Page 70 - Characterization and Properties of Petroleum Fractions

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50 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
a similar approach as the constants of Eq. (2.38) were ob-
tained. Similarly, Eq. (2.40) can be applied to hydrocarbon N C . Later, this type of correlation was used by other investiga-
tors to correlate T c and P c for n-alkanes and alkanols [58–60].
systems in the molecular weight range of 70–280, which is Based on the above discussion, M or T b may also be used in-
approximately equivalent to the boiling range of 30–350 C stead of N C . Equation (2.41) suggests that for extremely high
◦
(∼80–650 F). However, they may be used up to C 22 or molec- molecular weight hydrocarbons (M →∞), critical tempera-
◦
ular weight of 300 (∼boiling point 370 C) with good accuracy. ture or pressure approaches a finite value (T c∞ , P c∞ ). While
◦
In obtaining the constants, Eq. (2.40) was first converted into there is no proof of the validity of this claim, the above equa-
linear form by taking logarithm from both side of the equation tion shows a good capability for correlating properties of
and then using a linear regression program in a spreadsheet n-alkanes for the molecular weight range of interest in prac-
2
the constants were determined. The value of R (index of cor- tical applications.
relation) is generally above 0.99 and in some cases near 0.999. Based on Eq. (2.41), the following generalized correlation
However, when viscosity or CH parameters are used the R 2 was used to characterize hydrocarbons within each homolo-
values are lower. For this reason use of kinematic viscosity or gous hydrocarbon group:
CH weight ratio should be used as a last option when other pa- (2.42) ln(θ ∞ − θ) = a − bM c
rameters are not available. Properties of heavy hydrocarbons
are discussed in the next section. When Eq. (2.40) is applied The reason for the use of molecular weight was its avail-
to petroleum fractions, the choice of input parameters is de- ability for heavy fractions in which boiling point data may
termined by the availability of experimental data; however, not be available due to thermal cracking. For four groups
when a choice exists the following trends determine the char- of n-alkanes, n-alkylcycopentanes, n-alkylcyclohexanes, and
acterizing power of input parameters used in Eq. (2.39) or n-alkylbenzenes, constants in Eq. (2.42) were determined us-
(2.40): The first choice for θ 1 is T b , followed by M, and then ing experimental data reported in the 1988 edition of API-TDB
ν 38(100) , while for the parameter θ 2 the first choice is SG, fol- [2] and 1986 edition of TRC [21]. The constants for T M , T b , SG,
lowed by parameters I and CH. Therefore the pair of (M, SG) d 20 , I, T br (T b /T c ), P c , d c , ω, and σ are given in Table 2.6 [31].
is preferable to (M, CH) when the choice exists. Carbon number range and absolute and average absolute de-
viations (AAD) for each property are also given in Table 2.6.
Errors are generally low and within the accuracy of the ex-
2.3.3 Prediction of Properties of Heavy
Pure Hydrocarbons perimental data. Equation (2.42) can be easily reversed to
estimate M from T b for different families if T b is chosen as the
One of the major problems in characterization of heavy characterizing parameter. Then estimated M from T b can be
petroleum fractions is the lack of sufficient methods to predict used to predict other properties within the same group (fam-
basic characteristics of heavy hydrocarbons. As mentioned in ily), as is shown later in this chapter. Similarly if N C is chosen
the previous section, Eqs. (2.38) or (2.40) can be applied to as the characterization parameter, M for each family can be
hydrocarbons up to molecular weight of about 300. Crude oils estimated from N C before using Eq. (2.42) to estimate various
and reservoir fluids contain fractions with molecular weights properties. Application and definition of surface tension are
higher than this limit. For example, products from vacuum discussed in Chapter 8 (Sec 8.6).
distillation have molecular weight above this range. For such Constants given in Table 2.6 have been obtained from the
fractions application of either Eq. (2.38) or (2.40) leads to properties of pure hydrocarbons in the carbon number ranges
some errors that will affect the overall property of the whole specified. For T M , T b , SG, d, and I, properties of compounds
crude or fluid. While similar correlations may be developed up to C 40 were available, but for the critical properties values
for higher molecular weight systems, experimental data are up to C 20 were used to obtain the numerical constants. One
limited and most data (especially for critical properties for condition imposed in obtaining the constants of Eq. (2.42)
such compounds) are predicted values. As mentioned in the for the critical properties was the criteria of internal con-
previous section, the heavy hydrocarbons are more complex sistency at atmospheric pressure. For light compounds crit-
and two parameters may not be sufficient to correlate prop- ical temperature is greater than the boiling point (T br < 1)
erties of these compounds. and the critical pressure is greater than 1 atm (P c > 1.01325
One way to characterize heavy fractions, as is discussed in bar). However, this trend changes for very heavy compounds
the next chapter, is to model the fraction as a mixture of pseu- where the critical pressure approaches 1 atm. Actual data
docompounds from various homologous hydrocarbon fami- for the critical properties of such compounds are not avail-
lies. In fact, within a single homologous hydrocarbon group, able. However, theory suggests that when P c → 1.01325 bar,
such as n-alkanes, only one characterization parameter is suf- T c → T b or T br → 1. And for infinitely large hydrocarbons
ficient to correlate the properties. This single characterization when N C →∞ (M →∞), P c → 0. Some methods developed
parameter should be one of those parameters that best char- for prediction of critical properties of hydrocarbons lead to
acterizes properties in the vertical direction such as carbon T br = 1as N C →∞[43]. This can be true only if both T c and
number (N C ), T b ,or M. As shown in Table 2.4, parameters T b approach infinity as N C →∞. The value of carbon num-
SG, I 20 , and CH weight ratio are not suitable for this purpose. ber for the compound whose P c = 1 atm is designated by N .
∗
c
Kreglewiski and Zwolinski [57] used the following relation to Equation (2.42) predicts values of T br = 1at N for different
∗
c
correlate critical temperature of n-alkanes: homologous hydrocarbon groups. Values of N for different
∗
c
(2.41) ln(θ ∞ − θ) = a − bN 2/3 hydrocarbon groups are given in Table 2.7. In practical ap-
C plications, usually values of critical properties of hydrocar-
where θ ∞ represents value of a property such as T c at N C →∞, bons and fractions up to C 45 or C 50 are needed. However,
and θ is the value of T c for the n-alkane with carbon number of accurate prediction of critical properties at N ensures that
∗
c

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