United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/009 Mar. 1986
&ERA Project Summary
Design Scale-Up Suitability for
Air-Stripping Columns
Harold Wallman and Michael D. Cummins
An investigation was conducted to
determine the suitability of a design
scale-up from pilot-scale to full-scale
air-stripping columns used in the re-
moval of volatile organic compounds
from contaminated water supplies.
Forty-eight experimental runs were
made in packed columns of four dif-
ferent diameters (6,12,24, and 57 in.)
at air-to-water ratios ranging from 5:1
to 50:1. Water was used from the
Village of Brewster, New York, well
fields; this water was contaminated with
tetrachloroethylene, trichloroethylene,
and cis-1,2 dichloroethylene. Various
packing types (Vi-in., 1-in., and 3-in.
saddles and 2-in. TRI-PACKS*) were
used in the experimental runs.
The mass transfer coefficients gen-
erally increased with column diameter
— that is, mass transfer coefficients
obtained from a pilot column tend to be
conservative. Thus a full-scale column
designed from pilot data would tend to
be overdesigned. Such was the case
even when the pilot column had a
column diameter-to-packing size ratio
of 12:1 or 24:1.
The experimental mass transfer coef-
ficients were compared with values
calculated from the Onda mass transfer
coefficient model. Generally, the two
values were in reasonably good agree-
ment. Based on these results, it appears
that the Onda model tends to give a con-
servative design for a full-scale system.
Using a cost model developed by the
U. S. Environmental Protection Agency
(EPA), the 2-in. plastic TRI-PACKS (of
the packing types tested) gave the most
cost-effective design for a full-scale
•Mention of trade name* or commercial products
does not constitute endorsement or recommenda-
tion for use.
system. No operational problems were
encountered during subf reezing weather
otherthan rupture of some sample lines.
This Project Summary was developed
by EPA's Wafer Engineering Research
Laboratory, Cincinnati, OH, 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 Village of Brewster, New York, has
a serious groundwater contamination
problem — namely, their well fields are
badly contaminated with industrial chlo-
rinated solvents (tetrachloroethylene,
trichloroethylene, and cis-1,2 dichloroe-
thylene). A continuing program has been
under way to evaluate various approaches
of providing a potable water supply, such
as decontamination of water from the
existing well fields by air-stripping or
location of a new water supply source.
Air-stripping was selected as the most
cost-effective approach.
In 1982, a packed column pilot plant
(12-in.-diameter with 18 ft of 1 -in. pack-
ing) was erected at the Village well fields,
and an air-stripping test program was
conducted. This pilot column was de-
signed for 99% removal of tetrachloroe-
thylene at the average annual temperature
at Brewster using a design procedure
described in the technical literature and
augmented by EPA's Technical Support
Division (EPA-TSD). Test results were very
encouraging; the removal of tetrachlo-
roethylene exceeded 99% (with 1-in.
ceramic saddles at an air-to-water ratio
of 20:1).
EPA-TSD, which had developed a
computer program based on theory
-------
similar to the technical literature, under-
took a cooperative study with this pilot
plant, and the data were analyzed using
their program. More recently, EPA-TSD
tested a larger packed column pilot plant
(2-ft-diameter) at the Village well fields.
Based on the various studies conducted
by the Village's consulting engineer, a
decision was made to design and con-
struct a full-scale air-stripping column
for the Village's water supply. Since air-
stripping columns of three different sizes
would now be available (two pilot-scale
and one full-scale column), a proposal for
a cooperative research agreement was
made to EPA's Drinking Water Research
Division (EPA-DWRD) to conduct tests in
these columns with various packing
materials. At the request of EPA-DWRD,
a fourth column diameter (6-in.) was
added.
The principal objective of this coopera-
tive research agreement was to investi-
gate and confirm the scale-up capability
of an air-stripping packed column from
pilot-scale to full-scale module (design
capacity of 0.5 mgd). Secondary objectives
were as follows:
1. Develop engineering design guidelines
by evaluating mass transfer coeffi-
cients and Henry's coefficients in full-
scale and pilot-scale packed columns;
2. Evaluate the effect of cold weather
operation on the full-scale module (i.e.,
the effect of sub-freezing air tempera-
tures on operability and the effect of
low water temperatures on Henry's
coefficient);
3. Evaluate the limiting ratios of column
diameter-to-packing size for pilot
columns (i.e., are ratios of less than
12:1 feasible?);
4. Evaluate by means of a computer pro-
gram the economics of different pack-
ing sizes and operating conditions (i.e.,
the optimum range for air-to-water
ratio and other conditions to give
minimum life cycle cost); and
5. Document the installed equipment
cost of the air-stripping technique in a
full-scale module.
Description of Equipment
General Arrangement
Water can be supplied to the packed
columns from two old well fields (Well
Fields 1 and 2), two old gravel pack wells
(SG 1 and 2), two new gravel pack wells
(SG 3 and 4), and/or a rock well (Deep
Well 2). All of these Village wells are
contaminated with the synthetic chlori-
nated organics to some degree, with Well
Field 1 having the highest contamination
levels.
The study included three pilot-scale
columns and one full-scale air-stripping
column. Three of the columns (6-in., 12-
in., and 57-in. diameters) are hard-piped
installations; the EPA-TSD column (24-
in. diameter) was set up on a temporary
basis for its scheduled tests. A description
of the column construction is provided
below. A sketch showing a typical air-
stripping packed column is presented in
Figure 1.
Pilot-Scale and Full-Scale
Packed Columns
Each of the packed columns has similar
internal components:
(a) a liquid distributor above the pack-
ing at the top of the column, (b) wall
collectors (within the packing) to remove
water from the column wall and redis-
tribute it onto the packing, (c) a packing
support plate near the bottom of the
column, and (d) an air inlet below the
packing support plate. Sample tubes are
provided within the packing at 2-ft inter-
vals. In addition, sample taps are provided
for the water entering and leaving the
column. Instrumentation is provided for
measuring the air and water flows and
the air and water temperatures.
Water
Inlet
Figure 1.
Air Outlets
Liquid Distributor
Random Packing
Column Shell
Liquid Waif Wiper
Sample Collector
at 2 ft. Intervals
I
Packing
Support Plate
pD Air Inlet
U (Blower)
Cross section of a typical air-
stripping packed column. Village
of Brewster. New York
The packing height in each of the pilot
columns is 18 ft. The full-scale column
has a packing height of 17 ft 9 in. and
was designed for 99% removal of tetra-
chloroethyelene at an air-to-water ratio
of 33:1 (with 1 -in. plastic saddles).
Cost of Full-Scale Air-Stripping
Facility
The actual construction costs of the
full-scale column (57-in. diameter) are
tabulated in Table 1. These include costs
for the building (housing the air blowers,
pumps, and electrical controls), ancillary
equipment, sitework, and contractor's
overhead and profit. This air-stripping
facility has a nominal capacity of 600
gpm (0.86 MGD). Note that there are
many items and features (such as build-
ing, large clean/veil, backup blowers and
pumps, chemical feed system, etc.) that
may not be needed for locations with
different system operating conditions and
less severe weather conditions.
Outline of Test Runs
The packing materials tested were 1/2-in.
ceramic saddles, 1-in. and 3-in. plastic
saddles, and 2-in. plastic TRI-PACKS. The
planned experimental conditions were
selected to allow evaluation of: (a) dif-
ferent column diameters with the same
packing material (at the same air and
water velocities), and (b) different ratios
of column diameter to packing size (i.e.,
minimum ratio of column diameter to
packing size).
An outline of the planned test condi-
tions is presented in Table 2. Because of
budgetary considerations, the experi-
mental plan had to be limited to fit the
available funding level. In the case of
some of the test runs, the 5:1 and 10:1
air-to-water ratios could not be run be-
cause of water flow limitations that were
due either to insufficient pumping capac-
ity from the Village's well fields or to
excessive pressure drop in a column or
water feed line.
The air and water flow conditions
(loadings) used for the various packing
materials are shown in Table 3. These
flow conditions were selected to give a
calculated air pressure drop gradient of
1 /16 in. water column per foot of column
packing.
Operating Conditions and
Sample Results
Operating Conditions
Forty-eight experimental runs were
made in the four packed columns with
-------
Table 1. Construction Cost Of Full-Scale Air-Stripping Facility (1983 Dollars)'
Item Construction Cost (including installation)
Process equipment
Column shell
Column internals
Plastic saddle packing
Air blowers (two)
High service pumps (two)
Total process equipment
Air well (also building foundation)
Piping, valves, and appurtenances
Air ductwork and appurtenances
Chemical feed equipment
Instrumentation
Electrical
Building superstructure and sitework
Subtotal
Additional support equipment, piping, valves
and appurtenances for research operations
Total construction cost
$40,776.
4,620.
18,980.
$64.375.
$46,818.
25.OOO.
7,260.
7,OOO.
1,320.
49.103.
72.971.
$273,847
9,792.
$283,639
* Contractor's overhead and profit included.
Table 2. Outline of Experimental Plan
Column Diameter (in.)
12
24
57
Item
Packing types:
Saddles (in.)
TRI-PACKS (in.)
Packing height (ft)
Air-to-water
1/2&1
2
18
1 &3
2
18
1
2
18
1 &3
17.8
ratios 5:1
10:1
20:1
35:1
50:1
5:1
10:1
20:1
35:1
50:1
5:1
10:1
20:1
35:1
50:1
—
10:1
20:1
35:1
50:1
various packings and at various air-to-
water ratios. For purposes of data evalua-
tion, the runs were assigned an analysis
number (Table 4). All runs with the same
column and same packing material were
collectively referred to as a data group.
Water Sample Results
As noted previously, water samples
were collected for each run at the column
inlet, at approximately 2-ft intervals within
the packing, and at the column outlet.
Approximately 430 water samples were
collected and analyzed for these experi-
mental runs.
The results of these water analyses for
tetrachloroethylene, trichloroethylene,
and cis-1,2 dichloroethylene were plotted
as concentration profiles for each run. A
typical set of concentration profiles for
one run is shown in Figure 2.
Data Analysis and Discussion
Mass Transfer Coefficients
One set of mass transfer coefficients
resulting from analysis of the experi-
mental data is summarized in Table 5 for
tetrachloroethylene. Values for air-to-
water ratios of 20:1, 35:1, and 50:1 are
shown, since such ratios are typically
used for air-stripping of these volatile
organic compounds (VOC's). Similar re-
sults were obtained for trichloroethylene
and cis-1,2 dichloroethylene.
The mass transfer coefficients generally
increase as the column diameter in-
creases (with the same packing material).
This result is to be expected, since the
wall effect (i.e., channeling of water on
the inside of the column wall) is greater
with a smaller-diameter column. Of
special note, however, is the observation
that the mass transfer coefficient con-
tinued to increase as the column dia-
meter-to-packing size ratio was increased
from 12:1 and also from 24:1 (for the 1 -
in. saddles). Thus these results indicate
that using pilot-plant data to design a
full-scale column will result in a con-
servative design.
Full-Scale System Designs
In designing a full-scale packed column
system for a specific requirement (say,
99% removal of tetrachloroethylene), a
number of design parameters (such as
packing type, packing size, and air-to-
water ratio) can be varied to achieve the
same result. To select the cost-optimized
design parameters, a cost model has been
developed that estimates both the capital
and operating costs. With the data ob-
tained from the four different-diameter
columns, cost-optimized designs were
developed. The design criteria used were
as follows:
99% removal of tetrachloroethylene
350 gpm (0.5 MGD) design flow
9°C water temperature
5.8 0/kWh power cost
10% interest rate
1.2 safety factor for Henry's coefficient
1.2 safety factor for mass transfer
coefficient
The data in Table 6 summarize the
results for the 1 -in. plastic saddles and
air-to-water ratios of 20:1 to 50:1.
From these results, the cost-optimized
parameters in Table 7 would probably be
selected. Thus once again, a full-scale
system designed from pilot-plant data
will probably result in a conservative
design.
Note that the actual construction costs
for the 57-in. packed column system
(Table 1) are significantly higher than the
estimated capital costs predicted by the
cost model. This result is to be expected,
since there are many site-specific items
and features that are not included in the
cost model.
Onda Mass Transfer Coefficients
The mass transfer coefficients predicted
by the Onda correlation were compared
with the best fit experimentally derived
results for tetrachloroethylene, trichlo-
roethylene, and cis-1,2 dichloroethylene.
The two values were generally, but not
always, in reasonably good agreement.
-------
Table 3. Air and Water Flow Conditions for Plastic and Ceramic Saddles and TRI-PACKS
Air: Water Ratio Liquid Loading Air Loading
(volume basis) (gpm/ft3) (scfm/ft2)
Flow conditions for:
1-in. plastic saddles
5 45
10 38
20 24
35 16
50 13
3-in. plastic saddles and2-in. TRI-PACK
5 75
10 58
20 37
35 25
50 20
1/2 in. ceramic saddles
5 20.0
10 14.9
20 9.64
35 6.59
50 5.10
30
50
65
78
87
50
77
100
120
130
13.4
20.0
25.8
3O.8
34.1
Table 4. Operating Data Arranged for Data Analysis
Data Analysis
Group Number
1 1
2
3
4
5
6
7
2 8
9
10
11
12
3 13
14
15
16
17
4 18
19
20
21
22
5 23
24
25
6 26
27
28
29
30
Packing
Size
(in.)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.
1.
1.
1.
1
;.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
2.
2.
2.
2.
2.
Column
Diameter
(in.)
6
6
6
6
6
6
6
6
6
6
6
6
12
12
12
12
12
24
24
24
24
24
57
57
57
6
6
6
6
6
Air-
Water
Ratio
50.
34.
20.
10.
5.0
5.0
49.
50.
36.
21.
9.8
5.0
50.
36.
21.
9.9
5.0
49.
36.
20.
10.
5.0
53.
37.
22.
47.
35.
20.
9.9
5.1
Loading Rate
Air
0.17
0.16
0.13
0.10
0.067
0.067
0.17
0.44
0.40
O.33
0.25
0.15
0.44
0.39
0.33
0.25
0.15
0.44
0.39
0.32
0.26
0.15
0.47
0.40
0.34
0.65
0.61
0.51
0.39
0.26
Water
seel)
0.0034
0.0046
0.0067
0.0098
0.014
0.013
0.0035
0.0089
0.011
O.O16
0.026
0.031
0.0088
0.011
0.016
0.026
0.031
0.0089
O.O11
0.017
0.026
0.030
O.0088
0.011
0.016
0.014
O.017
0.025
0.039
0.051
Run
Number
13
9
8
7
6
12
10
46
45
44
48
47
43
42
41
40
39
35
34
33
32
31
38
37
36
18
2O
22
24
26
These results indicate that the Onda cor-
relation tends to give a conservative
design for a full-scale system.
Effect of Temperature on
Operabllity
Even though the experimental runs
were made during both winter and sum-
mer months, the water temperature
stayed within a fairly narrow range. The
water temperature entering the packed
columns ranged from approximately 9°
to 1 2° C over the course of all the runs.
This relatively constant temperature was
due, of course, to the consistency of the
groundwater temperature. In addition, the
ambient air temperature did not signifi-
cantly affect the water temperature within
the column.
The 57-in. column was run continu-
ously through periods of subfreezing
weather, and the low air temperatures
did not interfere with the operation of the
packed column. The only problem en-
countered with low temperatures was
with the copper tubing sample lines.
Some of these lines split open at night.
even though the sample valves were left
partly open. For any future designs, such
sample lines should be insulated to pre-
vent freezing.
Henry's Coefficient
Henry's coefficient, a physical-chemical
property that expresses the volatility of a
particular VOC, depends on the tempera-
ture and the molecular properties of the
VOC. For each of the experimental runs.
Henry's coefficient was determined.
An attempt was made to correlate
Henry's coefficient with temperature, but
it was unsuccessful because of scatter in
the data. Instead, a best-fit Henry's coef-
ficient was determined, and these values
were 0.30, 0.21 , and 0.094 atmosphere
for tetrachloroethylene, trichloroethylene,
and cis-1 ,2 dichloroethylene, respectively.
The inability to arrive at any satisfactory
correlation for Henry's coefficient may be
partly due to the relatively narrow range
of temperatures encountered, as dis-
cussed above.
Conclusions and
Recommendations
1 . The mass transfer coefficients
generally increased as the column dia-
meter increased. There did not appear to
be any cut-off point (i.e., the trend con-
tinued beyond column diameter-to-pack-
ing size ratios of 12:1). This trend is
attributed to a so-called wall effect, which
-------
7
a
9
10
TtbleS.
Column
Diameter
(in.)
6
12
24
57
6
12
24
12
57
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
Mass Transfer (
Packing
Type
1" Saddles
1" Saddles
1" Saddles
1" Saddles
2.
2.
2.
2.
2.
2.
2.
2.
2.
Co Co Co Co Co
3.
3.
3.
soefficit
12
12
12
12
12
24
24
24
24
12
12
12
12
12
57
57
57
51.
34.
20.
10.
5.0
49.
36.
19.
9.9
48.
35.
20.
48.
37.
49.
36.
20.
ants for Tetrachloroethyl
Column Diameter-
to-Packing Site
Ratio
2" TRI-PACKS
2" TRI-PACKS
2" TRI-PACKS
3" Saddles
3" Saddles
6:1
12:1
24:1
57:1
3:1
6:1
12:1
4:1
19:1
0.66
0.61
0.51
0.39
0.25
0.66
O.61
0.50
0.39
0.66
0.61
0.51
0.65
0.60
0.66
0.61
0.51
0.013
0.018
0.026
0.039
0.051
0.014
0.017
0.026
O.O39
0.014
0.017
0.026
0.014
0.016
0.014
0.017
0.025
17
19
21
23
25
30
29
28
27
2
4
5
15
16
14
3
11
ene
Mass Transfer Coefficients (sec.'lj
for Air-to-Water Ratios
20
0.0086
0.0012
O.OO15
0.035
0.015
0.016
0.013
0.0091
0.015
35
O.OO65
0.012
0.012
0.017
0.012
0.014
0.016
0.0064
0.010
50
0.0064
0.0078
0.0099
0.014
0.010
0.010
0.028
0.0066
0.0086
would be more pronounced in a small-
diameter column.
2. Because of the trend noted above, it
appears that using pilot-plant data to
design a full-scale column will result in a
conservative design.
3. Reasonably good agreement was
obtained between the experimentally
derived mass transfer coefficients and
those calculated from the Onda model.
These results indicate that the Onda cor-
relation tends to give a conservative
design for a full-scale system.
4. The 57-in. column was run con-
tinuously through periods of subfreezing
weather, and no operational problems
were encountered other than rupture of
some sample lines (even though the
sample valves were left open). In future
designs, such sample lines should be
insulated.
The full report was submitted in fulfill-
ment of Cooperative Agreement
CR810247 by the Village of Brewster,
New York, and Nathan L. Jacobson &
Associates under the sponsorship of the
U. S. Environmental Protection Agency.
0 123456
Location (Z) from Top of Packing fm)
1000
700
.o
I
I
o
0123456
Location (Z) from Top of Packing (m)
I
§
o
1000
100
(
to
1
.J.
~H
cis-1,2 Dichloroethylene
-t-
0123456
Location (Z) from Top of Packing (m)
Figure 2. Typical concentration profiles
112-inch column, 1-inch sad-
dles, and an air-to- water ratio of
21).
-------
Table & Design Parameters and Cost Estimates Resulting from Cost Model for 1-in. Plastic
Saddles
Column
Diameter
(in.)
6
6
6
12
12
12
24
24
24
57
57
57
Air-to-
Water
Ratio
SO.
36.
21.
50.
36.
21.
49.
36.
20.
53.
37.
22.
Diameter
(in.)
70.
63.
52.
70.
63.
52.
70.
63.
51.
70.
63.
53.
Packing .
Height
(ft)
27.
34.
40.
22.
18.
28.
17.
18.
24.
12.
13.
9.6
OUSl C
Capital
(K$)
140.
140.
130.
120.
110.
110.
110.
110.
100.
98.
94.
80.
surname [ ' Jo*
Operating
(K$/year)
7.2
7.4
7.2
6.6
5.6
6.0
6.1
5.7
5.5
5.4
5.0
4.1
Lfunars/
Production
(C/IOOOgal)
13.
13.
12.
12.
9.9
10.
11.
10.
9.6
9.2
8.8
7.4
Table 7. Cost-Optimized Parameters for 1-in. Plastic Saddles
Test
Column
Diameter
(in.)
6
12
24
57
Full-Scale Design
Air-to-
Water
Ratio
20:1
20:1
20:1
20:1
Column
Diameter
(in.)
52
52
51
53
Packing
Height
(ft.)
40
28
24
9.6
Est. Production
Cost
(C/IOOOgal)
12
10
9.6
7.4
•&U. S. GOVERNMENT PRINTING OFFICE:1986/646-l 16/20791 '20421F
-------
Harold Wallman is with Nathan L Jacobson & Associates, Chester, CT; the EPA
author Michael O. Cummins is with the EPA-Technical Service Division,
Cincinnati, OH.
J. Keith Carswell is the EPA Project Officer (see below).
The complete report, entitled "Design Scale-Up Suitability for Air-Stripping
Columns," {Order No. PB 86-154176/A S; Cost: $16.95, subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, v'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-86/009
-*,a,CT = 0.32H
0000329 PS
230 s of ARBOR STREET
CHICASO i
------- |