Model-based optimisation of biodiesel production from microalgae

This work presents a superstructure-based optimisation model to optimise the microalgae to biodiesel production flowsheet for the minimum net annualised life cycle cost (ALCC) of biodiesel. The model includes the important processing steps of converting microalgae into biodiesel, viz. microalgae growth, harvesting, lipid extraction, and transesterification of lipid. Different options to perform these steps are considered. The mass and volumetric balance for each process and equipment, and the equipment capacity limitations constitute the important model constraints. The decision variables include growth duration, medium, as well as the techniques and specifications to be followed in each of the downstream steps. The mixed integer linear programming model was applied to a case study of producing 30,000 kg/d biodiesel from Chlorella. The minimum ALCC was US $ 13.286/1 for the flowsheet and equipment details recommended by the model. Sensitivity analysis showed that lipid extraction was the most crucial step in the flowsheet. (C) 2016 Elsevier Ltd. All rights reserved

Integrated microalgae biorefinery: Impact of product demand profile and prospect of carbon capture

An integrated microalgae biorefinery producing fuels and value-added products needs to be optimally synthesized for realizing its potential. A large-scale optimization model considering biorefinery co-producing biodiesel, polar lipid, protein, and reduced sugar from microalgae has been previously developed, and biodiesel production without any co-products manufactured was found to be non-profitable. This work uses the model to study the impact of demands and selling prices of value-added co-products on the optimal biorefinery synthesis and the net annualized life cycle cost (ALCC) of biodiesel production. The simulation results showed that production of reduced sugar to the maximum extent resulted in an annualized profit of US$27.23/L of biodiesel production. Individual scenarios for mandatory production of protein and polar lipid resulted in an ALCC of US$15.11/L and US$13.20/L, respectively. The case of fluctuations in demand and selling prices of the end-products was also analyzed. Additionally, the potential of a biorefinery to sequester atmospheric carbon dioxide from a coal-fired 500 MW power plant, without any preferential product, is also modeled. Emission of carbon dioxide is considered to attract a penalty. For this study, with a low-value biodiesel selling price (US$0.48/L), the biorefinery captured 2.77% of the available carbon dioxide and recorded a net annual deficit of US$9.71 million. On the other hand, for a high price of biodiesel (US$8.74/L), 99.89% of the carbon dioxide was captured to have biodiesel and reduced sugar co-produced, yielding a net annual revenue of US$303.27 million. (c) 2017 Society of Chemical Industry and John Wiley & Sons, Lt

Optimization of integrated microalgal biorefinery producing fuel and value-added products

An integrated microalgal biorefinery is desirable from an economic standpoint but challenging to synthesize, due to diversity of options. This work uses a model-based optimization approach to address this challenge in a systematic manner. A superstructure of the integrated biorefinery is developed where biodiesel is considered as the main product, while polar lipid, protein, and carbohydrate are also processed to various value-added compounds. Mass balances, equipment capacity limitations, and cost functions corresponding to these processes constitute the constraints of the optimization problem. The decision variables include the process synthesis as well as process scheduling and operations-related decisions. A Mixed Integer Linear Programming (MILP) model was developed to minimize the net annualized life cycle cost (ALCC) of the biorefinery. For a scenario of 30 Mg/d production target of biodiesel, with no intermediate storage between the stages, the superstructure yielded an optimal biodiesel production cost of US$8.53/L and reduced sugar was selected as a co-product. Several cases were analyzed in terms of the decision making of the process on the upstream and downstream levels, as well as variations in scheduling strategies. Co-cultivation of the phototrophic and heterotrophic strains resulted in net ALCC of US$7.66/L, which was 10.2% less than the base case. Batch scheduling with various strategies were also investigated and the case with infinite intermediate storage coupled with debottlenecking reduced the net ALCC by 25% to US$6.4/L. (c) 2017 Society of Chemical Industry and John Wiley & Sons, Lt