JPH11217425A - Method and apparatus for continuous preparation of lactide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus and a method by which lactide can be prepared from lactic acid continuously for a long period of time and highly pure product lactide can be obtained on the premise that the amount of a catalyst for lactidation is as low as possible. SOLUTION: An apparatus for continuous preparation of lactide comprises lactidation reactor 13 which depolymerizes lactic acid oligomer, condenser 16 which condenses evaporated lactide and reflux condenser 15 between lactidation reactor 13 and condenser 16. A continuous preparation method involves continuously effecting the oligomerization step, lactidation step and lactide recovering step and, in distilling lactide in the lactide recovering step, refluxing accompanying oligomer and a catalyst by reflux condenser 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous production apparatus and a continuous production method of lactide, and more particularly, to a series of steps for producing lactide by depolymerizing lactide from lactic acid as a raw material and then depolymerizing the lactide. And a method for performing the above.

[0002]

2. Description of the Related Art Lactide is a raw material of polylactic acid which is a biodegradable polymer. Polylactic acid is used in the medical field because it is not only degraded in vivo but also has excellent mechanical properties, and is also degraded by microorganisms in the natural environment. Demand for lactide is increasing more and more, as it is expected to be used for various industrial and consumer applications.

As a method for producing the above lactide, there is a method described in JP-A-7-138253.
(1) a step of heating and concentrating lactic acid, (2) a step of dehydrating the concentrated lactic acid while refluxing to produce a lactic acid oligomer, and (3) a depolymerization and distillation of the lactic acid oligomer in the presence of a catalyst. A method has been proposed in which the steps are performed in a batch system.
However, in the above method, the temperature control in the reflux step is performed at 135 ° C.
From 150 ° C to 160 ° C, it is necessary to carry out the process while gradually raising the temperature.
The batch system is inefficient, has a large energy cost, and is difficult to process in large quantities. Further, the quality (particularly, lactide purity) is unstable because the composition in the reactor changes over time. Was pointed out.

As a method for producing lactide continuously instead of in a batch system, a method for continuously oligomerizing lactic acid is disclosed in Japanese Patent Application Laid-Open No. 9-316180. A method for performing this is disclosed in JP-A-9-110859. Therefore,
It is considered that by combining both methods, lactide can be produced consistently and continuously from lactic acid via lactic acid oligomers.

FIG. 1 shows an apparatus for producing lactide described in Japanese Patent Application Laid-Open No. 9-110859. Reference numeral 21 denotes a lactic acid oligomer tank, 22 denotes a reaction vessel, 23 denotes a condenser, and 24 denotes a lactide receiver. . According to the apparatus described above, the lactic acid oligomer stored in the lactic acid oligomer tank 21 is supplied to the reaction vessel 22 by the transfer pump P, and the electromagnetic valve is opened to allow 5 to 25% by weight of the catalyst to enter the catalyst hopper H. The lactic acid oligomer is heated in the reaction vessel 22 maintained at a predetermined pressure by the vacuum pump VP to synthesize lactide. The synthesized lactide is vaporized, condensed by the condenser 23 and stored in the lactide receiver 24.

[0006] However, in the above-mentioned continuous production method of lactide, there is a problem that the catalyst used for lactide conversion of lactic acid oligomer is deactivated and continuous operation must be stopped after about 100 hours. Therefore, there has been a demand for the development of a method and apparatus capable of continuously producing lactide over a long period of time.

Further, according to the above-mentioned method, the catalyst concentration needs to be extremely high at 5.1 to 25.0% (conventionally, the catalyst concentration is 0.1 to 25.0%).
(About 5 to 2.0% by weight), since the catalyst is mixed into the product lactide, the quality is reduced, and a catalyst removal step is indispensable.
A lactic acid oligomer of about 0% is also contained, and improvement in purity has been desired.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and is based on the premise that the amount of catalyst used for lactide formation is reduced as much as possible. Can produce lactide from lactic acid,
Moreover, an object of the present invention is to provide an apparatus and a method capable of obtaining a high-purity product lactide.

[0009]

Means for Solving the Problems The continuous production apparatus of the present invention which has solved the above problems is a continuous lactide production apparatus having a lactide conversion reactor for depolymerizing lactic acid oligomers and a condenser for condensing the evaporated lactide. And
The gist of the invention is that a reflux condenser is interposed between the lactide reactor and the condenser.

The continuous production apparatus of the present invention which has solved the above-mentioned problems is characterized in that lactic acid is heated and polymerized to form a lactic acid oligomer, and then the lactic acid oligomer is heated and depolymerized in the presence of a catalyst to convert lactide. A method for producing lactide to be produced, wherein an oligomerization step of continuously supplying a raw material mainly composed of lactic acid to an oligomerization reactor, dehydrating while heating and concentrating lactic acid, and continuously producing a lactic acid oligomer, A lactide-forming step in which a lactic acid oligomer continuously fed from the oligomerization reactor is depolymerized in the presence of a catalyst in a lactide-forming reactor to continuously produce lactide; The obtained lactide has a lactide recovery step of continuously recovering lactide by liquefaction by a condenser, the oligomerization step, Serial lactide step, performs continuously the lactide recovery step, when distilling the lactide in the lactide recovery step, it is an Abstract that is refluxed by reflux condenser oligomers and catalyst entrained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing lactide described in Japanese Patent Application Laid-Open No. 9-110859, continuous operation must be stopped after about 100 hours despite the use of a large amount of catalyst. As a result of investigations by the present inventors on the reason for the absence of the lactide, the catalyst is discharged out of the lactide formation reactor together with the lactide vapor in the lactide production process even when a large amount of the catalyst is used in the above method. I got the knowledge. Therefore, as a result of the present inventors' earnest studies, if a reflux condenser is provided between the lactide reactor and the condenser, and the catalyst accompanying vaporization of lactide is refluxed together with the lactic acid oligomer,
The inventors have found that a lactide-forming reaction can be carried out over a long period of time with a small amount of catalyst, and thereby succeeded in consistently and continuously producing high-purity lactide from lactic acid via lactic acid oligomers. .

Hereinafter, a method and an apparatus for continuously producing lactide according to the present invention will be described in detail with reference to FIG. The method for producing lactide according to the present invention is characterized in that it is produced continuously and continuously from lactic acid as a raw material to lactide as a product. Since it is produced as an intermediate product, the following (A) oligomerization step;
(B) lactide conversion step and (C) lactide recovery step.

A. 2. Oligomerization process In FIG. 2, 1 is a raw material tank, 2 is a preheater, 3 is a flash valve, 4 is a distillation column, 5 is an oligomerization reactor, 6 is a condenser, 7 is a distillate discharge tank, and 8 is external circulation. Type heat exchanger, 9 is an auxiliary oligomerization reactor, 10 is a stirrer, 11
Denotes vapor tubes, respectively.

The lactic acid stored in the raw material tank 1 is sent to a preheater 2 by a raw material feed pump P ₁ , heated to a predetermined temperature, and then supplied to a distillation column 4 via a flash valve 3.
The distillation column 4 is disposed above the oligomerization reactor 5, and the lactic acid flowing down into the oligomerization reactor 5 is heated and dehydrated and condensed into a lactic acid oligomer in the reactor.
The raw lactic acid generally contains about 10% of water, and water is also generated by a dehydration condensation (oligomerization) reaction. The water turns into steam by heating, rises together with the lactic acid vapor, reaches the distillation column 4 in which the tray or the packing is incorporated, and the lactic acid vapor liquefies and falls in the distillation column 4 to fall into the oligomerization reactor. The steam returned to 5 while passing through the distillation column 4 is led to the condenser 6. The steam is cooled and condensed by the condenser 6 to which a coolant such as cooling water is supplied, and is discharged out of the system through a distillate extraction tank 7. Also part of the water fed from the capacitor 6 to the distillate drainage unloading tank 7 is returned to the top of the distillation column 4 by a pump P _2, for the separation efficiency of the temperature control and water and the raw material component of said top Used.

The heater provided in the oligomerization reactor 5 may be a heating medium circulating heating system with a jacket structure or an electric heater heating system. In addition, shell / tube type heating medium circulation heating, electric heater heating, or the like can be employed.

In this way, the water contained in the raw material and the water in the steam generated in the oligomerization reactor 5 are taken out as steam from the top of the distillation column 4, and the water is removed from the inside of the oligomerization reactor 5. Since it is removed outside the system, the raw material from the supply system and the unreacted raw material in the vapor are concentrated and stored in the oligomerization reactor, the concentration of the raw material in the oligomerization reactor is increased, and the oligomerization is performed by the condensation reaction. Progresses.

The oligomerization reactor 5 is connected to an auxiliary oligomerization reactor 9 via an external heat exchanger 8. The external circulation type heat exchanger 8 and the auxiliary oligomerization reactor 9 have a heater (a heating medium circulation type heater with a jacket structure, an electric heater, or the like), and the internal liquid temperature is adjusted in the oligomerization reactor. By maintaining the liquid temperature higher than the temperature of the liquid, the flow of the liquid from the oligomerization reactor to the auxiliary oligomerization reactor and the external circulation type heat exchanger can be ensured. In addition, it is desirable that a stirrer 10 is disposed in the auxiliary oligomerization reactor 9 to stir the liquid in the vessel to increase the heat supply efficiency (however, the viscosity of the internal liquid in the auxiliary oligomerization reactor 9 is low. The stirrer is not necessary when the liquid circulates in the vessel by natural convection.)

The water and a part of lactic acid and lactic acid oligomer in the reaction solution circulating in the external circulation heat exchanger 8 and the auxiliary oligomerization reactor 9 are vaporized by heating by the external circulation heat exchanger 8, and the vaporized vapor Is returned to the distillation column 4 through a vapor tube 11 provided at the upper part of the auxiliary oligomerization reactor. The water in the returned steam is taken out as steam from the top of the distillation column as described above, and the raw materials are concentrated and returned to the reaction system again. In this way, the raw materials and oligomers are circulated through the oligomer reactor 5, the external circulation heat exchanger 8 and the auxiliary oligomerization reactor 9, and at the same time, water is distilled out from the top of the distillation column to concentrate and dehydrate the raw materials. The condensation (oligomerization) reaction proceeds.

Further, a certain amount of the liquid in the oligomerization reactor is returned to the raw material supply line by the oligomer circulation pump P ₃ through the bottom pipe, and sent to the preheater 2 together with the raw material, and the flash valve 3 is turned on. After that, it is supplied to the distillation column 4 again.

The distillation column 4, the oligomerization reactor 5, the external circulation heat exchanger 8, the auxiliary oligomerization reactor 9 and the like are provided with a vacuum pump VP _{1 provided} downstream of the condenser 6.
Is controlled to a constant pressure is depressurized by the pressure regulating valve V ₁ by.

The gas phase pressure in the oligomerization reactor is desirably set within a range where water can be distilled off without accompanying lactic acid or oligomer.
Lactic acid accompanies the distilled water, resulting in a raw material loss.
If it exceeds rr, water cannot be sufficiently distilled off, and the molecular weight of the oligomer does not increase.
It is desirable to set r or less.

The liquid phase temperature in the oligomerization reactor is:
It is desirable to set the range within which optical isomerization of lactic acid is suppressed and the dehydration / condensation reaction proceeds. If the temperature is lower than 150 ° C., the reaction rate is slow and inefficient, whereas if it exceeds 200 ° C., the optical isomerization proceeds. It is recommended that the oligomerization reaction be performed at a temperature of 150 ° C. or more and 200 ° C. or less because lactic acid easily accompanies the distilled water as well as the lactic acid.

The upper and lower portions of the oligomerization reactor 5 and the auxiliary oligomerization reactor 9, the distillation column 4, the preheater 2
Since the raw material supply pipe, the liquid feed pipe from the oligomerization reactor 9 bottom, a liquid feed pipe from the oligomer circulation pump P _3,
Circulation piping between the external circulation type heat exchanger 8 and an auxiliary oligomerization reactor 9, the suction and discharge piping oligomeric liquid feed pump P _4, vapor pipe is desirably heated and kept at an electric heater or a jacket or the like.

As described above, the oligomerization reactor according to the present invention is provided with a distillation column, and the reflux operation is performed in the distillation column to concentrate lactic acid and to dehydrate the reaction system. As a result, water (water contained in the raw lactic acid and condensed water generated in the oligomerization reaction) can be efficiently removed and the oligomerization reaction can be promoted, so that the molecular weight of the lactic acid oligomer is improved and the water content is reduced. Can be. In addition, lactic acid (= raw material loss) content in distillate is low,
Since the yield of lactide is improved and the reaction can proceed efficiently, the residence time in the reactor can be shortened and the isomerization reaction can be suppressed. Further, it has the advantage that the energy cost is small.

The oligomerization reactor according to the present invention is an external circulation heating type flash distillation can. The lactic acid raw material is continuously supplied to the reactor by a flash operation, and the reaction solution in the oligomerization reactor is continuously withdrawn. After external heating, it is returned to the reactor again by a flash operation. Therefore, the separation of water and lactic acid in the raw material is promoted, the residence time in the reactor required for oligomerization (to reach a sufficient average molecular weight) can be shortened, and the energy cost is reduced as compared with the case where dehydration is performed only by distillation. Can be suppressed.

Further, by providing the external circulation type heat exchanger 8, it is possible to supply a large amount of heat which is difficult by liquid heating in the oligomerization reactor 5 and the auxiliary oligomerization reactor 8, and to add heat required for the dehydration condensation reaction. be able to. Further, an ideal circulation state for the reaction operation / dehydration operation between the external circulation heat exchanger 8 and the auxiliary oligomer reactor 9 can be obtained by the thermosiphon effect. Accordingly, the provision of the external circulation heat exchanger 8 enables large-capacity processing (mass production).

The oligomer produced in the above polymerization reaction system is withdrawn at a predetermined liquid sending speed by the oligomer liquid sending pump P4, and is supplied to the reaction system in the lactide-forming step which is the next step.

B. Lactidation Step In FIG. 2, 12 indicates an external circulation heat exchanger, 13 indicates a lactide reaction reactor, and 14 indicates a stirrer.

The oligomer fed by the oligomerization feed pump P ₄ is sent to the lactide reactor 13 via the external circulation type heat exchanger 12. The lactide conversion catalyst is set in advance so that the catalyst concentration is within a predetermined range.
It is thrown into 3.

The external circulation type heat exchanger 12 and the lactide conversion reactor 13 each have a heater (a heating medium circulation type heater having a jacket structure, an electric heater, etc.). And the lactide vaporization reaction proceeds. Further, a stirrer 14 is provided in the lactide-forming reactor 13 so that the heat supply efficiency is increased by stirring the liquid in the reactor.

It is desirable that the gas phase pressure in the lactide conversion reactor is set within a range in which lactide is efficiently distilled. If the pressure exceeds 100 Torr, lactide volatilization hardly proceeds, so that the pressure is preferably 100 Torr or less. Recommended. Even if the pressure is too low, lactic acid oligomers are easily entrained in lactide, the purity of lactide in the distillate decreases, and the cost of equipment and equipment for high decompression increases. Good.

Further, the temperature in the lactide conversion reactor is desirably set within a range in which isomerization is suppressed and lactide generation and vaporization are promoted. If the temperature is lower than 170 ° C., the reaction rate is slow and inefficient. When the temperature exceeds ℃, the isomerization proceeds easily, and the oligomer is easily vaporized and entrained in the lactide. Therefore, it is desirable to carry out the lactide formation reaction by controlling the liquidus temperature to 170 to 220 ° C.

The lactide catalyst is tin, zinc, lead,
A known lactide reaction catalyst such as an alkali metal and these compounds or a mixture thereof may be appropriately selected and used, and an appropriate catalyst concentration varies depending on the type of the catalyst, but an addition amount of 5% or less is sufficient. For example, in the case of tin octylate as a typical lactide catalyst, 100
It may be used in an amount of about 0 to 20000 ppm.

The provision of the external circulation type heat exchanger 12 makes it possible to supply a large amount of heat which is difficult to heat by liquid heating in the lactide-forming reactor 13, and to add heat necessary for the lactide-forming reaction and lactide vaporization. Can be. Liquid in the lactide reactor 13 is fed to the outer circulation type heat exchanger 12 at a predetermined flow rate by the reaction liquid circulating pump P _5, it is possible to maintain the liquid circulating in the reaction system, a large volume processing It can be (but not the reaction liquid circulation pump P ₅ is required if the liquid is circulated by thermosiphon effect).

C. Lactide Recovery Step In FIG. 2, 15 is a reflux condenser, 16 is a lactide condenser, and 17 is a lactide extraction tank.

The vaporized lactide (containing a small amount of oligomer components) reaches the reflux condenser 15 through the upper piping of the lactide reactor 13.

The reflux condenser 15 may be a shell / tube type heat medium circulating heating type heat exchanger.
By introducing a heating medium at 80 ° C., lactide vapor is passed, and the lactic acid oligomer and catalyst can be liquefied and returned to the lactide reactor 13. The lactide vapor that has passed through the reflux condenser 15 is cooled and condensed by the lactide condenser 16, discharged through the lactide extraction tank 17 to the outside of the system, and becomes product lactide. The lactide condenser 16 is supplied with a refrigerant having a predetermined temperature capable of liquefying (but not solidifying) lactide vapor. In addition, the outer shell of the lactide condenser 16, the lactide extraction tank 17, and the pipe connecting these are maintained in a molten state of lactide by an electric heater or a heating medium circulation system with a jacket structure (the liquid state is maintained without vaporization / solidification). Keep) heated to the temperature.

The external circulation heat exchanger 12, the lactide reactor 13, the reflux condenser 15, the lactide condenser 16
And lactide extraction tank 17 and the like is reduced in pressure by the vacuum pump VP _2, is controlled to a predetermined pressure by the pressure regulating valve V _2.

Reference numeral 18 denotes a lactide trap for liquefying a small amount of lactide vapor leaking from the lactide condenser 16 to prevent the lactide vapor from flowing to a vacuum pump. If two lactide traps 18 are provided side by side as shown in FIG. 3 (18a, 18b), the flow path is alternately switched, and when one is clogged, the other is used, which hinders continuous operation. Lactide can be produced without causing any problems, which is desirable. The lactide traps 18a and 18b may be supplied with cooling water when solidifying lactide, and steam may be introduced when the solidified lactide is melted.

By performing the above operations, heat can be efficiently given to the reaction solution to promote the lactide formation reaction and the vaporization of the produced lactide, and the catalyst or oligomer other than lactide can be introduced into the reaction system. By collecting lactide while returning, high-purity lactide can be stably and continuously obtained.

The top and bottom of the lactide reactor 13, a circulation pipe between the external circulation heat exchanger 12 and the lactide reactor 13, the lactide reactor 13 and the reflux condenser 1
5, reflux condenser 15 and lactide condenser 1
The piping between 6 is protected at a predetermined temperature by an electric heater, a jacket or the like.

In place of the reflux cooler 15, a tray or packing is incorporated in the lower part, and an upper / lower two-stage concentrator comprising a shell / tube type heat medium circulation heating type heat exchanger in the upper part. The use of is preferred because the gas-liquid contact efficiency increases.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention, and any design changes based on the gist of the preceding and following aspects will be described. Are included within the technical scope of

[0044]

EXAMPLE Lactide was continuously produced under the conditions shown in Table 1 using a lactide production apparatus having the apparatus configuration shown in FIG. 2 and using tin octylate as a lactide-forming catalyst. Table 1
And in the columns of the reactor volume and the liquid temperature indicate, respectively: an oligomerization reactor, and an auxiliary oligomerization reactor.

The purity, optical purity and water content of the obtained lactide were measured, the average molecular weight and water content of the oligomer obtained in the oligomerization step were examined, and the lactic acid concentration of distilled water was measured. The results are shown in Table 2.

[0046]

[Table 1]

[0047]

[Table 2]

No. Examples 1 to 3 are Examples of the present invention, all of which have high purity and high optical purity, and have been operated continuously for 200 hours. However, uniform lactide could be produced stably.

No. No. 4 is a comparative example in which the gas phase pressure in the oligomerization reactor was too high, in which the average molecular weight of the oligomer was small and the purity of the obtained lactide was low.
No. No. 5 is a comparative example in which the liquid temperature in the lactide conversion reaction step was too high and the gas phase pressure was too high, and both the purity and the optical purity of lactide were low.

[0050]

According to the present invention, the lactide is produced continuously from lactic acid for a long period of time on the assumption that the amount of the catalyst used for lactide formation is minimized. Thus, it is possible to provide an apparatus and a method capable of obtaining a high-purity product lactide.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a conventional lactide manufacturing apparatus.

FIG. 2 is a schematic explanatory view showing a typical example of an apparatus for producing lactide according to the present invention.

FIG. 3 is a schematic explanatory view showing an apparatus configuration when two lactide traps are arranged in parallel.

[Description of Signs] 1 Raw material tank 2 Preheater 3 Flash valve 4 Distillation tower 5 Oligomerization reactor 6 Capacitor 7 Distillate discharge tank 8 External circulation type heat exchanger 9 Auxiliary oligomerization reactor 10 Stirrer 11 Vapor tube 12 External circulation heat exchanger 13 Lactidation reactor 14 Stirrer 15 Reflux cooler 16 Lactide condenser 17 Lactide extraction tank 18 Lactide trap 19 Lactide tank 21 Lactic acid oligomer tank 22 Reaction vessel 23 Condenser 24 Lactide receiver

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Koji Yamamoto 1-5-5 Takatsukadai, Nishi-ku, Kobe City Inside Kobe Research Institute, Kobe Steel Ltd. (72) Takaho Fujiura 1 Takatsukadai, Nishi-ku, Kobe-shi 5-5-5 Kobe Steel, Ltd.Kobe Research Institute

Claims (2)

[Claims] An apparatus for continuously producing lactide, comprising: a lactide conversion reactor for depolymerizing lactic acid oligomers; and a condenser for condensing the evaporated lactide, wherein reflux cooling is provided between the lactide conversion reactor and the condenser. A continuous production apparatus for lactide characterized by being provided with a vessel. 2. A method for producing lactide, wherein lactic acid is heated and polymerized to form a lactic acid oligomer, and then the lactic acid oligomer is heated and depolymerized in the presence of a catalyst to produce lactide. Is continuously supplied to the oligomerization reactor, and the lactic acid is heated and concentrated and dehydrated to continuously produce lactic acid oligomers; and the liquid is continuously sent from the oligomerization reactor. A lactide-forming step in which lactic acid oligomers are depolymerized in the presence of a catalyst in a lactide-forming reactor to continuously produce lactide; and lactide distilled from the lactide-forming reactor is liquefied by a condenser to form lactide. A lactide recovery step for continuously recovering the lactide, wherein the oligomerization step, the lactide conversion step, and the lactide recovery step are connected in series. , When distilling the lactide in the lactide recovery step, continuous process lactide, characterized in that for returning the reflux condenser oligomers and catalyst entrained with to perform.