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Hein, O. ; Jess, Andreas:
Bestimmung kinetischer Parameter exothermer Gas/Feststoff-Reaktionen mit der Zündpunktsmethode.
In: Erdöl, Erdgas, Kohle.
Bd. 116
(2000)
Heft 1
.
- S. 18-24.
ISSN 0179-3187
Abstract
The determination of kinetic constants of heterogeneous reactions is usually performed in tubular flow reactors under isothermal and stationary reaction conditions. This is often time-consumin and difficult. For example, in case of strong exothermic reactions, temperature gradients up to the ignition of the fixed-bed are hard to avoid, and therefore expensive reactors with internal or external gas recycle are needed. The effect ofignition can also be used to characterize the reactivity of solid fuels and the activity of catalysts, respectively. A well-known method for a simple, fast and accurate determination of the ignition temperature in a lab-scale tubular flow reactor is already described in literature [1-5]. Yet, the ignition point method is up to now only used as a qualitative measure for the (relative) reactivity of solid fuels and activity of catalysts. Therefore an attempt was made to extend this method towards a quantitative characterization ofthe kinetics of exothermic heterogeneous reaction systems, i. e. for a determination of the activation energy and the preexponential factor. The basic idea is thereby to alter the ignition point by a defined variation of the operation conditions such as particle diameter, heating rate as well as of the composition and flow rate of the reacting gas mixture. The resulting g data are then used to calculate the kinetic constants based on the theory of ignition of exothermic reactions (e. g. [6]). The combustion of different cokes as well as of carbon deposits on a Pt-catalyst were selected as model reactions for gas/solid-reactions. In addition, the oxidation of methane on a Pt-catalyst was investigated. The experiments on the influence of the operation conditions show that the gas composition - in case of combustion of coke the oxygen content - has the strongest influence on the ignition temperature. This method is therefore very suitable to determine kinetic constants. To prove the accuracy of this method, the kinetic data were also determined in an isothermal tubular flow reactor. The agreement between both methods is quite satisfactory, which indicates that the ignition point method is suitable for a fast and also a quiet accurate determination of kinetic contants of exothermic heterogeneous reactions.