Liquid-Wall Mass Transfer in Three-Phase Fluidised Beds with Annular Flow

摘要

Intimate mixing among the phases facilitates significant improvements in heat and mass transfer coefficients in two-phase flow systems in comparison to homogeneous flow. Introduction of a gas phase into a liquid-solid fluidised bed facilitates better circulation of the solids interacting physically with the reacting surface (outer wall of annular electrode). Thus, three-phase fluidisation has wide applications in chemical, petrochemical and biochemical processes. An experimental column with 44.4 mm I.D. with different annular copper rods serving as electrodes was used as the test section in the present study. The system chosen is an electrolyte-inert particles-nitrogen gas. The electrolyte is an equimolar ferri-ferro cyanide (redox) solution in the presence of an indifferent electrolyte. Diffusion controlled electrode reactions were employed to obtain limiting current densities and, hence, mass transfer coefficients. The liquid-wall mass transfer coefficient increased with increasing superficial gas velocity and reached a plateau. At constant superficial gas velocity k_L was found to be relatively independent of the variation in superficial liquid velocity. The k_L increased with increase in particle diameter and remained unaffected with equivalent diameter. The maximum value of k_L occurred when the gas to liquid mass velocity ratio is equal to 1.55 x 10~(-3). This finding helps in optimising the design and operating conditions of a three-phase fluidised bed. The mass transfer coefficient data were correlated in terms of Coulburn j_D-factor, particle Reynolds number Re_p, Froude number based on gas velocity Fr_g and gas-liquid mass velocity ratio G__(mr) as j_D = 7.971 G_(mr)Re_P~(0.03) FR_g~(-0.48).