386703 Optimal Design of an Integrated Algae-Based Biorefinery for Biodiesel, Astaxanthin and PHB Production
In this work, we propose an MINLP model for the design of an integrated microalgae-based biorefinery for the simultaneous production of biodiesel, astaxanthin and polyhydroxybutyrate (PHB), as well as a combined heat and power cycle. The biodiesel production process under study consists of four main stages namely: microalgae cultivation, harvesting and dewatering, lipid extraction and oil upgrading. Astaxanthin, a natural ketocarotenoid that is a secondary metabolite in several microalgae species, is a powerful antioxidant with application in nutraceuticals, pharmaceuticals, cosmetics and food industries. We analyze the possibility to maximize both astaxanthin and oil production from microalgae species like Haematococcus pluvialis, as their accumulation takes place under the same culture conditions (Huntley and Redalje, 2007). Astaxanthin can be produced after harvesting and dewatering, employing a drying and pulverization step (Li et al., 2011). On the other hand, PHB is a biopolymer that constitutes an alternative to fossil fuel based polymers, with similar properties. It also has promising uses as in drug delivery and food packaging and it is produced by microorganisms that can uptake glycerol as substrate. Glycerol, in turn, is a by-product in the biodiesel production process. In this way, the PHB production process is composed of three main steps: glycerol purification, fermentation, and PHB extraction. Residues from these processes, as well as glycerol, can be considered as an option to feed a power generation step. The produced energy is a significant alternative that can be used in the processing sections described before. In this work, we consider the anaerobic digestion of the algal biomass cake (after lipid extraction), using glycerol and waste paper as additional substrates. The addition of glycerol to the anaerobic digester, decreases PHB productivity but enhances methane yield, so a lower bound is set to the flowrate of glycerol submitted to the anaerobic digestion step (Garcia Prieto et al., 2014).
The proposed MINLP model is implemented in GAMS (Brooke et al., 2013) for net present value maximization. Model equality constraints include mass balances, yield equations and correlations, while inequality constraints include process specifications such as a requirement on produced CO2 to be less than the required amount for biomass generation in the photobioreactors. In this way, a global reduction of the CO2 emissions to the atmosphere is ensured, as the rest of the required amount of CO2 is provided by a flue gas stream from a thermoelectrical plant. The upper bound of CO2 is limited by the CO2that is produced in this plant, located next to Bahia Blanca city. Furthermore, the carbon to nitrogen ratio in the anaerobic digester must be between specific values for optimal operation. An upper bound for the waste paper amount fed to the anaerobic digester is needed. The constraint of this stream, which ensures the substrate requirements in the digester, is assumed to be the total amount of the domestic recycled paper in Bahia Blanca’s city. A lower bound constraint on the glycerol flowrate stream to the biodigester is required to ensure the amount of carbon necessary for biogas production.
Numerical results show that an important reduction in biodiesel production cost can be obtained for the integrated biorefinery scheme as compared to the biodiesel process from microalgae with power and heat generation unit by anaerobic digestion (0.98 $/kg biodiesel against 1.26 $/kg biodiesel). Even though the contribution of capital, raw material, utilities and operating and maintenance cost are quite similar, the production cost is lower, mainly due to revenues from PHB (6.25 $/kg), which is a value-added product with a selling price much higher than glycerol (0.26 $/kg), its main raw material. The integrated scheme including astaxanthin production is the optimal alternative as it includes an extra value-added product such as astaxanthin (7.78 $/kg).
Brooke, A.; Kendrick, D.; Meeraus, A.; Raman, R.; GAMS- A User’s Guide, GAMS Development Corporation: Washington, 2013.
Huntley, M. E.; Redalje, D.G. CO2 Mitigation and Renewable Oil from Photosynthetic Microbes: A new Appraisal. Mitigation and Adaptation Strategies for Global Change, 2007, 12, 573-608.
Garcia Prieto, C., F Ramos, V. Estrada, M. S. Diaz, Optimal Design of an Integrated Microalgae Biorefinery for the Production of Biodiesel and PHBs, Chemical Engineering Transactions, 2014
Li, J.; Zhu, D.; Niu, J.; Shen, S.; Wang, G. An Economic Assessment of Astaxanthin Production by large Scale Cultivation of Haematococcus pluvialis. Biotech. Adv. 2011, 29, 568-574.