(255c) Effects of System Pressure and Imposed Solids Mass Flux On Gas Bypassing in Deep Fluidized Beds of Group A Materials

Bubbling and turbulent beds are widely used in commercial processes because of their intimate gas-solids contacting. Fluidized beds have high mass transfer rates and rapid solids mixing that leads to a nearly homogeneous temperature in the bed. However, recent studies conducted in Plexiglas? units have revealed that significant bypassing of the fluidizing gas can occur in tall, fluidized beds containing Geldart Group A particles. Typically, the gas was observed to preferentially flow in one or more rapidly-moving streams of bubbles near the wall. The remainder of the bed was a mass of defluidized solids. If gas bypassing occurs in industrial beds, such as FCC regenerators, FCC strippers or in CFB combustors, it can result in afterburning in the freeboard, poor gas/solids contacting, poorly fluidized entrances to standpipes and the discharge regions of cyclone diplegs.

The gas bypassing phenomenon has been rarely reported in the literature, because laboratory beds are generally not tall enough and commercial units have limited instrumentation. Experimental work at PSRI has found that gas bypassing is due to increased gas compression at the bottom of tall fluidized beds. Reduction in the solids phase permeability at high gas compression ratios results in a biasing of the gas flow - even with proper grid and sparger designs. Previous studies have shown that gas bypassing can be eliminated or significantly reduced by increasing the fines content, increasing the gas velocity, lowering the bed height or installing properly-spaced horizontal baffles in the entire fluid bed.

Commercial fluidized beds often operate with solids flowing through the bed. PSRI has found that imposing a solids flux on the bed can promote gas bypassing. Solids fluxes over the range of 0 to 80 kg/s-m2 were imposed on a 0.9-meter diameter fluidized bed by continuously recycling the solids around the test unit. A Computational Fluid Dynamic (CFD) analysis of the imposed flux testing using the Barracuda^TM CFD code also showed that gas bypassing was more likely to occur at higher solids fluxes.

Increasing system pressure from atmospheric to 280 kPa was also found to lower or eliminate gas bypassing in a 0.6-meter diameter test unit. In both types of tests, gas bypassing was determined using traversing bubble probes and by high-frequency monitoring of the differential pressure fluctuations in the fluidized bed. This paper discusses the effects of pressure and imposed solids flux on gas bypassing in deep fluidized beds of Group A materials.