Surface reaction control

TABLE 7-2 Surface-reaction Controlling (Adsorptive Equilibrium Maintained of All Participants)... 

Students of professor R. G. Anthony at College Station, TX used a mechanism identical (by chance) to that in UCKRON for derivation of the kinetics. Yet they assumed a model in which the surface reaction controls, and had two temperature dependent terms in the denominator as 13,723 and 18,3 16 cal/mol. Multiplying both the numerator and the denominator with exp(-15,000) would come close to the Ea,/R about 15,000 cal/mol, with a negative sign, and a denominator similar to that in the previously discussed models. 

Example 10.4 Repeat Example 10.3, assuming now that the surface reaction controls the rate. 

Solution If the surface reaction controls the overall rate, all the active sites will be occupied. Assume that the entire surface is active so that it will be covered with a monolayer of A molecules. The surface area is 101(400) = 40,400m (4ha or 10 U.S. acres ). 

Kinetic Term The designation kinetic term is something of a misnomer in that it contains both rate constants and adsorption equilibrium constants. For thfe cases where surface reaction controls the overall conversion rate it is the product of the surface reaction rate constant for the forward reaction and the adsorption equilibrium constants for the reactant surface species participating in the reaction. When adsorption or desorption of a reactant or product species is the rate limiting step, it will involve other factors. 

The performance of a reactor for a gas-solid reaction (A(g) + bB(s) -> products) is to be analyzed based on the following model solids in BMF, uniform gas composition, and no overhead loss of solid as a result of entrainment. Calculate the fractional conversion of B (fB) based on the following information and assumptions T = 800 K, pA = 2 bar the particles are cylindrical with a radius of 0.5 mm from a batch-reactor study, the time for 100% conversion of 2-mm particles is 40 min at 600 K and pA = 1 bar. Compare results for /b assuming (a) gas-film (mass-transfer) control (b) surface-reaction control and (c) ash-layer diffusion control. The solid flow rate is 1000 kg min-1, and the solid holdup (WB) in the reactor is 20,000 kg. Assume also that the SCM is valid, and the surface reaction is first-order with respect to A. 

The rate of a solid catalyzed reaction, A2 surface reaction controlled. With the aid of the given is the dissociated or molecular species that reaction. The equilibrium constant is Ke = Pbe Pae With dissociation,... 

An ideal gas phase reaction, 2.A B, is surface reaction controlled and has the rate equation... 

Hydrogenation of octenes occurs with surface reaction controlling (Hougen Watson, Chemical Process Principles, p 943, 1947). The rate equation is... 

A situation, intermediate between a) and b) - a mixed transport-surface reaction controlled kinetics -may develop. 

Analogously, if cs c c the growth rate is surface reaction controlled. See Figure 1. 

Let us consider the data taken by Laible (LI) on the dehydration of normal hexyl alcohol at 450°F over a silica alumina catalyst. The single- and dualsite surface reaction controlled models applying to alcohol dehydration were discussed in Section V,A,2. We now consider, however, the functional forms given, for example, by Eq. (84), as probably being capable of describing the data, but do not restrict the Ct and C2 plots to a linear pressure dependence as before. Rather, we obtain an empirical pressure dependence from the... 

Based on Langmuir-Hinshelwood kinetics the rate expression for a first order reaction (A —> R) that is surface reaction-controlled becomes equal to the following expression [2] ... 

Fig. 6. Chemical reaction rate data for NH3 reacting with H3PO4 solution droplets, from Rubel and Gentry (1984a). The data are compared with theory for surface reaction control (S) and gas-phase diffusion control (D). Reprinted with permission from J. Aerosol Sci. 15,661-671, Rubel, G. O., and Gentry, J. W., Copyright 1984, Pergamon Press pic.

Fig. 2.3 Rate-limiting steps in mineral dissolution (a) transport-controlled, (b) surface reaction-controlled, and (c) mixed transport and surface reaction control. Concentration (C) versus distance (r) from a crystal surface for three rate-controUing processes, where is the saturation concentration and is the concentration in an infinitely diluted solution. Reprinted from Sparks DL (1988) Kinetics of soil chemical processes. Academic Press New York 210 pp. Copyright 2005 with permission of Elsevier...

Figure 7-11 Reactant concentration profiles within a porous catalyst pellet for situations where surface reaction controls aid vdiere pore difiusion controls the reactions.

Another important consequence of the constant rate of release diffusion model is that it mimics many of the features that have commonly been attributed to surface reaction (matrix dissolution) control. If one were to account for changes in surface area over time, the predicted long-term dissolution rate due to surface reaction control would also yield constant element release. In surface reaction controlled models, the invariant release rate with respect to time is considered to be the natural consequence of the system achieving steady-state conditions. Other features of experiments commonly cited as evidence for surface reaction control, such as relatively high experimental activation energies (60-70 kJ/ mol), could be explained as easily by the diffusion-control model. These findings show how similar the observations are between proponents of the two models it is only the interpretation of the mechanism that differs. 

Surface reaction controlled growth Diffusion-controlled growth... 

Mann and Rouleau (14) and Shapovalova et al. (18) studied the oxidation of 2-methylpropene to methacrolein in the absence of selenium dioxide, and they found that the oxidation of 2-methylpropene was a surface reaction controlling between adsorbed 2-methylpropene and weakly adsorbed oxygen. However, no theoretical explanation is given for this. 

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