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R. R. IRANI, H. L. VANDERSALL, and W. W. MORGENTHALER Monsanto Chemical Co., St. Louis 66, Mo.
Water Vapor Sorption In Flow C o n d i t i o n i n g a n d Cake Inhibition These studies provide further insight into the mechanism by which conditioning agents maintain flow properties of powders and granular materials
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G E N E R A L study (6) has suggested that Lvater vapor plays a major role in the caking of po\vders. As the conditioning agent ranges from 0.5 to 4% (weight) of the conditioned material, protection froin lvater vapor-type caking must be a surface phenomenon. The study reported here considered quantitatively the interaction of water vapor Ivith the surfaces of the agents and the conditioned materials. Direct adhesion of the conditioning agent particles onto the surface of the material was probably the major factor in the superiority of a conditioner in a specific application. Neither amount nor rate of Ivater vapor sorption on conditioning agents \vas useful in predicting conditioning effectiveness per se, as evidenced from comparison with previously published data (6). When water vapor leaks into closed containers, it is preferentially sorbed onto the conditioning agents by virtue of their large surface area.
Experimental
Materials. The conditioners. salt, and sugar were described previously (6). Water Sorption. Water sorption isotherms \\ere determined bv exposing samples to a controlled atmosphere. After equilibration, the amount sorbed was calculated as the difference between the moisture contents a t specified and Oyc relative humidity. Relative humiditv was controlled to within =t2Yc absolute and temperature to within 1 0 . 5 " C. I n a typical experiment, 2 grams of the material were placed in a humidity chamber for 24 hours; alter 18 to 20 hours no further change in lveight was observed, indicating equilibration. The equilibrium moisture content at 070 relative humidity and the same temperature \\ere determined by drying samples to a constant weight over preignited calcium sulfate. In the chamber,
temperature was controlled to 1 0 . 3 ' C. and relative humidity kept below 0.8%. Rate of Water Vapor Sorption. The rate of sorption data !\ere obtained bv suspending a sample. attached to a diff
