Plant and efficient process for producing lactide from lactic acid

Abstract

1. A method for producing lactide from lactic acid, the method comprising: a) providing lactic acid, a hydrocarbon solvent, and at least one catalyst to a reactor of a reactor system to form a reaction mixture in the reactor; b) reacting the reaction mixture such that at least a portion of the lactic acid is converted to lactide and water while distilling at least a portion of the reaction mixture to remove an azeotrope of water and hydrocarbon solvent from the reactor; c) separating at least a portion of the water from the azeotrope removed from the reactor in step b); d) separating at least a portion of the lactide from the reaction mixture; At least one catalyst comprises a homogeneous catalyst comprising an organometallic compound selected from the group consisting of tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates, and any mixture of two or more of these compounds.

Description

[Technical field]

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

[0002] Lactic acid is the monomer of polylactic acid. Homopolymers and copolymers of lactic acid are particularly interesting since they can be obtained from renewable resources and most are compostable and / or biodegradable. Moreover, the technical and physicochemical properties of these polymers are very close to those of polymers derived from fossil-based resources, which explains why these polymers are considered as very promising alternatives to polymers derived from fossil-based resources. Moreover, homopolymers and copolymers of lactic acid have a wide range of applications. For example, polylactic acid is used in the biomedical field in metallurgical implants, films such as packaging, fibers such as clothing, hygiene articles, carpets and disposable plastic products such as disposable cutlery or containers. Furthermore, polylactic acid is widely applied in composite materials such as fiber-reinforced plastics. Polylactic acid copolymers are an interesting alternative to polylactic acid homopolymers since the specific properties of the copolymer can be tailored to the intended application by appropriately selecting the comonomers and their relative amounts to each other and adjusting the appropriate molecular weight.

[0003] Generally, two alternative main methods are known for synthesizing polylactic acid homopolymers or polylactic acid copolymers. The first main method is to directly polycondense lactic acid, optionally with one or more comonomers such as caprolactone and / or glycolide, into the respective homopolymers or copolymers, for example, to directly polycondense lactic acid into polylactic acid homopolymers. However, this main method only results in low molecular weight (co)polymers, and is therefore limited to certain (co)polymers.

[0004] The second major method known to synthesize polylactic acid homopolymers or copolymers is the ring-opening polymerization of lactide (which is a cyclic diester of lactic acid) and, optionally, one or more other cyclic esters, such as glycolide (which is a cyclic diester of glycolic acid), lactones, etc. This is the preferred method today for the industrial production of polylactic acid homopolymers or copolymers. Lactide can be produced by condensation of two lactic acid molecules. Alternatively, lactide can be produced by first oligomerizing lactic acid and then subjecting the oligomers to a depolymerization reaction. For example, lactide is prepared by fermenting carbohydrates from biomass such as starch, sugar or corn to obtain lactic acid, then oligomerizing the lactic acid and then subjecting the oligomers to a depolymerization reaction. Similarly, optional other cyclic diester comonomers can be produced by condensation of two hydroxy acid molecules to a cyclic diester, for example, condensation of glycolic acid to glycolide, or intramolecular esterification of an aliphatic hydroxy acid, for example, in the case of a lactone, such as caprolactone. After purification, lactide and optionally one or more other cyclic diesters of hydroxy acids as comonomers are polymerized in the presence of a catalyst and optionally an initiator to form a high molecular weight polymer. Unreacted (co)monomers must be removed after polymerization to a final concentration of at least less than 0.5% by weight in order to obtain a product of marketable quality.

[0005] The synthesis of lactide from lactic acid by condensation of two lactic acid molecules into lactide and water must be carried out in the presence of a catalyst, otherwise the conversion rate will be unacceptably low. Furthermore, the catalyst must be selective in the sense that it enhances the conversion rate of two lactic acid molecules into lactide and water compared to the conversion rate of the thermodynamically favorable condensation of lactic acid into lactic acid oligomers. For this purpose, zeolite catalysts, such as acidic zeolites, are typically used to meet the aforementioned requirements.

[0006] For example, Dusselier et al., in Science, 2015, volume 349, pages 78 to 80, describe a method for producing lactide from lactic acid using an acidic zeolite and preferably an H-beta zeolite catalyst. More specifically, each method involves reacting a mixture containing lactic acid, water, o-xylene as a solvent and an H-beta zeolite catalyst at 110° C. under continuous reflux, which is performed by water-solvent distillation to remove a portion of the water produced during the reaction to promote the reaction. The specific zeolite catalyst is one that has high selectivity to obtain a high conversion rate of lactic acid to lactide with a relatively small amount of by-products such as linear lactic acid dimers and other linear lactic acid oligomers. Furthermore, the conversion rate of lactic acid to lactide is enhanced by the combination of using a zeolite catalyst and a solvent with a relatively high boiling point, i.e., o-xylene with a boiling point of about 139° C., which results in a higher conversion rate of lactic acid to lactide than a solvent with a lower boiling point such as toluene. The reaction mixture is continuously removed from the reactor, and then subjected to liquid-liquid extraction to separate the reaction mixture into an aqueous phase fraction containing water and by-products and an organic lactide-rich fraction, which is then crystallized to separate the lactide from the solvent. The fraction removed from the reactor by water-solvent distillation is phase separated in a settler to separate this fraction into water and solvent, and recycled to the reactor. However, this process has several disadvantages. First, the zeolite is not soluble in the solvent, and thus is a heterogeneous catalyst in this process. This has the disadvantage that the zeolite catalyst needs to be regenerated, which includes separation from the reaction mixture, followed by washing of the catalyst with additional solvent and separation. Furthermore, since the zeolite is not soluble in the solvent, the zeolite catalyst not only acts on the solvent phase, but also on the aqueous phase containing lactic acid oligomers and other by-products, which in principle unnecessarily produces more by-products than necessary. Another drawback is that, according to this document, the zeolite catalyst requires the use of o-xylene as a solvent in order to obtain a sufficiently high conversion of lactic acid to lactide.However, since o-xylene has a relatively low solubility for lactide and a high solubility for by-products such as lactic acid dimers or other lactic acid oligomers, a certain amount of lactide is lost in the aqueous phase obtained by liquid-liquid extraction, and a considerable amount of by-products is contained in the solvent phase obtained by liquid-liquid extraction. Summary of the Invention

[0007] In view of this, the object underlying the present invention is to provide a process for producing lactide from lactic acid, which makes it possible to rapidly obtain lactide in high purity and high yield with minimal capital costs for the plant and minimal operating costs.

[0008] According to the invention, the object is a process for producing lactide from lactic acid, comprising the steps of: a) providing lactic acid, a hydrocarbon solvent, and at least one catalyst to a reactor of a reactor system to form a reaction mixture in the reactor; b) reacting the reaction mixture in the reactor such that at least a portion of the lactic acid is converted to lactide and water while distilling at least a portion of the reaction mixture to remove an azeotrope of water and hydrocarbon solvent from the reactor; c) separating at least a portion of the water from the azeotrope removed from the reactor in step b); d) separating at least a portion of the lactide from the reaction mixture; This is accomplished by providing a method in which at least one catalyst comprises a homogeneous catalyst comprising an organometallic compound selected from the group consisting of tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates, and any mixture of two or more of these compounds.

[0009] This solution is based on the surprising finding that lactide can be obtained efficiently in high yield as well as in a rapid process with high purity by using a homogeneous catalyst comprising an organometallic compound selected from the group consisting of tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and any mixture of two or more of these compounds in a process comprising the steps of: feeding lactic acid, a hydrocarbon solvent and at least one catalyst to a reactor of a reactor system and forming a reaction mixture in the reactor; b) reacting the reaction mixture in the reactor while distilling at least a portion of the reaction mixture to remove an azeotrope of water and the hydrocarbon solvent from the reactor, reacting at least a portion of the lactic acid to form lactide and water; c) separating at least a portion of the water from the azeotrope removed from the reactor in step b); and d) separating at least a portion of the lactide from the reaction mixture. In particular, this process can be operated at low operating costs and requires only a plant with low capital costs. The use of a homogeneous catalyst comprising at least one organometallic compound selected from the group consisting of tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and any mixture of two or more of these compounds has the advantage that it has a particularly high selectivity and thus leads to a high conversion of lactic acid to lactide, with very small amounts of by-products such as linear lactic acid dimers and other lactic acid oligomers. This is due, among other things, to the fact that the catalyst is selectively soluble in a hydrocarbon solvent such as toluene but not in water, and therefore only catalyzes species that are soluble in the hydrocarbon solvent, keeping water-soluble species, i.e. lactic acid oligomers and acidic impurities, away from the reaction environment. Moreover, unlike heterogeneous catalysts such as zeolites, the homogeneous catalysts used in the process according to the invention can be easily separated from the hydrocarbon solvent by distillation and can be directly reused, so that there is no need to carry out a regeneration process with additional solvents and separations. Furthermore, the homogeneous catalysts used in the process according to the invention can be easily separated from the hydrocarbon solvent by distillation and ... This is because these catalysts, such as stannous octoate, stannous (II) 2-ethylhexanoate or stannous octoate (also known as Sn(Oct)2), are mostly FDA approved catalysts and therefore not toxicologically significant. A further advantage of the process according to the invention is that it can be carried out instead with high boiling point hydrocarbon solvents as in the aforementioned prior art, for example at high pressures of 0.2 MPa, and with low boiling point hydrocarbon solvents such as toluene, which have particularly high selectivity, i.e. very high solubility for lactide and particularly low solubility for by-products such as lactic acid oligomers. Thus, lactide is almost entirely contained in the organic solvent phase of the reaction mixture, while the by-products are almost entirely retained in the aqueous phase of the reaction mixture. Finally, the process according to the invention is flexible and can be carried out batchwise, semi-continuously or continuously.

[0010] A homogeneous catalyst in the sense of the present invention means any catalyst that is present in the same phase as the reactant molecules, i.e. in the reaction mixture containing the hydrocarbon solvent. In contrast to this, a heterogeneous catalyst is in a different phase from one or more of the reactants and often acts as a surface where the reaction can take place. Thus, a homogeneous catalyst in the sense of the present invention is any non-solid catalyst that has a high solubility in the hydrocarbon solvent used so that it dissolves during the reaction in the hydrocarbon solvent but not in the aqueous phase. Thus, according to the present invention, the catalyst used in step a) has a solubility in the hydrocarbon phase used at ambient temperature, i.e. 23° C., as well as at least 50% by weight, preferably at least 75% by weight, more preferably at least 95% by weight. In particular, homogeneous catalysts have a higher solubility in the hydrocarbon solvent than in water. Preferably, the solubility in water at 23° C. of the catalyst used in step a) is less than 10% by weight, more preferably less than 5% by weight, most preferably less than 1% by weight. Usually, homogeneous catalysts are not at all soluble in water, but decompose, for example, on contact with water. For example, tin octonoate, the most preferred homogeneous catalyst of the present invention, dissociates in water into 2-ethylhexanoate and Sn(II) ions, which spontaneously oxidize in water to form the insoluble inorganic solid Sn(IV)O2. In contrast to solid heterogeneous catalysts such as zeolites, the use of homogeneous catalysts avoids the steps of adsorbing reactants prior to the reaction and desorbing products after the reaction, which are usually mass transfer limited.

[0011] According to the present invention, the catalyst fed to the reactor in step a), i.e. the catalyst contained in the reactor during step b), comprises at least one organometallic compound selected from the group consisting of tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and any mixture of two or more of these compounds. Preferably, the catalyst comprises at least 50% by weight of said catalyst, more preferably at least 80% by weight of said catalyst, even more preferably at least 90% by weight of said catalyst, even more preferably at least 95% by weight of said catalyst, and most preferably consists of said catalyst. Thus, most preferably, no other catalysts than one or more of said organometallic compounds are used in steps a) and b).

[0012] Good results are particularly obtained when the at least one catalyst comprises or consists of at least one organometallic compound selected from the group consisting of tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and any mixture of two or more of these compounds. More preferably, all of the aforementioned carboxylates contain one or more C 2~14 -carboxylate groups, more preferably one or more C 6~14 Even more preferably, the at least one catalyst is an organometallic compound containing at least one tin carboxylate, preferably one or more C 2~14 -carboxylate groups, more preferably one or more C 6~14-carboxylate groups. Also more preferably, the at least one catalyst comprises or consists of tin octoate (i.e., tin 2-ethylhexanoate), tin oxalate or dibutyltin dilaurate. Most preferably, the at least one catalyst comprises or consists of tin octoate. Tin octoate has the advantage of being FDA approved. Furthermore, tin octoate is often used as a catalyst in the ring-opening polymerization reaction of lactide to polylactic acid, allowing for easy integration of the process for producing lactide from lactic acid according to the present invention with the subsequent process of reacting the lactide thus obtained to polylactic acid biopolymer.

[0013] In a further development of the inventive idea, it is proposed that the content of catalyst in the reaction mixture fed to the reactor in step a) is 0.01-1.00 mol %, preferably 0.02-0.50 mol %, more preferably 0.05-0.30 mol %, also more preferably 0.05-0.20 mol %, most preferably 0.08-0.12 mol %, based on the dry content of lactic acid in the reaction mixture.

[0014] According to the present invention, in step a), lactic acid, a hydrocarbon solvent and at least one catalyst are fed to a reactor of a reactor system to form a reaction mixture in the reactor. All of the aforementioned components may be added to the reactor as one mixture or separately from each other. In addition, further components, such as preferably water, and optionally one or more recycle streams, such as an aqueous stream containing a small amount of lactic acid and by-products obtained by purification of the water obtained in step d), may be fed to the reactor. More preferably, all components are mixed together before being fed to the reactor as one mixture in step a). For example, a hydrocarbon solvent may be mixed with the catalyst before the aqueous solution of lactic acid in water and one or more optional recycle streams are mixed to obtain a reaction mixture.

[0015] Preferably, in step a), a mixture comprising lactic acid, water, a hydrocarbon solvent and at least one catalyst is fed to the reactor. The content of lactic acid in the aqueous part of the reaction mixture is preferably in an amount of 60-80 wt. % based on the sum of lactic acid and water, and the content of lactic acid in the reaction mixture is preferably in an amount of 15-30 wt. % based on the content of the hydrocarbon solvent.

[0016] In principle, the present invention is not particularly limited with respect to the chemical nature of the hydrocarbon solvent, so long as it meets the aforementioned solubility requirements for the homogeneous catalyst together with the catalyst.

[0017] Furthermore, it is preferred that the hydrocarbon solvent used in the process according to the invention has a high solubility for lactide. More specifically, the solubility of lactide in the hydrocarbon solvent is at least 2% by weight, preferably at least 3% by weight, more preferably at least 4% by weight, even more preferably at least 4.5% by weight, also more preferably at least 6% by weight, more preferably at least 7.5% by weight, at room temperature, i.e. 23° C., and at ambient pressure.

[0018] Good results are obtained in particular with benzene or benzene derivatives, such as any alkyl substituted benzene, such as toluene, o-, m- or p-xylene, trimethylbenzene, tetramethylbenzene, pentamethylbenzene, hexamethylbenzene, monoethylbenzene, diethylbenzene, triethylbenzene, tetraethylbenzene, pentaethylbenzene, hexaethylbenzene, monopropylbenzene, dipropylbenzene, tripropylbenzene, propylbenzene, pentapropylbenzene, hexapropylbenzene and combinations thereof.

[0019] In a further development of the idea of ​​the present invention, it is suggested that the hydrocarbon solvent is an aromatic hydrocarbon solvent, i.e. a hydrocarbon compound or a mixture of two or more hydrocarbon compounds each having a boiling point of 80-130°C, more preferably 90-120°C, even more preferably 100-120°C.

[0020] Most preferably, the hydrocarbon solvent is toluene, which not only has very high selectivity, i.e., extremely high solubility for lactide and particularly low solubility for by-products such as lactic acid oligomers, but is also relatively inexpensive, especially significantly cheaper than high boiling point solvents such as xylene and isobutylbenzene.

[0021] According to certain preferred embodiments of the present invention, the reaction mixture is reacted in step b) of the process at a temperature of 100-180° C., more preferably at a temperature of 110-160° C., also more preferably at a temperature of 120-160° C., even more preferably at a temperature of 130-150° C. and most preferably at a temperature of 135-145° C. These temperatures are particularly preferred when toluene is used as the hydrocarbon solvent.

[0022] Furthermore, the reaction mixture is preferably reacted in step b) of the process at a pressure of 0.10-0.40 MPa, more preferably at a pressure of 0.15-0.30 MPa, even more preferably at a pressure of 0.15-0.25 MPa, even more preferably at a pressure of 0.18-0.22 MPa, most preferably at a pressure of 0.19-0.21 MPa. These pressures are particularly preferred when toluene is used as the hydrocarbon solvent.

[0023] Particularly good results are obtained when combining both of the aforementioned embodiments, i.e. reacting the reaction mixture in step b) of the process at a temperature of 100-180°C and a pressure of 0.10-0.40 MPa, more preferably at a temperature of 110-160°C and a pressure of 0.15-0.30 MPa, even more preferably at a temperature of 120-160°C and a pressure of 0.15-0.25 MPa, even more preferably at a temperature of 130-150°C and a pressure of 0.18-0.22 MPa, and most preferably at a temperature of 135-145°C and a pressure of 0.19-0.21 MPa. These temperature and pressure conditions are particularly preferred when toluene is used as the hydrocarbon solvent. Thereby, the aforementioned solubility advantages of toluene, i.e. very high solubility of lactide and very low solubility of by-products, are advantageous in combination with high temperature and high pressure conditions, since the reaction rate of the reaction of lactic acid to lactide and water is driven by temperature.

[0024] The present invention is not particularly limited by the number and type of reactors included in the reactor system. For example, the reactor may include one or more continuous stirred tank reactors, one or more loop reactors, one or more plug flow reactors, and any combination of one or more of the above-mentioned reactors. Preferably, the reactor of the reactor system to which lactic acid, a hydrocarbon solvent and at least one catalyst are fed in step a) is a continuous stirred tank reactor or a plug flow reactor. Most preferably, the reactor of the reactor system to which lactic acid, a hydrocarbon solvent and at least one catalyst are fed in step a) is a continuous stirred tank reactor.

[0025] The distillation in step b) may be realized in a distillation column, which is preferably located on top of the reactor, which is more preferably a continuous stirred tank reactor.

[0026] According to a preferred embodiment of the present invention, the reactor system comprises one reactor, i.e. no further reactor, more preferably a continuous stirred tank reactor as one reactor. In this embodiment, the reaction mixture is removed from the reactor and then subjected to the separation of step d).

[0027] According to an alternative embodiment of the present invention, the reactor system comprises two reactors in series with each other. Preferably, the first reactor, into which lactic acid, a hydrocarbon solvent and at least one catalyst are fed in step a), is a continuous stirred tank reactor, and the second reactor downstream of the first reactor is a continuous stirred tank reactor or a plug flow reactor, preferably a continuous stirred tank reactor. In this embodiment, the reaction mixture is removed from the first reactor and directed to the second reactor before being removed from the second reactor and subjected to the separation in step d).

[0028] In a further development of the idea of ​​the present invention, the reaction mixture is reacted in step b) in the reactor system for 0.1 to 10 hours, preferably 1 to 5 hours, more preferably 2 to 4 hours, most preferably 2.5 to 3.5 hours. This reaction time is the sum of the reaction times in all reactors contained in the reactor system. Thus, if the reactor system comprises three reactors, the residence time of the reaction mixture in the first reactor + the residence time of the reaction mixture in the second reactor + the residence time of the reaction mixture in the third reactor is the total time that the reaction mixture reacts in step b) in the reactor system. In this context, reacting means incubating the reaction mixture in the reactor for a certain residence time.

[0029] In the above-mentioned embodiment, particularly in step b) of the process, good results are obtained when the reaction mixture is reacted at a temperature of 110-160°C and a pressure of 0.15-0.30 MPa for 0.1-10 hours, more preferably at a temperature of 120-160°C and a pressure of 0.15-0.25 MPa for 1-5 hours, even more preferably at a temperature of 130-150°C and a pressure of 0.18-0.22 MPa for 2-4 hours, and most preferably at a temperature of 135-145°C and a pressure of 0.19-0.21 MPa for 2.5-3.5 hours.

[0030] In step d) of the method of the present invention, after the reaction mixture is removed from the reactor system, i.e., from the most downstream reactor of the reactor system, at least a portion of lactide is separated from the reaction mixture.This can be achieved, for example, by separating the reaction mixture, driven by gravity, into an organic phase containing the hydrocarbon solvent and lactide, and an aqueous phase containing water and by-products, before the lactide is separated from the organic phase by one or more separation techniques, such as crystallization, distillation, absorption, or any combination of any of these.The resulting aqueous phase can also be further purified by any of the separation techniques, such as one or more of the aforementioned separation techniques, to obtain a concentrated aqueous fraction containing pure water and by-products.The concentrated aqueous fraction containing by-products can be recycled to the feed to the reactor system.

[0031] According to a first particular preferred embodiment of the process of the present patent application, the separation of at least a portion of the lactide from the reaction mixture of step d), i.e. from the reaction mixture removed from the most downstream reactor of the reactor system, comprises the following substeps: d1) separating the reaction mixture in the first vessel into an organic phase and an aqueous phase; d2) extracting the organic phase obtained in step d1) with water in a liquid / liquid extraction step to obtain an organic phase and an aqueous by-product-containing phase; and d3) Separating the organic phase obtained in step d2) in a distillation column into a lactide-rich phase and a hydrocarbon solvent-rich phase, the hydrocarbon solvent-rich phase being preferably recycled into the reactor.

[0032] The first vessel used in step d1) may be a gravity separating vessel or settler in which the reaction mixture is driven into an organic phase and an aqueous phase.

[0033] Preferably, the liquid / liquid extraction step d2) is carried out in an extraction column. In order to obtain an efficient and rapid separation in step d2), in a further development of the idea of ​​the invention, it is proposed that the liquid / liquid extraction column used in step d2) comprises one or more coalescers, preferably one. Any type of coalescer can be used for this purpose, such as any type of electrostatic coalescer and / or any type of mechanical coalescer. For example, a mechanical coalescer comprising one or more structured corrugated metal or plastic sheets, a mechanical coalescer comprising two materials with a large difference in surface free energy to promote droplet coalescence of either phase, or a mechanical coalescer comprising a cartridge made of a selected fiber material, in which the liquid flows radially outward from the center of the cartridge, can be used. During the extraction of step d2), as many by-products and impurities as possible are removed from the organic phase by extraction with water. Preferably, the organic phase obtained in step d1) is fed to the liquid / liquid extraction column of step d2) at a first location and water is fed to the liquid / liquid extraction column at a second location, the second location being vertically above the first location.

[0034] During step d3), the organic phase obtained in step d2) is distilled in a distillation column to obtain a lactide fraction and a hydrocarbon solvent fraction. Preferably, the lactide fraction obtained in step d3) is further purified in at least one further distillation step to increase its purity to a desired level.

[0035] Furthermore, in order to minimize the amount of hydrocarbon solvent required for the process, it is preferred that the hydrocarbon solvent fraction obtained in step d3) is recycled and fed to the reactor of step a).

[0036] According to a further preferred embodiment of the present invention, a hydrolysis unit is arranged downstream of the liquid / liquid extraction column and is fluidly connected to the outlet of the liquid / liquid extraction column, the aqueous by-product-containing phase fraction comprises unreacted lactic acid and lactic acid oligomers, so that in a further step d4) in the hydrolysis unit, the lactic acid oligomers are hydrolyzed to shorter oligomers and lactic acid, or even exclusively to lactic acid. The hydrolysis in step d4) can be achieved by adding water and lactic acid to the residual fraction in the hydrolysis unit. The hydrolysis unit may also comprise an ion exchange catalyst, such as a solid acid catalyst or a solid base catalyst, preferably a solid acid catalyst, to enhance the reaction rate.

[0037] Preferably, the aqueous by-product-containing phase fraction obtained in the liquid / liquid extraction step d2), independently of whether it is subjected to the preferred treatment in the hydrolysis unit or not, is recycled to step a) and fed to the reactor.

[0038] Alternatively, in a second preferred embodiment of the process of the invention, the separation of at least a portion of the lactide from the reaction mixture of step d), i.e. from the reaction mixture removed from the most downstream reactor of the reactor system, may comprise the following substeps: d1) separating the reaction mixture in the first vessel into an organic phase and an aqueous phase; d2) separating the organic phase obtained in step d1) into a lactide fraction and a hydrocarbon solvent fraction in a second vessel; and d3) Separating the aqueous phase obtained in step d1) in a third vessel into water and a residual fraction containing unreacted lactic acid and lactic acid oligomers.

[0039] In order to obtain an efficient and rapid separation of the reaction mixture into organic and aqueous phases, in a further development of the idea of ​​the invention, it is proposed that the first vessel used in step d1) comprises one or more coalescers. For example, the first vessel may be a column comprising one or more coalescers. Any type of coalescer can be used for this purpose, such as any type of electrostatic coalescer and / or any type of mechanical coalescer. For example, a mechanical coalescer comprising one or more structured corrugated metal or plastic sheets, a mechanical coalescer comprising two materials with a large difference in surface free energy to promote droplet coalescence of either phase, or a mechanical coalescer comprising a cartridge made of a selected fiber material, in which the liquid flows radially outward from the center of the cartridge, can be used. Instead of one or more coalescers, the first vessel may comprise one or more mixer-settlers.

[0040] According to a further particularly preferred embodiment of the present invention, during the separation of step d1), the organic phase is also washed with water in order to remove as many by-products and impurities as possible from the organic phase. Preferably, for this purpose, the reaction mixture is fed to the first vessel of step d1) at a first position, and water is fed to the first vessel of step d1) at a second position and distributed across the cross-sectional plane of the first vessel by one or more liquid distributors, the second position being vertically above the first position. This allows the organic phase flowing upward, i.e. the light phase, to be washed with water, so that water-soluble by-products and impurities such as lactic acid oligomers are washed out of the organic phase.

[0041] According to a further embodiment of the invention, the second vessel used in step d2) is a distillation column, in which the organic phase is distilled to obtain a lactide fraction and a hydrocarbon solvent fraction.

[0042] The lactide fraction obtained in step d2) can be further purified in at least one additional distillation step to increase its purity to the desired level.

[0043] Preferably, the hydrocarbon solvent fraction obtained in step d2) is recycled and fed to the reactor in step a) in order to minimize the amount of hydrocarbon solvent required for the process.

[0044] Particularly, good results are obtained in step d3), in which the third vessel used in step d3) is a distillation column, in which the aqueous phase is distilled to obtain water and a residual fraction containing unreacted lactic acid and lactic acid oligomers. Alternatively, a reverse osmosis (RO) membrane unit can be used to remove water and recover unreacted lactic acid and its oligomers.

[0045] According to a particular preferred embodiment of the present invention, the hydrolysis unit is arranged downstream of the first or third vessel in fluid connection with the outlet of the fraction containing unreacted lactic acid and lactic acid oligomers, so that in a further step d4) of the hydrolysis unit, the lactic acid oligomers are hydrolyzed to shorter oligomers and lactic acid or exclusively to lactic acid. The hydrolysis of step d4) can be achieved by adding water and lactic acid to the residual fraction in the hydrolysis unit. The hydrolysis unit may also contain an ion exchange catalyst, such as a solid acid catalyst or a solid base catalyst, preferably a solid acid catalyst, to enhance the reaction rate.

[0046] Preferably, the residual fraction comprising unreacted lactic acid and lactic acid oligomers obtained in step d3), or even more preferably the residual fraction comprising unreacted lactic acid obtained in step d4), is recycled in step a) and fed to the reactor.

[0047] According to a further particular preferred embodiment of the present patent application, the separation of at least a portion of the water from the azeotrope in step c) comprises the following substeps: c1) condensing the azeotrope removed from the reactor in step b) to obtain a liquid stream; c2) Separating the liquid stream obtained in step c1) into a hydrocarbon solvent-rich fraction which is recycled to the distillation column above the reactor, and a water-rich fraction.

[0048] The separation of step c2) can be carried out by decantation, by using one or more coalescers, by adsorption, by distillation or by any other suitable separation technique. Preferably, the separation of step c2) is carried out by decantation in a vessel or settler, respectively.

[0049] Furthermore, in said first particularly preferred embodiment of the process according to the invention, it is preferred to feed the water-rich fraction obtained in step c2) alone or together with the aqueous phase obtained in step d1) as extractant to a liquid / liquid extraction step.

[0050] Alternatively, the separation of at least a portion of the water from the azeotrope carried out in step c) may comprise the following substeps: c1) partially or completely cooling the azeotrope to obtain a liquid stream, a portion of the liquid stream solvent being recycled to the distillation column above the reactor; c2) separating the portion of the liquid stream not recycled to the reactor into a water-rich fraction and a hydrocarbon solvent-rich fraction.

[0051] The separation of step c2) can be carried out by decantation, by using one or more coalescers, by adsorption, by distillation or by any other suitable separation technique. Preferably, the separation of step c2) is carried out by decantation.

[0052] Furthermore, in the aforementioned second particularly preferred embodiment of the process according to the invention, it is preferred to feed the hydrocarbon-rich solvent-rich fraction obtained in step c2) alone or together with the organic phase obtained in step d1) to the second vessel of step d2).

[0053] It is further preferred to feed the water-rich fraction obtained in step c2) alone or, in the above-mentioned first particularly preferred embodiment of the process according to the invention, together with the aqueous by-product-containing phase obtained in step d2) or, in the above-mentioned second particularly preferred embodiment of the process according to the invention, together with the aqueous phase obtained in step d1) to a third vessel in step d3).

[0054] As stated above, the process according to the invention is flexible and can be carried out batchwise, semi-continuously or continuously. Preferably, the process is carried out continuously.

[0055] Another aspect of the invention is a plant for producing lactide from lactic acid, comprising: a reactor system comprising at least one reactor, a distillation column being arranged at the top of the reactor so as to be able to distill the steam generated during the operation in the reactor into an azeotrope of water and hydrocarbon, the distillation column comprising an outlet line for the azeotrope of water and hydrocarbon solvent, the reactor system comprising a feed line and an outlet line for the reaction mixture, a separator connected to the outlet line for the water-hydrocarbon azeotrope of the distillation column for separating at least a portion of the water from the azeotrope removed through the outlet line, and - a separator connected to the outlet line for the reaction mixture The plant includes:

[0056] According to a preferred embodiment of the invention, the reactor system of the plant comprises one reactor, which is a continuous stirred tank reactor, on top of which a distillation column is placed. Preferably, a condenser is placed between the outlet line and the separator, which preferably comprises a recycle line for recycling the hydrocarbon solvent separated in the separator to the distillation column.

[0057] According to an alternative embodiment of the invention, the reactor system of the plant comprises two reactors in series with each other, the first reactor connected by a feed line is a continuous stirred tank reactor, on top of which a distillation column is placed, and the second reactor downstream of the first reactor is a continuous stirred tank reactor or a plug flow reactor, preferably a continuous stirred tank reactor, on top of which a distillation column is placed, both reactors being connected via lines to remove the reaction mixture from the first reactor to the second reactor during operation. In this embodiment, each of the distillation columns comprises an outlet line for the azeotrope of water and the hydrocarbon solvent, and the outlet line for the reaction mixture is placed in the second continuous stirred tank reactor. The outlet line for the azeotrope of water and the hydrocarbon solvent of the first distillation column is connected to a separator for separating at least a portion of the water from the azeotrope removed through the outlet line, while the outlet line for the azeotrope of water and the hydrocarbon solvent of the second distillation column is connected to another separator for separating at least a portion of the water from the azeotrope removed through the outlet line. Preferably, a condenser is disposed between each outlet line and the separator, and each separator preferably includes a recycle line for recycling the hydrocarbon solvent separated in the separator to the distillation column.

[0058] In a further development of the idea of ​​the present invention, it is suggested that the separator connected to the outlet line of the reaction mixture is a vessel or settler which separates gravity driven reaction mixture into organic and aqueous phases.

[0059] Alternatively, in particular when the aforementioned second preferred embodiment of the method of the invention is used for step d), the separator connected to the outlet line of the reaction mixture may comprise one or more coalescers. For example, the separator may be a column comprising one or more coalescers. Any type of coalescer may be used for this purpose, such as any type of electrostatic coalescer and / or any type of mechanical coalescer. For example, a mechanical coalescer comprising one or more structured corrugated metal or plastic sheets, a mechanical coalescer comprising two materials with a large difference in surface free energy to promote droplet coalescence of either phase, or a mechanical coalescer comprising a cartridge made of a selected fiber material, in which the liquid flows radially outward from the center of the cartridge, may be used. In order to be able to wash the organic phase with water during the separation of step d1) of the aforementioned second preferred embodiment of the method of the invention, it is preferred that the outlet line of the reaction mixture leads to the separator at a first location and the water inlet line leads to the separator at a second location, the second location being vertically above the first location, the water inlet line being connected to a distributor for distributing the water over the cross-sectional plane of the separator.

[0060] According to a first particular preferred embodiment of the plant of the present patent application, the plant for producing lactide from lactic acid comprises: a reactor system comprising a continuous stirred tank reactor with a distillation column arranged on top of it and a further continuous stirred tank reactor downstream of the continuous stirred tank reactor with a distillation column arranged on top of it, both reactors being connected via a line such that during operation a reaction mixture is removed from the continuous stirred tank reactor to the continuous stirred tank reactor, the distillation columns each comprising an outlet line for the azeotrope of water and the hydrocarbon solvent, the reactor system comprising a feed line and an outlet line for the reaction mixture, a first separator connected to the outlet line for the reaction mixture, the first separator being a vessel or a settler comprising an outlet line for the aqueous phase and an outlet line for the organic phase, a liquid / liquid extraction column comprising an inlet line for the water-rich stream, an inlet line connected to the outlet line for the organic phase of the first separator, an outlet line for the organic phase and an outlet line for the aqueous by-product-containing phase, preferably comprising one or more electrostatic coalescers and / or one or more mechanical coalescers, a condenser and a downstream separator connected to the outlet line for the azeotrope of the continuous stirred tank reactor, and a condenser and a downstream separator connected to the outlet line for the azeotrope of a further continuous stirred tank reactor, each of the separators comprising a recycle line leading back to the distillation column at the top of the corresponding continuous stirred tank reactor, and an outlet line for the water-rich fraction, wherein lines branching off from one or both of the outlet lines for the water-rich fraction are connected to the inlet line for the water-rich stream of the liquid / liquid extraction column, a further distillation column comprising an inlet line connected to the outlet line for the organic phase of the liquid / liquid extraction column, an outlet line for lactide, and an outlet line for the hydrocarbon solvent, preferably the outlet line for the hydrocarbon solvent being connected to the feed line of the reactor system Includes.

[0061] The plant of the first particular preferred embodiment preferably further comprises another distillation column comprising an inlet line connected to the outlet line for lactide of the further distillation column described above, the distillation column further comprising an outlet line for lactide and an outlet line for the hydrocarbon solvent.

[0062] In a further development of the idea of ​​the present invention, it is suggested that the hydrolysis unit of the liquid / liquid extraction column is connected to the outlet line for the aqueous by-product-containing phase of the liquid / liquid extraction column. The hydrolysis unit may include i) an inlet line for water and ii) an inlet line for lactic acid or an inlet line for a mixture of water and lactic acid. In addition, the hydrolysis unit may include an ion exchange catalyst such as a solid acid catalyst. Furthermore, the hydrolysis unit may include an outlet line for lactic acid and lactic acid oligomer-containing stream, and the outlet line may be connected to the feed line of the reactor system.

[0063] Alternatively, in a second preferred embodiment of the plant of the present invention, the plant for producing lactide from lactic acid comprises: a reactor system comprising a continuous stirred tank reactor with a distillation column arranged on top of it and a further continuous stirred tank reactor downstream of the continuous stirred tank reactor with a distillation column arranged on top of it, both reactors being connected via a line such that during operation a reaction mixture is removed from the continuous stirred tank reactor to the continuous stirred tank reactor, the distillation columns each comprising an outlet line for the azeotroping of water with the hydrocarbon solvent, the reactor system comprising a feed line and an outlet line for the reaction mixture, a first separator connected to an outlet line for the reaction mixture, the first separator being a column comprising one or more electrostatic coalescers and / or one or more mechanical coalescers, the outlet line for the reaction mixture leading to the column at a first location and a water inlet line leading to the separator at a second location, the second location being vertically above the first location, the water inlet line being connected to a distributor for distributing the water over the cross-sectional plane of the separator, the column further comprising an outlet line for the organic phase and an outlet line for the aqueous phase, a second separator connected to an outlet line from the distillation column for separating at least a portion of the water from the azeotrope removed through the azeotrope outlet line to obtain a water-rich fraction and a hydrocarbon solvent-rich fraction, the second separator comprising an outlet line for the water-rich fraction and an outlet line for the hydrocarbon solvent-rich fraction, and a further distillation column comprising an inlet line connected with an outlet line for the organic phase of the first separator and comprising an inlet line connected with an outlet line for the hydrocarbon solvent-rich fraction of the second separator, comprising an outlet line for lactide and an outlet line for the hydrocarbon solvent, the outlet line for the hydrocarbon solvent being preferably connected to the feed line of the reactor system; Includes.

[0064] The plant of the second preferred embodiment as described above preferably further comprises another distillation column comprising an inlet line connected to the outlet line for the aqueous phase of the first separator and connected to the outlet line for the water-rich fraction of the second separator, the distillation column further comprising an outlet line for water and an outlet line for the aqueous by-product-containing phase. The outlet line for the aqueous by-product-containing phase may be connected to the feed line of the reactor system.

[0065] In a further development of the idea of ​​this second preferred embodiment of the present invention, the hydrolysis unit is arranged downstream of the other distillation column, and the hydrolysis unit is connected to the outlet line of the aqueous by-product-containing phase. The hydrolysis unit may include an inlet line for water and an inlet line for lactic acid or an inlet line for a mixture of water and lactic acid. In addition, the hydrolysis unit may include an ion exchange catalyst, such as a solid acid catalyst. Furthermore, the hydrolysis unit may include an outlet line for lactic acid and lactic acid oligomer-containing stream, and the outlet line may be connected to the feed line of the reactor system.

[0066] Preferably, the plant of the aforementioned second preferred embodiment further comprises a third distillation column, which comprises an inlet line connected with the outlet line for lactide of the first distillation column, and comprises an outlet line for lactide, an outlet line for the hydrocarbon solvent, and an outlet line for a purge.

[0067] The invention will now be illustrated by way of example, but not limitation, figures. [Brief description of the drawings]

[0068] [Figure 1] 1 shows a schematic diagram of a plant according to an embodiment of the present invention. [Diagram 2] 1 shows a schematic diagram of a plant according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0069] The plant 10 shown in FIG. 1 comprises a reactor system 12 including a first continuous stirred tank reactor 14 and a second continuous stirred tank reactor 16 downstream of the first continuous stirred tank reactor 14. Both reactors 14, 16 are connected via a line 20 such that during operation of the plant 10, the reaction mixture from the first continuous stirred tank reactor 14 is led to the second continuous stirred tank reactor 16. At the top of the first continuous stirred tank reactor 14 is arranged a (first) distillation column 18 including an outlet line 22 for the azeotrope of water and the hydrocarbon solvent. Similarly, at the top of the second continuous stirred tank is arranged another (second) distillation column 19 including an outlet line 22' for the azeotrope of water and the hydrocarbon solvent. The reactor system 12 includes a feed line 24 and an outlet line 26 for the reaction mixture, with the feed line 24 connected to a line 34 for a mixture of lactic acid / water mixture with a recycled aqueous fraction containing by-products of the reaction such as lactic acid oligomers and lactic acid, an inlet line 30 for the catalyst, and an inlet line 32 for the hydrocarbon solvent.

[0070] Downstream of the reactor system 12, a first separator 35 is arranged, which is connected with the outlet line 26 for the reaction mixture from the second continuous stirred tank reactor 16, the first separator 35 including an outlet line 36 for the organic phase and an outlet line 37 for the aqueous phase. Furthermore, the plant 10 includes a liquid / liquid extraction column 42 including a mechanical coalescer 38, which is connected to the outlet line 36 for the organic phase of the first separator 35, such that the outlet line 36 leads to the lower part of the liquid / liquid extraction column 42 in a first position. Furthermore, the liquid / liquid extraction column 42 includes an inlet line 40 for the water-rich fraction, which leads to the liquid / liquid extraction column 42 in a second position vertically above the first position. Furthermore, the liquid / liquid extraction column 42 includes an outlet line 44 for the organic phase and an outlet line 46 for the aqueous phase, which is combined with the outlet line 37 for the aqueous phase to a recycle line 84. The recycle line 84 leads to a stirring vessel 83, which is also connected to an inlet line 28 for the lactic acid / water mixture and an (outlet) line 34 for a mixture of the lactic acid / water mixture and the recycled aqueous fraction.

[0071] Furthermore, the plant 10 includes, as second separators 48, 48', two settlers, one 48 connected to the outlet line 22 of the first distillation column 18 and the other 48' connected to the outlet line 22' of the second distillation column 19, for separating at least a portion of the water from the azeotrope removed through the azeotrope outlet lines 22, 22' via the condensers 49, 49'. Each of the settlers 48, 48' is connected to a condenser 49, 49' and to a line 52, 52' leading back to the top of the respective distillation column 18, 19. Thereby, during operation of the plant 10, the azeotrope removed from the first distillation column 18 through the outlet line 22 separates in the settler 48 into a water-rich fraction and a hydrocarbon solvent-rich fraction, which is returned to the first distillation column 18 through the line 52. Similarly, the azeotrope removed from the second distillation column 19 via outlet line 22' separates in settler 48' into a water-rich fraction and a hydrocarbon solvent-rich fraction, which is returned to the second distillation column 19 via line 52'. Both water-rich fractions removed from the settlers 48, 48' are combined in water-rich fraction line 50, which is branched off from line 40 before the remaining line 78 leads to a (third) separator 76. The third separator 76 includes an outlet line 82 for the organic phase and an outlet line 80 for the water.

[0072] In addition, plant 10 includes a third distillation column 56 that includes an inlet line 58 that is connected to outlet line 44 for the organic phase of liquid / liquid extraction column 42. Third distillation column 56 includes an outlet line 62 for lactide and an outlet line 64 for hydrocarbon solvent. Outlet line 64 for hydrocarbon solvent is connected via condenser 66, via recycle line 68, via buffer tank 70, and via line 32 to feed line 24 of reactor system 12. Also, an inlet line 72 for fresh hydrocarbon solvent from buffer tank 74 leads to buffer tank 70.

[0073] Finally, plant 10 further includes a fourth distillation column 86 that includes an inlet line 87 connected to outlet line 62 for lactide of the third distillation column 56, and includes an outlet line 88 for lactide, an outlet line 90 for the hydrocarbon solvent, and an outlet line 92 for the purge.

[0074] During operation of the plant 10, a reaction mixture containing water, lactic acid, catalyst and hydrocarbon solvent is fed to the first continuous stirred tank reactor 14 via a feed line 24. The lactic acid contained in the reaction mixture is reacted in the first continuous stirred tank reactor 14 to lactide and water, during which an azeotrope of water and hydrocarbon solvent is distilled in the first distillation column 18 and is removed from the first distillation column 18 via an outlet line 22. The reaction mixture in the first continuous stirred tank reactor 14 is continuously removed from the first continuous stirred tank reactor 14 and directed via a line 20 to the second continuous stirred tank reactor 16, where the reaction continues. During the reaction, an azeotrope of water and hydrocarbon solvent is distilled in the second distillation column 19 and is removed from the second distillation column 19 via an outlet line 22'. The reaction mixture in the second continuously stirred tank reactor 16 is continuously removed from the second continuously stirred tank reactor 16 and conducted via line 26 to a first separator 35 column. In the first separator 35, the reaction mixture is separated into an aqueous phase, which contains water and by-products / impurities from the reaction, and an organic phase. Additionally, the azeotrope of water and hydrocarbon solvent removed from the first and second distillation columns 18, 19 via lines 22, 22' is condensed in condensers 49, 49' from where it is conducted to second separators or settlers 48, 48', respectively. The azeotrope removed from the first distillation column 18 via the outlet line 22 separates in the settler 48 into a water-rich fraction and a hydrocarbon solvent-rich fraction, which is returned to the first distillation column 18 via the line 52, and the azeotrope removed from the second distillation column 19 via the outlet line 22' separates in the settler 48' into a water-rich fraction and a hydrocarbon solvent-rich fraction, which is returned to the second distillation column 19 via the line 52'. Both water-rich fractions removed from the settlers 48, 48' are combined in the water-rich fraction line 50, from where one part is led as an extractant to the liquid / liquid extraction column 42 via the line 40, and the other part is led to the third separator 76 via the line 78. The water-rich fraction separates in the third separator 76 into an organic phase, which is removed via the line 82, and pure water, which is removed via the line 80.In addition, the organic phase obtained in the first separator 35 is fed via line 36 to a liquid / liquid extraction column 42, which is fed with water via line 40 to extract the organic phase therein. The aqueous by-product-containing phase withdrawn from the liquid / liquid extraction column 42 via line 46 is fed via a recycle line 84, a stirred tank 83 and lines 34 and 24 to the first continuous stirred tank reactor 14 together with the aqueous phase withdrawn from the first separator 35 via line 37. In the stirred tank 83, the lactic acid oligomers contained in the aqueous by-product-containing phase fraction withdrawn from the liquid / liquid extraction column 42 via line 46 are hydrolyzed to shorter oligomers and lactic acid, or even exclusively to lactic acid. The purified organic phase withdrawn from the liquid / liquid extraction column 42 via line 44 is fed via line 58 to a third distillation column 56. The hydrocarbon solvent-rich fraction is separated into a hydrocarbon solvent fraction and a lactide fraction in third distillation column 56. The lactide fraction is removed from third distillation column 56 via line 62 and further purified in downstream distillation column 86, while the hydrocarbon solvent-rich fraction is removed from third distillation column 56 via line 64, condensed in condenser 66, and partially returned to first continuous stirred tank reactor 14 via line 68, buffer tank 70, line 32, and feed line 24.

[0075] The plant 10 shown in FIG. 2 comprises a reactor system 12 including a first continuous stirred tank reactor 14 and a second continuous stirred tank reactor 16 downstream of the first continuous stirred tank reactor 14. Both reactors 14, 16 are connected via a line 20 such that during operation of the plant 10, the reaction mixture from the first continuous stirred tank reactor 14 is led to the second continuous stirred tank reactor 16. At the top of the first continuous stirred tank reactor 14 is arranged a (first) distillation column 18 including an outlet line 22 for the azeotrope of water and the hydrocarbon solvent. Similarly, at the top of the second continuous stirred tank is arranged another (second) distillation column 19 including an outlet line 22' for the azeotrope of water and the hydrocarbon solvent. The reactor system 12 includes a feed line 24 and an outlet line 26 for the reaction mixture, with the feed line 24 connected to an inlet line 28 for a lactic acid / water mixture, an inlet line 30 for a catalyst, an inlet line 32 for a hydrocarbon solvent, and a recycle line 84 for an aqueous fraction containing by-products of the reaction such as lactic acid oligomers and lactic acid.

[0076] Downstream of the reactor system 12, a first separator 35 is arranged, which is connected with an outlet line 26 for the reaction mixture from the second continuously stirred tank reactor 16, the outlet line 26 leading to the lower part of the first separator 35 at a first location. More specifically, the first separator 35 is a column including three mechanical coalescers 38, 38', 38". Furthermore, the first separator 35 includes an inlet line 94 for wash water leading to the column of the first separator 35 at a second location vertically above the first location. The column of the first separator 35 includes a liquid distributor 96 for distributing the water introduced into the column of the first separator 35 during operation of the plant 10 over the cross section of the first separator 35. Furthermore, the column of the first separator 35 includes an outlet line 44 for the organic phase and an outlet line 37 for the aqueous phase.

[0077] Furthermore, the plant 10 includes a settler as second separator 48 connected with the outlet lines 22, 22' from the distillation columns 18, 19 for separating at least a portion of the water from the azeotrope removed through the azeotrope outlet lines 22, 22' via condensers 49, 49' to obtain a water-rich fraction and a hydrocarbon solvent-rich fraction during operation of the plant 10. The water-rich fraction is removed from the second separator 48 through an outlet line 50, whereas the hydrocarbon solvent-rich fraction is removed from the second separator 48 through an outlet line 52 connected to a buffer tank 54.

[0078] The plant 10 further comprises a third distillation column 56, which comprises an inlet line 58 connected to the outlet line 44 for the organic phase of the first separator 35. The inlet line 58 is also connected to an outlet line 60 of the buffer tank 54 for the hydrocarbon solvent-rich fraction. The third distillation column 56 comprises an outlet line 62 for lactide and an outlet line 64 for the hydrocarbon solvent. The outlet line 64 for the hydrocarbon solvent is connected via a condenser 66, via a recycle line 68, via a buffer tank 70 and via line 32 to the feed line 24 of the reactor system 12. An inlet line 72 for fresh hydrocarbon solvent from the buffer tank 74 also leads to the buffer tank 70.

[0079] In addition, plant 10 further includes a fourth distillation column 86 that includes an inlet line 87 connected to outlet line 62 for lactide of the third distillation column 56, and includes an outlet line 88 for lactide, an outlet line 90 for the hydrocarbon solvent, and an outlet line 92 for a purge.

[0080] Finally, the plant 10 further comprises a fifth distillation column 98, which comprises an inlet line 100 connected to the outlet line 37 for the aqueous phase of the column of the first separator 35 and to the outlet line 50 for the water-rich fraction of the second separator 48. The fifth distillation column 98 further comprises an outlet line 80 for water and an outlet line 102 for the aqueous by-product-containing phase. Downstream of the fifth distillation column 98 is arranged a hydrolysis unit 83, which further comprises an outlet line 85 connected to the outlet line 102 via a line 104 and returning to the fifth distillation column 98. The outlet line 102 is further connected via a recycle line 84 for the aqueous fraction comprising the by-products of the reaction to the feed line 24 of the reactor system 12.

[0081] During operation of the plant 10, a reaction mixture containing water, lactic acid, catalyst and hydrocarbon solvent is fed to the first continuous stirred tank reactor 14 via a feed line 24. The lactic acid contained in the reaction mixture reacts in the first continuous stirred tank reactor 14 to lactide and water, during which an azeotrope of water and hydrocarbon solvent is distilled in the first distillation column 18 and is removed from the first distillation column 18 via an outlet line 22'. The reaction mixture in the first continuous stirred tank reactor 14 is continuously removed from the first continuous stirred tank reactor 14 and directed to the second continuous stirred tank reactor 16, where the reaction continues. During the reaction, an azeotrope of water and hydrocarbon solvent is distilled in the second distillation column 19 and is removed from the second distillation column 19 via an outlet line 22'. The reaction mixture of the second continuously stirred tank reactor 16 is continuously withdrawn from the second continuously stirred tank reactor 16 and is conducted to the column of the first separator 35 via line 26. In the first separator 35, the reaction mixture is separated into an aqueous phase, containing water and by-products / impurities from the reaction, and an organic phase, which is washed by feeding water to the column of the first separator 35 via line 94 and distributing it across the cross-sectional plane of the first separator 35 by a liquid distributor 96. Furthermore, the azeotrope of water and hydrocarbon solvent withdrawn from the first and second distillation columns 18, 19 via lines 22, 22' is condensed in condensers 49, 49'. A part of the stream thereby obtained is recycled to the distillation columns 18, 19, while the remaining part of the stream is conducted to the second separator 48 or to a settler, respectively. In the second separator 48, the stream is separated into a water-rich fraction and a hydrocarbon solvent-rich fraction. The water-rich fraction is removed from the second separator 48 via outlet line 50, while the hydrocarbon solvent-rich fraction is removed from the second separator 48 via outlet line 52 through a buffer tank 54 together with the organic phase obtained in the first separator 35 via line 58 to a third distillation column 56. The hydrocarbon solvent-rich fraction is separated in the third distillation column 56 into a hydrocarbon solvent fraction and a lactide fraction.A lactide fraction is removed from the third distillation column 56 via line 62 and further purified in downstream distillation column 86, while a hydrocarbon solvent-rich fraction is removed from the third distillation column 56 via line 64, condensed in condenser 66 and returned to the first continuous stirred tank reactor 14 via line 68, buffer tank 70, line 32 and feed line 24. A water-rich fraction removed from the second separator 48 via outlet line 50 is conducted together with the aqueous phase removed from the column of the first separator 35 to the distillation column 98 via line 100. Water is removed from the distillation column 98 via line 80, while the aqueous by-product-containing phase is partially conducted from the distillation column 98 via line 102 to the hydrolysis unit 83, from where it is returned to the distillation column 98 via line 85, and the remaining portion of the aqueous by-product-containing phase is conducted to the first continuous stirred tank reactor 14 via lines 102, 84, 24. The lactic acid oligomers react with shorter lactic acid oligomers and / or lactic acid in hydrolysis unit 83, thus reducing the size and amount of lactic acid oligomers that are recycled to the first continuous stirred tank reactor 14 via lines 84, 24. [Explanation of symbols]

[0082] 10. Plant 12 Reactor System 14 First Reactor / Continuously Stirred Tank Reactor 16 Second Reactor / Continuously Stirred Tank Reactor 18 (First) Distillation Column 19 (Second) Distillation Column 20 Line for reaction mixture 22, 22' Exit line for azeotrope 24 Feed line to continuous stirred tank reactor 26 Exit line for reaction mixture 28 Inlet line for lactic acid / water mixture 30 Catalyst inlet line 32 Inlet line for hydrocarbon solvents 34 (Outlet) line for the mixture of lactic acid / water mixture and recycled aqueous fraction 35 First vessel / first separator 36 Outlet for organic phase 37 Outlet for aqueous phase 38, 38', 38" Mechanical Coalescer 40 Inlet line for water-rich fraction 42 Liquid / Liquid Extraction Column 44 Outlet line for organic phase 46 Outlet line for by-product-containing phase 48, 48' Second separator / settler 49, 49' Condenser 50 Line for water-rich fraction 52, 52' Outlet lines for the hydrocarbon solvent rich fraction 54 Buffer Tank 56 Third distillation column 58 Inlet line of third distillation column 60 Buffer tank outlet line 62 Exit line for lactide 64 Outlet line for hydrocarbon solvents 66 Condenser 68 Recycling Line 70 Buffer Tank 72 Inlet line for fresh hydrocarbon solvent 74 Buffer Tank 76 (Third) Separator of Residual Solvents and Other Impurities 78 Inlet line to the water-rich fraction separator 80 Water Outlet Line 82 Outlet line for organic phase 83 Stirred Vessel / Hydrolysis Unit 84 Recycling Line 85 Exit line from hydrolysis unit 86 (fourth) distillation column 87 Inlet line to fourth distillation column 88 Exit line for lactide 90 Outlet line for hydrocarbon solvents 92 Purge outlet line 94 Fresh water inlet line 96 Liquid distributor 98 Fifth distillation column 100 Entrance Line 102 Exit line for aqueous by-product containing phase 104 Line to Hydrolysis Unit

Claims

1. A method for producing lactide from lactic acid, comprising: a) supplying lactic acid, a hydrocarbon solvent and at least one catalyst to a reactor (14) of a reactor system (12) to form a reaction mixture in the reactor (14); b) reacting the reaction mixture in the reactor (14) such that at least a part of the lactic acid is converted into lactide and water while distilling at least a part of the reaction mixture so as to remove an azeotrope of water and the hydrocarbon solvent from the reactor (14); c) separating at least a part of the water from the azeotrope removed from the reactor (14) in step b); d) separating at least a part of the lactide from the reaction mixture, wherein the at least one catalyst comprises a homogeneous catalyst comprising an organometallic compound selected from the group consisting of tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and any mixture of two or more of these compounds.

2. At least one catalyst comprises a tin carboxylate, preferably one or more C ２～１４ -carboxylate groups, more preferably a tin carboxylate containing one or more C ６～１４ -carboxylate groups, and the method according to claim 1, wherein at least one catalyst is most preferably tin octoate.

3. The method according to claim 1 or 2, wherein the hydrocarbon solvent is an aromatic hydrocarbon solvent having a boiling point of 80 to 130°C, preferably 90 to 120°C, more preferably 100 to 120°C, and preferably the hydrocarbon solvent is toluene.

4. The method according to claim 1 or 2, wherein the reaction mixture reacts in step b) at a temperature of 100 to 180°C, preferably 110 to 160°C, more preferably 120 to 160°C, even more preferably 130 to 150°C, most preferably 135 to 145°C.

5. The method according to claim 1 or 2, wherein the reaction mixture reacts in step b) at a pressure of 0.10 to 0.40 MPa, preferably 0.15 to 0.30 MPa, more preferably 0.15 to 0.25 MPa, even more preferably 0.18 to 0.22 MPa, most preferably 0.19 to 0.21 MPa.

6. The method according to claim 1 or 2, wherein in step a), the reactor (14) to which lactic acid, the hydrocarbon solvent and at least one catalyst are supplied is a continuous stirred tank reactor, and in step b), the azeotropic distillation is carried out in a distillation column (18) arranged above the continuous stirred tank reactor (14).

7. The reactor system (12) includes two reactors (14, 16) in series with each other. In step a), the first reactor (14) to which lactic acid, a hydrocarbon solvent, and at least one catalyst are supplied is a continuous stirred tank reactor, and the second reactor (16) downstream of the first reactor (14) is a continuous stirred tank reactor. The reaction mixture is taken out from the second reactor (16) and, before being subjected to the separation in step d), the reaction mixture is taken out from the first reactor (14) and led to the second reactor (16). The method according to claim 1 or 2.

8. The reaction mixture reacts in the reactor system (12) for 0.1 to 10 hours, preferably 1 to 5 hours, more preferably 2 to 4 hours, and most preferably 2.5 to 3.5 hours in step b). The method according to claim 1 or 2.

9. At least partial separation of lactide from the reaction mixture in step d) is the following sub-steps: d １ Step of separating the reaction mixture in the first vessel (35) into an organic phase and an aqueous phase; d ２ ) Step d １ ) extracting the organic phase obtained in step ) with water in a liquid / liquid extraction step to obtain an organic phase and an aqueous by-product-containing phase, and d ３ d) in the distillation column (56) ２ d) separating the organic phase obtained in step d) into a lactide-enriched phase and a hydrocarbon solvent-enriched phase, and preferably recycling the hydrocarbon solvent-enriched phase into the reactor (14) The method according to claim 1 or 2, comprising.

10. Liquid / liquid extraction step d ２ ) is carried out in a liquid / liquid extraction column (42) comprising one or more coalescers (38), and as the coalescer (38), preferably an electrostatic coalescer (38), a mechanical coalescer (38), a mechanical coalescer (38) comprising one or more structured corrugated metal or plastic sheets, a coalescer comprising two materials with significantly different surface free energies to promote droplet coalescence of either phase, and / or a mechanical coalescer comprising a cartridge made of a selected fibrous material through which the liquid flows radially outwards from the center of the cartridge is used, the method according to claim 9.

11. Liquid / liquid extraction step d ２ The method according to claim 9, wherein the aqueous by-product-containing phase fraction obtained in step (d) is recycled in step (a) and supplied to the reactor (14).

12. At least partial separation of water from the azeotrope carried out in step c) is the following sub-steps: c １ b) condensing the azeotrope withdrawn from the reactor (14) in step b) to obtain a liquid stream c ２ c) Step of the process １ c) A step of separating the liquid stream obtained in step b) into a hydrocarbon solvent-rich fraction recycled to the distillation column (18) at the upper part of the reactor (14) and a water-rich fraction The method according to claim 1 or 2, comprising.

13. A plant (10) for producing lactide from lactic acid, A reactor system (12) including at least one reactor (14), wherein a distillation column (18) is arranged above the reactor (14) so that the vapor generated during operation in the reactor (14) can be distilled into an azeotrope of water and hydrocarbon. The distillation column (18) includes an outlet line (22) for the azeotrope of water and hydrocarbon solvent. The reactor system (12) includes a reactor system (12) including a supply line (24) and an outlet line (26) for the reaction mixture. A separator (48) connected to the outlet line (22) for the azeotrope of water and hydrocarbon of the distillation column (18) for separating at least a part of water from the azeotrope removed through the outlet line (22), and A separator (35) connected to the outlet line (26) for the reaction mixture The plant (10) comprising.

14. The reactor system (12) comprises one reactor (14) which is a continuous stirred tank reactor (14) with a distillation column (18) arranged on top, or the reactor system (12) comprises two reactors (14, 16) in series with each other, the first reactor (14) to which the feed line (24) is connected being a continuous stirred tank reactor (14) with a distillation column (18) arranged on top, the second reactor (16) downstream of the first reactor (14) being a continuous stirred tank reactor with a distillation column (19) arranged on top, and both reactors (14, 16) being connected via a line (20) so as to remove the reaction mixture from the first reactor (14) to the second reactor (16) during operation, the plant (10) according to claim 13.

15. The plant (10) comprises a reactor system (12) comprising a continuous stirred tank reactor (14) with a distillation column (18) arranged on top and, downstream of the continuous stirred tank reactor (14), a further continuous stirred tank reactor (16) with a distillation column (19) arranged on top, both reactors (14, 16) being connected via a line (20) so as to remove the reaction mixture from the continuous stirred tank reactor (14) to the continuous stirred tank reactor (16) during operation, the distillation columns (14, 16) each comprising an outlet line (22, 22') for an azeotrope of water and a hydrocarbon solvent, the reactor system (12) comprising a feed line (24) and an outlet line (26) for the reaction mixture, a first separator (35) connected to the outlet line (26) for the reaction mixture, the first separator (35) being a vessel comprising an outlet line (37) for the aqueous phase and an outlet line (37) for the organic phase, a liquid / liquid extraction column (42) comprising an inlet line (40) for the water-rich stream, an inlet line connected to the outlet line (36) for the organic phase of the first separator (35), an outlet line (44) for the organic phase, and an outlet line (46) for the aqueous by-product-containing phase, the liquid / liquid extraction column (42) preferably comprising one or more electrostatic coalescers (38) and / or one or more mechanical coalescers (38), A condenser (49) connected to the outlet line (22) for the azeotrope of the continuous stirred tank reactor (14) and a separator (48) downstream thereof, and a condenser (49') connected to the outlet line (22') for the azeotrope of a further continuous stirred tank reactor (16) and a separator (48') downstream thereof, each of the separators (48, 48') comprising a recycle line (52, 52') returning to the corresponding distillation column (18, 19) and an outlet line (50) for the water-rich fraction, and a line (40) branching from one or both of the outlet lines (50) for the water-rich fraction being connected to the inlet line (40) for the water-rich stream of the liquid / liquid extraction column (42), the condenser (49, 49') and the separator (48, 48') downstream thereof, A further distillation column (56) comprising an inlet line (58) connected to the outlet line (44) for the organic phase of the liquid / liquid extraction column (42), an outlet line (62) for lactide, and an outlet line (64) for the hydrocarbon solvent, preferably the outlet line (64) for the hydrocarbon solvent being connected to the supply line (24) of the reactor system (12), the distillation column (56) comprising the plant (10) according to claim 13 or 14.

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