Information Technology Reference
In-Depth Information
CHAPTER SIX
THE DATA REGRESSION SYSTEM
Aspen Plus's regression system can be used to fit various types of pure component
physical property data such as vapor pressure data, or a parameter for a new component
in the Unifac database, but its primary use is to customize thermodynamic models
dealing with vapor - liquid equilibrium (VLE) and liquid - liquid equilibrium (LLE).
Most of the equations of state and activity coefficient models commonly used are
described in detail by Poling et al. (2000), as are listings of textbooks and data sources
relating to fluid-phase equilibrium. Additional descriptions of models and variants may
be found in Aspen Plus's documentation.
When an ideal system is modeled, the equations and/or correlations that are used to
calculate the physical and thermodynamic properties of both phases are well defined
and do not require augmentation with experimental data; however, the opposite is true
when a real system is considered. In this regard there are two possibilities:
1. When both phases of a system can be represented by an equation of state, the
fundamental equation for vapor - liquid equilibrium for component
i
is given by
V
i
L
i
y
i
φ
=
x
i
φ
(6.1)
where,
y
and
x
are mole fractions of the vapor and liquid phases, respectively,
and
is a fugacity coefficient which depends on temperature, pressure, and
composition and is calculated from an equation of state.
2. When the vapor phase can be represented by an equation of state and the liquid
phase by an activity coefficient equation, the fundamental equation for vapor
equilibrium is
φ
= γ
i
x
i
f
i
y
i
φ
i
P
(6.2)
