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

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 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).

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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

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    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

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