to the downstream of the discharge valve a26 on the circulating material outlet pipe 35, a liquid phase outlet pipe B39 is connected downstream of the circulation valve c27, and a discharge valve d28 is installed on the liquid phase outlet pipe B39. A liquid phase discharge pipe A38 is connected upstream of the discharge valve a26 on the circulating material outlet pipe 35, and a discharge valve b25 is provided on the liquid phase discharge pipe A38. The liquid phase discharge pipe A38 is connected to the downstream of the discharge valve d28 on the liquid phase outlet pipe B39 downstream of the discharge valve a25, and its downstream is connected to the DMT discharge system 12.

[0052] Specifically, a temperature control valve f30 is connected downstream of the inner coil heat supplier 2.

[0053] Specifically, a temperature control valve g31 is connected downstream of the heater heat supplier 10 .

[0054] Specifically, the lower end of the heater 18 is connected to the heater heat supplier 11 , and the upper end of the heater 18 is connected to the heater heat recovery device 10 .

[0055] Specifically, the lower end of the outer coil 32 is connected to the outer coil heat supplier 2 , and the upper end is connected to the outer coil heat recovery device 5 .

[0056] like Figure 2 As shown, the outer coil 32 is connected to the DMT melting tank 14 to heat the pipe mouth.

[0057] The working principle of the melting device for recovering DMT from waste PET in the utility model is as follows:

[0058] The DMT slurry supplier 1 continuously pumps DMT slurry into a centrifuge 13. The DMT slurry primarily consists of DMT crystals and a small amount of liquid methanol. After centrifugation, the liquid phase enters the methanol refining section for recovery through the liquid methanol outlet 7. The resulting solid phase and a small amount of liquid methanol (40-60°C) are discharged by gravity into the DMT melting tank 14. To enhance melting efficiency, the DMT melting tank 14 is equipped with a vertical inner coil 33 and a horizontal inner coil 34. To prevent clogging caused by residual crystals on the inner walls and nozzles of the DMT melting tank 14, an outer coil 32 is installed outside the DMT melting tank 14. High-temperature steam is introduced into the outer coil 32 via the outer coil heat supplier 2 to heat the outer walls of the DMT melting tank 14 and prevent DMT crystallization on the walls and nozzles. Heat transfer oil at 240-260°C is introduced into the inner coil 33 via the inner coil heat supplier 4 as the heating medium for the DMT. In order to improve the heat transfer coefficient of the DMT melting tank 14 side, reduce the thermal resistance outside the coil, and thus strengthen the heat transfer process, the pump circulation stirring method is selected. The flow rate of the circulating material outlet pipe is 70-80 m 3 / h, with a circulation pump 16 of 25-30 m head. Open circulation valve b26 and circulation valve c27, close discharge valve a25 and discharge valve d28, and continuously circulate the slurry inside the DMT melting tank 14 to increase the turbulence of the slurry in the melting tank, thereby increasing the heat transfer rate between the material in the tank and the heat medium in the inner coil. At the same time, a double-pass heater 18 is set on the circulation pipeline. This can also ensure the fluidity of the material in the circulation pipeline while further heating and melting the DMT slurry. Heat transfer oil at 240℃-260℃ is introduced into the heater 18 through the heater heat supplier 11. The double-pass heater 18 can provide sufficient heat transfer area, which plays a good auxiliary role in melting the DMT and greatly reduces the consumption of heating medium in the DMT melting tank.

[0059] Since DMT has a room-temperature melting point of 140.6°C, the DMT in DMT melt tank 14 is completely molten by the time it reaches 150-155°C. At this point, the components in DMT melt tank 14 are primarily molten DMT, DMI, ethylene glycol, and partially evaporated methanol. If the material is discharged to the DMT distillation section at this point, the ethylene glycol in the material will be recovered from the top of the distillation column along with the DMT, affecting product purity. Therefore, it is necessary to ensure that the melted material is free of low-boiling-point substances (ethylene glycol and methanol). First, a gas-liquid separator 15 is installed based on the existing process. When the material in DMT melt tank 14 reaches a molten state, separator pump 20 is activated to pump the material into gas-liquid separator 15. The pressure is adjusted to 10-20 kPa (A) via post-pump pressure regulating valve e29. At an operating temperature of 180-190°C, the ethylene glycol vapor in gas-liquid separator 15 is discharged from the melting system through separator gas phase outlet pipe 21. At the same time, since DMT and ethylene glycol can produce a high-boiling-point product, HMT (diethylene terephthalate), under certain conditions, this can be used as a basis for removing ethylene glycol. The resulting HMT will separate into the still residue during the DMT distillation process, without affecting product quality. Multiple field experiments have shown that the reaction can occur when the system temperature is between 180-190°C. When the material reaches 150-155°C, the opening of temperature-controlled valve f30 is reduced to maintain the melting tank at an insulated state. The temperature-controlled valve g31, which supplies the thermal oil to heater 18, is opened to raise the temperature of the circulating material to 180-190°C. Since the liquid phase in gas-liquid separator 15 also contains unseparated ethylene glycol, the liquid phase from the gas-liquid separator is recirculated and heated via pipeline 13 along with the material in DMT melting tank 14. Based on the gas chromatography / mass spectrometry (GC / MS) data of the on-site material, when the material in the DMT melting tank 14 is maintained at 180-190° C. for 30 min-60 min, the EG in the material can be completely reacted to produce methanol and the heavy component HMT.

[0060] Since the material entering the DMT melting tank 14 contains some methanol, and methanol will be generated during the removal of ethylene glycol, although the boiling point of methanol at normal pressure is 64.7°C, the temperature in the DMT melting tank 14 is sufficient to vaporize the methanol, thereby removing most of the methanol in the material. However, there is still a situation where the methanol gas phase has difficulty breaking through the liquid film. Therefore, a nitrogen inlet N4 is provided below the melting tank. Nitrogen is supplied to the bottom of the DMT melting tank 14 through the nitrogen supplier 6 during the entire melting process. The nitrogen inlet N4 is connected to a gas distributor 39, which distributes the ejected gas in an irregular direction, thereby enhancing the turbulent effect of the gas-liquid two-phase flow, promoting the methanol gas phase at the bottom of the melting tank to escape faster with the nitrogen, and completely removing the methanol in the material.

[0061] When the slurry is completely melted and the light components of methanol and ethylene glycol are removed, the material can be discharged. In order to clear the molten material in the circulation pipeline and heat exchanger 18, reduce the probability of DMT crystallization and blockage in the heater 18 and the circulation pipeline, and reduce the frequency of pipeline and equipment cleaning, a drainage pipeline is set. At this time, the circulation pump 16, circulation valve b26 and circulation valve c27 are closed, and the discharge valve d28 is opened. At the same time, the discharge pump 17 and the discharge valve a25 are opened to discharge the molten DMT to the next refining unit.

[0062] Example 2

[0063] A melting device for recovering DMT from waste PET, specifically comprising a DMT slurry supplier 1, a centrifuge 13 connected to the DMT slurry supplier 1, one side of the centrifuge 13 connected to a liquid methanol outlet 7, and the other side of the centrifuge 13 connected to a material inlet N1 at the top of a DMT melting tank 14. A melting tank gas phase outlet N2 and a circulating material inlet N3 are also provided at the top of the DMT melting tank 14. The melting tank gas phase outlet N2 is connected to a melting tank gas phase outlet pipe 8. A nitrogen inlet N4, a circulating material discharge port N5, and a gas-liquid separator inlet N6 are provided at the bottom of the DMT melting tank 14. The nitrogen inlet N4 is connected to a nitrogen supplier 6 to supply nitrogen to the bottom of the DMT melting tank 14 during the melting process. The circulating material discharge port N5 is connected to a circulating material outlet pipe 35. A nitrogen separator is installed on the circulating material outlet pipe 35. Discharge valve a25, the circulation pump 16 is connected downstream of the discharge valve a25 on the circulating material outlet pipe 35, the circulation pump 16 is connected downstream of the circulation valve c27, the heater 18 is connected downstream of the circulation valve c27, and the other end of the heater is connected to the circulating material inlet N3 at the top of the DMT melting tank 14; the gas-liquid separator inlet N6 is connected to the separator pump 20, the separator pump 20 is connected downstream of the pressure regulating valve e29, the pressure detector 37 is connected downstream of the pressure regulating valve e29, the pressure detector 37 is directly connected to the gas-liquid separator 15, the top of the gas-liquid separator 15 is connected to the separator gas phase outlet pipe 9, the lower end of the gas-liquid separator 15 is connected downstream of the discharge valve a26 on the circulating material outlet pipe 35, a liquid phase outlet pipe B39 is connected downstream of the circulation valve c27, and a discharge valve d28 is installed on the liquid phase outlet pipe B39. A liquid phase discharge pipe A38 is connected upstream of the discharge valve a26 on the circulating material outlet pipe 35, and a discharge valve b25 is provided on the liquid phase discharge pipe A38. The liquid phase discharge pipe A38 is connected to the downstream of the discharge valve d28 on the liquid phase outlet pipe B39 downstream of the discharge valve a25, and its downstream is connected to the DMT discharge system 12.

[0064] like Figure 3-6As shown, nitrogen inlet N4 is connected to a gas distributor 39. This is an umbrella-shaped gas distributor composed of eight elliptical cylinders, fixed to the bottom of the DMT melting tank 14. This ensures that the position and structure of the gas distributor 39 do not affect the discharge of the molten DMT and that no molten DMT residue remains in the gas separator. Each elliptical cylinder of the gas distributor has a number of tongue-shaped holes arranged on either side of the elliptical cylinder: a small tongue-shaped hole side 391 and a large tongue-shaped hole side 392. The tongue-shaped holes on each side are of different sizes, but the tongue-shaped holes on the same side are of the same size. The tongue-shaped holes in the gas distributor 39 ensure that the gas ejected from the gas distributor 39 is irregularly oriented, thereby enhancing the turbulent flow of the gas and liquid phases and promoting the faster escape of the methanol gas phase from the bottom of the melting tank along with the nitrogen.

[0065] Specifically, the gas distributor consists of eight elliptical cylinders with a major axis a = 50 mm, a minor axis b = 25 mm, a length l = 650 mm, and an angle of 15° with respect to the horizontal. Each cylinder has 15 holes on the side with the small tongue-shaped hole, with γ = 135°, d = 18 mm, and t = 40 mm. The large tongue-shaped hole has 10 holes on the side with β = 45°, d´ = 28 mm, and t´ = 50 mm. The holes are located 30 mm from the gas inlet, with a tongue angle α = 30°.

[0066] Specifically, a temperature control valve f30 is connected downstream of the inner coil heat supplier 2.

[0067] Specifically, a temperature control valve g31 is connected downstream of the heater heat supplier 10 .

[0068] Specifically, the lower end of the heater 18 is connected to the heater heat supplier 11 , and the upper end of the heater 18 is connected to the heater heat recovery device 10 .

[0069] Specifically, the lower end of the outer coil 32 is connected to the outer coil heat supplier 2 , and the upper end is connected to the outer coil heat recovery device 5 .

[0070] Inside the DMT melting tank 14 , there are wrapped vertical inner coils 33 and vertical inner coils 34 , and outside the DMT melting tank 14 there is an outer coil 32 wound around the material inlet N1 and the material circulation outlet N5 .

[0071] Specifically, the bottommost outlet of the inner coil 32 is connected to the inner coil heat supplier 4 , and the topmost inlet is connected to the inner coil heat recovery device 3 .

[0072] The outer coil 32 is connected to the DMT melting tank 14 to heat the pipe mouth.

[0073] The working principle of the melting device for recovering DMT from waste PET in the utility model is as follows:

[0074] The DMT slurry supplier 1 continuously pumps DMT slurry into a centrifuge 13. The DMT slurry primarily consists of DMT crystals and a small amount of liquid methanol. After centrifugation, the liquid phase enters the methanol refining section for recovery through the liquid methanol outlet 7. The resulting solid phase and a small amount of liquid methanol (40-60°C) are discharged by gravity into the DMT melting tank 14. To enhance melting efficiency, the DMT melting tank 14 is equipped with a vertical inner coil 33 and a horizontal inner coil 34. To prevent clogging caused by residual crystals on the inner walls and nozzles of the DMT melting tank 14, an outer coil 32 is installed outside the DMT melting tank 14. High-temperature steam is introduced into the outer coil 32 via the outer coil heat supplier 2 to heat the outer walls of the DMT melting tank 14 and prevent DMT crystallization on the walls and nozzles. Heat transfer oil at 240-260°C is introduced into the inner coil 33 via the inner coil heat supplier 4 as the heating medium for the DMT. In order to improve the heat transfer coefficient of the DMT melting tank 14 side, reduce the thermal resistance outside the coil, and thus strengthen the heat transfer process, the pump circulation stirring method is selected. The flow rate of the circulating material outlet pipe is 70-80 m 3 / h, with a circulation pump 16 of 25-30 m head. Open circulation valve b26 and circulation valve c27, close discharge valve a25 and discharge valve d28, and continuously circulate the slurry inside the DMT melting tank 14 to increase the turbulence of the slurry in the melting tank, thereby increasing the heat transfer rate between the material in the tank and the heat medium in the inner coil. At the same time, a double-pass heater 18 is set on the circulation pipeline. This can also ensure the fluidity of the material in the circulation pipeline while further heating and melting the DMT slurry. Heat transfer oil at 240℃-260℃ is introduced into the heater 18 through the heater heat supplier 11. The double-pass heater 18 can provide sufficient heat transfer area, which plays a good auxiliary role in melting the DMT and greatly reduces the consumption of heating medium in the DMT melting tank.

[0075] Since DMT has a room-temperature melting point of 140.6°C, the DMT in DMT melt tank 14 is completely molten by the time it reaches 150-155°C. At this point, the components in DMT melt tank 14 are primarily molten DMT, DMI, ethylene glycol, and partially evaporated methanol. If the material is discharged to the DMT distillation section at this point, the ethylene glycol in the material will be recovered from the top of the distillation column along with the DMT, affecting product purity. Therefore, it is necessary to ensure that the melted material is free of low-boiling-point substances (ethylene glycol and methanol). First, a gas-liquid separator 15 is installed based on the existing process. When the material in DMT melt tank 14 reaches a molten state, separator pump 20 is activated to pump the material into gas-liquid separator 15. The pressure is adjusted to 10-20 kPa (A) via post-pump pressure regulating valve e29. At an operating temperature of 180-190°C, the ethylene glycol vapor in gas-liquid separator 15 is discharged from the melting system through separator gas phase outlet pipe 21. At the same time, since DMT and ethylene glycol can produce a high-boiling-point product, HMT (diethylene terephthalate), under certain conditions, this can be used as a basis for removing ethylene glycol. The resulting HMT will separate into the still residue during the DMT distillation process, without affecting product quality. Multiple field experiments have shown that the reaction can occur when the system temperature is between 180-190°C. When the material reaches 150-155°C, the opening of temperature-controlled valve f30 is reduced to maintain the melting tank at an insulated state. The temperature-controlled valve g31, which supplies the thermal oil to heater 18, is opened to raise the temperature of the circulating material to 180-190°C. Since the liquid phase in gas-liquid separator 15 also contains unseparated ethylene glycol, the liquid phase from the gas-liquid separator is recirculated and heated via pipeline 13 along with the material in DMT melting tank 14. Based on the gas chromatography / mass spectrometry (GC / MS) data of the on-site material, when the material in the DMT melting tank 14 is maintained at 180-190° C. for 30 min-60 min, the EG in the material can be completely reacted to produce methanol and the heavy component HMT.

[0076] Since the material entering the DMT melting tank 14 contains some methanol, and methanol will be generated during the removal of ethylene glycol, although the boiling point of methanol at normal pressure is 64.7°C, the temperature in the DMT melting tank 14 is sufficient to vaporize the methanol, thereby removing most of the methanol in the material. However, there is still a situation where the methanol gas phase has difficulty breaking through the liquid film. Therefore, a nitrogen inlet N4 is provided below the melting tank. Nitrogen is supplied to the bottom of the DMT melting tank 14 through the nitrogen supplier 6 during the entire melting process. The nitrogen inlet N4 is connected to a gas distributor 39, which distributes the ejected gas in an irregular direction, thereby enhancing the turbulent effect of the gas-liquid two-phase flow, promoting the methanol gas phase at the bottom of the melting tank to escape faster with the nitrogen, and completely removing the methanol in the material.

[0077] When the slurry is completely melted and the light components of methanol and ethylene glycol are removed, the material can be discharged. In order to clear the molten material in the circulation pipeline and heat exchanger 18, reduce the probability of DMT crystallization and blockage in the heater 18 and the circulation pipeline, and reduce the frequency of pipeline and equipment cleaning, a drainage pipeline is set. At this time, the circulation pump 16, circulation valve b26 and circulation valve c27 are closed, and the discharge valve d28 is opened. At the same time, the discharge pump 17 and the discharge valve a25 are opened to discharge the molten DMT to the next refining unit.

[0078] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent structural transformation made to the above embodiment based on the technical essence of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A melting device for refining DMT, characterized by comprising: DMT melting tank; The DMT slurry supplier is connected to a centrifuge, which is connected to the material inlet at the top of the DMT melting tank; The spiral heating pipes distributed on both sides of the DMT melting tank include a vertical inner coil inside the DMT melting tank, a horizontal inner coil located at the bottom of the DMT melting tank, and an outer coil located outside the DMT melting tank; A gas-liquid separator connected to the gas-liquid separator inlet at the bottom of the DMT melting tank; A nitrogen supplier connected to the nitrogen inlet at the bottom of the DMT melting tank.

2. A melting device for refining DMT according to claim 1, characterized in that: The circulating material discharge port at the bottom of the DMT melting tank is connected to a heater through a circulating material discharge pipe, and the heater is connected to the circulating material feed port at the top of the DMT melting tank through a circulating material feed pipe.

3. A melting device for refining DMT according to claim 2, characterized in that: The circulating material discharge pipe is provided with a circulating valve b and a circulating valve c.

4. A melting device for refining DMT according to claim 3, characterized in that: The top of the gas-liquid separator is connected to the separator gas phase outlet pipe through the separator gas phase outlet, and the bottom of the gas-liquid separator is connected to the downstream of the circulation valve b on the circulating material discharge pipe through the separator liquid phase outlet.

5. The melting device for refining DMT according to claim 3, characterized in that: The upstream of the circulating material discharge pipe circulation valve b is connected to a liquid phase discharge pipe A, the downstream of the circulating material discharge pipe circulation valve c is connected to a liquid phase discharge pipe B, and the liquid phase discharge pipe B is connected to the DMT discharge system.

6. The melting device for refining DMT according to claim 5, characterized in that: The liquid phase discharge pipe A is provided with a discharge valve a, the liquid phase discharge pipe is provided with a discharge valve d, and the downstream of the discharge valve a of the liquid phase discharge pipe A is connected to the downstream of the discharge valve d of the liquid phase discharge pipe B.

7. A melting device for refining DMT according to claim 1 or 2, characterized in that: A melting tank gas phase outlet is provided on the top of the DMT melting tank.

8. A melting device for refining DMT according to claim 1 or 2, characterized in that: The centrifuge is connected to a liquid-phase methanol outlet.

9. The melting device for refining DMT according to claim 2, characterized in that: The heating medium of the vertical inner coil and the vertical inner coil is heat transfer oil, and the heating medium of the heater is heat transfer oil.

10. The melting device for refining DMT according to claim 1, characterized in that: The heating medium of the outer coil is high-temperature steam.

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