Mercury Speciation and Distribution in an Egyptian Natural Gas Processing Plant

Unprocessed petroleum hydrocarbons often contain high concentrations of mercury (Hg), which can severely damage the metal components of a processing plant and pose a health risk to the workers and the natural environment. While Hg removal units can significantly reduce the Hg concentration in the export products, they are often installed in the final stage of the processing plant, thus failing to protect the production facilities. In this study, Hg distribution within a natural gas processing plant was studied in order to identify the most effective place for an Hg removal unit. Additionally, impact of sampling container materials and their acidification was evaluated and Hg species in the condensate were quantified. Total Hg concentration was significantly higher in all samples stored in the glass in comparison with plastic containers. However, the acidification effect of the containers was more pronounced for Hg in non-polar solutions. Interestingly, the assessment of Hg distribution within the gas plant showed that the export gas is being enriched in Hg, which concentration has risen from 1.25 to 4.11 µg/Sm3 during the processing steps. The 2nd stage separator was identified as the source of excess Hg, which partitioned from the liquid phase of condensate to the gas phase as a result of reduced operational pressure and temperature. The dominant Hg species found in the analyzed gas condensates were elemental Hg (Hg0) and inorganic Hg, with the methylmercury fraction comprising up to 18%. However, it was also found that the % fraction of individual Hg species varied along the plant units most probably as a result of Hg0 migration to the export gas. Therefore, in order to protect all treatment facilities from Hg contamination, the Hg removal unit should be installed after the 2nd stage compressor.