Melting regeneration method of DMT (Dimethyl Terephthalate) in chemical recovery process of waste PET (Polyethylene Terephthalate)

Abstract

The invention relates to the technical field of waste PET (Polyethylene Terephthalate) recovery, and discloses a DMT (Dimethyl Terephthalate) melting regeneration method in a waste PET chemical recovery process, which comprises the following steps: (1) centrifugally separating DMT slurry obtained in the waste PET chemical recovery process, introducing a solid phase into a melting tank, and heating; (2) when the melting tank begins to heat, the materials in the melting tank are introduced into a gas-liquid separator for gas-liquid separation, the separated liquid phase is introduced into a circulating pipeline, the circulating pipeline is provided with a double-tube-pass heater for heating, the liquid phase is introduced into the melting tank through the circulating pipeline, and the liquid phase and the materials in the melting tank are introduced into the circulating pipeline again for circulation; and (3) when the temperature of the material in the melting tank is maintained at 180-190 DEG C for 30-60 minutes, the molten DMT can be discharged and refined. According to the method, a gas-liquid separation method, an out-tank cyclic heating method and a tank bottom nitrogen filling and methanol removing method are combined, so that the loss rate of DMT is reduced, and the purity of a DMT product is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of waste PET recycling, in particular to a melting regeneration method of DMT in the process of chemical recycling of waste PET. Background Art

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

[0003] The invention patent with publication number CN 117486720 A discloses a DMT remelting recovery process in the chemical recycling process of waste textiles, comprising the following steps: (a) centrifugal separation: feeding the ethylene glycol / methanol / DMT mixture into a horizontal centrifuge for centrifugal separation, pumping the ethylene glycol / methanol liquid phase discharged from one outlet of the centrifuge to a crude methanol storage tank, and feeding the methanol / DMT solid-liquid mixture discharged from the other outlet of the centrifuge into a DMT remelting tank; (b) DMT remelting: heating and remelting DMT at 160-200° C., then discharging the material into a DMT gas-liquid separator for gas-liquid separation, and conveying the liquid phase obtained after the gas-liquid separation to a DMT distillation tank for distillation, and recovering the methanol volatilized from the top of the DMT remelting tank and the gas phase obtained after the gas-liquid separation to a methanol recovery system.

[0004] However, the DMT remelting tank in the above patent only uses internal heating equipment. Due to the high melting point of DMT, using conventional melting equipment in the DMT melting process can easily cause uneven melting of DMT, resulting in a large amount of heat loss and reduced production efficiency. Although the temperature in the melting equipment is higher than the boiling point of methanol, it is difficult for the methanol vapor at the bottom of the melting tank to break through the liquid layer and overflow from the gas phase outlet. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a method for melting and regenerating DMT in the process of chemical recycling of waste PET, which combines a gas-liquid separation method, an external circulation heating method, and a nitrogen filling method at the bottom of the tank to remove methanol, and applies them to the process of melting DMT, thereby reducing the loss rate of DMT and ensuring the purity of the DMT product.

[0006] The purpose of the present invention is achieved through the following technical solutions: The present invention provides a method for melting and regenerating DMT in a waste PET chemical recovery process, comprising the following steps: (1) centrifugally separating the DMT slurry obtained in the chemical recycling process of waste PET, passing the solid phase into a melting tank and heating it, and stopping the heating after heating to 150-160° C.; (2) When the melting tank starts to be heated, the material in the melting tank is passed into the gas-liquid separator for gas-liquid separation. After separation, the liquid phase is passed into the circulation pipeline. The circulation pipeline is equipped with a double-pass heater for heating. The set temperature is 180-190° C. The liquid phase is passed into the melting tank through the circulation pipeline and is passed out of the melting tank again to the circulation pipeline for circulation with the material in the melting tank; (3) When the material in the melting tank is maintained at 180-190°C for 30-60 minutes, the molten DMT can be discharged and refined.

[0007] The main components of DMT slurry are DMT crystals and a small amount of liquid methanol. The liquid phase after centrifugal separation enters the methanol refining section for recovery. The solid phase (containing a small amount of liquid methanol) obtained after centrifugation is discharged into the melting tank by gravity.

[0008] Since the melting point of DMT at room temperature is 140.6°C, when the temperature in the DMT melting tank reaches 150-155°C, the DMT inside is completely in a molten state. At this time, the components in the melting tank are mainly molten DMT, DMI, HMT, ethylene glycol and vaporized methanol that has not completely escaped. If the material is discharged into the DMT distillation section at this time, the ethylene glycol in the material will be recovered from the top of the distillation tower with the DMT, affecting the purity of the product. Therefore, it is necessary to ensure that there are no low-boiling point substances (ethylene glycol and methanol) in the melted material.

[0009] First, the present invention is provided with a gas-liquid separator connected to the melting tank. While the melting tank is heated, the material is transported to the gas-liquid separator for gas-liquid separation. When the melting temperature of DMT is not reached, the gas phase generated by the gas-liquid separation is also small. When the material in the melting tank is continuously heated to a molten state, the gas-liquid separation can generate more gas phase, and the gas phase is discharged from the DMT melting system through the gas phase pipeline. Secondly, the present invention is provided with a circulation pipeline connected to the melting tank and the gas-liquid separator. The material in the melting tank and the liquid phase after gas-liquid separation are all passed into the circulation pipeline for circulation heating. A double-pass heater is arranged on the circulation pipeline to ensure the fluidity of the material on the circulation pipeline while further heating and melting the material. The double-pass heating can provide sufficient heat transfer area, which plays a good auxiliary role in melting DMT and greatly reduces the consumption of heating medium in the DMT melting tank.

[0010] When the material in the melting tank reaches 150-160°C, the heating of the melting tank is stopped. Since the liquid phase of the gas-liquid separator also contains unseparated ethylene glycol, the liquid phase obtained through the gas-liquid separator is circulated and heated together with the material in the melting tank through the pipeline, and the material is heated to 180-190°C by heating the circulation pipeline. When the material in the melting tank is maintained at 180-190°C for 30-60 minutes, the ethylene glycol in the material can be completely reacted to generate gaseous methanol and heavy component HMT. The gaseous methanol can be discharged, and HMT will be separated into the kettle residue during the DMT distillation process, which will not affect the product quality. Therefore, the molten DMT can be discharged and refined.

[0011] Preferably, the DMT slurry comprises DMT crystals, methanol, ethylene glycol and some high-boiling impurities (such as HMT); the temperature of the solid phase obtained after centrifugal separation is 40-60°C.

[0012] Preferably, a heating coil is provided inside the melting tank, and heat transfer oil at 240-260° C. flows through the coil.

[0013] Preferably, the heating coil comprises a vertical inner coil arranged on the inner wall of the melting tank and a horizontal inner coil arranged on the bottom of the melting tank.

[0014] In order to improve the melting effect, vertical inner coils and bottom horizontal inner coils are set in the melting tank, and heat transfer oil at 240-260°C is passed into the coils in the melting tank as the heating medium of DMT.

[0015] Preferably, the outer wall of the melting tank is provided with a heating outer coil, and steam at 160-170° C. is passed through the coil.

[0016] In order to avoid blockage caused by residual crystals on the inner wall and pipe opening of the melting tank, an outer coil is set outside the DMT melting tank, and high-temperature steam is passed into the outer coil to heat the outer wall of the melting tank to avoid DMT crystallization on the wall and pipe opening.

[0017] Preferably, the circulation pipeline is provided with a double-pass heater, and the heating medium of the shell side of the double-pass heater is heat transfer oil at 240-260°C.

[0018] Preferably, the operating conditions of the gas-liquid separator are 10-20 kPa(A) and 180-190°C.

[0019] Under this operating condition, the gas-liquid separator can separate the ethylene glycol gas phase. Even if a small amount of ethylene glycol remains, since DMT and ethylene glycol can generate a high-boiling point product HMT (diethylene terephthalate) under this condition, ethylene glycol can also be removed based on this. The generated HMT will be separated into the kettle residue during the DMT distillation process and will not affect the product quality.

[0020] Preferably, the liquid level in the melting tank is level with that in the gas-liquid separator.

[0021] Preferably, the melting tank is stirred by a pump, and the flow rate of the circulating pump is 70-80m 3 / h, the head is 25~30m.

[0022] In order to improve the heat transfer coefficient on the melting tank side, reduce the thermal resistance outside the coil, and thus strengthen the heat transfer process, the pump circulation stirring method is selected. A circulation pump is set at the outlet of the melting tank to increase the turbulence of the slurry in the melting tank by continuously circulating the slurry inside the melting tank, thereby increasing the heat transfer rate between the material in the tank and the heat medium in the inner coil.

[0023] Preferably, nitrogen is continuously introduced into the bottom of the melting tank, and the gas phase in the tank is discharged from the top.

[0024] Since the material entering the melting tank contains some methanol and methanol will be generated in the process of removing ethylene glycol, although the boiling point of methanol under normal pressure is 64.7°C, the temperature in the melting tank is sufficient to vaporize methanol, thereby removing most of the methanol in the material, but it is still difficult for the methanol gas phase to break through the liquid film. Therefore, a nitrogen filling port is set at the bottom of the melting tank, and nitrogen is filled into the bottom of the melting tank during the entire melting process. Nitrogen with a certain pressure can entrain methanol gas phase to break through the liquid film resistance, thereby completely removing methanol from the material.

[0025] Preferably, the nitrogen has a pressure of 0.15-0.25 MPa and a temperature of 140-160°C.

[0026] Preferably, the nitrogen inlet is connected to a gas distributor through an air inlet pipe, and the gas distributor is an umbrella-shaped gas distributor composed of a plurality of elliptical cylinders, and a plurality of tongue-shaped holes are arranged on both sides of each elliptical cylinder, and the sizes of the tongue-shaped holes on both sides are different, while the sizes of the tongue-shaped holes on the same side are the same.

[0027] To ensure that the position and structure of the gas distributor will not affect the discharge of molten DMT and that there is no residue of molten DMT on the gas separator, an umbrella-shaped gas distributor connected to the nitrogen filling pipeline is installed at the bottom of the melting tank. To ensure that the gas ejected from the gas distributor 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 nitrogen, tongue-shaped holes are arranged on both sides of each elliptical column of the gas distributor. Preferably, the molten DMT is discharged through a DMT discharge pipe, and the DMT discharge pipe is connected to the circulation pipe.

[0028] 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 materials 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, a DMT discharge pipeline is set.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) Greatly shorten the melting time of DMT and greatly improve production efficiency; (2) It can completely remove methanol from the feed liquid, shorten the time for removing methanol, further increase the concentration of the target product DMT, and improve the product purity; (3) Reduce the loss rate of DMT and increase the DMT yield; (4) There is basically no clogging, saving manpower and time costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The figure is a schematic diagram of the device connection of the system used in the DMT melting recovery method of the present invention.

[0031] Figure 2 Schematic diagram of the structure of the gas distributor.

[0032] Figure 3 This is the distribution diagram of tongue-shaped holes on a single elliptical cylinder of the gas distributor.

[0033] Figure 4 Schematic diagram of the openings on both sides of the elliptical cylinder AA and BB on the gas distributor.

[0034] Figure 5 This is a comparison chart of the effect of stirring form on DMT melting time in Example 1.

[0035] Figure 6 This is a comparison chart of the effect of heating mode on DMT melting time in Example 2.

[0036] The accompanying drawings are marked as follows: 1, DMT slurry; 2, centrifuge; 3, melting tank; 4, gas-liquid separator; 5, circulation pipeline; 6, double-pass heater; 7, DMT discharge pipeline; 8, circulation pump I; 9, circulation pump II; 10, discharge pump; 11, gas phase outlet pipeline; 12, liquid phase outlet pipeline; 13, temperature control valve I; 14, temperature control valve II; 15, DMT discharge; 16, valve a; 17, valve b; 18, valve c; 19, valve d; 20, gas distributor; 20-1, one side of the small-sized tongue-shaped hole; 20-2, one side of the large-sized tongue-shaped hole; N1, feed port; N2, gas phase discharge port; N3, circulating material feed port; N4, nitrogen filling inlet; N5, circulating material discharge port; N6, gas-liquid separator inlet. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is described below with specific embodiments, but the protection scope of the present invention is not limited thereto: 1. Chemical recycling of waste PET The process of producing DMT from waste PET is as follows: (1) Waste PET, excess ethylene glycol, and catalyst are subjected to depolymerization reaction in a depolymerization reactor, and the reaction products mainly contain BHET, ethylene glycol, diethylene glycol, oligomers, and impurities in the raw materials (cotton, linen, dyes, DMI, etc.); (2) removing solid impurities in the above product by filtration, and the obtained BHET clear liquid enters the BHET concentration tower to remove light components such as ethylene glycol; (3) The concentrated BHET concentrate is subjected to an ester exchange reaction with methanol in an ester exchange reaction vessel, and the obtained reaction products include ethylene glycol, HMT, DMI, oligomers and other products in addition to the target product DMT; (4) After crystallization and centrifugation of the reaction product, most of the low-melting point substances can be removed to obtain DMT crystals with higher purity. However, it still contains a small amount of heavy components such as DMI and oligomers and methanol that cannot be removed by centrifugation. Distillation is used to remove the heavy components to obtain DMT slurry (including DMT crystals and methanol).

[0038] 2. Melting regeneration of DMT like Figure 1 As shown, the melting regeneration system of DMT includes a centrifuge 2, a melting tank 3, a gas-liquid separator 4 and a circulation pipeline 5.

[0039] The DMT slurry 1 obtained in the chemical recycling process of waste PET is centrifuged by a centrifuge 2, and the melting tank 3 is connected to the centrifuge 2. The solid phase obtained after centrifugation is passed into the melting tank 3 through the feed port N1 at the top of the melting tank 3, and the liquid methanol obtained after centrifugation enters the methanol refining section for recovery. The inner wall of the melting tank 3 is provided with a vertical inner coil, the bottom of the melting tank 3 is provided with a horizontal inner coil, and the outer wall of the melting tank 3 is provided with a heating outer coil. The inner coil is passed through a heat transfer oil of 240 to 260°C, and a temperature control valve Ⅰ13 is provided on the heat transfer oil inlet pipeline, and the outer coil is passed through a steam of 160 to 170°C. The melting tank 3 heats the materials inside the melting tank 3 through the coils arranged inside and outside. The bottom of the melting tank 3 is also provided with a nitrogen inlet N4, which is connected to the nitrogen inlet N4 through an air inlet pipe. A gas phase outlet N2 is provided at the top, and nitrogen is passed from the bottom while heating the melting tank 3.

[0040] The structure of the gas distributor 20 is as follows: Figure 2As shown, it is fixed to the bottom of the melting tank 3. The gas distributor 20 is an umbrella-shaped gas distributor 20 composed of 8 elliptical cylinders, the major axis a of the elliptical cylinder is 50-60 mm, the minor axis b is 25-30 mm, the length of the elliptical cylinder is l=550-720 mm, and the angle with the horizontal direction is 15-20°. A plurality of tongue-shaped holes are arranged on both sides of each elliptical cylinder of the gas distributor 20, such as Figure 3 As shown, there are two sides of the elliptical cylinder with small-sized tongue-shaped holes 20-1 (13 to 18 holes) and large-sized tongue-shaped holes 20-2 (10 to 15 holes). The tongue-shaped holes on both sides are of different sizes, but the tongue-shaped holes on the same side are of the same size. The opening is 20 to 30 mm away from the air inlet. Figure 4 As shown, the angle γ between the center line of the arc of the tongue-shaped hole on one side 20-1 of the small-sized tongue-shaped hole and the axis of the bottom end of the elliptical cylinder is 120-135°, the intersection of one end of the tongue root of the tongue-shaped hole is located on the center line of the elliptical cylinder section, the arc radius of the tongue-shaped hole is d=15-20mm, and the hole spacing (the distance between the arc centers of the two tongue-shaped holes) is t=40-45mm. The angle β between the center line of the arc of the tongue-shaped hole on one side 20-2 of the large-sized tongue-shaped hole and the axis of the bottom end of the ellipse is 45-60°, the intersection of one end of the tongue root of the tongue-shaped hole is located on the center line of the elliptical cylinder section, the arc radius of the tongue-shaped hole is d′=25-30mm, and the hole spacing (the distance between the arc centers of the two tongue-shaped holes) is t′=50-55mm. The tongue plate angle α on the tongue-shaped hole is the angle between the tongue plate and the AA axis, α=20-35°.

[0041] The melting tank 3 is connected to the circulation pipeline 5. The material in the melting tank 3 is passed into the circulation pipeline 5 from the circulation pipeline outlet N5 at the bottom. The circulation pipeline 5 is provided with a double-pass heater 6. The shell side of the circulation pipeline is provided with a heat transfer oil of 240-260°C. A temperature control valve Ⅱ14 is provided on the heat transfer oil inlet pipeline. After the material is heated through the circulation pipeline 5, it is passed into the melting tank 3 through the circulation pipeline feed port N3 at the top of the melting tank 3 to realize circulation. The circulation pipeline 5 is provided with a circulation pump Ⅰ8. The circulation pump Ⅰ8 is arranged near the outlet of the melting tank 3. Therefore, the melting tank 3 adopts pump circulation stirring. The flow rate of the circulation pump Ⅰ8 is 70-80m 3 / h, the head is 25~30m.

[0042] The melting tank 3 is also connected to the gas-liquid separator 4 through the gas-liquid separator inlet N6 provided at the bottom thereof, and a circulation pump II 9 is also provided on the connecting pipeline. The material in the melting tank 3 is passed into the gas-liquid separator 4 for gas-liquid separation, and the liquid levels in the melting tank 3 and the gas-liquid separator 4 are level. A gas phase outlet is provided at the top of the gas-liquid separator 4, and the gas phase passes out from the gas phase outlet after gas-liquid separation, and then is discharged from the melting system through the gas phase outlet pipeline 11. After gas phase separation, the liquid phase passes into the circulation pipeline 5 through the liquid phase outlet pipeline 12 connected to the gas phase separator, and is provided before the circulation pump I8 and passes into the circulation pipeline 5.

[0043] The system also includes a DMT discharge pipeline 7, which is connected to the circulation pipeline 5 after the circulation pump Ⅰ8, and the circulation pipeline 5 is provided with a discharge pump 10, and a valve d19 is provided before the discharge pump 10. In addition, a branch is provided between the discharge pump 10 and the valve d19 of the DMT discharge pipeline 7, which is connected to the circulation pipeline 5 and provided with a valve a16, and the connection point is provided before the circulation pump Ⅰ8, and a valve b17 is provided between the connection point and the circulation pump Ⅰ8. By adjusting the valve opening, the molten DMT discharge can be sent to the next refining unit through the DMT discharge pipeline 7.

[0044] The method for DMT melting recovery using the above system comprises the following steps: (1) The DMT slurry 1 obtained in the chemical recycling process of waste PET is centrifuged in a centrifuge 2, and the solid phase is fed into the melting tank 3 through the feed port N1. The heating device provided in the melting tank 3 heats the material. After heating to 150-160°C, the heating is stopped, and the opening of the temperature control valve Ⅰ13 on the heat transfer oil inlet pipeline is reduced to keep the melting tank 3 in a heat preservation state. Since the material entering the melting tank 3 contains some methanol, and methanol will be generated during the removal of ethylene glycol, a nitrogen inlet N4 is provided at the bottom of the melting tank 3. During the entire melting process, nitrogen is introduced into the melting tank through the nitrogen inlet N4 via the gas distributor 20 to remove methanol. The pressure of the nitrogen is 0.15-0.25MPa and the temperature is 140-160°C.

[0045] (2) When the melting tank 3 starts to be heated, the material in the melting tank 3 is passed into the gas-liquid separator 4 for gas-liquid separation. The operating conditions of the gas-liquid separator are 10-20 kPa (A) and 180-190°C. At the same time, the material is passed into the circulation pipeline 5 for circulation. The heating setting temperature in the circulation pipeline 5 is 180-190°C. After separation by the gas-liquid separator 4, the gas phase passes out of the melting system. After separation, the liquid phase passes into the circulation pipeline 5. The circulation pipeline 5 is provided with a double-pass heater 6 for heating. The liquid phase and the material passed out of the melting tank 3 are heated together through the circulation pipeline 5 and then passed into the melting tank 3 again. They are passed out of the melting tank 3 and circulated in the circulation pipeline 5 again. When the temperature of the melting tank 3 reaches 150-160°C and the heating is stopped, the opening of the temperature control valve II14 of the heat transfer oil passing into the double-pass heater 6 is increased.

[0046] (3) The material is heated through the circulation pipe 5. When the material in the melting tank 3 is maintained at 180-190°C for 30-60 min, the circulation pump I8, valve b17 and valve c18 are closed, and the valve d19, the discharge pump 10 and valve a16 are opened, and the molten DMT can be discharged through the DMT discharge pipe 7 for refining.

[0047] Example 1 (Influence of stirring form on DMT melting time) The method for DMT melting recovery using the above system comprises the following steps: (1) The DMT slurry obtained in the chemical recycling process of waste PET is centrifuged in a centrifuge, and the solid phase (temperature is 60°C, containing 16.7wt.% methanol, 72.6wt.% DMT, 10.2wt.% ethylene glycol, 0.5wt.% high boiling point impurities (HMT, etc.)) is fed into the melting tank from the feed port. The heating device provided in the melting tank heats the material, and the heating is stopped after heating to 160°C, and the opening of the temperature control valve I is reduced so that the melting tank is maintained in a heat preservation state at this time. At the same time, nitrogen is charged from the nitrogen charging inlet to the bottom of the melting tank during the entire melting process, and the nitrogen pressure is 0.2MPa and the temperature is 160°C.

[0048] Nitrogen is introduced into the tank through a gas distributor fixed at the bottom of the melting tank. The gas distributor is composed of 8 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 the horizontal direction. The number of holes on one side of the small-sized tongue-shaped hole of each elliptical cylinder is 15, γ = 135°, d = 18 mm, t = 40 mm. The number of holes on one side of the large-sized tongue-shaped hole is 10, β = 45°, d′ = 28 mm, t′ = 50 mm. The opening is 30 mm away from the air inlet, and the tongue angle α = 30°.

[0049] (2) When the melting tank starts to be heated, the material in the melting tank is passed into the gas-liquid separator for gas-liquid separation. The operating conditions of the gas-liquid separator are 15 kPa (A) and 180°C. At the same time, the material is passed into the circulation pipeline for circulation. The circulation pipeline is not equipped with a double-pass heater for heating. After separation by the gas-liquid separator, the gas phase is passed out of the melting system. After separation, the liquid phase is passed into the circulation pipeline. The liquid phase and the material passed out of the melting tank are heated together through the circulation pipeline and then passed into the melting tank again. They are passed out of the melting tank and circulated in the circulation pipeline again.

[0050] Example 1-1 to Example 1-3: There is no stirring in the melting tank and no stirring by the circulation pump I. The DMT slurry input amounts in each example are 3600kg / BX, 3800kg / BX and 4000kg / BX, respectively. The melting tank is heated and the melting time required to reach the set temperature is recorded.

[0051] Example 1-4 to Example 1-6: The agitator in the melting tank was used for stirring, but there was no circulation pump I for stirring. The DMT slurry input amounts in each example were 3600kg / BX, 3800kg / BX and 4000kg / BX, respectively. The melting tank was heated and the melting time required to reach the set temperature was recorded.

[0052] Example 1-7 to Example 1-9: There is no stirring in the melting tank, and the circulation pump I is used to realize pump circulation stirring. The turbulence of the DMT slurry in the melting tank is increased by external circulation of the pump. The pump flow rate is 70m 3 / h, the lift is 25m; the DMT slurry input amounts in each embodiment are 3600kg / BX, 3800kg / BX and 4000kg / BX respectively, and the melting time required to reach the set temperature is recorded by heating the melting tank.

[0053] Table 1 Effect of stirring form on DMT melting time As shown in Table 1 and Figure 5 As shown, it can be clearly observed from the data that the time required for melting the DMT slurry to the specified temperature using a non-stirring melting tank is about 40 to 50 minutes; installing an agitator in the tank can shorten the melting time to 30 to 35 minutes; pump circulation stirring can further shorten the melting time to 25 to 30 minutes.

[0054] Example 2 (Effect of heating form on DMT melting time) The method for DMT melting recovery using the above system comprises the following steps: (1) The DMT slurry obtained in the chemical recycling process of waste PET is centrifuged in a centrifuge, and the solid phase (temperature is 60°C, containing 16.7wt.% methanol, 72.6wt.% DMT, 10wt10.2wt.% ethylene glycol, 0.5wt.% high boiling point impurities (HMT, etc.)) is fed into a melting tank through a feed inlet. A heating device provided in the melting tank heats the material. After heating to 160°C, the heating is stopped, and the opening of the temperature control valve I is reduced so that the melting tank is maintained in a heat-insulating state at this time.

[0055] (2) When the melting tank starts to be heated, the material in the melting tank is passed into the gas-liquid separator for gas-liquid separation. The operating conditions of the gas-liquid separator are 15kPa (A) and 180°C. At the same time, it is passed into the circulation pipeline for circulation. The heating setting temperature in the circulation pipeline is 180°C. After separation by the gas-liquid separator, the gas phase passes out of the melting system. After separation, the liquid phase passes into the circulation pipeline. The circulation pipeline is equipped with a double-pass heater for heating. The liquid phase and the material passed out of the melting tank are heated together through the circulation pipeline and then passed into the melting tank again. They are passed out of the melting tank and circulated in the circulation pipeline again. When the temperature of the melting tank reaches 160°C and the heating is stopped, the opening of the temperature control valve II is increased.

[0056] (3) The material is heated through the circulation pipeline. When the material in the melting tank is maintained at 180°C for 60 minutes, the circulation pump I, valve b and valve c are closed, and valve d, the discharge pump and valve a are opened to discharge the molten DMT for refining.

[0057] Example 2-1 to Example 2-3: The melting tank was not provided with an external coil heating and the bottom was not filled with nitrogen. The DMT slurry input amount in each example was 3600 kg / BX, 3800 kg / BX and 4000 kg / BX respectively. The melting tank was heated and the melting time required to reach the set temperature was recorded. In addition, when the molten DMT was discharged, the DMT concentration and the methanol concentration were sampled and tested.

[0058] Example 2-4 to Example 2-6: The melting tank is not provided with an external coil heating, but the bottom is filled with nitrogen, the nitrogen pressure is 0.2MPa, the temperature is 160°C, and the gas distributor is set as in Example 1. The DMT slurry input amount in each example is 3600kg / BX, 3800kg / BX and 4000kg / BX respectively, and the melting tank is heated to record the melting time required to reach the set temperature. In addition, sampling is performed to detect the DMT concentration and methanol concentration when the molten DMT is discharged.

[0059] Example 2-7 to Example 2-9: The melting tank is provided with an external coil heating and nitrogen is filled at the bottom. The nitrogen pressure is 0.2 MPa and the temperature is 160°C. The gas distributor is provided in the same manner as in Example 1. The DMT slurry input in each example is 3600 kg / BX, 3800 kg / BX and 4000 kg / BX, respectively. The melting tank is heated and the melting time required to reach the set temperature is recorded. In addition, when the molten DMT is discharged, sampling is performed to detect the DMT concentration and the methanol concentration (it also contains high boiling point impurities, which are removed by distillation after discharge).

[0060] Table 2 Effect of heating form on DMT melting time Table 3 Comparison of melting conditions As shown in Table 2 and Figure 6As shown, if a circulating pipeline heater is set at the same time on the basis of pump circulation stirring (it will be more conducive to the melting process, and the melting time is only 20 to 25 minutes. Since the melting process proposed in the present invention focuses on the melting process of DMT slurry, the main components of DMT slurry are DMT crystals, a small amount of DMI, oligomers and other heavy components and a small amount of methanol. In Example 2, the effect of nitrogen filling on the composition is compared. Nitrogen filling is not performed at the bottom of the melting tank. A small amount of methanol (1.5 to 2wt%) still exists after melting. While nitrogen filling melting, the methanol in the feed liquid can be completely removed, and the concentration of the target product DMT is further increased.

[0061] As shown in Table 3, the concentration of DMT in the embodiment is further compared. Nitrogen filling at the bottom of the melting tank can effectively remove methanol and thus increase the concentration of DMT, and whether or not the melting tank outer coil is set will also cause different DMT loss rates. In the embodiment where the tank outer coil is not set, the pipe mouth is easy to clog, and the clogging cycle is about 85 days, which not only consumes manpower time to clean, but also causes the loss of DMT. The melting tank has an outer coil and basically no clogging will occur, and the DMT loss rate can be effectively reduced, and the yield of the target product can be improved.

[0062] Example 3 (Effect of holding time on ethylene glycol removal effect) The method for DMT melting recovery using the above system comprises the following steps: (1) The DMT slurry obtained in the chemical recycling process of waste PET is centrifuged in a centrifuge, and the solid phase (temperature of 60°C, containing 10wt.% ethylene glycol) is fed into the melting tank from the feed port. The heating device provided in the melting tank heats the material. After heating to 160°C, the heating is stopped, and the opening of the temperature control valve I is reduced so that the melting tank is maintained in a heat-insulating state. At the same time, during the entire melting process, nitrogen is charged from the nitrogen charging inlet to the bottom of the melting tank, the nitrogen pressure is 0.2MPa, the temperature is 160°C, and the gas distributor is set as in Example 1.

[0063] (2) When the melting tank starts to be heated, the material in the melting tank is passed into the gas-liquid separator for gas-liquid separation. The operating conditions of the gas-liquid separator are 15kPa (A) and 180°C. At the same time, it is passed into the circulation pipeline for circulation. The heating setting temperature in the circulation pipeline is 180°C. After separation by the gas-liquid separator, the gas phase passes out of the melting system. After separation, the liquid phase passes into the circulation pipeline. The circulation pipeline is equipped with a double-pass heater for heating. The liquid phase and the material passed out of the melting tank are heated together through the circulation pipeline and then passed into the melting tank again. They are passed out of the melting tank and circulated in the circulation pipeline again. When the temperature of the melting tank reaches 160°C and the heating is stopped, the opening of the temperature control valve II is increased.

[0064] (3) The material is heated through the circulation pipeline. When the material in the melting tank is maintained at 180° C. for 60 min (Example 3-1) or maintained at 180° C. for 20 min (Example 3-2), the circulation pump I, valve b and valve c are closed, and valve d, the discharge pump and valve a are opened to discharge the molten DMT.

[0065] Table 4 Effect of residence time on material composition As shown in Table 4, the material in the melting tank needs to be maintained at 180-190°C for a certain period of time to remove the ethylene glycol in the DMT slurry. If the ethylene glycol is not removed in time, the purity of the product DMT will be affected during the subsequent refining of DMT.

[0066] Example 4 (Effect of the Nitrogen Gas Distributor at the Bottom of the Melting Tank on the Methanol Removal Time) The method for DMT melting recovery using the above system comprises the following steps: (1) The DMT slurry obtained in the chemical recycling process of waste PET is centrifuged in a centrifuge, and the solid phase is fed into the melting tank from the feed port. The heating device provided in the melting tank heats the material, and the heating is stopped after heating to 160°C, and the opening of the temperature control valve I is reduced so that the melting tank is maintained in a heat preservation state. At the same time, nitrogen is charged from the nitrogen charging inlet to the bottom of the melting tank during the entire melting process, and the nitrogen pressure is 0.2MPa and the temperature is 160°C.

[0067] Example 4-1: No nitrogen distributor is provided at the bottom of the melting tank. Instead, nitrogen is supplied into the melting tank through 8 nitrogen inlets evenly distributed at the bottom of the melting tank. The methanol content in the gas phase is monitored by gas chromatography to 0wt.% to determine the methanol removal time.

[0068] Example 4-2: A gas distributor of the same type as described is set at the bottom of the melting tank, with the inclination angle β=γ=0° of the tongue-shaped hole, the angle α of the tongue piece=45°, the arc diameter of the tongue-shaped holes on both sides is 22 mm, the hole spacing is 50 mm, and the methanol content in the gas phase is monitored by gas chromatography to 0wt.% to determine the methanol removal time.

[0069] Example 4-3: A gas distributor of the same type as described is set at the bottom of the melting tank, with the inclination angles of the tongue-shaped holes β=45°, γ=135°, the tongue angle α=45°, the arc diameters of the tongue-shaped holes on both sides are 22 mm, the hole spacing is 50 mm, and the methanol content in the gas phase is monitored by gas chromatography to 0wt.% to determine the methanol removal time.

[0070] Example 4-4: The same type of gas distributor is set at the bottom of the melting tank, the inclination angle γ of the tongue-shaped hole on the 201 side is 135°, the arc diameter of the tongue-shaped hole is d=18mm, and the hole spacing is t=40mm; the inclination angle β of the tongue-shaped hole on the other side is 45°, the arc diameter of the tongue-shaped hole is d′=28mm, and the hole spacing is t′=50mm. The tongue angle α is 30°, and the methanol content in the gas phase is monitored by gas chromatography to 0wt.%, so as to determine the methanol removal time.

[0071] Table 5 Effect of gas distributor on methanol removal time Under the premise that the volume flow rate of nitrogen introduced is equal, the nitrogen removal time required for the four embodiments is shown in Table 5. A nitrogen gas distributor arranged at the bottom of the melting tank can shorten the time for methanol removal by 62-65% compared with that without a gas distributor. The inclination mode of the tongue-shaped hole of the gas distributor and the angle of the tongue also have obvious advantages in the methanol removal process. Compared with Embodiment 4-2 and Embodiment 4-3, Embodiment 4-4 (the gas distributor) shortens the methanol removal time by 40-60%.

[0072] In summary, the melting process in the present invention adds the following pipelines compared to the original melting process: (1) nitrogen filling pipeline at the bottom of the tank: it can effectively and completely remove the methanol gas phase in the material, thereby improving the quality of the product during the subsequent DMT distillation; (2) gas-liquid separator pipeline: it can remove part of the ethylene glycol by flash evaporation, and also can improve the quality of the product during the subsequent DMT distillation; (3) circulating pipeline DMT exhaust pipeline: it can effectively reduce the loss rate of the target product DMT, improve the DMT yield, and avoid the blockage of the circulating pipeline heater, thereby reducing its cleaning difficulty. It is known from the on-site operation that when the drain pipeline is not set, the relevant equipment often gets clogged, with a frequency of about once a year, and clearing the blockage consumes a lot of manpower and time; the molten material in the heater contains a large amount of DMT, and DMT is easy to crystallize when it is not circulated and heated, which is easy to clog the circulation pipeline and heater, affecting the production process. When the heater drain pipeline (DMT discharge pipeline 7) is set, the heater has not been blocked, which improves the production process of DMT to a certain extent. If the pipeline is not set, the material in the heater cannot be drained, but the material in the molten tank can be discharged through the circulation pipeline 5 and the pipeline where the valve a16 is located. At the same time, compared with the original melting process, an umbrella-shaped gas distributor composed of several elliptical cylinders is set, which can more efficiently remove the methanol in the melting tank without affecting the molten DMT discharge process, reduce the methanol removal time, and ensure product quality.

[0073] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the specification of the present invention, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for melting and regenerating DMT in a chemical recycling process of waste PET, characterized in that: The steps include: (1) The DMT slurry obtained in the chemical recycling process of waste PET is centrifuged and the solid phase is passed into a melting tank and heated. The heating is stopped after the temperature reaches 150-160°C; (2) When the melting tank starts to be heated, the material in the melting tank is passed into the gas-liquid separator for gas-liquid separation. After separation, the liquid phase is passed into the circulation pipeline. The circulation pipeline is equipped with a double-pass heater for heating. The set temperature is 180~190℃. The liquid phase is passed into the melting tank through the circulation pipeline and is passed out of the melting tank again to the circulation pipeline for circulation with the material in the melting tank; (3) When the material in the melting tank is maintained at 180-190°C for 30-60 min, the molten DMT can be discharged and refined.

2. The method for melting and regenerating DMT in the chemical recovery process of waste PET according to claim 1, characterized in that: The DMT slurry comprises DMT crystals, methanol and ethylene glycol; the temperature of the solid phase obtained after centrifugal separation is 40-60°C.

3. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to claim 1, characterized in that: The melting tank is provided with a heating coil inside, and heat transfer oil at 240-260° C. is passed through the coil.

4. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to claim 3, characterized in that: The heating coil comprises a vertical inner coil arranged on the inner wall of the melting tank and a horizontal inner coil arranged on the bottom of the melting tank.

5. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to any one of claims 1 to 4, characterized in that: The outer wall of the melting tank is provided with a heating outer coil, and steam at 160-170° C. is passed through the coil.

6. The method for melting and regenerating DMT in the chemical recovery process of waste PET according to claim 1, characterized in that: The operating conditions of the gas-liquid separator are 10~20 kPa (A), 180~190°C.

7. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to claim 1 or 6, characterized in that: The liquid level in the melting tank is level with that in the gas-liquid separator.

8. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to claim 1, characterized in that: The melting tank is stirred by a pump circulation, and the flow rate of the circulation pump is 70~80 m 3 / h, the head is 25~30 m.

9. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to claim 1 or 8, characterized in that: Nitrogen is continuously introduced into the bottom of the melting tank, and the gas phase in the tank is discharged from the top.

10. The method for melt regeneration of DMT in the chemical recovery process of waste PET according to claim 9, characterized in that: The pressure of the nitrogen is 0.15-0.25 MPa, and the temperature is 140-160°C.

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