A process for the production of di(2-propylheptyl) phthalate

Abstract

The application relates to the technical field of environment-friendly plasticizers, and discloses a production process of di(2-propylheptyl) phthalate. The process takes phthalic anhydride and 2-propylheptyl alcohol as raw materials, takes tetraisopropyl titanate as a catalyst, and sequentially completes esterification reaction, flash alcohol removal, neutralization and decolorization, and stripping alcohol removal and dehydration in a single reaction kettle, cancels the material transfer, material cooling, water washing and secondary heating process steps of the traditional two-kettle method. The application shortens the production cycle, reduces equipment investment and public engineering energy consumption, reduces sewage discharge, improves product quality stability, and prepares di(2-propylheptyl) phthalate products with high flash point, low volatility and excellent plasticizing performance.

Description

Technical Field

[0001] This invention relates to the field of environmentally friendly plasticizers, and in particular to a production process for di(2-propylheptyl) phthalate. Background Technology

[0002] Di(2-propylheptyl) phthalate (DPHP) is a novel environmentally friendly plasticizer developed in recent years, with the molecular formula C2. 28 H 46 O4, with a relative molecular weight of 446.66, is characterized by its high molecular weight, high flash point, extremely low volatility, and excellent plasticizing efficiency. Compared with traditional dioctyl phthalate (DOP), DPHP exhibits a nearly 20-fold difference in vapor pressure at 50°C, significantly improving the temperature resistance of plasticized products and reducing the risk of human contact. Simultaneously, its density is lower than that of DOP, diisononyl phthalate (DINP), and diisodecyl phthalate (DIDP), effectively improving the volumetric efficiency of plasticized products, making it an ideal upgraded alternative to traditional plasticizers.

[0003] As early as 2002, the article "Application and Performance Review of the Novel Plasticizer DPHP" in the 6th issue of the magazine "Plastic Additives" (pp. 36-39) provided a detailed introduction to the origin and application performance of DPHP. However, due to the lack of domestic manufacturers of 2-propylheptanol, market acceptance was low, and industrial production was not achieved in China at that time. In 2012, the first industrial-scale 2-propylheptanol plant in China was put into operation at Yangzi Petrochemical-BASF Co., Ltd. (information sourced from "Market Analysis of 2-propylheptanol" in Volume 28, Issue 6 of "Petrochemical Technology and Economy"). In 2013, domestic companies such as Jinling Petrochemical Chemical Plant No. 1 and Jiangsu Hongxin Chemical Co., Ltd. began constructing DPHP pilot production plants, all using the traditional two-stage reactor process. The core process of the traditional two-reactor process is as follows: In the esterification reactor, phthalic anhydride, 2-propylheptanol, and catalyst undergo esterification. After the acidity is within acceptable limits, the material is transferred to the processing reactor. In the processing reactor, the following steps are performed sequentially: cooling, neutralization with alkali water, washing with water, draining the water, reheating, and steam stripping to remove alcohol. Finally, after passing chemical analysis, the product is filtered to obtain the final product. This process has the following drawbacks: 1. The process is complicated and the production cycle is long. The material transfer between the esterification kettle and the treatment kettle and the multiple heating and cooling processes take up a lot of production time. The equipment occupies a large area and the equipment investment cost is high for the same production capacity. 2. The energy consumption is extremely high. The materials need to be cooled to about 85°C for neutralization and washing, and then heated to about 200°C for stripping. A large amount of heat energy is wasted, and the consumption of public utilities is high. 3. Significant environmental pressure: The process includes a water washing step, which generates a large amount of high-COD organic wastewater, resulting in high wastewater treatment costs and significant environmental risks. 4. Poor product quality stability: repeated material transfer, heating and cooling, and washing processes can easily lead to fluctuations in product color, acid value, volume resistivity, and other indicators. At the same time, it can easily trigger saponification and hydrolysis side reactions of esters, reducing product yield.

[0004] Therefore, it is necessary to develop a production process for di(2-propylheptyl) phthalate to address the aforementioned defects. Summary of the Invention

[0005] The purpose of this invention is to provide a production process for di(2-propylheptyl) phthalate that can shorten the production cycle, reduce equipment investment and energy consumption, reduce wastewater discharge and improve product quality.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A process for preparing di(2-propylheptyl) phthalate specifically includes the following steps: S1, Esterification reaction Open the alcohol addition valve and add the metered 2-propylheptanol to the reactor using the alcohol addition pump. After the alcohol addition is complete, start the stirrer and add the metered phthalic anhydride through the feed port. After closing the feed port, introduce heat transfer oil into the reactor coil to raise the temperature. When the material temperature reaches 120-130℃, the monoesterification reaction is completed. Phthalic anhydride reacts with 2-propylheptanol to produce mono-2-propylheptanyl phthalate. This reaction can proceed spontaneously without a catalyst. Continue heating until the liquid phase temperature of the material reaches 175-180℃, then add the metered tetraisopropyl titanate catalyst and continue heating. When the liquid phase temperature of the material reaches 215-235℃, slowly turn on the vacuum system and adjust the system vacuum degree between 0 and -0.07 MPa. Maintain the negative pressure and carry out the dieesterification reaction. Phthalic anhydride and 2-propylheptanol react under the action of the catalyst to produce di(2-propylheptanyl) phthalate and water. During the reaction, 2-propylheptanol forms an azeotrope with the water produced in the reaction. After being condensed by a packed tower at the top of the reactor and a condenser, the azeotrope enters an oil-water separator, where water is separated by gravity difference. The 2-propylheptanol is refluxed back into the reaction system, continuously driving the esterification reaction forward. The endpoint control indicators are as follows: liquid temperature of the material 220–235℃, vapor temperature at the top of the tower 195–210℃, acid value of the system ≤0.135% (calculated as KOH), and color of the material ≤70#. If the acid value is between 0.135% and 0.2%, and cannot be reduced to below 0.135% within half an hour, the process can proceed to the next step.

[0007] The molar ratio of phthalic anhydride to 2-propylheptanol is 1:2.5 to 2.9 (the theoretical reaction molar ratio is 1:2), and the weight ratio is 1:2.5 to 3.05 (the theoretical reaction weight ratio is 1:2.135). Excess 2-propylheptanol can, on the one hand, enhance the forward reaction and increase the conversion rate of phthalic anhydride, and on the other hand, act as a dehydrating agent to form an azeotrope with the water generated in the reaction to remove water from the system.

[0008] S2, flash evaporation deethanolation After the esterification reaction is successful, there is no need to transfer the material. The vacuum degree is increased directly in the original reactor. The vacuum degree is slowly adjusted according to the alcohol discharge rate and reflux alcohol temperature. The final system vacuum degree is not lower than -0.088MPa. The residual heat of the esterification reaction is used throughout the process to carry out flash evaporation under high temperature and negative pressure conditions to remove most of the excess 2-propylheptanol in the system. The removed alcohol is collected in the alcohol-water collection tank after condensation and can be recycled to the next reactor feed.

[0009] S3, neutralization and decolorization After flash evaporation, maintain the system under vacuum and control the material temperature to ≥180℃. Add the metered alkaline solution to the reactor through the feed line at the top of the reactor. Distribute the alkaline solution evenly into the material through the alkaline solution distribution plate at the bottom of the reactor to neutralize the unreacted mono-2-propylheptyl phthalate, while destroying the residual catalyst and improving the color of the product.

[0010] The alkaline solution is a sodium hydroxide aqueous solution with a mass concentration of 15-30%. The amount of material added is 0.5-0.9‰ of the total mass of the material. The specific amount is adjusted according to the actual acid value of the material. The alkaline solution is added within 5 minutes, and the endpoint of the neutralization reaction is controlled by the acid value of the material being ≤0.07mgKOH / g.

[0011] This step rapidly completes the neutralization reaction under high temperature and vacuum conditions, shortening the residence time of alkaline water in the material. This effectively inhibits the saponification side reaction of di(2-propylheptyl) phthalate, ensuring product yield.

[0012] S4, stripping, deethanolation, and dehydration After neutralization, there is no need to vent. Maintain the system vacuum at no less than -0.095 MPa and control the material temperature at 190-210℃. Introduce deionized softened water into the steam distribution plate in the reactor through the top pipeline. Using the principle of steam distillation, water and residual 2-propylheptanol form an azeotrope and are distilled off. After condensation, the water is separated, achieving deep de-alcoholization.

[0013] The flow rate of deionized softened water is 100–800 L / H, and the stripping time is 60–90 minutes. After stripping, nitrogen is introduced for 10 minutes, followed by dry evaporation for 10 minutes. Samples are then taken to test the product indicators. The qualified indicators are: flash point ≥ 210℃, acid value ≤ 0.07 mg KOH / g, and volume resistivity ≥ 1.0 × 10⁻⁶. 12 Ω·cm; If the volume resistivity is not up to standard, continue dry evaporation for 20 minutes under a vacuum of not less than -0.095MPa until the index is up to standard.

[0014] S5, Finished Product Filtration Diatomaceous earth filter aid is added to the stripped material and mixed evenly under negative pressure. The mixture is then filtered through a high-efficiency plate-type closed filter to remove activated carbon, filter aid, and other mechanical impurities. After filtration, the material is free of visible impurities, transparent, and anhydrous, thus obtaining di(2-propylheptyl) phthalate, which is then transported to the finished product storage tank for storage.

[0015] Compared with the prior art, the beneficial technical effects of the present invention are as follows: 1. This invention adopts a single-reactor process, which eliminates the material transfer step from the esterification reactor to the processing reactor, saving transfer time, material cooling time, water washing and settling time, and secondary heating time, thus shortening the production cycle, improving production efficiency, reducing equipment footprint, and lowering equipment investment costs.

[0016] 2. The process of the present invention utilizes the residual heat of the esterification reaction to complete the flash evaporation, neutralization and stripping processes in sequence, eliminating the heat loss from repeated heating and cooling of materials and reducing the energy consumption of public utilities; at the same time, the water washing process is eliminated, eliminating the need for the energy consumption and cost of supporting water washing water heating and wastewater treatment, thus reducing production costs.

[0017] 3. The traditional water washing process has been eliminated, and only a small amount of condensate wastewater is generated during the esterification reaction and stripping process. The amount of wastewater discharged is reduced, which lowers the cost of wastewater treatment and environmental risks, and meets the requirements of green chemical production. Detailed Implementation

[0018] The core of this invention is to provide a production process for di(2-propylheptyl) phthalate, which can shorten the production cycle, reduce equipment investment and energy consumption, reduce wastewater discharge and improve product quality.

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] All raw materials used in the embodiments of this invention are industrial-grade qualified products, and all testing equipment and methods used comply with national / industry standards: acid value is determined according to GB / T 1668-2008 "Determination of Acid Value and Acidity of Plasticizers"; flash point is determined according to GB / T 1671-2008 "Determination of Flash Point of Plasticizers - Cleveland Open Cup Method"; volume resistivity is determined according to GB / T 1672-1988 "Determination of Volume Resistivity of Liquid Plasticizers"; color is determined according to GB / T 1664-1995 "Determination of Appearance Color of Plasticizers"; purity is detected by gas chromatography.

[0021] Example 1 S1, towards 30m 3 14,000 kg of 2-propylheptanol was added to the reactor, and stirring was started. 5,000 kg of phthalic anhydride was then added. Heat transfer oil was introduced to raise the temperature, which was maintained at 125°C to complete the monoesterification reaction. The temperature was further increased to the liquidus temperature of 178°C, and 12 kg of tetraisopropyl titanate catalyst was added. The temperature was then raised to 220°C, and a vacuum was slowly introduced, adjusting the system vacuum to -0.05 MPa. The reaction was maintained under negative pressure. Water generated during the reaction was continuously separated, and the 2-propylheptanol was refluxed back to the reactor. After the reaction, samples were taken for testing. The acid value of the material was 0.12% (based on KOH), the liquid temperature was 228°C, the vapor temperature at the top of the column was 202°C, and the color was 45#, indicating that the esterification endpoint had been reached.

[0022] S2. After the esterification is qualified, the vacuum degree of the system is further increased to -0.090MPa. The residual heat of esterification is used for flash evaporation for 30 minutes to remove most of the excess 2-propylheptanol. The recovered alcohol is condensed and then sent to the alcohol storage tank.

[0023] S3. After flash evaporation, maintain the system vacuum at -0.090 MPa and the material temperature at 185℃. Within 5 minutes, add 11.2 kg of 20% sodium hydroxide aqueous solution through the top of the reactor. Stir and react for 10 minutes, then take a sample for testing. The acid value of the material is 0.04 mg KOH / g, reaching the neutralization endpoint.

[0024] S4. After neutralization, increase the system vacuum to -0.097 MPa, control the material temperature at 200℃, and introduce deionized softened water. Initially, the flow rate is controlled at 160 L / H, and as the amount of alcohol evaporated decreases, the water flow rate is gradually increased, eventually reaching 700 L / H. Stripping takes 75 minutes. After stripping, nitrogen is introduced for 10 minutes, followed by dry evaporation for 10 minutes. Samples are taken for testing. The product has a flash point of 221℃, an acid value of 0.03 mg KOH / g, and a volume resistivity of 1.8 × 10⁻⁶. 12 Ω·cm, all indicators are qualified.

[0025] S5. Add 20 kg of diatomaceous earth filter aid to the material, mix evenly under negative pressure, and then filter through a high-efficiency plate-type closed filter to obtain di(2-propylheptyl) phthalate.

[0026] In this embodiment, a final product of 15008 kg was obtained, with a product yield of 99.6% and a purity of 99.72% as determined by gas chromatography. No water washing wastewater was generated during the production process, only 1160 kg of condensate wastewater from the esterification and stripping processes.

[0027] Example 2 S1, towards 30m 3 14100 kg of 2-propylheptanol was added to the reactor, and stirring was started. 4850 kg of phthalic anhydride was then added. Heat transfer oil was introduced to raise the temperature, and the material was kept at 128°C to complete the monoesterification reaction. The temperature was further raised to the liquidus temperature of 175°C, and 12.5 kg of tetraisopropyl titanate catalyst was added. The temperature was further raised to 218°C, and a vacuum was slowly opened, adjusting the system vacuum to -0.06 MPa. The reaction was maintained under negative pressure. Water generated during the reaction was continuously separated, and the 2-propylheptanol was refluxed back to the reactor. After the reaction, samples were taken for testing. The acid value of the material was 0.11% (based on KOH), the liquid temperature was 225°C, the vapor temperature at the top of the column was 200°C, and the color was 40#, indicating that the esterification endpoint had been reached.

[0028] S2. After the esterification is qualified, the vacuum degree of the system is further increased to -0.091MPa. The residual heat of esterification is used for flash evaporation for 40 minutes to remove most of the excess 2-propylheptanol. The recovered alcohol is condensed and then sent to the alcohol storage tank.

[0029] S3. After flash evaporation, maintain the system vacuum at -0.091 MPa and the material temperature at 188℃. Within 5 minutes, add 11 kg of 25% sodium hydroxide aqueous solution through the top of the reactor. After stirring and reacting for 8 minutes, take a sample for testing. The acid value of the material is 0.035 mg KOH / g, reaching the neutralization endpoint.

[0030] S4. After neutralization, increase the system vacuum to -0.098 MPa, control the material temperature at 195℃, and introduce deionized softened water. Initially, control the flow rate at 200 L / H. As the amount of refluxed alcohol decreases, gradually increase the water flow rate, eventually controlling it to 800 L / H. Strip for 60 minutes. After stripping, introduce nitrogen for 10 minutes, dry evaporate for 10 minutes, and take samples for testing. The product has a flash point of 223℃, an acid value of 0.028 mg KOH / g, and a volume resistivity of 2.1 × 10⁻⁶. 12 Ω·cm, all indicators are qualified.

[0031] S5. Add 18 kg of diatomaceous earth filter aid to the material, mix evenly under negative pressure, and then filter through a high-efficiency plate-type closed filter to obtain di(2-propylheptyl) phthalate.

[0032] In this embodiment, the yield was 14,543 kg, with a yield of 99.5% and a gas chromatography purity of 99.6%. The production process only generated 1,090 kg of condensate wastewater and no water washing wastewater.

[0033] Example 3 S1, towards 30m 3 14210 kg of 2-propylheptanol was added to the reactor, and stirring was started. 4900 kg of phthalic anhydride was then added. Heat transfer oil was introduced to raise the temperature, and the material was kept at 130°C to complete the monoesterification reaction. The temperature was further raised to the liquid phase temperature of 180°C, and 11.5 kg of tetraisopropyl titanate catalyst was added. The temperature was further raised to 222°C, and a vacuum was slowly opened, adjusting the system vacuum to -0.07 MPa. The reaction was maintained under negative pressure. Water generated during the reaction was continuously separated, and the 2-propylheptanol was refluxed back to the reactor. After the reaction, samples were taken for testing. The acid value of the material was 0.10% (based on KOH), the liquid temperature was 232°C, the vapor temperature at the top of the column was 208°C, and the color was 35#, indicating that the esterification endpoint had been reached.

[0034] S2. After the esterification is qualified, the vacuum degree of the system is further increased to -0.092MPa. The residual heat of esterification is used for flash evaporation for 35 minutes to remove most of the excess 2-propylheptanol. The recovered alcohol is condensed and then sent to the alcohol storage tank.

[0035] S3. After flash evaporation, maintain the system vacuum at -0.092 MPa and the material temperature at 190℃. Within 5 minutes, add 8.5 kg of 30% sodium hydroxide aqueous solution through the top of the reactor. After stirring and reacting for 5 minutes, take a sample for testing. The acid value of the material is 0.03 mg KOH / g, reaching the neutralization endpoint.

[0036] S4. After neutralization, increase the system vacuum to -0.098 MPa, control the material temperature at 205℃, and introduce deionized softened water. Initially, control the flow rate at 240 L / H, gradually increasing it according to the alcohol output, eventually controlling it to 750 L / H. Strip for 90 minutes. After stripping, introduce nitrogen for 10 minutes, dry evaporate for 10 minutes, and take samples for testing. The product flash point is 225℃, acid value is 0.025 mg KOH / g, and volume resistivity is 2.5 × 10⁻⁶. 12 Ω·cm, all indicators are qualified.

[0037] S5. Add 25 kg of diatomaceous earth filter aid to the material, mix evenly under negative pressure, and then filter through a high-efficiency plate-type closed filter to obtain di(2-propylheptyl) phthalate product.

[0038] In this embodiment, 14,722 kg of finished product was obtained, with a yield of 99.7% and a gas chromatographic purity of 99.78%. The production process only generated 1,310 kg of condensate wastewater, with no water washing wastewater.

[0039] Comparative Example Using the same amount of raw materials as in Example 1, and following the conventional process, the specific steps are as follows: S1, towards 30m 314,000 kg of 2-propylheptanol was added to the reactor, and stirring was started. 5,000 kg of phthalic anhydride was added. Heat transfer oil was introduced to raise the temperature, and the material was kept at 125°C to complete the monoesterification reaction. The temperature was further raised to a liquid phase temperature of 178°C, and 14 kg of tetraisopropyl titanate catalyst was added. The temperature was continued until the final liquid temperature did not exceed 235°C. A vacuum was slowly opened, and the system vacuum was adjusted to -0.05 MPa. The reaction was maintained under negative pressure. Water generated during the reaction was continuously separated, and the 2-propylheptanol was refluxed back to the reactor. After the reaction, samples were taken for testing. The acid value of the material was 0.12% (based on KOH), the liquid temperature was 234°C, the vapor temperature at the top of the column was 202°C, and the color was 60#, indicating that the esterification endpoint had been reached.

[0040] S2. Transfer the qualified esterified material to 30m 3 Processing vessel; turn on the cooling coil to cool the material to 85℃; add 13kg of 20% sodium hydroxide aqueous solution to neutralize for 5 minutes, add 2000kg of washing water to wash for 5 minutes, stop stirring and let stand to settle for 30 minutes; after draining the water, reheat to 200℃; turn on the vacuum to -0.095MPa, introduce steam to strip and remove alcohol until the product flash point, acid value and volume resistivity are qualified.

[0041] S3. Transfer the qualified material into the pressure tank, add 20kg of diatomaceous earth filter aid, and filter it through a plate filter to obtain the finished product.

[0042] In this embodiment, 14,856 kg of finished product was obtained, with a product yield of 98.6% and a purity of 99.51% as determined by gas chromatography. The production process generated 2,015 kg of washing wastewater and 1,090 kg of esterification and stripping condensate wastewater, with a total wastewater discharge of 3,105 kg.

[0043] The performance indicators of the above embodiments are compared with those of the comparative examples, and the specific data are shown in the table below:

[0044] As shown in the table above, compared with the traditional two-reactor process, the one-reactor process of the present invention has a shorter production cycle, higher product yield, reduced wastewater discharge, lower total energy consumption, and the core indicators such as product purity, flash point, and volume resistivity are all superior to those of the traditional process.

[0045] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0046] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A production process for di(2-propylheptyl) phthalate, characterized in that: Includes the following steps: S1. Esterification reaction: Add 2-propylheptanol to the reactor, start stirring and add phthalic anhydride, raise the temperature to carry out monoesterification reaction; then add tetraisopropyl titanate catalyst, continue to raise the temperature to 215℃~235℃, slowly adjust the vacuum degree of the system to 0~-0.07MPa, and keep the reaction under negative pressure until the esterification endpoint. S2, Flash De-alcoholization: After the esterification reaction is qualified, keep the material temperature constant, increase the vacuum of the system to not less than -0.088MPa, and use the residual heat of esterification to carry out negative pressure flash evaporation; S3, Neutralization and Decolorization: After flash evaporation, maintain the vacuum state of the system, control the material temperature ≥180℃, and add alkaline solution to the reactor until the acid value of the material is qualified; S4. Stripping and dehydration: After neutralization, maintain the system vacuum at no less than -0.095 MPa, control the material liquid temperature at 190-210℃, and introduce deionized softened water into the reactor for steam stripping to remove residual 2-propylheptanol; after stripping, introduce nitrogen and perform dry evaporation until the product flash point, acid value, and volume resistivity are qualified. S5. Finished product filtration: Add filter aid to qualified materials, mix evenly under negative pressure, and then filter to remove solid impurities, obtaining di(2-propylheptyl) phthalate as the finished product.

2. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: In step S1, the molar ratio of phthalic anhydride to 2-propylheptanol is 1:2.5 to 2.9, and the weight ratio is 1:2.5 to 3.

05.

3. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: The monoesterification reaction temperature in step S1 is 120-130°C, and the liquid temperature is 175-180°C when the catalyst is added.

4. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: The control indicators for the esterification endpoint in step S1 are: material liquid temperature 220~235℃, vapor temperature 195~210℃, system acid value ≤0.135%, and material color ≤70#.

5. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: The alkaline solution mentioned in step S3 is a sodium hydroxide aqueous solution with a mass concentration of 15-30%, and the amount of material added is 0.5-0.9‰ of the total mass of the material. The addition time is controlled within 5 minutes. The neutralization endpoint control index is that the acid value of the material is ≤0.07mgKOH / g.

6. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: In step S4, the flow rate of the deionized softened water is 100-800 L / H, the stripping time is 60-90 minutes, nitrogen is introduced for 10 minutes, and dry evaporation is performed for 10 minutes.

7. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: The product qualification indicators in step S4 are: flash point ≥ 210℃, acid value ≤ 0.07 mg KOH / g, and volume resistivity ≥ 1.0 × 10⁻⁶. 12 Ω·cm.

8. The production process of di(2-propylheptyl) phthalate according to claim 1, characterized in that: The filter aid mentioned in step S5 is diatomaceous earth, and the filtration is carried out using a plate-type closed filter. The filtered material is free of visible impurities and is transparent and water-free.

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