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



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



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



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

and Materials. Philadelnhia. Part 24. Standard No. D-2006’. 15.

withdrawn 1976 I‘Annual Book of ASTM Standards: American Society for Testing

and 7aterials, Part 15, Proposed Standard for Asphalt

Composition’, 1979

‘Standards for Petroleum and Its Products, Standard No. IP

143/57’, Institute of Petroleum, London

Speight, J. G. App. Spectr osc. Rev. 1972, 5, 211 and references

therein

Yen, T. F. Am. Chem. Sot. Di v. Petrol. Chem., Preprints 1972,

17(4), F102 and references therein

Speight, J. G. Am . Chem. Sot. D iv . Petrol . Chem., Preprint s 1979,

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Brooks, J. D. and Taylor, G. H. ‘Chem. & Phys. of Carbon’, 1968,

4, p. 243, Marcel Dekker, New York

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Bland, W. F. and Davidson, R. L. (Editors) ‘Petroleum

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Mitchell , D. L. and Speight, J. G. Fuel 1973, 52, 149

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620 FUEL, 1984, Vol 63, May

factors influencing the separation of asphaltenes

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