Flow Mapping and Modeling of Liquid−Solid Risers

Abstract

The trend toward environmentally benign processing has resulted in novel chemistries and new reactor types. For alkylation of both aromatics and linear paraffins, with new solid acid catalysts, liquid−solid risers are one of the reactor types considered. As the fluid dynamics of liquid−solid risers is largely unknown, it has been investigated in this work in a 6-in.- (15-cm-) diameter “cold-flow” circulating fluidized-bed riser using noninvasive methods. The time-averaged solids holdup distribution is determined by γ-ray computed tomography (CT). The solids instantaneous and ensemble-averaged velocity patterns, as well as the solids residence time distribution in the riser, is obtained by computer-automated particle tracking (CARPT). Various measures of solids backmixing are evaluated including the eddy diffusivities and the overall axial dispersion coefficient. The obtained database is used for validation of a two-fluid Euler−Lagrange model, which is coupled with appropriate closures including the kinetic theory of granular solids. The model is shown to be capable of predicting the liquid and solids residence time distributions in the riser as well as the solids velocity and holdup pattern. The quantitative predictions from the two-dimensional axisymmetric simulation for the solids volume fraction and the time-averaged solids axial velocity radial distributions agree well with the experimental data. The model can be used to predict the extent of solids backmixing in the reactor.