Solid-Fluid Phase Equilibria forNatural Gas Processing at LowTemperatures

Abstract

Precipitation and deposition of solid components create potential risks of blocking gas passages in processes such as in LNG plants. To avoid such risks, experimental data and modelling of solid-fluid equilibrium should be used to optimize the design and operations. The objective of this work was to get a better understanding of the fundamentals of solid-fluid phase equilibrium. The specific focus of this work was to study solid-fluid phase behavior in systems of solid CO2, heavy hydrocarbons(HHC) and hydrate in equilibria with natural gas at low temperatures.

Experimental methods for measuring solid-fluid equilibrium data in natural gas systems at low temperatures were extensively reviewed, and important and practical issues for designing experimental systems were summarized. The frost points in the CO2-methane systems (CO2 mole fraction 0.108 to 0.542) were measured in this work. Meanwhile, in another experimental setup, the water content in the gas phase was measured in the hydratemethane and hydrate-natural gas systems down to temperature 238.15 K. These data, together with data from other researchers, were used to verify the thermodynamic models.

It is expensive and time-consuming to get experimental data at low temperatures, thus it is important to verify and use thermodynamic models to predict the solid-fluid phase behaviors. In the systems of solid CO2 and HHC in equilibrium with natural gas systems, the Soave-Redlich-Kwong (SRK) Equation of State (EOS) and simplified Perturbed-Chain Statistic Associating Fluid Theory (sPC-SAFT) EOS were used to calculate the fugacities in fluid phases. For solid phase, one fugacity model based on sublimation pressures and one model based on subcooled liquid were used. For correlating and predicting the hydrate behaviors, the Cubic-Plus-Association (CPA)EOS was used to model fluid phases and the hydrate-forming conditions were modelled by the solid solution theory of van der Waals and Platteeuw. Examples of applications of this work in LNG plant were illustrated, and the applications in other gas processing technologies were introduced. This work helped in better understanding and solving the solid blocking risks in low temperature gas treating processes.