(495b) On the Dynamics of Non-Coalescing Microscale Bubbles in Turbulent Flows for Intensification of Gas-Liquid Mass Transfer

In many gasâ??liquid contactors or reactors, mass transfer from the gas phase to the liquid phase is a major player in the reactorâ??s overall production and/or economics. The mass transfer rate can be enhanced either by a) increasing interfacial area a, b) intensifying the mass transfer coefficient k_L, or c) increasing the solubility of gas via higher pressure or complexing reagents. All three of these beneficial phenomena may be leveraged simultaneously by use of micron-scale bubbles. Therefore, to intensify reactor mass transfer rate, we studied the behavior of microbubbles (< 200 micron) in a 100L bubble column and in a 500 mL bench top column. Specifically, the stability of microbubbles against coalescence was studied under additions of surfactant (sodium dodecyl sulfate (SDS) [CH₃(CH₂)₁₁OSO^â??3 Na⁺]) and salt (KCl), both of which are known to decrease bubble coalescence (Parmar & Majumder 2014). According to previous literature, adding salt to the liquid phase in the presence of SDS has two different effects on the bubble surface tension and the hydrostatic repulsion between bubbles (Prosser & Franses, 2000): a) Na⁺ ions allow closer packing of surfactant molecules at the gas-liquid interface, which diminishes surface tension because surface tension is inversely proportional to the density of surfactant molecules at the surface, and b) decreases the repulsion between bubbles because the electrostatic Debye length decreases. Depending on the salt concentration, the stability can be changed from stable to unstable. Low concentrations of SDS (0-35) ppm w/w and KCl (0-75) ppm w/w were added to the solution. We observed that the presence of SDS up to 35 ppm w/w and KCl up to 75 ppm w/w did not alter surface tension by more than a few percent. However large change was observed in mean bubble size, k_La, and bubble coalescence rate. It was hypothesized that the stability of microbubbles was not from the slight decrease in the surface tension but from an increase in zeta potential. Zeta potential (ζ) was measured by high-speed video of the time-dependence of bubble motion inside a 50 cm tall bubble column with an oscillating electric field applied across the bubble column, similar to a method used previously by Takahashi (2005). Bubble size and coalescence rate were also measured. Results showed a significant increase in ζ-potential of bubbles as SDS and KCl concentrations were increased. An energy balance confirms ζ-potential to be the leading factor for decreased coalescence rates and decreased bubble size. In a test reactor, k_La and avalues increased both by 50% as SDS and salt concentrations was increased, likely due to an increase of interfacial area.

References:

Parmar, R. & Majumder, S. K. 2014 Hydrodynamics of microbubble suspension flow in pipes. Ind. Eng. Chem. Res. 53, 9, 3689â??3701.

Prosser, A.J. & Franses, E.I., 2000 Adsorption and surface tension of ionic surfactants at the airâ??water interface: review and evaluation of equilibrium models. Colloids and Surfacesâ?¨A: Physicochemical and Engineering Aspects. 178, 1â??40.

Takahashi, M. 2005 ζ Potential of Microbubbles in Aqueous Solutions: Electrical Properties of the Gas-Water Interface. J. Phys. Chem. B. 109, 21858-21864.