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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 Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00866 • Publication Date (Web): 20 Apr 2015 Downloaded from http://pubs.acs.org on April 25, 2015
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Environmental Science & Technology
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Equilibrium, Kinetics, and Spectroscopic Studies
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of SF6 Hydrate in NaCl Electrolyte Solution
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Youngrok Seo1,a, Donghyun Moon1,a, Changho Lee1, Jeong-Woo Park1, Byeong-Soo Kim1,
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Gang-Woo Lee2, Pratik Dotel3, Jong-Won Lee3, Minjun Cha4 and Ji-Ho Yoon*,1
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Busan 606-791, Korea,2EER&C Co., Ulsan 681-310, Korea, 3Department of Environmental
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Engineering, Kongju National University, Chungnam 330-717, Korea, 4Department of
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Energy and Resources Engineering, Kangwon National University, Gangwon 200-701, Korea.
Department of Energy and Resources Engineering, Korea Maritime and Ocean University,
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*To whom correspondence should be addressed. (phone) +82-51-410-4684; (fax) +82-51-
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403-4680; (e-mail)
[email protected].
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a
These authors contributed equally to this work.
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ABSTRACT
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Many studies have focused on desalination via hydrate formation; however, for their potential
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application knowledge pertaining to thermodynamic stability, formation kinetics and guest
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occupation behavior in clathrate hydrates need to be determined. Herein, the phase equilibria
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of SF6 hydrates in the presence of NaCl solutions (0, 2, 4, and 10 wt%) were monitored in the
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temperature range of 277 to 286 K and under pressures of up to 1.4 MPa. The formation
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kinetics of SF6 hydrates in the presence of NaCl solutions (0, 2, and 4 wt%) was also
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investigated. Gas consumption curves of SF6 hydrates showed that a pure SF6 hydrate system
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allowed fast hydrate growth as well as high conversion yield, while SF6 hydrate in the
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presence of NaCl solutions showed retarded hydrate growth rate as well as low conversion
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yield. In addition, structural identification of SF6 hydrates with and without NaCl solutions
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was performed using spectroscopic tools such as Raman spectroscopy and X-ray diffraction.
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The Raman spectrometer was also used to evaluate the temperature-dependent release
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behavior of guest molecules in SF6 and SF6 + 4 wt% NaCl hydrates. The results indicate that
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while SF6 hydrate starts to decompose at around 240 K, the escape of SF6 molecules in SF6 +
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4 wt% NaCl hydrate is initiated rapidly at around 205 K. The results of this study can provide
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a better understanding of guest-host interaction in electrolyte-containing systems.
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INTRODUCTION
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Gas hydrates are solid ice-like compounds that contain gas molecules in cavities constructed
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by hydrogen-bonded network structures. Based on differences in the sizes and shapes of these
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cavities, gas hydrates are commonly classified into three families: structure I (sI), structure II
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(sII), and structure H (sH) hydrates.1−3 Over the past 60 years, gas hydrates have been studied
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in energy-related fields. Gas hydrates have been considered troublemakers in the petroleum
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process, owing to the plugging of gas pipelines,1,4 which is mainly due to gas hydrate
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formation. Therefore, there have been intensive studies on avoiding hydrate plugging; these
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studies have used the injection of thermodynamic hydrate inhibitors (THIs), such as methanol
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and mono ethylene glycol, as well as of kinetic hydrate inhibitors (KHIs), such as polymers.5–
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transportation, owing to their ability to trap large quantities of gas under moderate
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conditions.9,10 Recent studies of the self-preservation effect have shown promising results on
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the enabling of gaseous guests to remain in hydrate cavities at atmospheric pressure and
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approximately ice-melting temperature conditions.11,12 In addition, notable hydrate-based
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energy applications, such as carbon capture and sequestration (CCS),13 refrigeration
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systems,14,15 storage of nature gas,16,17 selective separation of specific components from
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mixtures,18–21 and desalination22–25 could lead to gas hydrates becoming quite promising
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functional materials, closely involved in environmentally-friendly and/or energy-saving
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processes, which are of crucial importance in modern industry.
On the other hand, gas hydrates are also recognized as possible options for gas storage and
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Among other applications, gas hydrate-based desalination technologies have been
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studied over the last several decades. The desalination process through gas hydrate formation
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is based on the simple phase transition from liquid to solid; the formed gas hydrates are solid
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compounds. Therefore, a desalination process by gas hydrate formation can be easily coupled 3 ACS Paragon Plus Environment
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with the simple physical process of separation. This simple process can lead to hydrate-based
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desalination technology becoming an economical and efficient way to separate salts and other
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impurities from sea water. However, gas hydrates are normally formed under high pressure
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and low temperature conditions; thus, many researchers have tried to achieve an efficient way
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to form gas hydrates in moderate conditions.
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Simple CH4 or CO2 hydrate was initially tested for use in hydrate-based desalination
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technology; however, the cost of the pressurization of CH4 or CO2 and the cost of
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refrigeration are high, and thus there has been extensive research work to decrease the
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hydrate formation pressure and elevate the hydrate formation temperature.26,27 Propane
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(C3H8) and hydrofluorocarbon (HFC) gases were suggested for this purpose and studied over
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the past decades.28,29 Recently, Corak et al.30 suggested that cyclopentane (CP), because it
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could aid in the formation of simple CP hydrate, could be a good option for desalination. In
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addition, Cha and Seol31 reported that the co-use of CP and cyclohexane (CH) to make CO2
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hydrate could lead to promising results for hydrate-based desalination. The co-use of CP and
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CH for CO2 hydrate formation can provide great advantages in terms of energy consumption
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and reaction time.31 The finding of hydrate formers to improve the desalination process
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through hydrate formation is a key issue; it is important, but has so far been the subject of
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only limited work.
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Sulfur hexafluoride (SF6), because of its good electrical and chemical properties, has
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widely been used as a cleaning agent for semiconductor processing, as an insulating gas for
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underground cables, and as a covering gas in the foundry industry.32 However, SF6 is one of
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the most dominant greenhouse gases (GHGs) due to its significant global warming potential,
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which is 22,800 times larger than that of CO2. In addition, the atmospheric lifetime of SF6 is
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estimated to be on the order of 3200 years, which is extremely long compared with that of
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other GHGs such as CH4 (12 years) or CO2 (5~200 years).33 Therefore, it is urgently 4 ACS Paragon Plus Environment
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necessary to control the emission of SF6 from the variety of industrial applications that use
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SF6. SF6 is known as a sII hydrate former; it can form sII hydrate under milder conditions, i.e.,
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higher temperatures and lower pressures, than can CO2 or N2 hydrates.34 These results can
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make the hydrate-based process with SF6 a more promising option in terms of energy
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consumption and capital cost. Therefore, the application of SF6 could lead to the dual benefits
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of greenhouse gas control and desalination via gas hydrate formation. As a first task, we
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investigated the physicochemical characteristics of SF6 hydrates in the presence of NaCl
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solutions. The phase equilibria of SF6 hydrates with various concentrations of NaCl were
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measured to determine the shift of the stability boundary of the formed hydrates. The
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formation and dissociation kinetics were also monitored using a newly designed apparatus
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and temperature-dependent Raman spectroscopy. Furthermore, X-ray diffraction (XRD) and
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Raman spectroscopy were used to identify the crystal structure of SF6 hydrates with and
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without NaCl solutions.
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EXPERIMENTAL SECTION
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Apparatus and materials. A high-pressure equilibrium cell equipped with a magnetic drive
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and stirrer is used to determine the three-phase (H–Lw–V) equilibrium line of the SF6 + NaCl
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+ H2O system.35 The internal volume of the equilibrium cell is ca. 200 cm3. The cell is
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equipped with reinforced sight windows at the front and back for visual observation of the
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phase behavior in the hydrate and liquid phases at pressures of up to 30 MPa. A digital
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thermometer and a pressure gauge, with resolutions of 0.1 K and 0.001 MPa, respectively, are
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attached to the equilibrium cell for the measurement of temperatures and pressures. These
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sensors are always calibrated before the experiment runs. The temperature and pressure in the
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cell are measured with uncertainties of ±0.1 K and ±0.01 MPa, respectively. The kinetic 5 ACS Paragon Plus Environment
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system was newly designed and used in this work. There are two individual cells: one kinetic
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cell and one gas reservoir cell. The internal volumes of the kinetic cell and the gas reservoir
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are 200 and 1600 cm3, respectively. The kinetic cell is directly connected to the gas reservoir
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through a solenoid valve and two metering valves to constantly maintain the pressure in the
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kinetic cell during hydrate formation. The whole system is immersed in a water bath to
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maintain constant temperature. Thus, all kinetic experiments are performed at the desired
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isobaric and isothermal conditions. SF6 with a minimum purity of 99.9 mol% was supplied by
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Korea Standard Gas Co.; NaCl with a minimum purity of 99.5 mol% was supplied by Sigma-
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Aldrich Chemical Inc.
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Procedures. The experiment to determine the phase equilibrium is initiated by charging
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an evacuated cell with 60 cm3 of aqueous NaCl solutions (0, 2, 4 and 10 wt%). The cell is
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pressurized to desired pressures of up to 1.4 MPa of SF6. The cell temperature is controlled
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using an externally connected refrigerator/heater. Then, the cell contents are agitated by the
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magnetic drive and stirrer. Once the cell conditions are stabilized, hydrate nucleation is
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induced by cooling the system to about 5 K below the expected hydrate-formation
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temperature. The formation of SF6 hydrates is usually terminated within 15 hrs. After the
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hydrate forms completely, the system is kept constant for at least 6 hr. Then, dissociation of
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the formed hydrates is initiated by slowly elevating the system temperature by means of a
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digital temperature controller at a rate of 1 K per hour. When the system temperature has
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remained stable at least for 8 h after achievement of pressure stability in the presence of a
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minute amount of hydrate crystal, the temperature and pressure are considered as indicative
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of an equilibrium dissociation condition. A more detailed description of the experimental
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procedures and apparatus is presented elsewhere. 35 The kinetic cell is initially charged with
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aqueous solutions containing 90 g of water and balanced NaCl (0, 2, 4 and 10 wt%). Then, 6 ACS Paragon Plus Environment
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the whole system is evacuated to remove the air in the kinetic cell and gas reservoir. The gas
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reservoir and kinetic cell are charged with SF6 gas up to pressures of 1.1, and 0.6 MPa,
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respectively. The system temperature is kept constant at 270.2 K using the water bath, which
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has a temperature control system. The pressure in the kinetic cell is kept constant at 0.6 MPa
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by the solenoid valve, which is automatically actuated by the pressure control system. The
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hydrate formation is induced by agitating the magnetic drive; the pressure in the gas reservoir
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is monitored as a function of time. The gas consumption during hydrate formations is
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calculated using the pressure drop in the gas reservoir and the PVT relationship.
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Sample Analysis. Once the hydrate is formed, the sample is kept in contact with SF6 gas
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for at least two days to enable the complete reaction between water and the SF6 gas. After the
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complete reaction, the vessel is removed from the bath. Then, for the removal of the residual
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gas, the vessel is immediately immersed in a circulator (Jeio Tech., HTRC-10) at 258 K.
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After opening the vessel, the SF6 hydrate samples are collected for both Raman and XRD
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measurements. A customized Raman spectrometer emitting the second harmonic (532 nm)
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of an Nd:YAG laser (Excelsior) with a maximum power of 150 mW is used. The size of the
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laser spot incident to the samples is ~20 µm; the use of a spectrograph (SpectraPro, 2500i)
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and a multichannel air-cooled CCD detector (Princeton Instruments, PIXIS 100B) enables a
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spectral resolution of about 1.5 cm−1. Control of the sample temperature during Raman
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measurements is conducted using a temperature-controlled microscope stage (Linkam,
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THMS 600). For temperature-dependent Raman spectroscopic experiments, the sample
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temperature on the microscope stage is varied from 160 to 300 K at intervals of 5 K, with a
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temperature stability of
