Correlation of Mixture Vapor-Liquid Equilibria with the SPEADMD Model

The present work examines the accuracy of the SPEADMD molecular simulation methodology in correlating experimental data relative to a standard low-pressure database for testing VLE models. The database contains 104 binary systems categorized according to polarity and ideality. Although the database is somewhat small, it covers a broad range of chemical functionality, including halocarbons and carboxylic acids as well as hydrocarbons and alcohols. Six models were tested and compared for their characterization of these mixtures. Four standard models were evaluated to establish a basis for comparison: the Margules, NRTL, PR, and PRWS models. The SPEADMD model was evaluated in three forms. In its elementary form, the SPEADMD model includes∼10% deviations in vapor pressure because of the application of transferable potential functions in the molecular model. An alternative model is developed on the basis of SPEADMD combined with corrected vapor pressures and customized self-interaction parameter for pure compounds. This alternative is referred to as the SPEADCI model, in which CI stands for customized interactions. Results show that SPEADCI model provides accuracy similar to the NRTL and PRWS models, even though it includes only one adjustable parameter per binary system, whereas the NRTL model includes two and the PRWS models include three. Deviations in correlated bubble point pressure are roughly 1-2% for these models. The SPEADMD models have the advantage that transferable potentials can be applied for solvation interactions that are similar to the Kamlet-Taft interaction parameters. Accurate vapor-liquid equilibrium (VLE) estimation is important for many chemical processes, most notably distillation. Often, experimental data do not exist for VLE in the temperature range of interest. Therefore models to predict VLE have become important. The most basic model for VLE is Raoult’s Law,y i P )x i P i sat ,w herePis pressure,P i sat ,x i ,a ndy i are the vapor pressure and liquid and vapor mole fractions component ofi, respectively. Raoult’s Law assumes ideal mixing for all systems. However, most mixtures are nonideal, and thus more sophisticated methods are needed to explain mixture behavior. Two methods are commonly used for calculating VLE. The distinction between them is in the way the fugacity (φ) of the liquid is calculated. The fugacity of the vapor phase is always calculated using an equation of state (EOS). The fugacity of the liquid phase can be calculated by using either the same EOS used for the gas or by the activity coefficient (γ) method. 1 The SPEADMD model is based on Step Potential Equilibria And Discontinuous Molecular Dynamics. 2 It provides a bridge between molecular simulation methodology and a conventional approach to engineering phase behavior models. By discretizing the intermolecular potential model into a series of steps, a combination of discontinuous molecular dynamics (DMD) simulation and thermodynamic perturbation theory (TPT) can be adapted to achieve a high degree of leverage from each simulation. 3 The SPEADMD model begins with molecular simulation of pure fluids and mixtures. Simulation of every pure fluid is necessary to characterize the TPT contributions. Because of the adaptation of TPT for temperature effects, simulations of the reference fluid over the entire density range provide a complete EOS, customized for the particular molecular model at hand through the configurations generated by its particular size and shape. In principle, treating mixtures requires simulating each binary mixture over a range of compositions and densities. Gray and Elliott performed this kind of analysis for a broad range of binary mixtures and identified consistent trends in the perturbation contributions. 4 These trends can be expressed in the form of mixing rules with one binary interaction parameter (BIP) per binary system. Consequently, it is feasible to assume the validity of these mixing rules without exhaustively simulating every binary mixture, an added form of leverage derived from the SPEADMD approach. It is necessary to simulate each binary system to develop the connection between the site-site intermolecular potential and the macroscopic BIP, but explicitly making that connection is unnecessary when assessing the accuracy of the SPEADMD model for correlating a large VLE database. Hence the present work focuses on correlating the macroscopic BIPs in order to assess the level of accuracy that can be expected for an approached based on molecular simulations. Relating the macroscopic BIPs to the site-site BIPs would enable prediction of VLE solely on the basis of molecular structure. Developing predictive capability will be the subject of future work.