(146d) CFD-DEM Modeling the Effect of Column Size and Bed Height on Minimum Fluidization Velocity in Micro Fluidized Beds with Geldart B Particles

CFD-DEM
modeling the effect of column size and bed height on minimum fluidization
velocity in micro fluidized beds with Geldart B particles

Yupeng
Xu^a, Tingwen Li^a,b, Jordan Musser^a, Xiaoxing
Liu^c, Guangwen Xu^c, William A. Rogers^a

a.
National Energy Technology Laboratory, Morgantown, WV 26505, USA

b.
AECOM, Morgantown, WV 26505, USA

c.
State Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China

Micro
fluidized beds (MFBs) were first put forward by Potic et al. to refer to
fluidized beds with inner diameters of a few millimeters.  These are ideal
systems to study because they have a large wall area per unit reactor volume
which promotes excellent mass- and heat-transfer. Additionally, smaller systems
allow for measurement of small quantities of solid reactants. Applications of
micro-fluidized beds include the Micro Fluidized Bed Reaction Analyzer (MFBRA)
and Micro Membrane Fluidized Bed Reactor (MMFBR).

The
fluidization characteristics of a given kind of particles in MFBs are different
from those in ordinary-size fluidized beds due to much stronger wall effects.
From the viewpoint of operating and controlling a MFB kinetic analyzer, knowing
minimum fluidization and minimum bubbling velocities (Umf and Umb) enables
researchers to determine suitable gas velocities so that fluidization in the
MFB reactor can be maintained. Limited research has been conducted to
understand the fundamental hydrodynamics of micro-fluidized beds.

The
fluidization behavior of Geldart B particles in micro fluidized beds is
investigated numerically using Computational Fluid Dynamics coupled with
Discrete Element Method (CFD-DEM) available in the open-source Multiphase Flow
with Interphase eXchanges (MFIX) code. The effects of different bed diameters
(D) of 8 mm, 12 mm, 16 mm and various initial static bed heights (H) were
examined. It is found that both decreasing the column diameter and increasing
the bed height in a micro fluidized bed increases minimum fluidization velocity
(Umf). The observed overshoot in pressure drop that occurs before the onset of
fluidization decreases in magnitude with increasing column diameter, however
there is less sensitivity to bed height. Overall, the numerical results agree
qualitatively with existing theoretical correlations and experimental studies.
The simulations show that both column diameter and particle-wall friction
contribute to variations in minimum fluidization velocity. These two factors
are coupled and hard to separate. The detailed influences of wall friction on
minimum fluidization velocity are investigated for a prescribed column diameter
of 8 mm by varying the wall friction from 0 to 0.4. 

Fig.1. Influence of the column diameters on
the minimum fluidization velocity (static bed height of 6.88 cm)