JP3464905B2 - Continuous production apparatus and continuous production method of lactide - Google Patents

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 continuous production method for lactide, and more specifically, a series of steps for producing lactide by depolymerizing lactide from lactic acid as a raw material to lactic acid oligomer. The present invention relates to an apparatus and a method for performing the same.

[0002]

BACKGROUND OF THE INVENTION Lactide is a raw material for polylactic acid, which is a biodegradable polymer. Polylactic acid is used in the medical field because it is not only decomposed in the living body but also has excellent mechanical properties, etc., and it is decomposed by microorganisms in the natural environment. Demand for lactide is increasing more and more, as it is expected to expand to various industrial uses and consumer uses.

As a method for producing the above-mentioned lactide, there is a method described in JP-A-7-138253.
(1) heating and concentrating lactic acid, (2) dehydrating the concentrated lactic acid under reflux to form a lactic acid oligomer, and (3) depolymerizing the lactic acid oligomer in the presence of a catalyst and distilling it off. A method has been proposed in which the steps are performed in batches.
However, in the above method, the temperature control in the reflux step is 135 ° C.
It is necessary to gradually increase the temperature in stages from 1 to 150 ° C and 160 ° C, and the operation is complicated, and
Since it is a batch system, there are problems that it is inefficient, the energy cost is large, and it is difficult to process a large amount. Furthermore, the composition in the reactor changes over time, so that the quality (particularly lactide purity) is unstable. Was pointed out.

As a method for producing lactide continuously instead of a batch method, a method of continuously oligomerizing lactic acid is disclosed in JP-A-9-316180, and a lactic acid oligomer is continuously produced as lactide. A method of doing so is disclosed in Japanese Patent Laid-Open No. 9-110859. Therefore,
It is considered possible to consistently and continuously produce lactide from lactic acid via a lactic acid oligomer by combining both methods.

FIG. 1 shows a lactide production apparatus described in Japanese Patent Laid-Open No. 9-110859, wherein 21 is a lactic acid oligomer tank, 22 is a reaction vessel, 23 is a condenser, and 24 is a lactide receiver. . According to the above apparatus, the lactic acid oligomer stored in the lactic acid oligomer tank 21 is supplied to the reaction container 22 by the transfer pump P, and the electromagnetic valve is opened to allow the catalyst loading of 5 to 25% by weight of the catalyst loading hopper H. The lactic acid oligomer is heated in the reaction vessel 22 introduced from the above and kept 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.

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

Further, according to the above-mentioned method, the catalyst concentration is extremely high at 5.1 to 25.0% (conventionally, it was 0.
5 to 2.0% by weight), since the catalyst is mixed in the product lactide, the quality is deteriorated, the catalyst removal step is indispensable, and the product lactide obtained by the above method has 1
A lactic acid oligomer of about 0% was also contained, and improvement in purity was also desired.

[0008]

SUMMARY 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 should be minimized, and that the catalyst should be continuously used for a long period of time. Can produce lactide from lactic acid,
Moreover, it is an object of the present invention to provide an apparatus and a method capable of obtaining a highly pure product lactide.

[0009]

The continuous production apparatus of the present invention, which has solved the above-mentioned problems, is a continuous lactide production apparatus having a lactide-forming reactor for depolymerizing lactic acid oligomers and a condenser for condensing evaporated lactide. And
The gist is that a reflux condenser is provided between the lactide reaction reactor and the condenser.

Further, the continuous production apparatus of the present invention which has solved the above-mentioned problems, produces lactic acid oligomer by heat-polymerizing lactic acid, and then heats and depolymerizes the lactic acid oligomer in the presence of a catalyst to form lactide. A method for producing a lactide to be produced, which comprises continuously supplying a raw material mainly composed of lactic acid to an oligomerization reactor and continuously producing a lactic acid oligomer by dehydrating while concentrating the lactic acid under heating, The lactic acid oligomer continuously fed from the oligomerization reactor is depolymerized in the presence of a catalyst in the lactide reaction reactor to continuously produce lactide, and a lactide reaction is performed to distill the lactic acid oligomer from the lactide reaction reactor. The extracted lactide has a lactide recovery step of continuously recovering lactide by liquefying it with a condenser, and the above-mentioned 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 large amount of catalyst used. As a result of the present inventors' investigation of the reason why there is no such reason, in the above-mentioned method, even if a large amount of the catalyst is used, the catalyst is discharged along with the lactide vapor along with the lactide vapor in the lactide production process. I got the knowledge. Therefore, as a result of the inventors' earnest studies, if a reflux condenser is provided between the lactide reactor and the condenser, and the catalyst vaporized with lactide is refluxed together with the lactic acid oligomer,
It was found that the lactide-forming reaction could be carried out for a long period of time with a small amount of catalyst, and by doing so, it was possible to consistently and continuously produce high-purity lactide from lactic acid via a lactic acid oligomer. .

The continuous lactide production method and apparatus according to the present invention will be described in detail below with reference to FIG. The method for producing lactide according to the present invention is characterized by consistently and continuously producing 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,
It can be roughly divided into (B) lactide conversion step and (C) lactide recovery step.

A. 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 extraction tank, and 8 is an external circulation. Type heat exchanger, 9 is an auxiliary oligomerization reactor, 10 is a stirrer, 11
Indicate vapor tubes, respectively.

The lactic acid stored in the raw material tank 1 is sent to the preheater 2 by the raw material feed pump P ₁ , heated to a predetermined temperature, and then supplied to the distillation column 4 through the flash valve 3.
The distillation column 4 is disposed above the oligomerization reactor 5, and the lactic acid flowing down to the oligomerization reactor 5 is heated in the reactor and dehydrated and condensed to become a lactic acid oligomer.
Incidentally, the raw material lactic acid generally contains about 10% of water, and water is generated even in the dehydration condensation (oligomerization) reaction. These water become steam by heating and rise together with the vapor of lactic acid and reach the distillation column 4 in which a tray or packing is incorporated, and the vapor of lactic acid is liquefied in the distillation column 4 and falls to the oligomerization reactor. The water vapor returned to 5 while passing through the distillation column 4 is guided to the condenser 6. The water vapor is cooled and condensed by the condenser 6 to which a coolant such as cooling water is supplied, and is discharged to the outside of the system through the distillate water withdrawing tank 7. Further, a part of the water sent from the condenser 6 to the distillate extraction tank 7 is refluxed to the top of the distillation column 4 by the pump P ₂ , in order to control the temperature of the top and improve the separation efficiency of water and raw material components. Used.

The heater provided in the oligomerization reactor 5 may be a heating medium circulating heating system having a jacket structure or an electric heater heating system, and the liquid heating system of the preheater 2 is not limited to this. , Shell / tube type heating medium circulation heating, electric heater heating, etc. can be adopted.

In this way, the water contained in the raw materials and the water content 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 content is removed from the oligomerization reactor 5. Since it is removed outside the system, the raw materials from the feed system and the unreacted raw materials in the vapor are concentrated and stored in the oligomerization reactor, the concentration of the raw materials in the oligomerization reactor is enhanced, and the condensation reaction causes oligomerization. Progresses.

The oligomerization reactor 5 is in communication with the auxiliary oligomerization reactor 9 via the external circulation heat exchanger 8. The external circulation type heat exchanger 8 and the auxiliary oligomerization reactor 9 have a heater (heat medium circulation type heater having a jacket structure, electric heater, etc.), and the internal liquid temperature thereof is set in the oligomerization reactor. The temperature of the liquid is kept higher than the liquid temperature of (1) to ensure the flow of liquid from the oligomerization reactor to the auxiliary oligomerization reactor and the external circulation heat exchanger. In addition, it is desirable to dispose a stirrer 10 in the auxiliary oligomerization reactor 9 to stir the liquid in the container to improve heat supply efficiency (however, the viscosity of the liquid in the auxiliary oligomerization reactor 9 is low. , If the liquid circulates in the vessel by natural convection, the agitator is not required).

Moisture and a part of lactic acid or lactic acid oligomer in the reaction liquid circulating through the external circulation heat exchanger 8 and the auxiliary oligomerization reactor 9 are vaporized by heating by the external circulation heat exchanger 8 and vaporized vapor. Is returned to the distillation column 4 through a vapor pipe 11 provided at the upper part of the auxiliary oligomerization reactor. Moisture 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 material and the oligomer are circulated through the oligomer reactor 5, the external circulation heat exchanger 8 and the auxiliary oligomerization reactor 9, and the water is distilled from the top of the distillation column to concentrate and dehydrate the raw material. A condensation (oligomerization) reaction proceeds.

Further, a fixed 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 pipe at the bottom, and is sent to the preheater 2 together with the raw material and sent to the flash valve 3. After that, it is again supplied to the distillation column 4.

The distillation column 4, the oligomerization reactor 5, the external circulation heat exchanger 8, the auxiliary oligomerization reactor 9 and the like are vacuum pumps VP ₁ arranged downstream of the condenser 6.
The pressure is reduced by, and the pressure is adjusted to a constant pressure by the pressure adjusting valve V ₁ .

It is desirable that the gas phase pressure in the oligomerization reactor is set within a range in which water can be distilled off without being accompanied by lactic acid or oligomers.
Lactic acid accompanies distilled water, resulting in raw material loss, while 200T
If it exceeds orr, water cannot be distilled off sufficiently and the molecular weight of the oligomer does not increase, so 30 Torr or more and 200 Tor
It is desirable to be r or less.

The liquidus temperature in the oligomerization reactor is
It is desirable to set it in a range that suppresses optical isomerization of lactic acid and promotes dehydration / condensation reaction. Below 150 ° C, the reaction rate is slow and inefficient, while above 200 ° C, optical isomerization proceeds. Since lactic acid easily accompanies distilled water, it is recommended to control the oligomerization reaction at 150 ° C or higher and 200 ° C or lower.

Further, the top and bottom of the oligomerization reactor 5 and the auxiliary oligomerization reactor 9, the distillation column 4, the preheater 2
Subsequent raw material supply pipes, liquid feed pipes from the bottom of the oligomerization reactor 9, liquid feed pipes from the oligomer circulation pump P ₃ ,
The circulation pipe between the external circulation heat exchanger 8 and the auxiliary oligomerization reactor 9, the suction and discharge pipes of the oligomer feed pump P ₄ , and the vapor pipe are preferably heated and kept warm by an electric heater or a jacket.

Thus, the oligomerization reactor according to the present invention is equipped with a distillation column, and the reflux operation is performed in this distillation column to concentrate lactic acid and to dehydrate the reaction system. As a result, water (water contained in raw material 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. You can Also, the content of lactic acid (= raw material loss) in distillate water 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 an advantage that the energy cost is small.

The oligomerization reactor according to the present invention is an external circulation heating type flash distillation can, and the raw material lactic acid is continuously supplied to the reactor by a flash operation, and the reaction liquid 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 can be promoted, the reactor residence time required for oligomerization (reaching a sufficient average molecular weight) can be shortened, and the energy cost can be 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 with liquid heating in the oligomerization reactor 5 and the auxiliary oligomerization reactor 8, and to add the amount of heat required for the dehydration condensation reaction. be able to. Further, due to the thermosiphon effect, it is possible to obtain an ideal circulation state between the external circulation heat exchanger 8 and the auxiliary oligomer reactor 9 for reaction operation / dehydration operation. Therefore, by providing the external circulation heat exchanger 8, large capacity processing (mass production) becomes possible.

The oligomer produced by the above polymerization reaction system is extracted at a predetermined liquid feed rate by the oligomer feed pump P4 and supplied to the reaction system of the next step, the lactide conversion step.

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

The oligomer fed by the oligomerization feed pump P ₄ is sent to the lactide reaction reactor 13 via the external circulation heat exchanger 12. In addition, the lactide-forming catalyst is preliminarily adjusted 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 reaction reactor 13 each have a heater (heat medium circulation type heater having a jacket structure, an electric heater, etc.), and by heating the liquid, the lactide is heated. The chemical reaction and the vaporization reaction of lactide proceed. Further, a stirrer 14 is provided in the lactide reaction reactor 13, and the heat supply efficiency is increased by stirring the liquid in the reactor.

Further, the gas phase pressure in the lactide reaction reactor is preferably set in a range in which lactide is efficiently distilled, and when it exceeds 100 Torr, volatilization of lactide is difficult to proceed, and therefore, it should be 100 Torr or less. Recommended. Even if the pressure is too low, lactic acid oligomers are easily entrained in the lactide, the purity of lactide in the distillate is lowered, and the equipment / equipment cost for dealing with high pressure reduction increases. Good.

Furthermore, it is desirable to set the temperature in the lactide reactor to a range that suppresses isomerization and promotes lactide production and vaporization. If the temperature is lower than 170 ° C., the reaction rate is slow and inefficient. When the temperature exceeds ℃, isomerization is likely to proceed and the lactide is easily vaporized with the oligomer. Therefore, it is desirable to control the liquid phase temperature to 170 ° C or higher and 220 ° C or lower to carry out the lactide reaction.

The lactide-forming catalyst includes tin, zinc, lead,
It may be used by appropriately selecting from known lactide-forming reaction catalysts such as alkali metals and these compounds or mixtures thereof, and an appropriate catalyst concentration varies depending on the kind of the catalyst, but an addition amount of 5% or less is sufficient, For example, in the case of a typical tin octylate as a lactide-forming catalyst, 100
It suffices to use approximately 0 to 20000 ppm.

By providing the external circulation type heat exchanger 12, it is possible to supply a large amount of heat, which is difficult with liquid heating in the lactide reaction reactor 13, and to add the amount of heat necessary for the lactide reaction and the vaporization of lactide. You can The liquid in the lactide reaction reactor 13 is sent to the external circulation type heat exchanger 12 at a predetermined flow rate by the reaction liquid circulation pump P ₅ , and liquid circulation can be maintained in the reaction system, so that large-capacity treatment is possible. It is possible (however, if the liquid circulates due to the thermosiphon effect, the reaction liquid circulation pump P ₅ is not necessary).

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 component) reaches the reflux condenser 15 through the upper pipe of the lactide reaction reactor 13.

As the reflux condenser 15, a shell / tube type heat medium circulation heating type heat exchanger may be used.
By introducing the heating medium at 80 ° C., the lactide vapor can be passed, and the lactic acid oligomer and the 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 and is discharged to the outside of the system through the lactide withdrawing tank 17 to become a product lactide. The lactide condenser 16 is supplied with a refrigerant having a predetermined temperature capable of liquefying (but not solidifying) the lactide vapor. Further, the outer shell of the lactide condenser 16, the lactide extraction tank 17, and the pipes connecting them are kept in a molten state of lactide by an electric heater or a heating medium circulating system with a jacket structure (the liquid state is not vaporized and solidified). Keep) heated to temperature.

An external circulation heat exchanger 12, a lactide reaction reactor 13, a reflux condenser 15, a lactide condenser 16 are provided.
The lactide withdrawal tank 17 and the like are decompressed by a vacuum pump VP ₂ and controlled to a predetermined pressure by a pressure adjusting valve V ₂ .

Further, reference numeral 18 is a lactide trap, which prevents a small amount of lactide vapor leaking from the lactide condenser 16 from being liquefied and flowing to a vacuum pump. If two lactide traps 18 (18a, 18b) are installed side by side as shown in FIG. 3, the flow paths are alternately switched, and when one is blocked, the other is used, which hinders continuous operation. It is desirable that lactide can be produced without causing any trouble. In addition, cooling water may be caused to flow into the lactide traps 18a and 18b when the lactide is solidified, and steam may be introduced when the lactide captured by being solidified and melted.

By carrying out the above operation, heat can be efficiently applied to the reaction solution to promote the lactide reaction and the vaporization of the produced lactide, and the catalyst or oligomer which is a component other than lactide can be introduced into the reaction system. High-purity lactide can be stably and continuously obtained by collecting lactide while returning.

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

Further, in place of the reflux condenser 15, a tray and a packing are incorporated in the lower part, and a shell / tube type heat medium circulation heating type heat exchanger in the upper part is a demultiplexer having an upper and lower two-stage structure. Is preferable because the gas-liquid contact efficiency is increased.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any modification of the design of the present invention can be made in view of the gist of the preceding and the following. It is included within the technical scope of.

[0044]

Example Using the lactide production apparatus having the apparatus configuration shown in FIG. 2, tin octylate was used as the lactide-forming catalyst to continuously produce lactide under the conditions shown in Table 1. Table 1
In the column of reactor volume and liquid temperature, and indicate the: oligomerization reactor, and auxiliary oligomerization reactor, respectively.

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 the distilled water was measured. The results are shown in Table 2.

[0046]

[Table 1]

[0047]

[Table 2]

No. Nos. 1 to 3 are Examples of the present invention, and all have high purity and optical purity, and even after continuous operation for 200 hours, uniform lactide can be stably produced.

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

[0050]

EFFECTS OF THE INVENTION Since the present invention is constituted as described above, lactide is continuously produced from lactic acid over a long period of time on the premise that the amount of catalyst used for lactide formation is minimized. It has become possible to provide an apparatus and a method capable of obtaining a highly pure product lactide.

[Brief description of drawings]

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

FIG. 2 is a schematic explanatory view showing a representative example of a lactide manufacturing apparatus according to the present invention.

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

[Explanation of symbols] 1 raw material tank 2 preheater 3 flash valve 4 distillation tower 5 Oligomerization reactor 6 capacitors 7 Distilled water extraction tank 8 External circulation heat exchanger 9 Auxiliary oligomerization reactor 10 stirrer 11 vapor tubes 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 Capacitor 24 lactide receiver

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Yamamoto 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi Kobe Steel Works, Ltd., Kobe Research Institute (72) Inventor Kiho Fujiura 1-chome, Takatsuka, Nishi-ku, Kobe No. 5 No. 5 Kobe Steel Works, Ltd., Kobe Research Institute (56) References Tokukaihei 7-503490 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C08G 63/00 -63/91

Claims (2)

(57) [Claims] And 1. A lactide reactor for depolymerizing lactic acid oligomers, a continuous manufacturing apparatus of lactide having a capacitor for condensing lactide evaporation, during the lactide reactor and the capacitor, lactide
The catalyst that vaporizes with the gas is refluxed with the lactic acid oligomer.
A continuous lactide production apparatus, characterized in that a reflux condenser for the purpose is provided. 2. A method for producing lactide, wherein lactic acid is heat-polymerized to form a lactic acid oligomer, and then the lactic acid oligomer is heated in the presence of a catalyst to depolymerize to produce lactide. The raw material to be supplied is continuously supplied to the oligomerization reactor, and the oligomerization step of continuously producing lactic acid oligomer by dehydrating the lactic acid while heating and concentrating it is continuously fed from the oligomerization reactor. In the lactide reaction reactor, the lactic acid oligomer is depolymerized in the presence of a catalyst to continuously generate lactide, and the lactide distilled from the lactide reaction reactor is liquefied by a condenser to lactide. A lactide recovery step for continuously recovering the lactide. The oligomerization step, the lactide conversion step, and the lactide recovery step are combined. , 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.