Aqueous nonelectrolyte solutions — Part XX: Formula of structure I methane hydrate, congruent dissociation melting point, and formula of the metastable hydrate

• Published in: Canadian Journal of Chemistry
• Main Author: Glew, David N
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2003
• Subjects: Organic Chemistry; General Chemistry; Catalysis; Methane hydrate
• Online Access: http://dx.doi.org/10.1139/v03-156; http://www.nrcresearchpress.com/doi/pdf/10.1139/v03-156

Sixteen new measurements of high precision for structure I methane hydrate with water between 31.93 and 47.39 °C are shown to be metastable and exhibit higher methane pressures than found by earlier workers. Comparison of earlier measurements between 26.7 and 47.2 °C permit positive identification of the structure II and the structure I hydrates. Forty-nine equilibrium constants K p (h 1 [Formula: see text]l 1 g) for dissociation of structure I methane hydrate into water and methane, 32 between –0.29 and 26.7 °C for the stable hydrate and 17 between 31.93 and 47.39 °C for the metastable hydrate, are best represented by a three-parameter thermodynamic equation, which indicates a standard error (SE) of 0.63% on a single K p (h 1 [Formula: see text]l 1 g) determination. The congruent dissociation melting point C(h 1 l 1 gx m ) of metastable structure I methane hydrate is at 47.41 °C with SE 0.02 °C and at pressure 505 MPa. The congruent equilibrium constant K p (h 1 [Formula: see text]l 1 g) is 102.3 MPa with SE 0.2 MPa. ΔH° t (h 1 [Formula: see text]l 1 g) is 62 281 J mol –1 with SE 184 J mol –1 , and the congruent formula is CH 4 ·5.750H 2 O with SE 0.059H 2 O. At the congruent point, ΔV(h 1 [Formula: see text]l 1 g) is zero within experimental precision, and its estimate is 1.3 with SE 1.6 cm 3 mol –1 . The stability range of structure I methane hydrate with water extends from quadruple point Q(s 1 h 1 l 1 g) at –0.29 °C up to quadruple point Q(h 1 h 2 l 1 g) at 26.7 °C, and its metastability range with water extends from 26.7 °C up to the congruent dissociation melting point C(h 1 l 1 gx m ) at 47.41 °C. Key words: methane hydrate, clathrate structure I, metastability range, dissociation equilibrium constant, formula, congruent melting point, metastability of structure I hydrate.