Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Stripping ethanol and acetone from water with
modern packings
B. Anvaripour*, N. Yoswathana\ N. Ashtoif, A. Arrowsmith"
"School of Chemical Engineering, The University of Birmingham,
Birmingham B15 2TT, UK
^Department of Chemical Engineering, Mahidol University,
Salaya, Nakornpathom 73170, Thailand
Abstract
The stripping of volatile organic compounds plays an important role in ground
water remediation and modern packings give increased column performance. An
experimental comparison is made of the performance of two modern random
dumped packings, Rauschert Hi-flow and Lantec Lanpac in stripping ethanol or
acetone from aqueous solution by counter current flow. The studies were
carried out in a 1.5 ft diameter column packed to a depth of 9.5 ft with the gas
flow varying from 500 Ib./hr.fP to 2450 Ib./hr.fP and the liquid flow from 1950
Ib./hr.fP to 4500 Ib./hr.fP. Temperature was varied from 13 °C to 36 °C. The
gas flow rate was measured using an anemometer and the liquid flow with a
variable area rotameter. The concentration of the organic material in the
aqueous phase was measured by GC using FID. Observed values of the height
of a transfer unit varied between 2.6 and 125 feet.
1 Introduction
The stripping process has been used effectively to reduce the concentration of
contaminants of volatile organic compounds (VOCs) in water [1-14]. The
concentration of pollutant in the inlet water is in excess of its corresponding
equilibrium concentration in the air, thus providing a driving force for the
transfer of the contaminants from water to the air. The rate at which a VOC is
removed from water depends on several factors: air-to-water (A:W) ratio, water
loading rate, Henry's constant of the VOC, temperature of water and air, height
of packing, and packing type.
A high value of A:W ratio increases the stripping factor and should increase
the removal efficiency. However, as A:W ratio increases, the pressure drop also
increases and beyond the flooding point the air stripping will fail. Increasing the
water rate results in a higher value of wetted area. The higher the wetted area,
the greater the mass transfer and consequently the greater the removal
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
494 Water Pollution
efficiency. However, there is a diminishing improvement of wetted area with
increasing liquid rate [11] and at constant gas rate the pressure drop will
increase with increasing liquid loading. The greater the Henry's constant, the
easier the removal of the VOC. The minimum A:W ratio required to achieve a
certain removal depends greatly on the Henry's constant of the VOC of interest
and it is proportional to the reciprocal of the Henry's constant [9]. Temperature
strongly affects Henry's constant, the value increasing with temperature rise,
therefore also increasing the stripping efficiency. An increase in the packing
height increases the contact time between the air and the water and
consequently will result in an increased removal of VOCs.
Mass transfer takes place at the interface and therefore good contact
between the gas and the liquid is required. This is achieved by using a packing
with a large effective surface area per unit volume. Volumetric mass transfer
coefficient (K^a) values for a selected packing have to be determined from
experiments. There is very limited experimental data about the stripping
efficiency of modern packings and so the aim of this work is to investigate the
performance of two packings, namely 2 in. plastic Hi-flow and 2.3 in. plastic
Lanpac, by air stripping ethanol and acetone from water.
2 Theoretical background
The theory of a counter current packed column has been well documented in the
chemical engineering text [15]. According to this theory, the overall resistance
to mass transfer is equal to the sum of two resistances, a liquid-phase resistance
and a gas-phase resistance:
&=&+7<s
(1)
In accordance with two-phase resistance theory, the individual resistances are
defined as the reciprocals of their respective mass transfer coefficients. With the
assumption that Henry's law applies, the following equation is obtained:
K,a
L
krd
L
Hk^a
Lr
Performance of an air stripping column is calculated using the following four
basic equations derived from mass transfer theory:
(3)
#717 =
(4)
^
(5)
-
f
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Water Pollution
495
3 Experimental procedure
Figure 1 shows the process flow diagram used for the experiments. Water was
heated to the desired temperature by injecting steam and when the desired
temperature was established, ethanol or acetone was dissolved into the water.
The resulting solution was recycled by means of the pumps for about 30
minutes to ensure uniform concentration inside the tank.
The packed column stripper had a diameter of 1.5 ft and was randomly
packed with 9.5 ft of 2.3 in. Lanpac or 2 in. Hi-flow. Air was drawn into the
column by means of a fan at the exit of the column and the flow rate was
regulated by changing the area of duct exposed by the air inlet gate with a
movable plate. A digital anemometer (Testovent 4300) was moved across the
air inlet gate to measure the mean air inlet velocity over 20 seconds. Air was
varied form 500 Ib./hr.fP to 2450 Ib./hr.fP. Liquid flow rate was adjusted by
means of a valve and a rotameter provided after the feed pump respectively. The
flow rate was varied from 1950 Ib./hr.fP to 4500 Ib./hr.fP.
Temperatures of the inlet and outlet streams were measured by
thermometers. To achieve a steady state operation, the column was allowed to
operate for about 12-15 minutes before taking samples. Samples were taken
from the inlet and outlet liquid to determine the organic concentrations and
were analysed using a Perkin-Elmer 8500 GC, equipped with a flame ionisation
detector using an internal standard method. The column used for the GC was of
stainless steel (3.3 ft long with 0.0866 in. inside diameter) packed with 100-120
mesh Porapack Q
4 Results and discussion
Pressure drop
The experimental pressure drops are plotted in figure 2. The logarithmic plots of
the pressure drop per unit length of packing against gas loading for the dry
packings were straight lines of slope 1.56 and 1.55 for Lanpac and Hi-flow
respectively. The results in the two phase region also followed the same pattern
to that of dry packing before the loading points. Afterwards, the slopes of the
curves of pressure drop against gas rate at fixed liquid load increase with
increasing gas rates. Comparing the two packing, Lanpac shows a higher
pressure drop per unit length than Hi-flow at similar gas and liquid loadings.
The general trend of the pressure drop data of Lanpac was similar to
manufacturers data. However, Hi-flow shows a lower pressure drop.
Effect of temperature
Figure 3 illustrates the effect of liquid temperature on K^a. The overall
volumetric mass transfer coefficient increases with temperature, due to the
increase of Henry's law constant at higher temperatures. From this figure, which
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
496
Water Pollution
is a semi-log plot, it is clear that K^a is proportional to e°'°"" and e°'°**' for
acetone and ethanol respectively. The effect of temperature on K^a was more
for ethanol than acetone, probably due to the larger dependency of the Henry's
law constant of ethanol on temperature.
Stripping efficiency
Plots of the experimental overall height of a transfer unit (HTU) against liquid
load at fixed gas rates are shown in figures 4 to 7 for ethanol and acetone
respectively. As may be seen from these graphs, the HTU's of acetone and
ethanol depend on gas and liquid rates and therefore both liquid and gas films
resistances are important. At low gas rate, the HTU was approximately
independent of the liquid flow rate for ethanol. However, as the gas rate
increased, the dependency of HTU on liquid rate also increased. For acetone the
dependency of HTU on liquid at constant gas rate was larger than ethanol, due
to the larger predominance of liquid film resistance with respect to ethanol.
Figure 8 compares the performance of the two packings for acetone stripping. It
indicates that at low gas rate, Hi-flow gives lower HTU However, as the gas
rate increases, the performance of the two packings becomes similar.
5 Conclusions
1. With respect to Lanpac, Hi-flow showed a lower pressure drop per unit
length and therefore is capable of a higher gas handling capacity.
2. Within the range studied Hi-flow provided a better performance at low gas
rate. However, as the gas rate increases, the performance of Lanpac increases
until it becomes similar to that of Hi-flow
3. Air stripping of ethanol and acetone is a mixed film resistance controlled
process.
4. The stripping efficiency of both ethanol and acetone increase exponentially
with temperature.
Nomenclatures
a
Q,
C^
Q,
G
H
HTU
&G
KL
k^
specific interracial area (ftVfl?)
influent concentration of the VOC (Ib.mole/f?)
effluent concentration of the VOC (Ib.mole/f?)
molar density of the water (Ib.mole/fl?)
air loading rate (Ib.mole/hr.fF)
Henry's constant (atm)
height of transfer unit (ft)
individual gas-film mass transfer coefficient (Ib.mole/hr.atm.fF)
overall mass transfer coefficient (ft./hr)
individual liquid-film mass transfer coefficient (ft./hr)
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Water Pollution
L
NTU
PI
S
t
Z
497
liquid loading rate (Ib.mole/hr.fP)
number of transfer units (dimensionless)
the ambient pressure (atm)
stripping factor (dimensionless)
liquid temperature (° C)
packing height (ft)
References
1. Fang, C.S. & Khor, S.L. Reduction of volatile organic compounds in
aqueous solution through air stripping and gas-phase carbon adsorption,
Environmental Progress, 1989, 8, 270-278.
2. Ball, B.R. & Edwards, M.D. Air stripping VOCs from ground water: Process
design considerations, Environmental Progress, 1992, 11, 39-48.
3. Ball, W.P., Jones, M.D. & Kavanaugh, M.C. Mass transfer of volatile organic
compounds in packed tower aeration, Journal WPCF, 1984, 56, 127-136.
4. Gross, R.L. & TerMaath, S.G. Packed tower aeration strips trichloroethylene
from ground water, Environmental Progress, 1985, 4, 119-123.
5. Freiburger, E.J., Jacobs, T.L. & Ball, W.P. Probabilistic evaluation of
packed-tower aeration design for VOC removal, JournalAWWA, 1993, 85, 7386.
6. Lamarche, P. & Droste, R.L. Air-stripping mass transfer correlations for
volatile organics, Journal A WWA, 1993, 81, 78-89.
7. Thorn, E. J. & Byers, W.D. Limitations and practical use of a mass transfer
model for predicting air stripping performance, Environmental Progress, 1985,
12, 61-66.
8. Dzombak, DA, Roy, S.R. & Fang, H.J., Air-stripping design and costing
computer program, JournalAWWA, 1993, 81, 63-72.
9. Adams, J.Q. & Clark, R.M., Evaluating the costs of packed-tower aeration
and GAC for controlling selected organics, Journal AWWA, 1991, 79, 49-57.
10. Hand, D.W., Crittenden, J.C., Gehin, J.L. & Lykins Jr., B.W., Design and
evaluation of an air-stripping tower for removing VOCs from ground water,
^wrW,4fm%, 1986, 74, 87-97.
11. Nirmalakhandan, N., Lee, Y.H. & Speece, RE Designing a cost-effective
air- stripping process, Journal AWWA, 1987, 75, 56-63.
12. Nirmalakhandan, N., Speece, RE, Peace, J.L. & Jang, W, Operation of
counter-current air-stripping towers at higher loading rates, Water Res., 1993,
27, 807-1993.
13. Roberts, P.V., Hopkins, G.D., Munz, C & Riojas AH, Evaluating tworesistance model for air stripping of volatile organic contaminants in a countercurrent, packed column, Environ. Sci. Technol, 1985, 19, 164-173.
14. Nirmalakhandan, N., Jang, W. & Speece, R.E., Counter-current cascade airstripping for removal of low volatile organic contaminates, Water Res., 1990,
24,615-623.
15. Treybal, R.D., M%%» 7h3f#>r OperafzYms, McGraw-Hill, New-York, 1980.
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
498
Water Pollution
AIR FAN
AIR OUTLET
LIQUID INLET
If ROTAMETER
STEAM
PUMPS
A ~ I
IR INLET
STEAM
LIQUID OUTLET
Figure 1. Process flow diagram
Ors
a
g
N
5
LAJ MJ 3 \c ^^^*
#x4y
^
•
w
r& n •>"•
y—
V
y
I//
/A
H 4Lc)
W
/>
y
V
1000
100
10000
Gas loading (Ib./hr.ft^)
Figure 2. Pressure drop of both packings ( O L2=9200
Ib./hr.ft*, A Ll-2050 Ib./hr.ft^ x Dry column).
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Water Pollution
ET^AN(j)L (IJANIJAC)
10
15
20
25
30
35
40
Figure 3. Effect of temperature (• Ll=1950 Ib./hr.ft*
A L2-3220 Ib./hr.ft*, • L3-4510 Ib./hr.ft*)
1500
3000 3500 4000 4500 5000
L (Ib./hr.ff)
Figure 4. Effect of liquid rate on the HTU (* GM2000
Ib./hr.ft:, A G2-1380 Ib./hr.ft*, o G3=870 Ib./hr.ft*)
50
2000
2500
•
ETHAT^OL (LA^PAC)
40
.
\^
^ -^_ ^-/
20
^^^^3^
0
1500
2000
2500
G (Ib./hr.ft*)
Figure 5. Effect of gas rate on the HTU(• Ll=1950 Ib./hr.ft*,
A L2=3220 Ib./hr.ft*, • L3=4510 Ib./hr.ft*)
500
1000
499
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
500
Water Pollution
15
AC :ETC )NE (HI-: FLO W)
^10
<t^
S'
K
5 '
_._».----•' ,.-•-•-*"
A"
t
~4
/ -"~"~
^
A
1500
2000
2500
3000 3500 4000 4500 5000
L(lb./hr.fe)
Figure 6. Effect of liquid rate on the HTU (* Gl=2400
lb./hr.ft*, A G2-1650 Ib./hr.ft^, o G3-1000 Ib./hr.fF)
ID
ACETONE (H [-FLOW t
•
^>-^
-__
g
"" ^""•--.
-^:::
~ __ •
^ 10
D
^
K
5 -
"~~"~i
r> 500
1000
1500
2000
2500
G(lb./hr.fF)
Figure 7. Effect of gas rate on the #777 (" Ll=1950 Ib./hr.ft2,
A L2-3220 Ib./hr.ft*, • L3=4510 Ib./hr.ft^)
25
20
ACETONE STRAPPING
15
10
5
0
1500
2000
2500
3000 3500 4000 4500 5000
L(lb./hr.fe)
Figure 8. Comparison between the two packings (O Hi-flow,
A Lanpac,
G=1650 Ib./hr.ft^
G2-1000