Experimental Methane Hydrate Dissociation Conditions in Aqueous

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Experimental Methane Hydrate Dissociation Conditions in Aqueous Solutions of Lithium Salts Christoph Windmeier* and Lothar R. Oellrich Institut für Technische Thermodynamik und Kältetechnik, Karlsruher Institut für Technologie, 76128 Karlsruhe, Germany ABSTRACT: Gas hydrate dissociation pressures have been determined for methane in aqueous solutions of LiBr w = (0.05, 0.10, 0.15, and 0.2655) and LiCl w = (0.05, 0.10, 0.15, and 0.20) along the hydrate−liquid−gas equilibrium line in the temperature range (260 to 290) K and for pressures up to 22.5 MPa by applying an isochoric method. LiBr showed a more pronounced inhibition effect than LiCl as expected from the different degree of hydration caused by the different anion sizes.

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INTRODUCTION Gas hydrates, or more precisely clathrate hydrates, are ice-like crystalline substances in which water molecules form a cage-like structure, which is stabilized by enclosing small mostly nonpolar guest molecules. Formation conditions of gas hydrates typically are located at elevated pressures and temperatures ranging even above 290 K. A detailed understanding of clathrate formation and dissociation kinetics as well as calculation methods for accurately predicting hydrate phase equilibria are a prerequisite for the prevention of unwanted gas hydrate formation in natural gas production, processing, and transportation facilities. This also holds true for the production of natural gas from naturally occurring methane hydrate resources and for the application of gas hydrate properties to industrial separation processes. The past decades have provided numerous studies of gas hydrate equilibria in the presence of various salts of, for example, sodium,1−3 potassium, 3,4 magnesium,3−6 or calcium.3,6,7 In this work experimental incipient hydrate formation conditions for methane in aqueous solutions of lithium chloride (LiCl) and lithium bromide (LiBr) are presented. To the best of our knowledge, no equilibrium conditions of gas hydrates in the presence of lithium salts have been reported before in open literature. The obtained data may prove to be useful for the development of models suitable for the prediction of clathrate hydrate formation conditions in natural and artificial brines as, for example, required for designing natural gas drying processes or hybrid clathrate−absorption refrigeration cycles both using aqueous lithium salt solutions.

0.15, and 0.2655) and LiCl w = (0.05, 0.10, 0.15, and 0.20). Two concentrations of each salt were chosen such that they correspond to equal molar fractions of water xW. A description of the experimental setup together with the procedure applied is given elsewhere.6 Standard uncertainties u are u(T) = 0.01 K, u(p) = 25 kPa, and u(w) = 5·10−5. Purities and sources of the used substances are summarized in Table 1. All gases and chemicals were used as received. Additionally double deionized water with a residual conductivity of < 2·10−4 S·m−1 was taken from the institute supply. Table 1. Purities of the Used Substances source

mole fraction purity

methane LiCl LiBr

Messer GmbH Carl Roth GmbH Carl Roth GmbH

0.9995 0.995 0.995

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RESULTS AND DISCUSSION In Tables 2 to 4 the experimental phase equilibrium data obtained in this work are presented. Figure 1 shows a plot of the data together with a smoothed equilibrium line of pure methane hydrate.8 To be able to differentiate between the influences of the anion species on hydrate inhibition, two of the LiBr and LiCl concentrations were chosen to result in equal water molar fractions xW. Looking at the experimental data one can see a comparatively larger temperature depression of the hydrate equilibrium conditions in the LiBr system than in the corresponding LiCl system at the same water molar fraction

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EXPERIMENTAL PROCEDURE An isochoric method was applied to determine dissociation pressures of methane gas hydrate in aqueous solutions of LiCl and LiBr at four salt concentrations each: LiBr w = (0.05, 0.10, © 2014 American Chemical Society

substance

Received: November 18, 2013 Accepted: January 21, 2014 Published: February 3, 2014 516

dx.doi.org/10.1021/je4010036 | J. Chem. Eng. Data 2014, 59, 516−518

Journal of Chemical & Engineering Data

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Table 2. Experimental Dissociation Conditions for Methane Hydrate in Pure Watera

a

T/K

p/MPa

291.29 289.51 286.54 280.46 278.34 275.58 273.76

18.925 15.000 10.472 5.412 4.364 3.318 2.775

Table 4. Experimental Dissociation Conditions for Methane Hydrate in Aqueous LiCl Solutionsa T/K 274.84 281.80 285.10 287.25 288.81 w = 0.10, xW = 0.914

Table 3. Experimental Dissociation Conditions for Methane Hydrate in Aqueous LiBr Solutionsa p/MPa w = 0.05, xW = 0.979 273.53 279.18 284.36 273.03 289.00 290.52

3.632 8.475 12.543 16.054 18.200 21.544 w = 0.15, xW = 0.870

3.169 5.598 9.850 13.596 17.517 21.487

260.17 268.02 271.53 273.56 274.64

3.506 8.345 13.161 17.594 20.729 w = 0.20, xW = 0.825

w = 0.1, xW = 0.956 272.52 281.23 284.78 286.76 288.58

4.304 9.056 13.475 17.900 22.155

268.11 275.99 279.17 281.04 281.91 283.12

Standard uncertainties u are u(T) = 0.01 K and u(p) = 25 kPa.

T/K

p/MPa w = 0.05, xW = 0.956

261.69 3.512 8.776 13.404 17.447 22.417

15.846

a

Standard uncertainties u are u(T) = 0.01 K, u(p) = 25 kPa, and u(w) = 5·10−5.

w = 0.15, xW = 0.932 272.54 279.98 282.69 284.93

4.654 10.645 15.057 20.515 w = 0.2655, xW = 0.870

262.03 267.37 270.17 271.99 273.73

4.888 9.124 13.096 17.108 22.539

a

Standard uncertainties u are u(T) = 0.01 K, u(p) = 25 kPa, and u(w) = 5·10−5.

(LiBr w = 0.10 and LiCl w = 0.05: xW = 0.956; LiBr w = 0.2655 and LiCl w = 0.15: xW = 0.870). Since the degree of hydrate inhibition can be directly related to water activity, the observed trend is in agreement with general observations of the effect of ion size on the freezing point depression of aqueous systems containing bromide instead of chloride ions.9,10 At the higher concentration under consideration, this difference becomes even much more pronounced.

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Figure 1. Experimental methane hydrate equilibrium conditions in pure water and aqueous lithium salt solutions: ×, pure water, this work; −, pure water, smoothed from ref 8; □, w(LiBr) = 0.05; △, w(LiBr) = 0.10; ◇, w(LiBr) = 0.15; ○, w(LiBr) = 0.2655; ■, w(LiCl) = 0.05; ▲, w(LiCl) = 0.10; ◆, w(LiCl) = 0.15; ●, w(LiCl) = 0.20.

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS

*Tel.: +498974454404. Fax: +498974454981. E-mail: [email protected].

The equilibrium conditions of a methane hydrate in aqueous solutions of lithium salts were determined for the first time. LiBr shows a stronger inhibiting effect than LiCl at equal water molar fractions. This observation is in agreement to the comparatively lower water activity caused by the bromide anion compared to the chloride anion due to its larger size.10

Present Address

C.W.: Linde AG, Engineering Division, 82049 Pullach, Germany. Notes

The authors declare no competing financial interest. 517

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