Technical Feasibility of Biodiesel Production From Waste Cooking Oil: Comparison Between Electric Heating and Microwave Heating Process


 Base-catalyzed transesterification and conversion of waste cooking oil (WCO) into biodiesel is a renewable energy production technology with a wide range of applications. The most commonly used heating method is electric heating (EH). Microwave heating (MW) has the characteristics of high heat transfer efficiency and short preheating time, and has recently received attention in this field.This study compared effects of the alkali-catalyzed transesterification reaction of WCO under EH and MW processes. The maximum biodiesel yield of EH process appeared when the reaction temperature is 60 °C, the reaction time is 30 min, the molar ratio of alcohol to oil is 6:1, and the catalyst concentration is 1.0%, up to 93.4%. The maximum biodiesel yield obtained from MW technique is 80.66%, under the condition of 200W, 5min, 1wt. % KOH and the methanol/oil molar ratio of 9:1. The activation energy for CH and MW process are found to be 6 768 J·mol-1 and 503.4 J·mol-1, respectively. Microwave heating greatly reduced the activation energy of the reaction, as well as transesterification yield. Compared with other biodiesel producing process, EH process in this study has the advantages of high speed and low production cost, while biodiesel yield is slightly insufficient. This is likely due to the small amount of un-removed moisture contained in the WCO.


7
thermostat uses a column heating program: initial temperature 70 ° C, hold for 2 min, 10 ° C / 119 min to 180 ° C, then 5° C / min to 230 ° C, hold for 10 min; detector is FID, temperature is 120 260 ° C; hydrogen flow rate is 25 mL / min , The air flow rate is 300 mL / min. 121 The analytical internal standard for the quantitative determination was heptadecanoic methyl 122 ester. The peak areas of the internal standard and other esters were compared to quantify the 123 fatty acid methyl ester content in the biodiesel sample. The results were calculated according 124 to Eq.1. 125 (1) 126 Where C is the fatty acid methyl content (%);∑A is the total peak area; AEI is the peak area of 127 the heptadecanoic methyl ester; CEI is the concentration of heptadecanoic methyl ester in the 128 hexane (mg/L); VEI is the injection volume of heptadecanoic methyl ester solution (μL); m is 129 the mass of biodiesel sample (mg). 130

Kinetics study 131
In the transesterification reaction, stoichiometrically 1 mol of triglycerides (TG) reacts with 3 132 mol of MeOH to yield 3 mol of FAME and 1 mol of glycerol. The whole transesrification 133 reaction process can be written as Eq.2. 134 The geneal rate equation for the transesterification reaction can be expressed as Eq.3: 136 where t is reaction time, kr represents the rate content. 138 The rate equation can be rewritten as Eq.4: where t is reaction time, kr represents the rate content. 141 In the CH process, homogeneous base-catalyzed reaction, KOH is dissolved in methanol in 142 advance, and the effect of stirring makes the entire reaction system be regarded as a 143 homogeneous reaction. For the molar concentration of methanol is much higher than the 144 stoichiometric requirement during the whole reaction, the concentration of methanol can be 145 treated as a constant. And the second-order transesterification can be regarded as a pseudo-first 146 order reaction (Ramezani, Rowshanzamir et al. 2010, Zhang, Sheng et al. 2010. 147 The rate equation for a pseudo-first order model (n = 1) can be expressed as Eq.5: 148 As for the MW process, it has been reported that under certain microwave power conditions, 150 the reaction follows pseudo-second-order reaction kinetics (Yeong, Law et al. 2019 Excessive free fatty acids will react with the alkali catalyst KOH, and the water in the oil will 171 also promote the saponification reaction, reducing the yield of biodiesel produced by the 172 transesterification reaction. The threshold values of the acid value and moisture content of the 173 feedstock oil for homogeneous base-catalyzed transesterification are respectively 0.06% 174 (Canakci and Van Gerpen 1999) and 1% (2 mg KOH•g −1 ) (Maddikeri, Pandit et al. 2012). 175 According to Table 1, the acid value of all feedstocks is less than the threshold value moisture 176 content, which shows that the raw materials in the table are suitable for the production of 177 biodiesel by homogeneous base catalytic transesterification. Especially, after pretreatment, the 178 acid value of our WCO is 0.4 mg KOH•g −1 . It is possible to obtain a higher FAME yield 179 biodiesel in this study. 180 The density of the WCO is lower than other waste oils, and the saponification value is only 181 higher than that of Hekel fish oil, indicating that WCO has a longer carbon chain and higher 182 average molar mass. The low iodine value of WCO indicates that its degree of unsaturation is 183 low. The WCO has longer average carbon chain and lower degree of unsaturation, its biodiesel 184 would have a higher cetane number and strong oxidation stability. (

Biodiesel yield of EH process 195
In case of EH, the optimization of the factors including reaction temperature, reaction time, 196 catalyst amount and methanol/oil molar ratio. When the reaction time was in the range of 5-60min, the reaction was very fast in the first 5 212 min, then the process slowed down and tilled the maximum methyl content at 30min (Fig.2b). 213 To further increase the reaction time from 30 min to 60 min had no further positive effect on 214 the methyl content, even led to a small reduction. In this study, prolonging the reaction time 215 would slightly decrease the FAME yield, but previous studies had different results. Agarwal considering economy, 30min was still selected as the subsequent reaction. 223 When methanol content raising from 3:1 to 15:1, the FAME yield could remain above 75% 224 (Fig.2c). The improvement of methanol/oils molar ratio could drive the reaction process in the 225 forward direction. Besides, when methanol content gets higher in the system, the more 226 methanol can stay in the system when temperature increasing. But, extra methanol would 227 dissolve more glycerin, increasing the glycerol concentration in the reaction system. (Milano,228 Ong  RO stands for methanol/oil molar ratio; CA stands for catalyst amount. 248

Biodiesel yield of MW process 249
In case of MW, the optimization of the factors including microwave power, reaction time, 250 catalyst amount and methanol/oil molar ratio. Fig.3

Effect of methanol/oils molar ratio; (d) Effect of catalyst concentration. 260
The curves of MW process were very similar to those of the EH process, but there was 261 difference in value. The reaction time in MW technique was sustainably reduced, while the 262 amount of methanol required was higher. These was because of that microwave would affect 263 dipole rotation and ion migration, improve the thermal conductivity and convection current of the system temperature had already exceeded 80°C (Fig.4). Too high reaction temperature 269 made methanol boil violently, which seriously affected the condensation effect. Therefore, the 270 final yield was lower than EH process. 271 272

Fig.4 Temperature rising of the reaction system in MW process 273
Experimental yield data were given as an input of the analysis of variance (ANOVA) in table  274 3. According to the results, in MW process, the microwave power and reaction time would not 275 significantly affect the reaction yield. The significant terms influencing the biodiesel yield in 276 decreasing order are catalyst amount, and methanol/oil molar ratio. It was worth noting that the 277 probability value of catalyst amount were almost the same in EH process and MW process. 278

Kinetics Analysis 296
For CH and MW, the rate constants of the transesterification reaction under different 297 temperature or different microwave power were respectively determined by curve fitting the 298 reaction rate equations with the biodiesel yield. Performing kinetic calculations according to 299 2.5, the reaction rate constant of CH process was found to increase from 0.3479 min -1 to 0.4048 300 min -1 . The reaction rate constant at 60℃ was 0.3737. The reaction rate constant of MW process 301 increased from 3.623 min -1 to 9.965 min -1 . The reaction rate constant at 200W was 9.247 min -302 1 . It is worth noting that the reaction rate constant of the MW process reached the maximum 303 when the microwave power was 300W, and decreased at 500W. It is observed that the k value 304 was approximately 10 to 20 times higher respectively in the case MW process than that of CH. 305 The higher rate constant means a faster reaction rate, requiring lower activation energy. 306 A comparative study has been done on different biodiesel producing process as shown in table  316 6. The main advantage of homogeneous base-catalyzed transesterification is that the reaction 317 temperature is low, the reaction time is short, the catalyst and methanol are added in a small 318 amount, and the production cost can be saved. There is a large difference in FAME yield due 319 to the difference in raw materials. Compared with other research, this study still has the 320 advantages of high speed and low production cost, but FAME yield is slightly insufficient. This 321 is likely due to the small amount of un-removed moisture contained in the WCO. 322 The acid catalysed transesterification or two-step esterification-trans-esterification has a wider 323 application range, can adapt to the possible high free fatty acid content and high water content 324 of WCO, and can obtain high FAME yield under strong acid catalyzed conditions, but the 325 reaction conditions are harsh and the methanol consumption is larger. As compared to these 326 reported data, at present experiment work biodiesel yield 93% was obtained at 30min reaction 327 time, 60℃ reaction time and 6:1 methanol/ oil ratio for the composition and physical properties 328 of WCO in EH process. 329 The data in 3.2 shows that in this study, the transesterification yield of the MW process is lower 330 than that of the EH process. The possible reason is that methanol is more polar and has a strong 331 ability to absorb microwaves. The methanol in the system heats up rapidly, becoming a "hot 332 spot" in the reaction system, and rapidly evaporates above its boiling point, causing the 333 methanol concentration of the reaction system to drop, which affects subsequent 334 transesterification reactions proceed; while triglycerides are just the opposite. They are less 335 polar and have a weak microwave absorbing ability, which becomes a "cold spot" in the 336 reaction system, making the rate of reaction with methanol slower; in addition, with As the 337 reaction progresses, the polarity of the intermediate products and products of the reaction 338 system is smaller than the polarity of the initial reactants, which leads to the gradual decrease 339 of the thermal and non-thermal effects of the microwave. Compared with previous studies, the 340 transesterification yield of the MW process in this study is significantly lower (93.  The maximum FAME yield of CH process appeared at the reaction temperature is 60 °C, the 353 reaction time is 30 min, the molar ratio of alcohol to oil is 6:1, and the catalyst concentration 354 is 1.0%, reaches 93.4%. The maximum biodiesel yield obtained from MW technique is 80.66%, 355 under the condition of 200W, 5min, 1wt. % KOH and the methanol/oil molar ratio of 9:1. 356 Compared with the CH process, microwave heating can shorten the reaction time, but will 357 increase the consumption of methanol, and the final yield will also decrease. The best catalyst 358 percentages for CH process and MW process are the same. And one-way analysis of variance 359 results showed that the biodiesel yield was witnessed to be very sensitive to methanol/oil molar 360 ratio and catalyst weight % for both CH and MW techniques. 361 23 The activation energy of CH and MW were respectively determined by the pseudo-first-order 362 kinetic modeling and pseudo-second-order kinetic modeling. The activation energy for CH and 363 MW process are found to be 6 768 J·mol -1 and 503.4 J·mol -1 . Microwave heating greatly 364 reduced the activation energy of the reaction. However, the transesterification yield of the MW 365 process is lower than that of the CH process. 366 Compared with other biodiesel producing process, CH process in this study has the advantages 367 of high speed and low production cost, while FAME yield is slightly insufficient. This is likely 368 due to the small amount of un-removed moisture contained in the WCO.