Eley Rideal model
Apart from power law and Langmuir-Hinshelwood models, other kinetic models are also used to fit the data. EleyRideal model depicts the reaction mechanism in which one reactant species (say A) is adsorbed while the second reactant species (say B) is not adsorbed on the catalyst surface. The reaction then occurs when the passing gas molecules of B in gas phase directly reacts with the adsorbed species A. The schematic representation is shown in Fig. 1.
Fig. 1. Eley Rideal mechanism for reaction of adsorbed A with gas phase B producing product AB
Assuming that step 2, the reaction between adsorbed A and gas phase B is irreversible and the rate determining step, then the rate of reaction can be written as
 |
(9) |
The concentration of adsorbed A can be given as 
Now , 
Substituting value of CAS in equation (9)
If the products are chemisorbed then the reaction may become reversible as shown below.
Then the rate expression can be written as
The concentration of adsorbed product is 
Substituting CAS and CPS in rate expression,
Estimation of model parameters
The various parameters in the kinetic models such as rate constants, reaction orders, and equilibrium constants are derived by fitting the experimental data. The optimal values of the parameters in the rate equation of a heterogeneous reaction are determined traditionally by using gradient or direct search methods. For success of this method, a very good initial estimate of the parameters is required. This proves to be difficult in most cases. If the initial estimates are far from the global optima, then it is likely that the gradient or direct search method will not converge at all or will converge to local optima. Recently genetic algorithm (GA) is increasingly applied in estimation of kinetic parameters. The major advantage of this method is that it is not dependent on the initial estimate of the parameters. GA performs a multi-directional search.
Book Reference
• J. M. Smith, Chemical Engineering Kinetics, McGrawHill Book Company, 1981
• J.J. Carberry, Chemical and catalytic reaction Engineering, Dover Publications, 2001
• H. S. Fogler, Elements of Chemical reaction engineering, Prentice Hall of India,1999
• R. J. Farrauto and C. H. Bartholomew, Fundamentals of Industrial catalytic Processes, Blackie Academic & Professional, 1997