Ultra-high purity crystalline lactic acid and its preparation method and application

Abstract

The present invention relates to the field of lactic acid technology, and specifically discloses a kind of ultra-high purity crystalline lactic acid and its preparation method and application. The present invention carries out ultra-high vacuum evaporation crystallization on polymerization-grade liquid L-lactic acid at 6 ℃ ~ 10 ℃, 0.7kPa ~ 0.9kPa, adds L-lactic acid seed crystals when the lactic acid specific gravity is 1.34 ~ 1.35, and terminates crystallization until the lactic acid specific gravity is 1.38 ~ 1.39, thereby obtaining ultra-high purity crystalline lactic acid. The present invention utilizes the limited water solubility of lactic acid at low temperatures, and only utilizes ultra-high vacuum evaporation crystallization in one step under ultra-low temperature and ultra-high vacuum to obtain ultra-high purity crystalline lactic acid, which has a purity of more than 99.95%, a granular shape, a high crystallization rate, uniform and dense, high looseness, strong anti-caking performance, and excellent quality. The ultra-high purity crystalline lactic acid can be directly polymerized for the production of polylactic acid, which improves production efficiency, greatly reduces production costs, has excellent market competitiveness, and is suitable for large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of lactic acid, and in particular to ultra-high purity crystalline lactic acid and a preparation method and application thereof. Background Art

[0002] Polylactic acid (PLA), also known as polylactide, is a polyester polymer derived from lactic acid. It is a novel biodegradable material. It exhibits excellent biodegradability and is completely degraded by microorganisms in nature, ultimately producing carbon dioxide and water. This is highly beneficial for environmental protection and is a recognized environmentally friendly material for use in disposable products and biopharmaceuticals.

[0003] Polylactic acid (PLA) is made from starch, a renewable plant resource. The starch is saccharified to produce glucose, which is then fermented with a specific bacterial strain to produce high-purity lactic acid. Polylactic acid of a specific molecular weight is then synthesized through chemical synthesis. Currently, the typical lactic acid production process involves extracting crude L-lactic acid from the glucose fermentation broth. This is then refined through decolorization, purification, concentration, and short-path distillation to produce polymer-grade liquid L-lactic acid with a purity of 95% to 99%. However, existing purification processes cannot achieve high separation efficiency. Because many non-lactic acid impurities have a boiling point close to that of lactic acid, obtaining high-quality L-lactic acid with a purity exceeding 99.9% is difficult. Polymer-grade liquid L-lactic acid with a purity of 95% to 98% is generally used only as food-grade lactic acid. Polymer-grade liquid L-lactic acid with a purity of 98% to 99% can be used as a polymer-grade lactic acid raw material for the production of polylactic acid via the lactide route.

[0004] Currently, due to the insufficient purity and quality of existing L-lactic acid, direct polymerization using 98% to 99% L-lactic acid can disrupt the polymerization process due to impurities, resulting in a polymer molecular weight of only 5,000 to 10,000 Da, which is insufficient to meet the molecular weight requirements for high-molecular-weight polylactic acid (PLA for plastics, which has a molecular weight of several hundred thousand Daltons). The existing method for producing PLA from polymer-grade L-lactic acid (98% to 99% purity) first dehydrates and purifies the polymer-grade L-lactic acid to produce lactide, which is then polymerized to produce PLA. In terms of production cost, the cost of preparing lactide from L-lactic acid and then polymerizing it to PLA (a two-step process) is more than 1.5 times that of the one-step process of directly polymerizing L-lactic acid to produce PLA.

[0005] Therefore, there is an urgent need to develop a method for producing ultra-high purity lactic acid to meet the purity requirements of polylactic acid directly polymerized from lactic acid, thereby reducing the production cost of existing polylactic acid and improving the quality of polylactic acid. Summary of the Invention

[0006] To address the above problems, the present invention provides an ultra-high-purity crystalline lactic acid, a preparation method, and an application thereof. Using commercially available polymerization-grade liquid L-lactic acid as raw material, the present invention successfully produces ultra-high-purity, high-quality, uniformly dense crystalline lactic acid at ultra-low temperature and ultra-high vacuum by controlling the process parameters of ultra-high vacuum evaporation crystallization.

[0007] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0008] In a first aspect, the present invention provides a method for preparing ultra-high purity crystalline lactic acid, comprising the following steps:

[0009] Polymer-grade liquid L-lactic acid is subjected to ultra-high vacuum evaporation crystallization at 6°C to 10°C and 0.7kPa to 0.9kPa. L-lactic acid seed crystals are added when the lactic acid specific gravity is 1.34 to 1.35, and crystallization is completed when the lactic acid specific gravity reaches 1.38 to 1.39 to obtain ultra-high purity crystalline lactic acid.

[0010] Compared with the prior art, the method for preparing ultra-high-purity crystalline lactic acid provided by the present invention subjects polymer-grade liquid L-lactic acid to ultra-high vacuum evaporation crystallization at a specific temperature and absolute pressure. Utilizing the limited water solubility of lactic acid at low temperatures (6°C to 10°C), ultra-high-purity crystalline lactic acid can be obtained in a single step of ultra-high vacuum evaporation crystallization. The ultra-high-purity crystalline lactic acid has a purity of over 99.95%, is granular in shape, has a high crystallization rate, is uniform and dense, has a high degree of looseness, has strong anti-caking properties, and is of excellent quality.

[0011] Through extensive testing, the present invention discovered that lactic acid has excellent water solubility at room temperature (around 25°C) and is miscible with water in almost any ratio; however, below 10°C, the solubility of lactic acid in water is greatly reduced (at 10°C, 1g of water can only dissolve less than 5g of lactic acid). The present invention utilizes this characteristic of lactic acid to creatively control the various parameters of ultra-high vacuum evaporation crystallization (including temperature, absolute pressure, and specific gravity, etc.), and can successfully produce ultra-high purity crystalline lactic acid in a single step under an ultra-low temperature and ultra-high vacuum environment. This ultra-high purity crystalline lactic acid can be directly polymerized to produce polylactic acid, which greatly reduces production costs, improves production efficiency, has extremely excellent market competitiveness, and is suitable for large-scale production.

[0012] Preferably, the temperature of the ultra-high vacuum evaporation crystallization is 7° C. to 9° C., and the absolute pressure of the ultra-high vacuum evaporation crystallization is 0.75 kPa to 0.8 kPa.

[0013] Preferably, the ultra-high vacuum evaporation crystallization time is 4 hours to 5 hours.

[0014] For example, the evaporation rate of water from polymer-grade liquid L-lactic acid under ultra-high vacuum is controlled by controlling the heating rate of the heat source, thereby controlling the rate of increase in the specific gravity of the lactic acid to meet the aforementioned specific gravity of the lactic acid and the time required for ultra-high vacuum evaporation crystallization. The heat source can be heating steam at 80°C to 100°C. To save costs and reduce energy consumption, circulating water with a temperature above 20°C can also be used. The heat source temperature is sufficient to transfer heat to the material (polymer-grade liquid L-lactic acid).

[0015] The present invention can further control the solubility range of lactic acid by limiting the temperature of ultra-high vacuum evaporation crystallization and regulating the vacuum degree, so that the specific gravity of lactic acid is within a specific range and the corresponding supersaturation coefficient is 1.05 to 1.09, thereby ensuring good crystallization of crystalline lactic acid and improving the purity and quality of the crystalline lactic acid.

[0016] It should be noted that evaporation crystallization is a process that maintains supersaturation while evaporating water. The evaporation of water requires a heat source to transfer heat to the material. Under a stable vacuum, heating the material with a heat source will gradually evaporate the water in the material while the material temperature remains essentially stable.

[0017] Preferably, the purity of the polymer-grade liquid L-lactic acid is 98% to 99%.

[0018] Preferably, the amount of the L-lactic acid seed crystals added is 0.05‰ to 0.15‰ of the mass of the polymerization-grade liquid L-lactic acid.

[0019] Preferably, after the ultra-high vacuum evaporation crystallization is completed, the method further comprises: solid-liquid separation, and drying the obtained wet lactic acid crystals to obtain the ultra-high purity crystalline lactic acid.

[0020] It should be noted that the mother liquor obtained after solid-liquid separation can be recycled and refined by short-path distillation to obtain polymerization-grade liquid L-lactic acid again, which can be recycled to prepare ultra-high purity crystalline lactic acid.

[0021] For example, wet lactic acid crystals are dried at ultra-low temperatures to prevent the lactic acid crystals from melting.

[0022] For example, the moisture content of the ultra-high purity crystalline lactic acid is ≤0.1%.

[0023] Preferably, the crystallization rate of the ultra-high purity crystalline lactic acid is 48% to 55%.

[0024] Preferably, the purity of the ultra-high purity crystalline lactic acid is ≥99.95%.

[0025] For example, ultra-high vacuum evaporation crystallization is carried out in a vacuum evaporation crystallization tank.

[0026] In a second aspect, the present invention provides an ultra-high purity crystalline lactic acid, which is prepared by the method for preparing ultra-high purity crystalline lactic acid.

[0027] The ultra-high purity crystalline lactic acid provided by the present invention is granular in shape, has a purity of more than 99.95%, a high crystallinity, is uniform and dense, has high looseness, has strong anti-caking performance, is of excellent quality, and has outstanding market competitiveness.

[0028] In a third aspect, the present invention provides a polylactic acid, which is directly polymerized from the ultra-high purity crystalline lactic acid.

[0029] The polylactic acid provided by the present invention can be directly polymerized from ultra-high purity crystalline lactic acid without going through a lactide route, thereby greatly reducing the preparation cost, improving production efficiency, and significantly improving the quality of the polylactic acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a morphology picture of the ultra-high purity crystalline lactic acid in Example 1 of the present invention;

[0031] Figure 2 This is a morphology picture of the ultra-high purity crystalline lactic acid in Example 2 of the present invention;

[0032] Figure 3 This is a morphology picture of the ultra-high purity crystalline lactic acid in Example 3 of the present invention;

[0033] Figure 4 This is a morphology picture of the ultra-high purity crystalline lactic acid in Example 4 of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] The polymer-grade liquid L-lactic acid used as a raw material in the present invention is purchased from a Henan lactic acid production enterprise, and has a purity of 98% to 99%.

[0036] Example 1

[0037] This embodiment provides a method for preparing ultra-high purity crystalline lactic acid, comprising the following steps:

[0038] S1, start the vacuum control system of the vacuum evaporation crystallization tank (referred to as the crystallization tank), pump the crystallization tank to an absolute pressure of 20kPa to 30kPa, open the feed valve, and suck the polymerization-grade liquid L-lactic acid raw material into the crystallization tank.

[0039] S2, continue to evacuate the vacuum, and when the absolute pressure reaches 0.9 kPa, switch the vacuum control system to automatic. At this time, the temperature of the crystallizer is 8°C, and introduce heating steam into the steam drum of the crystallizer to control the evaporation rate of the water in the crystallizer to avoid violent boiling. Wait until the specific gravity of lactic acid in the crystallizer rises to 1.35, and add L-lactic acid seed crystal particles (the amount of L-lactic acid seed crystals added is 0.1‰ of the mass of polymerization-grade liquid L-lactic acid).

[0040] S3, control the rate of introduction of heating steam in the steam drum. After 4.5 hours, the specific gravity of lactic acid in the crystallizer gradually rises to 1.38. Stop introducing heating steam and the vacuum control system, then open the vacuum breaker valve to remove the vacuum in the crystallizer. When the absolute pressure is 80kPa~86kPa, open the discharge valve of the crystallizer to release the lactic acid paste.

[0041] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained wet lactic acid crystals are dried to a moisture content of ≤0.1% to obtain ultra-high purity crystalline lactic acid.

[0042] After testing, the ultra-high purity crystalline lactic acid prepared in this embodiment (see Figure 1 ) has a purity of 99.99% and a crystallinity of 51.2%.

[0043] Example 2

[0044] This embodiment provides a method for preparing ultra-high purity crystalline lactic acid, comprising the following steps:

[0045] S1, start the vacuum control system of the crystallizer, pump the crystallizer to an absolute pressure of 20kPa ~ 30kPa, open the feed valve, and suck the polymerization grade liquid L-lactic acid raw material into the crystallizer.

[0046] S2, continue to evacuate, and when the absolute pressure reaches 0.9 kPa, switch the vacuum control system to automatic. At this time, the temperature of the crystallizer is 7°C, and introduce heating steam into the steam drum of the crystallizer to control the evaporation rate of the water in the crystallizer to avoid violent boiling. Wait until the specific gravity of lactic acid in the crystallizer rises to 1.34, and add L-lactic acid seed crystal particles (the amount of L-lactic acid seed crystals added is 0.06‰ of the mass of polymerization-grade liquid L-lactic acid).

[0047] S3, control the rate of introduction of heating steam in the steam drum. After 4.9 hours, the specific gravity of lactic acid in the crystallizer gradually rises to 1.38. Stop introducing heating steam and the vacuum control system, then open the vacuum breaker valve to remove the vacuum in the crystallizer. When the absolute pressure is 80kPa~86kPa, open the discharge valve of the crystallizer to release the lactic acid paste.

[0048] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained wet lactic acid crystals are dried to a moisture content of ≤0.1% to obtain ultra-high purity crystalline lactic acid.

[0049] After testing, the ultra-high purity crystalline lactic acid prepared in this embodiment (see Figure 2 ) has a purity of 99.96% and a crystallinity of 48.3%.

[0050] Example 3

[0051] This embodiment provides a method for preparing ultra-high purity crystalline lactic acid, comprising the following steps:

[0052] S1, start the vacuum control system of the crystallizer, pump the crystallizer to an absolute pressure of 20kPa ~ 30kPa, open the feed valve, and suck the polymerization grade liquid L-lactic acid raw material into the crystallizer.

[0053] S2, continue to evacuate, and when the absolute pressure reaches 0.7 kPa, switch the vacuum control system to automatic. At this time, the temperature of the crystallizer is 6°C, and circulating water at 40°C is introduced into the steam drum of the crystallizer. The evaporation rate of the water in the crystallizer is controlled to avoid violent boiling. Wait until the specific gravity of lactic acid in the crystallizer rises to 1.35, and add L-lactic acid seed crystal particles (the amount of L-lactic acid seed crystals added is 0.15‰ of the mass of polymerization-grade liquid L-lactic acid).

[0054] S3, control the rate of introduction of heating steam in the steam drum. After 4.5 hours, the specific gravity of lactic acid in the crystallizer gradually rises to 1.39. Stop introducing heating steam and the vacuum control system, then open the vacuum breaker valve to remove the vacuum in the crystallizer. When the absolute pressure is 80kPa~86kPa, open the discharge valve of the crystallizer to release the lactic acid paste.

[0055] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained wet lactic acid crystals are dried to a moisture content of ≤0.1% to obtain ultra-high purity crystalline lactic acid.

[0056] After testing, the ultra-high purity crystalline lactic acid prepared in this embodiment (see Figure 3 ) has a purity of 99.97% and a crystallinity of 49.5%.

[0057] Example 4

[0058] This embodiment provides a method for preparing ultra-high purity crystalline lactic acid, comprising the following steps:

[0059] S1, start the vacuum control system of the crystallizer, pump the crystallizer to an absolute pressure of 20kPa ~ 30kPa, open the feed valve, and suck the polymerization grade liquid L-lactic acid raw material into the crystallizer.

[0060] S2, continue to evacuate, and when the absolute pressure reaches 0.8 kPa, switch the vacuum control system to automatic. At this time, the temperature of the crystallizer is 9°C, and circulating water at 40°C is introduced into the steam drum of the crystallizer. The evaporation rate of the water in the crystallizer is controlled to avoid violent boiling. Wait until the specific gravity of lactic acid in the crystallizer rises to 1.34, and add L-lactic acid seed crystal particles (the amount of L-lactic acid seed crystals added is 0.1‰ of the mass of polymerization-grade liquid L-lactic acid).

[0061] S3, control the rate of introduction of heating steam in the steam drum. After 4 hours, the specific gravity of lactic acid in the crystallizer gradually rises to 1.38. Stop introducing heating steam and the vacuum control system, then open the vacuum breaker valve to remove the vacuum in the crystallizer. When the absolute pressure is 80kPa~86kPa, open the discharge valve of the crystallizer to release the lactic acid paste.

[0062] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained wet lactic acid crystals are dried to a moisture content of ≤0.1% to obtain ultra-high purity crystalline lactic acid.

[0063] After testing, the ultra-high purity crystalline lactic acid prepared in this embodiment (see Figure 4 ) has a purity of 99.98% and a crystallinity of 52.7%.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing ultra-high purity crystalline lactic acid, characterized in that: The following steps are involved: Ultra-high vacuum evaporation crystallization of polymer-grade liquid L-lactic acid at 6°C-10°C and 0.7kPa-0.9kPa. L-lactic acid seed crystals are added when the specific gravity of the lactic acid is 1.34-1.35, and crystallization is completed when the specific gravity of the lactic acid is 1.38-1.39 to obtain ultra-high purity crystalline lactic acid. The purity of the polymerization-grade liquid L-lactic acid is 98% to 99%, the time for the ultra-high vacuum evaporation crystallization is 4 hours to 5 hours, and the amount of the L-lactic acid seed crystals added is 0.05‰ to 0.15‰ of the mass of the polymerization-grade liquid L-lactic acid.

2. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, wherein The temperature of the ultra-high vacuum evaporation crystallization is 7° C. to 9° C., and the absolute pressure of the ultra-high vacuum evaporation crystallization is 0.75 kPa to 0.8 kPa.

3. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, wherein After the ultra-high vacuum evaporation crystallization is completed, the process further includes: solid-liquid separation, and drying the obtained wet lactic acid crystals to obtain the ultra-high purity crystalline lactic acid.

4. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, wherein The crystallization rate of the ultra-high purity crystalline lactic acid is 48% to 55%.

5. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, wherein The purity of the ultra-high purity crystalline lactic acid is ≥99.95%.

6. An ultra-high purity crystalline lactic acid, characterized in that: The ultra-high purity crystalline lactic acid is prepared by the method for preparing the ultra-high purity crystalline lactic acid according to any one of claims 1 to 5.

7. A polylactic acid, characterized in that It is prepared by direct polymerization of the ultra-high purity crystalline lactic acid described in claim 6.

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