Chemical Engineering December 2019 - 56

TABLE 4. RECOMMENDED THERMODYNAMIC METHODS FOR VARIOUS APPLICATIONS
Type of system
Recommended property method
Glycol dehydration
Refinery crude tower (low pressure)
Refinery crude tower (high pressure)
Reformers and hydrotreaters
Sour water
Cryogenic gas-processing, natural gas
Air separation
Light hydrocarbons, petrochemicals
Aromatics
Ethylene towers
Reservoir systems
Steam systems
HF Alkylation
Aromatics plus non-aromatics
Hydrocarbon systems where water solubility in hydrocarbon
is important
Non-ionic, chemical applications
Amine system
* The formation of two liquid phases
(via liquid-liquid extraction, LLE)
The liquid-activity coefficient is essentially
a correction factor that attempts
to quantify deviations from
ideal solution behavior. It is a strong
function of composition and to a
lesser extent, temperature (thus accounting
for interactions between
molecules),
but
a much weaker
function of pressure. Since the activity
coefficients of the components in
a mixture are related to the composition,
they are not independent of
each other.
There are two principal approaches
for determining activity coefficients
for liquid mixtures:
1.
Excess
methods
2. Local compositional models
By estimating activity coefficients
using semi-empirical or semi-theoretical
equations and fitting adjustable
binary interaction parameters to
experimental data, VLE behavior can
be predicted.
Excess Gibbs free-energy methods
use excess Gibbs free energy
functions, which are analogous to
residual Gibbs free-energy functions
in that they mathematically (and
thermodynamically) relate deviations
from ideal solution behavior to
functions involving the activity coefficients
of the components in the liquid
phase. Examples of activity coefficient
methods that use the excess
Gibbs
free-energy
54
methodology,
include the regular solution model,
various forms of the Margules equaGibbs
free
energy
Peng
Robinson (PR), specialized package
Barun K10 (BK10), Grayson-Streed (GS)/GraysonStreed
Erbar(GSE), SRK/PR
GS/GSE, SRK/PR
GS/SRK/PR
PR, sour PR, or special packages
SRK/PR, Benedict-Webb-Rubin Sterling (BWRS)
SRK/PR
SRK/PR, Soave Redlich Kwong Kabadi Danner
(SRKKD) for high pressure
Ideal for low pressure, SRK/PR for pressure greater
than 2 atm
Lee Kesler Plocker (LKP), BWRS
PR/SRK
Steam tables
NRTL
NRTL/ UNIQUAC
SRKKD
Wilson, NRTL, UNIQUAC
EOS-based special packages
tion, and the Van Laar equation.
Local compositional models are
semi-empirical models based on the
concept that intermolecular forces
will cause non-random arrangement
of molecules in the liquid phase.
These models incorporate parameters
related to energies of interaction
and distribution tendencies of
molecules, and this increases their
general application and reliability
while also increasing the need for
binary interaction data and experimental
data. Examples of activity
coefficient methods that use local
compositional models include NRTL
methods, Wilson, universal quasichemical
(UNIQUAC)
methods
and more.
A word of caution when applying
liquid-activity-coefficient methods is
warranted here: Due to their empirical
nature and reliance on fitted parameters,
liquid-activity-coefficient
methods should be applied with
caution, giving heed to the following
criteria:
* Are components in the system
common and well understood?
* Are operating conditions relatively
moderate (that is, is the system at
low pressure, since fitted parameter
data available in commercial
software packages are usually
based on the assumption of ideal
gas conditions in equilibrium with
the liquid phase)?
* Are experimental data available
and valid for the applicable range
of operating conditions, either for
regression to tune binary interaction
parameters, or to verify the
predictions of the model?
If these criteria cannot be satisfied
with the available experimental data,
then the list of recommendations for
selecting an appropriate thermodynamic
property method should be
consulted to ascertain whether there
might be an appropriately modified
EOS to describe the system. Many
modified EOS are available to describe
a variety of common ideal
and partially non-ideal systems. In
general, EOS are less cumbersome
to use because of their ability to describe
both vapor-phase and ideal
liquid-phase behavior (unless modified
to account for non-ideal behavior),
and because they are less reliant
on parameters that are fitted for
a specific range of conditions. In any
case, there is no substitute for appropriate
experimental data for tuning
and checking model predictions.
Group contribution methods
When experimental
data
are not
available, various methods can be
used to predict activity coefficients
based on the theory that functional
groups within a molecule of any
component contribute uniquely to
the activity coefficient for that component.
These methods are approximate
because they rely on the following
two simplifying assumptions:
* That the contribution of a functional
group within any molecule is
the same, and
* That the contribution of each functional
group is independent of the
contribution of any other group
within a molecule
Examples of
these
group contribution
methods include UNIFAC
and ASOG. In order to apply the
UNIFAC method, all components
must be condensable and electrolyte
solutions cannot be modeled.
Given the assumptions associated
with these methods, in the absence
of experimental data for both the fitting
of adjustable parameters and
the comparison with model predictions,
such methods should be used
with extreme caution. From the author's
experience, however, it can be
stated that the data obtained from
the use of UNIFAC model is not as
accurate as the standard fitted values
obtained from the various simulation
packages.
A summary showing the general
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
DECEMBER 2019
http://WWW.CHEMENGONLINE.COM
Chemical Engineering December 2019

Table of Contents for the Digital Edition of Chemical Engineering December 2019
