Stabilising sunflower biodiesel with synthetic antioxidants

Abstract

The main objective of this project was to investigate the effect of synthetic antioxidants on the oxidation stability of sunflower biodiesel. Although the effect of antioxidants on the behaviour of edible plant oils has been widely investigated, less is known on the effectiveness, and possible synergistic combinations, of antioxidants on the stabilization of biodiesel also referred to as FAME (fatty acid methyl esters). For this project biodiesel was made at room temperature via the transesterification of sunflower oil with methanol in the presence of KOH as catalyst. Depending on the sunflower oil batch used, the ester content of the samples, as determined using GC-FID and 1H NMR analysis, ranged from 92 to 98 wt.% The progress of the transesterification reaction was monitored by 1H NMR and FTIR spectroscopy in addition to performing viscosity measurements. The oxidative stability was determined using the Rancimat method. The European Standard EN 14112 for the Rancimat method describes two procedures for determining the induction period. The automated method relies on finding the position of the peak in the second derivative of the conductivity vs. time curve while the manual method is based on the intersection of two tangents lines to the response curve. It was shown that the latter method can also be automated by a curve fitting approach based on a novel Rancimat response function. This analysis demonstrates that the induction period values determined by the two methods differ with the second derivative method returning slightly higher estimates for the induction period. Oxidation stability was investigated by stabilising the sunflower biodiesel using three different types of antioxidants and their combinations at a fixed dosage level of 0.15 wt.%. They included a hindered phenolic antioxidant, tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane (Anox 20), an amine-type antioxidant, poly(1,2-dihydro-2,2,4-trimethylquinoline) (Orox PK) and a phosphite-type antioxidant, tris(nonylphenyl) phosphite (Naugard P). When used alone, Anox 20 gave the highest stabilization factor followed by Orox PK and Naugard P. When used in binary or ternary formulations, no antagonistic effects were found. Anox 20 was the single most effective stabilizer of the three. However, synergistic improvement of stability was observed on partial substitution with Orox PK. The antioxidant Naugard P seemed to have no effect on the stabilization of sunflower biodiesel. The Rancimat induction period data was fitted with Scheff� polynomials. The optimum antioxidant mixture combination comprised Anox 20 and Orox PK in the mass ratio of ca.2:1. It was of interest to determine whether the Orox PK also synergises with other phenolic antioxidants. Therefore, combinations containing 33.3% Orox PK together with, butylated hydroxytoluene (BHT), tert-butylhydroxyquinone (TBHQ), 2,5-di-tert-butyl-1,4-dihydroxybenzene (DTBHQ), pyrogallol and propyl gallate were tested alone and compared to results obtained for the neat antioxidant on its own, keeping the total dosage level at 0.15%. All these antioxidants proved to be more effective stabilizers than Anox 20 in sunflower biodiesel. Only DTBHQ showed improved stabilization when used in combination with Orox PK. The antioxidants BHT, TBHQ, pyrogallol and propyl gallate showed decreased stabilization when combined with Orox PK.