Fig. 6 Comparison of shelf-life data (SLA) data
obtained from extrapolation of the Model A type equations to 25
°C (298.15 K) with shelf-life data [SL(1) and SL(3)] reported in
earlier studies (Dunn, 2008, 2020). Error bars for SLAdata are 95 % confidence intervals taken from Table 6. SeeFig. 1 for abbreviations
linear regression of IPR-T data and extrapolating the
resulting equations to T = 25 °C. However, the SL(1) data were obtained
from equations derived from experiments performed at three temperatures
in the ranges T = 90-110 °C (363.15-383.15 K) for CaME, PME, SME and T =
80-100 °C (353.15-373.15 K) for MeC18:2. In the present work,
SLA data were calculated from Model A type
equations derived from analysis of IPR-T data measured
at five or six measurement temperatures. As a consequence, variations
between SLA and SL(1) data from the earlier study were
observed. While SLA > SL(1) for CaME and
MeC18:2, the opposite trend was noted for PME, SME and MeC18:1. In the
cases where SLA > SL(1), deviations were
small, 200 h for CaME and 44 h for MeC18:2. In the remaining three
cases, deviations were larger with MeC18:1 demonstrating the largest
deviation (9,000 h), followed by PME (1,200 h) and SME (900 h).
Ranking the calculated SLA results in descending order
yielded the following:
MeC18:1 > PME > CaME > SME
> MeC18:2 (15)
This order generally agrees well with what would be expected based on
the degrees of unsaturation (DU) of PME, CaME, SME and MeC18:2, where DU
= 0.58, 1.28, 1.53 and 2.00 (calculated from results in Table
S1 in the supporting information). Predicting the shelf-life of FAME
generally does not correlate well with DU (Knothe, 2002) and this is
exemplified when considering that according to SLA,
MeC18:1 held the highest ranking above despite having DU = 1.00, a value
that was greater than the DU calculated for PME. Nevertheless, ranking
the SL(1) data reported earlier (Dunn, 2020) yielded similar results
with the remarkable exception that CaME [SL(1) = 2,560 h] and SME
(3,130 h) were switched in the rankings. The ranking from the earlier
work was curious since SME had a significantly higher total PUFAME
concentration (62.09 mass%) than CaME (26.77 %). Given the
aforementioned differences in the experimental method for acquiring and
analyzing IPR-T data to estimate SLA for
the present work and SL(1) in the earlier study, the change in the
ranking of CaME and SME suggests that the methodology can significantly
impact the outcomes of the analyses. Therefore, it was concluded that
the method employed in the present work (five to six
IPR-T data points over variable T-ranges) yielded more
reliable results than the method used in the earlier study, with respect
to the estimation of the shelf life of biodiesel at lower temperatures.
Two data points in Figure 6 were represented as ‘SL(3)’ results
reported in an earlier study (Dunn, 2008). This study was carried out
using a different instrument to measure the IP, which was generally
termed as the oil stability index (OSI). The experimental data were
measured taken at six temperatures in the range T = 60-80 °C
(333.15-353.15 K) for SME and eight temperatures in the range T = 60-95
°C (333.15-368.15 K) for MeC18:1. The SLA values for SME
and MeC18:1 were 12.9 and 5.0 times greater than their corresponding
SL(3) values. The most likely explanation for these poor comparisons was
the variation in instruments and experimental conditions employed to
measure the respective IP data for analysis.
The SLB data obtained from extrapolation of theModel B equations are presented graphically in Figure
7 . These data are presented in a separate figure mainly for convenience