Improved Joule Thomson equation of supercritical CO2-rich natural gas in separation system

Abstract

The rapid expansion of supercritical gas technology for high-content CO2 separation from natural gas is a promising avenue of research. However, CO2-rich natural gas cools immediately after being separated and expands when the CO2 dew point is reached in the absence of a refrigerant system. In our previous study, supercritical expansion experiments using various CO2 compositions revealed that the Joule–Thomson equation gives a significant absolute average error value of 16.28%. This paper describes corrections to the Joule–Thomson expansion equation under supercritical conditions with various CO2 concentrations. The results show that the trend of the expansion coefficient is highly dependent on the CO2 composition. Using an improved Joule–Thomson equation of state over a CO2 range of 25%–45% mol, the expansion coefficient tends to fall immediately when a rapid expansion occurs. For a supercritical fluid, the specific heat Cp depends on temperature, pressure, and density changes. The Van der Waals expansion coefficient profile is simulated using MATLAB, resulting in a correction factor of 1.17–1.32 being applied to the Cp value for CO2 concentrations of 25%–40% mol, whereby the absolute average error tends to zero. For CO2 concentrations of more than 40%, the Joule–Thomson equation cannot be applied because the expansion coefficient exhibits significant errors compared with the experimental data. The expansion coefficient does not directly determine the performance of supercritical expansion, but does affect the vapor fraction. Integrated production systems based on supercritical expansion are expected to produce an annual profit of around US$18 million from turbine expansion and US$489 million from the production of sweet gas with a purity of 96.6% and less than 2% mol CO2.