Thermal Decomposition of Metacinnabar (β-HgS) during Monoethylene Glycol Regeneration in Natural Gas Processing

Elemental mercury and mercury (Hg)-bearing particles may be present in gas and condensate from specific geologic reservoirs and be coproduced with them. In this study, we found that over 70% of the Hg mass in field monoethylene glycol (MEG) is present as 100-200 nm particulate β-HgS, and it is therefore important to understand the decomposition behavior of β-HgS in MEG to determine the partitioning of mercury species in liquid natural gas (LNG) plants. Thermal decomposition studies in MEG and MEG-water solutions showed that β-HgS decomposition to elemental mercury started at around 100 °C, which is significantly lower than the 200 °C required for β-HgS decomposition in an inert gas. Density functional theory calculations supported the experimental observations that β-HgS has a lower decomposition temperature in solvents than its counterpart without a solvent because the solvent interactions decrease the Hg-S bond strength. Thermal decomposition studies at 130 °C showed that increased water content and decreased β-HgS particle size significantly increased the decomposition rate, while some common additives in field MEG did not have a significant effect. Experiment results suggest the decomposition pathway of β-HgS in MEG/water includes dissolution to form dissolved Hg(II) ions, followed by reduction to form elemental mercury by reaction with MEG. This study highlights the strong effect of solvent on the thermal decomposition mechanism of β-HgS, improving our understanding of the fate and species of Hg in petrochemical processing.