Forces on Inserted Objects in Fluidized

Various objects are often inserted in fluidized beds for different purposes. For example, immersed tube bundles are used fluidized beds for heat transfer and baffles made of multiple inclined slats are widely employed in industrial fluidized bed reactors to split bubbles, suppress gas/solids back-mixing for higher conversion and better product selectivity. As well, probes are usually inserted in beds to monitor operating parameters. However, all these immersed internals are subject to forces during the ddifferent operation states in fluidized bed reactors. Erosion and excessive vibration caused by these forces may lead to internal failure, especially in industrial fluidized beds with hostile environment and needing several years of continuous operation. Therefore, the information of forces on internals under different operation states is very helpful for their reasonable strength design and long-period reliability.

In this study, forces on a horizontal slat inserted in the bed were summarized during different operating states, i.e. defluidized, start-up and steady fluidized states. During the starting-up of a fluidized bed, the bed will experience the above three different operating states and force acting on immersed objects are also different. The size of the slat was 300 mm (long) x 50 mm (wide)x 4 mm (thick). The forces on the slat were measured by adhering strain gauges on its surface. All experiments were carried out in a cold fluidized bed with a square cross-section of 300 mm×300 mm. The bed materials were fine FCC particles (Group A) and coarse silica sand particles (Group B). Other main influencing parameters considered in this study are gas velocity, installation level, inclination angle etc.

Typical transient stress signals on the slat measured under the three different operation states of FCC and silica sand beds are measured by the test slat. In defluidized state, the slat was subject to a downward force due to the weight of the upper un-fluidized particles. When the fluidized air was supplied, i.e. during start-up state, the slat encountered a very high upward force impulse. Then, when the beds were completely transformed into a steady fluidized state, the slat suffered continuous fluctuating forces induced by the dynamic gas-solids behaviors in beds. The magnitudes of the fluctuating force signals are several times smaller than that measured during start-up state, especially in FCC bed. The results indicate that, for the three different operation states, start-up process is the most dangerous condition for internals immersed in fluidized bed reactors. The designer should be paid more attention to this special operating condition. If the baffles immersed fluidized bed reactors without consideration of the special condition in design, the high transient load may lead to serious damage of immersed internals and their supports. On the other hand, if the situation happened in experimental unit, it also may result in damage of inserted expensive measurement instruments.