GCxGC-HRT for identification of compounds responsible for ... · Separation in the 2D separation...
Transcript of GCxGC-HRT for identification of compounds responsible for ... · Separation in the 2D separation...
GCxGC-HRT for identification of compounds responsible
for fouling during refining of petrochemical products
Rina van der Westhuizen
ChromSAAMS 2016
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The aim of the study was to evaluate GCxGC-HRT for identification of compounds responsible for soluble gum
formation during refining processes of petrochemical products.
Aim of study
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Background
Fouling is experienced in several environments in refineries, like the crude units, distillation streams, vapor
recovery units, catalytic crackers, hydrodesulphurization units, etc1.
Fouling causes clogging of process equipment such as heat exchangers, compressors, furnaces, reaction and
distillation systems
Causes loss of valuable product
Loss of expensive equipment
Deactivation of catalysts
Loss of production due to plant shutdowns
Fouling in petrochemical plants
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Background
Chemical mechanisms for fouling are very complex
Often the gummy masses or sediment are catalytically formed by the undesirable presence of metallic impurities.
Organic species such as olefins and other hydrocarbons may react with oxygenates and traces of dissolved oxygen
which can lead to fouling in process units2. Wallace (1964) ranked reactivity of hydrocarbons in autoxidation: Alkyl
aromatics > di-olefins, mono olefins> paraffins. Oxidation products include peroxides, aldehydes, acids and
ketones and heavier compounds, referred to as “Heavies” or “gum”.
Gum will precipitate from a bulk solution when its solubility limit is reached. The limit of gum solubility depends on
temperature, the nature of the gum, other species in the gum, etc.
Hydroperoxides, the intermediate products in autoxidation reactions are formed by reaction between oxygen and
hydrocarbons. They may accelerate sedimentation formation and lower the temperature of gum formation.
Residue formation in refineries – Literature study1,2
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Background
In order to get a fundamental understanding and to identify methods to reduce gum formation, it is essential to
identify the organic compounds responsible for gum formation
Gum and residues are heavy and not really suitable for GC analysis. We could only identify the main soluble gum
compounds in a boiling range suitable for GC. We subjected light olefinic material to high temperatures and
pressures for extended periods in order to form soluble gum (ASTM Method). We then distilled the product and
analysed the Heavy fraction suitable for HT-GCxGC-TOF-MS (< C45).
The mass spectra of the heavy boiling compounds can become highly complex and identification of compounds
very challenging. The mass spectra of compounds responsible for gum formation are seldom present in mass
spectral libraries and identification of compounds has to be done using first principal calculations.
Low resolution mass spectrometers, such as the TOF-MS and quadrupole instruments, provide nominal mass
spectra with integer mass values. Nominal mass spectra do not provide sufficient information on mass fragments
and molecular ions to unambiguously distinguish between hydrocarbons, oxygenated hydrocarbons, and other
hetero-atomic compounds
Analytical challenges
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Experimental
Light olefinic products were subjected to refining conditions of elevated temperatures and pressures, and distilled.
The distillation fraction between 200° - 550°C were analyzed (≈ C12-C45)
GCxGC-TOF-MS
GCxGC-HRT (Leco Africa)
GC
HPLC
NMR
ICP
Pilot plant studies, catalysts analyses, etc.
Sample preparation and analysis
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Experimental
GCxGC-TOF-MS
Main properties
The Leco Pegasus 4D GCxGC-TOFMS3 uses two columns in series with a duel-jet modulator to focus eluent eluting
from the primary column onto the secondary column; it’s peak capacity is the product of the peak capacities of the
two columns and is in the order of tens-of thousands
GCxGC provides structured 2D separations that assist with peak identification. Compounds with similar properties
are grouped together on the 2D contour plot in a roof-tile effect
Separation in the 2D separation plane is separated by polarity (Y-axis) and boiling point differences (X-axis)
The TOF-MS has high data acquisition rates (up to 500 spectra/s) to differentiate between closely eluting mass
spectra
TOF-MS is a low resolution mass spectrometer producing nominal (integer) mass spectra
Column Combination: Rxi-17Sil MS Max temp: 360 °C vs. Rxi-5 Max Temp: 350°C (Restek)
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Experimental
Mass Accuracy <1 ppm
Mass Range 10-1500 m/z
Resolving Power up to 50,000
Detection Limit 1 pg on-column
Linear Dynamic Range >3.5 orders
Data Acquisition Speed Up to 200 sps
Ionization EI, PCI
LECO Pegasus®-HRT3
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GCxGC-TOF-MS Results & Discussion
Comparison of GCxGC-TOF-MS contour plots of Soluble Gum A with a Heavy Oil showing the peaks in Gum A
eluting in the aliphatic region of the chromatogram
Gum A
Heavy Oil
Aliphatics
Aromatics
Oxygenated Hydrocarbons
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GCxGC-TOF-MS Results & Discussion - Gum A
Monomer region 1-50 min zoomed into
Gum A
1-50 min
Benzene
Hydrocarbons:
cyclic olefins
dienes, alkenylbenzenes
Oxygenates – alcohols
ethers, furans, acids, indenones, etc
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GCxGC-HRT Results & Discussion
Nominal mass Exact mass
Molecular
Formula
43 43.0548 C3H7 General mass fragment for paraffins
43.0184 C2H3O General mass fragment for ketones
58 58.0782 C4H10 Butane, paraffins
McLafferty
Rearrangement 58.0419 C3H6O Propanal/propanone, carbonyls
58.0055 C2H2O2 Formic acid, acids esters
160 160.1252 C12H16 Ethyldecalin
160.1463 C9H20O2 C9 Acetal
160.0524 C10H8O2 Aromatic acid or ester
160.0889 C11H12O Aromatic Aldehyde
Nominal vs. accurate masses
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GCxGC-HRT Results & Discussion
Peak 2988 - Structure Deduction from GCxGC-HRT Accurate Mass Spectra
Exact (Calculated) Mass 480.3792 400.3120 320.2496 240.1872 159.1170 81.0702
Accurate (Measured) Mass 480.3794 400.3108 320.2476 240.1862 159.1167 81.0700
Molecular Formula C36H48 C30H40 C24H32 C18H24 C12H15 C6H9
Mass Loss Molecular Ion 80.0635 80.0632 80.0614 81.0695 78.0467
Loss Fragment -C6H8 -C6H8 -C6H8 -C6H9 -C6H6
GCxGC-HRT: Proposed structure for Peak 2988 – Diels-Alder Hexamer - Hydrocarbons only
Compound
Exact Accurate
DeviationMass Mass
C36H48 480.3792 480.3794 0.0000
C35H48O 480.3466 480.3794 0.0068
C34H40O2 480.314 480.3794 0.0136
C34H40O3 480.2814 480.3794 0.0204
C32H32O2S 480.3168 480.3794 0.0130
C32H34NOS 480.3406 480.3794 0.0081
C32H38NOSi 480.3922 480.3794 0.0027
m/z 80 Cyclohexadiene
Methylcyclopentadiene
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Discussion
Conjugated Diene + Dienophile Cyclohexene system
The Diels-Alder cycloaddition reaction is governed by orbital symmetry considerations5. It entails the [4πS+2πS]
cycloaddition reaction between a conjugated diene in the S-Cis configuration and a substituted alkene (dienophile) to
form a substituted cyclohexene system.
There are no intermediates generated during the reaction (two π-bonds are exchanged for two σ-bonds)
Some aromatic and hetero-atomic compounds may also act as diene and/or dienophile in this reaction
The reaction may proceed in the reverse (retro-Diels-Alder reaction) under favorable conditions to again produce the
diene and dienophile derivatives. The Retro Diels-Alder products are not necessarily the same as the original reactants
Diels-Alder reactions – Literature study5-7
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Cyclic dienes can act as both diene and dienophile
Diels-Alder Polymerisation
+
+
+
++
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GCxGC-HRT
A proposed structure for Peak 3070 – Diels-Alder Pentamer
+ +
Peak 3070- Structure Deduction from Accurate Mass Spectra
Calculated Mass 400.3110 374.2954 359.2723 331.2407 317.2262 237.1634
Accurate (Measured) Mass 400.3108 374.2964 359.273 225.1634 317.2255 212.1546
Formula C30H40 C28H38 C27H35 C25H31 C24H29 C18H21
Mass Loss Molecular Ion 26.0154 15.0231 28.0316 14.0152 94.0773
Loss Fragment C2H2 -CH3 -C2H4 -CH2 -C7H10
C+
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Discussion
Oxo-Diels-Alder reactions may occur between conjugated dienes and α,β-unsaturated aldehydes to form
dihydropyrans
Aromatics are stabilized by their resonance energy and will need greater activation energy to take part in the Diels-
Alder reaction. They will react if the dienophile is extremely reactive or if the aromatic system is large enough that the
loss of one aromatic ring does not significantly destabilize the entire system, e.g. the middle ring in anthracene
Diels-Alder reactions – Literature Study5-7
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GCxGC-HRT
Proposed structure for Peak 1022 – Diels-Alder Dimer formed by aromatic and acid
Peak 1022- Structure Deduction from Accurate Mass Spectra
Calculated Mass 192.1144 177.0912 174.1041 158.1092 143.0861 128.0624 115.0546 115.0546 91.0546 43.0183
Accurate (Measured) Mass 192.1145 177.0911 174.1040 158.1091 143.0855 128.0621 115.0543 115.0543 91.0542 43.0179
Formula C12H16O2 C11H13O2 C12H14O C12H14 C11H11 C10H8 C9H7 C9H7 C7H7 C2H3O
Mass Loss Molecular Ion 15.0234 18.0105 15.9949 15.0236 15.0234 13.0078 28.0312
Loss Fragment -CH3 -H2O -O -CH3 -CH3 -CH -CH2=CH2
O
OH
O
OH+Compound
Exact Accurate Deviation
Mass Mass
C14H24 192.1878 0.0382
C13H20O 192.1514 0.0192
C12H16O2 192.1150 192.1145 0.0003
C11H12O3 192.0786 0.0187
C11H12OS 192.0609 0.0279
C12H16S 192.0973 0.0090
C11H12OS 192.0609 0.0279
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GCxGC-HRT
Peak 1022- Structure Deduction from Accurate Mass Spectra
Calculated Mass 272.2133 254.2028 239.1794 225.1638 212.1560 198.1404 185.1326 59.0492
Accurate (Measured) Mass 272.2124 254.2028 239.1794 225.1634 212.1546 198.1399 185.1325 59.0492
Formula C19H28O C19H26 C18H23 C17H21 C16H20 C15H18 C14H17 C3H7O
Mass Loss Molecular Ion 18.0105 15.0234 14.0156 13.0078 14.0147 13.0074
Loss Fragment -H2O -CH3 -CH2 -CH -CH2 -CH
Proposed structure for Peak 1918 – Diels-Alder Trimer with -OH group
OH OH
+ +
OH
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Discussion
Aldol Condensation and dehydration of Aldol Condensation products 8-10
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Results and Discussion
Shake up tests of standards in DEA at ≈123° for 24 hours
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Discussion
Aldol Condensation and dehydration of Aldol Condensation products 8-10
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Summary
● The accurate molecular mass and mass fragments, produced by the HRT, greatly enhanced compound identification for the heavy boiling
compounds in of the soluble gum. It was possible to distinguish between compounds containing oxygen atoms and pure hydrocarbons
which have very similar nominal mass spectra
● Soluble gum products in the olefinic product consist of oligomers up to and higher than the hexamer.
● The main oligomers contain polycyclic olefinic/dienic types of structures
● Oligomer peaks eluted in the non-polar or aliphatic region of the GCxGC chromatograms
● Each oligomeric unit was distributed over a range of carbon numbers but the main compounds differed by approximately 80 mass units,
indicating cyclohexadiene or methylcyclopentadiene addition between oligomers.
● It was deducted that the oligomers were formed by the Diels-Alder cycloaddition reaction between C6 cyclic dienes.
● Some monomers were found in the samples that were not expected in the heavy distillation cut. The monomers were identified as Retro-
Diels-Alder products. The compounds were cyclic olefins and dienes, benzene, a few alkenylbenzenes, as well as unsaturated oxygenates
like alcohols, acids, and ethers. The presence of oxygenated and aromatic monomers suggested that these compounds were present in
the higher oligomers as well.
● Cyclic dienes, aromatics and α,β-unsaturated oxygenates can act as both diene and dienophile in Diels-Alder reactions
Deductions from analytical results
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Summary
● The accurate mass spectra produced by the GCxGC-HRT analysis confirmed that some of the oligomers contained one or more oxygen
atoms
● It was shown that the oligomers were formed in the reaction between cyclic dienes (monomers) and higher Diels-Alder oligomers with α,β-
unsaturated oxygenated hydrocarbons like alcohols, acids and carbonyls.
● The α,β-unsaturated oxygenated hydrocarbons were probably formed in Aldol Condensation reactions
● Aromatics, incl. alkenylbenzenes, may act as dienes and react with α,β-unsaturated oxygenated hydrocarbons to form polycyclic
oxygenated hydrocarbon species. The resulting oligomers do not actually contain an aromatic ring
● The accurate mass spectra produced by the HRT greatly enhanced compound identification and enabled the distinction between pure and
oxygenated hydrocarbons
● GCxGC-TOF-MS, GC-SMB-MS and GCxGC-HRT provided supplementary information that enabled the identification of the main
compounds responsible for soluble gum formation in olefinic petrochemical products
Deductions from analytical results
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References
1. ECI Symposium Series, Vol RP5: Proceedings of 7th International Conference on Heat Exchanger Fouling and Cleaning – Challenges and
Opportunities, Editors Hans Müller-Steinhagen, M. Reza Malayeri, and A. Paul Watkinson, Engineering Conferences International, Tomar,
Portugal, July 1, 2007.
2. H.D. Willauer, D.R. Hardy, R.E. Morris, F.W. Williams, Navy Technology Centre for Safety and Survivability, Chemsitry Division, Report NRL/MR/6180—07-
9087, Washington, October 2007.
3. htpp:/www.leco.com/products/separation-science.com
4. A. Amirav, A. Gordin, M. Poliak, A.B. Fialkov, “Gas Chromatography Mass Spectrometry with Supersonic Molecular Beams”, J. Mass Spectrom. 43,141-
163 (2008).
5. K. Volhardt, C. Peter, N.E. Schore, Organic Chemistry: Structure and Function, New York, W.H. Feeman and Company, 2007
6. F. Carey, Advanced Organic Chemistry. 5th Ed. Springer, 2007.
7. F. Fringuelli, The Diels-Alder Reaction: Selected Practical Methods, John Wiley and Sons, 2002.
Diels-Alder publication
8. M.B. Smith, J. March, Advanced Organic Chemistry (5th Ed.), New York: Wiley Interscience, p1218-1223, (2001).
9. R. Mahrwald Modern Aldol Reactions 1,2 Weinheim, Germany: Wiley-VCH, p1218-1233.
10. L.G. Wade, “Organic Chemistry (6th ed.), Upper Saddle River, N.J: Prentice Hall, p 1056-1066, (2005).
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Acknowledgements
Leco Africa, Dr. Peter Gorst-Allman for inviting me to run a samples on the GCxGC-HRT at UP and training on the
instrument
Dr. Ryan Walmsley for providing pictures for my presentation, shake-up tests
Dr. Aletta Joubert, Heloise Winnan, Werner Greeff for supplying samples
Sasol, Dr. Riaan Bekker, Dr. Tracy Bromfield for supporting the study
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