Method for preparing waste oil-based vegetable insulating oil through manganese catalytic oxidation decoloration

Abstract

The invention discloses a method for preparing waste grease-based vegetable insulating oil through manganese catalytic oxidation decolorization, which comprises the following steps: by taking a complex formed by manganese salt and 2, 2 '-bipyridine derivative as a catalyst and taking a urea hydrogen peroxide compound as an oxidant, carrying out selective catalytic oxidation on pigment molecules in waste grease under mild conditions, thereby obtaining the waste grease-based vegetable insulating oil. The decolorization rate can reach 85%-92%; a small amount of activated clay is subsequently used for adsorbing and removing the residual metal catalyst and trace oxidation byproducts, and the high-quality vegetable insulating oil with various physicochemical indexes meeting the requirements can be obtained. The use amount of the urea hydrogen peroxide compound used in the method is small, compared with direct use of high-concentration hydrogen peroxide, the stability is higher, the oxidability is milder and more controllable, and the problems of grease emulsification, acid value increase, non-selective oxidation and the like caused by excessive hydrogen peroxide are effectively avoided. The method is mild in reaction condition, high in selectivity, low in catalyst and auxiliary material consumption and suitable for industrial production of the high-quality vegetable insulating oil.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of insulating oil preparation, and particularly relates to a method for preparing waste oil and fat-based plant insulating oil through manganese catalytic oxidation decolorization. BACKGROUND

[0002] Plant insulating oil, as an environmentally friendly and renewable liquid dielectric, is gradually replacing traditional mineral insulating oil and is widely used in high-voltage electrical equipment such as transformers and capacitors. Compared with mineral insulating oil, plant insulating oil has significant advantages such as biodegradability, high flash point, excellent thermal stability and environmental performance. However, the commercial application of plant insulating oil faces many technical challenges, one of which is the refining process of the raw material, especially the optimization of the decolorization process.

[0003] Waste oil and fat (including catering waste oil and plant oil refining by-products) can be used as raw material for preparing plant insulating oil due to its low cost and wide source. The use of waste oil and fat to prepare plant insulating oil not only realizes resource recycling, but also prevents waste oil and fat from re-entering the edible oil market from the source, which has significant economic and social benefits. However, such raw materials usually contain a large amount of complex pigment components, including conjugated polyene compounds (such as beta-carotene), metal porphyrin compounds (such as chlorophyll degradation products) and Maillard reaction products. These pigment molecules have a highly conjugated rigid structure and strong absorption in the visible light region, resulting in deep yellow to brown color of the oil and fat. This poses a high requirement for the decolorization process.

[0004] Traditional decolorization methods include physical adsorption and chemical oxidation. Physical adsorption usually uses adsorbents such as activated clay, diatomite or activated carbon. For example, Deng Runkang et al. (Guangdong Chemical Industry, 2015) used column chromatography silica gel adsorption, and the decolorization rate was only 73% to 83%; Wang Peng et al. (Grain and Oil, 2018) used activated clay at 120 ℃ for decolorization, and the best decolorization rate was 95.33%, but high temperature and large amount of adsorbent (10%) were required, which easily led to an increase in oil and fat acid value. Chemical oxidation can use hydrogen peroxide (Zhang Hai-xia et al., Journal of Beijing University of Chemical Technology, 2009), peroxy acid and other oxidizing agents. Xie Yu-qin et al. (Journal of Yulin University, 2019) used hydrogen peroxide-pulverized coal ash combined decolorization, and the decolorization rate was 80%, but the reaction temperature needed to be above 60 ℃, and excessive hydrogen peroxide easily caused oil and fat emulsification and acid value increase. It can be seen that traditional decolorization methods have obvious limitations: (1) poor selectivity of conjugated pigments in adsorption, incomplete decolorization; (2) harsh conditions in chemical oxidation, which easily destroys the structure of triglycerides and affects the insulating properties; (3) high energy consumption and easy occurrence of oil and fat side reactions due to high temperature. UV absorption analysis shows that the oil and fat treated by traditional methods still has obvious absorption peaks in the 650-700 nm region, indicating that the conjugated chromophore has not been completely removed.

[0005] Oxidative bleaching, as an emerging technology, can achieve more thorough bleaching effect by selectively destroying the conjugated structure of the chromophore. However, this technology faces many challenges in the implementation process: First, it is necessary to ensure the selectivity of the oxidation process, that is, only the conjugated structure of the pigment molecule is destroyed, and the triglyceride skeleton is not damaged, otherwise it will lead to excessive acid value (> 2.0 mg KOH / g) or reduced dielectric strength (< 40 kV); Second, the oxidation products must be converted into compounds with smaller molecular weight (MW < 200 Da) and stronger polarity (logP < 3), otherwise colloidal substances will block the filtration system; In addition, the metal catalyst needs to be strictly controlled to ensure that it can be effectively removed by the subsequent adsorbent.

[0006] To overcome the above-mentioned defects, improved decolorization methods have appeared in the prior art. For example, CN115197779A discloses a composite decolorization method for preparing electric power insulating oil from dark kitchen waste oil, which adopts a composite process of "preheating and initial addition of adsorbent --- vacuumizing and turning on UV light source radiation, while heating to high temperature and secondary addition of adsorbent --- cooling and filtering", which effectively destroys the pigment group by combining the effects of heat, UV radiation and two-step adsorption, significantly improving the decolorization efficiency of dark oil. However, this composite decolorization method involves multiple heating and cooling (for example, from 90 ℃ to 150 ℃ and then to 70 ℃), which consumes a lot of energy and increases the operating cost of large-scale production. At the same time, the ultraviolet irradiation system is complex, and the potential for industrial application is small. CN112725079A discloses a decolorization process for producing kitchen waste oil-based natural ester insulating oil, which first heats the kitchen waste oil to 95-100 ℃, then adds a decolorizing agent (1.0%-1.5% activated carbon and 3.5%-5.0% activated white clay), and filters after decolorizing for 2-3 hours under vacuum; then the preliminarily treated oil is heated to 60-80 ℃, 9%-10% hydrogen peroxide is added, and after reacting for 30-60 minutes, it is left to separate. This process can effectively remove stubborn pigments through the synergistic effect of adsorption and oxidation, but still has obvious defects: (1) the reaction temperature is high (all above 60 ℃, and even 95-100 ℃ for adsorption decolorization), which consumes a lot of energy and is prone to oil oxidation side reactions at high temperature; (2) the chemical oxidation step uses high-concentration hydrogen peroxide and a large amount (up to 20% of the mass of the oil), which poses a safety risk and is prone to cause oil acid value increase and emulsification; (3) the total amount of adsorbent is high (4.5%-6.5%), which increases the operating cost and solid waste treatment burden. In addition, Ling Xinlong et al. published "Research on Decolorization Process of Hogwash Oil" in China Oil and Fat (Vol. 33, No. 9, 2008), which discusses the hydrogen peroxide-activated white clay combined decolorization method. The study determines the optimal process conditions as follows: first add 3.5% hydrogen peroxide at 60 ℃ for 20 minutes, then add 5% activated white clay and stir at 60 ℃ for 25 minutes. The study points out that hydrogen peroxide can oxidize and degrade the structure of pigment molecules, while activated white clay mainly plays a physical adsorption role, and the combination of the two can improve the decolorization rate. However, this process also has problems such as high reaction temperature (60 ℃), incomplete decolorization, and quality deterioration of the oil caused by high temperature and excessive oxidizing agent. The study also found that the color of the decolored oil may continue to lighten after storage, indicating that the oxidation reaction may not be precisely controlled. Therefore, the existing technology still cannot economically, efficiently and safely decolorize dark kitchen waste oil to fully meet the standards of high-end electric power insulating oil under the premise of lower energy consumption, shorter working hours and simpler process.Developing a new type of decolorization process that achieves a better balance between decolorization efficiency, product performance, operating cost and process simplicity has become a key technical problem to be solved in the field. SUMMARY

[0007] The present application aims at the problems of high temperature, poor selectivity and low efficiency of existing decolorization methods, and provides a method for preparing waste oil-based plant insulating oil by manganese catalytic oxidation decolorization, which realizes selective oxidation of pigment molecules in waste oil at room temperature and guarantees the physical and chemical and electrical properties of insulating oil.

[0008] The method for preparing waste oil-based plant insulating oil by manganese catalytic oxidation decolorization provided by the present application comprises the following steps:

[0009] Step 1: Catalyst preparation

[0010] The manganese salt and the 2,2'-dipyridyl derivative are dissolved in an organic solvent at a molar ratio of 1:1.5-2.5, and reacted at 70-80 DEG C for 8-12 hours to prepare a manganese-dipyridyl complex.

[0011] Step 2: Waste oil pretreatment

[0012] The waste oil is subjected to alkali refining deacidification and dehydration treatment to make the acid value ≤1.0 mgKOH / g and the moisture content ≤0.5%.

[0013] Step 3: Catalytic oxidation decolorization

[0014] The manganese-dipyridyl complex of step 1 is added to the oil pretreated in step 2, and a urea hydrogen peroxide compound is added, and stirred at 20-40 DEG C for 2-4 hours.

[0015] Step 4: Adsorption refining

[0016] The activated clay is added to the oil after oxidation decolorization in step 3 at a mass fraction of 0.5%-1%, and stirred for 0.5-1.5 hours and then filtered.

[0017] Step 5: Vacuum dehydration

[0018] The oil after filtration in step 4 is dehydrated at a vacuum degree of -0.085 to -0.095 MPa and a temperature of 90-110 DEG C for 1-3 hours to make the moisture content ≤200 ppm, and a plant insulating oil is obtained.

[0019] Further, in the step 1, the manganese salt is selected from any one of manganese perchlorate hexahydrate, manganese acetate tetrahydrate, manganese sulfate monohydrate; the 2,2'-dipyridyl derivative is selected from any one of 4,4'-diamino-2,2'-dipyridyl, 4,4'-dimethyl-2,2'-dipyridyl, 4,4'-dicarboxy-2,2'-dipyridyl; and the organic solvent is selected from any one of acetonitrile, ethanol, acetone.

[0020] Further, in the step 3, preferably, the manganese-dipyridyl complex is added in an amount of 0.05% to 0.2% of the mass of the pretreated oil, and preferably, the urea-hydrogen peroxide compound is added in an amount of 0.5% to 2.0% of the mass of the pretreated oil.

[0021] In the step 3, the manganese-dipyridyl complex is used as a catalyst, and the urea-hydrogen peroxide compound is used as an oxidant, and selective oxygen active species are generated by the manganese-dipyridyl complex catalyzing H2O2, and conjugated chromophores are preferentially oxidized, such as nonpolar electron-rich olefins (such as the conjugated double bond in beta-carotene), heterocyclic compounds (such as the pyrrole ring of chlorophyll derivatives), and polycyclic aromatic chromophores (such as benzo[a]pyrene substances), and the reaction mechanism is as shown below:

[0022] [Mn II (L)] + H2O2 → [Mn IV =O(L)] + H2O

[0023] [Mn IV =O] + C=C→ [Mn II ] + C*-C* + -O -

[0024] Mn IV =O selectively attacks the double bond or conjugated system with high electron cloud density through an "oxygen transfer" mechanism, and electron transfer causes the pigment to be converted into an epoxidation product or an alcohol derivative.

[0025] In the step 4, the main purpose of adding activated clay is to adsorb and remove residual trace amounts of manganese catalyst and a small amount of possible polar oxidation byproducts in the reaction system, and not for adsorbing pigments (the pigments have been catalytically oxidized and decomposed in step 3). The amount of activated clay used in this step is much lower than the amount of activated clay required for adsorbing pigments in the traditional adsorption decolorization process, which is 3% to 10%.

[0026] In the step 5, the obtained plant insulating oil has an acid value ≤ 0.06 mgKOH / g, a dielectric loss factor ≤ 0.04, and a breakdown voltage ≥ 40 kV.

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

[0028] 1. The decolorization efficiency is significantly improved: UV absorption spectrum evaluation shows that the absorbance of the oil and fat after catalytic oxidation decolorization at the characteristic absorption peak of 650-700 nm is reduced by 85%-92%, and the decolorization rate is significantly higher than that of traditional adsorption method (40%-60%) and chemical oxidation method (65%-75%). This decolorization rate index clearly represents the effect of the catalytic oxidation step.

[0029] 2. Mild reaction conditions: High-efficiency decolorization can be achieved under mild conditions (20-40°C), avoiding oil and fat side reactions caused by high temperature, and reducing energy consumption.

[0030] 3. Good selectivity: The Mn-terpyridine complex catalyzed H2O2 generates Mn(IV)=O active species that specifically attacks the conjugated structure of the chromophore, achieving selective oxidation of the conjugated double bond. UV spectrum shows that the absorption peak at 650-700 nm after catalytic oxidation decolorization basically disappears, while no new absorption peak is added at 250-300 nm, indicating that the structure of triglyceride is well preserved.

[0031] 4. Low catalyst and auxiliary material consumption: The catalyst consumption is only 0.05%-0.2% of the mass of the oil and fat. The active clay used in subsequent refining has a very low dosage (0.5%-1%), which is far lower than the dosage of active clay used in traditional adsorption decolorization process (3%-10%) for adsorbing pigments, significantly reducing the treatment cost and solid waste generation.

[0032] 5. Superior oxidant system: Urea hydrogen peroxide complex is used as the oxidant, and its dosage (0.5%-2%) is much lower than that of high-concentration hydrogen peroxide. It is added in solid form, which is safe and convenient to operate, and slowly releases H2O2 in water, providing a mild and controllable oxidation environment, effectively avoiding problems such as oil emulsification, rapid increase of acid value, and non-selective oxidation caused by direct use of high-concentration hydrogen peroxide, improving the selectivity of the reaction and the quality of the product, and providing a new technical path for solving the decolorization problem of waste oil and fat in the preparation of plant insulating oil. DETAILED DESCRIPTION

[0033] The application will be further described in conjunction with the examples, but the scope of protection of the application is not limited to these examples.

[0034] Example 1

[0035] Step 1: Catalyst preparation

[0036] Manganese perchlorate hexahydrate 181 mg (0.5 mmol) and 4,4'-diamino-2,2'-bipyridine 186.2 mg (1.0 mmol) were dissolved in 20 mL acetonitrile, and the reaction was stirred at 75 °C for 12 hours. After the reaction was completed, the solvent was removed by concentration under reduced pressure to obtain 367 mg of manganese-bipyridine complex A as a brown solid, with a yield of 95%.

[0037] Step 2: Pretreatment of waste oil

[0038] The waste cooking oil was subjected to alkaline refining, deacidification and dehydration to obtain pretreated waste cooking oil with an acid value of 0.8 mgKOH / g, an absorbance of 1.85 at 650-700 nm, and a water content of ≤0.5%.

[0039] Step 3: Catalytic oxidation decolorization

[0040] To 100 g of pretreated waste cooking oil, 100 mg of manganese-bipyridine complex A and 1.26 g of urea hydrogen peroxide compound were added, and the reaction was stirred at 25 °C at a speed of 300 rpm for 3 hours.

[0041] Step 4: Adsorption refining

[0042] To the oil after decolorization in step 3, 0.8% of activated clay by mass was added, and after stirring for 1 hour, it was filtered.

[0043] Step 5: Vacuum dehydration

[0044] The oil after filtration in step 4 was dehydrated under a vacuum of -0.090 MPa and a temperature of 100 °C for 2 hours to obtain vegetable insulating oil with a water content of ≤200 ppm.

[0045] Example 2

[0046] In step 1 of this example, 122 mg (0.5 mmol) of manganese acetate tetrahydrate and 184 mg (1.0 mmol) of 4,4'-dimethyl-2,2'-bipyridine were dissolved in 20 mL of acetonitrile, and the reaction was stirred at 75 °C for 12 hours. After the reaction was completed, the solvent was removed by concentration under reduced pressure to obtain 298 mg of manganese-bipyridine complex B as a light green solid, with a yield of 93%. In step 3, manganese-bipyridine complex B was used instead of manganese-bipyridine complex A in the same amount, and the other steps were the same as in Example 1.

[0047] Example 3

[0048] In step 1 of the embodiment, 84.5 mg (0.5 mmol) of manganese sulfate monohydrate and 244.2 mg (1.0 mmol) of 4,4'-dicarboxy-2,2'-bipyridine were dissolved in 20 mL of ethanol, and the reaction was stirred at 75°C under reflux for 8 hours. After the reaction was completed, a green precipitate was obtained by filtration, and 298 mg of a light green solid manganese-bipyridine complex C was obtained after vacuum drying, with a yield of 91%. In step 3, the same amount of manganese-bipyridine complex C was used to replace the manganese-bipyridine complex A in Example 1, and the other steps were the same as in Example 1.

[0049] The oil after catalytic oxidative decolorization in step 3 of Examples 1-3 above was tested for decolorization rate and acid value change. The decolorization rate test method was as follows: the oil sample was diluted 10 times with n-hexane, and ultraviolet-visible spectrum scanning was performed in the range of 200-700 nm in a 1 cm quartz cuvette. The decolorization rate calculation formula was: decolorization rate (%) = (A0-A) / A0 x 100%, where A0 is the absorbance of the oil before catalytic oxidative decolorization at a wavelength of 650-700 nm, and A is the absorbance of the oil after catalytic oxidative decolorization at the same wavelength. The experimental results are shown in Table 1.

[0050] Table 1

[0051] As can be seen from Table 1, the catalytic oxidative decolorization method provided by the present application can achieve efficient and selective decolorization of waste oil at room temperature, with a decolorization rate of more than 88% in the catalytic oxidation step, and can well maintain the quality of the oil, with a small change in acid value, and is particularly suitable for the preparation of plant insulating oil.

[0052] The plant insulating oil obtained in step 5 of Example 1 above was further tested for physicochemical properties, and compared with a sample treated by a traditional adsorption method and the requirements of DL / T 1811-2018 standard, and the results are shown in Table 2.

[0053] Table 2

[0054] Note: In the table, the traditional adsorption method refers to a decolorization method using 5% activated carbon under vacuum at 90°C for 1 hour; all tests were performed under the same laboratory conditions, and each sample was tested 3 times to obtain the average value.

[0055] The test results show that the physicochemical properties of the plant insulating oil prepared by the catalytic oxidation decolorization method are all better than those of the sample treated by the traditional adsorption method, and all the test data meet the standard requirements of DL / T 1811-2018 "Plant Insulating Oil". The acid value (0.05 mgKOH / g) of the product of the application is significantly lower than the standard requirement (≤0.06 mgKOH / g), the dielectric loss factor (0.012) is lower than the standard requirement (≤0.04), and the breakdown voltage (72.5 kV) is higher than the standard requirement (≥40 kV), indicating that the method of the application can effectively prepare high-quality plant insulating oil.

[0056] In step 3 of the above example 1, the reaction was stirred at different temperatures at a speed of 300 rpm for 3 hours, and other steps were the same as those of example 1. The influence of reaction temperature on the decolorization effect was investigated, and the results are shown in table 3.

[0057] Table 3

[0058] As shown in table 3, the decolorization rate is poor when the temperature is too low (15 ℃), and although the decolorization rate can be slightly improved when the temperature is increased to 60 ℃, it will lead to obvious non-selective oxidation (new absorption peaks appear at 250-300 nm) and acid value increase. Compared with the traditional method, the catalytic oxidation decolorization method of the application has obvious advantages, and the decolorization rate of 91.9% can be reached at room temperature, and the acid value change is the smallest, indicating that the influence on the structure of triglyceride is the smallest.

Claims

1. A method for preparing waste oil-based vegetable insulating oil by manganese catalytic oxidation decolorization, characterized in that, Includes the following steps: Step 1: Catalyst Preparation Manganese salt and 2,2′-bipyridine derivative were dissolved in an organic solvent at a molar ratio of 1:1.5-2.5 and reacted at 70-80 °C for 8-12 hours to prepare manganese-bipyridine complexes. Step 2: Waste oil pretreatment Waste oils are subjected to alkali refining to remove acid and water, resulting in an acid value ≤1.0 mgKOH / g and a moisture content ≤0.5%. Step 3: Catalytic oxidation decolorization Add the manganese-bipyridine complex from step 1 to the oil pretreated in step 2, and add the urea-hydrogen peroxide complex. Stir and react at 20-40 °C for 2-4 hours. Step 4: Adsorption purification Add 0.5% to 1% of the mass of activated clay to the decolorized oil from step 3, stir for 0.5 to 1.5 hours, and then filter. Step 5: Vacuum dehydration The oil filtered in step 4 is dehydrated for 1 to 3 hours under a vacuum of -0.085 to -0.095 MPa and a temperature of 90 to 110 ℃ until the moisture content is ≤200 ppm, thus obtaining vegetable insulating oil.

2. The method for preparing waste oil-based vegetable insulating oil by manganese catalytic oxidation decolorization according to claim 1, characterized in that: In step 1, the manganese salt is selected from any one of manganese perchlorate hexahydrate, manganese acetate tetrahydrate, and manganese sulfate monohydrate; the 2,2′-bipyridine derivative is selected from any one of 4,4′-diamino-2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, and 4,4′-dicarboxy-2,2′-bipyridine.

3. The method for preparing waste oil-based vegetable insulating oil by manganese catalytic oxidation decolorization according to claim 1, characterized in that: In step 1, the organic solvent is selected from any one of acetonitrile, ethanol, and acetone.

4. The method for preparing waste oil-based vegetable insulating oil by manganese catalytic oxidation decolorization according to claim 1, characterized in that: In step 3, the amount of manganese-bipyridine complex added is 0.05% to 0.2% of the mass of the pretreated oil.

5. The method for preparing waste oil-based vegetable insulating oil by manganese catalytic oxidation decolorization according to claim 1, characterized in that: In step 3, the amount of urea-hydrogen peroxide complex added is 0.5% to 2.0% of the mass of the pretreated oil.

6. The method for preparing waste oil-based vegetable insulating oil by manganese catalytic oxidation decolorization according to claim 1, characterized in that: In step 5, the obtained vegetable insulating oil has an acid value ≤ 0.06 mgKOH / g, a dielectric loss factor ≤ 0.04, and a breakdown voltage ≥ 40 kV.

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