Experimental and theoretical studies of axial solid distributions in slurry bubble columns have been carried out extensively (Cova, 1966;Smith et al., 1986; Jean et al., 1989;Reilly et al., 1990). Most of the experimental works performed in this area have characterized the solid concentration profiles using water as the liquid phase. A limited amount of experimental data is available for slurry bubble columns with organic liquids (Bach and Pilhofer, 1970; Bhaga et al., 1971; Brian and Dyer, 1985;Badgujar et al., 1986b) and mixtures of immiscible liquids (Badgujar et al., 1986a).
The properties of the liquid phase are among the most important factors that affect the fluid dynamic behavior of bubble columns. It is a well known fact that mixtures of miscible liquids behave differently from pure liquids in bubble columns when the substances that comprise the mixture have different chemical structure. For instance, Bhaga et al. (1971) noted that the gas holdup of a mixture of two miscible organic liquids in two-phase bubble columns exhibits a maximum with respect to the concentration of one of the liquids. The difference of behavior between mixtures and pure liquids cannot be correlated in terms of the density, viscosity and surface tension. This implies that the empirical correlations available in the literature to predict gas holdup, which have been usually based on experiments with pure liquids, cannot be used for the prediction of gas holdup in bubble columns with liquid mixtures (Badgujar et al., 1986a;Pino et al., 1990). The main reason for this difference is the fact that one of the components in the mixture acts as a surfactant. In this case, the description of dynamic interfacial phenomena at the gas-liquid interface does not depend exclusively on common physical properties (density, viscosity, and surface tension), but also on the way that the surfactant is distributed over the gas-liquid interface. Recent attempts have been made to quantify this effect through measurable properties, such as dynamic surface tensions (Gorowara and Fan, 1990).
The complexities of the fluid dynamic of bubble columns described above become more evident when dealing with immiscible liquids. For instance, in a bubble column with two immiscible liquids, up to three different fluid-fluid interfaces might be present, each of which can be affected by the existence of surface-active substances. Bubble columns with two im-Correspondence concerning this note should be addressed to A. E. Saez.