Prediction of carbon dioxide gas hydrate formation conditions in aqueous electrolyte solutions

A methodology for predicting the incipient equilibrium conditions for carbon dioxide gas hydrates in the presence of electrolytes such as NaCl, KCl and CaCl₂ is presented. The method utilizes the statistical thermodynamics model of van der Waals and Platteeuw (1959) to describe the solid hydrate phase. Three different models were examined for the representation of the liquid phase: Chen and Evans (1986), Zuo and Guo (1991), and Aasberg-Petersen et al. (1991). It was found that the model of Zuo and Guo (1991) gave the best results for predicting incipient CO₂ gas hydrate conditions in aqueous single salt solutions. The model was then extended for prediction of CO₂ gas hydrates in mixed salts solutions. The predictions agree very well with experimental data.     

Thermodynamic stability, spectroscopic identification and cage occupation of binary CO2 clathrate hydrates

The hydrate phase behavior of CO₂/3-methyl-1-butanol (3M1B)/water, CO₂/tetrahydrofuran (THF)/water and CO₂/1,4-dioxane (DXN)/water was investigated using both a high-pressure equilibrium viewing cell and a kinetic pressure-temperature measurement system with a constant volume. The dissociation pressures of CO₂/3M1B/water were identical to those of pure CO₂ hydrate, indicating that CO₂ is not acting as a help gas for structure H hydrate formation with 3M1B, thus the formed hydrate is pure CO₂ structure I hydrate. The CO₂ molecules could be encaged in small cages of the structure II hydrate framework formed with both of THF and DXN. For a stoichiometric ratio of 5.56 mol% THF, we found a large shift of dissociation boundary to lower pressures and higher temperatures from the dissociation conditions of pure CO₂ hydrate. From the measurements using the kinetic pressure-temperature system, it was found that the solid binary hydrate samples formed from off-stoichiometric THF and DXN aqueous solutions are composed of pure CO₂ hydrate with a hydrate number n=7.0 and THF/CO₂ and DXN/CO₂ binary hydrates with a molar ratio of xCO₂·THF·17H₂O and xCO₂·DXN·17H₂O, respectively. The X-ray diffraction was used to identify the binary hydrate structure and Raman spectroscopy was measured to support the phase equilibrium results and to investigate the occupation of CO₂ molecules in the cages of the hydrate framework.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

In situ injection of THF to trigger gas hydrate crystallization: Application to the evaluation of a kinetic hydrate promoter

This paper investigates an original method to efficiently trigger gas hydrate crystallization. This method consists of an in situ injection of a small amount of THF into an aqueous phase in contact with a gas-hydrate-former phase at pressure and temperature conditions inside the hydrate metastable zone. In the presence of a CO₂–CH₄ gas mixture, our results show that the THF injection induces immediate crystallization of a first hydrate containing THF. This triggers the formation of the CO₂–CH₄ binary hydrate as proven by the pressure and temperature reached at equilibrium. This experimental method, which “cancels out” the stochasticity of the hydrate crystallization, was used to evaluate the effect of the anionic surfactant SDS at different concentrations, on the formation kinetics of the CO₂–CH₄ hydrate. The results are discussed and compared with those published in a recent article (Ricaurte et al., 2013), where THF was not injected but present in the aqueous phase from the beginning and at much higher concentrations.     

Phase equilibria and kinetic behavior of co<Subscript>2</Subscript> hydrate in electrolyte and porous media solutions: application to ocean sequestration of CO<Subscript>2</Subscript>

Understanding the phase behavior and formation kinetics of CO₂ hydrate is essential for developing the sequestration process of CO₂ into the deep ocean and its feasibility. Three-phase equilibria of solid hydrate, liquid water, and vapor were determined for aqueous mixtures containing CO₂ and NaCl/clay to examine the effect of both ocean electrolytes and sediments on hydrate stability. Due to the capillary effect by clay pores and inhibition effect by NaCl the corresponding hydrate formation pressure appeared to be a little higher than that required for simple and pure hydrate at specified temperature. In addition, the hydrate formation kinetics of carbon dioxide in pure water and aqueous NaCl solutions with or without clay mineral were also measured at various conditions. The formation kinetic behavior was found to be strongly influenced by pressure, temperature and electrolyte concentration. A simplified kinetic model having two adjustable parameters was proposed and the estimated results agreed well with the experimental data. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Extension of the electrolyte Trebble–Bishnoi EOS to mixed-salt systems,and application to vapour liquid and solid (hydrate) vapour liquid equilibrium calculations

This work examines the use of the electrolyte Trebble–Bishnoi equation of state (eTBEOS) in predicting high-pressure phase equilibria in the presence of aqueous mixed-salt solutions. The eTBEOS combines the Trebble–Bishnoi equation of state with a Born energy term, a mean spherical approximation term, and a cation solvation term. Shortcomings in the originally regressed set of eTBEOS parameters are identified and discussed, and a new set of equation of state parameters is subsequently regressed for 58 salts. In order to extend the eTBEOS to mixed-salt systems, two approaches for computing the EOS parameters of the ionic species are investigated. The first approach uses the originally regressed parameter set plus parameter mixing rules, where appropriate, whereas the second approach uses the newly regressed parameters and no mixing rules. In the prediction of osmotic coefficients in mixed-salt solutions, the maximum relative difference between the experimental and computed values was 5.70%. For the gas solubility predictions, data were available for the solubility of CO₂, CH₄, and N₂ in a small number of mixed electrolyte solutions; the maximum relative difference was 5.79%. Finally, CH₄, C₂H₆, C₃H₈, and CO₂ gas hydrate formation conditions were predicted in a number of mixed-salt solutions, with a maximum relative difference of 10.76%. Overall, it was seen that both approaches allowed for comparable accuracy in phase equilibrium calculations. However, in the case of solutions made from salt mixtures with common cations, the second approach consistently resulted in improved accuracy.     

Hydrate phase equilibria for gas mixtures containing carbon dioxide: A proof-of-concept to carbon dioxide recovery from multicomponent gas stream

Three-phase equilibrium conditions (aqueous liquid-hydrate-vapor) of CO₂-N₂ binary mixtures in the temperature range of 271.75 K to 284.25 K and the pressure range of 12 to 235 bar. In addition, three-phase (aqueous liquid-hydrate-vapor) behavior for CO₂-CH₄ mixture were measured in the temperature range of 272 to 284 K at the constant pressures of 15, 20, 26, 35 and 50 bar. In high concentration of CO₂, upper quadruple points were also measured. The obtained data indicates that three-phase equilibrium temperatures become higher with increasing concentration of CO₂. For the prediction of three-phase equilibrium, the vapor and liquid phases were treated by employing the Soave-Redlich-Kwong equation of state (SRK-EOS) with the second order modified Huron-Vidal (MHV2) mixing rule and the hydrate phase with the van der Waals-Platteeuw model. The calculated results showed good agreement with experimental data. The concentration of vapor and hydrate phases was also determined experimentally. This work can be used as the basic data for selective separation process by hydrate formation. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Separation and capture of carbon dioxide from CO2/H2 syngas mixture using semi-clathrate hydrates

The relation between anthropogenic emissions of CO₂ and its increased levels in the atmosphere with global warming and climate change has been well established and accepted. Major portion of carbon dioxide released to the atmosphere, originates from combustion of fossil fuels. Integrated gasification combined cycle (IGCC) offers a promising fossil fuel technology considered as a clean coal-based process for power generation particularly if accompanied by precombustion capture. The latter includes separation of carbon dioxide from a synthesis gas mixture containing 40 mol% CO₂ and 60 mol% H₂.A novel approach for capturing CO₂ from the above gas mixture is to use gas hydrate formation. This process is based on selective partition of CO₂ between hydrate phase and gas phase and has already been studied with promising results. However high-pressure requirement for hydrate formation is a major problem.We have used semiclathrate formation from tetrabutylammonium bromide (TBAB) to experimentally investigate CO₂ capture from a mixture containing 40.2 mol% of CO₂ and 59.8 mol% of H₂. The results shows that in one stage of gas hydrate formation and dissociation, CO₂ can be enriched from 40 mol% to 86 mol% while the concentration of CO₂ in equilibrium gas phase is reduced to 18%. While separation efficiency of processes based on hydrates and semi-clathrates are comparable, the presence of TBAB improves the operating conditions significantly. Furthermore, CO₂ concentration could be increased to 96 mol% by separating CO₂ in two stages.     

Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal

In the current work, molecular dynamics simulation is employed to understand the intrinsic growth of carbon dioxide and methane hydrate starting from a seed crystal of methane and carbon dioxide respectively. This comparison was carried out because it has relevance to the recovery of methane gas from natural gas hydrate reservoirs by simultaneously sequestering a greenhouse gas like CO₂. The seed crystal of carbon dioxide and methane hydrate was allowed to grow from a super-saturated mixture of carbon dioxide or methane molecules in water respectively. Two different concentrations (1:6 and 1:8.5) of CO₂/CH₄ molecules per water molecule were chosen based on gas–water composition in hydrate phase. The molecular level growth as a function of time was investigated by all atomistic molecular dynamics simulation under suitable temperature and pressure range which was well above the hydrate stability zone to ensure significantly faster growth kinetics. The concentration of CO₂ molecules in water played a significant role in growth kinetics, and it was observed that maximizing the CO₂ concentration in the aqueous phase may not result in faster growth of CO₂ hydrate. On the contrary, methane hydrate growth was independent of methane molecule concentration in the aqueous phase. We have validated our results by performing experimental work on carbon dioxide hydrate where it was seen that under conditions appropriate for liquid CO₂, the growth for carbon dioxide hydrate was very slow in the beginning.     

Metastable states during dissociation of carbon dioxide hydrates below 273 K

In this study, the dissociation of isolated carbon dioxide hydrate particles of sizes in the range 0.25–2.5 mm was investigated. It was found that below the ice melting point, the hydrates dissociated into supercooled water (metastable liquid) and gas. The formation of the liquid phase during CO₂ hydrate dissociation was visually observed, and the pressures of the hydrate dissociation into supercooled water and gas were measured in the temperature range 249–273 K. These pressures agreed well with the calculated data for the supercooled water–hydrate–gas metastable equilibrium (Istomin et al., 2006). In the P–T area on the phase diagram bounded by the ice–hydrate–gas equilibrium curve and the supercooled water–hydrate–gas metastable equilibrium curve, hydrates could exist for a long time because the metastable phase and their stability are not connected to the self-preservation effect. The growth of the metastable CO₂ hydrate film on the surface of supercooled water droplets formed during the hydrate dissociation was observed at pressure above the three-phase supercooled water–hydrate–gas metastable equilibrium pressure but still below the three-phase ice–hydrate–gas equilibrium pressure. It was found that the growth rate of the metastable CO₂ hydrate film was higher by a factor of 25 and 50 than that for methane hydrate and propane hydrate, respectively.     

THERMODYNAMICS OF ALKANOLAMINE + WATER SYSTEM

Design of gas treating processes requires knowledge of the vapor-liquid equilibrium behavior of the (acid gas + aqueous alkanolamine) system. The present study is focused on thermodynamics and associated nonideal behavior of binary MEA + H₂O, DEA + H₂O, and MDEA + H₂O systems, which is required to predict the vapor-liquid equilibrium of acid gases such as CO₂ and H₂S over aqueous alkanolamine solutions. Determination of binary interaction parameters and analytical prediction of infinite dilution activity coefficient, heats of solution at infinite dilution, the excess Gibbs free energy, and excess enthalpy for nonideal alkanolamine-water systems are the objectives of this study.     

High pressure measurements and molecular modeling of the water content of acid gas containing mixtures

Water content of three carbon dioxide containing natural gas mixtures in equilibrium with an aqueous phase was measured using a dynamic saturation method. Measurements were performed up to high temperatures (477.6 K = 400°F) and pressures (103.4 MPa = 15,000 psia). The perturbed chain form of the statistical associating fluid theory was applied to predict water content of pure carbon dioxide (CO₂), hydrogen sulfide (H₂S), nitrous oxide (N₂O), nitrogen (N₂), and argon (Ar) systems. The theory application was also extended to model water content of acid gas mixtures containing methane (CH₄). To model accurately the liquid‐liquid equilibrium at subcritical conditions, cross association between CO₂, H₂S, and water was included. The agreement between the model predictions and experimental data measured in this work was found to be good up to high temperatures and pressures. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3038–3052, 2015     

Study on the influence of SDS and THF on hydrate-based gas separation performance

Hydrate additives can be used to mitigate hydrate formation conditions, promote hydrate growth rate and improve separation efficiency. CO₂ + N₂ and CO₂ + CH₄ systems with presence of sodium dodecyl sulfate (SDS) or tetrahydrofuran (THF) are studied to analyze the effect of hydrate additives on gas separation performance. The experiment results show that CO₂ can be selectively enriched in the hydrate phase. SDS can speed up the hydrate growth rate by facilitating gas molecules solubilization. When SDS concentration increases, split and loss fraction increase initially and then decrease slightly, resulting in a decreased separation factor. The optimum concentration of SDS exists at the range of 100–300 ppm. As THF can be easily encaged in hydrate cavities, hydrate formation condition can be mitigated greatly with its existence. Additionally, THF can also strengthen hydrate formation. The THF effect on separation performance is related to feed gas components. CO₂ occupies the small cavities of type II hydrate prior to N₂. But the competitiveness of CO₂ and CH₄ to occupy cavities are quite fair. The variations of split fraction, loss fraction and separation factor depend on the concentration of THF added. The work in this paper has a positive role in flue gas CO₂ capture and natural gas de-acidification.     

Effects of Graphene Oxide Nanosheets and Al2O3 Nanoparticles on CO2 Uptake in Semi‐clathrate Hydrates

Gas hydrate/clathrate hydrate formation is an innovative method to trap CO₂ into hydrate cages under appropriate thermodynamic and/or kinetic conditions. Due to their excellent surface properties, nanoparticles can be utilized as hydrate kinetic promoters. Here, the kinetics of the CO₂ + tetra‐n‐butyl ammonium bromide (TBAB) semi‐clathrate hydrates system in the presence of two distinct nanofluid suspensions containing graphene oxide (GO) nanosheets and Al₂O₃ nanoparticles is evaluated. The results reveal that the kinetics of hydrate formation is inhibited by increasing the weight fraction of TBAB in aqueous solution. GO and Al₂O₃ are the most effective kinetic promoters for hydrates of (CO₂ + TBAB). Furthermore, the aqueous solutions of TBAB + GO or Al₂O₃ noticeably increase the storage capacity compared to TBAB aqueous solution systems.     

Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel

The performance of two gas/liquid contact modes was evaluated in relation to the rate of gas hydrate formation. Hydrate formation experiments were conducted for several gas mixtures relevant to natural gas hydrate formation in the earth (CH₄, CH₄/C₃H₈, CH₄/C₂H₆ and CH₄/C₂H₆/C₃H₈) and two CO₂ capture and storage (CO₂, CO₂/H₂/C₃H₈). One set of experiments was conducted in a bed of silica sand, saturated with water (fixed fed column) while the other experiment was conducted in a stirred vessel for each gas/gas mixture. Both sets of experiments were conducted at a constant temperature. The rate of hydrate formation is customarily correlated with the rate of gas consumption. The results show that the rate of hydrate formation in the fixed bed column is significantly greater and thereby resulted in a higher percent of water conversion to hydrate in lesser reaction time for all the systems studied.     

Modeling of CO2‐Hydrate Formation at the Gas‐Water Interface in Sand Sediment

Sub‐seabed geological storage of CO₂ in the form of gas hydrate is attractive because clathrate hydrate stably exists at low temperature and high pressure, even if a fault occurs by diastrophism like a big earthquake. For the effective design of the storage system it is necessary to model the formation of CO₂‐hydrate. Here, it is assumed that the formation of gas hydrate on the interface between gas and water consists of two stages: gas diffusion through the CO₂‐hydrate film and consequent CO₂‐hydrate formation on the interface, between film and water. Also proposed is the presence of a fresh reaction interface, which is part of the interface between the gas and aqueous phases and not covered with CO₂‐hydrate. Parameters necessary to model the hydrate formation in sand sediment are derived by comparing the results of the present numerical simulations and the measurements in the literature.     

Quantitative analysis of CO2 hydrate formation in porous media by proton NMR

Gas hydrate is a nonstoichiometric crystal compound formed from water and gas. Most nonvisual studies on gas hydrate are unable to detect how much water is converted to hydrates, and thus, the hydrate stoichiometry calculations are inaccurate. This study investigated the CO₂ hydrate formation process in porous media directly and quantitatively. The characteristics of the time-variable consumption of hydrate formation indicated a two-stage formation, hydrate enclathration and continuous occupancy. The enclathration stage occurred in the first 20 min of the formation when considerable heat is released. The continuous occupancy stage lasted longer than the hydrate enclathration because the empty cages in previously formed hydrates would also be occupied. The higher formation pressures can accelerate water consumption and increase cage occupancy. The compositions of completely formed CO₂ hydrates at 2.7, 3.0, and 3.3 MPa and 275.15 K were determined as CO₂·6.90H₂O, CO₂·6.70H₂O, and CO₂·6.49H₂O, respectively.     

A model of acid gas absorption/stripping using methyldiethanolamine with added acid

A nonequilibrium stage model was developed for the absorption and stripping of H₂S and CO₂ using aqueous methyldiethanolamine (MDEA). Heat and mass transfer are calculated for each stage assuming the liquid is well mixed and the gas moves in plug flow. The vapour-liquid equilibrium is represented by an empirical expression that was fit to experimental data. The mass transfer enhancement factor for CO₂ is based on the surface renewal theory with approximations made to the reaction term by the method of DeCoursey. Calculation of H₂S absorption assumes an instantaneous reaction rate at the gas/liquid interface and accounts for enhancement by equilibrium chemical reactions. Results were generated at Claus tail gas conditions using available equilibrium and rate data for 50 wt% MDEA. The amount of H₂S in the absorber outlet gas, or H₂S leak, was used to measure system performance. The base case resulted in a H₂S leak of 98 ppm with 20 absorber stages, 25 stripper stages, and a steam rate of 1.7 lb/gal solvent. Adding 0.05 equivalents of acid per mole of MDEA to the aqueous solution reduced the H₂S leak to 6 ppm and the steam rate to 1.2 lb/gal. Reducing the base case stripper pressure of 2.0 atm to 1.0 atm reduced the H₂S leak to 22 ppm. Analysis of McCabe-Thiele plots generated by the model showed that system performance improved after adding acid or reducing the stripper pressure because the H₂S equilibrium in the stripper was linearized.     

Vapour liquid equilibrium calculations for dilute aqueous solutions of CO2, H2S,NH3 and NaOH to 300°C

Henry's law constants for aqueous CO₂, H₂S and NH₃ up to 300°C have been recalculated from literature vapour pressure, enthalpy and heat capacity data. The high vapour pressure of water above 150°C causes significant solute-water interactions in the gas phase, which were calculated using the Peng-Robinson cubic equation of state. The results were combined with selected ionization constant data to derive a vapour-liquid equilibrium model for dilute solutions. The model reproduces experimental data for binary systems at solute molalities of up to 0.5 m at 100°C, 1.0 m above 250°C and ionic strengths below about 0.1 m.