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

Two techniques based on the measurements of the refractive index (nD) of the alcoholic phase at 20 °C ( n D , AL 20 ) 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 n D , AL 20 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.