Equations of State for Electrolyte Solutions
Sustainable process-product development & green chemistry
Modelling of Ionic Liquids (T1-1)
Keywords: Electrolyte, Equation of State, Model, Ion specific parameters
The properties of solutions containing electrolytes have usually been described by activity coefficient models. While significant results have been achieved using such models, they are far from perfect. Activity coefficient models do in general not have pressure dependency and can not be used for calculating the density of electrolyte solutions. Activity coefficient models are most accurate for binary and ternary solutions, but the accuracy usually decreases with an increasing number of components.
It is important to know the pressure dependency of the properties of electrolyte solutions in applications relating to the oil industry and the production of geothermal energy. The density of electrolyte solutions is an important property for process simulation. Industrial processes with natural brines typically contain many components, and a good accuracy in calculating the phase behavior of such solutions is therefore required.
In order to improve the thermodynamic modeling of electrolyte solutions we have examined the use of some equations of state for solutions containing electrolytes. Equations of state relate pressure, volume and temperature of a solution to each other. Flexible mixing rules for equations of state enable these to describe mixture properties from pure component properties. It is possible that some of the shortcomings of activity coefficient models for electrolyte solutions can be alleviated by introducing equations of state adapted for electrolyte solutions.
In this work we have examined different combinations of traditional equations of state and terms that take the long range electrostatic interactions of electrolyte solutions into account. The equations of state used for short range interactions are the Soave-Redlich-Kwong and the Peng-Robinson equations of state with or without a Wertheim association term. For the long range interactions we have used two different versions of the Mean Spherical Approximation term, a simplified Debye-Hückel term, and a Born term.
Ion specific interactions were determined for a multi component test system of ions in water. The pure component parameters for cubic equations of state are usually calculated from their critical state properties. For ions such critical state properties do not exist, the corresponding parameters were therefore considered as adjustable parameters and fitted to experimental data for electrolyte solutions. Two cubic equation of state parameters and an ion size parameter used in the Mean Spherical Approximation term and in the Born term were fitted.
The temperature dependence of the parameters offered another problem. A temperature dependence similar to the one used for the equation of state parameters of traditional components did not work well for ions and had to be modified.
Presented Monday 17, 11:15 to 11:33, in session Modelling of Ionic Liquid Systems (T1-1).
