Activation energy ethane pyrolysis

The pyrolysis of ethane proceeds with an activation energy of about 300 kJ/mol. How much faster is the decomposition at 650°C than at 500°C ... 

Kassel67 proposed an activation energy of 44 kcal. for the reaction on the basis of an uncertain mechanism for CH4 pyrolysis, and Bawn3 reported that ethane was not formed when methylene was (presumably) produced in the presence of methane at 300°C. by reaction of Na vapor with CH2Br2. 

Historically, of the various hydrocarbon pyrolyses, that of ethane has been the object of the greatest amount of investigation. As a consequence more is known about it than about the pyrolysis of any other hydrocarbon. Even more important, sufficient thermal and kinetic data are now available so that the rates (and activation energies) of various chain mechanisms can be calculated with about order of magnitude reliability. 

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

The remaining issue in the formulation of Arrhenius parameters concerns the size of combined activation energies and frequency factors that often appear in overall rate constants of mechanistic rate expressions. Since mechanistic constants are much more commonly encountered than elementary constants, we need to know if they too are subject to limits such as the ones described above. For example, in the pyrolysis of ethane (see Chapter 3) the mechanistic rate expression is ... 

An estimate of the overall activation energy of the cracking is obtained by substituting typical values of E for initiation (356 kj/mol), hydrogen abstraction (42 kJ/mol) radical decomposition (147 kJ/mol) and termination (0 kJ/mol) into (1.6.1.A-2). The result is 272.5 kj/mol. For the specific case of ethane pyrolysis, values given by Benson [1960, p.354] lead to Eo of 282.5 kj/mol. These Eo values are much lower than the E of initiation and the nominal values for breaking C-C-bonds. ... 

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