A continuous polymerization process for producing polylactic acid chips from lactide

Abstract

The present invention discloses a continuous polymerization process for producing polylactic acid chips from lactide, comprising the following steps: lactide in a lactide workshop and a devolatilization reactor enters a lactide buffer tank through a lactide pipeline; a portion of the lactide is circulated to the lactide buffer tank, and another portion of the lactide enters a lactide preheater; lactide and three catalysts enter a static mixer for full mixing; after mixing, the mixture enters a lactide prepolymerization reactor for ring-opening polymerization; the mixture is transported to a lactide polymerization system for polymerization reaction to generate a melt; a portion of the melt output from the polymerization system is circulated to a tubular reactor, and another portion is transported to a devolatilization reactor for devolatilization; after devolatilization, the melt is mixed with a modified additive and then transported to a granulation system by a devolatilization discharge pump, and the polymerization process is completed. The present invention has multiple devices in a closed system and is protected by high-purity nitrogen, so that the entire production process will not come into contact with air, and polylactic acid will not be oxidized or degraded.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical production, and in particular relates to a continuous polymerization process for producing polylactic acid chips from lactide. Background Art

[0002] Polylactic acid is a polymer material with good development prospects. It is not only degradable and biocompatible, but can also be produced and processed using most general processing equipment. The preparation methods of polylactic acid include: polycondensation, chain extension, and lactide ring-opening polymerization.

[0003] Preparation by polycondensation

[0004] The direct polycondensation of lactic acid belongs to bulk polymerization. The water produced by the polycondensation is distilled off during the polymerization. A suitable catalyst is added according to the reaction requirements. As the reaction proceeds, the vacuum degree and polymerization temperature are slowly increased. Since it is difficult to completely remove water molecules from the high-viscosity polymer mixture, the lactic acid polymer obtained by this method has only a low molecular weight. In addition, the uncontrollable stereoregularity during the polymerization process leads to poor mechanical properties of the polymer. Therefore, the direct polycondensation method is usually used when only a low molecular weight polylactic acid is required.

[0005] Chain extension preparation

[0006] High molecular weight polymers can be prepared by using chain extenders, which can connect low molecular weight polymers into high molecular weight polymers. Since the self-condensation of lactic acid produces low molecular weight polymers containing hydroxyl and carboxyl groups in equal molar concentrations, chain extenders preferentially react with the hydroxyl functional groups or carboxyl functional groups of the polymers. By using mixed monomers with bifunctional functional groups or multifunctional functional groups, polylactic acid with all hydroxyl ends or polylactic acid with all carboxyl ends can be obtained. Common chain extenders for polylactic acid include diisocyanates, dioxazolines, diepoxy resins, etc.

[0007] Lactide ring-opening polymerization

[0008] Direct polycondensation of lactic acid is the cheapest way to prepare polylactic acid, but in business, polylactic acid is usually prepared by ring-opening polymerization of lactide. Ring-opening polymerization of L-lactide is the best method for preparing high molecular weight polylactic acid because it has the possibility of precise chemical control and can change the properties of the final polymer in a more controllable way. This feature of ring-opening polymerization makes it suitable for large-scale production. The polymerization methods of lactide include melt polymerization, bulk polymerization, solution polymerization and suspension polymerization, but melt polymerization is the simplest and most reproducible method.

[0009] The preparation method currently used in industrial production is in contact with air during the production process, and the contact time is relatively long, which causes oxidation of the raw materials, resulting in a low yield of the final polymerization product. The larger polymerization reactor causes uneven reaction of the raw materials, affecting the low-consumption and high-output industrial production.

[0010] In view of this, the present invention is proposed. Summary of the invention

[0011] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a continuous polymerization process for producing polylactic acid chips from lactide. To solve the above technical problem, the basic concept of the technical solution adopted by the present invention is:

[0012] A continuous polymerization process for producing polylactic acid chips from lactide, comprising the following steps:

[0013] Step 1, unpolymerized lactide in the lactide workshop and the devolatilization reactor enters the lactide buffer tank through the lactide pipeline;

[0014] Step 2, under the action of the discharge pump, a part of the lactide is circulated to the lactide buffer tank, and the other part of the lactide enters the lactide preheater through the lactide filter;

[0015] Step 3, the preheated lactide and catalyst A, catalyst B, and catalyst C from three different catalyst metering pumps enter a static mixer for thorough mixing;

[0016] Step 4, after sufficient mixing, the mixture enters a lactide prepolymerization reactor to undergo ring-opening polymerization and stay there for a period of time;

[0017] Step 5, under the action of the prepolymerization discharge pump, the mixture is transported to a lactide polymerization system to undergo a polymerization reaction to generate a melt, wherein the lactide polymerization system is a tubular reactor;

[0018] Step 6, a part of the melt output from the polymerization system is circulated to the tubular reactor, and the other part is transported to the devolatilization reactor for devolatilization;

[0019] Step 7, the low-boiling substances and lactide gas from the devolatilization reactor are condensed and refluxed to the lactide buffer tank, and the non-condensable gas and low-boiling substances are sucked into the vacuum buffer tank for condensation and crystallization and then discharged to the sewage system;

[0020] Step 8, after the devolatilization, the melt is mixed with the modified additive and then transported to the granulation system by the devolatilization discharge pump, and the polymerization process is completed.

[0021] Furthermore, the lactide polymerization system comprises a 1# tubular reactor, a 2# tubular reactor, and a 3# tubular reactor connected in series in sequence, the heat medium temperature of the 1# tubular reactor, the 2# tubular reactor, and the 3# tubular reactor is controlled to be 200-220°C, and the diameter of the tubular reactor is less than 800mm.

[0022] Furthermore, the 1# tubular reactor includes two cavities connected in series, both cavities are provided with a thermal oil heating system, the 2# tubular reactor is provided with a thermal oil heating system, and the 3# tubular reactor includes two cavities connected in series, and share a thermal oil heating system.

[0023] Furthermore, the lower cavity jacket of the devolatilizer reactor and the devolatilizer screw jacket share a heat conduction heating system, and the heating system includes a heat medium circulation pump, a heat medium cooler, and a heat medium supplementary heating valve, and the heat medium temperature is controlled at 200°C.

[0024] Furthermore, the lactide pipeline and the lactide buffer tank are both provided with a warm water insulation jacket, the temperature of the insulation jacket is set to 90-95° C., and the lactide buffer tank is provided with nitrogen protection.

[0025] Furthermore, the catalyst in step 3 adopts a low-toxic and high-efficiency catalyst based on metal compounds such as calcium, magnesium, iron and zinc, and a part of the melt in step 6 is circulated to the 3# pipeline reactor.

[0026] Furthermore, the devolatilization reactor is provided with a vacuum system and a falling film evaporation heater, and the heating temperature is 185°C-200°C.

[0027] Furthermore, the polymerization catalyst A, polymerization catalyst B, and polymerization catalyst C are stored in storage tanks of three catalyst metering pumps respectively, and the catalyst storage tanks are provided with nitrogen protection and exhaust water seal tanks.

[0028] Furthermore, in step 7, the low-boiling substances crystallized in the buffer tank are washed with process spray water and discharged into the sewage system, the non-condensable gas is pumped away by a vacuum pump, and the pressure of the devolatilization reactor is controlled at 0.8 KPa.

[0029] Furthermore, the circulating heating heat medium temperature of the lactide preheater is 175°C to 200°C, and the lactide is heated to 150-160°C in the lactide preheater.

[0030] After adopting the above technical scheme, the present invention has the following beneficial effects compared with the prior art.

[0031] The lactide buffer tank, prepolymerization reactor, tubular reactor and polymerization devolatilization reactor of the present invention are all closed systems and protected by high-purity nitrogen. They will not come into contact with air during the entire production process, and polylactic acid will not be oxidized or degraded. The present invention circulates the outlet portion of the 3# tubular reactor to the inlet of the 3# tubular reactor to mix with the melt of the 2# tubular reactor, which is beneficial to the uniform quality of the polylactic acid melt, ensures the shortest reaction time required for sufficient polymerization, reduces the release amount of low molecular weight polymer acid, and meets the molecular weight and viscosity requirements required by the product.

[0032] The lactide prepolymerization reactor of the present invention is operated at high temperature and normal pressure, and the water generated by the reaction can be evaporated and removed as quickly as possible, which is conducive to the production of lactide prepolymer with a higher molecular weight, avoids material oxidation caused by air leakage during negative pressure operation, and is conducive to process optimization; the heating system of the present invention is provided with a split-range control of two heating and cooling circuits, which avoids overheating or overcooling of a heat-sensitive material such as polylactic acid caused by temperature fluctuation of the heat transfer oil.

[0033] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings are part of this application and are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention, but do not constitute an improper limitation of the present invention. Obviously, the drawings described below are only some embodiments. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0035] Figure 1 It is a schematic diagram of the process flow of the present invention;

[0036] Figure 2 It is a structural schematic diagram of the raw material preparation part of the present invention;

[0037] Figure 3 It is a schematic diagram of the structure of the preheating and static mixing part of the present invention;

[0038] Figure 4 It is a schematic diagram of the structure of the lactide polymerization system of the present invention;

[0039] Figure 5 It is a schematic diagram of the structure of the devolatilization reaction part of the present invention.

[0040] In the figure: 1200-D0213, lactide buffer tank; 1200-P0213, discharge pump; 1200-F0213 lactide filter; 1200-P0310, prepolymerization discharge pump; 1200-R0310, prepolymerization reactor; 1200-A0310, stirrer; 1200-A0311, feed mixer; 1200-P0312, prepolymerization heat medium circulation pump; 1200-E0312, prepolymerization heat medium cooler; 1200-E0311, lactide preheater; 1200-E0 318, heat medium cooler; 1200-P0319, heat medium heating circulation pump; 1200-P0313, C metering pump; 1200-P0314, B metering pump; 1200-P0315, A metering pump; 1200-D0313, C storage tank; 1200-D0314, B storage tank; 1200-D0315, A storage tank; 1200-D0316, water seal tank; 1200-P0321, heat medium heating circulation pump; 1200-P0322, heat medium heating circulation pump; 1200-P0 323, heat medium heating circulation pump; 1200-P0324, heat medium heating circulation pump; 1200-E0324, heat medium cooler; 1200-P0325, polymer circulation pump; 1200-A0325, static mixer; 1200-P0320, polymer discharge pump; 1200-R0320, 1# tubular reactor; 1200-R0321, 2# tubular reactor; 1200-R0322, 3# tubular reactor; 1200-P0410, devolatilization discharge pump; 1200-M0 410, devolatilizer screw; 1200-M0411, modifier feeding screw; 1200-P0411, heat medium circulation pump; 1200-E0411, heat medium cooler; 1200, P0412, heat medium circulation pump; 1200-E0412, heat medium cooler; 1200-E0413, primary condenser; 1200-E0414, secondary condenser; 1200-D0410, vacuum buffer tank; 1200-K0410, devolatilization vacuum pump; 1200-R0410, devolatilization reactor.

[0041] It should be noted that these drawings and textual descriptions are not intended to limit the conceptual scope of the present invention in any way, but are intended to illustrate the concept of the present invention for those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0043] In the description of the present invention, it should be noted that the directions or positional relationships indicated by terms such as “upper”, “lower”, “front”, “back”, “left”, “right”, “vertical”, “inside” and “outside” are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0044] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] Embodiment 1

[0046] like Figures 1 to 5 As shown, a continuous polymerization process for producing polylactic acid chips from lactide described in this embodiment includes the following steps:

[0047] Step 1, lactide in the lactide workshop and the devolatilization reactor enters the lactide buffer tank through the lactide pipeline;

[0048] Step 2, under the action of the discharge pump, a part of the lactide is circulated to the lactide buffer tank, and the other part of the lactide enters the lactide preheater through the lactide filter;

[0049] Step 3, the preheated lactide and catalyst A, catalyst B, and catalyst C from three different catalyst metering pumps enter a static mixer for thorough mixing;

[0050] Step 4, after sufficient mixing, the mixture enters a lactide prepolymerization reactor to undergo ring-opening polymerization and stay there for a period of time;

[0051] Step 5, under the action of the prepolymerization discharge pump, the mixture is transported to a lactide polymerization system to undergo a polymerization reaction to generate a melt, and the lactide polymerization system is a tubular reactor;

[0052] Step 6, a part of the melt output from the polymerization system is circulated to the tubular reactor, and the other part is transported to the devolatilization reactor for devolatilization;

[0053] Step 7, the low-boiling substances and lactide gas from the devolatilization reactor are condensed and refluxed to the lactide buffer tank, and the non-condensable gas and low-boiling substances are sucked into the vacuum buffer tank for condensation and crystallization and then discharged to the sewage system;

[0054] Step 8, the melt after devolatilization is mixed with the modifying additive and then transported to the granulation system by the devolatilization discharge pump, wherein the modifying additive is the heat stabilizer SEED, and the polymerization process is completed.

[0055] The lactide buffer tank is equipped with nitrogen protection, and the lactide pipeline and lactide buffer tank are equipped with hot water insulation jackets, and the temperature of the insulation jacket is set to 90-95℃. Under the action of the discharge pump, part of the lactide circulates to the lactide buffer tank, and the other part of the lactide enters the lactide preheater through the lactide filter. The purpose of lactide circulation to the lactide buffer tank is that when the lactide is interrupted from entering the preheater, the discharge pump continues to run without stopping to prevent the material from crystallizing and clogging in the pipeline. The filtered lactide enters the lactide preheater and is heated to 150-160℃, and is fully mixed with the polymerization catalysts A, B, and C from the catalyst metering pump in the pipeline and static mixer to enter the lactide prepolymerization reactor. The lactide polymerization catalyst is independently fed by a metering pump and a mass flowmeter, and the catalyst flow and the mass flow of lactide are automatically calculated and added at a constant ratio, and the polymer quality is stable. The lactide preheater heating system has a heat transfer oil circulation pump, a heat transfer medium cooler, and a heat transfer medium heating supplementary temperature regulating valve group. The circulating heating heat transfer medium temperature is controlled at 170℃~180℃. The heat transfer medium heating system of the lactide prepolymerization reactor is equipped with a split-range control of the heating and cooling circuits to avoid overheating or overcooling of the heat-sensitive material polylactic acid caused by fluctuations in the heat transfer oil temperature.

[0056] Polymerization catalyst A, polymerization catalyst B, and polymerization catalyst C are stored in the storage tanks of three catalyst metering pumps respectively. The catalyst storage tanks are provided with nitrogen protection and exhaust water seal tanks. The catalyst adopts low-toxic and high-efficiency catalysts based on calcium, magnesium, iron, and zinc. The low toxicity and high efficiency of the catalyst is relative to the high efficiency and toxicity of stannous octoate. The compound catalysts based on calcium, magnesium, iron, and zinc have the characteristics of low toxicity and high efficiency, which can meet the polymerization of lactide and lactone. The catalyst can increase the polymerization reaction rate and reduce the racemization rate to obtain a higher molecular weight polymer melt. A stirrer is provided in the lactide prepolymerization reactor. The catalyst and lactide can be fully mixed in the reactor, and the materials produce radial and axial flows, are evenly heated in the reactor, have consistent residence time, and have uniform polymer molecular weight. Lactide and the ring-opening polymerization catalyst undergo ring-opening polymerization reaction under the heating of the reactor jacket heat transfer oil. The reactor heating system is provided with a heat medium circulation pump, a heat medium cooler, and a heat medium supplementary temperature regulating valve group. The heating temperature of the prepolymerization reactor is controlled at 175-200°C. Lactide and the catalyst undergo a ring-opening polymerization reaction in the prepolymerization reactor. After staying for a certain period of time, they are transported to the No. 1 tubular reactor by the prepolymerization discharge pump. The outlet pressure of the prepolymerization discharge pump is greater than 8.0 MPa.

[0057] The lactide polymerization system includes a 1# tubular reactor, a 2# tubular reactor, and a 3# tubular reactor connected in series. The heat medium temperature of the 1# tubular reactor, the 2# tubular reactor, and the 3# tubular reactor is controlled at 200-220°C. The 1# tubular reactor includes two cavities connected in series, both of which are equipped with a thermal oil heating system. The 2# tubular reactor is equipped with a thermal oil heating system. The 3# tubular reactor includes two cavities connected in series, and the two cavities share a thermal oil heating system. Thermal oil heating coils are installed in the tubular reactors, which have high heat exchange efficiency with the materials, ensure uniform heating of the materials, and avoid overheating and degradation of the materials inside the reactor. The independent constant temperature thermal oil heating system with cooling and heating functions ensures the stability of the polymerization reaction temperature and reduces the influence of temperature changes on the thermal sensitivity of the polymer melt. The polymer circulation pump circulates part of the melt to the inlet of the 3# tubular reactor, and the other part is transported to the devolatilization reactor by the polymer discharge pump. Melt circulation to the entrance of the 3# tubular reactor is conducive to the uniform quality of the polylactic acid melt, ensuring the shortest reaction time required for sufficient polymerization, reducing the release of low molecular weight polymer acid, and achieving the molecular weight and viscosity requirements of the product. The three-stage tubular reactor is used for the polymerization of lactide. Tubular reactors of different diameters are designed according to different outputs. In principle, the smaller the diameter, the more conducive to the improvement of polymer quality. The diameter of the tubular reactor is flexibly adjusted according to production costs and basic product requirements during the production process. In this production process, preferably, the diameter of the tubular reactor is less than 800mm, the lactic acid polymer forms a plunger flow in the reactor without material dead zone, the polymer melt flows step by step in the reactor depending on the outlet pressure of the feed pump, and the melt flow rate is uniform. The tubular reactor has the characteristics of small diameter, uniform melt flow rate, and small radial temperature difference, which is conducive to obtaining high molecular weight polymers with narrow molecular weight distribution.

[0058] The polylactic acid melt from the lactide polymerization system first enters the falling film evaporation heater at the top of the devolatilization reactor. The falling film evaporator is equipped with a heating system, which includes a heat medium circulation pump, a heat medium cooler, and a heat medium heating supplementary regulating valve group. The heat medium heating temperature is controlled at 185-200°C. After the polylactic acid melt is heated by the high-temperature heat transfer oil when entering the devolatilization reactor, the falling film evaporator at the feed position can better remove the unreacted lactide and other low molecular weight components, while increasing the molecular weight of the polymer. The devolatilization reactor is equipped with a vacuum system. The low-boiling substances and lactide gas from the devolatilization reactor are condensed by the primary condenser and the secondary condenser to become lactide liquid and reflux to the lactide buffer tank, that is, return to step 1. The non-condensable gas and low-boiling substances are sucked into the vacuum buffer tank for further condensation and crystallization. The low-boiling substances crystallized on the buffer tank are washed with process spray water and discharged to the sewage system. The non-condensable gas is pumped away by the vacuum pump, and the pressure of the devolatilization reactor is controlled at 0.8KPa.

[0059] The polymer melt after devolatilization is extruded from the devolatilizer screw to the inlet of the devolatilization discharge pump, mixed with the modified additive from the additive feeding screw, and then transported to the granulation system by the devolatilization discharge pump. The lower cavity jacket of the devolatilizer reactor and the devolatilizer screw jacket share a heat conduction heating system, which includes a heat medium circulation pump, a heat medium cooler and a heat medium supplementary heating valve group, and the heat medium temperature is controlled at 200℃-220℃. The polylactic acid melt in the devolatilization reactor has a high molecular weight, high viscosity, and poor melt fluidity. A discharge screw is set at the bottom discharge port of the devolatilization reactor to ensure that the polylactic acid melt can be discharged smoothly from the bottom of the reactor. At the same time, it can improve the fluidity of the polylactic acid melt inside the devolatilization reactor and reduce the possibility of material stagnation inside the reactor. The pipelines of the devolatilization reactor discharge screw outlet and the devolatilization discharge pump inlet are equipped with additive feeding screws, where various functional additives can be added to meet the performance requirements of different product applications.

[0060] The lactide buffer tank, prepolymerization reactor, tubular reactor, and polymerization devolatilization reactor used are all closed systems and protected by high-purity nitrogen. The entire production process will not come into contact with the air, and polylactic acid will not be oxidized or degraded. The lactide prepolymerization reactor is operated at high temperature and normal pressure, and the water produced by the condensation reaction can be evaporated and removed as soon as possible, which is conducive to the production of lactide prepolymers with higher molecular weight. The normal pressure operation of the lactide prepolymerization reactor avoids material oxidation caused by air leakage during negative pressure operation. The air tightness requirements of the system are reduced, and the requirements for operation and control are reduced, which is conducive to process optimization.

[0061] The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment, it is not used to limit the present invention. Any technician familiar with this patent can make some changes or modify the technical contents suggested above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the solution of the present invention.

Claims

1. A continuous polymerization process for producing polylactic acid chips from lactide, characterized in that: The steps include: Step 1, unpolymerized lactide in the lactide workshop and the devolatilization reactor enters the lactide buffer tank through the lactide pipeline; Step 2, under the action of the discharge pump, a part of the lactide is circulated to the lactide buffer tank, and the other part of the lactide enters the lactide preheater through the lactide filter; Step 3, the preheated lactide and catalyst A, catalyst B, and catalyst C from three different catalyst metering pumps enter a static mixer for thorough mixing; Step 4, after sufficient mixing, the mixture enters a lactide prepolymerization reactor to undergo ring-opening polymerization and stay there for a period of time; Step 5, under the action of the prepolymerization discharge pump, the mixture is transported to a lactide polymerization system to perform a polymerization reaction to generate a melt, wherein the lactide polymerization system is a tubular reactor, wherein the lactide polymerization system comprises a 1# tubular reactor, a 2# tubular reactor, and a 3# tubular reactor connected in series in sequence; Step 6, a portion of the melt output from the lactide polymerization system is circulated to the inlet of the 3# tubular reactor through the outlet of the 3# tubular reactor to be mixed with the melt from the 2# tubular reactor; Another part of the melt output from the lactide polymerization system is conveyed to a devolatilization reactor for devolatilization; Step 7, the low-boiling substances and lactide gas from the devolatilization reactor are condensed and refluxed to the lactide buffer tank, and the non-condensable gas and low-boiling substances are sucked into the vacuum buffer tank for condensation and crystallization and then discharged to the sewage system; Step 8, after the devolatilization, the melt is mixed with the modified additive and transported to the granulation system by the devolatilization discharge pump, and the polymerization process is completed, wherein the lactide buffer tank, the lactide prepolymerization reactor, the tubular reactor, the lactide polymerization system and the devolatilization reactor are all closed systems and protected by high-purity nitrogen, and the lactide pipeline and the lactide buffer tank are both provided with a warm water insulation jacket, and the temperature of the warm water insulation jacket is set to 90-95°C.

2. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The heat medium temperatures of the 1# tubular reactor, the 2# tubular reactor and the 3# tubular reactor are all controlled to be 200-220°C, and the diameters of the tubular reactors are all less than 800mm.

3. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 2, characterized in that: The 1# tubular reactor comprises two cavities connected in series, both of which are provided with a thermal oil heating system. The 2# tubular reactor is provided with a thermal oil heating system. The 3# tubular reactor comprises two cavities connected in series, and both share a thermal oil heating system.

4. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The lower cavity jacket of the devolatilizer reactor and the devolatilizer screw jacket share a heat conduction heating system, which includes a heat medium circulation pump, a heat medium cooler, and a heat medium supplementary heating valve. The heat medium temperature is controlled at 200°C.

5. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The temperature of the circulating heating heat medium of the lactide preheater is 175°C to 200°C, and the lactide is heated to 150-160°C in the lactide preheater.

6. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The catalyst in step 3 is a low-toxic and high-efficiency catalyst based on calcium, magnesium, iron, and zinc metal compounds.

7. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The devolatilization reactor is provided with a vacuum system and a falling film evaporation heater, and the heating temperature is 185°C-200°C.

8. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The polymerization catalyst A, polymerization catalyst B and polymerization catalyst C are stored in storage tanks of three catalyst metering pumps respectively, and the catalyst storage tanks are provided with nitrogen protection and exhaust water seal tanks.

9. The continuous polymerization process for producing polylactic acid chips from lactide according to claim 1, characterized in that: The low-boiling substances crystallized in the buffer tank in step 7 are washed with process spray water and discharged to the sewage system, the non-condensable gases are pumped away by a vacuum pump, and the pressure of the devolatilization reactor is controlled at 0.8 KPa.

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