Quantitative Vacuum Distillation of Crude Oils to Give Residues

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Quantitative Vacuum Distillation of Crudes to give Residues Amenable to Asphaltene DeterminatorTM Coupled with Saturates, Aromatics, and Resins Separation Characterization Jeramie J Adams, John F. Schabron, and Ryan B. Boysen Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 14, 2015

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Quantitative Vacuum Distillation of Crudes to give Residues Amenable to Asphaltene DeterminatorTM Coupled with Saturates, Aromatics, and Resins Separation Characterization Jeramie J. Adams*, John F. Schabron, and Ryan Boysen Western Research Institute. 365 North 9th Street, Laramie, Wyoming, 82072, USA 307-721-2324. Fax 307-721-2345, [email protected]

ABSTRACT The automated Asphaltene Determinator coupled with saturates, aromatics, and resins (SARADTM) separation is a powerful characterization tool initially designed for the rapid and repeatable analysis of asphalt bitumen and petroleum residua. By virtue of the evaporative light scattering detector (ELSD)—and the fact that saturates and a portion of aromatics do not contain chromophores making them undetectable in the 500 or 700 nm detectors—the complete quantification of petroleum fractions by this method is somewhat restricted to samples that do not contain a significant amount of volatiles. Crude oils can contain more than 70% of volatile material which is not detected by the ELSD. To overcome the SAR-AD volatiles limitation, a quantitative vacuum distillation method was developed to capture the volatiles and produce unaltered residua that do not lose volatiles in the ELSD. To complete the characterization, the volatiles are analyzed for their saturate, aromatic, and reactive olefin content by proton nuclear magnetic resonance spectroscopy (1H NMR). The data from these two methods are combined in the form of a modified colloidal instability index to give a more complete profile of the original whole crude oils. INTRODUCTION Background. Blends consisting of heavy oil, bitumen from oil sands, crudes from subterranean fracturing, and partially upgraded blends are increasing in the diet as refinery feedstocks. Many refiners no longer have the luxury of upgrading relatively homogenous feedstock material from a single region over long periods due to increasing pressure to refine cheaper lower quality, “opportunity,” crudes and blends to boost profit margins.1,2 Blends are used with variable dayto-day compositions which can result in unexpected emulsion formation or fouling.2,3 Understanding the behavior of a feedstock through its composition—and ultimately the ability to optimize the behavior of the feedstock by tuning its composition—can improve the efficiency for processes such as desalting, distilling, coking, hydroprocessing, and other upgrading processes while minimizing fouling. Additional fouling problems can occur when the oils are subjected to thermal processes such as those experienced in preheat trains or while heating in atmospheric and vacuum distillation columns which can result in heat induced deposition. New characterization and process monitoring tools are crucial to optimize production and improve processing while ensuring that blends and new feedstocks will not become problematic during transport or at different junctures during the refining process. This understanding is also important to know how oil will behave in upstream processes starting within the reservoir and continuing through downstream upgrading processes.

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Partitioning of petroleum into saturates, aromatics, resins, and asphaltenes (SARA) is a classical approach to quantify petroleum’s composition by producing subfractions based upon adsorption affinity of its components on a given sorbent using a given solvent system. Traditional SARA separations are generally preceded by asphaltene precipitation with an aliphatic hydrocarbon solvent such as pentane, hexane, heptane, or isooctane. The insoluble precipitate material is defined as the asphaltenes. This first step is necessary due to the strong irreversible adsorption of asphaltenes onto typical column chromatography sorbents. The amount of asphaltenes can vary significantly between different precipitation and isolation methods since the amount of asphaltenes depends on solvent-to-oil ratio, temperature, time, and washing.4,5 Most of the molecules that report to the asphaltene subfraction possess a central pericondensed polyaromatic structure that absorb visible light (brown color) containing a few percent of heteroatoms of mainly N, O, S, and some chelating structures of Ni and V. Asphaltene component molecular core structures are decorated with aliphatic cycles and side chains and have an average molecular weight around 700 Daltons.6 At concentrations above 50-100 mg/L asphaltene component molecules are attracted to one another and form dissolved nanoaggregates. Increasing the concentration causes these nanoaggregates to further associate through to form suspended clusters of nanoaggregates. These nanoaggregates and clusters of nanoaggregates and their interplay with resins and the surrounding oil matrix are responsible for their dynamic supramolecular structure and colloidal-like behavior.7,8,9,10 Under certain conditions these clusters can become unstable leading to flocculation, and under poor circumstances precipitation will eventually occur. Asphaltenes are implicated in several undesirable refining issues such as emulsion formation, precipitation, heat induced fouling, clogging, and catalyst poisoning. The maltenes are defined as the soluble portion remaining after the asphaltenes are precipitated. This solution of hydrocarbon soluble material is typically separated further into saturates, aromatics, and resins (SAR) by normal phase liquid chromatography using column, thin layer, or rod chromatography techniques. Saturates consist of molecules which are mainly linear, branched and cyclic, aliphatic hydrocarbons and possibly a small amount of very small aromatic ring structures containing significant aliphatic side chains; there are very few if any heteroatoms found in this subfraction and this class of molecules generally appears colorless. Aromatics are usually yellow to light brown and contain small aromatic structures which may or may not be conjugated. These small aromatic structures are generally decorated with aliphatic cycles and chains and may contain a small amount of heteroatoms. Molecules which report to the resins are generally larger conjugated aromatic structures which contain more heteroatoms and are significantly brown in color. They are similar to asphaltenes except they are usually smaller and contain more aliphatic cycles or side chains and less heteroatoms making them more soluble and less interactive. Separations of oil using normal phase chromatography have been around for several decades, a more complete overview can be found elsewhere.11 For typical normal phase chromatography, the weight percent of each of the SAR fractions is determined by evaporating the eluent and weighing the residual materials. In their most basic form, these separation methods are limited to use with heavy oil materials such as residua and asphalt since they do not contain significant volatile components that would otherwise be lost during the solvent evaporation step.12 For most procedures there is no attempt to control the amount of volatiles lost during the evaporation step. Crudes which contain significant amounts of lighter material can be

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topped by distilling a portion of the lightest hydrocarbons. The results from lab to lab can vary significantly depending on the method used, which in turn leads to erroneous conclusions when using SARA analysis as a diagnostic and predictive method for crudes.5 There are methods which allow for the quantification of SARA fractions in situ the whole oil. These methods are generally based upon infrared (IR) and near infrared (NIR) spectroscopy and are coupled to a multivariate analysis to determine SARA components which were empirically derived from standard high performance liquid chromatography (HPLC) SARA separations data.13 However, there is no universal correlation that works for all oils. There are several powerful separation methods based upon HPLC,13,14,15,16,17 however most data are still obtained by chromatographically separating SARA fractions on a traditional open column. A recently developed asphaltene on-column precipitation and redissolution method has made it possible to develop a fully automated SARA separation without the added step of precipitating asphaltenes. The first step of the on-column method is precipitating asphaltenes on polytetrafluoroethylene (PTFE) known as the Asphaltene Determinator (AD) which separates asphaltenes into solubility subfractions.18 The asphaltenes are selectively dissolved with solvents of increasing strength to elute fractions of asphaltenes (cyclohexane, toluene, and methylene chloride:methanol (98:2)).18,19 By removing asphaltenes on the front end of the separation using the AD, the maltenes can then be conveniently separated into SAR fractions using three chromatographic columns to give the fully automated SAR-AD separation.20 The advantages of the SAR-AD are its high reproducibility for samples run in batch, it is fully automated, and it gives results and is fully regenerated in 4 hours. The separation is designed in such a way that the columns do not need to be changed between separations. SAR-AD data can be used for process control such as determining the onset of coke formation, it can be used to determine how close an oil is to coke formation, if the oil or blend contains pyrolysis components, or if oxidation has occurred.11,18,19,20 For the AD method, volatile oils can be analyzed, but some volatiles are lost in the evaporative light scattering detector (ELSD) when the solvent is evaporated to measure the amount of petroleum material dissolved in the particular solvent. Fortuitously, since the loss occurs only from the maltenes portion it is possible to compare the total ELSD peak areas of the sample to the total ELSD peak areas of a quality control vacuum residuum to make a mathematical correction for the volatiles lost. However, for the SAR-AD separation the volatile losses occur mainly from both the saturate and aromatic fractions and there is no straightforward method to mathematically determine which subfractions the volatiles were evaporated from. Typical methods to control volatiles loss from open column chromatographic separations, such as topping a crude by distilling components which boil below 200-300 °C,21,22 do not provide a deep enough cut to prevent significant loss of material in the ELSD. According to Robbins,23 to have 10% error within one standard deviation, 80% of the material must have a boiling point above 343 °C, which has been verified by others.24 A newly developed volatilesSARA separation (VSARA) takes into consideration the gravimetric amount of material lost during the SAR separation by carrying out repeated evaporations,12 however this method is time consuming and not amenable to the SAR-AD method.

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Distillation of a crude oil remains the preferred method to produce a residuum that is amenable to quantification by an ELSD. Ideally, the distillation of petroleum should not change the composition of the distillate or residue so that native SARA quantification can be obtained. This means that the distillation must be conducted in a controlled manner to prevent oxidation (often not considered in petroleum distillations) and bond cleavage through thermal cracking. To avoid thermal bond cleavage reactions, the temperature must stay well below pyrolysis of about 340 °C. Likewise, the composition of the oil can be catastrophically altered if the distillation procedure is not conducted under an inert atmosphere due to oxidation which consumes mainly aromatics and resins components and converts them into other resins or asphaltenes, respectively.25 Distillation of crude oil is a critical practice in the industry and is thoroughly understood since most the commercial products (distillates or bottoms) derived from petroleum originate from a distillation procedure. There are many well known methods for pilot scale and lab apparatus for distilling oil which take advantage of instrumentation to detect the onset of cracking, and inert gas atmospheres and vacuum to prevent oxidation. There are also methods which utilize dry ice to trap volatile components. Regarding standard methods, ASTM D86-12 is a method to distill petroleum products at atmospheric pressure which is not applicable to products containing appreciable quantities of residual material and suffers from significant volatiles loss for lighter distillates.26 ASTM D2892-13 is a standard method using a reflux column to determine crude oil distillation at ambient pressure followed by switching to vacuum to reach a corrected boiling point of about 400 °C. This method cannot accommodate petroleum mixtures with light ends such as light naphthas or mixture with initial boiling points above 400 °C and is complicated by potential pyrolysis. This method does however use dry ice traps to collect some of the lighter volatiles that bypass the condensers.27 To get higher distillation fractions ASTM D5236-13 gives a method to distill heavy hydrocarbon mixtures using a pot still at 0.5 mmHg to a corrected temperature up to 560 °C, but it cannot accommodate petroleum fractions with boiling points lower than 160 °C and employs a flask skin temperature of 400 °C which is well within the pyrolysis region of petroleum.28 Others have reported a vacuum distillation of oil at 0.75 mmHg but have lost naphtha material into the vacuum.29 None of these methods or other open literature methods in their current form are suitable for whole crudes or produce unaltered residues (due to oxidation or pyrolysis) that can be analyzed with and ELSD without losing a significant amount of volatile material. We report herein a vacuum distillation method that produces a residue that does not lose volatile material in the ELSD relative to a Lloydminster vacuum residuum quality control (QC). This method also allows the distillate to be collected without losing more than typically 1 wt% of the volatile material and without causing thermal cracking or oxidation of the residue. To compliment this method the aromaticity of the volatiles is measured by proton nuclear magnetic resonance (1H NMR) spectroscopy. The combined 1H NMR aromaticity and SAR-AD analysis are combined to give a variation of the colloidal instability index which is shown to relate to heat exchanger fouling tendencies. The presence of reactive olefins are also identified and partially characterized which can be a diagnostic for determining if crudes are synthetic blends containing pyrolysis components. EXPERIMENTAL SECTION

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Crude Oils and Reagents. Crudes 5, 7, 14, 15, 17, and Dilbit were mainly obtained from WRI’s industry partners who support the Heavy Oil Research Consortium. Prior to sampling the crudes were shaken vigorously to produce a homogenous sample. Solvents (heptane, cyclohexane, toluene, methylene chloride, and methanol) used for the chromatographic separations were HPLC grade from commercial sources and used as received. The silicone oil uses for the high temperature heat bath was a diphenyldimethyl silicone oil (DPDM-400) purchased from Clearco Products Company. SAR-AD Separation. Details for the separation have been previously published by Boysen and Schabron.11 The fully automated separation subdivides heavy oils and asphalts into chromatographic and asphaltene solubility fractions using only 2 mg of sample. The stationary phase column packings are not changed between separations. The separation is performed on a 1.0 gram portion of oil or asphalt which is diluted to 10 mL volume with chlorobenzene giving a 10 % solution (w:v) and a 20 µL aliquot of the solution (2 mg sample) is injected. In this publication slight modifications were made to the SAR-AD separation so that some of the more active brown aromatics material—due to extended conjugation and/or heteroatoms—is partitioned into a separate aromatics peak (aromatics-2). By separating this second aromatics peak from the resin and combining it with aromatics-1 the data almost perfectly track open column SARA data that exits in many historical databases. This modification is discussed in detail later. Vacuum Distillation. The distillation apparatus used was a custom vacuum jacketed Vigreux vacuum distillation apparatus (Chemglass CG-1246-10 is equivalent and been demonstrated to produced similar results) which prevents carryover of any bumped or splattered undistilled oil as well as having limited places for the distillate vapor to condense and get trapped. Circulating hot water or an electric heat gun was used to warm the condensing column to prevent waxes and other high melting point material to solidify in the distillation apparatus. Vacuum was supplied to the distillation apparatus via a high vacuum line capable of pulling to 5 microns (0.005 Torr). Approximately 35-60 g of crude oil was added to a pre weighted 100 mL round bottom flask fitted with a magnetic Teflon®-coated stir bar. Prior to attaching the flask containing the oil, the distillation apparatus and collection flask were evacuated and back flushed with nitrogen. A diagram of the distillation apparatus and overall distillation configuration is provided in Figure 1.

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Figure 1. Diagram showing the quantitative vacuum distillation apparatus. The flask with oil was attached to the distillation apparatus under nitrogen and a Dewar flask containing liquid nitrogen was used to partially cover the collection flask. After allowing the temperature of the collection flask to equilibrate, vacuum was gradually applied until the oil bubbled gently and the vacuum was isolated. The lightest volatile material was allowed to vacuum transfer into the collection flask, as the bubbling ceased the vacuum was increased until gentle bubbling was observed and the vacuum was again isolated to allow the vacuum transfer of more volatile material. The vacuum transfer procedure was repeated until the system was under full vacuum at about 7 microns (0.007 Torr) absolute pressure.

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After the system was under full vacuum a 100 °C silicone oil bath was gradually raised to contact the distillation flask. The distillation flask was allowed to equilibrate and some material was collected by distillation. After the amount of distillate and bubbling began to decrease the oil bath height was gradually raised until the crude oil began to reflux and more material began distilling. The vacuum was maintained at about 10 microns during this procedure (0.01 Torr). After the system reached equilibrium the temperature of the oil bath was increased to 250 °C. For most crude oils the maximum temperature of the distillate vapor was between 131-162 °C. After the vapor temperature of the distillate decreased by at least 50 °C from the maximum temperature the oil bath was removed. The distillate was kept frozen using liquid nitrogen while the residue was allowed to cool to ambient temperature with stirring. Stirring is important to maintain a homogenous residue otherwise distillate remaining in the distillation apparatus will condense onto the residue and not mix with it. After the distillation flask reached room temperature the liquid nitrogen was removed and the distillate flask was allowed to reach about 0 °C. Nitrogen gas was introduced into the distillation apparatus through the vacuum line and the residue and distillate flasks were removed from the distillation apparatus and capped with a rubber septum. The mass of the residue and distillate were measured. More viscous oils like crudes 5 and 7, which have been desalted and collected after the flash drums, needed to be gently warmed until they were fluid enough to be easily stirred with the magnetic stir bar and stir plate. Likewise, for the Dilbit it was found that after removing some volatiles during the vacuum transfer step it became significantly viscous and it was necessary to gently warm the oil to 80 °C to maintain adequate stirring. Nuclear Magnetic Resonance Spectroscopy Proton nuclear magnetic resonance 1 spectroscopy ( H NMR) spectra were acquired at the University of Wyoming using a Bruker DRX-400 instrument and 5 mm NMR tubes fitted with Teflon® valves purchased from New Era. A standard proton NMR pulse program was used with a delay time of 2 seconds. A portion of the distillates were added to the NMR tubes which were fitted with a custom flame sealed glass capillary containing CD2Cl2 purchased from Cambridge Isotope Laboratories (Figure 2).

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Figure 2. Diagram of 5 mm NMR tubes fitted with Teflon® valve filled with distillate (in the annular volume) and a sealed glass capillary containing CD2Cl2 (in the capillary volume). The 1H NMR spectra were referenced to the methylene chloride peak relative to tetramethyl silane (TMS) at 5.32 ppm. Integrations were carried out using Topspin 3.0 software and the bias and slopes were manually adjusted. The aromatic region was integrated from 6.5-9.3 ppm and the aliphatic region was integrated from -0.3-4.4 ppm. The capillary of CD2Cl2 provides a non-interfering internal standard for locking and shimming the instrument and referencing the spectra. RESULTS AND DISCUSSION Volatiles Losses in Analysis. A limitation to any traditional SARA method—and also the automated AD and SAR-AD (and also the Waxphaltene Determinator)30 methods—is that for samples containing significant volatile components, such as aliphatic hydrocarbons with about C25 or less, the lighter volatiles are lost when the solvent is evaporated to measure the weights of the components by gravimetry or ELSD. Since the volatile material is not detected there is a gap 8


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in the data called “volatiles loss”.11 These losses can be up to 60 weight percent or more, depending on the sample. To prevent volatiles loss into the vacuum pump, liquid nitrogen is used to cool the collection flask which also facilitates vacuum transfer and trapping of lighter volatiles when bringing the system under vacuum prior to heating the oil. This method efficiently traps the lightest volatile components and can theoretically trap hydrocarbons as low as C2, and efficiently hydrocarbons which are about C3-C4 in length, but it is bested suited for oils that contain hydrocarbons which are C5 and higher—which is typical of dead crudes—and prevents potentially dangerous build up of pressure (ethane boiling point is -89 °C and a melting point of 183 °C and nitrogen has a boiling point of -196 °C). Liquid nitrogen prevents the volatiles from back transferring and contaminating the resulting residue at the end of the distillation process. After the bulk of the vacuum transfer volatiles are removed from the distillation flask a silicone oil bath is applied to bring the temperature of the distillation flask up to 250 °C (higher temperatures can be obtained). When the temperature of the vapor reaches a maximum and decreases by about 50 °C the distillation can be stopped. At an absolute vacuum pressure of about 10 microns (0.01 mmHg), and assuming a perfectly insulated system, the theoretical maximum atmospheric equivalent boiling point that can be achieved is 598 °C. In practice, it has been observed over a rage of crude oils that at 10 microns the distillation vapor temperature reaches about 145 °C. This corresponds to a corrected atmospheric boiling point around 420 – 435 °C, which is sufficient to collect saturated hydrocarbons in the range of C25-C29. By making a cut in this region the resulting residue has virtually no ELSD volatiles loss when compared to the Lloydminster vacuum residuum QC. Some oils contain a significant amount of volatiles in the 145 °C range and in these cases distillation should be continued until no more volatiles are collected even if the temperature increases up to 165 °C. For example, Crude 17 (Minnelusa) gave a distillation maximum of 162 °C under these conditions. On the flipside, other oils such as diluted tar sand bitumen can give a lower maximum boiling point as was the case for Dilbit (131 °C), since they lack a significant amount of distillates in this region that can be adequately registered by the thermometer. Vacuum Distillation Results. The vacuum distillation involves two main steps: (1) a vacuum transfer of the lightest volatiles of petroleum crude oil followed by (2) vacuum distillation to remove enough volatile materials to provide a residuum suitable for SAR-AD analysis using the ELSD. In the current study, Crudes 5, 7, 14, 15, and 17 showed a broad distribution of volatile material during the distillation. Crudes 15 and 17 qualitatively contained significantly more lighter volatiles since a significant amount of material was collected during the vacuum transfer and during the initial stages of distillation. On the other hand Crudes 5 and 7 contained significantly less lighter volatile material since they were desalted crudes which were collected after flash drums. Crude 15 had a boiling point distribution between Crudes 5 and 7 and 15 and 17. The Canadian Dilbit on the other hand, had most of the volatiles come over during the vacuum distillation and early stages of distillation and very little material came over once the temperature of the distillation was increased to 250 °C. Dilbit is a bitumen that contains a diluent to reduce its viscosity so that it can be transported by pipeline. Therefore, Dilbit (and other synthetic crudes: bottoms from a bitumen upgrader facility combined with oil sands production, usually in a diluted form) is unique since it contains a large amount of lighter volatiles with a relatively narrow boiling point range and much less volatiles once the diluent is removed, 9


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compared to crude oils. Mass balance data and maximum actual distillation temperature at about 10 microns (0.01 mmHg) from the distillation of Crudes 5, 7, 14, 15, 17, and Dilbit are provided in Table 1.

Table 1. Mass balance and maximum vapor temperature distillation temperature data from the vacuum distillation of Crudes 5, 7, 14, 15, 17, and Dilbit.

Crude 5 7 14 15 17 Dilbit

Oil (g) 41.7513 40.3006 42.7272 56.9429 52.5085 34.3235

Distillate (g) Residue (g) 14.6503 26.7236 13.8965 26.1000 22.4674 19.9847 40.2857 16.2942 25.0006 27.1634 11.9549 21.9316

Mass Balance (g) Max Mass Fraction Mass Fraction (wt% loss) Distillation °C of Distillate of Residue 41.3739 (0.90) 152 0.351 0.640 39.9965 (0.75) 145 0.345 0.648 42.4521 (0.64) 145 0.526 0.468 56.5799 (0.64) 152 0.707 0.286 52.1640 (0.66) 162 0.476 0.517 33.8865 (1.27) 131 0.348 0.639

Proton NMR Aromaticities and Olefins. For lighter crude oils the volatiles component can be a significant portion which is often enriched in saturates and some aromatics. Regarding saturates, the ratio and type (linear vs. cyclic for saturates, and monoaromatic vs. polyaromatic and substituted vs. unsubstituted aromatics) can have a significant influence on the stability of the asphaltenes which affects their behavior in the reservoir, during transportation, refining, upgrading, and other properties of the oil or residue. Aromatics on the other hand, are generally much better at dissolving asphaltenes. When predicting the stability of oil by SARA fractionation, or any other analysis technique, if the volatiles are unaccounted for the stability of the oil matrix can be significantly over or underestimated, which can have an adverse effect on models for predicting asphaltene precipitation, emulsion formation, heat induced fouling, sediment formation, blending, deposition, heat induced fouling. Therefore, it is important to quantify some parameters of the distillate, especially in terms of its aromaticity. 1

H NMR data were acquired with a 400 MHz instrument using 5 mm NMR tubes fitted with Teflon valves as described in the experimental section. The aromatic region was integrated from 6.5-9.3 ppm and calibrated to one; the aliphatic region was integrated from -0.3-4.4 ppm, and olefins were observed in the rage of 4.5-6.3 ppm. Figure 3 shows the 1H NMR spectra for Crudes 5, 7, 14, 15, 17, and Dilbit showing the aromatic, the olefin, and the saturates regions separately. Regarding the aromatic portion of the spectra a sharp singlet is present in most of the distillates which corresponds to benzene.

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Figure 3. 1H NMR spectra of the aromatic (top) olefin (middle), and saturates (bottom) for volatiles vacuum transfer and vacuum distillation of Crudes 5, 14, 15, 17, and Dilbit.

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Aromaticities measured by NMR, either proton or carbon, represent the percent ratio of integrated aromatic protons or carbons to saturate protons or carbon, respectively. Aromaticities from proton (1H) NMR have been shown in the literature to correlate very well with carbon (13C) NMR aromaticities especially for crudes and lighter hydrocarbon fractions.31,32 Quantitative aromaticity data from the 1H NMR spectra (Har) can be used to calculate relative 13C NMR aromaticities (Car) using a relationship by Cookson (Car = [k((Har)-1-0.01)+0.01]-1, where k = 0.282).32 The Har and calculated Car values for the distillates are provided in Table 2. Cookson has showed over a wide range of petroleum and fractions that the empirically derived relationship is especially valid for samples where less than 50% of the sample has a boiling point above 320 °C which is the case for all the distillates produced from the quantitative distillation.32 Likewise, more recently others have similarly shown that there exists a linear relationship between Car and Har (the correlation is slightly different).31 From the 1H NMR spectra it is clear that Crude 14 and Dilbit contained olefins in the region δ ~ 4.8-6.2. These signals are very small compared to saturates and aromatics, so they were not included in the integrations. In future work, we intend to establish a method to include quantification of olefins. Table 2. 1H NMR data from the distillation of volatiles for Crudes 5, 7, 14, 15, 17, and Dilbit. 1

Crude 5 7 14 15 17 Dilbit

H NMR Saturates/ Aromatics 21.3 21.0 21.0 22.8 28.3 39.0

1

Har

H NMR %Saturates

Car

Olefins

4.48 4.55 4.55 4.20 3.41 2.50

95.52 95.45 95.45 95.80 96.59 97.50

14.27 14.45 14.45 13.46 11.14 8.33

No No Yes No No Yes

Another very important feature that can explain the thermal history of petroleum is the presence of olefins in the 1H NMR. Olefins are a reactive component within oil which can contribute significantly to fouling—especially at surface temperatures above 300 °C—which is commonly found in heat exchangers.33 It is known that several crudes contain significant amounts of native olefins but these olefins are generally internal type olefins. When more reactive α-olefins are present it is diagnostic that the crude contains pyrolysis components either from thermal cracking, catalytic cracking, hydrotreating, or from high temperature reservoirs. The 1H NMR spectra of Crude 14 contained a significant amount of internal and α-olefin resonances between 4.8-6.1 ppm. Olefins produced by pyrolysis give a distinct distribution of internal olefins and a significant amount of more reactive α-olefins.34,35 Judging by the distribution of internal and α-olefins compared to spectra found in the literature it appears that Crude 14 may be a blend containing lighter portions that were derived from a pyrolysis process.34,35 Figure 4 shows an expanded view of the 1H NMR spectra for the olefin region of Crude 14 and Dilbit distillates with partial olefin type assignments.

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Figure 4. 1H NMR spectra showing an expanded view of the olefin region with assignments based upon references 34 and 35. Other important information, besides aromaticity and olefins, can be gathered from the H NMR spectra. For instance, it was observed for the Dilbit that since it has been blended with a very light fraction of diluent, such as C3-C6, the methyl (CH3, δ ~ 1.2) proton resonances are significantly greater in intensity and integrated area relative to the methylene (CH2, δ ~ 1.6) proton resonances (Figure 3). This is not true for the other dead crudes in this study. Taking this into consideration, with the fact that there was very little later distillates and that there was a relatively large amount of residuum, can be used to help diagnose if a crude is a synthetic crude or a heavy oil/bitumen diluent blend. The proton NMR can also be used to estimate naphthenic species and CH2 groups adjacent to aromatics. 1

Modified SAR-AD Analysis. Following the publication of the original SAR-AD method,11 other researchers have expressed interest in tuning the method so that it would compare more closely with historical data from their particular open column separations. The modification was made by increasing the aromatics values by cutting into the resins fraction by using different heptane:toluene ratios to elute the aromatics-2 fraction. In the initial SAR-AD configuration, the aromatics pass through all four columns and when eluted they do not absorb visible light. These colorless aromatics indicate that this fraction is largely free from heteroatoms and/or very large aromatic structures. In this configuration some of the more polar aromatics were retained on the aminopropyl column (they are more polar since they are more strongly adsorbed to the sorbent). The glass bead and aminopropyl columns keep strongly adsorbing molecules from contacting the activated silica column. Thus, the activated silica column packing remains active and uncontaminated, this means that it does not need to be changed between injections. However, the aromatics from open column separations are typically several percent higher than

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the original SAR-AD aromatics and they traditionally contain some color, heteroatoms, and metals.36 The amino silica column retains most of these other more polar aromatics (these would elute with the open column separation aromatics), and in the original automated SAR-AD method they eluted with the resins fraction. To obtain SAR-AD data that are consistent with open column SARA data, the original SAR-AD resins fraction was split further into an aromatics-2 fraction and a resins fraction. After elution of the aromatics-1 fraction through the activated silica column, as in the original method, the aromatics-2 fraction is eluted by backflushing the amino silica and glass bead columns with the weak chromatographic solvent mixture of heptane:toluene (95:5) (v:v)—this heptane to toluene ratio can be changed to “tune” the separation to provide a weight percent resins fraction that compares with historical data. The remaining polar material is then eluted by backflushing these same two columns with methylene chloride:methanol (98:2) (v:v) as in the original version of the method. An example separation profile chromatogram for the modified configuration of the SARAD is provided in Figure 5 with the appropriate labels. By adding the area of the saturates-2 peak (contains some mono-aromatics with large alkyl side chains) to the area of the satrurates-1 peak a total saturates area is obtained similar to open column methods. Similarly, to better correlate with some open column SARA data—in which the cut between saturates and aromatics is often defined by the onset of absorbance at 254 nm—the areas of the aromatics-1 plus aromatics-2 peaks can be added together to give total aromatics values that are nearly identical to weight percent data from open column SARA separations run on silica gel. The aromatics-1 fraction contains smaller aromatics and is much less contaminated with heteroatoms since it does not adsorb light at 500 nm. This modified separation may be important to better characterize the oils and could provide more refined correlations, which we intend to explore further.

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Figure 5. Example modified SAR-AD separation profile for 2 mg residuum with labels indicating at which time the subfractions elute. Another advantage of the modified configuration of the SAR-AD separation, reported here, is that for most oils the saturates elute as two peaks. In the original SAR-AD configuration these were grouped together.11 The two peaks indicate a separation between more linear and branched type of saturates that are not retained by the columns (cholestane reports to saturates1), and a second peak that elutes through all four columns with heptane and is slightly retarded by the activated silica. Additionally, this material absorbs some light at 230 nm and 260 nm and very little at 290 and 310 nm indicating that it may contain some hydrocarbons with one or two aromatic ring structures with a significant amounts of alkyl side chains This fraction may also contain some saturates that are more cyclic (naphthenic) in character. It is known that oils that have a higher naphthenic character are better solvents for asphaltenes so this may be a gauge to help determine the solvent strength of the maltenes. An example data set collected from the distillation residue of crudes for the new modified SAR-AD configuration is given in Table 3.

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Table 3. Modified SAR-AD data for distillation residua for Crudes 5, 7, 14, 15, 17, and Dilbit. TPA stands for total pericondensed aromatics. Maltenes

Asphaltenes

Aromatics Crude Detector Saturates Nap Sat

CH 2Cl2:

1

2

Resins

CyC6

Total ELSD

Coke Index

Toluene MeOH Asphaltenes CyC6/CH 2Cl2

TPA

5

ELS 500 nm

19.8

10.3

21.4 0.4

22.4 9.4

12.7 19.1

2.1 16.8

10.8 48.8

0.5 5.5

13.4

4.4 3.0

18.8

7

ELS 500 nm

20.9

10.9

21.1 0.4

22.3 10.4

12.2 18.7

2.1 17.1

10.1 47.3

0.5 6.1

12.8

3.9 2.8

18.1

14

ELS 500 nm

24.4

12.3

20.7 0.4

19.6 11.7

9.6 16.5

2.7 20.1

10.5 47.5

0.3 3.8

13.5

9.6 5.3

18.8

15

ELS 500 nm

52.8

15.2

17.4 0.8

10.4 26.2

2.9 22.7

0.2 13.4

1.0 30.6

0.1 6.4

1.3

3.0 2.1

2.5

17

ELS 500 nm

28.9

12.9

20.5 0.3

19.3 11.7

6.8 16.5

1.7 16.5

9.3 48.6

0.6 6.5

11.6

3.1 2.5

16.2

Dilbit

ELS 500 nm

12.1

8.5

22.7 0.4

24.9 11.6

16.4 19.9

3.8 22.2

11.4 43.5

0.2 2.4

15.4

21.2 9.1

22.6

Separation profiles from the SAR-AD (and AD and Waxphaltene DeterminatorTM) take advantage of both an ELSD and a 500 nm optical absorbance spectrometer. The data between the two detectors are different from each other because the ELSD gives an approximate weight percent of material in each subfraction while the 500 nm detector gives the relative concentration of species that have aromatic chromophores capable of absorbing light at that wavelength in each subfraction. The coke index is a measure of the cyclohexane-soluble asphaltenes relative to methylene chlroride-soluble asphaltenes. Pyrolysis studies of residua have shown that as the ELSD coke index approaches about 2 or the 500 nm coke index approaches about 1 the oil becomes unstable with respect to coke formation; beyond these limits coke formation is imminent which is important for maximizing refinery upgrading efficency.18 The combination of ELSD and 500 nm data are used to calculate the total pericondensed aromatic content (TPA).19 The 500 nm data becomes important for samples where the asphaltenes are in very low concentrations and cannot be reliably detected by the ELSD (below 0.2). Further explanation between the differences in the ELSD and 500 nm data can be found in references 18 and 19.

Modified Colloidal Instability Index The colloidal instability (CII) or Gaestel Index relates the amounts of problematic asphaltenes and asphaltene destabilizing saturates to the components in the oil which are good at dissolving the asphaltenes—namely the aromatics and resins.37,38 The Gaestel Index or colloidal instability index is calculated from the weight percent of SARA components and is generally expressed as the ratio of saturates plus asphaltenes divided by the amount of aromatics plus

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resins. Colloidal instability index values have been used as predictors for various phenomena such as heat exchanger fouling,37,39,40,41,42,43 emulsion stabilization,44 and blending and physical properties of asphalt.45 Studies have shown very strong correlations between the CII for oils or blends and fouling.39,40 Generally, oils or blends with CII values around 1 or less foul much lower rate or not at all compared to those with higher CII values,37 and it was reported that the CII values correlated with fouling rates with less scatter than filterable solids or solubility blending numbers.41 Others have shown that the CII and solubility parameter difference can give equally good fits to fouling data since they correlate very strongly with each other.42 An early report showed that for some oils with a high CII value they could still be stable if the resins to asphaltene ratios were also high, therefore for oils with large resin to asphaltene ratios, the CII alone was not a good indicator of fouling. The same trend was reported for oil compatibility determined by solubility parameters from titrations.43 However, in a later study looking over a broad range of oils it was determined that the CII was a significantly better indicator of oil stability than the asphaltene to resin ratio and even slightly better than titration methods— however conclusions based upon the CII are only as reliable as the SARA data. The CII was found to give more reliable predictability for oils with low asphaltene content where other methods, including titration, fail. From this study it was determined that if the CII was below 0.7 the oils were stable, between 0.7-0.9 was an uncertain range, while values above 0.9 were unstable.39 A more robust modified colloidal instability index based upon highly repeatable SARAD data of the residue and aromaticity of the distillate is proposed for characterizing whole crudes. The overall scheme is shown in Figure 6. For simplicity, the Car aromaticity can be translated into a fraction amount of saturates by the following: 1-Car/100. This fraction of saturates will range between 0 and 1 which is multiplied by the weight fraction of the distillate (ωdist) to give the saturates content of the distillate (asphaltene destabilizing power of the distillate). The relative saturate contribution (1-Car/100) of the distillate is multiplied by ωdist and this value can then be added to the classical CII of the residue afforded by the SAR-AD ELSD fractions multiplied by the residue weight fraction (ωresid) to generate a modified colloidal instability index that takes into consideration all the components of the original whole oil (Equation 1). The modified colloidal instability index values for crudes 5, 7, 14, 15, 17, and Dilbit were calculated by this method and are given in Table 4.

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Crude Oil

Vacuum Transfer/Distillation

1 H NMR

SAR or SAR-AD of Residue

of Distillate

Saturates/Aromatics Ratio

Saturates

Aromatics

Resins

Asphaltenes

Figure 6. Analysis sequence for volatile crude oil separation with open column SAR, SARAD, or AD analysis. 1

H NMR data

SAR-AD or open column SARA data

஼ೌೝ

௔௦௣௛௔௟௧௘௡௘௦ା௦௔௧௨௥௔௧௘௦

ቁ + ߱‫ ݀݅ݏ݁ݎ‬ቀ CII = ߱݀݅‫ ݐݏ‬ቀ1 − ଵ଴଴

௥௘௦௜௡௦ା௔௥௢௠௔௧௜௖௦

(1)

ቁ

Table 4. Modified colloidal instability index using 1H NMR data from the volatiles fraction and the SAR-AD data from the distillation residua for Crudes 5, 7, 14, 15, 17, and Dilbit. Distillate Crude

Mass Fraction

5 7 14 15 17 Dilbit

0.35 0.34 0.53 0.71 0.48 0.35

Residue

13

C Saturates = (1-Car /100) 0.86 0.86 0.86 0.87 0.89 0.92

Mass Fraction 0.64 0.65 0.47 0.29 0.52 0.64 0.34830073

18

SAR-AD CII

Mod CII

0.77 0.80 1.01 2.25 1.15 0.56

0.79 0.81 0.92 1.26 1.02 0.68


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As saturates and asphaltenes increase relative to aromatics and resins, a smaller number indicates a more self-compatible oil. Similar to what was discussed above, we estimate that oil with a modified CII above about 0.9 will be unstable in regards to heat exchanger fouling. However there may be a slight rescaling due to how the saturates and aromatics content of the distillate are handled. We intent to publish fouling factor data in the future to validate the modified CII and optimum cut offs for high fouling and low fouling. It should be noted that the CII is only an accurate predictor of heat exchanger fouling if asphaltenes are the dominating mechanism of fouling. This is likely the case for well desalted crude in the absence of oxygen and other reactive species. It is possible that other mechanisms may dominate over asphaltene stability such as oxygen rich gums, solids, corrosion, and reactive species. Based solely on the modified CII, the asphaltene stability of the oils decreases and the relative fouling propensity is expected to increase in the following order: Dilbit, 5, 7, 14, 17, 15. In the future we intend to propose a model using a more optimized modified CII calculation by weighing SAR-AD fractions of saturates-1, saturates-2, aromatics-1, aromatics-2, and the Asphaltene Determinator asphaltene solubility profile data and consider the impact of solids content and type, salt content, olefins, and other reactive species. CONCLUSION A method was developed to vacuum distill a variety of crude oils to make the resulting residue amenable to ELSD quantification so that no volatile material is lost relative to a Lloydminster vacuum residuum QC. The distillation is carried out in a manner which prevents oxidation and thermal bond cleavage so that the distillate and residue accurately reflect the composition of the original whole oil. Data from highly repeatable SAR-AD separations were combined with the 1H NMR aromaticity of the distillate to generate a new modified colloidal instability index was defined which may be an accurate predictor for heat exchanger fouling. We intend to obtain and publish fouling factor data for these oils in the future to determine how well the modified colloidal instability index accurately predicts fouling. The previous configuration of the SAR-AD was modified so that it more closely reflects weight percent data from open column SARA separations performed on silica gel. 1H NMR data of the distillates were demonstrated to diagnose blends in the case of Crude 14 and Dilbit and to show the presence of reactive α-olefins indicating that these oils contained components which had underwent pyrolysis. ACKNOWLEDGMENTS The authors gratefully acknowledge the following Western Research Institute Heavy Oil Research Consortium cosponsors for samples, financial support, and fruitful discusions: ConocoPhillips, ExxonMobil, HTRI, Petrobras, Shell Canada, and Shell USA. ABBREVIATIONS AD, Asphaltene Determinator; ELSD, evaporative light scattering detector; HPLC, high performance liquid chromatography; PTFE, polytetrafluoroethylene; QC, quality control; SAR, saturates aromatics resins; SARA, saturates aromatics resins asphaltene; SAR-AD, saturates aromatics resins and Asphaltene Determinator; 1H NMR, proton nuclear magnetic resonance

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spectroscopy; 13C NMR, carbon nuclear magnetic resonance spectroscopy; Har, aromaticity determined by proton nuclear magnetic resonance spectroscopy; Car, aromaticity determined by proton nuclear magnetic resonance spectroscopy; ωdist, weight fraction of distillate; ωresid, weight fraction of residue.

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REFERENCES (1) Sayles, S.; Routt, D. M. Unconvential Crude Oil Selection and Compatibility. Digital Refining, Operations and Maintenance 2011, March 1000067, 1-11. (2) van den Berg, F. G. A.; Kapusta, S. D.; Ooms, A. C.; Smith, A. J. Fouling and Compatibility of Crudes as Basis for a New Crude Selection Strategy. Pet. Sci. and Tech 2003, 21, 557-568. (3) van den Berg, F. G. A.; Munsterman, E. H. Feedstock Effects in Fouling of Crude Oil Heat Exchangers. Proc. 4th International Conference on Petroleum Phase Behavior and Fouling 2003, Trondheim, Norway. (4) Andersen, S. I.; Keul, A.; Stenby, E. Variation in Composition of Subfractions of Petroleum Asphaltenes. Pet. Sci. and Tech 1997, 15, 611-645. (5) Kharrat, A. M.; Zacharia J.; Cherian, V. J.; Anyatonwu, A. Issues with Comparing SARA Methodologies. Energy Fuels 2007, 21, 3618-3621. (6) Mullins, O. C.; Sheu, E. Y.; Hammami, A.; Marshall, A. G. Asphaltenes, Heavy Oils, and Petroleomics; Springer: New York, 2007. (7) Gray, M. R.; Tykwinski, R. R.; Stryker, J. M.; Tan, X. Supramolecular Assembly Model for Aggregation of Petroleum Asphaltenes. Energy Fuels 2011, 25, 3125-3134. (8) Mullins, O. C. The Asphaltenes. Annu. Rev. Anal. Chem. 2011, 4, 393-418. (9) Mullins, O. C.; Sabbah, H.; Eyssautier, J.; Pomarantz, A. E.; Barre, L.; Andrews, A. B.; RuizMorales, Y.; Mostowfi, F.; McFarlane, R.; Goual, L.; Lepkowicz, R.; Cooper, T.; Orbulescu, J.; Leblanc, R. M.; Edwards, J.; Zare, R. N. Advances in Asphaltene Science and the Yen-Mullins Model. Energy Fuels 2012, 26, 3986-4003. (10) Mullins, O. C.; Seifert, D. J.; Zuo, J. Y.; Zeybek, M. Clusters of Asphaltene Nanoaggregates Observed in Oilfield Reservoirs. Energy Fuels 2013, 27, 1752-1761. (11) Boysen, R. B.; Schabron, J. F. The Automated Asphaltene Determinator Coupled with Saturates,Aromatics, and Resins Separation for Petroleum Residua Characterization. Energy Fuels 2013, 27, 4654–4661. (12) Wu, W.; Saidian, M.; Gaur, S.; Prasad, M. Errors and Repeatability in VSARA Analysis of Heavy Oils. SPE 146107 2012, 1-15. (13) Aske, N.; Kallevik, H.; Sjoblom, J. Determination of Saturate, Aromatic, Resin and Asphaltenic (SARA) Components in Crude Oils by Means of Infrared and Near-Infrared Spectroscopy. Energy Fuels 2001, 15, 1304-1312. (14) ASTM D4124-09. Standard Test Methods for Separation of Asphalt into Four Fractions. Annual Book of ASTM Standards, Vol. 4.03, 2012. 21


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Quantitative Vacuum Distillation of Crude Oils to Give Residues

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