William J. Thomson

The effects of steam and carbon dioxide on calcite decomposition using dynamic X-ray diffraction

The effects of steam (H2O) and carbon dioxide (CO2) on the rates of calcite decomposition have been studied using dynamic X-ray diffraction (DXRD). Because the DXRD technique allows for considerably better control of heat and mass transfer complications, intrinsic decomposition rates are obtained for temperatures between 440–560°C and up to H2O and CO2 pressures of 0.2135 and 0.00087 atm, respectively. It is shown that the observed enhancement effect of steam is related to its adsorptive properties which are faster and more significant than CO2 adsorption. This, in turn, leads to the hypothesis of a quantitative Langmuir-Hinshelwood model which is totally compatible with the kinetic data. Separate adsorption experiments show that the effect of steam pressure is limited due to adsorption saturation capacity and the model is used to predict the steam pressure required to produce 95% of the maximum effect of steam as a function of temperature.

Kinetic mechanisms for mullite formation from sol-gel precursors

The reaction kinetics for the formation of mullite (3Al 2 O 3 · 2SiO 2 ) from sol-gel derived precursors were studied using dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA). The reaction kinetics of diphasic and single phase gels are compared and different reaction mechanisms are found for each gel. Mullite formation in the diphasic gel exhibits an Avrami type, diffusion-controlled growth mechanism with initial mullite formation temperatures of about 1250 °C and an activation energy on the order 10 3 kJ/mole. On the other hand, mullite formation from the single phase gel is a nucleation-controlled process with an initial formation temperature of 940 °C and a much lower activation energy of about 300 kJ/mole.

Dynamic X-ray diffraction study of an unsupported iron catalyst in Fischer-Tropsch synthesis

Abstract In situ , Dynamic X-Ray Diffraction (DXRD) has been used to correlate changes in bulk catalyst composition with Fischer-Tropsch activity over unsupported iron catalysts. It was found that the specific iron carbide phase formed was a function of particle size and temperature, with e′-Fe 2.2 C formed over ξ-Fe 2.5 C for smaller particle sizes and lower temperatures. While catalytic activity initially increased with the increase in carbide formation, deactivation was observed to be coincident with the e′ → ξ iron carbide transformation. This and subsequent hydrogen etching experiments indicate that the carbon formed from this transformation may act as nucleation sites for subsequent carbon deposition via the Boudouard reaction. All of the data obtained in this study are consistent with the competition model of Fischer-Tropsch synthesis, but while the carbide model does not appear to be a viable mechanism, the carbide itself may act as the catalyst.

Reduction/oxidation of a high loading iron oxide catalyst

The reduction/oxidation of a high loading iron catalyst supported on γ-Al203 has been studied using dynamic X-ray diffraction (DXRD). In situ determination of changes in the lattice parameters as magnetite was reduced to iron indicated that aluminum was unevenly incorporated into the matrix of the supported iron oxide particles, leading to a form of metal-support interaction and manifested by reduction rates which are much slower than for unsupported magnetite. These results are corroborated in subsequent oxidation experiments by the fact that CO, oxidation rates of supported iron are much higher than for unsupported iron and the Fe203 phase formed in the former at 673 K is γ-Fe203 as opposed to α-Fe203 in the latter. The rate data are modeled and the results are shown to be consistent with the conclusion of nonuniform Al incorporation as well as with previous work on the oxidation of unsupported iron. The incorporation of AI in this Fe/Al203 catalyst is consistent with previous observations related to the form of metal-support interactions in Ni/Al2O3 and in Fe/SiO2.

The effect of sample preparation on the thermal decomposition of CaCO3

Abstract It is pointed out that many of the thermal analysis techniques commonly utilized to measure the rates of thermal decomposition reactions are plagued with problems of mass and energy transport limitations. Because the technique employed here, dynamic X-ray diffraction (DXRD), allows for in situ observations of the solid reactants, intermediates, and products, and for better control of the heat and mass transfer resistance, most of these problems are obviated. Results are presented which not only compare the dramatic differences in global reaction rates between TGA and DXRD experiments, but also demonstrate the effects of sample preparation on calcite decomposition. Specifically, it is shown that sample cleaning, which tends to remove nucleation sites, as well as sample de-gassing, which probably removes water vapor, can lead to very different calcite decomposition rates.

Pulsed gas-phase alkylation of isobutane/2-butene over sulfated zirconia

Abstract A pulsed-feed alkylation study of gas-phase 2-butene with isobutane was conducted over sulfated zirconia (SZ) to determine the effect of temperature and feed composition on the dominant reactions and deactivation mechanisms. For all conditions, high initial conversion of 2-butene to C 5 was observed, which is attributed to alkylation followed by rapid cracking on the strongest acid sites. These sites deactivated quickly, and then the steady-state catalyst behavior was observed to be different at different temperatures. During alkylation at 50°C, the catalyst deactivated after 40 pulses, which is attributed to the build-up of trimethylpentane (TMP) products that do not readily desorb from the catalyst surface at this temperature, and was verified when pulses of pure 2,3,4-TMP over SZ produced similar cracking results. At 100°C, the adsorbed alkylation and/or oligomerization products were very selectively cracked to form 2-butene, resulting in a net dehydrogenation of isobutane to 2-butene. At temperatures in the 150–250°C range, the product selection was dominated by high cracking rates, but the catalytic activity remained stable with nearly complete conversion of 2-butene at the highest temperatures. It was also found that the deactivated catalyst at 50°C could be regenerated by simply heating to 150°C, which apparently removed the adsorbed hydrocarbons by either volatilization or cracking.

MULLITE FORMATION FROM NONSTOICHIOMETRIC SLOW HYDROLYZED SINGLE PHASE GELS

A comparative dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA) study was performed in the investigation of mullite and spinel formation from slowly hydrolyzed single phase gels with Al/Si ratios ranging from 1/1 to 14/1. Both metastable tetragonal mullite and spinel were observed to form at temperatures 1250 °C. As the Al/Si ratio increases, both the tetragonal mullite and spinel formation temperatures decrease while the orthorhombic mullite formation temperature increases. Based on the Al/Si composition where the formation extents of tetragonal mullite and spinel were maximum, their compositions are estimated to be 2Al 2 O 3 · SiO 2 and 6A12 O3 · SiO 2 , respectively.

The influence of steam on mullite formation from sol-gel precursors

The effect of steam on the reaction kinetics of both mullite and metastable spinel formation from sol-gel precursors, having an Al/Si atomic ratio of 3/1, was studied using dynamic x-ray diffraction (DXRD) coupled with differential thermal analysis (DTA). Steam was observed to accelerate the nucleation of tetragonal mullite in single phase gels at ~980 °C. When single phase gels are formed with a faster hydrolysis step, the presence of steam also increases the nucleation of metastable spinel at this temperature, possibly due to the in situ formation of boehmite in locally enriched alumina regions at

Reaction kinetics of the gasification of Michigan Antrim oil shale char

Abstract Both CO 2 and steam gasification kinetics were determined for Michigan Antrim shale char using TGA techniques in combination with on-line gas chromatography. While the reaction kinetic expressions were similar to those previously derived for Green River Formation (western USA) and Swedish Ranstad shale chars, the gasification rates were found to be on the same order as those of Ranstad char but significantly lower than those of western shale char. In addition, the gas produced from steam gasification was found to be significantly higher in CO than the corresponding gas from western shale char. This and the presence of significant quantities of CO 2 lower the attractiveness of Antrim char as a source of synthesis gas or hydrogen.

Potassium catalysed gasification of Kentucky oil shale char

The steam gasification of a Kentucky oil shale char has been studied in a semi-batch fixed bed reactor. The effects of temperature (973–1173 K), catalyst loading (0–15% K2CO3) and pressure (up to 0.65 MPa) on the gasification rates and make-gas composition have been determined. Although gasification rates were increased by a factor of about three in the presence of 10% K2CO3, they were still significantly lower than those previously measured with western shale chars. The reason for this could be attributed to the relatively high hydrogen concentrations produced in the fixed-bed configuration since severe hydrogen inhibition has been previously reported for a similar shale char. There was no significant effect of the potassium catalysis on either the make-gas composition or the quantities of sulphur released during gasification. The only significant influence of pressure was to increase the methane make and this was independent of the catalyst loading.