Abstract

Background: Microalgae are a promising feedstock to produce biofuels such as biodiesel, since their culture does not compete with food crops. In order to make a microalgae biodiesel process cost effective, a biodiesel production process, which consists in separating the microalgae unsaponified lipids (such as carotenoids) from the biodiesel produced, has been tested.

Methods: In this process, the 1st step is an alkali reaction (saponification) between microalgae lipids and potassium hydroxide (KOH) (1.26 mmol OH-/g methanol), followed by a 2nd acid reaction step (esterification) using sulphuric acid (H2SO4) (1.65 mmol H+/g methanol) with a total methanol to lipid ratio of 13 mL/g. Between these 2 steps, a solvent extraction (hexane) separates the unsaponified lipids from the reaction mixture.

Results: This 2-step process resulted in a FAME yield of 91 g FAME/kg dry biomass, a biodiesel purity of 260 mmol FAME/100 g biodiesel and an unsaponified lipid yield of 170 g lipid/kg dry biomass. The 2-step process (with a hexane separation between both steps) tested in this study achieved a higher FAME yield and a higher biodiesel purity compared to a lipid crystallization separation. When crystallization with hexane at 0°C was tested at the same biodiesel production conditions as the 2-step process, a maximum FAME yield of 35 g FAME/kg dry biomass with a biodiesel purity of 52 mmol FAME/100 g biodiesel were obtained.

Conclusion: Despite the fact that this 2-step process produces an important amount of salt (4.8 kg K2SO4/kg FAME), for the best operating conditions tested, in comparison to a conventional vegetable oil-based biodiesel obtained by 1-step alkali homogeneous catalytic processes (0.03 kg K2SO4/kg FAME), it is effective and simple, and could help biodiesel from microalgae be cost effective.

Keywords: Biodiesel, crystallization, solvent extraction, lipids, microalgae, purification.

Graphical Abstract