Determination of Kinetic Parameters of the Thermal Dissociation of Methane

Kurzfassung

The solar thermal decomposition of methane could be an economically and ecologically beneficial process to produce hydrogen and particulate carbon. Aiming at the determination of general kinetic laws and parameters the thermal dissociation of methane was examined employing an alumina tubular reactor situated in an electric tube furnace. Nominal furnace temperatures in the range between 1200°C and 1600°C were set. Gas mixtures containing argon or helium as dilution gas and methane with a molar fraction between 2% and 10% were introduced into the reactor at an absolute pressure around 1 bar. The residence times ranged from 0.0115 s to 1.47 s. Temperature profiles along the reactor were measured with a thermocouple type S. Experimental results concerning the conversion of methane practically cover the full range from minor to total progress. Hydrogen was the main product of the decomposition. However, significant amounts of ethane, ethene, and especially ethyne formed part of the product flow. Seeding with carbon black featuring a specific surface similar to generated particles result in a significant increase of both, conversion of methane and yield of hydrogen. The laminar flow conditions at the inlet of and inside the reactor were assessed by means of simulations employing ANSYS and COMSOL Multiphysics. Diverse reactor models based on nested tube reactors were employed. The models either disregarded radial diffusion or implied ideal radial diffusion. A simplified kinetic model which takes the considered species into account and respects forward dehydrogenation reactions was engaged. Kinetic parameters were varied in order to minimize the model errors. Best agreement between the calculations and experimental findings was achieved for a reactor model featuring five nested tube reactors and neglecting radial diffusion. The respective decay of methane is characterized by a reaction order regarding methane of 1.283 and an activation energy of 510.1 kJ/mol. Low standard uncertainties of estimated parameter values were derived from the covariance matrix. Except for quantities associated with the same reaction, parameters showed only marginal correlation. Radial diffusion was found to be a key phenomenon difficult to assess properly. The probable presence of not considered high molecular intermediates and differing properties of generated carbon have been identified as limiting issues concerning a comprehensive kinetic approach including heterogeneous effects.