Calculating the chemical and phase equilibria of mercury in natural gas

Abstract Elemental mercury occurs naturally in traces in fossil fuels, such as crude oil and natural gas. Knowledge of the mercury solubility in natural gas is critical in order to avoid mercury drop-out, which can cause health, safety, and environmental issues during operation, maintenance, or equipment decommissioning. Moreover, mercury transformation to other forms, such as solid HgS, further complicates the management of mercury levels in a processing plant, as well as the design of mercury removal processes. Therefore, thermodynamic models and algorithms that can accurately describe the chemical and phase equilibria of mercury in natural gas are of paramount importance. In this work, a multiphase flash algorithm is implemented for calculating the solubility of elemental mercury in typical natural gas fluids. The algorithm can handle three- and even four-phase systems (vapor-liquid hydrocarbon-water-mercury), while a free-mercury assumption is proposed to accelerate solution speed. In addition, a simultaneous chemical and phase equilibrium algorithm is employed for studying a theoretical reaction between mercury and H2S that can lead to solid HgS formation in natural gas. Both algorithms are coupled with the UMR-PRU model, and new interaction parameters are estimated for groups Hg and H2S with CO2, N2, and hydrocarbons, yielding very satisfactory results. Despite the lack of experimental data for comparison, the study of mercury solubility and reaction in natural gas with the proposed algorithms leads to very useful qualitative results, which agree with field observations.