10.3.2 Is the Surface Reaction Rate-Limiting?

The rate of surface reaction is

10-26

image

Since we cannot readily measure the concentrations of the adsorbed species, we must utilize the adsorption and desorption steps to eliminate CC · S and CB · S from this equation.

From the adsorption rate expression in Equation (10-25) and the condition that kA and kD are very large by comparison with kS when surface reaction is limiting (i.e., rAD/kA ≃ 0),12 we obtain a relationship for the surface concentration for adsorbed cumene:

CC · S = KCPCCυ

In a similar manner, the surface concentration of adsorbed benzene can be evaluated from the desorption rate expression [Equation (10-29)] together with the approximation:

images

Single-site mechanism

Using image to find CB · S and CC · S in terms of partial pressures

Substituting for CB · S and CC · S in Equation (10-26) gives us

image

where the thermodynamic equilibrium constant was used to replace the ratio of surface reaction and adsorption constants, i.e.,

image

The only variable left to eliminate is Cυ:

Ct = Cυ + CB · S + CC · S

Site balance

Substituting for concentrations of the adsorbed species, CB · S, and CC · S yields

image

10-44

image

Cumene rate law for surface-reaction-limiting

The initial rate of reaction is

10-45

image

Figure 10-16 shows the initial rate of reaction as a function of the initial partial pressure of cumene for the case of surface reaction limiting.

Figure 10-16. Surface-reaction-limited.

image

At low partial pressures of cumene

image

and we observe that the initial rate will increase linearly with the initial partial pressure of cumene:

image

At high partial pressures

image

and Equation (10-45) becomes

image

and the initial rate is independent of the initial partial pressure of cumene.

If surface reaction were rate-limiting, the data would show this behavior.

..................Content has been hidden....................

You can't read the all page of ebook, please click here login for view all page.
Reset
52.86.227.103