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Performance of solvent mixtures for nonaqueous extraction of Alberta oil sands Krupal Pal, Lucas da Paz Nogueira Branco, Annabell Heintz, Phillip Choi, Qi Liu, Peter Rudolf Seidl, and Murray R Gray Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502882c • Publication Date (Web): 10 Mar 2015 Downloaded from http://pubs.acs.org on March 14, 2015
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Energy & Fuels
1 2
Performance of solvent mixtures for non-aqueous extraction of Alberta oil sands
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Krupal Pal1, Lucas da Paz Nogueira Branco2, Annabell Heintz1, Phillip Choi1, Qi Liu1, Peter R Seidl2, and Murray R Gray1,3
1. Institute for Oil Sands Innovation (IOSI), Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta 2. Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. 3. Hamad bin Khalifa University, Qatar Foundation
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Abstract
31 32
Non-aqueous extraction of Alberta oil sands is of great interest to developing an
33
alternative to the current hot water extraction process in order to eliminate the tailing
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ponds. Investigations have been conducted to evaluate the performance of solvent
35
mixtures to extract bitumen from a high grade oil sands ore. Solvent mixtures of
36
cycloalkane and n-alkanes were studied based on their Hildebrand solubility parameters
37
which affect bitumen recovery and fine solids migration during the extraction process,
38
and the results were compared with single solvents. Cyclohexane, cyclopentane and
39
methyl cyclopentane were selected as the cycloalkane solvents, and they were studied in
40
combination with n-alkane solvents such as n-heptane, n-hexane, or n-pentane to make up
41
a final solubility parameter between 16.65 and 16.45 (MPa)1/2 for the final solvent
42
mixture. It was observed that the solubility parameter of the solvent mixture has more
43
impact on the migration of fine solids in bitumen than the recovery of bitumen. The
44
amount of fine solids migrating into the bitumen product followed the order of
45
cycloalkane/n-heptane > cycloalkane/n-hexane > cycloalkane/n-pentane.
46 47 48 49 50 51 52 53
Keywords: oil sands, bitumen, solvent extraction, fine solids 2 ACS Paragon Plus Environment
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1. Introduction
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The depletion of light crude oil resources has directed the interest of researchers towards
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less accessible heavy oil resources, which require more processing. The Athabasca oil
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sands deposit located in north-eastern Alberta, Canada has proven reserves of 168 billion
58
barrels of recoverable bitumen. In 2012, 1.9 million barrels of raw bitumen was produced
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from the deposit per day, and the production is predicted to increase to 3.8 million barrels
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per day by 2022 1. The Alberta oil sands are a combination of sands, water, fine solids
61
(mostly clays) and bitumen, and the grade or bitumen content of the mined ore ranges
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from 7 to 14 wt%. Bitumen is denser and has higher viscosity as compared to crude oil.
63
The bitumen extracted from the Athabasca oil sands has the density close to water: 1000
64
– 1029 kg.m-3 and the viscosity in the range of 80 to 12000 Pa.s (at 20°C). Bitumen
65
contains a high carbon content in the range of 83.1 to 83.4 wt% 2, 3.
66 67
Shallow deposits of oil sands (less than 90 m) are processed by surface mining followed
68
by warm-water extraction. The ore deposits are not homogeneous, with considerable
69
variation in bitumen and fine solids contents (particles less than 44 µm in diameter),
70
which impacts the recovery of bitumen from the current warm-water extraction process 4.
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The primary shortcoming of this process is the accumulation of large volumes of wet
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tailings that remain after the bitumen is removed, and the amount of energy required to
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heat large volumes of fresh water and ore. The tailings are very difficult to reclaim and
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the large tailings ponds are a risk to wildlife and the north Alberta ecosystem 5. There is a
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need to develop new technologies in order to overcome these environmental
76
shortcomings. Candidate replacement technologies should enable the rapid return of the 3 ACS Paragon Plus Environment
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bitumen-free sands and minerals to the mine sites as backfill or in the form of stackable
78
gangue, and produce solid-free dry bitumen ( )
(5c)
#'
#'
87/ 8977
87/ 8977
87/ 8977
+ A2 )
(6)
is the mass of bitumen in the extraction product on the fines-free basis
= ) 3 (#' #$ ( #% )
(9)
0
# # > &
87/ 8977
%
).
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bitumen recovery of 100%, and working backwards from the measured carbon contents,
250
equation (3) gave 13.1% bitumen in the initial ore, which compares to the measured ore
251
grade of 13.5±1.1%.
252
3.1 Comparison of single solvents
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The non-aqueous extraction experiments were carried out in triplicate. Table 2
254
summarizes the performance of single solvents in these tests. The bitumen recovery
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varied by ±0.8% (average standard deviation of a series of triplicate experiments), while
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the variation in the concentration of fine solids was ±0.6 wt% of the produced bitumen.
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The average standard deviation of the carbon content of the fine solids was 0.8 wt%.
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Among the single solvents, the bitumen recovery (fines free basis) was similar to that of
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bitumen recovery obtained when centrifuged fine solids were not excluded for all the
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cases except for the case of n-heptane. Cyclohexane gave the best results for bitumen
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recovery of 97.4±0.8%, while n-heptane gave the worst recovery when the significant
262
amount of bitumen associated with the fine solids was subtracted (Table 2).
263 264
Table 2: Performance of single solvents for non-aqueous extraction of a high grade oil sands ore sample Solvent
265
Bitumen Recovery (%)
Centrifuged solid fines Carbon (wt%)
Bitumen Recovery (fines free basis)
Cyclohexane
97.9 ± 0.8
(wt% in bitumen product)* 5.8 ± 0.1
Cyclopentane
91.6±0.8
5.5 ± 0.8
11.6 ± 1.6 90.8 ± 1.0
Methylcyclopentane
84.8±1.2
8.9 ± 1.2
9.2 ± 0.5
n-heptane
96.0±0.63
32.6 ± 0.3
30.0 ± 0.8 81.5 ± 0.02
8 ± 0.3
97.4 ± 0.8
83.9 ± 1.2
* Mass percent of recovered bitumen on a solvent-free basis
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267
Based
on
the
bitumen
recovery,
the
ranking
of
single
solvents
was
268
cyclohexane>cyclopentane>>methylcyclopentane>n-heptane. The carbon content of all
269
of these samples of fine solids was consistent with an organic coating on the surface of
270
clay and mineral particles, which would make the solids more hydrophobic and increase
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their stability in suspension.11 The migration of fine solids into the bitumen product
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mainly happened during the second and third stages of washing on the vibrating sieve, as
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the fine solids had mostly settled out from the first supernatant in the 30 min standing
274
period. The high solids content in the extracted bitumen when n-heptane was used, at
275
32.6+0.3% wt% (Table 2), was much higher than the average fine solid content of 5-10
276
wt% when cyclohexane or cyclopentane was used.
277
When the bitumen in these fines was excluded from the calculation, the recovery
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decreased by 15 percentage points. The high carbon content of these fines indicated that
279
they contained precipitated asphaltene. When n-heptane was used for extraction,
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asphaltene precipitation was observed during the first settling as well as second and third
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stages of solvent washing. The fine solids collected from n-heptane extraction were
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washed with toluene, and it was found that the carbon content was reduced from circa 30
283
wt% prior to toluene wash to only 3 wt% after the toluene wash, confirming that more
284
bitumen components (most likely asphaltenes) were dissolved from the fine solids by
285
toluene which were not dissolved by n-heptane. Clearly, the use of pure n-alkane solvents
286
caused asphaltene precipitation, which reduced bitumen recovery. The kinetics of
287
precipitation could be important given the short contact times in these experiments
288
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used. The precipitated asphaltenes likely coated the surfaces of fine solids, causing more
290
of them to migrate to the bitumen product. If we subtract the bitumen from the mass of
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centrifuged fine solids in Table 2, we find that twice as much mineral fines migrated into
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the bitumen with n-heptane as with the other solvents. Cyclopentane also gave asphaltene
293
precipitation in the third stage of solvent washing, but the resulting solids content of the
294
bitumen was not increased as much as with n-heptane (Table 2).
295
For the cycloalkane solvents, a decrease in bitumen recovery was observed (Table 2) with
296
decrease in solubility parameter of the solvent (Table 1). Methylcyclopentane was not
297
considered for subsequent studies of solvent mixtures due to its lower bitumen extraction
298
recovery (less than 90%) and higher tendency to cause fines migration into the product as
299
compared to cyclohexane and cyclopentane. Both of these metrics are consistent with the
300
solubility parameter of methylcylcopentane, intermediate between cyclopentane and n-
301
heptane.
302
3.2 Mixtures of cycloalkane and paraffinic solvents
303
Experiments were carried out using solvent mixtures with various ratios of cyclohexane
304
and n-heptane. It was observed that while bitumen recovery was insensitive to solvent
305
composition in the range of 70-100% cyclohexane, at 96.9±1.1%, the content of fine
306
solids in the bitumen product was much more sensitive. The data of Figure 2 show that
307
the amount of centrifuged fine solids decreased as the ratio of cyclohexane in the solvent
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mixture was increased, to a distinct minimum of approximately 5% solids at 92.5%
309
cyclohexane, and then increased at higher cyclohexane ratios.
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The minimum in fine solids concentration corresponds to a solubility parameter of 16.65
311
(MPa)½ for the solvent mixture. In order to investigate the role of solvent composition,
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we examined several blends of cyclohexane or cyclopentane with paraffinic solvents (n-
313
heptane, n-hexane and n-pentane) at different ratios but maintained the solubility
314
parameters at 16.65 and 16.45 (MPa)1/2. The lower value of 16.45 (MPa)1/2 was chosen
315
because it corresponds to a cyclohexane concentration of 80 wt%, in the range of solvent
316
composition where fine solids content was insensitive to solvent composition but more
317
than double the minimum value at 92.5% cyclohexane (δ = 16.65 (MPa)1/2).
24 22
Fines in bitumen, wt%
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20 18 16 14 12 10 8 6 4 70
318 319 320 321
75
80
85
90
95
100
Cyclohexane in blend, wt% Figure 2: Percentage of centrifuged fine solids in the bitumen product using solvent mixtures of cyclohexane and n-heptane. The line shows the trend for mean values of duplicated experiments. 16 ACS Paragon Plus Environment
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All of the blends with cyclohexane gave bitumen recovery over 95%. Cyclohexane/n-
324
hexane blend exhibited the best results with bitumen recovery of 98 and 99.7% at a
325
solvent mixture solubility parameter of 16.65 and 16.45 (MPa)1/2, respectively. The
326
bitumen recovery was similar to or exceeded that obtained using cyclohexane alone. The
327
performance of blends of cyclopentane was consistently poorer, even at the same
328
solubility parameter of 16.45 (MPa)1/2 as illustrated in Figure 3. These data indicate that
329
solubility parameter alone is not sufficient to predict the extraction performance of
330
cycloalkane-paraffinic solvent blends. The amount of centrifuge solids in the bitumen
331
product was more sensitive to the solvent blend composition than recovery, as indicated
332
in Figure 4.
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334 335 336
Figure 3: Comparison of bitumen recovery (fines free basis) for mixtures of cyclopentane and cyclohexane with paraffinic solvents at a constant value of solubility parameter 16.45 (MPa)1/2.
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339 340 341 342
Figure 4: Percentage of centrifuged fine solids in extracted bitumen using cyclohexane and cyclopentane in mixtures with different paraffinic solvents. Where indicated, error bars are standard deviations of experiments in triplicate. The lines show linear regression for data of blends of constant solubility parameter.
343
344
Although the fine solids content in the bitumen ranged from 2 to 12% (on a solvent-free
345
basis), the carbon content of the centrifuged fine solids gave no trend and averaged
346
9.7±1.9%. Rather than collapsing onto a single result for a given value of the solubility
347
parameter, each series of blends gave a significant range of fine solids contents. The
348
amount of fine solids in the bitumen phase increased with the carbon number of
349
paraffinic solvents. In comparing the solvent blends for cyclohexane and paraffinic
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solvents at solubility parameter 16.65 and 16.45 (MPa)1/2, the fines migration into the
351
product differed, even though the bitumen recovery was insensitive (Figure 3). At a
352
higher solubility parameter of 16.65(MPa)1/2, the amount of fine solids migrating in the
353
bitumen product phase was less for all blends as compared to the solvent mixtures at
354
solubility parameter of 16.45 (MPa)1/2.
355
The blends of cyclopentane and paraffins at a solubility parameter of 16.45 (MPa)1/2 gave
356
similar solids contents in bitumen to the corresponding cyclohexane-paraffin mixtures
357
(Figure 4), even though the bitumen recovery was lower (Figure 3). The trend of fine
358
solids migration was similar to that obtained in cyclohexane-paraffin mixtures, in that the
359
amount of fine solids migration increases with increase in carbon number of the
360
paraffinic solvent.
361
The hypothesis for this study was that different solvent blends with the same solubility
362
parameter would give the same performance in both bitumen recovery and the
363
concentration of fine solids in the recovered bitumen. This expectation was based on the
364
observation that precipitation of asphaltenes from crude oils is independent of the solvent
365
composition, for mixtures of hydrocarbon solvents, and depends only on the solubility
366
parameter
367
trends
368
solubility parameter gives stable dispersions of both asphaltenes and asphaltene-coated
369
silica. These observations are consistent with force measurements between asphaltene
370
films. In toluene, the interactions are repulsive, while the asphaltene surfaces gave
371
adhesive forces in n-heptane 19.
13
18
. Dispersion of silica coated with asphaltene components follows similar
. In both cases, low solubility parameter gives precipitation, while higher
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None of the results of this study fit these expectations. The recovery of bitumen was
373
surprisingly insensitive to the addition of paraffinic solvents to cyclohexane over a wide
374
range of concentration. Cyclopentane blends were more sensitive, and definitely did not
375
give constant recovery of bitumen at a given solubility parameter of the blend (Figure 3).
376
The fine solids content of the bitumen at a given solubility parameter was very sensitive
377
to the components in the blend (Figure 4). Solubility parameter is a continuous function
378
of blend composition, and precipitation of asphaltenes in blends of cyclohexane and
379
heptane gives monotonic trends of the amount of precipitate versus the solubility
380
parameter of the solvent blend
381
repeatable minimum in the yield of fine solids in the bitumen extracted with blends of
382
cyclohexane and heptane was completely unexpected (Figure 2). A minimum at 92.5
383
wt% cyclohexane cannot be explained by solubility parameter alone. The sensitivity of
384
the migration of fines to solvent composition suggests that careful optimization of the
385
solvent blend, then maintaining it at the desired value would be beneficial in commercial
386
operation.
387
This study focused on cycloalkanes because of their desirable properties for solvent
388
extraction of Alberta oil sands, but these hydrocarbons have been studied relatively little
389
in the literature on asphaltenes and other colloidal dispersions. Mitchell and Speight
390
observed behavior that was completely consistent with solubility parameter for
391
cycloalkanes, methyl-cycloalkanes and ethyl-cycloalkanes. These solvents precipitated 0-
392
1.9 wt% of Athabasca bitumen in a 20:1 dilution, consistent with alkyl benzenes and
393
benzene-pentane blends in the same range of 16-17.5 MPa1/2. Cyclohexane was a strong
394
solvent, giving precipitation of only 6% of the n-heptane insoluble asphaltenes (0.7 wt%
20
. Consequently, the observation of a distinct and
18
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of the total bitumen). In contrast, Mannistu et al.20 found that 30% of Athabasca
396
asphaltenes could not be redissolved in cyclohexane. Cycloalkanes in blends may give
397
some differences in behavior from the heptane-toluene blends commonly used in
398
laboratory studies, but none of the prior observations would suggest a minimum in solids
399
carry-over into the bitumen product.
400
A minimum always indicates the presence of at least two opposing mechanisms, in this
401
case one to reduce the amount of fine solids passed into the bitumen as more heptane was
402
added, and a second to increase the amount of fine solids at less than 95% cyclohexane.
403
Over this narrow range of composition we can discount the physical properties of density
404
and viscosity as being nearly invariant. Addition of heptane reduces the solubility
405
parameter, which increases the precipitation of asphaltenes onto mineral surfaces, makes
406
the resulting asphaltenes more adhesive, and reduces the stability of dispersions of fine
407
solids. All of these trends are consistent with the reduction in solids in the bitumen over
408
the range 0-5% n-heptane. The laboratory method used to recover the bitumen was
409
designed to mimic an industrial process, so it incorporates not only sedimentation of
410
solids, but also washing of the solids on a shaking screen. This treatment accounts for a
411
significant fraction of the solids in the bitumen.11 Stronger binding of the fine solids to
412
the sands, either with residual moisture6, 11 or asphaltenes would help in retention. The
413
role of water is well documented, but under these filtration conditions the onset of
414
asphaltene precipitation increases the concentration of fine solids in the bitumen (Figure
415
2). The data show that increasing the precipitation of asphaltenes and making them more
416
adhesive on the surface of the fine solids increases the tendency of this material to pass
417
through the filter into the bitumen product. Changes in the pore structure of sand/fine
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solid/asphaltene mixture may play a role, and these interactions require further study to
419
develop effective separations for large-scale applications of solvent extraction to oil
420
sands.
421
4. Conclusions
422
Blends of a cycloalkane (cyclohexane or cyclopentane) with an n-alkane (n-pentane, n-
423
hexane or n-heptane) were used in the extraction of Athabasca oil sands. It was observed
424
that bitumen recovery did not follow a monotonous relationship with the solubility
425
parameter of the solvent blends. When blended to give the same solubility parameter, the
426
cyclohexane/n-hexane mixture was shown to result in higher bitumen recovery than the
427
cyclohexane/n-pentane and the cyclohexane/n-heptane mixtures.
428
However, with the same solubility parameter of the solvent blend, the fines contents in
429
the product bitumen followed a monotonous trend cycloalkane/n-heptane >
430
cycloalkane/n-hexane > cycloalkane/n-pentane. Blends of cyclohexane and n-heptane
431
give a minimum in the fines content at circa 93 wt% cyclohexane.
432 433
Notation
434
∆Hv
Enthalpy of vaporization (kJ/kmol)
435
M
Mass of the oil sands sample used in each extraction test (g)
436
Mc
Mass of the centrifuge solids (g)
437
Mt
Mass of extraction gangue (g)
438
R
Gas constant (kJ/K/kmol)
439
T
Temperature (K)
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440
V
Molar volume (m3/kmol)
441
Wo
Mass fraction of C in oil sands
442
Wb
Mass fraction of C in bitumen
443
Ws
Mass fraction of toluene insoluble C in oil sands
444
Ws’
Mass fraction of toluene insoluble C in fine solids
445
Wt
Mass fraction of C in the second gangue
446
Wc
Mass fraction of C in centrifuge solids
447
δi
Solubility parameter for individual pure solvent (MPa)1/2
448
δmix
Solubility parameter for mixtures (MPa)1/2
449
vi
Volumetric fraction of individual pure solvent
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450
451
Corresponding Author
452 453 454
Murray R. Gray Email:
[email protected] 455
Acknowledgment
456
The authors are grateful for the financial support provided by Natural Resources Canada
457
under the EcoEII program and the Institute for Oil sands Innovation (IOSI) at the
458
University of Alberta.
459
460
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Energy & Fuels
6. References
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