1. Mixing and flocculation
TVM 4145 Vannrenseprosesser / unit processes
Mixing and flocculation
Prof. TorOve Leiknes

2. Q
Chemical
Mixers
1. Rapid mixers - for rapid chemical reactions where the reactant needs to be added efficiently
2. Flocculators - when different phases need to be brought into contact (flocculation, aeration etc.)
Design of a mixer is dependent on the type of mixer and the purpose of the mixing.

3. What is mixing? Defined by 3 phenomena: 1. Molecular diffusion
Thermally induced Brownian motion
Functions of the degree of turbulence in the fluid body
(hydraulic induced motion)
2. Eddy diffusion
3. Nonuniform flow
The velocity gradient is proportional to the amount of energy dissipated in the fluid

4. Velocity gradient / Hydraulic gradient:
A power P has to be added to the system to maintain a velocity gradient x z y V
Laminar flow 
(one dimensional)
Shear Force:
Velocity gradient becomes:
Power in a hydraulic system:

5. 1. Rapid mixers (flash mixers)
coagulant
Principle:
Rapid mixing of a small chemical flow into a much larger water flow Q
homogenous distribution
distribution as quickly as possible
Rapid reaction
Mechanical mixers
Pneumatic mixers
Hydraulic mixers
Designed in relation to the mixing intensity

6. mixing = f (geometry, impeller)
1. Mechanical mixers:
mixing = f (geometry, impeller)
Correlations developed for Power input and Reynolds number
Power number
K is a characteristic constant

7. 2. Pneumatic mixers:
Rising bubbles cause circulation of the fluid and induce mixing
Diffuser aerators:
Rate of work is proportional to the weight of the fluid displace by the bubble
Calculating the power on the airflow rate and height gives:
Where: pa = atmospheric pressure (Pa)
Qa = airflow rate (m3/s)
h = height (m)

8. 3. Hydraulic mixers: Static mixers (Kenics): Venturi / in-line:
Dosage v V
Hydraulic jumps:

9. In general: Power input is expressed by the velocity gradient, G
P = Power input, kW
V = volume, m3
 = absolute viscosity, N.s/m2
General:
Time, s >40
G - value, s
Dimensionless number Gt to compare systems

10. 1. Perikinetic flocculation 2. Orthokinetic flocculation
Induce and encourage the formation of aggregates which can be separated from the liquid phase
1. Perikinetic flocculation
2. Orthokinetic flocculation
Destabilizing + movement with collision
Contact between particles and floc formation

11. 1. Perikinetic flocculation
no repulsive forces
no mixing/movement
Collisions due to Brownian motion
n = nr. particles in a given volume
kP= rate constant
Smoluchowski:
(1916)
D = diffusion constant
r = effective radius
n  and concentration   flocculation rate 
Flocculation rate  med mixing

12. 2. Orthokinetic flocculation
Mixing increases collision frequency → floc formation
Related to the velocity gradient G: b z R Ri Rj dz x
for collision
R = Rj + Ri
water past the particle
nr collisions in volume given ni and nj is nr of particles:

13. ↔ Typical values: Design of a flocculation tank:
Flocculation efficiency:
1. Design as close to PFR as possible
2. Mixing intensity should decrease
3. Theoretical retention should be minimum
4. Sludge recycle may be beneficial
5. Coagulant type and use of several ↔
(compare for correlation)
Typical values:
Chemical G-values (sek-1) Retention (min) Al Ca
FeII Ca
Al med polymer

14. Flocculation designs:
Paddle flocculators
Important parameters:
fluid velocity
paddle velocity
paddle dimensions (radius, distance etc.)
rate pf revolution

15. Pebble bed flocculators, Alabama flocculator ……
Pipe flocculators
Given:
Darcy-Weisbach equations:
Baffled flocculators
Defined by channel flow or headloss around the baffles:
Pebble bed flocculators, Alabama flocculator ……
And other innovative ideas!