Thermodynamic and kinetic influences of NaCl on HFC-125a hydrates and their significance in gas hydrate-based desalination
Introduction
When water molecules come into contact with pressurized gas at low temperatures, they can be crystallized into gas hydrates, which can trap gas molecules, called “guests”, in their hydrogen-bonded water frameworks, called “hosts” [1]. These non-stoichiometric crystalline compounds, called gas hydrates, have been actively studied in various science and engineering fields. Gas hydrates exist in the form of three different structures (structure I (sI), structure II (sII), and structure H (sH)) in nature and each structure consists of differently sized and shaped cages stabilized by van der Waals forces [2]. Although there are many factors that influence the structure of gas hydrates, the most crucial element to determine the structure is the size of the guest molecules. sI hydrates consists of 2 small (512) and 6 large (51262) cages in a unit cell and can capture only small molecules, such as methane (CH4) and carbon dioxide (CO2), whereas sII hydrates, composed of 16 small (512) and 8 large (51264) cages in a unit cell, can entrap larger molecules, such as propane, i-butane, hydrofluorocarbon (HFC) gases, and chlorofluorocarbon (CFC) gases. Much larger liquid hydrocarbons, such as methylcyclopentane and neohexane form sH hydrates, composed of 3 small (512), 2 medium (435663), and 1 large (51268) cages in a unit cell, in the presence of help gases for stabilization of crystal structure [1], [2], [3], [4], [5].
In the past, gas hydrates were considered as a nuisance in the oil and gas industry as they can undergo agglomeration into solid gas hydrates, leading to serious problems of flowline rupture and explosion during oil and gas production and transportation [6]. Therefore, thermodynamic and kinetic hydrate inhibitors have been actively investigated to avoid pipeline plugging caused by gas hydrate formation [7], [8], [9]. However, naturally occurring gas hydrates deposited in the deep-ocean sediments of continental margins or in permafrost regions are regarded as future energy sources because their total carbon content is estimated to be twice as high as the amount found in conventional fossil fuels, including coal, petroleum, and natural gas [10]. Recently, gas hydrates have also been used for CO2 capture and storage [11], [12], [13], [14], [15], [16], [17], [18]. In particular, the storage of CO2 in natural gas hydrates linked with natural gas production has been studied for both energy and environmental purposes [19], [20], [21], [22], [23], [24]. In contrast to previously used simple production methods, such as depressurization and thermal stimulation [25], [26], the CH4-CO2 replacement method has several significant advantages over conventional dissociation-based methods. Firstly, it does not induce the geological hazards associated with seafloor subsidence, because CH4 production and CO2 hydrate formation take place simultaneously in the sediment. Further, it works double duty, sequestering CO2 for global warming mitigation and exploiting CH4 for clean energy production [27], [28]. On the other hand, various greenhouse gases, including HFCs and SF6, can be subjected to gas hydrate-based separation, because these gases can be selectively captured in hydrate cages due to their lower hydrate equilibrium pressure at any given temperature [29], [30], [31], [32], [33].
Gas hydrate-based desalination is a viable technology in water treatment and purification, because salts and other impurities are excluded during hydrate formation [34], [35], [36]. Hydrate-based desalination is not significantly affected by the salt concentrations of the target solution because it is based on the phase transition between liquid and solid phases [37]. Therefore, it can be applied to the desalination or purification of not only sea water but also highly concentrated waste water and produced water. Owing to its capacity to remove highly concentrated salts, hydrate-based desalination can potentially overcome the limitations of conventional desalination methods, such as thermal distillation and reverse osmosis [38], [39]. Appropriate hydrate-forming substances, which can be readily incorporated in hydrate cages at milder pressure and temperature conditions and have lower solubility in water, should be selected for hydrate-based desalination for this technology to become more competitive [40]. Several possible guests, such as cyclopentane, SF6, and some HFCs have been studied for hydrate-based desalination with respect to their thermodynamic and kinetic aspects [34], [41], [42], [43]. In this study, HFC-125a (pentafluoroethane) was selected as a promising hydrate guest for desalination, because HFC-125a has a very low solubility in water and forms gas hydrates at low pressures. However, the hydrate phase equilibria, formation and dissociation behavior, and thermal properties of HFC-125a in the presence of salts are not very well known.
In this study, the hydrate phase equilibria of HFC-125a + NaCl (0, 3.5, and 8.0 wt%) + water mixtures were evaluated to verify the influence of NaCl on the thermodynamic stability of HFC-125a hydrates. In addition, the gas uptake and conversion of water to gas hydrates were investigated to elucidate the effect of NaCl on the hydrate-formation behavior. The dissociation enthalpy of HFC-125a hydrates in the presence of NaCl was measured using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). Structural identification and quantitative analysis of HFC-125a + NaCl hydrates were carried out by powder X-ray diffraction (PXRD) analysis. In particular, the dissociation behavior of HFC-125a hydrates in the presence of NaCl was observed by collecting temperature-dependent PXRD patterns at atmospheric pressure.
Section snippets
Materials
HFC-125a (pentafluoroethane) gas (purity of 99.9%) was supplied by RIGAS Co. (Republic of Korea). NaCl (purity of 99.5%) was purchased from Sigma-Aldrich Co. (USA). Double-distilled and deionized water was used for gas-hydrate formation. All the materials were used as received without further purification.
Phase equilibria
A specially designed high-pressure equilibrium cell with an internal volume of 250 cm3 was used to measure the three-phase (hydrate (H)-liquid water (LW)-vapor (V)) equilibria of
Phase equilibria of HFC 125a hydrates in the presence of NaCl
The thermodynamic stability of HFC-125a hydrates in the presence of NaCl is fundamental to understanding this process and setting up the operating conditions for a HFC-125a hydrate-based desalination process. As shown in Fig. 2 and Table 1, the three-phase equilibria (H-LW-V) of HFC-125a + NaCl (0, 3.5, and 8.0 wt%) hydrates were measured using a conventional isochoric (pVT) method to examine the thermodynamic influence of NaCl on HFC-125a hydrates. The phase equilibrium data of pure HFC-125a
Conclusions
The thermodynamic stability, gas uptake, dissociation enthalpy, structural identification, and dissociation behavior of HFC-125a + NaCl (0, 3.5, and 8.0 wt%) hydrates were comprehensively evaluated for their potential application in hydrate-based desalination processes. The three-phase (H-LW-V) equilibria of HFC-125a hydrates in the presence of NaCl (0, 3.5, and 8.0 wt%) measured by a conventional isochoric method indicated that the thermodynamic stability of HFC-125a hydrates is significantly
Acknowledgements
This research was supported by the Mid-career Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (2017R1A2B4005455) and also by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) through “Human Resources Program in Energy Technology” (No. 20164030201010) funded by the Ministry of Trade, Industry and Energy, Republic of Korea.
References (53)
- et al.
Gas storage in structure H hydrates
Fluid Phase Equilib.
(1998) - et al.
Hydrate plugging problems in undersea natural gas pipelines under shutdown conditions
J. Petrol. Sci. Eng.
(1991) - et al.
Effective kinetic inhibitors for natural gas hydrates
Chem. Eng. Sci.
(1996) - et al.
Energy-efficient natural gas hydrate production using gas exchange
Appl. Energy
(2016) - et al.
CO2 capture from flue gas using clathrate formation in the presence of thermodynamic promoters
Energy
(2017) - et al.
Phase equilibrium modeling of gas hydrate systems for CO2 capture
J. Chem. Thermodyn.
(2012) - et al.
Methane recovery from natural gas hydrate in porous sediment using pressurized liquid CO2
Energy Convers. Manage.
(2013) - et al.
Replacement of CH4 in the hydrate by use of liquid CO2
Energy Convers. Manage.
(2005) - et al.
Raman analysis on methane production from natural gas hydrate by carbon dioxide–methane replacement
Energy
(2015) - et al.
An experimental study on the productivity of dissociated gas from gas hydrate by depressurization scheme
Energy Convers. Manage.
(2010)
Enclathration of CHF3 and C2F6 molecules in gas hydrates for potential application in fluorinated gas (F-gas) separation
Chem. Eng. J.
Phase equilibria and azeotropic behavior of C2F6+N2 gas hydrates
J. Chem. Thermodyn.
A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+)
Desalination
Enhanced efficiency of salt removal from brine for cyclopentane hydrates by washing, centrifuging, and sweating
Desalination
Aqua Solaris – an optimized small scale desalination system with 40 litres output per square meter based upon solar-thermal distillation
Desalination
Energy consumption and economic evaluation of water desalination by hydrate phenomenon
Appl. Therm. Eng.
Effect of subcooling and amount of hydrate former on formation of cyclopentane hydrates in brine
Desalination
CH4-CO2 replacement occurring in sII natural gas hydrates for CH4 recovery and CO2 sequestration
Energy Convers. Manage.
Thermodynamic phase equilibria and cage occupancy of NF3 hydrate
Fluid Phase Equilib.
Dissociation behaviors of methane hydrate formed from NaCl solutions
Fluid Phase Equilib.
Methane storage in water frameworks: self-preservation of methane hydrate pellets formed from NaCl solutions
Appl. Energy
Clathrate Hydrates of Natural Gases
Fundamental principles and applications of natural gas hydrates
Nature
Gas hydrates: unlocking the energy from icy cages
J. Appl. Phys.
Complex gas hydrate from the Cascadia margin
Nature
Natural Gas Hydrates in Flow Assurance
Cited by (61)
Phase equilibrium conditions in carbon dioxide + cyclopentane double clathrate hydrate forming system coexisting with sodium chloride aqueous solution
2024, Journal of Chemical ThermodynamicsCO<inf>2</inf> competes with radioactive chemicals for freshwater recovery: Hydrate-based desalination
2024, Journal of Hazardous MaterialsSustainable freshwater recovery from radioactive wastewater by gas hydrate formation
2023, Chemical Engineering Journal