Process for the production of lactide

Abstract

The invention relates to a process for the continuous production of optically pure lactide from aqueous lactic acid by performing two reactive distillations followed by a main distillation column with a side draw wash column. The first reactive distillation is used to produce lactic acid oligomers and the second reactive distillation is used to depolymerize the lactic acid oligomers thus obtained into lactide. The crude lactide resulting from the second reactive distillation is further purified in the main distillation column with a side draw wash column.

Description

BACKGROUND

[0001] The present invention relates to a process for the continuous production of optically pure lactide by performing two reactive distillations followed by a main distillation column connected to a scrubbing column via a gas phase side draw. Lactide is the cyclic dimer of lactic acid and is useful as an intermediate for the production of high molecular weight polylactic acid. These polymers are useful in the biomedical industry and other applications, for example, because of the ability of these polymers to be biologically and hydrolytically degraded while forming environmentally acceptable degradation products, such that these polymers can be used as biodegradable packaging materials.

[0002] Examples of known processes for the synthesis of lactide include a step of concentrating the lactic acid feedstock to reduce the water content and promote the initiation of esterification between lactic acid molecules, a step of pre-polymerizing the lactic acid to form lactic acid oligomers during the removal of water produced by esterification, and a step of depolymerizing the lactic acid oligomers thus obtained into crude lactide. Processes for performing these concentration, pre-polymerization and depolymerization steps are known in the art, for example, U.S. Patent No. 6,326,458.

[0003] As is known in the art, lactic acid comprises two optical isomers, namely (R)-lactic acid and (S)-lactic acid. Thus, the formation of lactide from the enantiomers of lactic acid results in three stereoisomers having different geometric configurations, namely (R,R)-lactide (or D-lactide), (S,S)-lactide (or L-lactide) and (R,S)-lactide (or meso-lactide), respectively. In practice, the crude lactic acid fed to the system contains one of the two lactic acids selected from (S)-lactic acid and (R)-lactic acid as the major component. Thus, the crude lactide produced by depolymerization comprises a major portion of optically pure lactide (L-lactide or D-lactide), a minor portion of meso-lactide and a much smaller amount of the remaining third lactide.

[0004] While this three-step process described in U.S. Patent No. 6,326,458 enables the production of crude lactide from an aqueous solution of lactic acid, one disadvantage of the process is that the lactic acid is progressively exposed to increasing temperatures as it is concentrated and pre-polymerized to lactic acid oligomers. The starting lactic acid is typically of very high optical purity, with (S)-lactic acid being more commercially available. However, some racemization occurs under these conditions, for example, (S)-lactic acid is converted to (R)-lactic acid, which results in a loss of the major product, L-lactide, and an increase in the amount of meso-lactide in the crude lactide. This can create problems during the separation of meso-lactide from optically pure lactide, for example, L-lactide. Additional purification steps can be required prior to polymerization. Thus, it would be desirable to provide a more efficient process that minimizes the racemization of lactic acid and increases the yield of L-lactide.

[0005] The crude lactide from depolymerization contains not only lactide but also other impurities such as residual lactic acid, water, lactic acid oligomers and other reaction by-products. The molecular weight of the polylactic acid is controlled by the amount of hydroxyl impurities in the lactide. In particular, the presence of water, lactic acid and lactic acid oligomers in the lactide tends to slow down the polymerization and the resulting polylactic acid will not have a high molecular weight suitable for its use as a biodegradable polymer. It has been shown that it is possible to achieve the separation of the impurities from the L-lactide by distillation based on the difference in volatility between the components. The relative order of decreasing volatility of the main components of the crude lactide is water, lactic acid, meso-lactide, L-lactide, linear lactic acid dimer, which have boiling points of about 100°C, 215°C, 250°C, 255°C and 350°C, respectively, under atmospheric pressure, and even higher for linear lactic acid trimer, linear lactic acid tetramer, etc.

[0006] The crude lactide containing lactide, lactic acid, lactic acid oligomers and water is fed to a distillation column as described in US Patent No. 5236560, wherein pure lactide is removed from the side draw outlet of the distillation column in gaseous phase.

[0007] US Patent No. 10023553 describes a process wherein the crude lactide prepared by depolymerization is kept in a reaction vessel for a period of at least 5 hours prior to the distillation column, the aim being to reduce the lactic acid content at the expense of increasing the lactic acid oligomer content. Pure lactide is removed from the side draw outlet of the distillation column in gaseous phase.

[0008] While the pure gaseous phase lactide substantially free of lactic acid can be obtained as the side draw product from the distillation column described in US Patent No. 5236560 and US Patent No. 10023553, it still contains small amounts of meso-lactide and lactic acid oligomers. Part of the lactic acid oligomers are formed due to side reactions of lactic acid with lactide during the distillation process. The residual lactic acid oligomers in the pure lactide have a negative effect on the polymerization rate during the polymerization process, resulting in a relatively low molecular weight polylactic acid. In order to obtain practically pure L-lactide, the pure gaseous phase lactide is condensed and subjected to a further purification step, such as melt crystallization. The residual meso-lactide can be easily separated from the L-lactide by melt crystallization. However, the residual lactic acid oligomers are difficult to remove from the lactide by melt crystallization as they tend to be more viscous and stick to the surface of the lactide. Therefore, it is desirable to provide an efficient process for the production of highly purified lactide substantially free of lactic acid and lactic acid oligomers, which is beneficial for the preparation of high molecular weight polylactic acid. SUMMARY

[0009] It is an object of the present invention to develop a process based on two reactive distillations, i.e. a first reactive distillation for the concentration of lactic acid and the preparation of lactic acid oligomers and a second reactive distillation for the depolymerization of the lactic acid oligomers obtained thereby.

[0010] It is another object of the present application to provide a distillation column with a side draw scrubbing column attached for purifying crude lactide, through which a pure liquid lactide substantially free of lactic acid and lactic acid oligomers is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 is a schematic diagram representing a preferred lactide production system according to the present application. DETAILED DESCRIPTION

[0012] With respect to the conventional reaction sequence, lactic acid contained in the lactic acid is evaporated by subsequent heating in a lactic acid concentration apparatus followed by a lactic acid condensation apparatus, in which a lactic acid condensation (pre-polymerization) reaction can be carried out to produce lactic acid oligomers. According to the present application, the above two apparatuses, i.e. lactic acid concentrator and lactic acid condensation reactor, are replaced by a first reactive distillation. The expenditure for equipment and the space required for installing the first reactive distillation system is significantly reduced. Furthermore, the use of the first reactive distillation system has the advantage that the residence time of lactic acid is significantly reduced, which minimizes the racemization of lactic acid, thus increasing the yield of the target product L-lactide.

[0013] According to the present application, the first reactive distillation system preferably comprises at least a tank, a distillation column, a condenser and an evaporator. Depending on the process conditions, the orientation of the tank can be horizontal or vertical. The distillation column can be a conventional column or a divided wall column having a dividing wall separating the interior space of the column. The condenser can be any type commonly used in the chemical industry, including parallel flow and counter flow condensers. The evaporator not only provides the energy required for water evaporation, but also is the place where the lactic acid condensation reaction takes place.

[0014] The aqueous lactic acid solution can contain, for example, 0 to 50 wt% water and 50 to 100 wt% lactic acid, respectively. The temperature of the aqueous lactic acid solution is preferably in the range of 60 to 150°C, more preferably in the range of 100 to 150°C.

[0015] In the first reactive distillation column, lactic acid concentration and lactic acid condensation reactions for producing lactic acid oligomers can be carried out. The average molecular weight of the lactic acid oligomers obtained as a result of the above lactic acid condensation reaction is generally in the range of 300 to 10,000, preferably in the range of 450 to 5,000, more preferably in the range of 600 to 2,500.

[0016] With respect to the type of mass transfer elements installed in the distillation column of the first reactive distillation system, the present application is not particularly limited. Good results are obtained by using suitable mass transfer elements selected from the group consisting of tray, random packing, structured packing and any combination thereof. However, structured packing is particularly suitable as mass transfer element, having the advantage of reducing the column pressure drop and liquid holdup in the column. Preferably, the specific surface area of the structured packing is in the range of 50 to 1,000 m 2 / m 3 to 750 m 2 / m 3 , more preferably in the range of 125 m 2 / m 3 to 500 m 2 / m 3 .

[0017] According to the present application, the distillation column of the first reactive distillation is equipped with at least one evaporator. The evaporator can be any type common in the chemical industry, including falling film, forced circulation, thermosyphon, short tube, long tube vertical, long tube horizontal, etc., but is not limited thereto. However, due to its particularly reduced liquid hold-up and high heat transfer coefficient, a falling film evaporator is preferred, in order to minimize the residence time of lactic acid in the evaporator, thus reducing any adverse side reactions, such as racemization.

[0018] The distillation column of the first reactive distillation and its associated evaporator are preferably mounted, respectively, at the top of the tank, to form a single closed area in which lactic acid condensation and distillation occur. The aqueous lactic acid solution is preferably fed continuously to the inlet of the distillation column, which is located at a position between the upper and lower ends of the column. The reaction solution flows down along the long vertical tubes comprising the heat transfer and reaction zone, entering the top of the falling film evaporator, discharging a stream containing the gas phase from the vertical tube end, thus leaving the evaporation system. The gas-liquid two-phase stream flows directly into the connected tank, where the vapor is separated from the liquid. The separated vapor flows upward through the tank to the bottom of the distillation column mounted at the top, and the liquid from the distillation column is recycled to the tank. In order to prevent the breakage of the liquid film inside the tubes of the falling film evaporator, the evaporation of the reaction solution is generally less than 15 to 30 wt.%. The majority of the liquid at the bottom of the tank is recirculated via a transfer pump to the top of the falling film evaporator for continuous lactic acid condensation, while a small portion of the liquid at the bottom of the tank is fed to the subsequent depolymerization reactor. The reaction solution is generally heated at a temperature ranging from 120°C to 200°C (preferably from 150°C to 180°C), under a reduced pressure of 50 mbar or less (preferably 30 mbar or less).

[0019] In the process of the distillation column of the first reactive distillation, a concentration gradient is established within the column, with water being enriched in the rectifying section and high-boiling components such as lactic acid and lactic acid oligomers being enriched in the stripping section. The water contained in the aqueous lactic acid solution and the water produced during the lactic acid condensation process are distilled off as an overhead gas phase stream, which is condensed by a condenser, resulting in a condensate stream consisting essentially of water. A portion of the condensate stream is preferably refluxed into the column, while another portion of the condensate stream can be withdrawn. The gases not condensed in the condenser are removed by a vacuum system. The high-boiling fraction consisting essentially of lactic acid and lactic acid oligomers liquefied within the column is returned to the tank.

[0020] According to the present application, the depolymerization reactor is actually a second reactive distillation system comprising at least a tank, a distillation column, a condenser and a falling film evaporator. Also, in the second reactive distillation, the distillation column and its associated falling film evaporator are mounted directly on top of the tank, forming a separate enclosed area, within which depolymerization and distillation take place. The mass transfer elements installed in the distillation column include trays, random packings, structured packings and any combination thereof. However, structured packings are particularly suitable as mass transfer elements, with the advantage of reducing the column pressure drop and liquid holdup in the column. Preferably, the specific surface area of the structured packings is in the range of 50 m 2 / m 3 to 750 m 2 / m 3 , more preferably in the range of 125 m 2 / m 3 to 500 m 2 / m 3 . The condenser can be of any type commonly used in the chemical industry, including parallel flow and counter flow condensers.

[0021] A catalyst, for example stannous octoate, is added in the second reactive distillation and mixed with the lactic acid oligomers from the first reactive distillation, which mixture is fed as part of the reaction solution to the top of the falling film evaporator, where the lactide is generated and evaporated. A gas-liquid two-phase stream flows out from the bottom of the tubes of the falling film evaporator and directly into the connected tank, where the vapor is separated from the liquid. The separated vapor flows through the tank up to the bottom of the distillation column mounted on top, from which the liquid is recycled back to the tank. In order to prevent the liquid film inside the tubes of the falling film evaporator from breaking, the evaporation of the reaction solution is typically less than 15 to 30 wt.%. The majority of the liquid at the bottom of the tank is recirculated via a transfer pump to the top of the falling film evaporator to continue the depolymerization as part of the reaction solution, while a small portion of the liquid at the bottom of the tank is removed as a bleed stream, which contains the residual tin catalyst and metals precipitated from the system. The reaction solution is heated generally at a temperature in the range of 120°C to 250°C, preferably in the range of 150°C to 220°C, at a reduced pressure of below 100 mbar, preferably below 20 mbar. A top low-boiling distillate stream, i.e. crude lactide, consisting of a majority of L-lactide and some meso-lactide, lactic acid oligomers, residual water and lactic acid (e.g. 60 to 99 wt.% L-lactide, 0 to 15 wt.% meso-lactide, 0 to 10 wt.% lactic acid, 0 to 12 wt.% lactic acid oligomers and 0 to 3 wt.% water) is formed. A high-boiling fraction consisting essentially of unconverted lactic acid oligomers is returned to the tank.

[0022] The crude lactide from the overhead mounted distillation column of the second reactive distillation is fed to a subsequent main distillation column for purification of L-lactide. The crude lactide is fractionated based on the difference in volatility between the components. The relative order of decreasing volatility of the major components in the crude lactide is water, lactic acid, meso-lactide, L-lactide, and lactic acid oligomers. The lower volatility components, such as lactic acid oligomers, have higher boiling points than L-lactide and are concentrated at the bottom of the column and removed as a bottom product. The overhead product stream from the main distillation column contains most of the meso-lactide and a small amount of lactic acid and L-lactide. The lactide product with high purity L-lactide is withdrawn from the main distillation column as a gas phase side draw product.

[0023] The overhead vapor stream from the top of the main distillation column is condensed by a condenser to obtain a condensate stream enriched in meso-lactide. The vapor that is not condensed in the condenser is removed by the vacuum system. To efficiently remove other components from meso-lactide, a portion of the condensate stream is preferably refluxed into the column. Another portion of the condensate stream can be fed to another purification system, such as distillation, crystallization, or a combination thereof, to obtain pure meso-lactide.

[0024] The liquid bottom stream concentrated in the stripping section is withdrawn from the bottom of the main distillation column and subsequently split into a bottom product stream and a recycle stream. An increase in the lactic acid oligomer content in the bottom product stream is observed due to side reactions between lactide and residual lactic acid that occur at relatively high bottom temperature conditions. The bottom product stream is preferably refluxed to the second reactive distillation system as part of the reaction solution for depolymerization to produce lactide.

[0025] The lactide product stream withdrawn as a gas phase side draw product from the main distillation column is essentially free of water and lactic acid. However, it still contains a small amount of lactic acid oligomers due to side reactions between lactide and residual lactic acid that occur during distillation. The residual lactic acid oligomers in the lactide product have a negative impact on the polymerization rate in the polymerization process, resulting in relatively low molecular weight polylactic acid.

[0026] According to the present application, the gaseous phase side draw product stream of the main distillation column described above is directly fed to the bottom of a side draw scrubbing column with an overhead condenser and the bottom product from the side draw scrubbing column is refluxed to the main distillation column. In the side draw scrubbing column, L-lactide is separated from the remaining lactic acid oligomers. Pure L-lactide substantially free of lactic acid and lactic acid oligomers is obtained at the top of the side draw scrubbing column. The purity of L-lactide at the top of the side draw scrubbing column is ensured by a suitable method, for example by product analysis or online analysis, which accordingly influences the reflux ratio in the side draw scrubbing column. The amount of side draw product from the main distillation column is controlled, for example, by temperature measurement at a suitably sensitive point in the side draw scrubbing column, which controls the opening of a valve in the condensate line downstream of the overhead condenser of the side draw scrubbing column. Due to the associated influence on the liquid level and thus on the effective condensation area in the condenser, an automatic adjustment of the amount of side draw product from the main distillation column is achieved.

[0027] The main distillation column with a side draw scrubbing column is preferably carried out at low temperature and reduced pressure. The pressure at the top of the main distillation column is preferably in the range of 3 mbar to 25 mbar, more preferably in the range of 5 mbar to 15 mbar. The pressure at the bottom of the main distillation column is preferably in the range of 10 mbar to 35 mbar, more preferably in the range of 12 to 25 mbar.

[0028] The mass transfer elements installed in the main distillation column with a side draw scrubbing column consist of trays, random packings, structured packings and any combination thereof. Structured packings are particularly suitable as mass transfer elements, however, with the advantage of reducing the pressure drop of the column and the liquid holdup in the column. Preferably, the specific surface area of the structured packings is in the range of 125 m 2 / m 3 to 750 m 2 / m 3 , more preferably in the range of 250 m 2 / m 3 to 350 m 2 / m 3 . The condensers of the main distillation column and the side stream column can be of any type commonly used in the chemical industry, including parallel flow condensers and counterflow condensers.

[0029] Figure 1 A preferred lactide production system according to the present application is schematically shown, which comprises a first reactive distillation column 2, a condenser 4, a tank 9, a falling film evaporator 10, a pump 12, a second reactive distillation column 15, a condenser 17, a tank 22, a falling film evaporator 23, a pump 25, a main distillation column 29, a condenser 31, a pump 37, a falling film evaporator 39, a side draw scrubbing column 43 and a condenser 45.

[0030] A feed of aqueous lactic acid is continuously fed to a first reactive distillation column 2 via stream 1. Overhead vapors consisting essentially of water are removed via stream 3 and subsequently condensed in condenser 4. The condensate is divided into an overhead liquid product stream 8 distilled from the top, and a reflux stream 7 which is refluxed to the top of the first reactive distillation column 2. Uncondensed vapors are removed via stream 5. Lactic acid and lactic acid oligomers are concentrated at the bottom of column 2 and flow back to tank 9. The bottom stream 11 from the bottom of tank 9 is subsequently pumped via pump 12 and divided into a bottom product stream 14 which is combined with stream 26, and a recycle stream 13 which is fed to the top of falling film evaporator 10, partially evaporated and then flows into tank 9. Vapors are separated from the liquid in tank 9. The separated vapors flow up to the bottom of column 2 and liquid from distillation column 2 is recovered into tank 9.

[0031] The bottom product stream 14 combined with stream 26 and stream 41 mixed with depolymerization catalyst stream 28 is continuously fed to the top of falling film evaporator 23. Overhead vapors containing mostly lactide are removed via stream 16 and subsequently condensed in condenser 17. The condensate is divided into an overhead liquid product stream 21 distilled from the top, and a reflux stream 20 which is refluxed to the top of the second reactive distillation column 15. Uncondensed vapors are removed via stream 18. Unconverted lactic acid oligomers are concentrated at the bottom of column 15 and flow back to tank 22. The bottom stream 24 from the bottom of tank 22 is subsequently pumped via pump 25 and divided into a bottom product stream 27 which is a bleed stream, and a recycle stream 26 which is fed to the top of falling film evaporator 23, partially evaporated and then flows into tank 22. Vapors are separated from the liquid in tank 22. The separated vapors flow up to the bottom of column 15 and liquid from distillation column 15 is recovered into tank 22.

[0032] The overhead product stream 21 from the top of the second reactive distillation column 15 is fed into the main distillation column 29. Overhead vapor enriched with racemic lactide is discharged through stream 30 and subsequently condensed in condenser 31. The condensate is split into an overhead product stream 35 distilled from the top of the main distillation column and a reflux stream 34, which is returned to the top of the main distillation column. Uncondensed vapor is removed through stream 32. Lactic acid oligomers are concentrated at the bottom of the main distillation column 29 and discharged as bottom stream 36. Bottom stream 36 is then split into a bottom product stream 41 and a recirculation stream 38, which is merged with stream 26. The recirculation stream 38 is fed to the inlet of the falling film evaporator 39, partially evaporated, and then flows through stream 40 to the bottom of the main distillation column 29. A vapor-phase side-effect product stream 42, containing high-purity L-lactide, is discharged below the inlet stream 21 of the main distillation column 29 and fed to the bottom of the side-stream scrubbing column 43. The overhead vapor, which is essentially composed of L-lactide, is discharged through stream 44 and subsequently condensed in condenser 45. The condensate is separated into overhead liquid product stream 49, which is distilled off from the top, and reflux stream 48, which is returned to the top of the side-stream scrubbing column 43. Uncondensed vapor is removed through stream 46. Bottom product stream 50 is returned to the main distillation column 29.

[0033] The invention will now be described in more detail with reference to the accompanying drawings and examples.

[0034] Example

[0035] Example 1

[0036] Perform as Figure 1 The reactive distillation system of the first reactive distillation system according to an embodiment of the present invention is shown. Distillation column 2 has nine theoretical stages. An aqueous solution of lactic acid stream 1 (90 wt% lactic acid) at a mass flow rate of 250 kg / h is continuously fed into distillation column 2 at a temperature of 110°C. The feed inlet is located at theoretical stage 7. The rectifying section and stripping section of distillation column 2 each have a specific surface area of ​​441 m². 2 / m 3 and 250m 2 / m 3 Structured packing material serves as the mass exchange element. Water is removed by distillation simultaneously through dehydration of the lactic acid aqueous solution and the formation of water by lactic acid condensation. Falling film evaporator 10 provides the necessary heat and heats the reaction solution to 180°C. The overhead product stream 8, consisting of substantially pure water at a mass flow rate of 71 kg / h, is removed for further water treatment. The bottom product stream 14 contains the majority of lactic acid oligomers. The top and bottom pressures of distillation column 2 are 22 mbar and 27 mbar, respectively. The reflux ratio at the discharge point of overhead product stream 8 is 0.5:1. Titration of the acid content in the bottom product indicates that the average number-average molecular weight of the lactic acid oligomers is 800 g / mol.

[0037] Example 2

[0038] Perform as Figure 1 The reactive distillation of the second reactive distillation system according to an embodiment of the present invention is shown. Distillation column 15 has six theoretical stages. The bottom product stream 14 from the first reactive distillation is mixed with the catalyst (stannous octoate) stream 28 in a static mixer at a mass flow rate of 177 kg / h and a temperature of 180°C. Figure 1 The mixture (not shown) is combined with streams 26 and 41, and its reaction solution is fed to the top of falling film evaporator 23. A specific surface area of ​​125 m² is used. 2 / m 3 Structured packing material serves as the mass exchange element in distillation column 15. Lactic acid oligomers are depolymerized to produce lactide in falling film evaporator 23, which is simultaneously distilled off. The evaporator heats the reaction solution to 215°C. The overhead product stream 21, containing more than 85% by weight of L-lactide at a mass flow rate of 172 kg / h, is removed for further purification. The bottom product stream 27 is removed as a discharge stream. The top and bottom pressures of distillation column 15 are 7 mbar and 12 mbar, respectively. The reflux ratio at the discharge point of overhead product stream 21 is 0.3:1.

[0039] Example 3

[0040] Perform as Figure 1 The distillation shown is an embodiment of the invention, featuring a side-stream scrubbing tower for main distillation. A specific surface area of ​​345 m² is used. 2 / m 3 Structured packing serves as the mass exchange element for both columns. The main distillation column 29 has 35 theoretical stages, and the side-stream scrubbing column 43 has 6 theoretical stages. The overhead product stream 21 from the second reactive distillation, with a mass flow rate of 3550 kg / h at 107°C, is continuously fed into the main distillation column 29, with the feed inlet located at theoretical stage 9. The vapor-phase side-stream product stream 42 from the main distillation column 29 is discharged at theoretical stage 33 and fed to the bottom of the side-stream scrubbing column 43. The bottom product stream 50 from the side-stream scrubbing column 43 is refluxed into the main distillation column 29, with the feed inlet located at theoretical stage 33. In the main distillation column 29, the overhead product stream 35 is enriched with racemic lactide, and the bottom product stream 41 contains mostly lactic acid oligomers. The overhead product stream 49 from the side-stream scrubbing column 43 is essentially pure L-lactide and is used for subsequent polymerization. The top and bottom pressures of the main distillation column 29 are 8 mbar and 16 mbar, respectively. The reflux ratio at the discharge point of the top product stream 35 is 5.5:1. The energy consumption of the main distillation column 29 is 1.2 MW. The compositions of the different streams are listed in the table below.

[0041]

[0042] Comparative Example 3

[0043] Distillation was carried out in a distillation column for purification of lactide with a vapor side draw product. A structured packing with a specific surface area of 345 m2 / m 2 3 was used as mass exchange element in the column. The column had the same number of theoretical stages as the main distillation column 29 described in Example 3, i.e. a total of 35 theoretical stages. A top product stream 21 with a mass flow rate of 3550 kg / h at a temperature of 107°C was continuously fed to the distillation column, the feed inlet being located at the position of theoretical stage 9. A vapor side draw product stream from the distillation column was withdrawn at the position of theoretical stage 33 for further purification or polymerization. The top pressure and the bottom pressure of the distillation column were 8 mbar and 16 mbar, respectively. The reflux ratio at the withdrawal point of the top product stream was 5.5:1. The energy consumption of the distillation column with the vapor side draw product was 1.2 MW. The compositions of the different streams are listed in the table below.

[0044]

[0045] As described in the above examples according to the present application, two reactive distillations were carried out and then substantially pure L-lactide was obtained by a main distillation column with a side draw washing column. The production process of L-lactide according to the present application has the advantage that the investment cost is reduced, and high purity L-lactide substantially free of lactic acid oligomers is produced, which can be polymerized to high molecular weight polylactic acid without further purification. If an aqueous lactic acid solution containing a majority of (R)-lactic acid is used as the starting material, the process according to the present application will also be suitable for the production of pure D-lactide.​

Claims

1. A continuous process for the production of lactide from aqueous lactic acid solution, wherein, a) a first reactive distillation comprising at least a tank, a distillation column, a condenser and an evaporator, the distillation column and its associated evaporator are mounted on top of the tank to form a single closed area in which lactic acid concentration, lactic acid condensation and associated distillation take place, lactic acid concentration, lactic acid condensation and associated distillation take place in said closed area, i) the distillation column has a feed inlet for the aqueous lactic acid solution, the feed inlet is located at a position between the upper and lower end of the distillation column, ii) the evaporator is a falling film evaporator, iii) the tank is arranged horizontally or vertically, iv) the majority of the liquid at the bottom of the tank is recirculated as reaction solution via a delivery pump to the top of the falling film evaporator for continuous lactic acid condensation, while a small portion of the liquid is fed to a subsequent depolymerization process, v) the reaction solution enters from the top of the falling film evaporator, flows down along a long vertical tube comprising a heat and reaction zone, and flows out from the bottom of the tube in a gas-liquid two-phase form, which directly flows to the tank, wherein the vapor is separated from the liquid; vi) the separated vapor flows up via the tank to the bottom of the distillation column mounted on top and the liquid from the distillation column is recovered into the tank; and vii) water contained in the aqueous lactic acid solution and water produced during lactic acid condensation is distilled out as an overhead gaseous phase stream, high-boiling components consisting of lactic acid and lactic acid oligomers flow back into the tank after being liquefied in the distillation column, b) a second reactive distillation comprising at least a tank, a distillation column, a condenser and an evaporator, the distillation column and its associated evaporator are mounted on top of the tank to form a single closed area for the generation of crude lactide, depolymerization of the obtained lactic acid oligomers and associated distillation take place in said closed area; i) the evaporator is a falling film evaporator, ii) the majority of the liquid at the bottom of the tank is combined with the small portion from iv) of a) and is fed to the top of the falling film evaporator for continuous depolymerization; iii) a gas-liquid two-phase stream flows out from the bottom of the tube of the falling film evaporator and directly flows into the tank, wherein the vapor is separated from the liquid, iv) the separated vapor flows up via the tank to the bottom of the distillation column mounted on top and the liquid from the distillation column is recovered into the tank, v) the crude lactide consists of a majority of L-lactide and some meso-lactide, lactic acid oligomers, residual water and lactic acid as an overhead low-boiling distillate stream, and a high-boiling fraction consisting essentially of unconverted lactic acid oligomers flows back into the tank, c) a combination of a main distillation column and a side draw washing column, the crude lactide from v) of b) is fed to the main distillation column for the purification of L-lactide, pure liquid lactide substantially free of lactic acid oligomers is obtained at the top of the side draw washing column.

2. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The aqueous lactic acid solution comprises 0 to 50% by weight of water and 50 to 100% by weight of lactic acid. The aqueous lactic acid solution comprises 0 to 50% by weight of water and 50 to 100% by weight of lactic acid.

3. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The distillation performed in the distillation column in the first reactive distillation is carried out under reduced pressure below 50 mbar.

4. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The mass transfer elements of the distillation column in the first reactive distillation are selected from the group consisting of random packings, structured packings and any combination thereof.

5. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The distillation performed in the distillation column in the second reactive distillation is carried out under reduced pressure below 100 mbar.

6. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The mass transfer elements of the distillation column in the second reactive distillation are selected from the group consisting of random packings, structured packings and any combination thereof.

7. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The distillation performed in the main distillation column is carried out under reduced pressure below 35 mbar.

8. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The mass transfer elements of the main distillation column and of the side draw wash column are selected from the group consisting of random packings, structured packings and any combination thereof.

9. The continuous process for producing propiolactone from aqueous lactic acid according to claim 1, wherein, The bottom product stream of the main distillation column is refluxed to the second reactive distillation.

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