Arrhenius coefficients

In this problem we will consider the kinetics of this unimolecular isomerization reaction in a nitrogen bath gas at 1500 K, using several different theoretical treatments. The high-pressure Arrhenius coefficients for this reaction are A00 = 1 x 1014 s-1 and E0 = 45 kcal/mol. 

Thermochemical properties of gas-phase, surface, and bulk species are assumed to be available. This information is used in the calculation of the equilibrium constant, Eq. 11.110, and thus the reverse rate constant, Eq. 11.108. There is not a great deal of thermochemical data for species on surfaces, but techniques are becoming available for their calculation (e.g., Pederson et al. [310]). If surface reactions are specified to be irreversible, or if Arrhenius coefficients for the reverse rate constant are explicitly supplied, then the thermochemical data are not actually used. 

This expression for contains two unknown rate constants, k and ks. The Arrhenius coefficients for these rate constants were determined using the 140 °C conversion data from Figure 7.6. The parameters were estimated for both the case of chain transfer to monomer, and again for chain transfer to DH. Given that the two chain transfer models differ only in their predictions of Mw and that the fit was against conversion data, the optimum Arrhenius constants for both cases were the same ... 

Statistical evaluation of the peak maximum temperatures of the distillation bitumens results in means with coefficients of variation V = 1.01 % maximum. When using these means in the calculation, the following average values of the Arrhenius coefficients are found ... 

Table 4-80 Arrhenius coefficients and conversion for DSC pyrolysis in 1 bar argon Distillation bitumen...

If bitumens are regared as model substances for heavy residues, then their Arrhenius coefficients can serve as the basis for the calculation of the kinetics of pyrolysis reactions in thermal conversion processes (thermal cracking, visbreaking, hydrotreating etc.). Integration of the peak areas gives a value for the energy required for pyrolysis reactions. 

Table 4-81 Arrhenius coefficients and conversions in DSC pyrolysis in 1 bar argon for blown bitumen.

The individual data of the Arrhenius coefficients for the bitumens and their colloid components are shown in Tables 4-86 to 4-89. 

The very low variance of the means of the peak maximum temperatures permit their use in calculating the averages of the Arrhenius coefficients (Table 4-92). Again, there are only small differences between the average values of the activation energy E and the means of E from the individual data of the Tables 4-86 to 4-89, with no statistical significance. 

Table 4-92 Average values of the Arrhenius coefficients and the conversion for the pyrolysis of distillation bitumens in 10 bar methane.

There is no statistical difference in the Arrhenius coefficients between the bitumen and the dispersion medium, nor between the petroleum resins and the asphaltenes. The statisti-... 

Differences in the Arrhenius coefficients for the three components of the coUoid system (the dispersion medium, petroleum resins, and asphaltenes) do exist, but prove to be smaller than was at first assumed. 

The straight lines for the distillation bitumens and their colloid components in the plot of half life time, versus temperature run almost parallel, because the differences in the Arrhenius coefficients are small (Fig. 4-75). 

Even the corresponding peak temperatures of the blown bitumens show very small variances in the tests in 10 bar methane and also permit the calculation of statistical means. The resulting coefficients of variation are 3.0 % maximum. This is also true for the colloid components, except for the dispersion medium of the two bitumens 85/40 (sample III) and 85/25 (sample IV). Here again a weight loss caused by distillation even occurs under pressure with the consequence of low values for the activation energy and frequency factor. Only the data of the other three samples was included in the statistics. The average values of the Arrhenius coefficients calculated in this manner and the means of the conversion aie shown in Table 4-93. 

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