Pure Gas Hydrates Research Articles

Ionic Clathrate Hydrates of Tetraalkylammonium/phosphonium Salts: Structures, Properties, Some Applications, and Perspectives

Ionic clathrate hydrates (ICHs) belong to one of the groups of a wide class of clathrate hydrates. In the crystal structures of ICHs, cation–anion pairs of guest molecules, typically tetraalkylammonium/phosphonium salts, are included into the host framework of hydrogen-bonded water molecules. Because of their suitable phase change temperature and latent heat, ICHs are considered as promising phase change materials for application in cold energy storage and transportation. The presence of vacant cavities in ICHs structures, which can incorporate small gas molecules such as, for example, H2, CH4, CO2, N2, H2S, determines the possibility of their use for the separation and storage of gases. The formation of these double ICHs occurs under much milder pressure and temperature conditions compared to the pure gas hydrates of the same gases. Studying of the structural characteristics and thermophysical properties of ICHs and double ICHs with gases is the necessary basis for their effective applications in different practical ends. This work reviews information related to the new details and features of the structures of ICHs and double ICHs with gases revealed over the last decade or a little more as well as some thermophysical data such as hydrate numbers, temperatures and enthalpies of dissociation. This review focuses only on ICHs of tetrabutyl-, tetraisoamylammonium (TBA, TiAA), and tetrabutylphosphonium (TBP) salts, which are the most common representatives of this class of compounds. One of the sections is devoted to ICHs of TBA and TiAA with polymeric anions such as cross-linked polyacrylates. The purpose of the present paper is to review the studies that have been relatively little addressed in previous works.

Modeling of seismic attenuation in fracture-filling gas hydrate-bearing sediments and its application to field observations in the Krishna-Godavari Offshore Basin, India

Understanding the attenuation mechanism in gas hydrate-bearing sediments (GHBS) is essential for accurate quantification of natural gas hydrates, yet previous rock physics models fail to explain the observed attenuation suppression at seismic frequencies when hydrates occur as fracture-filling morphology. Here we present a new model to decipher the viscoelastic performance of fracture-filling GHBS, by treating it as a composite of host sediment and pure gas hydrate. We first characterize the attenuation in the host sediment without hydrate by the viscous fluid flow in swelling clay minerals. Then, based on laboratory data, we quantify the mechanical performance of pure gas hydrate, exhibiting that hydrate is a consolidated aggregate with properties different from unconsolidated host sediment. After incorporating fracture-filling gas hydrates into the host sediment, the properties of host sediment are significantly modified and the resulting model reproduces the suppressed attenuation observed in field. We apply this model to the field attenuation extracted from multichannel seismic data in the Krishna-Godavari Offshore Basin in India, where gas hydrate was found filled in fractures. The results show that the attenuation and hydrate concentration predicted by our model agree well with the field measurements, which suggests the promise of our model in detection and quantification of nature gas hydrates in clay-dominated sediments using seismic data.

Theoretical Prediction of the Occurrence of Gas Hydrate Stability Zones: A Case Study of the Mohe Basin, Northeast China.

Source rocks of the Mohe Basin, Northeast China are gas-prone and the organic matter has advanced to late oil-generation stages, producing condensate and natural gas. This provides suitable conditions for the Mohe Basin to become one of the most prolific terrestrial natural gas hydrate (NGH)-bearing areas in China. Knowing this, here we predict the depth and thickness of pure methane hydrate stability zones (HSZs) and gas hydrate stability zones (GHSZs) via simulating the hydrate-phase equilibrium and other formation P–T conditions. Furthermore, factors that have a major impact on the occurrence of HSZs are discussed. Results showed that the composition of gas (guest) molecules and the geothermal gradient are the two most controlling factors on HSZs. Moreover, it was found that a pure methane HSZ with a thickness of about 255 m can form in areas with a geothermal gradient of <1.5 °C/100 m, with top and bottom depth limits less than 493 m and greater than 748 m, respectively. In contrast, pure methane hydrates have difficulty forming, while hydrates from wet gas can form where there is a geothermal gradient of >1.6 °C/100 m. Furthermore, a wet gas HSZ with a thickness of at least 735 m can be expected when the geothermal gradient reaches 2.3 °C/100 m, with top and bottom depth limits at 115 and 850 m, respectively. Ultimately, a pure methane HSZ can still form in the abnormally high-pressured areas when the geothermal gradient is up to 2.0 °C/100 m. Overall, HSZs can occur due to the combined effect of formation temperature, pressure, and gas composition. Finally, based on the results from this study and drilling data, future successful hydrate drilling schemes can be implemented in the Mohe Basin and similar terrestrial areas.

The impact of ice on the tensile strength of unconsolidated sand - A model for gas hydrate-bearing sands?

Tensile strength is an important parameter when it comes to predictions of potential fracturing of sediments by natural processes such as the emplacement of ice or gas hydrate lenses, as well as anthropogenic fracturing or else the stability of engineering constructions such as boreholes. Yet, tensile strength (στ) measurements of unconsolidated ice-bearing or gas hydrate-bearing sands are scarce and affected by a large variability.In the course of the SUGAR project we successfully used ice as a model for pore-filling and “load-bearing” gas hydrate in sand to determine compressional wave velocity. We were thus able to verify comparable formation characteristics and morphologies of ice and gas hydrate within the pore space. As these are important values for the tensile strength of ice/hydrate-bearing sands, ice was also used as a model for hydrate-bearing sands, despite differences in the mechanical behavior and strength of pure ice and gas hydrate. Water-saturated sand cores with ice saturations (Sice) between 0 and 100% were tested at −6.8 °C. The varying Sice were a result of the freezing point depression caused by saline solutions of different concentrations. The στ was directly determined using a sleeve-fracturing test with an internal pressure that was created within the frozen samples. The setup was also adapted to fit a pressure vessel for tests using confining pressure.The correlation of Sice – στ shows an exponential increase of στ with Sice. Whereas at Sice < 60% the increase is small, it is large at Sice > 80%. In conjunction with the change in strength, the viscoelastic behavior changes. A clear peak strength occurs at Sice > 80%. We conclude that given 60% < Sice < 80% the pore-filling morphology of the ice converts into a frame-building habitus and at Sice > 80% the frame gains strength while the amount of residual water decreases. Tensile failure and cracking now exceed grain boundary sliding as the prevailing failure mode. The ice morphology in the sand is non-cementing and comparable to a gas hydrate-sand mixture.

Progress in Global Gas Hydrate Development and Production as a New Energy Resource

Natural gas hydrates have been hailed as a new and promising unconventional alternative energy, especially as fossil fuels approach depletion, energy consumption soars, and fossil fuel prices rise, owing to their extensive distribution, abundance, and high fuel efficiency. Gas hydrate reservoirs are similar to a storage cupboard in the global carbon cycle, containing most of the world's methane and accounting for a third of Earth's mobile organic carbon. We investigated gas hydrate stability zone burial depths from the viewpoint of conditions associated with stable existence of gas hydrates, such as temperature, pressure, and heat flow, based on related data collected by the global drilling programs. Hydrate‐related areas are estimated using various biological, geochemical and geophysical tools. Based on a series of previous investigations, we cover the history and status of gas hydrate exploration in the USA, Japan, South Korea, India, Germany, the polar areas, and China. Then, we review the current techniques for hydrate exploration in a global scale. Additionally, we briefly review existing techniques for recovering methane from gas hydrates, including thermal stimulation, depressurization, chemical injection, and CH4–CO2exchange, as well as corresponding global field trials in Russia, Japan, United States, Canada and China. In particular, unlike diagenetic gas hydrates in coarse sandy sediments in Japan and gravel sediments in the United States and Canada, most gas hydrates in the northern South China Sea are non‐diagenetic and exist in fine‐grained sediments with a vein‐like morphology. Therefore, especially in terms of the offshore production test in gas hydrate reservoirs in the Shenhu area in the north slope of the South China Sea, Chinese scientists have proposed two unprecedented techniques that have been verified during the field trials: solid fluidization and formation fluid extraction. Herein, we introduce the two production techniques, as well as the so‐called “four‐in‐one” environmental monitoring system employed during the Shenhu production test. Methane is not currently commercially produced from gas hydrates anywhere in the world; therefore, the objective of field trials is to prove whether existing techniques could be applied as feasible and economic production methods for gas hydrates in deep‐water sediments and permafrost zones. Before achieving commercial methane recovery from gas hydrates, it should be necessary to measure the geologic properties of gas hydrate reservoirs to optimize and improve existing production techniques. Herein, we propose horizontal wells, multilateral wells, and cluster wells improved by the vertical and individual wells applied during existing field trials. It is noteworthy that relatively pure gas hydrates occur in seafloor mounds, within near‐surface sediments, and in gas migration conduits. Their extensive distribution, high saturation, and easy access mean that these types of gas hydrate may attract considerable attention from academia and industry in the future. Herein, we also review the occurrence and development of concentrated shallow hydrate accumulations and briefly introduce exploration and production techniques. In the closing section, we discuss future research needs, key issues, and major challenges related to gas hydrate exploration and production. We believe this review article provides insight on past, present, and future gas hydrate exploration and production to provide guidelines and stimulate new work into the field of gas hydrates.

Nucleation curve of carbon dioxide hydrate

The first generation High Pressure Automated Lag Time Apparatus (HP-ALTA MkI) was used to measure the nucleation curves of Structure I (sI) – forming carbon dioxide hydrate. The instrument applied a large number of constant cooling ramps to a quiescent water sample contained in a glass sample cell under isobaric conditions and recorded maximum achievable subcooling distributions. A survival curves was constructed from the measured data and a nucleation curve was derived from the survival curve using the model-independent method we had recently reported. It was found that the nucleation rate of carbon dioxide hydrate was significantly greater than that of pure methane hydrate or natural gas hydrates.

Model for gas hydrates applied to CCS systems part II. Fitting of parameters for models of hydrates of pure gases

In this study comprising a series of three articles, the model for pure CO2 hydrate by Jäger et al. [Fluid Phase Equilib. 338 (2013) 100–113] has been improved and extended to other gases relevant in Carbon Capture and Storage (CCS) applications. The new model for eight pure gas hydrates is inspired by the currently most accurate model available for natural gas hydrates by Ballard and Sloan [Fluid Phase Equilib. 194 (2002) 371–383], which belongs to the family of van der Waals and Platteeuw (vdWP) models [Adv. Chem. Phys. 2 (1959) 1]. The new model is combined with highly accurate equations of state (EoS) in form of the Helmholtz energy for fluid phases and with Gibbs energy models for pure solid phases. Part I contains a critical analysis of the main parameters of the vdWP-based hydrate model. In this article (part II), the new hydrate model is described. A multi-property fitting algorithm is presented, which allows for simultaneous fitting of the model parameters to hydrate phase equilibrium data and to various types of hydrate composition data. Kihara potential parameters and reference parameters of the hydrate model were evaluated for eight selected hydrate formers; namely the CCS relevant gases carbon dioxide, argon, nitrogen, oxygen, carbon monoxide, and methane, and additionally ethane and propane. Experimental data used as an input to the fitting algorithm were weighted using uncertainty estimates. Estimation of the uncertainties of chemical potential along the three-phase coexistence lines including hydrates is a non-trivial task. Clearly, the uncertainty is dominated by various metastable and hysteresis effects, rather than by uncertainties of pressure and temperature measurements. For these reasons, some ad-hoc approaches have been developed. The uncertainty could straightforwardly be estimated in sparingly occurring regions covered by multiple data sources. Assuming a band of constant work of formation of hydrates along the three-phase coexistence lines then provided a plausible estimate of the uncertainties in a broad range of thermodynamic conditions. Usually, the Langmuir constant is fitted together with the reference state parameters of the hydrate model to three-phase hydrate formation data. In this work, a method is discussed that directly links the Langmuir constant to hydrate composition data. The performance of the new hydrate model dedicated to typical CCS gases in binary mixtures with water is demonstrated in part III. The model has been implemented in the software package TREND 2.0 by Span et al. [Thermodynamic Reference and Engineering Data 2.0. (2015) Lehrstuhl fuer Thermodynamik, Ruhr-Universitaet Bochum].

Erratum to: An application of rock physics modeling to quantify the seismic response of gas hydrate-bearing sediments in Makran accretionary prism, offshore, Pakistan

Naturally occurring gas hydrates are potential future energy source. A significant amount of gas hydrates is interpreted through seismic reflection data in the form of bottom simulating reflector (BSR) present in the sediments of the convergent continental margin of Pakistan. However, the seismic character of these hydratebearing unconsolidated sediments is not properly investigated. Since no direct measurements are available for quantitative estimation of gas hydrate and free gas in these sediments, therefore detailed knowledge of seismic velocities is essential. Seismic velocities of the gas hydrate-bearing sediments in the study area are estimated by using the effective medium theory and the fluid substitution modeling. The results show that the presence of gas hydrates increases the stiffness of the unconsolidated sediments; whereas the presence of free gas decreases the stiffness of these sediments. It is noted that seismic velocities and density of hydrate-bearing sediments are highly affected by saturation and distribution pattern of gas hydrates. The hydrate-bearing sediments seem to be characterized not only by high P-wave velocity (about 2800 m/s) but also by anomalously low S-wave velocity (about 850 m/s). As pure gas-hydrates have much higher seismic velocities than those of host sediments, presence of gas-hydrate increases the seismic velocities, whereas free-gas below the hydrate-bearing sediments decreases the velocities. Seismic reflection from the BSR exhibits a wide range of amplitude variation with offset characteristics, which depend upon the saturation and distribution of hydrates above and free gas below the BSR. We have also demonstrated that some attributes like acoustic and shear impedances, and AVO can be used as important proxies to detect gas hydrate saturation.

An approach to estimate gas hydrate saturation from 3-D heterogeneous resistivity model: A study from Krishna-Godavari basin, Eastern Indian offshore

A method is proposed to estimate gas hydrate saturation from three dimensional (3-D) heterogeneous model of resistivity simulated using resistivity log. Pure gas hydrates are highly resistive compared to the host sediments, and their presence in the pore space of sediments increase the resistivity of the formation. The anomalous increase of resistivity is used as a proxy for the delineation of gas hydrates using the resistivity log. A 3-D heterogeneous resistivity model has been constructed from one dimensional resistivity log in Krishna Godavari (KG) basin, eastern Indian offshore. The simulated model contains all small scale variation in resistivity of the reservoir and maintains all properties associated with covariance, like root mean square fluctuation, characteristic scales and fractal dimension of the observed log. We have estimated volumetric hydrate saturation using the three dimensional simulated model. The porosity used for estimating hydrate saturation is calculated from the simulated density field generated using the observed density log. Estimated average gas hydrate saturation is about 9.84% of the pore volume over a 1000 m × 1000 m x 131 m cubic meters area.

Development of a novel experimental apparatus for hydrate equilibrium measurements

A novel experimental apparatus was designed and constructed for measuring the three-phase equilibrium conditions of pure gas and gas mixture hydrates. The experimental apparatus can operate at pressures up to 20 MPa. The different parts of the experimental set-up are discussed and analyzed. Calibration issues are addressed and validation experimental measurements are reported at pressure conditions below 10 MPa for the case of pure gas hydrates. In addition to measuring the three-phase equilibrium pressure and temperature conditions with the current experimental apparatus, liquid phase analysis measurements are reported. In particular, the solubility of the gas in the aqueous phase is measured under three-phase equilibrium conditions. Gases of interest to the current study include CH4, and CO2. Both gases are of significant importance in industrial gas-storage and transportation applications, as well as gas-mixture separation applications.

Experimental procedure and results to measure the composition of gas hydrate, during crystallization and at equilibrium, from N2–CO2–CH4–C2H6–C3H8–C4H10 gas mixtures

Mixed Clathrate hydrates are an important issue in many fields, like flow assurance in the oil industry, as well as gas capture and storage, air conditioning, etc …Usually, studies of gas hydrates from hydrocarbon gas mixtures do not mention the volume of hydrate, nor the hydrate composition, and ternary or quaternary mixtures are not considered. Also, data involving propane and butane are limited.Therefore, we suggest both an experimental and modeling study of mixed clathrate hydrates from N2–CO2–CH4–C2H6–C3H8–C4H10 gas mixtures, in temperature range of [0.8–19 °C] and pressure range of [1.4–66 bars]. The experimental work provides 78 equilibrium points. Two procedures were used to perform crystallizations. The main procedure (71 equilibrium data) corresponds to a method at high crystallization rate (high supersaturation, or high ΔP). The first objective is to study the gas hydrates formation in usual dynamic conditions (start-up or reboot of an oil exploitation). The second method, 7 data, corresponds to a low rate of crystallization. This second objective is to investigate the impact of the speed of crystallization on the final equilibrium. For both procedures, P-T data are given, as well as the compositions of each phase at equilibrium.At last, thermodynamic modeling is used to compare the experimental results of both procedures. Kihara parameters of N2, CO2 and CH4 are taken from the literature, while parameters for C2H6 are regressed and given in this work.Results show a better agreement of the thermodynamic modeling with experimental study for pure gas hydrate and mixed gas hydrates at low crystallization rate. This observation suggests that mixed gas hydrates might form at thermodynamic equilibrium only at low crystallization rate.

Equilibrium Conditions for TBAB and TBAC Semiclathrates of Xenon and Argon

A constant-volume reactor was used for measuring the solid–vapor–liquid equilibrium conditions of semiclathrates formed in aqueous solutions of tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl ammonium chloride (TBAC). The TBAB and TBAC semiclathrates were formed from pure Ar and pure Xe. The experimental temperatures ranged from (284 to 303) K, the experimental pressures ranged from (266 to 6114) kPa, the concentration of TBAB ranged from wTBAB = (0.05 to 0.20), and the concentration of TBAC ranged from wTBAC = (0.05 to 0.20). As expected, at a given temperature, the pressure required to form TBAB and TBAC semiclathrates with argon and with xenon was much lower than the pressure that was required to form pure gas hydrates. From the equilibrium data, the enthalpy of formation was estimated to be between (133 and 188) kJ·mol–1 for semiclathrates formed from argon and (55 and 127) kJ·mol–1 for semiclathrates formed from xenon.

Sequentially sampled gas hydrate water, coupled with pore water and bottom water isotopic and ionic signatures at the Kukuy mud volcano, Lake Baikal: ambiguous deep-rooted source of hydrate-forming water

The isotopic and ionic composition of pure gas hydrate (GH) water was examined for GHs recovered in three gravity cores (165–193 cm length) from the Kukuy K-9 mud volcano (MV) in Lake Baikal. A massive GH sample from core St6GC4 (143–165 cm core depth interval) was dissociated progressively over 6 h in a closed glass chamber, and 11 sequentially collected fractions of dissociated GH water analyzed. Their hydrogen and oxygen isotopic compositions, and the concentrations of Cl– and HCO3 – remained essentially constant over time, except that the fraction collected during the first 50 minutes deviated partly from this pattern. Fraction #1 had a substantially higher Cl– concentration, similar to that of pore water sampled immediately above (135–142 cm core depth) the main GH-bearing interval in that core. Like the subsequent fractions, however, the HCO3 – concentration was markedly lower than that of pore water. For the GH water fractions #2 to #11, an essentially constant HCO3 –/Cl– ratio of 305 differed markedly from downcore pore water HCO3 –/Cl– ratios of 63–99. Evidently, contamination of the extracted GH water by ambient pore water probably adhered to the massive GH sample was satisfactorily restricted to the initial phase of GH dissociation. The hydrogen and oxygen isotopic composition of hydrate-forming water was estimated using the measured isotopic composition of extracted GH water combined with known isotopic fractionation factors between GH and GH-forming water. Estimated δD of −126 to −133‰ and δ18O of −15.7 to −16.7‰ differed partly from the corresponding signatures of ambient pore water (δD of −123‰, δ18O of −15.6‰) and of lake bottom water (δD of −121‰, δ18O of −15.8‰) at the St6GC4 coring site, suggesting that the GH was not formed from those waters. Observations of breccias in that core point to a possible deep-rooted water source, consistent with published thermal measurements for the neighboring Kukuy K-2 MV. By contrast, the pore waters of core St6GC4 and also of the neighboring cores GC2 and GC3 from the Kukuy K-9 MV show neither isotopic nor ionic evidence of such a source (e.g., elevated sulfate concentration). These findings constrain GH formation to earlier times, but a deep-rooted source of hydrate-forming water remains ambiguous. A possible long-term dampening of key deep-water source signatures deserves further attention, notably in terms of diffusion and/or advection, as well as anaerobic oxidation of methane.

Experimental Verification of Methane–Carbon Dioxide Replacement in Natural Gas Hydrates Using a Differential Scanning Calorimeter

The methane (CH4) - carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 - CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 - CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation.

Accurate prediction of H2S and CO2 containing sour gas hydrates formation conditions considering hydrolytic and hydrogen bonding association effects

An elegant thermodynamic model, considering the hydrogen bonding association effects in fluid phases by using cubic-plus-association equation of state (CPA EOS), electrostatic effects of hydrolytic dissociation of H2S and CO2 acidic gases according to the modified Pitzer–Debye–Hückle (PDH) model and an improved two step gas hydrate formation mechanism using Chen–Guo model is presented for accurate prediction of the pure and mixed sour gas hydrate formation conditions. The proposed CPA/Electrolyte/Chen–Guo model is quite reliable over wide ranges of temperatures, H2S and CO2 concentrations. The overall average absolute deviation between the predicted and measured sour gas hydrate equilibrium dissociation pressures regarding 149 data points comprising pure and multi-component sour gas mixtures, is about 3.32% which is considerably lower than the other thermodynamic models. The proposed model outperforms the other alternatives both in accuracy and generality and can be employed for accurate design of sour natural gas management and flow assurance systems and gas hydrate energy storage processes in oil and gas industries

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

Modeling of hydrate formation conditions for CH4, C2H6, C3H8, N2, CO2 and their mixtures using the PRSV2 equation of state and obtaining the Kihara potential parameters for these components

In this paper, we present a simple thermodynamic model to describe phase behavior of gas hydrates with CH4, C2H6, C3H8, N2, CO2 and their mixtures as guests. The model is based on equality of water fugacity in the liquid water and hydrate phases. The van der Waals and Platteeuw model is applied for calculating the fugacity of water in the hydrate phase. The Stryjek and Vera modification of Peng–Robinson (PRSV2) equation of state is used to evaluate the fugacity of water in the vapor and liquid phases. Binary interaction parameters are adjusted as a linear function of temperature. The Vapor–Liquid–Equilibria for the pure gas hydrates former have been predicted within 2.87% absolute average relative deviation and for the gas mixtures have been predicted within 3.43% absolute average deviation. The pure gas hydrate formation conditions have been predicted within 2.27% absolute average deviation. The hydrate formation conditions of gas mixtures have been predicted within AAD 3.72%. The results are in good agreement, demonstrating the reliability of the thermodynamic approach used in this work.

An accurate thermodynamic model to predict phase behavior of clathrate hydrates in the absence and presence of methanol based on the genetic algorithm

In this study, we present a simple thermodynamic model that predicts phase behavior of hydrates with CH4, C2H6, C3H8, N2, CO2, H2S and their mixtures as guests. The model is based on equality of water fugacity in the liquid water and hydrate phases. The van der Waals and Platteeuw model is applied for calculating the fugacity of water in the hydrate phase. The Stryjek and Vera modification of Peng–Robinson (PRSV2) equation of state is used to evaluate the fugacity of water in the vapor and liquid phases. In the absence of inhibitors, the pure gas hydrate formation conditions have been predicted within 2.15% absolute average relative deviation. The hydrate formation condition of gas mixtures have been predicted within AARD 4.02%. In the presence of organic inhibitors for pure gas 0.35% AART has been observed. Our results showed that the PRSV2 EOS is capable of prediction of hydrate formation condition with reasonable absolute average relative deviation (AARD) from experimental data.

Application of PHSC equation of state in prediction of gas hydrate formation condition

In this work the modified perturbed hard sphere chain equation of state (PHSC), coupled with Van der waals Platteuw model, has been used for prediction of gas hydrate formation (dissociation) conditions. The PHSC EOS has been modified using association and electrostatic contributions. In order to evaluate the capability of the PHSC EOS, the hydrate formation conditions of various pure as well as mixed gases in the presence and absence of the thermodynamic inhibitors such as methanol, ethanol, mono ethylene glycol(MEG), NaCl, KCl and CaCl2 have been studied. In the absence of inhibitors, the pure gas hydrate formation conditions have been predicted within 3.4% absolute average relative deviation. The hydrate formation condition of gas mixtures have been predicted within AARD 4.6%. In the presence of organic inhibitors and single electrolyte 6.1% and 5.8% AARD has been observed respectively. A total of 102 systems have been studied; our results showed that the PHSC EOS is capable of prediction of hydrate formation condition with reasonable absolute average relative deviation (AARD) from experimental data.

Modeling phase equilibria of semiclathrate hydrates of CH4, CO2 and N2 in aqueous solution of tetra-n-butyl ammonium bromide

Semiclathrate hydrates of tetra-n-butyl ammonium bromide (TBAB) offer potential solution for gas storage, transportation, separation of flue gases and CO2 sequestration. Models for phase equilibria for these systems have not yet been developed in open literatures and thus require urgent attention. In this work, the first attempt has been made to model phase equilibria of semiclathrate hydrates of CH4, CO2 and N2 in aqueous solution of TBAB. A thermodynamic model for gas hydrate system as proposed by Chen and Guo has been extended for semiclathrate hydrates of gases in aqueous solution of TBAB. A correlation for the activity of water relating to the system temperature, concentration of TBAB in the system and the nature of guest gas molecule has been proposed. The model results have been validated against available experimental data on phase equilibria of semiclathrate hydrate systems of aqueous TBAB with different gases as guest molecule. The extended Chen and Guo's model is found to be suitable to explain the promotion effect of TBAB for the studied gaseous system such as, methane, carbon dioxide and nitrogen as a guest molecule. Additionally, a correlation for the increase in equilibrium formation temperature (hydrate promotion temperature, ΔTp) of semiclathrate hydrate system with respect to pure gas hydrate system has been developed and applied to semiclathrate hydrate of TBAB with several gases as guest molecules. The developed correlation is found to predict the promotion effect satisfactorily for the system studied.