n
and
n
∫
−
=
n
n
M
M
Cn
Z
dM
M
F
1
)
(
……………………………………
(4.1)
Similarly, the mole fraction of any carbon group, or pseudo component, can be
determined by incorporating the appropriate boundaries in the above integral.
The molecular weight of SCN group
n
is determined by
∫
−
=
n
n
M
M
Cn
Cn
Z
M
MdM
M
F
1
)
(
……………………………
(4.2)
Similarly the molecular weight of any carbon group, or pseudo component, can
be determined by incorporating the appropriate
boundaries in the above
integral. The most widely used distribution function
is the gamma probability
function,
(
)
(
)
( )
[
]
α
β
β
η
η
α
α
Γ
⎥
⎦
⎤
⎢
⎣
⎡
⎥
⎦
⎤
⎢
⎣
⎡
−
−
−
=
−
M
M
M
F
exp
)
(
1
……………
(4.3)
Where,
( )
α
Γ
is
the gamma function,
η
is the
minimum molecular weight
included in the distribution,
α
and
β
determine the shape of the distribution
function with the mean
and the variance equal to
(
)
η
αβ
+
and
2
αβ
respectively.
TOPIC 4: Asphaltenes
12
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ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
Hence,
(
)
α
η
β
−
=
+
Cn
M
……………………………………
..
…
(4.4)
where
+
n
C
is the fraction of the fluid described by the continuous description.
The value of
η
controls the distribution skewness, the value
( )
α
Γ
can be
readily calculated for a given
α
using routine mathematical methods
(
Abramowitz and Stegun, 1972), while t
he parameters of the distribution
function,
α
and
Γ
are determined by regressing the available data (on the
fractions) of the heavy end.
In phase behaviour calculations, the distribution function is generally reduced
to a number of
pseudo-components
u
sing an integration method, such as the
Gaussian quadrature method (
Danesh, 1988).
The critical properties of the components required in the calculations are
generally estimated using correlations based on most commonly available
data such as molecular weight, boiling point and specific gravity (
Riazi and
Daubert, 1987).