Investigations have been carried out experimentally to study the effect of viscosity on wall-to-bed mass transfer coefficient in a three-phase fluidized bed. The system chosen was an electrolyte as liquid phase, glass spheres of different diameters as solid phase and inert nitrogen as gaseous phase. The electrolyte was an equimolar solution of 0.01 N potassium ferricyanide and potassium ferrocyanide with 0.4 N NaHCO₃/Na₂CO₃ as indifferent electrolyte. To vary the viscosity, Carboxy Methyl Cellulose (CMC) sodium salt in 0.1, 0.2 and 0.4 wt% was added to the electrolyte. The reaction considered was the reduction of ferricyanide ion. The experiment was conducted at a constant temperature of 25 ^oC at which the physical property values of the electrolyte were obtained from literature. The mass transfer coefficient was computed from the measured limiting current. It was found that the mass transfer coefficient was relatively independent of liquid velocity within the range covered in the present experiment. The k_L increased with increasing gas velocity and approached a constant value. k_L was also observed to increase with increasing particle size and decreased with increasing CMC concentration. The entire experimental data was correlated in terms of modified Coulburn j-factor and modified Reynolds number.

Intimate mixing, realization of isothermal conditions and uniform concentrations, achievement of high heat and mass transfer rates, attainment of good temperature control etc., lead to wide usage of three-phase fluidized beds as multiphase contacting devices. Applications of three-phase fluidization are in hydrogenation of liquid petroleum fractions, hydrodesulphurization of residual and heavy oils, benzene desulphurization, Fisher-Tropsch process, hydrocracking the oil to lighter fractions, coal conversion processes, coal liquefaction and pelletizing operations, catalytic hydrogenation of unsaturated fats, synthesis gas conversion processes, electrochemical processes, fermentation, cell cultivation, production of antibiotics and wastewater treatment. A detailed list of applications of three-phase fluidization is given by Fan (1989).

It is evident that the use of three-phase fluidized beds have immense potentialities in the design of efficient electrochemical cells (Morooka et al., 1980; Yasunishi et al., 1988; Lee et al., 1997 and Ramesh et al., 2008a and 2008b). Although most of the process fluids handled in the chemical industry are viscous in nature, the reported studies were primarily conducted with Newtonian liquids of narrow viscosity range. In view of this, the present investigation is planned to study the liquid-to-wall mass transfer in a gas-liquid-solid fluidized bed under diffusion-controlled conditions with varied concentrations of Carboxy Methyl Cellulose (CMC) sodium salt from 0 to 0.4 wt%. Addition of CMC renders the electrolyte solution more viscous. Literature survey revealed that very few liquid-to-wall mass transfer studies have been reported in literature (Morooka et al., 1980; Yasunishi et al., 1988; Lee et al., 1997; Ramesh et al., 2008a and 2008b and Ramesh et al., 2009). However, studies on the effect of viscosity on wall-to-bed mass transfer in gas-liquid-solid fluidized beds were found to be scarce (Ravi Sankar, 2009).

Chemical Engineering Journal, Three-Phase Fluidized Bed, Carboxy Methyl Cellulose, Coal Conversion Processes, Coal Liquefaction, Electrochemical Processes, Catalytic Hydrogenation, Electrochemical Cells, Empirical Modeling, Electrochemical Redox System, Regression Analysis.