Chemical Engineering February 2018 - 30

Cover Story
Distillation, Part 1:
Experimental Validation
of Column Simulations
A practical look at the need for validation, as well as conceptual considerations and a case study
Glenn Graham,
Pratik Pednekar
and Don Bunning
MATRIC
IN BRIEF
DIFFICULTIES
ASSOCIATED WITH
SIMULATIONS
CONCEPTS FOR
EXPERIMENTAL
VALIDATION
A CASE STUDY
T
his article addresses
the
need
for
experimental
validation of
computer-generated
simulations of distillation columns,
and discusses the main
concepts involved with experimental
validation. Also included
is a case study of a simulation
effort that initially yielded some
uncertainties. These uncertainties
were readily resolved with
batch laboratory testing.
Difficulties
Potential problems with physical property
models. Process simulation tools,
such as Aspen Plus, CHEMCAD, and ProSim
Plus, are indispensable for the design
and optimization of separation schemes
involving distillation columns. However, the
accuracy of their predictions depends highly
on the physical property models used in the
simulation, and it is not unusual to obtain
significantly different simulation results when
using different physical property models.
Some level of experimental testing is often
necessary to validate the simulation results.
The physical property models consist of
pure component properties, thermodynamic
equations and component-interaction parameters
used within the thermodynamic
equations. Accurate values
for
the
pure
component properties and the interaction
parameters, and the correct thermodynamic
equation are seldom known at the beginning
of a simulation effort.
Some software packages offer built-in
pure-component-property libraries containing
data for various components. In these
libraries, the parameters - such as those in
the Antoine equation - are regressed from
built-in experimental data. The user must determine
if the conditions of the simulation are
30
close enough to the conditions of the source
data to allow the data to be confidently utilized.
If not, literature data may be available at
the conditions of interest to allow regression
and estimation of new property parameters.
There
are
numerous
thermodynamic
equations that can be used to predict the
vapor-liquid equilibrium (VLE) or liquid-liquid
equilibrium (LLE) between various components.
The differences in the various thermodynamic
equations may be subtle and
the selection of the most appropriate equation
may not be obvious. The thermodynamic
equation selection is often based on
rule-of-thumb techniques. Other times, the
simulator default equation can be used, or
the equation may be obtained from similar
simulation work in the literature, which may
not have been validated. The interaction parameters
between the various components
that go into the thermodynamic equation
are often not available in the simulation software
for all of the binary pairs of chemicals.
These interaction parameters often must be
obtained from literature or estimated using
group contribution methods like UNIFAC
(universal
quasichemical
functional
group
activity coefficients) or developed from new
experimental testing.
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Chemical Engineering February 2018

Table of Contents for the Digital Edition of Chemical Engineering February 2018