Fig. 7 Comparison of shelf-life data (SLB) data
obtained from extrapolation of the Model B type equations to 25
°C (298.15 K) with shelf-life data [SL(2)] reported in an earlier
study (Dunn, 2020). Error bars for SLB data are 95 %
confidence intervals taken from Table 7. See Fig. 1for abbreviations
and to avoid overcrowding the results presented in Figure 6 .
Presented alongside the SLB data are shelf-life data
obtained from analysis of pressurized-differential scanning calorimetry
(P-DSC) data reported in an earlier study (Dunn, 2020), which are
denoted as ‘SL(2)’. The mathematical procedures for estimating the SL(2)
data are explained in more detail in the earlier work.
The SLA and SL(2) data were most comparable for SME,
MeC18:1 and MeC18:2, where the deviations were −1,400, 2,400 and 37 h,
respectively. On the other hand, deviations for CaME and PME were much
larger (−31,100 and −12,200 h). Similar comparison of
SLB and SL(2) data suggested that the values may be
comparable for MeC18:2 (deviation = 78 h). Otherwise, the deviations for
CaME, PME, SME and MeC18:1 were larger (−20,500, 18,200, 5,500 and
99,700 h). Analogous results were observed from comparison of
SLB and SL(1) results, where deviations for MeC18:2 were
lowest (85 h), while those for CaME, PME, SME and MeC18:1 were larger
(10,800, 29,200, 5,950 and 88,300 h).
Ranking the SLB values in Table 7 in descending
order yielded the following results:
MeC18:1 > PME > CaME > SME
> MeC18:2 (16)
This ranking was identical to the ranking shown noted in Eq. 15 for
SLA. The calculated SLB of MeC18:1 was
between one and two orders in magnitude greater than the
SLB values for CaME, PME and SME, and three orders in
magnitude greater than the SLB of MeC18:2. Compared with
the SLA and SLB rankings in the present
work, the SL(2) data reported in (Dunn, 2020) exhibited some variations,
where CaME had the highest value, followed by PME, MeC18:1, SME and
MeC18:2. This ranking did not agree well with those shown in Eqs. 15 and
16 for SLA and SLB. It is possible that
thermal degradation occurred in the non-isothermal P-DSC scans conducted
for the earlier study (Dunn, 2020). These effects may have altered the
reaction kinetics as the P-DSC cell temperature was ramped to higher
temperatures.
Earlier, it was noted that SLB was consistently greater
than SLA for all five FAME studied in the present work.
For CaME, PME and MeC18:1, deviations between these two values were more
than one order in magnitude. There may be some question on the
reliability of SLA and/or SLB data as
estimates for the shelf-life of biodiesel at low temperatures. For
example, SLA = 2,760 h and SLB = 13,400
h were estimated for CaME (see Tables 6 and 7 ). These
data convert to 115 d (16.4 wk) and 558 d (79.8 wk). Between the two
time periods, the latter appears to be more realistic for CaME, which
has a low total PUFAME concentration (26.77 mass%), if it is stored in
a dark space absent from moisture. Similar arguments apply to
SLB results for PME (1,430 d [205 wk]; total PUFAME
= 9.7 %), SME (378 d [54.0 wk]; total PUFAME = 62.09 %) and
MeC18:1 (4,500 d [643 wk]; no PUFAME). Both sets of results could be
realistic for MeC18:2 (SLA: 3.12 d [0.45 wk];
SLB: 4.83 d [0.69 wk]; 99.0 % PUFAME). Given these
considerations, it is concluded that the extrapolation of Model
B type equations to lower temperatures yielded more realistic
shelf-life results (SLB) than similar processing ofModel A type equations (SLA).