Experimental study of oxygen diffusion coefficients in clean water containing salt, glucose or surfactant: Consequences on the liquid-side mass transfer coefficients

This present paper proposes new investigations aiming at: (i) studying the effect on oxygen diffusion coefficients of the presence in clean water of some compounds usually encountered in biological media and (ii) quantifying their consequences on liquid-side mass transfer coefficients. The oxygen diffusion coefficients D were firstly measured in various synthetic liquid phases containing either salt (NaCl), sugar (glucose) or surfactant (sodium laurylsulphate). When compared to clean water, noticeable reductions of D were observed; the variation of D with the compound concentration C was modelled and found dependent on the nature of the compound added. In a second time, using the same liquid media, experiments on a train of bubbles rising in a quiescent liquid phase were carried out to determine the associated liquid-side mass transfer coefficients (kL). For all cases, as for diffusion coefficients, a decrease of kL with increasing C was clearly observed whatever the aqueous solutions. These findings firstly showed that, even if the properties of clean water (density, viscosity, surface tension) were not significantly changed by the addition of salts (NaCl), the liquid-side mass transfer coefficients could be, all the same, modified. For the aqueous solutions of glucose, the reduction of kL with diffusion coefficients D was well correlated, and mainly due to the change in viscosity with concentration. For surfactants, the hydrodynamic conditions (i.e. bubble Reynolds number) being almost kept constant for all concentrations, only the change in oxygen diffusion coefficients was thus responsible for the decrease of kL. The present study clearly confirmed the need to complete and/or account for the database related to oxygen diffusion coefficients in complex media, this condition being imperatively required to describe and to model appropriately the gas–liquid mass transfer phenomena.