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

Maintaining momentum 2

Copyright ©, 2016, SasolConfidential

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



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



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



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



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



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)



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



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



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



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



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



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



Cyclic dienes can act as both diene and dienophile

Diels-Alder Polymerisation

+

+

+

++



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+



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



GCxGC-HRT

Proposed structure for Peak 1022 – Diels-Alder Dimer formed by aromatic and acid


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



GCxGC-HRT


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



Discussion

Aldol Condensation and dehydration of Aldol Condensation products 8-10



Results and Discussion

Shake up tests of standards in DEA at ≈123° for 24 hours



Discussion

Aldol Condensation and dehydration of Aldol Condensation products 8-10



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



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



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).



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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