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Methane Formation (methane + formation)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Catalytic cracking, dehydrogenation, and aromatization of isobutane over Ga/HZSM-5 and Zn/HZSM-5 at low pressures

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2002
Yanping Sun
Isobutane cracking, dehydrogenation, and aromatization over Ga/HZSM-5 and Zn/HZSM-5 has been investigated in a Knudsen cell reactor and the kinetics of the primary reaction steps for isobutene and propene formation have been accurately determined. Although cracking is the dominant reaction channel, with propene and methane being primary products, methane formation is significantly less than propene formation. This indicates that a proportion of the cracking proceeds via Lewis acid attack at CC bonds, and not just via alkanium ion formation at Bronsted acid sites. This is particularly apparent over Zn/HZSM-5. Intrinsic rate constants for cracking, calculated from the rate of propene formation, are and for dehydrogenation, calculated from the rate of isobutene formation, are Large preexponential factors for cracking and dehydrogenation over Ga/HZSM-5 indicate that either the coverage of active sites is significantly less than the coverage of exposed sites or the intrinsic reaction step involves a large entropy change between reactant and transition state. For Zn/HZSM-5 the small preexponential factors suggest either small entropy changes during activation, perhaps initiated by Lewis acid sites, or a steady-state distribution of active and exposed sites is rapidly reached. Differences in intrinsic activation energies may reflect the ratio of Lewis and Bronsted acid sites on the respective catalyst surfaces. Aromatization is more prolific over Ga/HZSM-5 than over Zn/HZSM-5 under the low-pressure conditions. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 467,480, 2002 [source]

Prediction of flammability speciation for the lower alkanes, carboxylic acids, and esters

PROCESS SAFETY PROGRESS, Issue 1 2007
M. Palucis
Abstract A Gibbs energy minimization procedure is used to predict the flammability envelopes of alkanes, carboxylic acids, and acetates. In addition to providing the calculated adiabatic flame temperature (CAFT), the product profiles reveal regions of incomplete combustion products and the onset of methane formation above 0.0001 mole fraction. Temperatures at the predicted onset of methane production appear to be closely related to the temperature at the upper flammability limit (UFL). Although a fixed CAFT value could be related to the lower flammability limit (LFL), it was found that this was not the case with the UFL and only for acetic acid could a single CAFT value of 1200K be used to conservatively predict the flammable region. Rather, in general, a single CAFT value could not conservatively predict the upper flammable region. The predictions also reveal local maxima and minima in the concentrations of reaction products. These maps of incomplete combustion products for the flammability region predict that incomplete combustion products are produced at fuel/oxygen ratios significantly below the fuel/oxygen stoichiometric ratio. This region decreases as the limiting oxygen concentration is approached. © 2006 American Institute of Chemical Engineers Process Saf Prog, 2006 [source]

Syntrophic interactions among anode respiring bacteria (ARB) and Non-ARB in a biofilm anode: electron balances

BIOTECHNOLOGY & BIOENGINEERING, Issue 3 2009
Prathap Parameswaran
Abstract We demonstrate that the coulombic efficiency (CE) of a microbial electrolytic cell (MEC) fueled with a fermentable substrate, ethanol, depended on the interactions among anode respiring bacteria (ARB) and other groups of micro-organisms, particularly fermenters and methanogens. When we allowed methanogenesis, we obtained a CE of 60%, and 26% of the electrons were lost as methane. The only methanogenic genus detected by quantitative real-time PCR was the hydrogenotrophic genus, Methanobacteriales, which presumably consumed all the hydrogen produced during ethanol fermentation (,30% of total electrons). We did not detect acetoclastic methanogenic genera, indicating that acetate-oxidizing ARB out-competed acetoclastic methanogens. Current production and methane formation increased in parallel, suggesting a syntrophic interaction between methanogens and acetate-consuming ARB. When we inhibited methanogenesis with 50 mM 2-bromoethane sulfonic acid (BES), the CE increased to 84%, and methane was not produced. With no methanogenesis, the electrons from hydrogen were converted to electrical current, either directly by the ARB or channeled to acetate through homo-acetogenesis. This illustrates the key role of competition among the various H2 scavengers and that, when the hydrogen-consuming methanogens were present, they out-competed the other groups. These findings also demonstrate the importance of a three-way syntrophic relationship among fermenters, acetate-consuming ARB, and a H2 consumer during the utilization of a fermentable substrate. To obtain high coulombic efficiencies with fermentable substrates in a mixed population, methanogens must be suppressed to promote new interactions at the anode that ultimately channel the electrons from hydrogen to current. Biotechnol. Bioeng. 2009;103: 513,523. © 2009 Wiley Periodicals, Inc. [source]

Zero valent iron as an electron-donor for methanogenesis and sulfate reduction in anaerobic sludge

BIOTECHNOLOGY & BIOENGINEERING, Issue 7 2005
Srilakshmi Karri
Abstract Zero valent iron (ZVI) is a reactive media commonly utilized in permeable reactive barriers (PRBs). Sulfate reducing bacteria are being considered for the immobilization of heavy metals in PRBs. The purpose of this study was to evaluate the potential of ZVI as an electron donor for sulfate reduction in natural mixed anaerobic cultures. The ability of methanogens to utilize ZVI as an electron-donor was also explored since these microorganisms often compete with sulfate reducers for common substrates. Four grades of ZVI of different particle sizes (1.120, 0.149, 0.044, and 0.010 mm diameter) were compared as electron donor in batch bioassays inoculated with anaerobic bioreactor sludge. Methanogenesis was evaluated in mineral media lacking sulfate. Sulfate reduction was evaluated in mineral media containing sulfate and the specific methanogenic inhibitor, 2-bromoethane sulfonate. ZVI contributed to significant increases in methane production and sulfate reductioncompared to endogenous substrate controls. The rates of methane formation or sulfate reduction were positively correlated with the surface area of ZVI. The highest rates of 0.310 mmol CH4 formed/mol Fe0·day and 0.804 mmol SO reduced/ mol Fe0·day were obtained with the finest grade of ZVI (0.01 mm). The results demonstrate that ZVI is readily utilized as a slow-release electron donor for methanogenesis and sulfate reduction in anaerobic sludge; and therefore, has a promising potential in bioremediation applications. © 2005 Wiley Periodicals, Inc. [source]