Experimental determination and theoretical calculation for CO₂ liquid-solid phase equilibrium in PLNG

Objective Compared with traditional submarine pipelines, Floating Liquefied Natural Gas (FLNG) facilities are deemed more suitable for the exploitation of offshore natural gas. However, their current high cost presents challenges in terms of economic efficiency for some gas fields, limiting their widespread adoption and application to some extent. Pressurized Liquefied Natural Gas (PLNG) technology has emerged as a solution to this issue associated with FLNG. Under pressurization conditions, the liquefaction temperature of natural gas increases, leading to a rise in the solubility of impurities such as carbon dioxide. This property enables the adoption of a simplified natural gas pretreatment unit, and even the exclusion of this unit in some cases, for gas sources with a low CO₂ content. Understanding the liquidsolid phase equilibrium mechanism of CO₂ in PLNG is crucial for determining the gas-mass treatment indicators of PLNG.

Methods A novel liquid-solid phase equilibrium test setup was designed and built with visualization and continuous sampling functions. This setup was employed to experimentally determine the liquid-solid phase equilibrium of carbon dioxide. Furthermore, a theoretical calculation model of CO₂ solid solubility was developed, following the principle of liquid-solid phase equilibrium. The binary interaction coefficient was optimized using the genetic algorithm and test data.

Results In the test, the solubility of carbon dioxide solid in LNG exceeded 1.5% at approximately 162 K (equivalent to the saturated vapor pressure of pure methane at around 1.7 MPa). The calculations demonstrated improved accuracy in CO₂ solid solubility derived from the established model that was optimized using the genetic algorithm. For instance, considering the solubility of CO₂ in pure methane, the average relative percentage error between the results calculated using the optimized model and test data decreased significantly from 10.83% to 2.333 6%.

Conclusion This study provides a theoretical calculation model with high accuracy, which can be utilized as the foundation for establishing gas-mass indicators for carbon dioxide pretreatment under pressurized liquefaction conditions. The test setup developed in this research is well-suited for future explorations into the liquid-solid phase equilibrium of heavy hydrocarbon components in LNG within the PLNG temperature range. Additionally, this setup has the potential to contribute to the development of a robust impurity precipitation model.