Estimation of Compressibility and Other Thermodynamic Derivative Properties of CO2 Using Sonic Velocity for CCS and Enhanced Geothermal Applications
Authors: Ratnakar R. R. 
Published: August 2024

Reduction in Carbon-footprint has been gaining attention in variety of industries from manufacturing to energy due to the geopolitical pressures and climate related issues. Carbon capture and storage (CCS) and enhanced geothermal systems using CO2 as energy carrier are some of the possible decarbonization pathways. Process design for these options requires accurate estimation of thermochemical properties of CO2 at various temperature/pressure 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 heat capacities, enthalpy, entropy, sonic velocity, density, Joule-Thomson coefficient, and compressibility of CO2 that is applicable to wide range of pressure and temperature conditions.

The sonic velocity measurement is based on a pulse-echo technique while the density measurements were performed in a PVT cell. The subject measurements were conducted at two temperatures (300 and 311K), one below and the other one being above the critical temperature of CO2 (304K). The pressure points for the measurements range between 1 - 200 bar. Phase behavior is modeled using Peng and Robinson (1976, 1978) Equation of State (PR78-EOS) with Peneloux et al. (1982) volume-shift 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 evaluation of the heat capacities, enthalpy, entropy, speed of sound, Joule-Thomson coefficient and compressibility are directly obtained from EOS.

This work presents experimental and modeling results on sonic velocity and density of CO2 at two different temperatures (300 and 311K) within the pressure range of 1- 200 bar. An EOS-based framework, utilizing PR78 with Peneloux et al. volume shift, is developed to determine the CO2 properties (such as phase boundary, density, heat capacities, enthalpy, entropy, sonic velocity and compressibility) at extended pressure and temperature conditions. The main results of this study are as follows:

Experimental results on density and sonic velocity are aligned with the measured data found in the literature. Estimation of the CO2 properties from EOS-based framework agrees very well with the literature and newly presented data within, all within 1-3% relative error. 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 the discrete density data containing noise/fluctuations and as well as the nature of the compressibility being a derivative property. Most importantly, the framework is general, and applicable for the use of 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. Proposed framework leads to more accurate estimation of compressibility, density, sonic velocity, heat capacities, enthalpy and entropy.