DROP MECHANICS

An understanding of the performance of extraction equipment is furthered by an understanding of what may be going on inside individual drops. With the assumption of transfer of a solute A from a dispersed feed phase into a continuous solvent, as shown in Fig. 13, a concentration profile across the interface would appear to have a discontinuity (Fig. 14). The discontinuity is a consequence of the distribution coefficient, and reflects the general practice of choosing a solvent which has a greater preference for the solute than the feed phase has. If activities instead of concentrations were used, there would be no discontinuity at the interface. 

Transfer of solute across the interface can be assumed to be controlled by what happens through the immobilized films on both sides of the interface. Handles and Baronf have presented generalized correlations for the calculation of the individual inside and outside coefficients for mass transfer across these films. 

Small drops lead to more transfer area and better extraction, but to slower settling and less capacity. Thus, selection of extraction equipment frequently involves a compromise choice balancing efficiency against capacity. 

The terminal velocity of liquid drops is the same as solid spheres when the diameter is small. The drag coefficient versus Reynold s number can be recalculated to provide a diameter-free ordinate versus a velocity-free abscissa to facilitate direct solution, as shown in Fig. 15. With drops, a maximum velocity is attained, and this maximum has been correlated with a parameter based on physical properties of the system. 

The practical sequence of this phenomenon in column extraction is illustrated in Fig. 16. Drops larger than d won t travel any faster, so there is no capacity gain, and they have less specific area, so there will be an efficiency loss. Drops smaller than d will result in more extraction by providing more transfer area and a longer contact time, but at the potential expense of lower capacity. 

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Rotary wheels (see Fig. 14-87 ) Liquid is fed to a rotating surface The velocity that determines drop Mechanical complexity of rotating... 

If the mutual interaction of adsorbed particles is strong enough the surface diffusion can become organized and leads to a formation of islets of adsorbed molecules in the sense of the Volmer-Weber drop mechanism, or to a layer association, parallel to the crystalline surface, according to the Frank-van der Merve s mechanism. Alternatively, it can form a tridimensional organized region (Stranski-Krastanow s mechanism) similar to that of adsorption mixed crystals74). 

Figure 9 Contribution of Each Pressure Drop Mechanism to Total Pressure Drop.

Flow distribution Low pressure drop Mechanical strength... 

Particle size and shape pore diffusion, pressure drop) Mechanical stability abrasion, durability) Thermal stability catalyst lifetime, regenerability) interaction with active components selectivity, bifiinctional catalysts)... 

Formation of emulsions has been reviewed by Walstra (1983). In contrast to solid colloids, there appears to be much less control of the sizes of emulsion drops. Mechanical devices called homogenizCTs can reduce fat globules in milk to sizes where they take much longer to flocculate and cream to the top. Homog-enizers capable of making nearly uniform size drops are beginning to appear. 

A dropper insert is a dropping mechanism in the neck of the bottle (see Fig. 24.13). Many dropper inserts turn out not to meet the requirements of the Ph. Eur. the dropping speed is too high or they don t provide drops at all. The reproducibility of the drop weight is often poor. The drop size appears to depend for example on the angle at which the bottle is held and on the amount of fluid in the bottle. 

A spout cap is a dropping mechanism on top of the neck of the bottle. The most frequently used spout cap is the Zentrop dropper. This consists of a chlorobutyl mbber dropper mounted in a polypropylene screw cap and closed with a small polypropylene cap. In case of incompatibility of the liquid with the chlorobutyl rubber a Zentrop cap with a pol5q)ropylene dropper can be used. 

Khare VP, Greenberg AR, Krantz WB (2004) Investigation of the viscoelastic and transport properties of interfacially polymerized barrier layers using pendant drop mechanical analysis. J Appl Polym Sci 94(2) 558-568... 

The filter medium and element construction employed for specific hydraulic filters is chosen on the basis of filter performance required, acceptable pressure drop, mechanical strength, compatibility with the fluid being used, and other working... 

A further issue is the mechanical stability of both HMDE and MFE, which is in favour of the latter and, for instance, for hydrodynamic measurements with the rotating disc electrode (RDE, see Table 5.1 and reference (11)) or that in improvised shipboard laboratories, the reliable use of an HMDE with its dropping mechanism is hardly imaginable, perhaps, except for some special and purposely constructed automated analysers. ... 

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