DISCRETE CHARACTERIZATION OF COHESION IN GAS-SOLID FLOWS

by Jain, Kunal

Fluidization and the transport of solid particles either by gravity or by pneumatic means are used in a variety of industrial operations, including fluid catalytic cracking, fluid hydroforming and solid fuel processes such as coal gasification and liquefaction [1]. Despite the fact that a sizeable portion of gas-solid flows are cohesive in nature, the mechanics of cohesive flowing gas-particle systems is still poorly understood, and manipulation/control of the flow variables is still largely done on a trial-and-error basis. Cohesive forces between grains can arise from a variety of sources -- such as liquid bridge (capillary) forces, van der Waals forces, or electrostatic forces -- and may play a significant role in the processing of fine and/or moist powders. While recent advances have been made in our understanding of liquid-induced cohesion in quasi-static systems at the macroscopic level [2,3], in general, it is still not possible to directly connect this macroscopic understanding of cohesion with a microscopic picture of the particle properties and interaction forces. Moreover, conventional theories on gas-solid flows, make no attempt to distinguish between these modes of cohesion, despite clear qualitative differences (lubrication forces in wet systems or electrostatic repulsion are two good examples). In this work, we extend the previous work on discrete characterization tools of wet granular flows [4], using computations of gas-solid flows, in order to examine the transition from non-cohesive (dry) to cohesive (wet) behavior in gas-solid systems. Gas velocity and bridging liquid surface tension are varied to explore a range of the possible fluidization parameter space and a characterization criterion based on the the physical picture of liquid-induced particle-level cohesion is developed for gas-solid flows. Cohesion between wet particles has been modeled using the concept of liquid bridges. The characterization tool developed, namely the Granular Capillary Number, is tested by measuring both the minimum fluidization velocity as well as the mixing rate in fluidized systems. The systems explored here are traditionally thought to be cohesive but a marked difference is observed as the Granular Capillary Number changes.