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Web End = On the Initial Rate of FluidSolid Reactions
HONG YONG SOHN and DE-QIU FAN
It is argued in this paper that the initial rate should not be used for the measurement or analysis the kinetics of a uidsolid reaction, especially for a reaction in which the eect of pore diusion starts appearing even moderately as the reaction proceeds. Even in the absence of external mass transfer eects, it is shown in this work by rigorous mathematical analysis that the range of conditions where the initial rate represents the intrinsic kinetics is very narrow. For an initially non-porous solid in the absence of external mass transfer eects, the very initial rate should mathematically be the intrinsic rate even when pore diusion becomes important as the reaction proceeds. However, even in this case, the range of conditions for this statement is very limited. For the reaction of an initially porous solid, the rate at time zero is already aected by pore diusion unless its eect is negligible over the entire range of conversion. Furthermore, the initial reaction rates of porous solids reacting under large values of k/De ratio (chemical reactivity is much greater than the capacity for pore diusion) have an apparent rate constant of
p k De and thus pore diusion alone does not control the initial rate no matter how large the
eect of pore diusion is overall.
DOI: 10.1007/s11663-017-0940-x The Minerals, Metals & Materials Society and ASM International 2017
I. INTRODUCTION
FLUIDSOLID reactions play a major role in many important chemical and metallurgical processes. A large amount of work has been devoted to the kinetics of uidsolid reactions. The reader is referred to the literature[1] for a comprehensive discussion of this topic. The investigation of reaction kinetics often relies on the initial rates.[26] [Also see the numerous sources and sites found by googling with the term initial rate method.] The initial rate method is appropriate for the determination of kinetics of homogeneous reactions, often saving time and eort. But as will be shown in this paper, the initial rate should not be used for the measurement or analysis of the kinetics of a uidsolid reaction, especially for a reaction in which the eect of pore diusion starts...