The continuum treatment of gas-solid flows is similar to that of granular flows, in that the motion of the particles is often described by making an analogy with molecular motion in a gas (i.e., kinetic theory). Hence, many of the phenomena under investigation in the gas-particle systems parallel those being considered in granular systems (e.g., clustering, particle size distribution). Efforts in this area are focused on extending the constitutive theories developed for granular flows to gas-solid flows, and to use the resulting models for design, scale-up, and optimization of gas-solid systems.

Research Areas of Interest

Scale-up of Gas-Solid Systems. Experience in fluid catalytic cracking and other gas-solid processes has indicated that the scale-up of high-velocity fluidized beds is a complex and poorly understood process. To date, only three catalytic circulating fluidized bed systems had been successfully scaled to commercial units, and these scale-ups were achieved in a lengthy, trial-and-error manner (Matsen, 1997). The focus of this effort is to use existing models for gas-solid flows to gain insight into the scaling laws that have been proposed for such systems.

Effects of Clustering. Similar to granular flows, gas-solid flows are also characterized by particle clusters or streamers that continuously form and dissolve during the course of flow. Such transient fluctuations are analogous to those found in single-phase turbulent flows, and efforts in this area are aimed at the development of a time-averaged model for such flows.

Fluidization of Nonuniformly Sized Particles. The fluidization processes encountered in industry are typically polydisperse in nature. Although the distribution of particle sizes may simply be a property of the starting material, oftentimes a particular distribution is desired in order to improve the flow behavior. For example, FCC (fluidized catalytic cracking) operators have observed that the addition of fines to a monodisperse catalyst can decrease the deaeration time of a fluidized bed and reduce the bubble size of a bubbling bed. In order to fully exploit the benefits of a nonuniform size distribution, a more thorough understanding of the underlying mechanisms is needed. Research in this area involves numerical studies designed to quantify the effects of different size distributions on both local and overall flow behavior. Such results will provide a solid basis for detailed model development of polydisperse systems.

Fluidization of Cohesive Particles. The fluidization of particles with significant short-range, attractive forces is known to take on a markedly different character that the fluidization of non-cohesive particles. For example, achieving a uniformly fluidized bed is more challenging when cohesive particles are used. Research interests in this area include the development of a fundamental continuum model capable of predicting the flow behaviors specific to cohesive-particle systems.

References

Matsen, J. M., "Design and Scale-Up of CFB Catalytic Reactors," Circulating Fluidized Beds, Blackie Academic and Professional, New York, 489 (1997).