Estimation of Compressibility and Other Derivative Properties of CO2 Using Sonic Velocity for Carbon Capture and Storage and Enhanced Geothermal Applications

Summary Reduction in carbon footprint has been at the forefront of many manufacturing and energy industries. Carbon capture and storage (CCS) and enhanced geothermal systems (EGS) using CO2 as energy carrier are some of the possible decarbonization pathways. Process design and optimization for these options require accurate estimation of thermochemical properties of CO2 at various pressure and temperature (PT) conditions, in both subcritical and supercritical regions. The objective of this work is to present coupled experimental- and equation-of-state (EOS) modeling based on general framework to estimate various fluid properties, such as heat capacities, enthalpy, entropy, sonic velocity, density, Joule-Thomson coefficient, and compressibility of CO2 that is applicable to wide range of PT conditions. The sonic velocity measurement is based on a pulse-echo technique, while the density measurements were performed in a pressure/volume/temperature cell. The subject measurements were conducted at two temperatures (300 K and 311 K), one below and the other one being above the critical temperature of CO2 (304 K). The pressure points for the measurements range between 1 bar and 200 bar. Phase behavior is modeled using the Peng-Robinson EOS (Peng and Robinson 1976 and 1978; PR-78-EOS) with Péneloux et al. (1982) volume shift to accurately determine the CO2 density. First, the ideal part of the CO2 heat capacity is obtained from correlations available in literature, and the residual part is obtained using the EOS. After the evaluation of the heat capacities, other properties, such as enthalpy, entropy, speed of sound, Joule-Thomson coefficient, and compressibility, are directly obtained from the EOS. The modeling results were compared with both the newly generated and the literature experimental data on sonic velocity and density of CO2 at two different temperatures (300 K and 311 K) within the pressure range of 1–200 bar. The main results are as follows: (1) Experimental results on density and sonic velocity are aligned with the measured data found in the literature. (2) Estimation of the CO2 properties from the EOS-based framework agrees very well with the literature and newly presented data within 1–4% relative error. (3) Compressibility of the fluid is derived directly from the experimental measurements, bypassing the density-derivative-based approach, and hence, avoiding the significant errors associated with being derivative property as well as with discrete density data containing noise/fluctuations. (4) Most importantly, the framework is general and applicable for the use of any other EOS models and can also be extended to other fluid systems. Novelty of this work lies in new experimental data on sonic velocity and density of CO2 (especially at high pressures) as well as development of an EOS framework to determine thermodynamic properties of CO2 through sonic velocity. The proposed framework leads to more accurate estimation of fluid properties, such as compressibility, density, sonic velocity, heat capacities, enthalpy, and entropy.