Ultrahigh-purity crystalline lactic acid as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lactic acid, and particularly discloses ultra-high-purity crystalline lactic acid as well as a preparation method and application thereof. The method comprises the following steps: carrying out ultrahigh vacuum evaporative crystallization on polymer-grade liquid L-lactic acid at 6-10 DEG C and 0.7-0.9 kPa, adding an L-lactic acid seed crystal when the specific gravity of lactic acid is 1.34-1.35, and ending crystallization until the specific gravity of lactic acid is 1.38-1.39, thereby obtaining the ultrahigh-purity crystalline lactic acid. According to the method, the limited water solubility of the lactic acid at low temperature is utilized, the ultra-high-purity crystalline lactic acid can be obtained through one step of ultra-high vacuum evaporative crystallization at ultra-low temperature and ultra-high vacuum degree, the purity of the ultra-high-purity crystalline lactic acid is up to 99.95% or above, and the ultra-high-purity crystalline lactic acid is granular, high in crystallization rate, uniform and dense, high in looseness, high in anti-caking performance and excellent in quality. The ultra-high-purity crystalline lactic acid can be directly polymerized to be used for producing polylactic acid, the production efficiency is improved, the production cost is greatly reduced, and the ultra-high-purity crystalline lactic acid has extremely 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 obtained by polymerization of lactic acid as the main raw material. It is a new type of biodegradable material. Polylactic acid has good biodegradability and can be completely degraded by microorganisms in nature after use, eventually generating carbon dioxide and water without polluting the environment. This is very beneficial to environmental protection and is a recognized environmentally friendly material that can be used in disposable products and biopharmaceuticals.

[0003] Polylactic acid is made from starch extracted from renewable plant resources. The starch raw material is saccharified to obtain glucose, and then glucose and certain strains are fermented to produce high-purity lactic acid, and then polylactic acid of a certain molecular weight is synthesized by chemical synthesis. At present, the general production process of lactic acid is to extract crude L-lactic acid from glucose fermentation broth, and then obtain polymer-grade liquid L-lactic acid with a purity of 95% to 99% through refining methods such as decolorization, purification, concentration and short-path distillation. However, the existing purification process cannot achieve a high separation effect. Since many non-lactic acid impurities are close to the boiling point of lactic acid, it is difficult to obtain high-quality L-lactic acid with a purity higher than 99.9%. 95% to 98% polymer-grade liquid L-lactic acid is generally only used as food-grade lactic acid, and 98% to 99% polymer-grade liquid L-lactic acid can be used as a polymer-grade lactic acid raw material for the preparation of polylactic acid through the lactide route.

[0004] At present, due to the insufficient purity and quality of existing L-lactic acid, when 98% to 99% L-lactic acid is used for direct polymerization, impurities will interrupt the polymerization process, resulting in the molecular weight of the polymer can only reach 5000Da to 10000Da, which cannot meet the molecular weight requirements of high molecular weight polylactic acid (the molecular weight of polylactic acid for plastics is hundreds of thousands of Daltons). The existing method for preparing polylactic acid from polymerization-grade L-lactic acid raw materials (purity 98% to 99%) is to first dehydrate and purify the polymerization-grade L-lactic acid to obtain lactide, and then polymerize the lactide to generate polylactic acid. From the perspective of production cost, the production cost of first preparing lactide from L-lactic acid and then polymerizing it into polylactic acid (two-step method) is more than 1.5 times that of directly polymerizing L-lactic acid into polylactic acid (one-step method).

[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] In view of the above problems, the present invention provides an ultra-high purity crystalline lactic acid and a preparation method and application thereof. Using the existing commercially available polymerization-grade liquid L-lactic acid as a raw material, by controlling the process parameters of ultra-high vacuum evaporation crystallization, ultra-high purity, high quality, uniform and dense crystalline lactic acid is successfully produced under ultra-low temperature and ultra-high vacuum.

[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] The polymerization-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, and L-lactic acid seed crystals are added when the specific gravity of the lactic acid is 1.34 to 1.35, and the crystallization is terminated when the specific gravity of the lactic acid is 1.38 to 1.39, thereby obtaining ultra-high purity crystalline lactic acid.

[0010] Compared with the prior art, the preparation method of ultra-high purity crystalline lactic acid provided by the present invention performs ultra-high vacuum evaporation crystallization on polymerization-grade liquid L-lactic acid at a specific temperature and absolute pressure, and utilizes the limited water solubility of lactic acid at low temperatures (6°C to 10°C). Ultra-high purity crystalline lactic acid can be obtained by only one step of ultra-high vacuum evaporation crystallization. The purity of the ultra-high purity crystalline lactic acid is as high as 99.95% or more, the shape is granular, the crystallization rate is high, the uniformity and density are high, the looseness is high, the anti-caking performance is strong, and the quality is excellent.

[0011] The present invention has found through a large number of experiments that lactic acid has excellent water solubility at room temperature (about 25°C) and can be 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 and creatively controls 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 one step in an ultra-low temperature and ultra-high vacuum environment. The ultra-high purity crystalline lactic acid can be directly polymerized for the production of 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 4h to 5h.

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

[0015] The present invention can further control the solubility region 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 1.05-1.09, thereby ensuring good crystallization of crystalline lactic acid and improving the purity and quality of crystalline lactic acid.

[0016] It should be noted that evaporation crystallization is a process of evaporating water while maintaining supersaturation. The evaporation of water requires a heat source to transfer heat to the material. Under vacuum and stable conditions, heating the material with a heat source will gradually evaporate the water in the material, while the material temperature remains basically 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 process 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 purified 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 avoid melting of the crystalline lactic acid.

[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 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 a high looseness, has a strong anti-caking performance, is of excellent quality, and has excellent 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, while improving the production efficiency, and the quality of the polylactic acid can also be significantly improved. 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 solution 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 used to limit the present invention.

[0035] The raw material of the present invention, polymer-grade liquid L-lactic acid, 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, 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 heating steam is introduced into the drum of the crystallizer to control the evaporation rate of the water in the crystallizer to avoid violent boiling. Wait for the specific gravity of lactic acid in the crystallizer to rise to 1.35, and add L-lactic acid seed particles (the amount of L-lactic acid seed added is 0.1‰ of the mass of polymerization-grade liquid L-lactic acid).

[0040] S3, control the speed of introducing heating steam into 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, stop the vacuum control system, and 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 crystal paste.

[0041] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained lactic acid wet 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 (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 to 30kPa, open the feed valve, and suck the polymerization-grade liquid L-lactic acid raw material into the crystallizer.

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

[0047] S3, control the speed of introducing heating steam into 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, stop the vacuum control system, and 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 crystal paste.

[0048] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained lactic acid wet 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 (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 to 30kPa, open the feed valve, and suck the polymerization-grade liquid L-lactic acid raw material into the crystallizer.

[0053] S2, continue to evacuate, when the absolute pressure is 0.7 kPa, switch the vacuum control system to automatic, at this time the temperature of the crystallizer is 6 ° C, pass 40 ° C circulating water into the crystallizer drum, control the evaporation rate of the water in the crystallizer to avoid violent boiling, wait for the specific gravity of lactic acid in the crystallizer to rise to 1.35, add L-lactic acid seed particles (the amount of L-lactic acid seed added is 0.15‰ of the mass of polymerization-grade liquid L-lactic acid).

[0054] S3, control the speed of introducing heating steam into 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, stop the vacuum control system, and 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 crystal paste.

[0055] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained lactic acid wet 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 (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 to 30kPa, open the feed valve, and suck the polymerization-grade liquid L-lactic acid raw material into the crystallizer.

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

[0061] S3, control the speed of introducing heating steam into the steam drum. After 4 hours, the specific gravity of lactic acid in the crystallizer gradually rises to 1.38. Stop introducing heating steam, stop the vacuum control system, and 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 crystal paste.

[0062] S4, the lactic acid crystal paste is centrifuged to remove the mother liquor, and the obtained lactic acid wet 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 (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 modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing ultra-high purity crystalline lactic acid, characterized in that: The following steps are involved: The polymerization-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, and L-lactic acid seed crystals are added when the specific gravity of the lactic acid is 1.34 to 1.35, and the crystallization is terminated when the specific gravity of the lactic acid is 1.38 to 1.39, thereby obtaining ultra-high purity crystalline lactic acid.

2. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, characterized in that: 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, characterized in that: The ultra-high vacuum evaporation crystallization time is 4h to 5h.

4. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, characterized in that: The purity of the polymer-grade liquid L-lactic acid is 98% to 99%.

5. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, characterized in that: The amount of L-lactic acid seed crystals added is 0.05‰ to 0.15‰ of the mass of the polymerization-grade liquid L-lactic acid.

6. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, characterized in that: 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.

7. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, characterized in that: The crystallization rate of the ultra-high purity crystalline lactic acid is 48% to 55%.

8. The method for preparing ultra-high purity crystalline lactic acid according to claim 1, characterized in that: The purity of the ultra-high purity crystalline lactic acid is ≥99.95%.

9. An ultra-high purity crystalline lactic acid, characterized in that: The ultra-high purity crystalline lactic acid is prepared by the method for preparing ultra-high purity crystalline lactic acid according to any one of claims 1 to 8.

10. A polylactic acid, characterized in that: It is formed by direct polymerization of the ultra-high purity crystalline lactic acid described in claim 9.

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