Shelf-life of biodiesel by isothermal oxidation induction period at variable temperatures

Abstract

Biodiesel (fatty acid methyl esters [FAME]) is a renewable biomass-based diesel (BBD) fuel made from plant oils, animal fats and waste greases. One of the main disadvantages of biodiesel is its poor oxidative stability, which is caused by the presence of high concentrations of unsaturated FAME. When stored in fuel terminals, vehicle tanks and fuel systems, biodiesel can react with oxygen in ambient air, causing it to degrade, which can adversely affect its viscosity and ignition quality. The shelf-life (SL) of biodiesel is an important property that defines how long it can be stored at low temperatures. The objective of this work is to develop reliable mathematical models to estimate the SL of biodiesel at T = 25°C (298.15 K). This was done by measuring oxidation induction period with a Rancimat instrument (IP_R) at variable temperatures. The data were analyzed by linear regression to determine ln(IP_R) as a function of T (Model A) and T⁻¹ (Model B) for canola, palm and soybean oil FAME (CaME, PME and SME), methyl oleate (MeC18:1) and methyl linoleate (MeC18:2). Statistical analysis of the Model A and Model B type equations showed that all inferred equations were good fits of the experimental data (adjusted coefficients of determination, R² ≥ 0.985). The most dependable results were obtained from extrapolation of Model B type equations to predict the SL^B values. For CaME, PME, SME and MeC18:1, SL^B = 559.0, 1135, 378.3 and 4515 d were inferred. However, the reliability of SL^A (extrapolated from its Model A type equation) and SL^B values calculated for MeC18:2 (3.1 and 4.8 d) were questionable as estimates of its SL at 298.15 K.