Well-designed fluidized beds are assumed to have uniform gas and temperature distributions, which in many cases is true. However, some fluidized bed processes experience poor productivity and wide temperature variations despite the fact that traditional design criteria have been met. Particulate Solid Research, Inc. (PSRI) has found that deep or dense fluidized beds are prone to gas bypassing or jet streaming where a large gas jet develops and precesses around the walls of the fluidized bed. The remainder of the bed is either defluidized or poorly fluidized. In some cases, multiple gas jets can occur in the bed. This phenomenon is not typically seen in laboratory beds or pilot plants, as bed heights in these units are usually not high enough to produce this behavior. Gas bypassing tends to be an issue for large pilot plants and commercial units. Research at PSRI has shown that gas bypassing can be mitigated by decreasing the bed height, increasing the gas velocity, adding fines, increasing the freeboard pressure, or adding internals.

Recently, PSRI has found that the addition an imposed solids flux through a fluidized bed can trigger gas bypassing in what was previously a well-behaved fluidized bed. CFD studies suggest that the gas bypassing is stabilized in the region between the existing bed and the material entering the bed. It some cases the entering material can short-circuit the existing bed to a bottom standpipe and the freeboard region. Details of possible techniques for detecting gas-bypassing are presented.