Description of transport-reaction phenomena

The form in which the different processes are described reflects the assumptions of the model, which we briefly review in the succeeding text. 

2 Mass dispersion effective coefficients. Effects that result in a more complex flow pattern, such as higher fluxes near the wall due to higher void fraction, turbulent transverse velocity 

Froment [51] compiled several previous experimental works in Pcma -Re and Pe r - Re diagrams, where the Reynolds number is defined as Re = dpG/fi. In general, one may say that for 0.1 Re 1000, Pe a will take values between 0.5 and 2. Some correlations for the radial dispersion coefficient are of the form 

For 1 Re 1000, A should lie between 6 and 40, although for practical purposes, it is reasonable to assume [51] thatPe ,r varies in a range between 6 and 8. Experiments for a given system may be used to obtain the values for the coefficients. Froment [51] collected data from other authors that suggests Ak.7-15 and B = 19.4. Rase [52] also proposed that B = 19.4 for dpfdb 0.1, while B = 0 for larger values of this ratio. Moreover, a dependence on the Reynolds number is introduced  

Other expressions are available in Ref [57]. An alternative to the diffusion-like treatment of dispersion, in terms of a nonequilibrium approach, was given by BCronberg and Westerterp [58]. 

---