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Studying mercury partition in monoethylene glycol (MEG) used in gas facilities
Fuel 159 (2015) 917–924
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Studying mercury partition in monoethylene glycol (MEG) used in gas facilities Y.M. Sabri a, S.J. Ippolito a,⇑, J. Tardio a, P.D. Morrison b, S.K. Bhargava a,⇑ a b
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, Melbourne, VIC 3001, Australia Department of Applied Chemistry, School of Applied Sciences, RMIT University, Melbourne, VIC 3001, Australia
h i g h l i g h t s Mercury partition from industrial and laboratory grade monoethylene glycol (MEG) was studied. Mercury was more stable in the laboratory MEG. 0
MEG retained 0–60 ppb elemental mercury (Hg ) at pH 9 and 80 ppb at pH 6. 0
3.6% of spiked HgCl2 leaves as Hg , 10% as Hg
2+
and 86.4% stays in industrial-MEG.
Notably, mercury was found to ‘‘drop out’’ of industrial-MEG as particulate mercury.
a r t i c l e
i n f o
Article history: Received 1 December 2014 Received in revised form 29 May 2015 Accepted 14 July 2015 Available online 21 July 2015 Keywords: Monoethylene glycol (MEG) Mercury removal Mercury partition Speciation
a b s t r a c t Mercury partitioning from monoethylene glycol (MEG) during natural gas production is a highly complicated process. Understanding this process will help determine the distribution of mercury species within a given industrial process so that appropriate mercury removal control systems can be effectively managed. In this study, the partitioning of elemental mercury (Hg0) from the gaseous phase into MEG solutions is investigated under standard laboratory conditions. Additionally, the partitioning of dissolved ionic mercury (Hg2+) from MEG solutions into the gaseous phase is also investigated under atmospheric pressure and room temperature conditions. It was found that the solubility of Hg0 in MEG ranged from 0 to 60 ppb (ng/mL) with slight increase to 80 ppb when the pH was reduced from 9 to 6 at room temperature. On the other hand oxidized mercury (Hg2+) was found to be retained in the laboratory grade MEG and industrial samples of MEG (industrial-MEG). However, in the case of the industrial-MEG, the dissolved mercury (HgCl2 salt) was found to be unstable with approximately 3.6% and 10% converting within the solution and leaving in the gas phase as Hg0 and Hg2+, respectively, while the balance was retained in the MEG solution. The distribution was found to be highly pH dependent and was not observed when the experiments were repeated using laboratory grade MEG solutions. These findings help in better understanding the type and ratio of mercury species that are partitioned in and out of MEG in the gas processing facilities. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Abbreviations: weak-MEG, 50 wt.% MEG solution; strong-MEG, 90 wt.% MEG solution; Hg0, elemental mercury; ICP-MS, inductively coupled plasma mass spectroscopy; Industrial-MEG, monoethylene glycol provided by petroleum industry; Lab-MEG, laboratory reagent – monoethylene glycol; Salted-MEG, laboratory reagent monoethylene glycol with added salts; LPG, liquefied petroleum gas; Hg, mercury; MEG, monoethylene glycol; Hg(T), total mercury in glassware; Hg(B), total bound mercury in Tar residue; TOC, total organic carbon; sccm or ml/min, standard cubic centimetre per minute; VOCs, volatile organic compounds; XPS, X-ray photoelectron spectroscopy. ⇑ Corresponding authors. Tel.: +61 3 9925 3365; fax: +61 3 9925 2882. E-mail addresses: [email protected] (S.J. Ippolito), suresh.bhargava@ rmit.edu.au (S.K. Bhargava). http://dx.doi.org/10.1016/j.fuel.2015.07.047 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
The mercury content in many new gas fields is increasing as we are forced to explore deeper and lower quality reserves. Both elemental mercury (Hg0) and oxidized mercury (Hg+/Hg2+) species are present in many gas fields in countries such as Netherlands, USA, Malaysia and Canada just to name a few [1–3]. Depending on the gas field location, the oxidized mercury content can exist in the form of inorganic salts [1]. These salts can easily react with trace levels of H2S found in gas fields to produce HgS which is a relatively more stable, water insoluble
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and less reactive form of mercury that can be easily collected as solid precipitate and removed [4]. The various forms of mercury have also been previously demonstrated to react with each other or with halogens present within hydrocarbon solutions to produce stable Hg2+ 2 type compounds (i.e. Hg2Cl2) [5]. Knowledge of the type of mercury species present, their quantity and stability in different parts of the gas production process (i.e. pipeline transmission, processing, etc.) is difficult yet important due to their extreme toxicity and potential reactivity with process equipment [1]. Furthermore, the chemical and physical properties of each mercury species differ and must be considered in the evaluation of potential health effects and in the design of systems to remove mercury from natural gas. The main implications of mercury in gas production processes arises from its ability to contaminate catalysts, cause corrosion through liquid metal embrittlement of aluminium heat exchangers and impact other equipment that are made from zinc and copper metals [4,6,7]. Mercury related corrosion has resulted in a number of plant shutdowns in the past with multi-million dollar cost implications [1]. Additionally, mercury is toxic to humans and the environment in all its forms, particularly in the organic form known as organomercury species [6,8–12]. Due to the different physical and chemical properties of the various mercury species, it is of great interest to determine the type of mercury species and their stability at each stage of gas production process. This will help in the evaluation of potential health effects and in the design of equipment for efficient removal of mercury [6,7]. An important part of the gas production process where the partitioning and stability information of the different mercury species is of interest is during the transportation and processing of natural gas from the gas reservoir to onshore processing plants. In this process, monoethylene glycol (MEG) or similar agents are typically injected at the reservoir head to protect against formation of hydrates during gas transportation via pipelines. As the feed reaches the gas processing facility it is typically water laden, having approximately 50 wt.% water in addition to containing some other components, such as: salts (thermally stable and metal salts), corrosion products (iron carbonates, sulfides, etc.), mineral scale from the pipelines, heavy hydrocarbons and glycol degradation products [13]. Typically 50 wt.% water laden MEG is referred to as weak MEG and is separated at the front end of the gas processing facility. Due to the large volumes of hydrate inhibitors used in these processes, there is a strong economic drive to recycle (rather than replace) the MEG at the end of the process line by using a water removal step before recirculating it back to the reservoir head [14]. This regeneration process involves heating the weak MEG in a distillation column in order to boil off the water until the MEG solution only contains 10 wt.% water, which is known as strong MEG. The strong MEG is then recirculated out to the well head to repeat the cycle [15]. There is little information available in the public domain regarding the fate of mercury during the MEG recirculation process, making it difficult to determine how mercury partitions and what type of mercury species could be present. Here we investigate the partitioning behaviour of Hg0 and Hg2+ (in the form of HgCl2) to and from weak and strong MEG solutions prepared in the laboratory and an industrial MEG sample obtained from a natural gas processing plant, which are referred to as Lab-MEG and Industrial-MEG from here on, respectively. The MEG solutions were analysed using inductively coupled plasma mass spectroscopy (ICP-MS), total organic compounds (TOC), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). A comparison of the types of MEG provides an insight into how the Industrial-MEG constituents (salts, organics, etc.) effect mercury partition during natural gas transportation processes.
2. Experimental 2.1. Materials Laboratory reagents such as acids (HCl, HNO3, H2SO4), salts (i.e. NaCl, Na2SO4, HgCl2), certified HgCl2 standards, oxidizing agents (KMnO4) and monoethylene glycol (MEG) were purchased from Sigma Aldrich and used as received. A sample of circulated weak MEG from a natural gas processing plant (Industrial-MEG) was used as received, unless specified. The MEG solutions used in the experiments are described in Table 1. The Salted-MEG mix used was the same grade of MEG as that of the Lab-MEG mix, however some salt contaminants were added in order to closely match the salt content and pH of circulated Industrial-MEG (refer to Supplementary data, Table S1). In addition to our own analysis, the industrial MEG sample was supplied with a confidential certificate that showed the MEG was analyzed for mercury (and other inorganic and organic species) by National Association of Testing Authorities (NATA) accredited analytical company using Heavy Metals USEPA 6010/6020 method. The certificate showed insignificant amount of mercury (<0.1 ppb) in the Industrial MEG sample.
2.2. Analytical and characterisation methods 2.2.1. Inductively coupled plasma mass spectroscopy (ICP-MS) All solutions were analysed quantitatively using inductively coupled plasma mass spectroscopy (ICP-MS) from Agilent Technologies (7700x series) with ASX-520 series auto-sampler. The ICP-MS series used in this study addresses the requirements to be used to detect ultra-low concentrations of Hg in solution with minimal interference from organic contents that were present in Industrial-MEG [16]. The instrument had X-type lens with nickel cones and was tuned in helium mode. The analysis protocol utilised an integration time per mass of 1 s for Hg202 isotope (1 point/s), 3 replicates with a sweep/replicate setting of 100. Additionally a prepared mercury standard (used for ICP MS calibration) was analyzed every 3 samples to ensure correct operation of the instrument. Three internal standards (i.e. Tb159, Lu175 and Bi209 isotopes) were also used and were analyzed at an integration time of 0.3 s. The sample uptake was set at 40 s with a peristaltic pump speed of 0.3 rps. The stabilisation time was set at 60 s per sample with the peristaltic pump speed being set at 0.1 rps for this process. The probe from the auto-sampler was set to be rinsed twice from separate baths (each containing 5% nitric acid (HNO3)) while the peristaltic pump speed for this process was also set at 0.3 rps used were mercury was set to be analyzed for 2 s with 3 replicates while the internal standards were analyzed for 0.1 s. The instrument outputs the mean and the relative standard deviation (RSD) of the counts for each analyte and it was ensured that the RSD value was less than 5% for each data point considered for analysis. The background equivalent concentration (BEC) for mercury was found to be 0.2 ppb (ng/mL). All standards and samples (unless specified) prepared for ICP-MS analysis contained the recommended 2% HCl acid content.
Table 1 Description of the three different monoethylene glycol (MEG) solutions used throughout this study. MEG solution
Description of composition
pH
Lab-MEG
10, 50 or 90 wt.% monoethylene glycol (laboratory reagent) 50 wt.% Lab-MEG with added salts from Table S1 50 wt.% MEG supplied by industry
6
Salted-MEG IndustrialMEG
9 9
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Prior to analysing the samples with ICP-MS, the total organic compounds (TOC) in the samples were determined in order to ensure their content is below 1%. The instrument used to determine the TOC was a Sievers 820 Portable Total Organic Carbon Analyser. The instrument is calibrated every year by Amscorp using GE standards. The validation of the calibration is performed regularly every 6 months using GE validations solutions. Milli-Q water was analysed first on the instrument before every set of samples as a control. If Milli-Q gave an unusual reading, a validation was run. Samples were prepared for TOC analysis as follows: Lab-MEG (1% v/v) and Industrial-MEG (1% v/v) were diluted in Milli-Q water (as was done in samples which were analysed using ICP-MS) and analysed with the TOC instrument. It was observed that following preconditioning, the diluted Industrial-MEG samples had a TOC value of <1%, which was sufficiently low enough for safe and reliable analysis of the sample solutions by ICP-MS. 2.2.2. X-ray photoelectron spectroscopy (XPS) Solid tar residue (Tar) found in Industrial-MEG was digested in aqua regia (1:3 concentrated HCl:HNO3) before being diluted and analysed for their mercury content using ICP-MS. The Tar was also analysed using X-ray photoelectron spectroscopy (XPS). XPS characterisation of the tar residue was performed using a Thermo K-Alpha instrument at a pressure better than 1 10 9 Torr. The core level binding energies (BEs) were aligned with the adventitious C 1s binding energy of 285 eV. 2.2.3. Fourier transform infrared attenuated total reflectance (FTIRATR) All Fourier transform infrared attenuated total reflectance (FTIR-ATR) measurements in this study were performed using Perkin Elmer Spectrum 100 instrument equipped with micro ATR (diamond ATR crystal), in the range 650–4000 cm 1 region with a resolution of 2 cm 1. Each spectrum represents an average of 10 consecutive scans. The background spectra consisted of the base diamond under the same experimental conditions. 2.3. Experimental setups to investigate mercury partitioning In order to investigate the transfer of mercury from gas to liquid and also from liquid to gas, three setups (Hg0 delivery system, setup-1 and setup-2) were used. Each experiment was conducted with research laboratory equipment at atmospheric pressures as detailed in the proceeding sections. 2.3.1. Hg0 delivery system An elemental mercury (Hg0) vapor delivery system was first developed and calibrated so it generated repeatable Hg0 concentrations. A highly controllable Dynacal (Model 150C) system and permeation tubes (VICI, TX, USA) were used to generate Hg0 vapors in a repeatable fashion. This involved maintaining the permeation
vessel at a constant pressure flow of 200 sccm (200 ml/min) and using heated stainless steel/Teflon tubes that were compatible with Hg0 vapor. The concentrations of the Hg0 generated in the test stream were validated using acidic KMnO4 wet trapping method (similar to the Ontario-Hydro method [17]) which were analysed using ICP-MS. The mercury generator was calibrated over a period of 2 h prior to each experiment, however initial testing of the generators’ stability in reproducing Hg0 vapor concentrations was confirmed by testing it over different time periods (refer to Supplementary data, Fig. S1 and Table S2). It was observed that the generator produced 0.99 ± 0.02 mg/m3 of Hg0 balanced in dry N2 for all calibration times. 2.3.2. Setup-1: Transfer of mercury from gas to liquid phase Tests to investigate the extent of Hg0 vapor partitioning into the MEG solutions were conducted by bubbling Hg0 vapor stream (balanced in dry nitrogen) with a concentration of 1 mg/m3 (112 ppbv) through the MEG solutions at a flow rate of 200 sccm (standard cubic centimetre per minute or standard mL/min) for up to 17 h. The experimental setup (setup-1) is shown in Fig. 1. The chemical solutions that were placed in each glass impinger are listed in Table 2. In order to perform Hg0 mass balance, six impingers were used as modified version of the Ontario-Hydro method [17]. Impinger 1 contained the MEG to be saturated with Hg0. Impinger 2 was used to stop the foams produced in the MEG solution (impinger 1) from transferring into the proceeding impingers. Impinger 3 was used to stop the transfer of volatile organic compounds (VOCs) which were released from the MEG from entering into the KMnO4 impingers (impingers 4–6) as VOCs are known to decrease the efficiency of KMnO4 traps. This was achieved by using a diluted mixture (1:50) of H2O2:H2SO4 (1:3 ratio of concentrated oxidizers) in impinge 3. This impinger also oxidized and trapped some of the Hg0 which was not taken up by the MEG in impingers 1 or in the overflow trap (trap 2). The final 3 traps (impingers 4–6) contained H2SO4 and KMnO4 which were used to trap the remaining Hg0 within the gas stream. The chemicals in impingers 3–6 worked by oxidizing gas phase Hg0 to a soluble Hg2+ form. The solid tar
Table 2 List of glass impingers and the chemicals employed in setup-1 in order to trap the chemical of interest. Impinger No.
Chemical
Trapped species
1 2 3 4 5 6 MEG MEG-Tar
100 ml MEG Empty (acid wash) H2O2/H2SO4 KMnO4 KMnO4 KMnO4 MEG saturated with Hg0 Aqua regia wash of impingers 1 & 2
Hg0 Foam carry over VOCs/Hg0 Hg0 Hg0 Hg0 Hg0 Hg(B) partitioned to Tar
Industrial-MEG Fig. 1. Schematic of setup-1 for determining the partition of mercury from gas phase to liquid phase. All traps were analysed for mercury using ICP-MS.
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residue that remained in impingers 1 and 2 once the Industrial-MEG was emptied following the Hg0 saturation experiment is referred to as the Bottom-Tar in Table 2. This Tar residue was acid washed using 1 mL of aqua regia, diluted and analysed by ICP-MS. The mercury content that was partitioned to the tar residue is referred to as the total bound mercury (Hg(B)), which may include elemental and/or ionic mercury species. 2.3.3. Setup-2: Transfer of mercury from liquid to gas phase Experiments using setup-2 (see Fig. 2) were performed in order to understand the partition of mercury from the Industrial-MEG solution into a dry nitrogen gas stream. The test was conducted by bubbling the nitrogen gas stream through the Industrial-MEG solution which was spiked with 0.5 ppm Hg (as HgCl2) all at atmospheric conditions (1 atm). The total gas flow rate was set at 200 sccm for a period of 5 h and any evolved mercury was captured in a train of traps similar to one used in setup-1. The difference between setup-1 and setup-2 is the addition of two KCl traps in setup-2, which are used in order to trap any gas phase oxidized mercury (Hg+, Hg2+) species. All solutions were diluted and analysed using ICP-MS. 3. Results and discussions 3.1. Analysis of mercury in Lab-MEG solutions Analysis of mercury in complex solutions, particularly solutions containing organic compounds, can be problematic [1,7], hence measurement of mercury in MEG solutions was investigated prior to conducting partitioning tests. This involved ‘‘spiking’’ or adding mercury in a range of different MEG solutions and analysing the solutions using ICP-MS. Spike tests were conducted on the MEG solutions described in Table 1. The Lab-MEG and Salted-MEG (with Milli-Q water being used as the control sample) were diluted then spiked with HgCl2 and analysed by ICP-MS, the results of which are shown in Table 3. It was observed that ICP-MS has an error anywhere within ±3% for water (exp. 1–7) and ±5% for Lab-MEG (exp. 8–17) on most instances, which is expected given the combined error margins involved in spiking Hg from a known certified standard, performing dilutions and instrumental errors (such as BEC or memory effects within the analysis process). The relative standard deviation (RSD) values in the ICP-MS output data were all below the accepted 5% margin. It may be observed that the errors produced by ICP-MS for Salted-MEG (exp. 18–27) are not significantly different from that of the Lab-MEG, however it is worth noting that all the RSD values were negative, indicating under-reads from the expected Hg concentrations were occurring. Overall, the data indicates that up to 50 wt.% Lab-MEG and Salted-MEG samples can be analysed accurately for their mercury content using the ICP-MS technique.
3.2. Analysis of mercury in Industrial-MEG solutions The Industrial-MEG was initially unsuitable for ICP-MS analysis due to the strong odour that resulted from volatile organic compounds (VOCs), the presence of which was confirmed by FTIR-ATR (Fig. 3) and TOC analysis (refer to Supplementary data, Table S3). It may be observed that the Industrial-MEG contains organic compounds, however, a pre-treatment process involving open to air over the course of 3 days allowed for significantly reduced TOC content in Industrial-MEG thus making it acceptable for ICP-MS. The Industrial-MEG was pre-treated via opening cap container over the course of 3 days and, similar to Lab-MEG, was also diluted, spiked with HgCl2 and analysed using ICP-MS on the same day. The results obtained from the ICP-MS analysis is provided in Table 3 (exp. 28–33). As can be seen, significant under-readings were obtained when analysing freshly HgCl2 spiked Industrial-MEG samples by ICP-MS. Unlike the Lab-MEG and Salted-MEG, the Industrial-MEG was observed to introduce significantly large errors (under-reads) in the ICP-MS output when analysing for Hg content (see the standard deviation value shown in the last row for each MEG type). This indicates that mercury may have produced poor ionization when analysing by ICP-MS due to the matrix of the solution or mercury may have been unstable in Industrial-MEG solution and was undergoing precipitation. In order to determine whether mercury precipitation out of solution was the cause of the mercury content under-reading by ICP-MS, a long term mercury stability test in Industrial-MEG was performed. 3.3. Long term mercury stability in Industrial-MEG In order to determine the solubility and stability of HgCl2 in Industrial-MEG solutions, 0.5 ppm of Hg (as HgCl2) was dissolved in a 200 mL solution of Industrial-MEG. Thereafter the solution was periodically analysed by ICP-MS over a 17 day period using a 1:100 dilution ratio. The 1:100 dilution ratio was chosen as it produced the most accurate mercury content within the Industrial-MEG solution, both with and without the standard addition method (see Supplementary material, Tables S4–S6 and Figs. S2 and S3). The results are shown in Table 4. The data obtained clearly indicates that an error between the expected mercury concentration in the Industrial-MEG and the ICP-MS result increased with increasing number of days, showing a consistent increase in the extent of under readings in both (duplicates) Industrial-MEG solutions. On the other hand, the water sample and Lab-MEG mercury concentrations reported by ICP-MS were similar to that of the expected values throughout the entire 17 day test. The later result confirms that HgCl2 is unstable in the Industrial-MEG solution and was postulated to precipitate and
2+
Hg + Industrial-MEG
Fig. 2. Schematic of setup-2 for determining the partition of mercury from liquid phase to gas phase. All traps were analysed for mercury using ICP-MS.
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Y.M. Sabri et al. / Fuel 159 (2015) 917–924 Table 3 ICP-MS readings for water, Lab-MEG, Salted-MEG and Industrial-MEG diluted and spiked with HgCl2. Exp.
Sample
1 2 3 4 5 6 7
Standard Standard Standard Standard Standard Standard Standard
MEG (%) (H2O) (H2O) (H2O) (H2O) (H2O) (H2O) (H2O)
0 0 0 0 0 0 0
Sample dilution
Spiked Hg2+ (ppb)
ICP-MS [Hg] (ppb)
– – – – – – –
2 2 2 2 4 6 6
1.90 1.98 1.95 2.01 4.12 5.79 5.85
5.0 1.0 2.5 +0.5 +3.0 3.5 2.5
Error (%)
8 9 10 11 12 13 14 15 16 17
Standard deviation (%) Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab MEG Lab-MEG Lab-MEG
0.5 1 2 10 0.5 1 2 0.05 0.01 0.01
1:200 1:100 1:50 1:10 1:200 1:100 1:50 1:1000 1:10000 1:10000
2 2 2 2 6 6 6 5 5 5
1.89 1.93 1.85 1.9 5.68 5.81 5.70 5.20 5.25 5.10
±2.67 5.5 3.5 7.5 5.0 5.3 3.2 5.0 +4.0 +5.0 +2.0
18 19 20 21 22 23 24 25 26 27
Standard deviation (%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%)
0.25 0.5 1 5 50 0.25 0.5 1 5 50
1:200 1:100 1:50 1:10 1:1 1:200 1:100 1:50 1:10 1:1
2 2 2 2 2 6 6 6 6 6
1.87 1.99 1.88 1.82 1.95 5.82 5.97 5.80 5.72 5.81
±4.57 6.5 0.5 6.0 9.0 2.5 3.0 0.5 3.3 4.7 3.2
28 29 30 31 32 33
Standard deviation (%) Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG
0.5 1 2 0.5 1 2
1:200 1:100 1:50 1:200 1:100 1:50
2 2 2 6 6 6
2.03 1.68 1.49 5.16 6.08 5.13
±2.68 +1.5 16.0 25.5 14.0 1.3 14.5
Standard deviation (%)
±10.0
Transmittance
Industrial-MEG
Lab MEG CH CH bend
Water OH
4000
3000
OH bend
2000
1000
Wavenumber (cm-1) Fig. 3. FTIR ATR of Lab-MEG and Industrial-MEG samples.
‘‘drop out’’ of the Industrial-MEG solution to form a tar like residue on the bottom of the glassware which was not immediately visible to the naked eye. In order to visually demonstration the instability of mercury in Industrial-MEG solution, a relatively large amount of HgCl2 (4.9 g/L) was added to small volumes of Industrial-MEG (10 ml). Fig. 4 shows 5 vials containing Industrial-MEG with additional known chemical species. The first vial contained Industrial-MEG and H2O (to make up the volume to 15 mL) while the second vial contained Industrial-MEG and NaHCO3 (pH > 12). No visual changes were observed between these control vials, as expected. However on the addition of HgCl2 into identical control vials (vials 3 and 4), a significant dropout of presumably Hg rich solids can be observed where the kinetics of the dropout seems to increase in the presence of NaHCO3 (vial 4). When repeated with a vial containing Industrial-MEG and HCl (pH < 3) no drop out of solid material was observed when HgCl2 was added (vial 5)
Table 4 Stability of HgCl2 in water, Lab-MEG and Industrial-MEG over a 17 day period. Day
Expected Hg2+ (ppb)
Water
50% Lab-MEG [Hg] (ppb)
Industrial-MEG-1 [Hg] (ppb)
Industrial-MEG-2 [Hg] (ppb)
1 2 8 15 17
5 5 5 5 5
5.06 4.96 4.86 5.08 5.05
5.01 4.92 4.92 5.2 5.06
3.65 3.31 2.71 1.34 0.94
3.39 3.61 2.83 1.21 1.06
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Fig. 4. Addition of HgCl2/NaHCO3/HCl to Industrial-MEG. The images were taken 5 s following addition of (1) H2O, (2) NaHCO3, (3) HgCl2, (4) NaHCO3 and HgCl2, and (5) HCl. Upon addition of HCl, the pH of the Industrial-MEG solution had changed from 9 to 6.
indicating that the mercury precipitation and drop out is due to the high pH of the solution. The drop out of dissolved mercury out of solution at high pH is well known [18] and is also demonstrated in the Supplementary material of this manuscript (see Supplementary material, Fig. S4). The demonstration in Fig. 4 indicates that HgCl2 drops out of the Industrial-MEG solution due to its high pH of 9. Similar findings were also observed in the ICP-MS experiments conducted in Section 3.3 of this manuscript. Therefore it is postulated that the mercury could be unknowingly accumulating in the solid tar residue that is present in the Industrial-MEG. Two different approaches were used to show the presence of mercury in the solid tar residue of the supplied Industrial-MEG sample, which was initially presumed to be mercury free. The first approach involved scraping a small amount of tar residue from the original container holding the (untreated) Industrial-MEG and digesting it in aqua regia. The digested solution was then filtered, diluted to 100 ml. The analysis of this solution resulted in an ICP-MS reading of 6.2 ppb, thus confirming the presence of mercury within the
supplied Industrial-MEG tar residue. In contrary to this, the analysis of the as-supplied Industrial-MEG solution indicated no mercury was present, presumably as a result of complete drop out due to the long period of time (>30 days) between when the solution was used in the industrial processes and when the same Industrial-MEG was delivered and used in the these experiments. The second method to confirm the presence of mercury within the Industrial-MEG’s tar residue involved its analysis via XPS technique. Two regions (Hg4d and Hg4f core level) within XPS spectra which denote the binding energies of Hg atoms were investigated. Analyses of HgCl2 as a control as well as the Tar residue in the 4f core level (90–115 eV) are shown in Fig. 5a and b, respectively. Furthermore, the 4d core level (350–390 eV) spectra for a Hg contaminated Au film (used as control) and the Tar residue are shown in Fig. 5c and d. Following the de-convolution of the peaks, it can be observed that the mercury peaks are present in all spectra, thus, confirming the presence of mercury in the tar residue of the supplied Industrial-MEG samples. Since the sample in question was not spiked with mercury in the laboratory, the data suggests that Hg accumulated in Industrial-MEG sample while it was being used in the industrial process to extract natural gas. Therefore the removal of the tar residue from the MEG solution before it is re-used or re-circulated in a natural process could help in significantly reducing mercury content in the MEG. 3.4. Mercury partition 3.4.1. Hg0 partition into Lab-MEG In order to determine the saturation level of gas phase Hg0 in weak-MEG and strong-MEG, a nitrogen stream containing a known Hg0 vapor concentration of 1 mg/m3 was bubbled through the weak-MEG at 200 sccm for a period of 17 h. The experiments were carried out using a setup similar to that of setup-2, however the chemical contents in each dreschel bottle differed significantly. The dreschel bottle contents and the resulting mercury content in each solution are shown in Table 5. It can be observed that the
Fig. 5. XPS spectra of Hg4f core shell analysis in (a) HgCl2 salt (control) and (b) Tar present in Industrial-MEG; as well as Hg4d core shell analysis in (c) Au-Hg amalgam (control) and (d) Tar present in Industrial-MEG.
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Table 5 Hg0 saturation of Lab-MEG. The Hg0 concentration (obtained from the Hg0 generator calibration) prior to each test is presented in the last row of the table. Calibration was conducted prior to the experiment by using Hg0 delivery system setup shown in Fig. S1. Trap No.
Sample
Weak-MEG Hg0 Conc. (ppb)
Weak-MEG Hg0 Conc. (ppb)
Strong-MEG Hg0 Conc. (ppb)
1 2 3 4 5
50% MEG Water 50% MEG KMnO4 KMnO4
60 7 5 2942 9
4 74 10 2623 12
6 6 6 1909 12
1.04 1.09
1.00 1.06
0.713 0.843
Total [Hg0 ] in mg/m3 = Calibration [Hg0] in mg/m3 =
a a
Calibration conducted prior to experiment using Hg0 delivery system setup shown in Fig. S1.
Table 6 Hg0 saturation in Industrial-MEG at different conditions following 17 h of Hg0/N2 flow. ‘‘Not Bubbled’’ refers to experiments where Hg0 gas stream was only passed over the Industrial-MEG headspace and not bubbled into the solution. Exp. No.
Sample
Condition
pH
[Hg] (ppb)
1 2 3 4 5 6 7 8 9 10
Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG
Bubbled Bubbled Bubbled Bubbled Bubbled Not Bubbled Bubbled Bubbled Bubbled Not Bubbled
5–6 5–6 5–6 5–6 5–6 5–6 7–8 7–8 8–9 8–9
57.0 47.0 81.9 79.5 83.0 0.0 30.0 61.2 60.0 0.0
Table 7 Hg0 partition from Industrial-MEG after 5 h N2 flow using setup-2 shown in Fig. 2. The mass reported is within a precision of ±0.02 lg. Impinger No.
Chemical
1 100 ml MEG + 0.5 ppm Hg as HgCl2) 2 Empty (acid wash) 3 KCl 4 KCl 5 H2O2/H2SO4 6 KMnO4 7 KMnO4 8 KMnO4 Tar in impinger 1 Aqua regia Mass balance (total Hg recovered/total spiked Hg) =
weak-MEG and strong-MEG retains similar amounts of Hg0 to that of water. The amount of Hg0 observed to be retained in water is similar to the reported solubility of Hg0 in water (0–60 ppb) [19]. 3.4.2. Hg0 partition into Industrial-MEG A modified version of Hg0 delivery system was used to determine the Hg0 uptake in Industrial-MEG. This setup (as detailed in the Supplementary material section, Table S7) produced excellent mercury mass balance of 99.8%. The mercury uptake in numerous Industrial-MEG samples at different pH conditions is presented in Table 6. The pH of the Industrial-MEG was adjusted for each experiment by adding various amounts of HCl acid prior to conducting the 17 h exposure period at room temperature. It was observed that Hg0 saturation in Industrial-MEG ranges from 0 to 80 ppb (similar to Lab-MEG and water samples). This indicates that other species in Industrial-MEG does not influence the uptake of Hg0 vapor. Furthermore, if the gas was not bubbled into the solution and only passed over the headspace of the Industrial-MEG, no Hg0 solubility was observed indicating that long and vigorous mixing is required to dissolve Hg0 in Industrial-MEG. It is worth noting that the ICP-MS analysis could be affected when analyzing for
Trapped species
[Hg] (lg)
HgCl2 HgCl2 VOCs/Hg2+ Hg2+ Hg0 Hg0 Hg0 Hg0 Hg(B) partitioned to Tar
34.7 0 4.86 0.03 0.07 1.77 0 0 5.2 46.6 lg/50 lg
elemental mercury (as opposed to ionic mercury) by enhancing sample introduction efficiency due to the volatile nature of Hg0. However, it is quite possible that the residue in the industrial samples had reduced this effect as Hg0 is scavenged from the sample and this may have played a role in the excellent mass balance achieved for each data point in Table 6. Although this needs further investigation (i.e. develop and implement digestion methods for MEG samples) to confirm, it is in fact a further evidence that Hg0 is not stable in Industrial-MEG. 3.4.3. Mercury partitioning out of the Industrial-MEG (N2 bubbling) In order to determine the amount and type of mercury species that can partition out from the Industrial-MEG and into the gas stream, a total of 50 lg of Hg (as HgCl2) was spiked into the solution followed by bubbling a dry N2 stream (200 sccm) through the mercury spiked solution. The exit gas was trapped in a seven train trap of impingers. Impingers 4 and 5 contained 0.1 M KCl in order to trap any Hg2+ species. The results are presented in Table 7. It is observed that during the 5 h of dry N2 bubbling, 5 lg (10%) partitioned out of the Industrial-MEG as Hg2+, approximately 1.8 lg (or 3.6%) partitioned as Hg0 and the balance remained in the
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Industrial-MEG. The total mercury was measured to be 46.6 lg using ICP-MS which closely matched the amount of spiked Hg (50 lg). The presence of mercury in the bottom of the flask (10%) also confirms the build-up of mercury in the tar residue left on the glassware by the Industrial-MEG solution.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.07.047. References
4. Conclusions Hg partition to and from monoethylene glycol (MEG) solutions were studied at standard laboratory conditions It was found that mercury in the form of HgCl2 is stable within laboratory grade MEG, however was found to be unstable in the industrial MEG solution. Results indicated that the Hg slowly precipitated out of Industrial-MEG solution due to its high pH (c.a. 9) over extended periods of time (17 days). Although no Hg mercury was found to be present in the as supplied Industrial-MEG sample, XPS analysis of the tar residue implied that the mercury precipitates and accumulates in the tar residue. This finding may be useful when developing methods (i.e. filtration) to remove mercury content from Industrial-MEG solution, however further work in this area is warranted. Furthermore, it was found that Hg0 solubility in Industrial-MEG is similar to that of water, ranging from 0–60 ppb (lg/l) with slight increase to 80 ppb when pH is reduced from 9 to 6. Upon bubbling dry N2 for a period of 5 h through HgCl2 spiked Industrial-MEG, it was found that 3.6% leaves as Hg0, 10% as Hg2+ and the balance staying in the MEG solution. These findings could be useful in understanding the type and ratio of mercury species that could be potentially accumulated within a natural gas processing plant which uses MEG during the transportation and recovery processes.
Acknowledgements The authors acknowledge the RMIT microscopy and microanalysis facility (RMMF) for allowing the use of their comprehensive facilities to undergo this research project. The authors would also like to acknowledge the industrial partners for providing Industrial-MEG and for their financial support, feedback and contribution throughout the course of this project. The authors acknowledge the RMIT microscopy and microanalysis facility (RMMF) for allowing the use of their comprehensive facilities to undergo this research project.
[1] Shafawi A, Ebdon L, Foulkes M, Stockwell P, Corns W. Determination of total mercury in hydrocarbons and natural gas condensate by atomic fluorescence spectrometry. Analyst 1999;124:185–9. [2] Edmonds B, Moorwood RAS, Szczepanski R. Mercury partitioning in natural gas and condensates. In: GPA European Chapter Meeting, London; 1996. [3] Multiflash: Modelling Mercury.. [4] Evans M, Brown L. CAPE-OPEN models for Hg speciation and partitioning flowsheet development. In: American Institute of Chemical Engineers (ALCHE), Pittsburgh, PA; 2012. [5] Kaupp M, von Schnering HG. Origin of the Unique Stability of Condensed-Phase I II Hg2+ 2 . An ab Initio Investigation of M and M Species (M = Zn, Cd, Hg). Inorg Chem 1994;33:4179–85. [6] Snell JP, Qian J, Johansson M, Smit K. Stability and reactions of mercury species in organic solution. Analyst 1998;123:905–9. [7] Snell JP, Frech W, Thomassen 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 capillary gas chromatography-microwave-induced plasma atomic emission spectrometry. Analyst 1996;121:1055–60. [8] Qui J. Tough talk over mercury treaty. Nature 2013;493:144–5. [9] Sabri YM, Ippolito SJ, Bhargava SK. Support layer influencing sticking probability: enhancement of mercury sorption capacity of gold. J Phys Chem C 2013;117:8269–75. [10] Sabri YM, Ippolito SJ, Tardio J, Bhargava SK. Study of surface morphology effects on hg sorption–desorption kinetics on gold thin-films. J Phys Chem C 2012;116:2483–92. [11] Lubic N. Funding struggle for mercury monitoring. Nature 2009;459:620–1. [12] Sabri YM, Ippolito SJ, Al Kobaisi M, Griffin MJ, Nelson D, Bhargava SK. Investigation of Hg sorption and diffusion behavior on ultra-thin films of gold using QCM response analysis and SIMS depth profiling. J Mater Chem 2012;22:20929–35. [13] Diba KD, Guglielminetti M, Schiavo S. Glycol reclaimer. In: Offshore Mediterranean Conference (OMC), Ravenna, Italy; 2003. [14] Nazzer CA, Keogh J. Advances in glycol reclamation technology. In: Offshore technology conference, offshore technology conference, Houston, Texas; 2006. p. 7. [15] Psarrou MN, Jøsang LO, Sandengen K, Østvold T. Carbon dioxide solubility and monoethylene glycol (MEG) degradation at MEG reclaiming/regeneration conditions. J Chem Eng Data 2011;56:4720–4. [16] Sabri YM, Ippolito SJ, Tardio J, Hamid SBA, Bhargava SK. Mercury migration and speciation study during monoethylene glycol (MEG) regeneration processes. Ind Eng Chem Res 2015. [17] Laudal D, Nott B, Brown T, Roberson R. Mercury speciation methods for utility flue gas. Fresenius J Anal Chem 1997;358:397–400. [18] Knocke WR, Hemphill LH. Mercury(II) sorption by waste rubber. Water Res 1981;15:275–82. [19] Clever HL, Johnson SA, Derrick ME. The solubility of mercury and some sparingly soluble mercury salts in water and aqueous electrolyte solutions. J Phys Chem Ref Data 1985;14:631–80.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Studying mercury partition in monoethylene glycol (MEG) used in gas facilities Y.M. Sabri a, S.J. Ippolito a,⇑, J. Tardio a, P.D. Morrison b, S.K. Bhargava a,⇑ a b
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, Melbourne, VIC 3001, Australia Department of Applied Chemistry, School of Applied Sciences, RMIT University, Melbourne, VIC 3001, Australia
h i g h l i g h t s Mercury partition from industrial and laboratory grade monoethylene glycol (MEG) was studied. Mercury was more stable in the laboratory MEG. 0
MEG retained 0–60 ppb elemental mercury (Hg ) at pH 9 and 80 ppb at pH 6. 0
3.6% of spiked HgCl2 leaves as Hg , 10% as Hg
2+
and 86.4% stays in industrial-MEG.
Notably, mercury was found to ‘‘drop out’’ of industrial-MEG as particulate mercury.
a r t i c l e
i n f o
Article history: Received 1 December 2014 Received in revised form 29 May 2015 Accepted 14 July 2015 Available online 21 July 2015 Keywords: Monoethylene glycol (MEG) Mercury removal Mercury partition Speciation
a b s t r a c t Mercury partitioning from monoethylene glycol (MEG) during natural gas production is a highly complicated process. Understanding this process will help determine the distribution of mercury species within a given industrial process so that appropriate mercury removal control systems can be effectively managed. In this study, the partitioning of elemental mercury (Hg0) from the gaseous phase into MEG solutions is investigated under standard laboratory conditions. Additionally, the partitioning of dissolved ionic mercury (Hg2+) from MEG solutions into the gaseous phase is also investigated under atmospheric pressure and room temperature conditions. It was found that the solubility of Hg0 in MEG ranged from 0 to 60 ppb (ng/mL) with slight increase to 80 ppb when the pH was reduced from 9 to 6 at room temperature. On the other hand oxidized mercury (Hg2+) was found to be retained in the laboratory grade MEG and industrial samples of MEG (industrial-MEG). However, in the case of the industrial-MEG, the dissolved mercury (HgCl2 salt) was found to be unstable with approximately 3.6% and 10% converting within the solution and leaving in the gas phase as Hg0 and Hg2+, respectively, while the balance was retained in the MEG solution. The distribution was found to be highly pH dependent and was not observed when the experiments were repeated using laboratory grade MEG solutions. These findings help in better understanding the type and ratio of mercury species that are partitioned in and out of MEG in the gas processing facilities. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Abbreviations: weak-MEG, 50 wt.% MEG solution; strong-MEG, 90 wt.% MEG solution; Hg0, elemental mercury; ICP-MS, inductively coupled plasma mass spectroscopy; Industrial-MEG, monoethylene glycol provided by petroleum industry; Lab-MEG, laboratory reagent – monoethylene glycol; Salted-MEG, laboratory reagent monoethylene glycol with added salts; LPG, liquefied petroleum gas; Hg, mercury; MEG, monoethylene glycol; Hg(T), total mercury in glassware; Hg(B), total bound mercury in Tar residue; TOC, total organic carbon; sccm or ml/min, standard cubic centimetre per minute; VOCs, volatile organic compounds; XPS, X-ray photoelectron spectroscopy. ⇑ Corresponding authors. Tel.: +61 3 9925 3365; fax: +61 3 9925 2882. E-mail addresses: [email protected] (S.J. Ippolito), suresh.bhargava@ rmit.edu.au (S.K. Bhargava). http://dx.doi.org/10.1016/j.fuel.2015.07.047 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
The mercury content in many new gas fields is increasing as we are forced to explore deeper and lower quality reserves. Both elemental mercury (Hg0) and oxidized mercury (Hg+/Hg2+) species are present in many gas fields in countries such as Netherlands, USA, Malaysia and Canada just to name a few [1–3]. Depending on the gas field location, the oxidized mercury content can exist in the form of inorganic salts [1]. These salts can easily react with trace levels of H2S found in gas fields to produce HgS which is a relatively more stable, water insoluble
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and less reactive form of mercury that can be easily collected as solid precipitate and removed [4]. The various forms of mercury have also been previously demonstrated to react with each other or with halogens present within hydrocarbon solutions to produce stable Hg2+ 2 type compounds (i.e. Hg2Cl2) [5]. Knowledge of the type of mercury species present, their quantity and stability in different parts of the gas production process (i.e. pipeline transmission, processing, etc.) is difficult yet important due to their extreme toxicity and potential reactivity with process equipment [1]. Furthermore, the chemical and physical properties of each mercury species differ and must be considered in the evaluation of potential health effects and in the design of systems to remove mercury from natural gas. The main implications of mercury in gas production processes arises from its ability to contaminate catalysts, cause corrosion through liquid metal embrittlement of aluminium heat exchangers and impact other equipment that are made from zinc and copper metals [4,6,7]. Mercury related corrosion has resulted in a number of plant shutdowns in the past with multi-million dollar cost implications [1]. Additionally, mercury is toxic to humans and the environment in all its forms, particularly in the organic form known as organomercury species [6,8–12]. Due to the different physical and chemical properties of the various mercury species, it is of great interest to determine the type of mercury species and their stability at each stage of gas production process. This will help in the evaluation of potential health effects and in the design of equipment for efficient removal of mercury [6,7]. An important part of the gas production process where the partitioning and stability information of the different mercury species is of interest is during the transportation and processing of natural gas from the gas reservoir to onshore processing plants. In this process, monoethylene glycol (MEG) or similar agents are typically injected at the reservoir head to protect against formation of hydrates during gas transportation via pipelines. As the feed reaches the gas processing facility it is typically water laden, having approximately 50 wt.% water in addition to containing some other components, such as: salts (thermally stable and metal salts), corrosion products (iron carbonates, sulfides, etc.), mineral scale from the pipelines, heavy hydrocarbons and glycol degradation products [13]. Typically 50 wt.% water laden MEG is referred to as weak MEG and is separated at the front end of the gas processing facility. Due to the large volumes of hydrate inhibitors used in these processes, there is a strong economic drive to recycle (rather than replace) the MEG at the end of the process line by using a water removal step before recirculating it back to the reservoir head [14]. This regeneration process involves heating the weak MEG in a distillation column in order to boil off the water until the MEG solution only contains 10 wt.% water, which is known as strong MEG. The strong MEG is then recirculated out to the well head to repeat the cycle [15]. There is little information available in the public domain regarding the fate of mercury during the MEG recirculation process, making it difficult to determine how mercury partitions and what type of mercury species could be present. Here we investigate the partitioning behaviour of Hg0 and Hg2+ (in the form of HgCl2) to and from weak and strong MEG solutions prepared in the laboratory and an industrial MEG sample obtained from a natural gas processing plant, which are referred to as Lab-MEG and Industrial-MEG from here on, respectively. The MEG solutions were analysed using inductively coupled plasma mass spectroscopy (ICP-MS), total organic compounds (TOC), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). A comparison of the types of MEG provides an insight into how the Industrial-MEG constituents (salts, organics, etc.) effect mercury partition during natural gas transportation processes.
2. Experimental 2.1. Materials Laboratory reagents such as acids (HCl, HNO3, H2SO4), salts (i.e. NaCl, Na2SO4, HgCl2), certified HgCl2 standards, oxidizing agents (KMnO4) and monoethylene glycol (MEG) were purchased from Sigma Aldrich and used as received. A sample of circulated weak MEG from a natural gas processing plant (Industrial-MEG) was used as received, unless specified. The MEG solutions used in the experiments are described in Table 1. The Salted-MEG mix used was the same grade of MEG as that of the Lab-MEG mix, however some salt contaminants were added in order to closely match the salt content and pH of circulated Industrial-MEG (refer to Supplementary data, Table S1). In addition to our own analysis, the industrial MEG sample was supplied with a confidential certificate that showed the MEG was analyzed for mercury (and other inorganic and organic species) by National Association of Testing Authorities (NATA) accredited analytical company using Heavy Metals USEPA 6010/6020 method. The certificate showed insignificant amount of mercury (<0.1 ppb) in the Industrial MEG sample.
2.2. Analytical and characterisation methods 2.2.1. Inductively coupled plasma mass spectroscopy (ICP-MS) All solutions were analysed quantitatively using inductively coupled plasma mass spectroscopy (ICP-MS) from Agilent Technologies (7700x series) with ASX-520 series auto-sampler. The ICP-MS series used in this study addresses the requirements to be used to detect ultra-low concentrations of Hg in solution with minimal interference from organic contents that were present in Industrial-MEG [16]. The instrument had X-type lens with nickel cones and was tuned in helium mode. The analysis protocol utilised an integration time per mass of 1 s for Hg202 isotope (1 point/s), 3 replicates with a sweep/replicate setting of 100. Additionally a prepared mercury standard (used for ICP MS calibration) was analyzed every 3 samples to ensure correct operation of the instrument. Three internal standards (i.e. Tb159, Lu175 and Bi209 isotopes) were also used and were analyzed at an integration time of 0.3 s. The sample uptake was set at 40 s with a peristaltic pump speed of 0.3 rps. The stabilisation time was set at 60 s per sample with the peristaltic pump speed being set at 0.1 rps for this process. The probe from the auto-sampler was set to be rinsed twice from separate baths (each containing 5% nitric acid (HNO3)) while the peristaltic pump speed for this process was also set at 0.3 rps used were mercury was set to be analyzed for 2 s with 3 replicates while the internal standards were analyzed for 0.1 s. The instrument outputs the mean and the relative standard deviation (RSD) of the counts for each analyte and it was ensured that the RSD value was less than 5% for each data point considered for analysis. The background equivalent concentration (BEC) for mercury was found to be 0.2 ppb (ng/mL). All standards and samples (unless specified) prepared for ICP-MS analysis contained the recommended 2% HCl acid content.
Table 1 Description of the three different monoethylene glycol (MEG) solutions used throughout this study. MEG solution
Description of composition
pH
Lab-MEG
10, 50 or 90 wt.% monoethylene glycol (laboratory reagent) 50 wt.% Lab-MEG with added salts from Table S1 50 wt.% MEG supplied by industry
6
Salted-MEG IndustrialMEG
9 9
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Prior to analysing the samples with ICP-MS, the total organic compounds (TOC) in the samples were determined in order to ensure their content is below 1%. The instrument used to determine the TOC was a Sievers 820 Portable Total Organic Carbon Analyser. The instrument is calibrated every year by Amscorp using GE standards. The validation of the calibration is performed regularly every 6 months using GE validations solutions. Milli-Q water was analysed first on the instrument before every set of samples as a control. If Milli-Q gave an unusual reading, a validation was run. Samples were prepared for TOC analysis as follows: Lab-MEG (1% v/v) and Industrial-MEG (1% v/v) were diluted in Milli-Q water (as was done in samples which were analysed using ICP-MS) and analysed with the TOC instrument. It was observed that following preconditioning, the diluted Industrial-MEG samples had a TOC value of <1%, which was sufficiently low enough for safe and reliable analysis of the sample solutions by ICP-MS. 2.2.2. X-ray photoelectron spectroscopy (XPS) Solid tar residue (Tar) found in Industrial-MEG was digested in aqua regia (1:3 concentrated HCl:HNO3) before being diluted and analysed for their mercury content using ICP-MS. The Tar was also analysed using X-ray photoelectron spectroscopy (XPS). XPS characterisation of the tar residue was performed using a Thermo K-Alpha instrument at a pressure better than 1 10 9 Torr. The core level binding energies (BEs) were aligned with the adventitious C 1s binding energy of 285 eV. 2.2.3. Fourier transform infrared attenuated total reflectance (FTIRATR) All Fourier transform infrared attenuated total reflectance (FTIR-ATR) measurements in this study were performed using Perkin Elmer Spectrum 100 instrument equipped with micro ATR (diamond ATR crystal), in the range 650–4000 cm 1 region with a resolution of 2 cm 1. Each spectrum represents an average of 10 consecutive scans. The background spectra consisted of the base diamond under the same experimental conditions. 2.3. Experimental setups to investigate mercury partitioning In order to investigate the transfer of mercury from gas to liquid and also from liquid to gas, three setups (Hg0 delivery system, setup-1 and setup-2) were used. Each experiment was conducted with research laboratory equipment at atmospheric pressures as detailed in the proceeding sections. 2.3.1. Hg0 delivery system An elemental mercury (Hg0) vapor delivery system was first developed and calibrated so it generated repeatable Hg0 concentrations. A highly controllable Dynacal (Model 150C) system and permeation tubes (VICI, TX, USA) were used to generate Hg0 vapors in a repeatable fashion. This involved maintaining the permeation
vessel at a constant pressure flow of 200 sccm (200 ml/min) and using heated stainless steel/Teflon tubes that were compatible with Hg0 vapor. The concentrations of the Hg0 generated in the test stream were validated using acidic KMnO4 wet trapping method (similar to the Ontario-Hydro method [17]) which were analysed using ICP-MS. The mercury generator was calibrated over a period of 2 h prior to each experiment, however initial testing of the generators’ stability in reproducing Hg0 vapor concentrations was confirmed by testing it over different time periods (refer to Supplementary data, Fig. S1 and Table S2). It was observed that the generator produced 0.99 ± 0.02 mg/m3 of Hg0 balanced in dry N2 for all calibration times. 2.3.2. Setup-1: Transfer of mercury from gas to liquid phase Tests to investigate the extent of Hg0 vapor partitioning into the MEG solutions were conducted by bubbling Hg0 vapor stream (balanced in dry nitrogen) with a concentration of 1 mg/m3 (112 ppbv) through the MEG solutions at a flow rate of 200 sccm (standard cubic centimetre per minute or standard mL/min) for up to 17 h. The experimental setup (setup-1) is shown in Fig. 1. The chemical solutions that were placed in each glass impinger are listed in Table 2. In order to perform Hg0 mass balance, six impingers were used as modified version of the Ontario-Hydro method [17]. Impinger 1 contained the MEG to be saturated with Hg0. Impinger 2 was used to stop the foams produced in the MEG solution (impinger 1) from transferring into the proceeding impingers. Impinger 3 was used to stop the transfer of volatile organic compounds (VOCs) which were released from the MEG from entering into the KMnO4 impingers (impingers 4–6) as VOCs are known to decrease the efficiency of KMnO4 traps. This was achieved by using a diluted mixture (1:50) of H2O2:H2SO4 (1:3 ratio of concentrated oxidizers) in impinge 3. This impinger also oxidized and trapped some of the Hg0 which was not taken up by the MEG in impingers 1 or in the overflow trap (trap 2). The final 3 traps (impingers 4–6) contained H2SO4 and KMnO4 which were used to trap the remaining Hg0 within the gas stream. The chemicals in impingers 3–6 worked by oxidizing gas phase Hg0 to a soluble Hg2+ form. The solid tar
Table 2 List of glass impingers and the chemicals employed in setup-1 in order to trap the chemical of interest. Impinger No.
Chemical
Trapped species
1 2 3 4 5 6 MEG MEG-Tar
100 ml MEG Empty (acid wash) H2O2/H2SO4 KMnO4 KMnO4 KMnO4 MEG saturated with Hg0 Aqua regia wash of impingers 1 & 2
Hg0 Foam carry over VOCs/Hg0 Hg0 Hg0 Hg0 Hg0 Hg(B) partitioned to Tar
Industrial-MEG Fig. 1. Schematic of setup-1 for determining the partition of mercury from gas phase to liquid phase. All traps were analysed for mercury using ICP-MS.
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residue that remained in impingers 1 and 2 once the Industrial-MEG was emptied following the Hg0 saturation experiment is referred to as the Bottom-Tar in Table 2. This Tar residue was acid washed using 1 mL of aqua regia, diluted and analysed by ICP-MS. The mercury content that was partitioned to the tar residue is referred to as the total bound mercury (Hg(B)), which may include elemental and/or ionic mercury species. 2.3.3. Setup-2: Transfer of mercury from liquid to gas phase Experiments using setup-2 (see Fig. 2) were performed in order to understand the partition of mercury from the Industrial-MEG solution into a dry nitrogen gas stream. The test was conducted by bubbling the nitrogen gas stream through the Industrial-MEG solution which was spiked with 0.5 ppm Hg (as HgCl2) all at atmospheric conditions (1 atm). The total gas flow rate was set at 200 sccm for a period of 5 h and any evolved mercury was captured in a train of traps similar to one used in setup-1. The difference between setup-1 and setup-2 is the addition of two KCl traps in setup-2, which are used in order to trap any gas phase oxidized mercury (Hg+, Hg2+) species. All solutions were diluted and analysed using ICP-MS. 3. Results and discussions 3.1. Analysis of mercury in Lab-MEG solutions Analysis of mercury in complex solutions, particularly solutions containing organic compounds, can be problematic [1,7], hence measurement of mercury in MEG solutions was investigated prior to conducting partitioning tests. This involved ‘‘spiking’’ or adding mercury in a range of different MEG solutions and analysing the solutions using ICP-MS. Spike tests were conducted on the MEG solutions described in Table 1. The Lab-MEG and Salted-MEG (with Milli-Q water being used as the control sample) were diluted then spiked with HgCl2 and analysed by ICP-MS, the results of which are shown in Table 3. It was observed that ICP-MS has an error anywhere within ±3% for water (exp. 1–7) and ±5% for Lab-MEG (exp. 8–17) on most instances, which is expected given the combined error margins involved in spiking Hg from a known certified standard, performing dilutions and instrumental errors (such as BEC or memory effects within the analysis process). The relative standard deviation (RSD) values in the ICP-MS output data were all below the accepted 5% margin. It may be observed that the errors produced by ICP-MS for Salted-MEG (exp. 18–27) are not significantly different from that of the Lab-MEG, however it is worth noting that all the RSD values were negative, indicating under-reads from the expected Hg concentrations were occurring. Overall, the data indicates that up to 50 wt.% Lab-MEG and Salted-MEG samples can be analysed accurately for their mercury content using the ICP-MS technique.
3.2. Analysis of mercury in Industrial-MEG solutions The Industrial-MEG was initially unsuitable for ICP-MS analysis due to the strong odour that resulted from volatile organic compounds (VOCs), the presence of which was confirmed by FTIR-ATR (Fig. 3) and TOC analysis (refer to Supplementary data, Table S3). It may be observed that the Industrial-MEG contains organic compounds, however, a pre-treatment process involving open to air over the course of 3 days allowed for significantly reduced TOC content in Industrial-MEG thus making it acceptable for ICP-MS. The Industrial-MEG was pre-treated via opening cap container over the course of 3 days and, similar to Lab-MEG, was also diluted, spiked with HgCl2 and analysed using ICP-MS on the same day. The results obtained from the ICP-MS analysis is provided in Table 3 (exp. 28–33). As can be seen, significant under-readings were obtained when analysing freshly HgCl2 spiked Industrial-MEG samples by ICP-MS. Unlike the Lab-MEG and Salted-MEG, the Industrial-MEG was observed to introduce significantly large errors (under-reads) in the ICP-MS output when analysing for Hg content (see the standard deviation value shown in the last row for each MEG type). This indicates that mercury may have produced poor ionization when analysing by ICP-MS due to the matrix of the solution or mercury may have been unstable in Industrial-MEG solution and was undergoing precipitation. In order to determine whether mercury precipitation out of solution was the cause of the mercury content under-reading by ICP-MS, a long term mercury stability test in Industrial-MEG was performed. 3.3. Long term mercury stability in Industrial-MEG In order to determine the solubility and stability of HgCl2 in Industrial-MEG solutions, 0.5 ppm of Hg (as HgCl2) was dissolved in a 200 mL solution of Industrial-MEG. Thereafter the solution was periodically analysed by ICP-MS over a 17 day period using a 1:100 dilution ratio. The 1:100 dilution ratio was chosen as it produced the most accurate mercury content within the Industrial-MEG solution, both with and without the standard addition method (see Supplementary material, Tables S4–S6 and Figs. S2 and S3). The results are shown in Table 4. The data obtained clearly indicates that an error between the expected mercury concentration in the Industrial-MEG and the ICP-MS result increased with increasing number of days, showing a consistent increase in the extent of under readings in both (duplicates) Industrial-MEG solutions. On the other hand, the water sample and Lab-MEG mercury concentrations reported by ICP-MS were similar to that of the expected values throughout the entire 17 day test. The later result confirms that HgCl2 is unstable in the Industrial-MEG solution and was postulated to precipitate and
2+
Hg + Industrial-MEG
Fig. 2. Schematic of setup-2 for determining the partition of mercury from liquid phase to gas phase. All traps were analysed for mercury using ICP-MS.
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Y.M. Sabri et al. / Fuel 159 (2015) 917–924 Table 3 ICP-MS readings for water, Lab-MEG, Salted-MEG and Industrial-MEG diluted and spiked with HgCl2. Exp.
Sample
1 2 3 4 5 6 7
Standard Standard Standard Standard Standard Standard Standard
MEG (%) (H2O) (H2O) (H2O) (H2O) (H2O) (H2O) (H2O)
0 0 0 0 0 0 0
Sample dilution
Spiked Hg2+ (ppb)
ICP-MS [Hg] (ppb)
– – – – – – –
2 2 2 2 4 6 6
1.90 1.98 1.95 2.01 4.12 5.79 5.85
5.0 1.0 2.5 +0.5 +3.0 3.5 2.5
Error (%)
8 9 10 11 12 13 14 15 16 17
Standard deviation (%) Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab-MEG Lab MEG Lab-MEG Lab-MEG
0.5 1 2 10 0.5 1 2 0.05 0.01 0.01
1:200 1:100 1:50 1:10 1:200 1:100 1:50 1:1000 1:10000 1:10000
2 2 2 2 6 6 6 5 5 5
1.89 1.93 1.85 1.9 5.68 5.81 5.70 5.20 5.25 5.10
±2.67 5.5 3.5 7.5 5.0 5.3 3.2 5.0 +4.0 +5.0 +2.0
18 19 20 21 22 23 24 25 26 27
Standard deviation (%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%) Salted-MEG (50%)
0.25 0.5 1 5 50 0.25 0.5 1 5 50
1:200 1:100 1:50 1:10 1:1 1:200 1:100 1:50 1:10 1:1
2 2 2 2 2 6 6 6 6 6
1.87 1.99 1.88 1.82 1.95 5.82 5.97 5.80 5.72 5.81
±4.57 6.5 0.5 6.0 9.0 2.5 3.0 0.5 3.3 4.7 3.2
28 29 30 31 32 33
Standard deviation (%) Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG
0.5 1 2 0.5 1 2
1:200 1:100 1:50 1:200 1:100 1:50
2 2 2 6 6 6
2.03 1.68 1.49 5.16 6.08 5.13
±2.68 +1.5 16.0 25.5 14.0 1.3 14.5
Standard deviation (%)
±10.0
Transmittance
Industrial-MEG
Lab MEG CH CH bend
Water OH
4000
3000
OH bend
2000
1000
Wavenumber (cm-1) Fig. 3. FTIR ATR of Lab-MEG and Industrial-MEG samples.
‘‘drop out’’ of the Industrial-MEG solution to form a tar like residue on the bottom of the glassware which was not immediately visible to the naked eye. In order to visually demonstration the instability of mercury in Industrial-MEG solution, a relatively large amount of HgCl2 (4.9 g/L) was added to small volumes of Industrial-MEG (10 ml). Fig. 4 shows 5 vials containing Industrial-MEG with additional known chemical species. The first vial contained Industrial-MEG and H2O (to make up the volume to 15 mL) while the second vial contained Industrial-MEG and NaHCO3 (pH > 12). No visual changes were observed between these control vials, as expected. However on the addition of HgCl2 into identical control vials (vials 3 and 4), a significant dropout of presumably Hg rich solids can be observed where the kinetics of the dropout seems to increase in the presence of NaHCO3 (vial 4). When repeated with a vial containing Industrial-MEG and HCl (pH < 3) no drop out of solid material was observed when HgCl2 was added (vial 5)
Table 4 Stability of HgCl2 in water, Lab-MEG and Industrial-MEG over a 17 day period. Day
Expected Hg2+ (ppb)
Water
50% Lab-MEG [Hg] (ppb)
Industrial-MEG-1 [Hg] (ppb)
Industrial-MEG-2 [Hg] (ppb)
1 2 8 15 17
5 5 5 5 5
5.06 4.96 4.86 5.08 5.05
5.01 4.92 4.92 5.2 5.06
3.65 3.31 2.71 1.34 0.94
3.39 3.61 2.83 1.21 1.06
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Fig. 4. Addition of HgCl2/NaHCO3/HCl to Industrial-MEG. The images were taken 5 s following addition of (1) H2O, (2) NaHCO3, (3) HgCl2, (4) NaHCO3 and HgCl2, and (5) HCl. Upon addition of HCl, the pH of the Industrial-MEG solution had changed from 9 to 6.
indicating that the mercury precipitation and drop out is due to the high pH of the solution. The drop out of dissolved mercury out of solution at high pH is well known [18] and is also demonstrated in the Supplementary material of this manuscript (see Supplementary material, Fig. S4). The demonstration in Fig. 4 indicates that HgCl2 drops out of the Industrial-MEG solution due to its high pH of 9. Similar findings were also observed in the ICP-MS experiments conducted in Section 3.3 of this manuscript. Therefore it is postulated that the mercury could be unknowingly accumulating in the solid tar residue that is present in the Industrial-MEG. Two different approaches were used to show the presence of mercury in the solid tar residue of the supplied Industrial-MEG sample, which was initially presumed to be mercury free. The first approach involved scraping a small amount of tar residue from the original container holding the (untreated) Industrial-MEG and digesting it in aqua regia. The digested solution was then filtered, diluted to 100 ml. The analysis of this solution resulted in an ICP-MS reading of 6.2 ppb, thus confirming the presence of mercury within the
supplied Industrial-MEG tar residue. In contrary to this, the analysis of the as-supplied Industrial-MEG solution indicated no mercury was present, presumably as a result of complete drop out due to the long period of time (>30 days) between when the solution was used in the industrial processes and when the same Industrial-MEG was delivered and used in the these experiments. The second method to confirm the presence of mercury within the Industrial-MEG’s tar residue involved its analysis via XPS technique. Two regions (Hg4d and Hg4f core level) within XPS spectra which denote the binding energies of Hg atoms were investigated. Analyses of HgCl2 as a control as well as the Tar residue in the 4f core level (90–115 eV) are shown in Fig. 5a and b, respectively. Furthermore, the 4d core level (350–390 eV) spectra for a Hg contaminated Au film (used as control) and the Tar residue are shown in Fig. 5c and d. Following the de-convolution of the peaks, it can be observed that the mercury peaks are present in all spectra, thus, confirming the presence of mercury in the tar residue of the supplied Industrial-MEG samples. Since the sample in question was not spiked with mercury in the laboratory, the data suggests that Hg accumulated in Industrial-MEG sample while it was being used in the industrial process to extract natural gas. Therefore the removal of the tar residue from the MEG solution before it is re-used or re-circulated in a natural process could help in significantly reducing mercury content in the MEG. 3.4. Mercury partition 3.4.1. Hg0 partition into Lab-MEG In order to determine the saturation level of gas phase Hg0 in weak-MEG and strong-MEG, a nitrogen stream containing a known Hg0 vapor concentration of 1 mg/m3 was bubbled through the weak-MEG at 200 sccm for a period of 17 h. The experiments were carried out using a setup similar to that of setup-2, however the chemical contents in each dreschel bottle differed significantly. The dreschel bottle contents and the resulting mercury content in each solution are shown in Table 5. It can be observed that the
Fig. 5. XPS spectra of Hg4f core shell analysis in (a) HgCl2 salt (control) and (b) Tar present in Industrial-MEG; as well as Hg4d core shell analysis in (c) Au-Hg amalgam (control) and (d) Tar present in Industrial-MEG.
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Table 5 Hg0 saturation of Lab-MEG. The Hg0 concentration (obtained from the Hg0 generator calibration) prior to each test is presented in the last row of the table. Calibration was conducted prior to the experiment by using Hg0 delivery system setup shown in Fig. S1. Trap No.
Sample
Weak-MEG Hg0 Conc. (ppb)
Weak-MEG Hg0 Conc. (ppb)
Strong-MEG Hg0 Conc. (ppb)
1 2 3 4 5
50% MEG Water 50% MEG KMnO4 KMnO4
60 7 5 2942 9
4 74 10 2623 12
6 6 6 1909 12
1.04 1.09
1.00 1.06
0.713 0.843
Total [Hg0 ] in mg/m3 = Calibration [Hg0] in mg/m3 =
a a
Calibration conducted prior to experiment using Hg0 delivery system setup shown in Fig. S1.
Table 6 Hg0 saturation in Industrial-MEG at different conditions following 17 h of Hg0/N2 flow. ‘‘Not Bubbled’’ refers to experiments where Hg0 gas stream was only passed over the Industrial-MEG headspace and not bubbled into the solution. Exp. No.
Sample
Condition
pH
[Hg] (ppb)
1 2 3 4 5 6 7 8 9 10
Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG Industrial-MEG
Bubbled Bubbled Bubbled Bubbled Bubbled Not Bubbled Bubbled Bubbled Bubbled Not Bubbled
5–6 5–6 5–6 5–6 5–6 5–6 7–8 7–8 8–9 8–9
57.0 47.0 81.9 79.5 83.0 0.0 30.0 61.2 60.0 0.0
Table 7 Hg0 partition from Industrial-MEG after 5 h N2 flow using setup-2 shown in Fig. 2. The mass reported is within a precision of ±0.02 lg. Impinger No.
Chemical
1 100 ml MEG + 0.5 ppm Hg as HgCl2) 2 Empty (acid wash) 3 KCl 4 KCl 5 H2O2/H2SO4 6 KMnO4 7 KMnO4 8 KMnO4 Tar in impinger 1 Aqua regia Mass balance (total Hg recovered/total spiked Hg) =
weak-MEG and strong-MEG retains similar amounts of Hg0 to that of water. The amount of Hg0 observed to be retained in water is similar to the reported solubility of Hg0 in water (0–60 ppb) [19]. 3.4.2. Hg0 partition into Industrial-MEG A modified version of Hg0 delivery system was used to determine the Hg0 uptake in Industrial-MEG. This setup (as detailed in the Supplementary material section, Table S7) produced excellent mercury mass balance of 99.8%. The mercury uptake in numerous Industrial-MEG samples at different pH conditions is presented in Table 6. The pH of the Industrial-MEG was adjusted for each experiment by adding various amounts of HCl acid prior to conducting the 17 h exposure period at room temperature. It was observed that Hg0 saturation in Industrial-MEG ranges from 0 to 80 ppb (similar to Lab-MEG and water samples). This indicates that other species in Industrial-MEG does not influence the uptake of Hg0 vapor. Furthermore, if the gas was not bubbled into the solution and only passed over the headspace of the Industrial-MEG, no Hg0 solubility was observed indicating that long and vigorous mixing is required to dissolve Hg0 in Industrial-MEG. It is worth noting that the ICP-MS analysis could be affected when analyzing for
Trapped species
[Hg] (lg)
HgCl2 HgCl2 VOCs/Hg2+ Hg2+ Hg0 Hg0 Hg0 Hg0 Hg(B) partitioned to Tar
34.7 0 4.86 0.03 0.07 1.77 0 0 5.2 46.6 lg/50 lg
elemental mercury (as opposed to ionic mercury) by enhancing sample introduction efficiency due to the volatile nature of Hg0. However, it is quite possible that the residue in the industrial samples had reduced this effect as Hg0 is scavenged from the sample and this may have played a role in the excellent mass balance achieved for each data point in Table 6. Although this needs further investigation (i.e. develop and implement digestion methods for MEG samples) to confirm, it is in fact a further evidence that Hg0 is not stable in Industrial-MEG. 3.4.3. Mercury partitioning out of the Industrial-MEG (N2 bubbling) In order to determine the amount and type of mercury species that can partition out from the Industrial-MEG and into the gas stream, a total of 50 lg of Hg (as HgCl2) was spiked into the solution followed by bubbling a dry N2 stream (200 sccm) through the mercury spiked solution. The exit gas was trapped in a seven train trap of impingers. Impingers 4 and 5 contained 0.1 M KCl in order to trap any Hg2+ species. The results are presented in Table 7. It is observed that during the 5 h of dry N2 bubbling, 5 lg (10%) partitioned out of the Industrial-MEG as Hg2+, approximately 1.8 lg (or 3.6%) partitioned as Hg0 and the balance remained in the
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Industrial-MEG. The total mercury was measured to be 46.6 lg using ICP-MS which closely matched the amount of spiked Hg (50 lg). The presence of mercury in the bottom of the flask (10%) also confirms the build-up of mercury in the tar residue left on the glassware by the Industrial-MEG solution.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.07.047. References
4. Conclusions Hg partition to and from monoethylene glycol (MEG) solutions were studied at standard laboratory conditions It was found that mercury in the form of HgCl2 is stable within laboratory grade MEG, however was found to be unstable in the industrial MEG solution. Results indicated that the Hg slowly precipitated out of Industrial-MEG solution due to its high pH (c.a. 9) over extended periods of time (17 days). Although no Hg mercury was found to be present in the as supplied Industrial-MEG sample, XPS analysis of the tar residue implied that the mercury precipitates and accumulates in the tar residue. This finding may be useful when developing methods (i.e. filtration) to remove mercury content from Industrial-MEG solution, however further work in this area is warranted. Furthermore, it was found that Hg0 solubility in Industrial-MEG is similar to that of water, ranging from 0–60 ppb (lg/l) with slight increase to 80 ppb when pH is reduced from 9 to 6. Upon bubbling dry N2 for a period of 5 h through HgCl2 spiked Industrial-MEG, it was found that 3.6% leaves as Hg0, 10% as Hg2+ and the balance staying in the MEG solution. These findings could be useful in understanding the type and ratio of mercury species that could be potentially accumulated within a natural gas processing plant which uses MEG during the transportation and recovery processes.
Acknowledgements The authors acknowledge the RMIT microscopy and microanalysis facility (RMMF) for allowing the use of their comprehensive facilities to undergo this research project. The authors would also like to acknowledge the industrial partners for providing Industrial-MEG and for their financial support, feedback and contribution throughout the course of this project. The authors acknowledge the RMIT microscopy and microanalysis facility (RMMF) for allowing the use of their comprehensive facilities to undergo this research project.
[1] Shafawi A, Ebdon L, Foulkes M, Stockwell P, Corns W. Determination of total mercury in hydrocarbons and natural gas condensate by atomic fluorescence spectrometry. Analyst 1999;124:185–9. [2] Edmonds B, Moorwood RAS, Szczepanski R. Mercury partitioning in natural gas and condensates. In: GPA European Chapter Meeting, London; 1996. [3] Multiflash: Modelling Mercury.