Well-Mixed Stagnant Zones

The mass and heat transport model should be able to predict mass and energy fluxes through a gas/vapour-liquid interface in case a chemical reaction occurs in the liquid phase. In this study the film model will be adopted which postulates the existence of a well-mixed bulk and a stagnant transfer zone near the interface (see Fig. 1). The equations describing the mass and heat fluxes play an important role in our model and will be presented subsequently. 

Figure 5.2 The most common kinetic model used to estimate rates of gas exchange across the atmosphere-water boundary is the Stagnant Film Model. This model essentially has the following three regions of importance (1) a well-mixed turbulent atmospheric zone (PG) (2) a well-mixed thin-film liquid zone (PG) and (3) a laminar zone (A-B) separating the two turbulent regions. The thin-film is considered permanent with a thickness defined as z- (From Broecker and Peng, 1974, with permission.)...

Film theory predicts that the mass transfer coefficient for a phase (or the overall mass transfer coefficient) is proportional to the diffusion coefficient and inversely proportional to the thickness of the stagnant zone. The diffusion coefficient can be calculated from either the Wilke-Chang or the FSG equations. However, 6 is difficult (if not impossible) to determine. Hence, mass transfer coefficients are often determined from empirical correlations. Also, Film theory is based on the assumption that the bulk fluid phases are perfectly mixed. While this might approach reality for well-mixed turbulent systems, this is certainly not the case for laminar systems. 

The underlying idea for such a model is that only a fraction of the liquid flows in a more or less ordered way through the packing, while at each height there is a stagnant zone in which the liquid is well mixed and that exchanges mass with the flowing fraction. 

With respect to the above-given review, it certainly can be imagined that in multiphase catalytic reactions heat and mass transfer as well as phase mixing and stagnant zones have an influence on the chemical conversion in a very different way, depending on the operating variables of the reactor. But more than that, this behavior has consequences on scale-up procedures for these types of reactors, as the most appropriate scale-up procedure depends on mode of operation, flow regimes and the relative importance of mass transfer resistances compared to the chemical... 

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