Polylactic acid decomposition method, polylactic acid treatment method, polylactic acid decomposition system, and polylactic acid treatment system

Abstract

To provide a polylactic acid decomposition method capable of decomposing polylactic acid into lactic acid, while suppressing generation of lactic acid amide, capable of ready circular reuse of ammonia used in the decomposition of polylactic acid, and capable of efficient operation of a treatment system of polylactic acid.SOLUTION: A method of decomposing polylactic acid to produce lactic acid, in which a polylactic acid decomposition step is performed in which polylactic acid is impregnated into a treatment solution containing ammonia and lactic acid to decompose. An ammonia concentration is 29 μmol / ml or more and 534 μmol / ml or less in the treatment liquid at the time of polylactic acid decomposition, and the lactic acid concentration in the treatment liquid is 266 μmol / ml or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and system for degrading polylactic acid, and a method and system for processing polylactic acid. [Background technology]

[0002] Development of recycling technologies for waste plastics has been progressing for many years, and for example, chemical recycling methods have now been established that regenerate waste plastics into gases such as methane that can be used as chemical raw materials or fuel.

[0003] When producing methane from waste plastics using chemical recycling, syngas (a mixed gas mainly containing hydrogen and carbon monoxide) is first produced from the waste plastics, and the carbon monoxide in this syngas is then methanated. However, when syngas is produced from waste plastics, the waste plastics are decomposed at the molecular level, and there are cases where, for example, the amount of hydrogen required to methanate all of the carbon monoxide in the resulting syngas is insufficient, so separately produced hydrogen is often required.

[0004] At a time when a decarbonized society is desired, hydrogen is a relatively expensive energy source, so methane produced using large amounts of it is inevitably expensive, resulting in increased recycling costs.

[0005] Therefore, methods for efficiently obtaining methane from waste plastics have been studied, and for example, a technique described in Patent Document 1 has been proposed.

[0006] The technology described in Patent Document 1 involves impregnating an organic substance containing polylactic acid with a treatment liquid containing an organic acid salt or an inorganic acid salt of an amine compound such as ammonium carbonate, decomposing the polylactic acid into lactic acid as a monomer, and then subjecting this lactic acid to methane fermentation to obtain methane as biogas. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-144102 Summary of the Invention [Problem to be solved by the invention]

[0008] When polylactic acid is decomposed using ammonia as an amine compound, there is a risk of lactamide being produced by the reaction between ammonia and the lactic acid obtained by decomposition of polylactic acid. This lactamide can be converted into biogas by methane fermentation, just like lactic acid. However, ammonia is generated during the biogasification of lactamide, which may reduce the activity of methanogens. Furthermore, the ammonia must ultimately be removed by water treatment. Therefore, reusing the ammonia used in the decomposition process is important for efficient operation of the treatment system. In other words, to efficiently operate a polylactic acid treatment system that utilizes the decomposition of polylactic acid with ammonia, it is important to minimize the production of lactamide during the decomposition of polylactic acid and to make it easy to reuse ammonia in a cyclical manner.

[0009] However, the technology described in Patent Document 1 uses salts of amine compounds such as ammonium carbonate, and therefore substances derived from amine compound salts other than lactic acid and ammonia (e.g., substances derived from ammonium carbonate) are present in the treated product after the decomposition treatment. As a result, ammonia cannot be easily recovered from the treated product, and cyclical reuse of ammonia may be difficult. Therefore, the technology described in Patent Document 1 leaves room for improvement in terms of efficiently operating a polylactic acid treatment system.

[0010] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polylactic acid decomposition method and system that can decompose polylactic acid into lactic acid while suppressing the production of lactic acid amide, that allows the ammonia used in the decomposition of polylactic acid to be easily recycled in a cyclical manner, and that enables the efficient operation of the polylactic acid treatment system, as well as a polylactic acid treatment method and system that utilize the same. [Means for solving the problem]

[0011] The polylactic acid decomposition method according to the present invention for achieving the above object is characterized by the following features: A method for producing lactic acid by decomposing polylactic acid, comprising the steps of: a polylactic acid decomposition step in which the polylactic acid is impregnated in a treatment solution containing ammonia and lactic acid and decomposed; The ammonia concentration in the treatment solution during the polylactic acid decomposition step is 29 μmol / ml or more 134 μmol / ml or less, and the lactic acid concentration in the treatment solution is 5333 μmol / ml or more 8000μmol / ml or less The point is that.

[0012] The polylactic acid decomposition system according to the present invention for achieving the above object is characterized by the following features: A system for decomposing polylactic acid to produce lactic acid, a polylactic acid decomposition unit that decomposes the polylactic acid by impregnating it in a treatment solution containing ammonia and lactic acid; The polylactic acid decomposition unit is configured to treat a treatment solution containing ammonia at a concentration of 29 μmol / ml or more. 134 μmol / ml or less, and the lactic acid concentration in the treatment solution is 5333 μmol / ml or more 8000μmol / ml or less The point is that the structure is adjustable so that

[0013] As a result of extensive research into methods for decomposing polylactic acid, the inventors of the present application discovered that by impregnating polylactic acid with a treatment solution in which the ammonia concentration and lactic acid concentration are each within a predetermined range, polylactic acid can be decomposed into lactic acid while suppressing the production of lactamide, and that the ammonia used in the decomposition of polylactic acid can be easily reused in a cyclical manner, thereby completing the present invention.

[0014] According to the above characteristic configuration, by performing a decomposition process on polylactic acid using a treatment solution having an ammonia concentration of 29 μmol / ml or more and 534 μmol / ml or less and a lactic acid concentration of 266 μmol / ml or more, it is possible to decompose polylactic acid into lactic acid while suppressing the production of lactamide. In particular, by carrying out the decomposition treatment of polylactic acid using a treatment solution having an ammonia concentration of 29 μmol / ml or more and 134 μmol / ml or less and a lactic acid concentration of 5333 μmol / ml or more, no lactamide is produced and the amount of lactic acid produced is extremely large. Furthermore, the method uses ammonia and lactic acid, which constitutes the polylactic acid to be decomposed, without using amine compound salts such as ammonium carbonate, as has been used in the past. As a result, no substances derived from amine compound salts are present in the product after the decomposition treatment, making it easier than before to recover ammonia and to easily reuse the recovered ammonia in a cyclical manner, enabling efficient operation of a treatment system involving the decomposition of polylactic acid.

[0015] Furthermore, if the treated material contains substances derived from amine compound salts other than lactic acid and ammonia, these substances may affect the activity of methanogens. Therefore, in order to subject the treated material after the decomposition treatment to methane fermentation, not only ammonia removal but also removal of substances derived from amine compound salts is required. However, as described above, since no substances derived from amine compound salts are present in the treated material after the decomposition treatment, the treated material after the decomposition treatment can be subjected to methane fermentation without removal of substances derived from amine compound salts, which enables more efficient operation of the treatment system involving the decomposition of polylactic acid.

[0020] Further characteristic features of the polylactic acid decomposition method according to the present invention are: The pH of the treatment solution during the polylactic acid decomposition step is 0.5 or more and 1.5 or less.

[0021] Ammonia concentration is between 29 μmol / ml and 134 μmol / ml, and lactate concentration is 5333 μmol / ml or more 8000μmol / ml or less When a treatment solution having the formula (I) is used, the pH of the treatment solution becomes 0.5 or more and 1.5 or less, but lactamide is not produced and the amount of lactic acid produced becomes extremely large.

[0022] The polylactic acid processing method according to the present invention for achieving the above object is characterized by the following features: 1. A method for treating polylactic acid, comprising: a polylactic acid decomposition step in which the polylactic acid is decomposed by impregnating it in a treatment solution containing ammonia and lactic acid; an ammonia recovery step of recovering the ammonia from the treated product after the polylactic acid decomposition step; a lactic acid recovery step of recovering a portion of the lactic acid from the treated product after the polylactic acid decomposition step; a methane fermentation step of methane fermenting the lactic acid contained in the treated product obtained after the ammonia recovery step and the lactic acid recovery step, The ammonia concentration in the treatment solution during the polylactic acid decomposition step is 29 μmol / ml or more 134 μmol / ml or less, and the lactic acid concentration in the treatment solution is 5333 μmol / ml or more 8000μmol / ml or less and The ammonia recovered in the ammonia recovery step and the lactic acid recovered in the lactic acid recovery step are reused in the polylactic acid decomposition step.

[0023] Furthermore, the polylactic acid processing system according to the present invention for achieving the above object has the following characteristic configuration: 1. A system for processing polylactic acid, comprising: a polylactic acid decomposition unit that decomposes the polylactic acid by impregnating it with a treatment liquid containing ammonia and lactic acid; an ammonia recovery unit that recovers the ammonia from the treated product after the decomposition of the polylactic acid; a lactic acid recovery unit that recovers a portion of the lactic acid from the treated product after the polylactic acid is decomposed; a methane fermentation unit that performs methane fermentation on the lactic acid contained in the treated product after recovering the ammonia and the portion of the lactic acid, The polylactic acid decomposition unit is configured to treat a treatment solution containing ammonia at a concentration of 29 μmol / ml or more. 134 μmol / ml or less, and the lactic acid concentration in the treatment solution is 5333 μmol / ml or more 8000μmol / ml or less and configured to be adjustable so that The ammonia recovered in the ammonia recovery section and the lactic acid recovered in the lactic acid recovery section can be supplied to the polylactic acid decomposition section.

[0024] According to the above characteristic configuration, by performing a decomposition process on polylactic acid using a treatment solution having an ammonia concentration of 29 μmol / ml or more and 534 μmol / ml or less and a lactic acid concentration of 266 μmol / ml or more, it is possible to decompose polylactic acid into lactic acid while suppressing the production of lactamide. In particular, by carrying out the decomposition treatment of polylactic acid using a treatment solution having an ammonia concentration of 29 μmol / ml or more and 134 μmol / ml or less and a lactic acid concentration of 5333 μmol / ml or more, no lactamide is produced and the amount of lactic acid produced is extremely large. Furthermore, this method uses ammonia and lactic acid, which constitutes the polylactic acid to be decomposed, without using amine compound salts such as ammonium carbonate as in the past. As a result, no substances derived from amine compound salts are present in the product after the decomposition treatment, making it easier to recover ammonia than in the past, and the recovered ammonia and lactic acid can be reused for the decomposition of polylactic acid, enabling efficient operation.

[0025] Furthermore, as described above, if the treated material contains substances derived from amine compound salts other than lactic acid and ammonia, these substances may affect the activity of methanogens. Therefore, in order to subject the treated material after the decomposition treatment to a methane fermentation treatment, not only ammonia removal but also removal of substances derived from amine compound salts is required. However, with the above-described characteristic configuration, it is possible to recover a portion of the lactic acid from the treated material after the decomposition treatment and subject the remainder to methane fermentation without performing removal of substances derived from amine compound salts, thereby enabling more efficient operation of the system. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a diagram showing a schematic configuration of a lactic acid treatment system according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, a polylactic acid decomposition system, a polylactic acid decomposition method, a polylactic acid treatment system, and a polylactic acid treatment method according to one embodiment of the present invention will be described with reference to the drawings.

[0028] Fig. 1 is a diagram showing the schematic configuration of a polylactic acid processing system 1 according to one embodiment. As shown in Fig. 1, the polylactic acid processing system 1 is a system for processing polylactic acid, and includes a polylactic acid decomposition unit 2 that decomposes polylactic acid by immersing it in a processing solution containing ammonia and lactic acid, an ammonia recovery unit 3 that recovers ammonia from the processed product after the polylactic acid is decomposed, a lactic acid recovery unit 4 that recovers a portion of the lactic acid from the processed product after the polylactic acid is decomposed, and a methane fermentation unit 5 that performs methane fermentation on the lactic acid contained in the processed product after the ammonia and a portion of the lactic acid have been recovered.

[0029] The polylactic acid to be treated in the polylactic acid treatment system 1 is preferably derived from biomass, from the viewpoint of excellent decomposability in the polylactic acid decomposition section 2, but is not limited to this and may be derived from fossil fuels.

[0030] The polylactic acid processing system 1 according to this embodiment also includes an ammonia supply unit 6 that supplies ammonia to the polylactic acid decomposition unit 2, and a lactic acid supply unit 7 that supplies lactic acid to the polylactic acid decomposition unit 2.

[0031] The polylactic acid decomposition unit 2 includes a decomposition tank and the like, and in the decomposition tank, polylactic acid is impregnated with a treatment liquid containing ammonia, lactic acid, and water, whereby hydrolysis treatment of polylactic acid is carried out.

[0032] In this embodiment, water is supplied to the decomposition tank by a water supply means (not shown), and ammonia and lactic acid are appropriately supplied from an ammonia supply unit 6 and a lactic acid supply unit 7 before and during the hydrolysis treatment so that the ammonia concentration and lactic acid concentration in the treatment solution fall within predetermined ranges.

[0033] In the polylactic acid processing system 1 of this embodiment, the treatment solution in the decomposition tank can be adjusted to have an ammonia concentration of 29 μmol / ml or more and a lactic acid concentration of 266 μmol / ml or more. This allows the polylactic acid to be decomposed into lactic acid while suppressing the production of lactic acid amide during hydrolysis of the polylactic acid. To further suppress the production of lactic acid amide, the treatment solution in the decomposition tank preferably has an ammonia concentration of 267 μmol / ml or less. Furthermore, to suppress the production of lactic acid amide while increasing the amount of lactic acid produced, it is even more preferable that the ammonia concentration be 134 μmol / ml or less and the lactic acid concentration be 5333 μmol / ml or more. In this case, even if the pH of the treatment solution is 0.5 or more and 1.5 or less, the production of lactic acid amide is suppressed and the amount of lactic acid produced is significantly increased.

[0034] The timing for supplying ammonia or lactic acid to the degradation tank can be, for example, before the start of hydrolysis treatment or when the ammonia concentration or lactic acid concentration in the treatment liquid in the degradation tank becomes lower than the desired concentration, as determined by monitoring the ammonia concentration or lactic acid concentration in the treatment liquid in the degradation tank during the hydrolysis treatment.

[0035] The method of supplying ammonia to the decomposition tank may be any method, such as supplying ammonia water of a predetermined concentration to the decomposition tank, supplying liquefied ammonia, or supplying gaseous ammonia. In this embodiment, a method of supplying high-concentration ammonia water from the ammonia supply unit 6 to the decomposition tank is adopted. The ammonia supply unit 6 is configured to store ammonia water recovered in the ammonia recovery unit 3 (described later) in a storage tank, monitor the concentration of the ammonia water in the storage tank, and supply an appropriate amount of ammonia water so that the ammonia concentration in the treatment liquid in the decomposition tank becomes a desired value.

[0036] Lactic acid can be supplied to the decomposition tank by supplying a lactic acid solution or an aqueous lactic acid solution prepared to a predetermined concentration to the decomposition tank, but in this embodiment, a method is used in which a lactic acid solution is supplied from a lactic acid supply unit 7. The lactic acid supply unit 7 is configured so that lactic acid recovered in a lactic acid recovery unit 4 (described later) is stored in a storage tank, and is configured so that an appropriate amount of lactic acid can be supplied so that the lactic acid concentration in the treatment liquid in the decomposition tank reaches a desired value.

[0037] The pressure conditions for the hydrolysis treatment (i.e., the pressure inside the decomposition tank) are set in consideration of the amounts of ammonia and lactic acid in the decomposition tank and the amount of polylactic acid to be treated. The pressure conditions for the hydrolysis treatment are, for example, preferably 0 to 10 MPaG in gauge pressure, but more preferably 0 to 1 MPaG from the viewpoint of the apparatus structure.

[0038] The treatment time for the hydrolysis treatment is also set taking into consideration the amount of ammonia and lactic acid in the decomposition tank and the amount of polylactic acid to be treated, but is preferably 0.1 to 48 hours, and more preferably 1 to 12 hours, for example.

[0039] The temperature condition for the hydrolysis treatment (ie, the temperature inside the decomposition tank) is, for example, 40°C or higher, preferably 50 to 220°C.

[0040] Although not shown, the polylactic acid decomposition section 2 is also provided with a polylactic acid supply means for supplying a desired amount of polylactic acid to the decomposition tank.

[0041] In the polylactic acid decomposition unit 2 having the above-described configuration, polylactic acid is supplied to the decomposition tank, and the treatment solution in the decomposition tank is allowed to stand for a predetermined time in a state where the ammonia concentration and lactic acid concentration are adjusted to predetermined concentrations, whereby the polylactic acid is hydrolyzed into lactic acid. In this embodiment, the polylactic acid decomposition unit 2 in the polylactic acid treatment system 1 corresponds to the polylactic acid decomposition system.

[0042] The ammonia recovery unit 3 is configured to recover at least ammonia from the processed product (a mixture of ammonia, lactic acid, and water) after the polylactic acid decomposition treatment by a known method, and the recovered ammonia is stored in a storage tank of the ammonia supply unit 6 and reused in the polylactic acid decomposition unit 2.

[0043] The polylactic acid processing system 1 of this embodiment uses ammonia and lactic acid, which constitutes the polylactic acid to be hydrolyzed, but does not use amine compound salts such as ammonium carbonate. Therefore, no substances derived from amine compound salts are present in the product after hydrolysis. Therefore, the ammonia recovery unit 3 of this embodiment employs a method in which the product after polylactic acid hydrolysis in the polylactic acid decomposition unit 2 is heated and distilled to recover ammonia water. Therefore, in this embodiment, lactic acid remains in the product after ammonia recovery. Thus, in this embodiment, ammonia recovery does not require consideration of the presence of substances derived from amine compound salts, such as ammonium carbonate, in the product after hydrolysis, and ammonia recovery can be performed using a simple method. Note that the mode of recovering ammonia in the ammonia recovery unit 3 is not limited to recovery as ammonia water, and gaseous ammonia may also be recovered. Furthermore, when recovery as ammonia water is employed, the ammonia water recovered from the product after hydrolysis may be subjected to an appropriate distillation process before recovery into the storage tank of the ammonia supply unit 6, thereby adjusting the concentration of the ammonia water recovered in the storage tank to the desired ammonia concentration.

[0044] The lactic acid recovery section 4 is also configured to be able to recover a portion of lactic acid from the treated product after ammonia recovery by a known method.

[0045] As described above, in the polylactic acid processing system 1 of this embodiment, no substances derived from amine compound salts, such as ammonium carbonate, are present in the processed product after hydrolysis. Therefore, the lactic acid recovery unit 4 of this embodiment employs a method for recovering a portion of the lactic acid by recovering a portion of the lactic acid remaining after ammonia recovery. In this way, it is possible to recover a portion of the lactic acid simply by recovering a portion of the processed product (lactic acid) after hydrolysis without performing a process for removing substances derived from amine compound salts, and the remaining processed product (lactic acid) can be subjected to methane fermentation as is.

[0046] The methane fermentation section 5 is equipped with a fermenter and the like, in which lactic acid obtained by decomposition of polylactic acid undergoes methane fermentation to produce methane. The fermenter is configured to be able to adjust and maintain conditions suitable for methane fermentation of lactic acid.

[0047] The temperature inside the fermenter can be appropriately set within a wide temperature range depending on the type of microorganism used, and is not particularly limited, but is, for example, about 20 to 60°C.

[0048] The treatment time for lactic acid in the fermenter is set taking into consideration the amount of lactic acid, the type of microorganism used, the temperature in the fermenter, etc., and is, for example, about 1 to 10 days.

[0049] Although not shown, the methane fermentation section 5 is also equipped with an agitation means for agitating the fermentation tank using the biogas generated in the fermentation tank, and an extraction means for extracting the biogas containing methane and other gases generated in the fermentation tank to the outside, and each of these means can be constructed using known methods.

[0050] According to the methane fermentation section 5 having the above-described configuration, the remaining lactic acid after a portion of the lactic acid has been recovered in the lactic acid recovery section 4 is supplied into the fermentation tank, where methane fermentation of the lactic acid proceeds to produce methane, which is then taken out to the outside.

[0051] Next, a method for treating polylactic acid using the polylactic acid treatment system 1 (polylactic acid treatment method) will be described.

[0052] First, polylactic acid is supplied to the decomposition tank of the polylactic acid decomposition unit 2, and a treatment solution is prepared in the decomposition tank. Then, the temperature and pressure in the decomposition tank are adjusted to be within a predetermined range, and treatment is carried out for a predetermined time, thereby hydrolyzing the polylactic acid to lactic acid (polylactic acid decomposition step). In this embodiment, the polylactic acid decomposition step in the polylactic acid treatment method corresponds to the polylactic acid decomposition method.

[0053] In this embodiment, the treatment solution in the decomposition tank is prepared by supplying ammonia water and a lactic acid solution from the ammonia supply unit 6 and the lactic acid supply unit 7, and also by supplying water, so that the ammonia concentration in the treatment solution in the decomposition tank is 29 μmol / ml or more and 534 μmol / ml or less, and the lactic acid concentration is 266 μmol / ml or more.

[0054] In this way, by using a treatment solution with an ammonia concentration of 29 μmol / ml or more and 534 μmol / ml or less and a lactic acid concentration of 266 μmol / ml or more, polylactic acid can be decomposed into lactic acid while suppressing the production of lactamide during hydrolysis of polylactic acid. In the polylactic acid decomposition process, the amount of lactic acid in the decomposition tank gradually increases due to the decomposition of polylactic acid, causing a decrease in the relative ammonia concentration, which may make it impossible to maintain the desired concentration range (29 μmol / ml or more and 534 μmol / ml or less). Therefore, during the polylactic acid decomposition process, the ammonia concentration in the decomposition tank is monitored, and aqueous ammonia is supplied from the ammonia supply unit 6 as needed to maintain the ammonia concentration at the desired value.

[0055] Next, ammonia and water are separated from the treated product (a mixture of ammonia, lactic acid, and water) after the polylactic acid decomposition treatment step to recover ammonia water (ammonia recovery step). The recovered ammonia water is then stored in the storage tank of the ammonia supply unit 6 and is supplied to the decomposition tank as appropriate for reuse. That is, in this embodiment, the ammonia used for decomposing polylactic acid can be cyclically reused.

[0056] Thereafter, a portion of the lactic acid remaining as a processed product after the ammonia recovery step is recovered (lactic acid recovery step). The recovered lactic acid is then stored in a storage tank of the lactic acid supply unit 7 and is appropriately supplied to the decomposition tank for reuse. That is, in this embodiment, the lactic acid obtained by decomposition of polylactic acid can be used for decomposing polylactic acid.

[0057] Thereafter, the remaining lactic acid after the lactic acid recovery step is supplied to a fermenter in the methane fermentation section 5, where methane fermentation of the lactic acid is carried out to produce methane (methane fermentation step).

[0058] Then, after a predetermined time has elapsed since the lactic acid was supplied into the fermentation tank, the methane in the fermentation tank is discharged to the outside.

[0059] As described above, the polylactic acid processing system, polylactic acid decomposition system, polylactic acid processing method, and polylactic acid decomposition method according to the present embodiment can decompose polylactic acid to lactic acid while suppressing the formation of lactic acid amide during hydrolysis of polylactic acid. Furthermore, because the hydrolysis of polylactic acid is performed using ammonia and lactic acid, which constitutes the polylactic acid to be hydrolyzed, without using amine compound salts such as ammonium carbonate, no substances derived from amine compound salts are present in the hydrolysis product. Therefore, there is no need to recover ammonia due to the presence of substances derived from amine compound salts such as ammonium carbonate in the hydrolysis product, and ammonia recovery can be performed using a simple method. In other words, the polylactic acid processing system, polylactic acid decomposition system, polylactic acid processing method, and polylactic acid decomposition method according to the present embodiment can decompose polylactic acid to lactic acid while suppressing the formation of lactic acid amide, enabling the cyclical reuse of ammonia used in the decomposition of polylactic acid and efficient operation of the polylactic acid processing system.

[0060] [Experimental Example] The present invention will be described in more detail below based on experimental examples, although it goes without saying that the present invention is not limited to these examples.

[0061] When polylactic acid was hydrolyzed using ammonia and lactic acid, the presence or absence of lactamide formation and the increase in lactic acid were investigated depending on the ammonia and lactic acid concentrations.

[0062] Specifically, 28% aqueous ammonia (Kishida Chemical Co., Ltd.) and / or lactic acid (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to 1.5 ml of water in a 2 ml tube, and 20 mg of polylactic acid was added and mixed to achieve the ammonia concentration and lactic acid concentration shown in Tables 1 to 3. The mixture was then reacted for 2 hours at 70° C. The concentrations of lactamide and lactic acid were then measured, and the increase in the concentrations of lactamide and lactic acid from those before the start of the reaction was calculated.

[0063] Table 1 summarizes the increase in lactamide concentration. As shown in Table 1, regardless of the lactic acid concentration, lactamide is not produced when the ammonia concentration is 29.3 μmol / ml or more and 533.3 μmol / ml or less. The inventors of the present application have also confirmed that lactamide is particularly unlikely to be produced when the ammonia concentration is 29.3 μmol / ml or more and 266.7 μmol / ml or less. This indicates that when polylactic acid is hydrolyzed using ammonia and lactic acid, lactamide production can be suppressed by setting the ammonia concentration to approximately 29 μmol / ml or more and 534 μmol / ml or less, and that lactamide production can be further suppressed by setting the ammonia concentration to approximately 267 μmol / ml or less.

[0064] [Table 1]

[0065] Meanwhile, Table 2 summarizes the increase in lactic acid concentration. As can be seen from Table 2, the decomposition of polylactic acid to lactic acid is affected by the ammonia concentration and the lactic acid concentration. Specifically, when the ammonia concentration is 587.3 μmol / ml or higher, polylactic acid decomposes to lactic acid regardless of the lactic acid concentration. In contrast, when the ammonia concentration is lower than this, the decomposition of polylactic acid to lactic acid does not proceed at a lactic acid concentration of 33.3 μmol / ml or lower. It can also be seen that the decomposition of polylactic acid to lactic acid tends to proceed more easily as the lactic acid concentration increases.

[0066] [Table 2]

[0067] Table 3 summarizes the pH of the treatment solution before the start of the reaction. As can be seen from Tables 2 and 3, when the ammonia concentration is 29 μmol / ml or more and 134 μmol / ml or less and the lactic acid concentration is 5333 μmol / ml or more, the pH becomes extremely low at around 1, but decomposition of polylactic acid is likely to proceed.

[0068] [Table 3]

[0069] To summarize these results, when polylactic acid is hydrolyzed using ammonia and lactic acid, if the ammonia concentration in the treatment solution is 29 μmol / ml or more and 534 μmol / ml or less, and the lactic acid concentration is 266 μmol / ml or more, polylactic acid can be decomposed into lactic acid while suppressing the production of lactamide. Furthermore, since ammonia and lactic acid are used, but not ammonia compound salts, ammonia can be easily reused. Furthermore, if the ammonia concentration is 267 μmol / ml or less, the production of lactamide can be further suppressed.

[0070] Furthermore, if the ammonia concentration is 29 μmol / ml or more and 134 μmol / ml or less and the lactic acid concentration is 5333 μmol / ml or more, the decomposition of polylactic acid is facilitated, the amount of lactic acid produced is increased, and the production of lactamide is also suppressed. [Industrial Applicability]

[0071] The present invention can be applied to a method and system for decomposing polylactic acid, and a method and system for treating polylactic acid. [Explanation of symbols]

[0072] 1: Polylactic acid processing system 2: Polylactic acid decomposition section (polylactic acid decomposition system) 3: Ammonia recovery section 4: Lactic acid recovery section 5: Methane fermentation section

Claims

1. A method for producing lactic acid by decomposing polylactic acid, comprising the steps of: a polylactic acid decomposition step in which the polylactic acid is impregnated in a treatment solution containing ammonia and lactic acid and decomposed; The method for decomposing polylactic acid, wherein the ammonia concentration in the treatment liquid during the polylactic acid decomposition step is 29 μmol / ml or more and 134 μmol / ml or less, and the lactic acid concentration in the treatment liquid is 5333 μmol / ml or more and 8000 μmol / ml or less.

2. 2. The method for decomposing polylactic acid according to claim 1, wherein the pH of the treatment solution during the polylactic acid decomposition step is 0.5 or more and 1.5 or less.

3. 1. A method for treating polylactic acid, comprising: a polylactic acid decomposition step in which the polylactic acid is decomposed by impregnating it in a treatment solution containing ammonia and lactic acid; an ammonia recovery step of recovering the ammonia from the treated product after the polylactic acid decomposition step; a lactic acid recovery step of recovering a portion of the lactic acid from the treated product after the polylactic acid decomposition step; a methane fermentation step of methane fermenting the lactic acid contained in the treated product obtained after the ammonia recovery step and the lactic acid recovery step, the ammonia concentration in the treatment solution during the polylactic acid decomposition step is 29 μmol / ml or more and 134 μmol / ml or less, and the lactic acid concentration in the treatment solution is 5333 μmol / ml or more and 8000 μmol / ml or less, The method for treating polylactic acid, wherein the ammonia recovered in the ammonia recovery step and the lactic acid recovered in the lactic acid recovery step are reused in the polylactic acid decomposition step.

4. A system for decomposing polylactic acid to produce lactic acid, a polylactic acid decomposition unit that decomposes the polylactic acid by impregnating it in a treatment solution containing ammonia and lactic acid; The polylactic acid decomposition system is configured so that the ammonia concentration in the treatment liquid is 29 μmol / ml or more and 134 μmol / ml or less, and the lactic acid concentration in the treatment liquid is 5333 μmol / ml or more and 8000 μmol / ml or less.

5. 1. A system for processing polylactic acid, comprising: a polylactic acid decomposition unit that decomposes the polylactic acid by impregnating it with a treatment liquid containing ammonia and lactic acid; an ammonia recovery unit that recovers the ammonia from the treated product after the decomposition of the polylactic acid; a lactic acid recovery unit that recovers a portion of the lactic acid from the treated product after the polylactic acid is decomposed; a methane fermentation unit that performs methane fermentation on the lactic acid contained in the treated product after recovering the ammonia and the portion of the lactic acid, the polylactic acid decomposition unit is configured to be adjustable so that the ammonia concentration in the treatment liquid is 29 μmol / ml or more and 134 μmol / ml or less, and the lactic acid concentration in the treatment liquid is 5333 μmol / ml or more and 8000 μmol / ml or less; The polylactic acid treatment system is configured so that the ammonia recovered in the ammonia recovery section and the lactic acid recovered in the lactic acid recovery section can be supplied to the polylactic acid decomposition section.

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