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Air Stripping for VOC Removal - Advanced Topics
Dave FischerDave FischerQED Environmental Systems Inc.Ann Arbor, MI / San Leandro, CA
Copyright © QED Environmental Systems, Inc. 2007- 2011; all rights reserved.
Topic Overview
• Review - the Air Stripping process• The Impact of Fouling Conditions• Tower vs. Tray• Review - the QED Air Stripper Modeler• Extending the Model• Complicated Removal Design (VOC, THM,
ammonia)• Case Studies
Air StrippingMass transfer Process governed gby Henry’s Law
Counter-current flflow ensures efficient mass transfer throughout th ti fl ththe entire flow path
High air to water surface for transfersurface for transfer is created by the turbulent froth mixturemixture
Air Stripping
The froth in action.
Sliding Tray Type StripperMethodAir bubbles - froth and turbulent mixing creates mass transfer surface area
Advantages• Easy access• Less prone to fouling• Less prone to fouling• Less intrusive at site• Wide flow turn-down
Disadvantage• Requires higher pressure• Requires higher pressure blower (HP)
Selected QED Air Strippers
E-Z Tray E-Z Tray E-Z Tray E-Z TrayyModel 6.4(65 gpm max)
yModel 16.4(150 gpm max)
yModel 24.4(250 gpm max)
yModel 96.6(1000 gpm max)
Stacking Tray Stripper• Stacking tray strippers are a
series of stacked rectangular gboxes with bottom perforations
• Trays layers are sealed with k t d f t d t thgaskets and fastened together
with clamps around outer edges• Cleaning requires lifting trays g q g y
and breaking pipe connections, often requires two or more people or an overhead crane
Stacking Tray Design
people or an overhead crane• Requires access to all sides for
installation and maintenance g y g
More information at -- http://www.qedenv.com/davislf/
Successful Process Requirements• High air to water ratio (A/W)
#1. Process parameter• High surface area of contact
between air and water• Clean air (concentration gradient
Impact of dirty air
Clean air (concentration gradient driven process)
• Dissolved volatile organics in a water matrix (modeling valid forin a water matrix (modeling valid for levels < 25% of water solubility)
• No free-phase organics
Clean air Contaminated air
• No surfactants or other H lowering factors (dissolved polar organics)
• Stripper is levelStripper is level
Stripper Performance Impacts pp p
• Air or liquid flow restrictionsq• Significant water or air temperature changes• Free phase product or other sorptiveFree phase product or other sorptive
compounds that decrease stripping, such as organic solids
• Surfactants or other polar organic chemicals that can lower H for target organics
• Contaminated air
Air Stripping
T t i t th hi h
Some physical elements
• Temperature impacts the process – higher temperature = better stripping• Process temperature is roughly = water p g ytemperature• Freezing is not a concern for continuous operationoperation• Discharged air is saturated at the process temperature (consider condensation and thermal i t i t t t it )impacts on air treatment units)
Temperature EffectsThermal mass of water >> that of air – example (200gpm flow; 1300cfm air flow):
Water Temp (F) Air Temp (F) ProcessTemp (F)Water Temp (F)
55
55
Air Temp (F) ProcessTemp (F)
50
70
54.9
55.2
55
55
80
100
55.3
55.6
Hotter air is less dense (also for higher elevation) so stripping willHotter air is less dense (also for higher elevation) – so stripping will decrease – example (Tw = 55F; MTBE in = 10,000 ppb; 4-tray stripper)
Air Temp (F) MTBE (ppb)
40
50
80
4248
4343
454580 4545
Discharge Air
• Entrained water droplets and air at 100% RH at the process temperature• High efficiency mist eliminator for droplet removaleliminator for droplet removal• Improper demister sizing or fouling can cause water blow byby• Water knock out, downstream process i l ti t finsulation, etc. for condensation issues
Air FlowAir Flow
Types of Tray Air Stripper FoulingTypes of Tray Air Stripper Fouling
• Metal oxides1.Metal oxides• Hardness (scale)• Suspended solidsSuspended solids• Bio solids, slimes• Oils & Greases• Oils & Greases• Free phase non-
aqueous phaseaqueous phase liquids (NAPL)
1. CO2 stripping can cause a slight pH increase, leading to insoluble metal oxide formation
Bio FoulingBio Fouling
Example – pH adjustment to minimize inorganic tray fouling causeda fungus to rapidly develop a protective slime.
Tray Fouling – Knowing When to Clean
Normal stripper sump pressure = 4-6 inch H2O / tray stage
Tray Fouling – What Does it Look Like?
Expected performance impact is gradual as air flow decreases, due to tray fouling.
E-Z Tray® Advantages … Cleaning
E-Z Tray Tower Stacking TrayAir Strippers Air Strippers Air Strippers----------------------------------------------------------------------------------------------------
• Single person cleaning
• Packing access and removal is
• Major disassembly and multi person crew
difficult needed
Stripper CleaningStripper Cleaning• Cleaning frequency and effort is highly site-g q y g y
specific• Example -
– 1000ppm TDS, 260ppm total hardness, 0.03ppm iron - stripper requires cleaning every 3 weeks
Ti l E Z T i• Time to clean an E-Z Tray stripper– Two 1000gpm, E-Z Tray 96.6 units (8 doors, 48
trays) takes 8-10 minutes/tray to fully removetrays) takes 8-10 minutes/tray to fully remove, pressure wash and reinstall all the trays in this system
Fouling - Preventative Measures• Clean trays
– Backup tray set
g
• Sequestering agents (decrease cleaning frequency)
i i– inorganic polyphosphates
• Bio-foulingOzone etc– Ozone, etc.
• pH adjustment– In/out
P t i id ti• Pre-stripper oxidation and filtration
Tower StripperMethodThin film of water flows over a high surface area gpacking
AdvantagesAdvantages• Lower energy use in the air mover, due to lower overall pressurelower overall pressure drop
DisadvantagesDisadvantages• Flow turn-down difficult• Difficult to clean• Tall structure• Tall structure• Short circuiting
Tower Stripper
If fouling conditions d l th tdevelop, the tower can quickly loose mass transfer area. Small local areas ofSmall local areas of deposition can produce flow short circuiting that furthercircuiting that further limits available contact area.
Tray vs.Tower Stripper
• Hard to access for cleaning (high O&M costs)• Very tall structure (wind loading, thermal issues)g, )• Operating conditions difficult to observe
Complex design process• Complex design process due to structural issues• No web based performance model, models harder to use
E-Z Tray® vs. Tower O&M ExampleSite in Sturgis, MI treating 250gpm water containing:
1 1 1 t i hl th1,1,1-trichloroethanec-1,2-dichloroethylenehexachlorobutadienemethylene chloride
hth lnaphthalenetetrachloroethylene (PERC,PCE)trichloroethylene (TCE)
Oversized tower replaced with a 500gpm E-Z Tray 48.6 model
Historical tower cleaning with acid cost about $54,000/year
Pressure washing the E-Z Tray every 40-50 days estimated at $8,000/year
Modeling the ProcessWeb based Modelhttp://www.qedenv.com/modeler
Xin = aqueous concentration entering the air stripperXout = aqueous concentration exiting the air stripperYin = gas concentration entering the air stripperNth = number of theoretical trays in the air stripper The performance modeler is based on the designNth = number of theoretical trays in the air stripperS = stripping factorKh = Henry’s Law constantL = liquid flow rateG = gas flow rate
The performance modeler is based on the designprocedure discussed in -- Kibbey, T. C. G., K. F. Hayes andPennell, K.D., ‘‘Application of Sieve-Tray Air Strippers tothe 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.
1 – Pick Contaminants
Henry’s Constant (H)Larger H = more easily stripped (atm/mol frac)Larger H = more easily stripped (atm/mol-frac)
• vinyl chloride - 1245• TCE – 648
• MTBE - 32• acetone - 2 4TCE 648
• benzene - 309acetone - 2.4
2 – Other Information
Process Variables
• Temperatures (air and water)• Altitude (air density)• Flow rates (air and water)• Flow rates (air and water)
- Process impacts- Hydraulic impacts
3 – Pick a Stripper
• First pass – pick the stripper model that matches project flowproject flow
Metric units available on Model Site
4 – Contaminant Concentrations
• Concentration in ppb (ug/L) – 1000ppb = 1ppm• Each contaminant behaves independently
5 – Review Model Results
(URL listed to allow easy remodeling)
How certain field analytical results are modeled
TPH, DRO, GRO, Total-BTEX, TVPH, F1 – F4, C6-C10, etc. All represent groups of organic compounds, with ranges of Henry’s constant (H). A representative compound is used to stand for the
T i l tigroup. Typical practice:
• BTEX – modeled as benzene (lowest H out of the BTEX)• TPH modeled as either benzene (TPH GRO) or naphthalene• TPH – modeled as either benzene (TPH-GRO) or naphthalene
(TPH-DRO)• F1 (C6-C10) – model as GRO = benzene• F2 (C10-C18) – model as DRO = naphthaleneF2 (C10 C18) model as DRO naphthalene• F3 and above (>C16) = difficult to strip
This method carries RISK if the group actually has more lower H components than that of the representative. Model individual components if you need to meet specific targets.
We Can Help Model Special CasesFl l t t d i f i t i d l• Flow very close to rated maximum for a given stripper model
• Air flow conditions different than the standard (+/- 15-20%)• Strippers with a non-standard number of trays • Blended flow calculations• Strippers in series (use effluent from first model run as influent
for second)• Contaminants that are not listed in the model contaminant table• Calculation of “effective H” from field pilot data• Results less than 1ppb• Pilot cases where concentrations are >> 25% water solubility
Special Cases
Standard 4&6 Tray Custom 7 Tray Standard Series
Series – same air(like an 8 Tray) Parallel – different flow rates
Blended discharge
C li t d R l Sit tiComplicated Removal Situations
• Free phase NAPL• Surfactants / H altering non-strippable
components• THM Removal• Ammonia Removal
Free Phase Organics
• Dramatically lower removal efficiencyefficiency
• Can coat walls and accumulate in the sump to act as an ongoing VOC source
• Can cause partitioning effects where a percentage of certainwhere a percentage of certain VOCs are “sequestered” from the stripping process
Free Phase Organics - Partitioning
Contaminated Water
Stripping removes dissolved portion of contaminants – some NAPL moves through the system. Some organics may also partition into the free-
Organics re-equilibrate in the sample vial -partition into the free-
phase component.the sample vial -increasing the dissolved concentrations in the treated water.
Free Phase Organics - Example
90.00%
100.00%
Water + Hydrocarbon
60.00%
70.00%
80.00%
val)
30.00%
40.00%
50.00% pilotmodel
(% V
OC
Rem
ov
0 00%
10.00%
20.00%
Example – site handling tanker ballast water with a combination of
20.00% 40.00% 60.00% 80.00% 100.00%0.00%
(% Maximum Flow)
Example site handling tanker ballast water with a combination of free phase hydrocarbons and VOCs dissolved in water
Surfactants / Polar Organics
• Lower effective H for all contaminants due to solubilization of organic compounds by surfactant micelles.C f i• Can cause foaming
– Sometimes subtle (and not required when polar organics are present)
– Demister fouling and blower back-pressure increase– Control = Anti-foam additives (does not recover stripping
effectiveness )– Control = Knock-out tank prior to demister
Surfactants / Polar Organics – Example
Closed DOD site with low level dissolved TCE. Visual indication of
Normal froth Surfactant impacted froth
excessive foam in upper stripper trays. TOC 2-3X greater than sum of target organics.
Surfactants / Polar Organics – Example (cont.)
Field results show a consistent 60 85% reduction in stripping capability vsField results show a consistent 60-85% reduction in stripping capability vs. theoretical performance prediction for TCE.
THM Removal
• Trihalomethanes (THMs) can form in drinking water when disinfectant (chlorine) breaks down precursor organic compounds, normally organic solids
• Air stripping is an effective way to reduce THMs• Air stripping is an effective way to reduce THMs• THMs can re-form after stripping if organic precursors
are still available
Chloroform Removal
45
THM Removal
30
35
40
20
25
30
CHCl3 in CHCl3 out In – 24 hour Out – 24 hour
Chl
orof
orm
(ppb
)
5
10
15C
0 5 10 15 20 25 30 350
Time (days)
THM Removal – Possible Process
Successful THM removal process design will need toprocess design will need to account for remaining THM precursors, while providing sufficient residual disinfection. Clear well loop design or remote reservoir loop mayremote reservoir loop may provide the best solution.
Ammonia Removal
• Dissolved ammonia gas can be stripped from water• H is very low (very hard to strip) – requiring high A/W to
achieve significant removals• pH adjustment is required• pH adjustment is required• Pilot testing required to understand the balance
between ionic and dissolved gas forms
Ammonia Removal
pH adjusted > 11 prior to air stripperprior to air stripper, then adjusted back to required level
Model assumes the dissolved gasthe dissolved gas NH3
Ammonia RemovalFrom EPA article (ref. below) -- A/W of 300-500 cfm/gallon are typical for ammonia removal. Using QED’s normal dimensionless A/W – (ft3/min air / ft3/min water) this equates to A/W of 2200 - 3700.
Normal A/W for VOCs are 50-200.
The only way to achieve high A/W with the E Z TrayThe only way to achieve high A/W with the E-Z Tray systems is to lower the liquid flow rate.
Ammonia removal is viable for low liquid flow situations.
EPA Waste Water Technology Fact Sheet – Ammonia Stripping – EPA 832-F-00-019, Sept. 2000
Additional Site Information for Designg
• Site history of DNAPL and/or LNAPLP t th t 25% f t l bilit• Parameters that are > 25% of water solubility + are hard to strip (DRO, C12-C28 hydrocarbons, etc.)
• Is O&G above detection limit (is limit low enough)Is O&G above detection limit (is limit low enough)• Is there air contamination near the blower inlet• Does stable foam form if target water is shaken in a jar• Is there an offset between TOC and the target organics• Site history of surfactant use• Are high shear pumps used to capture the water (stable
emulsions of NAPL)
Pilot Testing
• Prepackaged, just add electricityjust add electricity
• Rental• Used for scale-upUsed for scale up
design• Allows H correction
from results when NAPLs, surfactants, t k t betc. are known to be
present
Case Study – Use of Pilot Data• Target contaminant = TCE at 140ppb• Model predicts <1ppb result (100% removal)
Field res lts sho 1 4ppb res lt (98 7 99% remo al)• Field results show 1.4ppb result (98.7-99% removal)• TOC checked – modeled contaminants add to 280ppb, TOC in is
1300ppb (TOC after stripper is 1100ppb)• There is about 1000ppb of unknown• Original tower stripper also never met modeling prediction –
supplier blamed fouled media• Slight abnormal foaming observed• Normal H for TCE is 648 (atm/mol-frac); pilot results show an
effective H between 97 and 236 (atm/mol-frac)• Used effective H values from pilot data to increase air to water ratio
(A/W) two stripper model steps to achieve target
Other Stripper ApplicationsOther Stripper Applications
H drogen S lfide eas to strip (pH needs to• Hydrogen Sulfide – easy to strip (pH needs to be dropped < pH = 6)
• (H = 545 atm/mol-frac)( )
• Radon removal – extremely easy to strip• (H = 4680 atm/mol-frac)
• Methane removal – extremely easy to strip• (H = 35390 atm/mol-frac)
Case Study 1
• VOC treatment of tanker ballast waterballast water
• Strippers replaced an aging activated sludge treatment process that was unable to handle changes in flow and concentrationconcentration
• Process string includes free-phase removal and airphase removal and air treatment
• Pilot testing used prior to design
Case Study 2• VOC reduction prior to SBR
treatment of pharmaceutical wastewater
• Stripper air flow rate much lower than flow from SBR
• Allowed smaller CATOX air treatment unitair treatment unit
Before After
Thermal Oxidizer
VOC
Thermal Oxidizer
Less VOCVOC
Treatment Plant Treatment Plant
Case Study 3 – Cheyenne, WY• Abandoned Atlas Missile
sites contaminated city wells with chlorinatedwells with chlorinated solvent
• US Army Corps is QED’s customer
• Strippers will treat city water during high demand, summer months (4000gpm capacity)capacity)
• Excellent equipment reliability required to ensure continuous water treatment
• System started June 2011
Questions?David Fischer
QED Environmental Systems IncQED Environmental Systems, Inc.
Tel: 800-624-2026E-mail: [email protected]
WEB:WEB:www.qedenv.com
