A high-speed method for obtaining kinetic data for exothermic or endothermic catalytic reactions under non-isothermal conditions illustrated for the ammonia synthesis

摘要

A detailed analysis of the method for fast acquisition of kinetic data by means of a polythermal-temperature-ramping reactor (PTR) as suggested by Wojciechowski in 1995 was performed. The ammonia synthesis reactio over a commercial Fe catalyst (BASF) was taken as an example. Kinetic data evaluation was based on a kinetic model as reported by Sehested et al. in 1999. As a result of data analysis, the experiemtnal molar rate of change of ammonia could be described adequately over a wide range of N_2 and H_2 partial pressures as well as temperature. The pre-exponential factors, activation energies and adsorption enthalpies are in reasonable agreement with data for promoted iron catalysts reported in literature. Accordingly, the PTR method was evaluated as a efficient tool for kinetic data analysis. The number of experiments including different initial conditions can be significantly diminished since PTR experiments have not to follow the strict sequence of varying one experimental parameter after the other by keeping all others constant. The collected exit concentrations, exit temperature and contact time are correlated by spline-functions which allow the extraction of functional relationships required for derivation of rate expressions. If the steady-state of the catalyst is rapidly attained, temperature ramping alollows fast continuous data acquisition. Thus, the PTR method has the potential of significantly reducing the time needed for determining reliable kinetics over a wide range of operating conditions.