Entrainment and elutriation

In any fluidized bed operating with a gas velocity above M /Some particles will be transported into the gas stream above the bed surface this is called entrainment. Some of these particles will be transported sufficiently so as to leave the fluidized bed column entirely and this mass of solids is then referred to as carry-over. However, the phenomenon is a little more complex. Fractionation or preferential separation of the bed particles occurs and this changes with height above the bed 

Elutriation is important in most industrial fluidized beds and is generally thought of as a disadvantage. In addition to the small particles which may be present in the initial particle size distribution, fines may be created in the course of operation by the attrition of bed particles. Elutriated particles usually need to be collected and recovered either because they represent the loss of product particles of a given size, because they must be separated from the exhaust gas for environmental reasons, or because of safety concerns there is a considerable risk of a dust explosion with very fine particles and perhaps especially so with many food particulates. Therefore the fluidized bed plant will require ancillary gas cleaning equipment such as a cyclone, filter or electrostatic precipitator to separate the fines from the gas. The loss of a particular size fraction from the bed may change fluidized bed behaviour and it then becomes important to return the fines to the bed continuously. 

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This section is a continuation of Section 21.3.2 dealing with pressure drop (-AP) for flow through a fixed bed of solid particles. Here, we make further use of the Ergun equation for estimating the minimum superficial fluidization velocity, ump In addition, by analogous treatment for free fall of a single particle, we develop a means for estimating terminal velocity, ur as a quantity related to elutriation and entrainment. 

Werther J, Hartge EU. Elutriation and entrainment. In Yang WC, editor. Handbook of fluidization and fluid-particle systems. New York Marcel Dekker 2003. p. 129-154. 

Since the particle population determines almost all relevant mechanisms in a fluidized bed system, attrition may thus strongly affect the performance of a fluidized bed process. For instance, the elutriation and entrainment effects (cf. Chapter 4 of this book), the heat transfer from bed to inserts (Molerus, 1992) or the conversion and selectivity of reactions (Werther, 1992) are affected either directly or through the bed hydrodynamics by the particle size distribution. Therefore some authors (such as Ray et al., 1987a) even claim that a fluidized bed with attritable materials... 

A schematic representation of this reactor model is shown in Figure 22.2. Particles of solid reactant B are in BMF, and fluid reactant A is uniform in composition, regardless of its flow pattern. The solid product, consisting of reacted and/or partially reacted particles of B, leaves in only one exit stream as indicated. That is, we assume that no solid particles leave in the exit fluid stream (no elutriation or entrainment of solid). This assumption, together with the assumption, as in the SCM, that particle size does not change with reaction, has an important implication for any particle-size distribution, represented by P(R). The implication is that P(R) must be the same for both the solid feed and the solid exit stream, since there is no accumulation in the vessel in continuous operation. Furthermore, in BMF, the exit-stream properties are the same as those in the vessel Thus, P(R) is the same throughout the system ... 

The CaO from CaCOs decomposition, the other new specie involved in the combustion mechanisms at LCL runs, is a solid porous material, which behaves like a fluid at the fluidised bed. That means that its interactions with radicals from coal are not limited and the radicals could be adsorbed [9] into its porous structure, hindering their total oxidation and, in consequence, promoting their interaction. This fact is corroborated by the Coronene formation in LCL experiments. Coronene (Co) is the most stable of the PAH studied and the radicals trend in their stabilization will be towards Co formation. Besides, as result of the fluidisation movement and the high temperature, CaO can be fragmented into smaller particles, elutriation and attrition phenomena, the smaller particles formed undergoing entrainment by the airflow. 

There has been a large number of experimental studies of entrainment and elutriation, and as in turbulent and fast beds there are considerable areas of disagreement amongst them. The main reason for this is the often wide disparity between equipment scale (particularly bed diameter) and the size and size distribution of the particles investigated. Comprehensive reviews have been given by Kunii and Levenspiel (1991) and Tasirin and Geldart (1998). 

At sufficiently high velocity of fluid upward through a bed of particles, the particles become entrained and do not settle that is, the particles are carried up with the fluid. Elutriation is the selective removal of particles by entrainment, on the basis of size. 

The elutriation velocity (of the fluid) is the velocity at which particles of a given size are entrained and carried overhead. 

The minimum elutriation velocity for particles of a given size is the velocity at incipient entrainment, and is assumed to be equal to the terminal velocity (ut) or free-falling... 

When bubbles burst at the surface of the fluidized bed, solid material carried along in their wake is ejected into the freeboard space above the bed. The solids are classified in the freeboard particles whose settling velocity ut is greater than the gas velocity fall back into the bed, whereas particles with u < u are elutriated by the gas stream. As a result, both the volume concentration of solids cy and the mass flow rate of entrained solids in the freeboard show a characteristic exponential decay... 

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