One-parameter modified nonrandom two-liquid (NRTL) activity coefficient model

• We proposed a new modification to the NRTL model which addresses the limitation of the original model.
• The proposed model reduces the NRTL model to a one-parameter model.
• The original and modified NRTL models provide comparable property representation capability.
• The parameter of the modified model was used to quantitatively classify VLE behaviors.

The nonrandom two-liquid (NRTL) model is utilized widely in phase equilibria calculations and employs three adjustable parameters (two interaction parameters and the non-randomness factor) that are determined through regression of experimental data for a specific binary vapor–liquid equilibrium (VLE) system. The two interaction parameters (a12 and a21) account for the difference between the pure-component liquid interactions (g11 and g22) and mixed-component liquid interactions (g12 and g21). The main disadvantage of the NRTL model is the strong correlation between the two parameters of the model.In this work, we propose a new modification to the NRTL model which addresses the limitation of the original model. The newly modified model recasts the formulation of the NRTL model parameters in such a way that the new parameters reflect two different characteristics. It also allows one of the two new model parameters to be generalized in terms of pure-component properties, which reduces the NRTL model to a one-parameter model. The original and modified NRTL models yield comparable precision in representing experimental equilibrium properties. Some of the benefits of our modification include easy generalizability of the parameters, ability to classify VLE behaviors based on a single model parameter and fewer convergence problems in parameter regressions.