Axial dispersion of liquid in concurrent gas-liquid downflow in packed beds.

Abstract

The axial dispersion in the liquid-phase of a gas-liquid downflow through a packed column was studied with air and water flowing concurrently in a 8.0 cm diameter column with glass spheres of 0.12, 0.26 and 0.43 cm. The impulse response was calculated numerically from the two signals measured at two cross-sections in the bed. The response was characterized by both a peak and a very long tail, and could be represented satisfactorily by using the PDE model in which the mass transfer between the dynamic and the stagnant holdups in addition to the axial dispersion in the dynamic holdup was taken into account. Four parameters appearing in the PDE model were determined by means of the time domain curve fitting method. Peclet number which was based on the actual velocity in the dynamic holdup ud had a constant value of 0.43 for Re''(=udρdp/μ) 400. The volumetric mass transfer coefficient was correlated as a function of particle size, liquid and gas velocities. The total liquid holdup agreed well with the literature data. The fraction of the dynamic holdup varied in the range of 0.6 to 0.95 depending on the particle size and the gas and the liquid velocities. The correlation of the dynamic liquid holdup was given graphically.