Kolmetz.com Thermo Paper Page 4
Introduction
There are many factors to consider when modeling or designing a distillation column
using a commercial process simulation package. One of the biggest concerns is correctly
choosing the thermodynamic package to use in simulation design work.
This paper will present some guidelines for selecting the correct thermodynamic method
when using a commercial process simulation package to model distillation equipment.
The authors will present a test case comparing operating data from a commercial
hydrocarbon column with results from three different process simulation packages. These
cases will be used to demonstrate the steps to obtain a good model using a process
simulator.
SIMULATION MODELING
Thermodynamic Properties
Thermodynamic properties are critical to the success of a simulation model. The
accuracy of a model depends on the ability of the thermodynamic package used to
estimate the physical properties of the components in the system. Sometimes poor
physical property data may prevent your simulation model from converging. (1)
Although many typical problems in the thermodynamics may arise from having missing
physical property parameters for the compounds in the system, this paper is going to
concentrate on using the simulation packages without any adjustments for these missing
parameters. The purpose of this paper is to compare results of various thermodynamic
packages for a specific system. The thermodynamic packages were chosen on the
expectation that they would be either good or not so good methods for the compounds
being used in the commercial column.
Vapor-Liquid Equilibrium
In the analysis of distillation columns and other vapor-liquid separation processes the
compositions of multicomponent vapor and liquid mixtures in equilibrium must be
estimated. The basis for vapor-liquid equilibrium calculations is the following
relationship.
)
,
,
(
)
,
,
(
i
V
i
i
L
i
y
P
T
f
x
P
T
f
=
where: T- temperature,
°F
P- pressure, psia
x
i
- liquid phase mole fraction
y
i
- vapor phase mole fraction
f
i
- fugacity of a species in a mixture
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