Size enlargement

In this chapter the different types of interparticle forces and their relative importance as a function of particle size are summarized. Liquid bridge forces, which are specifically important to granulation processes, are covered in more detail. The rate processes important to granulation (wetting, growth, consolidation 

Introduction to Particle Technology, 2nd Edition Martin Rhodes 2008 John Wiley Sons Ltd. ISBN 978-0-470 -01427-1 

There exist between all solids molecularly based attractive forces collectively known as van der Waals forces. The energy of these forces is of the order of 0.1 eV and decreases with the sixth power of the distance between molecules. The range of van der Waals forces is large compared with that of chemical bonds. The attractive force, Fvw, between a sphere and a plane surface as a result of van der Waals forces was derived by Hamaker (1937) and is usually presented in the form  

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Theoretical Strength of Agglomerates. Based on statistical-geometrical considerations, Rumpf developed the following equation for the mean tensile strength of an agglomerate in which bonds ate localized at the points of particle contact (9)  

When the void space in an agglomerate is completely filled with a Hquid (Fig. Ic), the capillary state of wetting is reached, and the tensile strength of the wet particle matrix arises from the pressure deficiency in the Hquid network owing to the concave Hquid interfaces at the agglomerate surface. This pressure deficiency can be calculated from the Laplace equation for chcular capillaries to yield, for Hquids which completely wet the particles  

It is evident, by comparing equations 2 and 3, that tensile strength in the pendular state is about one-tbhd that in the capillary state. Intermediate Hquid contents in the funicular state (Fig. lb) yield intermediate values that can be approximated as foUows  

Compression tests, in which agglomerates ate cmshed between parallel platens, ate probably most universal. To obtain reproducible and accurate results, the rate of loading and method of load appHcation must be strictly controUed. A variety of commercial testers ate available to allow this needed control over the compression process. Several means of distributing the load uniformly at the point of contact ate used, including covering the platen 

6 Purchase Costs of Other Chemical Processing Equipment 543 

The fundamentals upon which most of these operations are based are surface phenomena. Surfaces are attracted to each other by van der Waals forces surfaces are repelled by the electrochemical double layer or by steric hindrance. Surface energies, contact angles, and wetting are important. 

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Sitosterols SI units Sivola process Six-pack holders Six-row barley Size distribution Size enlargement... 

In pelletizing, the water—carbon slurry is contacted with a low viscosity oil which preferentially wets the soot particles and forms pellets that are screened from the water and homogenized into the oil feed to the gasification reactor (see Size enlargement). 

The gaseous ammonia is passed through electrostatic precipitators for particulate removal and mixed with the cooled gas stream. The combined stream flows to the ammonia absorber where the ammonia is recovered by reaction with a dilute solution of sulfuric acid to form ammonium sulfate. Ammonium sulfate precipitates as small crystals after the solution becomes saturated and is withdrawn as a slurry. The slurry is further processed in centrifuge faciHties for recovery. Crystal size can be increased by employing one of two processes (99), either low differential controUed crystallization or mechanical size enlargement by continuous compacting and granulation. 

See Size enlargement Size measurement of particles Size reduction. 

Classification of size enlargement methods reveals two distinct categories (8,39). The first is forming-type processes in which the shape, dimensions, composition, and density of the individual larger pieces formed from finely divided materials are of importance. The second is those processes in which creation of a coarse granular material from fines is the objective, and the characteristics of the individual agglomerates are important only in their effect on the properties of the bulk granular product. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

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