A temperature-and density-dependent effective diameter for PC-SAFT

Abstract

In this work, a simple effective diameter expression as a function of both temperature and density is presented. This new effective diameter is theoretically deduced by combining a modified Percus-Yevick (PY) cavity function with the square-shoulder soft-core repulsion of Chen and Kreglewski. Then, for illustrative purposes, this new expression was introduced into the perturbed chain-statistical association fluid theory (PC-SAFT EoS) to perform predictive thermodynamic calculations for simple hydrocarbon systems, and more complex mixtures such as polymers and petroleum reservoir fluids. The predictive calculations demonstrate that a temperature-and density- effective diameter only affects significantly the phase behavior of dense and complex systems and does not have an important impact on systems formed by simple molecules under ordinary conditions. With this new effective diameter, PC-SAFT generally tends to predict both asphaltene onset pressures (AOPs) and cloud-point pressures lower than the original EoS, which uses an effective diameter that depends on temperature but not upon density.