A semi-empirical molecular clustering based lattice model near to and far from the critical region

A semi-empirical molecular clustering based lattice fluid model is presented to improve the classical lattice model for volumetric properties in the critical region. This model is based on the two assumptions: (1) the Helmholtz energy is individually divided into classical and long-range density fluctuation contribution; (2) all molecules form cluster near the critical region due to long-range density fluctuation. To formulate such molecular clustering, we extended the Veytsman statistics originally developed for the cluster due to hydrogen bonding. The probability function in the statistics is modified to represent the characteristics of long-range density fluctuation vanishing far from critical region. The proposed fluctuation contribution was incorporated into the Sanchez–Lacombe model and the combined model with 6 adjustable parameters has been tested against experimental VLE data for polar and non-polar components. The combined model is found to good agreements with experimental vapor pressure, saturated density and supercritical PVT data.