$17.$ Scale$-$up challenges in fluidized bed reactors include maintaining consistent $\_\_\_\_\_\_\_$ dynamics and $\_\_\_\_\_\_\_$ transfer efficiency.

$18.$ Trickle bed reactors suffer from operational issues such as $\_\_\_\_\_\_\_$ and $\_\_\_\_\_\_\_$ under certain flow conditions.

$19.$ The gas $\_\_\_\_\_\_\_$ in a bubble column reactor is influenced by the rate of gas $\_\_\_\_\_\_\_$ and the properties of the liquid medium.

$20.$ In the drift flux model, the coefficient C adjusts for the distribution of the $\_\_\_\_\_\_\_$ phase relative to the $\_\_\_\_\_\_\_$ phase.