Shelf-life Results from Extrapolation of Models A and B Equations
Thermal degradation at temperatures above 60 °C can alter the mechanism of oxidation due to the solubility of oxygen decreasing as temperature increases (Frankel, 2005b). Measurement of the IPR works by the evolution of volatile organic acids from the heated oil which can be swept by the continuous flow or air into a test tube containing deionized water (Frankel, 2005c). The water-organic acid mixture is continuously monitored by a conductivity probe and increases are recorded as a function of time. The IPR is then calculated at the onset point where conductivity increases exponentially. Decreasing the measurement temperature hinders the formation of volatile organic acids. Therefore, elevated temperatures (60-140 °C) are necessary to generate them for the analysis of IPR. This condition is the main driving force for estimating the shelf-life of fatty derivatives such as biodiesel by extrapolating IPR-T data to lower temperatures.
Model A and Model B type equations obtained for the five FAME studied herein were extrapolated to estimate shelf-life data at T = 25 °C (298.15 K). These results are summarized under columns showing calculated data for Model A (SLA) andModel B (SLB) in Tables 6 and7 . The general procedure for both model types was to first calculate ln(IPA) or ln(IPB) using either Eq. 2 or Eq. 4, then take the inverse natural logarithm to obtain the SLA or SLB results for each FAME. Also presented in the tables are confidence intervals calculated at 95 % confidence levels.
Table 6 Shelf-life of FAME at T = 25 °C (298.15 K) calculated from Model A type equations (SLA)