Phase behavior of Safaniya vacuum residue

Fluid Phase Equilibria 380 (2014) 28–38

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Phase behavior of Safaniya vacuum residue Ala Bazyleva, Mildred Becerra, Dmytro Stratiychuk-Dear, John M. Shaw ∗ Department of Chemical and Materials Engineering, University of Alberta, 92 Avenue & 116 Street, Room 832, Edmonton, AB, Canada T6G 2G6

a r t i c l e

i n f o

Article history: Received 26 March 2014 Received in revised form 21 July 2014 Accepted 24 July 2014 Available online 4 August 2014 Keywords: Safaniya Vacuum Residue Phase Diagram

a b s t r a c t The phase behavior of Safaniya vacuum residue (Saudi Arabia) from (200 to 520) K at atmospheric pressure was investigated and the phase states were determined from nanofiltered samples with pentaneasphaltene mass fractions wA between 0.048 and 0.555 based on joint treatment of heat capacity data from differential scanning calorimetry and phase angle data from rheological measurements. SARA and elemental composition data of the oil are also considered and some assumptions on more detailed oil chemistry have been made based on a predictive heat capacity correlation. A pseudo binary phase diagram is constructed based on the behavior of retentate free permeate and permeate free retentate. Compositionally, these materials comprise pentane maltenes and asphaltenes respectively to a first approximation. These pseudo-components, partitioned at 473 K, are shown to behave independently. A broad phase transition arising in Safaniya vacuum residue phase behavior is shown to comprise overlapping but independent transitions in these two pseudo components. The temperature range of the combined glass + melting transition depends on sample thermal history (both long-term and short-term). Based on this representation, Safaniya vacuum residue comprises four phases drawn from crystalline and liquid maltene-rich phases as well as crystalline and liquid asphaltene-rich phases from 265 K to 360–370 K. It is also shown that the phase behavior of chemically separated pentane asphaltenes differs significantly from that of the nanofiltered asphaltene-rich material. The phase diagram of Safaniya vacuum residue is compared with that of Maya crude oil [M. Fulem, et al. Fluid Phase Equilib. 272 (2008) 32–41] and Athabasca bitumen [A. Bazyleva, et al. J. Chem. Eng. Data 56 (2011) 3242–3253]. The differences in the phase behavior of these oils are traced to differences in their composition. © 2014 Published by Elsevier B.V.

1. Introduction Heavy oils and bitumen are mixtures of innumerable hydrocarbon and non-hydrocarbon compounds with a broad distribution of molecular size, molecular structure, and elemental composition. It is currently infeasible to identify, quantify, and isolate all constituents. In order to simplify property analysis and interpretation, hydrocarbon resources are usually separated into the subfractions: saturates, aromatics, resins, and asphaltenes. These fractions possess diverse operational definitions that continue to be debated in the literature [1]. The first three subfractions are sometimes combined into a generalized class—maltenes. The phase diagrams of hydrocarbon resources such as heavy oil/bitumen are complex, and are expected to include several types of phase transformations: melting transitions, glass transitions (i.e., transitions from amorphous solid, or glass, to liquid), solid to liquid crystal transitions, dissolution of liquid crystals into a liquid phase, etc.

∗ Corresponding author. Tel.: +1 780 492 8236. E-mail address: [email protected] (J.M. Shaw). http://dx.doi.org/10.1016/j.fluid.2014.07.037 0378-3812/© 2014 Published by Elsevier B.V.

[2–4]. It should be emphasized that melting is a “true” transition from one separate phase (more or less ordered crystal) to another (liquid) with a distinct boundary between these two phases, while glass transition is a relaxation transition within a single amorphous phase, when system relaxation times become comparable to the timescale of a detection method. The observed phase transformations in heavy oil/bitumen resources and the conditions under which they arise are also expected to depend to some extent on the thermal and shear history of samples and are therefore irreversible in nature. Given these constraints, only phase behavior boundaries are identified for these resource samples. From a phase composition perspective, less information is available than for refined hydrocarbon products, irrespective of their source [5–8], and they must be viewed as ill-defined materials. The phase behavior of two heavy oils from different geographical locations (Canada and Mexico) has recently been studied in our laboratory [2,3]. Maya oil from Mexico has higher an H/C ratio and a lower asphaltene, heteroatom, and metals content than Athabasca bitumen (Canada), and its viscosity at room temperature is about four order of magnitude lower [2,9]. Their phase behaviors expressed as pseudo binaries where the constituents comprise

A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

nanofiltered maltene-rich and pentane asphaltene-rich materials separated at 473 K also differ markedly. For example, Maya crude oil is characterized largely by a first order phase transition (crystallization) upon cooling, while Athabasca bitumen undergoes a glass transition at low temperatures. Since phase-equilibrium and phase behavior data are crucial for all stages of process design, for process operation and optimization, and for process design software development, identifying characteristic phase behaviors and ways to track and model changes in phase behavior with the measureable properties and the origin of oil/bitumen resources is of significant interest. In this work, the phase behavior of Safaniya vacuum residue (Saudi Arabia) is studied using calorimetry and rheology to expand the needed phase behavior database. The measurements, conducted from 200 K to 520 K at atmospheric pressure, also complement prior studies where the nanoscale organization of Safaniya vacuum residue was evaluated from the perspective of nanofiltered [10], and chemically separated and reconstituted [11] oil samples. In this prior work, the size and nominal mass of the nano-scale structures present, and their temperature dependence in the vacuum residue were found to be insensitive to the method of partitioning. In the present work, phase behavior differences, between nanofiltered and chemically separated asphaltenes are also explored. 2. Experimental 2.1. Materials The Safaniya vacuum residue (SVR) sample, supplied by Loic Barre at IFPEN, originated from a Saudi Arabian field. Light fractions were removed by vacuum distillation. Sub samples with a wide range of asphaltene contents were prepared by solventfree nanofiltration of SVR at 473 K using membranes with 10 nm, 20 nm, and 50 nm nominal pore sizes. The nanofiltration apparatus description and operating procedure are described in detail in a previous publication [12]. The sub samples are also thoroughly described in a recent publication [10]. In the current work, only those sample and sub sample characteristics that are crucial for phase diagram generation and discussion – the elemental composition and asphaltene contents – are provided in Table 1. Other than the elemental compositions for pentane (C5) and heptane (C7) asphaltenes, determined using a Carlo Erba EA1108 Elemental Analyzer, the other analyses reported in Table 1 are reproduced from Ref. [10]. Permeates (P) are asphaltene depleted and retentates (R) are asphaltene rich respectively. In the tables, figures and text the nomenclature describing the sub samples is self-explanatory (e.g., P10 and R10 refer to permeate and retentate sub samples obtained using a 10 nm membrane). Asphaltene samples for composition analysis and calorimetric measurements were precipitated from the SVR sample with pentane (C5 asphaltenes) or heptane (C7 asphaltenes) at room temperature and atmospheric pressure at a solvent-to-oil volume to mass ratio of 40:1. The resulting mixtures were stirred overnight at room temperature and then filtered under vacuum through a 0.22 ␮m Millipore membrane. The flask and precipitate were washed with small volumes of solvent to eliminate residual oil. The membranes with precipitated asphaltenes were dried overnight at 333 K in a vacuum oven (∼9 kPa) and weighted. 2.2. Calorimetric measurements Specific isobaric heat capacities of SVR nanofiltered and asphaltene samples were measured in a differential scanning calorimeter TG-DSC 111 (Setaram, France) in the temperature range (200 to

29

520) K at a scanning rate of 5 K min−1 . The calorimeter was calibrated according to the recommendations developed by GEFTA [13–17], as detailed in Ref. [2]. The uncertainties in temperature and energy were established to be ±0.2 K and less than 2%, respectively. The uncertainty of the cp measurements was estimated to be: less than 5% in the temperature interval from (200 to 230) K, less than 3% from (220 to 300) K, and about 2% in the temperature range from (300 to 520) K. All heat capacity data were obtained using a continuous threestep method [18]. In this method, a measuring cell was scanned in the studied temperature range three times sequentially: empty, with a reference material (NIST SRM-720 sapphire in this work), and with an oil sample. An almost identical empty cell was in a reference tube to reduce DSC imbalance in all steps. The isothermal periods at the beginning and the end of each scan were set to 3600s (or 1800s when starting below 290 K). The typical mass of an oil sample was 30–60 mg (with uncertainty of 0.05 mg). Hermetically sealed stainless-steel cells, rated to a maximum pressure of 10 MPa, were used in all experiments. No mass loss was detected following any of the experiments, which confirms the tightness of the cell seals. Low-temperature cp measurements from (173 to 323) K were performed with liquid nitrogen cooling. Heat-flow stabilization was usually achieved at (200–210) K. Only measurements above (200–210) K are reported. High-temperature experiments from (293 to 523) K were conducted without liquid nitrogen cooling, and signal stabilization was attained at (305–310) K. In all cases, dry nitrogen gas was passed through the DSC tubes to eliminate possible impacts from air constituents (water condensation/freezing, oxidation, etc.). The heat-capacity data from these two stages were treated jointly to obtain self-consistent temperature dependences for heat capacity of samples from (200 to 520) K. 2.3. Rheological measurements Steady state and oscillatory rheological experiments were carried out in a Bohlin Gemini HR Nano rheometer (Malvern Instruments Limited, UK). This instrument has a torque range from 3 nN m to 200 mN m with a resolution better than 1 nN m. For these experiments, a parallel plate geometry (25 mm diameter) was used in conjunction with two temperature control units to ensure the reliability of the resulting data: (1) a Peltier plate assembly operating between (243 to 453) K with temperature stability of ±0.2 K, and (2) an extended temperature cell (ETC, forced gas oven temperature controller) with and without a low temperature extension (LTE, cooling with liquid nitrogen vapor) covering the temperature range from (123 to 823) K with stability better than ±0.2 K. The temperature calibration of both units was verified using certified viscosity standards N2700000SP, N74B, N1400B, and N115B (from CANNON Instrument Company, USA), and standard oils #12 and U3600 (from the Paint Research Association, UK). The uncertainty of the viscosity measurements was determined to be ±5% for the Peltier assembly and the ETC without liquid-nitrogen cooling, and better than 10% for the ETC with LTE. The repeatability of the rheological measurements was better than 1%. All experiments were conducted either under nitrogen atmosphere (ETC) or inside a solvent trap (Peltier unit) to minimize sample oxidation and evaporation. The gap between the plates was 1000 ␮m. The temperature sweep measurements were carried out at a heating rate of 3 K min−1 and under either stress or strain control depending on the temperature range. A constant frequency of 1 Hz was applied in the oscillatory mode for most measurements. Supplemental measurements with SVR were also made at 0.1 Hz and 10 Hz. Complex viscosity, shear moduli and phase angle were recorded for oscillatory experiments. A single steady shear experiment was also conducted for SVR at a constant stress of 5 Pa from 298 K to 423 K.

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Table 1 Composition of Safaniya vacuum residue related samplesa . Elemental analysis (wt.%)

Material

P10b P20b P50b SVRb R50b R20b R10b C5 asphaltenesc C7 asphaltenesc C5 maltenesb C7 maltenesb

˛, (mol g−1 )

H/C ratio

C

H

O

N

S

83.7 84.0 84.2 83.9 83.3 83.3 82.8 82.8 82.8 83.7 83.4

11.0 10.9 10.9 10.5 9.5 9.3 9.1 7.9 8.0 11.0 10.5

0.4 0.4 0.4 0.4 0.7 0.7 0.7 0.9 0.6 0.5 0.6

0.3 0.3 0.3 0.4 0.6 0.6 0.6 1.0 1.0 0.2 0.3

4.6 4.7 4.7 5.3 6.1 6.4 6.6 7.4 7.5 4.6 5.2

C5 asph.

C7 asph.

wt.% 1.56 1.55 1.54 1.48 1.37 1.33 1.30 1.13 1.15 1.56 1.50

0.180 0.180 0.179 0.175 0.166 0.164 0.162 0.151 0.152 0.180 0.176

4.8 6.2 7.8 22.9 47.9 53.3 55.5 100

0.7 1.4 2.8 13.4 29.6 35.5 36.7 100

a “P” and “R” stand for permeates and retentates, respectively (e.g., “P10” stands for the permeate obtained by nanofiltration through a membrane with the 10 nm pore size); ˛ is the similarity variable defined by Eq. (1); H/C ratio is the molar ratio of hydrogen to carbon. b Reproduced from Ref. [10]. c This work.

3. Results and discussion 3.1. Calorimetric properties Experimental heat capacity values for SVR and six nanofiltered SVR subsamples are shown in Figs. 1 and 2 for heating runs 1 and 2, respectively, performed successively in each case up to 523 K. Smoothed cp values for both runs with a 5 K step are provided in Tables S1 and S2 in the Supplementary materials. All seven heat capacity curves exhibit first- and second-order transitions similar to those observed with Maya crude oil [3] and Athabasca bitumen [2]. At low temperature, a typical glass-type transition is observed: at ≈210 K for permeates P10, P20, and P50, at ≈225 K for SVR, and at ≈240 K for R10, R20, and R50 (Fig. 1). At higher temperatures, the behavior of the permeate sub samples and SVR include a combination of glass transitions and multi-stage melting spanning from 250–260 K to ≈355 K. Although the heat-capacity shape in the melting region is similar for all permeates and SVR, the number and position of cp maxima vary within this group. The retentate sub samples possess local heat capacity maxima similar to the melting behavior of the permeate sub samples but starting at ≈310 K, with the last peak ending at ≈400 K. The size, shape and position differ from one retentate sub sample to another.

The thermal history effect was studied in two ways: (1) to evaluate short term impacts, samples were scanned between (300 and 523) K twice with a cooling rate of 5 K min−1 and minimal rest period between scans; (2) to evaluate long-term impacts, a second aliquot of permeate P20, prepared at the same time as the sample used to evaluate short term impacts and stored in a refrigerator, was scanned similarly but 7 months after the first aliquot. The results of the short term thermal history experiments, shown in Fig. 2, indicate that the heat capacity for the second heating (run 2) is generally higher than that for the first run (run 1). The melting anomaly is smaller for permeate sub samples and SVR sample during run 2 than during run 1 (smaller portion is crystalline). The melting peak for the retentate sub samples is broader during run 2 than run 1, indicating that larger fractions of these sub samples crystallize during cooling. The results for the long term thermal history experiment are summarized in Fig. 3 and show an additional sharp peak corresponding to a first order transition at ≈350 K. These results provide evidence of slow crystallization/re-crystallization occurring over a long period of time, and most probably this arises in the maltene fraction. The attribution of the sharp phase transition to melting is supported by the ratio Tonset /Tpeak ≈ 2/3. This is a typical ratio for Tg /Tmelt [19,20]. It should be noted that all of the samples were subject to the same thermal history prior to

2.6 2.5 2.4

cp / (J·K -1·g -1)

cp / (J·K -1·g -1)

2.2

1.9

1.6

2.0

1.8

1.3

1.0 200

2.2

250

300

350

400

450

500

T/K Fig. 1. Experimental specific heat capacity values for heating run 1: black line, P10 with wA = 0.048; pink line, P20 with wA = 0.062; green line, P50 with wA = 0.078; amber line, SVR with wA = 0.229; blue line, R50 with wA = 0.479; red line, R20 with wA = 0.533; cyan line, R10 with wA = 0.555. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1.6 300

340

380

420

460

500

T/K Fig. 2. Comparison of specific heat capacities from successive heating run 1 (solid lines) and heating run 2 (dashed lines) for selected nanofiltered SVR samples: black line, P10 with wA = 0.048; amber line, SVR with wA = 0.229; blue line, R50 with wA = 0.479; cyan line, R10 with wA = 0.555. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

2.6

31

2.7

2.5

2.4

2.4

cp / (J·K -1·g -1)

cp / (J·K -1·g -1)

2.1 2.3 2.2 2.1

1.5

1.2

2.0 1.9 310

340

370

400

T/K

430

460

490

3.2. Phase transition end point identification Well-defined heat-capacity baselines for solids and liquids are needed to determine the start and end points for the broad solid to liquid phase transition regions shown in Fig. 1. Accurate and predictive correlations for the specific heat capacity of organic solids and liquids [21–25], recently developed in our laboratory, are used for this purpose. These correlations are based on temperature and on the elemental composition of a sample, from which the value of a similarity variable, ˛, that is proportional to the number of atoms per unit mass is obtained: i

Mi

200

250

300

350

400

450

500

T/K

scanning. Stored samples were heated to 473 K during nanofiltration, and then cooled and stored at or below room temperature for at least three months. Clearly, the phase behavior of SVR is irreversible in the short-term and impacts on phase behavior based on the long-term thermal history of samples also influence outcomes.

n  w

0.9

520

Fig. 3. Comparison of specific heat capacities for P20 (wA = 0.062) due to sample aging: pink solid and dashed lines, cp measured in April 2011 on heating run 1 and 2, respectively; green solid and dashed lines, cp measured in November 2011 on heating run 1 and 2, respectively.

˛=

1.8

,

(1)

Fig. 4. Experimental vs predicted specific heat capacities for selected nanofiltered SVR samples: green lines, P50 with wA = 0.078; black lines, SVR with wA = 0.229; red lines, R20 with wA = 0.533; dashed lines, prediction from Eqs. (1) and (2) for solids; dotted lines, prediction from Eqs. (1) and (3) for liquids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

a22 = −0.3874, a31 = −9.8231 × 10−5 , and a32 = 4.182 × 10−4 are the parameters. These correlations are used to interpret the calorimetric data in two ways: absolute deviations from predicted values, and deviations from trends with temperature. These two uses are illustrated in Fig. 4. The solid correlation, Eq. (2), predicts the heat capacity of solid samples at low temperature and their trends with temperature quantitatively and this correlation was used directly to determine the beginning of glass-type transitions for all samples (Tonset in Table 2). The liquid-phase specific heat capacity correlation, Eq. (3), predicts trends accurately at high temperatures but over predicts sample heat capacity systematically as shown in Fig. 4. Eq. (3) is known to under predict liquid phase cp values for paraffinic compounds and over predict cp values for polycyclic aliphatic and aromatic compounds systematically [24,25]. Illustrative examples are presented in Fig. 5. Further, it is evident that (1) alkynes

i=1

where n is the number of element types in the sample, Mi is the molar mass of element i in g mol−1 , and wi is the mass fraction of element i in the sample. The solid heat-capacity baseline is defined as [21–23]: J K−1 g−1



= 3R A1 ˛ + A2 ˛2









 2  T



exp /T







  2 2 + C1 ˛ + C2 ˛ T

exp /T − 1



+ D1 ˛ + D2 ˛2 T 2 ,

(2)

where cp,corr,s is the specific solid-state heat capacity; T is the temperature in K; A1 = 1.3183 × 10−2 , A2 = 2.4938 × 10−1 , C1 = 2.6526 × 10−2 , C2 = −2.4942 × 10−2 , D1 = 2.5000 × 10−5 , D2 = −1.2300 × 10−4 and  = 151.8675 K are the parameters. The liquid heat capacity baseline, valid from 200 K to Tr ≈ 0.8, is defined as [24,25]: cp,corr,liq









J K−1 g−1 = 24.5 × a11 ˛ + a12 ˛2







+ a21 ˛ + a22 ˛2 T + a31 ˛ + a32 ˛2 T 2 ,



(3)

where cp,corr,liq is the specific liquid-state heat capacity; T is the temperature in K; a11 = −0.3416, a12 = 2.2671, a21 = 0.1064,

5

100 (cp,exp - cp,corr)/ cp,corr



cp,corr,s

10

0

-5

-10 cis-decalin 1-nonyne anthracene fluoroanthene perylene naphthalene

-15

-20 200

250

300

350

400

450

1,1'-bicyclopentyl trans-hydrindane phenanthrene pyrene benzo[a]pyrene

500

550

600

T/K Fig. 5. Relative deviations of the experimental liquid heat capacity values from the values predicted from correlation Eq. (3) for one alkyne and several polycyclic aliphatic and aromatic compounds: C9 H16 with ˛ = 0.201 (trans-hydrindane [26], 1nonyne [27]); C10 H18 with ˛ = 0.203 (cis-decalin [28], 1,1 -bicyclopentyl [29]); C10 H8 with ˛ = 0.140 (naphthalene [30]); C14 H10 with ˛ = 0.135 (anthracene [31], phenanthrene [32]); C16 H10 with ˛ = 0.129 (fluoranthene [33], pyrene [33]); C20 H12 with ˛ = 0.127 (perylene [34], benzo[a]pyrene [35]).

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A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

Table 2 Temperature boundaries and enthalpy for phase transitions in Safaniya oil samples obtained from DSC measurements at pressure p = 0.1 MPa a Material

P10 P20 P50 SVR R50 R20 R10 a

gl ◦

(fus h◦ − wgl cr h0 K )M /J g−1

Tend (K)

Tonset (Tg ) (K)

209 211 211 225 235 238 240

Run 1

Run 2

355 355 356 363 346b 353b 360b

366 372 372 362

50 49 48 50

c

d

c

d

c

d

Symbols: Tonset is the temperature corresponding to the beginning of the maltene glass transition (Tg ); Tend is the temperature corresponding to the end of maltene gl ◦

melting; (fus h◦ − wgl cr h0 K )M is the enthalpy value calculated by Eq. (7). The standard uncertainties in the derived temperatures and enthalpies are estimated to be 1 K and 1 J g−1 . b Tentative assignment due to asphaltene and maltene transition overlap. c The temperature was not determined due to significant overlap between maltene and asphaltene phase transitions. d The value was not estimated (see Section 3.5 for details).

cp,corr,liq









J K−1 g−1 = 24.5 × a10 + a11 ˛ + a12 ˛2









+ a20 + a21 ˛ + a22 ˛2 T + a30 + a31 ˛ + a32 ˛2 T 2

(4)

0.15

(a)

0.10

(cp,exp - cp,corr)/ (J·K -1·g -1)

have higher heat capacity than corresponding cyclic molecules with the same elemental composition (e.g., 1-nonyne vs transhydrindane); (2) fused polycyclic compounds have lower cp than corresponding compounds with separated rings (e.g., cis-decalin vs 1,1 -bycyclopentyl). Thus, rather than use Eq. (3) in a predictive mode, it was modified as suggested elsewhere [36] to account for this systematic deviation. Three correction parameters, a10 , a20 , and a30 , were introduced to obtain:

0.05 0.00 -0.05 -0.10 -0.15

3.3. Rheological responses The nature of the transitions observed in SVR and SVR sub samples was refined through phase angle and complex viscosity measurements. These results are summarized in Table 3 and illustrated in Figs. 7 and 8. All of the raw data are provided in the Supplementary materials as Tables S3–S5. The behavior of phase angle measurements depends on the nature of the transition and the number of constituents comprising a mixture. For a pure crystalline compound, the value approaches ≈0◦ for the solid state (elastic state) and ≈90◦ for the liquid state (viscous state) and

-0.20 300

350

400

T/K

450

500

0.15

(b)

0.10

(cp,exp - cp,corr)/ (J·K -1·g -1)

The parameters, a10 = 1.2908, a20 = −6.8263 × 10−3 , and a30 = 8.0711 × 10−6 , were obtained by performing a least squares fit to the experimental heat capacities (run 1) for P10, P20, and P50 from 360 to 520 K, where the maltenes are liquid and the impact of asphaltenes, irrespective of the phase state, is minor due to their small mass fraction. Fig. 6 shows deviations of the experimental cp values from the values predicted by Eq. (4) for all samples and for both the first and second heating cycles. From these deviations, the end of melting in maltenes (Tend ) is detected and the values are reported in Table 2. While the evaluation is straightforward for the permeate sub samples and SVR, Tend for retentates can be only be assigned tentatively due to the overlap between maltene and asphaltene phase transitions. All peaks arising at T > 360 K in P50 and the retentates are attributed to asphaltenes, as they do not arise in the maltene enriched sub samples. The end of melting in maltenes for run 2 is about 10 K higher than in run 1 (Table 2). This observation is attributed to additional crystallization of heavier fractions during cooling. The large peaks from 330 to 400–410 K appearing during run 2 in the retentate sub samples are attributed to the increase in crystallinity of asphaltenes during cooling. For both cases, the observed transitions appear first order but the designation “crystalline” may be viewed as tentative.

0.05

0.00

-0.05

-0.10 300

350

400

T/K

450

500

Fig. 6. Difference between experimental heat capacity and cp,corr,liq predicted from Eq. (4) for run 1 (a) and run 2 (b): black line, P10 with wA = 0.048; pink line, P20 with wA = 0.062; green line, P50 with wA = 0.078; amber line, SVR with wA = 0.229; blue line, R50 with wA = 0.479; red line, R20 with wA = 0.533; cyan line, R10 with wA = 0.555. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the values do not depend on the oscillatory frequency applied [3]. For this case, the sharp change in phase angle with temperature can be interpreted as the melting point of a compound. For a binary mixture where one of the components forms a crystalline solid, the phase angle approaches ≈0◦ when ∼33 vol.% of the mixture is liquid, approaches ≈45◦ when ∼50 vol.% of the mixture is liquid and approaches ≈90◦ when ∼67 vol.% of the mixture is liquid [3]. If a compound or mixture is amorphous, phase angle starts to

A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38 Table 3 Characteristic temperatures determined from phase-angle measurements at oscillatory frequency of 1 Hz and at pressure p = 0.1 MPa a T0 (K)

T45 (K)

T90 (K)

P10 P20 P50 SVR R50 R20 R10

245 249 257 261 298 303 310

270 273 276 285 336 356 355

301 309 327 343 410 427 432

90 80 70

phase angle / deg

Sample

33

60 50 40 30

a

Symbols: T0 , T45 and T90 are the temperatures corresponding the end of the low-temperature plateau (phase angle ≈0◦ ), to phase angle of 45◦ , and the beginning of the high temperature plateau (phase angle ≈90◦ ), respectively. The standard uncertainty in the derived temperatures is estimated to be 1 K.

20 10 0 240

90 80

phase angle / deg

280

300

T/K

320

340

360

380

Fig. 8. Temperature dependence of SVR phase angle at 0.1 Hz (blue circles), 1 Hz (green circles) and 10 Hz (red circles) oscillatory frequency. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

70 60 50

100

40 30

80

20 10 220

260

300

340

T/ K

380

420

460

/ vol.%

(a)

0

60

40

1.E+06

complex viscosity / (Pa·s)

260

(b) 20

1.E+05 1.E+04

0 200

250

300

350

400

450

T/K

1.E+03 1.E+02 1.E+01 1.E+00 220

260

300

340

T/ K

380

420

460

Fig. 7. Temperature dependence of phase angle (a) and complex viscosity (b) for: black circles, P10 with wA = 0.048; pink circles, P20 with wA = 0.062; green circles, P50 with wA = 0.078; amber circles, SVR with wA = 0.229; blue circles, R50 with wA = 0.479; red circles, R20 with wA = 0.533; cyan circles, R10 with wA = 0.555. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

deviate from ≈0◦ (elastic state) at the temperature corresponding to the end of the glass transition identified from calorimetric data, i.e.,: when the relaxation time of molecules reaches the oscillation frequency in the rheological experiments, and the phase angle exhibits a frequency dependence [2]. For SVR, the phase angle rise from the elastic-state plateau (T0 ) depends on the oscillatory frequency as shown in Fig. 8, where the values 255 K, 261 K, and 268 K correspond to 0.1 Hz, 1 Hz, and 10 Hz, respectively. T0 values obtained a 1 Hz are reported in Table 3 and correspond approximately to the end of the first heat capacity rise of the multi-stage phase transition regions visible in Fig. 1. Thus, the phase transition in the SVR related samples starts as a

Fig. 9. Liquid volume benchmarks from rheological and DSC experiments for permeate P10 with wA = 0.048 (blue), SVR with wA = 0.229 (red), and retentate R10 with wA = 0.555 (green): solid diamonds – apparent volume fraction of total liquid according to rheological experiments, asterisks – volume fraction of total liquid arising from maltenes from DSC experiments (asterisks, asphaltenes are assumed to be solid); open squares—calorimetrically determined glass transition in maltenes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

glass transition. The T90 values for the permeate sub samples and SVR (Table 3) are all below the calorimetrically determined end of maltene melting (Table 2). These observations confirm that SVR maltenes are fully liquid at temperatures above 360 K. Since the asphaltene content in the retentate sub samples is ∼50 wt.%, phase angle values approach the high-temperature plateau at temperatures well above the end of maltene melting, where a fraction of the asphaltenes have undergone a transition to liquid such that ∼67 vol.% of the sub samples is liquid. The temperatures corresponding to phase angle = 45◦ for the retentate sub samples are also important because they are sensitive to liquid volume fraction in a sample and suggest that the behavior of the asphaltene and maltene fractions are largely independent. The liquid fraction benchmarks related to the above analysis are presented visually in Fig. 9. While both analyses are at best semi quantitative, due to the overlapping first and second order phase transitions, the observations from these two independent experiments are consistent with one another. For example, extrapolation of the rheological liquid

34

A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

2.7 2.5

10000

2.3

1000 cp / (J·K -1·g -1)

complex or steady-rotation viscosity / (Pa·s)

100000

100 10

2.1 1.9 1.7

1

1.5 1.3

0.1 290

310

330

350

T/K

370

390

Fig. 10. Temperature dependence of complex viscosity (frequency of 1 Hz, 䊉) and steady-rotation viscosity (constant stress of 5 Pa, ×) for SVR.

phase volume data to low temperatures yields the glass transition temperature and at high temperature, the rheological data coincide with or extrapolate to the calorimetrically determined values. The detailed parsing of attributions to maltene-rich and asphaltenerich pseudo components requires more rigorous analysis of the calorimetry and rheology data. 3.4. Separation of maltene-rich and asphaltene-rich pseudo component effects The calorimetric and rheological data also provide points of departure for detailed analysis. From Fig. 7, when asphaltene content increases at fixed temperature, complex viscosity increases. This is expected for a suspension of asphaltene particles in a liquid maltene medium [10,11]. Further, the S-shaped phase angle curves shift to higher temperatures and becomes more stretched as the asphaltene content increases. The effect of asphaltene content on viscosity is several orders of magnitude and, according to the Cox–Merz rule [37], the values obtained in this work under oscillatory shear are comparable to those obtained previously under steady shear for reconstituted SVR samples [11] and for SVR in this work. The agreement among these data, shown in Fig. 10, is within 20%. The impact of asphaltenes on the calorimetric data is equally apparent. For example, in Fig. 1, heat capacity decreases and the observed glass-transition temperature increases with the increasing asphaltene content. In addition, the magnitude of the maltene melting transition diminishes and a new transition appears above 350 K as the asphaltene content increases. In order to separate the contributions of nanofiltered maltene-rich and asphaltene-rich pseudo components (nanofiltered maltenes and nanofiltered asphaltenes for short), a linear heat capacity model developed in a previous publication [2] was applied. Briefly, the heat capacity of a sub sample is defined on the basis of two pseudo components “nanofiltered maltene” and “nanofiltered asphaltene” fractions as:



cp,sample = wA × cp,A,app + (1 − wA ) × cp,M,app



(5)

where cp,sample is the heat capacity of a sample; cp,M,app and cp,A,app are the apparent heat capacities of the pseudo components, assuming they are independent; wA is the mass fraction of asphaltenes in the sample. While it is known that the C5 asphaltene definition more closely resembles the properties of nanofiltered asphaltenes [10,11], the elemental analysis of the permeate and retentate sub samples were extrapolated to the maltene and asphaltene axes to obtain estimates for the elemental composition, hydrogen to

340

310

410

370

400

T/K

430

460

490

520

Fig. 11. Apparent heat capacities of nanofiltered maltenes and asphaltenes: nanofiltered maltenes—heating run 1 (dark green solid line) and run 2 (light green solid line) derived using Eqs. (5) and (6); nanofiltered asphaltenes–heating run 1 (red solid line) and 2 (amber solid line) derived using Eqs. (5) and (6); measured values for chemically separated C5 and C7 asphaltenes—heating run 1 (black and grey solid lines, respectively); Liquid maltenes predicted using Eq. (4) (˛ = 0.180 mol g−1 )—green dotted line; Solid asphaltenes predicted using Eq. (2) (˛ = 0.151 mol g−1 )—red dashed line; liquid asphaltenes predicted using Eq. (4) (˛ = 0.151 mol g−1 )—red dotted line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

carbon ratio and the number of atoms per unit mass, ˛. These linearly extrapolated values that agree with all elemental composition data to within experimental error, are consistent with the C5 asphaltene definition but are inconsistent with the C7 asphaltene definition as shown in Table 4. Consequently, experimental cp values were fit to obtain estimates for cp,M,app and cp,A,app , based only on the C5 asphaltene definition by minimizing the objective function, Eq. (6), at each temperature:

  1  cp,sample,exp. − wA × cp,A,app + (1 − wA ) × cp,M,app 6 6

o.f. =

1 6

1 o.f. = |cp,sample,exp. − [wA × cp,A,app + (1 − wA ) × cp,M,app ]| 6 1

(6)

This analysis was conducted at T > 310 K only, where the behavior of the samples was most reliable. The absolute average deviation, shown as Fig. S1 in the Supplementary materials, is less than 0.02 J K−1 g−1 , and experimental heat capacity data for all sub samples are fit within the experimental uncertainty (±0.04 J K−1 g−1 ), except for sub sample R50 between 350 K and 400 K during run 1, where a large phase transition appears that is not present in any of the other samples. The results of the linear fits for nanofiltered maltenes and asphaltenes, from 310 K to 520 K, for runs 1 and 2 are shown in Fig. 11 along with experimentally measured heat capacities of chemically separated C5 and C7 asphaltenes (run 1). The regressed heat capacity values for nanofiltered maltenes are in agreement with values obtained from the liquid correlation Eq. (4), and reveal the end of melting at 358 K (run 1) and 370 K (run 2). No additional transitions are observed at higher temperatures in the nanofiltered maltenes. At lower temperatures, Table 2, extrapolation of the glass transition to maltenes (wA = 0) gives a C5 maltene glass transition at ∼208 K during run 1. So, the transition of maltenes from solid/glass to liquid spans a 150 K interval.

A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

35

Table 4 Safaniya vacuum residue asphaltene and maltene composition comparison. Elemental Analysis, wt.%

Material

C5a C5 extrapolatedb C7a C7extrapolatedb C5a C5 extrapolatedb C7a C7 extrapolatedb

Asphaltenes

Maltenes

a b

C

H

O

N

S

82.8 82.3 82.8 81.4 83.7 84.1 83.4 84.1

7.9 7.6 8.0 6.0 11.0 11.2 10.5 11.0

0.9 1.0 0.6 1.3 0.5 0.3 0.6 0.4

1.0 0.9 1.0 1.2 0.2 0.3 0.3 0.3

7.4 8.2 7.5 9.9 4.6 4.4 5.2 4.6

˛ (mol g−1 )

1.13 1.11 1.15 0.88 1.56 1.59 1.50 1.56

0.151 0.148 0.152 0.132 0.180 0.182 0.176 0.180

Experimental values. Obtained by linear extrapolation of the data from Table 1.

For nanofiltered asphaltenes extrapolating the low temperature glass transition (Table 2), observed during run 1 from 55.5 wt.% to the asphaltene axis, yields an estimated glass transition temperature for nanofiltered SVR asphaltenes of ≈265 K, a value comparable to the Tg value, 260 K, estimated directly from heat capacity for nanofiltered asphaltenes of Athabasca bitumen [2]. Other transitions during run 1, while clearly present, are less well defined. A tentative end of the phase transitions in nanofiltered asphaltenes during run 1 is set at ≈450 K. The heat capacity profile for run 2 is comparable except for a first order phase transition between 350 and 400 K that is superimposed on the broad transition. These phase behaviors contrast with those observed for chemically separated C5 asphaltenes, where a first order phase transition starting at ≈315 K overlaps with an exotherm (400 to 445 K) before ending at ≈490 K. This latter behavior is similar to the behavior of chemically separated Maya and Athabasca C5 asphaltenes [2,3]. So, even though the elemental composition of the chemically separated C5 asphaltenes and the nanofiltered asphaltenes are comparable, phase state changes occurring during chemical separation yield asphaltenes possessing a different phase state or phase state distribution than that arising in the parent oil. Thus colligative and other thermophysical properties of physically and chemically separated asphaltenes cannot be expected to be comparable. The heat capacities for the solid and liquid asphaltenes predicted using Eqs. (2) and (4), are also shown in Fig. 11. The solid heat capacity prediction provides quantitative a priori estimates and temperature trends for asphaltenes, while the heat capacity of asphaltenes in the liquid state (Fig. 11) is lower that the predicted by Eq. (4). 3.5. Analysis of maltene melting during run 1 By combining the results above, the phase transition from the solid to liquid for SVR maltenes is a combination of a second-order (glass) and a first-order (melting) transition with an unknown distribution between glass and crystal, starting at Tonset (Table 2) and ending at 358 K. The transition in asphaltenes starting at ≈265 K overlaps with maltene melting. The independence of phase behavior of the pseudo-components (maltenes and asphaltenes) in the nanofiltered SVR samples permits the calculation of the enthalpy of phase transition associated with the maltene but the phase transition enthalpies of the undefined crystalline and glass fractions cannot be parsed [2]:



H/C ratio

gl

◦

fus h◦ − wgl cr h0 K

 M

1 = (1 − wA )

⎡ 358 K   ⎣ cp,sample,exp. Tonset





358 K

−cp,sample,corr. dT − wA 265 K





⎤

cp,A,app − cp,A,corr. dT ⎦ ,

(7)

gl ◦

where fus h◦ is the specific enthalpy of fusion (melting); cr h0 K is the enthalpy difference between glass and crystal at T = 0 K; cp,sample,exp. and cp,sample,corr. are the specific experimental and solid-correlation heat capacities for the SVR sample from Eq. (2), respectively; cp,A,app and cp,A,corr. are the specific apparent (see Eq. (6)) and solid-correlation (Eq. (2)) heat capacities for C5 asphaltenes; wA is the C5 asphaltene fraction in the SVR sample; wgl is the fraction of glass maltenes at low temperatures on an asphaltene free basis. Since the cp,A,app was determined only from 315 to 520 K (Fig. 11), the heat capacity down to 265 K was extended linearly to the value predicted by the solid-correlation (Eq. (2)) for C5 asphaltenes. This assumption does not significantly increase gl ◦ the uncertainty of (fus h◦ − wgl cr h0 K )M , if calculated for samples with a low mass fraction of asphaltenes—P10, P20, P50, and SVR. The estimated values for these samples are given in Table 2 and agree within ±1 J g−1 (average of 49 J g−1 ). The enthalpy value gl ◦ (fus h◦ − wgl cr h0 K )M for the SVR maltenes is higher than that for the Athabasca bitumen (AB) maltenes [2] (36 J g−1 ) and lower than that for the Maya maltenes [3] (80 J g−1 ) because the maltene glass fraction sequence is Maya (wgl ≈ 0) < SVR (wgl between 0 and 1) < AB gl ◦

(wgl ≈ 1). Even though the range of cr h0 K values is known (40 to 100 J g−1 depending on the details of molecular structure and elemental composition [38–40]), fus h◦ for SVR cannot be estimated because wgl is unknown. 3.6. Phase diagrams of Safaniya vacuum residue Pseudo-binary (nanofiltered maltene–nanofiltered asphaltene) phase diagrams for Safaniya vacuum residue, based on the above analyses, are presented in Fig. 12a and b, respectively for runs 1 and 2. It should be noted that the glass and melting transitions attributed to nanofiltered asphaltenes are difficult to assess due to extrapolation from maximum wA values of 0.555. Hence, these phase boundaries above wA = 0.555 are considered tentative. Attributions below wA = 0.555 are more certain. During run 1 (Fig. 12a) the combined results from calorimetric and rheological measurements indicate the presence of four phases at low temperature. Above 208 K and 270 K respectively, the glass maltenes and asphaltenes transition to liquid. The calorimetrically determined glass transitions are shown as dashed lines, since there is no change in the number or state of phases arising from this type of transition, only a change in relaxation times [41,42]. These transitions are depicted in the phase diagram because they influence transport properties (viscosity, viscoelastic response) and limit the applicability of empirical property correlations at lower temperatures. With the completion of the melting of the maltenes, the number of phases present decreases to three at 360–370 K. The independence of phase behavior of the nanofiltered pseudo-components of SVR favors the hypothesis that asphaltenes remain nanodispersed colloids in maltenes [10,11], so that even above ∼450 K two separate phases co-exist. The phase diagram for run 2 (Fig. 12b) reported

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A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

500

550

450

500

400

450

LA

500 LM + LA

T/ K

LM + CM + LA+ CA

300

300

350

250

LM + CM + GA+ CA

LM + SA + LA

400

300 250

SA + LA

350

350

LM

LM + LA+ CA

400

LM + LA

250

LM + CM + SA + LA LM + C M

450

LM + CM + SA

200

(a) 150

150 0

0.2

0.4

wA

0.6

0.8

1

150

GM + CM

GM + CM + GA+ CA

SA

200

200

0.0

LM + LA

450

T/ K

450

400

400 LM + LA+ CA

(b) 300

300 0

0.2

0.4

wA

0.6

0.8

1

Fig. 12. Maltene–asphaltene phase diagram for Safaniya vacuum residue, run 1 (a) and run 2 (b): , experimental glass transition; , experimental end of the maltene melting. Abbreviations: GM , CM , and LM are glass, crystalline and liquid maltenes, respectively; GA , CA , and LA are glass, crystalline, and liquid asphaltenes. Solid lines depict phases (boundaries), dashed lines reflect glass to liquid transitions.

for 300 K and above, is qualitatively similar but the melting boundary for crystalline maltenes is higher and the melting boundary for the asphaltenes is lower than for run 1. Four phases coexist at low temperatures, three at intermediate temperatures and two at high temperatures. Thus, irrespective of short term thermal history, conventional correlations for thermophysical and transport properties are compromised at lower temperatures and in particular below temperatures where a fraction of the maltenes is glass or crystalline. 3.7. Phase diagram comparisons among heavy oils The phase diagram of SVR (Fig. 12a) is similar to that of Athabasca bitumen (Fig. 13). In both phases Diagrams first and second order transitions are observed. Overlap in the phase transitions of maltenes and asphaltenes, and the coexistence of up to four phases at equilibrium all arise. The only difference is that the fraction of crystalline maltenes is significantly larger for SVR. As the maltene compositions are similar (Table 5) and the H/C ratio of SVR is larger, this suggests that SVR maltene fraction is less aromatic and can more easily crystallize. The phase behavior of SVR is concurrently more complex than that of Maya crude oil (Fig. 14), which does not have a maltene glass transition and or an overlap between phase transformations in maltenes and

0.6

0.8

1.0

Table 5 Comparison of chemical composition (in wt.%) of heavy oils from three different locations. Component

LM + CM + LA+ CA

0.4

Fig. 13. Phase diagram for Athabasca bitumen, heating cycle 1. Abbreviations: GM , CM , and LM are glass, crystalline, and liquid maltenes, respectively; SA , GA , and LA are solid (either glass or solid or both), glass, and liquid asphaltenes. Solid lines depict phases (boundaries), dashed lines reflect glass-to-liquid transitions, dotted lines link transitions in chemically separated and nanofiltered pseudo components.

350

350

0.2

Mass fraction of asphaltenes(wA)

500

500

GM + CM + SA

Elemental composition C H N S O H:C ratio SARA analysis Saturates Aromatics Resins C5 asphaltenes C7 asphaltenes

Safaniya vacuum residue

Athabasca bitumen [2]

Maya crude oil [3]

83.9 10.5 0.4 5.3 0.4 1.48

83.2 9.7 0.4 5.3 1.7 1.39

84.5 11.3 0.3 3.3 1.2 1.59

10a 50a 26.6a 22.9 13.4

16.1 48.5 16.8 18.6

31.6 42.5 10.2 15.7

a Determined based on the C7 basis in contrast to the other oil, where the C5 basis was used. The values are reproduced from Ref. [42].

asphaltenes. For Maya crude, the maximum number of coexisting phases is only three. Again this difference is suggestive and can be attributed to the larger H/C ratio of the maltene constituents and the larger saturate fraction in Maya crude (Table 5), both of these constituents appear to crystallize more readily on cooling. At temperatures above the maltene phase transitions, specific heat capacities of SVR, Athabasca bitumen, and Maya crude oil relate to each other accordingly to their H/C ratios and similarity variable ˛ (Eq. (1)) supporting the conclusions of our previous work [21–25]: e.g., at 360 K–1.96 J K−1 g−1 , 1.92 J K−1 g−1 [2], and 1.99 J K−1 g−1 [3], respectively. Under conditions where these three heavy oils are encountered industrially, whether in reservoirs, pipelining, storage or processing, they comprise two to four phases. These phases arise from partitioning of asphaltene-rich material and maltene-rich material. The existence and the nature of these multiple phases, their spatial length scales, and their distribution within such hydrocarbon resource and resource fractions are rarely considered in process design, operation, or optimization. Anecdotally there is a tendency to attribute specific properties or processing problems to asphaltenes, however they may be

550

LA

A. Bazyleva et al. / Fluid Phase Equilibria 380 (2014) 28–38

LM + LA

500

SA + LA

LM

450 LM + SA + LA

400 350 LM + SA

SM + LM + SA

The authors thank Dr. Joelle Eyssautier and Dr. Loic Barre and IFP-EN for collaborating on the characterization of Safaniya vacuum residue. We appreciate their professionalism and generosity. The authors gratefully acknowledge financial support from the sponsors of the NSERC Industrial Research Chair in Petroleum Thermodynamics: Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta Innovates, KBR Energy and Chemical, Halliburton Energy Services, Imperial Oil Resources, ConocoPhillips Canada Resources Corp., Shell Canada Ltd., Nexen Inc., Virtual Materials Group (VMG), Total E&P Canada Ltd, BP Canada Energy Corporation. Appendix A. Supplementary data

200 SM + SA

SM

150

Acknowledgements

SA

250

LM + SM

300

37

0.0

0.2

0.4

0.6

0.8

1.0

Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.fluid.2014.07.037

Mass fraction of asphaltenes(wA) Fig. 14. Phase diagram for Maya oil, heating cycle 1. Abbreviations: SM and LM are solid and liquid maltenes respectively; SA and LA are solid and liquid asphaltenes, respectively.

defined. The phase diagrams presented here and in the prior related work with Athabasca bitumen and Maya crude oil underscore the need to revise such perspectives. For example, SVR, Athabasca bitumen and Maya oil behave as Newtonian fluids above approximately 330 K for SVR (viscosity of 130 Pa s, this work), 310 K for Athabasca bitumen (viscosity of 340 Pa s, [9]) and 280 K for Maya crude oil (viscosity of 1.2 Pa s, [9]. The respective asphaltene contents are 22.9 wt.%, 18.6 wt.% and 15.7 wt.% (Table 5). While it is easy to correlate viscosity, at fixed temperature, and Newtonian flow onset temperature with asphaltenes content, it is the maltene-rich fractions rather than the asphaltene-rich fractions that are undergoing a rapid transition to liquid at the respective transition temperatures. Thus there is only a causal relationship and hence a basis for correlation development between asphaltene content and viscosity. Non-Newtonian rheological behavior of these fluids arises at low temperatures because the maltene-rich materials become structured. As the temperatures associated with phase behavior transitions for maltenes are also a function of the thermal history of a fluid, phenomena are readily misattributed.

4. Conclusions A combination of rheological and calorimetric measurements for nanofiltered Safaniya vacuum residue permeates and retentates revealed the complex phase behavior of this oil, which includes independent and overlapping glass and melting transformations in the nanofiltered maltene and nanofiltered asphaltene pseudo components. These behaviors are irreversible within the time scale of the measurements and depend on the thermal history of samples. Safaniya vacuum residue comprises from two or four phases over the temperature range 250 K up to at least 520 K. This complex and irreversible phase behavior is consistent with that for other heavy hydrocarbon resources such as Athabasca bitumen (Canada) and Maya crude oil (Mexico) and would appear to be a general phenomenon, where the details of the phase diagrams vary from feedstock to feedstock. The complex and irreversible nature of the observed phase behavior constrains the applicability of traditional property prediction methods for production process design and process optimization for heavy oils in general and for the prediction of rheological properties in particular.

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