Kinetics and Reactor Design Principles of Volatile Fatty Acid Ketonization for Sustainable Aviation Fuel Production

Abstract

Ketonization of wet waste-derived carboxylic acids (volatile fatty acids, VFAs) constitutes the first step of a process to catalytically upgrade VFAs to an alkane sustainable aviation fuel blendstock. VFA ketonization has been demonstrated at near-theoretical yields at the lab scale, and robust operation of industrial-scale ketonization reactors is essential for the commercialization of VFA upgrading to sustainable aviation fuel. We present a ketonization kinetic study of hexanoic acid, a VFA model compound, over commercial ZrO2 and use the kinetic parameters derived from the study in an adiabatic packed-bed reactor simulation of hexanoic acid ketonization running to near-complete (98%) conversion. A key findings from the kinetic study is that ketonization rate is positive order in acid pressure at low (<10 kPa) pressures and transitions to zero order at higher pressures, conforming to a Langmuir-Hinshelwood surface coupling mechanism. Rates are inhibited by ketonization coproduct water but not by ketones themselves or coproduct CO2. Reactor simulations using these kinetics show that rate inhibition by water controls reactor size and that size requirements can be lessened by employing designs that allow for the removal of water from the partially converted acid stream.