(97b) Segregation in a Spouted Fluidized Bed

Spouted fluidized beds are used extensively for many types of solid-fluid contacting and reaction applications where uniform inter-phase contacting is important. While various commercial applications have been developed, the basic hydrodynamics and mixing patterns are not well characterized. Spouted beds are usually preferred where consistent and predictable exposure of solids to a gas phase is required such as in drying and coating operations. Circulation of all particles through the spout insures even exposure of the solids to the incoming gas. Particle segregation, however, can lead to major disruptions in the expected flow patterns and therefore poor or even failed process performance. We combine visual observation of the bed surface with a new method to measure internal solids circulation using magnetic particle tracking. The combined methods reveal conditions under which segregation may occur and the internal structure of the segregated bed.

Experimental Materials and Methods

We use our previously reported magnetic particle tracking method in a small three-dimensional bed. Safe inexpensive magnetic tracers and detectors are used thus avoiding the issues associated with past particle tracking methods. In this study, we used neodymium magnets embedded in plastic spherical particles and externally positioned Hall-effect magnetic field detectors to continuously locate the position of a tracer particle over time. The method takes advantage of the tendency of the tracer particles to align their magnetic axes with the earth's magnetic field like a compass needle. Proper positioning of the probes can take advantage of this tendency, which simplifies the data analysis.

We studied the solids flow and segregation behavior of various binary particle mixtures in a 39-mm-diameter spouted bed with a 45-degree conical bottom. Visual observation and video recording were used to observe overall segregation behavior. Magnetic particle tracking was then employed to reveal the sub-surface flow structure of segregated beds. Binary mixtures of closely sized glass beads were tested. Both the ratio of largest to smallest and the weight fraction of the mixtures were varied. Binary mixtures of particles with differing densities but nearly the same size were also tested.

Results and Discussion

Segregation of binary sized particles occurred under certain mixture and flow conditions. When segregated, the bed formed a top bubbling layer of the smaller particles above an internally circulating spouting bed of the coarser particles. The bed exhibited hysteresis in its flow behavior in which the superficial velocity of an initially well mixed spouting bed can be gradually reduced to a point where segregation occurs. The behavior from the segregated conditions is not reversible and the velocity must be increased appreciably before particle mixing occurs and the segregated bed of small particles disappears. The effect is evident when the ratio of larger to smaller particle sizes is about 3 to 1. It is not evident at a size ratio of 2 to 1. The segregation hysteresis varies with the weight fraction of the coarser material disappearing above and below certain limits.

Segregation in mixtures of nearly equally sized particles with different densities occurred as well. Under some conditions, a layer of the less dense particles formed on the surface. The denser material continued to spout in the central region while the less dense particle formed a torriodal vortex around the central spout. Unlike the size segregated bed, however, the density segregated beds did not exhibit hysteresis.