Gas absorption in mobile beds of spherical packings

Experiments have been conducted to investigate the hydrodynamics and mass transfer performance of some alternative mobile bed packings. The polypropylene packings used in this work were 50 x 38 mm oblate spheroids, 38 mm diameter plain spheres, 25 mm diameter plain spheres and 25 mm diameter slotted spheres. The perspex column was 22 cm internal diameter and 144 cm high and was fitted with a supporting grid having 72% open area. In the hydrodynamics studies, air and water were passed counter currently through the column. Bed pressure drop, mean expanded bed height, minimum expanding gas velocity and volume of liquid hold up for all of the packings were measured at different air and water rates and with two bed static heights of 10.5 and 16.5 cm. The general hydrodynamic behaviour of the fluidized packings has been observed and factors influencing pressure drop, liquid hold up, expanded bed height and minimum expanding velocity have been identified. Evidence for gas and liquid flow through the interiors of the slotted packings and giving rise to higher liquid hold up and a smoother quality of fluidization as compared with the other packings has been found. Plain spheres and oblate spheroid packings appeared to have almost identical hydrodynamic behaviour, although the oblate spheroid was found to show more slugging fluctuation in the bed height at high gas veloci ti e s . In mass transfer studies, dilute C02 (2% voV Vol) was absorbed into sodium hydroxide solutions and the interfacial area and liquid film transfer coefficient were thereby established employing the pseudo first order reaction model for the rate of absorption (known generally as the Danckwerts' model) (41). The experimental results on the Danckwerts' plot lay on a straight line and therefore confirmed the applicability of the Danckwerts' model from which (kL) and (a) could be estimated. The interfacial area per unit volume of expanded bed (a) and per unit volume static bed (as) was found to be higher for the slotted packings than for the plain sphere packings and the oblate spheroid packi ngs. The interfacial area per unit volume of liquid hold up (aL) was higher for the plain sphere packing than for the oblate spheroid and the slotted sphere packings, however, the (a) and (as) for the plain sphere packings were higher than for the oblate spheroid packings. The liquid film transfer coefficient for the slotted sphere packings was higher than for the other packings and was found to be almost the same for the plain sphere and the oblate spheroid packings. The operational mass transfer efficiency (defined as the ratio of the volumetric liquid film mass transfer coefficient to the fluid energy consumption in the bed) for the oblate spheroid packings was found to be higher than for the plain and slotted sphere packings. This efficiency ratio was also found to be higher for the slotted packings than for the plain sphere packings. Finally, the slotted packings appear to offer high volumetric mass transfer coefficients with a smoother quality of fluidization and homogeneous contacting of gas and liquid.