Bubble Coalescence and Break-Up in Air-Sparged Bubble Columns zy A phenomenological model is proposed for the rates of bubble coalescence and bubble break-up in turbulent gas-liquid dispersions. Bubble coalescence is modeled by considering bubble collisions due to turbulence, buoyancy, and laminar shear, and by analysis of the coalescence efficiency of collisions. Bubble break-up is analyzed in terms of bubble interactions with turbulent eddies. A method for the measurement of coalescence and break-up events in turbulent systems is described and used to test the validity of the proposed model. The measurement technique relies on the mixing of tracer gases within bubbles upon coalescence, in conjunction with Monte-Carlo simulations of coalescence events. Both distilled water and salt solutions are examined. Favorable agreement is found between the model and the individual coalescence and breakage rates, as well as with data obtained for the average bubble size and bubble size distribution. zyxwv Michael J. Prince Harvey W. Blanch Department of Chemical Engineering University of California Berkeley, CA 94720 Introduction zyxwvutsrqp Gas-liquid contactors find broad application as reactors and separation units in the chemical, mining, pharmaceutical and biochemical industries. In these systems the rate of transport of the gas to the liquid phase often limits productivity and is therefore a critical design criterion. Current design methods rely on empirical correlations which may not be confidently applied beyond the narrow range of operating conditions and geometries over which they were determined. The uncertainty in bubble column design arises from a lack of fundamental understanding of the hydrodynamics and rate processes which govern bubble size and thus interfacial area. Bubble size depends on a balance of coalescence and break-up rates in the vessel. No broadly applicable model for these rate processes in turbulent systems has yet been presented. This has been due to the inability to measure bubble coalescence and break-up in systems where high gas flow rates prevent direct visual observation. The objective of this study is to measure bubble coalescence and break-up rates in turbulent gas-liquid dispersions and to develop a model for these rate processes based zyxwvut on physicochemi- cal and hydrodynamic principles. The ability of the model to adequately predict the individual experimental rate of coales- Current address of M. J. Prince: Department of Chemical Engineering, BuckncU University. Lewisburg, PA 17837. cence and break-up is used to judge its validity. In addition, the model is tested against its ability to'predict bubble size and bubble size distributions. The coalescence process is analyzed by examination of bubble collision events and the likelihood of collisions resulting in coalescence. Bubble break-up is examined in terms of bubble interactions with turbulent eddies. In addition to the theoretical development, an experimental method for the measurement of bubble coalescence and break-up rates in turbulent systems is described. The effect of inorganic electrolytes on coalescence and break-up rates is also examined. Much of the current interest in the prediction of bubble size stems from the desire to predict oxygen transfer rates in aerobic biochemical reactors. In these systems the liquid media typically contains inorganic electro- lytes which provide minerals for microbial growth. Other applications where salts influence bubble size are mineral flotation and treatment of polluted saline waters. The addition of inorganic electrolytes has been found to have a marked effect on bubble size by a number of investigators (Marrucci and Nicodemo, 1967; Keitel and Onken, 1982; Oolman and Blanch, 1986a). Changes in bubble size may result from the influence of salts on either bubble break-up or bubble coalescence rates. In this work the degree to which each of these mechanisms is affected by salt concentration is examined and a model is developed to predict these rate processes in inorganic electrolyte solutions. While inorganic salts are only one form of AIChE Journal October 1990 Vol. 36, No. 10 1485