United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/023 Feb 1988
Project Summary
Performance and Modeling of a
Hot Potassium Carbonate Acid
Gas Removal System in
Treating Coal Gas
J. S. Staton, R. W. Rousseau, and J. K. Ferrell
Aqueous solutions of potassium
carbonate, with and without an amine
additive, were used as the acid gas
removal solvent in the Coal Gasifica-
tion/Gas Cleaning Test Facility at
North Carolina State University. The
acid gas removal system consisted of
a packed absorption column, one or
more flash tanks for intermediate
pressure reduction, and a packed
stripping column operated with a
reboiler. The removal of CO2, H2S,
COS, and other species from the crude
coal gas was studied, and data on the
distribution of these gases in regener-
ation exit streams were obtained.
Operating conditions for the selective
removal of sulfur species were also
examined.
A system model for chemical sol-
vents was developed and incorporated
into a simulation program. The model
was based on the mass transfer rate
of a key component, CO2, with the
assumption that non-key reactive com-
ponents affect the equilibrium of the
key component, but not its mass
transfer rate. The absorption and
stripping of non-key components were
assumed to be controlled by equilib-
rium between the gas and liquid phases
in these columns. An isothermal flash
model for chemical solvents was also
developed and included in the program.
The agreement between program
predictions and pilot plant data was
quite good, supporting the validity of
the model. Program simulations are
shown to provide insights into the
effects of changes in process variables
on system operation.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
As gasification processes have
developed in the U.S., it has become
apparent that improved technology in gas
cleaning is needed to meet current
environmental standards. As part of a
program to develop this technology, the
U.S. EPA sponsored a Coal Gasification/
Gas Cleaning Test Facility at North
Carolina State University. The final gas
cleanup in this facility is accomplished
by an acid gas removal system (AGRS)
in which a solvent is circulated through
a packed absorption column for removal
of acid gases from the coal gas Several
flash tanks and a packed stripping
column are used for regeneration of the
solvent. A process that removes C02 and
H2S from a gas stream is referred to as
an AGRS due to the acidic nature of these
species.
All previous work at the facility has
employed refrigerated methanol as the
acid gas removal solvent. The experimen-
tal work described in this report
employed an aqueous solution as potas-
sium carbonate, K2C03, as the AGRS
solvent. This process is referred to as the
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hot potassium carbonate process since
absorption and regeneration units are
typically operated at elevated tempera-
tures. The primary difference between
this solvent and methanol is that K2C03
removes the acid gases by a chemical
reaction, and is thus classified as a
chemical solvent, while methanol phys-
ically absorbs the acid gases, and is thus
termed a physical solvent. Another
important difference is the solubility of
organic compounds in the two solvents.
Hydrocarbons (e.g., methane, benzene,
and large cyclic compounds) are essen-
tially insoluble in the aqueous solution
of KzCOs, and will thus remain in the
product gas stream. Such compounds
are, however, highly soluble in methanol,
and they are absorbed and ultimately
distributed in all gas streams leaving the
regeneration units. These hydrocarbons
are important for their energy values, and
many are also considered environmen-
tally hazardous.
Major Objectives
The present work had two primary
objectives: (1) to evaluate the perfor-
mance of aqueous solutions of hot K2C03
in conditioning gases produced from
coal, and (2) to develop a model that could
be used in the design or simulation of
packed adiabatic absorbers and strippers
that use hot K2COa to condition gases
produced from coal. An experimental
program was conceived and executed to
satisfy the first objective directly and to
assist with the second. Several opera-
tional variables were purposefully
manipulated to examine the effect on
removal of CO2, H2S, and COS from the
gas exiting the absorber. The model
developed to describe adiabatic absorp-
tion and stripping was incorporated into
a system model that consisted of an
absorber, a flash tank, and a stripper, and
a computational procedure was devel-
oped to predict system performance.
Conclusions
As a result of the experimental and
modeling efforts, several conclusions
regarding the use of K2C03 can be drawn:
1. The model developed in this study
predicted gas flow rates and com-
positions that were in close agree-
ment with measured values.
2. Heat effects with the hot K2C03
process are less than with most
other acid gas removal solvents.
Calculations assuming isothermal
absorption, flash tank, and strip-
ping operations give accurate
results.
3. CO2 removal ranged from 38.1 to
98.4% at the conditions examined,
and was, in general, highly sensi-
tive to AGRS operating conditions
and solvent composition. The effect
of H2S and other coal gas species
on the mass transfer rate of C02
appeared to be small.
4. The absorption of COz was found
to be insensitive to solvent com-
positon when the absorption of C02
was controlled by equilibrium
between the gas and liquid phases
at the top of the absorber column.
5. The absorption of C02 was found
to be highly dependent on solvent
regeneration.
6. The regeneration of COa accom-
plished by flashing was negligible
at the conditions examined.
7. H2S removal ranged from 97.3 to
100% at the conditions examined
and was found to be insensitive to
solvent composition. The absorp-
tion of HzS was found to be con-
trolled primarily by equilibrium
between the gas and liquid phases
at the top of the absorption column.
8. COS removal ranged from 20.0 to
95.8% at the conditions examined
and appeared to be a function
primarily of absorber temperature.
9. H2S and COS may be selectively
removed with AGRS operating
conditions that include low K2CO3
concentrations, low absorber pres-
sures, and high absorber
temperatures.
10. The absorption of coal gas species
that do not participate in a reaction
in the solvent was found to be
insensitive to solvent composition
at the conditions examined. The
absorption of these species was
found to be controlled by equilib-
rium between the gas and liquid
phases at the bottom of the
absorber column.
11. An amine additive, commonly used
to increase the mass transfer rate
of C02, appeared to have no sig-
nificant effect on the fates of other
coal gas species. This additive |
appeared to increase the absorp-
tion of C02 slightly when such
absorption was controlled by equil-
ibrium between the gas and liquid
phases in the absorption column.
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J. S. Staton, R. W. Rousseau, and J. K. Ferrell are with North Carolina State
University. Raleigh. NC 27695-7905.
N. Dean Smith is the EPA Project Officer (see below).
The complete report, entitled "Performance and Modeling of a Hot Potassium
Carbonate Acid Gas Removal System in Treating Coal Gas," (Order No. PB
88-131 297/AS; Cost: $19.95) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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/S7-87/023
0000329 PS
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