Apparatus and efficient method for producing 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 less than 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 optionally devolatilizing the crude polylactic acid composition, d) subjecting the meso-lactide-enriched composition or a mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide to purification comprising at least one crystallization step to produce a purified polylactic acid composition and a composition comprising unreacted lactide, d) subjecting the meso-lactide-enriched composition or a mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide, the present invention relates to a method for purifying a meso-lactide-enriched composition, e) subjecting a meso-lactide-depleted composition or a mixture of a meso-lactide-depleted composition and 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, e) subjecting a meso-lactide-depleted composition or a mixture of a meso-lactide-depleted composition and a portion of a composition comprising unreacted lactide, the polymeric composition comprises at least a portion of the purified meso-lactide enriched composition produced in step d) and / or at least a portion of the purified meso-lactide depleted composition produced in step e), to produce a purified meso-lactide depleted composition, where the polymeric composition comprises at least a portion of the purified meso-lactide enriched composition produced in step d) and / or at least a portion of the purified meso-lactide depleted composition produced in step e).

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, an object of the present invention is to provide a method for producing polylactic acid, which first produces ultrapure lactides 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.

[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 less than 80 mol % (or 80 wt %) of 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 optionally 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 or a mixture of the meso-lactide-enriched 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-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 polymerizable composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d) and / or at least a portion of the purified meso-lactide-depleted composition produced in step e).

[0014] By the aforementioned method steps, not only can polylactic acid with high stereochemical purity be produced, which has a predetermined ratio of lactic acid stereoisomers, but the method 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. A further advantage of the method according to the invention is that, due to the crystallization step carried out during the method, 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, because these impurities would hinder the achievement of the desired molecular weight. Very pure polylactic acid with a predetermined stereoisomeric purity can be reliably produced with low content of water, lactic acid and other impurities. Preferably, the lactide acid content is, for example, less than 20 milliequivalents per 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. It is particularly advantageous to carry out the separation step b) according to the invention such that the meso-lactide-enriched composition comprises less than 80 mol % (or 80 wt %) of meso-lactide, based on the total lactide content of the meso-lactide-enriched composition. Therefore, the separation in step b) generally comprises one or more distillation steps, which can be carried out with relatively small separation devices (e.g. relatively small distillation columns) with relatively low energy consumption. In addition, it allows for a shorter residence time of the crude lactide composition in the separation device (e.g. distillation column), milder separation conditions (e.g. milder distillation conditions in terms of reduced temperature), which in turn leads to the formation of less undesirable by-products, such as 4-hydroxy-5-methyl-4-cyclopentene-1,3-dione (HMCP) during the separation step b). This is an important advantage, because the HMCP formed at a higher temperature during distillation is an impurity that causes polylactic acid to turn yellow, so it is an impurity particularly critical for polylactic acid. Since HMCP has only a slightly higher volatility relative to meso-lactide, it cannot be separated from meso-lactide during distillation. However, in addition to the reduction in the formation of HCMP due to possible milder separation conditions, the method according to the present invention further reliably removes the formed HCMP and other impurities whose boiling points are close to the boiling point of meso-lactide from the composition enriched with meso-lactide during the crystallization step d). In addition, the crystallization step e) ensures that the composition depleted in meso-lactide has a particularly high purity and contains (if any) trace impurities. For example, the composition depleted in meso-lactide can have a low free acid content, for example, a free acid content less than 2meq / kg.Even if the polymerization composition contains 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), the polymerization composition can achieve such a low free acid content of less than 2 meq / kg. Therefore, the produced lactide composition, in particular the produced purified meso-lactide-enriched composition, has a very high thermal stability, which allows the part of the lactide composition not required for the polymerization step (i.e., the part of the purified meso-lactide-enriched composition and the part of the purified meso-lactide-depleted composition) to be stored as a liquid or solid material for several months without deterioration.

[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). 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).

[0016] According to the present invention, in step d), the meso-lactide-enriched composition produced in step b) or the mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide obtained in the optional polylactic acid devolatilization step is subjected to a purification comprising at least one crystallization step. Likewise, in step e), the meso-lactide-depleted composition produced in step b) or the mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide obtained in the optional polylactic acid devolatilization step is subjected to a purification comprising at least one crystallization step. This covers all four possible combinations thereof, namely i) the meso-lactide-enriched composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) and the meso-lactide-depleted composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) are subjected to purification comprising crystallization in step d) and ii) the mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide obtained in the optional polylactic acid devolatilization step and the meso-lactide-depleted composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) are subjected to purification comprising crystallization in step d) and iii) the mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step and the meso-lactide-depleted composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) are subjected to purification comprising crystallization in step e) and iv) the mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step and the meso-lactide-depleted composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step ... The invention relates to a method of preparing the present invention which is characterized in that the present invention relates to a method of preparing the present invention which is characterized in that the meso-lactide-enriched composition is subjected to a purification comprising crystallization, iii) in step d) a meso-lactide-enriched composition (i.e., without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) and in step e) a mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step of the polylactic acid composition, iv) in step d) a mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step of the polylactic acid composition and in step e) a mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step of the polylactic acid composition. Regardless of whether a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step is added to the meso-lactide-enriched composition and / or the meso-lactide-depleted composition prior to purification by crystallization, the respectively purified or crystallized compositions are referred to herein as purified meso-lactide-enriched composition and purified meso-lactide-depleted composition, respectively.If a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step of the polylactic acid composition is added to the meso-lactide-enriched composition or the meso-lactide-depleted composition before purification, it is preferred that the mixture of the composition comprising unreacted lactide and the meso-lactide-enriched composition or the meso-lactide-enriched composition contains 1 wt.% to 40 wt.%, preferably 5 wt.% to 15 wt.% of lactide of the composition comprising unreacted lactide obtained in the optional devolatilization step, based on the total weight of lactide contained in the mixture.

[0017] 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 / or at least a portion of the purified meso-lactide-depleted composition produced in step e). Thus, during the polymerization, only the whole or a portion of the purified meso-lactide-enriched composition produced in step d) may be polymerized, or only the whole or a portion of the purified meso-lactide-depleted composition produced in step d) may be polymerized, or a mixture of firstly the whole or a portion of the purified meso-lactide-enriched composition produced in step d) and secondly the whole or a portion of the purified meso-lactide-enriched composition produced in step d) may be polymerized. Furthermore, even if less preferred, this also includes the embodiment in which a mixture of all or part of the purified meso-lactide-enriched composition produced in step d) (which may comprise lactide from the composition comprising unreacted lactide obtained in the optional devolatilization step, as described above) and part of the composition comprising unreacted lactide obtained in the optional devolatilization step (which may or may not have been 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 polymerized in step c). Furthermore, even though less preferred, this also includes the embodiment in which a mixture of all or part of the purified meso-lactide-depleted composition produced in step d) and a part of the composition comprising unreacted lactide obtained in the optional 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 comprising at least one crystallization step) is polymerized in step c), as well as the embodiment in which a mixture of all or part of the purified meso-lactide-enriched composition produced in step d), all or part of the purified meso-lactide-depleted composition produced in step d) and a part of the composition comprising unreacted lactide obtained in the optional 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 comprising at least one crystallization step) is polymerized in step c). Even though less preferred, the polymerized composition may also contain additional lactide from other sources. 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 include, and indeed preferably does include, non-lactide components that are helpful or necessary for the polymerization reaction, such as one or more catalysts, one or more initiators, and the like.

[0018] According to a first particularly preferred embodiment of the present invention, in step d) the meso-lactide-enriched composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) the meso-lactide-depleted composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and wherein the polymerized 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). Preferably, the polymerization composition comprises no more than 20 wt%, more preferably no more than 10 wt%, even more preferably no more than 2 wt%, still more preferably no more than 1 wt% of additional lactide in addition to the lactide from the purified meso-lactide-enriched composition and the purified meso-lactide-depleted composition, and most preferably comprises no additional lactide. The ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerization composition may be from 10:1 to 1:10 by weight, such as from 5:1 to 1:1 by weight. The polymerization reaction may comprise all of the purified meso-lactide-enriched composition produced in step d) or a portion of the purified meso-lactide-enriched composition produced in step d), such as from 10 wt% to 60 wt% thereof. Likewise, the polymerization reaction may comprise all of the purified meso-lactide-depleted composition produced in step d) or a portion of the purified meso-lactide-depleted composition produced in step d), for example 10% to 60% by weight thereof.

[0019] According to a second particularly preferred embodiment of the invention, in step d) a mixture of a meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide produced in the devolatilization is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) a meso-lactide-depleted composition (i.e., not mixed with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and wherein the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition. Preferably, the polymeric composition comprises not more than 20% by weight, more preferably not more than 10% by weight, even more preferably not more than 2% by weight, still more preferably not more than 1% by weight of further lactide in addition to the lactide from the purified meso-lactide-enriched composition and the purified meso-lactide-depleted composition, and most preferably comprises no further lactide. The ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerization composition may be from 10:1 to 1:10 by weight, for example from 5:1 to 1:1 by weight. However, it is preferred that in this embodiment at least 80 wt.-%, more preferably at least 90 wt.-%, even more preferably at least 98 wt.-%, even more preferably at least 99 wt.-%, most preferably all of the lactide contained in the polymerization composition comes from the purified meso-lactide-depleted composition, i.e. all or part of the purified meso-lactide-enriched composition produced in step d) is contained in the polymerization composition, but none of the purified meso-lactide-depleted composition produced in step e) is contained in the polymerization composition. The polymerization reaction may comprise all of the purified meso-lactide-enriched composition produced in step d) or part of the purified meso-lactide-enriched composition produced in step d), for example from 10 wt.-% to 60 wt.-% thereof.

[0020] According to a third particularly preferred embodiment of the present invention, in step d) the meso-lactide-enriched composition (i.e. without mixing with a portion of the composition comprising unreacted lactide obtained in the optional devolatilization step) is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) the mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and wherein the polymerized 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). Preferably, the polymerization composition comprises no more than 20 wt%, more preferably no more than 10 wt%, even more preferably no more than 2 wt%, still more preferably no more than 1 wt% of additional lactide in addition to the lactide from the purified meso-lactide-enriched composition and the purified meso-lactide-depleted composition, and most preferably comprises no additional lactide. The ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerization composition may be from 10:1 to 1:10 by weight, such as from 5:1 to 1:1 by weight. The polymerization reaction may comprise all of the purified meso-lactide-enriched composition produced in step d) or a portion of the purified meso-lactide-enriched composition produced in step d), such as from 10 wt% to 60 wt% thereof. Likewise, the polymerization reaction may comprise all of the purified meso-lactide-depleted composition produced in step d) or a portion of the purified meso-lactide-depleted composition produced in step d), for example 10% to 60% by weight thereof.

[0021] According to a fourth particularly preferred embodiment of the present invention, in step d) a mixture of a meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide produced in the devolatilization is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) a mixture of a meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and 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). Preferably, the polymeric composition comprises not more than 20% by weight, more preferably not more than 10% by weight, even more preferably not more than 2% by weight, still more preferably not more than 1% by weight of further lactide in addition to the lactide from the purified meso-lactide-enriched composition and the purified meso-lactide-depleted composition, and most preferably comprises no further lactide. The ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerization composition may be 10:1 to 1:10 by weight, for example 5:1 to 1:1 by weight. The polymerization reaction may comprise the entirety of the purified meso-lactide-enriched composition produced in step d) or a portion of the purified meso-lactide-enriched composition produced in step d), for example 10 wt % to 60 wt % thereof. Likewise, the polymerization reaction may comprise the entirety of the purified meso-lactide-depleted composition produced in step d) or a portion of the purified meso-lactide-depleted composition produced in step d), for example 10 wt % to 60 wt % thereof.

[0022] Furthermore, for all other possible embodiments except the above-mentioned specific preferred embodiments, the polymeric composition comprises i) at least a portion of the purified meso-lactide-enriched composition produced in step d) and / or at least a portion of the purified meso-lactide-depleted composition produced in step e). Furthermore, also for all other possible embodiments, the polymeric composition may comprise a mixture of i) at least a portion of the purified meso-lactide-enriched composition produced in step d) and / or at least a portion of the purified meso-lactide-depleted composition produced in step e) and ii) at least a portion of the composition comprising unreacted lactide. For example, the polymeric composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d), at least a portion of the purified meso-lactide-depleted composition produced in step e), and at least a portion of the composition comprising unreacted lactide.

[0023] 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.

[0024] Preferably, the catalyst content in the polymeric composition is from 10 to 10,000 ppm, more preferably from 50 to 1,000 ppm.

[0025] 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.

[0026] 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.

[0027] According to the present invention, the purification for producing a purified meso-lactide-enriched composition in step d) comprises at least one crystallization step. In principle, the present invention has no particular restrictions on the type of crystallization used in step d). Therefore, the crystallization step can 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 performed by a melt, i.e., using a starting material that has been melted without adding other components, such as solvents. Melt crystallization can be performed in the form of layer crystallization or in the form of suspension crystallization. In order to perform layer crystallization, a cooled surface is usually introduced into a melt of an optically active or optically inactive lactide used as a starting material. Thereafter, a crystal layer of enantiomerically enriched or non-enriched 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 performed 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 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 in meso-lactide or a composition enriched in 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 below the equilibrium solidification temperature of the liquid feed mixture, so that crystals enriched in meso-lactide are formed and deposited on the cooling outer surface of the plate. After crystallization is complete, the liquid mixture is completely removed from the static crystallization vessel, the cooling of the plates is stopped, and the plates are optionally heated so that the crystal layer formed on the outer surface of the plates melts, and the melt is then removed from the crystallization vessel to obtain a purified meso-lactide-enriched composition. In order to increase the purity of meso-lactide, the crystal layer can be sweated by slowly heating it to a temperature close to the melting temperature of meso-lactide so that they are partially melted before melting the crystals.

[0028] 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.

[0029] Furthermore, the present invention has no particular restrictions with regard 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 not 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.

[0030] 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.

[0031] 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).

[0032] 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.

[0033] 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 as a side fraction 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.

[0034] 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.

[0035] 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.

[0036] As described above, in step b), a meso-lactide-enriched composition is produced, which contains less than 80 mole % (or 80 wt %) of meso-lactide based on the total content of lactide. Similarly, according to the present invention, a meso-lactide-enriched composition refers to a corresponding composition produced during separation step b) having a meso-lactide content higher than that of the crude lactide composition fed to separation step b). In other words, if the crude lactide composition fed to separation step b) contains 15 wt % of meso-lactide based on the total content of lactide, the meso-lactide-enriched composition contains greater than 15 wt % to less than 80 wt % of meso-lactide based on the total content of lactide. When the meso-lactide-enriched composition contains 50 wt % to 80 wt %, more preferably 60 wt % to 80 wt %, and even more preferably 70 wt % to 80 wt % of meso-lactide, good results are particularly obtained. Furthermore, the meso-lactide-enriched composition preferably comprises 20 to 50 wt %, more preferably 20 to 40 wt %, still more preferably 20 to 30 wt % of the sum of L-lactide and D-lactide.

[0037] 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 wt.-% meso-lactide, based on the total lactide content, the composition depleted in meso-lactide contains less than 15 wt.-% 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 wt.-%, more preferably more than 90 wt.-% L-lactide, less than 2 wt.-%, more preferably less than 0.5 wt.-% D-lactide and less than 12 wt.-%, more preferably less than 5 wt.-%, most preferably more than 0 wt.-% to less than 5 wt.-% meso-lactide.

[0038] 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 selected from the following ones: tin compounds, such as SnO, SnCl2, Sn(octanoate)2, stannous octoate (II), Sb2O3, H2SO4, Bu2SnO, zinc oxide and any combination of two or more of the aforementioned catalysts. 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.

[0039] 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.

[0040] 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 sample concentrations of 1 mg / ml to 5 mg / ml in 1 ml HFIP according to the molecular weight of the sample, with a column temperature of 40° C., a temperature of the RI detector (refractive index) of 40° C., and a flow rate of 1 ml / min. As an instrument, a GPC Viscotek TDA max from Malvern Panalytical, UK, equipped with a Viscotek VE2001 solvent / sample module, a pre-column HFIP guard column (length 50 mm, inner diameter 8 mm), two columns (Viscotek HFIP6000M and HFIP3000, Viscotek, Switzerland; length 300 mm, inner diameter 8 mm) and a triple detector Viscotek TDA305 (RI, UV and viscometer) can be used. A calibration curve can be constructed with poly(methyl methacrylate) (PMMA) standards (Mn,max=50,352 g / mol, D of 1.023).

[0041] 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.

[0042] 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.

[0043] It is preferred that after the polymerization ring-opening polymerization, the unreacted lactide remaining in the polymerization 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. As described above, such removal of unreacted lactide is preferably 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 polymerization 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.

[0044] 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.

[0045] According to a further aspect, the present invention relates to an apparatus for producing polylactic acid, wherein the apparatus comprises:

[0046] 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,

[0047] 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,

[0048] iii) a crystallization unit comprising an inlet line for a composition depleted in meso-lactide or a mixture of a composition depleted in meso-lactide and a portion of the composition comprising unreacted lactide, and comprising an outlet line for a purified composition depleted in meso-lactide, and

[0049] iv) a polymerization reactor comprising an inlet line for a polymerization composition and an outlet line for a crude polylactic acid composition, and optionally 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,

[0050] Therein, the outlet pipeline of the purified meso-lactide-enriched composition of the crystallization unit ii) and / or the outlet pipeline of the purified meso-lactide-depleted composition of the crystallization unit iii) are connected to the inlet pipeline of the polymerization reactor.

[0051] According to a particularly preferred embodiment of the present invention, the outlet pipeline of the meso-lactide-enriched composition of the separation unit directly leads to the inlet pipeline of the meso-lactide-enriched composition of the crystallization unit ii), wherein the outlet pipeline of the meso-lactide-depleted composition of the separation unit directly leads to the inlet pipeline of the meso-lactide-depleted composition of the crystallization unit iii), 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, and wherein 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. Alternatively, the outlet pipeline of the purified meso-lactide-enriched composition of the crystallization unit ii) and the outlet pipeline of the purified meso-lactide-depleted composition of the crystallization unit iii) are merged with each other, and the merged pipeline is connected to the inlet pipeline of the polymerization reactor.

[0052] 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 outlet pipeline of the meso-lactide-depleted composition of the separation unit directly leads to the inlet pipeline of the meso-lactide-depleted composition of the crystallization unit iii), and 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.

[0053] 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.

[0054] 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).

[0055] 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.

[0056] 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.

[0057] The invention is then described by means of the accompanying illustrative and non-limiting drawings, in which:

[0058] Figure 1 A schematic diagram of an apparatus for producing polylactic acid according to an embodiment of the present invention is shown.

[0059] Figure 2 Shows Figure 1 Detailed schematic diagram of a static crystallizer for purifying a meso-lactide-enriched composition of the apparatus shown.

[0060] Figure 3 A schematic diagram of an apparatus for producing polylactic acid according to another embodiment of the present invention is shown.

[0061] Figure 4 Shows Figure 3 Detailed schematic diagram of a static crystallizer for purifying a meso-lactide-enriched composition of the apparatus shown.

[0062] 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 also 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, which leads to a second devolatilizer 86, which includes a vapor outlet line 88 and a polylactic acid outlet line 90.

[0063] 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 column 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 column 32 through a pipeline 40 as a side stream of the first distillation column 30. The pre-purified lactide stream is separated into an overhead stream and a bottom stream in the second distillation column 32, wherein the overhead stream is a meso-lactide-enriched composition and the bottom stream is a meso-lactide-depleted composition. The meso-lactide-enriched composition is fed to a static crystallizer 44 through a pipeline 42, while the meso-lactide-depleted composition is fed to a falling film crystallizer 48 through a pipeline 46. The purified meso-lactide-enriched composition is discharged from the static crystallizer 44 through a pipeline 52, and the residue is discharged as a purge stream through a purge line 58. Likewise, a purified meso-lactide-depleted composition is discharged from falling film crystallizer 48 via line 60, while a residue is discharged via line 66 and divided into a recycle stream, which is fed to second distillation column 32 via reflux line 70, and a purge stream, which is discharged via purge line 68. The purified meso-lactide-enriched composition is divided into a portion which is discharged from apparatus 10 via product line 54 as purified meso-lactide, and a portion which is fed to polylactic acid reactor 74 via lines 56 and 72, while the purified meso-lactide-depleted composition is divided into a portion which is discharged from apparatus 10 via product line 62 as purified L-lactide, and a portion which is fed to polylactic acid reactor 74 via lines 56 and 72. Lactide is polymerized into polylactic acid in polylactic acid reactor 74 in the presence of a catalyst and an initiator, which are fed to polylactic acid reactor 74 via inlet line 76. The crude polylactic acid stream is fed via line 78 to a first devolatilizer 80, wherein the crude polylactic acid stream is separated into a polylactic acid melt stream and a vapor stream comprising unreacted lactide and trace amounts of catalyst and initiator.The vapor fraction is discharged from the first devolatilizer 80 through pipeline 82, and the polylactic acid melt stream is fed to the second devolatilizer 86 through pipeline 84, wherein the polylactic acid melt stream is separated into a vapor stream containing unreacted lactide and trace amounts of catalyst and initiator, which is discharged from the second devolatilizer 86 through pipeline 88, and a purified polylactic acid, which is discharged from the device 10 through pipeline 90.

[0064] Figure 2 Schematically shows Figure 1 4 . The static crystallizer 44 of the apparatus 10 shown in FIG. 4 includes a first static crystallizer stage 92 and a second static crystallizer stage 92 '. The composition enriched in meso-lactide is fed to the first static crystallizer stage 92 of the static crystallizer 44 via line 42, where it is crystallized. During the crystallization process, meso-lactide is crystallized on the cooled surface of the first crystallizer stage 92, while the mother liquor depleted in meso-lactide remains. After the crystallization is completed, the mother liquor is discharged from the first crystallizer stage 92 via the purge line 58 as a purge, and the crystal layer of meso-lactide deposited during the first crystallizer stage 92 is subjected to a sweating step, wherein the sweating fraction (not shown) thus obtained is added to the purge stream discharged via 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 into the second crystallizer stage 92 ' via line 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 via line 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 via outlet line 52.

[0065] Figure 3 and Figure 4 The apparatus 10 and the static crystallizer 44 shown in FIG. 4 correspond to Figure 1 and Figure 2 Those, except Figure 3In the apparatus 10 shown in FIG. 1 , recycle lines 98, 98' and 98" are provided. 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. In this embodiment, the unreacted lactide from the first devolatilizer 80 is reused in the polymerization step carried out in the polylactic acid reactor 74. Any portion of the vapor stream withdrawn from the first devolatilizer 80 via the line 82 can be recycled to the static crystallizer 44 and / or the falling film crystallizer 48 via the recycle lines 98, 98' and 98".

[0066] Reference numerals

[0067] 10. Equipment

[0068] 12 Dehydration Unit

[0069] 14 Prepolymerization Reactor

[0070] 16 Depolymerization Reactor

[0071] 18 Lactic acid inlet line

[0072] 20 pipelines

[0073] 22 Return line

[0074] 24 Pipeline

[0075] 25 Water outlet pipeline

[0076] 26 Return line

[0077] 28 Clear the pipeline

[0078] 30 (First) Distillation Tower

[0079] 32 (Second) Distillation Tower

[0080] 34 pipeline

[0081] 36 Return line

[0082] 38 Return line

[0083] 40 pipeline

[0084] 42 pipeline

[0085] 44 Static Crystallizer

[0086] 46 pipeline

[0087] 48 Falling Film Crystallizer

[0088] 50 Return line

[0089] 52 Export pipeline

[0090] 54 Product pipeline

[0091] 56 Connecting pipeline

[0092] 58 Clear pipeline

[0093] 60 Export pipeline

[0094] 62 Product pipeline

[0095] 64 Connection pipeline

[0096] 66 Export pipeline

[0097] 68 Clear Line

[0098] 70 Return line

[0099] 72 Monomer inlet pipeline

[0100] 74. Polylactic acid reactor

[0101] 76 Inlet pipeline

[0102] 78 Export pipeline

[0103] 80 First Devolatilizer

[0104] 82 Steam export pipeline

[0105] 84 melt outlet pipeline

[0106] 86 Second Devolatilizer

[0107] 88 Steam export pipeline

[0108] 90 Polylactic acid export pipeline

[0109] 92 First static crystallization stage

[0110] 92' Second static crystallization stage

[0111] 94 pipeline

[0112] 96 pipeline

[0113] 98, 98', 98" recycle line from devolatilization

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 less than 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 optionally 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 or a mixture of the meso-lactide-enriched 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-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 polymerizable composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d) and / or at least a portion of the purified meso-lactide-depleted composition produced in step e).

2. The process of claim 1 , wherein in step d) the meso-lactide-enriched composition is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) the meso-lactide-depleted composition is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and wherein the polymerized 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).

3. The process of claim 1 , wherein in step d) the mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) the meso-lactide-depleted composition is subjected to a purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and wherein the polymerized composition comprises at least a portion of the purified meso-lactide-enriched composition.

4. The process of claim 1 , wherein in step d) the meso-lactide-enriched composition is subjected to purification comprising at least one crystallization step to produce a purified meso-lactide-enriched composition, wherein in step e) a mixture of the meso-lactide-depleted composition and a portion of the composition comprising unreacted lactide is subjected to purification comprising at least one crystallization step to produce a purified meso-lactide-depleted composition, and wherein the polymerized 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 according to any one of the preceding claims, wherein the polymerization composition comprises i) at least a portion of the purified meso-lactide-enriched composition produced in step d) and / or at least a portion of the purified meso-lactide-depleted composition produced in step e) and ii) at least a portion of a composition comprising unreacted lactide.

6. The method of claim 5, wherein the polymerization composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d), at least a portion of the purified meso-lactide-depleted composition produced in step e), and at least a portion of the composition comprising unreacted lactide.

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 four static crystallization stages, more preferably 2 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 as a side fraction 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 the meso-lactide-enriched composition or a mixture of the meso-lactide-enriched composition and a portion of the composition comprising unreacted lactide, and comprising an outlet line (52) for the 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 optionally 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, the outlet pipeline (52) of the purified meso-lactide-enriched composition of the crystallization unit ii) (44) and / or the outlet pipeline (60) of the purified meso-lactide-depleted composition of the crystallization unit iii) (48) are connected to the inlet pipeline (72) of the polymerization reactor (74).

14. The device (10) according to claim 13, wherein: i) the outlet line (42) of the meso-lactide-enriched composition of the separation unit is directly connected to the inlet line (42) of the meso-lactide-enriched composition of the crystallization unit ii) (44), wherein the outlet line (46) of the meso-lactide-depleted composition of the separation unit is directly connected to the inlet line (46) of the meso-lactide-depleted composition of the crystallization unit iii) (48), 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 wherein 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 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).

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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