GENERAL RESEARCH Solids Settling Velocity and Distribution in Slurry Reactors with Dilute Pseudoplastic Suspensions Davide Pinelli and Franco Magelli* DICMA-Department of Chemical, Mining and Environmental Engineering, University of Bologna, viale Risorgimento 2, 40136 Bologna, Italy The solids distribution was investigated in a tank stirred with multiple Rushton turbines. The analysis of the behavior of suspensions made of solid particles and non-Newtonian liquids allowed us to better single out the role of the most significant parameters that define the solids profiles: particle settling velocity is confirmed to be a key parameter for this process. It is shown that the same concept of terminal velocity in these systems is to be considered with care. The settling velocity in the stirred liquid is usually intermediate between the terminal settling velocity in the still liquid and the terminal settling velocity calculated with an effective liquid viscosity based on Metzner and Otto’s concept. A correlation between the first and last settling velocities with the ratio of Kolmogoroff length scale and particle diameter is in very good agreement with that determined previously for Newtonian liquids. 1. Introduction Mixing in stirred tanks is a common practice in many areas of the chemical and process industry. An under- standing of the complex fluid dynamic behavior prevail- ing in slurry agitated reactors and other equipment for solid-liquid treatment is crucial for rational design and reliable process operation. Three different topics are usually considered in the fluid dynamic context of solid- liquid systems, namely, off-bottom particle suspension, spatial solids distribution inside the equipment, and solids separation at the withdrawal tube. 1 This paper addresses the second of the above- mentioned items. The main results obtained in the analysis of particle-liquid interaction, which is a key phenomenon for solids distribution, are reviewed in the following section. Suffice it to mention here that most of these studies arrived at the conclusion that the particle settling velocity in a stirred medium, considered either as a real suspension property or as a model parameter, is usually less than the settling velocity in a still liquid (the so-called terminal velocity). The aim of this paper is to discuss this last aspect further. The investigation was conducted with dilute suspensions of spherical particles in pseudoplastic liquids in order that the liquid environment “seen” by the single particles and the nature of particle-liquid interaction are affected by nonlinear rheological behav- ior, while particle-particle interactions are minimized. The analysis was performed in a tank of high aspect ratio stirred with multiple impellers, which allows for the solids concentration gradients along the vertical axis to be maximized while those in the radial direction are neglected, thus making the study more straightforward. The experimental results are discussed in terms of the simple one-dimensional sedimentation-dispersion model with the particle velocity as a parameter. A limited number of ancillary experiments aimed at determining the terminal particle settling velocity and characterizing liquid macromixing in pseudoplastic fluids were also conducted and were instrumental in the above-mentioned analysis. 2. Background on Particle Settling Velocity in Stirred Media The first experimental analyses of particle-liquid interactions in stirred tanks date back to about 30 years ago. Different research groups 2-5 have concluded that the average particle settling velocity in a stirred liquid is 30-60% of the settling velocity in the quiescent liquid at rest, U t , thus implying a relevant drag coefficient increase. Although the techniques on which these results were based exhibit some limitations and the data themselves were relatively limited, the essence of these findings is consistent with turbulent statistical theory and with turbulence-related phenomena such as par- ticulate mass transfer. More recently, detailed LDV measurements have revealed that the particles actually lag or lead the liquid in the zones characterized by upward or downward flow, respectively. 6 On average, however, these last results do not seem to contradict the previous ones. Similarly, the drag coefficients of spheres falling in liquids have been reported to be affected by “external” turbulence (sometimes referred to as “free stream” turbulence), that is, turbulence other than that pro- duced by particle motion itself. 7 Although the form of this influence has been questioned recently, 8 the general effect remains undisputed. * Corresponding author: Dr. Franco Magelli, DICMA- Department of Chemical, Mining and Environmental Engi- neering, University of Bologna, viale Risorgimento 2, 40136 Bologna, Italy. Fax: +39-051-581200. Telephone: +39-051- 2093147. E-mail: franco.magelli@mail.ing.unibo.it. 4456 Ind. Eng. Chem. Res. 2001, 40, 4456-4462 10.1021/ie0010518 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/05/2001