Downstream processing of microalgal biodiesel production

Crop-based biodiesel has incurred international condemnation during recent years. Microalgae are strongly advocated as the most promising substitute for oil crops. However the commercialization of microalgal biodiesel is hindered by the high cost of microalgal feedstock and production through conventional processes. This thesis investigates the downstream processing of microalgal dewatering, lipid extraction, and transesterification. For microalgal dewatering, an energy-efficient vacuum belt filtration system with adaptable productivity is designed. For conventional lipid extraction-transesterification process, Soxhlet method and Bligh & Dyer method followed by alkali catalyzed transesterification are investigated by using avocados and microalgae Chlorella, respectively. The conventional methods are proved effective for laboratory scale biodiesel production. Avocado-based biodiesel production is patented by our group. Five transesterification parameters are determined, including water content, reaction time, temperature, catalysts, and molar ratio of methanol to oil. However these laboratory protocols are difficult to be scaled up and conventional extraction-transesterification process cannot lead to cost-competitive microalgal biodiesel. This thesis therefore designs a cost-effective, scalable process by integrating oil extraction and conversion into one step through in-situ transesterification. The co-solvent is the key to success. The co-solvent criteria for microalgal biodiesel production through in-situ transesterification are proposed. The overall biodiesel yield of microalgae Spirulina is determined by using Bligh & Dyer method and alkali catalyzed transesterification. The performance of the selected co-solvents of toluene, dichloromethane and diethyl ether, as well as other solvent combinations in in-situ transesterification is evaluated by the overall biodiesel yield. Among all the co-solvents, toluene/methanol system, 2 to 1 by volume ratio, demonstrates the highest efficiency which achieves a biodiesel yield of 76% of the overall biodiesel yield for the first in-situ transesterification cycle and 10% for the second in-situ transesterification cycle.