Aggregation and sedimentation in gas-fluidized beds of cohesive powders

Abstract

We present measurements on the settling velocity of gas-fluidized beds of fine cohesive powders. In the solidlike regime ͑solid volume fraction Ͼ c ) particles are static, sustained by enduring contacts. The settling is hindered by interparticle contacts and is a very slow process. In the fluidlike regime (Ͻ c ) permanent contacts no longer exist, and the bed displays a diffusive dynamics. The interparticle adhesive force leads to the formation of particle aggregates, and for this reason the sedimentation velocity exceeds the predicted value by empirical or theoretical laws on the settling of individual particles. We use an extension of the Richardson-Zaki empirical law for the settling of aggregates in the fluidlike regime to fit the experimental data. Aggregates are characterized by the number of aggregated particles N and by an effective radius R. The trend followed by these parameters with particle size is confirmed by direct visualization of the aggregates, and shows that cohesive effects become important when the adhesion force between particles is above particle weight. Results show that aggregates form open structures with a fractal dimension close to the predicted one in the diffusionlimited-aggregation model (Dϭ2.5).

Physics of Compaction of Fine Cohesive Particles

“…More generally, we may estimate that the critical shear force must be of order of the interparticle attractive force F max g , in close agreement with our previous experimental results [9]. Now we can explain why, for a constant F 0 (constant SAC), the size of our clusters measured in Ref.…”

“…A dry nitrogen atmosphere minimizes also capillary forces. Because of the strong interparticle attractive force as compared to particle weight, toner particles are clustered in the fluidlike regime [9]. According to our previous experimental results the typical number of particles per cluster N and typical ratio of cluster size to particle size depend on the ratio of attractive force to particle weight F 0 = m p g Bo g (granular Bond number).…”

“…Typical picture from the optical microscope of toner particles clustered in a nonaqueous liquid suspension that reminds one of a diffusion-limited aggregate. For this toner (12:7 m particle size and 10% SAC) the fractal dimension obtained from settling experiments in gas fluidization [9] is D ' 2:53. …”

Jamming Threshold of Dry Fine Powders

“…In a recent investigation however Nam et al [20] and obtain fluidized DLA clusters of 10 11 nanoparticles.) A nondimensional number that determines the size of the clusters is the ratio of the interparticle attractive force to particle weight (granular Bond number Bo g ) [19] g =N) is only slightly above 1 and quite independent of particle size. A Bond number of order 1 is roughly the limit that distinguishes free flowing individual grains from aggregative cohesive powders (see [19] for further discussion).…”

“…The number of particles per cluster (N) and the ratio of cluster size to d p (k) are obtained by means of sedimentation experiments (see Ref. [19] for details). In all the ranges of d p and SAC investigated the fractal dimension of the clusters D lnN= lnk approaches the diffusion-limited aggregation (DLA [6]) theoretical value D 2:5.…”