NUMERICAL EXPERIMENTATION - A TOOL TO CALCULATE THE AXIAL-DISPERSION COEFFICIENT IN DISCS AND DOUGHNUTS PULSED SOLVENT-EXTRACTION COLUMNS

In this paper, a correlation between axial mixing in a discs and dough nuts pulsed extraction column, the geometrical characteristics of the column, and the pulsation conditions is established by numerical exper imentation. The calculation method, based on the computational fluid d ynamics approach and detailed on an example, consists in evaluating th e axial dispersion coefficient from simulations of tracer mixing, tran sported by the turbulent pulsed flow in the column. Hydrodynamic chara cteristics of flow and tracer transport are calculated by the ESTET co de. The results concerning the mean velocity fields of the oscillatory turbulent flow show a transition from an unstable to a stable recircu lation regime. The stability of the recirculations and their size depe nd only on one geometrical parameter which is the distance between suc cessive discs and doughnuts. A quantitative analysis of the dispersion process is obtained from the time variation's study of the tracer clo ud's spatial moments. The results reveal that the distance between suc cessive discs and doughnuts is the key parameter governing axial mixin g in such columns. This work also confirms the proportionality observe d by other authors between the axial dispersion coefficient and the pu lsation intensity. Numerical results are compared with the experimenta l ones available. (C) 1997 Elsevier Science Ltd. All rights reserved.