Article Cite This: Energy Fuels 2017, 31, 11651-11659
pubs.acs.org/EF
Proof of Concept of High-Temperature Comprehensive TwoDimensional Gas Chromatography Time-of-Flight Mass Spectrometry for Two-Dimensional Simulated Distillation of Crude Oils Maximilian K. Jennerwein,†,‡,§ Markus S. Eschner,† Thomas Wilharm,† Ralf Zimmermann,‡,§ and Thomas M. Gröger*,‡ †
ASG Analytik-Service Gesellschaft mbH, 86356 Neusäß, Germany German Research Center for Environmental Health, Helmholtz Zentrum München, 85764 Oberschleißheim, Germany § Division of Analytical and Technical Chemistry, Institute of Chemistry, University of Rostock, 18059 Rostock, Germany ‡
S Supporting Information *
ABSTRACT: In this work, a reversed-phase high-temperature comprehensive two-dimensional gas chromatography time-offlight mass spectrometry (GC × GC−TOFMS) approach for the qualitative and quantitative analyses of crude oils will be presented. The proposed setup provides the best utilization of the two-dimensional separation space for carbon numbers between C10 and C60. Visual Basic Script (VBS) was successfully applied for data processing to achieve comprehensive classification of the main compound classes. On this basis, crude oils from different origins could be compared by their composition. Real distillation cuts following ASTM D2892 and ASTM D5236 were applied for the development of area-based templates representing virtual boiling point cuts. By this approach, a quantification of an artificial crude oil sample with a defined initial boiling point was evaluated versus the quantitative result according to ASTM D7169 (one-dimensional simulated distillation for high boiling samples, hereinafter 1D-SimDist), and by this, a two-dimensional simulated distillation (2D-SimDist) was successfully developed.
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techniques.13−16 Nitrogen- and sulfur-containing compounds are in the focus of most studies and only few studies address oxygen-containing compounds. Two recent works can be reported in this context, Li et al. accomplished a quantification of dibenzofurans and benzo[b]naphthofurans in crude oils,17 and Rohwer et al. presenting a high-temperature GC × GC analysis of oxidized paraffinic products.18 A sophisticated approach using several different detectors for the analysis of shale oil was reported in 2015, which provided detailed information about the chemical composition of this matrix.15 The authors combined the results of flame ionization detection (FID), sulfur chemiluminescence detection (SCD), nitrogen chemiluminescence detection (NCD), and time-of-flight mass spectrometry (TOFMS) to identify several heteroatomic compound classes in concentrations different from common crude oil. However, besides commonly known compound classes, further heteroatomic compound classes can be found with increasing boiling point and carbon number. Next to the qualitative composition, also the knowledge of the true boiling point (TBP) distribution and the composition of distillation feedstock is of high importance for refineries when products with a constant quality should be ensured. The determination of the boiling point distribution can be achieved using the common simulated distillation following ASTM D7169, but only rough information about the composition can be gained by this approach when a CNS-SimDis analyzer is
INTRODUCTION Crude oil takes a central role in human society and economy. However, while crude oil reservoirs as well as the quality of the produced oils are decreasing worldwide, the consumption of the main petrochemical products is still continuously increasing. Dealing with this issue is a major challenge for the petroleum industry. Heavier cuts and blends of different kinds of feedstock have to be upgraded using catalytic cracking and hydrocracking to satisfy this rising demand for fuels with a steady quality.1−7 The qualitative and quantitative analyses of different kinds of heteroatomic compounds within crude oil are of high relevance. The problems caused by these undesirable compounds are commonly known for their toxicity, carcinogenicity, and mutagenicity for humans and animals on the one hand and for catalyst poisoning, fouling, and corrosion during refinery processes on the other hand. Knowledge of not only the total heteroatomic content but also the chemical structures of the corresponding compounds is crucial for the production of petrochemical products. In recent studies with the focus on heteroatomic compounds in crude oils, scientists applied different sophisticated approaches to gain deeper insight in the composition of these compound classes. Hereby, the application of high-resolution mass spectrometry using different ionization techniques is very common. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS) is used in most cases8−12 often also in combination with previous highperformance liquid chromatography (HPLC) separation.8,10 Also, comprehensive two-dimensional gas chromatography (GC × GC) is used in combination with different detection © 2017 American Chemical Society
Received: June 26, 2017 Revised: September 25, 2017 Published: October 10, 2017 11651
DOI: 10.1021/acs.energyfuels.7b01799 Energy Fuels 2017, 31, 11651−11659
Article
Energy & Fuels applied.19 Some years ago, the first steps were made toward a correlation between GC × GC−TOFMS and common onedimensional simulated distillation (1D-SimDist) by Dutriez et al. for the characterization of vacuum gas oils (VGOs).3−5 The authors emphasize several limitations of the whole approach of a two-dimensional simulated distillation (2D-SimDist), because of the temperature limits of chromatographic columns, injector discriminations, cracking of petroleum compounds at high temperatures, and partial overlapping of compound families. The thoroughly chosen normal-phase column combination included a short first column with a high phase ratio (10 m DB1-HT, 0.32 mm × 0.1 μm), consequently resulting in a long modulation period of 20 s and FID. By this approach, a rough classification of saturates and aromatics up to tetra-aromatics could be achieved but only partial differentiation between n-, iso-, and cyclic alkanes was possible. Contrary to their considerations concerning pressure−temperature−volume (PTV) or on-column injection, hot split injection was chosen for their method development, leading to the described discriminations of high boiling compounds. Nevertheless, because the carbon numbers of the applied samples range between C20 and a maximum of C60, approximately 100% of all vaporizable compounds eluted from the column. Thus, quantification and correlation to 1D-SimDist could be achieved by normalizing the total area to 100% and converting to weight percent. Discrimination of the system was taken under account by the application of response factors for n-paraffin standard solutions. Apart from the mentioned short comings, the authors could show a good accordance of their developed method with 1D-SimDist following ASTM D2887 and conclude that, as long the stability of stationary phases is not improved for high temperatures, they have approached the elution limits for the use of GC × GC−TOFMS. However, the major part of the above-mentioned studies applied so-called normal-phase column combination. Already in 2006, Tran et al. showed in detail the advantages of a reversed-phase column combination with a polar first-dimension column and a nonpolar seconddimension column for the analysis of crude oils, petrochemical products, and environmental samples, such as oil spills.20 They concluded that, as a result of the increased retention of hydrocarbons in the second dimension, the two-dimensional separation space can be used more effectively and the separation itself can be improved. Only few scientists have followed this concept thus far, especially for the analysis of crude oils. A recent work was reported by Li et al. for the analysis of crude oils in 2015, including a detailed explanation of the comprehensive two-dimensional elution profile.21 The work presented here shows a new column combination and GC × GC−TOFMS setup, optimized for high-temperature measurements comprising a temperature range that has not been reported thus far. The thoroughly chosen column setup and GC parameters allow for a comprehensive analysis of the vaporable fraction of high-boiling samples, such as crude oils, heavy fuel oils, and vacuum gas oils. In contrast to previous studies, mass spectrometry (MS) was applied for detection to provide a detailed classification of the major compound classes using Visual Basic Script (VBS) for data processing. Automated classification tools, such as VBS, are necessary, despite the advantageous separation performance of GC × GC, because the number of overlapping substance classes is increasing along with the complexity of samples to be examined. The application of predefined functions and variable search parameters within VBS allows to search mass spectra fast and reliable for decisive
criteria, the assignment of spectra to corresponding substance classes and the generation of virtual boiling point cuts. In addition, measurements using high-resolution time-of-flight (HRTOF) with GC × GC separation were applied. By this approach, the higher mass resolution of the instrument provided the possibility to investigate the presence of further compound classes, which could not clearly be identified on the basis of the nominal mass and elution region. Furthermore, a correlation between standard simulated distillation methods following ASTM D7169 could be established. For this approach, different distillation fractions were produced in house following standard distillation processes ASTM D2892 and ASTM D5236. Finally, the concept of a 2D-SimDist using GC × GC−TOFMS, which combines the information about the boiling point distribution and the classification of different compound groups, could be realized and evaluated versus real boiling point cuts.
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EXPERIMENTAL SECTION
Samples. Two barrels of a light crude oil (CPC Blend) were provided by Gunvor Raffenerie Ingolstadt GmbH. The whole amount of crude oil was homogenized and portioned in several 5 L canisters. These portioned light crude oil samples were used for the production of different distillation fractions. In addition to the CPC Blend, several other crude oil samples from different origins were used for the development of the qualitative analysis using VBS, including Arabian Light crude oil, Norwegian Troll crude oil, Nigerian Furcado crude oil, Mittelplate crude oil, and a blend of North African crude oils. Methods. Several portions of the CPC Blend were distillated following ASTM D2892 and ASTM D5236, whereby different distillation cuts were generated for further method development. The distillation processes were performed on two different distillation units, Petrodist 100CC for distillations following ASTM D2892 and Petrodist 200CC for distillations following ASTM D5236, from the company Pilodist in Meckenheim, Germany. Distillation parameters are given in the Results and Discussion, and detailed information is given in the Supporting Information. According to the specification of the applied systems in theory, there is no minimum limitation for the temperature difference of distillation cuts, but from experience, it is known that the amount of overlap is increasing with narrowing distillation cuts. This effect could also be observed and determined for the produced cuts by simulated distillation following ASTM D7169 and is discussed in detail in the Results and Discussion. To achieve a good compromise between the narrowness of single distillation cuts and the amount of overlap, boiling point ranges of 30 °C were chosen, starting from 70, 80, and 90 °C (V1, V2, and V3), respectively (see also Table 1). In addition, a distillation was performed for the production of only two cuts with
Table 1. Overview of the Three Different Distillation Processes for the Production of Narrow Crude Oil Fractions and One Distillation Cut with Initial Boiling Point of 160 °C distillation process ASTM D2892
ASTM D5236
11652
temperature parameter
V1
V2
V3
V5
start temperature fraction collection (°C) end temperature fraction collection (°C) boiling point range of each fraction (°C) start temperature fraction collection (°C) end temperature fraction collection (°C) boiling point range of each fraction (°C)
70
80
90
15
370
380
390
160
30
145
370
380
390
580
560
570
30
DOI: 10.1021/acs.energyfuels.7b01799 Energy Fuels 2017, 31, 11651−11659
Article
Energy & Fuels Table 2. Results of the HT-GC × GC−TOFMS Measurements of the Applied Crude Oil Samples by Substance Class
n-/isoparaffins naphthenes dinaphthene polynaphthene hopanes/steranes alkylbenzenes naphthenobenzenes naphthalenes various diaromatics fluorenes triaromatics pyrenes/fluoranthenes tetra-aromatics alkylsulfides thianes thiophenes benzothiophenes dibenzothiophenes benzonaphthothiophenes carbazoles
Mittelplate (Iceland) (%)
CPC Blend (Kazakhstan) (%)
Arabian Light (Saudi Arabia) (%)
Troll (Norway) (%)
Forcado (Nigeria) (%)
North African blend (%)
49.66 18.84 4.41 0.07 0.41 13.48 4.62 3.19 0.43 0.35 0.23 0.01
