Gas diffusion into viscous and non-Newtonian liquids

Abstract

A quiescent technique has been developed to detemine the diffusion coefficients of carbon dioxide in water and viscous and non-Newtonian liquids. The rate of gas absorption was measured accurately as the pressure change of a fixed volume of gas by a micromanometer. Gas penetration analysis suggests that a plot of gas absorption rate against the square root of contact time should be linear. The plot revealed a distinct initial phase of molecular diffusion lasting for about 100 seconds when water was instantaneously exposed to carbon dioxide. This was followed by non-linear behaviour in which natural convection is driven by density gradients. The diffusion coefficient of carbon dioxide in water was found to agree well with the values reported in the literature. Mass transfer coefficients, k L, were determined for the interface in the convective regime. Diffusion without natural convection of CO 2 into viscous and pseudoplastic aqueous solutions appeared to be prolonged and proceeded at a slower rate. The onset of convection was suppressed considerably depending on the (apparent) viscosity of the solutions. A critical Rayleigh number was computed to characterise the onset of linear instability leading to natural convection. The critical times for stable diffusion were predicted from this critical Rayleigh number. Agreement with observed values is fair.