The influence of the diffusional phenomena on the apparent kinetics for nth‐order gas‐solid non‐catalytic reactions is studied assuming the unreacted‐core model. Flat‐plate, cylindrical and spherical particles are considered, the size of which may vary with extent of reaction. Specific methods of contacting of fluid with solid are taken into account, but their influence on the results are not remarkable. At high conversions of cylindrical and spherical particles, the process is always chemical reaction controlled.

By starting at the boundary condition of H = 1 at R = 0, as indicated by Equation (8), the N r --There is a slight difference in constants between Equation (A22) and Equation (17) owing to the slight difference in results of numerical integration. The difference can he shown by evaluation of H"DL from Equation (A19) and (A20). ThusThe comparable result used in Equations (12), ( l e ) , and ( 1 7 ) was the 0.642 value of White and Tallmadge ( 8 ) . This 1% difference is not important for the discussion under consideration.Comparison of the B value method with C value method indicates that:1. The B value places flow parameters into the static term, whereas the C value does not.2. The B value does not have a direct physical significance, whereas the C value signifies the curvature.3. The B value is a complicated function of H " , whereas C is simply equal to L".4. The B value method appears to be longer and less direct. 5. The C value provides for an analytical but approximate expression for the effect of DO (or p ) .6. The B value provides for an exact but numerical expression for the effect of DO without approximation.The only apparent advantage of the B method is noted in reason 6. The C value method was selected for use in deriving Equation ( 16) mainly because an analytical solution (reason 5 ) was desired, but also in order to present the simpler and more direct derivation (reasons 1 and 4 ) . Zeroth-Order Reactions in Heterogeneous Catalysis ROBERTO TARTARELLI and MlLVlO CAPOVANI lstituto di Chimiea Industriale ed Applicota dell'Universit~, Pisa, ItalyThe simultaneous zeroth-order reaction and diffusion in porous catalyst appeared in the classical paper of Wheeler ( 1 ) . Successive influence of volume change on catalyst effectiveness has been discussed by Weekman and Gorring ( 2 ) ; selectivity under diffusion limitation has been reported by Van de Vusse ( 3 ' ) , ' ana 'effectiveness functions have been used to describe selectivity and poisoning ( 4 ) . The zeroth-order reactions are special cases of power law kinetics (5, 6) and particular types of the LangmuirHinshelwood reactions, the treatment of which, in presence of diffusion limitation, has been given by Roberts and Satterfield (7).In the cited references the diffusion during reaction is characterized by an experimentally determined diffusivity, or a simple pore structure is assumed (single-pore model). This model (8, 9) has been extended by Mingle and Smith ( 1 0 ) and Carberry (11,12) to a more complex model (macro-micropore model) representing catalyst pills containing macropores created via pelletting of porous microspheres. The last may be visualized by a cylindrical pore (macropore) from which micropores branch along the length and at right angle to the macropore; Knudsen diffusion and molecular diffusion may be considered prevailing in the micropores and in the macropore, respectively. The macro-micropore effectiveness factor has been derived for isothermal irreversible and reversible first-order reactions (10, 11 ) and irreversible seco...