Attrition of particles

Vaux, W. G., Attrition of Particles in the Bubbling Zone of a Fluidized Bed, ... 

These parts are used in fluidized beds for various purposes. For example, gas distributors and various types of baffles are installed to decrease the size of the bubbles. Moreover, draft tubes are used to enhance gas or solid circulation. Other devices such as horizontal baffles limit circulation and backmixing of solids and gas. Horizontal or vertical tubes are used for heat management. Devices used to control or improve fluidization behavior, to improve fluidization of cohesive particles or to achieve solids recovery are within the various internals met in fluidized bed reactors (Kelkar and Ng, 2002). Immersed tubes in small diameter beds may lead to slugging. Furthermore, attrition of particle breakage may change the size distribution and possibly change the fluidization behavior. 

From a practical standpoint catalyst loss due to carryover with the gas stream from the reactor and regenerator may be an important problem. Attrition of particles decreases their size to a point where they are no longer fluidized, but move with the gas stream. It has been customary to recover most 6f these catalyst fines by cyclone separators and electrical precipitation equipment placed in the effluent lines from reactor and regenerator. 

Other practical considerations are attrition of particles caking of catalyst from malfunctioning of the reactor due to formation of tarry products (resulting sometimes in cakes as large as the reactor diameter) and the need to avoid premixing of reactants (particularly when they can form explosive mixtures) and fix their relative locations within the bed (e.g., in the chlorination of methane and ammoxidation of propylene). Refer to Doraiswamy and Sharma (1984) for further details. 

Hardness and attrition of particles and extrudates can be measured according to the ASTM hardness test or by measuring of the abrasion index. For activated carbons as catalyst carrier bulk and particle crushing strength tests have been developed. In Table 4 the hardness, the abrasion index and crushing strength are given. Extrudated carbons are extremely hard. 

Vaux WG. Attrition of particles in the bubbling zone of a fluidized bed. Proc Am Power Conf 40 793-802, 1978. 

External combustion for heating acts also as an afterburner Discharged after slow cooling Low rate of attrition of particles... 

Fluidized-bed catalytic reactors tend to generate loss of catalyst through attrition of the solid particles, causing fines to be generated. 

Particle Attrition. Distributor jets are a potential source of particle attrition. Particles are swept into the jet, accelerated to a high velocity, and smash into other particles as they leave. To reduce attrition at distributors, a shroud or larger-diameter pipe is often added concentric to the jet hole, as shown in Figure 15. The required length of the concentric shroud is given by the relation... 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

In order to describe these different mechanisms, various breakage functions have been proposed (Hill and Ng, 1995, 1996). For precipitation processes, a breakage function of the form given in equation (6.32) with h(v, Xk) being the discretized number fraction of particles broken from size v into size interval x, seems particularly suitable as both attrition - with a high probability - and particle splitting - with a low probability - are accounted for. 

The significance of this novel attempt lies in the inclusion of both the additional particle co-ordinate and in a mechanism of particle disruption by primary particle attrition in the population balance. This formulation permits prediction of secondary particle characteristics, e.g. specific surface area expressed as surface area per unit volume or mass of crystal solid (i.e. m /m or m /kg). It can also account for the formation of bimodal particle size distributions, as are observed in many precipitation processes, for which special forms of size-dependent aggregation kernels have been proposed previously. 

The functions of the filler and the binder are to provide physical integrity (density, attrition resistance, particle size distribution, etc.), a heat transfer medium, and a fluidizing medium in which the more important and expensive zeolite component is incorporated. 

Properties of particles that are conducive to smooth fluidization Include rounded or smooth shape, enough toughness to resist attrition, sizes In the range 50-500 im dia, a spectrum of sizes with ratio of largest to smallest In the range of 10-25. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

CO2 adsorption capacities with dry sorbents before and after attrition were shown in Fig.3. We found variation of CO2 adsorption capacity during operation by examining effect of attrition on adsorption capacity. So, adsorption experiments for each sorbent fluidized for 30hours were carried out. As a result, percentage losses of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but those of activated carbon and activated alumina were 8.3% and 8.1% respectively. This is because retention time of molecular sieve 5A and molecular 13X decreased due to elutriation of particle generated from attrition. 

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