The MAT of SCKO was carried out in methanol/oil mole ratios of 3, 6, 9, 12, and 15, along with KOH catalyst concentrations of 0.3, 0.6, 0.9, 1.2, and 1.5%, as well as microwave power values of 100, 200, 300, 400, and 500 W. The reaction time was either 1, 2, 3, 4, or 5 min. All experiments were designed based on changing one variable at a time and keeping the rest of the variables constant at their center point (i.e. microwave power of 300 W, methanol/oil mole ratio of 9, catalyst concentration of 0.9%, and reaction time of 3 min). Effects of each variable on the weight efficiency, purity, and final efficiency of the production of fatty acid methyl esters (FAME) were investigated and the optimal level for each parameter was determined. Eq. (1) was used for determining the amount of oil in each test.

A known quantity of catalyst (KOH) was initially dissolved in methanol and the resultant solution was then added to the SCKO. The reaction was carried out in a microwave oven (Samsung, 2450 MHz, model ME3410W), equipped with a condenser. The reaction was captured immediately by immersing the glass reactor in an ice bath. As the reaction was stopped, the product was kept in a separating funnel overnight, when biodiesel was separated from glycerol. The crude FAME remained in the upper phase, while the catalyst and unreacted methanol were situated in the lower glycerol phase, meaning that small amounts of catalyst, methanol, and glycerol were present in the upper phase (Dehghan et al., 2021). Excess methanol in the methyl ester phase was evaporated by a magnetic stirrer equipped with a condenser at 80 ^ºC for 30 min at 600 rpm (Azcan and Danisman, 2008). After separating the biodiesel phase, it was further washed with water to ensure a complete removal of glycerol, catalyst, and other contaminants. Then, a magnetic stirrer at the agitation speed of 400 rpm for 1 h was used to remove the remaining moisture at 60 °C (Alishahi et al., 2021). To determine the purity of the FAME, methyl laurate was used as internal standard. The weight efficiency, purity, and final efficiency of the resultant FAME were determined according to the following equations:

The kinematic viscosity, refractive index, and density of biodiesel were measured according to the guidelines of the American Society for Testing Materials (ASTM; D445), the AOCS Cc7-25 Official Method, and the AOCS 1a-64 Official Method, respectively (AOCS, 2000; ASTM, 2013; Golmakani et al., 2022). The fatty acid (FA) composition and color attributes of biodiesel were evaluated using a method described by Dehghan et al. (2019).

The kinematic viscosity, refractive index, density, fatty acid composition, and color attributes of the resultant FAME, produced with different transesterification methods, were measured according to section 2.2.2.

Cloud, flash, fire, and pour points were calculated according to the American Society for Testing Materials (ASTM, 2013). Also, a laser thermometer (TM-939, Lutron, Taiwan) was used for measuring the temperature.

The amounts of energy used in each step of the reactions per transesterification method, separation of methanol, washing, and drying were monitored using a digital electric energy meter (a watt-hour meter) at the entrance of the electrical power supply. The amount of energy (power consumption (W)) was determined and then multiplied by time to get the total energy consumption (Wh) (Eq. (5)) (Motasemi and Ani, 2012). The energy consumed in all stages was added together and by applying Eq. (6), the amount of energy consumed to produce 1 g of FAME was calculated as the relative energy consumption.

For producing 1 kWh of energy, 800 g of CO₂ entered the environment. CO₂ production and relative CO₂ production were measured according to the following equations:

Figure 1a shows variations in weight efficiency, purity, and final efficiency with respect to the reaction time. Within the first 4 min, the weight efficiency, purity, and final efficiency increased with the extension of the transesterification time. After 4 min (microwave power of 300 W, methanol/oil mole ratio of 9, and catalyst concentration of 0.9%), the weight efficiency, purity, and final efficiency decreased. Thus, 4 min was considered as an optimal reaction time. At the beginning of the process, due to the non-uniform distribution of methanol in the oil, the reaction was slow. However, the reaction rate increased with time. This means that in the initial stages, while the reactants had the least contact with the microwaves, FAME production was low in purity and efficiency (Sajjadi et al., 2014). Prolonging the reaction time above the optimal one led to a decrease in weight efficiency, purity, and final efficiency from several angles. Over time, the reversibility of the transesterification reaction caused an increase in the solubility of glycerol and the reaction slightly changed in the reverse direction, thereby resulting in by-products and reducing the production of FAME. Also, the long reaction time caused the reactants to overheat. Methanol evaporated from the reaction medium after reaching the boiling point and reduced the efficiency of FAME production. In addition, by increasing the reaction time, the costs related to the amount of energy required to carry out the reaction will also increase (Chen et al., 2012; Leung et al., 2010; Patil et al., 2011). Similarly, Azkan and Yilmaz (2013) reported the effects of reaction time on the final efficiency of FAME production from waste from frying oil.

Figure 1b shows variations in weight efficiency, purity, and final efficiency with respect to microwave power. The weight efficiency, purity, and final efficiency increased in response to the increase in microwave power up to 300 W (reaction time of 3 min, methanol/oil mole ratio of 9, and catalyst concentration of 0.9%). Maximum weight efficiency, purity, and final efficiency reached 97.14, 75.26, and 73.11%, respectively, when biodiesel production from the SCKO operated at 300 W. This can be justified by the fact that the increase in microwave power accelerated the electromagnetic wave transfer through the molecular components of the mixture and their energy spread at a higher rate within the reactant mixture. Thus, the final efficiency increased. However, by increasing the microwave power to above 300 W, the reactant mixture and the structure of organic compounds became susceptible to damage. Triglycerides broke down and were converted to FFA. An excessive increase in power rendered an intense and chaotic interaction between molecules, thereby reducing the formation rate of the final, intended product. Similarly, Zu et al. (2009) produced biodiesel using yellow horn (Xanthoceras sorbifolia Bunge.) oil and reported that at irradiation power of 500 W the highest efficiency was achieved in 6 min. However, when the power of 700 W was applied, the conversion efficiency of FAME began declining, because different raw materials have different appropriate irradiation power.

Figure 1c shows the variations in weight efficiency, purity, and final efficiency with respect to the methanol/oil mole ratio. The weight efficiency, purity, and final efficiency increased in response to the increase in methanol/oil mole ratio from 3 to 12 (reaction time of 3 min, microwave power of 300 W, and catalyst concentration of 0.9%). However, the weight efficiency, purity, and final efficiency decreased when the methanol/oil mole ratio increased from 12 to 15. Thus, the methanol/oil mole ratio of 12 was considered optimal. The highest weight efficiency, purity, and final efficiency of FAME (98.90, 80.52, and 79.63%, respectively) were obtained at the mole ratio of 12. An excessive increase in methanol in several aspects reduced its production efficiency to esterify the triglycerides to FAME. Since methanol is highly capable of absorbing microwaves, increasing its ratio causes a higher absorption of waves when sufficient amounts of methanol exist. The temperature of the reaction mixture increased with less intensity (Lin et al., 2014), while excess methanol made catalyst separation difficult at the end of the reaction. Increasing the methanol/oil mole ratio beyond a certain value increased the glycerol solubility and led to foam formation, thereby lowering the efficiency (Sharma et al., 2019). In addition, an excess of glycerol drove the shifted equilibrium towards the reactants, and, thus, lowered the efficiency of biodiesel conversion when glycerol remained in the solution (Mahlinda et al., 2017). In a similar study, Zhang et al. (2010) produced biodiesel using yellow horn oil and reported that the transesterification could be accelerated by increasing the amounts of methanol. The high mole ratio of methanol to oil could enhance the conversion efficiency of FAME. On the other hand, excessive methanol amounts reduced the concentrations of catalyst and reactant, which retarded the reaction and aggravated the recovery of the solvents.

Figure 1d shows variations in weight efficiency, purity, and final efficiency with respect to catalyst concentration. Weight efficiency, purity, and final efficiency increased when the amount of catalyst concentration increased from 0.3 to 1.2% (reaction time of 3 min, microwave power of 300 W, and methanol/oil mole ratio of 9). A higher catalyst concentration caused proper physical contact between the reactants that led to an increase in purity and final efficiency. Since increasing the catalyst concentration from 1.2 to 1.5% had no considerable impact on the efficiency and purity of FAME, the catalyst concentration of 1.2% was considered optimal. High amounts of alkaline catalyst increased the possibility of soap formation, which caused an emulsion to form between soap and water molecules. This emulsion entraps FAME and makes their separation difficult, so that some of them remain unrecovered (Atapour and Kariminia, 2011). An excessive increase in the catalyst concentration increased the kinematic viscosity of the mixture, created a gel, and led to problems in separating the glycerol phase (Sajjadi et al., 2014). In a similar study, Sharma et al. (2019) produced biodiesel using waste cotton-seed cooking oil, and reported that the excessive amount of heterogeneous catalyst increased washing time and decreased the formation of biodiesel as the reactant mixture became more viscous and thus, increased resistance to mass transfer. A suitable amount of catalyst reduces catalyst waste and avoids pollution in bodies of water.

Table 2 shows variations in the fatty acid composition of SCKO FAME with respect to different MAT variables. It seems that different microwave conditions had no significant effect on the transesterification of different fatty acids in terms of chain length and saturation degree. Thus, strong similarities existed between the percentages of fatty acids in the FAME produced under different MAT conditions.

Table 3 shows variations in the physical properties of SCKO FAME with respect to different MAT variables. The kinematic viscosity of SCKO was measured as 28.16 mm²/s in this study. The kinematic viscosity of the final FAME should be 1.9-6.0 centistokes (mm²/s), according to the ASTM 6751 (American Standard) and should be 3.5-5.0 centistokes (mm²/s), according to the EN 14214 (European standard) (Kantikar et al., 2011). As can be seen, there were significant differences among the viscosities of FAME produced at different microwave reaction times. Samples exposed to 1- and 2-min reaction times were more than 6.0 mm²/s and were outside the aforementioned limits. In contrast, other reaction times were within the permissible range of the defined standards. The best result (i.e. the lowest kinematic viscosity of 3.78 mm²/s) was obtained after 4 min of reaction time. By increasing the reaction time and increasing the purity of the produced FAME (i.e. decreasing the molecular weight), the process of kinematic viscosity changes declined and reached the lowest value after 4 min, but with a further increase in the reaction time and due to the purity reduction and an increase in the molecular weight, the kinematic viscosity increased. There was a negative correlation between kinematic viscosity and final efficiency (kinematic viscosity = (-0.069 × final efficiency) + 9.11, R² = 0.93 for reaction time; kinematic viscosity = (-0.086 × final efficiency) + 9.90, R² = 0.90 for microwave power; kinematic viscosity = (-0.11 × final efficiency) + 11.72, R² = 0.91 for mole ratio; kinematic viscosity = (-0.43 × final efficiency) + 30.30, R² = 0.98 for catalyst concentration).

The density of SCKO was 869.6 kg/m³ and increased after transesterification (Table 3). According to the EN standard, the density of FAME at 15 ^°C should be in the range of 860-900 kg/m³. All FAME were within the permitted range of the EN standard. The unsaturation degree had no significant effect on the transesterification reaction of the produced FAME. However, due to the conversion of primary triglycerides to FAME, the molecular weight of the final product decreased, whereas the density increased compared to the primary SCKO. There was a significant positive correlation between the density and the final efficiency (density = (0.59 × final efficiency) + 853.16, R² = 0.91 for reaction time; density = (0.62 × final efficiency) + 849.66, R² = 0.90 for microwave power; density = (0.68 × final efficiency) + 844.67, R² = 0.94 for mole ratio; density = (0.22 × final efficiency) + 870.82, R² = 0.98 for catalyst concentration). Therefore, the highest density of 900.01 kg/m³ was obtained after 4 min microwave power of 300 W, which is consistent with the results of Talebian-Kiakalaieh et al. (2013) regarding the FAME density of several types of vegetable oils.

The refractive index of SCKO was 1.479 and decreased after transesterification (Table 3). The lowest refractive index (1.460) was acquired and a strong negative correlation existed between the refractive index and the final efficiency (refractive index = (-0.0002 × final efficiency) + 1.4725, R² = 0.90 for reaction time; refractive index = (-0.0002 × final efficiency) + 1.4774, R² = 0.90 for microwave power; refractive index = (-0.0002 × final efficiency) + 1.14, R² = 0.90 for mole ratio; refractive index = (-0.0002 × final efficiency) + 1.47, R² = 0.94 for catalyst concentration). The obtained results are consistent with the research of Azcan and Yilmaz (2013) They reported the refractive index of waste frying oil to be 1.4710 and the resulting FAME to be 1.4575.

The highest purity and efficiency of FAME production were reflected in L* (89.67), a* (-3.67), and b* (34.67) values (Table 3). Initially, increasing the microwave power decreased the a* value (an increase in greenness or a decrease in redness) and b* values (a decrease in yellowness), but after the optimal point, both a ^* and b ^* values increased.

The highest weight efficiency, purity, and final efficiency of FAME production from SCKO were 99.03, 82.21, and 81.41, respectively, using MAT at an operating power of 300 W, methanol/oil mole ratio of 12, catalyst concentration of 1.2%, and reaction time of 4 min (Figure 2a).

The purities of FAME produced by different transesterification methods are shown in Figure 2b. It can be clearly seen that the highest purity was obtained after 4, 10, 40, and 120 min of MAT (86.47%), OAT (70.30%), UBAT (57.72%), and MSAT (60.16%) methods, respectively.

Figure 2c compares the efficiency of different transesterification methods. The process of changes in the efficiency correlated significantly with changes in purity. The highest efficiencies of different transesterification methods were 85.52% after 4 min MAT, 69.60% after 4 min OAT, 62.38% after 10 min UPAT, 57.88% after 40 min UBAT, and 54.79% after 120 min MSAT. The highest efficiency in the MSAT method was obtained after 120 min (54.79%), but prolonging the duration further than 120 min caused a decrease in both purity and efficiency.

Table 4 shows variations in the fatty acid composition of SCKO FAME with respect to MAT, OAT, UPAT, UBAT, and MSAT methods. As can be seen, the FA composition of samples produced with different transesterification methods were similar. The transesterification method had no selective effect on the FA in terms of chain length or degree of saturation.

Viscosity. Table 5 shows variations in the physicochemical properties of the produced FAME with respect to different transesterification methods. Except for the samples produced with the MSAT method, the properties of other transesterification methods were within the permissible limits of ASTM and EN standards. Since the highest purity and weight efficiency of triglycerides to lower molecular weight FAME were observed in the MAT method, the lowest kinematic viscosity was also obtained in this method. Among other transesterification methods, OAT, UPAT, and UBAT showed lower kinematic viscosity values than that of the MSAT method.

Density. The density is influenced by the weight efficiency of triglycerides to FAME and increases by increasing the purity and decreasing the molecular weight. The MAT method showed the highest density (Table 5). There were no significant differences among the density of FAME produced by OAT, UPAT, and UBAT methods in their optimal conditions. The lowest density occurred as a result of the MSAT method. In addition, the densities of FAME that were produced using all transesterification methods were within the allowed range of the EN standard.

Refractive index. Although no significant differences were observed among the FAME produced by different transesterification methods, the refractive index of the samples decreased by increasing the FAME production from triglycerides (weight efficiency), which resulted from the direct relationship between the refractive index and the length of the carbon chain. The lowest amount of FAME production and the highest refractive index were observed in samples produced by the MSAT method, whereas the highest FAME production and the lowest refractive index were observed in samples produced by the MAT method.

Color attributes. The color characteristics of FAME obtained by different transesterification methods are listed in Table 5. Accordingly, FAME obtained from the MAT method were more visually transparent than the other transesterification methods. FAME produced by MAT had the lowest a* and b* values.

Table 6 shows variations in thermal properties of the produced FAME with respect to different transesterification methods. There were no significant differences among different transesterification methods in terms of ignition and fire points (combustion points), but they differed from each other in terms of the drop and cloud points. The lower pour point of FAME produced by MAT, compared to other transesterification methods, indicates that they remained liquid at a lower temperature and were pumped more easily. After MAT, FAME produced by OAT and UPAT had the lowest drop points, respectively. There were no significant differences between the drop points in the UBAT and MSAT methods. Our results are consistent with previous findings on the transesterification of soybean oil using MSAT and MAT methods. The pour point of the MAT method was -18 ^°C, whereas the pour point of the MSAT method was -9 ^°C (Kanitkar et al., 2011). The lower pour point in MAT can be justified by the complete progress of the transesterification reaction as well as a kinematic viscosity reduction of the produced FAME, compared to the MSAT method. Regarding the cloud point, the optimal conditions for producing FAME by the MAT, UPAT, and UBAT methods showed the lowest crystal formation, temperature and a cloudy state, respectively. The cloud points of the produced FAME are consistent with previous results by Supalakpaniya et al., which involved measuring the cloud point of the FAME produced by MAT from crude palm oil and resulted in a cloud point at -8 ^°C (Suppalakpanya et al., 2010).

Table 7 shows the equivalent of energy consumption in different biodiesel production methods. The highest and the lowest energy consumptions of the reaction steps were related to the MSAT (227 Wh) and MAT (20 Wh) methods, respectively. The energy consumption of the purification steps of MAT, MST, OAT, UPAT, and UBAT methods were almost equal. According to Table 7, the lowest amount of relative energy consumption was attributed to the MAT method, followed by UPAT, OAT, UBAT, and MSAT methods, respectively.

Also, the OAT method reduced the reaction time to some extent due to the homogeneous energy transfer. In the UPAT method, due to the direct effect of the waves on the reactants and its strong mixing effect, the reaction speed was high, and, as a result, the energy consumption was low (Motasemi and Ani, 2012). In addition, the MSAT method emitted the largest amount of CO₂ into the environment.