TW201827502A - Method of recovering lactide - Google Patents

Abstract

A method of recovering the lactide by holding a resin mixture that contains the lactide under a reduced pressure, gasifying the lactide contained in the resin mixture, and introducing a gaseous mixture containing the gaseous lactide into a condensation line A while executing the vacuuming, wherein the condensation line A includes a plurality of condensers 71, 73, 75 arranged in series such that the cooling temperature decreases successively, and the lactide as well as impurities other than the lactide are separated from the gaseous mixture through the condensers.

Description

Method for recycling lactide

The present invention relates to a method for recovering lactide contained in a resin mixture.

Biodegradable plastics that are decomposed by the action of bacteria or fungi that excrete enzymes have attracted attention as a means to solve the abnormal increase in plastic waste with the increase in the use of plastic in recent years. Among such biodegradable plastics, polylactic acid has attracted attention as an aliphatic polyester which is easily obtained in industrial mass production and is also environmentally friendly, and various proposals have been made for use in a wide range of fields.

Polylactic acid (PLA) is a resin based on corn starch and other raw materials, and is a polymer of direct polycondensation using starch lactic acid fermentation product, L-lactic acid and D-lactic acid as monomers, and dimerization by the same. Polymers made from ring-opening polymerization of lactide. In order to decompose into water and carbon dioxide by microorganisms existing in nature, the polymer also focuses on biologically-circulating resins.

Recently, as a recycling system of polylactic acid, the chemical recycling method which can be used to decompose polylactic acid for reuse has attracted the most attention. In this method, polylactic acid is depolymerized by heating polylactic acid in the presence of a depolymerization catalyst, and then the obtained lactide is re-opened and polymerized to form polylactic acid and reused.

Apparatuses for recovering lactide from polylactic acid for such chemical recycling have been proposed, for example, in Patent Documents 1 and 2. In the devices proposed by these patent documents, the polylactic acid, the depolymerization catalyst, and the carrier resin are put into a biaxial extruder and melt-mixed, and the molten mixture is transferred to the exhaust chamber by the screw in the biaxial extruder ( Exhaust area), and then the lactide produced by the depolymerization of polylactic acid is vaporized in the exhaust chamber and separated from other components to be recovered. That is, lactide (molecular weight M = 144) produced by the depolymerization of polylactic acid has a boiling point of 255 ° C under standard atmospheric pressure, so it is difficult to separate it under atmospheric pressure. Therefore, by supplying a molten mixture containing a lactide produced by the depolymerization of polylactic acid and a depolymerization catalyst to an exhaust chamber maintained under reduced pressure, the boiling point of lactide can be reduced and the lactide can be crosslinked. The ester is recovered by gasification.

In the lactide recovery method implemented in such a recovery device, although gasified lactide is converted from a gas to a liquid by a cooling trap in a condenser, and then formed into a liquid to be recovered, a large amount of polymerization is invested. Lactic acid is industrially implemented to recover a large amount of lactide, and it is easy to cause piping clogging. Therefore, regular piping cleaning is required. That is, since the pipe pressure is temporarily returned to atmospheric pressure for piping cleaning, and then returned to a predetermined vacuum degree, there is a problem that it is time-consuming and difficult to continuously work. The case where foreign matter adheres to a curved portion or a concave portion of the piping and the like is particularly remarkable. In addition, although the present inventors have previously proposed a method for gasifying and recovering lactide produced by depolymerizing polylactic acid (see Japanese Patent Application No. 2015-240041, Japanese Patent Application No. 2016-015501, Japanese Patent Application No. 2016 -007553), but the piping clogging and the like described above have not been reviewed. [Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2010-126490 [Patent Document 2] Japanese Patent No. 5051729

[Problems to be solved by the invention]

Therefore, an object of the present invention is to provide a recovery method which can effectively suppress the problem of piping clogging when the lactide contained in the resin mixture is vaporized and the vaporized lactide is recovered by liquefying using a condenser. Another object of the present invention is to provide a lactide recovery method capable of cleaning pipes in a lactide recovery line without stopping the lactide recovery process. [Means to solve the problem]

The present inventors conducted many experiments on the clogging of the piping generated when the vaporized lactide was recovered by liquefaction, and found that the low molecular weight components contained in the gas mixture containing the vaporized lactide became the main cause. The present invention has completed the knowledge of foreign matter adhesion and accumulation in the piping.

According to the present invention, a method for recovering lactide is provided. The method is to maintain a resin mixture containing lactide under reduced pressure, vaporize the lactide contained in the resin mixture, and then vacuum suction the gas containing lactic acid. The lactide gas mixture is introduced into the condensing line to recover lactide, which is characterized in that: the above-mentioned condensing line arranges a plurality of condensers in series so that the cooling temperature is sequentially reduced, and the condensers are captured by the foregoing gas mixture through the condenser Collection of lactide and separation of impurities other than lactide.

In the lactide recovery method of the present invention, the following correspondence can be preferably performed. (1) The aforementioned lactide is produced by the depolymerization of polylactic acid. (2) The gas mixture is passed through a gas-liquid separation column for removing a lactic acid oligomer component, and is continuously introduced into the condensation line. (3) The first condenser, the second condenser, and the third condenser are sequentially arranged as the foregoing majority of condensers. (4) Vacuum suck the aforementioned gas mixture in a vacuum range of 0.1 to 8 kPaA. (5) The heat exchange temperature of the first condenser is set at 60 to 140 ° C. (6) The condensation line includes: an introduction line for introducing the gas mixture into the condensation line; and a termination line connected to a vacuum pump for vacuum suction, and further having a switching between the introduction line and the termination line The valve is connected to a parallel pipeline, and at least one condenser is provided in the parallel pipeline so as to include a condenser located at least on the most downstream side. The vacuum suction and cleaning pipeline is respectively connected to the branch of the parallel pipeline through a switching valve. Flow path. (7) The aforementioned gas mixture is allowed to flow into the first-stage path in the branch of the parallel pipeline to recover lactide, and at the same time, the vacuum suction and cleaning pipeline connected to another flow path in the branch of the parallel pipeline is operated to perform vacuum. Cleaning. (8) A first condenser is arranged in the introduction line, and the remaining condensers are separately arranged in the branched flow paths of the parallel lines. [Effect of the invention]

In the lactide recovery method of the present invention, although a gas mixture containing vaporized lactide is introduced into a condensation line, and gaseous lactide is liquefied and recovered in the condensation line, the condensation line has Most condensers, and most condensers are configured such that the cooling temperature is sequentially reduced. That is, gaseous lactide is liquefied and recovered in the initial condenser, and then foreign matter (unavoidable impurities) having a molecular weight lower than that of lactide is removed and removed by the subsequent condenser. Therefore, problems such as accumulation of low-molecular-weight foreign matter in the piping can be effectively suppressed, thereby effectively preventing clogging of the piping and malfunction of the vacuum pump used for vacuum suction.

In addition, in the present invention, a parallel pipeline that branches the above-mentioned condensation pipeline through a switching valve is formed, and the condensers can be installed in the parallel pipelines, respectively. In such a state, one of the parallel lines can be operated to perform the liquefaction of gasified lactide and the removal of low-molecular-weight foreign substances, and at the same time, the other parallel line is connected to a cleaning line through which the cleaning gas flows. For cleaning operations. That is, the pipeline can be cleaned without stopping the recovery of liquefied gaseous lactide and removing low-molecular-weight foreign matters. This makes it possible to recover the lactide continuously in a particularly efficient manner, which is extremely advantageous for the industrial implementation of recovering a large amount of lactide.

With reference to Fig. 1, a recovery device used for carrying out the lactide recovery method of the present invention will be described. Generally speaking, the recovery device is composed of an extruder (melt mixing device) 1, an exhaust chamber 3 connected to the extruder 1, a carrier resin recovery chamber 4 located below the exhaust chamber 3, and connected to the exhaust chamber 3. The collection device 5 and the carrier resin discharge extruder 6 connected to the carrier resin recovery chamber are configured. The capture device 5 includes a gas-liquid separation tower 51 and a condensing line A. The condensing line A is connected to a vacuum pump 7 for vacuum suction. That is, by driving the vacuum pump 7, the exhaust chamber 3 can be maintained at a predetermined pressure reduction degree.

In the present invention, first, using such a recovery device, polylactic acid, a catalyst for depolymerization, and a carrier resin are charged into a feeding funnel of the extruder 1, and then melt-mixed in a cylinder of the extruder 1 to decompose the polylactic acid. polymerization. Then, the molten mixture is supplied to the exhaust chamber 3, and in this exhaust chamber 3, lactide generated by the depolymerization of polylactic acid is vaporized. The vaporized lactide is introduced into a capture device 5 connected to the exhaust chamber 3. In the capture device 5, high-molecular-weight components (for example, oligomers) are removed by the gas-liquid separation column 51, and then, the gasified lactide is formed into a liquid by cooling in the condensation line A and recovered. The carrier resin is discharged from the carrier resin recovery chamber 4 below the exhaust chamber 3 through the carrier resin discharge extruder 6.

Polylactic acid used for recycling lactide can use industrial waste discharged from market recycled products (Post Consumer) and resin processing and manufacturing plants, or the specifications generated during the manufacturing steps of polylactic acid resin are not consistent Resin, etc. In addition, it may be a three-dimensional composite type in which L-polylactic acid (PLLA) and D-polylactic acid (PDLA) are mixed, or an intermediate type in which L-lactic acid units and D-lactic acid units are mixed in a molecular chain. Of course, it may be native polylactic acid. In addition, if a polylactic acid-based combination is used with a small amount of copolymerized unit combinations, for example, under the condition that 50 mol% or more of lactic acid units are used, it may include lactide and copolymerizable lactones, cyclic ethers, and cyclic It is a unit of amidines, various alcohols, and carboxylic acids.

Although it is preferable to use representative MgO as a catalyst for depolymerization of polylactic acid, alkaline earth metal oxides such as CaO, SrO, and BaO may also be used. In addition, stannous isooctanoate (Tin (II) 2-ethyl hexanoate) used for the polymerization catalyst and aluminum hydroxide (Al (OH) ₃ ) as a flame retardant can also be preferably used. In addition, these catalysts may be used in combination. This depolymerization catalyst lowers the depolymerization temperature of polylactic acid, and by using the depolymerization catalyst, the thermal decomposition of polylactic acid can be promoted to reduce the molecular weight of polylactic acid, such as the addition of the feeding funnel of extruder 1 Polylactic acid having a molecular weight of about 200,000 can be decomposed into lactide (molecular weight 144). In addition, MgO, aluminum-based catalysts, and the like also have the effect of suppressing the racemization (optical anisotropic reaction) phenomenon during thermal reactions.

The polylactic acid catalyst for depolymerization is usually used in an amount of 0.05 to 5 parts by mass per 100 parts by mass of the polylactic acid.

The carrier resin is used for conveying the melt of polylactic acid by a screw, and has a function as a sealing material between the cylinder of the extruder and the screw.

That is, although polylactic acid containing lactide may vary depending on its molecular weight, its melt viscosity is generally much lower than that of ordinary polymers. Therefore, the polylactic acid melt of the screw has a poor transfer efficiency, and there is a fear that the screw will become close to the idle state. Yu. Therefore, by using the carrier resin in combination, the viscosity of the molten resin containing the molten lactic acid in the extruder is increased, and the molten polylactic acid can be more efficiently transported by the screw. In addition, because the carrier resin has a higher melt viscosity than polylactic acid containing lactide, by using the carrier resin and polylactic acid in a certain amount or more, it is possible to maintain the molten mixture to fill the inner surface of the cylinder and the screw of the extruder. The molten mixture was conveyed by a screw while the space was between. That is, by using the carrier resin, the space between the cylinder inner surface and the screw can always be kept in a sealed state, thereby effectively reducing the pressure of the exhaust chamber 3. In addition, even if it is a carrier resin with a low melt viscosity, if it is a resin (PET, PC, PS, etc.) having a higher thermal decomposition temperature than the depolymerization temperature of PLA, it is not thermally decomposed by itself, so it can be transported by screw Lactic acid and its depolymerization (advance).

Such a carrier resin does not adversely affect the depolymerization of polylactic acid, and as long as it does not show reactivity to the lactide produced by the depolymerization of polylactic acid, various thermoplastic resins can be used. In general, although olefin resins such as polyethylene and polypropylene are preferably used, polyester resins such as polyethylene terephthalate (PET) and polycarbonate (PC) can also be preferably used. Such as polyester and styrene resin such as polystyrene (PS). In addition, for these resins, those having a molecular weight sufficiently exhibiting the above functions and having sufficient melt viscosity are used.

In the present invention, the above-mentioned carrier resin is usually set within an appropriate amount range in accordance with the specifications and the like of the device. For example, it is about 20 to 10,000 parts by mass per 100 parts by mass of polylactic acid, and more preferably 20 to 100 parts by mass, and is set to an amount that can secure screw conveyance and vacuum tightness.

The polylactic acid, the catalyst for depolymerization, and the carrier resin are put into a predetermined amount from the feeding funnel of the extruder 1 and melt-mixed in the cylinder of the extruder 1. That is, the inside of the cylinder is heated by a heater (not shown) provided to cover the cylinder of the extruder 1, and the screw is moved inside the cylinder while being melted and mixed while being stirred and transported at a temperature of 250 ° C or higher. Temperature depolymerizes polylactic acid. The extruder 1 usually uses a twin-screw extruder having two or more screws, and heats the inside of the cylinder to 250 ° C to 350 ° C for melt mixing. With the melt mixing, the polylactic acid starts to depolymerize and proceeds. Low molecular weight of polylactic acid.

Although the polylactic acid is reduced in molecular weight by the above-mentioned melt mixing to obtain a basic unit of polylactic acid, lactide (lactic acid dimer), the boiling point of the lactide at the standard atmospheric pressure is 255 ° C. In this state, it is difficult to perform stable gas capture with a high boiling point. In other words, lactide cannot be effectively and stably separated from the molten carrier resin in its liquid state. Therefore, it is necessary to introduce the molten mixture into the exhaust chamber 3 which is maintained under a reduced pressure, so as to make lactic acid. The boiling point of lactide decreases, which promotes gasification.

Explained with reference to FIG. 2 together with FIG. 1, the exhaust chamber 3 has a first screw conveying path 11, and a carrier resin recovery chamber 4 is arranged below the first screw conveying path 11, and the first screw conveying path 11 is upright at the same time The upper portion of the side wall 13 is connected to the collection tube 15 of the collection device 5. In addition, the top wall 17 of the exhaust chamber 3 has an inclined structure, and the observation window 19 is installed on the inclined portion, so the observation window 19 can often observe the inside of the exhaust chamber 3, especially the first screw conveying path. 11 of the state. In addition, the lower end portion of the observation window 19 extends to an outer portion of the side wall 13 which is erected upward by the first screw conveying path 11, and a storage tank 21 for a return liquid is provided on the lower side thereof. That is, since the storage tank 21 is separated from the first screw conveyance path 11 by the side wall 13 described above, the reflux liquid can be prevented from being mixed into the screw conveyance path 11.

In the exhaust chamber 3 thus constructed, the first screw conveying path 11 is composed of: one pair of first conveying screws 23a, 23b rotating toward the same direction; and a return screw 25 appropriately disposed above one of the first conveying screws 23a. ; And a cylinder wall (cylinder tube) 27 that houses the first transfer screws 23a and 23b.

The cylinder wall 27 is the cylinder wall extension of the extruder 1. Similarly, the first transfer screws 23a and 23b are the screw extensions of the extruder 1, and the molten mixture is transferred from the extruder 1 to FIG. The paper can be introduced into the exhaust chamber 3 in front of the paper. In addition, an appropriately arranged return screw 25 is selectively provided in the exhaust chamber 3, and is provided to be combined with the first conveying screw 23a and rotated in a direction opposite to the conveying screw 23a (the same direction at the biting position).

That is, the exhaust chamber 3 is decompressed by the vacuum suction of the vacuum pump 7. In addition, a heater (not shown) mounted on the cylinder wall 27 heats the inside of the first screw conveying path 11 to approximately 250 ° C to 350 ° C similarly to the cylinder portion in the extruder 1. Thereby, the lactide produced by depolymerizing the polylactic acid contained in the molten mixture introduced into the exhaust chamber 3 by the first conveying screws 23a, 23b extending in the first screw conveying path 11 is gasified and gasified. The lactide is introduced into the collection device 5 through the collection tube 15.

In addition, since the molten mixture conveyed by the screw contains a polylactic acid polymer having a high vapor pressure and is compressed while being introduced into the decompressed exhaust chamber 3, it expands in the exhaust chamber 3 and generates a screw 23a. , 23b resin block 30 in a floating state. Therefore, when the recovery device is continuously operated, the resin block 30 floating up by the pair of first transfer screws 23a and 23b is continuously generated in the exhaust chamber 3. The resin block 30 is mainly a shell made of a carrier resin, and when the resin block 30 grows and grows, it only becomes an obstruction that prevents the recovery of lactide gas, or the scattered resin block 30 enters through the collection tube 15 The inside of the collection device 5 therefore blocks the entire collection tube 15. That is, the resin block 30 causes vent-up.

The return screw 25 provided on the upper side of the first transfer screw 23a is provided to be rotated in a direction opposite to the first transfer screw 23a as understood from FIG. 2. Therefore, the resin block 30 in a state of being floated by the first screw conveying path 11 is returned to the first conveying screw 23a by the return screw 25, and then falls to the second screw conveying path 60, and is discharged together with the carrier resin. In this way, the return screw 25 has a function as a return member for returning the resin block 30 to the first screw conveying path 11, thereby suppressing the growth of the resin block 30, and thus effectively preventing the resin block 30 from being generated due to the growth of the resin block 30. The problem.

The rotation of the return screw 25 used as the return member may be a rotation synchronized with the first conveying screws 23a, 23b, or may be an asynchronous rotation. In addition, as long as the resin block 30 floating from the screw conveyance path 11 can be returned to the screw conveyance path 11, a member other than the return screw 25 may be used as the return member. For example, it can be preferably used so as not to prevent the molten mixture conveyed by the first conveying screws 23a and 23b from vaporizing and flowing into the lactide flow path of the collecting pipe 15 and covering the first conveying screws 23a and / or 23b. In the above method, a large number of paddle-shaped return members and an elliptical blade instead of a rotary shaft of a spiral blade are arranged as return members.

In addition, although the gas lactide comes into contact with the observation window 19 and the like and is cooled and reliquefied (ie, refluxed), exhaust gas blockage occurs, but the exhaust chamber 3 configured as described above can effectively prevent the Problems due to reflux. That is, when a molten mixture containing polylactic acid, a depolymerization catalyst, and a carrier resin is introduced into the exhaust chamber 3 through the extruder 1 through the first screw conveying path 11 and gasification of lactide is continuously performed, sometimes Condensed droplets 31 (ie, reflux liquid) are generated on the side of the observation window 19. When the droplet 31 drops to the first screw conveying path 11, a liquid film covering the surface of the first conveying screws 23 a and 23 b or the inner surface of the cylinder wall 27 passing through the conveying path 11 is formed, so that the molten mixture is liable to slip and the result is easy. The aforementioned resin block 30 is generated.

However, an observation window 19 is provided obliquely in the exhaust chamber 3 structured as shown in FIG. 2, and the condensed droplets 31 flow down along the observation window 19, and are then accommodated in the conveyance path through the side wall 13 and the first screw. 11 In the storage compartment 21 completely separated. That is, it is possible to effectively avoid the problem that the liquid droplet 31 drips into the first screw conveyance path 11 and promotes the resin mass 30. In addition, although the drop 31 dropped to the first screw conveying path 11 caused repeated gasification and liquefaction of lactide, and promoted the racemization of lactide, thus reducing the optical purity of lactide, but as described above Such a problem can also be effectively avoided in the exhaust chamber 3 having a structure.

In addition, the above-mentioned observation window 19 is preferably formed as a double window as shown in FIG. 2, and is mounted on the top wall 17 by a washer 35 having O-rings 33 a and 33 b. With such a structure, the heat-retaining property of the observation window 19 can be improved and condensation can be prevented, and thus the generation of reflux liquid can be effectively avoided.

In addition, a recovery line 37 for recovering the reflux liquid 31a stored in the storage tank 21 is provided at the bottom of the storage tank 21 that collects the liquid droplets 31 (return liquid), and a vacuum holding the exhaust chamber 3 is provided above the sidewall. The vacuum destruction / recovery line 39 may be used to destroy the vacuum. With this structure, the reflux liquid 31a stored in the storage tank 21 can be recovered.

The structure of the storage tank 21 is not limited to the structure shown in FIG. 2. For example, the bottom of the storage tank 21 is connected to a temporary capture tank through a capture line, and a vacuum destruction / recovery line is provided in the temporary capture tank. And the recovery line, the return liquid 31a stored in the storage tank 21 can be moved to the temporary collection tank through the collection line without destroying the vacuum system of the exhaust chamber 3 and recovered.

In addition, although the lactide vaporized by the exhaust chamber 3 is introduced into the trap device 5 through a trap tube 15 provided on the upper side of the side wall 13, the trap tube 15 extends obliquely upward as shown in FIG. 2 and The vacuum breakage prevention valve 50 is provided, so that the valve 50 can be opened and closed when there is an abnormality in the process operation.

In addition, it is preferable that an inlet portion of the trap tube 15 is provided with a storage tank 15a for receiving a reflux liquid. That is, it is preferable to form the structure which collects the liquefied reflux liquid in the collection pipe 15 in this storage tank 15a, and makes it fall into the screw conveyance path 11 without flowing. In addition, the storage tank 15a is also provided with a vacuum destruction / recovery line 15b and a recovery line 15c.

The gas mixture containing the vaporized lactide introduced into the capture device 5 from the capture tube 15 passes through the gas-liquid separation tower 51 and the condensing line A in the capture device 5, and then the lactide is formed into a liquid and the gas is produced from the gas. The mixture was recovered. In the present invention in which the recovery of lactide is thus performed, the pressure in the exhaust chamber 3 is maintained at a pressure of 0.1 to 8 KPaA by the vacuum suction of the vacuum pump 7, whereby the heated lactide is vaporized in the exhaust chamber 3. Next, the vaporized lactide is introduced into the trap device 5 from the trap tube 15 and recovered through the condensation line A. For example, when the pressure in the exhaust chamber 3 is lower than the above-mentioned range, since the degree of vacuum is too high, a large number of resin blocks are formed, and exhaust blockage is likely to occur. In addition, when the pressure in the exhaust chamber 3 is higher than the above range, the degree of vacuum is too low, so the boiling point of lactide is not sufficiently lowered, and the lactide is not sufficiently vaporized, which tends to reduce the recovery efficiency. In addition, in such a recovery operation, it is necessary to suppress the pressure fluctuation in the exhaust chamber 3 as much as possible. Specifically, the pressure fluctuation should be suppressed within ± 1KPaA. When this pressure fluctuation is excessively large, the boiling point of the lactide contained in the molten resin mixture changes greatly, and as a result, exhaust gas clogging is likely to occur and the recovery efficiency may decrease.

In the recovery device described above, the capture device 5 connected to the capture pipe 15 has, for example, a gas-liquid separation tower 51 as shown in FIG. 3, and the gas-liquid separation tower 51 is passed into the condensation line A, and the condensation line A is connected to the vacuum pump 7. That is, the gas mixture containing the gaseous lactide flowing in from the trap tube 15 is passed into a gas-liquid separation tower 51, and a high-molecular-weight component (for example, lactic acid) is removed by a mist eliminator provided in the gas-liquid separation tower 51. Oligomer), and then, by cooling in the condensing line A, liquefied gaseous lactide is recovered and low molecular weight components are removed. That is, the gas mixture containing the lactide trapped in the exhaust chamber 3 contains, in addition to lactide, various kinds of oligomers derived from lactic acid, polylactic acid, or a polymerization initiator mixed with a carrier resin. Molecular weight compounds, etc., these low molecular weight compounds must be removed. Specifically, the gas-recovered lactide passes through the gas-liquid separation tower 51 and is removed by a mist eliminator in the gas-liquid separation tower, and then is introduced into the first condenser 71 to liquefy only the lactide. The liquid lactide is recovered. Therefore, in order to efficiently supply the gaseous mixture from the exhaust chamber 3 to the gas-liquid separation tower 51 and the condensing line A, the gas-liquid separation tower 51 and the condensing line A should preferably be positioned higher than the exhaust chamber 3.

As can be seen from FIG. 3, in the condensing line A, the first condenser 71, the second condenser 73, and the third condenser 75 are arranged in series along the flow direction of the gas sucked by the vacuum pump 7. That is, the first condenser 71 is connected to the gas-liquid separation column 51 through the introduction line 81, the second condenser 73 is connected to the first condenser 71 through the connection line 83, and the third condenser 75 is connected through the connection line 85 The second condenser 73 is connected to the vacuum pump 7 through a termination line 87. Therefore, the gas mixture containing the gaseous lactide discharged from the exhaust chamber 3 passes through the gas-liquid separation column 51 and is sequentially introduced into the first condenser 71, the second condenser 73, and the third condenser 75.

The first condenser 71 liquefied lactide and recovered it from the gas mixture, and the heat exchange temperature in the first condenser 71 was set within an appropriate range according to the range of vacuum degree of vacuum suction. Generally speaking, the heat exchange temperature in the vacuum range of 0.1KPaA to 8KPaA is preferably 60 ° C to 140 ° C, and the heat exchange temperature in the vacuum range of 0.5KPaA to 4KPaA is more preferably 80 ° C to 90 ° C. When the heat exchange temperature is lower than the above range, liquefaction of low boiling point impurities may occur, and the purity of the recovered lactide may decrease. When the heat exchange temperature is higher than the above range, the lactide is difficult to liquefy, so There is a fear that the recovery efficiency of lactide may decrease.

In addition, as shown in FIG. 3, the first condenser 71 is connected to a lactide container 89, and the lactide liquefied by cooling in the first condenser 71 is trapped in the container 89, and the remaining The gas is introduced into the second condenser 73 through the connection line 83.

The second condenser 73 is used to remove low-molecular-weight compounds having a lower boiling point than lactide. Therefore, the second condenser 73 has a lower heat exchange temperature than the first condenser 71 and is generally set at about 40 to -20 ° C. The low-molecular-weight compound liquefied by cooling in the second condenser 73 is trapped in the container 90 and then discharged or recovered. In addition, the low molecular weight compound is removed and the gas cooled by the second condenser 73 is introduced into the third condenser 75 through the connection line 85.

The third condenser 75 is used to remove low-molecular-weight compounds (hard-aggregating components) having a lower boiling point. Therefore, its heat exchange temperature is lower than that of the second condenser 73, and it is generally set at an extremely low temperature of -40 to -60 ° C, forming a so-called cryogenic trap to liquefy compounds of lower molecular weight and passing through the third condenser 75 Remove the defogger, etc. In addition, the remaining gas passes through a filter 93 provided in the termination line 87 and is sucked out by the vacuum pump 7.

The first to third condensers 71, 73, and 75 provided in the condensation line A as described above may have a structure known per se as long as the heat exchange temperature is set within the above range. For example, the first condenser 71 and the second condenser 73 may use water (warm water) as a refrigerant, and the third condenser 75 that performs cooling of the cryogenic trap may use anhydrous ethanol as a refrigerant.

In this manner, the gas mixture from which the high molecular weight oligomer component is removed is introduced into the condensation line A through the gas-liquid separation column 51, and then the first condenser 71 is trapped in addition to liquefying the gaseous lactide contained in the gas mixture. In addition, by further using the second and third condensers 73 and 75 to remove the low molecular weight, impurities such as low molecular weight compounds in the condensation line A can be effectively prevented from being accumulated and piping clogging, so it can be effectively prevented. The impure operation of the vacuum pump 7 caused by such impurities.

Further, in the example of FIG. 3 described above, the second condenser 73 and the third condenser 75 are provided to remove low molecular weight compounds having a boiling point lower than that of lactide, but the present invention is not limited to this aspect. For example, a cooler having a different heat exchange temperature may be further provided between the second condenser 73 and the third condenser 75. In addition, the third condenser 75 (or the second condenser 73) may be omitted as occasion demands.

In addition, although the condensing line A forms a single series line in FIG. 3, and the first to third condensers 71, 73, and 75 are provided on one line, the introduction line 81 of the first condenser 71 and the third condenser Between 75 and the end line 87 of the vacuum pump 7, a branched parallel line can be set, and a condenser is installed in the parallel line, and the vacuum pumping of the lactide collection operation and the line cleaning operation of the vacuum pump 7 can be performed at the same time. Hereinafter, the pipeline for vacuum lactide trapping operation is referred to as a lactide trapping line, and the pipeline for cleaning operation is referred to as a vacuum suction cleaning line.

For example, as shown in FIG. 4, in this aspect, a switching valve M such as a three-way valve is provided in the middle of the connecting line 83 extending from the first condenser 71, and branched into parallel lines Xa, Xb at this portion. . The parallel lines Xa and Xb each extend to a switching valve V such as a three-way valve provided in the termination line 87 connected to the vacuum pump 7.

Here, a second condenser 73a and a third condenser 75a are sequentially arranged in one of the parallel lines Xa, and a second condenser 73b and a third condenser 75b are sequentially arranged in the other parallel line Xb. The above-mentioned second condensers 73a and 73b and third condensers 75a and 75b correspond to the second condenser 73 and the third condenser 75 in FIG. 2, respectively.

As can be seen from FIG. 4, in one of the parallel lines Xa branched from the switching valve M, a switching valve Ma is provided in the connecting line 83a extending from the switching valve M, and the connecting line 83a is connected to the second condenser 73a. In addition, the second condenser 73a is connected to the third condenser 75a through a connection line 85a, and a termination line 87a connected to the switching valve V is extended from the third condenser 75a. The termination line 87a is provided with a switching valve Ma '. Similarly, in another parallel line Xb, a switching valve Mb is provided in the connecting line 83b extending from the switching valve M, and the connecting line 83b is connected to the second condenser 73b. In addition, the second condenser 73b is connected to the third condenser 75b through a connecting line 85b, and a termination line 87b connected to the switching valve V is extended from the third condenser 75b. The termination line 87b is provided with a switching valve Mb '.

In addition, the switching valve Ma in the parallel line Xa and the switching valve Mb in the parallel line Xb are connected to a clean gas introduction line 101 having a pressure regulating valve 95. In addition, the switching valve Ma 'in the parallel line Xa and the switching valve Mb' in the parallel line Xb are connected to an exhaust line 109, which has a pressure regulating valve 103, a vacuum chamber 105, and a filter 111 and is provided for cleaning. Vacuum pump 107. Various valves provided in such parallel lines Xa, Xb, clean gas introduction line 101, and exhaust line 109 are opened and closed by a valve control device (not shown), whereby the gas discharged from the first condenser 71 The mixture flows into one of the parallel lines Xa, and when the low molecular weight compounds are removed by cooling in the second condenser 73a and the third condenser 75a, the cleaning gas is heated to, for example, room temperature or higher, preferably 50 ° C or higher. The other line Xb is connected in parallel for cleaning of piping and the like.

For example, in the example of FIG. 4, the switching valve M is opened on the parallel line Xa side (the parallel line Xb side is closed), and the switching valve V is controlled to be opened on the parallel line Xa side (the parallel line Xb side is closed). In addition, the clean gas introduction line 101 side of the switching valve Ma is closed, and then the exhaust line 109 side of the switching valve Ma 'is closed. Therefore, the gas mixture leaving the first condenser 71 is removed by the switching valve M through the second condenser 73a to remove the low molecular weight compounds, and then, the third condenser 75a is used to remove the lower molecular weight compounds, and after passing through the switching valve V, it passes The filter 93 is exhausted by the vacuum pump 7.

On the other hand, in another parallel line Xb, the clean gas introduction line 101 side of the switching valve Mb is opened, and then, after the exhaust line 109 side of the switching valve Mb ′ is opened, the pressure in the clean gas introduction line 101 is adjusted. The valve 95 is in an open state, and the pressure regulating valve 103 in the exhaust line 109 is also adjusted to an open state. Therefore, when the cleaning vacuum pump 107 is operated in this state, the cleaning gas is introduced into the parallel line Xb from the predetermined gas source through the cleaning gas introduction line 101, and then the connection line 83b, the second condenser 73b, the connection line 85b, the first After the three condensers 75b and the termination line 87b flow into the exhaust line 109 through the valve Mb ', they sequentially pass through the vacuum chamber 105 and the filter 111, and are discharged by the cleaning vacuum pump 107.

That is, the parallel line Xa is connected to the vacuum pump 7 and serves as a lactide trap line for performing a vacuum lactide trap operation. The gas passing through the first condenser 71 is passed through the second condenser 73a and the third The condenser 75a removes low molecular weight compounds. On the other hand, the parallel line Xb is connected to the cleaning vacuum pump 107 through the cleaning gas introduction line 101 and becomes a vacuum suction and cleaning line for performing cleaning operations. Without changing the vacuum degree of the Xa side line, the cleaning gas is used. Clean various piping and condenser of Xb side pipeline.

In addition, when performing cleaning on the parallel line Xa side, an operation completely opposite to the above is performed. That is, as shown in FIG. 5, the switching valve M is opened on the parallel line Xb side (the parallel line Xa side is closed), and the switching valve V is controlled to be opened on the parallel line Xb side (the parallel line Xa side is closed). In addition, the clean gas introduction line 101 side of the switching valve Mb is closed, and then, the exhaust line 109 side of the switching valve Mb ′ is closed. Therefore, the gas mixture leaving the first condenser 71 is removed by the switching valve M through the second condenser 73b to remove low molecular weight compounds, and then the third condenser 75b is used to remove lower molecular weight compounds, and after passing through the switching valve V, it passes The filter 93 is exhausted by the vacuum pump 7.

On the other hand, in another parallel line Xa, the clean gas introduction line 101 side of the switching valve Ma is opened, and then, after the exhaust line 109 side of the switching valve Ma ′ is opened, the pressure in the clean gas introduction line 101 is adjusted. The valve 95 is in an open state, and the pressure regulating valve 103 in the exhaust line 109 is also adjusted to an open state. Therefore, when the cleaning vacuum pump 107 is operated in this state, the cleaning gas flows into the parallel line Xa from the predetermined gas source through the cleaning gas introduction line 101, and then the connection line 83a, the second condenser 73a, the connection line 85a, the first After the three condensers 75a and the termination line 87a flow into the exhaust line 109 through the valve Ma ', they sequentially pass through the vacuum chamber 105 and the filter 111, and are discharged by the cleaning vacuum pump 107.

That is, on the parallel line Xb side, low-molecular-weight compounds are removed from the gas passing through the first condenser 71 by the second condenser 73b and the third condenser 75b, and at the same time, on the other parallel line Xa side, various kinds of gas are cleaned by clean gas Piping and condenser.

In this way, in the case where the parallel pipeline is installed, cleaning can be performed without stopping the process operation of the trapping device 5, so that the lactide can be continuously recovered for a long time, which is extremely industrially advantageous.

In addition, in FIGS. 4 and 5 described above, although the switching valve M is provided in the connecting line 83 connected to the first condenser 71 to form parallel lines Xa and Xb, the gas-liquid separation tower 51 and the first A switching valve M is provided in the introduction line 81 of the condenser 71 to form a parallel line. In addition, a switching valve M may be provided in the connecting line 85 connecting the second condenser 73 and the third condenser 75 to form a parallel line. However, when the first to third condensers 71, 73, and 75 are operated to recover lactide and remove low-molecular-weight compounds, it is most likely that impurities are attached to and accumulated in the piping are in the downstream region of the first condenser 71. Therefore, as shown in FIGS. 4 and 5, it is preferable to provide a switching valve M in the connecting line 83 connected to the first condenser 71 to form parallel lines Xa and Xb.

1‧‧‧ Extruder

3‧‧‧ exhaust chamber

4‧‧‧ Carrier resin recovery room

5‧‧‧trapping device

6‧‧‧ Extruder for discharging carrier resin

7‧‧‧vacuum pump

11‧‧‧The first screw conveying road

13‧‧‧ sidewall

15‧‧‧ collection tube

15a‧‧‧Storage slot

15b‧‧‧Vacuum destruction / recovery pipeline

15c‧‧‧Recovery pipeline

17‧‧‧ top wall

19‧‧‧observation window

21‧‧‧Storage trough

23a‧‧‧The first conveying screw

23b‧‧‧The first conveying screw

25‧‧‧fall back to the screw

27‧‧‧cylinder wall

30‧‧‧resin block

31‧‧‧ droplet

31a‧‧‧reflux

33a‧‧‧O ring

33b‧‧‧O ring

35‧‧‧washer

37‧‧‧ Recovery pipeline

39‧‧‧Vacuum destruction / recovery pipeline

50‧‧‧Vacuum damage prevention valve

51‧‧‧Gas-liquid separation tower

60‧‧‧Second screw conveying path

71‧‧‧first condenser

73‧‧‧Second condenser

73a‧‧‧Second condenser

73b‧‧‧Second condenser

75‧‧‧Third condenser

75a‧‧‧Third condenser

75b‧‧‧third condenser

81‧‧‧ import pipeline

83‧‧‧ Connected pipeline

83a‧‧‧connected pipeline

83b‧‧‧connected pipeline

85‧‧‧ Connected pipeline

85a‧‧‧connected pipeline

85b‧‧‧connected pipeline

87‧‧‧Ending the pipeline

87a‧‧‧End pipeline

87b‧‧‧End pipeline

89‧‧‧lactide container

90‧‧‧ container

93‧‧‧Filter

95‧‧‧ pressure regulating valve

101‧‧‧clean gas introduction pipeline

103‧‧‧ pressure regulating valve

105‧‧‧vacuum chamber

107‧‧‧Vacuum pump for cleaning

109‧‧‧Exhaust line

111‧‧‧ Filter

A‧‧‧Condensing pipeline

M‧‧‧ switching valve

Ma‧‧‧Switching valve

Ma'‧‧‧ switching valve

Mb‧‧‧ switching valve

Mb'‧‧‧ switching valve

V‧‧‧ switching valve

Xa‧‧‧ Parallel Pipeline

Xb‧‧‧ Parallel Pipeline

FIG. 1 is a schematic configuration diagram of a recovery device used to implement the recovery method of the present invention. FIG. 2 is a sectional structural view showing an exhaust chamber of the recovery device of FIG. 1. FIG. FIG. 3 is a diagram showing the simplest aspect of the capture device of the recovery device of FIG. 1. FIG. FIG. 4 is a diagram showing an example of a condensing line in the capture device of the recovery device of FIG. 1, and is a diagram showing a state in which a parallel line in the parallel line in the condensing line is operated to recover lactide. FIG. 5 is a diagram showing an example of a condensing line in the capture device of the recovery device of FIG. 1, and is a diagram showing a state in which another parallel line in the parallel line in the condensing line is operated to recover lactide.

Claims (9)

A method for recovering lactide, the method is to maintain a resin mixture containing lactide under reduced pressure, and gasify the lactide contained in the resin mixture, and then vacuum suction the gas containing gaseous lactide The mixture is introduced into a condensing line to recover lactide, which is characterized in that: the condensing line arranges a plurality of condensers in series so that the cooling temperature decreases sequentially, and the lactide is captured by the gas mixture through the condensers And the impurities other than lactide are separated. For example, the method for recovering lactide in item 1 of the patent application range, wherein the lactide is produced by depolymerization of polylactic acid. For example, the method for recovering lactide in item 2 of the patent application range, wherein the gas mixture is passed through a gas-liquid separation tower for removing lactic acid oligomer components, and is continuously introduced into the condensation line. For example, the method for recovering lactide in the first item of the patent scope, wherein the first condenser, the second condenser, and the third condenser are sequentially arranged as the majority condenser. For example, the method for recovering lactide in item 1 of the patent application range, wherein the gas mixture is vacuum-sucked in a vacuum range of 0.1 to 8 kPaA. For example, the method for recovering lactide in item 4 of the patent application range, wherein the heat exchange temperature in the first condenser is set to 60 to 140 ° C. For example, the method for recovering lactide in item 1 of the patent scope, wherein the condensation line includes: an introduction line for introducing the gas mixture into the condensation line; and a termination line connected to a vacuum pump for vacuum suction, and more There is a parallel pipeline branched through a switching valve between the introduction pipeline and the termination pipeline, and at least one condenser is provided in the parallel pipeline so as to include at least the condenser located at the most downstream side, and the vacuum suction cleaning pipeline A flow path of each branch of the parallel pipeline is connected through a switching valve. For example, the method for recovering lactide in item 7 of the scope of the patent application, wherein the gas mixture is allowed to flow into the first-stage path in the branch of the parallel pipeline for recovery of lactide, and at the same time, another one connected to the branch of the parallel pipeline The vacuum suction cleaning line of the first flow path operates to perform vacuum cleaning. For example, the method for recovering lactide in item 7 of the scope of patent application, wherein a first condenser is arranged in the introduction pipeline, and the remaining condensers are arranged in the flow paths of the branches of the parallel pipeline.