Nonuniformity of NaOH concentration and effective bubble diameter in CO 2 injection into aqueous NaOH solution
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INTRODUCTION
MASStransfer around a single gas bubble in liquid has been extensively studied, and the penetration theory was used for the analysis of the experimental data. I1'21It is important in metallurgical processes, on the other hand, to investigate the rate of mass transfer associated with bubble swarm and with the gas jet formed from a submerged nozzle. Themelis and Schmidt, t3J for example, injected CO gas into molten copper and obtained a correlation between the Reynolds number at the nozzle exit and the product of mass transfer coefficient and the surface area of bubbles per unit volume of the melt. Brimacombe et al. [4] injected SO2 gas into aqueous 0.09 mol 9 dm -3 H~O2 solution and analyzed the results together with those obtained by Themelis and Schmidt. Carbon dioxide gas was blown into 0.2 mol 9 dm -3 NaOH solution by Inada and Watanabe, rSJ and their results were used for the analysis of the vacuum oxygen decarburization (VOD) process. As mentioned above, the mass transfer data have often been explained in terms of the product of mass transfer coefficient, k~, and the surface area of bubbles per unit volume of the liquid, S. This was due to the difficulty in estimating the surface area of gas bubbles. The product Sk~ is significantly affected by the geometric dimension of the vessel, the operational conditions, and the physical properties of the liquid and gas. Thus, it is desired to estimate the surface area of bubbles and to determine the mass transfer coefficient which is independent of the geometric dimension of the vessel employed in the experiment. In a previous work, t61 mixed CO2-N2 gas was blown into deionized water from a submerged downward nozzle, and the absorption rate of CO2 was measured. From the measured distribution of bubble diameter, the total surface area of bubbles was calculated. The observed temperature dependence of the mass transfer coefficient coincided with that of the diffusivity of CO2 in deionized water. The mass transfer coefficient increased with the bubble frequency to a maximal value at a frequency of about six bubbles per secY. FUKUNAKA, Research Associate, M-F. JIANG and T. YAMAMOTO, Graduate Students, Z. ASAKI, Professor, and Y. KONDO, Emeritus Professor, are with the Department of Metallurgy, Kyoto University, Kyoto, Japan. Manuscript submitted March 28, 1986. METALLURGICAL TRANSACTIONS 13
ond. Then it decreased and attained a substantially constant value of 3 • 10-4 m 9 s-1 above the frequency of fifty to eighty bubbles per second. This was due to the intensified upward liquid flow induced by the bubble swarm. In the present work, mixed CO2-Nz gas was blown into aqueous NaOH solution by using the same vessel and the same nozzle as in the previous work. [6] The size distribution and rising velocity of bubbles were also measured. The reaction mechanism of dissolved CO2 with OH- ions is fairly well known. 18-141 The primary purpose of the present work is to evaluate the NaOH concentration difference between the plume zone and the bulk solution and the effec
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