United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S7-84-084 Sept. 1984
Project Summary
Development of a New Gravity
Sedimentation Process for
Dewatering Flue Gas Cleaning
Wastes
A. R. Tarrer
This report gives results of a project to
develop and test a novel system for
dewatering flue gas cleaning (FGC)
wastes at the pilot plant level. In this
new system, the clarification and
thickening functions are conducted in
separate, but interconnected, pieces of
equipment. The new system consists of
a lamella clarifier and a conventional
thickener that is smaller in diameter,
but deeper (than the thickener/clarifier
typically used to dewater FGC wastes),
connected by a recycle stream between
the two units to obtain a high degree of
flexibility and control of operating
conditions. Preliminary economic eval-
uation of this system indicates potential
savings of 10 perent of the total capital
costs and 6 percent of annual operating
costs for the FGC waste management/
disposal system.
In pilot testing of this system, a
completely new concept in thickener
operation, known as "bang-bang"
operation, evolved, in which the thick-
ener underflow rate is set as low as
possible without plugging the under-
flow lines. Periodically, the underflow
rate is increased briefly to remove ad-
ditional solids from the system at the
concentration established by the (pre-
viously set) low underflow rate. This
mode of operation appears to make it
possible to maintain the maximum
solids concentration in the underflow.
This Project Summary was developed
by EPA's Industrial Environmental
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
information at back).
Introduction
In the mid-1970's, EPA's Industrial
Environmental Research Laboratory in
Research Triangle Park, NC, initiated
several projects to solve problems
associated with slowly settling, difficult-
to-dewater flue gas cleaning (FGC) wastes.
These problems were primarily attributed
to small, thin "platelet" crystals of
calcium sulfite (CaS03-1/2H20) in these
wastes. When present in significant
quantities (more than a few percent), it
was difficult to achieve, greater than
about 30 percent solids in a gravity
settler, or about 50 percent solids with
vacuum filtration. The approaches used
to solve these problems included at-
tempting to make larger calcium sulfite
crystals, oxidation of the calcium sulfite
to calcium sulfate (CaSO4-2H2O), and
improving dewatering equipment design.
The project reported here involves
equipment design improvements, and
covers the period May 1, 1976, through
June 15, 1983.
Phase 1 - A preliminary evaluation
determined the direction of the pro-
gram; i.e., limited sedimentation and.
filtration were studied to disclose
which offered the better potential for
improving dewatering efficiencies.
Phase 2 - A detailed process synthesis
and development effort was conducted;
i.e., fundamental concepts in gravity
sedimentation and process dynamics
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were applied to develop a new system
for dewatering FGC wastes.
Phase 3 - A feasibility analysis was
done to estimate the maximum savings
possible with the use of the new
dewatering system. Also, equipment
designs, layouts, etc. were made for a
larger scale pilot evaluation of the new
dewatering system.
Phase 4 - A pilot-scale system was
constructed, transported, and installed
at the EPA/TVA pilot FGC facility at the
Shawnee Steam Plant near Paducah,
Kentucky. Shakedown studies revealed
needs for equipment improvement.
Equipment was modified in coopera-
tion with TVA, and final pilot-plant
tests were performed. Objectives of
these tests were to determine if the
clarification function of the unit could
be decoupled from its thickening
function, to determine if the unit was
capable of concentrating unoxidized
FGC wastes to a high solids content, to
identify design limitations, and to make
operational recommendations.
Phase 1 Results
In Phase 1, settling data were collected,
showing that the FGC wastes tested,
containing a significant percentage of
calcium sulfite (CaSO3-1 /2H2O), exhibit a
settling behavior similar to that of weakly
flocculating materials. Assuming that
FGC wastes do form weak floes while
settling, the floes appear to have a low
yield strength, and the gravitational force
(the weight) exerted by overlying layers of
solids in the settling medium compresses
underlying layers. This means that the
operating depth of the blanket of settling
waste in the thickener is an important
process parameter: the deeper the blan-
ket of settling wastes at the bottom of
the thickener, the higher the solids
concentration in the underflow; or, the
longer the residence time of solids in the
compression zone of the thickener, the
greater the dewatering. However, in
conventional FGC waste thickeners, a
deep blanket of wastes usually resulted in
a turbid overflow (insufficient clarifica-
tion). These findings showed promise that
the application of past studies in our
laboratories could lead to a less expen-
sive gravity sedimentation system for
dewatering FGC wastes.
Limited filtration studies involved a
0.09 m2 (1 ft2) plate and frame filter press
(see Figure 1). The filtration data collected
did not show much promise that contin-
ued study of the filtration operation itself
would yield much of an improvement over
conventional filtration. Therefore, an
Water
Inlet Pressure
to Filter Press
Air
Filter Press
Filtrate
Out
Plate
Frame
Figure 1. Filter press apparatus.
alternate strategy was chosen: to improve
the dewatering efficiency of the gravity
sedimentation system, thereby increas-
ing the solids concentration of the
influent stream to the filters and lowering
the filter dewatering load.
Phase 2 Results
In Phase 2, the major thrust of the
bench-scale part of this research effort
project was conducted. A new, more effi-
cient gravity sedimentation system for
dewatering FGC wastes was conceived,
designed, and tested on a laboratory pilot
scale. The system consisted of two units:
a laminar clarifier (more specifically, a
tube settler) and a smaller (in diameter)
but deeper, conventional thickener. The
units were connected uniquely,
decouple the clarification and the thi
ening functions of the system as much
possible. A recycle stream between t
two units was used to obtain a grea
degree of flexibility and control
operating conditions. The importa
operating parameters for the syslet
were investigated by a series of batch ai
continuous settling experiments. Tl
parameters included: the location of tl
level of the solid/liquid interface in tl
clarifier, the angle of inclination of tl
clarifier, and the flow rates of the fe<
and recycle streams.
The dewatering efficiency of the ne
system was evaluated on a labors^
pilot scale (see Figure 2). The pilot trnl
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ener was 15.24 cm (6 in.) in diameter and
about 4.6 m (15 ft) long. Tube settlers
(clarifiers) were used; these were 4.445
cm (1.75 in.) in inside diameter, 1.98 m
(6.5 ft) long, and inclined at an angle of
30°. The pilot tests showed the new sys-
tem to be effective in obtaining high
underflow concentrations (30 to 60
percent) and a clear overflow at high
throughput rates while requiring a
smaller settling than typical conventional
thickeners used in this application.
Phase 3 Results
In Phase 3, efforts were directed
toward an eventual scale-up of the
system. Sizing studies, based on a flux
plot developed to help estimate the
settling area required for a desired level
of final concentration, showed that the
new dewatering system would require
only about 59 percent of the area required
by conventional thickeners to achievethe
same amount of separation and concen-
tration. Alternately, for about the same
settling area, the degree of concentration
achieved is about 60 percent greater.
Preliminary economic studies were
also performed by TVA at the request of
EPA; these studies showed reductions in
capital costs of about 10 percent, and
reductions in annual operating costs of
about 6 percent, for an entire FGC waste
management/disposal system (not just
the dewatering portion of the system).
To evaluate the feasibility of the system
so that it could eventually be used on a
Feed
Feed Tank
Underflow
receiving
tank
Underflow
pump
Sample
point Recycle Pump
Splitter
Column Settler
Mixer
Clarifier
Thickener
Figure 2. Laboratory pilot scale clarifier/ thickener system.
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commercial scale, equipment design,
layout, etc. were made for a large pilot
scale version evaluation, and the pilot
dewatering system was fabricated. In
addition, studies were performed on the
effects of shear on the settling behavior of
the sludge. It was hypothesized and ex-
perimentally established that mixing of
the FGC wastes causes a more rapid set-
tling and a greater final concentration.
However, it was also established that vig-
orous mixing could fracture the solid
waste crystals, resulting in many fine
particles which would settle slowly.
Based on these results, a positive dis-
placement pump was selected for the re-
cycle stream from the clarifier to the
thickener.
Phase 4 Results
In Phase 4, the pilot dewatering system
was fabricated, shipped to TVA's Shawnee
Steam Plant near Paducah, Kentucky,
and installed at the EPA/TVA pilot FGC
facility for shakedown testing and final
pilot plant evaluation testing. The FGC
waste introduced to the system contained
a significant amount of calcium sulfite.
After the shakedown test, several
modifications were made to the pilot unit.
The main modification was adding more
settling plates in the clarifier. Other
modifications had to do with operational
considerations such as safety and ease of
operation. Figure 3 is a schematic of the
system.
The initial goal of the pilot tests was to
establish a baseline for comparison. At
baseline coditions, the underflow con-
centration was about 45 percent solids;
the underflow rate was set at about 10.6
to 11.4 l/min (2.8 to 3.0 gpm). The
average feed rate was about 49.2 to 53
l/min (13 to 14 gpm), with a solids
concentration of about 15 percent solids.
Prior to start-up, the thickener was
filled with solids from the existing
dewatering unit; the clarifier was not
prefilled with solids. As a result, during
the initial period of operation, solids
accumulated in the clarifier.
As the clarifier began to fill with solids,
it became necessary to increase the
thickener underflow rate periodically to
maintain a clear clarifier overflow. It was
discovered that the feed rate to the
system was being periodically increased
and corresponding increase in thickener
underflow rate was necessary to keep the
velocity in the clarifier below about 0.037
m/min (0.12ft/min). By adding a splitter
in the feedstream, the feed rate was held
fairly constant at about 49.2 to 53 l/min
(13 to 14 gpm), and the thickener
underflow rate could then be set as low as
possible without plugging the underflow
lines.
Each time the thickener underflow rate
was decreased, the underflow concentra-
tion increased significantly. When the
thickener underflow rate was decreased
from the baseline value of —10.6 l/min
(—2.8 gpm) to less than 3.8 l/min (1 gpm),
the underflow concentration increased to
as high as 56 percent solids.
No significant changes in the lamella
clarifier concentration was observed when
its underflow rate was decreased from
—76 l/min (—20 gpm) to as low as 5.7
l/min (1.5 gpm). However, a significant
response in separation efficiency (percent-
age of water in feedstream that left in
clear overflow) was observed; it increased
significantly each time the clarifier
underflow rate was decreased.
While operating at a very low thickener
underflow rate (—5.3 l/min or —1.4 gpm)
and a high underflow solids content (—53
percent), the underflow rate was increased
significantly for about 2 hours. During
this time, about 1.2 m (4 ft) of concentra-
ted sludge blanket was dumped. The
underflow rate was then reset at its
original low setting. This period of rapid
Slurry
Discharge
dumping, followed by resetting the
underflow rate to its original low level,
had little effect on the underflow solids
content, because of the relatively constant
solids concentration in the large compres-
sion zone in the thickener.
On the basis of this observation and the
observed strong inverse dependence of
underflow concentration on steady-state
(continuous) underflow rate, the concept
of "bang-bang" thickener operation was
proposed, in which the thickener under-
flow rate is set as low as possible without
the underflow lines plugging. Periodically,
the underflow rate is stepped up briefly to
maintain an overall solids flowrate in the
thickener. Using this mode of operation it
should be possible to maintain a maxi-
mum underflow concentration.
The dewatering system was observed
to be operationally versatile. Any possi-
ble plugging in the clarifier due to its
overdesign, for example, should be easily
avoided by simply increasing the clarifier
underflow rate. This could be done
without having to lower the overall
system underflow concentration because
further concentration would occur in the
thickener.
Flow
Meter
Stream No.
Description
Rate, Ib/hr
Rate, gpm
% Solids
Spec. Gravity
1
Slurry
Discharge
2186
4.00
15.0
1.093
2
Thickener
Overflow
3923
7.18
14.9
1.092
3
Clarifier
Underflow
2332
4.00
25.0
1.165
4
Thickener
Underflow
595
0.82
55.0
1.451
5
Clarifier
Overflow
1591
2.18
—
1.000
Figure 3. Large pilot dewatering unit.
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Conclusions
The FGC waste dewatering system
developed in this work in effect decouples
the clarification function from the
thickening function. Using this system, it
is possible to concentrate FGC wastes
containing significant quantities (greater
than a few percent) of calcium sulfite to a
high solids concentration (—56 percent)
without the addition of additives or vacuum
filters and with reasonably sized equip-
ment. Because the clarification and
thickening functions are effectively
separated, but at the same time intercon-
nected by "internal recycle," the system
offers flexibility for wide variations in
waste dewatering behavior created by
changes in FGC system operating condi-
tions. The system operates most effec-
tively in the "bang-bang" mode, with the
thickener underflow set, for most of the
"bang-bang" cycle, as low as possible
(without plugging the underflow lines).
On reaching the maximum feasible
concentration, the underflow rate is
increased significantly for a brief period,
then the cycle is repeated. Preliminary
economic evaluation of this system
indicates potential savings of 10 percent
of the total capital costs and 6 percent of
annual operating costs for the FGC waste
management/disposal system.
A. R. Tarrer is with Auburn University, Department of Chemical Engineering,
Auburn, AL 36849.
Julian W. Jones is the EPA Project Officer (see below).
The complete report, entitled "Development of a New Gravity Sedimentation
Process for Dewatering Flue Gas Cleaning Wastes, "(QrderNo. PB 84-231448;
Cost: $20.25, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
. S. GOVERNMENT PRINTING OFFICE: 1984/759-102/10689
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Environmental Protection
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