Page 116 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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96 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
section various types of composition of petroleum fractions
and different methods of their measurement are presented. Feedstock
n-Heptane (or n-Pentane)
3.1.5.1 Types of Composition
Based on the nature of petroleum mixture, there are sev-
eral ways to express the composition of a petroleum mixture.
Some of the most important types of composition are given
below: Asphaltenes Deasphaltened Oil
• PONA (paraffins, olefins, naphthenes, and aromatics) Acetone
• PNA (paraffins, naphthenes, and aromatics)
• PIONA (paraffins, isoparaffins, olefins, naphthenes, and aro-
matics)
• SARA (saturates, aromatics, resins, and asphalthenes)
• Elemental analysis (C, H, S, N, O) Resins Oils
Since most petroleum fractions are free of olefins, the hydro-
carbon types can be expressed in terms of only PINA and if Dimethylformamide
paraffins and isoparaffins are combined a fraction is simply
expressed in terms of PNA composition. This type of anal-
ysis is useful for light and narrow boiling range petroleum
products such as distillates from atmospheric crude dis-
tillation units. But the SARA analysis is useful for heavy Aromatics Saturates
petroleum fractions, residues, and fossil fuels (i.e., coal liq- FIG. 3.12—An all-solvent fractionation procedure. Reprinted
uids), which have high contents of aromatics, resins, and as- from Ref. [7], p. 267, by courtesy of Marcel Dekker, Inc.
phaltenes. The elemental analysis gives information on hy-
drogen and sulfur contents as well as C/H ratio, which are removed from the product [7]. ASTM [4] provides several
indicative of the quality of petroleum products. methods based on solvent separation to determine amounts
of asphaltenes. In ASTM D 2007 test method n-pentane is used
3.1.5.2 Analytical Instruments as the solvent, while in ASTM D 4124 asphaltene is separated
Generally three methods may be used to analyze petroleum by n-heptane. Schematics of these test methods are shown in
fractions. These are Figs. 3.13 and 3.14, respectively, as given by Speight [7]. As-
phaltenes are soluble in liquids with a surface tension above
• separation by solvents 25 dyne/cm such as pyridine, carbon disulfide, carbon tetra-
• chromatography methods chloride, and benzene [7].
• spectroscopic methods
The principle of separation by chromatography technique
The method of separation by solvents is based on solubil- was described in Section 3.1.1.3. If the mobile phase is gas
ity of some compounds in a mixture in a particular solvent. the instrument is called a gas chromatograph (GC), while for
The remaining insoluble compounds may be in a solid or an-
other immiscible liquid phase. This method is particularly Feedstock
useful for heavy petroleum fractions and residues contain- n-Pentane
ing asphaltenes, resins, and saturate hydrocarbons. The de-
gree of solubility of a compound in a solvent depends on the
chemical structure of both the solute and the solvent. If the
two structures are similar there is a greater degree of solubil-
ity. For example, high-molecular-weight asphaltenes are not Asphaltenes Deasphaltened Oil
soluble in a low-molecular-weight paraffinic solvent such as
n-heptane. Therefore, if n-heptane is added to a heavy oil, as- Clay
phaltenes precipitate while the other constituents form a solu-
ble solution with the solvent. If solvent is changed to propane,
because of the greater difference between the structure of
the solvent and the high-molecular-weight asphaltenes, more
asphaltenic compounds precipitate. Similarly if acetone is Resins Oils
added to a deasphalted oil (DAO), resins precipitate while
low-molecular-weight hydrocarbons remain soluble in ace- Silica Gel
tone. In Fig. 3.12 an all-solvent fractions procedure is shown
for SARA analysis [7].
One of the disadvantages of the all-solvent separation tech-
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
nique is that in some instances a very low temperature (0 to
−10 C) is required, which causes inconvenience in laboratory Aromatics Saturates
◦
operation. Another difficulty is that in many cases large vol-
umes of solvent may be required and solvents must have suffi- FIG. 3.13—The ASTM D 2007 procedure. Reprinted from
ciently low boiling point so that the solvent can be completely Ref. [7], p. 280, by courtesy of Marcel Dekker, Inc.

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