An efficient process for removing lactic acid and lactic acid oligomers from a crude lactide containing composition

Abstract

A process for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers comprises the step of subjecting the crude composition or a modified crude composition, which has been obtained by subjecting the crude composition to a phase separation so as to obtain as light phase the modified crude composition, to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water.

Description

[0001] Sulzer Management AG S13801 PEP

[0002] An efficient process for removing lactic acid and lactic acid oligomers from a crude lactide containing composition

[0003] The present invention relates to a process for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers.

[0004] Lactic acid is the monomer of polylactic acid. Homo- and copolymers of lactic acid are of particular interest, because they can be obtained from renewable resources and are mostly compostable and / or biodegradable. Moreover, the technological and physiochemical properties of these polymers come quite close to the properties of those polymers derived from fossil-based resources, which explains why these polymers are regarded as highly promising substitutes for the latter. Furthermore, homo- and copolymers of lactic acid have a wide range of applications. For instance, polylactic acid is used in the biomedical field in chirurgical implants, in films, such as e.g. in packagings, in fibers, such as e.g. for garments, in hygienic articles, in carpets and in disposable plastic products, such as e.g. disposable cutlery or containers. In addition, polylactic acid has found wide application in composite materials, such as in fiber-reinforced plastics. Polylactic acid copolymers are interesting alternatives to the polylactic acid homopolymers, since by appropriate selection of the comonomers and their relative amounts to each other and by adjusting an appropriate molecular weight, certain properties of the copolymers may be tailored to the intended use.

[0005] Generally, two alternative principal methods are known for synthesizing polylactic acid homo- or copolymers. The first principal method is the direct polycondensation of lactic acid optionally together with one or more comonomers, such as caprolactone and / or glycolide, to the respective homopolymer or copolymer, respec- tively, such as the direct polycondensation of lactic acid to polylactic acid homopolymer. However, this principal method only leads to low molecular weight (co)polymers and is thus limited to specific (co)polymers. The second principal method known for synthesizing polylactic acid homo- or copolymers is the ring- opening-polymerization of lactide (which is the cyclic diester of lactic acid) optionally with one or more other cyclic esters, such as glycolide (which is the cyclic diester of glycolic acid), lactones or the like. This is the preferred method nowadays for the industrial production of polylactic acid homo- or copolymers.

[0006] Lactide is often produced by condensation of two lactic acid molecules, such as by reacting a mixture containing organic solvent, such as toluene, lactic acid in a concentration of 100 to 200 g / l and optionally water at a temperature of 150 to 200°C. The obtained crude lactide containing composition typically contains lactide in a concentration of about 1 to 3% by weight, unreacted lactic acid in a concentration of about 1 to 3% by weight, lactic acid oligomers in a concentration of about 3 to 9% by weight and remainder to 100% by weight organic solvent.

[0007] In order to process the lactide, for instance by using it as starting material in a ringopening polymerization reaction to polylactic acid, the crude lactide containing composition needs to be purified by removing lactic acid and lactic acid oligomers. This is usually performed by firstly subjecting the crude lactide containing composition to a phase separation into a lower, heavy phase and into an upper, light phase and then secondly by subjecting the light phase to a liquid-liquid extraction with pure water as extracting agent. Since the crude lactide containing composition prepared by condensation reaction of two lactic acid molecules is obtained at a temperature of 150 to 200°C, at which the composition is a homogeneous solution, typically, the phase separation is performed by first cooling the crude lactide containing composition to a temperature of less than 100°C, such as about 60°C, in order to allow the crude lactide containing composition to phase separate into the heavy phase and light phase due to the different solubilities of lactic acid, lactic acid oligomers and lactide in the organic solvent. More specifically, lactic acid and short lactic acid oligomers containing one to three lactic acid units have a low solubility in the organic solvent, whereas medium sized lactic acid oligomers containing four to seven lactic acid units have medium solubility in the organic solvent and lactide as well as long lactic acid oligomers containing 8 or more lactic acids units have a good solubility in the organic solvent. Accordingly, after the phase separation a heavy phase, which contains mainly lactic acid as well as short lactic acid oligomers and lower amounts of medium sized lactic acid oligomers, and a light phase, which contains organic solvent, lactide, long lactic acid oligomers, some medium sized lactic acid oligomers and few short lactic acid oligomers and lactic acid, are obtained. The light phase is then extracted with pure water as extracting agent in a liquid- liquid extraction. Since the water and the organic solvent are not miscible, since lactic acid and short lactic acid oligomers have a high solubility in water, but a low solubility in organic solvent, since medium sized lactic acid oligomers have a medium solubility in both, organic solvent and water, and since lactide has a high solubility in organic solvent, but a low solubility in water, during the liquid-liquid extraction most of the lactic acid and short lactic acid oligomers migrate from the organic solvent into the extracting agent, whereas the medium sized lactic acid oligomers split between the organic solvent and the extracting agent and the lactide and long lactic acid oligomers remain in the organic solvent. Accordingly, an extract stream and a raffinate stream are obtained in the liquidliquid extraction, wherein the extract stream contains the extracting agent water, lactic acid, short lactic acid oligomers and about half of the medium lactic acid oligomers and the raffinate stream contains the organic solvent, lactide, long lactic acid oligomers and about half of the medium sized lactic acid oligomers. Hence, a disadvantage of the aforementioned process is that the medium sized lactic acid oligomers, i.e. lactic acid oligomers containing four to seven lactic acid units, cannot be efficiently removed from the crude lactide containing composition. In view of this, the object underlying the present invention is to provide a process for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers, which efficiently removes lactic acid as well as short and medium sized lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers.

[0008] In accordance with the present invention, this object is satisfied by providing a process for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers, wherein the process comprises the step of subjecting the crude composition or a modified crude composition, which has been obtained by subjecting the crude composition to a phase separation so as to obtain as light phase the modified crude composition, to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water so as to obtain a purified lactide stream.

[0009] This solution bases on the finding that by first extracting the crude composition or the modified crude composition with an aqueous solution containing lactic acid as first extracting agent and then by secondly extracting it, i.e. the raffinate stream obtained after the first extraction, with water as second extracting agent, not only lactic acid and short lactic acid oligomers containing one to three lactic acid units are efficiently removed from the (modified) crude composition, but in particular also the medium sized lactic acid oligomers containing four to seven lactic acid units. It has been found by the inventors of the present invention that the lactic acid contained in the first extracting agent increases the solubility of the medium sized lactic acid oligomers in the first extracting agent, but leads due to the lactic acid contained in the first extracting agent to an increase of the amount of lactic acid in the raffinate. Hence, in the first liquid-liquid extraction using the first extracting agent, namely aqueous solution containing lactic acid, significantly more of the medium sized lactic acid oligomers are extracted in comparison to using water as extracting agent, but the amount of lactic acid in the raffinate is increased. Since, however, water, i.e. the second extracting agent, has an excellent solubility for lactic acid and for the short lactic acid oligomers, the two liquid-liquid extractions, namely a first liquid-liquid extraction using aqueous solution containing lactic acid as first extracting agent and a second liquid-liquid extraction using water as second extracting agent, efficiently remove virtually all of the lactic acid, of the short lactic acid oligomers and of the medium sized lactic acid oligomers being contained in the (modified) crude lactide containing composition. All in all, after the two extractions a raffinate stream containing organic solvent, lactide and remaining long lactic acid oligomers is obtained, which does virtually not contain any lactic acid, short lactic acid oligomers and medium sized lactic acid oligomers, whereas the lactic acid, short lactic acid oligomers and medium sized lactic acid oligomers are contained in the extracts obtained during the two liquid-liquid extractions.

[0010] In accordance with the present invention, lactic acid oligomer denotes molecules, which contain 2 to 19 lactic acid units, whereas lactic acid polymer denotes molecules, which contain 20 or more lactic acid units.

[0011] In accordance with the present invention, the crude composition contains lactide, organic solvent, lactic acid and lactic acid oligomers. Preferably, the crude composition contains as lactic acid oligomers at least one medium sized lactic acid oligomer, i.e. a lactic acid oligomer containing four to seven lactic acid units, and preferably also at least one short lactic acid oligomer, i.e. a lactic acid oligomer containing one to three lactic acid units. For instance, at least 10% by weight and preferably at least 20% by weight of the lactic acid oligomers contained in the crude composition are medium sized lactic acid oligomers, whereas at least 10% by weight and preferably at least 20% by weight of the lactic acid oligomers contained in the crude composition are short lactic acid oligomers. More preferably, 20 to 70% by weight of the lactic acid oligomers contained in the crude composition are medium sized lactic acid oligomers and 20 to 70% by weight of the lactic acid oligomers contained in the crude composition are short lactic acid oligomers, wherein the sum of medium sized lactic acid oligomers and of short lactic acid oligomers of the lactic acid oligomers contained in the crude composition is preferably at least 60% by weight, more preferably at least 70% by weight, still more preferably at least 80% by weight and most preferably at least 90% by weight, with the remainder to 100% by weight being long lactic acid oligomers.

[0012] The present invention is not particularly limited concerning the amounts of the single ingredients in the crude composition. Good results are in particular obtained, when the crude composition contains, based on 100% by weight of the crude composition, 40 to 95% by weight and preferably 55 to 75% by weight of organic solvent, 0.5 to 17% by weight and preferably 2 to 8% by weight of lactide, 1 .0 to 60% by weight and preferably 10 to 40% by weight of sum of lactic acid and of lactic acid oligomers.

[0013] In terms of the “dry weight”, i.e. when disregarding the organic solvent contained in the crude composition, it is preferred that the crude composition contains, based on 100% by weight of the crude composition excluding the organic solvent, 20 to 40% by weight and preferably 24 to 35% by weight of lactide, 10 to 30% by weight and preferably 15 to 25% by weight of lactic acid and 40 to 80% by weight and preferably 50 to 70% by weight of lactic acid oligomers.

[0014] The present invention is also not particularly limited concerning the kind of organic solvent being contained in the crude composition. Preferably, the organic solvent has a solubility for lactide at 23°C of at least 50 g / l and more preferably of at least 100 g / l. In a further development of the idea of the present invention, it is suggested that the organic solvent contained in the crude composition is selected from the group consisting of Ci-20-alkanes, Ci-20-alkenes, Ci-20-alkynes, C6-24-cycloalkanes, C4-24- aromatic compounds, linear Ci-20-ethers, cyclic Ce-24-ethers, fatty alcohols, pH modifiers and arbitrary mixtures of two or more thereof. Good results are in particular obtained, when the organic solvent is a Ce-12-aromatic compound, more preferably is a Ce-s-aromatic compound and still more preferably is selected from the group consisting of benzene, toluene, xylene, styrene, ethyl benzene and arbitrary mixtures of two or more thereof. Most preferably, the organic solvent is toluene. Suitable examples for pH modifiers are inorganic salt, acids and bases.

[0015] The present invention is particularly suitable for processing a crude composition, which has been prepared by mixing an aqueous lactic acid solution with a lactic acid concentration of 50 to 95% by weight and preferably 65 to 90% by weight, based on 100% by weight of the aqueous lactic acid solution, with organic solvent in a weight ratio of 1 :5 to 1 :20 and preferably of 1 :8 to 1 : 12, heating the so obtained mixture to 100 to 200°C and reacting it at 100 to 200°C e.g. for 1 minute to 20 hours.

[0016] In accordance with a particularly preferred embodiment of the present invention, the process comprises the following steps: a) adjusting the temperature of the crude composition to 0 to 100°C, preferably to 40 to 80°C and more preferably to 50 to 65°C and allowing the crude composition to phase separate into a heavy phase and into a light phase as modified crude composition, b) subjecting the modified crude composition obtained in step a) to a liquidliquid extraction with an aqueous solution containing lactic acid so as to obtain a raffinate stream and an extract stream and c) subjecting the raffinate stream obtained in step b) to a liquid-liquid extraction with water so as to obtain a raffinate stream as purified lactide stream. It is preferred in the aforementioned embodiment that the modified crude composition obtained in step a) contains, based on 100% by weight of the modified crude composition, 60 to 99% by weight and preferably 85 to 95% by weight of organic solvent, 0.1 to 30% by weight and preferably 2 to 10% by weight of lactide, 0.5 to 10% by weight and preferably 1 to 4% by weight of lactic acid oligomers.

[0017] In terms of the “dry weight”, it is preferred that the modified crude composition contains, based on 100% by weight of the modified crude composition excluding the organic solvent, 20 to 60% by weight and preferably 30 to 50% by weight of lactide as well as 80 to 40% by weight and preferably 70 to 50% by weight of lactic acid oligomers.

[0018] In accordance with the present invention, the crude composition is subjected to a liquid-liquid extraction with an aqueous solution containing lactic acid as first extracting agent. Good results are in particular obtained, when the aqueous solution containing lactic acid contains 5 to 30% by weight and preferably 10 to 20% by weight of lactic acid. In addition to water and lactic acid, the aqueous solution containing lactic acid may contain up to 10% by weight of one or more impurities, such as one or more salts, e.g. sodium chloride, a C1-4-alcohol, e.g. ethanol, and others. Preferably, the aqueous solution contains not more than 5% by weight, more preferably not more than 1 % by weight, still more not preferably more than 0.1% by weight of impurities and most preferably no impurities at all. The aqueous solution containing lactic acid can comprise more than 5 vol% or wt%(% by weight) of water such as more than 7 or more than 10 vol% or wt% and less than 90 vol% or wt% of water such as less than 80 or less than 70 vol% or wt%, preferably 5-50, more preferably 5-25, even more preferably 5-15 and most preferably 5- 10 vol% or wt% of water. The pH value of the aqueous solution containing lactic acid is preferably 6 to 8, more preferably 6.5 to 7.5, still more preferably 6.8 to 7.2, yet more preferably 6.9 to 7.1 and most preferably about 7.0. Furthermore, in accordance with the present invention the crude composition is subjected to a liquid-liquid extraction with an aqueous solution containing water as second extracting agent. Good results are in particular obtained, when the water contains 10,000 ppm or less, preferably 1 ,000 ppm or less, more preferably 500 ppm or less, yet more preferably 100 ppm or less, still more preferably 50 ppm or less and most preferably 10 ppm or less of impurities. The pH value of the water is preferably 6 to 8, more preferably 6.5 to 7.5, still more preferably 6.8 to 7.2, yet more preferably 6.9 to 7.1 and most preferably about 7.0.

[0019] In accordance with the present invention, the crude composition or modified crude composition is subjected to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water so as to obtain a purified lactide stream. Both liquid-liquid extractions may be performed continuously or both liquid-liquid extractions may be performed discontinuously or one liquid-liquid extraction may be performed continuously and the other liquid-liquid extraction may be performed discontinuously. Preferably, both liquid-liquid extractions are performed continuously.

[0020] Furthermore, both liquid-liquid extractions may be performed in one liquid-liquid extraction column or in two subsequently arranged liquid-liquid extraction columns.

[0021] Preferably, each of the liquid-liquid extractions is performed at a temperature of at most 80°C, preferably of at most 70°C and more preferably at 30 to 60°C. In addition, it is preferred that each of the liquid-liquid extractions is performed at ambient pressure.

[0022] In accordance with a first particularly preferred embodiment of the present invention, the step of subjecting the crude composition or the modified crude composition to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water is performed in one liquid-liquid extraction column. The liquid-liquid extraction column may be a vertically arranged liquidliquid extraction column or a horizontally arranged liquid-liquid extraction column, wherein a vertically arranged liquid-liquid extraction column is preferred.

[0023] Good results are in particular obtained in this embodiment, when the crude composition or the modified crude composition is fed into the lower portion of a vertically arranged liquid-liquid extraction column, the water as second extracting agent is fed into the upper portion of the liquid-liquid extraction column and the aqueous solution containing lactic acid or concentrated lactic acid dissolving in the liquidliquid extraction column in the descending water and hence forming in the liquidliquid extraction column an aqueous solution containing lactic acid as first extracting agent is fed into the liquid extraction column at a middle position being located between the position, at which the crude composition or the modified crude composition is fed into the liquid-liquid extraction column, and the position, at which the water is fed into the liquid-liquid extraction column. During the operation, the water being fed as second extracting agent into the upper portion of the liquid-liquid extraction column flows downwardly, while the crude composition or the modified crude composition being fed into the lower portion of the liquid-liquid extraction column flows upwardly. In the upper portion of the liquid extraction column, where the upwardly flowing crude composition or the modified crude composition is contacted with the downwardly flowing water or second extracting agent, respectively, most of the remaining lactic acid, short sized lactic acid oligomers and medium sized lactic acid oligomers and also lactic acid and short lactic acid oligomers migrate from the crude composition or the modified crude composition into the second extracting agent. At the middle position of the liquid extraction column, at which the aqueous solution containing lactic acid or concentrated lactic acid is fed into the liquid extraction column, a two-phase mixture of an descending extract stream, which comprises the water as second extracting agent as well as lactic acid and lactic acid oligomers dissolved therein, and of an ascending raffinate stream, which comprises the crude composition or the modified crude composition, respectively, being depleted concerning lactic acid oligomers, is present. The descending extract stream may be withdrawn via an outlet line from the middle position of the liquid extraction column. However, preferably, the descending extract stream is not withdrawn via an outlet line from the middle position of the liquid extraction column, but is allowed to further flow downwardly within the liquid extraction column. In the first case, i.e. the embodiment, in which the descending extract stream is withdrawn via an outlet line from the middle position of the liquid extraction column, an aqueous solution containing lactic acid as first extracting agent is fed into the liquid extraction column at the middle position. However, in the preferred embodiment, in which the descending extract stream is not withdrawn via an outlet line from the middle position of the liquid extraction column, it is preferred that concentrated lactic acid is fed into the liquid extraction column at the middle position, which dissolves in the liquid extraction column in the descending aqueous extract stream, thereby forming an aqueous solution containing lactic acid as first extracting agent. In the lower portion of the liquid extraction column, where the upwardly flowing crude composition or the modified crude composition is contacted with the downwardly flowing first extracting agent, most of the medium sized lactic acid oligomers and also lactic acid and short lactic acid oligomers migrate from the crude composition or the modified crude composition into the first extracting agent. While purified lactide stream as raffinate stream is withdrawn from the upper portion of the liquid-liquid extraction column, an extract stream is withdrawn from the lower portion and optionally - as set out above - also from the middle position of the liquid-liquid extraction column. Lower portion of a liquidliquid extraction column means in this connection the portion, which extends, seen from the bottom of the liquid-liquid extraction column upwardly, from 0 to 20% of the height of the liquid-liquid extraction column, whereas upper portion of the liquid-liquid extraction column means the portion, which extends, seen from the bottom of the liquid-liquid extraction column upwardly, from 80 to 100% of the height of the liquid-liquid extraction column. As set out above, the aqueous solution containing lactic acid is preferably fed into the liquid extraction column at a middle position, which means a position being located between the position, at which the crude composition or the modified crude composition is fed into the liquid-liquid extraction column, and the position, at which the water is fed into the liquid-liquid extraction column. Good results are in particular obtained in this embodiment, when the aqueous solution containing lactic acid is fed into the liquid extraction column at a position being located between 40 to 70% and preferably between 40 to 60% of the distance between the position, at which the crude composition or the modified crude composition is fed into the liquid-liquid extraction column, and the position, at which the water is fed into the liquid-liquid extraction column.

[0024] In accordance with an alternative, second particularly preferred embodiment of the present invention, the step of subjecting the crude composition or the modified crude composition to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water is performed in two separate, subsequently arranged liquid-liquid extraction columns. Each of the two liquid-liquid extraction columns may be a vertically arranged liquid-liquid extraction column or a horizontally arranged liquid-liquid extraction column, wherein two vertically arranged liquid-liquid extraction columns are preferred.

[0025] Good results are in particular obtained in this embodiment, when the crude composition or the modified crude composition is fed into the lower portion of a first vertically arranged liquid-liquid extraction column, the aqueous solution containing lactic acid is fed as first extracting agent into the upper portion of the first liquidliquid extraction column, a raffinate stream is withdrawn from the upper portion of the first liquid-liquid extraction column and an extract stream is withdrawn from the lower portion of the first liquid-liquid extraction column. The raffinate stream being withdrawn from the first liquid-liquid extraction column is fed into the lower portion of a second vertically arranged liquid-liquid extraction column, whereas the water as second extracting agent is fed into the upper portion of the second liquid-liquid extraction column, a raffinate stream is withdrawn as purified lactide stream from the upper portion of the second liquid-liquid extraction column and an extract stream is withdrawn from the lower portion of the second liquid-liquid extraction column. The terms lower portion and upper portion of the liquid-liquid extraction column are defined as described above.

[0026] Independently, from whether the liquid-liquid extractions with the first extracting agent and the second extracting agent are performed in one liquid-liquid extraction column or in two separate liquid-liquid extraction columns, it is preferred that the liquid-liquid extraction is performed in one or two agitated extraction columns and more preferably in one or two agitated countercurrent extraction columns.

[0027] Each of the agitated liquid-liquid extraction column preferably comprises at least one and more preferably a plurality, such as 2 to 80 and still more preferably 5 to 40, agitated internals being disposed within the liquid-liquid extraction column. For instance, each of the agitated internals comprises one or more rotating shafts, each of which being connected with one or more agitators being preferably selected from the group consisting of discs, blades, paddles, turbine impellers, fins and arbitrary combinations of two or more of the aforementioned agitators.

[0028] Good results are in particular obtained, when each of the agitated liquid-liquid extraction columns comprises two or more compartments, which are separated from each other by static partition plates, wherein at least some and preferably all of the compartments comprise a rotating shaft comprising one or more agitators being preferably selected from the group consisting of discs, blades, paddles, turbine impellers, fins and arbitrary combinations of two or more of the aforementioned agitators. For instance, the liquid-liquid extraction column, in which the agitated liquid-liquid extraction is performed, may be embodied as a Kuhni column and the like.

[0029] As set out above, the step of subjecting the crude composition or modified crude composition, which has been obtained by subjecting the crude composition to a phase separation so as to obtain as light phase the modified crude composition, to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water so as to obtain a purified lactide stream leads to an at least essentially complete removal of lactic acid, of short lactic acid oligomers having one to three lactic acid units and of middle sized lactic acid oligomers having four to seven lactic acid units from the mixture of organic solvent and lactide. In particular, the process in accordance with the present invention allows to obtain a purified lactide stream, which contains, based on 100% by weight of the purified lactide stream, 90 to 99% by weight and preferably 97 to 99% by weight of organic solvent and 1 to 5% by weight and preferably 1 to 3% by weight by weight of lactide.

[0030] Preferably, the purified lactide stream contains, based on 100% by weight of the purified lactide stream, 1 % by weight or less, preferably 0.5% by weight or less and more preferably 0.2% by weight or less of lactic acid and lactic acid oligomers. The lactic oligomers are mainly long lactic acid oligomers, i.e. at least 70% by weight, preferably at least 80% by weight and more preferably at least 90% by weight of the lactic acid oligomers are long lactic acid oligomers, with the remainder to 100% by weight being short and middle sized lactic acid oligomers.

[0031] In terms of the “dry weight”, the purified lactide stream preferably contains, based on 100% by weight of the purified lactide stream excluding the organic solvent, 80% by weight or more, more preferably 85% by weight or more and most preferably 90% by weight or more of lactide, whereas the purified lactide stream preferably contains, based on 100% by weight of the purified lactide stream excluding the organic solvent, 20% by weight or less, more preferably 15% by weight or less and most preferably 10% by weight or less of lactic acid and lactic acid oligomers.

[0032] In order to remove the organic solvent and, if necessary, in order to further remove long lactic acid oligomers, it is proposed in a further development of the idea of the present invention that the purified lactide stream is subjected to one or more distillation steps and preferably to one or to two subsequent distillation steps so as to obtain a pure lactide stream.

[0033] Preferably, the pure lactide stream obtained after distillation contains, based on 100% by weight of the pure lactide stream, 90% by weight or more, preferably 95% by weight or more and more preferably 99% by weight or more of lactide.

[0034] Furthermore, it is preferred that the pure lactide stream obtained after distillation contains, based on 100% by weight of the pure lactide stream, 10% by weight or less, preferably 5% by weight or less and more preferably 1 % by weight or less of lactic acid and lactic acid oligomers.

[0035] Subsequently, the present invention is described by means of an illustrative, but not limiting figure, in which:

[0036] Fig. 1 is a schematic view of a plant for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers in accordance with one embodiment of the present invention.

[0037] Fig. 2 is a schematic view of a plant for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers in accordance with another embodiment of the present invention. Fig. 3a and 3b are schematic length-sectional views of a part of the liquid-liquid extraction columns of the plants shown in figures 1 and 2.

[0038] The plant 10 for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers shown in figure 1 comprises a vessel 12, a first liquid-liquid extraction column 14 and a second liquid-liquid extraction column 16. The vessel 12 comprises an inlet line 18 for crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers, an outlet line 20 for heavy phase and an outlet line 22 for light phase. The outlet line 22 for light phase of the vessel 12 is connected with an inlet being provided in the lower portion of the first liquid-liquid extraction column 14, wherein the first liquid-liquid extraction column 14 is further connected in its upper portion with an inlet line 24 for first extracting agent or an aqueous solution containing lactic acid, respectively, at its bottom with an outlet line 26 for extract and at its top with an outlet line 28 for raffinate. The outlet line 28 for raffinate of the first liquid-liquid extraction column 14 is connected with an inlet being provided in the lower portion of the second liquid-liquid extraction column 16, wherein the second liquid-liquid extraction column 16 is further connected in its upper portion with an inlet line 30 for second extracting agent or water, respectively, at its bottom with an outlet line 32 for extract and at its top with an outlet line 34 for raffinate or purified lactide stream, respectively.

[0039] During the operation of the plant 10, crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers, which has been obtained by condensation reaction of an aqueous lactic acid solution at a temperature of about 150°C, is fed via the inlet line 18 into the vessel 12. The temperature of the crude composition is adjusted in the vessel 12 to about 60°C and the crude composition separates in the vessel 12 into a heavy phase being withdrawn from the vessel via the outlet line 20 and into a light phase being withdrawn from the vessel via the outlet line 22. The light phase is led via line 22 into the lower portion of the first liquidliquid extraction column 14 and is contacted in the first liquid-liquid extraction column 14 with the first extracting agent (i.e. with aqueous solution containing lactic acid), which is fed via the inlet line 24 into the upper portion of the first liquid-liquid extraction column 14. During the contact of the light phase and the first extracting agent a part of the lactic acid, of the short lactic acid oligomers and most of the middle sized lactic acid oligomers migrate from the light phase and dissolve in the first extracting agent. The extract containing the first extracting agent and the dissolved lactic acid, short lactic acid oligomers and middle sized lactic acid oligomers is withdrawn from the first liquid-liquid extraction column 14 via the outlet line 26, whereas the raffinate containing the organic solvent and dissolved lactide, long lactic acid oligomers and remaining lactic acid, short lactic acid oligomers and middle sized lactic acid oligomers is withdrawn from the first liquid-liquid extraction column 14 via the outlet line 28. The raffinate stream of the first liquid-liquid extraction column 14 is led via an inlet into the lower portion of the second liquid-liquid extraction column 16 and is contacted in the second liquid-liquid extraction column 16 with the second extracting agent (i.e. with water), which is fed into the upper portion of the second liquid-liquid extraction column 16 via the inlet line 30. During the contact of the raffinate stream and the second extracting agent the lactic acid, the short lactic acid oligomers and remaining middle sized lactic acid oligomers migrate from the raffinate stream and dissolve in the second extracting agent. The extract containing the second extracting agent and the dissolved lactic acid, short lactic acid oligomers and middle sized lactic acid oligomers is withdrawn from the second liquid-liquid extraction column 16 via the outlet line 32, whereas the raffinate containing the organic solvent and dissolved lactide and possible remaining long lactic acid oligomers is withdrawn from the second liquid-liquid extraction column 16 as purified lactide stream via the outlet line 34.

[0040] The plant 10 for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers shown in figure 2 corresponds to that shown in figure 1 except that the two liquidliquid extraction columns 14, 16 of figure 1 are combined to one liquid-liquid extraction column 14 in figure 2. The liquid-liquid extraction column 14 shown in figure 2 comprises an outlet line 32 for extract at the middle position of the liquidliquid extraction column 14. During the operation of the plant an aqueous solution containing lactic acid as first extracting agent is fed into the middle position of the liquid-liquid extraction column 14 via the inlet line 24. As mentioned further above, the outlet line 32 for extract at the middle position of the liquid-liquid extraction column 14 may be omitted and is in fact preferably omitted. In this case, during the operation of the plant not an aqueous solution containing lactic acid, but concentrated lactic acid is fed into the middle position of the liquid-liquid extraction column 14 via the inlet line 24. The concentrated lactic acid mixes within the liquid-liquid extraction column 14 with the descending aqueous extract stream and thereby forms an aqueous solution containing lactic acid as first extracting agent. Preferably, the second extracting agent (i.e. with water), which is fed into the upper portion of the second liquid-liquid extraction column 16 via the inlet line 30 is provided in an amount of 5-25vol%, preferably 5-15vol%, most preferably 5-10vol% with respect to the incoming solvent streams.

[0041] Figures 3a and 3b show schematically length-sectional views of a part of the liquid-liquid extraction columns 14, 16 of the plants 10 shown in figures 1 and 2. Each of the liquid-liquid extraction columns 14, 16 is margined by a wall 36 and comprises a plurality of compartments 38, 38’, 38”, from which three are shown in figure 3a and one is shown in figure 3b. Each of the compartments 38, 38’, 38” is separated from adjacent compartments 38, 38’, 38” by means of a static partition plate 40, 40’, 40”, 40’”. Each of the static partition plates 40, 40’, 40”, 40’” is perforated (not shown), i.e. comprises several holes allowing liquid to pass from one side of the partition plate 40, 40’, 40”, 40’” to the other. Furthermore, the liquidliquid extraction column 14 comprises a rotating shaft 42 extending through each of the compartments 40, 40’, 40”, 40’” and being arranged, seen in the length section, in the center of the compartments 40, 40’, 40”, 40”’. The rotating shaft 42 comprises several turbine impellers as agitators 44, wherein each compartment 40, 40’, 40”, 40’” comprises two agitators 44. As shown in more detail in figure 3b, which shows the section of the liquid-liquid extraction column 14 highlighted in the box 46 of figure 3a, during the operation of the liquid-liquid extraction column 14 modified crude composition flows upwardly in the direction of arrows 48, 50’ through the liquid-liquid extraction column 14, whereas the extracting agent flows in the counter-direction downwardly as indicat- ed by the arrows 50, 48’. The mixture of the modified crude composition and of the extracting agent is agitated through the rotating agitators 44, wherein a rapid flow in the length direction is restricted by the perforated partition plates 40, 40’, 40”, 40’”, so that, seen in the length-section, an oval flow pattern as shown by the arrows 52, 52’ is generated. Thereby an intimate contact between the modified crude composition and the extracting agent is achieved leading to a particular efficient extraction of lactic acid, of short lactic acid oligomers and of medium sized lactic acid oligomers into the extracting agent.

[0042] Reference numerals

[0043] 10 Plant

[0044] 12 Vessel

[0045] 14 (First) liquid-liquid extraction column

[0046] 16 Second liquid-liquid extraction column

[0047] 18 Inlet line for crude composition

[0048] 20 Outlet for heavy phase

[0049] 22 Outlet for light phase

[0050] 24 Inlet line for first extracting agent (aqueous solution containing lactic acid)

[0051] 26 Outlet line for extract

[0052] 28 Outlet line for raffinate

[0053] 30 Inlet line for second extracting agent (water)

[0054] 32 Outlet line for extract

[0055] 34 Outlet line for raffinate (purified lactide stream)

[0056] 36 Wall of liquid-liquid extraction column

[0057] 38, 38’, 38” Compartments of liquid-liquid extraction column

[0058] 40, 40’, 40”, 40’” Static partition plate

[0059] 42 Rotating shaft

[0060] 44 Agitators

[0061] 46 Part of the liquid-liquid extraction column shown in figure 3b

[0062] 48, 50’ Arrow showing the flow direction of the modified crude composition

[0063] 50, 48’ Arrow showing the flow direction of the extracting agent

[0064] 52, 52’ Arrow showing flow pattern in the compartment

Claims

Claims:1 . A process for removing lactic acid and lactic acid oligomers from a crude composition containing lactide, organic solvent, lactic acid and lactic acid oligomers, wherein the process comprises the step of subjecting the crude composition or a modified crude composition, which has been obtained by subjecting the crude composition to a phase separation so as to obtain as light phase the modified crude composition, to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water so as to obtain a purified lactide stream.

2. The process in accordance with claim 1 , wherein the crude composition contains, based on 100% by weight of the crude composition, 40 to 95% by weight and preferably 55 to 75% by weight of organic solvent, 0.5 to 17% by weight and preferably 2 to 8% by weight of lactide, 1 .0 to 60% by weight and preferably 10 to 40% by weight of sum of lactic acid and of lactic acid oligomers.

3. The process in accordance with claim 1 or 2, wherein the organic solvent is selected from the group consisting of Ci-20-alkanes, Ci-20-alkenes, C1-20- alkynes, C6-24-cycloalkanes, C4-24-aromatic compounds, linear Ci-20-ethers, cyclic Ce-24-ethers, fatty alcohols, pH modifiers and arbitrary mixtures of two or more thereof, preferably is a Ce-12-aromatic compound, more preferably is a Ce-8-aromatic compound, still more preferably is selected from the group consisting of benzene, toluene, xylene, styrene, ethyl benzene and arbitrary mixtures of two or more thereof and most preferably is toluene.

4. The process in accordance with any of the preceding claims, wherein the crude composition has been prepared by mixing an aqueous lactic acid solution with a lactic acid concentration of 50 to 95% by weight and preferably65 to 90% by weight, based on 100% by weight of the aqueous lactic acid solution, with organic solvent in a weight ratio of 1 :5 to 1 :20 and preferably of 1 :8 to 1 :12, heating the so obtained mixture to 100 to 200° C and reacting it at 100 to 200°C for 1 minute to 20 hours.

5. The process in accordance with any of the preceding claims, wherein the process comprises the following steps: a) adjusting the temperature of the crude composition to 0 to 100°C, preferably to 40 to 80°C and more preferably to 50 to 65°C and allowing the crude composition to phase separate into a heavy phase and into a light phase as modified crude composition, b) subjecting the modified crude composition obtained in step a) to a liquid-liquid extraction with an aqueous solution containing lactic acid so as to obtain a raffinate stream and an extract stream and c) subjecting the raffinate stream obtained in step b) to a liquid-liquid extraction with water so as to obtain a raffinate stream as purified lactide stream.

6. The process in accordance with claim 1 or 5, wherein the modified crude composition contains, based on 100% by weight of the modified crude composition, 60 to 99% by weight and preferably 85 to 95% by weight of organic solvent, 0.1 to 30% by weight and preferably 2 to 10% by weight of lactide, 0.5 to 10% by weight and preferably 1 to 4% by weight of lactic acid oligomers.

7. The process in accordance with any of the preceding claims, wherein the aqueous solution containing lactic acid contains 5 to 30% by weight and preferably 10 to 20% by weight of lactic acid, up to 10% by weight and preferably up to 5% by weight of impurities and remainder to 100% by weight of the aqueous solution of water.

8. The process in accordance with any of the preceding claims, wherein the water contains 10,000 ppm or less, preferably 1 ,000 ppm or less, more preferably 500 ppm or less, yet more preferably 100 ppm or less, still more preferably 50 ppm or less and most preferably 10 ppm or less of impurities.

9. The process in accordance with any of the preceding claims, wherein the step of subjecting the crude composition or the modified crude composition to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water is performed in one liquidliquid extraction column.

10. The process in accordance with claim 9, wherein the crude composition or the modified crude composition is fed into the lower portion of a vertically arranged liquid-liquid extraction column, the water is fed into the upper portion of the liquid-liquid extraction column and the aqueous solution containing lactic acid or concentrated lactic acid is fed into the liquid extraction column at a middle position being located between the position, at which the crude composition or the modified crude composition is fed into the liquidliquid extraction column, and the position, at which the water is fed into the liquid-liquid extraction column, whereas purified lactide stream as raffinate is withdrawn from the upper portion and extract stream is withdrawn from the lower portion and optionally also from the middle position of the liquidliquid extraction column, wherein lower portion means the portion extending, seen from the bottom of the liquid-liquid extraction column upwardly, from 0 to 20% of the height of the liquid-liquid extraction column and upper portion means the portion extending, seen from the bottom of the liquidliquid extraction column upwardly, from 80 to 100% of the height of the liquid-liquid extraction column.11 . The process in accordance with any of claims 1 to 8, wherein the step of subjecting the crude composition or the modified crude composition to a liquid-liquid extraction with an aqueous solution containing lactic acid and then to a liquid-liquid extraction with water is performed in two separate, subsequently arranged liquid-liquid extraction columns, wherein preferably the crude composition or the modified crude composition is fed into the lower portion of a first vertically arranged liquid-liquid extraction column, the aqueous solution containing lactic acid is fed into the upper portion of the first liquid-liquid extraction column, a raffinate stream is withdrawn from the upper portion of the first liquid-liquid extraction column and an extract stream is withdrawn from the lower portion of the first liquid-liquid extraction column, wherein the raffinate stream being withdrawn from the first liquidliquid extraction column is fed into the lower portion of a second vertically arranged liquid-liquid extraction column, the water is fed into the upper portion of the second liquid-liquid extraction column, a raffinate stream is withdrawn as purified lactide stream from the upper portion of the second liquidliquid extraction column and an extract stream is withdrawn from the lower portion of the second liquid-liquid extraction column, wherein lower portion means the portion extending, seen from the bottom of the liquid-liquid extraction column upwardly, from 0 to 20% of the height of the liquid-liquid extraction column and upper portion means the portion extending, seen from the bottom of the liquid-liquid extraction column upwardly, from 80 to 100% of the height of the liquid-liquid extraction column.

12. The process in accordance with any of the preceding claims, wherein the liquid-liquid extraction is performed in one or two agitated countercurrent extraction columns, wherein each of the one or two agitated extraction columns comprises one or more rotating shafts, each of which comprising one or more agitators being selected from the group consisting of discs, blades, paddles, turbine impellers, fins and arbitrary combinations of two or more ofthe aforementioned agitators, wherein each of the one or more agitated extraction columns comprises two or more compartments, which are separated from each other by static partition plates, wherein at least some and preferably all of the compartments comprise a rotating shaft comprising one or more agitators being selected from the group consisting of discs, blades, paddles, turbine impellers, fins and arbitrary combinations of two or more of the aforementioned agitators.

13. The process in accordance with any of the preceding claims, wherein the purified lactide stream contains, based on 100% by weight of the purified lactide stream, 90 to 99% by weight and preferably 97 to 99% by weight of organic solvent and 1 to 5% by weight and preferably 1 to 3% by weight by weight of lactide, wherein the purified lactide stream preferably contains, based on 100% by weight of the purified lactide stream, 1% by weight or less, preferably 0.5% by weight or less and more preferably 0.2% by weight or less of lactic acid and lactic acid oligomers.

14. The process in accordance with any of the preceding claims, wherein the purified lactide stream is subjected to one or more distillation steps and preferably to one or to two subsequent distillation steps so as to obtain a pure lactide stream.

15. The process in accordance with claim 14, wherein the pure lactide stream contains, based on 100% by weight of the pure lactide stream, 90% by weight or more, preferably 95% by weight or more and more preferably 99% by weight or more of lactide, wherein the pure lactide stream preferably contains, based on 100% by weight of the pure lactide stream, 10% by weight or less, preferably 5% by weight or less and more preferably 1 % by weight or less of lactic acid and lactic acid oligomers.

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