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Cited by 14 publications
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Continuous-phase mass transfer coefficients and drop sizes in agitated vessels are correlated with operating variables and physical properties of liquid-liquid systems. In formulating mass transfer coefficients, a realistic mechanism has been developed, involving periodically varying rates of surface renewal associated with droplet circulation through varying degrees of turbulence around the vessel. Relationships are obtained for optimized design and scale-up. SCOPEThe rate of mass transfer between two immiscible liquids in mixing vessels depends on the concentration difference, the interfacial area, and the mass transfer coefficient. The comprehensive correlation of mass transfer coefficients and interfacial areas are therefore essential for the optimized design and scale-up of such equipment. Schindler and Treybal (1968) and Keey and Glen (1969) studied liquid-liquid mass transfer rates in agitated vessels.Both investigations were confined to single systems, so that comprehensive correlations over a range of physical properties were not established.The objects of this research are to measure and correlate such continuous-phase mass transfer coefficients and the corresponding drop sizes in the presence of mass transfer. Relationships for optimized design and scale-up will then be formulated. CONCLUSIONS AND SIGNIFICANCEThe continuous-phase mass transfer coefficient has been correlated and separate correlations for high and low interfacial tension systems have also been attempted for a better fit of the data.Comparison between our correlation for k, and expressions derived from the penetration theory with Kolmogoroff s time scale and from turbulent boundary layer theory show significant differences between the exponents on any given variable, suggesting the need for a new model. Accordingly, a theory based on a periodically varying rate of surface renewal has been developed and the average rate of surface renewal sa in this theory has been correlated.Droplet size was determined from photographs of the dispersion taken through a plane glass water pocket. A correlation was obtained for the Sauter-mean drop diameter when about 50% of the possible mass transfer had occurred; this was chosen as an average value during batch operation.These correlations have been used to develop relationships for optimized design and scale-up.This study included h e liquid-liquid systems, two sizes of six-flat-blade turbines, two vessel diameters, two principal liquid heights (T = H), impeller speeds between 3 and 8 rps, and dispersed-phase volume fractions between 0.03 and 0.09.
Continuous-phase mass transfer coefficients and drop sizes in agitated vessels are correlated with operating variables and physical properties of liquid-liquid systems. In formulating mass transfer coefficients, a realistic mechanism has been developed, involving periodically varying rates of surface renewal associated with droplet circulation through varying degrees of turbulence around the vessel. Relationships are obtained for optimized design and scale-up. SCOPEThe rate of mass transfer between two immiscible liquids in mixing vessels depends on the concentration difference, the interfacial area, and the mass transfer coefficient. The comprehensive correlation of mass transfer coefficients and interfacial areas are therefore essential for the optimized design and scale-up of such equipment. Schindler and Treybal (1968) and Keey and Glen (1969) studied liquid-liquid mass transfer rates in agitated vessels.Both investigations were confined to single systems, so that comprehensive correlations over a range of physical properties were not established.The objects of this research are to measure and correlate such continuous-phase mass transfer coefficients and the corresponding drop sizes in the presence of mass transfer. Relationships for optimized design and scale-up will then be formulated. CONCLUSIONS AND SIGNIFICANCEThe continuous-phase mass transfer coefficient has been correlated and separate correlations for high and low interfacial tension systems have also been attempted for a better fit of the data.Comparison between our correlation for k, and expressions derived from the penetration theory with Kolmogoroff s time scale and from turbulent boundary layer theory show significant differences between the exponents on any given variable, suggesting the need for a new model. Accordingly, a theory based on a periodically varying rate of surface renewal has been developed and the average rate of surface renewal sa in this theory has been correlated.Droplet size was determined from photographs of the dispersion taken through a plane glass water pocket. A correlation was obtained for the Sauter-mean drop diameter when about 50% of the possible mass transfer had occurred; this was chosen as an average value during batch operation.These correlations have been used to develop relationships for optimized design and scale-up.This study included h e liquid-liquid systems, two sizes of six-flat-blade turbines, two vessel diameters, two principal liquid heights (T = H), impeller speeds between 3 and 8 rps, and dispersed-phase volume fractions between 0.03 and 0.09.
Most of the work reported in the literature on liquidliquid mass transfer is for steady state conditions in extraction columns and spray towers (1, 3, 5, 10, 12, 14, 17, 2 0 ) . The reports have correlated the volumetric mass transfer coefficients with the flow rates of the continuous and dispersed phases. These results are useful for specifying the values of liquid flow rates for column operation, but more information is needed for complete elucidation of the basic mechanisms of liquid-liquid mass transfer in such systems. Of particular importance are data which will relate size of equipment to the rate of mass transfer, and no data on continuous flow liquid-liquid mass transfer systems are available for equipment of different size to provide such data.The use of continuous flow equipment allows the experimenter to reach a steady state and to make sufficient measurements to determine accurate material balances and rate coefficients. However in two-phase liquid-liquid mass transfer operations the equipment size itself is a very significant variable and interacts not only with fluid motion conditions but also with the physical properties of the fluids. Accordingly data are required for engineering design on a large scale. To evaluate the effect of size large scale continuous types of experiments would be highly desirable, but the amount of material to be handled and the size of auxiliary equipment necessary makes the cost of such studies much too high for practical consideration. Therefore a batch type of experiment is desired which will minimize the bulk of the equipment and material to be handled and still provide for sufficient accuracy so that the data may be translated to the performance of a continuouq flow operation. Then large size batch experiments can be made at far less cost than for large scale continuous operations and data obtained to elucidate the effects of size for continuous flow systems.Single-drop studies (4, 7, 18) have been used to develop both theoretical and empirical models to describe the relationship of mass transfer rates to the variables such as the fluid flow conditions, the physical properties of the liquids, etc. Such models describe the experimental results to a fair degree of accuracy, but their main weakness is that they are for single drops. When masses of drops are present, there is considerable coalescence and breakup of them causing the formation of new surface area as well as redistribution of the solute in the dispersed phase, hence the significant difference in overall rate transfer data between single and multiple drops in a turbulent field.In studies made with masses of drops the physical properties of the liquids and the flow motion of the system should be changed as independent variables because there is interaction between fluid motion and physical properties (16). Few such studies (6, 9, 1 1 ) have heretofore been made, perhaps because of the large number of interrelated variables which make an independent study of the variables difficult. Lewis (6) worked with an ap...
An extensive experimental study on the formation of liquid-liquid interfacial areas was conducted using flat-blade turbine impellers in standard mixing tank geometry. The interfacial areas were measured with a light probe for different combinations of volume fraction, continuous and dispersed phase physical properties, and mechanical mixing conditions. When two immiscible liquids are combined in a mixing tank, the interfacial area, or drop size, is usually of primary concern.In the operations of extraction, emulsion polymerization, and direct-contact heat transfer, the interfacial area directly affects the rates of interphase mass and heat transfer.Previous work in liquid-liquid mixing has shown that system variables such as volume fraction of dispersed phase, fluid properties, and the mechanical mixing conditions all have an effect on the interfacial area obtained, but the quantitative relationships have not been well established. Knowledge of these quantitative relationships is extremely important for: (1) the prediction of two-phase interfacial areas in new systems; (2) the optimization of existing operations; and (3) the scale-up of experimental or pilot plant units. Thus, there is a definite need for a better understanding of the liquid-liquid mixing process.In this study, the principle variables involved in the mixing process were varied systematically in order to identify those factors that have a significant influence on the interfacial area formed. The parameters varied include the volume fraction of dispersed phase, tank size, impeller size, impeller speed, and fluid properties of both the continuous and dispersed phases. Standard mixing tank geometry, baffles, and six-blade, flat-blade turbine impellers were used throughout (Rushton et al., 1950).A light probe described previously (McLaughlin and Rushton, 1973) was used to measure the interfacial areas of the dispersions. A total of 122 different combinations of volume fraction, tank size, impeller size, impeller speed, continuous phase properties, and dispersed phase properties were studied experimentally in a design that enabled interactions well as functional dependence of the above factors to be determined. In addition, 42 replicated runs of these conditions were used to strengthen the determined functional dependence of interfacial area on the above factors and establish the statistical validity of this model. CONCLUSIONS AND SIGNIFICANCEOur detailed experimental study of liquid-liquid mixing with flat-blade turbine impellers in standard mixing tank geometry has shown that the interfacial area per unit volume of dispersion is primarily a function of seven variables: a, u, p d , pd, pc, P/ V and U. Furthermore, statistical regressions on the experimental data identified those variables that significantly affect the area of the dispersions in a mixing tank. The results are presented as regression equations that represent the effects of the mixing Correspondence concerning this paper should be addressed to Roger E Eckert.Craig M. McLaughlin is with T...
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