Yang Fluidization, Solids Handling, and Processing

Pressure and Temperature Effects 135

jets at the grid which accelerate the particles and cause them to collide with others at a very high velocity.

Sishtla et al. (1989) reported that the particle attrition rate at the grid is proportional to the kinetic energy of the gas jets (ρgUj2) issuing from the grid. They varied both system pressure and temperature while maintaining gas velocity constant, and found that increasing system pressure resulted in an increase in the attrition rate (Fig. 16), while increasing system temperature decreased the rate of particle attrition (Fig. 17). They attributed this behavior to the change in gas density produced by changes in temperature and pressure. Kono (1981) also observed the same effects of temperature and pressure on mechanical attrition.

Time, h

Figure 16. The effect of system pressure on attrition at the grid. (Sishtla, et al.)

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1.8Particle Attrition in Cyclones

There can also be substantial particle attrition in cyclones in fluidized-bed systems because particles are accelerated at the inlet of the cyclone and impacted against the cyclone wall. Although there is little information on particle attrition in cyclones in the literature, it has been reported (Sishtla) that increasing system pressure decreases the attrition rate in cyclones operating with coal char. The mechanism by which this occurred was not determined.

Time, h

Figure 17. The effect of temperature on attrition at the grid. (Sishtla et al.)

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1.9Jet Penetration

Vertical jet penetration in fluidized beds increases significantly with system pressure as shown in Fig. 18. Vertical jets fluctuate in the vertical direction between a minimum and a maximum value. Most jet penetration correlations are given in terms of the maximum jet penetration length which is designated Lmax (Hirsan et al. 1980). Over a pressure range of 1 to 50 bar with jet nozzles up to 3.8 cm in diameter, Hirsan et al. (1980) developed the following empirical correlation for the maximum jet penetration.

This correlation predicts that the maximum vertical jet penetration into a fluidized bed varies with gas density to the 0.67 power, and decreases with increasing fluidizing gas velocity and increasing particle diameter.

Yang (1981) incorporated the high-pressure data of Hirsan et al. (1980) to produce the following correlation for Lmax

Yates et al. (1986) varied system pressure up to 20 bar with jet nozzles of 2 and 4 mm diameter, and also found that the maximum jet penetration varied significantly with pressure. They then developed the following correlation for the maximum jet penetration:

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which is similar in form to the correlation of Yang. Yates and Cheesman also incorporated high-temperature jet penetration data (obtained at temperatures up to 800°C) into this correlation.

Pressure, bar

Figure 18. The effect of pressure on jet penetration. (Knowlton and Hirsan.)

Findlay and Knowlton (1985) investigated the effect of gas viscosity on jet penetration by varying system temperature while maintaining gas density constant. This was accomplished by adjusting system pressure at

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Several other researchers have investigated the effects of pressure as well as temperature on the transition from bubbling/slugging to turbulent fluidization (Cai et al., 1989; Canada and McLaughlin, 1978; Yang and Chitester, 1988). Cai et al. (1989) varied system pressure over a range of 1 to 6 atmospheres for solids with different average particle sizes. They found that Uc decreased with system pressure. Yang and Chitester (1988) varied system pressure up to 64 atmospheres and also reported that increasing pressure caused a decrease in the velocity at which the transition from bubbling to turbulent fluidization occurred.

Gas Velocity

Figure 19. Bubbling-to-turbulent transition.

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Figure 20. The variance of bed pressure drop versus superficial gas velocity.

(Marzocchella & Salatino.)

Canada and McLaughlin (1978), working with 650-micron and 2600-micron particles over a pressure range of 1 to 10 atmospheres, found that Uk decreased significantly with pressure for both particle sizes (Fig. 21). They also reported that the ratio (Uk /Ut) remained relatively constant over this pressure range for both materials.

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Knowlton (1975), and Karri and Knowlton (1996), have also shown that the choking velocity for Group B solids decreases significantly with pressure. This is illustrated in Fig. 22 for 400-micron sand. As system pressure was increased from 1 to 31 bar, the choking velocity decreased from approximately 2.2 to 6.7 m/s for a mass flux of 210 kg/s-m2. This occurs because the carrying capacity of a gas increases as the density of the gas increases due to increased drag on the particles. This results in the pressure-drop-per-unit-length-vs.-velocity curves to be shifted to the left on the phase diagram for vertical pneumatic conveying as shown in Fig. 23.

Figure 22. The variation of choking velocity with pressure. (Knowlton.)