Valuable bioproducts from microalgae: A superstructure optimization approach

A superstructure to produce added-value products (pigment, omega-3, glycerol, biodiesel, biogas, and fertilizers) from three species of microalgae (Chlorella vulgaris, Haematococcus pluvialis, Nannochloropsis spp.) is developed in this study. The superstructure is converted into a mixed-integer nonlinear programming (MINLP) model. A block integration approach is used to drastically decrease the CPU times by reducing the number of variables, parameters, and constraints. The model is solved with Baron/AOA in AIMMS software, and the most promising production pathway is identified. For all three biorefineries (cultivating different microalgae), the most promising production pathways (in terms of cost-effectiveness) remain consistent. These pathways involve an open pond, sedimentation and flotation, flocculation without any dryer, sonication, organic solvent pigment extraction, n-butanol solvent lipid extraction, lipid production, and anaerobic digestion. Changing technologies of dewatering stages (flocculation to centrifugation and filter press) proposes the second and third cost-effective production pathways. The most profitable biorefinery cultivates Haematococcus pluvialis, with annual profits of 62 $/kg of microalgae. A high amount of valuable pigment produced by Haematococcus pluvialis leads to 22 times higher profits than Chlorella vulgaris and 47 times higher than Nannochloropsis spp. The Haematococcus pluvialis biorefinery produces approximately 500 *10³Kg of pigment bioproducts from 24 *10⁶Kg biomass by using 200 *10⁶Kg wastewater and 164 *10⁶Kg of carbon dioxide, annually. Ultimately, a sensitivity analysis is executed to confirm how the production of pigment, the price of this bioproduct, and day/ night ratio affect the profitability of microalgae biorefineries.