Gas Absorption in Stirred Gas-Liquid-Liquid Systems: Effect of Transferred Solute Solubility and Oil Phase Spreading Characteristics

Abstract

The effect on gas-liquid mass transfer of dispersing an oil phase in water in an aerated stirred tank is not predictable, because the mechanisms involved are not well understood. To try to elucidate these mechanisms, a set of experiments was carried out that included: (i) measurement of the effect of oil addition on gas dispersion properties, (ii) quantification of mass transfer of two solutes (heptane and oxygen) with very different solubilities in the liquid phases, and (iii) variation of the spreading characteristics of the oil phase, which consisted of mixtures of dodecane and heptane in varying concentrations. It was found that in the case of heptane mass transfer, when the oil spreading coefficient S changes from negative to positive, the outlet gas becomes practically saturated, corresponding to a several-fold increase in mass transfer coefficient. In the case of oxygen mass transfer, the effect of S is not as dramatic, but it is also quite significant. For a spreading oil phase (S > 0), the mass transfer coefficient decreases upon trace oil addition, going through a minimum as oil holdup increases, and then increasing steadily. In the case of a non-spreading oil phase, mass transfer coefficient initially increases with oil holdup increase, going through a maximum and then decreasing. Comparison between mass transfer coefficients for the two solutes indicates that k L a for heptane is larger than that of oxygen by a margin not explainable through the difference in diffusivities, which is evidence for a difference in transfer paths=mechanisms. A physical interpretation compatible with these results is offered.

Semi-Empirical Modeling of Gas–Liquid Mass Transfer in Gas–Liquid–Liquid Systems in Stirred Tanks

“…Experimental values of k L a obtained under these conditions of agitation were in fact not used in obtaining parameters for expressions (13)-(15) (Alves and Pinho, 2013). Figures 11 and 12 extend the range of simulations of E ¼ f(/), to higher and lower values of agitation speed N (at constant aeration v S ) and to higher and lower values of v S (at constant N).…”

“…In a recent experimental study, Alves and Pinho (2013) studied oxygen absorption in a stirred tank with both spreading (heptane) and non-spreading (dodecane) dispersed oil phases and found that E varies with oil holdup, /, in a significant different manner, as shown in Figure 1.…”

“…These models are inspired by hypothetical mechanisms suggested before (Alves and Pinho, 2013) for the two cases.…”

Application of hydrocarbon and perfluorocarbon oxygen vectors to enhance heterologous production of hyaluronic acid in engineered Bacillus subtilis

“…n-hexadecane) negatively or hardly affect k L a , although at lower vector concentrations k L a may increase due to the shuttling effect, through which vector droplets briefly contact the air bubble surface and subsequently transfer oxygen to the bulk aqueous phase (Alves & Pinho, 2013;Quijano & Hernandez, 2010). n-hexadecane) negatively or hardly affect k L a , although at lower vector concentrations k L a may increase due to the shuttling effect, through which vector droplets briefly contact the air bubble surface and subsequently transfer oxygen to the bulk aqueous phase (Alves & Pinho, 2013;Quijano & Hernandez, 2010).…”

“…Alternatively, spreading hydrocarbons (e.g., n-heptane) generally induce an initial decrease in k L a at low concentrations, followed by a gradual increase in k L a with increasing concentration (Alves & Pinho, 2013;Cents, Brilman, & Versteeg, 2001;Linek & Beneš, 1976;Yoshida, Yamane, & Miyamoto, 1970). An alternate/complementary explanation is that the vector film forming around air bubbles becomes unstable with increasing thickness as the vector concentration increases, resulting in the release of oxygen-rich droplets into the bulk aqueous phase (Alves & Pinho, 2013). An alternate/complementary explanation is that the vector film forming around air bubbles becomes unstable with increasing thickness as the vector concentration increases, resulting in the release of oxygen-rich droplets into the bulk aqueous phase (Alves & Pinho, 2013).…”