FLUIDIZATION OF NANOPOWDERS: EXPERIMENTS, MODELING, AND APPLICATIONS

Nanopowders are applied in a wide range of processes, and their use continues to increase. Fluidization is one of the best techniques available to disperse and process nanoparticles. However, nanoparticles cannot be fluidized individually; they fluidize as very porous agglomerates. Often, it is needed to apply an assisting method, such as vibration or microjets, to obtain proper fluidization. In this paper, we give a brief review of the developments in nanopowder fluidization

Tailoring Particle Mixtures for Fluidized Bed Reactors using High-Throughput Experimentation

The goal of the described project is to design mixtures of particles with optimal fluidization properties. Using high-throughput experimentation, a novel approach for hydrodynamic research, the relevant properties can be obtained in a limited period of time. This approach is demonstrated by measuring the hydrodynamic characteristics of typical Geldart B powders

Measuring the Gas-Solids Distribution in Fluidized Beds - A Review

This paper reviews techniques for measuring the voidage distribution in gas-solid fluidized beds, with a focus on the developments during the last ten years. Most attention is given to recent progress in tomography and pressure measurements, but visual observations, capacitance probes and optical probes are also covered

DEVELOPMENT OF A CONTINUOUS NANOPARTICLE COATING WITH ELECTROSPRAYING

Coating of micron-sized particles (host particles) with nanoparticles can result in modifying the surface properties of host particles leading to various applications. This work presents a novel concept of combining the electrospraying of nanoparticles onto the charged particles entrained out of the fluidized bed for developing a continuous coating process. By controlling the processes to effectively charge and contact particles using electric forces, we are able to fine-tune the properties of the obtained nano-coated particles

Long-term transients in fluidization of oxide nanoparticle agglomerates

Nanopowders are frequently fluidized for research during the last two decades. Interestingly, it was believed for a long time that nanopowders cannot be fluidized since they are classified as group C, very cohesive powders, in the Geldart diagram. For these powders the acting adhesion forces are too strong to allow fluidization. However, many studies showed that nanoparticles can be fluidized as micron-sized fractal agglomerates with very low densities and very high porosities [1]. The high porosities of the agglomerates are very attractive because most of the particle surface is accessible for mass transfer and reaction. The formation of the agglomerates in the fluidized bed is a dynamic process which includes collision, unfolding, breaking, and reagglomeration. This makes it likely that the agglomerate size distribution will change over time. On the other hand, long-duration fluidization of nanopowders is required for possible industrial applications such as coating or catalysis. Therefore, a better understanding of, the influence of time on the properties of fluidized nanoparticle agglomerates is crucial. Here we present a detailed analysis of the agglomerate size distribution over time during long-time fluidization of oxide nanoparticles. A settling tube set-up is used to investigate the agglomerate size distributions (see Fig. 1) as well as X-ray tomography which suggest stratification of the bed during long time fluidization (see Fig. 2). Further, the influence of the acting contact forces on the arising agglomerate size distributions was investigated. The results show that microscopic properties such as agglomerate size distribution can directly be linked to macroscopic properties as the bed expansion and that the time is a very important factor for the fluidization of nanopowders, because the bed dynamics changes strongly over time. Please click Additional Files below to see the full abstract

PLASMA-ENHANCED CHEMICAL VAPOUR DEPOSITION ON PARTICLES IN AN ATMOSPHERIC CIRCULATING FLUIDIZED BED

Plasma-enhanced chemical vapour deposition is an attractive technqiue to provide particles with a thin film. Applying a cold plasma enables us to work with temperature-sensitive materials. Using a CFB with an incorporated volume dielectric barrier discharge reactor we coated 20-30 m CuO particles with a thin SiOx layer

Pickup velocity of nanoparticles

This paper represents the first systematic study of the pneumatic conveying of nanoparticles. The minimum pickup velocity, Upu, of six nanoparticle species of different materials (i.e., silicon dioxide (SiO2), aluminum oxide (Al2O3) and titanium dioxide (TiO2)) and surfaces (i.e., apolar and polar) were determined by the weight loss method. Specifically, the weight loss method involves measuring the mass loss from the particle sample at various superficial gas velocities (U), and the Upu is the U value at which mass loss is zero. Nanoparticles were picked up as agglomerates rather than individually. Results show that (a) due to relative lack of hydrogen bonding, apolar nanoparticles have higher mass loss values at the same velocities, mass loss curves with accentuated S-shaped profiles, and lower Upu values; (b) among the three species, SiO2, which has the lowest Hamaker coefficient, exhibited the greatest discrepancy between apolar and polar surfaces with respect to both mass loss curves and Upu values; (c) Umf,polar/Umf,apolar was between 1 – 3.5 times that of Upu,polar/Upu,apolar due to greater extents of hydrogen bonding associated with Umf ; (d) Upu values are at least an order-of-magnitude lower than that expected from the well-acknowledged Upu correlation (1) due to agglomeration; (e) although nanoparticles should be categorized as Zone III (1) (or Geldart Group C (2)), the nanoparticles, and primary and complex agglomerates agree more with the Zone I (or Geldart Group B) correlation (Figure 1, whereby Rep* and Ar are the particle Reynolds number and Archimedes number, respectively (1)). In view of the importance of surface polarity on the pneumatic conveying of nanoparticles, more studies are on-going to further understand such surface effects. Please click Additional Files below to see the full abstract

Four Approaches to Structure Gas-Solid Fluidized Beds

Structuring fluidized beds can facilitate scale-up and increase conversion and selectivity by controlling the bubble size. We present four approaches to structure fluidized beds: oscillating the gas flow, distributing the gas injection, imposing an electric field to induce interparticle forces, and optimising distributed particle properties such as the size