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%.