Redesign of a High-Pressure, Single-Sinker Magnetic Suspension Densimeter to Measure Highly Accurate Densities for Fluids: Applications to Helium, Argon, and Helium + Methane Mixtures

Abstract

The petrochemical industry requires highly accurate equations of state (EoS) to calculate thermodynamic properties such as densities and calorific properties. However, the accuracy of an EoS depends upon the accuracy of the data used to construct it. Thus, a need exists for high accuracy p-p-T measurements. Multiple apparatus can provide high accuracy p-p-T measurements, but they do not operate over broad ranges of pressure and temperature. One apparatus that can operate over a broad range is a single sinker magnetic suspension densimeter (MSD). This work presents the redesign of the TAMU MSD.

This apparatus is a unique MSD because its pressure measurement range extends to 200 MPa. A system redesign has enabled the apparatus to achieve a temperature range of 300 to 500 K. The redesign entailed creating a new electrical heating system, heating shields, vacuum insulation, and new frame. Improvements for the measurement processes of the system include a new measurement sequence that reduces measurement time by approximately half.

After recommissioning the MSD, nitrogen measurements validated the system performance. After verifying system accuracy, measurements included two pure fluids, helium and argon, from 300 to 450 K up to 200 MPa. Additional measurements included three binary mixtures of methane + helium covering the same property ranges.

Finally, this work proposes a new approach to creating mixing rules for binary mixtures of “simple” molecules based upon a quadratic compositional dependence of the residual Helmholtz energy. This approach describes the contributions from interactions between unlike molecules with an interaction Helmholtz energy. A rational polynomial in density with coefficients having both temperature and compositional dependence describes these interactions within the accuracy of experimental measurements. This form is less complex than other mixture models that include exponential terms in density, thus the approach is more attractive for process modeling. Mixtures containing methane, ethane, nitrogen, carbon dioxide, argon, hydrogen, krypton and helium provide tests for the mixing rule.