Mechanism of enhanced gas absorption in presence of fine solid particles. Effect of molecular diffusivity on mass transfer coefficient in stirred cell

Desorption of oxygen and hydrogen from various liquids (water, 0.8 molar sodium sulphate solution) containing suspended particles of activated carbon at various solid loading was investigated. The desorption was used to avoid supersaturation effect which was observed during oxygen and hydrogen absorption into liquid saturated with nitrogen. Experiments were carried out in a stirred cell with flat gas–liquid interface at 30∘C and atmospheric pressure. An increase of kLkL upon addition of the particles was observed. Enhancement factor increases with increasing contact time of the particles with liquid reaching maximum steady-state value of approx. 3 after sufficiently long time (a few hours) regardless of solid loading (0.1–1kg/m3), agitator frequency (50–300min-1) and solute gas (O2,H2)(O2,H2). The results fit the correlation kL=Ce1/4Dn (e is specific power dissipated by agitator in liquid and D   is molecular diffusivity of gas absorbed) with the exponent n=23 for liquids without and n=12 for the liquids with the particles. It indicates that the interface is rigid in absence of particles and hinders the motion of liquid along the interface forming boundary layer while in the presence of particles the interface is completely mobile and surface renewal proceeds according to the penetration model. These results confirm a finding of Kaya and Schumpe (2005) that the enhancement of mass transfer in the cell at the presence of hydrophobic solids is due to clean-up of the interface from surfactants by their adsorption on hydrophobic solids rather than by a “shuttle mechanism” exerted by particles with a high adsorption capacity for the transfer component.