The effect of sodium caseinate concentration and processing conditions on bubble sizes and their break-up and coalescence in turbulent, batch air/aqueous dispersions at atmospheric and elevated pressures

The dispersion of air into aqueous solutions containing buffer and commercial sodium caseinate at concentrations from 0.01 to 2% w/w has been studied under batch conditions. The air volume fraction, Φ, were 0.05, 0.1 and 0.2 and dispersion was achieved by means of Rushton turbine type impellers in a vessel of 0.15 m diameter. Bubble size distributions were measured by a stereo microscope-video-computer technique as a function of time until equilibrium was reached after about 0.5 h following either a step increase or decrease in speed. The time to reach equilibrium was somewhat shorter for bubble break-up following a speed increase than for bubble coalescence following a speed decrease. However, somewhat surprisingly, at all concentration of Na-caseinate, coalescence occurred and at approximately the same rate. For the steady state conditions, the impact of agitation speed, air volume fraction and Na-caseinate concentration in terms of equilibrium surface tension could be accommodate by the equation: d ₃₂/D=0.027·(1+3.85·Φ)·We^-0.4 which is similar to equations found for batch liquid/liquid dispersions. Such an equation is not generally satisfactory for sparged gas-liquid systems. Some experiments were also conducted under pressure and these showed a decrease in d₃₂ at the same volume fraction of air as has also been found in bubble columns.