An Optimization Based Framework for the Design of Biofuels/Fossil Fuels Blendstocks

Biofuels have been identified as an important component in the development of a sustainable renewable energy pathway. With this goal in mind, significant efforts have been made to develop biofuels for the replacement of gasoline, diesel, and jet fuel. While in some cases biofuels can be used directly in engines, the most likely use of them is in blends with fossil fuels (e.g. ethanol is typically blended with gasoline at 10%, and biodiesel at 5% with diesel). It is likely that in the middle term, blending will remain common, especially if we consider that satisfying the whole fossil fuel demand using only bio-renewable sources is difficult. However, little attention has been paid to the opportunities that stem from the nature of the blending process. In particular, we note that it is possible to tailor the biofuel fraction such that, when blended with a lower quality fossil fuel, it produces a blend (biofuel plus fossil fuel) that satisfies all quality constraints and specifications. Having said that, designing biorefineries to produce such fuels is a challenging task. One alternative that has been gaining momentum is based on the use of ethanol upgrading [1]. This alternative is attractive because there are several chemistries associated with ethanol upgrading, providing access to a diverse palette of chemical components that can be used to tailor the properties of a biofuel [2].

In this work, we develop an optimization-based framework for the design of ethanol upgrading biorefineries for the production of fuels that are going to be used in blends with fossil fuels. This framework explicitly accounts for the properties of the fuels produced considering the blending with a fossil fuels stock, and it allows estimating the capital and operating costs associated with the chemistries used to upgrade ethanol. The framework uses a superstructure representation in which the selection of the chemistries and catalysts to be used in an upgrading strategy is explicitly modeled. Importantly, we include 29 alternative chemistries and 150 different catalysts, representative of the state-of-the-art for ethanol upgrading. The explicit treatment of the blending problem, coupled with an adequate representation of the process synthesis problem allows us to explore the different relations and trade-offs among the fossil fuel properties, the biodiesel properties, the chemistries, and catalysts selected, and the economics of the process. The model that we present is used to construct a complete technology map, identifying optimal biorefining technologies as a function of different parameters of interests like feedstock cost, and diesel fuel cetane number. We also discuss how changes in these parameters affect the fuel yield, composition, and properties.

1. Eagan, N. M., Kumbhalkar, M. D., Buchanan, J. S., Dumesic, J. A. & Huber, G. W. Chemistries and processes for the conversion of ethanol into middle-distillate fuels. Nat. Rev. Chem. 3, 223–249 (2019).
2. Sun, J. & Wang, Y. Recent advances in catalytic conversion of ethanol to chemicals. ACS Catal. 4, 1078–1090 (2014).