Environmental concerns have led many countries worldwide to apply stricter regulations on sulfur levels in liquid fuels. These regulations place many challenges for the refining industries. Separation process design to accomplish the removal of sulfur compounds requires knowledge of vapor-liquid equilibria (VLE) of sulfur compounds with hydrocarbons, particularly their activity coefficients at infinite dilution. However, the investigations concerning the systems containing sulfur compounds are scarce.

The original UNIFAC and UNIFAC-Dortmund group contribution models have been widely applied in the estimation of the VLE of organic mixtures, as it includes many functional groups in organic substances. However, the applications of the predictive models are limited since some important interaction parameters for sulfur and hydrocarbon functional groups are unavailable because of little or no experimental data available. When the UNIFAC and UNIFAC-Dortmund predictive models are not applicable, COSMO-RS can be used as an alternative method for the prediction of VLE. However, its predictions are often inadequate to describe the real behavior. Therefore, more measurements are needed to develop the estimation methods and the thermodynamics models in the dilute systems. The difficulties and the challenges of the VLE measurements for systems containing sulfur compounds are discussed.

The VLE of sulfur compounds and hydrocarbons published in the open literature have been reviewed and updated with our measurements. We have determined 36 VLE data sets of systems containing sulfur compounds found in petroleum refinery streams (mercaptans, sulfides, and thiophenes) in different hydrocarbons (alkanes, alkenes, cyclic, aromatic, and oxygenated) with a variety of apparatuses (recirculation still, comparative ebulliometer, and static total pressure). Measurements for determining phase equilibria of organic sulfur compounds will be continued and more data will be published in the near future.