Flocculation, orthokinetic

The natural process of bringing particles and polyelectrolytes together by Brownian motion, ie, perikinetic flocculation, often is assisted by orthokinetic flocculation which increases particle coUisions through the motion of the fluid and velocity gradients in the flow. This is the idea behind the use of in-line mixers or paddle-type flocculators in front of some separation equipment like gravity clarifiers. The rate of flocculation in clarifiers is also increased by recycling the floes to increase the rate of particle—particle coUisions through the increase in soUds concentration. 

Orthokinetic flocculation is induced by the motion of the Hquid obtained, for example, by paddle stirring or any other means that produces shear within the suspension. Orthokinetic flocculation leads to exponential growth which is a function of shear rate and particle concentration. Large-scale one-pass clarifiers used in water installations employ orthokinetic flocculators before introducing the suspension into the settling tank (see Water,... 

MUNICIPAL WATER treatment). Scale-up of orthokinetic flocculators, generally in the form of paddle devices, is based on the product of mean velocity gradient and time, for a constant volume concentration of the flocculating particles. 

Fig. 2. Schematic diagram of a rectangular basin clarifier having an orthokinetic flocculator where the feed is mixed with a flocculant.

Studies on orthokinetic flocculation (shear flow dominating over Brownian motion) show a more ambiguous picture. Both rate increases (9,10) and decreases (11,12) compared with orthokinetic coagulation have been observed. Gregory (12) treated polymer adsorption as a collision process and used Smoluchowski theory to predict that the adsorption step may become rate limiting in orthokinetic flocculation. Qualitative evidence to this effect was found for flocculation of polystyrene latex, particle diameter 1.68 pm, in laminar tube flow. Furthermore, pretreatment of half of the latex with polymer resulted in collision efficiencies that were more than twice as high as for coagulation. 

In summary, polymeric flocculants generally increase peri-kinetic flocculation rates compared with perikinetic coagulation rates. This is not necessarily true for orthokinetic flocculation, and experimental results in the literature are seemingly in conflict. Collision rate theory predicts that the polymer adsorption step may become rate limiting in orthokinetic flocculation. The present study was designed to elucidate the relationship between polymer adsorption rates and particle flocculation rates under orthokinetic conditions. 

Procedure. Initially, the flocculation procedure adopted was to add polymer to the clay suspension, stir rapidly for 15 seconds to ensure good mixing and then to flow the treated suspension through a 3 m length of coiled 1 mm diameter tubing. Tube flow is known to be an effective method of applying shear to a suspension and hence promoting orthokinetic flocculation (12). 

Orthoarsenic acid, 3 264—265 Orthoboric acid, 4 242t, 249—255 Orthoclase, hardness in various scales, l 3t Orthocortex, in wool fibers, 11 173 Orthodontics, superelastic and pseudoelastic SMA devices in, 22 350-351 Orthoesters, 10 498 Orthoferrites, 11 56t, 57 Orthogonal matrices, 6 27 ort/io-hydrogen, 13 759, 760—761 Orthokinetic flocculation, 11 631 22 56 Orthokinetic flocculator, 22 59 Orthomyxoviruses, 3 136—137... 

Among the primary collision mechanisms is Brownian flocculation, also termed perikinetic flocculation, which dominates for submicrometer particles at relatively high number densities. The second principal collision mechanism is that of velocity gradient flocculation, also termed orthokinetic flocculation, which dominates for particles of micrometer size and larger. Evidently, the presence of any stabilizer in the solution will reduce the number of particle encounters and subsequent floccing, as discussed in the last section, resulting in slow flocculation. In our discussion we shall separate the transport and stability problems by assuming that the suspension is completely destabilized, so flocculation occurs on encounter rapid flocculation). Our concern here is with the effect of the particle motion alone on the number of encounters between the suspended particles. 

An important effect in Equation (3.34) is that the collision radius enters as Rl and since we have approximated Rc = 2R it becomes 8R consequently this indicates that shearing is very sensitive to particle size. For this reason small particles are rather insensitive to shearing forces, whereas larger particles, for instance with R > 0.5 pm, can often be flocculated by stirring or shaking particularly at electrolyte concentrations close to the ccc. The sensitized coagulation which occurs in the presence of a velocity gradient is known as orthokinetic flocculation. 

In addition to the above mechanisms orthokinetic flocculation may be induced due to the shear produced inside the porous media. This may encourage entrapment as collection efiddmcy usually inqiroves with increase in suspmded particle size. The collection mechanisms are described in greater detail below. 

Oirect Filtration (Contact Flocculation). This is a mixture of filtration and flocculation in which the suspension entering the bed has been flocculated, but the floes have not fiiUy formed prior to bed entry. The technique is applied to suspensions of low concentration of colloidal matter. The shear induced inside the deep bed provides excellent conditions for orthokinetic flocculation. The main advantage of direct filtration is a reduction in the capital cost over the provision of a flocculator and separate vessel for solid-liquid... 

Perikinetic and Orthokinetic Flocculation. Particles in a dispersion are subjected under any circumstances to Brownian motion (the average velocity is distributed according to the Boltzmann exponential law) and hence, it is the absolute barrier height compared to the thermal energy kpT) that determines stability. This means that larger particles are more stable against thermal motion (perikinetic flocculation) than smaller particles. However, of practical importance during heterophase polymerization is the behavior of particles in dependence on the stirrer speed (orthokinetic flocculation), where their velocity depends on the... 

The important conclusion for heterophase polymerization where shear fields are applied is that in the case of electrostatically stabilized particles the tendency to coagulation increases with both increasing stirrer speed and particle size (particle mass). Furthermore, purely electrostatically stabilized particles are more stable in dispersion media of high permittivity (polar liquids or water) against orthokinetic flocculation. 

Gregory points out that if the shear rate, 10 s, is used to describe gentle agitation, the two rates are equal when the particle diameter is around 1 pm but orthokinetic flocculation becomes more significant with larger particles and higher shear rates. 

This is the simple Smoluchowski equation of orthokinetic flocculation, and shows that the rate of flocculation is second-order with respect to concentration, depends linearly on the velocity gradient and is proportional to the third power of the collision radius. Consequently, in these simple terms theory states that the rate of flocculation can be increased by ... 

A fuller theoretical discussion of the orthokinetic flocculation rate equations is given in Ives. Experimental tests of the kinetics of orthokinetic flocculation are described by Ives and Bhole and Ives and A1... 

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). 

Even when the particles are completely destabilized, with no repulsion energy barrier, there is evidence" that orthokinetic collisions will be reduced by particles rolling around one another without aggregating. This will multiply the orthokinetic rate (equation 4.42) by a factor of less than one, which will make the sizes of particles undergoing equal effective rates of perikinetic and orthokinetic flocculation smaller than those hsted above (which assumed that all collisions were permanent and effective). 

With the exception of sedimentation where differential settling velocities of floes create relative motion, hence orthokinetic flocculation, particle flocculators rely on the liquid drag past the particles to create the velocity gradients. 

Compared with the rapid development of new synthetic organic polymer flocculants, the developments in orthokinetic flocculations will be slower, as quite substantial hardware is usually involved. Certain trends which are apparent now may be developed further, or become more widely adopted. 

Tube settler modules are commonplace in setthng tanks, but their basic function is to increase the total plan area for settlement, as required by settling tank theory. However, these tube modules also act as orthokinetic flocculators, mainly in laminar flow, with Reynolds numbers of about 500. 

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