Correlation Between a Local Void Fraction and Dissolved Gas Concentration in a Bubble Column by Using a Newly Developed Photoelectric Optical Fiber Probe

A bubble column is frequently used in various industrial plants for the purpose of dissolving gases into liquid phases. Clarifying a relationship between the mass transfer mechanism and the structure of the bubbly flow is important in order to improve the efficiency of the gas absorption. In the present study, we discuss a correlation between the concentration transportation and the large-scale structure of the bubbly flow, by using a newly developed photoelectric optical fiber probe (POFP). The POFP has two functions of electrical concentration measurement and optical bubble measurement; i.e. measurement of the time-spatial correlation of the bubble swarms and the concentration transportation is achieved simultaneously by using a pair of the POFPs. A bubble column of 380 mm in inner diameter and 1500 mm in height was employed. We pumped a mixture gas of CO₂ and pure air into the bubble column through a perforated plate. Based on the fluctuation characteristics of the void fractions, the flow field in the column was divided into three zones: bottom, middle and upper zones. A long-period fluctuation of the void fractions fades out toward the upper zone of the bubble column. Simultaneously, we measured local CO₂ concentrations. In the bottom zone, the increase rate of the concentration significantly differed by position. In contrast, in the upper zone, little difference in the increase rate was observed by position. The differences in the increase rate of the CO₂ concentration faded out toward the upper zone. Furthermore, the fluctuations of the CO₂ concentration showed a very close correlation with the fluctuations of the void fraction. We will discuss correlations between the mass transportation characteristics and the large-scale structure of the bubbly flows (i.e. the gas-phase and liquid-phase large-scale structure).