Isochoric Pressure-search Method Research Articles

A Combined Experimental and Computational Study on the Effect of the Reactor Configuration and Operational Procedures on the Formation, Growth and Dissociation of Carbon Dioxide Hydrate

Clathrate hydrate-based technologies are considered promising and sustainable alternatives for the effective management of the climate change risks related to emissions of carbon dioxide produced by human activities. This work presents a combined experimental and computational investigation of the effects of the operational procedures and characteristics of the experimental configuration, on the phase diagrams of CO2-H2O systems and CO2 hydrates’ formation, growth and dissociation conditions. The operational modes involved (i) the incremental (step-wise) temperature cycling and (ii) the continuous temperature cycling processes, in the framework of an isochoric pressure search method. Also, two different high-pressure PVT configurations were used, of which one encompassed a stirred tank reactor and the other incorporated an autoclave of constant volume with magnetic agitation. The experimental results implied a dependence of the subcooling degree, (P, T) conditions for hydrate formation and dissociation, and thermal stability of the hydrate phase on the applied temperature cycling mode and the technical features of the utilized PVT configuration. The experimental findings were complemented by a thermodynamic simulation model and other calculation approaches, with the aim to resolve the phase diagrams including the CO2 dissolution over the entire range of the applied (P, T) conditions.

Experimental measurements and thermodynamic modeling of dissociation pressure of CO2 hydrate in sulfate solutions

Carbon capture and storage (CCS) has been a popular strategy to mitigate climate change and has attracted significant attention from both industry and academia. CO2 can be stored in the form of CO2 hydrate in deeper locations in an ocean, which makes it a potential option for CO2 sequestration. Dissolved salts in the ocean brine can significantly affect the dissociation pressure of CO2 hydrate. Sulfate salts are one of the most common salt species in the oceanic brine. Although various experimental studies have been conducted to investigate the phase behavior of CO2 hydrate in brine, few studies focus on the effect of sulfate salts on the dissociation pressure of CO2 hydrate. In this study, we build an in-house experimental setup to investigate the effect of monovalent and divalent sulfate salts on the dissociation pressure of CO2 hydrate. The dissociation pressure of CO2 hydrate in Na2SO4, K2SO4, MgSO4, and CuSO4 aqueous solutions is measured using the isochoric pressure search method at different concentrations over the temperature range of 274.36–––282.15 K and the pressure range of 1.50–––4.03 MPa. A hybrid methodology incorporating the Soave-Redlich-Kwong Equation of State (SRK EOS), the van der Waals-Platteeuw (vdW-P) model, and the Pitzer model is applied to predict the dissociation pressure of CO2 in sulfate solutions. In addition, the dissociation enthalpies of CO2 hydrate in these sulfate solutions are calculated using the Clausius-Clapeyron equation based on the measured dissociation points. The experimental results show that the dissociation pressure of CO2 hydrate in Na2SO4, K2SO4, and MgSO4 solutions is higher than that in pure water, and the dissociation pressure of CO2 hydrate in sulfate solutions increases with an increasing salt concentration. Conversely, CuSO4 barely affects the dissociation pressure of CO2 hydrate, which is mainly attributed to the lower molar concentration of the ions compared with the other salt solutions and the ion specificity of Cu2+. The prediction results are in alignment with the experimental data measured in this study, which proves the feasibility of the thermodynamic model in predicting the dissociation pressure of CO2 hydrate in sulfate solutions. Additionally, the calculated dissociation enthalpies of CO2 hydrate show a dependence on both temperature and salt concentration. It is also revealed that Cu2+ exhibits ion specificity in affecting the dissociation enthalpy of CO2 hydrate, likely due to its distinct ability to impact the cage occupancy of CO2 hydrate compared with the other ions. These findings enhance our understanding of the impact of sulfate salts on the dissociation behavior of CO2 hydrate and offer valuable insights for CO2 sequestration.

Thermal stabilities of CH4 and CO2 hydrates in quartz sands and modeling

Forming an impermeable hydrate bearing layer is a novel path to prevent CO2 leakage during marine carbon storage. To make an accurate description of the formation of impermeable hydrate bearing layer, the equilibria of CH4 and CO2 hydrates in quartz sands were measured in a temperature range from 273 to 283 K using isochoric pressure search method and HP-μDSC. The influence of particle size from 0.025 to 0.3 mm and water saturation from 5 to 30 % on the hydrate equilibrium were analyzed. Based on the typical hydrate equilibrium model, van Genuchten model was employed to describe the capillary effect. Results showed that particle size had negligible effect on the equilibria of CH4 and CO2 hydrates, but a decrease in water saturation was found to lead an exponential growth in temperature depression. As the water saturation reduced down to 10 % and below, the temperature depression surged from about 0.1 up to 0.89 K in maximum. The dissociation enthalpies of CH4 and CO2 hydrates were determined as 56.01 ± 1.29 and 64.98 ± 1.90 kJ/mol respectively. The reduction of water saturation from 30 to 5 % was found to enhance the dissociation enthalpy for CH4 hydrate, but no similar effect was observed for CO2 hydrate.

Probing the effect of amino acid-based ionic liquids on the CO2 (sI) and THF (sII) hydrate formation

In the present work, four acetyl glycinate-based ionic liquids (ILs) with distinct cations, namely 1-alkyl-3-methylimidazolium acetyl glycinate [C2mim][AcGly], ammonium acetyl glycinate [NH4][AcGly], N-(2-hydroxyethyl)-N,N,N-trimethyl ammonium N-acetylglycinate [N111(2OH)][AcGly] ≈ [Ch][AcGly], and N-Butyl-N,N-dimethyl-N-(2-hydroxyethyl) ammonium N-acetylglycinate [N411(2OH)][AcGly] ≈ [C4Ch][AcGly], were synthesized and tested for their effect on the formation of CO2 hydrates at elevated pressures and tetrahydrofuran (THF) hydrate formation at atmospheric pressure. The phase behaviour of the CO2 hydrates, sI type, was studied in the presence of the selected ILs using the isochoric pressure search method. The ILs shifted the phase boundary towards lower temperatures indicating a thermodynamic inhibition characteristic. The constant cooling method was used to determine the onset nucleation temperature of CO2 hydrates in the presence and absence of additives. The isothermal method was used to determine the induction time of CO2 hydrates in the presence of additives. The pressure drops in both experiments indicate the hydrate growth in the absence/presence of additives. Later, the THF hydrate crystal growth experiments at atmospheric pressure were carried out to understand the effect of additives on the sII-type hydrate growth. The crystal morphology also points out the mode of action of the tested additives on hydrate formation and growth. The extensive testing using the same additives at identical concentrations revealed the importance of the method and conditions, which become crucial in the absence of a standard screening procedure for kinetic hydrate additives.

CO2 hydrates phase behaviour and onset nucleation temperatures in mixtures of H2O and D2O: Isotopic effects

In this work, we report the CO2 hydrate phase equilibria in water (H2O), heavy water (D2O), and their binary mixtures following the isochoric pressure search method using a rocking cell apparatus. The phase behaviour was mapped within the temperature and pressure range of 276.32 – 284.80 K and 1.59 – 3.78 MPa, respectively. It was found that there is a difference of ∼ 2 K in the equilibrium line of CO2 hydrates formed in H2O and in D2O, respectively. The hydrate dissociation enthalpies obtained using the Clausius-Clapeyron equation indicate almost similar values formed either in D2O, H2O or their mixtures. These shifts in this equilibrium temperature were compared with the triphasic equilibrium temperature variation estimation obtained using Molecular Dynamics Simulations and a very good agreement with the experimentally obtained values was observed. Further, a constant cooling method was used to obtain the onset temperature of hydrate nucleation for these systems at 3.6 MPa. It has been found that during the cooling ramps, the nucleation always occurred in the vicinity of the temperature of maximum density (TMD) of the systems where water still retains some structuredness. The nucleation experiments also give information about the metastable zone width (MSZW) of the studied systems. The results reported in this work indicate the magnitude of the isotopic effect on CO2 hydrate formation and dissociation that may have implications towards the application of hydrate technology for separation and purification processes.

Phase Equilibrium of Hydrogen Clathrate Hydrates with Propylene Oxide and 1,2-Epoxycyclopentane

This study examined the phase behavior of hydrogen hydrates with additives containing oxygen-containing groups. The phase equilibrium of the H2 + propylene oxide (PO) + water hydrate system and the H2 + 1,2-Epoxycyclopentane (ECP) + water hydrate system was measured using the isochoric pressure search method. The mole fractions of PO and ECP were 0.056 and 0.028, respectively. The experimental data were determined in the temperature range of 273.90 to 285.20 K and the pressure range of 3.18 to 18.57 MPa. In the presence of PO and ECP, the hydrate phase equilibrium of the systems moved to a lower pressure and a higher temperature region compared to that of the hydrogen hydrates without additives. The addition of ECP with a mole fraction of 0.056 had the best promotion effect among all reported sII-type thermodynamic additives. The data obtained in this study also showed that the equilibrium was affected by the ether (c–o–c) functional group in epoxy heterocycle compounds, which was the effective functional group in changing the equilibrium conditions of hydrogen hydrates. The results provided a basis for hydrogen storage and transportation based on hydrate technology.

Hydrate phase equilibria of carbon dioxide/propane gas mixture in concentrated aqueous sodium chloride solutions: Experimental data and prediction

The information on hydrate equilibrium conditions of gas hydrate formers in concentrated NaCl solutions are the prerequisites for hydrate-based desalination methods. Hydrate dissociation conditions for CO2-C3H8 mixture, as a good candidate for this application, were measured for the temperature and pressure ranges of 269–283 K and 0.3–3.4 MPa, respectively using an isochoric pressure-search method. The used gas mixture was a solution of 10 mol% C3H8 + 90 mol% CO2 and NaCl concentrations varied between 8 and 12 wt%. The promoting effect of propane was obviously observed from the experimental data. This promoting effect nearly balanced the inhibitory effect of 12 wt% NaCl. A thermodynamic model was applied to predict the experimental data using SRK, PR, and VPT EOSs. All EOSs had accurate results for prediction of CO2 hydrate equilibrium pressures, with the best result by PR EOS. The overall average absolute percent deviation (AAPD) of PR EOS in pure water and 8 wt% NaCl solution were 5.29 and 3.8 %, respectively. For gas mixture (CO2-C3H8) in pure water, the best accuracy was obtained by using SRK EOS with AAPD of 4.44 %. However, the results of VPT and PR for gas mixture in NaCl solutions, were more accurate than SRK with AAPDs equal to 7.47 and 7.75 %, respectively.

Experimental measurements and thermodynamic modeling of hydrate dissociation conditions for CO2 + THF + MgCl2 + water systems

• Experimental hydrate dissociation conditions for (CO2 + THF + MgCl2 + Water) system are reported. • A thermodynamic model has been developed and its results are in acceptable agreement with the experimental data. • The addition of low-medium concentration THF to the system has a promotion effect. • The addition of MgCl2 has an inhibition effect In this study, experimental and thermodynamic modeling on hydrate dissociation conditions of CO 2 + water systems in the absence/presence of MgCl 2 + THF aqueous solutions at various concentrations were investigated. The ternary systems, including different aqueous solutions of tetrahydrofuran and magnesium chloride (0.05 mass fraction of MgCl 2 , 0.05 mass fraction of THF: solution A; 0.15 mass fraction of MgCl 2 , 0.05 mass fraction of THF: solution B; 0.05 mass fraction of MgCl 2 , 0.20 mass fraction of THF: solution C; 0.15 mass fraction of MgCl 2 , 0.20 mass fraction of THF: solution D) were conducted in the temperature range between 280.9 to 292.9 K and pressure ranges 0.92 to 3.89 MPa. An isochoric pressure–search method was applied to undertake all measurements. The results demonstrated that MgCl 2 acts as an inhibitor in all experiments, while THF acts as a promoter. Comparing mentioned data with the same system in the absence of MgCl 2 , a shift is observed toward the left in the equilibrium curve that this kind of shifting left is more observable at high pressures. In addition, a thermodynamic model was developed utilizing the Peng-Robinson equation of state with classical mixing rules and the UNIQUAC activity coefficient model to predict the equilibrium conditions of the aforementioned system. The model results implied a reasonable agreement with the experimental dissociation data with an average absolute deviation of pressure below 0.3%.

Experimental Kinetic Evaluation of Carbon Dioxide Hydrate-Based Concentration for Grape, Pineapple, and Bitter Melon Juices.

Hydrate-based technology has emerged as a promising approach to address the industry's energy demands and product quality challenges in the food industry. Despite reported successes in the literature where higher dehydration ratios were achieved, technological problems like slow formation rates and poor process scale-up economics need to be addressed. Moreover, with little hydrate formation data available, the major focus is on the technology's ability to remove water content, but studies on the kinetics of hydrate formation are scarce. In the present work, the effects of varying grape/pineapple/bitter melon juice water cuts (88.5 to 97.4 ± 2.53 wt %) on the formation kinetics of carbon dioxide (CO2) hydrates were investigated. Such information can provide insight into the possibile commercialization of the hydrate-based technology. The reported experimental data were determined using the isochoric pressure-search method in a high-pressure reactor at a target initial temperature from 274.15 to 276.15 K and varying initial pressures. Kinetic parameters were calculated using the relative kinetic models proposed in the literature. Lower relative values of investigated kinetic parameters and longer induction times were obtained at lower juice water cuts and lower degrees of subcooling. Despite observed inhibition effects, the study provides useful experimental and modeled kinetic data for filling the knowledge gap in understanding the controlling mechanism of CO2 hydrate formation. Therefore, it is believed that the reported findings may highlight some important practical aspects related to CO2 hydrate technology as an alternative juice concentration process.

Experimental Hydrate Phase Equilibrium Data Relevant to Bitter Melon, Pineapple, and Grape Juice Concentration.

One of the major challenges experienced by the fruit juice industry is the steady rise in energy costs. Hence, it is of industrial interest to find possible environmentally friendly measures that reduce energy consumption while cost-effectively maintaining the quality of manufactured products. Hydrate-based juice concentration technology can be used to overcome this challenge. In the present work, experimental hydrate phase equilibrium conditions of three systems involving juices (system 1, CO2 + grape juice; system 2, CO2 + pineapple juice; system 3, CO2 + bitter melon juice) were measured using an isochoric pressure search method. The temperature and pressure ranges for reported experimental data were 272.6–282.3 K and 1.17–3.85 MPa, respectively. Results have shown that a decrease in water cut from 98.3 to 88.5 ± 2.53 wt % could shift the hydrate phase equilibrium conditions toward higher pressures and lower temperatures. This proved that all investigated juices exhibited inhibitory effects on gas hydrate formation. To properly assess the energy requirements for this novel technology, molar hydrate dissociation enthalpies were estimated using the Clausius–Clapeyron relations under different measurement conditions. Finally, it was established that a hydrate-based fruit juice concentration technology would be a credible alternative to existing commercial technologies, on the basis of the dehydration ratio of 57% obtained in the present study.

Gas Hydrate Phase Equilibrium Data for the CO2 + TBPB + THF + Water System

In this study, phase equilibria of gas hydrates of CO2 in the presence of tetra n-butyl phosphonium bromide (TBPB) and tetrahydrofuran (THF) have been investigated. The experiments were performed using the isochoric pressure search method at varying compositions of promoters. The phase equilibrium data have been reported at 0.01, 0.05, and 0.1 mass fractions of TBPB in aqueous solutions at the temperature and pressure ranges of 278–295 K and 1.1–4.5 MPa, respectively. The results showed that adding the aqueous mixtures of TBPB and THF to the system changes the hydrate phase equilibrium conditions as expected. In comparison with the system including only TBPB, the promotion effect of the applied mixture is low at all studied concentrations of TBPB in the presence of 0.01 mass fraction of THF. However, at 0.05 mass fraction of THF, the hydrate equilibrium pressures at constant temperatures decrease dramatically.

Thermodynamic evaluation of inhibitors for methane-hydrate formation

The present paper presents an experimental study on the thermodynamic effect of three amines including (3-Aminopropyl)triethoxysilane (APTES), (3-Aminopropyl)triethoxysilane (APTMS), and Triethylenetetramine (TETA) on methane hydrates. Hydrate-Liquid-Vapour Equilibrium (HLwVE) data for methane hydrate were evaluated in the presence of 7 wt% and 20 wt% APTES, APTMS, and TETA, and with pressure ranging from 2.0 to 9.0 MPa. The thermodynamic effect of these amines on methane-hydrate formation has been investigated via measuring the methane-hydrate dissociation temperature, in applying an isochoric pressure-search method. The results reveal that APTES, APTMS, and TETA effectively shift the hydrate equilibrium curve to higher-pressure and lower-temperature regions for the methane + amine + water system. All of these three show thermodynamic-inhibition effects similar to the lower-concentration case (7 wt%), but with increasing concentration to 20 wt% for the thermodynamic-inhibition effect in the order APTES > APTMS > TETA. The average hydrate-suppression temperature (ΔT), as a ‘signature’ of thermodynamic-inhibition effect on methane hydrates measured for 7 wt% APTES, APTMS and TETA was 1.5, 1.5, and 1.3 K, respectively. ΔT increased with increasing amine concentration; these values were 5, 4.3, and 3.2 for 20 wt% APTES, APTMS, and TETA, respectively. Hydrate-onset time was further delayed due to the synergistic inhibition effect of inhibitors. The results show that these amines have a kinetic effect on gas-hydrate formation, such as for methane-hydrate formation in presence of APTMS: here, the hydrate formed after several cycles with the help of the memory effect. Moreover, the inhibition impact of these amines was compared with conventional hydrate thermodynamics, and found to be in the range of some and similar to ethylene glycol at 5 wt%. Furthermore, the addition of 5 wt% APTMS to sea salt not only enhances the thermodynamic inhibition effect, but also inhibits kinetically the formation of methane hydrate. For modelling of methane-hydrate dissociation conditions, the solid-solution theory of van der Waals and Platteeuw was applied for the hydrate phase along with the Peng–Robinson equation of state for the gas phase. Taken together, this study elucidates a novel class of thermodynamic hydrate inhibitors, which can control methane-hydrate formation efficiently.

Effect of Thiouronium-Based Ionic Liquids on the Formation and Growth of CO2 (sI) and THF (sII) Hydrates.

In this manuscript, two thiouronium-based ionic liquids (ILs), namely 2-ethylthiouronium bromide [C2th][Br] and 2-(hydroxyethyl)thiouronium bromide [C2OHth][Br], were tested at different concentrations (1 and 10 wt%) for their ability to affect CO2 (sI) and tetrahydrofuran (THF) (sII) hydrate formation and growth. Two different methods were selected to perform a thermodynamic and kinetic screening of the CO2 hydrates using a rocking cell apparatus: (i) an isochoric pressure search method to map the hydrate phase behavior and (ii) a constant ramping method to obtain the hydrate formation and dissociation onset temperatures. A THF hydrate crystal growth method was also used to determine the effectiveness of the ILs in altering the growth of type sII hydrates at atmospheric pressure. Hydrate–liquid–vapor equilibrium measurements revealed that both ILs act as thermodynamic inhibitors at 10 wt% and suppress the CO2 hydrate equilibria ~1.2 °C. The constant ramping methodology provides interesting results and reveals that [C2OHth][Br] suppresses the nucleation onset temperature and delays the decomposition onset temperatures of CO2 hydrates at 1 wt%, whereas suppression by [C2th][Br] was not statistically significant. Normalized pressure plots indicate that the presence of the ILs slowed down the growth as well as the decomposition rates of CO2 hydrates due to the lower quantity of hydrate formed in the presence of 1 wt% ILs. The ILs were also found to be effective in inhibiting the growth of type sII THF hydrates without affecting their morphology. Therefore, the studied thiouronium ILs can be used as potential dual-function hydrate inhibitors. This work also emphasizes the importance of the methods and conditions used to screen an additive for altering hydrate formation and growth.

Hydrate phase equilibrium of hydrogen-natural gas blends: Experimental study and thermodynamic modeling

Blending hydrogen (H2) into natural gas pipelines has become an effective way to transport H2 on a large scale. Due to the difference in properties between H2 and natural gas, the natural gas hydrate (NGH) phase equilibrium will be changed after blending H2, which influences the flow assurance in pipelines. In this work, hydrate phase equilibrium for CH4 + H2, CH4 + C2H6 + H2, and CH4 + C2H6 + C3H8 + H2 multicomponent systems had been measured at a temperature range from 274.24 K to 289.59 K by the isochoric pressure-search method. The results show the inhibition effect on CH4 hydrate formation of blending H2 to 20% is a little better than the 5.0 wt.%CH3OH solution. In addition, the Chen–Guo model was adopted to predict hydrogen-natural gas blends (HNGB) hydrates formation conditions, and model results have a good agreement with the experimental data. Average enthalpies of HNGB hydrates dissociation were also calculated utilizing the Clausius-Clapeyron equation, which was in the range of 68.02 kJ/mol and 90.17 kJ/mol. These studies provide basic data for flow assurance when transporting HNGB in pipelines.

The Effect of Various Concentrations of Tetra-n-butylammonium Fluoride on the Dissociation Enthalpy of Gas Hydrates

Semiclathrate hydrate formers such as tetra-n-butylammonium bromide (TBAB), chloride (TBAC) and fluoride (TBAF) are promising compounds that mild the thermodynamic conditions of gas hydrates, considerably. The Clausius-Clapeyron equation is employed in this manuscript to calculate the dissociation enthalpies of methane/carbon dioxide/nitrogen + TBAF semiclathrate hydrates. A 460 cm3 stirred batch reactor was used to measure the phase equilibria of gas + TBAF semiclathrate hydrates at various concentrations of tetra-n-butylammonium fluoride. The dissociation P-T data were obtained using an isochoric pressure-search method in the temperature range of 275.15 to 304.7 K and the pressure range of 0.53 to 10.24 MPa at 0.0 - 0.4482 mass fraction of TBAF. Investigating the obtained dissociation data showed that the addition of TBAF to the solution increases the amount of dissociation enthalpy of semiclathrate hydrates per mole of the hydrated gas. Increasing the mass fraction of tetra-n-butylammonium fluoride, showed a straight relation with the amount of dissociation enthalpy per mole of hydrated gas.

Insights into CO2 hydrates formation and dissociation at isochoric conditions using a rocking cell apparatus

In order to study the hydrate-liquid–vapour-equilibria (HLVE), the isochoric pressure search method is one of the most sought-after methods through which the characteristics of the hydrate formation and dissociation processes of a given hydrate-former substance can be traced. This method contains three temperature driven segments that are known as: (i) fast cooling step, (ii) isothermal step and (iii) a slow heating step applied to a given water + hydrate-former system under closed conditions. In this work, we have carried out a study to determine the effect of various variables on the formation/dissociation process of CO2 + H2O hydrate systems such as cooling rate, heating rate, isothermal segment temperature and its length, rocking rate, initial pressure and operating temperature. These variables may influence the characteristic shape of the p-T loops and thereby might affect the precise determination of kinetics and thermodynamics parameters. The experiments have been conducted using a rocking cell apparatus where nearly 50 experiments using 10 different versions of the isochoric pressure search method have been carried out. This work primarily shows practicalities of how to obtain the hydrate equilibrium curve more efficiently and accurately. The large number of experiments provide qualitative indications of the combined mass transfer and kinetics. This information might have implications towards the use of hydrate technology for energy and environmental related applications.

Natural gas hydrate stability conditions and water activity in aqueous solutions containing mono ethylene glycol (MEG) and salt: Experimental measurements and thermodynamic modeling

Gas hydrates can form in the presence of saline water produced from the gas and oil reservoirs under specific thermodynamic conditions. One of the best ways to prevent gas hydrate formation is to inject hydrate inhibitors like mono ethylene glycol (MEG) or methanol aqueous solutions in the pipelines. To design productive facilities and overcome operation faults, studying the stability conditions of natural gas hydrates in the presence of aqueous solutions of salt(s) and/or inhibitor(s) is necessary. However, a few studies have been conducted on stability conditions of natural gas hydrates while salt mixtures and MEG are present in aqueous solution. Here, the hydrate stability conditions of a synthetic natural gas in aqueous mixtures of 20 and 30 wt% of MEG, and 2.5 wt% NaCl + 2.5 wt% KCl were experimentally measured under 272–293 K and 3.30–12.70 MPa. An isochoric pressure-search method was employed to conduct the measurements. Then, a thermodynamic model was developed to estimate hydrate stability conditions in the presence of the aforementioned inhibitors. In this developed model, the aqueous phase fugacity is estimated using a combined form of the Cubic-Plus-Association Equation of State with the modified Debye-Hückel electrostatic term and the hydrate phase is modeled using the van der Waals and Platteeuw solid solution theory. To improve model accuracy, the aqueous phase activity was measured in various concentrations at atmospheric pressure and 298.15 K using a standard activity meter called Novasina LabMaster-aw neoAS, Switzerland. These measured data were used to correlate the activity coefficient parameters in the thermodynamic model. Finally, it is noted that the modeling results and the experimental data are in satisfactory agreement.

A robust and highly efficient phase boundary method for determining the thermodynamic equilibrium conditions of bulk gas hydrate systems

The phase equilibrium conditions of gas hydrate systems play a fundamental and essential role in establishing the feasibility of the various hydrate based technological applications. Although the classical isochoric and isothermal pressure search methods can provide accurate and reliable gas hydrate phase equilibrium data, these methods require researchers to devote tremendous time towards achieving each data point. Herein, we extend the phase boundary (PB) method for bulk hydrate systems and demonstrate it to be robust and highly efficient. The PB method involves a controlled pressure drop of the hydrate system to initiate the automatic trace of the pressure and temperature region upon dissociation of the hydrates. Three-phase equilibrium data for four different gas hydrate systems, namely pure methane (CH4), mixed CH4/1,3-dioxolane (DIOX), mixed hydrogen (H2)/DIOX, and mixed carbon dioxide (CO2)/DIOX, have been obtained using the PB method. Independent measurements made using the isochoric pressure search method have been obtained to validate the accuracy and reliability of the data obtained using the PB method.

Experimental determination of hydrate dissociation conditions in CO2 + hexane + dodecane system, + 1-propanol or + tetra-n-butyl ammonium bromide

Hydrate dissociation conditions for one four-component system and two five-component systems, the CO2 + hexane + dodecane + water system, the CO2 + hexane + dodecane + 1-propanol + water system, and the CO2 + hexane + dodecane + tetra-n-butyl ammonium bromide (TBAB) + water system, were measured in the pressure range of (1.5 to 4) MPa and temperature range of (283 to 290) K at (0.1, 0.2 and 0.3) mass fraction of 1-propanol or TBAB. The experimental measurements were performed using the isochoric pressure-search method in a high pressure blind cell. The hydrate dissociation conditions (temperature and pressure) for the CO2 + hexane + dodecane + water system were compared with experimental data available in international literature were good agreement between the data was founded which indicates the viability of the experimental technique reported in this study. The experimental uncertainties were determined for temperature and pressure as combined standard uncertainties and are 0.16 K and 0.04 MPa, respectively. The relative standard uncertainty in composition was estimated to be 0.0014. In order to verify the reliability of the experimental data containing 1-propanol, a thermodynamic consistency test was performed.

Effects of diethanolamine and ethylene glycol + diethanolamine aqueous solutions on methane hydrate stability conditions: Experimental measurements and thermodynamic modeling

In this work, a total of 83 dissociation conditions data of methane hydrate in the attendance of diethanolamine (DEA) and diethanolamine + ethylene glycol (EG) aqueous solutions were first experimentally measured: 45 data for DEA and 38 data for DEA + EG aqueous solutions. The data were measured using a stainless steel (SS–316) cell by employing an isochoric pressure-search method. The methane hydrate stability conditions in various aqueous systems containing 0.09, 0.15, 0.25 mass fractions of DEA, (0.05 mass fraction of DEA + 0.05 mass fraction of EG), (0.05 mass fraction of DEA + 0.10 mass fraction of EG), and (0.15 mass fraction of DEA + 0.10 mass fraction of EG) were measured in the pressure range of 3.23–8.38 MPa and the temperature range of 274.1–283.0 K. It was observed that DEA causes thermodynamic inhibition of methane hydrate formation, and using 0.25 mass fraction of DEA in an aqueous solution shifts the methane hydrate stability temperatures to about 4.2 K. Also, it was concluded that although DEA has weaker strength compared to EG, it can be used as a gas sweetening agent and thermodynamic hydrate inhibitor. For modeling purpose, the van der Waals–Platteeuw (vdWP) model was employed for the hydrate phase, the Peng-Robinson equation of state (PR EoS) was utilized to compute the methane fugacity in the gas phase, and the NRTL activity coefficient model was applied to calculate the nonideality of the aqueous phase. The AAEs (K) obtained from this thermodynamic model for DEA and DEA + EG aqueous solutions are 0.2 and 0.4, respectively.