The effect of solids on the dense phase gas fraction and gas-liquid mass transfer at conditions close to the heterogeneous regime in a mechanically agitated vessel

In the heterogeneous regime, there is a strong literature evidence (discussed herein) that solids can supplant small bubbles in the dense phase and reduce the gas hold-up. This work examines the effects of the addition of 205 μm glass ballotini on the gas hold-up and kLa in a 0.286 m diameter stirred tank operated under intense conditions (P/V ≥ 5 kW m-3) close to the heterogeneous regime and above the agitator speed corresponding to the just suspended point, NJS. The tests were carried out on two systems: air-water (coalescing) and air-0.2 M sodium sulphate (salt) solution (which resists coalescence in the bubble regime). For the air-water system it was observed that the overall gas plus solids hold-up remains approximately constant until all the small gas bubbles are supplanted and then increased in direct relation to the solids volume. The kLa mirrored the gas hold-up trend and decreased with a fall in gas hold-up. In the salt solution kLa decreased in direct relationship to the solids concentration, to 40% of the no solids value at around 19% solids by volume of dispersion. Dynamic engagement and disengagement experiments established that the salt solution behaves differently than water with the small bubbles initially growing in size rather than being coalesced directly into the large bubble population. © 2008 The Institution of Chemical Engineers.