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used here (Geldart's Group D powders), semicircular columns could provide information very similar to that from the circular ones.

3.4Fines Residence Time in Jetting Fluidized Beds

In operating a fluidized bed reactor such as a fluidized bed coal gasifier, fine particles tend to be elutriated from the fluidized bed. The elutriated fines, if not recovered, represent a significant carbon loss and thus a significant loss of reactor efficiency. In actual industrial practice, the fines are recycled back to the fluidized bed for further consumption. The location of the fines reinjection point into the fluidized bed reactor is important in order to maximize the consumption of fines in each pass. Otherwise, the fines will build up in the recycle loop and increase the heat load of the reactor operation. The fines reinjection location is selected to maximize the fines residence time in the bed and to provide an conducive environment for consumption, such as high temperature and an oxidizing atmosphere.

There is an abundance of literature on entrainment and elutriation of solid particles from operating fluidized beds of various sizes (see Wen and Chen, 1982; Colakyan and Levenspiel, 1984, for reviews). However, there are very few studies on fines residence time distribution in the bed and the effect of fines injection location. Geldart (1981) has measured the fines distribution in a bed of coarse particles with a bed-slumping technique and found that fines concentration increased toward the bed surface. Geldart and Pope (1983) also observed considerable interaction between the fines and the coarse particles not only in the bed but also in the freeboard as well. The only study found to be relevant to the subject under discussion here is that by Valenzuela and Glicksman (1983). They measured the residence time of fine iron powder (38–53 µm) in a two-dimensional bed of 700 µm glass beads and obtained a residence time between 5.8 and 6.5 seconds, depending on particle sizes, significantly larger than the gas residence time of 2.3 seconds. They concluded that because of the interaction between the fines and coarse particles in the bed, the fines residence time could be substantially longer than the gas residence time. Fines smaller than 10 µm usually percolate through the bed unhindered while the fines larger than 150 μm in size migrate due to the bulk movement of the coarse bed material.

Yang and Kearins (1987) describes the experiments carried out in a 30-cm diameter laboratory unit to study the fines residence time

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distribution in the bed by injecting -170 +230 mesh crushed acrylic fine particles (average particle size = 75 μm) into the bed at two different locations, with two different bed heights, and at three different bed operating conditions. The experimental results were corrected for the transitory time required from the bed surface to the particle collecting device by solving twenty-five simultaneous equations to arrive at the true fines residence time distribution in the bed. Experiments were carried out with fines injection both radially and coaxially. In the radial injection, the fines were injected into the jetting fluidized bed below the bed surface. In the coaxial injection, fines were injected through the central jet.

Some conclusions drawn from the experimental results are:

(i)The fines residence time in a jetting fluidized bed is of the order of 10 to 20 seconds. The transitory time of fines from the bed surface to the collecting device, the cyclone dipleg, dominates the characteristic particle collection time curves experimentally observed.

(ii)The initial collection of fines increases with increasing jet velocity and overall fluidizing gas velocity. This is true for both radial and coaxial injection of fines. However, the effect of jet velocity affects the coaxial fines injection more pronouncedly.

(iii)The rates of fines collection was the highest for the case where the fines were injected coaxially through the central jet. When the fines are injected radially into the bed, the rate of fines collection is higher for the experiments with lower bed height compared with that of higher bed height. Apparently, the elutriation mechanisms of fines are different and depend on the location of the fines injection. Substantial increase in fines collection rate for coaxial injection of fines is not surprising. The fines injected coaxially tend to concentrate in and around the jet region initially and are carried over by the eruptions of bubbles generated at the tip of the jet or by periodic penetration of the jet through the fluidized bed. The higher initial collection rate of fines for experiments carried out at lower bed height is indicative of a shorter fines residence time in the bed.

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(iv)Experiments performed during this study indicate that fines injected into the freeboard region above the bed stay in the freeboard region substantially longer than the simple transit time would indicate. This is consistent with the observation by Morooka et al. (1980) and Pemberton and Davidson (1984, 1986). Morooka et al. (1980) studied the behavior of FCC particles suspended in the freeboard region of a 12-cm I.D. fluidized bed and observed a flow pattern of internal circulation in the freeboard region. The fines traveled upward in the central region and downward closer to the wall. The holdup of circulating solids in the freeboard region was much larger than the net entrainment rate of solid particles. If this is the case, the design and configuration of the gas outlet in the freeboard region can have an important effect on the fines residence time distribution and holdup above the bed. The importance of the freeboard region for further reaction of fines merits greater emphasis in both experimental and theoretical studies.

3.5Scale-up Considerations

The development of commercial fluidized bed processors generally

requires intermediate stages of testing on physical models simulating commercial equipment. Simulation (or scale-up) criteria derived from fluidized bed momentum-conservation relations may be applied to determine the design and operating conditions for physical models discussed in Ch. 1 on Fluidized Bed Scale-up. Such criteria, while not totally established at this time, have been applied to simulate a pressurized fluidized bed gasifier having a large, vertical central jet for fuel feeding, combustion and gasification (Yang et al., 1995).

Physical models of commercial fluidized bed equipment provide an important source of design information for process development. A physical model of a commercial fluidized bed processor provides a small-scale simulation of the fluid dynamics of a commercial process. While commercial processes will typically operate at conditions making direct observation of bed fluid dynamics difficult (high temperature, high pressure, corrosive

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environment), a physical model is designed to allow easy observation (room temperature and pressure, nonreactive atmosphere, transparent vessel).

Cold flow studies have several advantages. Operation at ambient temperature allows construction of the experimental units with transparent plastic material that provides full visibility of the unit during operation. In addition, the experimental unit is much easier to instrument because of operating conditions less severe than those of a hot model. The cold model can also be constructed at a lower cost in a shorter time and requires less manpower to operate. Larger experimental units, closer to commercial size, can thus be constructed at a reasonable cost and within an affordable time frame. If the simulation criteria are known, the results of cold flow model studies can then be combined with the kinetic models and the intrinsic rate equations generated from the bench-scale hot models to construct a realistic mathematical model for scale-up.

The need for physical modeling of fluidized bed processors is dictated by the state-of-the-art of fluidized bed scale-up technology. In general, no rational procedure exists for scaling up a new fluidized bed processor concept that precludes the need for physical modeling. Many empirically developed “rules-of-thumb” for fluidized bed scale-up exist in specific areas of fluidized bed application which are not generally applicable. Existing mathematical modeling approaches are themselves based heavily on empirical descriptions of fluidized bed fluid dynamics. These bubbling bed models can be applied only where confidence exists for the empirical bubble flow description built into the model. Fluidized bed processors operate over such a broad range of fluid dynamic regimes that this confidence rarely exists for new concepts.

Yang et al. (1995) described the application of this scale-up approach. Comprehensive testing programs were performed on two relatively large-scale simulation units for a period of several years: a 30-cm diameter (semicircular) Plexiglas cold model and a 3-m diameter (semicircular) Plexiglas cold model, both operated at atmospheric pressure.

The results are highly significant to the development of fluidized bed technology because they represent a case study of a rational fluidized bed development approach, and because the extensive data generated are unique in their equipment dimensions, pushing existing models and correlations to new extremes and offering new insights into large-scale equipment behavior. The understanding of the hydrodynamic phenomena developed from the cold flow model studies and the analytical modeling reported was

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integrated with parallel studies that investigated coal gasification kinetics, ash agglomeration, char-ash separation and fines recycle to develop an integrated process design procedure.

3.6Applications

The primary applications for large-scale jetting fluidized beds are in the area of coal gasification as described by Yang et al. (1995), Kojima et al., (1995), and Tsuji and Uemaki (1994). Smaller scale applications are for fluidized bed coating and granulation to be discussed in Chapter 6.

NOTATIONS

B= buoyancy flux

C= tracer gas concentration

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L= height of draft tube, or height of downcomer, or distance between the distributor plate and the draft tube inlet

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μ= viscosity of the fluid

φ= volumetric solids loading

θ= jet half angle

α= angle of conical distributor plate

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Anagbo, P. E., “Derivation of Jet Cone Angle from Bubble Theory,” Chem. Eng. Sci., 35:1494 (1980)

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Blake, T. R., “Gas Jets in Fluidized Media, Turbulent Diffusion Flames, and Condensing Vapor Jets in Liquids,” Powder Technology (1996)

Botterill, J. S. M., George, J. S., and Besford, H., “Bubble Chains in Gas Fluidized Beds,” Chem. Eng. Symp. Series, 62(62):7 (1966)

Botterill, J. S. M., and Bessant, D. J., “The Flow Properties of Fluidized Solids,” Powder Tech., 8:213 (1973)

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Broadhurst, T. W., and Becker, H. A., “Measurement and Spectral Analysis of Pressure Fluctuations in Slugging Beds,” Fluidization Technology, Hemisphere Publishing Corporation, New York, 1:63 (1976)

Buchanan, R. H., and Wilson, B., “The Fluid-Lift Solids Recirculator,” Mech. Chem. Eng. Trans. (Australia), pp. 117 (1965)

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Curran, G. P., Pasek, B., Pell, M., and Gorin, E., “Pretreatment of Bituminous Coals for Pressure Gasification,” paper presented at the Fluidized Bed Combustion Symposium, American Chemical Society Meeting, Chicago (1973)

Davidson, J. F., and Harrison, D., Fluidized Particles, Cambridge University Press, England, (1963)

Decamps, F., Dumont, G., and Goossens, W., “Vertical Pneumatic Conveyor with a Fluidized Bed as Mixing Zone,” Power Tech., 5:299 (1971/1972)

Donsi, G., Massimilla, L., and Colantuoni, L., “The Dispersion of Axisymmetric Gas Jets in Fluidized Beds,” Fluidization, (J. R. Grace, and J. M. Matsen, eds.), Plenum Press, New York, pp. 297 (1980)

Ettehadieh, B., Yang, W. C., and Haldipur, G. B., “Motion of Solids, Jetting and Bubbling Dynamics in a Large Jetting Fluidized Bed,” Powder Tech., 54:243 (1988)

Fan, L. S., Hwang, S. J., and Matsuura, A., “Some Remarks on Hydrodynamic Behavior of a Draft Tube Gas-Liquid-Solid Fluidized Bed,” AIChE Symp. Ser., 80(234):91 (1984)

Foong, S. K., Barton, R. K., Ratcliffe, J. S., and Aust, F. I. E., Mech. Chem. Eng. Trans. (Australia), 7 (1975)

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