7 Asphaltenes in Petroleum Asphalts Composition and Formation MIECZYSLAW M. BODUSZYNSKI
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Department of Petroleum and Coal Chemistry, Polish Academy of Sciences, 1-go Maja 62, 44-100 Gliwice, Poland The effect of intermolecular associations of polar compounds upon formation of asphaltenes in straight-reduced and air-blown petroleum asphalts is discussed. Two separation schemes are used to evaluate asphalt composition: a separation method based on precipitation of asphaltenes followed by maltene separation on alumina; and a separation method based on removal of acids and bases using anion and cation exchange resins, respectively, followed by separation of neutrals on alumina. Asphaltenes appear to be agglomerates of highly polar compounds formed by intermolecular associations. Field ionization mass spectrometry (FIMS) molecular weight profiles of asphaltenes were found to be similar to those of whole asphalts, from 300 to 1800 molecular weight, and averaging about 1000. Asphaltenes' compositions vary because of the commonly used, chemically nonspecific precipitation method, which is based only upon solubility parameters. The increase in asphaltenes content upon air-blowing of asphalt results from chemical reactions leading to the formation of more strongly associating but not higher molecular weight compounds.
P
etroleum asphaltenes are c o m m o n l y v i e w e d as an undesired component of crude o i l that creates serious difficulties i n u p g r a d i n g of petroleum h e a v y ends. H o w e v e r , it is not often recognized that processing of petroleum residua also includes p r o d u c t i o n of asphalts where asphaltenes are not only a v e r y desired component but the component that determines, to a great extent, the physical properties of an asphalt. Current address: Chemistry Department, University of Wyoming, Laramie, Wyoming 82071 1
0065-2393/81 /0195-0119$05.00/0 ©1981 American Chemical Society In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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CHEMISTRY OF ASPHALTENES
P e t r o l e u m asphalts can be m a n u f a c t u r e d f r o m various crudes using different processes. Asphalt, b e i n g the highest b o i l i n g f r a c t i o n of p e t r o l e u m , c a n be m a n u f a c t u r e d b y v a c u u m distillation of c r u d e o i l or b y treatment of p e t r o l e u m residue w i t h propane under controlled conditions. Asphalt c a n also b e a product of an a i r - b l o w i n g process i n w h i c h p e t r o l e u m residue is contacted w i t h air at temperatures r a n g i n g f r o m 2 0 0 ° to 2 8 0 ° C . The
generally accepted
p r i m a r y subdivision of p e t r o l e u m
asphalts,
regardless of the process used for their manufacture, is a separation into asphaltenes a n d maltenes b y p r e c i p i t a t i o n w i t h l o w - b o i l i n g saturated h y d r o
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carbons ( i ) . Recently the η-heptane precipitate has been defined as asphal tenes (2). T h e asphaltene separation is based on the solubility of asphalt components a n d has l i m i t e d a n a l y t i c a l value because it does not produce c h e m i c a l l y defined fractions. Previous w o r k (3, 4, 5) w i t h maltenes
and
asphaltenes separated f r o m R o m a s h k i n o p e t r o l e u m asphalt showed that the same classes of polar compounds, acids, a n d bases, were present i n both fractions of the asphalt. M o r e recent w o r k (6) on h i g h v a c u u m residue f r o m the same p e t r o l e u m showed that the c o m p o u n d types that precipitate as asphaltenes are present i n the a c i d a n d base fractions a n d that these c o m p o u n d types are capable of association through h y d r o g e n b o n d i n g . Intermolecular associations also have d r a m a t i c effects on apparent average molecular weight measurements
b y vapor pressure osmometry. Association results i n erron
eously h i g h molecular weights of polar components, p a r t i c u l a r l y of asphal tenes. T h e purpose of this study is to investigate the composition a n d the formation
of
asphaltenes
i n straight-reduced
and
air-blown petroleum
asphalts. T h e f o l l o w i n g topics are discussed: asphalt solubility i n n o r m a l alkanes; effect of intermolecular associations on average m o l e c u l a r weight measurements b y vapor pressure osmometry; a n d f o r m a t i o n of asphaltenes i n straight-reduced a n d a i r - b l o w n asphalts. T h e c h e m i c a l separation m e t h o d based on reactivity was used to separate asphalts into c h e m i c a l l y m e a n i n g f u l fractions consisting of c o m p o u n d classes of different c h e m i c a l structure a n d reactivity. T h e compositional changes i n asphalts u p o n a i r - b l o w i n g , deter m i n e d b y the c h e m i c a l separation m e t h o d , were c o m p a r e d w i t h those deter m i n e d b y the precipitation m e t h o d . T h e emphasis of this study is o n the role of p o l a r compounds capable of intermolecular associations i n the f o r m a t i o n of asphaltenes.
Experimental Samples. Two straight-reduced asphalts from Romashkino-type (29° A P I grav ity) crude petroleum, 100 penetration (SR-100) and 125 penetration (SR-125), and three air-blown asphalts of different degrees of oxidation, 100, 50, and 25 penetration (AB-100, AB-50, and AB-25) were investigated. The air-blown asphalts were derived from SR-125. The properties of the asphalts are given in Table I.
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
7.
BODUSZYNSKi
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Petroleum Asphalt s T a b l e I. P r o p e r t i e s o f A s p h a l t s Asphalt
Properties
SR-100
Penetration at 2 5 ° C 100 g/5 s, 0.1 m m Softening point, R & B (°C)
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AB-100
AB-50
AB-25
102
125
97
52
25
44
37
46
58
76
Weight Carbon Hydrogen Sulfur Nitrogen Oxygen" Benzene insoluble (wt %) Asphaltenes ( C ) (wt%)
SR-125
Percent in Asphalt
85.3 10.6 3.3 0.6 0.2
85.3 10.2 3.7 0.7 0.1
85.7 10.3 3.3 0.5 0.2
86.0 10.4 2.9 0.5 0.2
86.1 10.1 3.0 0.4 0.4
