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.   References:

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