From a Dream to a Fact: Direct Measurement of Vanadium and Nickel

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Chapter 5

From a Dream to a Fact: Direct Measurement of Vanadium and Nickel Distribution in Crude Oil Cuts Fraction (800−1250 °F) M. M. Boduszynski,1 C. E. Rechsteiner,1 M. E. Moir,1 D. Leong,1 J. Nelson,2 L Poirier,1 and F. Lopez-Linares*,1 1Chevron

Energy Technology Company, 100 Chevron Way, Richmond, California 94801, United States 2Agilent Technologies, Inc., 5301 Stevens Creek Boulevard, Santa Clara, California 95051, United States *E-mail: [email protected].

High-Temperature Gas Chromatography coupled with Inductively Coupled Plasma Mass Spectrometry (HTGC-ICPMS) has shown to be a powerful technique to determine metal distribution as a function of boiling point. From the early days using the original prototype, the concept was proven and illustrated the potential of this technique for the vanadium and nickel fingerprint analysis of deep-cut vacuum gas oil (VGO) fractions (800-1250 °F). The technique shows that these elements are present at temperatures as low as 1040 °F and can be distributed in a narrow or broad boiling point range depending on the parent crude. With the advance of the technology, commercial instrumentation has become available that enable this technique to become routine. Different fractions from crude oils from around the world show that these elements can be distributed in different temperature ranges. Initial vanadium speciation analysis for a VGO fraction from a North American crude reveals that the most common porphyrin type structures are etioporphyrins. Finally, this technique can be extended to other elements beyond vanadium and nickel such as arsenic, selenium, iron, sulfur, iodine, among others to assess how they are distributed in crude oil by boiling point.

© 2018 American Chemical Society Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

1. Introduction It is well recognized in the oil and gas industry that elemental analyses have an essential role in knowing how the type and concentration of metals and non-metals present may impact the entire value chain. For daily upstream and downstream operations, the access to such information is a crucial factor to keep the business running. As an example, by knowing the nature of trace metals present in petroleum feedstocks, it is possible to plan its conversion into different products (1). In particular vanadium and nickel contents have important implications for the oil business; the V/Ni ratio has been used in geochemistry as a correlation parameter, and genetic indicator (2, 3), and V, Ni, and Fe have played an essential role as geochemical markers associated with porphyrins (4, 5). From the refiner’s point of view, the most critical metal-containing compounds are those having, V, Fe, and Ni. Although these metals are present in small amounts (up to 1 % by weight), they may negatively impact petroleum processing because they modify a catalyst’s activity that would ultimately lead to catalyst deactivation (5–8). Different approaches for elemental analysis in crude oil have been reported, and most of the methods usually are accomplished by using spectroscopic techniques after a sample preparation step (9). Details related to those preparation protocols used in petroleum analysis have been reported previously (10–13). Among them, conventional wet acid digestion methods still are the most common choice today for crude oil analysis on ICP-OES and ICP-MS instruments (9, 13), while direct injection of organic solutions has received more attention lately (14–23). The above methods pertain to obtaining the total concentration of the elements but do not produce speciation or classification according to physical or chemical properties. Boduszynski et al. (24) showed that a more in-depth distillation could yield additional high-value vacuum gas oil (VGO) for subsequent processing. From the refining point of view, the value of the increased yield is reduced by the cost of removal of the non-hydrocarbon constituents that may be contained in the VGO. Consequently, obtaining valuable information regarding the distribution of the non-hydrocarbon constituents became essential, since that information allows for the optimization of various refining processes. For obtaining information about hydrocarbon distribution, gas chromatography so far has been the workhorse in many petroleum operations. Simulated distillation (SimDis) is one of the most practiced techniques particularly in the refining industry due to its ability to provide fast information related to the yield distribution (boiling point versus amount) for crude feedstocks, intermediates, and products (25–29). The primary outcome from SimDis is the capability to access the carbon distribution and a hydrocarbon fingerprint of each crude oil and by extension the possibility to know sulfur and nitrogen distributions, which in turn would provide additional relevant information about the sample (29). Additionally, it has been reported that specific vanadium and nickel porphyrins can be analyzed in crude oils and derivatives by GC analysis at high temperatures using different detection methods such as MS or AED (29–34). 88 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

However, the possibility of coupling chromatography with a highly sensitive, multielement detector such as Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), or a multi-isotopic version like Inductively Coupled Plasma Mass Spectrometry (ICP-MS) would allow also the determination of the yield distribution of these elements present in the crude fractions as a function of boiling point. In the case of vanadium and nickel, the possibility to gather such information would have a significant impact on refining applications because it would help to optimize catalyst lifetime, process conditions, and product quality (24). To perform these analyses efficiently, certain experimental aspects need to be considered. First, the type of GC injector and column should be suitable for performing high-temperature SimDis analyses. Second, the transfer line needs to be able to quantitatively transport compounds whose boiling points exceed > 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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

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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