Diffusion Mass Transfer in Fluid Systems

Diffusion rates can be tremendously altered by chemical reactions. Indeed, these alterations are among the largest effects discussed in this book, routinely changing the mass fluxes by orders of magnitude. The effects of a chemical reaction depend on whether the reaction is homogeneous or heterogeneous. This question can be difficult to answer. In well-mixed systems, the reaction is heterogeneous if it takes place at an interface and homogeneous if it takes place in solution. In systems that are not well mixed, diffusion clouds this simple distinction, as detailed in Section 16.1.

The effects of chemical reactions are exemplified by the data for ammonia adsorption in water summarized in Fig. 17.0-1. The overall mass transfer coefficient, in cm/sec, is based on a liquid side driving force given in mol/cm3. The specific values shown are for a hollow-fiber membrane contactor, though similar values would be obtained in a packed tower or other more conventional apparatus.

The different mass transfer coefficients shown in Fig. 17.0-1 represent different forms of ammonia and different rate-controlling steps for mass transfer. At pH above 5, the mass transfer coefficient is small, somewhat less than typical values for liquids. Below pH 4, the mass transfer coefficient rises.