Dense phase gas-solid flows in circulating fluidized beds

Circulating fluidized beds (CFBs) are widely used in the chemical industry for fluid catalytic cracking and combustion applications. It is well known that under certain operating conditions, flow instabilities involving the entire CFB system can arise, leading to a breakdown in operation. However, the origin of such instabilities as well as the minimum physics needed in a mathematical model to capture these is not well understood. In this research such CFB loop instabilities have been experimentally and theoretically investigated.; The flow behavior of gas-Geldart type A particle mixtures in a pilot scale CFB under stable and unstable operating conditions has been examined. Stable dense phase flow is established at low aeration rates in the standpipe. At high aeration rates the flow becomes unstable, manifesting low frequency oscillations in the flow characteristics. Our results suggest that, under the conditions explored in the present study, this instability originates in the standpipe when the effect of frictional interactions becomes negligible. Any attempt to model this instability should consider the interaction between the various components of the CFB circuit, wall friction and the compressive yield stress of the particle assembly in the standpipe.; Careful measurements of pressure drop and bed height during fluidization-defluidization cycles in beds of different diameters have been used to quantitatively determine the compressive yield stress as a function of particle volume fraction for XL particles used in the CFB experiments. Analysis of the standpipe data reveals that the support provided by the standpipe wall can be estimated quantitatively from the standpipe holdup data and the estimated compressive yield stress.; A theoretical study of the flow of a gas-Geldart type A particle mixture in a uniformly aerated standpipe system has been presented. The theory is based on a one-dimensional treatment of the volume-averaged equations of motion. It is able to qualitatively capture many of the gas-particle flow features observed in our CFB experiments, such as the effect of aeration on the flow behavior and the operating conditions under which instabilities in the CFB can occur.; A frictional model for the rheology of a compressible granular material has been developed and numerical issues associated with its implementation in a CFD framework have been explored. The model is shown to capture the general features of dense phase flows in two test cases in a qualitatively correct manner.