Modeling spatially resolved data of methane catalytic partial oxidation on Rh foam catalyst at different inlet compositions and flowrates
Graphical abstract
Spatially resolved species and temperature profiles measured for a wide range of inlet stoichiometries and flowrates are compared with microkinetic numerical simulations to investigate the effect of transport phenomena on the catalytic partial oxidation of methane on Rh foam catalysts.
Experimental (left panel) and calculated (right panel) species profiles at a total inlet flowrate of F = 5 slpm and feed stoichiometries of C/O = 0.8, 1.0 and 1.3 (lighter to darker colors). CH4, O2 and H2, CO, CO2 mole fractions.
Research highlights
► New spatial profiles of CPOM at different C/O ratios and flowrates are available. ► The reactor model includes transport phenomena and a detailed surface chemistry. ► The model reproduces the experimental behavior well, both qualitatively and quantitatively. ► Sensitivity analyses to catalytic area and kinetic parameters are provided. ► Discussions on the dominant reaction regime and on the path to H2 formation have been touched.
Introduction
Methane catalytic partial oxidation (CPO) to synthesis gas is an alternative to other energy-intensive technologies for industrial exploitation of natural gas. Its optimal utilization requires a deep insight into the underlying chemistry. Even though the reaction has been investigated for the past two decades [1], [2], [3], [4], [5], there are still open questions and further scientific investigation is required. Methane oxidation kinetics are very complicated, yet they are the simplest among the hydrocarbon oxidation processes. Since both exothermic and endothermic reactions are involved, a temperature variation does not translate directly to a reactant conversion, and energy and mass balances are deeply coupled. Therefore, CPO is a challenging and fascinating application where chemistry and transport phenomena are intrinsically connected with each other, determining the final product distribution. Appropriate modeling is needed to reproduce these complex features and to derive a molecular understanding of the reaction mechanism.
Although there are many possible elementary reactions in the methane CPO system, the number of global reactions which may take place is not that high because the system is constrained to give only six products: CH4, CO2, CO, H2, H2O and O2 (higher hydrocarbons are commonly not detected). Some of the possible exothermic oxidation reactions ranging from total oxidation to partial oxidation are listed below.
After oxygen is consumed, there are only three global reactions which can take place (only two of them independent): steam reforming (SR), water gas shift (WGS) and dry reforming (DR), even if there is experimental evidence that DR does not occur on Rh and is a result of the combination of SR and WGS [6], [7], [8], [9].
The interplay between chemical and physical processes has been acknowledged to be a fundamental issue in the modeling [10], [11] of fast, exothermic reactions. Plug flow reactor (PFR) models are unsatisfactory for catalytic combustion applications because of the excessive simplifications in heat and mass transport; full computational fluid dynamics (CFD) models or at least lumped models accounting for transport phenomena [12], [13], [14] are required.
The reaction rate of species involved in very fast reactions will be dominated by the system’s mass transfer characteristics. However, for the other slower reactions, a good kinetic model and accurate assumption for active surface area are required. It is obvious that for an optimal kinetic study, all the species should be in kinetic control and also a safe determination of the reaction (surface) temperature is necessary. Only if the temperature is well captured by the model is a proper kinetic study possible. The dominant regime is investigated for oxygen and methane, at every flowrate, comparing the mass transfer coefficient and the kinetic constant of the pseudo-first-order reaction, i.e. the consumption rate divided by the concentration.
The reaction path influences and is in turn influenced by the temperature profile. This means that even if chemical kinetics are accurately described, incorrect species profiles will result unless a suitable temperature profile is used. However, the latter is not easy to model, because it is affected simultaneously by several physical transport processes. Heat released by the reaction [15] is transported by gas convection and conduction (minor gas phase influence of radiation) and solid conduction and radiation. Given the exponential temperature dependence of the kinetics on the one hand and the complexity of the phenomena contributing to the heat balance on the other hand, the kinetics will be more influenced by the temperature profile than vice versa. Therefore, a model capable of reproducing the temperature profile will reasonably predict species profiles, in particular far from the catalyst entrance.
At relatively long residence times, the kinetics slow down and the exit composition approaches equilibrium, which depends only on temperature. The authors showed [14] that also the PFR model correctly describes the exit composition, with the correct exit temperature, even if the onset of the calculated profiles diverges greatly from the experimental data at the inlet of the catalyst. Several studies have been published where a good description of the temperature profile led to a reasonable prediction of gas products, even without accounting for mass transfer limitations ([16] and partly in [17]) or for a detailed kinetic scheme [18], [19], [20]. On the other side, [21] provides an example of detailed modeling, accounting for both mass transfer and kinetics, that shows disagreement between calculated and measured product distribution, at the early stage of ignition, as well as of the temperature, presumably because of an inaccurate description of the energy balance. In our previous work [14], some underestimation of the solid temperature in the oxidation zone might be contributing to an underestimation of CO selectivity, which indeed increases with higher surface temperature [16]. This effect of temperature on CO selectivity is the combined result of a rapid desorption of CO and a low concentration of gas phase O2 at the catalyst surface due to transport resistances in the film, which could cause irreversible further oxidation to CO2. CO2 has a very low probability to re-adsorb on the surface and to dissociate back into CO(s) and O(s), which is reflected in the used mechanism by the very low sticking coefficient. As a consequence, CO2 reforming is rather unlikely and a direct path can be postulated for CO production. Contrary to CO2, water has a high probability to adsorb and react in the steam reforming reaction, so that both direct and indirect paths are possible. The route to hydrogen is one of the open issues that has been investigated over the years and will be discussed in light of the results available from the model. In the present work, we use the same model as in [14] to describe the profiles at different stoichiometries and flowrates and provide an additional sensitivity analysis on the most uncertain parameters, which are the catalytically active surface-to-volume ratio and the kinetic constants of the surface mechanism.
Section snippets
Experimental data
The reactor setup to measure spatially resolved species and temperature data was presented in detail in a previous work [22]. Composition profiles of species CH4, CO, CO2, H2 and O2 were measured with a capillary sampling technique and mass spectrometry (MS). The H2O profile was not measured; it was obtained from H and O balances, using a mean square error technique. Since the continuity equation can be applied to every single atomic species, both differences in the H and O atom balance can be
Numerical modeling: results and discussion
Spatially resolved experimental data as presented previously present a unique opportunity to pointwise validate numerical predictions from detailed models. Recently, new reactor configurations allowed 2D/3D inspection of the catalysts [26], [27], but for the first time, the described capillary sampling technique gave very high resolution species profiles in this geometry [28]. While it is certainly interesting and exciting to simply collect and discuss these profiles, the ability to compare
Discussion
The capillary sampling technique is very useful for gathering data in short contact time catalytic partial oxidation processes. The measured species and temperature profiles offer a unique opportunity for interpreting the data with detailed models, recognizing the contribution of physical and chemical phenomena, through the implementation of the more relevant transfer phenomena and the inclusion of a micro kinetic mechanism. Such a comparison allows in a first step the refinement of the model
Conclusions
In this work, original experiments at different C/O ratios in the feed and total flowrates are presented and critically compared with model predictions. We developed a pseudo-1D model, including solid, bulk gas and boundary layer mass and energy balances accounting for axial heat conduction and diffusion, and radiation. The micro kinetic model for CH4 oxidation on Rh, taken from the literature, includes adsorption, desorption and surface reactions.
The experimental data at different
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