Article pubs.acs.org/jced
Phase Equilibria and Dissociation Enthalpies of Tri‑n‑butylphosphine Oxide Semiclathrate Hydrates Incorporated with CH4, CO2, and H2 Jong-Ho Cha,† Eun Sung Kim,‡ Ki Sun Lee,‡ Jeong Won Kang,§ Jeong Won Kang,*,∥ and Ki-Sub Kim*,‡ †
National Energy Technology Laboratory, U.S. Department of Energy, Morgantown, West Virginia 26507, United States Department of Chemical and Biological Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju, Chungbuk 380-702, Republic of Korea § Department of Chemical and Biological Engineering, Korea University, 5-1 Anam-Dong, Sungbuk-Ku, Seoul 136-713, Republic of Korea ∥ Department of Transportation System Engineering, Graduate School of Transportation, Korea National University of Transportation, Uiwang-si, Gyeonggi-do 437-763, Republic of Korea ‡
ABSTRACT: We investigated the phase equilibrium boundary of tri-n-butylphosphine oxide (TBPO) semiclathrate hydrates incorporated with CH4, CO2, and H2. TBPO aqueous solutions with a molality (m) of (1.61 and 1.98) mol·kg−1 were used for hydrate formation, which corresponded to the clathrate structures of TBPO· 34.5H2O and 28H2O, respectively. The phase boundary at both concentrations was shifted to the promotion region represented by lower pressures and higher temperatures, compared to each simple gas hydrate. In particular, TBPO + CO2 double hydrate presented mild hydrate stabilization conditions of 0.95
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2.2. Apparatus and Experimental Procedure. A cylindrical vessel made of stainless steel 316 had an outer diameter of 6 cm and height of 5.5 cm (inner volume, Vinner of 155 cm3). A magnetic spin bar was placed in the vessel to agitate the fluid and hydrate crystals, and provide a fresh interface between gas and water/hydrate. A thermocouple Pt100Ω (Sungrim Industrial Co.) was inserted into the vessel to measure the temperature during hydrate formation/ dissociation processes. The pressure in the vessel was measured with a KELLER PA-21SR pressure transducer. The thermocouple and pressure transducer were connected to a Meps datalogger and a personal computer to record the temperature and pressure as functions of time. The temperature of the vessel was controlled using a thermostatic bath made of plexiglass in which coolant was circulated by a Jeiotech RW-0525G refrigerator. The hydrate equilibrium temperature and pressure were measured in an isochoric condition. TBPO aqueous solutions were prepared at stoichiometric composition of TBPO· 34.5H2O (m = 1.61 mol·kg−1) and 28H2O (m = 1.98 mol· kg−1). A total of 55 mL of aqueous solution was charged in the vessel, and gas was supplied from a cylinder through a pressure regulating valve into the cell. The vessel was purged one time with each gas to remove air in the head space and tubes. After the vessel was pressurized to the desired pressure, the temperature was decreased at a rate of 2 K·h−1 to form the hydrate. The agitation speed was maintained at 200 rpm. Hydrate formation was confirmed by both visual observation though a glass window and pressure depression in the pressure−time profiles. When system pressure reached a steady-state condition, the temperature was then slowly raised at steps of 0.2 K·h−1 to dissociate the hydrate crystals. The hydrate equilibrium points were determined at the intersection between two slopes; one sharply increased due to hydrate
Figure 1. Three phase (V-Lw-H) equilibrium (temperature T, pressure p) conditions. ●, this work, TBPO + CH4 double hydrate with m = 1.61 mol·kg−1; ○, this work, TBPO + CH4 double hydrate with m = 1.98 mol·kg−1; ▲, Mohammadi et al.,17 TBAB + CH4 double hydrate with m = 1.03 mol·kg−1; Δ, Mohammadi et al.,17 TBAB + CH4 double hydrate with m = 1.67 mol·kg−1; X, Lee et al.,18 THF + CH4 double hydrate with m = 3.26 mol·kg−1; ▼, CSMGem,19 CH4 simple hydrate. m: molality.
hydrate phase boundaries of TBPO + CH4 double semiclathrate hydrates. Both double hydrate systems show a drastic depression in pressure requirement at a designated temperature in the phase boundary curves, compared to CH4 simple hydrate. This result demonstrates that TBPO double hydrates are more stable than CH4 simple hydrates due to the occupation of TBPO in the cage framework; thus, CH4 can be incorporated into the water phase under lower pressure conditions at a given temperature. TBPO clathrate hydrates formed with CO2 and H2 were also stable under much lower pressure and higher temperature conditions than CO2 and H2 simple hydrates (Figures 2 and 3). Two different TBPO concentrations for the CH4 and CO2 gases used in this study almost overlapped with each other. Similar equilibrium conditions may be attributed to almost identical thermal stability of simple TBPO·34.5H2O and 28H2O at ambient pressure, as mentioned earlier. Table 2 lists the phase 3495
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Table 2. Three Phase (V-Lw-H) Equilibrium (Temperature T, Pressure p) Conditions for TBPO + CH4, CO2, and H2 Double Hydrates Prepared from TBPO Aqueous Solutions at m = (1.61 and 1.98) mol·kg−1a gas
T/K
p/MPa m = 1.61 mol·kg−1
CH4 285.4 288.7 292.7 294.8 295.9
1.05 1.78 3.85 5.01 6.92 m = 1.98 mol·kg−1
285.2 289.6 291.9 294.0 295.2 296.3
Figure 2. Three phase (V-Lw-H) equilibrium (temperature T, pressure p) conditions. ●, this work, TBPO + CO2 double hydrate with m = 1.61 mol·kg−1; ○, this work, TBPO + CO2 double hydrate with m = 1.98 mol·kg−1; ▲, Mohammadi et al.,17 TBAB + CO2 double hydrate with m = 1.03 mol·kg−1; Δ, Mohammadi et al.,17 TBAB + CO2 double hydrate with m = 1.67 mol·kg−1; X, Lee et al.,18 THF + CO2 double hydrate with m = 3.26 mol·kg−1; ▼, CSMGem,19 CO2 simple hydrate. m: molality.
0.51 1.91 3.35 4.83 6.26 7.75 m = 1.61 mol·kg−1
CO2 283.2 284.8 286.2 286.9 287.7 289.0
0.44 0.90 1.44 1.88 2.36 3.60 m = 1.98 mol·kg−1
283.1 285.0 286.3 287.0 287.9 288.6
0.46 0.90 1.40 1.85 2.40 3.05 m = 1.61 mol·kg−1
H2 281.4 281.6 282.3 282.9 283.6 283.9
1.04 2.05 4.11 6.15 8.09 10.42
a
m is the molality. Standard uncertainties u are u(T) = 0.1 K and u(p) = 0.02 MPa where the coverage factor k is 2.
Figure 3. Three phase (V-Lw-H) equilibrium (temperature T, pressure p) conditions. ●, this work, TBPO + H2 double hydrate with m = 1.61 mol·kg−1; ▲, Mohammadi et al.,17 TBAB + H2 double hydrate with m = 1.03 mol·kg−1; Δ, Hashimoto et al.,20 TBAB + H2 double hydrate with m = 2.07 mol·kg−1; ○, Hashimoto et al.,20 THF + H2 double hydrate with m = 3.26 mol·kg−1. m: molality.
tendency in which they became more stable in the order of TBAB, THF, and TBPO at temperatures > 285 K. The order of thermodynamic stabilities for double H2 hydrates coincided with that of the melting temperature of simple hydrates. TBPO + H2 double hydrate presented the phase equilibrium curve located between those of THF + H2 and TBAB + H2 hydrates. The dissociation temperature of the TBPO systems became higher in the order of CH4, CO2, and H2 double hydrates, which was different from that of the simple hydrates of CO2, CH4, and H2. The thermodynamic stability of simple hydrates with gaseous guests depended on the guest occupancy of both large 51262 or 51264 cages and small 512 cages. In contrast, that of the TBPO double hydrates was influenced by only the guest occupancy of the 512 cages because the combined cages are filled with TBPO. The 512 cages generally fit CH4 better than the CO2 molecules due to their proper molecular size and shape. Thus, CH4 molecules would afford higher 512 cage occupancy and lower chemical potential to the framework, which resulted in higher dissociation temperature and lower dissociation pressure for TBPO + CH4 double hydrates,
equilibrium conditions for TBPO + CH4, CO2, and H2 double hydrates. Figures 1, 2, and 3 also show the stability conditions for clathrate hydrates created by the occupation of other hydrate formers. The melting point of the simple hydrates is higher in the order of TBAB, TBPO, and THF at ambient pressure. However, THF hydrate presented the largest thermodynamic promotion effect, followed by TBPO and TBAB hydrates at equilibrium temperatures > 290 K, as CH4 molecules were incorporated into the hydrate phases. Note that the number of empty 512 cages of THF hydrate is larger than that of the TBAB and TBPO hydrates. Therefore, the TBPO and TBAB hydrates are less favorable than THF hydrate in terms of both storage capacity and hydrate formation conditions. In contrast, the clathrates with CO2 molecules show quite different promotion 3496
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CO2 would also be accommodated in the small 512 cages of TBPO hydrates as TBPO molecules were occupied in the combined cages during double hydrate formation. 3.3. Dissociation Enthalpies of TBPO + CH4/CO2 Double Hydrate. The dissociation enthalpies (ΔHd) of the TBPO + CH4/CO2 double hydrate were calculated from phase boundary data using a modified Clausius−Clapeyron equation:
compared to TBPO + CO2 double hydrate. The same trend was also found in the TBAB and THF double hydrate systems. 3.2. Spectroscopic Study Examining H2 Enclathration. The enclathration of gas molecules in the 512 cages was examined by dispersive Raman absorption spectra. Figure 4
ΔHd d ln P =− d(1/T ) ZR
(1)
where R is the gas constant and Z is the compressibility factor. The mean compressibility factor value determined by Pitzer’s equation was used for calculating dissociation enthalpy. All hydrate systems with CO2 and CH4 presented a good linear relationship between natural logarithmic phase equilibrium pressure and a reciprocal phase equilibrium temperature. The resulting dissociation enthalpies for TBPO/TBAB/THF + gaseous guest double hydrates and simple gas hydrates are summarized in Table 3. The semiclathrate hydrates with gaseous coguests showed higher dissociation enthalpy, compared to that of the corresponding simple gas hydrates. The TBPO + CO2 double hydrates showed dissociation enthalpies of (219.5 and 211.6) kJ·mol−1 at m = (1.61 and 1.98) mol·kg−1, respectively, which was very similar with both TBAB + CO2 double hydrates, whereas the enthalpies of TBPO + CH4 double hydrates were smaller than those of TBAB + CH4 double hydrates. Note that the TBAB + CO2 double hydrate has been suggested as a new refrigeration agent due to its suitable phase-change temperature and large heat of melting.12 Therefore, the results on dissociation enthalpy demonstrate that TBPO + CO2 double hydrate with phase change condition of ∼ 1 MPa below 285 K could also be employed for refrigeration and air-conditioning processes.
Figure 4. Raman spectrum of H−H vibron in TBPO + H2 double hydrate (top, the solid line) and TBPO simple hydrate (bottom, the dash line) prepared from TBPO aqueous solution with the molality of 1.61 mol·kg−1.
clearly shows two distinct peaks at 4131 and 4133 cm−1 in the TBPO + H2 double hydrate. The H−H vibron of H2 molecules can typically be found in the wavelength range between 4115 and 4135 cm−1,14 which distinguishes it from the higher wavelength for H2 enclathration in larger cages such as 51262 and 51264 cages. Thus, the peak can be assigned to H2 molecules enclathrated in small 512 cages where two split peaks are known to result from H−H vibron of ortho- and para-hydrogen molecule, respectively. The TBPO simple hydrate did not show any H−H vibrons, as expected. The Raman result revealed that H2 molecules selectively occupied the empty 512 cages, yielding the TBPO + H2 double clathrate hydrate. Comparing these Raman peak positions with THF + H2 double hydrate, the Raman peaks shifted to higher frequency region by about 7 cm−1,14 and had almost an identical wavenumber with the TBAB + H2 and TBAF + H2 double hydrates.15,16 Other gaseous molecules such as CH4 and
4. CONCLUSION Phase boundaries of TBPO + CH4/CO2/H2 semiclathrate hydrates were examined at TBPO molality of (1.61 and 1.98) mol·kg−1. The TBPO hydrate framework was confirmed to accommodate gaseous guest molecules in 512 cages, whereas the combined cages were filled with TBPO molecules. The TBPO semiclathrates incorporated with gaseous guest were more thermodynamically stable than the corresponding simple gas hydrates. In particular, the stability region of TBPO + CO2 double hydrate covered the pressure range < 1 MPa at
Table 3. Dissociation Enthalpies ΔHd for CH4 and CO2 Simple Hydrates and TBPO, TBAB, and THF Double Hydrates Incorporated with CH4 and CO2 Gasesa gas
hydrate former
m/mol·kg−1
Z
d ln P/d(1/T)
R2
ΔHd/kJ·mol−1
CH4
TBPO
1.61 1.98 1.03 1.67 3.26
0.933 0.927 0.891 0.877 0.918 0.866 0.899 0.904 0.948 0.934 0.919 0.847
−14894 −20476 −33329 −26401 −12000 −8364.9 −29366 −28152 −24846 −27220 −19312 −9996.5
0.9949 0.9902 0.9760 0.9968 0.9907 0.9984 0.9949 0.9994 0.9899 0.9996 0.9946 0.998
115.5 157.8 246.9 192.5 91.6 60.2 219.5 211.6 195.8 211.4 147.6 70.4
TBAB THF CO2
TBPO TBAB THF
a
1.61 1.98 1.03 1.67 3.26
m is the molality. 3497
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hydrophilic unit in the crystal structure of the clathrate tri-nbutylphosphine oxide 34.5-hydrate. J. Struct. Chem. (Engl. Transl.) 1982, 23 (3), 395−399; Zh. Struckt. Khim. 1982, 23 (3), 86−91. (14) Florusse, L. J.; Peters, C. J.; Schoonman, J.; Hester, K. C.; Koh, C. A.; Dec, S. F.; Marsh, K. N.; Sloan, E. D. Stable low-pressure hydrogen clusters stored in a binary clathrate hydrate. Science 2004, 306 (5659), 469−471. (15) Sakamoto, J.; Hashimoto, S.; Tsuda, T.; Sugahara, T.; Inoue, Y.; Ohgaki, K. Thermodynamic and Raman spectroscopic studies on hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrates. Chem. Eng. Sci. 2008, 63 (24), 5789−5794. (16) Trueba, A. T.; Radović, I. R.; Zevenbergen, J. F.; Kroon, M. C.; Peters, C. J. Kinetics measurements and in situ Raman spectroscopy of formation of hydrogen−tetrabutylammonium bromide semi-hydrates. Int. J. Hydrogen Energy 2012, 37 (7), 5790−5797. (17) Mohammadi, A. H.; Eslamimanesh, A.; Belandria, V.; Richon, D. Phase equilibria of semiclathrate hydrates of CO2, N2, CH4, or H2 + tetra-n-butylammonium bromide aqueous solution. J. Chem. Eng. Data 2011, 56, 3855−3865. (18) Lee, Y.-J.; Kawamura, T.; Yamamoto, Y.; Yoon, J.-H. Phase equilibrium studies of tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 hydrates. J. Chem. Eng. Data 2012, 57, 3543−3548. (19) CSMGem, version 1.10; software for calculating of multiphase equilibria of hydrate system; Center for Hydrate Research, Colorado School of Mines: Golden, CO 2007. (20) Hashimoto, S.; Murayama, S.; Sugahara, T.; Sato, H.; Ohgaki, K. Thermodynamic and Raman spectroscopic studies on H2 + tetrahydrofuran + water and H2 + tetra-n-butyl ammonium bromide + water mixtures containing gas hydrates. Chem. Eng. Sci. 2006, 61, 7884−7888.
temperature near 285 K, and their dissociation enthalpy was comparable with the TBAB + CO2 double hydrate. The results on the phase change behaviors demonstrated that TBPO + CO2 double hydrate could be employed as alternatives to conventional refrigerants.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: +82-31-462-8739. Fax: 8231-462-8734. *E-mail:
[email protected]. Phone: +82-43-841-5222. Fax: 8243-841-5220. Funding
This work was supported by the Industrial Strategic Technology Development Program (No. 10045068, Development of flow assurance and organic acid/calcium removal process for the production of offshore opportunity crude) of Korea Evaluation Institute of Industrial Technology funded by the Ministry of Trade, Industry & Energy (MI, Korea). Notes
The authors declare no competing financial interest.
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REFERENCES
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