DEM Numerical Investigation of Mixing Behaviors of Binary Mixtures Containing Non-Spherical Particles in a Fluidized Bed

DEM
numerical investigation of mixing behaviors of binary mixtures containing
non-spherical particles in a fluidized bed

Anxing Ren¹, Tianyu
Wang, Yurong He^1,*

¹ School of Energy Science and Engineering, Harbin Institute of Technology, 150001, China

rong@hit.edu.cn

As
a renewable and sustainable energy source, the conversion and utilization of
biomass are becoming increasingly popular worldwide, which can help to
alleviate the energy crisis and mitigate global warming. The thermochemical
processes of biomass such as combustion, pyrolysis and gasification are
considered to be the most widely used techniques for converting biomass into
energy or fuel. The fluidized bed has a series of advantages such as intense
mixing, large contact surface between the phases, and efficient heat and mass
transfer. Thus, the process of biomass conversion and utilization is usually
carried out in a fluidized bed^[1]. However, fluidization of biomass
particles is a difficult task because the non-spherical biomass particles are
generally large and have a low density. In the actual industrial processes, inert
particles such as silica sand, alumina, and calcite are often added to the
biomass to assist its fluidization^[2]. Therefore, understanding
mixing behaviors of binary mixtures in a fluidized bed has
specific significance for the design and optimization of related industrial
processes.

In
recent years, the macroscopic fluidization behaviors of the binary mixtures containing
non-spherical particles have been experimentally
investigated by many researchers. Fotovat et al.^[3]
investigated the effects of superficial gas velocity and the mixture
composition on the mixing/segregation behaviors by analyzing the circulatory
motion of the active tracer. Shao et
al.^[4] investigated the mixing behaviors in binary-component
and multi-components particulate systems and found that the density of
non-spherical particles has a more significant influence on the mixing
characteristics than particle size and shape. Boer et al.^[5] developed
a Digital Image Analysis technique to explore the particle distributions in the
fluidized bed and obtained qualitative and quantitative experimental results
which can be used for validation of numerical models concerning non-spherical
particle mixing.

Although
studies on the mixing behaviors of binary mixtures containing non-spherical
particles in fluidized beds have attracted attention, most studies focus on
macroscopic flow characteristics. It is still difficult to obtain detailed
information in the flow field through experimental research, such as the
microscopic motion characteristics of the particles. With the rapid
development of computer science and technology, numerical simulation has become
an effective method to study particle behaviors in fluidized beds.

In
this work, the Computational Fluid Dynamic-Discrete Element Method (CFD-DEM)
with consideration of rolling friction is
applied to investigate the mixing behaviors of binary mixtures containing
non-spherical particles in a fluidized
bed. The non-spherical particles are constructed by multi-sphere model^[6].
In the multi-sphere method, a single non-spherical particle is represented by a
composition
of
overlapping
spheres
of arbitrary size and the contact force and torque acting on a non-spherical
particle are calculated as the sum of contact forces and torques on its
spherical elements. The Gidaspow drag model^[7] is employed to describe
the momentum exchange between spherical particles and gas phase. The
Di Felice correlation^[8] and Hölzer/Sommerfeld model^[9] are
applied for non-spherical particles. The mixing processes of several rod-shaped
particles with different aspect ratios but the same volume and spherical
particles in the fluidized bed are simulated.  

Both macroscopic
and microscopic flow characteristics in the fluidized bed are investigated. The simulation
results demonstrate that the addition of spherical particles is advantageous
for the mixing and fluidization of the rod-shaped particles. The superficial gas
velocity has an obvious effect on particle flow behaviors and particle mixing
characteristics. The pressure drop and particle distribution in the fluidized
bed are discussed. In addition, the effect of particle aspect ratio, density
and size on flow and mixing characteristics is also analyzed.

Acknowledgement

This work was
financially supported by the National Natural Science Foundation of China
(Grant No. 91534112).

References

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