Drying method and system for full-biodegradable polyester chip

Abstract

The invention belongs to the field of polyester chip drying, and particularly relates to a drying method and system for full-biodegradable polyester chips. The drying system comprises a microwave preheater, a dryer connected with a discharge port of the microwave preheater, and a gas supply device used for continuously introducing gas into the microwave preheater and the dryer. The dew point testing equipment is arranged at an air outlet of the microwave preheater; and the dew point testing equipment is arranged at an air outlet of the dryer.

Description

Technical Field

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

[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 makes the fully biodegradable material contain carboxyl and hydroxyl groups, which are particularly sensitive to water and heat. Under high temperature and humid environment, the moisture in the fully biodegradable slices will cause the fully biodegradable polyester to undergo hydrolysis reaction, resulting in the depolymerization of the molecular chain and the generation of molecular chain segments with terminal carboxyl and terminal hydroxyl groups. When the fully biodegradable polyester is close to the glass transition temperature, the molecular weight decreases significantly with the increase of hydrolysis time. If the fully biodegradable polyester slices are directly packaged without drying, the moisture in the polyester slices will directly affect the shelf life of the product and thus affect the stability of downstream processing. Therefore, for the packaging of the slices and for the downstream melt processing, it is necessary to fully dry them first, strictly control the moisture content in the system, prevent excessive moisture from causing hydrolysis during the processing, avoid molecular chain breakage and molecular weight reduction, and thus affect the physical properties of the processed and formed products.

[0003] In the industrial production of fully biodegradable polyester, after the polymerization of fully biodegradable polyester is completed, air-cooled strands and underwater pelletizing are generally used for slicing. Air-cooled strands absorb moisture from the cooling air during the cooling process, and the moisture content of air-cooled stranded fully biodegradable polyester slices is generally around 0.06%; while centrifugal dehydration is generally used after underwater pelletizing, and the moisture content of fully biodegradable polyester slices is around 0.2%. Regardless of the slicing method used, if the fully biodegradable polyester slices are directly bagged and packaged, the moisture in the slices in the package is likely to cause hydrolysis.

[0004] Among the existing drying methods, the traditional hot air drying takes a long time, and 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] According to a first aspect of the present invention, a drying system for polyester chips is provided, the drying system 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 arranged at the air outlet of the microwave preheater and a dew point testing device arranged at the air outlet of the dryer.

[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 a gas 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 shell, and a material conveying device, a microwave irradiation device, and a material thickness homogenizing device disposed inside the shell;

[0009] The microwave radiating 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, which includes a shell, at least one hollow spiral conveying shaft disposed in the shell, and at least one transmission device disposed outside the shell, wherein one transmission device is connected to one 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 a gas-material channel is left between the partition and the shell;

[0011] The shaft body of the last-stage hollow screw conveyor shaft is hollow inside and has air holes on its surface. One end of the shaft body 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 screw 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 arranged in the cooling silo, and a plurality of through holes are arranged on the coil, one end of the coil is an air inlet end, and an air outlet is arranged at the upper part 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, using a first hot air flow to dry the polyester chip raw material under microwave radiation to obtain a predried polyester chip;

[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 gas 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 flow 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 1.5-6 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 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 less than 1 wt %, preferably 0.2 to 1 wt %. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It 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 It is a 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 a material thickness homogenizing device and a material conveying device in one or more embodiments of the present invention; wherein 17 is a material thickness homogenizing device, and 18 is a material conveying device.

[0039] Figure 4 It is a schematic diagram of a primary 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 5It is a schematic diagram of a two-stage single-axis 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'-rotating joint, 26'-air vent, 27'-transmission device, 28'-partition, 29'-air-feed channel. DETAILED DESCRIPTION

[0041] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition 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 be disclosed herein.

[0044] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are only for brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (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 used to limit the present invention to these embodiments or examples. On the contrary, all substitutes, improvements and equivalents of the methods and materials described in the present invention can be included in the scope limited by the claims.

[0047] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, 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 and 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 air flow entering the microwave 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 air flow entering the dryer to the mass of the predried polyester chips. For example, 1: (50-150) L / g means that 1 L of gas is passed through every 50-150 g 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 less water content in the gas at the outlet and the lower the gas dew point temperature; on the contrary, the smaller the gas-to-material ratio, the more water 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 by 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 the preset parameters, thereby achieving precise control of the temperature inside the microwave preheater during the drying process.

[0050] At a relatively high temperature, moisture in the environment (including some moisture released from the polyester chips) and moisture in the polyester chips are likely to cause hydrolysis of the polyester. The drying method of the present invention uses a microwave preheater to preheat the polyester chips, thereby greatly shortening the preheating time of the polyester chips and rapidly releasing some moisture from the polyester chips. The hot air convection in the microwave preheater and the dryer can not only take away the unreacted monomers and oligomers, but also take away some moisture released from the polyester chips, thereby reducing the occurrence of polyester hydrolysis. Moreover, 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 can reflect the moisture content in the environment in the microwave preheater, and the gas dew point temperature at the dryer outlet can reflect the moisture content in the environment in the dryer. By keeping the gas dew point temperatures at both outlets below -10°C, the moisture content in the environment in the system can be prevented from being too high.

[0051] Polyester chips 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 introducing 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 a 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, and the dryer 2 is provided with a feed inlet, a discharge port, an air inlet and an air outlet. As a further preferred solution, a dew point test device is provided at the air inlet of the microwave preheater 1 and at the air inlet of the dryer 2. Exemplarily, the dew point test 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 arranged 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 entering the microwave preheater 1 and / or the dryer 2 is controlled by setting the gas volume control valve a. Only one heater 9 can be arranged to heat the gas entering the microwave preheater 1 and the dryer 2.

[0055] In some embodiments, an interlayer is provided on the inner wall of the shell of the dryer 2, heat transfer oil is introduced into the interlayer, a heat transfer oil outlet c and a heat transfer oil inlet d are provided on the shell, and a heat insulation material is further wrapped on the outer surface of the shell of the dryer 2. The temperature of the heat transfer oil can be the same as the temperature of the gas, 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 of the dried material, and the material dried by the dryer 2 first enters the cooling silo 3, and is packaged after being cooled.

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

[0059] When the drying system of the present invention dries the raw material, the gas is heated to a set temperature in the heater 9, and then enters the microwave preheater 1 and the dryer 2 respectively. The material enters the microwave preheater 1 from the feed port of the microwave preheater 1, is preheated and dried by microwaves and hot convection gas, and then is discharged from the discharge port of the microwave preheater 1 into the dryer 2. After being further dried by hot convection gas in the dryer 2, the material is discharged from the discharge port of the dryer 2. Preferably, the material discharged from the discharge port of the dryer 2 enters the cooling silo 3, is cooled and further dried in the cooling silo 3, and then is discharged from the discharge port of the cooling silo 3.

[0060] Furthermore, rotary valves b are provided on the pipeline connecting the material raw material inlet and the feed port of the microwave preheater 1, the pipeline connecting the discharge port of the microwave preheater 1 and the feed port of the dryer 2, the pipeline connecting the discharge port of the dryer 2 and the feed port of the cooling silo 3, and the pipeline connecting the discharge port of the cooling silo 3 and the dried material outlet. The 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 further comprises a gas purification circulation device for purifying and recycling the gas flowing out of the microwave preheater 1 and the dryer 2; the gas purification circulation device comprises 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 air outlet of the microwave preheater 1 and the air outlet of 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 applications, the gas discharged from the 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 outlet of the microwave preheater 1 is mixed with the gas discharged from the outlet of the dryer 2 in the pipeline leading to the cyclone separator 4, and then enters 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 terminal end of the material conveying device 18.

[0067] Furthermore, the microwave preheater 1 further comprises a material thickness homogenizing device 17 disposed inside the housing and above the starting end of the material conveying device 18, and a side blocking block 16 disposed on the material conveying device 18 to prevent material from overflowing. The distance between the bottom surface of the material thickness homogenizing 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 homogenizing device 17 and the surface of the material conveying device 18. In some embodiments, the distance between the bottom surface of the material thickness homogenizing device 17 and the surface of the material conveying device 18 is 5 to 80 mm.

[0068] Exemplarily, the material thickness homogenizing device 17 is in the shape of an isosceles triangle, the base 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, and 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 play 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, 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 homogenizing device 17 can be supported on both sides of the material conveying device 18 by supporting legs. Since the plane is a triangle, a crossbeam can be added to the middle point of the feeding position as needed to make the support more stable. In other embodiments, the material thickness homogenizing device 17 can be arranged above the material conveying device 18 by a suspension rod fixed to the shell.

[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 may be a vertical dryer or a horizontal dryer. Exemplary vertical dryers include, but are not limited to, a vibrating countercurrent dryer, a tower dryer, and a fluidized bed dryer. Exemplary horizontal dryers include, but are not limited to, a single-shaft spiral stirring dryer, a double-shaft spiral stirring dryer, a multi-shaft spiral stirring dryer, and a drum dryer.

[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, and the material is not prone to "bridging" or agglomeration in a spiral vibration environment.

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

[0074] In some embodiments, the dryer 2 is a single-axis spiral stirring dryer, which includes a shell, at least one hollow screw conveying shaft arranged in the shell, and at least one transmission device arranged outside the shell, a transmission device is connected to a hollow screw conveying shaft, two adjacent hollow screw conveying shafts are separated by a partition, and the conveying directions of the two adjacent hollow screw 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 screw 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 screw conveying shaft close to the air inlet.

[0075] The gas enters the last stage of the hollow screw conveyor shaft from the air inlet arranged on the rotary joint, and flows out from the air vent of the last stage of the hollow screw 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 more 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 more single-shaft spiral stirring dryer are the same as its level, 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. The housing of the single-axis spiral stirring dryer is provided with a feed port 21, a discharge port 22 and an air outlet 24. The feed port 21 and the air outlet 24 are arranged above the end of the hollow spiral conveying shaft away from the air inlet 23, and the discharge port 22 is arranged below the end of the hollow spiral conveying shaft close to the air inlet 23. The hollow spiral conveying shaft includes a central shaft and conveying blades spirally wrapped around the central shaft. The interior of the central shaft is hollow and a plurality of vents 26 are arranged on the surface. The transmission device 27 drives the conveying blades to rotate by connecting with the central shaft, thereby promoting material transportation.

[0078] The material (such as polyester chips, fully biodegradable polyester chips) enters from the feed port 21 and exits from the discharge port 22 of the single-axis spiral stirring dryer. The low dew point gas enters from the air inlet 23 of the rotary joint 25 and is discharged from the vent 26 on the hollow spiral conveying shaft. It passes through the material layer in the spiral to exchange water and heat with the material and is finally discharged from the outlet 24. The material conveying direction is opposite to the main flow direction of the gas. The hollow spiral conveying shaft rotates under the drive of the transmission device 27, thereby promoting the material conveying. The material undergoes interface renewal under the action of the hollow spiral conveying shaft, and the material moves in a plug flow to prevent adhesion.

[0079] The structure of the two-stage or more 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 lower hollow spiral conveying shaft is located below the end of the upper hollow spiral conveying shaft, so that the upper and lower materials are conveyed in opposite directions; the channel formed between the partition 28' and the shell is an air channel 29', which is the discharge port of the upper single-axis spiral stirring dryer and the feed port of the next single-axis spiral stirring dryer, and also the air outlet of the next single-axis spiral stirring dryer and the air inlet of the upper 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 screw conveying shaft is hollow inside and has an air vent 26' on its surface. The gas in the next-stage hollow screw conveying shaft is discharged from the air vent 26' into the shell, passes through the material layer in the next-stage screw to perform hydrothermal exchange with the material, and the exchanged gas enters the material layer in the previous-stage screw 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 shell and the shell 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, wherein the polyester chips are dried using the above drying system, and the method comprises the steps of:

[0086] (1) in a microwave preheater, using a first hot air flow to dry the polyester chip raw material under microwave radiation to obtain a predried polyester chip;

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

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

[0089] The gas dew point temperature of the second hot gas 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] Further, 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 within a range consisting of any two values.

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

[0093] The wavelength of the microwave can 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 microwave can 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 top to bottom and exit from bottom to top, and the first hot air flow enters the microwave preheater from bottom to top.

[0095] In step (2), the gas dew point temperature of the second hot gas flow at the gas 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] Further, the gas dew point temperature of the second hot gas flow at the air inlet of the dryer is ≤ -40°C, such as -40°C, -45°C, -50°C, or within a range consisting of 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 stirring dryer. Preferably, the rotation speed of the single-shaft spiral stirring dryer is 2 to 50 rpm, preferably 5 to 30 rpm, for example 6 rpm. Preferably, the material filling rate in the single-shaft spiral stirring dryer is 5 to 95%, preferably 40 to 85%, for example 60%.

[0099] The first hot gas flow and the second hot gas flow are each independently selected from one or more of air, nitrogen and an inert gas. Preferably, the first hot gas flow and the second hot gas 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 sent to a cooling silo for cooling, and 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 pelletization is performed in a low dew point environment, and the gas dew point temperature of the low dew point environment 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. Further, the moisture content of the polyester chip raw material may be 0.03 to 0.3 wt%. Further, the melt index of the polyester chip raw material may be 10 to 80 g / 10 min (230° C., 2.16 kg). Further, 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 %. Further, the melt index of the dried polyester chips may be 3 to 50 g / 10 min (230° C., 2.16 kg). Further, 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 gas dew point temperature at the microwave preheater outlet and the gas dew point temperature at the dryer outlet can reduce the occurrence of hydrolysis reaction of polyester chips during the preheating and drying processes. However, the existing hot air drying usually only adjusts the relevant process parameters based on whether the moisture content of the discharge material meets the standard, the drying time is too long, and the adverse effects of moisture in the environment on polyester materials during the drying process are ignored.

[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 produce a 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, part of the monomers and oligomers, which can improve the quality / performance of the final product and enable continuous production.

[0110] 5. An interlayer is provided inside the shell of the dryer. Heat transfer oil is added to the interlayer for heating and insulation to 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 the dryer contains a small amount of monomers and oligomers. After passing through the catalytic oxidizer, the monomers and oligomers in the gas are removed, and then the gas is cooled and dehydrated. 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 consumption.

[0112] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples without specifying specific conditions are usually 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) Load 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 s under a weight of 2.16 kg, and obtain 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 cutting time interval of each segment.

[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 the sample and place it in a sealed container. Accurately add 15ml of hexafluoroisopropanol to dissolve it. After the dissolution is complete, transfer the test solution to a 100ml round-bottom (flat-bottom) flask, add 15ml of hexafluoroisopropanol in a sealed container and brush it clean. 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 of the above-mentioned reference solution and test solution respectively, 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, 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℃, the interlayer temperature of the tower dryer is 190℃, the drying time is 2h, the filling rate is 45%, 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 in the cooling silo is 60℃;

[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 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 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 2h, 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 slices, 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-axis spiral stirring dryer is air, the drying temperature in the single-axis 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 same drying system as in Example 1 is used to dry the polyglycolic acid slices. 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 are reduced, and the gas dew point temperature at the microwave preheater gas outlet and the gas dew point temperature at the dryer gas outlet are 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 relatively 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 material is directly sent to the tower dryer for drying in this comparative example.

[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 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 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] The difference between this comparative example and Example 1 is that the polyester chips are directly sent to the dryer for drying without being preheated by 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 also 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 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 arranged at the gas outlet of the microwave preheater and a dew point testing device arranged at the gas outlet of the dryer.

2. The drying system according to claim 1, characterized in that: 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. A gas volume control valve is arranged 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, characterized in that: The microwave preheater comprises a shell, and a material conveying device, a microwave radiating device and a material thickness homogenizing device arranged inside the shell; The microwave radiating 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, characterized in that: The dryer is a single-shaft 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, 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, and a gas material channel is left between the partition and the shell; The shaft body of the last-stage hollow screw conveyor shaft is hollow inside and has air holes on its surface. One end of the shaft body 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 screw 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 comprises 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 arranged in the cooling silo, and a plurality of through holes are arranged on the coil, one end of the coil is an air inlet end, and an air outlet is arranged 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 a drying system as claimed in any one of claims 1 to 6, characterized in that: The method comprises the steps of: (1) in a microwave preheater, using a first hot air flow to dry the polyester chip raw material under microwave radiation to obtain a predried polyester chip; (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 gas flow at the gas outlet of the dryer is ≤-10°C.

8. The drying method according to 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°C; 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; 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 drying method according to claim 7, characterized in that: 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 gas flow at the gas outlet of the dryer is -40°C to -10°C; The gas dew point temperature of the second hot gas 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 drying method according to any one of claims 7 to 9, characterized in that: The polyester chip raw material has the following characteristics: The particle size of the polyester chip raw material is ≥1.5 mm, preferably 1.5 to 6 mm; 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 %, preferably 0.2 to 1 wt %.

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