$21.$ Fluidized beds are characterized by their ability to maintain solid particles in a $\_\_\_\_\_\_\_\_$ state, facilitated by the upward $\_\_\_\_\_\_\_\_$ of fluid.

$22.$ The efficiency of gas$-$solid liquid contactors depends on the $\_\_\_\_\_\_\_\_$ distribution within the system and the efficiency of $\_\_\_\_\_\_\_\_$ transfer.

$23.$ Pressure drop in multiphase systems can be estimated using models that consider the $\_\_\_\_\_\_\_\_$ of all phases and the system's $\_\_\_\_\_\_\_\_$ characteristics.

$24.$ Bubble size and gas holdup in bubble column reactors are key parameters affecting the reactor's $\_\_\_\_\_\_\_\_$ efficiency and $\_\_\_\_\_\_\_\_$ dynamics.