Phase behavior of clathrate hydrates: a model for single and multiple gas component hydrates

Presented here is a model that accurately predicts equilibrium pressures as a function of temperature of hydrates with CH₄, C₂H₆, C₃H₈, N₂, H₂, and CO₂ and their mixtures as guests. The model parameters fit to a subset of the equilibrium pressure data for single guest hydrates allow the prediction of phase behavior in mixed guest hydrates. For single guest hydrates, our model improves upon the van der Waals and Platteeuw (vdWP) model with a percent absolute average deviation (%AAD) from all equilibrium pressure data of 5.7% compared to 15.1% for the vdWP model. Predictions of equilibrium pressures for all available mixed guest hydrates result in a 11.6%AAD with our fugacity-based model compared to 18.6% for the vdWP model. Also, our model leads to a prediction of the structure change of the methane-ethane hydrate within 5% of its known equilibrium composition in the vapor phase without any adjustment of its parameters. We have also found that at temperatures above , double occupancy of nitrogen in the large cavity of structure II hydrate is important for the prediction of accurate equilibrium pressures.     

Quantitative analysis of gas hydrates using Raman spectroscopy

A calibration protocol to quantify the compositional information of gas hydrates using Raman spectroscopy is proposed. Structure I pure CH₄‐, CO₂‐ and C₂H₆‐hydrates in their deuterated and hydrogenated forms with known cage occupancies were investigated by Raman spectroscopy. Raman scattering cross sections of CH₄ in the large and small cages were found to be very similar, but not identical. Some C₂H₆ bands of C₂H₆‐hydrate were tentatively reassigned or newly reported and assigned. Our results show that the relative cross sections of guest vibrational modes in the deuterated hydrate are in agreement with those in the hydrogenated hydrate, whereas they are considerably different from those in fluid phase. Using our Raman quantification factors, the relative cage occupancies can now be determined more reliably in CH₄‐hydrates. Moreover, with additional assumptions, the absolute cage occupancies, the bulk guest composition and hydration number of pure or mixed gas hydrates become accessible by Raman spectroscopy. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2155–2167, 2013     

Metastable states during dissociation of carbon dioxide hydrates below 273 K

In this study, the dissociation of isolated carbon dioxide hydrate particles of sizes in the range 0.25–2.5 mm was investigated. It was found that below the ice melting point, the hydrates dissociated into supercooled water (metastable liquid) and gas. The formation of the liquid phase during CO₂ hydrate dissociation was visually observed, and the pressures of the hydrate dissociation into supercooled water and gas were measured in the temperature range 249–273 K. These pressures agreed well with the calculated data for the supercooled water–hydrate–gas metastable equilibrium (Istomin et al., 2006). In the P–T area on the phase diagram bounded by the ice–hydrate–gas equilibrium curve and the supercooled water–hydrate–gas metastable equilibrium curve, hydrates could exist for a long time because the metastable phase and their stability are not connected to the self-preservation effect. The growth of the metastable CO₂ hydrate film on the surface of supercooled water droplets formed during the hydrate dissociation was observed at pressure above the three-phase supercooled water–hydrate–gas metastable equilibrium pressure but still below the three-phase ice–hydrate–gas equilibrium pressure. It was found that the growth rate of the metastable CO₂ hydrate film was higher by a factor of 25 and 50 than that for methane hydrate and propane hydrate, respectively.     

13C NMR analysis and gas uptake measurements of pure and mixed gas hydrates: Development of natural gas transport and storage method using gas hydrate

¹³C NMR spectra were obtained for pure CH₄, mixed CH₄+THF, and mixed CH₄+Neohexane hydrates in order to identify hydrate structure and cage occupancy of guest molecules. In contrast to the pure CH₄ hydrates, the NMR spectra of the mixed CH₄+THF hydrate verified that methane molecules could occupy only the small portion of 5¹² cages because the addition of THF, water-soluble guest component, to aqueous solution prevents the complete filling of methane molecules into small cages. Furthermore, from these NMR results one important conclusion can be made that methane molecules can’t be enclathrated at all in the large 5¹²6⁴ cages of structure II. In addition, gas uptake measurements were carried out to determine methane amount consumed during pure and mixed hydrate formation process. The moles of methane captured into pure CH₄ hydrate per mole of water were found to be similar to the full occupancy value, while the moles of methane captured into the mixed CH₄+THF hydrate per moles of water were much lower than the ideal value. The overall results drawn from this study can be usefully applied to storage and transportation of natural gas.     

Phase and kinetic behavior of the mixed methane and carbon dioxide hydrates

Large amounts of CH₄ are stored as hydrates on continental margins and permafrost regions. If the CH₄ hydrates could be converted into CO₂ hydrate, they would serve double duty as CH₄ sources and CO₂ storage sites in the deep ocean sediments. As preliminary investigations, both the phase behavior of CH₄ hydrates and kinetic behavior of CO₂ hydrate were measured at versatile conditions that can simulate actual marine sediments. When measuring three-phase equilibria (H-LW-V) containing CH₄ hydrate, we also closely examined pore and electrolyte effects of clay and NaCl on hydrate formation. These two effects inhibited hydrate nucleation and thus made the hydrate equilibrium line shift to a higher pressure region. In addition, the kinetic data of CO₂ hydrate in the mixtures containing clay and NaCl were determined at 2.0 MPa and 274.15 K. Clay mineral accelerated an initial formation rate of CO₂ hydrate by inducing nucleation as initiator, but total amount of formed CO₂, of course, decreased due to the capillary effect of clay pores. Also, the addition of NaCl in sample mixtures made both initial formation rate and total amount of CO₂ consumption decrease.     

Microscopic observations of clathrate-hydrate films formed at liquid/liquid interfaces. II. Film thickness in steady-water flow

A series of experiments were carried out to measure, with the aid of direct observations through a high-resolution optical microscope, the local thickness of a ring-shaped hydrate film formed over the surface of each discoid drop of HCFC-141b (CH₃CCl₂F) held stationary in a narrow liquid-water channel confined by two transparent, parallel plates, while the experimental system was controlled at a temperature below the hydrate/liquid-water/liquid-HCFC-141b equilibrium temperature, T_tri, at atmospheric pressure. It was revealed that the thickness may be significantly different from place to place over the same hydrate film and that the thickness at each location decreases with an increase in the water flow rate and with a temperature rise. To clarify the dependency of the local film thickness thus measured on the convective mass transfer of the hydrate-guest substance from the film surface to the water flow, we performed numerical simulations of the convective mass transfer and also chromatographic measurements of the solubility of HCFC-141b in liquid water so that we could predict the local mass transfer coefficient and the mass flux of the hydrate guest, HCFC-141b, at any location along the hydrate-film/liquid-water interface. Plotting the film-thickness data obtained at each temperature level against relevant predictions of the mass transfer coefficient or the mass flux, we found the film thickness to be nearly inversely proportional to the mass transfer coefficient or the mass flux. This finding supports the idea that the film thickness is determined by the balance between the rate of hydrate-crystal dissociation induced by the aforementioned convective mass transfer and the rate of hydrate-crystal formation depending on the liquid-water permeation into the hydrate film.     

Decomposition kinetics and recycle of binary hydrogen‐tetrahydrofuran clathrate hydrate

Decomposition kinetics and recycle of hydrogen–tetrahydrofuran (H₂–THF) clathrate hydrates were investigated with a pressure decay method at temperatures from 265.1 to 273.2 K, at initial pressures from 3.1 to 8.0 MPa, and at stoichiometric THF hydrate concentrations for particle sizes between 250 and 1000 μm. The decomposition was modeled as a two‐step process consisting of H₂ diffusion in the hydrate phase and desorption from the hydrate cage. The adsorption process occurred at roughly two to three times faster than the desorption process, whereas the diffusion process during formation was slightly higher (ca. 20%) than that during decomposition. Successive formation and decomposition cycles showed that occupancy seemed to decrease only slightly with cycling and that there were no large changes in hydrate structure due to cycling. Results provide evidence that the formation and decomposition of H₂ clathrate hydrates occur reversibly and that H₂ clathrate hydrates can be recycled with pressure. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

CO2 sequestration from coal fired power plants

The paper takes into consideration a new approach for CO₂ capture and transport, based on the formation of solid CO₂ hydrates.Carbon dioxide sequestration from power plants can take advantage of the properties of gas hydrates. The formation and decomposition of hydrates from various N₂-CO₂ mixtures has been studied experimentally in a 2 l reactor, to determine the CO₂ separation in terms of hydrate composition and residual CO₂ content in the reacted gas.Carbon dioxide acts as a co-former for the production of hydrates containing nitrogen, besides CO₂. The mixed hydrates that are obtained are less stable than simple CO₂ hydrates. When CO₂ content in the flue gas is higher than 30% by volume, the hydrates formed at 5 MPa are sufficiently concentrated (about 70% CO₂) and carbon dioxide reduction in the reacted gas is acceptable.The application of a process based on hydrate formation could be especially interesting (for CO₂ capture and transport) when connected to an oxy-coal combustion process; in this case the CO₂ content in the flue gas is very high and the hydrate formation is greatly facilitated.     

Carbonation of hydrated calcium silicates

Mass spectrometric-thermogravimetric analyses have been completed on decomposing calcium silicate hydrates produced in the presence of gaseous CO₂ and CO₃^2? ions. These data clearly show a structured evolution of water during decomposition of the calcium silicate hydrates. The evolution of CO₂ is complex. Most of it comes from the decomposition of CaCO₃, but peaks associated with two other processes are observed at temperatures lower than the CaCO₃ decomposition temperature. One of these may be attributed to surface desorption of CO₂, and the other to decomposition of the mineral scawtite, Ca₇Si₆O₂₁H₆. CO₃, formed during hydration. Because CO₂ and H₂O are evolved simultaneously, isothermal decomposition data must be interpreted with caution if meaningful activation energies are to be derived.     

Quantitative analysis of CO2 hydrate formation in porous media by proton NMR

Gas hydrate is a nonstoichiometric crystal compound formed from water and gas. Most nonvisual studies on gas hydrate are unable to detect how much water is converted to hydrates, and thus, the hydrate stoichiometry calculations are inaccurate. This study investigated the CO₂ hydrate formation process in porous media directly and quantitatively. The characteristics of the time-variable consumption of hydrate formation indicated a two-stage formation, hydrate enclathration and continuous occupancy. The enclathration stage occurred in the first 20 min of the formation when considerable heat is released. The continuous occupancy stage lasted longer than the hydrate enclathration because the empty cages in previously formed hydrates would also be occupied. The higher formation pressures can accelerate water consumption and increase cage occupancy. The compositions of completely formed CO₂ hydrates at 2.7, 3.0, and 3.3 MPa and 275.15 K were determined as CO₂·6.90H₂O, CO₂·6.70H₂O, and CO₂·6.49H₂O, respectively.     

Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal

In the current work, molecular dynamics simulation is employed to understand the intrinsic growth of carbon dioxide and methane hydrate starting from a seed crystal of methane and carbon dioxide respectively. This comparison was carried out because it has relevance to the recovery of methane gas from natural gas hydrate reservoirs by simultaneously sequestering a greenhouse gas like CO₂. The seed crystal of carbon dioxide and methane hydrate was allowed to grow from a super-saturated mixture of carbon dioxide or methane molecules in water respectively. Two different concentrations (1:6 and 1:8.5) of CO₂/CH₄ molecules per water molecule were chosen based on gas–water composition in hydrate phase. The molecular level growth as a function of time was investigated by all atomistic molecular dynamics simulation under suitable temperature and pressure range which was well above the hydrate stability zone to ensure significantly faster growth kinetics. The concentration of CO₂ molecules in water played a significant role in growth kinetics, and it was observed that maximizing the CO₂ concentration in the aqueous phase may not result in faster growth of CO₂ hydrate. On the contrary, methane hydrate growth was independent of methane molecule concentration in the aqueous phase. We have validated our results by performing experimental work on carbon dioxide hydrate where it was seen that under conditions appropriate for liquid CO₂, the growth for carbon dioxide hydrate was very slow in the beginning.     

Thermodynamic stability, spectroscopic identification and cage occupation of binary CO2 clathrate hydrates

The hydrate phase behavior of CO₂/3-methyl-1-butanol (3M1B)/water, CO₂/tetrahydrofuran (THF)/water and CO₂/1,4-dioxane (DXN)/water was investigated using both a high-pressure equilibrium viewing cell and a kinetic pressure-temperature measurement system with a constant volume. The dissociation pressures of CO₂/3M1B/water were identical to those of pure CO₂ hydrate, indicating that CO₂ is not acting as a help gas for structure H hydrate formation with 3M1B, thus the formed hydrate is pure CO₂ structure I hydrate. The CO₂ molecules could be encaged in small cages of the structure II hydrate framework formed with both of THF and DXN. For a stoichiometric ratio of 5.56 mol% THF, we found a large shift of dissociation boundary to lower pressures and higher temperatures from the dissociation conditions of pure CO₂ hydrate. From the measurements using the kinetic pressure-temperature system, it was found that the solid binary hydrate samples formed from off-stoichiometric THF and DXN aqueous solutions are composed of pure CO₂ hydrate with a hydrate number n=7.0 and THF/CO₂ and DXN/CO₂ binary hydrates with a molar ratio of xCO₂·THF·17H₂O and xCO₂·DXN·17H₂O, respectively. The X-ray diffraction was used to identify the binary hydrate structure and Raman spectroscopy was measured to support the phase equilibrium results and to investigate the occupation of CO₂ molecules in the cages of the hydrate framework.     

Extraction of methane hydrate energy by carbon dioxide injection-key challenges and a paradigm shift

Significant effort including field work has been devoted to develop a natural gas extraction technology from natural gas hydrate reservoirs through the injection of carbon dioxide. Natural gas hydrate is practically methane hydrate. The hypothesis is that carbon dioxide will be stored as hydrate owing to its favorable stability conditions compared to methane hydrate. Although the dynamics of the CO₂/CH₄ exchange process are not entirely understood it is established that the exchange process is feasible. The extent is limited but even if the CH₄ recovery is optimized there is a need for a CH₄/CO₂ separation plant to enable a complete cyclic sequence of CO₂ capture, injection and CH₄ recovery. In this paper we propose an alternative paradigm to the Inject (CO₂)/Exchange with (CH₄)/Recover (CH₄) one namely Recover (CH₄) first and then Inject (CO₂) for Storage.     

Liquid Salt Hydrate Acid Gas Absorbents: An Unusual Desorption Of Carbon Dioxide And Hydrogen Sulfide Upon Solidification

Abstract

It was recently reported that certain salt hydrate melts can function as pressure swing absorbents for acid gases. The utility of these salt hydrates derives from their large and reversible acid gas absorption capacities. Typical is the salt hydrate tetramethylammonium fluoride tetrahydrate which as a melt absorbs 0.30 mol CO₂/mol salt at 50[ddot]C and 100 kPa CO₂. It has now been discovered that the reactivity of some of these salt hydrate melts with CO₂ and H₂S exhibits an unusual and unexpected temperature dependence. When certain specific salt hydrate melts containing absorbed CO₂ were cooled to temperatures which resulted in solidification, CO₂ was spontaneously desorbed. For example, a sample of tetraethylammonium acetate tetrahydrate (TEAA) containing 0.15 mol CO₂/mol salt at 50[ddot]C and 102 kPa desorbed 90% of its bound CO₂ upon cooling to 26[ddot]C. Gas absorption and desorption are completely reversible, and the absorbent can be cycled by simply raising or lowering the temperature through the point of solidification. Similarly, a sample of TEAA containing 0.30 mol H₂S/mol salt at 50[ddot]C desorbed H₂S upon cooling to 10[ddot]C. A rationale for this unusual temperature-dependent desorption of acid gases from salt hydrates is presented. The modest decrease in temperature required for an abrupt release of gas from TEAA may supply a kind of “on/off” switch for gas absorption which may be of considerable value for the separation of acid gases, particularly H₂S, from process streams.     

Evidence for pore-filling gas hydrates in the sediments through morphology observation

To provide an evidence of natural gas hydrate occurrence state, a series of experiments on multiple growth and dissociation of 90.0% methane/10.0% propane hydrates at 1.3 MPa and 270.15 K were carried out in two sediments for morphology observation via a visible jacketed-reactor. The gas hydrate crystals were observed to form and grow on the surface of sediments at the initial growth. During the thermal decomposition, gas and liquid products had an unceasingly impact on the sediments, then gas/liquid–solid migration occurred, and a large number of cavitation appeared. In the later growth and dissociation experiments, the gas hydrate particles were in suspension or supporting states in the interstitial pore space between the sediment particles, indicating that the gas hydrate displayed a pore-filling characteristics. Through analyzing the distribution of gas hydrates and bubbles, it was found that the amount of gas hydrates distributed in the sediments was improved with multiple growth-dissociation cycle proceedings. Gas migration enhanced the sediment movement, which led to the appearance of the increasing quantity of gas bubbles in the sediments during cycles. Salts affected the growth of the gas hydrates and the migration of sediment grains, which also restricted the accumulation of gas bubbles in the sediments. According to the Raman analysis, the results showed that sII hydrates were formed for CH₄ and C₃H₈ gas mixtures in different sediments and solutions with hydration number of 5.84–6.53. The Salt restricted the access of gas into the hydrate cages.     

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.     

Impact of water film thickness on kinetic rate of mixed hydrate formation during injection of CO2 into CH4 hydrate

In this work, nonequilibrium thermodynamics and phase field theory (PFT) has been applied to study the kinetics of phase transitions associated with CO₂ injection into systems containing CH₄ hydrate, free CH₄ gas, and varying amounts of liquid water. The CH₄ hydrate was converted into either pure CO₂ or mixed CO₂?CH₄ hydrate to investigate the impact of two primary mechanisms governing the relevant phase transitions: solid‐state mass transport through hydrate and heat transfer away from the newly formed CO₂ hydrate. Experimentally proven dependence of kinetic conversion rate on the amount of available free pore water was investigated and successfully reproduced in our model systems. It was found that rate of conversion was directly proportional to the amount of liquid water initially surrounding the hydrate. When all of the liquid has been converted into either CO₂ or mixed CO₂?CH₄ hydrate, a much slower solid‐state mass transport becomes the dominant mechanism. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3944–3957, 2015     

Replacement of CH4 in Hydrate in Porous Sediments with Liquid CO2 Injection

The dynamics of the replacement of CH₄ in hydrate in porous sediments with liquid CO₂ was investigated using a self‐developed experimental apparatus at different temperatures and initial pressures. The pressure increases steadily as the replacement reaction processes. The amount of the replaced CH₄ is almost the same as that of the CO₂ forming hydrate in the early stage and gradually becomes somewhat less in the later stage. The initial pressure has minor effects on the replacement rate, and temperature reduction causes a lower replacement rate. The experimental results suggest that the replacement rate is not related to the region of the temperature‐pressure conditions but is mainly affected by the fugacity differences of CH₄ hydrate decomposition and CO₂ hydrate formation.     

Quantification of the risk for hydrate formation during cool down in a dispersed oil-water system

Gas hydrates are considered a nuisance in the flow assurance of oil and gas production since they can block the flowlines, consequently leading to significant losses in production. Hydrate avoidance has been the traditional approach, but recently, hydrate management is gaining acceptance because the practice of hydrate avoidance has become more and more challenging. For better management of hydrate formation, we investigated the risk of hydrate formation based on the subcooling range in which hydrates form by associating low, medium, and high probability of formation for a gas+oil+water system. The results are based on batch experiments which were performed in an autoclave cell using a mixture gas (CH₄: C₃H₈=91.9 : 8.1 mol%), total liquid volume (200 ml), mineral oil, watercut (30%), and mixing speed (300 rpm). From the measurements of survival curves showing the minimum subcooling required before hydrate can form and hydrate conversion rates for the initial 20 minutes, we developed a risk map for hydrate formation.