Combined effects of alcohol and electrolyte on mass transfer from single carbon-dioxide bubbles in vertical pipes

Abstract

Mass transfer rates, kL, of single carbon-dioxide (CO2) bubbles rising through vertical pipes filled with alcohol-electrolyte mixed aqueous solutions were measured to investigate the combined effects of alcohol and electrolyte on kL. 1-octanol and 1-heptanol, which are known to adsorb to the interface and behave like surfactant, were used. Sodium chloride (NaCl) was used for the electrolyte. The surface tension, σ, decreased with increasing the NaCl concentrations, CN, while keeping the alcohol concentration, CA, constant. Three combinations of CA and CN having the same values of σ were selected for the experimental conditions to avoid the effect of σ. The pipe diameters, D, were 12.5 and 18.2 mm. A wide range of bubble diameter covered various bubble shapes, i.e. ellipsoidal, cap, semi-Taylor and Taylor bubbles. The conclusions obtained are as follows: (1) the combination of CA and CN for the same σ does not affect the aspect ratios of ellipsoidal bubbles and the lengths, L, of Taylor bubbles, (2) the Sherwood numbers, Sh, of bubbles in alcohol-NaCl mixed aqueous solutions depend on the combination of CA and CN due to the change in the Schmidt number, Sc, even at the same σ for all the tested combinations of concentrations of impurities, and (3) the modified Sherwood number, ShD, of contaminated Taylor bubbles is well correlated in terms of the bubble Reynolds number, Sc, the dimensionless group for surfactant properties and L/dT, where dT is the transition bubble diameter at the transition from the ellipsoidal-cap bubble regime to the transition regime.