Particulate fluidization references

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. 

Hydrodynamic stability of uniform vertical suspension flow has been theoretically treated for more than 30 years (see reference [15,20,29,32-34,43,47], and also reference [48-36]). Much of this work has been undertaken when analyzing the important problem of reasons causing the transition from homogeneous (particulate) fluidization to nonhomogeneous (aggregative) fluidization, and subsequently, providing for the spontaneous origination in a fluidized bed of cavities (bubbles) almost devoid of particles. 

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiUary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular soflds. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the sohds become suspended in the air and move freely through the bed. 

Because of the inadequacies of the aforementioned models, a number of papers in the 1950s and 1960s developed alternative mathematical descriptions of fluidized beds that explicitly divided the reactor contents into two phases, a bubble phase and an emulsion or dense phase. The bubble or lean phase is presumed to be essentially free of solids so that little, if any, reaction occurs in this portion of the bed. Reaction takes place within the dense phase, where virtually all of the solid catalyst particles are found. This phase may also be referred to as a particulate phase, an interstitial phase, or an emulsion phase by various authors. Figure 12.19 is a schematic representation of two phase models of fluidized beds. Some models also define a cloud phase as the region of space surrounding the bubble that acts as a source and a sink for gas exchange with the bubble. 

The term impinging stream dryer (ISD) refers to a class of flash dryers in which moisture evaporation from wet particles or liquid droplets occurs in the impingement zone that develops as a result of the collision of two oppositely directed high-velocity gas streams, at least one of which contains the dispersed material to be dried. At the outset a distinction should be made from the impingement dryers in which gas jets are directed onto the web, sheet-form, or slablike materials (Mujumdar and Huang, 1995) or the jet-zone dryer in which a layer of particulates is pseudo-fluidized by a multiplicity of high-velocity airstreams exiting a number of perpendicularly oriented air nozzles (Kudra and Mujumdar, 1995). 

Loss of catalytic activity can occur in several ways, but firstly we will consider simple physical loss. Particulate catalysts used in agitated liquid phases or fluidized beds are liable to both wear and fracture through collisions of the particles with each other, the vessel walls and fittings. The process is usually referred to as attrition. The finest particles formed tend to escape the main separation or filtration equipment, and continual make-up of the catalyst charge is required. 

In essence, inequality dp /d(t) < 0 represents the condition of absolute thermodynamic instability for the pseudo-gas. Similarly, transformation of the pseudo-gas uniform state to the chaotic nonuniform state resembles, to all appearances, the well-known process of spinodal decomposition of thermodynamically unstable molecular and colloidal systems. Nothing like such a nonuniform state has ever been observed under conditions of incipient fluidization. This obviously calls into question the adequacy of nonmonotonous concentrational dependencies for fluidized bed particulate pressure which are occasionally derived in the literature (see, for example, reference [21]). 

The peas can be considered spherical and homogeneous in size. The ideal void-age = 0.4, can therefore, be used. Consulting any reference book (or appendix of some engineering textbook) listing physical properties of materials, at approximately -5°C the density of air is in the order of 1.3 kg/m while its viscosity is around 1.6 x 10 Pa s. The particulates are, obviously, large and so, the minimum fluidization velocity should be transposed from the expression of Re, , in Equation 7.14 ... 

Particles, whether solid or fluid, are usually dispersed within a fluid. The fluid phase that contains the particles is referred to as the continuous phase, sinee it is possible to move throughout that phase while remaining within it. The gap between the density of the particles, denoted by p, and the density /y of the eontinuous fluid phase is a key parameter of the particulate media under consideration, because this gap either intervenes directly in the study of mechanical separation (as in the case of gravitational or centrifugal separation or in fluidization) or because the mechanical stabihty of a deposited granular medium depends on it. The third essential parameter is the size > of the particles. The processes considered in this chapter are associated with a flow of the continuous fluid phase. The dynamic viscosity, of the continuous fluid phase is, therefore, also a parameter to be taken into account. Lastly, for fluid particles, the flow of the continuous phase generates a flow inside the fluid particles and, in that case, it is also necessary to introduce the dynamic viscosity inside the dispersed fluid particles. 

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