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The highly efficient purification capacity of rGO-zeolite composites for aged oil in transformer machines
⁎Corresponding author. tan.vuthi@hust.edu.vn (T. Tan Vu)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
This research attempts to develop an effective oil purification material composite comprised of reduced Graphene Oxide (rGO) and zeolite nanoparticles. The resultant composites have a high specific surface area (310–420 m2/g) and an appropriate average pore size (1.9–4 nm) for transformer oil purification applications. The oil purification capability of the rGO-zeolite composite was evaluated using a static adsorption test. The findings show that as-synthesised materials provide better acidity, breakdown voltage, viscosity, and color than the commercial zeolite material, ZSM-5, and other purification materials reported in the literature. Additionally, according to the purification results, the rGO-zeolite composite shows a better purification behaviour than the composite made of rGO and ZnO nanoparticles in the previous work. Furthermore, compared to virgin oil, the quality of refined oil in this work is practically identical. This result may be explained by the unique properties of the rGO-zeolite composite, such as its high BET specific surface area and the appropriate average pore size for aged oil purification. Furthermore, the homogeneous distribution of zeolite nanoparticles on the rGO flake may increase the residence period for the purification process, improving the purification efficiency of the rGO-zeolite composite.
Keywords
rGO-zeolite composite
Transformer oil
Purification process
Pore size
- rGO
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Reduce Graphene Oxide
- GO
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Graphene Oxide
- BET
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Brunauer-Emmett-Teller
- XRD
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X-ray diffraction
- SEM
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Scanning electron microscopy
- TEM
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transmission electron microscopy
Abbreviations
1 Introduction
Transformer oil plays an important role in the isolation of electric power transformers. It also serves as a heat transfer medium and prevents moisture damage to solid insulation. However, over time, various contaminants will accumulate in the transformer oil, altering its characteristics. Contaminants come from the breakdown of hydrocarbons in the oil, moisture in the air, or metallic compounds like iron, copper, and lead that come from the metallic parts of transformers (Vu et al. 2020).
Furthermore, organic acids are formed as a result of hydrocarbon degradation in the presence of oxygen in the air, increasing the acidity of oils. The acids may corrode the metal parts of the transformer, forming metallic oxide and causing oil sludge. Several scientists have also demonstrated that the dielectric strength of oil may be affected by the water generated in it (Q. Liu et al. 2019; Salvi y Paranjape 2017).
It is well known that when the performance insulating characteristic of oil is reduced, the transformer insulation will not function properly, posing a substantial danger of failure. To extend the life of the transformer oil, it must eliminate any foreign chemicals and impurities such as water and solid particles. Depending on the purification objective, several purification techniques, such as centrifugal purification, thermovacuum purification, or adsorption purification, were used for transformer oil purification (Rafiq, Lv, y Li 2016; Safiddine et al. 2019). However, there are various disadvantages of the use of thermovacuum and centrifugal purification, including a poor level of purification, complexity, and the requirement for constant control people presence. The most significant benefit of the adsorption approach is its simplicity. To remove gases, particles, and acids from transformer oil, single and multiple use filters with a high degree of filtration are utilized.
Recently, adsorbent materials such as activated bauxite(Oumert et al. 2017), carbonated calcium phosphate biopolymers (CACP) (C. Laurentino et al. 2007), activated carbon (Sulaiman, Noura, y Fardoun 2011), zeolite (Fofana et al. 2004), Kaolin clay (I Hafez et al. 2015), and ZnO-Graphene composites (Vu et al. 2020) have been widely used for the treatment of aged oil. However, there are many disadvantages to using the adsorbents indicated. Bauxite materials, for example, can produce a considerable quantity of waste (Oumert et al. 2017). Moreover, because the biopolymer CACP has a high manufacturing cost, the treatment cost in aged oil is higher (C. Laurentino et al. 2007). Adsorbents such as activated carbon, zeolite, and kaolin clay particles can clump together, decreasing purifying effectiveness (Vu et al. 2020). As a result, the materials should be combined with other materials to form a composite in order to reduce agglomeration issues. Newly, a combination of ZnO nanoparticles and reduced Graphene Oxide (rGO) has been used for aged oil purification in the transformer. The composite ZnO-rGO, according to the author, avoids agglomeration issues and has a greater purifying capacity than a commercial zeolite (Vu et al. 2020). However, ZnO nanoparticles are not a standard oil purification material. A question is raised: What happens if we create a composite consisting of popular oil purification materials, such as a zeolite, and Graphene for the transformer oil purification?
Due to its unique characteristics, Graphene-based materials is a good purification material, and its adsorption process is better than that of other adsorbents (Vinh et al. 2019; Vu et al. 2021b; (Vu et al., 2021a). Furthermore, the incomplete reduction process may create unreduced oxygen-containing functional groups and defects for rGO, which may give a good purifying capacity (Pham Van et al. 2019; Van Tuan et al. 2020; (Vu et al., 2021a). Additionally, because of the high BET surface area, rGO has the potential to be an excellent purification material (Vinh et al. 2019). Several studies have found that combining rGO with an inorganic material can result in a good composite for purification and separation purposes (Pham Van et al. 2019; Tuan et al. 2020; 2021; X. Yan et al. 2021; Yang et al. 2020; Cheng et al. 2019; L. Yan et al. 2021; Pham et al., 2022). Purification composites containing rGO and zeolite are frequently utilized, according to the literature (Farghali, Abo-Aly, y Salaheldin 2021; Khatamian et al. 2015).
Several methods for producing rGO-zeolite composites have recently been developed (Farghali, Abo-Aly, y Salaheldin 2021; Khatamian et al. 2015; Kim et al. 2018; Soni y Shukla 2019; Li et al. 2012; Qiu, Wang, y Li 2015). The hydrothermal process is the most widely used method for fabricating rGO-zeolite composites among these methods. In the presence of alkalinity and increased temperature, zeolite is produced, while Graphene Oxide (GO) is simultaneously reduced to rGO. As a result, a spherical composite of rGO-zeolite has been produced (Li et al. 2012). The time synthesis, on the other hand, takes a long time, even many days. And the elevated temperature and high pressure are also required.
In this work, we reduced GO in the presence of a commercial zeolite ZSM-5 to produce a structure rGO-zeolite composite with non-covalent Van der Waals forces and hydrogen bonding. The synthesis takes only a few hours, which might lower the fabrication cost. This method has advantages over others described in the literature, such as shorter synthesis times, the absence of hazardous chemicals, and low-pressure conditions. Thus, the fabrication cost may reduce when the technology is scaled. The as-synthesized rGO-zeolite composites are used to purify aged transformer oil to improve the life service of the oil in the transformer. The purification capability of the synthesized composites was compared to that of ZSM-5, a commercial zeolite material frequently used for purification. Furthermore, the comparison was also completed with the composite made of rGO and ZnO nanoparticles in the previous work (Vu et al. 2020). The result in this research may open a new purification route for the treatment of aged transformer oil.
2 Experimental details
2.1 The preparation of Graphene oxide
In this work, the modified Hummer technique (Vinh et al. 2019) was used to fabricate Graphene Oxide (GO). In a typical synthesis, 10 g of 99.5% pure graphite was combined with 5 g of sodium nitrate in a glass vessel, then 250 mL of 98.5% sulfuric acid was added under mechanical agitation. It is critical to emphasize that the combination should be cooled in an ice bath until the temperature reaches 5 °C.
At that instance, 35 g of potassium permanganate was little by little added into the prepared mixture.
To complete the oxidation of Graphite, the reaction is placed in a water bath for 3 h after adding the potassium permanganate. The oxidation process was then stopped using distilled water. The cleaning operation is then carried out with 15 mL of H2O2 (30 wt% in water) and 100 mL of HCl (20%). Finally, the graphite oxide slurry was produced by vacuum filtration and two days of drying at 40 °C.
The Graphite oxide was exfoliated using a 100 W ultrasonic bath. The exfoliating procedure takes one hour to complete. To separate the unexfoliated Graphite oxide, a separation step at 2500 rpm for 10 min in a centrifuge is required after the exfoliation process. The resume of the GO fabrication is presented in Fig. 1.
2.2 The preparation of rGO-zeolite composites
The reduction of GO to rGO (Vinh et al. 2019) and the deposition of zeolite ZSM-5 particles on the surface of rGO under hydrothermal conditions (Tan et al., 2019a) are two basic steps in the production of rGO-zeolite composites. A dispersion is made up of different amounts of ZMS-5 particles (1 g, 2 g, 3 g), 0.5 g ascorbic acid and a 100 mL of 5 g/L GO dispersion. The amount of GO was determined based on the outcome of the work (Vu et al. 2020). The prepared mixture was placed in a Teflon autoclave (150 mL) and subjected to three hours of hydrothermal treatment at 150 °C for 3 h.
After that, the obtained slurry rGO-zeolite composite was washed with abundant deionized water and dried at 60 °C for 5 h. The product obtained by changing the amount of zeolite were marked as Z-G-1 (1 g zeolite), Z-G-2 (2 g zeolite), and Z-G-3 (3 g zeolite), respectively. When the mass fraction of zeolite was over 3 g, the rGO-zeolite composites could not be prepared because zeolite is not uniformly dispersed in the GO dispersion. The preparation of rGO-zeolite composites is illustrated in Fig. 2.
2.3 Purification process
The aged transformer oil utilized in this work (five years of usage) was sourced from HABAC NITROGENOUS FERTILIZER & CHEMICALS COMPANY LIMITED's transformer substation (10 MVA, 30/10 kV). All the filtering tests were carried out at room temperature.
The oil purification procedure is carried out in static batches. According to (Vu et al. 2020), an optimal amount of adsorbent for oil treatment was a combination of 2 g rGO-zeolite composite mixed with 50 mL old oil.
For equilibrium adsorption, the mixture was continuously agitated for 1 h. After that, the rGO-zeolite composite was centrifuged at 700 rpm to remove it from the oil. The refined oils were given the following names based on the amount of zeolite used: Z-G-1, Z-G-2, and Z-G-3. Additionally, the separation of the adsorbents from the purifies oil can be performed by vacuum filtration or using a lab filter press. A given amount of adsorbents to the volume of aged oil for a pilot industrial operation is understudying.
The purification efficiency of the rGO-zeolite composite was compared a standard zeolite material, ZSM-5, a conventional zeolite material used to purify transformer oil. The zeolite ZSM-5 was purchased from Alfa Aesar, which has a 30:1 M ratio of SiO2 and Al2O3.
2.4 Characterization of the as-prepared rGO-zeolite composites
The X-ray diffraction (XRD) investigation was carried out on Bruker D8 Advance equipment utilizing Cu K radiation (=0.15406 nm) at 40 kV and 40 mA. Scanning electron microscopy (SEM, FEI Quanta FEG 650 model) and transmission electron microscopy (TEM, JEOL- JEM 1010 Microscope 300 kV) were used to examine the morphology of the composites. On a Micromeritics ASAP 2020 analyzer, the N2 isotherm analysis was performed. The specific surface area of the samples was estimated by the Brunauer–Emmett–Teller (BET) equation. The Barrett–Joyner–Halenda (BJH) method was employed to calculate the adsorption average pore of the material. The functional groups on the surface were characterized by Fourier transform infrared (FTIR) spectroscopy (FTIR 8400s Shimadzu).
2.5 Characterization of the refined oil
The International Electrotechnical Commission (IEC) and the International Organization for Standardization (ISO) standard criteria were used to characterize the refined oil. The following are the primary approaches used in oil characterization:
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The oil breakdown voltage is measured using the LEP OLT-80A tester,
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The oil acidity index is determined using the burette titrette. To evaluate the acid content of the oil, 10 g of aged oil is neutralized with potassium hydroxide (KOH). As a color indicator, phenolphthalein is used to regulate the endpoint of the titration process.
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Spectrophotometric Colorimeters OKATON C-105 is used to estimate the color index of the oil.
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The viscosity was determined by Ostwald viscometer at 30 °C.
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The water content (moisture) value is determined by METROHM 756 coulometer following IEC standard 60814.
To calculate the average value, all measurements were repeated three times. All of the tables now include the standard deviations of the measured values.
3 Results and discussion
3.1 Characterization of the rGO-Zeolite composites
3.1.1 Scanning electron Microscope (SEM) analysis
Fig. 3 illustrates the typical surface morphologies and microstructures of the composite rGO-zeolite with varied ratio mass between zeolite and rGO: Z-G-1 (Fig. 3 a, 3c); Z-G-2 (Fig. 3b, 3e) and Z-G-3 (Fig. 3c,3f), respectively. The low-resolution pictures demonstrate that the zeolite particles are evenly dispersed on the rGO surface. The zeolite is fully coated on the surface of rGO in sample Z-G-3 (Fig. 3c). Fig. 3d, 3e, and 3f show that the homogeneity distribution of zeolite on the wrinkle assembly of rGO is confirmed. This finding also demonstrates the complete conversion of GO into rGO with a sandwich structure.
3.1.2 Transmission Electron Microscope (TEM) analysis
The TEM analysis is essential for examining particle size and distribution in great detail (La et al., 2019; Vu et al., 2020). Three synthesized composites exhibit a characteristic wrinkle 3D structure with surface roughness at low resolution (Fig. 4a, 4b, 4c). Meanwhile, high-resolution TEM images (Fig. 4d, 4e, 4f) show that zeolite nanoparticles are distributed uniformly on the rGO surface.
Based on the SEM and TEM characterization results, it is reasonable to assume that zeolite nanoparticles were uniformly precipitated on the surface of the rGO during the GO reduction. The zeolite ZSM-5 particles successfully prevent rGO nanosheet aggregation, resulting in a 3D structure. The findings may give way to developing a new method for manufacturing rGO-zeolite composites.
3.1.3 The X-ray diffraction analysis
Fig. 5 represents the typical XRD pattern of three composites with a range of 2θ from 5° to 40°. Usually, GO has an identical diffraction peak at 9.88°, and in our work, the diffraction peak of GO is shifted to 25.8° for the three rGO-zeolite composites, corresponding to the (0 0 2) diffraction peak of rGO (Vinh et al. 2019). This finding shows that GO was completely reduced in the presence of ascorbic acid and zeolite. The XRD diffraction patterns of the three composites Z-G-1, Z-G-2, and Z-G-3 are highly comparable, indicating that zeolite is distributed uniformly over the surface of rGO. Furthermore, as the concentration of zeolites increases, the intensity of the XRD diffraction peaks also increases. The diffraction peaks 2θ = 7.95°, 8.73°, 14.75°, 23.11°, 23.86°, 24.54°, 27.64°, 30.13°, 35.53° related to the [0 1 1], [0 2 0], [0 3 1], [0 5 1], [3 0 3], [3 1 3] plane, which is matched to the type two interconnected channel systems (MFI) structure of ZSM-5 with highly crystalline degree (Ni et al. 2011).
3.1.4 FTIR spectroscopy
Fig. 6 illustrates the FTIR spectroscopy of the three rGO- zeolite composites and as-prepared GO sample, recording from 400 to 4000 cm−1. The characteristic absorption bands at 455 cm−1, 795 cm−1, 1100 cm−1, and 1220 cm−1 corresponded to T-O bending (T = Si and Al), external symmetric stretching, internal asymmetric stretching, and external asymmetric stretching, respectively (Mozgawa, Król, y Barczyk 2011). These absorption bands are matched to siliceous materials. The signal at 1430 cm−1 in the GO sample corresponds to the stretching vibration of the C-O carboxyl group (Vinh et al. 2019). However, this peak of the C-O carboxyl group disappeared in the three composites rGO-zeolite samples. The formation of rGO from GO may explain the changes observed in the FTIR spectra of the three composites.
During the reduction process, the reducing agent may open the ring of the epoxide group of GO structure, resulting in the production of C–OH and esters functional groups. As a result, the signal at 1550 cm−1 is associated with the asymmetric stretching vibration of COOH carboxylate groups(Vinh et al. 2019; V. Tan, Vinh, y Tung 2019), which was seen in all three composite samples. The peak at 3400 cm−1 is related to the stretching frequency of the O-H bond of both zeolite and rGO materials. According to the results, GO is fully reduced and linked with the functional groups of zeolites without any extra binder to form a composite.
3.1.5 N2 adsorption/desorption measurement
Isotherm characterizations can be used to evaluate the purifying capacity of the as-prepared composites. Fig. 7 depicts the N2 adsorption/desorption isotherms (BET results) of the three composite samples Z-G-1, Z-G-2, and Z-G-3. At relative pressure P/P0 greater than 0.45, the isotherm plots of the three samples show a comparable type IV isotherm with an H3-type hysteresis loop (Vu et al., 2021a; Vu et al. 2021b; Vu et al., 2021c). This finding reveals the presence of mesopores and slit-shaped pores in the three composite samples (Fig. 8), indicating that they are likely to be acceptable materials for specific purification applications (Vu et al. 2020).
The Z-G-3 composite has the largest specific surface area (420 m2/g), whereas the Z-G-1 and Z-G-2 composites have lower surface areas, 310 m2/g and 350 m2/g, respectively, according to BET results (Table 1).
| Material | SBET (m2/g) | L (nm) |
|---|---|---|
| Z-G-1 | 310 | 4 |
| Z-G-2 | 350 | 1.9; 2.3 |
| Z-G-3 | 420 | 2 |
| ZSM-5 (Fernández et al. 2014) | 364 | 1.76 |
Fig. 8 shows the pore size distribution of the three composites according to Barret-Joyner-Halenda (BJH). The average pore size of Z-G-1, Z-G-2, and Z-G-3 is 4 nm, 1.9, and 2.3 nm, 2 nm, respectively. According to the results, it can be inferred that all three composites are mesoporous materials based on BET results and average pore diameter values (Gupta and Khatri, 2017). Furthermore, numerous studies have shown that average pore diameters of less than 4 nm may be used for oil purification (Shihh-Hong et al., 2013).
Combining different amount of zeolite particles with rGO layers can explain the differences in BET values across the three composite samples. The dispersion of zeolite particles on the rGO surface may prevent the rGO layer from agglomerating (Pham Van et al. 2019), resulting in sandwich shape of the composite. The specific surface area of the composite may be increased when a high amount of zeolite is employed. Furthermore, the presence of zeolite particles may result in the development of micropores leading to the generation of pore walls (Park y Kwon 2020). Those results may provide an excellent refining capacity for purification of aged oil application.
3.2 Characterization of the refined oil
3.2.1 The acidity number
During oil deterioration, the oil acidity number generally rises, posing a risk of harm to the power transformer. As a result, lowering the oil acidity number is required to assure the power transformer's appropriate operation. Table 2 shows the acidity number of virgin oil, aged oil, and purified oils generated by rGO-zeolite composites. The acidity of the oil samples purified by ZSM-5 and rGO is lower than that of aged oils, but still higher than that of rGO-zeolite composites. Furthermore, the oil sample purified by the rGO-zeolite composite Z-G-2, which contains a medium quantity of zeolite, has an acidity number value that is extremely close to the virgin oil.
| Virgin oil | Aged oil | ZSM-5 | rGO | Z-G-1 | Z-G-2 | Z-G-3 |
|---|---|---|---|---|---|---|
| 0.15 ± 0.007 | 0.380 ± 0.01 | 0.262 ± 0.009 | 0.305 ± 0.01 | 0.246 ± 0.015 | 0.165 ± 0.007 | 0.205 ± 0.01 |
As a result, the dissolved acids were nearly wholly removed, employing a specific quantity of zeolite dispersed on the surface of the rGO, Z-G-2 sample. Because of the large concentration of ZSM-5 on the surface of rGO, Z-G-3 has a lower purifying efficiency than Z-G-2. The partial agglomeration of zeolite might explain this finding in the Z-G-3 sample.
3.2.2 Breakdown voltage
Because of the moisture, organic acids, and generated sludge in aged oil, it always has a lower breakdown voltage than virgin oil. In this work, the oil breakdown voltage tester KEP OLT-80A has been used to characterize the purified oil (according to IEC 60,156 standard measurement). The result showed that oil samples refined by rGO-zeolite composite have a better breakdown voltage than oil samples purified by the commercial zeolite ZSM-5 or as-prepared rGO (Table 3). Furthermore, the oil sample treated with Z-G-2 composite has a breakdown voltage close to that of virgin oil.
| Virgin oil | Aged oil | ZSM-5 | rGO | Z-G-1 | Z-G-2 | Z-G-3 |
|---|---|---|---|---|---|---|
| 75.0 ± 0.51 | 33.1 ± 0.73 | 66.2 ± 0.53 | 54.6 ± 0.42 | 67.6 ± 0.81 | 73.3 ± 0.46 | 71.5 ± 0.61 |
3.2.3 Viscosity
The viscosity in the power transformer should be quite low for a better cooling effect. The aged oil has a higher viscosity value than virgin oil due to the oil deterioration process. In Table 4, the viscosity values of the virgin, aged, and purified oil samples are presented. Again, the result reveals that the composites of zeolite and rGO possess better viscosity values than zeolite or rGO. In addition, the viscosity of the oil sample purified by Z-G-2 is similar to that of virgin oil.
| Fresh oil | Aged oil | ZSM-5 | rGO | Z-G-1 | Z-G-2 | Z-G-3 |
|---|---|---|---|---|---|---|
| 6.55 ± 0.07 | 8.57 ± 0.09 | 7.13 ± 0.11 | 7.24 ± 0.08 | 7.01 ± 0.09 | 6.68 ± 0.12 | 6.81 ± 0.11 |
3.2.4 Color
The virgin oil is colorless; the aged oil may have a dark color owing to impurities; hence, the oil color may be used to determine the degree of degradation. Following the determination of color ASTM D1500 standard, we utilized Spectrophotometric Colorimeters OKATON C-105 to determine the oil color of all the samples. The color of the purified oils improved significantly when the rGO-zeolite composite was used (Table 5). The color of the oil sample purified by Z-G-2 composite is the most similar to that of virgin oil. The breakdown voltage, acidity number, and viscosity value all agree with the color result.
| Virgin oil | Aged oil | ZMS-5 | rGO | Z-G-1 | Z-G-2 | Z-G-3 |
|---|---|---|---|---|---|---|
| 0.50 ± 0.006 | 3.12 ± 0.01 | 1.71 ± 0.09 | 1.82 ± 0.073 | 1.09 ± 0.008 | 0.610.005 | 0.99 ± 0.008 |
3.2.5 Water content
Water is one factor that affects the oil quality and transformer characteristics in all transformers. Water content should be at a level as low as possible, which is no more than 30 ppm. Following the determination of water content IEC standard 60814, we injected 1 g of the oil sample into the automatic titrator, and after the reaction, the water content was recorded. The water content of all the oil samples is illustrated in Table 6. It can be seen that the rGO-zeolite composites and zeolite have a higher moisture elimination capacity than the rGO sample. The results may be explained by the affinity of zeolite for water. Furthermore, the water content values of the oils treated by ZMS-5, Z-G-2, and Z-G-3 are identical.
| Virgin oil | Aged oil | ZMS-5 | rGO | Z-G-1 | Z-G-2 | Z-G-3 |
|---|---|---|---|---|---|---|
| 2 ± 0.5 | 29 ± 1 | 4 ± 0.3 | 8 ± 0.5 | 6 ± 0.6 | 4 ± 0.5 | 4 ± 0.4 |
3.3 Comparison of the refining capacity and the posible purification mechanism
From all the presented results above, it can be concluded that the as-prepared Z-G-2 composite sample offers the best performance in oil purification. As well, it's capacity compared with the ZnO-Graphene composite fabricated in (Vu et al. 2020) (Table 7). For the comparison, the similar quality of aged oil and same amount was used. The result shows that the rGO-zeolite composite prepared in this work exhibits a purified oil more similar to virgin oil than the one purified by ZnO-Graphene composite.
| Oils | Acidity (mg KOH/g, IEC 62021) | Breakdown voltaje (kV, IEC 60156) | Viscosity (40 °C, ISO 3104) | Color (ASTM D1500) | Water content (ppm, IEC 60814) |
|---|---|---|---|---|---|
| Virgin oil | 0.15 | 75 | 6.55 | 0.50 | 2 |
| Oil in (Vu et al. 2020) | 0.18 | 72 | 6.89 | 0.8 | – |
| Oil in this work | 0.165 | 73.3 | 6.68 | 0.61 | 4 |
The refining capacity of the as-synthesized rGO–zeolite composite was higher than that of the other purifying medium. This is mostly owing to the surface adsorption and ᴨ–ᴨ conjugate adsorption capabilities of rGO, which have a higher adsorption capacity, resulting in a greater purifying capacity(C. Liu et al. 2018). The zeolite ZSM-5 particles successfully prevent the rGO nanosheets from aggregating. The BET test results reveal that by layering the composite material with many slit-type mesopores, the composite membrane has a high specific surface area, allowing it to adsorb and retain particles in the aged oil, lowering the oil diffusion transfer resistance.
Fig. 9 shows a color comparison of four oils: aged oil, oil purified by ZSM-5 zeolite, virgin, and the purified Z-G-2 oil. The oil refined by the composite rGO- zeolite has less color than the oil purified by the commercial zeolite ZMS-5 and aged oil. Also, the purified Z-G-2 oil provides a color visually similar to virgin oil.
The aforementioned results show that the composite Z-G-2 purifies with exceptional efficiency when compared to commercial zeolites. The purified oil exhibits all of the characterisation values that are nearly identical to the virgin oil. This excellent result may be explained by the textural structure, which has a high specific surface area and a tiny pore size average, making it suitable for oil purification (Bockisch 1998). Furthermore, the homogeneous distribution of zeolite particles on the surface of rGO provides a similar sandwich structure with a low chance of collapsing in the filtering system (Dou et al. 2020).
4 Conclusion
The rGO- zeolite composite was created by hydrothermally reducing GO in the presence of ascorbic acid and ZMS-5 zeolite. Ascorbic acid acts as a reducing agent for GO and forms the interconnection between rGO and zeolite without the need for a binder. The SEM analysis reveals that the zeolite is distributed in a regular pattern over the surface of the rGO layer. The XRD diffraction pattern exhibits the high crystallinity of zeolite in the composite, and the GO is completely reduced into rGO when incorporating zeolite on its surface. The three composites synthesized by this new route present an elevated specific surface area (310–420 m2/g) compared with others synthesized by other approaches. Furthermore, all three composites are mesoporous materials with appropriate pore sizes (1.9–4 nm) for purifying the aged oil.
The as-synthesized rGO-zeolite composites were used as an adsorbent for oil purification in transformers. The results show that the synthesized composite has a considerable oil filtering capability when compared to commercial zeolite and the synthesized Graphene composites reported in the literature. Furthermore, the quality of purified oil is almost similar to that of virgin oil in terms of acidity (0.165 mg KOH/g), breakdown voltage (73.3 kV), colour (0.61), and viscosity (6.68, at 40 °C). These promising results may be attributed to the mesoporous structure, average pore size, and 3D sandwich structure of rGO-zeolite composites.
Acknowledgments
This research is funded by Vietnam Ministry of Education and Training under grant number B2020-BKA-562-22.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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