A continuous lactic acid polycondensation reactor and its use

Abstract

The application belongs to the technical field of polylactic acid synthesis, and discloses a lactic acid continuous polycondensation reactor and a method for using the same. The reactor has a cylindrical tank composed of multiple reaction tanks, and a screw stirring device is arranged in the cylindrical tank to promote the flow of reactants. A polymerization unit, a vibration unit and a vacuum condensation unit are arranged on the outer wall of the cylindrical tank. The exciting force from the vibration unit makes the components such as lactic acid, lactic acid polymer and catalyst in the reaction tank cavity more uniformly mixed, improves the heat and mass transfer effect in the lactic acid polymerization process, and increases the contact area between the reactants and the catalyst. The reactor and the method ensure the continuity and stability of lactic acid polymerization by designing multiple reaction tanks, and the screw stirring device and the vibration device solve the problems of poor mass and heat transfer and difficult removal of by-products in the later stage of melt polymerization, which is beneficial to the efficient polycondensation synthesis of polylactic acid.

Description

Technical Field

[0001] This invention relates to the field of polylactic acid synthesis technology, and more specifically, to a continuous polycondensation reactor for lactic acid and its application method. Background Technology

[0002] Currently, there are two main methods for synthesizing polylactic acid (PLA): lactide ring-opening polymerization and lactic acid melt polycondensation. Lactic acid ring-opening polymerization involves prepolymerizing lactic acid into cyclic lactide, followed by repeated recrystallization and purification before ring-opening polymerization. This method yields PLA with high molecular weight and good quality, but the synthesis process is complex, lengthy, has low yield, and high production costs. Lactic acid melt polycondensation, on the other hand, involves dehydrating and esterifying lactic acid monomers under high temperature and low pressure, thereby polycondensing to obtain PLA. This method does not use any organic solvents, has a simple synthesis process, short flow, high yield, and low production costs, making it suitable for large-scale industrial production. However, it is limited by the weak heat and mass transfer process of the batch stirred reactor, and the high viscosity of the system in the later stages of the reaction makes it difficult to remove byproducts. The synthesized PLA typically has disadvantages such as low molecular weight, wide molecular weight distribution, and dark color.

[0003] For the melt polycondensation process of lactic acid, the timely removal of reaction byproducts directly affects the extent to which the polycondensation reaction proceeds toward the target product. The flow mixing characteristics of the polycondensation reactor directly affect the component and temperature distribution of the material system within the reactor, thereby influencing the reaction progress and the quality of the target product. Therefore, enhancing heat and mass transfer during the continuous thickening process of the reaction system, improving the component and temperature distribution of the reaction system, homogenizing the residence time of the reaction system, and accelerating the removal of reaction byproducts to promote the reaction process are key issues that urgently need to be addressed for the precise preparation of high molecular weight PLA materials with controllable molecular weight distribution.

[0004] Based on the above problems, developing a continuous synthesis reaction equipment with continuous production process, high yield of reaction products, controllable chemical structure and morphology, and high efficiency in removing by-products is of great significance to promoting the development of polymerization reaction engineering. Summary of the Invention

[0005] This invention's continuous polycondensation reactor for lactic acid, through the integration of a screw agitator and a vibration device, combines a vibrational force field, a shear flow field, and a tensile flow field, resulting in excellent mixing and dispersion effects and interfacial renewal capabilities. This ensures the efficient removal of byproducts from high-viscosity polymers. Consequently, it solves the problems of intermittent batch production, poor stability, low production efficiency, low molecular weight, and wide molecular weight distribution inherent in existing lactic acid melt polycondensation technologies, achieving continuous and efficient synthesis of high-quality polylactic acid.

[0006] According to a first aspect of the present invention, a continuous lactic acid polycondensation reactor is provided, comprising a polymerization unit, a vibrating unit, and a vacuum condensing unit; the polymerization unit includes a cylindrical vessel and a screw agitator, the cylindrical vessel including a non-vibrating vessel and a plurality of vibrating vessels sequentially connected to the non-vibrating vessel, the screw agitator sequentially penetrating the non-vibrating vessel and each of the vibrating vessels; a feed inlet is provided on the non-vibrating vessel; the vibrating unit includes a plurality of excitation devices, each of the excitation devices being connected to the outer wall of each of the vibrating vessels;

[0007] The vacuum condenser unit includes a fractionation tower, a vacuum adsorption tank, and a vacuum pump; at least one of the cylindrical vessels is connected to the fractionation tower, which is connected to the vacuum adsorption tank and the vacuum pump in sequence via pipelines.

[0008] Preferably, the screw of the screw mixing device has a trapezoidal thread segment and a topological surface helical segment connected to each other, and the trapezoidal thread segment and the topological surface helical segment are alternately arranged.

[0009] Preferably, the screw agitator has multiple screws, and the screws mesh with each other.

[0010] Preferably, the number of cylindrical vessels is 2-20.

[0011] Preferably, the cylindrical vessel has a surrounding temperature control device on its side wall.

[0012] Preferably, the continuous lactic acid polycondensation reactor further includes a motor and a gearbox, the gearbox being connected to the screw agitator exposed at the non-vibrating vessel end, and the motor being connected to the gearbox.

[0013] According to another aspect of the present invention, a method for continuously polycondensing lactic acid in the said lactic acid continuous polycondensation reactor is provided, comprising the following steps:

[0014] (1) Set the reaction temperature of each of the cylindrical vessels, and the reaction temperature of each of the cylindrical vessels gradually increases from upstream to downstream of the material; then set the rotation speed of the screw agitator, the vacuum degree of the vacuum pump, the temperature of the fractionation tower, and the vibration frequency and amplitude of each of the excitation devices;

[0015] (2) After the lactic acid monomer and the catalyst are mixed evenly, they are added from the feed port. The periodic compression and release of the volume of the cylindrical reactor forces the contact area of ​​the lactic acid monomer, lactic acid polymer and catalyst to increase, thereby increasing the heat and mass exchange frequency. The by-product water generated by the reaction can be effectively removed by the fractionation tower, and the by-product lactide can be effectively refluxed by the fractionation tower, thereby inhibiting the side reaction and increasing the degree of forward reaction.

[0016] (3) After the reaction proceeds for 1 to 4 hours, polylactic acid can be obtained as a product.

[0017] Preferably, the reaction temperature range of the non-vibrating reactor is 100-120℃, and the reaction temperature of each of the vibrating reactors increases by 10-15℃ compared to the reaction temperature of the previous cylindrical reactor.

[0018] Preferably, the vacuum pump has a vacuum range of -0.1MPa to 0MPa, and the distillation tower has a temperature range of 80℃ to 100℃.

[0019] Preferably, the rotational speed of the screw agitator is 50-500 rpm; the vibration frequency of the excitation device is 100-1000 Hz, and the amplitude is 0.5-1.5 mm.

[0020] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0021] (1) The continuous polycondensation reactor for lactic acid of the present invention has a special mechanical structure (i.e., the setting of screw stirring device and vibrating unit). The combination of vibration force field, shear flow field and stretching flow field gives it excellent mixing and dispersion effect and interface renewal ability. The setting of multiple cylindrical vessels makes the step-by-step increase of temperature in the cylindrical vessels conform to the characteristics of lactic acid melt polycondensation reaction, ensuring that different temperatures correspond to different polymerization stages. The introduced fractionation column ensures efficient removal / reflux of by-products (water / lactide) from high viscosity polymer. The continuous polycondensation reaction method of the present invention can solve the problems of intermittent batch production, poor stability, low production efficiency, low molecular weight and wide distribution in the existing lactic acid polycondensation process, and realize the continuous and efficient synthesis of high-quality polylactic acid.

[0022] (2) The lactic acid continuous polycondensation reactor of the present invention introduces the force field of the vibration of the vibrating vessel and the screw stirring device to make the material in the polymerization process turbulent, which greatly increases the collision frequency of each component, shortens the reaction time, and improves the uniformity of the product. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the lactic acid continuous polycondensation reactor of the present invention.

[0024] Figure 2 for Figure 1 Schematic diagram of the screw structure and its different cross sections.

[0025] The labels and corresponding component names in the diagram are as follows: 1-Motor, 2-Gearbox, 3-Feeder, 4-Non-vibrating vessel, 5-Vibrating vessel I, 6-Temperature control device I, 7-Fracturing tower I, 8-Observation port, 9-Temperature control device II, 10-Vibrating vessel II, 11-Fracturing tower II, 12-Temperature control device III, 13-Vibrating vessel III, 14-Vacuum adsorption tank, 15-Discharge port, 16-Vacuum pump, 17-Screw agitator, 18-Trapezoidal threaded section of the screw, 19-Elastic flange, 20-Vibration device I, 21-Topological curved spiral section of the screw, 22-Vibration device II, 23-Vibration device III. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0027] This invention provides a continuous lactic acid polycondensation reactor, comprising a polymerization unit, a vibrating unit, and a vacuum condensing unit. The polymerization unit includes a cylindrical vessel and a screw stirring device. The cylindrical vessel includes multiple reaction vessels, which are divided into vibrating vessels and non-vibrating vessels according to their functions, and the reaction vessels are connected by flanges. The screw is horizontally installed inside the cylindrical vessel. The vibrating unit includes an excitation device connected to the outer wall of the vibrating vessel. The vacuum condensing unit includes a vacuum pump, a fractionation tower, and a vacuum adsorption tank.

[0028] like Figure 1 As shown, the lactic acid continuous polycondensation reactor of the present invention includes a drive unit, a polymerization unit, and a vibration unit.

[0029] The drive unit includes a motor 1 and a gearbox 2. The polymerization unit includes a cylindrical reactor and a screw agitator. The cylindrical reactor includes multiple reactors, which are classified according to their functions as a non-vibrating reactor 4, a vibrating reactor I 5, a vibrating reactor I 10, and a vibrating reactor III 13. Each reactor is connected by a flexible flange 19. The vibration unit includes an excitation device I 20, an excitation device I 11 22, and an excitation device III 23. The drive unit is connected to the non-vibrating reactor 4 via an output shaft. The screw agitator 17 is installed inside the cylindrical reactor. The excitation devices I 20, I 11 22, and III 23 are respectively connected to the outer walls of the vibrating reactors I 5, I 10, and III 13. The vacuum condenser unit includes a fractionation tower I 7, a fractionation tower I 11, a vacuum adsorption tank 14, and a vacuum pump 16.

[0030] Specifically, such as Figure 2As shown, the screw mixing device 17 includes a trapezoidal threaded section 18 of the screw and a topological curved spiral section 21 of the screw connected alternately, and the topological curved spiral section 21 of the screw has an eccentricity e.

[0031] Specifically, the vibration unit includes excitation device I20, excitation device I122, and excitation device III23, which are connected to the outer walls of vibrating vessel I5, vibrating vessel I110, and vibrating vessel III13.

[0032] Temperature control devices I6, I10, and III13 are installed on the outside of each vibrating vessel. The non-vibrating vessel 4 has a feed inlet connected to the feeder 3. Vibrating vessel I5 has a fractionation tower I7 and an observation port 8, while vibrating vessel I10 has a fractionation tower I11. Vibrating vessels I5, I10, and III13 are fixedly connected to the output shafts of excitation devices I20, I122, and III23, respectively, which generate the excitation force. During operation, the feeder 3 transports the reactants to the non-vibrating vessel 4. Fractionation towers I7 and I11, along with the vacuum pump 16, can promptly discharge the byproducts (water) generated during the reaction and allow the lactide generated by the side reaction to flow back to the cylindrical vessel to suppress the side reaction. A vacuum adsorption tank 14 is positioned between fractionation towers I7 and I11 and the vacuum pump 16 to prevent small molecule gases from damaging the vacuum pump 16.

[0033] During screw rotation, as the material is conveyed from the feeder 3 to the trapezoidal thread section 18 of the screw, the reactants and catalyst are thoroughly mixed under the stirring of the screw. During the mixing and conveying process, excitation devices I20, I122, and III23 apply vibrational force fields to vibrating vessels I5, I10, and III13 respectively, thereby forcing the vibrating vessels to vibrate up and down in a direction perpendicular to the screw axis. Byproducts (water / lactide) generated during polymerization can be discharged / recirculated in a timely manner through fractionation towers I7 and I11 and vacuum pump 16, thereby promoting the forward reaction and suppressing side reactions. In addition, the observation port 8 allows for real-time observation of the state of the reactants in the cylindrical vessel. Under the synergistic effect of screw rotation and vibrating vessel vibration, the material is subjected to periodic volume compression and release in the vertical direction, which increases the interfacial contact area of ​​lactic acid, lactic acid polymer, and catalyst, and improves the heat and mass exchange frequency, greatly promoting the forward reaction of lactic acid polycondensation. In addition, the stepwise increase in temperature of the cylindrical reactor conforms to the characteristics of lactic acid melt polycondensation reaction, ensuring that different temperatures correspond to different polymerization stages.

[0034] In some embodiments, the number of cylindrical vessels is 2-20, and they are arranged in parallel on the same horizontal plane.

[0035] In some embodiments, the flanges connecting the various reactors can be flexible or rigid flanges to enable segmented or overall vibration of the polycondensation reactor.

[0036] In some embodiments, the maximum amplitude of the cylindrical vessel is 1.5 mm; the maximum vibration frequency range of the cylindrical vessel is 1000 Hz.

[0037] In some embodiments, the screw has both trapezoidal and curved structures that mesh with each other.

[0038] In some embodiments, the cylindrical vessel may have an inlet and an outlet, the inlet being connected to a feeder; and the outlet being connected to a ball valve.

[0039] In some embodiments, the cylindrical vessel is provided with a surrounding temperature control device on its sidewall.

[0040] In some embodiments, a fractionation tower is provided on the cylindrical vessel, and the fractionation tower is connected in sequence to a vacuum adsorption tank and a vacuum pump via pipelines.

[0041] In some embodiments, the vacuum condenser unit includes a vacuum adsorption tank, wherein the number of vacuum adsorption tanks may be one or two.

[0042] In some embodiments, the cylindrical vessel is provided with an observation device, which includes an observation window.

[0043] The present invention also proposes a method for preparing polylactic acid using the above-mentioned continuous lactic acid polycondensation reactor, the method comprising the following steps:

[0044] (1) Set the temperature of the continuous lactic acid polycondensation reactor;

[0045] (2) After the equipment temperature reaches the set temperature, set the screw speed, equipment vacuum degree, distillation tower temperature, vibration frequency and amplitude;

[0046] (3) After the lactic acid monomer and the catalyst are mixed evenly, they are fed into the extruder by the feeder at a certain rate. The periodic compression and release of the volume of the cylindrical vessel cavity forces the contact area of ​​the interface between lactic acid, lactic acid polymer and catalyst to increase, the heat and mass exchange frequency is improved, and the by-products generated by the reaction can be effectively removed / refluxed through the fractionation tower, which inhibits the side reaction and improves the degree of forward reaction.

[0047] (4) After the reaction has proceeded for 1 to 4 hours, the product polylactic acid can be obtained by opening the outlet.

[0048] The present invention will be described in detail below through embodiments.

[0049] Example 1

[0050] The screw speed is set to 50 rpm, and the vacuum degree is reduced from 0 to -0.1 MPa at a rate of -0.0015 MPa / min within 1 hour. The temperatures of non-vibrating vessel 4, vibrating vessel I 5, vibrating vessel II 10, and vibrating vessel III 13 are 100℃, 130℃, 150℃, and 160℃, respectively. The amplitudes of excitation devices 20, 22, and 23 are 0.5, 0.8, and 0.9 mm, respectively, and the vibration frequencies are 100, 300, and 600 Hz, respectively. The temperatures of fractionation towers 7 and 11 are 80℃.

[0051] Close the discharge port 15, open the feed port of the feeder 3, and add 5 kg of 90% pure L-lactic acid and 0.6 wt% SnCl2 into the feeder 3. Set the feeder 3 rate to 1.5 kg / h. After reacting for 2 hours, stop the reaction, open the discharge port 15, and obtain polylactic acid with a weight-average molecular weight of 43486 and a PDI of 1.4.

[0052] Example 2

[0053] The screw speed is set to 80 rpm, and the vacuum degree is reduced from 0 to -0.1 MPa at a rate of -0.0015 MPa / min within 1 hour. The temperatures of non-vibrating vessel 4, vibrating vessel I 5, vibrating vessel II 10, and vibrating vessel III 13 are 100℃, 130℃, 150℃, and 160℃, respectively. The amplitudes of excitation devices 20, 22, and 23 are 0.6, 0.8, and 1 mm, respectively, and the vibration frequencies are 600, 800, and 800 Hz, respectively. The temperature of fractionation towers 7 and 11 is 80℃.

[0054] Close outlet 15, open feed inlet 3, and add 8 kg of 90% pure L-lactic acid and 0.8 wt% SnCl2 into the feeder. Set feeder 3 speed to 2 kg / h. After 3 hours of reaction, stop the reaction, open outlet 15, and polylactic acid with a weight average molecular weight of 71484 and PDI of 1.7 is obtained.

[0055] Example 3

[0056] The screw speed is set to 100 rpm, and the vacuum degree is reduced from 0 to -0.1 MPa at a rate of -0.0015 MPa / min within 1 hour. The temperatures of non-vibrating vessel 4, vibrating vessel I 5, vibrating vessel II 10, and vibrating vessel III 13 are 110℃, 140℃, 160℃, and 180℃, respectively. The amplitudes of excitation devices 20, 22, and 23 are 1, 1.2, and 1.4 mm, respectively, and the vibration frequencies are 600, 1000, and 1000 Hz, respectively. The temperature of fractionation towers 7 and 11 is 80℃.

[0057] Close outlet 15, open feed inlet 3, and add 8 kg of 90% pure L-lactic acid and 1 wt% SnCl2 into the feeder. Set feeder 3 speed to 2 kg / h. After 3 hours of reaction, stop the reaction, open outlet 15, and polylactic acid with a weight average molecular weight of 97814 and PDI of 1.6 is obtained.

[0058] Comparative Example 1

[0059] This comparative example uses a traditional polyester reactor to prepare polylactic acid for comparison, and includes the following steps:

[0060] 3.5 kg of 90% pure L-lactic acid and 1 wt% SnCl2 were added to a 5 L polyester reactor. The stirrer speed was set to 100 rpm, and the vacuum degree was reduced from 0 to -0.1 MPa at a rate of -0.0015 MPa / min over 1 hour. The reactor temperature was 160 °C, and the temperature of the pipeline heating cable was set to 80 °C. The reaction was terminated after 2 hours, yielding polylactic acid with a weight-average molecular weight of 3846 and a PDI of 2.1.

[0061] Comparative Example 2

[0062] This comparative example uses a traditional polyester reactor to prepare polylactic acid for comparison, and includes the following steps:

[0063] 3.5 kg of 90% pure L-lactic acid and 1 wt% SnCl2 were added to a 5 L polyester reactor. The stirrer speed was set to 100 rpm, and the vacuum degree was reduced from 0 to -0.1 MPa at a rate of -0.0015 MPa / min over 1 hour. The reactor temperature was 160 °C, and the temperature of the pipeline heating cable was set to 80 °C. The reaction was terminated after 8 hours, yielding polylactic acid with a weight-average molecular weight of 7971 and a PDI of 1.9.

[0064] Comparative Example 3

[0065] This comparative example uses a traditional polyester reactor to prepare polylactic acid for comparison, and includes the following steps:

[0066] 3.5 kg of 90% pure L-lactic acid and 1 wt% SnCl2 were added to a 5 L polyester reactor. The stirrer speed was set to 100 rpm, and the vacuum degree was reduced from 0 to -0.1 MPa at a rate of -0.0015 MPa / min over 1 hour. The reactor temperature was 170 °C, and the temperature of the pipeline heating cable was set to 80 °C. The reaction was terminated after 16 hours, yielding polylactic acid with a weight-average molecular weight of 11031 and a PDI of 2.2.

[0067] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A continuous lactic acid polycondensation reactor, characterized in that, The system includes a polymerization unit, a vibratory unit, and a vacuum condenser unit. The polymerization unit comprises a cylindrical reactor and a screw agitator. The cylindrical reactor includes multiple reaction vessels, classified according to function as a non-vibratory reactor, vibratory reactor I, vibratory reactor II, and vibratory reactor III, all connected by flexible flanges. The screw agitator sequentially passes through the non-vibratory reactor and each vibratory reactor. The screw of the screw agitator has interconnected trapezoidal thread segments and topologically curved helical segments, which are alternately arranged. The topologically curved helical segments of the screw exhibit eccentricity. A feed inlet is provided on the non-vibratory reactor. The vibratory unit includes multiple excitation devices, each connected to the outer wall of each vibratory reactor. The vacuum condenser unit includes a fractionating tower I, a fractionating tower II, a vacuum adsorption tank, and a vacuum pump; the vibrating vessel I is equipped with a fractionating tower I and an observation port, the vibrating vessel II is equipped with a fractionating tower II, and the vacuum adsorption tank is placed between the fractionating tower I, the fractionating tower II, and the vacuum pump. The fractionating tower I and the fractionating tower II are connected to the vacuum adsorption tank and the vacuum pump in sequence through pipelines.

2. The continuous lactic acid polycondensation reactor as described in claim 1, characterized in that, The screw agitator has multiple screws, and the screws mesh with each other.

3. The continuous lactic acid polycondensation reactor as described in claim 1, characterized in that, Temperature control device I, temperature control device II, and temperature control device III are respectively installed on the outside of the vibrating vessel I, vibrating vessel II, and vibrating vessel III.

4. The continuous lactic acid polycondensation reactor as described in claim 1, characterized in that, The continuous lactic acid polycondensation reactor also includes a motor and a gearbox. The gearbox is connected to the screw agitator exposed at the non-vibrating vessel end, and the motor is connected to the gearbox.

5. A method for continuous lactic acid polycondensation using the lactic acid continuous polycondensation reactor according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Set the reaction temperature of each of the reactors, and the reaction temperature of each reactor gradually increases from upstream to downstream of the material; then set the rotation speed of the screw agitator, the vacuum degree of the vacuum pump, the temperature of the fractionation tower, and the vibration frequency and amplitude of each of the excitation devices; (2) After the lactic acid monomer and the catalyst are mixed evenly, they are added from the feed port. The periodic compression and release of the volume of the vibrating kettle I, vibrating kettle II and vibrating kettle III forces the contact area of ​​the interface between the lactic acid monomer, the lactic acid polymer and the catalyst to increase, thereby increasing the heat and mass exchange frequency. The by-product water generated by the reaction can be effectively removed by the fractionation tower, and the by-product lactide can be effectively refluxed by the fractionation tower, thereby inhibiting the side reaction and increasing the degree of forward reaction. (3) After the reaction proceeds for 1 to 4 hours, polylactic acid can be obtained.

6. The method for continuous polycondensation of lactic acid as described in claim 5, characterized in that, The reaction temperature range of the non-vibrating reactor is 100-120℃, and the reaction temperature of each of the vibrating reactors increases by 10-15℃ compared to the reaction temperature of the previous reactor.

7. The method for continuous polycondensation of lactic acid as described in claim 5, characterized in that, The vacuum pump has a vacuum range of -0.1MPa to 0MPa, and the distillation tower has a temperature range of 80℃ to 100℃.

8. The method for continuous polycondensation of lactic acid as described in claim 5, characterized in that, The rotational speed range of the screw agitator is 50~500 rpm; the vibration frequency range of the excitation device is 100-1000 Hz, and the amplitude range is 0.5-1.5 mm.

Images