Removal of heavy metals from hydrocarbons

Abstract

The research presented in this thesis focused on enhancing the reactivity of elemental sulfur by utilizing ionic liquids composed of quaternary ammonium and phosphonium organic salts paired with alkyl carboxylate anions of various lengths. This approach facilitated the formation of active sulfur species, such as hexapolysulfides and radical polysulfides. These reactive sulfur species were employed to oxidize and complex various mercury species present in hydrocarbon, water, and mixtures of both matrices, leading to the precipitation of mercury as β-HgS. The ionic liquids also functioned as phase transfer agents, enabling the migration of active anionic sulfur species between hydrocarbon and aqueous phases. Significant variations in sulfur solubility were observed depending on the anion chain length, revealing a defined solubility limit. Colour change was observed as sulfur dissolved in the ionic liquids, transitioning from yellow sulfur to orange-red and finally dark green. Corrosion testing revealed negligible corrosion rates for carbon steel for sulfur-containing quaternary ammonium based ionic liquids, though some surface corrosion was observed in corresponding phosphonium-based ionic liquids after exposure to moisture. Mercury extraction efficiencies for sulfur-containing ionic liquids were compared with conventional aqueous sulfides (e.g. sodium sulfide, etc.) across mercury containing matrices, including natural gas condensate, produced water and mixtures of both. Mercury extraction was evaluated using various mercury species comprised of elemental mercury, molecular mercury (e.g. mercury(II) chloride) and ionic mercury (e.g. mercury(II) nitrate). Aqueous sulfides achieved up to 99% mercury removal in water and 78% in hydrocarbons. While sulfur-containing ionic liquids, particularly those with shorter alkyl chains, demonstrated superior and consistent mercury extraction efficiencies exceeding 97%, driven by the presence of highly reactive sulfur species. These sulfur-containing ionic liquids outperformed conventional aqueous sulfides, reducing residual mercury concentrations from ca. 1,000 μg/kg to as low as < 1 μg/kg. Mercury extraction resulted in the formation of the black, beta-polymorph of mercury(II) sulfide (i.e. β-HgS). Microscopy and particle size distribution analysis identified the β-HgS as nanoparticles with spherical morphology and a tendency to form aggregates. The addition of polymeric flocculants (both cationic and anionic) led to the formation of larger β-HgS flocs in water. Computational fluid dynamics analysis was performed to determine the migration behavior of β-HgS particles and flocs in a three-phase separator and established ≤ 10 μm as the threshold size to enable proper settling within the separator. Overall, the findings highlight the potential of sulfur-containing ionic liquids as highly effective agents for mercury abatement in the oil and gas industry, offering a promising alternative to conventional removal technologies.

Thesis is embargoed until 31st July 2030.

Date of Award	Jul 2025
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Petronas Research
Supervisor	John Holbrey (Supervisor) & David Rooney (Supervisor)