United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/044 Dec. 1985
4>EPA Project Summary
Mathematical Modeling of
Emissions from Cooling Towers
Using Coal Gasification
Wastewater
D. A. Green, K. E. Leese, and W. J. McMichael
The report describes a computer
program that calculates atmospheric
emissions from counterflow cooling
towers when using pretreated coal
gasification wastewaters as tower
makeup water. Air stripping and biolog-
ical oxidation are both incorporated into
the mathematical model as possible
contaminant removal mechanisms. Con-
firmation of model predictions is dem-
onstrated by a 30-cm square by 91 -cm
high laboratory cooling tower using
both simulated and real pretreated coal
gasification wastewaters. Stripping of
volatile components is shown to be the
primary contaminant removal mechan-
ism. Cycles-of-concentration (i.e., the
ratio of makeup water flowrate to
blowdown flowrate) is the major tower
operating parameter influencing the
amount of volatile contaminants re-
maining in the blowdown.
This Project Summary was developed
by EPA's 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 infor-
mation at back).
Introduction
A computer program was developed
which calculates the atmospheric emis-
sions of contaminants in the makeup
wastewater as a function of counterflow
cooling tower operating conditions. The
stripping of volatile components is esti-
mated using Antoine-type apparent vapor
pressure data. Biological oxidation is
incorporated in the model as a removal
mechanism for organics based on sus-
pended growth kinetics developed for
coal gasification wastewater treatment.
These kinetic data have been adapted to
apply to fixed film oxidation by organisms
growing on the surface of the tower
packing.
A study of potential emissions from
cooling towers operated with partially
treated wastewater from coal gasification
plants was conducted to estimate the
magnitude of these emissions and to
determine the effect of cooling tower
operating conditions on these emissions.
Two laboratory scale cooling systems
were operated. A 5-cm (2 in.) diameter
packed column was built and operated
first, and variables such as liquid-to-gas
ratio, inlet liquid temperature, cycles of
concentration, and makeup composition
were investigated. The relatively low gas
throughput of this column made it feas-
ible to substitute nitrogen for air during
selected tests.
After the 5-cm diameter cooling tower
had been tested, a 30-cm (12 in.) square
by 91-cm (36 in.) high cooling tower,
representing a scaleup factor of approx-
imately 46, was operated. The testing
program for the larger cooling tower was
designed to establish the influence of
cycles of concentration and liquid-to-gas
ratio on cooling tower emissions under
conditions which would inhibit biological
oxidation of organics and under condi-
tions (packing type/area, liquid/gas ve-
locities) more representative of commer-
cial towers.
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The results of the laboratory exper-
iments indicated that cooling tower oper-
ating parameters, such as liquid-to-gas
ratio and inlet water temperature, have
little influence on atmospheric emissions
of contaminants at given cycles of con-
centration. This is consistent with the
mathematical model which links the
evaporation of water to the stripping of
contaminants by the ratio of the apparent
vapor pressure of the contaminant to the
vapor pressure of water. While both of
these quantities change with tempera-
ture, the ratio of the two quantities is
nearly constant over the range of cooling
tower temperatures for several of the
contaminants investigated in this study.
The cycles-of-concentration (i.e., the ratio
of the makeup flowrate to the blowdown
flowrate, assuming insignificant drift loss)
is the primary influence on the amount of
volatile contaminants in the blowdown.
By fixing the fraction of the makeup water
which is to be evaporated, the fraction of
some of the contaminants in the makeup
which will be evaporated is also fixed. The
capacity of the tower for transfer of water
between the liquid and gas phases is an
input to the mathematical model which
characterizes the cooling tower. This
capacity is also used to describe the
transfer of contaminants between phas-
es.
Operation of the 5-cm diameter labor-
atory cooling tower with nitrogen instead
of air did not produce a significant change
in the mass of phenol, methanol, and
acetone recovered in the cooling tower
blowdown. With both chemical and bio-
logical oxidation mechanisms eliminated,
the composition of the circulating coolant
did not change at given cycles of concen-
tration, implying that these mechanisms
are of minor importance. This was
observed both when actual and when
synthetic pretreated wastewater was
used as makeup. Similarly, operation
under conditions designed to promote
bioactivity did not produce a significant
change in the recovery of volatile organics
in the tower blowdown. While the math-
ematical model predicts that a substantial
amount of phenol could be removed
through biological oxidation, this mech-
anism would not have a major effect on
the amount of phenol recovered in the
blowdown. Phenol removed by biological
oxidation would, however, decrease the
driving force for stripping of phenol and
thus decrease air emissions. The more
volatile components (methanol, acetone,
and acetonitrile) are almost entirely re-
moved by stripping: this is to be expected
in the presence or in the absence of
bioactivity.
Based on concentrations of contami-
nants in the tower blowdown, no signif-
icant differences were observed in scaling
up from the 5-cm diameter tower to the
30-cm square tower. Model predictions
of the role of bioactivity on organic
contaminant removal are very sensitive
to the specific surface area of tower
packing material. Thus, predictions of the
potential role of biological oxidation in
contaminant removal are much higher for
the 5-cm diameter column than for the
30-cm square column because the pack-
ing used in the 5-cm diameter column
had a much greater specific surface area
than the packing used in the 30-cm
square column. Application of the cooling
tower model to a much larger experi-
mental system with lower surface area
predicts a reduced but measurable degree
of biological oxidation of phenol. More
volatile components are predicted to be
almost completely stripped (i.e., not bio-
logically oxidized) under actual cooling
tower operating conditions.
D. Green, K. Leese. and W. McMichael are with Research Triangle Institute,
Research Triangle Park, NC 27709.
N. Dean Smith is the EPA Project Officer (see below).
The complete report, entitled "Mathematical Modeling of Emissions from Cooling
Towers Using Coal Gasification Wastewater." (Order No. PB 86-118 940/AS;
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
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