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Phase Equilibrium Conditions of Tetrabutyl Ammonium Nitrate + CO2, N2, or CH4 Semiclathrate Hydrate Systems Jian-Wei Du,†,‡ De-Qing Liang,*,† Dong-Liang Li,† Yu-Feng Chen,†,‡ and Xin-Jun Li† †
Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P.R. China ‡ China Graduate University of Chinese Academy of Science, Beijing 100049, P.R. China ABSTRACT: Phase equilibrium conditions for semiclathrate hydrates formed from tetrabutyl ammonium nitrate (TBANO3) + CO2, N2, or CH4 + water system were investigated under high-pressure in a “full view” sapphire cell. An isochoric equilibrium stepheating pressure-search method was used for the determination of the hydrate phase transition points through plotting the points of intersection of hydrate-liquid vapor (H-L-V) to liquid vapor (L-V) experimentally. All the experimental equilibrium data were obtained in the presence of TBANO3 solution at ammonium salt mass fraction of 0.3941 corresponding to a stoichiometric composition for TBANO3 3 26H2O. Measurements were performed for TBANO3 + CO2 + water, TBANO3 + N2 + water, and TBANO3 + CH4 + water system in the temperature range of (281.5 to 290.9) K and in the pressure range of (1.19 to 32.89) MPa. The three-phase equilibrium lines of TBANO3 + N2 and TBANO3 + CO2 semiclathrate hydrate shifted the hydrate stability region to lower pressure and higher temperature than that of the pure N2 and CO2 hydrate, respectively. However, the addition of TBANO3 made TBANO3 + CH4 semiclathrate hydrate phase equilibrium curve at higher pressure than the pure CH4 hydrate.
1. INTRODUCTION Pure CO2, N2, and CH4 hydrate are known as a kind of nonstoichiometric crystalline inclusion compounds composed of a hydrogen-bonded water host lattice and guest molecules.1 Normally, the usual gas hydrate structures are those of structure I (sI), structure II (sII), and structure H (sH), where either structure is composed of a certain number of cavities formed only by water molecules.2 However, some semiclathrate formers such as tetraalkylammonium or tetraalkylphosphonium salts can form an unusual structure with water.3 Indeed, hydrates encapsulating these semiclathrate formers are specifically called semiclathrate hydrates.4 Different from the usual gas hydrates, semiclathrate hydrates include compounds which are not only inserted as guest gas molecules but also form a part of the clathrate cage structure with the hydrogen bonds network built by the water molecules.5 To lower the gas hydrate formation pressure at atmosphere temperature is a key issue for the industrial application of gas (e.g., H2 or natural gas) storage or mixed gases separation (e.g., CO2, N2, and CH4) using hydrate as medium material. Semiclathrate hydrates contain empty cages capable of entrapping some guest gas molecules and thus have lower equilibrium pressure than the corresponding pure gas hydrates. So, adding a semiclathrate former into the binary gas + water system forming semiclathrate hydrate to stabilize the hydrate cages is a relative valid method to obtain lower hydrate synthesis pressures. Some semiclathrate formers have been thoroughly verified by experiment to form semiclathrate hydrate and were used to store gases and purify mixed gases. For example, the research on tetrabutylammonium bromide semiclathrate hydrate (TBAB), tetrabutylammonium fluoride (TBAF), tetrabutylphosphonium bromide (TBPB), tetrabutylammonium chloride (TBACl), tetrabutyl ammonium nitrate (TBANO3), and so on as help guest r 2011 American Chemical Society
molecules have been reported in ternary system of TBAB + water + H2S,6,7 TBAB + water + N2,8 TBAB + water + CH4,8 TBAB + water + CO2,8 TBAB + water + H2,9,10 TBAF + water + H2,10 TBPB + water + CO2,11 TBANO3 + water + CO2,11 TBPB + water + H2,12 and TBACl + water + H2.12 These experimental results are critical in gas storage and separation via hydrate technology. However, there is neither research available in the literature nor the thermodynamic equilibrium data for the system of TBANO3 + water + N2 and TBANO3 + water + CH4. Furthermore, phase relation as a function of different gases with the same semiclathrate former of TBANO3 for a wide range of P-T conditions is vital to understand gas enclathration phenomena, thus it can provide an important tool for validation of predictive models for semiclathrate hydrate. In the present study, phase equilibrium measurements were implemented to obtain the three-phase hydrate equilibrium data of TBANO3 + water + CO2, TBANO3 + water + N2, and TBANO3 + water + CH4 systems at TBANO3 mass fraction of 0.3941 in the temperature range of (281.5 to 290.9) K and in the pressure range of (1.19 to 32.89) MPa, respectively. The experimental data for the TBANO3 + water + CO2 system were measured over an extended pressure range uninvestigated below CO2 liquefaction pressure point. Meanwhile, the novel experimental data on the equilibrium conditions for semiclathrate hydrate of TBANO3 + water + N2 and TBANO3 + water + CH4 were reported. The results generated from this work may be useful for gas storage and mixed gases separation. Received: April 8, 2011 Accepted: September 15, 2011 Revised: September 15, 2011 Published: September 15, 2011 11720
dx.doi.org/10.1021/ie200380j | Ind. Eng. Chem. Res. 2011, 50, 11720–11723
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Table 1. List of the Materials Used for the Experiments component
purity
supplier
phase
CH4
99.999%
Foshan Huate Gas Co.
gas
N2
99.999%
Foshan Huate Gas Co.
gas
CO2
99.999%
Foshan Huate Gas Co.
gas
TBANO3
98%
Shanghai Adamas Reagent Co., Ltd.
solid
deionized
liquid
water
Table 2. Semiclathrate Hydrate Phase Equilibrium Data in the Systems of TBANO3 + Water + Each of the Three Gases (CO2, N2, and CH4) phase Lw +H+V> Lw +V
T/K
P/MPa
TBANO3 + CO2 + water 281.5
1.19
281.9
1.45
282.2
1.68
282.4
1.96
282.6 283.0
2.21 2.63
283.1
2.77
TBANO3 + N2 + water 282.9
17.55
284.1
21.03
285.3
24.84
286.1
28.84
286.7 TBANO3 + CH4 + water
32.79
283.3
4.81
284.5
6.97
286.1
9.95
287.4
13.69
288.5
17.34
289.3
21.27
289.8 290.9
25.05 32.89
2. EXPERIMENTAL SECTION 2.1. Materials Preparation. The suppliers and purity of chemicals used in the present study were listed in Table 1. CO2, N2, CH4, and TBANO3 were used without any further purification. Deionized water was used after careful degassing. In this work, the aqueous solutions used were mixtures of deionized water and TBANO3. Here, to prepare 0.3941 mass fraction of TBANO3 solution, the required amounts of each component were predetermined (5.912 g of TBANO3 and 9.088 g of water), corresponding to a stoichiometric composition for TBANO3 3 26H2O. Then 9.088 g of water was placed in a 50 mL flask and 5.912 g of TBANO3 was added in later. Finally, a stopper was placed on the flask for two hours completely dissolved before the solution was inhaled into the vacuum cell. The mass of each component was measured by the mass balance throughout the process. 2.2. Experimental Apparatus and Procedure. The apparatus used in the present study was made by Sanchez Technologies Company. The apparatus allowed measurement of phase equilibrium
Figure 1. Phase equilibrium data for 9, pure CO2 hydrate, ref 1; 0, semiclathrate TBANO3 + water + CO2 hydrate, ref 11; b, this work (semiclathrate TBANO3 + water + CO2 hydrate); solid line and dotted line, best fit to experimental data.
curves within the pressure range of (0.1 to 40) MPa with an uncertainty of (0.024 MPa and the temperature ranging from 253 to 399 K with an uncertainty of (0.1 K. The equilibrium cell was a “full view” sapphire cell. The cell consists of a sapphire tube sealed at the top end with a stainless steel flange. The test contents were mixed through a stirrer, which was driven by both a dc motor located at the end of the piston and a magnetic coupling mounted outside the cell. The data from the acquisition system were saved at preset sampling intervals on a computer. A detailed schematic figure of the apparatus was described in ref 13. The experimental procedures in this work were the same as that described in detail by Du et al.14 16 For the ternary system of CO2, N2, or CH4 + TBANO3 + water, the semiclathrate hydrate dissociation points for initial aqueous TBANO3 concentrations of 0.3941 mass fraction were determined by isochoric equilibrium step-heating pressure-search method with the assistant of visual observation through the “full view” sapphire cell. This procedure resulted in reliable and repeatable measurements.17
3. RESULTS AND DISCUSSION The phase equilibrium data for CO2 + TBANO3 + water system at the stoichiometric composition for TBANO3 3 26H2O were generated and presented in Table 2 and plotted in Figure 1 together with data reported by Mayoufi et al.11 using the DSC method different from this work. Figure 1 showed that TBANO3 acted as semiclathrate hydrate former which would have lower formation pressure than that of the pure CO2 hydrate. The dissociation conditions of CO2 semiclathrate hydrate shifted to lower pressures or higher temperatures due to the presence of TBANO3 in the system. As shown in Figure 1, the presence of TBANO3 can decrease the CO2 semiclathrate hydrate phase equilibrium pressure by half than that of pure CO2 hydrate in the experimental temperature range. Meanwhile, TBANO3 + CO2 semiclathrate hydrate phase equilibrium pressure increases monotonically and moderately with the temperature increasing continuously at the hydrate phase equilibrium curve. As seen from Figure 1, the data in this work deviating with the literature11 value at the turning point may be caused by a different measuring method applied in the experiment. 11721
dx.doi.org/10.1021/ie200380j |Ind. Eng. Chem. Res. 2011, 50, 11720–11723
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Figure 2. Phase equilibrium data for b, pure N2 hydrate, ref 18;O, this work (semiclathrate TBANO3 + water + N2 hydrate); solid line and dotted line, best fit to experimental data.
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The opposite trend occurs on the CO2 + water + TBANO3 system as the phase equilibrium pressure variation of CO2 semiclathrate hydrate relative to pure CO2 hydrate reduces from 2.20 MPa at 281.5 K to 1.54 MPa at 283.1 K. Experimental phase equilibrium data of semiclathrate hydrate for CH4 + water + TBANO3 system obtained in the present work are gathered in Table 2 and illustrated in Figure 3 comparing with pure CH4 hydrate phase equilibrium data at the same temperature range. Figure 3 shows that the influence of TBANO3 on semiclathrate hydrate formation in CH4 + water + TBANO3 system is sharply different from the effect of TBANO3 on semiclathrate hydrate formation in both CO2 + water + TBANO3 system and N2 + water + TBANO3 system. In the experiment application of TBANO3 appears ‘inhibition’ effect other than ‘promotion’ effect just as thermodynamic inhibitors which means that the pure CH4 hydrate thermodynamic formation conditions is more ‘milder’ than the semiclathrate TBANO3 + CH4 hydrate at higher pressures. The semiclathrate TBANO3 + CH4 hydrate is more stable than pure CH4 hydrate at low pressures. Arjmandi et al.8 found the similar phenomenon in CH4 + water + TBAB system. At approximately 9.87 MPa, the phase boundaries coincide.
4. CONCLUSIONS In this work, the phase conditions of TBANO3 and CO2, N2, or CH4 semiclathrate hydrates were measured in the temperature range of (281.5 to 290.9) K and in the pressure range of (1.19 to 32.89) MPa at TBANO3 mass fraction of 0.3941. The results showed that semiclathrates of TBAB and CO2 or N2 studied in this work are more stable than pure gas hydrate, respectively, but this changed for semiclathrates formed with TBANO3 and CH4. These results may provide valuable information on the practical applications for gas storage and mixed gases separation. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +(86)20-87057669. Fax: +(86)20-87057669. E-mail:
[email protected].
Figure 3. Phase equilibrium data for b, pure CH4 hydrate, ref 1; O, pure CH4 hydrate, ref 19; +, pure CH4 hydrate, ref 20;2, pure CH4 hydrate, ref 21; 0, pure CH4 hydrate, ref 22; 4, pure CH4 hydrate, ref 23; 9, this work (semiclathrate TBANO3 + water + CH4 hydrate); solid line and dotted line, best fit to experimental data.
The semiclathrate hydrate equilibrium data of TBANO3 + water + N2 measured systematically were presented in Table 2 and Figure 2 together with the pure N2 hydrate phase equilibrium data. The results demonstrated that the presence of TBANO3 can ‘help’ the N2 incorporate into the semiclathrate hydrate cages more easily, which meant that formation of semiclathrate TBANO3 + N2 hydrate can occur at lower pressures and higher temperatures due to the presence of TBANO3 in the system. As presented in Figure 2, the semiclathrate hydrate formed is more stable than the pure N2 hydrate, and with the pressure increasing this trend is more obvious. As can be seen in Figure 2, the final phase equilibrium pressure of N2 + TBANO3 semiclathrate hydrate reduced 26.97 MPa lower than pure N2 hydrate at about 282.9 K, whereas for the same system, the final phase equilibrium pressure drop increases to 31.31 MPa at 286.7 K.
’ ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (50876107), NSFC-Guangdong Union Foundation (U0933004), National Basic Research Program of China (2009CB219504), and CAS Program (KGCX2-YW-805) ’ REFERENCES (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press, Taylor & Francis Group: Boca Raton, 2007. (2) Mohammadi, A. H.; Richon, D. Phase equilibria of clathrate hydrates of methyl cyclopentane, methyl cyclohexane, cyclopentane or cyclohexane + carbondioxide. Chem. Eng. Sci. 2009, 64, 5319–5322. (3) Fowler, D. L.; Loebenstein, W. A.; Pall, D. B.; Kraus, C. A. Some Unusual Hydrates of Quaternary Ammonium Salts. J. Am. Chem. Soc. 1940, 62, 1140–1142. (4) McMullan, R. K.; Jeffrey, G. A. Hydrates of the Tetra-n-butyl and Tetra-i-amyl Quaternary Ammonium Salts. J. Chem. Phys. 1959, 31, 1231–1234. (5) Deschamps, J.; Dalmazzone, D. Hydrogen Storage in Semiclathrate Hydrates of Tetrabutyl Ammonium Chloride and Tetrabutyl Phosphonium Bromide. J. Chem. Eng. Data 2010, 55, 3395–3399. 11722
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