Kinetics ethane activation energy

Hydrocarbon conversions can. In general, be represented fairly well by first-order reaction kinetics, and the conversion levels for runs made with a constant hydrocarbon flow rate as a general rule Increased significantly as the temperature Increased. Figures 1, 2, and 3 show typical results for ethane, propane, ethylene, and propylene. Based on first-order kinetics, the activation energies for ethane, propane, ethylene, and propylene were determined In the various reactors tested. In the Vycor reactor, these activation energies were approximately 51, 57, 56, and 66 k cal/g mole respectively. They were much lower In metal reactors especially after the reactor was oxidized. In a relatively new and unoxidized Incoloy reactor, the activation energies were 15, 47, 27, and 26 k cal/g mole respectively. 

The kinetics of the two reactions show that the desirable TML reaction has a higher activation energy than does the undesirable ethane reaction. Therefore the yield of TML is maximized by running the reaction at high temperatures. Of course, an additional and very significant advantage of high-temperature operation is shorter batch times (increased production rate). 

The kinetics of alkane hydrogenolysis, that is, the dependence of rate on reactant concentration, have been the subject of numerous studies, and much effort has been devoted to devising rate expressions based on the Langmuir-Hinshelwood formalism to interpret them. Reactions of ethane, propane, and n-butane with H2 on EUROPT-3 and -4 have been carefully studied, with the surfaces in either as clean a state as possible, or deliberately carbided [21, 22] kinetic measurements at different temperature permitted adsorption heats and true activation energies to be obtained. There were two surprises (but like all surprises they were obvious afterwards) ... 

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. 

The reaction is often initiated by photolysis of bromine. The hydrogen-abstraction step is rate-limiting, and the product composition is governed by the selectivity of the hydrogen abstraction. The enthalpy requirement for abstraction of hydrogen from methane, ethane (primary), propane (secondary), and isobutane (tertiary), by bromine atoms are -1-16.5, -1-10.5, -1-7.0, and -1-3.5 kcal/mol, respectively. These differences are reflected in the activation energies, and there is a substantial kinetic preference for hydrogen abstraction in the order tertiary > secondary > primary. Structural features that promote radical stability by delocalization, such as phenyl, vinyl, or carbonyl substituents, also lead to kinetic selectivity in radical brominations. 

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