Riazi-Daubert extended: MW = 20.486 * T R 1.26007 * S 4.98308 * e (1.165E-04*T R -7.78712*S +1.1582E-03*T R *S) (3)
or MW = 42.965 * T K 1.26007 * S 4.98308 * e (2.097E-04*T K -7.78712*S+2.08476E-03*T K *S) (4)
T R = MeABP (mean average boiling point) , in °R (degree Rankin )
T K = MeABP (mean average boiling point) , in K (degree Kelvin )
S = Specific gravity at 60 °F (= 15.6°C)
Equation (1) and (2) fail to properly predict properties for hydrocarbons above C 25 , with boiling point > 400°C (750°F). For heavier oil samples equation (3) and (4) are recommended. These relationships are valid in the ranges:
Molweight: 70 to 700 kg/kmol
MeABP: 20 - 560°C (90 to 1050 °F)
API: 14° to 93°
Specific gravity: 0.630 to 0.973
The average error is said to be about 7%.
Eq. (2) and (4) are used to calculate the molecular weight for a number of specific gravities and boiling points and the results, given with °C and °F, are shown in the figures below
See also Average boiling point from gravity and molecular weight and Molecular Weight - Gases and Vapors
Example 1: Molecular weight of naphtha
Calculate the molecular weight of a naphtha with specific gravtity, S = 0.763 and a MeABP of 292°F
Naphtha is in the low boiling point range, and equation (1) can be applied.
First, °F are converted to °R:
MeABP = 292°F + 460 = 752°R
Average MW = 4.5673E-05 * 752 2.1962 * 0.763 -1.0164 = 124.6 kg/kmol
Applying equation (3) gives
Average MW = 20.486 * 752 1.26007 * 0.763 4.98308 * e (1.165E-04*752 -7.78712*0.763 +1.1582E-03*752*0.763) = 124.8 kg/kmol
The results from Example 1 are given as a red X in the figures. The difference between molecular weight of naphtha from Eq (1) and (3) is 0.2 %.
Example 2: Molecular weight of a light gas oil from a distillation curve
D86 used to distille a gas oil sample showes:
10vol% recovered at 255 °C
30vol% recovered at 280 °C
50vol% recovered at 303 °C
70vol% recovered at 325 °C
90vol% recovered at 351 °C
The sample gravity is measured to be of 31.4°API
First, API gravity must be converted to specific gravity :
S, specific gravity = 141.5/(31.4°API +131.5) = 0.869
Then, VABP (volume average boiling point) and the distillation curve slope is calculated:
VABP = 0.2*(255+280+303+325+351) = 303 °C.
Slope = (351-255)/80 = 1.2 °C/% recovered.
MeABP can then be found by using correction factors :
MeABP = VABP + (-5)°C = 303-5 = 298°C
°C is then converted to K : MeABP = 298°C + 273 = 571 K
Finally, by applying equation (2) the molweight can be calculated:
Average MW = 1.6607E-04 * 571 2.1962 * 0.869 -1.0164 = 217 kg/kmol
Applying equation (4) gives
Average MW = 42.965 * 571 1.26007 * 0.869 4.98308 * e (2.097E-04*571-7.78712*0.869+2.08476E-03*571*0,869) = 232 kg/kmol
The difference between MW of gas oil from Eq (2) and (4) is 6.5 %.
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