The "fluidization" reflects the conferring of fluid-like properties onto the solid particles, much as the term liquefaction is used to denote the act of making liquid or conferring liquid properties. Fluidization occurs when gas or liquid is forced to flow vertically through a bed of particles at such a rate that the buoyed weight of the particles is completely supported by the drag force imposed by the fluid. When fluidization occurs, the bed of particles gains the appearance and many properties of a true liquid or fluid. Almost all liquid- fluidized beds expand smoothly without bubbles when the superficial velocity of the fluidizing fluid is increased beyond Umf is has commonly been called "particulate" or homogeneous Fluidization , which occurs for almost all gas fluidized beds, including all those of practical importance. The high-velocity fluidized bed, extending in a riser reactor, consists of a dense bottom bed, a transition zone, and a transport zone.