Ethane Hydrate Research Articles

Study of ethane hydrate formation kinetics using the chemical affinity model with and without presence of surfactants

Abstract In this article, we examine ethane hydrate formation kinetics with and without the presence of various surfactants. Firstly, the influences of stirring rate and initial pressure without the presence of surfactants are studied. The effects of four surfactants containing Sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), Polyoxyethylene (20) sorbitanmonopalmitate (Tween® 40), and TritonX-100 (TX-100) on ethane hydrate formation kinetics were experimentally investigated. Then the chemical affinity model is applied to model the ethane hydrate formation kinetics with and without surfactants. The kinetic parameters of the chemical affinity model were computed for the ethane hydrate formation with and without surfactants. We will see that the results of the modeling are in good agreement with the experimental data.

Effects of process variables on the initial gas hydrate formation rate: The case of ethane hydrate formation in the absence or presence of SDS kinetic promoter

The ethane (C2) gas hydrate formation kinetic at the onset of solution turbidity in the absence or presence of sodium dodecyl sulfate (SDS) kinetic promoter is experimentally and theoretically investigated. The time dependent heterogeneous nucleation and growth kinetics are simultaneously employed for the determination of the initial rate of gas hydrate formation. It is shown that the effective diffusion coefficient of ethane in the solution is one of the most important influencing physical properties regarding gas hydrate growth process. At the SDS concentration of 500ppm, the maximum ethane effective diffusion coefficient and the highest conversion rate are obtained. The C2 hydrate–liquid interfacial tension according to the suggested heterogeneous nucleation mechanism is about 17.3mJm−2 for all cases. The overall average absolute deviations between predicted and measured C2 consumption rates are about 0.63% and 0.75% regarding non-promoted and promoted gas hydrate formation processes respectively.

New Observations and Insights into the Morphology and Growth Kinetics of Hydrate Films

The kinetics of film growth of hydrates of methane, ethane, and methane-ethane mixtures were studied by exposing a single gas bubble to water. The morphologies, lateral growth rates, and thicknesses of the hydrate films were measured for various gas compositions and degrees of subcooling. A variety of hydrate film textures was revealed. The kinetics of two-dimensional film growth was inferred from the lateral growth rate and initial thickness of the hydrate film. A clear relationship between the morphology and film growth kinetics was observed. The shape of the hydrate crystals was found to favour heat or mass transfer and favour further growth of the hydrate film. The quantitative results on the kinetics of film growth showed that for a given degree of subcooling, the initial film thicknesses of the double hydrates were larger than that of pure methane or ethane hydrate, whereas the thickest hydrate film and the lowest lateral growth rate occurred when the methane mole fraction was approximately 0.6.

Gas hydrate phase equilibrium in porous media: An assessment test for experimental data

In this communication, experimental dissociation data for carbon dioxide, methane, and ethane hydrates in the presence of pure water and different types of porous media (mesoporous silica gel, porous glass, and silica gel) were assessed, in order to detect any suspect or doubtful data. The Leverage statistical model algorithm was used in which determination of the statistical hat matrix, sketching of the Williams plot, and calculation of the residuals of selected correlation results are included. Gas hydrate dissociation conditions in porous media were predicted using of the least squares support vector machine (LSSVM) algorithm. This study provides an overview on the quality of existing experimental data (outlier detection), as well as the applicability domain of the model employed.

A unified approach for description of gas hydrate formation kinetics in the presence of kinetic promoters in gas hydrate converters

The kinetic promoters have found wide applications in enhancing the rate of energy conversion and storage via gas hydrate formation processes. Effects of different kinetic promoters such as anionic surfactants sodium dodecyl sulfate (SDS), dodecylbenzene sulfonic acid (DBSA), and sodium dodecyl benzene sulfonate (SDBS); cationic surfactants, Cetyl trimethyl ammonium bromide (CTAB), dodecyl trimethyl ammonium bromide (DTAB) and non-ionic surfactants, alkylpolyglucoside (APG), dodecyl polysaccharide glycoside (DPG), TritonX-100 (TX100) on methane (CH4), ethane (C2H6) and propane (C3H8) gas hydrate formation processes are investigated in this work. A macroscopic kinetic model based on the time variations of reaction chemical potential is also presented for global description of gas hydrate formation processes. Experimental gas hydrate formation data are employed to validate the proposed kinetic model. Effects of promoter’s concentrations and agitation intensities on the gas consumption profiles are also investigated. A universal correlation and a unified kinetic map have been proposed for macroscopic description of gas hydrate formation kinetics in the presence or absence of kinetic promoters. According to the presented unified kinetic map, a unique region of gas hydrate formation is identified for the first time. For negligible amounts of kinetic promoters, the presented region disappears and approaches to a unique path at high agitation intensities. The presented unified approach is very useful for global understanding of gas hydrate formations processes in the absence or presence of kinetic promoters.

Equilibrium of Ethane Hydrate Formation in Carbon Pores

The formation of hydrates in carbon pores was utilized to store natural gas, and ethane is the second major component of natural gas; therefore, ethane hydrate formation in carbon pores is studied in comparison with that of methane. Ethane is a major raw material of ethylene in the petrochemical industry; therefore, its mixture with ethylene is of importance and the formation condition of ethane hydrates was compared also with that of ethylene. The equilibrium isotherms of ethane on wet carbons with different water contents as well as the isotherms for different temperatures were collected, from which the formation pressures of ethane hydrates as well as the enthalpy change of ethane hydrate formation were determined. It was shown that the largest volumetric capacity for ethane storage is observed at the water load that just about fully fills pore spaces. Ethane hydrates were formed in wet activated carbons at about same pressures as in water media and the formation pressure is much lower than that required for methane hydrates formation.

Phase Equilibria of Clathrate Hydrates of Ethane + Ethene

Dissociation conditions for simple and mixed hydrates of ethane and ethene (ethylene) have been measured using the isochoric pressure-search technique. Simple hydrate dissociation data for ethane and ethene measured in this work are compared to existing literature data. The agreement between the measured ethane hydrate dissociation data with the literature data is acceptable while some deviations between the measured ethene hydrate dissociation data and those reported in the literature are observed. All new data for simple and mixed hydrates of ethene and ethane are well-correlated using an empirical correlation proposed by Adisasmito and co-workers. However, agreement between the experimental data with the predictions of the CSMGem model is found to be unsatisfactory. A gas chromatography technique combined with the isochoric-pressure search method is used to measure composition of the vapor phase in equilibrium with the hydrate and aqueous phases inside the hydrate stability region. The feed compositions investigated were 0.174, 0.420, and 0.810 mole fractions of ethane. In addition, dissociation points were found to be in the vicinity of the ethane hydrate dissociation conditions regardless of the initial content of ethane in the feed. It was also found that temperature does not have a considerable effect on ethane content of the vapor phase in equilibrium with hydrate and aqueous phases inside the hydrate stability region at given pressures.

Theory Research on Desalination of Brackish Water Using Gas Hydrate Method

In allusion to some methods on desalination of brackish water such as distillation, electrodialysis method and reverse osmosis, this paper introduced the mechanism, advantages and disadvantages of these methods. It is convinced that these methods have higher energy consumption. Temperature is much lower annually in alpine region of China, it provides a good condition for hydrate formation. It also provides the possibility for the desalinating process of brackish water using hydrate methods in alpine region. This paper proposed a new method of desalinating brackish water. Based on the model of van der Waals-Platteeuw, we calculated the phase equilibrium conditions of methane, ethane and propane hydrate in NaCl solution combined parameter equation which was used to calculate the Langmuir constant presented by DU Ya and GUO Tian-min and Pitzer equation used to water activity in the system of electrolyte solution. The result shows that it is possible to desalinate brackish water using environment cold quantity. It also shows that propane is more suitable for brackish water desalination among methane, ethane and propane. It is very significantly important to seek energy-saving and environment-friendly methods for the process of desalinating brackish water with energy shortage increasingly intensifying and with awareness of human being enthancing on energy conservation and emission reduction.

Prediction of hydrate dissociation conditions for alkanes in the presence of alcohol and electrolyte solutions using ion-based statistical associating fluid theory

Ion-based statistical associating fluid theory (SAFT2), coupled with van der Waals and Platteeuw theory, is applied to predict methane, ethane, and propane hydrate dissociation conditions in the presence of ethanol, ethylene glycol (MEG), and glycerol, as well as mixed MEG and electrolyte solutions, including NaCl, KCl, and CaCl2. Ion-based SAFT2 is found to give accurate predictions for these complex systems. The effects of temperature and concentration on hydrate dissociation conditions are well captured.

Equilibrium Conditions of Ethane Hydrates in the Presence of Aqueous Solutions of Alcohols, Glycols, and Glycerol

Equilibrium Conditions of Ethane Hydrates in the Presence of Aqueous Solutions of Alcohols, Glycols, and Glycerol

First-principles investigation on the structural stability of methane and ethane clathrate hydrates

Clathrate hydrates are characterized by their hydrogen-bond linked cavities that can encapsulate guest molecules. First-principles calculations were performed in this study to compare the prototypical structures of methane and ethane hydrates. The relative stability of singly and doubly occupied structures was studied in terms of their structural variations and binding energies. The preferred structures under various gaseous components (methane, ethane, and their mixture) were identified and compared with experimental evidences. The influence of tetrahydrofuran (THF) on the formation of methane and ethane hydrates was rationalized from computations on the methane–ethane–THF-hydrate systems.

Dissociation Behavior of C2H6 Hydrate at Temperatures below the Ice Point: Melting to Liquid Water Followed by Ice Nucleation

The dissociation of C(2)H(6) hydrate particles by slow depressurization at temperatures slightly below the ice melting point was studied using optical microscopy and Raman spectroscopy. Visual observations and Raman measurements revealed that ethane hydrates can be present as a metastable state at pressures lower than the dissociation pressures of the three components: ice, hydrate, and free gas. However, they decompose into liquid water and gas phases once the system pressure drops to the equilibrium boundary for supercooled water, hydrate, and free gas. Structural analyses of obtained Raman spectra indicate that structures of the metastable hydrates and liquid water from the hydrate decay are fundamentally identical to those of the stable hydrates and supercooled water without experience of the hydration. These results imply a considerably high energy barrier for the direct hydrate-to-ice transition. Water solidification, probably induced by dynamic nucleation, was also observed during melting.

Compressibility of Gas Hydrates

Experimental data on the pressure dependence of unit cell parameters for the gas hydrates of ethane (cubic structure I, pressure range 0-2 GPa), xenon (cubic structure I, pressure range 0-1.5 GPa) and the double hydrate of tetrahydrofuran+xenon (cubic structure II, pressure range 0-3 GPa) are presented. Approximation of the data using the cubic Birch-Murnaghan equation, P=1.5B(0)[(V(0)/V)(7/3)-(V(0)/V)(5/3)], gave the following results: for ethane hydrate V(0)=1781 Å(3) , B(0)=11.2 GPa; for xenon hydrate V(0)=1726 Å(3) , B(0)=9.3 GPa; for the double hydrate of tetrahydrofuran+xenon V(0)=5323 Å(3) , B(0)=8.8 GPa. In the last case, the approximation was performed within the pressure range 0-1.5 GPa; it is impossible to describe the results within a broader pressure range using the cubic Birch-Murnaghan equation. At the maximum pressure of the existence of the double hydrate of tetrahydrofuran+xenon (3.1 GPa), the unit cell volume was 86% of the unit cell volume at zero pressure. Analysis of the experimental data obtained by us and data available from the literature showed that 1) the bulk modulus of gas hydrates with classical polyhedral structures, in most cases, are close to each other and 2) the bulk modulus is mainly determined by the elasticity of the hydrogen-bonded water framework. Variable filling of the cavities with guest molecules also has a substantial effect on the bulk modulus. On the basis of the obtained results, we concluded that the bulk modulus of gas hydrates with classical polyhedral structures and existing at pressures up to 1.5 GPa was equal to (9±2) GPa. In cases when data on the equations of state for the hydrates were unavailable, the indicated values may be recommended as the most probable ones.

Dissociation Termination of Methane–Ethane Hydrates in Temperature‐Ramping Tests at Atmospheric Pressure below the Melting Point of Ice

Dissociation Termination of Methane–Ethane Hydrates in Temperature‐Ramping Tests at Atmospheric Pressure below the Melting Point of Ice

Metastability of Ethane Clathrate Hydrate Induced by [Co(NH3)6]3+Complex

The metal complex of [Co(NH 3 ) 6 ] 3+ is introduced to C 2 H 6 hydrate to confirm its possible inclusion in hydrogen-bonded water cages and the occurrence of metastable structure. The 13 C NMR spectra of C 2 H 6 +([Co(NH 3 ) 6 ]Cl 3 + 6NaOH in D 2 O) hydrate confirmed a new peak at 6.5 ppm matching with C 2 H 6 in sII-L cages. The retarded appearance of metastable sII phase is due to brine rejection of the cobalt complex occurring during solution freezing. The anions of OH and F were found to be incorporated in the host water cage framework, providing proton-deficient sites. The ionic conductivity of the frozen [Co(NH 3 ) 6 ] 3+ solution increased up to 20-fold after ethane hydrate formation, implying the incorporation of F ― into the host lattice. A notable finding of this work is that the metastability occurs only when the cobalt complex is in the presence of anions such as OH ― and F ― .

Modeling of methane and ethane hydrate formation kinetics based on non-equilibrium thermodynamics

In this study, experimental data of the kinetics of methane and ethane hydrate formation at constant volume were collected. The experiments were carried out in a batch reactor at different temperatures and pressures. The property of chemical affinity was used in the modeling of hydrate formation rate in a constant volume process. In this model a macroscopic driving force was defined which only needed the initial (experimental condition, temperature, and pressure) and final conditions (equilibrium conditions); thus, this model did not have the limitations of microscopic models, such as heat and mass transfer coefficients or population of particles, which may differ for each experiment. The experiments were carried out at temperatures 273, 274, 275.5, and 276 K for methane and 272, 273, 274, and 275 K for ethane with different initial pressures. The parameters of the model, Ar and tK, were obtained for each experiment, and the results show that the parameter of for each gas has a constant value. Subsequently, parameters of the model were used to predict experimental data and the variation of pressures with time. The results indicated that this model can well predict constant volume experimental data for crystals I hydrate former.

Gas Hydrate Phase Equilibrium in the Presence of Ethylene Glycol or Methanol Aqueous Solution

In this communication, experimental hydrate dissociation data for the ethane, carbon dioxide, or hydrogen sulfide + ethylene glycol + water system are first reported at 0.1, 0.2, 0.35, and 0.5 mass fractions of ethylene glycol in aqueous solution. The experimental data were generated using an isochoric pressure-search method. The experimental hydrate dissociation data are compared with the corresponding literature data and the experimental dissociation data for ethane, carbon dioxide, or hydrogen sulfide simple hydrates in the presence of pure water reported in the literature. A discussion is made on the reliability of the few sets of experimental hydrate dissociation data reported in the literature for the ethane, carbon dioxide, or hydrogen sulfide + ethylene glycol + water system. We finally report experimental hydrate dissociation data for the methane + methanol + water system at 0.55 and 0.65 mass fractions of methanol in aqueous solution. The experimental hydrate dissociation data are compared with the corresponding literature data and the experimental dissociation data for methane hydrates in the presence of pure water reported in the literature. It is shown that the few sets of experimental hydrate dissociation data reported in the literature for the methane + methanol + water system at high concentrations of methanol in aqueous solution may not be very reliable. A discussion is made on the inhibition effect of methanol at its very high concentration in aqueous solution.

Phase Equilibria of Ethane Hydrate in MgCl2 Aqueous Solutions

In this work, phase equilibrium data for ethane hydrate in the presence of MgCl2 aqueous solutions were reported at concentrations of (0.05, 0.3, 0.5, 0.8, and 1.2) mol·dm−3 MgCl2 in the temperature and pressure ranges of (276.2 to 286.0) K and (1.12 to 2.70) MPa, respectively. Measurements were made using the isochoric pressure-search method. The results showed that the addition of MgCl2 caused the phase equilibrium temperatures to be lowered. The difference of phase equilibrium temperatures between pure water and a solution at a given pressure increased with the increasing concentration of MgCl2.

Changes in structure and preferential cage occupancy of ethane hydrate and ethane-methane mixed gas hydrate under high pressure

Structural changes and preferential cage occupancies were examined for ethane hydrate and ethane-methane mixed gas hydrates with five compositions in a pressure range of 0.2 to 2.8 GPa at room temperature. X-ray diffractometry and Raman spectroscopy showed the following structural changes. The initial structure, structure I (sI), of ethane hydrate was retained up to 2.1 GPa without any structural change. For the mixed hydrates, sI was widely distributed throughout the region examined except for the methane-rich and lower pressure regions, where sII and sH appeared. Above 2.1 GPa ethane hydrate and all of the mixed hydrates decomposed into ice VI and ethane fluid or methane-ethane fluid, respectively. The Raman study revealed that occupation of the small cages by ethane molecules occurred above 0.1 GPa in ethane hydrate and continued up to decomposition at 2.1 GPa, although it was thought that ethane molecules were contained only in the large cage.

Modeling of phase transition sI−sII in binary gas hydrates of methane and ethane in dependence on composition of gas phase

Calculations of phase transformations ‘structure sI–structure sII’ for binary methane–ethane hydrates were performed in the framework of molecular level model. The conditions of hydrate formation in equilibrium with gas phase and ice have been determined at different gas-phase compositions and pressures. It was shown that even at very low ethane concentration in the gas phase (about 0.5%) hydrate structure sII becomes more stable than sI. Regions of stability in T, P plane of sI and sII structures for mixed hydrates have been predicted. For mixed hydrates with ethane content about 5% in gas phase pressure-induced phase transformation at low temperatures is predicted.