Grupo 4 elutriação

Powder Technology 106 Ž1999. 110–118[pic 1]

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www.elsevier.comrlocaterpowtec[pic 2]

Modelling fluidized bed elutriation of fine particles

D. Santana, J.M. Rodr´ıguez 1, A. Mac´ıas-Mach´ın )

Grupo EMA, ETSII de Las Palmas, UniÕersidad de Las Palmas de Gran Canaria, Tafira Baja s r n, Las Palmas de Gran Canaria, Canary Islands, 35017, Spain

Received 30 October 1998; received in revised form 10 March 1999; accepted 10 March 1999[pic 3]

Abstract

Experimental work with Geldart groups A and C powders was carried out in a 6.4-cm i.d. column fluidized batchwise at superficial velocities between 0.24 and 0.6 m sy1. A new model was proposed to account for elutriation and attrition based on the assumption that the generation of fines by attrition is a nonlinear function of time and depends on the percentage of agglomerated fines. Elutriation rate constants and attrition rates were evaluated at various particle mixing ratios and it could be observed that the entrainment rate at low air velocities was affected by interparticles adhesion forces. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Elutriation; Fluidized beds; Entrainment; Modelling[pic 4]

1. Introduction

The  understanding of  the  elutriation  process  of  solid particles has become increasingly important in recent years not only owing to the increasing use of fluidized bed techniques in industrial plant operations but also because of the convenience of this process as a method of particle generation used for the proper design of separation equip- ment such as cyclones, granular filters, scrubbers, ceramic filters and others w1x.

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forces are shown to have an important influence on the elutriation rate.

Geldart et al. w9x considered that the addition of a small

quantity of Geldart-C particles Žcohesive difficult-to-flui- dize. caused a decrease in elutriation due to fines adhering to the large particles, working together instead of working independently. On the contrary, if a large quantity of Geldart-C particles were added, the bed would become cohesive, thus, decreasing in bubbling and increasing in formation of channels.

Several investigators have made important contributions

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Smolders  and  Baeyens

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w11x

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conducted  studies  of  en-

to  the  understanding  of  the  mechanism  of  elutriation. Initial results were presented in Refs. w2–4x. These investi- gators studied the rate of elutriation from beds of two different particle sizes. System variables such as the gas velocity, particle size and concentration of fines, solid and gas properties, etc., were empirically related to the elutria- tion constant.

Other relevant research in this field has been carried out by Kunii and Levenspiel w5x,  Geldart et al. w6x,  Wen and Chen w7x and Yates and Newton w8x. Later, Geldart et al. w9x  and Baeyens et al. w10x presented results on the elutriation from beds of fine powders, where interparticles adhesion

) Corresponding author.  Tel.:  q34-928-45-1900;  Fax:  q34-928-45-

1846; E-mail: [e-mail]

1 On leave from University of Salamanca, Spain.

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trainment of Geldart groups A and C powders showing that  the  entrainment rate  at  low  velocities  was  strongly reduced by interparticles adhesion forces when a vibrating bed and different freeboard geometries were used.

Jaraiz et al. w12x used the vibration of the fluidized bed

and showed vibration is useful in fluidized beds with very small particles, smaller than about 30 mm, because rela- tively strong interparticles forces develop which lead to particle agglomeration with channelling of gas and mini- mum fluidizing velocities much higher than predicted by theory.  Fortunately, vibration of  the  fluidized bed  over- comes this poor contacting by helping to break up those agglomerates. Channelling diminishes or disappears, and a higher pressure drop across the bed is observed indicating better gasrsolid contacting and more complete suspension of the particles.

Whether breakage of particles in a fluidized bed occurs by abrasion or fragmentation depends upon a wide range

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0032-5910r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 9 9 . 0 0 0 5 7 - 1[pic 6]

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of factors which include particle morphology and particle strength as well as fluid-dynamic-induced forces exerted on the particles in suspension w13x. Ayazi Shamlou et al. w14x  indicate that the breakage of bed material can occur by purely hydrodynamic effects in the bed and Arena et al. w15x indicate that the attrition during the fluidized combus- tion of a bituminous coal depends on fluidizing velocity and size of sand and coal in the bed.

In this paper, the attrition term is used to define the process in which small particles adhered to bigger ones and small particle agglomerates are dispersed due to the action  of  hydrodynamic  forces.  The  particles  are  later found in the bed in the form of free fines.

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equation, supposing that attrition rate constant or fines generation constant would be constant with time. Finally, Liu and Kimura w19x  used this later model, dividing the fines remaining in the bed into three states Žfreely moving fines, fines attached to large particles and fines agglomera- tions..