Phospholipid transesterification in sub-/super-critical methanol with the presence of free fatty acids

Abstract

Phospholipids and free fatty acids (FFA), along with triglycerides, are naturally formed constituents in unrefined vegetable oils and other plant lipids. Presence of phospholipids and FFA in such oils can cause processing difficulties, such as saponification and decrease in catalytic efficiency, thus lead to an adverse process efficiency in the transesterification of such oils for biodiesel production. This phenomenon was also observed in our previous study on converting microalgal lipids to fatty acid methyl esters (FAME) via in situ transesterification. This study aimed at further exploring the transesterification of phospholipids and investigating the effects of FFA presence and other processing conditions in biodiesel production from plant oils in sub- and/or super-critical methanol (SubCM/SCM). Experiments were carried out in a batch reactor in SubCM/SCM under various conditions without addition of catalysts. Pure chemicals of lecithin and stearic acid were used as the model compounds for phospholipids and FFA, respectively. The product yield (FAME in mol%) of the phospholipids after transesterification, as affected by the presence of FFA under different conditions, was selected as the respond factor to determine the process efficiency. Experimental results showed that phospholipids can be converted into FAME in such a process. Transesterification of phospholipids is largely affected by the interactive effect of operating temperature and reaction time. The increase in product yield is proportional to the increases in temperature and/or reaction time. The maximum product yield of 68.1mol% was achieved at 250°C and 120min without the presence of FFA. The product yield started to level off once the system reached the SCM state. When temperature was held at 290°C for 30min, the product yield dropped to 33.6mol%. Another phenomenon observed is that the presence of FFA enhances considerably the lipid conversion. The study revealed that phospholipids can be converted to FAME with a highest product yield of 93.9mol% at 250°C for 120min in SCM without catalysts and with the presence of FFA. However, the FFA enhancement became less significant when the system was operated for a longer period of time than 120min.