Article pubs.acs.org/EF
Development of a High-Pressure Bubbling Sampler for Trace Element Quantification in Natural Gas Maxime Cachia,†,‡,§ Brice Bouyssière,† Hervé Carrier,‡ Hervé Garraud,∥ Guilhem Caumette,*,§ and Isabelle Le Hécho*,† †
CNRS/ UPPA, UMR 5254, Institut des Sciences Analytiques et de Physico-Chimie pour l'environnement et les Materiaux, Laboratoire de Chimie Analytique Bio-Inorganique et Environnement (LCABIE-IPREM), 64000 Pau, France ‡ CNRS/TOTAL/UPPA, Laboratoire des Fluides Complexes et leurs Reservoirs-IPRA, UMR 5150, 64000 Pau, France § Transport et Infrastructures Gaz France (TIGF), 40 Avenue de l’Europe, CS 20522, 64010 Pau Cedex, France ∥ Sobegi, Avenue du Lac, BP 58, 64150 Mourenx, France S Supporting Information *
ABSTRACT: A high-pressure bubbling sampler has been developed to trap and preconcentrate metals and metalloids from natural gas. This high-pressure sampler was designed to work at pressures up to 100 bar and be directly plugged into distribution and transportation networks. It consists of three vials in series, which contain 50 mL of metal trapping solution and the gas flows at the network pressure with a flow rate up to 40 L/min. The trapping solutions for mercury and other metals are permanganate/ sulfuric acid or nitric acid/hydrogen peroxide according to standards EN 13211 and EN 14385. The sampler design, development, and validation steps are presented in this work. First, the trapping vials were tested in the laboratory, where argon gas was spiked with mercury at two different pressures that represented the distribution and transportation networks. The results show that more than 96% of the metal was trapped from the gas phase into the solution for both tested pressures. Moreover, more than 90% of the trapped metal was found in the first vial, which shows the good efficiency of the traps. Finally, the highpressure bubbling sampler was tested in three field campaigns of natural gas sampling from a transportation network at 60 bar. Each sampling was performed for 5 days with a flow rate of 20 L/min, which makes a total volume of 140 Nm3 of sampled gas. The gas flowed through three vials of 50 mL, which makes a final preconcentration factor of 3 × 106. The trapping solutions were analyzed for trace metal concentrations using inductively coupled plasma mass spectrometry. The concentrations were 10−1 ng/ Nm3 for barium, from 10−1 to 5 ng/Nm3 for tin, from 0.9 to 10 ng/Nm3 for arsenic and copper, and from 1 to 101 ng/Nm3 for aluminum, selenium, and zinc. The efficiency of the traps and the low measured concentrations make this high-pressure bubbling sampler a useful tool for trace element analyses in natural gas.
1. INTRODUCTION Natural gas is the third highest fossil energy consumed in the world because of its abundant reserves and lower carbon dioxide emissions compared to other fossil fuels.1 It represents almost one-fourth of the fossil energy consumption in the world,2 and its consumption is expected to increase in future years.3 Natural gas contains mainly methane (90−95%mol), ethane, propane, and/or other hydrocarbons (7%mol), and inorganic gaseous compounds, such as carbon dioxide, hydrogen sulfur, dinitrogen, helium, and water, with a total volume percentage of 3−15%mol.4,5 Inorganic compounds may also be found at trace levels, such as mercury, arsenic, zinc, nickel, tin, copper, or vanadium.6−10 Knowing the trace metal composition of natural gas is crucial because of its potential effects on industrial infrastructures11 and the environment during underground storage or through its presence in the exhausts of gas power plants.12 Until now, there has been no regulation for the metal composition limits in natural gas, except for mercury, whose maximum allowed total concentration is 1 μg/m3. The few works on this subject only examined mercury and arsenic. Indeed, four different organic arsenic compounds have been © XXXX American Chemical Society
reported in the literature, namely, trimethylarsine, dimethylethylarsine, methyldiethylarsine, and triethylarsine, with concentrations of approximately 5−6 μg/m3.13 These arsenic compounds were also detected in gas from offshore rigs by Krupp and co-workers,7 with concentrations up to 16 μg/m3. Trimethylarsine (TMA) was identified as the main arsenic compound of natural gas. Mercury was also reported at concentration levels of 1−200 μg/m3 according to Ryzhov et al.,14 which reached 200−500 μg/m3 in specific geological formations.15 The most exhaustive analysis, which was reported by Duoyi and co-workers, contains 51 elements at trace level (alkali metals, alkaline earth metals, rare metals, transition metals, quasi-metals, and halogens) in a natural gas sample from China with concentrations from 0.001 to 30 μg/m3.9 These metals are present in trace concentrations, which require performant sampling and analysis techniques. To precisely determine the concentrations of these trace level metals, different sampling and analytical techniques have been developed. Krupp and co-workers used three different Received: November 17, 2016 Revised: February 3, 2017 Published: March 6, 2017 A
DOI: 10.1021/acs.energyfuels.6b03059 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Principle of the high-pressure bubbling sampler.
(AAS)29 or inductively coupled plasma mass spectrometry (ICP−MS).7 Overall, the metal trapping techniques only work at atmospheric pressure and often require an intermediary step before analysis. In the present work, a high-pressure bubbling sampler is developed to directly sample trace metal elements from natural gas in gas installations from the distribution or transportation networks. The sampler works at pressures of 1− 100 bar and was developed to sample in EX zone 0. Working in pressurized networks gives a faster preconcentration of metals in the trapping solution and saves time compared to the regular sampling procedures. The development of the sampler is presented, which includes the selection of components, tests in the laboratory with mercury for the trapping efficiency, and results of metal concentrations from three field campaigns from transportation networks, which represent the most difficult and innovating sampling conditions.
techniques to sample arsenic from gas: gas cylinder at 1000 psi pressure, adsorption tubes, and bubbling system under atmospheric pressure.7 In this case, the gas sample was depressurized before analysis. Xu et al. were inspired by the bubbling system of Krupp and co-workers and optimized a bubbling process, which included filling absorption vials with nitric acid or silver nitrate for arsenic trapping.16 The bubbling process consists of several absorption stages to increase arsenic trapping at atmospheric pressure. With this process, the absorption efficiency was 40%.16 Duoyi and co-workers used two other techniques at atmospheric pressure: air extracting collection and adsorption collection,9 which enabled the successful trapping and identification of 49 and 51 compounds, respectively. Because few investigations were conducted for natural gas sampling, we searched for other techniques in the literature for metal trapping in similar matrices, such as fumes, biogas, and air. Generally, two phases are considered for sampling techniques: gaseous phase and particular phase. The choice of sampling technique depends upon the sampled phase. The most common systems to sample biogas or synthetic gas are bags made from different matter, such as PETP/AL/PE,17,18 nalophan,19 or polypropylene.19 Despite the easy use,20 this sampling technique has disadvantages. First, it is not suitable for gaseous samples under pressure and, consequently, cannot preconcentrate gases with notably low analyte concentrations. Second, sampling gases in bags can present loss of compounds over time because of the adsorption on the bag walls or small leaks.21 Prior to analysis, the gas is filtered with glass fibers, nylon, quartz, polyvidilene fluoride, or polycarbonate filters.22−25 In other cases, sampling techniques were developed to trap and analyze a specific element with a specific adsorption gel or bubbling solution.26−28 Krupp and Xiu used silver nitrate or nitric acid for arsenic trapping,7,16 whereas mercury was trapped with a permanganate/sulfuric acid solution. The collected samples were usually analyzed with furnace absorbance atomic spectrometry (FAAS),22,23 atomic absorbance spectrometry
2. MATERIALS AND METHODS 2.1. Reagents. The trapping solutions were prepared in Millipore Milli-Q water (18.2 MΩ). The reagents were selected with analyticalgrade purity or above. The metal trapping solution was prepared with 70% nitric acid from Baker and 30% hydrogen peroxide from Scharlau. The mercury trapping solution was prepared with concentrated sulfuric acid at 97% from Merck and >99% potassium permanganate from Fluka. 2.2. High-Pressure Bubbling Sampler: Working Principle. The high-pressure bubbling sampler was designed to work at a pressure of 1−100 bar with flow rates up to 40 L/min. The system circulates natural gas through adequate solutions to trap and preconcentrate metals at trace concentrations. The schematic diagram and a photo of the high-pressure bubbling sampler are shown in Figures 1 and 2, respectively. The main part of the bubbling sampler is a group of three highpressure bubbling traps (HPBTs), which contain the trapping solution. The inside part of the HPBT is a 150 mL Teflon cylinder to make cleaning easy and avoid metal sorption. It is circled by a stainless cylinder to resist working pressures up to 100 bar. A manometer monitors the pressure during the sampling phase. Details of the HPBT are presented in Figure 3. B
DOI: 10.1021/acs.energyfuels.6b03059 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
At the sampling site, the high-pressure bubbling sampler is connected to the pipe via a Swagelock connection with a 7 m transfer line. The gas injected into the sampler crosses a condenser and a filter [polyvinylidene fluoride (PVDF, 0.10 μm, classic filter], which is put up in a support (inverted coalescing filter housing SV127) to remove water and particles. Then, the gas flows through three high-pressure bubbler traps (HR-100, Berghof), which are assembled in series and numbered HPBT1, HPBT2, and HPBT3 in the text. The traps are filled with 50 mL of trapping solution. The solutions are (i) 1%w permanganate + 5%w sulfuric acid to trap mercury according to the norm NF EN 1321130 and (ii) 10%w nitric acid + 5%w hydrogen peroxide to trap other metals according to the norm NF EN 14385.31 To maximize the exchanges between the trapping solution and gas phase, the gas enters the HPBT through a Teflon tube with a fully immersed sinter head, which is designed to generate small-sized bubbles. After the three HPBTs, a heater/pressure reducer depressurizes the gas at atmospheric pressure and a rotameter (Yokogawa RAMC) measures the sampled volume. At the exit of the sampler, the gas flow is connected to a venting pipe. All components of the bubbling sampler were certified for working in EX zone 0 according to the European classification of hazardous areas. 2.3. Methodology for Laboratory Tests. The efficiency of the HPBTs was investigated in the laboratory with pure argon (5.0, Linde), which was spiked with metallic mercury (Hg0) by crossing a permeation cell that contained a drop of mercury 0 (Figure 4). Thus, the mercury concentration in the gas depends upon the selected pressure, temperature, and flow rates. These parameters were maintained constant during the experiment to guarantee a stable mercury concentration in the gas. The mercury concentration in the gas was controlled by a gold trap before the trapping experiment. All measured concentrations each 45 min throughout the experiment were averaged to represent the mercury value in gas during the experiment ([Hg]av gas,in). Then, the spiked gas crossed the three HPBTs in series, which were filled with the specific solution to trap mercury. A flow meter (MassView MV-102, Bronkhorst) was placed at the end of the system to control the flow rate during the experiment, and the mercury concentration in the gas after the HPBTs was measured by mercury trapping on gold traps ([Hg]av gas,out). The experiment was run for 10 h to preconcentrate sufficient mercury in the trapping solutions for analysis. Two sets of experiments were performed at two selected pressures, namely, 6 and 50 bar, which represent the working conditions of
Figure 2. Photo of the high-pressure bubbling sampler.
Figure 3. Pictures of the (a) closed and (b) opened bubbling trap.
Figure 4. Experimental setup to test the efficiency of the HPBTs. C
DOI: 10.1021/acs.energyfuels.6b03059 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
where cHg is the mercury concentration in air (ng/mL), T is the absolute temperature (K), A and B are different constants, with A being −8.134 459 741 and B being 3240.871 534 K. The parameters of the Sir Galahad atomic fluorescence spectrometer are presented in Table 1. The limit of quantification of the CVAFS was 0.2 ng.
natural gas distribution and transportation networks. Before each experiment, a blank of argon was run through the HPBTs. The trapping solutions were analyzed at the end of each blank and each experiment ([Hg]sol). The efficiency of the HPBTs was demonstrated by measuring the mercury concentrations in the gas before and after trapping and in the trapping solution from each HPBT. The mass balance of mercury was calculated with eqs 1 and 2. Equation 1 provides the amount of mercury in the gas phase that enters the system (nHg,gas entrance), and eq 2 sums the amount of mercury trapped in the HPBTs and that remaining in the gas phase (nHg,sol + nHg,gas exit)
nHg,gas entrance = c Hg,gas entranceVgas
Table 1. Parameters of the Sir Galahad CVAFS for the Analysis of Mercury from the Gold Traps parameter Sir Galahad Trap time before heating heating time cooling time Amasil Trap time before heating heating time cooling time Measuring Mode integration of the peak baseline check filter factor
(1)
nHg,sol + nHg,gas exit = c Hg,sol HPBT1VHPBT1 + c Hg,sol HPBT2VHPBT2 + c Hg,sol HPBT3VHPBT3 + c Hg,gas exitVgas
(2)
where cHg,gas entrance is the mercury concentration in the gas at the entrance (μg/L), cHg,gas exit is the mercury concentration in the gas at the exit (μg/L), VHPBT1/2/3 is the volume of the bubbling solution in each bubbling trap (L), and cHg,sol HPBT1/2/3 is the mercury concentration in each solution of the bubbling trap (μg/L). The standard deviations of nHg,gas entrance and (nHg,sol + nHg,gas exit) were calculated using eqs 3 and 4, respectively.
std nHg,gas entrance = std c Hg,gasVgas + std Vgasc Hg,gas
(3)
= std c Hg,sol HPBT1VHPBT1 + c Hg,sol HPBT1std VHPBT1 + std c Hg,sol HPBT2VHPBT2 + c Hg,sol HPBT2 std VHPBT2 + std c Hg,sol HPBT3VHPBT3 + c Hg,sol HPBT3std VHPBT3 (4)
2.4. Methodology for On-Site Sampling. The high-pressure bubbling sampler was tested in three field campaigns to analyze the natural gas from the transportation network. Sampling was performed at the transport network pressure of 60 bar directly on the installations in an EX zone 0 area. The gas flow rate was 20 L/min, and the sampling lasted 1 day for mercury and 5 days for other metals to have a final sampled volume of approximately 40 and 140 Nm3, respectively. The gas flow rate and time sampling were selected to (i) find a compromise between field sampling feasibility, (ii) detect metals at concentration levels in natural gases, and (iii) have a suitable gas representation. Then, the trapping solutions were transported to the laboratory for ICP−MS analysis. 2.5. Analytical Procedure. 2.5.1. Mercury Analysis in Reference Gas. Mercury from the spiked argon gas was sampled in absorbance tubes and analyzed using a cold vapor atomic fluorescence spectrometer (CVAFS). The absorbance tubes were filled with a mixture of gold particles and sand (the Amasil trap tubes were purchased from PSAnalytical) and are referred to in this paper as gold traps. This sampling technique based on amalgamation between gold and mercury was used according to the norm ISO 6978-2:2003.32 The absorbance tubes were placed in front of and behind the HPBTs, as shown in Figure 4. After sampling, the absorbance tubes were heated and analyzed using the CVAFS (Sir Galahad of PSAnalytical) for mercury desorption and quantification following the normalized methods ISO 6978-2:200332 and ASTM D6350,33 respectively. A calibration curve was realized with gaseous mercury in contact with air in a vial. Its concentration in the vial was calculated using the PSAnalytical equation considering the temperature (eq 5)
c Hg = (3216522.61/T ) × 10−(A + [B / T ])
30 s 60 s 190 s peak area 1 1
2.5.2. Mercury and Metal Analysis in the Trapping Solutions. Mercury in the trapping solutions was analyzed by ICP−MS (Agilent 7500). The analytical parameters of ICP−MS are listed in the Supporting Information. A calibration curve was realized with concentrations of 0.1−10 μg/L. Rhodium (1 μg/L) was used as the internal standard and added to each solution. The limits of quantification were 0.6 μg/L. The metals in the trapping solutions were analyzed by ICP−MS (Agilent 7500). The analytical parameters of ICP−MS are listed in the Supporting Information. The calibration curve was 0.05−20 μg/L. The limits of quantification were 10−1 μg/L for aluminum and manganese, 10−2 μg/L for selenium, barium, zinc, copper, nickel, and chromium, and 10−3 μg/L for molybdenum, silver, cadmium, tin, lead, vanadium, and arsenic. Before each experiment, the trapping solutions were analyzed to ensure the absence of metal contamination. The detected concentrations were below the quantification limits.
std(nHg,sol + nHg,gas exit)
+ std c Hg,gas exitVgas + c Hg,gas exit std Vgas
30 s 20 s 110 s
3. RESULTS AND DISCUSSION 3.1. Efficiency Measurement of Bubbling Traps at the Laboratory Scale. The efficiency of the three bubbling traps in preconcentrating metals from a gas was tested at 6 and 50 bar with mercury-spiked argon gas. The results of both experiments are shown in Table 2. First, the mercury concentrations of the blank and mercuryspiked argon were 10−3 and 10−1 μg/L at the entrance and exit of the HPBTs, respectively, for both 6 and 50 bar. Hence, the amount of mercury in the blank was negligible compared to that in mercury-spiked argon. After the gas-spiked bubbling, [Hg]av gas,out was negligible for both pressures; at the exit of the HPBTs, the mercury concentration corresponded with the incompressible background level that is linked to the presence of ultratraces in the gold trap and/or the system. These concentrations were at least 10−3 μg/L, which is on the same order of magnitude as the blank. Moreover, the mercury concentration in gas was measured before ([Hg]av gas,in) and after ([Hg]av gas,out) the HPBTs by mercury trapping in the gold traps. Both gold traps were identically analyzed with CVAFS. The results are presented in Table 2 and show that the concentration ratio between
(5) D
DOI: 10.1021/acs.energyfuels.6b03059 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Table 2. Average Mercury Concentration in Argon before ([Hg]av gas,in) and after ([Hg]av gas,out) the HPBTs and in the Permanganate/Sulfuric Acid Solution ([Hg]sol) for Both Experiments at 6 and 50 bar experiment
blank (argon)
pressure (bar) [Hg]av gas,in (μg/L) [Hg]av gas,out (μg/L)
2a
trapping efficiency (%) HPBT1 HPBT2 [Hg]sol (μg/L) HPBT3
2b
argon spiked with mercury
6 0.00005 ± 0.00005 0.0005 ± 0.0001
50 0.00012 ± 0.00005 0.0006 ± 0.0002
6 0.047 ± 0.038 0.002 ± 0.001
50 0.019 ± 0.003 0.0008 ± 0.0005
1.20 ± 0.05
