Arrhenius plot transport limitation

Figure 5.37. Arrhenius plot illustrating the effect on the apparent activation energy of pore diffusion and transport limitations through the stagnation layer surrounding a catalyst...

Describe qualitatively the consequences of transport limitations on the apparent activation energy of a catalytic process by using an Arrhenius plot. What is the best temperature to run this reaction in an industrial application ... 

The pyrolytic regime is distinguished from the (diffusion-limited) dialytic regime by the effect of the input gas flow rate on the deposition rate. Both transport-controlled regimes occur at high temperatures, where the slopes of the Arrhenius plot decrease (Figure 6.12). In the case of small equilibrium constants, the slope of the Arrhenius plot in the pyrolytic regime equals the reaction heat, as will be shown below. 

An important factor in an FT slurry process is the mass transfer outside the catalyst particles, in particular the transport of synthesis gas components from the gas bubbles to the liquid. Satterfield and Huff (1980) have analysed previously published studies of the FT synthesis in slurry reactors and concluded that, in particular at higher reaction temperatures, significant gas-liquid mass transfer limitations do occur, as evidenced by the curvature of the Arrhenius plots. 

Because JPS is limited by reaction kinetics and mass transport a dependency on the HF concentration cHf and the absolute temperature Tcan be expected. An exponential dependence of JPS on cHf has been measured in aqueous HF (1% to 10%) using the peak of the reverse scan of the voltammograms of (100) p-type electrodes. If the results are plotted versus 1/7) a typical Arrhenius-type behavior... 

Reaction limitation obtains if the rate coefficients of the surface reactions [(4), (5), (6)] are very low compared to the transport steps. Chemical reactions are thermally activated and rates of simple reactions follow Arrhenius behavior. Thus a plot of the logarithm of the deposition rate vs. 1/T is a straight line in the temperature range corresponding to the reaction-limited regime. The slope of this line is determined by the adsorption heats of the reactants and the activation energy of the rate-determining surface reaction step. 

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