Analysis of solid flow patterns and mixing in gas/solid flow systems.

摘要

Gas/solid flow systems are an essential part of many chemical processes, and contributions to the understanding of the behavior of such flow systems can significantly enhance the design and, in turn, the productivity of such processes.; The objective of this research was to obtain a better understanding of gas/solid flow at different fluidization regimes. To achieve this objective, multiphase hydrodynamic models in CFX-F-3D and Fluent were modified to include proper governing and constitutive equations along with appropriate boundary and initial conditions to compute gas/solid flow patterns in the actual fluidized systems. Furthermore, an experimental technique based on light remittance of phosphorescent tracer particles was developed and tested in a small-scale bubbling fluidized bed.; The transient multiphase flow equations along with CFX-F3D and Fluent computational fluid dynamic (CFD) computer codes were able to simulate gas/solid flow patterns at different fluidization regimes. Two- and three-dimensional bubbling fluidized beds with a gas jets distributor were simulated, and a bubbling regime was predicted. Uniform gas distribution also predicted a continuously bubbling flow behavior. A power spectrum analysis of a fluidized bed with a different gas distributor design showed a main frequency of oscillation of about I Hz, typical of bubbling regimes. Furthermore, the uniform gas inlet distributor showed the most stable fluidization regime.; The riser section of a circulating fluidized bed (CFB) was also simulated, and the solid flux and pressure drop profiles compared well with the pilot-scale experimental data (Knowlton et al. 1995). The core-annular flow regime that is usually observed experimentally was predicted using this model. The mixing patterns and the hydrodynamics of the gas/solid flow were in agreement with the experimental observations. The effects of grid size as well as the inlet and outlet conditions in 2-D and 3-D systems were investigated. It was found that inlet and outlet gas and solid designs and flow conditions have a very significant impact on solid flow behavior, clusters formation, and solid concentration along the riser as well as wall regions.; The implementation of the kinetic theory approach added strength to the multiphase flow model by allowing the direct theoretical determination of the solid viscosity and pressure. The solid turbulence quantities such as granular temperature and viscosity were computed and compared qualitatively with the experimental data.; In the experimental part of this research, phosphorescent particles were used as tracer to study the mixing in a bubbling fluidized bed. This technique was successfully applied to a small-scale fluidized bed was proven to work, and real time light intensity data were correlated to tracer concentration by the established calibration curves.