Drying method and system for fully biodegradable polyester chips

Abstract

The present invention relates to the field of polyester chip drying, specifically to a method and system for drying fully biodegradable polyester chips. The drying system includes a microwave preheater, a dryer connected to the microwave preheater's discharge port, a gas supply device for continuously supplying gas to the microwave preheater and the dryer, and dew point testing equipment located at the microwave preheater's and dryer's outlets.

Description

Technical Field

[0001] The invention belongs to the field of polyester chip drying, and in particular relates to a drying method and system for fully biodegradable polyester chips. Background Art

[0002] Fully biodegradable polyester is an aliphatic polyester with a simple, regularly repeating ester bond structure. The presence of ester bonds in the molecular chain results in the presence of carboxyl and hydroxyl groups in fully biodegradable materials, making them particularly sensitive to water and heat. At high temperatures and in humid environments, moisture within the fully biodegradable polyester chips can cause hydrolysis, leading to molecular chain depolymerization and the formation of 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 without drying, the moisture content in the chips directly affects the shelf life of the product and, consequently, the stability of downstream processing. Therefore, prior to packaging and for downstream melt processing, thorough drying of the chips is essential, and strict control of the moisture content in the system is essential to prevent excess moisture from causing hydrolysis during processing, which can lead to molecular chain breakage and molecular weight loss, and thus affect the physical properties of the final product.

[0003] In the industrial production of fully biodegradable polyester, after polymerization, the pellets are typically sliced ​​using either air-cooled strands or underwater pelletizing. Air-cooled strands absorb moisture from the cooling air during the cooling process, resulting in a moisture content of approximately 0.06% for the pelletized pellets. Centrifugal dehydration is typically used after underwater pelletizing, resulting in a moisture content of approximately 0.2%. Regardless of the slicing method used, if the pellets are directly bagged and packaged, the moisture in the pellets can easily lead to hydrolysis.

[0004] Among the existing drying methods, traditional hot air drying takes a long time, while the vacuum drying method, which is widely used, cannot be produced continuously in the industrial production process. Therefore, it is urgent to provide a new drying method to solve the above problems. Summary of the Invention

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

[0006] In a first aspect of the present invention, a drying system for polyester chips is provided, comprising 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, and a dew point testing device disposed at the microwave preheater outlet and a dew point testing device disposed at the dryer outlet.

[0007] In one or more embodiments, the gas supply device includes a heater, which is respectively connected to the air inlet of the microwave preheater and the air inlet of the dryer, and an air volume 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 irradiation device, and a material thickness homogenizing device disposed inside the housing;

[0009] The microwave radiation device is used to radiate microwaves to the surface of the material conveying device. The material thickness homogenizing device is arranged above the starting end of the material conveying device. 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, comprising a housing, at least one hollow spiral conveying shaft disposed within the housing, and at least one transmission device disposed outside the housing, wherein one transmission device 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, and a gas-material channel is left between the partition and the housing;

[0011] The shaft body of the last-stage hollow spiral conveyor shaft is hollow inside and has air holes on its surface. One end of the shaft is connected to a rotary joint provided with an air inlet, and the other end is connected to the transmission device. The discharge port of the dryer is arranged below one end of the last-stage hollow spiral conveyor shaft close to the air inlet.

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

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

[0014] In one or more embodiments, the drying system further comprises a cooling silo connected to the discharge port of the dryer, wherein a spiral coil is provided in the cooling silo, and a plurality of through holes are provided on the coil, one end of the coil is an air inlet end, and an air outlet is provided at the upper portion of the cooling silo;

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

[0016] The second aspect of the present invention provides a method for drying polyester chips using the drying system according to the first aspect of the present invention, the method comprising the steps of:

[0017] (1) in a microwave preheater, drying the polyester chip raw material using a first hot air flow under microwave radiation to obtain pre-dried polyester chips;

[0018] (2) drying the pre-dried polyester chips in a dryer using a second hot air flow to obtain dried polyester chips;

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

[0020] The gas dew point temperature of the second hot air flow at the gas outlet of the dryer is ≤-10°C.

[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 flow at the gas 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 gas 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 flow at the gas 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 gas 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 particle size of the polyester chip raw material is ≥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 polyester chip raw material has a melt index of 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 dried polyester chips have a melt index of 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 less than 1 wt%, preferably 0.2 to 1 wt%. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 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-thermal oil outlet, d-thermal oil inlet.

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

[0038] Figure 3 It 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 is the material thickness homogenizing device, and 18 is the material conveying device.

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

[0040] Figure 5Schematic diagram of a two-stage single-shaft spiral stirring dryer in one or more embodiments of the present invention; wherein 21'-feed port, 22'-discharge port, 23'-air inlet, 24'-air outlet, 25'-rotary joint, 26'-air vent, 27'-transmission device, 28'-partition, 29'-gas channel. DETAILED DESCRIPTION

[0041] To facilitate understanding of the features and effects of the present invention by those skilled in the art, the following provides a general description and definition of the terms and expressions used in the specification and claims. Unless otherwise indicated, all technical and scientific terms used herein have the ordinary meanings as understood by those skilled in the art regarding the present invention. In the event of conflict, the definitions in this specification shall prevail.

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

[0043] Herein, “comprising,” “including,” “containing” and similar terms encompass the meanings of “consisting essentially of” and “consisting of,” for example, when “A comprises B and C” is disclosed herein, “A consists essentially of B and C” and “A consists of B and C” should be deemed to have been disclosed herein.

[0044] Throughout this document, all features, such as values, amounts, amounts, and concentrations, specified in numerical ranges or percentage ranges are provided for simplicity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to encompass and specifically disclose all possible subranges and individual values ​​within those ranges (including integers and fractions).

[0045] In this document, unless otherwise specified, percentage refers to mass percentage and ratio refers to mass ratio.

[0046] Herein, when describing embodiments or examples, it should be understood that they are not intended to limit the present invention to these embodiments or examples. On the contrary, all alternatives, modifications and equivalents of the methods and materials described herein are encompassed within the scope defined by the claims.

[0047] In this document, for the sake of brevity, not all possible combinations of the various technical features in each embodiment or example are described. Therefore, as long as there are no contradictions in the combination of these technical features, the various technical features in each embodiment or example can be combined in any way, and all possible combinations should be considered to be 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 flow entering the microwave preheater to the mass of the polyester chips. The gas-to-material ratio entering the dryer is the ratio of the volume of the second hot gas flow entering the dryer to the mass of the predried polyester chips. For example, 1:(50-150) L / g means 1 L of gas is passed through for every 50-150 g of material. The gas-to-material ratio entering the microwave preheater or dryer can be adjusted based on the dew point temperature at the outlet of each device. A higher gas-to-material ratio results in lower water content in the outlet gas and a lower dew point temperature. Conversely, a lower gas-to-material ratio results in higher water content in the outlet gas and a higher dew point temperature. The outlet dew point temperature 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 cause polyester hydrolysis. The drying method of the present invention uses a microwave preheater to preheat the polyester chips, significantly shortening the preheating time of the polyester chips while rapidly removing some moisture from the polyester chips. Hot air convection within the microwave preheater and the dryer not only removes unreacted monomers and oligomers, but also removes some moisture released from the polyester chips, thereby reducing the occurrence of polyester hydrolysis. Furthermore, the drying method of the present invention also requires controlling the gas dew point temperature at the microwave preheater outlet and the gas dew point temperature at the dryer outlet. The gas dew point temperature at the microwave preheater outlet reflects the moisture content in the environment within the microwave preheater, and the gas dew point temperature at the dryer outlet reflects the moisture content in the environment within the dryer. By maintaining the gas dew point temperature at both outlets below -10°C, the moisture content in the environment within the system can be prevented from being too high.

[0051] Polyester chip drying system

[0052] like Figure 1 As shown, the present invention provides a drying system for polyester chips, comprising a microwave preheater 1, a dryer 2 connected to the discharge port of the microwave preheater 1, a gas supply device for continuously supplying gas into the microwave preheater 1 and the dryer 2, and a dew point testing device arranged at the gas outlet of the microwave preheater 1 and the dew point testing device arranged at the gas outlet of the dryer 2.

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

[0054] The gas supply device includes a heater 9, which is respectively connected to the air inlet of the microwave preheater 1 and the air inlet of the dryer 2. A gas volume control valve a is provided 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. By providing the gas volume control valve a, the gas flow entering the microwave preheater 1 and / or the dryer 2 is controlled. Only one heater 9 can be provided 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 an interlayer into which thermal oil is introduced. The shell is provided with a thermal oil outlet c and a thermal oil inlet d. The outer surface of the dryer 2 shell is further wrapped with a thermal insulation material. The temperature of the thermal 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 comprises a cooling silo 3 connected to the discharge port of the dryer 2. The cooling silo 3 is provided with a feed port and a discharge port. The discharge port of the dryer 2 is connected to the feed port of the cooling silo 3. The discharge port 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 and is then packaged after cooling.

[0057] Furthermore, a spiral coil 31 is provided in the cooling silo 3, and a plurality of through holes are provided on the coil 31. One end of the coil 31 is an air inlet end, and the other end of the coil 31 can be closed or open, and an air outlet is provided at the upper part of the cooling silo.

[0058] Low-temperature, dry gas enters coil 31 through the air inlet of cooling silo 3 (i.e., the air inlet end of coil 31), flows out of coil 31 through the through hole, exchanges heat and further moisture with the material in cooling silo 3, and then flows out through the air outlet at the top of cooling silo 3. The gas flowing out of cooling silo 3 has an extremely low water content and can be mixed with the gas entering the drying system through the gas inlet in the pipeline leading to heater 9, and then enters heater 9, realizing gas recycling.

[0059] When the drying system of the present invention dries 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 port of microwave preheater 1, is preheated and dried by microwaves and the hot convection gas, and then is discharged from the discharge port of microwave preheater 1 into dryer 2. Within dryer 2, it is further dried by the hot convection gas and then discharged from the discharge port of dryer 2. Preferably, the material discharged from the discharge port of dryer 2 enters cooling silo 3, is cooled and further dried within cooling silo 3, and is then discharged from the discharge port of cooling silo 3.

[0060] Furthermore, a rotary valve b is provided on the pipe connecting the material raw material inlet and the feed port of the microwave preheater 1, the pipe connecting the discharge port of the microwave preheater 1 and the feed port of the dryer 2, the pipe connecting the discharge port of the dryer 2 and the feed port of the cooling silo 3, and the pipe connecting the discharge port of the cooling silo 3 and the dried material outlet. The rotary valve b can be used to adjust the flow direction and flow rate of the material.

[0061] Furthermore, the drying system of the present invention includes a gas purification and circulation device for purifying and recycling the gas flowing out of the microwave preheater 1 and 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, which are sequentially connected to the cyclone separator 4. The air outlets of the microwave preheater 1 and the dryer 2 are respectively connected to the cyclone separator 4, and the air outlet of the fan 8 is connected to the air supply device.

[0062] Preferably, the air outlet of the fan 8 is connected to the air inlet end 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 actual application, the gas discharged from the air outlet of the microwave preheater 1 enters the cyclone separator 4, is separated from the entrained solid and liquid impurities in the cyclone separator 4, then enters the catalytic oxidizer 5, removes organic small molecule impurities in the catalytic oxidizer 5, then enters the cooler 6, is cooled in the cooler 6, then enters the dehumidifier 7, is dehumidified in the dehumidifier 7, then enters the fan 8, the gas flows out from the fan 8, enters the cooling silo 3, exchanges heat and moisture with the material in the cooling silo 3, and then enters the heater 9.

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

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

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

[0067] Furthermore, microwave preheater 1 includes a material thickness equalizing device 17 disposed within the housing, above the starting end of material conveyor 18, and side blocks 16 disposed on material conveyor 18 to prevent material from overflowing. The distance between the bottom of material thickness equalizing device 17 and the surface of material conveyor 18 is adjustable, thereby adjusting the thickness of the material on material conveyor 18. In some embodiments, the distance between the bottom of material thickness equalizing device 17 and the surface of material conveyor 18 is 5 to 80 mm.

[0068] Exemplarily, the material thickness homogenizing device 17 is shaped like an isosceles triangle, the base of which is perpendicular to the conveying direction of the material conveying device 18. The material first passes through the waist of the isosceles triangle and then passes through the base of the isosceles triangle. The angle of the vertex angle of the isosceles triangle is ≤120°. Setting the vertex angle of the isosceles triangle formed by the material thickness homogenizing device 17 to below 120° can have a good diversion effect on the incoming material. Preferably, the vertex angle of the isosceles triangle formed by the material thickness homogenizing device 17 is 120°. After the material to be dried (such as polyester chips) is fed into the material thickness homogenizing device 17, it can be evenly spread on the surface of the material conveying device 18, 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 equalizing 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 middle point of the feed point as needed to provide more stable support. In other embodiments, the material thickness equalizing device 17 can be installed above the material conveying device 18 by a suspension rod 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 embodiment, 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, double-shaft spiral stirring dryers, multi-shaft spiral stirring dryers, and drum dryers.

[0072] Preferably, the tower dryer and fluidized bed dryer are provided with a stirring assembly to keep the material dynamically dry during the drying process, thereby preventing the material from sticking; alternatively, the vertical dryer is a vibrating countercurrent dryer, in which the material is less likely to "bridge" or agglomerate under a spiral vibration environment.

[0073] Within dryer 2, the gas flows in the opposite direction to the material. For example, in a vertical dryer, low-dew-point hot gas enters through the air inlet at the bottom of the dryer, passing through the material bed and creating a countercurrent flow to dry the material, creating a "reciprocating motion" within the dryer.

[0074] In some embodiments, the dryer 2 is a single-axis spiral stirring dryer, which includes a shell, at least one hollow spiral conveying shaft arranged in the shell, and at least one transmission device arranged outside the shell, a transmission device is connected to a hollow spiral conveying shaft, 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, and an air material channel is left between the partition and the shell; the shaft body of the last-stage hollow spiral conveying shaft is hollow inside and has air holes on the surface, one end of which is connected to a rotary joint provided with an air inlet, and the other end is connected to the transmission device, and the discharge port of the dryer is arranged below one end of the last-stage hollow spiral conveying shaft near the air inlet.

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

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

[0077] The structure of the first-stage single-axis spiral stirring dryer is as follows: Figure 4As shown in the figure, the housing of the single-shaft spiral agitator dryer is provided with a feed inlet 21, a discharge inlet 22, and an air outlet 24. The feed inlet 21 and the air outlet 24 are located above the end of the hollow spiral conveyor shaft away from the air inlet 23, while the discharge inlet 22 is located below the end of the hollow spiral conveyor shaft near the air inlet 23. The hollow spiral conveyor shaft comprises a central shaft and conveying blades spirally wound around the central shaft. The central shaft is hollow and has a plurality of vents 26 on its surface. A transmission device 27, connected to the central shaft, drives the conveying blades to rotate, thereby promoting material transportation.

[0078] Material (e.g., polyester chips, fully biodegradable polyester chips) enters the single-shaft spiral agitator dryer through feed port 21 and exits through discharge port 22. Low-dew-point gas enters through inlet 23 of rotary joint 25 and exits through vents 26 on the hollow spiral conveyor shaft. It passes through the material layer within the spiral, exchanging heat and water with the material before being discharged through outlet 24. The material is conveyed in the opposite direction of the main gas flow. Driven by a transmission device 27, the hollow spiral conveyor shaft rotates, further driving the material. The material's interface is updated under the action of the hollow spiral conveyor shaft, and the material moves in a plug flow to prevent adhesion.

[0079] The structure of the two-stage or above single-shaft spiral stirring dryer is similar, such as Figure 5 As shown, taking a two-stage single-axis spiral stirring dryer as an example, each stage includes a hollow spiral conveying shaft and a transmission device 27'. The two-stage hollow spiral conveying shafts are stacked up and down 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 hollow spiral conveying shaft of the lower stage is located below the end of the hollow spiral conveying shaft of the upper stage, so that the conveying directions of the upper and lower materials are opposite; the channel formed between the partition 28' and the shell is the air channel 29', which is the discharge port of the upper-stage single-axis spiral stirring dryer and the feed port of the lower-stage single-axis spiral stirring dryer, and also the air outlet of the lower-stage single-axis spiral stirring dryer and the air inlet of the upper-stage single-axis spiral stirring dryer.

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

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

[0082] Preferably, the gap between the portion of the hollow spiral conveying shaft closest to the housing and the housing is less than 1 mm.

[0083] Preferably, the spiral pitch of the hollow spiral conveying shaft is 100-500 mm, preferably 250-350 mm, for example 320 mm.

[0084] Drying method

[0085] The present invention provides a method for drying polyester chips, which uses the above drying system to dry the polyester chips. The method comprises the following steps:

[0086] (1) in a microwave preheater, drying the polyester chip raw material using a first hot air flow under microwave radiation to obtain pre-dried polyester chips;

[0087] (2) drying the pre-dried polyester chips in a dryer using a second hot air flow to obtain dried polyester chips;

[0088] The gas dew point temperature of the first hot air flow at the outlet of the microwave preheater is ≤-10°C;

[0089] The gas dew point temperature of the second hot air flow at the gas outlet of the dryer is ≤-10°C.

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

[0091] Furthermore, the gas dew point temperature of the first hot gas flow at the gas inlet of the microwave preheater is ≤-40°C, such as -40°C, -45°C, -50°C, or a range between any two values.

[0092] In step (1), the first hot air flow convection dries the polyester chip raw material. The gas-to-material ratio entering the microwave preheater can be 1: (50-150) L / g, for example, 1: 90 L / g, 1: 130 L / g, 1: 150 L / g, or a range between any two numerical values. The drying time in the microwave preheater can be 1-15 minutes, for example, 4 minutes, 8 minutes, 12 minutes, or a range between any two numerical values. The drying temperature in the microwave preheater can be 100-200°C, for example, 120°C, 150°C, 180°C, or a range between any two numerical values.

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

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

[0095] In step (2), the gas dew point temperature of the second hot air flow at the outlet of the dryer is -40°C to -10°C, for example, -40°C, -20°C, -10°C, or within a range consisting of any two values.

[0096] Furthermore, the gas dew point temperature of the second hot air flow at the air inlet of the dryer is ≤-40°C, such as -40°C, -45°C, -50°C, or a range between any two values.

[0097] In step (2), a second hot air flow convection drying is used to pre-dry the polyester chips. The gas-to-material ratio entering the dryer can be 1: (10-100) L / g, for example, 1: 20 L / g, 1: 60 L / g, 1: 80 L / g, or between any two numerical ranges. In the dryer, the drying temperature can be 100-200°C, for example, 120°C, 150°C, 180°C, or between any two numerical ranges. The drying time in the dryer can be 0.5-2h, for example, 1h, 1.5h, 2h, or between any two numerical ranges. Preferably, the temperature of the heat transfer oil in the dryer interlayer, 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 above-mentioned single-shaft spiral agitator dryer. Preferably, the single-shaft spiral agitator dryer has a rotational speed of 2 to 50 rpm, preferably 5 to 30 rpm, for example 6 rpm. Preferably, the material filling rate within the single-shaft spiral agitator dryer is 5 to 95%, preferably 40 to 85%, for example 60%.

[0099] The first hot air flow and the second hot air flow are each independently selected from one or more of air, nitrogen and an inert gas. Preferably, the first hot air flow and the second hot air flow 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. Polyglycolic acid chips are copolymers containing repeating units of glycolic acid.

[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, such as 20°C, 40°C, 50°C, or in a range between any two values; preferably 30-60°C.

[0102] In some embodiments, the pelletized polyester chip raw material is directly conveyed to a microwave preheater; preferably, the pelletizing process is carried out in a low dew point environment, where the gas dew point temperature is ≤-30°C.

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

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

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

[0106] 1. Microwave is a method of internal heating, which increases the temperature of water inside the material (such as polyester chips) and causes it to leave the material. Using microwave to preheat the material can quickly increase the material temperature, and has the advantages of fast heating speed and small heat loss.

[0107] 2. Preheating the material (such as polyester chips) with microwaves before sending it into the dryer for drying can shorten the heating and drying time of the material in the dryer, thus greatly saving energy consumption.

[0108] 3. Controlling the dew point temperature at the microwave preheater outlet and the dryer outlet can reduce hydrolysis of polyester chips during the preheating and drying processes. Existing hot air drying processes typically adjust process parameters based solely on whether the output moisture content meets standards, leading to excessively long drying times and ignoring the adverse effects of ambient moisture on the polyester material during the drying process.

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

[0110] 5. There is an interlayer inside the dryer shell. Adding heat-conducting oil into the interlayer for heating and insulation can make the temperature inside the dryer more balanced.

[0111] 6. The drying system of the present invention is equipped with a gas purification 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 completely removed. After cooling, the gas is dehydrated and the pure gas can be reused. In addition, 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 described below in conjunction with specific examples. It should be understood that these examples are intended to illustrate the present invention only and are not intended to limit the scope of the invention. The experimental methods in the following examples, for which specific conditions are not specified, are generally based on conventional conditions or the conditions recommended by the manufacturer. Unless otherwise stated, percentages and parts are calculated by weight.

[0113] Test method:

[0114] 1. Moisture test

[0115] Refer to GB / T37191-2018 Determination of moisture content of raw rubber, Karl Fischer method.

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

[0117] (1) Set the test temperature of the test instrument to 230°C and preheat the instrument; (2) Place 6-8 g of dry polymer into a barrel through a funnel and compact the material with a piston; (3) After heating for 4 min, cut a section every 10 seconds under a 2.16 kg weight, obtaining a total of ten sections; (4) Weigh the mass of each sample and calculate its MFR.

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

[0119] 3. Particle size test

[0120] Place the particles in a 4-10 mesh sieve and use mechanical vibration or manual method to make the particles pass through the sieve until the desired particles are obtained.

[0121] 4. Residual monomer test

[0122] Take about 0.5g of sample and place it in a sealed container. Accurately add 15ml of hexafluoroisopropanol to dissolve it. After dissolution is complete, transfer the test solution to a 100ml round-bottom (flat-bottom) flask, add 15ml of hexafluoroisopropanol and brush it clean in a sealed container. Transfer it to the flask, accurately add 10ml of acetone, shake to precipitate the polymer, shake to dissolve, filter, and take the filtrate; take 1μL each of the above-mentioned reference solution and test solution, inject them into the gas chromatograph, record the chromatogram, and calculate the content of residual small molecules.

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

[0124] In the embodiment, the tower dryer and the first-stage single-axis 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] use Figure 1 The drying system shown dries polyglycolic acid chips, wherein the dryer 2 is specifically a tower dryer.

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

[0128] The gas entering the tower dryer is air, the drying temperature in the tower dryer is 190°C, the interlayer temperature of the tower dryer is 190°C, the drying time is 2 hours, the filling rate is 45%, the dew point temperature of the gas at the dryer gas inlet is -50°C, the dew point temperature of the gas at the dryer gas outlet is -35°C, and the temperature in the cooling silo is 60°C;

[0129] The moisture content of the polyester chips after drying 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 slices were dried using the same drying system as in Example 1.

[0132] The gas entering the microwave preheater is air, the drying temperature in the microwave preheater is 200°C, 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°C, and the gas dew point temperature at the microwave preheater gas outlet is -30°C;

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

[0134] The moisture content of the polyester chips after drying 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] use Figure 1 The drying system shown is used to dry polyglycolic acid chips, wherein the dryer 2 is specifically a tower dryer with stirring.

[0137] The gas entering the microwave preheater is air, the drying temperature in the microwave preheater is 100°C, 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°C, and the gas dew point temperature at the microwave preheater gas outlet is -10°C;

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

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

[0140] Example 4

[0141] use Figure 1 The drying system shown dries polyglycolic acid chips, wherein the dryer 2 is specifically a one-stage single-axis 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 in the microwave preheater is 150°C, 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°C, and the gas dew point temperature at the microwave preheater gas outlet is -20°C;

[0143] The gas entering the single-shaft spiral stirring dryer is air, the drying temperature in the single-shaft spiral stirring dryer is 150°C, the material filling rate in the dryer is 50%, the stirring rate is 20 rpm, the dryer interlayer temperature is 150°C, the drying time is 2h, the gas dew point temperature at the dryer gas inlet is -50°C, the gas dew point temperature at the dryer gas outlet is -20°C, and the temperature in the cooling silo is 50°C;

[0144] The moisture content of the polyester chips after drying 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 was 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 microwave preheater gas outlet and the gas dew point temperature at the dryer gas outlet were both controlled at 0°C.

[0147] The moisture content of the polyester chips after drying 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 polyester chips after drying 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 the microwave preheater 1, that is, the polyester chip raw materials are directly fed into the tower dryer for drying.

[0150] The gas entering the tower dryer is air, the drying temperature in the tower dryer is 190℃, the interlayer temperature of the tower dryer is 190℃, the drying time is 2h, the dew point temperature of the gas at the dryer gas inlet is -50℃, the dew point temperature of the gas at the dryer gas outlet is -35℃, and the temperature in the cooling silo is 60℃;

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

[0152] This comparative example differs from Example 1 in that the polyester chips are directly fed into the dryer for drying without undergoing preheating in a microwave preheater. The drying temperature in the dryer and the gas dew point temperature at the gas outlet are the same as those in Example 1. The results show that the moisture content of the polyester chips after drying is relatively high, the residual monomers are relatively high, and the melt index is significantly higher than the melt index of Example 1.

Claims

1. A polyester chip drying system, characterized in that: The drying system includes a microwave preheater, a dryer connected to the discharge port of the microwave preheater, a gas supply device for continuously supplying gas into the microwave preheater and the dryer, and a dew point testing device arranged at the gas outlet of the microwave preheater and the dew point testing device arranged at the gas outlet of the dryer.

2. The drying system according to claim 1, wherein: The air supply device includes a heater, which is respectively connected to the air inlet of the microwave preheater and the air inlet of the dryer. An air volume 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 according to claim 1, wherein: The microwave preheater includes a shell, and a material conveying device, a microwave radiating device and a material thickness homogenizing device arranged inside the shell; The microwave radiation device is used to radiate microwaves to the surface of the material conveying device. The material thickness homogenizing device is arranged above the starting end of the material conveying device. 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 according to claim 1, wherein: The dryer is a single-shaft spiral stirring dryer, comprising a housing, at least one hollow spiral conveying shaft disposed within the housing, and at least one transmission device disposed outside the housing, wherein one transmission device 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, and an air-material channel is left between the partition and the housing; The shaft body of the last-stage hollow spiral conveyor shaft is hollow inside and has air holes on its surface. One end of the shaft is connected to a rotary joint provided with an air inlet, and the other end is connected to the transmission device. The discharge port of the dryer is arranged below one end of the last-stage hollow spiral conveyor shaft close to the air inlet.

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

6. The drying system according to claim 5, characterized in that The drying system further comprises a cooling silo connected to the discharge port of the dryer, wherein a spiral coil is provided in the cooling silo, and a plurality of through holes are provided on the coil, one end of the coil is an air inlet end, and an air outlet is provided at the upper part of the cooling silo; The air outlet of the fan is connected to the air inlet end 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 by using the drying system according to any one of claims 1 to 6, characterized in that: The method comprises the steps of: (1) in a microwave preheater, drying the polyester chip raw material using a first hot air flow under microwave radiation to obtain pre-dried polyester chips; (2) drying the pre-dried polyester chips in a dryer using a second hot air flow to obtain dried polyester chips; The gas dew point temperature of the first hot gas flow at the gas outlet of the microwave preheater is ≤-10°C; The gas dew point temperature of the second hot air flow at the gas outlet of the dryer is ≤-10°C.

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

9. The method according to claim 7, wherein Step (2) has one or more of the following characteristics: In the dryer, the drying temperature is 100-200°C; The gas dew point temperature of the second hot air flow at the outlet of the dryer is -40°C to -10°C; The gas dew point temperature of the second hot air flow at the air inlet of the dryer is ≤-40°C; The drying time in the dryer is 0.5 to 2 hours.

10. The method according to any one of claims 7 to 9, wherein The polyester chip raw material has the following characteristics: The particle size of the polyester chip raw material is ≥1.5mm; The moisture content of the polyester chip raw material is 0.03-0.3wt%; The melt index of the polyester chip raw material is 10 to 80 g / 10 min (230° C., 2.16 kg); The residual monomer content of the polyester chip raw material is 1 to 5 wt%; And the dried polyester chips have the following characteristics: The moisture content of the dried polyester chips is less than 0.005wt%; The melt index of the dried polyester chips is 3 to 50 g / 10 min (230°C, 2.16 kg); The residual monomer content of the dried polyester chips is less than 1 wt%.

11. The method according to any one of claims 7 to 9, wherein: The particle size of the polyester chip raw material is 1.5-6 mm.

12. The method according to any one of claims 7 to 9, wherein The residual monomer content of the dried polyester chips is 0.2-1 wt%.

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