Apparatus and efficient method for producing polylactic acid using lactide obtained from devolatilization of polylactic acid

Abstract

A process for producing polylactic acid comprising the steps of: a) providing a crude lactide composition comprising meso-lactide and at least one of L-lactide and D-lactide, b) isolating a meso-lactide enriched composition and a meso-lactide depleted composition from the crude lactide composition, wherein the meso-lactide-enriched composition comprises at least 80 mol% of meso-lactide, based on the total content of lactide, c) polymerizing a polymeric composition comprising meso-lactide and at least one of L-lactide and D-lactide into a crude polylactic acid composition and devolatilizing the crude polylactic acid composition, d) subjecting at least a portion of the meso-lactide-enriched composition and the composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, and d) subjecting at least a portion of the meso-lactide-enriched composition and the composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, e) subjecting the meso-lactide-depleted composition or a mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide to purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, wherein the polymeric composition comprises at least a portion of the purified meso-lactide enriched composition produced in step d).

Description

[0001] The present invention relates to a method and a device for producing polylactic acid from lactide, preferably a method and a device for continuously producing polylactic acid from lactide.

[0002] The polymer of lactic acid is particularly interesting because they can be obtained by renewable resources and are mostly compostable and / or biodegradable. In addition, the technical properties and physicochemical properties of these polymers are very close to the properties of those polymers derived from fossil-based resources, which explains why these polymers are regarded as the latter's highly promising substitutes. In addition, the polymer of lactic acid has a wide range of applications. For example, polylactic acid is used for surgical implants, films (for example packaging), fibers (for example clothing), sanitary products, carpets and disposable plastic products (for example disposable tableware or containers) in biomedical fields. In addition, polylactic acid has been widely used in composite materials (for example fiber-reinforced plastics).

[0003] Generally, there are two main alternative methods known for synthesizing polylactic acid. The first main method is to directly polycondense lactic acid into polylactic acid. However, this main method can only produce low molecular weight polymers and is therefore limited to specific polymers.

[0004] The second main method known for synthesizing polylactic acid is the ring-opening polymerization of lactide, which is a cyclic diester of lactic acid. This is the preferred method for the current industrial production of polylactic acid. Lactide can be produced by the condensation of two lactic acid molecules. Alternatively, lactide can be produced by first prepolymerizing lactic acid and then subjecting the oligomers or prepolymers to a depolymerization reaction, respectively. For example, lactide is prepared by fermenting carbohydrates from biomass (e.g., starch, sugar, or corn) to produce lactic acid, then prepolymerizing lactic acid, and then subjecting the oligomers to a depolymerization reaction. The depolymerization reaction of lactic acid oligomers to lactide is an equilibrium reaction. In order to drive the reaction in the direction of lactide, lactide must be removed from the system. In a continuous depolymerization, wherein lactic acid oligomers are continuously fed to a reactor, the crude lactide is evaporated under vacuum. The synthesis reactor is preferably a distillation column, wherein the synthesis occurs primarily in the sump of the distillation column, but also in other sections where the lactide composition and lactic acid oligomers are in an unbalanced state. The depolymerization reactor produces a crude lactide stream comprising lactide and lactic acid, lactic acid oligomers and other impurities. The crude lactide stream is usually purified, and then the purified lactide is polymerized to form a crude polylactic acid composition, such as a crude high molecular weight polylactic acid composition, in the presence of a catalyst and an optional initiator. After polymerization, unreacted lactide must be removed from the crude polylactic acid composition to a final concentration less than at least 0.5% by weight to obtain a product of saleable quality. The removal of unreacted lactide and other low-boiling components of the crude polylactic acid composition is usually completed by devolatilization. More specifically, the crude polylactic acid composition is subjected to one or more devolatilization steps, which are carried out at elevated temperatures and reduced pressures. In the devolatilization process, unreacted lactide and other low-boiling and near-boiling components of lactide, components formed as polymerization by-products in the polylactic acid reactor, are removed from the polylactic acid as a vapor fraction, thereby obtaining a vapor composition containing unreacted lactide and a liquid fraction containing a purified polylactic acid melt. The vapor composition containing unreacted lactide is then condensed and can be recycled to the polymerization reaction.

[0005] Lactic acid is chiral and exists in the form of the enantiomers L-lactic acid and D-lactic acid (also known as S-lactic acid and R-lactic acid, respectively). Therefore, the dimer of lactic acid, lactide, exists in the form of three stereoisomers, namely L-lactide (or S,S-lactide, respectively), D-lactide (or R,R-lactide, respectively) and meso-lactide (or S,R-lactide, respectively). The properties of polylactic acid, such as the processing properties, crystallization properties and degradation behavior of polylactic acid, depend on the structure and composition of the polymer chain, in particular the ratio of the L-stereoisomer to the D-stereoisomer of lactic acid. The stereochemical structure of polylactic acid can be changed by copolymerization of a mixture of L-lactide, meso-lactide and D-lactide, resulting in amorphous or semi-crystalline polymers of high molecular weight with a melting point in the range of 130°C to 185°C. In addition, homopolymers consisting only of L-lactic acid or D-lactic acid monomers can be produced. For example, isotactic polylactic acid homopolymers containing only L-lactic acid monomers are semi-crystalline materials with the highest melting point, while polylactic acid copolymers containing L-lactic acid and D-lactic acid monomers with a relatively high D-stereoisomer content exhibit lower melting points and slower crystallization behavior. Therefore, a method is needed that can easily and reliably produce polylactic acid with a predetermined stereoisomeric structure from lactide.

[0006] In view of this, the object of the present invention is to provide a method for producing polylactic acid, which first produces ultrapure lactide with high stereochemical purity, i.e., a composition of meso-lactide and a composition of L-lactide and / or D-lactide, and then polymerizes one or both of these lactide compositions into polylactic acid having a predetermined ratio of two lactic acid stereoisomers. In addition, the method should be flexible so that the ratio of the three lactide stereoisomers fed to the polymerization reactor can be easily changed, so that, for example, the production of a first specific polylactic acid having a first ratio of D-lactide and L-lactide can be quickly and easily converted to the production of a second specific polylactic acid having a second ratio of D-lactide and L-lactide. Finally, the method should be economical, i.e., have a high yield and low operating cost per unit of polylactic acid produced.

[0007] According to the present invention, this object is met by providing a method for producing polylactic acid, the method comprising the following steps:

[0008] a) providing a crude lactide composition comprising meso-lactide and at least one of L-lactide and D-lactide,

[0009] b) separating a meso-lactide-enriched composition and a meso-lactide-depleted composition from the crude lactide composition, wherein the meso-lactide-enriched composition comprises at least 80 mol % (or 80 wt %) meso-lactide, based on the total lactide content,

[0010] c) polymerizing a polymeric composition comprising meso-lactide and at least one of L-lactide and D-lactide into a crude polylactic acid composition, and devolatilizing the crude polylactic acid composition to produce a purified polylactic acid composition and a composition comprising unreacted lactide,

[0011] d) subjecting the meso-lactide-enriched composition and at least a portion of the composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition,

[0012] e) subjecting the composition depleted in meso-lactide or a mixture of the composition depleted in meso-lactide and a portion of the composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified composition depleted in meso-lactide,

[0013] wherein the polymerisation composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d).

[0014] By the aforementioned process steps, not only can polylactic acid with high stereochemical purity be produced, which has a predetermined ratio of lactic acid stereoisomers, but the process is also flexible so that the ratio of the three lactide stereoisomers fed to the polymerization reactor can be easily changed, so that, for example, the production of a first specific polylactic acid with a first ratio of D-lactide and L-lactide can be quickly and easily converted to the production of a second specific polylactic acid with a second ratio of D-lactide and L-lactide. Therefore, it is particularly advantageous that at least a portion of the unreacted lactide obtained after devolatilization of the crude polylactic acid composition, i.e., the composition comprising unreacted lactide, is (re)used as purified monomer for polymerization in the process. That is, the composition containing unreacted lactide obtained after devolatilization of the crude polylactic acid composition has a relatively high total content of L-lactide and D-lactide and a relatively low content of meso-lactide, so that the ratio of the three lactide stereoisomers in the polymerized composition can be reliably adjusted as needed by adjusting the amount of the composition containing unreacted lactide fed to the polymerization step c) relative to the amount of the meso-lactide-enriched composition and optionally relative to the amount of the meso-lactide-depleted composition. In addition, by simply changing the amount of the composition containing unreacted lactide fed to the polymerization step c) relative to the amount of the meso-lactide-enriched composition and optionally relative to the amount of the meso-lactide-depleted composition, the ratio of the three lactide stereoisomers fed to the polymerization reactor can be quickly and easily changed. A further advantage of the process according to the invention is that, due to the crystallization step carried out during the process, the lactide stereoisomers fed to the polymerization reactor are ultrapure, i.e. the lactide is at least substantially free of hydroxyl (including hydroxycarboxylic acid) impurities, such as water, lactic acid and other impurities, since these impurities would prevent the desired molecular weight from being achieved. With low contents of water, lactic acid and other impurities, very pure polylactic acid with a predetermined stereoisomeric purity can be reliably produced. Preferably, the acid content of the lactide is, for example, less than 20 milliequivalents / kilogram (meq / kg), more preferably not more than 15 meq / kg, even more preferably not more than 10 meq / kg, still more preferably not more than 5 meq / kg, and most preferably less than 2 meq / kg. In particular, the lactide stereoisomers fed to the polymerization reactor as a polymeric composition are substantially free of by-products formed during the separation step b) and during the polymerization due to the relatively high temperatures during these steps, such as 4-hydroxy-5-methyl-4-cyclopentene-1,3-dione (HMCP). This is an important advantage, since HMCP, which is formed at higher temperatures during distillation and polymerization, is an impurity that causes yellowing of polylactic acid and is therefore a particularly critical impurity for polylactic acid. Since HMCP has only a slightly higher volatility relative to meso-lactide, it cannot be separated from meso-lactide during the distillation process.However, the method according to the invention further reliably removes the formed HCMP and other impurities having a boiling point close to the boiling point of meso-lactide from the meso-lactide-enriched composition during the crystallization step d). In addition, the crystallization step e) ensures that the meso-lactide-depleted composition has a particularly high purity and contains, if any, trace amounts of impurities. For example, the meso-lactide-depleted composition can have a low free acid content, for example a free acid content of less than 2 meq / kg. Such a low free acid content of less than 2 meq / kg can be achieved for the polymeric composition even if the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d) and at least a portion of the purified meso-lactide-depleted composition produced in step e). Thus, the lactide composition produced, in particular the purified meso-lactide-enriched composition produced, has a very high thermal stability, which allows the part of the lactide composition not required for the polymerization step to be stored as a liquid or solid material (i.e., the part of the purified meso-lactide-enriched composition and the part of the purified meso-lactide-depleted composition) for several months without deterioration. Finally, the process according to the invention is particularly economical, since it is characterized by a high yield and low operating costs per unit of polylactic acid produced (due to the (re)utilization of the composition comprising unreacted lactide obtained after devolatilization of the crude polylactic acid composition).

[0015] According to the present invention, a meso-lactide-enriched composition refers to a corresponding composition produced during separation step b) having a higher meso-lactide content than the crude lactide composition fed to separation step b). According to the present invention, a meso-lactide-enriched composition comprises at least 80 mol% (or 80 wt%) of meso-lactide based on the total content of lactide. Similarly, according to the present invention, a meso-lactide-depleted composition refers to a corresponding composition produced during separation step b) having a lower meso-lactide content than the crude lactide composition fed to separation step b). A purified meso-lactide-enriched composition refers to a composition obtained in step d), which comprises at least a portion of a meso-lactide-enriched composition that has undergone at least one crystallization step and at least a portion of a composition comprising unreacted lactide obtained by devolatilizing a crude polylactic acid composition that has undergone at least one crystallization step.

[0016] According to the present invention, in step d), at least a portion of the meso-lactide-enriched composition produced in step b) and the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to purification including at least one crystallization step. As an embodiment, this includes mixing the meso-lactide-enriched composition produced in step b) and at least a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition, and then feeding the mixture to at least one crystallization step of step d). However, in an alternative embodiment, the meso-lactide-enriched composition produced in step b) can be subjected to at least one crystallization step of step d), and separately therefrom, at least a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step of step d), and then at least a portion of the two purified streams are mixed with each other to form a purified meso-lactide-enriched mixture. In the latter embodiment, the meso-lactide-enriched composition produced in step b) and at least a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition can be fed separately from each other to the same crystallization step, where the purified meso-lactide-enriched mixture is purified and taken out from the crystallization step. Alternatively, the meso-lactide-enriched composition produced in step b) and at least a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition can be fed separately from each other to different crystallization steps, for example, the meso-lactide-enriched composition produced in step b) is fed to the first crystallization step, and at least a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is fed to the second crystallization step. In this variant, the crude polylactic acid composition is purified in a first crystallization step and removed from the first crystallization step, while the unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is purified in a second crystallization step and removed from the second crystallization step, and then the two purified streams are mixed to a purified meso-lactide-enriched mixture. In other words, "subjecting a meso-lactide-enriched composition and at least a portion of a composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition" means that at the end of step d), a purified composition (expressed as a purified meso-lactide-enriched mixture) is obtained, which comprises at least a portion of a meso-lactide-enriched composition that has been purified by crystallization and at least a portion of an unreacted lactide composition obtained in a devolatilization step that has been purified by crystallization.

[0017] In step e), the meso-lactide-depleted composition produced in step b) or a mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide obtained in the devolatilization step of crude polylactic acid is subjected to a purification comprising at least one crystallization step.

[0018] The method according to the invention encompasses all four possible combinations of the aforementioned embodiments, namely i) in step d), the meso-lactide-enriched composition and, separately therefrom, at least a portion of the composition comprising unreacted lactide obtained in the devolatilization step are subjected to respective independent crystallization steps, and the two compositions are then mixed with one another to a purified meso-lactide-enriched composition, and in step e), the meso-lactide-depleted composition (i.e., without being mixed with a portion of the composition comprising unreacted lactide obtained in the devolatilization step) is subjected to at least one crystallization step, ii) in step d), the mixture of the meso-lactide-enriched composition and at least a portion of the composition comprising unreacted lactide obtained in the devolatilization step of crude polylactic acid is subjected to at least one crystallization step, and in step e), the meso-lactide-depleted composition (i.e., without being mixed with a portion of the composition comprising unreacted lactide obtained in the devolatilization step) is subjected to at least one crystallization step. a crystallization step, iii) in step d), the meso-lactide-enriched composition and, separately therefrom, at least a portion of the composition comprising unreacted lactide obtained in the devolatilization step are subjected to respective independent crystallization steps, and the two compositions are then mixed with each other to a purified meso-lactide-enriched composition, and in step e), the mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide obtained in the devolatilization step of a crude polylactic acid composition is subjected to at least one crystallization step, and iv) in step d), the mixture of the meso-lactide-enriched composition and at least a portion of the composition comprising unreacted lactide obtained in the devolatilization step of a crude polylactic acid composition is subjected to at least one crystallization step, and in step e), the mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide obtained in the devolatilization step of a crude polylactic acid composition is subjected to at least one crystallization step. Regardless of whether a portion of the composition comprising unreacted lactide obtained in the devolatilization step is added to the meso-lactide-depleted composition before purification by crystallization in step e), the respectively purified or crystallized composition is referred to herein as purified meso-lactide-depleted composition.

[0019] For all the above-mentioned embodiments and for all the subsequently mentioned embodiments, it is preferred that, if two or more devolatilization steps are carried out with the crude polylactic acid composition, the composition comprising unreacted lactide obtained in the devolatilization step is preferably the corresponding composition obtained in the first devolatilization step.

[0020] According to the present invention, the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d). This means that the polymeric composition must comprise at least a portion of the purified meso-lactide-enriched composition produced in step d), and in addition, it may or may not comprise at least a portion of the purified meso-lactide-depleted composition produced in step e). Preferably, the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d) and at least a portion of the purified meso-lactide-depleted composition produced in step e).

[0021] Good results are particularly obtained when the polymerizable composition comprises from greater than 0% to 25% by weight of the purified meso-lactide-enriched composition produced in step d) and from 75% to less than 100% by weight of the purified meso-lactide-depleted composition produced in step e). More preferably, the polymerizable composition comprises from 0% to 20% by weight of the purified meso-lactide-enriched composition produced in step d) and from 0% to 100% by weight of the purified meso-lactide-depleted composition produced in step e).

[0022] It is preferred that the ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerizing composition is from 10:1 to 1:10 by weight, such as from 5:1 to 1:1 by weight.

[0023] In a further development of the concept according to the invention, it is proposed that in step d) 10 to 100 wt. %, preferably 50 to 100 wt. %, of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step.

[0024] Preferably, in step d), the weight ratio of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition subjected to at least one crystallization step is 0 wt % to 90 wt %, preferably 30 wt % to 80 wt %, based on the sum of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition and the meso-lactide-enriched composition subjected to at least one crystallization step.

[0025] According to a further preferred embodiment of the present invention, in step e), 0 to 90 wt. %, preferably 50 to 90 wt. %, of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step.

[0026] If a portion of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is added to the meso-lactide-depleted composition in step e) before purification, it is preferred that the weight ratio of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition subjected to at least one crystallization step is 0% to 50% by weight, preferably 0 to 20% by weight, based on the sum of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition and the meso-lactide-depleted composition subjected to at least one crystallization step.

[0027] Furthermore, according to the present invention, a polymerization composition is used in step c), which comprises at least a portion of the purified meso-lactide-enriched composition produced in step d) and optionally at least a portion of the purified meso-lactide-depleted composition produced in step e). Thus, during the polymerization, only all or a portion of the purified meso-lactide-enriched composition produced in step d) may be polymerized, or a mixture of first all or a portion of the purified meso-lactide-enriched composition produced in step d) and secondly all or a portion of the purified meso-lactide-depleted composition produced in step e) may be polymerized. Furthermore, even though it is less preferred, this also includes the following embodiments: a mixture of all or part of the purified meso-lactide-enriched composition produced in step d) (which contains lactide from the composition containing unreacted lactide obtained in the devolatilization step, as described above) and a part of the composition containing unreacted lactide obtained in the devolatilization step (which may or may not be purified before polymerization and which is added to the already purified meso-lactide-enriched composition, i.e. after purification including at least one crystallization step) is optionally polymerized in step c) with all or part of the purified meso-lactide-depleted composition produced in step d) (which may or may not contain lactide from the composition containing unreacted lactide obtained in the devolatilization step, as described above). In other words, a part of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition can also be added to the polymerized composition in an unpurified form, even though this is less preferred. The polymerized composition can also contain lactide from further sources, even though it is less preferred. However, it is preferred that the polymerization composition used in step c) comprises at least 60 wt%, preferably at least 80 wt%, more preferably at least 90 wt%, even more preferably at least 95 wt%, still more preferably at least 98 wt%, still more preferably at least 99 wt%, most preferably at least 100 wt% of lactide from the purified meso-lactide-enriched composition and the purified meso-lactide-depleted composition, based on the total weight of lactide contained in the polymerization composition. In addition to lactide, the polymerization composition may also comprise, and indeed preferably does comprise, non-lactide components that are helpful or necessary for the polymerization reaction, such as one or more catalysts, one or more initiators, etc.

[0028] As described above, the polymer composition used in step c) preferably comprises at least one catalyst for the ring-opening reaction of lactide to produce polylactic acid. In principle, the present invention has no particular restrictions on the chemical nature of the catalyst used. If the catalyst is at least one organometallic compound, good results are particularly obtained. When the catalyst is at least one organometallic compound, the organometallic compound contains a metal selected from magnesium, titanium, zinc, aluminum, indium, yttrium, tin, lead, antimony, bismuth and any combination of two or more of the aforementioned metals, good results are particularly obtained. The at least one organometallic compound preferably contains a residue selected from alkyl, aryl, halide, oxide, alkanoate, alkoxide and any combination of two or more of the aforementioned groups as an organic residue. More preferably, the catalyst is at least one organometallic compound comprising aluminum and / or tin as a metal. Still more preferably, the catalyst is at least one organometallic compound selected from the following: tinoctoate, tetraphenyltin, trimethoxybutyltin, dibutyltin oxide, aluminum isopropoxide, Al(Oi-Pr) p , where 1≤p≤3, Et3-pAl(O(CH2)2X) p , wherein 1≤p≤3, α, β, γ, δ, ε tetraphenylporphyrin aluminum (TPPIAIX) and any combination of two or more of the foregoing compounds. Particularly suitable catalysts are stannous octoates, such as tin(II) 2-ethylhexanoate.

[0029] Preferably, the catalyst content in the polymeric composition is from 10 to 10,000 ppm, more preferably from 50 to 1,000 ppm.

[0030] According to a further particularly preferred embodiment of the present invention, the polymerization composition used in step c) comprises at least one initiator, more preferably together with the aforementioned at least one catalyst. The at least one initiator is preferably a hydroxyl compound, more preferably, a hydroxyl compound selected from the following: monohydroxyl compounds, dihydroxyl compounds, trihydroxyl compounds, tetrahydroxyl compounds and any combination of two or more of the aforementioned compounds. By means of the functionality of the at least one hydroxyl compound, the design of the resulting copolymer can be adjusted. If a monohydroxyl compound is used, a linear copolymer will be produced, while by using one or more dihydroxyl compounds, trihydroxyl compounds and / or tetrahydroxyl compounds, a branched copolymer can be produced. When the at least one initiator is selected from 2-ethylhexanol, 1-decanol, C 10 -C 20- monohydroxy aliphatic alcohols, benzyl alcohol, p-phenylbenzyl alcohol, ethylene glycol, propylene glycol, 1,4-butanediol, poly(ethylene glycol) having a weight average molecular weight of 200 to 10,000 g / mol, 2-hydroxymethyl-1,3-propane, glycerol, polyglycerol having a weight average molecular weight of 100 to 1,000 g / mol, trihydroxybenzene (phloroglucinol), trimethylolpropane and its dimer, pentaerythritol and its dimer, and any combination of two or more of the foregoing compounds, particularly good results are obtained.

[0031] Preferably, the initiator content in the polymeric composition is 0.1 to 50 meq or less and 0.1 to 50 mmol / kg, respectively, more preferably 0.5 to 40 meq or mmol / kg, respectively.

[0032] According to the present invention, the purification for producing a purified meso-lactide-enriched composition carried out in step d) comprises at least one crystallization step, which may include 1 to 10 crystallization stages, such as one crystallization stage, or 2 to 10, preferably 2 to 4 crystallization stages. In principle, the present invention is not particularly limited in terms of the type of crystallization used in step d). Therefore, the crystallization step may be any melt crystallization step, such as a static crystallization step or a dynamic crystallization step, such as a falling film crystallization step or a suspension crystallization step. The term "melt crystallization" is known to those skilled in the art and is described in detail, for example, in GF Arkenbout, Melt Crystallization Technology, Lancaster / Pa., Technomic Publ. Co., 1995. In the context of the present invention, this should be understood to refer to crystallization carried out by a melt, i.e., using an already melted starting material without adding other components, such as a solvent. Melt crystallization can be carried out in the form of layer crystallization or in the form of suspension crystallization. In order to carry out layer crystallization, a cooled surface is usually introduced into a melt of optically active or optically inactive lactide used as a starting material. After this, a crystal layer of enantiomerically enriched or unenriched lactide crystals is formed on the cooled surface, which can then be separated from the remaining melt. The thus obtained crystallized enriched lactide can be melted again. Subsequently, this operation can be repeated as desired to improve purity and yield. However, when the purification carried out in step d) includes at least one static crystallization step, good results are particularly obtained. Static crystallization has the advantage that it economically purifies compounds with a relatively high content in the crystallization liquid. During static crystallization, the liquid phase does not move, so crystals form and grow in the static liquid phase. More specifically, a typical static crystallizer includes a plurality of walls, such as plates, or tubes or finned tubes, which can be cooled and heated by circulating a heat transfer medium inside the plate. At the beginning, the static crystallizer is filled with a liquid feed mixture, i.e., a mixture of a composition enriched with meso-lactide or a composition enriched with meso-lactide and a part of a composition comprising unreacted lactide, so that the plate contacts the liquid feed mixture. Then, the plate of the static crystallization container is cooled to a temperature lower than the equilibrium solidification temperature of the liquid feed mixture, so that the crystals enriched in meso-lactide are formed and deposited on the cooled outer surface of the plate. After the crystallization is complete, the liquid mixture is completely removed from the static crystallization container, the cooling plate is stopped, and the plate is optionally heated so that the crystal layer formed on the outer surface of the plate is melted, and then the melt is removed from the crystallization container to obtain a purified meso-lactide-enriched composition. In order to improve the purity of meso-lactide, the crystal layer can be sweated by slowly heating the crystal layer to a temperature close to the melting temperature of meso-lactide, so that they are partially melted before the crystals are melted.

[0033] As described above, the purification carried out in step d) to produce a purified meso-lactide-enriched composition comprises at least one crystallization step, which may include 1 to 10 crystallization stages, such as one crystallization stage, or 2 to 10 crystallization stages, preferably 2 to 4 crystallization stages, which are most preferably static crystallization stages. If the meso-lactide-enriched composition produced in step b) and the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition are purified by crystallization and preferably static crystallization, respectively, and then at least part of the two purified streams are mixed to the purified meso-lactide-enriched composition, then step d) comprises two or more, preferably 2 to 10, more preferably 2 to 4 crystallization stages, which are most preferably static crystallization stages.

[0034] Typically, in separation step b), the meso-lactide-enriched composition is obtained as an overhead vapour composition of a distillation column. If so, the meso-lactide-enriched composition is preferably condensed before being fed as a liquid composition to at least one crystallization step.

[0035] Furthermore, the present invention has no particular restrictions as to the type of crystallization used in step e). More specifically, the crystallization step may be any melt crystallization step, such as a static crystallization step or a dynamic crystallization step, such as a falling film crystallization step or a suspension crystallization step. However, particularly good results are obtained when the purification carried out in step e) comprises at least one dynamic crystallization step, more preferably at least one falling film crystallization step. In contrast to static crystallization, the liquid phase is moved or agitated during dynamic crystallization. Dynamic crystallization, in particular falling film crystallization, has the advantage that it economically purifies compounds whose content in the crystallization liquid is relatively low.

[0036] Preferably, the at least one dynamic crystallization step preferably comprises one to four dynamic crystallization stages, more preferably one dynamic crystallization stage. Particularly preferably, the at least one dynamic crystallization step comprises one to four falling film crystallization stages, most preferably one falling film crystallization stage.

[0037] Furthermore, the present invention is not particularly limited as to the type of separation technology used in step b). Preferably, step b) comprises one or more distillation steps, wherein each of these distillation steps is carried out in a distillation column, wherein the crude lactide composition is fed to the distillation column of the first distillation step (if more than one distillation step is included), or to the distillation column of one distillation step (if only one distillation step is included). The meso-lactide-enriched composition is produced as an overhead fraction, while the meso-lactide-depleted composition is produced as a bottom fraction or side fraction of the distillation column of the last distillation step (if more than one distillation step is included), or the distillation column of one distillation step (if only one distillation step is included).

[0038] According to a preferred embodiment of the invention, step b) comprises only one distillation step, i.e. is carried out with only one distillation column. The crude lactide composition is fed to a distillation column, wherein a composition enriched in meso-lactide is taken out of the distillation column as an overhead fraction, and a composition depleted in meso-lactide is taken out of the distillation column as a bottom fraction. This embodiment is particularly suitable if the crude lactide composition contains relatively low amounts of oligomers (e.g. lactic acid oligomers), for example if the crude lactide composition contains at most 3% by weight of oligomers or preferably at most 1% by weight of oligomers.

[0039] In a further development of the concept of the invention, it is proposed that step b) comprises two distillation steps, i.e. is carried out with two consecutive distillation columns, wherein each distillation step is carried out in one distillation column, wherein the crude lactide composition is fed to a first distillation column, wherein the crude lactide composition is preferably separated into an overhead fraction comprising light compounds, a bottom fraction comprising heavy compounds and a side fraction comprising meso-lactide. The meso-lactide-rich side fraction of the first distillation column is fed to a second distillation column, wherein the meso-lactide-rich fraction is further purified, so that a meso-lactide-enriched composition is produced as an overhead fraction, and a meso-lactide-depleted composition is produced as a side fraction or a bottom fraction. Preferably, the meso-lactide-enriched side fraction of the first distillation column is fed to a second distillation column, wherein a meso-lactide-enriched composition is produced as an overhead fraction, and a meso-lactide-depleted composition is produced as a side fraction or a bottom fraction. This embodiment is particularly suitable if the crude lactide composition comprises substantial amounts of oligomers (eg lactic acid oligomers), for example if the crude lactide composition comprises at least 10 wt% oligomers or preferably at least 3 wt% oligomers.

[0040] Preferably, a portion of the purified meso-lactide-enriched composition produced in step d) is recycled as a side stream to the last distillation column if more than one distillation column is present, or to the distillation column if only one distillation column is present.

[0041] In step a), a crude lactide composition comprising meso-lactide and at least one of L-lactide and D-lactide is provided. Preferably, the content of meso-lactide in the crude lactide composition is 1 to 20 wt %, preferably 3 to 10 wt % of meso-lactide, based on the total content of lactide, with the remainder to 100 wt % being L-lactide and D-lactide.

[0042] As described above, in step b), a meso-lactide-enriched composition is produced, which contains at least 80 mol% (or 80% by weight) of meso-lactide based on the total content of lactide. Likewise, according to the present invention, a meso-lactide-enriched composition refers to a corresponding composition produced during separation step b) having a higher meso-lactide content than the crude lactide composition fed to separation step b). In other words, if the crude lactide composition fed to separation step b) contains 15% by weight of meso-lactide based on the total content of lactide, the meso-lactide-enriched composition contains more than 15% by weight, but at least 80% by weight of meso-lactide based on the total content of lactide. Good results are particularly obtained when the meso-lactide-enriched composition contains more than 80% by weight, more preferably more than 80% by weight to 99% by weight, and most preferably more than 80% by weight to 95% by weight of meso-lactide. Furthermore, the meso-lactide-enriched composition preferably contains less than 20 wt% of the sum of L-lactide and D-lactide.

[0043] Similarly, according to the present invention, a composition depleted in meso-lactide means that the corresponding composition produced during separation step b) has a lower meso-lactide content than the crude lactide composition fed to separation step b). In other words, if the crude lactide composition fed to separation step b) contains 15% by weight of meso-lactide, based on the total lactide content, the composition depleted in meso-lactide contains less than 15% by weight of meso-lactide, based on the total lactide content. Furthermore, it is preferred that the composition depleted in meso-lactide produced in step b) contains more than 85% by weight, more preferably more than 90% by weight of L-lactide, less than 2% by weight, more preferably less than 0.5% by weight of D-lactide and less than 12% by weight, more preferably less than 5% by weight of meso-lactide, based on the total lactide content.

[0044] In the further development of the concept of the present invention, it is proposed that step a) comprises prepolymerized lactic acid to produce lactic acid oligomers, and then the lactic acid oligomers are subjected to depolymerization reaction to obtain the step of crude lactide composition. For example, the crude lactic acid composition subjected to prepolymerization comprises lactic acid greater than 70 wt %, more preferably greater than 80 wt %. More specifically, it is preferred that the lactic acid portion (i.e., water-free and other impurities) of the crude lactic acid composition comprises L-lactic acid greater than 97 wt % and D-lactic acid of 0.1 wt % to 3 wt %, wherein the total content of L-lactic acid and D-lactic acid of the lactic acid portion of the crude lactic acid composition is preferably 97 wt % to 99.9 wt %. The catalyst suitable for depolymerization reaction is a tin compound, such as SnO, SnCl2, Bu2SnO, Sn(Oc)2, and any combination of two or more of Sb2O3, H2SO4, ZnO and the aforementioned catalyst. The prepolymerization is preferably carried out at a temperature of 150 to 200° C., while the depolymerization is preferably carried out at a temperature of 190 to 240° C. and at a pressure of less than 10 mbar.

[0045] According to the particularly preferred embodiment of the present patent application, it is preferred that the number average molecular weight of the polylactic acid produced in the polymerization process is at least 5,000g / mol, preferably at least 15,000g / mol, more preferably at least 25,000g / mol, still more preferably at least 35,000g / mol, most preferably at least 45,000g / mol. The upper limit of the number average molecular weight of the polylactic acid produced in the ring-opening polymerization process is preferably 110,000g / mol, but can also be 90,000g / mol or 80,000g / mol. The suitable range of the number average molecular weight of the polylactic acid produced in the ring-opening polymerization process can be, for example, 10,000 to 20,000g / mol, 20,000 to 30,000g / mol, 30,000 to 50,000g / mol or 50,000 to 80,000g / mol.

[0046] According to the present invention, the number average molecular weight and weight average molecular weight (M n and M w) is determined by gel permeation chromatography using poly(methyl methacrylate) standards and a sample concentration of 1 mg / ml to 5 mg / ml according to the molecular weight of the sample in 1 ml of hexafluoroisopropanol (HFIP), wherein the column temperature is 40° C., the temperature of the RI detector (refractive index) is 40° C., and the flow rate is 1 ml / min. As an instrument, a GPC Viscotek TDA max from Malvern Panalytical, UK, equipped with a Viscotek VE 2001 solvent / sample module, a pre-column HFIP guard column (length 50 mm, internal diameter 8 mm), two columns (Viscotek HFIP6000M and HFIP3000, Viscotek, Switzerland; length 300 mm, internal diameter 8 mm) and a triple detector Viscotek TDA 305 (RI, UV and viscometer) can be used. A calibration curve can be constructed using poly(methyl methacrylate) (PMMA) standards (Mn,max = 50,352 g / mol, D of 1.023).

[0047] The polydispersity index, ie, the ratio of Mw / Mn, of the polylactic acid produced during the ring-opening polymerization may be preferably 1 to 3, more preferably 1 to 2, and most preferably 1 to 1.5.

[0048] In a further development of the concept of the invention, it is proposed that the polylactic acid produced in the ring-opening polymerization process has a yellowness index below 40, preferably below 30, more preferably below 15, even more preferably below 10, still more preferably below 5, most preferably below 3. According to the invention, the yellowness index is measured according to ASTM E313.

[0049] It is preferred that after the polymerization ring-opening polymerization, the unreacted lactide remaining in the polymer product is removed to a final concentration of preferably less than at least 0.5% by weight, more preferably less than 0.3% by weight, in order to obtain a product of saleable quality. According to the present invention, such removal of unreacted lactide is achieved by at least one devolatilization step carried out at an elevated temperature (e.g., between 190° C. and 230° C.) and a reduced pressure (e.g., less than 5 mbar (absolute pressure)). For example, a two-stage devolatilization process can be performed in order to obtain the desired degree of lactide removal, thereby obtaining a polymer of the desired quality. In order to stop the polymerization reaction, an inhibitor is preferably added to the polymer product at the end of the polymerization and before or after the first devolatilization step. In order to maximize the yield of polymer product per unit amount of lactide feed, it is further preferred to recover unreacted lactide after devolatilization, for example, by condensation, and then optionally recycle the condensed composition containing unreacted lactide to the meso-lactide-enriched composition and / or recycle to the meso-lactide-depleted composition and / or recycle separately to the polymer composition. In particular, the addition of one or more effective inhibitor additives at the end of the polymerization reaction is preferred for efficient devolatilization. Additives and / or other polymers may be mixed and / or blended in one or more units for mixing and / or blending additives into the product stream before the first devolatilization step, between the first and second devolatilization steps or after the second devolatilization step to improve the mechanical, rheological and / or thermal properties of the final polymer product.

[0050] The composition containing unreacted lactide obtained during the devolatilization of the crude polylactic acid composition or the composition containing unreacted lactide obtained during multiple devolatilization steps of the crude polylactic acid composition respectively has a relatively high amount of L-lactide and D-lactide based on the sum of lactides and a relatively low amount of meso-lactide. Preferably, if more than one devolatilization step is performed, the composition containing unreacted lactide obtained during at least the first devolatilization step of the crude polylactic acid composition has a meso-lactide content of at most 25% by weight based on the sum of lactides, or if only one devolatilization step is performed, the composition containing unreacted lactide obtained during one devolatilization step of the crude polylactic acid composition has a meso-lactide content of at most 25% by weight based on the sum of lactides.

[0051] Furthermore, the final polymer product stream may be cooled in a cooler and then pressed through a granulator or pelletizer, respectively, or through another shaping unit.

[0052] According to a further aspect, the present invention relates to an apparatus for producing polylactic acid, wherein the apparatus comprises:

[0053] i) a separation unit comprising an inlet line for a crude lactide composition, an outlet line for a meso-lactide-enriched composition and an outlet line for a meso-lactide-depleted composition,

[0054] ii) a crystallization unit comprising an inlet line for a meso-lactide-enriched composition or a mixture of a meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide, and comprising an outlet line for a purified meso-lactide-enriched composition,

[0055] iii) a crystallization unit comprising an inlet line for a mixture of a composition depleted in meso-lactide and at least a portion of a composition comprising unreacted lactide, or comprising two inlet lines, one for a composition enriched in meso-lactide and another for at least a portion of a composition comprising unreacted lactide, and further comprising an outlet line for a purified composition depleted in meso-lactide, and

[0056] iv) a polymerization reactor comprising an inlet line for a polymerization composition and an outlet line for a crude polylactic acid composition, and further comprising a devolatilization unit comprising an inlet line for a crude polylactic acid composition, an outlet line for a purified polylactic acid composition, and an outlet line for a composition comprising unreacted lactide,

[0057] Therein, the outlet line of the purified meso-lactide-enriched composition of the crystallization unit ii) and optionally the outlet line of the purified meso-lactide-depleted composition of the crystallization unit iii) are connected to the inlet line of the polymerization reactor.

[0058] According to a particularly preferred embodiment of the present invention, the inlet line of the meso-lactide-enriched composition of the crystallization unit ii) is connected to the outlet line of the meso-lactide-enriched composition of the separation unit and the outlet line of the composition containing unreacted lactide of the devolatilization unit, wherein the outlet line of the meso-lactide-depleted composition of the separation unit directly leads to the inlet line of the meso-lactide-depleted composition of the crystallization unit iii), wherein the outlet line of the purified meso-lactide-enriched composition of the crystallization unit ii) is connected to the inlet line of the polymerization reactor, and optionally the outlet line of the purified meso-lactide-depleted composition of the crystallization unit iii) is connected to the inlet line of the polymerization reactor. For example, the outlet line of the purified meso-lactide-enriched composition of the crystallization unit ii) and the outlet line of the purified meso-lactide-depleted composition of the crystallization unit iii) are combined with each other, and the combined line is connected to the inlet line of the polymerization reactor.

[0059] According to an alternative particularly preferred embodiment of the present invention, the inlet pipeline of the meso-lactide-enriched composition of the crystallization unit ii) is connected to the outlet pipeline of the meso-lactide-enriched composition of the separation unit and to the outlet pipeline of the composition comprising unreacted lactide of the devolatilization unit, wherein the inlet pipeline of the meso-lactide-depleted composition of the crystallization unit iii) is connected to the outlet pipeline of the meso-lactide-depleted composition of the separation unit and to the outlet pipeline of the composition comprising unreacted lactide of the devolatilization unit, wherein the outlet pipeline of the purified meso-lactide-enriched composition of the crystallization unit ii) is connected to the inlet pipeline of the polymerization reactor, optionally the outlet pipeline of the purified meso-lactide-depleted composition of the crystallization unit iii) is connected to the inlet pipeline of the polymerization reactor.

[0060] In a further development of the concept of the invention, it is proposed that the crystallization unit ii) comprises at least one dynamic crystallizer, preferably one to four dynamic crystallizers, more preferably one dynamic crystallizer. Preferably, at least one dynamic crystallizer, i.e. preferably one to four dynamic crystallizers or more preferably one dynamic crystallizer, is (one or more) falling film crystallizers.

[0061] According to a further preferred embodiment of the present invention, the apparatus further comprises two distillation columns, wherein the upstream of the two distillation columns comprises an inlet pipeline for the crude lactide composition, a top outlet pipeline, a bottom outlet pipeline and a side outlet pipeline, wherein the side outlet pipeline of the upstream of the two distillation columns is connected to the side inlet pipeline of the downstream of the two distillation columns, wherein the downstream of the two distillation columns further comprises a top outlet pipeline, a bottom outlet pipeline and a side outlet pipeline, wherein the top outlet pipeline of the downstream of the two distillation columns is connected to the inlet pipeline of the crystallization unit ii), and wherein the side outlet pipeline of the downstream of the two distillation columns is connected to the inlet pipeline of the crystallization unit iii).

[0062] Furthermore, it is preferred that the apparatus further comprises a prepolymerization reactor, the prepolymerization reactor comprising an inlet pipeline for lactic acid and an outlet pipeline for lactic acid oligomers, wherein the apparatus further comprises a depolymerization reactor, the depolymerization reactor comprising an inlet pipeline and an outlet pipeline for a crude lactide composition, the inlet pipeline being connected to the outlet pipeline for lactic acid oligomers of the prepolymerization reactor, and the outlet pipeline for the crude lactide composition being connected to the inlet pipeline of the separation unit.

[0063] Preferably, the outlet line of the crude lactide composition is connected to the inlet line of the crude lactide composition upstream of the two distillation columns.

[0064] The invention is then described by means of the accompanying illustrative and non-limiting drawings, in which:

[0065] Figure 1A schematic diagram of an apparatus for producing polylactic acid according to an embodiment of the present invention is shown.

[0066] Figure 2 Shows Figure 1 Detailed schematic diagram of a static crystallizer for purifying a meso-lactide-enriched composition of the apparatus shown.

[0067] Figure 3 Shows Figure 1 A detailed schematic diagram of an alternative static crystallizer suitable for purifying a meso-lactide-enriched composition of the apparatus shown.

[0068] Figure 1The apparatus 10 shown comprises a dehydration unit 12, a prepolymerization reactor 14 and a depolymerization reactor 16 connected in series, wherein the dehydration unit 12 comprises an inlet line 18 for lactic acid. One outlet of the dehydration unit 12 is connected to the inlet of the prepolymerization reactor 14 via a line 20, while the prepolymerization reactor 14 is further connected to the dehydration unit 12 via a reflux line 22 and to the inlet of the depolymerization reactor 16 via a line 24. Furthermore, the dehydration unit 12 comprises a water outlet line 25. The depolymerization reactor 16, in turn, is connected to the prepolymerization reactor 14 via a reflux line 26 and further comprises a purge line 28. Downstream of the depolymerization reactor 16, two distillation columns 30, 32 are arranged, wherein the outlet of the depolymerization reactor 16 is connected to the inlet of the first distillation column 30 via a line 34, the first distillation column 30 is connected to the prepolymerization reactor 14 via a reflux line 36, to the depolymerization reactor 16 via a reflux line 38 and to the second distillation column 32 via a line 40. The second distillation tower 32 includes a pipeline 42 connected to a static crystallizer 44 at its top, a pipeline 46 connected to a falling film crystallizer 48 at its side, and a reflux line 50 connected to the first distillation tower 30 at its bottom. The static crystallizer 44 includes an outlet line 52, which is divided into a product line 54 and a connecting line 56. In addition, the static crystallizer 44 includes a purge line 58. In turn, the falling film crystallizer 48 includes a first outlet line 60, which is divided into a product line 62 and a connecting line 64, and a second outlet line 66, which is divided into a purge line 68 and a reflux line 70, and the reflux line 70 leads to the second distillation tower 30. The connecting lines 56 and 64 are combined into a monomer inlet line 72, which leads to a polylactic acid reactor 74. In addition, an inlet line 76 for a catalyst and an initiator leads to the polylactic acid reactor 74. The polylactic acid reactor 74 further includes an outlet line 78, which leads to a first devolatilizer 80. The first devolatilizer 80 includes a vapor outlet line 82 and a melt outlet line 84, the melt outlet line 84 leading to a second devolatilizer 86, and the second devolatilizer 86 includes a vapor outlet line 88 and a polylactic acid outlet line 90. In addition, recycle lines 98, 98' and 98" are provided in the apparatus 10. More specifically, the recycle line 98 branches off from the vapor outlet line 82 of the first devolatilizer 80, which in turn is divided into recycle lines 98, 98' and 98". The recycle line 98' leads to the falling film crystallizer 48, while the recycle line 98" leads to the static crystallizer 44.

[0069] During operation of the apparatus 10, lactic acid is continuously fed to the dehydration unit 12 via inlet line 18, where the lactic acid is dehydrated. The dehydrated lactic acid is introduced into the prepolymerization reactor 14 via line 20, where the lactic acid is prepolymerized to produce lactic acid oligomers, and the water that has been separated from the lactic acid in the dehydration unit 12 is discharged therefrom via line 25. The lactic acid oligomers are introduced into the depolymerization reactor 16 via line 24, where the lactic acid oligomers are depolymerized into a lactide mixture, which typically contains meso-lactide, L-lactide, and D-lactide. Residual lactic acid is recycled to the prepolymerization reactor 14 via line 26 and a purge stream is discharged from the depolymerization reactor 16 via line 28, while the lactide mixture is introduced from the depolymerization reactor 16 into the first distillation column 30 via line 34. In the first distillation tower 30, light substances are separated from lactide as an overhead stream and refluxed to the prepolymerization reactor 14 through a pipeline 36, while the remaining lactic acid oligomers are refluxed to the depolymerization reactor 16 as a bottom stream through a pipeline 38, and the pre-purified lactide stream is passed into the second distillation tower 32 through a pipeline 40 as a side stream of the first distillation tower 30. The pre-purified lactide stream is separated into an overhead stream and a bottom stream in the second distillation tower 32, wherein the overhead stream is a composition enriched in meso-lactide, and the bottom stream is a composition depleted in meso-lactide. The meso-lactide-enriched composition is fed to a static crystallizer 44 through a pipeline 42, and the meso-lactide-depleted composition is fed to a falling film crystallizer 48 through a pipeline 46. In addition, the unreacted lactide from the first devolatilizer 80 is reused in the polymerization step performed in the polylactic acid reactor 74. Part of the vapor stream discharged from first devolatilizer 80 through line 82 is recycled to static crystallizer 44 and falling film crystallizer 48 via recycle lines 98, 98' and 98". A purified meso-lactide-enriched composition is discharged from static crystallizer 44 through line 52, and the residue is discharged as a purge stream through purge line 58. Likewise, a purified meso-lactide-depleted composition is discharged from falling film crystallizer 48 through line 60, and the residue is discharged through line 66 and divided into a recycle stream and a purge stream, the recycle stream being fed to second distillation column 32 through reflux line 70, and the purge stream being discharged through purge line 68. The purified meso-lactide-enriched composition is divided into a portion discharged from the apparatus 10 through product line 54 as purified meso-lactide and a portion fed to the polylactic acid reactor 74 through lines 56 and 72, while the purified meso-lactide-depleted composition is divided into a portion discharged from the apparatus 10 through product line 62 as purified L-lactide and a portion fed to the polylactic acid reactor 74 through lines 56 and 72. Lactide is polymerized to polylactic acid in the presence of a catalyst and an initiator in the polylactic acid reactor 74, which are fed to the polylactic acid reactor 74 through an inlet line 76.The crude polylactic acid stream is fed to a first devolatilizer 80 via line 78, wherein the crude polylactic acid stream is separated into a polylactic acid melt stream and a vapor stream containing unreacted lactide and trace amounts of catalyst and initiator. The vapor fraction is discharged from the first devolatilizer 80 via line 82, and the polylactic acid melt stream is fed to a second devolatilizer 86 via line 84, wherein the polylactic acid melt stream is separated into a vapor stream containing unreacted lactide and trace amounts of catalyst and initiator discharged from the second devolatilizer 86 via line 88 and purified polylactic acid discharged from the device 10 via line 90.

[0070] Figure 2 Schematically shows Figure 1 The static crystallizer 44 of the apparatus 10 shown in FIG. 4 includes a first static crystallization stage 92 and a second static crystallization stage 92'. The composition enriched in meso-lactide and the composition containing unreacted lactide obtained during the devolatilization of the crude polylactic acid composition are fed to the first static crystallization stage 92 of the static crystallizer 44 through lines 42 and 98", and the mixture is crystallized therein. During the crystallization process, meso-lactide is crystallized on the cooled surface of the first crystallization stage 92, while a mother liquor depleted in meso-lactide remains. After the crystallization is completed, the mother liquor is discharged from the first crystallization stage 92 as a purge through the purge line 58, and the crystal layer of meso-lactide deposited during the first crystallization stage 92 is subjected to a sweating step, in which the sweating fraction (not shown) thus obtained is added to the purge stream discharged through the purge line 58. Thereafter, the crystal layer is subjected to a sweating step. The first crystalline fraction of the purified meso-lactide composition is melted to obtain a first crystalline fraction of the purified meso-lactide composition. The first crystalline fraction of the purified meso-lactide composition is introduced into the second crystallization stage 92' through pipeline 94, where it is crystallized. During the crystallization process, meso-lactide is crystallized on the cooled surface of the second crystallization stage 92', while the mother liquor depleted in meso-lactide remains. After the crystallization in the second crystallization stage 92' is completed, the mother liquor is discharged and fed to the first crystallization stage 92 through pipeline 96. Thereafter, the crystal layer deposited in the second crystallization stage 92' is subjected to a sweating step, and then the crystal layer deposited in the second crystallization stage 92' is melted to obtain a purified meso-lactide-enriched stream, which is discharged from the static crystallizer 44 through the outlet pipeline 52.

[0071] Figure 3 Shows Figure 1 A detailed schematic diagram of an alternative static crystallizer 44 of the apparatus shown is suitable for purifying a meso-lactide-enriched composition. Figure 3 The static crystallizer 44 shown corresponds to Figure 2The static crystallizer shown in FIG. 4 is a static crystallizer, except that it includes a third static crystallization stage 92″ and a fourth static crystallization stage 92″′ in addition to the first static crystallization stage 92 and the second static crystallization stage 92′. The crystalline fraction of the purified meso-lactide composition obtained in the first static crystallization stage 92 is introduced into the second crystallization stage 92′ through a pipeline 94, where it is crystallized. During the crystallization process, meso-lactide is crystallized on the cooled surface of the second crystallization stage 92′, while a mother liquor depleted in meso-lactide remains. After the crystallization in the second crystallization stage 92′ is completed, the mother liquor is discharged and fed to the first crystallization stage 92 through a pipeline 96. Thereafter, the crystal layer deposited in the second crystallization stage 92′ is subjected to a sweating step, and the crystal layer deposited in the second crystallization stage 92′ is subsequently melted to obtain a purified meso-lactide-enriched stream, which is discharged from the static crystallizer 44 through an outlet pipeline 100. From line 100, a line 102 is branched, through which a portion of the purified meso-lactide-enriched stream obtained in second crystallization stage 92' can be discharged from apparatus 10, while the remainder thereof is introduced into outlet line 52 via line 100. In turn, a crystalline fraction of the purified D-lactide- and L-lactide-enriched composition obtained in third static crystallization stage 92" is introduced into fourth crystallization stage 92'" via line 96" for crystallization. During the crystallization, the D-lactide- and L-lactide-enriched composition is crystallized on the cooled surface of fourth crystallization stage 92'", while a mother liquor depleted in D-lactide and L-lactide remains. After the crystallization in the fourth crystallization stage 92'" is completed, the mother liquor is discharged and fed to the third crystallization stage 92'" through the pipeline 94". Thereafter, the crystal layer deposited in the fourth crystallization stage 92'" is subjected to a sweating step, and then the crystal layer deposited in the fourth crystallization stage 92'" is melted to obtain a purified D-lactide and L-lactide-enriched stream, which is discharged from the static crystallizer 44 through the outlet pipeline 100'. A pipeline 102' is branched from the pipeline 100', and a part of the purified D-lactide and L-lactide-enriched stream obtained in the fourth crystallization stage 92'" can be discharged from the device 10 through the pipeline 102', while the remaining part thereof is introduced into the outlet pipeline 52 through the pipeline 100'.

[0072] The meso-lactide-enriched composition is fed to the first static crystallization stage 92 of the static crystallizer 44 through the pipeline 42 and crystallized therein, while the composition containing unreacted lactide obtained in the devolatilization process of the crude polylactic acid composition is fed to the third static crystallization stage 92" of the static crystallizer 44 through the pipeline 98" and crystallized therein. The lactide-enriched fraction obtained in each of the first, third and fourth crystallization stages 92, 92", 92"' is introduced into the corresponding upstream crystallization stage 92', 92', 92" through the pipeline 94, 94', 94", and the purified lactide-enriched fraction obtained in the second crystallization stage 92' is discharged from the second crystallization stage 92' through the pipeline 100. A pipeline 102 is branched from the pipeline 100, and a part of the purified lactide-enriched fraction obtained in the second crystallization stage 92' can be discharged from the device 10 through the pipeline 102, and the remaining part thereof is introduced into the outlet pipeline 52 through the pipeline 100. The lactide-depleted mother liquor obtained in each of the second, first and third crystallization stages 92', 92, 92" is introduced into the corresponding downstream crystallization stage 92, 92", 92" through pipelines 96, 96', 96", while the purified lactide-enriched fraction obtained in the second crystallization stage 92' is discharged from the second crystallization stage 92' through pipeline 100. A pipeline 102 is branched from pipeline 100, through which a part of the purified lactide-enriched fraction obtained in the second crystallization stage 92' can be discharged from the device 10, while the remaining part thereof is introduced into the outlet pipeline 52 through pipeline 100.

[0073] During the crystallization process, meso-lactide is crystallized on the cooled surface of the first crystallization stage 92, while a mother liquor depleted in meso-lactide remains. After the crystallization is completed, the mother liquor is discharged from the first crystallization stage 92 as a purge through the purge line 58, and the crystal layer of meso-lactide deposited during the first crystallization stage 92 is subjected to a sweating step, wherein the sweating fraction (not shown) thus obtained is added to the purge stream discharged through the purge line 58. Thereafter, the crystal layer is melted to obtain a first crystalline fraction of a purified meso-lactide composition. The first crystalline fraction of the purified meso-lactide composition is introduced through the pipeline 94 and crystallized therein. During the crystallization process, meso-lactide is crystallized on the cooled surface of the second crystallization stage 92', while a mother liquor depleted in meso-lactide remains. After the crystallization in the second crystallization stage 92' is completed, the mother liquor is discharged and fed to the first crystallization stage 92 through the pipeline 96. Thereafter, the crystal layer deposited in the second crystallization stage 12 is subjected to a sweating step, and then the crystal layer deposited in the second crystallization stage 92 ′ is melted to obtain a purified meso-lactide-enriched stream, which is discharged from the static crystallizer 44 through the outlet pipeline 52.

[0074] Reference numerals

[0075] 10. Equipment

[0076] 12 Dehydration Unit

[0077] 14 Prepolymerization Reactor

[0078] 16 Depolymerization Reactor

[0079] 18 Lactic acid inlet line

[0080] 20 pipelines

[0081] 22 Return line

[0082] 24 Pipeline

[0083] 25 Water outlet pipeline

[0084] 26 Return line

[0085] 28 Clear the pipeline

[0086] 30 (First) Distillation Column

[0087] 32 (Second) Distillation Column

[0088] 34 pipeline

[0089] 36 Return line

[0090] 38 Return line

[0091] 40 pipeline

[0092] 42 pipeline

[0093] 44 Static Crystallizer

[0094] 46 pipeline

[0095] 48 Falling Film Crystallizer

[0096] 50 Return line

[0097] 52 Export pipeline

[0098] 54 Product pipeline

[0099] 56 Connecting pipeline

[0100] 58, 58' Clear Line

[0101] 60 Export pipeline

[0102] 62 Product pipeline

[0103] 64 Connection pipeline

[0104] 66 Export pipeline

[0105] 68 Clear Line

[0106] 70 Return line

[0107] 72 Monomer inlet pipeline

[0108] 74. Polylactic acid reactor

[0109] 76 Inlet pipeline

[0110] 78 Export pipeline

[0111] 80 First Devolatilizer

[0112] 82 Steam export pipeline

[0113] 84 melt outlet pipeline

[0114] 86 Second Devolatilizer

[0115] 88 Steam export pipeline

[0116] 90 Polylactic acid export pipeline

[0117] 92 First static crystallization stage

[0118] 92' Second static crystallization stage

[0119] 92” Third Static Crystallization Stage

[0120] 92”’ Fourth static crystallization stage

[0121] 94, 94', 94" pipeline

[0122] 96, 96', 96" pipeline

[0123] 98, 98', 98" recycle line from devolatilization

[0124] 100, 100' pipeline

[0125] 102, 102' pipeline

Claims

1. A method for producing polylactic acid, comprising the following steps: a) providing a crude lactide composition comprising meso-lactide and at least one of L-lactide and D-lactide, b) separating a meso-lactide-enriched composition and a meso-lactide-depleted composition from the crude lactide composition, wherein the meso-lactide-enriched composition comprises at least 80 mol % of meso-lactide, based on the total lactide content, c) polymerizing a polymeric composition comprising meso-lactide and at least one of L-lactide and D-lactide into a crude polylactic acid composition, and devolatilizing the crude polylactic acid composition to produce a purified polylactic acid composition and a composition comprising unreacted lactide, d) subjecting the meso-lactide-enriched composition and at least a portion of the composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, e) subjecting the meso-lactide-depleted composition or a mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, wherein the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d).

2. The process according to claim 1, wherein in step d) the meso-lactide-enriched composition produced in step b) is mixed with at least a portion of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition and then fed as a mixture to at least one crystallization step of step d).

3. The process according to claim 1, wherein in step d) the meso-lactide-enriched composition produced in step b) is subjected to at least one crystallization step of step d), and separately therefrom at least a portion of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step of step d), and then at least portions of the two purified streams are mixed with each other to form a purified meso-lactide-enriched mixture.

4. The method of any one of the preceding claims, wherein the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d) and at least a portion of the purified meso-lactide-depleted composition produced in step e).

5. The method of claim 5, wherein the ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerization composition is from 10:1 to 1:10 by weight, such as from 5:1 to 1:1 by weight.

6. The process according to any one of the preceding claims, wherein in step d), 10 to 100% by weight, preferably 50 to 100% by weight, of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step, and in step e), 0 to 90% by weight, preferably 50 to 90% by weight, of the composition comprising unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step.

7. The method according to any one of the preceding claims, wherein the polymerizing composition comprises at least one catalyst and / or at least one initiator, wherein the catalyst is preferably at least one organometallic compound comprising aluminum or tin as metal, and wherein the catalyst is more preferably at least one organometallic compound selected from the group consisting of stannous octoate, tetraphenyltin, trimethoxybutyltin, dibutyltin oxide, aluminum isopropoxide, Al(Oi-Pr) p , where 1≤p≤3, Et3-pAl(O(CH2)2X) p , wherein 1≤p≤3, α, β, γ, δ, ε tetraphenylporphyrin aluminum (TPPIAIX) and any combination of two or more of the foregoing compounds, and wherein at least one initiator is preferably selected from monohydroxy compounds, dihydroxy compounds, trihydroxy compounds, tetrahydroxy compounds and any combination of two or more of the foregoing compounds, more preferably the at least one initiator is selected from 2-ethylhexanol, C 10 -C 20 - monohydroxy aliphatic alcohols, benzyl alcohol, p-phenylbenzyl alcohol, ethylene glycol, propylene glycol, 1,4-butanediol, polyethylene glycol having a weight average molecular weight of 200 to 10,000 g / mol, 2-hydroxymethyl-1,3-propane, glycerol, polyglycerol having a weight average molecular weight of 100 to 1,000 g / mol, trihydroxybenzene, trimethylolpropane and dimers thereof, pentaerythritol and dimers thereof, and any combination of two or more of the foregoing compounds.

8. The process according to any one of the preceding claims, wherein the purification of step d) comprises at least one static crystallization step, wherein the at least one static crystallization step preferably comprises one to ten static crystallization stages, preferably one crystallization stage or 2 to 10 crystallization stages, more preferably 2 to 4 static crystallization stages.

9. The method according to any one of the preceding claims, wherein the purification in step e) comprises at least one dynamic crystallization step, wherein the at least one dynamic crystallization step preferably comprises one to four dynamic crystallization stages, more preferably one dynamic crystallization stage, wherein the dynamic crystallization is preferably falling film crystallization.

10. The process according to any one of the preceding claims, wherein step b) comprises at least one distillation step, each of which is carried out in a distillation column (30, 32), wherein the crude lactide composition is fed to the distillation column (30, 32) of the first distillation step, and wherein the meso-lactide-enriched composition is produced as an overhead fraction and the meso-lactide-depleted composition is produced as a bottom fraction or a side fraction of the distillation column (30, 32) of the last distillation step.

11. The process of claim 10, wherein step b) comprises two distillation steps, each of which is carried out in a distillation column (30, 32), wherein in a first distillation column (30, 32), the crude lactide composition is separated into an overhead fraction, a bottom fraction and a side fraction, wherein the side fraction of the first distillation column (30, 32) is fed to a second distillation column (30, 32), wherein the meso-lactide-enriched composition is produced as an overhead fraction and the meso-lactide-depleted composition is produced as a side fraction or a bottom fraction.

12. The process according to claim 10 or 11, wherein a portion of the purified meso-lactide-enriched composition produced in step d) is recycled as a side stream to the last distillation column (30, 32).

13. An apparatus (10) for producing polylactic acid, wherein the apparatus (10) comprises: i) a separation unit comprising an inlet line (10) for a crude lactide composition, an outlet line (42) for a meso-lactide-enriched composition and an outlet line (46) for a meso-lactide-depleted composition, ii) a crystallization unit (44) comprising an inlet line (42) for a mixture of a meso-lactide-enriched composition and at least a portion of a composition comprising unreacted lactide, or comprising two inlet lines (42, 98"), one (42) for the meso-lactide-enriched composition and the other (98") for at least a portion of the composition comprising unreacted lactide, and further comprising an outlet line (52) for a purified meso-lactide-enriched composition, iii) a crystallization unit (48) comprising an inlet line (46) for the composition depleted in meso-lactide or a mixture of the composition depleted in meso-lactide and a portion of the composition comprising unreacted lactide, and comprising an outlet line (60) for the purified composition depleted in meso-lactide, and iv) a polymerization reactor (74) comprising an inlet line (72) for a polymerization composition and an outlet line (78) for a crude polylactic acid composition, and further comprising a devolatilization unit comprising an inlet line (78) for a crude polylactic acid composition, an outlet line (90) for a purified polylactic acid composition, and outlet lines (82, 88) for a composition comprising unreacted lactide, Wherein, at least the outlet line (52) of the purified meso-lactide-enriched composition of the crystallization unit ii) (44) and optionally the outlet line (60) of the purified meso-lactide-depleted composition of the crystallization unit iii) (48) are connected to the inlet line (72) of the polymerization reactor (74).

14. The device (10) according to claim 13, wherein: i) the inlet line (42) of the meso-lactide-enriched composition of the crystallization unit ii) (44) is connected to the outlet line (42) of the meso-lactide-enriched composition of the separation unit and to the outlet line (82, 98") of the composition comprising unreacted lactide of the devolatilization unit, wherein the outlet line (46) of the meso-lactide-depleted composition of the separation unit directly leads to the inlet line (60) of the meso-lactide-depleted composition of the crystallization unit iii) (48), and wherein the outlet line (52) of the purified meso-lactide-enriched composition of the crystallization unit ii) (44) is connected to the inlet line (72) of the polymerization reactor (74), and optionally the outlet line (60) of the purified meso-lactide-depleted composition of the crystallization unit iii) (48) is connected to the inlet line (72) of the polymerization reactor (74), or ii) the inlet line (42) of the meso-lactide-enriched composition of the crystallization unit ii) (44) is connected to the outlet line (42) of the meso-lactide-enriched composition of the separation unit and to the outlet line (82, 98") of the composition containing unreacted lactide of the devolatilization unit, wherein the inlet line (60) of the meso-lactide-depleted composition of the crystallization unit iii) (48) is connected to the outlet line (46) of the meso-lactide-depleted composition of the separation unit and the outlet pipeline (82, 98') of the devolatilization unit for a composition comprising unreacted lactide, wherein the outlet pipeline (52) of the purified meso-lactide-enriched composition of the crystallization unit ii) (44) is connected to the inlet pipeline (72) of the polymerization reactor (74), and optionally the outlet pipeline (60) of the purified meso-lactide-depleted composition of the crystallization unit iii) (48) is connected to the inlet pipeline (72) of the polymerization reactor (74).

15. The apparatus (10) according to claim 13 or 14, wherein the apparatus (10) further comprises two distillation columns (30, 32), wherein the upstream of the two distillation columns (30, 32) comprises an inlet line (34), a tower top outlet line (36), a tower bottom outlet line (38), and a side outlet line (40) for a crude lactide composition, wherein the side outlet line (40) of the upstream (30) of the two distillation columns is connected to the side inlet line (40) of the downstream (32) of the two distillation columns. The downstream of the two distillation towers (32) further comprises a top outlet pipeline (42), a bottom outlet pipeline (50) and a side outlet pipeline (46), wherein the top outlet pipeline (42) of the downstream of the two distillation towers (32) is connected to the inlet pipeline (42) of the crystallization unit ii) (44), and wherein the side outlet pipeline (46) of the downstream of the two distillation towers (32) is connected to the inlet pipeline (46) of the crystallization unit iii) (48).

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