A two-dunenslonal gram model has been developed to descnbe solid-gas reactlons m a cyhndncal pellet of firute length The grams are assumed to be spherical NumerIcal solutions of the model have been compared to udimte cyhnder soluttons. mfimte slab soluhons, and the approximate mfimte cyhnder and mfimte slab solutions devetoped by Szekely ef al (Szekely J , Evans J W and Sohn H Y , Gas-.Wrd Reactrons Academic Press, New York 1976) Pellet SIZS and reactor conchtlons under which one or more of these solutions IS vahd have been mchcated The accuracy of the other models depends on whether they are to be used to determme rate constants from lmtml rate data or whether they are to be used to relate converslon to time #en the Thede-type reactlon modulus Q = L[3(1-l O)K/D,rr] '12, has a value 2 2 with a cyhnder half-length of L = 0 2 cm and a cyhnder radms of R,, = 0 8 cm the conversIon levels after about 8 mm are 40% by an mfimte slab model, 50% by the 2-D model and 65% by the approxunate miimte slab soluhon If one determmes a rate constant from the mltlal slope of the rate curve, then the m&Me slab model mves a value 14% high The approximate mtirute slab solution 1s mappropnate for the latter purpose Slmrlar results are obtamed when the cyhnder radms Is much smaller than the half-length Pellets with RJL > 4 or <O 25 can be described by mfnnte slab or mlimte cyhnder models, respectwely, d the d&uslonal resistance 1s not large