(70d) Agglomerate Growth and Breakup in Wet Fluidized Beds

Wet fluidized beds, gas-solid fluidized beds with a small amount of
liquid added, are used extensively in the oil and gas, pharmaceuticals and food
industries. The liquid can be used to either facilitate agglomeration, rapid
heat transfer or chemical reaction. Agglomeration occurs because liquid bridges
form between particles, creating a cohesive force. The cohesive force provided
by these liquid bridges depends on the surface tension, viscosity and contact
angle of the liquid, as well as the amount of liquid in the bridge. Due to the
industrial importance of wet fluidized beds, several experimental (1?5) and computational (6?9) studies have been undertaken to determine the effects
of these parameters on individual forces between two particles (5, 8) as well as hydrodynamics (1, 3, 7, 9), agglomerate size distribution (6) and drag force (2) in wet fluidized beds.

 

In many processes, liquid is injected into the bed at local sites, immediately
creating large wet agglomerates near the injection sites while the rest of the
particles are left largely dry. Depending on the process and its purpose, it
can either be desirable for the agglomerate to grow, absorbing dry particles as
it collides with them, or breakup due to interaction with surrounding gas and
particles. Agglomerates can grow upon collision with other particles because
liquid which has migrated from the center of the agglomerate to the surface can
form a liquid bridge with colliding particles, engulfing the particle.
Alternatively, drag force from surrounding gas and collisions with particles
can impart stress on certain sections of agglomerates causing the agglomerates
to break-up.

 

Here, we present the results of a computational study in which large
wet agglomerates are immersed in fluidized beds of dry particles and allowed to
interact dynamically with the surrounding gas and particles. We use the
computational fluid dynamics ? discrete element method (CFD-DEM) (10) to simulate individual particles in a Lagrangian fashion and gas
dynamics on an Eulerian grid. Liquid loading, viscosity and surface tension are
accounted for to track the amount of liquid on each particle and in each
pendular bridge, as well as the cohesive force provided by liquid bridges (11) and the finite rate of liquid transfer between particles and bridges (12). We vary relevant liquid, particle and gas-flow parameters to identify
parameter spaces which lead to break-up and growth of agglomerates.
Additionally, through analysis of the simulation output, we determine the physical
mechanisms by which agglomerates can grow and break up.

 

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