Eq. (2) is also applicable to hydrocarbons having molecular weight range of 70-300, but with less accuracy
T b = Average boiling point, in Kelvin
M = Average molecular weight
S = Specific gravity at 60 °F (= 15.6°C)
See also Molecular weight from average boiling point and gravity
Eq. (1) and (2) are used to calculate T b for a number of specific gravities and molecular weights and the results, in °C and °F, are given in the figures below.
Example 1: Average boiling point of naphtha
Calculate the average boiling point of a naphtha with specific gravtity, S = 0.763 and a molecular weight of 125.
Naphtha is in the low molecular weight range, and equation (1) should be applied.
ABP = 3.76587*e (3.7741E-03*125+2.98404*0.763-4.25288E-03*125*0.763) *125 0.40167 *0.763 -1.58262 = 418 K
Converted to °C and °F : 418 K = 145°C = 293 °F
Applying equation (2) gives
ABP = 9.3369*e (1.6514E-04*125+1.4103*0.763-7.5152E-04*125*0.763) *125 0.5369 *0.763 -0.7276 = 423 K = 150°C = 302°F
NB! For low molecular weights (< 300) this ABP is consider to be less accurate than calculated from Eq. (1).
Example 2: Average boiling point of a vacuum gas oil.
Calculate the average boiling point of a vacuum gas oil of gravity of 16.7°API and an average molecular weight of 391.
The vacuum gas oil has a molecular weight > 300, and Eq. (2) must be applied.
But first, we have to convert the API gravity to specific gravity :
S, specific gravity = 141.5/(16.7°API +131.5) = 0.955
ABP = 9.3369*e (1.6514E-04*391+1.4103*0.955-7.5152E-04*391*0.955) *391 0.5369 *0.955 -0.7276 = 737 K = 464°C = 867°F
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