Dissolution rate of spherical carbon dioxide bubbles in strong alkaline solutions

The dissolution rate of single carbon dioxide (CO,) bubbles in a strong alk aline solution was investigated experimentally and numerically. We develope d a system that uses a charged-coupled device (CCD) camera coupled with a m icroscope to track the rising bubble and we photographed the rising CO, bub bles in 0.01-1 M sodium hydroxide (NaOH) solutions. From these photographs we measured the bubble size and the rising speed, and from this data we est imated the drag coefficient, C-D, and the Sherwood number, Sh, for CO, bubb les dissolving in NaOH solutions with simultaneous chemical reactions. Assu ming chemical equilibrium at the bubble gas-liquid interface, we also estim ated the dissolution rate of bubbles in alkaline solutions using numericall y estimated dissolution rates in water. Comparing the numerical and experim ental results indicates that chemical equilibrium is not achieved at the bu bble surface because the values of the calculated Sh were larger than the m easured Sh. Next, we numerically estimated C-D and Sh corresponding to the "stagnant cap model" by directly solving the Navier-Stokes and the convecti on-diffusion equations for a CO, bubble dissolving in a strong alkaline sol ution with simultaneous chemical reactions. We assumed that chemical reacti ons near the bubble were nonequilibrium. We included the species source-sin k terms for the chemical reactions in the convection-diffusion equation. We compared these results with the measured rising speed and dissolution rate . This comparison shows that the experimental and numerical results are in good agreement and that the dissolution rate with chemical reactions can be estimated within about 10% of measured values, even for nonequilibrium che mical reactions near the bubble. (C) 2000 Elsevier Science Ltd. All rights reserved.