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The investigation of the chemical constitution of petroleum heavy fractions such as resins and asphaltenes is hindered by their complex nature. The first known scientific application of asphaltene is by the French chemist Niépce. According to experts at the Getty Conservation Institute, the first surviving photograph by Niepce (1826-7) currently in Austin, Texas has been made exposing a thin layer of bitumen brushed on pewter plate. When exposed about 8 hours in the camera obscura, the image could be produced by dissolving unexposed bitumen in oil of lavender.
The classic definition of asphaltenes is based upon the solution properties of petroleum residuum in various so lvents. The word asphaltene was coined in France by J.B. Boussingault in 1837. Boussingault described the constituents of some bitumens (asphalts) found at that time in Eastern France and in Peru. He named the alcohol insoluble, essence of turpentine soluble solid obtained from the distillation residue "asphaltene", since it resembled the original asphalt.
Marcusson in1945 classified asphaltenes and resins as follows : (i) Neutral resins are defined as the insoluble fraction in alkalies and acids and it is completely miscible with petroleum oils, including light fractions; (ii) Asphaltenes are defined as inso luble fraction in light gasolines and petroleum ether. In contrast to resins, the asphaltenes are precipitated in the presence an excess ether; (iii) Asphaltogenic acid is defined as the soluble fraction in alkaline solutions and in such solvents as benze ne.
Recently, asphaltene is defined by chemists as the part precipitated by addition of a low-boiling paraffin solvent such as normal-pentane and benzene soluble fraction whether it is derived from carbonaceous sources such as petroleum, coal, or oil shale. S ince asphaltogenic compounds are present in petroleum in insignificant quantities, resins and asphaltenes are most important compounds of petroleum. There is a close relationship between asphaltenes, resins, and high molecular weight polycyclic hydrocarbo ns. In nature, asphaltenes are hypothesized to be formed as a result of oxidation of natural resins. On the contrary, the hydrogenation of asphaltic compound products containing neutral resins and asphaltene produces heavy hydrocarbon oils, i.e., neutra l resins and asphaltenes are hydrogenated into polycyclic aromatic or hydroaromatic hydrocarbons. They differ, however, from polycyclic aromatic hydrocarbons by presence of oxygen and sulfur in varied amounts.
On heating above 300-400 oC, asphaltenes are not melted, but decompose, forming carbon and volatile products. They react with sulfuric acid forming sulfonic acids, as might be expected on the basis of the polyaromatic structure of these components. The co lor of dissolved asphaltenes is deep red at very low concentration in benzene as 0.0003 % makes the solution distinctly yellowish. The color of crude oils and residues is due to the combined effect of neutral resins and asphaltenes. The black color of som e crude oils and residues is related to the presence of asphaltenes which are not properly peptized.
Our knowledge of the asphaltenes is very limited. Asphaltenes are not crystallized and cannot be separated into individual components or narrow fractions. Thus, the ultimate analysis is not very significant, particularly taking into consideration that the neutral resins are strongly adsorbed by asphaltenes and probably cannot be quantitatively separated from them. Not much is known of the chemical properties of asphaltenes. Asphaltenes are lyophilic with respect to aromatics, in which they form highly s cattered colloidal solutions. Specifically, asphaltenes of low molecular weight are lyophobic with respect to paraffins like pentanes and petroleum crudes. There have been considerable efforts by analytic chemists to characterize the asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous sources. A number of investigators have attempted to postulate model structures for asphaltenes, resins, and other heavy fractions based on physical and chemical methods.
Physical methods include IR, NMR, ESR, mass spectrometry, x-ray, ultracentrifugation, electron microscopy, VPO, GPC, etc. Chemical methods involve oxidation, hydrogenation, etc.
Prof. T.F. Yen in 1974 suggested asphaltene structure which is deduced from microscopic and macroscopic analysis, showing their micro- and macro-molecular bonding. From chemical methods, Yen postulated the micro-structure of asphaltene in which the aroma tic nuclei of petroleum asphaltene are of a peri-condensed nature and various structural parameters such as aromaticity, substitution extent, etc. can specify a given asphaltene structure with hypothetical empirical formula C74H87NS2O. By physical approa ches such as dissociation-association, charge transfer, excitation, defect center, etc., Yen also postulated the macrostructure of asphaltene. One of the properties of peri-condensed polynuclear aromatic systems is the attraction between the p-electron s ystems. For petroleum asphaltenes, in general, the stacking is circa five layers as found by x-ray diffraction.
NOTE: Flocs and aggregates of asphaltene which are caused due to the addition on non-polar solvents must not be confused with asphaltene micelles which may be formed by the addition of large amounts of polar (or aromatic) solvents to the crude oil.
The physical and physico-chemical properties of asphaltenes are different from those of neutral resins. The molecular weight of asphaltenes is very high ranging from approximately 1,000 to 2,000,000. The reported molecular weight of asphaltene varies cons iderably depending upon the method and conditions of measurement. A major concern in reporting molecular weights is the association / aggregation of asphaltenes which can exist at the conditions of the method of measurement. Vapor pressure osmometry (VP O) has become the prevalent method for determining asphaltene molecular weights. However, the value of the molecular weight from VPO must be weighed carefully since, in general, the measured value of the molecular weight is a function of temperature, the solvent molecular properties. Reported molecular weights from ultracentrifuge and electron microscope studies are high. To the contrary, those from solution viscometry and cryoscopic methods are low.
By careful work of electron micrography with rapid lyophilization, the size of asphaltene is found to be 20-30 by Dickie and co-workers in 1968. According to Dwiggens (1965) and Pollack and Yen (1970) in native oil and solutions, the asphaltene partic le size can be doubled. If the internal structure of a particle is known then, technically, one can predict its size by taking into account the bond lengths.
Asphaltenes and resins are two of the several, but important, heavy organics present in petroleum fluids. Their exact molecular structures are not generally known in a particular oil field and they could vary from well to well. In developing the know- how to mitigate and prevent heavy organics deposition from petroleum fluids exact knowledge of the molecular structures of all the heavy organics present in a particular oil field are not necessary. What we need to know is their various macroscopically d etectable roles in such depositions as it is discussed in a short course entitled Experimental Methods in Characterizing Petroleum Fluids and Heavy Organics - Field and Labora tory Techniques offered by the UIC/TRL.
The mathematical models developed at the UIC/TRL using polydisperse statistical thermodynamics, mechanics, kinetics and transport for phase transitions, interactions, aggregations and depositions, ( ASPHRAC), requires to identify these certain macroscopic measurable properties which could then quantify the role of the heavy organics in a petroleum arterial blockage problem.
The importance of the molecular structures of asphaltenes and resins to the practicing engineers is similar to the importance of the knowledge of a cardiologist about the structure of cholesterol present in the arteries of a patient.
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Last update: 02/17/02
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