United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-86/024 Sept. 1986
£EPA Project Summary
Comparison of Pilot Plant Data
with Predictions from the U.S.
EPA SPRAYMOD Computer
Program
Wayne T. Davis, Gregory D. Reed, and George P. Partridge
This report compares SO2 removal
predictions of the EPA-developed
SPRAYMOD computer program with 12
different data sets collected at the Uni-
versity of Tennessee with a 1000 cfm
(28.3 m3/min) lime spray dryer operat-
ing without recycling of the baghouse
collected solids. The experimental re-
sults were obtained with spray dryer
operating conditions of: SO2 concentra-
tion, 633-3178 ppm; inlet gas tempera-
ture, 123-171°C (254-339°F); and ap-
proach to saturation, 8-20°C (14-36°F).
The predicted SO2 removal efficiency
depended on SO2 concentration and/or
slurry concentration as well as the
user's choice of the model reaction rate
coefficient (KR). For large KR (>105 cm3/
mole-s), both the backmix and plug
flow models were insensitive to most
operating conditions and overpredicted
the observed efficiency. With KR=0, the
plug flow model predicted lower effi-
ciency, and its sensitivity to operating
conditions more closely approached
that seen in the pilot spray dryer. De-
tailed analyses .of predicted versus
measured efficiencies indicated the
model (KR=0) to be less sensitive to
lime slurry concentration and SO2 con-
centration than the observed perform-
ance. The model underpredicted effi-
ciency at low slurry concentrations and
overpredicted efficiency at high slurry
concentrations. Recommendations for
improving the model are suggested.
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
The use of computer models for sul-
fur dioxide (S02) removal processes al-
lows the user to predict conditions
which may not have been tested in
spray dryer/fabric filter applications.
This report compares the U.S. EPA-
developed SPRAYMOD computer pro-
gram with 12 different data bases col-
lected at the University of Tennessee on
a 1000 cfm (28.3 m3/min) spray dryer
with a calcium hydroxide [Ca(OH)2]
slurry. Results are presented from a
variety of operating conditions for the
spray dryer without recycle, including:
S02 concentration:
Inlet temperature:
633-3178 ppm
123-171 °C (254-
339°F)
Approach to satura-
tion (AT): 8-20°C(14-36°F)
The SPRAYMOD computer program
is a simulation program that mathemat-
ically models the removal of S02 in a
spray dryer. The model developed by
Research Triangle Institute under con-
tract to EPA, is a simultaneous heat and
mass transfer model that predicts the
simultaneous evaporation of a droplet
and the absorption/reaction of S02 with
Ca(OH)2 in the evaporating droplet. The
S02 removal is modeled by two mecha-
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nisms corresponding to wet and dry
particle stages. A bulk volume reaction
rate coefficient (KR) for the entire
droplet/particle is used for the dry parti-
cle stage. The model includes the disso-
lution kinetics of the Ca(OH)2 particles
within the droplet and has both backmix
and plug flow reaction options. The
pilot spray dryer/baghouse test facility
used a slipstream of the University of
Tennessee's stoker fired boilers. The
pilot plant system consisted of a slaking
system, a control room, a 7 ft (213 cm)
diameter spray drying chamber with a
variable speed Stork-Bowen AA-6 spray
machine (5,000 - 21 ,OOO rpm), and a 6 in.
(15 cm) diameter centrifugal atomizer
with six nozzle inserts. A baghouse,
used to collect the product leaving the
spray dryer, was periodically cleaned to
minimize the pressure drop.
After conducting preliminary sensitiv-
ity analyses of the model and determin-
ing appropriate internal conditions for
operation of the model, data analyses
were conducted by comparing the mea-
sured efficiency versus stoichiometric
ratio, curve for each data base to the
model predicted efficiency versus stoi-
chiometric ratio curve.
Ca(OH)2 particles and diffusion of S02
and calcium in the spray dryer slurry
droplets. In addition, determining ap-
propriate values of KR for inclusion in
the model as a function of moisture con-
tent would eliminate the need for as-
suming a value of KR.
W. Davis, G. Reed, and G. Partridge are with the University of Tennessee,
Knoxville, TN 37996.
Theodore G. Brna is the EPA Project Officer (see below).
The complete report, entitled "Comparison of Pilot Plant Data with Predictions
from the USEPA SPRAYMOD Computer Program." (Order No. PB 86-216
769/AS; Cost: $11.95, 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, NC27711
Conclusions and
Recommendations
The ability of SPRAYMOD to predict
measured efficiencies was observed to
depend on the operating conditions of
S02 concentration and/or slurry concen-
tration as well as the user's choice of the
model reaction rate coefficient (KR). De-
tailed comparative analyses were con-
ducted of the predicted versus mea-
sured efficiencies for various assumed
values of KR. The model was observed
to be less sensitive to the effect of slurry
concentration and SO2 concentration
than the observed data. The model
tended to underpredict at low slurry
concentrations and overpredict at high
slurry concentrations. This conclusion
suggests that there is a lack of sensitiv-
ity in the model to handle the change in
Ca(OH)2 dissolution and/or diffusion
rate of reactants in the slurry droplet re-
sulting from changes in the slurry con-
centration.
Further development could improve
the model's ability to simulate spray
dryer performance. Specifically, devel-
opment could include modifying the
dissolution equations in the model to
account for potential effects of slurry
concentration on the dissolution of
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