Factors Influencing the Separation of Asphaltenes
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Transcript of Factors Influencing the Separation of Asphaltenes
7/28/2019 Factors Influencing the Separation of Asphaltenes
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Asphaltenes in heavy petroleum feedstocks: J. G. Speight et al.
RESULTS AND DISCUSSION
The present definition of asphaltenes is based on the
solution properties of petroleum residua, bitumens and
the like in various solvents 2m4 ut there has been scientific
effort to define asphaltenes in terms of molecular
structures5-‘. Nevertheless, it should be recognized that
asphaltenes (from whatever the source) are, in fact, a
solubility class (Figure 1) and that the definition is an
operational one; that is, asphaltenes are soluble in light
aromatics such as benzene or toluene and insoluble in
light paraffins such as pentane, hexane, heptane, etc.
It is, therefore, not surprising that there are standard
methods for asphaltene separation using either n-pentane
or n-heptane (Tables I and 2). Indeed, these standards for
asphaltene separation have been described in exact detail
but there are many variations that can be employed
without even considering the variations in
precipitant l O 12. Although the ‘classification’ of
asphaltenes as pentane-asphaltenes, hexane-asphaltenes,
heptane-asphaltenes and the like is an easy way of
identifying the precipitant employed, this method
‘classification’ does not identify any potential differences
in asphaltene character.
If n-pentane is substituted for n-heptane as the
separating medium the respective yields of asphaltenes
Table 1 Analytical procedures for the determination of petroleum asphaltenes usrng pentane
Sample/ Solvent volume/
solution container/
Test No./title Solvent(s)
Solvent(s)/g-r
heated Sample sire
Standing Repeatability,’
filter Techntque sample time reproducibilrty
ASTM D893-69 n-Pentane
(Procedure A) commer-
Insolubles in cial
Used Lubri- grade
eating Oils
(1 O/3/69)
To 65 ? 5“C 10. 0 t 0.1 g
suspend all
solids, filter
through
150 m
before add-
ing solvent/
room tem-
perature
Syncrude Ana- n-Pentane, If necessary/ 2.5 g
lytrca Method commer- no
5.1 cial ben-
Analysis for zene, ACS
Asphaltenes reagent)
in Bitumen..
ASTM D2006- n-Pentane
70 commer-Characteristic cial
Groups in
Rubber Exten-
der and Pro-
cessrng Oils
by Precipita-
tion Method
(2/27/70)
(Discon-
tamed)
ASTM D2007- n-Pentane,
75 commer-
Characteristic cial
Groups in
Rubber Exten-
der and Pro-cessing Oils by
the Clay-Gel
Adsorption
Chromatb-
graphic
Method
(8/29/75)
Yes/no l . Ot 0. 1 g
No/yes 10 f 0.5 g
3 h max. 0.0-l .O wt%;
0.07 wt%;
Over 1 .O wt%
10% of mean/
0.0-l .o wt%;
0.10 wt%
Over 1 .O wt%,
15% of mean
2h Standard devia-
tion to.14 wt%
- 100 ml less -Shake centrifuge 10 ml nC3 g-t
sample tube, don’t let
- 100 ml cen- stand more than
trifuge tube 3 h, centrifuge
- Centrifuge @ 600-700 ref
for 20 min
- Decant to 3 ml
- Resuspend in
50 ml and
repeat
- Resuspend rn
50 ml and
repeat
-Dry@105i
3’C for 30 mm
in oven- 1 ml benzene/ - Dissolve in ben- 1 ml 82 g-t
g sample 40 zene 40 ml nCs g-t
ml n-pentanel -Warm if neces-
1 mlbenzene sary
- 300 ml Erlen- - Add n-pentane,
meyer flask stopper and shake
- Buchner with for 5 min
medium pore - Allow to settle
fritted glass for 2 h; occa-
filter sional shaking,
rn dark
- Vacuum filter
- Rinse flask; wash
- Dry at 105’C
- 5 ml n-pen- - Add n-pentane 50 ml nCs g-1
tene -Stand for 15 h- 125 ml weigh--filter
ing flask - Rinse flask three
- Medium times with lo-
weight, rapid 20 ml each time
filtering -Wash
filter paper
15 h 0.1 wt%/O.l wt%
- 100 ml n-pen- - Add n-pentane lOmlnCsg-1
tane mix well
- 250 ml coni- -Warm for few
cal flask seconds
- Fine porosity - Let stand 30 min
filter disk - Rinse with 10and 20 ml
solvent
-Wash with 50 ml
- Dry by aspirat-
ing air through
disk for 45 min
30 min 0.1 wt%lO.l wt%
FUEL, 1984, Vol63, May 617
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Asph altenes in heavy petro leum feedstocks: J. G. Speight et al.
Table 2 Analytical procedures for the determination of petroleum asphaltenes using heptane
Sample/ Solvent volume/
solution container/ Solvent(s)/g-’ Standing Repeatability/
Test No./trtle Solvent(s) heated Sample srze filter Technique sample time reproducibility
IP 143157
Normal
HeptaneInsolubles
ASTM D3279
76
Normal
Heptane
Insolubles
(9124176)
IP 143177
Asphaltenes
Precipitation
wrth normal
Heptane
Proposed
Methods
for As-
phalt Com-
positron
Analysis
(ASTM)
(May 1977)
n-Heptane Sltghtly/no/ Air blown
>99+ mol% warm for
(Pure grade) filtering
n-Heptane Slightly/
99 min reflux
mol%
(Pure grade)
n-Heptane No/reflux
toluene
(or
benzene)
n-Heptane If needed/ 11-139
99+ mol% ves Asphalt
Pure grade +O.Ol g
asphalt 0.5 -
0.6 gBinders
0.7-0.8 g
Gas oil .-
fuel 011 1 .O-
1.2 g fall
to.1 mg)
Air blown
asphalt
0.5-0.6 g
Binders
0.7-0.8 g
Fuel oils
1.0-l .2 g(all +O.l mg)
UptolOgto
give asphal-
tenes about
0.25 g;
to.01 g
_ IOOmln- - Add nC7, heat, 100 ml nC7 g-t 1 h cool- 0.5 wt%/l .O wt%
heptane per sonic vibrate for ing
1 g sample lo-15 min- 250 ml beaker - Cool for 1 h
- Gooch Crucr- - Vacuum filter
ble with at 38-49’C
glass filter -Wash wrth 3 10
pad 934-AH ml portions
(Iluribut) nC7- Dry in oven @
107YI for
15 min_ 100 ml nC7 - Add nC7, re- 100 ml nC, g-l 1 h cool-
per 1 g sample flux with mag- ing
- 250 ml Erlen- netic mixing
meyer flask 15-30 min
-Gooch Cruc- - Cool for 1 h
rble wrth glass - Vacuum filter at
filter pad 38-49°C934-N I -Wash with 3
(Huribut) IO ml portions
nC7_ Dry @ 107’C
for 15 mm
_ 30 ml per 1 g - Add nC7. reflux 30 ml nC7 g-t Cool for 10/20%
sample for 1 h 1.5-2.5
- Flask - Cool In stopper h in
- Whatman No. flask for 1.5- dark
42 filter 2.5 h in dark
paper - Filter through
Whatman No.
42 paper
- Extract paper
with fresh re-
fluxrng nC7 for1 h
- Reflux with 30-
60 ml toluene
(benzene) until
solids drssolved
from paper
- Evaporate tol-
uene benzene)
in water bath
- Dry at IOO-
11 O’C for
30 min
- lOOmIn- - Add nC7. stir on 100 ml nC7/1 Overnight
Heptane/l ml steam bath 1 h ml
asphalt - Lower, set aside
- 2 dm3 Erhlen- (not heated)
meyer flask overnight
-General pur- - Vacuum filter
pose qualita- - Decant oil/C7
tive filter first, then filter
paper on solids
12 cm Buch- - Resuspend asphal-
ner Funnel tenes in 500 ml
beaker with
150 mot nC7
on steam bath
- Refilter
- Dry at 200°F
under N2
not only differe markedly i1 but so do the aromaticity
(H/C atomic ratio) and the molecular weight* of the
asphaltene (Figure 2). Indeed, the current work has
* Asphaltene molecular weight is considered here to be a series of
relative numbers; absolute molecular weights are difficult to
define13-15.
identified other parameters that also influence the yieldand character of the asphaltenes.
For example, the feedstock/paraffin ratio plays an
important role in determining not only the amount of
asphaltenes separated (Figure 3) but also the quality.
Asphaltenes are generally recognized as brown to dark-
brown powdery materials but use of ‘insufficient amounts
618 FUEL, 1984, Vol 63, May
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Asphaltenes in heavy petroleum feedstocks: J. G. Speight et al.
1.5-
.Pz 1.4-
5i
.o 1.3-E
o2
?z 1.2-=
,”
Pa 1.1 -
-1000
E.P
2‘m
-5000 $
r”
-10000
i.oJ 12 4 6 6 10
Carbon Atoms in Paraffin
Figure 2 Relationship of asphaltene aromaticity (and molecular
weight - vpo/CaHs) to carbon number of the paraffin
10 20 30 40 50
Volume of Paraffin/Volume of Feedstock
Figure 3 Relationship of asphaltene yield to paraffin/feedstock
ratio. Time, 16 h
of the paraffin (e.g. 6 20 ml g ’ feed) produces materials
that are semi-solid (resembling propane asphalts) or
black, shiny solids. The latter, in fact, are asphaltenes that
have retained resin material that can only be removed by
repetition of the treatment on these products.
In this respect, the standard methods differ in the
amounts of paraffin that are recommended for the
separation. For example, amounts of pentane varying from
lo-50 ml g - ’ of feed (Table 1) are recommended. One
method which employs benzene as a ‘thinner’
recommends 40 ml pentane per gram of feed. In essence,
the use of benzene may be considered to negate the
influence of the large excess of pentane and effectively
doubles the quantity of feedstock, thereby effectively
reducing the ratio to 20 ml pentane g ’ feedstock. This is
not enough to guarantee efficient separation of the
asphaltenes (Figure 3).
1 ,
4 a 12 16 24
However, the standard methods which employ heptane Time, Hr.
all recommend > 30 ml heptane g- ’ feedstock. Since, Figure 4 Relationship of asphaltene yield to feedstock/paraffin
heptane (in work parallel to that reported here) has also contact time. 30 ml pentane per gram bitumen
been found to produce similar results to the use of
pentane, the amounts of paraffin per gram feedstock
recommended by these standard methods (Table 2)
ensure efficient asphaltene separation.
Another important parameter that has been identified
is the time required for the separation to be complete.
Contact times of the order of z 8 h are required for stable
asphaltene yields (Figure 4) but contact times between the
‘precipitant’ and the oil must be of the order of 12-16 h to
ensure reproducible yields of asphaltenes (Figure 4).
Shorter contact times produce asphaltenes that resemble
propane asphalts i.e., semi-solid/solid black materials. In
addition, it should be noted that when the asphaltenes are
allowed to remain in contact with the supernatant liquid
for > 16 h, adsorption of ‘resin’ material can occur from
the liquid on to the asphaltenes which will be difficult to
remove by washing.
Although time has been shown to be a very important
parameter for asphaltene separation (Figure 4) the
standard methods (Tables I and 2) do not agree. For
example, for the pentane separation (Table 1) the
recommended time varies from 0.5-15 h but heptane
(Table 2) the recommended times vary from 1 h (after
heating) to overnight (16-24 h). With only one exception
for pentane and one for heptane, the importance of the
‘standing’ time has been grossly underestimated (Tables I
and 2) and could lead to erroneous yield and to materials
which may also influence characterization methods.
Other effects, such as the use of heat to cause
coagulation of the asphaltene particles, are also recom-
mended (Tables 1 and 2) but caution is advised if the
solutions are to be hot-filtered. An increase in tem-
perature can cause a decrease in the solubility of asphaltic
material in the hydrocarbon” thereby adding ‘resin’
material to the asphaltene precipitate.
Whilst the data reported here refer to the use of n-
pentane as the precipitating medium, similar phenomena
(with exception of wt% yield) were also noted when n-
heptane was employed as the precipitating medium. Note
here that some workers prefer to use n-heptane because
this paraffin maintains high molecular weight resins/low
FUEL, 1984, Vol 63, May 619
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Asph altenes in heavy petro leum feedstocks: J. G. Speight et al.
molecular weight asphaltenes in solution and the yield
of asphaltenes is virtually stable (cf. use of C, and C,,,;
reference 11). However, the precipitation of the high
molecular weight resins/low molecular weight
asphaltenes by n-pentane is a benefit and leads to
overall higher material balance when the deasphalted
oil is subsequently fractionated by adsorption
chromatography. It is this particular ‘C5-C, difference’
asphaltene fraction which has a tendency to be strongly
adsorbed by any one of a series of chromatographic
adsorbents.
CONCLUSIONS
Asphaltenes are difficult to define even when a standard
method of precipitation is employed. The many variations
in the recommended procedures (Tables 1 and 2) may all
have some influence not only upon the yield but also upon
the chemical nature of this complex fraction. Indeed,
asphaltenes are not only a complex chemical fraction but
also a complex physical fraction that is extremely difficult
to define whether they arise from petroleum16m1R or coal-
derived liquids .9 The complex nature of the multitude of
heavy feedstocks currently in use makes the establishment
of a truly standard method of asphaltene separation
essential. Whilst the acceptance of a general method of
asphaltene determination will be difficult, it is the only
means by which exact comparisons of published data can
be made. This would require the use of high purity
solvents as well as recognition of the intricacies of the
method. This latter is particularly important now that the
heavier feedstocks are of increasing significance and at a
time when most researchers have modifed an alreadyexisting technique to satisfy differences in feedstock
character or even availability of materials.
The present data illustrate the need to satisfy the
various identifiable parameters that are operative in
asphaltene separation. The most important parameter is
the hydrocarbon employed for the separation. This aspect
of the separation is still open to discussion since there is a
preference for pentane if the deasphalted oil is to be
further subdivided by adsorption chromatography and
adsorbent hold-up is to be minimal. However, heptane
separation would be chosen if a more stable asphaltene
(Figure 2) is desired.
The feedstock :hydrocarbon ratio has been identified asan important parameter that has generally been
recognized in the standard methods for heptane (Table 2)
but is completely lacking in recognition in the standard
methods for pentane (Table I). In addition, ‘settling’ time
has received too little recognition as a procedural
parameter and has been identified as an important aspect
of the separation.
Part of the problem lies in the utilization of various
standard methods designed to separate asphaltenes from
feedstocks other than heavy oils. Since there is now the
tendency to apply these methods (as written) to heavy oils
and residua, some modification of the methods is required
for efficient (and reproducible) separation of asphaltenes
from the heavier feedstocks. Thus, to ensure such a
requirement, conditions such as: 1. 230 ml paraffin* g-’
(ml) of heavy oil; 2. 28 h but preferably 620 h
paraffin/feed contact time; and 3. all operations to be
carried out at room temperature, are necessary.
* As similarly noted, the data described herein relate the use of n-
pentane but n-heptane behaves similarly; the trend, however, is (for a
variety of reasons including safety) to the use of n-heptane; hexane has
long since ceased to be used because of safety (health) aspects
REFERENCES
1
2
3
11
12
13
14
1516
17
18
19
Rostler, F. S. in ‘Bituminous Materials: Asphalts, Tars, and
Pitches’ (Ed. A. J. Hoberg) Volume II: Asphalts, Wiley
Interscience, New York, 1965
‘Annual Book of ASTM Standards; American Society for Testing
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withdrawn 1976 I‘Annual Book of ASTM Standards: American Society for Testing
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Composition’, 1979
‘Standards for Petroleum and Its Products, Standard No. IP
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Speight, J. G. App. Spectr osc. Rev. 1972, 5, 211 and references
therein
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620 FUEL, 1984, Vol 63, May