Dynamic modeling and simulations of the behavior of a fixed‐bed reactor‐exchanger used for CO₂ methanation

Abstract

A multidimensional heterogeneous and dynamic model of a fixed-bed heat exchanger reactor used for CO 2 methanation has been developed in this work that is based on mass, energy and momentum balances in the gas phase and mass and energy balances for the catalyst phase. The dynamic behavior of this reactor is simulated for transient variations in inlet gas temperature, cooling temperature, gas inlet flow rate, and outlet pressure. Simulation results showed that wrong-way behaviors can occur for any abrupt temperature changes. Conversely, temperature ramp changes enable to attenuate and even fade the wrong-way behavior. Traveling hot spots appear only when the change of an operating condition shifts the reactor from an ignited steady state to a non-ignited one. Inlet gas flow rate variations reveal overshoots and undershoots of the reactor maximum temperature.2) Wall thermal conductivity, k w (stainless steel) 17 W m 21 K 21 Catalyst thermal conductivity, k s 9 0.87 W m 21 K 21 Catalyst particle density, q s 58 1274 kg m 23 472

Transient Flow Rate Ramps for Methanation of Carbon Dioxide in an Adiabatic Fixed‐Bed Recycle Reactor

“…Consequently, the cold portion of the catalyst bed between both steady states has to be heated during the transient phase, which induces a local temperature minimum moving wave‐like downstream until the new steady‐state is reached. In contrast to polytropically operated reactors, the minimum reaches the reactor outlet. The shape of the temperature profile also broadens while moving toward the reactor outlet, even though axial dispersion was not considered directly in the heat and mass balance.…”

“…With the aim to reduce the costs of the hydrogen storage by minimizing upstream storage capacities (up to 8% of the total investment costs can be saved), the unsteady‐state operation of the exothermic methanation reaction is demanded. However, the occurring phenomena and operation strategies under dynamic conditions are not exhaustively addressed in scientific literature thus far, but receive more attention in recent years . One promising approach is adiabatically operated fixed‐bed reactors, which show a good partial and excess load behavior, since the chemical equilibrium is reached within the residence time.…”

“…Consequently, a front with an elevated reactant concentration reaches the hotter part of the reactor and a hot spot is formed by the heat release of the excess reactants . Applied to the unsteady‐state methanation, van Doesburg and de Jong as well as Try et al detected the wrong‐way behavior for changes of the inlet temperature. Similarly, Sun and Simakov reported moving thermal fronts propagating in the reactor.…”

“…However, the occurring phenomena and operation strategies under dynamic conditions are not exhaustively addressed in scientific literature thus far, but receive more attention in recent years. [6][7][8][9][10][11][12][13][14][15][16][17] One promising approach is adiabatically operated fixed-bed reactors, which show a good partial and excess load behavior, since the chemical equilibrium is reached within the residence time. The variation of the inlet volumetric flow rate indeed leads to a transition of the axial steady-state profiles, though without changing the outlet temperature and composition.…”

Applying Reaction Kinetics to Pseudohomogeneous Methanation Modeling in Fixed‐Bed Reactors

“…However, when simulating large chemical reactors or complex geometries, shorter computation times are desired. Therefore, continuum models, which do not account for the detailed structure of the bed, are used frequently in methanation applications . In continuum models, balances can be accounted for by a single phase in pseudohomogeneous reactor models , or by the two phases separately in heterogeneous reactor models .…”