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Vegetable oil refinery waste containing acid oil is used as an inexpensive feedstock for producing biodiesel by microwave-assisted esterification (MAE) method. Effects of some main variables such as free fatty acid:methanol molar ratio (1:1, 1:5, and 1:10), reaction time (5, 30, and 60 min), and catalyst concentration (1%, 2%, and 3%) on physicochemical properties of produced biodiesel are investigated. Optimum reaction conditions of MAE are free fatty acid:methanol molar ratio of 1:10, reaction time of 60 min, and a catalyst concentration of 3%, while having 95.79% conversion yield. By increasing the conversion yield of the biodiesel, density and color brightness increase, while viscosity and refractive index decrease. There are no significant differences between physicochemical and heating properties of biodiesel produced by MAE and magnetic stirrer esterification (MSE) methods. Meanwhile, energy consumption of MAE method is almost four times lower than that of MSE. MAE as a promising alternative to the conventional esterification method can be considered as an energy-efficient method for producing biodiesel from inexpensive vegetable oil refinery waste. Practical applications: Acid oil is an inexpensive by-product of alkali refining in vegetable oil plants that would pollute the environment if not rendered safely. In this study, MAE is used to convert acid oil to biodiesel as a practical process for bringing alkali refining waste into production cycle. Acid oil can provide a reduction in the cost of biodiesel production. In addition, application of energy-efficient MAE method can facilitate the economical production of biodiesel.

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The ohmic-assisted esterification method was compared and contrasted with the conventional esterification method for biodiesel (fatty acid methyl esters) production from vegetable oil refinery waste containing high free fatty acids. The reaction variables were free fatty acid:methanol molar ratio (1:1, 1:5, 1:10, and 1:15), catalyst concentration (1%, 2%, and 3%) and reaction time (5, 30, and 60 min). By increasing the conversion yield of free fatty acids to fatty acid methyl esters, density increased while viscosity and refractive index decreased. Optimum reaction conditions were a molar ratio of 1:10 and a catalyst concentration of 3% after 60 min of esterification reaction, while having a 95.74% conversion yield. There was no significant difference between fatty acid methyl esters produced with ohmic-assisted esterification and conventional esterification methods in terms of fatty acid profile, physicochemical and heating properties. Meanwhile, energy consumption by the conventional esterification method was about 25% higher than that of ohmic-assisted esterification. In fact, ohmic-assisted esterification can be considered as a green, cost-effective alternative method for the production of biodiesel from vegetable oil refinery waste.