influence of chemical composition on the physical

2nd Mercosur Congress on Chemical Engineering 4 Mercosur Congress on Process Systems Engineering th

INFLUENCE OF CHEMICAL COMPOSITION ON THE PHYSICAL CHARACTERISTICS OF PAVING ASPHALTS L.O. Oyekunle Department of Chemical Engineering, University of Lagos

Abstract. The simplest general compositional model considers asphalt to be made up of asphaltenes, heavy oils and resins. In the present study the extent of similarity between the properties and composition of various types of asphalt, obtained from different sources has been carefully explored. It shows how ductility, penetration and softening point are related to the chemical nature of paving asphalts in the range 41 – 58 softening points. Asphalt properties are found to be a direct function of its chemical constituents. The relationship between softening point and asphaltene content was found to be linear while asphalts of the same softening point are easily distinguished by the content of the resins. Evaluation of the two asphalt indices: the asphaltene index (IA) and the Gaestel index (IC) reveals that they both vary linearly with the composition of asphaltenes and resins thus confirming their suitability in the characterization of asphalts. Comparison of the two indices shows that IC is more suitable in estimating the colloidal stability of different groups of asphalts. The observed correlations can be found useful in the areas of asphalt blending and rejuvenating through the appropriate selection of asphalt components. Keywords: Paving Asphalts, Group Chemical Composition, Correlation of Properties.

1. Introduction Asphalt is an extremely versatile material and its usage is widespread from paving, road building, roofing to protective coating and hydraulic structures. Asphalt has these and various other uses as a result of certain properties it possesses like its ductility, durability, cohesiveness, its adhesive nature and its waterproof nature. These properties vary greatly among different types of asphalt just as petroleum differs according to where it is obtained. Asphalt is of particular interest because of its many engineering and industrial applications. Almost all the asphalts used in the world are refined from petroleum. Such asphalts are produced in a variety of types and grades ranging from hard brittle solids to almost water-thin viscous liquids. Processes employed in asphalt manufacture include atmospheric-vacuum distillation, air-oxidation, solvent extraction or precipitation, chemical treatment and blending of individual stocks (Nelson, 1958; Broome, 1973).

1.1. Physical properties of asphalts Asphalts differ in their physical properties due to the nature of their crude oil sources and the operations involved in their production. They are characterized by a large number of standard tests, which include ductility, penetration, softening point, specific gravity, asphaltene content and viscosity. Other important determinations are temperature susceptibility, cold fragility, and viscosity before and after aging, interfacial _______________________________________ Address: Department of Chemical Engineering, University of Lagos, Lagos 101017, Nigeria. E-mail: [email protected]

1


tension, adhesion, electrical resistance and impermeability to water (Gun, 1973; Jennings and Pribanic, 1989; Ruan et al., 2003). Ductility. The ductility of asphalt is a measure of its capacity to elongate or stretch and it is an indication of the material to flow. A briquette of the asphalt is pulled apart at a uniform rate at a specified temperature, and the elongation before rupture takes place measured in centimetres (ASTM D113). Penetration. Penetration is measured with a penetrometer by which a standard needle is applied to the sample. It is the distance in tenths of a millimetre that a needle penetrates vertically into the sample under fixed conditions of temperature, load and time (ASTM D5). Softening point. Softening of asphalts does not take place at a definite temperature. As temperature rises they gradually and imperceptibly change from a brittle or exceedingly thick, slow flowing material to a softer, less viscous liquid. The softening point is defined as the temperature at which a disk of the sample held within a horizontal ring is forced downward a distance of 2.54 cm under the weight of a steel ball (ASTM D 36). 1.1. Group chemical composition The composition of asphalts and other heavy fractions has been the subject of several investigations over the years and it is still currently receiving much attention (Gun, 1973; Marvillet, 1975; Hagen, 1989; Andersen and Speight, 2001; Demirba, 2002; Baginska and Gawel, 1987, 2004; Cong et al., 2004; Trejo et al, 2004). The composition of asphalt is very complex but separation into different chemical groups has been made by both physical and chemical methods. The fractions so obtained are made up of poly-nuclear aromatics and hetero-aromatic molecules (asphaltenes), viscous naphtheno-aromatic hydrocarbons (heavy oils), heterocyclic and polar compounds (resins). The relative amount of asphaltenes, oils and resins obviously depends on the origin of the source crudes, the refinery treatment and the finishing processes (Gawel, 1993; Barre et al., 1997). Asphaltenes. Asphaltenes are large, discrete solid inclusions of asphalts. They are hard, non-plastic high molecular weight compounds ranging between 1200 and 200,000. They are responsible for the presence of structure in asphalts and for their non-Newtonian rheological behavior. The asphaltenes are the most highly polar molecules, insoluble in low molecular weight normal paraffins and are classified by the precipitating solvent with different solvents precipitating different amount of asphaltenes. The asphaltenes are the thickening agent or structuring agent and they impart strength, stiffness and colloidal structure in asphalt. Heavy oils. Maltenes (or Petrolenes) are the remaining portion of the asphalt material after the precipitation of asphaltenes with the normal paraffins. It consists of two fractions, heavy oils and resins (Gun, 1973, Cong et al., 2004). These oils are the liquid part of the asphalt and consist of normal-, iso- and cyclo-paraffins and condensed naphthenes with some alkyl aromatics. The aromatic portion is mostly naphtheno-aromatic hydrocarbons with three or four naphthenic rings per molecule and is non-polar. The oils have a key feature of dispersing polar agglomerations of asphaltenes and resins. Thus this component is very important, as it is a colorless liquid, soluble in most solvents and it is responsible for viscosity and fluidity of the asphalt. Resins. The resins are chemically very similar to the asphaltenes and they are a transition from oils to asphaltenes. They are semisolid or solid at room temperature, fluid when heated and brittle when cold. The 2


resins consist of mainly polycylic molecules containing saturated, aromatic and hetero-aromatic rings and heteroatoms in various functional groups. The resins are not as polar as the asphaltenes and their molecular weight ranges from 300 to 2000. These resins provide adhesion, ductility, malleability and plasticity

1.3. Previous studies on asphalts A study of the composition of three paving asphalts obtained from Russian crude oils and having very close softening points (48-49 0C) revealed that asphalts are clearly distinguished by their group chemical composition (Oyekunle, 1980). There were differences between the contents of oils, resins and asphaltenes for each sample. The asphaltene content alone did not fully explain the rheological behavior of the asphalts while other contributory factors were asphaltenes/resin ratio and the degree of aromaticity of the oils. Residual asphalts obtained from 25 samples of Nigerian light, medium and heavy crude oils were chemically treated by air-oxidation and sulfurization (Oyekunle and Onyehanere 1990, 1991a, 1991b; Oyekunle and Adesanya, 2000; Oyekunle and Onwa, 2000; Oyekunle et al., 2004). The chemical treatment brought about changes in chemical composition, increase in molar mass, manifold increases in asphaltene content and more than 20% rise in softening point. Aim of the study. Asphalt is extensively used and thereby requires that scientists investigate its structure, composition and properties. It is of interest to determine the extent of similarity between the properties and composition of asphalts obtained from several locations. The aim of the present study is to further assess the impact of the group chemical composition on the properties of asphalt. The paper summarizes the work that has been conducted on several Russian asphalts (Gun, 1973) and five fresh Chinese asphalts (Yang et al., 2003) showing the relationship of chemical composition with the physical properties of asphalts.

2.

Sources of Experimental Data The data used in this work were obtained from a number of experimental studies as shown in Table 1.

Table 1. Sources of data for the different asphalt samples. Group A B C D

Type of asphalt Russian pilot plant Russian pilot plant Russian refinery Chinese Fresh

Samples 6 7 4 5

Softening points 41, 46, 54.5, 58, 77 & 99.5 45, 45, 49, 49, 50.5, 50.5 42, 45, 49.5 &53.5. 43.5, 44.6, 45.1, 45.7 &45.8

Reference Gun,1973 (p. 227) Gun, 1973 (p. 225) Vinogradov,1977 Yang et al., 2003

2.1. Materials and methods. The materials investigated were oxidized Russian asphalts as shown in Table 1. The five fresh paving asphalts were obtained from heavy Chinese crude oil. The determination of general physical properties was made according to the ASTM methods. Asphaltenes were determined by n-heptane insolubles (ASTM D3279) while the contents of resins, aromatics and the saturates were evaluated by liquid chromatography. 3


3. Results and discussion 3.1. Physical characteristics and chemical composition of asphalts Figure 1A shows the plots of ductility and penetration as a function of softening point (S. P.) for oxidized asphalts (S.P.= 41-58) together with the profiles of the group chemical composition: asphaltenes, oils and resins (Group A). As expected, asphalts with high asphaltene content have lower penetration values while there is a linear relationship between asphaltene content and softening point. There is no observed correlation between ductility and other asphalt properties. Ductility, penetration and softening point plots as recorded in Figure 1B for typical paving asphalts (S.P.= 45-53) present an interesting picture of how the physical properties are related to the chemical nature of asphalts (Group B). The profiles for both ductility and resins content show similar trends. It is clearly seen here that the ductility of asphalt increases with increase in the resins content and decreases with the increase in the contents of both asphaltenes and oils. Penetration at both 25 0C and 0 0C decrease simultaneously with increasing softening point and with the rise in asphaltene content. The data presented in Figures 1A and 1B

80 60 40 20 0 -20 -40 -60 -80 -100

250 200 150 100 50 0 Resins Ductility

Asphaltenes Pen @0

Oils Pen@25

Penetration (0.1mm)

Chemical composition (%) & Ductility (mm)

confirm the fact that the empirical properties are not adequate enough to characterize asphalts.

-50 -100

40

45

50

55

60

Softening point ( 0C) Figure 1A. Properties of asphalts (Group A)

60

140 45

40

49

45

49

50.5

50.5

120 100 80

30

60

20

40

10

20

0

Softening point Asphaltenes Ductility

-10 -20 0

1

2

Oils Pen@25

3 4 5 Aspalt sam ples

Resins Pen@0

0 -20

Penetration (0.1mm)

Chemical composition (%), Ductility (cm)

50

53

-40 6

7

8

Fig 1B. Properties of asphalts (Group B)

4


Asphaltene content and softening point.

Figure 1C shows the relationships recorded between the

asphaltene contents, oils and the softening point for asphalts in Group A. The plots obtained for the other groups of asphalts (B-D) are exactly similar. The softening point increases with increase in asphaltene content and decreases with the oil content in a linear fashion. This property relationship can be employed in improving the characteristic properties of asphalts during blending. Resins and ductility. The relationship between ductility and other properties could not be easily determined in Figures 1A and 1B while asphalts with the same softening point (50.5 0C) vary widely in ductility (Fig. 1B). Figure 1D shows the non-linear relationship of ductility and resins content for two groups of asphalts (Groups A and B). It is quite obvious that it is the quantity of resins that determines the ductility of an asphalt; the

Asphaltene content (%)

higher the content of resins, the greater the ductility of the asphalt.

60 50 40 30 20

Asphaltenes

10

Oils

0 40

50

60

70

80

90

100

Softening point (OC) Fig. 1C. Relationship between asphaltene content, oils and softening point (Group A)

3.2. Influence of asphalt indices on properties The most important structure forming element of asphalts are the asphaltenes, whose quantity and nature of interaction with resins and oils largely determine the rheological properties. Asphalt is considered to be a

80

140

70

120

60

100

50

Group A

40

Group B

80 60

30 20

Ductility, Group B (cm)

Ductility, Group A (cm)

polymer solution in which the oils as the liquid part are the solvent and the asphaltenes the solute. It is

40 25

26

27

28

29

30

31

32

33

Resins content (%) Fig. 1D. Relationship betw een ductility and resins content

commonly accepted in petroleum chemistry that asphaltenes form micelles, which are stabilized by adsorbed resins kept in solution by the aromatics. Two key parameters that control the stability of asphaltene micelles are the ratio of aromatics to saturates and that of resins to saturates and that of resins to asphaltenes. When 5


these ratios decrease, asphaltene micelles will coalesce and form larger aggregates. These two ratios are explored in the evaluation of the relationship the group chemical composition and the physical properties of asphalts. They are expressed in terms of two indices: the asphaltene index and the Gaestel index. Asphaltene Index, IA: This is the changing rate of asphaltene content and it is calculated using normal heptane asphaltene precipitation (ASTM D3279). IA =

Asphaltenes + Re sin s Saturates + Aromatics

(1)

2

IA

1.6 1.2

Group A Group B Group C

0.8 0.4 0 10

15

20

25

30

35

40

Asphaltene content (%) Figure 2A. Plot of IA and asphaltene content

The plots of IA against the asphaltenes and resins content are reported in Figures 2A and 2B respectively. A linear relationship is recorded and as expected IA increases with asphaltene content and decreases with the content of resins for the different groups of asphalts. The slopes of the plots obtained from the trendline equations as shown in Table 2 can be used to evaluate the colloidal stability of the asphalts. The values recorded here for the variation of IA with both the asphaltenes and resins show that the Group B asphalts is the most stable

1.6

1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4

1.2

Group B Group C Group A

0.8

IA (Groups A, C)

IA (Group B)

(0.007 and - 0.0401).

0.4 23

25

27

29

31

33

Resins content (% ) Fig. 2B. Plot of IA and resins content

IC (Groups A & B)

0 .8

0 .9

0 .6 0 .8 G ro u p A G ro u p B G ro u p C

0 .4 0 .2

0 .7 0 .6

0 10

15

20

25

30

35

A s p h a lte n e c o n te n t (% ) F ig . 2 C . P lo t o f I C a n d a s p h a lte n e c o n te n t

40

IC (Group C)

1

1

6


Table 2. Slopes of the various indices plots Figure 2A IA /asphaltenes 0.0395 0.0070 0.0216

Groups A B C

Figure 2B IA / resins - 0.0857 - 0.0401 - 0.0435

Figure 2C IC /asphaltenes 0.0305 0.0273 0.0216

Figure 3 IA & I C 1.0305 1.0095 0.7312

Gaestel Index, IC: The Gaestel index is the dispersing capability of maltenes to asphaltenes (Yang et al., 2003; Baginska and Gawel, 2004; Gawel and Baginska, 2004). It is also evaluated using n-heptane asphaltenes. IC =

Saturates + Asphaltenes Re sin s + Aromatics

( 2)

Figure 2C describes the relationship between Gaestel Index (IC) and asphaltenes for three groups of asphalts. The colloidal stability of asphalt is evaluated by the value of IC: as its value increases, the colloidal stability decreases. The index is therefore very helpful in comparing the stability of asphalt samples. The linear plots confirm the composition-property relationship of these asphalts and the suitability of the asphalt indices in the characterization of asphalts. It is clearly seen in Figure 2C that the colloidal stability decreases with increase in asphaltene content. Comparison of the values of the slopes obtained in Table 2 reveals that the Group C 1.2

1.4

1

1.2

Group A

1

Group B

0.8

0.8 0.6

IA (Group C)

IA (Groups A, B)

1.6

Group D

0.6 0.3

0.5

0.7

0.9

0.4 1.1

IC Fig. 3. Plot of IA and IC

asphalts are the most stable while the asphalts in Group A are the least stable. Consequently, the stability of all the asphalts can be improved by reducing the content of asphaltenes. This is in contrast to the results obtained with slopes of IA plots in Figures 2A and 2B and could be easily traced to the definitions of the two indices: IA and IC. Figure 3 showing the direct correlation between the asphaltene index (IA) and the Gaestel index (IC) is a confirmation of the suitability of the various composition parameters employed in the study of asphalts. The colloidal stability of the asphalt groups is easily compared using Figure 3 in conjunction with Table 2; the trend in stability is Group C>Group B>Group A. It shows that the two indices can be conveniently used to evaluate the stability of individual asphalts. However, IA alone is not adequate enough to confirm the trend in the colloidal stability of a group of asphalts, the more suitable parameter is the stability index IC.

7


4. Application of the results Asphalt is usually specified in several grades differing in hardness or softening point and depending on the end use. Production of marketable grades is mostly achieved by blending two or more basic grades. The results obtained in this study showing the relationship between asphalt composition and properties can be very useful in the areas of asphalt blending and rejuvenating.

4.1. Asphalt blending Asphalts obtained from air-blowing, deasphalting, cracking etc are mixed together in tanks equipped with coils for air agitation or with mechanical stirrer during blending. The procedure enables the final characteristics of the asphalt to be adjusted by an appropriate selection of component asphalts. The knowledge of asphalt composition will provide some useful guide in product blending. The linearity of the plots of asphaltene content and softening point show that the hardness of the asphalt can be improved practically not only by blending hard and soft asphalt grades but by appropriately altering the contents of asphaltenes and oils (Madrid, 2000). The different plots of asphalt indices can also be used to determine the type of blends required.

4.2. Asphalt rejuvenating It is characteristic of asphalts, in paving mixtures of all types, to change in composition with time (Madrid, 2000; Yang et al., 2003; Gawel and Baginska, 2004). This process, referred to as aging, is generally a gradual one with the rate and degree of change dependent on the original chemical composition of the asphalt, its environmental conditions, and length of exposure to weathering. The asphalt colloidal stabilities deteriorate clearly with resins and asphaltenes as the more unstable constituents. Addition of suitable components such as selected fractions of petroleum oils and resins, at the appropriate time during the aging process can stop aging by reconstitution of the weathered asphalt and returning it to its original condition. This is accomplished by restoring the balance of the two major constituents in asphalt: asphaltenes and maltenes (Vallerga, 1987; Pruett, 1987; Boyer, 2000). Using the different Indices-composition plots can be found helpful in the choice of the rejuvenators needed to revitalize the aged asphalts.

CONCLUSION

An analysis of the properties and chemical composition of asphalts produced as residual products in petroleum refining and used primarily as a binder in road paving formulations has been conducted. The relationship between asphalt composition and the results of standard tests has been carefully explored and some useful correlations have been developed. The results show that the behavior of asphalt is greatly affected by the distribution of its constituent chemical groups. The following conclusions can be drawn from the present study: 8


(1) The behavior of an asphalt is greatly affected by the distribution of its constituent chemical groups. (2) The physical properties are not adequate enough to distinguish asphalts of narrow softening point ranges. The most widely varying physical parameter is ductility, whose value is significantly determined by the content of resins present in the particular asphalt. (3) A good knowledge of the chemical composition of asphalts and a detailed understanding of their colloidal structures can provide a useful guide for distinguishing between asphalts, which have been handled in different ways. (4) The Gaestel Index (IC) is found to be the more suitable parameter compared with the asphaltene index (IA) in the estimation of the colloidal stability of a group of asphalts. (5) The observed correlations between the asphalt indices and composition can be conveniently applied to modify the properties of asphalts thus validating the importance of compositional studies.

REFERENCES Andersen, S. I., Speight, J.G. (2001). Petroleum Resins: Separation, Character and Role in Petroleum. Petroleum Science and Technology, 19, 1.

ASTM Standards, (1998). American Society for Testing and Materials, Philadelphia, Annual Handbook on Petroleum Products and Lubricants. D5, D36, D113, D3279.

Baginska, K. Gawel, I. (1997). The Chemical Structure and Properties of Bitumens of Different Origin. Petroleum and Coal, 39, 50.

Baginska, K. Gawel, I. (2004). Effect of Origin and Technology on the Chemical Composition and Colloidal Stability of bitumens. Fuel Processing Technology, 85, 1453 Barre, L., Espinat, D., Rosenberg, E. and Scarsella, M. (1997). Colloidal Structure of Heavy Crudes and Asphaltenes. Oil & Gas Science and Technology, IFP, 52, 161.

Boyer, R.E. (2000). Asphalt Rejuvenators: Fact or Fable. Transportation Systems 2000 (TS2K) Workshop, San Antonio, Texas, Feb. 28 - March 3 Broome, D.C., (1973). Bitumen, in Modern Petroleum Technology, Hobson, G.D., Pohl, H. (eds). 4th ed., IP: Applied Science Publishers Ltd, Barking, Essex, England. 805. Cong, Y., Huang, W., Liao, K. Liu, G., Yang, Y. and Liu, J. (2004). Study on the Composition and Structure of Liaoshu Asphalt. Petroleum Science and Technology, 22, 455. Demirba, A. (2002). Physical and Chemical Characterizations of Asphaltenes from Different Sources. Petroleum Science and Technology, 20, 485.

Gawel, I. (1987). Structural Investigation of Asphalts Produced from Paraffinic-base Crude Oil by Different Methods. Fuel, 66, 618.

Gawel, I. (1993). Relationship Between the Production Method and the Properties and Composition of Bitumens. In Proceedings of the 5th Eurobitume Congress, Stockholm, Sweden, 175.

Gawel, I., Baginska, K. (2004). Effect of the Chemical Nature on Susceptibility of Asphalt to Aging. Petroleum Science and Technology, 22, 1261.

Gun, R.B. (1973). Chapters I –III, Petroleum Bitumens (in Russian), Moscow: Published by Khimiya.

9


Halagen, A.P., Johnson, M.P. and Pandolf, B.B. (1989).

13C

NMR Studies on Roadway Asphalts. Fuel Science &

Technology International, 7, 9, 1289.

Jennings, P.W. and Pribanic, J.A.S. (1989). A Perspective on Asphalt Chemistry Research and the Use of HP-GPC Analysis. Fuel Science & Technology International, 7, 1269. Madrid, R.C., Davison, R.R. and Glover, C.J. (2000). Compositional Evaluation of Asphalt Binder Recycling Agents. Petroleum Science and Technology, 18, 153.

Marvillet J., (1975). Influence of Asphalt Composition on its Rheological Behavior. Proc. Assoc. Asphalt Paving Technology, 44, 416.

Nelson, W.L. (1958). Petroleum Refinery Engineering, 4th ed., Mc Graw-Hill, New York. Oyekunle, L.O. (1980) Rheological Properties of Petroleum Bitumens. Proceedings of the Nigerian Society of Chem. Engrs. University of Benin, Benin, Nigeria. 56.

Oyekunle, L.O., Adesanya, A.A. (2000). Chemical Transformation of Residual Asphalts. Petroleum Science and Technology, 18, 91.

Oyekunle, L.O., Fasae, H.A., Imafidon, J.U., Soile, O.T., Bankole, A.A., Bello, K.A. (2004). Studies on Nigerian Crudes IV. Improvement of Asphaltic Properties of Residual Asphalts, Petroleum Science & Tech., 22, 1227. Oyekunle, L.O., Onwa, J.U. (2000). Catalytic Treatment of Petroleum Asphalts. Petroleum Science & Tech., 18, 81. Oyekunle, L.O., Onyehanere, L.N. (1990). Upgrading of Residual Asphalts: Kinetics of Asphaltene Generation. ANSTI 3rd Chemical Engineers Seminar, Dar-es Salaam, Tanzania. 239

Oyekunle, L.O., Onyehanere, L.N. (1991a). A Kinetic Study of the Sulphurization of Residual Asphalts. Chemical Engineering Communication. 105, 53.

Oyekunle, L.O., Onyehanere, L.N. (1991b). Studies on Nigeria Crudes. Analysis of Sulphurized Asphalts. Fuel Science and Technology International, 9, 681.

Pruett, R.L. (1987). The Role of Rejuvenators in Asphalt Pavement Maintenance. 5thAnnual Texas Airport Operators Conference, April 9.

Ruan, Y, Davison, R. R. and Glover, C.J. (2003).An Investigation of Asphalt Durability: Relationships Between Ductility and Rheological Properties for Unmodified Asphalts. Petroleum Science and Technology, 21, 231. Trejo, F., Centeno, G., Ancheyta, J. (2004). Precipitation, Fractionation and Characterization of Asphaltenes

from

Heavy and Light Crude Oils. Fuel, 83, 2169. Vallerga, B.A. (1987). Emulsified Petroleum Oils and Resins in Reconstituting Asphalts in Pavements. http://www.bridgeasphalt.com Vinogradov, G.V., Isayev, A.I., Zolotarev, V.A. and Verebskaya, E.A. (1977). Rheological Properties of Road Bitumens, Rheologica Acta, 16, 266. Yang, P., Cong, Q. and Liao, K. (2003). Application of Solubility Parameter Theory in Evaluating the Aging Resistance of Paving Asphalts Petroleum Science and Technology, 21, 1843.

10