Conditions and thermophysical properties for transport of hydrocarbons and natural gas at high pressures: Dense phase and anomalous supercritical state

Abstract

Hydrocarbons and natural gas, like other fluids exhibit anomalous behavior in the vicinity of their critical points (Pc, Tc), where flow oscillations, thermal instabilities, and decreased heat transfer can occur. This work, based on Peng-Robinson equations of state, demonstrates that in the case of mixtures, the cricondenbar, cricondentherm, and a part of the dense phase belong to this anomalous region. Consequently, the most desirable conditions for the transport of supercritical natural gas (SNG) are: above the critical pressure and critical temperature, above the cricondenbar and cricondentherm, and beyond the anomalous state. For methane, such conditions exist at pressure, P > 5 MPa and temperature, T > - 30 °C whereas for average composition of natural gases from US/Canada, West Asia, and North Sea, P ≥ 6 MPa and T > −30 °C may be appropriate/safe for SNG transport. Corresponding gray zones which will require special design considerations are also identified. Reduced pressure, (P/Pc) of 1.15 and reduced temperature, (T/Tc) of 1.25 and 1.4 may be appropriate to delimit the Unsafe Zone/Gray Zone and Gray Zone/Safe zone. In addition, the specific volume and kinematic viscosity show asymptotic behavior at high SC pressures that can have important implications in the transport of hydrocarbons and natural gas. Moreover, the critical temperature, cricondentherm, and anomalous region of natural gas can be moved to lower temperatures by adding one or more modifier gases with low Tc, e.g., methane, nitrogen, and argon; or, higher using gases with high Tc such as ethane, propane, and carbon dioxide. The pipelines carrying SNG, with Tc-modifications if needed, can therefore pass through polar, tropical, arid, and desert-like conditions.