Particle packing structure

Figure 2.11. Representative flow through a packed column. (A) Simplified diagram of column with "uniform" particles. (B) Representative diagram of column with experimental particles. How tortuous the path becomes depends on the particle packing structure. Plug flow usually results.

Several structured packing elements have been compared with regard to catalyst holdup, heat transfer performance, and pressure drop [120]. The results indicate that using catalyst coating gave lower pressure drop than packed beds but had a much lower catalyst inventory per reactor volume. On the other hand, a particle-packed structure exploited the advantages of structured flow vdiile not sacrificing much in catalyst holdup compared to a randomly packed bed. This alternative retained the favorable pressure drop characteristics so that smaller particle-sized catalysts could be used. Despite their lower... 

Monolithic materials with highly controlled iimer surfaces such as well-defined hierarchical pores can substitute many of the areas where particle-packed structure has been playing an important role. Their low flow resistance (high permeability) and enhanced accessibility to the nano-scaled surfaces in liquid phase are advantageous to every re-action/separation/purification processes. Chromatography in larger dimensions such as preparative and process chromatography will be also benefited by the use of monolithic columns prepared in appropriate module stmctures. 

Current theories do not consider the effect of particle packing structure (i.e. bulk density) on sintering behaviour. Particle packing is a major process variable. One example concerning the inconsistency of theory and reality concerns the theoretical result that densification kinetics should increase with decreasing particle size. Common experience indicates that powders with a very small crystallite size (e.g. <0.1 pm) can be very difficult to pack and densify. It is now commonly accepted that strong. 

From the electron micrographs, assuming that PVAc particles in the latex are the same size, the formation model of the porous film from the latex film can be illustrated as in Fig. 3 [19]. When the latex forms a dried film over minimum film-forming temperature, it is concluded that PVA coexisted in the latex and is not excluded to the outside of the film during filming, but is kept in spaces produced by the close-packed structure of PVAc particles. 

The formation of a 3D lattice does not need any external forces. It is due to van der Waals attraction forces and to repulsive hard-sphere interactions. These forces are isotropic, and the particle arrangement is achieved by increasing the density of the pseudo-crystal, which tends to have a close-packed structure. This imposes the arrangement in a hexagonal network of the monolayer. The growth in 3D could follow either an HC or FCC struc-... 

In a sense each monolithic column is unique, or produced as a product of a separate batch, because the columns are prepared one by one by a process including monolith formation, column fabrication, and chemical modification. Reproducibility of Chro-molith columns has been examined, and found to be similar to particle-packed-silica-based columns of different batches (Kele and Guiochon, 2002). Surface coverage of a Chromolith reversed-phase (RP) column appears to be nearly maximum, but greater silanol effects were found for basic compounds and ionized amines in buffered and nonbuffered mobile phases than advanced particle-packed columns prepared from high purity silica (McCalley, 2002). Small differences were observed between monolithic silica columns derived from TMOS and those from silane mixtures for planarity in solute structure as well as polar interactions (Kobayashi et al., 2004). 

All the spheres in a layer were supported by two spheres of the layer below and the column wall, creating a stable packing structure. As the tube-to-particle diameter ratio of the bed was only four, the entire packing structure was controlled by the influence of the wall. Nevertheless, the packing was divided into an immediate wall layer and a central section, but this should not be taken to imply that the central structure was not wall influenced. Although a three-sphere planar structure would almost fit within the nine-sphere wall layer, there was just not enough room at the same axial coordinate. When, however, the... 

When we want to look at the connection between the flow behavior and the amount of heat that is transferred into the fixed bed, the 3D temperature field is not the ideal tool. We can look at a contour map of the heat flux through the wall of the reactor tube. Fig. 19 actually displays a contour map of the global wall heat transfer coefficient, h0, which is defined by qw — h0(Tw-T0) where T0 is a global reference temperature. So, for constant wall temperature, qw and h0 are proportional, and their contour maps are similar. The map in Fig. 19 shows the local heat transfer coefficient at the tube wall and displays a level of detail that would be hard to obtain from experiment. The features found in the map are the result of the flow features in the bed and the packing structure of the particles. 

We have assumed that a random packed structure is more likely at high rates with a distribution of floe sizes. The volume fraction of floes will depend upon the floe packing fraction, giving rise to a floe diameter 2af with particles per floe ... 

The form of the above equations suggests that the only properties of the bed on which the pressure gradient depends are its specific surface S (or particle size d) and its voidage e. However, the structure of the bed depends additionally on the particle size distribution, the particle shape and the way in which the bed has been formed in addition both the walls of the container and the nature of the bed support can considerably affect the way the particles pack. It would be expected, therefore, that experimentally determined values of pressure gradient would show a considerable scatter relative to the values predicted by the equations. The importance of some of these factors is discussed in the next section. 

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