DEM Simulation on Particle Agglomerates Flow Behaviors during Drying Process in a Bubbling Fluidized Bed

DEM Simulation on Particle Agglomerates Flow
Behaviors during Drying Process in a Bubbling Fluidized Bed

Tianqi
Tang¹, Biao Wang¹, Yurong He^1,*

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

rong@hit.edu.cn

Dependence on fossil
fuels as primary energy source has caused a serious energy crisis and
environmental problems. Renewable sources, such as biomass, are effective
resources to provide energy for the human population. Meanwhile, bubbling
fluidized beds play an important role in biomass combustion, and biomass is an
important clean fuel. Besides,
bubbling fluidized beds have been widely used in drying,
combustion and gasification. Owing
to their unique advantages, bubbling fluidized beds have attracted great
attention from researchers. However, most studies have
focused on dry particle systems. Cohesive liquids cannot
be neglected in relevant industrial processes. For example, wet grinding is
employed to reduce dust and energy loss during alumina production.[1]Moreover, the flow
characteristics of particles change greatly under the effects of a cohesive
liquid between particles[2]. Thus,
the flow behaviors of particles in bubbling fluidized beds are important and
require further investigation.

The
gas-solid two-phase flow behaviors are complex and unique in bubbling fluidized
beds. As far, bubbling fluidized beds have attracted more and more attention
from researchers. For the experiment, Taghipour et
al.[3]
have conducted experiment to investigate the hydrodynamics of a two-dimensional
bubbling fluidized bed reactor, including solid-phase kinetic energy
fluctuation and pressure drop. Radmanesh et al.[4]
studied gasification of beech wood particles in a bubbling fluidized bed
gasifier, and found that pyrolysis is an important step in the overall
gasification model that can determine the distribution of products. With the development of
computational fluid dynamics, numerical simulation has become an effective
method of further investigating the gas-solid hydrodynamic characteristics in a
gas-solid fluidized bed. Since 1993, a modified
soft-sphere model has been used, which was improved by Tsuji et al. [5]
based on the model proposed by Cundall
and Strack[6].
The discrete element method (DEM) has been widely used to investigate complex
granular flow[7],
and also plays an important role in the analysis of particle flow behaviors in
mixers[8], V-blender[9]
and
rotating drum[10].
Furthermore, DEM was also used to explore particle flow behaviors in bubbling
fluidized beds. Gui et al.[11]
investigated gas–solid flow behaviors in a bubbling fluidized bed with immersed
tubes, and found that particle back-mixing motion and bubble motion seem to
bear a strong correlation with each other. Rhodes et al.[12]
investigated the mixing characteristics in a bubbling fluidized bed, and
pointed that the rate of solids mixing increases with increasing gas velocity. However, most numerical
simulation were limited to dry granular systems.

Few
investigations have been conducted on wet particle flow behaviors, especially
those considering heat and mass transfer behaviors processing. In fact, wet
particle systems couldnft be neglected in industrial processing, especially
considering heat and mass transfer characteristics. Moreover, heat and mass
transfer is important in industrial processing, such as particle drying. In
this study, the module of heat and mass transfer has been added. For
the heat transfer process, conduction, convection, radiation and liquid
evaporation are considered. Besides, the mass transfer between liquid coated on
particles and gas are taken into account. Moreover, the effect of gas
temperature on wet particle system drying process has been conducted. The distribution
of heat transfer coefficient in the bubbling fluidized bed has been
investigated. From numerical simulation results, it could be
obtained that particles formed agglomerates under the effect of cohesive
liquid. With the effect of fluidization gas, agglomerates disappeared with the
evaporation of cohesive liquid. The wet particle system converted into dry
particle system. With an increases of gas temperature, the rate of drying
processing increased gradually. In the bed middle, the heat and mass transfer
rates were higher than those near the walls due to higher gas and solid
momentum exchange. It could be obtained that the momentum exchange has a strong
effect on mass and heat exchange.

Acknowledgement

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

References