Effects of Aging on Asphaltene Deposit Composition Using Ultrahigh

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Effects of Aging on Asphaltene Deposit Composition Using UltrahighResolution Magnetic Resonance Mass Spectrometry (MRMS) Estrella Rogel, Matthias Witt, and Michael E. Moir Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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Effects of Aging on Asphaltene Deposit Composition Using Ultrahigh-Resolution Magnetic Resonance Mass Spectrometry (MRMS) Estrella Rogel,*,1 Matthias Witt,2 Michael E. Moir.1 1Chevron

Energy Technology Company, Richmond, CA 94801, USA.

2Bruker

Daltonik GmbH, 28359 Bremen, Germany.

* To whom correspondence should be addressed. Telephone: (510)-242-1725. E-mail: [email protected]

ABSTRACT

In this work, the compositional changes of asphaltene deposits as a function of time were analyzed. It has been demonstrated using Ultrahigh-Resolution Magnetic Resonance Mass Spectrometry (MRMS) that the precipitation of asphaltenes is a dynamic process where the composition of the deposit show variations that are linked to the exchange of molecules between deposit and fluid. The detailed analysis of the nature of the species that were exchanged during this process indicates that the more aromatic and richer in heteroatoms molecules are less soluble and become enriched in the deposit as it ages, as the less aromatic with fewer heteroatoms go back into the fluid. It was also found that the oxygen-containing species that precipitate are less aromatic and have smaller

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sizes than those that do not contain oxygen. This is an indication that for these molecules, hydrogen bond and dipole-dipole interactions might play a significant role in their solubility, while for the species that do not contain oxygen, the driven force for precipitation is aromaticity. These findings support a previously proposed model that suggests that during aging, the aggregates become more organized as the more soluble molecules are expelled, while others, the less soluble molecules, are incorporated into the precipitated material.


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INTRODUCTION Among the several factors that affect asphaltene precipitation, time plays one of the most important roles. The appearance of asphaltene flocs outside of the colloidal range (>1 ) can take hours and, in some cases, even months after the addition of heptane.1 Furthermore, time also has a great influence on the amount of precipitated asphaltene,2 on its chemical characteristics,3,4 and on its structure.5 Increasing the contact time between the crude oil and the precipitant agent increases the asphaltene amount.2,4 Deposited asphaltenes become more hydrogen deficient,4 and they form more compact solids.6 Asphaltene precipitation under real conditions, either during upstream or downstream operations, is also influenced by time or aging. Studies of the evolution of deposits as a function of time,4,7 indicated that the deposits get enriched in the least soluble asphaltenes as they aged. This behavior has also been observed during the analysis of deposits from upstream operations.810

In some of these analyses,8,10 it was shown that asphaltenes extracted from deposits are less

soluble than those extracted from the parent crude oil. These observations are compatible with the decrease in the hydrogen content as the deposits aged. Understanding the effect of time is particularly relevant for the evaluation of methodologies focused on asphaltene determination and characterization. It is essential for operations in the oilfield, especially during solvent operation injections or crude oil blending. On the other hand, in the refinery or in the oilfield, the amount, chemical characteristics, and hardiness of the asphaltene deposits influence the selection of the removal treatment and its success. In a previous work,4 we analyzed several physicochemical properties that are strongly linked to precipitation behavior such as hydrogen deficiency, solubility distribution, and apparent


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molecular size as a function of time. In the present work, we evaluate how the detailed compositional and chemical characteristics of the deposit vary with time. Our main objective is to understand the progression of molecules that precipitate and are re-dissolved as the deposit aged. To this end, we used Magnetic Resonance Mass Spectrometry (MRMS) also known as Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). This technique has enabled the characterization of the compositional distributions of crude oils and asphaltenes.11-15 Although quantification in complex systems is not possible, FT-ICR MS has been used successfully to compare the evolution of petroleum materials during chemical reactions16-20 as well as deposits formed under different conditions.8,14,21 Besides, it is important to consider that crude oils contain a wide variety of different species, and the detection of molecules with different characteristics depends on the applied ionization technique.13,22 In the present study, we used atmospheric pressure photo ionization (APPI) as well as laser desorption ionization (LDI). Both techniques can efficiently ionize polycyclic aromatic compounds whether they show basic, acidic, or neutral characteristics.22,23 Regarding asphaltene analysis, it has been shown that the molecular information provided by APPI is in qualitative agreement with data obtained using more conventional methods.15,24 For instance, it was found that the use of APPI data to calculate bulk properties such as H/C molar ratios, average molecular weights, and densities for asphaltene solubility fractions matched exceptionally well the tendencies shown by other experimental values.24 It was also found that the more hydrogen deficient the asphaltene fraction, the better the predictions obtained using APPI data.15,24 Our main goal is to evaluate the evolution as a function of time of the precipitated asphaltenes based on changes in the compositional space. Of special interest are the changes that occur in asphaltenic deposits in the bottom of tanks. These deposits form when incompatible oils are added


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to the same tank. Also, the work is focused on the study of a solvent situation close to the stability boundary; this is, a condition where the solvent power was relatively high. It is expected that this state is closer to the real solvent power conditions in the industry when asphaltene precipitation begins.

EXPERIMENTAL SECTION Materials. The studied crude oil has a medium density with API of 30.4 and an asphaltene content of 2.43 wt. % using the standard method ASTM D-6560.25 Heptane HPLC grade was used in all the experiments. Asphaltene Precipitation. Asphaltenes were separated from the crude oil by mixing 10 mL of crude oil with 20 mL of heptane (heptane/crude oil ratio: 2) and shaking for 5 min. After blending, solutions were kept in static conditions up to 4 weeks (672 h). Blends were filtered using a Teflon membrane filter (0.2 ) after 1, 24, 168 and 672 h. The filtered cake was not washed with additional heptane after filtration. This last part is key to the goal of this work: to replicate as close as possible to a real situation. On the other hand, it seems that precipitation procedures where asphaltenes are not washed with additional solvent are more repeatable.26 We ran samples in duplicates and found that the amount of precipitate differed an average of 10 % between duplicates. Once collected, the filtered cake was dried under nitrogen flow. Carbon and Hydrogen analysis of precipitated material was carried out with a Carlo Erba model 1108 analyzer. In this analysis, the standard deviation depends on the element: C (0.11) and H (0.15). Elemental Analysis of the samples is shown in Table 1.


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Table 1. Elemental Analysis of the asphaltene deposits. Time

Elemental Analysis

(h) C

H

H/C

(wt. % )

(wt. %)

1

84.62

10.87

1.54

24

84.08

10.00

1.43

168

83.38

9.85

1.42

672

83.11

9.53

1.38

FT-ICR MS Analysis. The samples were analyzed using a solariX 2xR MRMS system (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 7 T refrigerated actively shielded superconducting magnet (Bruker Biospin, Wissembourg, France) and the ParacellTM analyzer cell. The spectra were acquired in quadrupole phase detection.27 The Apollo II Dual ESI/MALDI ion source was used. Samples were analyzed using positive ion mode for both ionization methods, APPI and LDI. The transient length of the mass spectrometric measurements was 2.9 seconds. Half-sine apodization was applied, and spectra were processed in absorption mode resulting in a resolving power of 1,300,000 at m/z 400. The spectra were externally calibrated with NaTFA clusters by electrospray ionization in positive ion mode. Spectra were single point calibrated during acquisition with a known mass (lock mass calibration). The final spectrum was additionally


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internally calibrated in DataAnalysis 5.0 (Bruker Daltonics) with the hydrocarbon series for APPI and LDI using quadratic calibration. All RMS mass errors of the internal calibration were below 150 ppb. The RMS mass errors of the calculated molecular formulas of all compound classes for the APPI and LDI measurements were in average below 170 ppb. Samples were prepared by diluting them 1:20 in toluene as stock solution. The stock solution was diluted 1:1000 in 10/90 CH3OH/toluene for final spray solution (50 ppm) for APPI measurements. This solution was directly injected to the APPI source with a syringe pump at a flow rate of 50 L/min. The ion accumulation time was 10 ms, and 300 single scans were added for final mass spectrum. For LDI measurements the stock solution as prepared without any addition of matrix on a stainless steel MALDI plate. The laser energy was set low to reduce ion fragmentation. 300 single scans were added for final LDI mass spectra. Internal mass calibration, spectral interpretation, and export of mass lists were performed using DataAnalysis 5.0 (Bruker Daltonics). The analysis of the data including calculation of molecular formulas and relative abundances of compound classes were performed using PetroOrg 10.0 (Florida State University). Elemental composition assignment was based on Kendrick mass defect sorting. A maximum mass error of 0.5 ppm and a maximum number of heteroatoms of N=3, O=3, and S=3 were allowed for molecular formula calculation. Double-bond- equivalents (DBE) were calculated using the standard equation.28 The isotopic peaks (13C, 34S, etc.) were calculated in the algorithm of the PetroOrg software. Protonated species and radical cations compound classes were calculated separately in the PetroOrg software. Weighted average intensities were calculated using both protonated species and radical cations. Double-bond- equivalence (DBE) values were calculated using the following equation:


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DBE = c -h/2 + n/2 + 1

(1)

For the elemental formula CcHhNnOoSs. In this manuscript, weighted and unweighted averages are calculated for several properties. For example, weighted (Xw) and unweighted (Xu) averages for oxygen content were calculated according to:

(2)

=

(3)

=

Where Ii is the intensity of molecule i, while Oi is the number of the oxygen in molecule i and N represents the total number of molecules. In previous studies, it was found that average values obtained for asphaltenes15,24 and asphaltenic deposits24 showed a good correlation with the bulk properties of these materials determined by conventional techniques. RESULTS AND DISCUSSION Class Distributions. Class distribution plots are shown in Figure 1 based on weighted average intensities considering radical cations as well as protonated species generated by positive APPI. In this plot, HC classes, as well as classes containing one heteroatom, become less abundant in the asphaltene deposit as time passes. The opposite happens for most classes containing 2 or more heteroatoms. Similar results are found when positive LDI is used (Figure 1S in supporting information). Figure 2 shows relative abundances for classes containing different numbers of heteroatoms (APPI) as a function of time. In this plot, the relative abundance of species containing more than one heteroatom increases, while the relative abundance of those containing less than


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DBE and Carbon Number. Figure 4 shows a comparison of the average DBE for the distributions of molecules (calculated based on weighted relative abundances) using positive APPI as a function of time. There is an increase in the average DBE as the deposit ages. These results agree with the decrease in the molar H/C ratio shown in Table 1 and also with a previous analysis of the evolution of n-heptane asphaltenes. In contrast, average carbon numbers, also shown in Figure 4, do not show a clear tendency. The comparison of the C and DBE distributions can be found in the SI (Figures 3S and 4S). Changes due to aging of the deposit are comparatively smaller than those previously reported when different solvent/crude oil ratios were used.21 14.2

38.5

14.0 38.0 13.8 37.5

13.6

C

DBE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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13.4

37.0

13.2 36.5

DBE 13.0

C

12.8

36.0 0

100

200

300

400 Time (h)

500

600

700

800

Figure 4. Weighted average DBE and C as a function of time in the asphaltene deposit. Determined using positive APPI. All classes show an increase in average DBE as the time increases while carbon numbers show slight variations. Therefore, aromaticity increases with age of deposit. Average aromaticity calculated as DBE/(C+N)30 shows an increase from 0.34 (1 h) to 0.38 (672 h). Average aromaticity values follow similar tendencies, whether they are weighted or unweighted averages.


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Dissolved and precipitated species. In a previous work,4 it was shown that the precipitation of asphaltenes is a complex process that involves not only aggregation but also the reorganization of the aggregates. It was found that kinetics plays a large role in the composition of deposits, and 24 h does not seem to be enough time to reach a steady state at the conditions of these experiments.4 Based on these results, a molecular model for precipitation was proposed that involves, at short times, the formation of disordered larger aggregates once the non-solvent is added to the crude oil. In these aggregates, formed in local environments, a large variety of molecules are present, including some that cannot be strictly considered as asphaltenes. Next, the process of formation of more organized domains in the aggregates starts. It was hypothesized that this process is driven by I I non-covalent interactions and possible hydrogen bonding to a lower extent. This process involves the expelling of soluble molecules with a lower capability to engage in I I interactions or hydrogen bonding interactions as well as the incorporation of less soluble molecules.4 Analyzing the species that appear and disappear from the precipitated material can provide clues to a better understanding of the kinetic mechanism of precipitation based on the changes of the precipitated material and shed some light about the aging of deposits during upstream operations where high temperatures are not a factor as they implied the chemical modification of the deposits. Figure 5 shows the total deposit recovered as a function of the time. In this plot, there is an initial decrease in the amount of deposit from 1 to 24 h and, after that, a steady increase. An earlier study3 reports that the maximum amount of precipitate during the first 24 h was obtained a very short time after the n-hexane or n-heptane was added to a crude oil (29.7 API). More recently, the same tendency was reported for a solvent/sample ratio of 1.5.4 In this study, it was observed that the deposit contains a large amount of maltenes on it. It was found that the maltene amount decreases substantially during the first 24 h and is the reason for the observed initial decrease. This


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behavior was not observed at larger ratios (>5). In the present work, as mentioned before, the filtered cakes were not washed based on our goal to replicate real precipitation conditions. However, it is important to mention that the evaluation of asphaltene content as a function of time in the deposits (see Figure 5S) shows that the amount of asphaltenes increases over the whole time. This is the same tendency that has been reported for precipitated asphaltenes1 that are obtaining washing with additional amounts of solvent. Curiously, the number of assigned species seems to correlate with the increase in the content of deposit, as shown in Figure 5. The number of assigned species can be an indication of the

3.50

27000

3.00

25000

2.50

23000

2.00

21000

1.50

19000 Deposit amount

1.00

Number of identified species

complexity of the precipitated material, and it seems to increase as the deposit ages.

Amount of deposit collected (wt. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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17000

Number of identified species

0.50

15000 0

200

400 Time (h)

600

800

Figure 5. Collected amount and number of assigned species in the deposit as a function of time. Lines added as a visualization aid. The comparison of the species present at each time in the deposit can be used to evaluate the exchange of molecules that it is expected from the amount deposited as a function of time shown in Figure 5. By comparing the lists of detected species, it is possible to determine the species that


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are back in the solvent (dissolve) and those that precipitate during specific periods of time. Of course, only compounds that disappeared or appeared can be quantified. Because of this limitation, it is not possible to evaluate every compound that participates in this exchange unless it appears or disappears completely. In Figure 6, the number of species dissolved and precipitated during the three periods studied is shown: 1 to 24 h, 24 h to 168 h, and 168 to 672 h. During the first 24 h after the beginning of the precipitation more species get back to the fluid than precipitate. After the other two time periods, the opposite is observed, more species precipitate than go back into the fluid. In principle, these results agree with the one observed in Figure 5, during the first 24 h the amount of deposit decreases and, therefore, the number of species that dissolved is larger than the number of species that appeared in the deposit at the end of the 24 h. For the other two time periods (24-168 h, 168-671 h), the number of species that precipitate is larger than those that get dissolved as expected from the increase in the total amount of deposit after 24 h shown in Figure 5.

6000 Dissolved species 5000

Number of species

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Precipitated Species

4000

3000

2000

1000

0 1-24

24-168 Time (h)

168-672

Figure 6. Exchange of species per time period. Lines added as a visualization aid.


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Analysis of the characteristics of the species that go back into the fluid or precipitate support the general tendencies observed in previous sections: enrichment of species with more heteroatoms and more aromatic as the deposit ages. Figure 7 shows a comparison of the species that are exchanged between the deposit and the solvent for the three time periods described above. This plot indicates that the species that precipitate are always more aromatic and contain a larger number of heteroatoms than those that are dissolved into the fluid during the same period. These observations support the model proposed previously,4 in which the initial formation of disordered larger aggregates is followed by the evolution into more organized domains. It was postulated that this process of evolution involved the expulsion of soluble molecules with a lower capability to engage in I I or hydrogen bonding interactions as well as the incorporation of molecules that can engage more efficiently in these interactions. Large aromaticity and large heteroatom content in the molecules that precipitate make efficient I I interactions more possible as well as hydrogen bonding and dipole-dipole interactions.

Therefore, these molecules are less soluble in

hydrocarbons than those that dissolved in the fluid. According to the data, after 4 weeks of aging, around 25 % of species are new. In other words, they were not part of the initially precipitated material. Figure 8 illustrates the fraction of molecules originally precipitated that remain in the deposit after aging. This plot, together with Figure 6 seems to suggest that equilibrium has not been reached even after 4 weeks.


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0.48 0.46 Dissolved species

0.44


DBE/(C+N)

0.42 0.40 0.38 0.36 0.34 0.32 0.30 0.28 1-24

24-168 Time (h)

168-672

2.50 2.40 Heteroatoms per molecule (O+N+S)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dissolved species

2.30


2.20 2.10 2.00 1.90 1.80 1.70 1.60 1.50 1-24

24-168 Time (h)

168-672

Figure 7. Comparison of dissolving and precipitating species: a) Aromaticity (DBE/(C+N)) and b) Number of Heteroatoms per molecule. Lines added as a visualization aid.


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1.00

0.95

Fraction of Molecules

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.90

0.85

0.80

0.75

0.70 0

100

200

300

400 Time (h)

500

600

700

800

Figure 8. Fraction of molecules that remain in the deposit after aging. Line added as a visualization aid. Analysis of individual classes comparing the new species at the end of the precipitation (672 h) with the ones that disappear from the original deposit (1 h) showed remarkable differences between both sets of species, depending on their class. Figures 9 and 10 compare the species that are in the deposit at 672 h, but not at 1 h. These species are labeled as “precipitated species”, since it can be assumed that precipitate during the period 1h to 672 h. Additionally, the species that are present at 1 h, but not at 672 h are labeled as “dissolved species” because it can be assumed that they went back into the fluid. Gaps in compositional space show in Figures 9 and 10 are occupied by species that are present in both deposits (1 and 672 h). Figure 9 shows typical compositional plots for S-containing species. These plots indicate that new species in the aged deposit are more aromatic, having larger DBEs than those that were dissolved in the fluid during the aging process. Several classes show similar behavior, for example,


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HC, N1, and N1S1, among others. In contrast, species containing oxygen showed a different pattern, as demonstrated by Figure 10.

Figure 9. Comparison of the compositional space for sulfur containing species that appear in the aged deposit (672 h) with the ones that go back to the fluid from the initially precipitated material (1 h). a) class S1, b) class S2, c) class S3 and d) class S4 In Figure 10, it is noticeable that a small number of species containing oxygen go into the liquid in contrast to a large number that precipitate after several weeks. This behavior contrasts with the one observed for the other classes. Also, the oxygen-containing species that appear in the deposit occupy the whole compositional space. This is also different from what it is observed in


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Figure 9 where the species that appear in the aged deposit are clustered in the high DBE region. Although oxygen-containing molecules that precipitate show higher average aromaticity than similar molecules that go into the fluid, they are, in general, less aromatic than molecules belonging to other classes. This is a significant difference concerning oxygen containing species, indicating that for these molecules, a large DBE is not the only reason for precipitation. This behavior suggests that an important driven force for aggregation and precipitation for these molecules is not just I I non-covalent interactions, but hydrogen bonding or dipole-dipole interactions. Additionally, molecular sizes for species represented in Figure 10 are smaller than for those shown in Figure 9. In general, it was found that species appearing in the aged deposit have smaller sizes and larger DBE than those that disappear independently of the class, however, oxygen-containing species show the lowest C and DBE average values indicating that the presence of oxygen in these species changes their behavior significantly. These changes are linked to the solubility of these molecules in hydrocarbons as shown in Figure 11. Summarizing, the more aromatic molecules and those richer in heteroatoms are less soluble and become enriched in the deposit as it ages, as the less aromatic with fewer heteroatoms go back to the fluid. It is noteworthy that the process might take months to reach equilibrium as the results of this work suggest in agreement with previous reports slow kinetics for asphaltene precipitation.


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Figure 10. Comparison of the compositional space for oxygen containing species that appear in the aged deposit (672 h) with the ones that go back to the fluid from the initially precipitated material (1 h). a) class O1, b) class S1O1 and c) class N1O1


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CONCLUSIONS Using MRMS, it was found that during aging, an exchange of molecules takes place between the precipitated material and the solution. In this exchange, the more aromatic molecules and those richer in heteroatoms go into the precipitated material, while the ones that go back into the solution are less aromatic and contain fewer heteroatoms. These results support the asphaltene precipitation mechanism previously proposed.4 In this mechanism, disordered larger aggregates are formed once the non-solvent (i.e., heptane) is added to the crude oil. These initial aggregates contain a large variety of different molecules and continue changing in composition as time goes by. The model proposed that during aging, the aggregates become more organized as the more soluble molecules are expelled, while others, the less soluble molecules, are incorporated into the precipitated material. The MRMS results also revealed that species appearing in the aged deposit have smaller sizes and larger DBE than those species that disappear independently of the class. However, oxygencontaining species show lower C and DBE average values compared to non-oxygen containing species, indicating that the presence of oxygen significantly changes the behavior of the molecules. This finding suggests that for molecules containing oxygen atoms, hydrogen bonding and/or dipole-dipole interactions play an essential role in their solubility behavior in hydrocarbons. Molecules containing other heteroatoms do not show this tendency. Finally, it is important to point out that, although MRMS APPI does not allow quantification, it is useful in helping the qualitative interpretation of compositional changes that take place within a system. In this case, it has been a key method in finding the link between observed macroscopic changes of precipitated asphaltenes and their compositional evolution.


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Effects of Aging on Asphaltene Deposit Composition Using Ultrahigh

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