February 14, 2013
How do cohesive particles in fluidized beds interact, and how does this affect the functioning of the beds? This fundamental question is of great practical interest to a number of industries, leading an international chemical company to award a grant of $1.5M to ChBE Professor Christine Hrenya, along with collaborators Al Weimer and Dan Schwartz, to model the gas fluidization of cohesive solid particles.
While larger particles fluidize in beds relatively easily, smaller cohesive particles with diameters of 1-50mm generally exhibit poor fluidization due to the non-negligible role of van der Waals forces and their tendency to form agglomerates. Relatively little is known about the flow characteristics of these smaller particles, as most industrial fluidized-bed operations and fluidization research have focused on larger particles.
“Flow conditions for cohesive particles must be optimized to achieve adequate bed performance,” says Hrenya. “For instance, at lower velocities, vertical channeling results in low gas-solid contacting and poor mixing within the bed. This leads to reduced reaction rates and heat transfer. At higher velocities, cohesive particles typically agglomerate and fluidize. Unfortunately, these higher velocities can have a negative impact on entrainment.”
The goal of the ChBE team is to develop a reliable predictive model of the hydrodynamics in gas-fluidized beds with cohesive particles. To do this, they will use a combination of both mathematical modeling and experiments.
The Hrenya group’s computational fluid dynamic model will utilize first principles with no adjustable parameters to predict gas-solid hydrodynamics when the particles have cohesion.
Explains Hrenya, “Our goal is to predict agglomerate and bubble sizes and distributions, determine whether these characteristics vary with scale, and provide detailed predictions of the bed hydrodynamics.”
Experiments conducted in the Weimer and Hrenya labs will focus on the fluidization of cohesive particles, particularly local information on agglomerate size. For the first time, laser-sheet images will provide resolution down to the primary particle level, which is crucial for high-accuracy model validation of agglomeration levels. Dan Schwartz will provide expertise on the AFM measurements of particle cohesion.
While industrial grants are relatively uncommon for fundamental modeling efforts, Hrenya and team were bestowed their industrial sponsor’s biggest academic grant ever awarded.