Page 242 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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222 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
5.8 GENERALIZED CORRELATION
FOR PVT PROPERTIES OF T1: IML 17:42 where V r sat is the reduced saturation volume (V sat /V c ) and T r is
the reduced temperature. To improve this generalized corre-
LIQUIDS—RACKETT EQUATION lation a third parameter such as Z c can be used and Rackett
[61] suggested the following simple form for V r sat versus T r :
Although cubic EOS and generalized correlations discussed sat
above can be used for both liquid and vapor phases, it was (5.120) V r sat = V = Z c (1−T r ) 2/7
mentioned that their performance for the liquid phase is weak V c
especially when they are used for liquid density predictions. This equation is in fact a generalized correlation for saturated
For this reason in many cases separate correlations have been liquids and it is in dimensionless form. Later Spencer and
developed for properties of liquids. As can be seen from Fig. Danner [62] modified this equation and replaced parameter
5.1, the variation of P with V for an isotherm in the liquid Z c with another parameter called Rackett parameter shown
phase is very steep and a small change in volume of liquid, a by Z RA :
big change in pressure is needed. In addition it is seen from

this figure that when the pressure is near the saturation pres- (5.121) V sat = RT c Z n n = 1.0 + (1.0 − T r ) 2/7
sure, liquid volume is very close to saturation volume. In this P c RA
section the Rackett equation, which is widely used for pre-
diction of saturated liquid densities, is introduced for pure Values of Z RA are close to the values of Z c and they are re-
substances and defined mixtures. Then the method of predic- ported by Spencer and Adler [63]. For some selected com-
tion of liquid densities at high pressures is presented. pounds, values of Z RA are given in Table 5.12 as reported by
the API-TDB [59]. A linear relation between Z RA and ω simi-
lar to Eq. (5.112) was proposed based on the initial values of
5.8.1 Rackett Equation for Pure Component Rackett parameter [64].
Saturated Liquids
(5.122) Z RA = 0.29056 − 0.08775ω
If Eq. (5.6) is applied at the saturation pressure, P sat we have
r
It should be noted that the API-TDB [59] recommends values
(5.118) V sat = f 1 (T, P sat ) of Z RA different from those obtained from the above equation.
Since for any substance, P sat depends only on temperature Usually when the value of Z RA is not available, it may be re-
thus the above equation can be rearranged in a reduced form placed by Z c . In this case Eq. (5.121) reduces to the original
as Rackett equation (Eq. 5.120). The most accurate way of pre-
dicting Z RA is through a known value of density. If density of
(5.119) V sat = f 2 (T r ) a liquid at temperature T is known and is shown by d T , then
r
TABLE 5.12—Values of Rackett parameter for selected compounds [59].
No. Z RA No. Z RA
Paraffins Olefins
1 Methane 0.2880 32 Ethene (ethylene) 0.2813
2 Ethane 0.2819 33 Propene (propylene) 0.2783
3 Propane 0.2763 34 1-Butene 0.2735
4 n-Butane 0.2730 35 1-Pentene 0.2692
5 2-Methylpropane (isobutane) 0.2760 36 1-Hexene 0.2654
6 n-Pentane 0.2685 37 1-Heptene 0.2614
7 2-Methylbutane (isopentane) 0.2718 Di-olefin
8 2,2-Dimethylpropane (neopentane) 0.2763 38 Ethyne (acetylene) 0.2707
9 n-Hexane 0.2637
10 2-Methylpentane 0.2673 Aromatics
11 n-Heptane 0.2610 39 Benzene 0.2696
12 2-Methylhexane 0.2637 40 Methylbenzene (toluene) 0.2645
13 n-Octane 0.2569 41 Ethylbenzene 0.2619
14 2-Methylheptane 0.2581 42 1,2-Dimethylbenzene (o-xylene) 0.2626
15 2.3,4-Trimethylpentane 0.2656 43 1.3-Dimethylbenzene (m-xylene) 0.2594
16 n-Nonane 0.2555 44 1.4-Dimethylbenzene (p-xylene) 0.2590
17 n-Decane 0.2527 45 n-Propylbenzene 0.2599
18 n-Undecane 0.2500 46 Isopropylbenzene (cumene) 0.2616
19 --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`--- n-Dodecane 0.2471 47 n-Butylbenzene 0.2578
20 n-Tridecane 0.2468 48 Naphthalene 0.2611
21 n-Tetradecane 0.2270 49 Aniline 0.2607
22 n-Pentadecane 0.2420 Nonhydrocarbons
23 n-Hexadecane 0.2386 50 Ammonia 0.2466
24 n-Heptadecane 0.2343 51 Carbon dioxide 0.2729
25 n-Octadecane 0.2292 52 Hydrogen 0.3218
26 n-Nonadecane 0.2173 a 53 Hydrogen sulfide 0.2818
27 n-Eicosane 0.2281 54 Nitrogen 0.2893
Naphthenes 55 Oxygen 0.2890
28 Cyclopentane 0.2709 56 Water 0.2374
29 Methylcyclopentane 0.2712 57 Methanol 0.2334
30 Cyclohexane 0.2729 58 Ethanol 0.2502
31 Methylcyclohexane 0.2702 59 Diethylamine (DEA) 0.2568
a Calculated from Eq. (5.123) using specific gravity.

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