Mechanism of gas absorption enhancement in presence of fine solid particles in mechanically agitated gas-liquid dispersion. Effect of molecular diffusivity

The effect of fine particle addition in physical gas desorption and absorption with fast reaction (sulphite oxidation in the presence of a cobalt catalyst) has been studied in a stirred cell with a flat gas-liquid interface and mechanically agitated gas-liquid bubble dispersion in a wide range of stirring speeds. Activated carbon and TiO2 were used at low loadings (0.5-1 kg m(-3)). The desorption was used to avoid supersaturation effect which was observed during oxygen and hydrogen absorption into liquid saturated with nitrogen. Using two gases with sufficiently different diffusivity (O-2, H-2), the effect of molecular diffusivity on the mass transfer coefficient was estimated in the form k(L) - D-n, with the exponent n indicating the surface mobility accompanying the effect of the particles. The value n = 2/3 indicates a fully rigid and the value 1/2 a fully mobile mass transfer interface. Chemisorption experiments confirmed that the particles do not affect mass transfer area of the agitated dispersion. After addition of particles, kL for physical desorption from bubbles in dispersion was increased by 10-30% in water and by 20-60% in sulphate solution with decreasing agitation rate. In the stirred cell, the increases were much higher reaching 200% and 230% for water and sulphate solution, respectively. The exponent n exhibited a significant decrease in the presence of particles ranging from 10% (for dispersion in water) to 33% (for stirred cell in sulphate solution). The decrease in n encountered in dispersion indicated the transition from a partially mobile to a fully mobile surface. The reduction of kL was interpreted through the physicochemical effect of surfactants removal from the gas-liquid interface by activated carbon particles. The results have confirmed that the mechanism of mass transfer enhancement in the presence of fine particles, based on the removal of surface contaminants from the liquid by adsorption onto the hydrophobic surface of the particles, as suggested by Kaya and Schumpe [2005. Surfactant adsorption rather than "shuttle effect"? Chemical Engineering Science 60, 6504-6510] for stirred cell, is valid also for the enhancement of absorption rate from bubbles in mechanically agitated dispersion. (c) 2008 Elsevier Ltd. All rights reserved.