Mercury Migration and Speciation Study during Monoethylene Glycol

Apr 28, 2015 - The partitioning of mercury from the distillation process used to regenerate water-laden monoethylene glycol (MEG) in the natural gas p...
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Mercury Migration and Speciation Study during Monoethylene Glycol Regeneration Processes Y. M. Sabri,† S. J. Ippolito,*,† J. Tardio,† S. Bee Abd Hamid,‡ and S. K. Bhargava*,† †

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, Melbourne, Victoria 3001, Australia ‡ Department Nanotechnology & Catalysis Research Center (NANOCAT), Institute of Postgraduate Studies (IPS), University of Malaya, third floor, Block A, 50603 Kuala Lumpur, Malaysia S Supporting Information *

ABSTRACT: The partitioning of mercury from the distillation process used to regenerate water-laden monoethylene glycol (MEG) in the natural gas processing industry is a highly complicated process. The experiments detailed in this paper concerned the transfer and partitioning of mercury species in an unpressurized laboratory scale process at a distillation temperature of 170 °C. Experiments were conducted with both laboratory grade MEG solutions and industry based samples obtained from a natural gas processing facility. It was found that ionic mercury (Hg2+) was not stable in MEG samples taken from the natural gas processing facility due in part to the high pH (pH >8) and the various additives as well as the buildup of inorganic compounds within a recirculated industrial MEG sample. That is, Hg2+ can accumulate in the solid tar residue as a precipitated mass as well as decompose and partition out of the MEG solution as gas phase elemental mercury (Hg0). Significantly, it was found that approximately 50% of the spiked Hg2+ in an industry based sample is reduced and leaves as Hg0 during the MEG regeneration process, whereas only 10% leaves as Hg0 in laboratory based samples. The partition percentage is highly dependent on the salt and organic content present in the MEG solution. The results from these types of studies could potentially lead to more costeffective mercury treatment technologies for natural gas processing industries as well as improve the understanding of how potential mercury contamination of recirculating MEG behaves during natural gas recovery processes.

1. INTRODUCTION Mercury and its compounds are present in many gas fields worldwide with levels ranging from 5 × 10−6 to 4.4 g/m3 in some gas fields.1 Even trace amounts of mercury can contaminate precious metal catalysts downstream as well as accumulate in equipment parts and have adverse effects on the processing of natural gas streams (e.g., liquid-metal embrittlement (LME) of aluminum heat exchangers). Furthermore, mercury is also well-known to be toxic to both humans and the environment.2−5 Hence there is significant interest in understanding the chemistry of mercury in natural gas transportation and refinery processes in order to take appropriate mitigation steps. In the natural gas recovery processes, mercury species can be found in the hydrate inhibitor (such as monoethylene glycol or similar dewatering agents) that are typically injected in the reservoir at the start of the pipeline to assist with transporting of the gas to onshore processing facilities.6,7 This water-laden MEG (sometimes referred to as weak MEG) can contain ∼50 wt % water by the time it arrives at the processing facility. It also contains components such as corrosion products (iron carbonates, sulfides, etc.), salts (thermally stable and metal salts), heavy hydrocarbons, glycol degradation products, and mineral scales from the pipelines.8,9 Due to the high cost of replacing the large amounts of hydrate inhibitor used within the natural gas processing system, there is a strong economic drive to recycle and recirculate the MEG.10,11 Essentially, the effectiveness of MEG as a hydrate inhibitor depends to a large extent on the cleanliness of the regeneration system; © 2015 American Chemical Society

therefore, regular monitoring of MEG concentration, salt content, solids content, pH stabilization, and iron content is advocated.9 A common process for regenerating the weak MEG involves heating the MEG in a distillation column to boil off the water until it contains ∼10 wt % water, which is known as strong MEG.11 The bottom of the column is usually operated between 120 and 200 °C and just above the boiling point of water at the top (usually ∼110 °C).11 Although higher temperatures at the bottom of the column allow for higher recovery of the MEG, it can also cause thermal degradation and is thus avoided.12 During natural gas transportation, the elemental and inorganic forms (oxidized species such as ions from salts) of mercury present in the natural gas can partition into MEG and accumulate as the recirculation process continues.13 The presence of organic mercury compounds (i.e., methylmercury, ethylmercury, etc.) is rarely expected in reservoirs, and if the compounds are present, they are also expected to accumulate in the MEG solution.1 The mercury content from the MEG can transfer to the water vapor and dissolved gas content of the MEG during the regeneration process, thus representing a critical emission point for mercury into the surrounding environment. There is little information available on the fate of mercury during the MEG regeneration cycle, making it difficult to determine what factors influence the Received: Revised: Accepted: Published: 5349

February April 28, April 28, April 28,

4, 2015 2015 2015 2015 DOI: 10.1021/acs.iecr.5b00492 Ind. Eng. Chem. Res. 2015, 54, 5349−5355

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Industrial & Engineering Chemistry Research

Figure 1. Schematic of the experimental setup used for the distillation process of MEG solutions to determine the mercury partition behavior from the bottoms MEG into the gas phase. The solutions from the heated flask and the train of six traps were analyzed for their mercury content using ICP-MS.

capture efficiencies. A train of six jacketed glass impinges was used where water circulating in the jacket was kept at 3.5 ± 0.1 °C. The distillation experiments were performed by placing 200 mL of MEG solution spiked with 100 μg of Hg (as HgCl2) into a 500 mL flask which was heated using a heating mantle. The mercury content was added from a validated mercury standard having pH ∼7, ensuring the mercury concentration in each prepared solution was 500 ppb. This solution was heated in order to convert the weak MEG into strong MEG. The heating mantle was maintained at 170 ± 1 °C by an in-built temperature controller with varying degrees of insulation around the 500 mL flask and overhead connector. This maintained the temperature of the weak MEG mixtures in the flask at 170 °C and between 100 and 110 °C at the overhead connector leading to the speciation traps (similar bottoms and overheads operating temperatures used in industry, however under laboratory scale). The distillation process took about 2− 2.5 h to complete (i.e., reach ∼10 wt % water content) depending on the type of MEG that was being studied. A Teflon magnetic stirrer inside the MEG solution was set at 150 rpm. The generated vapor that reached the overhead area of the setup was directed toward the train of chemical traps using dry nitrogen as the carrier gas. The carrier gas was injected into the headspace of the heating flask at a flow rate of 200 sccm (mL/ min) to ensure that the partitioned vapor from the MEG solution transferred to the train of traps in order to allow for the mercury species present in the vapor to be identified. The relative amounts of mercury released from the MEG solution and that retained in the resulting strong MEG were determined by using ICP-MS. In order to perform mass balance, each experiment was conducted in triplicate due to the problematic issue of analyzing mercury in organic based solutions by ICPMS. This allowed for the mass balance to be repeated and confirmed, thereby also confirming the repeatability of the obtained results. Furthermore, each of the three replicates for each sample analyzed was ensured to have less than 5% relative standard deviation before being considered for analysis. The points at which mercury analysis was performed using ICP-MS (marked with black stars in Figure 1) included the strong MEG in the 500 mL flask at the end of the experiment,

mercury partitioning process. Additionally, it is not known whether oxidized mercury can convert to elemental mercury and escape via the overhead of the distillation column during the MEG regeneration process. Here we investigate the partitioning behavior of mercury during the regeneration process of laboratory grade MEG (referred to as Lab-MEG) and the actual industrial MEG provided by a natural gas processing plant (referred to as Industrial-MEG). A comparison of the types of MEG provides an insight into how the Industrial-MEG constituents (salts, organics, etc.) effect mercury partition during MEG regeneration processes.

2. EXPERIMENTAL SECTION 2.1. Materials. Laboratory reagents such as acids (HNO3, HCl, H2SO4), salts (i.e., HgCl2, NaCl, Na2SO4), 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 (referred to as Industrial-MEG) was used as received. The MEG solutions used for distillation experiments are described in the Supporting Information (Tables S1 and S2). Salted-MEG mix used the same grade of MEG used in the LabMEG mix; however, the salt content and pH were adjusted to closely match the measured values present in the recirculated Industrial-MEG provided by a natural gas processing plant. 2.2. Experimental Setups To Investigate Mercury Partitioning. A laboratory scale distillation process operating at atmospheric pressure was set up (Figure 1) and tried to replicate the operating temperature (∼170 °C) used in the plant scale MEG regeneration process. The transfer and partitioning of mercury from the MEG solutions to the gaseous phase were investigated by capturing the evolved mercury in a set of chemical traps and analytically measuring the mercury level in each trap as well as the remaining mercury present in the MEG solution in order to obtain a full mass balance. In order to overcome some of the issues with trapping the evolved mercury, a modified version of the Ontario Hydro method14 was used which required specially designed traps (Dreschel bottles) to degrade any volatile organic compounds present in Industrial-MEG without influencing the mercury speciation and 5350

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Industrial & Engineering Chemistry Research the tar residue which remained on the walls of the glass flask, the overhead connector line (CL), and the six impingers. The chemical content of each impinger and their role in the type of mercury species they were designed to trap are presented in the Supporting Information (Table S3). The CL glassware was also rinsed with aqua regia and the solution was collected, diluted, and analyzed for its mercury content, which is reported as Hg(CL) and could include elemental and/or ionic mercury. The transferred (gas phase) mercury species captured in the six impinger train were separated by using 0.1 M KCl in impingers 1 and 2 in order to trap any ionic mercury (Hg+/ Hg2+). 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 impinger 3. This impinger was also expected to oxidize and trap some of the elemental mercury (Hg0) vapor; therefore mercury captured in this trap was designated as Hg0. The final three traps (impingers 4−6) contained H2SO4 and KMnO4 which were used to trap the remaining Hg0 within the gas stream. Three impingers in series were used as the MEG solutions containing mercury were heated at 170 °C; thus high concentrations of mercury may be reached in the flue gas and so a three-trap system will guarantee that next to no amount of mercury escapes the trap solutions. The chemical content in impingers 3−6 worked by oxidizing the gas phase Hg0 to a soluble (Hg2+) form. The regenerated MEG following the distillation experiments was also analyzed using ICP-MS following a 1:100 dilution. Residual tar that remained in 500 mL glass flask once the regenerated MEG (or strong MEG) was emptied, was extracted via an additional acid wash using 1 mL of aqua regia (1:3 concentrated HNO3:HCl). The mercury content that was partitioned to tar residue is referred to as the bound mercury (Hg(B)), which may include elemental and/or ionic mercury species. 2.3. Analytical Method: Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). All solutions were analyzed quantitatively using inductively coupled plasma mass spectroscopy (ICP-MS) from Agilent Technologies (7700x series) with an ASX 520 series autosampler. The instrument had an X-type lens with nickel cones and was tuned in helium mode. The development of ICP-MS nebulizers for mercury analysis is ongoing, and detection limits as low as 100 parts per trillion (ppt or ng/L) have been reported with designs developed over two decades ago.15 The ICP-MS instrument used in this study was certified for a mercury detection limit of 1 ppt. The low detection limit for this instrument was achieved by analyzing clean acidic solutions (no other chemical species present) containing mercury alone. However, due to the highly contaminated industry derived solutions used in this study, the Hg background equivalent concentration (BEC) value that determines the lowest measurable concentrations of mercury was found to be ∼0.2 parts per billion (ppb or μg/L). Numerous quality controls were followed in order to ensure the soundness of the experimental methodology which involved using mercury specific analysis settings. The analysis protocol utilized an integration time per mass of 1 s for 202Hg isotope (1 point/s) and three replicates with a sweep/replicate setting of 100. Additionally, a prepared mercury standard (used for ICPMS calibration) was analyzed every three samples to ensure correct operation of the instrument. Three internal standards

(i.e., 159Tb, 175Lu, and 209Bi 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 stabilization 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 autosampler 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. Mercury was set to be analyzed for 2 s with three 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. Prior to analysis, 0.1 mL of concentrated HNO3 was added to each, blank, standard and sample in order for the mercury content to remain stable within the sample matrix. This acidification procedure ensured no loss of mercury occurred between the completion of distillation experiments and sample analysis protocols. This enabled conducting of the regeneration experiments and determination of the Hg mass balance throughout the system. The mass balances were performed once confidence in the analytical method and instrument (ICP-MS) was achieved. The validation of the analytical method for mercury analysis using ICP-MS is presented in the Supporting Information (Tables S4 and S5). The ICP-MS uncertainty data for measuring the mercury content of water, Lab-MEG, salted-MEG, and Industrial-MEG are also presented in the Supporting Information (Table S6). Mercury memory effects are common with analytical instruments. In the case of ICP-MS, long after a sample containing mercury has been nebulized into a conventional spray chamber, a significant residual mercury signal can be detected, indicating its failure to return to baseline, and can lead to cross-contamination with the next sample. In order to avoid this memory effect, three blank solutions (i.e., Milli-Q water containing 5% HNO3) were analyzed following a mercurycontaining sample. The data was only analyzed if the blank solution prior to the mercury-containing sample had returned a mercury reading of 0 ± 0.02 ppb. In addition, it has been reported that nonspectroscopic interferences are caused by the presence of matrix (main) elements and/or the concentration of nitric acid and other mineral acids, which may cause mercury signal suppression as well as enhancement.16,17 The mechanism of both processes is not well understood, but the occurrence of this detrimental effect can be detected by using spiked samples.9 In order to determine if such matrix effects occurred with the analytical method employed in this study, a set of experiments were carried out where the mercury content in the Industrial-MEG solution was determined using standard addition (SA) method as shown in the Supporting Information (Tables S4 and S5). It was found that spiking of the samples was not necessary given that the Industrial-MEG was diluted 100 times. In order to ensure that chemical matrix effects did not occur during sample analysis, the ICP-MS instrument mercury calibration standards were prepared with a closely matched chemical matrix (i.e., MEG, KMnO4, and acid contents) and analysis of the experimental samples was only carried out when a coefficient of determination (R2) for the calibration curve of ≥0.9990 was achieved. 5351

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Industrial & Engineering Chemistry Research Table 1. Amount and Type of Mercury Partitioned during Lab-MEG Distillationa trap no. 1 2 3b 4 5 6 bottoms flask

chemical

trapped species

expt 1 [Hg] (μg)

expt 2 [Hg] (μg)

expt 3 [Hg] (μg)

0.1 M KCl 0.1 M KCl H2O2/H2SO4 KMnO4 KMnO4 KMnO4 MEG bottoms acid wash

Hg2+ Hg2+ Hg0 Hg0 Hg0 Hg0 Hg2+ Hg(CL)

0.05 0 1.62 5.52 0.6 0.5 83.8 1.4 93.49

0.1 0 2.82 10.8 1.5 0.8 93 1.72 110.74

0.05 0 10.4 3.6 0 0 94.4 0.87 109.32

total (out of 100 μg) a

The starting mercury content was 100 μg (500 ppb in 200 mL). The mass reported is within a precision of ±0.02 μg. bStops transfer of organics.

Table 2. Amount and Type of Hg Partitioned during Salted-MEG Distillation Processa trap no. 1 2 3b 4 5 6 bottoms flask

chemical

trapped species

expt 1 [Hg] (μg)

expt 2 [Hg] (μg)

expt 3 [Hg] (μg)

0.1 M KCl 0.1 M KCl H2O2/H2SO4 KMnO4 KMnO4 KMnO4 bottoms MEG acid wash

Hg2+ Hg2+ Hg0 Hg0 Hg0 Hg0 Hg2+ Hg(CL)

0.35 2.5 44.4 49.2 0.6 0.6 0.13 0 97.78

0.36 0.01 54.5 9.27 0.23 0.05 25.5 0.82 90.74

1.41 0.82 41.83 19.3 0.81 0.23 27.5 5.2 97.09

total (out of 100 μg) a

The starting mercury content was 100 μg (500 ppb in 200 mL). The mass reported is within a precision of ±0.02 μg. bStops transfer of organics.

3. RESULTS AND DISCUSSION 3.1. Laboratory MEG (Lab-MEG). To establish a control experiment, several stock solutions (each with a total volume of 200 mL) were prepared containing 50% laboratory grade MEG with Milli-Q water. Each solution was spiked with 100 μg of mercury in the form of HgCl2 from a validated solution having pH ∼7. This step ensured the solutions contained a mercury concentration of 500 ppb, with the low concentration being similar to industrial conditions. Three identical solutions prepared were then regenerated, each at 170 °C, followed by the analysis of the rinse solutions from the overhead CL and the round-bottom flask, as well as each of the speciation traps by ICP-MS. The regenerated MEG following the distillation experiments was also analyzed using ICP-MS. The mercury remaining in the MEG solution is reported as Hg2+ since Hg0 is not expected to be present in MEG that has been heated to 170 °C. The results obtained for the Lab-MEG distillation experiments are shown in Table 1. It is observed that on average ∼90% of the spiked Hg (as HgCl2) remains in the MEG solution while ∼10% partitions out as Hg0. It is also found that most of the mercury retained in the Lab-MEG solution was homogeneously distributed throughout the solution. That is, less than 2 μg of Hg was found when an acid wash of the 500 mL flask was performed. Given the low quantity of mercury measured in the acid wash, it may have come from the leftover residue of the Lab-MEG after it was poured out and is expected to be Hg2+ in the Lab-MEG residue. 3.2. Salted Laboratory MEG (Salted-MEG). To determine if the salts and pH level of the MEG had any effect during the distillation process, three stock solutions (each with a volume of 200 mL) were prepared containing Salted-MEG with 500 ppb Hg (as HgCl2) to determine if the salt content and pH of the solution affected the behavior of the Hg. Three identical solutions were then regenerated, each at 170 °C, followed by

the analysis of the rinse solutions from the overhead CL and the round-bottom flask, as well as each of the speciation traps by ICP-MS. The results obtained for the Salted-MEG distillation experiments are shown in Table 2. It can be observed from Table 2 that the majority of HgCl2 spiked in the Salted-MEG is reduced to Hg0 during the distillation process. On average, ∼80% of the mercury is reduced and partitioned as Hg0, while ∼2% is partitioned as Hg2+ with the balance staying dissolved in the Salted-MEG solution. This indicates that the presence of salts (shown in Supporting Information, Table S2) in the MEG solution may be promoting the reduction of mercury from Hg2+ into Hg0. It can also be seen that no mercury is found in the remaining MEG in experiment 1 shown in Table 2. However, ∼26 and ∼28 μg of mercury are captured in experiments 2 and 3 in the MEG. This suggests that, due to the relatively high pH of the MEG solution, the mercury content present is either reduced to Hg0 vapor or remains in the solid particles at the bottom of the solution. When comparing the results from the with Lab-MEG (Table 1) with the Salted-MEG (Table 2), it can be observed that the salts in the MEG promote the release of mercury from the solution in the form of Hg0 vapor as evidenced by the increase in mercury levels measured in impinger traps 3 and 4. 3.3. Industrial-MEG. Recirculated MEG which was obtained from a natural gas processing plant was obtained and used as is. Three separate stock solutions (each with a volume of 200 mL) were prepared and spiked with 500 ppb Hg (as HgCl2). The three separate solutions were regenerated under identical conditions at 170 °C. ICP-MS analysis of the rinse solutions from the overhead CL and the round-bottom flask, as well as both the bottoms and each of the traps in the overheads (each of the speciation traps), was performed. Given the results obtained with the control experiments (Lab-MEG and Salted-MEG), the mercury remaining in the regenerated MEG solution at the end of the distillation experiments was 5352

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Industrial & Engineering Chemistry Research designated as Hg2+. This assumption is also in line with work which indicates that Hg0 has similar solubility/stability in MEG to that in water and is therefore not expected to be present in MEG, especially when the solution has been heated to 170 °C under agitation. The mercury measured from the rinse solution used to wash the 500 mL flask after pouring out the MEG solution is unknown and reported as total bound Hg partitioned to tar (Hg(B)). The results obtained following the distillation of the Industrial-MEG in the 500 mL flask are shown in Table 3. Table 3. Amount and Type of Mercury Partitioned during Industrial-MEG Distillation Processa trap no. 1 2 3b 4 5 6 bottoms flask

total (out of 100 μg)

chemical 0.1 M KCl 0.1 M KCl H2O2/ H2SO4 KMnO4 KMnO4 KMnO4 MEG bottoms acid wash (tar)

expt 1 (μg)

expt 2 (μg)

Hg2+

0.4

0.1

0.2

Hg2+

0.6

0.1

0

Hg0

n/a

3.6

24.4

Hg0 Hg0 Hg0 Hg2+

29 12 2 35

32.5 5.8 1.6 16.9

37 1.5 0.4 10.4

Hg(B) partitioned to tar

17.3

31.2

27

96.3

91.8

100.9

trapped species

expt 3 (μg)

Figure 2. Mercury mass balance of the MEG regeneration process. The numbers are based on the averages obtained for the experimental data presented in Tables 1−3.

content and presence of organic compounds in MEG play an important part in determining mercury speciation behavior and their fate during MEG distillation processes. Hg2+ (in the form of HgCl2) is unstable in the MEG sample due to the presence of salts and organic species. Approximately 50% of the added Hg2+ was observed to leave as Hg0 during the Industrial-MEG regeneration process, whereas the remaining was retained in the regenerated MEG solution. The fact that 80% of Hg2+ partitioned out of the Salted-MEG as Hg0 shows that the organic content in the Industrial-MEG slightly increases the ionic mercury remaining in solution; however, a significantly larger quantity is precipitated into the tar residue as bound mercury (Hg(B)). In order to determine the variance in mercury recovery (mass balance) obtained for the three experiments performed on each MEG type, the coefficients of the variance (CoVs) for the data in Tables 1−3 was calculated. It was found that the CoVs for Lab-MEG, Salted-MEG, and Industrial-MEG were 9.2, 4.1, and 4.7%, respectively. The relatively higher CoV of the Lab-MEG mass balance is attributed to the mercury partitioning behavior in this solution. That is, most of the mercury content is observed to remain in the regenerated MEG (MEG bottoms) which is the solution that introduces relatively larger errors (compared to other parts of the distillation apparatus) when analyzed for its mercury content using ICPMS, as a result of the high organic content (MEG) of the solution (see Supporting Information, Table S6). On the other hand, the Salted-MEG and Industrial-MEG experiments showed a better recovery of mercury as most of the mercury ended up in impinger trap 4, the bottoms, and the acid wash tar. These parts of the apparatus had relatively low organic content, so a low variability (within ±5% of ICP-MS specifications) was observed in Tables 2 and 3. The results confirm that the analytical method developed and used in this study is well-suited for determining the amount of mercury that is partitioned in and out of Industrial-MEG.

The starting mercury content was 100 μg (500 ppb in 200 mL). The mass reported is within a precision of ±0.02 μg. bStops transfer of organics. a

It can be observed that ∼50% of the HgCl2 originally spiked in the Industrial-MEG is reduced to Hg0 during the distillation process, while the remaining is retained in the Industrial-MEG solution. A significantly reduced amount of Hg2+, when compared to the results from the Lab-MEG and Salted-MEG tests, is observed in the speciation traps. These findings indicate that the organics and other impurities present in the IndustrialMEG play a major role in retaining the Hg2+ in the bottoms when compared to the results obtained for the Lab-MEG and/ or Salted-MEG solutions. Furthermore, the variability in the mercury content of the regenerated MEG among the three experiments conducted also suggests that the mercury in the Industrial-MEG became distributed in a nonhomogeneous fashion in the solution. This is similar to the results presented for Salted-MEG in section 3.2 where the discrepancy was attributed to the relatively high pH of the Industrial-MEG solution and in this case also the presence of insoluble particles (tar) in the MEG solution during distillation experiments. Furthermore, a significant portion of the mercury that remained in the 500 mL flask was recovered in the acid wash of the flask following the MEG regeneration experiment, thus indicating that Hg2+ “drop out” had occurred, forming insoluble mercury particulates on the flask walls. This is evidenced by the high Hg level measurements in the acid wash solution used to dissolve the tar residue deposited on the walls of the 500 mL glass flask. 3.4. MEG Distillation Mass Balance Summary. A summary of the overall approximate mass balances observed from the experimental data shown in Tables 1−3 is presented in Figure 2. It may be observed from Figure 2 that the salt 5353

DOI: 10.1021/acs.iecr.5b00492 Ind. Eng. Chem. Res. 2015, 54, 5349−5355

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4. CONCLUSIONS The distillation of 200 mL solution containing either laboratory MEG, salted laboratory MEG, or industry provided MEG spiked with 500 ppb Hg (as HgCl2) was performed in triplicate at 170 °C and atmospheric pressure. It was found that Hg2+ (as HgCl2) is unstable in the MEG sample due to the presence of salts and organic species. Approximately 50% of the added Hg2+ was observed to leave as Hg0 during the MEG distillation process, whereas the remaining was retained in the regenerated MEG solution. Furthermore, it was established that pH, salt content, and the presence of organic compounds in MEG can play a significant role in influencing the fate of Hg0 and Hg2+ species during the MEG regeneration process. These initial results with the specified experimental conditions (i.e., atmospheric pressure) show that mercury partition during MEG regeneration is a highly complicated process. It should be noted that the results presented in this study are specific to the experimental conditions outlined within each section, and that the interpretation of these results should be considered with regard to the operating temperatures, flow rate, pressure, and mercury concentrations at which the experiments were conducted. However, these types of studies bridge the knowledge gap in how mercury contamination of recirculating MEG behaves during natural gas recovery processes.

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ASSOCIATED CONTENT

S Supporting Information *

MEG types and their chemical contents, role of each impinger trap in the experiments, analytical method validation, and ICPMS uncertainty calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00492.

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Industrial-MEG = monoethylene glycol provided by petroleum industry Lab-MEG = laboratory reagent monoethylene glycol LPG = liquefied petroleum gas Hg = mercury MEG = monoethylene glycol Hg(CL) = total mercury in glassware Hg(B) = total bound mercury in tar residue TOC = total organic carbon sccm or mL/min = standard cubic centimeters per minute VOCs = volatile organic compounds XPS = X-ray photoelectron spectroscopy

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +61 3 9925 2673. *E-mail: [email protected]. Tel.: +61 3 9925 3365. Author Contributions

The paper was written through contributions of all authors. S.K.B. supervised the research. Y.M.S., J.T., and S.J.I. have conducted the experiments. S.B.A.H. has analyzed some of the mercury speciation data. All authors analyzed the data and reviewed the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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.

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NOMENCLATURE weak MEG = 50 wt % MEG solution strong MEG = 90 wt % MEG solution Hg0 = elemental mercury ICP-MS = inductively coupled plasma mass spectroscopy 5354

DOI: 10.1021/acs.iecr.5b00492 Ind. Eng. Chem. Res. 2015, 54, 5349−5355

Article

Industrial & Engineering Chemistry Research Amniotic Fluid by Inductively Coupled Plasma Mass Spectrometry. J. Toxins 2014, 1, 1.

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DOI: 10.1021/acs.iecr.5b00492 Ind. Eng. Chem. Res. 2015, 54, 5349−5355