AN EXPERIMENTAL AND THEORETICAL STUDY OF ISOTHERMAL CONCENTRATION FORCING, USING HIGH-PRESSURE AMMONIA SYNTHESIS AS AN EXAMPLE

Enhanced production rates over the optimal steady state rate are possible in chemical reactors operated under forced-concentration cycling conditions. This study investigates such production-rate enhancements, using the high-pressure ammonia synthesis reaction as an example.; Square-wave concentration forcing experiments have been performed in isothermal, fixed-bed, plug-flow reactors, near industrial pressures. We find that, within the parameter ranges explored, cycling between pure reactants results in decreased time-averaged production rates, while mixture cycling under appropriate conditions results in improvements over the optimal steady-state rate.; Mathematical modelling approximations which simplify the plug-flow reactor equations are considered, and the phenomenon of enhanced chemisorption under reaction conditions is addressed with a novel effective-driving-pressure model.; The kinetics of ammonia synthesis on promoted industrial iron catalysts are not well understood. Although classic models, such as the one proposed by Temkin and Pyzhev (1940), fit steady-state data and are widely used in commercial reactor design, they fail to describe observed reactor dynamics. A phenomenological adsorption/desorption (A/D) model is developed, the parameters of which are fit to experimental data from a differential reactor. This model predicts observed experimental integral reactor steady-state and forced-cycling behavior for several temperatures and pressures. Moreover, the model obeys thermodynamic equilibrium constraints. The existence of "stored" nitrogen in the bulk phase of iron catalysts is confirmed, but we show that it does not directly affect either steady-state or dynamic ammonia production. Also, several potential pitfalls in kinetic modelling are exposed.