Diagnostic techniques of mixing effectiveness: the effect of shear and elongation in drop production in mixing tanks

• Published in: Chemical engineering science 1992, Vol.47 (6), p.1401-1410
• Main Authors: Collias, Dimitris I., Prud'homme, Robert K.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Exact sciences and technology; Mixing
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/0009-2509(92)80285-K

Results on drop-size distributions produced in mixing tanks using Rushton turbine, anchor and blade impellers are presented. The goal is to assess the relative importance of shear and elongational flows on the drop breakup process. The ratio of the dispersed- to continuous-phase viscosity was varied from 0.08 to 11.9 by varying the viscosity of the dispersed phase. The interfacial tension was varied from 0.00327 to 0.0118 N/m. Correlations of drop diameter as a function of the Weber number, percentage of the organic (dispersed) phase and viscosity ratio of dispersed to continuous phase are presented for the Rushton turbine and blade impeller, and they are valid for Reynolds numbers from 750 to 2700. Shear and elongational flows have different efficiencies in breaking liquid drops: at viscosity ratio of one, shear and elongation contribute equally to breakup but, at a higher viscosity ratio, only elongational flow produces breakup. This allows us to quantify the relative strengths of shear and elongational flows in complex geometries by considering the largest surviving drops. Using drop breakup data, we found that the anchor impeller produced higher elongation rates than the Rushton turbine, and the blade impeller produced the same elongation rates as the Rushton turbine. The anchor impeller was found to produce the smaller sized drops per unit of specific power input; but the Rushton turbine produced the smallest drops when the three impellers are considered at the same rotational speed. Finally, the turbulence intensity factor for the anchor and the blade impellers was found to be 2.9 and 2.0 times that of the Rushton turbine, respectively.