Baffle flocculators

Ni, X., Cosgrove, J.A., Cumming, R.H., Greated, C.A., Murray, K.R. and Norman, P. (2001). Experimental smdy of flocculation of bentonite and alcaligenes eutrophus in a batch oscillatory baffled flocculator. Trans IchemE, Vol. 79, Part A. 

Black Liquor Soap Recovery. Black Hquor soap consists of the sodium salts of the resin and fatty acids with small amounts of unsaponifiables. The soap is most easily separated from the black Hquor by skimming at an intermediate stage, when the black Hquor is evaporated to 25% soHds (7). At this soHds level, the soap rises in the skimmer at a rate of 0.76 m/h. At higher soHds concentrations, the tall oil soap is less soluble, but higher viscosity lowers the soap rise rate and increases the necessary residence times in the soap skimmer beyond 3—4 hours. The time required for soap recovery can be reduced by installing baffles, by the use of chemical flocculants (8,9), and by air injection into the suction side of the soap skimmer feed pump. Soap density is controUed by the rate of air injection. Optimum results (70% skimmer efficiency) are obtained at a soap density of 0.84 kg/L (7 lb/gal). This soap has a minimum residual black Hquor content of 15% (10—12). 

Flocculation units should have multiple compartments and should be equipped with adjustable speed mechanical stirring devices to permit meeting changed conditions. In spite of simplicity and low maintenance, non-mechanical, baffled basins are undesirable because of inflexibility, high head losses, and large space requirements. Mechanical flocculators may consist of rotary, horizontal-shaft reel units as shown in Figure 9. 

Equipment typically consists of concentric circular compartments for mixing, flocculation and settling. Velocity gradients in the mixing and flocculation compartments are developed by turbine pumping within the unit and by velocity dissipation at baffles. For ideal flexibihty it is desirable to independently control the intensity of mixing and sludge scraper drive speed in the different compartments. 

Gao, S., et al. (1998). Experimental investigation of bentonite flocculation in a batch oscillatory baffled column. Separation Science Technol. 33, 14, 2143-2157. 

So for most cases of fluid motion caused by stirring or baffled flow the perikinetic rate dominates for particles less than 1 pm diameter, and the orthokinetic rate dominates for particles larger than 1 pm. For example, at G = 10 s when the particle radius has increased to 5 pm (diameter 10 pm) orthokinetic flocculation will proceed 1000 times faster than perikinetic (equation 4.50). 

Velocities in baffled channels are usually between 0.25 and 0.5 m s , and retention time varies between 10 and 60 min. However, a disadvantage of baffled channels is that their performance varies with flow rate, whereas mechanical flocculators can be operated independently of flow rate. 

Calculate the number of baffles required in a round-the-end baffled channel under average conditions of water velocity, v = 0.3ms, t = 30min (1800 s) and Gt for flow-through flocculation = 5 x 10 ... 

Such pipe flocculators tend to create rather high velocity gradients at practical flow rates and need to be very long to give the required detention times. Therefore it may be useful to utilize an existing pipe as a flocculator, but it is not economic to build one for the purpose, as a concrete channel with baffles would be more compact and cost less. 

At the other extreme, the static mixer, comprising labyrinth baffles in a tubular module, can only provide flocculation with no retention. 

Flocculation oscillatory flow conditions require lower shear rates than stirred tanks to cause a given degree of flocculation. Furthermore, the narrower shear rate range in OBRs enhances control of floe size and morphology. Ni et al. (2001) demonstrated that oscillatory baffled devices could require as httle as 5% of the average shear rate in an equivalent stirred tank to provide the same flocculation rate, in this case for bentonite with Alcaligenes eutrophus. 

Slurry feedpipe 2 Flocculant solution 3 Baffled feedwell 4 Clarification zone 5 Bed (solids)... 

Good mixing and heat transfer can be obtained without internal baffles if marine propellers are mounted at an angle off the vertical. Such a reactor system is described by Pettelkou and Ehrig (8). They have also suggested non-geometric scale-up to preserve heat transfer surface as reactor volume increases. This Bayer patent (8) claims considerable success in the polymerization of chloroprene which is a system quite prone to flocculation, wall polymer formation and excessive reaction rates. 

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