$21.$ 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.