United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/030 Nov. 1985
<8?EPA Project Summary
Steam Stripping of Fixed-Bed
Gasification Wastewaters
F. D. Skinner and B. J. Hayes
Laboratory- and bench-scale steam
stripping tests were conducted using
gas liquor from a fixed-bed coal gasifier
at the Department of Energy's Morgan-
town Energy Technology Center. The
gas liquor was pretreated by solvent
extraction (for phenol removal) and fil-
tered prior to stripping. This report pre-
sents the results of the wastewater
stripping tests and provides engineer-
ing and environmental data for the de-
sign of steam strippers for fixed-bed
gasification wastewaters. The labora-
tory tests were performed primarily to
determine the effect of pH on contami-
nant removals. During the bench-scale
tests, samples of influent, effluent, and
overhead vapor and condensate were
analyzed for a number of species of po-
tential environmental concern (dis-
solved gases, sulfur and nitrogen spe-
cies, trace metals, organics, and other
water quality parameters). Mass trans-
fer coefficients for ammonia, carbon
dioxide, and hydrogen sulfide stripping
were calculated.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Research Laboratory, Research Tri-
angle 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 or-
dering information at back).
Introduction
The raw gas liquor resulting from
fixed-bed coal gasification processes
contains a number of contaminants,
some of which are present in relatively
high concentrations. These include tars
and oils, dissolved organic (especially
phenols), dissolved gases (e.g., NH3,
HCN, H2S, C02), and both suspended
and dissolved inorganics. Removal of
these contaminants in a multiple step
wastewater treatment system has been
included in many proposed commercial
coal gasification plant designs.
Steam stripping for the removal (and,
in some cases, recovery) of dissolved
ammonia and acid gases is a treatment
process which is common to most of
these designs. Capital and operating
costs for steam stripping systems can
account for a significant portion of the
overall cost of wastewater treatment in
coal gasification plants. There is a need,
therefore, to develop design data that
can be used to maximize the cost effec-
tiveness of this process. In addition, the
outlet streams need to be characterized
to evaluate the effects of contaminants
on downstream process performance.
This report presents the results of
laboratory- and bench-scale stripping
work, using wastewater obtained from
a fixed-bed gasifier at the Department of
Energy's Morgantown Energy Technol-
ogy Center (DOE-METC).
Objectives and Approach
The principal objectives of the
wastewater stripping study were:
• To provide data characterizing the
various stripper outlet streams for
species of environmental interest
with respect to potential impacts on
downstream process performance
and environmental effects.
• To develop mass transfer data that
could be used in the design of a
steam stripper for wastewater from
fixed-bed gasifiers following pre-
treatment by solvent extraction, fil-
tration, and pH adjustment (if
needed).
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To meet these objectives, two series
of tests were performed. First laboratory-
scale screening tests were conducted to
determine the effect of wastewater pH
on the removal of dissolved ammonia,
H2S, C02, HCN, and the residual phenol
remaining after pretreatment by solvent
extraction using methyl isobutyl ketone
(MIBK). These tests were conducted by
heating a measured quantity of pH-
adjusted wastewater to 95-100°C in a
distillation flask. Gas (air or nitrogen)
was sparged through the contents of
the flask, and the ammonia concentra-
tion and pH of the wastewater were
measured over a 270-minute period.
After each run the contents of the flask
were analyzed for pH, conductivity, total
alkalinity, ammonia, cyanide (free and
total), phenol, sulfide, sulfite, sulfate,
and thiocyanate.
The results of the laboratory tests
were used to determine the influent pH
for the second series of tests, performed
using a bench-scale steam stripping ap-
paratus. The stripper was a 0.1 m (4 in.)
diameter stainless steel column packed
with 1.2 m (4 ft) of 0.6 mm (1/4-in.) ce-
ramic Intalox saddles. Solvent-
extracted wastewater was preheated to
90°C in an electric heater and pumped
into the top of the column. Stripping
steam entered below the packing and
flowed countercurrent to the waste-
water. Concentric tube heat exchangers
were used to condense and cool the
overhead vapor and to cool the stripped
effluent. Samples were obtained peri-
odically during each run of the various
inlet and outlet streams for analysis of
the environmental and performance
parameters of interest. The principal in-
dependent variable during these tests
was the steam to wastewater influent
flow ratio.
Results and Conclusions
Laboratory-Scale Results
• Greater than 90% removals of dis-
solved ammonia and alkalinity (due
to dissolved CO2) were obtained by
stripping the extracted METC
wastewater at a pH of 8.6 (existing
after solvent extraction) and higher.
Ammonia removal increased from
about 92% to over 99.9% as the ini-
tial wastewater pH was raised from
8.6 to 11.0. A decrease in C02 re-
moval efficiency from over 99 to
96% was observed when increasing
pH from 8.6 to 11.0.
• Dissolved H20 removals decreased
from 80 to 50% as the initial pH was
increased from 8.6 to 11.0. Cyanide
removals were between 10 and
20%; most of the cyanide content of
this wastewater was present as
fixed (and therefore non-strippable)
cyanide at the pHs evaluated. It is
likely that some of the free cyanide
initially in the wastewater had been
converted to fixed cyanide and/or
thiocyanate, and the removal may
be higher for "fresh" wastewater.
• Removal of the small amount of
phenol (total) remaining in the ex-
tracted wastewater was found to be
less than 20%. No clear trends were
observed as the pH was increased.
Technical questions remain regard-
ing phenol stripping for unextracted
wastewater or wastewater having
phenol levels closer to those ex-
pected from commercial gasifier
systems.
• Because of its buffering capacity
(due to HC03/CO3 alkalinity), the
wastewater required a significant
quantity of lime to raise its pH from
8.6 to 11.0. In order to go from pH
8.6 to 9.5, 470 milliequivalents
(meq) of lime per liter was required:
to go from pH 8.5 to 11, nearly 1200
meq of lime per liter was needed.
The buffering capacity of the waste-
water is readily reduced by steam
stripping of the dissolved CO2.
• Two-stage stripping would likely be
required to remove all (or nearly all)
of the dissolved ammonia and acid
gas species (expecially H2S and
HCN) from this wastewater.
• Stripping the wastewater at a pH of
8.6 produced significant quantities
of solids that collected on the sur-
faces of the equipment and led to
plugging problems. Increasing the
pH to 9.5 or higher by lime addition
significantly reduced the plugging.
The solids are likely ammonium
salts, possibly ammonium carbon-
ate or ammonium carbamate; how-
ever, the solids were not analyzed.
Bench-Scale Tests:
Environmental
• Thiocyanate, sulfate, fluoride, and
chloride are not removed by steam
stripping. These contaminants will
be found in the stripper effluent
stream.
• Trace elements detected in stripper
outlet streams appear to be largely
system contaminants, possibly
from the column, ceramic packing,
and the lime added for pH adjust-
ment. It appears that some of the
volatile trace elements (e.g., ar-
senic, selenium, and antimony) are
stripped to some extent. This has
implications for the potential envi-
ronmental impacts of the stripper
overheads and effluent streams. For
example, it may be possible to re-
duce the amounts of some toxic
trace elements that'might otherwise
concentrate in brines produced by
downstream evaporators; however,
this potential was not investigated.
• Phenols were the major organic
species found in the wastewater. 2,
4-dimethyl phenol was largely
stripped and was found principally
in the overhead condensate. Other
phenols (e.g., phenol, cresol, and
other xylenols) were only partially
stripped and are found in both the
effluent and overhead condensate.
• Hydrocarbon analyses of the over-
head vapor were hampered by the
relatively high concentration of
residual methyl isobutyl ketone
(MIBK) from the solvent extraction
process. Toluene and xylene were
not detected in any of the samples,
and benzene was detected (at
1.1 ppmv) in only one set of the
samples collected on charcoal. The
residual solubility of MIBK in water
is significant (reportedly about 2%
by weight). Some other organics
may be present in the MIBK layer
produced as a result of condensing
the stripper overhead stream. The
solvent layer was not analyzed in
this work.
• The presence of significant quanti-
ties of solvent vapor in the stripper
overhead vapor stream has poten-
tial impacts on the downstream pro-
cesses that may be used to remove
H2S and other acid gas species from
this stream.The residual solvent
concentration after extraction/inert
gas stripping seen in this study is
probably not representative of com-
mercial operations. In a commercial
extraction system, solvent recovery
would be more efficient, not only to
reduce the possibility of problems
with downstream processes, but
also to reduce solvent makeup re-
quirements. However, more effi-
cient solvent stripping would likely
produce additional streams contain-
ing species stripped from the raffi-
nate (including ammonia and hy-
drogen sulfide).
• Carbonyl sulfide was detected in all
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Table 1. Component Removal Summary for Bench-Scale Stripping Tests"
% Removal
Run Date
9 '25
9/27
77/7
77/2
77/56
Steam'lnfluent
kg/m3
133 ± 7
298 ±31
282 ± 44
459 ± 49
297 ± 34
Influent
pH
9.03 ± 0.05
9.02 ±0.15
9.14 ± 0.03
9.77 ± 0.06
8.45 ± 0.04
NH3
83.6
94.3
91.6
95.0
31.0
±3.8
± 1.5
±4.5
± 1.7
±25.6
CO2
93.3 ±
98.3 ±
98.6 ±
99.0 ±
81.6 ±
Sulfide
0.5
0.6
0.2
0.1
5.1
30.9
17.7
65.1
69.4
-7.6
+-
-+-
•+-
+
+
77.9
38.7
9.1
16.6
51.8
Total
Cyanide
18.9 ± 4.2
23.8 ± 13.6
59.7 ±3.5
16.6 ±41.1
23.7 ±21.0
Total
Phenols
-5.2 ± 5.4
44.9 ±9.7
-72.8 ±28.6
4.2 ± 31.8
46.7 ± 12.4
3Values shown are mean ± sample standard deviation. All runs performed using 1.2 m (4 ft) of packing.
b11/5 run performed using effluent collected from previous stripping runs at similar steam/influent ratios.
overhead vapor samples at concen-
trations of about 0.04 ppmv in the
two-pass stripper run and from 1 to
5 ppmv in the single-pass runs. Car-
bon disulfide was the only other sul-
fur species detected (1 to 32 ppmv).
Bench-Scale Tests:
Performance
• Contaminant removals consistent
with the results of the laboratory-
scale tests were found for ammo-
nia, C02, and H2S. Data scatter pre-
cluded the development of
meaningful correlations for HCN
and phenol (total) removal as a
function of the steam to influent
ratio. The component removals are
summarized in Table 1.
• Contaminant removals were found
to increase with increasing steam to
wastewater ratio up to 250-300 kg
steam/m3 wastewater. Higher ratios
produced no statistically significant
improvement in contaminant re-
movals. There would appear to be
little incentive to operate at a steam
to wastewater ratio higher than
about 250 kg/m3.
• Overall volumetric mass transfer
coefficients (l
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F. D. Skinner andB. J. Hayes are with Radian Corporation, Austin, TX 78766.
William J. Rhodes is the EPA Project Officer (see below).
The complete report, entitled "Steam Stripping of Fixed-Bed Gasification
Wastewaters," (Order No. PB 85-247 450; Cost: $16.95, subject to change) 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/S7-85/030
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