Particles segregation

The availabihty of spray-dried lactose, microcrystaUine cellulose, and other excipients allows for the use of granular rather than powdered phases. This eliminates some of the problems of particle segregation according to size (demixing) and even flow to the die. Direct compression eventually may be the preferred method of tablet preparation. 

Particle Segregation Mechanisms. Segregation is the process by which an assembly of soHd particles separates as it is being handled. This often results in cosdy quaUty control problems due to the waste of raw or finished materials, lost production, increased maintenance, and capital costs required to retrofit existing faciUties. 

The primary mechanisms responsible for most particle segregation problems are sifting, particle velocity, air entrainment, particle entrainment, and dynamic effects (14). 

Correcting Particle Segregation. The main techniques to consider when segregation problems are present are to change the material, change the process, or change the design of the equipment. 

Rowe, P.N., Nienow, A.W. and Agbim, A.J., The mechanisms by which particles segregate in gas fluidised beds binary systems of near-spherical particles, Trans. Inst. Chem. Engrs., 50 (1972a) 310-323. 

Figure A-1 Illustration of three modes of particle segregation.

Separating Particle segregation based on size alone, without any significant particle size reduction (commonly referred to as screening or bolting)... 

Sometimes, as in the case of particle segregation on fractals (e.g., the planar Sierpinski gasket discussed in Section 6.1) this effect indeed is self-evident [88-90]. Its analytical treatment for particle accumulation was presented in [91, 92] we reproduce here simple mesoscopic estimates following these papers. Particle concentrations obey the kinetic equations... 

The A + jf 2 —>0 surface reaction island formation and complete particle segregation... 

Despite the potential advantages of the procedure, sampling coal in motion may suffer from disadvantages such as (1) it is not always possible to penetrate the full depth of the coal cascading out of the car (2) attempts to penetrate the stream result in sample scoop overflow (3) increment collection is limited to the exposed surface at the sides of the car (4) moisture is often higher at the sides of the car than for the entire contents of the car (5) flow rates are highly variable and (6) disproportionate amounts of coarse coal are often collected because the coarse particles segregate and roll down the exposed surface. 

D. McGlinchey, P. Marjanovic, M. G. Jones, S. Cook, Particle segregation in pneumatic conveying lines, IMechE Conference Transactions, June 2000, pp. 331-340. 

McKee, S. L., Parker, D. J., and Williams, R. A., Visualization of size-dependent particle segregation in slurry mixers using positron emission tomography, in "Frontiers in Industrial Process Tomography" (D. M. Scott, and R. A. Williams Eds.), pp. 249-259. AIChE and Engineering Foundation, New York (1995). 

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