Alkali-Modified Boehmite for Acid Removal in Waste Transformer Oil

doi:10.21203/rs.3.rs-3120960/v1

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Alkali-Modified Boehmite for Acid Removal in Waste Transformer Oil

https://doi.org/10.21203/rs.3.rs-3120960/v1

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Transformer oil plays a critical role in ensuring the insulation and normal operation of power transformers. However, during operation, the transformer oil would undergo oxidation and degradation, leading to the formation of acids and polar compounds that increase its acidity and degrade its performance. Recycling and reclamation of aged transformer oil are necessary to prolong transformer lifespan and mitigate risks. Adsorption separation is a preferred method for regeneration, offering simplicity and high efficiency. This study focuses on impregnated boehmite as an adsorbent for acidity removal in waste transformer oil. The effects of impregnation amount and calcination temperature on acidity removal are investigated, along with a comparative analysis of other adsorbents. The findings contribute to the development of sustainable methods for regenerating transformer oil, ensuring efficient and environmentally friendly operations.

Transformer oil

boehmite

adsorption separation

Transformer oil plays a crucial role in the insulation of power transformers, ensuring their normal operation under both physical and chemical stresses[1]. During the operation of a transformer, transformer oil is exposed to a combination of electrical and thermal stresses, the presence of oxygen, and the components of the coil core. This exposure lead to a series of oxidation and degradation reactions in the transformer oil, resulting in the formation of acids and polar compounds[2]. During this process, the acidity of the oil increases, leading to corrosion of the metallic components inside the transformer. Additionally, the formation of sludge could impede heat transfer between the core/coils and the surfaces of the oil tank/radiators, thereby affecting the thermal dissipation capability of the system[3]. This further exacerbates the deterioration of both the transformer insulating oil and the electrical-grade paper insulation. Even in extreme cases, aged transformer oil can cause irreversible damage to the paper insulation and other components of the transformer[4]. Hence, after aging, transformer oil must be either replaced or subjected to recycling and reclamation processes in order to prolong the lifespan of transformers. This is necessary to ensure the continued efficient operation of transformers and mitigate potential risks associated with the use of aged transformer oil[3, 5]. Currently, the global annual consumption of transformer oil exceeds 160 billion liters. Considering that waste oil could cause severe damage to water sources, soil[6], and air within ecosystems[7], recycling and reclamation of aged transformer oil is a more ideal choice compared to direct replacement. Methods for the reuse of transformer oil include incineration for energy recovery[8], solvent extraction, and vacuum distillation for purification of waste oil. However, these methods have drawback s such as air pollution, difficulties in solvent recovery, and complex operation procedures[9]. Compared to these methods, adsorption separation method is considered as the preferred approach for transformer oil regeneration due to its simplicity in operation and high efficiency in separating contaminants[10, 11].

In adsorption separation technology field, the development of low-cost, high-efficiency, and environmentally friendly adsorbents is a key focus. As reported by Hafez et al.[12], the use of kaolin clay as an adsorbent to remove acidity, water, and other polar contaminants from transformer oil has shown significant improvement in parameters that characterize the aging of transformer oil, including acid content, breakdown voltage, viscosity, and water content. Similarly, Safiddine et al.[13] successfully developed a new adsorbent by combining four different adsorbents: activated carbon, silica gel, alumina, and bentonite. They employed a combination of adsorption separation technology, centrifugation, and dehydration to regenerate waste transformer oil and impart it with properties similar to fresh oil. This approach has effectively extended the lifespan of transformer oil. In addition, there have been studies on the adsorption of impurities in waste transformer oil using molecular sieves[14, 15].

It is commonly recognized that boehmite (AlOOH) is an important precursor for the synthesis of γ- Al₂O₃[16]. After being subjected to high-temperature calcination, boehmite undergoes dehydration and transforms into γ- Al₂O₃, with the removal of structural water[17]. γ- Al₂O₃, known for its high specific surface area and porous structure, finds numerous applications in industrial catalysis. Additionally, due to these characteristics, γ- Al₂O₃ can also be utilized for the adsorption of organic pollutants. While there have been numerous studies on the use of alkali metal carbonates impregnated on Al₂O₃ supports for carbon dioxide capture[18–20], there is limited reporting on the application of such carbon dioxide adsorbents for the regeneration of waste transformer oil.

In this study, we used boehmite as the initial carrier and subjected the impregnated boehmite to calcination after K₂CO₃ impregnation. The focus of this study was to investigate the effect of the amount of K₂CO₃ impregnation and the calcination temperature of the adsorbent on the removal of acidity in waste transformer oil. The effect of calcination temperature on the phase transformation of boehmite and its influence on alkalinity were investigated. In addition, a comparative analysis was conducted to assess the efficacy of the impregnated and calcined boehmite with K₂CO₃ for acidity removal in waste transformer oil, in comparison with impregnated and calcined alumina, activated carbon, and bentonite.

2.1 Materials.

Both benzoic acid (SHANGHAI LINGFENG CHEMICAL REAGENT CO., LTD), potassium hydroxide (ALADDIN REAGENT CO., LTD), potassium carbonate (SHANGHAI LINGFENG CHEMICAL REAGENT CO., LTD) and bromothmol blue (SHANGHAI D&B Biological Science and Technology CO., LTD) are of analytical grade. All chemicals are dried without secondary processing. Bentonite (ANJI GAOCHAO ACTIVE WHITE EARTH CO., LTD.), Al₂O₃(ZIBO HENG HUAN ALUMINIUM CO., LTD), activated carbon and boehmite (consisting of 97.736% boehmite, 0.008% Fe₂O₃, 0.036% Na₂O, 2.1%SiO2, 0.12% SO₄²⁻; CHALCO SHANDONG ADVANCED MATERIAL Co., LTD) are all used as adsorbent carriers.

The waste transformer oil for experiments in the laboratory were obtained from Gaoqiao Petrochemical Company of SINOPEC, Shanghai. The acidity of the waste transformer oil falls within the range of 0.022 mg KOH/g-0.027 mg KOH/g.

2.2 Absorbent Preparation.

The adsorbent loaded with K₂CO₃ was synthesized using the excess impregnation method. A specified quantity of K₂CO₃ (16%, 33%, 50%, 75% of the mass of the carrier, respectively) was dissolved in deionized water to form a solution. The adsorbent carrier was then immersed in the solution and stirred at room temperature for 1 hour. Subsequently, the mixture was subjected to vacuum drying at 110°C for 12 hours in a vacuum drying oven. Following the drying process, the mixture was transferred to a muffle furnace and subjected to calcination at different temperatures (550℃, 650℃, 750℃, 850℃, respectively) for a duration of 3 hours. The resultant material obtained after calcination served as the adsorbent.

2.3 Experimental Setup and Procedure. Prior to conducting the experiment, the acid value of the waste transformer oil raw material was determined using standard methods. A specific amount of the waste transformer oil was weighed and transferred to a beaker, followed by the addition of 1g of the adsorbent and a magnetic stirrer. The mixture was stirred at a speed of 550 rpm for a duration of 12 hours. It is worth noting that, during the screening process to identify the optimal adsorbent, the oil-to-adsorbent ratio employed was 1:30. Similarly, for screening the optimal mass of impregnated potassium carbonate, the oil-to-adsorbent ratio used was 1:150. Likewise, for screening the optimal calcination temperature, the oil-to-adsorbent ratio used was 1:250.

Subsequent to the adsorption process, the adsorbed waste transformer oil was subjected to centrifugation. The acid value of the treated oil sample was then measured again. A comparative analysis of the acid values before and after adsorption was performed to evaluate the efficacy of the adsorbents.

2.4 Characterization of Adsorbents.

2.4.1. X-ray diffraction analysis (XRD)

The composition of crystalline structure of all samples were analyzed by a D8 Advance X-ray diffractometer operating at 40 kV and 30 mA. The target and wavelength were Cu Kα and 1.54 Å, respectively. The XRD measurements were performed from the scanning range of 3^° to 85^° at the rate of 1^°/min and the step size of 0.02^°.

2.4.2. Brunauer-Emmett-Teller(BET) and Barrett-Joyner-Halenda(BJH)

The specific surface area, total pore volume, and average pore sizes of all samples were measured on a JW-ZQ200 material suction and desorption instrument from JWGB (Beijing JWGB Sci &Tech Co., ltd.). Initially all samples were degassed in situ at 150°C for 2 h to remove physically absorbed moisture on the sample. Then the specific surface area and average pore diameter were measured by the injection of nitrogen gas at the temperature of -196℃. Mesopore properties of the sample, such as average pore sizes and pore size distributions, were derived from the desorption branch of the isotherm by adopting the Barren-Joyner-Halenda method.

2.4.3. Fourier Transform Infrared (FTIR)

FTIR spectroscopy analysis was conducted using a Nicolet-6700 Fourier Transform Infrared (FTIR) spectrophotometer to investigate the types of surface chemical bonds present in the adsorbent. The adsorbent was mixed with KBr at a mass ratio of 1:100. The analysis was performed within the range of 4000 − 400 cm^− 1 with a resolution of 4 cm^− 1.

2.4.4. Measurement of Alkalinity of Adsorbents

To determine the alkalinity of the adsorbents, the Hammett titration method was employed. Firstly, 0.1g of the adsorbent was accurately weighed and placed in a beaker. Anhydrous ethanol was then added to the beaker, allowing the adsorbent to soak in the solution. To ensure the uniform distribution of the adsorbent in the solvent, the mixture was subjected to ultrasonication for 30 minutes. Subsequently, 0.5ml of thymolphthalein bromothymol blue indicator was measured and added to the solution. For the titration process, a standard benzoic acid solution (0.02mol/L) was utilized. The benzoic acid solution was carefully titrated into the beaker containing the adsorbent solution until the endpoint was reached. The endpoint of the titration was indicated by a color change in the solution from green to yellow. The volume of benzoic acid consumed during the titration was recorded. This method enabled the quantification of the alkalinity of the adsorbents based on the volume of benzoic acid required for neutralization.

$$Alkalinity=\frac{Normality of benzoic acid*volume benzoic acid}{Weight of Adsorbents}$$

2.4.5. Determination of Acid Value in Transformer Oil

The acid value of transformer oil is determined using a precise titration method. Initially, 10g of the transformer oil sample is carefully measured and placed in a suitable container. To perform the titration, a solution of KOH in ethanol with a known concentration is prepared and filled into a burette. Alizarin Blue 6B, a color indicator, is added to the transformer oil, imparting a red color to the mixture. The titration process begins by slowly adding the KOH solution from the burette into the container, while continuously stirring the mixture. As the KOH solution reacts with the acidic components present in the transformer oil, the color of the mixture gradually changes from red to blue. The addition of the KOH solution is continued until a persistent blue color is observed for a brief duration, indicating the endpoint of the titration has been reached. At this point, the volume of KOH solution consumed during the titration is recorded. By knowing the concentration of KOH in ethanol, the acid value of the transformer oil can be calculated using the recorded volume of KOH solution. This meticulous titration method, employing Alizarin Blue 6B as the color indicator, ensures accurate determination of the acid value of the transformer oil, providing valuable insights into its quality and composition.

$$Acid Value=\frac{Normality of KOH*volume KOH ethanol solution}{Weight of transformer oil}$$

3.1. Physicochemical Characterization of Adsorbents

3.1.1. XRD Analysis

X-ray diffraction (XRD) analysis was performed to explore the crystalline structure changes of boehmite during the calcination process and the successful loading of K₂CO₃ onto boehmite. The results are presented in Fig. 1. By comparing the major diffraction peaks of raw material with the standard XRD pattern of boehmite (JCPDS Card 21-1307), we found that all the sharp and undefined peaks correspond to the pure and non-crystalline phase of boehmite and no impurity peaks were observed. In addition, the broadening reflections of the diffraction peaks are due to the presence of boehmite crystals in the boehmite powder[21]. During the calcination process, it was found that the structure of boehmite remained intact as long as the calcination temperature was below 650℃. However, once the temperature exceeded 650℃, the structure of boehmite started to undergo significant changes and eventually got completely destroyed. This led to the generation of Al₂O₃. Furthermore, after subjecting boehmite to calcination at 650℃ for a duration of 3 hours, a notable transformation occurred. The boehmite was completely converted into γ-Al₂O₃, this conversion was evidenced by the appearance of peaks at 37°, 46°, and 67° in the XRD pattern[22]. The XRD pattern provided clear evidence of the presence of γ-Al₂O₃, indicating the successful transformation of boehmite into this crystalline phase at the given calcination conditions. This observation highlights the temperature-dependent behavior of boehmite during the calcination process and demonstrates the successful conversion of boehmite into γ-Al₂O₃ at a specific temperature and duration, as verified by the XRD pattern analysis[23]. With the further increasing of calcination temperature, the γ-Al₂O₃ would transform into other transitional alumina phases such as δ-Al₂O₃, θ-Al₂O₃ and β-Al₂O₃. Hence, the XRD pattern is not specific to a crystalline alumina phase, but a mixture of different phases of alumina. Only when the calcination temperature reaches above 1000℃, fully crystalline phase of α-Al₂O₃ would be formed[24]. Through XRD analysis of the adsorbents after impregnation and calcination, we found that the addition of K₂CO₃ resulted in changes in the crystal structure of boehmite. As the concentration of K₂CO₃ increased, the crystal grain size of Al₂O₃ in the catalyst decreases, and the characteristic peaks in the XRD gradually disappeared. They disappeared completely when the concentration of added K₂CO₃ reached 30%. However, with further increase in the concentration of K₂CO₃, new characteristic diffraction peaks appeared at 2θ = 32°, which can be attributed to the accumulation of excess K₂CO₃ on the surface of boehmite[25]. This finding further confirm the successful loading of K₂CO₃ onto boehmite and the formation of new active centers in conjunction with the hydroxyl groups on the surface of boehmite.

3.1.2. Surface Area, Pore Volume, and Average Pore Diameter.

The BET surface area, total pore volume, and average pore diameter of the fresh material/modified supporting material are presented in Table 1. Due to the blocking of the boehmite pores during the impregnation process with K₂CO₃, an increase in the impregnation amount of potassium carbonate results in a reduction in the specific surface area and pore volume of the adsorbent. Furthermore, we investigated the influence of calcination temperature on the specific surface area, pore volume, and average pore diameter of the adsorbent while keeping the impregnation amount of potassium carbonate constant (50% K₂CO₃). With the increase in calcination temperature, there is a noticeable reduction in the specific surface area and pore volume of the adsorbent, while the average pore diameter increases. This can be attributed to the changes in the wall crystal structure that form the macrostructure[26]. With the increase in calcination temperature, boehmite undergoes a transformation into γ-alumina. Subsequently, the free water, crystalline water, interlayer water, and other water species are removed, causing the collapse of the pore structure. As a result, small pores are transformed into larger ones. The dehydrated alumina consists of a mixture of η, δ, and θ alumina phases[24]. Figure 2 illustrates the decrease in pore volume and the increase in average pore diameter of the adsorbent with increasing calcination temperature. The pore size distribution (PSD) of the boehmite carrier impregnated with K₂CO₃ is significantly different from that of unimpregnated boehmite. The first peak on the PSD almost disappears, and the second peak shifts to larger pore sizes. This indicates that potassium carbonate preferentially fills the smaller alumina pores[27]. It is also observed that the impregnated adsorbent, after calcination, exhibits a broader distribution of mesopore sizes. These mesopores enhance the dispersion of K₂CO₃ within the pores, facilitating the contact between the adsorbent and the acids present in the oil during the acid removal process[20]. Figure 3 presents the N₂ adsorption-desorption isotherms of all the adsorbents. All the curves can be classified as type Ⅳ isotherms, which are typical characteristics of mesoporous materials. Furthermore, the adsorption-desorption hysteresis of all samples occurs in the range of 0.4 to 1.0 P/P₀, indicating a capillary condensation phenomenon in the pore structure of the adsorbents[28]. The pore morphology exhibits a bottle-like shape, characteristic of mesoporous materials[29].

Table 1

Comparison of specific surface area, average pore diameter, and total pore volume of adsorbents with different K₂CO₃ impregnation mass and different calcination temperatures.
adsorbent	Adsorbent name	K₂CO₃ loading (wt%)	Surface area (m²/g)	Pore volume (cm³/g)	Average Pore diameter(nm)
fresh boehmite	A	0	379.19	1.18	11.52
boehmite calcined at 750℃	A(750)	0	373.38	1.45	13.97
16% K₂CO₃/boehmite calcined at 750℃	16KA(750)	16	200.43	0.95	18.11
33% K₂CO₃/boehmite calcined at 750℃	33KA(750)	33	90.10	0.58	23.55
50% K₂CO₃/boehmite calcined at 750℃	50KA(750)	50	59.00	0.46	27.73
75% K₂CO₃/boehmite calcined at 750℃	75KA(750)	75	42.09	0.34	27.05
50% K₂CO₃/boehmite	50KA	50	106.62	0.49	15.23
50% K₂CO₃/boehmite calcined at 350℃	50KA(350)	50	103.32	0.57	20.44
50% K₂CO₃/boehmite calcined at 550℃	50KA(550)	50	54.48	0.41	27.26
50% K₂CO₃/boehmite calcined at 650℃	50KA(650)	50	49.83	0.37	27.35

3.1.3 Thermogravimetric analysis (TGA) of boehmite after K₂CO₃ impregnation

The first accelerated weight loss peak observed around 100°C can be attributed to the removal of surface-bound structural water in boehmite and crystalline water in K₂CO₃[25]. Additionally, the presence of a second accelerated weight loss peak observed between 300–400°C can be attributed to the decomposition of KAl(OH)₂CO₃. The decomposition temperature of KAl(OH)₂CO₃ is approximately 320°C. During the impregnation process, the reaction between potassium carbonate, boehmite, and water results in the formation of KAl(OH)₂CO₃[30]. However, when the temperature reaches the decomposition temperature range, KAl(OH)₂CO₃ decomposes, leading to the observed weight loss. This decomposition process is responsible for the weight loss peak observed in that temperature range during thermal analysis. Since there was no observed accelerated weight loss peak indicating the transformation of boehmite to aluminum oxide, it can be inferred that the duration of high temperature during the thermal analysis was relatively short. This suggests that the transformation from boehmite to Al₂O₃ may not have been fully completed within the limited time of the thermal analysis. It is possible that a longer duration of high temperature is required to achieve a complete transformation from boehmite to Al₂O₃.

3.1.4. FTIR analysis

The changes in surface functional groups of the adsorbent were determined using FTIR spectroscopy, and the results are shown in Fig. 5. The broad peak observed between 3427 cm^− 1 and 3115 cm^− 1 in the uncalcined boehmite arises from the combined vibrations of the stretching of the Al-O-H bond[31] and the intermolecular hydrogen bonding. The peak observed at 1071 cm^− 1 is attributed to the symmetric bending vibration of the Al-O-H bond in boehmite. The disappearance of these peaks after undergoing high-temperature calcination represents the transformation of boehmite into Al₂O₃. After impregnation with potassium carbonate, peaks corresponding to the CO₃²⁻ ions appear at 1544 cm^− 1 and 864 cm^− 1[20]. This further confirms the loading of K₂CO₃ on the boehmite surface. When the calcination temperature exceeds 750°C, significant changes are observed in the infrared spectrum of the adsorbent. Specifically, the carbonate ion peak and the hydroxyl peak representing the surface hydroxyl groups of aluminum oxide at 1400 cm^− 1 disappear. This indicates a loss of carbonate species and a reduction in the concentration of surface hydroxyl groups. Instead, two new peaks appear at 1372 cm^− 1 and 1451 cm^− 1, suggesting the presence of new functional groups or chemical species on the adsorbent surface. This transformation leads to a decrease in the alkalinity of the adsorbent. When the adsorbent completes the adsorption process for the regeneration of the waste transformer oil, a decrease in the intensity of the CO₃²⁻ ion peak confirms the involvement of K₂CO₃ in the adsorption of the waste transformer oil. Additionally, new absorption peaks appeared in the FTIR spectrum. The absorption peaks at 2924 cm^− 1 and 2851 cm^− 1 correspond to the stretching vibration of C-H bonds in the -CH₂- group, and the peak at 1378 cm_− 1 corresponds to the deformation vibration of the CH₃- group. The new absorption peaks observed at 3618 cm^− 1 and 3529 cm^− 1 after adsorption are attributed to the -OH groups in carboxylic acid functional groups present in the transformer oil. The above results indicate the adsorption of polar functional groups on the adsorbent, but no carboxylic acid groups were observed. This may be attributed to the excessively low acidity concentration in the sample. The presence of these functional groups indicates the attachment of organic acids generated from the aging of transformer oil and oil-paper insulation to the adsorbent[32].

3.2 Impact of Modification Conditions of Bentonite on Adsorption Performance

During the initial stage of our research, we conducted screening experiments using various adsorbents, including bentonite, activated carbon, Al₂O₃, and boehmite. All adsorbents were impregnated with a 16wt% K₂CO₃ and calcined at a temperature of 550°C. It should be noted that the bentonite was calcined at a lower temperature of 400℃ specifically for this process. The adsorption process was carried out with a reagent-to-oil ratio of 1:30. The adsorption results are shown in Fig. 6. After comparison, it was found that boehmite exhibited significantly better performance compared to traditional adsorbents used for transformer oil regeneration.

The acid removal performance of the modified boehmite adsorbent was investigated with respect to different K₂CO₃ immersion masses and calcination temperatures. The results are presented in Fig. 7. The influence of immersion mass of K₂CO₃ on the adsorption performance was investigated at a calcination temperature of 750°C. The results are depicted in Fig. 7a. As the immersion mass of K₂CO₃ increased from 0 to 50wt%, the acid value of the adsorbed transformer oil decreased from 0.015 mg KOH/g to complete removal of acidity in the transformer oil. Correspondingly, the alkaline dosage of the adsorbent exhibited a sharp increase. Based on the comparison of acid removal performance with the alkaline dosage of the adsorbent, it was generally observed that adsorbents with higher alkaline dosages demonstrated superior adsorption performance. In other words, increasing the alkaline dosage of the adsorbent resulted in enhanced acid removal capabilities. This observation suggests that the alkaline components in the adsorbent play a crucial role in neutralizing and removing acids effectively. The higher alkaline dosage provides a greater number of active sites for acid adsorption and neutralization, leading to improved performance in acid removal. However, as the immersion mass of K₂CO₃ further increased, the adsorption performance deteriorated. The acid value of the adsorbed transformer oil increased, despite a slight improvement in the alkaline dosage of the adsorbent. This could be attributed to the increase in the loading amount of K₂CO₃. On the surface of the bentonite carrier, a threshold is reached where a monolayer dispersion is formed. When the loading amount further increases, excessive active components cluster on the surface of the boehmite, reducing the dispersion of active components on the carrier's surface. As a result, the specific surface area of the catalyst decreases, which can be observed through BET characterization, leading to a decrease in adsorption performance[33]. The influence of calcination temperature on the alkaline dosage of modified bentonite was investigated under a K₂CO₃ immersion mass of 50w%. The results are presented in Fig. 7b. With the continuous increase in calcination temperature, the alkaline centers of the adsorbent gradually become more apparent[34], reaching the maximum alkaline dosage at 650 ℃. With the gradual increase in calcination temperature, the alkaline centers of the adsorbent become more prominent, reaching their maximum alkaline dosage at 650°C. It is worth noting that at this temperature, boehmite undergoes a transformation into γ-alumina, as confirmed by X-ray diffraction (XRD) phase analysis. It is generally recognized that γ-alumina possesses stronger adsorption and separation capabilities compared to boehmite. Therefore, at 650°C, we believe that the maximum alkaline dosage corresponds to the highest proportion of γ-alumina in the adsorbent, which is the primary reason for achieving the optimal acid removal performance at this temperature. However, as the calcination temperature further increases, the alkaline dosage of the adsorbent starts to decrease. Simultaneously, the adsorbent exhibits a decrease in its ability to adsorb acidity in transformer oil. This is due to the further increase in temperature, which leads to the transformation of γ-alumina into an intermediate phase known as transitional alumina[35]. Consequently, the content of γ-alumina decreases. During this phase transition, the Al³⁺ ions migrate from tetrahedral to octahedral sites, and oxygen ions redistribute within the face-centered cubic fcc lattice. Excessive temperatures during calcination can lead to the sintering of potassium carbonate, thermal decomposition of potassium carbonate, or reactions between potassium carbonate and the carrier material. These factors can potentially result in a reduction of the alkaline dosage of the adsorbent. These findings have been confirmed in the aforementioned infrared characterization.

The color of the oil in a power transformer is indicative of its deterioration. Fresh oil typically has a colorless appearance, so the level of oil degradation can be assessed by observing its color after being used in power transformers. Aged oil tends to exhibit a dark color due to the presence of impurities accumulated during its utilization. The darkening of the oil color is a visual indication of the degradation and contamination that has occurred over time[36]. The changes in the color of the adsorbent and waste transformer oil before and after the transformer oil regeneration adsorption are illustrated in Fig. 8. In the initial state, the waste transformer oil appears dark in color, possibly due to impurities and degradation byproducts present in the oil. The adsorbent, on the other hand, exhibits its original color in white. After the transformer oil regeneration adsorption, noticeable color changes can be observed in the adsorbent. The color of the adsorbent changes from white to yellow, and there is also the occurrence of agglomeration, indicating that it has adsorbed the waste transformer oil and its associated impurities. This change in color reflects the effectiveness of the adsorption process in removing contaminants from the waste transformer oil.

In this study, the use of impregnated boehmite as an adsorbent for acidity removal in waste transformer oil was investigated. The effects of impregnation amount and calcination temperature on acidity removal were studied, and a comparative analysis with other adsorbents was conducted. The results demonstrated that impregnated boehmite showed promising performance in removing acidity from transformer oil.

The X-ray diffraction (XRD) analysis confirmed the successful transformation of boehmite into γ-Al2O3 after calcination at 650℃ for 3 hours. The physicochemical characterization of the adsorbents provided insights into their structural changes and surface properties. The experimental results showed that the amount of K2CO3 impregnation and the calcination temperature significantly influenced the acidity removal efficiency of the adsorbent. The optimal impregnation amount was found to be 50% of the mass of the carrier, and the optimal calcination temperature was 650℃. Under these conditions, impregnated boehmite achieved a high acidity removal efficiency. Comparative analysis with other adsorbents, including alumina, activated carbon, and bentonite, revealed that impregnated boehmite outperformed these materials in terms of acidity removal. This highlights the potential of impregnated boehmite as a sustainable and effective adsorbent for regenerating waste transformer oil.

The findings of this study contribute to the development of environmentally friendly methods for recycling and reclamation of aged transformer oil. By removing acidity and polar compounds, the lifespan of transformers can be prolonged, ensuring their efficient and safe operation. The adsorption separation method using impregnated boehmite offers simplicity, high efficiency, and cost-effectiveness for transformer oil regeneration. Further research can explore the optimization of impregnation and calcination parameters, as well as investigate the long-term performance and stability of impregnated boehmite in real transformer oil regeneration processes. Additionally, the impacts of impregnated boehmite on other oil properties, such as breakdown voltage and viscosity, can be studied to comprehensively assess its effectiveness as an adsorbent.

Overall, this study provides valuable insights into the use of impregnated boehmite for acidity removal in waste transformer oil, contributing to the development of sustainable and efficient methods for transformer oil regeneration.

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No competing interests reported.

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Alkali-Modified Boehmite for Acid Removal in Waste Transformer Oil

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Version 1

Abstract

Figures

1. Introduction

2. Experimental and Methods

2.1 Materials.

2.2 Absorbent Preparation.

2.4 Characterization of Adsorbents.

2.4.1. X-ray diffraction analysis (XRD)

2.4.2. Brunauer-Emmett-Teller(BET) and Barrett-Joyner-Halenda(BJH)

2.4.3. Fourier Transform Infrared (FTIR)

2.4.4. Measurement of Alkalinity of Adsorbents

2.4.5. Determination of Acid Value in Transformer Oil

3. Results and Discussion

3.1. Physicochemical Characterization of Adsorbents

3.1.1. XRD Analysis

3.1.2. Surface Area, Pore Volume, and Average Pore Diameter.

3.1.3 Thermogravimetric analysis (TGA) of boehmite after K₂CO₃ impregnation

3.1.4. FTIR analysis

3.2 Impact of Modification Conditions of Bentonite on Adsorption Performance

4. Conclusion

References

Additional Declarations

Status:

Version 1

Alkali-Modified Boehmite for Acid Removal in Waste Transformer Oil

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental and Methods

2.1 Materials.

2.2 Absorbent Preparation.

2.4 Characterization of Adsorbents.

2.4.1. X-ray diffraction analysis (XRD)

2.4.2. Brunauer-Emmett-Teller(BET) and Barrett-Joyner-Halenda(BJH)

2.4.3. Fourier Transform Infrared (FTIR)

2.4.4. Measurement of Alkalinity of Adsorbents

2.4.5. Determination of Acid Value in Transformer Oil

3. Results and Discussion

3.1. Physicochemical Characterization of Adsorbents

3.1.1. XRD Analysis

3.1.2. Surface Area, Pore Volume, and Average Pore Diameter.

3.1.3 Thermogravimetric analysis (TGA) of boehmite after K2CO3 impregnation

3.1.4. FTIR analysis

3.2 Impact of Modification Conditions of Bentonite on Adsorption Performance

4. Conclusion

References

Additional Declarations

Status:

Version 1

3.1.3 Thermogravimetric analysis (TGA) of boehmite after K₂CO₃ impregnation