Hydrocarbon Processing - June 2021 - 40

Process Optimization
TABLE 2. Typical properties of pure components and petroleum cuts that can be blended for gasoline
Light straight-run naphtha
Blend vol %
Gravity, °API
Specific gravity, SpGr.
Aromatics, vol.%
Olefins, vol. %
RVP, psi
RON
MON
(R + M) / 2
J = R - M
MS = MON × sensitivity
O = Vol% olefins
A = vol% aromatics
Gasoline blending. Different gasolines (alkylates, reformates,
polymerate, crackate, straight runs, etc.) are blended
along with various additives to boost the performance value of
gasoline. Additives include octane enhancers, metal deactivators,
antioxidants, anti-knock agents, gum and rust inhibitors,
and detergents, and are added during and/or after blending
to provide specific properties not inherent in hydrocarbons.
However, the blend should be to specifications and the two
essential properties on which blends are critically constituted
are vapor pressure and octane number. The vapor pressure of
a mixture can be estimated by Raoult's law, but scant information
on the molecular composition of a blend does not permit
it; and laborious experimentation for evaluating molecular
composition is unwise.
A gasoline blending example includes three blend stocks
and two specifications to produce regular gasoline (87 RON)
for both summer (9 psi RVP) and winter (15 psi RVP).
Non-linear programming. Non-linear blending rules can
more closely match the physics of the problem. Depending on
the availability of olefin and aromatic contents in the blended
components, the octane number of the blend can be calculated
using the linear mixing rule method with a correction factor.1
As an example, octane blending models using the values of
coefficients a and b using the model referenced here gives Eqs.
17 and 18:
R = R + 0.03324 RJ - R × J⎦⎤ + 0.00085 O2
⎣⎡
⎣⎡
⎢⎣
⎢⎣
⎡( ) - O2
M = M + 0.04285 MJ - M × J⎦⎤ + 0.00066 O2
⎡( ) - A2
6.32 × 10-7 A2
⎢⎣
⎤
⎥⎦
⎤
⎥⎦
⎡( ) - O2
⎤
⎥⎦
(17)
−
(18)
Product qualities are estimated through correlations that
depend on the quantities and the properties of the blended
components. Mixing rules and these correlations are employed
to estimate the blend properties (e.g., specific gravity, RVP, viscosity,
flash point, pour point, cloud point, aniline point). The
octane number for gasoline is correlated with corrections based
40 JUNE 2021 | HydrocarbonProcessing.com
RVP = 7.13(
R = 0.33(
M = 0.33(
)
33
81.8
0.6634
2.2
0.9
10.8
63.7
61.2
62.5
2.5
Mid-cut reformate
67
32.8
0.8612
94.2
0.6
1
109.3
100.4
104.9
8.9
42.29
0.7956
63.84
0.699
7.13
94.25
87.46
90.86
6.79
96.39
87.62
92.01
8.77
on aromatic and olefin content. The desired property of the
blend is determined by Eq. 19:
PBlend =
i =1
n
qi Pi
i =1
n
where Pi
(19)
qi
is the value of the property of component i, and qi
can be volume fraction xvi
is the
mass, volume or molar flowrate of component i contributing to
the total amount of the finished product.
For example, qi
; therefore, the denominator
in Eq. 19 = 1. Eq. 19 assumes that the given property
is additive (or linear). Additive properties include specific gravity,
boiling point and sulfur content. However, properties such
as RVP, viscosity, flash temperature, pour point, aniline point
and cloud point are not additive. TABLE 2 lists typical properties
of pure components and petroleum cuts that can be blended for
gasoline to meet market specifications.
The API gravity, RVP and average octane number for a
33/67 blend of light straight-run gasoline and mid-cut reformate
can be determined using the Eqs. in the following section.
Since these values are vol%, they can be directly calculated as
volume averages using Eqs. 20 and 21:
A = 0.33(
O= 0.33(
o = 0.33(
°API =
) 2.2(
) 0.9(
(
) + 0.67(
) + 0.67(
) 94.2(
) 0.6(
) 0.6634) + 0.67(
141.5
SpGr
131.5 =
) 10.8(
(
) = 63.8
) = 0.7
) 0.8612) = 0.7958
141.5
0.7958
The volume average of the RVP is (Eqs. 24-27):
(RVP)1.25 = 0.33(
)1.25 + 0.67(
1
1.25
= 4.81
) 63.7(
) 61.2(
) + 0.67(
) + 0.67(
)(109.7) = 94.3
)(100.4) = 87.5
) 1( )1.25 = 7.13
131.5 = 46.3
(20)
(21)
API gravity cannot be directly calculated as a volume average,
but specific gravity can by using Eqs. 22 and 23:
(22)
(23)
(24)
(25)
(26)
(27)
Volume average octane blending
100
Non-linear octane blending
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