Page 140 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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120 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
while for gaseous mixtures there is no significant difference
mixture and air at STP can be calculated from Eq. (3.47).
between these two methods. T1: IML 14:23 where M g is the gas molecular weight. Density of both a gas
M gas P sc
3.4.2 Gas Mixtures ρ gas 83.14T sc M gas
(3.48) SG g = = =
As discussed earlier the gases at atmospheric pressure condi- ρ air M air P sc M air
tion have much larger free space between molecules than do 83.14T sc
liquids. As a result the interaction between various like and where sc indicates the standard condition. Molecular weight
unlike molecules in a gaseous state is less than the molecular of air can be calculated from Eq. (3.48) with molecular
interactions in similar liquid mixtures. Therefore, the role of weight of its constituents obtained from Table 2.1 as M N2 =
composition on properties of gas mixtures is not as strong as 28.01, M O2 = 32.00, and M Ar = 39.94. With composition given
in the case of liquids. Of course the effect of composition on as y N2 = 0.78, y O2 = 0.21, and y Ar = 0.01, from Eq. (3.1) we
properties of gas mixtures increases as pressure increases and get M air = 28.97 g/mol. Equation (2.6) can be derived from
free space between molecules decreases. The role of compo- substituting this value for M air in Eq. (3.49). In practical cal-
sition on properties of dense gases cannot be ignored. Under culations molecular weight of air is rounded to 29. If for a gas
low-pressure conditions where most gases behave like ideal mixture, specific gravity is known its molecular weight can be
gases all gas mixtures regardless of their composition have the calculated as
same molar density at the same temperature and pressure. As
it will be discussed in Chapter 5, at the standard conditions (3.49) M g = 29SG g
(SC) of 1.01325 bar and 298 K (14.7 psia and 60 F), most gases where SG g is the gas specific gravity. It should be noted that
◦
behave like ideal gas and RT/P represents the molar volume values of specific gravity given for certain gases in Table 2.1
of a pure gas or a gas mixture. However, the absolute density are relative to density of water for a liquefied gas and are
varies from one gas to another as following:
different in definition with gas specific gravity defined from
M mix P Eq. (2.6).
(3.47) ρ mix =
83.14T
3
where ρ mix is the absolute density of gas mixture in g/cm , 3.5 PREDICTION OF THE COMPOSITION
M mix is the molecular weight of the mixture in g/mol, P is OF PETROLEUM FRACTIONS
pressure in bar, and T is the temperature in kelvin. Equation
(3.1) can be used to calculate molecular weight of a gas mix- As discussed earlier the quality and properties of a petroleum
ture, M mix . However, the mole fraction of component i in a fraction or a petroleum product depend mainly on the
gas mixture is usually shown as y i to distinguish from com- mixture composition. As experimental measurement of the
position of liquid mixtures designated by x i . From definition composition is time-consuming and costly the predictive
of mole and volume fractions in Section 1.7.15 and use of methods play an important role in determining the quality
Eq. (3.47) it can be shown that for ideal gas mixtures the of a petroleum product. In addition the pseudocomponent
mole and volume fractions are identical. Generally volume
and mole fractions are used interchangeably for all types of method to predict properties of a petroleum fraction requires
gas mixtures. Composition of gas mixtures is rarely expressed the knowledge of PNA composition. Exact prediction of all
in terms of weight fraction and this type of composition has components available in a petroleum mixture is nearly im-
very limited application for gas systems. Whenever composi- possible. In fact there are very few methods available in the
tion in a gas mixture is expressed only in percentage it should literature that are used to predict the composition. These
be considered as mol% or vol%. Gas mixtures that are mainly methods are mainly capable of predicting the amounts (in
composed of very few components, such as natural gases, it is percentages) of paraffins, naphthenes, and aromatic as the
possible to consider them as a single pseudocomponent and main hydrocarbon groups in all types of petroleum fractions.
to predict properties form specific gravity as the sole param- These methods assume that the mixture is free of olefinic hy-
eter available. This method of predicting properties of nat- drocarbons, which is true for most fractions and petroleum
ural gases is presented in Chapter 4 where characterization products as olefins are unstable compounds. In addition to the
PNA composition, elemental composition provides some vital
of reservoir fluids is discussed. The following example shows information on the quality of a petroleum fraction or crude
derivation of the relation between gas phase specific gravity oil. Quality of a fuel is directly related to the hydrogen and
and molecular weight of gas mixture.
sulfur contents. A fuel with higher hydrogen or lower carbon
content is more valuable and has higher heating value. High
Example 3.14—Specific gravity of gases is defined as the ra- sulfur content fuels and crude oils require more processing
tio of density of gas to density of dry air both measured at cost and are less valuable and desirable. Methods of predict-
the standard temperature and pressure (STP). Composition ing amounts of C, H, and S% are presented in the following
of dry air in mol% is 78% nitrogen, 21% oxygen, and 1% ar- section.
gon. Derive Eq. (2.6) for the specific gravity of a gas mixture.

Solution—Equation (2.6) gives the gas specific gravity as 3.5.1 Prediction of PNA Composition
Parameters that are capable of identifying hydrocarbon types
M g
(2.6) SG g = are called characterization parameters. The best example of
28.97

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