Abstract
The aim of this study was to develop a microalgal cultivation system for intracellular lipid production and a solid catalytic process for biodiesel production. The motile intensity of microalgae, which was strongly related to specific growth rate, was used as the index for selecting a suitable cultivation medium. It was observed that the color of microalgae changed from green in the growth phase to yellow in the stationary phase during cultivation porcesses, mainly due to the changes of the biochemical composition in the microalgae. The color was analyzed by image analysis and an RGB (Red, Green, and Blue) model was developed to correlate the change in color to chlorophyll a and lipid contents of microalgae. The model was used to predict the chlorophyll a and lipid contents of microalgae and the experimental results were well correlated by the model (R2 > 0.94). This colorimetric method was more suitable in monitoring chlorophyll a and lipid contents in the microalgae as compared to the standard analytical methods. The culture condition for lipid production differs from that of cell growth; it is possible to separate the growth and the production phases into two separate stages. The maximum lipid accumulation occurs under the nitrogen-limited growth. A two-stage cultivation strategy was developed. The microalgae was cultivated in a nitrogen-sufficient phase for cell growth and transferred into a nitrogen-deficient phase to induce the lipid production. The production of lipid obtained from the two-stage process was 2.82-fold higher than that from the traditional single stage cultivation. Furthermore, a solid catalytic process for converting microalgal lipid into biodiesel was developed by using soybean oil as the source. Fatty acids obtained from enzymatic hydrolysis of soybean oil were esterified with methanol by using strong acidic cation-exchange resin as a heterogeneous catalyst. The kinetic study of esterification was carried out with different levels of catalyst loading, reaction temperature and molar ratio of methanol to fatty acids. The experimental data can be well-correlated by a second order kinetic equation. The kinetic model was used to predict the optimal operating conditions at a fatty acids conversion of 99%. The optimal operating conditions numerically calculated by the kinetic model were found to be reactants molar ratio of 14.9, reaction temperature of 99oC and reaction time of 9.5 h. The predicted results were verified with experimental results and the relative error was less than 0.5%.