Method and Prediction of Transition Velocities in a Circulating Fluidized Bed's Riser

A series of experiments were conducted using four different particles (812 mu m cork, 60 mu m glass beads, 180 mu m glass beads, and 750 mu m phosphorescent polyethylene beads) in a 0.3 m diameter circulating fluidized bed (CFB) cold model to evaluate the transitional velocities for different transport flow regimes. Transitional velocities were obtained by measuring the time required to empty out all solids from the riser of the CFB after cutting off solids flow under various gas velocities after attaining maximum solids loading. This method has been shown to involve less influence of the operating conditions such as solid inventory, solid flow rate, and solids feeding system and provide the limiting responses for the specific geometric configuration tested. Correlations were developed to predict these transitional velocities in terms of gas and solids properties in NETL's cold flow test facility. Drag coefficients were then determined using force balance at each of these transitions. The resulting expressions for the transitions were compared with the literature and the best available expressions to assess their validity and test their range of applicability. It is concluded that the transient method allows researchers to identify transition velocities between transport regimes based upon dynamic riser response and apparent gas solids drag and can thus readily identify the potential for rapid buildup of solids inventory in the riser.