Prediction of Gas-hydrate Equilibrium, Stability and Kinetic Nucleation in Porous Media

Natural gas-hydrates are crystalline inclusion compounds with gas molecules (guest compounds) trapped within a host lattice formed by water molecules in an ice-like hydrogen-bonded framework. Natural gas-hydrates have the potential to become an important carbon-based resource addressing the increasing energy demand, and they pose a risk in terms of climate change. Accurate estimates of gas-hydrates global inventory, understanding of formation and dissociation processes of gas-hydrates, and evaluation of their environmental impact require models that accurately describe gas-hydrate stability in sediments and predict gas-hydrate kinetic nucleation processes. The hypothesis driving this work is that incorporation of selected sediment properties, i.e., surface energies and pore diameter, can lead to more accurate predictions of hydrate equilibrium, stability and nucleation in porous media. In this work, a model for gas-hydrate equilibrium in porous media was developed from basic thermodynamic principles and tested against available experimental data published in the scientific literature. The proposed model predicts reported experimental data with high accuracy for the range of pore sizes (3.4 ~ 24.75 nm) of different materials reported in the literature. It was found that the wettability of the pore surface affects the shape of the hydrate phase inside the pore and consequently influences the equilibrium pressures of gas-hydrates formed in porous media. A predictive macroscopic mathematical model describing the kinetic nucleation of gas-hydrates was developed based on Classical Nucleation Theory (CNT) in order to formulate correction factors for three types of interfaces mostly encountered in natural sediments (gas-liquid interface, liquid-solid interface and three-phase boundary lines). This approach, which incorporates the interfacial properties of sediments, can efficiently provide a fundamental understanding on the dependence of the formation mechanism of gas hydrates on a wide range of interfacial properties (wettability, substrate size, interfacial tension). The model predicts that hydrate nucleation is energetically favorable on confined surfaces with smaller contact-angle values, i.e., hydrophilic surfaces. Comparison between different types of interfaces leads to the conclusion that the nucleation of gas hydrates preferentially occurs in larger sediment pores. At the beginning of methane hydrate formation, for example, hydrate will preferentially nucleate at the gas-liquid interface. With the increase of hydrate volume or growth of the hydrate phase, the center of crystal growth moves towards the liquid-solid interface. In natural systems, gas hydrates form first on the concave liquid/solid interface and gas/liquid interface in sandstone sediments, gas/liquid interface and gas/liquid/solid triple boundary line in clay sediments and gas/liquid interface in pipeline with oil droplets. The inclusion of sediment properties in the model for gas-hydrate equilibrium in sediments predict experimental data within a margin of %AAD lower than 2%, a significant improvement upon previous modeling attempts. Additionally, the inclusion of sediment properties in the models for kinetic nucleation of gas hydrates result in mathematical models that capture the qualitative information obtained from examination of gas-hydrate core samples. Therefore, the hypothesis of the present work was proven

Effects of spherical quasi-crystal on microstructure and mechanical properties of ZA155 high zinc magnesium alloy

Effects of spherical quasi-crystal contained in Mg-Zn-Y-Mn master alloy on the microstructure and as-cast mechanical properties of ZA155 high zinc magnesium alloy have been investigated by means of optical microscopy, XRD, SEM, EDS, tensile test, impact test and hardness test. Experimental results show that the addition of spherical quasi-crystal contained in the Mg-Zn-Y-Mn master alloy into the ZA155 high zinc magnesium alloy resulted in grain refinement of the matrix, changing the morphologies of φ-Al2Mg5Zn2 phase and τ-Mg32(Al, Zn)49 phase from continuous net-like structures to discontinuous strip-like structure and blocky one, respectively. In the present research, the best comprehensive mechanical properties of reinforced ZA155 high zinc magnesium alloy has been obtained when 5.0wt% spherical quasi-crystal was introduced from the Mg-Zn-Y-Mn master alloy into the target alloy system. In such case, the room-temperature tensile strength reached 207 MPa, about 23% higher than that of the base alloy; the impact toughness peaked at 5.5 J/cm2, about 40% higher than that of the base alloy; and the elevated-temperature tensile strength reached 203 MPa, indicating improved heat resistance

An Experimental Examination of the Effects of the Nature and Direction of Hypertext Links on Trust Transfer between Organizations

Trust transfer occurs when a person bases his or her initial trust in an unknown target (a person, group, or organization) on the relationship between that unknown target and another source of trust (e.g., a well known organization). This study examines how hypertext links between organizations on the World Wide Web influence this process. A hypertext link between a trusted organization and an unknown organization has been shown to increase the extent to which consumers perceive that there is a relationship between the two organizations. Perceptions regarding the relationship between organizations have been shown to influence trust in the unknown organization. In the current study, these past findings are extended by examining how the type of link (an advertising link versus a link sent to a partner) and the direction of the link (from a trusted organization to an unknown organization or vice versa) influence the trust transfer process

A Correlation-Based Optical Flowmeter for Enclosed Flows

A low-cost flowmeter would be very useful in a wide variety of monitoring situations. This article discusses the development of such a flowmeter based on optical components and its testing with water in an enclosed flow system. The sensor consisted of two sets of LEDs and phototransistors spaced 4 cm apart, monitoring the optical properties of the fluid at upstream and downstream locations, respectively. A small amount of dye was injected into the flow, which caused a change in the optical properties of the fluid at both locations. The time required for this change to move from the upstream to the downstream locations was determined using the biased estimate of the cross-covariance between the upstream and downstream signals. The velocity was then calculated using this time difference and the known distance between the locations. Tests were conducted at fluid velocities from 0.125 to 4.5 m s-1, and separate results were calculated using phototransistors located 45° and 180° from the LEDs. The mean percent error was between 5% and 0% for individual measurements using the 180° phototransistors at velocities from 0.5 to 4.5 m s-1 and between 2% and -8% for measurements using the 45° phototransistors in the same velocity range. Error increased when the velocity was reduced to 0.5 m s-1 and was greater than 20% at 0.125 m s-1 for both sets of phototransistors. A regression model was developed to correct the velocity estimate. This regression model was validated by conducting an independent test of the sensor under the same conditions. After using the regression model for calibration, errors in the validation set were between 9.1% and -5% for the 180° phototransistors and between 10.5% and -3.6% for the 45° phototransistors for the entire velocity range tested (0.125 to 4.5 m s-1). Finally, the cross-correlation coefficient for each measurement was calculated to determine the degree of similarity between the signals recorded by the phototransistors at the upstream and downstream locations. The cross-correlation coefficient was higher at lower velocities and higher for measurements using the 180° phototransistors

Global bifurcation of positive solutions for a class of superlinear elliptic systems

We are concerned with the global bifurcation of positive solutions for semilinear elliptic systems of the form −∆u = λ f(u, v) in Ω, −∆v = λg(u, v) in Ω, u = v = 0 on ∂Ω, where λ ∈ R is the bifurcation parameter, Ω ⊂ RN, N ≥ 2 is a bounded domain with smooth boundary ∂Ω. We establish the existence of an unbounded branch of positive solutions, emanating from the origin, which is bounded in positive λ-direction. The nonlinearities f , g ∈ C 1 (R × R,(0, ∞)) are nondecreasing for each variable and have superlinear growth at infinity. The proof of our main result is based upon bifurcation theory. In addition, as an application for our main result, when f and g subject to the upper growth bound, by a technique of taking superior limit for components, then we may show that the branch must bifurcate from infinity at λ = 0