Particulate solids, when packed within a container, form an arrangement allowing for the void space between particles to be filled by a gas, normally air. Any particular fluid flowing at low velocities through a porous bed of solid particles, for example, in a packed tower, does not cause the particles to move. The fluid passes through the small, tortuous channels, losing pressure energy. The pressure drop in a stationary bed of solids can be described by fundamental fluid mechanics theory. The form of the curve of pressure drop against velocity for the fixed and fluidized beds should provide considerable amount of information on structure of the bed. The transference of gas between dense and bubble phases within a fluidized bed is an important factor, which must be considered in designing fluidized beds as reactors. Spouted beds feature special advantages compared with normal fluidized beds when the particles, or rather particulates, to be fluidized cover sizes over the few millimeters range.