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Hydrogen Partial Pressures (hydrogen + partial_pressure)

Distribution by Scientific Domains

Chemistry	60%

Selected Abstracts

Performance and population analysis of a non-sterile trickle bed reactor inoculated with Caldicellulosiruptor saccharolyticus, a thermophilic hydrogen producer

BIOTECHNOLOGY & BIOENGINEERING, Issue 5 2009
J.W. van Groenestijn
Abstract Non-axenic operation of a 400 L trickle bed reactor inoculated with the thermophile Caldicellulosiruptor saccharolyticus, yielded 2.8 mol,H2/mol hexose converted. The reactor was fed with a complex medium with sucrose as the main substrate, continuously flushed with nitrogen gas, and operated at 73°C. The volumetric productivity was 22 mmol,H2/(L,filterbed,h). Acetic acid and lactic acid were the main by-products in the liquid phase. Production of lactic acid occurred when hydrogen partial pressure was elevated above 2% and during suboptimal fermentation conditions that also resulted in the presence of mono- and disaccharides in the effluent. Methane production was negligible. The microbial community was analyzed at two different time points during operation. Initially, other species related to members of the genera Thermoanaerobacterium and Caldicellulosiruptor were present in the reactor. However, these were out-competed by C. saccharolyticus during a period when sucrose was completely used and no saccharides were discharged with the effluent. In general, the use of pure cultures in non-sterile industrial applications is known to be less useful because of contamination. However, our results show that the applied fermentation conditions resulted in a culture of a single dominant organism with excellent hydrogen production characteristics. Biotechnol. Bioeng. 2009;102: 1361,1367. © 2008 Wiley Periodicals, Inc. [source]

Modeling of Kinetic Expressions for the Reduction of NOx by Hydrogen in Oxygen-Rich Exhausts Using a Gradient-Free Loop Reactor

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 6 2003
E. Frank
Abstract The reduction of NOx by hydrogen under lean conditions is investigated in a gradient-free loop reactor. Using this computer-controlled reactor, the reaction rates can be measured under exact isothermal conditions. Systematic variation of the input concentrations of hydrogen, nitric oxide, oxygen as well as reaction temperature provides a complete data set of reaction rates for the given reaction system. A number of kinetic rate expressions were evaluated for their ability to fit the experimental data by using toolboxes of MATLAB. The temperature influence on reaction rate constants and adsorption equilibrium constants were correlated simultaneously using Arrhenius and van't Hoff equations, respectively. The kinetic rate expression based on a Langmuir-Hinshelwood-type model describes the data and the model can be improved by introducing a correction term in square root of hydrogen partial pressure over the range of conditions investigated. [source]

Bioenergetics of the formyl-methanofuran dehydrogenase and heterodisulfide reductase reactions in Methanothermobacter thermautotrophicus

FEBS JOURNAL, Issue 1 2003
Linda M. I. De Poorter
The synthesis of formyl-methanofuran and the reduction of the heterodisulfide (CoM-S-S-CoB) of coenzyme M (HS-CoM) and coenzyme B (HS-CoB) are two crucial, H2 -dependent reactions in the energy metabolism of methanogenic archaea. The bioenergetics of the reactions in vivo were studied in chemostat cultures and in cell suspensions of Methanothermobacter thermautotrophicus metabolizing at defined dissolved hydrogen partial pressures (,pH2). Formyl-methanofuran synthesis is an endergonic reaction (,G°, = +16 kJ·mol,1). By analyzing the concentration ratios between formyl-methanofuran and methanofuran in the cells, free energy changes under experimental conditions (,G,) were found to range between +10 and +35 kJ·mol,1 depending on the pH2 applied. The comparison with the sodium motive force indicated that the reaction should be driven by the import of a variable number of two to four sodium ions. Heterodisulfide reduction (,G°, = ,40 kJ·mol,1) was associated with free energy changes as high as ,55 to ,80 kJ·mol,1. The values were determined by analyzing the concentrations of CoM-S-S-CoB, HS-CoM and HS-CoB in methane-forming cells operating under a variety of hydrogen partial pressures. Free energy changes were in equilibrium with the proton motive force to the extent that three to four protons could be translocated out of the cells per reaction. Remarkably, an apparent proton translocation stoichiometry of three held for cells that had been grown at pH2<0.12 bar, whilst the number was four for cells grown above that concentration. The shift occurred within a narrow pH2 span around 0.12 bar. The findings suggest that the methanogens regulate the bioenergetic machinery involved in CoM-S-S-CoB reduction and proton pumping in response to the environmental hydrogen concentrations. [source]

Kinetic study of carbon nanotubes synthesis by fluidized bed chemical vapor deposition

AICHE JOURNAL, Issue 2 2009
R. Philippe
Abstract Multi-walled carbon nanotubes (MWCNTs) have been produced with high selectivity by fluidized bed catalytic chemical vapor deposition from ethylene on Fe/Al2O3 catalysts. The influence of operating parameters such as deposition duration, temperature, ethylene and hydrogen partial pressures, and iron loading on MWCNT productivity, process selectivity, characteristics of final powders, and chemical composition of the outlet gases has been analyzed. Using gas phase chromatography, methane and ethane have been detected, whatever are the conditions used. Between 650 and 750°C, no catalyst deactivation occurs because nucleation remains active all along the synthesis, thanks to the explosion of the catalyst grains. Above 650°C, ethane itself produces MWCNTs, whereas methane does not react in the temperature range, 550,750°C. The formation of MWCNTs induces marked bed expansions and sharp decreases of grain density. Apparent kinetic laws have been deduced from the collected data. The apparent partial orders of reaction for ethylene, hydrogen, and iron were found to be 0.75, 0, and 0.28, respectively. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source]

In situ gravimetric monitoring of decomposition rate on the surface of (0001) c-plane sapphire for the high temperature growth of AlN

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007
K. Akiyama
Abstract The thermal stability of (0001) sapphire was investigated at atmospheric pressure using the in situ gravimetric monitoring (GM) method. The weight change of a sapphire substrate was monitored at various hydrogen partial pressures in carrier gas (P) at temperatures over 1200 °C. Although the sapphire substrate was stable up to 1450 °C in an inert carrier gas (P = 0.0 atm), sapphire decomposition started to occur at 1200 °C in H2 carrier gas (P = 1.0 atm). Moreover the activation energy and order of reaction for sapphire surface decomposition changed at approximately 1300 °C. These results indicate that the rate-limiting reaction for sapphire decomposition shifts near 1300 °C. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]