JPH10101777A - Recirculation method and apparatus in direct polycondensation apparatus for polylactic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover and recirculate components to be recirculated, including a lactide and low-molecular lactic acid compds. and excluding water produced as a by-product, in a high yield to thereby increase the ultimate mol.wt. of polylactic acid. SOLUTION: Components to be recirculated, including a lactide and low- molecular lactic acid compd., and water are produced as by-products in producing polylactic acid by polycondensing a lactic and monomer in a batch-type polycondensation vessel 1. Vapors of these by-products are cooled in a crystallization and recirculation apparatus 7 to a crystallization temp. of the lactide. Though the lactide and the low-molecular lactic acid compds. crystallize, the apparatus 7 is not blocked since it has a self-cleaning function due to two screws 9 installed in a cylinder 8 in such a manner that the flights 9a engage with each other. Thus, the components to be recirculated except water produced as a by-product can be recovered in a high yield and forcibly recirculated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a direct polycondensation reaction apparatus for producing polylactic acid by subjecting a lactic acid monomer to a polycondensation reaction under a reduced-pressure atmosphere. The present invention relates to a reflux method and apparatus in a polylactic acid direct polycondensation reaction apparatus for refluxing a component to be refluxed such as a molecular compound into a polycondensation reaction tank.

[0002]

2. Description of the Related Art Polylactic acid can be easily and chemically recycled into cyclic dimer lactide by hydrolysis or thermal decomposition, and has excellent transparency and rigidity. It is used as a material for general molded articles as well as medical materials because it is possible to design grades. As a method for producing the polylactic acid, there are known methods roughly described below.

(A) When a hydroxycarboxylic acid is dehydrated and polycondensed to produce a high molecular weight polyhydroxycarboxylic acid, by-product water and low-boiling components produced by the dehydration polycondensation reaction can be efficiently removed from the reaction system. To remove well, a large amount of solvent is added to the reaction system, and the by-product water and low-boiling components are separated by vacuum suction together with the by-product water to the outside of the reaction system using a vacuum pump or the like. (See JP-A-6-298913).

(B) As shown in FIG. 4, a weight-average molecular weight of 1,
Lactic acid prepolymer P ₁ of 000～5,000 as a raw material, the weight-average molecular weight of 30,000 by continuous polycondensation reactor 110 having the same configuration as screw extruders used the lactic acid prepolymer P ₁ in plastic extrusion ~ 1
A method for producing 00,000 polylactic acid.

[0005] The method, the lactic acid prepolymer P ₁ is introduced from the hopper 111, there is from the catalyst supply port 114 with the addition of catalyst to dehydration polycondensation reaction under a reduced pressure atmosphere of nitrogen gas, by-product Vacuum pump 1
The vacuum pipe 1 is connected to a plurality of vent ports 115 provided in the cylinder 112 by vacuum suction.
Then, components to be refluxed such as lactide and low molecular weight compounds of lactic acid other than by-product water are cooled and liquefied by the reflux pipe 116 and refluxed into the continuous polycondensation reaction device 110. The solvent that is not liquefied in the reflux pipe 116 is cooled and liquefied in the cooling pipe 117 disposed on the downstream side and is returned through the trap 118, and the by-product water is discharged out of the system through the by-product water trap 119.

On the other hand, the produced polylactic acid P ₂ is
3 and stored in the storage tank 121.

[0007]

However, among the above-mentioned prior arts, (a) uses a large amount of solvent, so that a large amount of solvent remains in the product polylactic acid, and it is difficult to separate and purify the product. However, there is a problem that the cost is high. In addition, since the polycondensation reaction of polylactic acid has a reaction equilibrium between the monomer and the prepolymer, and between the lactide and the polymer, the prepolymer or the polymer must be maintained unless the lactide concentration in the reaction solution exceeds a certain level. However, there is a problem that the rate of the depolymerization reaction becomes high, and the increase in the amount does not progress.

On the other hand, (b) uses an inexpensive lactic acid prepolymer, so that the production cost of polylactic acid can be reduced. However, when the cooling temperature of the reflux tube is lowered to completely liquefy components to be refluxed such as lactide (which may contain a solvent in some cases) and low molecular weight compounds of lactic acid in the reflux tube, lactide (freezing point: 95-98 ° C) crystallizes and becomes blocked in the reflux tube. Conversely, if the cooling temperature of the reflux tube is set higher to prevent lactide crystallization, the cooling effect at the center of the reflux tube will be worsened, and lactide and lactic acid low molecular weight compounds passing through the center will be deteriorated. The components to be refluxed are not liquefied but are scattered outside the system as vapor, which not only does not increase the reaction rate, but also makes it difficult for the reaction equilibrium to shift to the chain-bonding reaction side (polymer side), and the reached molecular weight Cannot be made too large.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is directed to cooling a component to be refluxed containing a low molecular weight compound of lactide and lactic acid excluding by-produced by-product water. It is an object of the present invention to realize a reflux method and apparatus in a direct polycondensation reaction apparatus for polylactic acid, which is capable of increasing the attainable molecular weight of polylactic acid, which is a product, by recovering and recovering the product in high yield and refluxing. It is.

[0010]

In order to achieve the above object, a reflux method in a polylactic acid direct polycondensation reaction apparatus of the present invention comprises producing a polylactic acid by subjecting a lactic acid monomer to a polycondensation reaction under a reduced-pressure atmosphere. Lactide excluding by-produced water in a polycondensation reaction tank in a direct polycondensation reaction apparatus, a method for recovering a component to be refluxed containing a low molecular weight compound of lactic acid, and refluxing the component to the polycondensation reaction tank. By cooling the vapor generated by the polycondensation reaction to a temperature at which lactide is crystallized in a crystallization reflux apparatus having a self-cleaning function, the components to be refluxed excluding the by-product water produced as a by-product are The present invention is characterized in that lactide and the low molecular weight compound of lactic acid are crystallized by cooling and forcibly refluxed.

In the above method, the pressure of the reduced-pressure atmosphere is preferably in the range of 0.1 to 50 mmHg.

The reflux apparatus in the polylactic acid direct polycondensation reaction apparatus of the present invention includes a polycondensation reaction tank for performing a polycondensation reaction of a lactic acid monomer under a reduced pressure atmosphere, and a by-product water by-produced in the polycondensation reaction tank. A reflux device for recovering and refluxing components to be refluxed including lactide except for lactic acid and low molecular weight compounds of lactic acid,
In a reflux device in a polylactic acid direct polycondensation reactor provided with a decompression device connected to a downstream side of the reflux device, a cylinder having a cooling device for cooling the reflux device to a temperature at which the lactide crystallizes. And a crystallization recirculation device having a self-cleaning function provided with two screws arranged in a state in which flights are engaged with each other in the cylinder, and a rotation driving means for rotating each of the two screws. It is characterized by the following.

Further, the polycondensation reaction tank may be a batch type polycondensation reaction tank for performing a polycondensation reaction of lactic acid monomer by batch processing.

Further, the polycondensation reaction tank is provided with a cylinder having a plurality of heating zones, a screw disposed in the cylinder, and a rotation driving means for rotating the screw. It may be composed of a continuous polycondensation reaction tank for performing a condensation reaction.

[0015]

[Function] Since a crystallization reflux apparatus having a self-cleaning function is used, even if a low molecular weight compound of lactide and lactic acid is crystallized by cooling to a temperature at which lactide crystallizes, the crystallized lactide and lactic acid The components to be refluxed such as lactide and low molecular weight compounds of lactic acid, excluding by-product water, can be completely recovered without being clogged with the low molecular weight compound and forcedly refluxed into the polycondensation reaction tank. As a result, the reaction equilibrium in the polycondensation reaction shifts to the chain bonding reaction side (polymer side), and the reached molecular weight increases.

In the present invention, the pressure of the reduced-pressure atmosphere is 0.1.
When the content is within the range of 1 to 50 mmHg, the components to be refluxed including the low molecular weight compounds of lactide and lactic acid can be refluxed even if recovered in high yield.

If the pressure of the reduced pressure atmosphere is lower than 0.1 mmHg, the cost of equipment is increased in order to make the structure of the polycondensation reaction tank and the exhaust capacity of the pressure reducing means withstand this, and the components to be refluxed are formed as by-products. Spatters out of the system with water. Conversely, if it is higher than 50 mmHg, the efficiency of removing by-product water decreases.

[0018]

Embodiments of the present invention will be described with reference to the drawings.

First, a first embodiment of a reflux apparatus in a polylactic acid direct polycondensation reaction apparatus according to the present invention will be described.

As shown in FIG. 1, a reflux device E ₁ in the direct polycondensation reactor polylactic acid, a covered batch polycondensation reaction tank 1 with jacket outer peripheral surface is not shown, batch polycondensation reaction tank A crystallization reflux device 7 whose discharge port side is connected to one vent port 6, and a knockout drum 15 connected to a suction port 12 on the opposite side of the crystallization reflux device 7 via a suction pipe 14 a. , A cooling trap 17 connected to the knockout drum 15 via a suction pipe 14b, and the cooling trap 17 is connected to a suction side of the vacuum pump 14 via a suction pipe 14c.

That is, downstream of the crystallization reflux unit 7, a pressure reducing means comprising a knockout drum 15, a cooling trap 17, and a vacuum pump 14 is connected in sequence.

The crystallization reflux device 7 includes a cylinder 8 having a cooling jacket (not shown), which is a cooling means for cooling to a temperature at which lactide is crystallized, provided on the outer wall surface, and a flight 9a meshed with the cylinder 8. Two screws 9 arranged in a state, and a motor 1 serving as a rotation driving means for rotating the two screws 9 in the same direction or in the opposite direction.
0, and a connection portion 11 between each screw 9 and the motor 10 is provided with a Wilson seal, a labyrinth seal, a mechanical seal, and the like, so that airtightness is maintained in a reduced pressure state. Thus, a cooling medium whose temperature is adjusted to a temperature at which lactide is crystallized from the inlet 13a is introduced into the cooling jacket and circulated and supplied so as to be discharged from the outlet 13b, thereby cooling the lactide and lactic acid. Even if the molecular compound is crystallized, the vent port 6 is formed by the two screws 9 rotating in the same or opposite directions.
The batch-type polycondensation reaction tank 1 is conveyed toward the reactor and is not clogged with the crystallized low molecular weight compounds of lactide and lactic acid, and collects the components to be refluxed other than the by-product water without leaving it.
It can be forcibly refluxed inside. That is, the crystallization reflux device 7 has a self-cleaning function.

The batch type polycondensation reaction tank 1 circulates and supplies a heat medium whose temperature has been adjusted to a predetermined temperature from the introduction path 1b of the jacket and discharges the heat medium from the discharge path 1c. The temperature can be maintained, and the agitator 2 composed of a ribbon-shaped agitating blade 2a integrally provided on the rotating shaft 2b is rotated by the motor 5 to agitate the lactic acid monomer, the catalyst and the solvent that have been introduced. It is configured so that a lactic acid monomer can undergo a polycondensation reaction by batch processing.

At the bottom of the batch type polycondensation reaction tank 1, there is provided a discharge path 1a interposed with an on-off valve 4 and a gear pump 3, so that polylactic acid can be discharged after the completion of the batch processing.

Next, a reflux method in the polylactic acid polycondensation reaction apparatus using the reflux apparatus in the polylactic acid polycondensation reaction apparatus shown in FIG. 1 will be described.

The vacuum pump 14 was started to reduce the pressure inside the system, and the lactic acid monomer, solvent, and catalyst were introduced into the batch type polycondensation reaction tank 1, and the temperature was adjusted to a predetermined temperature through the introduction path 1 b of the jacket. While the inside of the batch type polycondensation reaction tank 1 is maintained at a predetermined temperature by circulating and supplying the heating medium, the polycondensation reaction is performed by stirring with the stirrer 2.

By the above, polylactic acid is produced,
At the same time, the components to be refluxed, including the low molecular weight compounds of lactide and lactic acid, and by-product water are vaporized under reduced pressure, and are sucked into the crystallization reflux device 7.

The components to be refluxed contained in the vapor sucked into the crystallization reflux device 7 are cooled to a temperature at which lactide is crystallized by the cooling medium introduced from the inlet 13a of the cylinder jacket. And
Some of the low molecular weight compounds of lactide and lactic acid are crystallized and the remaining components are liquefied, and are conveyed to the vent port 6 by the two screws 9 rotating in the same or opposite directions, and are reliably conveyed to the vent port 6. From 6, it is forcibly refluxed to the batch type polycondensation reaction tank 1. On the other hand, the vapor of the by-product water is sucked into the cooling trap 17 through the knockout drum 16a and liquefied, and is discharged into the discharge passage 15 through the on-off valves 16 and 18, respectively.
a and 17a are discharged out of the system.

In this embodiment, the components to be refluxed, including the low molecular weight compounds of lactide and lactic acid, excluding the by-produced by-product water, are crystallized and liquefied in the crystallization reflux apparatus 7 without being stirred, and are batched. Forcibly refluxed into the polycondensation reaction tank 1, the polycondensation reaction shifts to the polymer side and the molecular weight is 5
0000 or more polylactic acid can be produced with good yield.

As is apparent from the above description, the present invention is directed to a method in which lactide, a component to be refluxed containing a low molecular weight compound of lactic acid, and a vapor comprising by-product water are produced in the batch type polycondensation reaction tank 1. It is led to a crystallization reflux device, in which the lactide is cooled to a temperature at which lactide is crystallized, and lactide and a low-molecular compound of lactic acid are crystallized, thereby removing lactide and low-molecular compounds of lactic acid excluding by-product water. The components to be refluxed, including, are forcibly refluxed.

In particular, when batch processing is carried out in a batch type reaction tank, a solvent is added to facilitate the removal of by-product water and to efficiently remove heat of reaction from the reaction solution, and the components in the reaction solution are uniformly mixed. It is desirable that the mixture be brought into contact with and stirred.
This solvent has a boiling point of 15 depending on the reaction temperature and the degree of vacuum.
It is good to use what is 0-350 degreeC, for example, diphenyl ether.

Next, a modification of the reflux device in the polylactic acid polycondensation reaction apparatus according to the present invention will be described.

As shown in FIG. 2, the reflux apparatus E ₂ in the polylactic acid direct polycondensation reaction apparatus of this modification is different from the crystallization reflux apparatus 7 of the first embodiment shown in FIG. A crystallization reflux apparatus 20 having substantially the same configuration is connected in reverse to the vent port 6 of the batch-type polycondensation reaction tank 1, and a discharge port 12b for discharging a component to be refluxed in addition to the suction port 12a is provided on the discharge side. The discharge port 12b is connected to a heat retaining tank 21 via a discharge pipe 24a, and the heat retaining tank 21 and a reflux port provided with a check valve 23 of the batch type polycondensation reaction tank 1 are connected to a plunger pump. Reflux path 24b with 22 interposed
Are connected by. Except for this, the second embodiment is the same as the first embodiment shown in FIG.

In this modification, lactide crystallized,
The low molecular compound of lactic acid is supplied from the discharge port 12b to the discharge pipe 24.
a, is temporarily stored in a heat retaining tank 21 kept at a predetermined temperature, and is forcibly refluxed into the batch type polycondensation reaction tank 1 by a plunger pump 22 via a reflux path 24b.

It should be noted that the heat retaining tank 21 is
A heat medium whose temperature has been adjusted to a predetermined temperature is circulated and supplied to a jacket (not shown) provided on the outer wall surface of the outer surface so as to be introduced from the introduction path 21a and discharged from the discharge path 21b, thereby maintaining the predetermined temperature. It is configured to be able to.

Next, a description will be given of a second embodiment of a reflux apparatus in a polylactic acid direct polycondensation reaction tank according to the present invention.

As shown in FIG. 3, the reflux device E ₃ in the polylactic acid direct polycondensation reaction apparatus of the present embodiment includes a cylinder 51 having a plurality of heating zones, and a screw 52 disposed in the cylinder 51. A rotational driving means including a motor 56 with a speed reducer 55 for rotating the screw 52, and a labyrinth seal, a mechanical seal, and the like are provided at a connecting portion 57 between the speed reducer 55 and the screw 52 to reduce the pressure in the decompressed state. Is provided with a continuous polycondensation reaction tank 50 configured to maintain airtightness.

The cylinder 51 is provided with a plurality of vent ports 51a to 51c (three in this embodiment) spaced from each other, and each of the vent ports 51a to 51c is provided with the first embodiment shown in FIG. The crystallization reflux units 53a to 53c having the same configuration as that of the crystallization reflux unit 7 are connected to each other, and the suction ports 58a to 5c on the opposite side to the discharge side of each of the crystallization reflux units 53a to 53c.
8c, the knockout drum 1 is connected via the suction line 14a.
5 is connected. Other than this, the first type shown in FIG.
Since the configuration is the same as that of the embodiment, the same portions are denoted by the same reference numerals, and description thereof will be omitted.

In this embodiment, since the continuous type polycondensation reaction tank 50 has a strong mixing / stirring power and a surface renewing power with respect to the material such as lactic acid monomer supplied from the supply port 54 of the cylinder 51, Polylactic acid can be continuously produced without adding lactic acid.

In this embodiment, the single-screw screw 52 is replaced with a continuous-type polycondensation reaction tank 50 by a twin-screw extruder having two screws in which a flight 52a is engaged with each other in a cylinder. It goes without saying that a continuous polycondensation reaction tank similar to that of the apparatus can be used.

[0041]

Since the present invention is configured as described above, it has the following effects.

Among the steam generated in the polycondensation reaction tank, components to be refluxed such as lactide and low molecular weight compounds of lactic acid, excluding by-product water, are cooled to the temperature at which the lactide crystallizes, and lactide and lactic acid are removed. Can be crystallized, and these components to be refluxed can be recovered in a high yield and refluxed into the polycondensation reaction tank without dissipating outside the system. As a result, the polycondensation reaction shifts to the chain bonding reaction side (polymer side), and polylactic acid having a molecular weight of 50,000 or more can be produced with high yield.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a first embodiment of a reflux apparatus in a polylactic acid direct polycondensation reaction apparatus according to the present invention.

FIG. 2 is an explanatory view showing a modification of the first embodiment shown in FIG.

FIG. 3 is an explanatory view showing a second embodiment of a reflux apparatus in the polylactic acid direct polycondensation reaction apparatus according to the present invention.

FIG. 4 is an explanatory diagram of a conventional polylactic acid polycondensation reaction apparatus.

[Explanation of symbols]

 Single batch type polycondensation reaction tank 1a, 15a, 17a Discharge path 2 Stirrer 3 Gear pump 4,16,18 On-off valve 5,10,56 Motor 6,51a, 51b, 51c Vent port 7,20,53a, 53b, 53c Crystallization reflux device 8,51 Cylinder 9,52 Screw 9a, 52a Flight 11,57 Connecting part 12,12a, 58a, 58b, 58c Suction port 14 Vacuum pump 14a, 14b, 14c Suction line 15 Knockout drum 17 Cooling trap 50 Continuous polycondensation reaction tank

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Noriaki Hashimoto 1-6-1, Funakoshi Minami, Aki-ku, Hiroshima City, Hiroshima Prefecture Inside Japan Steel Works, Ltd. (72) Inventor Kunihiko Koyanagi 1-chome, Funakoshi-minami, Aki-ku, Hiroshima City, Hiroshima Prefecture 6-1 Japan Steel Works Co., Ltd. (72) Inventor Takeshi Fukushima 1-6-1, Funakoshi Minami, Aki-ku, Hiroshima-shi, Hiroshima Japan Steel Works Co., Ltd.

Claims (5)

[Claims] 1. A lactide excluding by-product water produced in a polycondensation reaction tank in a direct polycondensation reaction apparatus for producing a polylactic acid by subjecting a lactic acid monomer to a polycondensation reaction under reduced pressure atmosphere.
A method for recovering a component to be refluxed including a low molecular weight compound of lactic acid and refluxing the recovered component to the polycondensation reaction tank, wherein lactide is crystallized in a crystallization reflux device having a self-cleaning function for vapor generated by the polycondensation reaction. By cooling to a temperature at which the by-product is produced, the components to be refluxed except the by-produced by-product water, the lactide and the low-molecular compound of the lactic acid are cooled and crystallized, thereby forcibly refluxing. Reflux method in a direct polycondensation reaction apparatus for polylactic acid. 2. The pressure of the reduced pressure atmosphere is 0.1 to 50 mm.
The reflux method in a polylactic acid direct polycondensation reactor according to claim 1, wherein the temperature is within the range of Hg. 3. A polycondensation reaction tank for subjecting a lactic acid monomer to a polycondensation reaction under a reduced pressure atmosphere, and reflux containing a low molecular weight compound of lactide and lactic acid excluding by-product water produced in the polycondensation reaction tank. A reflux device for recovering and refluxing the components, and a reflux device in a polylactic acid direct polycondensation reactor equipped with a decompression means connected downstream of the reflux device, wherein the reflux device crystallizes the lactide. A cylinder having cooling means for cooling to a temperature, two screws arranged in the cylinder in a state in which flights are engaged with each other, and rotation driving means for rotating the two screws respectively. A reflux device in a polylactic acid direct polycondensation reaction device, comprising a crystallization reflux device having a self-cleaning function. 4. A reflux device in a polylactic acid direct polycondensation reaction apparatus according to claim 3, wherein the polycondensation reaction tank comprises a batch type polycondensation reaction tank for performing a polycondensation reaction of lactic acid monomers by batch processing. . 5. The polycondensation reaction tank includes a cylinder having a plurality of heating zones, a screw disposed in the cylinder, and a rotation driving unit for rotating the screw. The reflux apparatus in a polylactic acid direct polycondensation reaction apparatus according to claim 3, comprising a continuous polycondensation reaction tank for performing a polycondensation reaction.