Korean J. Chem. Eng., 26(1), 168-174 (2009) SHORT COMMUNICATION 168 † To whom correspondence should be addressed. E-mail: ahaghighi@semnan.ac.ir Equation of state for the systems containing aqueous salt: Prediction of high pressure vapor-liquid equilibrium Hadi Baseri, Mohammad Nader Lotfollahi, and Ali Haghighi Asl † Chemical Engineering Department, Semnan University, Semnan, Iran (Received 12 March 2008 • accepted 15 July 2008) Abstract −An equation of state (EOS), which is based upon contributions to the Helmholtz energy, is presented for systems containing aqueous electrolyte solutions at high pressure. The Peng-Robinson equation of state is used to pro- vide the Helmholtz energy of a reference system. The electrolyte terms consist three terms containing a modified Debye- Hückel term for long-range electrostatic interactions, the Born energy contribution for electrostatic works and a Margules term for short-range electrostatic interactions between ions and solvents. The binary and ternary interaction parameters of the equation of state are obtained by experimental osmotic coefficient data. Systems that were studied here are (water+ NaCl+SC-CO 2 ), (water+NH 4 Cl+SC-CO 2 ), (water+Na 2 SO 4 +SC-CO 2 ) and (water+methanol+NaCl+SC-CO 2 ). It is found that the proposed equation of state is able to accurately represent the experimental data over a wide range of pressure, temperature and salt concentration. Key words: Equation of State, Electrolyte Solution, Vapor-liquid Equilibrium INTRODUCTION Salt containing and more specifically aqueous electrolyte solu- tions are involved in many industrial processes, such as extractive distillation, solution crystallization, ion exchange, waste water treat- ment, liquid-liquid extraction and many other processes. Supercrit- ical carbon dioxide is also used in systems containing (supercritical carbon dioxide, solvent and salt). Several data series of these sys- tems have been found in the literature. To mention only a few, Wander and coworkers (1989) measured the CO 2 solubility in pure water and in a 1% wt aqueous NaCl solution at pressures up to 10 MPa and temperature range from 353 K up to 473 K. Koschel et al. [1] studied the dissolution of carbon dioxide in water and in aqueous solution of sodium chloride by measuring the heat of mixing, ∆H mix , of a supercritical gas with the liquid phase. Prutton and Savage [2] published the data of solubility of CO 2 in water and effect of salts CaCl 2 on the concentration of phases at 348, 373 and 393 K and pressures up to 70 MPa. For ambient pressure conditions, several activity coefficient mod- els have been developed to represent the vapor liquid equilibrium (VLE) in mixed solvent-electrolyte systems [3-7]. But these mod- els are not suitable for systems containing supercritical components. An alternate description of high-pressure phase equilibria is pro- vided by a single equation of state to calculate the fugacity of each species in each phase. Such equations of state do not have any prob- lem to select a truly useful standard state for the supercritical com- ponents. However, the widely used equations for high-pressure phase equilibrium calculations, such as the cubic equations of state, have to be developed with systems containing electrolytes. Efforts along these lines were reported in several works (Donohue et al., 1991; [8-11]). Donohue and coworkers [13] developed the extended perturbed- anisotropic-chain-theory. Their theory consists of ten contributions to the Helmholtz energy. Seven contributions of their proposed EOS are needed to describe the nonionic interactions, and the remaining three are needed to consider the ionic interactions. Clarke [8] used an equation of state based on contributions to the Helmholtz energy from a non-electrolyte term and three electrolyte terms. The non- electrolyte term comes from the Trebble-Bishnoi equation of state, and the electrolyte terms consist of a Born energy term, a mean spheri- cal approximation term and a newly developed hydration term. Raatschen and coworkers [9] developed an equation of state to describe the phase equilibrium of the water+methanol+lithium bro- mide system. They applied the hard-sphere equation of Boublik and Mansoori in combination with a Lennard-Jones potential to describe nonionic systems. They used the Born equation, a Debye-Hückel term, and a modified Pitzer equation to describe the ionic interac- tions. Harvey and Prausnitz [10] presented a procedure for super- imposing ionic effects on a conventional equation of state for non- electrolytes. They used a modification of Born’s equation to describe charging the ions, in combination with mean spherical approxima- tion to describe ion-ion interactions. They calculated an adjustable salt/solvent parameter from osmotic-coefficient data and obtained good results for phase equilibria in a natural-gas/brine system at high pressure. Liu et al. [12] combined an electrolyte perturbation theory with the mean spherical approximation theory and the statis- tical associating fluid theory to derive an equation of state for aque- ous electrolyte systems. Zuo and Guo [13] applied a three-parameter cubic equation of state by Patel and Teja [14] combined with an excess Gibbs energy term to find a new mixing rule for some model parameters. They used a Debye-Hückel term for electrostatic inter- actions. Lu and coworkers [15] used a model for electrolyte solutions. Their model consists of the cluster equation theory, with which the osmotic coefficient and activity coefficient in the dilute concentra- tion range, can be predicted by using the Pauling ionic diameter. Collinet and Gmehling [16] proposed the volume translated Peng-