from first principles PP-I-1

PP-III-84Catalytic Reaction Dynamics Caused by Ions Mobility in SolidsOstrovskii N.M.Hipol a.d., Odžaci, Serbianikolaj.ostrovski@hipol.rsThe mobility of ions and atoms in solid catalysts is manifested as the diffusion of oxygen inmetal oxides (V, Cr, Mn, Fe, Mo); of carbon in metal carbides (Fe, Ni); and of hydrogen inmetals and hydrides (Pd, Ti, La). It happens when reagents oxidize or reduce the surface layerof the catalyst. Then the solid bulk tends “to lick its wounds” by diffusion of atoms or ionsfrom the bulk to surface (in reducing medium) or in opposite direction (in oxidizing medium).Certainly, ions mobility (for example oxygen) is marked only in oxides of variable level ofmetal oxidation (V 2 O 3 -V 2 O 5 , Cr 2 O 3 -CrO 3 ) in contrast to Al 2 O 3 or SiO 2 .Since ions diffusion in bulk is caused by reaction on the surface, it affects the state of surface,and consequently affects the reaction dynamics. Because the capacity of bulk is much higherthan that of the surface, ions mobility increases the working period of the catalyst. It makesacceptable the non-stationary cyclic regime of the process with the reaction period and thecatalyst recovery period. In oxidation processes such a regime provides usually higherselectivity than the steady-state regime [1, 2].In this report, the region in which the diffusion of ions in catalyst bulk affects the transientregime of catalytic reaction is examined, and corresponding ions diffusivity is estimated.The complex effect of ions diffusion in catalyst bulk and reagents diffusion in pores is alsoconsidered. It is shown that the pellet effectivity is considerably affected by ions diffusion in akinetic region of the reaction. On the contrary, in a diffusion region, the pellet effectivity onlyslightly depends on ions diffusion.The considered problem, takes on a special significance in the case of reactor with a fluidizedcatalyst bed. Such a reactor is the most convenient option for cycle periodic regime of theprocess, with an internal or external circulation of the catalyst. The interval of ions diffusivityis estimated that affects total conversion in the reactor and average activity of the catalyst.References:[1] N.M. Ostrovskii. Catalyst deactivation kinetics. Moscow, Nauka. 2001, 334 p.[2] N.M. Ostrovskii, S.I. Reshetnikov, Chem. Eng. Journ. 107 (2005) 141.[3] N.M. Ostrovskii, F. Kenig, Chem. Eng. Journ. 107 (2005) 73.241