FLASH MIXING

What is flash mixing?

"Flash mixing” is called operations of mixing water to be treated with reagents. The objective pursued is coagulation, which involves the destabilization of colloidal particles by neutralizing their electrical charges.

?Issue:

It is to homogeneously mix the reagents (metal salts of aluminum or iron) in water to be treated. This mixture must be quick since the coagulation time is very short and it must be avoided reagents react and precipitate.

?A very general design parameter, encrypts the average gradient G generated by mechanical agitation between 100 and 1000 s-1

Background:

?The need to generate a high degree of agitation, has led to use impellers that produce a high degree of turbulence such as Rushton turbines, flat blades turbines, etc.

?

Current solution:

Using thin-profile impellers has the advantage of high performance. Following the principle of generating a minimum turbulence sufficient and the recirculation of the entire fluid mass through it, they achieve the same mixing as with a general turbulence of the fluid mass, but with much lower installed power.

Design parameters in flash mixing

?TM mixing time must be equal or less than the retention time of the fluid in the deposit TR????????????????????

??????????????????????????????????????TM ≤ TR

?The TR ranges from 15 seconds to 3 minutes. Based on the assumption that liquids with similar viscosity to water are mixed by passing 3 times through the turbulent zone, where TC is the time for one full circulation, we have:

TM = 3TC?????????????? V: Volume of Fluid

TC = V / Qc????????????Qc: Flow Circulation generated by the agitator.

TR = V/Qa??????????????Qa: Flow of water entering the tank.

TM ≤ TR?????3Tc ≤ TR????????????3V/Qc ≤ V/Qa?????????????

Qc ≥ 3Qa

For diameter ratios employed, the flow rate of circulation generated Qc is about twice the pumping flow impeller rate Qb.

Qc = 2Qb

D/T = 0,28 to 0,4???????D: Impeller diameter??????????T: Tank diameter

Qb ≥ 1,5Qa

Impeller must be downflow or upflow?

The impeller must operate in countercurrent, that generating a flow opposite to the natural water flow in the tank, as shown in figures.

It must be avoided the incoming water flow directly hits the propeller since it will cause disequilibrating efforts that can shorten the life of the agitator. This problem is corrected by baffles as shown in the last figure.