Page 118 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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98 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS

FIG. 3.16—HP made GC–MS model 5890 Series II. (courtesy of Chemical Engineering --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
Department at KU).
chains (C H) and the nature of any polynuclear aromatic compounds an integrated LC–MS unit may be suitable for
systems [7]. analysis of mixtures; however, use of LC–MS is more difficult
Another type of analysis of petroleum fractions to iden- than GC–MS because in LC–MS solvent must be removed
tify molecular groups is by spectrometric methods such as from the elute before it can be analyzed by MS. A GC–MS
mass spectrometry (MS). In general, there is a difference be- instrument from Hewlett Packard (HP) is shown in Fig. 3.16
tween spectroscopy and spectrometry methods although in Another type of separation is by SEC or GPC, which can
some references this difference is not acknowledged. Spec- be used to determine molecular weight distribution of heavy
troscopy refers to the techniques where the molecules are ex- petroleum fractions as discussed in Section 3.1.3. Fractions
cited by various sources, such as UV and IR, to return to their are separated according to their size and molecular weight
normal state. Spectrometry refers to the techniques where and the method is particularly useful to determine the amount
the molecules are actually ionized and fragmented. Evolution of asphaltenes. Asphaltenes are polar multiring aromatic
of spectroscopic methods comes after chromatography tech- compounds with molecular weight above 1000 (see Fig. 1.2).
niques, nonetheless, and in recent decades they have received It is assumed that in this molecular weight range only aro-
considerable attention. While volatile and light petroleum matics are present in a petroleum fraction [8].
products can be analyzed by gas chromatography, heavier
and nonvolatile compounds can be analyzed and identified by 3.1.5.3 PNA Analysis
spectrometric methods. One of the most important types of As determination of the exact composition of a petroleum
spectrometry techniques in analysis of petroleum fractions is fraction is nearly an impossible task, for narrow boiling range
mass spectrometry (MS). In this method, masses of molecular petroleum fractions and products a useful type of compo-
and atomic components that are ionized in the gaseous state sitional analysis is to determine the amounts of paraffins
by collision with electrons are measured. The advantage of (P), naphthenes (N), and aromatics (A). As mentioned be-
MS over other spectrometric methods is its high reproducibil- fore, most petroleum products are olefin free and PNA anal-
ity of quantitative analysis and information on molecular type ysis provides a good knowledge of molecular type of mixture
in complex mixtures. Mass spectrometry can provide the most constituents. However, some analyzers give the amount of
detailed quantitative and qualitative information about the isoparaffins and olefins as well. These analyzers are called
atomic and molecular composition of organic and inorganic PIONA analyzer, and a Chrompack Model 940 PIONA ana-
compounds. However, use of MS is limited to organic com- lyzer is shown in Fig. 3.17. An output of this type of analyzer
pounds that are stable up to 300 C (570 F). At higher tem- is similar to the GC output; however, it directly gives wt% of
◦
◦
peratures thermal decomposition may occur and the anal- n-paraffins, isoparaffins, olefins, naphthenes, and aromatics.
ysis will be biased [7]. Through MS analysis, hydrocarbons The composition is expressed in wt%, which can be converted
of similar boiling points can be identified. In the MS analy- to mole, weight, and volume fractions as will be shown later
sis, molecular weight, chemical formula of hydrocarbons, and in this chapter.
their amounts can be determined. The most powerful instru-
ment to analyze petroleum distillates is the combination of 3.1.5.4 Elemental Analysis
a GC and an MS called GC–MS instrument, which separates The main elements present in a petroleum fraction are car-
compounds both through boiling point and molecular weight. bon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur
For heavy petroleum fractions containing high-boiling-point (S). The most valuable information from elemental analysis

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