Evaporation, rate through surface films

Evaporation through surface films. Rideal1 has shown that surface films, both expanded and condensed, retard the rate of evaporation of water considerably. This retardation is not, however, at all easy to observe since the natural evaporation of water is already enormously retarded by the resistance caused by the slow diffusion of the evaporated molecules away from the surface. The real rate of evaporation can only be observed in a perfect vacuum Rideal obtained, by evacuating to just above the pressure at which water boils, a rate from clean water surfaces about 0 4 per cent, of the theoretical rate into a vacuum. Under these conditions, both expanded and condensed films appreciably lowered the rate, the maximum diminution being about 50 per cent. 

The rates of evaporation of water through unimolecular fi has been examined by Hedestrom and by the writer. It is foi that the rate of evaporation of water through unimolecular fi of fatty acids on the surface of water is relatively slow in compari to the high rate of evaporation calculated with the aid of Herz-Knudsen equation. The rate is approximately inveri proportional to the surface film pressure F as is indicated fi the following figures. 

The complete failure of the 10% hexane-squalane mixture to spread is disconcerting until the rates of exhaustion of the various liquids from a thin film are examined quantitatively. Making reasonable assumptions about evaporation rates and diffusional transport of the volatile component through the solution, it can be shown that half of the hexane would be lost from a film 10 microns thick in 1 second. Half exhaustion of hexadecane from an equally thick film would require several hours. Thus the surface tension gradients produced by the evaporation of hexane disappear before any detectable flow of the viscous squalane has occurred. 

It is difficult to distribute the entire product liquid to achieve the same thin liquid film in all tubes and to avoid the disrapture of the liquid film Surface tension effects (the surface tension can increase or decrease in the film downward flow) are decisive for the film stability (Zuiderweg and Harmens 1958 Ford and Missen 1968). Cocurrent once-through rising film evaporators can be used for a complete evaporation of the feed. However, fouling may occur at high evaporation rates. 

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