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Comparison of preparation methods for the determination of metals in petroleum fractions (1000°F +) by Microwave Plasma-Atomic Emission Spectroscopy. Laura Poirier, Jenny Nelson, Greg Gilleland, Steve Wall, Lidia Berhane, and Francisco A Lopez-Linares Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00654 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017
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Comparison of preparation methods for the determination of metals in petroleum fractions (1000°F +) by Microwave Plasma-Atomic Emission Spectroscopy. Laura Poirier1, Jenny Nelson*2, Greg Gilleland2, Steve Wall2, Lidia Berhane1 and Francisco Lopez-Linares*1 1 2
Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94801, USA Agilent Technologies, Inc., 5301 Stevens Creek Blvd, Santa Clara, California 95051, USA
ABSTRACT Microwave induced plasma atomic emission spectroscopy (MP-AES) with nitrogen gas was employed for the determination of trace elements in petroleum fraction with a boiling point above 1000°F, using direct dilution in an organic solvent. Nitrogen produces robust plasma for the analysis of fractions having API range from -2° up to 9.7° and elemental composition as follows: S (1.0 at. % up to 5.5 wt. %) and N (200 mg kg-1 up to 14000 mg kg-1). Satisfactory limit of quantification and spike recoveries at low and high concentration levels were determined for Na, K, Ca, V, Fe, Ni and Mo present in different residua samples. The recoveries obtained from the analysis of three QC test materials were within + - 10% of the actual and or certified values. It was found that V, Fe and Ni are the most predominant elements present in these samples and the effect of the source of sustained plasma gas as well as sample preparation method was focused on the quantification of these three elements. Using direct dilution method, V calculated recoveries using NIST 8505 CRM, (104 % and 109%), are found independently of the plasma sustained gas source (argon vs. nitrogen). Comparing sample preparation methods (wet ashing vs. direct dilution), recoveries of 99% and 109% were determined. Furthermore for Ni, regardless that NIST 8505 CRM reference value is not available, the calculations using our measured value of 51 mg kg-1, indicates that that nickel
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quantification is not sensitive to either sample preparation or detection methods, giving recovery of 100 % in all cases. For Fe, the quantification is susceptible to a sample preparation method; the presence of microparticles which is possible to have it in such fractions cannot be homogeneously dispersed into the organic media, leading to bias low in most cases. It seems better results are obtained using wet acid digestion. Finally, it was shown that using a nitrogen-based plasma (MP-AES) versus the argonbased plasma (ICP-OES), excellent agreement in all cases for vanadium and nickel was determined, indicating that MP-AES represents an alternative for analysis of these sample types of complex samples.
1. INTRODUCTION
The world energy consumption has been increasing, and fossil fuels and its derivatives provide the most available source to supply the high energy demands.1 Recently, it has been recognized that heavy oil and bitumen reserves are the most available energy source, and their uses are expected to increase in the future.2 Typically, heavy hydrocarbons such as bitumen and heavy oil comprise more than 50% (w/w) of a distillation residue (1000 °F+),2,3 which become the feedstock for further upgrading processes that lead to higher yields of valuable distillates.1 Therefore, these feeds need to be fully characterized in term of bulk properties such as boiling point distribution, American Petroleum Institute (API) gravity, viscosity, and metal content. This information allows refineries to make daily process decisions in a safe and timely manner. In particular for vacuum residues, density and viscosity are the dominant measurements. Nevertheless, the ability to measure carbon, hydrogen, nitrogen, sulfur and metals are becoming critical.2
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Metal analysis of these fractions has been done using different strategies; commonly, they are separated from the hydrocarbon matrix by converting them into the aqueous based sample by acid assisted digestion. Typically, nitric and hydrochloric acid combined with hydrogen peroxide are employed to solubilize metals prior analysis by Flame Atomic Absorption Spectroscopy (FAAS) or by plasma techniques such as Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).4-7 Conventional ashing and refluxing methods have been widely used for many years in the analytical laboratories for petroleum industry as common practices8 as well as the use of closed microwave (MW) oven systems has been introduced to reduce the long timing sample preparation step for complex samples and nowadays to become a regular method in such facilities.6-10 By the incorporation of a high-pressure microwave system, many innovative and rapid analytical methods were developed and applied particularly to heavy feedstocks.7,9-11 The high operating temperature and pressure attainable in microwave digestion systems ensure complete decomposition of complex petroleum crude oils and petrochemical products, with no residual organic content. Some industry standard methods for the determination of metals such as iron, nickel, vanadium, and sodium in crude oils and residual fuels are available. ASTM D5708-12, ASTM D5863-13, and ASTM D7876-14 use wet acid digestion, direct dilution in an organic solvent, as well as microwave assisted acid digestion as the chosen sample preparation method and FAAS, ICP-OES, and Atomic Absorption Spectrometry (AAS) as the detection techniques. In our group, we routinely analyze by direct dilution in organic solvents (o-xylene and Premisolv), most of the crude oils samples and derivatives; and for non-distillable fractions, conventional wet acid, ashing or microwave assisted acid digestions are our top preparation
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methods, having ICP-OES as the primary detection technique. Very recently, we reported the results gathered by direct introduction of crude oil samples (from 9.5° up to 51°) diluted in oxylene, analyzed by ICP-OES and ICP-MS. Advantages of this methodology were presented therein as well as the common strategies used to overcome potential challenges running this type of samples. Loss of sensitivity due to sample dilution, different spectral and matrix interferences from the sample and solvent can be minimized by proper instrument tuning and carbon deposits reduction on the torch and detector/cone interfaces by introducing oxygen as an auxiliary gas in combination with argon15. The data obtained from both instruments were comparable in term of precision, relative standard deviation and recoveries for Ni, V, Fe, Ca. Additionally, we reported the application of Microwave Plasma-Atomic Emission Spectroscopy, MP-AES, as an alternative detection technique for the elemental analysis of crude oils, using direct dilution in an organic solvent as a preparation method16. Samples evaluated covered API’s from 7−38° among other properties such as H/C ratio, N and S content, and it was shown that the plasma sustained by nitrogen is comparable to a standard argon based system and it was capable of handling this type of crude oils. However, petroleum fractions, 1000 °F+ like vacuum residua, the possibility of using this sample introduction method into plasma instruments to determine vanadium, nickel and sulfur has been investigated for many years and some problem associated with solubility of high wax content that could be present in this fraction leading to inaccurate results.17 In this work, the determination of iron, nickel, and vanadium in crude oil fractions, typically with distillation temperature above 1000 °F, was evaluated by using MP-AES and comparing the results with those obtained with traditional detection techniques such as ICP-OES. Taking into consideration that applications of this technology in the analysis of heavy petroleum feedstocks
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have not been explored so far, we decided to investigate if the nitrogen plasma in a combination of direct dilution in organic solvents could perform as efficient to the analog ICP-OES method. Since a lower plasma temperature is produced using nitrogen, it is important to determine if direct dilution method is suitable for the type of sample selected for this work, characterized by high asphaltenes, S, N, V, and Ni contents as well as broad molecular weight distribution which could lead towards different solubility behavior depending the organic solvent used. For that reason, two sample preparation methods such as wet acid digestion and direct dilution in an organic solvent were evaluated using nitrogen and argon plasma sources.
2. EXPERIMENTAL 2.1. Reagents. Trace metal grade purity chemicals were used without further purification. 2.1.1 Direct Dilution. Conostan S21+K 885 mg kg-1 organosoluble standard (Quebec, Canada), dispersant (Chevron Oronite, Richmond, CA, USA), mineral oil (Fisher Scientific, Fair Lawn, NJ, USA), NIST 1634c trace elements in fuel oil Certified Reference Material, CRM (Gaithersburg, MD, USA; Certificate of Analysis (C of A), December 31st, 2020). NIST 8505 Vanadium in crude oil (Gaithersburg, MD, USA), o-xylene (Fisher Scientific, Fair Lawn, NJ, USA), and Scandium (2000 mg kg-1) Conostan (Quebec, Canada) 2.1.2 Wet Ash Digestion. SPEX multi-element standards (Metuchen, NJ USA), SPEX Scandium 10,000 mg kg-1 (Metuchen, NJ USA); Sulfuric Acid, TraceMetal Grade, 93 to 98%, H2SO4, w/w (Fisher Scientific, Fair Lawn, NJ, USA); Nitric Acid, TraceMetal Grade, 67 to 70%, HNO3, w/w (Fisher Scientific, Fair Lawn, NJ, USA); Hydrochloric acid, TraceMetal Grade, 34 to 37 %, HCl w/w
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(Fisher Scientific, Fair Lawn, NJ, USA); and water 18.2 MΩ.cm resistivity (MilliQ water, Millipore, Billerica, MA, USA). 2.2. Instrumentation. Direct injection of o-xylene solutions used an Agilent 4200 MP-AES (Agilent Technologies, Santa Clara, CA, USA), with an Agilent 4107 Nitrogen Generator (Agilent Technologies, Santa Clara, CA, USA), and a Thermo Radial ICAP 6000 Series ICPOES (Thermo Fisher Scientific, Fremont, CA, USA). On the MP-AES, a MicroMist nebulizer and glass double-pass cyclonic spray chamber were used for the sample introduction system. Also, an external module accessory for gas control (EGCM) was used to inject a controlled low flow of air into the plasma to prevent carbon deposits from building up in the torch and reducing background emissions. Thermo Radial ICAP 6000 Series ICP-OES, equipped with ESI SC4DX autosampler (Omaha, NE, USA) with a Teflon carbon fiber 0.8 mm sipper probe, the conventional ICAP 6000 peristaltic pump, solvent-resistant Viton tubing and a Noordermeer V-Groove nebulizer. The samples were introduced in o-xylene solutions through peristaltic pump tubing with a diameter of 0.64 mm and into the nebulizer by using different pump speeds for rinsing and analyzing the samples. IsoMist spray chamber from Glass Expansion (Pocasset, MA, USA), was employed to cool our quartz twister spray chamber to 5 °C, and a Thermo radial D-torch with an outer ceramic tube and a quartz 1.0 mm injector. Digestates from the wet ash process were run using a Thermo Radial ICAP 6000 Series ICP-OES (Thermo Fisher Scientific, Fremont, CA, USA) which was equipped with a Burgener T2100 Nebulizer (Burgener Research Inc, Mississauga, Ontario, Canada).
Twister spray
chamber (Glass Expansion, Pocasset, MA, USA), Thermo radial-torch with a quartz outer tube and a quartz 1.5 mm injector. See Table 1 for exact instruments parameters. For ICP-OES, the
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samples can be run with or without oxygen addition. For the present work, oxygen mixed up to 10% prevents any carbon deposit formation found after extended periods of analysis.18-19 Negligible carbon formation on our cones and torches were observed after 24 h of continuing operation. The wavelengths selected for the analytes examined in this work had the highest signal intensity and the lowest spectral interferences obtained while performing the analysis on these samples evaluated in this work.15 Thermo Scientific Type 6000 furnace (8 segment programmable, Thermo Fisher Scientific, Fremont, CA, USA) was employed for wet ashing procedure. Temperature program used is as follows: 25°C to 163°C/1 °C/min/120 min // 282 °C/1 °C/min /600 min // 610 °C/1 °C/min/720 min //50 °C/10 °C/min/10 min. 2.3 Calibration standards preparation. The o-xylene diluted calibration standards for the MP-AES and ICP-OES were prepared from Conostan organosoluble standards diluted to concentrations as discussed below. For the MP-AES and ICP-OES, a diluent was prepared to add a given quantity of matrix modifier (mineral oil and dispersant), and Sc (2000 mg kg-1) spiked at 5 mg kg-1 as an internal standard dissolved in a certain amount of o-xylene. This diluent solution was used to make the standards and dilute the samples. A dilution by weight of Conostan S21+K 885 mg kg-1 standard with the above diluent was used to create calibration standards at 1, 2, 5 and 10 mg kg-1. The diluent was also used as a blank for the calibration. Continuing calibration verification standards (CCV’s) were used on all instruments to determine if the sample analysis is within our control limits. For MP-AES and ICP-OES, the CCV standard was diluted from 885 mg kg-1 S-21+K Conostan standard, (from a different lot number than used for the calibration standards, on a weight basis) of the diluent. The 885 mg kg1
S-21+K standard was diluted to approximately 5 mg kg-1 (1:177), then, the CCV would be at
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the midpoint of the calibration curve. The aqueous calibration standards for the ICP-OES were prepared from SPEX multi-element standards. Standards were made by volume and spiked with 2.5% v/v nitric acid, 2.5% v/v hydrochloric acid and 5 mg kg-1 Sc as an internal standard, and diluted to volume with MilliQ water. Calibration standards were created at 1, 20, 50 and 100 mg kg-1. The blank for the calibration was made by spiking the MilliQ water with 2.5% v/v nitric acid, 2.5% v/v hydrochloric acid, and 5 mg kg-1 Sc.
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Table 1. MP-AES and ICP-OES Experimental conditions Instrument Conditions
4200 MP-AES
EGCM setting Pump rate Tubing Read time Number of replicates Sample uptake delay Stabilization delay Background correction Sample flow rate Rinse flow rate Peri-pump speed analysis Peri-pump speed rinse Forward power Plasma gas flow rate Auxiliary gas flow rate Carrier gas flow rate Optional O2 gas flow rate Sampling depth/radial
Low 5 Organic 3s 3 55 s 10 s Auto 238 µL/min 4.5 mL/min 5 rpm 80 rpm
6000 ICP-OES Organic Aqueous
40 rpm 85 rpm 1350 W 16 L/min 2 L/min 0.42 L/min 0.04 L/min 12 mm
Peri-pump speed rinse
40 rpm
Peri-pump speed analysis
50 rpm
Spray chamber temp Elements wavelength (nm)
5 °C Fe 238.204 V 292.460 Mo 202.030 Mo 589.59 Ni 221.647 Ca 184.00 Na 589.59 K 766.49 Sc 227.318 Sc 255.237
Internal standard
Fe 259.940 V 311.070 Mo 313.259 Mo 317.034 Ni 341.476 Ca 396.847 Na 588.995 K 769.897 Sc 335.372
13 rpm 13 rpm 1150 W 12 L/min 0.5 L/min 0.62 L/min 0 L/min 10-12 mm
Not monitored Fe 238.204 V 309.310 Mo 202.031 Mo 589.59 Ni 231.600 Ca 315.089 Na 589.60 K 766.49 Sc 227.318 Sc 255.237
2.4. Samples. API, as well as elemental composition, were used to select ten residue samples; API values span from -2° up to 9.7°, and elemental composition as follows: S (1.0 wt. % to 5.5 wt. %) and N (3000 mg kg-1 to 14000 mg kg-1). For direct dilution, these samples were diluted from 1:20 up to 1:200 based on the estimated metal concentrations using the o-xylene diluent (described in
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section 2.3), following the same methodology previously reported for crude oil samples.15 The dilutions of the residue samples were chosen to fit within the concentration range that would be covered by the calibration range. All samples were shaken overnight in a mechanical shaker to guarantee the residue samples were completely dissolved. To ensure this, the samples were visually inspected to make sure no material stayed over the walls of the vials. If any residue was found, a vortex would be used to promote a better homogenization of the sample. The dilutions were prepared less than 24 hours before running. For wet acid digestion, a weighed portion of a heated and mixed sample (1 to 5 g) is transferred into a quartz ashing vessel. H2SO4 (93-98 % w/w, 1-2 ml) is added, and the sample can react on a hot plate at 100 °C for four h. After the reaction, the quartz ashing vessel is placed into a programmable ashing furnace, and the ashing sequence is started until reaching a max temp of 610 °C, using a temperature program described in section 2.2. Once it is completed, HCl (34 to 37 % w/w, 6 ml), HNO3 (67 to 70 % w/w, 2 ml) are added to the quartz ashing vessel, and the acid dissolution is placed on a hot plate 100 °C for 1h. An internal standard (Sc, 5 mg kg-1) is added to the acid solution and brought to volume final volume (25 ml) with MilliQ water, before analysis on an ICP-OES.
3. RESULTS AND DISCUSSION 3.1. MP-AES instrument performance: method detection limit (MDL) and limit of quantification (LOQ). As a first step, we decided to determine the method detection limits, MDL (3σ), and limit of quantification, LOQ, per previous reports.15,16 A blank solution (diluent containing a matrix modifier and internal standard) was measured ten times and the results are shown in Table 2.
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Table 2, list the elements of interest in residue samples such as Ca, Fe, K, Mo, Na, Ni and V. Because they can be present in such fractions as metalloporphyrins (V, Ni), transition metals complexes with tetradentate mixed ligands (V, Ni, Fe), carboxylic salts of the polar functional groups (Mo, Na, K, Ca) as colloidal minerals such as silica and NaCl20 or introduced as additive during crude oil production. As shown in Table 2, using MP-AES, low LOQ values can be achieved for this matrix using direct dilution method. The internal standard ratio was also monitored (data not shown) to demonstrate that the instrument drift during this measurement does not exceed 1% and how the signal of each analyte is compensated along the run.
Comparing these results with those
obtained previously by ICP-OES, ran by direct dilution methodology employed in this work,15 equivalent values are achieved in most of the elements. The results presented in Table 2 also indicates that effectively, the vacuum residua samples can be introduced into the plasma via direct dilution by using a diluent containing o-xylene in a combination of mineral oil and a dispersant. This approach can be extended toward others organic solvents such as kerosene as well as Premisolv. The efficiency of the nitrogen plasma generated by MP-AES allows for proper MDL and LOQ values for residue analysis by direct dilution with a solvent such as o-xylene.
Table 2. Method detection limits and the limit of quantification (µg kg-1) with a blank solution, using MP-AES. Blank solution (MP-AES) MDL LOQ
Na K Ca V Fe Ni Mo 588.995 nm 769.897 nm 396.847 nm 311.070 nm 259.940 nm 341.476 nm 313.259 nm 3.28 110.05 1.61 5.23 9.40 2.93 2.08 10.95 366.85 5.37 17.44 93.97 9.78 6.94
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3.2. QC test. Three separate Quality Control (QC) recovery criteria were examined. First, a CCV sample at the midpoint concentration of the calibration (5 mg kg-1) was repeatedly analyzed, and the determined recovery was within ±10% of the actual value. Then a certified reference sample S21 + K 885 mg kg-1 was run. Finally, certified reference sample NIST 1634c Trace Elements in Fuel Oil was analyzed seven times for V (28.19 mg kg-1) and Ni (17.54 mg kg-1); only recoveries were within ±10%. The QC results presented in Table 3 indicates that the MP-AES can achieve a high level of recoveries (except K in S21+K was high), using all QC As it was demonstrated for crude oil analysis using MP-AES16, these results also
standards.
show that MP-AES can also be used as an alternative for residua analysis via direct dilution.
Table 3. QC average results of a CCV sample, S21+K standard sample and NIST 1634c Trace Elements in Fuel Oil CRM, determined by MP-AES
Solution label
Na
K
Ca
588.995 nm 769.897 nm 396.847 nm -1
V
Fe
Ni
Mo
311.070 nm 259.940 nm 341.476 nm 313.259 nm
5 mg kg CCV SD RSD (%) recovery (%)
4.98 0.08 1.61 99.60
5.15 0.15 2.91 103.00
4.96 0.11 2.22 99.20
5.09 0.19 3.73 101.80
5.11 0.24 4.70 102.20
5.07 0.11 2.17 101.40
5.12 0.16 3.13 102.40
885 mg kg-1 S-21 SD RSD (%) recovery (%)
939 9 1 106
1015 13 1 115
918 13 1 104
987 8 1 112
965 9 1 109
940 7 1 106
944 8 1 107
NIST 1643 c SD RSD (%) recovery (%)
28.90 0.21 0.73 102.52
17.70 0.13 0.73 100.91
3.3. Petroleum fractions > 1000 °F+ analysis. A variety of samples such as vacuum residue, heavy crude oil, deasphalted crude oil containing different levels of sulfur (1.0 at. % up to 5.5 wt. %) and nitrogen (200 mg kg-1 up to 14000 mg kg-1) were analyzed to determine if the feed
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properties could impact the plasma, causing a potential bias.15 Table 4 shows how the metal concentrations differ among samples. The samples were diluted with the o-xylene diluent, 1:10 when possible; otherwise, when the concentration of any element was >100 mg kg-1, the dilution factor was increased from 1:20 up to 1:200 depending on the highest elemental concentration in the sample. The measurement associated relative standard deviations did not exceed 8. For this set of samples, all could be analyzed on the MP-AES system. However, for samples S1, S2, S7, S8, S10 it was not possible to quantify Na because of the dilution factor used (1:50-1:200); Na injected concentration was too low to be quantified at the same time with the high-level of Ca, V, Fe, and Ni present in these samples. The previous is one example where one dilution could not fit all elements within the effective calibration range.
Table 4. Measured concentrations of metals present in residua (mg kg-1) using MP-AES. Solution label S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 NIST 8505
Na
K
Ca
V
Fe
Ni
Mo
588.995 nm 769.897 nm 396.847 nm 311.070 nm 259.940 nm 341.476 nm 313.259 nm
