Phase Equilibrium Conditions for the Double Semiclathrate Hydrate

Sep 11, 2014 - To our best knowledge, there is no study on the phase equilibrium ...... Lee , J. D.; Seo , Y. Thermodynamic and spectroscopic identifi...
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Phase Equilibrium Conditions for the Double Semiclathrate Hydrate Formed with Tetraamylammonium Chloride Plus CH4, CO2, or N2 Ling-Li Shi†,‡ and De-Qing Liang*,† †

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

ABSTRACT: Phase equilibrium conditions of the CH 4 , CO 2 , or N 2 semiclathrate hydrates formed with tetraamylammonium chloride (TAAC: C20H44NCl) aqueous solution at 0.328 mass fraction, corresponding to the stoichiometric composition of TAAC·38H2O, were experimentally measured. The experimental data were obtained in the temperature range of (279.9 to 292.8) K and pressure range of (2.11 to 16.40) MPa by employing an isochoric pressure-search method. The results showed that double semiclathrate hydrates of TAAC + CH4, TAAC + CO2, and TAAC + N2 all had an obvious advantage in thermal stability compared to the corresponding pure CH4, CO2, and N2 hydrates. In addition, some comparisons were made between our data and experimental equilibrium data from the open literature for semiclathrate hydrates formed with various semiclathrate hydrate formers.

1. INTRODUCTION Gas hydrates are a group of ice-like crystalline compounds formed by the incorporation of guest molecules of suitable size and shape, such as CH4, CO2, and N2, into host water frameworks combined through hydrogen-bond under specified conditions of low temperatures and elevated pressures.1 In the past decade, great attention has been paid to the examination of hydrate-based industrial applications in energy and environmental fields, such as the storage and transportation of natural gas,2,3 CO2 capture and storage,4,5 energy storage,6,7 and so on. However, high pressure/low temperature conditions are still the main concern about the mentioned applications. To reduce the pressure required for hydrate formation, one of the promising methods is to make use of gas hydrate promoters, such as tetrabutylammonium bromide (TBAB: C16H36NBr),8−12 tetrabutylammonium chloride (TBAC: C16H 36NCl),13−16 tetrabutylammonium fluoride (TBAF: C16H36NF),14,17−21 tetrabutylammonium nitrate (TBANO3: C16H36NNO3),22−24 and tetrabutylphosphonium bromide (TBPB: C16H36PBr).25−30 Normally, hydrates formed with these kinds of promoters are also called semiclathrate hydrates. Recently, due to their moderate phase equilibrium conditions and improved selectivity of the cages for encaging gas molecules, more attention was focused on semiclathrate hydrates-based industrial applications such as storage and transportation of natural gas, separation of gases from industrial exhaust and carbon dioxide sequestration. Actually, semiclathrate hydrates were first reported by Fowler31 in 1940 and later analyzed through detailed X-ray structural analysis by Jeffrey,32 who named them as semiclathrate hydrates (SCH). These kinds of hydrates share many of the physical and structural properties of ordinary hydrates. The primary difference is that the guests in SCH not only occupy cages © XXXX American Chemical Society

but also act as part of the cage structure while those in ordinary hydrates do not participate in forming hydrate lattices. To take the unit cell of TBAB SCH as an example, the host water− anion framework, composed of six dodecahedral (512, D) cavities, four tetrakaidecahedral cavities (51262, T), and four pentakaidecahedral cavities (51263, P), is formed by 76 water molecules and two anions (Br−).33 Two cations ((C4H9)4N+) are situated in four-compartment cavities of 2T·2P with part of the cage structure broken. Meanwhile, the empty D-cages are capable of incorporating suitable gas molecules, which permits SCH the ability of gas separation and storage under moderate conditions. To design effective and efficient processes of hydrate-based industrial applications, the phase equilibrium condition is one of the key parameters and should be obtained. Many scientific reports have shown the equilibrium conditions of SCH formed with TBAB + CH4 + H2O, TBAB + CO2 + H2O, TBAB + N2 + H2O, TBAC + CH4 + H2O, TBPB + N2 + H2O, and TBPB + CO2 + H2O. However, no such study has been reported on one SCH former, tetraamylammonium chloride (TAAC: C20H44NCl). According to the studies of Mcmullan,34 Dyadin,35 and Aladko,36 TAAC could form hydrates of TAAC·27H2O, TAAC·32H2O, and TAAC·38H2O under atmospheric pressure, with melting points of 302.65 K, 302.85 K, and 302.95 K, respectively. The third TAAC hydrate (TAAC·38H2O) is formed based on the water−anion framework of hexagonal structure I, whose unit cell is similar to TBAB hydrate and composed of 4T·4P·6D·76H2O. In addition, it is noteworthy that the melting point of TAAC·38H2O is also higher than those of many known pure SCH formed without Received: June 25, 2014 Accepted: September 2, 2014

A

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in the laboratory and TAAC are used for solution preparation with the help of a Shanghai Jingke electronic analytical balance, whose reading uncertainty is ± 0.1 mg. All the materials are used without any further purification. 2.2. Experimental Apparatus and Procedure. The experimental apparatus was designed by Sanchez Technologies Company for accurate measurements of hydrates’ phase equilibria and is shown in Figure 1. The apparatus has been described in detail in our previous works.22,23,26,38 The core of the apparatus, the equilibrium cell, is a “full view” sapphire variable-volume cell with a movable piston and a sapphire tube sealed at the top end with a stainless steel flange. It is suitable for measurements within the temperature range of (253 to 399) K and pressure up to 40 MPa. The fluid is mixed through a stirrer driven by both a DC motor located at the end of the piston and a magnetic coupling mounted outside the cell. The stirring rate was maintained at 180 rpm. All of the temperature data, measured by a platinum resistance thermometer with an uncertainty of ± 0.1 K, and pressure data, measured by a TF01 400A absolute pressure transducer with an uncertainty of ± 0.024 MPa, are collected and saved at preset sampling intervals on a computer. The measurement was performed by employing an isochoric pressure-search method. Prior to each test, the cell was thoroughly cleaned three times with deionized water and the aqueous solution, respectively. Then the cell was evacuated and approximately 20 mL of TAAC (0.328 mass fraction) aqueous solution was introduced into the vacuum cell by an injector. After being pressurized up to the desired pressure with CH4, CO2, or N2, the cell was cooled to form hydrates, which could be determined by an abrupt pressure depression and visual observation. Then the cell was heated very gradually and step by step (0.1 K each), and each temperature was maintained for at least 6 h to achieve a steady equilibrium state in the cell. During the process, the temperature and pressure were

gases, such as TBAB·38H2O, TBAC·38H2O, and TBAF· 38H2O, whose melting points under atmospheric pressure are 285.8 K, 288.3 K, and 300.75 K, respectively.21,36,37 It indicates that TAAC has the potential ability to enlarge the stable region of gas hydrates by a larger scale. To our best knowledge, there is no study on the phase equilibrium conditions for TAAC + CH4, + CO2, or + N2 hydrates, which can provide valuable information for the design of effective industrial processes and optimization of thermodynamic models. In the present work, we systematically measured the equilibrium conditions for the double SCH formed with CH4, CO2, or N2 in the presence of TAAC (0.328 mass fraction) aqueous solution. The range of temperature and pressure for the measurement are from (279.9 to 292.8) K and (2.11 to 16.40) MPa, respectively.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Table 1 lists the purities and suppliers of materials used in this work. Deionized water made Table 1. List of the Materials Used for the Experiments chemical tetraamylammonium chloride (TAAC: C20H44NCl) CH4

purity

unit

> 0.98

mass fraction

0.999

mole fraction

CO2

0.9999

mole fraction

N2

0.99999

mole fraction

deionized water

18

MΩ (resistivity)

supplier Tokyo Chemical Industry Co., Ltd. Guangzhou Yigas Gases Co., Ltd. Guangzhou Shiyuan Gases Co., Ltd. Guangzhou Shiyuan Gases Co., Ltd. Laboratory-made

Figure 1. Schematic diagram of the experimental apparatus. DPT, differential pressure transducer; RTD, resistance temperature detector; GC, gas chromatography; V1 to V9, valves. B

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measured continuously and plotted. Eventually, the full dissociation point of the hydrate was determined by the sharp change in the slope of the plotted curve. An entire experiment required approximately 5 days, and the procedure was repeated to obtain the hydrate equilibrium conditions.

3. RESULTS AND DISCUSSION Phase equilibrium data of double TAAC + CH4, TBAC + CO2, and TAAC + N2 hydrates measured at w = 0.328, where w denotes TAAC mass fraction, are summarized in Table 2 and plotted with equilibrium data of pure CH4,39,40 CO2,40−42 and N243−45 hydrates from the open literature in Figures 2 to 4, respectively. Table 2. Equilibrium Data of Semiclathrate Hydrates for the TAAC Aqueous Solution (0.328 Mass Fraction) + CH4, + CO2, or + N2 Systemsa system

T/K

P/MPa

TAAC + CH4 + H2O

287.4 288.9 290.2 291.2 292.1 292.8 282.3 282.9 283.7 284.4 284.7 284.9 279.9 281.4 282.6 283.9 284.8 285.6

5.97 7.85 9.86 11.74 13.92 15.86 2.11 2.42 2.92 3.53 3.85 4.29 6.21 8.15 10.15 12.10 14.03 16.40

TAAC + CO2 + H2O

TAAC + N2 + H2O

Figure 2. Phase equilibrium data for various systems of CH4 + H2O + semiclathrate hydrate formers: ●, pure CH4 hydrate, ref 40; ◆, pure CH4 hydrate, ref 39; ■, TAAC (TAAC·38H2O, w = 0.328), this work; △, TBPB (TBPB·35H2O, w = 0.35), ref 28; ○, TBAB (w = 0.385), ref 12; +, TBAC (TBAC·30H2O, w = 0.3396), ref 13; ▽, TBAF (TBAF· 29.7H2O, w = 0.328), ref 19.

a Uncertainties u are u(w) = ± 0.002, u(T) = ± 0.1 K, u(P) = ± 0.024 MPa.

As shown in Figure 2, TAAC, acting as the SCH former, significantly decreases the CH4 hydrate equilibrium pressures by about (3.00 to 4.60) MPa, which is up to 40 % when compared with the pure CH4 hydrate equilibrium pressure at the same experimental temperature.40 It leads the hydrate equilibrium conditions to a more stabilized region, represented by lower pressures and higher temperatures. In Figure 3, the stable region of pure CO2 hydrates is also enlarged by TAAC. For example, the equilibrium temperature of TAAC + CO2 hydrate is 282.9 K at 2.42 MPa while that of pure CO2 hydrate is 278.9 K at 2.42 MPa. Figure 4 presented the equilibrium conditions for N2 hydrates formed with TAAC. It is observed that TAAC increases equilibrium temperatures of pure N2 hydrates consistently by about 12.0 K. For example, the equilibrium temperature of TAAC + N2 hydrate is 285.6 K at 16.40 MPa, which is obviously higher than that of pure N2 hydrate (273.2 K at 16.31 MPa). The reason for the stabilization effect shown in these three systems might be that the addition of TAAC changed the hydrate structures to SCH and increased the total amount of encaged guest species. The cation is located in four-compartment cavities, and gas molecules are trapped in small D cages. Thus, relatively more cavities are filled with guest species in (TAAC + CH4/CO2/N2

Figure 3. Phase equilibrium data for various systems of CO2 + H2O + semiclathrate hydrate formers: ●, pure CO2 hydrate, ref 41; ◆, pure CO2 hydrate, ref 42; ▲, pure CO2 hydrate, ref 40; ■, TAAC (TAAC· 38H2O, w = 0.328), this work; △, TBPB (TBPB·32H2O, w = 0.371), ref 26; ○, TBAB (TBAB·26H2O, w = 0.4), ref 10; +, TBAC (TBAC· 30H2O, w = 0.34), ref 47; ▽, TBAF (TBAF·29.7H2O, w = 0.328), ref 19; ×, TiAAB (w = 0.1403), ref 48.

+ H2O) systems than those in (CH4/CO2/N2 + H2O) systems. Since hydrate stability is partly dependent on the fractional occupancy of hydrate lattices,46 the thermodynamic stability is increased. This phenomenon has also been observed and studied in many double semiclathrate hydrate systems, such as double CH4 + TBAB hydrates. Figures 2 to 4 also show comparisons between the phase equilibrium data generated in this study and those from open literature for gas hydrates formed in the presence of various SCH formers, such as TBPB,26,28 TBAB,10,12,18 TBAC,13,47 TBAF,18,19 and TiAAB.48 As shown, all of the SCH formers have substantially lowered the pure gas hydrate formation pressures, indicating that they all have similar stabilization effects on gas hydrates. The stronger the stabilization effect is, C

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Nevertheless, TAAC hydrates, capable of stabilizing hydrates of CH4, CO2, and N2, are potentially applicable in gas storage and separation processes.

4. CONCLUSIONS The three phase equilibria of the double TAAC semiclathrate hydrates with a guest gas (CH4, CO 2 , or N 2 ) were experimentally measured to determine the stability conditions. The experiments were conducted by employing an isochoric pressure-search method at temperatures from (279.9 to 292.8) K and pressures from (2.11 to 16.40) MPa with TAAC aqueous solution of 0.328 mass fraction, corresponding to TAAC· 38H2O. The results showed that the additive of TAAC could enlarge the stable region of pure gas hydrates, and its stabilization effect was weaker than that of TBAB, TBPB, TBAC, and TBAF. The data generated in this work could provide valuable information for gas storage and separation processes.

■

Figure 4. Phase equilibrium data for various systems of N2 + H2O + semiclathrate hydrate formers: ●, pure N2 hydrate, ref 43; ◆, pure N2 hydrate, ref 44; ▲, pure N2 hydrate, ref 45; ■, TAAC (TAAC·38H2O, w = 0.328), this work; △, TBPB (TBPB·32H2O, w = 0.371), ref 26; ○, TBAB (TBAB·26H2O, w = 0.4), ref 18; +, TBAC (TBAC·30H2O, w = 0.34), ref 47; ▽, TBAF (TBAF·28.6H2O, w = 0.3366), ref 18; ×, TiAAB (w = 0.1403), ref 48.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 20 8705 7669. Fax: +86 20 8705 7669. Funding

the lower are the values to which the SCH formers decrease the equilibrium pressures. Among them, the effect of TAAC is the weakest and the strong-to-weak order of their stabilization effects is TiAAB, TBAF, TBAC, TBPB, TBAB, and TAAC. The stabilization effect is mainly due to the hydrogen bonding interaction between the cation and anion in the special structure of SCH. The main difference between TAAC and other chemicals is the cation. For TAAC and TiAAB, their cations are (C5H11)4N+. For TBPB, its cation is (C4H9)4P+. And for other chemicals, their cations are (C4H9)4N+. In addition, for the tetrabutylammonium series, the strong-toweak order is the same as the order of halogen group elements (F, Cl, and Br). Thus, the cation is assumed to have greater effect on the trend of stabilization effects, and the anion has minor impact. It is noteworthy that the strong-to-weak order of stabilization effect is different from the high-to-low order of their melting temperatures under atmospheric pressure, which is TAAC, TiAAB, TBAF, TBAC, TBAB, and TBPB.21,30,36,37 Hydrates of tetraamylammnonium halides are reported to melt at higher temperatures compared to those of the tetrabutylammonium series.36 It was reported that the melting temperatures of SCH without gases showed the strength of hydrogen bonding interaction between the cation and the anion, and they also presented the potential ability of SCH formers to enlarge stable regions of gas hydrates. In SCH, the hydrogen-bonding interaction between the cation and the anion is stronger than van der Waals forces between guest gas and host water lattice. Thus, the equilibrium conditions of the double SCH with guest gases are supposed to be similar to those of pure SCH without guest gases. In our study, the results are not consistent with this assumption. We suppose that the difference of TAAC in the two orders might be caused by three factors in the SCH system with guest gases: (1) partial filling of the small cavities in the host lattice with “guest” water molecules, (2) the decrease of the activity of TAAC in the liquid phase, and (3) weakness of the van der Waals forces between the host and guest. This assumption still needs more study, and the reason is the subject of future study.

This work was supported by the National Natural Science Foundation of China (51376182), CAS Program (KGZD-EW301), 863 Program (2012AA061403−03). Notes

The authors declare no competing financial interest.

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