Phase Stability of Semiclathrate Hydrates of Carbon Dioxide in

Article pubs.acs.org/jced

Phase Stability of Semiclathrate Hydrates of Carbon Dioxide in Synthetic Sea Water Kranthi K. Godishala,† Jitendra S. Sangwai,*,† Nagham A. Sami,‡,§ and Kousik Das† †

Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India Department of Petroleum Engineering, College of Engineering, University of Baghdad, Baghdad, Iraq § Department of Chemical Engineering, AC Tech Campus, Anna University, Chennai 600 025, India ‡

ABSTRACT: Experimental studies are carried out on semiclathrate hydrate of carbon dioxide (CO2) in tetra-n-butylammonium bromide (TBAB) for varying concentrations of TBAB (0.05, 0.10, and 0.20 mass fraction) + sodium chloride (NaCl) (0.035 and 0.10 mass fraction) in an aqueous system. The three-phase equilibrium (H-LW-V) data are generated for quaternary system of CO2 + TBAB + H2O + NaCl and are not available in the open literature. The competing effect of TBAB and NaCl at different concentrations on phase behavior of semiclathrate hydrate equilibrium is studied. It is found that the inhibition effect of salt is much more pronounced at higher pressures compared to lower pressure conditions. It is observed that the inhibiting effect of the NaCl is suppressed by the promoting effect of semiclathrate hydrates of CO2 in TBAB. Although there is a shift in hydrate equilibrium curve toward inhibition zone compared to that of the same system in the absence of salt, this system is more stable than the hydrate of pure CO2 in a similar environment. The study, in general, shows that the semiclathrate hydrates of CO2 are more stable than the hydrates of pure CO2 in the real environments containing salts, thus promising their use for safe carbon capture and sequestration (CCS) application.

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INTRODUCTION Clathrates (gas hydrates) are ice-like structures in which gas molecules (guest) are trapped inside the cages of water molecules (host).1,2 Carbon capture and sequestration (CCS) shows a potential solution to control the greenhouse gas effect. The majority of the enhanced greenhouse gas effect is due to carbon dioxide (CO2) emissions, mainly attributed to anthropogenic activities. Various methods, such as chemical absorption in amines or in the form of gas hydrates, are being explored for the capture of CO2 and sequestration in geological media such as depleted reservoirs.1 CO2 in the form of hydrates may be used to store gas safely at reservoir conditions rather than in their pure form. In gas hydrates, the gas molecules (guests) are trapped in water cavities (host) which are bonded due to hydrogen bond between them and are formed typically at conditions of low temperatures and higher pressures.2 A more economic way of forming hydrates could be by using the thermodynamic promoters such as tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), and so forth. These compounds form semiclathrate hydrates with water which are stable sufficiently at low pressure conditions as compared to gas hydrates. TBAB in water forms a semiclathrate hydrate which shows close physical and structural properties as true clathrate hydrates. Even though promoters like tetrahydrofuran (THF) can depress hydrate formation temperature drastically, they are more volatile in nature compared to TBAB.3 © 2013 American Chemical Society

To develop a TBAB semiclathrate-based CO2 capturing process, thermodynamic equilibrium data of these semiclathrate hydrate compounds are essential. Arjmandi et al.4 presented equilibrium data for pure gas semiclathrate hydrates of CO2, nitrogen (N2), and methane (CH4) in TBAB for varying mass fractions. They showed that hydrate stability increases with the increase in TBAB concentration. Mohammadi and Richon5 studied the phase equilibrium of semiclathrate hydrates of hydrogen sulfide (H2S) and methane in 0.05 mass fraction of TBAB aqueous solution for temperature ranges of (290.9 to 295.7) K and (283.6 to 290.1) K, respectively. Mohammadi et al.6 measured semiclathrate hydrate equilibrium conditions of CO2, CH4, N2, and hydrogen (H2) in TBAB aqueous solution in a temperature range of (277.7 to 294.7) K and pressures up to 15.49 MPa using an isochoric pressure search method. It was observed that TBAB promotes hydrate formation substantially up to a certain concentration, after which it inhibits further hydrate formation. Meysel et al.7 measured the three-phase equilibrium conditions of semiclathrate hydrate formed from gas mixtures of (CO2 + N2) in TBAB. The experiments were carried out at temperature range of (281 to 290) K, pressure range of (1.9 to 5.9) MPa, and with varying mass fractions of TBAB, namely, 0.05, 0.10, and 0.20. The results show that Received: February 21, 2013 Accepted: March 18, 2013 Published: March 27, 2013 1062

dx.doi.org/10.1021/je400186b | J. Chem. Eng. Data 2013, 58, 1062−1067

Journal of Chemical & Engineering Data

Article

Figure 1. Schematic diagram of the experimental apparatus.

as requirement of high pressure, low temperature, and low formation rate of hydrates have been a major obstacle for industrial application so far. Nevertheless, the use of semiclathrate hydrate formers like TBAB that decreases the hydrate formation pressure is an important economic aspect. As there are reasonable environmental concerns for other promoters like THF, the use of TBAB could be a promising alternative.21 Salts/electrolytes such as sodium chloride (NaCl), calcium chloride (CaCl2), potassium chloride (KCl), magnesium chloride (MgCl2), and so forth, lower the activity of water in the liquid phase, thus resulting in the formation of gas hydrates at low temperature and high pressure condition (inhibition) as compared to their formation in the absence of salts. As naturally occurring water contains dissolved salts, it is essential to study the phase equilibrium of semiclathrate hydrates of CO2 in the presence of salts. Sabil et al.22 investigated the contending effects of NaCl (and other metal halides) and THF on the phase equilibrium of hydrates of quaternary system containing, CO2 + THF + NaCl + H2O. It was observed that the promoting effect of THF negate the inhibiting effect of salts. This may be due to the salting out effect of NaCl in THF hydrates, especially at lower concentration of sodium chloride. However, in comparison to gas hydrates and the effect of salts on their stability, the thermodynamic equilibrium data on semiclathrate hydrates of TBAB and other promoters and their selective studies on slats are rare and need investigations. In this work phase equilibrium data for hydrate, liquid water, and vapor (H-LW-V) for CO2 and TBAB + NaCl aqueous solution are measured for various concentrations of TBAB. A typical concentration of 0.035 and 0.10 mass fraction of NaCl in base fluid (water) is chosen so as to reflect oceanic salinity and real conditions in several applications during CO2 sequestration, natural gas storage, and inhibition of semiclathrate hydrates of gases for other potential applications.

TBAB acts as a thermodynamic promoter for the system of gaseous mixture. Belandria et al.8,9 experimentally measured the equilibrium dissociation data and the equilibrium composition of the gas phase for the system containing varying feed concentration of CO2 + N2 + TBAB semiclathrate hydrate system containing 0.05 and 0.30 mass fraction TBAB using an isochoric pressure-search method. It was observed that at a given temperature semiclathrate hydrates were formed at lower dissociation pressures than those for gas hydrates and the pressure generally decreased as the concentration of TBAB increased. It is also observed that, under reasonable operating conditions, TBAB semiclathrate hydrate can be use to separate CO2 from highly concentrated gas. Mohammadi et al.10,11 studied the semiclathrate hydrate dissociation conditions for CO2 + N2 and TBAB (0.05, 0.10, and 0.15 mass fraction) and CO2 + H2/CH4 in TBAB (with 0.05 and 0.10 mass fraction), respectively, and with varying concentrations of guest gases. The phase equilibrium measurements showed a high promotion effect of TBAB at lower concentration. Also, the operating conditions were significantly lower for semiclathrate hydrate of TBAB. Duc et al.12 conducted phase stability experiments on carbon dioxide semiclathrate hydrates for varying TBAB mass fraction from 0.0495 to 0.65 . Lin et al.13 studied the phase equilibrium and the enthalpy during dissociation of semiclathrate hydrates of carbon dioxide formed in the presence of TBAB. Li et al.14 conducted experiments for semiclathrate hydrates of carbon dioxide for 0.05 and 0.10 mass fraction of TBAB. Ye and Zhang15 studied the phase equilibrium of CO2 in TBAB aqueous solution. Though several studies on experimental investigation were carried out on phase equilibria of semiclathrate hydrate systems, models for the same have been developed very recently.16−20 All the models were tested for simple system of semiclathrate hydrate containing single gas as guest molecules. Further development on modeling aspects of semiclathrate hydrate are required and, thus, call for more reliable phase equilibrium data with complex semiclathrate hydrate system containing mixture of gases for successful practical applications.7−12 Economical aspects such

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EXPERIMENTAL DETAILS Experimental Setup and Materials. A schematic of the experimental setup used for this study is shown in Figure 1. 1063



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preflushed at least twice with CO2 at about 0.1 MPa to ensure that no air remains in the cell and also to remove dissolved gases in the solution. Finally the CO2 gas is filled up to a desired pressure. The stirrer speed is set around 400 rpm for all of the experimental runs. The three phase (H-LW-V) semiclathrate hydrate equilibrium conditions are determined using an isochoric method.7,23,24 In this method, the threephase equilibrium line is crossed by decreasing the temperature of the system while keeping the volume constant. This causes a proportional decrease in system pressure due to thermal contraction of the gas mixture. Semiclathrate hydrate formation is observed by marked pressure drop and increase in temperature (as hydrate formation is an exothermic process). After hydrate formation, the system temperature is increased slowly, and hydrates are dissociated with a slow heating rate of about 0.2 K/h to get the equilibrium conditions. The similar heating rate is used by several other researchers4,12,23 and found to be efficient in determining the correct equilibrium conditions of the hydrate system. It is to be noted that initially the gas− liquid thermodynamic equilibrium is attained outside the semiclathrate hydrate stability region before the start of the experiment. The result obtained from the isochoric method is a loop-like pressure temperature trace as shown in Figure 2 for

The core part of the experimental set up is a one liter high pressure reactor with a working pressure of 10 MPa and is made of stainless steel vessel (SS-316). The reactor is installed with pressure transducers and temperature sensor, platinum (Pt-100). A high-performance magnetic stirrer is used with the stirrer speed range of (200 to 1000) rpm to ensure adequate gas−liquid contact. The agitation speed of 400 rpm provides sufficient agitation that is needed to overcome liquid phase metastability in the region of thermodynamic hydrate stability and to reduce the mass and heat transfer resistances limiting the rate of hydrate formation and dissociation. The outer part of the reactor is jacketed to facilitate temperature control using circulation of the water−glycol mixture as a heat/cold carrier from the Julabo water bath (model FP 50). The pressure transducer is calibrated using dead weight pressure testing apparatus in the range of (1 to 7) MPa. The temperature sensor, Pt-100, is A type according to DIN 43760 and calibrated using American Society for Testing and Materials (ASTM) 1137 procedure prior use. The uncertainty of the instrument in measuring pressure and temperatures is ± 0.005 MPa and ± 0.05 K, respectively. The mass of chemicals was measured using a high-precision balance (Radwag AS-220/X) with an uncertainty of ± 0.00004 mass fraction. Carbon dioxide gas (0.995 mass fraction purity) was supplied by Bhuruka Gas Agency, Bangalore, India. The chemicals used in this work, mainly, TBAB (0.99 mass fraction purity) and NaCl (0.999 mass fraction purity), were obtained from Sisco Research Laboratories Pvt. Ltd., Mumbai and Finar Chemicals Ltd., Ahmadabad, respectively. Deionized water obtained from Millipure deionized setup is used to make TBAB solution as required. Chemicals used in present work are listed with the source and purity in Table 1. Table 1. Chemicals Used in the Present Study mass fraction purity

chemical name

source

carbon dioxide tetra-n-butylammonium bromide sodium chloride deionized water

Bhuruka Gas Agency, Bangalore Sisco Research Laboratories Pvt. Ltd., Mumbai Finar Chemicals Ltd., Ahmadabad Millipure deionized setup

0.995 0.990 0.999

Figure 2. Isochoric PT (pressure temperature) trace for the CO2 + TBAB (aqueous) mixture in the presence of inhibitor (NaCl). The region: (a-b) cooling, (b-c) formation of semiclathrate hydrates, (c-de) heating, (e-f) slow heating (0.2 K/h) region.

Experimental Procedure. The reactor is first evacuated using a vacuum pump to remove atmospheric gases present and thoroughly cleaned using deionized water before the aqueous solution of TBAB + NaCl is filled. Different concentrations of aqueous solutions used in this work are given in Table 2 (all of the mass fractions are with respect to water). The system is

CO2 + TBAB (aqueous) semiclathrates in the presence of inhibitor (NaCl) obtained in this study for the sample case of 0.20 mass fraction TBAB + 0.035 mass fraction NaCl in the aqueous system. The equilibrium dissociation point is calculated by taking the intersection between three phase (HLW-V) hydrate dissociation line and the two phase thermal contraction line of the fluid mixture (V-L). The equilibrium temperatures and pressures were determined with an expanded uncertainty of ± 0.109 K and ± 0.021 MPa, respectively. A similar procedure is adopted for different runs of the experiment carried out in this work (as in Table 2).

Table 2. Details on the Number of Experiments and Data Points N for Various Mass Fractions, w, of TBAB and NaCl in Aqueous Solution for the Range of Equilibrium Pressure, P, and, Temperature, T, Generated in This Work wTBAB 0.05

wNaCl

T/K

0.035 (279.96 to 285.4) 0.10 (280.5 to 284.12) 0.10 0.035 (283.96 to 287.7) 0.10 (283.65 to 286.92) 0.20 0.035 (284.42 to 289.4) 0.10 (284.85 to 287.57) total no. of experimental runs/data points

p/MPa

N

(0.44 to 2.8) (0.6 to 3.01) (0.82 to 3.64) (0.85 to 3.59) (0.83 to 3.94) (0.84 to 3.82)

6 5 5 5 5 5 31

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RESULTS AND DISCUSSION The results obtained by experimental studies on semiclathrate hydrates of CO2 for various concentrations of TBAB (0.05, 0.10, and 0.20 mass fraction) and NaCl (0.035 and 0.10 mass 1064



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fraction) are reported and discussed in detail. The procedure for determining the experimental equilibrium hydrate dissociation conditions is first verified for the phase equilibrium data for pure CO2 hydrates and for CO2 semiclathrate hydrates in TBAB for the 0.05 mass fraction case. The obtained results found to match well with the literature data2,4,23 and are not presented here for the sake of simplicity. The competing effect of NaCl (inhibitor) and TBAB (promoter) on CO2 hydrate phase equilibria is investigated and reported in the temperature range of (279 to 290) K and pressures up to 4 MPa. For this purpose, the measured semiclathrate equilibrium data for quaternary (H2O + CO2 + TBAB + NaCl) system are compared with the data from open literature14,23 for the ternary (H2O + CO2 + TBAB) system and are compared with experimental data reported from open literature for pure CO2 gas hydrate2 and pure CO2 hydrates in the present of inhibitor (NaCl)2 for concentrations of 0.03 and 0.10 mass fraction. Table 3 shows the three phase (H-LW-V) equilibrium data for various TBAB and NaCl mass fractions for semiclathrate hydrates of CO2. Figures 3, 4, and 5, respectively, show the comparison of the equilibrium data generated in this work with that of the data available in the literature at 0.05,

Figure 3. Experimental results for CO2 semiclathrate hydrates with 0.05 mass fraction TBAB and NaCl (0.035, 0.10 mass fraction). Experimental points: □, 0.05 mass fraction TBAB;14 ◊, pure CO2 hydrate;2 ○, CO2 + 0.03 mass fraction NaCl;2 △, CO2 + 0.10 mass fraction NaCl;2 ●, 0.05 mass fraction TBAB + 0.035 mass fraction NaCl (this work); ▲, 0.05 mass fraction TBAB + 0.10 mass fraction NaCl (this work).

Table 3. Experimental Values of Three Phase (H-LW-V) Equilibrium Pressure, P, and Temperature, T, for Various Mass Fractions, w, of TBAB and NaCl in Aqueous Solution for Semiclathrate Hydrates of CO2a wTBAB

wNaCl

T/K

p/MPa

0.05

0.035

285.4 284.6 284.1 283.1 282.0 279.9 284.1 283.4 283.2 281.6 280.5 287.7 287.2 286.4 285.3 283.9 286.9 286.2 285.7 284.9 283.6 289.4 288.2 287.3 287.4 284.4 287.5 287.4 286.6 285.7 284.8

2.80 2.24 1.84 1.40 0.89 0.44 3.01 2.48 1.83 1.18 0.60 3.64 2.82 2.14 1.56 0.82 3.59 2.68 2.14 1.40 0.85 3.94 2.89 2.14 1.49 0.83 3.82 2.94 2.04 1.42 0.84

0.10

0.10

0.035

0.10

0.20

0.035

0.10


0.10, and 0.20 mass fraction TBAB aqueous solution with and without salt, NaCl, and hydrate of CO2 gas for the purpose of validation. The shaded symbols represent the data generated in this work, and the unshaded symbols correspond to the literature data. It is evident from the figures that the experimental data generated for CO2 + TBAB + NaCl system follow a similar trend to that of the literature values.2,14,23 From Figure 3, for 0.05 mass fraction of TBAB, the inhibiting effect of salt, NaCl, is more pronounced for higher concentrations of NaCl in the system. At around 285 K, the depression in the semiclathrate hydrate formation is observed to be around 25 % for 0.035 mass fraction NaCl and 50 % for the case of 0.10 mass fraction NaCl in the system. For the case of 0.10 mass fraction TBAB (Figure 4), the depression in pressure for semiclathrate hydrates, at about 290 K, is observed to be around 15 % for 0.035 mass fraction NaCl and 40 % for the case of 0.10 mass fraction NaCl in the system. For the case of 0.20 mass fraction TBAB (Figure 5) the semiclathrate hydrate depression in

T and P were determined with an expanded uncertainty of Uc(T) = ± 0.109 K and Uc(P) = ± 0.021 MPa, respectively. a

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hydrates of CO2 in TBAB, although there is a shift in hydrate equilibrium curve toward inhibition zone compared to that of the same system in the absence of salt. The inhibition effect of salt was much pronounced at higher pressures compared to lower pressure conditions. As the concentration of NaCl is increased, the hydrate promoting effect of TBAB is found to decrease. Moreover, it is observed that the inhibition effect of NaCl is observed to be less in TBAB semiclathrate hydrates of CO2 as compared to that of hydrate formation in pure water. The study, in general, shows that the semiclathrate hydrates of CO2 are expected to be more stable than the hydrates of pure CO2 in the real environments containing salts, thus promising their use for safe CCS application.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-44-22574825. (Office) Fax: +91-44-2257-4802. Funding

Financial support from Earth System Science Organization, Ministry of Earth Sciences, Government of India, through NIOT, Chennai, India (grant: OEC/10-11/105/NIOT/JITE) and IIT Madras (grand: OEC/10-11/530/NFSC/JITE) is gratefully acknowledged.

pressure, at about 290 K, is observed to be 3 % for 0.035 mass fraction NaCl and about 30 % for 0.10 mass fraction NaCl in the system. However, the inhibiting effect of NaCl for the case of 0.035 and 0.10 mass fraction in the system for all concentrations of TBAB (0.05, 0.10, and 0.20 mass fraction) is less than 3 to 5 % in all the cases at lower pressures and temperatures. In general, as the concentration of TBAB in the system increases from 0.05 to 0.20 mass fraction, the inhibiting effect of NaCl decreases. Moreover, it is observed that the inhibition effect of NaCl is observed to be less in TBAB semiclathrate hydrates of CO2 as compared to that of hydrate formation in pure water. For the case of pure hydrates of CO2, the effect of NaCl for similar concentrations of 0.035 mass fraction and 0.10 mass fraction depresses the formation pressure of hydrate by more than 50 % for the equilibrium temperatures ranging from (270 to 280) K. Additionally, the promoting effect of TBAB is found to suppress the inhibiting effect of salt although there is a shift in the hydrate equilibrium curve toward the inhibition zone compared to that of the same system in the absence of salt. In general, the inhibition effect of salt was much pronounced at higher pressures compared to lower pressure conditions. As the concentration of NaCl is increased, the hydrate promoting effect of TBAB is found to decrease. The inhibition effect of NaCl is low for the CO2 + TBAB system compared to that of the system in the absence of TBAB. From the above discussion, the semiclathrate hydrates of CO2 are expected to be more stable than the hydrates of pure CO2 in the real environments containing salts, thus promising their use for safe CCS application.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors would like to thank the Director, National Institute of Ocean Technology (NIOT) and NIOT-IITM cell for the encouragement towards setting up the laboratory facilities.

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REFERENCES

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CONCLUSIONS In this work experiments are conducted on semiclathrate hydrates of CO2 in TBAB for 0.05, 0.10, and 0.20 mass fractions of TBAB in the presence of 0.035 and 0.10 mass fractions of NaCl. The three-phase equilibrium (H-LW-V) data are generated for the quaternary system of CO2 + TBAB + H2O + NaCl which is not available in the open literature. The competing effects of TBAB and NaCl at different concentrations on phase behavior of semiclathrate hydrate equilibrium have been studied. It is observed that the inhibiting effect of the NaCl is suppressed by the promoting effect of semiclathrate 1066



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(9) Belandria, V.; Mohammadi, A. H.; Eslamimanesh, A.; Richon, D. Compositional Analysis of the Gas Phase for the CO2 + N2 + Tetra-nbutylammonium Bromide Aqueous Solution Systems under Hydrate Stability Conditions. Chem. Eng. Sci. 2012, 84, 40−47. (10) Mohammadi, A. H.; Eslamimanesh, A.; Belandria, V.; Richon, D.; Naidoo, P.; Ramjugernath, D. Phase Equilibrium Measurements for Semi-Clathrate Hydrates of the (CO2 + N2 + Tetra-nButylammonium Bromide) Aqueous Solution System. J. Chem. Thermodyn. 2012, 46, 57−61. (11) Mohammadi, A. H.; Eslamimanesh, A.; Richon, D. Semiclathrate Hydrates Phase Equilibrium Measurements for the CO2 + H2/CH4 + Tetra-n-butylammonium Bromide Aqueous Solution System. Chem. Eng. Sci. 2013, http://dx.doi.org/10.1016/j.ces.2013. 01.063. (12) Duc, N. H.; Chauvy, F.; Herri, J. M. CO2 Capture by Hydrate Crystallizationa Potential Solution for Gas Emission of Steelmaking Industry. Energy Convers. Manage. 2007, 48, 1313−1322. (13) Lin, W.; Delahaye, A.; Fournaison, L. Phase Equilibrium and Dissociation Enthalpy for Semi-Clathrate Hydrate of CO2 + TBAB. Fluid Phase Equilib. 2008, 264, 220−227. (14) Li, S.; Fan, S.; Wang, J.; Lang, X.; Wang, Y. Semiclathrate Hydrate Phase Equilibria for CO2 in the Presence of Tetra-nbutylammonium Halide (Bromide, Chloride, or Fluoride). J. Chem. Eng. Data 2010, 55, 3212−3215. (15) Ye, N.; Zhang, P. Equilibrium Data and Morphology of Tetra-nbutylammonium Bromide Semiclathrate Hydrate with Carbon Dioxide. J. Chem. Eng. Data 2012, 57, 1557−1562. (16) Paricaud, P. Modeling the Dissociation Conditions of Salt Hydrates and Gas Semiclathrate Hydrates: Application to Lithium Bromide, Hydrogen Iodide, and Tetra-n-butylammonium Bromide + Carbon Dioxide Systems. J. Phys. Chem. B 2011, 115, 288−299. (17) Joshi, A.; Mekala, P.; Sangwai, J. S. Modeling Phase Equilibria of Semiclathrate Hydrates of CH4, CO2 and N2 in Aqueous Solution of Tetra-n-butylammonium Bromide. J. Nat. Gas Chem. 2012, 21, 459− 465. (18) Mohammadi, A. H.; Belandria, V.; Richon, D. Use of an Artificial Neural Network Algorithm to Predict Hydrate Dissociation Conditions for Hydrogen + Water and Hydrogen + Tetra-nbutylammonium Bromide + Water Systems. Chem. Eng. Sci. 2010, 65, 4302−4305. (19) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D. Thermodynamic Modeling of Phase Equilibria of Semi-clathrate Hydrates of CO2, CH4, or N2+Tetra-n-butylammonium Bromide Aqueous Solution. Chem. Eng. Sci. 2012, 81, 319−328. (20) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D. Corrigendum to “Thermodynamic Modeling of Phase Equilibria of Semi-clathrate Hydrates of CO2, CH4, or N2 + Tetra-n-butylammonium Bromide Aqueous Solution”. Chem. Eng. Sci. 2012, 84, 381. (21) Spenser, D. F.; Currier, R. P. Methods of Selectively Separating CO2 from a Multicomponent Gaseous Stream using CO2 Hydrate Promoter. U.S. Patent 6.352.576, 2002. (22) Sabil, K. M.; Romàn, V. R.; Witcamp, G. J.; Peters, C. J. Experimental Observations on the Competing Effect of Tetrahydrofuran and an Electrolyte and the Strength of Hydrate Inhibition Among Metal Halides in Mixed CO2 Hydrate Equilibria. J. Chem. Thermodyn. 2010, 42, 400−408. (23) Joshi, A.; Sangwai, J. S.; Das, K.; Sami, N. A. Experimental Investigations on the Phase Equilibrium of Semiclathrate Hydrates of Carbon Dioxide in TBAB with Small Amount of Surfactant. Int. J. Energy Environ. Eng. 2013, 4, 11. (24) Belandria, V.; Mohammadi, A. H.; Richon, D. Phase Equilibria of Clathrate Hydrates of Methane + Carbon Dioxide: New Experimental Data and Predictions. Fluid Phase Equilib. 2010, 296, 60−65.

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Phase Stability of Semiclathrate Hydrates of Carbon Dioxide in

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