(570c) A Superstructure Optimization Approach for the Design of Ethanol Upgrading Processes to Distillate Range Fuels

The use of ethanol as a sustainable fuel alternative to fossil fuels although well established is not exempt of challenges. First, ethanol can only be blended in fuels at levels below 10%. With an increasing offer of ethanol, coupled with the potential to have lignocellulosic ethanol production in the near future, one question that arises is how can we can find strategies to upgrade ethanol to other molecules such that this 10% constraint can be relaxed. A second important challenge stems from the fact that ethanol is mainly used to replace gasoline. Since the production of ethanol is likely to increase, it would be beneficial to find strategies to upgrade ethanol into the middle distillate range components which can also be used for diesel. In response to these challenges a growing body of literature dealing with the catalytic upgrading of ethanol to other fuel molecules has been produced in the last 30 years. Numerous chemistries have been explored (e.g. Guerbet coupling, dehydration, condensation etc.), and a large number of catalysts has been developed. The corpus of this literature is so vast that it is not trivial to answer the question of which strategy is optimal for the production of a particular fuel.

Accordingly, the goal of this work is to use process systems engineering strategies, and specifically of superstructure based optimization techniques, to address some of the important questions in this area. Specifically, we present a hierarchical superstructure optimization approach with the aim of finding optimal pathways for ethanol upgrading into fuels. Importantly, we formulate the problem such that three decision levels are considered simultaneously. In the first level we decide the species whose chemical transformation is allowed in a particular fuel production strategy (e.g. ethanol and ethylene). In the second level, we decide the specific chemistries that will be used to transform each of the species selected in the first level (e.g. ethanol dehydration, and ethylene oligomerization). Finally, in the third level, we select a specific process to realize the chemistry selected in the second level; this process is characterized by a specific catalyst and separation strategy. The contributions of this work are twofold: methodologically we introduce a new superstructure optimization strategy that can be used for the design of fuel upgrading strategies; from an application perspective, we present a complete and comprehensive analysis of the different strategies used for ethanol upgrading and provide insights into the major cost drivers for different ethanol upgrading strategies.