United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/017 Aug. 1987
Project Summary
Regeneration of Refrigerated
Methanol in Conditioning Gases
from Coal
J. K. Ferrell, J. S. Staton, R. W. Rousseau, and K. J. Games
The report gives results of an exami-
nation of various methods of solvent
regeneration in an acid gas removal
system (AGRS) coupled to a fluidized-
bed coal gasifier. (Earlier research on
acid gas removal using refrigerated
methanol had shown that, when a high
purity gas is desired as a product gas.
the most critical step in the process is
solvent regeneration.) The composition
of the absorber exit gas stream (the
sweet gas) obtained from each system
configuration studied was used as a
basis for comparing the various
schemes.
For the systems studied, the ability of
the acid gas removal system to produce
a conditioned gas with low levels of
H2S and CO2 was found to be governed
primarily by the purity of the solvent
entering the absorber, and thus by
regeneration conditions. These results
are believed to be general for refrig-
erated methanol systems and, together
with mathematical models developed
as a part of the project, can provide a
basis for selecting an optimum con-
figuration for an acid gas removal sys-
tem. The fate of the various trace
compounds produced in the gasifier was
determined, and a design method for
predicting the exit stream in which these
compounds leave the AGRS was pro-
posed.
This Project Summary was developed
by EPA'a Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
Research on acid gas removal using
refrigerated methanol carried out at North
Carolina State University has shown that,
when a high purity gas is desired as a
product gas, the most critical step in the
process is solvent regeneration. This
research examined various methods of
solvent regeneration in an acid gas re-
moval system (AGRS) coupled to a
fluidized-bed coal gasifier. The composi-
tion of the absorber exit gas stream (the
sweet gas) obtained from each system
configuration studied was used as a basis
for comparing the various schemes.
Under conditions used in these studies,
it was found that the absorption of acid
gases in the AGRS was governed primarily
by how well the solvent entering the
absorber was stripped of dissolved gases.
It is likely that solvent regeneration plays
such a role in any acid gas removal
system, especially one using a physical
solvent and where the absorber packing
height or number of stages is sufficient
to cause the exit gas from the absorber to
be near equilibrium with the entering
solvent.
Objectives
As a consequence of this observation,
a major objective of the research reported
here was to evaluate various schemes
for solvent regeneration. In particular,
the AGRS was operated with and without
multiple flash tanks between the absorber
and stripper, and with stripping by
nitrogen or heat supplied in a reboiler at
the base of the stripper. The study was
conducted using Texas lignite as the
gasifier feed, and refrigerated methyl
alcohol in the acid gas removal system.
-------
The data were used to evaluate the effect
of system configuration on the quality of
the conditioned gas leaving the absorber
of the AGRS.
Also examined was the effect of adding
water to the methyl alcohol solvent.
Operation with small amounts of water
in the solvent is important since it can
accumulate in the solvent in an industrial
operation. The solubilities of the acid
gases in water are much less than in
methyl alcohol, and thus the presence of
water could have a major effect on AGRS
operation.
Results
Although a conditioned gas containing
less than 1.5 mole % C02 and less than 5
ppm carbonyl sulfide (COS), was produced
at all operating conditions and with all
regeneration configurations used in this
study", the concentration of H2S in the
product gas stream varied significantly.
Specifically, a product gas essentially free
of H2S was obtained only with the use of
high temperature regeneration (use of
the reboiler methanol in the stripper), or
a very high flow of N2 stripping gas. The
major results of the study are summarized
in Table 1.
The experimental results of this study
provided information on the relationship
between absorber operation and regen-
eration. For all of the work reported here,
the height of packing in the absorber was
sufficient to bring the absorber exit gas
nearly to equilibrium with the entering
solvent. Absorption was thus controlled
by equilibrium between the gas and liquid
phases in the column rather than by
mass transfer. The experimental results
may be generalized by an understanding
of the operation of an absorber under
these conditions. Most industrial absorp-
tion and stripping columns operate in
this mode as a result of the practice of
oversizing packing heights in design.
Graphical methods are a convenient
tool for understanding the mass balance
relationships and equilibrium constraints
in absorption and stripping operations.
Consider the equilibrium and operating
lines given in Figure 1 for absorption
with a physical solvent. The operating
line represents conditions as they actually
exist in the column, while the equilibrium
line represents conditions at equilibrium
between the solvent and the gas. The
slope of the operating line is directly
related to the liquid-to-gas ratio, while
the slope of the equilibrium line is the
ratio of the Henry's Law constant to the
column pressure. The separation between
the two lines represents the driving force
Table 1. Acid Gas Removal Summary
Composition, mole %
Run No.
AMIL-
1
2
3
7
11
14
16
19
21
Regeneration Scheme
1 flash tank and strp N2
flow 0.03 scm/min strp liq
temp 20. 7°C solvent rate
4.8 L/min (standard
conditions)
1 flash tank and strp N2
flow 0.03 scm/min strp liq
temp 26.6°C solvent rate
4.8 L/min
1 flash tank and strp N2
flow 0.06 scm/min strp liq
temp31.9°C solvent rate
4.8 L/min
1 flash tank and strp N2
flow 0.06 scm/min strp liq
temp 31.9°C solvent rate
3.3 L/min
2 flash tanks and strp N2
flow 0.03 scm/min flash 2
liq temp 27.5°C strp liq
temp23.7°C
1 flash tank and strp
reboiler steam 7. 7 kg/hr
strp temp 70°C
3 flash tanks, no strp flash
2 liq temp 24.9°C flash 3
liq temp 17.8°C
3 flash tanks, no strp flash
2 liq temp -10.9°C flash 3
liq temp -1 .60°C
same as AMIL-2. 10%
water. 90% methanol
CO2
27.73
25.47
27.51
28.67
26.46
27.38
27.10
25.56
25.4
%JUUI Uaa
H2S
0.243
0.258
0.304
0.214
0.152
0.258
0.247
0.179
0.234
COS
0.0058
0.0078
0.0039
0.0029
0.0038
0.0034
0.0041
0.0025
0.0051
o weei uss
^n w c
I* L/2 ti2
-------
for mass transfer. Thus, when the oper-
ating line and the equilibrium line inter-
sect, the two phases are in equilibrium
and no mass transfer can take place. This
condition is commonly referred to as a
pinched condition in the column. If a
pinch occurs, additional packing height
will not result in improved separation of
this species. For the experimental condi-
tions of this study, the absorption of all
major coal gas species was controlled by
pinched conditions.
If the slope of the equilibrium line is
greater than that of the operating line
and an intersection occurs, it is clear
from Figure 1 that the intersection will be
at the top of the diagram, which repre-
sents the bottom of the column. If the
slope of the operating line is greater, the
intersection will occur at the bottom of
the diagram (the top of the column). Since
the slope of the equilibrium line decreases
with increasing solubility, the absorption
of species that have a high solubility in
methyl alcohol will be controlled by equili-
brium at the top of the absorber if a pinch
occurs. Conversely, the absorption of
species that are slightly soluble in methyl
alcohol will be controlled by equilibrium
at the bottom of the absorber.
When absorption is controlled by a
pinch at the bottom of the column, the
concentration of solute in the absorber
exit gas is highly sensitive to its con-
centration in the absorber feed gas. If,
however, absorption is controlled by a
pinch at the top of the column, the con-
centration of solute in the absorber exit
gas is governed primarily by its con-
centration in the liquid stream entering
the absorber, or by the effectiveness of
the solvent regeneration.
Table 2 gives the relative solubilities of
the major coal gas species in methyl
alcohol. As seen, the acid gas species
CO2, H2S, and COS are highly soluble
and their absorption is very sensitive to
regeneration. Other major components
are, however, only slightly soluble and
are insensitive to regeneration.
Simulation
The regeneration of methyl alcohol may
be simulated by flash tank and stripping
algorithms developed as part of this pro-
ject. An adiabatic flash tank algorithm for
methyl alcohol and the major coal gas
species was developed that gave predic-
tions that were in good agreement with
pilot plant data. In addition, an algorithm
for the multicomponent stripping of major
coal gas species from methyl alcohol was
developed that also agreed well with pilot
plant data. Trace components may be
included in these algorithms if equilibrium
and physical property data are available
or may be predicted. These models may
be used independently to predict the
composition of methyl alcohol exiting a
regeneration operation, or in conjunction
with the absorpotion model to simulate a
complete acid gas removal system. Al-
though, these algorithms were developed
for the methyl alcohol solvent, they may
be modifed to simulate absorption and
regeneration of coal gas species in other
physical solvents as well.
Table 2. Relative Solubilities in
Methanol at-40° C
Gas
H,S
COS
CO2
CH4
CO
N2
H2
Solubility of
Gas/ Solubility of H2
2540
1555
430
12
5
2
1
at conditions to give the maximum
absorption possible.
For the compounds that did not ac-
cumulate in the methanol solvent and
therefore have good mass balance clo-
sures, it should be possible to predict the
concentration of the compound in the
gas phase resulting from a flash operation.
A simple flash model, using Henry's Law
constant from the literature, was used to
do this. The mass balance closures for
the aliphatic hydrocarbons were usually
good, and an example of the prediction
for ethylene and ethane is shown in
Figure 2. The agreement between pre-
dicted and observed is quite good for
ethylene and ethane, and is reasonably
good for the other hydrocarbons that were
sufficiently volatile so that they did not
accumulate in the solvent.
Table 3 shows the trace compounds
produced by the gasif ier and measured in
this study, together with the lower detec-
tion limits for each species and the ranges
of concentrations found in the AGRS
feed gas. The relatively large variations in
the concentrations of most trace compo-
nents in the sour gas can probably be
attributed to changes in reactor conditions
and coal composition. Ten runs were
selected for experiments to determine
the effect of solvent regeneration condi-
tions on the fate of trace components.
For these runs the absorber was operated
Table 3. Trace Components In the AGRS Feed Gas
Trace Component
Thiophene
CHjSH
CMSH
CS2
CM
CM
CM
CM
CM
CjHfo
Benzene
Toluene
Ethylbemene
P-Xylene
M-Xylene
0-Xylene
Lower
Detection
Limit, ppm
3
2
2
1
20
20
20
20
20
20
8
8
3
2
2
2
Flange of Concentration
in A GftS Feed Gas
ppm
11-26
19-45
ND-2
ND-5
1278 - 3729
1809 - 5581
331 - 1290
125 - 595
125 - 626
44 - 186
165 - 1040
24 - 242
ND-21
NO- 10
ND-21
NO -4
ND - None Detected
-------
0)
8
o
------- |