A techno-economic analysis of biodiesel biorefineries: Assessment of integrated designs for the co-production of fuels and chemicals

In this work we explore the concept of integrated biorefineries, and we examine alternative schemes for the co-production of biofuels (biodiesel) and chemicals (succinic acid). Four different biorefinery schemes considering the different uses of crude glycerine from the biodiesel process are simulated and compared: (i) the disposal of crude glycerine as a waste, (ii) the purification (through distillation) of crude glycerine to 80%, (iii) the purification of glycerine to 95%. and (iv) the production of succinic acid from glycerine through fermentation. For the latter, we consider the bioprocess that converts the glycerol to succinate, and a downstream separation process that purifies and crystallises our product to the final succinic acid crystals. To apply complex kinetics for the fermentation we have linked Aspen Plus (2006.5) with Matlab (R2007b), where we have used the experimentally-based unstructured model from Vlysidis et al., 2009 and 2010. We first determine the operating parameters of the fermentor that have a significant effect on the economics of this scenario, i.e. the cycle time of the batch fermentation and the water flowrate entering the bioreactor. Subsequently, we perform single- and multi-objective optimisation to maximise the profit and/or to minimise the environmental impact of the overall process. We then analyse and compare the economics of the four different biorefinery schemes by using well-known profitability and/or emission criteria. Furthermore, we carry out sensitivity analysis that takes into consideration price variations for the most important materials and we extract firm conclusions about the profitability of each scenario. It is found that succinic acid co-production can enhance the profit of the overall biorefinery by 60% for a 20 years plant lifetime. These results indicate the importance of glycerol when it is utilised as a key renewable building block for the production of commodity chemicals.