general question test format: what types of questions should we expect? 7 multiple choice questions...
TRANSCRIPT
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General Question
• Test format: What types of questions should we expect? • 7 multiple choice questions (5 points
each)• Short problems (similar to previous
prelims)
• How far should we be able to extrapolate beyond what is in the notes?• You should understand the
fundamental concepts and be able to talk about those concepts with your friends• Algebra and arithmetic are expected
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Design of the float valve
The float valve has an orifice that restricts the flow of the chemical and that affects the head loss. Different float valves have different sizes of orifices.
2vc orQ A g h
Chemical Feed Tank
h
2or
vc
QA
g h
Why are we concerned with the orifice head loss? Where else is there head loss?Is this a minimum or a maximum orifice diameter?
2
4or
or
DA
4
2or
vc
QD
g h
Why not increase Dh?
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H
20191817161514131211109876543210
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Open channel supercritical flowDrop tube
Dosing tubes
Stock Tank of coagulant
Lever
LFOM
Float
Constant head tank
Float valve
Slider
Purge valves
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H
20191817161514131211109876543210
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Decrease dose
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Increase dose
H
20191817161514131211109876543210
05101520253035404550556065707580859095100
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H
20191817161514131211109876543210
05101520253035404550556065707580859095
100
Half flow
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20191817161514131211109876543210
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Plant off
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Dosing tubes
• What is the purpose of• What is the purpose of the dosing
tubes?• What is the design constraint for
the maximum flow rate in the dosing tubes?• Why does the flow through the
dose controller increase if the plant flow rate increases?
2 2
4L Error
Maxe
h gDQ
K
2 L Error
Maxe
h gV
K
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What are examples of major and minor losses?
• Major: Caused byshear with the solid surface• Pipe walls• Flocculator baffle surfaces
(insignificant)
• Minor: Flow expansions (analogous to pressure drag)• Orifice, elbow, valve, any place where
streamlines are diverging!
VelocityShear (wall on fluid)
2
2e
Vh K
g
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Origin of
• How is this equation used for rapid mix?• How is this equation used for
flocculator design?
• Draw a picture. What are the parameters in each case?
3
Jet JetMax
Jet
V
D
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Orifice Diameter to Obtain Target Mixing (Energy
Dissipation Rate, Kolmogorov Length Scale)
3
2
4RoundJet
JetMax
Jet
QD
D
Orifice vc JetA A Orifice vc JetD D
3
7
1
3
4 1RoundJetOrifice
vcMax
QD
Substitute for DJet and solve for DOrifice
The orifice must be smaller than this to achieve the target energy dissipation rate
3
RoundJet JetMax
Jet
V
D
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1 10 100 10000.1
1
10
100
1000
10000
Turbidity due to clay (NTU)
Ene
rgy
diss
ipat
ion
rate
(m
W/k
g)
Energy dissipation rate for uniform
nanoglob application
• We need an energy dissipation rate of approximately 3 W/kg to ensure uniform application of nanoglobs to colloids!
• Below 10 NTU the mixing in the flocculator should be adequate!
1 10 100 10000
50
100
150
200
Turbidity due to clay (NTU)
Cla
y se
para
tion
dis
tanc
e (μ
m)
13
6Colloid
Separation ColloidColloid
L DC
1 10 100 1000 100000
50
100
150
200
Energy dissipation rate (mW/kg)
Kol
mog
orov
leng
th s
cale
(μ
m)
13 4
KL
3
43
4
6
MinRM
ColloidColloid
Colloid
DC
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How does aggregation occur?
• Coat flocs with nanoglobs of coagulant precipitate to make them sticky, • then provide energy to create
velocity gradients • Collisions!!!!
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Collision time dependence on floc
diameter11
3 2
23
1 6 1
6c
Floc
t
0
21 1 22 33 23
0
1 6
6
Fractal
viscous
D
c Floc
dt
d
0
3
0
FractalD
Floc Floc
d
d
Why does one of them have a (d/do) term and the other doesn’t?
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Successful Collision Models
113 2
23
1 6 1
6c
Floc
t
11 2 39
89
1 6 1
6Floc
c
Floc
dt
1
2234.8
viscousc FlocN t
13
89
25.6
inertialc FlocFloc
N td
viscous inertia
Time for one collision
Number of successful collisions
is fractional surface coverage of colloids with adhesive nanoglobsdFloc is perhaps mean floc size of flocs that are capturing colloids
For completeness we should probably include a correction for hydrodynamic effects that make it difficult for non porous particles to approach closely. This may increase the time for the first few collisions when flocs aren’t very porous
234.8
viscousc FlocN Gt
cc
tN
t Number of collisions is equal
to time over collision time
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When does the first order reaction rate apply to
flocculation?Colloid
Colloidc
dCkC
dN
The big question! What is C?
The bigger question! Why this disappointing result?
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Flocculation Model: Integrating and tracking
residual turbidityColloid
Colloidc
dCkC
dN
Colloidc
Colloid
dCkdN
C
0
Colloid
Colloid
CColloid
cCColloid
dCk dN
C
0
ln Colloidc
Colloid
CkN
C
The change in colloid concentration with respect to the potential for a successful collision is proportional to the colloid concentration (the fraction of the colloids swept up is constant for a given number of collisions)
Integrate from initial colloid concentration to current colloid concentration
Separate variables
Classic first order reaction with number of successful collisions replacing time
1
2234.8
viscousc FlocN t
13
89
25.6
inertialc FlocFloc
N td
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8
3
*2
9
1
3
0
9log( ) 8
8 9Coll
C
C
oapture
id
oag
V
tC W
d
ep
Proposed Turbulent Flocculation
Sedimentation Model (missing phase 1)
Energy dissipation rate
Flocculation time
Fractional surface coverage of colloid by coagulant
Initial floc volume fraction
Sedimentation tank capture velocity
Sedimentation velocity of ???
-log(fraction remaining)
Characteristic colloid size
Lambert W Function
What does the plant operator control? ________
What does the engineer control? __________
1
3t CaptureV
What changes with the raw water? __________𝜙0
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Review
• Why is it that doubling the residence time in a flocculator doesn’t double pC* for the flocculator?• Why does increasing floc
volume fraction decrease the time between collisions?• Which terms in the model are
determined by the flocculator design?
8
3
*2
9
1
3
0
9log( ) 8
8 9Coll
C
C
oapture
id
oag
V
tC W
d
ep
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Reflection Questions
• What are some alternate geometries?• How else could
you generate head loss to create flocs?
0.6S
0.4S
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Design steps:Given H,W, Q, eMax, and y
• Calculate S
• Calculate ae
• Calculate yB, NB, and heTotal
BB
N
1
3B
Max
K QS
H W
For H/S>5For H/S<5
2
2eTotal B B
Vh N K
g
3
4
1
4
1 PlaneJet
VCBaffle Max
QS
W
3
4
2PlaneJet
VCBaffle B
H
K S
2
12 4 6
4B
B
K H
Why only one yB?
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Cool Equations
LHg
1
3
1
6
1
Max
Ultimately I get to pick three values when designing a flocculator.I’m willing to use 40 cm of head loss and would like to provide 35 m2/3 of collision potential. I’ll design an efficient flocculator. What is the required residence time and the maximum energy dissipation rate?
What do you expect for the residence time?
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Cool Equations
1
2
1
6
HL 40cm 35m
2
3
g HL( )
1
3
3
2
124s
2 13 3
LgH
LgH
3
2
1
3LgH
I’m willing to use 40 cm of head loss and would like to provide 35 m2/3 of collision potential. I’ll design an efficient flocculator. What is the required residence time and the maximum energy dissipation rate?
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What is the maximum energy dissipation rate?
Aveg HL
32
mW
kg
Max 2 Ave 63mW
kg
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Reflection Questions
• How does the collision potential in a flocculator change with flow rate?• What is the average
energy dissipation rate in a flocculator?• What is the maximum
energy dissipation rate for small H/S?
12 4 6
4B
B
K H
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A few Reflections
• The energy dissipation rate that we are using in design is the maximum (in the core of the energy dissipation zone right after the expansion)
• This implicitly suggests that the break up of flocs is the most important constraint
• It is also possible that total head loss (average energy dissipation rate) or maintaining the flocs in suspension is more important
max
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A few Design Guidelines for
• High • High head loss• Small flocs• Keeps flocs in suspension• Slightly lower required residence time
• Perhaps goal is to produce flocs that have sedimentation velocities similar to upflow velocity in sedimentation tank for rapid formation of floc blanket
• Tradeoffs without clear guidance (yet)
maxmax
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Reflection Questions
• What are some alternate geometries?• How else could
you generate head loss to create flocs?
0.6S
0.4S
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Estimate Residence Time Directly
• Use the collision potential equation to solve for residence time• Need to know H/S to do
this!
1
3
1
6
1
3
1
6
1
3
1
2
1
1
Max
Max
Max
1
2
1
3Max
Est70m
2
32
10mW
kg
1
3
7.7min
If H/S<5
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Reflection Questions
• What is the relationship between potential energy loss and the average energy dissipation rate in a flocculator?• How did AguaClara get
around the 45 cm limitation?
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Review
• What is the symbol for collision potential?• Approximately how much
collision potential is required for a flocculator?• What is e?• How does the non uniformity
of e influence efficiency of energy use?
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• Two changes to for nonuniformity
• We currently design based on
1
3
Max
1
3Max
8
3
*2
9
1
3
0
9log( ) 8
8 9Coll
C
C
oapture
id
oag
V
tC W
d
ep
1
3t
1
3t
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What controls ɛ in a flocculator flocculator?
• What is ɛ controlled by?• What change in geometry is required
to change ɛ?• Flocculator design!• What controls collision potential?
• What controls energy dissipation rate?
Energy dissipation rate, residence time, number of baffles
3
Jet JetMax
Jet
V
D
Baffle spacing
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Flocculator Efficiency
H/S = 4
H/S = 10
Can you clarify the H/S ratio limit?
Why does a transition happen at H/S=5?
Transition marks the geometry where the jet has the opportunity to fully expand before going around the next bend
Energy dissipation zone limited by HEnergy dissipation zone limited by S