Suitable Density Determination for Heavy Hydrocarbons by Solution

ARTICLE pubs.acs.org/EF

Suitable Density Determination for Heavy Hydrocarbons by Solution Pycnometry: Virgin and Thermal Cracked Athabasca Vacuum Residue Fractions Lante Carbognani,* Lante Carbognani-Arambarri, Francisco Lopez-Linares, and Pedro Pereira-Almao Alberta Ingenuity Centre for In Situ Energy (AICISE), Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada ABSTRACT: Density is an important parameter for understanding molecular packing, stability, and reactivity of petroleum fractions. The determination of density for extremely viscous residual fractions measured at high temperature (reduced viscosity) is difficult and prone to error if results are extrapolated to lower temperature ranges. This problem is addressed in the present study with fractions derived from virgin and visbroken Athabasca vacuum residua. Solution pycnometry (toluene solvent) was studied and demonstrated a feasible, fast, simple, and reliable technique, applicable to a wide variety of petroleum materials, including asphaltenes and vacuum residua. Reported densities were affected to the third decimal position [American Petroleum Institute (API) gravities found reliable to (0.15° API]. The density for mixtures of residual oil fractions was determined to be an additive property. Athabasca vacuum residue solvent deasphalting and thermal cracked fractions are studied. Thermal cracking was observed to increase asphaltene densities to values as high as those displayed by coals, suggesting that highly aromatic-condensed structures represent these species better. Further application of the methodology is illustrated by monitoring Athabasca bitumen upgrading at bench scale.

1. INTRODUCTION Bulk density for hydrocarbon fractions is an important parameter for understanding stability issues of dense asphaltenes,1 correlations between carbon content, density, porosity, and gasification reactivity of coals,2 and assessing oxidation/cracking balances during asphalt oxidation.3 Density has been found as a convenient tuning parameter for building representative average molecular structures via molecular dynamics simulation.4 Density changes has been described for detection of onset precipitation of solids.5 Methods for density determination of petroleum and derived products based on different properties have been standardized, as described by Speight.6 Different measurement principles are described within these standard procedures, among them: hydrometry (ASTM D287, ASTM D1298, ASTM D1657, and IP 160), pycnometry (ASTM D70, ASTM D941, ASTM D1217, ASTM D1480, and ASTM D1481), fluid displacement (ASTM D712), digital density meter (ASTM D4052 and IP 365), and digital density analyzer (ASTM D5002). When solid samples or extremely viscous materials, such as vacuum residua, are the analytes of interest, serious drawbacks arise because low viscosity (fluidity) is required for determinations using these systems. High temperatures can be set up for bringing samples to fluidity. Vacuum residua normally become mobile in the 120
150 °C range. Asphaltenes normally liquefy above ∼205 °C.7 However, nonlinear extrapolation of densities from these temperature ranges to routine ambient set points, i.e., 15
25 °C, is a serious drawback from using high temperatures during the measurements.8 Instead, wide-mouth glass pycnometers and water + surfactant as a displacing fluid are standard techniques described for solid asphalts.9 However, nonmiscible displacing fluids, such as water or n-heptane, have been determined to pose several problems as well as trapped gas bubbles r 2011 American Chemical Society

release. Gas (helium) pycnometers have also been described for both types of solid or viscous materials.10,11 Helium pycnometry is accepted to provide true densities of solid materials, such as coals, because this probe of small-molecular dimensions guarantees its access to the whole porous space of samples and is a noninteracting fluid with the solid matrix.12,13 Aiming to run several samples simultaneously at near ambient conditions (15
25 °C temperature range), glass pycnometry was evaluated during this work for the determination of density of cumbersome materials, such as sticky vacuum residua and solid asphaltenes. The present work contemplates the use of a fluid able to dissolve the analytes of interest, aiming to provide the following solutions: (1) Overcoming the solid/sticky nature of the samples, (2) use of small samples, (3) simultaneous determination of several samples, (4) simplicity and inexpensive assemblies implied, and (5) reasonable accuracy and precision. Toluene was determined to be a convenient solvent for the intended goal. Determined densities were found accurate to the third decimal position, variability that was found enough for monitoring purposes. Several applications are described in this paper with Athabasca bitumen-derived fractions; in particular, the effect from mild thermal cracking on Athabasca residua asphaltenes was studied in greater detail.

2. EXPERIMENTAL SECTION 2.1. Studied Samples. Selected samples from bench-scale experiments were studied in this work. These will be directly described within the Results and Discussion. Received: May 25, 2011 Revised: July 11, 2011 Published: July 15, 2011 3663

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Table 1. Densities for Reference Materials accepted density

determined density (toluene

temperature (°C)

[(g/mL at T (°C)]

solvent; g/mL at T (°C)]

Athabasca bitumen

15.6

0.9031a

0.9023

atmospheric GO

Athabasca bitumen

15.6

0.9212a

0.9201

vacuum GO

Lloydminster bitumen

15.6

0.9605a

0.9612

Sigma-Aldrich (P/N 32287)

15.6

0.6871a

0.6885

o-xylene

Sigma-Aldrich (P/N 34886)

15.6

0.8833a

0.8821

1-methyl-naphthalene

Sigma-Aldrich (P/N MS7006)

20

1.0058b

1.0127

Sigma-Aldrich (P/N 147141) Aldrich (P/N 12.833-3)

20 20

1.0253b 1.1749c

1.0349 1.0878

biphenyl

Fisher (P/N 0-1421)

20

1.04b

1.0420

phenantrene

Sigma-Aldrich (P/N 11409)

23

1.1793c

1.1386

phenantrene

Sigma-Aldrich (P/N 11409)

25

1.1749c

1.1970d

analyte

analyte group type

light GO

oil distillate

n-heptane

apolar liquid

nahpthalene fluorene

apolar solid

pyrene

b

Sigma-Aldrich (P/N185515)

23

1.271

Aldrich (P/N 40284-2)

23

0.8781a

0.8800

butyl-acetate

Aldrich (P/N 40284-2)

15.6

0.8839a

0.8870

1-octanol 1-octanol

Fisher (A-402) Fisher (A-402)

23 15.6

0.8230a 0.8289a

0.8185 0.8241

2-methoxy-methyl-ethyl ether

Aldrich (P/NM1410-2)

23

0.9477a

0.9484

2-methoxy-methyl-ethyl ether

Aldrich (P/NM1410-2)

15.6

0.9538a

0.9589

3-pentanone

Aldrich (P/N 12760-4)

23

0.8112a

0.8184

BDH (P/N 273381)

18

1.111b

1.1295

camphor

Fluka (P/N 21300)

25

0.99b

0.9609

camphor

Fluka (P/N 21300)

23

0.9649c

0.9628

acridine acridine

Fluka (P/N 01640) Fluka (P/N 01640)

20 20

1.005b 1.005b

1.1971 (avg) 1.2694c

20

1.005b

1.0114d

butyl-acetate

polar liquid

benzophenone

polar solid

acridine a

origin

b

Fluka (P/N 01640) c

1.2033

d

Neat. Handbook (see ref 21). C7-displacer pycnometry (see refs 1 and 4). MeOH solution pycnometry.

Deasphalted oils (DAOs or maltene fractions) and asphaltene fractions isolated from virgin and thermal cracked Athabasca vacuum residua are also studied. Mild thermal cracking (visbreaking) for these samples was previously reported.14 n-Heptane (nC7) and n-pentane (nC5) alkane precipitants were used during deasphalting operations, with experimental deasphalting conditions being reported before.14 Isolated asphaltenes were not solvent-washed. Washing is reported to change asphaltene properties, depending upon the washing approach followed.15 One set of C7-precipitated Venezuelan asphaltenes was analyzed by glass pycnometry with three different displacing fluids: n-heptane, acetonitrile, and helium. Asphaltene properties and origin can be found elsewhere.16,17 Pycnometry using non-solvent displacer fluids was reported for the studied Venezuelan asphaltenes,1,4 and helium pycnometry for selected samples was carried out within IFP facilities at RueilMalmaison (Paris, France).18 Asphaltene samples from the 550 °C+ vacuum residue from Venezuela’s Cerro Negro heavy petroleum have been analyzed for solvent swelling19 and are discussed within the Results and Discussion in the present study. 2.2. Glass Pycnometers. Two commercially available glass pycnometers were used in this study. Both are glass systems of about 11 mL capacity. The more complex device (model 1) is provided with an internal temperature probe (thermometer), manufacturers P/N 16628 029 (KIMAX) and 15123R-10 (KIMBLE-CHASE). The simpler model (2) is manufactured by ACE Glass Incorporated (NJ, P/N 5475-05). More details on pycnometry were recently disclosed.20 2.3. Pycnometer Calibration. Deionized water was used for pycnometer calibration. Its density values taken from the literature21 at 10, 20, 30, 50, and 70 °C are 0.9997, 0.9982, 0.9957, 0.9980, and 0.9778

g/mL, respectively. Water density at 15.6 °C was calculated to be 0.9990 g/mL, by interpolation. A Mettler model AB304-5 balance accurate to 0.1 mg was used for all determinations. 2.4. Solvents. Toluene [high-performance liquid chromatography (HPLC) grade, >99.9%] from Sigma-Aldrich (P/N 354866-44L) was used as a solvent medium for the present study. CHCl3 (>99.8%, Sigma-Aldrich, P/N 480150-4L), tetrahydrofuran (THF, >99.0%, Sigma-Aldrich, P/N 360589-4L), and n-heptane (Sigma-Aldrich, P/N 32287) were used as received for comparison to toluenedetermined density values. 2.5. Thermostated Recirculating Bath. A VWR 28 L refrigerated recirculator bath model 1180S was used for this study. The temperature can be controlled to (0.1 °C. Deionized water was used as recirculation fluid. This particular bath allows for inclusion of up to 40 pycnometers for carrying out simultaneous determinations; however, reasonably only six can be simultaneously handled by a single operator in a routine analysis session. 2.6. Reference Materials. Three bitumen distillates and several commercially available standard compounds were selected for validation of determined densities via toluene pycnometry. Table 1 presents the origin and properties for these reference materials. 2.7. Routine Pycnometry Protocol. About 2
3 g of vacuum residua or 1.5 g of asphaltenes were weighted (to the nearest 0.1 mg) inside 20 mL and/or 2 ounce vials. About 5 mL of toluene was added, and the vials were then capped. A battery of samples was prepared, and the vials were left overnight in a shaker. Quantitative transfer of the solutions was achieved by repeated toluene rinsing (about 5  0.5 mL). Pycnometers were sonicated for a couple of minutes in an ultrasound bath (Branson model 3510). Pycnometers were immersed in the 3664

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thermostatted bath, and achievement of the setup temperature was verified with one model 1 pycnometer filled with toluene for such a purpose. Thermostating is usually performed for about 30 min. During this period, repeated toluene additions were systematically carried out over each pycnometer with the aid of a syringe and needle inserted though the capillary tube. When pycnometers were thermostatted and totally filled, they were capped, dried, and weighted. Density values [at T (°C)] were determined using eq 1 δs ¼ m=ðPv
tm=δtÞ

ð1Þ

where δs is the sample density (g/mL), m is the sample mass (g), Pv is the pycnometer volume (mL, determined with water), tm is the toluene mass (g), δt is the toluene density (g/mL), and T is the temperature Specific gravity (60/60 °F) was calculated correcting determined density values by water density at 60 °F (15.6 °C), and this parameter was used for determining American Petroleum Institute (API) gravity using eq 2. Density reported as API gravity follows a modified Baume scale, with this being common practice within the oil industry.6 API gravity ðdegÞ ¼ ð141:5=specific gravity at 60=60 °FÞ
131:5 ð2Þ

2.8. Bitumen Upgrading Conversion. Heavy end conversions during upgrading studies of a wide range distillation cut and one thermal cracked residue were defined via high-temperature simulated distillation (HTSD). The Athabasca 250
516 °C fraction was the selected distillate. The Athabasca vacuum residue thermal cracked under different severities was the residual studied sample. ASTM D7169 was adopted for these calculations.22 Equation 3 presents how conversion is calculated for fractions boiling above one set temperature limit when converted into lighter distillates. percent weight conversion ¼ ðpercent weight ½IBP > T ð°CÞ in feedstock
percent weight ½IBP > T ð°CÞ in productÞ =ðpercent weight ½IBP > T ð°CÞ in feedstockÞ  100

ð3Þ

where IBP is the initial boiling point and T is the temperature cut point. 2.9. Viscosity Determination. Reported viscosities were determined with a cone-plate Brookfield viscometer model RV DV-II +PROCP. Setup temperatures were maintained with a recirculating glycol bath (Brookfield model TC-102). 2.10. Microcarbon (MCR) Analysis. MCR was determined following a published in-house procedure.23

3. RESULTS AND DISCUSSION 3.1. Proposal of Solution Pycnometry for Density Determination of Solid/Viscous Petroleum Samples. Routine

practiced hygrometer24 and digital densimeter25 density determinations are not suitable for extremely viscous materials, such as oil vacuum residua, unless high temperatures are set up for bringing samples into the liquid phase. Fluidification for these samples requires temperatures in the 120
150 °C range for oil residua and more than 200 °C for asphaltene samples.7 Extrapolation of densities determined around these temperatures to routine ambient ranges (15
25 °C) is not straightforward. Reportedly, nonlinear effects preclude reliable and easy achievable linear extrapolation.8 Gas pycnometry has been described for coping with the cited problems, with applications being described in the literature.10
13 However, this technique is more complex, expensive, and nonroutine in most petroleum laboratories.

Figure 1. Densities for nC7-precipitated asphaltenes. Results were determined with helium or glass pycnometry with nC7 and CH3CN fluids. Analyzed Venezuelan asphaltenes are described elsewhere.16
18

Use of non-solvent displacer fluids in glass pycnometry has been standardized for asphalt samples, selecting water plus a wetting surfactant as the displacing fluid.9 Also, a non-solvent, such as n-heptane, the common precipitant alkane used for deasphalting, has been described in the literature for pycnometry density determination.1,4 However, these pycnometry methods based on water or non-solvent displacer liquids have been found to randomly fail from a series of causes. First, filling pycnometers with highly viscous and sticky materials, such as some oil residua, is extremely cumbersome. Second, releasing trapped air bubbles proved difficult, even submitting the mixtures to ultrasonication. One further drawback of using nonsolvent displacer fluids is the fact that density values determined with different fluids have been observed to differ. One set of asphaltene samples was analyzed by helium pycnometry plus glass pycnometry with n-heptane and acetonitrile. Figure 1 presents the findings from these experiments. Samples analyzed with helium and nC7 were observed to display higher values when the data were gathered by gas pycnometry. These results are a priori expected, because the displacer fluid (helium) is an small molecule able to penetrate the whole porous space of the samples, independent of pore sizes or interlamellar spacing.12,13 Size effects have been reported during density determination of coal samples with different fluids.2,12 On the other hand, the set of samples analyzed with nC7 and CH3CN displacer fluids showed an opposite trend; i.e., density results achieved with acetonitrile were lower compared to those obtained with nC7. These results are contrary to the expectations based on the estimated lower molecular cross-section for CH3CN. However, non-size effects, such as these determined with asphaltene samples, have been reported for coals determined with methanol, hexane, and mercury,26 and even with hydrogen that reportedly is able to become adsorbed over coal samples.12 Complex interactions, such as those observed herein, have been described in experiments in which carbon nanotubes are filled with water and acetonitrile, showing varying interactions that depend upon tube dimensions plus solvent mobility, polarity, and surface tension.27,28 From the preceding findings, different non-solvents can produce different density values for the same sample, hampering in such a way their characterization. Another issue that complicates the use of non-solvent displacing fluids can be envisaged from published results. Asphaltene samples have been determined to swell when exposed to organic solvents.19 This effect is similar to coal swelling induced by 3665

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Figure 2. Solvent swelling of nC7-precipitated asphaltenes. Asphaltenes were isolated from virgin and oxidized (48 h under air bubbling) Cerro Negro heavy petroleum 550 °C+ vacuum residue. hf and ho are the final (solvent equilibrated) and initial asphaltene packed height, respectively. Swelling details are presented elsewhere.19

different organic solvents or solvent mixtures.29
33 Liquids contacting solid asphaltenes exert osmotic pressure that promotes solvent uptaking until this effect is balanced by the restoring elastic forces of the network. Solvent swelling of asphaltenes isolated from vacuum residua asphaltenes is presented in Figure 2. Dramatic volumetric changes are observed to occur after an equilibration period of 24 h. These asphaltenes were observed to almost double its volume (expansion to about 100% additional volume) when an apolar solvent, such as nC7, is the material contacting the sample. To the best of our knowledge, this phenomenon has not been studied further since the publication of the paper.19 No information about the kinetics of asphaltene swelling is available at present. Densities using nC7 as displacing fluid are routinely determined within 1 h after the samples contact the solvent and are thermally equilibrated.1,4 Occurrence of volume expansion during density measurement cannot be verified or discarded unless kinetics of volume expansion are determined. At present, swelling effects on determined sample densities are unknown because volumetric changes during density measurement have not been studied. All of these reasons justify the search for methods for density determination less susceptible to secondary effects. The conceived approach validated here relies on the use of solution pycnometry. 3.2. Validation of Solution Pycnometry. Solution pycnometry must address the feasibility of volume expansion/contraction from mixing of analytes with the chosen solvent. Solution ideality must be guaranteed; i.e., volumes of mixtures should be additive. This presumption is reasonably achieved if the solvent polarity is similar to the sample polarity. This follows the widely accepted principle of “like dissolves like”. Toluene is conceived as a good candidate fulfilling the desired properties because (i) it displays similar polarity to oil samples (the dielectric constant for toluene34 is 2.4 and within the range of 2.2
2.6 for oils35), (ii) it is a good solvent for petroleum components, even solids, such as asphaltenes,36 (iii) its boiling point (BP of 110.61 °C at 1 atm) is reasonably high for decreasing evaporation drawbacks during pycnometry determinations,21 and (iv) water solubility in toluene is very low (about 330 ppm),21 in this way, avoiding humidity problems by solvent evaporation during pycnometry determination.

Figure 3. Correlation between known densities for reference materials and determined values via toluene pycnometry. Analyzed materials are described in Table 1. Outlayers detected in panel A were deleted, and a better correlation is presented in panel B. Analyte group-type sets are illustrated in panel B.

Validation of toluene solution pycnometry was initially verified by comparing values determined for selected known reference materials. Figure 3 presents the results achieved. Solution pycnometry with the toluene solvent provides density values that match those determined for most analyzed materials. Interestingly, most determined polar compound densities are observed to match, which was not expected a priori. However, three solids were observed to deviate, i.e., pyrene, fluorene, and acridine. The widespread deviation of the later is particularly noticeable, which is expectable for polar compounds dissolved in an apolar solvent. However, no explanations can be forwarded at present for the behavior of the two apolar polyaromatics, i.e., fluorene and pyrene, whose solution density was determined to be lower than reported (Table 1). Available evidence points toward an opposite trend; i.e., their solution density should be larger instead of lower than reported, on the basis of the following premise. Strong intermolecular packing for these two solids is expected to occur because they do not have alkyl substitutions. A rich alkyl periphery has been evidenced to disrupt molecular packing, facilitating solubilization phenomena.37 At present, it is speculated that tight molecular packing of these unsubstituted aromatics results in compact, lower volume entities that are not relaxed in toluene. From the preceding, because density is calculated from 3666

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Energy & Fuels the ratio of mass/volume, with volume being lower, it should result in larger determined density values. These previous findings raise concerns for the samples of interest in the present study, i.e., solids, such as asphaltenes. To better understand if toluene pycnometry for solid asphaltenes is reliable, two experiments were carried out as described in the ensuing paragraphs. First, the absence of volumetric changes when mixing asphaltenes and toluene was verified. Three asphaltene toluene solutions were prepared, spanning the range from 1.0 to 1.8 g in about 11 mL. This concentration range covers the routine determination of asphaltene densities, implying about 1.5 g/test (see the Experimental Section). Results presented in Figure 4 suggest that mixtures of asphaltenes and toluene in these proportions follow mixture ideality; i.e., no volumetric changes were determined after mixing. Ideal mixing of the studied petroleum asphaltenes with toluene is believed to happen because asphaltene molecules are surrounded by abundant alkyl moieties that preclude their extended intermolecular interactions and the formation of large solid aggregates difficult to dissolve. Typical Athabasca asphaltenes on average contain 43 carbon atoms/100 carbons,38 suggesting that more than half of these molecules bear paraffinic substituents that preclude the formation of large solid aggregates. However, it must be said that these substituent effects are not expected to avoid the formation of nanosized aggregates commonly reported in the literature.39,40 A second proof for asphaltene ideal mixing was carried out by experimentally determining the density of separated maltenes/ asphaltenes via toluene pycnometry. The density for the starting feedstock (ATVR, i.e., Athabasca vacuum residue) was compared to the density calculated from the separated deasphalting fractions using nC5 and nC7 solvents, considering their massic abundances (mass fractions, F). Results determined at 15.6 °C are presented in the following expressions: δATVR ¼ 1:0554 g=mL ≈ δC7 asphaltene  FC7 asphaltene þ δC7 maltene  FC7 maltene ¼ 1:1801  0:1765 þ 1:0341  0:8235 ¼ 1:0598 δATVR ¼ 1:0554 g=mL ≈ δC5 asphaltene  FC5 asphaltene þ δC5 maltene  FC5 maltene ¼ 1:1825  0:2250 þ 1:0196  0:7750 ¼ 1:0563 From the preceding, calculated densities agree with the determined density for the whole Athabasca residue with an absolute maximum deviation of 0.005 g/mL. This indicates that (i) maltene/asphaltene densities are additive and (ii) toluenedetermined densities are reasonably representative and accurate. Mixing ideality verified with the two experiments discussed before indicates that the density for the samples of interest, i.e., asphaltenes and vacuum residua, can be determined reliably via toluene solution pycnometry. It is presumed that the presence of small quantities of insoluble materials, such as waxes or coke, does not affect these measurements, provided that they are quantitatively transferred into the glass pycnometers by thorough rinsing of the sample containers. Repeatability for any analytical methodology is always a matter of concern. For toluene solution pycnometry, this parameter was evaluated with three oil distillates plus n-heptane and o-xylene as

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Figure 4. Additivity verification of asphaltene/toluene solution volumes. An industrial C5-precipitated asphaltene from Athabasca vacuum residue (average density of 1.1424 at 15.6 °C) was studied.

analytes. Determinations were carried out with three different pycnometers, running at least three replicas in each case. Results indicated (not shown in this paper) that variations were observed in the third or fourth decimal positions. The worst-case scenario, i.e., errors affecting the third position ((0.001 g/mL), translates into a repeatability of (0.15 API gravity values. In addition, analysis repeatability was evaluated with replicated analysis of nC5-deasphalted Athabasca 550 °C+ residue and its corresponding steam catalytic cracking products. The nonvolatile fractions separated after processing within a hot separator device are the samples included in Figure 5. Reasonable API repeatabilities are determined for the processed samples, with standard deviations (SDs) spanning the 0.01
0.11 range. Only the feedstock was observed to produce a larger variability (SD of 0.18). The precisions found are deemed appropriate for process monitoring purposes. The development of the present methodology was undertaken mainly for handling cumbersome residual samples, such as the viscous residua discussed herein. 3.3. Applications of Toluene Pycnometry to Maltene/ Asphaltene Samples Separated from Vacuum Residua. Solid asphaltene and maltene fractions (DAO) derived from vacuum residua are among the most difficult samples for density determination. Air bubbles removal from solids is a frequent problem affecting measurements. The viscous and sticky nature of residual DAO fractions is another problem. Three sets of samples selected from these hydrocarbon types were studied in this section to illustrate the application of toluene glass pycnometry. The sets comprised: (1) one virgin Athabasca vacuum residue (ATVR) and its nC5 and nC7 DAO/asphaltene fractions, (2) nC5 and nC7 asphaltenes isolated from ATVR visbroken residua produced under varying severities, and (3) nC7 DAO/asphaltene fractions isolated from an ATVR, determined at different temperatures. Figure 6 presents the density values determined for the first two sets of studied samples. Mild thermal cracking (visbreaking14) was involved in these studies. Determined density values at 15.6 °C for the whole residue fell in between its asphaltene and maltene fractions. In preceding paragraphs, calculations were made showing that these density values are additive. It is worth noting that virgin C7 maltenes are slightly denser that C5 maltenes, and unexpectedly, virgin C5 and C7 asphaltenes showed similar density values. Another interesting finding from 3667

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Figure 5. API gravity replicate determinations (n g 3) for the feedstock and topped heavy bottoms derived from steam catalytic cracking of deasphalted Athabasca 550 °C+ residue. The tie line (SD) is appended only as a visual aid.

this figure is that visbroken C7 asphaltenes are always denser that visbroken C5 asphaltenes, suggesting that the co-precipitated resins lower the density values for the later. One final interesting aspect shown in this figure is the value of visbroken asphaltenes isolated from a product obtained near instability conditions, i.e., a P value near 1.1 [visbreaking conversion of 28.5% (w/w)14]. Densities around 1.30 g/mL are determined for these asphaltenes, which resemble typical density values for coals.2,12,13,26 Packed structures resembling typical polyaromatic hydrocarbons are suggested from the determined densities (for example, pyrene density is 1.274 g/mL, and benzo[a]pyrene is 1.351 g/mL). The discussed results suggest that dense visbroken asphaltenes can be better represented by condensed molecular architectures described as “like your hand” based in the combined picture obtained from several characterization techniques.41 These condensed and dense materials are also described as “island types”42 and have been identified as the most troublesome fractions during oil production,1,17 showing high aromaticity1 and possessing low H/C atomic ratios and large condensed aromatic ring systems, as indicated by simulation.4 Results presented in Figure 6 for visbroken asphaltenes compared to virgin materials further indicate that thermal cracking induces an increase in density values for the asphaltene fractions as a function of visbreaking severity. It appears that a plateau is reached after thermal cracking proceeds beyond conversions of about 23 wt %. The most important presumed reaction occurring in this process is cracking of alkyl appendages (“pendants”) from aromatic cores43 and/or weak bond scission (for example, sulfide and disulfide bridges).44,45 Alkyl appendages are rich in hydrogen and show density values typical of small alkane distributions centered at about C4
C12, i.e, about 0.75
0.80 g/mL. Their release from the heavy products can rationalize the observed increase in density for the remaining products from visbreaking. Temperature changes can bring severe density changes to vacuum residua, as recently published for Athabasca 500 °C+ vacuum residue.46 Densities spanning from about 1.01 g/mL at 100 °C to 0.88 g/mL at 350 °C are presented therein. In the present study, density values for nC7 maltene/asphaltene fractions isolated from the Athabasca vacuum residue were determined to assess the contribution of these two components to the overall density of Athabasca vacuum residue. The temperature range for these determinations was restricted to

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Figure 6. Density of nC7 Athabasca vacuum residue and its isolated virgin and visbroken maltenes/asphaltenes.

the interval within which toluene can be used as a convenient solvent that does not disrupt the measurements by the production of bubbles, i.e., 10
70 °C. Higher temperatures are expected to induce bubbling within the sample solutions (toluene boils at 110.6 °C/1 atm21). Figure 7 presents the results achieved. The findings indicate that asphaltenes displayed an approximated constant density of 1.170 ( 0.007 g/ mL within the evaluated temperature range. On the other hand, the DAO fraction showed a steady decrease of about 0.025 g/ mL for the spanned 60 °C temperature interval tested. From these results, it seems that the important changes in density are brought by the liquid components of the residue, at least within the temperature range for these measurements. Within the 10
70 °C temperature range, asphaltenes are solids and not prone to noticeable density changes. If an attempt was made to extrapolate asphaltene density determined at one of the above set temperature values and then to use tables to calculate densities at other temperatures, this experimentally determined constancy is not verified by extrapolation because the tables were developed for liquid samples.47 One question that remains open is what happens if the temperature is increase above ∼205 °C, where Athabasca asphaltenes reportedly behave as liquids.7 Glass pycnometry with higher boiling solvents (for example, xylene and tetraline of ca. 140 and 207 °C, respectively) should be used. Eventually, another technique instead of glass pycnometry must be followed to answer this question, with gas (helium) pycnometry being a suitable option. 3.4. Solution Pycnometry with Different Organic Solvents. The present study described toluene solution pycnometry of residual fractions, including isolated asphaltenes. From recent papers39,40 and considering a deemed high experimental concentration of asphaltenes in solution (about 15 wt/vol), aggregates are believed to better describe the mixture state; i.e., densities are being determined for dispersed materials instead of true dissolved substances. Dispersibility effects on determined densities were investigated by preparing asphaltene “solutions” in three different good solvents: toluene (chosen for the present study), CHCl3, and THF. In addition, displacement with nheptane was compared for the set of studied Athabasca virgin and visbroken asphaltenes. Achieved results are presented in Figure 8. Figure 8 indicates that determined density values agree for the three tested good solvents. However, values determined with 3668

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Table 2. Steam Catalytic Cracking of a 250
516 °C Athabasca Distillate Using an Ultradispersed Catalyst Formulation

Figure 7. Density of nC7 asphaltenes/maltenes isolated from ATVR, as a function of the temperature.

Figure 8. Density of C7 virgin and visbroken Athabasca residue asphaltenes determined by solution or displacement pycnometry. Solutions were prepared with toluene, THF, and CHCl3 solvents. Displacement pycnometry was carried out with n-heptane. Replicated analysis was carried out only with the toluene solvent.

n-heptane differed by approximately a factor of 0.96. The later finding confirmed previous observations made with asphaltenes isolated from other crude oils instead of Athabasca bitumen (see the first section of the Results and Discussion); i.e., different solvent displacers provide different density values. Restricted access to the porous space of the solids by different solvent displacers is probably the main cause; however, other interactions discussed above can also play different roles.2,12,26
28 The fact that asphaltene dispersions in three different good solvents provided the same density values is considered as preliminary proof that fundamental asphaltene units are being determined in these experiments. An analogy can be established with helium in gas pycnometry; i.e., solution pycnometry is believed to provide the most representative value of heavy, polar oil components. Further research should be carried out to fully understand these aspects. 3.5. Application of Toluene Pycnometry To Process Reaction Monitoring. Toluene-based glass pycnometry was conceived for handling cumbersome samples, such as asphaltenes and sticky vacuum residua. However, the possibility of running several samples simultaneously opens the feasibility of using it for routine monitoring of samples derived from upgrading studies. One example presented in Table 2 illustrates this possibility. Upgrading a wide Athabasca distillate spanning from 250 to 516 °C is described, with the aim being to decrease viscosity to levels that allow us to use the products as diluents for transportation

conversion at 350 °C+

MCR

sample

(%, w/w)

(%, w/w)

API (deg)

25 °C (cP)

feed 1

0.0 23.1

0.16 1.59

16.4 18.9

216 36.2

2

29.5

2.24

19.8

17.8

3

36.0

3.30

19.3

15.5

viscosity at

of diluted bitumen. Multimetallic ultradispersed catalysts for steam catalytic cracking were used in these experiments.48 Conversion (as defined in eq 3) is calculated in this case as the appearance of atmospheric distillates under a set limit of 350 °C. It is noticed how API gravity determination was able to detect the onset of product degradation that occurred when severity reached a conversion of 36 wt %. Density reversal has also been reported during high-severity thermal processing of whole bitumen.49 In the present example, the conversion level at which API reversal was determined corresponds to a MCR content of 3.3 wt %, and no practical viscosity gains were achieved from these severity levels. The example illustrates how API (density) can be selected as another of the feasible parameters for characterizing oil fractions during upgrading processing.

4. CONCLUSION Pycnometry of toluene sample solutions was studied and demonstrated as a feasible, simple, and reliable technique, applicable to highly viscous or solid samples, such as vacuum residua and asphaltenes. Reported API gravities were found reliable to (0.15° API; i.e., densities are affected to the third decimal position. Density was determined to be an additive parameter for heavy bitumen fractions, such as asphaltenes plus maltenes. Athabasca vacuum residua nC7 asphaltene was determined to display an approximate constant density within the 10
70 °C range, while its corresponding maltenes showed a decrease of 0.025 g/mL in this temperature interval. Mild thermal cracking (visbreaking) was determined to increase Athabasca visbroken asphaltene densities to levels as high as those displayed by coals (1.3 g/mL). Condensed aromatic architectures account for these density ranges better. Solution pycnometry is set up for routine analysis, illustrated with one example describing the catalytic steam cracking of one Athabasca distillate. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: 1-403-210-9707. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge funding from the Alberta Ingenuity Centre for In Situ Energy (AICISE) and the facilities provided by the Schulich School of Engineering, University of Calgary, Canada. Manuel F. Gonzalez is thanked for his earlier involvement in this research. Dr. Estrella Rogel from Chevron-ETC is acknowledged for helpful discussions. Funding derived from PDVSA-Intevep 1990
1995 projects managed by Drs. Orlando Rivas and Alejandro Izquierdo is acknowledged for analysis of 3669

dx.doi.org/10.1021/ef200780d |Energy Fuels 2011, 25, 3663–3670

Energy & Fuels Venezuelan asphaltenes. Drs. Charles Bardon and Emmanuel Behar from IFP-France provided helium pycnometry analyses. Dr. A. Hassan is thanked for MCR analysis. Mariana Trujillo and Pablo Gonzalez are acknowledged for routine pycnometry analysis. Members of the pilot-plant facilities of the Schulich School of Engineering are acknowledged for providing studied Athabasca samples.

’ REFERENCES (1) Carbognani, L. Energy Fuels 2001, 15, 1013–1020. (2) Ng, S. H.; Fung, D. P. C.; Kim, D. Fuel 1984, 63, 1564–1569. (3) Glozman, E. P.; Akhmetova, R. S. Chem. Technol. Fuels Oils 1974, 10 (7), 540–542. (4) Rogel, E.; Carbognani, L. Energy Fuels 2003, 17, 378–386. (5) Buckley, J. S.; Wang, J.; Creek, J. L. Solubility of the least soluble asphaltenes. In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 16, pp 401
437. (6) Speight, J. G. Handbook of Petroleum Analysis; Wiley Interscience: New York, 2001; Vol. 159, Series of Monographs on Analytical Chemistry. (7) Gray, M. R.; Assenheimer, G.; Boddez, L.; McCaffrey, W. C. Energy Fuels 2004, 18, 1419–1423. (8) Mochinaga, H.; Onozuka, S.; Kono, F.; Ogawa, T.; Takahashi, A.; Torigoe, T. Properties of oil sands and bitumen in Athabasca; The Canadian Society of Exploration Geologists CSPG
CSEG
CWLS Convention; Calgary, Alberta, Canada, May 14
17, 2006. (9) American Society for Testing and Materials (ASTM). ASTM D2320. Standard Test Method for Density (Relative Density) of Solid Pitch (Pycnometer Method); ASTM: West Conshohocken, PA, 2008. (10) Keng, E. Y. H. Method and apparatus for volume measurement. U.S. Patents 3,453,881, 1969 and 5,585,861, 1971. (11) Keng, E. Y. H. Powder Technol. 1969, 3 (3), 179–180. (12) Huang, H.; Wang, K.; Bodily, D. M.; Hucka, V. J. Energy Fuels 1995, 9, 20–24. (13) Saha, S.; Sharma, B. K.; Kumar, S.; Sahu, G.; Badhe, Y. P.; Tambe, S. S.; Kulkarni, Y. P. Fuel 2007, 86, 1594–1600. (14) Carbognani, L.; Gonzalez, M. F.; Pereira-Almao, P. Energy Fuels 2007, 21, 1631–1639. (15) Alboudwarej, H.; Jakher, R. K.; Svercek, W Y.; Yarranton, H. W. Pet. Sci. Technol. 2004, 22 (5 and 6), 647–664. (16) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13 (2), 351–358. (17) Carbognani, L.; Espidel, J.; Izquierdo, A. Characterization of asphaltenic deposits from oil production and transportation operations. In Asphaltenes and Asphalts; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 2000; Part 2, Developments in Petroleum Science, Vol. 40, Chapter 13, pp 335
362. (18) Petr
oleos de Venezuela S.A. (PDVSA)-Intevep. Research Projects Asphaltene Deposition and Inhibition of Solid Deposition; PDVSAIntevep: Caracas, Venezuela, 1992. (19) Carbognani, L.; Rogel, E. Energy Fuels 2002, 16, 1348–1358. (20) Carbognani, L.; Lopez-Linares, F.; Carbognani-Arambarri, L.; Pereira-Almao, P. Prepr. Pap.
Am. Chem. Soc., Div. Fuel Chem. 2011, 56 (1), 29–32. (21) CRC Handbook of Chemistry and Physics 2003
2004; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2003. (22) American Society for Testing and Materials (ASTM). ASTM D7169. Standard Test Method for Determination of Boiling Point Distribution of Samples with Resid such as Crude Oil and Atmospheric Resids by High Temperature Gas Chromatography; ASTM: West Conshohocken, PA, 2011 (23) Hassan, A.; Carbognani, L.; Pereira-Almao, P. Fuel 2008, 87, 3631–3639. (24) American Society for Testing and Materials (ASTM). ASTM D1298. Standard Test Method for Density, Relative Density (Specific

ARTICLE

Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method; ASTM: West Conshohocken, PA, 2005. (25) American Society for Testing and Materials (ASTM). ASTM D4052. Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter; ASTM: West Conshohocken, PA, 2009. (26) Toda, Y. Fuel 1972, 51, 108–112. (27) Chaban, V. Chem. Phys. Lett. 2010, 496, 50–55. (28) Chaban, V. Chem. Phys. Lett. 2010, 500, 35–40. (29) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9 (2), 324–330. (30) Liotta, R.; Brown, G.; Isaacs, J. Fuel 1983, 62, 781–791. (31) Larsen, J. W.; Gurevich, I. Energy Fuels 1996, 10, 1269–1272. (32) Yun, Y.; Suuberg, E. M. Energy Fuels 1998, 12, 798–800. (33) Iino, M. Fuel Process. Technol. 2000, 62, 89–101. (34) Static Dielectric Constant of Pure Liquids and Binary Liquid Mixtures: Supplement to IV/6 (Landolt-B€ornstein: Numerical Data and Functional Relationships in Science and Technology; Springer: Berlin, Germany, 2008; Vol. IV/17. (35) Goual, L.; Firoozabadi, A. AIChE J. 2002, 48 (11), 2646–2663. (36) Wiehe, I. A.; Liang, S. K. Fluid Phase Equilib. 1996, 117, 201–210. (37) Carbognani, L; Rogel, E. Pet. Sci. Technol. 2003, 21 (3 and 4), 537–556. (38) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Bitumens and Heavy Oils: Alberta Energy Research Institute (AERI): Calgary, Alberta, Canada, 2003. (39) Friberg, S. E. Micellization. In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 7, pp 189
203. (40) Andersen, S. I. Association of petroleum asphaltenes and the effect on solution properties. In Handbook of Colloids and Surface Chemistry; Birdi, K. S., Ed.; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2009; Chapter 18, pp 703
718. (41) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and weight by time-resolved fluorescence depolarization. In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 2, pp 17
62. (42) Sabbah, H.; Morrow, A. L.; Pomerantz, A. E.; Zare, R. N. Energy Fuels 2011, 25 (4), 1597–1604. (43) Wiehe, I. A. Process Chemistry of Petroleum Macromolecules; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2008. (44) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (45) Dutta, R. P.; McCaffrey, W. C.; Gray, M. R. Energy Fuels 2000, 14, 671–676. (46) Li, X. S.; Elliot, J. A. W.; McCaffrey, W. C.; Yan, D.; Li, D.; Famulak, D. J. Colloid Interface Sci. 2005, 287, 640–646. (47) American Society for Testing and Materials (ASTM). Adjunct to ASTM D1250-04. Standard Guide for Use of the Petroleum Measurement Tables (ADJD1250CD); ASTM: West Conshohocken, PA, 2004. (48) Pereira-Almao, P.; Ali-Marcano, V.; Lopez-Linares, F.; Vasquez, A. Ultradispersed catalyst compositions and methods of preparation. CA Patent 2,630,365 and U.S. Patent 7,897,537, 2007. (49) Al-Sawabi, M.; San Yip, W.; de Bruijn, T. Prepr. Pap.
Am. Chem. Soc., Div. Fuel Chem. 2001, 56 (1), 24–25.

3670

dx.doi.org/10.1021/ef200780d |Energy Fuels 2011, 25, 3663–3670