Application of thermogravimetric analysis method for the characterisation of products from triglycerides during biodiesel production

Abstract

In this present work, thermogravimetric analysis (TGA) was used to study the thermal degradation of a range of lipids and lipid-derived compounds associated with the production of biodiesel. Thereafter, the procedure was used to successfully quantify the compounds of three process streams from a biodiesel plant. Relevant organic chemicals involved in biodiesel production chemistry, including glycerol, oleic acid (fatty acid), palmitic acid (fatty acid), rapeseed oil (model triglyceride) and fatty acid methyl esters (FAMES) have been studied to determine their volatilisation/thermal degradation patterns. The developed method was then applied for the quantitative characterisation of three samples from a 3-stage biodiesel production plant, including two in-process samples and the final biodiesel product. The method was able to clearly distinguish between two main sets of compounds namely, early - mid volatiles (glycerol, fatty acids and fatty acid methyl esters) and late volatiles (incompletely converted and unreacted triglycerides). In addition, the FAMES in the industrial samples were extracted into petroleum ether and analysed by gas chromatography - mass spectrometry (GC/MS), with good agreement between the two analytical methods. For instance, GC/MS analysis showed that the three industrial samples contained 31.2 ± 0.1 wt%, 60.6 ± 0.2 wt% and 91 ± 0.53 wt% of FAMES, respectively. Similarly, the TGA method gave the FAMES contents of the three samples as 33.9 ± 0.4 wt%, 57.8 ± 0.2 wt% and 85.3 ± 0.52 wt%. This study shows that TGA is a fast and simple method for accurately monitoring the triglyceride conversion stages and the purity of the final product during biodiesel production, without the need for extensive sample preparations and expensive standard solutions.