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.     

Gauche conformation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates

In the present study, measurements and analyses were made of the High-Power Decoupling (HPDEC) solid-state ¹³C NMR spectra of structure-H (sH) methane hydrates with isopentane, one of the simplest and smallest acyclic large guest molecules, and methylcyclohexane (MCH), a commonly used cyclic guest molecule that is larger than isopentane. From the spectroscopic information, clear and definite evidence for the molecular conformation of acyclic guest molecules that are sufficiently small so as to be entrapped into the structure-H large cage (sH-L) was expected. The ¹³C NMR chemical shift change was additionally checked through the use of a hydrogen-hydrogen steric perturbation model. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities. The present results strongly suggest that the guest position and structure in hydrate cages are greatly influenced by both short-range interactions between guest molecules and cage frameworks and long-range interactions between small and large guests. Accordingly, cage dynamics must be carefully considered when a specific sH hydrate is designed and synthesized for the purpose of tuning material properties.     

Equilibrium and crystallographic measurements of the binary tetrahydrofuran and helium clathrate hydrates

Clathrate compounds are crystalline materials formed by a physical interaction between host and relatively light guest molecules. Various types of nano-sized cages surrounded by host frameworks exist in the highly unique crystalline structures and free guest molecules are entrapped in an open host-guest network. Recently, we reported two peculiar phenomena, swapping and tuning, naturally occurring in the hydrate cages. Helium, one of the smallest light guest molecules, must be the challengeable material in the sense of physics and moreover possesses versatile applications in the field of superconductivity technology and thermonuclear industry. In this regard, we attempted for the first time to synthesize helium hydrates at moderate temperature and pressure conditions. According to inclusion phenomena, helium itself normally cannot form clathrate hydrates due to being too small molecularly without the help of hydrate former molecules (sI, sII, and sH formers). In this study, the hydrate equilibria of the binary clathrate hydrate containing tetrahydrofuran, helium, and water were determined at 2, 3, 5.56 THF mol%. Direct volumetric measurements were also carried out to confirm the exact amount of helium captured in the hydrate cages. Finally, the crystalline structure of the formed mixed hydrates was identified by powder X-ray diffraction, resulting in structure II.     

Hydrate phase equilibria for gas mixtures containing carbon dioxide: A proof-of-concept to carbon dioxide recovery from multicomponent gas stream

Three-phase equilibrium conditions (aqueous liquid-hydrate-vapor) of CO₂-N₂ binary mixtures in the temperature range of 271.75 K to 284.25 K and the pressure range of 12 to 235 bar. In addition, three-phase (aqueous liquid-hydrate-vapor) behavior for CO₂-CH₄ mixture were measured in the temperature range of 272 to 284 K at the constant pressures of 15, 20, 26, 35 and 50 bar. In high concentration of CO₂, upper quadruple points were also measured. The obtained data indicates that three-phase equilibrium temperatures become higher with increasing concentration of CO₂. For the prediction of three-phase equilibrium, the vapor and liquid phases were treated by employing the Soave-Redlich-Kwong equation of state (SRK-EOS) with the second order modified Huron-Vidal (MHV2) mixing rule and the hydrate phase with the van der Waals-Platteeuw model. The calculated results showed good agreement with experimental data. The concentration of vapor and hydrate phases was also determined experimentally. This work can be used as the basic data for selective separation process by hydrate formation. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

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.     

Phase equilibria and kinetic behavior of co<Subscript>2</Subscript> hydrate in electrolyte and porous media solutions: application to ocean sequestration of CO<Subscript>2</Subscript>

Understanding the phase behavior and formation kinetics of CO₂ hydrate is essential for developing the sequestration process of CO₂ into the deep ocean and its feasibility. Three-phase equilibria of solid hydrate, liquid water, and vapor were determined for aqueous mixtures containing CO₂ and NaCl/clay to examine the effect of both ocean electrolytes and sediments on hydrate stability. Due to the capillary effect by clay pores and inhibition effect by NaCl the corresponding hydrate formation pressure appeared to be a little higher than that required for simple and pure hydrate at specified temperature. In addition, the hydrate formation kinetics of carbon dioxide in pure water and aqueous NaCl solutions with or without clay mineral were also measured at various conditions. The formation kinetic behavior was found to be strongly influenced by pressure, temperature and electrolyte concentration. A simplified kinetic model having two adjustable parameters was proposed and the estimated results agreed well with the experimental data. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

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.     

Kinetics of absorption of carbon dioxide in aqueous piperazine solutions

In the present work the absorption of carbon dioxide into aqueous piperazine (PZ) solutions has been studied in a stirred cell, at low to moderate temperatures, piperazine concentrations ranging from 0.6 to , and carbon dioxide pressures up to 500 mbar, respectively. The obtained experimental results were interpreted using the DeCoursey equation [DeCoursey, W., 1974. Absorption with chemical reaction: development of a new relation for the Danckwerts model. Chemical Engineering Science 29, 1867-1872] to extract the kinetics of the main reaction, 2PZ+CO₂→PZCOO^-+PZH⁺, which was assumed to be first order in both CO₂ and PZ. The second-order kinetic rate constant was found to be at a temperature of , with an activation temperature of . Also, the absorption rate of CO₂ into partially protonated piperazine solutions was experimentally investigated to identify the kinetics of the reaction . The results were interpreted using the Hogendoorn approach [Hogendoorn, J., Vas Bhat, R., Kuipers, J., Van Swaaij, W., Versteeg, G., 1997. Approximation for the enhancement factor applicable to reversible reactions of finite rate in chemically loaded solutions. Chemical Engineering Science 52, 4547-4559], which uses the explicit DeCoursey equation with an infinite enhancement factor which is corrected for reversibility. Also, this reaction was assumed to be first order in both reactants and the second-order rate constant for this reaction was found to be at 298.15 K.     

Experimental Study of Methane Hydrate Decomposition Kinetics

Experimental investigations of methane hydrate dissociation kinetics were performed. The test rig consists of a stirred reactor equipped with particle size analysis. The observed dissociation rates were found to be about one order of magnitude faster than previously reported by others. A mass transfer control of the dissociation process is proposed to dominate in the proximity of a dispersed hydrate‐liquid interface. The results are relevant for the design of processes employing dispersed gas hydrates in chemical engineering and the production of natural gas from dispersed deposits.     

Kinetics of carbon dioxide sorption on potassium-doped lithium zirconate

Potassium-doped lithium zirconate (Li₂ZrO₃) sorbents with similar crystallite but different aggregate sizes were prepared by a solid-state reaction method from mixtures of Li₂CO₃, K₂CO₃, and ZrO₂ of different particle sizes. Carbon dioxide sorption rate on the prepared Li₂ZrO₃ sorbents increases with decreasing sorbent aggregate size. It is the size of the aggregate, not the crystallite, of Li₂ZrO₃ that controls the sorption rate. Temperature effect on CO₂ sorption is complex, depending on both kinetic and thermodynamic factors. A mathematical model based on the double-shell sorption mechanism was established for CO₂ sorption kinetics and it can fit experimental data quite well. Above 500°C, the rate-limiting step of CO₂ sorption is the diffusion of oxygen ions through the ZrO₂ shell formed during the carbonation reaction. Oxygen ion conductivities in the ZrO₂ shell were obtained by regression of the experimental CO₂ uptake curves with the model and are consistent with the literature data.     

Kinetics and rate of enzymatic hydrolysis of cellulose in supercritical carbon dioxide

Experiments were carried out on the application of supercritical fluid to the hydrolysis of cellulose by the enzyme, cellulase. The stability of cellulase was sustained at the pressures of up to 160 atm for 90 min at 50 ‡C in supercritical carbon dioxide. In the hydrolysis of cellulose the glucose yield was 100% at supercritical condition. Kinetic constants of hydrolysis at supercritical condition were increased as compared to those at atmospheric condition. The hydrolysis reaction was found competitively inhibited by glucose at supercritical condition.     

Structure and thermal expansion of natural gas clathrate hydrates

We report on the structural properties of natural gas hydrate crystals from the Sea of Okhotsk. Using powder X-ray diffraction (PXRD), it was determined that sediments from four locations contained type I gas hydrate, which encage mostly methane (96-98%) and a small amount of carbon dioxide. For all hydrates, the lattice constant was estimated to be at 113 K, which approximately equals that of pure methane hydrate. The result is in good agreement with the structure of artificially synthesized methane + carbon dioxide mixed-gas hydrates. These results suggest that the lattice constant of the natural gas hydrate does not change due to a change of CO₂ gas content. In addition, the thermal expansion of the sampled hydrate was measured for the temperature range of 83-173 K, and the resulting density of the hydrate crystal at 273 K was estimated to be . These results are essential for applying natural gas hydrates as an alternative natural fuel resources.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

Kinetics and modeling of carbon dioxide absorption into aqueous solutions of piperazine

In this work, the kinetics of the reaction between CO₂ and aqueous piperazine (PZ) have been estimated over the temperature range of 298-313 K from the absorption data obtained in a wetted wall contactor. The absorption data are obtained for the PZ concentrations of 0.2- and for CO₂ partial pressures up to 5 kPa. A coupled mass transfer-kinetics-equilibrium mathematical model based on Higbie's penetration theory has been developed with the assumption that all reactions are reversible. The model is used to estimate the rate constants from the experimental data for absorption of CO₂ in aqueous PZ. The estimated rate constants of this study are in good agreement with those reported in the literature.     

Spectroscopic analysis of carbon dioxide and nitrogen mixed gas hydrates in silica gel for CO2 separation

In this study solid-state NMR spectroscopy was used to identify structure and guest distribution of the mixed N₂ + CO₂ hydrates. These results show that it is possible to recover CO₂ from flue gas by forming a mixed hydrate that removes CO₂ preferentially from CO₂/N₂ gas mixture. Hydrate phase equilibria for the ternary CO₂–N₂–water system in silica gel pores were measured, which show that the three-phase H–L_w–V equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO₂ in the vapor phase decreased. ¹³C cross-polarization (CP) NMR spectra of the mixed hydrates at gas compositions of more than 10 mol% CO₂ with the balance N₂ identified that the crystal structure of mixed hydrates as structure I, and that the CO₂ molecules occupy mainly the abundant 5¹²6² cages. This makes it possible to achieve concentrations of more than 96 mol% CO₂ gas in the product after three cycles of hydrate formation and dissociation.     

Spectroscopic evidences of the double hydrogen hydrates stabilized with ethane and propane

Hydrogen molecules are known to occupy the small cages of structure I (sI) and II (sII) hydrates with the aid of coguests, leading to the highly stable state of their crystalline framework. For the first time, we synthesized the double hydrogen hydrates incorporated with ethane and propane that play a special role as the hydrate promoters or stabilizers. The resulting hydrate structures cage occupancy was identified by the spectroscopic methods of the PXRD and solid-state NMR. In addition, direct GC analysis confirmed that the encaged hydrogen amounts are 0.127 for sI ethane and 0.370 for sII propane at 120 bar and 270 K. The proper hydrate thermodynamics particularly focusing on the cage occupancy estimated that 0.17 and 0.33 wt% of hydrogen are observed in small cages of sI and sII hydrates. The overall spectroscopic and physicochemical analysis strongly imply that the sII cages act as much more favorable sites than sI cages in storing hydrogen.     

Storage capacity of hydrogen in tetrahydrothiophene and furan clathrate hydrates

The storage capacity of hydrogen in the tetrahydrothiophene and furan hydrates was investigated by means of pressure-volume-temperature measurement. The hydrogen-absorption rate of tetrahydrothiophene and furan hydrates is much larger than that of tetrahydrofuran hydrate in spite of same crystal structure (structure-II). The storage amount of hydrogen at 275.1 K is about 1.2 mol (hydrogen)/mol (tetrahydrothiophene or furan hydrate) (∼0.6 mass%) at 41.5 MPa, which is coincident with that of tetrahydrofuran hydrate.     

Phase equilibria of clathrate hydrates of methyl cyclopentane, methyl cyclohexane, cyclopentane or cyclohexane+carbon dioxide

In this work, experimental dissociation data for clathrate hydrates of methyl cyclopentane, methyl cyclohexane, cyclopentane or cyclohexane+carbon dioxide are reported at different temperatures. The experimental data were generated using an isochoric pressure-search method. The reliability of this method is examined by generating new dissociation data for clathrate hydrates of methyl cyclopentane+methane and comparing them with the experimental data reported in the literature. The acceptable agreement demonstrates the reliability of the experimental method used in this work. The experimental data for all measured systems are finally compared with the corresponding literature data in the absence of the above mentioned cyclic compounds to identify their promotion effects.     

Solubility of astaxanthin in supercritical carbon dioxide

The solubility of astaxanthin in carbon dioxide was measured under supercritical conditions of a pressure range from 80 to 300 bar, and temperature range from 303 to 333 K, by using a dynamic flow-type. The solubility of astaxanthin increasing from 0.42×10⁻⁵ to 4.89×10⁻⁵ with higher temperature and pressure maintains certain density of supercritical carbon dioxide. The solubility data obtained were applied to the Chrastil model, based on the density of carbon dioxide. The data fitted well with the Chrastil model at most experimental conditions.     

注CO2驱油技术的室内评价方法研究

注CO2驱油作为一种有效提高采收率的方法,在油气田开采中已经占据越来越重要的地位。文章阐述了CO2驱油机理,介绍了对应的驱油方式,其中着重系统地总结出了现行测定及评价注CO2驱参数的各种室内试验方法。