Phase Equilibria Of Semiclathrate Hydrates Research Articles

Modeling Phase Behavior of Semi-Clathrate Hydrates of CO2, CH4, and N2 in Aqueous Solution of Tetra-n-butyl Ammonium Fluoride

Abstract Semi-clathrate hydrates are members of the class of clathrate compounds. In comparison with clathrate hydrates, where the networks are formed only by H2O molecules, the networks of semi-clathrate hydrates are formed by mixtures of H2O and quaternary ammonium salts (QASs). The addition of QASs to the solution enables to improve the formation of semi-clathrate hydrates at much milder conditions comparing to clathrate hydrates. In this work, we study the phase equilibria of semi-clathrate hydrates of CH4, CO2, and N2 gas in an aqueous solution of tetra-n-butyl ammonium fluoride (TBAF). An extension of the Chen–Guo model is proposed as a thermodynamic model. The Peng–Robinson equation of state (PREOS) was applied to calculate the fugacity of the gas phase and in order to determine the water activity in the presence of TBAF, a correlation between the system temperature, the TBAF mass fraction, and the nature of the guest molecules has been used. These equations were solved simultaneously and through optimizing tuning parameters via the Nelder–Mead simplex algorithm. The results are compared to experimental data and good agreement is observed.

Modeling Dissociation Pressure of Semi-Clathrate Hydrate Systems Containing CO2, CH4, N2, and H2S in the Presence of Tetra-n-butyl Ammonium Bromide

Abstract In this study, the phase equilibria of semi-clathrate hydrates of methane (CH4), carbon dioxide (CO2), nitrogen (N2), and hydrogen sulfide (H2S) in an aqueous solution of tetra-n-butyl ammonium bromide (TBAB) were modeled using a correlation based on a two-stage formation mechanism: a quasi-chemical reaction that forms basic semi-clathrate hydrates and adsorption of guest molecules in the linked cavities of the basic semi-clathrate hydrate. The adsorption of guest molecules was described by the Langmuir adsorption theory and the fugacity of the gas phase was calculated by Peng–Robinson (PR) equation of state (EOS). The water activity in the presence of TBAB was calculated using a correlation, dependent on temperature, the TBAB mass fraction, and the nature of the guest molecule. These equations were coupled together and form a correlation which was linked to a genetic algorithm for optimization of tuning parameters. The results showed an excellent agreement between model results and experimental data. In addition, an outlier diagnostic was performed for finding any possible doubtful data and assessing the applicability of the model. The results showed that more than 97 % of the data were reliable and they were in the applicability domain of the model.

Phase Equilibrium Data for Semiclathrate Hydrate of Synthesized Binary CO2/CH4 Gas Mixture in Tetra-n-butylammonium Bromide Aqueous Solution

Hydrate-based gas separation(HBGS) technology based on tetra-n-butyl ammonium bromide (TBAB) semiclathrare hydrate can be utilized in CO2 separation from biogas. The phase equilibria of semiclathrate hydrates are crucial for the successful industrial application of HBGS technology. In this work, the phase equilibrium data of TBAB semiclathrate hydrate with a synthesized binary CO2/CH4 mixed gas (0.5 CO2 and 0.5 CH4 in mole fraction) were reported at five different TBAB concentrations (0.001, 0.01, 0.05, 0.1, and 0.2 mass fraction). The experiments were conducted in the temperature range (278.1 to 292.6) K and pressure range (2.17 to 8.16) MPa using an isochoric method. The agreement between the experimental data and the data reported in the literature indicated the reliability of the apparatus and method. The results showed that there were no obvious thermodynamic promotion effects on the CO2/CH4 mixed gas hydrate phase equilibrium conditions when the mass fraction of TBAB was less than 0.01, while there ...

Modeling study on phase equilibria of semiclathrate hydrates of pure gases and gas mixtures in aqueous solutions of TBAB and TBAF

Quaternary ammonium salts (QASs) such as tetrabutyl ammonium bromide (TBAB) and fluoride (TBAF) can form semiclathrate hydrates with water molecules at atmospheric pressure. Recently, the semiclathrate hydrates have attracted more and more interests for their wide applications, such as the storage and transportation of natural gas or coal bed methane, separation of low-boiling gases and carbon dioxide sequestration at mild conditions. In this work, the modeling investigations were conducted for the TBAB and TBAF semiclathrate hydrates with different gases such as CH4, CO2, N2, and their mixtures. The Chen-Guo hydrate model was extended to the semiclathrate hydrate systems. The thermodynamic properties of the gas phase were described with the Patel-Teja equation of state (PT EOS). The PT EOS combined with a Debye-Huckel electrostatic contribution term was chosen to describe the thermodynamic properties of the electrolyte solution. The parameters in the EOS for ions were determined by fitting the thermodynamic properties of the electrolyte solution. The parameters of TBAB and TBAF in the hydrate model were obtained from correlating the equilibrium data of semiclathrate hydrates. The extensive tests show that the proposed model results are in acceptable agreement with the experimental data on phase equilibria of semiclathrate hydrate systems.

Phase equilibria of semi-clathrate hydrates of tetra-n-butyl phosphonium bromide at atmospheric pressure and in presence of CH4 and CO2 + CH4

Gas separation and gas storage involving semi-clathrate formation are promising processes that can be used in many industrial fields. Among them, there is the possibility to purify the biogas, which is a gas mixture composed essentially of carbon dioxide (CO2) and methane (CH4). The investigations of the feasibility of these processes require knowledge about the phase diagram of the involved system. This work presents the phase diagrams of the systems of tetra-n-butyl phosphonium bromide (TBPB) + H2O, CH4 + TBPB + H2O and CO2 + CH4 + TBPB + H2O. The dissociation temperatures of the semi-clathrate phase were determined at a mass fraction of TBPB varying from 0.05 to 0.35 and at a 1–8 MPa pressure range. For CH4 + TBPB + H2O systems the measured temperatures varied from 280.8 to 293.1 K. The dissociation temperature of the semi-clathrate formed with TBPB has also been obtained in the case of CH4 + CO2 mixtures, the proportion of the gases being close to what is observed in typical biogases.

Experimental measurement and thermodynamic modelling of phase equilibria of semi-clathrate hydrates of (CO2 + tetra-n-butyl-ammonium bromide) aqueous solution

Comprehensive studies on semi-clathrate hydrates phase equilibria are still required to better understand characteristics of this type of clathrates. In this communication, new experimental data on the dissociation conditions of semi-clathrate hydrates of {carbon dioxide+tetra-n-butyl-ammonium bromide (TBAB)} aqueous solution are first reported in a wide range of TBAB concentrations and at different pressures and temperatures. A thermodynamic model is then proposed to predict the dissociation conditions of the semi-clathrate hydrates for the latter system. The (hydrate+TBAB) aqueous solution (H+Lw) phase equilibrium prediction is considered based on Gibbs free energy minimization approach. A modified van der Waals–Platteeuw solid solution theory developed based on the (H+Lw) equilibrium information is employed to predict the dissociation conditions of semi-clathrate hydrates of carbon dioxide+TBAB. The properties of the aqueous solution are estimated using the AMSA-NRTL electrolyte model (considering the association and hydration of ions). The Peng–Robinson equation of state is used for estimating the gas/vapour phase properties. Results show that the proposed model satisfactorily predicts the experimental values with an average absolute relative deviation of approximately 13%.

Thermodynamic model of phase equilibria of tetrabutyl ammonium halide (fluoride, chloride, or bromide) plus methane or carbon dioxide semiclathrate hydrates

In this work, based on the van der Waals–Platteeuw (vdW–P) theory, a thermodynamic approach is proposed to determine the phase equilibria of semiclathrate hydrates formed with tetrabutyl ammonium halide (fluoride, chloride, or bromide) and guest gas (CH4 or CO2). The Peng–Robinson equation of state (PR-EoS) and electrolyte-Non-Random Two-Liquid (e-NRTL) activity model are employed to obtain the fugacity of gaseous hydrate formers and activity coefficients of species in the aqueous phase, respectively. In addition, two modifications for evaluations of vapor pressure of water in the empty hydrate lattice and Langmuir constants relating to the salt concentrations in aqueous solution and temperature are applied. It is shown that the model results are in acceptable agreement with the experimental data on phase equilibria for studied systems over wide temperature, pressure, and salt concentration ranging from 280.1 to 304.8K, from 0.172 to 9.932MPa, and from 0.02 to 0.35, respectively.

Phase equilibria of semiclathrate hydrates for methane + tetra n-butylammonium chloride (TBAC), carbon dioxide + TBAC, and nitrogen + TBAC aqueous solution systems

The equilibria of semiclathrate hydrates for the systems of tetra-n-butylammonium chloride (TBAC)+water+gas (methane, carbon dioxide, or nitrogen) and TBAC + water were measured. The experiments were performed in the temperature range of 277.1–293.2K and the pressure range of 1.02–9.83MPa at 0.05, 0.15, and 0.22mass fraction of TBAC. An isochoric pressure-search method was employed to generate the phase equilibrium data. The experimental results show that the presence of TBAC moderates the hydrate formation (dissociation) conditions remarkably (in the concentration ranges studied in this work). Moreover, pressure reduction is dependent on TBAC concentration. By increasing the concentration of TBAC from 0.05mass fraction to 0.22mass fraction, its promotion effect increases and the pressure–temperature curves of the double gas+TBAC semiclathrate shift to the stabilized regions. The isobaric dissociation temperature of TBAC+methane semiclathrates is close to that of TBAC+carbon dioxide semiclathrates.

Phase equilibrium of semiclathrate hydrates of methane in aqueous solutions of tetra-n-butyl ammonium bromide (TBAB) and TBAB–NaCl

Phase equilibria of semiclathrate hydrates are important for their successful engineering applications due to more favorable process conditions compared to classical gas hydrate systems. Though sufficient information on the phase equilibria of semiclathrate hydrates of methane (CH4) in tetra-n-butyl ammonium bromide (TBAB) seems to be available, there are pronounced disagreements on the phase equilibrium data, particularly for 0.05 and 0.20 mass fraction (w) of TBAB. In this work, experimental studies are carried out to generate the equilibrium pressure (P) and temperature (T) for hydrates and semiclathrate hydrates of CH4 in an aqueous solution containing wTBAB=0.05 and 0.20 at P and T range of 1.02–13.73MPa and 281.63–294.54K, respectively. This study tries to clarify the discrepancy of published data in the literature and their reliability. Additionally, we present interesting insights into the phase behavior of semiclathrate hydrate of methane in TBAB based on the formation and dissociation curves observed in the experiments. It is observed that there existed two equilibrium points during the dissociation of semiclathrate hydrates of methane in TBAB; one closely corresponds to the pure methane hydrate phase stability curve and the second one to the semiclathrate hydrate system of methane. In addition phase equilibrium data is generated for the quaternary system of CH4+TBAB+H2O+NaCl for wNaCl=0.03 and 0.10 and wTBAB=0.05 and 0.20 in an aqueous solution at a P and T range of 1.65–20.71MPa and 281.19–296.38K, respectively. This is not yet available in the open literature. It is observed that NaCl inhibits the semiclathrate hydrate formation of CH4 in TBAB for wNaCl=0.03 and 0.10 in wTBAB=0.20 in the system. However, a promotion effect is observed for wNaCl=0.03 in wTBAB=0.05. This study calls for more detailed investigations on the effect of salts on semiclathrate hydrate systems, which may find potential use in engineering applications.

Experimental and Modeling Study on Phase Equilibria of Semiclathrate Hydrates of Tetra-n-butyl Ammonium Bromide + CH4, CO2, N2, or Gas Mixtures

Tetra-n-butyl ammonium bromide (TBAB) is most-widely used as a hydrate promoter in the separation of flue gases and CO2 sequestration. Because of the complicacy of semiclathrate hydrate formed by TBAB aqueous solution and gases, few researchers have investigated the thermodynamic modeling of phase equilibria of semiclathrate hydrates. In this work, the CH4–N2 hydrate dissociation conditions were determined in TBAB solution first, and then a thermodynamic model for gas hydrate proposed by Chen and Guo was extended for semiclathrate hydrates of gas mixtures in TBAB aqueous solution. The Patel-Teja equation of state (PT EoS) was used for calculation of the fugacity of the gas phase. To evaluate the activity of water and activity coefficients of TBAB in TBAB aqueous solution, a correlation on the basis of existing osmotic coefficient and activity coefficient values is employed. For modeling the hydrate phase, parameters for sII hydrates proposed in the Chen–Guo hydrate model were used to calculate the fugacit...

Thermodynamic modeling of phase equilibria of semi-clathrate hydrates of CO2, CH4, or N2+tetra-n-butylammonium bromide aqueous solution

Prediction of phase equilibria of semi-clathrate hydrates has been very rarely investigated in the literature. In this work, a thermodynamic model is proposed for representation/prediction of phase equilibria of semi-clathrate hydrates of the CO2, CH4, or N2+tetra-n-butylammonium bromide (TBAB) aqueous solution. For modeling the hydrate phase, the van der Waals–Platteeuw (vdW–P) solid solution theory is used, revised with two modifications for evaluations of Langmuir constants and vapor pressure of water in the empty hydrate lattice, in which these values are supposed to be a function of TBAB concentration in aqueous solution. The Peng–Robinson (PR-EoS) equation of state along with re-tuned parameters of Mathias–Copeman alpha function is applied for calculation of the fugacity of gaseous hydrate former. For determination of the activity coefficient of the non-electrolyte species in the aqueous phase, the Non-Random Two-Liquid (NRTL) activity model is used. To calculate the mean activity coefficients of the electrolyte portion, a correlation on the basis of existing osmotic coefficient and activity coefficient values is employed. It is shown that the presented model results are in acceptable agreement with the experimental semi-clathrate hydrate dissociation data investigated in this work.

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.