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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.

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For product quality purposes, the drying of some delicate materials is not easily feasible without the realization of a tight, well-designed controller. This study presents the results of a model predictive control for the drying of a thin fiber sheet moving through an electric infrared dryer, whose process dynamics are described by a minimum realization, previously developed. The simulation results indicate that the MPC multivariable controller is able to handle process interactions and produced good performance for tracking set point changes in the humidity and temperature of the material exiting the dryer and also for rejecting measured stochastic disturbances in the inlet humidity stream.