Draft Tube Operated As A Fluidized Bed

The pressure balance for the dense phase in the downcomer in the circulating fluidized system shown in Fig. 2 can be expressed as  

A similar expression can be written for the pressure balance in the draft tube as  

It was experimentally confirmed, using capacitance probes, that the solids flow down the downcomer at close to minimum fluidization, thus ebd = 0. 

The bubble voidage in the draft tube, ebr, was calculated on the basis of the velocity of a rising gas slug relative to its surrounding solids. The total gas superficial velocity in the draft tube, lJjn can be derived to be 

The slug velocity, Usl, is defined as the rising velocity of the slug relative to the particle velocity at its nose and can be expressed as 

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Figure 2. Recirculating fluidized bed concept—draft tube operated as a fluidized bed.

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