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Dave Fischer
QED Environmental Systems Inc.
Ann Arbor, MI / San Leandro, CA
Copyright © QED Environmental Systems, Inc. 2007, 2008, 2009; all rights reserved.
Unique Advantages of Tray
Strippers for VOC Removal Under
Fouling Conditions
• Description of the Air Stripping process
• Air stripping for fouling conditions
• Unique E-Z Tray® advantages
• E-Z Tray product features/benefits
• The QED Air Stripper Modeler
• Design Principles/Oddities
• Pilot Testing
• Case Studies
Topic Overview
Air Stripping
A process that removes or
“strips” volatile organic
compounds from
contaminated water by
contacting clean air with
contaminated water
across a high surface
area, causing the volatile
compounds to move from
the water into the air.
Process is governed by
Henry‟s Law.
Driving dissolved volatile
organic contaminants from
water into air.
Air Stripping
Counter-current flow
means the cleanest air
contacts the cleanest
water. This ensures
efficient mass transfer
throughout the entire
flow path.
Air Stripping
Air water contact happens
in the active tray areas.
High air to water surface
for transfer is created by
the turbulent froth mixture.
Air Stripping
The froth in action.
Tower Stripper
MethodThin film of water flows
over a high surface area
packing
Advantages
• Lower energy use in
the air mover, due to
lower overall pressure
drop
Disadvantages
• Flow turn-down difficult
• Difficult to clean
• Tall structure
Tower Stripper
If fouling conditions
develop, the tower
can quickly loose
mass transfer area.
Small local areas of
deposition can
produce flow short
circuiting that further
limits available
contact area.
Sliding Tray Type Stripper
Method
Air bubbles - froth and
turbulent mixing creates
mass transfer surface area
Advantages
• Easy access
• Less prone to fouling
• Less intrusive at site
• Wide flow turn-down
Disadvantage
• Requires larger blower
Types of Fouling
• Iron oxides
• Hardness (scale)
• Suspended solids
• Bio solids, slimes
• Oils & Greases
• Free phase DNAPL
E-Z Tray Tower Stacking Tray
Air Strippers Air Strippers Air Strippers
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E-Z Tray® Advantages … Cleaning
• Single
person
cleaning
• Packing
access and
removal is
difficult
• Major
disassembly and
multi person crew
needed
Lightweight, front slide out trays
provide easy access. Trays range
from 24-73 lbs and are about 24
inches wide with lengths ranging
from 2.4 to 6 feet
E-Z Tray® Design
Single person
maintenance saves on
cleaning and labor
costs -- made possible
by front access
serviceabilityStripper
Blower
Tray
E-Z Tray® Advantages … Footprint
E-Z Tray Tower Stacking Tray
Air Strippers Air Strippers Air Strippers
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• Reduced footprint
for installation and
maintenance
• Small footprint but
very tall structure
often required
• Lots of space needed
for disassembly, lifting
from all sides, pipe
disconnection and
tray stage stacking
E-Z Tray® Advantages … Monitoring
E-Z Tray Tower Stacking Tray
Air Strippers Air Strippers Air Strippers
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• Easy process
monitoring and
inspection, even
while in operation
• Condition of packing
and air flow
distribution are very
difficult to observe
• Difficult or impossible
to observe air and
liquid flow distribution
during operation
E-Z Tray® Advantages … Modeling
E-Z Tray Tower Stacking Tray
Air Strippers Air Strippers Air Strippers
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• Easily modeled
online by customer
to help process
evaluation
• More complex design
process due to
structural aspects,
assistance normally
required
• Online modeler
not offered
One piece shell with single
gasketed door cuts down on
possible air leaks
Clear front door gives easy
visual check of air stripper
performance
Front access cleaning means
piping stays put – no need to
break connections
E-Z Tray® Design
Ancillary Equipment
• Pressure Switch
• Liquid Flow Meter
• Air Flow Meter
• Bag Filters
• Solenoid Valve
• Tank
• Skid
• Blower
• Infeed Pump
• Discharge Pump
• Gravity Drain
• Control Panel
• Level Switches
QED Air Strippers (specifications)
An extensive
range of
models to
match liquid
flow and
removal
needs.
QED Air Strippers
E-Z Tray
Model 6.4
E-Z Tray
Model 16.4
E-Z Tray
Model 24.4
E-Z Tray
Model 96.6
Modeling the Process
Xin = aqueous concentration entering the air stripper
Xout = aqueous concentration exiting the air stripper
Yin = gas concentration entering the air stripper
Nth = number of theoretical trays in the air stripper
S = stripping factor
Kh = Henry’s Law constant
L = liquid flow rate
G = gas flow rate
Web based Model
http://www.qedenv.com/modeler
The performance modeler is based on the design
procedure discussed in -- Kibbey, T. C. G., K. F. Hayes and
Pennell, K.D., „„Application of Sieve-Tray Air Strippers to
the Treatment of Surfactant-Containing Wastewaters‟‟,
AIChE Journal, Vol. 47, No. 6, June 2001. Also -- Perry, R.
H., and D. W. Green, Perry’s Chemical Engineer’s Hand-
book, 7th ed., McGraw-Hill, New York 1997.
Henry‟s Constant (H)
Larger H = more easily stripped (atm/mol-frac)
• vinyl chloride - 1245
• TCE – 648
• benzene - 309
• MTBE - 32
• acetone - 2.4
1 – Pick Contaminants
• Temperatures (air and water)
• Altitude (air density)
• Flow rates (air and water)- Process impacts
- Hydraulic impacts
2 – Other Information
Process Variables
• First pass – pick the stripper model that matches
project flow
3 – Pick a Stripper
• Concentration in ppb (ug/L) – 1000ppb = 1ppm
• Each contaminant behaves independently
4 – Contaminant Concentrations
5 – Review Model Results
(URL listed to allow easy remodeling)
• Dissolved volatile organics
in a water matrix
• No free-phase organics
• Clean air (concentration gradient
driven)
• High surface area of contact
between air and water
• High air to water ratio
• Sufficient contact time
• No surfactants or other H lowering
factors (dissolved polar organics)
• Stripper is level
Impact of dirty air
Successful Process Requirements
Clean air Contaminated air
Stripper Performance
• Air and liquid flow issues
• Air flow restrictions
• Water/air temperatures
• Free phase product or other sorptive
compounds that decrease stripping
• Surfactants or other polar organic chemicals
that can influence H
Free Phase Product
• Coats walls
• Residual source in sump
• Co-dissolution issues
• Interferes with performance
Surfactants / Polar Organics
• Lowers effective H for all
contaminants
• Foam can lead to demister fouling
• Foaming
– Anti-foam additives (stripping
effectiveness not recovered)
– Knock-out tank prior to demister
Example - closed DOD site with low level dissolved TCE. Visual
indication of non typical foam in upper stripper trays. Consistent
60-85% reduction in stripping capability vs. theoretical
performance prediction.
H
(atm/mol-frac)
0
200
400
600
800
TCE
benzene
MTBE
polar organic
impacted TCE
Temperature Effects
• Higher water temperature = better stripping
• Higher air temperature = small impact on water
temperature
• Example:
Water Flow
Water Temp
79.6 FAir Temp
200 gpm
80 F
50 F
Equilibrium
Air Temp
Air Temp
100 F
120 F
80.3 F
80.5 F
Air Flow 1300 cfm
• Clean trays
– Backup tray set
• Sequestering agents
(decrease cleaning
frequency)
– inorganic
polyphosphates
• Bio-fouling
– Ozone, etc.
• pH adjustment
– In/out
• Pre-stripper oxidation
and filtration
Fouling - Preventative Measures
Bio Fouling
Example – pH adjustment to minimize inorganic tray fouling
caused a bio-component to rapidly develop a protective slime.
Inorganic Stripper Applications
• Ammonia – hard to strip • (H = 0.7-5.5 atm/mol-frac)
• Hydrogen Sulfide – easy to strip • (H = 545 atm/mol-frac)
• Radon removal – extremely easy to strip• (H = 4680 atm/mol-frac)
Ammonia Removal
pH adjusted > 11
prior to air stripper,
then adjusted back
to required level
Model assumes
the dissolved gas
NH3
Hydrogen Sulfide Removal
pH adjusted < 6
prior to stripping
then adjusted back
to required level
Model assumes
the dissolved gas
H2S5 6 7 8 9 10 11
0
10
20
30
40
50
60
70
80
90
100
H2S
HS-
S2-
pH
%%
Pilot Testing
• Prepackaged,
just add electricity
• Rental
• Can scale flows
correctly
• Allow H correction
from results when
NAPLs, surfactants,
etc. are known to be
present
Case Study 1
• VOC reduction prior to SBR
treatment of pharmaceutical
wastewater
• Stripper air flow rate much lower
than flow from SBR
• Allowed smaller CATOX
air treatment unit
VOC
Treatment Plant
Before
Treatment Plant
After
Thermal Oxidizer
Less VOCVOC
Case Study 2
• Gasoline spill in
high value
residential area
• Space for process
equipment limited
• Need for low
profile - sound
and appearance
• Strippers housed
in metal shipping
containers
Case Study 3
• Large industrial
chlorinated cleanup
• Strippers one component
in a multi-unit / multi-
phase process string
• EZ-Tray design allowed
space saving stripper
placement
• Strippers used in series
for higher removal
Liquid In
Liquid Out
Air In
Air Out
• Air strippers are effective at removing dissolved
volatile organic compounds from water
• The primary process factor is air to water ratio
• The process can be modeled using on-line
computer tools -
http://www.qedenv.com/modeler
• Air stripping equipment needs to be maintained
to ensure continued design removals
Summary
Questions?
David FischerQED Environmental Systems, Inc.
Tel: 800-624-2026
E-mail: [email protected]
WEB:
www.qedenv.com
