Impact of internal transport limitations on kinetic studies

4 Impact of internal transport limitations on kinetic studies 

In a study of intrinsic kinetics, if data are taken in a laboratory PER where intraparticle transport resistances are appreciable, and if this circmnstance is arbitrarily neglected, experimental results will lead to falsified reaction kinetics  

Although the reaction rate is nth order in the absence of internal mass transfer limitations, the order determined from global rate data affected by pore diffusion will be ((n -t l)/2] instead of the true order of the surface reaction, for example, 3/2 instead of 2. The intrinsic reaction order will be observed only when n = 1. 

When data are taken under isothermal conditions at several temperatures in order to calculate the activation energy, the apparent rate constants evaluated will include the square root of (kyDe) at each temperature. Since D , is not very sensitive to temperature changes, the Ea calculated from the Arrhenius equation will be equal to one-half the Ea of the surface reaction, provided that external transport limitations do not exist 

Another point to be considered in the estimation of Ea is to select the temperature range carefully. At low temperatures, the reaction rate constant ky is low so the value of p is also low, leading to rjKl. However, ky increases exponentially with temperature while is not so responsive to temperature changes. Therefore, as the reaction temperature is gradually increased in successive runs, q starts increasing and eventually rj may drop below unity. Consider as an example [12] a flat plate catalyst with a 0.06 cm size factor and a of 0.070 cm s for a first-order reaction ky of 0.84 s at 499 K, t] is calculated as 0.99. If the Ea of the surface reaction is 110 kj mol , ky is calculated as 70.3 s at 599 K at this temperature, with the same size factor and values, r is estimated as 0.50. This trend continues with further increases in temperature, until rj becomes inversely proportional to p. 

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