Stagnant film theory

Effects of Total Pressure on Uq and The influence of total system pressure on the rate of mass transfer from a gas to a licniid or to a solid has been shown to be the same as would be predicted from stagnant-film theory as defined in Eq. (5-285), where... 

Fig. 45.1 Schematic presentation of the two stagnant films theory applied to gas—liquid systems, indicating the concentration profiles on both sides of the interface.

The stagnant film theory was developed by Nernst (1904). In this theory, a stagnant film exists on both sides of the interface, as illustrated in Figure 8.8. The thickness of the film is controlled by turbulence and is constant. 

Figure 8.8. Concentration gradient at the interface as assumed by the stagnant film theory.

In practice, the contact time t is not known except in special cases in which the hydrodynamics are clearly defined. This is somewhat similar to the case of the stagnant-film theory in which the unknown quantity is the thickness of the stagnant layer 5 (in film theory, the liquid-phase mass-transfer coefficient is given by k L = DJ5). 

The different solute concentrations on the feed and permeate sides are linked to the volumetric permeate flux in terms of the concentration polarization model, which is based on the stagnant film theory [14] ... 

In this section the classical heat and mass transfer theories are examined. The singular surface jump conditions for the primitive quantities, as derived in the framework of the standard averaging procedures, are approximated by the classical chemical engineering stagnant film theory normally used in chemical reactor models. The relevant transport phenomena solutions and the classical theories on heat and mass transfer considering both low- and high mass transfer rates are summarized in the subsequent subsections. 

The stagnant film theory was developed by Nemst in 1904. Because no flow can exist at the solid interface, a stagnant or laminar film of fluid (i.e., hydraulic film) of thickness, 5, was assumed to exist, and beyond this film the fluid was assumed to be turbulent. 

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