Pelletization processes powder layering

With a double-walled centrifugal granulator, the process is carried out with the inner wall in the open or closed position (Fig. 2)P With powder layering, the inner wall is closed so that simultaneous application of liquid and powder could proceed until the pellets have reached the desired size. The inner wall is then raised, and the spheres enter the drying zone. The pellets are lifted by the fluidization air up and over the inner wall back into the forming zone. The cycle is repeated until the desired residual moisture level in the pellets is achieved. 

Nastruzzi, C Cortesi, R. Esposito, E. Genovesi, A. Spa-doni. A. Vecchio, C. Menegatti, E. Influence of formulation and process parameters on pellet production by powder layering technique. AAPS Pharm. Sci. Tech. 2000,1 (2), 1-22. 

Extrusion is the process of heating practically only TPs that may be in the shape of powders, beads, flakes, pellets, or combinations of these forms. This plastic enters the extruder s hopper. The extruder utilizes a plasticator [spiral screw that rotates within a heated barrel (cylinder)] to melt the plastic (Chapter 3). The melted plastic is then forced through a die to produce the desired continuous product shape. Figure 5.1 shows a very simplified schematic of the extrusion process. Information on dies (mono-layer and coextruded) used in extrusion is in Chapter 17. 

The experiments illustrated in Figure 21.4 however, were carried out with 4 g of material because, as was mentioned before, the aim was not to elucidate the reaction pathway or the kinetics parameters of the pyrolytic reaction, but to provide know how about the microwave pyrolysis process. Therefore as can be seen in the figure, the fastest degradation was achieved with the laminate because of its smaller thickness (plastic layer 90-150 p,m) in comparison with the average diameter of the HOPE powder (150 p,m) and pellets (3 mm diameter, 1 mm high). 

The necessary porosity for thicker layers was introduced by appropriate current densities [321-323], by co-deposition of composites with carbon black [28, 324] (cf. Fig. 27), by electrodeposition into carbon felt [28], and by fabrication of pellets from chemically synthesized PPy powders with added carbon black [325]. Practical capacities of 90-100 Ah/kg could be achieved in this way even for thicker layers. Self-discharge of PPy was low, as mentioned. However, in lithium cells with solid polymer electrolytes (PEO), high values were reported also [326]. This was attributed to reduction products at the negative electrode to yield a shuttle transport to the positive electrode. The kinetics of the doping/undoping process based on Eq. (59) is normally fast, but complications due to the combined insertion/release of both ions [327-330] or the presence of a large and a small anion [331] may arise. Techniques such as QMB/CV(Quartz Micro Balance/Cyclic Voltammetry) [331] or resistometry [332] have been employed to elucidate the various mechanisms. 

Lamellar double hydroxides are part of, or are precursors of, a more general family of compounds designated as pillared layered structures (PLS). We quote some terms used to describe them (Mitchell 1990 Van Damme 1990). Pillared Layered Structures are nanocomposite materials prepared by linking molecules or colloids to a layered host. They exhibit a remarkably broad spectrum of structural, chemical, electronic, ionic, optical and magnetic properties, and provide supermesh host structures in which chemical reactions or physical processes can proceed under gas-phase conditions, but at liquid/solid state densities. They can be shaped as powders, pellets, or supported or self-standing Aims, and can be dispersed in solid or liquid matrices. 

In some processes coated catalysts are employed. They are made by coating massive solid grains with a thin and very i rous layer of carrier powder with the catalytic material. These are then dried and sintered, subjected to reduction, etc. Catalyst pellets result with a thin porous outer layer that is catalytically active. Coated catalysts are particularly attractive when the catalytic material is both expensive and very active (e.g., noble metal catalysts). 

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