(143f) Studies on Ionic Mass Transfer with Coaxially Placed String of Spheres as Turbulence Promoter in Fluidized Beds

STUDIES ON IONIC MASS TRANSFER WITH COAXIALLY PLACED STRING OF SPHERES AS TURBULENCE PROMOTER IN FLUIDIZED BEDS

Rajesh Kumar. P., Raju. CH. A. I., Rajendra Prasad. P. and Sarveswara Rao. S. Department of Chemical Engineering, College of Engineering, Andhra University, Visakhapatnam-530 003, Andhra Pradesh, India.

ABSTRACT

The process of improving the performance of a mass transfer system is referred to as mass transfer augmentation, enhancement or intensification. Several works in heat and mass transfer were carried out adopting the various augmentation techniques for economic operation, reduced equipment size and increased output. The use of surface promoters, swirl generators, insert promoters, vibration of the reacting surface, additives and pulsation of fluids are some of the methods adopted by earlier investigators for process augmentation. Several studies on the use of insert promoters were reported in literature. Notable among them are coiled wires and twisted tapes. The turbulence created is due to of swirl flow generated and its interaction with the axial flow. Displaced promoters like discs, spheres and cones etc. are placed at equi distance on a central rod and fixed coaxially in the conduit. The turbulence created is due to the obstruction to flow. Rough surfaces also create turbulence because of the protrusions on the surface and increase in transfer rates are also due to increase in transfer surface area. In all the above cases the augmentation in transfer coefficients is due to the turbulence generated in a flowing fluid. The use of fluidizing solids augments the heat and mass transfer coefficients by reducing thermal or concentration gradients in a flow medium. Insertion of promoters in fluidized beds further augments the coefficients while also improving the quality of fluidization. Several works on heat transfer in liquid fluidized beds were reported in literature. The present study is envisaged to obtain higher augmentation by employing a coaxially placed string of spheres in fluidized beds. Diffusion controlled electrochemical redox reaction for the system potassium ferri-ferro cyanide couple was chosen for the study. The work is based on 1200 limiting current and pressure drop measurements by varying the geometric parameters such as the diameter of the sphere, the distance between the spheres, the diameter of the fluidized solids, the solids concentration and the dynamic parameter, the flow rate. The study reveals that the mass transfer coefficient increases with increase in diameter of the sphere and the diameter of the fluidizing particle. The spacing between the spheres shows negligible effect on mass transfer coefficient. The augmentation in mass transfer coefficients due to the insertion of string of spheres in fluidized beds is found to be 18 fold when compared to the smooth tube flow without any turbulence promoter. The data are correlated using JD , Rep/(1- ) and geometric parameters of the promoter. The mass transfer data are correlated as : JD = 0.057 [Rep / (1- )]-0.27 (dp/D)0.213 (ds/D)0.043 (S/D)?0.023 The momentum transfer data are correlated as :

R(h+) = 0.87 [Rep / (1-e)]-0.116 (ds/D)0.194 (dp/D)-0.358 (S/D)?0.008

The mass transfer factors in the present study are compared with the earliest works.