Increasing gas prices, limited fossil fuel resources and U.S. dependence on foreign oil make research in alternative fuels a priority. The feasibility of producing fuels from microalgae is economically dependent on improvements in lipid productivity by the algae. The research presented here focuses on the development of a mathematical model to describe the biomass and lipid productivity in a continuously-operated photobioreactor (PBR) system.
Five different cell lysing methods were evaluated for the purpose of improving the methods of analysis of lipid synthesis. The two most promising methods were found to be mortar and pestle and organic solvent cell fractionation methods.
Four types of batch experiments were performed with Scenedesmus dimorphus to determine key reactor model parameters: maximum cell growth rate (µmax), yield (Yx/s), and Monod constant (Ks). Two of the experiments were performed with varying initial sodium nitrate concentrations, for the purpose of more accurately obtaining the Monod parameters. The data was analyzed using three methods: differential/linear least-square, initial substrate and nonlinear. Nearly all of the results had non-reliable error results because most data yielded an ill-conditioned Covariance Matrix. Based on the results obtained by initial substrate method, the Ks was determined to be about 0.005 +/ 0.01 g/L, the maximum growth rate to be 0.7+/- 0.1 day-1 and the yield to range between 1.2 and 2.7 gcell/gsubstrate.
The values found in this research, although preliminary, were used to formulate an approximate steady-state model of a two- PBR system, with first reactor used for maximizing biomass and utilizing substrate, and the second reactor for accumulating lipids. The fed substrate concentration and the dilution rate of the first reactor were estimated to be (So and D here) 1g/L and 0.65 day-1 for optimal biomass productivity. The dilution rate obtained for the second reactor suggests that the volumes ratio of the 2nd to the 1st reactor is about 3.25. A large second reactor would have a negative economic impact. Based on the results obtained here it is suggested that a concentration step be implemented between the first algae growth reactor and the second lipid accumulation reactor.
The growth and substrate kinetics along with experimental measurements of growth, nitrogen yield, and extraction methods are anticipated to help further microalgae based fuel Research and Development and thus become the foundation for scale-up of algae-based systems.