Hydrate Equilibrium Conditions Research Articles

High pressure rheological studies of methane hydrate slurries formed from water-hexane, water-heptane, and water-decane multiphase systems

High pressure rheological properties of methane hydrate slurries entail the high-pressure rheometer that can deliver a suitable mixing to form in situ methane hydrate from the multiphase system. However, the hydrate formation was extremely challenging in conventional cup and bob geometry due to its plane surface. In this work, modified Couette geometry has been used in a high pressure rheometer to study in situ formation and dissociation of methane hydrate slurries formed from multiphase water-hexane (C6H14), water-heptane (C7H16) and water-decane (C10H22), with varying water-to-liquid hydrocarbon ratios (50:50, 30:70 v/v). Rheological properties such as viscosity, flow curve measurements, viscoelastic properties have been measured during hydrate formation and dissociation. Transient viscosity measurements are carried out at 10 MPa and 275.15 K at a shear rate of 1000 s−1. It was observed that the viscosity increases significantly during hydrate induction. Viscosity profile has shown bobbing behavior during the course of hydrate formation indicating in situ morphological changes (agglomeration and breakage of hydrate crystals) during hydrate formation. Flow curves indicate strong shear thinning behavior. Viscosity profile of methane hydrate slurries were also observed for each case during the hydrate dissociation using thermal stimulation. The peak in the viscosity has been identified near hydrate equilibrium condition during dissociation. Dynamic viscoelastic properties indicate solid behavior of methane hydrate slurries. The present study provides significant contribution on the rheology of methane hydrate slurries formed from multiphase systems in the context of flow assurance applications.

Effect of n-dodecane on equilibrium dissociation conditions of carbon dioxide hydrate

The effect of n-dodecane on the phase equilibrium conditions of carbon dioxide hydrate is investigated in this work. The phase equilibrium conditions for (carbon dioxide + n-dodecane + water) mixtures were measured under constant volume conditions using a stepwise heating method. This experiments were carried out in the different n-dodecane mass fractions of 0.3333, 0.4286, 0.5000 and 0.6000, respectively. The experimental results shows that the carbon dioxide hydrate phase equilibrium pressures with small amount addition of n-dodecane are lower than that of the system of pure carbon dioxide at higher temperatures. But as the addition mass fraction of n-dodecane increases, the hydrate formation temperature-pressure equilibrium curve shifts to the left at higher pressures. Moreover, excessive addition of n-dodecane inhibits carbon dioxide hydrate formation, rather than promoting it.

Experimental measurement and thermodynamic modeling of methane hydrate phase equilibria in the presence of chloride salts

The phase equilibria of hydrate are necessary to study the gas control/production from marine sediments. Considering the abundance of chloride salts in the ocean, the phase equilibrium conditions of CH4 hydrate in 3.5 wt% NaCl, KCl, CaCl2 and MgCl2 solutions were experimentally investigated at the temperature and pressure ranges of 281.9 K~287.1 K and 7.33 MPa~14.02 MPa, respectively. Then the reliability of the obtained data was validated by three thermodynamic consistency criteria. In addition, a new calculation model was proposed based on the Chen-Guo model and the Hu-Lee-Sum (HLS) correlation for hydrate prediction. And the prediction accuracy of the proposed model was described by the absolute average deviation of temperature (AADT). Results show that the inhibition effect of MgCl2 is stronger than that of other salts. Also, the experimental data obtained in this study can meet all the requirements of the criteria. Furthermore, the prediction results from the proposed model well agree with the experimental data. And the proposed model shows a good performance in predicting the phase equilibria of CH4 hydrate in the systems containing single chloride salt and mixed chloride salts with AADT of 0.54 K and 0.82 K, respectively.

Experimental study and thermodynamic modeling of CCl4 + O2 and CCl4 + N2 hydrate equilibria

Nitrogen and oxygen are the main components of air. Liquid nitrogen is an alternative to fossil fuels. However, little attention has been paid to this energy resource. Liquid oxygen is not a fuel in itself, however, it can be used as an oxidizer to run vehicles. In this respect, gas hydrate technology can be of interest, as applicable media for nitrogen/oxygen transport or nitrogen/oxygen separation. Both nitrogen and oxygen have the tendency to form hydrates, especially at high pressures, with double occupancy of the large cavities of structure sII. Thermodynamic hydrate promoters (THPs) can be used to facilitate the formation of nitrogen or oxygen hydrates at more moderate equilibrium conditions. For this purpose, CCl4 can be a potential candidate. In this contribution, hydrate equilibrium conditions for binary mixtures of nitrogen + CCl4 (50 data points) and oxygen + CCl4 (41 data points) were experimentally measured. This was done using three different experimental equipment, namely: a tensimeter, the Cailletet apparatus, and a high-pressure autoclave. A thermodynamic model, according to the van der Waals-Platteeuw (vdW-P) solid solution theory, was then employed to predict the hydrate equilibrium temperatures of the aforementioned mixtures. The fugacities of the components in the mixtures were calculated through the Peng-Robinson (PR) EoS and the classical van der Waals mixing rules. The Kihara potential model was applied to represent the guest-host interactions. The results reveal that the model has the ability to predict the hydrate equilibrium temperatures with the average absolute deviation (AAD) of 0.32 K. The fractional occupancy calculations indicate that the CCl4 molecules mostly occupy the large cages and do not enter the small cages of sII hydrates because of their size, while nitrogen and oxygen have small contributions in the filling of the large cages of sII.

Toward a Robust, Universal Predictor of Gas Hydrate Equilibria by Means of a Deep Learning Regression.

Due to offshore reservoirs being developed in ever deeper and colder waters, gas hydrates are increasingly becoming a significant factor when considering the profitability of a reservoir due to flow disruptions, equipment, and safety hazards arising from the hydrate plug formation. Due to low-dosage hydrate inhibitors such as kinetic inhibitors competing with traditional thermodynamic inhibitors such as methanol, accurate information regarding the hydrate equilibrium conditions is required to determine the optimal hydrate control strategy. Existing thermodynamic models can prove inflexible regarding parameter adjustment and the incorporation of new data. Developing a multivariate regression model capable of generalizing hydrate equilibria over a wide range of conditions, with results competing with thermodynamic models is worthwhile. A multilayer perceptron neural network of three hidden layers has undergone supervised training means of a backpropagation to accurately predict uninhibited hydrate equilibrium pressure for a range of gas mixtures with nine input features, excluding hydrogen sulfide and electrolytes, from a dataset of 1209 equilibrium points, 670 of which are multicomponent gases, sampled in a rigorous data sampling campaign from existing experimental studies. Statistical significance of results has been emphasized, with models validated using 10-fold cross-validation and holdout validation, facilitating hyperparameter optimization without overfitting, while stratified holdout ensures testing a wide range of conditions. The developed model has proven to outperform two popular thermodynamic models. Various scoring metrics are used, with an average cross-validated R2 of 0.987 ± (0.003). An R2 of 0.993 and mean absolute percentage error of 5.56% are yielded for holdout validation. Auxiliary models are included to determine the multicomponent prediction capability and dependency on individual data sources. Multicomponent data prediction has proven successful; results prove that the model accurately generalizes hydrate equilibria and is well suited to predicting unseen data. Positive results are largely insensitive to exact model parameters, thus indicating a robust, replicable methodology.

Effect of 1-Propanol and TBAB on Gas Hydrates Dissociation Conditions for CO2 + Hexane + Water Systems

Hydrate equilibrium conditions of carbon dioxide in the presence of hexane in aqueous solutions of 1-propanol/tetra-n-butyl ammonium bromide (TBAB) have been studied in this Article. The hydrate di...

Phase stability conditions for clathrate hydrate formation in (fluorinated refrigerant + water + single and mixed electrolytes + cyclopentane) systems: Experimental measurements and thermodynamic modelling

Phase equilibrium conditions (dissociation data) for clathrate hydrates (gas hydrates) were studied for systems involving fluorinated refrigerants + water + single and mixed electrolytes (NaCl, CaCl2, MgCl2 and Na2SO4) at varying salt concentrations in the absence and presence of cyclopentane (CP). The ternary systems for (R410a or R507) + water + CP were performed in the temperature and pressures ranges of (279.8–294.4) K and (0.158–1.385) MPa, respectively. Measurements for {R410a + water + (NaCl or CaCl2) + CP} were undertaken at salt concentrations of (0.10, 0.15 and 0.20) mass fractions in the temperature and pressure ranges of (278.4–293.7) K and (0.214–1.179) MPa, respectively. The temperature and pressure conditions for the (R410a + water + Na2SO4) system were investigated at salt concentration of 0.10 mass fraction in range of (283.3–291.6) K and (0.483–1.373) MPa respectively. Measurements for {(R410a or R507) + water + mixed electrolytes NaCl, CaCl2, MgCl2} were undertaken at various salt concentrations of (0.002–0.15) mass fractions in the temperature and pressure ranges of (274.5–292.9) K and (0.149–1.119) MPa in the absence and presence of CP, for which there are no published data related to mixed salt and a promoter. The phase equilibrium measurements were performed using a non-visual isochoric equilibrium cell and the pressure-search technique. This study was focused on obtaining equilibrium data that can be utilized to design and optimize for water desalination process and the development of a Hydrate Electrolyte–Cubic Plus Association (HE–CPA) Equation of State based model. The results indicate hydrate dissociation pressure reduction/hydrate dissociation temperature increase up to ambient conditions in the presence of promoter (CP). The experimental results were then modelled. The modelling results are in good agreement with the measured hydrate dissociation data.

Thermodynamic modelling on methane hydrate equilibrium condition in the presence of electrolyte inhibitor

Abstract In hydrate equilibrium conditions at given pressure, only one temperature is presence which is the equilibrium temperature. Therefore, in this work, thermodynamic modelling was employed to predict the effect of TEACl on methane hydrate equilibrium temperature. The hydrate equilibrium temperature are modelled using the thermodynamic model proposed by Dickens and Quinby-Hunt adapter from earlier work of Pieroen. The model was established from the conventional thermodynamic theory of salts are capable to interrupt the water activity. Based on the theory, cation and anion of TEACl is able to form a hydrogen bond with water hence hindrance the hydrate cage formation and further disrupts the encapsulation of methane gas molecules. Acceptable agreement between the experimental data and the model results is observed with less than 1.0 K deviation. In comparison, the thermodynamic model is used to predict the literature experimental data and deviation is found to be at 0.6 K. Furthermore, the average absolute deviation (AAD) is found to be over predicted of 0.15 K, 0.18 K and 0.32 K for 1 wt%, 5 wt% and 10 wt% of TEACl concentrations, respectively. The accuracy of the model indicates of all the studied system in the presence of TEACl serves an excellent groundwork to extend the application of the model.

Prediction of phase equilibrium of methane hydrate below 272.2 K based on different equations of state

By revealing that a key parameter D in Chen-Guo model is related to the temperature T, a new temperature-dependent parameter D(T) trained by the reported experimental data is proposed with Chen-Guo model to improve the prediction accuracy of phase equilibrium conditions of methane hydrate in the range of 145.75 K- 272.2 K. Four equations of state (EOS) such as Redlich-Kwong (RK), Soave- Redlich-Kwong (SRK), Peng-Robinson (PR) and Patel-Teja (PT) are chosen to calculate the gas fugacity respectively and the corresponding prediction accuracy is evaluated by the average absolute deviation percentage (AADP). The process of training D(T) based on the Chen-Guo model employed with SRK EOS is described in detail first, and the prediction results show that the corresponding prediction accuracy is greatly improved, whose AADP decreases from 10.83% to 3.31%. In order to verify that training D(T) is an effective way to improve the prediction accuracy, the same procedure is also used to the Chen-Guo models employed with RK, PR and PT EOS respectively, and the corresponding AADP decreases from ∼11% to ∼3%. In addition, the prediction accuracy according to the proposed D(T) is also evaluated by other temperatures different from the data used for training D(T) to verify its feasibility and effectiveness.

A Generalized Correlation for Predicting Ethane, Propane, and Isobutane Hydrates Equilibrium Data in Pure Water and Aqueous Salt Solutions.

Hydrate formation can cause serious problems in hydrocarbon exploration, production, and transportation, especially in deepwater environments. Hydrate‐related problems affects the integrity of the deepwater platforms, leads to equipment blockages, and also increases operational costs. In order to solve these problems, salts are used as thermodynamic inhibitors and also mixed with the drilling fluids in most drilling processes. A comprehensive understanding of hydrate formation in aqueous salt solutions is vital to overcome these problems. Statistical thermodynamic models are commonly used to predict hydrate formation conditions in different aqueous solutions. However, these models involve rigorous computations and are restricted to certain conditions. They give inaccurate predictions of hydrate equilibrium conditions for high‐temperature, high‐pressure, and high‐salinity systems. Therefore, it is paramount to develop a simple‐to‐use and reliable prediction tool. In this work, an empirical correlation is developed and successfully used to predict the equilibrium conditions of ethane, propane, and isobutane hydrates in pure water and aqueous solutions of sodium chloride, potassium chloride, calcium chloride, and magnesium chloride. Experimental data on hydrate formation conditions for these components are regressed and a generalized correlation is obtained. The predictions in this work show excellent agreement with all the experimental data in the literature.

Experimental Study on Hydrate Formation with Synthesized CH4/CO2/N2 Ternary Mixtures

The variations in the equilibrium conditions of the hydrate phase between different gas components can be employed for gas separation. The use of hydrate-based gas separation technologies for CH4 recovery from biogas has gained prominence. In this study, the formation of a ternary mixture gas hydrate with CH4, CO2, and N2 was studied. The amount of gas consumed and the hydrate formation rate were calculated using pressure and temperature variations during the reaction. Gas composition changes, the CH4 recovery factor, and CO2 split ratio were the key parameters considered to evaluate the technical feasibility of HBGS. Results indicated that the formation of the mixture gas hydrate depends on the driving force between the hydrate equilibrium conditions and operating conditions. The process of hydrate formation can be divided into two stages: Most of the CO2 formed the hydrate in the first stage, and both CH4 and N2 may have entered the hydrate structure in the second stage. For a given gas, the CO2 separat...

Universal correlation for gas hydrates suppression temperature of inhibited systems: III. salts and organic inhibitors

In Parts I and II, the Hu‐Lee‐Sum (HLS) correlation was developed and demonstrated to have an inherent generality and simplicity to predict structures I and II gas hydrates suppression temperature for both single and mixed salts systems. Utilization of thermodynamic hydrate inhibitors (THIs) is common practice in industry to prevent hydrate formation. Therefore, the HLS correlation is extended to predict hydrate suppression temperature with either single or mixed THIs, including any combination of inorganic salts and organic inhibitors. It is verified that hydrate equilibrium conditions with organic inhibitors also shows nearly independence of the water activity on temperature. In addition, additive contribution of salts and organic inhibitors to the water activity are used to determine the hydrate suppression temperature. A comprehensive comparison between the literature data and predicted results demonstrate the reliability of the HLS correlation for any combination of inorganic salts and organic inhibitors. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4097–4109, 2018

Phase equilibrium for argon clathrate hydrate at the temperatures from 197.6 K to 274.1 K

Abstract The phase equilibrium conditions of clathrate hydrate of argon were measured by the isochoric procedure at the temperatures from 197.6 K to 274.1 K. Argon hydrate would have industrial applications including a working medium of the hydrate-based heat pump systems as well as influence the analysis of the past climate change on the Earth. Nevertheless, the equilibrium conditions of argon hydrate were limited in the literatures and not reported below the temperature 264 K. The lowest and the highest measured equilibrium pressure is the pressure 0.825 MPa at the temperature 197.6 K and the pressure 9.881 MPa at the temperature 274.1 K. The equilibrium pressure increases with an increase in temperature. The reliability of the measured equilibrium conditions is supported by the internal consistency of those and the coincidence of the experimentally determined quadruple point (vapor + hydrate + ice + aqueous phases in equilibrium) temperature with the water freezing temperature theoretically predicted. The discussion is given on the comparison of the equilibrium data measured in this study and those reported in the literature.

Thermodynamic modeling of methane hydrate formation in the presence of imidazolium-based ionic liquids using two-step hydrate formation theory and CPA EoS

In this study, thermodynamic modeling is performed to predict the P-T equilibrium conditions of methane gas hydrate formation in presence of ionic liquids. The imidazolium-based ionic liquids including [EMIM][HSO4], [EMIM][EtSO4], [BMIM][BF4], [OH-EMIM][BF4], [BMIM][Cl] and [BMIM][Br] are modeled through two-step hydrate formation theory of Chen and Guo coupled with CPA EoS for calculating the fugacities of species in liquid and gas phase. The CPA pure component parameters for ionic liquids and also the majority of binary interaction parameters are adjusted using the liquid density and binary VLE data, respectively. The thermodynamic modeling is carried out in the presence of 0.1 weight fractions of all the studied imidazolium based ionic liquids. Moreover, the effect of three various concentrations of [BMIM][BF4] on the P-T equilibrium diagram of methane hydrate are predicted at 0.1, 0.15 and 0.20 weight fractions of this ionic liquid. The results indicate that the CPA EoS can satisfactorily model the phase behavior of aqueous solutions of imidazolium based ionic liquids considering 2B scheme for the association term. In addition, the results indicate good predictions of the proposed model for methane hydrate equilibrium conditions in the presence of imidazolium-based studied ionic liquids including concentration changes such that the absolute average deviations of methane hydrate formation pressures is obtained below 4.2%.

Phase Equilibrium of Isotopologue Methane Hydrates Enclathrated CH3D and CD4

Hydrate equilibrium conditions formed from deuterated methane isotopologues (CH3D and CD4) were measured over a temperature range of (273 to 278) K and a pressure range of (2.6 to 4.2) MPa. The phase boundary of CH3D and CD4 hydrates was shifted to higher pressures and lower temperatures than that of the CH4 hydrate. Crystal structure and cage occupancies of gas hydrate crystals formed from CH3D and CD4 were characterized by Raman spectroscopy. The isotope effect of methane hydrates revealed in this study can be used to explain the isotopic fractionation of hydrogen in methane during the formation of clathrate hydrates observed in natural settings, as reported by previous studies.

Analysis of three-phase equilibrium conditions for methane hydrate by isometric-isothermal molecular dynamics simulations.

To develop prediction methods of three-phase equilibrium (coexistence) conditions of methane hydrate by molecular simulations, we examined the use of NVT (isometric-isothermal) molecular dynamics (MD) simulations. NVT MD simulations of coexisting solid hydrate, liquid water, and vapor methane phases were performed at four different temperatures, namely, 285, 290, 295, and 300 K. NVT simulations do not require complex pressure control schemes in multi-phase systems, and the growth or dissociation of the hydrate phase can lead to significant pressure changes in the approach toward equilibrium conditions. We found that the calculated equilibrium pressures tended to be higher than those reported by previous NPT (isobaric-isothermal) simulation studies using the same water model. The deviations of equilibrium conditions from previous simulation studies are mainly attributable to the employed calculation methods of pressure and Lennard-Jones interactions. We monitored the pressure in the methane phase, far from the interfaces with other phases, and confirmed that it was higher than the total pressure of the system calculated by previous studies. This fact clearly highlights the difficulties associated with the pressure calculation and control for multi-phase systems. The treatment of Lennard-Jones interactions without tail corrections in MD simulations also contributes to the overestimation of equilibrium pressure. Although improvements are still required to obtain accurate equilibrium conditions, NVT MD simulations exhibit potential for the prediction of equilibrium conditions of multi-phase systems.

The formation of carbon dioxide hydrate from water sorbed by coals

Experiments on the formation and dissociation of carbon dioxide gas hydrate formed from water that was adsorbed in two sub-bituminous coals and anthracite were carried out. Preliminarily, the coal samples were studied using various physicochemical methods. It was demonstrated that the conditions of the hydrate decomposition in coal do not differ substantially from the equilibrium conditions of carbon dioxide hydrate. In all the cases, the conversion of water to hydrate increases with increasing the pressure of the hydrate former and the initial humidity of coal samples. At the same time, it turned out that the amount of unreacted water in the sample also increases with an increase in the initial humidity of coal sample. A phenomenological model explaining the observed features of the hydrate formation in coal was proposed on the basis of the data obtained. In this model, the hydrate formation in coal is considered as a two-stage process: (1) a partial displacement of coal-sorbed water from the pore space into macropores and on the coal surface due to competitive sorption of carbon dioxide, and (2) the formation of hydrate from water droplets formed in the first stage.

Experimental measurement and thermodynamic modeling of equilibrium condition for natural gas hydrate in MEG aqueous solution

Comprehensive information about the hydrate formation and dissociation conditions is necessary for design and operation of different processes in the petroleum industry. This study investigated the natural gas hydrate equilibrium conditions in the presence of mono ethylene glycol aqueous solution as a widely used thermodynamic hydrate inhibitor in the oil and gas industries. Stepwise heating method was used for the hydrate formation and dissociation tests in a 250 cm3 stainless steel equilibrium cell. To validate and verify the performance of the apparatus and the method used, methane hydrate equilibrium data were measured and compared to a number of selected experimental data in the literature. This verification showed that the experimental procedure was accurate. The equilibrium temperatures of synthetic natural gas (SNG) hydrates in the absence of MEG aqueous solutions at various pressures of 3.95, 4.49, 5.36, 6.50, 6.85, and 8.20 MPa were 289.9, 290.9, 291.7, 293.1, 293.5 and 294.5 K, respectively. The equilibrium data for natural gas hydrate in the presence of 0.10 and 0.20 mass fractions of MEG in the water for pressure range of 3.90–8.20 MPa were also comparatively measured. The results showed that the natural gas hydrate equilibrium temperature decreased about 2.5 and 5 K in the presence of 0.10 and 0.20 mass fractions of MEG, respectively. The modified statistical thermodynamic model based on van der Waals and Platteeuw solid solution theory and the PVTsim software were applied to predict and evaluate hydrate equilibrium conditions. Thermodynamic model predictions matched reasonably well with the experimental results.

Modeling of gas hydrate phase equilibria: Extremely randomized trees and LSSVM approaches

This paper involves an innovative computer-based methodology called Extremely Randomized Trees (Extra Trees) to estimate the hydrate formation/dissociation conditions of different gases in the presence of pure water or water-based solutions of salt(s) and/or alcohol(s) for the first time. A smart model is likewise established founded on the least square version of support vector machine (LSSVM). For modeling purpose, an extensive databank containing up more than 1840 experimental data on equilibrium conditions of clathrate hydrate formation/dissociation have been gathered from literature (1940 to 2016). The gathered databank covers extensive variety of pressure-temperature conditions at liquid water-hydrate-vapor or ice-hydrate-vapor equilibria. Error analysis revealed that each of the suggested models replicate the targets with R2% of greater than 96. The forecasts of the evolved Extra Trees and LSSVM models are in good arrangement with corresponding experimental records with the average absolute relative deviation percent (%AARD) identical to 0.04 and 0.32, in that order. Nevertheless, fault examination showed the LSSVM model cannot oppose Extra Trees in expressing the hydrate phase equilibria for the studied gases.

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.