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 pf mixer and the purpose of the mixing. TVM 4145 Vannrenseprosesser TorOve Leiknes

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 TVM 4145 Vannrenseprosesser TorOve Leiknes

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: TVM 4145 Vannrenseprosesser TorOve Leiknes

Designed in relation to the mixing intensity 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 TVM 4145 Vannrenseprosesser TorOve Leiknes

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 TVM 4145 Vannrenseprosesser TorOve Leiknes

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) TVM 4145 Vannrenseprosesser TorOve Leiknes

3. Hydraulic mixers: Static mixers (Kenics): Venturi / in-line: Dosage v V Hydraulic jumps: TVM 4145 Vannrenseprosesser TorOve Leiknes

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 20 30 40 >40 G - value, s-1 1000 900 790 700 Dimensionless number Gt to compare systems TVM 4145 Vannrenseprosesser TorOve Leiknes

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 TVM 4145 Vannrenseprosesser TorOve Leiknes

1. Perikinetic flocculation no repulsive forces no mixing/movement Collisions due to Brownian motion n = nr. particles in et given volume kP= rate constant Smoluchowski: (1916) D = diffusion constant r = effective radius n  and concentration   flocculation rate  Flocculation rate  med mixing TVM 4145 Vannrenseprosesser TorOve Leiknes

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: TVM 4145 Vannrenseprosesser TorOve Leiknes

↔ 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 40-50 15-25 10 40 Ca 30-40 15-20 10 20 FeII Ca 25-35 10-20 10 20 Al med polymer 50-70 30-40 10 30 TVM 4145 Vannrenseprosesser TorOve Leiknes

Flocculation designs: Paddle flocculators Important parameters: fluid velocity paddle velocity paddle dimensions (radius, distance etc.) rate pf revolution TVM 4145 Vannrenseprosesser TorOve Leiknes

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! TVM 4145 Vannrenseprosesser TorOve Leiknes