The stirred-vessel geometry at the pilot scale was scaled up to attain geometric parameters at the full scale. This scale-up process, to a large extent, depends on the mixing process parameters. It is essential to maintain the same mixing time at the pilot and full scale.
Appropriate scaling of the pilot-scale process was carried out to ensure the mixing time at the two scales was unchanged. The full-scale stirred-vessel configuration performance then was examined using tier two methods. Table 4 depicts the performance parameters at full scale.
The final configuration at the pilot scale and full scale was analyzed in detail using tier three methods. These methods are based on the solution of Navier-Stokes equations to predict stirred vessel behavior.
Computational fluid dynamics (CFD) is one such method. This method is useful in evaluating detailed flow patterns in complex geometries and in situations in which tier two methods are not applicable. CFD methods involve the resolution of conservation equations of mass, momentum and energy at thousands of locations within the flow domain. A CFD solution provides full-field data; flow variables at every location in the domain are available; and a graphical representation of the flow can be created.
For the stirred-vessel study, CFD was applied to identify regions of low or excessively high flows and attain information on localized flow behavior. Fig. 1 depicts the CFD computed velocity distribution in the stirred tank at the pilot-scale, and Fig. 2 depicts the CFD computed velocity distribution in the vessel at the full scale. The nondimensionalized velocity fields at the two scales, as depicted in Fig. 3 and Fig. 4, are similar, indicating the proper scaling from pilot scale to full scale was achieved.
The multi-tiered solution strategy provides information at various scales. It also can be applied to solve other problems associated with stirred vessels. This information can be assimilated for selected processes to generate specific guidelines for design and scale-up of stirred vessels. Scale-up or scale-down study for any stirred tank mixing process can be carried out using these techniques.
Pordal is a staff consultant and Matice is principal and team leader at SES-Process Technology Group, Mason, Ohio. Contact them at (513) 336-6701.
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