Department of Chemical and Petroleum Engineering, Sharif University of Technology, P. O. Box 11365-1465 Tehran, I.R. IRAN
In this work the effect of electrolytes on hydrate formation was investigated. To do so, a new model was used in predicting the hydrate formation conditions in presence of both single and mixed electrolyte solutions. The new model is based on the van der Waals - Platteeuw hydrate equation of state. In order to evaluate the values for the activity of water in electrolyte solutions the simplified version of the Ghotbi-Vera Mean Spherical Approximation (SGV-MSA) model was used. According to the SGV-MSA model the ions in the solutions are considered as charged hard spheres with different sizes. The values of the parameters for the SGV-MSA model are independent of temperature and depend only on the nature as well as concentration of electrolytes studied in this work. These parameters were obtained using the experimental data for the mean ionic activity coefficient of the single electrolyte solutions at 298.15 K. In the case of the mixed electrolyte solutions a new mixing rule was introduced to obtain the activity of water in aqueous electrolyte solutions. The results show that the proposed model can predict the hydrate formation for the systems containing single or mixed electrolyte solutions with good accuracy compared to the experimental data available in the literature. In addition, the results obtained from the proposed model were favorably compared with those obtained from the previously used models.
 Dendy, E., Sloan, Jr.,”Clathrate Hydrate of Natural Gases”, First Edition, New York, Marcel Dekker Inc. (1990).
 Ripmeester, J. A., Tse, J. S., Rateliffe, C. I., A Clathrate Hydrate of Carbon Monoxide, Nature,328, p. 469 (1987).
 Nasrifar, Kh., Moshfeghian, M., Maddox, R. N., Prediction of Equilibrium Condition for Gas Hydrate Formation in the Mixture of Both Electrolyte and Alcohol, Fluid Phase Equilibria,146, p. 1(1998).
 Nasrifar, Kh. and Moshfeghian, M., A Model for Prediction of Gas Hydrate Formation in Aqueous Solutions Containing Electrolytes and/or Alcohol, J. Chem. Thermodynamics,33, p. 999 (2001).
 Parrish William, R. and Prausnitz John, M., Dissociation Pressure of Gas Hydrate formed by Gas Mixture, Ind. Eng. Chem. Process Des. Develop.,11 (1), p. 26 (1972).
 Van der Waals, J. H., Platteeuw, J. C., Clathrate Solutions, Advanced in Chemical Physics,2 (1) (1959).
 Ng. H. J. , Robinson, D. B., The Measurement and Prediction of Hydrate Formation in Liquid Hydrocarbon- Water System, Ind. Eng. Chem. Fundam., 15, p. 293 (1976).
 Holder, G. D., Zetts, S.P., Pradhan N., Phase Behavior in Systems Containing Clathrate Hydrates, Rev. Chem. Eng.,5, p. 1 (1988).
 John, V. T., Papadopoulos, K. D. and Holder, G. D., Generalized Model for Predicting Equilibrium Conditions for Gas Hydrates, AIChE J., 31 (2), p. 252 (1985).
 Englezos, P., Bishnoi, P.R., Prediction of Gas Hydrate Formation in Aqueous Electrolyte Solutions, AIChE J., 34 (10), p. 1718 (1988).
 Tohidi, B., Burgass, R. W., Danesh, A., Todd, A. C., Hydrate Inhibition Effect of Produced ater: Part 1-Ethane and Propane Simple Gas Hydrates, SPE26701, p. 255 (1993).
 Javanmardi, J., Moshfeghian, M., Maddox, R. N., Simple Method for Predicting Gas-Hydrate Forming Conditions in Aqueous Mixed-Electrolyte Solutions, Energy and Fules,12, p. 219 (1998).
 Ghotbi, C., Taghikhani, V. and Azimi, G., Correlation of the Mean Ionic Activity Coefficient in 1:1 Electrolyte Solutions Using the GV-MSA Model, Iranian J. of Sci. & Tech.,27 (B2), p. 1 (2003).
 Prausnitz , J.M., Lichtenthaler, R.N., Gomes De Azevedo, E., “Molecular Thermodynamics”, New Jersey, Prentice Hall Inc., Englewood Cliffs (1999).
 Ghotbi, C., Vera, J., Extension to Mixtures of Two Robust Hard-sphere Equations of State Satisfying the Ordered Closed-packed Limit, The Canadian J. of Chem. Eng.,79, p. 678 (2001).
 Lu Jiu-Fang , Yu Yang-Xin and Li Yi-Gui, Modi-fication and Application of the Mean Spherical Approximation Method, Fluid Phase Equilibria,85, p. 81 (1983).
 Patwardhan, V.S., Kumar, A., A Unified Approach for Prediction of Thermodynamic Properties of Aqueous Mixed-Electrolyte Solutions. I. Vapor pressure and Heat of Vaporization, AIChE J.,32, p. 1419 (1986).
 Dholabhai, P. D, Englezos, P., Kalogerakis, N. and Bishnoi P. R., Equilibrium Conditions for Methane Hydrate Formation in Aqueous Mixed Electrolyte Solutions, The Canadian J. of Chem. Eng.,69, p. 800 (1991).
 Roo, J. D., Peters, C. J., Lichtenthaler, R. N. and Diepen, G. A. M., Occurrence of Methane Hydrate in Saturated and Unsaturated Solutions of Sodium Chloride and Water in Dependence of Temperature and Pressure, AIChE J., 29 (4), p. 651 (1983).
 Kharrat, M., Dalmazzone, D., Experimental Deter-mination of Stability Conditions of Methane Hydrate in Aqueous Calcium Chloride Solutions Using High Pressure Differential Scanning Calorimetry,J. Chem. Thermodynamics,35, p. 1489 (2003).
 Rossi, L. F. S. and Gasparetto, C. A., Prediction of Hydrate Formation in Natural Gas Systems, SPE22715, p. 545 (1991).
 Mei Dong-Hai, Liao Jian, Yang Ji-Tao and Guo Tian-Min, Hydrate Formation of a Synthetic Natural Gas Mixture in Aqueous Solutions Containing Electrolyte , Methanol and (Electrolyte + Methanol), J. of Chem. and Eng. Data,43 (2), p. 178 (1998).
 El Guendouzi Mohamed, Dinane Abderrahim and Mounir Abdelfetah, Water Activities, Osmotic and Activity Coefficients in Aqueous Chloride Solutions at T=298.15 K by the Hygrometric Method, J. Chem. Thermodynamics,33, p. 1059 (2001).
 Harned, H. S. and Owen, B. B., “Physical Chemistry of Electrolyte Solutions”, 3rd Edition, Reinhold Pub. Co. (1958).
 Weast Robert, C., “Handbook of Chemistry and Physics”, 54th Edition, Published by CRC Press, (1973-74).
 Goldberg, R. N., Nuttall, R. L. and Staples, B. R., Evaluated Activity and Osmotic Coefficients for Aqueous Solutions : Iron Chloride and the Bi- Univalent Compounds of Nickel and Cobalt, J. Phys. Chem. Ref. Data,8 (4), p. 923 (1979).
 Goldberg, R. N., Evaluated Activity and Osmotic Coefficients for Aqueous Solutions : Thirty- Six Uni-Bivalent Electrolytes, J. Phys. Chem.Ref.Data,10 (3), p. 671 (1981).
 Dinane Abderrahim, El Guendouzi Mohamed and Mounir Abdelfetah, Hygrometric Determination of Water Activities , Osmotic and Activity Coefficients of (NaCl + KCl) (aq) at T=298.15, J. Chem. hermodynamics,34, p. 423 (2002).
 Dinane Abderrahim , Mounir Abdelfetah, Water Activities , Osmotic and Activity Coefficients in Aqueous Mixture of Sodium and Magnesium Chlorides at 298.15 K by the Hygrometric Method, Fluid Phase Equilibria,206, p. 13 (2003).
 Pankaj D . Dholabhai, Nicolas Kalogerakis and P.Raj Bishnoi, Equilibrium Conditions for Carbon Dioxide Hydrate Formation in Aqueous Electrolyte Solutions , J. Chem. Eng. Data, 38, p. 650 (1994).