Enhancing biodiesel production from soybean oil using ultrasonics
Biodiesel, or fatty acid alkyl ester, is a non-toxic, biodegradable and renewable source of energy. It is mostly produced from vegetable oils. Other sources include animal fat, oils obtained from algae, fungi, bacteria, etc. Esterification of oil with alcohol in the presence of moderate temperature (60 yC) and a catalyst to produce fatty acid alkyl esters is known as transesterification. The transesterification process, facilitated by mechanical mixing, is commercially used to produce biodiesel.
In industry, biodiesel is characterized using proton nuclear magnetic resonance spectroscopy (1H NMR spectroscopy). In the first section of this study, thermogravimetric analysis (TGA) is demonstrated as a potential method for monitoring biodiesel production by transesterification of soybean oil with methanol. Soybean oil and commercially available biodiesel were mixed in varying proportions by weight as standards. In addition, mixtures of different biodiesel/soybean oil ratios were also created by periodically interrupting base-catalyzed transesterification of soybean oil with methanol. The mixtures produced by both approaches were analyzed with TGA and the results were then compared with analytical data obtained by NMR spectroscopy. It was found that results from both analytical methods were in good agreement ( ±5 %). Thus TGA is a simple, convenient, and economical method for monitoring biodiesel production for screening characterization.
In the second section of this study, the effect of ultrasonics on biodiesel production is studied by applying ultrasonics during transesterification reaction to the reactants, i.e., soybean oil and methanol, in the presence of sodium hydroxide as a catalyst. Ultrasonics are sound waves of a high frequency above the audible range of to humans. Ultrasonic energy was applied in two different modes: pulse and continuous sonication. Soybean oil was mixed with methanol and sodium hydroxide and was sonicated at three amplitudes (60 ympp, 120 ympp and 180 ympp) in pulse mode (5 s on/ 25 s off). In the continuous mode, the same reaction mixture was sonicated at 120 ympp for 15 s. The reaction was monitored for biodiesel yield by stopping the reaction at selected time intervals and analyzing the biodiesel content by TGA. The results were compared to a control group where the same reactant composition was allowed to react by mechanical stirring at 60 yC without ultrasonic treatment. It was observed that ultrasonic treatment resulted in a 96 % yield (percent conversion to biodiesel) in less than 90 s using the pulse mode; compared to 30 to 45 minutes for the control sample. In continuous sonication, the highest biodiesel conversion of 86 % was obtained at 15 s. It was also found that significantly less energy was used to produce biodiesel with ultrasonics compared to conventional heating and stirring.
Thus biodiesel can be produced at a faster rate and the high efficiency of commercial methods can be maintained by using ultrasonics.