Optimization of an integrated algae-based biorefinery for the production of biodiesel, astaxanthin and PHB

In this work, we address the optimal design of an integrated microalgae-based biorefinery through the formulation of a mixed integer nonlinear programming model for the production of biodiesel and potential high-added value products. Main bioproducts are poly (hydroxybutyrate) (PHB) and astaxanthin. A combined heat and power cycle to transform biogas generated by the anaerobic digestion of waste streams is also included in the superstructure. Mass and energy balances are formulated for the biorefinery. Different alternatives for PHB extraction are taken into account. The anaerobic digestion model accounts for detailed composition of the different feed streams. Detailed capital cost models for process equipment are formulated and implemented in GAMS to maximize net present value (NPV). Results show that the production of astaxanthin and PHB provides a way to make biodiesel production economically feasible. Open pond and surfactant-chelate are selected for microalgae cultivation and PHB extraction method, respectively. Biodiesel price can be reduced to $0.48 due to incomes from astaxanthin and PHB sales. Also, an economic sensitivity analysis is performed. Further comparison between domestic and international cost conditions is carried out, showing higher NPV in the domestic case.