Entrance Effect and Gas-Film Mass-Transfer Coefficient in a Large-Diameter Packed Column

Abstract

The effects of the bed height and the entrance section below the packing support on mass transfer in a 1.2-m diameter packed bed of 50-mm ceramic Intalox saddles were investigated under various gas flow rates from 1957 to 7828 kg‚h -1 ‚m -2 and liquid flow rates from 12 200 to 46 700 kg‚h -1 ‚m -2 . The entrance effect accounted for 17% of the overall water vapor transferred from moist air to a calcium chloride solution in the tower, regardless of the packing height. For a 0.91-m high bed, the average mass-transfer coefficient was directly proportional to the gas rate and was proportional to the liquid rate to the power of 0.24. For a 1.8-m high bed, the average mass-transfer coefficient was proportional to the gas rate to the power of 0.89 and appeared to be independent of the liquid rate. The height of a transfer unit (HTU) for 50-mm ceramic Intalox saddles remained relatively constant under various gas rates used in the present study. The HTU was about 0.5 m for the short bed and 0.8 m for the tall bed.

Dispersion-Concentric Model for Mass Transfer in a Packed Bed with a Countercurrent Flow of Gas and Liquid

“…This portion of mass transfer was lumped into the overall mass transfer over the bed. 12 On the other hand, the concentration of water vapor in the air outlet predicted from the model was free of the entrance effect; hence, it was higher than the experimental value obtained with the air-calcium chloride solution system. For the air-water system in the 0.61 m diameter column filled with 38-mm ceramic Intalox saddles, the gas entrance section was designed in such a way to provide only a small space where gas and liquid were in contact prior to the packed bed.…”

“…The mass transfer in the entrance section may be a significant portion of the overall mass transfer over the whole column as observed in a regularly designed column with a large entrance section. 12 The column was filled with 38-mm (1.5-in.) ceramic Intalox saddles.…”

“…Thus, the mass-transfer rate, and hence the average mass-transfer coefficient (K ga ), may increase. 12 Therefore, the model was also evaluated at different liquid rates.…”

“…For the air-water and air-CMC solution systems used in the present study, the mass-transfer process was gas-phase controlled; i.e., the main resistance to mass transfer resided in the gaseous phase. 12 Water was evaporated and transferred to the air. The concentration of water vapor at the gas-liquid interface was the saturated concentration of water vapor in air at the prevailing liquid temperature.…”

Performance of a pilot-scale cross-flow rotating packed bed in removing VOCs from waste gas streams

“…As pointed out by Waldie [24] as well as Doan and Fayed [25], the height of a transfer unit (HTU) is a simple but representative design parameter for comparing the performance of a packed bed. The relationship between the height of a transfer unit and K G a can be taken as follows [25]:…”