Methane Ethane Research Articles

Unusual kinetic inhibitor effects on gas hydrate formation

Abstract Gas hydrate formation experiments were conducted with a methane–ethane mixture at 273.7 or 273.9 K and 5100 kPa and using water droplets or water contained in cylindrical glass columns. The effect of kinetic inhibitors and the water/solid interface on the induction time for hydrate crystallization and on the hydrate growth and decomposition characteristics was studied. It was found that inhibitors GHI 101 and Luvicap EG delayed the onset of hydrate nucleation. While this inhibition effects has been reported previously some unusual behaviour was observed and reported for the first time. In particular, the water droplet containing GHI 101 or Luvicap EG was found to collapse prior to nucleation and spread out on the Teflon surface. Subsequently, hydrate was formed as a layer on the surface. Catastrophic growth and spreading of the hydrate crystals was also observed during hydrate formation in the glass columns in the presence of the kinetic inhibitor. Finally, when polyethylene oxide (PEO) was added into the kinetic inhibitor solution the memory effect on the induction time decreased dramatically.

Enhancement of the performance of gas hydrate kinetic inhibitors with polyethylene oxide

Inclusion of polyethylene oxide into a kinetic inhibitor solution was found to enhance the performance of the inhibitor. Polyethylene oxide is a commercially available high molecular weight polymer that is not a kinetic inhibitor by itself. The hydrate formation experiments in the presence of the various inhibitor solutions were conducted in a vessel in a semi-batch manner at constant pressure and using methane and methane–ethane gas mixtures. In some experiments a non-aqueous liquid phase ( n-heptane) was also present. The induction time, the gas uptake and the temperature were measured. It was found that the induction time is prolonged in the presence of the additive by an order of magnitude in some cases compared to the inhibitor only.

Hydrogen-enriched natural gas blend oxidation under high-pressure conditions: Experimental and detailed chemical kinetic modeling

For the first time, experimental results were obtained for the kinetics of oxidation of hydrogen/natural gas mixtures in a fused silica jet-stirred reactor operating at 10 atm, over the temperature range 900–1200 K for equivalence ratios of 0.3, 0.6, and 1. The concentration profiles of the reactants, stable intermediates and the final products were measured by probe sampling followed by on-line FTIR analyses and off-line GC-TCD/FID analyses. The addition of hydrogen in variable concentrations significantly increases the reactivity of the natural gas blend used (methane–ethane 10:1), particularly under fuel-lean conditions. The present experiments were modeled by means of a detailed chemical kinetic reaction mechanism (97 species involved in 732 reversible reactions). We obtained an overall good agreement between the present data and the modeling. According to the proposed kinetic scheme, the enhanced oxidation of methane by hydrogen proceeds through an increased production of OH. The increase in hydrogen initial concentration boosts the formation of HO 2 radicals at low temperature, yielding higher concentrations of H 2 O 2 and OH. The oxidation of hydrogen, methane and ethane mostly proceeds via reaction with OH. The following sequence of reactions summarizes the mechanism yielding the observed reactivity increase in hydrogen-enriched mixtures: CH 3 + H 2 ⇒ CH 4 + H ; H + O 2 ⇒ HO 2 ; 2 HO 2 ⇒ H 2 O 2 + O 2 ; H 2 O 2 ⇒ 2 OH ; HO 2 + H ⇒ 2 OH ; OH + H 2 ⇒ H 2 O + H followed by reactions of hydrocarbons with OH. The proposed kinetic scheme was also used to simulate the burning velocities of methane–hydrogen–air mixtures over the pressure range 1–5 atm.

Methane–ethane and methane–propane hydrate formation and decomposition on water droplets

Gas hydrate formation and decomposition on water droplets using an 89.4% methane—10.6% ethane mixture, and a 90.1% methane—9.9% propane mixture were carried out in a new apparatus suitable for morphology studies. As expected the induction time was found to be much shorter when the water had hydrate memory. All droplets nucleated simultaneously and the droplet size and shape had no noticeable effect on induction time and macroscopic crystal growth morphology for hydrates from the methane–ethane mixture. However, the surface of the hydrate crystals from methane–propane had a “hairy-like” appearance which changed to a smooth surface over time. Moreover, the smaller droplets during hydrate reformation showed an extensive hydrate growth and looked like snow-flakes. Sequential pictures generated by time-lapse videos showed that the time required for hydrate to cover the water droplet surface ranged from 10 to 23 s and was shorter when there was gas-phase agitation (mixing). The growth is postulated to occur in two stages. The first stage lasts about 10–23 s and growth takes place laterally. Growth takes place at the hydrate/gas and the hydrate/water interfaces during the second stage. The implication of the findings for process design of hydrate formation vessels is also discussed.

Experimental investigation on flame and detonation quenching: applicability of static flame arresters

The present investigation was aimed at demonstrating the pertinent use of a flame/detonation arrester for the safety in the process industry. The experimental data provide information on the performance of a static flame arrester. The experiments were conducted in a 3-m long and 28-mm i.d. tube. The arrester, itself consists in 2-mm i.d. tubes arranged in a honeycomb type structure which is located in the middle of the test tube. This is aimed at promoting heat losses during the passage of the flame. The experiments were conducted with fuel-rich mixtures of methane–ethane–ethylene at initial pressures in the range of 0.1–0.6 MPa. These very rich mixtures were more or less diluted in inert, namely, N2, as well as a mixture of CO2 and Ar, in order to match with compositions that are currently used in the process industry. The experimental results show that this device is appropriate for flame quenching but, in most cases, detonations were not successfully arrested. These unsuccessful operations of the device occurred for flames at a high velocity, namely for Re numbers of the order of or greater than 15,000.

On the thermodynamic stability and structural transition of clathrate hydrates

Gas mixtures of methane and ethane form structure II clathrate hydrates despite the fact that each of pure methane and pure ethane gases forms the structure I hydrate. Optimization of the interaction potential parameters for methane and ethane is attempted so as to reproduce the dissociation pressures of each simple hydrate containing either methane or ethane alone. An account for the structural transitions between type I and type II hydrates upon changing the mole fraction of the gas mixture is given on the basis of the van der Waals and Platteeuw theory with these optimized potentials. Cage occupancies of the two kinds of hydrates are also calculated as functions of the mole fraction at the dissociation pressure and at a fixed pressure well above the dissociation pressure.

Experimental and detailed kinetic modeling study of hydrogen-enriched natural gas blend oxidation over extended temperature and equivalence ratio ranges

The reduction of CO, CO 2, and NO x emissions from combustion in gas turbines can be achieved using hydrogen-enriched fuels operation under very fuel-lean conditions where the emission of NO x is strongly reduced. New experimental results were obtained for the kinetic of oxidation of hydrogen–natural gas mixtures in a fused silica jet-stirred reactor operating at 1 atm, over the temperature range 900–1450 K. Probe sampling followed by on-line FTIR analyses and off-line GC-TCD/FID analyses allowed the measurement of concentration profiles for the reactants, stable intermediates, and final products. Hydrogen addition in variable fractions strongly increases the reactivity of the natural gas blend considered (methane–ethane–propane). The effect is more pronounced in fuel-lean conditions. A detailed kinetic modeling of the present experiments was performed. An overall reasonable agreement between the present data and the modeling was obtained. According to the proposed model, the sensitization of the oxidation of methane by hydrogen proceeds through an increased production of OH radicals by increasing the importance of the reaction H + HO 2 ⇒ OH + OH. The increase in hydrogen initial concentration favors the formation of HO 2 radicals at low temperature, resulting in increased concentrations of OH and H 2O 2. The oxidation of hydrogen, when added, and that of methane, ethane, and propane mostly proceed via reaction with OH. The following sequence of reactions summarizes the mechanism yielding to the observed reactivity increase in hydrogen-enriched mixtures: H + O 2 ⇒ HO 2; HO 2 + H ⇒ 2OH; 2HO 2 ⇒ H 2O 2; H 2O 2 ⇒ 2OH; and OH + H 2 ⇒ H 2O + H followed by reactions of hydrocarbons with OH.

Simple group contribution theory for adsorption of alkanes in nanoporous carbons

This paper develops a simplified form of the group-contribution theory of Russell and LeVan (Chem. Eng. Sci. 51 (1996) 4025) for the prediction of adsorption equilibria of n-alkanes on activated carbon. Molecules are separated into elements or groups, and adsorbate group-adsorbent interactions are the determining factor for adsorption equilibrium. The adsorbent surface is considered to be heterogeneous with a single dimensionless energy distribution. Model parameters are determined for pure-component adsorption of methane, ethane, n-butane, and n-hexane on BPL activated carbon at multiple temperatures. The resulting parameters are then used to predict adsorption equilibrium of propane and n-octane at multiple temperatures and mixtures of methane and n-hexane and ethane and n-hexane. The model gives a more accurate prediction of mixture adsorption than the ideal adsorbed solution theory. The model is also used to describe methane, propane, and n-pentane adsorption on BAX activated carbon (Ind. Eng. Chem. Res. 40 (2001) 338) and methane, ethane, and propane on Columbia Grade L carbon (Ind. Eng. Chem. 42 (1950) 1315) at multiple temperatures. The model is shown to describe adsorption equilibria of alkanes on all three activated carbons with good accuracy and can also be used as a predictive tool for components on which no experimental data are available.

Gas solubility measurement and modeling for methane–water and methane–ethane–n-butane–water systems at low temperature conditions

In this communication, experimental measurements and thermodynamic modeling of solubilities of methane and a hydrocarbon gas mixture (94% methane + 4% ethane + 2% n-butane) in water at low temperature conditions are reported. Methane solubility measurements have been conducted at a temperature range of 275.11–313.11K and pressures up to 18MPa. The solubility of the individual components in the gas mixture was measured from 278.14 to 313.12K and pressures up to 14.407MPa. A static–analytic apparatus, taking advantage of a pneumatic capillary sampler is used for fluid sampling. The Valderrama modification of the Patel–Teja equation of state combining with non-density dependent mixing rules were used for modeling gas solubilities in water. The data generated in this work are compared with the literature data and the predictions of the thermodynamic model, demonstrating the reliability of the techniques used in this work.

Kinetics of Partial Oxidation of Alkanes at High Pressures: Oxidation of Ethane and Methane–Ethane Mixtures

The kinetics and product yield in oxidation of ethane and ethane–nitrogen and methane–ethane mixtures in the pressure range 10–70 atm are studied. The possibility of producing liquid organic products at high yield at a pressure of 30 atm and less is demonstrated. The promoting effect of methane on ethane oxidation to form methanol is detected.

Production of Hydrogen and Carbon Nanostructures by Non-oxidative Catalytic Dehydrogenation of Ethane and Propane

Nanoscale, binary, M−Fe (M = Mo, Ni, or Pd) catalysts supported on alumina have been shown to be very effective for the decomposition of undiluted methane to yield hydrogen and multiwalled nanotubes. The same suite of catalysts has been tested for non-oxidative catalytic dehydrogenation of undiluted ethane and propane. After prereduction at 700 °C, all three binary catalysts exhibited significantly lower decomposition temperatures and longer time-on-stream performances than either the nonmetallic alumina support or a mono-metallic Fe/alumina catalyst. Ethene, propene, water, and carbon oxides were absent, and only hydrogen, methane, and unreacted ethane or propane were observed in the product stream. The carbon deposit left on the catalyst bed showed two distinct forms, depending on the reaction temperature. Above 600 °C, the carbon deposits were in the form of multiwalled nanotubes with some irregularly spaced capping of the innermost walls. At or below 500 °C reactor temperature, carbon nanofibers with ...

Experimental and modeling study of the oxidation of natural gas in a premixed flame, shock tube, and jet-stirred reactor

New experimental results have been obtained for the oxidation of a stoichiometric premixed synthetic natural gas flame at low pressure (10.6 kPa). Mole fraction profiles of stable and reactive species and temperature were measured using molecular beam/mass spectrometry, gas chromatography, and thermocouple techniques. A detailed reaction mechanism has been evaluated by comparison of computed and measured species mole fraction profiles. Good agreement was obtained for most molecular species (reactants, intermediate species, and final products). The proposed mechanism was also validated by modeling the laminar flame speeds of methane/air, ethane/air, and propane/air flames reported in the literature over a wide range of equivalence ratios. The model reproduces correctly the experimental values. The model was also used to simulate new experimental results obtained in a jet-stirred reactor (JSR) at atmospheric pressure and variable equivalence ratio ( Φ=0.75:1.0:1.5) for the oxidation of methane–ethane mixtures representative of natural gas. The ignition delays of methane–oxygen–argon and methane–ethane–oxygen were modeled. Sensitivity analyses and reaction path analyses, based on species rates of reaction, were used to interpret the results, especially the effect of higher alkanes on the oxidation of natural gas.

Molecular simulation of binary vapour–liquid equilibria with components differing largely in volatility

In this work the Gibbs–Duhem integration method is used in combination with molecular simulation in the semigrand ensemble to simulate vapour–liquid equilibria for the binary mixtures ethane–methane and carbon dioxide–dimethylsulphoxide (DMSO). In both binary mixtures, the two components have a large difference in relative volatility. The starting point for tracing the vapour–liquid phase envelope is either calculated in a pure species NVT Gibbs ensemble simulation or in a pure species NpT+test particle simulation. The chosen method depends on the vapour pressure of the pure species. An equation is presented that relates the initial slope of the Clapeyron equation to an ensemble average in the Gibbs ensemble. The NpT+test particle method is slightly modified in order to improve the convergence characteristics when one of the coexisting phases is a (very) dense phase. Configurational-bias methods are used in the NVT Gibbs ensemble to enhance the acceptance of particle exchanges, and in the NpT+test particle method to grow test particles in a dense liquid phase. Force-biased molecular translations and torque-biased molecular rotations are used to enhance the diffusion of molecules in state space. The work shows the capability of the combination of the NpT+test particle method and Gibbs–Duhem integration to predict vapour–liquid equilibria of binary systems with components differing largely in volatility.

Exact statistical mechanical one-dimensional lattice model of alkane binary mixture adsorption in zeolites and comparision with Monte-Carlo simulations

Adsorption isotherms obtained by Monte-Carlo simulation for ethane–methane mixtures in the zeolite silicalite show a highly unusual structure with a shoulder at low coverage and adsorption preference reversal at high pressures whose interpretation is uncertain. To understand this behaviour an exact calculation of the statistical mechanics of a lattice model of alkane mixture adsorption in zeolites has been undertaken. The lattice model reproduces all of the novel features of the Monte-Carlo isotherms, allowing us to infer that these unusual characteristics arise from a combination of molecular reorientation of ethane molecules, and displacement of ethane by methane at higher pressures.

Impact of aerosols present in Titan's atmosphere on the CASSINI radar experiment

Simulations of Titan's atmospheric transmission and surface reflectivity have been developed in order to estimate how Titan's atmosphere and surface properties could affect performances of the Cassini radar experiment. In this paper we present a selection of models for Titan's haze, vertical rain distribution, and surface composition implemented in our simulations. We collected dielectric constant values for the Cassini radar wavelength (∼2.2 cm) for materials of interest for Titan: liquid methane, liquid mixture of methane–ethane, water ice, and light hydrocarbon ices. Due to the lack of permittivity values for Titan's haze particles in the microwave range, we performed dielectric constant ( ε r ) measurements around 2.2 cm on tholins synthesized in laboratory. We obtained a real part of ε r in the range of 2–2.5 and a loss tangent between 10 −3 and 5×10 −2. By combining aerosol distribution models (with hypothetical condensation at low altitudes) to surface models, we find the following results: (1) Aerosol-only atmospheres should cause no loss and are essentially transparent for Cassini radar, as expected by former analysis. (2) However, if clouds are present, some atmospheric models generate significant attenuation that can reach −50 dB, well below the sensitivity threshold of the receiver. In such cases, a 13.78 GHz radar would not be able to measure echoes coming from the surface. We thus warn about possible risks of misinterpretation if a “wet atmosphere” is not taken into account. (3) Rough surface scattering leads to a typical response of ∼−17 dB. These results will have important implications on future Cassini radar data analysis.

Experimental and kinetic modeling study of the reduction of NO by hydrocarbons and interactions with SO 2 in a JSR at 1 atm ☆

The reduction of nitric oxide (NO) by a mixture of methane, ethylene and acetylene with and without addition of SO 2 has been studied in a fused silica jet-stirred reactor operating at 1 atm in simulated conditions of the reburning zone. The temperatures were ranging from 800 to 1400 K. In these experiments, the initial mole fractions of NO and SO 2 were 0 or 1000 ppm, that of methane, ethylene and acetylene were, respectively, 2400, 1200 and 600 ppm. The equivalence ratio has been varied from 0.5 to 2.5. It was demonstrated that the reduction of NO varies as the temperature and that for a given temperature, a maximum NO reduction occurs slightly above stoichiometric conditions. The addition of SO 2 inhibited the process of reduction of NO under the present conditions. The present results generally follow those obtained in previous studies involving simple hydrocarbons or natural gas as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (1006 reversible reactions and 145 species). An overall reasonable agreement between the present data and the modeling was obtained. Also, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethane, ethylene, a natural gas blend (methane–ethane 10:1). The kinetic modeling indicates that the reduction of NO proceeds via the following sequence of reactions: HCCO+NO=HCNO+CO; HCCO+NO=HCN+CO 2; HCN+O=NCO+H; HCN+O=NH+CO; HCN+H=CN+H 2; HCNO+H=HCN+OH; CN+O 2=NCO+O; NCO+H=NH+CO; NCO+NO=N 2O+CO; NCO+NO=CO 2+N 2; NH+NO=N 2O+H; NH+NO=N 2+OH. The inhibition of this process by SO 2 is explained by the sequence of reactions H+SO 2+M=HOSO+M and HOSO+H=SO 2+H 2 that acts as a termination process: H+H+M=H 2+M.

A study of volatile organic sulfur emissions causing urban odors

Levels of hydrogen sulfide and sulfur containing organic compounds were studied in the air at the deltas of the polluted creeks in the city of Izmir, Turkey in summer 2001. High concentrations of these malodorous compounds were measured in the air samples. Presence of these compounds in the air was connected with the dark appearance and rising gas bubbles in the studied segments of the creeks. These creeks were like open sewers carrying wastewaters from the industry and residential areas into the inner Izmir Bay until September 2001. Within the scope of this study organic sulfur compounds such as methane thiol, ethane thiol, 2-propane thiol, 2-butane thiol, dimethylsulfide, dimethyldisulfide, thiophene, diphenylsulfide and hydrogen sulfide were studied in the air at selected urban sites where odor nuisance was recognized. Flux measurements from polluted surfaces were preferred rather than direct ambient air measurements. Organic sulfur emission fluxes from the creek surfaces were found above the values reported in the literature. Their concentrations and fluxes were higher in June field program. A limited number of measurements of reduced sulfur compound emission concentrations from the wastewater treatment plant equalization tank and the sludge drying beds as well as the landfill soil surface were also included in the study. Concentrations of total organic sulfur compounds and certain individual components such as dimethylsulfide and hydrogen sulfide in emitted gases from river surfaces were correlated with ambient SO 2 concentrations.

Temperature dependence of polyhedral cage volumes in clathrate hydrates

The polyhedral cage volumes of structure I (sI) (carbon dioxide, methane, trimethylene oxide) and structure II (sII) (methane–ethane, propane, tetrahydrofuran, trimethylene oxide) hydrates are computed from atomic positions determined from neutron powder-diffraction data. The ideal structural formulas for sI and sII are, respectively, S2L6 · 46H2O and S16L'8 · 136H2O, where S denotes a polyhedral cage with 20 vertices, L a 24-cage, and L' a 28-cage. The space-filling polyhedral cages are defined by the oxygen atoms of the hydrogen-bonded network of water molecules. Collectively, the mean cage volume ratio is 1.91 : 1.43 : 1 for the 28-cage : 24-cage : 20-cage, which correspond to equivalent sphere radii of 4.18, 3.79, and 3.37 Å, respectively. At 100 K, mean polyhedral volumes are 303.8, 227.8, and 158.8 Å3 for the 28-cage, 24-cage, and 20-cage, respectively. In general, the 20-cage volume for a sII is larger than that of a sI, although trimethylene oxide is an exception. The temperature dependence of the cage volumes reveals differences between apparently similar cages with similar occupants. In the case of trimethylene oxide hydrate, which forms both sI and sII, the 20-cages common to both structures contract quite differently. From 220 K, the sII 20-cage exhibits a smooth monotonic reduction in size, whereas the sI 20-cage initially expands upon cooling to 160 K, then contracts more rapidly to 10 K, and overall the sI 20-cage is larger than the sII 20-cage. The volumes of the large cages in both structures contract monotonically with decreasing temperature. These differences reflect reoriented motion of the trimethyelene oxide molecule in the 24-cage of sI, consistent with previous spectroscopic and calorimetric studies. For the 20-cages in methane hydrate (sI) and a mixed methane–ethane hydrate (sII), both containing methane as the guest molecule, the temperature dependence of the 20-cage volume in sII is much less than that in sI, but sII is overall larger in volume. PACS Nos.: 82.75, 61.66H, 65.40D, 61.12

Measured temperature and pressure dependence of Vp and Vs in compacted, polycrystalline sI methane and sII methane–ethane hydrate

We report on compressional- and shear-wave-speed measurements made on compacted polycrystalline sI methane and sII methane–ethane hydrate. The gas hydrate samples are synthesized directly in the measurement apparatus by warming granulated ice to 17°C in the presence of a clathrate-forming gas at high pressure (methane for sI, 90.2% methane, 9.8% ethane for sII). Porosity is eliminated after hydrate synthesis by compacting the sample in the synthesis pressure vessel between a hydraulic ram and a fixed end-plug, both containing shear-wave transducers. Wave-speed measurements are made between –20 and 15°C and 0 to 105 MPa applied piston pressure. PACS No.: 61.60Lj

Temperature, pressure, and compositional effects on anomalous or "self" preservation of gas hydrates

We previously reported on a thermal regime where pure, polycrystalline methane hydrate is preserved metastably in bulk at up to 75 K above its nominal temperature stability limit of 193 K at 0.1 MPa, following rapid release of the sample pore pressure. Large fractions (>50 vol.% ) of methane hydrate can be preserved for 2–3 weeks by this method, reflecting the greatly suppressed rates of dissociation that characterize this "anomalous preservation" regime. This behavior contrasts that exhibited by methane hydrate at both colder (193–240 K) and warmer (272–290 K) isothermal test conditions, where dissociation rates increase monotonically with increasing temperature. Here, we report on recent experiments that further investigate the effects of temperature, pressure, and composition on anomalous preservation behavior. All tests conducted on sI methane hydrate yielded self-consistent results that confirm the highly temperature-sensitive but reproducible nature of anomalous preservation behavior. Temperature-stepping experiments conducted between 250 and 268 K corroborate the relative rates measured previously in isothermal preservation tests, and elevated pore-pressure tests showed that, as expected, dissociation rates are further reduced with increasing pressure. Surprisingly, sII methane–ethane hydrate was found to exhibit no comparable preservation effect when rapidly depressurized at 268 K, even though it is thermodynamically stable at higher temperatures and lower pressures than sI methane hydrate. These results, coupled with SEM imaging of quenched sample material from a variety of dissociation tests, strongly support our earlier arguments that ice-"shielding" effects provided by partial dissociation along hydrate grain surfaces do not serve as the primary mechanism for anomalous preservation. The underlying physical-chemistry mechanism(s) of anomalous preservation remains elusive, but appears to be based more on textural or morphological changes within the hydrate material itself, rather than on compositional zoning or ice-rind development. PACS Nos.: 82.30Lp, 81.40Gh, 81.40Vw, 68.37Hk, 83.80Nb