In a process for removing SO ₂ from flue gas, gas at 410°C and 1.2 atm is passed upward through a fluidized bed containing 1.5-mm spherical particles of Al ₂ 0 ₃ impregnated with copper. The particle density is 2,300 kg/m ³ and the gas density and viscosity are almost the same as for air.

(a) Predict the minimum fluidization velocity.

(b) For an initial bed height of 1 m, the expanded bed height should be no greater than 1.5 m. Explain why it is difficult to predict the gas velocity that will cause this amount of expansion.