United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/004 May 1987
&EPA Project Summary
Vapor/Liquid Equilibria of
Constituents from Coal
Gasification in Refrigerated
Methanol
Te Chang, R. M. Rousseau, and J. K. Ferrell
A thermodynamic framework was es-
tablished for developing a model of the
phase equilibria of mixtures of meth-
anol and the major constituents found
in gases produced from coal. Two ap-
proaches were used to model the equi-
librium behavior: (1) an equation of
state was used to describe both gas
and liquid phases, and (2) an equation
of state was used to describe the gas,
while a solution model involving activ-
ity coefficients was used to describe the
liquid. The approach chosen for each
species was based on the component
volatility. An experimental apparatus
was constructed to obtain data against
which the model predictions could be
tested. The apparatus was evaluated by
comparison of experimental P-T-x data
on mixtures of carbon dioxide (CO2)
and methanol with those from the liter-
ature. The comparison was favorable.
P-T-x-y data on mixtures of CO2,
methanol, and water, and mixtures of
COfe nitrogen, and methanol at temper-
atures in a range of -30 to 25°C and
pressures up to 54 atm (5472 kP) were
obtained. Comparisons of the calcu-
lated and measured values of bubble
point pressures and/or liquid composi-
tions of the dissolved gases were satis-
factory.
This Project Summary was devel-
opad by EPA's Air tnd Energy engineer-
ing Research Laboratory. Research Tri-
angle Park, NC, to announce key
findings of the research project that Is
nimy documented In a separata report
of the same title (see Project Report
ordering Information at back).
Introduction
The objective of this effort was to de-
velop a thermodynamic framework that
could be used to describe the equi-
librium behavior of methanol with com-
pounds found in the gas produced from
coal. The approach that was taken di-
vided the constituents into three
groups: Group I consisted of supercriti-
cal components. Group II consisted of
compounds that are normally gases at
the conditions of interest, and Group III
contained compounds that are liquids
at the conditions of interest. This divi-
sion was used to select either an equa-
tion of state or an activity coefficient for-
mulation of the equilibrium criteria.
Once this selection was made, appropri-
ate parameters in the formulation were
evaluated from binary data obtained
from the literature and experiments that
were part of this study. With the pa-
rameters evaluated in this way, it is pos-
sible to predict the behavior of systems
containing any number of the compo-
nents from Group I, II, and/or III.
Accomplishments/Results
A vapor/liquid equilibrium (VLE) ap-
paratus was developed along with a
sampling technique and analytical
method to obtain equilibrium data on
systems having high gas solubilities.
Good data were obtained from this ap-
paratus for C02 solubilities in methanol
with and without an inert gas (nitrogen),
and in mixtures of methanol and water.
However, low solubilities of nitrogen in
methanol cannot be measured accu-
rately.
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Bubble point pressure variance pro-
vided a useful objective function in the
parameter search procedure for both
equation-of-state and activity coeffi-
cient methods. This search procedure,
which includes a bubble point pressure
calculation, does not require measure-
ment of vapor-phase composition.
Mathias' polar correction factor im-
proved the calculation of vapor pres-
sures with the Soave-Redlich-Kwong
(SRK) equation of state for polar com-
pounds of methanol and water, but this
factor may not be necessary for ethyl
mercaptan and dimethyl sulfide.
Temperature-dependent binary
interaction parameters in the extended
SRK equation of state have greatly im-
proved the accuracy of correlations of
methanol-containing binary VLE sys-
tems consisting of constituents from
coal gasification. Temperature-
independent binary interaction parame-
ters are satisfactory for gas/gas mix-
tures. These parameters are applicable
in a broad range of temperatures.
Phase equilibrium calculations using
the extended SRK equation of state
were satisfactory for mixtures of meth-
anol/gas systems as long as the gas
component had a mole fraction in the
liquid less than 0.6. They did not pro-
vide satisfactory predictions for those
systems at high gas concentrations in
the liquid phase. However, absorption-
stripping processes that condition syn-
thetic gas mixtures operate at liquid cir-
culation rates that maintain the levels of
dissolved gases below those at which
the equation of state loses its accuracy.
The extended SRK equation of state
tends to predict false liquid/liquid/gas
(LLG) three-phase equilibria in metha-
nol/light-hydrocarbon and methanol/
acid-gas systems. This is believed to be
caused by the quadratic mixing rules for
interaction parameters.
The use of the optimized parameter
sets enables the equation of state to
predict the behavior of a methanol-
containing multicomponent system;
comparisons between experimental
data and bubble point pressure calcula-
tions for methanol/H2/N2, methanol/CO/
N2, and methanol/CO2/H2O mixtures
were good.
A simple gas solubility calculation
using the extended SRK equation of
state was effective in calculating mixed-
gas solubilities in a pure solvent and
pure-gas solubilities in a mixed solvent.
The activity coefficient models using
the four-suffix Margules, Wilson, and
UNIQUAC equation are excellent for bi-
nary systems that contain condensable
components (those from Groups II and
III). They are especially useful in de-
scribing the vapor/liquid equilibrium
behavior of systems containing volatile
liquids; e.g., methanol, mercaptans,
and sulfides. The determined parame-
ters in these models may be used to
describe the multi-component system
without further adjustment, but there is
no guarantee for describing the behav-
ior of a system containing a supercriti-
cal component.
The Wilson and UNIQUAC equations
are superior to the four-suffix Margules
equation for most of the systems stud-
ied. The former two equations have a
good built-in temperature-dependent
relationship and do not predict false
phase separations for methanol/acid-
gas and methanol/light-hydrocarbon
systems.
Recently developed equations of
state—which include density depend-
ent mixing rules based on the two-fluid,
local composition theories—should be
useful in future applications describing
the vapor/liquid equilibrium behavior of
systems having components covering a
broad range of densities or with polar or
hydrogen-bonding components. Coal-
produced gas that is conditioned with a
physical solvent is an example of such a
system.
T. Chang, R. Rousseau, and J. Ferrell are with North Carolina State University,
Department of Chemical Engineering, Raleigh. NC 27695,
N. Dean Smith is the EPA Project Officer (see below).
The complete report, entitled "Vapor/Liquid Equilibria of Constituents from Coal
Gasification in Refrigerated Methanol," (Order No. PB 87-165 627/AS; Cost:
$ 18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-87/004
0000329 PS
*
CHICAGO
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