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Modeling of continuous particle classification in a liquid medium Chen, Aihua
Abstract
Previous studies of sedimentation, wherein particles fall under the influence of gravity through a fluid in which they are suspended, are first reviewed. This technique can be used to separate particles having different settling velocities. Various hydrodynamic models, including a batch, a differential and a continuous model, of the particle segregation and classification in liquid fluidized beds for binary and polydisperse systems are described. A stochastic model, the Markov Chain Model, recently written for the liquid classifier by Zhang (1998) of our group, is also introduced. The emptying phenomenon encountered in industrial classifiers can be explained in terms of a limiting voidage in the column of the classifier, below which the particles cannot move downwards and be removed from the bottom of the classifier. Continuous classification of particles by size is studied in a solid- liquid classifier of 191 mm diameter and 1540 mm height under steady and unsteady state conditions, similar in geometry to industrial units. Spherical glass beads of uniform density, and Rosin-Rammler particle size distributions, were employed in the tests. The mean particle diameter and the operating conditions were dynamically similar to those used in industry. During the particle classification operation, a dense suspension of particles in water enters the classifier through a radial feed port near the top, while water without particles is injected upwards from the bottom. A relatively dilute stream containing mostly small particles is taken off the top as an overflow stream, while an underflow stream enriched in coarser particles is removed near the base of the column. Differential hydrodynamic models are developed to describe the steady and the unsteady state motions of the particles and liquid, based on mass and momentum conservation laws and knowledge of the sedimentation behavior of individual species within the mixture. The boundary conditions require that the particle concentration in the overflow stream be equal to the concentration at the top of the classifier and the particle concentration at any vertical level in the discharge stream equal the horizontally adjacent concentration in the column of the classifier. The correlation of Di Felice (1994) is used to calculate the drag force on the particles, and the particle dispersion force is introduced according to the concept of Thelen and Ramirez (1999). Any turbulence in the flow is taken into account indirectly via an axial dispersion coefficient, assumed to be uniform throughout the classifier. This sole fitted parameter is correlated in terms of the relevant dimensionless parameters. The degree of classification becomes better with increasing feed voidage, feed flow rate and fluidizing liquid flow rate, but is worse at higher underflow discharge rates. The performance of the classifier is better for a broad than for a narrow particle size distribution of the feed stream. The classification can be improved by increasing the height of the cylindrical zone. Predictions of the model agree reasonably well with experimental results.
Item Metadata
Title |
Modeling of continuous particle classification in a liquid medium
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2000
|
Description |
Previous studies of sedimentation, wherein particles fall under the influence of gravity
through a fluid in which they are suspended, are first reviewed. This technique can be
used to separate particles having different settling velocities. Various hydrodynamic
models, including a batch, a differential and a continuous model, of the particle
segregation and classification in liquid fluidized beds for binary and polydisperse systems
are described. A stochastic model, the Markov Chain Model, recently written for the
liquid classifier by Zhang (1998) of our group, is also introduced. The emptying
phenomenon encountered in industrial classifiers can be explained in terms of a limiting
voidage in the column of the classifier, below which the particles cannot move
downwards and be removed from the bottom of the classifier.
Continuous classification of particles by size is studied in a solid- liquid classifier
of 191 mm diameter and 1540 mm height under steady and unsteady state conditions,
similar in geometry to industrial units. Spherical glass beads of uniform density, and
Rosin-Rammler particle size distributions, were employed in the tests. The mean particle
diameter and the operating conditions were dynamically similar to those used in industry.
During the particle classification operation, a dense suspension of particles in water
enters the classifier through a radial feed port near the top, while water without particles
is injected upwards from the bottom. A relatively dilute stream containing mostly small
particles is taken off the top as an overflow stream, while an underflow stream enriched
in coarser particles is removed near the base of the column.
Differential hydrodynamic models are developed to describe the steady and the
unsteady state motions of the particles and liquid, based on mass and momentum
conservation laws and knowledge of the sedimentation behavior of individual species
within the mixture. The boundary conditions require that the particle concentration in the
overflow stream be equal to the concentration at the top of the classifier and the particle
concentration at any vertical level in the discharge stream equal the horizontally adjacent
concentration in the column of the classifier. The correlation of Di Felice (1994) is used
to calculate the drag force on the particles, and the particle dispersion force is introduced
according to the concept of Thelen and Ramirez (1999). Any turbulence in the flow is
taken into account indirectly via an axial dispersion coefficient, assumed to be uniform
throughout the classifier. This sole fitted parameter is correlated in terms of the relevant
dimensionless parameters.
The degree of classification becomes better with increasing feed voidage, feed
flow rate and fluidizing liquid flow rate, but is worse at higher underflow discharge rates.
The performance of the classifier is better for a broad than for a narrow particle size
distribution of the feed stream. The classification can be improved by increasing the
height of the cylindrical zone. Predictions of the model agree reasonably well with
experimental results.
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Extent |
9344997 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-07-23
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0058981
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2000-11
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.