Peptization studies of asphaltene and solubility parameter spectra

Peptization studies of asphaltene so~u~i~ity parameter spectra Hsienjen

Lian, Jiuun-Ren

and

Lin and Teh Fu Yen

Envjro~~en~al and Civil Engineering, University of Southern California, 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)

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 floorcovering materials, electrical insulation products, brakelining 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 Figure 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 Asphalt

OOl6-2361/94/03/0423-06 0 1994 Butte~ort~-~eine~ann

Ltd.

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

Fuel 1994 Volume 73 Number 3

423

Peptization and solubility spectra of asphaltene:

Hsienjen

Lian et al. 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.

Monomerc sheets

EXPERIMENTAL Solubility parameter Table 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 studied

The source,

refinery

processes

and types of asphalt

samples

Multdamellar

it

!

Veslck

Llqud cryslal or gel - lW.ax M

FIOC N 20,ooo run

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

Sample

Source

Refinery

AAMAAA-1 ABA-l

West Texas Lloydminister West Texas Intermediate/ West Texas Sour

Solvent Distillation Air blown

Asphalt Table 1

Liquefaction

of bituminous

coal via various

Molecular weight

Structural formula

solvents

% yield

Insoluble

Maltbenes

Aspbaltene / Preaspbaltene

OH

Cresol

138

32

Tetralin

132

50

176

82

Gas Oil

Figure 2

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

424

TOlUeIIe Insoluble

Soluble

phenol

Fuel 1994

Volume

73 Number

3

Sol Sol-gel Gel

I

Propane

Naphthalene

o-Cyclohexyl

Types

PeIltaIle Soluble

Solvent

processes

Resins

Fractionation

Soluble

Asphaltenes

and classification

Table 3 Solubility parameter mixture of solvents commonly _.

values used

Insoluble

Preasphaltenes

scheme for asphalt fractions

for a number

Solvents/mixture

Solubility

n-Pentane n-Hexane n-Pentanelcyclohexane Cyclohexane Carbon tetrachloride Toluene Chloroform Carbon disulfide Carbon disulfide/pyridine Pyridine Carbon disulfide/butanol Pyridine/butanol Butanol

7.0 7.4 7.8 8.2 8.6 8.9 9.4 10.1 10.5 10.9 11.0 11.2 11.3

of solvents

and

parameter

(6)

Peptization Table 4 pentane

Surfactants

examined

for the peptization

Molecular

Nonyl

220

CH,(CH&

phenol

weight

Structural

Compound

of asphaltenes

in

formula

0 -9

OH

Stearic acid

284

CH,(CH,),,COOH

Hexadecylamine

241

CH,(CH,),,NH,

Resins

8OC-1300

? may include with hydroxyl, and mercapto

aromatics amino, imino 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 peptization test. The use of dilute concentrations of resin is due to the limit of the usable range of the spectrophotometric method.

and solubility

spectra of asphaltene:

RAE,,\ 112

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-

9

:

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/,(& -

Lian et al.

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

RESULTS AND DISCUSSION The solubility parameter, 6, is defined as the positive square root of cohesive-energy density (potential energy per unit volume):

Hsienjen

8

t

I

-

Pyridine

_

Dichloromethane: Methanol’

-

Dioxane

-

THF2

-

Benzene

-

Ethyl acetate or toluene

-

Cyclohexane

i;

-

Ethyl

Z

_

n-hexane

-

Petroleum

-

; 0,

ether

R

2 ether3

n-pentane

-t -

cBaxterville’l petroleum asphaltene

Propane [isobutane)

.z z IY

2

_ 1. 95.5 (V:Vl 2. Tetrahydrofuran

3. 4.

Estimated Calculated

for

various

mixture value

b2124142

where V,,,is the total volume of the mixture, $1 and 42

Figure 3 fractions

Solubility

parameters

solvents

and

crude

Fuel 1994 Volume 73 Number 3

425

Peptization and solubility spectra of asphaltene: Hsienjen Lian et al

70. 60. 504om 30. A

2om lo0, 5

AA 6

7

8

9

10

Solubility

11

Parameter

12

13

14

15

(6)

Figure 4 The solubility parameter spectra for (+) and the corresponding asphaltene (---A---)

AAM-

asphalt

80 3 a ). c = cl 'G

70

iz

30 40

60 50

20

0

5

6

7

8

9

10

Solubility

11

Parameter

Figure 5 The solubility parameter and the corresponding asphaltene

12

13

14

15

(6)

spectra for AAA-1 asphalt (---A---)

(+)

80 'zi‘ b ). c E

70 60 50

'U 2

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 hexadecylamine, 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

30 40 20

0

5

6

7

8

9

10

Solubility Figure 6 The solubility parameter and the corresponding asphaltene

11

Parameter

12

13

14

-I

15

(6)

spectra for ABA-l asphalt (---A---)

(-0)

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. parameters

426

Fuel 1994

Volume

73 Number

3

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

Peptization

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

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 hydrogenation 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

surfactant

and solubility

spectra of asphaltene:

Hsienjen

Lian et al.

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~ spectra of asp~a~~ene: ~sienje~ Lian et al. REFERENCES 1

2

3 4 5

6 7

8

9 10 11 12

428

American Society for Testing and Materials. ‘Standards on Petroleum Products and Lubricants’, ASTM D8, Philadelphia, 1989, pp. 104-109 The Asphalt Institute. ‘The Asphalt Handbook: Modern Asphalt Usage’, Manual Series no. 4, College Park, MD, 1989, pp. 10-l 1 Yen, T. F. Fuel Sci. Technol. Int. 1992, 10, 723 Schwaeer. I. and Yen. T. F. Fuel 1978. 57. 100 Altgel
Fuel 1994 Volume 73 Number 3

13 14

15 16 17 is 19

20 21 22

Brown, A. B., Sparks, J. W. and Smith, F. M. J. Cofloid Sci. 1957, 12, 283 Yen, T. F. in ‘Proceedings of the First Pan Pacific Synfuel Conference, Tokyo, Vol. 2, The Japan Petroleum Institute, Tokyo, 1982, pp. 547-557 Weinberg, V. L. and Yen. T. F. Fuel 1980, 59, 287 Wong, E. and Yen, T. F. Energy Sources 1988, 10, 201 Lian, H. J., Lee, C. Z. H., Wang, Y. Y. and Yen, T. F. J. Planar Chromarogr. (Heidelberg) 1992, 5, 263 Weinberg, V. A., White, J. I. and Yen, T. F. Fuel 1983,62,1903 Yen, T. F. in ‘Future of Heavy Crude Oils & Tar Sands’ (Eds R. F. Mever and C. T. Steele). McGraw-Hill. New York. 1981. pp. 174-i79 Gonzalez. G. and Middea, A. Cofloids and Surfaces 1991, 52, 207 Yen, T. F. in ‘Proceedings of the Internatjonal Conference on Chemistry of Bitumen, Rome, Italy’, Vol. 1, University of Wyoming Research Corp., Laramie, WY, 1991, pp. 382-407 Dickie, J. P. and Yen, T. F. ACS Petrol. Div., Preprint 1968, 13(2), F140-3

23 24 25 26

Chan. M., Sharma, M. M. and Yen, T. F. Ind. Eng. C/tern. Process Design Der. 1982, 21, 580 Sadeghi, K. M., Sadeghi, M. A., Kuo, J. F., Jang, L. K. and Yen, T. F. Energy Sources 1990, 12(2), 147 Jang, L. K., Sharma. M. M., Chang, Y. I., Chan, M. and Yen, T. F. AIChE, Symp. Ser. No. 212 1982, 78, 97 Yen. T. F. and Famanian, P. A. ‘Petroleum recovery process using natire petroleum surfactant’, US Pat. 4 232 738, 1980