Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Reactors The Case of the Chlorine Contact Tank.

Monroe L. Weber-Shirk

School of Civil and

Environmental Engineering

ReactorsReactors

The Case of the Chlorine Contact Tank

OutlineOutline

The Regulations The Goal Contact Tanks Characterizing a Contact Tank: Tracers Reactor Theory

CMFRCMFR in Series1-D Advective Dispersion Equation

Building a Better Contact Tank

Disinfection CT CreditsDisinfection CT Credits

Contact time (min)

chlorine pH 6.5 pH 7.5

(mg/L) 2°C 10°C 2°C 10°C

0.5 300 178 430 254

1 159 94 228 134

To get credit for 99.9% inactivation of Giardia:

Inactivation is a function of _______, __________________, and ___________.

concentrationtimepH temperature

Contact Time DefinitionContact Time Definition

The contact time for purposes of chlorine disinfection is defined by the EPA asThe time that it takes for 10% of the mass of a

tracer pulse to arrive at the effluent of the tank Or equivalently, the time it takes for the

effluent concentration to reach 10% of the influent concentration after a tracer is added continuously to the influent

EPA Contact Time CreditEPA Contact Time Credit

* 0.1t at F

Baffling Condition

Baffle Factor (BF)

Extent of Baffles Typical Unit Processes

Unbaffled (CMFR)

0.1 No baffles, agitated basin with low length to width ratio, high inlet and outlet flow velocities

Clearwell, storage tank, no perforated inlet or outlet, inlet or outlet submerged.

Poorly baffled

0.3 Single or multiple unbaffled inlets and outlets, no intrabasin baffles

Many conventional sedimentation basins. Storage tanks with two or three baffles.

Average 0.5 Baffled inlet or outlet with some intrabasin baffles

Some (few) sedimentation basins. Highly baffled storage tanks.

Superior 0.7 Perforated inlet baffles, serpentine or perforated intrabasin baffles, outlet weir or perforated launders

Filters. Contact tanks with serpentine baffling

Perfect (PFR) 1.0 Very high length to width ratio (pipeline flow), perforated inlet, outlet and intrabasin baffles

Sections of pipe ten times longer than their diameter.

The Meaning of Life…for Contact Tanks

The Meaning of Life…for Contact Tanks

Minimally – To meet EPA regulationsBetter – To obtain as high a contact time as

possible with a given tankOr – To build as small a tank as possible

that meets the EPA regulations

Serpentine Chlorine Contact TanksSerpentine Chlorine Contact Tanks

Baffle Factor of________.0.5

Distribution Tank (Honduras)Distribution Tank (Honduras)

How would you model this tank?

Baffle Factor of________.0.1

Characterize a Tank:Tracer Studies

Characterize a Tank:Tracer Studies

Tracers Desirable properties Candidates Measuring techniques

Choosing a tracer concentrationMeasurement range Interferences

Density matching Pulse vs. Step

Reactor Theory: CMFRReactor Theory: CMFR

0.100.00.2

0.40.60.8

1.01.2

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

FE

F

t* at F=0.1

r in

dCC C Q

dt

trt

tr tr

C

C e

* *

*

rt t

ttr tr

CE

C e

*

* *

*

0

t

t tF E dt

r = reactortr = tracert = time

Reactor Theory: Series CMFRReactor Theory: Series CMFR

0.26

0.0

0.2

0.4

0.6

0.8

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

FE

F

t* at F=0.1

0.720.0

0.5

1.0

1.5

2.0

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

FE

F

t* at F=0.1

N=2

1

1 !

NtNNN rt

tr tr

C N t

C N e

*

*

1*

1 !

NtNN

N t

NE t

N e

0.870.0

1.0

2.0

3.0

4.0

5.0

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

FE

F

t* at F=0.1

N=20

N=100

1-D Advective Dispersion Equation1-D Advective Dispersion Equation

0.460.0

0.1

0.2

0.3

0.4

0.5

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

FE

F

t* at F=0.1

*

2*

* *

1exp

t

t PePeE

t t

d

ULPe

D * tU

tL

2

C(x,t) expdd

M x

D tA D t

Pe = 2

Note: This reactor has more dispersion than a series CMFR with N = 2, but it has a longer contact time!

1-D Advective Dispersion Extremes: High Dispersion

1-D Advective Dispersion Extremes: High Dispersion

What is going on here?

Why is the contact time so good?

Why is F so small at 3 residence times?

Hey! That’s not fair!

0.610.00.1

0.10.20.2

0.30.3

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

F

E

F

t* at F=0.1

(Pe = 0.1)

1-D Advective Dispersion Extremes:Low Dispersion

1-D Advective Dispersion Extremes:Low Dispersion

Approaches plug flow!

0.960.0

5.0

10.0

15.0

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

F

E

F

t* at F=0.1

(Pe = 2000)

CMFR in series ≡ Advective Dispersion

CMFR in series ≡ Advective Dispersion

0.840.00.5

1.01.52.0

2.53.0

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

F

E

F

t* at F=0.1

2Pe N

(Pe = 100)

0.820.00.5

1.01.52.0

2.53.0

0.0 1.0 2.0 3.0

t*

E

0

0.2

0.4

0.6

0.8

1

FE

F

t* at F=0.1

(N = 50)

*

2*

* *

1exp

t

t PePeE

t t

*

*

1*

1 !

NtNN

N t

NE t

N e

They both approach plug flow!

Physical Models: How do you build reactors that approach plug flow?

Physical Models: How do you build reactors that approach plug flow?

Many CMFR in seriesHigh Peclet number

Laminar pipe flowTurbulent pipe flowPorous media flow

Eliminating “Dead volume”Requires more mixing!Turbulent pipe flow: Serpentine channelsTurbulent jets: Perforated baffles

A Baffling DesignA Baffling Design

Baffle perforation sizeTurbulent JetsHead loss

Mean circulation patternsSerpentine vs. series CMFR

Risk of dead volumes

Experiment Design OptionsExperiment Design Options

Baffle perforations (pattern, diameter, number)

Head lossJet Reynolds numberReactor flow rateReactor depthPorous mediaPacking material

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