Fluidization, characteristic regions

The solids circulation pattern and solids circulation rate are important hydrodynamic characteristics of an operating jetting fluidized bed. They dictate directly the solids mixing and the heat and mass transfer between different regions of the bed. 

Figure 28 shows that the pressure drop of a conical bed reaches a maximum value at the initial fluidization point (GR)i, and it drops at higher fluid rates. Inherent in this drooping pressure-drop characteristic lies the instability of conical bed operation, especially with gas as the fluidizing medium, for as soon as fluid rate reaches (GR) , the decrease in pressure drop induces higher flow from a compressible medium. As the pressure expends itself, fluid flow drops to even lower values, only to permit reaccumulation of pressure because of reversion to the higher pressure-drop region of the system. 

To account for the intrinsic characteristics of particle-fluid two-phase flow in fast fluidization, the particles and the fluid are considered to interact with each other on both a micro-scale and meso-scale level to produce local or meso-scale heterogeneity (phases), and the overall fluid-particle system interacts with the equipment boundaries on a much larger scale to produce macro-scale heterogeneity (regions). 

As noted by the correlation of Kim et al.,58 any quantitative evaluation of the holdup characteristics of a three-phase fluidized bed must consider the phenomenon of bed contraction.137 The wakes of the bubbles rising in a three-phase fluidized bed consist of a particle-free liquid immediately below the bubble (commonly known as liquid wake ) and a lower region of particles and liquid which apparently moves with liquid (known as particulate region ). Particulate elutrition is caused by this particulate region, whereas bed contraction is caused by the liquid wakes. 

The main objective of the test program is to burn processed MSW in a fluidized-bed boiler environment to investigate combustion characteristics, heat transfer, corrosion and erosion, and control parameters. The tests, are being conducted in a CPC-owned 0.7 sq m fluidized-bed combustor which has been reworked to incorporate water tubes for heat extraction in both the bed region and exhaust. Additional tubes will be air cooled to simulate boiler temperatures and to observe erosion and corrosion phenomena, detailographic examination will be made of these materials. 

The bubble phase formed by gas in excess of that required for the onset of fluidization. The bubble is usually surrounded by a cloud of gas-soM mixture and is characterized by an indentation caused by suction due to the upward movement of the bubble. The solids that flU up this region are called the wake. The bubbles are usually large and move faster than the surrounding emulsion gas flowing at u p thus giving rise to the cloud. This behavior is usually characteristic of Geldart A and B particles (see Section 11.4.2.2). 

Although, for example, Fig. 4.48A and D shows very similar characteristics, the difference in fluidization velocity causes the shape of the stagnant zones to the left and right to be different, and with 140% background fluidization particles are ejected into the freeboard region to a larger extent. 

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