(207c) Measurement of Dynamic Forces in Fluidized Bed Systems

The internals in fluidized bed systems, such as heat exchanger tube bundles and baffles trays, which are utilized for improving fluidized bed operations, are subject to dynamic buffeting forces caused by rising bubbles and particle motion. Understanding the nature of these forces are industrially important in the structural design since excessive vibrations and erosion in tubes can eventually lead to their failure. The forces are dynamic in nature due to the small residence time of bubbles when they come into contact with the internals. To quantify these forces, a piezoelectric force sensor, which has a high response time and a constant sensitivity over a wide range of temperatures and pressures, was mounted in a 2-D Plexiglas column 30 cm wide and 2.5 cm deep for a visual depiction of the phenomenon. Tests were conducted with fluid catalytic cracking catalyst particles and glass beads. A single bubble was injected at a certain time interval and the motion of the bubble was synchronized in real time with the response from the initial thrust of the bed, the bubble roof, and the bubble wake. Tests were also done to quantify the forces during the startup and subsequent fluidization of the bed. The glass beads, which were bigger in size and moving with a greater momentum, generated greater forces when they came into contact with the sensor. There were a few events, which produced enormous amounts of dynamic forces per unit area, close to about 170 kPa at the center of the bed. Such forces and excessive vibrations caused by fast moving bubbles are an important component when designing fluidized bed systems to mitigate internal structural damage.