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the mixing rules given by
a =
i
x i x j a i a j ( 1 k i,j )
(6.9)
j
b =
x i b i
(6.10)
i
Modifications of the mixing rules for some variants may be found in Aspen Plus
documentation. The introduction of the k ij parameter, one for each binary pair in a
mixture, improves the ability of the equations of state to fit experimental data. These
values, which are available from a variety of sources (e.g., Knapp et al., 1982), are
included in the Aspen Plus database and may be used within the temperature and
pressure ranges given in Aspen Plus's documentation. There is rarely a need to fit the
k ij parameter. As an example, Figure 6.1 shows the k ij parameters for the six binary
pairs in the system nitrogen - methane - propane - n -pentane. Note that the upper and
lower bounds of the working temperature ranges are given.
If the need arises to correlate a binary system not in the database, a set of PVT data
with system vapor and liquid compositions is required. Aspen Plus implements the k ij
parameter with two terms:
k ij
= a ij + b ij T
(6.11)
with the introduction of the b i,j term for temperature dependency. Some variants have
additional terms which may be selected during the setup of the regression.
Figure 6.1
Soave - Redlich - Kwong Parameters.
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