Effect of Carbon Chain Length of Organic Salts on the

(5) These have included tetra-n-butyl ammonium salts: tetra-n-butyl ammonium bromide (TBAB: (C4H9)4NBr);(20, 27-29) tetra-n-butyl ammonium chloride (T...
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Effect of Carbon Chain Length of Organic Salts on the Thermodynamic Stability of Methane Hydrate Yuan Su,†,‡ Stefano Bernardi,‡ Debra J. Searles,‡,§ and Liguang Wang*,† †

School of Chemical Engineering, The University of Queensland, Brisbane, Qld. 4072, Australia Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld. 4072, Australia § School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Qld. 4072, Australia ‡

ABSTRACT: This study presents the phase equilibrium conditions for methane hydrate with one of the following organic ammonium salts differing in carbon chain length: tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB), tetrapropylammonium bromide (TPrAB), tetrabutylammonium bromide (TBAB), and tetrapentylammonium bromide (TPeAB). The hydrate phase equilibrium measurements were conducted for a temperature range of 278.94−291.85 K and pressure range of 4.79−14.32 MPa using the step-heating pressure search method. The addition of TBAB or TPeAB shifts the phase equilibria of the semiclathrate hydrates (SCHs) of CH4 to a lower pressure and higher temperature zone. At a given temperature, increasing the mole fraction of TBAB and TPeAB from 0.294 mol % to 0.620 mol % made the shift in phase equilibrium conditions greater. At a given dosage, TBAB consistently outperformed TPeAB in thermodynamically promoting methane hydrate formation. TMAB, TEAB, or TPrAB slightly shifts the phase equilibrium conditions to a higher pressure and lower temperature region. We analyzed the hydrate phase equilibrium data for TMAB, TEAB, and TPrAB using the colligative property equation and compared them with the phase equilibrium data of a CH4 and salt water system. The results suggest that these three organic salts have a small hydrate inhibiting effect that is comparable to NaCl. Promotion of the formation of CH4 hydrate by TBAB and TPeAB indicates that these additives provide a means to store CH4 at moderate pressure conditions, which could lower the cost of pressure reduction in hydrate formation. In contrast, TMAB, TEAB, and TPrAB could be used for prevention of formation of hydrates in systems where the use of NaCl is unsuitable.

1. INTRODUCTION Clathrate hydrates are solid crystalline compounds mainly formed by hydrogen bonding of water molecules that, under favorable conditions of low temperature and elevated pressure, are stabilized by appropriate gas guest molecules such as CO2, N2, CH4, C2H6, C3H8, H2, etc., or organic compounds such as acetone and tetrahydrofuran.1−8 In accordance with the different sizes and shapes of the cavities within the clathrate lattice, there are three crystallographic hydrate structures: two cubic structures I (sI) and II (sII) and a hexagonal structure H (sH). Structure I and structure II are both commonly identified during the production and processing of oil and gas.9 The hydrate structures are comprised of different combinations of five polyhedral cavities: pentagonal dodecahedron (512), tetrakaidecahedron (51262), hexakaidecahedron (51264), irregular dodecahedron (435663), and icosahedron (51268).2 Methane forms sI hydrate which consists of 2 small cages (512) and 6 large cages (51262).2,9 The capability of engaging diverse gas molecules with specific sizes by gas hydrates is considered a promising technique for practical applications such as gas storage,10−12 transportation,13,14 and separation from gas mixtures.4,15,16 Nevertheless, the hydrate-based technology © 2016 American Chemical Society

enhancement for gas processing has been hampered by their slow formation kinetics and the requirement of elevated pressures in order to attain hydrate formation.17−19 To reduce the phase equilibrium pressures required to form solid gas hydrates for hydrate-based gas storage, utilization of different chemical additives as thermodynamic promoters (e.g., tetrahydrofuran,2 tetra-n-butyl ammonium bromide,20 cyclopentane, 21 etc.), is one of the most promising options.5,19,20,22−26 Recently, a number of organic quaternary ammonium salts have been investigated for the formation of semiclathrate hydrates (SCHs) with various gases under relatively low pressures and high temperatures.5 These have included tetra-n-butyl ammonium salts: tetra-n-butyl ammonium bromide (TBAB: (C4H9)4NBr);20,27−29 tetra-n-butyl ammonium chloride (TBAC: (C4H9)4NCl);30−32 tetra-n-butyl ammonium fluoride (TBAF: (C4H9)4NF)6 and tetra-n-butyl ammonium hydroxide (TBAOH: (C4H9)4NOH)33 as well as organic phosphonium salts: tetra-n-butyl phosphonium broReceived: March 1, 2016 Accepted: April 8, 2016 Published: April 20, 2016 1952

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mide (TBPB: (C4H9)4PBr)34,35 and tri-n-butylphosphine oxide (TBPO: (C4H9)3OP).19 SCHs and ordinary clathrate hydrates share similarities in morphological, physical, and structural aspects.8 In the absence of guest gas molecules, some of these organic salts can form unusual clathrate hydrate structures involving a water−anion framework with vacant dodecahedral voids and large cavities accommodating the cations with covalently bonded alkyl chains at atmospheric pressure,6,33,36−40 which are unlike conventional hydrates.41 For instance TBAB SCH, which has a hydration number of 26, contains 10 dodecahedron, 16 tetrakaidecahedron, and 4 pentakaidecahedron cages.42 The large (tetrakaidecahedron and pentakaidecahedron) cavities contain the cations (TBA+) of TBAB, whereas small (dodecahedron) voids remain unoccupied. Therefore, these small dodecahedral (512) frameworks are potentially capable of selectively encapsulating diverse guest gases (or auxiliary gases), depending on gas molecule sizes.6,38,41,43 As mentioned above, the SCHs do not form the same well-defined clathrate hydrate structures as common gas hydrates, because they contain elements that not only occupy the clathrate cages as guest molecules, but also participate in establishing the hydrogen-bonding network of the water molecules.41,42 For instance, the cations (TBA+) of numerous tetra-alkylammonium salts are basically embedded in SCH crystal cavities, while the halide anions of the incorporated salts, such as Cl− and Br−, substitute water molecules of the hydrate crystal structure.8,18,23,44 Shimada et al.26 demonstrated that TBAB SCH could be formed from water molecules under ambient pressure and room temperature, and their X-ray single-crystal analysis revealed that TBAB hydrate has small unoccupied dodecahedral voids, which can potentially store small gas molecules, such as CH4, CO2, and N2. The phase equilibrium data of Arjmandi et al.27 indicated that, under relatively low pressures, TBAB stabilizes the SCHs containing various gases more effectively than typical clathrate hydrates (sI or sII). TBAB is one of the commonly known quaternary ammonium salts, and it has been recognized as a favorable hydrate promoter that can enlarge the gas hydrate stability zone.26 Note that TBAB, TBAF, and TBAC have the same carbon chain length, but different tetraalkylammonium halides, Br−, F−, and Cl−. These organic salts have the same water-structure-forming cation (RA+) that has a certain degree of hydrophobicity.45 It has been suggested that the hydrophobic nature of this specific structure might trigger water molecules to arrange orderly around the dissolved solutes.45−48 Aladko et al.49 also found that the hydrophobic guest molecules with different sizes and shapes, such as CH4, Xe, and H2,2 had different clathrate-making capabilities, which means that the replacement of the cation leads to changes not only in the SCH’s stability but also in the clathrate crystal structure and hydrate composition. The binary (two-component) system of tetramethylammonium bromide (TMAB) + water was found to have two phases in the stable zone: ice and TMAB salt and to be lacking stable hydrates. However, it formed three TMAB hydrates in the metastable area at different concentrations with different melting points.50 The tetraethylammonium bromide (TEAB) + water binary system was investigated by Aladko,51 and differential thermal analysis showed that the TEAB hydrate had an analogous crystalline structure to tetraethylammonium chloride (TEAC). In addition, the cation (Et4N+) of TEAB was presumed to embed into the networks formed by hydrogen bonding of water molecules with unoccupied 4258 cavities.

According to the results from Aladko,52 tetrapropylammonium bromide (TPrAB) can form a hydrate with water at atmospheric pressure, having a dissociation temperature around 273.15 K, and the hydrate might involve small vacant cavities which guest gases with appropriate sizes could fit in, whereas little equilibrium data of TPrAB + H2O + gas (e.g., CH4) SCH has been reported. Tetrapentylammonium bromide (TPeAB) has been used as a hydrate inhibition synergist, cooperating with polyvinylcaprolactam to efficiently hinder the growth of tetrahydrofuran hydrate.53 Aladko et al.49 deemed that the melting points of tetra-n-amylammonium hydrates were noticeably lower than the corresponding halide salts with butyl and isoamyl radical. As reported by Aladko et al.,49 tetraisopentylammonium bromide (TiPeAB) or tetraisoamylammonium bromide (TiAAB) could form a clathrate crystal structure with water, and the dissociation point of (iC5H11)4NBr·38H2O hydrate is 301.45 K.49,54,55 The melting points of the mentioned tetraalkylammonium halide hydrates, TBAF·28.6H2O (300.75 K), TBAC·30H2O (288.19 K), and TBAB·38H2O (285.8 K), are relatively lower than TiPeAB· 38H2O hydrate (301.45 K). However, TiPeAB needs to be synthesized via Menshutkin’s reaction using triisopentylamine and isopentyl bromide, refluxed in acetonitrile for more than 24 h,54 which is more costly and time-consuming than the preparation of TPeAB. Shi and Liang56 measured the phase equilibria of tetraamylammonium chloride (TAAC: (C5H11)4NCl) + CH4 and stated that TAAC + CH4 SCH could form at much lower pressures than pure CH4 hydrate, which suggests that TAAC is promising for hydrate-based capture of CH4 from gas mixtures under appropriate thermodynamic conditions. Recently, Shi and Liang8 reported the phase equilibria of tetraamylammonium bromide (TAAB: (C5H11)4NBr) or tetrapentylammonium bromide (TPeAB: (C5H11)4NBr) + CH4 SCH, which provided a quantitative analysis and fundamental insight for understanding TPeAB double SCH. Therefore, TPeAB is supposed to be a promising hydrate promoter and can efficiently accelerate hydrate formation for industrial applications. No systematic research has been done into investigating the effect of carbon chain length in tetraalkylammonium salts on hydrate formation. In the present work, we studied four lesserknown organic ammonium salts, tetramethylammonium bromide (TMAB: (CH3)4NBr), tetraethylammonium bromide (TEAB: (C 2 H 5 ) 4 NBr), tetrapropylammonium bromide (TPrAB: (C3H7)4NBr), and tetrapentylammonium bromide (TPeAB: (C5H11)4NBr), and compared their results with that of TBAB. The results provide important data indicating that if a quaternary ammonium salt with an appropriate carbon chain length is added to the system, the pressure required to form hydrates can be reduced. This could lead to substantial reductions in the cost and time required for hydrate formation, thereby advancing the development of hydrate-based gas processing technologies. The results also provide insight into the effects of quaternary salts on clathrate hydrate formation and decomposition.

2. EXPERIMENTAL SECTION 2.1. Experimental Materials. TMAB with 98% (mass fraction) purity, TEAB with 98% (mass fraction) purity, TPrAB with 99% (mass fraction) purity, TBAB with 99% (mass fraction) purity, and TPeAB with 99% (mass fraction) purity were purchased from Sigma-Aldrich. Figure 1 shows the molecular structures of these ammonium salts. TMAB, TEAB, 1953

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The reactor was washed with deionized water seven times and dried out ahead of being loaded with TMAB, TEAB, TBAB, TPrAB, or TPeAB aqueous solution. Thirty grams of a salt solution with the composition listed in Table 1 was Figure 1. Schematic of the molecular structures of tetraalkylammonium bromides (TMAB, TEAB, TPrAB, TBAB, and TPeAB).

Table 1. Compositions of Aqueous Salt Solutionsa

and TPrAB have shorter hydrophobic tails than TBAB ((C4H9)4NBr), whereas TPeAB has longer hydrophobic tails. Deionized water (DI) was used to prepare the aqueous solutions of each additive. CH4 gas (99.995%) was supplied by Coregas. All of these materials were used as received. 2.2. Experimental Apparatus and Procedure. The experimental testing system is shown in Figure 2, and the

composition (mol %)

a

composition (wt %)

molality (mol·kg−1 solvent)

sample

salt

water

salt

water

solution

TMAB TEAB TPrAB TBAB TPeAB TBAB TPeAB

0.620 0.620 0.620 0.620 0.620 0.294 0.294

99.380 99.380 99.380 99.380 99.380 99.706 99.706

5.07 6.79 8.45 10.05 11.60 5.01 5.84

94.93 93.21 91.55 89.95 88.40 94.99 94.16

0.347 0.347 0.347 0.347 0.347 0.163 0.163

Uncertainties u are u(x) = ± 0.001 mol %.

weighed by an electrical balance with a reading uncertainty of ±0.01 g (concentration uncertainty = ±0.001 mol %) and then poured into the reactor. A vacuum pump was applied to the whole system, except for the reactor, for 5−10 min. Then the reactor with the testing solution was degassed for approximately 60 s prior to commencing the hydrate experiments. The influence caused by vacuuming on the concentration of the testing solution was considered negligible. The measurements of phase equilibria of gas hydrates were performed for the temperature range of 278.94−291.85 K and pressure range of 4.79−14.32 MPa using the isochoric equilibrium step-heating pressure search method.57−66 The hydrate dissociation point was determined from the pressure−temperature trace following the experimental procedure described in ref 19. Figure 3 exhibits a typical pressure−temperature trace for the SCH of 0.62 mol % TPrAB + CH4) obtained in the present study, from which the hydrate dissociation point was determined.2,64 Figure 2. Schematic diagram of the high pressure apparatus for phase equilibrium measurements, modified from Du and Wang.19

details have been given in our previous study.34 Briefly, the current measurements of gas hydrate phase equilibria were carried out using a homemade nonvisual 102 mL gas hydrate reactor, which is a stainless steel cylindrical vessel. The internal diameter and internal depth of the reactor are 38 mm and 90 mm, respectively. The reactor was submerged within a glycol− water bath with the temperature controlled precisely at 0.01 K. The liquid bath was enclosed with two layers made of thermal insulating materials to guarantee that the temperature stability of the constant-temperature steps maintained at 0.1 K. A resistance temperature detector (RTD) sensor, which was jacketed by a matched thermowell, was inserted a distance equal to three-quarters of the depth of the reactor, and the accuracy of the temperature of hydrate phase or aqueous solution measurement was ±0.03 K. The measurements of the pressure of the gas phase inside the reactor were accomplished by a pressure transducer which has measurement uncertainty of ±0.01 MPa. To agitate the tested liquid adequately, a magnetically driven stirrer was applied at a rotation speed of 600 rpm (equal to a Reynolds number of ∼7000) to achieve a sufficient mixing. A gas booster was utilized to pressurize the feeding gas from gas cylinder and feed it into the reactor. A data acquisition system was used to collect experimental data at the interval of 10 s.

Figure 3. Typical pressure−temperature trace for determination of hydrate dissociation point of methane gas (0.62 mol % TPrAB aqueous solution + CH4).

The phase equilibria measurements were carried out for the SCHs of CH4 in the aqueous solutions of TMAB (0.62 mol %), TEAB (0.62 mol %), TPrAB (0.62 mol %), TBAB (0.294 mol % and 0.62 mol %), and TPeAB (0.294 mol % and 0.62 mol %). The considerations leading to selection of the above concentrations include (i) low-dosage hydrate promoters/ 1954

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inhibitors attract special attention in industrial/technological applications; (ii) a reliable concentration of aqueous solution of quaternary ammonium salts can be achieved. When the mole fraction of TPeAB exceeds 0.62 mol %, TPeAB powder would only partly dissolve in the water like tiny oil-drops due to the solubility and miscibility.45,67

3. RESULTS First, the accuracy and reliability of the current experimental apparatus and constant-volume procedure were confirmed via the comparison of hydrate phase equilibria for (i) 0.62 mol % TBAB solution + CH4 and (ii) 0.294 mol % TBAB solution + CH4 between the present work and the corresponding data from the literature.19,20,27,29,61,64,68 The phase equilibrium conditions measured in the present work for 0.62 mol % TBAB solution + CH4 semiclathrate hydrate are shown in Table 2 and Figure 4. The phase equilibrium data of 0.294 mol

Figure 4. Phase equilibrium data for 0.62 mol % (10 wt %) TBAB + CH4 semiclathrate hydrate: ●, this work (isochoric equilibrium stepheating pressure search method); light blue ◇, Du and Wang19 (the same method); green △, Sun and Sun20 (the same method); blue □, Arjmandi et al.27 (the same method); red ○, Mohammadi et al.64 (the same method).

Table 2. Phase Equilibrium Data of CH4 SCHs Formed with TMAB, TEAB, TPrAB, TBAB, or TPeAB Aqueous Solution at 0.62 mol % (x)a system

T/K

P/MPa

TMAB

287.80 286.45 284.59 282.64 278.94 287.67 286.10 284.29 282.65 279.29 287.24 286.22 284.31 282.15 279.41 291.85 291.04 289.95 291.72 290.43 289.07 287.63 285.92

13.31 11.60 9.60 7.66 5.33 14.05 11.55 9.58 7.64 5.61 13.52 11.67 9.60 7.56 5.66 9.70 8.08 5.93 14.12 11.75 9.69 7.69 5.73

TEAB

TPrAB

TBAB

TPeAB

Table 3. Phase Equilibrium Data of CH4 SCHs Formed with TBAB or TPeAB Aqueous Solution at 0.294 mol % (x)a system

T/K

P/MPa

TBAB

290.28 288.77 287.11 289.89 289.22 287.72 286.42 285.35

10.71 7.40 4.79 14.32 12.66 10.68 8.71 6.80

TPeAB

Uncertainties u are u(x) = ±0.001 mol %, u(T) = ±0.03 K, u(P) = ±0.01 MPa. a

a Uncertainties u are u(x) = ±0.001 mol %, u(T) = ±0.03 K, u(P) = ±0.01 MPa.

% TBAB solution + CH4 semiclathrate hydrate are shown in Table 3 and Figure 5. Our phase equilibrium data are in good agreement with the literature, confirming that our experimental setup and procedure has met the necessary requirements for determining the hydrate phase equilibria. We measured the phase equilibrium conditions of CH4 + TMAB/TEAB/TPrAB/TBAB/TPeAB + H2O aqueous solution systems to compare the hydrate stabilization effects of TMAB, TEAB, TPrAB, TBAB, and TPeAB at x = (0.620 mol %). The results for x = 0.62 mol % are summarized in Table 2 and plotted in Figure 6. Also plotted in Figure 6 is a curve representing the polynomial fit to the experimental data of pure CH 4 hydrate dissociation conditions reported in the literature.2,29,69−71 As shown, at a given TMAB/TEAB/

Figure 5. Phase equilibrium data for 5 wt % (0.294 mol %) TBAB + CH4 semiclathrate hydrate: ●, this work (isochoric equilibrium stepheating pressure search method); blue ×, Sun and Sun20 (the same method); yellow × with vertical line, Sangwai and Oellrich29 (the same method); light blue ◇, Mohammadi and Richon;61 green □, Mohammadi et al.64 (the same method).

TPrAB/TBAB/TPeAB concentration, the equilibrium pressure increased steadily with the rising temperature. The hydrate stability zone of 0.62 mol % TBAB solution + CH4 was 1955

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Figure 7. Experimental phase equilibrium data for three systems: − , pure water + CH4, polynomial fit to data from Sloan and Koh,2 Sangwai and Oellrich,29 Maekawa and Imai,69 Nixdorf and Oellrich,70 and Gayet et al.;71 green ▲, 0.294 mol % TBAB solution + CH4, this work; red ◆, 0.294 mol % TPeAB solution + CH4, this work.

Figure 6. Experimental phase equilibrium data for six systems: −, pure water + CH4, polynomial fit to data from Sloan and Koh,2 Sangwai and Oellrich,29 Maekawa and Imai,69 Nixdorf and Oellrich,70 and Gayet et al.;71 light blue ▶, 0.62 mol % TMAB solution + CH4, this work; green ■, 0.62 mol % TEAB solution + CH4, this work; red ●, 0.62 mol % TPrAB solution + CH4, this work; yellow ▲, 0.62 mol % TBAB solution + CH4, this work; blue ◆, 0.62 mol % TPeAB solution + CH4, this work.

substantially larger than that of 0.62 mol % TPeAB solution + CH4, which in turn was considerably larger than that of 0.62 mol % TMAB/TEAB/TPrAB solution + CH4. Compared to CH4 + H2O hydrate, addition of TMAB/TEAB/TPrAB slightly shifted the phase equilibrium curve to the upper left region, where the temperature is lower and the pressure is higher, suggesting that TMAB, TEAB, and TPrAB act as hydrate inhibitors. By contrast, the curve of 0.62 mol % TPeAB solution + CH4 is below that of pure CH4 hydrate but is above that of 0.62 mol % TBAB solution + CH4, which demonstrates that TPeAB can promote CH4 hydrate formation, but to a less extent than TBAB. We also measured the hydrate dissociation conditions of TBAB and TPeAB at a lower concentration to compare the thermodynamic stability conditions, and the results of 0.294 mol % TBAB + CH4 and 0.294 mol % TPeAB + CH4 SCHs are shown in Table 3 and Figure 7. As shown, the stability area of TBAB SCH is larger than that of TPeAB SCH, confirming that the promotion effect of TPeAB on CH4 hydrate formation is weaker than that of TBAB.

Figure 8. Comparison of experimental phase equilibrium data between TBAB and TPeAB at different concentrations: −, pure water + CH4, polynomial fit to data from Sloan and Koh,2 Sangwai and Oellrich,29 Maekawa and Imai,69 Nixdorf and Oellrich,70 and Gayet et al.;71 red ▲, 0.294 mol % TPeAB solution + CH4, this work; green ●, 0.62 mol % TPeAB solution + CH4, this work; orange ×, 0.294 mol % TBAB solution + CH4, this work; light blue × with vertical line, 0.62 mol % TBAB solution + CH4, this work.

equilibrium pressure of SCH with 0.294 mol % TBAB was even lower than that of SCH with 0.62 mol % TPeAB, suggesting that TBAB is a more effective hydrate promoter than TPeAB. Possible reasons for this phenomenon include: (a) different strength of the guest−host interaction; (b) different extent of reduced activity of salts in the aqueous solution; (c) the generation of distorted unusual water cages caused by larger guest molecules; and/or (d) difference in the amount of water molecules partially occupying the empty lattices within the host cavities as guests.8 Norland and Kelland72 reported that one of the alkyl groups in TPeAB perforated a 51264 cage on the surface of a type II hydrate clathrate structure, and two other pentyl groups in TPeAB lay in frameworks on the surface of hydrate where normally the further 51264 cavities should be built up, implying that these cavities were only partially established, and the pentyl groups in TPeAB were trapped or embedded in the unusual cages on hydrate surface. Norland and Kelland72 also found that, when the size of the hydrate

4. DISCUSSION 4.1. Hydrate Promoters (TBAB and TPeAB). The thermodynamic promotion effects of TBAB and TPeAB were influenced by their aqueous solution concentrations. Table 2 and Figure 8 show that increasing the concentrations of TPeAB and TBAB from 0.294 mol % to 0.62 mol % would enlarge the SCH stability zone. At a given temperature, the equilibrium pressure at which melting of the hydrate occurred became lower with an increase in the concentration of the solution. At 287.63 K, for example, the phase equilibrium pressure decreased by 2.99 MPa from 10.68 MPa for 0.294 mol % TPeAB solution to 7.69 MPa for 0.62 mol % TPeAB solution. A similar observation was made for TBAB SCH. At 290 K, the phase equilibrium pressure reduced by 4.8 MPa, from 10.7 MPa for 0.294 mol % TBAB solution to 5.9 MPa for 0.62 mol % TBAB solution. Note that, at a given temperature, the phase 1956

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particle was smaller than the critical nucleation size, the nuclei growth of hydrate could be energetically inadequate, and then TPeAB would detach relatively easily from the surface of the hydrate nuclei, rather than embed into the surface. When larger than the critical nucleation size, TPeAB molecules were probably trapped on the hydrate surface, and then incomplete hydrate cavities constructed encircling the pentyl groups. However, an obstruction would be produced to the new growth of a structure II hydrate owing to the remaining pentyl groups53,72,73 because the pentyl groups in TPeAB are relatively long and incapable of generating strong van der Waals forces with the hydrate surface cavities. This would result in distortion of the hydrate cages and disruption of the formation of new cages, with detachment from the surface occurring easily.74,76 Norland and Kelland72 reported that modeling studies found that tetrahexylammonimum bromide (THAB), which has six carbons in each carbon chain, could not easily become embedded in the surface of structure I or II hydrate because it has long alky groups which reduce its capacity to have strong van der Waals attractions to stabilize the hydrate structure. These modeling studies shed light on understanding the structure/function relationship for designing better thermodynamic promoters for methane hydrate. An effect of carbon chain length is also found in CH4 + H2O + TAAC and CH4 + H2O + TBAC systems. At a given concentration (35 wt % TAAC56 and 33.96 wt % TBAC56,75), TBAC has promoted formation of CH4 hydrate more than TAAC, which is consistent with the trend that we observed in the present study. We also compared our measured phase equilibrium data with those of CH4 + H2O + TBAC30,77 and CH4 + H2O + TBAF6,78 systems based on mass fraction (see Figure 9). Because of lack of data, it is not possible to make the comparison based on molar concentration, despite being desirable. As shown in Figure 9, at a given concentration, the methane hydrate stabilization effects among TBAF, TBAC, TBAB, and TPeAB aqueous solutions followed the order TBAF > TBAB > TBAC > TPeAB. At atmospheric pressure, without gas molecules involved in the clathrate hydrate formation, the melting temperatures of SCHs of TBAF, TBAC, TBAB, and TPeAB would follow the same order: TBAF > TBAB > TBAC > TPeAB.47 This kind of consistency was also observed by others.8,23 4.2. Hydrate Inhibitors (TMAB, TEAB, and TPrAB). Based on our equilibrium data, TMAB, TEAB, and TPrAB all have a slight inhibition effect on CH4 hydrate formation, similar to NaCl which suggest that this depression in melting point could be a colligative effect. Dissolved salt is known to depress the freezing point of water. For instance, the freezing points of NaCl−H2O and KCl−H2O solutions are lower than that of pure water.79 Colligative properties depend only on the properties of the solvent; in other words, freezing point depression is only associated with the solute concentration in the solution, irrespective of the chemical nature of the solute.80,81 The freezing point depression observed in salt solutions at atmospheric pressure can be directly applied in modeling the natural gas hydrate system.82 In the present work, we calculated the freezing point depression of CH4 hydrate in the presence of TMAB, TEAB, or TPrAB using the following equation,83,84 ΔTf = Tf (solution) − Tf (pure solvent) = −iK f ms

Figure 9. Comparison of experimental phase equilibrium data of CH4 SCHs formed with TBAF, TBAC, TPrAB, TBAB or TPeAB: (a) w = 0.294 mol % (5 wt %) of TBAB: − , pure water + CH4, polynomial fit to data from Sloan and Koh,2 Sangwai and Oellrich,29 Maekawa and Imai,69 Nixdorf and Oellrich,70 and Gayet et al.;71 blue ◇, 0.361 mol % (5 wt %) TBAF solution + CH4, Mohammadi et al.;6 red □, 0.339 mol % (5 wt %) TBAC solution + CH4, Sun and Liu;30 green △, 0.294 mol % (5 wt %) TBAB solution + CH4, Sangwai and Oellrich;29 orange ▲, 0.294 mol % (5 wt %) TBAB solution + CH4, this work; blue ◆, 0.294 mol % (5.8 wt %) TPeAB solution + CH4, this work; (b) w = 0.62 mol % (10 wt %) of TBAB:, pure water + CH4, polynomial fit to data from Sloan and Koh,2 Sangwai and Oellrich,29 Maekawa and Imai,69 Nixdorf and Oellrich,70 and Gayet et al.;71 blue ◇, 0.759 mol % (10 wt %) TBAF solution + CH4, Lee et al.;78 red □, 0.715 mol % (10 wt %) TBAC solution + CH4, Kamran-Pirzaman et al.;77 green △, 0.62 mol % (10 wt %) TBAB solution + CH4, Arjmandi et al.;27 orange ▲, 0.62 mol % (10 wt %) TBAB solution + CH4, this work; blue ◆, 0.62 mol % (11.6 wt %) TPeAB solution + CH4, this work.

freezing point of the pure solvent, i is the number of ions in the solute (2 in this case), Kf is the cryoscopic constant for the solvent, and ms is molality (moles of solute per kilogram of solvent)the concentration of the solution. Over a pressure range of approximately 4−10 MPs, the freezing point depression of CH4 hydrate in the presence of 5.44 wt % NaCl (ms = 0.984 mol/kg water) was found to be about 2.30 K,85 so Kf of the solvent (water with dissolved methane) was determined to be approximately 1.17 K kg/mol. Using this result, the calculated freezing point depression of CH4 hydrate in the presence of TMAB, TEAB, or TPrAB at a given concentration (ms = 0.347 mol/kg) would be predicted to be 0.81 K if it was purely a colligative effect. Figure 10 presents the

(1)

where ΔTf is the freezing point depression, Tf (solution) is the freezing point of the solution, Tf (solvent) is the normal 1957

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TPeAB. We report and compare semiclathrate hydrate phase equilibrium conditions for the temperature range 278.94− 291.85 K and pressures up to 14.32 MPa for pure CH4 + TMAB/TEAB/TPrAB/TBAB/TPeAB aqueous solutions at different concentrations. It was observed that the addition of TBAB or TPeAB allowed the dissociation conditions of the semiclathrate hydrates (SCHs) of the CH4 hydrate to shift to higher temperature/lower pressure hydrate stability zone. Both TBAB and TPeAB are thermodynamic promoters for CH4 hydrate, and their promotion effect would increase with increasing the dosage, and TBAB appears to be a more effective promoter than TPeAB. TMAB, TEAB, and TPrAB are thermodynamic inhibitors for CH4 hydrate, which could be accounted for by the colligative properties. Overall, the different carbon chain lengths of TMAB, TEAB, TPrAB, TBAB, and TPeAB play a significant role in determining the thermodynamic stability of the SCHs of CH4.

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Figure 10. Comparison between experimental phase equilibrium data with calculated freezing temperatures for three systems: Experimental data (solid line and symbols): blue −, pure water + CH4, polynomial fit to data from Sloan and Koh,2 Sangwai and Oellrich,29 Maekawa and Imai,69 Nixdorf and Oellrich,70 and Gayet et al.;71 light blue ▲, 0.3466 mol·kg−1 (0.62 mol %) TMAB solution + CH4, this work; green ■, 0.3466 mol·kg−1 (0.62 mol %) TEAB solution + CH4, this work; red ●, 0.3466 mol·kg−1 (0.62 mol %) TPrAB solution + CH4, this work; Calculated freezing point data: orange ×, seawater + CH4, Tishchenko et al.;86 blue  , 0.3466 mol·kg−1 (0.62 mol %) TMAB solution + CH4, this work; black − − − − , 0.3466 mol·kg−1 (0.62 mol %) TEAB solution + CH4, this work; red − · −, 0.3466 mol·kg−1 (0.62 mol %) TPrAB solution + CH4, this work.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +61 7 336 57942. Fax: +61 7 336 54199. Funding

This work was supported by Australian Research Council under the Discovery Projects scheme (No. 1092846). Y.S. gratefully acknowledges The University of Queensland International Scholarship. Notes

The authors declare no competing financial interest.

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predicted and measured freezing point data of CH4 hydrate in the presence of TMAB, TEAB, and TPrAB at 0.347 mol/kg. The phase equilibrium data of CH4 + seawater system at this concentration using a model developed by Tishchenko et al.86 is also plotted in Figure 10 for comparison. It can be seen that the experimental equilibrium data for the CH4 + TMAB/ TEAB/TPrAB + H2O systems are very close to the equilibrium data for the CH4 + seawater system and are in good agreement with the freezing points calculated via eq 1. It is, therefore, suggested that the dissociation point differences between pure CH4 hydrate and CH4 + TMAB/TEAB/TPrAB + H2O systems can be accounted for by considering it as a colligative property. Shimada et al.44 carried out an experiment to analyze the crystal structure of TBAB semiclathrate hydrate, C16H36N+·Br−· 38H2O, and they found that Br− and water molecules cooperate to form the hydrate framework structure. Data on the crystal structure of TPeAB, TPrAB, TEAB, or TMAB hydrate is limited; therefore, it is unclear if the Br− anion in TPeAB, TPrAB, TEAB, or TMAB can also replace water molecules and form hydrate structures via hydrogen bonding. It is therefore uncertain if weaker hydrogen bonds between Br− anions and water molecules in TPeAB, TPrAB, TEAB, or TMAB hydrates than those in TBAB hydrates is the reason for a less efficient hydrate stabilization effect of TPeAB, TPrAB, TEAB, or TMAB. Further structural analysis needs to be done to better understand the differences between the hydrates studied in the present work.

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5. CONCLUSIONS In this work, the effect of the size of the alkyl groups of organic salts on methane hydrate stability was studied by measuring the hydrate phase equilibria of TMAB, TEAB, TPrAB, TBAB, and 1958

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