Granule size

Advantages of pan granulation are ease of granule size control, hardness of granule, and simplicity of pollution control. 

D. Critical hmit of granule size cm in energy required for rebound ... 

The effect of miid-penetratiou rate and the extent of penetration on granule-size distribution for drum granulation was shown by Gluba et... 

Powder Hand. Pmc., 2, 323 (1990).] Increasing penetration rate increased granule size and decreased asymmetry of the granule-size distribution. 

FIG. 20-68 Mechanisms of granule coalescence for low- and high-deformability systems. Rebound occurs for average granule sizes greater than the critical granule size D. K = deformability. 

Sfo is the Stokes number based on initial nuclei diameter do [Adetayo et al.. Powder Tech., 82, 37 (1995)]. Extent (/cf), depends logarithmically on binder viscosity and inversely on agitation velocity. Maximum granule size depends hnearly on these variables. Also, (/cf ), has been observed to depend hnearly on liquid loading y. Therefore, the maximum granule size depends exponentially on liquid loading. Fig. 20-73 illustrates this normahzation of extent (/cf), for the drum granulation of hmestone and fertilizers. 

Growth rate is very sensitive to liquid content for narrow initial-size distributions, with increases in liquid content for fine powders leading to an approximate exponential increase in granule size. For low-viscosity hquids, granulation occurs when very Hose to the saturation of the granule. This leads to the following equation to estimate moisture requirements [Capes, Pai ticle Size Enlargement, Elsevier (1980)] ... 

Continuous fluid-bed granulators are used in the fertilizer and detergent industries. For fertilizer apphcations, near-size grannies are recycled to control the granule size distribution. Dust is not recycled directly, but first remelted or slurried in the liquid feed. 

Coalescence Coalescence is the most difficult mechanism to model. It is easiest to write the population balance (Eq. 20-71) in terms of number distribution by volume n v) because granule volume is conserved in a coalescence event. The key parameter is the coalescence kernel or rate constant P(ti,i ). The kernel dictates the overall rate of coalescence, as well as the effect of granule size on coalescence... 

Modehng growth where deformation is significant is more difficult. It can be assumed that a critical cutoff size exists tr> which determines which combination of granule sizes are capable of coalescence, based on their inertia. When the harmonic average of sizes of two colliding... 

Analytical solutions for self-preseivdng growth do exist for some coalescence kernels and such benavior is sometimes seen in practice (Fig. 20-97). Roughly speaking, self-preseivdng growth implies that the width of the size distribution increases in proportion to mean granule size, i.e., the width is uniquely related to the mean of the distribution. 

The suspension polymerization of 65% acrylamide aqueous solution dispersed in n-hexane (aqueous phase -hexane = 1 5) in the presence of a stabilizer (sorbitan monostearate, 1.4% with respect to -hexane) and an initiator (2,2 -azo-bis-A/, A/ -dimethyleneisobutylamide chloride) carried out at 65°C for 3 h, with subsequent holding at 110°C, yields a powdered product with the granule size of 0.5 mm, while the addition of Na2S04... 

Bentonite is the name for a hydrous aluminum silicate comprised principally of the clay mineral montmorillonite, notable for its ability to swell in water and to form a very low-permeability seal." It is available as powder, granule (chip), or pellets. Powder and granule sizes are produced by processing after mining. Bentonite powder... 

Fig. 7 Effect of binder concentration and method of addition on granule size. (From Ref. 46.)...

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