Mixing times for process vessels with aspect ratios greater than one

Abstract Stirred tank reactors are one of the most important and common pieces of equipment used in speciality, pharmaceutical, and agrichemical processes. It is also typical for these to be operated at large aspect ratios; however, there is very little information in the open literature about mixing times in vessels with aspect ratios greater than one. This paper aims to provide new information in this area that will enable the design of better reactors. Electrical resistance tomography is used to monitor the mixing time of systems with aspect ratios higher than one. The mixing time has been measured on vessels of 0.914 and 0.610 m diameter with Cowles disc, Rushton turbine and mixed flow type impellers to aspect ratios of 2. The current correlation of choice, by Grenville and Nienow (2004) , has been compared with the results and found to under predict the mixing time at aspect ratios greater than one. The exponent on the H / T term has been explored and it has been found that this varies with agitator type, this information has never been shown before. The affect of adding a second impeller on the mixing time and flow pattern is also investigated. Adding a second Rushton turbine creates zoning in the vessel which impedes the mixing; this can be visualised using electrical resistance tomography.

[1]  P. Hansen.  The discrete picard condition for discrete ill-posed problems , 1990 .

[2]  Johannes Tramper,et al.  Basic Bioreactor Design , 1991 .

[3]  Trevor A. York.  Status of electrical tomography in industrial applications , 2001, J. Electronic Imaging.

[4]  Alvin W. Nienow,et al.  On impeller circulation and mixing effectiveness in the turbulent flow regime , 1997 .

[5]  J. Couderc,et al.  Study by laser Doppler anemometry of the turbulent flow induced by a Rushton turbine in a stirred tank: Influence of the size of the units—I. Mean flow and turbulence , 1988 .

[6]  P. Ayazi Shamlou,et al.  Processing of Solid–Liquid Suspensions , 1993 .

[7]  J. Joshi,et al.  Liquid-phase mixing in stirred vessels: turbulent flow regime , 2003 .

[8]  P. Hansen.  Regularization,GSVD and truncatedGSVD , 1989 .

[9]  Joachim Schöberl,et al.  NETGEN An advancing front 2D/3D-mesh generator based on abstract rules , 1997 .

[10]  M. Jahoda,et al.  CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part II Vessel with Three and Four Impellers , 2005 .

[11]  Adam J. Kowalski,et al.  An electrical resistance tomography method for determining mixing in batch addition with a level change , 2010 .

[12]  G. Montante,et al.  CFD simulations and experimental validation of homogenisation curves and mixing time in stirred Newtonian and pseudoplastic liquids , 2005 .

[13]  Michael Yianneskis,et al.  Direct determination of energy dissipation in stirred vessels with two‐point LDA , 2005 .

[14]  E. L. Paul,et al.  Handbook of Industrial Mixing: Science and Practice , 2003 .

[15]  Alvin W. Nienow,et al.  Mixing in large-scale vessels stirred with multiple radial or radial and axial up-pumping impellers: modelling and measurements , 2000 .

[16]  Octave Levenspiel,et al.  New scale-up and design method for stirrer agitated batch mixing vessels , 1976 .