The application of drop size distribution and discrete drop mass transfer models to assess the performance of a rotating disc contactor

Abstract

A method has been developed to calculate the overall mass transfer coefficient in an agitated liquid extraction column containing a wide range of drop sizes. The method applied the drop size distribution diagram to estimate the volume percentage of stagnant, circulating, and oscillating drops in the drop population. Individual mass transfer coefficients were then evaluated for the corresponding drop state, using the different singledrop mass transfer models, after which the overall coefficient K,,1 was calculated as the fractional sum of the individual coefficients and their proportion in the drop population. These estimated mass transfer coefficients have been compared with results obtained from a large rotating disc extractor 0.45 m in diameter and 6.75 m high, extracting acetone between water and Clairsol (a parafinic hydrocarbon, principally decane). Good agreement was obtained in the majority of the experiments when the Rose-Kintner correlation was applied to evaluate the dispersed phase mass transfer coefficient of the oscillating drops. In all experiments agreement between the calculated and experimental overall mass transfer coefficients was improved by evaluating the concentration driving force by applying Simpson's rule, thereby introducing corrections for the variation in flowrate along the column and for possible backmixing. SCOPEIn all liquid extraction equipment the dispersed phase exists in the form of drops. In order to analyze the performance of this type of equipment the assumption is made that these drops are spherical and of uniform size. This simplifies the application of discrete drop models of mass transfer to the assessment of equipment performance. Olney (1964) pointed out that such an assumption would lead to serious error, since a distribution of drop sizes always exists in each compartment, or column section, in all extraction equipment. In most mechanically agitated extraction contactors the drop size distribution is the result of competing effects, namely, the generation of new drops through breakup by shear or local turbulence in the bulk flow, and the coalescence due to interactions between the drops. This size distribution is bounded by an upper limit or maximum drop size (Mugele and Evans, 1951) and a lower limit, or minimum drop size, dependent upon the prevailing breakup processes (Olney, 1964). Mass transfer during drop passage through the continuous phase is significantly affected by these hydrodynamic effects, i.e., whether the liquid inside the drops is stagnant, circulating, or oscillating. This has a pronounced effect on the mechanism and therefore rate of mass transfer. The dispersion in an agitated contactor may contain all these types of drops in significant quantities and they must be included in the analysis of the performance of this type of equipment, otherwise reasonable comparison with practical results cannot be expected.At present the correlations of mass transfer rate and mass transfer coefficient are based on single-drop models; the effects of the presence ...

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