Effect of crystallization time
Time has an influence on the mass transfer between the solid and liquid
phases in the crystallization process (Morales-Medina et al., 2016).
Thus, four crystallization times (4 h, 8 h, 12 h and 16 h) were
investigated with the SPOMFs to methanol ratio of 1:15 at -20 °C. It was
observed that the change of crystallization time had a significant
influence on the purity and yield of palmitoleic acid as shown in Figure
2c. Overall, the purity of palmitoleic acid increased with time, whereas
the yield decreased with time. When the crystallization time ranged from
4 to 16 h, the purity of palmitoleic acid ranged from 27.17% to
41.55%. There were only slight increases in palmitoleic acid purity
from 12 h to 16 h, suggesting that crystallization was almost complete
at 16 h. Longer time will lead to the crystallization of the palmitoleic
acid, reducing its yield significantly. Add what time is the
best….
Secondary crystallization of
liquid fraction 1
After completing the optimization of crystallization conditions at the
first stage, the fatty acid composition of liquid fraction 1 was
obtained under the optimal conditions. As shown in Table 2, the liquid
fraction 1 contained low contents of saturated fatty acids, with
palmitic acid being the most abundant, accounting for 9.37% of the
total fatty acids. It is important that the fatty acids having ≤16
carbons are removed as much as possible from the liquid fraction because
subsequent molecular distillation can only separate palmitoleic acid
from the C18 fatty acids, but cannot separate palmitoleic acid from
palmitic acid. However, it has been proven that multistage fractionation
can further improve the purity (Jin et al., 2017). Based on these
considerations, secondary crystallization of liquid fraction 1 was
performed to remove palmitic acid.
A liquid fraction 2 was collected after re-crystallization at -40 °C. As
shown in Table 2, the purity of palmitoleic acid in the liquid fraction
2 by secondary crystallization was 45.22%, with a yield of 95.49%. The
content of palmitic acid was significantly reduced, from 9.37% to
2.13% in this fraction. Therefore, re-crystallization led to a
significant increase of palmitoleic acid content in the liquid fraction,
from 40.55% to 45.22%. In addition, the oleic acid content in the
liquid fraction 1 significantly differed from that in the liquid
fraction 2, and no statistically significant differences were found in
the proportions of linoleic acid and linolenic acid in solution after
secondary crystallization. It should
be noted that these contents are relative percentages. If one fatty acid
is removed, others will tend to increase in relative percentage values.
Overall, secondary crystallization
significantly decreased the
proportion of saturated fatty acids in the liquid phase, and thus the
unsaturated fatty acid content in liquid fraction 2 reached 97.52%
after two steps of crystallization. Finally, we had obtained
sufficiently enriched material for further purification by molecular
distillation.