A method for preparing aromatic hydrocarbon oil by using kitchen waste oil

Abstract

The application provides a method for preparing aromatic hydrocarbon oil by using kitchen waste oil, comprising the following steps: a raw material pretreatment step: the kitchen waste oil is sequentially subjected to standing clarification, multi-stage filtration, hot water washing and demulsification dehydration treatment; a hydrolysis reaction step: the pretreated kitchen waste oil is reacted with water to obtain a fatty acid mixture; a catalytic aromatization step: the fatty acid mixture obtained by hydrolysis is mixed with a composite catalyst, and the mixture is reacted in a fixed bed reactor to obtain an aromatization reaction product; a product separation step: the aromatization reaction product is condensed and cooled to, and water and oil phases are separated by oil-water decantation, the oil phase is extracted with an extractant to obtain aromatic hydrocarbons, and finally, light aromatic hydrocarbons and diesel oil fractions are obtained by fractional distillation. The kitchen waste oil with complex components is efficiently converted into high-value aromatic hydrocarbon oil, and the core problems of catalyst deactivation, low product separation efficiency and poor process stability in the prior art are systematically solved while realizing high-value utilization of resources.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization technology of waste resources, specifically relating to a method for preparing aromatic oil from waste cooking oil. Background Technology

[0002] Waste cooking oil, a mixture of waste animal and vegetable oils emitted by the catering industry, has a complex and varied composition. It contains not only triglycerides and free fatty acids, but also food residues, water, salt, detergents, and small molecules such as aldehydes, ketones, and acids produced during oil degradation. This complex composition poses a significant challenge to the effective utilization of waste cooking oil.

[0003] Currently, the main methods for treating waste cooking oil include the production of biodiesel, industrial grease, and soap. Among these, transesterification is the most widely used technology for biodiesel production, but this process has significant limitations. First, the free fatty acids in waste cooking oil undergo saponification with alkaline catalysts, not only consuming the catalyst but also producing emulsification, leading to difficulties in product separation. Second, the presence of water hydrolyzes ester bonds, reducing biodiesel yield. Third, the unsaturated fatty acids in waste cooking oil are prone to polymerization during processing, forming gum that clogs equipment. Most importantly, the added value of biodiesel products is limited, and their market competitiveness is significantly affected by fluctuations in fossil diesel prices.

[0004] In recent years, some researchers have attempted to convert waste cooking oil into fuel or chemical feedstock through processes such as catalytic cracking and hydrogenation. For example, noble metal catalysts are used for aromatization reactions to prepare aromatics. However, these methods face multiple technical obstacles in practical applications. Impurities in waste cooking oil can rapidly deactivate noble metal catalysts, and frequent catalyst replacement significantly increases production costs. Simultaneously, traditional molecular sieve catalysts are prone to losing activity due to carbon buildup during waste cooking oil conversion, requiring frequent regeneration. Furthermore, existing processes have poor adaptability to raw materials; the composition of waste cooking oil from different sources varies greatly, leading to frequent adjustments to process parameters and difficulty in ensuring product stability. Another prominent problem is the difficulty in product separation. Due to the complexity of the reaction products, the separation efficiency of aromatics from other hydrocarbons is low, affecting the purity and value of the final product.

[0005] Therefore, it is necessary to design a method for preparing aromatic oils from waste cooking oil. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, a method for preparing aromatic oils from waste cooking oils is provided.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing aromatic oil from waste cooking oil, the method comprising the following steps: Raw material pretreatment steps: The waste cooking oil is sequentially processed through settling and clarification, multi-stage filtration, hot water washing, and demulsification and dehydration. Hydrolysis reaction steps: Mix pretreated kitchen waste oil with water at a mass ratio of 1:0.5-5, and react at 160 to 260°C and 1 to 4 MPa pressure for 1 to 6 hours to obtain a fatty acid mixture; Catalytic aromatization step: The fatty acid mixture obtained by hydrolysis is mixed with a composite catalyst and reacted in a fixed-bed reactor at 300 to 480 °C and 0.1 to 3 MPa pressure for 0.5 to 8 hours to obtain the aromatization reaction product; Product separation steps: The aromatization reaction product is cooled to 20 to 60°C by condensation, and the water and oil phases are separated by oil-water decantation. The oil phase is used to extract aromatics with an extractant, and finally, light aromatics and diesel fractions are obtained by fractionation.

[0008] The multi-stage filtration includes 100 to 200 mesh coarse filtration and 300 to 500 mesh fine filtration; the hot water washing uses 80 to 95°C hot water and the washing time is 30 to 120 minutes; the amount of demulsifier added in the demulsification and dehydration is 0.1-1% of the mass of kitchen waste oil, the demulsification temperature is 70 to 90°C, and the demulsification time is 1 to 3 hours.

[0009] In the hot water washing step, a cleaning aid is also added. The amount of the cleaning aid added is 0.5-2% of the mass of waste cooking oil. The cleaning aid is one of sodium citrate, sodium silicate, or sodium carbonate.

[0010] In the hydrolysis reaction step, a segmented temperature control is adopted. First, the reaction is carried out at 160 to 180℃ for 0.5 to 1 hour, then the temperature is increased to 200 to 220℃ for 1 to 2 hours, and finally the temperature is increased to 240 to 260℃ for 0.5 to 1 hour.

[0011] In the hydrolysis reaction step, 0.5-3% of the mass of waste cooking oil is added as an acidic catalyst, wherein the acidic catalyst is p-toluenesulfonic acid.

[0012] In the catalytic aromatization step, the preparation of the composite catalyst includes the following specific steps: Preparation of modified molecular sieve: HZSM-5 molecular sieve is mixed with rare earth metal nitrate solution, the rare earth metal loading is 2-6% of the mass of HZSM-5 molecular sieve, and the mixture is impregnated for 12 to 24 hours, dried at 100 to 120℃ for 2 to 4 hours, and calcined at 500 to 650℃ for 2 to 5 hours to obtain modified molecular sieve. Preparation of modified clay: Mix attapulgite clay with a hydrochloric acid solution of 2% to 8% by mass at a mass ratio of 1:10-30, stir at 60 to 80°C for 30 to 90 minutes, wash until neutral, and then dry. Catalyst molding: 30 to 50 parts by weight of modified molecular sieve, 25 to 35 parts by weight of alumina and 25 to 35 parts by weight of modified clay are mixed, 5 to 10 parts by weight of binder and 2 to 5 parts by weight of pore-forming agent are added, kneaded evenly and then extruded into shape, dried at 100 to 120°C for 2 to 4 hours and calcined at 500 to 650°C for 2 to 5 hours to obtain composite catalyst.

[0013] The mass ratio of the fatty acid mixture to the composite catalyst is 1:8-25.

[0014] A hydrogen donor is also added during the catalytic aromatization step. The hydrogen donor is one of formic acid, methanol, or tetrahydronaphthalene, and the amount of hydrogen donor added is 0.1-0.5% of the mass of waste cooking oil. The extraction operation uses a composite extractant, which is a mixture of dimethyl sulfoxide and sulfolane in a mass ratio of 1:1-3. The extraction temperature is 40 to 70°C, and the mass ratio of the composite extractant to the oil phase is 1-2:1.

[0015] In the product separation step, the fractionation adopts a dual-tower system. The first tower separates light aromatics at 60 to 120°C, and the second tower separates diesel fraction at 150 to 250°C. The heavy oil at the bottom of the towers is recycled back to the catalytic aromatization step for processing.

[0016] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. The raw material pretreatment step of this invention can systematically remove food residues, salts, detergents, and small-molecule polar substances produced by oil degradation from waste cooking oil. This multi-stage synergistic purification effect significantly reduces the risk of catalyst poisoning and deactivation in subsequent hydrolysis and aromatization reactions.

[0017] 2. A staged temperature control strategy was adopted in the hydrolysis reaction step, that is, hydrolysis was first initiated at a relatively mild temperature, and then the temperature was gradually increased to drive the reaction to proceed completely. This gradual temperature field control helps to smoothly break the triglyceride molecular chains, effectively suppresses the excessive decomposition or polymerization side reactions of fatty acids caused by local overheating, thereby improving the target yield of the C8 to C18 fatty acid mixture.

[0018] 3. Introducing p-toluenesulfonic acid as an acidic catalyst during the hydrolysis stage can provide sufficient protons to efficiently attack the ester bonds of glycerides, accelerate the hydrolysis kinetics of fatty acid formation, significantly shorten the reaction time, and reduce overall energy consumption.

[0019] 4. The composite catalyst uses rare-earth metal-modified HZSM-5 molecular sieve as the main active component. Its acidic sites are modulated, retaining the acid strength required to promote carbocation reactions while moderately reducing the density of strong acid centers through the introduction of rare-earth elements. This characteristic facilitates the aromatization reaction pathway while inhibiting excessive cracking and carbon deposition. The alumina and acid-modified clay in the catalyst not only provide suitable mechanical strength and pore structure as structural supports, but their surface properties also synergistically enhance the reaction with the modified molecular sieve, jointly optimizing reactant diffusion and product desorption behavior.

[0020] 5. Adding a hydrogen donor during the aromatization stage can release active hydrogen and promptly saturate the free radicals or carbocations formed by unsaturated intermediates. This not only stabilizes the reaction pathway and guides the formation of aromatics, but also effectively inhibits the reaction trend of olefin polymerization and even coking, playing a positive role in maintaining the long-term activity of the catalyst.

[0021] 6. In the product separation stage, a composite extractant composed of dimethyl sulfoxide and sulfolane is used, which can maintain high extraction efficiency for aromatics within a wider operating window. Furthermore, by adjusting the ratio of the two extractants, the separation selectivity for aromatics with different carbon numbers can be finely optimized, thereby ensuring the purity of the final aromatic product. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In the specific embodiments of this application, the sources of various main raw materials are briefly described as follows: Sodium citrate: Hebei Lihua Biotechnology Co., Ltd. Sodium silicate: Shandong Kuoda Biotechnology Co., Ltd. Sodium carbonate: Weifang Haizhiyuan Chemical Co., Ltd. p-Toluenesulfonic acid (monohydrate): Hunan Huibaishi Biotechnology Co., Ltd. HZSM-5 molecular sieve: Dalian Haixin Chemical Co., Ltd. Rare earth metals: Cerium oxide from China Rare Earth Group Alumina: Longkou Donghai Alumina Co., Ltd. Attapulgite: Lingshou County Fuda Mineral Products Processing Plant Dimethyl sulfoxide (DMSO): Guangzhou Yuanda New Materials Co., Ltd. Sulfolane: Zibo Xinrong Chemical Technology Co., Ltd. Formic acid: Shandong Asder Technology Co., Ltd. Methanol: Shandong Jiutai Chemical Technology Co., Ltd. Tetrahydronaphthalene Wuhan Rongcan Biotechnology Co., Ltd. The technical solution of this application is as follows: A method for preparing aromatic oil from waste cooking oil, the method comprising the following steps: Raw material pretreatment steps: The waste cooking oil is sequentially processed through settling and clarification, multi-stage filtration, hot water washing, and demulsification and dehydration. Hydrolysis reaction steps: Mix pretreated kitchen waste oil with water at a mass ratio of 1:0.5-5, and react at 160 to 260°C and 1 to 4 MPa pressure for 1 to 6 hours to obtain a fatty acid mixture; Catalytic aromatization step: The fatty acid mixture obtained by hydrolysis is mixed with a composite catalyst and reacted in a fixed-bed reactor at 300 to 480 °C and 0.1 to 3 MPa pressure for 0.5 to 8 hours to obtain the aromatization reaction product; Product separation steps: The aromatization reaction product is cooled to 20 to 60°C by condensation, and the water and oil phases are separated by oil-water decantation. The oil phase is used to extract aromatics with an extractant, and finally, light aromatics and diesel fractions are obtained by fractionation.

[0024] The multi-stage filtration includes 100 to 200 mesh coarse filtration and 300 to 500 mesh fine filtration; the hot water washing uses 80 to 95°C hot water and the washing time is 30 to 120 minutes; the amount of demulsifier added in the demulsification and dehydration is 0.1-1% of the mass of kitchen waste oil, the demulsification temperature is 70 to 90°C, and the demulsification time is 1 to 3 hours.

[0025] In the hot water washing step, a cleaning aid is also added. The amount of the cleaning aid added is 0.5-2% of the mass of waste cooking oil. The cleaning aid is one of sodium citrate, sodium silicate, or sodium carbonate.

[0026] In the hydrolysis reaction step, a segmented temperature control is adopted. First, the reaction is carried out at 160 to 180℃ for 0.5 to 1 hour, then the temperature is increased to 200 to 220℃ for 1 to 2 hours, and finally the temperature is increased to 240 to 260℃ for 0.5 to 1 hour.

[0027] In the hydrolysis reaction step, 0.5-3% of the mass of waste cooking oil is added as an acidic catalyst, wherein the acidic catalyst is p-toluenesulfonic acid.

[0028] In the catalytic aromatization step, the preparation of the composite catalyst includes the following specific steps: Preparation of modified molecular sieve: HZSM-5 molecular sieve is mixed with rare earth metal nitrate solution, the rare earth metal loading is 2-6% of the mass of HZSM-5 molecular sieve, and the mixture is impregnated for 12 to 24 hours, dried at 100 to 120℃ for 2 to 4 hours, and calcined at 500 to 650℃ for 2 to 5 hours to obtain modified molecular sieve. Preparation of modified clay: Mix attapulgite clay with a hydrochloric acid solution of 2% to 8% by mass at a mass ratio of 1:10-30, stir at 60 to 80°C for 30 to 90 minutes, wash until neutral, and then dry. Catalyst molding: 30 to 50 parts by weight of modified molecular sieve, 25 to 35 parts by weight of alumina and 25 to 35 parts by weight of modified clay are mixed, 5 to 10 parts by weight of binder and 2 to 5 parts by weight of pore-forming agent are added, kneaded evenly and then extruded into shape, dried at 100 to 120°C for 2 to 4 hours and calcined at 500 to 650°C for 2 to 5 hours to obtain composite catalyst.

[0029] The mass ratio of the fatty acid mixture to the composite catalyst is 1:8-25.

[0030] A hydrogen donor is also added during the catalytic aromatization step. The hydrogen donor is one of formic acid, methanol, or tetrahydronaphthalene, and the amount of hydrogen donor added is 0.1-0.5% of the mass of waste cooking oil. The extraction operation uses a composite extractant, which is a mixture of dimethyl sulfoxide and sulfolane in a mass ratio of 1:1-3. The extraction temperature is 40 to 70°C, and the mass ratio of the composite extractant to the oil phase is 1-2:1.

[0031] In the product separation step, the fractionation adopts a dual-tower system. The first tower separates light aromatics at 60 to 120°C, and the second tower separates diesel fraction at 150 to 250°C. The heavy oil at the bottom of the towers is recycled back to the catalytic aromatization step for processing.

[0032] The present invention will be described in detail below through examples and comparative examples, but the scope of protection of the present invention is not limited to these examples. Unless otherwise specified, the chemical reagents and raw materials used in the following examples and comparative examples are all conventional commercially available products.

[0033] Example 1 First, the raw materials undergo pretreatment. The collected waste cooking oil is allowed to stand and clarify for 24 hours, followed by multi-stage filtration, with a 200-mesh sieve for coarse filtration and a 400-mesh sieve for fine filtration. Next, hot water washing is performed at 80℃ for 120 minutes. Sodium citrate, a detergent additive, is added to the hot water washing process at a concentration of 2% of the waste cooking oil's mass. Following this, demulsification and dehydration are carried out, with a demulsifier added at 0.1% of the waste cooking oil's mass, at a demulsification temperature of 90℃ for 1 hour.

[0034] In the hydrolysis reaction step, pretreated waste cooking oil and water were mixed at a mass ratio of 1:5 and reacted in a high-pressure reactor. A staged temperature control method was used: the first stage was at 180℃ for 0.5 hours, the second stage at 200℃ for 2 hours, and the third stage at 240℃ for 1 hour. The reaction pressure was set at 4 MPa, and the total reaction time was 6 hours. p-Toluenesulfonic acid was added as an acidic catalyst at a concentration of 3% of the mass of the waste cooking oil.

[0035] In the catalytic aromatization step, a composite catalyst is first prepared. For the modified molecular sieve, the rare earth metal loading is 6% of the mass of HZSM-5 molecular sieve, the impregnation time is 12 hours, the drying temperature is 120℃, the drying time is 2 hours, and the calcination temperature is 650℃, with a calcination time of 2 hours. For the modified clay, the hydrochloric acid solution concentration is 8%, the mass ratio of attapulgite to hydrochloric acid solution is 1:30, the stirring temperature is 60℃, and the stirring time is 90 minutes. For catalyst molding, the following amounts are used: 50 parts by mass of modified molecular sieve, 25 parts by mass of alumina, 35 parts by mass of modified clay, 10 parts by mass of binder, and 2 parts by mass of pore-forming agent. The drying temperature is 120℃, the drying time is 2 hours, and the calcination temperature is 650℃, with a calcination time of 2 hours. The fatty acid mixture obtained from hydrolysis is mixed with the composite catalyst at a mass ratio of 1:25 and reacted in a fixed-bed reactor at 480℃ and 0.1 MPa pressure for 8 hours. Add formic acid as a hydrogen donor, at a rate of 0.5% of the mass of waste cooking oil.

[0036] In the product separation step, the aromatization reaction product is condensed to 20°C, and the oil phase is extracted with a composite extractant, which is a mixture of dimethyl sulfoxide and sulfolane in a mass ratio of 1:3. The extraction temperature is 40°C, and the mass ratio of the composite extractant to the oil phase is 2:1. A dual-tower system is used for fractionation, with the first tower at 120°C and the second tower at 150°C. The heavy oil from the bottom of the towers is recycled back to the catalytic aromatization step.

[0037] Example 2 In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows: In the raw material pretreatment, a 100-mesh sieve is used for coarse filtration, and a 500-mesh sieve is used for fine filtration. Hot water washing uses 95℃ water for 30 minutes, with sodium silicate added as a detergent aid at 0.5% of the weight of the waste cooking oil. In the demulsification and dehydration process, a demulsifier is added at 1% of the weight of the waste cooking oil, at a demulsification temperature of 70℃, for 3 hours.

[0038] In the hydrolysis reaction step, waste cooking oil and water are mixed at a mass ratio of 1:0.5. The temperature is controlled in stages: the first stage is at 160℃ for 1 hour, the second stage is at 220℃ for 1 hour, and the third stage is at 260℃ for 0.5 hours. The reaction pressure is 1 MPa, and the total reaction time is 1 hour. The amount of p-toluenesulfonic acid, an acidic catalyst, added is 0.5% of the mass of waste cooking oil.

[0039] In the catalytic aromatization step, during the preparation of the composite catalyst, the rare earth metal loading was 2%, the impregnation time was 24 hours, the drying temperature was 100℃, the drying time was 4 hours, and the calcination temperature was 500℃, with a calcination time of 5 hours. In the preparation of the modified clay, the hydrochloric acid solution concentration was 2%, the mass ratio of attapulgite clay to hydrochloric acid solution was 1:10, the stirring temperature was 80℃, and the stirring time was 30 minutes. During catalyst molding, the amount of modified molecular sieve was 30 parts by mass, alumina was 35 parts by mass, modified clay was 25 parts by mass, binder was 5 parts by mass, and pore-forming agent was 5 parts by mass. The drying temperature was 100℃, the drying time was 4 hours, and the calcination temperature was 500℃, with a calcination time of 5 hours. The mass ratio of the fatty acid mixture to the composite catalyst was 1:8, and the aromatization reaction was carried out at 300℃ and 3 MPa pressure for 0.5 hours. The amount of methanol added as the hydrogen donor was 0.1% of the mass of waste cooking oil.

[0040] In the product separation step, the aromatization reaction product is condensed to 60°C. The composite extractant is a mixture of dimethyl sulfoxide and sulfolane in a 1:1 mass ratio. The extraction temperature is 70°C, and the mass ratio of the composite extractant to the oil phase is 1:1. The temperature of the first fractionation column is 60°C, and the temperature of the second column is 250°C.

[0041] Example 3 In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows: In the raw material pretreatment, a 150-mesh sieve is used for coarse filtration, and a 300-mesh sieve is used for fine filtration. Hot water washing uses 87.5℃ water for 75 minutes, with sodium carbonate added as a detergent aid at 1.25% of the mass of waste cooking oil. In the demulsification and dehydration process, a demulsifier is added at 0.55% of the mass of waste cooking oil, at a demulsification temperature of 80℃, for 2 hours.

[0042] In the hydrolysis reaction step, waste cooking oil and water were mixed at a mass ratio of 1:2.75. The temperature was controlled in stages: the first stage was at 170℃ for 0.75 hours, the second stage at 210℃ for 1.5 hours, and the third stage at 250℃ for 0.75 hours. The reaction pressure was 2.5 MPa, and the total reaction time was 3.5 hours. The amount of p-toluenesulfonic acid, an acidic catalyst, added was 1.75% of the mass of the waste cooking oil.

[0043] In the catalytic aromatization step, during the preparation of the composite catalyst, the rare earth metal loading was 4%, the impregnation time was 18 hours, the drying temperature was 110℃, the drying time was 3 hours, and the calcination temperature was 575℃, with a calcination time of 3.5 hours. In the preparation of the modified clay, the hydrochloric acid solution concentration was 5%, the mass ratio of attapulgite to hydrochloric acid solution was 1:20, the stirring temperature was 70℃, and the stirring time was 60 minutes. During catalyst molding, the following were used: 40 parts by mass of modified molecular sieve, 30 parts by mass of alumina, 30 parts by mass of modified clay, 7.5 parts by mass of binder, and 3.5 parts by mass of pore-forming agent. The drying temperature was 110℃, the drying time was 3 hours, and the calcination temperature was 575℃, with a calcination time of 3.5 hours. The mass ratio of the fatty acid mixture to the composite catalyst was 1:16.5, and the aromatization reaction was carried out at 390℃ and 1.55 MPa pressure for 4.25 hours. The amount of tetrahydronaphthalene added as the hydrogen donor was 0.3% of the mass of waste cooking oil.

[0044] In the product separation step, the aromatization reaction product is condensed to 40℃. The composite extractant is a mixture of dimethyl sulfoxide and sulfolane in a mass ratio of 1:2. The extraction temperature is 55℃, and the mass ratio of the composite extractant to the oil phase is 1.5:1. The temperature of the first fractionation column is 90℃, and the temperature of the second column is 200℃.

[0045] Comparative Example 1 In this comparative example, the similarities with Example 1 will not be repeated, and the differences are as follows: The hot water washing and detergent addition steps are omitted in the raw material pretreatment, and demulsification and dehydration are carried out directly.

[0046] Comparative Example 2 In this comparative example, the similarities with Example 2 will not be repeated, and the differences are as follows: Instead of using segmented temperature control in the hydrolysis reaction, the reaction was directly carried out at a constant temperature of 260℃ for 3 hours.

[0047] Comparative Example 3 In this comparative example, the similarities with Example 3 will not be repeated, and the differences are as follows: In the catalytic aromatization step, unmodified ordinary HZSM-5 molecular sieves were used instead of the composite catalyst, and no hydrogen donor was added.

[0048] Comparative Example 4 In this comparative example, the similarities with Example 1 will not be repeated, and the differences are as follows: Instead of using a composite extractant, dimethyl sulfoxide (DMSO) is used as a single solvent for product separation.

[0049] Comparative Example 5 In this comparative example, the similarities with Example 2 will not be repeated, and the differences are as follows: In the catalytic aromatization step, the mass ratio of fatty acid to catalyst is 1:50.

[0050] Performance Test Results and Analysis To evaluate the effectiveness of this invention, performance was assessed using general testing methods. Fatty acid yield was determined by acid-base titration, expressed as a percentage of the mass of hydrolyzed fatty acids to the mass of the raw material, waste cooking oil. Aromatic hydrocarbon yield was analyzed by gas chromatography, expressed as a percentage of the mass of aromatic hydrocarbons in the aromatized product to the mass of the feed fatty acids. Catalyst lifetime was determined through continuous operation experiments, recording the time required for catalyst activity to decrease to 80% of its initial value. Product purity was analyzed by gas chromatography-mass spectrometry (GC-MS) to determine the content of the target aromatic hydrocarbons in the aromatic hydrocarbon product. Separation efficiency was determined by calculating the recovery rate of aromatic hydrocarbons in the oil phase after extraction. All tests were repeated three times, and the average value was taken. The results are shown in Table 1.

[0051] As shown in Table 1, the three examples all exhibited excellent performance in terms of fatty acid yield, aromatic hydrocarbon yield, catalyst lifetime, product purity, and separation efficiency, with similar values. This indicates that the method of the present invention can maintain stable performance under different parameter combinations. Example 1 achieved a fatty acid yield of 92.5% and an aromatic hydrocarbon yield of 78.3%. This was mainly due to the synergistic effect of hot water washing and detergent in the raw material pretreatment, which effectively removed food residues, salts, and polar impurities from waste cooking oil, providing clean raw materials for the hydrolysis reaction. Simultaneously, the segmented temperature control smoothly broke down the triglyceride molecular chains, inhibiting excessive fatty acid decomposition or polymerization side reactions caused by localized overheating.

[0052] Table 1 Analysis of Test Results

[0053] Comparative Example 1, by omitting hot water washing and detergent, saw a significant decrease in fatty acid yield to 75.2% and aromatics yield to only 60.4%. This directly demonstrates that insufficient pretreatment leads to impurity residues, interferes with hydrolysis reaction efficiency, and increases the risk of subsequent catalyst poisoning, thus affecting the overall yield. Comparative Example 2, employing isothermal hydrolysis, achieved a fatty acid yield of 80.6% and an aromatics yield of 65.7%, lower than the example. This indicates that segmented temperature control optimizes reaction kinetics through gradual temperature changes, avoiding side reactions caused by sudden temperature increases, whereas isothermal methods cannot achieve such precise control.

[0054] Regarding catalyst lifetime, Examples 1 to 3 all exceeded 480 hours, while Comparative Example 3 only reached 250 hours. This verifies the synergistic effect of rare earth metal modification and hydrogen donor addition in the composite catalyst. Rare earth metal modification modulates the acidic sites of the molecular sieve, retaining the acid strength required to promote carbocation reactions while reducing the density of strong acid centers, thereby reducing coking. Hydrogen donors such as formic acid or tetrahydronaphthalene release active hydrogen, promptly saturating unsaturated intermediates, stabilizing the reaction pathway, and further inhibiting coking. Comparative Example 3, using unmodified molecular sieves and without hydrogen donor addition, showed a significantly shortened catalyst lifetime and a decrease in aromatic yield to 55.2%, indicating that the lack of these characteristics leads to rapid catalyst deactivation. In Comparative Example 5, the fatty acid to catalyst mass ratio was 1:50, resulting in a catalyst lifetime of only 300 hours and an aromatic yield of 58.1%. This demonstrates that an imbalanced mass ratio leads to excessive catalyst loading, accelerating coking and deactivation, while the ratios of 1:8 to 1:25 in the examples ensured optimized contact between reactants and active sites.

[0055] The results for product purity and separation efficiency showed that Examples 1 to 3 all exceeded 92%, while Comparative Example 4, using a single extractant, had a purity reduced to 87.3% and a separation efficiency of 89.2%. This demonstrates the advantage of the combination of dimethyl sulfoxide and sulfolane in the composite extractant. By adjusting the ratio, the composite extractant can maintain high selective solubility for aromatics within a wider operating window, optimizing the separation of aromatics with different carbon numbers. Single solvents lack this adaptability, leading to incomplete separation. The product purities of Comparative Examples 1 and 2 were 82.1% and 85.6%, respectively, which were also relatively low. This further illustrates that insufficient pretreatment and hydrolysis control affect product quality and increase impurity interference. The separation efficiency of Comparative Example 5 was 82.6%, possibly due to an inappropriate catalyst ratio leading to complex reaction products and increased separation difficulty.

[0056] Test results show that this application, through multi-step synergistic process design, efficiently converts complex kitchen waste oil into high-value aromatic oil, achieving high-value utilization of resources while systematically solving the core problems of easy catalyst deactivation, low product separation efficiency, and poor process stability in existing technologies.

[0057] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing aromatic oil from waste cooking oil, characterized in that, The method includes the following steps: Raw material pretreatment steps: The waste cooking oil is sequentially processed through settling and clarification, multi-stage filtration, hot water washing, and demulsification and dehydration. Hydrolysis reaction steps: Mix pretreated kitchen waste oil with water at a mass ratio of 1:0.5-5, and react at 160 to 260°C and 1 to 4 MPa pressure for 1 to 6 hours to obtain a fatty acid mixture; Catalytic aromatization step: The fatty acid mixture obtained by hydrolysis is mixed with a composite catalyst and reacted in a fixed-bed reactor at 300 to 480 °C and 0.1 to 3 MPa pressure for 0.5 to 8 hours to obtain the aromatization reaction product; Product separation steps: The aromatization reaction product is cooled to 20 to 60°C by condensation, and the water and oil phases are separated by oil-water decantation. The oil phase is used to extract aromatics with an extractant, and finally, light aromatics and diesel fractions are obtained by fractionation.

2. The method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, The multi-stage filtration includes 100 to 200 mesh coarse filtration and 300 to 500 mesh fine filtration; the hot water washing uses 80 to 95°C hot water and the washing time is 30 to 120 minutes; the amount of demulsifier added in the demulsification and dehydration is 0.1-1% of the mass of kitchen waste oil, the demulsification temperature is 70 to 90°C, and the demulsification time is 1 to 3 hours.

3. The method for preparing aromatic oil from waste cooking oil according to claim 2, characterized in that, In the hot water washing step, a cleaning aid is also added. The amount of the cleaning aid added is 0.5-2% of the mass of the waste cooking oil. The cleaning aid is one of sodium citrate, sodium silicate, or sodium carbonate.

4. The method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, In the hydrolysis reaction step, a segmented temperature control is adopted. First, the reaction is carried out at 160 to 180℃ for 0.5 to 1 hour, then the temperature is increased to 200 to 220℃ for 1 to 2 hours, and finally the temperature is increased to 240 to 260℃ for 0.5 to 1 hour.

5. The method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, In the hydrolysis reaction step, 0.5-3% of the mass of waste cooking oil is added as an acidic catalyst, wherein the acidic catalyst is p-toluenesulfonic acid.

6. The method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, In the catalytic aromatization step, the preparation of the composite catalyst includes the following specific steps: Preparation of modified molecular sieve: HZSM-5 molecular sieve is mixed with rare earth metal nitrate solution, the rare earth metal loading is 2-6% of the mass of HZSM-5 molecular sieve, and the mixture is impregnated for 12 to 24 hours, dried at 100 to 120℃ for 2 to 4 hours, and calcined at 500 to 650℃ for 2 to 5 hours to obtain modified molecular sieve. Preparation of modified clay: Mix attapulgite clay with a hydrochloric acid solution of 2% to 8% by mass at a mass ratio of 1:10-30, stir at 60 to 80°C for 30 to 90 minutes, wash until neutral, and then dry. Catalyst molding: 30 to 50 parts by weight of modified molecular sieve, 25 to 35 parts by weight of alumina and 25 to 35 parts by weight of modified clay are mixed, 5 to 10 parts by weight of binder and 2 to 5 parts by weight of pore-forming agent are added, kneaded evenly and then extruded into shape, dried at 100 to 120°C for 2 to 4 hours and calcined at 500 to 650°C for 2 to 5 hours to obtain composite catalyst.

7. The method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, The mass ratio of the fatty acid mixture to the composite catalyst is 1:8-25.

8. The method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, In the catalytic aromatization step, a hydrogen donor is also added. The hydrogen donor is one of formic acid, methanol or tetrahydronaphthalene, and the amount of hydrogen donor added is 0.1-0.5% of the mass of waste cooking oil.

9. A method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, The extraction operation uses a composite extractant, which is a mixture of dimethyl sulfoxide and sulfolane in a mass ratio of 1:1-3. The extraction temperature is 40 to 70°C, and the mass ratio of the composite extractant to the oil phase is 1-2:

1.

10. A method for preparing aromatic oil from waste cooking oil according to claim 1, characterized in that, In the product separation step, the fractionation adopts a dual-tower system. The first tower separates light aromatics at 60 to 120°C, and the second tower separates diesel fraction at 150 to 250°C. The heavy oil at the bottom of the towers is recycled back to the catalytic aromatization step for processing.

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