Successful Approach to Mimic the Solvent Power of Maltenes Based

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A successful approach to mimic the solvent power of maltenes based on SARA analysis, solvatochromic- and solubility parameters Luzia P Novaki, Raphael Lira, Michelle M. N. Kwon, Marcia Cristina Khalil de Oliveira, Francis A Meireles, Gaspar Gonzalez, and Omar A. El Seoud Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04064 • Publication Date (Web): 11 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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A successful approach to mimic the solvent power of maltenes based on SARA analysis, solvatochromic- and solubility parameters

Luzia P. Novaki,a Raphael Lira;a Michelle M. N. Kwon,a Marcia C. K. de Oliveira,b,* Francis A. Meireles,b Gaspar Gonzalez,c Omar A. El Seoud,a,* a- Institute of Chemistry, the University of São Paulo, 748 Prof. Lineu Prestes Av., 05508-000 São Paulo, SP; e-mail: [email protected] b- Flow assurance laboratory, Petrobras Research Center, (CENPES), Av. Horácio Macedo, 950, Cidade Universitária, 21941-915 Rio de Janeiro, RJ email: [email protected] c- Federal University of Rio de Janeiro , 21941-598 Rio de Janeiro, RJ

Abstract We extended the oil compatibility model to the dissolution of asphaltenes (Asps) in maltenes from ten crude oils (COs). As scales for the power of solvents of interest, vide infra, we used solvatochromic parameters, calculated from the Uv-Vis spectra of solvatochromic compounds (probes), Hildebrand/Hansen solubility parameters, and the colloidal instability index of COs. As the colors of maltenes or asphaltene-free crude oils CO(Asp-free) were too dark to permit recording the absorption spectra of the probes, we formulated models for these fractions, (MCO(Asp-free)). They were composed of low molar mass hydrocarbons, namely cis and trans decalines, isooctane, 1-methylnaphthalene and- as model for resins- benzothiazole/n-octyl-1-naphthoate. We based formulations of these MCO(Asp-free) on SARA analysis of the COs, and elemental analysis of the corresponding resins. We validated MCO(Asp-free) as models for the corresponding CO(Aspfree)

by showing that the correlation between Hildebrand solubility parameter (δt) of (COs)

and δt for MCO(Asp-free) is linear with a slope close to unity. Regarding Asp dissolution, we show that the correlations between log (dissolved Asp, mass%) and each of the following solvent descriptors is linear: empirical polarity of MCOs(Asp-free); (δt) of COs; colloidal instability index of COs. Furthermore, the multiple correlation between log (dissolved Asp, mass-%) and other solvatochromic parameters showed that solvent dipolarity and ACS Paragon Plus Environment

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polarizability are important factors for Asp dissolution, in agreement with our previous results on Asp dissolution in pure solvents. The formulation of a model that successfully mimics maltenes is potentially very useful, e.g., in rationalizing the efficiency of certain classes of additives employed for Asp stabilization.

Abbreviations and acronyms

Asp:

Asphaltene

CII

Colloidal instability index of a crude oil.

CO:

Crude oil

CO(Asp-free)

Asphaltene-free crude oil (or maltene).

ET(HxQMBu2)

Empirical solvent polarity in kcal/mol, based on HxQMBu2.

HxQMBu2

(E-2,6-di-tert-butyl-4-(2-(1-hexylquinolin-1-ium-4-

yl)vinyl)phenolate), a solvatochromic probe for calculation of the empirical solvent polarity parameter ET(probe). MCO(Asp-free)

Formulated model for Asphaltene-free crude oil

MCO(Asp&Resin-free)

Formulated model for Asphaltene- and resin free crude oil

MNI

1-Methyl-5-nitroindoline, a solvatochromic probe for calculation of

solvent Lewis basicity. NI

5-Nitroindoline, a solvatochromic probe for calculation of solvent

Lewis basicity r2

Regression correlation coefficient

SARA

Saturates, aromatics, resins, and asphaltenes of a crude oil.

ΣQ2

Sum of the squares of the residuals.

Introduction The potential problems caused by flocculation followed by sedimentation of asphaltenes (Asps) during petroleum extraction, transport and refining are the impetus for the sustained interest in studying different aspects of Asps, inter alia, experimental methods for determination of Asp stability,1-5 determination of Asp structure from different crude oils (COs),6-11 introduction of models for Asps, 12-18 and theories regarding factors that contribute to their stability, etc.19-25. The complexity of Asp structure, the ACS Paragon Plus Environment

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multiplicity of the interactions of Asps with maltenes (complex mixture of hydrocarbons plus resins, hereafter designated CO(Asp-free)) mean that a general theory to evaluate the importance of Asp-maltene interactions to Asp stability is a distant goal. Instead, we hope to identify general guidelines that serve, e.g., in predicting the stability of COs during their extraction, transport, refining, and choose between different additives to enhance Asp stability, where required. Several scales were introduced to evaluate Asp stability, including the colloidal instability index, CII;1,26 the Heithaus (P) parameter;27-30 the toluene-equivalence (TE) parameter and the related Bureau of Mines correlation index.

31,32

Although the

experimental parts of these determinations and the subsequent calculations are relatively simple, their empirical nature does not offer much insight into the delicate balance of forces whose perturbation leads to flocculation and sedimentation of Asps.22 One of the strategies that are employed for a better understanding Asp stability is to use the oil compatibility model. The latter rests on the assumption that Asp stability is assured when Asps (the solute) and maltenes (the solvent) are “compatible”.

33,34

The

problem is reduced to finding an appropriate scale for quantifying the “solvent power” of maltenes, and then test a collection of maltenes to evaluate the success/generality of this model. A distinct merit of this approach is that it addresses the complex problem of Asp stability in terms of solute/solvent interactions on the molecular level. This is done by using solvent descriptors whose values are experimentally accessible. Examples of these descriptors are Hildebrand and Hansen solubility parameters, recently showed- solvatochromic parameters.

40,41

35-39

and- as we

The term solvatochromism refers to

the effect of the solvent, e.g., maltenes on the Uv-Vis spectra, absorption or emission of solvatochromic substances (hereafter designated as “probes”). There is an intramolecular charge-transfer within the probe whose strength (hence the value of λmax) is affected by probe-solvent interactions. We use this solvent effect on the spectrum of the probe to calculate the corresponding solvatochromic parameter. For example, we calculate the empirical (or total) polarity of the solvent by using the probe HxQMBu2 and the equation:

ET(HxQMBu2) (kcal/mol) = 28591.5/λmax (nm)

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where λmax is the wavelength of the intramolecular charge transfer within the probe, i.e., that from the phenolate oxygen to the quaternary nitrogen. Figure 1 shows the structure of the probes of interest in the present work.

Figure 1: Molecular structures of the solvatochromic probes employed in the present work: (a) (E-2,6-di-tert-butyl-4-(2-(1-hexylquinolin-1-ium-4-yl)vinyl)phenolate, HxQMBu2), (b) β-carotene, (c) 2-(N,N-dimethylamino)-7-nitro- 9H-fluorene (DMANF) (d) 5nitroindoline (NI) and (e) 1-methyl-5-nitroindoline (MNI).

Note that ET(probe) is the sum of specific (e.g., hydrogen-bonding) and nonspecific (e.g., dispersion forces) probe-solvent interactions as shown by Eq 2: 42

ET(probe) = a SA + b SB + d SD + p SP

(2)

where (S) refers to solvent (or mixture of solvents) whose Lewis acidity, Lewis basicity, dipolarity, and polarizability are given by the solvent descriptors (A, B, D, P) respectively and (a, b, d, p) are the corresponding regression coefficients. Eq 3 is the corresponding version of Hansen´s solvent solubility parameters:

δt2 = δAB2 + δD2 + δP2

(3)

The terms AB, D and P refer to (combined) hydrogen bond donation (Lewis acidity) and acceptance (Lewis basicity), dipolar, and dispersion properties of the solvent,


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respectively. The symbols that we use in Eq 3 are different from those originally suggested by Hansen; 36 we use them here for consistency with Eq 2. Employing a set of 38 molecular solvents we recently showed that the values of ET(HxQMBu2) correlate linearly with the corresponding Hildebrand (total) solubility parameters (δt). Likewise, the solvatochromic-based descriptors for solvent Lewis acidity/basicity, dipolarity, and polarizability correlated linearly with the corresponding Hansen solubility parameters. Therefore, we showed that solvatochromic- and solubility parameters are equivalent in accounting for solvent effects.41 Compared with the solubility parameters, the solvatochromic counterparts have the merit of simplicity of the experimental part, and that we treat Lewis acidity and Lewis basicity separately. We obtain information regarding the medium effect on a phenomenon of interest (e.g., Asp dissolution) by applying equations 2 or 3 to the phenomenon in several solvents and examining the sign -positive or negative- and magnitude of the regression coefficients. For example, application of Eq 2 to the reaction of 1-methylimidazole with 1bromohexane in 10 solvents, at 40 °C resulted in the following regression coefficients: a = - 3.79; b = + 20.89; d/p = +56.36. This means that solvent Lewis basicity and dipolarity/polarizability enhances this SN2 reaction; the latter solvent descriptor is more important. On the other hand, solvent Lewis acidity decreases the reaction rate, probably because of diazole solvation by hydrogen-bonding.

43

Equation 3 was applied to

determine the contribution of these parameters to the stability of six COs. 44,45 Likewise, we correlated both scales (solvatochromic and Hansen) with the solubility of Asps in 28 molecular solvents by using Eq 4 and concluded that solvent dipolarity and polarizability are the relevant solvent descriptors. 41

log (dissolved Asps; mass-%) = a/b (SA + SB) + d SD + p SP

(4)

In the present work, we extended the above-mentioned approach to maltenes. Specifically, we wanted to probe the effect of CO(Asp-free) composition on the dissolution/stability of the corresponding Asps, and to correlate this dissolution with solvent descriptors. The dark colors of the latter fractions precluded recording the Uv-Vis spectra of the probes. Therefore we formulated models for maltenes, MCO(Asp-free) whose composition was based on SARA analysis (Saturate, Aromatic, Resin, and Asphaltene) of ACS Paragon Plus Environment

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the precursor COs, and elemental analysis (C/H/N/S) of the corresponding resins. The components of MCO(Asp-free) were mixtures of isooctane (2,2,4-trimethylpentane), cis and trans decalines, 1-methylnaphthalene, benzothiazole and (n-octyl)-1-naphthoate. First we validated the use of this MCO(Asp-free). We then showed that the solubility of Asps in MCO(Asp-free) correlate linearly with: experimental ET(HxQMBu2) of MCO(Asp-free); with experimental δt of CO and SARA-based CII. Additionally, multiple regression analysis with the solvatochromic parameters listed in Eq. 2 indicated that dipolarity and polarizability are important to the dissolution of Asps, in agreement with our previous investigation on solubility of Asps in pure solvents. The relevance of our results to inhibition of Asp precipitation is briefly discussed.

2- Experimental Section 2.1-Equipment We used Fritsch, Laborette 17 model, operating at 35 kHz for sample sonication. The refractive indices (n ± 0.00001) of the crude oils and mixtures were determined with Rudolph Research J357 digital refractometer, operating at 488 nm, all measurements were recorded at 25 οC. We used Bruker Vector-22 and Varian Innova-300 spectrometers to obtain IR and 1H NMR spectra, respectively. Elemental analysis was carried out on Perkin Elmer 2400 series II CHN analyzer at the Analytical Facilities of the Institute of Chemistry, USP, São Paulo.

2.2-Chemicals We purchased the chemicals from Acros, Merck, or Synth (São Paulo) and purified them as recommended elsewhere. 46

2.3- Crude oils and their fractions The crude oil samples were from Petrobras offshore production fields in the Campos Basin, Brazil. Their SARA analyses were supplied by Petrobras, using chromatographic analysis (ASTM D5186)

47

and the IP-143/84 method.

48,49

Results of these analyses are

listed in Table 1 along with the calculated, SARA-based CII of the COs. The corresponding


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asphalt-free and resin-free fractions (CO(Asp&Res-free)) were supplied by Petrobras and were obtained using high pressure fractionation unit. 49

Table 1: Crude oil designation; results of SARA analysis (mass %), and calculated colloidal instability indexes, CII.

Crude oil

Saturate

Aromatic

Resin

Asphaltene

CII

PB-359

42.2

28.9

27.3

1.6

0.78

PB-574

44.2

34

21.3

0.5

0.81

PB-758

58.6

23.4

18

0.5

1.43

PB-1069

66.4

21.0

11.3

1.3

2.10

PB-1081

41.8

35.2

22.3

0.75

0.74

PB-1091

64.4

19.8

15.1

0.7

1.87

PB-1093

52.3

24.5

22.5

0.70

1.13

PB-1100

48.6

21.3

28.6

1.2

1.00

PB-1154

77.6

16

6.4

0.5

3.49

PB-1157

28.0

35.3

33.6

3.1

0.45

designation

An asphaltene-rich solid was obtained from the deasphalting unit of Petrobras. The Asps were obtained from this residue by repeated extraction with n-heptane as described elsewhere.41 Briefly, 200 g Asp-rich solid were suspended in 400 mL n-heptane, sonicated for 4h, the n-heptane separated by decantation; this step was repeated 3 times. The residual solid was the extracted with n-heptane for 32 h (Soxhlet), and dried under reduced pressure until constant mass. Elemental analysis: C, 83.05%; H, 8.60%; N, 1.64%; and S, 5.35%. The H/C (atomic) ratio is 1.23, which is in the same range as that found for Asphs extracted from crude samples, e.g., from Brazil (1.18),49 Mexico (1.14),50 Venezuela (1.10-1.20),51 and USA (0.95-1.14).52

2.4- Synthesis of n-octyl 1-naphthoate ACS Paragon Plus Environment

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We prepared this ester as follows: 1-naphthoic acid (12.00 g; 69.7 mmol) was reacted with thionyl chloride (150 ml, 2.1 mol) under reflux, 3h, followed by removing excess thionyl chloride. Under cooling (ice-bath) the crude 1-naphthoyl chloride (15.58 g, 81.8 mM) was slowly added to a mixture of 1-octanol (9.97g, 76.6 mmol), triethylamine (8.39 g, 83 mmol) and dicloromethane (180 mL). The reaction mixture was stirred for two hours at 0oC, and then overnight at room temperature. Hexane (50 mL) was added, the white precipitate (triethylammonium chloride) filtered, and the volatiles removed. Dichloromethane (50 mL) was added and the resulting yellowish solution was washed with dilute HCl, dilute NaOH, water and dried (anhydrous MgSO4). The solvent was evaporated, and the residual oil was distilled (b.p. 240°C at 1.0 mmHg). Yield “75 %” of a pale buff color liquid. IR data, neat sample (frequency in cm-1, band attribution): 3087 (aromatic C-H), 2928 (aliphatic C-H); 1716 (C=O); 1244 (C-O). 53 The structure and hydrogen atom numbering of this ester is shown in Figure SI-1 . 1H NMR (CDCl3) n-octyl 1-naphthoate: Hg 8.99 (1H, d, J=8.6Hz); Ha 8.22 (1H, dd, J=8.6 and J=1.3Hz); Hc 8.01 (1H, d, J= 8.3Hz); Hd 7.88 (1H, d, J=8.1 Hz); Hf 7.63 (1H, td, J= 8.6 and 8.1Hz); Hb and He 7.47-7,56 (2H, overlapping peaks); Hh 4.44 (2H, t, J=6Hz); Hi 1.85 (2H, m); Hj 1.32-1.46 (10H, overlapping peaks); Hk 0.93 (3H, t, J=7Hz). 54,55 Elemental analysis: Calculated for C19H24O2: C% (80.24); H% (8.51), analyzed C% (80.06); H% (8.65).

2.5- Formulation of models for asphaltene and resin-free crude oils MCO(Asp&Res-free) and asphaltene-free crude oils (MCO(Asp-free)) Each MCO(Asp&Res-free) was formulated by mixing decalines, isooctane and 1methylnaphtalene as given in Table SI-1. Samples of MCO(Asp-free) were obtained from the same hydrocarbons plus a model for the resins, made of equimolar mixture of benzothiazole and n-octyl-1-naphthoate, see Table SI-2.

2.6- Spectroscopic Determination of the solvatochromic parameters ET(HxQMBu2), SB, SD and SP. We used the probes shown in Figure 1 to determine the following solvatochromic parameters: HxQMBu2, ET( HxQMBu2); NI and MNI (SB), DMANF (SD) and β-carotene (SP).41,56-58 Aliquots of the probe solution in acetone were pipetted into small glass vials, followed by evaporation of acetone at room temperature, under reduced pressure, in the ACS Paragon Plus Environment

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presence of P4O10. The solvent mixture whose solvatochromic parameter is to be determined was added (1mL), the probe was dissolved, the solution transferred into a cuvette with appropriate path-length (0.5-2 cm); see experimental conditions in Table SI3. The absorbance of the probes was recorded at 25 °C, 120 nm/min, using Shimadzu UV 2550 spectrophotometer in the range of 350-550nm for β-carotene, DMANF, NI and MNI and 500-700nm for HxQMBu2. Values of λmax were calculated from the first derivative of the absorption spectra; values of ET(HxQMBu2) in kcal/mol were calculated from Eq 1. The equations employed to calculate SB, SD and SP from the values of λmax of the solvatochromic probes are given in the Calculations section of SI. Calculated values of SB, SD, and SP of MCO(Asp-free) are listed in Table-4, vide infra. We also calculated ET(HxQMBu2) of CO(Asp&Res-free). Due to the dark color of these samples, we proceeded as follows: solid HxQMBu2 (4 mg) was added to 2.5 mL of CO(Asp&Res-free), containing 10 mg NaHCO3 (to neutralize any acid traces present). The mixture was sonicated (1h), centrifuged at 4400g for 30 min, the supernatant filtered (PTFE filter, 0.45 μm membrane), and the solution absorbance was recorded as indicated above, using 0.1 cm path length cell, and CO(Asp&Res free) in 0.1 cm cell in the reference beam.

2.7- Determination of Hildebrand solubility parameters (δt) of Crude oils from refractive index measurements We prepared several mixtures (10) each containing 0.5 g of (CO) and variable volumes (0.5 to 1.75 mL) of n-heptane. Each mixture was homogenized (vortex mixer) during 2 minutes, left overnight, and the refractive index (n) of the supernatant determined. Each experiment was carried out in triplicate and the average value of (n) was calculated. Values of (δt) were calculated from the values of (n) as given in the Calculations section of SI. The correlation between the refractive index of CO and volume of n-heptane/g of crude oil is shown in Figure SI-2.

2.8- Determination of asphaltene solubility in MCO(Asp-free): The models prepared in item 2.5 were employed as follow: we weighted ca 350 mg of asphaltenes (see item 2.3 above) in 2 mL Eppendorf polypropylene tube; added 1 mL of MCO(Asp-free); agitated the tube at room temperature, first for 2 min using a vortex ACS Paragon Plus Environment

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mixer, then for 5 h using a tube rotator (Glas-Col model 099A RD4512, 60 rpm), left the tube overnight at room temperature. The tubes were centrifuged for 1 h at 16400 g (Incibrás Spin 1), and the supernatant was filtered through a PTFE membrane (0.45 μm). A certain mass of the filtered solution (ca. 0.3- 0.5 g) was dried in a vacuum oven until constant mass (T = 80 °C; 12-36 h). The solubility of dissolved Asp was calculated in mass%, i.e., dissolved Asp in g/100 g of the filtered asphaltene solution. Each experiment was carried out at least in duplicate.

3- Results and Discussion Note: calculations of all solvent parameters are given in the Calculations section of supporting Information

3.1-Information sought from use of the solvatochromic probe HxQMBu2 As shown in Figure 1, HxQMBu2 possess a dinuclear heterocyclic ring, branched and normal alkyl chains, and two sites for specific and non-specific interactions, namely the phenolate oxygen and the quaternary nitrogen, respectively.

59

Therefore, this probe is

expected to be sensitive to precisely the same (specific and nonspecific) interactions that affect the stability of Asps.

60-63

In other words, we hoped to gain information on the

quality of maltenes as solvents for Asps from the solvatochromic response of this probe.

3.2-The need for- and suitability of model for CO(Asp-free) As mentioned above, we decided to test whether the dissolution of Asps in CO(Aspfree) can

be correlated with the quality of the latter as solvent, as quantified by: (i) “global”

solvent descriptors, including ET(HxQMBu2) and δt; (ii) individual solvent descriptors, e.g., those shown in Eq 4 , with CO(Asp-free) substituting (S). Application of this equation requires that we have the solvatochromic data for CO(Asp-free). This was not feasible because (dark colored) CO(Asp-free) absorbed strongly in the same Uv-Vis spectral region of the probes shown in Figure 1. This turned determination of these solvatochromic parameters for unfeasible. Facing this problem, we decided to develop mixtures that mimic asphaltene-free crude oils, here after designated MCO(Asp-free). We based such formulation essentially on the results of SARA analysis of the corresponding COs, and


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elemental analysis (C/H/N/S) of the corresponding resins. We discuss below this formulation, and give evidence for its suitability as model for CO(Asp-free). We based the model on the assumption that maltenes are mixtures of simple, low molar mass hydrocarbons plus resins. As model for the former, we employed mixtures of 1-methyl naphthalene, isooctane and decalines as aromatic and aliphatic components, respectively. As model for the resins we used an equimolar mixture of benzothiazole and n-octyl-1-naphoate. Each MCO(Asp-free) sample was prepared by mass so that is has the same (SARA) component of the SARA analysis of the corresponding crude oil, see Tables 1 and SI-2. Regarding this formulation, the following is relevant: (i)- There are several examples in the literature for use of mixtures of low molar mass hydrocarbons as models for CO(Asp&Resin-free), including mixtures of n-decane, cyclohexane, toluene, and dioxane;

64

and mixtures of n-heptane, n-octadecane, n-tricosane plus a

petroleum fraction (b.p. = 230–400°C); 65 (ii) Table-2 shows the elemental analysis results for petroleum resins, and some suggested resin models.

Table-2: Elemental analysis of some petroleum resins and models for resins Entry

Country of crude oil origin

%N

%S

%O

H/C atom

Refer-

ratio

ence

1.26-1.49

52

1.31-1.51

66

1.41-1.48

67

1.36-1.39

68

1.39

7

Crude oil-based resins 1

2

3

4

5

USA

USA; Saudi Arabia; Argentina

Canada

USA, Saudi Arabia

Saudi Arabia

0.87-

1.76-

1.94-

1.38

3.36

8.04

0.81-

0.88-

1.53-

1.52

6.91

2.77

0.5-

4.8-

2.7-

0.9

6.0

4.6

0.66-

1.9-

2.3-

1.89

5.95

2.9

0.78

5.02

2.25

Models for resins


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Structure of resin component(s)

%N

%S

%O

H/C

Reference

6

3.3

7.7

7.6

1.16

Presen t work

7

3.2

14.8

--

1.58

69, 70

8

--

16.2

--

0.77

71

As shown in Table 2, the %N of our model (entry 6) is practically the same as the model given in entry 7. Likewise, the %S of our model is not far from crude oil-based resins (entries 3-5) and is less than that in entries 7-8. In variance with the last entries, our model contains oxygen; its H/C atom ratio is close to the range given in entries 1-5. Therefore, elemental analysis of our resin model is in the range of resins from several distinct crude oils, as well as other suggested resin models.

3.3-Validation of our model for CO(Asp-free) For convenience, we discuss this validation by considering first MCO(Asp&Resin-free) because this is a fraction of MCO(Asp-free), then we address MCO(Asp-free) . As indicated above, we were able to record the Uv-Vis spectra of HxQMBu2 both in CO(Asp&Resin-free) and MCO(Asp&Resin-free) hence calculate the corresponding values of ET(HxQMBu2). Based on the data listed in Table 3, a plot of ET(HxQMBu2) for CO(Asp&Resin-free) versus ET(HxQMBu2) for MCO(Asp&Resin-free) gave a straight line with a slope of 0.98 and correlation coefficient 0.93, see Figure 2.


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Figure 2: Correlation between ET(HxQMBu2) for CO(Asp&Resin-free) and ET(HxQMBu2) for MCO(Asp&Resin-free); slope 0.98, r2= 0.93. Values of ET(HxQMBu2) are in kcal/mol.

This shows that the probe-solvent interactions are practically the same in both solvents, i.e., MCO(Asp&Resin-free) mimics CO(Asp&Resin-free).


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[14]

Table 3. Crude oil designation, properties of crude oil fractions, properties of models for crude oil fractions, and mass-% dissolved asphaltenes in MCO(Asp-free).(a) Crude oil

Properties of crude oil fractions

Properties of models for crude oil fractions

designation

Dissolved Asphaltenes, mass-% in MCO(Asp-free)

δ CO

ET(HxQMBu2) of

ET(HxQMBu2) of

ET(HxQMBu2) of

(MPa0.5)

CO(Asp&Resin- free)

MCO(Asp&Resin-free)

MCO(Asp-free)

PB-359

19.0

47.0

47.2

PB-574

18.9

47.4

47.8

PB-758

18.4

47.7

47.7

PB-1069

17.9

47.3

47.4

PB-1081

18.9

47.6

47.4

PB-1091

18.2

47.6

47.5

PB-1093

18.6

48.0

48.1

PB-1100

18.6

47.8

47.8

PB-1154

17.3

48.1

48.2

PB-1157

18.4

47.5

47.5

(a)ET(HxQMBu2) values in kcal/mol.


46.6 46.9 47.1 47.3 46.7 47.3 46.8 46.8 47.9 46.4

25.56 23.84 27.06 16.82 30.06 21.20 22.55 24.44 6.88 34.95

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Due to the above-mentioned problem of the dark color of CO fractions we correlated

δt(CO) versus δtMCO(Asp-free).The former was calculated from the onset of asphaltene precipitation from the crude oil with n-heptane (Table 3), whereas the latter was calculated from δt of its components as given in the Calculations section of SI. This correlation gave a straight line with slope = 1.28 and correlation coefficient = 0.82 (Figure SI-3). Considering these results, we satisfactorily validated the use of MCO(Asp-free) as model for maltenes.

3.4-Can we evaluate the power of asphaltene dissolution by hydrocarbon mixtures in terms of their solvent descriptors? At the outset we comment on Asp dissolution because any particle with diameter < 0.45 µ (the average pore size of the PTFE membrane employed) will pass through the filter and considered, therefore, as dissolved Asp, although it is certainly a nanoaggregate, see, e.g., the discussion on the Yen−Mullins Asp aggregation model.72 Additionally, there are kinetic effects associated with the precipitation and aggregation of Asps; reaching thermodynamic equilibrium may require days in some cases.

73

Considering these aspects we decided to use a filtration membrane with small pores instead of filter paper,74 agitated the Asp suspension for 5h, left it overnight and centrifuged it at the same g-force employed by Fogler et al.73 In summary, we made effort to ensure consistent Asp dissolution data. We show below the obtained correlations between mass-% dissolved Asps and solvent descriptors; these are shown in Table-4. The plot of Figure-3 is based on empirical solvent polarity of MCO(Asp-free); Figure-4 is that based on the solubility parameters of the crude oil, whereas we use CII of (CO) as descriptor in Figure-5. The latter property was calculated from SARA analysis, using Eq. (5), as given elsewhere:75

CII= (% saturates + % asphaltenes) / (% aromatics + % resins)


(5)

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Table 4: Calculated solvatochromic parameters for models of asphalt-free crudes, MCO(Asp-free) , at 25 οC.

MCO(Asp-free) formulated

Solvatochromic Parameters

based on SARA of SB

SD

SP

PB-359

0.211

0.586

0.822

PB-574

0.494

0.475

0.756

PB-758

0.505

0.625

0.788

PB-1069

0.473

0.281

0.773

PB-1081

0.430

0.519

0.799

PB-1091

0.577

0.546

0.761

PB-1093

0.510

0.410

0.794

PB-1100

0.515

0.461

0.790

PB-1154

0.269

0.081

0.717

PB-1157

0.428

0.519

0.795

All plots are straight lines with good correlation coefficients. Figures-3 and Figure-5 have

negative

slopes

for the

following reasons: HxQMBu2

shows

positive

solvatochromism, i.e, the value of λmax increases as a function of increasing solvent polarity. As shown by Eq 1, more polar solvents have smaller values of ET(HxQMBu2), leading to the negative slope shown in Figure-3. From the definition of CII, solvents with high aromatic content, i.e., efficient solvents for Asp dissolution have smaller CII. The conclusion from Figure-3 to Figure-5 is the following: more polar solvents dissolve Asps better; this efficiency is directly related to experimentally accessible solvent descriptors.


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Figure-3: Correlation of ET(HxQMBu2) (in kcal/mol) of MCO(asp-free) and log(dissolved asphaltene, mass-%) in MCO(asp free). r2 = 0.84. (N=10)

Figure-4: Correlation of Hildebrand solubility parameter of CO and log(dissolved asphaltene, mass-%) in MCO(asp free).; r2 = 0.84 (N=9)

Figure-5: Correlation between colloidal instability index (CII) of CO(Asp-free) and log(dissolved asphaltene, mass-%) in MCO(Asp free); r2 = 0.92 (N=10) ACS Paragon Plus Environment

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We took this analysis a step further by calculating the dependence of log (dissolved Asp, mass-%) on the solvatochromic parameters of MCO(Asp-free), where recording the UvVis spectra of all probes shown in Figure 1 is feasible. The Eq employed was:

log (dissolved Asps; mass-%) = b SB + d SD + p SP

(6)

That is, we considered that solvent Lewis acidity is not contributing; Lewis basicity is operative because of the presence of heteroatoms in the resin model, SD and SP are- as expected- contributing. In order to compare the regression coefficients of Eq (6) directly, we used reduced solvatochromic parameters (i.e., on a scale that varies between 0 and 1); Eq (7) was obtained:

log (dissolved Asps, mass-%) = (5.84 ± 0.9) + (0.13 ± 0.10) SBred + (0.31 ± 0.15) SDred + (0.33 ± 0.16) SPred , r2: 0.885 ΣQ2 = 0.081

(7)

The regression coefficients of all parameters are positive, i.e., basic, dipolar, and polarizable maltenes stabilize Asps; dipolarity and polarizability are important solvent properties and agreement with previously literature.41,44,76,33

4-Conclusions The compatibility model is very useful in predicting the stability of Asps. We have successfully used solvatochromic parameters, Hildebrand/Hansen solubility parameters and CII as convenient “scales” for assessing the power of maltenes to dissolve Asps. We formulated models for maltenes by mixing simple hydrocarbons (cis and trans-decalines, isooctane and 1-methylnaphthalene) and resin components (benzothiazole and n-octyl-1naphthoate). We based this formulation on SARA analysis of the crude oils and elemental analysis (C/H/N/S) of the corresponding resins. We validated these formulations as models for maltenes. Correlations of Asp solubility with the descriptors of (CO) and MCO(Asp-free) showed that solvent dipolarity and polarizability are important factors that contributes to Asp dissolution. Note that the latter descriptor includes the stacking of aromatic rings that contributes to Asp stability and Asp-maltene interactions.33 The ACS Paragon Plus Environment

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formulation of a model that successfully mimics maltenes is potentially very useful, e.g., in rationalizing the efficiency of certain classes of additives employed for Asp stabilization.

Acknowledgements L. P. Novaki, R. Lira, M. M. N. Kwon thank Petrobras for research fellowships. We thank the Institute of Chemistry of the University of São Paulo for making its research facilities available to us. O. A. El Seoud thanks FAPESP (grant 2014/22136) and CNPq for research productivity fellowship (307022/2014-5).

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Figure 1 203x96mm (300 x 300 DPI)


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Energy & Fuels

Figure 2 289x202mm (150 x 150 DPI)


Energy & Fuels 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

Figure 3 289x202mm (150 x 150 DPI)


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Energy & Fuels

Figure 4 289x202mm (150 x 150 DPI)


Energy & Fuels 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

Figure 5 289x202mm (150 x 150 DPI)


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Successful Approach to Mimic the Solvent Power of Maltenes Based

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