Mass transfer to particles: Part I. Suspended in agitated tanks

Abstract

Mass transfer coefficients were measured for particles suspended in agitated, baffled tanks.The coefficients for a given particle size and a given fluid vary with the 0.10 to 0.15 power of the power dissipated per unit volume. The coefficients, which cover a one hundred fold range, are about 1.5 to 8 times those predicted from the correlations for fixed particles if the terminal velocity is used to calculate the particle Reynolds number. The measured effects of particle diameter, diffusivity, viscosity, and density difference cannot be described by a simple exponential equation or by previous empirical correlations. The effects of these variables can be explained by using a slip-velocity theory combined with a modified penetration theory.Studies of the rate of mass transfer to suspended particles are pertinent to many processes. For example in crystallization there is always a drop in solute concentration from the bulk of the solution to the face of a growing crystal. In some cases the crystal growth rate is limited by the rate of diffusion to the surface, but even when diffusion is not controlling, the mass transfer resistance should be used to calculate the actual surface concentration for any fundamental study of the crystallization mechanism. Similarly in reactions with suspended solid catalysts the diffusion of reactants to the catalyst surface influences and may limit the over-all reaction rate. Data for transfer to suspended solids are also valuable for studies of mass transfer between agitated 0uid phases, since very small drops or bubbles usually act as rigid spheres. Even for large drops traces of surface-active impurities may inhibit drop circulation and reduce the external transfer coefficients almost to those for rigid spheres ( 5 ) .Only a few of the forty or more studies of transfer to particles in stirred tanks will be discussed here; some deal mainly with large pellets which are not completely suspended, and many are restricted to unbaffled or partially baffled vessels of a particular shape. Several recent studies in fully baffled tanks with standard turbine impellers should have given consistent results, but the scatter of the data and the large differences in effects reported by different workers make the published correlations very uncertain. For example the reported effect of stirrer speed on the transfer coefficient has ranged from N o (2, 17) to No." (1, 1 3 ) , and the reported effects of fluid properties cover almost as wide a range of exponents ( 1 ) . 15) is not justified, and the use of the stirrer diameter instead of the particle diameter in both the Sherwood and Reynolds numbers indicates that the correlations have no fundamental significance and cannot be extrapolated.The aims of this study were to develop a more reliable correlation for suspended particles based on a wider range of variables and more accurate measuring techniques and also to explain the measured effects with existing or new theories of mass transfer. The mass transfer study also seemed a promising way of learning ...

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