Ternary Methane Research Articles

Calorimetric Assessment of Ternary Methane–Carbon Dioxide–Tetrahydrofuran (CH4–CO2–THF) Hydrates: Application in Storage and Transport of CO2 Lean Natural Gas

Calorimetric Assessment of Ternary Methane–Carbon Dioxide–Tetrahydrofuran (CH₄–CO₂–THF) Hydrates: Application in Storage and Transport of CO₂ Lean Natural Gas

Experimental Measurement of Multiple Hydrate Structure Formation in Binary and Ternary Natural Gas Analogue Systems by Isochoric Equilibrium Methods

Traditionally, it is commonly assumed that a single gas hydrate (or clathrate hydrate) structure/phase is formed in natural gas systems—e.g., “s-II natural gas (NG) hydrates” or “s-I methane hydrates”—based on that which is understood to be the most thermodynamically stable at incipient phase boundary conditions. This applies with respect to both studies of hydrates in natural sediments, and in the case of hydrocarbon production operations, including the testing of low-dosage hydrate inhibitors (LDHIs). Here, we present the results of experimental studies of simple propane (C₃), binary methane–propane (C₁–C₃), and ternary methane–ethane–propane (C₁–C₂–C₃) hydrate systems, aimed at investigating phase behavior at higher subcoolings where significant water conversions to clathrates—and associated gas fractionation—might be expected to result in the formation of more than one hydrate structure. Measurements were made by means of established, reliable, isochoric equilibrium step-heating/cooling approaches. In the single guest propane system, pressure–temperature (PT) conditions follow the gas hydrate phase boundary, in agreement with the univariant hydrate + gas + water (H + G + W) equilibrium. By contrast, binary and ternary mixes clearly show PT behavior consistent with the growth of at least two and four hydrate phases, respectively, with these forming sequentially on cooling, and dissociating in the corresponding reverse order upon heating. In-house hydrate model (HydraFLASH) predictions support experimental observations, pointing to processes being gas fractionation driven; the mixed C₃–C₁ or C₃–C₂–C₁ s-II type hydrates formed first close to the phase boundary preferentially incorporating the most strongly clathrate cage (5¹²6⁴) stabilizing propane, making the remaining gas leaner. Once C₃ is largely consumed, trends agree with less stable C₂–C₁ structures (s-II followed by s-I) then growing (where ethane is present), with this, like C₃ uptake, making remaining free gas increasingly of almost pure C₁. This ultimately drives systems toward the final simple, single guest C₁ s-I clathrate formation as the well-established univariant phase boundary for this structure is reached. As observed phase behavior arises because of gas fractionation, it can be expected to occur during hydrate formation in all mixed gas systems of relatively fixed composition (at constant volume or pressure). The findings highlight that care should be taken in the application of isochoric methods to multicomponent gas systems, given the potential for numerous (rather than just one) clear changes in heating curve slopes as different hydrate structures form/dissociate. In addition, results show that extending equilibrium steps well into the hydrate region can reveal important information on clathrate phase behavior with potentially significant implications for issues such as gas production from hydrates in sediments, hydrate technologies for gas capture/separation/storage in the energy industry, and flow assurance in hydrocarbon production operations.

Experimental study of impurity freeze-out in ternary methane + ethane + benzene mixtures with applications to LNG production

The freeze-out of impurities in LNG production can pose significant operational risks and lead to costly blockage-induced plant shutdowns. Study of ternary and higher-order mixtures, which are more analogous to LNG, present the critical tests of thermodynamic models in terms of their utility and predictive accuracy. In this work, a visual CryoSolids apparatus was upgraded to allow analytical measurements of solvent composition in multi-component systems where a solid phase is present at equilibrium. The analytical system, which included a ROLSI sampling valve and capillary together with a gas chromatograph, was successfully commissioned and used to measure melting temperatures and solvent compositions of a ternary mixture containing methane, ethane, and benzene at temperatures down to 125 K and pressures up to 6 MPa. The effect on the solubility of benzene by adding ethane to the solvent was investigated by varying the ethane mole fraction from 0 to 0.96. The resulting temperature at which benzene melted into the liquid solvent decreased from 246 K at 4.7 MPa to 125 K at 5.7 MPa. The comparison of results with the ThermoFAST model showed that it could describe the new data with a deviation less than 1 K for ethane liquid phase mole fractions from 0 <xC2< 0.5, which increased to less than 4 K for 0.85 <xC2< 0.97. This result indicates that ThermoFAST satisfactorily represents SFE conditions in LNG mixtures of industrial relevance but needs improvement to cover wider ranges of composition.

Phase equilibrium studies of high-pressure natural gas mixtures with toluene for LNG applications

To prevent possible freeze out in the main cryogenic heat exchanger (MCHE) used in liquefied natural gas (LNG) plants, new and accurate phase equilibrium data are required to improve the predictive reliability of existing models, in particular cubic equations of state (EOS). In this work, the vapor-liquid equilibrium (VLE) of a ternary methane + propane + toluene (methylbenzene) mixture was studied over a wide range of conditions with toluene as the minor component in both the liquid and vapor phases. Measurements were conducted along different isochoric paths at temperatures between (213 and 298 K) and pressures up to 8.3 MPa, to obtain data at conditions relevant to the operation of LNG scrub columns. The measured VLE data were compared to results calculated with the HYSYS Peng Robinson (PR) equation of state (EOS) that is used widely in LNG industry. The amount of toluene in the vapor phase was found to be under-predicted by the HYSYS PR EOS by an average of around 77% at lower temperatures, with the error increasing as temperature and toluene concentration decreased. The current work demonstrates that the HYSYS PR EOS as well as other cubic EOS substantially under-predict the possible toluene content of saturated vapours that could be present in the overhead of the LNG scrub column. Using the ThermoFAST model recently developed and optimised for the calculation of solid-liquid equilibrium conditions in LNG production, this work further demonstrates that the 77% increase in the toluene content of a saturated vapor entering the MCHE, corresponds to a 7 K increase in the solid formation temperature, which could significantly increase the likelihood of a blockage in the MCHE and thus possible shutdown of the LNG plant. The experimental and modelling work presented here underscores the importance of improving predictions of the allowable threshold concentration of heavy components in fluids entering cryogenic heat exchangers in LNG plants.

Prediction of viscosity-density dependence of liquid methane+n-butane+n-pentane mixtures using the molecular dynamics method and empirical correlations

We present a study of viscosities of n-pentane and binary and ternary methane–n-butane–n-pentane mixtures in liquid state at the temperature 360 K by the non-equilibrium molecular dynamics simulations with a modified fully flexible version of the all-atom TraPPE-EH force field. The Batchinski correlation η=C/(v−b) is used to describe the viscosities of pure n-pentane. The calculated viscosity-density dependence for liquid n-pentane is in agreement with the experimental data. We show that the viscosity-volume dependencies of liquid binary and ternary mixtures can be fitted by the Batchinski's equation as well, and the coefficients for the mixtures are obtained by linear mixing of the coefficients for pure components. The model can be used for the interpretation and interpolation of the experimental data on viscosities of hydrocarbon liquids. The Batchinski-like behavior of viscosity of liquid mixtures in some density range can be used as a test for more complex correlations, which we show by comparing the hard sphere correlation and the expanded fluid correlation for hydrocarbons.

Solubility of Methane and Carbon Dioxide in the Aqueous Phase of the Ternary (Methane + Carbon Dioxide + Water) Mixture: Experimental Measurements and Molecular Dynamics Simulations

New experimental measurements (at 10 MPa and 323.15 K) and molecular dynamics (MD) simulations (at 10 MPa and 323.15 K, 50 MPa and 344.25 K, and 50 MPa and 375.5 K) are reported in order to measure/predict the aqueous solubilities of the ternary methane–carbon dioxide–water system under two-phase (vapor–liquid) equilibrium conditions. The TIP4P/ice, TraPPE-UA, and OPLS-UA force fields for water, carbon dioxide, and methane, respectively, are used, according to the modifications introduced in the recent study by Michalis et al. (Phys. Chem. Chem. Phys., 2016, 18, 23538–23548) for the binary methane–carbon dioxide hydrate. The MD calculated values for the solubility are in good agreement with the newly available experimental measurements. The molecular dynamics simulations clearly indicate that the solubility of each gas decreases by the addition of the other gas. The particular conclusion is in excellent agreement with the conclusion obtained from the new experimental measurements reported in the current s...

Two- and three-phase equilibrium experimental measurements for the ternary CH4 + CO2 + H2O mixture

Experimental measurements are reported for the ternary methane + carbon dioxide + water mixture under three-phase hydrate equilibrium conditions utilizing a recently-introduced experimental apparatus. In addition to measuring the three-phase equilibrium pressure and temperature conditions, vapor and liquid phase analysis measurements are also reported. In particular, we performed gas chromatographic analysis of the vapor phase and the solubility of each gas in the aqueous phase was measured under three-phase equilibrium conditions. Additionally, two-phase equilibrium solubility measurements are also reported at pressure and temperature conditions that are in the close neighborhood of hydrate-formation conditions.

Direct phase coexistence molecular dynamics study of the phase equilibria of the ternary methane-carbon dioxide-water hydrate system.

Molecular dynamics simulation is used to predict the phase equilibrium conditions of a ternary hydrate system. In particular, the direct phase coexistence methodology is implemented for the determination of the three-phase coexistence temperature of the methane-carbon dioxide-water hydrate system at elevated pressures. The TIP4P/ice, TraPPE-UA and OPLS-UA forcefields for water, carbon dioxide and methane respectively are used, in line with our previous studies of the phase equilibria of the corresponding binary hydrate systems. The solubility in the aqueous phase of the guest molecules of the respective binary and ternary systems is examined under hydrate-forming conditions, providing insight into the predictive capability of the methodology as well as the combination of these forcefields to accurately describe the phase behavior of the ternary system. The three-phase coexistence temperature is calculated at 400, 1000 and 2000 bar for two compositions of the methane-carbon dioxide mixture. The predicted values are compared with available calculations with satisfactory agreement. An estimation is also provided for the fraction of the guest molecules in the mixed hydrate phase under the conditions examined.

Simulating the capture of CO2 from natural gas: New data and improved models for methane + carbon dioxide + methanol

The simulation of carbon capture unit operations often involves predicting the vapor liquid equilibrium (VLE) for mixtures containing polar, non-polar and quadrupolar compounds. In this work, we investigate how well a simple cubic equation of state (EOS) can predict the results of new low temperature, high-pressure VLE measurements of the ternary methane+carbon dioxide+methanol system, which is important to the Rectisol process used for capturing CO2 from natural gas. The ternary p, T, x measurements presented here are the first such data for this system reported in the open literature. First, predictions made with the Peng Robinson (PR) EOS as implemented in a commercial process simulator were compared to binary p, T, x data measured in this work and also taken from the literature. Significant deviations were found between the measured liquid mole fractions and those predicted with the EOS using the default binary interaction parameters (BIP): the relative standard errors were 39%, 77% and 17% for the methane+methanol, methane+carbon dioxide and methanol+carbon dioxide binaries, respectively. Regression of the PR EOS to the binary VLE data by adjusting the BIPs improved the liquid phase mole fraction predictions for the ternary mixture data by a factor of about 2.5 for methane and methanol. However, improvement by a factor of 4.4 in the carbon dioxide liquid mole fraction was achieved by describing the carbon dioxide+methanol binary with an asymmetric composition and temperature dependent mixing rule and tuning the BIPs therein to VLE data for this binary over a wide temperature range. This reduced the standard error in the liquid phase CO2 mole fractions predicted for the ternary mixture using the optimized model by 79% relative to the default model.

An Examination of the Ternary Methane + Carbon Dioxide + Water Phase Diagram using the SAFT-VR Approach

In this work, the molecular based Variable Range Statistical Associating Fluid Theory (SAFT-VR) has been used to estimate the global phase equilibria diagram of the ternary mixture water + carbon dioxide + methane, over a wide pressure and temperature range. An accurate determination of the phase equilibria of this mixture is relevant in Petrophysics, as, for instance, in enhanced natural gas recovery from low permeability reservoirs (the so-called tight gas reservoirs), or in geology, as it is the basic composition of many geological fluids. A previous study on the phase behavior of the binary mixtures involved is presented, using in a transferable manner the characteristic molecular parameters for the three molecules involved. The ternary mixture presents a very rich and complex phase behavior, with a wide region of the thermodynamic space of phases (at higher pressures) presenting a large gap of ternary liquid-liquid equilibria, that upon descending pressures leads to the transition to a three-phase liquid-liquid-vapor equilibria region, and both regions are separated by a continuous critical end point line. The ability of the theory to describe this complex multicomponent mixture phase transition with a reduced and physically sound set of characteristic parameters must be underlined.

Dew Points of Ternary Methane (or Ethane) + Carbon Dioxide + Water Mixtures: Measurement and Correlation

Experimental measurements of dew points for seven methane + carbon dioxide + water mixtures in the pressure range of 1.1 × 105−60.5 × 105 Pa in the temperature range of 243.1−288.1 K, and four ethane + carbon dioxide + water mixtures at pressures of 1.1 × 105−20.3 × 105 Pa and temperatures of 252.2−288.4 K, were determined. The experimental results obtained on the ternary systems were analyzed in terms of a predictive excess function−equation of state (EF−EOS) method, which reproduced experimental dew-point temperature data within an absolute average deviation (AAD) of 0.1−2.1 K. The experimental results obtained for the studied mixtures at pressures of >5 × 105 Pa were also compared to a predictive equation of state (EOS) model. It reproduced experimental dew-point temperature data within AAD values of 0.9−2.1 K.

Dew points of ternary methane + water + methanol. Measurement and correlation

A water and methanol dew point generation device has been built up and tested. Experimental measurements of dew point for ternary methane + water + methanol were carried out between 1.01 × 105 Pa to 97.84 × 105 Pa and the temperature range from 248.71 to 282.93 K. A EOS-CR method (equation of state – chemical reticular) reproduces very accurately the experimental curves independently of the pressure and temperature ranges.Key words: dew point, experimental method, equation of state, excess function.

Dew points of ternary methane+ethane+butane and quaternary methane+ethane+butane+water mixtures: measurement and correlation

Dew points for ternary methane+ethane+butane and quaternary methane+ethane+butane+water mixtures were determined experimentally between 4.77×10 5 and 99.45×10 5 Pa and at temperatures from 250.92 to 288.54 K. The experimental dew point curves of the mixtures with water were reproduced quite accurately with an excess function–equation of state method, independent of the temperature and pressure ranges.

Dew points of ternary methane + water + methanol. Measurement and correlation

Dew points of ternary methane + water + methanol. Measurement and correlation

Tricritical phenomena in quasi-binary mixtures. V. New measurements on ternary methane systems

Phase equilibria have been studied in ternary mixtures of methane with 2,2- and 2,3-dimethylbutane near the tricritical point. The quasi-binary approximation allows the path to the tricritical point to be defined experimentally, making it possible to study the details of the phase behavior as the tricritical point is approached. The decrease in the differences in temperature and pressure between the upper and lower critical end points is consistent with the asymptotic classical theory of tricritical phenomena; an extended classical theory is necessary to account for the shape of the coexistence curves.