Comparison of Biodiesel Production by a Supercritical Methanol Method from Microalgae Oil Using Solvent Extraction and Hydrothermal Liquefaction Processes

The production of biodiesel from bio-oils using two different processes was investigated. The bio-oils were produced via either freeze drying followed by extraction with chloroform/methanol solvents (solvent-extracted oil) or employing a subcritical (573 K, 10 MPa, and 30 min) hydrothermal liquefaction process (HTL oil). Both oils were derived from the diatome Fistulifera solaris JPCC DA0580 and were converted to fatty acid methyl esters (FAMEs) via transesterification/esterification reactions with supercritical methanol (SCM) at relatively mild conditions (593 K and 13 MPa). The impact of the reaction time (10–60 min), methanol/oil molar ratio (42:1 and 21:1), and water content (0–5 wt % based on the weight of the oil) on the FAME yield was investigated. FAME yields from HTL oil (75–80% after 30 min) were higher than corresponding experiments with solvent-extracted oil (48–64% after 30 min) under all investigated conditions. This was attributed to the HTL oil containing more free fatty acids (FFAs), in which SCM promotes the conversion of this oil to FAMEs. Decreasing the ratio of methanol/oil had little impact on the FAME yield from the HTL oil. Adding up to 2.5 wt % water (on the basis of the weight of the oil) to the oil-SCM mixture also had little impact on the FAME yield from either oil. However, adding 5.0 wt % water decreased the FAME yield from both oils, particularly from the solvent-extracted oil.