Electrical conductivity has been used to measure local gas holdup in the bubbling zone (holdup C&30 %) and froth zone (holdup 695 %) ofa flotation column. Models have been developed to cover the full range of dispersed phase volume fraction, based on tortuosity or effective path between electrodes. Tortuosity was calculated from both statistical and geometrical properties of the bubble-liquid system. The statistical model is shown to hold adequately for the entire gas holdup range tested (&95%). Gas and liquid holdup in such systems as bubble and foam fractionation columns and froth flotation columns is of fundamental importance in order to estimate interstitial liquid flow, residence time distribution, bubble coalescence or to determine reflux ratio, wash water addition or interface level, for design, scaleup or control purposes. Typically the gas and liquid holdup in bubble columns has been determined by comparing directly the height of aerated liquid with that of liquid without aeration. In the case of fluidized two phase systems, the solid volume fraction can be estimated from the bed expansion (Turner, 1976). The problem becomes more complicated in the case of froth beds. The use of radioactive tracers which are well suited to holdup or density measurements (Desai and Kumar, 1983; Steiner et al., 1977) has the objection of safety and cost. Another technique is to cut a section of a three phase froth column and trap the dispersed materials (Flint, 1974; Watson and Grainger-Allen, 1974). An alternative method to determine holdup, by using electrical conductivity, has been used for foam columns (Fanlo and Lemlich, 1965; Shih and Lemlich, 1971), packed bubble columns (Achwal and Stepanek, 1975), fluid&d beds (Turner, 1976; Dhanuka and Stepanek, 1978; Begovich and Watson, 1978) and for drainage of granular packed beds (Shirato et al., 1981). Despite the wide interest in the use of conductivity techniques to measure holdup, as far as is known no general model has been reported which successfully describes the froth zone, i.e. a zone with a high concentration of non-conducting phase. Attempts to generalize the classical Maxwell (1873) theory are reviewed by Turner (1976).
A new approach to estimate local holdup from conductivity measurements is introduced here, based on the concept of tortuosity.