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)