Analysis of the effect of small amounts of liquid on gas–solid fluidization using CFD‐DEM simulations

Abstract

Gas-solid fluidization involving small amounts of liquid is simulated using a CFD-DEM model. The model tracks the amount of liquid on each particle and wall element and incorporates finite rates of liquid transfer between particles and pendular liquid bridges which form between two particles as well as between a particle and a wall element. Viscous and capillary forces due to these bridges are modeled. Fluidization-defluidization curves show that minimum fluidization velocity and defluidized bed height increase with Bond number (Bo), the ratio of surface tension to gravitational forces, due to cohesion and inhomogeneous flow structures. Under fluidized conditions, hydrodynamics and liquid bridging behavior change dramatically with increasing Bo, and to a lesser extent with capillary number, the ratio of viscous to surface tension forces. Bed fluidity is kept relatively constant across wetting conditions when one maintains a constant ratio of superficial velocity to minimum fluidization velocity under wet conditions. V C 2017 American Institute of Chemical Engineers AIChE J, 63: 5290-5302, 2017

Linking discrete particle simulation to continuum properties of the gas fluidization of cohesive particles

“…Using the least squares method, α coh, 1 for the best fitting is shown to decrease (from 785 to 682) with the increase of B o (see the legend of Figure ). This may be ascribed to the decreased particle velocity for the increase of B o in fluidization of particles; for the particles with decreased velocity and increased B o , they may have more enduring contacts with their neighbors, which may more easily lead to the negative tensile pressure and result in a smaller α coh, 1 .…”

Magnetic resonance imaging of gas–solid fluidization with liquid bridging

“…On a larger scale, this leads to greater heterogeneity in gas and particle flow, affecting many aspects of fluidization hydrodynamics, which in turn alters heat and mass transport as well as chemical reactions. Previous studies have shown that the addition of small amounts of liquid alters the oscillations in pressure drop across the bed, slows the speed of particles, and increases the minimum fluidization velocity . Studies have also sought to map the behavior of wet fluidized beds into regimes based on the amount of liquid added as well as the surface tension and viscosity of the liquid; these regimes have included shifts in the Geldart grouping of the particles and the growth or breakup of agglomerates .…”

“…Studies have also sought to map the behavior of wet fluidized beds into regimes based on the amount of liquid added as well as the surface tension and viscosity of the liquid; these regimes have included shifts in the Geldart grouping of the particles and the growth or breakup of agglomerates . Despite decades of studies, key questions remain open in the field, including the relative importance of liquid loading, surface tension, and viscosity of the liquid, the non‐dimensionalization of liquid bridge behavior based on force‐ or energy‐based analyses and the validity of approximating a wet and dry bed of behaving similarly if the ratio of superficial velocity to minimum fluidization velocity ( U/U mf ) is kept constant . Boyce provides a review of prior work in wet fluidization, highlighting open questions and areas for future work.…”

“…Difficulties in fully understanding the effects of liquid bridging on fluidization hydrodynamics stem in part from a lack of temporally and spatially resolved experimental data on gas and particle motion in wet fluidized beds. Previous studies have been limited to device‐scale measurements, such as pressure oscillations and fluidization curves, measurements on pseudo‐2D fluidized beds using optical imaging and tomographic measurements on 3D beds of tracer particles, which are unable to capture behavior across the entire device on a temporally resolved level. In recent years, magnetic resonance imaging (MRI) has been able to generate detailed maps of both particle and gas motion in fluidized beds.…”

Random loose packing of small particles with liquid cohesion

“…In addition to van der Waals and electrostatic interactions, the capillary and viscous forces introduced by the interstitial liquid between adjacent particles are also considerably important and ubiquitous, which results in extremely different dynamic behavior of wet particles from that of dry particles . Seville et al compared different interparticle forces and concluded that the capillary force could be several orders of magnitude stronger than gravity, electrostatics, and van der Waals forces, thus the local and global properties of random packing of granular materials are also suggested to be significantly influenced by the addition of liquid.…”