Mixing indices allow scale-up of stirred tank slurry reactor conditions for equivalent homogeneity

• Published in: Chemical engineering research & design 2020-01, Vol.153, p.865-874
• Main Authors: Harrison, S.T.L., Kotsiopoulos, A., Stevenson, R., Cilliers, J.J.
• Format: Article
• Language: English
• Subjects: Chemical reactions; Chemical reactors; Electric resistance; Electrical resistance tomography (ERT); Homogeneity; Impeller speed; Impellers; Scale-up; Slurries; Solids concentration; Solids suspension; Specific power; Tip speed; Tomography; Vessels
• ISSN: 0263-8762; 1744-3563
• DOI: 10.1016/j.cherd.2019.10.049

•Mixing indices may be used to reduce energy needed to achieve slurry homogeneity.•Suspension quality most consistent in smaller vessels than in larger vessels.•Higher P/V ratios are achieved in smaller vessels with similar impeller tip speeds.•Inconsistencies in mixing attributed to limited mixing in the axial direction.•Less homogeneous suspension at low solids loading than at high loadings. The influence of reactor scale, impeller tip speed and specific power on the overall homogeneity (multi-phasic mixing) of two dimensionally similar stirred tanks of 50mm and 220mm internal diameter (ID) are compared using data collected from electrical resistance tomography for 5 and 15%v/v 600–800μm particle suspensions. The data collected is used to quantify the suspension quality of the system by defining mixing indices in the axial MIz and radial MIr directions as well as the overall mixing index MIo. Analyses across the vessels indicated that axial and radial homogeneity improved with increasing impeller tip speed for both large and small vessels with more consistent suspensions observed in the former. Improved homogeneity was consistently found in the 10–20% (v/v) solids loading range (data only shown for 15%) than at 5%, across both reactors. In either vessel, optimum homogeneity was achieved at impeller tip speeds ca. 20% lower than the critical suspension speed, as shown previously. Analysis of the local concentration of particles in the lowest region of the vessels indicated that absolute homogeneity was unattainable as the decreasing local concentration displayed an asymptotic character with increasing power per unit volume. The suspension quality during reactor scale-up was relatively consistent with specific power ratios P/V, while the same degree of homogeneity was not achieved when the impeller tip speed was kept constant.