Flocculation in turbulent flow

Polymer Adsorption and Particle Flocculation in Turbulent Flow... 

Thomas, D.G., Transport characteristics of suspensions Part IV. Friction loss of concentrated-flocculated suspensions in turbulent flow, AIChE Journal, 8, pp. 266-71 (1962). 

Stock composition, kinetics of adsorption and hydrodynamic shear dictate the point at which a cationic polymer is added to a papermaking furnish in order to induce flocculation. Flocculation of cellulose fibers in turbulent flow proceeds very rapidly and is completed in less than two seconds.120-123 Flocks form due to charge interactions through a patch-type or a bridging-type mechanism. However, these flocks will be sensitive to shear force and deflocculation and reflocculation might occur. 

In pipes at higher Reynolds numbers, ranging from 2000 to 25 000, and therefore in turbulent flow, Doll has demonstrated the mixing and flocculation conditions in laboratory systems. These employed silica particle suspensions, flocculated with three different cationic polymers, and the turbulent root mean square velocity gradient (G) characterized the influence of flow rate on reaction rate. In West Berlin, a practical pipe flocculator has been used prior to the entry of treated waste water into a sedimentation tank, based on the concepts set out in section 4.10.3, with the pipe set out in a snake-like pattern on the ground surface. 

The standard friction factor chart (Fig. 48.6) can also be used with Romr- Please note that two liquids having similar k and n values in laminar flow may not necessarily behave similarly to one another in turbulent flow, as would be the case if you compared a flocculated clay suspension with a polymer solution. 

It follows directly that a slurry which is flowing horizontally cannot keep its particles in suspension unless the flow is turbulent, or unless it is an extremely stiff flocculated mud, and even in turbulent flow a minimum velocity may be required to prevent the accumulation of a sediment along the bottom of a tube or conduit. Such sedimentation can occur in a reactor blanket vessel in regions where the net velocity is extremely low. 

Continuous flocculation in stirred tanks shows similar trends with regard to agitation/mixing conditions as for batch systems. The overall performance of continuous flocculation processes tends to be inferior to that which can be obtained in batch systems. This is attributed to a closer approach to equilibrium in the continuous case. In-line flocculation, by injection of polymer into turbulent flow in a pipe, is widely practised in industry and offers certain advantages. Multiple addition at several points along the length of a pipe is preferred to single-point addition. 

Miihle K. (1993), Floe stability in laminar and turbulent flow, In Coagulation and flocculation theory and... 

In recent years other colloid systems—such as microemulsions—have been found to exhibit a wide range of structures [81,82]. We can observe spontaneous phase separation, flocculation and formation of complex bicontinuous structures after the formation of these colloidal systems. It is not possible to form a colloidal system, whether in a polymeric matrix, in water, or in an organic solvent, without a supercritical input of energy, which is provided by turbulent flow conditions during the formation of microemulsions or melt fracture conditions [86] during the formation of colloidal systems in polymers. It seems that a general principle for the behaviour of multiphase systems has been found. 

In most practical cases, the flow in flocculators is turbulent, fluctuating rapidly in both position and time. Where the flow may be laminar in tube flocculators or lamella separators or deep bed filters, the velocity gradients are not uniform but parabolic or quasi-parabolic in form. 

The power P can be calculated in terms of the head lost in flow through a pipe, a tube module flocculator, lamella separators or fixed (deep bed) or fluidized bed flocculators, whether in laminar or turbulent flow. It can also be estimated for paddle flocculators and stirred beakers (jar test apparatus) on the basis of paddle drag, or torque on the drive shaft. Such practical cases are dealt with in section 4.10. 

The full-scale analogue of tube flocculation is in flocculation in pipes, although the flow would be turbulent, not laminar this is discussed later in section 4.10.3. It may also be considered that flocculation in inclined tube modules is similar, as these are often in laminar or transitional flow. 

The natural turbulence of flow through a pipe can create velocity gradients leading to flocculation. In the laboratory, small-bore tubes operate in laminar flow (section 4.9.2) with Reynolds numbers less than 2000. In practical pipe flow the situation is turbulent and head loss is given by the Darcy-Weisbach equation 4.66. 

Although Equation 5-29 has been extensively used, it has its own limitations. Measur ing the power exponent " n" in laminar flow tests and then trying to apply it to turbulent flows is asking for trouble, particularly for cases when / < 0.5. Hey wood and Richardson (1978) showed that pumping flocculated clays yielded higher experimental values of friction coefficient than those predicted by Dodge and Metzner (1959), particularly when the value of n had been obtained at low shear stress. 

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