Chapter 4 - Experimental observations of voidage in gas fluidized beds

Abstract

This chapter discusses the experimental observations of voidage in gas fluidized beds. Voidage is an important quantity in determining the performance of a gas-fluidized bed. It affects the degree of gas-solid contact and thus, has a direct influence on the chemical reactions occurring in the bed and on the bed's heat and mass transfer characteristics. The magnitude of the voidage in the region of the emulsion phase surrounding a rising bubble in fluidized beds of both Group A and B powders has been a matter of debate since the earliest days of research into fluidization. The voidage of the emulsion phase of a bubbling bed may be measured in a straightforward way by the means of “bed-collapse” method. This technique involves stopping abruptly the flow of gas to a fluidized bed and measuring the rate of collapse of the bed surface. The assumption is that after a short initial period during which the gas bubbles are expelled from the system, the bed surface moves downwards at a constant velocity as a result of particle sedimentation and only ceases to move when the packed-bed state has been reached.