The periodic transient kinetics method for investigation of kinetic process dynamics under realistic conditions: Methanation as an example

Frequency Response Analysis of the Unsteady-State CO/CO2 Methanation Reaction: An Experimental Study

“…Between the experimental runs the catalyst is regenerated for 4 h under flowing H2 (125 mL STP min −1 ), where the catalyst remained at least for 1 h at 603 K. The data evaluation is performed as described in detail in our recent publication introducing the periodic transient kinetics method. 6 In particular, the external standard Ne is used for determining the outlet molar flow rate. Therefore, the volume variation factor 𝛼 is calculated from the measured (𝑥 Ne ) and the initial (𝑥 Ne,0 ) molar fraction of Ne at the reactor outlet according to eq.…”

“…Hence, the methanation reactor needs to be operated under unsteady-state conditions for such cases with respect to fluctuations in the feed gas composition and volumetric flowrate, which induce a complex non-linear interaction of kinetic processes in the methanation reactor occurring at different length scales. 1,6 Moreover, the input fluctuations can appear stochastically over a wide frequency range, 1 which may result in a complex reactor behavior depending on the input frequencies. 7 Dynamic changes in the inlet composition, however, may also affect the thermal behavior of the methanation reactor, since the hydrogenation of CO and CO2 is highly exothermic.…”

“…19 In order to unfold the unsteady-state reactor behavior, the evaluation of the frequency-dependent reactor response, also referred to as frequency response, is in the focus of the present contribution assuming isothermal conditions. The frequency response can, in principle, be classified based on the ratio between the cycle period duration 𝜏 and the characteristic relaxation time 𝜏 C into the following three regimes: 6,20,21 • quasi-steady-state (qss) regime (𝜏 ≫ 𝜏 C ): The reactor response is able to reach steady-state in each half-period of the forced periodic input, since the kinetic processes are fast enough compared to the cycle period. Thus, the amplitude of the forced input signal is not damped by the system.…”

“…However, for experimental validation of the theoretical predictions a sophisticated experimental setup and respective procedures are required, as we pointed out, recently. 6 The aforementioned NFR method was applied to the methanation reaction and predicts rate enhancement for CO and CO2 methanation. 29,30 For CO methanation various authors have confirmed experimentally that a higher CH4 yield is possible under periodic operation compared to optimized steady-state conditions.…”

“…Therefore, we vary the frequency of the inlet gas composition by switching between a COx-rich and a COx-lean feed gas and evaluate the corresponding reactor response using the Periodic Transient Kinetics (PTK) method introduced in our recent work. 6 In particular, the whole range of CO/CO2 ratios are covered experimentally in order to deduce the effect of competing ad-/desorption and reaction steps involved in simultaneous CO/CO2 methanation on the dynamic response. The observed effect of the forcing frequency is discussed with respect to mechanistic aspects characteristic for both CO and CO2 hydrogenation, whose behavior is considered to be distinct from each other as presumed from our previous steady-state experiments 5 .…”