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Pure SF6 and SF6-N2 Mixture Gas Hydrates Equilibrium and Kinetic Characteristics EUN KYUNG LEE,† JU DONG LEE,‡ HYUN JU LEE,† BO RAM LEE,§ YOON SEOK LEE,† SOO MIN KIM,† HYE OK PARK,‡ YOUNG SEOK KIM,‡ Y E O N G - D O P A R K , | A N D Y A N G D O K I M * ,† School of Materials Science and Engineering, Pusan National University, Jangjeon 2-dong, Geumjeoung-gu, Busan, 609-735, Republic of Korea, Dongnam Technology Service Division, Korea Institute of Industrial Technology, Jisa-dong, Gangseo-gu, Busan, 618-230, Republic of Korea, Department of Chemical Engineering, Pohang University of Science and Technology, San 31 Hyojadong, Namgu, Pohang, 790-784, Republic of Korea, and Department of Advanced Materials Engineering, Dong-Eui University, Busan 341-714, Korea
Received May 6, 2009. Revised manuscript received August 21, 2009. Accepted August 24, 2009.
Sulfur hexafluoride (SF6), whether pure or mixed with inexpensive inert gas, has been widely used in a variety of industrial processes, but it is one of the most potent greenhouse gases. For this reason, it is necessary to separate and/or collect it from waste gas streams. In this study, we investigated the pure SF6 and SF6-N2 mixture gas hydrates formation equilibrium as well as the gas separation efficiency in the hydrate process. The equilibrium pressure of SF6-N2 mixture gas was higher than that of pure SF6 gas. Phase equilibrium data of SF6-N2 mixture gas was similar to SF6 rather than N2. The kinetics of SF6-N2 mixture gas was controlled by the amount of SF6 at the initial gas composition as well as N2 gas incorporation into the S-cage of structure-II hydrate preformed by the SF6 gas. Raman analysis confirmed the N2 gas incorporation into the S-cage of structure-II hydrate. The compositions in the hydrate phase were found to be 71, 79, 80, and 81% of SF6 when the feed gas compositions were 40, 65, 70, and 73% of SF6, respectively. The present study provides basic information for the separation and purification of SF6 from mixed SF6 gas containing inert gases.
Introduction Gas hydrates are stable crystalline compounds formed physically by water and natural gas molecules (CH4, CO2, N2, SF6, etc.) at appropriate pressures and temperatures. Molecules of water, through hydrogen bonding, form a framework containing relatively large cavities that can be occupied by the guest molecules with favorable shapes and sizes. The hydrate structures are thermodynamically stabilized through nonbonded interaction between the encaged gas molecules * Corresponding author phone: +82-51-510-2478; fax: 82-51-5120528; e-mail:
[email protected]. † Pusan National University. ‡ Korea Institute of Industrial Technology. § Pohang University of Science and Technology. | Dong-Eui University. 10.1021/es901350v CCC: $40.75
Published on Web 09/08/2009
2009 American Chemical Society
and the water lattice. These hydrates are known to crystallize into 3 structures depending on the nature and size of the guest molecules (1-3). The most typical lattice structures are structure-I (space group Pm3n) and structure-II (space group Fd3m) cubic structures with unit cell formulas 2S6M · 46H2O and 16S8L · 136H2O, respectively. After the discovery of hydrate self-preservation (4, 5), which allows hydrate to remain metastable at atmospheric pressure and a few degrees below the ice point, scientists have become interested in studying the storage and transportation of gases in the forms of hydrates (6-9). Sulfur hexafluoride (SF6) gas has good electrical and chemical insulating properties as well as heat transfer properties. For these reasons, SF6 gas has been used as insulating gas in electrical transformers, cleaning gas in semiconductors manufacturing processing, and covering gas in the foundry process. However, SF6 mixture gas containing inexpensive inert gas has been widely used in the industrial sector instead of pure SF6 gas to eliminate some of the problems associated with pure SF6 and also to reduce the cost. Among the various SF6 mixture gas investigated so far, SF6-N2 mixture gas appears to be the most promising for technical applications. For example, SF6-N2 mixture gas containing 50-60% SF6 has a dielectric strength of up to about 85-90% compared to that of pure SF6 (10). However, SF6 gas is one of the most potent greenhouse gases (GHGs) that cause significant global warming, and has been, thus, blanketed into the Kyoto Protocol (11). The global warming potential of SF6 is 22,200 times larger than CO2 and it remains in the air for 3200 years (N2O 114 years, CH4 12 years) (12). For these reasons, it is necessary to separate and/ or collect the waste SF6 gas. To control the emission rates of this greenhouse gas and prevent global warming, developments of an efficient recovery and recycling process are underway. Liquefaction and/or cryogenic distillation could be applicable to the separation of SF6 gas from nitrogen mixtures due to the large differences in their boiling points. However, these processes are highly energy-intensive processes due to the cooling or pressurizing the feed gaseous mixtures. Membrane separation could be a more promising option in terms of power consumption. However, when polymeric membranes are used for gaseous mixtures, nitrogen is more permselective mainly due to its smaller molecular size; so such a process may not be suitable for the enrichment of SF6 gas from mixtures with high nitrogen concentration (13). Recently, a new process for separation of gas mixtures using clathrate hydrate compounds has been proposed (14). The basic mechanism of the separation process is a selective partition of the target component between the hydrate phase and the gaseous phase. It is known that SF6 would form hydrates under milder conditions, i.e., higher temperatures and lower pressures, than nitrogen or oxygen. Hence, high selectivity could be expected for the separation of such greenhouse gases from mixtures accompanied with nitrogen or oxygen (15). By following up the previous pure SF6 kinetic study (16), we investigated the SF6-N2 mixture gas hydrate formation equilibrium as well as the gas separation efficiency in the hydrate process.
Experimental Section Apparatus. The experimental apparatus used in this study is shown in Figure 1. It consists of a reactor (R), which is a high-pressure vessel equipped with temperature and pressure VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Experimental apparatus.
FIGURE 2. Temperature and pressure trace for determination of hydrate equilibrium point (initial temperature and pressure were 284.15 K and 0.4 MPa). control systems. A supply vessel (SV) acts as a reservoir that supplies gas into the reactor during the kinetic experiments. All vessels are immersed in an insulated bath. An Omega copper-constant thermocouple is inserted into the cell to measure the liquid temperatures during the kinetic experiments. The reactor (R), made of 316 stainless steel, has a volume of 350 cm3. Mixing of the cell contents was accomplished using a magnetic stir bar magnetically coupled to a set of rotating magnets placed directly underneath the cell. A baffle arrangement was used in the crystallizer to prevent vortex formation and enhance mixing of the crystallizer contents. In this experiment, 135 mL of deionized water was used to investigate hydrate equilibrium points and kinetics. Phase Equilibrium. Pure SF6 and SF6-N2 mixture gases containing 35 and 60% N2 were used to investigate equilibrium points with deionized water. Typical dissociation pattern for this system is shown in Figure 2. The volume was kept constant and the temperature was changed during the experiment. The nucleation and dissociation steps were repeated at least twice to remove the hysteresis phenomenon using fresh gas and water with each cycle. The reactor cell was then pressurized to the desired pressure while the temperature was kept constant. As the cell temperature was lowered, the pressure decreased linearly without hydrate 7724
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formation occurring (from point A to B) due to the gas contraction as well as increased gas solubility upon cooling at constant volume. We kept the cooling rate of about 40 K/hour (from point A to C). Neither gas nor water was added to the system during the experiment. At point B, the hydrates started to form and the pressure dropped rapidly to point C. The catastrophic growth was observed from point B to C. Hydrate dissociation began when the cell was heated from point C but hydrate was still remaining until Point D. Between points C and x, the cell temperature was increased, and waited at least 5 h until reaching equilibrium condition. To avoid obtaining an erroneous dissociation temperature and pressure, the dissociation part of the loop must be performed at a heating rate sufficiently slow to allow the system to reach equilibrium (17, 18). For this reason, we kept the heating rate of about 0.1 K/hour from point x to D. Finally, the hydrate equilibrium condition (or hydrate dissociation temperature and pressure) was determined at point D (19). Kinetics. Pure SF6 and SF6-N2 mixture gases containing 10, 27, 30, 35, and 60% N2 were used to investigate the kinetic characteristics under the same driving force condition. The difference between the experimental pressure and hydrate equilibrium pressure is considered as the driving force in this study. A number of driving forces, such as fugacity or chemical potential, generally are used (20). However, we considered the pressure difference as the driving force because the variation of pressure and fugacity was relatively small in our low-pressure conditions (0.6-0.75 MPa). Prior to each experimental run, the reactor was flushed at least three times with the hydrate-forming gas to remove any residual air. Before the hydrate formation started, we tried to minimize undesired gas dissolution. To minimize the diffusion of gas dissolving in water, the whole system was kept at a desired temperature condition in advance. Subsequently, the reactor was filled with SF6 and SF6-N2 gases until the desired pressure was obtained. Once the temperature was stabilized, the magnetic stir bar in the reactor was started and that point was set to time zero. To choose a suitable stirring rate we considered two factors: higher gas consumption rate to reduce experimental time, and stable gas consumption rate, which ensures any undesirable changes in the system. We decided that 450 rpm is the most suitable stirring rate in our experimental apparatus because there were no variations in temperature and gas consumption rate.
FIGURE 3. Pure SF6 and SF6-N2 mixture gas hydrates phase equilibrium.
FIGURE 4. Pure SF6 and SF6-N2 mixture gas hydrates formation rates under the same driving force.
SF6 and SF6-N2 gas were automatically supplied from the supply vessel, and the pressure in the reactor was kept constant with the help of a metering valve and automatic pressure controller. The number of moles of gas consumed as a function of time and temperature were recorded. The data acquisition system scanned the pressure and temperature at every second and then recorded the average values at every 10 s. At any given time, the number of moles of gas that have been consumed is the difference between the number of moles of the gas at time t ) 0 and the number of moles of the gas at time t present in the supply vessel and is given by the eq 1 where VSV is the volume of the supply vessel, including the tubing, and z is the compressibility factor, which is calculated by Pitzer’s correlations (21).
The equilibrium pressure difference between SF6-N2 mixture gas and pure SF6 gas increased as the formation temperature increased. Figure 4 shows the SF6 gas and SF6-N2 mixture gas hydrate formation rates in pure water under the same driving force condition. Table 1 summarizes the detailed kinetic experimental conditions. The data are given after induction time. SF6-N2 mixture gas showed higher hydrate formation rate compared to that of pure SF6 gas. The SF6 and N2 gases were reported to form structure-II hydrate and occupy L-cage and S-cage, respectively (22, 24). N2 can also occupy the large cavity, but provides insignificant stability to the large cavity. For this reason, two molecules can occupy the 51264 cavity at high pressures (19). However, our experiment conditions were extremely lower than hydrate equilibrium conditions of N2. Also, SF6 gas is known to form hydrate easily due to its milder hydrate formation condition compared to that of N2 gas. SF6 gas was reported to form structure-II hydrate and occupy L-cage, and this formation formed S-cage naturally. Therefore, higher hydrate formation rate of SF6-N2 mixture gas compared to that of pure SF6 gas is probably due to the N2 gas incorporation into the S-cage of structure-II hydrate which is preformed by the SF6 gas. As shown in Figure 4, the hydrate formation rates gradually increased as the amount of N2 addition increased up to 27%. On the other hand, the hydrate formation rates of SF6-N2 mixture gas containing more than 30% of N2 were lower than that of those containing 27% of N2. This is possibly due to the relatively small amount of S and L-cage cavities of structure-II hydrate preformed by the SF6 gas which resulted from the relatively small amount of SF6 at the initial gas composition. Therefore the kinetics of SF6-N2 mixture gas hydrate were affected by the amount of SF6 hydrate formed initially, as well as N2 incorporated into hydrate preformed by the SF6 gas. Raman spectroscopy was also used to analyze the structure characteristics of pure SF6 and SF6-N2 mixture gases. Typical Raman spectra of the SF6-N2 mixture gas hydrate are shown in Figure 5. Raman peaks at around 769 cm-1 and 2326 cm-1 correspond to the symmetric S-F and N-N stretching vibration in the hydrate phase, respectively (22, 25). The Raman peak at around 2329 cm-1 corresponds to the N-N stretching vibration in gas phase. Raman analysis confirmed the N2, SF6 gas incorporation into the S-cage, L-cage of structure-II hydrate preformed by the SF6 gas. The gas compositions in the hydrate were analyzed by using gas chromatography to estimate the separation efficiency. Table 1 summarizes the detailed kinetic experimental conditions with the separation efficiency data. Figure 6 shows the SF6-N2 mixture gas compositions of the initial
P ( zRT )
n ) VSV
0
P ( zRT )
- VSV
t
(1)
To measure the composition of the gas phase in the reactor as well as in the hydrate, a Varian CX 3400 gas chromatograph (GC) was connected online with the reactor and automated with a PC. Initial feed gases as well as the residual gases were analyzed at each cycle. The pressure in the reactor was quickly brought down to the atmospheric pressure and the hydrates were allowed to completely dissociate. The gas which evolved from the decomposed hydrate was collected and analyzed using GC. These results of hydrate composition were used to calculate the separation efficiency and the rough occupancy of SF6-N2. During the kinetic experiments, pure SF6 hydrate and SF6-N2 hydrate were analyzed to determine structure of the hydrate phase and the existence of SF6 and N2 gases into hydrate cavity using fiber optic based Raman spectroscopy (Sentinel, BRUKER) with a multichannel detector. The wavelength and power level were 532 nm and 70mW, respectively. Unilab II Probe, having 60 mm working distance, was used and analyzed in situ through the quartz window from the outside of the reactor.
Results and Discussion Figure 3 shows the phase equilibrium curves for pure SF6 gas and SF6-N2 mixture gas hydrate systems. In case of pure SF6 gas, the phase equilibrium data was almost identical to those of previously reported by K. Sugahara et al. (22). Also, phase equilibrium data of SF6-N2 mixture gas was similar to SF6 rather than N2’s equilibrium pressure and temperature of 20.1 MPa and 275.6 K, respectively (23). As shown in Figure 3, the equilibrium pressure of SF6-N2 mixture gas was higher than that of pure SF6 gas and the equilibrium pressure gradually increased as the amount of N2 addition increased.
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TABLE 1. Kinetic Experimental Conditions along with Conversion Rates and Composition Analyses initial composition [%] (SF6:N2)
residual composition [%] (SF6:N2) hydrate Composition [%] (SF6:N2) temperature [k]/pressure [MPa] driving force ((Pexp - Peq) [MPa]) hydration numbera consumption mole conversion rate (%) measurement time (min) a
40:60
65:35
70:30
73:27
100:0
16:84 71:29 276/0.75 0.45 12.07 0.135 21.77 800
25:75 79:21 276/0.65 0.45 13.43 0.13 23.21 250
29:71 80:20 276/0.65 0.45 13.6 0.1 18.18 200
34:66 81:19 276/0.65 0.45 13.77 0.113 19.82 200
100:0 100:0 276/0.6 0.45 17 0.08 18.18 900
Hydration number (n) ) water molecules per guest, assumed SF6 gas fully occupied L-cage cavity of structure-II.
FIGURE 5. Raman spectra of SF6-N2 mixture gas hydrates (Raman peaks at around 769 cm-1 and 2326 cm-1 correspond to the symmetric S-F and N-N stretching vibration in the hydrate phase). Therefore, the data presented in this study have useful information on the basic concept of separation and capturing processes. Although this greenhouse gas would form hydrates under much lower pressure and higher temperature conditions than the accompanying component (N2), the SF6 capturing speed was found to be low. Therefore, further studies such as investigation of better gas/water contacting mode like a coflow reactor that was developed to maximize hydrate production by injecting water droplets from a capillary tube into liquid gas and kinetic promoter are necessary to achieve effective separation (26).
Acknowledgments FIGURE 6. SF6-N2 mixture gas compositions of the initial feed gas, residual gas, and gas in the hydrate (hydrates were formed under the same driving force). feed gas, residual gas, and gas hydrate. The composition in the hydrate was found to be 71, 79, 80, and 81% of SF6 when the feed gas composition was 40, 65, 70, and 73% of SF6, respectively. The relatively large amount of SF6 incorporation into hydrate is due to the milder hydrate formation condition of SF6 gas compared to that of N2 gas. The composition analysis also indicates the N2 gas incorporation into the S-cage of structure-II hydrate. In this study, we investigated the hydrate equilibria and kinetics of pure SF6 and SF6-N2 gas mixtures to evaluate the SF6 separation efficiency from gas mixtures. The separation process is based on the equilibrium partition of the components between the gaseous phase and the hydrate phase. In addition, the hydrate processes for separating these greenhouse gases are strongly related with hydrate kinetics. 7726
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We gratefully acknowledge financial support from the Ministry of Knowledge and Economy (MKE) in Korea and Korea Institute of Industrial Technology (KITECH), and this work is the outcome of a Manpower Development Program.
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