Performance and optimization studies of oil extraction from Nannochloropsis spp. and Scenedesmus obliquus

Abstract

The amount of water removed from harvested algae and the required quantity of solvent for efficient oil recovery impact the overall economic viability of transforming algae to bioenergy. Thus, in this study, the effect of total solids (TS) content and solvent-to-biomass ratio on oil recovery from Nannochloropsis oculata and Nannochloropsis salina (both referred to in this work as Nannocloropsis spp. and representative of marine species of algae) and Scenedesmus obliquus (a freshwater specie) was studied using modified Bligh & Dyer (B-D) method and Soxhlet extraction (with either hexane or ethanol as solvent). Adopting the modified B-D method, a maximum oil content exists between 10 and 20 wt% TS. Oil recovered with ethanol (with no pretreatment) and hexane (on acid hydrolyzed algae) with thimble increases with both TS (varied from 10 to 20 wt%) and solvent-to-biomass ratio (varied from 10 to 25). Conversely, oil yield increases with solvent-to-biomass ratio but reduces with TS due to enhanced mixing and mass transfer, increase in partition coefficient of lipids during downstream separation at lower TS, and implementation of hexane extraction without thimble. Hexane extraction was subsequently performed on acid hydrolyzed algae in a scale-up model and a 98% performance was achieved while about 60% of algal oil at the optimum data point was transformed to biodiesel with no significant impact by the solvent-to-biomass ratio or scale-up. Ethanol recovers mostly polar lipids and potentially forms an azeotrope with water which makes green diesel production less favorable and solvent recoverability challenging, respectively while the use of a large quantity of chloroform is unsafe. Thus, ethanol extraction and modified Bligh-Dyer methods were not performed in the scale-up model. Results from the application of our techno-economic analysis (TEA) model indicate that predicted sale price of Hydrotreated Algal Oil (HTAO) at each of the oil extraction data points decreases with increase in the oil content of algae, using a base case of 10000 barrel per day of HTAO with credits from animal feed and nutraceutical production. At the optimum data point, the predicted sale price of HTAO is $8.59 per gallon.