Flocculation in Flow through Pipes & In-Line Mixers

Flocculation in Flow through Pipes & In-Line Mixers

flocculant_dilution_systemIt is common industrial practice to add polymeric flocculants directly into slurry streams during flow through pipes. However, very little information is available on the specific requirements for such polymer addition systems and their design and operation appear to be largely intuitive.

In-Line Pipe Flocculant Mixer Test

The flocculation tests were performed in a Kenics static mixer. The mixer consists of a series of fixed helical elements surrounded by a tubular housing. The static mixers used were made from either stainless steel or plastic and obtained from the Kurt J. Lesker Company, Clairton, PA and Cole Parmer Instrument Company, Niles, IL, respectively. The mixers are manufactured in modules with standard diameters and lengths. The diameters of the element used were 1.9 cm (L/D =1.5) and 1.27 cm (L/D=1).

The clay suspensions were prepared by dispersing the required weight of solid in the appropriate solution in an 11 liter mixing tank. This suspension was stirred at 460 rpm for 35 minutes after which the tank was placed in an ultrasonic bath for 7 minutes. The suspension was then transferee! to a 52 liter mixing tank, diluted to 3% solids by weight and stirred at 330 rpm for one hour.

flocculation experimental set-up

The flocculation tests were started by pumping the suspension and the polymer solution into the mixer at a designated flowrate. After the system reached a steady state, i.e., after about three mean residence times in the mixer, the flocculated product was collected in a glass cylinder. Initial settling rates were measured directly in the cylinder.

How to dilute flocculant before addition

Investigations of polymer-induced flocculation in agitated tanks have shown that performance depends not only on the type and amount of polymer added but also on mixing conditions: especially mixing time and agitation intensity. While these two can be varied independently in a stirred tank, the agitation intensity in a pipe or static mixer is controlled by the flow velocity which, for a given mixer length, also determines the mixing time.

Apparently the large flocs produced close to the point of polymer addition are slowly degraded as they move along the pipe. Turbidity generally passes through a minimum at some intermediate mixing time suggesting that the removal of small suspended particles occurs somewhat more slowly than the growth of large flocs. Eventually, however, the release of small particles by floe degradation leads to a net increase in turbidity.

flocculation experimental slurry flowrate

flocculation comparison of supernatant turbidities

The results shown in Figures 2-5 suggest that

flocculation effect of polymer dosage

flocculation effect of slurry flowrate

overall performance might be enhanced through multiple addition of the polymer at several locations along the mixer. In this way, floc degradation might be reduced by further polymer addition while sufficient mixing time can be allowed for turbidity reduction.

flocculation polymer additions


flocculation in flow through pipes and in-line mixers