Determination of Volumetric Properties Using Refractive Index Measurements for Nonpolar Hydrocarbons and Crude Oils

Abstract

A novel method to evaluate volumetric properties, namely the thermal expansivity (αP) and the isothermal compressibility (κT) for nonpolar hydrocarbon systems using refractive index measurements, is presented in this work. New expressions for αP and κT are derived from the Lorentz–Lorenz equation and the One-Third rule, respectively. A further simplified expression for αP is proposed requiring only refractive index data and molecular weight for calculation. Densities and refractive indices of 12 pure nonpolar hydrocarbons, 6 hydrocarbon mixtures, and 3 crude oils are measured at temperatures from 283.15 K up to 343.15 K and at 0.1 MPa. The measured refractive indices are used to calculate αP for a wide range of temperatures using the proposed method, and the measured densities are used to calculate αP for comparison. Reported densities and refractive indices of benzene at 298.15 K and at pressures up to 90 MPa are used for κT evaluations with the proposed method. Values of αP and κT calculated from refractive index measurements are in good agreement with experimental data and those determined from densities. This work aims to establish the foundation for experimental methods to determine volumetric properties of nonpolar hydrocarbon systems based on refractive index measurements. A high temperature and high pressure refractometer is expected to have multiple advantages over conventional techniques for density measurements, which include but are not limited to smaller amounts of sample needed, simpler calibration, faster measurement, and cells that are corrosion-resistant (i.e., sapphire glass).