| This dissertation presents a predictive, two-dimensional, time-averaged hydrodynamic model for a circulating fluidized bed (CFB) riser operating in the fast fluidization regime with downward flow of gas and solids near the wall. The basis for the model is a substantially augmented core-annulus approach with both gas and solids ascending in the core and descending in the annulus. Rigorous development of the mass and momentum conservation equations results in a novel material interchange scheme between the core and annulus, for both phases, and unprecedented sophistication in the pressure drop calculation. In addition to predicting the axially varying core radius and pressure, the model predicts the axially and radially varying solids mass flux and velocity, gas mass flux and velocity, and voidage given the riser geometry, inlet temperature, pressure and solids mass flux, and physical properties of the gas and solids. With the use of a neoteric simulator, extensive comparisons between the model predictions and published experimental data demonstrate that the model is successful in representing the hydrodynamics in a CFB riser.; A coupled hydrodynamic and kinetic reaction model for a CFB riser reactor stems from a slightly simplified version of the elaborate two-dimensional hydrodynamic model. The reactor model also manifests itself in the simulator and generates residence time distribution functions for both gas and solids phases that deviate markedly from plug flow conditions, which emulates experimental observation. Derivation of the energy conservation equation and its incorporation into the coupled hydrodynamic-kinetic reaction model demonstrates that near isothermal operation of a CFB riser reactor is possible, even for highly exothermic reactions. Simulations also confirm that, for a reversible reaction, the conversion in a CFB riser reactor with downward flow of gas and solids near the wall is much less than the conversion in a comparable dense phase pneumatic transport reactor that behaves as a plug flow reactor. |
