Yang Fluidization, Solids Handling, and Processing

146 Fluidization, Solids Handling, and Processing

Riser Radial Location, cm

Riser Radial Location, in

Figure 25. The effect of pressure on radial density distribution in a riser.

1.11 Cyclone Efficiency

Rosin et al. (1932) derived a simple expression for cyclone efficiency for a fixed cyclone diameter based on a force balance on the particle at the cyclone inlet, i.e.,

This expression indicates that cyclone efficiency should not change with gas density because (ρp - ρg) is insensitive to changes in pressure. However, cyclone efficiency should decrease with gas viscosity because it is harder for a particle to travel to the cyclone wall in a more viscous fluid.

There have been relatively few literature articles reporting on the observed effects of temperature and pressure on cyclone efficiency.

Pressure and Temperature Effects 147

Parker et al. (1981) conducted tests with a 5 cm diameter cyclone at temperatures up to 700°C with 20 micron fly ash. They found that cyclone efficiencies decreased with temperature as expected. Patterson and Munz (1989) investigated the effect of temperature on cyclone efficiency up to 1700°C in a 10-cm-diameter cyclone, and also found that cyclone efficiency decreased with temperature. However, this effect was only seen at very small particle sizes (below about 12 microns) where cyclones become relatively inefficient.

According to Eq. (17), pressure should have a negligible effect on cyclone efficiency. However, Parker et al. (1981) reported that gas density caused cyclone efficiency to increase over the pressure range of 2 to 13 atmospheres. Knowlton and Bachovchin (1978) varied system pressure up to 50 atmospheres and found that, at low loadings for a conventional cyclone, the efficiency increased with increasing pressure. At higher loadings, the opposite effect was noticed. As with the studies investigating the effect of temperature, the effect of pressure in both studies was only seen at particle sizes less that about 12 microns. Therefore, for particles greater than about 10 to 15 microns, cyclone efficiency seems be insensitive to gas properties.

NOTATIONS

E= Cyclone efficiency

F= Weight fraction of 0–44 micron particles in mixture

148 Fluidization, Solids Handling, and Processing

g= Gravitational constant

h= Overall bed-to-surface heat transfer coefficient

ε= Voidage

μ= Gas viscosity

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154 Fluidization, Solids Handling, and Processing

As discussed in other chapters of this book, two-phase flows of gas and particles occur with different flow regimes. The mechanisms for heat transfer and the resulting heat transfer coefficients are strongly affected by the different flow characteristics, resulting in different design correlations and predictive models for each flow regime. This chapter will deal with the two most often encountered flow regimes:

•Bubbling dense fluidization

•Circulating fast fluidization

2.0BUBBLING DENSE FLUIDIZATION

2.1Hydrodynamic Characteristic

The key characteristic of fluidized beds is the levitation of solid

particles against gravity by shear-drag of the flowing gas. The resulting two-phase mixture of gas and particles behaves as a “fluid” in its response to pressure gradients. Upward flow of gas past solid particles can be classified into several fluidization regimes. At low flow rates, the gas percolates through the void spaces between stationary particles; this is termed flow through packed beds. With increasing gas velocity, a point is reached when the drag force between particles and fluid counterbalances the gravitational force on the particles resulting in suspension of particles by the upward flowing gas. This is the point of minimum fluidization, marking the transition from packed to fluidized beds. Particles of Geldart (1973) classification A or B (diameters in range of 50 to 500 μm and densities in range of 0.3 to 3 gm/cm3) can be smoothly fluidized. For such particles, gas velocities above the minimum fluidization value result in bubbling fluidization wherein some fraction of the gas flows as discrete bubbles through the mobile mixture of suspended particles. The upper limit of gas velocity for bubbling dense fluidization occurs when interfacial drag becomes sufficient to entrain the particles and transport them upward through the vessel. Thus bubbling fluidization is bounded between gas velocities for minimum fluidization and terminal transport.

Using the Ergun (1952) equation for the interfacial friction factor, Wen and Yu (1966) derived the following general equation to estimate the minimum fluidization superficial velocity Umf for spherical particles: