Fluidized bed voidage

The fluidized-bed voidage at minimum bubbling conditions (emb) also has been shown to increase with increasing temperature (Fig. 7) and increasing pressure (Fig. 8). 

In particulate fluidization, for us > u[m and Rep <10, the relationship between the fluidized bed voidage and velocity can be derived from the Ergun equation (McCabe et al., 1983) ... 

In this equation, Re is tlie particle Reynolds number based on the minimum superficial velocity for fluidization. Moreover, for fixed-beds, we can set = 1 and sf = s. The correlation is applicable for void fractions between 0.4 and 0.8 with particle density up to 480 lb/ft3. Note that by changing the Rep number, the fluidized bed voidage ef is changed. 

It should be noted that in the material balances, the fluidized bed voidage should be used instead of fixed-bed voidage, and that the appropriate mass transfer correlations should be used for particulate fluidization. 

FIGURE 4-11 Results of calibrating optic fiber probe in liquid-solid fluidized bed (voidage between 0.6 to 0.9) (Qin and Lin, 1982). 

Botterill JSM, Teoman Y, Yuregir KR. The effect of operating temperature on the velocity of minimum fluidization bed voidage and general behaviour. Powder Technology 31 101-110, 1982. 

A nozzle level only for steam injection provides the minimum fluidization (bed voidage >0.4) and permits an independent moderator supply. Further nozzle levels for the injection of steam and oxygen are situated above it [125,137]. 

Fig. 3. Zenz plot. Correlation of bed voidage, S, the volume fraction of the fluidized bed that is occupied by gas, for values of S from 1.0 to 0.5, and dimensionless velocity and particle properties, where p — p )/(3 PgU ). The horizontal lines represent the different values of e = 0.5...

Eor turbulent and fast-fluidized beds, bubbles are not present as distinct entities. The following expression for bed voidage, bed occupied by gas, where U is in m/s, has been suggested (17) ... 

Before the bed can become fluidized, however, the particles must dislodge from their packed state, which expands the bed. Thus, the porosity (s) in Eqs (14-5) and (14-9) is not the initial packed bed porosity but the expanded bed porosity at the point of minimum fluidization ( ml ), i.e., the minimum bed voidage in the bed just prior to fluidization. Actually, the values of C and C2 in Eq. (14-8) that give the best results for fluidized beds of uniform spherical particles have been found from empirical observations to be ... 

Horio s scaling law derivation was based on the requirement that two similar circulating fluidized beds have equal values of voidage distribution, dimensionless core radius, gas splitting to core and annulus, solid splitting to core and annulus, and cluster voidage. The CAFM equations were then examined to determine how these requirements could be met. 

Daw, C. S., and Harlow, J. S., Characteristics of Voidage and Pressure Signals from Fluidized Beds using Deterministic Chaos Theory, Proc. 11th Int. Conf. FluidizedBedComb., 2 777 (1991)... 

Zhang, W., Tung, Y., and Johnsson, F., Radial Voidage Profiles in Fast Fluidized Beds of Different Diameters, Chem. Eng. Sci., 46(12) 3045 (1991)... 

The Hydrodynamic Theory of fluidized bed stability was proposed by Foscolo and Gibilaro who adapted the stability principle of Wallis. They postulated that a fluidized bed is composed of two interpenetrating fluids. One fluid is the gas phase, and the solids phase is also considered as a continuous fluid phase. In this theory, voidage disturbances in the bed propagate as dynamic and kinetic waves. The stability of the fluidized bed depends upon the relative velocities of these two waves. The velocities of the kinetic wave (ue) and the dynamic wave (nj are ... 

Louge, M., Lischer, J., and Chang, H., Measurements of Voidage Near the wall of a Circulating Fluidized bed Riser, Powder Tech., 62 269-276 (1990)... 

Silva, E. M. V., Ferreira, M. C., and Freire, J. T., Mean Voidage Measurements and Fluid Dynamics Analysis in a Circulating Fluidized Bed with a Spouted Bed Type Solids F eeding System, Proc. of2nd European Thermal-Sciences and 14th UIT National Heat Transfer Conference, Rome (1996)... 

Thus, the three-dimensional voidage distribution in a fast fluidized bed can be determined, semi-empirically as our understanding stands at the present, from the physical properties of the solids and the gas and the operating variables. 

Tung, Y., Li, J., and Kwauk, M., Radial Voidage Profiles in a Fast Fluidized Bed, Proc. of Third China-Jpn. Symp. on Fluidization, p. 139, Beijing, China (1988)... 

Wang, N., Li, Y., Zheng, X., and Kwauk, M., Voidage Profiling for Fast Fluidization, First Intern. Conf. Circulating Fluidized Beds, Halifax, Canada (1985)... 

We next consider the volume fractions of the various regions in the fluidized bed. The volume fraction of bubbles, fb, m3 bubbles (m3 bed)-1, can be assessed from the point of view of either voidage or velocity. In terms of voidage, if we assume the void fraction... 

A first-order reaction, A -> products, is conducted in a fluidized-bed reactor. Before fluidization, the bed is 1.8 m deep (Lpa), with a voidage (epa) of 0.42. The feed gas (P/ = 0.45 kgm-3 p,y = 4.2 x 10-5 Pa s) flows through the bed at a velocity three times the minimum fluidization velocity. The following data are also available ... 

Calculate the pressure drop (-AP/kPa) for flow of fluid through a fluidized bed of solid particles based on the following data the difference in density between solid particles and fluid is 2500 kg m-3 at mf conditions, the bed voidage (em/) is 0.4, and the bed depth (Lm ) is 1.2 m. State any assumptions) made. 

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