Bitumens Liquid Chromatography

Table 1 Unit molecular weight of asphaltenes

Method

Particle weight

Researcher

Viscosity

900-

4000

Fischerand Schrem (1959)

High angle X-ray

805-

-3360

Yen etal. (1961)

Light absorption

1000-

4000

Markhasin et al. (1966)

Electron microscopy

3440-

5430

Dickie et al (1969)

VPO

2950-

-8130

Dickie and Yen (1967)

VPO

2000-

4600

Dickie and Yen (1967)

VPO

1220-

-2160

Speight etal. (1985)

Cryoscopic, naphthalene

1700

Hilman and Barnett (1937)

Cryoscopic, phenathrene

2500

Boyd and Montgomery (1962)

Cryoscopic, benzene

5000

6000

Katz (1934)

solutions. The results are influenced by solvent polarity, asphaltene concentration and the temperature used. Different molecular weights of asphaltenes can be obtained by different instruments. For example, the data obtained from solution viscosity and cryo-scopic method are low, whereas those obtained from the ultracentrifuge and electron microscope are high. Vapour pressure osmometry (VPO) is considered to be the most accurate method at present when a good solvent is used to disperse the asphaltenes. Because the solvent affects the degree of asphaltene aggregation, the true molecular weights of asphaltenes are generally much lower than those measured. Some of these results are listed in Table 1.

The asphaltene content is a variable value depending on the relative amounts and characteristics of the source material and on the procedure adopted for separation. The physical and chemical characteristics are of considerable importance: the higher boiling point components of crude oil can precipitate in certain solvents, whereas the lower boiling point components (cyclic and aromatic compounds) are soluble in solvents. Most of the studies on bitumens and asphalts are concentrated on groups of compounds with particular physical and chemical properties. Depending on the solubility in certain solvents, asphalt or bitumen is generally fractionated into four main fractions: saturates, aromatics, resins and asphaltenes. The polarity of these fractions increases in the following order: saturates < aromatics < resins < asphal-tenes. A detailed scheme for bitumen fractionation is shown in Figure 1 and explained as follows:

1. Gas oil is propane-soluble and «-pentane-insoluble.

2. Resins are «-pentane-soluble but propane-insoluble.

3. Asphaltenes are toluene-soluble but «-pentane insoluble.

4. Preasphaltenes are insoluble in both «-pentane and toluene.

Asphaltenes are multipolymer systems containing a great variety of building blocks. The statistically average molecule contains a flat sheet of condensed aromatic systems that may be interconnected by sulfide, ether, aliphatic chains or naphthenic ring linkages. Gaps and holes appear as defect centres in the aromatic systems most likely involving free radicals. Heterocyclic atoms may be coordinated to transition

Figure 1 Fractionation scheme of bitumen or asphalt.

metals such as vanadium, nickel and iron. Approximately five of these sheets are associated by n-n interaction. They are stacked one above the other: the distance between the sheets ranges from 0.36 to 0.38 nm, giving an overall height for a stack of 1.6-2.0 nm; the average sheet diameter appears to be 0.85-1.5 nm. The hypothetical structure of an as-phaltene is shown in Figure 2.

Resins are considered smaller analogues of asphal-tenes with a much lower molecular weight. The resins contain aromatic compounds substituted with longer alkyl chains and a greater number of side chains attached to the rings than asphaltenes. The combination of the saturated and aromatic characteristics of resins stabilizes the colloidal nature of asphaltenes present in the oil.

Figure 2 (A) The classification of asphaltene structure. (B) The structure of asphaltene stack. (C) The structrure of one sheet of asphaltene stack.

The gas oils constitute the lowest molecular fraction of the asphalt and serve as the dispersion medium for the peptized asphaltenes. Structurally, gas oils consist mostly of naphthenic-aromatic nuclei with a greater proportion of side chains than the resins. Alkyl naphthenes predominate and straight chain al-kanes are rarely present. The naphthenic content is 15-50%, with naphthenics containing two to five nuclei per molecule.

The fractionation techniques for crude oil involve solvent fractionation and precipitation, column chromatography, ion exchange and complexation methods. No matter what method is used, the important step is the isolation of asphaltenes. Of course, all these separation processes are carried out on the heavy fractions of petroleum only; usually the volatile fractions have been removed by distillation.

The most popular method of column chromatogra-phy is the fractionation into saturates, aromatics, resins and asphaltenes (SARA). Solvent fractionations have been used in the industry since 1930, becoming widely accepted between 1950 and 1970. Recently, centrifugal thin-layer chromatography (TLC) has been used to separate crude oil successfully. This method reduces the separation time and cost, as well as preserving each fraction for further study. TLC with flame ionization detector (TLC-FID) can achieve quantitative analysis quickly and simply. The combination of solvent fractionation by silica gel and ion exchange chromatography can achieve a more detailed separation showing the complexity of petroleum. The polar fraction of the crude oil is separated by ion exchange. For chromatographic methods for bitumen or asphalt separation (SARA method, centrifugal TLC, TLC-FID and the combination of silica gel chromatography and ion exchange) are described in detail in the following sections.

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