Dense phase recirculation

The use of recirculating fluid beds has caused considerable interest in dense phase vertical conveying. These units are indeed dense phase transport systems with a significant amount of recirculation taking place. 

Knowlton has cautioned on the difference between small diameter and large diameter systems for pressure losses. The difference between these systems is especially apparent for dense phase flow where recirculation occurs and wall friction differs considerably. Li and Kwauk (1989, 1989) have also studied the dense phase vertical transport in their analysis and approach to recirculating fluid beds. Li and Kwauk s analysis included the dynamics of a vertical pneumatic moving bed upward transport using the basic solid mechanics formulation. Some noncircular geometries were treated including experimental verification. The flows have been characterized into packed and transition flows. Accurate prediction of the discharge rates from these systems has been obtained. 

The sketch on the far right of Fig. 7.88 identifies a spouted bed [B.56], Typically, to obtain the spouted bed condition, gas is introduced in a relatively narrow area in the bottom center of the particle bed (Fig. 7.89a). Movement of the particles in such an arrangement is caused by a vertical, steady axial jet and is rather regular. The bed particles circulate much like a water fountain they are carried-up in the central spout as a dilute phase until they loose their momentum and fall back onto the top of the bed towards the outer periphery. The particles then recirculate back down as a dense moving bed, are directed back into the gas stream by the normally conical base of the apparatus, and begin again their upward flow. 

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