Optimization of biodiesel synthesis using ultrasound and mechanical stirring methods: a comparative study

Mechanical stirring (MS) and ultrasound (US) frequency are two key factors commonly used to reduce mass-transfer resistance during the reaction between methanol (MeOH) and triglycerides (TG), facilitating the efficient production of biodiesel (BD) from various feedstocks such as rapeseed oil and Jatropha curcas oil. In this study, a comparative model analysis has been conducted to evaluate the performance efficacy of optimum mechanical stirring (MS) and optimum ultrasound (US) frequency using rapeseed oil biodiesel production. Sensitivity and uncertainty analyses have been conducted using Latin hypercube sampling (LHS) and Partial Rank Correlation Coefficients (PRCCs) to identify the key kinetic and transport parameters influencing biodiesel yield. An optimal control framework has also been applied to regulate MS speed and US frequency over time to overcome initial mass-transfer resistance between oil and methanol. The influence of mixing intensity on biodiesel conversion has been examined at different temperatures and MeOH:oil molar ratios using mass-transfer correlations for both MS and US frequency. Numerical results show that, at a MeOH:TG molar ratio of 6:1 and temperature of 50 °C, optimal US frequency achieves a maximum biodiesel conversion of 97.67% within 40 min, whereas optimal MS attains 95.32% conversion after 60 min. The results further indicate that ultrasound provides faster mass-transfer enhancement and a superior production profile compared to mechanical stirring. This study addresses two key questions: whether rapeseed oil is a suitable feedstock for biodiesel production, and which mixing strategy, MS or US frequency, more effectively minimizes the mass-transfer resistance over time.