Fluidization particulate

Particulate fluidization, where the fluidizing medium is usually a liquid, is characterised by a smooth expansion of fhe bed. Liquid-solid fluidized beds are used in continuous crystallisers, as bioreactors in which immobilised enzyme beads are fluidized by the reactant solution and in physical operations such as the washing and preparation of vegetables. The empirical Richardson-Zaki equation (Richardson 

Figu re 1.22 Richardson-Zaki plot measurement of minimum fluidizing velocity for particulate fluidization. 

The Richardson-Zaki equation has been found to agree with experimental data over a wide range of condifions. Equally, if is possible fo use a pressure drop-velocity relationship such as Ergun to determine minimum fluidization velocity, just as for gas-solid fluidizafion. An alternative expression, which has the merit of simplicify, is fhaf of Riba ef al. (1978) 

For a bed with Group A particles, bubbles do not form when the gas velocity reaches Umf. The bed enters the particulate fluidization regime under this condition. The operation under the particulate fluidization regime is characterized by a smooth bed expansion with an apparent uniform bed structure for Umf U Umb, where Umb is the superficial gas velocity at the minimum bubbling condition. The height of the bed expansion in terms of a can be estimated by [Abrahamsen and Geldart, 1980a] 

The gas velocity for the upper bound of the particulate fluidization regime is Umb. The operational range of the particulate fluidization regime is quite narrow, as shown in Fig. 9.6. It is seen that for U Umf, Hf monotonically increases with U until the minimum bubbling point is reached. Further increase in the gas velocity leads to a decrease in the bed height as bubbling fluidization begins. 

The particulate fluidized regime will occur when the following criterion is met [Wilhelm and Kwauk, 1948] 

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Particulate Fluidization Fluid beds of Geldart class A powders that are operated at gas velocities above the minimum fluidizing velocity (L/, y) but belowthe minimum bubbhngvelocity (L/, i) are said to be particulately fluidized. As the gas velocity is increased above L/, y, the bed further expands. Decreasing (p, — Py), d and/or increasing increases the spread between L/, yand U, b until at some point, usually at high pressure, the bed is fully particulately fluidized. Richardson and Zald [Trans. Inst. Chem. Eng., 32, 35 (1954)] showed that U/U = E , where /i is a function of system properties, = void fraction, U = superficial fluid velocity, and Uj = theoretical superficial velocity from the Richardson and Zald plot when = 1. 

As mentioned above batch crystallizers are usually simple vessels provided with some means of mechanical agitation or particulate fluidization. These have the effect of reducing temperature and concentration gradients, and maintain crystals in suspension. Baffles may be added to improve mixing and heat exchange or vacuum systems may be added, as appropriate. Various design combinations are available and some are illustrated in Figure 7.1. 

Y. T, Shah, Design Parameters for Mechanically Agitated Reactors Mooson Kwauk, Particulate Fluidization An Overview... 

While Fig. 56 demonstrates, from modeling, the disparate nature between G/S and L/S fluidization, Fig. 57 shows continuity in particle-fluid behavior through properly selected intermediate systems, thus reconciling through theory the phenomenological discrimination between aggregative and particulate fluidization. 

Figure 57. Computation results showing gradual transition from particulate fluidization to aggregative fluidization. (Li, 1987.)...

Kwauk, M., Particulate Fluidization in Chemical Metallurgy, Scientia Sinica, 16 407 (1973)... 

Particulate composites, 26 754-755 ceramics processing, 5 653-654 extensional modulus of, 26 777 fabrication of, 26 766 Particulate emission limits, 13 183 Particulate emissions, reducing, 11 689 Particulate fillers, ll 302t, 303 for rubber, 21 772, 773 Particulate fluidized-bed regime, 11 801 Particulate impurities in gases, 13 464 in high purity gases, 13 466 Particulate matter (PM), 1 798-801 ... 

It has not been possible to cover all aspects of the principles of fluidization. A number of comprehensive texts on fluidized bed behaviour are available and inevitably I have drawn heavily on these. The reader who wishes to go into greater depth about the fundamental mechanisms at work in fluidized beds should consult those works by Davidson and Harrison (1971), Botterill (1975), Davidson, Clift and Harrison (1985), Kunii and Levenspiel (1991) and more recently Gibilaro (2001). Full references can be found at the end of Chapter 1. In addition, I have concentrated on gas-solid fluidized beds somewhat to the exclusion of liquid-solid fluidization although an indication of how particulate fluidization can be applied to biochemical reactors is given in Chapter 7. 

Main characteristic Ideal for fluidization particulate fluidization is possible Only bubbling fluidization Difficult to fluidize Coarse solids... 

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) ... 

The following analysis holds for Type B fluidization and for Type A bubbling fluidization, when the region of particulate fluidization is so small that it can be ignored. In the framework of the two-phase model (see the subsection Hydrodynamic modeling of bubbling fluidization), the bed expansion in terms of the fraction of the bed occupied by bubbles is... 

In Figure 3.54, it is clear that while approaching the limit of Group B particles (large particle density and particle size), uhm approaches wfm, as expected. On the other hand, the difference between the two velocities is increased constantly for smaller particles and lower particle density, while it is very high for dp<0. 045 mm where A0 45 = 1. This could be useful in the case where an expanded region of particulate fluidization is desirable in gas-solid systems. 

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