Critical analysis of Zwietering correlation for solids suspension in stirred tanks

Abstract Design specifications for solids suspension (just suspended speed-Njs) in stirred tanks are currently based on the original form of the Zwietering correlation. There are several limitations of this correlation which can lead to poor predictions. The limitations, however, are not clearly defined. In this paper we review the reported limitations of applicability of the Zwietering correlation in detail, and propose ways to predict Njs which extend some of these limits. New data is reported and a detailed analysis of the effects of off-bottom clearance and solids concentration is presented. These results show two things: first that the effect of off-bottom clearance cannot be explicitly accounted for using the Zwietering equation form. S values that exactly match the geometry of the system must be used. Second, the current exponent on solids concentration is applicable only up to 2 wt% solids (X = 2). Three new exponents are proposed based on the particle type: 0.17, 0.23, and 0.32. Using a modified exponent of 0.23, which represents the entire data set with some scatter, Njs can be predicted up to 35 wt% solids (X = 54). A new correlation that is a combination of Zwietering and Baldi, and is robust in scale-up based on constant power/mass is proposed.

[1]  G. Micale,et al.  Assessment of Particle Suspension Conditions in Stirred Vessels by Means of Pressure Gauge Technique , 2002 .

[2]  Jos Derksen,et al.  Effect of geometry on the mechanisms for off-bottom solids suspension in a stirred tank , 2012 .

[3]  A. W. Nienow,et al.  Particle-gas-liquid mixing in stirred vessels. I: Particle-liquid mixing , 1983 .

[4]  Giancarlo Baldi,et al.  Complete suspension of particles in mechanically agitated vessels , 1978 .

[5]  Gül Özcan-Taşkin.  Effect of scale on the draw down of floating solids , 2006 .

[6]  A. W. Etchells,et al.  Prediction of just suspended speed for mixed slurries at high solids loadings , 2013 .

[7]  A. W. Nienow,et al.  Suspension of solid particles in turbine agitated baffled vessels , 1968 .

[8]  A. A. Shaikh,et al.  Solids suspension in stirred tanks with pitched blade turbines , 2003 .

[9]  J. B. Fasano,et al.  Effect of solids loading on agitator just‐suspended speed , 2013 .

[10]  Suzanne M. Kresta,et al.  Design rules for suspending concentrated mixtures of solids in stirred tanks , 2011 .

[11]  J. B. Fasano,et al.  The influence of solid properties on the just-suspended agitation requirements of pitched-blade and high-efficiency impellers , 1994 .

[12]  P. Armenante,et al.  Determination of correlations to predict the minimum agitation speed for complete solid suspension in agitated vessels , 1998 .

[13]  Th.N. Zwietering.  Suspending of solid particles in liquid by agitators , 1958 .

[14]  K. J. Bittorf,et al.  Three-dimensional wall jets: Axial flow in a stirred tank , 2001 .

[15]  Alvin W. Nienow,et al.  THE EFFECT OF VISCOSITY ON PARTICLE SUSPENSION IN AN AERATED STIRRED VESSEL WITH DIFFERENT IMPELLERS AND BASES , 2009 .

[16]  W. Roy Penney,et al.  Solid–Liquid Mixing , 2004 .

[17]  O. Molerus,et al.  Suspension of solid particles in agitated vessels , 1987 .

[18]  Alvin W. Nienow,et al.  Particle suspension in the turbulent regime: The effect of impeller type and impeller/vessel configuration , 1996 .

[19]  Kevin J. Myers,et al.  The influence of baffle off‐bottom clearance on the solids suspension performance of pitched‐blade and high‐efficiency impellers , 1992 .

[20]  P. Armenante,et al.  Effect of low off-bottom impeller clearance on the minimum agitation speed for complete suspension of solids in stirred tanks , 1998 .

[21]  K. J. Bittorf,et al.  Prediction of Cloud Height for Solid Suspensions in Stirred Tanks , 2003 .

[22]  Alvin W. Nienow,et al.  Comparing Impeller Performance for Solid-Suspension in the Transitional Flow Regime with Newtonian Fluids , 1999 .

[23]  František Rieger,et al.  Suspension of Solid Particles , 1994 .