Article pubs.acs.org/EF
Study of the Size Distribution of Sulfur, Vanadium, and Nickel Compounds in Four Crude Oils and Their Distillation Cuts by Gel Permeation Chromatography Inductively Coupled Plasma HighResolution Mass Spectrometry Alain Desprez,†,‡,§ Brice Bouyssiere,*,† Carine Arnaudguilhem,† Gabriel Krier,‡ Lionel Vernex-Loset,‡ and Pierre Giusti§ †
CNRS/UPPA, UMR 5254, LCABIE, Hélioparc, 2 Avenue de Président Angot, 64053 Pau, France UL, LCP-A2MC, 1 bd Arago, Technopôle Metz 2000, 57070 Metz, France § TOTAL Raffinage Chimie, TRTG, BP 27, 76700 Harfleur, France ‡
S Supporting Information *
ABSTRACT: The size distribution of sulfur, vanadium, and nickel was determined for four crude oils and their distillation cuts using gel permeation chromatography (GPC) coupled to inductively coupled plasma high-resolution mass spectrometry (ICP HR MS). The results show a trimodal distribution of vanadium and nickel compounds in the crude oils, the atmospheric residues, and the vacuum residues and, for sulfur compounds, either a mono- or bimodal distribution depending upon the distillation cut considered. A correlation exists between the sulfur fraction retention times and the temperature cuts of the distillation for a temperature below 560 °C and also between the viscosity of the crude oils and the proportion of trapped sulfur compounds in a higher boiling temperature fraction. The thermic treatment applied for the distillation increases the aggregation of low- and medium-molecular-weight compounds of vanadium and nickel into higher molecular weight aggregates between the crude oil on the one hand and the atmospheric residue and vacuum residue on the other hand, especially when the crude oil has a high total sulfur content.
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has been recently improved using ICP MS detection,16 and the use of ICP high-resolution (HR) MS enabled the simultaneous detection of metals and sulfur by the resolution of the isobaric interferences.17 In recent years, GPC ICP MS has emerged as an interesting technique for the study of the speciation of sulfur and metals in petroleum products and during treatment processes.15−19 This study is presenting the repartition and size distribution of sulfur, vanadium, and nickel using GPC ICP HR MS in four complete distillation series, allowing us to compare the mobility and changes in the aggregation of these elements between the different cuts and between four very different crude oils from various geographical regions. This work also introduces a relationship between the viscosity of the crude oils and their sulfur speciation that would require the study of more samples to be validated.
INTRODUCTION Sulfur and metals, such as vanadium and nickel, are known to be present in crude oils, with their distillation cuts in concentrations going up to 10% for sulfur and hundreds of parts per million (ppm) for metals.1,2 The study of the total concentrations and molecular size distributions of those three elements is of great interest because it is characteristic of the crude oil geochemical origin3,4 and it is also a crucial parameter when choosing the porosity of the catalysts used during the conversion of heavy fractions (atmospheric residue and vacuum residue) to transportation fuels.1,5−7 Spectrometric techniques, such as atomic absorption spectrometry (AAS), X-ray fluorescence (XRF), inductively coupled plasma (ICP) atomic emission spectrometry (AES), and more recently ICP mass spectrometry (MS), now allow for the routine determination of the total concentration of metals and sulfur.2,8,9 As for the sulfur and metal speciation, the use of chromatographic separation in combination with element-specific detection has been one of the main approaches. Gas chromatography (GC) remains the primary technique for the study of crude oil and lower boiling temperature fractions.10,11 However, because of its limitation when it comes to high boiling temperature compounds, gel permeation chromatography (GPC) has become widely used12−15 and, here, is used to study the complete distillation series of four crude oils. The detection after liquid chromatography of petroleum products is typically achieved using graphite furnace atomic absorption spectrometry (GFAAS), ICP AES, or ICP MS. The detection sensitivity © 2014 American Chemical Society
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MATERIALS AND METHODS
Instrumentation. A Thermo Scientific Element XR double focusing sector field inductively coupled plasma mass spectrometer (Thermo Fisher, Germany) was used at a resolution of 4000 (medium resolution) to access spectrally interfered isotopes of 32S, 51V, and 60 Ni.9 The mass spectrometer was equipped with a quartz injector (1.0 mm inner diameter), a Pt sampler cone (1.1 mm orifice diameter), and a Pt skimmer cone (0.8 mm orifice diameter). A flow of 0.08 mL/min Received: March 14, 2014 Revised: May 14, 2014 Published: May 20, 2014 3730
dx.doi.org/10.1021/ef500571f | Energy Fuels 2014, 28, 3730−3737
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Article
of O2 was continuously added to the nebulizer Ar gas flow. The mass calibration was optimized daily at resolutions of 300 and 4000.9 A tuning solution containing 1.0 ng g−1 of Ag, Al, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, In, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Sc, Si, Sn, Ti, V, Zn, and Y in tetrahydrofuran (THF) was used to optimize the instrument settings (Table 1). Mass offset values were implemented in the acquisition method to compensate for the mass drift related to the hysteresis of the magnet.
Table 1. HR ICP MS Experimental Conditions for GPC Coupling inductively coupled plasma RF power plasma gas flow rate auxiliary gas flow rate nebulizer gas flow rate O2 gas flow rate mobile phase flow rate (splitted from 1 L min−1) acquisition parameters medium resolution detection mode measured isotopes number of points per isotope mass and search window integration window sampling time total acquisition time scan type integration type
1500 W 16 L min−1 0.90 L min−1 0.60 L min−1 0.08 L min−1 38 μL min−1 4000 triple 32 S, 51V, and 60Ni 10 150% 60% 0.1 s 90 min EScan average
Figure 1. Description of the analyzed distillation cuts.
described by McKenna et al.24 and the dilution factor has an influence on the size distribution of those aggregates.16 However, on the basis of the work by Merdrignac et al.22 using the same low dilution factor for all of our samples, we place ourselves in a “stable association state”, so that we can compare the size distributions obtained with similar aggregation conditions. GPC and ICP MS Detection. The separations were carried out using three Waters (Waters Corporation, Milford, MA) styrene− divinylbenzene gel permeation columns (7.8 mm inner diameter × 300 mm length): HR4 (particle size, 5 μm; exclusion limit, 600 000 Da of polystyrene equivalent), HR2 (particle size, 5 μm; exclusion limit, 20 000 Da), and HR0.5 (particle size, 5 μm; exclusion limit, 1000 Da), connected in series. A Styragel guard column (4.6 mm inner diameter × 30 mm length) was used before the three mentioned columns to protect and increase their lifetime. A sample of 20 μL was injected and eluted isocratically at a flow rate of 1 mL min−1 of THF for 90 min. A post-column splitter is used to divide the flow between a low flow outlet of 40 μL/min to feed ICP MS and a high flow outlet of 960 μL/ min that is used to collect fractions, or they will go to waste. With the exclusion being based on the hydrodynamic volumes of the compounds, the system was calibrated using a mix of polystyrene (PS) standards ranging from 2630 kDa to 381 Da, which gives us the calibration polynomial expressing the relationship between the molecular weight (Mw in equivalent polystyrene) and the retention time (t in minutes).
The mass spectrometer was fitted with a modified DS-5 microflow total consumption nebulizer (CETAC, Omaha, NE) mounted with a laboratory-made jacketed spray chamber to allow for thermostatting at 60 °C by a water/glycol mixture using a Neslab RTE-111 (Thermo Fisher Scientific, Waltham, MA) temperature-controlled bath circulator. Both the nebulizer and heated spray chamber have been described elsewhere.20,21 The carrier solution in GPC was delivered using a Dionex high-performance liquid chromatography (HPLC) system constituted of an UltiMate 3000 microflow pump, a UltiMate 3000 autosampler, and a low port-to-port dead-volume microinjection valve. The total element analyses were performed using X-ray fluorescence for S, V, and Ni for concentrations between 1 and 300 ppm. An internal method using atomic absorption spectroscopy based on ASTM D5863 was used to determine concentrations below 2 ppm after dry ashing of the samples. Reagents, Samples, and Solutions. For the GPC experiments, HPLC-grade THF (Scharlau, Spain) was used for the solutions and sample dilutions and as the mobile phase. Samples analyzed are four crude oils and their distillation cuts. Crude oil 1 (CO1) was from Africa; crude oil 2 (CO2) was from northern Europe; crude oil 3 (CO3) was from Russia; and crude oil 4 (CO4) was from South America. The naming and description of the distillation cuts are presented in Figure 1. Note that fraction a, the atmospheric residue (AR), with a boiling temperature above 375 °C is fractionated between fraction b, the vacuum residue (VR), with a boiling temperature above 560 °C and fraction c, the vacuum gas oil (VGO), with a boiling temperature between 375 and 560 °C. Fractions d, e, f, and g have boiling temperatures ranging from 230 to 375 °C, from 145 to 230 °C, from 75 to 145 °C, and from 15 to 75 °C, respectively. The distillation is carried out at atmospheric pressure up to 350 °C and under vacuum. For the GPC analyses, a precise amount of sample (between 0.01 and 0.1 g) was weighted and an adequate mass of THF was added to reach a final concentration of 1% (100-fold mass dilution). At that concentration and in that solvent, there is aggregation within our samples22,23 because we are well above the 500 ng/mL threshold
log(M w ) = −1.88 × 10−3t 3 + 1.27 × 10−1t 2 − 3.10t + 30.55 A built-in software was used to integrate the signal recorded. An Excel file was used for the deconvolution of the sulfur chromatograms by summing Gaussian curves.
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RESULTS AND DISCUSSION Total Concentration Analyses. The total concentration analyses of sulfur, vanadium, and nickel in the 32 samples were performed and are presented in Table 2 along with the distillation yields, elemental mass balances, and repartition of the three studied elements in the different distillation cuts. The four crude oils selected for this study were because they originate from four different geographical locations and represent the four combinations possible between high and low sulfur content and between high and low acidity. Moreover, they have various vanadium and nickel total concentrations. 3731
dx.doi.org/10.1021/ef500571f | Energy Fuels 2014, 28, 3730−3737
Energy & Fuels
Article
Table 2. Results of the Total Element Analyses of Crude Oils and Distillation Cutsa CO boiling temperature (°C) Africa CO1
northern Europe CO2
Russia CO3
South America CO4
yield (%, S V Ni TAN V/Ni viscosity yield (%, S V Ni TAN V/Ni viscosity yield (%, S V Ni TAN V/Ni viscosity yield (%, S V Ni TAN V/Ni viscosity
m/m)
m/m)
m/m)
m/m)
100 5210 7 18 1.6 0.39 105.0 100 3030 1.97 1.17 0.10 1.69 5.7 100 12900 36 10 0.06 3.60 15.3 100 31900 344 69 2.75 4.99 2448.5
AR
VR
VGO
d
e
f
g
>375
>560
375−560
230−375
145−230
75−145
15−75
7470 13 29
26.2 9190 (46) 29 (99.8) 71 (99.8)
0.45
0.41
6280 4 3
12.0 9280 (38) 15 (99.3) 9 (98.8)
1.33
1.67
21000 74 21
20.5 26600 (43) 172 (99.9) 49 (99.9)
3.52
3.51
42000
mass balance: fractions/crude oil
33.6 5950 (38)
