Method for separating and purifying long-chain fatty acid

Abstract

The invention discloses a method for separating and purifying long-chain fatty acid, which takes rapeseed oil containing nervonic acid as a raw material for separation and purification, and comprises the following steps: carrying out methyl esterification on the rapeseed oil to obtain mixed fatty acid methyl ester; the mixed fatty acid methyl ester is subjected to primary distillation through a thin film evaporation technology, a light component D1 and a heavy component W1 are obtained respectively, and the carbon number of the light component D1 is smaller than or equal to 18; the heavy component W1 is subjected to secondary distillation through a short-path distillation technology, a light component D2 and a heavy component W2 are obtained, and the carbon number of the light component D2 is 22; distilling the heavy component W2 for three times by using a short-path distillation process to respectively obtain a light component D3 and a heavy component, the carbon number of the light component D3 being 24; performing acidification reduction to obtain a corresponding fatty acid product and a nervonic acid crude product; and carrying out crystallization separation on the nervonic acid crude product to obtain nervonic acid. The content of trans-acid in the nervonic acid product is lower than 1%, which is obviously superior to that of the conventional separation and purification process.

Description

Technical Field

[0001] The invention belongs to the technical field of separation and purification, and specifically relates to a method for separating and purifying long-chain fatty acids. Background Art

[0002] Long-chain fatty acids usually refer to fatty acids with more than 12 carbon atoms in the carbon chain, which are divided into: ① saturated fatty acids, such as stearic acid and palmitic acid (C16-0); ② unsaturated fatty acids, such as oleic acid (C18), erucic acid (C22-1) and nervonic acid (C24-1). Among them, nervonic acid (nervonic acid) is scientifically named: cis-15-tetradecenoic acid, referred to as cis C24:1. It is a cis Omega-9 type long-chain monoolefin fatty acid naturally present in animals and plants. Because it was first isolated from shark brain and cow brain, it is also called shark acid. Nervonic acid is an important component of the brain nerve cell membrane, and plays an important role in increasing the activity of brain nerves and preventing the weakening of brain nerves. At present, nervonic acid has been widely used in medicine and health food as a key raw material.

[0003] The production and research of neuraminic acid mainly focus on the extraction and purification of animal and plant oils rich in neuraminic acid. Such raw materials mainly include garlic fruit, rapeseed oil, maple seed oil, etc. Although these raw materials all contain a certain amount (1-40%) of neuraminic acid, the only neuraminic acid raw material that meets regulatory requirements and is recognized in the announcement of my country's National Health Commission is rapeseed oil. In order to obtain a neuraminic acid product with a purity of more than 85%wt that meets application requirements from rapeseed oil with a neuraminic acid content of only 1-4%, and at the same time realize the recovery of other useful fatty acids, the development of related separation processes is crucial.

[0004] Patent document with publication number CN101760327B discloses a method for preparing nervonic acid using rapeseed oil containing nervonic acid as raw material. Although nervonic acid with a purity of more than 95%wt is obtained by high-pressure hydrolysis, high-temperature distillation, and solvent crystallization, there are two problems: (1) due to the long residence time of the material under high temperature conditions, cis acid isomerization occurs, and the trans acid in the product increases. Although the purity of tetracosenoic acid (C24:1) is measured to be 95%-97% according to GB5009.168-2016, the actual purity of cis-15-tetracosenoic acid is only 85%-91% according to GB5009.527-2016, and the content of trans acid is as high as 4%-12%, and the presence of a large amount of trans acid has an adverse effect on the quality and safety of downstream medical and health products. (2) The purity of the crude nervonic acid obtained by distillation in the patent document is relatively low (only 40%-50%), which increases the difficulty of the subsequent solvent crystallization process and has high energy consumption costs.

[0005] In view of this, this patent application is filed. Summary of the invention

[0006] In order to solve the technical problems that the method mentioned in the background technology obtains a high content of trans structure of neuraminic acid and the purity of the crude neuraminic acid is low, the present invention application provides a novel method for separating and purifying long-chain fatty acids. The trans acid content in the neuraminic acid product obtained by this method is low (less than 1%), the purity of the crude neuraminic acid reaches more than 65%, and various fatty acid products with other carbon numbers are also separated. The process is simple and economically feasible.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for separating and purifying long-chain fatty acids, using rapeseed oil containing nervonic acid as a raw material for separation and purification, comprises the following steps:

[0009] (1) methyl esterifying rapeseed oil to obtain mixed fatty acid methyl esters;

[0010] (2) Separation of low-carbon fatty acids: The mixed fatty acid methyl ester is distilled once by thin film evaporation process to obtain a light component D1 and a heavy component W1, respectively, wherein the carbon number of the light component D1 is less than or equal to 18;

[0011] Among them, the light component D1 is obtained at the top of the thin film evaporator, and its main components are methyl oleate and methyl stearate, while the heavy component W1 is obtained at the bottom of the thin film evaporator, and the heavy component W1 is a high boiling point component with a carbon number greater than 18;

[0012] (3) Separation of C22 acid methyl ester: The heavy component W1 is subjected to secondary distillation using a short-path distillation process to obtain a light component D2 and a heavy component W2, respectively, wherein the carbon number of the light component D2 is 22;

[0013] Among them, the light component D2 is obtained at the top of the short-path distiller, and its main components are erucic acid and docosanoic acid, while the heavy component W2 is obtained at the bottom of the short-path distiller, and the heavy component W2 is a high-boiling component with a carbon number greater than 22;

[0014] (4) Separation of C24 acid methyl ester: The heavy component W2 is distilled three times using a short-path distillation process to obtain a light component D3 and a heavy component, respectively, wherein the carbon number of the light component D3 is 24;

[0015] Among them, the light component D3 is obtained at the top of the short-path distiller, and its main components are nervonic acid and tetracosanoic acid, while the heavy component in this process is obtained at the bottom of the short-path distiller;

[0016] (5) reducing the obtained light components D1, D2, and D3 to obtain corresponding fatty acid products and crude nervonic acid;

[0017] (6) The crude product of neuraminic acid is subjected to crystallization separation to obtain neuraminic acid product D4.

[0018] like Figure 1 As shown in , in the present invention, rapeseed oil is first methylated, and then distilled three times to separate low-carbon fatty acids, C22 acid methyl ester, and C24 acid methyl ester in sequence, and finally acidified and reduced, and separated by one crystallization, to obtain nervonic acid products and other fatty acid products of corresponding carbon numbers. In the process of separating fatty acids, the present invention uses a thin film evaporation process to separate low-carbon fatty acids, and short-path distillation processes are used for C22 acid methyl ester and C24 acid methyl ester, respectively. The long-chain fatty acids in rapeseed oil are separated by this "thin film evaporation + secondary short-path distillation" method, which can effectively reduce the temperature and high-temperature residence time of the separation process, and avoid the large-scale production of trans acids. At the same time, the crude nervonic acid is only crystallized and separated once, which greatly improves the product yield and reduces the production cost.

[0019] At the same time, in the present invention, the light components obtained in each process stage are detected by gas chromatograph, wherein the light components D1 and D2 are detected with reference to GB5009.168-2016, and the light components D3 and product D4 are detected with reference to GB5009.257-2016.

[0020] Preferably, the fatty acids in the rapeseed oil: the mass proportion of fatty acid components with carbon numbers less than or equal to 18 is 40%-50%, the mass proportion of fatty acid components with carbon numbers of 22 is 40%-50%, and the mass proportion of fatty acid components with carbon numbers of 24 is 1%-4%.

[0021] Preferably, the process of methyl esterification in step (1) is:

[0022] The raw material rapeseed oil is heated to 80-150°C and subjected to an ester exchange reaction with methanol to obtain mixed fatty acid methyl esters;

[0023] The mass ratio of methanol: rapeseed oil is (0.3~1):1.

[0024] The amount of methanol selected in the present invention is excessive so as to completely achieve the methyl esterification of fatty acids.

[0025] Preferably, an antioxidant is added to the mixed fatty acid methyl ester obtained in step (1), and then the fatty acids are separated;

[0026] The antioxidant is any one of antioxidant 1010, BHT, tea polyphenols, and phytic acid;

[0027] The dosage of the antioxidant accounts for 0.01% to 0.5% of the mass of the rapeseed oil.

[0028] The present invention adds an antioxidant to the mixed fatty acid methyl ester, thereby combining the "antioxidant protection" measure, and selects an antioxidant with suitable chemical properties, and enters the "thin film evaporation + short-range distillation" process along with the entire logistics process, thereby further inhibiting the generation of trans acids in the process.

[0029] Preferably, the first distillation in step (2) is carried out under the conditions of a pressure of 10-20 Pa and a temperature of 160-168° C., and the residence time of the first distillation is 150-240 s.

[0030] Preferably, the secondary distillation in step (3) is carried out at a temperature of 170-180° C. and a pressure of 1-10 Pa, and the residence time of the secondary distillation is 60-120 s.

[0031] Preferably, the three distillations in step (4) are carried out at a temperature of 160-170° C. and a pressure below 2 Pa, and the residence time of the three distillations is 20-30 s.

[0032] Preferably, the process of acidification reduction in step (5) is:

[0033] The light components D1, D2 and D3 are respectively saponified with potassium hydroxide ethanol solution, and the saponification reaction is stopped when the pH is greater than 13, and then dilute sulfuric acid is added to acidify to a pH less than 3, thereby obtaining the corresponding fatty acid products and crude nervonic acid.

[0034] Preferably, the crystallization and separation process in step (6) is: dissolving the crude neuraminic acid in petroleum ether or n-hexane or ethyl acetate or acetone, with the mass ratio of the crude neuraminic acid to the solvent being 1:2 to 1:10, slowly controlling the temperature to -5°C to 10°C, and performing a cooling crystallization; after the crystallization is completed, it is separated by extraction, and then the organic solvent is removed under low-temperature vacuum conditions to obtain the neuraminic acid product D4.

[0035] Preferably, in step (2), a thin film evaporator is used for thin film evaporation, and the evaporation area of ​​the thin film evaporator is 2m 2 , external condenser;

[0036] In steps (3) and (4), a short-path distiller is used for short-path distillation. The evaporation area of ​​the short-path distiller in step (3) is 1 m 2 , built-in condenser;

[0037] The evaporation area of ​​the short-path distiller used in step (4) is 0.1 m 2 , built-in condenser.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] The present invention uses a thin film evaporation process to separate low-carbon fatty acids during the separation of fatty acids, and uses short-path distillation processes for C22 acid methyl ester and C24 acid methyl ester, respectively. The "thin film evaporation + secondary short-path distillation" method is used to separate long-chain fatty acids in rapeseed oil, which can effectively reduce the temperature and high-temperature residence time of the separation process, and avoid the large-scale generation of trans acids. At the same time, the crude nervonic acid is only crystallized and separated once, which greatly improves the product yield and reduces the production cost.

[0040] The purity of each fatty acid product obtained by the preparation method of the present invention reaches more than 80%, which meets the market product demand. At the same time, the purity of the crude neuraminic acid reaches more than 65%. The purity of the cis-structured neuraminic acid in the neuraminic acid product is ≥90%wt, and the content of the trans-structured neuraminic acid is less than 1%, which is significantly better than the existing separation and purification process. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0042] Figure 1 A schematic flow chart of a method for separating and purifying long-chain fatty acids provided in an embodiment of the present invention.

[0043] Figure 2 This is the gas chromatogram of the C18 fatty acid obtained in Example 1.

[0044] Figure 3 This is the gas chromatogram of the C22 fatty acid (i.e., erucic acid) obtained in Example 1.

[0045] Figure 4 This is the gas chromatogram of the neuraminic acid product obtained in Example 1.

[0046] Figure 5 This is the gas chromatogram of the neuraminic acid product obtained in Example 2.

[0047] Figure 6 This is the gas chromatogram of the neuraminic acid product obtained in Comparative Example 1.

[0048] Figure 7 This is the gas chromatogram of the neuraminic acid product obtained in Comparative Example 2. DETAILED DESCRIPTION

[0049] 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 embodiments. The illustrative embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0050] Embodiment 1:

[0051] According to Figure 1 The method shown in , the specific process is:

[0052] (1) 105.3 kg of raw rapeseed oil (fatty acid composition: carbon number ≤ C18: 44.1%, C20: 6.7%, C22: 47.7%, C24: 1.5%) was heated to 80° C. and subjected to ester exchange reaction with 40.1 kg of methanol under normal pressure reflux to obtain 99.7 kg of mixed fatty acid methyl esters. 100.0 g of antioxidant 1010 was then added and mixed uniformly to obtain a mixed solution.

[0053] (2) The mixed solution was fed into the thin film evaporator #1 at a flow rate of 100 kg / h and distilled once at a pressure of 20 Pa and a temperature of 165°C. The average residence time of the liquid was 3 min, and the weight of C18 fatty acid methyl ester in the light component D1 obtained at the top of the tower was 46.2 kg; the heavy component obtained at the bottom of the tower was a high boiling point component W1 with a carbon number greater than 18.

[0054] (3) The heavy component W1 is sent to the 1# short-path distiller for secondary distillation at a temperature of 180°C and 5 Pa. The average residence time of the liquid is 1.5 min. The main component of the light component D2 obtained at the top of the tower is fatty acid methyl ester with a carbon number of 22, with a total amount of 51.1 kg; the heavy component obtained at the bottom of the tower is the high-boiling-point component W2 with a carbon number greater than 22.

[0055] (4) The heavy component W2 is sent to the 2# short-path distiller and distilled three times at a temperature of 170°C and 2 Pa, with a residence time of 20-30 seconds. The main component of the light component D3 obtained at the top of the tower is fatty acid methyl ester with a carbon number of 24 (weight is 2.2 kg), and the heavy component obtained at the bottom of the tower is high-boiling point W3 (containing antioxidant).

[0056] (5) The light components D1, D2, and D3 are respectively saponified with potassium hydroxide ethanol solution. When the pH is greater than 13, the saponification reaction is stopped. Then, dilute sulfuric acid is added to acidify to a pH less than 3, thereby obtaining the corresponding fatty acid products and crude nervonic acid.

[0057] Finally, a mixed fatty acid with a carbon number of 18 (39.4 kg, purity of 81.34%, gas chromatogram see Figure 2 ), C22 fatty acid (44.4kg, purity 86.32%, gas chromatogram see Figure 3) and crude nervonic acid (1.9 kg, purity 63.5%).

[0058] (6) Crystallization separation: The crude nervonic acid product was fully dissolved in acetone at a mass ratio of 1:5, and the temperature was slowly controlled to 0°C for a cooling crystallization. After the crystallization was completed, the filtrate was separated by extraction, and the obtained filtrate was the nervonic acid solution. The obtained nervonic acid solution was desolventized under low temperature vacuum conditions (50°C, -0.09Mpa) to obtain 1.2kg of nervonic acid product.

[0059] The gas chromatogram of the nervonic acid product obtained in Example 1 is shown in Figure 4 , and the gas chromatography detection data of the obtained neuraminic acid product are given in Table 1.

[0060] Embodiment 2:

[0061] (1) 103.2 kg of raw rapeseed oil (fatty acid composition: ≤C18: 44.1%, C20: 6.4%, C22: 47.3%, C24: 2.2%) was heated to 90°C and subjected to transesterification reaction with 45.1 kg of methanol under normal pressure reflux to obtain 100.2 kg of mixed fatty acid methyl esters, and then 120.0 g of antioxidant 1010 was added and mixed evenly.

[0062] (2) The mixed solution was fed into the thin film evaporator #1 at a flow rate of 100 kg / h and distilled once at a pressure of 15 Pa and a temperature of 160°C. The average residence time of the liquid was 2.5 min, and the weight of C18 fatty acid methyl ester in the light component D1 obtained at the top of the tower was 46.5 kg; the heavy component obtained at the bottom of the tower was a high boiling point component W1 with a carbon number greater than 18.

[0063] (3) The heavy component W1 is sent to the 1# short-path distiller for secondary distillation at a temperature of 175°C and 5 Pa. The average residence time of the liquid is 1 min. The light component D2 obtained at the top of the tower is mainly composed of 22-carbon fatty acid methyl ester, with a total amount of 50.9 kg; the heavy component obtained at the bottom of the tower is a high-boiling-point W2 with a carbon number greater than 22.

[0064] (4) The heavy component W2 is sent to the 2# short-path distiller and distilled three times at a temperature of 165°C and 1 Pa, with a residence time of 20-30 seconds. The light component D3 obtained at the top of the tower is mainly composed of 24-carbon fatty acid methyl ester (weight 2.6 kg), and the heavy component obtained at the bottom of the tower is high-boiling point W3 (containing antioxidant).

[0065] (5) The light components D1, D2, and D3 are respectively saponified with potassium hydroxide ethanol solution. When the pH is greater than 13, the saponification reaction is stopped. Then, dilute sulfuric acid is added to acidify to a pH less than 3, thereby obtaining the corresponding fatty acid products and crude nervonic acid.

[0066] Finally, a mixed fatty acid with a carbon number of 18 (39.7 kg, purity of 83.7%), C22 fatty acid (44.2 kg, purity of 86.5%) and crude neuraminic acid (2.3 kg, purity of 66.5%) were obtained.

[0067] (6) Crystallization separation: The crude nervonic acid product was fully dissolved in acetone at a mass ratio of 1:7, and the temperature was slowly controlled to 0°C for a cooling crystallization. After the crystallization was completed, the filtrate was separated by extraction, and the obtained filtrate was the nervonic acid solution. The obtained nervonic acid solution was desolventized under low temperature vacuum (50°C, -0.09Mpa) to obtain 1.4kg of nervonic acid product.

[0068] The gas chromatogram of the nervonic acid product obtained in Example 2 is shown in Figure 5 , and the gas chromatography detection data of the obtained neuraminic acid product are given in Table 1.

[0069] Comparative Example 1:

[0070] According to Example 5 of patent document CN101760327B, the purity of nervonic acid obtained was 96.2% (tested according to GB5009.168-2016). However, according to GB5009.168-2016, it was found that the purity of cis-15-tetradecenoic acid was only 90.07%, of which the trans acid content was as high as 4.68%.

[0071] The gas chromatogram of the nervonic acid product obtained in Comparative Example 1 is shown in Figure 6 , and the gas chromatography detection data of the obtained neuraminic acid product are given in Table 1.

[0072] Comparative Example 2:

[0073] According to Example 1 of patent document CN101760327B, the purity of nervonic acid obtained was 95.3% (tested according to GB5009.168-2016). However, according to GB5009.168-2016, it was found that the purity of cis-15-tetradecenoic acid was only 90.97%, of which the trans acid content was as high as 5.12%.

[0074] The gas chromatogram of the nervonic acid product obtained in Comparative Example 2 is shown in Figure 7 , and the gas chromatography detection data of the obtained neuraminic acid product are given in Table 1.

[0075] Table 1 Content (%) of nervonic acid products obtained in Examples and Comparative Examples

[0076] name serial number C22-1 Anti C22-1 Shun C24-0 C24-1 Anti C24-1 Shun Other Total Example 1 J22092326 0.04 0.08 3.76 0.02 92.76 3.34 Example 2 J22092327 0.00 0.11 2.49 0.15 93.73 3.52 Comparative Example 1 J22110121 0.3 0.48 1.32 4.68 90.07 3.23 Comparative Example 2 J22091102 0.12 0.23 0.83 5.12 90.97 2.73

[0077] C22-1 in Table 1 represents erucic acid, and C24-0 represents tetracosanoic acid. Other total items include fatty acids with a carbon number of 18 or less. And each data in Table 1 represents the purity value of each component in the corresponding product.

[0078] According to Table 1, in the neuraminic acid product of Example 1, the purity of the C24-1 cis structure is 92.76%, in the neuraminic acid product of Example 2, the purity of the C24-1 cis structure is 93.73%, and the purity of neuraminic acid (C24-0, C24-1 trans, C24-1 cis) reaches 96.37%, which is greater than 90.0%. The content of the C24-1 trans structure in Examples 1 and 2 is both lower than 1.0%, which is at a relatively low level. The content of the C24-1 trans structure obtained by the methods of Comparative Examples 1 and 2 is higher. It can be seen that the purity of the C24-1 cis structure in Comparative Examples 1 and 2 is about 90.0%, which is lower than the purity of the cis structure in Examples 1 and 2.

[0079] The above specific implementation methods further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation methods of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. 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 separating and purifying long-chain fatty acids, characterized in that: The method uses rapeseed oil containing nervonic acid as raw material for separation and purification, comprising the following steps: (1) methyl esterifying rapeseed oil to obtain mixed fatty acid methyl esters; (2) Separation of low-carbon fatty acids: The mixed fatty acid methyl ester is distilled once by thin film evaporation process to obtain a light component D1 and a heavy component W1, respectively, wherein the carbon number of the light component D1 is less than or equal to 18; (3) Separation of C22 acid methyl ester: The heavy component W1 is subjected to secondary distillation using a short-path distillation process to obtain a light component D2 and a heavy component W2, respectively, wherein the carbon number of the light component D2 is 22; (4) Separation of C24 acid methyl ester: The heavy component W2 is distilled three times using a short-path distillation process to obtain a light component D3 and a heavy component, respectively, wherein the carbon number of the light component D3 is 24; (5) reducing the obtained light components D1, D2, and D3 to obtain corresponding fatty acid products and crude nervonic acid; (6) The crude product of neuraminic acid is subjected to crystallization separation to obtain neuraminic acid.

2. A method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: The fatty acids in the rapeseed oil include: fatty acid components with carbon numbers less than or equal to 18 accounting for 40%-50% by weight, fatty acid components with carbon numbers of 22 accounting for 40%-50% by weight, and fatty acid components with carbon numbers of 24 accounting for 1%-4% by weight.

3. A method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: The process of methyl esterification in step (1) is: The raw material rapeseed oil is heated to 80-150°C and subjected to an ester exchange reaction with methanol to obtain mixed fatty acid methyl esters; The mass ratio of methanol: rapeseed oil is (0.3~1):

1.

4. A method for separating and purifying long-chain fatty acids according to claim 3, characterized in that: An antioxidant is also added to the mixed fatty acid methyl ester obtained in step (1), and then the fatty acids are separated; The antioxidant is any one of antioxidant 1010, BHT, tea polyphenols, and phytic acid; The dosage of the antioxidant accounts for 0.01% to 0.5% of the mass of the rapeseed oil.

5. The method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: The first distillation in step (2) is carried out under the conditions of a pressure of 10-20 Pa and a temperature of 160-168° C., and the residence time of the first distillation is 150-240 s.

6. The method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: During the secondary distillation in step (3), the temperature is 170-180° C. and the pressure is 1-10 Pa. The residence time of the secondary distillation is 60-120 s.

7. A method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: The three distillations in step (4) are carried out at a temperature of 160-170° C. and a pressure below 2 Pa, and the residence time of the three distillations is 20-30 s.

8. The method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: The process of acidification reduction in step (5) is: The light components D1, D2 and D3 are respectively saponified with potassium hydroxide ethanol solution, and the saponification reaction is stopped when the pH is greater than 13, and then dilute sulfuric acid is added to acidify to a pH less than 3, thereby obtaining the corresponding fatty acid products and crude nervonic acid.

9. A method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: The crystallization separation process in step (6) is as follows: dissolving the crude nervonic acid in petroleum ether or n-hexane or ethyl acetate or acetone, with the mass ratio of the crude nervonic acid to the solvent being 1:2 to 1:10, slowly controlling the temperature to -5°C to 10°C, and performing a cooling crystallization; After the crystallization is completed, it is separated by extraction, and then the organic solvent is removed under low temperature vacuum conditions to obtain the neuraminic acid product D4.

10. The method for separating and purifying long-chain fatty acids according to claim 1, characterized in that: In step (2), a thin film evaporator is used for thin film evaporation, and the evaporation area of ​​the thin film evaporator is 2m 2 , external condenser; In steps (3) and (4), a short-path distiller is used for short-path distillation. The evaporation area of ​​the short-path distiller in step (3) is 1 m 2 , built-in condenser; The evaporation area of ​​the short-path distiller used in step (4) is 0.1 m 2 , built-in condenser.

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