Irreversible surface-reaction-limited rate laws

Table 10-4. Irreversible Surface-Reaction-Limited Rate Laws...

Table 10-5 gives the forms of rate laws for different reaction mechanit that are irreversible and surface-reaction-limited. 

If surface reaction is assumed to be rate limiting and irreversible (and no adsorbed inerts are involved), the overall rate expression for consumption of A becomes -rA = A aCa/(1 + KaCa + KbCb), where k is the surface reaction rate constant and Ka and A b are adsorption equilibrium constants. If the surface is only sparsely covered, i.e., KaCa + KbCb 1, this can be approximated as simply va kKACA = k CA-This illustrates how a simple power law rate expression can apply, under some circumstances, for what is actually a relatively complex mechanism. 

In the original work on this reaction by Papp et al.. over 25 models wet tested against experimental data, and it was concluded that the precedin mechanism and rate-limiting step (i.e., the surface reaction between adsorbe toluene and gas) is the correct one. Assuming that the reaction is essential) irreversible, the rate law for the reaction on clinoptilolite is... 

Figure 15-1 Total pressure dependence of the best pseudo-first-order kinetic rate constant when a first-order rate law approximates a Hougen-Watson model for dissociative adsorption of diatomic A2 on active catalytic sites. Irreversible triple-site chemical reaction between atomic A and reactant B (i.e., 2Acr - - Bcr -> products) on the catalytic surface is the rate-limiting step. The adsorption/desorption equilibrium constant for each adsorbed species is 0.25 atm. ...

Notice that the molar density of key-limiting reactant A on the external surface of the catalytic pellet is always used as the characteristic quantity to make the molar density of component i dimensionless in all the component mass balances. This chapter focuses on explicit numerical calculations for the effective diffusion coefficient of species i within the internal pores of a catalytic pellet. This information is required before one can evaluate the intrapellet Damkohler number and calculate a numerical value for the effectiveness factor. Hence, 50, effective is called the effective intrapellet diffusion coefficient for species i. When 50, effective appears in the denominator of Ajj, the dimensionless scaling factor is called the intrapellet Damkohler number for species i in reaction j. When the reactor design focuses on the entire packed catalytic tubular reactor in Chapter 22, it will be necessary to calcnlate interpellet axial dispersion coefficients and interpellet Damkohler nnmbers. When there is only one chemical reaction that is characterized by nth-order irreversible kinetics and subscript j is not required, the rate constant in the nnmerator of equation (21-2) is written as instead of kj, which signifies that k has nnits of (volume/mole)"" per time for pseudo-volumetric kinetics. Recall from equation (19-6) on page 493 that second-order kinetic rate constants for a volnmetric rate law based on molar densities in the gas phase adjacent to the internal catalytic surface can be written as... 

The experimental quantity used to characterize heterogeneous reaction rates is the "reaction probablity", y, which is defined as the fractional collision frequency that leads to reactive loss. Kinetic data for the generally irreversible reactive uptake of trace gas species on condensed surfaces are expressed in terms of uptake experiments, where the disappearance of the species under consideration and/or the appearance of one or more reaction products has been observed. Such processes may not be rate limited by Henry s law constraints, however the fate of the uptake reaction products may be subject to saturation limitations. 

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