We compare Rh and Pt as catalysts for the partial oxidation of methane to syngas at millisecond contact times. The basis for the comparison are species and temperature profiles, with a spatial ...resolution of about 300 μm measured along the centerline of an adiabatically operated metal-coated
α-Al
2O
3 foam using a capillary sampling technique with mass spectrometric species measurement. Gas temperature profiles are measured with a thermocouple. Investigated stoichiometries range from C/O
=
0.6 to 2.6 at constant flow rate of 4.7 slpm and atmospheric pressure. Rh and Pt are compared with respect to (i) profile development at syngas stoichiometry, (ii) profile development at varying stoichiometries from C/O
=
0.6–2.6, (iii) product selectivities and yields in the oxidation zone, (iv) contribution of partial oxidation and steam reforming to the final syngas yield, (v) mass transport limitations, and (vi) approach to thermodynamic equilibrium. Independent of C/O and metal, all profiles show an oxidation zone and a steam-reforming zone. H
2 and CO are formed in the presence of gas-phase oxygen by partial oxidation and in the absence of gas-phase oxygen by steam reforming. CO
2 reforming is not observed. At the same C/O, H
2 and CO selectivities and yields are higher in the oxidation zone on Rh than on Pt. As the C/O ratio increases, the catalyst temperature decreases and selectivities to H
2 and CO in the oxidation zone decrease. The decrease is larger on Pt than on Rh. Because Rh is also the better steam-reforming catalyst, H
2 and CO yields are generally higher on Rh than on Pt. The rate of O
2 conversion at the catalyst entrance is largely mass transport-controlled on Rh but not on Pt. In the oxidation zone on Pt, the methane CPO is kinetically controlled with a constant reaction rate. An average O
2 mass transport coefficient is calculated and compared with literature values on foam catalysts. Finally, exit species flow rates and temperatures are compared with thermodynamic calculations at constant pressure and enthalpy. Rh brings the methane oxidation close to equilibrium if
C/O
⩽
1.0
, whereas Pt reaches equilibrium only at very high catalyst temperatures if
C/O
⩽
0.7
. At higher C/O, deviations from equilibrium are observed mainly because steam-reforming slows, but also because water–gas shift equilibrium is not established.
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.
CH
4
+
O
2
=
H
2
+
CO
+
H
2
O
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). CH
4, O
2 and H
2, CO, CO
2 mole fractions.
Display omitted
► 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 H
2 formation have been touched.
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. In agreement with the experimental data, the species profiles calculated at different C/O inlet stoichiometries show that both partial oxidation products (H
2, CO) and total oxidation products (H
2O, CO
2) are formed in the presence of oxygen. At the leaner stoichiometries, both oxygen and methane react in the diffusive regime at the catalyst entrance. At the richest methane stoichiometry (high C/O), surface temperatures are lower and methane consumption is only partly determined by transport. For all stoichiometries, a kinetically controlled regime prevails in the downstream reforming zone after O
2 is fully consumed. The effect of increasing the flowrate shifts all species profiles downstream and also slightly modifies the shapes of the axial profiles, due to the different effectiveness of heat and mass transfer. Despite enhanced mass transfer and increased surface temperature, the shortened contact time causes a reduced CH
4 conversion at high flowrates. The effect of flowrate on the dominant regime is investigated, for both reactants, comparing the resistances calculated in the pure transport regime and in the pure kinetic regime. From a chemical point of view, the model allows for the analysis of the reaction path leading to hydrogen. Due to inhibition of H
2O re-adsorption, it can be proven that H
2 can be a primary product even in the presence of gas phase O
2. The analysis of the surface coverages shows analogous effects on the profiles when decreasing C/O or increasing flow, because in both cases the surface temperature is increased. Syngas selectivity was also evaluated, both from measured and calculated profiles. S
H2 is well described by the model at each stoichiometry and flowrate, while S
CO is underestimated in every case. From this work, it is also indicated that the Rh catalyst works with CO (measured) selectivities higher than equilibrium. Carbon dioxide only forms in the oxidation zone, for C/O
=
1 and 1.3, but in the rest of the catalyst zone, there is no further production despite what would be expected from equilibrium. This confirms Rh does not catalyze the water gas shift reaction. On the other hand, at C/O
=
0.8, this reaction becomes active, due to the higher temperature, and the CO
2 is also produced in the reforming zone. This suggests that CO
2 will not rise after the oxidation section if the surface temperature is kept sufficiently low. Sensitivity analyses to the active catalytic surface and to the kinetic parameters are provided.
Hard X-ray micro-tomography is a powerful tool to reveal the internal structure of thick objects in a non-destructive manner. For synchrotron applications and practical lab works with students, a 2D ...detector based on a thin YAG:Ce scintillator optically coupled to a new generation high frame rate (100 fps) low noise sCMOS camera has been developed and characterized in depth on the METROLOGIE and PSICHE beamlines of the SOLEIL synchrotron. The Detector gain, Modulation Transfer Function, Noise Power Spectrum and Detective Quantum Efficiency have been measured and compared with analytical model. A tomography reconstruction was performed on small insects to demonstrate the performance of this X-ray imaging detector.
A 2 m long 18 mm period Cryogenic Permanent Magnet Undulator (CPMU) has been constructed at SOLEIL. Praseodymium was chosen instead of Neodymium magnetic material, because of the absence of the Spin ...Reorientation Transition phenomenon. The use of Pr2Fe14B with a remnence Br of 1.35 T at room temperature enables to increase the peak magnetic field at 5.5 mm minimum gap, from 1.04 T at room temperature to 1.15 T at a cryogenic temperature of 77 K. The magnetic field reaches 1.91 T at a gap of 3 mm in case of FELs applications. Different corrections were performed first at room temperature to adjust the phase error, the electron trajectory and to reduce the multipolar components. A dedicated magnetic measurement bench to check the magnetic performance of the undulator at low temperature has been designed and assembled inside the vacuum chamber. The results of the magnetic measurements at low temperature and at room temperature are compared. The CPMU has been installed and commissioned in the storage ring.
The mechanical behavior of human hair fibers is determined by the interactions between keratin proteins structured into microfibrils (hard
α-keratin intermediate filaments), a protein sulfur-rich ...matrix (intermediate filaments associated proteins), and water molecules. The structure of the microfibril-matrix assembly has already been fully characterized using electron microscopy and small-angle x-ray scattering on unstressed fibers. However, these results give only a static image of this assembly. To observe and characterize the deformation of the microfibrils and of the matrix, we have carried out time-resolved small-angle x-ray microdiffraction experiments on human hair fibers stretched at 45% relative humidity and in water. Three structural parameters were monitored and quantified: the 6.7-nm meridian arc, which is related to an axial separation between groups of molecules along the microfibrils, the microfibril’s radius, and the packing distance between microfibrils. Using a surface lattice model of the microfibril, we have described its deformation as a combination of a sliding process and a molecular stretching process. The radial contraction of the matrix is also emphasized, reinforcing the hydrophilic gel nature hypothesis.
Shaping a YAG scintillator crystal into a truncated-ball lens enables to image with a high numerical aperture its front surface, where the image converted from x-rays to visible is localized. Hence, ...both resolution and luminosity gains can be expected. Moreover if the plane surface is set at the Young-Weierstrass point of the spherical refractive surface, stigmatic imaging is achieved. On this principle, we have constructed an imaging detector from a 10 mm diameter YAG:Ce sphere and a long working distance plane-apochromatic microscope objective which does not limit the numerical aperture. The effective numerical aperture of the built system is 1.08, giving a Rayleigh resolution limit of 0.3 μm. Images of test objects (diatoms) have been recorded, in contact mode, with 103 eV. Periodic features of 0.4 μm pitch are visible on these images. The field of view is close to 200μm. The device is intended as an aid for X-ray optics fine-tuning and characterization.
We experimentally and theoretically studied cellulose pyrolysis at high temperature and short residence time. We investigated the gas phase chemistry with dedicated experiments and feeding ...intermediates. Results have been also compared with equilibrium calculations, both single (gas) phase and allowing for solid C formation. Our aim was to understand the cellulose degradation mechanism and particularly the role of gas phase chemistry. We provided evidence of a simplified mechanism, where CO formation is a first, fast step that can be related to levoglucosan ring opening, while H2 comes from a totally different route, based on hydrocarbon reforming reactions, which also provide further CO. In addition, butadiene was identified as a key intermediate in the decomposition sequence. The different paths and rates of CO formation and H2 formation explain why the ratio of CO to H2 is not constant, particularly at short residence time. A two-stage process or longer contact time is required, if aiming at syngas production.
Using spatially resolved measurements of temperature and concentration, we critically analyzed the chemistry and transport limitations in the partial oxidation of methane (POM) reaction carried out ...on Rh, supported on a foam catalyst. The analysis was based on two models, both sharing a detailed surface chemistry but with different gas–surface transport processes. The simulation neglecting transport limitations correctly predicts the outlet concentrations, apparently because of the approach to equilibrium, but significant disagreement was found along the catalysts, particularly in the initial region, demonstrating the existence of regions in which strong diffusive limitations prevail. We developed a pseudo-1D model that can differentiate the species and temperature in the bulk of the gas and at the surface and describe heat (including radiation) and mass transport through correlations with ad hoc parameters based on experimental studies. With this model, we correctly predicted the profiles along the reactor for all species. Only CO
2 had a relevant relative error, but its composition was very low. The solid temperature was well reproduced as well, whereas the gas temperature was somewhere higher than the experimental temperature, possibly due to overestimation of the heat transport coefficient. Analysis of the transport limitations found that O
2 and H
2O had large concentration gradients between gas and surface due to their involvement in the total oxidation, which is a very fast reaction. The analysis thus demonstrated that production and consumption rates at the catalytic surface were frequently sufficiently high so as to enter a diffusive regime. Accordingly, we highlight the need to augment the implementation of detailed surface chemistry with some accounting of the transport processes of both mass and heat. In addition, we show that the Chilton–Colburn analogy can be seriously misleading under these conditions of locally fast heterogeneous kinetics.
A honeycomb monolith with a Pt/γ-Al
2O
3 catalyst has been tested in order to highlight and explain the bifurcation behavior of catalytic methane (partial) oxidation under process conditions, ...elucidating the light off behavior. With CH
4/O
2=1 (rich mixture) and different heating/cooling policies it has been shown that hysteresis occurs only after complete ignition of the catalyst. The dependence of hysteresis on the feed composition was studied, showing that it disappears with lean mixtures, has its maximum for slightly richer than stoichiometric mixtures and then decreases with much richer composition. At the same time, the ignition temperature lowers with increasing CH
4/O
2 ratio. Finally, we suggest a procedure to decrease the ignition temperature by taking advantage of the hysteretic behavior, gradually tuning the feed composition, with the only limit given by the thermal insulation capabilities.
In this work, steam reforming of natural gas is investigated with an application to a small-scale hydrogen production unit of industrial interest, coupled to PEM fuel cells. We carried out a ...thermodynamic analysis to estimate the efficiency of this overall process, which consists of a reformer reactor and two water gas shift reactors. We compare two different configurations: physical (PSA) or chemical (PROX) removal of CO from syngas. We find a similar global efficiency for both processes, although the PSA version has a relatively better performance.
PDF
