Relating speed of sound and echo amplitude with biodiesel manufacture

Biodiesel has a potential not completely exploited as substitute of diesel oil, despite it is known as biodegradable, nontoxic, and has a low-emission profile. The aim of this paper is to propose an ultrasonic method as a tool for real-time monitoring transesterification reactions by using low-power ultrasound based on a pulse/echo scheme. Two catalytic routes were tested, both using degummed soybean oil as source and KOH as catalyser. The difference between catalytic routes was the type of alcohol used (methanol and ethanol). Three different reaction times were studied: 10, 20, and 30 min. Low power (less than 100 mW) ultrasonic time-gated bursts were applied to the reactional media at 1 MHz. The eco was measured during the reaction, and small amounts of the reaction media were collected every 2 min for chemical analysis. Time of flight and echo waveform amplitude were the ultrasonic parameter of interest. The amplitude of the ultrasonic waveform was related with the variation of the ethanol-transesterification reaction rate and could help in qualitatively monitoring that reaction. Furthermore, density measurements where consistent with the variations observed on time of flight, confirming the possibility of monitoring the reaction using ultrasonic parameters. The behaviour of the ultrasonic parameters varied depending on the alcohol and the presence or not of heating during the reaction. As a common rule, the stabilization of the ultrasonic parameters indicates that the maximum yield for each catalytic route is achieved, indicating that the reaction should be stopped for maximum energy efficiency. Results disclose that the proposed method is a feasible way to monitor the reactions of biodiesel during its production, in real time, and with relatively low-cost equipment. Further development shall be done to use it in industrial scale, mainly regarding the ultrasound power and frequency to be employed.