Article pubs.acs.org/jced
Experimental Determination of CCl4 Hydrate Phase Equilibria up to High Pressures Alireza Shariati,† Geert H. Lameris,‡ and Cor J. Peters*,§,∥ †
Natural Gas Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Molla Sadra Street, Shiraz 71345, Iran ‡ DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands § Separations Technology Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ∥ Chemical Engineering Department, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates ABSTRACT: A number of hydrate phase boundaries of the binary system of tetrachloromethane (CCl4) + water were measured experimentally at several temperatures and from low pressures up to 89.25 MPa. These hydrate phase boundaries included hydrate−ice−vapor, hydrate−liquid CCl4−vapor, hydrate−water−vapor, hydrate−solid CCl4−liquid CCl4, hydrate−solid CCl4−water, and hydrate−liquid CCl4−water. From the points of intersections of the different boundaries, the three quadruple points of hydrate−ice−water−vapor, hydrate−liquid CCl4−water−vapor, and hydrate−solid CCl4−liquid CCl4−water were determined as (273.18 K, 4.72 kPa), (273.71 K, 5.25 kPa), and (273.35 K, 55.94 MPa), respectively.
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INTRODUCTION Clathrate hydrates are solid crystals consisting of a lattice of water molecules. The crystal lattice has some types of cavities, which are large enough to contain molecules of moderate sizes. The structure of the lattice is such that it is not stable in itself, but encloses molecules of other substances, mostly apolar ones, to become stable. Because the “host lattice” encloses the “guest molecules”, these types of solids are called inclusion compounds or clathrates. In 1810, Davy1 was the first to mention the existence of chlorine hydrate. Since then, it has been shown that a number of substances can form hydrates. Hydrates can be classified based on their crystal structures. The three most important hydrate structures are called structure I, structure II, and structure H. The crystal systems of structures I and II are cubic, and for structure H, it is hexagonal. Each unit cell of structure I is built of two small cavities of pentagonal dodecahedron and six large cavities of tetrakaidecahedron. Each unit cell of structure II is built of 16 small cavities of pentagonal dodecahedron and 8 large cavities of hexakaidecahedral, and each unit cell of structure H has three small cavities of pentagonal dodecahedron, two medium cavities of irregular dodecahedron, and one large cavity of icosahedron.2 Clathrate hydrates occur at low temperatures and/or at high pressures. Since the clathrate hydrates can block the gas flow, their occurrence is an obstacle in a natural gas pipeline and should thus be prevented. However, several useful applications have also been suggested in the industries which promote the use of hydrates. One such application is the separation of hydrate forming substances from nonforming hydrate sub© XXXX American Chemical Society
stances, by entrapping them in a separable solid (hydrate) phase. On the basis of this idea, Seo and Lee3,4 proposed the separation of chlorinated hydrocarbons from aqueous solutions by the formation of hydrates. The chlorinated hydrocarbons investigated were classified as volatile organic compounds (VOCs). VOCs are used as solvents in various processes and are among the most common pollutants emitted by industry. Seo and Lee studied the thermodynamic feasibility of hydrate formation for dichloromethane (CH2Cl2), 1,1,1-trichloroethane (CH3CCl3), and tetrachloromethane (CCl4) using carbon dioxide (CO2), methane (CH4), and nitrogen (N2), as help gases. Their results suggested that the hydrate structure of the above-mentioned chlorinated hydrocarbons is structure II. They verified that the small cages of the hydrate structures were occupied by only help gases, while the large cages of structure II were simultaneously occupied by a large fraction of the studied chlorinated hydrocarbons and a relatively small fraction of the help gases. Muromachi et al.5 examined CCl4 and 1,1-dichloro1-fluoroethane (CCl2FCH3) hydrates for separating oxygen (O2) and ozone (O3) from one another. However, O3 reacted with CCl2FCH3, and they could measure only limited and qualitative data for the system containing CCl2FCH3. Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: July 11, 2014 Accepted: October 23, 2014
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vacuum pump is melted off. During the experiments, the tensimeters are kept at a constant temperature in a thermostated bath. The temperature fluctuations are smaller than 0.05 K. Four tensimeters can be set inside a thermostated bath at the same time, therefore, multiple measurements can be carried out. To assist the establishment of equilibrium, mixtures containing a solid phase were stirred. For this purpose, the vessels contained magnetic stirrers which could be moved by magnets. These magnets were operated by an electric motor and a set of gears. The criterion of equilibrium is an equal reading of at least two tensimeters, which may take several weeks to establish. The tensimeters contained CCl4 and water, therefore, a three-phase equilibrium of hydrate−liquid CCl4−vapor is monovariant. The hydrate phase can be distinguished from ice by its greater specific gravity, which is in between those of water and liquid CCl4. The three-phase equilibrium pressure was measured at each temperature based on the visual observation of three phases in equilibrium. B. High Pressure Measurements. The high-pressure measurements were performed within a window-autoclave. This apparatus has been described in detail in several publications.10,11 It consists of a thick metallic vessel, in which the glass equilibrium cell is placed within. The autoclave itself, is inside a thermostated bath and has two windows. Using these windows, the equilibrium phases can be seen visually at each condition. One equilibrium cell can be set inside the autoclave surrounded by a thermostated bath at each time. Magnets move a small steel ball inside the system for the purpose of mixing. The temperature measurements are carried out using a platinum resistance thermometer, which is covered by a steel jacket, reaching into the glass equilibrium cell, near the system. The liquid surrounding the glass equilibrium cell at low temperatures is a mixture of alcohol, water, and ethylene glycol in the proportions of 1:1:1, and is stirred by the same mechanism which moves the magnets. In this way, the difference between the real temperature and the measured one is kept as small as possible. The dissociation pressure of the hydrate is determined by lowering the pressure and allowing the hydrate to dissociate until the last hydrate particle disappears. This is determined visually.
If chlorinated hydrocarbons are to be used as hydrate formers in industrial purposes, such as the two above-mentioned applications, experimental information on the hydrate phase behavior of such systems is necessary. Dyadin et al.6 investigated the hydrate phase behavior of the three chlorinated methane derivatives of CH2Cl2, trichloromethane (CHCl3), and CCl4. They observed that at very high pressures, up to 5000 bar, the clathrate hydrates of structure II can be transformed to another structure, which they believed to be structure I. Their study was presented only in graphical form, and the one table presented in their work was limited to the coordinates of the quadruple points of the CH2Cl2, CHCl3, and CCl4 clathrate hydrates. In this study, we have experimentally measured the hydrate formation phase boundaries of the system water + CCl4 at several temperatures from low pressures up to 89.25 MPa. Liquid CCl4 and water have small mutual solubilities in oneanother. Goldman and Krishnan7 have reported the solubilities and Henry’s Law constants of water in CCl4. The mole fraction of water in CCl4 at 25 °C is 83.6·10−5. Therefore, CCl4 is essentially a water-insoluble hydrate former.
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MATERIALS AND METHODS Water was demineralized and distilled in our laboratory. CCl4 was of analytical grade (purity 99.9 %) and was purchased from Merck. All the materials were used without further purification. Table 1 presents the purities of substances used in this study. Table 1. Purities of the Substances Used in This Work substance
supplier
purity
tetrachloromethane water
Merck TU Delft
99.9 % distilled
The temperature measurements were made with two types of accurate thermometers: a quartz thermometer (Dymec) with digital readout and a platinum resistance thermometer with resistance bridge (Bleeker). The accuracy in temperature measurement was better than 0.02 K. Pressure measurements were done with the aid of mercury manometers (up to 500 kPa), Bourdon gauges (DRD-Feinmessungmanometer) and dead weight gauges (Barnet and ‘t Hart) up to 100 MPa. In the low pressure range, the mercury manometers were read with a cathetometer (Bleeker). The inaccuracy in the pressure readings was as follows: at pressures up to 20 kPa about 2.5· 10−02 kPa; in the range from 20 kPa up to 500 kPa about 0.5 kPa; from 500 kPa up to 3 MPa about 10 kPa, and from 3 MPa up to 90 MPa about 0.05 MPa. A. Low-Pressure Measurements. The low-pressure hydrate boundaries of CCl4 + H2O were measured with a tensimeter, which is a glass instrument consisting of a vessel and a mercury manometer. Readers are referred for further details regarding this apparatus elsewhere.8,9 The filling of the tensimeter is carried out as follows: The glass vessel is filled with a few millimeters of the substance to be investigated. Then the concerning orifices are closed by melting the glass and the tensimeter is evacuated. During evacuation, the vessel is cooled with liquid nitrogen. After a few minutes, the valve of the vacuum pump is closed, and the cooling of the vessel is stopped and the contents are brought to room temperature to allow the solved or included gases to escape. This procedure is repeated several times. After this, the mercury is degassed. Finally the manometer is filled with mercury and the connection to the
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RESULTS Chlorinated hydrocarbons are, in general, immiscible in water; therefore, they produce a second liquid phase in contact with water. CCl4 and water form a stable hydrate which can be in equilibrium with a combination of ice, liquid CCl4, water, and vapor phases. In this work, the following equilibrium phase boundaries were investigated with the tensimeter and the window autoclave for the binary mixture of water + CCl4: (1) equilibrium between hydrate (H), ice (I) and vapor (V) phases (H−I−V) using the tensimeter (2) equilibrium between hydrate, liquid CCl4 (L1) and vapor phases (H−L1−V) using the tensimeter (3) equilibrium between hydrate, water (L2), and vapor phases (H−L2−V) using the tensimeter (4) equilibrium between hydrate, solid CCl4 (S) and liquid CCl4 phases (H−S−L1) using the window autoclave (5) equilibrium between hydrate, water and liquid CCl4 phases (H−L1−L2) using the window autoclave (6) equilibrium between hydrate, solid CCl4, and water phases (H−S−L2) using the window autoclave B
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Table 4. Low-Pressure Phase Boundary of H−L2−V for the Water + CCl4 System
Tables 2, 3, and 4 present the measured data for the binary system of CCl4 + water for the low-pressure phase boundaries Table 2. Low-Pressure Phase Boundary of H−I−V for the Water + CCl4 System
a
T/Ka
P/kPab
255.93 255.95 258.18 259.41 260.76 262.51 262.54 264.64 264.66 267.14 267.17 269.09 269.13 269.61 270.68 271.66
1.47 1.43 1.69 1.87 2.10 2.41 2.37 2.76 2.72 3.29 3.28 3.65 3.69 3.76 4.04 4.34
a
T/Ka
P/kPab
271.25 272.22 273.04 273.20 273.46 273.63 273.72
3.24 3.91 4.59 4.74 5.00 5.17 5.26
u(T) = ± 0.02 K. bu(P) = ± 0.025 kPa.
u(T) = ± 0.02 K. bu(P) = ± 0.025 kPa.
Table 3. Low-Pressure Phase Boundary of H−L1−V for the Water + CCl4 System
a
T/Ka
P/kPab
255.93 258.17 258.18 259.41 260.75 260.76 262.54 262.55 264.66 264.67 267.16 267.17 269.12 269.14 271.24 272.20 273.01 273.19 273.45 273.62 273.70
1.80 2.08 2.02 2.22 2.46 2.44 2.77 2.75 3.10 3.10 3.63 3.62 4.02 4.03 4.61 4.86 5.09 5.13 5.21 5.26 5.28
Figure 1. Hydrate equilibrium phase boundaries of ◇, H−I−V; ○, H−L1−V; and △, H−L2−V in the system CCl4 + water.
H−I−L2−V is at 273.18 K and 4.72 kPa; the quadruple point of H−L1−L2−V is at 273.71 K and 5.25 kPa. Tables 5, 6, and 7 present the measured data for the binary system of CCl4 + water for the high-pressure phase boundaries of H−S-L1, H−S-L2, and H−L1-L2, respectively. Figure 2 shows these phase boundaries graphically. The point of intersection of the three phase boundaries represents the quadruple point of H−S−L1−L2. This quadruple point has the temperature of 273.35 K and 55.94 MPa. From the steep slope of the H−L1−L2 curve, it may be concluded that the volume change for the melting process of H → L1 + L2 is less than zero at high pressures. This would correspond to an expected difference in the compressibility of the two liquids on the one hand, and of the solid on the other. We expect the H−S−L1 curve to nearly coincide with the melting curve of pure CCl4, as can be expected according to the insignificant solubility of water in CCl4. The negative sign of the slope of the H−S−L2 curve corresponds to the negative sign of the volume change between the hydrate phase and the liquid phase, according to the Clapeyron equation. Dyadin et al.6 reported that under pressure, structure II hydrates are replaced by hydrates that they believe to be structure I. For example, for CCl4, they reported that there is a quadruple point of the type H (structure II)−H(structure I)−
u(T) = ± 0.02 K. bu(P) = ± 0.025 kPa.
of H−I−V, H−L1−V and H−L2−V, respectively. Figure 1 shows these data graphically. The point of intersection of the H−L1−V and H−L2−V curves is the quadruple point of the type H−L1-L2−V. The point of intersection between the H−I− V and H−L2−V curves is the quadruple point of type H−I-L2− V. The quadruple points are unique for every hydrate former. When a mixture of two hydrate formers is present, the quadruple point of H−L1-L2−V evolves into a line. On the basis of our experiments, these quadruple points have the following temperatures and pressures: The quadruple point of C
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Table 5. High-Pressure Phase Boundary of H−S−L1 for the Water + CCl4 System
a
T/Ka
P/MPab
253.65 255.59 257.56 258.25 259.66 260.33 262.22 263.59 263.66 265.06 266.33 267.95 267.97 268.76 269.27 269.80 270.71 270.73 272.18 272.36 272.60 272.84 273.17 273.18
8.09 13.09 18.24 20.25 23.78 25.55 30.64 34.03 34.61 38.09 41.52 45.84 45.99 47.65 49.62 50.79 53.44 53.04 57.55 58.14 57.89 58.78 60.40 60.20
Figure 2. Hydrate equilibrium phase boundaries of □, H−S-L1; ○, H− S-L2; and △, H−L1-L2 in the system CCl4 + water.
addition, three types of quadruple points of this system, in which four phases are in equilibrium and therefore are invariant, have been presented. This type of experimental study is necessary when the separation of chlorinated hydrocarbons from aqueous solutions is to be designed using hydrate formation.
u(T) = ± 0.02 K. bu(P) = ± 0.05 MPa.
Table 6. High-Pressure Phase Boundary of H−S-L2 for the Water + CCl4 System
a
T/Ka
P/MPab
272.55 272.85 273.05 273.15
89.25 79.52 74.66 69.69
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*E-mail:
[email protected].
u(T) = ± 0.02 K. bu(P) = ± 0.05 MPa.
Notes
The authors declare no competing financial interest.
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Table 7. High-Pressure Phase Boundary of H−L1−L2 for the Water + CCl4 System
a
AUTHOR INFORMATION
Corresponding Author
T/Ka
P/MPab
273.30 273.41 273.53 273.68 273.74 273.78 273.80
60.55 50.40 42.51 32.80 22.65 11.62 0.10
ACKNOWLEDGMENTS Alireza Shariati would like to thank both Shiraz University and Eindhoven University of Technology for giving him the opportunity to collaborate on this research.
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REFERENCES
(1) Davy, H. On a Combination of Oxymuriatic Gas and Oxygen Gas. Philos. Trans. R. Soc. 1811, 101, 155−162. (2) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; 3rd ed.; CRC Press: Boca Raton, FL, 2008. (3) Seo, Y.; Lee, H. A New Hydrate-Based Recovery Process for Removing Chlorinated Hydrocarbons from Aqueous Solutions. Environ. Sci. Technol. 2001, 35, 3386−3390. (4) Seo, Y.; Lee, H. Phase Behavior and Structure Identification of the Mixed Chlorinated Hydrocarbon Clathrate Hydrates. J. Phys. Chem. B 2002, 106, 9668−9673. (5) Muromachi, S.; Nakajima, T.; Ohmura, R.; Mori, Y. H. Phase Equilibrium for Clathrate Hydrates Formed from an Ozone + Oxygen Gas Mixture Coexisting with Carbon Tetrachloride or 1,1-Dichloro-1fluoroethane. Fluid Phase Equilib. 2011, 305, 145−151. (6) Dyadin, Y. A.; Zhurko, F. A.; Mikina, T. V.; Udachin, R. K. Clathrate Formation in Binary Aqueous Systems with CH2Cl2, CHCl3, and CCl4 at High Pressures. J. Inc. Phenom. Mol. Recog. Chem. 1990, 9, 37−49.
u(T) = ± 0.02 K. bu(P) = ± 0.05 MPa.
S−L2 at 75 MPa and 273.55 K. The positive slopes of the P−T curves of melting, that Dyadin et al.6 reported as evidence for structural change in hydrates, were not observed in our investigation. Such a change in the structure of the hydrates could not be investigated using our high-pressure equilibrium cell on the basis of visual measurements.
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CONCLUSION In this work, hydrate phase equilibria of the binary system of CCl4 + water was studied from low up to high pressures. In D
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(7) Goldman, S.; Krishnan, T. R. The Henry’s Law Constants of Water in the Binary Mixtures of Benzene, Carbon Tetrachloride and Cyclohexane at 25 °C. J. Sol. Chem. 1976, 5, 693−707. (8) Aaldijk, L. Monovariant Gas Hydrate Equilibria in the System of Xenon−Water. Ph.D. Thesis, Delft University of Technology, The Netherlands, 1971. (9) Broers, P. M. A.; De Roo, J. L.; Diepen, G. A. M. Vapour− Pressure Measurements for Beryllium Sulfate + Water. J. Chem. Thermodyn. 1976, 8, 83−91. (10) De Loos, T. W.; Wijen, A. J. M.; Diepen, G. A. M. Phase Equilibria and Critical Phenomena in Fluid (Propane + Water) at High Pressures and Temperatures. J. Chem. Thermodyn. 1980, 12, 193−204. (11) Shariati, A.; Peters, C. J. High-Pressure Phase Behavior of Systems with Ionic Liquids: Measurements and Modeling of the Binary System Fluoroform + 1-Ethyl-3-methylimidazolium Hexafluorophosphate. J. Supercrit. Fluids 2003, 25, 109−117.
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