- Email: [email protected]
Synthesis of trimethylolpropane fatty acid triester as a high performance electrical insulating oil
Industrial Crops & Products 142 (2019) 111834
Contents lists available at ScienceDirect
Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Synthesis of trimethylolpropane fatty acid triester as a high performance electrical insulating oil
T
⁎
Kaizheng Wanga, Feipeng Wanga,b, , Jian Lia, Zhengyong Huanga, Ziyi Loua, Qiuhuang Hana, Qi Zhaoa, Kelin Hua a b
State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai 200092, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Medium chain fatty acid Trimethylolpropane esters Bio-based industrial products Electrical insulating oil
The trimethylolpropane esters (TME) based high performance electrical insulating oil was synthesized by esterification of the mixture of C6, C8 and C10 acids blend with trimethylolpropane (TMP). The conversion yields of TME is maximized at catalyst (SnCl2) dosage of 0.8 wt.% and the TMP:acid molar ratio of 3.2:1 at 140 °C. The electrical performance is significantly improved by a purpose-oriented purification process which is structured by washing with ultrapure water and absorption with molecular sieve. The obtained insulating oil shows much reduced viscosity (23.3 mm2 s−1) and much enhanced AC breakdown voltage (72.6 kV) comparing with the commercial natural ester insulating oil FR3® (34 mm2s−1 and 63 kV). The high performance of the insulating oil is also manifested by quite low pour point (−45 °C) and high flash point (248 °C) which are superior to those values of mineral insulating oil (−30 °C and 170 °C). The remarkable oxidation stability of the insulating oil is metered by the oxidation induction time (OIT) 2275 min at 130 °C which should be attributed to the elimination of C]C double bonds and β-H atoms.
1. Introduction Conventional mineral insulating oils could cause serious problems due to their low flash point which has been evidenced accounting for several fire accidents. The poor degradation ability of mineral oils has led to disposal problems which against environmental laws and regulations in many countries. During the past three decades, natural ester (NE) insulating oils have been receiving increasing attention due to their distinctive characteristics such as biodegradable, renewable, and fire safety(Ab Ghani et al., 2017). However, the NE insulating oils, such as FR3® and BIOTEMP (Oommen et al., 1998) oils, are still away from satisfying the request for application in ultra-high voltage and large power transformers due to its high viscosity, low oxidation stability and high pour point. The high viscosity lowers the effective heat transfer which limits the lifetime and load capacity of power transformers. The poor oxidation stability results in increased aging products during longterm operation. Most of the aging products are highly polar, such as moisture, acid and gases, which should lead to significant decreasing of dielectric breakdown strength and increasing of dielectric loss as well the lowered safety level. Last but not the least, the pour point depressants is not capable of enabling NE insulating oils with pour point
below −20 °C. This has limited the application of NE insulating oil in cold regions such as northeast China, northern Europe, Siberia and northern regions in Canada (Sitorus et al., 2016). The free-breathing transformers account for about 90% of distribution transformers in the word, which indicates the oxidation stability of insulating oils is critical because this property determines the safety level (Ab Ghani et al., 2017). The oxidation induction time (OIT) of NE insulating oils is recognized as being determined by the C]C bonds and ®-hydrogen atoms in triglycerides (Gryglewicz et al., 2003; Oommen, 2002). To eliminate the effect of ®-hydrogen atoms, the glycerol in the natural esters has been replaced by polyols contained no ®-hydrogen atoms, e.g. trimethylolpropane (TMP) (Hamid et al., 2016), neopentyl glycol (Fernandes et al., 2018), or pentaerythritol (Padmaja et al., 2012). The price of TMP is acceptable and the temperature of melting point relatively low. Therefore, it is widely used in the production of synthetic esters. Selective hydrogenation (Pinto et al., 2013) and epoxidation (Abdelmalik et al., 2011; Adhvaryu and Erhan, 2002) are used to reduce the effect of C]C bonds on the oxidation stability of esters. However, C]C bonds in polyol esters cannot completely eliminate to ensure relative low pour point. In order to reduce the viscosity of ester insulating to remove heat
⁎ Corresponding author at: State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (F. Wang).
https://doi.org/10.1016/j.indcrop.2019.111834 Received 18 March 2019; Received in revised form 27 March 2019; Accepted 29 September 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
generated by electrical losses, Bertrand and Lauzevis (2013) have achieved ester insulating oil with quite low viscosity of 17 mm²s−1 from raw rapeseed oil via replacing glycerol by 2-ethyl-1-hexanol during the transesterification. However, the dielectric loss was twice as that of FR3® insulating oil and the flash point dropped from ca. 310 to 175 °C. Sitorus et al. (2016) have reduced the viscosity of Jatropha curcas oil to a very low level which is comparable to that of mineral oil (MO), but the very low flash point of 140 °C (30 K lower than that of MO) should prohibit the oil from the application in power transformers. Apparently, these researches only focus on reducing the viscosity of ester insulating oil, but not guarantee the high flash point of ester insulating oils. So far it is lacking of an effective route to synthesis ester insulating oil with both high flash point and oxidation stability, as well with low viscosity and pour point. In this work, the transesterification of the medium chain saturated fatty acids (C6-C10 acids) with TMP is utilized to synthesize an ester insulating oil with high flash point and oxidation stability, low pour point and reduced viscosity. The reaction condition parameters including temperature, the dosage of catalyst and the molar ratio of acid to TMP are optimised.
Table 1 The main properties of TME: A is the TME after removing the catalyst and residual CA and TMP, B is further processed from A by three times washing and molecular sieve absorption. Property −3
(20 °C ASTM D-4052) Density / g cm Pour point / ºC (ASTM D 97) Acid value / mgKOH·g−1 (ASTM D 974-2008) Flash point / °C (GB/T 261-2008) Viscosity / mm2s−1 (40 °C ASTM D455) Tanδ/ % (90 °C IEC 60247) Permittivity (90 °C IEC 60247) Resistivity / Ω·cm (90 °C IEC 60247) AC Vb / kV (2.5 mm GB/T 507)
2.1. Materials C6, C8 and C10 acids (purity better than 98%) were purchased from China Agri-Industries Holdings Ltd. The mineral insulating oil (MO) of naphthenic type (DT25 PetroChina) and NE insulating oil (FR3® Cargill) was used as received. Analytical grade (Aladdin Co. Ltd., China): trimethylolpropane [2-ethyl-2-(hydroxymethyl)-1,3-propanediol], xylene, ethanol, potassium hydroxide (KOH). Chromatographical grade (Aladdin Co. Ltd., China): deuterated chloroform (CDCl3) with internal standard tetramethyl silane (0.03% v/v). Stannous chloride (purity of 98 wt.%), alkali blue 6B and molecular sieves (0.3 nm) were also obtained from Aladdin Co. Ltd., China. 2.2. Synthesis of trimethylolpropane esters Trimethylolpropane (TMP) and C6, C8 and C10 acids (CA) were mixed with catalyst stannous chloride and xylene (300 ml) in a 1 L, three-necked, round-bottomed flask. The flask was equipped with a Dean-Stark cap to collect the azeotropic distillation of water–xylene. The reaction was accompanied by magnetic stirring at 600 rpm in nitrogen gas. After 5 h reaction, the stannous was removed by centrifugation at 4000 rpm followed by low pressure distillation at 150–160 °C and 0.2–0.6 kPa to remove the CA and TMP residuals. The generated water collected by Dean-Stark cap was used to monitor the reaction. The esterification reactions were also monitored by the acid value measurement according to standard (ASTM D 9742008). Approximately 0.1 g of the reactant was mixed with 50 mL ethanol and the mixture was boiled for 5 min in a flask equipping with a condenser to eliminate CO2 before adding 0.5 mL Alkali blue 6B (1:50 g/mL of ethanol). The acid value Av was determined by using 0.05 M KOH solution during the titration. The completion (endpoint) of the titration was signaled by the color changing from blue to light red. The Av was determined by Eq. (1)
MKOH × N × V G
B
0.931 −46 0.15 220 22.3 183 4.5 108 10
0.934 −45 0.0017 248 23.3 2 3.2 1013 72.6
The stannous of products were removed by the centrifugal machine (4000 rpm, 10 min) and the residual CA and TMP were distilled at 150160 °C and 0.2-0.6 kPa. As shown in Table 1, despite these processes were carried out to purify the products, the dielectric loss cannot meet with the application in the transformer (IEC 61099 Tanδ < 3%). The little impurity significantly degradation of electrical properties of the insulating oil, therefore, TME needs to be purified further. Ultrapure water (Resistivity at 25 °C > 18.18 MΩ cm) was used to reduce the ionic and polar impurities. After washing three times by ultrapure water, molecular sieve was added into the products under by stirring for 1 h to reduce the water content. In addition, 0.25 μm filter membrane was used to filter the solid impurity in the products. After 3 times the of filtration, the products were dried in a vacuum for 48 h (50 Pa, 90 °C). The cooling process was performed in the vacuum oven to avoid contact with the surrounding air. The comparison of the main properties of products before and after three times washing and the absorption of the molecular sieve as shown in Table 1. The physical properties variate a little but the electrical properties significantly change after a series of purification processes. Therefore, purification processes are essential for using the TME in transformers. Though the ionic and polar impurities become less after three times washing, the residual water in the TME still degrade the electrical properties. So vacuum drying and molecular sieve were adopted to remove the water in TME. As shown in Fig. 1, molecular sieve has been successful in reducing the dielectric loss of TME.
2. Experiment
Av =
A
(1)
where V is the volume of KOH solution used for titration, MKOH and N are KOH molecular weight concentration, respectively. G is the mass of the reactant. The conversion was calculated from
conversion(%) = (Av0 − Avt )/Av0 × 100
(2) Fig. 1. The measured frequency (10−1 – 104 Hz) dependence of loss factor tanδ at 20 °C for the TME insulating oils before and after molecular sieve adsorption.
where Av0 and Avt are the acid value of the sample at time zero and t, respectively. 2
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
Fig. 2. Effect of the SnCl2/CA mass ratio on TME yield. Reaction condition: TMP/ CA molar ratio: 3.2:1; reaction temperature: 140 °C.
Fig. 4. Effect of molar ratio of TMP to CA on TME yield. Reaction condition: SnCl2/CA mass ratio: 8 wt.%; reaction temperature: 140 °C.
3. Results and discussion
bubbles hinder the contact between the catalyst and the reactants, which is indicated by a decrease in reaction rate and conversion shown in Fig. 3.
3.1. Optimization of conditions for the high yield of TME 3.1.1. Effect of catalyst dosage To optimize the effect of catalyst dosage, the investigation of catalyst (SnCl2) effect on the esterification was carried out. The dosage of catalyst was varied from 0 to 1.2 wt.%, The reaction was performed at 140 °C, 3.2:1 M ratio of TMP to CA and 5 h of reaction. As shown in Fig. 2, the dosage of catalysts as the important factor obviously affect on esterification reaction. The proper catalyst loading is 0.8 wt.%. Increasing the catalyst loading further (above 0.8 wt.%) affects the conversion little. The conversion of TME at 96.8% was achieved at 0.8 wt. % stannous chloride after 5 h.
3.1.3. Effect of the molar ratio of TMP to CA The TME yield could be elevated by acid value of the solution. To study the effect of the molar ratio, the various molar ratios of TMP to CA (1:3.0 to 1:3.8) were performed. Increasing the molar ratio enhances the conversion and reaction rate increased because of it is a reversible reaction. As shown in Fig. 4, the proper molar ratio is 1:3.2. Further increasing CA loading further (above 3.2:1 TMP/ CA molar ratio) does not affect the conversion significantly. The maximum conversion yields of TME is achievable at the optimum reaction conditions at the temperature of 140 °C, catalyst (SnCl2) dosage of 0.8 wt.% and the TMP: acid molar ratio of 3.2:1.
3.1.2. Effect of the reaction temperature The reaction rate and conversion of TME affected significantly by the temperature. The range of reaction temperature was varied from 120 °C to 160 °C. Fig. 3 shows that the reaction rate increases with the increase of reaction temperature. The reaction rate is slow at 120 °C and the conversion was 75.4% at 120 °C after 5 h of reaction. It is observed that reaction rate and conversion increase with increasing of reaction temperature that varies from 120 °C to 140 °C (cf. Fig. 3). However, when the reaction temperature reaches 150 °C, the vaporization of much xylene (boiling point 138.5 °C) causes of many bubbles. The
3.2. Analysis of products The structure the product was detected using an NMR spectroscopy (AVANCE 500 spectrometer, Bruker). Samples were dissolved in deuterated chloroform (CDCl3) with internal standard Tetramethyl Silen (TMS) at 25 °C, at 6 mg/ml concentration. Fig. 5 shows the 1H NMR spectra of TME. Based on information from the software ChemDraw, six methylene protons (–O–CH2–) exit in TME molecule. The Main protons in the molecule are listed below: 1H NMR (CDCl3, ™ ppm): 0.85-0.92 [d,
Fig. 3. Effect of reaction temperature on TME yield. Reaction condition: TMP/ CA molar ratio: 3.2:1; SnCl2/CA: 8 wt.%.
Fig. 5. 1H NMR spectra of TME. 3
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
convection heat transfer. In addition, low viscosity will promote the insulating oil flow into the narrow passage in the windings to prevent local overheating. The low viscosity of insulating oil will accelerate the initial impregnation of insulating paper. Therefore, the low viscosity of insulating oil is important for the transformer. The viscosity of TME is about 50% below FR3®, which means the new insulating oil (TME) enhance the heat transfer of transformers. Insulating oils were mainly used to quench arc discharges and transfer heat dissipating heat. Therefore, For a comparison with FR3®, the relative lower viscosity means TME-immersed transformer has a longer lifetime and larger load capacity of power transformers. 3.3.2. Pour point The pour point of the insulating is just the temperature at which the oil can flow. The pour point reflects the low temperature fluidity of the insulating oil to a certain extent. To ensure that the transformer can work safely when it stops running and restarts, it is necessary to choose insulating oil with good low temperature fluidity. The pour point of TME is -45 °C. NE oils have higher pour point than TME typically in the range 29–32 K.
Fig. 6. FTIR spectra of TME.
12H, 3×(–CH3)], 1.56-1.65 [t, 6H, 3×(–CO–CH2–CH2)], 2.30 [t, 6H, 3×(–CO–CH2–)], 4.0 [s, 6H, 3×(–O–CH2–)] Fig. 6 shows the FTIR spectra of TME. The stretching of C]O causes an obvious peak exists at 1739 cm−1 the stretching of CO] (Soares et al., 2008). CeC(O)O]e stretching vibrations produce a peak at 1154 cm−1. The existence of the peak at 2856 cm−1 and 2928 cm−1 is attributed to the asymmetric vibration and symmetric stretching of the CH2 group, respectively.
3.3.3. Flash point The flammability of insulating oil concerns the safety of the transformer. The transformer may cause rapid temperature rise due to thermal failure or electrical failure, which may cause explosion burning and insulating oil leakage due to the relative low flash point of MO. The burning and leakage cause a significant threat to the ecological environment and social stability. Many accidents about MO filled transformer explosion have occurred in recent years. Therefore, the insulating oil with high flash point is important for the transformer. The insulating oil cycles in the transformer which is an enclosed tank. Therefore, it is prescribed to measure the flash point of the insulating oil by the closed cup method. The flash point was measured by the closed-cup method as standard GB/T 261-2008. The comparative values for different insulating fluids are shown in Table 2. It was observed that the closed-cup flash point of TME has more than about 78 °C compared to that of mineral insulating oil.
3.3. Physical and electrical properties The viscosity and pour point increase with the increase of molecular weight and branching. However, increasing molecular weight increases flash points. A medium chain saturated fatty acids (C6-C10 acids) replace the long chain fatty acids in NE insulating oil to decrease the viscosity and ensure a relatively high flash point. Moreover, most of the NE insulating oils are from camellia seed, soybean and rapeseed. These edible oils used as insulating oil burden the food supply. C6-C10 acids from coconut oil and castor oil are being on mass production stage and cannot burden the food supply. To eliminate the effect of ®-hydrogen, TMP was use to synthesis TME. The oxidation resistance of these esters was easy to predict due to the absence of double bonds and ®-hydrogen into the molecule. Table 2 shows the comparison of main properties MO, FR3® and TME.
3.3.4. Oxidation stability Pressure differential scanning calorimetry can measures the oxidation induction time (OIT) quickly and accurately in terms of exothermal effect (Tan et al., 2002; Xu et al., 2014). Xu et al. (2014) found that the OIT of NE insulating oil at 130 °C is greater than 15 min, which can guarantee the reliability of measurement based on ASTM D6186. Therefore the OIT was measured at 130 °C. A differential scanning calorimeter (DSCQ20 TA) measured the OIT of ester insulating oil. 5.0 ± 0.5 mg of ester oil was weighed into open aluminum pans. The isothermal temperature was programmed at 130 °C and oxygen (99.9 wt.%) was passed through the sample at 60 ml/min. The OIT of soybean oil and rapeseed oil are 85 and 94 min, respectively. The OIT of TME is 2275 min, which is much longer than that of soybean and rapeseed oil.
3.3.1. Viscosity The viscosity of insulating oil determines the heat transfer rate of transformers. The relative high viscosity of NE insulating oil cannot transfer the heat generated by the loss of the transformer, which limits the lifetime and load capacity of power transformers. Convection heat transfer is the most important way in cooling transformer. The low viscosity of the insulating obviously enhances the
3.3.5. Electrical properties The AC breakdown voltage (Vb) is the most critical and commonly used parameter for estimating the insulating performance of the insulating oil. The AC Vb was measured as the standard GB/T 507. The electrode gap and voltage rise rate are 2.5 mm and 2 kV/s, respectively. The AC Vb is the average of 7 measured data points. The breakdown voltages of synthetic ester (TME), NE insulating oil (FR3®), and MO at ambient temperature were shown in Table 2. AC breakdown voltage of TME is about 72.6 kV/2.5 mm, higher than (around 12%) that of MO and FR3®. The impurities (e.g. particles, water and gas bubbles) have a significant effect on the AC Vb of insulating oil (Wang et al., 2018). Therefore, the measured AC breakdown voltage of an insulating oil
Table 2 Comparison of main properties mineral, FR3® and TME oil. Property
MO
FR3®
TME
Density/g cm−3 (20 °C ASTM D-4052) Pour point/ºC (ASTM D 97) Acid value /mgKOH g−1 (ASTM D 974-2008) Fash point /ºC (GB/T 261-2008) Viscosity/mm2s−1 (40 °C ASTM D455) Tanδ/% (90 °C IEC 60247) Permittivity (90 °C IEC 60247) Resistivity/Ω cm (90 °C IEC 60247) AC Vb/kV (2.5 mm GB/T 507)
0.84 −30 0.03 170 9.7 0.03 2.2 1014 60
0.92 −21 0.04 316 34 3 2.8 1013 65
0.934 −45 0.02 248 23.3 2 3.2 1012 72.6
4
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
performance ester insulating oil TME is expected to find wide applications in size reduced and high safety level electrical insulating equipment which requires good cooling efficiency, high oxidation stability, high flash point, etc. Acknowledgments The authors gratefully acknowledge the NSFC (51425702 and 51321063), 973 program (2015CB251003), Project 111 of the Ministry of Education, China (B08036), as well the support from Chongqing (CXTDX201601001) and the Opening Project of Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology (ammt2017A-3). References Ab Ghani, S., Muhamad, N.A., Noorden, Z.A., Zainuddin, H., Abu Bakar, N., Talib, M.A., 2017. Methods for improving the workability of natural ester insulating oils in power transformer applications: a review. Electr. Power Syst. Res. https://doi.org/10.1016/ j.epsr.2017.10.008. Abdelmalik, A.A., Abbott, A.P., Fothergill, J.C., Dodd, S., Harris, R.C., 2011. Synthesis of a base-stock for electrical insulating fluid based on palm kernel oil. Ind. Crops Prod. 33, 532–536. https://doi.org/10.1016/j.indcrop.2010.11.019. Adhvaryu, A., Erhan, S.Z., 2002. Epoxidized soybean oil as a potential source of hightemperature lubricants. Ind. Crops Prod. 15, 247–254. https://doi.org/10.1016/ S0926-6690(01)00120-0. Bertrand, Y., Lauzevis, P., 2013. Development of a low viscosity insulating liquid based on natural esters for distribution transformers. 22nd Int. Conf. Exhibit. Electr. Distrib. 1–4. https://doi.org/10.1049/cp.2013.0701. Fernandes, K.V., Papadaki, A., da Silva, J.A.C., Fernandez-Lafuente, R., Koutinas, A.A., Freire, D.M.G., 2018. Enzymatic esterification of palm fatty-acid distillate for the production of polyol esters with biolubricant properties. Ind. Crops Prod. 116, 90–96. https://doi.org/10.1016/j.indcrop.2018.02.058. Gryglewicz, S., Piechocki, W., Gryglewicz, G., 2003. Preparation of polyol esters based on vegetable and animal fats. Bioresour. Technol. 87, 35–39. https://doi.org/10.1016/ S0960-8524(02)00203-1. Hamid, H.A., Yunus, R., Rashid, U., Choong, T.S.Y., Ali, S., Syam, A.M., 2016. Synthesis of high oleic palm oil-based trimethylolpropane esters in a vacuum operated pulsed loop reactor. Fuel 166, 560–566. https://doi.org/10.1016/j.fuel.2015.11.022. Martins, M.A.G., 2010. Vegetable oils, an alternative to mineral oil for power transformers- experimental study of paper aging in vegetable oil versus mineral oil. IEEE Electr. Insul. Mag. 26, 7–13. https://doi.org/10.1109/MEI.2010.5599974. Oommen, T.V., 2002. Vegetable oils for liquid-filled transformers. IEEE Electr. Insul. Mag. 18, 6–11. https://doi.org/10.1109/57.981322. Oommen, T.V., Claiborne, C.C., Walsh, E.J., 1998. Introduction of a new fully biodegradable dielectric fluid, textile, fiber film. In: Indust. Tech. Conf. Charlotte, North California. pp. 1–4. https://doi.org/10.1109/TEXCON.1998.679223. Padmaja, K.V., Rao, B.V.S.K., Reddy, R.K., Bhaskar, P.S., Singh, A.K., Prasad, R.B.N., 2012. 10-Undecenoic acid-based polyol esters as potential lubricant base stocks. Ind. Crops Prod. 35, 237–240. https://doi.org/10.1016/j.indcrop.2011.07.005. Pinto, F., Martins, S., Gonçalves, M., Costa, P., Gulyurtlu, I., Alves, A., Mendes, B., 2013. Hydrogenation of rapeseed oil for production of liquid bio-chemicals. Appl. Energy 102, 272–282. https://doi.org/10.1016/j.apenergy.2012.04.008. Rafiq, M., Lv, Y.Z., Zhou, Y., Ma, K.B., Wang, W., Li, C.R., Wang, Q., 2015. Use of vegetable oils as transformer oils – a review. Renew. Sust. Energ. Rev. 52, 308–324. https://doi.org/10.1016/j.rser.2015.07.032. Sitorus, H.B.H., Setiabudy, R., Bismo, S., Beroual, A., 2016. Jatropha curcas methyl ester oil obtaining as vegetable insulating oil. IEEE Trans. Dielectr. Electr. Insul. 23, 2021–2028. https://doi.org/10.1109/TDEI.2016.7556474. Soares, I.P., Rezende, T.F., Silva, R.C., Castro, E.V.R., Fortes, I.C.P., 2008. Multivariate calibration by variable selection for blends of raw soybean oil/biodiesel from different sources using fourier transform infrared spectroscopy (FTIR) spectra data. Energy Fuels 22, 2079–2083. https://doi.org/10.1021/ef700531n. Tan, C.P., Che Man, Y.B., Selamat, J., Yusoff, M.S.A., 2002. Comparative studies of oxidative stability of edible oils by differential scanning calorimetry and oxidative stability index methods. Food Chem. 76, 385–389. https://doi.org/10.1016/S03088146(01)00272-2. Wang, K., Wang, F., Li, J., Zhao, Q., Wen, G., Zhang, T., 2018. Effect of metal particles on the electrical properties of mineral and natural ester oils. IEEE Trans. Dielectr. Electr. Insul. 25, 1621–1627. https://doi.org/10.1109/tdei.2018.006909. Xu, Y., Qian, S., Liu, Q., Wang, Z.D., 2014. Oxidation stability assessment of a vegetable transformer oil under thermal aging. IEEE Trans. Dielectr. Electr. Insul. 21, 683–692. https://doi.org/10.1109/TDEI.2013.004073.
Fig. 7. Comparisons of breakdown voltage (Vb) of insulating oils with different water content.
mostly represents the oil quality. As shown in Fig. 7, it is apparent that the MO is highly sensitive to the water, which is indicated that the AC Vb decreases rapidly with the increase of water content. The water content has little effect on AC Vb of FR3® before critical value (340 mg/ L). The AC Vb decreases rapidly with the increase of water content after the critical value. The effect of water content on AC Vb of TME oil is similar to that of FR3®. However, the critical value is 460 mg/L which is much larger than the critical value of FR3®. The rate of AC Vb decrease become smaller after critical value than that of FR3®. Free water and dissolved water are the two formations exist in the insulating oil. Hydrogen bonds are easy to form between polar and water molecule (Rafiq et al., 2015). Therefore, the weak polarity insulating oil (FR3® and TME) can dissolve more water than that of nonpolar MO. Therefore, AC breakdown voltages decrease rapidly as the increase of moisture contents (cf. Fig. 7). The permittivity of TME is 3.2 which is higher than that of FR3® (2.8), which means the water tolerance of TME is higher than that of NE insulating oil. This capability identified that TME would be able to absorb more moisture from insulating paper and thus extend the lifetime of the transformer filled with TME (Martins, 2010). 4. Conclusion This paper synthesized a high performance ester insulating oil TME from the transesterification of saturated fatty acids with TMP. The conversion yields of TME is maximized at catalyst (SnCl2) dosage of 0.8 wt.% and the TMP:acid molar ratio of 3.2:1 at 140 °C. With the designed purification process structured by washing with ultrapure water and absorption with molecular sieve, high performance TME oil was obtained. Comparing to the very short oxidation induction time (OIT = 85 min) for commercial FR3® ester insulating oil, the TME oil shows very much enhanced oxidation stability (OIT = 2275 min). The TME oil has low viscosity of 23.3 mm2s−1 and pour point of -45 °C, both are quite lower than those of FR3® oil. The water tolerance of AC breakdown voltage for TME oil is much better than that of FR3® oil (cf. Fig. 7). Furthermore, the AC breakdown voltage of TME is about 72.6 kV/2.5 mm, around 12% higher than that of MO and FR3®. The TME oil is suitable for high safety level applications because of its high flash point at 248 °C which is higher than that (170 °C) of MO. The high
5
Contents lists available at ScienceDirect
Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Synthesis of trimethylolpropane fatty acid triester as a high performance electrical insulating oil
T
⁎
Kaizheng Wanga, Feipeng Wanga,b, , Jian Lia, Zhengyong Huanga, Ziyi Loua, Qiuhuang Hana, Qi Zhaoa, Kelin Hua a b
State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai 200092, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Medium chain fatty acid Trimethylolpropane esters Bio-based industrial products Electrical insulating oil
The trimethylolpropane esters (TME) based high performance electrical insulating oil was synthesized by esterification of the mixture of C6, C8 and C10 acids blend with trimethylolpropane (TMP). The conversion yields of TME is maximized at catalyst (SnCl2) dosage of 0.8 wt.% and the TMP:acid molar ratio of 3.2:1 at 140 °C. The electrical performance is significantly improved by a purpose-oriented purification process which is structured by washing with ultrapure water and absorption with molecular sieve. The obtained insulating oil shows much reduced viscosity (23.3 mm2 s−1) and much enhanced AC breakdown voltage (72.6 kV) comparing with the commercial natural ester insulating oil FR3® (34 mm2s−1 and 63 kV). The high performance of the insulating oil is also manifested by quite low pour point (−45 °C) and high flash point (248 °C) which are superior to those values of mineral insulating oil (−30 °C and 170 °C). The remarkable oxidation stability of the insulating oil is metered by the oxidation induction time (OIT) 2275 min at 130 °C which should be attributed to the elimination of C]C double bonds and β-H atoms.
1. Introduction Conventional mineral insulating oils could cause serious problems due to their low flash point which has been evidenced accounting for several fire accidents. The poor degradation ability of mineral oils has led to disposal problems which against environmental laws and regulations in many countries. During the past three decades, natural ester (NE) insulating oils have been receiving increasing attention due to their distinctive characteristics such as biodegradable, renewable, and fire safety(Ab Ghani et al., 2017). However, the NE insulating oils, such as FR3® and BIOTEMP (Oommen et al., 1998) oils, are still away from satisfying the request for application in ultra-high voltage and large power transformers due to its high viscosity, low oxidation stability and high pour point. The high viscosity lowers the effective heat transfer which limits the lifetime and load capacity of power transformers. The poor oxidation stability results in increased aging products during longterm operation. Most of the aging products are highly polar, such as moisture, acid and gases, which should lead to significant decreasing of dielectric breakdown strength and increasing of dielectric loss as well the lowered safety level. Last but not the least, the pour point depressants is not capable of enabling NE insulating oils with pour point
below −20 °C. This has limited the application of NE insulating oil in cold regions such as northeast China, northern Europe, Siberia and northern regions in Canada (Sitorus et al., 2016). The free-breathing transformers account for about 90% of distribution transformers in the word, which indicates the oxidation stability of insulating oils is critical because this property determines the safety level (Ab Ghani et al., 2017). The oxidation induction time (OIT) of NE insulating oils is recognized as being determined by the C]C bonds and ®-hydrogen atoms in triglycerides (Gryglewicz et al., 2003; Oommen, 2002). To eliminate the effect of ®-hydrogen atoms, the glycerol in the natural esters has been replaced by polyols contained no ®-hydrogen atoms, e.g. trimethylolpropane (TMP) (Hamid et al., 2016), neopentyl glycol (Fernandes et al., 2018), or pentaerythritol (Padmaja et al., 2012). The price of TMP is acceptable and the temperature of melting point relatively low. Therefore, it is widely used in the production of synthetic esters. Selective hydrogenation (Pinto et al., 2013) and epoxidation (Abdelmalik et al., 2011; Adhvaryu and Erhan, 2002) are used to reduce the effect of C]C bonds on the oxidation stability of esters. However, C]C bonds in polyol esters cannot completely eliminate to ensure relative low pour point. In order to reduce the viscosity of ester insulating to remove heat
⁎ Corresponding author at: State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (F. Wang).
https://doi.org/10.1016/j.indcrop.2019.111834 Received 18 March 2019; Received in revised form 27 March 2019; Accepted 29 September 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
generated by electrical losses, Bertrand and Lauzevis (2013) have achieved ester insulating oil with quite low viscosity of 17 mm²s−1 from raw rapeseed oil via replacing glycerol by 2-ethyl-1-hexanol during the transesterification. However, the dielectric loss was twice as that of FR3® insulating oil and the flash point dropped from ca. 310 to 175 °C. Sitorus et al. (2016) have reduced the viscosity of Jatropha curcas oil to a very low level which is comparable to that of mineral oil (MO), but the very low flash point of 140 °C (30 K lower than that of MO) should prohibit the oil from the application in power transformers. Apparently, these researches only focus on reducing the viscosity of ester insulating oil, but not guarantee the high flash point of ester insulating oils. So far it is lacking of an effective route to synthesis ester insulating oil with both high flash point and oxidation stability, as well with low viscosity and pour point. In this work, the transesterification of the medium chain saturated fatty acids (C6-C10 acids) with TMP is utilized to synthesize an ester insulating oil with high flash point and oxidation stability, low pour point and reduced viscosity. The reaction condition parameters including temperature, the dosage of catalyst and the molar ratio of acid to TMP are optimised.
Table 1 The main properties of TME: A is the TME after removing the catalyst and residual CA and TMP, B is further processed from A by three times washing and molecular sieve absorption. Property −3
(20 °C ASTM D-4052) Density / g cm Pour point / ºC (ASTM D 97) Acid value / mgKOH·g−1 (ASTM D 974-2008) Flash point / °C (GB/T 261-2008) Viscosity / mm2s−1 (40 °C ASTM D455) Tanδ/ % (90 °C IEC 60247) Permittivity (90 °C IEC 60247) Resistivity / Ω·cm (90 °C IEC 60247) AC Vb / kV (2.5 mm GB/T 507)
2.1. Materials C6, C8 and C10 acids (purity better than 98%) were purchased from China Agri-Industries Holdings Ltd. The mineral insulating oil (MO) of naphthenic type (DT25 PetroChina) and NE insulating oil (FR3® Cargill) was used as received. Analytical grade (Aladdin Co. Ltd., China): trimethylolpropane [2-ethyl-2-(hydroxymethyl)-1,3-propanediol], xylene, ethanol, potassium hydroxide (KOH). Chromatographical grade (Aladdin Co. Ltd., China): deuterated chloroform (CDCl3) with internal standard tetramethyl silane (0.03% v/v). Stannous chloride (purity of 98 wt.%), alkali blue 6B and molecular sieves (0.3 nm) were also obtained from Aladdin Co. Ltd., China. 2.2. Synthesis of trimethylolpropane esters Trimethylolpropane (TMP) and C6, C8 and C10 acids (CA) were mixed with catalyst stannous chloride and xylene (300 ml) in a 1 L, three-necked, round-bottomed flask. The flask was equipped with a Dean-Stark cap to collect the azeotropic distillation of water–xylene. The reaction was accompanied by magnetic stirring at 600 rpm in nitrogen gas. After 5 h reaction, the stannous was removed by centrifugation at 4000 rpm followed by low pressure distillation at 150–160 °C and 0.2–0.6 kPa to remove the CA and TMP residuals. The generated water collected by Dean-Stark cap was used to monitor the reaction. The esterification reactions were also monitored by the acid value measurement according to standard (ASTM D 9742008). Approximately 0.1 g of the reactant was mixed with 50 mL ethanol and the mixture was boiled for 5 min in a flask equipping with a condenser to eliminate CO2 before adding 0.5 mL Alkali blue 6B (1:50 g/mL of ethanol). The acid value Av was determined by using 0.05 M KOH solution during the titration. The completion (endpoint) of the titration was signaled by the color changing from blue to light red. The Av was determined by Eq. (1)
MKOH × N × V G
B
0.931 −46 0.15 220 22.3 183 4.5 108 10
0.934 −45 0.0017 248 23.3 2 3.2 1013 72.6
The stannous of products were removed by the centrifugal machine (4000 rpm, 10 min) and the residual CA and TMP were distilled at 150160 °C and 0.2-0.6 kPa. As shown in Table 1, despite these processes were carried out to purify the products, the dielectric loss cannot meet with the application in the transformer (IEC 61099 Tanδ < 3%). The little impurity significantly degradation of electrical properties of the insulating oil, therefore, TME needs to be purified further. Ultrapure water (Resistivity at 25 °C > 18.18 MΩ cm) was used to reduce the ionic and polar impurities. After washing three times by ultrapure water, molecular sieve was added into the products under by stirring for 1 h to reduce the water content. In addition, 0.25 μm filter membrane was used to filter the solid impurity in the products. After 3 times the of filtration, the products were dried in a vacuum for 48 h (50 Pa, 90 °C). The cooling process was performed in the vacuum oven to avoid contact with the surrounding air. The comparison of the main properties of products before and after three times washing and the absorption of the molecular sieve as shown in Table 1. The physical properties variate a little but the electrical properties significantly change after a series of purification processes. Therefore, purification processes are essential for using the TME in transformers. Though the ionic and polar impurities become less after three times washing, the residual water in the TME still degrade the electrical properties. So vacuum drying and molecular sieve were adopted to remove the water in TME. As shown in Fig. 1, molecular sieve has been successful in reducing the dielectric loss of TME.
2. Experiment
Av =
A
(1)
where V is the volume of KOH solution used for titration, MKOH and N are KOH molecular weight concentration, respectively. G is the mass of the reactant. The conversion was calculated from
conversion(%) = (Av0 − Avt )/Av0 × 100
(2) Fig. 1. The measured frequency (10−1 – 104 Hz) dependence of loss factor tanδ at 20 °C for the TME insulating oils before and after molecular sieve adsorption.
where Av0 and Avt are the acid value of the sample at time zero and t, respectively. 2
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
Fig. 2. Effect of the SnCl2/CA mass ratio on TME yield. Reaction condition: TMP/ CA molar ratio: 3.2:1; reaction temperature: 140 °C.
Fig. 4. Effect of molar ratio of TMP to CA on TME yield. Reaction condition: SnCl2/CA mass ratio: 8 wt.%; reaction temperature: 140 °C.
3. Results and discussion
bubbles hinder the contact between the catalyst and the reactants, which is indicated by a decrease in reaction rate and conversion shown in Fig. 3.
3.1. Optimization of conditions for the high yield of TME 3.1.1. Effect of catalyst dosage To optimize the effect of catalyst dosage, the investigation of catalyst (SnCl2) effect on the esterification was carried out. The dosage of catalyst was varied from 0 to 1.2 wt.%, The reaction was performed at 140 °C, 3.2:1 M ratio of TMP to CA and 5 h of reaction. As shown in Fig. 2, the dosage of catalysts as the important factor obviously affect on esterification reaction. The proper catalyst loading is 0.8 wt.%. Increasing the catalyst loading further (above 0.8 wt.%) affects the conversion little. The conversion of TME at 96.8% was achieved at 0.8 wt. % stannous chloride after 5 h.
3.1.3. Effect of the molar ratio of TMP to CA The TME yield could be elevated by acid value of the solution. To study the effect of the molar ratio, the various molar ratios of TMP to CA (1:3.0 to 1:3.8) were performed. Increasing the molar ratio enhances the conversion and reaction rate increased because of it is a reversible reaction. As shown in Fig. 4, the proper molar ratio is 1:3.2. Further increasing CA loading further (above 3.2:1 TMP/ CA molar ratio) does not affect the conversion significantly. The maximum conversion yields of TME is achievable at the optimum reaction conditions at the temperature of 140 °C, catalyst (SnCl2) dosage of 0.8 wt.% and the TMP: acid molar ratio of 3.2:1.
3.1.2. Effect of the reaction temperature The reaction rate and conversion of TME affected significantly by the temperature. The range of reaction temperature was varied from 120 °C to 160 °C. Fig. 3 shows that the reaction rate increases with the increase of reaction temperature. The reaction rate is slow at 120 °C and the conversion was 75.4% at 120 °C after 5 h of reaction. It is observed that reaction rate and conversion increase with increasing of reaction temperature that varies from 120 °C to 140 °C (cf. Fig. 3). However, when the reaction temperature reaches 150 °C, the vaporization of much xylene (boiling point 138.5 °C) causes of many bubbles. The
3.2. Analysis of products The structure the product was detected using an NMR spectroscopy (AVANCE 500 spectrometer, Bruker). Samples were dissolved in deuterated chloroform (CDCl3) with internal standard Tetramethyl Silen (TMS) at 25 °C, at 6 mg/ml concentration. Fig. 5 shows the 1H NMR spectra of TME. Based on information from the software ChemDraw, six methylene protons (–O–CH2–) exit in TME molecule. The Main protons in the molecule are listed below: 1H NMR (CDCl3, ™ ppm): 0.85-0.92 [d,
Fig. 3. Effect of reaction temperature on TME yield. Reaction condition: TMP/ CA molar ratio: 3.2:1; SnCl2/CA: 8 wt.%.
Fig. 5. 1H NMR spectra of TME. 3
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
convection heat transfer. In addition, low viscosity will promote the insulating oil flow into the narrow passage in the windings to prevent local overheating. The low viscosity of insulating oil will accelerate the initial impregnation of insulating paper. Therefore, the low viscosity of insulating oil is important for the transformer. The viscosity of TME is about 50% below FR3®, which means the new insulating oil (TME) enhance the heat transfer of transformers. Insulating oils were mainly used to quench arc discharges and transfer heat dissipating heat. Therefore, For a comparison with FR3®, the relative lower viscosity means TME-immersed transformer has a longer lifetime and larger load capacity of power transformers. 3.3.2. Pour point The pour point of the insulating is just the temperature at which the oil can flow. The pour point reflects the low temperature fluidity of the insulating oil to a certain extent. To ensure that the transformer can work safely when it stops running and restarts, it is necessary to choose insulating oil with good low temperature fluidity. The pour point of TME is -45 °C. NE oils have higher pour point than TME typically in the range 29–32 K.
Fig. 6. FTIR spectra of TME.
12H, 3×(–CH3)], 1.56-1.65 [t, 6H, 3×(–CO–CH2–CH2)], 2.30 [t, 6H, 3×(–CO–CH2–)], 4.0 [s, 6H, 3×(–O–CH2–)] Fig. 6 shows the FTIR spectra of TME. The stretching of C]O causes an obvious peak exists at 1739 cm−1 the stretching of CO] (Soares et al., 2008). CeC(O)O]e stretching vibrations produce a peak at 1154 cm−1. The existence of the peak at 2856 cm−1 and 2928 cm−1 is attributed to the asymmetric vibration and symmetric stretching of the CH2 group, respectively.
3.3.3. Flash point The flammability of insulating oil concerns the safety of the transformer. The transformer may cause rapid temperature rise due to thermal failure or electrical failure, which may cause explosion burning and insulating oil leakage due to the relative low flash point of MO. The burning and leakage cause a significant threat to the ecological environment and social stability. Many accidents about MO filled transformer explosion have occurred in recent years. Therefore, the insulating oil with high flash point is important for the transformer. The insulating oil cycles in the transformer which is an enclosed tank. Therefore, it is prescribed to measure the flash point of the insulating oil by the closed cup method. The flash point was measured by the closed-cup method as standard GB/T 261-2008. The comparative values for different insulating fluids are shown in Table 2. It was observed that the closed-cup flash point of TME has more than about 78 °C compared to that of mineral insulating oil.
3.3. Physical and electrical properties The viscosity and pour point increase with the increase of molecular weight and branching. However, increasing molecular weight increases flash points. A medium chain saturated fatty acids (C6-C10 acids) replace the long chain fatty acids in NE insulating oil to decrease the viscosity and ensure a relatively high flash point. Moreover, most of the NE insulating oils are from camellia seed, soybean and rapeseed. These edible oils used as insulating oil burden the food supply. C6-C10 acids from coconut oil and castor oil are being on mass production stage and cannot burden the food supply. To eliminate the effect of ®-hydrogen, TMP was use to synthesis TME. The oxidation resistance of these esters was easy to predict due to the absence of double bonds and ®-hydrogen into the molecule. Table 2 shows the comparison of main properties MO, FR3® and TME.
3.3.4. Oxidation stability Pressure differential scanning calorimetry can measures the oxidation induction time (OIT) quickly and accurately in terms of exothermal effect (Tan et al., 2002; Xu et al., 2014). Xu et al. (2014) found that the OIT of NE insulating oil at 130 °C is greater than 15 min, which can guarantee the reliability of measurement based on ASTM D6186. Therefore the OIT was measured at 130 °C. A differential scanning calorimeter (DSCQ20 TA) measured the OIT of ester insulating oil. 5.0 ± 0.5 mg of ester oil was weighed into open aluminum pans. The isothermal temperature was programmed at 130 °C and oxygen (99.9 wt.%) was passed through the sample at 60 ml/min. The OIT of soybean oil and rapeseed oil are 85 and 94 min, respectively. The OIT of TME is 2275 min, which is much longer than that of soybean and rapeseed oil.
3.3.1. Viscosity The viscosity of insulating oil determines the heat transfer rate of transformers. The relative high viscosity of NE insulating oil cannot transfer the heat generated by the loss of the transformer, which limits the lifetime and load capacity of power transformers. Convection heat transfer is the most important way in cooling transformer. The low viscosity of the insulating obviously enhances the
3.3.5. Electrical properties The AC breakdown voltage (Vb) is the most critical and commonly used parameter for estimating the insulating performance of the insulating oil. The AC Vb was measured as the standard GB/T 507. The electrode gap and voltage rise rate are 2.5 mm and 2 kV/s, respectively. The AC Vb is the average of 7 measured data points. The breakdown voltages of synthetic ester (TME), NE insulating oil (FR3®), and MO at ambient temperature were shown in Table 2. AC breakdown voltage of TME is about 72.6 kV/2.5 mm, higher than (around 12%) that of MO and FR3®. The impurities (e.g. particles, water and gas bubbles) have a significant effect on the AC Vb of insulating oil (Wang et al., 2018). Therefore, the measured AC breakdown voltage of an insulating oil
Table 2 Comparison of main properties mineral, FR3® and TME oil. Property
MO
FR3®
TME
Density/g cm−3 (20 °C ASTM D-4052) Pour point/ºC (ASTM D 97) Acid value /mgKOH g−1 (ASTM D 974-2008) Fash point /ºC (GB/T 261-2008) Viscosity/mm2s−1 (40 °C ASTM D455) Tanδ/% (90 °C IEC 60247) Permittivity (90 °C IEC 60247) Resistivity/Ω cm (90 °C IEC 60247) AC Vb/kV (2.5 mm GB/T 507)
0.84 −30 0.03 170 9.7 0.03 2.2 1014 60
0.92 −21 0.04 316 34 3 2.8 1013 65
0.934 −45 0.02 248 23.3 2 3.2 1012 72.6
4
Industrial Crops & Products 142 (2019) 111834
K. Wang, et al.
performance ester insulating oil TME is expected to find wide applications in size reduced and high safety level electrical insulating equipment which requires good cooling efficiency, high oxidation stability, high flash point, etc. Acknowledgments The authors gratefully acknowledge the NSFC (51425702 and 51321063), 973 program (2015CB251003), Project 111 of the Ministry of Education, China (B08036), as well the support from Chongqing (CXTDX201601001) and the Opening Project of Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology (ammt2017A-3). References Ab Ghani, S., Muhamad, N.A., Noorden, Z.A., Zainuddin, H., Abu Bakar, N., Talib, M.A., 2017. Methods for improving the workability of natural ester insulating oils in power transformer applications: a review. Electr. Power Syst. Res. https://doi.org/10.1016/ j.epsr.2017.10.008. Abdelmalik, A.A., Abbott, A.P., Fothergill, J.C., Dodd, S., Harris, R.C., 2011. Synthesis of a base-stock for electrical insulating fluid based on palm kernel oil. Ind. Crops Prod. 33, 532–536. https://doi.org/10.1016/j.indcrop.2010.11.019. Adhvaryu, A., Erhan, S.Z., 2002. Epoxidized soybean oil as a potential source of hightemperature lubricants. Ind. Crops Prod. 15, 247–254. https://doi.org/10.1016/ S0926-6690(01)00120-0. Bertrand, Y., Lauzevis, P., 2013. Development of a low viscosity insulating liquid based on natural esters for distribution transformers. 22nd Int. Conf. Exhibit. Electr. Distrib. 1–4. https://doi.org/10.1049/cp.2013.0701. Fernandes, K.V., Papadaki, A., da Silva, J.A.C., Fernandez-Lafuente, R., Koutinas, A.A., Freire, D.M.G., 2018. Enzymatic esterification of palm fatty-acid distillate for the production of polyol esters with biolubricant properties. Ind. Crops Prod. 116, 90–96. https://doi.org/10.1016/j.indcrop.2018.02.058. Gryglewicz, S., Piechocki, W., Gryglewicz, G., 2003. Preparation of polyol esters based on vegetable and animal fats. Bioresour. Technol. 87, 35–39. https://doi.org/10.1016/ S0960-8524(02)00203-1. Hamid, H.A., Yunus, R., Rashid, U., Choong, T.S.Y., Ali, S., Syam, A.M., 2016. Synthesis of high oleic palm oil-based trimethylolpropane esters in a vacuum operated pulsed loop reactor. Fuel 166, 560–566. https://doi.org/10.1016/j.fuel.2015.11.022. Martins, M.A.G., 2010. Vegetable oils, an alternative to mineral oil for power transformers- experimental study of paper aging in vegetable oil versus mineral oil. IEEE Electr. Insul. Mag. 26, 7–13. https://doi.org/10.1109/MEI.2010.5599974. Oommen, T.V., 2002. Vegetable oils for liquid-filled transformers. IEEE Electr. Insul. Mag. 18, 6–11. https://doi.org/10.1109/57.981322. Oommen, T.V., Claiborne, C.C., Walsh, E.J., 1998. Introduction of a new fully biodegradable dielectric fluid, textile, fiber film. In: Indust. Tech. Conf. Charlotte, North California. pp. 1–4. https://doi.org/10.1109/TEXCON.1998.679223. Padmaja, K.V., Rao, B.V.S.K., Reddy, R.K., Bhaskar, P.S., Singh, A.K., Prasad, R.B.N., 2012. 10-Undecenoic acid-based polyol esters as potential lubricant base stocks. Ind. Crops Prod. 35, 237–240. https://doi.org/10.1016/j.indcrop.2011.07.005. Pinto, F., Martins, S., Gonçalves, M., Costa, P., Gulyurtlu, I., Alves, A., Mendes, B., 2013. Hydrogenation of rapeseed oil for production of liquid bio-chemicals. Appl. Energy 102, 272–282. https://doi.org/10.1016/j.apenergy.2012.04.008. Rafiq, M., Lv, Y.Z., Zhou, Y., Ma, K.B., Wang, W., Li, C.R., Wang, Q., 2015. Use of vegetable oils as transformer oils – a review. Renew. Sust. Energ. Rev. 52, 308–324. https://doi.org/10.1016/j.rser.2015.07.032. Sitorus, H.B.H., Setiabudy, R., Bismo, S., Beroual, A., 2016. Jatropha curcas methyl ester oil obtaining as vegetable insulating oil. IEEE Trans. Dielectr. Electr. Insul. 23, 2021–2028. https://doi.org/10.1109/TDEI.2016.7556474. Soares, I.P., Rezende, T.F., Silva, R.C., Castro, E.V.R., Fortes, I.C.P., 2008. Multivariate calibration by variable selection for blends of raw soybean oil/biodiesel from different sources using fourier transform infrared spectroscopy (FTIR) spectra data. Energy Fuels 22, 2079–2083. https://doi.org/10.1021/ef700531n. Tan, C.P., Che Man, Y.B., Selamat, J., Yusoff, M.S.A., 2002. Comparative studies of oxidative stability of edible oils by differential scanning calorimetry and oxidative stability index methods. Food Chem. 76, 385–389. https://doi.org/10.1016/S03088146(01)00272-2. Wang, K., Wang, F., Li, J., Zhao, Q., Wen, G., Zhang, T., 2018. Effect of metal particles on the electrical properties of mineral and natural ester oils. IEEE Trans. Dielectr. Electr. Insul. 25, 1621–1627. https://doi.org/10.1109/tdei.2018.006909. Xu, Y., Qian, S., Liu, Q., Wang, Z.D., 2014. Oxidation stability assessment of a vegetable transformer oil under thermal aging. IEEE Trans. Dielectr. Electr. Insul. 21, 683–692. https://doi.org/10.1109/TDEI.2013.004073.
Fig. 7. Comparisons of breakdown voltage (Vb) of insulating oils with different water content.
mostly represents the oil quality. As shown in Fig. 7, it is apparent that the MO is highly sensitive to the water, which is indicated that the AC Vb decreases rapidly with the increase of water content. The water content has little effect on AC Vb of FR3® before critical value (340 mg/ L). The AC Vb decreases rapidly with the increase of water content after the critical value. The effect of water content on AC Vb of TME oil is similar to that of FR3®. However, the critical value is 460 mg/L which is much larger than the critical value of FR3®. The rate of AC Vb decrease become smaller after critical value than that of FR3®. Free water and dissolved water are the two formations exist in the insulating oil. Hydrogen bonds are easy to form between polar and water molecule (Rafiq et al., 2015). Therefore, the weak polarity insulating oil (FR3® and TME) can dissolve more water than that of nonpolar MO. Therefore, AC breakdown voltages decrease rapidly as the increase of moisture contents (cf. Fig. 7). The permittivity of TME is 3.2 which is higher than that of FR3® (2.8), which means the water tolerance of TME is higher than that of NE insulating oil. This capability identified that TME would be able to absorb more moisture from insulating paper and thus extend the lifetime of the transformer filled with TME (Martins, 2010). 4. Conclusion This paper synthesized a high performance ester insulating oil TME from the transesterification of saturated fatty acids with TMP. The conversion yields of TME is maximized at catalyst (SnCl2) dosage of 0.8 wt.% and the TMP:acid molar ratio of 3.2:1 at 140 °C. With the designed purification process structured by washing with ultrapure water and absorption with molecular sieve, high performance TME oil was obtained. Comparing to the very short oxidation induction time (OIT = 85 min) for commercial FR3® ester insulating oil, the TME oil shows very much enhanced oxidation stability (OIT = 2275 min). The TME oil has low viscosity of 23.3 mm2s−1 and pour point of -45 °C, both are quite lower than those of FR3® oil. The water tolerance of AC breakdown voltage for TME oil is much better than that of FR3® oil (cf. Fig. 7). Furthermore, the AC breakdown voltage of TME is about 72.6 kV/2.5 mm, around 12% higher than that of MO and FR3®. The TME oil is suitable for high safety level applications because of its high flash point at 248 °C which is higher than that (170 °C) of MO. The high
5