JP4994314B2 - Method and apparatus for synthesizing lactide and polylactic acid - Google Patents

Description

  The present invention relates to a method and apparatus for synthesizing lactide or polylactic acid.

  In general, in the synthesis of a polymer, as the temperature history becomes longer, the quality of the final polymer may become a problem because the raw material or the polymer is partially thermally decomposed. In addition, when an asymmetric carbon is present in the raw material molecule among the polymers, a part of the optical isomerization occurs with the temperature history, and quality degradation of the final polymer may be a problem. Polylactic acid, which is one of polymers synthesized by a ring-opening polymerization reaction, is a polyester made from lactic acid, which is one of hydroxycarboxylic acids, as a raw material. One method of synthesizing polylactic acid with a weight average molecular weight of tens of thousands to hundreds of thousands from lactic acid is to condense lactic acid to produce a lactic acid oligomer, and then add a catalyst such as tin 2-ethylhexanoate, antimony oxide, etc. There is a method in which lactide (a dimer of lactic acid) is produced by adding a depolymerization reaction under reduced pressure to produce a cyclic condensate, and a catalyst such as tin octylate is added to the lactide to perform ring-opening polymerization. . Lactic acid oligomers can be considered as low molecular weight polylactic acid having a weight average molecular weight of, for example, about 1000 to 5000, but it is known that the molecular weight is difficult to increase due to the equilibrium of the reaction simply by performing condensation. In many cases, the above method via lactide is employed. Even in the above method, there is a possibility of thermal decomposition due to the prolonged temperature history of each process, and the same thing can be said, for example, by converting glycolic acid to glycolide, which is a cyclic dimer, and performing ring-opening polymerization. This also applies to polymers of other hydroxycarboxylic acids such as polyglycolic acid synthesized in this way, and inclusion of thermal decomposition products is undesirable because it can cause deterioration of physical properties such as coloring of the product polymer. In the case of polylactic acid, since asymmetric carbon is contained in the lactic acid molecule that forms the polymer skeleton, the optical history of the depolymerization process causes partial optical isomerization (production of D-lactic acid) through the enolization reaction. There is. Polylactic acid usually synthesizes homopolymers using L-lactic acid as a starting material. However, when a part of the lactic acid skeleton constituting polylactic acid is optically isomerized, the crystallinity of the polymer decreases depending on the amount. For this reason, it is known that the polymer physical properties deteriorate, such as a melting point reduction and hydrolysis resistance reduction.

  For this reason, in the method described in Patent Document 1, depolymerization is performed by bringing a lactic acid oligomer into contact with a catalyst in a tank-type reaction tank under a reduced pressure environment, and the difference in boiling point is utilized from the crude lactide generated thereby. A purification process for separating meso-lactide (lactide made of L-lactic acid and D-lactic acid) by distillation and increasing the purity of LL-lactide is installed. Other purification methods include melt crystallization using a difference in melting point and solvent crystallization using a difference in solubility in a solvent. However, in any method, a part of LL-lactide is lost in removing the generated pyrolyzate or meso lactide in the purification step. For this reason, the yield of a raw material falls, and this contributes to the price increase of a product polymer, and development of the technique which suppresses the thermal decomposition and optical isomerization in a depolymerization process is required. Further, in the method described in Patent Document 2, since the optical isomerization reaction is a kind of thermal deterioration, the depolymerization reaction is completed in a short time to suppress the optical isomerization. A centrifugal thin film evaporator having a quantity control function is applied as a reaction apparatus. Centrifugal thin film evaporator continuously feeds the raw material liquid from one end of the reaction tank, lifts the raw material liquid by the centrifugal force of the stirring blade, forms a thin film along the inner wall of the reaction tank, and continuously discharges the generated steam from the other end. At the same time, the concentrated liquid is continuously discharged from the drain hole. The film thickness at that time is determined by the width between the inner wall of the tank and the stirring blade, and is usually several mm to several tens mm. Centrifugal thin-film evaporators are not reactors, but evaporators, and can increase the evaporation area and heat transfer coefficient, so the process of concentrating products sensitive to temperature, such as fruit juice, meat and seafood extracts, in a short time, Or it is suitable for the process which condenses and collect | recovers generated steam in order to obtain the scent component of tea or coffee as a product. In the latter case, when most of the raw material liquid can become product vapor, it is important to reduce the drain amount of the residue (concentrated liquid) as much as possible from the viewpoint of improving the raw material yield. The same can be expected for the example in the above document where the evaporator is regarded as a continuous depolymerization reactor for the production of lactide vapor. In addition, as described in this document, when product vapor is generated from the reaction liquid by a chemical reaction via an additive such as a catalyst, in addition to the above, in order to stabilize the quality and production amount of the product vapor, It is important to reduce the thickness of the additive while uniformly dispersing the additive in the reaction solution at a constant concentration. At that time, the reaction residue after sufficient lactide has already been generated is poor in reactivity, and it is difficult to recover product vapor from the residue simply by homogenization, and it is discharged and discarded as drain.

  On the other hand, from the viewpoint of material recycling of waste plastics, Patent Document 3 discloses that polylactic acid having a weight average molecular weight of 5000 or more is heated in a reduced pressure environment using a horizontal twin-screw stirrer or horizontal twin-screw extruder. There is a description of a technique for recovering lactide by polymerization and using it for polymerization of polylactic acid. Horizontal twin-screw stirrers and horizontal twin-screw extruders (hereinafter represented by horizontal twin-screw stirrers) are usually applied to continuous polycondensation of polyester such as polyethylene terephthalate. By-products such as ethylene glycol accompanying polymerization reactions The evaporation area for the object can be made relatively large. In addition, even if the reactivity decreases as the viscosity increases due to the progress of the polymerization reaction, the surface renewal effect diffuses and migrates by-products from the bulk of the reaction solution to the evaporation surface to promote devolatilization and reduce the reactivity. It has a function to improve. This method can be applied not only to the recycling field, but also to the above-described depolymerization step from the viewpoint of the effect of reducing optical isomers, and it is considered to be effective for those having reduced reactivity such as residues. However, in the method of this document, the thickness of the liquid film during the operation of the apparatus cannot be made as thin as the method using the centrifugal thin film evaporation apparatus of Patent Document 2 and cannot be made constant, so there is a problem in controllability. . For this reason, it is thought that the reduction effect of an optical isomer is small compared with the system of patent document 2. FIG. From the above, it has been technically difficult to realize a depolymerization step with a high raw material yield.

Japanese Patent No. 3258324 JP 2007-100011 A Japanese Patent No. 3503127

  An object of the present invention is to provide a method and apparatus for producing and recovering crude lactide product vapor from lactic acid oligomers, which can recover the product vapor with a high raw material yield.

  As a result of intensive studies to solve the above problems, the present inventors conducted a depolymerization reaction in a multistage continuous reaction in the production of crude lactide vapor by depolymerization of lactic acid oligomers, and produced each stage. Water, unreacted lactic acid, and low molecular weight lactic acid oligomers are separated from the crude lactide vapor by partial condensation, and returned to the concentration process of raw lactic acid and the condensation process of concentrated lactic acid, and the crude lactide after removing impurities is tanked. It was found that optically active lactide, particularly LL-lactide, can be recovered with high raw material yield by purifying these separately in each stage.

  In addition, by applying a horizontal twin-shaft stirrer that can process low-reactivity residues in the subsequent stage in a multistage continuous reaction, the product quality such as production speed and optical purity is stabilized at a high level. It was found that drainage can be reduced to improve raw material yield.

  Furthermore, it has been found that the product quality such as production speed and optical purity can be improved to a higher level by applying the centrifugal thin film evaporation apparatus as a reaction apparatus in front of the stage to which the horizontal twin-shaft is applied.

That is, the present invention includes the following inventions.
(1) A method for producing lactide by depolymerizing a lactic acid oligomer,
The method as described above, wherein the depolymerization reaction is performed in a multistage continuous reaction, impurities are separated from the crude lactide vapor generated in each stage, and the crude lactide after the impurity separation is purified separately for each stage.
(2) The method according to (1), wherein a horizontal biaxial stirrer is used in the second and subsequent stages in the multistage continuous reaction.
(3) The method according to (1) or (2), wherein a centrifugal thin film evaporator is used in at least one stage of the multistage continuous reaction, and a horizontal biaxial stirrer is used in the subsequent stage.

(4) A method for producing polylactic acid from lactic acid,
A lactic acid condensation step of condensing lactic acid to synthesize a lactic acid oligomer,
A depolymerization step for producing lactide by depolymerizing the obtained lactic acid oligomer by the method according to any one of (1) to (3), and ring-opening polymerization for synthesizing polylactic acid by ring-opening polymerization of lactide The method comprising the steps of: refluxing impurities separated from the crude lactide vapor generated in the depolymerization step to the lactic acid condensation step or the upstream step.

(5) A lactide production apparatus for synthesizing lactide by depolymerization of a lactic acid oligomer,
Multi-stage continuous depolymerization equipment,
A partial reduction apparatus for separating impurities from the crude lactide vapor generated in each stage of a multistage continuous depolymerization apparatus;
A tank for recovering the crude lactide after the impurity separation, and a purification device for purifying the crude lactide after the impurity separation, wherein the fractionation device and the tank are at least one downstream of each stage of the multistage continuous depolymerization apparatus. The devices are installed one by one.

(6) The apparatus according to (5), wherein the multistage continuous depolymerization apparatus includes a horizontal biaxial stirrer in the second and subsequent stages.
(7) The apparatus according to (5) or (6), wherein the multistage continuous depolymerization apparatus includes a centrifugal thin film evaporation apparatus, and further includes a horizontal biaxial stirrer.
(8) A polylactic acid production apparatus for synthesizing polylactic acid from lactic acid,
A lactic acid condensing apparatus for synthesizing lactic acid oligomers by condensing lactic acid,
The lactide production apparatus according to any one of (5) to (7), for synthesizing lactide by depolymerization of a lactic acid oligomer,
The apparatus comprising: a ring-opening polymerization apparatus that synthesizes polylactic acid by ring-opening polymerization of lactide; and a system for refluxing impurities separated by a partial condensation apparatus of the lactide production apparatus to a lactic acid condensation apparatus or an upstream apparatus. .

  According to the present invention, high quality lactide can be synthesized in high yield.

  In the present invention, the lactic acid oligomer is a concept including a lactic acid polymer having a molecular weight of about 50,000 from a dimer of lactic acid, but the molecular weight of the lactic acid oligomer charged into the depolymerization apparatus in the production of lactide is the number average molecular weight. In general, it is 150 to 10,000, preferably 500 to 5,000.

  In the present invention, lactide means a cyclic ester produced by dehydrating two water molecules from two lactic acid molecules. Lactic acid includes both L-lactic acid and D-lactic acid. The present invention is particularly suitable for the production of optically active lactides, particularly LL-lactide.

  Polylactic acid means a polymer mainly composed of lactic acid. Poly L-lactic acid homopolymer, poly D-lactic acid homopolymer, poly L / D-lactic acid copolymer, and other ester bond formation on these polylactic acids. For example, it includes a copolymer component such as hydroxycarboxylic acid, lactone, copolymerized polylactic acid obtained by copolymerizing dicarboxylic acid and diol, and a mixture of additives as a secondary component. Examples of additives include antioxidants, stabilizers, ultraviolet absorbers, pigments, colorants, inorganic particles, various fillers, mold release agents, plasticizers, and the like. The addition ratio of these copolymerization components and additives is arbitrary, but the main component is lactic acid or derived from lactic acid, and the copolymerization components and additives are preferably 50% by weight or less, particularly preferably 30% or less.

  In the present invention, continuous reaction or continuous has the meaning normally used in the art, and when the raw material supply and the product discharge are at least partially overlapped, or the raw material supply is continuously performed. Including the case where the product is discharged continuously.

  The present invention continuously supplies the lactic acid oligomer, which is a raw material liquid, in a molten state, performs contact with a catalyst as necessary while heating and places it in a reduced pressure environment to perform depolymerization, and recovers lactide vapor as a product. To do. It should be noted that the liquid in the reaction vessel is altered by depolymerization and is in a state different from the original raw material liquid, but in this specification, these are not distinguished, and the raw material liquid or reaction Expressed in liquid terms.

  In order for the lactic acid oligomer to be in a molten state, the heating temperature needs to be higher than its melting point. Although melting | fusing point changes with the polymerization degree distribution etc. of a lactic acid oligomer, it is 100-250 degreeC normally, Preferably it is 180-220 degreeC.

  In the present invention, the depolymerization reaction of the lactic acid oligomer is performed in a multistage continuous reaction. That is, the depolymerization reaction is carried out in a multistage continuous depolymerization apparatus. The continuous depolymerization apparatus refers to a reaction apparatus for performing a depolymerization reaction by a continuous reaction, and the multistage continuous depolymerization apparatus refers to a plurality of the reaction apparatuses being connected.

  In the present invention, impurities are separated from the crude lactide vapor generated in each stage of the multistage continuous depolymerization reaction, and the crude lactide after the impurity separation is purified separately for each stage. That is, in the present invention, the continuous depolymerization apparatus includes a fractionation device for separating impurities from the generated crude lactide vapor, a tank for recovering the crude lactide after impurity separation, and the crude lactide after impurity separation is purified. The fractionation device and the tank are installed at least one downstream of each stage of the multistage continuous depolymerization device. The purification apparatus refers to an apparatus for purifying a target optically active lactide from lactide produced by a depolymerization reaction, and preferably a racemic mixture containing DL-lactide and DD-lactide generated by optical isomerization in the production of LL-lactide. Refers to an apparatus for purifying LL-lactide from

  There may be a plurality of purification apparatuses, but by installing at least one tank for recovering the crude lactide after the impurity separation downstream of each stage of the multistage continuous depolymerization apparatus, The crude lactide can be purified separately for each stage, which eliminates the need to install a plurality of expensive purification apparatuses.

  The crude lactide generated in the continuous depolymerizer on the front side has a lower production rate of optical isomers for a larger production amount. Therefore, there is little loss of product lactide (LL-lactide) due to disposal with optical isomers in the purification process. As in the present invention, the loss of product lactide when refining crude lactide from each stage separately is small compared to the case where the crude lactide from all stages is mixed and purified at once, so the raw material yield is low. improves.

  The continuous depolymerization apparatus has at least a reaction tank, a lactic acid oligomer supply port, a lactide discharge port, and a residue discharge port. A normal thermometer is also installed. The inside of the continuous depolymerization apparatus in each stage is usually depressurized by a vacuum pump installed via a partial pressure reduction apparatus, a lactide vapor condenser, and an impurity condenser. The molten lactic acid oligomer as a raw material and the previous residue liquid (drain) are continuously supplied into the subsequent depolymerization apparatus. It is desirable that the retention amount of the reaction liquid in the apparatus is measured by a retention amount measuring device as necessary, and the supply amount and the drain discharge amount are adjusted so as to be constant.

  As a heating method in the continuous depolymerization apparatus, a method usually used in this technical field can be used. For example, a method of installing a heat medium jacket on the outer periphery and heating by heat transfer through the reaction vessel wall surface, or There is a method of heating through a heat medium by heat exchange installation inside the apparatus, and these may be used alone or in combination.

  The continuous depolymerization apparatus is usually equipped with a depressurization apparatus. The depolymerization of the lactic acid oligomer is usually performed at 120 to 250 ° C., preferably 180 to 220 ° C. in a reduced pressure environment of 100 Torr or less, preferably 10 Torr or less. Perform the reaction. Lactide is generated as a gas by the depolymerization reaction.

  In the depolymerization reaction, a catalyst for the depolymerization reaction may be added as necessary. As the catalyst, conventionally known ones can be used, for example, a catalyst containing at least one metal or metal compound selected from the group consisting of Group IA, IVA, IVB and VA of the periodic table is used. Can do. Examples of those belonging to Group IVA include organotin catalysts (for example, tin lactate, tin tartrate, tin dicaprylate, tin dilaurate, tin dipalmitate, tin distearate, tin dioleate, tin α-naphthoate). , Β-naphthoic acid tin, 2-ethylhexanoic acid tin, etc.), and powdered tin. Examples of those belonging to Group IA include alkali metal hydroxides (for example, sodium hydroxide, potassium hydroxide, lithium hydroxide), alkali metal and weak acid salts (for example, sodium lactate, sodium acetate, sodium carbonate). Sodium octylate, sodium stearate, potassium lactate, potassium acetate, potassium carbonate, potassium octylate, etc.), alkali metal alkoxides (eg sodium methoxide, potassium methoxide, sodium ethoxide, potassium ethoxide, etc.) Can be mentioned. Examples of those belonging to Group IVB include titanium compounds such as tetrapropyl titanate and zirconium compounds such as zirconium isopropoxide. Examples of those belonging to Group VA include antimony compounds such as antimony trioxide. Among these, an organotin catalyst or a tin compound is particularly preferable from the viewpoint of activity.

  If the catalyst is a liquid or powdery solid, it is added to the molten lactic acid oligomer by a catalyst addition apparatus usually used in the art, and then the mixed liquid is continuously supplied to the continuous depolymerization apparatus, and the remaining liquid after depolymerization May be continuously discharged, or may be directly added to the continuous depolymerization apparatus and brought into contact with the molten lactic acid oligomer supplied continuously in the continuous depolymerization apparatus. From the viewpoint of maintaining reactivity, it is possible to supply the catalyst in each stage of the continuous depolymerization apparatus as necessary, but only the catalyst added to the first stage of the continuous depolymerization apparatus can be used until the final stage. The reaction may proceed. In addition, when the catalyst is solid, it may be supported on at least a part of the surface in contact with the molten lactic acid oligomer by an apparatus or structure in the continuous depolymerization apparatus. While this method eliminates the need for a catalyst addition device, the actual contact area with the molten lactic acid oligomer is reduced compared to the above method, resulting in a slow depolymerization reaction. There are disadvantages such as the need to maintain equipment and structures.

  In the present invention, it is preferable to use a horizontal biaxial stirrer as a continuous depolymerization apparatus in the second and subsequent stages in the multistage continuous reaction. Therefore, in the present invention, the multistage continuous depolymerization apparatus preferably includes a horizontal biaxial agitator in the second and subsequent stages.

  In the present invention, it is preferable to use a centrifugal thin film evaporator in at least one stage of the multistage continuous reaction, and to use a horizontal biaxial stirrer in the subsequent stage. Therefore, in the present invention, the multistage continuous depolymerization apparatus preferably includes a centrifugal thin film evaporator, and further includes a horizontal biaxial stirrer in the subsequent stage.

  In the present invention, the horizontal twin-screw stirrer as the continuous depolymerization apparatus includes a horizontal twin-screw extruder, and is installed so that the two rotating shafts of the stirrer in the reaction tank are substantially horizontal with respect to the ground. In the reaction tank, a device having a supply port for supplying a reaction liquid containing a molten lactic acid oligomer at one end in the rotation axis direction and a discharge port for taking out the reaction liquid at the other end is indicated. The term “substantially horizontal to the ground” does not mean that the rotation axis of the stirring device is strictly horizontal, and the angle between the ground, that is, the horizon and the rotation axis is usually −5 ° to 5 °. °, preferably -1 ° to 1 °, more preferably 0 °. About the shape of a reaction tank etc., it does not restrict | limit in particular, What is normally used in this technical field can be used. Typical examples include glasses blade polymerizers and lattice blade polymerizers of Hitachi Plant Technology. The discharge port is preferably located below the rotation axis of the stirring device. The mixer installed in the horizontal biaxial agitator is not particularly limited as long as the agitation is performed by rotation around a rotation axis arranged substantially horizontally with respect to the ground. For example, a two-shaft mixer that meshes with each other in which two or more stirring blades such as a circle, an oval, a triangle, a quadrilateral, a multi-leaf, a glasses, and a lattice are placed on the rotation axis at intervals. Can be mentioned. A biaxial or more mixer that meshes with each other is preferable from the viewpoint of the self-cleaning action because it can prevent the reaction liquid from adhering to the rotating shaft of the mixer and the reaction tank. In the case of using a biaxial mixer having a plurality of stirring blades, it is preferable to rotate each rotating shaft in the opposite direction.

  In the horizontal biaxial stirrer, the retention amount of the reaction liquid in the apparatus is measured by a retention amount measuring device as necessary, and the supply amount and the drain discharge amount are adjusted to be constant. In the horizontal biaxial stirrer, the catalyst diffuses due to the surface renewal effect generated by the stirring blades rotating in opposite directions, and is mixed with the reaction liquid inside the apparatus. The reaction liquid is pulled by stirring blades rotating in opposite directions to form a liquid film. The outer surface of the apparatus casing is heated with a heat medium or the like, whereby the reaction liquid is heated, and evaporation of lactide generated by contact with the catalyst is promoted. The bubbles of lactide vapor are diffused and transferred from the reaction liquid bulk layer toward the evaporation surface due to the surface renewal effect generated by the stirring blade, and are discharged to the outside of the reaction liquid. Thereafter, the steam is supplied to a partial condensation apparatus to separate water, unreacted lactic acid, and low molecular weight volatile lactic acid oligomers, and preferably refluxed to the concentrated lactic acid oligomerization step. The lactide vapor from which moisture, unreacted lactic acid, and low molecular weight volatile lactic acid oligomers have been removed is then fed to a lactide vapor condenser. Here, the lactide vapor is condensed and recovered and transferred to the purification process. Vapor exiting the lactide vapor condenser enters the impurity condenser where it is liquefied. Condensed impurities are usually discarded. The gas exiting the impurity condenser is discharged out of the system via a vacuum pump. Further, a liquid level gauge using a liquid head, a dielectric constant of a molten lactic acid oligomer, a differential pressure, a gamma ray, or the like can be applied to a residence measuring device of a horizontal biaxial stirrer. After the residual liquid whose reactivity has already decreased in the first stage continuous depolymerization apparatus is discharged as drain, it is depolymerized with a horizontal twin-screw stirrer that can improve the reactivity by having a surface renewal effect in the second stage. By attaching to, lactide can be further recovered, so that the raw material yield is improved.

  In the present invention, the centrifugal thin film evaporation apparatus refers to a liquid to be concentrated supplied to the inner surface of the apparatus casing by centrifugal force to form a liquid film, and promotes evaporation by heating from the outer surface of the apparatus casing. Is. Centrifugal force is applied to the liquid to be concentrated through a rotating blade and a rotor with blades installed in a fixed device casing. By using this apparatus, the equipment scale can be made compact, and the liquid film thickness can be controlled by controlling the gap width between the blade and the casing and the rotation of the blade. Either a horizontal or vertical centrifugal thin film evaporator may be used. In any case, it is possible to retain an additive such as a catalyst in the apparatus and disperse the catalyst in the molten lactic acid oligomer by operating the rotor and blades. Therefore, even if the molten lactic acid oligomer is continuously supplied from one end of the apparatus, the retention amount can be kept constant if it matches the discharge amount of lactide vapor. Not necessarily. If the supply amount is insufficient compared to the discharge amount, that is, if it is confirmed that the liquid film is thinned, the supply amount of the raw material liquid may be increased so as to obtain a predetermined liquid film thickness. On the other hand, if it is large, it is confirmed that the leakage of the raw material liquid from the liquid film is suppressed by the weir behind the stirring blade and the liquid film becomes thicker. Therefore, the supply amount of the raw material liquid can be reduced so that the predetermined liquid film thickness is obtained. That's fine. The retention amount increases due to a decrease in efficiency of the depolymerization reaction accompanying a decrease in the catalyst activity and a residue accumulation after the depolymerization. These are continuously or intermittently discharged from the drain hole. In accordance with this, the catalyst is added continuously or intermittently into the apparatus. From such a viewpoint, it is desirable to install a nozzle for supplying an additive such as a catalyst in the raw material liquid supply system or reaction tank. Also, regarding the location of the drain hole, when draining continuously, it must be behind the agitating blade. It may be. In this case, the weir for suppressing the leakage of the raw material liquid from the liquid film can be installed as close to the stirring blade as possible, and the leakage space of the raw material liquid can be significantly reduced. There are various methods for measuring the liquid film thickness. One of them is to measure the pressure applied to the inner wall of the reaction tank by the centrifugal force. The pressure measurement includes a method in which a level gauge is installed on the top of the outer surface of the reaction tank and the liquid level is observed, a method in which a pressure sensor is installed and a voltage corresponding to the pressure is measured.

  In particular, when a vertical centrifugal thin film evaporator is applied, whether the raw material liquid is supplied from the upper part or the lower part of the reaction tank, the stirring blade is partially provided with a screw function to resist the liquid flow due to gravity. It is desirable to generate lift for holding the liquid film.

  In the centrifugal thin film evaporator, the retention amount of the reaction liquid in the device is measured by a retention amount measuring device, the supply amount is adjusted to be constant, and leakage to the outside of the stirring blade is suppressed by the weir at the rear of the stirring blade. Is done. In the centrifugal thin film evaporator, the catalyst is mixed with the reaction liquid by the rotation of the rotor blades and is pressed against the inner surface of the apparatus casing by centrifugal force to form a liquid film along the shape of the inner surface. The outer surface of the apparatus casing is heated with a heat medium or the like, whereby the liquid film is heated, and evaporation of lactide generated by contact with the additive is promoted. The lactide vapor is discharged out of the centrifugal thin film evaporator and supplied to the lactide condenser. Here, the lactide vapor is condensed and recovered and transferred to the purification process. Vapor exiting the lactide condenser enters the impurity condenser where it is liquefied. Condensed impurities are usually discarded. The gas exiting the impurity condenser is discharged out of the system via a vacuum pump. For centrifugal thin film evaporators, the above method is a method of applying centrifugal force by rotating the blades in a fixed device casing, and the blades are usually plate-like ones parallel to the rotating shaft and are installed radially from the rotating shaft. In other cases, the wing is twisted to form a screw. The centrifugal thin film evaporator may be any one whose rotation axis is horizontal to the ground, vertical, or an intermediate angle. For the residence amount measuring device, for example, a liquid level meter using a liquid head, a dielectric constant of a molten lactic acid oligomer, a differential pressure, etc. can be applied. Unlike a horizontal twin-screw stirrer, a depolymerization apparatus using a centrifugal thin film evaporator is not necessarily sufficient for improving the reduced reactivity of the residue that is processed on the downstream side, but it is highly reactive. On the other hand, the reaction solution on the front stage side has a larger effect of suppressing the formation of optical isomers than the horizontal biaxial stirrer. In a multistage continuous depolymerization system, by installing a centrifugal thin film evaporator on the front side of the horizontal twin-shaft stirrer, waste generated can be reduced while suppressing the formation of optical isomers, thus further improving the raw material yield. can do.

  As for the partial reduction device, the lactide condenser, and the impurity condenser, a surface condenser in which the above-mentioned refrigerant indirectly contacts with a metal pipe is desirable. This is because when lactide comes into direct contact with a refrigerant containing water, it decomposes to produce an acid. This hinders the progress of the polymerization reaction as an acid catalyst and may cause corrosion of materials such as a condenser. The impurity condensate may also contain a low melting point, such as an optical isomer component, which also generates an acid when contacted with water, and may cause corrosion as well. In the case where a refrigerant inert to lactide is used, it is not limited to the above, but in that case, it is necessary to sufficiently dry the refrigerant to reduce moisture.

  The multistage continuous depolymerization apparatus may include a normal tank type agitator as disclosed in Patent Document 1 in addition to a horizontal biaxial agitator and a centrifugal thin film evaporator. The tank type stirrer is characterized in that the retention amount, that is, the production amount per unit volume, is larger than that of the horizontal biaxial stirrer and the centrifugal thin film evaporator. For this reason, when constructing a three-stage continuous depolymerization process using the above three types of equipment, for example, a continuous depolymerizer is installed in the order of a tank type agitator, a centrifugal thin film evaporator, and a horizontal biaxial agitator. It is conceivable to implement the method.

In one embodiment, the present invention is a method of producing polylactic acid from lactic acid,
A lactic acid condensation step of condensing lactic acid to synthesize a lactic acid oligomer,
The obtained lactic acid oligomer comprises the above lactide production step, and a ring-opening polymerization step of synthesizing polylactic acid by ring-opening polymerization of lactide, and the impurities separated from the crude lactide vapor generated in the depolymerization step are subjected to a lactic acid condensation step or The present invention relates to a method characterized by returning to an upstream process.

The method for producing the polylactic acid is as follows:
A polylactic acid production apparatus for synthesizing polylactic acid from lactic acid,
A lactic acid condensing apparatus for synthesizing lactic acid oligomers by condensing lactic acid,
The lactide production apparatus of the present invention for synthesizing lactide by depolymerization of lactic acid oligomers,
Implemented by a ring-opening polymerization apparatus for ring-opening polymerization of lactide to synthesize polylactic acid, and an apparatus equipped with a system for refluxing impurities separated by a partial condensation apparatus of a lactide production apparatus to a lactic acid condensation apparatus or an upstream apparatus. it can.

  In the lactic acid condensation step of synthesizing a lactic acid oligomer by condensing lactic acid, any lactic acid produced by a conventionally known method may be used, but lactic acid having a low water content is preferred. By using such lactic acid, the process for evaporating and concentrating the water contained in the lactic acid can be shortened, which is advantageous in terms of cost.

  The water originally contained in lactic acid is preferably removed by heating and evaporating. The water contained in the raw lactic acid may be removed together with the water produced by the condensation reaction of lactic acid in the condensation step. However, after the water is removed from the raw lactic acid in advance and concentrated, the lactic acid is condensed. You may attach to a process.

  In the former case, the lactic acid condensation reaction is carried out by directly transporting the raw lactic acid to the lactic acid condensing device. In the latter case, the lactic acid concentrating device and the lactic acid condensing device are connected in series, and the lactic acid is heated by the lactic acid concentrating device in the previous stage. Then, after the water is evaporated, the obtained lactic acid concentrate is transported to a lactic acid condensation apparatus to perform a lactic acid condensation reaction. A lactic acid supply device may be installed in front of the lactic acid condensation device or the lactic acid concentration device, and lactic acid may be supplied to any of the devices from here.

  In the lactic acid condensing apparatus, lactic acid oligomer is produced by heating lactic acid. In the lactic acid condensation reaction, the molecular weight of the obtained lactic acid oligomer is set to a molecular weight suitable for introduction into the above-mentioned continuous depolymerization apparatus. That is, the reaction is performed so that the molecular weight of the obtained lactic acid oligomer is usually 150 to 10,000, preferably 500 to 5,000 in terms of number average molecular weight. The lactic acid condensation reaction is usually carried out by gradually raising the temperature to 160 to 220 ° C., preferably 170 to 200 ° C., usually at a pressure of 100 Torr or less, preferably 10 Torr or less, more preferably 1 Torr or less. By shortening the heating time as much as possible, thermal decomposition of lactic acid and oligomer can be suppressed.

  In the lactic acid condensation reaction, a catalyst for the lactic acid condensation reaction may be added as necessary. As the catalyst, a conventionally known catalyst can be used. For example, an organic tin-based catalyst (for example, tin lactate, tin tartrate, tin dicaprylate, tin dilaurate, tin dipalmitate, tin distearate, diolein) Acid tin, α-naphthoic acid tin, β-naphthoic acid tin, octylic acid tin, etc.) and powdered tin. When the lactic acid supply device is installed, these catalysts may be added in advance in the lactic acid supply device.

  The lactic acid oligomer obtained by the condensation reaction may be once accumulated in a lactic acid oligomer supply apparatus or the like as a buffer tank and then transported to the continuous depolymerization apparatus, or directly transported to the continuous depolymerization apparatus. In the case where the depolymerization reaction is continuously performed, it is preferable that the lactic acid oligomer is accumulated in the lactic acid oligomer supply apparatus and then continuously transported to the continuous depolymerization apparatus.

  The ring-opening polymerization reaction of lactide is carried out in a ring-opening polymerization apparatus by heating at 120 to 250 ° C., preferably 120 to 200 ° C. in an inert gas atmosphere. Polylactic acid is produced by the ring-opening polymerization reaction.

  As the catalyst for the ring-opening polymerization reaction, the same catalyst as that for the depolymerization reaction may be used as necessary. A conventionally known catalyst can be used as the catalyst, for example, a catalyst made of a metal or a metal compound selected from the group consisting of Group IA, Group IIIA, Group IVA, Group IIB and Group VA of the periodic table is used. it can. Among these, it is preferable to use a tin compound such as tin octylate or an antimony compound such as antimony trioxide. The amount of these catalysts used is about 1 to 2000 ppm, preferably 5 to 1500 ppm, and more preferably about 10 to 1000 ppm with respect to lactide.

  In the ring-opening polymerization reaction, a polymerization initiator for the ring-opening polymerization reaction may be added as necessary for the purpose of adjusting the molecular weight. As the polymerization initiator, a substance having a hydroxyl group such as alcohols such as 1-dodecanol can be used.

  Impurities separated by the fractionation apparatus in the depolymerization of lactide are refluxed to the lactic acid condensation step or an upstream step, for example, a lactic acid concentration step. That is, the impurities separated by the fractionation device of the lactide production device are refluxed to the lactic acid condensation device or an upstream device. The system for that purpose usually includes a valve, a liquid feed pump, and piping. Thereby, water, lactic acid, and low molecular weight lactic acid oligomers produced as impurities in the depolymerization step can be recovered and reused, and the yield of lactide and polylactic acid with respect to raw lactic acid can be further improved.

  The present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited thereto.

Example 1
FIG. 1 shows one embodiment of all steps of polylactic acid synthesis using the multistage continuous depolymerization method and apparatus of the present invention. In this embodiment, the polylactic acid synthesis step includes a raw material monomer (lactic acid) concentration step, a concentrate condensation step, a lactic acid oligomer depolymerization step, a lactide purification step, a ring-opening polymerization step, a residual monomer treatment step, a polymer ( Polylactic acid) drying step. These may be omitted depending on the properties of the polymer and the necessity of the process, or other steps may be added during the process. The residual monomer treatment step can be selected from methods such as monomer desorption / removal in a reduced pressure environment and solid phase polymerization at a temperature below the melting point.

  FIG. 2 shows an embodiment in which the multistage continuous depolymerization method of the present invention is carried out in three stages in the order of a tank type agitator, a centrifugal thin film evaporator, and a horizontal biaxial agitator. Any type of depolymerization apparatus may be changed as necessary. The number of stages may be two or more, and one of them may be omitted or the number of stages may be increased. Others can be omitted as necessary.

  After the valve 5 is closed and the discharge from the drain pipe 6 is stopped, the needle valve 1 is opened, the liquid feed pump 2 is operated, and the concentrated lactic acid is supplied to the lactic acid condensation apparatus 4 through the pipe 3. The oligomerization step may be either a continuous type or a batch type. In addition, in the present embodiment, one tank type stirrer is assumed for the lactic acid condensing apparatus 4, but any type such as a horizontal stirrer may be used, and a plurality of lactic acid condensing apparatuses may be connected in series and in parallel. When the continuous method is applied, it is desirable to apply an appropriate back pressure by the needle valve 1. In order to send concentrated lactic acid, it is desirable to heat the pipe 3 to room temperature to 200 ° C. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used.

  The inside of the lactic acid condensing apparatus 4 is in a reduced pressure environment at a temperature of 100 ° C. or higher and 200 ° C. or lower. By treating the lactic acid condensing device for a predetermined time in this environment, a condensation reaction occurs between lactic acid molecules, and lactic acid having a molecular weight of several hundred to several tens of thousands. An oligomer is formed. For example, a lactic acid oligomer having a weight average molecular weight of about 600 is produced by treatment under conditions of 180 ° C., 30 torr and residence time of 2 hours, and a weight average molecular weight of 1200 is produced by treatment under conditions of 200 ° C., 30 torr and residence time of 2 hours. A degree of lactic acid oligomer is formed. The method for heating the inside of the lactic acid condensation apparatus 4 may be any method such as heating from the outside with a heat medium jacket or heating medium heating by installing a heat exchanger inside the apparatus. The internal pressure of the apparatus is reduced by the vacuum pump 14. Water is generated along with the condensation reaction between the lactic acid molecules, which becomes water vapor and reaches the refluxing device 8 through the pipe 7. It is desirable to heat the inside of the pipe 7 at a temperature of 100 ° C. or higher and 200 ° C. or lower. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. The reflux unit 8 separates the water vapor, the lactic acid and the condensate transferred together with the water vapor. The temperature of the reflux device 8 is preferably 50 ° C. or higher and 100 ° C. or lower in order to condense while maintaining a temperature at which lactic acid and its condensate are not fixed, and not to condense water vapor. Lactic acid and its condensate are condensed and returned to the lactic acid condensing apparatus 4 through the pipe 11. The water vapor reaches the condenser 10 through the pipe 9. In the condenser 10, the water vapor is condensed, and the condensed water is stored in the condensed water recovery tank 17 through the pipe 16 by opening the valve 123. The exhaust gas after the water vapor is removed passes through the pressure reducing valve 12 and the pipe 13 and is discharged from the vacuum pump 14 through the pipe 15 to the outside of the system. During the operation of the lactic acid condensation apparatus 4, the valves 24 and 18 are closed, and the drainage from the pipes 19 and 25 is stopped. In the case of continuous operation, the valve 20 is opened and the liquid feed pump 21 is continuously operated, and the lactic acid oligomer is sent to the lactic acid oligomer supply tank 23 through the pipe 22. In the case of operation by batch operation, the valve 20 is closed and opened when the oligomerization is completed, the liquid feed pump 21 is operated, and the lactic acid oligomer is sent to the lactic acid oligomer supply tank 23 through the pipe 22. In that case, since the piping 22 does not fix a lactic acid oligomer, it is desirable to heat the inside at a temperature of 100 ° C. or higher and 200 ° C. or lower. The heating method at that time is preferably external heating using a heat medium jacket. Since the lactic acid oligomer supply tank 23 does not fix the lactic acid oligomer, it is desirable to heat the inside at a temperature of 100 ° C. or higher and 200 ° C. or lower. The heating method at that time may be any method such as external heating using a heat medium jacket or heating medium heating by installing a heat exchanger inside the apparatus. The lactic acid oligomer is continuously supplied to the first-stage continuous depolymerization device 29 through the pipe 28 by the liquid feed pump 27 while opening the valve 26 and applying back pressure by the needle valve 124 as necessary. At that time, it is desirable that the pipe 28 is heated at a temperature of 100 ° C. or higher and 200 ° C. or lower. The heating method at that time is preferably external heating using a heat medium jacket.

  The continuous depolymerization device 29 is a tank-type reaction device. In addition to the lactic acid oligomer continuously supplied from the pipe 28, the valve 31 is opened to open the inside of the catalyst tank 30 through the needle valve 125 and the pipe 33 by the catalyst transfer pump 32. Receives continuous supply of catalyst. The catalyst may be either liquid or solid, but in the case of a liquid catalyst, the catalyst transfer pump 32 is a liquid supply pump, and it is desirable to apply a back pressure as required by the needle valve 125. The continuous depolymerization apparatus 29 is in a reduced pressure environment at a temperature of 180 ° C. or higher and 220 ° C. or lower. The pressure is preferably 10 torr or less. By contacting and mixing the lactic acid oligomer and the catalyst for a predetermined time in this environment, a depolymerization reaction of the lactic acid oligomer occurs to produce lactide. The method for heating the inside of the continuous depolymerization device 29 may be any method such as heating from the outside with a heat medium jacket or heating medium heating by installing a heat exchanger inside the device. Further, the vacuum in the apparatus is performed by the vacuum pump 49. The produced lactide evaporates from the liquid surface of the lactic acid oligomer as moisture, lactic acid, and a low molecular weight lactic acid oligomer, and reaches the partial reduction device 35 through the pipe 34. The piping 34 is desirably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the adhesion of the crude lactide. About the method of heating, the heating from the outside by a heat medium jacket is desirable. In the partial reduction device 35, impurities such as moisture, lactic acid, and low molecular weight lactic acid oligomers are condensed and roughly separated from the lactide vapor. The separated lactide vapor is supplied to the lactide condenser 37 through the pipe 36. The pipe 36 is desirably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the adhesion of crude lactide. About the method of heating, the heating from the outside by a heat medium jacket is desirable. The condensed liquid of impurities passes through the valve 38 and is refluxed from the pipe 40 to the lactic acid condensing apparatus 4 by the liquid feed pump 39. In addition, the place which recirculates must be the lactic acid condensation apparatus 4 like a present Example, or the upstream of a plant rather than it. This is to remove moisture in the impurities and collect and reuse lactic acid and low molecular weight lactic acid oligomers for oligomerization. In the lactide condenser 37, the crude lactide vapor is cooled to a temperature close to the melting point of lactide of 95 to 98 ° C., condensed and recovered. During continuous operation of the continuous depolymerization apparatus 29, the valve 41 is opened, and the crude lactide condensate is supplied to the first crude lactide recovery tank 44 through the pipe 43 by the pump. At that time, the valve 110 is closed. The optical purity of the crude lactide is about 90 to 95%. The exhaust gas after the crude lactide separation contains lactide, moisture, lactic acid, low molecular weight lactic acid oligomer, etc. that have not been separated, and reaches the impurity condenser 46 through the pipe 45. The pipe 45 is preferably heated to 50 ° C. or higher and 100 ° C. or lower in order to prevent adhesion of lactide, moisture, lactic acid, low molecular weight lactic acid oligomers and the like. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. In the impurity condenser 46, impurities such as lactide, moisture, lactic acid, and low molecular weight lactic acid oligomer are condensed and separated from the exhaust gas, and discharged from the pipe 50 through the pressure reducing valve 47 and the pipe 48 and from the pipe 50 by the vacuum pump 49. Considering the melting point of meso lactide, the impurity condenser 46 is kept at 50 ° C. to 90 ° C. to condense impurities. Condensate is discharged from the pipe 137 by opening the valve 136. During continuous operation of the continuous depolymerizer 29, the valve 51 is opened, and the residue of the reaction solution is continuously supplied from the continuous depolymerizer 29 to the second-stage continuous depolymerizer 54 through the pipe 53 by the liquid feed pump 52. The It is desirable to heat the inside of the pipe 53 at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent adhesion of the residual liquid. About the method of heating, the heating from the outside by a heat medium jacket is desirable.

  The continuous depolymerization apparatus 54 is a reaction apparatus to which a centrifugal thin film evaporation apparatus is applied. In addition to the residual liquid of the continuous depolymerization apparatus 29 that is continuously supplied from the pipe 53 (hereinafter also referred to as lactic acid oligomer), a valve is optionally used. By opening 31, the catalyst transfer pump 55 receives the continuous supply of the catalyst in the catalyst tank 30 through the needle valve 126 and the pipe 56. The catalyst may be either liquid or solid, but in the case of a liquid catalyst, the catalyst transfer pump 55 is a liquid supply pump, and it is desirable to apply a back pressure as required by the needle valve 126. Since the lactic acid oligomer in this stage is a residual liquid of the continuous depolymerization apparatus 29, the catalyst has already been added and mixed, and the addition of the catalyst here is not always necessary. The continuous depolymerizer 54 is in a reduced pressure environment at a temperature of 180 ° C. or higher and 220 ° C. or lower. The pressure is preferably 10 torr or less. By contacting and mixing the lactic acid oligomer and the catalyst for a predetermined time in this environment, a depolymerization reaction of the lactic acid oligomer occurs to produce lactide. About the method of heating the inside of the continuous depolymerization apparatus 54, the heating from the outside by a heat-medium jacket is desirable. Further, the vacuum inside the apparatus is performed by the vacuum pump 132. The produced lactide evaporates from the liquid surface of the lactic acid oligomer as moisture, lactic acid, and a low molecular weight lactic acid oligomer, and reaches the partial reduction device 58 through the pipe 57. The pipe 57 is preferably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the adhesion of crude lactide. About the method of heating, the heating from the outside by a heat medium jacket is desirable. In the partial reduction device 58, impurities such as moisture, lactic acid, and low molecular weight lactic acid oligomer are condensed and roughly separated from the lactide vapor. The separated lactide vapor is supplied to the lactide condenser 60 through the pipe 59. The pipe 59 is preferably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the adhesion of the crude lactide. About the method of heating, the heating from the outside by a heat medium jacket is desirable. The impurity condensate passes through the valve 61 and is refluxed from the pipe 63 to the lactic acid condensing device 4 by the liquid feed pump 62. In addition, the place which recirculates must be the lactic acid condensation apparatus 4 like this Example, or the upstream of a plant rather than it like this example. In the lactide condenser 60, the crude lactide vapor is cooled to a temperature close to the melting point of lactide of 95 to 98 ° C., condensed and recovered. During continuous operation of the continuous depolymerization apparatus 54, the valve 64 is opened, and the crude lactide condensate is supplied to the second crude lactide recovery tank 67 through the pipe 66 by the pump 65. At that time, the valve 111 is closed. The optical purity of the crude lactide is about 90 to 95%. The exhaust gas after the crude lactide separation contains lactide, moisture, lactic acid and the like that cannot be separated, and reaches the impurity condenser 129 through the pipe 128. The pipe 128 is preferably heated to 50 ° C. or higher and 100 ° C. or lower in order to prevent adhesion of lactide, moisture, lactic acid, low molecular weight lactic acid oligomer, and the like. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. In the impurity condenser 129, impurities such as lactide, moisture, and lactic acid are condensed and separated from the exhaust gas, and are discharged from the pipe 133 by the vacuum pump 132 through the pressure reducing valve 130 and the pipe 131. The impurity condenser 129 considers the melting point of meso-lactide and keeps the temperature at 50 ° C. to 90 ° C. to condense the impurities. The condensate is discharged from the pipe 135 by opening the valve 134. During continuous operation of the continuous depolymerization apparatus 54, the valve 68 is opened, and the reaction liquid residue is continuously supplied from the continuous depolymerization apparatus 54 to the third-stage continuous depolymerization apparatus 71 through the pipe 70 by the liquid feed pump 69. The It is desirable to heat the inside of the pipe 70 at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent adhesion of the residual liquid. About the method of heating, the heating from the outside by a heat medium jacket is desirable.

  The continuous depolymerization apparatus 71 is a reaction apparatus to which a horizontal biaxial stirrer is applied. In addition to the residual liquid of the continuous depolymerization apparatus 54 that is continuously supplied from the pipe 70 (hereinafter also referred to as a lactic acid oligomer), a valve is provided as necessary. By opening 31, the catalyst transfer pump 72 continuously receives the catalyst in the catalyst tank 30 through the needle valve 127 and the pipe 73. The catalyst may be either liquid or solid, but in the case of a liquid catalyst, the catalyst transfer pump 72 is a liquid supply pump, and it is desirable to apply a back pressure as required by the needle valve 127. Since the lactic acid oligomer in this stage is the residual liquid of the continuous depolymerization units 29 and 54, the catalyst has already been added and mixed, and the addition of the catalyst here is not always necessary. The continuous depolymerization apparatus 71 is in a reduced pressure environment at a temperature of 180 ° C. or higher and 220 ° C. or lower. The pressure is preferably 10 torr or less. By contacting and mixing the lactic acid oligomer and the catalyst for a predetermined time in this environment, a depolymerization reaction of the lactic acid oligomer occurs to produce lactide. About the method of heating the inside of the continuous depolymerization apparatus 71, the heating from the outside by a heat-medium jacket is desirable. Further, the pressure reduction inside the apparatus is performed by a vacuum pump 89. The produced lactide evaporates from the liquid surface of the lactic acid oligomer as moisture, lactic acid, and a low molecular weight lactic acid oligomer, and reaches the partial reduction device 75 through the pipe 74. The pipe 74 is preferably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the adhesion of crude lactide. About the method of heating, the heating from the outside by a heat medium jacket is desirable. In the partial reduction device 75, impurities such as moisture, lactic acid, and low molecular weight lactic acid oligomer are condensed and roughly separated from the lactide vapor. The separated lactide vapor is supplied to the lactide condenser 77 through the pipe 76. The piping 76 is desirably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the adhesion of crude lactide. About the method of heating, the heating from the outside by a heat medium jacket is desirable. The condensed liquid of impurities passes through the valve 78 and is refluxed from the pipe 80 to the lactic acid condensing apparatus 4 by the liquid feed pump 79. In addition, the place which recirculates must be the lactic acid condensation apparatus 4 like this Example, or the upstream of a plant rather than it like this example. In the lactide condenser 77, the crude lactide vapor is cooled to a temperature close to the melting point of lactide of 95 to 98 ° C., condensed and recovered. During continuous operation of the continuous depolymerization apparatus 71, the valve 81 is opened, and the crude lactide condensate is supplied to the third crude lactide recovery tank 84 through the pipe 83 by the pump 82. At that time, the valve 112 is closed. The optical purity of the crude lactide is 90% or less. The exhaust gas after the crude lactide separation contains lactide, moisture, lactic acid and the like that cannot be separated, and reaches the impurity condenser 86 through the pipe 85. The pipe 85 is preferably heated to 50 ° C. or higher and 100 ° C. or lower in order to prevent adhesion of lactide, moisture, lactic acid and the like. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. In the impurity condenser 86, impurities such as lactide, moisture, and lactic acid are condensed and separated from the exhaust gas, and discharged from the pipe 90 by the vacuum pump 89 through the pressure reducing valve 87 and the pipe 88. The impurity condenser 86 considers the melting point of meso-lactide and keeps the temperature at 50 ° C. to 90 ° C. to condense the impurities. The condensate is released from the pipe 92 by opening the valve 91. During continuous operation of the continuous depolymerization apparatus 71, the valve 93 is opened, and the residue of the reaction solution is continuously discharged from the continuous depolymerization apparatus 71 through the pipe 95 by the liquid feed pump 94 to the residue collection tank 96. The pipe 95 is preferably heated at a temperature of 100 ° C. or higher and 200 ° C. or lower in order to prevent the residue liquid from adhering. About the method of heating, the heating from the outside by a heat medium jacket is desirable.

  The crude lactide recovered in the first to third crude lactide recovery tanks 44, 67, 84 is individually processed by the purifier 116. Hereinafter, although it demonstrates in the procedure refine | purified in order from the crude lactide of the upper stage side, it is not necessary to necessarily perform a refinement | purification process from the upper stage side. First, only the valve 110 is opened with the valves 111, 112, 104, 105, 106 closed, and the crude lactide in the first crude lactide recovery tank 44 is supplied to the purifier 116 through the pipes 114, 115 by the pump 113. To do. The purification device 116 may be either a batch type or a continuous type. In addition, it is desirable that the pipes 114 and 115 are heated to 100 ° C. or higher in order to prevent rough lactide adhesion. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. In the refiner 116, impurities such as water, lactic acid, and lactic acid oligomer and meso-lactide (L-lactic acid and a cyclic dimer of D-lactic acid that is an optical isomer thereof) are separated from the crude lactide and discharged through a pipe 118. The pipe 118 is desirably heated to 50 to 100 ° C. in order to prevent such adhesion. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. Emissions are both collected and reused and discarded. The purified lactide is discharged through the pipe 117. It is desirable that the pipe 117 is heated to 100 ° C. or higher in order to prevent the purified crude lactide from adhering. As a heating method, there is a method using an electric heater, a steam trace, a heat medium jacket, or the like, but any method may be used. Next, only the valve 111 is opened with the valves 110, 112, 104, 105, 106 closed, and the crude lactide in the second crude lactide recovery tank 67 is transferred to the purifier 116 through the pipes 120, 115 by the pump 119. Supply. Further, like the pipe 114, the pipe 120 is desirably heated to 100 ° C. or higher in order to prevent rough lactide adhesion. Similarly to the above, the purifier 116 separates impurities such as water, lactic acid, and lactic acid oligomer from the crude lactide and meso lactide and discharges them through the pipe 118. The purified lactide is discharged through the pipe 117. Then, with the valves 110, 111, 104, 105, 106 closed, only the valve 112 is opened, and the crude lactide in the third crude lactide recovery tank 84 is supplied to the purifier 116 through the pipes 122, 115 by the pump 121. To do. Further, like the pipe 114, the pipe 122 is desirably heated to 100 ° C. or higher in order to prevent rough lactide adhesion. Similarly to the above, the purifier 116 separates impurities such as water, lactic acid, and lactic acid oligomer from the crude lactide and meso lactide and discharges them through the pipe 118. Purified lactide (LL-lactide, purity 99% or more) is discharged through the pipe 117. The purified lactide synthesized as described above becomes a raw material for polylactic acid synthesis by ring-opening polymerization.

  Purified lactide was synthesized by the apparatus shown in Example 1. As a result, the yield of purified lactide was about 70% based on the number of moles of lactic acid. In addition, the polylactic acid ring-opening polymerized using this lactide had a weight average molecular weight of about 200,000 and a coloration of 2.5 in terms of b value.

(Comparative example)
Using the same lactic acid oligomer as in Example 1, batch depolymerization was performed in a conventional tank reactor. As a result, the yield of purified lactide was about half that of Example 1 on the basis of the number of moles of lactic acid.

  From the above, it is possible to obtain a high-quality and high-yield cyclic condensate such as lactide with little thermal decomposition including optical isomerization by the method of the present invention, and through this, a polymer such as high-quality polylactic acid. Was obtained with high efficiency.

It is a figure which shows the whole process of the polymer synthesizer using this invention. It is a figure which shows one Embodiment of the multistage continuous depolymerization apparatus of this invention.

Explanation of symbols

1, 124, 125, 126, 127 ... Needle valves 2, 21, 27, 39, 42, 52, 62, 65, 69, 79, 82, 94, 113 ... Liquid feed pumps 3, 6, 7 9, 11, 13, 15, 16, 19, 22, 25, 28, 33, 34, 36, 40, 43, 45, 48, 50, 53, 56, 57, 59, 63, 66, 70, 73 74, 76, 80, 83, 85, 88, 90, 93, 95, 114, 115, 117, 118, 120, 122, 128, 131, 133, 135, 137 ... Piping 4 ... Lactic acid condensation Device 5, 18, 20, 24, 26, 31, 38, 41, 51, 61, 64, 68, 78, 81, 91, 93, 104, 105, 106, 110, 111, 112, 123, 134, 136 ... Valve 8 ... Refluxer 0 ... Condensers 12, 47, 87, 130 ... Pressure reducing valves 14, 49, 89, 132 ... Vacuum pump 17 ... Condensed water recovery tank 23 ... Lactic acid oligomer supply tanks 29, 54, 71 ... Continuous depolymerization device 30 ... Catalyst tanks 32, 55, 72 ... Catalyst transfer pumps 35, 58, 75 ... Reduction devices 37, 60, 77 ... Lactide condensers 44, 67, 84 ... Rough lactide recovery tanks 46, 86, 129 ... Impurity condenser 96 ... Residue recovery tank 116 ... Purification equipment

Claims (8)

A method for producing lactide by depolymerizing a lactic acid oligomer,
The method as described above, wherein the depolymerization reaction is performed in a multistage continuous reaction, impurities are separated from the crude lactide vapor generated in each stage, and the crude lactide after the impurity separation is purified separately for each stage.   The method according to claim 1, wherein a horizontal twin-screw stirrer is used in the second and subsequent stages in the multistage continuous reaction.   The method according to claim 1 or 2, wherein a centrifugal thin film evaporator is used in at least one stage of the multistage continuous reaction, and a horizontal twin-screw stirrer is used in the subsequent stage. A method for producing polylactic acid from lactic acid,
A lactic acid condensation step of condensing lactic acid to synthesize a lactic acid oligomer,
The depolymerization process which manufactures a lactide by depolymerizing the obtained lactic acid oligomer by the method of any one of Claims 1-3, and the ring-opening polymerization process which synthesize | combines polylactic acid by ring-opening polymerization of lactide Wherein the impurities separated from the crude lactide vapor generated in the depolymerization step are refluxed to the lactic acid condensation step or an upstream step. A lactide production apparatus for synthesizing lactide by depolymerization of a lactic acid oligomer,
Multi-stage continuous depolymerization equipment,
A partial reduction apparatus for separating impurities from the crude lactide vapor generated in each stage of a multistage continuous depolymerization apparatus;
A tank for recovering the crude lactide after the impurity separation, and a purification device for purifying the crude lactide after the impurity separation, wherein the fractionation device and the tank are at least one downstream of each stage of the multistage continuous depolymerization apparatus. The devices are installed one by one.   6. The apparatus according to claim 5, wherein the multistage continuous depolymerization apparatus includes a horizontal twin-screw stirrer in the second and subsequent stages.   The apparatus according to claim 5 or 6, wherein the multi-stage continuous depolymerization apparatus includes a centrifugal thin film evaporator, and further includes a horizontal twin-screw stirrer. A polylactic acid production apparatus for synthesizing polylactic acid from lactic acid,
A lactic acid condensing apparatus for synthesizing lactic acid oligomers by condensing lactic acid,
The lactide production apparatus according to any one of claims 5 to 7, for synthesizing lactide by depolymerization of a lactic acid oligomer,
The apparatus comprising: a ring-opening polymerization apparatus that synthesizes polylactic acid by ring-opening polymerization of lactide; and a system for refluxing impurities separated by a partial condensation apparatus of the lactide production apparatus to a lactic acid condensation apparatus or an upstream apparatus. .

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