A simplified statistical associating fluid theory (SAFT) equation of state is developed for associating and nonassociating compounds. The new equation of state has the same number of adjustable parameters as the original SAFT equation, but is simpler. For pure compounds, this equation of state can correlate the vapor pressure and liquid density with errors equal to those of the original SAFT equation. Both self-associating and cross-associating binary mixtures were used to test the simplified SAFT equation of state. With a single binary adjustable parameter, the new equation results in better correlations of vapor-liquid equilibria than the original SAFT equation for most mixtures. However, for some mixtures, both the original and simplified SAFT equations of state fail to provide a satisfactory description. The optimized binary parameter of the simplified SAFT equation of state is found to be linearly dependent on temperature for each mixture. Consequently, the binary parameter computed from several data sets can be used to accurately predict the vapor-liquid equilibrium at temperatures other than those for which experimental data are available.
Two UNIFAC association models are developed using Wertheim's
theory of association rather
than a chemical theory. In both models the activity coefficient is
the sum of combinatorial,
residual, and association contributions. The UNIFAC
group-contribution model is used for the
combinatorial and residual terms, and two different types of
association models are considered.
The UNIFAC-AG model uses functional-group-based association, while
the UNIFAC-AM model
considers association to occur between molecules. For associating
mixtures containing acids,
alcohols, or water, both activity coefficient models provide better
predictions of binary vapor−liquid equilibria than the original UNIFAC model. Of those models,
the UNIFAC-AM model
led to the best predictions. The association term was also added
to the more recent, modified
UNIFAC model and briefly tested with vapor−liquid equilibrium data
for the acetic acid +
heptane and acetic acid + butanol mixtures. It was found that,
for vapor−liquid equilibria, the
modified UNIFAC + association model is only slightly better than the
UNIFAC-AM model.
However, the modified UNIFAC + association model has four more
adjustable parameters than
the UNIFAC-AM model. Therefore, use of the UNIFAC-AG or UNIFAC-AM
models is
recommended.
In this work P−T−x−y
vapor liquid equilibrium data were obtained for the acetic acid +
octane mixture
at two temperatures, 323.15 and 343.15 K. This mixture is very
nonideal due to the association of acetic
acid, and the data could only be correlated with sufficient accuracy by
using two recently proposed activity
coefficient models that include association in both the liquid and
vapor phases.
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