Article pubs.acs.org/jced
Equilibrium Data of (tert-Butylamine + CO2) and (tert-Butylamine + N2) Clathrate Hydrates Zong-Cai Xu†,‡ and De-Qing Liang*,† †
Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: In this work, experimental hydrate dissociation pressures of the (tert-butylamine + CO2) and (tert-butylamine + N2) systems were reported at (0.005, 0.015, 0.03, 0.056 and 0.093) mole fraction of tertbutylamine with measurements made in the temperature range of (273.5 to 283.3) K and in the pressure range of (2.27 to 13.79) MPa. The equilibrium data were generated using an isochoric pressure-search method. The hydrate dissociation data for the (CO2 + water) system were obtained in advance and compared with experimental data reported in the literature, and the acceptable agreements demonstrate the reliability of the experimental method and apparatus employed in this work. Dissociation experimental results indicated that tertbutylamine had an inhibition effect on the dissociation conditions of carbon dioxide, shifting the dissociation conditions of CO2 hydrate to lower temperatures/higher pressures, while for the (tert-butylamine + N2 + water) system, tert-butylamine showed a hydrate promotion effect, which meant higher temperatures/lower pressures would be required for the formation of hydrate. The inhibition/promotion effect was the highest at 0.0556 mole fraction of tertbutylamine within the experimental range.
1. INTRODUCTION Clathrate hydrates are nonstoichiometric crystalline compounds, in which “guest” molecules such as methane, nitrogen, and carbon dioxide are trapped in the three-dimensional latticelike cages of hydrogen-bonded water molecules under suitable conditions of temperatures and pressures.1,2 Normally, there are three kinds of gas hydrates, structure I (s I), structure II (s II), and structure H (sH), which are different in cage sizes and shapes.1,3 In addition, there are a variety of more complex clathrate hydrate structures, labeled as types III−VII4 and structure T (sT).5 Besides academic interest, hydrate-related technologies such as gas storage and transportation, gas separation, seawater desalination, separation of close-boiling point compounds,6,7 and CO2 capture have been attracting more attention. Not only being an attractive gas separation technology, CO2 capture by hydrate formation also can be applied for disposing of CO2 in the ocean.8,9 To make the process more feasible, some additives introduced into the (water + CO2) system were investigated. It was found that the introduction of addition agent such as tetran-butylammonium bromide (TBAB),10−17 tetrahydrofuran (THF),18 tetrabutylphosphonium bromide (TBPB),19,20 and tetrabutyl ammonium nitrate (TBAN)21 could “help” the CO2 molecules incorporate into the hydrate cages more easily. tert-Butylamine, known as an additive, is also a potential stabilizing agent, whose molecules can form a special kind of hydrate structure designated as sVI.4 The stoichiometric © 2014 American Chemical Society
composition of the sVI hydrate is 16-tert-butylamine·156H2O with 16 large cages (43596273) occupied by tert-butylamine molecules, and 12 small cages (4454) remain empty.22 Phase equilibria of the (tert-butylamine + water + CH4/H2) systems have been measured,23,24 and the results indicated that the introduction of tert-butylamine to both systems could lead to a more moderate hydrate forming condition. Few thermodynamic equilibrium data have been reported for systems of (tertbutylamine + water + N2) and (tert-butylamine + water + CO2), which could be beneficial for CO2 capture and gas storage via hydrate technology. Using an isochoric pressure-search method,25,26 the hydrate phase equilibrium data of the (tert-butylamine + CO2) and (tert-butylamine + N2) systems were measured at (0.005, 0.015, 0.03, 0.056, and 0.093) mole fraction of tert-butylamine in the temperature range of (273.0 to 283.3) K and in the pressure range of (2.18 to 13.79) MPa.
2. EXPERIMENTAL SECTION 2.1. Chemicals. The suppliers and purities of the fluid chemicals used in the experiments were shown in Table 1. Carbon dioxide, nitrogen, and tert-butylamine were used without any further purification. High-purity deionized water Received: October 17, 2013 Accepted: January 2, 2014 Published: January 8, 2014 476
dx.doi.org/10.1021/je400922j | J. Chem. Eng. Data 2014, 59, 476−480
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point at which the P−T curve slope sharply changed. One hydrate dissociation P, T value was acquired for one experimental run.
Table 1. List of the Materials Used for the Experiments component
purity
tert-butylamine
w = 0.995
N2 CO2 water
x = 0.9999 x = 0.9999 deionized
supplier Shanghai Jingchun Reagent Co., Ltd. Fushan Kede Gas Co., Ltd. Fushan Kede Gas Co., Ltd.
phase liquid
3. RESULTS AND DISCUSSION The experimental hydrate dissociation pressures of the (water + CO2) system are presented in Table 2 and plotted in Figure 2
gas gas liquid
Table 2. Hydrate Dissociation Pressures of Carbon Dioxide in Pure Watera
was used. All masses of the components were measured by an electronic balance with an uncertainty of ± 0.1 mg. 2.2. Apparatus. A schematic of the apparatus is shown in Figure 1. The equilibrium data of nitrogen and carbon dioxide
a
phase
T/K
P/MPa
Lw−H−V
278.7 279.4 280.0 280.6 281.3
2.39 2.61 2.86 3.13 3.42
Uncertainties u are u(T) = ± 0.1 K and u(P) = ± 0.01 MPa.
Figure 1. Schematic diagram of experimental apparatus. DA, data acquisition; GC, gas cylinder; PC, personal computer; PT, pressure transducer; PRV, pressure regulating valve; MS, magnetic stirrer; TS, temperature sensor; HLTTC, high and low temperature test chamber; SS, stirring seed; R, reactor; VP, vacuum pump.
were measured by an isochoric pressure-search method.25,26 The main part of the experimental apparatus was a stainless steel cell, the volume of which was about 25 mL. The vessel can stand pressure up to 20 MPa. A magnetic stirrer under the cell drove the stirrer inside the cell to facilitate equilibrium. A platinum resistance thermometer (PT100) was inserted into the cell through a pressure seal to measure the temperature with a maximum uncertainty of ± 0.1 K. The pressure was measured with a pressure transducer (CYB-20S) with a range (0 to 16) MPa with an uncertainty of ± 0.01 MPa. The experimental data from the platinum resistance thermometer and pressure transducer were stored and displayed on a computer through an Agilent data collector. 2.3. Experimental Method. The equilibrium data were generated using an isochoric pressure-search method.25,26 Before the experiment began, the reaction cell was thoroughly washed three times with deionized water and then adequately dried in an air oven. The cell was charged with 10 g of tertbutylamine solutions of known mole fraction, then sealed and evacuated to remove air inside the vessel. Then, an appropriate amount of gas (N2 or CO2) was charged into the vessel from a high pressure cylinder, and the cell was placed into the temperature controlled bath. The temperature was lowered until the formation of hydrate, noticed by an abrupt temperature increase and pressure drop. Next, the temperature was increased in steps of 0.1 K. The temperature was kept constant for at least one hour between each step to guarantee equilibrium at each temperature step. Finally, the P−T diagram for hydrate decomposition and formation was plotted, from which the hydrate dissolution point was determined to the
Figure 2. Phase equilibrium data for CO2 hydrate in pure water. △, ref 27; □, ref 28; ○, ref 29; +, ref 30; ●, this work.
together with some experimental data reported in the literatures,27−30 verifying the reliability of the experimental procedure and apparatus used in this work. Figure 3 indicates the deviations of the experimental results in this work and those reported in the literature from the third-order polynomial empirical formula by fitting the experimental data sourced from this work. The formula of the deviation is defined as 100(Pexp − Pcal)/Pcal, where Pcal is the pressure calculated by the fitted equation mentioned above and Pexp is the experimental equilibrium pressure. Figure 3 shows that the data deviation of this work is within the experimental uncertainty. The absolute average deviation between the data of this work and literature is 1.1 %. Phase equilibrium data of (tert-butylamine + water + CO2) system are listed in Table 3 and plotted in Figure 4 at (0.005, 0.03, 0.056, and 0.093) mole fraction of tert-butylamine. The finding shown in Figure 4 indicates that the presence of tertbutylamine in aqueous solution prevents the dissociation of CO2 hydrate, shifting the dissociation conditions of CO2 hydrate to lower temperatures/higher pressures. The plotted P−T curves of (0.005, 0.03, and 0.056) mole fraction reveals that the inhibition effect of tert-butylamine on CO2 hydrate increases as an increase of tert-butylamine mole fraction. The corresponding equilibrium pressures is the highest at 0.056 477
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Figure 4. Hydrate phase equilibria of the CO2 + tert-butylamine + water systems: ○, pure CO2, ref 29; ■, tert-butylamine 0.005 mole fraction; ●, tert-butylamine 0.03 mole fraction; ◆, tert-butylamine 0.056 mole fraction; □, tert-butylamine 0.093 mole fraction.
Figure 3. Pressure deviations of the experimental data from the calculative value at a given temperature for CO2 hydrate in pure water. △, ref 27; □, ref 28; ○, ref 29; +, ref 30; ●, this work; dashed lines, uncertainty of the present measurement.
Table 4. Hydrate Phase Equilibrium Data in the System of tert-Butylamine + N2 + H2O System at Mole Fraction x and Mass Fraction w of tert-Butylaminea
Table 3. Hydrate Phase Equilibrium Data in the System of tert-Butylamine + CO2 + H2O System at Mole Fraction x and Mass Fraction w of tert-Butylaminea x
w
T/K
P/MPa
0.005
0.020
0.030
0.112
0.056
0.194
0.093
0.294
277.9 278.8 279.6 280.3 281.1 274.4 275.4 276.5 277.4 278.3 273.8 274.6 275.6 276.6 277.0 273.4 274.1 275.2 275.7 276.8
2.27 2.55 2.82 3.12 3.50 2.14 2.40 2.72 3.09 3.47 2.43 2.74 3.12 3.55 3.74 2.31 2.56 2.90 3.18 3.61
Uncertainties u are u(x) = ± 0.001, u(w) = ± 0.001, u(T) = ± 0.1 K, and u(P) = ± 0.01 MPa. a
mole fraction of tert-butylamine, and the equilibrium pressure of 0.093 mole fraction case is almost the same as that of 0.056. Thus greater than a certain mole fraction of tert-butylamine does not give a noticeable effect on the equilibrium pressures. Compared with the selected equilibrium data of CO2 hydrate, the presence of 0.056 mole fraction tert-butylamine maximally raises the hydrate equilibrium pressure of the (water + CO2) system by approximately 1.4 MPa at T = 275 K. Phase equilibrium data of the (tert-butylamine + N2 + water) system are listed in Table 4 and plotted in Figure 5. The hydrate equilibrium pressures are measured at (0.005, 0.015, 0.03, 0.056, and 0.097) mole fraction of the tert-butylamine. In contrast to the (tert-butylamine + water + CO2) system, Figure
x
w
T/K
P/MPa
0.005
0.020
0.015
0.058
0.030
0.112
0.056
0.194
0.093
0.294
273.5 274.1 275.1 275.8 276.5 279.0 279.6 280.3 281.0 281.7 279.1 280.3 281.4 282.3 283.0 280.3 281.0 281.7 282.4 283.1 280.2 281.4 282.3 282.7 283.3
11.08 11.43 12.22 13.01 13.79 9.06 9.68 10.30 11.14 11.65 6.01 7.12 8.04 9.15 9.69 6.02 6.54 7.14 7.71 8.40 6.13 7.01 7.75 8.23 8.85
Uncertainties u are u(x) = ± 0.001, u(w) = ± 0.001, u(T) = ± 0.1 K, and u(P) = ± 0.01 MPa. a
5 demonstrates that the presence of tert-butylamine shows a promotion effect on N2 hydrate, shifting the dissociation conditions of N2 hydrate to higher temperatures/lower pressures. Moreover, the promotion effect increases with an increase of tert-butylamine mole fraction. Among all of the experimental data, the promotion effect reaches a maximum when the mole fraction is 0.056. The same phenomenon to the (tert-butylamine + water + CO2) system appears that the curve of dissociation conditions of N2 hydrate in the presence of 478
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86 20 8705 7669. Fax: +86 20 8705 7669. E-mail:
[email protected]. Funding
This work was supported by the National Natural Science Foundation of China (51176192), CAS Program (KGZD-EW301), and 863 Program (2012AA061403-03). Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 5. Hydrate phase equilibria of the N2 + tert-butylamine + water systems: ○, pure N2, ref 31; ◆, tert-butylamine 0.005 mole fraction; ■, tert-butylamine 0.015 mole fraction; ●, tert-butylamine 0.03 mole fraction; ◇, tert-butylamine 0.056 mole fraction; ▲, tert-butylamine 0.093 mole fraction.
0.093 mole fraction almost overlaps that in the presence of 0.056 mole fraction. Compared with the selected equilibrium data of N2 hydrate, the maximum reduction of the hydrate equilibrium pressure of the (water + N2) system due to the presence of tert-butylamine (0.056 mole fraction) is approximately 30 MPa around T = 281 K, proving the great promotion effect of tert-butylamine on the (water + N2) system. As shown in Figure 4 and Figure 5, among the measured data groups the inhibition/promotion effect is the highest at 0.0556 mole fraction of tert-butylamine. A possible explanation for the inhibition effect of tert-butylamine on CO2 hydrate can be given that, when CO2 hydrate cages form, there are no tertbutylamine molecules inside the cages, so tert-butylamine molecules play a similar role as halogen ions. While in the N2 + tert-butylamine system, the reason why tert-butylamine acts as a promoter may be explained by tert-butylamine molecules participated in forming the mixed sII hydrate. The internal structure of tert-butylamine hydrates studied in this paper are still uncertain, and a more detailed study and analysis of the hydrate structure would require direct measurements by suitable physical techniques (e.g., P-XRD, Raman spectroscopy, NMR).
4. CONCLUSIONS In this work, experimental hydrate dissociation pressures of the (tert-butylamine + CO2) and (tert-butylamine + N2) systems are measured at (0.005, 0.015, 0.03, 0.056, and 0.093) mole fraction of tert-butylamine in the temperature range of (273.0 to 283.3) K and in the pressure range of (2.18 to 13.79) MPa. The equilibrium data are generated using an isochoric pressuresearch method. Through comparison and analysis, tert-butylamine is found to perform an inhibition action on the dissociation conditions of carbon dioxide, while a promotion effect is shown in the (tert-butylamine + water + N2) system. The strongest inhibition/promotion effect is observed at 0.056 mole fraction of tert-butylamine. 479
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