ABSTRACT
An Energy-Based
Model of Gas-Solids Transport in a Riser
Speaker:
Jun You
Advisor:
Dr. Chao Zhu
A simple and reliable
method to estimate the solids holdup distribution and solids residence time in a
gas-solid riser flow is essential to the optimum design and efficient operations
of riser reactors. The traditional approach of equating the local solids holdup
to the pressure drop in a riser overlooks the effects of solids acceleration and
energy dissipation in the acceleration and dense phase transport regions. The
energy dissipation in these regions is mainly due to the interfacial friction
between interstitial gas and suspended solids, inter-solids collisions, as well
as solids-wall frication. Most momentum-based models fail to account for the
energy dissipation of inter-solids collisions, and the models using the simple
granular kinetic theory fail to account for the energy dissipation in
micro-sliding or rolling from off-center inter-solids collisions.
These works presents an
energy-based mechanistic model to analyze the partitions of the axial gradient
of pressure by solids acceleration, collision-induced energy dissipation and
solids holdup in gas-solid riser flows. Based on this model, the correct
estimation of axial distributions of solids holdup and solid velocity are
obtained. Our analysis shows that the effect of solids acceleration on the
pressure drop can be significant in a range of moderate solids holdup (typically
from 3.5% to 12% by solids volume fraction) whereas the effect of energy
dissipation becomes important in the dense phase transport region (typically
when the solids volume fraction above 5%). The exemplified results indicate
that the traditional approach of equating the local solids holdup to the
pressure drop overestimates the solids holdup by an error up to 30% in the
acceleration and dense phase transport regions in typical gas-solid riser flow
applications.
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