Accurate Density Measurements of Co2 at the Vicinity of the Critical Point by a Single-Sinker Densimeter

Abstract

This study presents a renovated single-sinker magnetic suspension densimeter system for density measurements at temperature from 298.15 to 423.15 K and pressure up to 35 MPa. The expanded uncertainties (k = 2) are 25 mK for temperature, 4 kPa for pressure, and 0.05% for density. The system was first used to measure density of nitrogen at temperature from 308.15 to 423.15 K and pressure up to 20 MPa, yielding an average absolute relative deviation of 0.05% compared to the reference multi-parameter equation of state (EoS) of Span and Wagner. Density of carbon dioxide was then measured mainly at the vicinity of the critical point, where the reference EoS deviates from data in literature. Vapor pressure and liquid density at saturated conditions were also determined. The new experimental data were compared to data from literature, the reference EoS of Span and Wagner and the crossover EoS with strict asymptotic singular behavior and critical exponent at critical point by Yang et al.. The results shows that the new data generally yield good agreement with most of literature data, while both EoS underestimate density at the vicinity of the critical point. The EoS of Yang et al., as compared to EoS of Span and Wagner, provides a more accurate description of critical-point behavior while yielding similar accuracy in density and pressure calculation.

Keywords: density, carbon dioxide, near-critical, single-sinker magnetic suspension densimeter