Mechanism of the Intensification of a Heterogeneous Reduction Reaction with the Liberation of Gas Bubbles

Abstract

The degree of intensification is estimated for heterogeneous reduction and dissolution reactions by solving the problem of describing the hydrodynamics, mass transfer, and shielding of a reaction surface upon the formation and action of gas-phase products in the form of bubbles. Some theoretical postulates on the formation of gas bubbles in the course of a heterogeneous solid–liquid–gas reaction are considered. Based on the analogy with boiling and bubbling heat hydraulics, theoretical dependences are derived for the bubble growth rate in the presence of a liquid reagent microsublayer. The analytical dependences derived in this work are in qualitative agreement with the empirical relationships of other authors. Due to the complexity of the problems, no special emphasis is laid on quantitative interpretation, but quantitative coincidence may be attained via the introduction of small corrections. The dynamics of a heterogeneous solid–liquid–gas reaction with mass transfer between a liquid reagent and a solid reducer is studied with consideration for the specific features of the hydrodynamics of gas–liquid media. Some relationships for estimating the reagent concentration and the surface-area fraction shielded by liberated bubbles are derived. Estimates show that the effect of shielding is negligible. This seems to be the reason shielding is not taken into account in the technical calculations of reactions with the liberation of bubbles. Reaction stability is investigated to reveal that gas–liquid structures providing the alternation of shielded and unshielded reaction surface areas are formed under equilibrium, thus creating the gas content gradient, which provokes the motion of liquid-reagent layers with intense stirring. An analysis of criterial equations for mass transfer on a reaction surface gives the dependence, which qualitatively and, partially, quantitatively characterizes mass-transfer processes and coincides with the experimental dependence. The results of this work may be used to form the regimes offering the optimal operation of technological reactors.