Kinetics Study of the Solvent Extraction of Lipids from Chlorella vulgaris

The aim of this investigation was to reduce the chemical hazard and possibly the production time of biodiesel from microalgae by investigating the process of hexane lipid extraction. This extraction process is energy-intensive and time-consuming. Hexane, a toxic solvent, is traditionally used for microalgae lipid extractions. The goals of this project were to (1) evaluate the less hazardous heptane as a substitute for hexane and (2) determine the kinetics of the extraction process for hexane and heptane at different solvent volume to dry microalgae mass ratios (R, mL solvent/g dry microalgae). To accomplish the first goal, the project included growth, monitoring, and harvesting of Chlorella vulgaris microalgae in reverse osmosis water and analyzing the lipid extraction using hexane and heptane. The second goal was accomplished by proposing a kinetics model of lipid extraction, measuring the hexane and heptane extracted lipids as function of the extraction time and fitting the data to the kinetic model. Using a spectrophotometer, it was determined that hexane and heptane extract similar material. For both solvents, the extraction process followed a first order model: τ dY/dt = (K-Y), where Y is the grams of lipid extracted per 100 gram dry microalgae, K is the maximum gram of lipid which can be extracted in the process per 100 gram dry microalgae, t is the extraction time (min), and τ is the time constant for the process (min). A smaller τ implies a faster extraction process, while a higher K indicates a higher lipid extraction yield. It was found that for hexane with R = 5, K = 2.75 and τ = 10. Increasing R to 30 amplifies K to 3.90 and τ to 20. For heptane with R = 5, K = 1.80 and τ = 2. When R was increased to 30, K rose to 2.61 and τ to 7. For a given R, heptane extraction is faster, but hexane extraction has a higher maximum yield.