Fluid Phase Equilibria 360 (2013) 146–155 Contents lists available at ScienceDirect Fluid Phase Equilibria j our na l ho me pa ge: www.elsevier.com/locate/fluid Prediction of stability limits and critical conditions of binary liquid mixtures containing nitro-methane using the NRTL model Muhammad Qasim, Naif A. Darwish ∗ Department of Chemical Engineering, American University of Sharjah, P.O. Box 26666, Sharjah, United Arab Emirates a r t i c l e i n f o Article history: Received 3 April 2013 Received in revised form 2 August 2013 Accepted 4 September 2013 Available online 12 September 2013 Keywords: Theory of liquids Solutions Stability Supercritical fluids Computation a b s t r a c t The stability limit (spinodal) curves and critical points for binary liquid solutions of nitro-methane with 21 different compounds are determined by employing the rigorous stability and criticality thermodynamic criteria using the NRTL model. The NRTL binary interaction parameters used are those resulting from the regression of the available experimental binodal data in the literature. All systems considered in this work show upper critical solution temperatures (UCST) with a very clear convergence of both the spinodal and the binodal curves at the mixture critical point. The predicted mixture critical temperatures, together with the available binodal data were tested for compliance with the universal criticality theory, which implies that as a certain “field variable” (e.g., T - T c ) approaches the critical value the behaviour of “order parameters” (such as  L -  G ) follow a simple power law with universal exponents. The predicted critical exponents for two order parameters of relevance to the liquid–liquid systems, that is the difference in mole fraction of species 1 in the two coexisting phases ( x ′ 1 - x ′′ 1 ) and the susceptibility (), which is related to the chemical potential of species 1 were found consistent with those reported in the literature from other order parameters like magnetization and density difference. © 2013 Elsevier B.V. All rights reserved. 1. Introduction A detailed knowledge of liquid–liquid equilibrium (LLE) for binary and multicomponent systems is required for the design of important chemical engineering separation and extraction pro- cesses [1,2]. Efficient design, operation and control for such unit operations require thorough understanding of thermodynamic sta- bility limits and criticality conditions of the involved chemical species. As a result, the need for predicting and generating phase diagrams, stability limits, and critical loci for liquid mixtures is becoming of increasing popularity [3]. Liquid–liquid equilibria for a wide variety of binary partially miscible liquid systems have been studied over a wide temperature range [4–9]. Ternary systems, however, have been investigated at limited ranges of tempera- tures and in many cases over a single temperature [10]. Data on quaternary systems and above are very scarce [11]. The NRTL model, introduced by the end of the 1960s by Renon and Prausnitz [12], is one of the most frequently employed tools in correlating experimental data of phase equilibria [13]. Usually, temperature-dependent parameters for this model can be corre- lated using experimental equilibrium data to allow reconstruction of the phase envelope for liquid–liquid equilibrium (LLE) [14]. ∗ Corresponding author. Tel.: +971 6 5152401; fax: +971 6 5152979. E-mail address: ndarwish@aus.edu (N.A. Darwish). This paper focuses on the liquid–liquid equilibrium of binary mixtures containing nitro-methane, an important industrial chem- ical. One of the most important direct uses for nitro-methane is in the stabilization of halogenated hydrocarbons, which are widely used for degreasing, dry cleaning and for cleaning semiconductors and lenses [15]. For example, tetrachloroethylene is often mixed with nitro-methane to form a stable binary mixture that is com- monly used for dry cleaning of fabrics [16]. Nitro-methane is also frequently used as a polar solvent for cellulose esters [17]. In addition, nitro-methane is widely used as a solvent in many indus- trial applications. For example, the properties of nitro-methane make it an interesting solvent for capillary electrophoresis espe- cially for lipophilic analytes that are not sufficiently soluble in water [18]. Moreover, paint strippers and paint removers such as dichloroethane are often formulated with nitro-methane for effective paint removal [19]. Liquid mixtures of nitro-methane with other compounds are also used as fuels in racing cars and in model engines. For example, a blend of nitro-methane with methanol can be used to produce increased power output over reg- ular fuels [20]. Furthermore, nitro-methane is often blended with refrigerants, such as fluoro-olefins, in order to avoid decomposi- tion of the refrigerant [21,22]. Therefore, the importance of binary and multicomponent liquid mixtures involving nitro-methane requires thorough understanding of liquid–liquid phase equilib- rium behaviour. In this paper, the spinodal curves, or equivalently, the stability limits for binary liquid solutions containing nitro-methane have 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.09.001