Mercury Species in Natural Gas Condensate

Azman Bin Shafawi

January 1999

Department of Environmental Sciences, University of Plymouth, Plymouth, U.K.

in collaboration with

Petroliam Nasional Bhd, PETRONAS, Malaysia

and

P.S. Analytical Ltd., U.K.

The presence of ultra-trace levels of mercury in industrial gas and condensate streams is a cause of both environmental and production concern. The toxic nature of the element, in all forms, together with its ability to shut-down large processing plants dictates a need for its accurate and precise measurement.

The study which investigated the recovery of various mercury species, spiked into synthetic and real condensate samples using conventional and new digestion and/or extraction techniques showed recovery was dependent upon the speciation. Using the most efficient technique, L-cysteine with persulphate, recoveries of over 90 % were obtained for diphenyl mercury, ethyl and phenyl mercury chlorides and mercury chloride. The recovery of 15 % for the important dimethyl mercury species limits the use of this technique.

A novel technique has been developed for the determination of total mercury in complex liquid hydrocarbons. Samples (up to 1.0 ml) were vaporised (400°C) and swept through a gold-coated silica trap maintained at 200°C, which retained all mercury species and discarded the matrix. The trap when heated to 900°C released the mercury for measurement by atomic fluorescence spectrometry (AFS). The recoveries for eight mercury species spiked (10 to 50 ng ml -1) into toluene and condensate were generally over 90 %. The instrumental limit of detection (LOD) was 11 pg. The total mercury content of gas condensates, gasolines and heavy oils were determined.

Gas chromatography coupled, via a pyrolysis interface, with AF detection was able to determine mercury species in gas condensate, at picogram levels (LOD: 2.5 to 7 pg) using a direct sample injection procedure. For a given column system the positive identification and quantification of up to eight mercury species was obtained. A maximum injector temperature of 125°C was recommended, to avoid the conversion of species. Mass balance calculations show a strong correlation between the total mercury content and the sum of the lower dialkyl mercury species, for all condensate samples studied.

Three commercially available mercury removal systems, A, B and C produced a reduction in the mercury content of hydrocarbon streams under pilot plant conditions. The two stage system, 'A', produced a minimum of 30 % conversion from organomercury to elemental mercury after the hydrogenation reaction in stage 1. While elemental mercury was adsorbed by the stage 2 reactor, the organomercury species were not removed. The single stage adsorber 'B' showed 100 % removal efficiency for three dialkyl mercury species in liquid hydrocarbon streams. The removal efficiency for adsorber 'C' was species dependent. Two common condensate species gave values of 50 to 80 % removal efficiency while the third species showed time-dependent bleed-off.

© 1999 by Azman Bin Shafawi. All Rights Reserved

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