Process for treating lactide feed

Abstract

A process for treating a lactide feed to form a lactide stream having a high configuration purity and a high chemical purity, comprising the steps of:-subjecting a feed comprising L-lactide, meso-lactide, water, lactic acid and optionally other components to a first separation step, thereby obtaining a stream comprising lactic acid and water and one or more crude lactide streams; -subjecting the crude lactide stream comprising L-lactide and meso-lactide resulting from the first separation step to a first crystallization step, thereby obtaining a purified L-lactide stream and a residual stream comprising L-lactide and meso-lactide; -subjecting the residue stream comprising L-lactide and meso-lactide resulting from the first crystallization step to a second separation step, thereby obtaining at least a second L-lactide stream and a meso-lactide stream; -subjecting the meso-lactide stream resulting from the second separation step to a second crystallization step, thereby obtaining a purified meso-lactide stream and a further residue stream.

Description

[0001] This invention relates to a method for processing lactide feed. This invention also relates to a method for preparing polylactide.

[0002] Polylactide (also known as polylactic acid or PLA) is a polymer used in a wide range of applications, from medical applications to packaging and disposable tableware. Polylactide is a polymer derived from lactic acid. Lactic acid is a chiral molecule and therefore exists as (S)-lactic acid or (R)-lactic acid, with (S)-lactic acid being the predominant form in nature. Commercially available polylactide typically contains a high proportion of (S)-lactic acid units and is usually obtained through the ring-opening polymerization of (primarily) L-lactide (a dimer of (S)-lactic acid).

[0003] Lactose is typically synthesized from oligolactic acid via a condensation reaction to form lactic acid oligomers. These oligomers are then depolymerized to form crude lactose containing L-lactide ((S,S)-lactide), D-lactide ((R,R)-lactide), and meso-lactide ((S,R)-lactide). When oligomers are synthesized from (S)-lactic acid, the resulting oligomers will primarily contain (S)-lactic acid units. This depolymerization of oligomers will again generate crude lactose, with L-lactide being the predominant stereoisomer. D-lactide and meso-lactide are produced in the presence of (R)-lactic acid, which can be present in the original feedstock or formed through racemization.

[0004] L-lactide, D-lactide, and mesolactide can be fed into the polymerization reactor in various proportions. The proportion of lactide fed to the polymerization reactor determines the ratio of (S)-lactic acid units to (R)-lactic acid units in the polymer, because each L-lactide molecule contributes two (S)-lactic acid units to the polymer, each D-lactide molecule contributes two (R)-lactic acid units, and each mesolactide molecule contributes one (S)-lactic acid unit and one (R)-lactic acid unit. The ratio of (S)-lactic acid units to (R)-lactic acid units in the polymer is very important because it largely determines the physical properties of the polylactide. Polylactide that mainly contains (S)-lactic acid units and, for example, 10% (R)-lactic acid units will, for example, have lower crystallinity than polylactide that contains only (S)-lactic acid units.

[0005] However, the ratio of (S)-lactic acid units to (R)-lactic acid units in the polymer is not the only factor affecting the physical properties of polylactide. Another important factor is the presence of impurities in the lactide stream fed to the polymerization reactor, such as hydroxyl-containing impurities. The presence of hydroxyl-containing impurities in the polymerized lactide stream reduces the average molecular weight of the resulting polylactide. Impurities can also affect the color of polylactide, its thermal stability, and its suitability for food contact applications.

[0006] Many methods for processing lactide feed have been disclosed in this field.

[0007] For example, WO2014180836 describes a process for recovering and preparing racemic lactide from a stream containing crude lactide, which includes the following steps: a. Pass the initial crude lactide stream through a first distillation step to obtain a top stream, a bottom stream, and a side stream containing mainly L-lactide and mesolacide. b. Recover the side stream and subject it to a melt crystallization step to obtain a first purified stream mainly containing L-lactide and an exhaust stream mainly containing mesolacide and L-lactide; c. Recycle the top stream from step (a) and the discharge stream from step (b); d. Pass the top stream and the discharge stream through a second distillation step to obtain a second purified stream containing L-lactide and mesolactide.

[0008] WO2014 / 139730 describes a process for purifying lactide, which includes the following steps: (a) Separating a lactide-containing stream into one or more lactide-containing vapor fractions and one or more lactide-containing liquid fractions; (b) The lactide-containing vapor fraction obtained in step (a) is condensed to obtain lactide condensate; (c) subjecting at least a portion of the lactide-containing condensate obtained in step (b) to melt crystallization to obtain a purified lactide-containing stream and a residual stream; and (d) Recover the purified lactide-containing stream obtained in step (c).

[0009] However, given the increasing interest in polylactide in many applications, there is a need in the art for a method of processing lactide feed that, on the one hand, produces a lactide stream with high configurational and chemical purity, and on the other hand, is carried out in an efficient and economical manner. Here, high configurational purity refers to a high content of the desired stereoisomers. High chemical purity refers to a low content of non-lactide contaminants. This invention provides such a process. Summary of the Invention

[0010] This invention relates to a method for processing lactide feed, comprising the following steps: - The feed containing L-lactide, mesolactide, water, lactic acid, and optional other components is subjected to a first separation step, which results in the formation of a stream containing lactic acid and water, and one or more crude lactide streams. - At least a portion of the crude lactide stream containing L-lactide and racemic lactide produced by the first separation step is subjected to a first crystallization step, which produces a purified L-lactide stream and a residual stream containing L-lactide and racemic lactide. - The residual stream containing L-lactide and mesolactide produced by the first crystallization step is subjected to a second separation step, which produces at least a second L-lactide stream and a mesolactide stream. - At least a portion of the racemic lactide stream produced by the second separation step is subjected to a second crystallization step, resulting in a purified racemic lactide stream and another residual stream.

[0011] It has been found that the specific sequence of steps of the present invention can generate L-lactide streams and mesolacide streams with high chemical and configurational purity, while the process can be operated in a flexible and economical manner. Further advantages of various embodiments of the method of the present invention will become apparent in the following description. Detailed Implementation

[0012] The invention will now be discussed in more detail. Reference will be made to the following figures. These figures are for illustrative purposes only; the invention is not limited thereto or thereby restricted.

[0013] Figure 1 A first embodiment of the process according to the present invention is shown, comprising a feed comprising L-lactide, mesolactide, water and lactic acid undergoing a first separation step to obtain a crude lactide stream; the crude lactide stream undergoing a first crystallization step to obtain a purified L-lactide stream and a residual stream containing L-lactide and mesolactide; the residual stream undergoing a second separation step to obtain a second purified L-lactide stream and a mesolactide stream; and the mesolactide stream undergoing a second crystallization step to obtain a purified mesolactide stream and another residual stream.

[0014] Figure 2 A second embodiment of the process according to the invention is shown, wherein the first separation step consists of a two-step process.

[0015] Figure 3 A third embodiment of the process according to the invention is shown, wherein a stream containing mesolaclide obtained from the first step of the first separation step is provided to a second separation step, and a stream containing L-lactide is obtained from the first separation step.

[0016] Figure 4 A fourth embodiment of the process according to the present invention is shown, wherein the first separation step and the second separation step consist of a two-step process.

[0017] Figure 5A fifth embodiment of the process according to the invention is shown, wherein the high L-lactide content stream obtained from the second separation step is recycled to the first crystallization step, and the high mesolacide content stream is conveyed to the second crystallization step.

[0018] Figure 6 A sixth embodiment of the process according to the invention is shown, wherein the stream containing L-lactide, mesolactide and contaminants obtained from the second distillation step is conveyed to a further separation step.

[0019] Figure 7 A seventh embodiment of the process according to the invention is shown, wherein the lactic acid feed undergoes a polycondensation step to obtain low molecular weight polylactic acid, and the low molecular weight polylactic acid obtained in the polycondensation step undergoes a depolymerization step to obtain a lactide synthesis mixture.

[0020] Regarding the attached image, please note the following: The accompanying drawings are intended to illustrate the invention. The invention is not limited thereto or thereby restricted.

[0021] The embodiments shown in the various figures can be combined unless they are mutually exclusive. The figures are flowcharts illustrating the process according to the invention. Reactor apparatus is not shown in the figures. For example, when a separation step is shown as a single step, it can be carried out in more than one reactor. Conversely, different steps can be carried out in the same unit. For the same reason, the various lines are intended to show how components flow from one reaction step to another. They do not represent actual structures.

[0022] The accompanying drawings do not always show all elements of the process according to the present invention.

[0023] The accompanying drawings do not show all the cleaning or replenishing flows that may be present in the actual execution of the process according to the invention, although it will be apparent to those skilled in the art that cleaning and replenishing flows may be necessary in practice to maintain stable operation.

[0024] The invention will be explained with reference to the accompanying drawings, but the invention is not limited thereto or thereby limited.

[0025] First separation step In the first step, a feed comprising L-lactide, mesolactide, water, lactic acid and optional other components is subjected to a first separation step, which results in the formation of a stream comprising at least lactic acid and water, and one or more crude lactide streams.

[0026] The feed provided to the first separation step may be referred to as the lactide synthesis mixture. The lactide synthesis mixture typically contains 75% to 95% by weight of L-lactide (based on the total weight of lactide in the stream), preferably 80% to 90% by weight. The lactide synthesis mixture may contain up to 5% by weight of D-lactide (based on the total weight of lactide in the mixture), for example, 0.01% to 5% by weight (based on the total weight of lactide in the mixture), preferably up to 1% by weight. The lactide synthesis mixture typically contains 1% to 25% by weight of mesolacide (based on the total weight of lactide in the mixture), preferably 2.5% to 10% by weight. The total lactide content of the lactide synthesis mixture provided to the first separation step is typically 75% to 99.5% by weight (based on the total weight of the mixture), particularly 85% to 98% by weight. In addition to lactide, the mixture also contains lactic acid, typically in an amount of 0.01% to 10% by weight, particularly 0.5% to 5% by weight. The feed also contains water, typically in amounts from 0.01 wt% to 4 wt%, particularly from 0.1 wt% to 2.5 wt%. The feed composition, particularly the lactide content and composition, can be determined using methods known in the art, including HPLC, for example using a water / acetonitrile mixture as eluent, and UV detection.

[0027] The lactide synthesis mixture may contain other components, such as acidic impurities like lactyl lactic acid, succinic acid, and acetic acid. Acidic impurities may be present in the crude lactide stream in amounts such that the crude lactide stream has a free acid content of at least 20 meq / kg (milliequivalents per kilogram), particularly at least 50 meq / kg, and / or up to 150 meq / kg. As an upper limit, acidic impurities may be present in the crude lactide stream in amounts up to 250 meq / kg. The free acid content, as used herein, can be determined by titration, for example, using sodium methoxide or potassium methoxide in anhydrous methanol.

[0028] The first separation step is typically a distillation step, as distillation is an efficient method for separating lactic acid and water from lactide. However, the first separation step can also be, for example, a pre-crystallization step. The first separation step can be carried out in a single unit, but it can also be carried out in multiple units in series. For example, in one embodiment, the first separation step is carried out in a two-step operation, where a feed is provided to a first separation unit (e.g., a distillation column), where a stream containing water, lactic acid, and other volatile components is separated from a stream containing the majority of lactide components. The stream containing the lactide components is at least partially provided to a second separation unit (typically another distillation unit), where a different lactide stream can be generated.

[0029] The first separation step generates one or more crude lactide streams. The determination of the number of crude lactide streams to be generated in this step depends particularly on the composition of the lactide fraction in the lactide synthesis mixture, and especially on the amount of meso lactide relative to L-lactide.

[0030] Specifically, as will be discussed below, the crude lactide stream containing L-lactide and racemic lactide produced by the first separation step is subjected to a first crystallization step, which produces a first purified L-lactide stream and a residual stream containing L-lactide and racemic lactide. The composition of the crude lactide stream supplied to this first crystallization step is typically as follows: • L-lactide: 80% to 98% by weight, particularly 85% to 95% by weight, more preferably 90% to 95% by weight, based on the total weight of the crude lactide stream. • D-lactide: 0% to 5% by weight of the total weight of the crude lactide stream. • Mesolactide: 1% to 25% by weight, particularly 2% to 12% by weight, more preferably 5% to 10% by weight, based on the total weight of the crude lactide stream. • Lactic acid oligomers: 0% to 7.5% by weight, particularly up to 5% by weight, based on the total weight of the crude lactide stream. • Other organic hydroxy acids: less than 2% by weight, preferably less than 1% by weight, based on the total weight of the crude lactide stream. • Free acid content: 0% to 5% by weight, preferably up to 2% by weight, more preferably up to 1.5% by weight, even more preferably up to 1% by weight, even more preferably up to 0.5% by weight, based on the total weight of the crude lactide stream.

[0031] Therefore, in one embodiment, the first separation step produces a crude lactide stream having a composition suitable for direct supply to the first crystallization step, such as the composition described above. Furthermore, the total lactide content and L-lactide content of the crude lactide stream are higher than the total lactide content and L-lactide content of the lactide synthesis mixture.

[0032] In one embodiment, a first separation step produces a single crude lactide stream containing L-lactide and mesolacide, which is then subjected to a first crystallization step. This can be particularly attractive when the amount of mesolacide is relatively limited compared to the amount of L-lactide. In this embodiment, the single crude lactide stream preferably has the composition described above.

[0033] In another embodiment, the first separation step generates a racemic lactide stream and a crude lactide stream containing both L-lactide and racemic lactide. In this case, the crude lactide stream containing both L-lactide and racemic lactide is then subjected to a first crystallization step. Preferably, this crude lactide stream has the composition described above for the feed to be provided to the first crystallization step. The racemic lactide stream typically has a higher racemic lactide content than the crude lactide stream containing both L-lactide and racemic lactide to be provided to the first crystallization step. Typically, the racemic lactide stream will contain 0% to 10% by weight of lactic acid and 60% to 90% by weight of racemic lactide, calculated by total weight. Furthermore, the racemic lactide stream may contain 0% to 30% by weight of L-lactide, calculated by total weight.

[0034] This implementation scheme is advantageous because it ensures a high proportion of L-lactide in the crude lactide stream supplied to the first crystallization step, as well as a low proportion of amorphous material, thereby preventing unnecessary unit loading and improving separation efficiency and yield.

[0035] As described above, the first separation step can be performed in a two-step operation, wherein the lactide synthesis mixture is fed to a first separation unit (e.g., a distillation column), where a stream containing water, lactic acid, and other volatile components is separated from a stream containing the majority of lactide components. In one embodiment, a portion of the stream containing the majority of lactide components is fed to a second separation unit (typically another distillation unit), where a different lactide stream is generated, while another portion of the stream is fed directly to the first crystallization step.

[0036] This implementation is particularly attractive when the lactide synthesis mixture contains relatively high amounts of water, lactic acid and / or meso lactide, for example when the lactide undergoes a relatively high degree of racemization, or when the processing volume is relatively high.

[0037] The stream containing water, lactic acid, and other volatile components may also contain some mesolaclide. In one embodiment, the stream containing water, lactic acid, and other volatile components obtained from the first separation step, or from the first step and / or the second step of the first separation step, is provided to the second separation step. In this way, a higher yield of mesolaclide is obtained.

[0038] In one embodiment, a stream containing L-lactide is also removed from the first separation step. Preferably, based on the total weight of the stream, the L-lactide-containing stream contains at least 85% by weight of L-lactide, more preferably at least 90% by weight of L-lactide, and even more preferably at least 95% by weight of L-lactide. The L-lactide-containing stream typically contains some D-lactide. Preferably, based on the total weight of the stream, the L-lactide-containing stream contains at most 15% by weight of D-lactide, more preferably at most 10% by weight, and even more preferably at most 5% by weight of D-lactide. The composition of the L-lactide-containing stream removed from the first separation step is suitable for providing the stream to the PLA manufacturing process.

[0039] Preferably, the crude lactide stream is condensed and obtained in liquid form as part of the first separation step. This is advantageous because the condensation of the crude lactide stream can remove highly volatile impurities (such as aldehydes and pyruvic acid), resulting in a purer crude lactide stream.

[0040] First crystallization step In the process according to the invention, a crude lactide stream containing L-lactide and mesolacide, generated by the first separation step, is provided to the first crystallization step. As described above, the stream provided to the first crystallization step typically comprises: • L-lactide: 80% to 98% by weight, particularly 85% to 95% by weight, and more preferably 90% to 95% by weight, based on the total weight of the crude lactide stream. • Mesolactide: 1% to 25% by weight, particularly 2% to 12% by weight, based on the total weight of the crude lactide stream. • Other components as described above.

[0041] A first crystallization step is performed to produce a first purified L-lactide stream and a residual stream containing L-lactide and mesolacide. The first crystallization step is carried out at a temperature of 60°C to 100°C, preferably 70°C to 90°C. Generally, a relatively low temperature is required when the L-lactide stream contains a relatively low concentration of L-lactide, and vice versa.

[0042] The purpose of the first crystallization step is to produce a high-purity L-lactide fraction. The L-lactide fraction produced in this step, based on the total weight of the first purified L-lactide stream, typically contains at least 90% by weight of L-lactide, particularly at least 94% by weight, more especially at least 96% by weight, or even at least 98% by weight of L-lactide.

[0043] Preferably, the free acid content of the first purified L-lactide stream is <30 meq / kg, more preferably <15 meq / kg, and even more preferably <5 meq / kg.

[0044] Typically, the first purified L-lactide stream will contain a higher amount of L-lactide than the L-lactide-containing stream obtained from the first separation step.

[0045] The residual stream from the first crystallization step contains racemic lactide that did not crystallize in the crystallization step, and typically also contains L-lactide. The residual stream may also contain lactic acid and lactic acid oligomers, as well as other organic acids, such as other organic hydroxy acids. Since the purpose of this step is to produce high-purity L-lactide, the crystallization conditions will be selected to ensure this. Therefore, it is acceptable that the residual stream from the first crystallization step still contains L-lactide.

[0046] In one embodiment, the residual stream comprises, by weight of the total residual stream, 60% to 90% L-lactide, 5% to 40% mesolactide, 0% to 10% lactic acid oligomer, and 0.5% to 2% other organic acids.

[0047] The first crystallization step can be performed in two steps, wherein a crude lactide stream containing L-lactide and racemic lactide, produced by the first separation step, is fed to a pre-crystallizer where pre-crystallization occurs, resulting in an L-lactide-rich stream and a residual stream containing racemic lactide and L-lactide. The L-lactide-rich stream is then fed to a second crystallizer, resulting in a first purified L-lactide stream and a residual stream containing racemic lactide and L-lactide.

[0048] At least a portion of the residual stream undergoes a second separation step. Another portion of the residual stream from the pre-crystallizer and / or the first crystallizer can be recycled and mixed into the crude lactide feed for another crystallization cycle, can be used as feed for another crystallization batch, or can even be recycled further upstream, for example, by mixing into the lactide synthesis mixture.

[0049] In addition, the L-lactide obtained from the first crystallization step can be recrystallized to achieve the required yield and / or purity of the purified L-lactide stream.

[0050] Preferably, the free acid content of the purified L-lactide stream is < 10 meq / kg, more preferably < 5 meq / kg, and even more preferably < 3 meq / kg.

[0051] Second separation step At least a portion of the residual stream from the first crystallization step is fed to a second separation step, which produces at least a second L-lactide stream and a mesolacide stream. The second separation step is typically a distillation step. The second separation step can be carried out in a single unit, but it can also be carried out in multiple units in series. This will depend on the composition of the feed supplied to the second separation step.

[0052] The residual stream supplied to the second separation step contains meso lactide that did not crystallize in the first crystallization step, as well as L-lactide. The residual stream may also contain lactic acid and lactic acid oligomers, as well as other organic acids, such as other organic hydroxy acids.

[0053] The second separation step produces a second L-lactide stream and a mesolacide stream. The second L-lactide stream generated in this step is generally relatively pure, although not necessarily as pure as the purified L-lactide generated in the first crystallization step. In one embodiment, the second L-lactide stream contains at least 80% by weight of L-lactide, particularly at least 90% by weight, more particularly at least 96% by weight, or even at least 98% by weight of L-lactide, based on the total weight of the second L-lactide stream. The second L-lactide stream may contain from 0% by weight to 20% by weight of mesolacide, based on the total weight of the second L-lactide stream. The second L-lactide stream may also additionally contain small amounts of lactic acid, D-lactide and other low molecular weight lactic acid oligomers, organic acids and / or hydroxy acids other than lactic acid. Preferably, the second L-lactide stream contains < 0.5% by weight of lactic acid and < 0.5% by weight of lactic acid oligomer, calculated based on the total weight of the second L-lactide stream. Preferably, the second L-lactide stream contains < 5 meq / kg of lactic acid, calculated based on the total weight of the second L-lactide stream.

[0054] The second L-lactide stream can be processed as needed, for example, as discussed for the purified L-lactide produced by the first crystallization step. In one embodiment, the second L-lactide stream can be fed as feed to another crystallization step, such as the first crystallization step discussed above. This can further purify the L-lactide. In another embodiment, a third L-lactide stream is generated in the second separation step as a bottom stream, which can be used as feed to the crystallization step. Since the primary function of this crystallization step is to recover L-lactide from the bottom stream, this increases the overall yield of L-lactide.

[0055] The second separation step also produces a racemic lactide stream. This stream will be provided to the second crystallization step, discussed below. The composition of the racemic lactide stream generated in the second separation step and provided to the second crystallization step is generally as follows: 80% to 98% by weight of racemic lactide (particularly 85% to 94% by weight), 0% to 20% by weight of L-lactide, trace amounts of low molecular weight, high-boiling-point lactic acid oligomers, and organic acids other than lactic acid, calculated by the total weight of the racemic lactide stream. The free acid content of the racemic lactide stream is preferably 30 meq / kg to 500 meq / kg.

[0056] The second separation step may also produce a light fraction containing, for example, water, lactic acid, relatively small amounts of L-lactide and mesolactide, and small-molecule organic acids (e.g., acetic acid). Depending on its composition, this fraction may be discharged from the process or recycled wholly or partially to earlier steps, particularly to the first separation step, or to the lactide manufacturing step, as will be discussed below.

[0057] The second separation step may also generate a bottom fraction containing high-boiling-point contaminants, L-lactide, and relatively high amounts of lactic acid oligomers with a degree of polymerization greater than 3 (i.e., oligomers containing three or more lactic acid monomers). Depending on its composition, this fraction may be discharged from the process or recycled, wholly or partially, to an earlier step, regardless of whether it has undergone intermediate separation.

[0058] In one embodiment, the bottom fraction is subjected to a further crystallization step, in which another L-lactide-containing stream and another purge stream are generated.

[0059] Second crystallization step As described above, the racemic lactide stream generated in the second separation step is provided to the second crystallization step. In the second crystallization step, the racemic lactide crystallizes out of the system, thereby generating a purified racemic lactide stream. The purified racemic lactide stream generated in this step typically contains at least 90% by weight of racemic lactide, particularly at least 94% by weight, more particularly at least 96% by weight, or even at least 98% by weight of racemic lactide, based on the total weight of the purified racemic lactide stream. The free acidity of the purified racemic lactide stream can be less than 50 meq / kg, preferably less than 25 meq / kg, and particularly less than 10 meq / kg.

[0060] The second crystallization step also produces another residual stream. The composition of this other residual stream is typically as follows: based on the weight of the other residual stream, 60% to 90% by weight of meso-lactide and 0% to 30% by weight of L-lactide.

[0061] Depending on the crystallization conditions, the other residual stream may also contain 0% to 10% by weight of lactic acid and other organic acids. The other residual stream may be discharged, or it may be recycled wholly or partially to an earlier step, such as a first separation step, a first crystallization step, or a second separation step. Optionally, it may undergo an intermediate separation step before being recycled to an earlier step.

[0062] The racemic lactide stream supplied to the second crystallization step can be crystallized by melt crystallization. For example, the racemic lactide stream can be crystallized at a temperature of 30°C to 53°C, preferably 35°C to 45°C. The selected temperature depends on the concentration of racemic lactide in the racemic lactide stream. Generally, a relatively low temperature is required when the racemic lactide stream contains a relatively low concentration of racemic lactide. Preferably, the second crystallization step is performed only once.

[0063] Another residual stream from the second crystallization step can be treated as needed. Depending on its composition, it can be discharged from the system, or it can be recycled wholly or partially to other steps, regardless of whether it undergoes intermediate separation. This other residual stream can also be hydrolyzed to industrial-grade lactic acid with low stereochemical purity.

[0064] General instructions on how the different steps can be performed. As described above, the separation step can be carried out in a single step or multiple steps, in a single unit or multiple units. For example, when the first separation step is carried out in a two-step operation, the two steps of the first separation step can be carried out in the same reactor or in different reactors. The first separation step can also be carried out in the same reactor as the one used to produce the lactide synthesis mixture.

[0065] Distillation is generally an attractive separation method, but other separation methods may be applicable depending on the compound to be separated. Examples of such further separation methods include extraction, adsorption and absorption on ion exchange columns or carbon columns or functionalized resins.

[0066] When the separation step is performed in multiple steps, these steps can be based on the same separation mechanism, such as distillation, but can also be combined to depend on different separation mechanisms, such as a combination of distillation and extraction steps.

[0067] Examples of suitable apparatus include distillation columns, wall separators, extraction columns, absorption columns, sublimation apparatus, etc.

[0068] Based on the guidance given above regarding the various fractions to be generated in the separation steps, those skilled in the art can select appropriate separation steps, apparatus, and process conditions.

[0069] The method according to the invention comprises (at least) two crystallization steps.

[0070] The crystallization step can be performed using conventional crystallization techniques, such as solvent crystallization and melt crystallization. Conventional apparatus can be used, such as layer crystallization apparatus (e.g., falling film crystallizers and static crystallizers), or suspension crystallizers (e.g., forced circulation crystallizers, scraped wall crystallizers, OSLO crystallizers, and baffle plate crystallizers). While layer crystallization recovery is carried out by reheating to melt the crystals, suspension crystallization typically requires solid / liquid separation techniques, such as centrifuges, cyclone separators, filters, or washing columns. Based on the guidance given above regarding the crystallization step, those skilled in the art can select appropriate crystallization steps, apparatus, and process conditions.

[0071] Further steps As discussed above, the feed provided to the first separation step comprises L-lactide, mesolactide, water, lactic acid, and optional other components. This feed can be derived, for example, from a process for producing lactide from lactic acid or from the depolymerization of PLA. As mentioned above, lactide is typically synthesized by oligolactic acid to form lactic acid oligomers via a polycondensation reaction. These oligomers are then depolymerized to form a lactide composition comprising L-lactide, D-lactide, and mesolactide. The process according to the invention may further comprise the steps of subjecting lactic acid to a polycondensation step to produce lactic acid oligomers, and then subjecting the lactic acid oligomers to a depolymerization step to produce a crude lactide feed comprising L-lactide, mesolactide, water, lactic acid, and optional other components.

[0072] In the first step, low molecular weight polylactic acid (PLA) is formed by the polycondensation of lactic acid. Lactic acid is typically obtained from bioprocesses and usually has high optical purity. Depending on the source, it may contain at least 90% by weight L-lactide, particularly at least 95% by weight, and even more particularly at least 98% by weight L-lactide. The polycondensation reaction can be carried out as is known in the art. It typically involves subjecting the lactic acid to subatmospheric pressure (e.g., 50 mbar - 500 mbar) and elevated temperature (e.g., 100°C - 200°C) to initiate polymerization by removing water. The resulting average degree of polymerization is typically between 5 and 20. The low molecular weight PLA obtained in the first step is then subjected to a depolymerization step to convert the low molecular weight PLA into lactide. Depolymerization is also known in the art. It is typically carried out in the presence of a catalyst. Metal-containing catalysts are frequently used, particularly those based on tin, zinc, aluminum, lead, antimony, calcium, and magnesium, for example in the form of halide salts or organic acid salts (such as fatty acid salts). Commercially, bis(2-ethylhexanoate)tin(II) is commonly used. Typical concentrations of lactide synthesis catalysts range from 20 ppm (parts per million) to 2000 ppm. Reaction conditions include temperatures from 160°C to 260°C, pressures from 5 mbar to 100 mbar, and residence times from 10 minutes to 8 hours.

[0073] The lactic acid-containing stream generated in the process according to the invention (e.g., in the first or second separation step) can be recycled to the polycondensation step. If desired, a stream containing mesolacide can be conveyed to a downstream crystallization step. For example, a mesolacide-rich stream with a relatively low amount of L-lactide produced by the first separation step can bypass the first crystallization step and be conveyed to the second separation step. Furthermore, streams with high L-lactide content can be recycled, for example, to further increase the L-lactide content of the stream. For this purpose, for example, a second L-lactide stream can be recycled to the first crystallization step. In addition, lactide-containing streams derived from external sources (e.g., PLA preparation) can be sent to the first crystallization step, the second separation step, and / or the second crystallization step.

[0074] L-lactide and mesolactide generated through the various steps of the process according to the invention can be processed as needed. In one embodiment, at least a portion of the L-lactide produced by the first crystallization step, or at least a portion of the mesolactide produced by the second crystallization step, or a combination of at least a portion of the L-lactide produced by the first crystallization step and at least a portion of the mesolactide produced by the second crystallization step, is provided to a polymerization step to form a polymer comprising lactide units. Depending on whether other monomers are added, the polymer may be a polylactide homopolymer or a polylactide copolymer. In the latter case, other monomers may be polymerizable with the lactide monomer. Examples of other monomers include glycolide and ε-caprolactone, thereby forming poly(lactide-co-glycolic acid), poly(lactide-co-ε-caprolactone), and poly(lactide-co-glycolic acid-co-ε-caprolactone).

[0075] The L-lactide and mesolactide prepared according to the process of the present invention can also be used to prepare high-purity lactic acid. They can also be applied to other applications, such as the preparation of coatings, sealants, adhesives, resins, and hot melt adhesives or esters.

[0076] This application is prepared in connection with a method for processing lactide feed containing L-lactide as the major lactide isomer. However, it will be apparent to those skilled in the art that the same process described herein is applicable to lactide feed containing D-lactide as the major lactide isomer, and that the term "L-lactide" used herein can be replaced with "D-lactide," and vice versa. It will also be apparent to those skilled in the art that such a process for processing lactide feed containing D-lactide as the major isomer falls within the scope of this application.

[0077] As will be apparent to those skilled in the art, different embodiments of the invention can be combined, unless they are mutually exclusive. When amounts, concentrations, sizes, and other parameters are expressed in the form of ranges, preferred ranges, upper limits, lower limits, or preferred upper and lower limits, it should be understood that any range obtained by combining any upper or preferred value with any lower or preferred value is also specifically disclosed, regardless of whether the resulting range is explicitly mentioned in the context.

[0078] Furthermore, it should be noted that the title is provided for the reader's convenience and does not limit the invention. Additionally, the terms "stream," "feed," and "fraction" are used interchangeably herein, and the stream may be withdrawn or provided in whole or in part to subsequent steps, even if not explicitly stated otherwise.

[0079] Exemplary Implementation The present invention is illustrated by the following figures, but is not limited thereto or thereby restricted.

[0080] Figure 1 A general schematic diagram of the process of this invention is provided. Figure 1 In this process, a feed comprising mesolactone, L-lactide, water, lactic acid, and optionally other components is provided to a separation step (2) via line (1). The separation step (2) produces a stream (3) containing lactic acid and water, and a crude lactide stream (4) containing L-lactide and mesolactone. The stream (4) containing L-lactide and mesolactone is provided to a first crystallization step (5). The crystallization step (5), which may consist of one or more stages, produces high-purity L-lactide, which is withdrawn via line (6). The L-lactide content of the L-lactide stream is typically at least 90%, particularly at least 95%, more particularly at least 98%, and in some embodiments at least 99% (based on total lactide). The crystallization step (5) also produces a residual stream (7) containing L-lactide and mesolactone. The residual stream (7) is provided to a second separation step (8). The second separation step (8) produces a second L-lactide stream (9) and a mesolacide stream (10). The mesolacide stream (10) is fed to a second crystallization step (11), where the mesolacide is crystallized in purified form and removed from the process via line (12). Another residual stream is removed via line (13).

[0081] Figure 2 Some aspects of the invention are shown in more detail below. Figure 2In this embodiment, the separation step consists of two distillation steps (22) and (24). In distillation step (22), lactic acid and water are separated and taken out through line (3). The effluent from the first distillation step (22) has a high lactide content. It is taken out through line (23) and provided to the second distillation step (24). The second distillation step (24) generates a lactide stream rich in meso lactide, which is taken out through line (25). The second distillation step (24) also generates a crude lactide stream (4) containing L-lactide and meso lactide, which is provided to the first crystallization step (5). Thus, in this embodiment, at least some meso lactide has been separated from the lactide fraction before the first crystallization step (5). This is advantageous when the feed has a relatively high meso lactide content, because in this case, removing some meso lactide before the first crystallization step reduces the load on the first crystallization step (5). The lactide-rich stream extracted through pipeline (25) can be fed to a second separation step (8), where it undergoes further separation. This embodiment is shown in Figure 2 This implementation is advantageous because, in this way, the amount of racemic lactide in the crude lactide stream (4) containing L-lactide and racemic lactide supplied to the first crystallization step (5) is reduced. Therefore, the first crystallization step (5) can be carried out more efficiently. Furthermore, the L-lactide present in the lactide stream removed through pipeline (25) can be recovered when the stream is conveyed to the second separation step (8). This helps to increase the overall yield of L-lactide.

[0082] Alternatively, depending on its composition, the lactide stream rich in racemic lactide taken out through line (25) can also be directly supplied to the second crystallization step (11). This embodiment (not shown in Figure (2)) is particularly attractive when the racemic lactide stream (25) taken out from the second distillation column (24) has a relatively high racemic lactide content and, in particular, a relatively low L-lactide content. In this way, the first crystallization step can be carried out more efficiently, thereby increasing the overall yield of racemic lactide.

[0083] Figure 3 It shows Figure 2A variation of the illustrated embodiment. Besides lactic acid and water, the stream taken from the first distillation step (22) of the first separation step via pipeline (3) still contains mesolactone. This stream (3) is provided to the second separation step (8), where it undergoes further separation. In this way, the mesolactone yield of the process is increased. Furthermore, a stream (14) containing L-lactide is taken from the second distillation column (24). Stream (14) has a relatively high L-lactide content, but typically also contains some impurities, such as some D-lactide. However, the composition of stream (14) is generally suitable for supplying to the PLA manufacturing process. Taking stream (14) from the second separation step results in a smaller volume of stream (4) supplied to the first crystallization step (5). Therefore, the first and second crystallization steps, as well as the second separation step, can use smaller equipment, thereby reducing process costs.

[0084] Figure 4 It was shown as Figure 2 Another variation of the implementation method. In Figure 4In the embodiment, the second separation step (8) consists of two distillation steps (81) and (83). The residual stream (7) from the first crystallization step (5) is provided to the first distillation step (81). In the illustrated embodiment, this also applies to the stream (25) rich in meso lactide. However, as discussed above, this stream is only optional. In the distillation step (81), the light fraction containing water, lactic acid, and optional other volatile components is separated through line (85). Depending on its composition, it may be recycled to other process steps (with or without intermediate purification) or discharged from the process. The fraction containing heavy components (e.g., lactide oligomers generated during the process) is taken out through line (84). Depending on its composition, it may be recycled to other process steps (with or without intermediate purification) or discharged from the process. The first distillation step (81) also produces a stream with a high lactide content. This stream is taken out through line (82) and provided to the second distillation step (83). The second distillation step (83) produces a second L-lactide feed, which is taken out through line (9). The feed rich in mesolactide is taken out through line (10) and provided to the second crystallization step (11). The second distillation step (83) may also produce a bottom fraction (not shown), which is taken out from this step and provided to, for example, the first distillation step (81) or otherwise disposed of. Similarly, the second distillation step (83) may also produce a top fraction (not shown), which is taken out from this step and provided to, for example, the first distillation step (81) or otherwise disposed of. The advantage of performing the second separation step in two sequential steps (especially as two sequential distillation steps) is that, since the light and heavy fractions are separated in the first distillation step, both the L-lactide stream produced in this step and the stream provided to the second crystallization step can be relatively pure. Therefore, the mesolactide stream can be efficiently processed in the second crystallization step, and L-lactide of better quality is obtained compared to embodiments in which the second separation step consists of a single step.

[0085] Figure 5 It shows Figure 4 Variations. In Figure 5 In one embodiment, the first distillation step (81) of the second separation step (2) also generates a racemic lactide stream (86), which is directly fed to the second crystallization step (11) and bypasses the second distillation step (83). Whether this embodiment is attractive depends on how the first distillation step (81) can be operated. Figure 5 In the illustrated embodiment, the second distillation step (83) also produces stream (87) with a relatively high L-lactide content, but still containing some contaminants. Stream (87) has a higher contaminant content compared to the L-lactide stream (9). Therefore, it benefits from further purification, and Figure 5In the embodiment shown, it is recycled to the first crystallization step (5). By directly supplying the racemic lactide stream (86) to the second crystallization step (11), the total yield of racemic lactide is increased, and the contaminant content of the stream (87) is reduced.

[0086] Figure 6 Another embodiment is shown, in which the second separation step is performed in two steps, namely distillation steps (88) and (92). In the embodiment shown in the figure, the residual stream (7) from the first crystallization step (5) is provided to the first distillation step (88). In the embodiment shown, this also applies to the stream (25) rich in mesolacide. The distillation step (88) produces a stream rich in mesolacide, which is provided to the second crystallization step (11) via line (10). In the embodiment shown, the distillation step (88) produces a light fraction stream (89), which can be removed from the process or, for example, recycled to a lactide manufacturing step not shown. The distillation step (88) also produces a stream containing a large amount of L-lactide, some mesolacide, and some contaminants. This stream is sent to the second distillation step (92) via line (91). In step (92), the purified L-lactide is separated and removed via line (93). In the embodiment shown in the attached figure, the fraction containing racemic lactide is taken out through line (95) and recycled back to the first distillation step (88) with the aim of directing the racemic lactide in the fraction to the second crystallization step (11). In the embodiment shown in the attached figure, the second distillation step (92) also produces a stream (94) with a relatively high L-lactide content, but still containing some contaminants. Stream (94) has a higher contaminant content compared to the L-lactide stream (93). Therefore, it benefits from further purification, and Figure 6 In the embodiment shown, the product is recycled to the first crystallization step (5). The L-lactide stream (93) can be sent to a crystallization step, such as the first crystallization step (5) or the second crystallization step (not shown). This embodiment is advantageous because it results in higher yields and higher purity of both mesolactide and L-lactide.

[0087] Figure 7 One embodiment of the manufacturing feed (1) is shown, as it can be provided to the method of the present invention. Figure 7In this embodiment, lactic acid is supplied to the polycondensation step (102) via line (101), where water is removed via line (103) to prepare lactic acid oligomers. The lactic acid oligomers are then conveyed to the depolymerization step (105) via line (104). In the depolymerization step (105), lactide is formed, which then enters the process via line (1) and is supplied to the separation step (2). As mentioned above, the various boxes in the figures are not intended to represent individual process steps. For example, the depolymerization step may be performed in a step where a first separation step is also carried out. In this case, a single step would encompass both the depolymerization step (105) and the separation step (2). In this case, there would be no explicit feed line (1). The depolymerization step (105) may generate a bottom flow, in Figure 7 The bottom stream (106) is shown in the middle. This stream may contain lactic acid oligomers and contaminant compounds. It can be treated as needed. In one embodiment, it can be hydrolyzed to form industrial-grade lactic acid. The depolymerization step (105) can also generate a light distillate stream, in Figure 7 The light distillate fraction (107) is shown in the diagram. The light distillate fraction (107) will typically contain lactic acid. Depending on the conditions in the depolymerization step (105), it may also contain some meso-lactide. The light distillate fraction (107) can be processed as needed. In one embodiment, it is fed to the polycondensation step (102). It can also be fed to the lactic acid feed (101), such as... Figure 7 As shown, regardless of whether a hydrolysis step (not shown) is performed to convert lactide or other lactic acid oligomers in the stream into lactic acid.

Claims

1. A method for processing lactide feed, comprising the following steps: - A feed comprising L-lactide, mesolactide, water, lactic acid, and optionally other components is subjected to a first separation step, which results in the formation of a stream comprising lactic acid and water, and one or more crude lactide streams. - At least a portion of the crude lactide stream containing L-lactide and racemic lactide produced by the first separation step is subjected to a first crystallization step, which produces a purified L-lactide stream and a residual stream containing L-lactide and racemic lactide. - The residual stream containing L-lactide and mesolactide produced by the first crystallization step is subjected to a second separation step, the second separation step producing at least a second L-lactide stream and a mesolactide stream. - At least a portion of the racemic lactide stream produced by the second separation step is subjected to a second crystallization step, resulting in a purified racemic lactide stream and another residual stream.

2. The method according to claim 1, wherein the first separation step and / or the second separation step includes a distillation step.

3. The method according to claim 1 or 2, wherein the first separation step and / or the second separation step is a multi-step process, preferably wherein the first separation step and / or the second separation step is a two-step process.

4. The method according to any one of the preceding claims, wherein the first crystallization step and / or the second crystallization step is a multi-step process, preferably wherein the first crystallization step and / or the second crystallization step is a two-step process.

5. The method according to any one of the preceding claims, wherein the stream rich in mesolacide obtained from the first separation step is provided to the second separation step or the second crystallization step.

6. The method according to any one of the preceding claims, wherein the lactide stream containing L-lactide is condensed as part of the first separation step.

7. The method according to any one of the preceding claims, wherein the lactide-containing stream obtained by the second separation step is provided to the first crystallization step.

8. The method according to any one of the preceding claims, wherein another residual stream obtained from the second crystallization step is provided to the first separation step, the first crystallization step, and / or the second separation step.

9. The method according to any one of the preceding claims, wherein the first separation step produces a single lactide stream, or The first separation step produces a meso lactide stream and a crude lactide stream containing L-lactide and meso lactide.

10. The method according to any one of the preceding claims, wherein the meso lactide stream generated by the first separation step is provided to the second separation step or the second crystallization step.

11. The method according to any one of the preceding claims, wherein the second L-lactide stream is provided to the first crystallization step.

12. The method according to any one of the preceding claims, wherein the feed comprising L-lactide, mesolactide, water, lactic acid and optional other components comprises 75% to 95% L-lactide based on the total weight of lactide.

13. The method according to any one of the preceding claims, wherein the feed comprising L-lactide, mesolactide, water, lactic acid and optional other components comprises 75% to 99.5% total lactide based on the total weight of the feed.

14. The method according to any one of the preceding claims, wherein the feed comprising L-lactide, mesolactide, water, lactic acid and optional other components has a free acid content of at least 20 meq / kg and / or at most 150 meq / kg.

15. The method according to any one of the preceding claims, wherein prior to the first separation step, the following is performed: - The step of subjecting lactic acid to a condensation polymerization step to produce lactic acid oligomers, and - The lactic acid oligomer obtained in the polycondensation step is subjected to a depolymerization step to produce a feed comprising L-lactide, mesolactide, water, lactic acid and optional other components.

16. The method of claim 15, wherein the lactic acid-containing stream generated by the first separation step is provided to the step of subjecting the lactic acid to a polycondensation step.

17. The method according to claim 15 or 16, wherein the depolymerization step and the first separation step are performed in the same reactor.

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