Mass Transfer in Laminar Flow at a Rotating Disk Electrode in Suspensions of Inert Particles: II . Theoretical Evaluation of Experimental Results

Abstract

In a previous publication, we studied mass transfer at a rotating disk electrode in "inert" particle suspensions. We now account for the variations of the limiting diffusion current with numerous parameters by different phenomena. These involved either the topography of the interface through adhesion of the particles to the electrode or the mass exchanges in the diffusion layer through changes in the velocity field and/or diffusion coefficient. Each phenomenon, considered separately, gives rise to one or more linear segments for the variations of current with the rotation speed ft, in logarithmic coordinates. Considering all the invoked phenomena, we would expect more segments than the four observed in experimental log I vs. log gt plots. Analyzing the consequences of each phenomenon in the whole f~ range allows one to disregard some of them for certain ft values and to account for most of the experimental results. At low ft values, two predominant phenomena, with antagonistic effects, explain the deviations from the one-phase media behavior: the reduction of the exchange area and the increase of the apparent diffusion coefficient through changes in the size of aggregates along the streamlines. Moreover. for small-size particles or for particle concentrations sufficiently high, deviations from newtonian behavior are also observed. At intermediate and high ~ values, the predominant phenomenon is the variation of the diffusion coefficient with the shear rate. Depending on the ratio of the diffusion layer thickness over the particle size (g/a), two different laws can describe this variation.In the first paper of this series (1), we determined, using an electrochemical method, the influence of several parameters on mass transfer in two-phase media. These parameters are inherent in the particle, the medium, and the rotating disk electrode (RDE). The fact that the influence of each of them is closely dependent on the influence of the others suggests that various phenomena must take place at the interface, with shear rate dependent relative weights.We present here a general model in order to take into account most of our experimental results and to interpret the "particle-wall-flow" interactions which control the mass transfer to the wall.The particles are known to induce discrepancies on mass transfer with respect to the supernatant. Three different approaches have been used in the literature to account for these discrepancies: a hydrodynamic approach, a diffusional approach, and a third approach in which blocking of the exchange area is invoked. The two first approaches disregard the role of the electrode surface, and lead to an increase of mass transfer. The third approach only takes into account the phenomena at the interface and leads to a decrease of mass transfer. This last approach ignores the particle-induced perturbations of the volume-related phenomena.The first and most microscopic approach was employed in the case of the rotating disk electrode (2, 3). It consists in solving with important approximations th...

The influence of suspended particles on the mass transfer at a rotating disc electrode. Non-conducting particles

“…Another explanation for the origin of the critical rotation speed is given by Leitel et al [28] and Caprani et al [16][17][18][19]. The occurrence of one or more critical values of ~ is attributed to partial blockage of the electrode surface by adhesion of particles and removal of adsorbed species at higher rotation speeds.…”

“…The rate of mass transfer of an electrode process can be enhanced via increased convection of the electrolyte, or by introducing turbulence promoters [1], gas bubbles [2,3], and solid particles [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The influence of glass spheres was measured by Leitel et al [4] and Roha [5] for a RDE (rotating disc electrode) and by Smith et al [6] for an electrode in a tube wall.…”

“…Caprani and co-workers [16][17][18] investigated the influence of suspensions of SiC, A1203, B4C and red blood cells with a rotating-disc electrode, and gave a theoretical model [19]. A1203 suspensions were studied in a Couette cell by Deslouis et al [20].…”

“…Also, surface renewal [5] due to the scraping of the particles or penetration into the diffusion layer cannot explain the large increase of mass transfer. The most acceptable model is based on the rotation of the particles in the diffusion layer [12][13][14][15][16][17][18][19][20], which is caused by the rotation component of the shear field in the viscous boundary layer. This effect can be expressed [12][13][14] in terms of an effective diffusion coefficient which is determined by the shear rate (~) and the volume fraction, qS, of the particles.…”

“…The model of Hyman [15] is often used to calculate the limiting diffusion current [16][17][18][19][20]. For the rotating disc electrode this model predicts a linear /~-f~" relation at high rotation speeds with n = 7/6 [16,17,19]. Caprani et al [17] found experimentally that n = 0.5 for both low and high rotation rates, while at the intermediate rates n = about 0.6 was found.…”