Hydrogen Consumption Rate Research Articles

Hydrogenases in sulfate-reducing bacteria function as chromium reductase.

The ability of sulfate-reducing bacteria (SRB) to reduce chromate VI has been studied for possible application to the decontamination of polluted environments. Metal reduction can be achieved both chemically, by H(2)S produced by the bacteria, and enzymatically, by polyhemic cytochromes c(3). We demonstrate that, in addition to low potential polyheme c-type cytochromes, the ability to reduce chromate is widespread among [Fe], [NiFe], and [NiFeSe] hydrogenases isolated from SRB of the genera Desulfovibrio and Desulfomicrobium. Among them, the [Fe] hydrogenase from Desulfovibrio vulgaris strain Hildenborough reduces Cr(VI) with the highest rate. Both [Fe] and [NiFeSe] enzymes exhibit the same K(m) towards Cr(VI), suggesting that Cr(VI) reduction rates are directly correlated with hydrogen consumption rates. Electron paramagnetic resonance spectroscopy enabled us to probe the oxidation by Cr(VI) of the various metal centers in both [NiFe] and [Fe] hydrogenases. These experiments showed that Cr(VI) is reduced to paramagnetic Cr(III), and revealed inhibition of the enzyme at high Cr(VI) concentrations. The significant decrease of both hydrogenase and Cr(VI)-reductase activities in a mutant lacking [Fe] hydrogenase demonstrated the involvement of this enzyme in Cr(VI) reduction in vivo. Experiments with [3Fe-4S] ferredoxin from Desulfovibrio gigas demonstrated that the low redox [Fe-S] (non-heme iron) clusters are involved in the mechanism of metal reduction by hydrogenases.

Kinetic Studies on Karlıova Coal

In this study, reaction kinetics of the liquefaction of Karliova coal in a process development unit having three reactors in series have been studied at temperatures of 530–570°C and pressures of 15–25 MPa. It is shown that the rate of hydrogen consumption can be expressed as a function of the concentrations of coal and catalyst, hydrogen partial pressure, reaction temperature, and residence time, and is controlled by the rates of hydrogenation of polynuclear aromatic components and the rates of formation and stabilization of radicals. The relative contribution of these reactions, at any temperature, determines the influence of the hydrogen partial pressure on the rate of the hydrogen consumption. The kinetics of the decomposition reactions of coal to preasphaltene, asphaltene, and oil also have been studied. The apparent activation energies determined are 20 kJ/mol for coal to preasphaltene, 40 kJ/mol for preasphaltene, 66 kJ/mol for asphaltene to oil, and 174 kJ/mol for oil to gases.

Effects of C18 long chain fatty acids on glucose, butyrate and hydrogen degradation

The effects of linoleic (C18:2), oleic (C18:1), and stearic (C18:0) acids on glucose, butyrate and hydrogen degradation were investigated at 21°C using a culture unacclimated to long-chain fatty acids (LCFAs). Diethyl ether was used to facilitate precise addition of LCFAs and provide adequate dispersion in cultures. Butyrate degradation was affected by diethyl ether but minimal effects were observed on hydrogen and glucose consumption. In the presence of oleic and stearic acids, the glucose consumption rate was similar but was ∼50% lower in the presence of linoleic acid. The effect of a mixture of 100 mg l −1 of each individual LCFA (300 mg l −1 total LCFA) was approximately the same as 100 mg l −1 linoleic acid alone, suggesting no synergistic inhibition of glucose degradation. Butyric acid degradation was more severely inhibited by the LCFAs with inhibition becoming more severe with the addition of double bonds to the LCFA. Furthermore, mixtures of LCFAs synergistically inhibited butyric acid degradation compared to the results with individual LCFAs. In contrast, although lower hydrogen consumption rates were observed in cultures receiving oleic and linoleic acids compared to cultures receiving stearic acid, inhibition by all three acids individually or in mixture was limited. The introduction of LCFAs into a system may severly inhibit intermediate acid degradation while having little effect on acid production.

Hydrogenation of palm olein catalyzed by polymer stabilized Pt colloids

Polyvinylpyrolidone stabilized Pt colloids with different polymer: Pt ratios with average particle sizes of between 1.4 and 2.9 nm and narrow size distribution were found to be active for the hydrogenation of palm olein at 35°C and atmospheric pressure. The initial hydrogen consumption rate was high, corresponding to the hydrogenation of the linoleate and slowed down thereafter, corresponding to the oleate hydrogenation. The rate constants for the two competing reactions were determined and the selectivity calculated to be between 10 and 100. The particle size of the Pt clusters influences the rate of the diene much more significantly than the monoene. The smaller the average particle size, the higher is the rate for the diene and hence the higher the selectivity. The trans-isomer formation was comparative low and was found to be a consequence of the partial saturation of the diene rather than the isomerization of the monoene.

Study of constancy of hydrogen-consuming flora of human colon.

The constancy of the hydrogen consuming flora of the human colon was studied in 15 healthy subjects via two measurements obtained 18 to 36 months apart. Hydrogen disappearance rate and the major products of H2-consuming bacteria, methane and sulfide, were measured during incubation of fecal homogenates with excess hydrogen and sulfate. In 11/15, the hydrogen consumption rate and the predominant hydrogen-consuming pathway (methanogenesis, sulfate reduction, or neither) remained constant. However, major shifts in these pathways were observed in four subjects, with two losing and two gaining the ability to produce methane. Methanogenesis was associated with the highest hydrogen consumption rate. This study demonstrates that clinically unrecognizable, major alterations of the colonic flora occur in healthy subjects. Understanding of the factors responsible for these alterations might allow for therapeutic manipulation of the colonic flora.

Kinetics of the hydrogenation of 2,4-dinitrotoluene over a palladium on alumina catalyst

The chemistry and kinetics of the catalytic hydrogenation of 2,4-dinitrotoluene (2,4-DNT) over a palladium on alumina catalyst have been investigated. The catalyst consists of porous cylindrical pellets of 4.2 × 4.2 mm in which a shell of Pd has penetrated to a depth of the order of 100 μm. The experiments have been performed in a three-phase batch reactor under isothermal and isobaric conditions. Two reactor temperatures of 308 and 345 K and a hydrogen pressure range 0.5–4 MPa have been chosen. Employing a chromatographic method, analyses of the reaction mixtures have been carried out and the reaction pathway has been studied—three stable intermediates have been identified. We have concluded that for both types of catalyst, independent of the palladium carrier, the chemistry of the conversion of DNT into DAT is the same. Mass transfer experiments have shown that external mass transfer resistances can be neglected. However, internal diffusion limitations had to be taken into account. To study the intrinsic reaction kinetics, a series of measurements with a finely crushed catalyst of particle diameter lower than 40 μm have been carried out. To describe the kinetics, a Langmuir—Hinshelwood model based on adsorption of hydrogen and organic species on different active sites has been applied. The kinetics parameters have been determined and compared with data for a Pd on carbon catalyst. The influence of the internal mass transfer resistances on the reaction rates has been quantitatively described with an effectiveness factor. At higher temperatures an additional series of experiments with pellets has been performed at 371 K in order to obtain an impression of the possibilities to extrapolate our correlations outside the experimental region. A good agreement between experimental and calculated hydrogen consumption rates is observed.

Hydrogen consumption rate in a soil as an indication of compaction and gas-diffusion status

H2 consumption rates were significantly correlated with penetrometer measurements of soil samples at different compaction levels. By measuring the ratio of the H2 consumption of an undisturbed soil sample to that of a sieved soil, it is possible to obtain, in a rapid and simple way, an indication of the gas-diffusion status of the soil.

MASS TRANSFER EFFECTS IN LIQUEFACTION OF COAL BY SRC-II PROCESS PART II STUDY OF BUBBLE COLUMN REACTORS

The effects of hydrogen mass transfer resistance in large-scale SRC-II bubble column reactors (BCR), over large ranges of process variables, are studied. Due to the interactive effects of mass transfer resistance and gas hold up, the hydrogen consumption or liquid yield in a BCR has a maximum with respect to the specific mixing power. Under normal SRC-II process conditions a superficial gas velocity of about 0.01 m/s represents the optimum with respect to the hydrogen consumption or liquid yield. In general, the product quality requirement rather than the rate of hydrogen consumption determines the minimum specific mixing power requirement. Increase in hydrogen partial pressure can be used to reduce the level of mixing power required to maintain the desired product quality. Interrelations between mass transfer and gas hold effects and the variations in hydrogen concentration in slurry over large ranges of process conditions are also illustrated. This work provides some bases for the selection of reactor dimensions and process conditions for an SRC-II bubble column reactor (BCR).

MASS TRANSFER EFFECTS IN LIQUEFACTION OF COAL BY SRC-II PROCESS PART I EFFECT OF MIXING POWER

A correlation between the volumetric hydrogen mass transfer coefficient (K La) in SRC-II process slurry and specific power consumption in mixing (E/V R) is developed. This correlation is independent of reactor configuration and it is also in very good agreement with the literature correlations for KL a. It is unified with an intrinsic hydrogen reaction rate expression to simulate the rate of hydrogen consumption in the SRC-II process. The simulation is validated by experimental data from an SRC-II continuous stirred tank reactor (CSTR). The mass transfer effects are interpreted and related to changes in product quality and a reduction in hydrogen reaction rate. Critical mixing power, required to maintain kinetically controlled operation of the process, is suggested.

Direct Measurement of the Hydrogenase Activity of Anaerobic Microorganisms

An electro-chemical method of direct measurement of the concentrations of dissolved hydrogen in the mixed culture of anaerobic microorganisms was used for the determination of the rates of hydrogen production and consumption. A certain modification of the Sweet's conception has been used in the design of the device. Experiments demonstrate that the hydrogenase activity expressed as a hydrogen consumption rate corresponds well with the activity of electron carrier and with the concentration changes of particular volatile fatty acids.

Dissolved hydrogen and methane in Saanich Inlet, British Columbia

Hydrogen, methane, and relevant microbiological and hydrographic observations were made to study the processes responsible for the observed distributions of the gases in Saanich Inlet, a seasonally anoxic fjord. Concentrations of surface waters were up to 22 (H 2) and 13 (CH 4) times atmospheric saturation. Below the surface hydrogen fell to undersaturation, rising to above saturation in the anoxic layer. Methane increased to a maximum at 30 m, and after a minimum at 125 m, increased greatly in the anoxic layer. Microbes cultured from inlet surface water produced hydrogen under experimental conditions, not by nitrogen fixation but while apparently denitrifying. Large numbers of protozoa present may provide anaerobic microniches for hydrogen and methane production, as might the intestinal tracts of the large populations of higher organisms in the inlet. Methane profiles in the upper 50 m are typical of waters outside the inlet, indicating regional sources and sinks. Hydrogen production and consumption rates, inferred to be rapid in the anoxic waters, nearly balance so that only slight increases in hydrogen occur there.

Kinetic studies on Victorian brown coal hydroliquefaction

Reaction kinetics of the liquefaction of Victorian brown coal in a process development unit (PDU) having three reactors in series have been studied at temperatures of 430–470°C, and pressures of 15–25 MPa. It is shown that the rate of hydrogen consumption can be expressed as a function of the concentrations of coal and catalyst, hydrogen partial pressure, reaction temperature and residence time, and is controlled by the rates of hydrogenation of polynuclear aromatic components, and the rates of formation and stabilization of radicals. The relative contribution of these reactions, at any temperature, determine the influence of the hydrogen partial pressure on the rate of the hydrogen consumption. The kinetics of the decomposition reactions of brown coal to preasphaltene, asphaltene and to oil also have been studied. The apparent activation energies determined are 25 kJ mol −1 for the brown coal to preasphaltene, 50 kJ mol −1 for preasphaltene to asphaltene, 76 kJ mol −1 for asphaltene to oil, and 184 kJ mole −1 for oil to gases.

Kinetics of the Hydrogenation of Fructose with Raney‐Nickel

AbstractThe kinetics of the hydrogenation of fructose have been studied at different conditions. The initial hydrogen consumption rate is independent of the fructose concentration and can be described as a function of the hydrogen pressure. The relations can be understood assuming that hydrogen and fructose are adsorbed on different type of active sites according to a Langmuir‐Hinshelwood model, that a reaction between adsorbed species of atomic hydrogen and fructose is rate controlling, and that the reaction is retarded by adsorption of a product formed during the reaction.

Reaction of Hydrogen with Uranium

The reaction of hydrogen with uranium to produce uranium hydride was studied in the temperature range 96°–400°C at pressure levels of of mercury (where is system pressure and is the plateau dissociation pressure of the uranium hydride product). Reaction rates followed the linear law. At a given , the linear rate increased with increasing temperature in the range 96° to about 250°C and decreased with increasing temperature from about 250° to 400°C. Variation of the linear rate with temperature and pressure in the range 96° to about 250°C is given by the equation: where is in units of ml/cm2/sec and is in units of mm Hg. In the range from about 250° to 400°C an empirical relationship between the reaction rate and pressure is proposed where the rate is a function of . However, since none of the reaction mechanisms was determined, the significance of the pressure dependencies is not known. Two different initial reactions were obtained. One in the range 96deg; to about 250°C at of 70 and 150 mm Hg and 96°–400°C at of 430 mm Hg showed a gradual increase in the rate of hydrogen consumption until the rate became linear. The other initial reaction followed the parabolic rate law in the range of about 250°–400°C at of 70 and 150 mm Hg.