by chemical reaction, Hg0


FUNCTIONAL SPECIATION OF NON-VOLATILE, SOLUBLE FORMS OF MERCURY IN HYDROCARBON LIQUIDS (LIGHT DISTILLATES)


Mercury in the Oil and Gas Industry


Mercury is a naturally occuring contaminant found in almost all oil and gas reservoirs. Typically, the mercury is present primarily in an elemental (metallic) form (Hg0


), but will often react to form other species during production and processing.


The production of oil and gas yields a complex, multiphase fl uid which, during the production process, may be subjected to variations in temperature, pressure, phase, pH and redox potential. Although the full reaction mechanisms of mercury throughout this production process are not fully understood, it is known that mercury species partition into different phases of the produced fl uids as conditions change. For example, whilst elemental mercury (Hg0


) has a relatively high boiling point (357°C), due to its high


vapour pressure, it demonstrates characteristics associated with volatile compounds, with some partitioning into the gas phase during gas and oil separation.


The liquid (condensate or oil) phase may contain a complex mixture of mercury species (1), including Hg0


(predominantly HgS), soluble ionic mercury (Hg2+ )2 typically only Hg0 .


, insoluble mercury species ) and some


non-ionic / organic mercury compounds e.g. dimethyl mercury ((CH3


Hg) and mercury thiolates (RS Hg–SR). In the gas fraction, will be present (2, 3)


Mercury concentrations in the oil range from < 0.1 µg/kg up to 20 mg/kg (4)


up to 5000 µg/m3


Processing of gas and hydrocarbon liquids that are rich in mercury can be problematic and needs to be monitored and managed on an ongoing basis. The three main issues of concern resulting from the presence of mercury are:


• Corrosion: primarily liquid metal embrittlement of aluminum components


• Catalytic poisoning: an issue for downstream refi ning processes


• Environmental issues: includes worker exposure and release to the environment


Mercury forms amalgams with other metals, particularly aluminium, and has the potential to cause corrosion of welds, cryogenic components, aluminium based heat exchangers, compressor seals/ stems/seats and pump shafts made from copper alloys. There have been some widely publicised incidents where mercury corrosion of aluminium heat exchangers has caused catastrophic failures leading to plant shutdowns and, in one unfortunate case, resulted in several fatalities (6).


Why is Mercury Speciation Required in Liquid Hydrocarbons?


As a consequence of the issues arising from the presence of mercury, even at parts per billion (µg/kg) concentrations, it may be necessary to incorporate a mercury removal system into the production/process design. There are several options for removal of mercury from liquid hydrocarbons but, most commonly, mercury removal beds (MRBs) are employed. MRBs are generally fi lled with media that removes,


Figure 1: Schematic of purge and trap apparatus


whilst in the gas phase, concentrations from < 0.01 µg/m3 have been reported (5).


Aims of the Study The aims of this initial study conducted by Qa3 were to (i) evaluate


the use of a fi ne sintered frit during the purge stage and the effect on the recovery of Hg0


(ii) identify the most effi cient aqueous


solution for extraction of soluble ionic species (iii) quantify the extraction effi ciency of various inorganic and organic ionic mercury species and (iv) evaluate the impact that the partitioning behaviour of the species under study may have on the accuracy of quantifi cation of the non-ionic organic and ionic species.


Experimental Purge and Trap (Figure 1)


Synthetic deoxygenated condensates were spiked in turn with a known concentration of (i) Hg0


(ii) (CH3 chloride (HgCl2 )2 Hg and (iii) mercuric ). An aliquot of the spiked condensate was decanted


into a Drechsel bottle, held at 0°C and purged with nitrogen at a fl ow rate of 500 mL/min for a period of 30 minutes using a fi ne sintered frit to provide a greater contact area between purge gas and sample.


only; however, a signifi cant proportion


of any particulate forms of mercury may be removed physically. As other forms of soluble mercury will almost certainly be present, speciation is necessary in order to estimate the expected effi ciency and appropriateness of a standard MRB, prior to installation.


Molecular Speciation Techniques


Due to the losses of mercury that occur over time when hydrocarbon samples are stored, even in inert coated sampling bottles (7), and the relatively quick transformation of mercury from one species to another in liquid hydrocarbons (8), it is desirable to perform speciation analysis on site immediately after sampling. This excludes the use of molecular speciation techniques such as GC-ICP-MS (9) and SEC-ICP-HR-MS (10) since portable instruments are not available.


The international standard test method UOP 938-10 describes a procedure for the functional speciation of soluble mercury in hydrocarbon liquids. The precise application of this method on site or in an offshore environment is diffi cult as there are often unyielding time constraints that necessitate adaptations. Modifi cations are also necessary to accommodate direct analysis of samples with very high mercury concentrations and the use of combustion/AAS analysers supplied by alternative manufacturers; however, the method provides a base from which on-site mercury speciation methodology can be developed.


UOP 938-10 utilises the chemical and physical properties of soluble mercury species in particulate-free samples to categorise them into (i) total soluble (ii) elemental (iii) non-ionic organic and (iv) ionic species. The initial fi ltering of the sample provides a route for the quantifi cation of insoluble mercury species. However, UOP 938-10 provides no information on the extraction effi ciency of individual ionic species and very little documentation can be found in the literature to support the assumption that the extraction employed will yield a complete separation and consistently accurate quantifi cation of the non-ionic organic and ionic species.


Clearly, a more comprehensive understanding of the effi ciency of extraction of the commonly found ionic mercury species would provide the necessary information for assessing how much of each compound is included in each of the two categories and assessing the suitability of the mercury removal technologies available without having to instigate a live trial. It would also help identify where the mercury species may be expected to partition within the hydrocarbon processing system.


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