Effect of Radial Mass-transport Limitations

Mass transport of the dosed component oxygen from the membrane to the center of the catalyst bed is studied in this section. The boundary conditions for the component mass balances were given in Table 5.3. In setting up these BCs it was assumed that the membrane walls are impermeable for every component except oxygen and an inert gas (nitrogen). This constraint is valid for membrane materials with low permeability and dominating convective transport through the membrane. 

Reactor models accounting for radial porosity profile were compared with models using the averaged bed-porosity value. Isothermal conditions were applied in order to rule out thermal effects on the concentration profiles. To check the need for two-dimensional models the results were compared with that obtained by using the pseudohomogeneous one-dimensional reactor model (Eqs. (5.1)-(5.4)). 

Summarizing, the effect of the porosity profile on the integral reactor performance is rather small for the conditions studied. This can, however, change for systems with kinetics more sensitive to the educt concentration (higher reaction orders). In comparison with the ID model results it was found that the simple model overpredicts the achievable intermediate yields in the PBMR. Consequently, radial mass-transfer limitations can not be neglected if more precise predictions are required. 

---