$17.$ True$/$False: Agitated vessels are used in gas$-$solid liquid mixtures primarily to enhance mixing and mass transfer.

$18.$ True$/$False: The pressure drop in packed beds is not influenced by the properties of the packed solids and fluid flow rates.

$19.$ True$/$False: The modification of the drift flux model for liquid$-$liquid flows in mini channels ignores the effects of surface tension.

$20.$ True$/$False: Fluidized beds operate on the principle that solid particles can be suspended by upward$-$flowing fluid.