Internal pressure and solubility parameter as a function of pressure

Abstract

The main goal of this work was to measure the solubility parameter of a complex mixture, such as a crude oil, especially as a function of pressure. Thus, its definition is explained, as well as the main approximations generally used in literature. Then, the internal pressure is investigated, since it is presented as an alternative of the solubility parameter. In this work, the assumption that internal pressure is a measure of the physical solubility parameter was made, i.e. representing the dispersion and polar forces. As for the pressure influence, it was seen that internal pressure reaches a maximum contrary to solubility parameter. An indirect method was chosen to estimate internal pressure, using thermal expansivities (determined by microcalorimetry) and isothermal compressibilities (determined by density measurements). The uncertainty is within 2% for the expansivity and 0.1% for the density. Five pure compounds (four hydrocarbons and 1 alcohol) were investigated at 303.15 K and up to 30 MPa, as well as a dead crude oil. The “physical” solubility parameter is slightly increasing with pressure (up to 0.8 MPa 1/2 for cyclohexane) and, at 0.1 MPa, the difference with literature data is less than 1 MPa 1/2 for hydrocarbons. On the contrary, the difference reaches 9 MPa 1/2 for ethanol as expected, due to the presence of hydrogen bonding. A dead crude oil was also studied and its solubility parameter is within the expected range. Two cubic equations of states (Peng–Robinson and Soave–Redlich–Kwong) were able to approximate the “physical” solubility parameter of n-heptane (within 0.2 MPa 1/2), providing that the volumes were measured and used as input. The Peng–Robinson equation gave somewhat better results.