Mass transfer from single carbon-dioxide bubbles in electrolyte aqueous solutions in vertical pipes

Mass transfer rates, kL, of single carbon-dioxide (CO2) bubbles rising through vertical pipes filled with electrolyte aqueous solutions were measured to investigate the effects of the presence of electrolyte on kL. Sodium chloride (NaCl) was used for electrolyte and its concentration was varied from 3.5 to 14 wt.%. The pipe diameters, D, were 12.5, 18.2 and 25.0 mm and the bubble diameter, d, ranged from 5 to 25 mm. The diameter ratio, λ(= d/D), was varied from 0.2 to 1.7, to cover various bubble shapes, i.e. ellipsoidal, cap, semi-Taylor bubbles and Taylor bubbles. The conclusions obtained are as follows: (1) kL in the NaCl aqueous solutions decreases with increasing the NaCl concentration mainly due to the reduction of the diffusion coefficient of CO2 in the liquid phase, (2) the Sherwood numbers, Sh, of Taylor bubbles of L/D > 1 in the NaCl aqueous solutions can be well evaluated using the available Sherwood number correlation for clean Taylor bubbles, where L is the length of a Taylor bubble, (3) Sh of ellipsoidal, cap, semi-Taylor bubbles and Taylor bubbles of L/D ≤ 1 in the clean water and NaCl aqueous solutions can be well correlated in terms of the Peclet number and the dimensionless bubble diameter d∗ = d/dT, where dT is the bubble diameter at the transition from the ellipsoidal-cap bubble regime to the semi-Taylor bubble regime, and (4) the Sh correlations give good predictions for long-term bubble dissolution processes in NaCl aqueous solutions.