Abstract:
Gas mixing is studied in fluidized beds of large particles. The
bed is 0. 483 m by 0. 127 m (19 in. by 5 in. ) in cross section and has
transparent plexiglass panels on the front and back. A tube matrix
made of twenty-seven 50.8 mm (2 in.) diameter plexiglass cylinders
fixed in an equitriangular pitch (with the pitch to diameter ratio equal
to 2) is used to study the effects of tubes. Experiments are done in a
bed with and without tube array. Sand and dolomite particles with
mean particle diameter of 1.3 mm (0. 051 in. ) to 4.0 mm (0. 157 in.)
are used. Gas velocities are varied from minimum fluidization
velocity to an excess velocity of 2.5 m/s.
Two large particle fluidization regimes are described. The
experimental evidence is presented in support of the existence of slow
bubble and exploding bubble regimes. A semi-theoretical equation
establishing the boundary between these two modes of fluidization is
derived.
A slow bubble regime is encountered in the bed in which the
interstitital velocity of the gas exceeds the rising velocity of bubbles.
Here the gas uses the bubble as a convenient shortcut on its way
through the bed.
The exploding bubble regime is reached at higher superficial gas
velocities when the bubble growth rate is of the same magnitude as the
bubble rise velocity. Large pressure drop oscillations, gross gas
bypassing, defluidization of some of the particles and rapid bubble
growth are characteristics of the exploding bubble regime.
A new criterion is suggested for distinguishing between the fast
and the slow bubble regimes. The criterion is derived as a relationship
between two non-dimensional groups.
The expansion of the bed of large particles with and without tube
array is also studied. Theoretical equations are proposed for correlating
relative bed voidage versus relative excess gas velocity.
They are based on the two-phase theory and all are developed as a
special case of one general equation.
The equation derived for the slow bubble regime fits the experimental
data of this study better than other existing correlations. The
equation developed for the fast bubble regime compares favorably with
literature data for fine particles. A special case of the general
equation can be developed for stationary bubbles and for the slugging
regime.
It is found that there is little difference in expansion of beds with
and without a tube array at low excess gas velocities. However, for
higher excess gas velocities expansion is considerably greater for
beds with a tube array. Exploding bubbles in a bed without tubes are
responsible for this difference.
The dispersion of tracer gas injected continuously through a line
source above the distributor plate is determined for time-averaged
concentration measurements. The tracer concentration at points
within the bed is successfully predicted using a single-phase model
with interstitial gas velocity (based on average bed voidage) as a
characteristic fluidization velocity. The model is insensitive to axial
dispersion and depends only on radial dispersion coefficients. The
radial dispersion coefficient does not depend on either horizontal or
vertical position in the bed and is a strong function of excess gas
velocity.
Considerable difference is found in tracer dispersion in beds
with and without tube array.
A new model, called the meandering flow model, is proposed
for gas flow through fluidized beds of large particles. The concept of
meandering flow is developed on the basis of actual physical movement
of gas.
The series of peaks observed in tracer concentration data
records are explained by a bulk lateral movement of gas induced by
large bubbles. A simple mathematical technique is suggested for the
analysis of tracer data. As a result of the application of the meandering
flow model the turbulent and meandering dispersion coefficients
are defined. Meandering of fluid through the bed does not contribute
to gas mixing, and consequently the meandering dispersion coefficient
has to be subtracted from the overall radial dispersion coefficient.
Only the turbulent dispersion should be used in the evaluation of the
extent of gas mixing in fluidized beds of large particles, which contributes
to gas phase combustion.