Effect of Material of Nanoparticle on Mass Transfer Enhancement and a Convective Diffusion Model to Predict the Enhancement

While particles smaller than the thickness of the diffusion film have been known to enhance rates of interfacial mass transfer [1], a relatively new result is the discovery that nanoparticles in suspension show enhancements that far exceed the earlier reported enhancements, and without any apparent adsorptive or reactive effects[2]. Different mechanisms for the enhancements have been speculated upon, but there is a paucity of data on different nanoparticulate materials, collected in a systematic way on model contactors so that rational comparisons may be made. In this work, enhancement in Carbon dioxide absorption in water has been studied using SiO₂ and TiO₂ nanoparticles using the same capillary tube apparatus for which previous results of Fe₃O₄ were reported. For 0.4% silica particles and 0.0118% TiO₂ nanoparticles, 165% and 155% enhancement was observed respectively. A phenomenological convective diffusion model has been proposed to explain the observed effects of particle size, holdup and material density. The model accounts for the overall effect of the Brownian (and any diffusiophoretic) motion of the nanoparticles on the surrounding fluid in terms of an ‘effective’ convective velocity, which is determined from the experimental data and correlated to the modified Sherwood Number proposed earlier [2], volume fraction of Nanoparticles and a solid Reynolds number R_p. This model provides a good fit to the data from wetted wall column and capillary tube experiment for iron oxide from the previous literature, as well as for the data on silica and Titanium dioxide nanoparticles from this work, the average error being 8.3%.