Centrifugal recovery of solvent after biphasic wet extraction of lipids from a concentrated slurry of Nannochloropsis sp biomass

Wet extraction of lipids from a microalgae slurry requires efficient recovery of the solvent. The use of a non-polar solvent such as hexane allows physical recovery of the solvent by centrifugation, avoiding energy intensive distillation of the aqueous slurry. However, the kinetics and mechanisms of centrifugal separation of solvent from microalgae are yet to be investigated. In this study, hexane was used to perform biphasic extraction of lipids from a concentrated slurry of ca 50% ruptured Nannochloropsis sp. cells. The hexane droplet size of the resulting emulsion and the viscosity of continuous phase were characterised. The slurry of ruptured cells was highly viscous and shear-thinning. The hexane droplets were < 50 mu m in diameter. The rate of separation of hexane during centrifugation at different centrifugal forces was monitored in real time using a centrifuge fitted with optics (LUMiFuge). It was concluded that the rate of hexane recovery was limited by coalescence rather than droplet migration (creaming). A step-change increase in the rate of hexane separation was found to occur at a critical force of approximately 550 g. This was attributed to the initiation and propagation of droplet-droplet coalescence within the emulsion phase above a critical compressive force. Despite the high viscosity of the continuous phase, droplet movement was not limiting, possibly due to collective migration of the hexane droplets. Due to coalescence rather than droplet migration being limiting, as well as the complex rheology of the microalgae slurry, it is not possible to confidently estimate the required capacity of large scale centrifuges from batch experiments performed at a laboratory scale.