A New Activity Coefficient Model For Aqueous Alkali-Earth Metal Solution At High Temperature And Pressure

With the wide and successful application of hydrothermal synthesis of new nano-materials, selective synthesis and supercritical oxidation, aqueous solutions at high temperature and pressure are good media for many organic and inorganic reactions. So the thermodynamic properties of aqueous solutions under extreme conditions catch more and more interests nowadays. Electrolyte solutions are widely used in chemical engineering, metallurgy, environmental protection, geology and oceanography etc. After Debye Hückel who laid a foundation on the theory of electrolyte solution in 1920s, many other theories and models were proposed during the last century. But most of them can only be applicable within the limited of temperature, pressure and low concentration of electrolytes. Regarding the wide range of conditions in practical engineering application and the difficulty of carrying out experiments at extreme conditions, it is important and essential to build a new model which is applicable at high temperature, high pressure, and high concentration and can predict properties under unknown conditions.In this thesis, a review is firstly given on the basis of the theories of electrolyte solutions and the models of activity coefficients. When describing the non-ideal properties of electrolytes, the interactions of the long range force between ions, the short range force between species, the ion solvations and the associations which play an important role at high temperature and high pressure musst be taken into account at the same time, and meanwhile the popular relationships of these actions with temperature and pressure should also be considered. Among various models, the short range force with the Margules form which was proposed by Pitzer and Simonson and the association model in consideration of one association degree which was proposed by Pitzer and Y. G.. Li are selected to build the new model where two association degrees are considered for aqueous alkali-earth metal solutions. In this new model the explicit physical meanings of the parameters are defined.In order to satisfy the consistency of thermodynamics, the parameters of the new model are regressed by activities of water, and then are used to calculate activity coefficients of solutes. The researches on MgCl2, SrCl2, BaCl2 at 196.2bar and 523.15~623.15K show that this new model is applicable. The research on CaCl2 at 196.2~588.6bar, 523.15~623.15K and at atmosphere, 273.15~373.15K also shows that the calculated values agree well with experimental data. Avoiding the shortcoming of former models that can't be used to MgCl2, CaCl2, SrCl2 and BaCl2 at the same time, the new model can be applied to calculate both the activities of water and the activity coefficients of the solute of all these electrolyte solutions even to high concentrations.Based on the previous work of quantized calculations made by our research group, the parameters of CaCl2 solution in this model at different conditions are fitted as the function of temperature and pressure. Using the function of the same form the activity of water and the mean activity coefficient of the solute are predicted and compared with experimental data. The result shows that the predicted values are in good agreement with experiment data.The new model proposed in this paper can be used to calculate the activity coefficients of alkali-earth metal solutions from atmosphere, lower temperature (1.013bar, 273.15K) to higher pressure and temperature (588.6bar, 623.15K) with good accuracy, Furthermore it also can be used to predict the properties of any condition within the range of the experimental data covered. Compared with the formers' work, this work and the model proposed are both new and may be useful in complicated practical engineering.