Drying method and system for fully biodegradable polyester chips

Abstract

The present invention belongs to the field of polyester chip drying, and specifically relates to a drying method and system for fully biodegradable polyester chips. The drying system comprises a microwave preheater, a dryer connected to a discharge port of the microwave preheater, a gas supply device for continuously introducing gas into the microwave preheater and the dryer, a dew-point test device provided at a gas outlet of the microwave preheater, and a dew-point test device provided at a gas outlet of the dryer.

Description

Drying method and system for fully biodegradable polyester chips Technical Field

[0001] This invention belongs to the field of polyester chip drying, specifically relating to a drying method and system for fully biodegradable polyester chips. Background Technology

[0002] Fully biodegradable polyester is an aliphatic polyester with a simple, regular, repeating ester bond structure. The presence of ester bonds in the molecular chain gives fully biodegradable materials carboxyl and hydroxyl groups, making them particularly sensitive to water and heat. In high-temperature and humid environments, the moisture within the fully biodegradable chips will cause hydrolysis, leading to depolymerization of the molecular chains and the generation of molecular chain segments with terminal carboxyl and hydroxyl groups. As the fully biodegradable polyester approaches its glass transition temperature, its molecular weight decreases significantly with increasing hydrolysis time. If fully biodegradable polyester chips are packaged directly without drying, the moisture content within the chips directly affects the product's shelf life and consequently the stability of downstream processing. Therefore, for both chip packaging and downstream melt processing, thorough drying is essential, and the moisture content in the system must be strictly controlled to prevent excessive moisture from causing hydrolysis during processing, avoiding molecular chain breakage and molecular weight reduction, and ultimately affecting the physical properties of the finished product.

[0003] In the industrial production of fully biodegradable polyester, after polymerization, the polyester is generally spun into chips using two methods: air-cooled pelletizing and underwater pelletizing. Air-cooled pelletizing absorbs moisture from the cooling air during the cooling process, resulting in fully biodegradable polyester chips with a moisture content of approximately 0.06%. Underwater pelletizing typically involves centrifugal dehydration, resulting in fully biodegradable polyester chips with a moisture content of approximately 0.2%. Regardless of the spunting method, if the fully biodegradable polyester chips are directly bagged, the moisture within the chips inside the packaging can easily lead to hydrolysis.

[0004] Existing drying methods, such as traditional hot air drying, are time-consuming, while the widely used vacuum drying method cannot be used for continuous production in industrial processes. Therefore, there is an urgent need to provide a new drying method to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide a drying method and system for fully biodegradable polyester chips.

[0006] In a first aspect, the present invention provides a drying system for polyester chips, the drying system comprising a microwave preheater, a dryer connected to the outlet of the microwave preheater, a gas supply device for continuously introducing gas into the microwave preheater and the dryer, and a dew point testing device disposed at the outlet of the microwave preheater and the outlet of the dryer.

[0007] In one or more embodiments, the gas supply device includes a heater connected to the air inlet of the microwave preheater and the air inlet of the dryer, respectively, and a gas flow control valve is provided on the pipeline connecting the heater and the microwave preheater, and / or on the pipeline connecting the heater and the dryer.

[0008] In one or more embodiments, the microwave preheater includes a housing, and a material conveying device, a microwave radiation device, and a material thickness homogenization device disposed inside the housing;

[0009] The microwave radiation device is used to radiate microwaves onto the surface of the material conveying device. The material thickness homogenizing device is located above the starting end of the material conveying device, and the distance between the bottom surface of the material thickness homogenizing device and the surface of the material conveying device is adjustable.

[0010] In one or more embodiments, the dryer is a single-shaft spiral stirring dryer, which includes a shell, at least one hollow spiral conveying shaft disposed within the shell, and at least one transmission device disposed outside the shell. One of the transmission devices is connected to one of the hollow spiral conveying shafts. Adjacent hollow spiral conveying shafts are separated by a partition, and the conveying directions of adjacent hollow spiral conveying shafts are opposite. An air-material channel is provided between the partition and the shell.

[0011] The last stage hollow screw conveyor shaft is hollow inside and has ventilation holes on its surface. One end of it is connected to a rotary joint with an air inlet, and the other end is connected to the transmission device. The outlet of the dryer is located below the end of the last stage hollow screw conveyor shaft near the air inlet.

[0012] In one or more embodiments, the drying system further includes a gas purification and recycling device for purifying and recycling the gas flowing out of the microwave preheater and the dryer.

[0013] The gas purification and circulation device includes a cyclone separator, and a catalytic oxidizer, a cooler, a dehumidifier, and a fan connected in sequence to the cyclone separator; the outlet of the microwave preheater and the outlet of the dryer are respectively connected to the cyclone separator, and the outlet of the fan is connected to the gas supply device.

[0014] In one or more embodiments, the drying system further includes a cooling hopper connected to the outlet of the dryer, the cooling hopper being provided with a spiral coil and a plurality of through holes, one end of the coil being an air inlet and an air outlet being provided at the upper part of the cooling hopper;

[0015] The air outlet of the fan is connected to the air inlet of the coil, and the air outlet of the cooling silo is connected to the air supply device.

[0016] A second aspect of the present invention provides a method for drying polyester chips using the drying system described in the first aspect of the present invention, the method comprising the steps of:

[0017] (1) In a microwave preheater, under microwave radiation, the polyester chip raw material is dried using a first hot air stream to obtain pre-dried polyester chips.

[0018] (2) The pre-dried polyester chips are dried in a dryer using a second hot air stream to obtain dried polyester chips;

[0019] The gas dew point temperature of the first hot gas stream at the outlet of the microwave preheater is ≤-10℃;

[0020] The gas dew point temperature of the second hot gas stream at the outlet of the dryer is ≤-10℃.

[0021] In one or more embodiments, the drying temperature in the microwave preheater is 100–200°C.

[0022] In one or more embodiments, the gas dew point temperature of the first hot gas stream at the outlet of the microwave preheater is -40°C to -10°C.

[0023] In one or more embodiments, the gas dew point temperature of the first hot gas stream at the inlet of the microwave preheater is ≤-40°C.

[0024] In one or more embodiments, the drying time in the microwave preheater is 1 to 15 minutes.

[0025] In one or more embodiments, the drying temperature in the dryer is 100–200°C.

[0026] In one or more embodiments, the gas dew point temperature of the second hot gas stream at the outlet of the dryer is -40°C to -10°C.

[0027] In one or more embodiments, the gas dew point temperature of the second hot gas stream at the air inlet of the dryer is ≤-40°C.

[0028] In one or more embodiments, the drying time in the dryer is 0.5 to 2 hours.

[0029] In one or more embodiments, the polyester chip raw material has a particle size ≥1.5 mm, preferably 1.5 to 6 mm.

[0030] In one or more embodiments, the moisture content of the polyester chip raw material is 0.03 to 0.3 wt%.

[0031] In one or more embodiments, the melt index of the polyester chip raw material is 10 to 80 g / 10 min (230°C, 2.16 kg).

[0032] In one or more embodiments, the residual monomer content of the polyester chip raw material is 1 to 5 wt%.

[0033] In one or more embodiments, the moisture content of the dried polyester chips is <0.005 wt%.

[0034] In one or more embodiments, the melt index of the dried polyester chips is 3 to 50 g / 10 min (230°C, 2.16 kg).

[0035] In one or more embodiments, the residual monomer content of the dried polyester chips is <1 wt%, preferably 0.2 to 1 wt%. Attached Figure Description

[0036] Figure 1 is a schematic diagram of a drying system in one or more embodiments of the present invention; wherein, 1-microwave preheater, 2-dryer, 3-cooling silo, 31-coil, 4-cyclone separator, 5-catalytic oxidizer, 6-cooler, 7-dehumidifier, 8-fan, 9-heater, a-gas volume control valve, b-rotary valve, c-heat transfer oil outlet, d-heat transfer oil inlet.

[0037] Figure 2 is a schematic diagram of a microwave preheater in one or more embodiments of the present invention; wherein, 11-feed inlet, 12-discharge outlet, 13-air inlet, 14-air outlet, 15-microwave radiation device, 16-side baffle block, 17-material thickness homogenization device, and 18-material conveying device.

[0038] Figure 3 is a partial top view of the material thickness homogenizing device and the material conveying device in one or more embodiments of the present invention; wherein, 17-material thickness homogenizing device, 18-material conveying device.

[0039] Figure 4 is a schematic diagram of a single-axis spiral stirring dryer in one or more embodiments of the present invention; wherein, 21-inlet, 22-outlet, 23-air inlet, 24-air outlet, 25-rotary joint, 26-ventilation hole, 27-transmission device.

[0040] Figure 5 is a schematic diagram of a two-stage single-shaft spiral stirring dryer in one or more embodiments of the present invention; wherein, 21'-inlet, 22'-outlet, 23'-air inlet, 24'-air outlet, 25'-rotary joint, 26'-ventilation hole, 27'-transmission device, 28'-partition plate, 29'-air-material channel. Detailed Implementation

[0041] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0042] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0043] In this document, the terms “contains,” “includes,” “containing,” and similar terms encompass the meanings of “basically composed of” and “composed of.” For example, when this document discloses “A contains B and C,” “A is basically composed of B and C” and “A is composed of B and C” should be considered as having been disclosed in this document.

[0044] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0045] Unless otherwise specified, percentages refer to mass percentages and proportions refer to mass ratios in this article.

[0046] In this document, when describing embodiments or examples, it should be understood that it is not intended to limit the invention to those embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein are covered within the scope defined by the claims.

[0047] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0048] In this article, the gas-to-material ratio refers to the ratio of gas to material entering the microwave preheater or dryer. The gas-to-material ratio entering the microwave preheater is the ratio of the volume of the first hot gas stream entering the preheater to the mass of the polyester chip raw material. The gas-to-material ratio entering the dryer is the ratio of the volume of the second hot gas stream entering the dryer to the mass of the pre-dried polyester chips. For example, 1:(50~150)L / g means that 1L of gas is introduced for every 50~150g of material. The gas-to-material ratio entering the microwave preheater or dryer can be adjusted according to the gas dew point temperature at the outlet of each device. The larger the gas-to-material ratio, the lower the moisture content in the gas at the outlet, and the lower the gas dew point temperature; conversely, the smaller the gas-to-material ratio, the higher the moisture content in the gas at the outlet, and the higher the gas dew point temperature. The gas dew point temperature at the outlet can be measured using a dew point meter.

[0049] In this paper, the microwave preheater is equipped with an advanced temperature sensing and control system, which can monitor the material temperature in real time and automatically adjust the microwave output power according to preset parameters, thereby achieving precise control of the temperature inside the microwave preheater during the drying process.

[0050] At higher temperatures, moisture in the environment (including some moisture released from the polyester chips) and moisture within the polyester chips can easily lead to hydrolysis of the polyester. The drying method of this invention uses a microwave preheater to preheat the polyester chips, which greatly shortens the preheating time of the polyester chips and can quickly remove some moisture from the polyester chips. The hot air convection in the microwave preheater and dryer can not only remove unreacted monomers and oligomers, but also remove some of the moisture released from the polyester chips, reducing the occurrence of polyester hydrolysis. Moreover, the drying method of this invention also needs to control the gas dew point temperature at the outlet of the microwave preheater and the outlet of the dryer. The gas dew point temperature at the outlet of the microwave preheater reflects the moisture content in the environment inside the microwave preheater, and the gas dew point temperature at the outlet of the dryer reflects the moisture content in the environment inside the dryer. By keeping the gas dew point temperature at both outlets below -10°C, the excessive moisture content in the system can be prevented.

[0051] Drying system for polyester chips

[0052] As shown in Figure 1, the present invention provides a drying system for polyester chips, including a microwave preheater 1, a dryer 2 connected to the outlet of the microwave preheater 1, a gas supply device for continuously introducing gas into the microwave preheater 1 and the dryer 2, and a dew point testing device installed at the outlet of the microwave preheater 1 and the outlet of the dryer 2.

[0053] The microwave preheater 1 is provided with a feed inlet, a discharge inlet, an air inlet, and an air outlet, and the dryer 2 is provided with a feed inlet, a discharge inlet, an air inlet, and an air outlet. As a further preferred embodiment, dew point testing equipment is provided at the air inlet of both the microwave preheater 1 and the dryer 2. Exemplarily, the dew point testing equipment is an online dew point meter.

[0054] The gas supply device includes a heater 9, which is connected to the air inlet of the microwave preheater 1 and the air inlet of the dryer 2 respectively. A gas flow control valve a is installed on the pipeline connecting the heater 9 and the microwave preheater 1, and / or on the pipeline connecting the heater 9 and the dryer 2. The gas flow rate entering the microwave preheater 1 and / or the dryer 2 is controlled by the gas flow control valve a. Only one heater 9 is needed to heat the gas entering the microwave preheater 1 and the dryer 2.

[0055] In some embodiments, the inner wall of the dryer 2 shell is provided with a jacket, through which heat transfer oil is circulated. The shell is provided with a heat transfer oil outlet c and a heat transfer oil inlet d. The outer surface of the dryer 2 shell is further wrapped with insulation material. The temperature of the heat transfer oil can be the same as or higher than the temperature of the gas, preferably the same as the temperature of the gas.

[0056] In some embodiments, the drying system of the present invention further includes a cooling silo 3 connected to the outlet of the dryer 2. The cooling silo 3 is provided with an inlet and an outlet. The outlet of the dryer 2 is connected to the inlet of the cooling silo 3, and the outlet of the cooling silo 3 is the outlet for the dried material. The material dried by the dryer 2 first enters the cooling silo 3, is cooled, and then packaged.

[0057] Furthermore, a spiral coil 31 is provided inside the cooling silo 3, and several through holes are provided on the coil 31. One end of the coil 31 is the air inlet, and the other end of the coil 31 can be closed or open. An air outlet is provided at the top of the cooling silo.

[0058] Low-temperature, dry gas enters the cooling silo 3 through the air inlet (i.e., the air inlet end of the coil 31), then flows out of the coil 31 through the through-hole, where it exchanges heat and moisture with the material inside the cooling silo 3, before flowing out through the air outlet at the top of the cooling silo 3. The gas flowing out of the cooling silo 3 has extremely low moisture content and can also mix with the gas entering the drying system from the gas inlet in the pipe leading to the heater 9, before entering the heater 9 again, thus achieving gas recycling.

[0059] In the drying system of this invention, when drying raw materials, the gas is heated to a set temperature in heater 9, and then enters microwave preheater 1 and dryer 2 respectively. The material enters microwave preheater 1 through the feed inlet, is preheated and dried by microwaves and hot convection gas, and then exits from the discharge outlet of microwave preheater 1 into dryer 2. In dryer 2, it is further dried by hot convection gas and then exits from the discharge outlet of dryer 2. Preferably, the material discharged from the discharge outlet of dryer 2 enters cooling silo 3, is cooled and further dried in cooling silo 3, and then exits from the discharge outlet of cooling silo 3.

[0060] Furthermore, rotary valves b are installed on the pipes connecting the raw material inlet and the inlet of microwave preheater 1, the pipes connecting the outlet of microwave preheater 1 and the inlet of dryer 2, the pipes connecting the outlet of dryer 2 and the inlet of cooling silo 3, and the pipes connecting the outlet of cooling silo 3 and the outlet of dried material. Rotary valves b can be used to adjust the flow direction and flow rate of the material.

[0061] Furthermore, the drying system of the present invention also includes a gas purification and circulation device for purifying and recycling the gas flowing out of the microwave preheater 1 and the dryer 2; the gas purification and circulation device includes a cyclone separator 4, and a catalytic oxidizer 5, a cooler 6, a dehumidifier 7, and a fan 8 connected in sequence to the cyclone separator 4. The gas outlets of the microwave preheater 1 and the dryer 2 are respectively connected to the cyclone separator 4, and the gas outlet of the fan 8 is connected to the gas supply device.

[0062] Preferably, the air outlet of the fan 8 is connected to the air inlet of the coil 31 in the cooling silo 3, and the air outlet at the upper end of the cooling silo 3 is connected to the air supply device.

[0063] In practical applications, the gas discharged from the outlet of the microwave preheater 1 enters the cyclone separator 4, where it is separated from the entrained solid and liquid impurities. Then it enters the catalytic oxidizer 5, where small organic molecule impurities are removed. Then it enters the cooler 6, where it is cooled down. Then it enters the dehumidifier 7, where it is dehumidified. Then it enters the fan 8, where the gas flows out and enters the cooling silo 3. In the cooling silo 3, it undergoes heat and moisture exchange with the material, and then enters the heater 9.

[0064] Similarly, the gas discharged from the outlet of dryer 2 enters cyclone separator 4, where it is separated from the entrained solid and liquid impurities. Then it enters catalytic oxidizer 5, where small organic molecule impurities are removed. Then it enters cooler 6, where it is cooled. Then it enters dehumidifier 7, where it is dehumidified. Then it enters fan 8, where the gas flows out and enters cooling silo 3. In cooling silo 3, it undergoes heat and moisture exchange with the material and then enters heater 9.

[0065] As a preferred option, the gas discharged from the outlet of the microwave preheater 1 and the gas discharged from the outlet of the dryer 2 are mixed in the pipe leading to the cyclone separator 4, and then enter the cyclone separator 4. In this way, only one gas purification and circulation device needs to be set up.

[0066] As shown in Figures 2-3, in the above embodiment, the microwave preheater 1 includes a housing, an inlet 11, an outlet 12, an air inlet 13, and an air outlet 14 disposed on the housing, and a material conveying device 18 and a microwave radiation device 15 disposed inside the housing; the microwave radiation device 15 is used to radiate microwaves to the surface of the material conveying device 18, the inlet 11 and the air outlet 14 are disposed above the starting end of the material conveying device 18, and the outlet 12 and the air inlet 13 are disposed below the ending end of the material conveying device 18.

[0067] Furthermore, the microwave preheater 1 also includes a material thickness equalization device 17 disposed inside the housing and above the starting end of the material conveying device 18, and a side baffle block 16 disposed on the material conveying device 18 to prevent material overflow. The distance between the bottom surface of the material thickness equalization device 17 and the surface of the material conveying device 18 is adjustable, and the thickness of the material on the material conveying device 18 can be adjusted by adjusting the distance between the bottom surface of the material thickness equalization device 17 and the surface of the material conveying device 18. In some embodiments, the distance between the bottom surface of the material thickness equalization device 17 and the surface of the material conveying device 18 is 5–80 mm.

[0068] For example, the material thickness homogenizing device 17 is shaped like an isosceles triangle, with its base perpendicular to the conveying direction of the material conveying device 18. The material passes through the legs of the isosceles triangle first and then through its base, with the apex angle of the isosceles triangle ≤ 120°. Setting the apex angle of the isosceles triangle formed by the material thickness homogenizing device 17 to below 120° can effectively divert the incoming material. Preferably, the apex angle of the isosceles triangle formed by the material thickness homogenizing device 17 is 120°. After the material to be dried (e.g., polyester chips) is fed, it can be evenly spread on the surface of the material conveying device 18 under the material thickness homogenizing device 17, and the thickness of the spread can be adjusted by the height set by the material thickness homogenizing device 17.

[0069] Furthermore, the material thickness equalization device 17 can be supported on both sides of the material conveying device 18 by support legs. Since the plane is triangular, a crossbeam can be added at the midpoint of the feed point as needed to make the support more stable. In some other embodiments, the material thickness equalization device 17 can be installed above the material conveying device 18 by a hanger fixed to the housing.

[0070] The shell of the microwave preheater 1 is made of glass fiber; and / or the material conveying device 18 is made of Teflon.

[0071] In the above embodiments, the dryer 2 can be a vertical dryer or a horizontal dryer. Exemplary vertical dryers include, but are not limited to, vibrating countercurrent dryers, tower dryers, and fluidized bed dryers. Exemplary horizontal dryers include, but are not limited to, single-shaft spiral stirring dryers, dual-shaft spiral stirring dryers, multi-shaft spiral stirring dryers, and drum dryers.

[0072] Preferably, the tower dryer and fluidized bed dryer are equipped with a stirring component that keeps the material dynamically dry during the drying process, which can prevent the material from sticking together; or, the vertical dryer is a vibrating counter-current dryer, in which the material is less likely to "bridge" or clump together under the spiral vibration environment.

[0073] Inside dryer 2, the gas flow direction is opposite to the material flow direction. For example, in a vertical dryer, low dew point hot gas enters from the air inlet at the bottom of the dryer, passes through the material layer, and flows in the opposite direction to the material for drying. The material can generate "reciprocating motion" inside the drying equipment.

[0074] In some embodiments, the dryer 2 is a single-shaft spiral stirring dryer, which includes a shell, at least one hollow spiral conveyor shaft disposed inside the shell, and at least one transmission device disposed outside the shell. The transmission device is connected to one hollow spiral conveyor shaft. Adjacent hollow spiral conveyor shafts are separated by partitions, and the conveying directions of adjacent hollow spiral conveyor shafts are opposite. An air-material channel is left between the partitions and the shell. The shaft of the last hollow spiral conveyor shaft is hollow inside and has ventilation holes on its surface. One end of the shaft is connected to a rotary joint with an air inlet, and the other end is connected to the transmission device. The outlet of the dryer is located below the end of the last hollow spiral conveyor shaft near the air inlet.

[0075] Gas enters the last stage hollow spiral conveyor shaft through the air inlet on the rotary joint, and flows out through the vent hole of the last stage hollow spiral conveyor shaft into the shell to dry the material. Finally, it flows out from the air outlet of the single-shaft spiral stirring dryer. The conveying direction of the material in the same stage inside the shell is opposite to the mainstream flow direction of the gas.

[0076] Specifically, the single-shaft spiral stirring dryer can be a single-stage, two-stage, or higher single-shaft spiral stirring dryer. The number of hollow spiral conveying shafts and the number of transmission devices contained in the single-stage, two-stage, or higher single-shaft spiral stirring dryer are the same as its stage number, and one transmission device is connected to one hollow spiral conveying shaft.

[0077] The structure of a single-shaft spiral agitator dryer is shown in Figure 4. The shell of the single-shaft spiral agitator dryer is equipped with an inlet 21, an outlet 22, and an air outlet 24. The inlet 21 and outlet 24 are located above the end of the hollow spiral conveyor shaft furthest from the air inlet 23, while the outlet 22 is located below the end of the hollow spiral conveyor shaft closest to the air inlet 23. The hollow spiral conveyor shaft includes a central shaft and conveying blades spirally wound around the central shaft. The central shaft is hollow inside and has several ventilation holes 26 on its surface. A transmission device 27, connected to the central shaft, drives the conveying blades to rotate, thereby propelling the material conveyor.

[0078] Materials (such as polyester chips and fully biodegradable polyester chips) enter through the feed inlet 21 and exit through the discharge outlet 22 of the single-shaft spiral stirred dryer. Low dew point gas enters through the air inlet 23 of the rotary joint 25 and exits through the vent 26 on the hollow spiral conveyor shaft. It passes through the material layer inside the spiral and exchanges water and heat with the material, finally exiting through the air outlet 24. The material conveying direction is opposite to the mainstream gas flow direction. The hollow spiral conveyor shaft rotates under the drive of the transmission device 27, thereby pushing the material to be conveyed. The material undergoes interface renewal under the action of the hollow spiral conveyor shaft, and the material moves in a plug flow motion to prevent adhesion.

[0079] The structure of two-stage or higher single-shaft spiral stirring dryers is similar, as shown in Figure 5. Taking a two-stage single-shaft spiral stirring dryer as an example, each stage includes a hollow spiral conveying shaft and a transmission device 27'. The two hollow spiral conveying shafts are stacked one on top of the other and separated by a partition 28'. The conveying directions of the upper and lower hollow spiral conveying shafts are opposite. The starting end of the lower hollow spiral conveying shaft is located below the end of the upper hollow spiral conveying shaft, so that the material conveying directions of the upper and lower stages are opposite. The channel formed between the partition 28' and the shell is the air-material channel 29', which is the discharge port of the upper single-shaft spiral stirring dryer and the inlet of the lower single-shaft spiral stirring dryer, as well as the air outlet of the lower single-shaft spiral stirring dryer and the air inlet of the upper single-shaft spiral stirring dryer.

[0080] The air inlet 23' of the entire single-shaft spiral stirring dryer is located on the rotary joint 25' connected to one end of the next-stage hollow spiral conveyor shaft, and the discharge port 22' is located below the end of the next-stage hollow spiral conveyor shaft near the air inlet 23'. That is, the discharge port 22' and the air inlet 23' are located on the same side of the single-shaft spiral stirring dryer; the feed inlet 21' and the air outlet 24' are located above the previous-stage hollow spiral conveyor shaft and on the same side as the discharge port 22' and the air inlet 23' (when the single-shaft spiral stirring dryer is an even-numbered single-shaft spiral stirring dryer). Only the central shaft of the next-stage hollow screw conveyor is hollow inside and has a vent hole 26' on its surface. The gas in the next-stage hollow screw conveyor is discharged from the vent hole 26' into the shell, passes through the material layer inside the next-stage screw, and exchanges water and heat with the material. After the exchange, the gas enters the material layer inside the previous-stage screw through the gas-material channel 29' and exchanges water and heat with the material. Finally, it is discharged from the outlet 24'. The material conveying direction of each stage is opposite to the mainstream gas flow direction.

[0081] Preferably, the diameter of the vent holes 26 and 26' is 0.1 to 1 mm, more preferably 0.4 to 0.6 mm or 0.2 to 0.8 mm.

[0082] Preferably, the gap between the part of the hollow spiral conveyor shaft closest to the housing and the housing is <1mm.

[0083] Preferably, the helical pitch of the hollow screw conveyor shaft is 100-500 mm, more preferably 250-350 mm, for example 320 mm.

[0084] Drying method

[0085] This invention provides a method for drying polyester chips, using the drying system described above to dry the polyester chips, the method comprising the following steps:

[0086] (1) In a microwave preheater, under microwave radiation, the polyester chip raw material is dried using a first hot air stream to obtain pre-dried polyester chips.

[0087] (2) In a dryer, the pre-dried polyester chips are dried using a second hot air stream to obtain dried polyester chips;

[0088] The gas dew point temperature of the first hot gas stream at the outlet of the microwave preheater is ≤-10℃;

[0089] The dew point temperature of the second hot gas stream at the outlet of the dryer is ≤-10℃.

[0090] Preferably, in step (1), the gas dew point temperature of the first hot gas flow at the outlet of the microwave preheater is controlled to be -40℃ to -10℃, for example -40℃, -35℃, -30℃, or between any two values.

[0091] Furthermore, the gas dew point temperature of the first hot gas stream at the inlet of the microwave preheater is ≤-40℃, for example -40℃, -45℃, -50℃, or within a range of any two values.

[0092] In step (1), the first hot air stream convectively dries the polyester chip raw material. The air-to-material ratio entering the microwave preheater can be 1:(50~150)L / g, for example 1:90L / g, 1:130L / g, 1:150L / g, or within any range of two values. The drying time in the microwave preheater can be 1~15min, for example 4min, 8min, 12min, or within any range of two values. The drying temperature in the microwave preheater can be 100~200℃, for example 120℃, 150℃, 180℃, or within any range of two values.

[0093] The wavelength of microwaves can be 1mm to 1m, preferably 50mm to 150mm, such as 80mm, 120mm, 140mm, or within any range of two values. The power of microwaves can be 140 to 160kW, such as 145kW, 150kW, 155kW, or within any range of two values.

[0094] In practical applications, polyester chips enter from the top and exit from the bottom inside the microwave preheater, while the first hot gas flow enters from the bottom and exits from the top inside the microwave preheater.

[0095] In step (2), the gas dew point temperature of the second hot gas stream at the outlet of the dryer is -40℃ to -10℃, for example -40℃, -20℃, -10℃, or between any two values.

[0096] Furthermore, the gas dew point temperature of the second hot gas stream at the air inlet of the dryer is ≤-40℃, for example -40℃, -45℃, -50℃, or within a range of any two values.

[0097] In step (2), a second hot air stream is used for convection drying of the pre-dried polyester chips. The air-to-material ratio entering the dryer can be 1:(10~100)L / g, for example 1:20L / g, 1:60L / g, 1:80L / g, or within any range of two values. The drying temperature in the dryer can be 100~200℃, for example 120℃, 150℃, 180℃, or within any range of two values. The drying time in the dryer can be 0.5~2h, for example 1h, 1.5h, 2h, or within any range of two values. Preferably, the temperature of the heat transfer oil in the dryer jacket, the drying temperature in the microwave preheater, and the drying temperature in the dryer are set to be the same.

[0098] In a preferred embodiment, the dryer is the aforementioned single-shaft spiral agitator dryer. Preferably, the rotational speed of the single-shaft spiral agitator dryer is 2–50 rpm, more preferably 5–30 rpm, for example, 6 rpm. Preferably, the material filling rate inside the single-shaft spiral agitator dryer is 5–95%, more preferably 40–85%, for example, 60%.

[0099] The first and second hot gas streams are each independently selected from one or more of air, nitrogen, and inert gases. Preferably, the first and second hot gas streams are the same gas.

[0100] In some embodiments, the polyester chips are polyglycolic acid chips; preferably, the polyester chips are fully biodegradable polyglycolic acid chips. The polyglycolic acid chips are copolymers containing repeating glycolic acid units.

[0101] In a preferred embodiment, the dried polyester chips are fed into a cooling silo for cooling. The temperature of the cooling silo can be ≤60°C, for example, 20°C, 40°C, 50°C, or between any two of these values; preferably 30 to 60°C.

[0102] In some implementations, the polyester chips after pelleting are directly fed into a microwave preheater; preferably, the feeding is carried out in a low dew point environment with a gas dew point temperature ≤ -30°C.

[0103] For example, the particle size of the polyester chip raw material can be ≥1.5 mm; preferably 1.5–6 mm or 2–6 mm. Further, the moisture content of the polyester chip raw material can be 0.03–0.3 wt%. Further, the melt flow index of the polyester chip raw material can be 10–80 g / 10 min (230°C, 2.16 kg). Further, the residual monomer content of the polyester chip raw material can be 1–5 wt%.

[0104] The moisture content of the dried polyester chips can be <0.005 wt%. Furthermore, the melt flow index of the dried polyester chips can be 3–50 g / 10 min (230°C, 2.16 kg). Furthermore, the residual monomer content of the dried polyester chips can be <1 wt%.

[0105] The embodiments of the present invention have the following beneficial effects:

[0106] 1. Microwaves are a method of internal heating that raises the temperature of water inside materials (such as polyester chips) and causes it to leave the material. Using microwaves to preheat materials can quickly raise the material temperature and has the advantages of fast heating speed and low heat loss.

[0107] 2. Preheating materials (such as polyester chips) with microwaves before sending them into a dryer can shorten the heating and drying time of the materials in the dryer, thus saving energy.

[0108] 3. Controlling the gas dew point temperature at the outlet of the microwave preheater and the dryer can reduce the occurrence of hydrolysis reactions in polyester chip raw materials during the preheating and drying processes. However, existing hot air drying methods typically adjust process parameters only based on whether the moisture content of the output material meets the standard, resulting in excessively long drying times and neglecting the adverse effects of environmental moisture on the polyester material during the drying process.

[0109] 4. The drying method of the present invention dries polyester chips under a low dew point gas and ensures that the low dew point hot air flow in the dryer is in a circulating state. The low dew point gas and the material flow in opposite directions, which can ensure that the material is heated evenly in the dryer. During the drying process, the hot air flow continuously removes moisture, some monomers and oligomers, which can improve the quality / performance of the final product and enable continuous production.

[0110] 5. The dryer shell has an internal jacket, and the addition of heat-conducting oil for heating and insulation can make the temperature inside the dryer more uniform.

[0111] 6. The drying system of the present invention includes a gas purification and circulation device (cyclone separator, catalytic oxidizer, cooler, dehumidifier, etc.). The high-temperature gas discharged from the microwave preheater and dryer contains a small amount of monomers and oligomers. After passing through the catalytic oxidizer, the monomers and oligomers in the gas are removed. After cooling, the gas is dehydrated and the purified gas can be reused. Furthermore, the purified gas first passes through a cooling silo to cool the material, which can increase the gas temperature and further save energy.

[0112] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. Percentages and parts are by weight unless otherwise stated.

[0113] Testing method:

[0114] 1. Moisture test

[0115] Refer to GB / T37191-2018 Determination of Moisture Content in Raw Rubber, Karl Fischer Method.

[0116] 2. Melt Flow Rate (MFR) Test

[0117] (1) Set the test temperature of the test instrument to 230℃ and preheat the instrument; (2) Put 6-8g of dry polymer into the material bucket through the funnel and compact the material with the piston; (3) After heating for 4 minutes, cut a piece every 10 seconds under a weight of 2.16kg to get a total of ten pieces; (4) Weigh the mass of each sample and calculate its MFR.

[0118] MFR = 600W / t(g / 10min), where W is the average mass of each sample segment and t is the cutting time interval between each segment.

[0119] 3. Particle size test

[0120] The particles are placed in a sieve with a mesh size of 4 to 10, and the particles are passed through the sieve by means of mechanical vibration or manual means until the desired particles are obtained.

[0121] 4. Residual monomer test

[0122] Take approximately 0.5 g of sample and place it in a sealed container. Accurately add 15 ml of hexafluoroisopropanol to dissolve it. After dissolution, transfer the test solution to a 100 ml round-bottom (flat-bottom) flask. Add 15 ml of hexafluoroisopropanol to the sealed container and rinse it thoroughly. Transfer the flask to the flask and accurately add 10 ml of acetone. Shake to precipitate the polymer, shake to dissolve, filter, and collect the filtrate. Take 1 μL each of the above reference solution and test solution and inject them into the gas chromatograph. Record the chromatogram and calculate the content of residual small molecule substances.

[0123] The raw materials for the following specific embodiments are: polyglycolic acid chips with a particle size of 2.8 mm, a moisture content of 0.2%, a melt index of 80 g / 10 min (230 °C, 2.16 kg), a residual monomer content of 3%, and a material density of 1.5 g / cm³ before entering the drying system. 3 Bulk density 1 g / cm³ 3 .

[0124] In this embodiment, the tower dryer and the single-shaft spiral stirring dryer are both non-standard customized, and the dew point testing equipment is a LY60SP intelligent dew point meter.

[0125] Example 1

[0126] The polyglycolic acid chips were dried using the drying system shown in Figure 1, wherein the dryer 2 is specifically a tower dryer.

[0127] The gas entering the microwave preheater is air. The drying temperature inside the microwave preheater is set to 190℃, the material thickness to 10mm, the microwave wavelength to 120mm, the microwave power to 150kW, the preheating time to 6min, the gas dew point temperature at the microwave preheater gas inlet to -50℃, and the gas dew point temperature at the microwave preheater gas outlet to -35℃.

[0128] The gas entering the tower dryer is air. The drying temperature inside the tower dryer is 190℃, the jacket temperature of the tower dryer is 190℃, the drying time is 2 hours, the filling rate is 45%, the gas dew point temperature at the gas inlet of the dryer is -50℃, the gas dew point temperature at the gas outlet of the dryer is -35℃, and the temperature inside the cooling silo is 60℃.

[0129] The moisture content of the dried polyester chips was measured to be 0.0025%, the melt index was 23 g / 10 min, and the residual monomer was 0.3%.

[0130] Example 2

[0131] The polyglycolic acid chips were dried using the same drying system as in Example 1.

[0132] The gas entering the microwave preheater is air. The drying temperature inside the microwave preheater is 200℃, the material thickness is 5mm, the microwave wavelength is 120mm, the microwave power is 150kW, the preheating time is 5min, the gas dew point temperature at the microwave preheater gas inlet is -50℃, and the gas dew point temperature at the microwave preheater gas outlet is -30℃.

[0133] The gas entering the tower dryer is air. The drying temperature inside the tower dryer is 200℃. The jacket temperature of the tower dryer is 200℃. The drying time is 0.5h. The filling rate is 11%. The gas dew point temperature at the gas inlet of the dryer is -50℃. The gas dew point temperature at the gas outlet of the dryer is -35℃. The temperature inside the cooling silo is 50℃.

[0134] The moisture content of the dried polyester chips was measured to be 0.003%, the melt index was 27 g / 10 min, and the residual monomer was 0.5%.

[0135] Example 3

[0136] The polyglycolic acid chips were dried using the drying system shown in Figure 1, wherein the dryer 2 is specifically a tower dryer with stirring.

[0137] The gas entering the microwave preheater is air. The drying temperature inside the microwave preheater is 100℃, the material thickness is 5mm, the microwave wavelength is 120mm, the microwave power is 150kW, the preheating time is 1min, the gas dew point temperature at the microwave preheater gas inlet is -50℃, and the gas dew point temperature at the microwave preheater gas outlet is -10℃.

[0138] The gas entering the tower dryer is air. The drying temperature inside the tower dryer is 100℃, the stirring rate is 30 rpm, the jacket temperature of the tower dryer is 100℃, the drying time is 2 hours, the filling rate is 45%, the gas dew point temperature at the gas inlet of the dryer is -50℃, the gas dew point temperature at the gas outlet of the dryer is -10℃, and the temperature inside the cooling silo is 50℃.

[0139] The moisture content of the dried polyester chips was measured to be 0.0042%, the melt index was 50 g / 10 min, and the residual monomer content was 0.8%.

[0140] Example 4

[0141] The drying system shown in Figure 1 is used to dry polyglycolic acid chips. Specifically, the dryer 2 is a single-stage single-shaft spiral stirring dryer. The diameter of the vent hole of the hollow spiral conveying shaft is 0.5 mm, the spiral pitch of the hollow spiral conveying shaft is 320 mm, and the gap between the hollow spiral conveying shaft and the shell is 0.5 mm.

[0142] The gas entering the microwave preheater is air. The drying temperature inside the microwave preheater is 150℃, the material thickness is 15mm, the microwave wavelength is 120mm, the microwave power is 150kW, the preheating time is 3min, the gas dew point temperature at the microwave preheater gas inlet is -50℃, and the gas dew point temperature at the microwave preheater gas outlet is -20℃.

[0143] The gas entering the single-shaft spiral stirring dryer is air. The drying temperature inside the single-shaft spiral stirring dryer is 150℃, the material filling rate inside the dryer is 50%, the stirring speed is 20 rpm, the temperature of the dryer jacket is 150℃, the drying time is 2 hours, the gas dew point temperature at the dryer gas inlet is -50℃, the gas dew point temperature at the dryer gas outlet is -20℃, and the temperature inside the cooling hopper is 50℃.

[0144] The moisture content of the dried polyester chips was measured to be 0.004%, the melt index was 39 g / 10 min, and the residual monomer was 0.6%.

[0145] Comparative Example 1

[0146] The polyglycolic acid chips were dried using the same drying system as in Example 1. The difference between this comparative example and Example 1 is that the gas-to-material ratio entering the microwave preheater and the gas-to-material ratio entering the dryer were reduced, and the gas dew point temperature at the outlet of the microwave preheater and the outlet of the dryer were both controlled to be 0°C.

[0147] The moisture content of the dried polyester chips was measured to be 0.0047%, the melt index was 57 g / 10 min, and the residual monomer was 1.4%. Although the moisture content of the dried polyester chips was below 0.005%, the residual monomer was high, and the melt index was significantly higher than that of Example 1.

[0148] Comparative Example 2

[0149] The difference between the drying system used and the drying system in Example 1 is that the drying system in this comparative example does not include microwave preheater 1. That is, the polyester chip raw material is directly fed into the tower dryer for drying in this comparative example.

[0150] The gas entering the tower dryer is air. The drying temperature inside the tower dryer is 190℃, the jacket temperature of the tower dryer is 190℃, the drying time is 2 hours, the gas dew point temperature at the dryer gas inlet is -50℃, the gas dew point temperature at the dryer gas outlet is -35℃, and the temperature inside the cooling silo is 60℃.

[0151] The moisture content of the dried polyester chips was measured to be 0.012%, the melt index was 73 g / 10 min, and the residual monomer content was 2.3%.

[0152] The difference between this comparative example and Example 1 is that the polyester chips were not preheated by a microwave preheater and were directly fed into the dryer for drying. The drying temperature inside the dryer and the gas dew point temperature at the gas outlet were the same as in Example 1. The results showed that the moisture content of the dried polyester chips was higher, the residual monomer was higher, and the melt index was also significantly higher than that of Example 1.

Claims

1. A drying system for polyester chips, characterized in that, The drying system includes a microwave preheater, a dryer connected to the outlet of the microwave preheater, a gas supply device for continuously introducing gas into the microwave preheater and the dryer, and a dew point testing device installed at the outlet of the microwave preheater and the outlet of the dryer.

2. The drying system as described in claim 1, characterized in that, The gas supply device includes a heater, which is connected to the air inlet of the microwave preheater and the air inlet of the dryer respectively. A gas flow control valve is provided on the pipeline connecting the heater and the microwave preheater, and / or on the pipeline connecting the heater and the dryer.

3. The drying system as described in claim 1, characterized in that, The microwave preheater includes a shell, and a material conveying device, a microwave radiation device, and a material thickness homogenization device disposed inside the shell. The microwave radiation device is used to radiate microwaves onto the surface of the material conveying device. The material thickness homogenizing device is located above the starting end of the material conveying device, and the distance between the bottom surface of the material thickness homogenizing device and the surface of the material conveying device is adjustable.

4. The drying system as described in claim 1, characterized in that, The dryer is a single-shaft spiral stirring dryer, which includes a shell, at least one hollow spiral conveying shaft disposed inside the shell, and at least one transmission device disposed outside the shell. One of the transmission devices is connected to one of the hollow spiral conveying shafts. Two adjacent hollow spiral conveying shafts are separated by a partition, and the conveying directions of the two adjacent hollow spiral conveying shafts are opposite. An air-material channel is left between the partition and the shell. The last stage hollow screw conveyor shaft is hollow inside and has ventilation holes on its surface. One end of it is connected to a rotary joint with an air inlet, and the other end is connected to the transmission device. The outlet of the dryer is located below the end of the last stage hollow screw conveyor shaft near the air inlet.

5. The drying system according to any one of claims 1-4, characterized in that, The drying system also includes a gas purification and circulation device for purifying and recycling the gas flowing out of the microwave preheater and the dryer. The gas purification and circulation device includes a cyclone separator, and a catalytic oxidizer, a cooler, a dehumidifier, and a fan connected in sequence to the cyclone separator; the outlet of the microwave preheater and the outlet of the dryer are respectively connected to the cyclone separator, and the outlet of the fan is connected to the gas supply device.

6. The drying system as described in claim 5, characterized in that, The drying system also includes a cooling hopper connected to the outlet of the dryer. The cooling hopper is equipped with a spiral coil with several through holes. One end of the coil is an air inlet, and an air outlet is provided at the top of the cooling hopper. The air outlet of the fan is connected to the air inlet of the coil, and the air outlet of the cooling silo is connected to the air supply device.

7. A method for drying polyester chips using a drying system as described in any one of claims 1-6, characterized in that, The method includes the following steps: (1) In a microwave preheater, under microwave radiation, the polyester chip raw material is dried using a first hot air stream to obtain pre-dried polyester chips. (2) The pre-dried polyester chips are dried in a dryer using a second hot air stream to obtain dried polyester chips; The gas dew point temperature of the first hot gas stream at the outlet of the microwave preheater is ≤-10℃; The gas dew point temperature of the second hot gas stream at the outlet of the dryer is ≤-10℃.

8. The drying method as described in claim 7, characterized in that, Step (1) has one or more of the following characteristics: In the microwave preheater, the drying temperature is 100-200℃; The gas dew point temperature of the first hot gas stream at the outlet of the microwave preheater is -40℃ to -10℃. The gas dew point temperature of the first hot gas stream at the air inlet of the microwave preheater is ≤-40℃; The drying time in the microwave preheater is 1 to 15 minutes.

9. The drying method as described in claim 7, characterized in that, Step (2) has one or more of the following characteristics: The drying temperature in the dryer is 100-200℃; The dew point temperature of the second hot gas stream at the outlet of the dryer is -40℃ to -10℃; The dew point temperature of the second hot gas stream at the air inlet of the dryer is ≤-40℃; The drying time in the dryer is 0.5 to 2 hours.

10. The drying method according to any one of claims 7-9, characterized in that, The polyester chip raw material has the following characteristics: The polyester chip raw material has a particle size ≥1.5mm, preferably 1.5~6mm; The moisture content of the polyester chip raw material is 0.03-0.3 wt%. The melt flow index of the polyester chip raw material is 10-80 g / 10 min (230℃, 2.16 kg); The residual monomer content of the polyester chip raw material is 1-5 wt%. The dried polyester chips have the following characteristics: The moisture content of the dried polyester chips is <0.005 wt%. The melt index of the dried polyester chips is 3-50 g / 10 min (230℃, 2.16 kg); The residual monomer content of the dried polyester chips is <1 wt%, preferably 0.2 to 1 wt%.

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