ISOBARIC HEAT CAPACITY OF SUPERCRITICAL FLUIDS: EXPERIMENTAL MEASUREMENTS AND MODELING

Abstract

Fluids at high temperature and pressure are commonplace across engineering applications. These fluids are sometimes the product of a harsh environment, as in subsurface operations such as oil and gas development or geothermal energy production, or at other times, exploited for their desirable transport and thermodynamic properties in, for example, chemical extractions and material synthesis. At these elevated conditions, many single and multi-component fluids exist in a supercritical state, where small changes in temperature and pressure cause significant property variations without induction of a phase change. A fluid’s isobaric heat capacity (Cp), a property important to most thermal processes, fluctuates strongly in the supercritical region, ranging from a few times the ideal gas state Cp to infinity exactly at the critical point for pure fluids. Accurately capturing the thermophysical behavior of supercritical pure fluids and mixtures is necessary for optimal design and efficient operation of numerous engineering processes that utilize these substances. The main goal of this research was to increase understanding of isobaric heat capacity property changes in supercritical fluids. The first step in a threefold approach was to design and construct a calorimeter for precise and accurate measurements of Cp in the supercritical region. Second was to carry out experimental Cp measurements for pure fluids and representative fluid mixtures in pressure-temperature-composition regions with only limited available data. Systems studied included: CO2-methanol, common in supercritical fluid extraction and chemical processing; CO2-decane, relevant to tertiary recovery in oil and gas applications; and R1234yf, a low-global-warming-potential drop-in replacement refrigerant for R134a which is currently used in heat pumps and power cycles. And finally, the third step was to use equations of state and molecular simulations to extend Cp data and predict behavior in the supercritical region. A flow calorimeter was built to operate over a wide range of temperatures (20-150 °C), pressures (1-300 bar), and densities (1-1000 kg/m3). By precisely placing both the measurement devices and the heating element in direct contact with the fluid, and by limiting experimental heat losses through vacuum insulation and immersion of the entire apparatus in a fluidized thermal bath, the calorimeter achieved ±1% accurate measurements of Cp. To further our ability to model mixtures at supercritical conditions, measurements of CO2-methanol were compared to Monte Carlo molecular simulation predictions of Cp. The average absolute deviation, when compared to experiment, of the simulation results is comparable with the current state-of-the-art equation of state (4% versus 3%, respectively). However, the molecular simulations were significantly less correct predicting Cp in the near critical region where Cp was most sensitive to small changes in temperature and pressure. This region of greatest sensitivity, called the “heat capacity ridge,” was mapped onto temperature-pressure-composition coordinates for CO2-methanol and CO2-decane using experimental measurements. For single component fluids and binary mixtures, the subtle difference in the relationship between a fluid’s heat capacity ridge and its critical point was demonstrated.