Effect of molecular distillation temperature on palmitoleic acid
enrichment
Molecular distillation can be used for the separation of mixtures with
different molecular weight and partial vapor pressure, and for the
separation of homologs (Zhang et al., 2018). Compounds with different
mean free paths are separated under vacuum condition, which decreases
the evaporation temperature and the residence time (Solaesa et al.,
2016; Zhang et al., 2018). Table 2 shows that the oleic acid in the
liquid fraction 2 was 45.06% of the total fatty acids, which was
approximately the same as the proportion of palmitoleic acid. Oleic acid
is similar in structure to palmitoleic acid, with the former having two
more carbons but with both containing one double bond. These two fatty
acids are difficult to separate by crystallization. Thus, molecular
distillation was applied to enrich palmitoleic acid in the liquid
fraction 2.
The chain length and the number of double bonds of a fatty acid affect
its boiling point, and the distillation order is palmitoleic acid
> oleic acid ≈ linoleic acid (Zhang et al., 2013). Thus,
oleic acid is expected to remain in the residue while palmitoleic acid
would be collected in the distillate under a proper temperature. In a
study by (Solaesa et al., 2016), saturated fatty acids such as myristic
acid and palmitic acid were evaporated in the distillate at 125 °C under
a vacuum level of 10−3 mbar. Thus, considering the
fatty acid composition difference in the feedstock in this study,
evaporation temperature was first set at 120 °C to investigate the
effect of separation. Unfortunately, this led to almost all products
going in the distillate. At 95 °C, some separation was achieved, but the
yield of the distillate was 17.79%.
Table 3 shows that as the temperature was increased, a greater yield
(about 65%) of palmitoleic acid was obtained. At 100 °C to 110 °C, the
purity of the distillate was ~54%, while it was 54.18%
at 100 °C with a yield of 63.64%. Accordingly, 100 °C was the best
temperature for distillation. Thus, by combining the above steps of
crystallization and distillation, a 54.18% pure liquid of palmitoleic
acid was obtained, in an overall yield of 56.31%.