Fluid flow in pachuca (air-agitated) tanks: Part I. Laboratory-scale experimental measurements
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INTRODUCTION
GAS-agitated reactors are used in a number of process industries. In the chemical and biochemical industries, gas- (generally air-) agitated reactors are used extensively. Here, they are known as bubble columns or airlift reactors and are used in a wide range of processes, such as coal liquefaction, polymerization, air oxidation of organic and inorganic compounds, fermentation, wastewater treatment, etc. In the metallurgical industry, where they are commonly known as "Pachuca" or "Brown" tanks, they are used as leaching vessels in the hydrometallurgical production of nonferrous metals (notably, gold, uranium, zinc, and copper). Argon stirring of molten steel in ladles is another example of the use of gasagitated reactors in the metallurgical industry. Pachuca tanks are generally classified in terms of the draft tube, as shown in Figure 1. Two major differences among Pachuca tanks, bubble columns, and argon-stirred ladles are size and geometry. Another major difference is that argon-stirred ladles contain molten steel. Pachuca tanks range in diameter from 5 to more than 10 m. Argonstirred ladles seldom exceed 2 m in diameter, and bubble columns are usually even smaller. Also, Pachuca tanks have conical bottoms, while argon-stirred ladles and bubble columns are usually flat bottomed. At values ranging from 1.5 to 4 (including the conical bottom), the height-to-diameter ratios of Pachuca tanks fall between those of argon-stirred ladles and bubble columns. The principal objectives in the operation of gas-agitated
R. SHEKHAR, Postdoctoral Student, and J.W. EVANS, Chairman and Professor of Metallurgy, are with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted March 2, 1989. METALLURGICAL TRANSACTIONS B
reactors are the suspension of particles, mixing of reagents, mass transfer between the solution and particles, and mass transfer between air bubbles and the slurry. Flow regime (bubbly, churn-turbulent, slug, etc.), bubble size distribution, coalescence characteristics, gas holdup, and gas-liquid interfacial area are some of the important process parameters that affect the operation of air-agitated reactors. Therefore, extensive studies have been carried out, primarily in laboratory-scale bubble columns, to determine the effect of different operating parameters, such as superficial air velocity, tank dimensions, tank design, and physical properties of the liquid/ slurry on the process parameters described above. These studies, summarized by Shah and coworkers, I1J have provided rough guidelines for the modification of bubble columns to achieve high process efficiency in different applications. More recently, Koide and coworkers, [2,3,4] Weiland, tSl Miyahara and coworkers, [6] and Jones I7] have conducted experiments that shed light on the effect of tank geometry and operational parameters on air holdup, mass transfer, particle suspension, and recirculation velocity in bubble columns. However, to date, precise guidelines for the
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