DEVELOPMENT OF WASTE COOKING OIL METHYL ESTERAS
POTENTIAL ELECTRICAL INSULATING FLUID FOR POWER
TRANSFORMER
Imran Sutan Chairul, Norazhar Abu Bakar, Md Nazri Othman, Sharin Ab Ghani
and Muhammad Nazori Deraman
High Voltage Engineering Research Laboratory, Centre for Robotics and Industrial Automation, Fakulti Kejuruteraan Elektrik, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka, Malaysia
Email: [email protected]
ABSTRACT
Due to toxicity and non-biodegradability of petroleum-derived mineral insulating oil, the use of vegetable-based oils such natural esters as insulating liquid is on grow. Although natural esters have higher flash point compared to mineral insulating oil, its high viscosity is not suitable for existing distribution transformer with natural cooling system. Thus, low-viscosity esters derived from various vegetable-based oils have been developed. In this study, waste cooking oil methyl ester (WCOME) is proposed as potential low-viscosity insulating fluid for transformer. Waste cooking oil (WCO) is cheaper vegetable-based oil relative to crude vegetable oil. It is also abundantly available as 50,000 tonnes were reported being produced in Malaysia each year. WCOME is produced via catalysed transesterification reaction between WCO and methanol usingpotassium hydroxide (KOH). The physical (density, flash point, pour point, viscosity), chemical (water content, acidity) and electrical (breakdown voltage) properties of WCOME are presented and discussed. Results indicated that transesterification reaction produced a low viscosity WCOME fluid (14.19 mm2/s) that possessed a dielectric breakdown voltage (BdV) of 30 kV, which is 50% above the IEEE C57.147 BdV’s requirement for new natural ester fluids. Hence, the WCOME has a potential to be used as electrical insulating liquid for transformer.
Keywords: breakdown voltage, low viscosity fluid, waste cooking oil, transesterification, waste cooking oil methyl ester.
INTRODUCTION
Natural ester insulating (NEI) fluids are derived from seed oils [1]-[3], therefore the properties of NEI oils are reflects to the fatty acids composition of the NEI based-oil. NEI oils have good dielectric properties [2], [4], [5] and can extend the life of cellulose material [6]-[8] but it’sviscosity is higher compared to mineral insulating oil. Thus, NEI’s required modification to lower its viscosity such as through chemical modification [3]for application upon existing operated distribution transformer.
Several researchers had proposed esterification reaction method to reduce the viscosity of an ester oil. P. Thomas et al. [9] developed a dielectric liquid from karanji oil. The oil was esterified with methanol and refluxed for 8 to10 hours using a base catalyst. Methyl ester of karanji oil (MEKO) produced from this esterification was then refined by passing it through alumina and earth material. As results, MEKO properties obtained were 65 kV for electric strength and 8 mm2/s for kinematic viscosity. On the other hand, Y. Bertrand and P. Lauzevis [10]produced a low viscosity insulating liquid (LVIL) for distribution transformers by using a mixture of oleic rapeseed oil, fatty mono-esters and 0.3% 2, 6-ditert-butyl-p-cresol (DBPC). The fatty mono-esters were produced via transesterification reaction between rapeseed oil and 2-ethyl-1-hexanol. The breakdown voltage and kinematic viscosity of the developed LVIL are 74 kV and 17 mm2/s respectively. Alternatively, A. A. Abdelmalik et al. [11] synthesized a palm kernal oil epoxy methyl ester (PKOEME) from an epoxidized palm kernel oil through base catalyzed transesterification reaction. The break
down voltage of the produced PKOEME insulating oil is 42.58 kV while its kinematic viscosity is 6.14 mm2/s. Other than that, H. B. H. Sitorus et al. [12] produced jatropha curcas methyl ester oil (JMEO) through neutralization and alkali base catalyzed esterification reaction followed by water treatment process. The average AC breakdown voltage and kinematic viscosity for JMEO are 87 kV and 10.45 mm2/s. respectively. Other researchers, M.C. Menkiti et al.[13] produced a modified terminalia catappa kernel oil (MTCKOc) through acid catalyzed esterification followed by base catalyzed transesterification reaction. As for oxidation prevention, acetic acid (AA) and citric acid (CA) additives were added into MTCKOc, thus creating two more varieties of insulating liquid; (1) MTCKOc blended with 0.2% AA is labeled as MTCKOd while (2) MTCKOc blended with 0.15% AA and CA is named MTCKOe. The dielectric strength for MTCKOc, MTCKOd and MTCKOe are 33.95, 41.08 and 48.55 kV respectively while the viscosity is 14.1, 11.84 and 10.29 mm2/s in the same order. The physicochemical and electrical properties of the reported modified ester oils is tabulated as in Table-1.
WASTE COOKING OIL
WCO is obtained after edible vegetable oils were used several times for frying. WCO is selected for this study because it is 2 to 3 times cheaper [14] compared to crude vegetable-based oil [15], which could reduce the
processing cost. WCO is also abundantly available since it is estimated that 50, 000 tonnes of WCO [16] were being produced in Malaysia annually while European countries generates approximately up to 10 million tonnes of WCO each year [17].
Table-1. Properties of modified ester oils.
Reference [9] [10] [11] [12] [13] [13] [13]
Property Unit MEKO LVIL PKOEME JMEO MTCKOc MTCKOd MTCKOe
Density g/cm3 0.870 0.890 - 0.896 0.852 0.840 0.840
Flash point °C 295 175 - 191 255 265 275
Pour point °C - -30 -10 0 -5 -5 -5
Viscosity mm2/s 8 17 6.14 10.45 14.1 11.84 10.29
Water content mg/kg - - - 64.91 1 0.9 0.9
Acidity mg KOH/g - 0.12 - 0.0708 0.857 0.845 0.844
Breakdown voltage kV 65 74 42.58 87 33.95 41.08 48.55
The availability of WCO are due to increment of food consumption which parallel to the growth of human population [17]. In addition, the use of WCO could mitigate the issues of "competing for food with mankind" and "excessive logging of primitive forest for planting crops"[18] that raised while using edible and non-edible vegetable-based oils respectively.
Environmentally, waste cooking oil discarded into drain can possibly pollute the water resources and clogged the sewage system. Thus, by utilizing waste cooking oil, it could reduce water pollution [19] and reduce the cost of treating sewage [20].
TRANSESTERIFICATION
Transesterification reaction is a process to separate fatty acids of triglyceride (vegetable-based oil) that attached to a glycerol, thus forming fatty acid esters and glycerol. A molecular representation of a trans-esterification reaction is shown in Figure-1 [21]. The fatty acids of triglyceride are represented as R1, R2 and R3.
Figure-1. Molecular representation of a transesterification reaction [21].
Several techniques of transesterification reaction via catalyst have been developed [22], [23]. Basically, two (2) type of catalyst are used, either alkalior acid. The selection of an effective technique for transesterification reaction is depends on the percentage of free fatty acids (%FFA) of vegetable-based oil used. This is because an alkali catalyzed transesterification is effective if %FFA is less than 5% [22]Otherwise, the acid and solid catalyzed
transesterification should be applied because these techniques are independent of % FFA. Other than choosing an effective technique, there are four (4) parameters that should be controlled [23] to ensure high yield of the produced esters. The parameters are (1) ratio of alcohol to triglyceride, (2) catalyst concentration (wt%), (3) reaction temperature (°C) and (4) reaction time (hours).
A. V. Tomasevic and S. S. Siler-Marinkovic [19] obtained methyl esters from used frying oils through homogeneous transesterification reaction via alkali catalyst. It is found that, at 1wt% of potassium hydroxide (KOH), reaction temperature at 25 °C, molar ratio of 6 to 1 between methanol and used frying oil as well as 30 minutes of reaction temperature; the used frying oils were sufficiently transesterified. It is also found that methyl esters have much lower viscosities than the used frying oils. On the other hand, Y. Wang et al. [24] transesterified WCO producing methyl esters via acid catalyst. The methyl esters yield was 90% at 4 wt% of sulphuric acid, molar ratio of 20 to 1 between methanol and WCO, 10 hours and 95 °C of reaction time and temperature respectively. Alternatively, F. H. Alhassan et al. [25] obtained 96.5% methyl ester yield via transesterification of used frying oil using solid acid catalyst. The optimum reaction parameters are 3 wt% of ferric–manganese doped sulphated zirconia nanoparticle solid acid catalyst, 20 to 1 of molar ratio between methanol and used frying oil, 3 hours of reaction time and 180 °C of reaction temperature. Additionally, the reaction was done at agitation speed of 600rpm.
It can be seen that alkali catalyst transesterification required small amount of catalyst, low ratio of methanol to oil, low reaction temperature as well as low reaction time compared to acid and solid acid transesterification reaction provided that the %FFA of oil under investigation is less than 5%. Therefore, in this study, WCO with less than 5% of %FFA will be selected
CH2-O-CO-R1
CH-O-CO-R2 3 ROH
CH2-O-CO-R3
+ Catalyst
CH2-OH
CH-OH
CH2-OH
+
R-O-CO-R1
R-O-CO-R2
R-O-CO-R3
whereas transesterification reaction via alkali catalyst will be adapted.
METHODOLOGY
Methodology adapted in this study is summarized in a flow chart, as shown in Figure-2.
Sample preparation of waste cooking oil
Initially, a liter of WCO was filtered through a filter paper via filtration process. The filter paper pore size is 0.2µm while the %FFA of the WCO is 1.41%; less than 5%. %FFA can be calculated via WCO's acidity by using the following equation (1) [26]:
(1)
Then, the filtered WCO were heated at 120 °C for 10 minutes to reduce its water content. The low %FFA and reduction of water content in WCO are necessary to ensure an effective alkali catalyzed transesterification reaction [22]. After that, the WCO were cooled down to 60 °C prior to transesterification reaction.
Transesterification reaction
In this study, transesterification reaction using alkali catalyst is adapted. Initially, 7g of KOH were dissolved into 250ml of methanol in a 500ml erlenmeyer flask forming a potassium methoxide solution. Next, the cooled down WCO at 60 °C and the potassium methoxide solution were mixed in a 2L beaker. The mixture then was stirred for 5 minutes. After that, the 2L beaker were wrapped in aluminium foil and stored in a thermos for 48 hours so that the transesterification reaction could be continued. After 48 hours of reaction, there were 2 layers of liquid being formed in the 2L beaker. Top layer was waste cooking oil methyl esters (WCOME) whereas bottom layer was glycerol. There were also excessive methanol and KOH in both layers.
Figure-2. Flow chart of the methodology adopted in this study.
As methanol and KOH are soluble in water, the top layer WCOME were washed using hot water (>90°C) to discard the excessive methanol and KOH from WCOME. The washing process were repeated six times. Next, the washed WCOME were heated at 120 °C for 10 minutes to reduce its water content followed by being cooled down to room temperature. The cooled WCOME properties were then assessed according to IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers (IEEE C57.147).
99 . 1 %FFA Acidity
Start
Selection of waste cooking oil (WCO): % FFA < 5%
Transesterification reaction using alkali catalyst: potassium hydroxide (KOH)
Physicochemical properties measurement: (1) density, (2) flash point, (3) pour point, (4)
viscosity and (5) acidity
< 200 ppm?
Water content measurement
Breakdown voltage measurement
Water treatment
process Waste cooking oil methyl ester
(WCOME)
Physicochemical and electrical properties
Physical properties (density, flash point, pour point and viscosity)
Density, flash point, pour point and viscosity of the oil samples were measured based on ISO 12185, ISO 2719, IEC 3016 and ASTM D7042 respectively at a laboratory that accredited to ISO/IEC 17025. All measurements were done in the best knowledge of the personal concern by following very stringent procedures. Superior then verified the measurement results as such they were undeniable and disputed.
Chemical properties (acidity and water content) Acidity or total acid number (TAN) was measured based on the amount of potassium hydroxide (in mg) required to neutralize hydrogen ions (H+) in 1 g of oil sample. Acidity for all oil samples was measured in accordance with ASTM D974 [27] by using 848 Titrino Plus (Metrohm). Water content of oil sample was determined using a coulometer based on the Karl Fischer titration method. The method is based on the oxidation of sulphur dioxide by iodine in methanolic hydroxide solution. Measurement of water content in oil was done based on ASTM D1533[28] by using899 Karl Fischer coulometer (Metrohm).
Electrical property (breakdown voltage)
Breakdown voltage (BdV)of oil sample was measured according to ASTM D1816[29] by usingOTS60PB Portable Oil Tester (Megger). Two (2) semi-spherically capped VDE electrodeswith a gap distance of 1.0 mm was used for the measurement. The voltage was increased gradually at a rate of 0.5 kV/s until breakdown occurs. The minimum volume of oil sample is 350 ml. In this study, fives (5) BdV measurements were performed on each oil samples.
Water treatment process
A low value of BdV primarily indicates the existence of contaminants[30]such water in a liquid. Thus, a water treatment process on the liquid is necessary so that the measured BdV will reflected the liquid’s ability to withstand electric stress. Hence, in this study, water treatment process on WCOME was done by bubbling nitrogen gas (N2) [31]into WCOME for 30 minutes.
RESULTS
Results in this study are focusing on the appearances, physicochemical and electrical properties of WCO and WCOME. The results were assessed according to the IEEE C57.147.
Appearances
Figure 3(a) and 3(c) show colour of WCO and WCOME respectively. Notice that the colour of WCO is dark brown while WCOME is light brown. On the other hand, Figure-3(b) shows 2 layer of liquids. The top layer is WCOME whereas bottom layer is glycerol with both layers consist of KOH and excessive methanol.
Density
Density of WCO and WCOME are 0.9136 g/cm3 and 0.8966 g/cm3 respectively as shown in Figure-4. Both values satisfy the density requirement as per IEEE C57. 147 which is the density should be less than or equal to 0.96 g/cm3.
Flash point
Figure-5 shows the flash point of WCO and WCOME. Flash point of WCO is 269 °C whereas the WCOME is 184 °C. Although these values do not satisfy the flash point requirement as per IEEE C57.147 which is the flash point should be more than or equal to 275 °C; but both values are above 145 °C; fulfilling the flash point requirement based on ASTM D3487.
Pour point
Both pour point of WCO and WCOME are -28 °C as shown in Figure-6. These values satisfy the pour point requirement as per IEEE C57. 147 which is the pour point should be less than or equal to -10 °C.
Viscosity
Figure-7 shows the viscosity at 40 °C of WCO and WCOME the viscosity of WCO is 40.84mm2/s whereas WCOME is 14.19 mm2/s. Hence, both values satisfies the viscosity at 40 °C requirement as per IEEE C57.147 which is should be less than or equal to 50 mm2/s.
Acidity
Acidity of WCO is 2.7972 mgKOH/g whereas WCOME has a lower acidity of 0.2561 mgKOH/g as shown in Figure-8. It is notice that both values do not satisfies the acidity requirement of new natural ester fluids as per IEEE C57. 147 which is the acidity should be less than or equal to 0.06 mg KOH/g.
Water content
Figure-9 shows the water content of WCO, WCOME and WCOME (after water treatment). The water content of WCO is 1,036.5 ppm whereas WCOME is 257.6 ppm. On the other hand, the water content of WCOME (after water treatment) is 156.4 ppm. As the water content’s requirement as per IEEE C57.147 is should be less than or equal to 200 ppm; it is notice that only WCOME (after water treatment) fulfilled the water content limits.
Breakdown voltage
DISCUSSIONS
The results obtained from the measurements on physicochemical and electrical properties of WCO and WCOME are summarized in Table-2.
As WCO was exposed to repeated frying and oxidation processes, the by-product such dissolved decay product from these processes can be indicated by darker colour such dark brown as shown in Figure-3(a).
WCO and WCOME is a vegetable-based oils, hence its composition normally consists five (5) main types of chains.
(a) (b) (c)
Figure-3. (a) WCO, (b) layer of raw WCOME (top) and glycerol (bottom), (c) WCOME.
Figure-4. Density (g/cm3) of WCO and WCOME.
Figure-5. Flash point (°C) of WCO and WCOME.
Figure-6. Pour point (°C) of WCO and WCOME.
The chains are palmitic, stearic, oleic, linoleic, and linolenic [23]. The unsaturated (double bond) alkyl chains esters such oleic and linoleic acids correlates well with the cold flow performance [32]. Therefore, the WCO and WCOME has low pour point.
0.885 0.89 0.895 0.9 0.905 0.91 0.915
WCO WCOME
Den
sity
(
g
/cm
3 )
0 50 100 150 200 250 300
WCO WCOME
Flas
h
p
o
in
t
(°
C
)
-30 -25 -20 -15 -10 -5 0
WCO WCOME
P
o
u
r
p
o
in
t (
°C
)
Table-2. Physicochemical and electrical properties of WCO and WCOME samples.
Property Unit
Specification for Mineral Insulating Oil
[33], [34]
Specification for Natural Ester Fluids
[30], [35]
WCO WCOME
Density g/cm3 ≤ 0.91 ≤ 0.96 0.9136 0.8966
Flash point °C ≥ 145 ≥ 275 269 184
Pour point °C ≤ -40 ≤ -10 -28 -28
Viscosity mm2/s ≤ 12 ≤ 50 40.84 14.19
Acidity mg KOH/g ≤ 0.03 ≤ 0.06 2.7972 0.2561
Water content mg/kg ≤ 35 ≤ 200 1036.5 156.4a
Breakdown voltage kV ≥ 20 ≥ 20 7 30a
a
: after water treatment
Figure-7. Viscosity at 40 °C(mm2/s) of WCO and WCOME.
Figure-8. Acidity (mg KOH/g) of WCO and WCOME.
Figure-9. Water content (ppm) of WCO, WCOME and WCOME (after water treatment).
Figure-10. Breakdown voltage (kV) of WCO and WCOME (after water treatment).
According to F. Ma and M. A. Hanna[36], transesterification is a sequential reaction. Initially, triglycerides are reduced to diglycerides followed by the diglycerides are reduced to monoglycerides. Finally, the monoglycerides are reduced to fatty acid esters. In this study, the viscosity of WCOME (fatty acid esters) is lower than WCO (triglycerides) by 62.25%. The low viscosity of WCOME might be resulting from the decrement of ester chain length [37] because the glycerol of triglycerides have been discarded thus only fatty acid esters remained. On the other hand, it is expected that the 0
10 20 30 40 50
WCO WCOME
Vis
co
sity
at
4
0
°
C
(m
m
2/s
)
0 0.5 1 1.5 2 2.5 3
WCO WCOME
A
cid
ity
(
m
g
KOH/g
)
0 200 400 600 800 1000 1200
WCO WCOME WCOME
(after water treatment)
W
ater
co
n
ten
t
(p
p
m
)
0 10 20 30 40
WCO WCOME (after water treatment)
B
rea
k
d
o
w
n
v
o
ltag
e
(k
WCO show higher viscosity due to oxidation process that has changed its composition [38].
It can be seen that the acidity difference between WCO and WCOME is 2.5411 mgKOH/g, which corresponds to 90.84% difference due to transesterification reaction. According to IEEE C57.147, it is found that the acidity of WCOME (2.7972 mg KOH/g) is higher than the prescribed limit for new natural ester fluids which is should be less than or equal to 0.06 mg KOH/g. Fortunately, the acidity of WCOME is within suggested limits for continued use of in-service natural ester fluids in oil-immersed transformer which is should be less than or equal to 0.30 mg KOH/g[30].
It is notable that the water content of WCO is 1,036.5 ppm. On the other hand, WCOME's water content is 257.6 ppm, a reduction of 75.15% relative to the water content of WCO. Alas, both oil samples' water content is higher than 200 ppm; a maximum limit for new natural ester fluids [30]. Thus, water content of WCOME were then being reduced via water treatment process producing WCOME (after water treatment) with 156.4 ppm of water content, a value below 200 ppm.
The transesterification reaction and water treatment process have resulting in higher BdV of WCOME (after water treatment): 30 kV compared to BdV of WCO: 7 kV). The low BdV of WCO is expected because the WCO has high water content (1,036.5 ppm). As for WCOME (after water treatment), 30kV is 50% higher than the minimum dielectric breakdown voltage required for new natural ester fluids which is 20 kV[30]. Thus, the finding shows the advantage of transesterification reaction in enhancing the dielectric breakdown voltage of oil.
CONCLUSIONS
WCOME was obtained viaalkali catalysed transesterification reaction between methanol and WCO using KOH. The reaction resulting in change of oil colour; from dark brown (WCO) to light brown (WCOME). It is also found that the reaction has resulting to a lower physicochemical property (density, flash point, pour point, viscosity, acidity and water content) of WCOME relative to WCO; the original based-oil used in the reaction. Water treatment process has resulting the WCOME (after water treatment) to have a water content of 156.4 ppm, an acceptable value according to IEEE C57.147. Hence, as the water content is low, it is found that the BdV of WCOME (after water treatment) is 30 kV, 50% more than the minimum requirement as per IEEE C57.106.Based on the results obtained in this study, it can be suggested that the WCOME has a potential to be insulating fluids for transformers since it helps tackle issues related to high viscosity of aged NEI oils – however, much work is needed to reduce the acidity value of WCOME.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support provided by the Ministry of Higher Education Malaysia (MoHE) and Universiti Teknikal Malaysia
Melaka (UTeM) under the grant:(FRGS/1/2017/TK04/ FKE-CERIA/F00332). The authors amiably thank Ms Nur Lidiya Muhammad Ridzuan and Ms Siti Nadzirah Binti Norhan from Faculty of Electrical Engineering, UTeM, Malaysia for providing assistance on the preparation and measurements of the materials used in this study.
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