Isothermal phase equilibria for the (xenon + cyclopropane) mixed-gas hydrate system

Isothermal three-phase equilibria of gas, aqueous, and hydrate phases for the {xenon (Xe) + cyclopropane (c-C₃H₆)} mixed-gas hydrate system were measured at two different temperatures (279.15 and 289.15) K. The structural phase transitions from structure-I to structure-II and back to structure-I, depending on the mole fraction of guest mixtures, occur in the (Xe + c-C₃H₆) mixed-gas hydrate system. The isothermal pressure–composition relations have two local pressure minima. The most important characteristic in the (Xe + c-C₃H₆) mixed-gas hydrate system is that the equilibrium pressure–composition relations exhibit the complex phase behavior involving two structural phase transitions and two homogeneous negative azeotropes. One of two structural phase transitions exhibits the heterogeneous azeotropic-like behavior.     

Isothermal phase equilibria and structural phase transition in the carbon dioxide + cyclopropane mixed-gas hydrate system

The isothermal phase equilibria of the carbon dioxide + cyclopropane mixed-gas hydrate system were investigated by means of static temperature measurement and Raman spectroscopic analysis. Raman spectra indicated that the crystal structure of the carbon dioxide + cyclopropane mixed-gas hydrate changes from structure-I to structure-II and back to structure-I with an increase of the equilibrium carbon dioxide composition at 279.15 K, while each simple gas hydrate belongs to structure-I at the temperature. Whereas, unlike 279.15 K, no structural phase transition occurs along the isothermal stability boundary at 284.15 K.     

Isothermal phase equilibria for the (HFC-32 + HFC-134a) mixed-gas hydrate system

Isothermal phase equilibria (pressure-composition relations in hydrate, gas, and aqueous phases) in the {difluoromethane (HFC-32) + 1,1,1,2-tetrafluoroethane (HFC-134a)} mixed-gas hydrate system were measured at the temperatures 274.15 K, 279.15 K, and 283.15 K. The heterogeneous azeotropic-like behaviour derived from the structural phase transition of (HFC-32 + HFC-134a) mixed-gas hydrates appears over the whole temperature range of the present study. In addition to the heterogeneous azeotropic-like behaviour, the isothermal phase equilibrium curves of the (HFC-32 + HFC-134a) mixed-gas hydrate system exhibit the negative homogeneous azeotropic-like behaviour at temperatures 279.15 K and 283.15 K. The negative azeotropic-like behaviour, which becomes more remarkable at higher temperatures, results in the lower equilibrium pressure of (HFC-32 + HFC-134a) mixed-gas hydrates than those of both simple HFC-32 and HFC-134a hydrates. Although the HFC-134a molecule forms the simple structure-II hydrate at the temperatures, the present findings reveal that HFC-134a molecules occupy a part of the large cages of the structure-I mixed-gas hydrate.     

Hydrate phase equilibrium and structure for (methane + ethane + tetrahydrofuran + water) system

The separation of methane and ethane through forming hydrate is a possible choice in natural gas, oil processing, or ethylene producing. The hydrate formation conditions of five groups of (methane + ethane) binary gas mixtures in the presence of 0.06 mole fraction tetrahydrofuran (THF) in water were obtained at temperatures ranging from (277.7 to 288.2) K. In most cases, the presence of THF in water can lower the hydrate formation pressure of (methane + ethane) remarkably. However, when the composition of ethane is as high as 0.832, it is more difficult to form hydrate than without THF system. Phase equilibrium model for hydrates containing THF was developed based on a two-step hydrate formation mechanism. The structure of hydrates formed from (methane + ethane + THF + water) system was also determined by Raman spectroscopy. When THF concentration in initial aqueous solution was only 0.06 mole fraction, the coexistence of structure I hydrate dominated by ethane and structure II hydrate dominated by THF in the hydrate sample was clearly demonstrated by Raman spectroscopic data. On the contrary, only structure II hydrate existed in the hydrate sample formed from (methane + ethane + THF + water) system when THF concentration in initial aqueous solution was increased to 0.10 mole fraction. It indicated that higher THF concentration inhibited the formation of structure I hydrate dominated by ethane and therefore lowered the trapping of ethane in hydrate. It implies a very promising method to increase the separation efficiency of methane and ethane.     

Phase equilibria for H2 + CO2 + H2O system containing gas hydrates

Isothermal phase equilibrium (pressure–composition in the gas phase) for the ternary system of H₂ + CO₂ + H₂O has been investigated in the presence of gas hydrate phase. Three-phase equilibrium pressure increases with the H₂ composition of gas phase. The Raman spectra suggest that H₂ is not enclathrated in the hydrate-cages and behaves only like the diluent gas toward the formation of CO₂ hydrate. This fact is also supported by the thermodynamic analysis using Soave–Redlich–Kwong equation of state.     

Raman spectroscopic studies on methane + tetrafluoromethane mixed-gas hydrate system

Raman spectra of intramolecular vibration mode for each guest species in the methane + tetrafluoromethane (CF₄) mixed-gas hydrate crystal have been measured at 291.1 K. Both of pure guest species generate the structure-I hydrate in the present pressure ranges. Isothermal phase-equilibrium curve exhibits two discontinuous points around the equilibrium methane compositions (water-free) in the gas phase of 0.3 and 0.8. At the above points, the Raman spectra of both guest molecules have been drastically changed. One of the most important findings is that the crystal of methane + tetrafluoromethane mixed-gas hydrate shows the structural phase-transition (from the structure-I to the structure-II and back to the structure-I) caused by composition changes.     

Thermodynamic modeling for clathrate hydrates of ozone

We report a theoretical study to predict the phase-equilibrium properties of ozone-containing clathrate hydrates based on the statistical thermodynamics model developed by van der Waals and Platteeuw. The Patel–Teja–Valderrama equation of state is employed for an accurate estimation of the properties of gas phase ozone. We determined the three parameters of the Kihara intermolecular potential for ozone as a = 6.815 · 10⁻² nm, σ = 2.9909 · 10⁻¹ nm, and ε · k_B⁻¹ = 184.00 K. An infinite set of ε–σ parameters for ozone were determined, reproducing the experimental phase equilibrium pressure–temperature data of the (O₃ + O₂ + CO₂) clathrate hydrate. A unique parameter pair was chosen based on the experimental ozone storage capacity data for the (O₃ + O₂ + CCl₄) hydrate that we reported previously. The prediction with the developed model showed good agreement with the experimental phase equilibrium data within ±2% of the average deviation of the pressure. The Kihara parameters of ozone showed slightly better suitability for the structure-I hydrate than CO₂, which was used as a help guest. Our model suggests the possibility of increasing the ozone storage capacity of clathrate hydrates (∼7% on a mass basis) from the previously reported experimental capacity (∼1%).     

Hydrate phase equilibrium for the (hydrogen + tert-butylamine + water) system

The three-phase equilibrium conditions of ternary (hydrogen + tert-butylamine + water) system were first measured under high-pressure in a “full view” sapphire cell. The tert-butylamine–hydrogen binary hydrate phase transition points were obtained through determining the points of intersection of three phases (H–L_w–V) to two phases (L_w–V) experimentally. Measurements were made using an isochoric method. Firstly, (tetrahydrofuran + hydrogen) binary hydrate phase equilibrium data were determined with this method and compared with the corresponding experimental data reported in the literatures and the acceptable agreements demonstrated the reliability of the experimental method used in this work. The experimental investigation on (tert-butylamine + hydrogen) binary hydrate phase equilibrium was then carried out within the temperature range of (268.4 to 274.7) K and in the pressure range of (9.54 to 29.95) MPa at (0.0556, 0.0886, 0.0975, and 0.13) mole fraction of tert-butylamine. The three-phase equilibrium curve (H + L_w + V) was found to be dependent on the concentration of tert-butylamine solution. Dissociation experimental results showed that tert-butylamine as a hydrate former shifted hydrate stability region to lower pressure and higher temperature.     

The partition coefficients of ethylene between vapor and hydrate phase for methane + ethylene + THF + water systems

The vapor-hydrate equilibria were studied experimentally in detail for CH₄ + C₂H₄ + tetrahydrofuran (THF) + water systems in the temperature range of 273.15–282.15 K, pressure range of 2.0–4.5 MPa, the initial gas–liquid volume ratio range of 45–170 standard volumes of gas per volume of liquid and THF concentration range of 4–12 mol%. The results demonstrated that, because of the presence of THF, ethylene was remarkably enriched in vapor phase instead of being enriched in hydrate phase for CH₄ + C₂H₄ + water system. This conclusion is of industrial significance; it implies that it is feasible to enrich ethylene from gas mixture, e.g., various kinds of refinery gases or cracking gases in ethylene plant, by forming hydrate.     

Thermodynamic stability of structure-H hydrates of methylcyclopentane and cyclooctane helped by methane

The four-phase equilibria were measured for the methylcyclopentane+methane+H₂O hydrate system (274.28–287.40 K, 1.75–9.34 MPa) and the cyclooctane+methane+H₂O hydrate system (274.08–288.57 K, 1.60–9.33 MPa). Each structure-H hydrate has the lower equilibrium pressure than the pure methane structure-I hydrate in the temperature range of the present work. The isothermal equilibrium pressures of both methylcyclopentane and cyclooctane hydrates are slightly higher than that of methylcyclohexane hydrate.     

Phase equilibrium measurements for clathrate hydrates of flue gas (CO2 + N2 + O2) in the presence of tetra-n-butyl ammonium bromide or tri-n-butylphosphine oxide

This paper reports the measured hydrate phase equilibria of simulated flue gas (12.6 vol% CO₂, 80.5 vol% N₂, 6.9 vol% O₂) in the presence of tetra-n-butyl ammonium bromide (TBAB) or tri-n-butylphosphine oxide (TBPO), at (0, 5 and 26) wt%, respectively. The measurements of the phase boundary between (hydrate + liquid + vapor) (H + L + V) phases and (liquid + vapor) (L + V) phases were performed within the temperature range (275.97 to 293.99) K and pressure range (1.56 to 18.78) MPa with using the isochoric step-heating pressure search method. It was found that addition of TBAB or TBPO allowed the incipient equilibrium hydrate formation conditions for the flue gas to become milder. Compared to TBAB, TBPO was largely more effective in reducing the phase equilibrium pressure.     

Phase equilibrium measurements and the tuning behavior of new sII clathrate hydrates

We suggest two types of new amine-type sII formers: pyrrolidine and piperidine. These guest compounds fail to form clathrate hydrate structures with host water, but instead have to combine with light gaseous guest molecules (methane) for enclathration. First, two binary clathrate hydrates of (pyrrolidine + methane) and (piperidine + methane) were synthesized at various amine concentrations. ¹³C NMR and Raman analysis were done to identify the clathrate hydrate structure and guest distribution over sII-S and sII-L cages. XRD was also used to find the exact structure and corresponding cell parameters. At a dilute pyrrolidine concentration of less than 5.56 mol%, the tuning phenomenon is observed such that methane molecules surprisingly occupy sII-L cages. At the critical guest concentration of about 0.1 mol%, the cage occupancy ratio reaches the maximum of approximately 0.5. At very dilute guest concentration below 0.1 mol%, the methane molecules fail to occupy large cages on account of their rarefied distribution in the network. Direct-release experiments were performed to determine the actual guest compositions in the clathrate hydrate phases. Finally, we measured the clathrate hydrate phase equilibria of (pyrrolidine + methane) and (piperidine + methane).     

Phase equilibrium measurements for semi-clathrate hydrates of the (CO2 + N2 + tetra-n-butylammonium bromide) aqueous solution system

The application of semi-clathrate hydrate formation technology for gas separation purposes has gained much attention in recent years. Consequently, there is a demand for experimental data for relevant semi-clathrate hydrate phase equilibria. In this work, semi-clathrate hydrate dissociation conditions for the system comprising mixtures of {CO₂ (0.151/0.399 mole fraction) + N₂ (0.849/0.601 mole fraction) + 0.05, 0.15, and 0.30 mass fraction tetra-n-butylammonium bromide (TBAB)} aqueous solutions have been measured and are reported. An experimental apparatus which was designed and built in-house was used for the measurements using the isochoric pressure-search method. The range of conditions for the measurements was from 277.1 K to 293.2 K for temperature and pressures up to 16.21 MPa. The phase equilibrium data measured demonstrate the high hydrate promotion effects of TBAB aqueous solutions.     

Modeling hydrate formation conditions in the presence of electrolytes and polar inhibitor solutions

In this paper, a new predictive model is proposed for prediction of gas hydrate formation conditions in the presence of single and mixed electrolytes and solutions containing both electrolyte and a polar inhibitor such as monoethylene glycol (MEG), diethylene glycol (DEG) and triethylene glycol (TEG). The proposed model is based on the γ–φ approach, which uses modified Patel–Teja equation of state (VPT EOS) for characterizing the vapor phase, the solid solution theory by van der Waals and Platteeuw for modeling the hydrate phase, the non-electrolyte NRTL-NRF local composition model and Pitzer–Debye–Huckel equation as short-range and long-range contributions to calculate water activity in single electrolyte solutions. Also, the Margules equation was used to determine the activity of water in solutions containing polar inhibitor (glycols). The model predictions are in acceptable agreement with experimental data. For single electrolyte solutions, the model predictions are similar to available models, while for mixtures of electrolytes and mixtures of electrolytes and inhibitors, the proposed model gives significantly better predictions. In addition, the absolute average deviation of hydrate formation pressures (AADP) for 144 experimental data in solutions containing single electrolyte is 5.86% and for 190 experimental data in mixed electrolytes solutions is 5.23%. Furthermore, the proposed model has an AADP of 14.13%, 5.82% and 5.28% in solutions containing (Electrolyte + MEG), (Electrolyte + DEG) and (Electrolyte + TEG), respectively.     

Dissociation behaviour of (tetra-n-butylammonium bromide + tetra-n-butylammonium chloride) mixed semiclathrate hydrate systems

The thermal properties of {tetra-n-butylammonium bromide + tetra-n-butylammonium chloride (TBAB + TBAC)} mixed semiclathrate hydrates prepared from aqueous solutions were investigated by dissociation temperature measurements and differential scanning calorimetry (DSC). The maximum dissociation temperature of the mixed hydrate crystals at 0.1 MPa is 288.5 K for x_TBAB = 0.2 {mole fraction of TBAB to (TBAB + TBAC)}, which is higher than that of the pure hydrates {T = (285.5 and 288.2) K for TBAB and TBAC hydrates, respectively}. In addition, the dissociation enthalpies of the mixed hydrates are higher than those of the pure hydrates {(5.55 ± 0.06) kJ ⋅ mol⁻¹ H₂O for pure TBAB hydrate and (5.30 ± 0.05) kJ ⋅ mol⁻¹ H₂O for pure TBAC hydrate}, with a maximum of (5.95 ± 0.12) kJ ⋅ mol⁻¹ H₂O recorded at approximately x_TBAB = 0.4. It was therefore suggested that the crystal distortion in (TBAB + TBAC) mixed hydrates, caused by replacing water molecules by both bromide and chloride anions, was smaller than that observed for each pure hydrate. Consequently, the hydration numbers in the mixed hydrates were hypothesized to be slightly higher than those of the pure hydrates.     

Measurements of methane hydrate equilibrium in systems inhibited with NaCl and methanol

Natural gas hydrates are ice-like inclusion compounds that form at high pressures and low temperatures in the presence of water and light hydrocarbons. Hydrate formation conditions are favorable in gas and oil pipelines, and their formation threatens gas and oil production. Thermodynamic hydrate inhibitors (THIs) are chemicals (e.g., methanol, monoethylene glycol) deployed in gas pipelines to depress the equilibrium temperature required for hydrate formation. This work presents a novel application of a stepwise differential scanning calorimeter (DSC) measurement to accurately determine the methane hydrate phase boundary in the presence of THIs. The scheme is first validated on a model (ice + salt water) system, and then generalized to measure hydrate equilibrium temperatures for pure systems and 0.035 mass fraction NaCl solutions diluted to 0, 0.05, 0.10, and 0.20 mass fraction methanol. The hydrate equilibrium temperatures are measured at methane pressures from (7.0 to 20.0) MPa. The measured equilibrium temperatures are compared to values computed by the predictive hydrate equilibrium tool CSMGem.     

Phase equilibria in ternary (carbon dioxide + tetrahydrofuran + water) system in hydrate-forming region: Effects of carbon dioxide concentration and the occurrence of pseudo-retrograde hydrate phenomenon

In the present work, the three- and four-phase hydrate equilibria of (carbon dioxide (CO₂) + tetrahydrofuran (THF) + water) system are measured by using Cailletet equipment in the temperature and pressure range of (272 to 292) K and (1.0 to 7.5) MPa, respectively, at different CO₂ concentration. Throughout the study, the concentration of THF is kept constant at 5 mol% in the aqueous solution. In addition, the fluid phase transitions of L_W–L_V–V → L_W–L_V (bubble point) and L_W–L_V–V → L_W–V (dew point) are determined when they are present in the ternary system. For comparison, the three-phase hydrate equilibria of binary (CO₂ + H₂O) are also measured. Experimental measurements show that the addition of THF as a hydrate promoter extends hydrate stability region by elevating the hydrate equilibrium temperature at a specified pressure. The three-phase equilibrium line H–L_W–V is found to be independent of the overall concentration of CO₂. Contradictory, at higher pressure, the phase equilibria of the systems are significantly influenced by the overall concentration of CO₂ in the systems. A liquid–liquid phase split is observed at overall concentration of CO₂ as low as 3 mol% at elevated pressure. The region is bounded by the bubble-points line (L_W–L_V–V → L_W–L_V), dew points line (L_W–L_V–V → L_W + V) and the four-phase equilibrium line (H + L_W + L_V + V). At higher overall concentration of CO₂ in the ternary system, experimental measurements show that pseudo-retrograde behaviour exists at pressure between (2.5 and 5) MPa at temperature of 290.8 K.     

Enthalpies of dissociation of clathrate hydrates of carbon dioxide,nitrogen, (carbon dioxide + nitrogen), and (carbon dioxide + nitrogen + tetrahydrofuran)

A calorimetric technique is described for measuring the enthalpy of dissociation liberated from solid hydrates. In this study, the enthalpies of dissociation were determined at T =  273.65 K andp =  0.1 MPa for simple and mixed hydrates of carbon dioxide, nitrogen, (carbon dioxide  +  nitrogen), and (carbon dioxide  +  nitrogen  +  tetrahydrofuran) using an isothermal microcalorimeter. The addition of tetrahydrofuran (THF) promoted hydrate stability and increased the number of guest molecules encaged in the small and large cavities of the hydrate lattice, resulting in lower enthalpy of dissociation, compared with structure II hydrate. The composition ratio of guest molecules did not affect the enthalpy of dissociation, which was found to be nearly constant for the same mixture.     

Pressure dependent Raman studies in the K2Mo2O7·H2O crystal

The pressure dependent Raman scattering in the potassium molybdenum oxide hydrate crystal, K₂Mo₂O₇·H₂O, was measured. The high pressure Raman study showed, that the compound remains in the triclinic structure within the 0.0–3.81 GPa range and undergoes a structural phase transition between 3.81 and 4.13 GPa. This particular phase transition is most likely connected with changes in the Raman spectrum, in which the number of modes associated originally with the stretching vibrations in the MoO₅ and MoO₆ units is increased. However, the phase at atmospheric pressure shows bands due to the presence of only one equivalent site, while in the high-pressure phase, two bands are associated with the stretching modes. Continuing the pressure evolution up to 17.04 GPa, two further phase transitions occurred in this crystal in the 6.3–8.1 GPa and the 12.3–14.0 GPa range, respectively. The Raman spectra measured at about 17.04 GPa presented a crystal structure, which experienced a pre-amorphization with a total loss of all lattice modes. This particular result is indicative that this material may have undergone a complete amorphization at pressures larger than 17.04 GPa. Then, the reversible character in the triclinic P-1 (C_i¹) structure was recovered after releasing the pressure.     

Hydrate phase equilibrium in the system of (carbon dioxide + 2,2-dimethylbutane + water) at temperatures below freezing point of water

The four-phase equilibrium conditions of (vapor + liquid + hydrate + ice) were measured in the system of (CO₂ + 2,2-dimethylbutane + water). The measurements were performed within the temperature range (254.2 to 270.2) K and pressure range (0.490 to 0.847) MPa using an isochoric method. Phase equilibrium conditions of hydrate formed in this study were measured to be at higher temperatures and lower pressures than those of structure I CO₂ simple hydrate. The largest difference in the equilibrium pressures of structure I CO₂ hydrate and the hydrate formed in the present study was 0.057 MPa at T = 258.3 K. On the basis of the four-phase equilibrium data obtained, the quintuple point for the (ice + structure I hydrate + structure H hydrate + liquid + vapor) was also determined to be T = 266.4 K and 0.864 MPa. The results indicate that structure H hydrate formed with CO₂ and 2,2-dimethylbutane is stable exclusively at the temperatures below the quintuple temperature.