The role of the Boudouard and water-gas shift reactions in the methanation of CO or CO2 over Ni/γ-Al2O3 catalyst

The Boudouard and the water-gas shift reactions were studied at different temperatures between 453 and 490 K over a Ni/γ-Al2O3 catalyst in a Carberry batch reactor using various mixtures of CO, H2 and CO2. The activity of the Boudouard reaction was found to be low, compared to the water-gas shift reaction, and diminished over time, suggesting that the temperature was too low for significant activity after an initiation period of CO adsorption. Furthermore, the rate of the Boudouard reaction has been reported to decrease in the presence of H2O and H2. The water-gas shift reaction was found to be the main reaction responsible for the production of CO2 in a mixture of CO, H2 and H2O in the batch reactor. The ratio of the total amount of CO consumed to the total amount of CO2 produced showed that the catalyst was also active towards hydrogenation, where the rate of the hydrogenation reaction was very much faster than the water-gas shift reaction. The resulting ratio of pH2 to pCO was found to be extremely low, probably leading to the production of long-chain hydrocarbons. The stoichiometry of the overall reaction was such that for every mole of mole of CO2 produced, 1.5 mol of CO was consumed in the batch reactor. Kinetic studies were performed in the batch reactor. An Eley-Rideal mechanism was found to provide a good agreement with the experimental results over a wide range of partial pressures of steam and CO

Kinetic studies of CO2 methanation over a Ni/γ-Al2O3 catalyst using a batch reactor

The methanation of CO2 was investigated over a wide range of partial pressures of products and reactants using a gradientless, spinning-basket reactor operated in batch mode. The rate and selectivity of CO2 methanation, using a 12 wt% Ni/γ–Al2O3 catalyst, were explored at temperatures 453–483 K and pressures up to 20 bar. The rate was found to increase with increasing partial pressures of H2 and CO2 when the partial pressures of these reactants were low; however, the rate of reaction was found to be insensitive to changes in the partial pressures of H2 and CO2 when their partial pressures were high. A convenient method of determining the effect of H2O on the rate of reaction was also developed using the batch reactor and the inhibitory effect of H2O on CO2 methanation was quantified. The kinetic measurements were compared with a mathematical model of the reactor, in which different kinetic expressions were explored. The kinetics of the reaction were found to be consistent with a mechanism in which adsorbed CO2 dissociated to adsorbed CO and O on the surface of the catalyst with the rate-limiting step being the subsequent dissociation of adsorbed CO

11-interval PFG pulse sequence for improved measurement of fast velocities of fluids with high diffusivity in systems with short T2*

Magnetic resonance (MR) was used to measure SF6 gas velocities in beds filled with particles of 1.1 mm and 0.5 mm in diameter. Four pulse sequences were tested: a traditional spin echo pulse sequence, the 9-interval and 13-interval pulse sequence of Cotts et al. (1989) and a newly developed 11-interval pulse sequence. All pulse sequences measured gas velocity accurately in the region above the particles at the highest velocities that could be achieved (up to 0.1 m s-1). The spin echo pulse sequence was unable to measure gas velocity accurately in the bed of particles, due to effects of background gradients, diffusivity and acceleration in flow around particles. The 9- and 13-interval pulse sequence measured gas velocity accurately at low flow rates through the particles (expected velocity < 0.06 m s-1), but could not measure velocity accurately at higher flow rates. The newly developed 11-interval pulse sequence was more accurate than the 9- and 13-interval pulse sequences at higher flow rates, but for velocities in excess of 0.1 m s-1 the measured velocity was lower than the expected velocity. The increased accuracy arose from the smaller echo time that the new pulse sequence enabled, reducing selective attenuation of signal from faster moving nuclei

Nuclear magnetic resonance measurements of velocity distributions in an ultrasonically vibrated granular bed

This paper was accepted for publication in the journal Philosophical Transactions of The Royal Society A-Mathematical Physical and Engineering Sciences and the definitive published version is available at http://dx.doi.org/10.1098/rsta.2013.0185We report the results of nuclear magnetic resonance (NMR) imaging experiments on granular beds of mustard grains fluidised by vertical vibration at ultrasonic frequencies. The variation of both granular temperature and packing fraction with height was measured within the three-dimensional cell for a range of vibration frequencies, amplitudes and numbers of grains. Small increases in vibration frequency were found – contrary to the predictions of classical ‘hard-sphere’ expressions for the energy flux through a vibrating boundary – to result in dramatic reductions in granular temperature. Numerical simulations of the grain-wall interactions, using experimentally-determined Hertzian contact stiffness coefficients, showed that energy flux drops significantly as the vibration period approaches the grain-wall contact time. The experiments thus demonstrate the need for new models for ‘soft-sphere’ boundary conditions at ultrasonic frequencies

Compressed sensing reconstruction improves sensitivity of variable density spiral fMRI

Purpose Functional MRI (fMRI) techniques that can provide excellent blood oxygen level dependent contrast, rapid whole brain imaging, and minimal spatial distortion are in demand. This study explored whether fMRI sensitivity can be improved through the use of compressed sensing (CS) reconstruction of variable density spiral fMRI. Methods Three different CS-reconstructed 1-shot variable density spirals were explored (corresponding to 28%, 35%, and 46% under-sampling), and compared with conventional 1-shot and 2-shot Archimedean spirals acquired using matched echo time and volume repetition time. fMRI maps were reconstructed with or without CS MRI and sensitivity was compared using identically matched voxels. Results The results demonstrated that an l 1-norm based CS reconstruction only led to an increase in functional contrast when applied to 28% under-sampled data. A whole brain t-contrast map revealed that 2-shot uniformly sampled spiral and 28% under-sampled spiral data reconstructed with CS yield equivalent sensitivity, even with matched echo time and volume repetition time Conclusion VD spiral exhibits a useful operating range, in the region of 25-30% under-sampling, for which CS reconstruction can be used to increase the sensitivity of fMRI to brain activity. Using CS, VD acquisitions achieve the same sensitivity as 2-shot Archimedean acquisitions, but require only a single shot. Magn Reson Med 70:1634-1643, 2013. \ua9 2013 Wiley Periodicals, Inc. Copyright \ua9 2013 Wiley Periodicals, Inc.Peer reviewed: YesNRC publication: Ye

Nuclear magnetic resonance measurements of velocity distributions in an ultrasonically vibrated granular bed

We report the results of nuclear magnetic resonance (NMR) imaging experiments on granular beds of mustard grains fluidised by vertical vibration at ultrasonic frequencies. The variation of both granular temperature and packing fraction with height was measured within the three-dimensional cell for a range of vibration frequencies, amplitudes and numbers of grains. Small increases in vibration frequency were found – contrary to the predictions of classical ‘hard-sphere’ expressions for the energy flux through a vibrating boundary – to result in dramatic reductions in granular temperature. Numerical simulations of the grain-wall interactions, using experimentally-determined Hertzian contact stiffness coefficients, showed that energy flux drops significantly as the vibration period approaches the grain-wall contact time. The experiments thus demonstrate the need for new models for ‘soft-sphere’ boundary conditions at ultrasonic frequencies