Unique Advantages of Tray Strippers for VOC …Air Stripping Counter-current flow means the cleanest...

transcript

Dave Fischer

QED Environmental Systems Inc.

Ann Arbor, MI / San Leandro, CA

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

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

E-Z Tray® Advantages … Footprint

----------------------------------------------------------------------------------------------------

• 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

----------------------------------------------------------------------------------------------------

• 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

----------------------------------------------------------------------------------------------------

• 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.

(atm/mol-frac)

benzene

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

Equilibrium

Air Temp

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

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

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

Treatment Plant

Before

Treatment Plant

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: dfischer@qedenv.com

www.qedenv.com

Unique Advantages of Tray Strippers for VOC …Air Stripping Counter-current flow means the cleanest...

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