Hsienjen & Yen- Peptization Studies of Asphaltene and Solubility Parameter Spectra, 1994

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Peptization studies of asphaltene and

so~u~i~ity parameter spectra

Hsienjen Lian, Jiuun-Ren Lin and Teh Fu Yen

Envjro~~en~al and Civ il Engineeri ng, Uni versit y of Southern Cali forni a, Los Angeles,

CA 9~089-2537~ USA

Received 75 May 7992; revised 4 January 7993)

Asphaltene particles are dispersed in gas oil (saturates and aromatics) with resins as peptizing agents in

the asphalt system. The interaction between resin and asphaltene micelles is not well understood. In the

present study, aromatic hydrocarbons are proved to be a good dispersed medium for peptization tests by

the solubility parameter approach. The partial precipitation of asphaltene in a fixed amount of aromatic

hydrocarbon system (such as toluene), with gradual additions of paraffinic hydrocarbon (such as pentane),

in the presence of various surfactants has been studied. These surfactants affect the asphaltene precipitation,

either by acceleration or by retardation, depending on the structural types and quantities of the surfactants.

We have found that the nature of resin serves as a good peptizing agent (interfacial agent) since the polar

fractions of resin also contain surfactants (amphiphiles). Due to this peptizing function, resins can be

applied to enhance oil recovery or lengthen paving asphalt life.

(Keywords: peptization; asphalt; asphaltenes)

Asphalt

is a dark brown to black cementitious material,

solid or semisolid in consistency. The predominating

constituents are bitumens which occur in nature, or are

obtained as residue of refining petroleum’. Asphalts

possess special properties such as: impermeability to

water; pronounced adhesive and cohesive properties;

susceptibility to temperature changes and deformation

in service; excellent abrasion resistance; chemical

resistance to acids, alkalis, air, ground water, corrosive

soil conditions, etc. Asphalts can thus be used for paving,

roofing, road joint materials, crack fillers, coatings

materials (canal linings, water-proofing cements, pipe

dips, sound-deadening products), tiling and floor-

covering materials, electrical insulation products, brake-

lining products, etc2. In the United States about 70%

of all oil asphalts are consumed by the road-paving

industry, with some 20% consumed by roofing

manufacturers and another 10% consumed by special

usage manufacturers.

The asphalt system has a colloidal nature and is not

a true solution3. It can be fractionated into saturates,

aromatics, resins and asphaltenes by the solvent fraction

methods4, saturates-aromatics-resins-asphaltenes (SARA)

method5, or thin-layer chromatography ft.1.c.) method6.

The polarity of these four fractions roughly increases in

the order of saturates, aromatics, resins and asphaltenes.

In its natural state, asphaltene exists in asphalt systems as

an oil-external (Winsor’s terminology) or reversed micelle

(see

Figur e 1)‘.

The polar groups are oriented toward

the centre, which can be comprised of water, silica (or

clay) or metals (V, Ni, Fe, etc.). The driving force of the

polar groups assembled toward the centre originates from

hydrogen bonding, charge transfer or even salt formation.

This oil-external micelle system can be reversed to

oil-internal, water-external micelles (usually called

Hartley micelIes)*. An aggregate of asphaltene particles

with adsorbed resins can form a supermicelle, and oil

may be occluded between supermicelles as an intermicellar

medium. Upon further aggregation the supermicelles can

coalesce into giant supermicelles, and can even gradually

grow into a liquid crysta19*‘o.

From the above observation, it can be noted that

reversed micelles are predominant in asphalt systems with

a higher asphaltene content. Three different types of

asphalt,

such as sol (micelle, supermicelle, giant

supermicelle), sol-gel (supermicelle, giant supermicelle),

gel (liquid crystal) asphalt, can be defined. Most of the

paving asphalts belong to the sol-gel type, and roofing

asphalts belong to the gel (air blown) type.

In asphalt systems, asphaltene micelles are present as

discrete or colloidally dispersed particles in the oily phase.

Although the asphaltenes themselves are insoluble in gas

oil (saturates and aromatics), they can exist as fine or

coarse dispersions, depending on the resin content. The

resins are part of the oily medium, but they have polarity

and molecular weight higher than gas oil. These

properties enable the molecules to be easily adsorbed

onto the asphaltene micelles and to act as a peptizing

agent of the colloid stabilizer by hydrogen bonding or

charge neutralization”.

Age hardening (molecular structuring) of paving

asphalt has been a major concern in road maintenance

for many years. This reversible phenomenon can produce

large changes in the flow properties of asphalt without

altering the chemical composition of the asphalt

molecules. Brown et uL~‘*‘~ studied this reversible

molecular structuring (steric hardening) by rheological

methods. Very little work has been conducted in the

OOl6-2361/94/03/0423-06

0 1994 Butte~ort~-~eine~ann Ltd. Fuel 1994 Volume 73 Number 3

423



Pepti zati on and solubil it y spectra of asphalt ene: Hsienjen Li an et al.

Monomerc sheets

Multdamellar

it

Veslck

FIOC Llqud cryslal

N 20,ooo run

or gel - lW.ax M

Figure 1 Association, aggregation and coalescence of micelles to form

vesicles and precipitates (floes). Circle denotes polar functional groups,

e.g. S, N and 0

Table 1 Liquefaction of bituminous coal via various solvents

Solvent

Molecular

weight

Structural

formula

% yield

Naphthalene

Cresol

OH

138

32

Tetralin

132

o-Cyclohexyl phenol

176

50

82

interim years, however, and no one has approached this

topic as a colloidal chemistry problem. Previous studiesI

have shown that the amphipathic structure of the solvents

is related to the percentage of liquefaction of a bituminous

coal (see Table I). The present paper illustrates the fact

that colloidal nature, and how it changes with time, is

controlled by the chemistry of its components, especially

the asphalteneeresin ratio. To prove the fact that resin

is the peptizing agent in the asphalt system it is important

to select a solvent that can dissolve asphaltene. For

this reason, determining the solubility parameter of

asphaltenes, as well as a third component (surfactant or

peptizing agent) that can improve the asphaltene

dispersion in the system by the solubility parameter

approach, is being attempted.

EXPERIMENTAL

Solubil i ty parameter

Tabl e 2 shows the three types of asphalt from different

sources and refinery processes used in this experiment.

All the samples were supplied by the Strategic Highway

Research Program (SHRP).

All asphaltene samples were isolated by pentane using

the solvent fraction method (see Figure 2)4. All the

solvents, including mixtures, used in these experiments

were reagent grade, and the solubility parameters of all

solvents are listed in

Table 3.

The pentane solvent used

was n-pentane.

For each run, 0.5 g of asphaltene with 10 ml solvent

were placed in a flask, then agitated by a magnetic stir

bar for 30 min at room temperature. Finally, the

precipitation of solutes was filtered out by Whatman No.

1 filter paper. The solubility parameter of asphaltene was

determined by miscibility. Determination of solubility

parameter spectra is based on a method developed by

Weinberg and Yen15.

Table 2 The source, refinery processes and types of asphalt samples

studied

Sample

AAM-

AAA-1

ABA-l

Source

West Texas

Lloydminister

West Texas Intermediate/

West Texas Sour

Refinery processes Types

Solvent

Sol

Distillation Sol-gel

Air blown Gel

Asphalt

I

PeIltaIle

Soluble

Insoluble

Maltbenes

Aspbaltene /

Preaspbaltene

Propane

TOlUeIIe

Soluble

Insoluble

Soluble

Insoluble

Gas Oil Resins Asphaltenes Preasphaltenes

Figure 2 Fractionation and classification scheme for asphalt fractions

Table 3 Solubility parameter values for a number of solvents and

mixture of solvents commonly used

_.

Solvents/mixture

Solubility parameter (6)

n-Pentane

7.0

n-Hexane

7.4

n-Pentanelcyclohexane

7.8

Cyclohexane

8.2

Carbon tetrachloride 8.6

Toluene

8.9

Chloroform

9.4

Carbon disulfide

10.1

Carbon disulfide/pyridine

10.5

Pyridine

10.9

Carbon disulfide/butanol

11.0

Pyridine/butanol

11.2

Butanol

11.3

424 Fuel 1994 Volume 73 Number 3



Peptization and solubility spectra of asphaltene: Hsienjen Lian

et al.

Table 4 Surfactants examined for the peptization of asphaltenes in

pentane

Compound

Molecular weight

Structural formula

Nonyl phenol

220

CH,(CH& 0 OH

-9

Stearic acid

284

Hexadecylamine 241

Resins

8OC-1300

CH,(CH,),,COOH

CH,(CH,),,NH,

? may include aromatics

with hydroxyl, amino, imino

and mercapto groups

Peptization test

First, traditional solvent fractionation methods were

used to remove soluble impurities from the precipitates

(asphaltenes and preasphaltenes). The precipitates were

then dissolved in a toluene solution to remove the

precipitated preasphaltenes. Finally, relatively pure

asphaltenes were redissolved in toluene to obtain a

100 ppm concentration of asphaltene solution16.

The formulae and properties of the various surfactants

assayed as peptizing agents are listed in Table 4 in which

resins were isolated by a preparative TLC method17.

Toluene was used as a solvent in this experiment.

Asphaltene precipitation from toluene solutions was

tested by adding pentane, and was carried out using a

100 ppm asphaltene solution containing 0.5% (by

weight) nonyl phenol, stearic acid or hexadecylamine.

After 20 min agitation the solutions were left at room

temperature for 3 h and, afterwards, centrifuged at 3000

rev min-’ for 30 min. The absorbance of supernatant

was determined at 400 nm by a double beam Varian u.v.-

visible spectrophotometer. Different concentrations

(0.5% and 1%) of nonyl phenol were then used to repeat

the peptization test following the same procedure.

Finally, two different resins (AAA-1 and AAX-1) were

used to compare with nonyl phenol at 50 ppm

concentration of surfactants, for performing the peptiza-

tion test. The use of dilute concentrations of resin is due

to the limit of the usable range of the spectrophotometric

method.

RESULTS AND DISCUSSION

The solubility parameter, 6, is defined as the positive

square root of cohesive-energy density (potential energy

per unit volume):

RAE,,\ 112

where

AE

is the energy change for complete isothermal

vaporization of the saturated liquid to the ideal gas state

and

V

is the molar volume of the liquid.

For a material to be dissolved in a solvent, the free

energy change, AG,,

of the process must be negative

where AG,= AH,- TAS,. Since the entropy change,

AS,, is always positive, the heat of mixing, AH,,,,

determines whether or not dissolution will occur.

According to the Hidebrand-Schatchard theory, the heat

of mixing is given by

AH, = 1/,(& -

b2124142

where

V

s the total volume of the mixture, 1 and 42

are the volume fractions, and 6, and J2 are the solubility

parameters of the solvent and solute, respectively.

Therefore, one can only make the change of free energy

negative if the heat of mixing is small or negligible, such

as the case when 6, is equal to 6,.

Many methods can be used to determine the solubility

parameter of unknown components. In the present study,

we estimated the solubility parameter of unknown

compounds by measuring the solubility in a number of

solvents whose 6 values are known18.

In order to test the substance’s solubility with various

solubility parameters of solvents, the individual solvent

as well as the mixed solvent are used. The solubility

parameter for single or mixed solvent can be generalized

as

where 6, and 6, are the solubility parameters of the pair

of solvents, and 41 and 42 are the volume fractions of

the individual solvent from the pair. From the above

theory, the solubility parameters of different asphalt

fractions can be easily analysed. Figure 3 shows the

solubility parameter of different fractions of asphaltlg.

The solubility parameter spectra of asphalt and

asphaltenes for three different colloidal types of asphalt

(AAM-1, AAA-1 and ABA-l) are obtained.

Figure 4

shows that the solubility parameters of AAM- asphalt

and asphaltenes are in the range of 7.4-10.4 and

8.6-10.4, respectively. Without doubt, the composition

of asphalt is more complex than that of asphaltenes.

Therefore, the solubility parameter of asphalt has a wider

range than that of the asphaltenes.

Figures 5

and 6 indicate the solubility parameters of

AAA-1 and ABA- 1 asphalt and asphaltenes, respectively.

The solubility parameters of AAA-1 asphalt and

asphaltenes are in the range of 8.2-10.4 and 8.6-10.1;

the solubility parameters of ABA-l asphalt and

asphaltenes are in the range of 7.9-10.4 and 8.6-10.1.

Similar to the trend shown in Figure 4 the solubility

12 -

11 -

10 -

-N

_ 9-

?

=

- Pyridine

_ Dichloromethane:

Methanol’

- Dioxane

- THF2

- Benzene

- Ethyl acetate

or toluene

cBaxterville’l

petroleum

;

asphaltene

0,

9

t

- Cyclohexane

i;

:

8

-

Ethyl ether

Z

R

I

_

n-hexane

2

- Petroleum ether3

n pentane

-t

.z

z

IY

2

-

Propane

[isobutane)

1. 95.5 (V:Vl 3. Estimated mixture

2. Tetrahydrofuran 4. Calculated value

Figure 3 Solubility parameters for various solvents and

fractions

_

crude

Fuel 1994 Volume 73 Number 3

425



Pept izat ion and solub i l i ty sp ectra of asphaltene: Hsienjen L ian

et al

70.

60.

50-

4om

30.

2om

A

l o-

0,

AA

5 6 7 8 9 10 11 12 13

14 15

Solubility Parameter (6)

Figure 4 The solubility parameter spectra for AAM- asphalt

(+) and the corresponding asphaltene (---A---)

80

3

a

70

-

) .

60

c

=

cl

50

G

i z 400

20

0

5 6 7

8 9 10 11 12 13

14 15


Figure 5 The solubility parameter spectra for AAA-1 asphalt (+)

and the corresponding asphaltene (---A---)

80

z i ‘

b

70

-

) .

60

c

E 50

U

2 400

20

0

5

6 7 8 9 10 11 12 13

14


-I

15

Figure 6 The solubility parameter spectra for ABA-l asphalt (-0)

and the corresponding asphaltene (---A---)

parameters of these two asphalts have a wider range than

that of the asphaltenes.

From the above results, it can be summarized that the

solubility parameters of different asphalts are in the range

of 7.4-10.4, and those of asphaltenes are in the range of

8.6-10.4. From

Figures 4 to

6, we can easily observe

significant differences for three different types of asphalts.

Compared to asphaltenes, all spectra for the three

samples are similar, except for the tail part. This may

indicate that the difference in the composition of

asphaltenes in the three asphalt types is not significant.

It can be demonstrated that the significantly different

ranges of solubility parameters for three asphalts are due

to the different compositions of four fractions in these

three different types of asphalt, which are dissolved in

various degrees of the solvent. Furthermore, it proves

that the selection of toluene for the peptization test is

within the range of 8.6-10.4. These results are more

concise than those results shown on Figure 3 because of

the more narrow range of the solubility parameter of

asphaltene.

Gonzalez and Middea” have proved that the addition

of oil soluble surfactants to an asphaltene-toluene

solution can keep asphaltene more stable in solution

when the heptane volume is increased. Their results also

illustrate that functional groups, such as aromatic groups

or hydroxyl groups, play an important role for the

peptization of asphaltene in toluene solution.

In this paper we attempted to simulate colloidal

asphalt systems to perform peptization tests (precipitation

tests).

Figure 7

shows that adding 0.5% by weight of

nonyl phenol, stearic acid and hexadecylamine exhibits

different concentrations of asphaltenes in solution when

pentane volumes are above 50%. At a 70% volume of

pentane, the concentration of asphaltenes in 0.5%

hexadecylamine solution was less than the solution

without addition of surfactants. The difference was about

6.2 wt%, meaning that hexadecylamine could be a

flocculation agent. The efficiency of different surfactants

for peptization tests increases in the order of hexa-

decylamine, no addition of surfactant, stearic acid and

nonyl phenol. Doubtlessly, nonyl phenol is the best

peptizing agent tested in this experiment.

In order to correlate peptizing efficiency with the

concentration of surfactants, nonyl phenol was tested at

different concentrations, such as 0.5% and 1% by weight.

The results are shown in Figure 8. The precipitation of

asphaltenes at 60% volume of pentane in solution has a

15 wt% difference between 1.0 wt% nonyl phenol

solution and original (no additive) solution, and the

difference increases to 27 wt% when the pentane volume

reaches 80% in solution. It is clear that the peptizing

efficiency increases with increasing concentration of

surfactants.

Lastly, two different resins (AAA-1 and AAX-1) were

compared with nonyl phenol to determine which

Figure 7 The comparison of different surfactants (nonyl phenol,

stearic acid, hexadecylamine) on the precipitation of AAA-1 asphaltenes

by pentane (100 ppm asphaltene for initial solution). -m-, nonyl

phenol;

-+-, stearic acid; &-, no amphiphile; +,

hexadecylamine

426 Fuel 1994 Volume 73 Number 3



Peptization and solubility spectra of asphaltene: Hsienjen Lian et al.

b io

io

Penta”:(0,:;

BO d0

Figure 8 The comparison of two different concentrations (0.5% and

I%) of nonyl phenol on the precipitation of AAA-1 asphaltenes by

pentane (100 ppm asphaltene for initial solution). +, 0.5% nonyl

phenol; ~

+ ,

no amphiphile; b, 1 O% nonyl phenol

Figure 9 The comparison of adding nonyl phenol and two different

resins on the precipitation of AAA-1 asphaltenes by pentane (50 ppm

Fsphaltene for initial solution); surfactant concentration, 50 ppm each.

a, nonyl phenol; -+--, AAA-I resin; &, AAX- resin

surfactant

was the best peptizing agent. From

Figure 9

the peptizing efficiency of resins is seen to be better than

nonyl phenol. Although AAA-1 resins seem to have a

higher peptizing efficiency than AAX- resins, these two

curves are very close. Notice also the fact that resins

tested are at a concentration two orders of magnitude

lower than other surfactants.

The effectiveness of a peptizing agent may be directly

linked to its structure. The fact that o-cyclohexyl phenol

is a better solvent for the hydrogenation of coal is similar

to the fact that nonyl phenol is a better peptizing agent

for asphaltene. A closer examination of this reveals that

both molecules contain aromatics, hydroxyl groups and

hindered or zigzagged configurated C, to C, hydrocarbon

skeletons. These are necessary elements to be effective in

the interaction with associated molecules in a micelle or

cluster. The simplest picture may be that the aromatic

part of the approaching molecule may be easily inserted

into the asphaltene stacks” due to the n--71association.

Once associated, the bulkiness of the paraffinic or

naphthenic portion of the peptizing agent, or hydro-

genation solvent, may force the asphaltene sheets19 apart

due to the strong anchoring properties of hydrogen bonds

to the polar centres within the asphaltene system. In this

analysis the function and mechanism of the hydrogenation

solvent and peptizing agent are the same, whether or not

the substrate is the asphaltene of a coal or of a petroleum

derived asphalt.

This discussion is linked to the ordinary surfactant

screening process. A good amphiphile or surfactant, from

either synthetic or natural (e.g. microbial) origin, is one

in which its molecular design contains an aromatic head

with a bulky tail. The use of hydrogen bonding is essential

if the surfactant is to be used with heavy end of fossil

fuels since they all contain heterocyclic atomic centres

with lone pairs of electrons available for donation.

Certainly the shifts of resin-asphaltene ratio, or the

reassemblages of the molecules within micelles by

surfactants to create more resins or even gas oil fractions,

are not chemical changes. All these association

phenomena including clustering, aggregation2’q21, etc.

are physical reassemblages or restructurings of the age

hardening problem. Peptization can be viewed as a

control for the re-establishment of resin-asphaltene

ratio, and in this context peptization may control the

age hardening of asphalt.

Previous publications 22 have indicated that petroleum

resins contain excessive heterocyclic atoms; some

molecules contain multiple sulfur, nitrogen and oxygen

atoms. Many of the resins do contain hydroxyl, amino

or imino, and mercapto functions23, and many bitumen

molecules contain mercaptans24. There is no doubt that

certain fractions of resin (e.g. the polar resin) will contain

sufficient acidic hydrogens (replaceable protons) as

modified by the adjacent heterocyclic atoms. Therefore,

certain petroleum resins appear to be very good peptizing

agents.

The fact that caustic flooding is effective for enhanced

oil recovery of medium heavy crude is simply the

modification of the asphaltene by in situ surfactants

formed by the alkalis with the active acidic constituents

within the polar resin of that petroleum25. The injection

of the resin for further enhanced oil recovery is obviously

adequate2’j.

From the solubility parameter experiments, we find

that different compositions in asphalts have a different

miscibility in solvents because different fractions exhibit

different miscibilities in solvents, and the miscibilities of

different asphaltenes in solvents are slightly different

because different compositions of asphaltenes are not

dissolved to the same degree in solvents.

Evidently, from the peptization experiments the lower

limit of the asphaltene solubility parameter is about 8.0

by using mix-solvent formulation. We also find that

surfactants with a molecular weight of at least 220 (e.g.

nonyl phenol) can be adsorbed by the asphaltene

molecule as a peptizing agent. The results indicate that

the interactions are not restricted to the polar groups,

but the 7~ electrons of the aromatic and naphthenic

portions in the asphaltenes may act as electron donors

for hydrogen bonds with hydroxyl groups of the

surfactants. Resins have proven to be the best peptizing

agent in asphalt colloidal systems due to their high

molecular weight and high aromatic, naphthenic portion

and hydroxyl group. Synthetic (reconstituted) asphalts

may be made by increasing resin fractions which were

isolated from original asphalts to solve the age hardening

problem. Through this method one can prolong the

paving asphalt life. Also, in this manner resin can be used

as a surfactant for enhanced oil recovery.

Fuel 1994 Volume 73 Number 3 427



Pap~ization and sol~b ~lit~ spec tra of asp~a~~ene: ~sienje~ Lian

et al.

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4 8

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