(583a) Evaluation of Multi-Scale Models for Ethylene Epoxidation in a Fixed Bed Reactor

In multi-scale pseudo-continuum fixed bed reactor models, the transport parameters lump several phenomena at various length scales. The question of whether the models are adequate to predict reactor performance is usually addressed by comparison to experimental data. The comparison is compromised by uncertainties in the reaction kinetics and in the correlations used for the effective transport parameters.

Particle-resolved computational fluid dynamics (PRCFD) simulations of highly exothermic ethylene partial oxidation in a fixed bed were made for a random packed bed of 807 spheres at tube-to-particle diameter ratio 5.96. This generated detailed 3D benchmark solutions at both the tube and particle scales, to evaluate heterogeneous pseudo-continuum reaction engineering models using the same kinetics and fluid/particle properties. The effective transport parameters were obtained from PRCFD runs in the same bed of particles without reaction. So transport properties directly from the fixed bed being modeled were used, instead of introducing uncertainty by choosing one of the many literature correlations. The 3D PRCFD simulations showed detailed pictures of temperature (see Fig. 1, flow left to right), species and reaction rates down to the level of a catalyst particle.

The 2-D pseudo-continuum reactor model gave a high reaction rate compared to the PRCFD simulations (see Fig. 2). Unlike with previous studies of endothermic steam methane reforming (SMR), manipulation of the fluid-particle heat and mass transfer coefficients did not improve agreement. In addition, again in contrast to SMR, agreement was not helped by the inclusion of radial voidage and velocity profiles, but was improved by adjusting the radial heat transfer model to give the correct total heat flux. These divergent results arose from the differences between heating from the strong heat flux applied to the tube wall in SMR, and from the weaker heat source supplied by reaction in the main fixed bed in ethylene oxidation.