(473e) CFD-DEM Simulation of Hydrodynamics in Wet Gas-Solid Fluidized Beds

It
is common in many industries, including energy, pharmaceuticals and polymer
production, for small amounts of liquid to be injected into gas-solid fluidized
beds. This liquid can serve as a reactant or facilitate agglomeration or heat
transfer. The liquid often covers the surface of particles and proceeds to form
cohesive liquid bridges between particles, ultimately leading to the
agglomeration. The cohesive force caused by small amounts of liquid can alter
hydrodynamics dramatically, since agglomerates, rather than individual
particles, need to be fluidized. In extreme cases, liquid bridging and
agglomeration can lead to local or device-scale defluidization. Despite this
industrial importance, the effect of wetness on hydrodynamics is not fully
understood. A limited number of experimental studies show differences in bed
fluidity [1,2], particle
velocities  [3–6], bed
height  [1],
bubble size  [6,7] and
minimum fluidization velocity  [6] caused
by liquid bridging in gas-solid fluidized beds.

Various
efforts have been undertaken to model the effect of wetness on fluidization
computationally. Most work has involved an Euler-Lagrange approach often
referred to as the Computational Fluid Dynamics – Discrete Element Method
(CFD-DEM)  [8] in
which each individual particle is modeled, gas flow is simulated on a CFD grid
and the two phases are linked via a drag law. In this approach, contacts
between particles are modeled, often with a coefficient of restitution
dictating the level of inelasticity in the collisions. A number of studies have
accounted for wetness by lowering this coefficient of restitution  [9,10],
since the cohesive force provided by liquid bridges will act to slow the
particles from rebounding off of one another after a collision. A fundamental
issue with approach is that liquid bridging does not merely make collisions
more inelastic; liquid bridges provide cohesion necessary to keep particles
together for long periods of time and form agglomerates. To address this issue
with a modified coefficient of restitution approach, a few more detailed
studies have directly modeled forces arising from liquid bridges  [11,12].
One issue with these studies is that they have assumed liquid bridges to form
instantaneously. In fact, the surface tension and viscosity of the liquid will
affect the rate at which liquid pours from the surface of a particle into a
liquid bridge  [13], and
in certain cases, particles can rebound off of one another before a significant
liquid bridge has had time to form.

In
this study, we model the effects of liquid on fluidization hydrodynamics using
a CFD-DEM model in which liquid bridges are modeled directly and liquid bridges
form at a finite rate. We simulate laboratory-sized fluidized beds and
investigate the effects of liquid content, viscosity and surface tension on bed
height, particle velocity, bubble behavior and minimum fluidization velocity
for comparison with effects seen in the literature.

References:

[1]    S. L. McDougall, M. Saberian, C. Briens, F.
Berruti, and E. W. Chan, Int. J. Chem. React. Eng. 2, (2004).

[2]    S. McDougall, M.
Saberian, C. Briens, F. Berruti, and E. Chan, Chem. Eng. Process. Process
Intensif. 44, 701 (2005).

[3]    V. S. Sutkar, N.
G. Deen, A. V. Patil, E. A. J. F. Peters, J. a. m. Kuipers, V. Salikov, S.
Antonyuk, and S. Heinrich, AIChE J. 61, 1146 (2015).

[4]    Q. Zhang, Y.
Zhou, J. Wang, B. Jiang, Y. Yang, S. Stapf, C. Mattea, and Q. Gong, Chem. Eng.
Technol. n/a (2015).

[5]    M. L. Passos and
A. S. Mujumdar, Powder Technol. 110, 222 (2000).

[6]    Y. Zhou, C. Ren,
J. Wang, and Y. Yang, AIChE J. 59, 1056 (2013).

[7]    J. Ma, D. Liu,
and X. Chen, Ind. Eng. Chem. Res. 55, 624 (2016).

[8]    Y. Tsuji, T.
Kawaguchi, and T. Tanaka, Powder Technol. 77, 79 (1993).

[9]    P. Darabi, K.
Pougatch, M. Salcudean, and D. Grecov, Powder Technol. 214, 365 (2011).

[10]  V. S. Sutkar, N.
G. Deen, J. T. Padding, J. a. m. Kuipers, V. Salikov, B. Crüger, S. Antonyuk,
and S. Heinrich, AIChE J. 61, 769 (2015).

[11]  T. Mikami, H.
Kamiya, and M. Horio, Chem. Eng. Sci. 53, 1927 (1998).

[12]  M. Girardi, S.
Radl, and S. Sundaresan, Chem. Eng. Sci. 144, 224 (2016).

[13]  M. Wu, J. G.
Khinast, and S. Radl, in (Barcelona, 2014).