(79c) Non-Geometric Scale-up to Meet Mixing Requirements

Scale-up is a term most often used for mixing processes, because mixing is about both process results and fluid flow patterns. The interaction between process results and flow patterns can be a complicated relationship. Simple rules apply to some applications and are often combinations of factors related to both fluid motion and mixing intensity. As mixer geometry changes with different vessel sizes and impeller types, flow patterns change. As flow patterns change, so does mixing intensity and often process results.

Keeping important mixing parameters the same in different size vessels requires maintaining some similarities in flow patterns and mixing intensity to achieve comparable process results. As mixer and vessel geometry changes between the pilot scale and the production equipment, adjustments need to be made in the mixing behavior.

Geometric similarity provides a convenient way of achieving similar flow patterns, but large scale stirred tanks rarely have the same geometry as the pilot plant equipment. Finding ways of maintaining similar flow patterns and mixing intensity in not only different size equipment, but with different tank geometries and impeller types requires multiple adjustments to achieve desired results.

A systematic scale-up method combining geometric similarity and non-geometric adjustments provides a step-by-step process to observe and confirm changes in mixing intensity and flow patterns. No single method always works, but the chances of success increase when the effects of changes can be estimated and observed by basic mixing calculations. A multi-step method for non-geometric mixing scale-up will be presented.