Peter Englezos

Kinetics of formation of methane and ethane gas hydrates

Abstract An intrinsic kinetic model with only one adjustable parameter is proposed for the formation of methane and ethane gas hydrates. Experimental formation data were obtained in a semi-batch stirred tank reactor. The experiments were conducted at four temperatures from 274 to 282 K and at pressures ranging from 0.636 to 8.903 MPa. The kinetic model is based on the crystallization theory, while the two-film theory model is adopted for the interfacial mass transfer. Experiments were performed at various stirring rates to define the kinetic regime. The study reveals that the formation rate is proportional to the difference in the fugacity of the dissolved gas and the three-phase equilibrium fugacity at the experimental temperature. This difference defines the driving force which incorporates the pressure effects. The gas consumption rate is also proportional to the second moment of the particle size distribution. The rate constants indicate a very weak temperature dependence.

Patterned superhydrophobic metallic surfaces.

This work shows that after creating certain dual scale roughness structures by femtosecond laser irradiation different metal alloys initially show superhydrophilic behavior with complete wetting of the structured surface. However, over time, these surfaces become nearly superhydrophobic with contact angles in the vicinity of 150 degrees and superhydrophobic with contact angles above 150 degrees. The contact angle hysteresis was found to lie between 2 and 6 degrees. The change in wetting behavior correlates with the amount of carbon on the structured surface. The explanation for the time dependency of the surface wettability lies in the combined effect of surface morphology and surface chemistry.

Kinetics of gas hydrate formation from mixtures of methane and ethane

Abstract Experimental data on the kinetics of formation of gas hydrates from three mixtures of gaseous methane and ethane are reported. the experiments were conducted in a semi-batch stirred tank reactor at temperatures from 273 to 284 K and of pressures from 0.68 to 5.60 MPa. An intrinsic kinetic model for the growth of the gas hydrate is proposed. It is extension of the model for pure component hydrate formation. The model is based on the crystallization theory coupled with the two-film theory for the gas absorption into the liquid phase. the model does not contain any adjustable parameters. The kinetic rate constants which appear in the model are those obtained previously from pure component formation data. The results indicate that the formation rate is proportional to a lienar combination of the differences in the fugacities of the dissolved gases and their three-phase equilibrium fugacities at the experimental temperature. The effect of the mixture composition is taken into account indirectly through the computation of the three-phase equilibrium conditions and of the fugacities. the total gas consumption rate is proportional to the second moment of the particle size distribution.

Gas hydrates: A cleaner source of energy and opportunity for innovative technologies

The global energy system is characterized by a gradual de-carbonization and move to cleaner burning technologies: from wood to coal to oil and to natural gas. A final destination characterized by the term“hydrogen economy” is desired. Gas hydrate found in the earth’s crust is considered a source of natural gas that is essentially 100% methane (CH4) gas. Natural gas hydrate estimates worldwide range from 10,000 to 40,000 trillion cubic meters (TCM). Efforts are underway to exploit this resource. These methane hydrates in the earth’s crust also have the potential to be a significant factor in global climate change. Moreover, gas hydrates offer opportunities for the development of innovative technologies (separation of CO2 from CO2/N2 and CO2/H2 mixtures, CO2 sequestration, natural gas transportation and storage and H2 storage). In this work we assess the progress towards exploitation of gas hydrates as a resource for methane (cleaner energy) and summarize the state of the art with respect to the role of gas hydrates in the development of innovative technologies.

Femtosecond laser irradiation of metallic surfaces: effects of laser parameters on superhydrophobicity

This work studies in detail the effect of femtosecond laser irradiation process parameters (fluence and scanning speed) on the hydrophobicity of the resulting micro/nano-patterned morphologies on stainless steel. Depending on the laser parameters, four distinctly different nano-patterns were produced, namely nano-rippled, parabolic-pillared, elongated sinusoidal-pillared and triple roughness nano-structures. All of the produced structures were classified according to a newly defined parameter, the laser intensity factor (LIF); by increasing the LIF, the ablation rate and periodicity of the asperities increase. In order to decrease the surface energy, all of the surfaces were coated with a fluoroalkylsilane agent. Analysis of the wettability revealed enhanced superhydrophobicity for most of these structures, particularly those possessing the triple roughness pattern that also exhibited low contact angle hysteresis. The high permanent superhydrophobicity of this pattern is due to the special micro/nano-structure of the surface that facilitates the Cassie-Baxter state.

Physics of ice friction

Although the study of friction has a long history, ice friction has only been investigated during the last century. The basic physical concepts underlying the different friction regimes, such as boundary, mixed, and hydrodynamic friction are also relevant to ice friction. However, these friction regimes must be described with respect to the thickness of the lubricating liquidlike layer on ice. In this review the state of knowledge on the physics of ice friction is discussed. Surface melting theories are introduced. These theories attempt to explain the existence and nature of the liquidlike surface layer on ice at any temperature and without any load applied. Pressure melting, as the long-time explanation for the ease of ice friction, is discussed, together with the prevailing theory of frictional heating. The various laboratory setups for ice friction measurements are presented as well as their advantages and disadvantages. The individual influence of the different parameters on the coefficient of ice fr...

Two-Stage Clathrate Hydrate/Membrane Process for Precombustion Capture of Carbon Dioxide and Hydrogen

A hybrid process for the capture of CO2 and H2 from a treated fuel gas mixture is presented. It consists of two hydrate crystallization stages operating at 273.7 K and 3.8 and 3.5 MPa, respectively. The CO2-lean stream from the first stage is directed to a membrane separation unit whereas the CO2-rich one is directed to the second hydrate stage. These operating pressures at the crystalli- zation stages are possible by adding 2.5% by mole propane. Propane enables the reduction in the hydrate formation pressure and thus reduces the cost associated with the compression of the fuel gas. The two hydrate stages would operate at 7.5 and 3.5 MPa without adding propane. This work provides the relevant kinetic data, as well as the separation efficiency and recoveries achieved.

Laser-Patterned Super-Hydrophobic Pure Metallic Substrates: Cassie to Wenzel Wetting Transitions

A femtosecond laser was used to create microstructures on very pure metal surfaces. The irradiated samples initially showed super-hydrophilic behavior. With time and exposure to ambient air the contact angle increased to about 160° with very low hysteresis. The surfaces supported the Cassie and Wenzel wetting states, depending on the technique used to deposit the water droplets. The created surface morphologies were idealized with a geometric model that is an assembly of densely packed cylindrical pillars with semispherical caps. Using this geometric model for calculation of the surface roughness, a theoretical Young contact angle of about 99° was calculated for all samples from the Wenzel and Cassie–Baxter equations. While the value of 99° significantly differs from the measured hydrophilic contact angles on the polished pure metallic samples, it indicates that a laser-induced surface reaction must be responsible for the evolution of contact angles to super-hydrophobic ones and that this phenomenon is independent of the type of metal.

Molecular Modeling of the Dissociation of Methane Hydrate in Contact with a Silica Surface

We use constant energy, constant volume (NVE) molecular dynamics simulations to study the dissociation of the fully occupied structure I methane hydrate in a confined geometry between two hydroxylated silica surfaces between 36 and 41 Å apart, at initial temperatures of 283, 293, and 303 K. Simulations of the two-phase hydrate/water system are performed in the presence of silica, with and without a 3 Å thick buffering water layer between the hydrate phase and silica surfaces. Faster decomposition is observed in the presence of silica, where the hydrate phase is prone to decomposition from four surfaces, as compared to only two sides in the case of the hydrate/water simulations. The existence of the water layer between the hydrate phase and the silica surface stabilizes the hydrate phase relative to the case where the hydrate is in direct contact with silica. Hydrates bound between the silica surfaces dissociate layer-by-layer in a shrinking core manner with a curved decomposition front which extends over a 5-8 Å thickness. Labeling water molecules shows that there is exchange of water molecules between the surrounding liquid and intact cages in the methane hydrate phase. In all cases, decomposition of the methane hydrate phase led to the formation of methane nanobubbles in the liquid water phase.

Application of the ATR-IR Spectroscopic Technique to the Characterization of Hydrates Formed by CO2, CO2/H2 and CO2/H2/C3H8

The spectroscopic investigation of CO(2)-containing clathrate hydrates is complicated because techniques such as Raman spectroscopy cannot distinguish cage populations. (13)C NMR spectroscopy also has some complications as the isotropic chemical shifts do not change for the different CO(2) cage populations. It is known that CO(2) molecules in the different phases relevant to hydrates give unique infrared vibrational frequencies; however, so far only thin cryogenic films prepared at low pressure have been studied with IR transmission spectroscopy. In this study, hydrates from CO(2), CO(2)/H(2), and CO(2)/H(2)/C(3)H(8) mixtures were synthesized in a high-pressure attenuated total reflection (ATR) cell and in situ infrared spectroscopy was performed at -50 degrees C to distinguish the vibrational frequencies from CO(2) in small and large cages in the resultant hydrate and in the other CO(2)-containing phases. Quantitative estimates of cage occupancies and hydration numbers are provided as based on the analysis of the IR spectra and knowledge of hydrate gas composition from gas chromatography.