Article pubs.acs.org/est
Equilibrium, Kinetics, and Spectroscopic Studies of SF6 Hydrate in NaCl Electrolyte Solution Youngrok Seo,†,∥ Donghyun Moon,†,∥ Changho Lee,† Jeong-Woo Park,† Byeong-Soo Kim,† Gang-Woo Lee,‡ Pratik Dotel,§ Jong-Won Lee,§ Minjun Cha,# and Ji-Ho Yoon*,† †
Department of Energy and Resources Engineering, Korea Maritime and Ocean University, Busan 606-791, Korea EER&C Company, Ulsan 681-310, Korea § Department of Environmental Engineering, Kongju National University, Chungnam 330-717, Korea # Department of Energy and Resources Engineering, Kangwon National University, Gangwon 200-701, Korea
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ABSTRACT: Many studies have focused on desalination via hydrate formation; however, for their potential application, knowledge pertaining to thermodynamic stability, formation kinetics, and guest occupation behavior in clathrate hydrates needs to be determined. Herein, the phase equilibria of SF6 hydrates in the presence of NaCl solutions (0, 2, 4, and 10 wt %) were monitored in the temperature range of 277−286 K and under pressures of up to 1.4 MPa. The formation kinetics of SF6 hydrates in the presence of NaCl solutions (0, 2, and 4 wt %) was also investigated. Gas consumption curves of SF6 hydrates showed that a pure SF6 hydrate system allowed fast hydrate growth as well as high conversion yield, whereas SF6 hydrate in the presence of NaCl solutions showed retarded hydrate growth rate as well as low conversion yield. In addition, structural identification of SF6 hydrates with and without NaCl solutions was performed using spectroscopic tools such as Raman spectroscopy and X-ray diffraction. The Raman spectrometer was also used to evaluate the temperature-dependent release behavior of guest molecules in SF6 and SF6 + 4 wt % NaCl hydrates. The results indicate that whereas SF6 hydrate starts to decompose at around 240 K, the escape of SF6 molecules in SF6 + 4 wt % NaCl hydrate is initiated rapidly at around 205 K. The results of this study can provide a better understanding of guest−host interaction in electrolytecontaining systems.
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(CCS),13 refrigeration systems,14,15 storage of nature gas,16,17 selective separation of specific components from mixtures,18−21 and desalination,22−25 could lead to gas hydrates becoming quite promising functional materials, closely involved in environmentally friendly or energy-saving processes, which are of crucial importance in modern industry. Among other applications, gas hydrate-based desalination technologies have been studied over the past several decades. The desalination process through gas hydrate formation is based on the simple phase transition from liquid to solid; the formed gas hydrates are solid compounds. Therefore, a desalination process by gas hydrate formation can be easily coupled with the simple physical process of separation. This simple process can lead to hydrate-based desalination technology becoming an economical and efficient way to separate salts and other impurities from seawater. However, gas hydrates are normally formed under high-pressure and lowtemperature conditions; thus, many researchers have tried to
INTRODUCTION Gas hydrates are solid ice-like compounds that contain gas molecules in cavities constructed by hydrogen-bonded network structures. On the basis of differences in the sizes and shapes of these cavities, gas hydrates are commonly classified into three families: structure I (sI), structure II (sII), and structure H (sH) hydrates.1−3 Over the past 60 years, gas hydrates have been studied in energy-related fields. Gas hydrates have been considered troublemakers in the petroleum process, owing to the plugging of gas pipelines,1,4 which is mainly due to gas hydrate formation. Therefore, there have been intensive studies on avoiding hydrate plugging; these studies have used the injection of thermodynamic hydrate inhibitors (THIs), such as methanol and monoethylene glycol, as well as of kinetic hydrate inhibitors (KHIs), such as polymers.5−8 On the other hand, gas hydrates are also recognized as possible options for gas storage and transportation, owing to their ability to trap large quantities of gas under moderate conditions.9,10 Recent studies of the selfpreservation effect have shown promising results on the enabling of gaseous guests to remain in hydrate cavities at atmospheric pressure and approximately ice-melting temperature conditions.11,12 In addition, notable hydrate-based energy applications, such as carbon capture and sequestration © 2015 American Chemical Society
Received: Revised: Accepted: Published: 6045
February April 16, April 20, April 20,
18, 2015 2015 2015 2015 DOI: 10.1021/acs.est.5b00866 Environ. Sci. Technol. 2015, 49, 6045−6050
Article
Environmental Science & Technology
experiment runs. The temperature and pressure in the cell are measured with uncertainties of ±0.1 K and ±0.01 MPa, respectively. The kinetic system was newly designed and used in this work. There are two individual cells: one kinetic cell and one gas reservoir cell. The internal volumes of the kinetic cell and the gas reservoir are 200 and 1600 cm3, respectively. The kinetic cell is directly connected to the gas reservoir through a solenoid valve and two metering valves to constantly maintain the pressure in the kinetic cell during hydrate formation. The whole system is immersed in a water bath to maintain constant temperature. Thus, all kinetic experiments are performed at the desired isobaric and isothermal conditions. SF6 with a minimum purity of 99.9 mol % was supplied by Korea Standard Gas Co.; NaCl with a minimum purity of 99.5 mol % was supplied by Sigma-Aldrich Chemical Inc. Procedures. The experiment to determine the phase equilibrium is initiated by charging an evacuated cell with 60 cm3 of aqueous NaCl solutions (0, 2, 4, and 10 wt %). The cell is pressurized to desired pressures of up to 1.4 MPa of SF6. The cell temperature is controlled using an externally connected refrigerator/heater. Then, the cell contents are agitated by the magnetic drive and stirrer. Once the cell conditions are stabilized, hydrate nucleation is induced by cooling the system to about 5 K below the expected hydrate formation temperature. The formation of SF6 hydrates is usually terminated within 15 h. After the hydrate forms completely, the system is kept constant for at least 6 h. Then, dissociation of the formed hydrates is initiated by slowly elevating the system temperature by means of a digital temperature controller at a rate of 1 K per hour. When the system temperature has remained stable for at least for 8 h after achievement of pressure stability in the presence of a minute amount of hydrate crystal, the temperature and pressure are considered as indicative of an equilibrium dissociation condition. A more detailed description of the experimental procedures and apparatus is presented elsewhere.35 The kinetic cell is initially charged with aqueous solutions containing 90 g of water and balanced NaCl (0, 2, and 4 wt%). Then, the whole system is evacuated to remove the air in the kinetic cell and gas reservoir. The gas reservoir and kinetic cell are charged with SF6 gas up to pressures of 1.1, and 0.6 MPa, respectively. The system temperature is kept constant at 270.2 K using the water bath, which has a temperature control system. The pressure in the kinetic cell is kept constant at 0.6 MPa by the solenoid valve, which is automatically actuated by the pressure control system. The hydrate formation is induced by agitating the magnetic drive; the pressure in the gas reservoir is monitored as a function of time. The gas consumption during hydrate formations is calculated using the pressure drop in the gas reservoir and the PVT relationship. Sample Analysis. Once the hydrate is formed, the sample is kept in contact with SF6 gas for at least 2 days to enable the complete reaction between water and the SF6 gas. After the complete reaction, the vessel is removed from the bath. Then, for the removal of the residual gas, the vessel is immediately immersed in a circulator (Jeio Tech., HTRC-10) at 258 K. After the vessel is opened, the SF6 hydrate samples are collected for both Raman and XRD measurements. A customized Raman spectrometer emitting the second harmonic (532 nm) of an Nd:YAG laser (Excelsior) with a maximum power of 150 mW is used. The size of the laser spot incident to the samples is ∼20 μm; the use of a spectrograph (SpectraPro, 2500i) and a multichannel air-cooled CCD detector (Princeton Instruments,
achieve an efficient way to form gas hydrates in moderate conditions. Simple CH4 or CO2 hydrate was initially tested for use in hydrate-based desalination technology; however, the cost of the pressurization of CH4 or CO2 and the cost of refrigeration are high, and thus there has been extensive research work to decrease the hydrate formation pressure and elevate the hydrate formation temperature.26,27 Propane (C3H8) and hydrofluorocarbon (HFC) gases were suggested for this purpose and studied over the past decades.28,29 Recently, Corak et al.30 suggested that cyclopentane (CP), because it could aid in the formation of simple CP hydrate, could be a good option for desalination. In addition, Cha and Seol31 reported that the combination of CP and cyclohexane (CH) to make CO2 hydrate could lead to promising results for hydrate-based desalination. The combination of CP and CH for CO2 hydrate formation can provide great advantages in terms of energy consumption and reaction time.31 The finding of hydrate formers to improve the desalination process through hydrate formation is a key issue; it is important, but has so far been the subject of only limited work. Sulfur hexafluoride (SF6), because of its good electrical and chemical properties, has been widely used as a cleaning agent for semiconductor processing, as an insulating gas for underground cables, and as a covering gas in the foundry industry.32 However, SF6 is one of the most dominant greenhouse gases (GHGs) due to its significant global warming potential, which is 22 800 times larger than that of CO2. In addition, the atmospheric lifetime of SF6 is estimated to be on the order of 3200 years, which is extremely long compared with that of other GHGs such as CH4 (12 years) or CO2 (5−200 years).33 Therefore, it is urgently necessary to control the emission of SF6 from the variety of industrial applications that use SF6. SF6 is known as a sII hydrate former; it can form sII hydrate under milder conditions, that is, higher temperatures and lower pressures, than can CO2 or N2 hydrates.34 These results can make the hydrate-based process with SF6 a more promising option in terms of energy consumption and capital cost. Therefore, the application of SF6 could lead to the dual benefits of greenhouse gas control and desalination via gas hydrate formation. As a first task, we investigated the physicochemical characteristics of SF6 hydrates in the presence of NaCl solutions. The phase equilibria of SF6 hydrates with various concentrations of NaCl were measured to determine the shift of the stability boundary of the formed hydrates. The formation and dissociation kinetics were also monitored using a newly designed apparatus and temperature-dependent Raman spectroscopy. Furthermore, X-ray diffraction (XRD) and Raman spectroscopy were used to identify the crystal structure of SF6 hydrates with and without NaCl solutions.
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EXPERIMENTAL SECTION Apparatus and Materials. A high-pressure equilibrium cell equipped with a magnetic drive and stirrer is used to determine the three-phase (H−Lw−V) equilibrium line of the SF6 + NaCl + H2O system.35 The internal volume of the equilibrium cell is ca. 200 cm3. The cell is equipped with reinforced sight windows at the front and back for visual observation of the phase behavior in the hydrate and liquid phases at pressures of up to 30 MPa. A digital thermometer and a pressure gauge, with resolutions of 0.1 K and 0.001 MPa, respectively, are attached to the equilibrium cell for the measurement of temperatures and pressures. These sensors are always calibrated before the 6046
DOI: 10.1021/acs.est.5b00866 Environ. Sci. Technol. 2015, 49, 6045−6050
Article
Environmental Science & Technology PIXIS 100B) enables a spectral resolution of about 1.5 cm−1. Control of the sample temperature during Raman measurements is conducted using a temperature-controlled microscope stage (Linkam, THMS 600). For temperature-dependent Raman spectroscopic experiments, the sample temperature on the microscope stage is varied from 160 to 300 K at intervals of 5 K, with a temperature stability of
