(85b) Linking Defluidization Behavior of Solid Particles in Humid Environments to Micro-Scale Forces

Humidity results in small capillary bridges that condense between particles and induce cohesive behavior. To better understand the effects of humidity-induced cohesion, defluidization experiments are investigated under increasing relative humidity (RH) levels, the results of which are explained via particle-level predictions for the resulting capillary force. The sensitivity of the defluidization curve is captured by a new defluidization velocity U_df, which is intended to be similar to the complete fluidization velocity used to characterize defluidization of polydisperse beds. In particular, U_df characterizes the curvature of the defluidization plot (pressure drop vs. velocity) arising from humidity and leading to a partially fluidized state between the packed bed and fully-fluidized regions. The partially fluidized region, as characterized by U_df, is more sensitive to increasing levels of cohesion than the packed-bed region. An increase in RH results in a U_df that gradually increases approximately linearly, followed by an abrupt (non-linear) increase at RH ~55%. Following this abrupt increase in U_df,, the bed transitions from Group A to Group C behavior over a narrow range of increasing RH levels. To explain these macro-scale (many-particle) results, micro-scale (two-particle) theories for capillary forces are employed, in which measured values (i.e. not assumed or fitted) of particle surface roughness are implemented. From this physical description, it is found that the cohesive energy, rather than the maximum cohesive force or Bond number, predicts the same qualitative trend with increasing RH as U_df (for RH < 55%). Moreover, a hypothesis is presented to explain the transition form Group A to Group C behavior that occurs over RH ~60-65%; namely, the previously observed adsorption of water onto glass surfaces with liquid-like layers rather than crystal/ice-like layers.