Agitators forces

Diffusion filtration is another contributor to the process of sand filtration. Diffusion in this case is that of Brownian motion obtained by thermal agitation forces. This compliments the mechanism in sand filtration. Diffusion increases the contact probability between the particles themselves as well as between the latter and the filter mass. This effect occurs both in water in motion and in stagnant water, and is quite important in the mechanisms of agglomeration of particles (e.g., flocculation). 

There are very many situations in which well-defined patterns of convection can be established, and analytical expressions for vf derived. Such situations usually involve forced convection, in which the movement of the liquid is determined by rotation, agitation, forced flow over a flat surface, etc. Once the functional form of vf is known, solutions for c as a function of x are sought so that values of the current can be found and compared with those obtained experimentally. 

From this description, it is obvious how agitation and buoyant effects of the soil could speed up this mechanism (in fact, roll up cannot occur at all unless buoyant or agitation forces act on the soil), but soil-removal rate is a kinetic question and will not be pursued further here. 

Agglomerate strength plays an even more fundamental role in that it determines whether or not nucleation and growth can occur at all in an agitated moist powder. To survive and grow, the cohesive forces present in a nucleus which are responsible for its formation must be able to withstand the destructive agitation forces of its environment. Once formed, the final size reached by the agglomerate represents a balance between these same cohesive and destructive forces. 

Internal agitation (forced circulation) performs several functions. In addition to assuring uniform temperature and density in the solution it helps to assure an even flow of solvent around the work parts and rapid removal of the contaminant. This results in a shorter cleaning cycle. In addition, forced circulation assures uniform heating and cooling of critical parts during the process cycle. 

In lead oxide (litharge) production, molten lead is intermittently pumped into a reactor where a series of rakes, mounted on a shaft which turns at 200 rpm, violently agitate the metal. The agitation forces the molten metal to form particles and improves gas-metal reactions. The reactions are exothermic oxidation. Air pulled... 

All of the molecules in a solution are subjected to agitation forces, known as Brownian motion, that tend to make them occupy the maximum amount of available space. A solid that dissolves in a liquid is dispersed throughout the entire volume and is thus uniformly distributed. The Brownian motion of colloidal particles is slower. If they are put into the bottom of a container, they diffuse very slowly through the mass of the liquid. 

Brownian motion disperses the particles. In [15.14], 3c(i) designates the instantaneous displacement of a particle from an original positioa These displacements are governed by the fundamental law of dynamics. The forces applied to the particle are the thermal agitation force of the Brownian motion and the friction force exerted by the flow, which opposes the relative movement of the particle with respect to the fluid. We therefore write ... 

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

Lime Soda. Process. Lime (CaO) reacts with a dilute (10—14%), hot (100°C) soda ash solution in a series of agitated tanks producing caustic and calcium carbonate. Although dilute alkaH solutions increase the conversion, the reaction does not go to completion and, in practice, only about 90% of the stoichiometric amount of lime is added. In this manner the lime is all converted to calcium carbonate and about 10% of the feed alkaH remains. The resulting slurry is sent to a clarifier where the calcium carbonate is removed, then washed to recover the residual alkaH. The clean calcium carbonate is then calcined to lime and recycled while the dilute caustic—soda ash solution is sent to evaporators and concentrated. The concentration process forces precipitation of the residual sodium carbonate from the caustic solution the ash is then removed by centrifugation and recycled. Caustic soda made by this process is comparable to the current electrolytic diaphragm ceU product. 

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

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