Separation of SF6 from Gas Mixtures Using Gas Hydrate Formation

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Environ. Sci. Technol. 2010, 44, 6117–6122

Separation of SF6 from Gas Mixtures Using Gas Hydrate Formation INUK CHA,† SEUNGMIN LEE,† JU DONG LEE,‡ GANG-WOO LEE,§ AND Y O N G W O N S E O * ,† Department of Chemical Engineering, Changwon National University, 9 Sarim-dong, Changwon, Gyeongnam 641-773, Republic of Korea, Green Technology Center, Korea Institute of Industrial Technology, 421 Daun-dong, Jung-gu, Ulsan 681-802, Republic of Korea, and R&D Center, Yoo Sung Co., LTD, 248 Dangwol-ri, Onsan-eup, Ulju-gun, Ulsan 689-892, Republic of Korea

Received February 11, 2010. Revised manuscript received July 10, 2010. Accepted July 13, 2010.

This study aims to examine the thermodynamic feasibility of separating sulfur hexafluoride (SF6), which is widely used in various industrial fields and is one of the most potent greenhouse gases, from gas mixtures using gas hydrate formation. The key process variables of hydrate phase equilibria, pressure-composition diagram, formation kinetics, and structure identification of the mixed gas hydrates, were closely investigated to verify the overall concept of this hydratebased SF6 separation process. The three-phase equilibria of hydrate (H), liquid water (LW), and vapor (V) for the binary SF6 + water mixture and for the ternary N2 + SF6 + water mixtures with various SF6 vapor compositions (10, 30, 50, and 70%) were experimentally measured to determine the stability regions and formation conditions of pure and mixed hydrates. The pressure-composition diagram at two different temperatures of 276.15 and 281.15 K was obtained to investigate the actual SF6 separation efficiency. The vapor phase composition change was monitored during gas hydrate formation to confirm the formation pattern and time needed to reach a state of equilibrium. Furthermore, the structure of the mixed N2 + SF6 hydrate was confirmed to be structure II via Raman spectroscopy. Through close examination of the overall experimentalresults,itwasclearlyverifiedthathighlyconcentrated SF6 can be separated from gas mixtures at mild temperatures and low pressure conditions.

Introduction Gas hydrates are nonstoichiometric crystalline compounds formed when “guest” molecules of suitable size and shape are incorporated in well-defined cages in the “host” lattice made up of hydrogen-bonded water molecules. These compounds exist in three distinct structures, structure I (sI), structure II (sII) and structure H (sH), which contain differently sized and shaped cages. The sI and sII hydrates consist of two types of cages, while the sH hydrate consists of three types of cages (1). Gas hydrates have been of great concern in the energy and environmental fields. Oil and gas * Corresponding author phone: 82-55-213-3757; fax: 82-55-2836465; e-mail: [email protected]. † Changwon National University. ‡ Korea Institute of Industrial Technology. § Yoo Sung Co., LTD. 10.1021/es1004818

 2010 American Chemical Society

Published on Web 07/27/2010

companies are trying to avoid pipeline plugging due to hydrate formation during production and transportation. Large masses of natural gas hydrates deposited in the permafrost and sediment of the continental margins are regarded as future energy resources (1). Gas hydrates are also suggested as an attractive way of storing large quantities of gases such as natural gas and hydrogen because each volume of CH4 gas hydrate can contain as much as 170 volumes of CH4 gas at standard temperature and pressure conditions (2, 3). Recent investigations suggest that carbon dioxide produced from fossil fuel-fired power plants and industry is possibly sequestered as gas hydrates in the deep ocean or in the natural gas hydrate layer (4, 5). Hydratebased separation concepts have also been proposed for recovering target gases or organic contaminants from gaseous or aqueous mixtures (6-9). In the present study, hydrate-based sulfur hexafluoride (SF6) separation from gas mixtures was suggested as a novel SF6 separation and recovery method. SF6-containing gases are widely used in industry because SF6 has good electrical insulating properties. Gas mixtures of SF6 and N2 are used as an insulating filler gas for underground cables, a protective gas cushion when casting magnesium, and an etching agent in the semiconductor industry (10). However, SF6 is a serious greenhouse gas because of its extremely long lifetime in the atmosphere (3200 years) and significant global warming potential which is 23900 times higher than that of CO2, even though the absolute emissions of SF6 are much lower than those of CO2 (11). Therefore, there is an urgent need to develop a separation and recovery method of SF6 from gas mixtures to prevent further release into the atmosphere. One of the most promising options is the hydrate-based SF6 separation method because SF6, whose hydrate equilibrium condition is remarkably milder than N2, is expected to be enriched in the hydrate phase, resulting in high selectivity of SF6 in the hydrate phase. It was reported that in total energy consumption the hydrate process is more preferable at temperature conditions lower than 281 K when compared with liquefaction, which is potentially in competition (12). To the best of our knowledge, no research on the systematic and thermodynamic approach of the feasibility of hydratebased SF6 separation appears in the literature, even though a few studies have covered the energy consumption estimation and the limited results of hydrate phase equilibrium measurement and kinetics for pure SF6 and SF6 + N2 hydrates (12-14). In the present study, we attempted to check the thermodynamic feasibility of the hydrate-based SF6 separation process in various ways. First, the hydrate phase equilibria for the binary SF6 + water mixture and for the ternary N2 + SF6 + water mixtures with various SF6 vapor compositions (10, 30, 50, and 70%) were experimentally measured to determine the stability region of the pure and mixed hydrates. Second, on the basis of the hydrate phase equilibria, the pressure-composition diagram at two different temperatures of 276.15 and 281.15 K was obtained to examine the actual separation efficiency. Third, the vapor phase composition change was monitored during gas hydrate formation at different initial conditions to confirm the formation pattern and time for reaching a state of equilibrium. Furthermore, the structures of the pure SF6 and mixed N2 + SF6 hydrates were confirmed via Raman spectroscopy.

Experimental Section Phase Equilibrium. The pure SF6 gas and the gas mixtures of N2 + SF6 (10, 30, 50, and 70%) were supplied by Daesung VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic diagram of the experimental apparatus. Special Gas Co. (Republic of Korea). A schematic diagram of the experimental apparatus used in this study is shown in Figure 1. The experimental apparatus for the hydrate phase equilibria was specially designed to accurately measure the hydrate dissociation pressures and temperatures. The equilibrium cell was made of 316 stainless steel and had an internal volume of about 250 cm3. Two sapphire windows equipped in the front and back of the cell allowed the visual observation of phase transitions that occurred inside the equilibrium cell. The cell content was vigorously agitated by an impeller-type stirrer. Before each experimental run, the equilibrium cell was flushed at least three times with the hydrate-forming gas to remove any residual air or mixed gas. The experiment for the hydrate-phase equilibrium measurements began by charging the equilibrium cell with about 80 cm3 of ultra high purity water. After the equilibrium cell was pressurized to the desired pressure with pure or mixed N2 + SF6 gas, the whole main system was slowly cooled to a temperature lower than the expected equilibrium one. Because of thermal contraction, the cell pressure was slightly decreased by decreasing the temperature at a cooling rate of 1 K/h. Then, an abrupt pressure depression was observed at the stage of hydrate crystal growth after nucleation. When the pressure depression due to hydrate formation reached a steady-state condition, the temperature was increased in 0.1 K steps, with sufficient time, and accordingly the cell pressure was increased with hydrate dissociation. After all the hydrates were dissociated with increasing temperature, the cell pressure was again slightly increased due to thermal expansion. The H-LW-V equilibrium points at each vapor composition were determined as the intersection between the hydrate dissociation and thermal expansion lines, as shown in Figure 2. Pressure-Composition Diagram. To measure the compositions of the vapor and hydrate phases, we installed a sampling valve (Rheodyne, Model 7010, USA) with a loop volume of 20 µL connected to a gas chromatograph (HP 5890II, USA) through a high-pressure metering pump (Eldex, USA). A thermal conductivity detector (TCD) and a Porapak Q column (Supelco, USA) were used as the detector and the column, respectively. To measure the H-LW-V equilibrium compositions at a specified temperature, we charged an excess of water (about 180 cm3) into the cell to avoid the complete conversion of water into hydrate, i.e., H-V condition. In the condition of excess water and vigorous agitation, once nucleation occurs and hydrate formation proceeds at a specified temperature, the pressure of the system decreases and spontaneously approaches that of H-LW-V equilibrium after a sufficiently long time. The final state of the threephase (H-LW-V) equilibrium was also confirmed by visual observation. For each experimental run after nucleation, the 6118

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FIGURE 2. Pressure-temperature trace for determination of equilibrium dissociation point for the N2 + SF6 (30%) + water mixture. equilibrium cell with an excess of water and vigorous agitation was left as it was for 24 h to reach a state of equilibrium at a specified temperature. Then, vapor phase composition was analyzed by gas chromatograph after the vapor phase was circulated through a sampling line with a high-pressure metering pump to equilibrate both compositions of the cell and loop for more than 10 min. After it was confirmed that there was no SF6 composition change in vapor phase according to reaction time, the corresponding composition of the hydrate phase in coexistence with the vapor and liquid water phases was also measured by gas chromatograph after the vapor phase had been evacuated with a vacuum pump to 0.005 MPa within 10 s, and the entire hydrate phase was dissociated at 298.15 K. Vapor Phase Composition Change. An experiment to monitor the vapor phase composition change during hydrate formation was also carried out under excess water conditions. The stirring rate was kept at 300 rpm, which ensured the absence of water entrainment into the sampling line and external mass transfer resistance. Under isothermal and isobaric conditions, the reaction time was counted just after the nucleation of hydrate crystals. The isobaric condition was achieved by using a microflow syringe pump (ISCO, 500D, USA) operated in constant pressure mode. As the system

FIGURE 3. Hydrate phase equilibria for the N2 + SF6 + water mixtures. pressure decreases due to hydrate crystal growth, the piston of a syringe pump instantaneously moves upward to compensate for the pressure depression and as a result keeps the system pressure constant. During hydrate formation, the vapor phase was analyzed via an online gas chromatograph at a time interval of 20-50 min. A high-pressure metering pump was also used to circulate the vapor phase through a sampling line. The hydrate formation process, including induction, nucleation, and growth, was also carefully confirmed by visual observation. Raman Spectroscopy. For the Raman Spectra of the pure SF6 and mixed N2 + SF6 hydrates, a high-pressure reactor that is almost the same shape as that used for equilibrium measurement with an internal volume of about 350 cm3 was used. It was made of 316 stainless steel and equipped with two quartz windows in the front and back sides of the cell. A fiber optic-based Raman spectrometer (Sentinel, Bruker, Germany) with a multichannel detector was used to identify the structure of the pure SF6 and mixed SF6 + N2 hydrates. The light source for excitation was an integrated diode laser whose wavelength and power level were 532 nm and 70 mW, respectively. A UniLab II probe having a 60 mm working distance was used for in situ analysis through the quartz window from outside of the reactor.

Results and Discussion In industrial applications, SF6 is usually used in a gas mixture with N2 and sometimes becomes a component in gas mixtures with N2 during the blowing process after usage despite initially pure SF6. The SF6 composition in the gas mixture can vary depending on each application. The main purpose of the present study is to suggest a new hydrate-based SF6 separation process and to verify the feasibility of this process. For these reasons, the three-phase H-LW-V equilibria for the binary SF6 + water mixture and for the ternary N2 + SF6 + water mixtures with various SF6 vapor compositions (10, 30, 50, and 70%) were experimentally measured to confirm the thermodynamic validity of the process. The equilibrium measurements for the mixed gas hydrates were carried out over wide temperature and pressure ranges of 275-288 K and 0.2-3.0 MPa, depending on the vapor phase compositions. The results are shown in Figure 3 and listed in Table 1. Figure 3 also includes the hydrate equilibrium data

obtained by Sugahara et al. (13) and Dyadin et al. (15) for pure SF6 hydrate, which were in good agreement with our results. As shown in Figure 3, the H-LW-V lines of the ternary N2 + SF6 + water mixtures generally appeared considerably nearer to that of the binary SF6 + water mixture than that of the binary N2 + water mixture. N2, which has a small molecular size, and SF6, which has a relatively large molecular size, are known to form sII hydrates (1, 13). However, N2 can occupy the small and large cages of sII hydrate, while SF6 can occupy only the large cages because of its large molecular size. The presence of SF6 with a large molecular size in the large cages of the sII mixed hydrate caused the mixed N2 + SF6 hydrates to stabilize significantly despite the extremely high equilibrium pressure of N2 hydrate. Although the experimental determinations for the ternary N2 + SF6 + water mixtures were restricted to the H-LW-V phase boundary, especially for the binary SF6 + water mixture, the upper quadruple point (Q2), where two H-LW-V and H-LW-LSF6 phase boundaries intersect and thus four phases (H, LW, LSF6, and V) coexist, was measured and is presented in Figure 3. This point is located very closely along the saturation vapor pressure curve of SF6. As shown in Figure 3, the equilibrium pressure of pure N2 hydrate is very high, at more than 15 MPa at 273 K, while pure SF6 hydrate has very mild (almost atmospheric) equilibrium pressure at temperatures lower than 276 K. The concept for the hydrate-based gas separation was derived from the extremely large difference in equilibrium pressures between N2 and SF6 hydrates. When SF6 and N2 molecules compete with each other in occupying the hydrate lattice at conditions where the mixed N2 + SF6 hydrate can form, SF6 whose equilibrium pressure condition is much lower than N2 can be preferentially entrapped in the hydrate lattice, resulting in high selectivity of SF6 in the hydrate phase. The larger the equilibrium pressure difference that exists between two gases, the better the separation efficiency that can be expected through the hydrate-based gas separation process. With the fundamental information on hydrate phase equilibria presented in Figure 3, the actual thermodynamic validity and separation efficiency were closely examined through a pressure-composition diagram. The pressurecomposition diagram of the ternary N2 + SF6 + water mixtures measured at 276.15 and 281.15 K is shown in Figure 4. The experiment for the pressure-composition diagram was conducted under excess water conditions to analyze accurate compositions at H-LW-V equilibrium points. When gas hydrate formation occurs at conditions that are relatively lacking in water, the final state of the system will be hydrate-vapor (H-V) equilibrium, and thus, the final vapor phase composition will be dependent on the initial water amount. However, it should be noted that the experiment for general hydrate equilibrium pressure-temperature measurement has no relation to the amount of water. In the present study, for the experiment of the pressure-composition diagram the final state was intentionally set to be H-LW-V equilibrium at excess water conditions. At the final H-LW-V equilibrium state the system has three phases (hydrate, liquid water, and vapor) and three components (N2, SF6, and water). Therefore, the number of degrees of freedom becomes two, and accordingly, the system is divariant. If the pressure and temperature of the system are fixed, the corresponding compositions of the hydrate and vapor phases should be automatically determined according to the phase rule. From the pressure-temperature diagram shown in Figure 3, the vapor phase compositions in the pressure-composition diagram shown in Figure 4 can be approximately predicted because all the experimental data represented in Figure 3 were based on H-LW-V equilibrium. From the pressurecomposition diagram shown in Figure 4, it is possible to find the hydrate formation pressure and composition of dissociVOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Hydrate Phase Equilibrium Data for the N2 + SF6 + Water Mixtures N2 + SF6 (10%)

N2 + SF6 (30%)

N2 + SF6 (50%)

N2 + SF6 (70%)

pure SF6

T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

276.25 278.76 280.31 282.08 283.42 284.69

0.82 1.228 1.604 2.04 2.629 3.027

275.81 278.38 280.92 284.13 286.11 286.62

0.289 0.462 0.821 1.366 2.014 2.378

275.75 279.83 281.54 283.82 285.67 286.81

0.198 0.462 0.630 1.033 1.534 1.993

275.92 280.51 283.31 284.53 285.53

0.17 0.454 0.83 1.082 1.373

281.25 283.54 285.40 286.68 287.11a

0.433 0.767 1.193 1.659 1.852a

a The upper quadruple points (Q2), where two H-LW-V and H-LW-LSF6 phase boundaries intersect, and thus, four phases (H, LW, LSF6, and V) coexist.

FIGURE 4. Pressure-composition diagram for the N2 + SF6 + water mixtures. ated gas corresponding to a given vapor phase composition at a specified temperature and thus to determine the number of steps required to achieve the desired SF6 composition through hydrate-based separation. A gas mixture having concentrations of 50% SF6 and 50% N2 can form hydrates with water at 0.27 and 0.8 MPa at 276.15 and 281.15 K, respectively, and as a result formed hydrates are expected to contain 85% SF6 and 77% SF6 at 276.15 and 281.15 K, respectively. The degree of SF6 enrichment in the hydrate phase increased with decreasing hydration formation temperature. At 276.15 K only one step of gas hydrate formation and dissociation can offer a highly enriched SF6 composition of 85% from the initial gas mixture of 50% SF6. Furthermore, only one additional step, which can be conducted at much lower equilibrium pressure, makes it possible to obtain almost pure SF6. The composition change behavior in the vapor phase according to the reaction time is shown in Figures 5 and 6. As shown in Figure 5, two systems with different initial vapor compositions (30 and 50%) finally converged into the same composition at 276.15 K and 0.8 MPa. According to the phase rule mentioned earlier, if the pressure and temperature of the system were fixed under excess water conditions, a final composition should be automatically determined corresponding to fixed pressure and temperature conditions, regardless of the initial composition. By the same principle, as shown in Figure 6, two systems with the same initial vapor composition of 50% finally diverged into two different compositions corresponding to each 6120

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FIGURE 5. Composition change behavior in the vapor phase for two different initial gas compositions (30 and 50% SF6) during the mixed N2 + SF6 hydrate formation at 276.15 K and 0.8 MPa.

FIGURE 6. Composition change behavior in the vapor phase for two different pressure conditions (0.6 and 0.8 MPa) during the mixed N2 + SF6 hydrate formation at 276.15 K.

FIGURE 7. Raman spectra of the mixed N2 + SF6 (30%) gas, pure SF6 hydrate, and mixed N2 + SF6 (30%) hydrate. pressure condition at 276.15 K. As can be expected from the equilibrium pressure-temperature diagrams of Figure 3, at a specified temperature, a lower final vapor composition was observed at higher pressure conditions. In the case of a larger driving force, which is defined here as the difference between the initial and final compositions, the composition change was faster until reaching a final composition. Raman spectra of N2 +SF6 gas, pure SF6 hydrate, and N2 + SF6 hydrate are represented in Figure 7. Structure identification of mixed gas hydrates is very helpful to estimate the process scale and to design reactor volume in a hydrate-based separation process because each gas hydrate structure consists of differently sized and shaped cages and accordingly has different gas storage capacity. In the present study, Raman spectroscopy, known to be a simple and nondestructive form of analysis (17), was used to examine the structural aspects of gas hydrate at in situ temperature and pressure conditions. Raman peaks of N2 + SF6 gas mixture were observed at 773 and 2329

cm-1. A Raman peak detected at 773 cm-1 can be assigned to the symmetric S-F stretching vibration for SF6 gas molecules, while a Raman peak at 2329 cm-1 is assigned to the N-N stretching vibration for N2 gas molecules. For pure SF6 hydrate, a Raman peak was detected at 769 cm-1, which indicates that SF6 molecules can occupy only the large cages of sII hydrate (13). From the result that the peak position for the S-F stretching vibration in the mixed N2 + SF6 hydrate was the same as that in the pure SF6 hydrate, it can be concluded that the mixed N2 + SF6 hydrate is also sII. It should be noted, however, that a Raman peak at 2324 cm-1 is assigned to N2 molecules captured in the small and large cages of the sII mixed gas hydrate. According to Hinsberg et al. (18) and Sugahara et al. (19), pure N2 hydrate, known to form sII, also shows only one peak for the N-N stretching vibration at 2324 cm-1 due to the small molecular size of N2, even though N2 molecules occupy the small and large cages of sII hydrate. A more detailed analysis of the cage occupancy behavior of N2 in sII mixed gas hydrate should be investigated using more sophisticated methods such as neutron diffraction and X-ray diffraction. The conceptual diagram of the hydrate-based SF6 separation process is schematically shown in Figure 8. If the gas mixture of N2 + SF6 enters a hydrate formation unit with temperature and pressure conditions appropriate for gas hydrate formation, solid hydrates enriched with SF6 are formed and readily separated from the coexisting liquid phase. Then, solid hydrates are transferred to a hydrate dissociation unit with temperature and pressure conditions adjusted for dissociation, and finally, highly concentrated SF6 can be recovered from dissociated hydrates. To obtain almost pure SF6 from a gas mixture, we found that one additional cycle may be sometimes necessary depending on the initial SF6 composition. The suggested process seems to be very simple from an operational standpoint because no special facilities requiring sensitive and complex functions are needed. The operation can be carried out over mild temperature (275-288 K) and low pressure (0.2-3.0 MPa) ranges depending on the vapor phase compositions. In most cases, the operation can be conducted at pressures lower than 1.0 MPa. This process can be also applied to the separation and recovery of other target gases from gas mixtures without changing the basic concept. In spite of the thermodynamic validity and advantages of the hydrate-based SF6 separation process as stated above, future challenges still remain to commercialize this concept. As shown in Figures 5 and 6, the slow formation rate of hydrate containing SF6 could be an obstacle to the hydrate-based SF6 separation process. To overcome this inherent difficulty and achieve efficient separation, we find further research needs to be attempted in the future (4, 20, 21) on kinetic promoters such as surfactants, which can enhance the hydrate formation rate remarkably by adding only a very small amount, and on novel reaction modes such as spraying or static mixers, which can improve the gas-water contacting area. In addition, to

FIGURE 8. Conceptual diagram of the hydrate-based SF6 separation process. VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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treat larger amounts of gas streams, further research on continuous operation should also be carried out in the near future.

Acknowledgments This work was supported by the Human Resources Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Government Ministry of Knowledge and Economy (20104010100060) and is also supported by the Basic Science Research Program through the National Research Foundation by Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0069569). This study is also financially supported by Changwon National University in 2009-2010.

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