Salting-out of methane in single-salt solutions at 25°C and below 800 psia |
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Authors: | Ronald K Stoessell Patricia A Byrne |
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Institution: | Coastal Studies Institute, Louisiana State University Baton Rouge, LA 70803 U.S.A. |
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Abstract: | Aqueous solubilities of methane at 25°C have been determined in single-salt solutions equilibrated with a CH4 gas phase at 350, 550, and 750 psia. Measurements were made over a range of ionic strengths in NaCl, KCl, CaCl2, MgCl2, Na2SO4, K2SO4, MgSO4, Na2CO3, K2CO3, NaHCO3, and KHCO3 aqueous solutions.At 25°C and constant pressure and methane fugacity, methane solubilities were largely controlled by the stoichiometric ionic strength, I, and the cation of the salt. Except for an increased salting-out due to HCO3?, the anion effect was relatively insignificant. Different but consistent solubility trends were followed in monovalent and divalent cation salt solutions as a function of I. Solubilities increased in salt solutions having a common anion in the following cation sequence: Na+ < K+ ? Ca2+ < Mg2+.The molal salting coefficient, km, for each salt was constant under the experimental conditions of the study, km is defined by where , the molal activity coefficient, is the methane solubility ratio () measured at constant temperature, pressure, and CH4 fugacity. Single-salt km values are as follows: 0.121, NaCl (4m); 0.121, Na2SO4 (1m); 0.118, Na2CO3 (1.5m); 0.146, NaHCO3 (0.5m); 0.101, KCl (4m); 0.108, K2SO4 (0.5m); 0.111, K2CO3 (2m); 0.145, KHCO3 (0.5m); 0.071, CaCl2 (2m); 0.063, MgCl2 (2m); and 0.066, MgSO4 (1.5m) where the molalities in parentheses refer to the maximum salt concentrations used in this study. |
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