Is the Adsorption of Cumene Rate-Limiting

To answer this question we shall assume that the adsorption of cumene is indeed rate-limiting, derive the corresponding rate law, and then check to see if it is consistent with experimental observation. By assuming that this (or any other) step is rate-limiting, we are considering that the reaction rate constant of this step (in this ease k/ ) is small with respeet to the specific rates of the other steps (in this case ks and The rate of adsorption is 

Since we eannot measme either C or Cc.g, we must replace these variables in the rate law with measmable quantities for the equation to be meaningful. 

For adsorption-limited reactions, is small and k and are large. Consequently, the ratios r /k and rjy/ko are very small (approximately zero), whereas the ratio is relatively large. The surfaee reaction rate expression is 

Strictly speaking one should compare the product kp Pc with and kn. (See Sum- 

For adsorption-limited reactions the surface specific reaction rate ks is large by comparison and we can set 

Strictly speaking, one should compare the product with As and ko- 

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Combining Equations (10-41) and (10-37), we find that the rate law for the catalytic decompositon of cumene, assuming that the adsorption of cumene is the rate-limiting step, is... 

If the cumene decompostion is adsorption limited, then the initial rate will be linear with the initial partial pressure of cumene as shown in Figure 10-12. 

Cumene is cracked in a recycle reactor over commercial H-ZSM5 extrudates during a pulse experiment. The results are compared to those obtained from steady state measurements. A linear model for diffusion, adsorption and reaction rate is applied to reactants and products. In contrast to literature it is shown that if the Thiele modulus is greater than 5, the system becomes over parameterised. If additionally adsorption dynamics are negligible or not measurable, only one lumped parameter can be extracted, which is the apparent reaction constant found from steady state experiments. The pulse experiment of cumene is strongly diffusion limited showing no adsorption dynamics of cumene. However, benzene adsorbed strongly on the zeolite and could be used to extract transient model parameters. 

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