Method and apparatus for producing lactide

Abstract

The present invention relates to a method for continuously producing optically pure lactide, which includes a first reactive distillation system, a second reactive distillation system, a main distillation system and a side-draw reflux. For each of the four systems, a novel horizontal top-mounted condenser is used to reduce the total pressure drop and thus reduce the side reactions associated with high bottom temperature. In addition, a wiped film evaporator, a short-path evaporator or a combination thereof is used to concentrate the purge stream from the second reactive distillation system in order to remove metal contaminants contained in the purge stream and depolymerize the contained unconverted lactic acid oligomers into crude lactide, all of which provide further benefits to lactide production.

Description

[Technical field]

[0001] The present invention relates to a method for the continuous production of optically pure lactide based on reactive distillation. The present invention also relates to an apparatus for producing lactide, including a novel condenser that produces very low pressure loss during operation. Furthermore, an apparatus for concentrating a purge stream from the bottom of a second reactive distillation system is described. Similarly, the intended use of both these apparatuses and the method is described. [Background technology]

[0002] Lactide is a cyclic dimer of lactic acid and can be used as an intermediate in the production of high molecular weight polylactic acid. These polymers are useful in the biomedical industry and other applications due to their ability to be biologically and hydrolytically degraded while forming environmentally acceptable degradation products, making them useful, for example, as degradable packaging materials.

[0003] An example of a known method for synthesizing lactide includes concentrating raw lactic acid in a lactic acid concentrator to reduce the water content and promote the initiation of esterification between lactic acid molecules, prepolymerizing lactic acid in a prepolymer reactor to produce lactic acid oligomers while removing the water generated by esterification, and depolymerizing the obtained lactic acid oligomers to crude lactide in a depolymerization reactor. Methods for carrying out the above concentration, prepolymerization and depolymerization are known in the art, for example, in U.S. Pat. No. 6,326,458.

[0004] It is well known in the art that lactic acid contains 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 with different geometric structures, distinguished as (R,R)-lactide (or D-lactide), (S,S)-lactide (or L-lactide) and (R,S)-lactide (or meso-lactide). In reality, the crude lactic acid fed to the system contains one of two types of lactic acid selected from (S)-lactic acid and (R)-lactic acid as the main component. Thus, the crude lactide produced by depolymerization contains a large amount of optically pure lactide (L-lactide or D-lactide), a small amount of meso-lactide, and an even smaller amount of the remaining third lactide.

[0005] Although crude lactide can be obtained from an aqueous solution of lactic acid by such a three-step method described in US Pat. No. 6,326,458, one drawback of this process is that the lactic acid is increasingly exposed to high temperatures as it is concentrated and prepolymerized to lactic acid oligomers. The starting lactic acid is usually of very high optical purity, with (S)-lactic acid being more commercially available. However, some racemization occurs under these conditions, e.g., (S)-lactic acid as the main component is converted to (R)-lactic acid, resulting in a loss of the main product L-lactide and an increase in the amount of meso-lactide in the crude lactide. This can cause problems in separating meso-lactide from optically pure lactides, e.g., L-lactide. An additional purification step may be required before the L-lactide is polymerized.

[0006] Furthermore, as described in US Patent No. 6,326,458, the water vapor from the lactic acid concentrator and the prepolymer reactor is condensed in two separate condensers, respectively, and the water vapor inevitably carries some lactic acid. The carried lactic acid is preferably separated from the condensate after passing through the condenser, and recycled to the lactic acid concentrator or the prepolymer reactor. However, a large amount of heat energy is required to recover the lactic acid from the condensate.

[0007] The above drawbacks are mitigated by replacing the two devices, the lactic acid concentrator and the prepolymer reactor, with one device, namely a reactive distillation system, which shortens the residence time of lactic acid and facilitates the separation of water from lactic acid. The reactive distillation system preferably includes at least a pot, a distillation column, a condenser and an evaporator. The evaporator not only provides the energy required for the evaporation of water, but is also the place where the lactic acid condensation (prepolymerization) reaction takes place. In the distillation column, a concentration gradient is formed as water is concentrated in the rectification section and high boiling components such as lactic acid and lactic acid oligomers are concentrated in the stripping section. The water contained in the aqueous solution of lactic acid and the water produced during the lactic acid condensation are distilled off as an overhead product stream consisting essentially of water.

[0008] The introduction of a distillation column can increase the pressure drop in the reactive distillation system, resulting in increased operating temperatures of the associated evaporators and pots. However, this problem is mitigated by proper selection of mass transfer elements suitable for the distillation column. Conventional condensers associated with distillation columns typically cause a pressure drop of about 5-20 mbar, so it would be desirable to provide a condenser with a very low pressure drop to reduce the reaction temperature of the reactive distillation system, and therefore reduce lactic acid racemization.

[0009] Similarly, a second reactive distillation system has been proposed for the depolymerization reactor, which includes at least a pot, a distillation column, an evaporator and a condenser with very low pressure drop.

[0010] The crude lactide from the second reactive distillation system 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 polylactic acid is controlled by the amount of hydroxyl impurities in lactide. In particular, the presence of water, lactic acid and lactic acid oligomers in lactide tends to retard polymerization, and the resulting polylactic acid does not have a high molecular weight suitable for its use as a biodegradable polymer. It is shown that separation of impurities from L-lactide can be achieved by distillation based on the difference in volatility between the components. The order of relative decrease in volatility of the main components in crude lactide is water, lactic acid, meso-lactide, L-lactide, lactic acid dimer, and their boiling points at atmospheric pressure are about 100°C, 215°C, 250°C, 255°C, and 350°C, respectively, and the boiling points of lactic acid trimer, lactic acid tetramer, etc. are even higher.

[0011] As described in US Patent No. 5,236,560, crude lactide containing lactide, lactic acid, lactic acid oligomers and water is fed to a distillation column and purified lactide is withdrawn in vapor form from a side draw of the distillation column. US Patent No. 8,569,517 proposes separating crude lactide through a dividing wall distillation column in which purified lactide in liquid form is obtained from the main fractionation zone on the other side of a vertical dividing wall.

[0012] Purified lactide that is substantially free of lactic acid can be obtained from the distillation column described in US Pat. No. 5,236,560 and US Pat. No. 8,569,517, but still contains small amounts of meso-lactide and lactic acid oligomers. Some of the lactic acid oligomers are formed by the side reaction of lactic acid and lactide exposed to high temperatures in the distillation process. Residual lactic acid oligomers in the purified lactide adversely affect the polymerization rate, resulting in the production of relatively low molecular weight polylactic acid. To obtain substantially optically pure lactide, the purified lactide is subjected to further purification steps, such as melt crystallization. By melt crystallization, the residual meso-lactide can be easily separated from the lactide. However, it is difficult to remove the residual lactic acid oligomers from lactide by melt crystallization, because the lactic acid oligomers are more viscous and tend to adhere to the surface of the lactide.

[0013] The above problems will be overcome or at least alleviated by the introduction of a small column, i.e., a side-draw reflux, connected downstream of the main distillation column with a vapor side-draw. The vapor side-draw stream withdrawn from the main distillation column is fed directly to the bottom of the side-draw reflux, which is equipped with a top condenser, and the bottom product from the side-draw reflux is refluxed to the main distillation column. An optically pure lactide stream, substantially free of lactic acid and lactic acid oligomers, is obtained at the top of the side-draw reflux. Similarly, in order to reduce the bottom temperature of the main distillation column and to reduce the side reactions of lactic acid and lactide, the main distillation column and the side-draw reflux are individually equipped with novel condensers that experience low pressure losses during operation.

[0014] As crude lactide is produced, some high boiling or non-volatile contaminants present in the feed to the entire system are believed to be concentrated in the pot of the second reactive distillation system. Such contaminants include catalyst residues and metals accumulated due to corrosion of stainless steel in the system. The yield of optically pure lactide is observed to decrease in the lactide formation process over a period of time without a purge stream at the bottom of the pot of the second reactive distillation system. This indicates that metal contaminants are detrimental to lactide production and that the incorporation of a purge stream is required.

[0015] Preferably, the purge stream is not recycled to the second reactive distillation system to prevent the accumulation of metal contaminants. It is desirable to provide an apparatus that can heat and concentrate the purge stream to recover lactide by depolymerization of the lactic acid oligomers contained in the purge stream, while removing most of the metals present in the purge stream for further processing for recovery of the metals by any known means. Summary of the Invention

[0016] One object of the present invention is to develop a process and apparatus for producing optically pure lactide based on two reactive distillations and two conventional distillations, in which a novel condenser is used for each distillation to reduce the total pressure drop and to reduce the racemization of lactic acid and the side reactions of lactic acid with lactide. The novel condensers are horizontal top-mounted (HTM) condensers welded or connected directly to the top of each distillation column.

[0017] Another object of the present invention is to provide a method and apparatus for removing accumulated metal contaminants to increase lactide production in the second reactive distillation, specifically, the apparatus includes a wiped film evaporator, a short path evaporator, or a combination thereof. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of a preferred lactide production system according to the present invention. [Diagram 2] FIG. 1 is a schematic diagram of a preferred HTM condenser according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In the conventional reaction sequence, the water contained in lactic acid is evaporated by heating in the lactic acid concentrator, and then the lactic acid condensation (prepolymerization) reaction is carried out in the lactic acid condenser to produce lactic acid oligomers. According to the present invention, the above two devices, i.e., the lactic acid concentrator and the prepolymer reactor, are replaced with a first reactive distillation system. The equipment cost and space required for the installation of the first reactive distillation system are greatly reduced. In addition, the use of the first reactive distillation system has the advantage of shortening the residence time of lactic acid, reducing the racemization of lactic acid, and thus increasing the yield of optically pure lactide.

[0020] The aqueous solution of lactic acid as a feedstock may contain, for example, 0 to 50% by weight of water and 50 to 100% by weight of lactic acid, respectively. The temperature of the aqueous solution of lactic acid is preferably in the range of 60 to 150°C, more preferably in the range of 100 to 130°C.

[0021] In the first reactive distillation, lactic acid concentration and lactic acid condensation reaction proceed, and lactic acid oligomers are produced. The average molecular weight of the lactic acid oligomers obtained by the lactic acid condensation reaction is generally 300 to 10,000, preferably 450 to 5,000, and more preferably 600 to 2,500.

[0022] According to the present invention, the first reactive distillation system preferably includes at least a pot, a distillation column, a condenser and an evaporator. The orientation of the pot may be horizontal or vertical depending on the process conditions. The distillation column may be a conventional column or a dividing wall column with a dividing wall dividing the inner space of the column. The evaporator not only provides the energy required for the evaporation of water, but also is the place where the lactic acid condensation reaction takes place.

[0023] The present invention is not particularly limited by the type of mass transfer element installed in the distillation column of the first reactive distillation system. Good results can be obtained by using an appropriate mass transfer element selected from the group consisting of trays, random packing, structured packing, and any combination thereof. On the other hand, structured packing is particularly suitable as a mass transfer element since it has the advantage of reducing the pressure loss in the distillation column and the liquid retention in the column. The specific surface area of ​​the structured packing is preferably 50 to 750 m. 2 / m 3 More preferably, it is in the range of 125 to 500 m. 2 / m 3 is in the range.

[0024] The distillation column in the first reactive distillation comprises at least one evaporator. The evaporator may be a type of evaporator commonly found in the chemical industry, including, but not limited to, a falling film evaporator, a forced circulation evaporator, a thermosiphon evaporator, etc. Falling film evaporators are particularly preferred for reducing the residence time of lactic acid and reducing undesirable side reactions such as racemization of lactic acid, due to their low liquid retention and high heat transfer rate.

[0025] The conventional condenser is usually connected to the distillation column via a connection elbow and piping. A pressure loss in the range of 5-20 mbar is usually caused by the conventional condenser and its connecting piping and fittings during the distillation process. In order to promote the removal of water and the condensation reaction between lactic acid molecules, the first reactive distillation is preferably carried out under a reduced pressure of 25 mbar or less. Most of the water vapor generated during the operation is condensed in the condenser under the reduced pressure achieved by the vacuum system, for example at a preferred pressure of 20 mbar by maximizing the use of cold water as a cooling medium. Considering the pressure loss caused by the distillation column and the conventional condenser, the pot and evaporator pressures are in the range of 30-45 mbar, which also indicates that the condensation reaction temperature in the evaporator is higher than the desired temperature and the racemization of lactic acid may be relatively high accordingly.

[0026] As shown in FIG. 2, the HTM condenser is a modified horizontal shell and tube heat exchanger, typically having one shell passage for the vapor to be condensed rising from the top of the distillation column, and multiple tube passages for the cooling medium, preferably ranging from 2 to 8. The HTM condenser includes a vertical baffle installed in the center of the shell at an inclination of about 3 to 7 degrees, which separates the inner space of the shell into a tube-arranged section above and an empty section below. In the tube-arranged section, two vertical single segment baffles (baffles #1 and #2) are separately connected to the vertical baffles at the left and right ends, and another three vertical single segment baffles (baffles #3, #4, and #5) are installed between the two vertical single segment baffles. The vapor flowing upward from the top of the distillation column to the vapor inlet of the horizontal top-mounted HTM condenser is guided in two opposite directions by the vertical baffles, and then directed by five vertical baffles for condensation. Steam condensation occurs by heat exchange with the cooling medium flowing in the tubes in the area formed by the vertical baffle and the five vertical single segment baffles, and to a lesser extent in the area outside the left and right ends of the vertical baffle. Most of the condensate formed flows across the inclined vertical baffle through the gap between the vertical baffle and the inner wall of the shell into the empty space of the shell, while the remaining condensate falls directly into the empty space outside the left and right ends of the vertical baffle. All the condensate is finally collected in the annular section around the vapor inlet and is divided into an overhead liquid product stream that is distilled off from an outlet at a lower position in the annular section and an internal reflux stream that is taken from an outlet at a higher position and fed back to the top of the column. Uncondensed vapor is removed through a vapor outlet at the top of the HTM condenser.

[0027] Due to its special geometric configuration, a pressure drop of less than 2 mbar can be achieved during operation by a properly designed HTM condenser. Therefore, an HTM condenser is proposed instead of a conventional condenser to condense the vapor rising from the top of the first reactive distillation column to minimize the operating pressure of the evaporator and pot. Furthermore, an HTM condenser has the advantage of being less expensive to install than a conventional condenser, since fewer fittings and piping are used.

[0028] In the first reactive distillation, a distillation column with a horizontal top-mounted HTM condenser and an evaporator are preferably mounted on the top of the pot, respectively, to form a single enclosed area in which lactic acid condensation and distillation take place. An aqueous solution of lactic acid is continuously fed to an inlet located between the top and bottom of the distillation column. The reaction solution enters from the top of the falling film evaporator, flows down a long vertical tube containing a heat transfer zone and a reaction zone, and exits from the bottom of the tube in the form of a gas-liquid two-phase. The two-phase flow flows directly to the connected pot, where the vapor is liberated from the liquid. The liberated vapor flows upward to the bottom of the top-mounted distillation column, and the liquid is collected in the pot. In order to prevent the liquid film inside the tube of the falling film evaporator from being destroyed, the evaporation of the reaction solution is generally less than 15-30 wt.%. Most of the liquid taken out from the bottom of the pot as the reaction solution is recycled to the top of the falling film evaporator via a transfer pump for continuous condensation of lactic acid, and a small part of it is fed to the next depolymerization reactor. The reaction solution is usually heated to a temperature in the range of 120 to 200°C, preferably in the range of 150 to 180°C.

[0029] In the first reactive distillation process, a concentration gradient is formed in the column as water is concentrated in the rectification section and high boiling components such as lactic acid and lactic acid oligomers are concentrated in the stripping section. The water contained in the aqueous solution of lactic acid and the water produced during lactic acid condensation are distilled off as an overhead vapor stream, which is condensed by an HTM condenser to obtain an internal reflux stream flowing downward and an overhead product stream consisting essentially of water, which is discarded. The vapor not condensed in the HTM condenser is removed by a vacuum system. The high boiling fraction consisting essentially of lactic acid and lactic acid oligomers liquefied in the column can be refluxed to the pot.

[0030] According to the present invention, a second reactive distillation system is used for the depolymerization reactor, which system includes at least a pot, a distillation column, an HTM condenser and a falling film evaporator. Similarly, in the first reactive distillation, a distillation column with a horizontal top-mounted HTM condenser and its associated falling film evaporator are mounted directly on the top of the pot, respectively, to form a single enclosed area in which the depolymerization and distillation take place. Structured packing is particularly suitable as a mass transfer element for the distillation column, since it has the advantage of reducing the pressure drop of the column and the liquid holdup in the column. The specific surface area of ​​the structured packing is preferably 50 to 750 m 2 / m 3 More preferably, it is in the range of 125 to 500 m. 2 / m 3 is in the range.

[0031] In the second reactive distillation, a catalyst such as tin dioctoate is added and mixed with the lactic acid oligomers from the first reactive distillation, and this mixture is fed as part of the reaction solution to the top of the falling film evaporator, where lactide is produced and evaporated. The gas-liquid two-phase flow leaves the bottom of the falling film evaporator tube and flows directly to the connected pot, where the vapor is liberated from the liquid. The liberated vapor flows upward through the pot to the bottom of the top-mounted distillation column, and the liquid is collected in the pot. In order to prevent the liquid film inside the falling film evaporator tube from being destroyed, the evaporation of the reaction solution is generally less than 15-30% by weight.

[0032] The reaction solution is heated in an evaporator at a reduced pressure of less than 100 mbar, preferably less than 10 mbar, at a temperature in the range of 120-250° C., preferably in the range of 150-220° C. An overhead low-boiling distillate stream is formed, i.e. crude lactide consisting of mostly L-lactide and some meso-lactide, lactic acid oligomers, residual water and lactic acid, e.g. 60-99% by weight of L-lactide, 0-15% by weight of meso-lactide, 0-10% by weight of lactic acid, 0-12% by weight of lactic acid oligomers and 0-3% by weight of water. The high-boiling fraction consisting essentially of unconverted lactic acid oligomers flows back into the pot.

[0033] Most of the liquid removed from the bottom of the pot as part of the reaction solution is recycled to the top of the falling film evaporator via a transfer pump for the continuous depolymerization reaction, and a small portion of it is removed as a purge stream. The purge stream contains most of the unconverted lactic acid oligomers and small amounts of high boiling point or non-volatile contaminants, such as catalyst residues and metals accumulated by corrosion of stainless steel. It is desirable to remove most of the metals present in the purge stream while the unconverted lactic acid oligomers are depolymerized to crude lactide. This lactide is recovered and mixed with the overhead product stream from the second reactive distillation for further purity confirmation.

[0034] According to a preferred embodiment of the present invention, a wiped film evaporator is used to concentrate the purge stream. The wiped film evaporator is also called a thin film evaporator or an agitated thin film evaporator and typically includes a jacketed shell, an agitator, a droplet separator and a drive unit. A heating medium flows in the heating jacket and provides the thermal energy required for the depolymerization of lactic acid oligomers in the purge stream and the evaporation of volatile components, including the crude lactide produced. The agitator, driven by the drive unit, is equipped with a paddle, wiper or scraper and is arranged in the shell. In this way, the purge stream fed to the evaporator through the inlet is distributed uniformly as a film over the entire inner surface of the heating jacket. The evaporated components enter the droplet separator attached to the top of the shell to remove entrained liquid before leaving the evaporator through the vapor outlet to an external condenser for condensation. The least volatile components, including tin catalyst residues and metal contaminants, are removed through the liquid outlet and sent to a subsequent process to recover metals.

[0035] Alternatively, taking into consideration the pressure loss of the steam flowing from the surface of the heating jacket to the external condenser, an internal condenser is placed within the shell at a short distance from the surface of the heating jacket to reduce the pressure loss, thus, in effect, a short-path evaporator is obtained.

[0036] The crude lactide from the second reactive distillation is sent to the next main distillation column for purification of L-lactide. The crude lactide is fractionated based on the difference in volatility of each component. The order of relative decreasing volatility of the major components in the crude lactide is water, lactic acid, meso-lactide, L-lactide, and lactic acid oligomers. Less volatile components, such as lactic acid oligomers, which have higher boiling points than L-lactide, are concentrated at the bottom of the column and removed as the bottoms product. The overhead product stream from the main distillation column contains mostly meso-lactide and small amounts of lactic acid and L-lactide. The lactide product, which contains high purity L-lactide, is withdrawn as a vapor side draw from the main distillation column.

[0037] The overhead vapor stream at the top of the main distillation column is condensed by an HTM condenser to obtain a condensate stream rich in meso-lactide. Vapors not condensed in the condenser are removed by a vacuum system. Preferably, a portion of the condensate stream is refluxed into the column, and another portion is subjected to an additional purification system, such as distillation, crystallization, or a combination thereof, to obtain pure meso-lactide.

[0038] The liquid bottom stream concentrated in the stripping section is removed from the bottom of the main distillation column, and then divided into a bottom product stream and a recycle stream.The increase in the lactic acid oligomer content in the bottom product stream is observed due to the side reaction between lactide and residual lactic acid under the condition of relatively high bottom temperature.The bottom product stream is preferably refluxed to the second reaction distillation system as part of the reaction solution for depolymerization.

[0039] The lactide product stream taken as a vapor side draw of the main distillation column is substantially free of water and lactic acid. However, due to side reactions between lactide and residual lactic acid in the distillation process, the lactide product stream still contains a small amount of lactic acid oligomers. The residual lactic acid oligomers in the lactide product stream adversely affect the polymerization rate, resulting in the production of relatively low molecular weight polylactic acid.

[0040] According to the invention, the vapor side-draw stream of the main distillation column is fed directly to the bottom of a side-draw reflux vessel equipped with an HTM condenser, and the bottom product from the side-draw reflux vessel is refluxed to the main distillation column. In the side-draw reflux vessel, L-lactide is separated from residual lactic acid oligomers, and purified L-lactide, substantially free of lactic acid and lactic acid oligomers, is obtained at the top of the side-draw reflux vessel.

[0041] The main distillation column and the side draw reflux are preferably operated at low temperature and reduced pressure. The pressure at the top of the main distillation column is preferably in the range of 3 to 25 mbar, more preferably in the range of 5 to 15 mbar. The pressure at the bottom of the main distillation column is preferably in the range of 10 to 35 mbar, more preferably in the range of 12 to 25 mbar.

[0042] The mass transfer elements installed in the main distillation column and the side draw reflux vessel are composed of trays, random packing, structured packing and any combination thereof. However, structured packing is particularly suitable as a mass transfer element since it has the advantage of reducing the pressure drop and liquid holdup in the column. The specific surface area of ​​the structured packing is preferably 125 to 750 m. 2 / m 3 More preferably, it is in the range of 250 to 350 m. 2 / m 3 is in the range.

[0043] 1 shows a schematic diagram of a preferred lactide production system according to the present invention, which includes a first reactive distillation column 2, an HTM condenser 3, a pot 7, a falling film evaporator 8, a pump 10, a second reactive distillation column 13, an HTM condenser 14, a pot 18, a falling film evaporator 19, a pump 21, a wiped film evaporator 24, an external condenser 27, a main distillation column 31, an HTM condenser 32, a pump 37, a falling film evaporator 39, a side-draw reflux 43, and an HTM condenser 44.

[0044] An aqueous solution of lactic acid is continuously fed to the first reactive distillation column 2 via stream 1. The overhead vapor, consisting essentially of water, is removed and then condensed in the HTM condenser 3. The condensate is split into an overhead liquid product stream 6, which is distilled off from the top of the distillation column 2, and an internal reflux stream 5, which is fed back to the top of the distillation column 2. Uncondensed vapor is removed via stream 4. Lactic acid and lactic acid oligomers are condensed at the bottom of the column 2 and flow back to pot 7. Bottoms stream 9, which is removed from the bottom of pot 7, is then transferred via pump 10 and split into bottoms product stream 12 and recycle stream 11, which is fed to the top of the falling film evaporator 8 and partially evaporated before flowing to pot 7. The vapor is released from the liquid in pot 7. The released vapor flows upwards to the bottom of the column 2 and the liquid in the column 2 returns to pot 7.

[0045] Bottom product stream 12 is mixed with depolymerization catalyst stream 30, which mixture is mixed with streams 22 and 41 and fed continuously to the top of falling film evaporator 19. Overhead vapors containing most of the lactide are removed and then condensed in HTM condenser 14. The condensate is split into overhead liquid product stream 17, which is distilled off from the top of distillation column 13, and internal reflux stream 16, which is fed back to the top of distillation column 13. Uncondensed vapors are removed through stream 15. Unconverted lactic acid oligomers are condensed at the bottom of column 13 and flow back to pot 18. Bottom stream 20, which is removed from the bottom of pot 18, is then transferred via pump 21 and split into purge stream 23 and recycle stream 22. In falling film evaporator 19, the liquid reaction solution is partially evaporated before flowing to pot 18. Vapor is released from the liquid in pot 18. The released vapor flows upward to the bottom of column 13 and the liquid within column 13 returns to pot 18.

[0046] Purge stream 23 is fed to the inlet of wiped film evaporator 24. Vapor produced from wiped film evaporator 24 is condensed in external condenser 27 via stream 26. The condensate from condenser 27 is mixed with overhead product stream 17 and uncondensed vapor is removed via stream 28. The least volatile components are removed from wiped film evaporator 24 via stream 25.

[0047] Overhead product stream 17 from distillation column 13 is fed to main distillation column 31. Meso-lactide-rich overhead vapor is removed and then condensed in HTM condenser 32. The condensate is split into overhead product stream 35, which is distilled off from the top of main distillation column 31, and internal reflux stream 34, which is fed back to the top of main distillation column 31. Uncondensed vapor is removed via stream 33. Lactic acid oligomers are concentrated in the bottom of main distillation column 31 and removed as bottoms stream 36. Bottoms stream 36 is then split into bottoms product stream 41 and recycle stream 38, which is fed to the inlet of falling film evaporator 39 and partially vaporized before flowing to the bottom of main distillation column 31 via stream 40. Vapor side-draw stream 48 removed from main distillation column 31 is fed to the bottom of side-draw reflux 43. Overhead vapor in side-draw reflux 43, consisting essentially of L-lactide, is removed and then condensed in HTM condenser 44. The condensate is split into an overhead liquid product stream 47 and an internal reflux stream 46 which is fed back to the top of side-draw reflux 43. Uncondensed vapor is removed via stream 45. Bottoms product stream 49 from side-draw reflux 43 is refluxed to main distillation column 31.

[0048] The invention will now be described in more detail below with reference to the figures and examples. EXAMPLES

[0049] Example 1 Distillation was carried out by a first reactive distillation system according to an embodiment of the present invention shown in Figure 1. The distillation column 2 had a total of 9 theoretical stages. A stream 1 of an aqueous solution of lactic acid (90 wt% lactic acid) with a mass flow rate of 4600 kg / h at a temperature of 110 °C was continuously fed to the distillation column 2, whose feed inlet was located at theoretical stage 7. A stream 1 of an aqueous solution of lactic acid (90 wt% lactic acid) with a specific surface area of ​​441 m 2 / m 3 and 250m 2 / m 3 Structured packings of the formula (I) were used as mass exchange elements in the rectification and stripping sections of the distillation column 2, respectively. The reaction solution was heated in a falling film evaporator 8 to a temperature of 180° C. The bottom product stream 12 contained mostly lactic acid oligomers.

[0050] The HTM condenser 3 contained a total of 815 tubes with a length of 4500 mm and an outside diameter (OD) of 25.4 mm. The inside diameter (ID) of the shell of the HTM condenser 3 was 2000 mm. The total surface area of ​​the HTM condenser 3 was 282 m 2 The water vapor rising from the top of the distillation column 2 was fed directly to the vapor inlet of the HTM condenser 3, where it was condensed by heat exchange with the cooling medium flowing in the tubes. The condensate was collected in an annular section around the vapor inlet and divided into an overhead liquid product stream 6, which was distilled off at a lower point in the annular section, and an internal reflux stream 5, which was fed back to the top of the column 2. The uncondensed vapor stream 4 was removed through the vapor outlet at the top of the HTM condenser 3. An overhead product stream 6, consisting essentially of pure water, with a mass flow rate of 1240 kg / h was removed for further water treatment. The pressure losses caused by the HTM condenser 3 and the distillation column 2 were 1.9 mbar and 4.3 mbar, respectively, which, when added together, gave a total system pressure loss of 6.2 mbar. When the pressure of the vapor outlet stream 4 was set to 18.5 mbar by the vacuum system, the pressure of the evaporator 8 and pot 7 during operation was 24.7 mbar.

[0051] Example 2 A reactive distillation was carried out according to a second reactive distillation system according to an embodiment of the present invention shown in Figure 1. The distillation column 13 had a total of six theoretical stages. The bottom product stream 12 from the first reactive distillation was mixed with the catalyst (tin dioctoate) stream 30 in a static mixer (not shown in Figure 1) and combined with streams 22 and 41 to form a mixture of reaction solutions. This mixture had a mass flow rate of 3500 kg / h and was fed to the top of a falling film evaporator 19. A 1000 ml distillation column with a specific surface area of ​​125 m 2 / m 3 A structured packing having a mass exchange element of the distillation column 13 was used. The crude lactide was distilled off during its production by depolymerization of lactic acid oligomers in a falling film evaporator 19, in which the reaction solution was heated to a temperature of 215° C.

[0052] The HTM condenser 14 contained a total of 605 tubes with a length of 4000 mm and an OD of 19.05 mm. The shell ID of the HTM condenser 14 was 1700 mm. The total surface area of ​​the HTM condenser 14 was 139 m 2 The crude lactide vapors rising from the top of the distillation column 13 were fed directly to the vapor inlet of the HTM condenser 14, where they were condensed by heat exchange with a cooling medium flowing in the tubes. The condensate was collected in an annular section around the vapor inlet and divided into an overhead liquid product stream 17, which was distilled off at a lower point in the annular section, and an internal reflux stream 16, which was fed back to the top of the column 13. The uncondensed vapor stream 15 was removed through a vapor outlet at the top of the HTM condenser 14. The overhead product stream 17, which was over 85 wt. % L-lactide and had a mass flow rate of 3035 kg / h, was removed for further purification. The pressure losses caused by the HTM condenser 14 and the distillation column 13 were 1.7 mbar and 2.9 mbar, respectively, and adding these pressure losses resulted in a total pressure loss of 4.6 mbar in the system. The pressure of the vapour outlet stream 15 was set to 5 mbar by the vacuum system, resulting in a pressure of 9.6 mbar in the evaporator 19 and pot 18 during operation.

[0053] Example 3 A purge stream 23 of 400 kg / h was fed to the wiped film evaporator 24. The evaporated components comprising the crude lactide left the wiped film evaporator 24 and entered the external condenser 27 via stream 26, where condensation took place. The condensate stream 29 had a mass flow rate of 360 kg / h and entered the overhead product stream 17. Uncondensed vapors were removed via stream 28. The least volatile components comprising tin catalyst residues and metal contaminants were removed via stream 25 for further processing. The operating temperature and pressure of the wiped film evaporator 24 were set in the range of 200-230° C. and 6 mbar, respectively.

[0054] Example 4 Distillation was carried out using a main distillation column equipped with a side-draw reflux according to one embodiment of the present invention shown in Figure 1. The main distillation column 31 has a total of 35 theoretical stages, and the side-draw reflux 43 has a total of 6 theoretical stages. 3550 kg / h of feed at a temperature of 107°C was continuously fed to the main distillation column 31 with the feed inlet at theoretical stage 9.

[0055] The HTM condenser 32 contained a total of 360 tubes with a length of 4000 mm and an OD of 25.4 mm. The shell ID of the HTM condenser 32 was 1600 mm. The total surface area of ​​the HTM condenser 32 was 110 m 2 During operation, the total pressure drop caused by the HTM condenser 32 was 1.6 mbar.

[0056] The HTM condenser 44 contained a total of 242 tubes with a length of 3500 mm and an OD of 25.4 mm. The shell ID of the HTM condenser 44 was 1100 mm. The total surface area of ​​the HTM condenser 44 was 65 m 2 During operation, the total pressure drop caused by the HTM condenser 44 was 1.2 mbar.

[0057] As described in the above embodiments of the present invention, the use of an HTM condenser instead of a conventional condenser is particularly useful for reducing the total pressure drop of the system and correspondingly reducing the operating temperature of the system. Additionally, a wiped film evaporator is utilized for removing metal contaminants contained in the purge stream and for recovering crude lactide by depolymerization of unconverted lactic acid oligomers contained therein.

Claims

1. A process for the continuous production of optically pure lactide from an aqueous solution of lactic acid, comprising a first reactive distillation system, a second reactive distillation system, a main distillation system and a side-draw reflux, wherein a novel horizontal top-mounted (HTM) condenser is used to provide low pressure drop to reduce side reactions associated with high bottom temperatures.

2. 10. The method of claim 1 , wherein the HTM condenser is welded or connected directly to the top of a distillation column of the first reactive distillation system.

3. 10. The method of claim 1 , wherein the HTM condenser is welded or connected directly to the top of a distillation column of the second reactive distillation system.

4. 10. The method of claim 1, wherein the HTM condenser is welded or connected directly to the top of the main distillation system.

5. 10. The method of claim 1 , wherein the HTM condenser is welded or connected directly to the top of the side-draw reflux.

6. The HTM condenser comprises: a cylindrical shell and a number of tubes attached inside the shell; a vertical baffle installed in the center of the shell at an inclination of 3 to 7 degrees and separating the inner space of the shell into a tube-arranged section above it and an empty section below it, in which two vertical single segment baffles are separately connected to the vertical baffle at left and right ends in the tube-arranged section, and another three vertical single segment baffles are installed between the two vertical single segment baffles; 2. The process of claim 1 including a vapor inlet at the bottom of the HTM condenser for vapor rising from the top of the distillation column, an uncondensed vapor outlet at the top of the HTM condenser, an annular section around the vapor inlet for accumulating condensate, an overhead liquid product outlet at a lower location in the annular section, and an internal reflux outlet at an upper location in the annular section.

7. 10. The method of claim 1, wherein the HTM condenser has one shell passage for the vapor to be condensed and multiple tube passages for the cooling medium.

8. 7. The method of claim 6, wherein in the HTM condenser, vapor from the top of the distillation column is guided by the vertical baffle and the five vertical baffles and exchanges heat with a cooling medium flowing inside the tubes.

9. 7. The method of claim 6, wherein in the tube arrangement section of the HTM condenser, steam condensation occurs in an area formed by the vertical baffle and the five vertical single segment baffles, and to a lesser extent in areas outside the left and right ends of the vertical baffle.

10. 7. The method of claim 6, wherein in the HTM condenser, most of the condensate formed crosses the inclined vertical baffle and flows through a gap between the vertical baffle and an inner wall of the shell into the empty space of the shell, and the remaining condensate falls directly from outside the left and right ends of the vertical baffle into the empty space.

11. 7. The method of claim 6, wherein in the HTM condenser, all condensate formed eventually collects in the annular section around the steam inlet.

12. 10. The method of claim 1, wherein the HTM condenser has a pressure drop of less than 2 mbar during operation.

13. An apparatus for continuously producing optically pure lactide from an aqueous solution of lactic acid, comprising a first reactive distillation system, a second reactive distillation system, a main distillation system and a side-draw reflux, using a novel horizontal top-mounted (HTM) condenser to reduce pressure drop and thereby reduce side reactions associated with high bottom temperatures, using a concentrator for concentrating a purge stream from the second reactive distillation system to remove metal contaminants contained in the purge stream and prevent metals from accumulating in the system, and recovering crude lactide by depolymerization of unconverted lactic acid oligomers.

14. The apparatus of claim 13 , wherein the concentrator comprises a wiped film evaporator and an external condenser.

15. 15. The apparatus of claim 14, wherein the crude lactide vapor produced in the wiped film evaporator is condensed in the external condenser.

16. 14. The apparatus of claim 13, wherein in the concentrator, the concentrated fraction contains metal contaminants and is sent to a subsequent process for recovering metals.

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