Discussion
A packed bed of 480 kg solid particles having particle size of 0.15 mm and density of 800 kg m−3 is fluidized using air at 25 °C and 1 atmospheric pressure. If the cross section of the empty bed is 0.45 m2 and voidage at minimum fluidizing condition is 0.5, then the minimum height of the fluidized bed, in m is
Answer: C
The minimum height of the fluidized bed is about 2.7 m.
At minimum fluidization the solids occupy the fraction \((1 - \varepsilon_{mf})\) of the bed volume, so the solid mass fixes the bed height:
\[ \begin{aligned} m &= \rho_{s} A L_{mf} (1 - \varepsilon_{mf}) \\ L_{mf} &= \frac{480}{800 \times 0.45 \times 0.5} \\ &= \frac{480}{180} = 2.67\ \text{m} \end{aligned} \]
The particle size and the air temperature and pressure are needed for the minimum fluidizing velocity, not for the bed height. Ignoring the voidage gives 1.33 m, and the larger values would need a smaller area or a higher voidage than given.
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