Monitoring of the methanolysis reaction for biodiesel production by off-line and on-line refractive index and speed of sound measurements

•The refractive index of the alcoholic phase is very sensitive to the oil conversion.•The speed of propagation of sound in the reaction mixture is sensitive as well.•Both physical properties increase with the oil conversion mainly due to the glycerol.•Good linear relationships have been found under wide ranges of operating conditions.•Real-time monitoring using the speed of sound is feasible and first reported here.

Two techniques based on the measurements of the refractive index (nD) of the alcoholic phase at 20 °C (nD,AL20) and the speed of propagation of sound (us  ) in the reaction mixture are presented for monitoring the methanolysis of vegetable oils. Application of these techniques for the real-time monitoring of the synthesis of biodiesel is described for the first time. Size exclusion chromatography has been used as reference method. Both nD,AL20 and us increase linearly with the oil conversion due to the contribution of the glycerol formed. Good correlations have been found whose coefficients depend on the initial methanol/oil ratio and in the case of us also on the temperature Due to flow instabilities in the refractometer chamber, monitoring based on nD has been possible only off-line whereas the measurement of us allowed an effective, fast, accurate and cheap real-time monitoring of the process.

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