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Energy & Fuels 2006, 20, 180-186
Identification and Properties of Mercury Species in Crude Oil† S. Mark Wilhelm,* Lian Liang, and David Kirchgessner Mercury Technology SerVices, 23014 Lutheran Church Road, Tomball, Texas 77377 ReceiVed May 11, 2005. ReVised Manuscript ReceiVed September 6, 2005
The identities and properties of mercury species in crude oil were investigated. Dimethylmercury (DMHg) and elemental mercury (Hg0) were independently quantified as volatile species evaporated from a single crude oil using selective trapping. Henry’s law constants of Hg0 and DMHg in toluene were determined by measuring concentrations in vapor/liquid equilibrium over a range of temperatures. Of the two, elemental mercury, as a dissolved species in the two-phase toluene liquid/vapor systems, has the higher fugacity and Henry’s constant. Nonvolatile forms of mercury associated with asphaltenes were identified using differential solubility methods. Asphaltene mercury was found to be a significant fraction of the total mercury concentration in the crude oils that were examined. The newly developed analytical protocols have assisted in understanding how mercury and its compounds distribute in petroleum processing.
1. Introduction Mercury, in several of its chemical forms, is a trace component of all fossil fuel source materials including coal, petroleum, natural gas, gas condensates, oil shale, and bitumen.1-3 The chemical nature of mercury in such materials is of interest for a variety of reasons but most notably because it is important to understand the fate of the various species as they partition to fuels and waste streams in processing.4 Practical considerations also exist because mercury affects equipment integrity,5 catalyst performance,6 product quality, and the health and safety of workers.7 In addition, the chemical behavior of mercury species in crude oil is of interest to analysts dedicated to method development. Mercury in crude oil can be operationally speciated into the following categories.8 • Category 1 which has the volatile mercury defined as that fraction that can be sparged from fresh crude oil samples by * To whom correspondence should be addressed. E-mail: smw@ hgtech.com. † Disclaimer: The views expressed by individual authors are their own and do not necessarily reflect those of the U.S. Environmental Protection Agency. Mention of trade names, products, or services does not convey, and should not be interpreted as conveying, official EPA approval, endorsement, or recommendation. (1) Wilhelm, S. M.; Bloom, N. S. Mercury in Petroleum. Fuel Process. Technol. 2000, 63, 1. (2) Brown, T. D.; Smith, D. N.; Hargis, R. A.; O’Dowd, W. J. Mercury Measurement and Its Control: What We Know, Have Learned, and Need to Further Investigate. J. Air Waste Manage. Assoc. 1999, 6, 1. (3) Fox, J. P. Distribution of Mercury during Simulated in situ Oil Shale Retorting. EnViron. Sci. Technol. 1985, 19, 316. (4) Wilhelm, S. M. An Estimate of Mercury Emissions to the Atmosphere from Petroleum. EnViron. Sci. Technol. 2001, 35, 4704. (5) Wilhelm, S. M. Methods to Combat Liquid Metal Embrittlement by Mercury in Cryogenic Aluminum Heat Exchangers. In Proceedings of the 73rd Annual ConVention of the Gas Processors Association, New Orleans, LA, March 7-9, 1994; Gas Processors Association: Tulsa, OK, 1994. (6) Menon, R. Mercury and its Impact on Ethylene Purification Catalysts. In Proceedings AIChE 2005 Spring National Meeting, Atlanta, GA, April 2005; American Institute of Chemical Engineers: New York, 2005. (7) Wilhelm, S. M. Avoiding Exposure to Mercury during Inspection and Maintenance Operations in Oil and Gas Processing. Process Saf. Prog. 1999, 118, 178. (8) Bloom, N. S. Analysis and Stability of Mercury Speciation in Petroleum Hydrocarbons. Fresenius’ J. Anal. Chem. 2000, 366, 438.
inert gas at ambient temperature. Volatile mercury is determined by difference of measured total mercury concentration (THg) before and after sparging and by mass balance to mercury trapped on gold from sparge vapor. • Category 2 which has the insoluble forms defined as that fraction that can be separated from dissolved forms by simple filtration (0.45 µm filter typically). Suspended mercury is determined by the difference of THg measured before and after filtration and by mass balance to mercury in filtered solids obtained by digestion or combustion. • Category 3 which has the purely dissolved forms defined as the difference between THg of a homogenized oil sample and THg obtained after filtration. Category 3 includes dissolved volatile forms. • Category 3a which has the water-soluble forms defined as that amount of category 3 that can be extracted from the oil phase by water or brine. Water-extracted mercury is determined by difference of measured THg before and after extraction and by mass balance to mercury in water phase. • Category 3b which has the nonvolatile and non-watersoluble forms defined as the difference between the concentration of all dissolved forms (Category 3) and volatile species (Category 1) and water-soluble forms obtained from brine extraction (Category 3b). Volatile mercury, as defined in Category 1, includes elemental mercury (Hg0) and dialkylmercury (DAHg, dimethyl-, diethyl-, ethylmethyl-, etc.) which exist in oil as dissolved species. Elemental mercury is known to be one of the volatile mercury species in crude oil because it has been physically observed as a condensed liquid in distillation trays in refineries and as condensed liquid in cryogenic equipment used to separate LPG (liquefied petroleum gas) derived from crude oil. Both elemental and dialkylmercury have been identified in gas condensates using chromatographic techniques.9,10 Whether elemental mer(9) Schickling, C.; Broekaert, J. Determination of Mercury Species in Gas Condensates by On-line Coupled HPLC and CVAA Spectrometry. Appl. Organomet. Chem. 1995, 9, 29. (10) Tao, H.; Murakami, T.; Tominaga, M.; Miyazaki, A. Mercury Speciation in Natural Gas Condensate by Gas Chromatography-Inductively Coupled Plasma Mass Spectrometry. J. Anal. At. Spectrom. 1998, 13, 1085.
10.1021/ef0501391 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/10/2005
Mercury Species in Crude Oil
Energy & Fuels, Vol. 20, No. 1, 2006 181 Table 1. Speciation of Mercury in Hydrocarbon Matrixesa
ref 9 14 15
method
matrix
HPLC-CVAA operational CGC-MIP-AES
gas condensate
10
GC-MIP-AES GC-FAPES HPLC-CVAA operational GC-ICP-MS
17 8
GC-ICP-MS operational
12
GC-CVAF operational
16
Hg0
DAHg
Hg2+
HgS
CH3Hg
X (90%)
gas condensate
X (10%) X
gas condensate
X
crude oil gas condensate naphtha condensate crude oil gas condensate crude oil
X
X
X X
X
X (1%) X
(Hg0 + Hg2+)
X
X
X
X
others
X
X X
suspended Hg Hg adsorbed to particulate
a AES, atomic emission spectrometry; FAPES, furnace-atomization plasma-emission spectrometry; CGC, capillary gas chromatography; HPLC, highperformance liquid chromatography; CVAA, cold-vapor atomic absorption; ICP, inductively coupled plasma; CVAF, cold-vapor atomic fluorescence; MIP, microwave-induced plasma; GC, gas chromatography; MS, mass spectroscopy.
cury and dialkylmercury compounds are the exclusive volatile mercury species in produced liquid hydrocarbons is unknown but, to date, no other class of volatile species has been identified. The definition of volatility is important, however. In the context of this discussion, volatility refers to the ability to evaporate readily from crude oil or gas condensate at ambient temperature as opposed to volatility in distillation. Almost all mercury compounds will volatilize from solution if heated to sufficiently high temperatures. The chemical identity of the suspended fraction (Category 2) remains uncertain. Because meta-HgS has been identified as a suspended component of water produced in conjunction with oil and gas that contains mercury,11 one could reasonably infer that HgS is at least one component of the mercury fraction suspended in oil. HgS is essentially insoluble in both oil and water and evidently some separates by gravity to the water phase in the primary separation immediately downstream of wellhead production. Whatever HgS that may be present is typically amorphous and very finely divided and thus elusive to crystallographic identification. Sequential leaching techniques applied to oil sediment suggest that the mercury species in oil sediment are chemically inert to mild acid (HNO3) extraction inferring that HgS, HgSe, or both could be constituents of crude oil sediment.12 The natural abundance of HgS in crude oil is suspect, however, because H2S is a constituent of all crude oils and the oxidation of H2S by air during sampling may allow HgS to form artificially by reaction with elemental sulfur. Elemental mercury does not react spontaneously with H2S in oil but will react with elemental sulfur, the oxidation product of O2 and H2S.
2H2S + O2 f 2H2O + 2S0
(1)
Hg0 + S f HgS
(2)
Hg0 + H2S f no reaction
(3)
Speciation of mercury in crude oil and gas condensates has been attempted using a variety of methods. Table 1 summarizes (11) Chongprasith, P. Mercury Assessment in Thailand; Submitted to United Nations Environment Programme (UNEP); Chemicals Ministry of Science Technology and Environment: Thailand, 2001. (12) Corns, W. T. Mercury Speciation in Crude oil and Natural Gas Condensates using Cold Vapor Atomic Fluorescence Spectrometry. Proceedings of the 8th International Conference on Mercury as a Global Pollutant, Ljubljana, 2004.
the methods and species that have been identified. The origin and method of collection of liquid samples are very poorly documented in the literature, and one is left to wonder about sampling procedures, containers, and processing steps now known to be important to preservation of the natural distribution of species in crude oil.13 In summary, speciation of mercury in gas condensates and crude oil has identified dissolved elemental mercury, dissolved dialkylmercury, dissolved mono-alkylmercury, dissolved loosely complexed ionic mercury, and a suspended fraction that is chemically different from the identified dissolved forms. The prevalence of dialkylmercury in liquid hydrocarbons also is important because of its extreme toxicity and ability to incorporate into blood transdermally.7 Thus, there can be concern for the safety of oil field workers who handle oils or condensates having elevated total mercury concentrations, if DMHg is found to be abundant and prevalent. Field measurements of mercury in natural gas do not differentiate forms of mercury, thus there always exists some uncertainty as to whether organic species are a significant fraction of the total amount of mercury measured in gas streams. Likewise, monitoring procedures for mercury vapor in air, as part of health and safety requirements, also do not distinguish volatile species. For all of the above stated reasons, development of techniques to measure DAHg in both the laboratory and field environments and in both gaseous and liquid matrixes was considered an important objective. A primary objective of the work discussed was to understand the volatility of dialkylmercury relative to elemental mercury to assist prediction of its partition in gas/liquid separations and its evolution from liquids found in oil and gas processing. To predict vapor and liquid concentrations in chemical processes, empirical measurements of the fugacities of DMHg and Hg0 in hydrocarbon vapor/liquid 2-phase, 2-component systems are needed. From phase concentration measurements, Henry’s Law constants and enthalpies of evaporation were calculated. For the dilute solutions and low partial pressure conditions examined, the convention that vapor phase fugacity is equivalent to partial pressure has been adopted. Concentrations in vapor phase are reported for convenience, but they can be converted to partial pressure using an ideal gas approximation if need be. (13) Wilhelm, S. M.; Kirchgessner, D. A.; Liang, L.; Kariher, P. Sampling and Analysis of Mercury in Crude Oil. J. ASTM Int. 2005, In press, Paper 12985.
182 Energy & Fuels, Vol. 20, No. 1, 2006
Wilhelm et al.
Table 2. Typical Composition and Metals Content of Asphaltenes20 Maya
Khafji
Iranian Light
C H S N O H/C
elemental (wt %) 82.0 82.2 7.5 7.6 7.1 7.6 1.3 0.9 1.2 1.1 1.10 1.11
83.2 6.8 5.9 1.4 1.5 0.98
Ni V
metals (ppm, wt ) 390 200 1800 550
390 1200
density
1.18
1.17
1.17
The identities and chemical behaviors of mercury species that partition to residual streams in refining are important for the task of predicting environmental fate. Separate from the mercury categorization, crude oil can be separated into four chemically distinct hydrocarbon fractions: saturates, aromatics, asphaltenes, and resins.19 Asphaltenes exist in crude oil as both dissolved molecules (micelles) and suspended colloidal material. Asphaltenes, operationally defined as the nonvolatile and polar fraction of petroleum that is insoluble in normal alkanes, are known to hold metals, especially nickel and vanadium, in relatively high concentrations and in near exclusivity as compared to other fractions. Table 2 gives the composition of the asphaltene fractions of three typical crude oils.20 Of interest in the present study was the extent to which mercury also may be included in the asphaltene fraction. 2. Experimental Section 2.1. Methods and Calibration Procedures. Experiments followed the procedures specified in U.S. EPA Method 1631 (analysis of total Hg) and U.S. EPA Method 1630 (speciation of Hg)21,22 in all situations where applicable. The equipment, standards, reagents, materials, calibrations, and the rationale for their employment are detailed in EPA method documents and additionally discussed in previous publications by the authors.23-28 A summary description (14) Snell, J. P.; Frech, W.; Thomasson, Y. Performance Improvements in the Determination of Mercury Species in Natural Gas Condensate Using an On-line Amalgamation Trap or Solid-phase Micro-extraction with GCMIP-AES. Analyst 1996, 121, 1055. (15) Frech, W.; Baxter, D.; Bakke, B.; Snell, J.; Thomasson, Y. Determination and Speciation of Mercury in Natural Gases and Gas Condensates. Anal. Comm. 1996, 33, 7H. (16) Zettlitzer, M.; Scholer, R.; Falter, R. Determination of Elemental, Inorganic and Organic Mercury in North German Gas Condensates and Formation Brines. In Proceedings of Symposium: Oil and Gas Chemistry; Society of Petroleum Engineers: Houston, Texas, 1997; SPE Paper No. 37260. (17) Bouyssiere, B.; Bacco, F.; Savary, L.; Lobinski, R. Analytical Methods for Speciation of Mercury in Gas Condensates. Oil Gas Sci. Technol. 2000, 55, 639. (18) Snell, J.; Johansson, M.; Frech, W.; Smit, K. Stability and Reactions of Mercury Species in Organic Solution. Analyst 1998, 123, 905. (19) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1991. (20) Tanaka, R.; Winans, R. E.; Hunt, J. E.; Thiyagarajan, P.; Sato, S.; Takanohashi T. Aggregation Of Petroleum Asphaltenes In Three Different Crude Oils. In ACS Fuel Chemistry DiVision Preprints; American Chemical Society: Washington DC, 2001; p 359. (21) U.S. EPA Method 1631 Revision E: Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry, August 2002, EPA-821-R-02-019. (22) U.S. EPA Method 1630, Methyl Mercury in Water by Distillation, Aqueous Ethylation, Purge and Trap, and CVAFS, January 2001, EPA821-R-01-020. (23) Liang L.; Horvat, M.; and Danilchik, P. Novel Analytical Methods for Determination of Low Level of Total Mercury in Petroleum and its Products by Gold Amalgamation CVAFS. Science Total EnViron. 1996, 187, 57-64.
follows highlighting aspects of the present work that are not specified in the EPA methods or that differ from those previously reported. Caution! DMHg used in this work is extremely toxic and especially dangerous as a neat liquid or concentrated solution. Inhalation and transdermal absorption of minute quantities are lethal. Rigorous safety procedures, equivalent in scope to those specified in ref 22, were employed and should be by anyone doing similar work. 2.1.1. For total mercury analyses (THg) of hydrocarbon liquids, all Hg species, other than Hg0, were converted, either through BrCl oxidation/SnCl2 reduction23 or via air combustion,24 to Hg0. For BrCl extraction, Hg0 was then sparged from solution using purified N2 and collected on gold amalgamation traps. Amalgamated Hg on a gold trap was thermally desorbed to Hg0 vapor in N2 (cold vapor) and carried to an atomic fluorescence (AF) detector (BR III, Brooks Rand, Seattle, WA). The combustion method used purified air as the oxidation and carrier gas. Quantification in both cases was by peak area using single stage gold amalgamation. In the text, the BrCl method is referred to as extraction/CVAF and the combustion method as combustion/CVAF. 2.1.2. DMHg in vapor was collected on Carbotrap or Tenax traps, each selected on the basis of trapping efficiency at the temperature of the experiment.27 Carbotrap and Tenax traps were modified from those previously described26,27 to improve collection efficiencies by increasing the length of the material in the trap from 2 to 4 cm. A glass T-connector was used for syringe loading of gas samples onto traps (see Figure 2). Low flow rates were found to be critical to trapping efficiency. The traps containing DMHg were heated with flowing Ar to desorb organic species into a GC column for separation. Separated and eluted species were carried through a thermal decomposition column, converted to Hg0, and then carried to the AF detector.26 2.1.3. Gold traps were used to trap Hg0 in gas streams. The methods used to construct and verify the efficiencies of gold traps have been discussed previously.28 2.1.4. A DMHg working standard and serial dilutions derived from it were used to spike toluene for the fugacity measurements. The standard was constructed by placing 5 g of liquid DMHg standard (Strem Chemicals, Newburyport, MA) into 40 mL of methanol (note safety concerns for concentrated DMHg solutions). DMHg working standards were constructed by serial dilution with toluene. The DMHg concentration of working standards was determined by sparging aliquots (0.03 mL) from reagent water bubblers onto gold traps. Gold traps were calibrated against Hg0 standards that were generated by SnCl2 reduction of Hg2+ aqueous standards and collected on gold traps. The mean of 7 measurements of DMHg working standard in toluene was found to be 2528 ng/ mL (RSD 4.2%). 2.1.5. A Hg0 working standard was prepared by placing 0.5 g of liquid elemental Hg (electronic grade, 99.9999%, Aldrich, Milwaukee, WI) in 35 mL of paraffin oil in a 40 mL glass vial with a Teflon-lined cap. The vial was shaken for 24 h and placed in a refrigerator at 3 °C for at for 24 h. Aliquots (0.03 mL) were sparged from glass impingers containing reagent water and collected on (24) Liang, L.; Lazoff, S.; Horvat, M.; Swain, E.; Gilkeson, J. One Step Determination of Mercury in Crude Oil and Related Products Using a Simple Lab Built Thermal Decomposition System. Fresenius’ J. Anal. Chem. 2000, 367, 8. (25) Liang, L.; Horvat, M.; Fajon, V.; Prosenc, N.; Li, H.; Pang, P. Comparison of Improved Combustion/Trap Technique to Wet Extraction Methods for Determination of Mercury in Crude Oil and Related Products by Atomic Fluorescence. Energy Fuels 2003, 17, 1175. (26) Liang, L.; Kichgessner, D. A.; Wilhelm, S. M.; Kariher, P.; Pang, P. Occurrence of Dimethylmercury in Organic Solvents. Mater. GeoenViron. 2004, 51, 1968. (27) Liang, L.; Horvat, M.; Bloom, N. S. An Improved Speciation Method for Mercury by GC/CVAFS after Aqueous Phase Ethylation and RoomTemperature Pre-Collection. Talanta 1994, 41, 371. (28) Liang L.; Bloom N. S. Determination of Total Mercury by SingleStage Gold Amalgamation with Cold Vapour Atomic Spectrometric Detection. J. Anal. At. Spec. 1993, No. 4, 591.
Mercury Species in Crude Oil
Figure 1. Apparatus for trapping volatile species.
gold traps. Gold traps were calibrated in the same manner as described in section 2.1.4. The mean of 7 measurements of Hg0 in paraffin oil working standard was found to be 1271 ng/mL (RSD 3.1%). 2.1.6. A monomethylmercury standard solution was prepared in 2-propanol as described previously.23 Working solutions were prepared by serial dilution of the stock solution with CH2Cl2. The monomethylmercury standard was used to calibrate the combustion/ CVAF detector. 2.2. Extraction of Mercury from Liquid Hydrocarbon Matrixes.23 A 4.0 mL hydrocarbon aliquot was transferred by pipet to a 40 mL glass vial with a Teflon-lined cap; 6.0 mL of BrCl and 6.0 mL of concentrated HCl were added. The mixture was shaken vigorously for 30 min. after which an aliquot (0.1-3.0 mL) of the aqueous phase was transferred via pipet for analysis using procedures specified in EPA 1631 (SnCl2 reduction, purge and trap, CVAF detection). Total mercury concentrations (THg) were converted from µg/L into µg/kg using hydrocarbon densities measured separately. 2.3. Digestion Procedure for Analysis of Filters. Filters were digested with BrCl in a 60 mL Teflon vial for 3 h at 75 °C. A digestate aliquot was analyzed using procedures specified in EPA 1631. 2.4. Evaporation of Hg0 and DMHg. The procedure to identify volatile mercury species followed the methods of Liang26 for determination of dimethylmercury and elemental mercury in vapor. An open oil-sample vial (40 mL) was placed in a cylindrical vessel (150 mL volume, 11.7 cm height, 4.1 cm i.d., Figure 1). The vessel containing the crude oil sample vial was then purged with nitrogen for varying periods of time at a rate of 165 mL/min. The purge gas exiting the chamber flowed sequentially through a Tenax trap and gold trap. The Tenax trap adsorbs organic or organometallic mercury compounds but not appreciable elemental mercury.26 The gold trap amalgamates both DAHg and Hg0. After the chamber was purged, the gold trap was analyzed by thermal desorption/ CVAF. The Tenax trap was heated under flowing argon to desorb mercury species onto a gas chromatography column with a CVAF detector (column 1 m, 85 °C, carrier gas flow rate 50 mL/min, DMHg peak retention time 2.7-2.8 min, Hg0 peak retention time 1.7 min). The particular crude oil selected for the experiment had a natural concentration of total mercury close to 600 ppb and was selected to have a sufficient quantity of evaporated mercury for measurement. 2.5. Fugacity of Elemental and Dialkylmercury. Three-port vessels (Figure 2) were constructed, each having a volume of 163.9 mL (61.2 mL vapor volume and 102.7 mL liquid volume at 23 °C). The vessels were configured with Teflon-lined septa through which aliquots were removed using syringes. A vapor-liquid equilibrium was established at each temperature by swirling the liquid periodically over a 1 h period. Headspace vapor was collected using a volumetric syringe and injected onto carbon traps (for
Energy & Fuels, Vol. 20, No. 1, 2006 183
Figure 2. Apparatus for volatile mercury species determination. Table 3. Blank Values for Asphaltene Experiments blank name
THg (mean of duplicates)
unit
filter filtration heptane toluene BrCl extraction hot BrCl digestion
0.00029
