Enzyme catalysis synchronous desulfurization process for preparing biodiesel from kitchen waste oil

Abstract

The application relates to the technical field of renewable bioenergy, in particular to an enzyme catalysis synchronous desulfurization process for preparing bio-diesel from kitchen waste oil, which comprises the following steps: putting pretreated kitchen waste oil, immobilized lipase, a cross-linked chitosan porous microsphere composite adsorbent loaded with zinc oxide and iron oxide and methanol into the same reactor, and performing one-pot ester exchange reaction under the condition of 40-50 DEG C through the mode of step-by-step addition of methanol, so that deep desulfurization is synchronously completed in the reaction process, that is, the core contradiction that the existing desulfurization scheme cannot be applied to the one-pot process due to the incompatibility with the enzyme is solved.

Description

Technical Field

[0001] This application relates to the field of renewable bioenergy technology, and more specifically, it relates to an enzyme-catalyzed simultaneous desulfurization process for producing biodiesel from kitchen waste oil. Background Technology

[0002] As an important raw material for biodiesel, the resource utilization of waste cooking oil has always been strongly supported by national policies. However, during the cooking and collection process, waste cooking oil is generally mixed with sulfur-containing organic matter, including thiols, thioethers, thiophenes and their derivatives. The total sulfur content is usually in the range of tens to hundreds of ppm, which seriously restricts the quality upgrading and market application of biodiesel.

[0003] If the aforementioned organic sulfur compounds are not removed during processing, they will cause the final biodiesel products to emit sulfur oxides during combustion. This will not only corrode engine metal parts but also poison exhaust aftertreatment catalysts. Currently, the national standard requires the sulfur content of automotive diesel to be below 10 ppm, while in high-end special applications, the sulfur content must be controlled below 3 ppm.

[0004] The related technology discloses a method for pretreatment of kitchen waste oil, which uses Sm2O3 / ZrO2 solid acid catalyst to pretreat kitchen waste oil. In the pretreatment step, in-situ esterification of free fatty acids and selective adsorption and removal of sulfur-containing compounds are completed simultaneously. The sulfur content of the pretreated oil can be reduced to below 1 ppm and the acid value can be reduced to below 1.5 mg KOH / g.

[0005] While this scheme boasts outstanding technical specifications, it is essentially a typical two-step process. The "pretreatment desulfurization and acid reduction" and "enzyme-catalyzed ester exchange" must be carried out sequentially in two independent reaction stages or reactors. Therefore, this process inevitably leads to the following problems:

[0006] First, it requires two independent reaction equipment sets and supporting separation and conveying devices, resulting in high equipment investment costs. Second, material transfer between processes leads to increased energy consumption, operational complexity, and extended production cycles. Third, there is a risk of loss and secondary pollution during material transfer.

[0007] Therefore, some companies have tried to integrate all the above reaction processes into a single reactor and complete them simultaneously, which is the so-called "one-pot" process. However, when the catalyst and lipase are mixed in the same reactor, the activity retention rate of the lipase is significantly lower than that in its stepwise process, and the sulfur content of the product cannot meet the ultra-low sulfur standard. The analysis shows that this is related to the negative impact of the catalyst on the enzyme.

[0008] Based on this, this application provides an enzyme-catalyzed simultaneous desulfurization process for biodiesel production from kitchen waste oil. By designing a composite adsorbent that is highly compatible with lipase to replace the original catalyst, it achieves for the first time the simultaneous and efficient execution of enzyme-catalyzed transesterification reaction and deep adsorption desulfurization in a "one-pot" process. Summary of the Invention

[0009] To address the aforementioned technical issues, this application provides an enzyme-catalyzed simultaneous desulfurization process for converting kitchen waste oil into biodiesel. Through improvements to the adsorbent and supporting processes, the compatibility problem between high-performance catalysts and lipases can be effectively overcome, simplifying the process from a "two-step method" to a "one-pot method".

[0010] In the first aspect, this application provides an enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel, employing the following technical solution:

[0011] An enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel includes the following steps:

[0012] A. First, pre-treat the waste cooking oil;

[0013] B. The pretreated kitchen waste oil, immobilized lipase, composite adsorbent and methanol are put into the same reactor and carried out a one-pot transesterification reaction under stirring and constant temperature conditions.

[0014] Simultaneous desulfurization occurs during the reaction process;

[0015] The composite adsorbent is a cross-linked chitosan porous microsphere loaded with zinc oxide and iron oxide;

[0016] The methanol is added in steps, that is, 40% to 60% of the total amount of methanol is added initially, and the remaining methanol is added in 1 to 3 times during the reaction process.

[0017] C. After the reaction is complete, the immobilized lipase and composite adsorbent are separated and recovered, and the mixture is allowed to stand and separate to obtain the crude ultra-low sulfur biodiesel product.

[0018] More preferably, the composite adsorbent is prepared by the following method:

[0019] Step a: First, chitosan is made into spheres by reverse suspension method, and then cross-linked with epichlorohydrin to obtain cross-linked chitosan porous microspheres.

[0020] Step b: Immerse the cross-linked chitosan porous microspheres obtained in step a in an aqueous solution containing zinc salt and iron salt, so that the cross-linked chitosan porous microspheres swell and adsorb zinc ions and iron ions;

[0021] Step c: Under alkaline conditions, an in-situ precipitation reaction is carried out to uniformly deposit zinc hydroxide and iron hydroxide inside and on the surface of the cross-linked chitosan porous microspheres.

[0022] Step d: By drying and dehydrating at low temperature, zinc hydroxide and iron hydroxide are converted into zinc oxide nanoparticles and iron oxide nanoparticles, respectively, thus obtaining the composite adsorbent.

[0023] Furthermore, in step b, the zinc salt is zinc sulfate heptahydrate and the iron salt is ferric chloride hexahydrate.

[0024] More preferably, the particle size of the composite adsorbent is 75-187.5 μm;

[0025] The loading of zinc oxide is 6-12 wt%, and the loading of iron oxide is 3-7 wt%.

[0026] More preferably, the mass ratio of the pretreated kitchen waste oil, immobilized lipase, composite adsorbent and methanol is 100:(6-10):(3-5):(10-18).

[0027] Further preferred conditions for the one-pot transesterification reaction in B are: reaction temperature 40-50℃, stirring rate 150-250 rpm, and total reaction time 18-30 h.

[0028] More preferably, the immobilized lipase is Novozym 435, its carrier is acrylic resin, and the enzyme protein is Candida antarcticis lipase B.

[0029] Secondly, this application provides a composite adsorbent, which adopts the following technical solution:

[0030] The composite adsorbent is a cross-linked chitosan porous microsphere loaded with zinc oxide and iron oxide;

[0031] The particle size of the composite adsorbent is 75-187.5 μm;

[0032] The cross-linked chitosan porous microspheres are made by cross-linking chitosan with epichlorohydrin through reverse-phase suspension. The loading of zinc oxide is 6-12 wt%, and the loading of iron oxide is 3-7 wt%.

[0033] Thirdly, this application provides an application of a composite adsorbent in the field of oil desulfurization. The resulting composite adsorbent is also suitable for simultaneous desulfurization of other reaction systems, wherein the reaction system comprises immobilized lipase, composite adsorbent, and / or other optional functional additives.

[0034] In summary, this application has the following beneficial effects:

[0035] 1. Under the premise of being compatible with lipase activity, this application realizes the simultaneous completion of transesterification reaction and deep desulfurization in one reactor, eliminating the need for a separate pretreatment desulfurization reactor and its supporting catalyst separation, material cooling, intermediate storage tank and transfer pump in the traditional process, further reducing the number of equipment and management costs of biodiesel production lines;

[0036] 2) By using chitosan as a carrier, this application fundamentally eliminates the negative interference of adsorbents or catalysts on enzymes, ensuring that the transesterification reaction can always maintain a high conversion rate in the case of a one-pot reaction cycle of tens of hours and multiple recycling scenarios. The yield of fatty acid methyl esters is consistently greater than or equal to 95%, and the enzyme activity retention rate is still as high as 91-93% after 5 batches of continuous use.

[0037] 3) The composite adsorbent in this application, with its triple-relay desulfurization mechanism of "coordination-oxidative conversion-chemical adsorption", effectively overcomes the competitive interference from multiple components such as oils, methanol, glycerol, water, and fatty acids contained in the reaction medium, as detailed below:

[0038] First, the abundant pore structure and surface amino groups in chitosan microspheres have a pre-enrichment effect on organic sulfides, which can effectively increase the local concentration.

[0039] Secondly, the highly dispersed zinc oxide nanoparticles within the pores provide Lewis acid sites, forming stable irreversible zinc-sulfur coordination adsorption with the lone pair electrons of sulfur atoms in thiols and thioethers.

[0040] Meanwhile, in the presence of trace amounts of water in the reaction system, iron oxide nanoparticles can induce the generation of hydroxyl radicals with mild oxidizing ability on their surface, which can oxidize chemically inert and difficult-to-adsorb sulfide compounds in situ into sulfoxides or sulfones with significantly enhanced polarity. These oxidation products are then firmly captured by the high density of free amino groups on the chitosan molecular chain through acid-base neutralization and hydrogen bonding.

[0041] The above three desulfurization mechanisms work synergistically to ensure that the sulfur content of the product is stable at less than or equal to 3 ppm even in such a complex reaction environment. In most cases, it can be as low as about 2 ppm, which fully meets the most stringent market standard requirements.

[0042] 4) The core raw materials of the composite adsorbent in this application, namely chitosan, zinc sulfate heptahydrate, and ferric chloride hexahydrate, are all domestically produced bulk chemical raw materials with stable sources and low prices. They do not involve any rare earth elements, precious metals, or toxic and harmful reagents. Moreover, the entire preparation process of the adsorbent is carried out in an atmospheric pressure and aqueous system without the need for any high-temperature calcination or high-pressure reaction equipment.

[0043] In summary, this application has successfully overcome the technical bottleneck of existing high-performance desulfurization catalysts being unable to be applied to the simplified "one-pot" process due to their fundamental conflict with enzymes by constructing a chitosan-based bimetallic composite adsorbent at the molecular level. The "one-pot" process not only ensures product quality but also has the comprehensive advantages of high efficiency, economy, safety, and environmental friendliness. Detailed Implementation

[0044] The present application will be further described in detail below with reference to the embodiments.

[0045] Main raw material sources and specifications

[0046] Unless otherwise specified, the raw materials used in the following preparation examples and embodiments are all commercially available industrial grade or analytical grade products, and their specific sources and specifications are as follows:

[0047] Chitosan: Industrial grade, degree of deacetylation greater than or equal to 95%, viscosity 100-200 mPa·s, purchased from Shandong Aokang Biotechnology Co., Ltd.

[0048] Immobilized lipase: trade name Novozym 435, purchased from Novozymes (China) Biotechnology Co., Ltd., with an initial enzyme activity of approximately 10,000 PLU / g;

[0049] Liquid paraffin: density 0.84-0.86 g / mL, purchased from Shanghai Titan Technology Co., Ltd.

[0050] Sorbitan monooleate: purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0051] Diatomaceous earth: industrial grade, purchased from Jilin Yuantong Mining Co., Ltd.;

[0052] Epichlorohydrin: purity ≥ 99.5%, purchased from Sinopharm Chemical Reagent Co., Ltd.;

[0053] Waste cooking oil: Mixed waste cooking oil collected from multiple catering enterprises in the Yangtze River Delta region. The initial total sulfur content was 186 ppm, the acid value was 12 mgKOH / g, and the water and impurities content was 0.8%.

[0054] It should be clarified that these are only representative sample indicators. The method of this application has good applicability to kitchen waste oil from various sources with an initial total sulfur content of 50-500 ppm, an acid value of 5-25 mgKOH / g, and a water and impurity content of less than or equal to 2%.

[0055] Preparation Example 1

[0056] A composite adsorbent is prepared by the following method:

[0057] Step a: First, chitosan is made into spheres by reverse suspension method, and then cross-linked with epichlorohydrin to obtain cross-linked chitosan porous microspheres. The specific operating conditions are as follows.

[0058] a1. Add 950 mL of deionized water and 20 mL of glacial acetic acid to a 2 L glass beaker. Set the stirring speed to 300 rpm and stir until homogeneous. Then, slowly add 50 g of chitosan powder and continue mechanically stirring at 300 rpm for 4 hours until the chitosan is completely dissolved, resulting in a transparent and viscous chitosan acetate solution with a chitosan mass concentration of approximately 2%.

[0059] a2. Add 2.5 L of liquid paraffin and 50 mL of sorbitan monooleate emulsifier to a 5 L three-necked glass flask. Set the stirring speed to 400 rpm and preheat the system to 60°C by water bath heating. Maintain this temperature for 30 minutes to stabilize the oil phase system.

[0060] a3. The prepared chitosan acetate solution is added dropwise to the stirred oil phase at a constant rate of 10 mL / min using a peristaltic pump, while the mechanical stirring speed is increased to 600 rpm. High-speed shear dispersion is used to form a stable water-in-oil emulsion system. After all the chitosan solution has been added, stirring continues at 600 rpm for 1 hour to ensure the emulsion droplets are fully stabilized.

[0061] a4. Reduce the stirring speed to 300 rpm and operate in a fume hood. Use a constant pressure dropping funnel to add 15 mL of epichlorohydrin as a crosslinking agent dropwise to a three-necked flask. After the addition is complete, raise the water bath temperature to 70°C and maintain the crosslinking reaction at this temperature for 4 hours.

[0062] a5. After the reaction is complete, stop heating and stirring, let the mixture in the three-necked flask stand and separate into layers. After the oil phase and the aqueous phase are completely separated, discard the upper oil phase (liquid paraffin layer). The lower layer is the product containing cross-linked chitosan microspheres, and then wash it.

[0063] Wash with acetone three times, and filter out the washing liquid after each wash.

[0064] Wash twice with anhydrous ethanol, and filter out the washing liquid after each wash.

[0065] Finally, wash repeatedly with plenty of deionized water and filter until there is no oil floating on the surface of the washing liquid and the pH value of the washing liquid is neutral as tested by pH test paper.

[0066] a6. The washed wet cross-linked chitosan microspheres are passed through a standard inspection sieve for particle size classification. First, pass through an 80-mesh standard sieve and collect the undersize material. Then, pass the undersize material through a 200-mesh standard sieve and collect the oversize material. Spread the classified microspheres evenly into a forced-air drying oven and dry at 60°C for 24 hours to obtain cross-linked chitosan porous microspheres, denoted as CTS-M.

[0067] Step b: Immerse the cross-linked chitosan porous microspheres obtained in step a in an aqueous solution containing zinc and iron salts, under the following specific operating conditions:

[0068] b1. Weigh 24.7 g of zinc sulfate heptahydrate and 13.5 g of ferric chloride hexahydrate, add them to a 500 mL glass beaker, then add 300 mL of deionized water to the beaker, turn on the magnetic stirrer and stir until the solid salts are completely dissolved, to obtain a transparent, orange-red mixed aqueous solution of metal salts.

[0069] b2. Weigh 100 g of the CTS-M cross-linked chitosan porous microspheres prepared in a, immerse them completely in the prepared metal salt mixed aqueous solution, set the stirring speed to 150 rpm, and stir slowly at room temperature for 2 hours to allow the cross-linked chitosan porous microspheres to fully swell in the salt solution, and utilize the free amino groups on the chitosan molecular chain to complex and adsorb zinc ions and iron ions.

[0070] Step c: Under alkaline conditions, an in-situ precipitation reaction is carried out to uniformly deposit zinc hydroxide and iron hydroxide inside and on the surface of the cross-linked chitosan porous microspheres. The specific operating conditions are as follows:

[0071] c1. While maintaining a stirring speed of 150 rpm, slowly add a 2 mol / L sodium hydroxide aqueous solution to the suspension system using a constant pressure dropping funnel. The total amount added is approximately 80 mL. Adjust the pH of the entire suspension system to between 9.5 and 10.0.

[0072] c2. After the addition is complete, continue stirring at 150 rpm for 1 hour. Light brown precipitates can be observed to gradually form on the surface of the microspheres and in the suspension, proving that zinc hydroxide and iron hydroxide are precipitating in situ. Place the resulting suspension in a constant temperature water bath at 60°C and let it stand for 3 hours to allow the precipitate particles to grow and stabilize.

[0073] Step d: Zinc hydroxide and iron hydroxide are converted into zinc oxide nanoparticles and iron oxide nanoparticles, respectively, by low-temperature drying and dehydration. The specific operating conditions are as follows:

[0074] d1. After aging, the suspension is filtered under reduced pressure and the filter cake is repeatedly washed with deionized water. After each wash, the filter cake is dried and deionized water is added again until the conductivity of the filtrate after the last wash is less than 20 μS / cm as measured by a conductivity meter, to ensure that free salt ions and excess alkali are fully removed.

[0075] d2. Place the washed filter cake into a forced-air drying oven and dry at 80℃ for 2 hours to remove free water. Then raise the temperature to 105℃ and continue drying for 6 hours, so that the zinc hydroxide and iron hydroxide deposited in the microsphere channels are converted into zinc oxide and iron oxide nanoparticles respectively during the drying process.

[0076] d3. After drying, the product was naturally cooled to room temperature. The resulting dark yellow-brown microsphere product (117.2 g) is the cross-linked chitosan porous microsphere composite adsorbent loaded with zinc oxide and iron oxide in this application, denoted as CTS-M@Zn / Fe. The loading of zinc oxide in the composite adsorbent was approximately 8.5 wt%, and the loading of iron oxide was approximately 5.2 wt%.

[0077] Performance testing

[0078] The biodiesel products and related indicators obtained in each embodiment and comparative example were selected as test objects. Then, the total sulfur content of the products, the yield of fatty acid methyl esters, and the activity retention rate of immobilized lipase after multiple batches of use were tested. The specific test methods are as follows:

[0079] 1) Total sulfur content of the product: The total sulfur content of the product was determined by ultraviolet fluorescence method. The standard for the test was SH / T 0689. The results were reported in ppm. 1 ppm is equivalent to 1 mg / kg.

[0080] 2) Fatty acid methyl ester yield test: The yield was determined by gas chromatography according to EN 14103, and the results were reported as a percentage by mass.

[0081] 3) Activity retention rate test of immobilized lipase after multiple batches: The immobilized lipase (mixture with composite adsorbent) collected by filtration in step 3 was directly put into the next batch of reaction without any regeneration treatment, and this process was repeated for 5 batches.

[0082] Then, the initial activity of the fresh enzyme and the residual activity of the recovered enzyme after the 5th batch reaction were measured. The activity was measured using the standard olive oil emulsification method, with the initial fresh enzyme activity as 100%, and the average activity retention rate after the 5th batch was calculated and reported as a percentage.

[0083] 4) Regeneration performance test of composite adsorbent: The composite adsorbent (a mixture of the composite adsorbent and the immobilized lipase, which can be roughly separated by sieving) obtained by filtration in step 3 is washed 3 times with anhydrous ethanol. After each washing, the ethanol washing liquid is removed by vacuum filtration. Finally, the filter cake is placed in a 105℃ forced-air drying oven and dried to constant weight to complete the regeneration.

[0084] The regenerated composite adsorbent was then reused in the same reaction as in the examples or comparative examples, and five regeneration and reuse experiments were conducted consecutively. The sulfur content of the biodiesel product obtained after the fifth regeneration cycle was examined to evaluate the regeneration performance and long-term stability of the adsorbent, in ppm.

[0085] Example

[0086] Example 1

[0087] An enzyme-catalyzed simultaneous desulfurization process for producing biodiesel from waste cooking oil uses mixed waste cooking oil collected from multiple catering enterprises in the Yangtze River Delta region as raw material. The initial total sulfur content is 186 ppm, acid value is 12 mgKOH / g, and water and impurity content is 0.8%. The specific process steps are as follows:

[0088] A. First, heat the waste cooking oil to 60°C, then filter it through a plate and frame filter press pre-coated with diatomaceous earth to remove insoluble impurities such as solid food residue and mud. Then, transfer the filtered oil to a vacuum dehydration tank, heat it to 105°C, and dehydrate it under vacuum of -0.095 MPa for 30 minutes to remove dissolved and emulsified trace amounts of water. After dehydration, the pretreatment is completed, and the oil is cooled to 45°C for later use.

[0089] B1. First, add 200 kg of kitchen waste oil that has been pretreated in step A above, 8 kg of CTS-M@Zn / Fe composite adsorbent prepared in Preparation Example 1, and 16 kg of immobilized lipase Novozym 435 to a 500 L enamel reactor in sequence.

[0090] B2. Next, set the stirring speed to 200 rpm, and control the temperature inside the reactor at 45℃ using a jacketed water bath, with a temperature fluctuation range of ±2℃;

[0091] The total amount of methanol to be fed is 30 L, and it will be added in stages:

[0092] The first batch of methanol, 16 L, was added at the start of the reaction. The second batch of methanol, 7 L, was added at the 6th hour of the reaction. The third batch of methanol, 7 L, was added at the 12th hour of the reaction.

[0093] B3. The reaction time is calculated from the time the first batch of methanol is added. The transesterification reaction is carried out continuously for 24 hours. Stirring and constant temperature are maintained throughout the reaction to simultaneously complete deep desulfurization.

[0094] C1. After the reaction is completed, all the materials in the reactor are filtered under reduced pressure through a closed pressure filter. The filter medium is an industrial filter cloth with a pore size of 10 μm. The filter cake is a mixture of immobilized lipase and composite adsorbent, which is collected for later recycling and regeneration.

[0095] C2. The filtrate is transferred to a jacketed separatory tank and left to stand for 60 minutes under heat preservation. The liquid will naturally separate into two layers. The lower dark glycerol phase is discharged from the valve at the bottom of the separatory tank. The upper light methyl ester layer (i.e., crude biodiesel) is washed once with an equal volume of deionized water to remove residual trace amounts of glycerol and soap. Then, it is dried and dehydrated under vacuum to obtain the ultra-low sulfur crude biodiesel product.

[0096] Comparative Example 1

[0097] An enzyme-catalyzed simultaneous desulfurization process for producing biodiesel from kitchen waste oil differs from Example 1 in that, in step B, 200 kg of pretreated kitchen waste oil, 16 kg of immobilized lipase Novozym 435, and 30 L of methanol are added to the reactor. All other steps and parameters are the same as in Example 1.

[0098] Comparative Example 2

[0099] An enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel differs from Example 1 in that the CTS-M@Zn / Fe composite adsorbent in step B is replaced with an equal amount of cross-linked chitosan microsphere adsorbent loaded with only a single metal oxide, as detailed below:

[0100] Comparative Example 2A: Cross-linked chitosan microspheres loaded only with zinc oxide, namely CTS-M@Zn, were used. The zinc oxide loading was consistent with that in Preparation Example 1, which was 8.5 wt%. Other steps and parameters were the same as in Example 1.

[0101] Comparative Example 2B: Cross-linked chitosan microspheres loaded only with iron oxide, namely CTS-M@Fe, were used. The iron oxide loading was consistent with that in Preparation Example 1, which was approximately 5.2 wt%. All other steps and parameters were the same as in Example 1.

[0102] Comparative Example 3

[0103] An enzyme-catalyzed simultaneous desulfurization process for biodiesel production from kitchen waste oil differs from Example 1 in that the CTS-M@Zn / Fe composite adsorbent used in step B is replaced with an adsorbent of equal amounts of activated carbon loaded with zinc oxide and iron oxide.

[0104] The adsorbent is prepared by taking commercially available coconut shell activated carbon particles with a particle size range similar to that of CTS-M obtained in Preparation Example 1, and loading them according to the same amount of metal salt, impregnation process, alkaline precipitation conditions and drying temperature as in Preparation Example 1, so that the loading of zinc oxide and iron oxide is as close as possible to the level of Preparation Example 1.

[0105] Comparative Example 4

[0106] An enzyme-catalyzed simultaneous desulfurization process for biodiesel production from kitchen waste oil differs from Example 1 in that the methanol addition method in step B is changed from adding it in three batches to adding it all at once at the start of the reaction. All other steps and process parameters are exactly the same as in Example 1.

[0107] Comparative Example 5

[0108] An enzyme-catalyzed simultaneous desulfurization process for biodiesel production from kitchen waste oil differs from Example 1 in that the CTS-M@Zn / Fe composite adsorbent in step B is replaced in equal amounts with the Sm2O3 / ZrO2 solid acid catalyst prepared in Example 1 of CN121160404A, while other steps and parameters remain the same.

[0109] Biodiesel products obtained in Example 1 and Comparative Examples 1-5, as well as relevant indicators in the process, were selected as test objects. The total sulfur content of the products, the yield of fatty acid methyl esters, and the activity retention rate of immobilized lipase after multiple batches of use were then tested. The test results are recorded in the table below.

[0110] Table: Performance test results of Example 1 and Comparative Examples 1-5

[0111] Test Project Group Sulfur content of the product (ppm) Fatty acid methyl ester yield (%) Lipase activity retention rate (%) in 5 batches Sulfur content (ppm) of the product after the 5th regeneration Example 1 2.1 96.5 92 2.8 Comparative Example 1 168 96.8 94 / Comparative Example 2A 12 96.2 92 19 Comparative Example 2B 45 96.0 78 68 Comparative Example 3 18 95.2 78 25 Comparative Example 4 2.3 89.1 56 3.1 Comparative Example 5 7.5 91.5 42 24.8

[0112] As can be seen from the data in the table above, the proposed method can effectively reduce the total sulfur content of biodiesel products made from kitchen waste oil to below 3.0 ppm, with only 2.1 ppm, while the fatty acid methyl ester yield is as high as 96.5%, indicating that the transesterification reaction is carried out relatively completely.

[0113] Regarding the lifespan of the immobilized lipase, the enzyme in Example 1 maintained an activity retention rate of 92% after five consecutive batches of use, demonstrating excellent stability in use.

[0114] After five regenerations and reuses, the composite adsorbent could still control the sulfur content of the product at 2.8 ppm. This demonstrates that the chitosan-based composite adsorbent prepared in this application has better regeneration recovery performance and long-term desulfurization stability compared to comparative examples 1-5. Specific analysis is as follows:

[0115] By comparing with Comparative Example 1 without the addition of composite adsorbent, it can be seen that the core of this application to achieve deep desulfurization relies on composite adsorbent. Under the same reaction conditions, the sulfur content of its product is as high as 168 ppm, which is only slightly lower than the initial sulfur content of 186 ppm of the feed oil. The reason for this is that trace amounts of sulfides are carried away during the separation of the glycerol phase.

[0116] Comparison of Adsorbents 2A and 2B loaded with a single metal oxide shows that neither met the ultra-low sulfur standard. Comparative Example 2A used CTS-M@Zn, and the sulfur content of the product only decreased to 12 ppm; Comparative Example 2B used CTS-M@Fe, and the sulfur content of the product only decreased to 45 ppm.

[0117] It is evident that there is a significant synergistic effect between the two active components, zinc oxide and iron oxide, in this application. The reason for this can be inferred to be that zinc oxide provides strong coordination adsorption for thiols, and the hydroxyl radicals induced by iron oxide oxidize inert sulfides into polar sulfoxides or sulfones, which are then captured and fixed by the amino groups on the chitosan carrier. The entire mechanism is indispensable.

[0118] By comparing with Comparative Example 3, which replaced the carrier with activated carbon, it can be seen that after the carrier was replaced, even if the same metal oxide was loaded, the desulfurization effect was significantly reduced, and the sulfur content of the product could only be reduced to 18 ppm, which was far worse than 2.1 ppm in Example 1.

[0119] More importantly, its enzyme activity retention rate was only 78%, significantly lower than 92% in Example 1. This shows that chitosan is not a simple choice as a carrier material. The abundant amino and other functional groups on its molecular chain not only have the effect of pre-enrichment and chemical adsorption of organic sulfur compounds, but more importantly, it exhibits high biocompatibility with lipase proteins. This is the decisive factor that enables this scheme to achieve high desulfurization efficiency and high enzyme activity retention in a one-pot system.

[0120] By comparing Comparative Example 4, which changed the way methanol was added, it can be seen that in the one-pot process of this application, the stepwise addition of methanol is a necessary process feature to ensure the successful implementation of the entire technical solution. In Comparative Example 4, where methanol was added directly, the yield of fatty acid methyl esters dropped sharply from 96.5% in Example 1 to 89.1%, and the enzyme activity retention rate plummeted from 92% to 56%.

[0121] The reason for this can be inferred to be that high concentrations of methanol can inhibit lipase. This application overcomes this problem effectively by using a specific stepwise alcohol addition operation in conjunction with a biocompatible chitosan-based adsorbent, thus ensuring the efficient reaction and long-term stability of the enzyme.

[0122] By comparing Comparative Example 5, which uses Sm2O3 / ZrO2 solid acid catalyst to replace the composite adsorbent, it can be seen that the Sm2O3 / ZrO2 solid acid catalyst is only suitable for stepwise processes. When it is directly applied to the simplified "one-pot" reaction system advocated in this application, its performance deteriorates sharply.

[0123] The product had a sulfur content as high as 7.5 ppm, which could no longer meet the ultra-low sulfur standard of less than or equal to 3 ppm; the lipase activity retention rate was only 42% in 5 batches, far lower than its performance in the stepwise method, and also far lower than the 92% in Example 1 of this application; the sulfur content of the adsorbent after 5 regenerations was as high as 24.8 ppm, and the regeneration performance was also poor. The reasons for this are as follows:

[0124] The design logic of existing high-performance catalysts is for independent pretreatment steps and does not take into account the requirement of coexistence with biological enzymes. Therefore, they are not compatible with one-pot process in principle. However, this application breaks through the technical bottleneck of mutual interference when existing catalysts coexist with enzymes by redesigning the carrier material and loading mode of the adsorbent at the molecular level. For the first time, it makes it possible to achieve ultra-deep desulfurization and maintain high enzyme activity in a simplified "one-pot" process.

[0125] It should also be noted that, due to limitations in testing costs, patent priority time limits, and the general understanding of parameter response patterns among those skilled in the art, this specification only uses Example 1 as a representative example for detailed experimental verification, and does not conduct repeatable tests on other parameter adjustment schemes one by one.

[0126] However, based on the fully disclosed composite adsorbent preparation mechanism, bimetallic synergistic desulfurization reaction principle, and response law of stepwise alcohol addition process to various parameters in this specification, those skilled in the art can reasonably expect without objection that excellent results can be obtained within the preferred parameter range.

[0127] The above are all modifications that can be made to this embodiment without contributing any inventive step, or solutions that clearly constitute technical teaching, after reading this specification. However, as long as they are within the scope of the claims of this application, they should be protected by patent law.

Claims

1. An enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel, characterized in that, Includes the following steps: A. First, pre-treat the waste cooking oil; B. The pretreated kitchen waste oil, immobilized lipase, composite adsorbent and methanol are put into the same reactor and carried out a one-pot transesterification reaction under stirring and constant temperature conditions. Simultaneous desulfurization occurs during the reaction process; The composite adsorbent is a cross-linked chitosan porous microsphere loaded with zinc oxide and iron oxide; The methanol is added in steps, that is, 40% to 60% of the total amount of methanol is added initially, and the remaining methanol is added in 1 to 3 times during the reaction process. C. After the reaction is complete, the immobilized lipase and composite adsorbent are separated and recovered, and the mixture is allowed to stand and separate to obtain the crude ultra-low sulfur biodiesel product.

2. The enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel according to claim 1, characterized in that, The composite adsorbent is prepared by the following method: Step a: First, chitosan is made into spheres by reverse suspension method, and then cross-linked with epichlorohydrin to obtain cross-linked chitosan porous microspheres. Step b: Immerse the cross-linked chitosan porous microspheres obtained in step a in an aqueous solution containing zinc salt and iron salt, so that the cross-linked chitosan porous microspheres swell and adsorb zinc ions and iron ions; Step c: Under alkaline conditions, an in-situ precipitation reaction is carried out to uniformly deposit zinc hydroxide and iron hydroxide inside and on the surface of the cross-linked chitosan porous microspheres. Step d: By drying and dehydrating at low temperature, zinc hydroxide and iron hydroxide are converted into zinc oxide nanoparticles and iron oxide nanoparticles, respectively, thus obtaining the composite adsorbent.

3. The enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel according to claim 2, characterized in that, In step b, the zinc salt is zinc sulfate heptahydrate and the iron salt is ferric chloride hexahydrate.

4. The enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel according to claim 2, characterized in that, The particle size of the composite adsorbent is 75-187.5 μm; The loading of zinc oxide is 6-12 wt%, and the loading of iron oxide is 3-7 wt%.

5. The enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel according to claim 2, characterized in that, The mass ratio of the pretreated kitchen waste oil, immobilized lipase, composite adsorbent and methanol is 100:(6-10):(3-5):(10-18).

6. The enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel according to claim 2, characterized in that, The conditions for the one-pot transesterification reaction in B are: reaction temperature 40-50℃, stirring rate 150-250 rpm, and total reaction time 18-30 h.

7. The enzyme-catalyzed simultaneous desulfurization process for converting waste cooking oil into biodiesel according to claim 2, characterized in that, The immobilized lipase is Novozym 435, its carrier is acrylic resin, and the enzyme protein is Candida antarcticis lipase B.

8. The composite adsorbent prepared by the method according to any one of claims 1-7, characterized in that, The composite adsorbent is a cross-linked chitosan porous microsphere loaded with zinc oxide and iron oxide; The particle size of the composite adsorbent is 75-187.5 μm; The cross-linked chitosan porous microspheres are made by cross-linking chitosan with epichlorohydrin through reverse-phase suspension. The loading of zinc oxide is 6-12 wt%, and the loading of iron oxide is 3-7 wt%.

9. The application of the composite adsorbent as described in claim 8 in the field of oil desulfurization.

10. A simultaneous desulfurization reaction system, characterized in that, It comprises immobilized lipase and the composite adsorbent as described in claim 8.