Waste oil pretreatment method for hydrocracking preparation of biological aviation kerosene

Abstract

The invention relates to the technical field of biomass energy and green chemical industry, in particular to a waste grease pretreatment method for preparing biological aviation kerosene through hydrocracking, which comprises the following steps: pre-settling kitchen waste grease to obtain pre-settled grease, adding liquid lipase, methanol and demineralized water into the pre-settled grease, and carrying out hydrocracking to obtain pre-settled grease; performing two-stage enzymatic reaction: performing first-stage enzymatic reaction, namely continuously reacting until the acid value is reduced to 3 mg KOH / g or below; secondary enzymatic reaction: supplementing methanol, and reacting until the total content of diglyceride and monoglyceride is 5-15wt%; settling the reaction liquid to obtain a light-phase oil layer, adding crude methyl ester and hot demineralized water into the light-phase oil layer, carrying out coupled extraction washing and water phase removal to obtain an upper-layer clarified oil phase, and drying to obtain the biological aviation kerosene raw material oil; the method has the advantages of effectively controlling the distribution of glyceride in the waste grease, deeply removing metal ions, improving the aviation kerosene yield in the subsequent hydrocracking process and prolonging the service life of the catalyst.

Description

Technical Field

[0001] This application relates to the technical fields of biomass energy and green chemical engineering, and in particular to a method for pretreatment of waste oils for hydrocracking to produce bio-aviation kerosene. Background Technology

[0002] To address climate change, the aviation industry's demand for bio-jet fuel is increasingly urgent. Producing bio-jet fuel from waste cooking oil offers significant environmental and economic benefits. However, waste cooking oil has a complex composition, containing large amounts of free fatty acids (FFA), colloids, phospholipids, and metal ions (such as Ca, Mg, K, and Na) as impurities. These impurities, especially metal ions and colloids, can cause irreversible poisoning and coking deactivation of subsequent hydrocracking catalysts, severely limiting the production efficiency and economic viability of bio-jet fuel.

[0003] Currently, common pretreatment methods largely draw inspiration from biodiesel processes, which involve converting oils (including FFA) into fatty acid methyl esters (FAME) through acid / base catalysis or enzyme catalysis. For example, existing technologies disclose a method for producing biodiesel using immobilized lipases, but its process objective is to maximize triglyceride conversion (>98%) to obtain high-purity fatty acid methyl esters (FAME). When this product is directly used in hydrocracking, its single component may lead to excessive cracking, and the process lacks specific optimization for metal ion removal, making it difficult to meet the long-term stability requirements of hydrocracking catalysts for feedstocks.

[0004] Therefore, developing a pretreatment method for waste oils that can deeply purify and regulate the composition of raw materials and is specifically suitable for harsh hydrocracking conditions has become a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] In order to effectively control the distribution of glycerides in waste oils and remove metal ions, improve the jet fuel yield in subsequent hydrocracking processes and extend catalyst life, this application provides a waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking.

[0006] This application provides a method for pretreatment of waste oils used in the hydrocracking process to produce bio-aviation kerosene, employing the following technical solution: A method for pretreating waste oils used in the hydrocracking process to produce bio-aviation kerosene, comprising the following steps: S1. Pre-sedimentation of waste cooking oil to separate water and solid impurities, resulting in pre-sedimented oil; S2. Add liquid lipase, methanol, and demineralized water to the pre-sedimented oil to carry out a two-stage enzymatic reaction, as follows: Primary enzymatic reaction: Continue the reaction until the acid value drops below 3 mg KOH / g; Secondary enzymatic reaction: methanol is added, and the reaction continues to obtain a reaction solution. The total content of diglycerides and monoglycerides in the reaction solution is controlled at 5-15 wt%. S3. The reaction solution is allowed to settle, the heavy phase is separated, and a light phase oil layer is obtained; S4. Add crude methyl ester and hot demineralized water to the light phase oil layer, perform coupled extraction and washing, separate the aqueous phase, and obtain the upper clear oil phase. S5. Dry the upper clarified oil phase to obtain bio-aviation kerosene feedstock.

[0007] By adopting the above technical solution, this application defines five core steps of the pretreatment method. Its overall effect lies in achieving synergistic optimization of waste oil in two dimensions: "precise control of chemical composition" and "deep removal of impurities," through a specific process combination. Specifically, S1 pre-sedimentation, as a preliminary physical purification, effectively removes most of the free water, emulsified water, and suspended solids in the waste oil through heating and settling, laying the foundation for subsequent deep treatment of dissolved impurities (such as metal ions) and avoiding interference from these impurities on subsequent enzymatic reactions. The two-stage enzymatic reaction of S2 is one of the core innovations of this invention. Its effect is not to pursue the complete conversion of triglycerides (>98%) as in traditional processes, but rather to intentionally stabilize the total content of diglycerides (DG) and monoglycerides (MG) in the final product within the range of 5-15 wt% by controlling the reaction process in stages (first-stage rapid acid value reduction stage, second-stage glyceride distribution regulation stage). This "incomplete conversion" strategy yielded unexpected technical benefits: in the subsequent hydrocracking process, the retained MG and DG, as medium- to long-chain oxygen-containing compounds, could synergistically interact with fatty acid methyl esters (FAME), providing a more suitable hydrolysis and cracking pathway. This effectively suppressed the excessive generation of light gases such as methane and ethane, thereby significantly increasing the selectivity of the target product, aviation kerosene (C8-C16), from approximately 40% to over 50% (e.g., 52.1% in Example 1). The sedimentation separation in S3 removed the glycerol, residual catalyst, and some water-soluble impurities generated during the reaction, purifying the oil phase. The coupled extraction in S4 is another key innovation. Utilizing a "crude methyl ester-hot water" synergistic system, crude methyl ester acts as a diluent and co-solvent, reducing the viscosity of the oil phase and disrupting the micelle structure, thus fully exposing the encapsulated metal ions. Hot water, on the other hand, increases the diffusion rate and solubility of the ions. This synergistic process achieves highly efficient extraction of alkali and alkaline earth metal ions, significantly reducing the total metal content of the final feedstock oil to below 10 ppm (5 ppm in Example 1), far superior to conventional water washing processes (32 ppm in Comparative Example 1). This deep purification effect directly translates into strong protection for downstream expensive and sensitive hydrocracking catalysts (such as Pt-Pd / USY), significantly mitigating metal poisoning and coking problems. For example, the catalyst carbon deposition in Example 1 (2.1%) is far lower than that in Comparative Example 1 (5.8%), and it is expected to extend the catalyst's single-cycle life by more than 30%. Finally, drying in S5 removes trace amounts of moisture, ensuring the stability of the feedstock oil. In summary, the overall solution of this application has generated a synergistic effect of "1+1>2", which not only improves the jet fuel yield, but also greatly enhances the economy and sustainability of the process.

[0008] Preferably, in step S2, before the two-stage enzymatic reaction, the amounts of liquid lipase, methanol, and deionized water added are 1-3 wt%, 25-35 wt%, and 2-4 wt%, respectively.

[0009] Preferably, in S2, the amount of methanol added in the secondary enzymatic reaction is 5-10 wt%.

[0010] By employing the above technical solution, the initial and supplementary amounts of methanol in the two-stage reaction were precisely controlled. These specific material ratios are key operational parameters for achieving the "precise component control" effect described in S2. The initial methanol amount (25-35 wt%) forms an optimal reaction environment with the lipase and deionized water, ensuring that the first-stage reaction can quickly and efficiently reduce the acid value of the high-acid-value feedstock to below 3 mg KOH / g, creating stable starting conditions for the second-stage reaction. The supplementary methanol amount (5-10 wt%) precisely controls the depth of the second-stage reaction. Insufficient methanol will lead to incomplete reaction, resulting in excessively high MG and DG content (e.g., 13.5% in Example 2), which may slightly reduce the jet fuel yield (49.8%); excessive methanol tends to lead to complete conversion, resulting in excessively low MG and DG content (<2%), which is detrimental to jet fuel selectivity (41.5% in Comparative Example 1). Therefore, by limiting the methanol supplementary amount and precisely controlling MG and DG within the optimal range, it is the core guarantee for achieving a high jet fuel yield.

[0011] Preferably, the total content of diglycerides and monoglycerides in the reaction solution obtained from the secondary enzymatic reaction is 8-12 wt%.

[0012] By adopting the above technical solution, the total content of MG and DG is further limited to the preferred range of 8-12 wt%. Experimental verification shows that this range is key to achieving optimal technical performance. Example 1 (total MG and DG content = 10.2%) achieved the highest jet fuel yield (52.1%) and the lowest catalyst coking (2.1%). In contrast, the yields of Comparative Example 1 (total MG and DG content <2%) and Example 2 (total MG and DG content = 13.5%) both decreased. This demonstrates that 8-12 wt% is a preferred range within which the retained oxygen-containing components optimally control the hydrocracking reaction pathway, maximizing the target fraction yield while avoiding the potential negative effects of excessive retention.

[0013] Preferably, the temperature of the two-stage enzymatic reaction is 40-55℃, and the rotation speed is 100-300 rpm.

[0014] By employing the above technical solutions, temperature is the most crucial factor affecting enzyme activity and stability. A suitable temperature ensures high catalytic efficiency and a high reaction rate, while preventing enzyme inactivation due to excessive heat. Stirring speed affects the mass transfer efficiency of the reaction system. Stirring at an appropriate speed ensures sufficient contact between methanol and the oil, allowing the reaction to proceed uniformly and preventing localized excess or deficiency of methanol. This is essential for achieving uniform and precise control of MG and DG content throughout the entire reaction system. This parameter range is fundamental to ensuring efficient and controllable reaction.

[0015] Preferably, in S4, the amount of crude methyl ester added is 10-30 wt%.

[0016] Preferably, the metal content in the crude methyl ester is <5 ppm.

[0017] Preferably, the amount of hot demineralized water added in S4 is 8-12 wt%.

[0018] Preferably, the temperature of the hot demineralized water in S4 is 60-80℃.

[0019] By adopting the above technical solution, key parameters of the S4 coupled extraction step were defined. Specifically, the amount of crude methyl ester added was limited to 10-30 wt%. The main function of crude methyl ester is dilution and solubilization, and its dosage directly affects the demetallization effect. Too little dosage results in insufficient dilution and micelle disruption; too much dosage is uneconomical. Furthermore, the crude methyl ester itself must have a metal content of <5 ppm. This is a prerequisite for preventing secondary contamination and ensuring a low metal content in the final product, reflecting the rigor of process control. The dosage (8-12 wt%) and temperature (60-80℃) of the hot demineralized water were limited. Hot water is the receiving phase for metal ions, and its dosage and temperature together determine the extraction efficiency. Sufficient dosage ensures extraction capacity, while increasing the temperature significantly reduces the viscosity of the aqueous phase, increases the ion diffusion coefficient and solubility, thereby greatly improving the removal rate. Comparative Example 2 (using only hot water, without crude methyl ester) had a metal content of 15 ppm, while this application achieves 5 ppm, demonstrating the importance of parameter synergy. The combination of these parameters is the direct reason for achieving the outstanding effect of "deep demetallization" (total metals <10ppm or even <5ppm).

[0020] Preferably, in step S5, the drying method for the upper clarified oil layer is vacuum drying, and the vacuum drying temperature is 70-95℃.

[0021] By employing the above technical solution, the purpose of this step is to completely remove the trace amounts of water remaining in the oil phase after water washing. The presence of water can harm the subsequent hydrocracking catalyst. Drying under vacuum conditions at an appropriate temperature can efficiently evaporate and remove water without causing thermal degradation of the oil (such as oxidation or polymerization), ensuring the stable quality of the final bio-aviation kerosene feedstock and meeting the feed requirements of the hydrocracking unit.

[0022] In summary, this application includes at least one of the following beneficial technical effects: 1. This application breaks away from the conventional thinking of "complete conversion," achieving synergistic optimization of waste oils in two dimensions: "precise control of chemical composition" and "deep removal of impurities." Through a two-stage enzymatic reaction design, the effect is not to pursue the complete conversion of triglycerides (>98%) achieved in traditional processes, but rather to intentionally stabilize the total content of diglycerides (DG) and monoglycerides (MG) in the final product within the range of 5-15 wt% by controlling the reaction process in stages (a first-stage rapid acid value reduction stage and a second-stage glyceride distribution control stage). This "incomplete conversion" strategy brings unexpected technical benefits: in the subsequent hydrocracking process, these retained MG and DG, as medium- to long-chain oxygenated compounds, can synergistically interact with fatty acid methyl esters (FAME), providing a more suitable hydrogenolysis and cracking pathway, effectively inhibiting the excessive generation of light gases such as methane and ethane, thereby significantly increasing the selectivity of the target product, aviation kerosene (C8-C16) fraction, from the conventional approximately 40% to over 50%.

[0023] 2. Coupled extraction is another core innovation. Utilizing a "crude methyl ester-hot water" synergistic system, crude methyl ester acts as a diluent and co-solvent, reducing oil phase viscosity and disrupting micellar structure, thus fully exposing encapsulated metal ions. Hot water, on the other hand, increases ion diffusion rate and solubility. The synergistic effect achieves highly efficient extraction of alkali and alkaline earth metal ions, significantly reducing the total metal content of the final feedstock oil to below 10 ppm, far superior to conventional water washing processes. This deep purification effect directly translates into strong protection for downstream expensive and sensitive hydrocracking catalysts (such as Pt-Pd / USY), significantly mitigating metal poisoning and coking problems, and is expected to extend catalyst single-cycle life by more than 30%. In summary, the overall scheme of this application produces a synergistic effect of "1+1>2," greatly enhancing the economics and sustainability of the process while improving jet fuel yield. Detailed Implementation

[0024] The following provides a more detailed description of this application in conjunction with specific details. raw material

[0025] All products used in this application are common commercial products. The initial acid value range of the waste cooking oil applicable to the embodiments of this application is 20-40 mg KOH / g, and the total metal content is <300 ppm. The specific embodiments are subject to testing. Example

[0026] Example 1 A method for pretreating waste oils used in the hydrocracking process to produce bio-aviation kerosene includes the following preparation steps: S1. Raw material pre-sedimentation and impurity removal 1.0 ton of kitchen waste oil (initial acid value 35.2 mg KOH / g, total metal content 185 ppm) was heated to 65°C in a heating tank and left to stand for 24 hours. The heavy phase wastewater and suspended solids at the bottom of the heating tank were separated and discharged. The total mass of the heavy phase wastewater and suspended solids was 5.3% of the kitchen waste oil, thus obtaining pre-sedimented oil. S2, multi-stage enzymatic reaction Add 2.0 wt% liquid lipase (Novozymes, Lipozyme TL IM), 30 wt% methanol and 3 wt% deionized water to the pre-precipitated oil, and carry out the following two-stage enzymatic reaction at 50°C and 200 rpm. Primary enzymatic reaction: lasted 2.5 hours, and the acid value dropped to 2.1 mg KOH / g when sampled; Secondary enzymatic reaction: 8 wt% methanol was added, and the reaction continued for 6 hours. After the reaction was completed, the reaction solution was obtained. Samples were taken and analyzed by GC. The product contained 86.5% FAME, 3.1% triglycerides (TG), and a total content of 10.4% diglycerides (DG) and monoglycerides (MG). S3, Sedimentation after reaction The reaction solution was transferred to a settling tank and settling at 50°C for 6 hours. The lower layer of glycerol-water-methanol mixed heavy phase was discharged. The total mass of the heavy phase was 12.1% of the kitchen waste oil, and a light phase oil layer was obtained. S4, Coupled Extraction for Deep Demetallization Add 20wt% crude methyl ester (crude fatty acid methyl ester, metal content 4ppm) and 10wt% 70℃ hot deionized water to the light phase oil layer, stir for 20min, let stand for 2h to separate the layers, drain the lower aqueous phase, and obtain the upper clear oil phase. S5. Separation and Drying The upper clarified oil phase was vacuum dried at 90℃ and -0.095MPa for 2h to obtain bio-aviation kerosene feedstock, denoted as Sample-Inv1. The feedstock parameters were as follows: acid value: 0.5mg KOH / g; total metal content: 5ppm; MG+DG content: 10.2wt%.

[0027] Example 2 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the time for the secondary enzymatic reaction in S2 is 5 hours, while the remaining steps are the same as in Example 1. The resulting bio-aviation kerosene feedstock is denoted as Sample-Inv2. The test results are as follows: total metal content: 6 ppm; MG+DG content: 13.5 wt%.

[0028] Example 3 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the amount of crude methyl ester added in S4 is 10 wt%, while the remaining steps are the same as in Example 1.

[0029] Example 4 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the amount of crude methyl ester added in S4 is 30 wt%, while the remaining steps are the same as in Example 1.

[0030] Example 5 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the amount of hot demineralized water added in S4 is 5 wt%, while the remaining steps are the same as in Example 1.

[0031] Example 6 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the amount of hot demineralized water added in S4 is 15 wt%, while the remaining steps are the same as in Example 1.

[0032] Example 7 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the temperature of the hot demineralized water in S4 is 60°C, while the remaining steps are the same as in Example 1.

[0033] Example 8 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that the temperature of the hot demineralized water in S4 is 80°C, while the remaining steps are the same as in Example 1.

[0034] Comparative Example 1 A pretreatment method for waste oil used in the preparation of bio-aviation kerosene via hydrocracking differs from Example 1 in that, after S1, 2.0 wt% of the same liquid lipase and 38 wt% methanol are added to the pre-sedimented oil (in one step), and the reaction is carried out at 50°C and 200 rpm for 10 h to ensure a triglyceride conversion rate >98%. Then, after reaction and sedimentation separation according to S3 of Example 1, a conventional water washing process is used: 10 wt% of 25°C demineralized water is added, the mixture is stirred and allowed to separate into layers, the lower aqueous phase is discharged, and the mixture is vacuum dried at 90°C and -0.095 MPa for 2 h to obtain bio-aviation kerosene feedstock, denoted as Sample-Cp1. The test results are: acid value: 0.8 mg KOH / g; total metal content: 32 ppm; MG+DG content: <2 wt%.

[0035] Comparative Example 2 A waste oil pretreatment method for preparing bio-aviation kerosene by hydrocracking differs from Example 1 in that coupled extraction is omitted and conventional water washing is used instead. Specifically, crude methyl ester is not added in step S4, and only 10wt% of 70℃ hot demineralized water is used for washing. The remaining steps are the same as in Example 1. The resulting bio-aviation kerosene feedstock is denoted as Sample-Cp2. The test results are as follows: total metal content: 15ppm; MG+DG content: 10.1wt%.

[0036] Performance testing Detection methods / test methods Feedstock oils were prepared according to the pretreatment methods of the examples and comparative examples, and then evaluated in the same hydrocracking microreactor. The catalyst was an industrial Pt-Pd / USY catalyst, and the reaction conditions were: temperature 380°C, hydrogen partial pressure 5 MPa, and weight hourly space velocity 1.0 h⁻¹. -1 The test results are shown in Table 1.

[0037] Table 1. Test Results (After 100 hours of stable operation)

[0038] As can be seen from the test data in Table 1, the bio-aviation kerosene feedstock obtained by the pretreatment method of this application, after 100 hours of stable operation, has a jet fuel fraction yield of 49.8 wt% or higher, with a maximum of 52.4 wt%, while the catalyst carbon deposition is 2.8 wt% or lower, with a minimum of 2.0 wt%. This method can improve the jet fuel fraction while extending the catalyst life.

[0039] Sample-Inv1 vs Sample-Cp1: Compared with the conventional full conversion process (Comparative Example 1), this application (such as Example 1) improves jet fuel yield by 10.6 percentage points, which is a significant improvement. Simultaneously, catalyst carbon deposition is reduced by 64%, fully demonstrating the dual advantages of "component regulation" and "deep demetallization".

[0040] Sample-Inv1 vs Sample-Cp2: Both have similar MG+DG contents, but the metal content of this invention is lower (5 ppm vs 15 ppm). This allows this application to achieve similarly high yields (52.1% vs 50.5%) while significantly lower catalyst carbon deposition (2.1% vs 3.9%), demonstrating the unique and inventive contribution of "coupled extraction" to deep demetallization and catalyst protection.

[0041] Sample-Inv2: When the MG+DG content is slightly higher than the preferred range (8-12wt%), the jet fuel yield begins to decrease slightly, indicating that controlling the MG+DG content at 8%-12% is the best choice.

[0042] In addition, by further adjusting the key process parameters in S4 through Examples 3-8, the waste oil pretreatment method can be made more stable, and the total MG+DG content and total metal content of the bio-aviation kerosene feedstock can be controlled within an appropriate range.

[0043] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A method for pretreatment of waste oils used in the hydrocracking process to produce bio-aviation kerosene, characterized in that: It includes the following steps: S1. Pre-sedimentation of waste cooking oil to separate water and solid impurities, resulting in pre-sedimented oil; S2. Add liquid lipase, methanol, and demineralized water to the pre-sedimented oil to carry out a two-stage enzymatic reaction, as follows: Primary enzymatic reaction: Continue the reaction until the acid value drops below 3 mg KOH / g; Secondary enzymatic reaction: methanol is added, and the reaction continues to obtain a reaction solution. The total content of diglycerides and monoglycerides in the reaction solution is controlled at 5-15 wt%. S3. The reaction solution is allowed to settle, the heavy phase is separated, and a light phase oil layer is obtained; S4. Add crude methyl ester and hot demineralized water to the light phase oil layer, perform coupled extraction and washing, separate the aqueous phase, and obtain the upper clear oil phase. S5. Dry the upper clarified oil phase to obtain bio-aviation kerosene feedstock.

2. The method for pretreatment of waste oils used in the preparation of bio-aviation kerosene by hydrocracking according to claim 1, characterized in that: In S2, before the two-stage enzymatic reaction, the amounts of liquid lipase, methanol, and deionized water added are 1-3 wt%, 25-35 wt%, and 2-4 wt%, respectively.

3. The waste oil pretreatment method for hydrocracking to produce bio-aviation kerosene according to claim 1, characterized in that: In S2, the amount of methanol added during the secondary enzymatic reaction is 5-10 wt%.

4. The method for pretreatment of waste oils used in the hydrocracking production of bio-aviation kerosene according to claim 1, characterized in that: The total content of diglycerides and monoglycerides in the reaction solution obtained from the secondary enzymatic reaction is 8-12 wt%.

5. The waste oil pretreatment method for hydrocracking to produce bio-aviation kerosene according to claim 1, characterized in that: The temperature for the two-stage enzymatic reaction is 40-55℃, and the rotation speed is 100-300 rpm.

6. The method for pretreatment of waste oils used in the hydrocracking production of bio-aviation kerosene according to claim 1, characterized in that: In S4, the amount of crude methyl ester added is 10-30 wt%.

7. A method for pretreatment of waste oils used in the hydrocracking production of bio-aviation kerosene according to claim 1, characterized in that: The metal content in the crude methyl ester is <5 ppm.

8. The method for pretreatment of waste oils used in the hydrocracking production of bio-aviation kerosene according to claim 1, characterized in that: The amount of hot demineralized water added in S4 is 8-12 wt%.

9. A method for pretreatment of waste oils used in the hydrocracking production of bio-aviation kerosene according to claim 1, characterized in that: The temperature of the hot demineralized water in S4 is 60-80℃.

10. A method for pretreatment of waste oils used in the hydrocracking production of bio-aviation kerosene according to claim 1, characterized in that: In step S5, the drying method for the upper clarified oil layer is vacuum drying, and the vacuum drying temperature is 70-95℃.

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