Cellular convection induced by absorption

Abstract

The influence of cellular convection, induced by oxygen absorption in sodium sulphite solution, upon the mass transfer coefficient was studied experimentally both in a mechanically agitated non-aerated vessel (MANV), where absorption proceeds through the liquid surface, and in a mechanically agitated gas—liquid dispersion (MAD). It was found that the relation derived for correlation of the enhancement factor of the mass transfer coefficient for physical absorption as suggested in [1, 4] is not suitable for correlating the results obtained in the MANV. The probable mechanism of cellular convection occuring in this system is density driven, induced by concentration gradients. An empirical formula was obtained for the critical value of Rayleigh number. It was deduced from the formula that the onset of cellular convection does not depend on hydrodynamic conditions prevailing in the liquid phase within the whole range of Reynolds number used (3·4 × 10 3 to 5·3× 10 4). The enhancement factor R of the physical mass transfer coefficient caused by cellular convection is suitably described by the relation R = ( Ra/ Ra cr ) u . The value of the exponent y is not constant. We present the correlation of experimentally determined values of y. For values of Reynolds number not higher than 3 × 10 4 the value of y is constant and equal to 0·73. For higher values of Re the value of y is lower ( y = 5·33 × 10 20 Re −4·66). It is shown that it would be more suitable to correlate the exponent y by the ratio of the time scale of initiation of cellular convection and thatof surface renewal. It was proved experimentally, that the absorption of oxygen into sodium sulphite solution in the MAD does not induce cellular convection in the studied range of oxygen absorption rated.