The effect of suspended solids on mass transfer in electrochemical systems

Technical Report · Sun Nov 01 00:00:00 EST 1987

OSTI ID:5612303

Andersen, P K; Tobias, C W; Muller, R H

Enhancement of mass-transfer rates by suspended, inert solid particles (glass or polymer particles) was studied using a rotating-disk electrode (RDE). The effects of solids concentration, particle size, and shear rate on limiting currents were measured for the reduction of ferricyanide ion. The data show evidence of two different transport mechanisms, which depend on particle size. For particles less than or equal to the thickness of the mass-transfer boundary layer, the Sherwood number Sh is a function of a Peclet number Pe: Sh = ..cap alpha..Pe/sup m/, where ..cap alpha.. and m are functions of the solids volume fraction. To apply to the RDE system, it was necessary first to solve the transport equations assuming that the diffusivity varies with radial position on the disk. The analysis yields I/sub L/*/I/sub L/ proportional to (..cap alpha..Pe/sub R//sup m/)/sup 2/3/ where I/sub L/*/I/sub L/ is the ratio of limiting current obtained with particles to the limiting current without, and Pe/sub R/ is the Peclet number evaluated at the outer radius of the electrode (r = R). For particles much larger than the thickness of the mass-transfer boundary layer, transport enhancement results from a particle-depleted ''slip'' layer at the electrode surface. The shear rate is higher inside the slip layer than in a homogeneous suspension; this thins the mass-transfer boundary layer. If this layer is of constant thickness, and the r- and theta-velocity gradients are increased by the same factor, it can be shown that the limiting current is related to the torque required to turn the disk: I/sub L/*/I/sub L/ = (T*/T)/sup 1/3/ where T* and T are the torques on the disk with and without particles, respectively. Measurements show that this equation underestimates the limiting current by as much as 60%, while failing to account for the observed increase in current density with electrode size. The torque measurements were also useful in estimating the mixing-power requirements.

• Research Organization: Lawrence Berkeley Lab., CA (USA)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 5612303
• Report Number(s): LBL-24522; ON: DE88005269
• Resource Relation: Other Information: Thesis (Ph.D.). Portions of this document are illegible in microfiche products. Thesis. Submitted by P.K. Andersen to Univ. of California, Berkeley
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ELECTROCHEMISTRY
MASS TRANSFER
FERRICYANIDES
REDUCTION
BOUNDARY LAYERS
CURRENT DENSITY
ELECTRIC CURRENTS
ELECTRODES
GLASS
MICROSPHERES
PARTICLE SIZE
PMMA
POLYMERS
ROTATION
SLIP
SUSPENSIONS
TORQUE
CHEMICAL REACTIONS
CHEMISTRY
COMPLEXES
CURRENTS
DISPERSIONS
ESTERS
IRON COMPLEXES
LAYERS
MOTION
ORGANIC COMPOUNDS
ORGANIC POLYMERS
POLYACRYLATES
POLYVINYLS
SIZE
TRANSITION ELEMENT COMPLEXES
400400* - Electrochemistry