Method for refining biodiesel from kitchen waste oil

Abstract

The invention provides a method for preparing biodiesel from kitchen waste oil, and relates to the field of waste resource utilization, an efficient and environment-friendly process is constructed through the following scheme: (1) magnetic nano pretreatment: adsorbing polar impurities (the removal rate is greater than 95%) by 20-50nm silane modified Fe3O4 particles in an ultrasonic-assisted manner, and dehydrating until the removal rate is less than 0.3% to solve catalyst poisoning; (2) bifunctional catalysis-microwave coupling: the ZnO (at) MCM-41-SO3H catalyst is in acid-base site coordination, grease with FFA less than or equal to 20% is converted in one step, microwave heating is performed for 30-45 min, and the conversion rate is greater than or equal to 98%; (3) DES green separation: a choline chloride-ethylene glycol solvent replaces water washing, the emission of wastewater is reduced by 80%, DES (greater than or equal to 95%) and high-purity glycerol (greater than or equal to 98%) are recovered through low-temperature distillation, and the energy consumption is reduced by 40%; and (4) by-products are fully utilized: magnetic particles are regenerated, residual liquid is used as fuel oil, and zero emission is realized. The scheme has the advantages of high conversion (more than or equal to 98%), wide adaptation (FFA is less than or equal to 20%), low consumption (reduced by 40%) and less wastewater (reduced by 80%), the catalyst can be reused for more than or equal to 10 times, and an efficient path is provided for waste oil recycling.

Description

Technical Field

[0001] The present invention relates to the field of waste resource utilization, and in particular to a method for refining biodiesel from waste cooking oil. Background Art

[0002] Waste cooking oil is an important raw material for the preparation of biodiesel, but the impurities it contains, such as solid particles, free fatty acids (FFA), metal ions, saponification products, etc., lead to problems in traditional refining processes such as inefficient pretreatment, limited catalytic reaction, high pollution and high energy consumption in separation.

[0003] The traditional "filtration / centrifugation + acid washing" process only removes large impurities, but nanoscale polar impurities (such as saponified products and metal ions) and emulsified water are difficult to remove, resulting in the blocking of catalyst active sites. For example, the amphiphilic nature of saponified products easily forms a stable emulsion system, resulting in an FFA removal rate of less than 30% and a water content of more than 1%, which directly affects the efficiency of subsequent reactions.

[0004] When FFA exceeds 5%, traditional liquid base catalysis methods emulsify the system due to saponification side reactions, making it difficult to separate the biodiesel and glycerol, and causing a sharp drop in conversion rate (<70% at FFA = 10%). Existing technologies require a step-by-step esterification and transesterification process, which is lengthy (total time > 4 hours) and expensive. Furthermore, the acid catalyst corrodes equipment and produces waste acid, making it difficult to adapt to the wide fluctuations in FFA in actual oils (5%-20%).

[0005] Liquid catalysts (such as NaOH and H2SO4) are highly active but cannot be recycled. They generate 5-10 tons of wastewater for every ton of biodiesel produced, which puts great pressure on environmental protection. Although solid catalysts (such as CaO) avoid wastewater, their active sites are easily poisoned by metal ions (>50ppm), the reaction time is as long as 2-3 hours, and they can be reused less than 5 times, which is inefficient.

[0006] Traditional water washing to remove catalysts requires 30-50 times the volume of the reaction liquid, resulting in high wastewater treatment costs; glycerol recovery relies on high-temperature distillation (>120°C), with energy consumption accounting for >30% and purity <95%, limiting high value-added utilization.

[0007] Therefore, there is an urgent need to develop a new process with efficient pretreatment, wide acid value adaptability, recoverable catalyst, and low-carbon separation, so as to break through the technical bottleneck and promote the industrial application of resource utilization of waste cooking oil. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for refining biodiesel from waste cooking oil to solve the technical defects pointed out in the background technology.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] A method for refining biodiesel from waste cooking oil comprises the following steps:

[0011] Step S1: heating the waste cooking oil to 50-60° C., adding Fe3O4 magnetic nanoparticles with surface modified silane coupling agent, ultrasonically dispersing for 10-15 minutes, and then separating by magnetic field to obtain pretreated oil;

[0012] Step S2: mixing the pretreated oil and methanol at a molar ratio of 1:6-8, adding a bifunctional catalyst ZnO@MCM-41-SO3H, and conducting an ester exchange reaction at 120-150°C for 30-45 minutes under microwave assistance. After the reaction, the catalyst is recovered by a magnetic field;

[0013] Step S3: adding 10-15% of the volume of the reaction solution of choline chloride-ethylene glycol deep eutectic solvent to the reaction solution, stirring for 10 minutes and then standing to separate the layers, and separating the upper layer of crude biodiesel and the lower layer of choline chloride-ethylene glycol deep eutectic solvent-glycerol mixture;

[0014] Step S4: performing reduced pressure distillation on the choline chloride-ethylene glycol deep eutectic solvent-glycerol mixed solution, controlling the pressure to be less than 10 kPa and the temperature to be lower than 80° C., and recovering the choline chloride-ethylene glycol deep eutectic solvent and high-purity glycerol.

[0015] Furthermore, the particle size of the Fe3O4 magnetic nanoparticles is 20-50nm, the silane coupling agent is γ-aminopropyltriethoxysilane, and its dosage is 5-10% of the mass of Fe3O4; the added amount of the Fe3O4 magnetic nanoparticles is 0.5-1wt% of the mass of the pretreated oil, and the ultrasonic frequency is 20-40kHz.

[0016] Furthermore, in step S2, the preparation steps of the bifunctional catalyst ZnO@MCM-41-SO3H include:

[0017] Step S21: preparing an MCM-41 mesoporous molecular sieve carrier by a hydrothermal method, with a calcination temperature of 550-600° C. and a calcination time of 6-8 h;

[0018] Step S22: loading ZnO on an MCM-41 carrier at a loading amount of 10-20 wt%, calcining at a temperature of 400-500° C. for 3-5 h;

[0019] Step S23: introducing -SO3H acidic sites through chlorosulfonic acid modification, with the modification reaction time being 1 to 2 hours.

[0020] Furthermore, in step S2, the microwave power of the microwave-assisted reaction is 300-600 W, the amount of the bifunctional catalyst ZnO@MCM-41-SO3H is 3-5% of the mass of the pretreated oil, and the catalyst can be reused ≥10 times after separation by the magnetic field.

[0021] Furthermore, in step S3, the molar ratio of choline chloride-ethylene glycol deep eutectic solvent is 1:2, and the mixture is prepared by stirring at 60-80° C. until uniform and transparent; the vacuum distillation recovery rate of the choline chloride-ethylene glycol deep eutectic solvent-glycerol mixture is ≥95%, and the purity of the recovered glycerol is ≥98%.

[0022] Furthermore, the method further includes a by-product processing step: washing and regenerating the magnetic nanoparticles separated in step S1 with a 0.1M HCl solution, and using the washing liquid for preparing a fertilizer additive; and using the distillation residue in step S4 as a fuel oil component.

[0023] Furthermore, the free fatty acid content of the waste cooking oil is ≤20%, and after the transesterification reaction in step S2, the biodiesel conversion rate is ≥98%.

[0024] Furthermore, in the microwave-assisted transesterification reaction in step S2, the stirring speed in the reactor is 200-400 r / min.

[0025] Furthermore, in step S3, when stirring after adding the choline chloride-ethylene glycol deep eutectic solvent, the stirring mode is intermittent stirring, stirring for 2 to 3 minutes each time, with an interval of 1 to 2 minutes, and stirring for a total of 10 minutes.

[0026] Furthermore, when preparing the MCM-41 mesoporous molecular sieve carrier, the molar ratio of ethyl orthosilicate, hexadecyltrimethylammonium bromide, sodium hydroxide and deionized water is 1-2:0.1-0.2:0.2-0.4:50-100.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention provides a method for refining biodiesel from waste cooking oil. Through magnetic nanoparticle-enhanced pretreatment, bifunctional catalyst-coupled microwave-assisted reaction, deep eutectic solvent (DES) green separation process and full recovery of by-products, an efficient and environmentally friendly closed-loop production process is constructed.

[0029] The impurity removal rate of the present invention is greater than 95%, the biodiesel conversion rate is greater than 98%, and the reaction efficiency is more than three times higher than that of the traditional process; it breaks through the high acid value limit, simplifies the process and reduces costs by 30%; wastewater discharge is reduced by 80%, energy consumption is reduced by 40%, glycerol purity is greater than or equal to 98%, and the catalyst can be reused for greater than or equal to 10 times; zero by-product emissions, and the recycling of magnetic nanoparticles and DES meet the requirements of green chemistry and circular economy, and has significant industrial application value and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 This is a logic block diagram of a method for refining biodiesel from waste cooking oil according to the present invention. DETAILED DESCRIPTION

[0032] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] The present invention relates to a method for refining biodiesel from waste cooking oil. The method is comprehensively and practicably described below in conjunction with detailed experimental operations and parameter settings.

[0034] Example 1

[0035] 1. Raw Materials Preparation

[0036] Waste cooking oil from local catering companies was collected and placed in a sealed container, labeled as sample A. A preliminary analysis of sample A showed that its free fatty acid (FFA) content was 15%, its moisture content was 1.2%, and it contained a small amount of solid impurities.

[0037] Prepare analytical grade methanol, γ-aminopropyltriethoxysilane, Fe3O4 magnetic nanoparticles (particle size about 20 nm), tetraethyl orthosilicate (TEOS), hexadecyltrimethylammonium bromide (CTAB), sodium hydroxide, zinc oxide (ZnO), chlorosulfonic acid, choline chloride, ethylene glycol and other chemical reagents.

[0038] 2. Preprocessing Steps

[0039] Heating grease: Accurately weigh 1000g of sample A and place it in a 2L reactor equipped with a stirrer and a thermometer. Turn on the heating device and slowly heat the grease to 50°C. Stir at a speed of 100r / min during the heating process to ensure uniform temperature.

[0040] Adding magnetic nanoparticles: Weigh 5g of Fe3O4 magnetic nanoparticles surface-modified with γ-aminopropyltriethoxysilane, at a rate of 0.5wt% based on the weight of the oil. The silane coupling agent modification method is as follows: Disperse the Fe3O4 magnetic nanoparticles in an ethanol solution, add γ-aminopropyltriethoxysilane (5% of the Fe3O4 mass, i.e., 0.25g), and stir at 60°C for 3 hours. Then, centrifuge, wash, and dry to obtain the modified magnetic nanoparticles. Add these to the reactor.

[0041] Ultrasonic dispersion: Turn on the ultrasonic equipment, set the ultrasonic frequency to 20kHz, and ultrasonically disperse for 10 minutes. During the ultrasonic process, maintain the stirring speed at 100r / min to ensure that the magnetic nanoparticles are fully mixed with the oil and fat, and absorb impurities such as metal ions and saponification.

[0042] Magnetic field separation: Magnetic nanoparticles are separated from the oil using a strong external magnetic field (magnetic field strength of approximately 1 T) to obtain pre-treated oil. The separation time is approximately 5 minutes, ensuring that the magnetic nanoparticles are completely separated.

[0043] 3. Preparation of bifunctional catalyst ZnO@MCM-41-SO3H

[0044] Preparation of the MCM-41 mesoporous molecular sieve support: 1 mol of tetraethyl orthosilicate (TEOS), 0.1 mol of hexadecyltrimethylammonium bromide (CTAB), 0.2 mol of sodium hydroxide, and 50 mol of deionized water were added to a 5-liter reactor and stirred at 100°C for 24 hours. After the reaction, the product was filtered, washed to neutrality, and then calcined at 550°C for 6 hours to obtain the MCM-41 mesoporous molecular sieve support.

[0045] ZnO loading: ZnO was loaded onto the MCM-41 support using an impregnation method. The MCM-41 support was immersed in a zinc nitrate solution (ZnO loading of 10 wt%), stirred at room temperature for 12 hours, dried at 100°C for 12 hours, and finally calcined at 400°C for 3 hours to obtain ZnO@MCM-41.

[0046] -SO3H acidic site introduction: ZnO@MCM-41 was placed in a three-necked flask, chlorosulfonic acid was added, and the reaction was carried out at 80°C for 1 hour. After the reaction, it was washed with deionized water to neutrality and dried to obtain the bifunctional catalyst ZnO@MCM-41-SO3H.

[0047] 4. Transesterification step

[0048] Material Mixing: Add the pretreated oil and methanol at a molar ratio of 1:6 to a 2L reactor equipped with a microwave and a stirrer. The amount of methanol added is calculated to be approximately 227g.

[0049] Add catalyst: weigh 3% of the mass of pretreated oil and fat and add the bifunctional catalyst ZnO@MCM-41-SO3H (i.e. 30 g) into the reactor.

[0050] Microwave-assisted reaction: The microwave power was set at 300 W, the reaction temperature was set at 120°C, and the stirring speed in the reactor was set at 200 rpm. The transesterification reaction was carried out for 30 minutes. The temperature and pressure were monitored in real time during the reaction to ensure stable reaction conditions.

[0051] Catalyst recovery: After the reaction is completed, the catalyst is separated from the reaction solution using an external magnetic field (magnetic field strength is about 1 T). The recovered catalyst is washed three times with methanol and dried for later use.

[0052] 5. Separation Step

[0053] Preparation of DES: Choline chloride and ethylene glycol were added into a 500 mL flask at a molar ratio of 1:2 and stirred at 60°C until the mixture was homogeneous and transparent to obtain a choline chloride-ethylene glycol deep eutectic solvent (DES).

[0054] DES addition and stirring: Add 10% DES by volume to the reaction solution and stir intermittently for 2 minutes each time, followed by a 1-minute rest, for a total of 10 minutes. The stirring speed is 150 r / min.

[0055] After stirring, the reaction solution was transferred to a separatory funnel and allowed to stand for 30 minutes to allow the upper layer of crude biodiesel and the lower layer of DES-glycerol mixture to fully separate.

[0056] 6. Recycling steps

[0057] Vacuum distillation: Transfer the DES-glycerol mixture to a vacuum distillation apparatus and perform vacuum distillation at a pressure below 10 kPa and a temperature below 80°C. Collect the DES and glycerol fractions during the distillation process, and record the distillation time and yield.

[0058] Product analysis: The recovered DES and glycerol were subjected to purity analysis. The purity of DES was detected by gas chromatography, and the purity of glycerol was detected by high performance liquid chromatography.

[0059] 7. By-product treatment steps

[0060] Regeneration of magnetic nanoparticles: Wash the magnetic nanoparticles separated in step 2 three times with 0.1 M HCl solution for 10 minutes each wash, then rinse with deionized water until neutral. Dry and reuse. Collect the washing solution and use it to prepare the fertilizer additive.

[0061] Utilization of distillation residue: The distillation residue from step 6 is collected and subsequently utilized as a fuel oil component.

[0062] After testing, the biodiesel conversion rate of the cooking waste oil after the above treatment reached 98.2%, the vacuum distillation recovery rate of the DES-glycerol mixture was 95.5%, and the purity of the recovered glycerol was 98.1%. After the catalyst was reused 10 times, the biodiesel conversion rate could still reach more than 95%.

[0063] Example 2

[0064] 1. Raw Materials Preparation

[0065] Another sample of waste cooking oil from a different restaurant was collected and placed in a sealed container, labeled Sample B. Analysis of Sample B revealed a free fatty acid (FFA) content of 20%, a moisture content of 1.5%, and a slightly higher solid impurity content than Sample A.

[0066] Prepare chemical reagents of the same specifications and purity as in Example 1.

[0067] 2. Preprocessing Steps

[0068] Heating grease: Accurately weigh 1000g of sample B and place it in a 2L reactor. Turn on the heating device and heat the grease to 60°C. Stir at a speed of 120r / min during the heating process.

[0069] Add magnetic nanoparticles: Weigh 10g of Fe₃O₄ magnetic nanoparticles (approximately 50nm in diameter) surface-modified with γ-aminopropyltriethoxysilane, based on 1wt% of the oil mass. The silane coupling agent modification method is the same as in Example 1, but the amount used is 10% of the Fe₃O₄ mass, i.e., 1g. Add these to the reactor.

[0070] Ultrasonic dispersion: Turn on the ultrasonic equipment, set the ultrasonic frequency to 40 kHz, ultrasonic dispersion for 15 min, and the stirring speed to 120 r / min.

[0071] Magnetic field separation: Use an external strong magnetic field (magnetic field strength of about 1.2T) to separate magnetic nanoparticles from oil and fat to obtain pretreated oil and fat. The separation time is about 6 minutes.

[0072] 3. Preparation of bifunctional catalyst ZnO@MCM-41-SO3H

[0073] Preparation of the MCM-41 mesoporous molecular sieve support: 2 mol of tetraethyl orthosilicate (TEOS), 0.2 mol of hexadecyltrimethylammonium bromide (CTAB), 0.4 mol of sodium hydroxide, and 100 mol of deionized water were added to a 5-L reactor and stirred at 100°C for 24 hours. After the reaction, the product was filtered, washed to neutrality, and then calcined at 600°C for 8 hours to obtain the MCM-41 mesoporous molecular sieve support.

[0074] ZnO loading: ZnO was loaded onto the MCM-41 support using an impregnation method. The MCM-41 support was immersed in a zinc nitrate solution (ZnO loading of 20 wt%), stirred at room temperature for 12 hours, dried at 100°C for 12 hours, and finally calcined at 500°C for 5 hours to obtain ZnO@MCM-41.

[0075] -SO3H acidic site introduction: ZnO@MCM-41 was placed in a three-necked flask, chlorosulfonic acid was added, and the reaction was carried out at 80°C for 2 hours. After the reaction, it was washed with deionized water to neutrality and dried to obtain the bifunctional catalyst ZnO@MCM-41-SO3H.

[0076] 4. Transesterification step

[0077] Material mixing: Add the pretreated oil and methanol to a 2L reactor at a molar ratio of 1:8. Calculated methanol addition amount is approximately 303g.

[0078] Add catalyst: weigh 5% of the mass of pretreated oil and fat and add the bifunctional catalyst ZnO@MCM-41-SO3H (i.e. 50 g) into the reactor.

[0079] Microwave-assisted reaction: The microwave power was set at 600 W, the reaction temperature was set at 150°C, and the stirring speed in the reactor was set at 400 rpm. The transesterification reaction was carried out for 45 minutes. The temperature and pressure changes were closely monitored during the reaction.

[0080] Catalyst recovery: After the reaction is completed, the catalyst is separated from the reaction solution using an external magnetic field (magnetic field strength is about 1.2 T). The recovered catalyst is washed three times with methanol and dried for later use.

[0081] 5. Separation Step

[0082] Preparation of DES: Choline chloride and ethylene glycol were added into a 500 mL flask at a molar ratio of 1:2, and stirred at 80°C until homogeneous and transparent to obtain a choline chloride-ethylene glycol deep eutectic solvent (DES).

[0083] DES addition and stirring: Add DES to the reaction solution at a volume of 15% of the reaction solution, stirring intermittently for 3 minutes each time, followed by a 2-minute rest, for a total of 10 minutes. The stirring speed was 180 r / min.

[0084] After stirring, the reaction solution was transferred to a separatory funnel and allowed to stand for 40 minutes to allow the upper layer of crude biodiesel and the lower layer of DES-glycerol mixture to fully separate.

[0085] 6. Recycling steps

[0086] Vacuum distillation: Transfer the DES-glycerol mixture to a vacuum distillation apparatus and perform vacuum distillation at a pressure below 10 kPa and a temperature below 80°C. Record the distillation time and fraction yield.

[0087] Product analysis: The recovered DES and glycerol were subjected to purity analysis using the same analytical method as in Example 1.

[0088] 7. By-product treatment steps

[0089] Regeneration of magnetic nanoparticles: Wash the magnetic nanoparticles separated in step 2 three times with 0.1 M HCl solution for 10 minutes each wash, then rinse with deionized water until neutral. Dry and reuse. Collect the washing solution and use it to prepare the fertilizer additive.

[0090] Utilization of distillation residue: The distillation residue from step 6 is collected and subsequently utilized as a fuel oil component.

[0091] After testing, the biodiesel conversion rate of the cooking waste oil after the above treatment reached 98.5%, the vacuum distillation recovery rate of the DES-glycerol mixture was 96%, the purity of the recovered glycerol was 98.3%, and after the catalyst was reused 10 times, the biodiesel conversion rate could still reach more than 95.5%.

[0092] It can be seen from the above two examples that the method of the present invention can effectively treat waste cooking oil with a high FFA content and achieve efficient conversion to biodiesel. At the same time, the catalyst can be recycled and the by-products are reasonably treated. It has good economic and environmental benefits and is feasible for practical application.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0094] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for refining biodiesel from waste cooking oil, characterized in that: The following steps are involved: Step S1: heating the waste cooking oil to 50-60° C., adding Fe3O4 magnetic nanoparticles with surface modified silane coupling agent, ultrasonically dispersing for 10-15 minutes, and then separating by magnetic field to obtain pretreated oil; Step S2: mixing the pretreated oil and methanol at a molar ratio of 1:6-8, adding a bifunctional catalyst ZnO@MCM-41-SO3H, and conducting an ester exchange reaction at 120-150°C for 30-45 minutes under microwave assistance. After the reaction, the catalyst is recovered by a magnetic field; Step S3: adding 10-15% of the volume of the reaction solution of choline chloride-ethylene glycol deep eutectic solvent to the reaction solution, stirring for 10 minutes and then standing to separate the layers, and separating the upper layer of crude biodiesel and the lower layer of choline chloride-ethylene glycol deep eutectic solvent-glycerol mixture; Step S4: performing reduced pressure distillation on the choline chloride-ethylene glycol deep eutectic solvent-glycerol mixed solution, controlling the pressure to be less than 10 kPa and the temperature to be lower than 80° C., and recovering the choline chloride-ethylene glycol deep eutectic solvent and high-purity glycerol.

2. The method according to claim 1, wherein: The particle size of the Fe3O4 magnetic nanoparticles is 20-50nm, the silane coupling agent is γ-aminopropyltriethoxysilane, and its dosage is 5-10% of the mass of Fe3O4; the addition amount of the Fe3O4 magnetic nanoparticles is 0.5-1wt% of the mass of the pretreated oil, and the ultrasonic frequency is 20-40kHz.

3. The method according to claim 1, wherein: In step S2, the preparation steps of the bifunctional catalyst ZnO@MCM-41-SO3H include: Step S21: preparing an MCM-41 mesoporous molecular sieve carrier by a hydrothermal method, with a calcination temperature of 550-600° C. and a calcination time of 6-8 h; Step S22: loading ZnO on an MCM-41 carrier at a loading amount of 10-20 wt%, calcining at a temperature of 400-500° C. for 3-5 h; Step S23: introducing -SO3H acidic sites through chlorosulfonic acid modification, with the modification reaction time being 1 to 2 hours.

4. The method according to claim 1, wherein: In step S2, the microwave power of the microwave-assisted reaction is 300-600 W, the amount of the bifunctional catalyst ZnO@MCM-41-SO3H is 3-5% of the mass of the pretreated oil, and the catalyst can be reused ≥10 times after separation by the magnetic field.

5. The method according to claim 1, wherein: In step S3, the molar ratio of the choline chloride-ethylene glycol deep eutectic solvent is 1:2, and the mixture is prepared by stirring at 60-80° C. until it is uniform and transparent; the vacuum distillation recovery rate of the choline chloride-ethylene glycol deep eutectic solvent-glycerol mixture is ≥95%, and the purity of the recovered glycerol is ≥98%.

6. The method according to claim 1, wherein: Also includes by-product processing steps: The magnetic nanoparticles separated in step S1 are washed and regenerated with a 0.1 M HCl solution, and the washing solution is used to prepare a fertilizer additive; The distillation residue in step S4 is used as a fuel oil component.

7. The method according to any one of claims 1 to 6, characterized in that: The free fatty acid content of the waste cooking oil is ≤20%, and after the transesterification reaction in step S2, the biodiesel conversion rate is ≥98%.

8. The method according to claim 1, characterized in that In the microwave-assisted transesterification reaction of step S2, the stirring speed in the reactor is 200-400 r / min.

9. The method according to claim 1, characterized in that In step S3, when stirring after adding the choline chloride-ethylene glycol deep eutectic solvent, the stirring mode is intermittent stirring, stirring for 2 to 3 minutes each time, with an interval of 1 to 2 minutes, and stirring for a total of 10 minutes.

10. The method according to claim 3, characterized in that When preparing the MCM-41 mesoporous molecular sieve carrier, the molar ratio of ethyl orthosilicate, hexadecyltrimethylammonium bromide, sodium hydroxide and deionized water is 1-2:0.1-0.2:0.2-0.4:50-100.

Images