Reaction-limited regime estimation

Because rates of reduction by Fe° vary considerably over the range of treatable contaminants, it is possible that there is a continuum of kinetic regimes from purely reaction controlled, to intermediate, to purely mass transport controlled. Fig. 9 illustrates the overlap of estimated mass transport coefficients (kmt) and measured rate coefficients (kSA). The values of kSA are, in most cases, similar to or slower than the kmi values estimated for batch and column reactors. The slower kSA values suggest that krxu < kml, and therefore removal of most contaminants by Fe° should be reaction limited or only slightly influenced by mass transport effects (i.e., an intermediate kinetic regime). 

Estimate the maximum temperature at the center of a catalytic pellet at large intrapellet Damkohler numbers in the diffusion-limited regime when the thermal energy generation parameter P = 2 (i.e., exothermic chemical reaction), the temperature on the external surface of the pellet is surface = 300 K, and the average pore size is greater than 1 tim. 

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

Estimate the limiting diffusion-reaction regimes under the prevailing lab reactor conditions for heterogeneous reactions, and use the appropriate lab reactor model. When possible, operate the reactor under kinetic control. 

After the partition step the model involves association of the enzyme with the substrate and subsequent dimerization before activation. The proportion of activated enzyme ( /) was assumed to be very small in relation to the total amount of enzyme at the interface ( -i- E/). Enzyme activation was the rate-limiting step for the reaction. The activation was slow (k3et=3xl0 s ) compared with the rate of catalysis in the steady-state regime (kcat=4xl0 s ), the being latter estimated on the basis of classic Michaehs-Menten analysis. The authors conclude that the lag time in SMase catalysis is due to an equihbrium between rnonomolecular and bi-molecular activated states of the enzyme in the interface, and to a slow activation process. 

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