A method for extracting diaminotoluene from polyurethane soft foam alcoholysis product and its application

Abstract

The invention provides a method for extracting diaminotoluene from polyurethane soft foam alcoholysis products. The purification scheme is to extract diaminotoluene from a mixture of diaminotoluene and an alcoholysis agent, and the separated alcoholysis agent byproduct can be recycled; the diaminotoluene produced by the invention has high purity, good color, does not contain diaminotoluene tar, can be used as a raw material for producing TDI, a coating curing agent, a special polyether initiator, a pharmaceutical intermediate, etc., and has obvious application value.

Description

Technical Field

[0001] The invention belongs to the field of polyurethane soft foam degradation and recycling, and specifically relates to a method for extracting diaminotoluene from polyurethane soft foam alcoholysis products. Technical Background

[0002] According to statistics, China produces more than 1 million tons of waste polyurethane soft foam every year. These waste soft foams are mainly landfilled or burned for power generation, which is neither environmentally friendly nor economical. With the rise of the concept of carbon emission reduction, people are increasingly aware that garbage is also a resource, a valuable renewable and recyclable resource that is misplaced. Therefore, many scientific research institutes and industries (API, ISOPA, Dow Chemical, BASF, Covestro) are promoting or studying the degradation and closed-loop utilization of polyurethane soft foam to reduce the carbon footprint of polyurethane materials.

[0003] The product composition of TDI-based polyurethane soft foam after degradation is complex, mainly containing the soft foam polyether polyol, diaminotoluene and alcoholysis agent used initially. The alcoholysis product contains diaminotoluene, which is harmful to the environment and has obvious harm to the human body. The aromatic amine contains primary amine groups and has very high reactivity, resulting in poor quality of alcoholysis products, insufficient performance and limited application. The measures to reduce the impact of diaminotoluene mainly include reducing production and converting it into other components. For example, patent CN102504331A reports the use of polycarbonate diol as an alcoholysis agent, and the aminolysis reaction of carbonate bonds and primary amines is used to eliminate the negative impact of aromatic primary amines such as diaminotoluene on the quality of alcoholysis products. For example, patent CN107286369A reports the use of monobasic or polybasic organic carboxylic acids as acidolysis agents to neutralize the by-product diaminotoluene and produce low amine value recovery polyols. For example, patent CN102585286A reports the use of anhydrides or compounds with at least one carboxyl group as treatment agents to post-treat a small amount of amine components in alcoholysis products. For example, patent CN109320764A reported the use of hexamethylenetetramine to convert aromatic amine impurities into aldehydes and ammonia. For example, patent CN1399659A reported four methods for reducing aromatic amines, one is to use organic carboxylic acids or acid anhydrides to precipitate aromatic amines, the second is to add isocyanate compounds to form allophanates, the third is to add epoxides for chain extension and addition to convert them into polyols, and the fourth is to add urea compounds to form uret.

[0004] However, to date, there is no reported solution for extracting aromatic amines, such as diaminotoluene, from alcoholysis products as products or raw materials.

[0005] In order to solve the above technical problems, the present invention develops a method for extracting diaminotoluene from polyurethane soft foam alcoholysis products. The extracted diaminotoluene has the characteristics of high purity, good color, extremely low water content, and no diaminotoluene tar. It can be used as a raw material for producing TDI, a coating curing agent, a special polyether initiator, a pharmaceutical intermediate, etc., and has industrial application value. Summary of the invention

[0006] The object of the present invention is to provide a method for extracting diaminotoluene from polyurethane soft foam alcoholysis products. The purification scheme is to extract diaminotoluene from a mixture of diaminotoluene and an alcoholysis agent, and the separated alcoholysis agent byproduct can be recycled.

[0007] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solution:

[0008] A method for extracting diaminotoluene from polyurethane soft foam alcoholysis products, the method comprising the following steps:

[0009] S1. Separation of alcoholysis products: separating the alcoholysis agent and diaminotoluene mixed components in the alcoholysis product of polyurethane soft foam by vacuum distillation;

[0010] S2, reaction precipitation and separation: adding liquid organic acid chloride to the mixed components in S1, reacting with diaminotoluene in the mixed components to precipitate, filtering to obtain an alcoholysis agent acidified with amide salt and hydrogen chloride;

[0011] S3, regeneration of alcoholysis agent and preparation of hydrochloric acid aqueous solution: nitrogen stripping is used to extract hydrogen chloride from the alcoholysis agent acidified with hydrogen chloride in S2 to obtain a pure alcoholysis agent by-product, and at the same time, nitrogen and hydrogen chloride streams are introduced into water to form a hydrochloric acid aqueous solution for standby use;

[0012] S4, regeneration of organic acid chloride: dissolving the amide salt obtained in S2 in the hydrochloric acid aqueous solution prepared in S3, reacting to obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated organic acid chloride and the upper layer is the diaminotoluene aqueous solution, and after liquid separation, the upper and lower phase components are obtained respectively;

[0013] S5. Preparation of diaminotoluene: remove water from the diaminotoluene aqueous solution obtained in S4 by distillation under reduced pressure to obtain a high-purity diaminotoluene product.

[0014] In the present invention, the amide salt is formed by the reaction of diaminotoluene and an organic acid chloride;

[0015] It should be particularly pointed out that the polyurethane soft foam alcoholysis product used in the present invention can be prepared by conventional methods in the art, specifically: using a binary or ternary small molecule polyol as an alcoholysis agent, a metal hydroxide or peroxide or an organic metal compound as a catalyst, and degrading TDI-based polyurethane soft foam to obtain a mixture containing a soft foam polyether polyol, an alcoholysis agent and diaminotoluene.

[0016] In the present invention, the alcoholysis product in step S1 is subjected to reduced pressure distillation to obtain an alcoholysis agent and diaminotoluene mixed component, wherein the alcoholysis agent has a mass proportion of 30-80% and the diaminotoluene has a mass proportion of 20-70%.

[0017] The temperature for the reduced pressure distillation of the alcoholysis product is 180-220° C., most preferably 200-210° C., and the absolute pressure is 1-5 kPa, most preferably 1-2 kPa.

[0018] In the present invention, the alcoholysis agent in step S1 is a polyol with a molecular weight of 100-3000 and a functionality of 2-3, including but not limited to ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, glycerol, polyethylene oxide / propyl ether glycol, etc., preferably diethylene glycol, dipropylene glycol, and more preferably dipropylene glycol.

[0019] In the present invention, the organic acid chloride in step S2 is a substance in liquid state at room temperature, including but not limited to acetyl chloride, chloroacetyl chloride, oxalyl chloride, benzoyl chloride, preferably acetyl chloride and benzoyl chloride, and most preferably benzoyl chloride.

[0020] In the present invention, the concentration of the hydrochloric acid aqueous solution formed by passing the nitrogen and hydrogen chloride streams into water is between 33-36wt%, preferably 35-36wt%, and most preferably 36wt%.

[0021] In the present invention, the diaminotoluene aqueous solution in step S5 is subjected to reduced pressure distillation to remove water, the temperature of the reduced pressure distillation is 40-60° C., most preferably 45-55° C., and the absolute pressure is 1-3 kPa, most preferably 2 kPa.

[0022] In the present invention, the diaminotoluene obtained by the above method has a purity of more than 99.50%, a snow-white appearance, and no diaminotoluene tar is detected. The obtained diaminotoluene can be used as a raw material for producing TDI, a coating curing agent, a special polyether initiator, a pharmaceutical intermediate, etc.

[0023] The beneficial effects of the present invention are:

[0024] (1) A simple, effective and low-cost method for purifying soft foam alcoholysis products to obtain diaminotoluene was developed.

[0025] (2) The obtained diaminotoluene has high purity and snow-white appearance, and no diaminotoluene tar is detected.

[0026] (3) Diaminotoluene can be used as raw material for the production of TDI, coating curing agent, special polyether initiator, pharmaceutical intermediate, etc.

[0027] (4) The alcoholysis agent can be recycled and reused, reducing the production cost of the polyurethane alcoholysis degradation process. DETAILED DESCRIPTION

[0028] Specific implementation examples of the present invention are given below to illustrate the present invention in detail, but these embodiments are not to be construed as limiting the present invention in any sense.

[0029] Sources of raw materials and testing equipment:

[0030] The polyurethane soft foam alcoholysis product comes from the laboratory of Wanhua Chemical Central Research Institute.

[0031] Drying equipment: Use fume hood or general blower drying oven for drying.

[0032] Alcoholysis agent, organic acid chloride, industrial grade, Sinopharm Group.

[0033] Example 1

[0034] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, containing 53% of soft foam polyether polyol, 14% of alcoholysis agent ethylene glycol and 33% of diaminotoluene, and distill at 200° C. and 2 kPa absolute pressure to obtain 4.7 kg of a mixture of alcoholysis agent and diaminotoluene.

[0035] S2. Add 7.63 kg of benzoyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a brown-yellow solid precipitate, and filter to obtain amide salt and hydrogen chloride-acidified ethylene glycol, wherein the mass of dry hydrogen chloride is 1.98 kg.

[0036] S3, pass nitrogen to strip out the hydrogen chloride from the ethylene glycol acidified with hydrogen chloride in S2 to obtain pure ethylene glycol by-product, and at the same time pass the tail gas stream into 33.52 kg of water to form a 36% saturated hydrochloric acid aqueous solution.

[0037] S4. Dissolve the amide salt obtained in S2 in the saturated hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated benzoyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0038] S5. Remove the water from the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 50° C. and an absolute pressure of 1.5 kPa to obtain a high-purity diaminotoluene product.

[0039] Product indicators: Snow-white appearance, purity 99.80%, no diaminotoluene tar detected.

[0040] Example 2

[0041] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 60% of soft foam polyether polyol, 15% of alcoholysis agent diethylene glycol and 25% of diaminotoluene, and distill it at 180° C. and 5 kPa absolute pressure to obtain 4.0 kg of a mixture of alcoholysis agent and diaminotoluene.

[0042] S2. Add 3.22 kg of acetyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a light yellow solid, and filter to obtain amide salt and diethylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 1.50 kg.

[0043] S3, pass nitrogen to strip out the hydrogen chloride from the diethylene glycol acidified by hydrogen chloride in S2 to obtain pure diethylene glycol by-product, and at the same time pass the tail gas stream into 2.75 kg of water to form a 35% hydrochloric acid aqueous solution.

[0044] S4. Dissolve the amide salt obtained in S2 in the hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated acetyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0045] S5. Remove the water from the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 40° C. and an absolute pressure of 1 kPa to obtain a high-purity diaminotoluene product.

[0046] Product indicators: Snow-white appearance, purity 99.71%, no diaminotoluene tar detected.

[0047] Example 3

[0048] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 70% of soft foam polyether polyol, 24% of alcoholysis agent dipropylene glycol and 6% of diaminotoluene, and distill it at 220° C. and an absolute pressure of 1 kPa to obtain 3.0 kg of a mixture of the alcoholysis agent and diaminotoluene.

[0049] S2. Add 1.248 kg of oxalyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a dark yellow solid, and filter to obtain amide salt and dipropylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 0.36 kg.

[0050] S3, pass nitrogen to strip out the hydrogen chloride from the dipropylene glycol acidified with hydrogen chloride in S2 to obtain pure dipropylene glycol by-product, and at the same time pass the tail gas stream into 0.73 kg of water to form a 33% hydrochloric acid aqueous solution.

[0051] S4. Dissolve the amide salt obtained in S2 in the aqueous hydrochloric acid solution prepared in S3, and stir at room temperature for 30 minutes to obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated oxalyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0052] S5. Remove the water content of the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 60° C. and an absolute pressure of 2 kPa to obtain a high-purity diaminotoluene product.

[0053] Product indicators: Snow-white appearance, purity 99.85%, no diaminotoluene tar detected.

[0054] Example 4

[0055] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 60% of soft foam polyether polyol, 20% of alcoholysis agent dipropylene glycol and 20% of diaminotoluene, and distill it at 205° C. and an absolute pressure of 1.5 kPa to obtain 4.0 kg of a mixture of alcoholysis agent and diaminotoluene.

[0056] S2. Add 4.62 kg of benzoyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a brown-yellow solid precipitate, and filter to obtain amide salt and dipropylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 1.27 kg.

[0057] S3, pass nitrogen to strip out the hydrogen chloride from the dipropylene glycol acidified with hydrogen chloride in S2 to obtain pure dipropylene glycol by-product, and at the same time pass the tail gas stream into 2.26 kg of water to form a 36% saturated hydrochloric acid aqueous solution.

[0058] S4. Dissolve the amide salt obtained in S2 in the saturated hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated benzoyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0059] S5. Remove the water content of the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 50° C. and an absolute pressure of 2.0 kPa to obtain a high-purity diaminotoluene product.

[0060] Product indicators: Snow-white appearance, purity 99.91%, no diaminotoluene tar detected.

[0061] Example 5

[0062] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 30% of soft foam polyether polyol, 49% of alcoholysis agent diethylene glycol and 21% of diaminotoluene, and distill it at 210° C. and an absolute pressure of 2.0 kPa to obtain 7.0 kg of a mixture of alcoholysis agent and diaminotoluene.

[0063] S2. Add 4.858 kg of benzoyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a brown-yellow solid precipitate, and filter to obtain amide salt and diethylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 1.334 kg.

[0064] S3, pass nitrogen to strip out the hydrogen chloride from the diethylene glycol acidified by hydrogen chloride in S2 to obtain pure diethylene glycol by-product, and at the same time pass the tail gas stream into 2.37 kg of water to form a 36% saturated hydrochloric acid aqueous solution.

[0065] S4. Dissolve the amide salt obtained in S2 in the saturated hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated benzoyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0066] S5. Remove the water content of the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 50° C. and an absolute pressure of 2.0 kPa to obtain a high-purity diaminotoluene product.

[0067] Product indicators: Snow-white appearance, purity 99.89%, no diaminotoluene tar detected.

[0068] Comparative Example 1

[0069] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 50% of soft foam polyether polyol, 35% of alcoholysis agent dipropylene glycol and 15% of diaminotoluene, and distill it at 210° C. and an absolute pressure of 2.0 kPa to obtain 5.0 kg of a mixture of the alcoholysis agent and diaminotoluene.

[0070] S2. Add 3.47 kg of benzoyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a brown-yellow solid precipitate, and filter to obtain amide salt and dipropylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 0.953 kg.

[0071] S3, pass nitrogen to strip out the hydrogen chloride from the dipropylene glycol acidified with hydrogen chloride in S2 to obtain pure dipropylene glycol by-product, and at the same time pass the tail gas stream into 2.63 kg of water to form a 17% hydrochloric acid aqueous solution.

[0072] S4. Dissolve the amide salt obtained in S2 in the hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated benzoyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0073] S5. Remove the water content of the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 50° C. and an absolute pressure of 2.0 kPa to obtain a diaminotoluene product.

[0074] Product indicators: yellow-brown appearance, purity 68.57%, tar content 31.43%.

[0075] Comparative Example 2

[0076] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 30% of soft foam polyether polyol, 49% of alcoholysis agent diethylene glycol and 21% of diaminotoluene, and distill it at 210° C. and an absolute pressure of 2.0 kPa to obtain 7.0 kg of a mixture of alcoholysis agent and diaminotoluene.

[0077] S2. Add 4.858 kg of benzoyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a brown-yellow solid precipitate, and filter to obtain amide salt and diethylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 1.334 kg.

[0078] S3, pass nitrogen to strip out the hydrogen chloride from the diethylene glycol acidified by hydrogen chloride in S2 to obtain pure diethylene glycol by-product, and at the same time pass the tail gas stream into 2.11 kg of water to form a 17% hydrochloric acid aqueous solution.

[0079] S4. Dissolve the amide salt obtained in S2 in the saturated hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated benzoyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0080] S5. Remove the water content of the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 50° C. and an absolute pressure of 2.0 kPa to obtain a high-purity diaminotoluene product.

[0081] Product indicators: yellow-brown in appearance, purity 58.41%, tar content 41.59%

[0082] Comparative Example 3

[0083] S1: Weigh 10 kg of polyurethane soft foam alcoholysis product, which contains 50% of soft foam polyether polyol, 35% of alcoholysis agent dipropylene glycol and 15% of diaminotoluene, and distill it at 210° C. and an absolute pressure of 2.0 kPa to obtain 5.0 kg of a mixture of the alcoholysis agent and diaminotoluene.

[0084] S2. Add 3.47 kg of benzoyl chloride to the mixed components in S1, stir at room temperature for 30 minutes to precipitate a brown-yellow solid precipitate, and filter to obtain amide salt and dipropylene glycol acidified with hydrogen chloride, wherein the mass of dry hydrogen chloride is 0.953 kg.

[0085] S3, pass nitrogen to strip out the hydrogen chloride from the dipropylene glycol acidified with hydrogen chloride in S2 to obtain pure dipropylene glycol by-product, and at the same time pass the tail gas stream into 5.72 kg of water to form a 10% hydrochloric acid aqueous solution.

[0086] S4. Dissolve the amide salt obtained in S2 in the hydrochloric acid aqueous solution prepared in S3, stir at room temperature for 30 minutes, and obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated benzoyl chloride organic phase, and the upper layer is the diaminotoluene aqueous solution. After separation, the upper and lower phase components are obtained respectively.

[0087] S5. Remove the water from the diaminotoluene aqueous solution obtained in S4 by reduced pressure distillation at 50° C. and an absolute pressure of 2.0 kPa to obtain a diaminotoluene product.

[0088] Product indicators: yellow-brown appearance, purity 57.14%, tar content 42.86%.

[0089] Those skilled in the art will appreciate that, based on the teachings of this specification, some modifications or adjustments may be made to the present invention, and these modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A method for extracting diaminotoluene from a polyurethane soft foam alcoholysis product, the method comprising the following steps: S1. Separation of alcoholysis products: separating the alcoholysis agent and diaminotoluene mixed components in the alcoholysis product of polyurethane soft foam by vacuum distillation; S2, reaction precipitation and separation: adding liquid organic acid chloride to the mixed components in S1, reacting with diaminotoluene in the mixed components to precipitate, filtering to obtain an alcoholysis agent acidified with amide salt and hydrogen chloride; S3, regeneration of alcoholysis agent and preparation of hydrochloric acid aqueous solution: nitrogen stripping is used to extract hydrogen chloride from the alcoholysis agent acidified with hydrogen chloride in S2 to obtain a pure alcoholysis agent by-product, and at the same time, nitrogen and hydrogen chloride streams are introduced into water to form a hydrochloric acid aqueous solution for standby use; S4, regeneration of organic acid chloride: dissolving the amide salt obtained in S2 in the hydrochloric acid aqueous solution prepared in S3, reacting to obtain a phase-separated mixed liquid, wherein the lower layer is the regenerated organic acid chloride and the upper layer is the diaminotoluene aqueous solution, and after liquid separation, the upper and lower phase components are obtained respectively; S5. Preparation of diaminotoluene: remove water from the diaminotoluene aqueous solution obtained in S4 by distillation under reduced pressure to obtain a high-purity diaminotoluene product.

2. The method according to claim 1, characterized in that In the alcoholysis product of step S1, the alcoholysis agent accounts for 30-80% by weight, and the diaminotoluene accounts for 20-70% by weight.

3. The method according to claim 1, characterized in that The temperature for the reduced pressure distillation of the alcoholysis product in step S1 is 180-220° C. and the absolute pressure is 1-5 kPa.

4. The method according to claim 1, characterized in that The temperature of the reduced pressure distillation of the alcoholysis product in step S1 is 200-210° C. and the absolute pressure is 1-2 kPa.

5. The method according to any one of claims 1 to 4, characterized in that: In the step S1, the alcoholysis agent is a polyol with a molecular weight of 100-3000 and a functionality of 2-3, including ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, glycerol, and polyethylene oxide / propyl ether glycol.

6. The method according to any one of claims 1 to 4, characterized in that: The organic acyl chloride in step S2 is a substance in liquid state at room temperature, including acetyl chloride, chloroacetyl chloride, oxalyl chloride, and benzoyl chloride.

7. The method according to any one of claims 1 to 4, characterized in that: In step S3, the concentration of the hydrochloric acid aqueous solution formed by the nitrogen and hydrogen chloride streams being introduced into water is between 33-36 wt %.

8. The method according to claim 6, characterized in that In step S3, the concentration of the hydrochloric acid aqueous solution formed by passing the nitrogen and hydrogen chloride streams into water is between 35-36 wt %.

9. The method according to any one of claims 1 to 4, characterized in that: In step S5, the diaminotoluene aqueous solution is subjected to reduced pressure distillation to remove water, and the temperature of the reduced pressure distillation is 40-60° C. and the absolute pressure is 1-3 kPa.