Gas holdup and volumetric mass transfer coefficient in bubble columns with dilute alcohol solutions

Abstract

Applications of bubble columns in different fields of chemical industry and in diverse gas-liquid systems has shown that their behavior often depends very strongly on the nature of the liquid phase. The liquid phase in bioreactions and in coal liquefaction, which are important areas of bubble column use, can be simulated fairly well with dilute alcohol solutions (Schiigerl et al., 1977). However, the literature offers a limited quantity of the information on this subject. Schiigerl et al. investigated gas holdup in these systems for relatively low superficial gas velocities. They proposed an empirical correlation for the prediction of gas holdup. Oels et al. (1 978) observed a significant increase of gas holdup in alcohol solutions, which is in accordance with the results of Schiigerl and his coworkers. Higher gas holdup in alcohol solutions was also observed by Hikita et al. (1980). Kelkar et al. (1983) investigated the effect of the addition of aliphatic alcohols on gas holdup and noted an increase of the gas holdup with an increase of alcohol chain length. As the decrease of surface tension in the presence of alcohols was not sufficient to explain the increase of the gas holdup, they supposed that in the presence of alcohols the bubbles become more rigid and hence have low rise velocities, resulting in a bubble flow regime up to the superficial gas velocities of 0.08-0.10 m/s.As for gas holdup, the literature provides a very limited amount of data for the volumetric mass transfer coefficient (k,a) in dilute alcohol solutions. Schiigerl et al. (1977), Oels et al. (1 978), and Voigt and Schiigerl (1 979) measured the mass transfer coefficient in aqueous alcohol solutions. The effect of addition of alcohols on kLa was explained by means of coalescence promoting and hindering properties of the liquid medium. These authors did not propose any correlation for prediction of kLa. ExperimentalExperiments were carried out in a glass bubble column of 0.10 m ID and 2.50 m tall, with a single sparger of 4 mm ID. Air was always used as a gas phase, and for the liquid phase sohtions of water and various alcohols were used: methanol, ethanol, i-propanol, and n-butanol. The concentration of all solutions was 0.5%; for methanol and butanol, 1% aqueous solutions were also used. Gas holdup was measured by the simple hydrostatic head method. The concentration of dissolved oxygen was measured by oxygen electrode. All experiments were carried out at room temperature, 23°C 2". Results and DiscussionExperimental results for gas holdup, cg, are shown in Figure 1. cg increases with the increase of gas superficial velocity and depends significantly on the type of alcohol added. The gas holdup increases in the order water < methanol < ethanol < ipropanol < n-butanol, which is in agreement with the observations of other authors (Schiigerl et al., Oels et al., Kelkar et al.). We obtained lower values of cg than these authors, which can be explained by the use of a less efficient gas sparger in our column. By comparison of their own experime...

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