A continuous hydrolysis process for a by-product polymer

Abstract

The present application belongs to the technical field of polylactic acid production, and relates to a continuous hydrolysis method of by-product polymers. The by-product polymer melt and the hydrolysis medium are simultaneously transported into a pre-hydrolysis device for pre-hydrolysis to obtain a pre-hydrolysis product; the pre-hydrolysis product and the hydrolysis medium are simultaneously transported into a hydrolysis device for hydrolysis to obtain a hydrolysis product. The by-product polymer is a polymer produced in the process of converting lactic acid into lactide, and the by-product polymer can also be replaced by waste polylactic acid. The present application degrades the by-product polymer into lactic acid multimers with good water solubility through pre-hydrolysis, and then completely hydrolyzes the lactic acid multimers into lactic acid and lactic acid dimers through homogeneous hydrolysis. The heterogeneous process and the homogeneous process of the hydrolysis reaction are distinguished, the continuity of the overall unit operation is facilitated, and the final hydrolysis product is ensured to be free of unhydrolyzed high molecular polymers.

Description

Technical Field

[0001] This invention belongs to the field of polylactic acid production technology and relates to a continuous hydrolysis method for by-product polymers. Background Technology

[0002] Polylactic acid (PLA) is a biodegradable material that can be completely degraded into water and carbon dioxide under appropriate conditions, and is considered one of the most promising alternatives to traditional petroleum-based materials. To date, continuous production of PLA has employed a two-step process: first, lactic acid is used as a raw material to produce lactic acid dimer—lactide; then, lactide undergoes catalytic ring-opening polymerization to obtain PLA resin.

[0003] In the production process of lactide, especially in the process unit of lactic acid oligomer catalytic cracking to produce lactide, a certain amount of polymer is inevitably produced as a byproduct due to the existence of parallel reactions. This byproduct polymer has the following characteristics: (1) low molecular weight (weight average molecular weight Mw < 2.0 × 10⁻⁶). 4 (2) The composition is complex. This part of the polymer usually contains 5.0 to 10.0% lactide, several times the concentration of the catalyst in the feed, as well as high-temperature carbides and impurities after enrichment. If this part of the by-product polymer is not discharged from the production unit in time, the catalyst and impurities will easily accumulate, which will cause the product indicators to deteriorate; if it is directly discarded, it will increase the production material consumption and need to be treated as hazardous waste. In addition, the above-mentioned substances contained in the by-product polymer, especially the catalyst, are extremely difficult to separate in the melt state. Therefore, the polymer must be completely degraded before the corresponding separation operation can be carried out. At present, the disposal schemes for the reuse of by-product polymers mainly include: (1) hydrolysis into lactic acid oligomers (Mw 0.8 to 3.0 × 10 3 (a) The product is directly returned to the process unit for producing lactide; (2) The polymer that will eventually be discharged from the production system is completely hydrolyzed into lactic acid and some lactic acid dimers, which are then separated, purified and reused, with only catalysts, high-temperature carbides, impurities, etc. being discharged.

[0004] When polylactic acid (PLA) undergoes complete hydrolysis, the amount of water added typically exceeds the theoretical value (25% of the polymer mass). Although excess water can promote hydrolysis, this excess water needs to be removed during reuse (in the lactic acid oligocondensation process), increasing energy consumption. Furthermore, PLA hydrolysis has an "induction period" (usually ranging from 0.5 to 2 hours), characterized by the absence of lactic acid or lactic acid polymers in the hydrolysis medium during the initial stage of the intermittent hydrolysis reaction. However, as the reaction time increases, lactic acid or lactic acid polymers become detectable in the hydrolysis medium, and their concentration gradually increases until equilibrium is reached. This is because the degradation of PLA in the initial stage of the hydrolysis reaction occurs only at the phase interface (solid-liquid or liquid-liquid two-phase). As the molecular weight of the degradation products decreases, they dissolve in the hydrolysis medium, leading to a homogeneous hydrolysis reaction and accelerating the hydrolysis process. For continuous production conditions, this phenomenon results in PLA and the hydrolysis medium remaining in a heterogeneous reaction state in the reactor, slowing the hydrolysis process and hindering the operation of subsequent separation units.

[0005] Therefore, finding an efficient hydrolysis method that can meet the requirements of continuous operation is an urgent problem to be solved in this technical field. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a continuous hydrolysis method for by-product polymers, addressing the shortcomings of the prior art.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] This invention discloses a continuous hydrolysis method for by-product polymers, comprising the following steps:

[0009] (1) The by-product polymer melt and the first hydrolysis medium are simultaneously fed into the pre-hydrolysis device for pre-hydrolysis to obtain the pre-hydrolysis product;

[0010] (2) The pre-hydrolysis product obtained in step (1) and the second hydrolysis medium are simultaneously fed into the hydrolysis device for hydrolysis to obtain the hydrolysis product;

[0011] The by-product polymer is a polymer produced during the production process of converting lactic acid into lactide.

[0012] The by-product polymer is a composition of polylactic acid and other substances, wherein the other substances are any one or more of L-lactide, D-lactide, meso-lactide, lactic acid and lactic acid oligomers; and the degree of polymerization of the lactic acid oligomers is 2 to 4.

[0013] The by-product polymer melt is formed by the by-product polymer at a temperature of 130°C to 180°C.

[0014] In this case, the pre-hydrolysis product obtained in step (1) does not need to be separated and purified, and can be directly hydrolyzed in the second hydrolysis medium.

[0015] In some embodiments, the first hydrolysis medium and the second hydrolysis medium are the same substance or different substances; the first hydrolysis medium and the second hydrolysis medium are water or a mixture of water and lactic acid; the concentration of lactic acid in the mixture of water and lactic acid is 0.1 to 10.0 wt%.

[0016] In some embodiments, in step (1), the mass flow ratio of the by-product polymer melt to the first hydrolysis medium is 1000:25 to 1000:50.

[0017] In some embodiments, preferably, in step (1), the mass flow ratio of the by-product polymer melt to the first hydrolysis medium is 1000:25, 1000:35 or 1000:50.

[0018] In some embodiments, in step (1), the pre-hydrolysis device is a tubular reactor with an inner coil or a static reactor with a jacket.

[0019] In some embodiments, in step (1), the pre-hydrolysis temperature is 120-140°C, the pre-hydrolysis pressure is 0.2-0.4 MPa, and the material residence time is 5-10 min.

[0020] In some embodiments, the mass flow ratio of the by-product polymer melt in step (1) to the second hydrolysis medium in step (2) is 1000:275 to 1000:450.

[0021] In some embodiments, preferably, the mass flow ratio of the by-product polymer melt in step (1) to the second hydrolysis medium in step (2) is 1000:275, 1000:350 or 1000:450.

[0022] In some embodiments, in step (2), the hydrolysis device is a fully mixed flow reactor or a plug flow reactor.

[0023] In some embodiments, in step (2), the hydrolysis temperature is 120-140°C, the hydrolysis pressure is 0.2-0.4 MPa, and the material residence time is 115-170 min.

[0024] In some embodiments, the by-product polymer described above is replaced with waste polylactic acid.

[0025] The term "waste polylactic acid" refers to waste polylactic acid (PLA) generated during the production of polylactic acid resin or to products manufactured using PLA resin with a content of 90 wt% or higher that are discarded after use. For example, PLA resin particles that cannot be sold as products due to slight defects in the granulation stage during production (such as during debugging or quality control), or products made primarily or entirely of PLA, are discarded after use; hence the term "waste PLA."

[0026] The hydrolysis products obtained by the above-mentioned continuous hydrolysis method contain water, lactic acid, and lactic acid dimers.

[0027] Invention concept: The above-mentioned continuous hydrolysis method first degrades the by-product polymer into a water-soluble lactic acid polymer through pre-hydrolysis, and then completely hydrolyzes the lactic acid polymer into lactic acid and lactic acid dimer through homogeneous hydrolysis. The heterogeneous process and homogeneous process in the hydrolysis reaction are distinguished, which facilitates the continuity of the overall unit operation and ensures that the final hydrolysis product does not contain unhydrolyzed polymer.

[0028] Beneficial effects:

[0029] (1) The present invention degrades the by-product polymer into a lactic acid polymer with good water solubility through pre-hydrolysis, and then completely hydrolyzes the lactic acid polymer into lactic acid and lactic acid dimer through homogeneous hydrolysis. The heterogeneous process and homogeneous process in the hydrolysis reaction are distinguished, which facilitates the continuity of the overall unit operation and ensures that the final hydrolysis product does not contain unhydrolyzed polymer.

[0030] (2) The present invention adopts a pre-hydrolysis process. Through the mass transfer enhancement of the pre-hydrolysis device, the “induction period” time is shortened and the hydrolysis reaction efficiency is improved, so that the overall residence time of the hydrolysis process is within 120 to 180 minutes.

[0031] (3) By adding the hydrolysis medium in stages, the total mass of the hydrolysis medium added is reduced to a minimum of 30 wt% of the mass flow rate of the by-product polymer, which is close to the theoretical amount. At the same time, the water content in the hydrolysis product is <17 wt%, which is close to the water content of existing commercial lactic acid (8-12 wt%). After simple separation of the impurities contained therein, it can be used as a raw material for the production of lactide.

[0032] (4) The hydrolysis process of the present invention does not require the addition of an extra catalyst, which is environmentally friendly. Attached Figure Description

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer.

[0034] Figure 1This is a schematic diagram of the process flow for the continuous hydrolysis method of the by-product polymer of this invention. Detailed Implementation

[0035] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, but this is not intended to limit the scope of protection of this application.

[0036] The by-product polymers used in the embodiments of this invention are polymers produced as a byproduct during the synthesis of lactide. Specifically, lactide is synthesized from lactic acid oligomers through cyclization and cleavage under the action of a catalyst. The specific synthesis method of these by-product polymers can be found in the literature (W. Huang, et al., Polym. Degrad. Stab., 101, 2014, 18-23). ​​These by-product polymers include polylactic acid, and contain any one or more of L-lactide, D-lactide, meso-lactide, lactic acid, and lactic acid oligomers with a degree of polymerization ranging from 2 to 4. The weight-average molecular weight of polylactic acid ranges from 10362 to 29857, and the molecular weight distribution ranges from 2.1 to 4.0.

[0037] In this embodiment of the invention, the by-product polymer melt is formed by the by-product polymer at a temperature of 130°C to 180°C. Since the by-product polymer has different melting points due to its molecular weight, the temperature range for forming the melt is defined as a temperature range.

[0038] A schematic diagram of the process flow for the continuous hydrolysis method of the by-product polymer of this invention is shown below. Figure 1 As shown.

[0039] Comparative Examples 1-6

[0040] Comparative Examples 1-6 involved feeding the by-product polymer and the hydrolysis medium together into a fully stirred-flow hydrolysis reactor. By adjusting the liquid level within the reactor, the content of each component in the hydrolysis products was investigated at different residence times. Specific implementation methods and results are as follows:

[0041] The by-product polymer melt (weight-average molecular weight Mw and molecular weight distribution index PDI data are detailed in Table 1) was pumped at a flow rate of 1000 kg / h via a gear pump and mixed with a 0.1 wt% lactic acid aqueous solution at a flow rate of 400 kg / h via a centrifugal pump into a mixed flow reactor for hydrolysis. The material temperature in the mixed flow reactor was 140℃, and the pressure was 0.3 MPa. The material residence times were adjusted to 30 min, 60 min, 90 min, 120 min, 150 min, and 180 min (comparative examples 1 to 6), respectively. The content of each component in the final hydrolysis product is detailed in Table 1.

[0042] Table 1. Data on by-product polymers and hydrolysis products in the comparative examples.

[0043]

[0044] Note: LA stands for lactic acid, L2A stands for lactic acid dimer, and LnA stands for lactic acid polymers and polymers with a degree of polymerization greater than 2.

[0045] The experimental results of Comparative Examples 1 to 6 show that, under continuous operation conditions, due to the presence of heterogeneous hydrolysis, the lactic acid content in the hydrolysis products is only in the range of 5.62% to 28.83%, while the content of lactic acid polymers with a degree of polymerization of 2 or higher is in the range of 38.67% to 62.28%, indicating that the hydrolysis efficiency is low.

[0046] Examples 1-3

[0047] Examples 1-3 involved feeding the by-product polymer and the hydrolysis medium together into a pre-hydrolysis reactor, and by adjusting the flow rate of the hydrolysis medium, the degree of polymerization of the polymer in the pre-hydrolysis product was investigated. Specific implementation methods and results are as follows:

[0048] The by-product polymer melt (weight-average molecular weight Mw and molecular weight distribution index PDI data are detailed in Table 2) was pumped at a flow rate of 1000 kg / h via a gear pump and then pumped together with a 0.1 wt% lactic acid aqueous solution at flow rates of 25.0 kg / h, 35.0 kg / h, and 50.0 kg / h (for Examples 1-3, respectively) via a centrifugal pump into a jacketed static reactor for pre-hydrolysis reaction. The jacket heat transfer medium temperature was adjusted to maintain a material temperature of 140°C, a pressure of 0.3 MPa, and a material residence time of 10 min in the static reactor. The degree of polymerization of the polymer in the final pre-hydrolysis product is detailed in Table 2.

[0049] Table 2 Data on by-product polymers and pre-hydrolysis products in Examples 1-3

[0050]

[0051]

[0052] Note: LA represents lactic acid, L2A represents lactic acid dimer, LnA represents lactic acid polymers and polymers with a degree of polymerization greater than 2, and DPN represents the highest number-average degree of polymerization of lactic acid polymers in the hydrolysis medium.

[0053] Lactic acid polymers with a degree of polymerization less than 10 exhibit good water solubility; therefore, the amount of hydrolysis medium added in step (1) pre-hydrolysis is in the range of 25–50 kg / h. Examples 4–9 of this invention continue to implement the entire hydrolysis process using the amount of hydrolysis medium added in the pre-hydrolysis process obtained in Examples 1–3.

[0054] Example 4

[0055] (1) By-product polymer melt (weight average molecular weight Mw, molecular weight distribution index PDI data are detailed in Table 3) is transported at a flow rate of 1000 kg / h through a gear pump and pure water at a flow rate of 25 kg / h through a centrifugal pump. It first enters a tubular reactor with an inner coil for pre-hydrolysis reaction. The material temperature in the tubular reactor with an inner coil is 120℃, the pressure is 0.2 MPa, and the material residence time is 5 min to obtain the pre-hydrolysis product.

[0056] (2) The pre-hydrolyzed product obtained in step (1) is pumped together with pure water at a flow rate of 275 kg / h via a centrifugal pump through a gear pump at a flow rate of 1025 kg / h and then fed into a plug flow reactor for hydrolysis. The material temperature in the plug flow reactor is 120℃, the pressure is 0.2 MPa, and the material residence time is 170 min. The composition of the final hydrolysis product is detailed in Table 3.

[0057] Example 5

[0058] (1) By-product polymer melt (weight average molecular weight Mw, molecular weight distribution index PDI data are detailed in Table 3) is transported at a flow rate of 1000 kg / h through a gear pump and lactic acid aqueous solution with a lactic acid concentration of 5 wt% is transported at a flow rate of 35 kg / h through a centrifugal pump. First, they enter a tubular reactor with an inner coil for pre-hydrolysis reaction. The material temperature in the tubular reactor with an inner coil is 130℃, the pressure is 0.3 MPa, and the material residence time is 7 min to obtain the pre-hydrolysis product.

[0059] (2) The pre-hydrolyzed product obtained in step (1) is pumped at a flow rate of 1035 kg / h via a gear pump and a lactic acid aqueous solution with a lactic acid concentration of 5 wt% is pumped at a flow rate of 350 kg / h via a centrifugal pump into a plug flow reactor for hydrolysis. The material temperature in the plug flow reactor is 130℃, the pressure is 0.3 MPa, and the material residence time is 150 min. The composition of the final hydrolysis product is detailed in Table 3.

[0060] Example 6

[0061] (1) By-product polymer melt (weight average molecular weight Mw, molecular weight distribution index PDI data are detailed in Table 3) is transported at a flow rate of 1000 kg / h through a gear pump and lactic acid aqueous solution with a lactic acid concentration of 10 wt% is transported at a flow rate of 50 kg / h through a centrifugal pump. First, they are transported together into a tubular reactor with an inner coil for pre-hydrolysis reaction. The material temperature in the tubular reactor with an inner coil is 140℃, the pressure is 0.4 MPa, and the material residence time is 10 min to obtain the pre-hydrolysis product.

[0062] (2) The pre-hydrolyzed product obtained in step (1) is pumped at a flow rate of 1050 kg / h via a gear pump and a 10 wt% lactic acid aqueous solution is pumped at a flow rate of 450 kg / h via a centrifugal pump into a plug flow reactor for hydrolysis. The material temperature in the plug flow reactor is 140℃, the pressure is 0.4 MPa, and the material residence time is 115 min. The composition of the final hydrolysis product is detailed in Table 3.

[0063] Example 7

[0064] (1) By-product polymer melt (weight average molecular weight Mw, molecular weight distribution index PDI data are detailed in Table 3) is transported at a flow rate of 1000 kg / h through a gear pump and lactic acid aqueous solution with a lactic acid concentration of 0.1 wt% is transported at a flow rate of 25 kg / h through a centrifugal pump. First, they enter a jacketed static reactor for pre-hydrolysis reaction. The temperature of the jacket heat medium is adjusted so that the material temperature in the static reactor is 140℃, the pressure is 0.4 MPa, and the material residence time is 5 min to obtain the pre-hydrolysis product.

[0065] (2) The pre-hydrolyzed product obtained in step (1) is pumped at a flow rate of 1025 kg / h via a gear pump and a lactic acid aqueous solution with a lactic acid concentration of 0.1 wt% is pumped at a flow rate of 275 kg / h via a centrifugal pump into a mixed flow reactor for hydrolysis. The material temperature in the mixed flow reactor is 140℃, the pressure is 0.4 MPa, and the material residence time is 115 min. The composition of the final hydrolysis product is detailed in Table 3.

[0066] Example 8

[0067] (1) By-product polymer melt (weight average molecular weight Mw, molecular weight distribution index PDI data are detailed in Table 3) is transported at a flow rate of 1000 kg / h through a gear pump and lactic acid aqueous solution with a lactic acid concentration of 10 wt% is transported at a flow rate of 35 kg / h through a centrifugal pump. First, they enter a jacketed static reactor for pre-hydrolysis reaction. The temperature of the jacket heat medium is adjusted so that the material temperature in the static reactor is 130℃, the pressure is 0.3 MPa, and the material residence time is 7 min, to obtain the pre-hydrolysis product.

[0068] (2) The pre-hydrolyzed product obtained in step (1) is pumped at a flow rate of 1035 kg / h via a gear pump and a 10 wt% lactic acid aqueous solution is pumped at a flow rate of 350 kg / h via a centrifugal pump into a mixed flow reactor for hydrolysis. The material temperature in the mixed flow reactor is 130℃, the pressure is 0.3 MPa, and the material residence time is 150 min. The composition of the final hydrolysis product is detailed in Table 3.

[0069] Example 9

[0070] (1) By-product polymer melt (weight average molecular weight Mw, molecular weight distribution index PDI data are detailed in Table 3) is transported at a flow rate of 1000 kg / h through a gear pump and lactic acid aqueous solution with a lactic acid concentration of 5 wt% is transported at a flow rate of 50 kg / h through a centrifugal pump. First, they enter a jacketed static reactor for pre-hydrolysis reaction. The temperature of the jacket heat medium is adjusted so that the material temperature in the static reactor is 120℃, the pressure is 0.2 MPa, and the material residence time is 10 min to obtain the pre-hydrolysis product.

[0071] (2) The pre-hydrolyzed product obtained in step (1) is pumped at a flow rate of 1050 kg / h via a gear pump and a 5 wt% lactic acid aqueous solution is pumped at a flow rate of 450 kg / h via a centrifugal pump into a mixed flow reactor for hydrolysis. The material temperature in the mixed flow reactor is 120℃, the pressure is 0.2 MPa, and the material residence time is 170 min. The composition of the final hydrolysis product is detailed in Table 3.

[0072] Table 3 Data on by-product polymers and hydrolysis products in Examples 4-9

[0073]

[0074] Note: LA represents lactic acid, L2A represents lactic acid dimer, and lactic acid polymers and polymers with a degree of polymerization greater than 2 were not detected.

[0075] The experimental results of Examples 4 to 9 show that when the total amount of hydrolysis medium added reaches 500 kg / h, the water content in the hydrolysis product reaches 16.78%. Therefore, the amount of hydrolysis medium added in the optimized step (2) is in the range of 275 to 450 kg / h.

[0076] This invention provides a concept and method for the continuous hydrolysis of by-product polymers. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A continuous hydrolysis process of a by-product polymer, characterized in that, The method comprises the following steps: (1) feeding the by-product polymer melt and the first hydrolysis medium into a pre-hydrolysis device simultaneously to perform pre-hydrolysis, and obtaining a pre-hydrolysis product; (2) feeding the pre-hydrolysis product obtained in step (1) and the second hydrolysis medium into a hydrolysis device simultaneously to perform hydrolysis, and obtaining a hydrolysis product; The by-product polymer is a polymer produced in the process of converting lactic acid into lactide; In step (1), the pre-hydrolysis device is a tubular reactor with an inner coil or a static reactor with a jacket; In step (1), the mass flow ratio of the by-product polymer melt to the first hydrolysis medium is 1000:25-1000:50; In step (1), the mass flow ratio of the by-product polymer melt to the second hydrolysis medium in step (2) is 1000:275-1000:450; The by-product polymer is hydrolyzed into lactic acid and lactic acid dimer; The first hydrolysis medium and the second hydrolysis medium are the same substance or different substances; the first hydrolysis medium and the second hydrolysis medium are water or a mixture of water and lactic acid; the concentration of lactic acid in the mixture of water and lactic acid is 0.1-10.0 wt%; In step (1), the pre-hydrolysis temperature is 120-140℃, the pre-hydrolysis pressure is 0.2-0.4 MPa, and the material residence time is 5-10 min; In step (2), the hydrolysis temperature is 120-140℃, the hydrolysis pressure is 0.2-0.4 MPa, and the material residence time is 115-170 min; In step (2), the hydrolysis device is a complete mixing flow reactor or a plug flow reactor.

2. The continuous hydrolysis process of claim 1, wherein, The by-product polymer is replaced by waste polylactic acid.

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