WO2015080402A1 - Method for producing lactide using multi-tube falling film reactor and agitated thin film reactor - Google Patents

Abstract

The present invention relates to a method for producing lactide comprising an evaporation concentration step (first step) for a lactic acid solution, a lactic acid oligomerization step (second step) and a lactic acid oligomer depolymerization step (third step), wherein the second step is performed in a multi-tube falling film reactor and the third step is sequentially performed in the multi-tube falling film reactor and an agitated thin film reactor. According to the present invention, a multi-tube falling film device in which a liquid-phase reactant with a high viscosity is equally distributed to individual vertical pipes of the multi-tube falling film device even at a low pressure, and 100% of the liquid-phase reactant flows only along inner walls of the vertical pipes in a uniform thickness in a circumferential direction, is applied to the all of the above-mentioned unit steps, thereby continuously producing lactide with high yield rate and high efficiency. Furthermore, the depolymerization step is performed sequentially in the multi-tube falling film reactor and the agitated thin film reactor, thereby rarely having a deposition of carbon material even after extended operation periods, thus an operation pause period for device cleansing can be extended so as to provide excellent process efficiency and economic benefits.

Description

Lactide Manufacturing Method Using Bundled Falling Reactor and Agitated Thin Film Reactor

The present invention relates to a method for producing lactide from lactic acid using a multi-tube falling film reactor and a thin film reactor, and more particularly, to prepare a lactic acid oligomer from lactic acid, and to sequentially produce a multi-tube falling film reactor and a stirred thin film reactor. It relates to a method for continuously producing lactide by depolymerization using

Lactide, a cyclized compound of lactic acid dimer, is used as a raw material for producing polylactide, a biodegradable polymer. Due to its molecular structure, the method of directly cyclizing water by taking water out of two lactic acid molecules is the simplest method. However, in general, lactic acid is prepolymerized into an oligomer having a weight average molecular weight of 1,000-3,000, followed by a metal catalyst. Lactide is prepared by depolymerization again under the following. In more detail about the above-described depolymerization reaction, when the lactic acid oligomer is heated to a temperature of 200 ° C. or higher in the presence of a catalyst, the molecular chain ends of the oligomer proceed with cyclization and chain cutting by back-biting. The lactide is obtained by a reactor in which lactide is produced and separated.

The first polymerization step, lactic acid polymerization, is a kind of esterification reaction in which the carboxyl group of the lactic acid molecule reacts with alcohol groups of other lactic acid molecules to give water and increase molecular weight. Therefore, the use of acid catalysts such as sulfuric acid as in the general esterification reaction promotes the reaction. At high temperatures, the acid catalyzes the reaction without an external catalyst, and the reaction reaches equilibrium unless the by-product water is removed. .

The difference from the low-molecular esterification reaction is that the rate of water removal dominates the reaction rate because the viscosity of the reaction system continues to increase as the degree of polymerization increases. Therefore, a falling film evaporator or a stirred thin film evaporator capable of rapidly evaporating water may be used as a reactor for lactic acid small polymerization. Membrane evaporators, in which liquid flows down in the form of a thin film of less than 1 mm in thickness, are extremely fast in mass transfer and heat transfer.

The depolymerization reaction of the second reaction step, lactic acid oligomer, can also be carried out particularly advantageously in the reactor of the falling film evaporator type. This is because not only can the lactide generated by the reaction be separated quickly to prevent the reverse reaction, but also it can supply the enormous heat of reaction necessary for cutting the chain of the oligomer at a high speed.

However, the conventional stirring thin film evaporator has a limitation in manufacturing a large-diameter device due to the eccentricity of the rotor blade is not used for bulk chemical production. In addition, the conventional falling film evaporator in which the spray nozzle is generally used has a problem in spraying a high viscosity material such as lactic acid oligomer of the present invention with a nozzle at a high temperature.

USP 5,274,073, USP 6,229,046 and their related patents, USP 6,875,839, and USP 8,053,584, evaporative concentration of aqueous lactic acid solution, preparation of oligomer from concentrated lactic acid and / or ring to obtain lactide from oligomer Although the use of various types of evaporators and / or reactors in the cyclizing depolymerization stage is disclosed, reactors of the falling film evaporator type are used only in the concentration of lactic acid and / or in the cyclic depolymerization stage.

In general, the falling film evaporator requires the liquid to flow down the inner wall surface (or outer wall surface) of the 100% vertical pipe to a uniform thickness in the circumferential direction and flow down to increase the efficiency of the device. Spraying high-viscosity liquids with nozzles in a tube falling film evaporator to distribute them evenly in individual tubes is difficult, even with high energy. Even if sprayed, a significant amount of droplets will fall downward through the space inside the tube rather than the inner wall of the vertical tube, reducing the evaporation efficiency. In order to reprocess droplets falling into the space, the prior art uses a circulation pump to circulate the liquid below the evaporator upwards.

In a falling film evaporator in which liquid is dispersed by a spray nozzle, the liquid is concentrated in a so-called circulation operation method. However, there are quite a few evaporation and chemical reactions that require a one-pass operation, which requires one pass through the evaporator and then withdraws into the product. For example, evaporative concentration and chemical reaction of materials with low thermal stability. The depolymerization reaction of the lactic acid oligomer is also a reaction requiring one pass operation rather than a cyclic operation. When the lactic acid oligomer is heated to a high temperature, depolymerization reaction in which molecular weight decreases due to cyclization and chain cleavage to lactide, and on the contrary, polymerization reaction in which coke-like substances in which oxygen and hydrogen atoms are released continue to react to generate carbon. This occurs competitively, if circulating them, carbon will deposit in the metal tube of the falling film apparatus. Therefore, the conventional nozzle spray falling film apparatus is not suitable for use in the depolymerization reaction of lactic acid oligomer.

Meanwhile, Korean Patent Application No. 10-2012-0090098 (filed Aug. 17, 2012, unpublished) discloses the production of lactide, which performs a depolymerization step as well as a depolymerization step of lactic acid and a depolymerization step in a multi-drop falling-film reactor. It describes a method, in which a single liquid dispersing device is installed at the top of each vertical tube of the bundle tube membrane reactor to allow liquid to flow only on the wall so that no reactant falls into the space inside the vertical tube. A falling film reactor is used in which the reactants flow evenly around the inner wall of the vertical tube.

Using the bundle downfall reactor including the liquid dispersing device described above can produce lactide from lactic acid in high yield, high efficiency and continuously, but after the depolymerization reaction is carried out continuously, the carbonaceous material in the lower portion of the reactor bundle It was found that it was calm. Since the deposition of such carbonaceous materials interferes with the heat transfer required for depolymerization and the movement of the falling film, the yield of lactide is decreased, and a new method for solving such problems is needed.

[ Prior Art Document ]

[ Patent Literature ]

(Patent Document 1) USP 5,274,073

(Patent Document 2) USP 6,229,046

(Patent Document 3) USP 6,875,839

(Patent Document 4) USP 8,053,584

It is an object of the present invention to apply a new multi-layer drop-down membrane reactor in which a liquid flows to 100% of the entire surface of a unit process in a lactide manufacturing method consisting of evaporative concentration, small polymerization and depolymerization of an aqueous solution of lactic acid, and thus high conversion rate of lactic acid and While achieving a high yield of lactide, it is also an object of the present invention to provide a method for preventing the deposition of carbonaceous materials that may occur when performing such continuous depolymerization for a long time.

The inventors have carried out the condensation step of lactic acid and / or the cyclic depolymerization step using a multi-tube descending membrane reactor equipped with a liquid dispersing device that allows liquid to flow only on the wall on top of each individual vertical tube. By carrying out the depolymerization step using a single tube falling-film reactor and a stirred thin film reactor, not only can the lactide be produced from lactic acid in high yield, high efficiency and continuously, but also if the depolymerization reaction is performed continuously for a long time The present invention was found to be able to prevent the deposition of carbonaceous material at the bottom of the present invention.

According to the present invention, in the process for producing lactide through evaporation concentration of lactic acid aqueous solution, small polymerization of lactic acid and depolymerization of lactic acid oligomer, the number of individual pieces of the multi-tube falling film device at low pressure of the liquid reactant having high viscosity Lactide can be produced in high yield, high efficiency and continuously by applying a multi-tube descending film device distributed evenly to the straight pipe and flowing down to only 100% vertical pipe inner wall surface and down to circumferentially even thickness. In addition, since the depolymerization process is performed sequentially in a multi-tube falling film reactor and a thin film reactor, there is almost no deposition of carbonaceous material even during long-term operation, thus prolonging the operation stop period for cleaning the device, thereby providing excellent process efficiency and economic efficiency. Do.

1 is a schematic diagram of a multiple-tube drop membrane reactor of the present invention,

Figure 2 is a perspective view showing the combination and arrangement of the support plate, liquid dispersion plate, gas-liquid dispersion tube and gas hole of the present invention,

3 is AA ′ Cross-sectional view of the bundle tube falling membrane reactor,

4 is BB ′ A cross-sectional view of the bundle tube falling membrane reactor viewed in the direction of the arrow.

In the first object of the present invention is a method for producing lactide comprising the step of evaporation and concentration of lactic acid aqueous solution (step 1), the lactic acid small polymerization step (step 2) and the lactic acid oligomer depolymerization step (step 3), step It is to provide a method for producing lactide, characterized in that 1 and / or step 2 is carried out in a bundle downfall reactor, and step 3 is carried out in a bundle downfall reactor and stirred thin film reactor.

According to one embodiment of the invention, the preparation method comprises the following steps (1) to (3):

(1) lactic acid aqueous solution was concentrated by evaporation at a pressure of 50-700 torr and a temperature of 80-120 ° C;

(2) lactic acid concentrated to 1% or less of water content was polymerized into oligomer at a temperature of 190 to 205 ° C; And

(3) The obtained lactic acid oligomer is depolymerized at a pressure of 5-100 torr and a temperature of 200-270 ° C.

According to a preferred embodiment of the invention, the depolymerization step may comprise the following steps 3 and 4:

(3-1) The obtained lactic acid oligomer is fed into a bundle tube falling-film reactor to depolymerize firstly until the yield of lactide is approximately 70-90%, preferably 75-85%,

(4) supplying the unreacted oligomer obtained in the first depolymerization to the stirred thin film reactor to depolymerize secondly.

According to a preferred embodiment of the present invention, the method further comprises the step of purifying the mixture which is discharged in the vapor phase in the depolymerization step and comprises a lactide, water, lactic acid and an oligomer thereof, which is the following step 5, step 6 and / or step 7 may include:

(5) a first purification step of removing water and lactic acid from a mixture comprising water, lactic acid and oligomers thereof to obtain a mixture comprising lactide and oligomers,

(6) a second purification step of removing oligomers from the mixture comprising lactide and oligomer and obtaining lactide, and

 (7) a third purification step of separating and purifying meso-lactide, D- and / or L-lactide from lactide.

The present invention is explained in more detail below.

In the lactide manufacturing process of producing lactide through evaporative concentration of lactic acid solution, small polymerization and depolymerization of lactic acid, the evaporative concentration of step 1, the small polymerization of step 2 and the depolymerization of step 3 all have a mass transfer rate and a heat transfer rate. It is a physical and chemical process that requires the use of large falling film devices.

In step 1, when the water is evaporated by applying heat to the aqueous solution of lactic acid having a concentration of 20-90%, the viscosity of the lactic acid is gradually increased, so that the rate of water evaporation and the latent heat of evaporation energy are gradually reduced. As a result, it takes a long time to remove more than 99% of the water and concentrate it to a high concentration, so that a membrane reactor such as a falling-film evaporator can be used. In addition, in the small polymerization reaction of lactic acid of step 2 and the depolymerization reaction of step 3, since rapid mass transfer and heat transfer are required for rapid recovery of lactide, which is water and a product, which is a reaction by-product, a film-type reaction such as a falling-film evaporator It is preferable to use a device.

The inventors of the present invention provide a method for efficiently producing lactide using the bundle drop film device of FIG. 1 applicable to evaporative concentration of the lactic acid aqueous solution of step 1, small polymerization of lactic acid of step 2, and / or depolymerization of step 3. Was completed.

The multi-tube falling film reactor that can be used in the present invention is a reactor as shown in Figure 1, the name of the invention "manufacturing method for lactide using a multitude falling film reactor}" The reactor described in Korean Patent Application No. 10-2012-0090098 (filed August 17, 2012) filed as an example may be exemplified, which application is incorporated herein by reference.

1 illustrates a bundle tube falling membrane reactor that can be used in the present invention with reference to the bundle tube falling membrane reactor, the bundle tube 1 consisting of one or more vertical tubes and a jacket comprising the same ( 2), the upper bundle pipe flange 4a in which the upper end of the bundle pipe is inserted and welded, the lower bundle pipe flange 4b in which the lower end of the bundle pipe is inserted and welded, and the upper bundle pipe flange described above ( 4a) an upper blocking flange 6a positioned above and having a pupil 8 and a liquid inlet 7a, and a lower blocking flange 6b installed below the lower bundle tube flange 4b and having a liquid outlet 7b. ), And further, further comprises the following components:

A gas-liquid dispersion flange 5 provided between the above-mentioned upper bundle tube flange 4a and the upper shut-off flange 6a;

-Falling film formation having the same axis, diameter, number and arrangement as the bundle tube 1, which is inserted into the gas-liquid dispersion flange 5 and joined in the same manner as welding and has a gas hole 12 in the tube wall. Tube 11;

A support plate 10 which is installed to cover the upper end inlet of the above-described falling film forming tube 11 and in which the hole 14 is drilled so that the supplied liquid can flow into the falling film forming tube 11;

It is fixed to the lower end of the above-described support plate 10 is to be inserted into the inside of the above-described falling film generating tube 11, the liquid dispersion plate having a predetermined interval in the circumferential direction with the inner wall of the falling film generating tube (11) (9).

According to one embodiment of the invention, lactide may be prepared according to the following steps 1-7.

First, lactic acid is usually prepared in an aqueous solution of lactic acid content of 15-20% in a bioreactor. In some cases, lactic acid may be preconcentrated to lactic acid content of 50-70% and supplied to Step 1.

Step 1 is a step of evaporating the lactic acid aqueous solution. The lactic acid aqueous solution of 15-70% of lactic acid is concentrated by evaporation to a lactic acid content of 95-99% at a temperature of 100-120 ° C. and a pressure of 50-200 torr. , Preferably in a falling-film reactor.

Step 2 is a step of small polymerization of the oligomer from the evaporated lactic acid, in a falling film reactor at a temperature of 180-210 ° C., preferably 190-205 ° C. and a pressure of 200-760 torr, preferably 500-700 torr. Can be done. The resulting oligomers produced in the prepolymerization stage may generally have a weight average molecular weight of 1000-3000, specifically 1000-2000.

Step 3 is a step of first depolymerization of the small polymerized lactic acid oligomer, supplying the lactic acid oligomer to the bundle tube falling membrane reactor to perform the first depolymerization until the lactide yield is about 70-90%. The first depolymerization is generally carried out in the presence of a metal oxide catalyst, such as SnO, at a temperature of 220-270 ° C., preferably 230-250 ° C. and a pressure of 5-50 torr, preferably 5-30 torr. do. The yield of lactide can be adjusted by adjusting the oligomer dose and / or reaction temperature.

Products such as water, lactic acid, oligomer and lactide obtained in step 3, the lactide is discharged in the form of a vapor phase mixture is sent to the distillation column and purified as necessary, the unreacted oligomer is discharged to the bottom of the bundle tube Collected at the bottom of the reactor, and back to the membrane reactor of step 4.

Step 4 is a second step of depolymerization of the unreacted oligomer, the thin film reactor, preferably at a temperature of 230-270 ℃, preferably 240-260 ℃ and a temperature of 5-50 torr, preferably 5-30 torr It is carried out in a stirred thin film reactor. In the aforementioned stirring thin film reactor, the stirring blade has a vertical tilt angle of 80-90 degrees for smooth spreading and flowing down of high viscosity unreacted oligomers, and has an optimum thin film thickness of 1 mm or less and 1-20 minutes, preferably 1 The process can be carried out with an oligomer residence time of -10 minutes.

Products such as water, lactic acid, oligomers and lactide obtained in step 4, the lactide is discharged in the form of a vapor phase mixture is sent to the distillation column and purified as needed, unreacted oligomers, carbonaceous residues, etc. Collected to the bottom of the evaporator may be discharged out of the reactor. The above-mentioned carbonaceous residue includes a carbon deposit produced by repeating the polymerization reaction while hydrogen and oxygen atoms in the oligomer are disconnected.

Step 5 is a first purification step of lactide, and the vapor phase mixture discharged in steps 3 and 4 is distilled in a first distillation column at a temperature of 100-150 ° C. and a pressure of 5-30 torr, such as water, lactic acid, and the like. The mixture can be separated off to the top and a mixture such as lactide and oligomer can be recovered at the bottom.

Step 6 is a secondary purification step of lactide, in which the bottom product separated in step 5 is distilled, for example, in a second distillation column at a temperature of 100-160 ° C. and a pressure of 5-30 torr, so that the lactide is formed on the tower. It collect | recovers and mixtures, such as an oligomer, can be collect | recovered in a tower bottom.

Step 7 is a third purification step of the lactide, and the lactide mixture separated in step 6 is distilled, for example, in a third distillation column at a temperature of 100-160 ° C. and a pressure of 5-30 torr, and meso-lactide The column can be recovered and L- or D-lactide can be recovered at the bottom.

The distillation temperature and pressure for the purification are not critical and can be selected from conditions known in the art. Such distillation temperatures and pressures may be appropriately selected by those skilled in the art in view of other process environments or device conditions.

Hereinafter, the manufacturing process of lactide according to the present invention, specifically steps 1 to 3, can be described in more detail below, with reference to the carried out in the bundle tube reinforcement membrane and / or stirred thin-film reactor.

In the evaporation concentration step of step 1, the lactic acid aqueous solution is supplied to the falling film evaporator via a liquid inlet 7a attached to the upper blocking flange 6a. The feed flow can be adjusted by the pump as needed. The lactic acid aqueous solution entering the liquid inlet 7a flows into the hole 14 of the support plate through the cavity 8 between the upper shut-off flange 6a and the gas-liquid dispersion flange 5. The pupil 8 between the upper shut-off flange 6a and the gas-liquid dispersion flange 5 is a result of lathe machining of the lower face of the shut-off flange, and if the height is 2 mm or less, a high pressure is applied, whereas if it is 6 mm or more, it is detained. Since the amount of lactic acid aqueous solution is increased, it is preferable to process the bottom surface of the blocking flange so that the height of the pupil is 2-6 mm.

The liquid dispersing device is composed of a liquid dispersion plate 9 for controlling the direction of the flow so that the lactic acid aqueous solution flows to the inner wall surface of the falling film forming pipe 11 and the support plate 10 for supporting it. The liquid dispersion plate 9 is fixed to the support plate 10 by a fixing mechanism such as a bolt and is smaller than the inner diameter of the falling film generating tube 11 so as to be inserted into the falling film generating tube, and the supporting plate 10 is The diameter is larger than the inner diameter of the falling film generating pipe 11 is installed so as to cover the falling film generating pipe, the upper surface of the falling film generating pipe (11) to the depth forming a plane with the top surface of the gas-liquid dispersion flange (5) The structure is inserted. Since the liquid dispersing device comprising the support plate 10 and the liquid dispersing plate 9 fixed thereto is installed in a detachable manner in the falling film device, the liquid dispersing plate can be replaced as necessary.

One or more holes or grooves, that is, holes 14, having a diameter of 2-5 mm are drilled in the support plate 10 vertically. In order for the lactic acid aqueous solution to flow well into the falling film generating tube, four holes 14 are preferable as shown in FIG. 2. The lactic acid aqueous solution flowing through the hole 14 and reaching the upper surface of the liquid dispersion plate 10 flows horizontally to change the direction of movement downward at the circumferential end portion of the liquid dispersion plate, thereby to lower the film forming tube 11. Ride down the inner wall of the house.

The falling-film forming tube 11 is a short-length bundle tube arranged on the gas-liquid dispersion flange 5 in the same axis, the same diameter, the same number, and the same arrangement as the vertical tube 1 forming the bundle tube. A gas hole 12 through which water vapor or an external carrier gas can pass is drilled on the wall at a position lower than the liquid dispersion plate. The number of gas holes 12 drilled in the falling film generating tube is not particularly limited. However, more gas holes and larger diameters greatly affect the behavior of the liquid drop film, so the number of gas holes is less than 10 and the diameter is preferably 2-5 mm.

The falling film generating pipe 11 and the vertical pipe 1 are arranged so that the central axis is aligned so that the lactic acid aqueous solution flowing down the inner wall of the falling film generating pipe flows down to the wall surface of the vertical pipe without any obstacle. When the lactic acid solution flowing down the inner wall surface of the falling film enters the vertical bundle tube heated to 80-120 ° C., water vapor evaporates as water evaporates. At this time, the generated water vapor may be discharged to the lower portion of the bundle pipe, or may be discharged to the gas entrance 13a through the gas hole 12. The method of discharging the former to the bottom of the bundle tube is cocurrent operation because lactic acid and water vapor flow in the same direction in the vertical tube, and the method of discharging water through the latter gas entrance 13b is the opposite direction of the vapor and lactic acid. As it flows, it becomes reverse flow operation. The lactic acid aqueous solution evaporation apparatus of the present invention is capable of both cocurrent flow and countercurrent operation.

The vertical bundle tube 1 has its upper and lower portions fixed by welding to the bundle tube flanges 4a and 4b, and a jacket 2 is installed outside thereof so that the heat medium oil is provided in the space between the bundle tube and the jacket. By circulating, it is possible to heat the vertical bundle tube to a uniform temperature.

In Figure 3 is described that the four vertical pipes are installed, but the number of vertical pipes can be installed from one to several thousand depending on the supply amount of the lactic acid aqueous solution without particular limitation. This is because the bundle pipe falling film device of the present invention is relatively simple in structure and the bundle pipe and the gas and liquid dispersing device are separated, thereby enabling precise processing, respectively.

In particular, the upper part of the falling film evaporator composed of the upper shut-off flange 6a, the gas-liquid dispersion flange 5, and the upper bundle tube flange 4a can be precisely processed by a machine tool so that the liquid dispersion plate 9, the support plate 10, The falling film generating pipe (11) and the vertical pipe (1) can be produced to match the central axis. By this structure, the lactic acid aqueous solution flows uniformly in the inner wall of the 100% vertical tube and in the circumferential direction of the vertical tube, so that the water evaporates with the maximum efficiency retained by the apparatus.

The diameter of the liquid dispersion plate 9 inserted into the falling film generating tube 11 is an important variable for determining the distribution efficiency of the lactic acid aqueous solution to the individual vertical tubes. If the diameter of the liquid dispersion plate is too small and the gap between the inner wall of the falling film forming tube 11 and the side of the liquid dispersion plate is large, the pressure drop is small and the lactic acid aqueous solution flows only to a specific vertical tube without being evenly distributed among the multiple bundle tubes. May occur. On the contrary, the diameter of the liquid dispersion plate is similar to the inner diameter of the falling-film forming tube, so if the tolerance or gap is too small, the pressure drop increases, so that the effect of evenly distributing the lactic acid solution to the individual vertical tubes may be increased. In order to solve this problem, a pump having a large discharge pressure must be used. In the falling film evaporator of the present invention, the liquid dispersion plate can be attached and detached, and the pressure drop of the liquid dispersion plate can be arbitrarily adjusted by adjusting its diameter.

The optimum spacing between the falling film forming tube and the liquid dispersion plate is 0.05-2 mm, more preferably 0.1-1 mm, for evaporative concentration of the lactic acid aqueous solution.

The vertical tube may be advantageous in terms of evaporation rate at higher temperature, but at a high temperature of 120 ° C. or higher, water and lactic acid form an azeotropic point, resulting in a large amount of lactic acid loss. Thus, the temperature is lower than 120 ° C., more preferably 50-700. It is recommended to operate at 80-120 ℃ temperature under reduced pressure condition. The water content of the lactic acid aqueous solution may be any concentration between 20-90%. In evaporating water, the circulating operation of circulating the lactic acid aqueous solution under the less concentrated evaporator to the vertical bundle tube using a pump may be used, but considering the physical properties and evaporation rate of the lactic acid to be used for the small polymerization reaction, the number and drop of the bundle tube will be reduced. It may be more desirable to carry out in a single pass operation by simultaneously controlling the residence time of the membrane.

The lactic acid solution concentrated to 1% or less of the water content in the falling film evaporator is transferred to the small polymerization reactor to proceed with the small polymerization reaction of Step 2. The small polymerization reactor may use a multi-tube drop membrane device of a different type and / or length of the drop membrane evaporator and the vertical tube as the reactor.

The temperature of the prepolymerization reaction of step 2 is 170-210 ° C., and the optimum temperature is 190-205 ° C. At temperatures below 180 ° C., the reaction rate is too slow, while at temperatures above 210 ° C., depolymerization may occur and result in the loss of the desired molecular weight oligomer.

The small polymerization reaction of step 2 may be carried out at once by raising the temperature of the falling film reactor to 190-205 ° C., but the lactic acid having a concentration of 99% or more concentrated in the falling film evaporator is directly 190-205 ° C. at 80-120 ° C. When the reaction is performed by heating, the conversion of lactic acid is not high due to the azeotropic evaporation of lactic acid, so it is better to perform the reaction temperature in two stages. The rapid progress of the prepolymerization decreases the conversion rate because unreacted lactic acid is evaporated and lost together with the byproduct water.

Preferred stepwise small polymerization temperatures are (2-1) step 150-170 ° C. and step 2 190-205 ° C. That is, lactic acid of 80-120 ° C. or more concentrated in the falling film evaporator of step 1 was raised to 150-170 ° C. as a first step of 2 and reacted first in a falling film reactor having the above structure, ( As a step 2-2, the second reaction in another bundle tube falling-film reactor heated to 190-205 ° C. can produce a lactic acid oligomer having a desired molecular weight at a high conversion rate.

Here, when the catalyst of the depolymerization reaction of step 3 in advance in the lactic acid small polymerization reaction of step (2-1) and the small polymerization reaction proceeds, lactide as a final product can be more efficiently produced.

As catalysts for the depolymerization of lactic acid oligomers, metal compounds such as metal oxides, metal chlorides and metal organic compounds are generally used. When these catalysts are added to a high temperature liquid lactic acid oligomer, they react with the oligomer or lactic acid. Because of the conversion to metal lactate, the catalytic activity of the depolymerization reaction is similar to any compound used. This is because at high temperatures all take the form of liquid metal lactate. If the depolymerization catalyst is liquid at these reaction temperature conditions, there are several advantages over the solid particle phase. Since the catalyst is uniformly dispersed in the reactant in the size of molecular units, the efficiency of the catalyst is increased, and it is easy to transfer the reactant and the product to the pump. The only pump that can supply quantitative reactants at high temperature above 150 ℃ is gear pump. If the catalyst is solid rather than liquid, the gear pump cannot be used due to abrasion and damage.

According to the inventors, the metal compound catalyst of the depolymerization reaction of step 3 hardly reacts with the oligomer or more of the trimer even at a high temperature of 200 ° C. It is well converted to metal lactate in the conditions present to some extent. Therefore, when the small polymerization reaction of step 2 of the present invention is divided into steps (2-1) and (2-2) as described above, the depolymerization catalyst is added to step (2-1) in which lactic acid and water still exist. As a result, the metal powder or particulate catalyst can be converted into a liquid metal lactate catalyst.

In the small polymerization reaction of the steps (2-1) and (2-2), when the reaction is carried out under reduced pressure at atmospheric pressure or lower, the evaporation rate of the byproduct water increases, so that the oligomer can be produced more effectively. Suitable pressure ranges are 500-760 torr for the small polymerization of step (2-1) and 300-760 torr for the small polymerization of step (2-2). In the small polymerization reaction of step (2-1), if the pressure is 500 torr or less, the rate of evaporation of the monomer lactic acid is increased and the lactic acid conversion rate is lowered.

The optimum weight average molecular weight of the lactic acid oligomer prepared under the above temperature and pressure conditions is between 1000-3000. When the molecular weight of the oligomer exceeds 3000, the amount of carbon produced in the depolymerization step of step 3 increases, so that the yield of lactide decreases. Falls. It is more advantageous for the depolymerization reaction of step 3 that oligomers having a molecular weight of 1000-3000 have a narrow distribution rather than a wide molecular weight distribution. Therefore, it may be preferable to prepare the oligomer in the one-pass operation rather than the cyclic operation in which the molecular weight distribution becomes large in the small polymerization reaction of the steps (2-1) and (2-2). The residence time of the falling film in the single pass operation, that is, the reaction time can be controlled by the number and length of the vertical tubes and the supply speed of the reactants.

The oligomer obtained in the small polymerization reactor of step 2 may be transferred to a falling film depolymerization reactor by a pump to perform step 3.

Step 3 is a first depolymerization of the small polymerized lactic acid oligomer, wherein the lactic acid oligomer is fed to the bundle tube falling-film reactor, until a substantial portion of the lactic acid oligomer is converted to lactide, for example the conversion rate of the oligomer and / or Or the first depolymerization is carried out until the yield of lactide reaches at least about 60%, in particular at least 70%, preferably 75-85%. The first depolymerization is generally carried out at a temperature of 200-270 ° C., especially 220-270 ° C., preferably 230-260 ° C. and 5-100 torr, specifically 5-50 torr, preferably 5-30 torr. At a pressure of, it is carried out in the presence of a metal oxide catalyst such as tin oxide, such as any SnO. The yield of lactide can be adjusted by adjusting the oligomer dose and / or reaction temperature.

Products such as water, lactic acid, oligomer and lactide obtained in step 3, the lactide is discharged in the form of a vapor phase mixture is sent to the distillation column and purified as necessary, the unreacted oligomer is discharged to the bottom of the bundle tube Collected at the bottom of the reactor, and back to the membrane reactor of step 4.

Depolymerization of the lactic acid oligomer is a typical reaction requiring a single pass operation with a short residence time. If the residence time of the reactants is long, the amount of carbon produced is increased by thermal polymerization because carbon particles are deposited on the reactor wall if circulated through the reactor.

In step 3, since the viscosity of the lactic acid oligomer having a molecular weight of 1000-3000 has a range of 10-100 cP at 200 ° C, the interval between the inner wall of the falling film forming tube and the liquid dispersion plate is appropriately 0.1-1.5 mm. . By continuously supplying lactic acid oligomer to the liquid dispersing device as a pump and depolymerizing the oligomer in one pass operation, it is possible to disperse the high viscosity evenly on the inner wall surface of the 100% vertical tube even without applying high pressure. The lactide can be prepared.

Step 4 is a second step of depolymerization of the unreacted oligomer, the thin film reactor, preferably at a temperature of 230-270 ℃, preferably 240-260 ℃ and a temperature of 5-50 torr, preferably 5-30 torr It is carried out in a stirred thin film reactor.

In the aforementioned stirring thin film reactor, the stirring blade has a vertical tilt angle of 80-90 degrees for smooth spreading and flowing down of high viscosity unreacted oligomers, and has an optimum thin film thickness of 1 mm or less and 1-20 minutes, preferably 1 The process can be carried out with an oligomer residence time of -10 minutes.

Products such as water, lactic acid, oligomers and lactide obtained in step 4, the lactide is discharged in the form of a vapor phase mixture is sent to the distillation column and purified as needed, unreacted oligomers, carbonaceous residues, etc. Collected to the bottom of the evaporator may be discharged out of the reactor. The above-mentioned carbonaceous residue includes a carbon deposit produced by repeating the polymerization reaction while hydrogen and oxygen atoms in the oligomer are disconnected.

In one embodiment of the present invention, the thin film reactor used in step 4 may be a thin film reactor or evaporator of the type which can be used for film or thin film formation, for example a disk thin film reactor, a stirred thin film reactor. And the like can be mentioned. Specifically, an agitated thin film reactor capable of dropping a high viscosity material to form a film may be mentioned, and the above-described agitator blade may be disposed to be inclined with respect to the horizontal, for example, 45 degrees or more, specifically 45 It may have an inclination angle of -90 degrees, especially 60 degrees or more, preferably 80 to 90 degrees.

The method of transferring the mixture containing the unreacted oligomer from the bundle tube falling-film reactor of step 3 to the stirred thin film reactor of step 4 is not particularly limited, and pump transfer and the like can be mentioned.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the scope of the present invention is not limited to the embodiment.

Example 1 Lactic Acid Concentration (Step 1) and Small Polymerization (Step 2)

A 20% aqueous L-lactic acid solution was supplied to a 4-bundle falling film evaporator containing 4 tubes of 300 cm in length and 2 cm in diameter at a flow rate of 25 ml / min. The water was evaporated and concentrated to 98%, followed by 300 cm in length. In order to prepare an oligomer, the lactic acid was polymerized by transferring the 18-bundle tube falling-film small polymerization reactor in which 18 tubes having an inner diameter of 2 cm were embedded.

The 4-bundle falling film evaporator was operated at a temperature of 120 ° C., a pressure of 50 torr, and an 18-bundling falling film reactor at a temperature of 205 ° C. and a pressure of 600 torr, respectively. The 4-bundle evaporator and the 18-bundle reactor were equipped with a circulation pump to transfer the liquid dropped to the lower portion to the upper portion where the dispersion plate was installed at a rate of 200 ml / min for circulation.

The residence time of lactic acid in the 4-bundle evaporator is 40 minutes. The residence time of the oligomer in the 18-bundle small polymerization reactor is 60 minutes. As a result of analyzing the product flowing out from the 18-bundle reactor, the weight average molecular weight of the prepared lactic acid oligomer was 1,350, and the mass balance was 99% when the mass balance was calculated after the reaction was completed.

Comparative Example 2: Depolymerization (Step 3) in a Single Tube Falling Film Reactor

1 wt% of tin octoate catalyst was added to the lactic acid oligomer having a molecular weight of 1350 prepared in Example 1, and stirred well, followed by 18-bundle containing 18 tubes of 300 cm in length and 2 cm in inner diameter at 15 ml / min flow rate. L-lactide was prepared by depolymerizing the lactic acid oligomer while feeding into a falling-film depolymerization reactor. The temperature of the 18-bundle depolymerization reactor was maintained at 250 ° C. and the pressure at 10 torr. The mixture vapor consisting of lactide, water, lactic acid and oligomers discharged to the top of the depolymerization reactor was converted to a liquid using a cooler, and the tin-based catalyst and oligomer residues dropped to the bottom of the reactor were stored in a tank connected to the bottom of the reactor. After 5 hours depolymerization, the yield of lactide including meso- and L-lactide was 93%.

Comparative Example 3 Depolymerization (Step 3) in a Single Tube Falling Film Reactor

The lactic acid oligomer having a molecular weight of 1350 prepared in Example 1 was subjected to a depolymerization reaction for 80 hours in the same manner as in Example 2. As a result of measuring the yield of lactide over time, the yield of lactide gradually decreased with time as 20 hours 93%, 40 hours 92%, 60 hours 90%, 80 hours 88%. It was found that carbonaceous material was deposited at the bottom of the depolymerization reactor bundle tube after the reaction. Since the deposition of such carbonaceous materials impeded the heat transfer and the movement of the falling film required for depolymerization, the yield of lactide seemed to drop gradually.

Example 4 First Depolymerization (Step 3) and Second Depolymerization (Step 4)

1 wt% of tin octoate catalyst was added to the lactic acid oligomer having a molecular weight of 1350 prepared in Example 1, and stirred well, followed by 18-bundle falling-film depolymerization containing 18 tubes having a length of 300 cm and an inner diameter of 2 cm at a flow rate of 25 ml / min. After supplying to the reactor, one-step depolymerization reaction was carried out, and the depolymerized oligomer and catalyst mixture withdrawn from the bottom of the reactor were transferred to a stirred thin film reactor having an area of 1.0 m ² , and the two-step depolymerization reaction was performed for a total of 80 hours.

The temperature of the 18-bundle falling membrane reactor was maintained at 250 ° C., the pressure was 10 torr, and the temperature of the stirred thin film depolymerization reactor was 250 ° C., and the pressure was 10 torr. The stirring blade of the two-stage thin-film depolymerization reactor was installed with a vertical angle of about 88 degrees so that the lactic acid oligomer was forced to the bottom.

As a result of measuring the yield of lactide with time, the yield of lactide in the 1st stage 18-bundle drop membrane reactor remained constant up to 80 hours, and the overall yield of lactide including the 2nd stage thin film reactor was also maintained. It was constant at 96%. After the reaction for 80 hours, the inner wall surface of the bundle tube of the 18-bundle drop membrane reactor was clean without depositing carbon material.

Example 5

Cholactide prepared in Example 4 was purified to high purity by performing three-step distillation in a packed column type distillation apparatus under the distillation conditions of Table 1 below.

Table 1

Zolactide was supplied to the first stage distillation column at a flow rate of 20 ml / min, and continuously distilled for 8 hours to obtain L-lactide having a purity of 99.9% at the bottom of the three stage distillation apparatus.

The present invention can be used industrially in the manufacturing industry of lactide and the industry using lactide.

[ Description of the Code ]

1: vertical tube or bundle 2: jacket

4a: upper bundle tube flange 4b: lower bundle tube flange

5: gas-liquid dispersion flange

6a: upper shutoff flange 6b: lower shutoff flange

7a: liquid inlet 7b: liquid outlet

8: pupil 9: liquid dispersion plate

10: support plate 11: falling film generating tube

12: gas hole 13a, 13b: gas entrance

14: Hole 16: O-ring

Claims (12)

1.       A method for producing lactide comprising an evaporative concentration step of lactic acid aqueous solution (step 1), a small polymerization step of lactic acid (step 2), and a depolymerization step of lactic acid oligomer (step 3), wherein step 2 is a multi-drop falling-film reactor Step 3 is carried out in the manufacturing method of the lactide, characterized in that carried out sequentially in the bundle-down film reactor and stirred thin film reactor.
2. The method of claim 1, wherein the manufacturing method comprises the following steps (1) to (3):

   (1) step 1 by evaporating the lactic acid aqueous solution at a pressure of 50 ~ 700 torr and a temperature of 80 ~ 120 ℃;

   (2) polymerizing lactic acid concentrated to 1% or less in water to oligomer at a temperature of 190 to 210 ° C;

   (3) depolymerization of the obtained lactic acid oligomer at a pressure of 5 ~ 100 torr and a temperature of 200 ~ 270 ℃ 3.
3. The method of claim 1, wherein the depolymerization step comprises the following steps 3 and 4.

   (3) feeding the lactic acid oligomer obtained in step 2 to the bundle tube falling-film reactor to depolymerize first until the yield of lactide is 70-90%,

   (4) supplying the unreacted oligomer obtained in the first depolymerization to the thin film reactor to depolymerize secondarily.
4.       The method of claim 1, wherein the manufacturing method further comprises the step of purifying the mixture discharged in the vapor phase in the depolymerization step and containing a mixture of lactide, water, lactic acid and oligomers thereof.
5. The method of claim 1, wherein the purification method comprises one or more of the following steps (5), (6) or (7):

   (5) a first purification step of removing water and lactic acid from a mixture comprising water, lactic acid and oligomers thereof to obtain a mixture comprising lactide and oligomers,

   (6) a second purification step of removing oligomers from the mixture comprising lactide and oligomer and obtaining lactide, and

    (7) a third purification step of separating and purifying meso-lactide, D- and / or L-lactide from lactide.
6.       The method of claim 1, wherein the small polymerization step 2 is the first small polymerization at a pressure of 500 ~ 760 torr and a temperature of 150 ~ 170 ℃ 2-1 and at a pressure of 300 ~ 760 torr and a temperature of 190-210 ℃ The process of dividing into 2-2 of the second small polymerization, and the above-described steps 2-1 and 2-2 is a method for producing lactide, characterized in that proceeded sequentially in a separate bundle tube falling membrane reactor, respectively.
7.       7. The method of claim 6, wherein the depolymerization catalyst is added in step 2-1.
8.       The method according to any one of claims 1 to 4, wherein the liquid dispersing device which allows liquid to flow only on the wall is installed on top of each vertical tube of the bundle tube falling membrane reactor, so as to drop into the space inside the vertical tube. A method for producing lactide, wherein the steps are performed using a falling-film reactor in which there is no reactant and the reactant evenly flows down circumferentially on the inner wall of the vertical tube.
9. 9. The bundle bundle according to claim 8, wherein the bundle drop membrane reactor includes a bundle tube (1) consisting of one or more vertical tubes, a jacket (2) outside the bundle tube, and an upper end in which the upper end of the bundle tube is inserted and welded. A tube flange 4a, a lower bundle tube flange 4b in which the lower end of the bundle tube described above is inserted and welded, and positioned above the upper bundle tube flange 4a described above, having a pupil 8 and a liquid inlet 7a. Lactide comprising an upper blocking flange 6a, a lower blocking flange 6b installed below the lower bundle tube flange 4b and having a liquid outlet 7b, and further comprising the following components: Manufacturing Method:

   A gas-liquid dispersion flange 5 provided between the above-mentioned upper bundle tube flange 4a and the upper shut-off flange 6a;

   A falling film generating tube 11 having the same axis, diameter, number and arrangement as the bundle tube 1, which is inserted and welded to the gas-liquid dispersion flange 5 described above and which has a gas hole 12 in the tube wall;

   A support plate 10 which is installed to cover the upper end inlet of the above-described falling film forming tube 11 and in which the hole 14 is drilled so that the supplied liquid can flow into the falling film forming tube 11; And

   It is fixed to the lower end of the above-described support plate 10 is to be inserted into the inside of the above-described falling film generating tube 11, the liquid dispersion plate having a predetermined interval in the circumferential direction with the inner wall of the falling film generating tube (11) (9).
10.       10. The method of claim 9, wherein the distance between the side surface of the support plate (10) and the inner wall of the drop film forming tube (11) is 0.05 to 2 mm.
11.       The method of claim 1, wherein the agitated thin film reactor is a stirred thin film reactor having a rotating blade capable of dropping a high viscosity material to form a film.
12. 12. The method of claim 11, wherein the rotating blade has a vertical tilt angle of at least 45 degrees.

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