CONCLUSIONS
The shelf-life of biodiesel is an important property that determines how
long it can be stored at low temperatures at fuel terminals and in
vehicle tanks and fuel systems. The present work was organized with the
objective of developing mathematical equations for estimating the
shelf-life of biodiesel at T = 25 °C (298.15 K). To meet this objective,
two types of models, Model A and Model B , were
obtained from regression analysis where ln(IPR) was
expressed as a linear function of T and T−1 (Eqs. 1
and 3). While the experimental IPR data were acquired
with measurement temperatures in the range 50-140 °C (323.15-413.15 K),
the shelf-life data (SLA and SLB) were
calculated by extrapolating the model equations to T = 298.15 K (Eqs. 2
and 4).
Both Model A and Model B type equations demonstrated
good correlation between ln(IPR) and T (R² ≥ 0.985) and
T−1 (R² ≥ 0.989) and predicting IPAand IPB values within corresponding measurement
temperature ranges for the five FAME (CaME, PME, PME, MeC18:1 and
MeC18:2) studied in this work. Confirmation analysis of these results
showed that calculated IPB data from the Model
B equations were more accurate than IPA (fromModel A equations), with respect to measured
IPR data. Four of the five FAME exhibited lower MAD and
RMSD values for calculated results from the Model B type
equations (exception: MeC18:1).
The shelf-life results from extrapolation of the model equations showed
that results from Model B (SLB) were
consistently greater than results from Model A(SLA) equations. The results for CaME, PME, MeC18:1 and
MeC18:2 demonstrated that SLB values exceeded
SLA values by an order in magnitude. While Model
A is traditionally used to estimate shelf-life data in the fats and
oils industry, this work showed that extrapolating Model B type
equations appeared to yield more realistic shelf-life estimates for
CaME, PME, SME and MeC18:1. The estimated shelf-life results for MeC18:2
(SLA = 74.8 h and SLB = 116 h) seemed
too low for realistic storage at T = 298.15 K.
Future studies on this topic might include acquiring more experimental
data for Ea and Z0 factors for FAME
mixtures under conditions similar to those used to measure
IPR at variable temperatures. Acquiring such data would
be helpful in the development of kinetic modeling approaches to estimate
shelf-life data for biodiesel at low temperatures. For example, an
earlier study (Dunn, 2020) suggested that the autocatalysis model f(α) =
α½(1 − α) may work well for FAME mixtures that are
high in MeC18:1 concentration (Dunn, 2020).