Plant and efficient method for producing polylactic acid using lactide obtained by devolatilization of polylactic acid

Abstract

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, the meso-lactide-enriched composition containing at least 80 mol-% meso-lactide, based on the total lactide content; c) polymerizing a polymerization composition containing meso-lactide and at least one of L-lactide and D-lactide to form a crude polylactic acid composition, and devolatilizing the crude polylactic acid composition to produce a purified polylactic acid composition and a composition containing unreacted lactide; d) subjecting the meso-lactide-enriched composition and at least a portion of the composition that includes unreacted lactide to purification, including 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 that includes unreacted lactide, to purification, including at least one crystallization step, to produce a purified meso-lactide-depleted composition. Including, A method of producing polylactic acid, wherein the polymerization composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d).

Description

[Technical field]

[0001] The present invention relates to a process and a plant for the production of polylactic acid from lactide, preferably a continuous production plant. [Background technology]

[0002] Lactic acid polymers are of particular interest because they are obtained from renewable resources and are largely compostable and / or biodegradable. Moreover, the technical and physicochemical properties of these polymers are very close to those of polymers obtained from fossil resources, so that these polymers are considered as very promising alternatives to fossil resources. Furthermore, lactic acid polymers are used in a wide range of applications. For example, polylactic acid is used in the biomedical field, such as in surgical implants, films such as packaging materials, textiles such as clothing, sanitary products, carpets, disposable plastic products such as disposable tableware and containers, etc. Furthermore, polylactic acid is also widely applied in composite materials such as fiber-reinforced plastics.

[0003] In general, two main alternative methods are known for the synthesis of polylactic acid. The first main method is the direct polycondensation of lactic acid to polylactic acid. However, this main method only gives low molecular weight polymers and is limited to certain polymers.

[0004] The second major method known for the synthesis of polylactic acid is the ring-opening polymerization of lactide, a cyclic diester of lactic acid. This is currently the preferred method for 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 oligomer or prepolymer, respectively, to a depolymerization reaction. For example, lactide is prepared by fermenting carbohydrates from biomass such as starch, sugar, or corn to produce lactic acid, prepolymerizing the lactic acid, and then subjecting the oligomer to a depolymerization reaction. The depolymerization reaction of lactic acid oligomers to lactide is an equilibrium reaction. To drive the reaction toward lactide, lactide must be removed from the system. In continuous depolymerization, where lactic acid oligomers are continuously fed to the reactor, the crude lactide is evaporated under vacuum. The synthesis reactor is preferably a distillation column, in which case synthesis takes place mainly in the sump of the distillation column, but also in other parts where the lactide composition is not in equilibrium with the lactic acid oligomers. The depolymerization reactor produces a crude lactide stream containing lactide and lactic acid, lactic acid oligomers, and other impurities. This crude lactide stream is usually purified, and then the purified lactide is polymerized in the presence of a catalyst and, optionally, an initiator to form a crude polylactic acid composition, such as a crude high molecular weight polylactic acid composition. After polymerization, in order to obtain a product of marketable quality, it is necessary to remove unreacted lactide from the crude polylactic acid composition to a final concentration of at least less than 0.5% by weight. Removal of unreacted lactide and other low boiling components of the crude polylactic acid composition is usually performed by devolatilization. More specifically, the crude polylactic acid composition is subjected to one or more devolatilization steps carried out at elevated temperature and reduced pressure. During devolatilization, unreacted lactide and other low-boiling and close-boiling components of lactide, components formed as by-products of polymerization in the polylactic acid reactor, are removed as a vapor fraction from the polylactic acid to obtain 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 may be recycled to the polymerization reaction.

[0005] Lactic acid is chiral and exists in the enantiomeric forms L- and D-lactic acid, also called S- and R-lactic acid, respectively. Thus, the dimer of lactic acid, i.e. lactide, exists in three stereoisomeric forms: 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 its processing properties, crystallization properties, and degradation behavior, depend on the structure and composition of the polymer chain, especially the ratio of the L- and D-stereoisomers of lactic acid. The stereochemical structure of polylactic acid can be modified by copolymerization of a mixture of L-lactide, meso-lactide, and D-lactide, resulting in high molecular weight amorphous or semicrystalline polymers with melting points in the range of 130-185 °C. In addition, homopolymers consisting of only L- or D-lactic acid monomers can also be produced. For example, isotactic poly(lactic acid) homopolymers containing only L-lactic acid monomers are semicrystalline materials with the highest melting points, whereas poly(lactic acid) copolymers containing L-lactic acid and D-lactic acid monomers with comparable high D stereoisomer content exhibit lower melting points and slower crystallization behavior. Therefore, a method is desired that can easily and reliably produce poly(lactic acid) with a defined stereoisomeric structure from lactide.

[0006] With this in mind, the object of the present invention is to provide a method for producing polylactic acid by first producing an ultra-pure lactide with high stereochemical purity, i.e. a composition of meso-lactide and a composition of L-lactide and / or D-lactide, and then polymerizing one or both of these lactide compositions to produce polylactic acid having a predetermined ratio of the two stereoisomers of lactic acid. Furthermore, the process should be flexible so that it can be easily modified with respect to the ratio of the three stereoisomers of lactide fed to the polymerization reactor, 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 changed to the production of a second specific polylactic acid having a second ratio of D-lactide and L-lactide. Finally, the process should be economical, i.e. characterized by a high yield per unit of polylactic acid produced and low operating costs.

[0007] According to the present invention, this object is achieved by providing a method for producing polylactic acid, the method comprising the steps of: a) providing a crude lactide composition comprising meso-lactide and at least one of L-lactide and D-lactide; b) separating from the crude lactide composition a meso-lactide-enriched composition and a meso-lactide-depleted composition, the meso-lactide-enriched composition containing at least 80 mol-% (or 80 wt. %) meso-lactide, based on the total lactide content; c) polymerizing a polymerization composition containing meso-lactide and at least one of L-lactide and D-lactide to form a crude polylactic acid composition, and devolatilizing the crude polylactic acid composition to produce a purified polylactic acid composition and a composition containing unreacted lactide; d) subjecting at least a portion of the meso-lactide-enriched composition and the composition that includes unreacted lactide to purification, including 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 that includes unreacted lactide, to purification, including at least one crystallization step, to produce a purified meso-lactide-depleted composition. Including, The polymerized composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d).

[0008] The above process steps not only allow the production of polylactic acid with high stereochemical purity at a given lactic acid stereoisomer ratio, but also the method is flexible and can be easily changed with respect to the ratio of the three stereoisomers of lactide fed to the polymerization reactor, 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 changed to the production of a second specific polylactic acid with a second ratio of D-lactide and L-lactide. For this reason, it is particularly advantageous to (re)use in the process at least a portion of the unreacted lactide obtained after devolatilization of the crude polylactic acid composition, i.e., the composition containing unreacted lactide, as a purified monomer for polymerization. That is, the composition containing unreacted lactide obtained after devolatilization of the crude polylactic acid composition has, in total, an equally high content of L-lactide and D-lactide and an equally low content of meso-lactide, so that the ratio of the three lactide stereoisomers in the polymerization composition can be reliably adjusted as required by adjusting the amount of the composition containing unreacted lactide relative to the amount of the meso-lactide-rich composition fed to the polymerization step c) and, optionally, relative to the amount of the meso-lactide-depleted composition. Furthermore, the ratio of the three lactide stereoisomers fed to the polymerization reactor can be quickly and easily changed by simply changing the amount of the unreacted lactide-containing composition relative to the amount of the meso-lactide-rich composition fed to the polymerization step c), and, optionally, the amount of the unreacted lactide-containing composition relative to the amount of the meso-lactide-depleted composition. A further advantage of the process according to the invention is that - due to the crystallization step carried out during the process - the lactide stereoisomer fed to the polymerization reactor is ultra-pure, i.e. the lactide is at least essentially free of hydroxyl (including hydroxycarboxylic acid) impurities such as water, lactic acid and other impurities, which would prevent the achievement of the desired molecular weight. The low content of water, lactic acid and other impurities ensures the production of very pure polylactic acid with the desired stereoisomeric purity.The acid content of lactide is, for example, preferably less than 20 milliequivalents per kilogram (meq / kg), more preferably less than 15 meq / kg, even more preferably less than 10 meq / kg, even more preferably less than 5 meq / kg, and most preferably less than 2 meq / kg. In particular, the lactide stereoisomers fed to the polymerization reactor as the polymerization composition are substantially free of by-products such as 4-hydroxy-5-methyl-4-cyclopentene-1,3-dione (HMCP) formed during the separation step b) and during polymerization due to the relatively high temperatures during these steps. This is an important advantage, since HMCP formed during distillation and polymerization at high temperatures is an impurity that causes yellowing of polylactic acid and is therefore a particularly important impurity for polylactic acid. HMCP is only slightly more volatile than meso-lactide and therefore cannot be separated from meso-lactide during distillation. However, the method of the invention further ensures that during crystallization step d) the formed HCMP and other impurities having boiling points close to that of meso-lactide are removed from the meso-lactide-enriched composition. Furthermore, crystallization step e) ensures that the meso-lactide-depleted composition is particularly pure and contains only trace amounts of impurities. For example, the meso-lactide-depleted composition may have a low free acid content, such as less than 2 meq / kg. Such a low free acid content of less than 2 meq / kg can be achieved even when the polymerization 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 compositions produced, and in particular the purified meso-lactide-enriched compositions produced, have a very high thermal stability and can be stored as liquid or solid materials (i.e., purified meso-lactide-enriched compositions and purified meso-lactide-depleted compositions) for several months without degradation of the parts of the lactide compositions not required for the polymerization process. Finally, the process according to the invention is particularly economical since - due to the (re)use of the composition comprising unreacted lactide obtained after devolatilization of the crude polylactic acid composition - it is characterized by a high yield per unit of polylactic acid produced and low operating costs.

[0009] By meso-lactide-enriched composition, we mean that the respective composition produced during separation step b) has a higher meso-lactide content than the crude lactide composition fed to separation step b). According to the present invention, the meso-lactide-enriched composition contains at least 80 mole % (or 80 weight %) of meso-lactide, based on the total lactide content. Similarly, by meso-lactide-depleted composition, we mean that the respective composition produced during separation step b) has a lower meso-lactide content than the crude lactide composition fed to separation step b). By purified meso-lactide-enriched composition, we mean the composition obtained in step d) that contains at least a portion of the meso-lactide-enriched composition that has been subjected to at least one crystallization step and at least a portion of the composition containing unreacted lactide obtained by devolatilization of the crude polylactic acid composition that has been subjected to at least one crystallization step.

[0010] According to the present invention, in step d), the meso-lactide-rich 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 are subjected to purification including at least one crystallization step. In one embodiment, this involves mixing the meso-lactide-rich composition produced in step b) with 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 in step d). However, in another embodiment, it is also possible to subject the meso-lactide-rich composition produced in step b) to at least one crystallization step in step d) and separately subject at least a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition to at least one crystallization step in step d), and then mix at least a portion of both purification streams together to form a purified meso-lactide-rich mixture. In the latter embodiment, the meso-lactide-rich composition produced in step b) and at least a portion of the composition containing unreacted lactide obtained in the volatilization step of the crude polylactic acid composition are supplied separately to the same crystallization step, purified therein, and a purified meso-lactide-rich mixture is withdrawn from the crystallization step. Alternatively, the meso-lactide-rich composition produced in step b) and at least a portion of the composition containing unreacted lactide obtained in the volatilization step of the crude polylactic acid composition may be supplied separately to different crystallization steps, for example, the meso-lactide-rich composition produced in step b) is supplied to a first crystallization step, and at least a portion of the composition containing unreacted lactide obtained in the volatilization step of the crude polylactic acid composition is supplied to a second crystallization step. In this variation, the crude polylactic acid composition is purified in a first crystallization step and is withdrawn 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 is withdrawn from the second crystallization step, and then both purified streams are combined into the purified meso-lactide-enriched mixture.In other words, "subjecting at least a portion of a composition that includes a meso-lactide-enriched composition and unreacted lactide to purification, including at least one crystallization step, to produce a purified meso-lactide-enriched composition" means that at the end of step d), a purified composition is obtained, denoted as a purified meso-lactide-enriched mixture, that includes at least a portion of the meso-lactide-enriched composition that has been purified by crystallization, and at least a portion of the unreacted lactide composition obtained in the devolatilization step that has been purified by crystallization.

[0011] In step e), either the meso-lactide-depleted composition produced in step b) or a mixture of the meso-lactide-depleted composition and a portion of the composition containing unreacted lactide obtained in the devolatilization step of crude polylactic acid is subjected to purification including at least one crystallization step.

[0012] The method according to the invention covers all four possible combinations of the aforementioned embodiments, namely, i) in step d), the meso-lactide-enriched composition and, separately, at least a portion of the composition containing unreacted lactide obtained in the devolatilization step are each subjected to a separate crystallization step, and then both compositions are mixed with each other to obtain a purified meso-lactide-enriched composition, and in step e), the meso-lactide-depleted composition (i.e., not mixed with the portion of the composition containing unreacted lactide obtained in the devolatilization step) is subjected to at least one crystallization step; and ii) in step d), a mixture of the meso-lactide-enriched composition and at least a portion of the composition containing 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., not mixed with the portion of the composition containing unreacted lactide obtained in the devolatilization step) is subjected to at least one crystallization step. ) is subjected to at least one crystallization step; iii) in step d), the meso-lactide-enriched composition and, separately, at least a portion of the composition containing unreacted lactide obtained in the devolatilization step are each subjected to a separate crystallization step, and then both compositions are mixed with each other to obtain a purified meso-lactide-enriched composition; in step e), a mixture of the meso-lactide-depleted composition and 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; and iv) in step d), a mixture of the meso-lactide-enriched composition and 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, and in step e), a mixture of the meso-lactide-depleted composition and 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. Regardless of whether a portion of the composition containing unreacted lactide obtained in the devolatilization step is added to the meso-lactide-depleted composition prior to purification by crystallization in step e), the purified or crystallized composition, respectively, is referred to herein as a purified meso-lactide-depleted composition.

[0013] In all of the embodiments described above and all of the embodiments described below, when the devolatilization step is performed two or more times using a crude polylactic acid composition, the composition containing unreacted lactide obtained in the devolatilization step is preferably the respective composition obtained in the first devolatilization step.

[0014] According to the invention, the polymerization composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d). This means that the polymerization composition essentially comprises at least a portion of the purified meso-lactide-enriched composition produced in step d), and in addition, may or may not also comprise at least a portion of the purified meso-lactide-depleted composition produced in step e). Preferably, the polymerization composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d), as well as at least a portion of the purified meso-lactide-depleted composition produced in step e).

[0015] In particular, good results are obtained when the polymerization composition comprises from 0 to greater than 25 weight percent of the purified meso-lactide-enriched composition produced in step d), and from 75 to less than 100 weight percent of the purified meso-lactide-depleted composition produced in step e).More preferably, the polymerization composition comprises from 0 to 20 weight percent of the purified meso-lactide-enriched composition produced in step d) and from 0 to 100 weight percent of the purified meso-lactide-depleted composition produced in step e).

[0016] It is preferred that the ratio of purified meso-lactide-enriched composition to purified meso-lactide-depleted composition in the polymeric composition is from 10:1 to 1:10 by weight, such as from 5:1 to 1:1 by weight.

[0017] In a further development of the idea according to the present invention, it is proposed to subject the composition containing 10 to 100% by weight, preferably 50 to 100% by weight, of unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition to at least one crystallization step in step d).

[0018] Preferably, in step d), the weight ratio of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition to be subjected to at least one crystallization step is 0 to 90% by weight, preferably 30 to 80% by weight, based on the total of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition and the meso-lactide-enriched composition to be subjected to at least one crystallization step.

[0019] According to a further preferred embodiment of the present invention, the composition containing 0 to 90% by weight, preferably 50 to 90% by weight, of unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition is subjected to at least one crystallization step in step e).

[0020] When a portion of the composition containing unreacted lactide obtained in the volatilization step of the crude polylactic acid composition is added to the meso-lactide-depleted composition in step e) before purification, the weight ratio of the composition containing unreacted lactide obtained in the volatilization step of the crude polylactic acid composition to be subjected to at least one crystallization step is preferably 0 to 50% by weight, more preferably 0 to 20% by weight, based on the total of the composition containing unreacted lactide obtained in the volatilization step of the crude polylactic acid composition and the meso-lactide-depleted composition to be subjected to at least one crystallization step.

[0021] Further according to the invention, in step c) a polymerization composition is used that includes 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 then all or a portion of the purified meso-lactide-depleted composition produced in step e) may be polymerized. Furthermore, even if less preferred, this also includes an embodiment in which a mixture of all or a portion of the purified meso-lactide-rich composition produced in step d) (which, as described above, includes lactide from the composition containing unreacted lactide obtained in the devolatilization step) and a portion of the composition containing unreacted lactide obtained in the devolatilization step (which may or may not have been purified before polymerization, i.e., added to the already purified meso-lactide-rich composition after purification, including at least one crystallization step) is polymerized in step c) together with all or a portion of the purified meso-lactide-depleted composition produced in step d) (which, as described above, may or may not include lactide from the composition containing unreacted lactide obtained in the devolatilization step). In other words, a portion of the composition containing unreacted lactide obtained in the devolatilization step of the crude polylactic acid composition may be added to the polymerization composition in an unpurified form, even if less preferred. The polymerization composition may also include additional lactide from other sources, even if less preferred. However, it is preferred that the polymerization composition used in step c) contains at least 60% by weight, preferably at least 80% by weight, more preferably at least 90% by weight, even more preferably at least 95% by weight, even more preferably at least 98% by weight, even more preferably at least 99% by weight, and most preferably at least 100% by weight, of lactide from the purified meso-lactide-enriched composition and lactide from 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, and preferably does, include non-lactide components useful or necessary for the polymerization reaction, such as one or more catalysts, one or more initiators, etc.

[0022] As mentioned above, the polymerization composition used in step c) preferably comprises at least one catalyst for the ring-opening reaction of lactide to polylactic acid. The invention is in principle not particularly limited with respect to the chemical nature of the catalyst used. In particular, good results are obtained when the catalyst is at least one organometallic compound. Particularly good results are obtained when the catalyst is at least one organometallic compound comprising a metal selected from the group consisting of magnesium, titanium, zinc, aluminum, indium, yttrium, tin, lead, antimony, bismuth, and any combination of two or more of the aforementioned metals. The at least one organometallic compound preferably comprises, as organic residue, a residue selected from the group consisting of alkyl groups, aryl groups, halides, oxides, alkanoates, alkoxides, and any combination of two or more of the aforementioned groups. More preferably, the catalyst is at least one organometallic compound comprising aluminum and / or tin as metal. Even more preferably, the catalyst is selected from the group consisting of tin octoate, tetraphenyltin, butyltin trimethoxide, dibutyltin oxide, aluminum isopropoxide, Al(Oi-Pr) p [In the formula, 1≦p≦3], Et3-pAl(O(CH2)2X) p [wherein 1≦p≦3], α, β, γ, δ, ε tetraphenylporphinatoaluminum (TPPIAIX), and at least one organometallic compound selected from the group consisting of any combination of two or more of the aforementioned compounds. Particularly suitable as catalysts are tin octoates, such as tin(II) 2-ethylhexanoate.

[0023] The catalyst content in the polymer composition is preferably 10 to 10,000 ppm, and more preferably 50 to 1,000 ppm.

[0024] 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 at least one catalyst as described above. The at least one initiator is preferably a hydroxy compound, more preferably a hydroxy compound selected from the group consisting of monohydroxy compounds, dihydroxy compounds, trihydroxy compounds, tetrahydroxy compounds, and any combination of two or more of the aforementioned compounds. The functional group of the at least one hydroxy compound allows the design of the resulting copolymer to be tailored. When a monohydroxy compound is used, linear copolymers are produced, whereas branched copolymers can be produced by using one or more dihydroxy compounds, trihydroxy compounds, and / or tetrahydroxy compounds. The at least one initiator is preferably 2-ethylhexanol, 1-decanol, C 10 ~C 20 Particularly good results are obtained when the glycerol is selected from the group consisting of monohydroxy fatty alcohols, benzyl alcohol, p-phenylbenzyl alcohol, ethylene glycol, propylene glycol, butane-1,4-diol, poly(ethylene glycol) having a weight average molecular weight of 200 to 10,000 g / mol, 2-hydroxymethyl-1,3-propane, glycerin, polyglycerol having a weight average molecular weight of 100 to 1,000 g / mol, trihydroxybenzene (phloroglucinol), trimethylolpropane and its dimers, pentaerythritol and its dimers, and any combination of two or more of the above compounds.

[0025] The content of the initiator in the polymerizable composition is preferably 0.1 to 50 meq or 0.1 to less than 50 mmol / kg, and more preferably 0.5 to 40 meq or mmol / kg, respectively.

[0026] According to the present invention, the purification carried out in step d) to produce a purified meso-lactide-enriched composition comprises at least one crystallization step, which may comprise 1 to 10 crystallization stages, for example 1 crystallization stage, or 2 to 10, preferably 2 to 4 crystallization stages. In principle, the present invention is not particularly limited with respect to the type of crystallization used in step d). The crystallization step may therefore 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 the skilled person 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 mean crystallization carried out from the melt, i.e. with an already molten starting material, without the addition of further components, such as, for example, a solvent. Melt crystallization can be carried out in the form of layer crystallization or in the form of suspension crystallization. To carry out layer crystallization, a cooling surface is usually introduced into the melt of optically active or inactive lactide used as starting material. A crystalline layer of lactide crystals, either enriched or not enriched in the enantiomer, is then formed on the cooled surface and can be separated from the remaining melt. The crystalline enriched lactide thus obtained can be melted again. This operation can then be repeated as many times as necessary to increase the purity and yield. However, particularly good results are obtained when the purification carried out in step d) comprises at least one static crystallization step. Static crystallization has the advantage that it allows economical purification even when the crystallization liquid contains relatively large amounts of the compound. During static crystallization, the liquid phase does not move, so that the crystals form and grow in the static liquid phase. More specifically, a typical static crystallization apparatus comprises multiple walls, such as plates, tubes or finned tubes, which can be cooled and heated by circulating a heat transfer medium inside the plates.At the start, the static crystallizer is filled with the liquid feed mixture, i.e., the meso-lactide-enriched composition, or a mixture of the meso-lactide-enriched composition and a portion of the composition containing unreacted lactide, and the plate is contacted with the liquid feed mixture. The plate of the static crystallization vessel is then cooled to a temperature below the equilibrium freezing temperature of the liquid feed mixture, and crystals enriched in meso-lactide are formed and deposited on the cooled outer surface of the plate. After crystallization is completed, the liquid mixture is completely removed from the static crystallization vessel, the cooling of the plate is terminated, and the plate is optionally heated to melt the crystal layer formed on the outer surface of the plate, and then the melt is removed from the crystallization vessel to obtain the purified meso-lactide-enriched composition. To increase the purity of meso-lactide, the crystal layer can be sweated by slowly heating it to a temperature close to the melting point of meso-lactide to partially melt the crystals and then melting them.

[0027] As described above, the purification carried out in step d) to produce a purified meso-lactide-enriched composition includes at least one crystallization step, which can include 1 to 10 crystallization stages, for example 1 crystallization stage or 2 to 10 crystallization stages, preferably 2 to 4 crystallization stages, and most preferably a static crystallization stage. When 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 are separately subjected to purification by crystallization, preferably static crystallization, and then at least a portion of both purification streams are mixed into the purified meso-lactide-enriched composition, step d) includes 2 or more, preferably 2 to 10, more preferably 2 to 4 crystallization stages, and most preferably a static crystallization stage.

[0028] Typically, the meso-lactide-enriched composition is obtained in separation step b) as an overhead vapor composition of the 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] The invention is also not particularly limited with respect to the type of crystallization used in step e). More specifically, 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. 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, in dynamic crystallization the liquid phase is moved or agitated. Dynamic crystallization, in particular falling film crystallization, has the advantage that it allows economic purification of compounds that are present in relatively small amounts in the crystallization liquid.

[0030] Preferably, the at least one dynamic crystallization process comprises 1 to 4 dynamic crystallization stages, more preferably 1 dynamic crystallization stage. Particularly preferably, the at least one dynamic crystallization process comprises 1 to 4 falling film crystallization stages, most preferably 1 falling film crystallization stage.

[0031] The invention is also not particularly limited with respect to the type of separation technique used in step b). Preferably, step b) includes one or more distillation steps, each of which is carried out in a distillation column, and the crude lactide composition is fed to the distillation column of the first distillation step if multiple distillation steps are included, or to the distillation column of the first distillation step if only one distillation step is included. The meso-lactide-enriched composition is produced as an overhead fraction, and the meso-lactide-depleted composition is produced as the bottoms or side stream fraction of the distillation column of the last distillation step if multiple distillation steps are included, or as the bottoms or side stream fraction of the distillation column of the first 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 in only one distillation column. The crude lactide composition is fed to the distillation column, the meso-lactide-enriched composition is withdrawn from the distillation column as an overhead fraction, and the meso-lactide-depleted composition is withdrawn from the distillation column as a bottoms fraction. This embodiment is particularly suitable when the crude lactide composition contains relatively small amounts of oligomers, such as lactic acid oligomers, e.g., when the crude lactide composition contains at most 3% by weight oligomers, or preferably at most 1% by weight oligomers.

[0033] As a further development of the idea of ​​the present invention, it is proposed that step b) comprises two distillation steps, i.e. is carried out in two successive distillation columns, each distillation step being carried out in a distillation column, and the crude lactide composition is fed to the first distillation column, and the crude lactide composition is preferably separated into an overhead fraction containing light compounds, a bottoms liquid containing heavy compounds, and a side fraction containing meso-lactide. The meso-lactide-enriched fraction of the first distillation column is fed to a second distillation column, where the meso-lactide-enriched fraction is further purified, producing a meso-lactide-enriched composition as the overhead fraction and a meso-lactide-depleted composition as the side fraction or bottoms. Preferably, the meso-lactide-enriched side fraction of the first distillation column is fed to a second distillation column, where a meso-lactide-enriched composition is produced as the overhead fraction and a meso-lactide-depleted composition is produced as the side fraction or bottoms. This embodiment is particularly suitable when the crude lactide composition contains a comparable amount of oligomers, such as lactic acid oligomers, e.g., when the crude lactide composition contains at least 10% by weight oligomers, preferably at least 3% by weight 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 multiple distillation columns are present, or to the distillation column if only one distillation column is present.

[0035] In step a), a crude lactide composition is provided which comprises meso-lactide and at least one of L-lactide and D-lactide. Preferably, the content of meso-lactide in the crude lactide composition is from 1 to 20% by weight, based on the total lactide content, preferably from 3 to 10% by weight of meso-lactide, the remainder to 100% by weight being L-lactide and D-lactide.

[0036] As mentioned above, in step b) a meso-lactide-enriched composition is produced which contains at least 80 mole % (or 80 weight %) of meso-lactide based on the total lactide content. Also according to the invention, a meso-lactide-enriched composition means that the respective composition produced during separation step b) has 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 weight % meso-lactide based on the total lactide content, the meso-lactide-enriched composition contains more than 15 weight % but at least 80 weight % meso-lactide based on the total lactide content. Particularly good results are obtained when the meso-lactide-enriched composition contains more than 80 weight %, more preferably more than 80-99 weight %, most preferably more than 80-95 weight % meso-lactide. Additionally, the meso-lactide enriched composition preferably has less than 20% by weight of the sum of L-lactide and D-lactide.

[0037] Similarly, in the present invention, a meso-lactide-depleted composition means that the respective 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, then the meso-lactide-depleted composition contains less than 15% by weight of meso-lactide, based on the total lactide content. Furthermore, it is preferred that the meso-lactide-depleted composition produced in step b) contains at least 85% by weight, more preferably at least 90% by weight, L-lactide, less than 2% by weight, more preferably less than 0.5% by weight, D-lactide, and less than 12% by weight, more preferably less than 5% by weight, of meso-lactide, based on the total lactide content.

[0038] As a further development of the idea of ​​the present invention, it is proposed that step a) comprises a step of prepolymerizing lactic acid to produce lactic acid oligomers, and then subjecting the lactic acid oligomers to a depolymerization reaction to obtain a crude lactide composition. For example, the crude lactic acid composition subjected to the prepolymerization contains 70% by weight or more, more preferably 80% by weight or more, of lactic acid. More specifically, the lactic acid portion of the crude lactic acid composition (i.e., the portion not containing water and other impurities) contains more than 97% by weight of L-lactic acid and 0.1-3% by weight of D-lactic acid, and the total content of L-lactic acid and D-lactic acid in the lactic acid portion of the crude lactic acid composition is preferably 97-99.9% by weight. Suitable catalysts for the depolymerization reaction include tin compounds such as SnO, SnCl2, Bu2SnO, Sn(Oc)2, as well as Sb2O3, H2SO4, ZnO, and any combination of two or more of the above catalysts. The prepolymerization reaction is preferably carried out at a temperature of 150-200° C., while the depolymerization reaction is preferably carried out at a temperature of 190-240° C. and a pressure of less than 10 mbar.

[0039] According to certain preferred embodiments of the present patent application, the number average molecular weight of the polylactic acid produced during polymerization is preferably at least 5,000 g / mol, preferably at least 15,000 g / mol, more preferably at least 25,000 g / mol, even more preferably at least 35,000 g / mol, and most preferably at least 45,000 g / mol. The upper limit of the number average molecular weight of the polylactic acid produced during ring-opening polymerization is preferably 110,000 g / mol, but may be 90,000 g / mol or 80,000 g / mol. The suitable range of the number average molecular weight of the polylactic acid produced during ring-opening polymerization may be, for example, 10,000 to 20,000 g / mol, 20,000 to 30,000 g / mol, 30,000 to 50,000 g / mol, or 50,000 to 80,000 g / mol.

[0040] According to the present invention, the number average molecular weight and weight average molecular weight (M n and M w ) was measured by gel permeation chromatography using poly(methyl methacrylate) standards and sample concentrations of 1-5 mg / ml in 1 ml of hexafluoroisopropanol (HFIP) depending on the molecular weight of the sample, with a column temperature of 40 °C, a RI detector (refractive index) temperature of 40 °C, and a flow rate of 1 ml / min. The instrument used was a GPC Viscotek TDA max from Malvern Panalytical, UK, equipped with a Viscotek VE 2001 solvent / sample module, a pre-column HFIP guard (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 TDA 305 (RI, UV, viscometer). The calibration curve was based on poly(methyl methacrylate) (PMMA) standards (Mn, max = 50,352 g / mol, It can be created using the .tif2025513636000001.tif61701.023) file.

[0041] The polydispersity index, ie, Mw / Mn ratio, of the polylactic acid produced during the ring-opening polymerization is preferably 1-3, more preferably 1-2, and most preferably 1-1.5.

[0042] In a further development of the idea of ​​the present invention, it is suggested that the polylactic acid produced during the ring-opening polymerization has a Yellowness Index of less than 40, preferably less than 30, more preferably less than 15, even more preferably less than 10, even more preferably less than 5, and most preferably less than 3. In the present invention, the Yellowness Index is measured according to ASTM E313.

[0043] Polymerization After the ring-opening polymerization, it is preferable to remove the unreacted lactide remaining in the polymerization product until the final concentration is preferably at least less than 0.5% by weight, more preferably less than 0.3% by weight, in order to obtain a product of marketable quality. In the present invention, the removal of unreacted lactide is achieved by at least one devolatilization step carried out at high temperatures, for example between 190°C and 230°C, and at reduced pressure, for example less than 5 mbar (absolute). For example, a two-stage devolatilization process can be carried out to achieve the required degree of lactide removal and thereby obtain a polymer with the required quality. In order to terminate the polymerization reaction, it is preferable to add an inhibitor 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 lactide feed amount, it is further preferable to recover the unreacted lactide after devolatilization, for example by condensation, and then, optionally, to recycle the condensed composition containing unreacted lactide to the meso-lactide-enriched composition and / or the meso-lactide-depleted composition and / or the polymerized composition separately. In particular, for efficient devolatilization, it is preferred to add one or more effective suppressor additives at the end of the polymerization reaction.Before the first devolatilization step, between the first and second devolatilization steps, or after the second devolatilization step, additives and / or other polymers can be mixed and / or blended in one or more units for mixing and / or blending additives into the product stream to improve the mechanical, rheological, and / or thermal properties of the final polymer product.

[0044] The composition containing unreacted lactide obtained during the devolatilization of the crude polylactic acid composition, or the composition containing unreacted lactide obtained during the devolatilization step of the crude polylactic acid composition, each have a similarly high amount of L-lactide and D-lactide and a similarly low amount of meso-lactide, based on the total amount of lactide. Preferably, at least the composition containing unreacted lactide obtained during the first devolatilization step of the crude polylactic acid composition has a meso-lactide content of up to 25% by weight, based on the total amount of lactide, when two or more devolatilization steps are performed, or when only one devolatilization step is performed.

[0045] Additionally, the final polymer product stream may be cooled in a cooler and then compressed through a granulator or pelletizer, respectively, or another molding unit.

[0046] According to a further aspect, the present invention provides a plant for producing polylactic acid, comprising: i) a separation unit including 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; ii) a crystallization unit including an inlet line for the meso-lactide-enriched composition or a mixture of the meso-lactide-enriched composition and a portion of the composition including unreacted lactide, and an outlet line for the purified meso-lactide-enriched composition; iii) a crystallization unit that includes an inlet line for a mixture of the meso-lactide-depleted composition and at least a portion of the composition that includes unreacted lactide, or that includes two inlet lines, one for the meso-lactide-enriched composition and one for at least a portion of the composition that includes unreacted lactide, and that further includes an outlet line for the purified meso-lactide-depleted composition; iv) a polymerization reactor including an inlet line for the polymerization composition and an outlet line for the crude polylactic acid composition, and further including a devolatilization unit including an inlet line for the crude polylactic acid composition, an outlet line for the purified polylactic acid composition, and an outlet line for the composition containing unreacted lactide; Including, In one embodiment, the present invention relates to a plant in which an outlet line for the purified meso-lactide-enriched composition of crystallization unit ii), and optionally an outlet line for the purified meso-lactide-depleted composition of crystallization unit iii), are connected to the inlet line of the polymerization reactor.

[0047] According to certain preferred embodiments of the invention, the inlet line for the meso-lactide-enriched composition of crystallization unit ii) is connected to the outlet line for the meso-lactide-enriched composition of the separation unit and the outlet line for the composition comprising unreacted lactide of the devolatilization unit, the outlet line for the meso-lactide-depleted composition of the separation unit is connected directly to the inlet line for the meso-lactide-depleted composition of crystallization unit iii), the outlet line for the purified meso-lactide-enriched composition of crystallization unit ii) is connected to the inlet line of the polymerization reactor, and optionally, the outlet line for the purified meso-lactide-depleted composition of crystallization unit iii) is connected to the inlet line of the polymerization reactor. For example, the outlet line for the purified meso-lactide-enriched composition of crystallization unit ii) and the outlet line for the purified meso-lactide-depleted composition of crystallization unit iii) are joined to each other, and the joined lines are connected to the inlet line of the polymerization reactor.

[0048] According to another particularly preferred embodiment of the invention, the inlet line for the meso-lactide-enriched composition of crystallization unit ii) is connected to the outlet line for the meso-lactide-enriched composition of the separation unit and to the outlet line for the composition comprising unreacted lactide of the devolatilization unit, the inlet line for the meso-lactide-depleted composition of crystallization unit iii) is connected to the outlet line for the meso-lactide-depleted composition of the separation unit and to the outlet line for the composition comprising unreacted lactide of the devolatilization unit, the outlet line for the purified meso-lactide-enriched composition of crystallization unit ii) is connected to the inlet line of the polymerization reactor, and optionally, the outlet line for the purified meso-lactide-depleted composition of crystallization unit iii) is connected to the inlet line of the polymerization reactor.

[0049] As a further development of the idea of ​​the present invention, it is proposed that the crystallization unit ii) comprises at least one dynamic crystallizer, preferably 1 to 4 dynamic crystallizers, more preferably one dynamic crystallizer. Preferably, the at least one dynamic crystallizer, i.e. preferably 1 to 4 dynamic crystallizers or more preferably one dynamic crystallizer, is a falling film crystallizer.

[0050] According to a further preferred embodiment of the invention, the plant further comprises two distillation columns, the upstream of which comprise an inlet line for the crude lactide composition, a top outlet line, a bottom outlet line and a side outlet line, the side outlet lines upstream of the two distillation columns being connected to the side feed inlet lines downstream of the two distillation columns, and the downstream of the two distillation columns further comprising an top outlet line, a bottom outlet line and a side outlet line, the top outlet lines downstream of the two distillation columns being connected to the inlet line of the crystallization unit ii), and the side outlet lines downstream of the two distillation columns being connected to the inlet line of the crystallization unit iii).

[0051] Moreover, it is preferred that the plant further comprises a prepolymerization reactor including an inlet line for lactic acid and an outlet line for lactic acid oligomers, and that the plant further comprises a depolymerization reactor including an inlet line connected to the outlet line for lactic acid oligomers of the prepolymerization reactor and an outlet line for the crude lactide composition connected to the inlet line of the separation unit.

[0052] Preferably, the outlet line for the crude lactide composition is connected to the inlet line for the crude lactide composition upstream of the two distillation columns.

[0053] The invention will now be described with the aid of non-limiting exemplary figures. [Brief description of the drawings]

[0054] [Figure 1] FIG. 1 shows a schematic diagram of a polylactic acid production plant according to one embodiment of the present invention. [Diagram 2]FIG. 2 shows a detailed schematic diagram of a static crystallizer used to purify the meso-lactide-enriched composition of the plant shown in FIG. [Diagram 3] FIG. 2 shows a detailed schematic diagram of an alternative static crystallizer suitable for purifying the meso-lactide-enriched composition of the plant shown in FIG.

[0055] The plant 10 shown in Figure 1 comprises a dehydration unit 12, a prepolymerization reactor 14 and a depolymerization reactor 16 in series, the dehydration unit 12 comprising an inlet line 18 for lactic acid. One outlet of the dehydration unit 12 is connected to the inlet of the prepolymerization reactor 14 by a line 20, while the prepolymerization reactor 14 is connected to the dehydration unit 12 via a return 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. Meanwhile, the depolymerization reactor 16 is connected to the prepolymerization reactor 14 via a return line 26 and further comprises a purge line 28. Downstream of the depolymerization reactor 16, two distillation columns 30, 32 are provided, and the outlet of the depolymerization reactor 16 is connected to the inlet of the first distillation column 30 via a line 34, and the first distillation column 30 is connected to the prepolymerization reactor 14 via a return line 36, to the depolymerization reactor 16 via a return line 38, and to the second distillation column 32 via a line 40. The second distillation column 32 includes a line 42 connected to a static crystallizer 44 at its top, a line 46 connected to a falling film crystallizer 48 at its side, and a return line 50 connected to the first distillation column 30 at its bottom. The static crystallization apparatus 44 includes an outlet line 52 that branches into a product line 54 and a connecting line 56. Furthermore, the static crystallization apparatus 44 includes a purge line 58. Meanwhile, the falling film crystallizer 48 includes a first outlet line 60, which branches into a product line 62 and a connecting line 64, and a second outlet line 66, which branches into a purge line 68 and a return line 70 and leads to the second distillation column 30. The connecting lines 56 and 64 are joined to a monomer inlet line 72, which is connected 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 leading to a first volatilizer 80. The first volatilizer 80 includes a vapor outlet line 82 and a melt outlet line 84, which leads to a second volatilizer 86, which includes a vapor outlet line 88 and a polylactic acid outlet line 90. Furthermore, the plant 10 is provided with recycle lines 98, 98', 98''.More specifically, a recycle line 98 branches off from the vapor outlet line 82 of the first devolatilizer 80 and branches into recycle lines 98, 98', 98''. The recycle line 98' leads to the falling film crystallizer 48, and the recycle line 98'' leads to the static crystallizer 44.

[0056] During operation of the plant 10, lactic acid is continuously fed via inlet line 18 to the dehydration unit 12, where the lactic acid is dehydrated. The dehydrated lactic acid is conducted via line 20 to the prepolymerization reactor 14, where the lactic acid is prepolymerized to produce lactic acid oligomers, while water separated from the lactic acid in the dehydration unit 12 is withdrawn therefrom via line 25. The lactic acid oligomers are conducted via line 24 to the depolymerization reactor 16, where the lactic acid oligomers are depolymerized to a lactide mixture typically including meso-lactide, L-lactide and D-lactide. Meanwhile, the remaining lactic acid is recycled to the prepolymerization reactor 14 via line 26, a purge stream is withdrawn from the depolymerization reactor 16 via line 28, and the lactide mixture is conducted from the depolymerization reactor 16 to the first distillation column 30 via line 34. In the first distillation column 30, the lights are separated from the lactide as an overhead stream and returned to the prepolymerization reactor 14 via line 36, while the remaining lactic acid oligomers are returned to the depolymerization reactor 16 as a bottom stream via line 38, and the prepurified lactide stream is conducted to the second distillation column 32 via line 40 as a side stream of the first distillation column 30. The prepurified lactide stream is separated into an overhead stream and a bottom stream in the second distillation column 32, the overhead stream being a meso-lactide-rich composition and the bottom stream being a meso-lactide-depleted composition. The meso-lactide-rich composition is fed to a static crystallizer 44 via line 42, and the meso-lactide-depleted composition is fed to a falling film crystallizer 48 via line 46. Additionally, the unreacted lactide from the first devolatilizer 80 is recycled to the polymerization step carried out in the polylactic acid reactor 74. Via recycle lines 98, 98', and 98'', a portion of the vapor stream withdrawn from first devolatilizer 80 via line 82 is recycled to static crystallizer 44 and falling film crystallizer 48. A purified meso-lactide-enriched composition is withdrawn from static crystallizer 44 via line 52, while the residue is withdrawn as a purge stream via purge line 58.Similarly, a purified meso-lactide-depleted composition is withdrawn from falling film crystallizer 48 via line 60, while the residue is withdrawn via line 66 and split into a recycle stream that is fed to second distillation column 32 via return line 70, and a purge stream that is withdrawn via purge line 68. The purified meso-lactide-rich composition is split into a portion that is withdrawn from plant 10 via product line 54 as purified meso-lactide and a portion that is fed to polylactic acid reactor 74 via lines 56 and 72, while the purified meso-lactide-depleted composition is split into a portion that is withdrawn from plant 10 via product line 62 as purified L-lactide and a portion that is fed to polylactic acid reactor 74 via lines 56 and 72. The lactide is polymerized in polylactic acid reactor 74 to polylactic acid in the presence of a catalyst and an initiator that are fed to polylactic acid reactor 74 via inlet line 76. The crude polylactic acid stream is fed via line 78 to a first volatilizer 80 where it 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 withdrawn from the first volatilizer 80 via line 82, and the polylactic acid melt stream is fed via line 84 to a second volatilizer 86 where it is separated into unreacted lactide and a vapor stream containing small amounts of catalyst and initiator withdrawn from the second volatilizer 86 via line 88, and purified polylactic acid withdrawn from the plant 10 via line 90.

[0057] Figure 2 shows diagrammatically the two stages of static crystallizer 44 of plant 10 shown in Figure 1. Static crystallizer 44 includes a first static crystallization stage 92 and a second static crystallization stage 92'. The meso-lactide-enriched composition and a composition comprising unreacted lactide obtained during the devolatilization of the crude polylactic acid composition are fed via lines 42 and 98'' to first static crystallization stage 92 of static crystallizer 44, where the mixture is crystallized. During crystallization, meso-lactide crystallizes on the cooled surfaces of first crystallization stage 92, while meso-lactide-depleted mother liquor remains. After the crystallization is completed, the mother liquor is discharged as a purge from the first crystallization stage 92 via purge line 58, while the crystalline layer of meso-lactide accumulated during the first crystallization stage 92 is subjected to a sweating step, in which the resulting sweated fraction (not shown) is added to the purge stream discharged via purge line 58. The crystalline layer is then melted to obtain a first crystallized fraction of the purified meso-lactide composition. The first crystallized fraction of the purified meso-lactide composition is conducted via line 94 to the second crystallization stage 92', where it is crystallized. During the crystallization, the meso-lactide is crystallized on the cooled surface of the second crystallization stage 92', while the meso-lactide-depleted mother liquor remains. After the crystallization in the second crystallization stage 92' is completed, the mother liquor is discharged and fed via line 96 to the first crystallization stage 92. The layer of crystals deposited in the second crystallization stage 92′ is then subjected to a sweating process, after which the layer of crystals deposited in the second crystallization stage 92′ is melted to obtain a purified meso-lactide-enriched stream, which is withdrawn from static crystallizer 44 via exit line 52.

[0058] FIG. 3 shows a detailed schematic diagram of an alternative static crystallizer 44 suitable for purifying the meso-lactide-enriched composition of the plant shown in FIG. 1. Static crystallizer 44 shown in FIG. 3 corresponds to static crystallizer 44 shown in FIG. 2, except that in addition to a first static crystallization stage 92 and a second static crystallization stage 92', it includes a third static crystallization stage 92'' and a fourth static crystallization stage 92'''. The crystallized fraction of the purified meso-lactide composition obtained in the first static crystallization stage 92 is conducted via line 94 to the second crystallization stage 92', where it is crystallized. During crystallization, meso-lactide is crystallized on the cooled surface of the second crystallization stage 92', while the meso-lactide-depleted mother liquor remains. After crystallization in the second crystallization stage 92' is finished, the mother liquor is discharged and fed via line 96 to the first crystallization stage 92. The crystal layer deposited in the second crystallization stage 92' is then subjected to a sweating step, after which the crystal layer deposited in the second crystallization stage 92' is melted to obtain a purified meso-lactide-enriched stream, which is withdrawn from static crystallizer 44 via outlet line 100. A line 102 branches off from line 100, through which a portion of the purified meso-lactide-enriched stream obtained in the second crystallization stage 92' is withdrawn from plant 10, and the remaining portion is conducted via line 100 to outlet line 52. The crystallized fraction of the purified D- and L-lactide-enriched composition obtained in the third static crystallization stage 92'' is then conducted via line 96'' to a fourth crystallization stage 92''', where it is crystallized. During crystallization, the D- and L-lactide-enriched composition crystallizes on the cooled surface of the fourth crystallization stage 92, while the D- and L-lactide-depleted mother liquor remains. After crystallization in the fourth crystallization stage 92''' is finished, the mother liquor is drained and fed via line 94''' to the third crystallization stage 92'''. The crystal layer deposited in the fourth crystallization stage 92''' is then subjected to a sweating step, after which the crystal layer deposited in the fourth crystallization stage 92''' is melted to obtain a purified D- and L-lactide-enriched stream, which is withdrawn from the static crystallizer 44 via outlet line 100'.From line 100' there branches line 102' through which a portion of the purified D- and L-lactide-enriched stream obtained in the fourth crystallization stage 92''' is withdrawn from plant 10, the remaining portion being conducted via line 100' to outlet line 52.

[0059] The meso-lactide-enriched composition is fed via line 42 to a first static crystallization stage 92 of static crystallizer 44, where it is crystallized, while a composition containing unreacted lactide obtained during the devolatilization of the crude polylactic acid composition is fed via line 98'' to a third static crystallization stage 92'' of static crystallizer 44, where it is crystallized. The lactide-enriched fractions obtained in each of the first, third, and fourth crystallization stages 92, 92'', 92''' are conducted via lines 94, 94', 94'' to respective upstream crystallization stages 92', 92', 92'', while a purified lactide-enriched fraction obtained in the second crystallization stage 92' is withdrawn from the second crystallization stage 92' via line 100. From the line 100, a line 102 branches off, through which a part of the purified lactide-rich fraction obtained in the second crystallization stage 92' is withdrawn from the plant 10, and the remaining part is led via line 100 to the outlet line 52. The lactide-depleted mother liquors obtained in the second, first and third crystallization stages 92', 92 and 92'', respectively, are led via lines 96, 96' and 96'' to the respective downstream crystallization stages 92, 92'', and 92''', while the purified lactide-rich fraction obtained in the second crystallization stage 92' is withdrawn from the second crystallization stage 92' via line 100. From the line 100, a line 102 branches off, through which a part of the purified lactide-rich fraction obtained in the second crystallization stage 92' is withdrawn from the plant 10, and the remaining part is led via line 100 to the outlet line 52.

[0060] During crystallization, meso-lactide crystallizes on the cooled surfaces of first crystallization stage 92, while the meso-lactide-depleted mother liquor remains. After completion of crystallization, the mother liquor is discharged as a purge from first crystallization stage 92 via purge line 58, while the crystalline layer of meso-lactide accumulated during first crystallization stage 92 is subjected to a sweating step, in which the resulting sweated fraction (not shown) is added to the purge stream discharged via purge line 58. The crystalline layer is then melted to obtain a first crystallized fraction of the purified meso-lactide composition. The first crystallized fraction of the purified meso-lactide composition is conducted via line 94, where it is crystallized. During crystallization, meso-lactide crystallizes on the cooled surfaces of second crystallization stage 92', while the meso-lactide-depleted mother liquor remains. After crystallization in second crystallization stage 92' is completed, the mother liquor is drained and fed via line 96 to first crystallization stage 92. The crystal layer deposited in second crystallization stage 12 is then subjected to a sweating process, after which the crystal layer deposited in second crystallization stage 92' is melted to obtain a purified meso-lactide-enriched stream, which is withdrawn from static crystallizer 44 via outlet line 52. [Explanation of symbols]

[0061] Reference code 10. Plant 12 Dehydration unit 14 Prepolymerization reactor 16 Depolymerization reactor 18 Lactic acid inlet line 20 Line 22 Return line 24 Line 25 Drain line 26 Return Line 28 Purge Line 30 (First) Distillation Column 32 (Second) Distillation Column 34 Line 36 Return Line 38 Return Line 40 Line 42 Line 44 Static crystallizer 46 Line 48 Falling film crystallization device 50 Return Line 52 Exit Line 54 Product Lines 56 Connection Lines 58, 58' purge line 60 Exit Line 62 Product Lines 64 connection lines 66 Exit Line 68 Purge Line 70 Return Line 72 Monomer inlet line 74 Polylactic Acid Reactor 76 Entrance Line 78 Exit Line 80 First volatilizer 82 Steam outlet line 84 Melt Outlet Line 86 Second devolatilizer 88 Steam Outlet Line 90 Polylactic acid outlet line 92 First static crystallization stage 92' Second static crystallization stage 92'' Third static crystallization stage 92'' Fourth static crystallization stage 94, 94', 94'' lines 96, 96', 96'' lines 98, 98', 98'' Recycle line from devolatilization 100, 100' Line 102, 102' Line

Claims

1. a) A step of providing a crude lactide composition comprising meso-lactide and at least one of L-lactide and D-lactide, b) A step of separating a meso-lactide-enriched composition and a meso-lactide-depleted composition from a crude lactide composition, wherein the meso-lactide-enriched composition contains at least 80 mol-% meso-lactide based on the total lactide content, c) A step of polymerizing a polymer composition containing meso-lactide and at least one of L-lactide and D-lactide onto a crude polylactic acid composition, and defoliating the crude polylactic acid composition to produce a composition containing a purified polylactic acid composition and unreacted lactide, d) A step of subjecting at least a portion of the meso-lactide-enriched composition and the composition containing unreacted lactide to purification including at least one crystallization step to produce a purified meso-lactide-enriched composition, e) A process of purification, including at least one crystallization step, of a meso-lactide depleted composition or a mixture of a meso-lactide depleted composition and a portion of a composition containing unreacted lactide, in order to produce a purified meso-lactide depleted composition. Includes, A method for producing polylactic acid, wherein the polymerization composition comprises at least a portion of the purified meso-lactide-enriched composition produced in step d).

2. The method 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 containing unreacted lactide obtained in the defoliation step of the crude polylactic acid composition, and the mixture is then supplied to at least one crystallization step in step d).

3. The method 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, at least a portion of the composition containing unreacted lactide obtained in the defoliation step of the crude polylactic acid composition is subjected to at least one crystallization step of step d), and thereafter, at least a portion of both purified streams are mixed together to form a purified meso-lactide-enriched mixture.

4. The method according to any one of claims 1 to 3, wherein the polymerization 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 claim 4, wherein the ratio of the purified meso-lactide-enriched composition to the purified meso-lactide-depleted composition in the polymerization composition is 10:1 to 1:10 by weight, for example, 5:1 to 1:1 by weight.

6. The method according to any one of claims 1 to 3, wherein 10 to 100% by weight, preferably 50 to 100% by weight, of the composition containing unreacted lactide obtained in the defoliation step of the crude polylactic acid composition is subjected to at least one crystallization step in step d), and 0 to 90% by weight, preferably 50 to 90% by weight, of the composition containing unreacted lactide obtained in the defoliation step of the crude polylactic acid composition is subjected to at least one crystallization step in step e).

7. The polymerization composition comprises at least one catalyst and / or at least one initiator, the catalyst preferably being at least one organometallic compound comprising aluminum or tin as the metal, and more preferably being tin octoate, tetraphenyltin, butyltin trimethoxide, dibutyltin oxide, aluminum isopropoxide, Al(O-i-Pr) ｐ [In the formula, 1≦p≦3], Et ３ -pAl(O(CH) ２ ) ２ X) ｐ [wherein 1 ≤ p ≤ 3], at least one organometallic compound selected from the group consisting of α, β, γ, δ, ε tetraphenylporfinatoaluminum (TPPIAIX) and any combination of two or more of the aforementioned compounds, wherein the at least one initiator is preferably selected from the group consisting of monohydroxy compounds, dihydroxy compounds, trihydroxy compounds, tetrahydroxy compounds and any combination of two or more of the aforementioned compounds, and more preferably the at least one initiator is 2-ethylhexanol, C １０ ~C ２０ The method according to any one of claims 1 to 3, wherein a compound is selected from the group consisting of monohydroxyfatty alcohols, benzyl alcohol, p-phenylbenzyl alcohol, ethylene glycol, propylene glycol, butane-1,4-diol, 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 its dimers, pentaerythritol and its dimers, and any combination of two or more of the aforementioned compounds.

8. The method according to any one of claims 1 to 3, wherein the purification in step d) comprises at least one static crystallization step, the at least one static crystallization step preferably comprises 1 to 10 static crystallization steps, preferably 1 or 2 to 10 crystallization steps, and more preferably 2 to 4 static crystallization steps.

9. The method according to any one of claims 1 to 3, wherein the purification of step e) comprises at least one dynamic crystallization step, the at least one dynamic crystallization step preferably comprises 1 to 4 dynamic crystallization steps, more preferably 1 dynamic crystallization step, and the dynamic crystallization is preferably flow-through film crystallization.

10. The method according to any one of claims 1 to 3, wherein step b) comprises at least one distillation step, each performed in a distillation column (30, 32), the crude lactide composition being fed into the distillation column (30, 32) of the first distillation step, the meso-lactide enriched composition being produced as the top fraction, and the meso-lactide depleted composition being produced as the bottom fraction or side fraction of the distillation column (30, 32) of the final distillation step.

11. The method according to claim 10, wherein step b) comprises two distillation steps performed in distillation columns (30, 32), in which the crude lactide composition is separated into a top fraction, a bottom fraction, and a side fraction in the first distillation column (30, 32), the side fraction from the first distillation column (30, 32) is fed to the second distillation column (30, 32), where a meso-lactide-enriched composition is produced as the top fraction and a meso-lactide-depleted composition is produced as the side fraction or bottom fraction.

12. The method according to claim 10, wherein a portion of the purified meso-lactide-enriched composition produced in step d) is recycled as a side flow to the final distillation columns (30, 32).

13. A plant (10) for producing polylactic acid, i) A separation unit including 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 two inlet lines (42, 98'') for a mixture of a meso-lactide-enriched composition and at least a portion of a composition containing unreacted lactide, one of which (42) is for the meso-lactide-enriched composition and the other (98'') is for at least a portion of a composition containing unreacted lactide, and further comprising an outlet line (52) for the purified meso-lactide-enriched composition, iii) A crystallization unit (48) comprising an inlet line (46) for a meso-lactide depleted composition or a mixture of a meso-lactide depleted composition and a portion of a composition containing unreacted lactide, and an outlet line (60) for a purified meso-lactide depleted composition, 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, the polymerization reactor (74) further comprising a defoliation 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 containing unreacted lactide. Includes, A plant (10) for producing polylactic acid, wherein at least an outlet line (52) for a purified meso-lactide-enriched composition of crystallization unit ii) (44), and optionally an outlet line (60) for a purified meso-lactide-depleted composition of crystallization unit iii) (48), are connected to the inlet line (72) of a polymerization reactor (74).

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

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

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