Process system for synthesizing polylactic acid through direct polycondensation by taking special corn as raw material

Abstract

The invention relates to a process system for synthesizing polylactic acid through direct polycondensation by taking special corn as a raw material, and belongs to the field of preparation of biodegradable materials. According to the method, firstly, the problem of impurity residues in a polylactic acid synthesis system is solved from a corn raw material, that is, special corn in which the binding force of starch and binding protein is small and impurities such as the starch and the binding protein are easy to separate is selected as a raw material to prepare high-purity starch and glucose, and then a biological fermentation method is used for preparing a lactic acid monomer with extremely high purity; secondly, the lactic acid monomer with extremely high purity is directly subjected to polycondensation to synthesize polylactic acid under the synergistic effect of microwaves, ultrasonic waves, an energy storage catalytic material and mechanical stirring, and the polycondensation reaction is promoted to be carried out towards the positive direction of polymerization through the methods of gradual polycondensation, flash evaporation drainage, pipeline chain extension prepolymerization, screw extrusion polymerization, negative pressure exhaust, enhanced screw extrusion polymerization and the like; the purpose of preparing high-purity and large-molecular-weight polylactic acid at low cost is achieved, and meanwhile the comprehensive utilization value of corn is increased.

Description

Technical Field

[0002] The invention belongs to the field of bio-based degradable materials, and specifically relates to a process system for directly polycondensing polylactic acid using special corn as a raw material. Technical Background

[0004] Polylactic acid has excellent biodegradability, compatibility and absorbability. It is a non-toxic and non-irritating synthetic polymer material that can be used as packaging materials, fibers and non-woven fabrics, etc. It can be used in the fields of extrusion, injection molding, film drawing, spinning, etc. It can also be used in medical fields such as disposable infusion tools, non-disassembly surgical sutures, drug sustained-release packaging, artificial fracture internal fixation materials, and tissue repair materials. It is gradually developing into a basic bulk raw material necessary for national economic and social development.

[0005] There are generally two ways to synthesize polylactic acid. One is the polylactic acid ring-opening polymerization method (also known as the two-step method), and the other is the direct polycondensation of lactic acid into polylactic acid (also known as the one-step method). The polylactic acid ring-opening polymerization method is mainly divided into two parts. First, the lactic acid molecules are dehydrated to generate low molecular weight lactic acid oligomers, and then, they are depolymerized at 180-260°C to generate crude lactide. Crude lactide contains impurities such as water, lactic acid, and lactic acid oligomers. The purity and yield of lactide will directly affect the quality and production cost of polylactic acid. After the crude lactide is purified, purified lactide is obtained. Finally, the purified lactide is ring-opened to obtain high molecular weight polylactic acid. The prepared polylactic acid has a high molecular weight and good mechanical strength, but the polylactic acid ring-opening polymerization process is complicated and the process is lengthy, so the cost is high and the production efficiency is relatively low, which limits its application development. The production process of synthesizing polylactic acid by direct polycondensation is relatively simple, and it can reduce production costs, and it is easy to achieve large-scale industrial production, which has great potential. However, in the later stage of the polycondensation reaction, as the viscosity of the reaction system continues to increase, it becomes more difficult to remove the water, small molecular by-products and some impurities produced by the polycondensation reaction, which hinders the polycondensation reaction from proceeding in an orderly manner in the positive direction. The molecular weight of the prepared polylactic acid is generally low, and it is not easy to meet the relative molecular weight and purity requirements of polylactic acid. At this time, the molecular weight of the polycondensation product can be increased by increasing the temperature, high vacuum and other conditions, but high-temperature coking reactions will occur to produce coking by-products, resulting in an increase in the degree of hybridization of the polycondensation product, and the polymerization product needs to be further purified, which will greatly increase the cost. Therefore, the most important thing in the process of direct polycondensation of lactic acid into polylactic acid is to reduce the content of impurities and the generation of by-products in the reaction system, effectively remove the water produced by the polycondensation reaction, and promote the reaction to proceed in the polymerization direction to increase the molecular weight and purity of polylactic acid.

[0006] The core problem of reducing the content of impurities and the generation of by-products in the reaction system of direct polycondensation of lactic acid into polylactic acid is that the impurities remaining in the lactic acid monomer cannot be brought into the polycondensation reaction system. The type and content of impurities in the lactic acid monomer directly affect the process route and equipment system for the preparation of polylactic acid, and further affect the production cost of polylactic acid. The selection of lactic acid monomers with extremely high optical purity (≥99.5%) and extremely high chemical purity (≥98.0%) is an effective measure to fundamentally solve the content of impurities and the generation of by-products in the reaction system of direct polycondensation of lactic acid into polylactic acid.

[0007] The residual impurities in the lactic acid prepared by the biological fermentation method mainly come from glucose, and the residual impurities in glucose depend on the type and content of impurities in corn starch. The content of impurities in corn starch depends on the separation effect of corn binding protein and starch. Corn starch granule binding protein is a type of protein that is naturally located on the surface of starch granules or (and) tightly bound inside, which is obviously different from the alcohol-soluble protein and storage protein inside the plant. It is difficult to effectively separate the binding protein and starch with general protein solvents and separation methods. Due to the diverse types of corn used to produce starch, uneven textures, and inconsistent quality standards, it is extremely difficult to control starch production, making it difficult to control the content of protein and other impurities in starch, and incompletely separating starch from protein and other impurities. Although this starch containing residual binding protein and other organic matter, metal ions and other impurities can produce lactic acid that meets food standards, to prepare high-purity polylactic acid, it is necessary to solve the problems of purification of lactic acid and the output of some by-products in the polylactic acid synthesis process, which will lead to process difficulties and high production costs in the subsequent production of polylactic acid.

[0008] Since the high-quality L-lactic acid production technology is blocked by foreign countries, the domestic independent innovation capability of key technologies for the industrialization of L-lactic acid is insufficient. In order to obtain high-purity lactic acid monomers, a lengthy purification process needs to be added, which greatly increases the production cost. Li Rongjie and others have developed a high-purity L-lactic acid industrial preparation technology of "strain cultivation, efficient fermentation and membrane separation + continuous ion exchange + molecular distillation". Among them, the "membrane separation + continuous ion exchange + molecular distillation" purification process passes the solution containing lactic acid through a membrane to remove organic impurities such as large molecular proteins and sugars, and then removes inorganic salts through continuous ion exchange, and then through concentration and molecular distillation, the separation of lactic acid and other trace impurities is achieved, thereby obtaining a relatively pure lactic acid solution.

[0009] In summary, it can be seen that in recent years, although my country's polylactic acid industry has achieved great results in technology, equipment and other aspects, and has formed a complete industrialization technology route, the polylactic acid biodegradable material products that are recognized as alternatives to petrochemical plastics have not been favored by the market, and plastic pollution has not been effectively curbed. The fundamental reason is that the price of polylactic acid biodegradable materials is high and the market is difficult to accept. Under the condition that the production technology of polylactic acid tends to be perfect and mature, there are two reasons for the high price of polylactic acid biodegradable materials. One is the organic matter (macromolecule protein, sugar, etc.), inorganic salts, other polar small molecule impurities and catalyst residues in the lactic acid solution, which are the main factors causing the difficulty of polylactic acid synthesis and high production costs; the other is that with the continuous increase of viscosity in the reaction system, it is more and more difficult to discharge the water produced by the polycondensation reaction, which hinders the polycondensation reaction from proceeding in an orderly manner in the positive direction, and brings great difficulties and high production costs to the preparation of high-purity, high-molecular-weight polylactic acid. Summary of the invention

[0012] In view of the above problems, the present invention provides a process system for directly condensing and synthesizing polylactic acid using special corn as raw material. The first purpose is to solve the problem of impurity residues in the polylactic acid synthesis system starting from corn raw materials, that is, to select special corn with low starch binding protein content, low binding force, and easy separation of impurities such as starch and binding protein as raw materials to prepare high-purity starch and glucose, and then use biological fermentation to prepare extremely high-purity lactic acid monomers. The extremely high-purity lactic acid monomers are directly condensed and synthesized into polylactic acid under the synergistic effect of microwaves, ultrasound, energy storage catalytic materials and mechanical stirring. Through methods such as gradual condensation, flash distillation drainage, pipeline chain extension prepolymerization, spiral extrusion polymerization, negative pressure exhaust, and enhanced spiral extrusion polymerization, the condensation reaction is promoted to proceed in the positive direction of polymerization, thereby achieving the purpose of preparing high-purity, high-molecular-weight polylactic acid at low cost.

[0013]

[012] The present invention provides a process system for directly polycondensing polylactic acid using special corn as raw material, which comprises at least the following steps:

[0014] Step (1) is to prepare ultra-high purity lactic acid by using selected special corn as raw material and adopting biological fermentation method to prepare high purity lactic acid monomer.

[0015] Step (2), polycondensation, the lactic acid polycondensation system is decomposed into a number of reaction units at different temperatures under the synergistic effect of microwaves, ultrasound, heating oil, mechanical stirring and energy storage catalytic pellets, and the water generated by the polycondensation reaction of each polycondensation unit is flash evaporated and separated from the reaction system to obtain lactic acid oligomers.

[0016] Step (3), prepolymerization, the lactic acid oligomer obtained in step (2) is transported to a tubular prepolymerization reactor by a melt pump. A chain extender is added in proportion according to the flow rate of the melt pump, and prepolymerization is carried out under stirring, solid energy storage catalyst material, and microwave action to obtain a prepolymer melt.

[0017] Step (4), polymerization, introducing the prepolymer melt obtained in step (3) into a polymerization reactor, and performing screw extrusion polymerization under the synergistic effect of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials to obtain a polymer melt.

[0018] Step (5), exhausting, the polymer melt obtained in step (4) is subjected to negative pressure exhaust under the coordination of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials, so that the molecular structure of the polymer melt obtained in step (4) is tighter, and a high-density polymer melt is obtained.

[0019] Step (6), enhanced polymerization, the high-density polymer melt obtained in step (5) is subjected to enhanced extrusion polymerization and extrusion pelletization under the synergistic action of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials to obtain polylactic acid particles with high optical purity and high molecular weight.

[0020]

[014] The special corn is a special corn variety bred by conventional breeding methods, and has the following specific characteristics: low content of starch-bound protein in the grains, low degree of protein polymerization, easy release of starch granules, easy separation of bound protein and starch, high purity of the prepared starch, low protein residue and impurities, and high-purity lactic acid monomer can be produced without lengthy separation process. The high-purity lactic acid monomer can be used as a raw material to synthesize high-purity, high-molecular-weight polylactic acid by direct polycondensation, laying the foundation for low-cost production of polylactic acid biodegradable material products.

[0021] Furthermore, the special corn described in

[014] is characterized in that the yield per mu is greater than 600 kg, the crude starch content of the corn kernels is about 80%, the amylopectin content in the crude starch is greater than 95%, and the average starch output rate is greater than 75%.

[0022] Furthermore, the corn starch prepared using the special corn described in

[014] contains less than 0.05% protein and less than 0.5% of the total content of other impurities.

[0023]

[015] The microwaves described generally refer to electromagnetic waves with a wavelength between 1 mm and 1 m (frequency 300-300000 MHz), and the most commonly used frequencies are 915 and 2450 MHz. The basic properties of microwaves are generally penetrability, selectivity and reflectivity, and they have a special promoting effect in many chemical reactions. Compared with conventional heating chemical reactions, the chemical reaction speed under microwave radiation is fast, which can be increased to 10-10000 times of the conventional method, and stirring at the molecular level can be achieved, with high reaction yield and few side reactions. Moreover, since different substances respond differently after microwave irradiation, microwaves can make the reaction highly selective; the role played by microwaves is not only a unique intramolecular heating method, but also a non-thermal effect in the reaction. Microwaves can act directly on substances, and in many organic reactions, solvents as heat transfer media do not need to be used, which can reduce and eliminate the use of harmful solvents; microwave heating can be precisely controlled and used, which is conducive to realizing automated operation. Microwave heating is very safe under certain technical specifications and standards. Furthermore, microwave synthesis technology has the characteristics of high heat, low energy consumption, environmental protection and easy operation, and is currently widely used in the field of organic chemistry. Microwave synthesis also has an important manifestation in esterification reactions. It can replace catalysts and accelerate the reaction rate through microwave heating. Furthermore, microwaves can greatly accelerate the dehydration of lactic acid and polycondensation reactions and the separation rate of polar small molecule impurities. Due to the characteristics of selective heating of microwaves, different substances produce different thermal effects. Although water molecules, polar small molecule impurities and lactic acid are all polar molecules, their dielectric loss factors are very large and they have strong absorption capacity for microwaves. The dielectric constant of water is 78.3 and the dielectric constant of lactic acid is 22.0, but their ability to absorb microwaves is different. Therefore, according to the difference in microwave absorption capacity of water, other polar small molecule impurities and lactic acid, the power density of microwaves can be controlled, and water and polar small molecule impurities can be evaporated and separated from the lactic acid polycondensation system at low temperature, reducing the probability of side reactions caused by the presence of water and polar small molecule impurities.

[0024]

[015] The ultrasonic wave is a sound wave with a frequency higher than 20000Hz. When the ultrasonic wave propagates in the medium, the interaction between the ultrasonic wave and the medium causes the medium to undergo physical and chemical changes, thereby generating a series of mechanical, thermal, electromagnetic and chemical ultrasonic effects. For example, mechanical effect: the mechanical action of ultrasonic wave can promote the emulsification of liquid, liquefaction of gel and dispersion of solid; cavitation effect: when ultrasonic wave acts on liquid, a large number of small bubbles can be generated. The small bubbles formed by cavitation will continuously flip, grow or suddenly burst with the vibration of the surrounding medium. When bursting, the surrounding liquid suddenly rushes into the bubbles to generate high temperature and high pressure, and at the same time generates shock waves; thermal effect: due to the high frequency and high energy of ultrasonic wave, it can produce significant thermal effect when absorbed by the medium; chemical effect: the action of ultrasonic wave can promote or accelerate certain chemical reactions. Ultrasonic wave can transfer mass and mix more effectively, so that the catalyst is better dispersed in the reactants. Cavitation can cause monomers to split or generate reaction intermediates, which is easier to promote ring-opening reactions, with fast reaction rates, high relative molecular mass and concentrated distribution.

[0025] Furthermore, the energy storage catalytic pellets described in

[015] are both microwave absorbing energy storage materials and solid catalysts, and are made by sintering materials such as silicon carbide, four-needle zinc oxide whiskers, nano-SnO2, nano-monazite, tourmaline, etc., and the pellet diameter is 3 to 5 mm.

[0026]

[025] The silicon carbide described is a high-quality microwave absorbing heat generation energy storage material.

[0027]

[025] The nano-tin dioxide (SnO2) is an important intrinsic semiconductor oxide with excellent optical properties, electrical properties and chemical stability. It is a widely used energy storage and catalytic material and is also the preferred catalyst for the preparation process of polylactic acid.

[0028]

[025] The four-needle zinc oxide whisker has special tip nanoactivity and catalytic effect and strong wave absorption and heating effect; zinc oxide is also an important catalyst for the preparation of polylactic acid.

[0029]

[025] The monazite, whose Chinese name is "cerium phosphate lanthanum ore" (Ce, La, Nd, Th) PO4, contains about 65% rare earth oxides. It is a kind of rare earth mineral with unique catalytic superiority.

[0030]

[025] The tourmaline has the characteristics of permanent spontaneous electrode, piezoelectricity and pyroelectricity, can generate infrared rays and microcurrent, can electrolyze water and make water molecules smaller, which is conducive to accelerating the extraction of water molecules from the reaction system; tourmaline has a good adsorption capacity for heavy metal ions, can adsorb the residual metal ions in the polylactic acid preparation system, and reduce the residual metal ion content in the reaction system.

[0031]

[016] The chain extender is an epoxy polymerization type chain extender PZYC-06, and the added amount is 0.1 to 0.6% of the weight of the lactic acid oligomer.

[0032]

[012] The present invention provides a process system for directly condensing and synthesizing polylactic acid using special corn as raw material, which also includes: a horizontal lactic acid polycondensation kettle, a tubular prepolymerization reactor, and a polymerization reactor.

[0033] The horizontal lactic acid polycondensation kettle described in

[032] is a device for specifically implementing step (2) of the process system for directly polycondensing polylactic acid using special corn as raw material described in

[012] , comprising a microwave resonance cavity, a polycondensation reaction cavity, a mechanical stirring system, an ultrasonic device, a heating oil system, a moisture extraction system, etc.

[0034]

[033] The polycondensation reaction chamber is composed of a plurality of polycondensation reaction units, each of which is sealed and buckled together by two upper and lower U-shaped grooves. The lower U-shaped groove is made of 316L or 310S stainless steel and coated with a wear-resistant nano-coating. The upper U-shaped groove is made of quartz ceramic.

[0035]

[034] The plurality of polycondensation reaction units are connected to each other, and two adjacent polycondensation reaction units are separated by a partition, and a plurality of circular holes are opened in the middle and lower part of the partition, and a mechanical stirring device connects the polycondensation reaction units in series.

[0036]

[034] Each of the polycondensation reaction units has stirring blades and energy storage catalytic pellets, the bottom of each polycondensation reaction unit is connected to the corresponding heating oil jacket, each polycondensation reaction unit is also equipped with an ultrasonic device at the bottom and lower side, the middle and upper part of each polycondensation reaction unit is a microwave resonance cavity, microwave magnetrons are installed on both sides of the microwave resonance cavity, the upper U-shaped groove of each polycondensation unit made of quartz ceramic is located at the bottom center of the microwave resonance cavity, a water vapor outlet is installed at the top of each polycondensation reaction unit, the water vapor outlet passes through the microwave resonance cavity and is connected to the negative pressure extraction system, and each polycondensation reaction unit is equipped with an infrared temperature measuring device.

[0037] The aluminum diamond infrared temperature measuring and heat-conducting elements on both sides of the upper part of the lower U-shaped groove of the polycondensation unit described in

[034] correspond to the far-infrared temperature measuring device. The materials of the previous polycondensation reaction unit can be sequentially introduced into the next polycondensation reaction unit. The mechanical stirring system adopts electromagnetic clutch transmission. The lactic acid inlet of the polycondensation reaction chamber is arranged at the front end of the synthesis chamber and is connected to the lactic acid supply pipeline; the outlet of the polycondensate generated by the polycondensation reaction is arranged at the tail of the polycondensation synthesis chamber and is connected to the melt pump.

[0038]

[032] The tubular prepolymerization reactor is a device for specifically implementing step (3) of the process system for directly polycondensing polylactic acid using special corn as raw material as described in

[012] , comprising a prepolymerization pipeline, a mechanical stirring system, energy storage catalytic pellets, a heating oil system, a chain extender filling system, etc.

[0039]

[038] The prepolymerization pipeline is divided into a plurality of prepolymerization units, and the plurality of prepolymerization units are connected to each other. Two adjacent prepolymerization units are separated by a partition, and a plurality of circular holes are opened in the middle and lower part of the partition. A mechanical stirring device connects the prepolymerization units in series.

[0040]

[039] Each of the prepolymerization units is provided with energy storage catalytic pellets. The material inlet of the prepolymerization pipeline is arranged at the front end of the prepolymerization pipeline and is connected to the polycondensate supply melt pump. The outlet of the prepolymerization pipeline is arranged at the rear end of the prepolymerization pipeline and is connected to the melt pump.

[0041] The polymerization reactor described in

[032] is a device for specifically implementing steps (4) to (6) of the process system described in

[012] for directly polycondensing polylactic acid using special corn as raw material.

[0042]

[032] The polymerization reactor comprises: a cylinder body, a stirring extrusion assembly, a vacuum receiving cavity, a microwave resonance cavity, a vacuum exhaust port, a heating oil jacket and an ultrasonic vibrator, etc.

[0043] The cylinder body of the polymerization reactor described in

[042] is composed of two upper and lower semicircular grooves sealed together, and the cylinder body located in the microwave resonance cavity described in

[042] is made of quartz ceramics.

[0044] The stirring extrusion assembly described in

[042] is located in the cylinder of the polymerization reactor described in

[043] , and the feed chamber is located at the front end of the polymerization reactor described in

[032] .

[0045]

[042] The stirring and extruding assembly includes: a stirring motor, a stirring and extruding shaft, a stirring blade sleeve, a conveying spiral sleeve, an extrusion spiral sleeve, a conveying spiral sleeve, an extrusion spiral sleeve assembly and an extrusion grate plate.

[0046] The stirring blade sleeve, conveying spiral sleeve, extrusion spiral sleeve, conveying spiral sleeve, and extrusion spiral sleeve assembly described in

[045] are sequentially mounted on the stirring shaft, and the manufacturing material thereof is the same as the material of the energy storage catalytic pellets described in

[025] .

[0047]

[032] The polymerization reactor is divided into a polymerization zone, a degassing zone and an enhanced polymerization zone. The polymerization zone is located at the front end of the polymerization reactor, the deep polymerization zone is located at the rear end of the polymerization reactor, and the degassing zone is located in the middle of the polymerization reactor.

[0048]

[046] The polymerization zone of the polymerization reactor includes: a stirring section, a feeding section, an extrusion polymerization section, and a feeding section.

[0049] The exhaust zone of the polymerization reactor described in

[046] is located after the feeding section described in

[048] , and includes a stirring section and an exhaust port.

[0050]

[046] The enhanced polymerization zone of the polymerization reactor is located behind the exhaust zone, and includes: a feed section, an extended enhanced extrusion polymerization section, a feeding section and a discharge granulation section.

[0051] The microwave resonance cavity

[042] is located above the feed section of the polymerization zone to the end of the enhanced polymerization zone of the polymerization reactor, and the bottom edge is located at the center line of the cylinder of the polymerization reactor

[043] , and is connected to the cylinder of the polymerization reactor

[043] .

[0052] The vacuum exhaust port described in

[042] is located in the exhaust zone of the polymerization reactor described in

[047] , and the material of the horn-shaped component located in the microwave resonance cavity is quartz ceramic.

[0053] The heating oil jacket described in

[042] is located below the polymerization reactor described in

[032] , and its upper side is located at the center line of the cylinder body of the polymerization reactor described in

[042] , and is connected to the cylinder body of the polymerization reactor described in

[042] .

[0054] The ultrasonic vibrator described in

[042] is located below the polymerization reactor described in

[042] , and passes through the heating oil jacket described in

[042] to be connected to the cylinder body described in

[042] .

[0055] Specifically, the special corn described in

[042] is used as a raw material to prepare high-purity starch with a protein content of less than 0.05% and a total content of other impurities of less than 0.5%. The high-purity starch is saccharified and then used to prepare lactic acid using a biological fermentation method to obtain a high-purity lactic acid monomer with an optical purity of ≥99.5% and a chemical purity of ≥98.0%.

[0056] The lactic acid monomer described in

[055] is continuously pumped into the horizontal lactic acid polycondensation reactor described in

[032] , and the polycondensation reaction is gradually carried out in the polycondensation reaction units at different temperatures under the synergistic effect of microwaves, ultrasonic waves, heating oil, mechanical stirring and energy storage catalytic pellets. The water generated by the polycondensation reaction in each polycondensation unit is flash-evaporated and separated from the reaction system to obtain lactic acid oligomers. The polycondensation reaction temperature is 130°C to 160°C, the pressure is 15 to 20KPa, and the polycondensation time is 4.5 hours.

[0057] The lactic acid oligomer prepared in

[056] is continuously transported to the tubular prepolymerization reactor described in

[032] by a melt pump. A chain extender is added in proportion according to the flow rate of the melt pump, and prepolymerization is carried out under stirring, solid energy storage catalytic material, and microwave action to obtain a prepolymerized melt. The prepolymerization temperature is 120-150°C, the pressure is 10-15KPa, and the prepolymerization reaction time is 2-4 hours.

[0058] The prepolymer melt obtained in

[057] is pumped into the polymerization zone of the polymerization reactor by a melt pump, and polymerized under the combined action of stirring, extrusion, microwave, ultrasonic wave and solid energy storage catalyst material to obtain a polymer melt. The polymerization temperature is 120-150°C, the pressure is 10-15KPa, and the time is 1-3 hours.

[0059] The polymer melt obtained in step

[058] is sent to the exhaust zone described in step

[046] by the conveying screw of the polymerization reactor, and is exhausted under negative pressure under the synergistic effect of ultrasound and microwaves, so that the molecular structure of the polymer melt obtained in step (4) is more compact, and a high-density polymer melt is obtained. The exhaust zone temperature is 130-160° C. and the pressure is 5-10 KPa.

[0060] The high-density polymer melt obtained by

[059] is sent from the feed screw of the enhanced polymerization zone to the enhanced extrusion polymerization section, the feed section and the discharge granulation section for deep polymerization and granulation to obtain polylactic acid particles with high optical purity and high molecular weight. The enhanced polymerization reaction time is 3 to 5 hours.

[0061] Beneficial Effects

[0062] The present invention provides a process system for directly polycondensing polylactic acid using special corn as raw material, which has the following beneficial effects:

[0062] Since the protein polymerization degree of the special corn selected is low and the binding force between the protein and starch is small, high-purity starch with less protein residue and impurities can be separated without lengthy separation and purification processes, reducing the residual impurities such as large molecular binding proteins, organic matter, metal ions, etc. in the preparation process of lactic acid monomers, reducing the hybridization rate of crude lactic acid monomers, and alleviating the pressure of subsequent purification and refining of crude lactic acid. Lactic acid monomers with an optical purity of ≥99.5% and a chemical purity of ≥98.0% can be prepared without lengthy purification and refining processes, laying a foundation for optimizing the process flow of producing high-purity polylactic acid and reducing production costs. The high-purity lactic acid monomer prepared by using the high-purity starch can be directly polycondensed to synthesize polylactic acid under the synergistic effect of microwave, ultrasound, energy storage catalytic material and mechanical stirring. The process for preparing polylactic acid is simple, the production process is continuously controllable, the mass transfer and heat transfer are uniform, the vacuum degree is low, no powder catalyst is used, no catalyst residue is left, and the moisture and other polar by-products generated in the polycondensation process are completely discharged; the prepared polylactic acid has high comprehensive yield, few impurities, high purity and high molecular weight, which can not only greatly reduce the production cost of polylactic acid, but also increase the comprehensive utilization value of corn. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 A process flow chart of a process system for directly polycondensing polylactic acid using special corn as raw material

[0065] Figure 2 Schematic diagram of the partial cross-section structure of the horizontal lactic acid polycondensation reactor

[0066] Figure 3 Schematic diagram of the partial cross-section structure of a tubular prepolymerization reactor

[0067] Figure 4 Schematic diagram of partial cross-section structure of polymerization reactor

[0068] Reference numerals list

[0069] 100 Special corn with easy separation of impurities such as starch and binding protein

[0070] 200 High-purity lactic acid made from special corn

[0071] 300 horizontal lactic acid polycondensation kettle

[0072] 301 Horizontal lactic acid polycondensation kettle polycondensation unit microwave resonant cavity

[0073] 302 horizontal lactic acid polycondensation reactor polycondensation reaction chamber

[0074] 3021 horizontal lactic acid polycondensation kettle first polycondensation reaction unit

[0075] 3022 horizontal lactic acid polycondensation kettle second polycondensation reaction unit

[0076] 3023 horizontal lactic acid polycondensation kettle third polycondensation reaction unit

[0077] 3024 horizontal lactic acid polycondensation kettle fourth polycondensation reaction unit

[0078] 3025 horizontal lactic acid polycondensation kettle fifth polycondensation reaction unit

[0079] 3026 horizontal lactic acid polycondensation kettle sixth polycondensation reaction unit

[0080] 3027 horizontal lactic acid polycondensation kettle seventh polycondensation reaction unit

[0081] 3028 horizontal lactic acid polycondensation kettle eighth polycondensation reaction unit

[0082] 3029 horizontal lactic acid polycondensation kettle ninth polycondensation reaction unit

[0083] 303 Horizontal lactic acid polycondensation reactor polycondensation reaction chamber 302 water vapor outlet

[0084] 304 horizontal lactic acid polycondensation reactor partition between adjacent polycondensation units

[0085] 305 horizontal lactic acid polycondensation reactor polycondensation reaction lactic acid oligomer outlet

[0086] 306 Horizontal lactic acid polycondensation kettle polycondensation unit ultrasonic vibrator

[0087] 307 horizontal lactic acid polycondensation kettle with heating oil jacket

[0088] 3071 Horizontal lactic acid polycondensation reactor first polycondensation reaction unit 3021 heating oil jacket

[0089] 3072 Horizontal lactic acid polycondensation kettle second polycondensation reaction unit 3022 heating oil jacket

[0090] 3073 Horizontal lactic acid polycondensation kettle third polycondensation reaction unit 3023 heating oil jacket

[0091] 3074 Horizontal lactic acid polycondensation reactor fourth polycondensation reaction unit 3024 heating oil jacket

[0092] 3075 horizontal lactic acid polycondensation kettle fifth polycondensation reaction unit 3025 heating oil jacket

[0093] 3076 Horizontal lactic acid polycondensation kettle sixth polycondensation reaction unit 3026 heating oil jacket

[0094] 3077 Horizontal Lactic Acid Polycondensation Reactor 7th Polycondensation Reaction Unit 3027 Heating Oil Jacket

[0095] 3078 Horizontal lactic acid polycondensation kettle eighth polycondensation reaction unit 3028 heating oil jacket

[0096] 3079 horizontal lactic acid polycondensation kettle ninth polycondensation reaction unit 3029 heating oil jacket

[0097] 308 Horizontal Lactic Acid Polycondensation Reactor Mechanical Agitator Shaft

[0098] 309 horizontal lactic acid polycondensation kettle lactic acid import

[0099] 310 horizontal lactic acid polycondensation kettle mechanical stirring magnetic coupling

[0100] 311 horizontal lactic acid polycondensation reactor energy storage catalytic pellets

[0101] 312 Horizontal lactic acid polycondensation kettle mechanical stirring blade

[0102] 313 Horizontal lactic acid polycondensation kettle insulation material

[0103] 400 Tubular Prepolymerization Reactor

[0104] 401 Tubular Prepolymerization Reactor Lactic Acid Oligomer Import

[0105] 402 Tubular Prepolymerization Reactor Mechanical Stirring Shaft

[0106] 403 Tubular Prepolymerization Reactor Prepolymerization Pipeline

[0107] 404 Tubular Prepolymerization Reactor Sealed Shell

[0108] 405 Tubular Prepolymerization Reactor Heating Oil Jacket

[0109] 406 Tubular Prepolymerization Reactor Mechanical Stirring Blade

[0110] 407 Tubular prepolymerization reactor partition between adjacent prepolymerization units

[0111] 408 Tubular Prepolymerization Reactor Energy Storage Catalytic Pellets

[0112] 409 Tubular prepolymerization reactor steam outlet

[0113] 410 Tubular prepolymerization reactor prepolymerization reactant outlet

[0114] 500 polymerization reactor

[0115] 501 polymerization reactor polymerization zone

[0116] 50101 polymerization reactor polymerization zone prepolymerization reactant inlet

[0117] 50102 Polymerization reactor polymerization zone water vapor outlet

[0118] 50103 polymerization reactor polymerization zone prepolymerization reactant feed silo

[0119] 50105 polymerization reactor cylinder

[0120] 50104 polymerization reactor polymerization zone stirring shaft sleeve assembly

[0121] 50106 polymerization reactor polymerization zone feed screw sleeve assembly

[0122] 50107 polymerization reactor polymerization zone extrusion polymerization spiral sleeve assembly

[0123] 50108 Polymerization reactor polymerization zone feeding spiral sleeve assembly

[0124] 502 Polymerization reactor exhaust area

[0125] 50201 Exhaust port of exhaust zone of polymerization reactor

[0126] 50202 Polymerization reactor exhaust zone stirring sleeve assembly

[0127] 503 polymerization reactor enhanced polymerization zone

[0128] 50301 polymerization reactor enhanced polymerization zone feed screw sleeve assembly

[0129] 50302 polymerization reactor enhanced polymerization zone lengthened extrusion polymerization spiral sleeve assembly

[0130] 50303 polymerization reactor enhanced polymerization zone feeding spiral sleeve assembly

[0131] 50304 polymerization reactor intensified polymerization zone extrusion cutting granulation components

[0132] 504 Polymerization Reactor Metal Cylinder and Quartz Ceramic Cylinder Assembly Connecting Sleeve (1)

[0133] 505 Polymerization Reactor Microwave Resonant Cavity

[0134] 506 polymerization reactor cylinder quartz ceramic cylinder assembly

[0135] 507 Polymerization reactor metal cylinder and quartz ceramic cylinder assembly connection sleeve (2)

[136] 508 Polymerization reactor heating oil jacket

[0137] 509 Polymerization Reactor Ultrasonic Vibrator

[0138] 510 Polymerization Reactor Stirring Screw Shaft

[0139] 511 Polymerization Reactor Stirring Screw Shaft Motor

[0140] Specific implementation

[0141] The following is combined with Figure 1 , Attachment Figure 2 , Attachment Figure 3 and attached Figure 4 , describe the specific process and flow of the present invention in detail.

[0142] The process system for directly synthesizing polylactic acid by polycondensation using special corn as raw material described in the present invention comprises: a horizontal lactic acid polycondensation kettle 300, a tubular prepolymerization reactor 400 and a polymerization reactor 500.

[0143]

[142] The horizontal lactic acid polycondensation kettle 300 comprises: a microwave resonance cavity 301, a polycondensation cavity 302, a water vapor outlet 303 of the polycondensation cavity 302, a partition 304 between adjacent polycondensation units, a polycondensation reaction lactic acid oligomer outlet 305, a polycondensation unit ultrasonic vibrator 306, a polycondensation heating oil jacket 307, a mechanical stirring shaft 308, a lactic acid inlet 309, a mechanical stirring magnetic coupling 310, energy storage catalytic pellets 311, a mechanical stirring blade 312, and a thermal insulation material 313.

[0144]

[143] The condensation chamber 302 includes: a first condensation reaction unit 3021, a second condensation reaction unit 3022, a third condensation reaction unit 3023, a fourth condensation reaction unit 3024, a fifth condensation reaction unit 3025, a sixth condensation reaction unit 3026, a seventh condensation reaction unit 3027, an eighth condensation reaction unit 3028, and a ninth condensation reaction unit 3029.

[0145]

[143] The heating oil jacket polycondensation reaction unit 307 comprises: a heating oil jacket 3071 of the first polycondensation reaction unit 3021, a heating oil jacket 3072 of the second polycondensation reaction unit 3022, a heating oil jacket 3073 of the third polycondensation reaction unit 3023, a heating oil jacket 3074 of the fourth polycondensation reaction unit 3024, a heating oil jacket 3075 of the fifth polycondensation reaction unit 3025, a heating oil jacket 3076 of the sixth polycondensation reaction unit 3026, a heating oil jacket 3077 of the seventh polycondensation reaction unit 3027, a heating oil jacket 3078 of the eighth polycondensation reaction unit 3028, and a heating oil jacket 3079 of the ninth polycondensation reaction unit 3029.

[0146] The present invention sets the polycondensation chamber 302 of the horizontal lactic acid polycondensation kettle 300 described in

[143] to be, but not limited to, 9 polycondensation reaction units. Each polycondensation reaction unit is sealed and buckled together by two upper and lower U-shaped grooves. The lower U-shaped groove is made of 316L stainless steel and coated with a wear-resistant nano-coating. The upper U-shaped groove is made of quartz ceramic. The upper half of each polycondensation reaction unit is placed in the corresponding resonant cavity 301. The 9 polycondensation reaction units are connected to each other, and two adjacent polycondensation reaction units are separated by a partition 304. Several circular holes are opened in the middle and lower part of the partition. The mechanical stirring shaft 308 connects the polycondensation reaction units in series. Each polycondensation reaction unit has a stirring blade 312 and an energy storage catalytic spherical particle 311. The bottom of each polycondensation reaction unit is connected to the corresponding heating oil jacket. The bottom and lower side of each polycondensation reaction unit are also installed with an ultrasonic vibrator 306. Above each polycondensation reaction unit is a microwave resonant cavity 301, microwave magnetrons are installed on both sides of the microwave resonant cavity, a water vapor outlet 303 is installed on the top of each polycondensation reaction unit, the water vapor outlet is connected to the negative pressure extraction system, and each polycondensation reaction unit is installed with an infrared temperature measuring device. The aluminum diamond infrared temperature measuring heat conducting elements on both sides of the upper part of the lower U-shaped groove correspond to the far-infrared temperature measuring device. The materials of the previous polycondensation reaction unit can be introduced into the next polycondensation reaction unit in sequence. The mechanical stirring shaft 308 is driven by an electromagnetic clutch 310. The lactic acid inlet 309 of the polycondensation reaction unit is located at the front end of the polycondensation chamber 302 and is connected to the lactic acid supply pipeline; the outlet 305 of the polycondensate generated by the polycondensation reaction unit is located at the tail of the polycondensation synthesis chamber and is connected to the melt pump.

[0147]

[142] The tubular prepolymerization reactor 400 comprises: a tubular prepolymerization reactor lactic acid oligomer inlet 401, a tubular prepolymerization reactor mechanical stirring shaft 402, a tubular prepolymerization pipeline 403, a tubular prepolymerization reactor sealing shell 404, a tubular prepolymerization reactor heating oil jacket 405, a tubular prepolymerization reactor mechanical stirring blade 406, a partition 407 between adjacent prepolymerization units of the tubular prepolymerization reactor, a tubular prepolymerization reactor energy storage catalytic pellet 408, a tubular prepolymerization reactor water vapor exhaust port 409 and a tubular prepolymerization reactor prepolymerization reactant outlet 410.

[0148]

[142] The tubular prepolymerization reactor 400 is a tubular structure. The present invention sets the prepolymerization pipeline 403 of the tubular prepolymerization reactor to be, but not limited to, 11 adjacent prepolymerization units. The adjacent prepolymerization units are separated by a partition 407. The middle and lower parts of the partition 407 are provided with several circular hole channels. The mechanical stirring shaft 402 connects the prepolymerization units in series. Each prepolymerization unit has a mechanical stirring blade 406 and an energy storage catalytic pellet 408. The material inlet 401 of the prepolymerization pipeline 403 is arranged at the front end of the prepolymerization pipeline 403 and is connected to a melt pump for supplying polycondensate; the outlet 410 of the prepolymerization pipeline is arranged at the rear end of the prepolymerization pipeline and is connected to a melt pump. A heating oil jacket 405 is provided between the prepolymerization pipeline 403 and the sealed housing 404.

[0149]

[142] The polymerization reactor 500 comprises: a polymerization zone 501 of the polymerization reactor, an exhaust zone 502 of the polymerization reactor, an enhanced polymerization zone 503 of the polymerization reactor, a connecting sleeve (1) 504 for connecting the metal cylinder body of the polymerization reactor and the quartz ceramic cylinder body assembly, a microwave resonant cavity 505 of the polymerization reactor, a quartz ceramic cylinder body assembly of the polymerization reactor 506, a connecting sleeve (2) 507 for connecting the metal cylinder body of the polymerization reactor and the quartz ceramic cylinder body assembly, a heating oil jacket 508 of the polymerization reactor, an ultrasonic vibrator 509 of the polymerization reactor, a stirring screw shaft 510 of the polymerization reactor and a stirring screw shaft motor 511 of the polymerization reactor.

[0150]

[149] The polymerization zone 501 of the polymerization reactor comprises: a prepolymer reactant inlet 50101 of the polymerization reactor polymerization zone, a water vapor outlet 50102 of the polymerization reactor polymerization zone, a prepolymer reactant feed bin 50103 of the polymerization reactor polymerization zone, a polymerization reactor cylinder body 50105, a stirring shaft sleeve assembly 50104 of the polymerization reactor polymerization zone, a feeding spiral shaft sleeve assembly 50106 of the polymerization reactor polymerization zone, an extrusion polymerization spiral shaft sleeve assembly 50107 of the polymerization reactor polymerization zone, and a feeding spiral shaft sleeve assembly 50108 of the polymerization reactor polymerization zone.

[0151]

[149] The exhaust zone 502 of the polymerization reactor comprises: an exhaust port 50201 of the exhaust zone of the polymerization reactor and a stirring shaft sleeve assembly 50202 of the exhaust zone of the polymerization reactor.

[0152]

[149] The polymerization reactor enhanced polymerization zone 503 comprises: a polymerization reactor enhanced polymerization zone feeding spiral sleeve assembly 50301, a polymerization reactor enhanced polymerization zone extended extrusion polymerization spiral sleeve assembly 50302, a polymerization reactor enhanced polymerization zone feeding spiral sleeve assembly 50303, and a polymerization reactor enhanced polymerization zone extrusion cutting granulation assembly 50304.

[0153] The polymerization reactor 500 is divided into a polymerization zone 501, a degassing zone 502 and an enhanced polymerization zone 503. The polymerization zone 501 is the front part of the polymerization reactor 500, the deep polymerization zone 503 is the rear part of the polymerization reactor, and the degassing zone 502 is located in the middle part of the polymerization reactor.

[0154] The stirring screw shaft 510 passes through the polymerization zone 501, the exhaust zone 502 and the enhanced polymerization zone 503. In the polymerization zone 501, the stirring screw shaft 510 is equipped with a polymerization zone stirring shaft sleeve assembly 50104, a polymerization zone feeding screw shaft sleeve assembly 50106, a polymerization zone extrusion polymerization screw shaft sleeve assembly 50107 and a polymerization zone feeding screw shaft sleeve assembly 50108; in the exhaust zone 502, the stirring screw shaft 510 is equipped with a stirring shaft sleeve assembly 50202; in the enhanced polymerization zone 503, the stirring screw shaft 510 is equipped with an enhanced polymerization zone feeding screw shaft sleeve assembly 50301, an enhanced polymerization zone lengthened extrusion polymerization screw shaft sleeve assembly 50302, and an enhanced polymerization zone feeding screw shaft sleeve assembly 50303.

[0155] All sleeve components installed on the stirring screw shaft 510 described in

[169] are made of the same material as the energy storage catalytic pellets described in

[015] .

[0156] The upper part of the polymerization reactor cylinder 50105 located from the middle of the polymerization zone 501 to the tail of the enhanced polymerization zone 503 is a quartz ceramic cylinder assembly 506. The metal assembly of the polymerization reactor cylinder 50105 in the polymerization zone 501 and the quartz ceramic cylinder assembly 506 are connected by a metal cylinder assembly and quartz ceramic cylinder assembly connecting sleeve (1) 504. The metal assembly of the polymerization reactor cylinder 50105 in the polymerization zone 503 and the quartz ceramic cylinder assembly 506 are connected by a metal cylinder assembly and quartz ceramic cylinder assembly connecting sleeve (2) 507.

[0157] The polymerization zone prepolymer reactant feed bin 50103 is located at the front of the polymerization zone, the polymerization zone prepolymer reactant inlet 50101 is located at the side of the polymerization zone prepolymer reactant feed bin 50103, and the polymerization zone water vapor outlet 50102 is located at the top of the polymerization zone prepolymer reactant feed bin 50103.

[0158] The polymerization reactor microwave resonant cavity 505 is located in the upper middle part of the position of the extrusion polymerization spiral sleeve assembly 50107 in the polymerization zone 501 to the feeding spiral sleeve assembly 50303 in the enhanced polymerization zone 503, and its bottom edge is located at the midline position of the polymerization reactor cylinder body 50105 described in

[034] , and is connected to the metal part of the cylinder body 50105 of the polymerization reactor described in

[041] .

[0159] The exhaust port 50201 of the exhaust zone is located directly above the stirring shaft sleeve assembly 50202 of the exhaust zone 502. The bell-mouth assembly of the exhaust port 50201 of the exhaust zone in the microwave resonance cavity is made of quartz ceramic material, and the lower end is sealed and connected to the opening at the corresponding position of the polymerization reactor cylinder 50105.

[0160] The intensified polymerization zone extrusion cutting and granulation assembly 50304 is located at the tail end discharge port of the intensified polymerization zone 503.

[0161] The heating oil jacket 508 is located below the polymerization reactor cylinder 50105, and its upper end is located at the center line of the polymerization reactor cylinder 50105, and is connected to the metal part of the polymerization reactor cylinder 50105 and the bottom of the microwave resonance cavity

[041] .

[0162] The ultrasonic vibrator 509 is located below the cylinder 50105 system of the polymerization reactor and is connected to the cylinder through the heating oil jacket. Specific implementation plan

[0164] Specialty corn cultivation

[0165]

[021] The special corn is a special corn variety selected by conventional breeding methods and planted under the guidance of experts.

[0166] Preparation of high purity lactic acid monomer

[0167] The special corn described in

[165] is used as raw material to separate high-purity corn starch with a protein content of less than 0.05% and a total content of other impurities less than 0.5%. After saccharification, a biological fermentation method is used to prepare lactic acid monomers with an optical purity of ≥99.5% and a chemical purity of ≥98.0%.

[0168] Polycondensation

[0169] The lactic acid monomer with an optical purity of ≥99.5% and a chemical purity of ≥98.0% prepared by

[167] is pumped into the polycondensation chamber 302 through the lactic acid inlet 309 of the polycondensation reaction unit, and is introduced into each polycondensation unit in turn through the circular hole channel on the partition 304 through two adjacent polycondensation reaction units. Under the synergistic effect of the microwave energy of the microwave resonance cavity 301, the ultrasonic vibrator 306, the heating oil, the stirring blade 312 and the energy storage catalytic pellet 311, the polycondensation reaction is gradually carried out. The polycondensation reaction time is 4.5 hours, the pressure is 10-15KPa, and the temperature is 130℃-160℃. The reaction temperature of each polycondensation unit is provided by the corresponding heating oil jacket. Under normal circumstances, the temperature difference between the oil temperature of the heating oil jacket and the temperature in the reactor is more than 10℃. Therefore, the oil temperature of the heating oil jacket is actually controlled to be 5℃ higher than the polycondensation reaction temperature. The heat shortage of the corresponding polycondensation reaction unit is adjusted by the heat energy provided by the microwave source of the corresponding microwave resonator. The water generated by the polycondensation reaction of each polycondensation unit is separated from the reaction system by flash evaporation and discharged from the water vapor outlet 303 to obtain lactic acid oligomers. The pumping flow rate of lactic acid is adjusted so that no water is discharged from the eighth and ninth polycondensation units.

[0170] The polycondensation reaction conditions of the 9 polycondensation units described in

[146] are:

[0171]

[167] The lactic acid is introduced into each polycondensation reaction unit in turn and gradually undergoes polycondensation reaction. The energy storage catalytic pellets 311 in each polycondensation reaction unit are turned over in the reaction system with the stirring of the mechanical stirring blades 312. Lactic acid continuously forms a new reaction interface on the surface of the energy storage catalytic pellets 311. The rare earth, tin dioxide, zinc oxide and other catalysts in the energy storage catalytic pellets 311 are in uniform contact with the system and have a good catalytic effect. The lactic acid polycondensation reaction generates lactic acid low molecular polymers and water. The tourmaline in the energy storage catalytic pellets 311 electrolyzes the water and makes the water molecules small. The surface of the energy storage catalytic pellets 311 is covered with the feed liquid of the lactic acid polycondensation reaction system to form a liquid film. Under the action of mechanical stirring, the energy storage catalytic pellets 311 are thrown above the liquid surface of the reaction system. Under the appropriate microwave power radiation, the microwaves selectively heat the highly polar water. The water on the liquid surface of the polycondensation reaction unit and the liquid film on the surface of the energy storage catalytic pellets 311 is instantly vaporized and flashed and discharged from the steam outlet. The energy storage catalytic pellet 311 absorbs microwaves and heats up rapidly. The energy storage catalytic pellet 311 falls back into the liquid phase reaction system and brings heat energy into the reaction system. The liquid film on the surface of the energy storage catalytic pellet 311 after the water is evaporated is peeled off under the action of mechanical stirring and ultrasound, so that the reaction system is heated more evenly, the mass transfer and heat transfer effect is increased, and the rare earth, tin dioxide, zinc oxide and other catalysts in the energy storage catalytic pellet are in uniform contact with the system, which has a good catalytic effect, and a dynamic reaction interface is continuously formed on the surface of the energy storage catalytic pellet 311 to generate lactic acid oligomers and water. The tourmaline in the energy storage catalytic pellet electrolyzes the water and makes the water molecules small. Under the action of mechanical stirring, the energy storage catalytic pellet 311 is brought to the upper part and above the reaction system again. The surface area of ​​the vaporized product of the polycondensation reaction system is greatly increased. The selective heating of water molecules by microwaves causes the generated water to flash evaporate instantly; and because the energy storage catalytic pellet is a high-temperature sintered ceramic material, the catalytic material will not be dispersed into the reaction system. Therefore, there will be no catalyst residue in the reaction system, and some side reactions caused by catalyst residue in the reaction system will not exist. In this way, the polycondensation reaction proceeds in an orderly manner in the positive direction.

[0172] Furthermore, in each polycondensation reaction unit, polycondensation reactions in different temperature ranges and different reaction time periods are carried out in sequence. The polycondensation reaction of the previous polycondensation unit is the basis for the polycondensation reaction of the next polycondensation unit, and the polycondensation reaction of the next polycondensation unit is the continuation of the polycondensation reaction of the previous polycondensation chamber unit. The main function of the front polycondensation chamber is to evaporate the free water and bound water contained in lactic acid out of the polycondensation reaction system at a temperature of 135°C to 140°C, and start a preliminary polycondensation reaction. The polycondensation chambers from the middle to the rear end are synchronously and orderly carried out in stages with the orderly increase of temperature and the increase of reaction time, and the water generated by the polycondensation reaction is immediately flash evaporated and leaves the reaction system.

[0173] Furthermore, under the combined action of microwave, ultrasound, heating oil, mechanical stirring and energy storage catalytic pellets, the lactic acid polycondensation system has no catalyst residue, the reaction system is rapidly dehydrated, and the lactic acid oligomers can be effectively inhibited from developing into macromolecules, so that the entire polycondensation reaction process gradually and orderly moves the reaction equilibrium toward the positive direction. By adjusting the pumping flow rate of lactic acid and the microwave power, the eighth and ninth polycondensation units are free of water discharge, and the obtained high-purity lactic acid polycondensation product is continuously pumped into the tubular prepolymerization reactor 400 for prepolymerization reaction.

[0174] Prepolymerization

[0175] The high-purity lactic acid polycondensate prepared in

[173] is continuously pumped into the prepolymerization pipeline 403 through the material inlet 401 by a melt pump, and enters each prepolymerization unit distribution in turn through the circular hole channel of the partition 407 for prepolymerization reaction. The chain extender is added in proportion according to the flow rate of the melt pump. The chain extender is an epoxy polymerization type chain extender PZYC-06, and the addition amount is 0.1 to 0.6% of the mass of the high-purity lactic acid polycondensate.

[0176]

[175] The high-purity lactic acid polycondensate in the prepolymerization pipeline 403 is prepolymerized under the combined action of stirring, solid energy storage catalytic pellets and chain extender. The water vapor generated by the prepolymerization is discharged from the water vapor outlet 409 under negative pressure to obtain a prepolymerized melt. The prepolymerization temperature is 120-150° C., the pressure is 10-15 KPa, and the prepolymerization reaction time is 2-4 hours.

[0177] Polymerization

[0178] The prepolymer melt obtained in the tubular prepolymer reactor 400 is fed to the polymerization zone prepolymer reactant inlet 50101 by a melt pump, and the polymerization zone prepolymer melt feed bin 50103 is maintained at a temperature of 130-150°C and a pressure of 10-15KPa, and the reaction time is 1-3 hours. The water vapor is discharged through the polymerization zone water vapor outlet 50102.

[0179] The lactic acid prepolymer in the polymerization zone prepolymer melt feed bin 50103 is stirred by the polymerization zone stirring sleeve assembly 50104, pushed by the polymerization zone feeding spiral sleeve assembly 50106, and extruded by the polymerization zone extrusion polymerization spiral sleeve assembly 50107 to obtain polylactic acid melt.

[0180]

[179] The polylactic acid melt obtained by polymerization is pushed by the feeding spiral sleeve assembly 50108 of the polymerization zone to the exhaust zone 502, and is stirred and exhausted by the stirring sleeve assembly 50202 of the exhaust zone 502. It is then pushed by the feeding spiral sleeve assembly 50301 of the intensified polymerization zone, extruded and intensified by the extended extrusion polymerization spiral sleeve assembly 50302 of the intensified polymerization zone, and then pushed by the feeding spiral sleeve assembly 50303 of the intensified polymerization zone to the extrusion, cutting and granulation assembly 50304 of the intensified polymerization zone for cutting and granulation to obtain high molecular weight polylactic acid. The temperature is maintained at 130-160°C, the pressure is 5-10KPa, and the reaction time is 3-5 hours.

[0181] Example

[0182] Example 1

[0183] The special corn variety described in

[020] is selected and professionally planted under the guidance of experts to obtain corn kernels with a crude starch content of about 80%, of which the amylopectin content is greater than 95%, and the average starch yield is greater than 75%. The corn kernels are used as raw materials to separate high-purity corn starch with a protein content of less than 0.05% and a content of other impurities less than 0.5%, and a fermentation method is used to prepare high-purity lactic acid monomers with an optical purity of ≥99.5% and a chemical purity of ≥98.0%. The lactic acid monomer is decomposed into nine reaction units with different temperatures under the synergistic effect of microwave, ultrasonic wave, heating oil, mechanical stirring and energy storage catalytic pellets, and gradually undergoes polycondensation reaction. The reaction time of each polycondensation unit is 30 minutes, the polycondensation reaction time is 4.5 hours, the pressure is 12KPa, the temperature is 130℃~160℃, the temperature difference between the first to the sixth reaction units is 5℃, the reaction temperature of the seventh, eighth and ninth polycondensation units is 160℃, the water generated by the polycondensation reaction of each polycondensation unit is flashed and separated from the reaction system, the ninth polycondensation unit has been discharged without water, and the obtained lactic acid oligomer is 2100Da. The obtained lactic acid oligomer is transported to a tubular prepolymerization reactor by a melt pump. According to the melt pump flow rate, epoxy polymerization type chain extender PZYC-06 is added in proportion, and the addition amount is 0.20% of the mass of high-purity lactic acid polycondensate, and prepolymerization is carried out under stirring, solid energy storage catalytic material and microwave system to obtain a prepolymer melt. The prepolymerization temperature is 145°C and the pressure is 12KPa. The reaction time is 2 hours. The obtained prepolymer melt is introduced into a polymerization reactor and subjected to spiral extrusion polymerization under the synergistic effect of mechanical stirring, microwaves, ultrasound and solid energy storage catalyst materials to obtain a polymer melt. The polymerization temperature is 145°C, the pressure is 10KPa, and the polymerization time is 2.5 hours. The obtained polymer melt is then subjected to negative pressure exhaust under the synergy of mechanical stirring, microwaves, ultrasound and solid energy storage catalyst materials to make the molecular structure of the polymer melt tighter and obtain a high-density polymer melt. Finally, the obtained high-density polymer melt is subjected to enhanced extrusion polymerization and extrusion pelletization under the synergistic effect of mechanical stirring, microwaves, ultrasound and solid energy storage catalyst materials. The polymerization temperature is 155°C, the pressure is 5KPa, and the reaction time is 4 hours to obtain polylactic acid particles with a high optical purity of 99.63% and a molecular weight of 370,000 Da.

[0184] Example 2

[0185] The special corn variety described in

[020] is selected and professionally planted under the guidance of experts to obtain corn kernels with a crude starch content of about 80%, of which the amylopectin content is greater than 95%, and the average starch yield is greater than 75%. The corn kernels are used as raw materials to separate high-purity corn starch with a protein content of less than 0.05% and a content of other impurities less than 0.5%, and a fermentation method is used to prepare high-purity lactic acid monomers with an optical purity of ≥99.5% and a chemical purity of ≥98.0%. The lactic acid monomer is decomposed into nine reaction units with different temperatures under the synergistic effect of microwave, ultrasonic wave, heating oil, mechanical stirring and energy storage catalytic pellets, and gradually undergoes polycondensation reaction. The reaction time of each polycondensation unit is 20 minutes, the polycondensation reaction time is 3 hours, the pressure is 15KPa, the temperature is 130℃~160℃, the temperature difference between the first to the sixth reaction units is 5℃, the reaction temperature of the seventh, eighth and ninth polycondensation units is 160℃, the water generated by the polycondensation reaction of each polycondensation unit is flashed and separated from the reaction system, the ninth polycondensation unit has been discharged without water, and the obtained lactic acid oligomer is 1980Da. The obtained lactic acid oligomer is transported to a tubular prepolymerization reactor by a melt pump. According to the melt pump flow rate, epoxy polymerization type chain extender PZYC-06 is added in proportion, and the addition amount is 0.20% of the mass of high-purity lactic acid polycondensate, and prepolymerization is carried out under stirring, solid energy storage catalytic material and microwave system to obtain a prepolymer melt. The prepolymerization temperature is 135°C, the pressure is 10KPa, and the reaction time is 2 hours. The obtained prepolymer melt is introduced into a polymerization reactor, and spiral extrusion polymerization is carried out under the synergistic effect of mechanical stirring, microwaves, ultrasound and solid energy storage catalyst materials to obtain a polymer melt. The polymerization temperature is 135°C, the pressure is 10KPa, and the polymerization time is 2.5 hours. The obtained polymer melt is then vented under negative pressure under the synergy of mechanical stirring, microwaves, ultrasound and solid energy storage catalyst materials to make the molecular structure of the polymer melt tighter and obtain a high-density polymer melt. Finally, the obtained high-density polymer melt is subjected to enhanced extrusion polymerization and extrusion pelletizing under the synergistic effect of mechanical stirring, microwaves, ultrasound and solid energy storage catalyst materials. The enhanced polymerization temperature is 155°C, the pressure is 5KPa, and the reaction time is 4 hours to obtain polylactic acid particles with a high optical purity of 99.51% and a molecular weight of 310,000 Da.

[0186] Comparative Example

[0187] Comparative Example 1

[0188] In Example 1 of the patent 201811068025.X of He Kai et al., "A method for preparing high molecular weight polylactic acid by a twin-screw extruder", each synthetic polylactic acid material is premixed uniformly according to the following components. The components and weight proportions of the raw materials are as follows: 100 parts of polylactic acid, 0.2 parts of antioxidant, 0.1 parts of catalyst, and 0.1 parts of lubricant; the premixed materials are uniformly added to the feed port of the extruder under nitrogen protection, and vacuum is drawn at the exhaust port at the same time. The screw speed is controlled at 120r / min. After 15 hours of polymerization reaction in the twin-screw extruder, the product is extruded through a metering pump and cooled into a solid state in the air. The extrusion temperatures are: 150-160°C, 165-175°C, 175-185°C, 185-190°C, 180-185°C, and the die temperature is 210-220°C. The number average molecular weight of the obtained polylactic acid product is 4.7×10 5 .

[0189] Comparative Example 2

[0190] 85% industrial lactic acid is selected from the market. Under the synergistic effect of microwave, ultrasonic wave, heating oil, mechanical stirring and energy storage catalytic pellets, it is decomposed into nine reaction units with different temperatures to gradually carry out polycondensation reaction at 130℃~160℃. The reaction time of each polycondensation unit is 45min, the polycondensation reaction time is 4.5 hours, the pressure is 12KPa, the temperature is 130℃~160℃, the temperature difference between the first to the sixth reaction units is 5℃, the reaction temperature of the seventh, eighth and ninth polycondensation units is 160℃, the water generated by the polycondensation reaction of each polycondensation unit is flashed and separated from the reaction system, the ninth polycondensation unit has no water discharged, and the obtained lactic acid oligomer is 1890Da. The obtained lactic acid oligomer is transported to the tubular prepolymerization reactor by a melt pump. According to the melt pump flow rate, epoxy polymerization chain extender PZYC-06 is added in proportion, and the addition amount is 0.20% of the mass of high-purity lactic acid polycondensate, and prepolymerization is carried out under stirring, solid energy storage catalyst material, and microwave system; a prepolymer melt is obtained. The prepolymerization temperature is 135°C, the pressure is 12KPa, and the reaction time is 2 hours. The obtained prepolymer melt is introduced into the polymerization reactor, and spiral extrusion polymerization is carried out under the synergistic action of mechanical stirring, microwave, ultrasound and solid energy storage catalyst material to obtain a polymer melt. The polymerization temperature is 135°C, the pressure is 10KPa, and the polymerization time is 2 hours. The obtained polymer melt is then vented under negative pressure under the synergy of mechanical stirring, microwave, ultrasound and solid energy storage catalyst material to make the molecular structure of the polymer melt tighter and obtain a high-density polymer melt. Finally, the obtained high-density polymer melt was subjected to enhanced extrusion polymerization and extrusion pelletization under the synergistic action of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials. The polymerization temperature was 155°C, the pressure was 5KPa, and the reaction time was 4 hours to obtain polylactic acid particles with a high optical purity of 90.08% and a molecular weight of 110,000 Da.

[0191] Obviously, the above embodiments are only examples for clear explanation, and are not limitations of the embodiments. For those of ordinary skill in the art, other different forms of changes or variations can be made on the basis of the above description. For example, on the basis of the above description, polylactic acid fibers, polylactic acid-glycolic acid copolymers (PLGA) are prepared, and polyglycolide is prepared using the horizontal lactic acid polycondensation kettle, tubular prepolymer reactor, and polymerization reactor described above. It is not necessary and impossible to list all the embodiments here. The obvious changes or variations derived from this are still within the scope of protection of the present invention.

[0192] The structures, proportions, sizes, etc. illustrated in the drawings of the specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no substantial technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effects and purposes that can be achieved by the present invention, should still fall within the scope of the technical contents disclosed by the present invention. At the same time, the terms such as "upper", "lower", "front", "back", "middle" and so on quoted in this specification are only for the convenience of description, and are not used to limit the scope of the implementation of the present invention. The change or adjustment of their relative relationship should also be regarded as the scope of the implementation of the present invention without substantially changing the technical contents.

Claims

1. A process system for directly polycondensing polylactic acid using special corn as raw material, characterized in that: The steps include: Step (1) is to prepare ultra-high purity lactic acid, using special corn from which impurities such as starch and bound protein can be easily separated as raw material, and using a biological fermentation method to prepare lactic acid monomers with an optical purity of ≥99.5% and a chemical purity of ≥98.0%. Step (2), polycondensation, the lactic acid polycondensation system is decomposed into a number of reaction units at different temperatures under the synergistic effect of microwaves, ultrasound, heating oil, mechanical stirring and energy storage catalytic pellets, and the water generated by the polycondensation reaction of each polycondensation unit is flash evaporated and separated from the reaction system to obtain lactic acid oligomers. Step (3), prepolymerization, the lactic acid oligomer obtained in step (2) is transported to a tubular prepolymerization reactor by a melt pump. A chain extender is added in proportion according to the flow rate of the melt pump, and prepolymerization is carried out under stirring, solid energy storage catalyst material, and microwave action to obtain a prepolymer melt. Step (4), polymerization, introducing the prepolymer melt obtained in step (3) into a polymerization reactor, and performing screw extrusion polymerization under the synergistic effect of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials to obtain a polymer melt. Step (5), exhausting, exhausting the polymer melt obtained in step (4) under negative pressure under the coordination of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials, so that the molecular structure of the polymer melt obtained in step (4) is more compact, and a high-density polymer melt is obtained. Step (6), enhanced polymerization, the high-density polymer melt obtained in step (5) is subjected to enhanced extrusion polymerization and extrusion pelletization under the synergistic action of mechanical stirring, microwaves, ultrasound and solid energy storage catalytic materials to obtain polylactic acid particles with high optical purity and high molecular weight.

2. The special corn according to step (1) of claim 1, characterized in that: The degree of protein polymerization is low, starch granules are easy to release, and the bound protein and starch are easy to separate. The prepared starch has high purity, less protein residue and less impurities. High-purity lactic acid monomer can be produced without lengthy separation process and directly used in the synthesis of polylactic acid.

3. The process system for directly synthesizing polylactic acid by polycondensation using special corn as raw material according to claim 1, characterized in that: It includes a horizontal lactic acid polycondensation reactor, a tubular prepolymerization reactor and a polymerization reactor.

4. Further, according to the horizontal lactic acid polycondensation kettle of claim 5, it is characterized in that: It includes a polycondensation reaction chamber, a mechanical stirring system, a microwave resonance cavity, an ultrasonic device, a heating oil system, a moisture extraction system, etc.

5. The polymerization reactor according to claim 5, characterized in that It includes a cylinder body, a stirring and extruding component, a vacuum receiving cavity, a microwave resonance cavity, a vacuum exhaust port, a heating oil jacket and an ultrasonic vibrator, etc.

6. Further, the polymerization reactor according to claim 3 is characterized in that: The polymerization reactor is divided into a polymerization zone, a degassing zone and an enhanced polymerization zone. The polymerization zone is located at the front end of the polymerization reactor, the deep polymerization zone is located at the rear end of the polymerization reactor, and the degassing zone is located in the middle of the polymerization reactor.

7. Further, according to the chain extender described in step (3) of claim 1, it is characterized in that The chain extender is an epoxy polymerization type chain extender PZYC-06, and the added amount is 0.1-0.6% of the mass of the lactic acid oligomer.

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