Single drop experiments with liquid test systems: A way of comparing two types of mechanically agitated extraction columns

Abstract

This paper deals with a comparative analysis between a rotary agitated column (a Kühni column) and a pulsed sieve-plate column, based on the description of the basic mechanisms of drop transport and drop breakage, which are sufficient for the prediction of the global behaviour of the columns, as long as coalescence can be neglected. Laboratory-scale devices were designed, in which single drop experiments could be performed to determine the parameters of the transport and breakage laws that are representative of the phase system and of the column type involved. In the case of the pulsed sieve-plate column, a one-compartment cell was sufficient, whereas in the case of the Kühni contactor, a three-compartment cell was required. These devices were fully automated using microcomputer control and data processing and are devoted to a routine working. Experiments were carried out with liquid-liquid test systems recommended by the EFCE The statistical results are expressed in terms of the following: mean drop residence time in a column compartment; axial mixing relative to the drop; breakage probability and consequently breakage rate; number of daughter drops and daughter drop size distribution. The comparative analysis demonstrates that description of a pulsed column is formally easier than that of a Kühni column. Quite surprisingly, the main difficulty lies in the transport term: in a pulsed column, mixing essentially acts vertically and it is then possible to correlate directly the relative drop velocity and the terminal drop velocity; in contrast, in a Kühni column, the flow patterns are more complicated and more heterogeneous (radially and axially) and it is therefore difficult to derive correlations between drop velocity, drop diameter, agitation intensity and geometry of the plates and impellers. Finally, the results of global efficiency in both columns are presented, according to the resulting remarks on drop transport and drop breakage.