Mixed factors, processes, variables

There is no constant scale-up factor for each specific mixing system/process [29]. The two independent impeller variables come from speed, diameter, or power, because once the impeller type/style has been selected. 

There is an intimate and often very complex interrelation between the performance of an agitated vessel and the system geometry, the properties of the processed materials, and the operating conditions. In some cases the true complexity of a mixing situation may not be appreciated. Particularly in scale-up problems, the physical nature of the mixing in a process may be as important a factor as the proper reaction temperature or reactant ratios. Thus, another objective of this review is to demonstrate that agitation is a process variable of major significance. 

Xj coded variable for the factor i. It is also used to describe the axis for the factor i in coded factor space, for both independent (process) variables and mixture components. In the case of a mixed problem (of mixture and process components), we use it for the mixture components only, and identify the process variables by Z/. xj level of Xi in the model for an experiment. 

Perhaps a major factor is the handling of batches. For instance, pharmaceutical plants usually handle fixed sizes for which integrity must be maintained (no mix-ing/splitting), while solvent or polymer plants handle variable sizes that can be split and mixed. Similarly, different requirements on processing times can be found in different industries depending on process characteristics. For example pharmaceutical applications might involve fixed times due to FDA regulations, while solvents or polymers have times that can be adjusted and optimized with process models. 

First ask yourself if there is any role for fluid shear stresses in determining and obtaining the desired process result. About half of the time the answer will likely be no. That is the percentage of mixing processes where fluid shear stresses either have no effect or seem to have no effect on the process result. In these cases, mixer design can be based on pumping capacity, blend time, velocities and other matters of that nature. Impeller type location and other geometric variables are major factors in these types of processes. 

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