Experimental Study On The Dynamics And Mass Transfer Of CO₂ Bubbles Rising In Viscoelastic Fluids

The formation of gas bubbles and their subsequent rise due to buoyancy are very important fundamental phenomena that dictate the hydrodynamics of gas-liquid systems and the overall performance of bubble column reactors.Carbon dioxide（CO₂）is a relatively common gas in the chemical industry with moderate to high solubility in a wide range of fluids.The behavior of CO₂ bubbles in both Newtonian and non-Newtonian fluids differs from majority of gas bubbles due to their solubility.Many researchers have investigated the bubble rise dynamics and mass transfer of CO₂ in liquid media.However,majority of these studies focused on Newtonian fluids when in reality,most fluids encountered in industrial applications are non-Newtonian.This research focuses on the bubble rise dynamics and mass transfer of CO₂ bubbles in non-Newtonian media.The influence of bubble characteristic parameters such as shape,size,velocity and trajectory as well as the rheological properties of the solution on single CO₂bubbles rising in quiescent viscoelastic polyacrylamide solutions of various weight percentages was studied via high speed photography and digital image processing technology.It was found that a random and an abrupt fall and rise in bubble velocity occurred as the bubble velocity decreased along the journey.This is the first time this observation is reported.In the most viscoelastic medium where this phenomenon occurred,CO₂ bubbles decreased in diameter by at least 20%over 1m of height.A shape change occurred from convex to a cusp-like（tear drop）shape with trailing ends during the process.It is evident that the shape transition,and the presence of strong elastic forces are essential elements for this phenomenon to occur.In addition,bubble diameter increased with increase in nozzle size.Bubble deformation,path deviations and rise velocity decreased with increase in polymer concentration and decrease in bubble diameter.The bubble rising dynamics in tap water was correlated using three dimensional parameters,Reynolds number（Re）,Weber number（We）and Morton number（Mo）.In the viscoelastic fluids,the bubble behavior was correlated by Re,We and De（Deborah number）,and the prediction by a developed empirical correlation considering the combined effect of fluid elasticity and shear-thinning property showed a satisfactory agreement with experimental data in all liquids studied.The mass transfer coefficient,k _L,of single CO₂ bubbles rising in water and polyacrylamide aqueous solutions was investigated,and the influence of bubble size and rheological properties of the solution was examined.It was found that experimental mass transfer coefficient values were much higher in water than in polymer solutions.Also,experimental k _Lvalues in water approached theoretical values predicted by Higbie’s correlation while Fr?ssling’s correlation closely predicted the k _L values of CO₂ bubbles rising in aqueous polyacrylamide solutions.It was deduced that the surface of the bubble in water was highly mobile whereas an immobile gas-liquid interface prevailed on the bubble rising in the polymer solution.The mass transfer coefficient in the polymer solution was drastically reduced due to low bubble velocity and enhanced contamination of the bubble’s surface.