Dispersed-phase holdup in liquid-liquid emulsions generated by high-strength bottom gas injection
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I.
INTRODUCTION
THE mixing of two immiscible liquids to create a liquid-liquid emulsion is very important in many phase-contacting operations. Many metallurgical processes use such emulsions. Two methods are generally used to mix two immiscible phases: one is mechanical agitation and the other is bottom gas injection. The very high temperatures involved make it difficult to use the mechanical agitation method in pyrometatlurgical processes. Therefore, bottom gas injection is playing an increasingly important role in many new smelting processes. Examples include the QBOP steelmaking process, the QSL leadmaking process,m and the Hismelt steelmaking process.t2J In the mixing system, knowledge of mass- and heattransfer rates is important in designing a reactor. Estimation or correlation of mass and heat transfer rates requires intbrmation on the interfaeial area between the two immiscible phases. The interracial area per unit volume of emulsion is related to the mean drop size and the volume traction of the dispersed phase (holdup) by the following equation: 6q~ In this article, the dispersed-phase holdup & shall be defined excluding the gas bubbles from consideration. In mechanically agitated liquid-liquid systems, the dispersed-phase holdup is the same as the charged volume ratio of the two liquid phases, because the content of a vessel is usually completely mixed. Therefore, the variation of drop size with the mixing conditions is sufficient to calculate the interfacial area and to characterize the system, ts.41 In gas-agitated systems, the holdup of the dispersed phase within the plume zone varies with position and the operating conditions. Much work has been performed on the bottom gas-in-
jected liquid-liquid systems, including the measurement of mass-transfer rates and the drop size distribution.tS-9-~ Zaidi and Sohn t91 measured the Sauter mean diameter of water in kerosene in the bottom gas-injected liquid-liquid system and correlated its variation with operating conditions against the Weber number. However, very little work has been done on the dispersed-phase holdup in such a system. In this work, a cold model study was performed to determine the effects of various operating conditions on the dispersed-phase holdup in a bottom gas-injected liquid-liquid system. Dimensional analysis was used to determine the dimensionless groups for correlating the measured holdup data. Although a cold model system was used in this work, the correlation developed in terms of dimensionless variables containing properties and operating conditions that affect the dispersed-phase holdup will be useful in estimating the holdup values in real systems. It has been shown that in terms of dimensionless variables, the operating conditions in the type of cold model system used in this work can more than cover the operating conditions of the real processes mentioned previouslyY 01There have been many previous investigations in which useful information regarding mass transfer and phase dispersion between slag and molten metal (or matte
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