(165c) An Energy-Based Model of Gas-Solids Transport in a Riser

A simple and reliable method to estimate the solids holdup distribution and solids residence time in a gas-solid riser flow is essential to the optimum design and efficient operations of riser reactors. The traditional approach of equating the local solids holdup to the pressure drop in a riser overlooks the effects of solids acceleration and energy dissipation in the acceleration and dense phase transport regions. The energy dissipation in these regions is mainly due to the interfacial friction between interstitial gas and suspended solids, inter-solids collisions, as well as solids-wall frication. Most momentum-based models fail to account for the energy dissipation of inter-solids collisions, and the models using the simple granular kinetic theory fail to account for the energy dissipation in micro-sliding or rolling from off-center inter-solids collisions. This paper presents an energy-based mechanistic model to analyze the partitions of the axial gradient of pressure by solids acceleration, collision-induced energy dissipation and solids holdup in gas-solid riser flows. Based on this model, the correct estimation of axial distributions of solids holdup and solid velocity are obtained. Our analysis shows that the effect of solids acceleration on the pressure drop can be significant in a range of moderate solids holdup (typically from 3.5% to 12% by solids volume fraction) whereas the effect of energy dissipation becomes important in the dense phase transport region (typically when the solids volume fraction above 5%). The exemplified results indicate that the traditional approach of equating the local solids holdup to the pressure drop overestimates the solids holdup by an error up to 30% in the acceleration and dense phase transport regions in typical gas-solid riser flow applications.