Carbon dioxide properties
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Thermodynamic Properties of Carbon Dioxide
Carbon dioxide (CO₂) exhibits well-characterized thermodynamic properties, including entropy, enthalpy, Gibbs free energy, and phase behavior. Analytical expressions for entropy, enthalpy, and Gibbs free energy have been developed, providing highly accurate predictions across a wide temperature range (300–6000 K), with deviations from standard databases being less than 1% for all three properties 3. Modern equations of state, such as those based on the Helmholtz free energy, accurately represent CO₂’s thermal properties, phase behavior, speed of sound, and heat capacities across a broad range of temperatures (from the triple point up to 1100 K) and pressures (up to 800 MPa), with uncertainties as low as ±0.03% for density and ±1.5% for isobaric heat capacity in key regions 5. The phase diagram of CO₂ is well understood, showing its existence as a gas, liquid, solid, or supercritical fluid depending on temperature and pressure 6.
Transport Properties: Viscosity, Thermal Conductivity, and Diffusivity
CO₂’s transport properties—viscosity, thermal conductivity, and self-diffusivity—have been extensively studied. Representative equations for viscosity and thermal conductivity cover temperatures from 200 K to 1500 K and pressures up to 100 MPa, with uncertainties as low as ±0.3% for viscosity in dilute gas near room temperature and up to ±5% for thermal conductivity in the liquid phase 1. Reduced state correlations for these properties show average deviations of 2.0% for viscosity, 1.4% for thermal conductivity, and 5.9% for self-diffusivity when compared to experimental data, even near the critical point 2. Molecular simulations confirm good agreement with experimental values for viscosity and phase coexistence densities, though some discrepancies remain for thermal conductivity at low densities 4.
Physical Properties: Density, Specific Heat, and Compressibility
The density, specific heat, and compressibility factor of CO₂ are crucial for applications such as pipeline transport and industrial processes. These properties vary significantly with temperature and pressure, especially in mixtures or under supercritical conditions. Understanding these variations is essential for safe and efficient transport and utilization of CO₂, particularly in high-pressure pipelines and gas fields 7. Accurate property tables and models are available to support engineering calculations and system design 5.
Supercritical Carbon Dioxide: Unique Characteristics
When CO₂ is brought above its critical temperature and pressure, it becomes a supercritical fluid (sCO₂), exhibiting a combination of gas-like and liquid-like properties. Supercritical CO₂ has low viscosity, high diffusivity, and high density, making it valuable for applications in extraction, dyeing, pharmaceuticals, power generation, and heat transfer. Its high solubility and easy compressibility further enhance its industrial utility 8.
Estimation Methods for Thermodynamic Properties
Several methods exist for estimating the thermodynamic properties of CO₂ and its derivatives. Group additivity and semi-empirical quantum-chemistry methods are widely used for calculating standard enthalpies of formation and Gibbs energies, with semi-empirical methods providing higher accuracy and robustness for a wide range of CO₂-based products 9.
CO₂ in Mixtures and Refrigeration Applications
CO₂ is often used in mixtures with other gases, such as in refrigeration systems. The properties of CO₂-containing mixtures, including density and phase behavior, have been measured and modeled with high precision, supporting the development of environmentally friendly refrigerants and efficient refrigeration cycles 10.
Conclusion
Carbon dioxide’s properties—thermodynamic, transport, and physical—are well understood and accurately modeled across a wide range of conditions. This knowledge underpins its safe and efficient use in industrial processes, energy systems, and environmental applications, with ongoing research refining property correlations and expanding their applicability to new technologies and mixtures 12345678+2 MORE.
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