Highly stable nanobubbles in the reduction of apparent viscosity of liquids during UF process

Abstract

Nano-bubbles (NBs) have attracted more attention in various chemical processes due to their different physicochemical properties from macro-bubbles. However, little attention has been paid to the variations in physicochemical properties of the liquids caused by NBs. In this study, stable NBs in the range of 120–360 nm (mean diameters of 221 nm) are generated at an air pressure of 0.3 MPa. Meanwhile, the surface tension of H2O and sodium dodecyl sulfate (SDS) solution decrease with increasing amounts of NBs, indicating that NBs altered the physicochemical properties of liquids. Consistent with the variations in surface tension, the apparent viscosity of pure H2O and 400 mg L−1 sodium alginate (SA) solutions decreased by 5.6 % and 6.6 % after bubbling for 180 min, respectively, which can be attributed to the long retention time of NBs in the mixture of NB-liquids. The correlations between the apparent viscosity of the gas–liquid mixture (μ) and the gas holdups (V) are then deduced as μ=μ0(1-αV), where μ0 is the apparent viscosity of liquid in the absence of NBs and α is the correction factor. The CFD results further confirm that the apparent viscosity of the gas–liquid mixture decreases with increasing gas holdups. During the UF process, NBs significantly increase the normalized flux of NB-H2O from 1.00 to 1.35 and from 0.72 to 1.02 for NB-10 mg L−1 SA solution, which from the other side confirms that NBs decrease the apparent viscosity of the gas–liquid mixture, thus improving the normalized flux of filtrates. This study provides valuable clues for the significance of NBs in chemical processes.