Structure and composition of CO2/H2 and CO2/H2/C3H8 hydrate in relation to simultaneous CO2 capture and H2 production

Gas hydrates from a (40/60 mol %) CO₂/H₂ mixture, and from a (38.2/59.2/2.6 mol %) CO₂/H₂/C₃H₈ mixture, were synthesized using ice powder. The gas uptake curves were determined from pressure drop measurements and samples were analyzed using spectroscopic techniques to identify the structure and determine the cage occupancies. Powder X‐ray diffraction (PXRD) analysis at ?110°C was used to determine the crystal structure. From the PXRD measurement it was found that the CO₂/H₂ hydrate is structure I and shows a self‐preservation behavior similar to that of CO₂ hydrate. The ternary gas mixture was found to form pure structure II hydrate at 3.8 MPa. We have applied attenuated total reflection infrared spectroscopic analysis to measure the CO₂ distribution over the large and small cavities. ¹H MAS NMR and Raman were used to follow H₂ enclathration in the small cages of structure I, as well as structure II hydrate. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Characterization of gas hydrates with PXRD, DSC, NMR, and Raman spectroscopy

Structure I (sI) and H (sH) hydrates containing methane were synthesized and characterized with PXRD, DSC, NMR, and Raman spectroscopy. Three well-known large molecule guest substances (LMGSs) were selected as sH hydrate formers: 2,2-dimetylbutane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The solid phase analysis confirmed the presence of sH hydrate whenever a LMGS was present. The presence of a non-hydrate former (n-heptane) did not affect the methane hydrate structure or cage occupancies. Ice to hydrate conversion was limited when the LMGS amount was less than stoichiometric and synthesized at low methane pressure, but nearly complete conversion was achieved with temperature ramping and excess LMGS. The methane occupancies were found to depend on the type of LMGS and increased with pressure. The hydrate with TBME was found to have the smallest methane content followed by the hydrates with NH and MCH. Both NMR and Raman identified methane and LMGS signals from the hydrate phase, however, the cage occupancy values of sH hydrate can only be obtained from NMR spectroscopy. The hydrate structures, ice to hydrate conversion, gas content in hydrate and cage occupancy from the various measurements are consistent with each other.     

Enclathration of hydrogen by organic-compound clathrate hydrates

The properties of hydrogen enclathration by cyclic ethers and acetone clathrate hydrates were investigated by powder X-ray diffraction, Raman spectroscopic analysis and volumetric analysis. Powder X-ray diffraction profiles indicate that the hydrates are structure-II hydrates. The variation in lattice constant by hydrogen occupation was investigated. This result indicates that inclusion of H₂ atom within empty small cage changes size of host cages depending on type of guest molecule. Raman results show that the samples formed binary clathrate hydrate of hydrogen and each organic compound. The amount of encaged H₂ was found to be comparable to that of H₂–THF binary hydrate. The trend of the changes for lattice constants is not related to the amount of encaged H₂. These results suggest that the organic compounds investigated in this study can be used as alternatives to THF for H₂ enclathration.     

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 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.     

Notes on the dissolution rate of gas hydrates in undersaturated water

An elementary model for the dissolution of pure hydrate in undersaturated water is proposed that combines intrinsic decomposition within a desorption film and the subsequent diffusion of the released hydrate guest species into bulk water. Applying the proposed approach to recently published measurements of the decomposition rates of methane (CH₄) and carbon dioxide (CO₂) hydrates in deep seawater suggests that the concentration of the hydrate guest species at the interface between desorption film and diffusive boundary layer may be much lower than ambient solubility. Calculations, however, fail to account for the observed proportionality of decomposition rate with solubility for both CH₄ and CO₂ hydrates. This may indicate a limitation in the range of applicability of published formulas for intrinsic hydrate decomposition rates.     

Hydrogen and carbon dioxide adsorption with tetra‐n‐butyl ammonium semi‐clathrate hydrates for gas separations

Gas adsorption rates of H₂, CO₂, and H₂‐CO₂ gas mixture (H₂/CO₂ = 3.4) with tetra‐n‐butyl ammonium salt (bromide, chloride, and fluoride) semi‐clathrate hydrate particles were measured at 269 K to assess their properties for gas separation. Equilibrium gas occupancies in the S‐cages of the particles were in order of (high to low) for hexagonal structure‐I, tetragonal structure‐I, and superlattice of cubic structure‐I structures with the maximum fractional occupancy by CO₂ being about 40%. The CO₂ diffusion rate depended on the anion size of the salt, which is attributed to distortion of the S‐cage that is close to the molecular size of CO₂. Simulations of semi‐clathrate hydrate particles with theory showed that H₂/CO₂ selectivities could be as high as 36 (3.0 mol% TBAF) and that selectivities for an ideal membrane (3.3 mol% TBAF) could be >100 (269 K, 0.3–4.5 MPa). Semi‐clathrate hydrates have wide application as separation media for gas mixtures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 992–1003, 2015     

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     

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.     

Thermodynamic model for predicting phase equilibria of simple clathrate hydrates of refrigerants

In this communication, a thermodynamic model is presented for the study of the phase equilibria of clathrate hydrates of simple refrigerants. The van der Waals–Platteeuw solid solution theory is used to model the hydrate phase while it is assumed that vapor phase is an ideal gas of refrigerant ignoring its water content and the aqueous phase is considered as pure water (activity coefficient=1) ignoring aqueous solubility. The results through this model are successfully compared with the experimental data reported in the literature for clathrate hydrates of four refrigerants namely C₂H₂F₄ (1, 1, 1, 2-tetrafluoroethane or HFC-134a or R-134a), C₂H₄F₂ (1, 1-difluoroethane or HFC-152a or R-152a), CH₂F₂ (difluoromethane or HFC-32 or R-32), and C₂H₃Cl₂F (1,1-dichloro-1-fluoroethane or HCFC-141b or R-141b).     

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.     

Hydrogenation of carbon dioxide on cobalt catalysts – activation,deactivation, influence of carbon monoxide

A batch reactor directly combined with an ultrahigh vacuum apparatus, which is equipped with facilities for catalyst preparation and Auger electron spectroscopy, was used to answer some questions which had arisen in recent studies concerning carbon dioxide hydrogenation on pure metallic and supported Co catalysts. Both oxygen incorporated during oxidation/reduction cycles and carbon deposited when CO₂ is hydrogenated penetrate deep into the bulk. This kind of carbon can easily be hydrogenated. CO strongly hinders the reduction of the oxidized Co surface in the H₂/CO₂ reaction mixture (4 : 1). CO hydrogenation is favoured over CO₂ hydrogenation and leads to a higher percentage of C₂ to C₄ hydrocarbons as compared with CH₄ formation.     

Spectroscopic observation of H<Subscript>2</Subscript> migration in structure-I clathrate hydrate

We demonstrate the spectroscopic observation of H₂ migration in the binary structure-I (sI) clathrate hydrate. The H₂ molecules captured into sI small cage (sI-S) at lower temperature migrate to sI large cage (sI-L) through shared pentagonal face of 5¹²6² cage. The hexagonal faces of 5¹²6² cage provide the windows essential for creating continuous diffusion paths for H₂ molecules. It is essential to realize that the vacant channels formed by the linkage of specific cages can play an important role in guest diffusion pathways and occupancy occurring in a complex clathrate hydrate matrix.     

Compositional and structural identification of natural gas hydrates collected at site 1249 on ocean drilling program leg 204

In contrast to the structural studies of laboratory-grown gas hydrate, this study has been performed on naturally grown clathrate hydrates from the sea floor. The PXRD pattern of natural gas hydrate shows that the sample had a structure I hydrate. The¹³C NMR spectrum was obtained for the natural gas hydrate sample in order to identify the cage occupancy of guest molecules and determine the hydration number. The NMR spectrum reveal that the natural gas hydrates used in this study contain only methane with no noticeable amount of other hydrocarbons. The existence of two peaks at different chemical shifts indicates that methane molecules are encapsulated in both large and small cages. In addition, Raman spectroscopic analysis is also carried out to identify natural hydrates and compared with the NMR results. Investigating the composition and structure of natural gas hydrates is essential for applying natural gas hydrates as a novel energy source.     

Enhancement of the performance of kinetic inhibitors in the presence of polyethylene oxide and polypropylene oxide for binary mixtures during gas hydrate formation in a flow mini‐loop apparatus

The objective of this work is to demonstrate the impact of the polyethylene oxide (PEO) and polypropylene oxide (PPO) on the performance of gas hydrate kinetic inhibitors for binary mixtures during gas hydrate formation in a flow mini‐loop apparatus. PEO and PPO are commercially available polymers that they have been considered to be unable to exhibit kinetic hydrate inhibition (KHI) by their self. Prevention of gas hydrate formation experiments in the presence of the KHIs solutions were conducted in a flow mini‐loop apparatus manner under suitable pressures and temperature conditions for binary gaseous mixtures including 70% CH₄/30% C₃H₈, 30% CH₄/70% C₃H₈, 70% CH₄/30% i‐C₄H₁₀, and 30% CH₄/70% i‐C₄H₁₀. In the experiments, induction time for crystallisation of gas hydrate formation and gas consumption rate are investigated in systems without KHI, containing KHI only (such as polyvinylpyrrolidone (PVP) and L ‐tyrosine) and PEO or PPO together with KHI. Pressure is maintained at a constant value during experimental runs by means of required gas make‐up. The addition of a KHI into system delayed the onset of hydrate crystal nucleation. Furthermore, addition of the PEO or PPO to a KHI solution was found to enhance the performance of KHI. In addition, under the same pressure temperature hydrate formation conditions the induction time is longer when the PPO is present. Thus, inclusion of PPO into a KHI solution shows a higher enhancement in its inhibiting performance compare to PEO. © 2011 Canadian Society for Chemical Engineering     

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.     

Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel

The performance of two gas/liquid contact modes was evaluated in relation to the rate of gas hydrate formation. Hydrate formation experiments were conducted for several gas mixtures relevant to natural gas hydrate formation in the earth (CH₄, CH₄/C₃H₈, CH₄/C₂H₆ and CH₄/C₂H₆/C₃H₈) and two CO₂ capture and storage (CO₂, CO₂/H₂/C₃H₈). One set of experiments was conducted in a bed of silica sand, saturated with water (fixed fed column) while the other experiment was conducted in a stirred vessel for each gas/gas mixture. Both sets of experiments were conducted at a constant temperature. The rate of hydrate formation is customarily correlated with the rate of gas consumption. The results show that the rate of hydrate formation in the fixed bed column is significantly greater and thereby resulted in a higher percent of water conversion to hydrate in lesser reaction time for all the systems studied.     

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.     

Thermodynamic and kinetic study of fluorinated gas hydrates for water purification

This study investigated and compared the thermodynamic stability, kinetic behaviour, and effectiveness of a water purification process using pentafluoroethane (HFC125a) and 1,1,1,2-tetrafluoroethane (HFC134a) as guest molecules. The hydrate phase equilibria of each fluorinated gas (F-gas) in pure water and NaCl solution were predicted using the Hu-Lee-Sum correlation, which agreed well with the experimental results from our previous studies. Under the same subcooling temperature of 3 K (at 0.3 MPa), the rate of hydrate growth with HFC134a was faster than that of HFC125a in the absence or presence of NaCl. In situ Raman spectroscopy confirmed that the HFC134a and HFC125a molecules occupy only a large cage of structure II hydrate. The Raman shifts of C H and C C bands in all phases (gas, liquid, and hydrate phases) of HFC125a shifted to higher wavelengths than those of HFC134a due to the increase in the number of fluorine atoms. The change in the salinity was studied to evaluate the effectiveness of an F-gas hydrate-based water purification process. In addition, the desalination efficiency of the HFC134a and HFC125a hydrates was compared by separating hydrate crystals from the slurries. The results showed that the desalination efficiency (or total dissolved solids removal efficiency) of HFC134a hydrate was higher than that of HFC125a hydrate. This study proves the importance of the water purification process using hydrates.     

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.