Characterizing Kinetics in Three-Phase Reactors for Conversion of Renewable Feedstocks

Three phase reactors such as trickle beds have long been used in industry, but the modeling of their behavior remains an active area of research. As processes for catalytic conversions of biorenewable substrates develop, these three phase reactors will play a key role because of low volatility and thermal fragility of reactants and products. Among the most significant challenges in three phase reactor use is the application of laboratory and pilot-scale reactor data to commercial-scale reactor design. This is because laboratory systems typically run at much lower liquid mass velocities than commercial units, such that partial wetting and mass transport strongly affect reaction rates and mask critical catalyst performance parameters such as selectivity. In the past, remedies for this shortcoming include addition of fine particles to aid in liquid wetting and use of recirculating systems. In this work, we demonstrate modes of trickle bed operation wherein intrinsic reaction rates are observed, as evidenced by direct comparison with measured reaction rates in a stirred batch system. Conditions for eliminating both intraparticle and external mass transfer, while ensuring full wetting of the catalyst in the laboratory reactor, are described using lactic acid hydrogenation to propylene glycol as a model system. The ability to translate batch reactor data, which is readily collected, to continuous three-phase reactor design reconciles an ongoing challenge to the use of trickle bed systems for characterizing renewable feedstock catalytic systems.