Discrete agglomeration

Spheronizing As mentioned several times before, feeds suitable for extrusion must be somewhat plastic. In fact, many wet mixtures which, during compaction, become too pasty for use in any other agglomeration equipment can be successfully densified in extruders and shaped into discrete agglomerates. Such products are still easily formable and, therefore, can be further treated in so-called spheronizing equipment to yield uniform round particles. 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

It is often observed that crystals do not exist as discrete entities but, especially when viewed under the optical microscope, appear to comprise agglomerated particles. Even a cursory glance at the literature quickly shows that the subject of agglomeration spans several disciplines including crystallography, materials... 

Hill, P.J. and Ng, K.M., 1996a. New discretization procedure for the agglomeration equation. American Institution of Chemical Engineers Journal, 42(3), 727. 

Kapur and Fuerstenau (K6) have presented a discrete size model for the growth of the agglomerates by the random coalescence mechanism, which invariably predominates in the nuclei and transition growth regions. The basic postulates of their model are that the granules are well mixed and the collision frequency and the probability of coalescence are independent of size. The concentration of the pellets is more or less fixed by the packing... 

This additional Eq. (18) was discretized at the same resolution as the flow equations, typical grids comprising 1203 and 1803 nodes. At every time step, the local particle concentration is transported within the resolved flow field. Furthermore, the local flow conditions yield an effective 3-D shear rate that can be used for estimating the local agglomeration rate constant /10. Fig. 10 (from Hollander et al., 2003) presents both instantaneous and time-averaged spatial distributions of /i0 in vessels agitated by two different impellers color versions of these plots can be found in Hollander (2002) and in Hollander et al. (2003). 

In order to coat each particle in a discrete manner and avoid agglomeration, the coating fluid must often be atomized more finely and in a more controlled manner than in the case where tablets are coated in a typical pan process. 

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