A crystallization drying device and method for PLA chips

Abstract

The application belongs to the field of crystallization drying of low-melting-point PLA chips, and particularly relates to a crystallization drying device and method for PLA chips. The device comprises a pre-crystallization tank, a crystallization bed and a drying tower which are sequentially connected through a material pipeline, and a cooling fluidized bed is arranged between the crystallization bed and the drying tower. The PLA chips after preliminary crystallization in the pre-crystallization tank and crystallization in the double-air-channel crystallization bed are subjected to cooling treatment, and the PLA chips are cooled to below the glass transition temperature, so that the PLA chips entering the drying tower are not deformed and not adhered.

Description

Technical Field

[0001] This invention belongs to the field of crystallization and drying of low-melting-point PLA chips, and specifically relates to a crystallization and drying apparatus and method for PLA chips. Background Technology

[0002] With the demand for biodegradable polylactic acid (PLA) and biodegradable polybutylene adipate (PBAT) increasing rapidly in the past two years, the production capacity of PLA, which is made from fermented plant starch such as corn and sugar beets, is severely insufficient.

[0003] The polymerized PLA melt needs to be sliced ​​in an underwater rotary granulator to obtain ellipsoidal particles. At this stage, the PLA particles are still polymerized semi-finished products with a relatively high temperature (approximately 70℃~80℃). They must undergo a crystallization and drying process to achieve a certain degree of crystallinity and moisture content before they can be packaged and used. However, during the drying process, material accumulation and adhesion frequently occur inside the tower. This problem has not been fundamentally solved, making it difficult to carry out large-scale drying in the tower, and the efficiency is also low. Summary of the Invention

[0004] The purpose of this invention is to solve the problems existing in the prior art by providing a crystallization and drying apparatus and method for PLA slices.

[0005] The technical solution of this invention is as follows:

[0006] A crystallization and drying apparatus for PLA slices includes a pre-crystallization tank, a crystallization bed, and a drying tower connected in sequence by a material pipeline. The material pipeline between the crystallization bed and the drying tower is also equipped with a cooling fluidized bed.

[0007] Here, due to the low glass transition temperature (60-65℃) of PLA, the temperature of the PLA slices after preliminary crystallization in the pre-crystallization tank and crystallization in the crystallization bed is much higher than the glass transition temperature of PLA slices. At this time, the PLA slices are like soft candy, amorphous, and easily deformed and stuck together due to their own weight and mutual compression. In severe cases, they will form large lumps. Therefore, it is necessary to cool the crystallized PLA slices. Through the cooling fluidized bed and cooling circulation system in this invention, the PLA slices are cooled to below the glass transition temperature to stop the crystallization of PLA slices, so that the surface of the material hardens. This ensures that the material does not stick or deform when it enters the drying tower in the next step.

[0008] Furthermore, the crystallization bed is a dual-channel crystallization bed, which is provided with a channel outlet and a channel inlet. The channel inlet includes a first inlet and a second inlet. A heating circulation system is sequentially arranged on the gas pipeline from the channel outlet to the channel inlet of the dual-channel crystallization bed, consisting of a first cyclone separator, a first centrifugal fan and a second centrifugal fan connected in parallel, a first electric heater connected to the first centrifugal fan, and a second electric heater connected to the second centrifugal fan. The first inlet is connected to the gas pipeline of the first centrifugal fan, and the second inlet is connected to the gas pipeline of the second centrifugal fan. Preferably, the controlled temperature of both the first electric heater and the second electric heater connected to the dual-channel crystallization bed is 90℃~100℃.

[0009] A dual-channel crystallization bed connected to a heating circulation system can heat PLA slices to improve their crystallinity. Here, the dual-channel crystallization bed increases the heating and ventilation volume, improving heating and ventilation efficiency, thus increasing the contact surface with the PLA slices and improving crystallization efficiency. Furthermore, the dual-channel crystallization bed allows for separate temperature control in different areas within the crystallization bed, facilitating temperature adjustment in these different regions.

[0010] Furthermore, the cooling fluidized bed is provided with a cold air outlet to a cold air inlet, and a cooling circulation system consisting of a second cyclone separator, a third centrifugal fan, and a cooler is sequentially arranged on the gas pipeline from the cold air outlet to the cold air inlet of the cooling fluidized bed. Preferably, the temperature of the cooler connected to the cooling fluidized bed is controlled to be no greater than 40°C.

[0011] After being cooled by a fluidized bed, the surface of the PLA slices hardens rapidly, forming a glassy state. The heat distortion temperature of the PLA pellets increases after they reach the exfoliation state. At this point, the heat distortion temperature of the PLA slices is close to the melting temperature. Therefore, it can effectively prevent the PLA slices from softening and sticking together in the drying tower.

[0012] Furthermore, the drying air inlet at the bottom of the drying tower is equipped with a drying system consisting of a fan, a condenser, a dehumidifier, and a third electric heater. The temperature of the third electric heater, which is connected to the drying tower, is controlled at 70°C to 80°C.

[0013] Furthermore, a first rotary valve is installed on the material pipeline between the pre-crystallization tank and the dual-channel crystallization bed, and a second rotary valve is installed on the material pipeline between the cooling fluidized bed and the dual-channel crystallization bed.

[0014] This invention also provides a method for crystallizing and drying PLA slices. The temperature of the dual-channel crystallizer is controlled at 90℃~100℃ (the crystallization temperature of PLA slices is generally considered to be between 90℃ and 110℃; within this range, the higher the temperature, the faster the crystallization rate. As the crystallinity of the PLA slices increases, the softening temperature of the slices also changes. In this embodiment, 90℃~100℃ is the preferred temperature while ensuring that the PLA slices do not deform or stick together). The cooling fluidized bed cools the PLA slices, causing the surface of the PLA slices to harden rapidly and form a glassy state. The drying temperature in the drying tower is 70℃~80℃. Since the PLA slices have already undergone increased crystallinity and cooled to a glassy state, the softening temperature is close to the melting temperature of the PLA slices. Therefore, even with long-term high-temperature drying at 70℃~80℃, softening and sticking of the PLA can be avoided. The specific method is as follows:

[0015] A method for crystallizing and drying PLA slices, characterized by comprising the following steps:

[0016] 1) The slices are transported to a pre-crystallization tank for pre-crystallization;

[0017] 2) The slices in the pre-crystallization tank are sequentially fed into the crystallizer for heating and crystallization;

[0018] 3) The heated and crystallized PLA slices are transported to a cooling fluidized bed for cooling and temperature reduction.

[0019] 4) The cooled PLA slices are sent to the drying tower, where hot dry air flowing from the bottom to the top of the drying tower dries the slices that are moving down from the top of the drying tower.

[0020] Furthermore, in step 2), the temperature in the crystallizer is 90-100℃.

[0021] Furthermore, the crystallinity of the PLA slices after heating and crystallization in step 2) is 30%-40%.

[0022] Furthermore, in step 3), the temperature in the cooling fluidized bed is less than 65°C, preferably less than 60°C, and more preferably less than 40°C.

[0023] Furthermore, in step 3), the temperature of the dry, hot air is 70-80°C.

[0024] Here, PLA slices are crystalline polymer materials, and their heat distortion temperature is mainly affected by the crystallization region. When the crystallinity is within a certain range (e.g., 30%-40%), the heat distortion temperature is close to the melting temperature. The melting temperature of PLA slices is around 170℃, meaning that adhesion and agglomeration will only occur when the crystallized PLA slices are reheated to around 170℃. In this crystallization bed, the initial PLA slices are first heated to 90℃-100℃ to gradually increase their crystallinity. By the time they exit the crystallization bed, they already have a high degree of crystallinity. Then, they are rapidly cooled to a preferred temperature of 40℃ in a cooling fluidized bed, causing the surface of the PLA slices to harden rapidly and form a glassy state. At this point, the heat distortion temperature of the glassy PLA slices is close to its melting temperature. Therefore, when the cooled and crystallized PLA slices re-enter the drying tower, adhesion / agglomeration and clumping are less likely to occur when they are piled up or heated to 70℃-80℃.

[0025] Specifically: ① In the pre-crystallization tank, the residual temperature is higher than the glass transition temperature of the PLA slices, so the crystallinity of the PLA slices will increase; ② The heating temperature in the dual-channel crystallization bed is 90-100℃, which is higher than the crystallization temperature of the PLA slices, so the crystallinity continues to increase; ③ When the crystallinity of the PLA slices increases to a certain range (such as 30%-40%), it is cooled to below 40℃ in a cooling fluidized bed, causing the surface of the PLA slices to harden rapidly and form a glassy state. At this time, the heat distortion temperature of the PLA slices is close to the melting temperature (150-170℃); ④ The cooled PLA slices enter the drying tower for drying. Although the temperature of the drying tower is 70-80℃, the heat distortion temperature of the PLA slices is much higher than the temperature of the drying tower, so the slices will not soften and stick together.

[0026] In fact, the heat distortion temperature of PLA slices shows a trend of first increasing and then decreasing with the increase of crystallinity. When the crystallinity of PLA slices is 30%-40%, its heat distortion temperature reaches its extreme value. Therefore, in this invention, the crystallinity of PLA slices is preferably controlled at 30%-40%. The crystallinity can be adjusted by controlling the heating time in the dual-channel crystallization bed. In this application, it is preferred that the PLA slices be heated in the dual-channel crystallization bed for 40-45 minutes. Here, it is preferred that the cooling temperature of PLA slices in the cooling fluidized bed is not greater than 40°C, but the cooling temperature is not limited to not greater than 40°C. The cooling temperature can be set below the glass transition temperature of PLA slices (60-65°C). The purpose is to allow the crystallized slices to cool down rapidly to form a glassy state before proceeding to the next step of drying.

[0027] Compared with existing technologies, the advantages of this device and method are as follows: Due to the low glass transition temperature (60-65℃) of PLA slices, the PLA slices, after preliminary crystallization in the pre-crystallization tank and crystallization in the dual-channel crystallization bed, undergo cooling treatment to cool the PLA slices below the glass transition temperature, ensuring that the PLA slices entering the drying tower do not deform or stick together. Furthermore, it can process not only high-melting-point PLA but also the low-melting-point PLA that has emerged in the last two years, perfectly matching the production capacity of the polymerization section. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the connection relationship in this invention.

[0029] In the picture:

[0030] Pre-crystallization tank 1001, first rotary valve 1002, dual-channel crystallization bed 1003, first cyclone separator 1004, first centrifugal fan 1005A, second centrifugal fan 1005B, first electric heater 1006A, second electric heater 1006B.

[0031] Second rotary valve 2001, cooling fluidized bed 2002, second cyclone separator 2003, third centrifugal fan 2004, cooler 2005;

[0032] Drying tower 3001, filter screen 3002, fan 3003, condenser 3004, dehumidifier 3005, third electric heater 3006. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be further described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. For clarity and brevity, not all features of the actual implementation are described in the specification. However, it should be understood that many implementation-specific decisions must be made in the development of any such actual embodiment in order to achieve the developer's specific goals.

[0034] It should also be noted that the elements and features described in one drawing or embodiment of the present invention may be combined with the elements and features shown in one or more other drawings or embodiments. In order to avoid obscuring the present invention with unnecessary details, only the device structure closely related to the solution according to the present invention is described in the drawings and description.

[0035] Example 1: As Figure 1As shown, a crystallization and drying device for PLA slices includes a pre-crystallization tank 1001, a dual-channel crystallization bed 1003, a cooling fluidized bed 2002, and a drying tower 3001, which are connected in sequence by material pipelines. A first rotary valve 1002 is installed on the material pipeline between the pre-crystallization tank 1001 and the dual-channel crystallization bed 1003, and a second rotary valve 2001 is installed on the material pipeline between the cooling fluidized bed 2002 and the dual-channel crystallization bed 1003.

[0036] The dual-channel crystallization bed 1003 is provided with an air inlet and an air outlet. The air inlet includes a first air inlet and a second air inlet. The first air inlet is located near the first rotary valve 1002, and the second air inlet is located near the middle of the dual-channel crystallization bed 1003. Preferably, both the first and second air inlets are located at the bottom of the dual-channel crystallization bed. A heating circulation system is provided between the air outlet and the air inlet. The heating circulation system includes a first cyclone separator 1004, a centrifugal fan, and an electric heater, which are connected in sequence by a gas pipeline. The centrifugal fan includes a first centrifugal fan 1005A and a second centrifugal fan 1005B connected in parallel, and the electric heater includes a first electric heater 1006A and a second electric heater 1006B connected in parallel. The first centrifugal fan 1005A and the first electric heater 1006A are connected to the first air inlet, and the second centrifugal fan 1005B and the first electric heater 1006B are connected to the second air inlet.

[0037] The cooling fluidized bed 2002 is provided with an air inlet and an air outlet. The air outlet and the air inlet are connected in sequence by a gas pipeline to a second cyclone separator 2003, a third centrifugal fan 2004, and a cooler 2005. The air outlet of the cooling fluidized bed 2002 is connected to the second cyclone separator 2003, and the air outlet of the cooler 2005 is connected to the air inlet of the cooling fluidized bed 2002, thereby forming a cooling circulation system. An exhaust port is provided on the gas pipeline between the second cyclone separator 2003 and the third centrifugal fan 2004.

[0038] The drying air inlet at the bottom of the drying tower 3001 is connected to a drying system consisting of a filter screen 3002, a fan 3003, a condenser 3004, a dehumidifier 3005, and a third electric heater 3006.

[0039] The PLA slice crystallization and drying device is controlled by a PLC controller (not shown in the figure). The first electric heater 1006A and the second electric heater 1006B, connected to the dual-channel crystallization bed 1003, are both controlled at temperatures between 90°C and 100°C. In this embodiment, the controlled temperatures of the first electric heater 1006A and the second electric heater 1006B are 90°C and 100°C, respectively. The temperature of the first electric heater 1006A is lower because the slices without any crystallization first fall into zones one and two, where there are more raw slices, and higher temperatures would easily cause them to stick together. Zones one and two are mainly to increase the softening point of the slices. The temperature in zones three and four can be increased to the highest crystallization rate temperature of the slices. At this temperature, the slices will no longer stick together, achieving the goal of complete crystallization of the slices within a specified time.

[0040] The temperature of the cooler 2005 connected to the cooling fluidized bed 2002 is controlled to be no more than 40°C. In this embodiment, the temperature of the cooler 2005 is controlled to be 40°C.

[0041] The temperature of the third electric heater 3006, which is connected to the drying tower 3001, is controlled at 70°C to 80°C. In this embodiment, the temperature of the third electric heater 3006 is controlled at 70°C.

[0042] The present invention also provides a method for crystallizing and drying PLA slices, comprising the following steps:

[0043] 1) The slices are transported to the pre-crystallization tank 1001;

[0044] 2) The slices in the pre-crystallization tank 1001 are sequentially fed into the dual-channel crystallization bed for heating and crystallization;

[0045] 3) The heated and crystallized PLA slices are transported to a cooling fluidized bed 2002 for cooling and temperature reduction.

[0046] 4) The cooled PLA slices are sent to the drying tower 3001, where hot dry air flowing from the bottom to the top of the drying tower 3001 dries the slices that are moving down from the top of the drying tower 3001.

[0047] The controlled temperature of the dual-channel crystallization bed is 90℃~100℃, the cooling temperature of the PLA slices in the cooling fluidized bed is no greater than 40℃, and the drying temperature in the drying tower 3001 is 70℃~80℃.

[0048] To more clearly demonstrate the present invention, a detailed operational description of the crystallization and drying apparatus and system for PLA slices is provided below, in conjunction with... Figure 1As shown, PLA slices from the pelletizing system enter the pre-crystallization tank 1001. In the pre-crystallization tank 1001, the PLA slices remain in the pre-crystallization tank 1001 for about 20 to 25 minutes using their own residual heat. During this process, the PLA slices crystallize, and the color changes from transparent to slightly blurry.

[0049] The damping cone (see the damping cone disclosed in patent document CN217248846U) provided in the pre-crystallization tank 1001 can prevent the PLA slices in the middle of the pre-crystallization tank from flowing too fast, so that the flow in the middle and the periphery is consistent, ensuring the first-in-first-out and uniform residence time of the PLA slices. This makes the PLA slices that enter the dual-channel crystallization bed 1003 through the first rotary valve 1002 have almost the same degree of crystallinity.

[0050] Here, the capacity of the pre-crystallization tank 1001 determines the residence time of the PLA slices in the pre-crystallization tank 1001. The temperature of the PLA slices entering the pre-crystallization tank 1001 from the upstream pelletizing system is higher than the glass transition temperature of the PLA slices. Therefore, there cannot be too many PLA slices in the pre-crystallization tank 1001. They can only stay in the pre-crystallization tank 1001 for about 20 to 25 minutes. Otherwise, if the residence time is too long, deformation and adhesion will occur. During this process, the PLA slices undergo a crystallization reaction due to accumulation and preheating, and the color changes from transparent to slightly blurry.

[0051] The dual-airflow crystallization bed 1003 has five zones, such as... Figure 1 As shown, zones one, two, three, four, and five are spaced apart by weir plates from left to right. Since PLA crystallizes relatively slowly and takes a long time, the two air ducts correspond to the four zones, meaning each air duct corresponds to two zones. This is used to set different crystallization temperatures for different zones. The temperatures of zones one and two at the feed need to be set lower, while the temperatures of zones three and four and the adjacent discharge outlet are set higher than the feed zone temperature. This is beneficial for PLA crystallization and prevents the slices from clumping.

[0052] Here, the temperature settings for zones one and two of the feed need to be relatively low because the uncrystallized slices fall into zones one and two first, where there are more raw slices, and if the temperature is too high, they will easily stick together. Zones one and two are mainly to increase the softening point of the slices. The temperature in zones three and four can be increased to the highest crystallization rate temperature of the slices. At this point, the slices will no longer stick together, achieving the goal of complete crystallization of the slices within the specified time.

[0053] The four zones of the dual-air-duct crystallization bed use two air inlets. Zones 1 and 2, which are closer to the feed inlet, are connected to the first air inlet. Zones 3 and 4, which are closer to the discharge outlet, are connected to the second air inlet. The two zones at the discharge end have smaller areas because the chips have partially crystallized by the time they reach zones 3 and 4. Zones 1 and 2 have larger areas because they need to achieve a mixing ratio of raw and cooked materials, such as a 1:9 ratio.

[0054] Zone 5 is connected to the discharge port of the dual-channel crystallization bed 1003. The weir plate increases the residence time of the slices in the dual-channel crystallization bed 2003 and makes the mixing of raw and cooked materials more uniform. Each of the two air inlets of the dual-channel crystallization bed is equipped with a butterfly valve, which controls the air velocity and volume at the air inlet.

[0055] The hot air temperatures of the two air ducts are set by the corresponding first electric heater 1006A and second electric heater 1006B, respectively. The hot air directly exchanges heat with the PLA slices in the dual-air duct crystallization bed 1003, raising the PLA slices to 90℃~100℃ for crystallization reaction. The hot air that has lost heat is removed by the first cyclone separator 1004 and then pressurized by the corresponding first centrifugal fan 1005A and second centrifugal fan 1005B before flowing back through the corresponding first electric heater 1006A and second electric heater 1006B for heating, and the PLA slices are heated in a cycle. Here, the air intake passage formed by the first centrifugal fan 1005A and the first electric heater 1006A is connected to the air intake ports of the first and second zones in the dual-channel crystallization bed 1003, and the air intake passage formed by the second centrifugal fan 1005B and the second electric heater 1006B is connected to the air intake ports of the third and fourth zones in the dual-channel crystallization bed 1003. In this embodiment, the air intake temperature of the first and second zones of the dual-channel crystallization bed 1003 is lower than that of the third and fourth zones. For example, the first and second zones are rapidly heated zones with a temperature of 90°C, while the third and fourth zones have a temperature of 100°C. The air velocity and air volume of the first and second zones must be greater than those of the third and fourth zones. The main reason is that the PLA slices in the first and second zones must be blown up by the air, such as in a bouncy, fluidized state, and flow sequentially to the discharge port to prevent stagnation and clumping. At a temperature of 90-100℃, the residence time of PLA granules in the dual-channel crystallization bed 1003 is controlled at 40-45 minutes, which can control the crystallinity of PLA slices at 30%-40%.

[0056] In the dual-channel crystallization bed 1003, PLA slices exhibit a "jumping" leaping state under the action of hot air. As the feed rate continues to enter, the leaping PLA slices jump over the weir plate and enter the second zone from the first zone of the dual-channel crystallization bed 1003, then from the second zone to the third and fourth zones. After passing the last weir plate in the fourth zone, they enter the discharge pipe and then enter the cooling fluidized bed 2002 through the second rotary valve 2001. Compared with the long-term high-temperature drying in the drying tower 3001, the slices in the dual-channel crystallization bed 1003 have a shorter residence time and are in a "jumping" leaping state. Therefore, the slices are less likely to stick together in the dual-channel crystallization bed 1003.

[0057] The PLA slices entering the cooling fluidized bed 2002 exhibit a fluctuating state on the bed surface. The "cold air" below 40°C, regulated by the cooler 2005, comes into direct contact with the hot PLA slices in the cooling fluidized bed 2002, cooling the PLA slices to the range of 50°C to 55°C before they enter the drying tower 3001 for drying.

[0058] Because the PLA slices, after initial crystallization in the pre-crystallization tank 1001 and crystallization in the dual-channel crystallization bed 1003, are already at temperatures far exceeding the glass transition temperature of PLA slices, these slices, resembling soft candy, are amorphous. Directly entering the drying tower for prolonged high-temperature drying can easily lead to deformation and adhesion due to their own weight and mutual compression, potentially forming large lumps. Therefore, in this embodiment, the crystallized PLA slices undergo cooling treatment. Since PLA slices have a low glass transition temperature (60-65°C), they are cooled to a temperature range of 50-55°C below the glass transition temperature to harden them. This ensures that the PLA slices entering the drying tower 3001 do not deform or stick together.

[0059] The hot air that exchanges heat with PLA slices in the cooling fluidized bed 2002 is then de-dusted by the second cyclone separator 2003, pressurized by the third centrifugal fan 2004, and then enters the cooler 2005 to cool down. After the air temperature drops to below 40°C, it enters the cooling fluidized bed 2002 again, and the cycle continues.

[0060] The PLA slices entering the drying tower 3001 flow downwards in a plunger flow pattern. The formation of the plunger flow is due to the damping cone and air inlet distributor in the drying tower 3001 (see the damping cone and air inlet distributor disclosed in patent document CN217248846U). This not only distributes the incoming hot air evenly across the entire cross-section, but its unique conical design also hinders the flow of material in the center of the drying tower. This is the key to forming a stable plunger flow between the PLA material in the center of the drying tower 3001 and the surrounding PLA material, ensuring that the PLA material has the same residence time in the drying tower 3001. The hot air entering the drying tower 3001 is filtered by filter 3002, pressurized by Roots blower 3003, pre-dehumidified by pre-condenser 3004, further dehumidified by dehumidifier 3005, and then heated by third electric heater 3006. The air exiting dehumidifier 3005 has a dew point of -75°C. The dehumidifier 3005 used here is an atmospheric pressure, energy-saving dehumidifier, which saves about 30% to 35% energy compared to a high-pressure dehumidifier when processing the same amount of air. The hot, dry air at a temperature of 70°C to 80°C entering the drying tower 3001 forms a counter-current flow with the PLA slices. Finally, it enters the cooling circulation system with a cooling fluidized bed 2002 through the feed inlet pipe at the top of the drying tower 3001, and is discharged into the atmosphere through the exhaust port of the cooling circulation system.

[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A crystallization and drying apparatus for PLA slices, characterized in that... The system includes a pre-crystallization tank, a crystallization bed, and a drying tower connected sequentially by material pipelines. A cooling fluidized bed is installed on the material pipeline between the crystallization bed and the drying tower. The crystallization bed is a dual-channel crystallization bed, which is provided with a channel outlet and a channel inlet. The channel inlet includes a first inlet and a second inlet. The dual-channel crystallization bed has five zones, which are arranged from left to right as zones one, two, three, four, and five, and are separated by weir plates. Zones one and two, which are closer to the feed inlet, are connected to the first inlet, and zones three and four, which are closer to the discharge outlet, are connected to the second inlet. The areas of zones three and four, which are located at the discharge end, are smaller than those of zones one and two.

2. The crystallization and drying apparatus for PLA slices according to claim 1, characterized in that: A heating circulation system is provided between the air outlet and the air inlet of the air duct. The heating circulation system includes a first cyclone separator, a centrifugal fan, and an electric heater connected in sequence by a gas pipeline. The centrifugal fan includes a first centrifugal fan and a second centrifugal fan arranged in parallel, and the electric heater includes a first electric heater and a second electric heater arranged in parallel. The first centrifugal fan and the first electric heater are connected to the first air inlet, and the second centrifugal fan and the first electric heater are connected to the second air inlet.

3. The crystallization and drying apparatus for PLA slices according to claim 1, characterized in that: The cooling fluidized bed is provided with a cold air outlet to a cold air inlet, and a cooling circulation system consisting of a second cyclone separator, a third centrifugal fan, and a cooler is sequentially arranged on the gas pipeline from the cold air outlet to the cold air inlet of the cooling fluidized bed.

4. The crystallization and drying apparatus for PLA slices according to claim 1, characterized in that: The drying air inlet at the bottom of the drying tower is equipped with a drying system consisting of a fan, condenser, dehumidifier, and third electric heater.

5. The crystallization and drying apparatus for PLA slices according to claim 1, characterized in that: A first rotary valve is installed on the material pipeline between the pre-crystallization tank and the dual-channel crystallization bed, and a second rotary valve is installed on the material pipeline between the cooling fluidized bed and the dual-channel crystallization bed.

6. A method for crystallizing and drying PLA slices using a crystallization and drying apparatus according to any one of claims 1-5, characterized in that, Includes the following steps: 1) The slices are transported to a pre-crystallization tank for pre-crystallization; 2) The slices in the pre-crystallization tank are sequentially fed into the crystallizer for heating and crystallization, so that the crystallinity of the PLA slices is 30%-40%; 3) The heated and crystallized PLA slices are transported to a cooling fluidized bed for cooling and temperature reduction. 4) The cooled PLA slices are sent to the drying tower, where hot dry air flowing from the bottom to the top of the drying tower dries the slices that are moving down from the top of the drying tower.

7. The method for crystallizing and drying PLA slices according to claim 6, wherein the temperature of the crystallizer is controlled at 90℃-100℃.

8. The method for crystallizing and drying PLA slices according to claim 6, wherein the cooling temperature of the material in the cooling fluidized bed is less than 65°C.

9. The method for crystallizing and drying PLA slices according to claim 8, wherein the cooling temperature of the material in the cooling fluidized bed is less than 40°C.

10. The method for crystallizing and drying PLA slices according to claim 6, wherein the drying temperature in the drying tower is 70℃-80℃.

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