The Boduszynski continuum : contributions to the understanding of the molecular composition of petroleum 9780841232945, 0841232946, 9780841232938

Table of contents :
Content: Heavy fraction and asphaltene characterization --
The Boduszynski Continuum model --
Distillation.

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The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum

ACS SYMPOSIUM SERIES 1282

The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum Cesar Ovalles, Editor Chevron Energy Technology Company Richmond, California

Michael E. Moir, Editor Chevron Energy Technology Company Richmond, California

Sponsored by the ACS Division of Energy and Fuels

American Chemical Society, Washington, DC Distributed in print by Oxford University Press

Library of Congress Cataloging-in-Publication Data Names: Ovalles, Cesar, editor. | Moir, Michael E., 1956- editor. | American Chemical Society. Division of Energy and Fuels. Title: The Boduszynski continuum : contributions to the understanding of the molecular composition of petroleum / Cesar Ovalles, editor (Chevron Energy Technology Company, Richmond, California), Michael E. Moir, editor (Chevron Energy Technology Company, Richmond, California) ; sponsored by the ACS Division of Energy and Fuels. Description: Washington DC : American Chemical Society, [2018] | Series: ACS symposium series ; 1282 | Includes bibliographical references and index. Identifiers: LCCN 2018024930 (print) | LCCN 2018030475 (ebook) | ISBN 9780841232938 (ebook) | ISBN 9780841232945 Subjects: LCSH: Petroleum--Analysis. | Boduszynski, Mieczyslaw M., 1940- | Chemistry, Technical. Classification: LCC TP691 (ebook) | LCC TP691 .B555 2018 (print) | DDC 665.5--dc23 LC record available at https://lccn.loc.gov/2018024930

The paper used in this publication meets the minimum requirements of American National Standard for Information Sciences—Permanence of Paper for Printed Library Materials, ANSI Z39.48n1984. Copyright © 2018 American Chemical Society Distributed in print by Oxford University Press All Rights Reserved. Reprographic copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Act is allowed for internal use only, provided that a per-chapter fee of $40.25 plus $0.75 per page is paid to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. Republication or reproduction for sale of pages in this book is permitted only under license from ACS. Direct these and other permission requests to ACS Copyright Office, Publications Division, 1155 16th Street, N.W., Washington, DC 20036. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

Foreword The ACS Symposium Series was first published in 1974 to provide a mechanism for publishing symposia quickly in book form. The purpose of the series is to publish timely, comprehensive books developed from the ACS sponsored symposia based on current scientific research. Occasionally, books are developed from symposia sponsored by other organizations when the topic is of keen interest to the chemistry audience. Before agreeing to publish a book, the proposed table of contents is reviewed for appropriate and comprehensive coverage and for interest to the audience. Some papers may be excluded to better focus the book; others may be added to provide comprehensiveness. When appropriate, overview or introductory chapters are added. Drafts of chapters are peer-reviewed prior to final acceptance or rejection, and manuscripts are prepared in camera-ready format. As a rule, only original research papers and original review papers are included in the volumes. Verbatim reproductions of previous published papers are not accepted.

ACS Books Department

Contents Preface .............................................................................................................................. ix

Section I: Heavy Fraction and Asphaltene Characterization 1.

Asphaltenes, What Art Thou? ................................................................................ 3 Michael E. Moir

2.

The Importance of Mass Balances: Case Studies of Evaluation of Asphaltene Dispersants and Antifoulants ............................................................ 25 Cesar Ovalles, Estrella Rogel, Harris Morazan, and Michael E. Moir

3.

Equivalent Distillation: A Path to a Better Understanding of Asphaltene Characteristics and Behavior ................................................................................ 51 Estrella Rogel, Michael Roye, Janie Vien, and Matthias Witt

4.

Characterization of Heavy Petroleum Fractions by NMR Techniques ............ 73 Ajit Pradhan, Cesar Ovalles, and Michael Moir

5.

From a Dream to a Fact: Direct Measurement of Vanadium and Nickel Distribution in Crude Oil Cuts Fraction (800−1250 °F) ..................................... 87 M. M. Boduszynski, C. E. Rechsteiner, M. E. Moir, D. Leong, J. Nelson, L Poirier, and F. Lopez-Linares

Section II: The Boduszynski Continuum Model 6.

The Compositional and Structural Continuum of Petroleum from Light Distillates to Asphaltenes: The Boduszynski Continuum Theory As Revealedby FT-ICR Mass Spectrometry ........................................................... 113 Martha L. Chacón-Patiño, Steven M. Rowland, and Ryan P. Rodgers

7.

Plausible Locus for Large Paraffinic Compounds in the Boduszynski Continuous Composition Petroleum Model ...................................................... 173 Lante A. Carbognani Ortega

8.

Effect of Bitumen Molecular Transformation during Upgrading on Asphaltenes Chemistry and Compatibility ........................................................ 189 Parviz Rahimi

vii

Section III: Distillation 9.

Attainable Product Yield Distribution Curve: A Roadmap to Crude Oil Composition .......................................................................................................... 205 Carl E. Rechsteiner

10. Correlation of Thermogravimetry and High Temperature Simulated Distillation for Oil Analysis: Thermal Cracking Influence over both Methodologies ....................................................................................................... 223 Lante A. Carbognani Ortega, Josune Carbognani, and Pedro Pereira Almao Editors’ Biographies .................................................................................................... 241

Indexes Author Index ................................................................................................................ 245 Subject Index ................................................................................................................ 247

viii

Preface Dr. Mieczyslaw M. Boduszynski (Mietek) was the recipient of the 2016 George A. Olah Award in Hydrocarbon or Petroleum Chemistry and the 2017 ENFL Distinguished Researcher Award. Dr. Boduszynski was honored for his significant contributions to the understanding of the molecular composition of petroleum and his profound influence on a generation of petroleum and fuel chemists. This book is based on the symposium titled “ENFL Distinguished Researcher Award in Honor of Mieczyslaw M. Boduszynski”, sponsored by the Energy and Fuel Division (ENFL), held at the 253rd ACS National Meeting & Exposition, April 2-6, 2017, in San Francisco. Other authors were invited to participate to complement the multiples technical areas impacted by Boduszynski’s work during his illustrious career. Dr. Boduszynski always advocated that a better understanding of petroleum molecular composition is key to predicting its properties and behavior. Such information is valuable for chemists, geologists, and chemical and petroleum engineers working in all areas of the petroleum value chain from exploration and production to refining. In a demonstration of Mietek’s influence, this book presents an in-depth account of Boduszynski’s work from the point of view of his collaborators and peers. The impacts on operations and current practices in the industry are discussed as well. This monograph covers three broad topics of Mietek’s outstanding career, Heavy Fraction and Asphaltene Characterization, The Boduszynski Continuum Model, and Distillation. Since his days as a graduate student in Poland, Mietek dedicated himself to the understanding of petroleum composition especially that of heavy ends; this means the isolation and characterization of the heaviest portions of the petroleum in a rational and consistent fashion. In Chapter 1, Dr. Michael Moir discusses the origin of the term asphaltene, the history of the science behind, and how the current state of knowledge aligns with the model of petroleum proposed by Boduszynski. In 1980, Dr. Boduszynski caused a considerable stir in his landmark conference publication “Asphaltenes, where are you?” and inspired the title of Dr. Moir’s chapter (Chapter 1) “Asphaltenes, What Art Thou?”. Next, in Chapter 2, Dr. Cesar Ovalles and coworkers present a chapter dedicated to the importance of mass balances (also called material balances) for the understanding of petroleum chemistry and to ensure the successful outcome of experimentation. During his illustrious career, Dr. Mietek Boduszynski devoted considerable time and effort to analyzing and understanding mass balances during his laboratory testing and experimentation. Following his teaching, several case studies are presented for the evaluation of asphaltene dispersants and antifoulants. ix

The primary goal is to show how mass balances were crucial to assess the validity of the new asphaltene determination method, the reliability of the data, and to evaluate the relative effectiveness of the asphaltene-dispersant and antifoulant additives. In Chapter 3, Dr. Estrella Rogel et al. present a series of correlations that link chemical composition with the solubility and thermal behavior of asphaltenes. In the late 80s and early 90s, Dr. Boduszynski used sequential elution fractionation to demonstrate that non-distillable fractions followed the same patterns as distillation cuts and that solubility could be used to perform an equivalent distillation for non-distillable materials. By using new data and some already published in the literature, Rogel and coworkers find that high hydrogen deficiencies and uneven solubility fraction distributions are the main contributors to asphaltene precipitation. Following the footsteps of Boduszynski, in Chapter 4, Dr. Ajit Pradhan et al. describes the latest developments in nuclear magnetic resonance (NMR) to characterize heavy petroleum fractions and heterogeneous catalysts. They use a combination of the extended Brown and Ladner method and surrogate molecules to monitor structural changes resulting from hydroprocessing of petroleum feeds. They also employ Diffusion Ordered Spectroscopy (DOSY) to characterize heavy petroleum fractions and asphaltenes by their size, shape, mass, and charge. Finally, they use Diffusion Ordered Spectroscopy (DOSY) and Dynamic Nuclear Polarization (DNP) to characterize heterogeneous catalysts and heavy petroleum fractions. In Chapter 5, Dr. Francisco Lopez-Linares and co-workers determine vanadium and nickel contents and distributions as a function of boiling point of vacuum gas oil by using High-Temperature Gas Chromatography coupled with Inductively Coupled Plasma Mass Spectrometry (HTGC-ICP-MS). This line of work was inspired by a late-afternoon comment from Dr. Boduszynski to Dr. Michael Moir, “Do you know what we need? We need a method to measure metal distribution versus temperature”. Dr. Moir took up the challenge, and with the help of the glass and machine shops, designed a suitable quartz torch and transfer-line. The rest is history. This brings us to the second broad topic covered in this monograph and perhaps one of the Mietek’s most significant contribution to petroleum chemistry. This topic is called “The Boduszynski Continuum Model.” In his pioneering work, Dr. Boduszynski postulated that petroleum is a continuum of compounds having a wide variety of structural and elemental compositions. The model proposed that, unlike conventional wisdom at the time, petroleum transitions from individual molecules to high molecular weight components (i.e., asphaltenes), and that the molecular composition of petroleum is a continuously variable series of molecules without abrupt transitions. His work in this area forms the basis of the current model of petroleum. He predicted that the high-boiling residue after distillation was, in fact, a continuous extension of low molecular weight components, rather than (as was then thought) high-molecular weight “polymers.” He had great difficulty in getting his ideas published, but has been overwhelmingly vindicated by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. In Chapter 6, Dr. Ryan Rodgers and co-workers review the work x

carried out at the National High Magnetic Field Laboratory in the chemical and compositional analysis of petroleum. The material discussed clearly demonstrated the validity of The Boduszynski Continuum Model from the low molecular weight compounds up to asphaltenes. Their results create a molecular-level map with the relationships between carbon number, aromaticity (DBE), heteroatom content, chemical functionality, ring number, asphaltene structures (island and archipelago), and boiling points. In Chapter 7, Lante Carbognani, from the University of Calgary, carries out a literature review on the existence of large molecular weight paraffinic compounds in petroleum streams. He finds that sample handling, diluted conditions, and high set up temperatures are mandatory for the successful characterization of such compounds. Also, he proposes a plausible locus within The Boduszynski Continuum Model for these large molecular weight paraffins within high carbon number (C90-C215 carbon atoms) and low Double Bond Equivalent (DBE 1000°F into the ICP plasma. Third, the transfer line must be inert and needs to maintain excellent temperature control. These requirements can only be met using a transfer line and ICP injector that is uniformly heated up to the dry plasma. Also, the ICP-MS itself must be able to control common interferences, notably sulfur. Efforts oriented to develop a heated transfer line from GC to plasma source spectrometers for the analysis of metals were carried out by different research groups, and a good review can be found elsewhere (35–39). Particularly for petroleum applications, high-temperature gas chromatography (HTGC) was coupled efficiently to inductively coupled plasma mass spectrometry (ICP-MS) for the determination of porphyrins from oil shales (38). The in-house, directly heated stainless-steel capillary interface was used to introduce the eluent into the ICP torch, which allowed them the capability of determining cobalt, chromium, iron, nickel, titanium, vanadium and zinc metalloporphyrin fingerprints (38). More recently, we have demonstrated the capability to couple GC using a commercial transfer line which can control temperature accurately (40). In that work, both the transfer line and ICP injector were lined with highly inert Sulfinert® stainless steel as well as independently heated and controlled by the GC. Additionally, an argon makeup gas which helps convey the GC effluent to the ICP was preheated in the GC oven to the current GC oven temperature. Finally, the high flow rate of heated argon through the transfer line minimized the time spent between the GC and ICP torch to less than 100 milliseconds, which in turn significantly reduces the opportunity for interaction between the eluting compounds and the transfer line. These factors allow reproducible and reliable results for the speciation of vanadium and nickel in crude oils and related fractions. The uses of this hyphenated technique in the petroleum business have been extended to the determination of different alkyl lead species in fuels (37), arsenic species in natural gas (41), and mercury species in petroleum hydrocarbons (42, 43). GC-ICP-MS is currently being used as a routine technique in R&D facilities as well as in commercial laboratories (44). In this work, we present our findings gathered from seventeen years working on crude oil characterization and products by high-temperature gas chromatography coupled with ICP-MS (HTGC-ICP-MS). Briefly, we will provide a historical overview on the development of this technique-focused particularly 89

towards the determination of vanadium and nickel in crude oil fractions with a boiling point between 800-1250 °F and then discuss the significant finds of characterizing such fractions using this technique.

2. Experimental Section 2.1. Materials Reagent grade ACS carbon disulfide (Fisher Scientific, USA) was used for all dilutions of the HT SIMDIS SD-SS3E-05 standard (Separation Systems, INC; Gulf Breeze, Florida, USA), 2,3,7,8,12,13,17,18- octaethyl-21H, 23H- porphyrin vanadium oxide (Sigma Aldrich; USA), 2,3,7,8,12,13,17,18- octaethyl-21H, 23Hporphyrin Nickel (Sigma Aldrich; USA), vanadyl (IV) etioporphyrin (III) (Strem Chemical, USA), 5,10,15,20-Tetraphenyl-21H,23H-porphine vanadium(IV) oxide (Sigma-Aldrich; USA), 5,10,15,20-Tetraphenyl-21H,23H-porphine nickel(II) (Sigma-Aldrich; USA), and crude oil fractions selected from worldwide regions, having a boiling point range between 800-1250 ˚F. The boiling points of n-paraffin are available from ASTM Test Method D 6352; the n-alkane boiling points are reported to the nearest whole number in either degree Celsius or degrees Fahrenheit. To transform the GC retention time of an n-alkane mixture to an equivalent boiling point, Analytical Controls Boiling Point calibration mixture (C5-C28) combined with Analytical Controls Polywax 655 calibration standard (C20-C100, PAC, Houston, USA) was employed to cover the carbon number range from C5 to C110. The observed n-alkane equivalent boiling points for the vanadium and nickel porphyrins were above 1000 ˚F. 2.2. Instrumentation Initial development: An HP model 6890 series GC (Wilmington, DE), was used for the separation of the crude oil fractions. The GC was interfaced with a heated interface produced in-house to an Elan 6100 ICP-MS (Perkin-Elmer, Norwalk, CT, USA). Data were collected using the Elan 6100 ICP-MS software and converted to industry standard AIA files using TurboChrom Workstation (Perkin-Elmer, Norwalk, CT). Simulated distillation results for GC-ICP-MS were calculated using Simdist2000 software (Envantage Analytical Software, Cleveland, OH distributed by Scientific Software Inc., Pleasanton, CA). Later, a commercial configuration was commissioned at Chevron Technology Center: Agilent Technologies (Agilent Technologies, Santa Clara, CA, USA) model 7890 series GC was interfaced to an Agilent 7700x ICPMS (Agilent Technologies, Tokyo, Japan) through a commercially available heated GC-ICPMS interface (Agilent Technologies, Santa Clara, CA, USA). Heated argon as a makeup gas was required to assist the flow of species through the heated transfer line (the Ar gas is preheated by passing through a stainless-steel coil mounted in the GC oven). Separation of n-paraffin compounds, as well as the vanadium and nickel present in such fractions, was performed on a high temperature simulated distillation column, DB-HT-SIMDIS, 5 m, 0.530 mm id., 0.15 micron (Agilent 90

Technologies Inc). Typical instrument conditions for the HTGC-ICP-MS are indicated in Table 1.

Table 1. TYPICAL 7700xICP-MS AND 7980 GC EXPERIMENTAL CONDITIONS ICP-MS conditions

7700x

Forward power

1200-1500 W

Plasma gas flow rate

15 L/min

Auxiliary gas flow rate

0.9-1.5 L/min

Make up gas

0.1 L/min

Carrier gas flow rate

0.45-1.5 L/min

Sampling depth/radial viewing height

6-8 mm

Elements isotope

12C, 13C, 51V,58Ni,60Ni

GC parameters

7980 GC

Column

DB-HT-SIMDIS

GC carrier gas flow rate

20 ml He/min (constant flow mode)

Oven temperature

40 °C initial, ramped at 15°C min-1to 200°C, ramped at 5°C min-1 to 430°C and held for 5 min(104 °F initial, ramped at 59°F min-1to 392°F, ramped at 41°F min-1 to 806°F and held for 5 min)

Split/Splitless Inlet

100 °C initial, ramped at 15 °C min-1to 340 °C, and held for 45 min(212 °F initial, ramped at 59 °F min-1to 644 °F, and held for 45 min)

Sample injection (µL)

2

Transfer line temperature

350°C/662 °F

ICP injector temperature

350°C/662 °F

2.3. Procedure A detailed procedure for the technique was reported elsewhere (40). Typical instrumental conditions and isotopes measured using a commercial unit are given in Table 1. To prepare the crude oil samples for analysis into the HTGC-ICPMS, an aliquot (~0.1g) was dissolved in carbon disulfide (~1.0-2.0 g). The samples were vortexed for total dissolution and prepared daily. The HT-SIMDIS standard used as received from the manufacturer (Separation Systems, Inc, 100 Nightingale Lane Gulf Breeze, FL) and warmed up gently (around 104 ˚F / 40 °C) before being 91

injected. Depending on the analysis time (24 h), the standard is injected one more time to check any considerable variation of the retention time as well as signal intensity.

3. Results and Discussion 3.1. A Brief of History of in-House Development for HT-GC-ICP-MS At the beginning of the nineties, an opportunity to produce additional highvalue vacuum gas oil (VGO) was identified by extending the distillation cut point using the new deep-cut assay technique, at the expense of low-value residua (23). By using a high vacuum, short-path distillation to fractionate atmospheric residue, a series of VGO’s and residua having progressively higher cut points were prepared (23). As an example of using this methodology, a VGO yield was increased from 22.9 to 36.9 wt.%. To gain more VGO yield (up to 52.1 wt. %) the temperature of the atmospheric equivalent boiling point (AEBP) needs to be increased to 1075˚F. If the cut point temperature is increased, the quality of VGO tends to be reduced. For example, sulfur and nitrogen content, MCR and metals tend to increase notably in the VGO with an increased cut point, reaching the maximum concentration of the residua (5–7, 45). Regardless of the metals present in VGO and residua, most of the metalcontaining compounds contain vanadium, nickel, and iron. Even if they are present in small amounts, they can still be detrimental to petroleum processing, leading to rapid catalyst poisoning among other problems (5, 7, 8). Therefore, knowledge of the metal concentration of the whole feedstocks prior distillation is required to predict how much metal will be present in the high boiling point fractions. Having a more direct determination of vanadium and nickel in fractions cut up to 1150 °F is of interest because these elements need to be removed in the conversion and upgrading processes. To accomplish that, in late 1990’s, a team was formed to explore the possibilities of a direct measurement of vanadium and nickel distributions by interfacing high-temperature gas chromatography simulated distillation (HTGC-SimDis) with inductively coupled plasma mass spectrometry (ICP-MS) or atomic emission detection (AED). At that time, although GC-ICP-MS applications had been reported in the literature for a decade (33–38), there were no commercially available instruments, and the reported high-temperature applications were limited. Considering this, the first task was to develop an HTGC interface. The original de-mountable torch from the ICP-MS vendor just could not handle temperatures higher than 150°C (302°F) because it contained low-melting plastic components. A one-piece quartz torch was designed by Michael Moir (46), to withstand temperatures of up to 440 °C (824 °F) which implied that all polymeric materials were excluded from the design. An initial prototype was made in the Chevron glass shop by Phil Sliwoski to overcome this high-temperature problem. The initial design that was used to build the torch to fit the existing torch holder of the Perkin-Elmer Elan 6100 ICP-MS is presented in Figure 1 (46). 92

Figure 1. In-house Design of One-piece Quartz Torch for HTGC-ICP-MS. Reproduced with permission from reference (46). Copyright 2018 Chevron.

The prototype was carefully designed to be adapted to the current instrument, minimizing the potential impact on the ICP-MS performance. By incorporating inside the torch injector an SS tubing that contains the capillary column coming from the GC, it could ensure that all eluting compounds from the GC were transported efficiently into the plasma zone. Once the torch was designed and built, Moir designed the interface by considering the type of materials, temperature limits, flow-rates, and temperature stability needed. Phil Johnson made the design a reality (46). The schematic of the transfer line designed is shown in Figure 2. The entire assembly was wrapped with fiberglass thermal insulation, with an outer wrapping of aluminum tape and wire screen to minimize RF leakage (46).

Figure 2. Schematic for In-house transfer line design by M. Moir. Reproduced with permission from reference (46). Copyright 2018 Chevron. 93

The position of the torch with respect to the cone orifice of the ICP-MS spectrometer was adjusted for maximum sensitivity. Additionally, the argon flow rate through the torch tip was optimized by using a 10 mg/L Indium standard in water. A standard nebulizer attached to the one-piece quartz torch was used. Then the transfer line was connected to an HP6890 equipped with a programmable temperature vaporization inlet suitable for performing simulated distillation and one-piece torch in ICP-MS. After this step was accomplished, the prototype was ready for initial testing. The initial simulated distillation conditions used for the first trial are listed in Table 2. They were based upon ASTM D2887-97 and ASTM D6352-98. Simulated distillation analysis using a flame ionization detector (FID) was performed on this instrument to verify that this GC could produce reliable data using conventional FID detection.

Table 2. GC CONDITIONS USED TO EVALUATE VGO ANALYSIS BY HTGC-ICP-MS Oven conditions Initial temperature

40 °C/104 °F

Initial time

0 min

Programmed temperature rate

10 °C/min

Final temperature

440 °C/824 °F

Final time

5 min

Total run time

45 min

Injector conditions Initial temperature

100°C/212 °F

Initial time

0 min

Programmed temperature rate

10 °C /min

Final temperature

440 °C/824 °F

Final time

11 min

Column conditions Column

0.53 ID X 5 m Chrompak Ultiimetal HT simdis

Flow rate

20 ml/min at 50° C/122 °F

Column head pressure

3.2 psi at 50 °C/ 122 °F

Then, a correlation of the retention time axis was accomplished using a standard containing n-alkanes from C-8 to C-100, as it is shown in Figure 3. The ICP-MS conditions used for the analysis of VGO samples are shown in Table 3. 94

Table 3. ICP-MS CONDITIONS USED TO EVALUATE VGO’s BY HTGC-ICP-MS Dwell time (ms)

Isotope 12C 13C 51V

200

56Fe 58Ni 60Ni

Number of scans

1904

Run time (min)

45.08

ICP parameters Nebulizer gas flow rate (SCFH air)

2

Auxiliary gas flow rate (L/min)

1.05

Plasma gas flow rate (L/min)

15

RF power (W)

1250

MS parameter Lens voltage (V)

3

Analog stage voltage (V)

-1875

Pulse stage voltage (V)

1100

Quadrupole rod offset

0

Cell rod offset

-12

Discriminator threshold

60

Cell path voltage std

-17

A breakthrough was accomplished by successfully interfacing HTGC-SimDis with an ICP-MS detection system and demonstrating the feasibility of direct measurement of vanadium and nickel content in such cut fractions. Presented in Figure 4, is the fully assembled configuration used for the analysis of VGO samples. During heating, it was required to ensure the in-house made transfer line was homogeneously thermally isolated to minimize issues of possible cold spots. Additionally, a labor-intensive effort was required to switch the transfer line assembly for different operational ICP-MS modes at that time. Nevertheless, the initial results were promising which encouraged the team to go further. Then, the chromatographic robustness was determined by selecting a VGO cut and 95

following carbon, vanadium and nickel chromatograms. Figure 5 shows the results of three replicates, and each run is overlayed which allows for a qualitative inspection of chromatographic robustness and reproducibility.

Figure 3. n-Alkane retention time standard used to correlate retention time with boiling point.

Figure 4. In-house HTGC-ICP-MS system by the year 2000.

96

Figure 5. Chromatograms for three replicates of 1 wt.% VGO cut in carbon disulfide: a) carbon, b) vanadium and c) nickel.

Aside from differences in intensity due to variability in sample size, chromatographic retention time and peak features were acceptably reproducible. The proof of concept was considered accomplished, and the technique was deemed ready to be used on a routine basis. By summer of 2000, a project was initiated to develop the application of HTGC-SimDis-ICP-MS (from now on, HTGC-ICP-MS) to perform direct measurements of vanadium and nickel in deep cut VGO samples.

3.2. Crude Oil Fraction Analysis by HTGC-ICP-MS Several VGO samples from a variety worldwide crude oils were analyzed using this in-house configuration. One of the significant pieces of information obtained was the metal fingerprint as a function of the boiling point. In general, it was observed that different types of metal-containing components were present in these cuts. In Figure 6, two different VGO samples are presented as examples of the information that was obtained using this technique; the retention time is converted to boiling point and is plotted vs. metal content and the distillation cumulative yield %. First, for VGO 1, it is observed that vanadium signal/area is higher than that of nickel, consistent with the total values obtained by bulk analysis performed by conventional ICP-OES (V: 295 mg kg-1; Ni: 37 mg kg-1) as it is shown in Figure 6. This bulk analysis was performed before running HTGC-ICP-MS. Second, the technique allows for identifying and tracking the metal evolution with the temperature, critical information for the deep-cut assay method. Third, it provided an initial indication of the different metal-containing compounds present in this fraction. 97

Figure 6. V and Ni signature and TBP curve for deep-cut (900-1150 °F) VGO’s.

For VGO 2, Figure 6 reveals that the nickel signal is a higher than that of vanadium, aligned with the bulk values determined by ICP-OES analysis (V:7 mg kg-1; Ni: 25 mg kg-1). Interestingly, the vanadium and nickel patterns show similar signatures. In both examples, it is shown that the metals are concentrated from 1050 °F up to 1250 °F. Also, it is shown that the technique applies to characterization of deep-cut vacuum gas oil (VGO) fractions with a final boiling point (FBP) not exceeding 1350°F. These results suggest that a potential extension of the boiling point cut would allow obtaining more yield of valuable product with a reduced quantity of metals. 98

With the first-generation transfer line developed in-house, and using the GC-ICP-MS configuration presented in Figure 4, this technique allowed the characterization in detail of VGO’s derived from many crudes during the year 2000 until 2008 and improved the understanding of the metal distribution with boiling point. Later, with advances in the technology, a commercial instrument became available on the market and later was installed in our facility. Figure 7 presents the current unit available in our laboratory for HTGC-ICPMS: a) Agilent GC 7980A with high-temperature SimDis capability, b) GC-ICPMS interface. One of the significant advantages of this set up is the improved transfer line design which reduces the hands-on labor requirement from hours to minutes. Also, the new transfer line displays more uniform heating that reduces potential cold spots in the interface, it is very stable, and can reach a temperature of 400 °C /752°F.

Figure 7. Commercial instrumentation: a) Agilent GC 7980A and 7700x ICP-MS b) GC-ICP-MS interface.

Figure 8 shows the robustness experiment carried out using this commercial instrumentation during a routine analysis of different VGO samples. The available carbon HT SIMDIS SD-SS3E-05 standard that is used for ASTM D-7169 was injected, and the 13C signal was followed every day during five consecutive days. As shown, good resolution is observed over four days, and variation of retention time is not appreciable. Day 1 and Day 3 results correspond to the same standard vial, whereas in D2 was used an old standard to determine any variability. Overall, the results reflect good system robustness that provides confidence in the analysis, particularly over lengthy periods of time. After determining system stability, various samples were analyzed. 99

Figure 8. Chromatographic robustness using D-7169 carbon standard.

An extension of these studies is shown in Figure 9, where carbon, vanadium and nickel fingerprints are presented for two-common porphyrin compounds found in petroleum samples (47–61). From this figure, one main advantage using ICP-MS as a detection technique can be observed. We can use the 13C isotope signal like a flame ionization detector to track carbon evolution simultaneously with vanadium and nickel. From Figure 9, it is easy to see that both elements are related because they have the same retention time. The same approach can be extended to any feedstock, and as an example, in Figure 10 the signatures for the three elements from North American deep-cut VGO sample are shown. As can be seen in Figure 10, it is possible to obtain a rapid assessment of the carbon distribution and at the same time, at which temperature vanadium and nickel start to elute. Additionally, the diversity of vanadium and nickel compounds present that are distributed in a particular boiling point range can be seen. Prior running this analysis, the bulk metal content was determined by ICP-OES, and later the signal intensity correlated with the concentration. It can be seen for example, that both metals start to be detected at 1040 °F, with several vanadium compounds 100

detected up to 1200 °F while for nickel, most of the compounds could be found up to approximately 1140 °F. This information is critical because it can inform the process engineers of the optimal temperature ranges for the distillation cut.

Figure 9. Vanadium and nickel octaethyl-etioporphyrin signature. 101

Figure 10.

13C, 51V

and 60Ni signature on North American deep-cut (850-1150 °F) VGO.

Four deep-cut VGO’s obtained for crude oils from four different sources were analyzed by this technique. Figure 11 shows the results for vanadium distribution as a function of boiling point. It is quite evident that the vanadium distribution and species vary depending on the parent crude. As an example, for VGO2 and VGO3, the vanadium start to be detected around 1015 °F, with VGO2 being a narrower distillation cut in comparison to VGO3 (1015-1100 °F). Additionally, VGO3 contains more vanadium compounds distributed over a wider range than VGO2. Moreover, for VGO1 and VGO4, it can be seen that they contain higher boiling vanadium compounds based on their initial boiling points (around 1040 °F). On the other hand, the vanadium distribution for VGO1 spans from 1050 °F until 1190 °F whereas VGO4 contains vanadium compounds boiling above 1200 °F, close to the limit of chromatographic separation capability for this technique (1250 °F). Significant differences in the vanadium distribution as a function of the feed are observed, essential information to have prior to distillation. The power of this characterization technique for element distribution is clearly demonstrated. The same kind of information was obtained for nickel (data not shown). In general, nickel distributions show the same trends described for vanadium. To have some idea of potential metal species that could be present in these feeds, an initial vanadium and nickel speciation activity was undertaken. By using commercial vanadium and nickel etioporphyrin compounds, examples of the metal compounds commonly found in petroleum (47–61), and tetraphenyl vanadyl porphyrin, that has high MW and boiling point, were analyzed using 102

the same experimental conditions. In Figure 12, the results for the vanadium compounds are overlaid with the VGO2 sample, which comes from a North American crude. As observed, the retention time of the etioporphyrin compounds matches at least two of the peaks present in the feed.

Figure 11.

51V

signature on four deep-cut (850-1200 °F) VGO’s.

Figure 12. Vanadium speciation on crude deep-cut VGO2. 103

The initial result suggests that octaethyl-vanadyl porphyrin would elute around 1090 °F and the ethyl-methyl derivative would elute a lower boiling point. Increasing the molecular weight of the porphyrin from 599.70 g/mol to 679.66 g/mol like tetraphenyl-vanadyl porphyrin would result in detection around 1175 °F. A critical aspect is that tetraphenyl vanadyl porphyrin is not naturally present in crude petroleum oil. The same findings were observed in other VGO cuts from different crude oil sources. Another critical piece of information towards complete vanadium speciation is that moving from the octaethyl porphyrin species (MW: 599.70 g/mol) towards the ethyl/methyl porphyrin structure (MW: 543.61 g/mol) there is a decrease in the boiling point of about 5 °F. This is an indication that dealkylation of the porphyrin structure would lead to vanadyl compounds having a lower boiling point. More effort to characterize vanadyl and nickel porphyrins compounds based on boiling point is in progress. An additional practical application is the monitoring of feed quality for a hydroprocessing unit. Over three months, the vanadium and nickel distributions as a function of the boiling point were monitored for quality control purposes. As shown in Figure 13, vanadium and carbon distributions were monitored and plotted on the same graph. It is noticed that in general, the vanadium distribution remains almost the same (from 1050 °F to 1200 °F). However, it was observed that in January, the feed showed a carbon distribution displaced to the higher boiling point than that observed in the previous month.

Figure 13. Monitoring 13C, 51V signature of product from a hydroprocessing unit.

Information like can be used to determine any potential process variation as well as potential indicators of catalyst activity/selectivity based on the type of feed entering to the unit. 104

The information presented in the preceding section is qualitative, and efforts oriented in the quantification are in progress. Determination of vanadium and nickel limit of detection (LOD) and limit of quantification (LOQ) are contemplated as well as the figures of merit. Challenges associated with a robust quantification, such as mass recovery, quantification strategy among others have been identified. Work addressing these points is in progress. HTGC-ICP-MS has shown to be a powerful technique for the characterization of element distributions as a function of boiling point. The examples illustrate the potential of this technique, and it can be extended to other elements such as arsenic, selenium, iron, sulfur, iodine among others to help to understand the fate of metals found in petroleum. Indeed, experiments performed in our facilities confirm that metals can be monitored routinely as a part of product quality control. Still, other potential applications of this technique are currently being developed and will be reported elsewhere.

4. Conclusions High-temperature GC-ICP-MS (HTGC-ICP-MS) has shown to be a robust and reproducible technique for the analysis of metals in VGO fractions. The technique can efficiently determine the presence of elements such as vanadium and nickel as a function of boiling point. This information is key to determining the cut point that provides a higher yield of VGO while not exceeding the permissible metals content. The technique can be used for monitoring of the quality of feed entering hydroprocessing units, providing information regarding the quality of the feed that the catalysts will be converting to products. With this information, some predictability of catalyst performance can be obtained. A critical outcome of this development is that vanadium and nickel can be present as different compounds that could have a narrower or broader boiling point range depending on the type of crude. Initial speciation analysis reveals that some porphyrin structures such as etioporphyrins are present in a VGO cut originated from a North American crude. Further work will reveal the relationship between porphyrin structure and boiling point.

Acknowledgments Chevron Energy Technology Company is acknowledged for funding and permission to publish this work

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Section II: The Boduszynski Continuum Model

Chapter 6

The Compositional and Structural Continuum of Petroleum from Light Distillates to Asphaltenes: The Boduszynski Continuum Theory As Revealedby FT-ICR Mass Spectrometry Martha L. Chacón-Patiño,1 Steven M. Rowland,1,2 and Ryan P. Rodgers*,1,2,3 1National

High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States 2Florida State University Future Fuels Institute, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States 3Department of Chemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States *E-mail: [email protected].

Forty years ago, Boduszynski published the first part in a series of manuscripts that addressed the composition, and compositional progression of heavy oil. It concluded that crude oil composition increases gradually and continuously in aromaticity, molecular weight, and heteroatom content as a function of boiling point. Remarkably, the Continuum Model was inferred from field ionization mass spectral data that lacked the requisite resolution to uniquely identify elemental compositions across the observed mass range. However, combined with boiling point trends of light distillate classes, molecular weight distributions, boiling point cuts, and chromatography, Boduszynski assembled a series of manuscripts that described the Continuum Model of Petroleum. Herein, we revisit the same topic and employ the same analytical methods, only now armed with state-of-the-art Fourier Transform Ion Cyclotron Resonance Mass Spectrometers (FT-ICR MS) that can readily resolve and uniquely identify

© 2018 American Chemical Society

molecular formulas to tens-of-thousands of individual petroleum species in a single analysis. We tested the Continuum Model for hundreds-of-thousands of species identified by mass spectrometry, from light distillates to asphaltenes, and found no deviations. In the process, we collected data that supports the low molecular weight of petroleum (< 2000 Da), defines the maltene continuum, highlights the effect of aggregation on mass spectral analysis, identifies and overcomes selective ionization, and confirms that asphaltenes are composed of abundant island and archipelago structures. In this chapter, we review FT-ICR MS petroleum characterization efforts that led to the conclusion that the Boduszynski Continuum Model is correct and thus applicable to both the compositional and structural continuum of petroleum.

1. Introduction The effective conversion of petroleum distillates into valuable products through refinery processes requires comprehensive knowledge of structure and composition (1). The task is not simple; it requires combined efforts from several disciplines: chemical separations, analytical instrumentation, petroleum/organic chemistry, and “big-data” analysis (2–4). The difficulties largely arise from the increased molecular complexity of the high-boiling fractions. As the boiling point increases, the molecular weight distribution broadens and the heteroatom content increases (5). Thus, the compositional complexity of high-boiling distillates, “non-distillables”, and asphaltenes is immense, and thus requires the use of ultrahigh-resolution Fourier Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR MS) to obtain accurate molecular level information not provided by “bulk” techniques such as elemental analysis or nuclear magnetic resonance spectroscopy (6). FT-ICR MS, discovered 45 years ago by Marshall and Comisarow (7, 8), has enabled a detailed molecular-level understanding of the composition of petroleum (9). In the early 2010s, FT-ICR MS demonstrated that carbon number, aromaticity, heteroatom content and complexity, gradually and continuously increase as a function of boiling point, known as the Boduszynski continuum (10). Remarkably, Mieczyslaw M. Boduszynski proposed this compositional continuum principle nearly 30 years prior. He was able to overcome the limitations of analytical instrumentation at that time and proposed a unified model to describe the molecular composition of petroleum distillate fractions (5). Boduszynski based his model on atmospheric equivalent boiling point (AEBP), bulk elemental analysis, and molecular weight measurements by field desorption / ionization low-resolution mass spectrometry (5, 11). He proposed rules to describe the dependence of boiling point on molar mass and elemental composition of petroleum species. Summarized, the Boduszynski model dictates that diverse compounds with similar molecular weight cover a broad range of boiling point; conversely, a narrow distillation cut contains a 114

broad molar mass range (5, 11). Figure 1 illustrates the principle, as proposed by Boduszynski (5). For a given homologous series, for example, paraffins (CnH2n+2) shown on the far left, the boiling point increases continuously as a function of molecular weight. The same is true for all homologous series, however, compounds that contain naphthenic rings, fused aromatic moieties, and/or heteroatom-containing functionalities, are offset to higher boiling point. Thus, for a defined boiling point range (x-axis), the compounds with the highest molecular weight are paraffins, followed by naphthenic species, aromatic hydrocarbons, polar heteroatom-containing compounds, and finally polar/polyaromatic species. Armed with homologous series plotted in such a manner, combined with molecular weight distributions obtained by FD-MS, Boduszynski realized that for a defined boiling point, the vertical progression from the saturates downward, one compound family to the next (Figure 1), resulted in a decrease in ~2-3 carbon atoms per molecule. Hence, S- and N-containing aromatics should exhibit ~2-3 fewer carbon atoms than their hydrocarbon counterparts. Boduszynski predicted that for a given boiling point, a polycyclic aromatic compound, substituted with multiple heteroatomic functionalities, would have ~6-7 fewer carbon atoms than the analogous hydrocarbon. This premise captured two remarkable compositional features of petroleum. First, as the boiling increases, the mass range will broaden. Second, on the basis of bulk H:C ratio, MS data, and heteroatom concentration, the molecular weight of asphaltenes (operationally defined as the petroleum fraction insoluble in n-pentane/n-heptane but soluble in toluene/benzene) should not exceed ~2000 g/mol (5, 12–14). The Boduszynski model reveals compositional trends that demonstrate that the terms “heavy” and “high boiling” are incorrectly used as synonyms for “high molecular weight.” Boduszynski demonstrated that the high boiling point of distillation residues is a consequence of increased intermolecular interactions, derived from a higher concentration of polyaromatic moieties and heteroatom-containing functionalities capable of acid/base interactions and hydrogen bonding. Thus, Boduszynski challenged the idea that high boiling equaled high molecular weight. He went on to address the “non-distillable” nature of distillation residues/asphaltenes and used chemical separations to extend the characterization of petroleum. In 1987, Boduszynski found that once a distillation residue is separated, specific solubility fractions are, in fact, distillable. Boduszynski and coworkers hypothesized that all compounds in crude oil (those not in an aggregated state) are soluble in heptane and only aggregated species exhibit insolubility (5, 15–17).

2. FT-ICR Mass Spectrometry Enables a Definitive Test of the Boduszynski Continuum Model Petroleomics emerged in the early 2000s as a Proteomics counterpart, with the goal to predict the behavior and economic value of petroleum from detailed molecular-level information (18–21). The term was jokingly suggested by Dr. Carol Nilsson, in a hallway conversation at the National High Magnetic Field Laboratory, but the term stuck and has been used regularly ever since. 115

Petroleomics was not a new idea at the time the term was coined; it is rooted in the work of Quann and Jaffe (17), and was renewed with the recent developments in high field FT-ICR MS and soft ionization techniques. The ultra-high resolving power and mass accuracy, only accessible by FT-ICR MS, allow forif she the baseline resolution and the molecular formula assignment of tens of thousands of compounds within a single petroleum sample. Such performance is crucial, as complex fractions derived from heavy oils, such as asphaltenes, can exhibit more than 500 peaks in less than one nominal mass. Ultrahigh resolving power (m/Δm50% > 750,000 at m/z 500) is critical for the resolution and the accurate identification of isobaric peaks, which differ in composition by, for example, 12C4 versus SH313C and 12C3 versus SH4, which differ in mass by 1.1 and 3.4 mDa (19). In routine FT-ICR MS analysis of heavy petroleum derived fractions, it is possible to resolve and confidently assign elemental compositions to over 25,000 mass spectral peaks in a single sample, by one ionization method (22). By taking advantage of the mass accuracy and the spacing patterns between spectral peaks, CH2 (14.01565 Da) and H2 (2.01565 Da) units, it is possible to deconstruct the compositional complexity into hundreds of overlapping homologous series (23).

Figure 1. Effect of molecular structure on boiling point. Reproduced with permission from reference (5). Copyright 1987 American Chemical Society. (see color insert) 116

Figure 2. Top panel: Positive APPI 9.4 T FT-ICR mass spectrum of the interlaboratory sample Petrophase 2017 Asphaltenes, with characteristic spacing patterns of 14.01565 Da and 1.01565 Da. Bottom panel, left: High resolving power, mass accuracy, and spacing trends allow for the elemental assignment of thousands of peaks. Bottom panel, right: Representation of molecular level information by a color-coded isoabundance contoured plot of DBE versus carbon number; the relative abundance is normalized within the compound class N1, which refers to all the species that contain C, H, and only one N atom. (see color insert)

Figure 2 presents a positive-ion Atmospheric Pressure Photo-ionization (APPI) FT-ICR mass spectrum of an interlaboratory asphaltene sample (Petrophase 2017 Asphaltenes), (Top panel) (24). A mass spectral zoom inset exposes two predominant spacing patterns, given by additions of CH2 and H2 (23, 25). Specialized software algorithms take advantage of the mass accuracy provided by FT-ICR MS and the spacing patterns to generate peak lists with the assigned elemental compositions (Bottom panel, left) (26). The vast amount of molecular level information, derived from a single mass spectrum, is represented in color-coded isoabundance contoured plots of double-bond equivalents (DBE, number of rings + double bonds in a molecule) versus carbon number (Bottom 117

panel, right) for a specific class of molecules. DBE is plotted on the y-axis and is a measure of the aromaticity of the compounds (DBE = C – H/2 + N/2 + 1), where C, H, and N are the numbers of carbon, hydrogen, and nitrogen in the molecular formula. Thus, a higher DBE denotes a higher aromaticity. Carbon number is plotted on the x-axis; and at constant DBE, an increasing carbon number denotes a greater CH2 content, hence increasing the extent of alkyl substitution. The color scale represents the relative abundance, which is usually normalized within each compound class (27, 28). Several publications highlight the advantage of DBE versus carbon number plots to compare and sort samples, track the effect of refinery process on petroleum-derived materials, and evaluate the performance of chromatographic separations applied to petroleum (29–33).

3. Compositional and Structural Continuum of Petroleum Revealed by FT-ICR MS 3.1. A Molecular-Level Test of the Boduszynski Model Through the FT-ICR MS Analysis of Athabasca Bitumen HVGO Distillates A conclusive demonstration of the Boduszynski continuum model requires ultrahigh resolution mass spectrometry to determine the elemental composition of each ionizable compound from petroleum distillation fractions (10). Impressively, based on low-resolution mass spectrometry data, Boduszynski proposed a general model to describe the composition of petroleum, with the general premise that different compounds with similar molar masses occupy a wide boiling point range. Conversely, a narrow boiling cut can exhibit a wide molar mass range (5). Figure 3 illustrates the progression of the compositional space of the hydrocarbon class from Athabasca Bitumen heavy vacuum gas oil (HVGO) distillates. From the initial boiling point (IBP)-343 °C fraction to the boiling cut 500-538 °C, there is a sequential increase in the abundance weighted average carbon number from 20 to 40 carbon atoms. A close examination of the carbon number distribution reveals an important compositional feature, more evident for the distillation cuts 375400 °C, 400-425 °C, 425-450 °C, and 450-475 °C: homologous series with high DBE are slightly shifted toward lower carbon number. This behavior illustrates a fundamental principle of the Boduszynski model: within a distillation cut, high DBE species must exhibit lower carbon number and vice versa (5, 10).

3.1.1. High Boiling Point Due To Increased Heteroatom Content Figure 4 shows the contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for the Athabasca Bitumen distillation cuts 343-375 ºC, 375-400 ºC, 400-425 ºC, and 425-450 ºC. It illustrates the effect of increased heteroatom content on the progression of carbon number within a distillation cut. Each sulfur addition, HC class to S1, and S1 to S2, leads to a decrease of ~2-3 carbon atoms within a given boiling cut. It is important to consider that compounds that exhibit a higher aromaticity or higher content of heteroatoms such as S, N, and O exhibit stronger attractive intermolecular forces and thereby concentrate in 118

high boiling cuts, but the increased heteroatom content lowers their carbon number relative to the HC class (5, 34). For example, the class HC of the distillation cut 425-450 °C, exhibits an abundance-weighted average carbon number of 30. The addition of one sulfur atom decreases the average carbon number to 28; finally, the addition of a second sulfur atom decreases the average carbon number to 26 (10).

Figure 3. Color-coded isoabundance contoured plots of DBE versus carbon number for the hydrocarbon (HC) class for Athabasca bitumen HVGO distillate cuts (IBP-343 °C to 500-538 °C). Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

The Boduszynski continuum model predicts that for a given carbon number range, the addition of heteroatomic functionalities should increase the boiling point (5, 11, 13). Figure 5 demonstrates this premise. For a fixed range of carbon number (~20-35), each additional sulfur atom increases the boiling point by approximately 25 °C. Thus, the classes S1 and S2 boil between 400-425 ºC and 425-450 ºC. Importantly, both S2 and S1O1 classes are di-heteroatomic; however, the increased dipole moment due to oxygen (increased polarity) leads to a higher boiling point for the O1S1 species, which also exhibit much lower DBE values than the HC and S1-2 counterparts. Boduszynski predicted that a higher concentration of heteroatoms, such as N and O, promotes stronger intermolecular associations, such as hydrogen bonding and acid/base interactions, which result in an increased boiling point (5, 10). 119

Figure 4. Color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for Athabasca bitumen HVGO distillate cuts (343-375 °C, 375-400 °C, 400-425 °C, and 425-450 °C). Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

3.1.2. High Boiling Point Due To Heteroatom-Containing Functionalities Electrospray ionization (ESI) performed in negative-ion mode enables access to polar/acidic species, capable of hydrogen bonding (35–37). Figure 6 presents a comparison of the compositional space of the HC and S1 classes accessed by positive APPI, and the class O2 by negative electrospray ionization, for the distillation cuts 425-450 °C and 475-500 °C. Boduszynski and Altgelt proposed that for a given distillation cut, the presence of acidic functionalities must decrease aromaticity and carbon number (13, 38, 39). Thus, within a given boiling range, O2-containing species contain fewer carbon atoms and display much lower DBE values (aromaticity) than non-oxygenated species. For instance, the distillation cut 475-500 °C contains O2 species that exhibit a marked reduction in aromaticity, from DBE = 9 to DBE = 5, compared with the hydrocarbon class. A decrease in 3-4 units of DBE suggests that O2 species could have ~1 aromatic ring and/or ~1-4 naphthenic rings less than their hydrocarbon counterparts, which decreases the strength of London dispersion forces and potential π-π interactions (5, 10). However, the loss in DBE is offset by the ability to hydrogen bond. 120

Figure 5. Color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC, S1, S2, O1S1 with a fixed range of carbon atoms (~20-35) for Athabasca bitumen HVGO distillate cuts (343-375 °C, 375-400 °C, 400-425 °C, and 425-450 °C). The increased heteroatom content (S1 and S2) and finally the dipole moment induced by oxygen (O1S1) progressively increase the boiling point range at a constant carbon number range. Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

Figure 6. Color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC (positive APPI), S1 (positive APPI) and O2 (negative ESI) for the distillation cuts 425-450 °C and 475-500 °C. Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert) 121

3.1.3. The Continuous Progression of DBE Values as a Function of Increasing Boiling Point Demand Naphthenic Structures The characterization of “non-distillable” heteroatom-containing petroleum compounds, necessitates a comprehensive understanding of the molecular structure of species from low-boiling cuts. Boduszynski and Altgelt hypothesized that the continuum model, rooted in the “distillable” petroleum fractions, could be linearly extended to explain the molecular composition of distillation residues and asphaltenes (13, 38, 39). Figure 7 illustrates an application of the Boduszynski continuum model to understand petroleum structure. Figure 7 (top row) highlights the progression of DBE and carbon number for the class S1 as the boiling point increases for Athabasca Bitumen distillation cuts (10). For particular distillation cuts, homologous series with DBE values of 3, 6, 9, and 12 exhibit a prominent relative abundance when compared with intermediate values such as 2, 4, and 5. It is well known that S1 species with DBE of 3 are most likely alkylated thiophene derivatives. The addition of one and two benzene rings yields benzothiophene (DBE = 6) and dibenzothiophene (DBE = 9) homologous series, which are well recognized as abundant (stable) compounds in distillation cuts from sulfur-enriched crude oils (40–42). Further addition of benzene rings, following a catacondensed growing pattern (structures a and b, Figure 7 bottom panel), produce core structures with DBE 12 and 15 (43–45). The bottom DBE versus carbon number plot in Figure 7 is the combination of the top panel plots; it contains the molecular composition of the S1 species for all the HVGO cuts derived from Athabasca bitumen. Interestingly, the combined plot does not exhibit a prominent abundance of “magic” DBE values (DBE = 3, 6, 9, 12, 15, 17, …). Given that alkenes are not native structures in virgin crude oils, the intermediate DBE values of 4-5, 7-8, and 10-11, must be derived from cycloalkane ring addition to the aromatic core structures (46–48). Figure 7 is evidence of the tremendous structural diversity of petroleum and emphasizes that the degree and abundance of cycloalkane substitutions cannot be ignored in the structural continuum. The fact that high-boiling distillation cuts and vacuum residues yield considerable amounts of distillable 1-4 ring alkyl- and cycloalkylaromatics after thermal cracking processes, suggests that island-type structures are not always the dominant or sole structural motif in petroleum. A more indepth discussion about the debate of island- versus archipelago-type structures is found later in this chapter. The structural diversity of petroleum, highlighted in Figure 7, suggests the potential existence of species with multiple aromatic cores bridged by cycloalkane or heteroatom-containing 5-membered rings. If this hypothesis is correct, thermal cracking and/or mild pyrolysis would break these alkyl / cycloalkyl bridges to produce distillable products extensively reported for high-boiling distillation cuts, petroleum asphaltenes, and vacuum residues (10, 49–52). Thus, if the Boduszynski Model is correct, the structural diversity of these distillates provides insight into abundant structural motifs present in higher boiling distillates and residua.

122

Figure 7. Upper panel: Progression of the compositional space of the class S1 with an increase of boiling point for Athabasca Bitumen HVGO distillation cuts (343-375 °C, 375-400 °C, 400-425 °C, and 425-450 °C). Lower panel: Combined DBE versus carbon number plots for the class S1 for all Athabasca Bitumen distillation cuts. The lack of the distinctive DBE “magic” numbers, combined with the absence of alkenes in virgin crudes, requires the presence of naphthenic rings. Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

3.2. Extending the Boduszynski Continuum Model to the Limit of Distillation: FT-ICR MS of Heavy Vacuum Gas Oils from a Middle Eastern Heavy Crude Oil 3.2.1. Progression of Molecular Weight, Heteroatom Content, Carbon Number, and Aromaticity from 371 °C to the Limit of Distillation Up to this point, there is a clear correlation between the boiling range of light distillates and the continuous progression of carbon number, aromaticity, and the N, S, and O content of heteroatom-containing functionalities. The analysis highlighted the importance of cycloalkane substitution, through the disappearance 123

of “magic” DBE numbers as boiling point increased. In this section, there is a focus on distillation fractions that boil above 371 °C in an effort to demonstrate extension of the continuum model to the limit of distillation. Figure 8 (left) presents the molecular weight distributions (MWD) of the distillation cuts 371-510 °C, 510-538 °C, 538-593 °C and 593+ °C from a Middle Eastern Heavy crude oil. Figure 8 demonstrates that the center of the molecular weight distribution shifts towards higher m/z values as the boiling point increases. Although very similar to the results Boduszynski initially used to develop his model, the analytical capabilities afforded by FT-ICR MS allow assignment of the elemental compositions to nearly all (> 95%) mass spectral peaks detected. Combined with advances in soft ionization sources such as ESI, APCI and APPI, and continued progress of ultrahigh-resolution mass spectrometry, modern analytical methods allow for a definitive test of the Boduszynski Model to the limit of distillation and into the “non-distillables” (2, 5, 53, 54). Figure 8 also shows the heteroatom class distribution for all ionized/detected species with a relative abundance above 1%. In general, there is a decrease in the abundance of low-heteroatom containing compounds with an increased boiling point. Concurrently, the relative abundance of poly-heteroatomic classes such as N1S1, S2, and S3 increases as a function of boiling point. As postulated in the Boduszynski model, heteroatoms such as sulfur, nitrogen, and oxygen increase the strength of intermolecular interactions; thus, high boiling distillation cuts and residues are enriched with polyheteroatom-containing species (5, 13, 39, 55). Figure 9 presents the contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for the distillation series and the residue from the Middle Eastern heavy crude oil. For the hydrocarbon class, the progressive increase in boiling point yields a gradual increase in the abundance weighted average of carbon number and DBE. The compositional progression starts with the distillation cut 371-510 °C, with an average carbon number of 36 and DBE = 10. The following two fractions (510-538 °C and 538-593 °C) exhibit an increase of 6 and 16 carbon atoms, with only a slight increase in 2 DBE values. Finally, the “non-distillable” residue (593+ °C) is notably “heavier” with an average carbon number of 63, and more aromatic, with an average DBE of 16, as it is not a true boiling “cut”. The average increase of 6 DBE units suggests the addition of 2-fused aromatic rings to the core structures. Greater numbers of fused-aromatic rings lead to stronger intermolecular interactions, such as π-π stacking, which result in a higher boiling point (56, 57). The Boduszynski continuum model also predicts that distillation residues (and asphaltenes) have high boiling points due to a greater concentration of heteroatom-containing species that promote stronger interactions and nanoaggregation, and not necessarily because of higher molecular weight (5, 38, 39). Figure 9 also illustrates the impact of heteroatom content in the carbon number of high boiling point distillates. The effect of the addition of one and two sulfur atoms is more pronounced for the residue 593+ °C. The addition of two S-containing functionalities decreases the abundance weighted average carbon number from 63 to 56, as hypothesized by Boduszynski (5, 55). Thus, the Boduszynski Continuum Model accurately describes the molecular progression of high boiling and non-distillable species. 124

Figure 8. Molecular weight distributions (MWD) derived from positive APPI FT-ICR mass spectrometry (Left), and heteroatom class distribution (Right) of the distillation cuts 371-510 °C, 510-538 °C, 538-593 °C and 593+ °C from a Middle Eastern Heavy crude oil. Reproduced with permission from reference (55). Copyright 2010 American Chemical Society. (see color insert)

3.2.2. Implications of the Boduszynski Continuum Model for the Molecular Composition of Asphaltenes The Boduszynski continuum model predicts that the molecular weight of most petroleum compounds is less than 2000 g/mol. The reason for the “non-distillable” nature of asphaltenes is rooted in the increased concentration of aromatics and oxygen/nitrogen-containing functionalities (and perhaps sulfur as sulfides), and not in ever-increasing molecular weight. However, to be consistent with the continuum model, the increase of carbon number and aromaticity for “non-distillable” species should follow a linear extrapolation of the molecular compositions (determined by FT-ICR MS), established by the distillable fractions. The conventional wisdom at the time was to simply extrapolate to greater carbon number, but that is inconsistent with the continuum model proposed by Boduszynski. The discussion below aims to highlight the inconsistency in the extrapolation of DBE and carbon number from distillation data and the observed, molecular-level data from atmospheric / vacuum residues and asphaltenes. 125

Figure 10 illustrates the extrapolation of distillable compositional space (orange line) to an improbably high molecular weight, 1 MDa. The hydrogen to carbon ratio obtained by such extrapolation is ~1.4. Thus, a linear extrapolation of the distillable (maltene) continuum to greater carbon number neither accounts for the widely accepted H:C ratio of asphaltene samples (H:C ~0.90-1.15) nor their recently recognized low molecular weight (55, 58–63). If asphaltenes share the same carbon number range as their maltenic counterparts, their molecular compositional space must be displaced to greater DBE (aromaticity). So, do asphaltenes share the same carbon number range as maltenes?

Figure 9. APPI derived color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for “heavy” distillation cuts 371-510 °C, 510-538 °C, 538-593 °C, and 593+ °C, form a Middle Eastern heavy crude oil. Reproduced with permission from reference (55). Copyright 2010 American Chemical Society. (see color insert) 126

Figure 10. Combined DBE versus carbon number plot for the class S1 for the distillation cuts 371-510 °C, 510-538 °C, 538-593 °C, and 593+ °C, from a Middle Eastern heavy crude oil. The linear extrapolation of the continuum trend (orange dotted line) to higher carbon numbers cannot account for bulk asphaltene composition and confirmed molecular weight (55). However, extrapolation to higher DBE (aromaticity), within the same carbon number range, yields acceptable H:C ratios (red dotted line). Reproduced with permission from reference (55). Copyright 2010 American Chemical Society. (see color insert)

It is important to keep in mind that the history of asphaltene characterization is highlighted by controversy and ambiguous debates on molecular weight and structure (14, 64–66). For more than 50 years of asphaltene research, the Petroleum Community hypothesized that a high molecular weight should account for the “non-distillable” nature, strong aggregation and deposition, and increased content of N, O, S, and V/Ni in asphaltenes (14, 67–71). Boduszynski disagreed, and suggested that they share the same carbon number range as maltenes. Since 2000, results from time-resolved fluorescence depolarization (60, 72), atmospheric pressure mass spectrometry (73–76), and two-step laser mass spectrometry (77, 78), converged on molecular weights between ~250-1200 127

g/mol with an average around at 750 g/mol. Thus, the “low” molecular weight of petroleum asphaltenes, hypothesized by Boduszynski 30 years ago, is correct. Boduszynski provided experimental support of the initial premise of Dean and Whitehead, who speculated that 2000 g/mol was the upper molecular weight limit for most petroleum compounds (79, 80). As previously discussed, Figure 10 shows the rationale for predicting the compositional space of asphaltenes based on a linear projection of the molecular composition of the distillable species. The orange line highlights the abundance weighted average of H:C ratio of the S1 species in all distillation cuts and the 593+ ºC residue from Middle Eastern Heavy crude oil. The extrapolation of the orange line to molecular weights greater than 1 MDa results in a decrease in H:C as a function of increasing carbon number. However, at 1 MDa, the projected H:C ratio is ~1.39, which is considerably greater than the measured bulk H:C ratio for asphaltenes (0.90 800,000 at m/z = 500) and mass accuracy less than 1 ppm. Moreover, it must be performed quickly, on a chromatographic time scale. Such capabilities are only currently possible at the highest magnetic fields available for FT-ICR MS, those equipped with 21 T superconducting magnets (162). Successful demonstration of online HPLC/MS of petroleum samples was recently presented and exposes a wealth of structurally defined, molecular-level information in continued support of the Boduszynski Model (163–165). The mass spectral total ion chromatogram (TIC) was shown to match the evaporative light scattering detector response, but is the sum of ~1400 high resolution mass spectra acquired over the 80 minute LC separation. Abundant species are detected for 158

all the structures targeted in the analysis: saturates, 1-ring, 2-ring, 3-ring, 4-ring, 5+ring/polars, and finally, sulfides. Summation of the mass spectra across each structurally defined elution windows recapitulates the results obtained by off-line fraction collection followed by MS analysis and exposes the Boduszynski defined trends highlighted earlier in this chapter. Specifically, the ~3 carbon shift to lower carbon number between the HC and S1 class (within the same ring fraction), and within the HC and S1 classes, there is a ~30 carbon shift (to lower carbon number) as one progresses from the saturates (HC), sulfides (S1 class) through the increasing ring number fractions, to the 5+ring fraction. Such analysis captures the Boduszynski defined relationships between tens-of-thousands of petroleum species, across multiple heteroatom classes, in a structurally defined manner, in a single analysis. Most exciting, the compositional information provided by the method exposes the Boduszynski trends within a ring class. The 1-ring aromatic elution period begins with low DBE species and ends with species 5 DBE higher (the addition of 5 naphthenic rings), but as Boduszynski predicted, the higher DBE species contain 10 fewer carbons than those at lower DBE. Thus, all of the previously presented molecular-level results are now attainable within a ring class, as well as between ring classes, and the entire experiment takes less than 2 hours.

Acknowledgments The work presented herein is attributed to the petroleum scientific community, and specifically to all of the authors and coauthors of the referenced manuscripts. It was made possible by the gracious and continued support of Dr.’s Parviz Rahimi and Andrew T. Yen who provided invaluable samples and discussions over the years. Work supported by NSF Division of Materials Research (DMR- 1157490), the Florida State University, the Florida State University Future Fuels Institute, and the State of Florida. We would like to give a special thanks to Marianny Y. Combariza for providing the South American crude oil and Pierre Giusti and the organizers of Petrophase 2017 for providing the Petrophase 2017 asphaltene.

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Chapter 7

Plausible Locus for Large Paraffinic Compounds in the Boduszynski Continuous Composition Petroleum Model Lante A. Carbognani Ortega* Catalysis and Adsorption for Fuels and Energy, University of Calgary, Calgary, Alberta T2N 1N4, Canada *E-mail: [email protected].

Literature presenting evidence on the existence of petroleum large molecular weight paraffinic compounds spanning the C90-C215 carbon atoms is reviewed. The use of conventional handling and characterization techniques of the oil samples could lead to inadequate or misleading information. High temperature liquid chromatography isolation and characterization techniques and the use of soft ionization mass spectrometry techniques were shown mandatory for their analysis. Both positive and negative properties over petroleum fractions provided by such hydrocarbons were identified. A plausible locus within the Boduszynski continuous molecular composition petroleum model is advanced.

Introduction The existence of large molecular mass paraffinic compounds was known decades ago, mostly related to oil field deposits. Alkanes in the vicinity of C57 (i.e., carbon atoms/molecule) were reported (1). One seminal article published in 1985 by Dr. Boduszynski reported on the existence of large cycloparaffins (“naphthenes” ) spanning up to C90, as illustrated in Figure 1 (2). Adoption of soft mass spectrometry ionization techniques such as Field Ionization Mass Spectrometry (FIMS) and Field Desorption Mass Spectrometry (FDMS) allowed for the detection of these compounds that otherwise were routinely fragmented when analyzed with the standard high energy electron impact ionization techniques, widely followed by most researchers at that time. Two important © 2018 American Chemical Society

concepts were advanced in this article (2): 1) The inadequate and/or misleading information provided by average structures of hydrocarbon complex mixtures; 2) The continuous distillation patterns determined for petroleum components, where distillation properties were influenced by the weak and strong intermolecular forces affecting molecules (London, dipole-dipole, dipole-induced dipole and H-bonding).

Figure 1. FIMS analysis of the 1150-1369°F saturates fraction isolated from Kern River Petroleum. Reproduced with permission from reference (2). Copyright 1985 American Chemical Society. Alkane mixtures are indistinctly identified like “paraffins”, “saturates” and “waxes” in the oil-related literature, the latter term describing large isomers able to crystallize under ambient conditions. Paraffins interact only via weak intermolecular forces (London) and thus, are the largest compounds present in a defined distillation cut that comprises additional low Molecular Weight (MW) polar, acidic and basic compounds. The preceding aspects were thoroughly covered in the well-known monograph on heavy petroleum fractions published by Drs. Altgelt and Boduszynski in 1993 (3).

Recently Published Evidence Supporting the Existence of Large MW Alkanes in Petroleum As mentioned in the previous section, FDMS proved to be a key analytical technique for the characterization of large MW paraffins, a fact recognized by authors studying the nature of waxes (4), and sludges containing high MW alkanes (boiling points above 1,250°F) (5). Results presented by Musser and Kilpatrick suggested the possible existence of paraffins spanning the C70-C215 174

range (i.e.,~1,000-3,000 a.m.u.), as illustrated in Figure 2 (4). Thomson et al. showed the existence of alkanes spanning up to C62 (Z series +2) together with monocycloalkyl (Z series +0, up to C62) and monoaromatic waxes (Z series -6, up to C81) within 1,250°F+ waxy sludges isolated from the USA Strategic Petroleum Reserve (US-SPR) (5). These results are deemed interesting because the later discussion in this chapter will point out that mixtures of these hydrocarbon families often occur within waxy paraffinic deposits. Figure 3 illustrates the findings reported by Thomson et. al. (5)

Figure 2. FDMS Spectrum for San Joaquin Valley waxes, showing microcrystalline character. Reproduced with permission from reference (4). Copyright 1998 American Chemical Society.

Figure 3. FDMS spectrum of >1250°F wax from Cavern B. Reproduced with permission from reference (5). Copyright 1989 American Chemical Society. 175

The advent of commercial High Temperature Gas Chromatography (HTGC) at the beginning of the 1990s, allowed researchers to apply the technique to the study of waxy deposits. Thomson et al. reported the existence of waxy components larger than C100 within the >600°C fraction isolated from one US-SPR storage cavern, as presented in Figure 4 (6). HTGC was widely applied by other research groups during the 1990s decade to study waxy solid deposits (7–10). Paraffinic compounds up to C70 were reported in these studies. The use of unheated autosamplers despite temperature programmable cold on-column injectors for sample introduction into the GC was described, is believed by the author of the present chapter as the reason precluding the detection of larger MW components. The preceding practice induced losses of large paraffinic compounds which adhered to the walls of vials and/or syringe tips since warm and diluted conditions were not carefully maintained. One interesting feature of waxy solid deposits studied in these articles is the existence of bimodal and even multimodal compound distributions (7–10), feature also observed by the author when relying on High Temperature Size Exclusion Chromatography (HT-SEC) of solid deposits retrieved from paraffinic oil storage tanks (11). One further important aspect described by Biao et al. pertains to the use of hot xylene for extraction of large MW waxes (9). The preceding discussion emphasises the importance of high temperature as a fundamental parameter for isolation and analysis of large petroleum paraffinic compounds.

Figure 4. HTGC chromatogram of a > 600°F wax from Cavern B sludge. *C100 appended over the chromatogram based on the reported data. Reproduced with permission from reference (6). Copyright 1992 Wiley-VCH Verlag GmbH & Co. KGaA. Researchers from India provided an important body of information over solid sludges sampled from the bottom of paraffinic oils storage tanks in the 1990s decade (12–15). Combined separation/spectroscopy techniques like distillation, solvent precipitation, chromatographic separation, solvent extraction, urea adduction, simulated distillation, elemental analysis, and Nuclear Magnetic Resonance spectroscopy (NMR) were used by these authors to study the nature 176

of the studied solids. A summary of the most important results for the aims of the present article ensues: -

-

-

Paraffinic components spanning the C70-C125 carbon range were described, the largest showing melting points above 90°C High proportions of n-alkanes (60-90 wt%) were determined in selected studied fractions via urea adduction Cycloalkane and aromatic waxy components were observed present in the waxy solids. Concomitant hydrogen deficiencies were determined within these fractions Combined presence of naphtheno-aromatic structures (i.e., hydrogen deficient compounds) plus the high complexation capacity of urea adduction, indicate the possibility that molecules long alkyl appendages were included within urea host helices, leaving outside their non-linear moieties Bimodal and even trimodal wax distributions were observed in some of the studied cases

The preceding results are considered important because in the experience of the author of the present article, these coincide in many aspects with results gathered for paraffinic solids studied in relation to oil production and storage operations within the Eastern and Maracaibo basins in Venezuela, during the 1990s. These will be addressed in the next section.

Large MW Saturates Isolation via High Temperature Liquid Chromatography (HTLC) and Characterization via High Temperature Size Exclusion Chromatography (HT-SEC) A systematic study of oilfield solid deposits indicated the necessity of developing isolation techniques that guarantee samples integrity, i.e., achieve complete recovery of hydrocarbon components (16). High temperatures were determined necessary for waxy deposits, which often showed high melting temperature ranges spanning the 90-100ºC, thus requiring solvents like i-octane or xylene for their complete extraction. The final protocol set up for complete solid wax recovery from field deposits was reported by Garcia (17), comprising sequential Soxhlet extraction with solvents of increasing boiling temperatures: I) CHCl3/MeOH:95/5 vol., II) toluene/2-propanol:95/5 vol., III) xylene/n-butanol:95/5 vol. After wax extraction was achieved, further separation of the saturates fraction from other co-extracted components was accomplished by applying HTLC separation (18). Paraffinic compounds, i.e., alkane soluble saturates fractions were not retained on the column packing and were observed with refractive index detectors as one single initial band eluting from the column with apolar solvents like n-heptane or i-octane. The key for successful isolation of saturates was to be able to dissolve/disperse waxy components in hot solvents to avoid wax crystals to grow and adhere to any surface contacting them (vials, pipettes, syringes…). A rule of thumb for success was to visually observe the 177

absence of cloudiness in the prepared sample solutions. On-column deposition of dissolved samples in hot solvent was carried out over medium-pressure columns packed with different type of adsorbents, as described in detail elsewhere (18). Complete recovery of paraffins was thus achieved by guaranteeing high MW alkanes elution in hot liquid media, i.e., they were not allowed to crystallize under the combined effects of dilution and high temperature. Figure 5 illustrates successful complete isolation of paraffinic fractions carried out under increasing temperatures, up to 90ºC.

Figure 5. Chromatographic elution of waxes from Attapulgus clay packed columns, carried out at different temperatures. n-heptane used for 25, 45 and 75 ºC; i-octane for 90ºC elution.

Interesting behavior of “waxes” isolated from commercial asphalt used in paving operations was observed when applying HTLC separation (19). It was found that these “waxes” comprised compounds bearing naphtheno-aromatic moieties linked to alkyl functionalities (19). The presence of these functionalities on the studied waxes provided them with intermediate polarity properties, between those for pure alkanes and pure aromatics, thus leading these fractions to elute within ranges in-between those typical for saturates/aromatics (19). The preceding findings suggest that complex waxy mixtures like those existing in solid field deposits and also in distillation vacuum residua, often include different hydrocarbon types like mentioned before in the discussion related to Figure 3 (cycloparaffinic waxes) and in relation to waxy tank bottoms gathered in India’s facilities (naphtheno and aromatic-alkanic waxes, addressed in the previous section) (12–15). 178

Setting up high temperatures for handling very large paraffinic components was shown in the preceding discussion to be mandatory for avoiding losses during their isolation and purification. HT-SEC was then developed with the purpose of characterizing large alkane mixtures (20). High resolution silica columns (80Å pores) eluted with hot toluene delivered with highly accurate syringe pumps and very stable-very sensitive evaporative type detectors, allowed the analysis of large wax components. Elution examples for n-paraffin standards, one asphalt paraffinic fraction and a commercial polyethylene wax (“Polywax 655”) routinely used for calibration of high temperature simulated distillation (21), are presented in Figure 6. The included chromatograms visually show the presence of large paraffinic components in the asphalt fractions, reaching up to 113 °C / molecule. The Polywax 655 displays components within the 20-79 carbon atoms, which is the most abundant range observed in ASTM D7169 simulated distillation chromatograms (21); low abundance C79-C110 components normally observed in GC (21), precluded their detection by HT-SEC, indicating this technique to be less sensitive than HTGC for minor components of complex mixtures.

Figure 6. HT-SEC chromatograms for selected samples. *Numbers identified n-paraffins used for calibration purposes. Cn: show the carbon numbers determined at the described elution points. Saturates fractions from different samples were isolated via HTLC as described before (18). Fractions from a paraffinic oil (M12S) were isolated under varying temperatures from a column packed with Attapulgus clay and, further characterized via HT-SEC. Figure 7 presents the HT-SEC results achieved. The whole sample does not show saturates larger than C60, possibly by their inherent low concentration, as discussed before. Interesting is the fact that with a temperature increase for the HTLC isolation step (i.e., 45 and 75ºC) larger components sequentially appeared, reaching up to C157 at 75ºC. Existence of large waxy hydrocarbons reaching limits beyond those published by others (about C90-C120) (2, 5, 6, 12–15), was thus demonstrated with these HT-SEC 179

examples, agreeing with a previous report that suggests the existence of large alkanes spanning a range from C70 to about C215, as determined via FDMS (4). Appearance of small components at the highest tested temperature of 90 ºC (Figure 7), initially was difficult to explain; however, further studies confirmed this finding and provided a plausible explanation for the phenomenon, as discussed in the ensuing paragraph.

Figure 7. HT-SEC chromatograms for M12S paraffinic oil and its separated fractions by HTLC carried out at 25ºC, 45ºC, 75ºC, and 90ºC. HT-SEC carried out with toluene eluent @ 60°C. Waxy components from the tank bottom sludge produced from the same oil described above (M12B deposit originating from the M12S oil) were further separated via HTLC using columns packed with different adsorbents. Figure 180

8 presents the HT-SEC results achieved for fractions isolated from silica or asphaltenes packed columns. Three interesting features were shown with these examples: I) HTLC isolation under increasing temperatures sequentially provided larger components identified via HT-SEC, II) Small components appeared at the largest tested temperature of 90 ºC, III) Multimodal compound distributions as determined via HT-SEC, were observed in all studied cases. Elution of small components at the largest evaluated temperature (90ºC) is believed to have occurred because these were able to enter the porous space of packed adsorbents (Attapulgus clay, silica gel or asphaltenes), being then trapped by the largest components that solidified when the sample was deposited over the adsorbent, before starting the HTLC elution process. When the larger solid components finally eluted under high-temperature regimes, the smaller compounds were then able to exit the pores of the packings. The results discussed in the present section are evidence of the existence of paraffinic fractions spanning the C90-C215 range, the upper limit inferred from published results from other authors via FDMS (4). The ensuing section will address some ideas on the possible implications derived from the existence of such large paraffins in oils.

Figure 8. HT-SEC chromatograms for M12B waxy deposit and its separated fractions by HTLC carried out from silica gel or Orinoco Asphaltenes packed columns. HTLC carried out at 25ºC, 45ºC, 75ºC, and 90ºC. HT-SEC carried out with toluene eluent @ 60°C. 181

Implications Derived from the Existence of Large MW Paraffins in Oils Understanding and assessing the amount and nature of large MW paraffinic compounds is of paramount importance as they impact the properties of some oil fractions. Three possible consequences are believed derive from the presence of large paraffins in petroleum:

I)

Positive effects: one example presented in a study case where the paraffinic waxes isolated from Boscan asphalt (see Figure 6), were spiked over asphalts from this as well as other crudes, showing improved performance for the doped materials, i.e., better elasticity and resistance to cracking under low set up temperatures (22). II) Negative effects: since large MW paraffinic compounds solidify when temperatures attain lower values than their crystallization ranges. Solid deposition in storage tanks and pipelines flow disruption have been discussed in this regard (5–7, 11–17, 19, 20). III) Feasible effects derived from the resulting properties of wax-asphaltenes composite materials. The field of wax-asphaltenes composite materials has received the attention of many authors in the past decades (10, 23–27). Thermal maturation differences of occluded-protected paraffins versus adsorbed materials more prone to maturation have been recently addressed in the open literature (28–31). Cycloparaffins like steranes and hopanes are among the biomarker molecules studied in the preceding studies (28–31). Surprising enhanced solubilities have been reported for wax-asphaltene composite solids (23, 24) implying that many aspects of these composite materials are worth studying in greater detail. Example of increased asphaltenes solubility provided by waxes is presented in Figure 9. Highly “insoluble asphaltenes” having 0.9 H/C atomic ratio and 0.65 carbon aromaticity were observed to improve solubility when admixed with waxy alkanes in the example presented, providing values beyond those calculated based on the solubilities for the pure blended components.

Locus of Large MW Alkanes within the Boduszynski Continuous Molecular Petroleum Model The preceding discussion from the article is evidence of the existence of large paraffinic compounds in petroleum oils. As predicted long time ago by Boduszynski (2, 3), these are the largest compounds within defined distillation cuts, because they are mostly affected by the weakest London forces, thus requiring large carbon backbones that made thus possible the existence of noticeable intermolecular interactions by a large number of atoms present. One fundamental property allowing their isolation either from solid deposits or from distillation residua is their low solubility parameter (they appear as the alkane 182

soluble “saturates” fraction in HTLC). A C90-C215 spanned carbon range was discussed in the preceding section of this article, based on published reports (2, 4, 6, 14, 18, 20). Presence of unsaturation (H-deficiency) due to naphtheno and aromatic moieties was determined by different authors in waxy deposits (2, 5, 12, 19). Long alkane moieties (urea adductables) were also reported (14, 15). It is the belief of this author that the field of large petroleum paraffinic compounds and very large molecular mass alkanes has rarely been addressed in the past since the mandatory high temperatures and diluted ranges required for handling these compounds have been neglected. Figure 10 suggests a plausible locus for such compounds within the Boduszynski continuous molecular model of petroleum.

Figure 9. Dissolution of asphaltene / microcrystalline wax:75/25:wt/wt composite material as a function of Toluene solvent eluted volume. Experiments carried out at 25ºC.

Advanced petroleomics techniques now available should be applied to unravel the nature of such large paraffinic compounds (32). Mixtures of multiple paraffin classes (Z: 2, 0, -4, -6,…) (2, 5), plus the observed multimodal nature of waxy mixtures discussed in the preceding sections (11, 12, 18, 20), suggest as a possibility that their large molecular mass can be a consequence of reaction or even n-merization processes, i.e., reactive coupling of smaller paraffinic moieties to produce the large structures detectable and discussed in the preceding. Speculation on the feasible coupling of reactive groups like those present in long alkenones (34), or unsaturated biomolecules like squalene (35), interacting with insaturated cyclic biomarkers like sterenes and hopenes (35), represents an intriguing idea in this regard. 183

Figure 10. Visualization of the heavy end (C20+) from the Petroleum Continuum Model as proposed by Boduszynski. Adapted with permission from reference (33). Copyright 2013 American Chemical Society.

Conclusions High molecular weight paraffinic compounds have been shown to exist in petroleum. Paraffinic compounds spanning the C90-C215 carbon atoms were discussed in these regards. Handling and analysis performed under dilute conditions and high set up temperatures were found to be mandatory for their successful characterization; absence of these conditions has precluded more frequent study of these oil components. Both positive and negative practical effects were discussed in relation to the studied compounds. A plausible locus within the Boduszynski continuous molecular model of petroleum is proposed. However, better understanding deserves further efforts with advanced petrolemic techniques to address the point and unravel their, so far, practically unknown nature.

Acknowledgments The author wishes to acknowledge the support and helpful discussions held with former colleagues working in the areas of asphaltenes, solid deposition and flow-assurance: Drs. A. Izquierdo, O. Leon, M. C. Garcia. J. Espidel, F. Cassani, O. Rivas and Mr. M. Orea. Funding provided by Intevep S.A.-Affiliate of Venezuela’s National Oil Company (PDVSA) during the 1990s, allowed to carry out the experimental part of the described research. Dr. P. Pereira-Almao from the University of Calgary is thanked for the opportunity to keep working in areas related to oil characterization, thus enabling the continuous adventure of getting to know the complex nature of petroleum oils in greater detail. Dr. Maria 184

Josefina Perez-Zurita is acknowledged for helpful suggestions for improving the manuscript. Finally, Dr. M. M. Boduszynski is acknowledged for inspiring published research that gave origin to studies like the one covered in the present chapter.

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14. Fazal, S. A.; Zarakpar, S. S.; Joshi, G. C. Studies on Sludge from Storage Tank of Waxy Crude Oil. Part II: Solvent Fractionation. Fuel Sci. Technol. Int. 1995, 13, 1239–1249. 15. Fazal, S. A.; Rai, R.; Joshi, G. C. Characterization of Sludge Waxes form Crude Oil Storage Tanks Handling Offshore Crude. Fuel Sci. Technol. Int. 1997, 15, 755–764. 16. Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes, and Asphalts, 2; Yen, T. F., Chilingarian G. V., Eds.; Developments in Petroleum Series, 40B; Elsevier Science B.V: New York, 1993; pp 335−362. 17. Garcia, M. C. Paraffin Deposition in Oil Production. Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, Feb. 13−16, 2001 (SPE 64992, pp 1-7). 18. Carbognani, L.; Orea, M. Studies on Large Crude Oil Alkanes. I: High Temperature Liquid Chromatography. Fuel Sci. Technol. Int. 1999, 17, 165–187. 19. Carbognani, L.; DeLima, L.; Orea, M.; Ehrmann, U. Studies on Large Crude Oil Alkanes. II: Isolation and Characterization of Aromatic Waxes and Waxy Asphaltenes. Fuel Sci. Technol. Int. 2000, 18, 607–634. 20. Carbognani, L. Fast monitoring of C20-C160 crude oil alkanes by size-exclusion-chromatography-evaporative light scattering detection performed with silica columns. J. Chromatogr. A 1997, 788, 63–73. 21. American Society for Testing and Materials (ASTM). ASTM D7169. Standard Test method for Boiling Point Distribution of Samples with Resid such as Crude Oil and Atmospheric Resids by High Temperature Gas Chromatography; ASTM: West Conshohocken, PA, 2011. 22. Carbognani, L.; Duarte, D.; Rosales, J.; Villalobos, J. Isolation and Characterization of Paraffinic Components from Venezuelan Asphalts. Effects of Paraffin Dopants on Rheological Properties of Some Asphalts. Pet. Sci. Technol. 1998, 16, 1085–1111. 23. Garcia, M. C.; Carbognani, L. Asphaltenes-Paraffins Structural Interactions. Effect on Crude Oil Stability. Energy Fuels 2001, 15, 1021–1027. 24. Carbognani, L.; Rogel, E. Solid Petroleum Asphaltenes Seem Surrounded by Alkyl Layers. Pet. Sci. Technol. 2003, 21, 537–556. 25. Venkatesan, R.; Ostlund, J-A.; Chawa, H.; Wattana, P.; Nyden, M.; Scott Fogler, H. The Effect of Asphaltenes on the Gelation of Waxy Oils. Energy Fuels 2003, 17, 1630–1640. 26. Kriz, P.; Andersen, S. Effects of Asphaltenes on Crude Oil Wax Crystallization. Energy Fuels 2005, 19, 948–953. 27. Mahmoud, R.; Gierycz, P.; Solimando, R.; Rogalski, M. Calorimetric Probing of n-Alkane-Petroleum Asphaltenes Interactions. Energy Fuel 2005, 19, 2474–2479. 28. Chacon-Patino, M. L.; Vesga-Martinez, S. J.; Blanco Tirado, C.; OrregoRuiz, J. A.; Gomez-Escudero, A.; Combariza, M. Y. Exploring Occluded Compounds and Their Interactions with Asphaltene Networks Using HighResolution Mass Spectrometry. Energy Fuel 2016, 30, 4550–4561.

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Chapter 8

Effect of Bitumen Molecular Transformation during Upgrading on Asphaltenes Chemistry and Compatibility Parviz Rahimi* Upgrading Solutions Inc., 4751-151 Street, Edmonton, Alberta Canada T6H 5N9 *E-mail: [email protected].

The chemistry of thermal conversion of bitumen and heavy oil is complex not only because bitumen consists of a variety of different molecular structures, but also the thermal chemistry is not selective and many reaction pathways lead to the production of a complex mixture of products. It has been recognized through the genius of Mieczyslaw (Mietek) Boduszynski’s continuum petroleum model that one can relate the molecular structure of these complex hydrocarbons to their boiling points. Further, with the recent advancement in analytical instrumentation such as FT-ICR mass spectrometry the chemical structure of these complex hydrocarbons can be identified and structural reactivity and phenomenon of molecular interactions such as asphaltenes aggregation can be illustrated with confidence. It is important to realize that any changes in the molecular structure of bitumen at the primary stage of conversion to produce synthetic crude, would have a significant impact on downstream processing and handling. In this chapter methods developed for measuring asphaltenes stability and solubility parameters will be reviewed. Factors influencing fouling tendency of crudes because of blending or processing as well as correlations between chemical composition and crude fouling propensity will be discussed.

© 2018 American Chemical Society

Introduction The current surplus of crude oil, partly because of advancement in technology such as fracking, most likely will not last and the great value of heavy oils and bitumen, once more will be realized in a near future. Technology advancement for the extraction has made the production of these heavy hydrocarbons more economical. Further, with the development of advanced analytical technique it is now possible to characterize, in detail, the heaviest fractions of petroleum feedstocks such as asphaltenes. All these progress in production, transportation, upgrading and refining of heavy oils would not have been possible without fundamental understanding of their complex molecular composition and structure (1–6). Asphaltenes, the heaviest fraction of petroleum have been studied for years and by many research organizations and Universities (7–11). These molecules which were once thought to have molecular weight in the range of millions, consists of individual molecules of less than a thousand Daltons each that have been implicated in problems associated with petroleum production, transportation, refinery and upgrading processes. Asphaltenes, because of the nature of their molecular composition tend to form aggregates even at very low concentration in solutions and as such, it is very difficult to determine the nature of molecular structure of asphaltenes. Nevertheless, these molecules are solubilized in the oil matrix (colloidal in nature) by the solvent power of resins and aromatics present and the saturated fractions of the petroleum play an unstabilizing role and may cause asphaltenes to precipitate (12–14). The controversy on the reversibility or irreversibility of asphaltenes aggregation has recently been discussed in detail by Morozov, et al. (15) There is also a recent study that addresses the controversial effect of temperature of on asphaltenes aggregation and stability (16). While in nature the asphaltenes are stable in petroleum, they can come out of solutions as result of physical or chemical processes and cause significant operational issues. Blending 30vol% diluent with bitumen for pipeline transportation sound a trivial task but, if it is done in a wrong order can cause local asphaltenes precipitation. The same argument can be applied when blending different crudes. If crudes are incompatible precipitation of asphaltenes can cause major operational problems in the processing units. Asphaltenes can also become unstable in the oil matrix by chemical reaction during thermal and catalytic conversion. In this multi-multi reaction pathways, thermal energy converts bigger and high molecular weight components to smaller and lower molecular weight fractions. During these chemical transformations, asphaltenes solubility is decreased to the extent that it may no longer be soluble in the media and precipitate. The detailed mechanism of heavy oil upgrading can be found in references (11, 17). In a previous study (18), it was demonstrated that it is possible to separate asphaltenes into different sub-groups using column chromatography. By using different stationary phases, Silica and Aluminum, and changing solvent polarity, asphaltenes from Cold Lake vacuum bottoms were separated into four different fractions (A1-A4) with different properties. The molecular weight of these fractions changed from A1= 1800 g/mole to A4 = 7000 g/mole indicating that it is possible to “selectively” separate different asphaltenes fractions. It was further 190

shown, in this paper, that heaviest asphaltenes fraction, A4 produced more coke than the lighter asphaltenes fraction A1. Because of the foregoing discussion, determination of solubility parameters of petroleum feedstocks, which is directly related to asphaltenes stability in the oil matrix, should be given very high analytical priority and include in all crude assays. There are a number of different methods for measuring crude solubility parameters or compatibility, and a major section of this chapter is allocated to describe these techniques. Further, application of oil compatibility in blending, processing/upgrading, and refining/fouling will be investigated.

Asphaltenes Stability/Compatibility The stability of asphaltenes in petroleum depends not only on the chemical properties of surrounding media but also the method that the oil was produced. Both chemical and physical processes can change the concentration of stabilizing components such as resins and aromatics and result in destabilizing asphaltenes leading to flocculation and finally precipitation. It is important to have a firm knowledge of the state of asphaltenes in oil throughout of the petroleum supply chain, from production, transportation, refining, and upgrading to storage. Once the degree of stability of asphaltenes in an oil is determined, appropriate action can be taken to prevent costly plant shutdown and/or unscheduled maintenance.

Method of Measurement In recent years there are a number of automated methods that were developed for the determination of solubility parameters. All these methods are based on optical detection of asphaltenes as a result of the addition of a paraffinic solvent. 1- ASTM D-7112 – Porla -Finish Measurement System (Po/RFmax and SBN/ IN), Xylene or Toluene, Heptane 2- ASTM D-7060 -Equivalent to Shell SMS 2715 manual method (Po/FRmax), 1-methylnaphthalene, Hexadecane 3- ASTM D-7157 _Rofa France (S-value, So/RFmax), Toluene, Heptane 4- ASTM D-6703 - Western Research Institute (Po/FRmax), Toluene, Isooctane In these methods, the solubility parameter Po (solvent power to dissolve asphaltenes) and FRmax (the difficulty to dissolve asphaltenes in the oil matrix) can be obtained from a straight line obtained by plotting: Flocculation ratio, FR = VT/(VT + VH) vs Dilution number, 1/X = Voil/(VT + VH) at minimum three oil concentration where, VT = volume of toluene, VH = volume of heptane and Voil = volume of oil. 191

For oils with asphaltenes, two manual oil compatibility method can be found in the literature one developed by Irwin Wiehe and another by Shell Global Solutions (13, 19). Although the detail procedures of these methods are not publicly available, but like the automated methods, the initial stage involves addition of paraffinic solvent such as heptane or 1- methyl naphthalene to the oil to observe asphaltenes under microscope (13, 19). Later, Wiehe and Rahimi published a number of papers in which they described the determination of solvency of oils/streams, diluent and condensates that have no asphaltenes (20, 21). In 2016 CCQTA (Canadian Crude Quality Technical Association) initiated a project to develop a new manual oil compatibility method that is completely different from the existing methods. An example of this method is shown in Table 1. In this method, asphaltenes are deliberately precipitated, as shown in microscopic picture (Figure 1), by adding a large amount of paraffinic solvent such as heptane and in subsequent steps, toluene is added until there are no asphaltenes visible under a light microscope. Using three different oil concentrations, flocculation ratios and dilution numbers are calculated (Table 2) and are plotted as shown in Figure 2. From the straight line we can then calculate all solubility parameters (Table 3):

FRmax = Maximum flocculation ratio 1/Xmin = Reciprocal Critical Dilution Xmin = Heptane consumption of undiluted oil, mL/g oil (Critical Heptane Dilution) Pa = Peptizability of asphaltenes = 100- FRmax P-value = 1+ Xmin, also P-value = Po/FRmax Po = Peptizing power of the oil matrix = FRmax (1+Xmin)

Figure 1. Floculation of asphaltenes after addition of heptane.

192

Table 1. Flocculation Titration for Crude X (Manual CCQTA Method) 1 g oil ± 0.01

2 g oil ±0.01

3 g oil ± 0.01

VH

VT

FR

VH

VT

FR

VH

VT

FR

9

1.0

10

9

1.0

10

9

1.0

10

9

1.5

14.3

9

1.3

12.6

9

1.2

11.8

9

2.0

18.2

9

1.6

15.1

9

1.5

14.3

9

2.2

19.6

9

2.0

18.2

9

1.9

17.4

VH = Volume of heptane.

VT = Volume of toluenee.

Table 2. Dilution Ratio and Flocculation Ratio Oil, mL

1/X

FR

1

8.9

19.6

2

18.2

18.2

3

27.5

17.4

1/X = 100*Voil/(VH+VT), FR = 100* VT/(VH+VT).

Figure 2. Flocuulation ratio vs dilution number for crude x.

193

Table 3. Solubility Parameters Calculations FRmax

20.55

1/Xmin

174.15

Xmin

0.57

Pa

79.45

P-value

1.57

Po

32.35

Po/FRmax

1.57

Table 4. Crude Blending A Po

Frmax

Crude oil 1

139

46

Crude oil 2

90

30

Vol% 1

Vol% 2

Pomix

0

100

90

10

90

94.9

2.06

20

80

99.8

2.17

30

70

104.7

2.28

40

60

109.6

2.38

50

50

114.5

2.49

60

40

119.4

2.60

70

30

124.3

2.70

80

20

129.2

2.81

90

10

134.1

2.92

100

0

139

3.02

Po/Frmax

Application in Blending Crudes with different properties are blended for transportation by pipeline or rail to be stored or processed at refineries. Heavy crudes/bitumen with low viscosity must be blended with diluents or natural gas condensates to meet pipeline specification. Recently, because of increase in production of shale oil and possible blending/processing with other crudes, there has been an unprecedented number of incidents that resulted in some crudes to be contaminated with precipitated asphaltenes. These incidents have happened most likely, because some crudes 194

with asphaltenes have been in contact with high paraffinic crudes and/or the order of blending of paraffinic crudes with asphaltenic crudes has not been followed. The solubility numbers of different oils, Po and FRmax obtained from different methods can be used to determine if the crudes are compatibility at any ratios or incompatible, at specific ratio. From the mixing rule (PoA X VA + PoB X VB)/(VA+VB), as demonstrated in ASTM D 7060, we can determine the solubility of the blend (PoMix). Further, the solubility index, P-value is calculated by dividing PoMix/FRMax. For any crude or blends to be stable the Po/FRMax should be greater than 1 and preferably around 1.2. Table 4 and Table 5 illustrate the results of blending for different crudes at different ratios. The results in Table 4 show that blending two relatively stable crudes results in mixtures that are stable at any ratio. However, in Table 5, where a crude or diluent with no asphaltenes is blended with a relatively stable crude, resulting in some blends that are incompatible at certain ratios, i.e., Po/FRMax 24 the crude would fail the fouling test. Oil sands bitumen and crudes derived by diluting bitumen with diluent (dilbits) are very stable and have very low fouling tendencies as determined using the Alcor fouling rig. As was discussed in the previous section, the stability of thermally converted bitumen is decreased as the severity of the process is increased for the light oils with low asphaltenes and relatively good solubility (compatibility). The fouling tendency was shown to be substantially higher in part because of the presence of “contaminants” (27). Fouling was shown to be significantly less for some crudes when contaminants were removed. It was shown that the contaminated crude oil, besides precipitated asphaltenes, contains different metals as well as phosphorous (crudes containing precipitated asphaltenes are known as self-incompatible) (30). From the SARA analysis of the light crudes, The Colloidal Instability Index (CII) was calculated which shows all crudes are highly foulant, all have CII greater than 1. (Table 6).

Although according to Table 6 all the crudes have high fouling tendency, there is a good correlation between the CII and ΔT. From Table 6, a simple calculation showed that r2 = 0.91 For cracked petroleum products, the fouling propensity is mainly because of the presence of unstable asphaltenes but also to olefin/diolefin. Fouling tendencies of streams containing olefin/diolefin compounds such as coker gas oil, has been reported (29). The cracked gas oil (CGO) has high fouling tendency in part because contain large amounts of diolefins as indicated by high diene number = 4.48 gI2/ 100 g oil. 197

Table 6. Fouling and CII of Light Crudes Crudes

CII

ΔT

A

2.6

96

B

2.3

69

C

1.7

54

D

1.8

55

E

2.3

76

CII = Collidal Instability Index. t and at time 0.

ΔT = Difference between the Outlet temperature at time

It must be recognized that in each refinery equipment where there is a potential of fouling there should be some mitigation strategy been developed to mitigate fouling. If fouling is not properly managed, the fouling precursors can eventually form coke. The initial stage of coke formation is development of mesophase or liquid crystals which can be observed under microscope with polarized light (31–33). Using Hot Stage Microscopy, Gentzis and Rahimi investigated the onset of mesophase formation from Athabasca bitumen as function of boiling point (34). It was shown as the boiling point increased from 525°C to 675°C the onset of mesophase formation did not change significantly. “However, when these materials were thermally treated, the amount of coke produced increased as a function of boiling point. The results are consistent with the fact as boiling point of the fractions increased the amount of asphaltenes in these fractions also increased which are then converted to coke during thermal processing.

Conclusions The importance of feedstock characterization in understanding the chemistry of heavy oil conversion has been described. The changes in product characteristics, both physical and chemical, as a function of process options and process severity can be measured in detail that result in maximization of product yields and reducing unscheduled shutdown and maintenance. This chapter concentrated mostly on the chemistry of asphaltenes conversion and the effect on product stability. Different methods and techniques used for measuring asphaltenes stability were reviewed briefly. The development of a new manual oil compatibility method was described. The applications of oil compatibility in crude blending, upgrading and fouling were described in detail. It was shown that the refinery heat exchanger fouling of light crudes with low asphaltenes were mainly because of the presence of contaminants rather than compatibility. For these crudes, the fouling propensity correlated with Colloidal Instability Index (CII). Hot stage microscopy has shown to be a powerful tool to observe formation 198

and development of mesophase under the upgrading reaction conditions. Further, microscopic examination of residue and solid deposits obtained after process shutdown can reveal the mechanism that led to the formation of deposits/coke and develop mitigation strategies.

References 1.

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25. Wiehe, I. A.; Kennedy, R. J.; Dickakian, G. Fouling of Nearly Incompatible Oils. Energy Fuels 2001, 15, 1057–1058. 26. Brons, G.; Rudy, T. M. In Fouling of Whole Crude oils on Heated Surface; Proceedings of the 4th International Conference on Heat exchanger foulingFundamental approaches and technical solutions, Davos, Switzerland. July 8−13, 2001; Müller-Steinhange, E., Ed.; PP Publico publications: Germany, 2002; pp 249−257. 27. Fan, Z.; Rahimi, P.; McGee, R.; Wen, Q.; Alem, T. Investigation of Fouling Mechanisms of a Light Crude Oil Using an Alcor Hot Liquid Process Simulator. Energy Fuels 2010, 24, 6110–6118. 28. Rahimi, P.; Wiehe, I.; Alem, T.; Oh, Y. D. In Investigation of Fouling of Crudes with low Asphaltenes. Presentation at the AIChE/ACS joint meeting, New Orleans, LA, Apr 6−10, 2008. 29. Fan, Z.; Rahimi, P.; Alem, T.; Eisenhawer, A.; Arboleda, P. Fouling Characteristics of Hydrocarbon Streams Containing Olefins and Conjugated Olefins. Energy Fuels 2011, 25, 1182–1190. 30. Wiehe, I. A. Self-Incompatible Crude oils and Converted Petroleum Resids. J. Dispersion Sci. Technol. 2004, 25, 333–339. 31. Parker, R. J.; McFarlane, R. A. In Mitigation of Fouling in Bitumen Furnaces by Pigging; Proceedings of 1st international conference on petroleum phase behavior and fouling, AIChE Spring National Meeting, Houston, TX, Mar 14−18, 1999. 32. Parker, R. J.; McFarlane, R. A. Mitigation of Fouling in Bitumen Furnaces by Pigging. Energy Fuels 2000, 14, 11–13. 33. Gentzis, T.; Parker, R. J.; McFarlane, R. A. Microscopy of Fouling Deposits in Bitumen Furnaces. Fuel 2000, 79, 1173–1184. 34. Rahimi, P.; Gentzis, T.; Taylor, E.; Carson, D.; Nowlan, V.; Coté, E. The Impact of cut points on Processability of Athabasca Bitumen. Fuel 2001, 80, 1147–1154.

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Section III: Distillation

Chapter 9

Attainable Product Yield Distribution Curve: A Roadmap to Crude Oil Composition Carl E. Rechsteiner* CRechsteiner Consulting, LLC, 1712 Putnam Way, Petaluma, California 94954, United States *E-mail: [email protected].

Crude oil is an exceedingly complex mixture of organic compounds. Advances in separations and measurement technologies, especially the utilization of Gas Chromatography (GC), allow robust measurement of the distribution of material within a crude oil from C3 up to ca. C110 to C120 calibrated against normal paraffin standards. The focus of this chapter is on the application of Detailed Hydrocarbon Analysis (DHA) for light ends and GC Simulated Distillation (SimDis) for distillable fractions and residuum. The chapter discusses the concepts behind these technologies, the robustness of these measurements, and the importance of the composite yield distribution curve obtained from combining these measurements.

Background Among the many contributions of Mieczyslaw M. Boduszynski (Mietek) to the understanding of petroleum composition, arguably the most significant is his “Petroleum Molecular Composition Continuity Model”. A concise description of this model is found in the references (1). This model was also presented at the 13th International Conference on Petroleum Phase Behavior, and Fouling (2). Mietek has also published numerous articles and a seminal book on this matter (3–8). The model proposes that, unlike conventional wisdom that petroleum transitions from individual molecules to extremely high molecular weight © 2018 American Chemical Society

components (i.e., asphaltenes), the molecular composition of petroleum is a continuously variable series of molecules without abrupt transitions. Recent work by A.G.Marshall and R.P. Rodgers, et al. provides validation for this model (9, 10). Mietek defined a temperature scale (n-paraffin atmospheric equivalent boiling point, AEBP) as a yardstick for tracking these continuous changes. Some important concepts behind this choice are: 1. The boiling points for normal paraffins are well defined. 2. Mathematical techniques can be used to extend this scale far beyond what current technology can measure, e.g., to C200 (11). 3. This yardstick provides a mechanism to interpolate physical and chemical properties and allow direct comparison of different crudes and crude fractions on a consistent basis. Further, this choice allows compositional information to be discussed with members of Upstream operations (molecular weight or carbon number) or Downstream operations (boiling point) in terms that they prefer.

Historical Perspective The work reported here spans many years (decades, in fact). Several of the relevant measurement technologies were either non-existent or in a gestational phase and developed during these efforts. The gas chromatographic methodologies of Simulated Distillation and Detailed Hydrocarbon Analysis were developed, modified, improved, and standardized during this period. A refinery fractionates crude oil into a series of boiling point range cuts by distillation for subsequent processing into products, e.g., motor gasoline, jet fuel, diesel fuel, etc. Distillation is one of the few industrial scale processes that can handle the amount of crude oil converted into products in a refinery. For example, a small refinery may process 50,000 barrels per day of crude oil. That means that it is processing over 2 million gallons of crude every day of operation. The value of any crude oil to refiners is determined by that specific crude oil’s chemical and physical properties and its yield in each product boiling point range and that refinery’s specific configuration. A crude assay is used to measure the yield and properties of crude fractions for valuation purposes in a standardized way and then build a description of the whole crude oil.

Deep-Cut Distillation ASTM methods, D-2892 for atmospheric distillation and D-5236 for vacuum distillation (12, 13), are standard distillation methods for crude oil assays. ASTM method D-2892 is used to produce low boiling crude oil fractions up to 400°C AET (Atmospheric Equivalent Temperature) which can be further analyzed. The method specifies that the vapor temperature at the collection point must not exceed 310 °C, and as needed the vapor pressure can be reduced from atmospheric to 100 mm of mercury (Hg). Fractions are collected in a crude oil assay from the D-2892 method might be: Start to 350 °F, 350-450 °F, 450-550 °F, 550-650 °F, and atmospheric residuum. 206

ASTM D-5236 specifies a vacuum distillation that can start with the atmospheric residuum from the D-2892 distillation and then produce crude oil fractions up to 1050 °F under reduced pressure, 1 mm of Hg. However, the exact endpoint is highly crude dependent. The maximum pot still temperature is not to exceed 400 °C, and some crude oils must be well below this temperature to avoid thermal cracking of the residue. Use of a molecular distillation allows collecting fractions to much higher endpoints (14). In molecular distillation, the pressure is significantly reduced, coupled with a short residence time for contact between a thin film of the feed and the heated surface allows for efficient mass and heat transfer. This means that higher boiling components can be produced without experiencing thermal cracking. Figure 1 shows a molecular distillation unit from UIC GmbH that has been used to produce fractions with endpoints up to 1250-1350 °F.

Figure 1. Molecular Distillation Unit, model KDL-5.

Access to these much higher boiling point fractions allows a more accurate assay (interpolation of the physical and chemical properties rather than extrapolation) particularly for crude oil that cannot reach the 1050 °F target boiling point with D-5236 and facilitates evaluation of the higher boiling fractions. This topic will be illustrated in a later section. 207

Detailed Hydrocarbon Analysis In an ideal world, one could compute the value of any crude by summing up the assigned value of each molecule in the crude. Due to the overwhelming number of compounds in crude (the number of isomers of C60H122 is on the order of 1022) (15), this is not possible. However, within limits, this can be done at much lower boiling points using the detailed hydrocarbon analysis methodology (DHA). In DHA, the fraction of interest is separated and quantified using high-resolution gas chromatography (GC). Prior to industrial standardization of the DHA methodology, a variety of approaches were used to measure the concentration of individual components in crude oil fractions, process intermediates, and finished products. Industry standardization took several forms. ASTM Method D-5134 was approved in 1990 for olefin-free samples up to n-nonane (ca. 300°F) (16). All species are eluting after n-nonane were lumped into a single group. Three methods, ASTM Methods D-6729 (17), D-6730 (18), and D-6733 (19), were originally approved in 2001. These extended DHA to include specific oxygen-containing compounds with boiling point ranges up to 225°C (about the boiling point for n-eicosane). Such methods come with limitations. 1) Coelution increases as the carbon number/boiling point increases. Table 1 shows the number of paraffin isomers as a function of carbon number/boiling point. 2) The inclusion of olefins in the method vastly increases the number of isomers and possible coelutions. 3) As the carbon number increases, there is a lack of standards for each isomer, along with their individual chemical and physical properties. For example, at C6 and above, olefins are only identified by their carbon number. Similarly, paraffins above C9 and aromatics (except for specific condensed ring cores like naphthalene) above C10 are only identified by their carbon number. Generic identifications, while used for economic assessments, are less precise and increase errors in valuations.

Table 1. Number of Isomers of Paraffins as a Function of Carbon Number Range Carbon Number Range

Number of Isomers

C1 - C9

35

C1 - C12

355

C1 - C15

4347

C1 - C18

60523

Figure 2 shows the GC traces for 3 consecutive crude oil fractions. This figure clearly shows the increase in coelutions as the carbon number/boiling point increases. For the 350-450°F fraction, even the distinct peaks are riding upon other coeluting compounds. Figures 3 and 4 expand and label specifically identified peaks in the Start to 250°F and 250-350°F fractions. 208

Figure 2. Impact of boiling point range on GC of consecutive boiling range fractions from a single crude oil. Start to 250°F about C8 (bottom), 250-350°F about C8 to C10 (middle), and 350-450°F about C10 to C13 (top).

Figure 3. Lowest boiling fraction (Start to 250°F) showing limited coelutions, mostly between C7 (toluene) and C8. 209

Figure 4. Middle boiling fraction (250 to 350°F) where components identified only by carbon number and type (paraffin, naphthene, and aromatic) starts between C9 and C10.

Simulated Distillation SimDis is a GC based method that provides yield distribution information comparable to physical distillation. Compared to physical distillation, it is faster, more reproducible, uses smaller sample sizes, and has lower workforce requirements. During the time of these studies, SimDis methodologies evolved to cover not only subsets of the distillable range up to 1000°F but also to include residuum containing materials (including certain crudes). Table 2 lists some of the ASTM SimDis methods, their initial date of approval, and their range of coverage. Note, ASTM Method D-6352 was reissued in 2005 with an expanded carbon number/ boiling point range.

Table 2. ASTM VGO SimDis Method Comparisons ASTM Method

BP Range °F

Carbon # Range

Initially Issued

D-2887

~100-1000

5-44

1973

D-6352

345-1292

10-90

1998

D-7213

~100-1140

7-60

2005

4 – 100/110/120

2005

D-6352

210

The limitations concerning residuum-containing fractions are of particular interest. To dissolve the residuum containing samples for measurement, carbon disulfide is used as a solvent. That limits the accuracy in the C4 to C8 range due to coelution of the hydrocarbons with the solvent. A second issue concerns the upper-temperature limit for calibration. Typically, a Polywax standard is used as a stationary phase but one may not be able to identify n-paraffins above C100, C110, or C120. Issues with the early versions of this method will be discussed later. Figure 5 compares the yield curve for a start to 350°F crude oil fraction obtained by DHA and by SimDis. Minor differences are seen between the two yield curves. These discrepancies are explainable due to the difference between the high-resolution GC data from DHA and the much lower resolution from the SimDis measurement.

Figure 5. Comparison of DHA versus SimDis Yield curves for a Start to 350°F crude fraction. Tag indicates whether SimDis measured temperature is above or below that from DHA.

Figure 6 shows the chromatographic data for fractions obtained from a 30°API crude. In addition to the data for the fractions, a composite yield curve is also overlaid. The top is on the boiling point scale (AEBP) while the bottom is on the chromatographic retention time scale. Note, the highest boiling fraction (950-1100°F) was obtained from the molecular distillation of the 950°F+ vacuum residua. 211

Figure 6. SimDis Chromatograms for 8 fractions from a 30°API crude and the composite from those fractions on the AEBP scale (top) and the retention time scale (bottom).

Figure 7 and 8 modify the data in Figure 6 by removing some of the fractions. In figure 7, the Start-to-350°F fraction is removed while in Figure 8 the fractions below 650oF are removed. The chromatographic data for the whole liquid product for the 350-650°F and the 650-1100°F fractions and the chromatogram from the compositing of the individual fractions are virtually identical. For reference, table 3 compares basic properties for the 11°API crude. 1) As measured on the crude oil, calculated from the ASTM D-2892 distillation (fractions below 650°F plus the atmospheric resid), and that calculated from all of the cuts including the 1100°F+ residua. 2) On the atmospheric residua and the calculation from the 650°F+ fractions. 3) On the wide 650-1100°F gas oil. Differences between the values are consistent with the experimental errors associated with the measurement methods. 212

Figure 7. GC chromatograms for consecutive VGOs from 350 to 1100°F and their composite yield distribution curve on the AEBP scale (top) and the retention time scale (bottom).

213

Figure 8. GC chromatograms for consecutive VGOs from 650 to 1100°F and their composite yield distribution curve on the AEBP scale (top) and the retention time scale (bottom).

214

Table 3. 11.0°API Crude Oil Properties. Measured and Calculated from Fractions Properties / Fractions

Crude Measured

Crude Calculated D-2892 Basis

Crude Calculated Cut 1- Cut-n

Gravity, °API

11.0

11.1

11.5

Sp. Gr. 60F/60F, g/mL

0.9930

0.9921

0.9899

Sulfur, wt%

1.54

1.53

1.59

Nitrogen, wt%

0.86

0.88

0.87

Hydrogen, wt%

11.10

10.51

10.90

Vanadium, ppm

92.55

97.52

88.82

Nickel, ppm

93.57

95.50

87.08

9.44

9.35

MCR, wt%

Table 4 shows the same as measured versus as computed for this crude’s atmospheric residuum (AR) and its wide VGO.

Table 4. 11.0°API AR and VGO Properties. Measured and Calculated from Fractions Properties/Fractions

650°F+ Measured

650°F+ Calculated

650-1100 Measured

650-1100 Calculated

Gravity, °API

7.6

7.8

10.8

11.1

Sp. Gr. 60F/60F, g/mL

1.0173

1.0161

0.9944

0.9921

Sulfur, wt%

1,72

1.8

1.61

1.61

Nitrogen, wt%

1.09

1.08

0.75

0.74

Hydrogen, wt%

10.21

10.74

11.07

1.00

Vanadium, ppm

120.6

110.67

29.99

28.82

Nickel, ppm

118.10

109.16

49.35

47.25

MCR, wt%

11.68

11.53

3.79

3.85

Figure 9 compares the yield curve obtained on the VGO from the atmospheric residua from a 29.7°API crude with the composite of the cuts from a deep cut assay of that VGO (20). The VGO endpoint is above 1250°F, and the yield curves are identical. 215

Figure 9. Comparison of yield distribution curves for 650-1250°F VGO and a composite from 8 fractions.

Examples In 1995, Mietek and coworkers published a paper titled “Deep-cut assay reveals additional yields of high-value VGO.” Using the combination of distillation and chromatographic methods (ASTM D-2892, D-5236, molecular distillation and sequential extraction fractionation) with physical/chemical measurements (21), they illustrated the compositional changes that accompany cutting deeper into an atmospheric residuum. The use of the boiling point scale allows reporting information for these fractions on a consistent, comparable basis. As the cut-point between the respective VGO and the residua increases, the yield of the VGO increases. Other vital properties also increase but at a slower rate. For example, the sulfur content in a VGO with a cut-point of 1179°F is lower than that of the atmospheric residuum starting material. Likewise, the content of nickel, vanadium, and Microcarbon residue is all reduced compared to the starting material. Table 5 shows these changes across a series of VGOs with increasing cut points. 216

Table 5. Changes to VGO Properties as VGO-Residuum Cut-Point Increases for a 30.9°API Crude Oil Cut-point, °F

650°F+

650-919°F

650-1066 °F

650-1179 °F

Gravity, °API

14.1

21.1

19.5

18.2

Sp. Gr., 60F/60F, g/mL

0.9718

0.9273

0.9371

0.9452

Carbon, wt%

86.25

86.32

86.33

86.61

Hydrogen, wt%

11.4

12.62

12.30

11.86

Sulfur, wt%

1.97

1.33

1.48

1.67

Nitrogen, wt%

0.37

0.12

0.17

0.21

Vanadium, ppm

54.9